Microsystems Technology: Fabrication, Test & Reliability

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Microsystems Technology

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Microsystems Technology Fabrication, Test & Reliability

edited by Jumana Boussey

KOGAN PAGE SCIENCE

London and Sterling, VA

First published in France in 2002 by Hermes Science Publications entitled 'Microsystem Technology' First published in Great Britain and the United States in 2003 by Kogan Page Science, an imprint of Kogan Page Limited Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, storec1 or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licences issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned addresses: 120 Pentonville Road London N1 9JN UK www.koganpagescience.com

22883 Quicksilver Drive Sterling VA 20166-2012 USA

© Lavoisier, 2002 © Kogan Page Limited, 2003 The right of Jumana Boussey to be identified as the editor of this work has been asserted by her in accordance with the Copyright, Designs and Patents Act 1988. ISBN 1 9039 9647 3

British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library.

Library of Congress Cataloging-in-Publication Data Microsystems technology: fabrication, test and reliability / edited by Jumana Boussey. p. cm. ISBN 1-903996-47-3 1. Microelectromechanical systems. I. Boussey, Jumana. TK7875.M58 2003 621.381-dc22

2003015993

Typeset by Kogan Page Printed and bound in Great Britain by Biddies Ltd, Guildford and King's Lynn www.biddies.co.uk

Contents Foreword Jumana Boussey 1. From Microelectronics to Microtechnology Robert Aigner

vii

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2. An Overview of Surface Micromachining Dominique Collard, Hiroyuki Fujita, Hiroshi Toshiyoshi, Bernard Legrand and Lionel Buchaillot

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3. Bulk Micromachining and MEMS Packaging Masayoshi Esashi

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4. Electrostatic Micro-actuators Dominique Collard, Hiroyuki Fujita, Hiroshi Toshiyoshi, Bernard Legrand and Lionel Buchaillot

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5. The LIGA Microfabrication Technique Wolfgang Menz and Jürgen Mohr

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6. A Review of Wafer Bonding Stefan Bengtsson

147

7. Single-crystal Silicon Micro-opto-electro-mechanical Devices Tarik Bourouina, Philippe Helin and Olivier Francais

173

8. Permanent Magnets for MAGMAS Orphee Cugat

195

9. RF MEMS for the Mobile Communications Era: Present and Future Adrian M. Ionescu 10. From Microelectronics to Integrated Microsystems Testing Salvador Mir and Benoit Chariot

213

241

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11. Reliability and Failure Analysis Issues in MEMS Ingrid De Wolf

263

12. CoventorWare™ MEMS Design Methodology Christian Dupiller

281

Index

293

Foreword Since the late nineties, microsystems technologies (MST) have evolved from research subjects into relatively mature industrial applications. The high added value provided by the embedded microsensors and/or microactuators in industrial products, e.g. airbags and inkjet printer heads, is undoubtedly a very challenging motivation for R&D actors. Additionally, the use of integrated microsystems reduces their unit cost, improves their reliability and multiplies their functionalities. To some extent, the use of microsystems is believed to have become the only solution for some of today's problems (a solution that we cannot yet imagine, at a scale we cannot see). But MST industry is unique, in that it encompasses various microfabrication techniques and requires multiple types of expertise, from specific design to custom packaging via a large number of collective fabrication procedures. This publication assembles selected papers derived from three different modules: The first gives a review of the more currently used techniques in microsystems fabs. After an interesting comparison between microelectronics and microsystems technologies, surface and bulk micromachining techniques are addressed and illustrated by several concrete examples including sensors and electrostatically actuated microactuators. The LIGA microfabrication technique is reviewed with emphasis on relevant examples especially for microfluidic applications. Finally, wafer assembly techniques by wafer bonding methods is exposed in the case of bare and microstructured wafers. The second module was conceived to assess all the above mentioned techniques by case studies dealing with the major applications on the market (RF-MEMS, MOEMS, Magnetics microsystems). The third part focuses on packaging, testing, reliability and failure analysis techniques. We hope that this volume will be a reference for the microsystems scientific community and will modestly contribute to promoting this dynamic domain. The guest editors are pleased to acknowledge the Ministere de la Jeunesse, de 1'Education nationale et de la Recherche for its financial funding through the Universite Europeenne d'Ete 2002. Jumana Boussey MIGAS, IMEP-INPG Grenoble, France


Chapter 1

From Microelectronics to Microtechnology Robert Aigner Infineon Technologies, Munich, Germany

1. Introduction A large number of MST/MEMS1-based products for automotive application have successfully entered the market during the past decade. Pressure, acceleration and mass-flow sensors are offered by many suppliers. Yaw-rate sensors for roll-over detection, vehicle dynamics control and navigation systems are expected to become the next big opportunity for automotive MEMS. InkJet printers have become the biggest MEMS application in terms of unit sales. In optical networks MEMS have become a key technology and enabler for broadband optical data transmission. In contrast to these relatively mature markets the applications of MST/MEMS in the fields of mobile communications and bioMEMS are just emerging. The number of mobile phones produced in 2002 is expected to be around 400 Million units. The mobile phone manufacturers face an ongoing pressure to miniaturize and reduce costs. For these reasons they consider MST/MEMS as a possible solution. Examples include silicon microphones, RF-switches, resonators and passive RF-components. bioMEMS have attracted a lot of attention during the past years specially in the USA, and many approaches are under evaluation. MST/MEMS components for automotive and communication applications have in common that they are usually manufactured using methods closely related to semiconductor processing. This approach has proven to be the most cost efficient in large volume production. All big MST/MEMS suppliers have a solid background in semiconductor manufacturing and many of them have converted former IC fabs into MEMS fabs.

1.1. Materials used in microtechnologies The most prominent materials used in MEMS are Naturally inherited from IC manufacturing: Mono-crystalline and poly-crystalline silicon; metals such as aluminium, tungsten, copper (and in some cases gold); dielectric layers such as 1. MST = Microsystem Technology, EMS = Micro Electro Mechanical System.

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silicon oxide, silicon nitride; polymers such as imide and epoxy. The introduction of new materials is a real challenge because very often there is no reliable set of material parameters available and reliability is uncertain. Contamination of other processes is also a big concern. Deposition equipment does often not fulfil semiconductor standards. At least 95% of all MST/MEMS components on the market today use only the very traditional and conventional materials mentioned above. These materials are well understood in terms of material parameters and reliability issues. Instead of introducing new materials, the manufacturers have been quite creative in modifying the processing sequence in order to construct mechanical structures like membranes, beams, springs and plates with the desired functions. Some future applications will need new materials, even though the introduction will be a slow and evolutionary process. Methods to characterize material parameters will be reviewed in Chapter 2.

1.2. Processes used in microelectronics and in microtechnologies Lithography, etching and patch-processing are the basis of all mainstream MEMS technologies. (Only a few special applications rely on laser machining, ionbeam machining or conventional micro-fabrication methods; these devices are typically only found in low-volume applications and niche markets.) The concept of building MEMS on wafers yields a high reproducibility and tight tolerances at lowest possible cost. The only significant drawback of using a similar approach to microelectronics (for example CMOS) is that large manufacturing volume is required to justify capital costs and cost-of-ownership for equipment and infrastructure. Regarding infrastructure, it should be mentioned that MEMS manufacturing requires a clean room. As structures are generally larger in MEMS than in most advanced CMOS processes the clean-room rule (particle specification) can be somewhat relaxed. While CMOS processes follow an aggressive shrink path in order to increase performance and reduce costs the MEMS community is less interested in the smallest feature sizes. The reason can be found in the fact that the scaling of mechanical behavior does not always favor the smallest possible solution. On the other hand many parameters that never were in the focus of microelectronics are of substantial importance for MEMS. A good example of such a parameter is stress and stress gradient in layers. Freestanding beams and cantilevers made from polysilicon without special precaution will deform as a result of stress gradients. The same polySi layer would most probably not cause any problem in a CMOS process. Mechanical parameters, including fracture strength and fatigue, are not well controlled in CMOS but they determine if a MEMS structure will work at all and if variations in this parameter may reduce yield to zero. Regarding lithography, MEMS sometimes need thicker photoresist layers in order to extend the etching time or to overcoat larger topology steps. Thicker resist layers or deeper etching will in general limit the minimum feature size of structures to values which are easily

From Microelectronics to Microtechnology

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accessible by traditional optical lithography. Many MEMS devices on the market today work with minimum feature sizes between 0.8 and 2 fim, which was state-ofthe-art in microelectronics around 1985. The same is true regarding wafer size; the majority of MEMS manufactures are today using 6 inch wafers, many still use 4 inch. There is no clear trend to go for 8 inch except in those cases where MEMS processes are made on top of prefabricated CMOS wafers from 8 inch 1C fabs. As lithography is also relaxed in MEMS it makes a lot of sense to fill old 1C fabs with MEMS products. Examples for this approach can be found around the world: Analog Devices (USA), Texas Instruments (USA), Motorola (USA), APM (Taiwan), Infineon (D), AustriaMicrosystems (A). Even though many pieces of equipment can be reused to manufacture MEMS there is still a lot of expertise and investment required to start a MEMS fab. MEMS processes can be divided into two fundamental techniques: 1.2.1. Bulk-micromachining The mechanical structures are made out of the substrate "bulk" wafer by etching from the front and/or backside of the wafer. Usually this is done by wet or dry etching of monocrystalline silicon. The structures can be made mechanically robust and the processes required are relatively simple. There are limits in functionality (because handling of wafers with holes in it is troublesome) and drawbacks regarding volume manufacturing on standard equipment. 1.2.2. Surface-micromachining The mechanical structures are made out of thin films which have been deposited on top of a so called "sacrificial layer". After structuring the mechanical layer the sacrificial layer is removed by a highly selective underetch. The result is a moveable structure that is anchored to the substrate only at certain points. This method works well together with CMOS processes and equipment because at least the wafer backside is as usual. The most prominent pair of materials used for surface micromachining is polysilicon as structural layer and silicon oxide as sacrificial layer. The release etch can be done with excellent selectivity in hydrofluoric acid based etches. Both materials are also widely used in mainstream microelectronics. The most critical issues in surface micromachining are stress in the layers to be used and the so called stiction problem. Stiction is a risk whenever MEMS structures dry up after they have been wetted. Capillary forces can be much stronger than the mechanical restoring force and the result can be permanent "bonding" of structures that leads to mechanical malfunction. Depending on the method for sacrificial layer etching the stiction problem can already occur during the manufacturing process. MEMS devices that are in direct contact with humid air must deal with "in use stiction" which can occur when moisture condenses at the surface. There are several methods to fix the stiction

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problem: During manufacture one can use dry etching to remove the sacrificial layer, thus avoiding wetting of the structure. Dry etching does unfortunately not work on all kinds of sacrificial layers and is usually quite expensive. Or one can replace the etch or solvent using liquid (supercritical) CO2 in a pressure vessel; CO2 can be brought into gaseous form without occurrence of capillary forces. Another method is to use freeze drying or to use temporary mechanical fixtures made of polymer to avoid stiction. None of the methods mentioned above really helps against "in-use stiction" in devices such as silicon microphones. Advanced MEMS processes use coating layers that will make the surfaces extremely hydrophobic or they use extremely small needle-shaped "dimples" in order to fix the stiction problem. The effect of the "dimples" is simply to reduce the area of potential touching by several orders of magnitude which will reduce the required restoring forces accordingly. Stress and stress gradients in thin-film layers are a real challenge for surface micromachining. Stress can have different causes which are relevant to MEMS: - the layer is deposited at elevated temperatures and this layer has a different thermal expansion coefficient from the substrate. A built-in stress is the result. - grain structure and growth in polycrystalline layers causes stress. This is relevant to polySi and most metals. Grains can grow and change the stress even at later stages of the processing. Grain structure is one of the main reasons for stress gradients. Stress in polySi can be adjusted to sufficiently low values by varying the doping concentration/type and the deposition temperature and curing profile that follows the deposition. Stress control in metals can be a very delicate business because recrystallization and change of stress behavior starts already at moderate temperatures. The recipes to make low-stress layers are usually kept secret. In general it can be stated that excessive curing after deposition helps. But in many processes there is a strict limit as to how long the curing may take and at which temperature it may be carried out in order to keep the device alive. Both bulk and surface micromachining technologies co-exist on the market and have their specific advantages [6]. While it is relatively simple and cheap to process wafers in bulk micromachining, the back-end and packaging is often expensive and there is limited potential for miniaturization. On the other hand surface micromachining gives more freedom to create complicated structures and it allows extreme miniaturization and full system level integration. The development and equipment is usually several times as expensive as for bulk micromachining. In applications where size is not as important as cost we will have both technologies present for a long time. It is remarkable that some manufacturers use combined processes [3]. BioMEMS which need microfluidic channels could be made efficiently using combined processes too. RF-MEMS will most likely be made mainly with surface micromachining or related processes.


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1.3. Wafer bonding Another speciality of some MEMS processes is the need to bond wafers together. MEMS have definitely pioneered wafer-bonding technologies which now migrate also into pure microelectronic applications and into the field of packaging. Both for bulk and surface micromachining wafer bonding is used mainly to encapsulate the mechanical structures and thus protect them from environmental influences, but in some cases each of the wafers to be bonded together contribute to the mechanical function (for example in micro-valves). In bulk-micromachined pressure sensors a bonding process is used to generate a reference pressure at the backside of the sensitive membrane. Special equipment for aligning the wafers of a bond-pairs to each other and for the bonding process itself is required. There are many different methods to bond wafers together which have significant differences in complexity temperature required, strength of the bond, flatness requirement for the wafers and achievable hermeticity. The mostly used processes are: a) Gold eutectic bonding: moderate temperature (400°C) is sufficient. Not applicable to devices which could suffer from gold contamination. b) Silicon fusion bonding and anodic bonding: requires very high temperatures which is prohibitive for wafers with metal-interconnects. c) Glass-frit bonding or gluing: applicable to devices with less stringent specs for hermeticity d) Surface activated bonding: Very attractive method because it is practically a cold process but still yields very strong bonds. The requirements for local and global flatness of the bond-surfaces usually need pre-treatment of the wafers by chemical mechanical polishing (CMP). This can be difficult for some types of MEMS structures unless sacrificial layer etching is done after CMP. When using wafer bonding just to protect surface micromachined structures during assembly and packaging the chip area consumed for the bonding ring around the structures is often larger than the structures itself.

1.4. Deep silicon trench etching Some types of MEMS (for example lateral actuators) rely on high aspect ratio structures. Those structures require deep etching with almost vertical side walls. Wet etching cannot fulfil this requirements. Plasma etching is widely used in the manufacturing of microelectronics. Standard equipment is optimized with respect to lowest possible particle generation, uniformity of etch-rate and selectivity. In contrast, an ideal tool for etching high aspect ratio MEMS structures needs very high etch rate and aims for smooth and vertical side-walls. A lot of progress has been made to provide the MEMS community with equipment and processes that are well suited for this purpose. The famous "Bosch-process" uses a modulated gas composition to get a suitable compromise of fast etching into vertical direction while

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maintaining an etch protection for the side wall. An alternative approach is to use a cryogenic-process in which the wafer is kept at a very low temperature and thus the scattering of the incoming ion-bombardment gets drastically reduced. Etching rates of 10μm/min are possible but if the side walls really need to be vertical and smooth the state-of-the-art today is closer to 3 μm/min. Fast processes typically generate sloped side walls or suffer from large erosion of photo-resist. Difficulties arise from the fact that wider trenches etch faster than smaller trenches and etching rates depend on the percentage of open area to be etched away. This usually results in strict design rules for the structures.

1.5. Monolithic versus hybrid integration There is one question that comes up in any principal discussion about how to implement MEMS into a system: "To integrate or not to integrate". What is meant is the difficult decision to make a monolithic (one-chip) solution or to go for a hybrid (multi-chip) approach. The problem is that there is no definite and general answer to this question. Depending on following questions/criteria a decision should be made: - How important are costs in the application (usually very important)? Or is performance and size the driver? High performance devices (for example digital mirror devices from Texas Instruments) sometimes would not work in hybrid integration or would be too large. Monolithic integration requires high volume applications (or if this is not fulfilled a customer who does not care about the price); - Complexity of the process(es) and possible synergies: It is in general true that two very complex processes (for example BiCMOS + RF-Switch) should not be made on top of each other unless there are very strong synergies in the processes; - Yield of combined process versus yield of hybrid assembly process: combining processes on one wafer means multiplying the respective yields. This can turn out to be a killer for monolithic integration. On the other hand yield losses at an early stage of processing are less expensive than yield losses during assembly. For hybrid assembly the "known good die (KGD)" issue is very important; for some two-chip MEMS-products a test of the MEMS chip is very difficult; - Chip size ratio: If a small MEMS structure that requires a complicated process is monolithically integrated into a simple CMOS process then the cost for this combined chip can be a lot more expensive than the sum of the chips for a two-chipsolution. If the size ratio is other way round then the additional cost can easily be worth the savings in assembly costs; - Flexibility is clearly an argument supporting hybrid integration. CMOS processes advance very quickly from one generation to the next one. It will be very expensive to start the embedding of the MEMS process again and again.


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2. Characterization of materials 2.1. Fracture strength A thermal actuator for the determination of polysilicon fracture strength and investigation of its long-term stability is briefly reviewed here. The actuator consists of two narrow beams, which expand due to electrical heating, and a cold plate to which a short tensile fracture beam is attached. The structure is very small (300μm long), and can be characterized purely by electrical testing. The method is suitable for tensile and compressive materials, and allows the integration of the test structure for on-wafer tests in process control. Using a finite element solver, a model for the temperature and stress distribution has been established. Tensile fracture strength for polySi has been found to be (3.1 ± 0.4) GPa. Long-term investigations have not yet shown any relevant mechanical ageing effects on polySi. Fracture strength and long-term stability are crucial quantities which directly affect the reliability of MEMS. Polysilicon layers are widely used for building MEMS devices, but so far there has been no possibility to investigate both parameters simultaneously using one single microstructure tested by purely electrical measurements. In certain cases, such as e.g., materials with tensile prestress the fracture strength can be extracted from long microstructures that will break when exceeding a certain length. Other methods use macroscopic set-ups with large structures with a size of several millimeters. But process control requires small test structures, which can be placed on chip close to the sensor and actuator elements of a MEMS product and which have purely electrical readout. Therefore we developed a novel actuator which exhibits the following features: - Dedicated to the determination of fracture strength; - Suitable for investigations of long-term stability; - Only electrical testing required; - Low power consumption; - Suitable for tensile and compressive polysilicon; - Very small dimensions; - High reproducibility and; - Suitable for on-chip integration and hence, process control; Concept for a thermal actuator: Using an electrothermally heated U-shaped structure the requirements can be met with a small actuator as illustrated in Figure 1. In order to generate sufficient stress for fracture we use long electrothermally heated beams and a short fracture beam. While the heated beams get hot, the temperature in the fracture beam has to remain

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constant. This is achieved by using long and narrow heated beams and a short and wide plate to which the fracture beam is attached. The thermal beams are heated up by conducting electric current through pads. The short fracture beam is also connected to a separate pad to detect breakage. Using the finite element method to simulate, the temperature distribution across the actuator can be calculated as shown in Figure 2.

Figure 1. Thermal actuator for measurement of fracture strength

Figure 2. Temperature distribution on thermal actuator as calculated by FEM

Electrical current through heating beam [A] Figure 3. Fracture test for several beam length


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Deflection can be measured using a microscope using mechanical amplification by a lever as illustrated in Figure 4. From this one can determine the fracture strength accurately enough. More details about this method are given in [16].

Figure 4. SEM view of the thermal actuator used for fracture strength measurement. Insert: mechanical amplification by a lever

2.2. Long-term stability of standard CMOS materials An extensive characterization of standard CMOS materials for surfacemicromachined applications after deposition, during conditioning and under accelerating conditions such as high temperature and humidity was carried out. We used several measuring procedures, techniques and structures to determine, compare and analyze the parameters. We investigate P- and B-doped polysilicon, silan- and TEOS-plasmaoxide, BPSG, plasmanitride and aluminum. Storing BPSG, silan- and TEOS-plasma-oxide layers under humid conditions leads to a significant exponential increase in compressive stress. Typical methods used to characterize material parameters for MEMS applications: - waferbow-measurement (to determine stress) - surface acoustic wave characterization (to measure Young's Modulus) - membrane buckling measurements (stress and Young's Modulus) These methods are schematically shown in following diagram:

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Young's modulus [GPa] Figure 5. Mechanical material parameters for several commonly used materials in CMOS

We revealed during our studies that oxides undergo serious changes in stress when stored in an humid environment. This can cause strong drift in MEMS if oxides are of mechanical relevance in a structure and no means of protection is used. In Figure 6, stress evolution versus storing time, under several storage environments for three types of oxides is given. For more detailed result we refer to [17].


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Figure 6. Change of stress in humid environment

3. Application fields for MEMS 3.1. Automotive application fields for MEMS The following overview lists the most important fields for MEMS in present and future cars [1, 2, 3, 4, 5]. 3.1.1. Engine and transmission control MAP (manifold absolute pressure): This "classical" MEMS device in a car was introduced in 1979. It is characterised by large volume production and the existence of many suppliers. bulk micromachined piezoresistive sensors are dominating, surface micromachining gaining market share. MAF (manifold air flow): is gaining importance because MEMS-based solutions enable to measure directional air-flow. Its characterised by medium volume production. Mainly bulk micromachined thermal sensors. BAP (barometric air pressure): is required for precision engine control. Its characterised by medium volume production. Same technology as MAP. Fuel injection: Injection nozzles and valves can be called a MEMS-device only in some cases. Very specialized technology is used for these applications. More important for MEMS is the trend towards gasoline direct injection (GDI) and highpressure diesel systems. They require cheap pressure sensors with a full-scale of 300 bar and 2000 bar respectively. Technology: bulk or surface micromachined, extreme pressures use ceramic types today. Exhaust sensing: MEMS based sensors have up to now failed to compete with conventional oxygen sensors. Breakthrough expected for many years. Challenge: Robustness and costs.

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Engine torque: Beneficial for engine diagnostics and control of automatic transmission. So far no cheap system on market. Future market. Ignition diagnostics: Knock sensors are widely used. Intelligent spark plugs under development. Pressure sensors for combustion chamber technically challenging. Uncertain market for MEMS. 3.1.2. Safety systems Antilock braking: Wheelspeed sensors are used in present systems, mainly magnetic sensors. Hydraulic pressure measurement is emerging. Future systems will rely on signals from low g accelerometers and gyroscopes, system will merge with vehicle dynamics control. Possibly traction measurement in individual tires will become feasible. Front airbag: Accelerometers with typically 50g range are used in present systems. Today the most important single application for MEMS in automotive, production volume approx. 35 M units per year. Dramatic price erosion > 15% per year during the last 5 years. Bulk micromachined sensors are loosing market share because surface micromachined devices are featuring self-test. Side-airbag: Standard in most cars already. Mainstream goes for acceleration sensing, alternatively pressure sensing can be applied (see examples). Seat occupancy detection: Prerequisite for advanced restraint systems to improve timing of belt-pretension and airbags, detection of a person's size and position. Limited production volume today but highly important for the future. Optical systems compete with mechanical solutions (force profile measurement in seats). Vehicle dynamics control: Requires gyroscopes and low g accelerometers for detection of inertial motion. Specs are demanding and price targets are tough, still a highly interesting market. Today no system is on market that meets price targets, but many (if not all) MEMS companies are working on solutions. Role-over detection: Head airbags will protect passengers during role over accidents in future cars. Gyroscopes and low g Accelerometers are required. Same situation as in vehicle dynamics control. Tire pressure monitoring: Emerging market with some challenges. Pressure sensors need to be ultra-low power because of battery lifetime. Pre-crash detection: Passenger restraint systems will benefit from data collected before a crash occurs. Radar sensors compete with optical systems. Cost and performance will be crucial for opening bigger markets. 3.1.3. Navigation and driver information systems GPS support: Gyroscopes have been proposed as supplement for GPS signals if RF reception is bad inside buildings or in urban canyons. Performance of gyroscopes is extremely demanding and it is not yet clear if wheelspeed sensors are a cheap alternative for low-end systems.


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Microdisplays: It is expected that future driver information systems will project pictures and data onto the windshield. Cheap solutions are not yet feasible and many technical problems have to be solved. Night vision systems: First systems are entering the market. Technology is based on large IR-sensor arrays. Extremely demanding technology and very limited market. 3.1.4. Auxiliary systems Air conditioning: Pressure sensors in the 40bar range are used in large numbers for controlling climate compressors. Typically bulk micromachined sensors. Future systems might apply dew-point sensors for fan control. Keyless entry systems: Biometric Identification may become important in automotive applications. Fingertip sensing has a potential to meet automotive requirements best (see examples). Electric power assisted steering (EPAS): Requires precise, reliable and cheap torque and position sensors in the steering column. Optical systems, magnetic systems and strain gauges compete in this application. Highly interesting application for MEMS, but technically demanding. Active noise cancellation: Future application which requires phased array of microphones in proximity to potential noise sources. Some potential for MEMS in this field. Automatic cruise control: Requires the most complex sensors and systems from above. Gyroscopes, accelerometers, radar sensors, camera systems, IR arrays. Unlikely to be in volume production before 2010 for technical and legal reasons. 3.2. Application fields of MEMS in mobile communication 3.2.1. RF-front-end (between antenna and transceiver) RF switches (electrostatic micro-relays) will be used to switch the antenna between different bands and modes with little loss and ultra-low power consumption (see examples). Micromechanical resonators will be used as filters/duplexers and will replace conventional RF filters [13]. High Q inductors: MEMS technology will also be used to improve the performance of integrated inductors. 3.2.2. Man-machine interface Silicon microphones used in phased arrays will enable achievement of better sound quality in noisy environments by adjustable directivity.

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Microdisplays and microcameras have a potential to be incorporated into mobile Phones for video transmission and internet access. Biometric identification devices such as fingertip sensors will be required for ebusiness applications. 3.3. Application fields of MEMS in computer peripherals The dominant application for MEMS today is the inkjet printhead. Hundreds of millions are shipped every year. Another big opportunity for MEMS is the growing demand for high-density magnetic data storage. Advanced hard disk concepts utilize MEMS actuators to write extremely narrow tracks on hard disks and they apply accelerometers for active position control which reduces interference by mechanical vibrations. A big application for MEMS will also be the display and micro-display arena.

3.4. MEMS in optical communication MEMS are considered a key technology for optical telecommunication because they enable to build optical cross connects and an optical switch matrix. In the field of dense wavelength division multiplexing (DWDM) some MEMS-related structures serve as filters. micro machining is also used to build alignment structures for optical fibers.

3.5. Chemical and biological applications of MEMS Many kinds of chemical sensors have been developed based on MEMSprocesses, amongst them sensors for combustible gases and toxic substances. In many cases the MEMS structure serves just as a heater and thermal platform because many of those sensors are operated at elevated temperatures. Some sensors for chemical substances are based on IR spectroscopy. In this cases MEMS can used as spectrometers. The term BioMEMS includes sensing of biological properties (for example DNA) as well as handling of biological substances. For the handling of biological substances microfluidic MEMS structures are highly important.


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4. Examples of MEMS in production and development 4.1. Crash sensor for side airbag application Side impact sensors are more demanding than frontcrash sensors because there is very little crumple zone at the sides of a car. A firing decision must be made quicker (typically within 10ms). Accelerometers in the central unit have serious disadvantages regarding speed. Accelerometers mounted in the impact zone are widely used, even though there have been many reported cases of false deployment. The most demanding thing is to distinguish between a real crash and just a football hitting the car. An alternative approach for side impact sensing is to detect pressure waves inside the door. The signal strength from a pressure sensor is significantly larger and signals are more characteristic for the type of crash occurring. A cheap but reliable pressure sensor is required for this application. Infineon has developed the fully integrated surface micromachined BiCMOS pressure sensor KP100 for this purpose; it is in large volume production since 1998 [9, 13]. Processing of the sensor cells adds less then 10% costs to a standard 0.8 μrn BiCMOS process. Membranes are made from gate poly on top of field oxide (which serves as sacrificial layer). Only standard equipment is used throughout the whole process. The chip features complete digital calibration and compensation and provides output of digital data by a serial interface. The area used for membrane cells is less than 10% of the complete chip area. The package used in this case is an 8-pin open premolded SMT package (D-SOF 8), the chip and bond-wires are covered by a soft silicon gel after bonding. Package outline is (7 x 7 x 3) mm3.

Figure 7. Cross-section of a surface micromachined pressure sensor cell showing compliant and stiff parts of the membrane as well as the air gap

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Figure 8. Pressure sensor process

- starts with a CMOS wafer that is finished up to gate-polySi and poly-poly capacitors; - a MEMS process for etching the air-gap cavity is inserted into the standard process flow; — sealing of the etching holes with doped reflow oxide (BPSG), in the same process used for planarizing the transistor topology; - proceed with the standard CMOS metal processes and passivation; - finalize the pressure sensor cell by defining compliant and stiff regions of the membrane by removing passivation layers.

Figure 9. Membranes of KP100 monolithic BiCMOS pressure sensor for side impact detection (Infineon)


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Figure 10. KP100 monolithic BiCMOS pressures in package size: 7 x 7 x 3 mm3 (Infineon)

The pressure range is 0.6 to 1.3 bar, absolute tolerance is 5% full scale. With advanced digital calibration schemes less than 1 % tolerance can be achieved, which makes this technology suitable for MAP and BAP applications. Higher pressure ranges can be realized with minor changes in processing.

4.2. Silicon microphone As compared to an electret microphone a silicon microphone offers the advantage to be insensitive to temperature shocks which occur for example during SMT-reflow-soldering. There is no drift in sensitivity or degradation. Si microphones can thus be mounted on PCBs without the work-arounds today required for Electrets - an enormous customer benefit. Furthermore Si microphones allow one to build phased arrays of microphones which would enable one to realize adaptive directivity. In noisy environments this is a key technology to achieve good speech quality. Manufacturing of silicon microphones is a very delicate business because very thin ( 400nm, 1mm diameter) and sensitive membranes must be used to achieve sufficient deflection for a given sound pressure. These membranes tend to break if they are handled improperly. Polysilicon is again the material of choice and a combined bulk and surface micromachining process is used [14].

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Figure 11. Schematic cross-section of a silicon microphone

Figure 12. Spidernet-shaped membrane of a silicon microphone, membrance diameter: 1 mm

4.3. RF-MEMS switch RF-switches have gained importance during the past years because most mobile Phones feature multiple frequency bands and thus need switches to connect the antenna to different parts of the transceiver. The European GSM system need an additional antenna-switch which connects the antenna to either the receiver or the transmitter. Conventional switch diodes are used in most systems today but have the serious drawback that they need considerable amount of bias current and show a rather nonlinear behavior. RF-MEMS switches are mechanical micro-relays which do not show nonlinear effects. As they are driven by electrostatic actuation they have practically zero power consumption.


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One of the challenges is to achieve fast (10 μs .. 100 us) switching times with actuation voltages smaller than 10V. Our composite torsional switch features a low mass Polysilicon actuator with miniature metal contacts placed at both ends. The small moving mass enables increased speed without requiring high voltages. The torsional principle enables an electrostatic push-pull operation for the contacts helping again to increase speed and improve reliability. The device is compatible to standard-IC processing [15].

Figure 13-a. Actuator of a torsional microswitch, size of moving plate: 100 μm2, neutral position

Figure 13-b. Switch state 1

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Figure 13-c. Switch state 2 of torsional actuator. Actuation voltage: 10V. Pictures made by interferometric profilometer

4.4. Fingertip sensor for identification The number of stolen cars has been dramatically reduced with the introduction of immobilizer systems during the last years. But keys can get lost or be stolen. Biometric identification systems will protect cars even better and will enable cars to automatically adopt for a specific user's size, seat and mirror position as well as driving habits. Voice recognition or fingertip recognition are feasible approaches. Ultra-compact CMOS devices for fingerprint sensing are on the market for identification purposes from Infineon [7, 8]. These are arrays of 65000 individual capacitive electrodes at the surface of a specially passivated CMOS chip, which measure the distance from electrode to skin at 65000 pixels simultaneously. Typically 80 gray-levels resolution are achieved by an AD converter at each pixel.

Figure 14. Image of fingerlines captured with Infineon's CMOS fingertip sensor. Resolution: 500 dpi


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The sensors are delivered with a software package that runs on DSPs or PCs for feature extraction and pattern recognition. It is important that the system is robust enough to survive scratch and rubbing tests but also ESD air discharge up to 8 kV. Special methods guaranty that fingertip is fake-proof up to a level which cannot be achieved with keys or pin codes.

Figure 15. CMOS fingertip module, size: ( 1 8 x 2 1 x 1.5) mm3

4.5. Airbag accelerometer using a cavity micromachining technology "Cavity-micromachining" is a technology that enables one to ship MEMS products like inertial sensors as flip chips or in any other low cost package. Moveable structures are buried inside cavities below the wafer surface. The protection against mechanical and environmental influences is done with deposited layers on wafer level instead of using wafer bonding. Therefore the chips are much smaller and can be made thin enough to fit into any cheap standard package or module. The technology to be presented here is somewhat related to surface micromachining in terms of materials used (polySi, sacrificial oxides, CMOSmetalizations, ...) and geometric dimension of structures (μm range thickness, 100 .. 500μm length). Concerning the processing there is one significant difference: a roofing layer above the mechanically active layer is formed at a quite early stage of the process. After sacrificial layer etching and after a sealing procedure this hermetic roof will protect moving structures from any mechanical and environmental influence for the rest of the processing, testing, dicing, mounting and also during operation. This approach was named "cavity micromachining" because front and backsides of the processed wafers look exactly the same as ordinary CMOS wafers no holes, no fragile structures, no problem with particles and humidity - the

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moveable structures are buried a few μm below the passivation layers inside cavities. The complete process module "cavity micromachining" was developed in a 6 inch BiCMOS production facility without using any exotic piece of equipment. After fabrication of the BiCMOS devices the micromachining process module is introduced into the process flow by etching a 2 μm deep recess into the silicon wafer. Next step is the deposition of a thin polySi, a sacrificial silicon oxide and a 4 μm thick polySi layer, used for fabricating the mechanical elements. The thin polySi layer, used in the mechanically active region as interconnect and anchor layer is identical to Poly2 of the standard BiCMOS process, typically used for fabrication of poly-poly-capacitors. The first sacrificial oxide (below the layer forming the seismic mass) is structured to define the anchor regions of the sensor element. A 4 μm polySi with optimized stress and stress gradient is structured by trench etching and subsequently refilled with oxide. The aspect ratio of the trenches is 4:1. A third oxide is deposited and structured providing a second sacrificial layer for the "roof polySi layer. The roof is anchored on pillars incorporating the 4 μm layer. Both sacrificial oxides are etched away through small holes in the roof polySi. HF gas etching is applied in order to avoid sticking. The process is carried out in a commercial PTFE chamber under atmospheric pressure. During the etching the formation of a thin water film or even droplets can occur. In order to avoid this effect, pulsed etching at room temperature is used. The holes in the roof polySi are afterwards closed by an optimized sealing process using passivation layers. After cavity sealing the backend processing (contact hole, Metal1, Via, Metal2, Pad) of the standard BiCMOS-process is carried out without any modifications. The thermal budget of active CMOS devices is not violated by additional layer depositions or anneals. Therefore monolithic integration is possible for applications where best performance at smallest size is most important. Monolithic as well as "mechanics only" wafers have been processed.

Figure 16. Schematic cross-section of "cavity micromachined" accelerometer


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Figure 17. SEM-picture of a cleaved wafer showing combfingers, pillars and the roof

As a demonstrator for cavity micromachining a 2-axis capacitive accelerometer has been realized and tested [18]. The demonstrator shows a performance suitable for automotive front airbag systems. The size of the sensor chip is less than 2mm2 per axis. In contrast to devices applying wafer-bonding there is very little area wasted at the edges of the chips and around the bonding pads. Wafers were thinned down to 380μm without any special precautions and diced on standard equipment. Sensor-chips were connected to an ASIC by wire bonding. The ASIC features a switched capacitor sigma-delta converter, digital calibration of sensitivity and offset, temperature compensation, self-test, digital filtering and a bi-directional serial interface. The chips were injection-molded into lead-frame based plastic packages with only 1.5 mm total thickness. During the molding procedure a pressure peak of 100 bar can occur. Even under those severe conditions the roofs above the moveable structures did not show signs of damage as long as a sufficient number of pillars was used (see Figure 17). Potential new applications for inertial sensors in consumer markets, mobile communication and IT-peripherals will require a cost level even lower than automotive suppliers have to face today. Obviously, this is a big challenge for suppliers of MEMS. Cavity micromachining has the potential to reduce the cost of inertial sensors significantly because the chips are much smaller and do not require any capping wafer or package at all. It was demonstrated that wafers can be thinned down using a standard process and that dicing is feasible with high yield. Standard flip chip methods are directly applicable without any modifications. It has also been demonstrated that properly designed roofs with a sufficient number of pillars withstand injection molding into plastic packages at 100 bar without damage. While a two-chip-solution is more flexible to adopt to new applications (and also easier to manufacture) a monolithic integration of the MEMS structure with CMOS circuitry

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will enable us to build accelerometers including digital signal processing into a volume of 1 mm3.

5. Market for microsystems as compared to microelectronics Microsystems are in an early state of maturity, which compares to the maturity of microelectronics in 1985. The NEXUS Market Studies I and II highlight the bright future of microsystems with a steady and aggressive growth for the next years. It should be pointed out that the numbers given in the NEXUS study are not suitable to judge the sales potential of a component supplier or MEMS foundry because the authors evaluated the net price of a finalized product enabled by MST and not only the real MST turnover. In case of the inkjet printheads the numbers given by NEXUS include the ink container and in case of the heart pacemaker the complete implantable unit (not just the accelerometer it contains) was considered as MST related turnover. Other studies come up with market figures that are typically a factor 4 lower. From the hype respectively "gold fever" in optical switches we have seen in year 2000 we can learn one thing: As soon as MST will generate sales that compare to the microelectronic market we have today there will also be ups and downs with dramatic effects for suppliers and their employees. But MST is a prospering growing field in the long term and therefore young engineers should be encouraged to step into this field.

6. References [1] Grace R.H., "The Growing Presence of MEMS and MST in Automotive Applications", Sensors, September 1999, pp. 89-96. [2] Sparks D.R., "Applications of MEMS Technology in Automotive Sensors and Actuators", International Symposium on Micro-mechatronics and Human Science, 1998, pp. 9-15. [3] Marek J., "Microsystems in Automotive Applications", AMAA '98, Berlin, pp. 43-49. [4] Jeromski G., "Long-term Outlook for MEMS: New Applications and Markets", Solid State Technology 42, 1999, 7. [5] Bryzek J., "Converting MEMS Technology into Profits", Proc. SPIE (USA), Micromachining and Microfabrication Process Technology IV, Santa Clara, 21-22 Sept. 1998, Vol 3511. [6] Lang W., "Reflections on the Future of Microsystems", Sensors and Actuators 72 (1999), pp. 1-15. [7] "FingerTip Sensor for Biometric Identification", Hierold C., Aigner R., Proc. Commercialization of Microsystems, July 1999, Dortmund.


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[8] Hierold C., Aigner R., "FingerTip Sensor for Biometric Identification", Proc. Commercialization of Microsystems, July 1999, Dortmund. [9] Kapels H., Oppermann K.G., Steger M., Hierold C., Werner W.M., Timme H.J., "Full Integration of a Pressure-sensor System into a Standard BiCMOS Process", T.Scheiter, Sensors and Actuators A, 67, 1998, pp. 211-214. [11] Kapels H., et.al., "Measuring Fracture Strength and Long-Term Stability of Polysilicon with a Novel Surface-micromachined Actuator", Proc. Eurosensors XIII, Sept. 12-15 1999, p. 189. [12] Kapels H., et.al, "Full Characterization and Long-term Stability of Standard CMOS Materials", Proc. Eurosensors XIII, Sept. 12-15 1999, p. 197. [13] Bever T., Gussmann V., "Pressure Sensor Speed up Airbag Release", Components 4/98, p. 12. [13] Clark T.-C. Nguyen, "Micro-electro-mechanical Devices for Wireless Communications", Proceedings MEMS '98, Heidelberg, January 25-29, 1998, pp. 1-7. [14] Bever T., Aigner R., Burrer C., Dehe A., Draxlmayr D., Fuldner M., Pettenpaul E., Schmitt S., Timme H. J., "BICMOS Compatible Silicon Microphone Packaged as Surface Mount Device", Sensors, EXPO, Conference Paper, 1999. [15] Plotz F., Michaelis S., Fattinger G., Aigner R., Noe R., "Performance and Dynamics of a RF MEMS Switch", Proceedings of Transducers'01, Munich, Germany, June 10-14 2001, 4C1-09P. [16] Kapels H., Urscher J., Aigner R., Sattler R., Wachutka G., Binder J., "Measuring Fracture Strength and Long-term Stability of Polysilicon with a Novel Surfacemicromachined Thermal Actuator by Electrical Wafer-Level-Testing", Proceedings of Eurosensors XIII, Sept. 12-15 1999, The Hague, pp. 379-382. [17] Kapels H., Maier-Schneider D., Schneier R., Hierold C., "Full Characterization and Long-term Stability of Standard CMOS Materials for Integrated Micromechanical Applications", Proceedings of Eurosensors XIII, Sept. 12-15 1999, The Hague, pp. 393396. [18] Aigner R., Oppermann K.G., Kapels H., Kolb S., "Cavity-micromachining Technology: Zero-Package Solution for Inertial Sensors", Proceedings of Transducers '01, Munich, Germany, June 10-14 2001, 1C3-03.


Chapter 2

An Overview of Surface Micromachining Dominique Collard, Hiroyuki Fujita and Hiroshi Toshiyoshi CIRMM, Institute for Industrial Science, University of Tokyo, Japan

Bernard Legrand and Lionel Buchaillot Institut d'Electronique de Microelectronique et de Nanotechnologie, Villeneuve d'Ascq, France

1. Introduction Surface micromachining and the intimately associated electrostatic actuation are two basic key technologies that have convinced a large number of researchers to push microsystems development ahead, resulting in the impact and results we recognise nowadays. Surface micromachining is an efficient and relatively simple way to realise mobile microstructures on the surfaces of support wafers using processing steps that are compatible with silicon integrated circuits technology. These mobile microstructures are either sensor parts or microactuators that lead to microsystems, once associated with control electronic. The basic principle of surface micromachining and electrostatic actuation are depicted in Figure 1. Starting from a substrate, most commonly silicon, 2 layers are successively deposited, namely, the sacrificial layer and the structural layer, Figure l(a). After a photolithography (resist coating, exposure and development), the resist is patterned according to the microstructure geometry. Then, the structural layer is etched and only the area protected by the resist remains, Figure l(b). Finally, the resist is stripped away and the sacrificial layer is partially dissolved to release the microparts from the substrate. In the case of Figure 1, a cantilever is formed that is separated from the substrate by a gap corresponding to the thickness of the sacrificial layer. If a voltage is now applied to this cantilever, the substrate being grounded, attractive electrostatic forces pull the cantilever toward the substrate. This cantilever can be efficiently actuated thanks to the reduced air gap (0.1-3 μm) that produces a strong enough electric field and resulting forces given reasonable bias (10-200 V).

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Figure 1. Principle of surface micromachining and electrostatic actuation

A pioneer work that combined surface micromachining, electrostatic force and motion sensing have been published by Nathanson et al. in 1967. This first microsystem demonstrator, called the resonant gate transistor is presented in Figure 2. The device includes a MIS (metal insulator semiconductor) transistor in which the gate has been released from the substrate and behaves mechanically as a clamped/free cantilever. Under the free edge of this cantilever, an electrode has been incorporated for vertical electrostatic actuation of the gate. The gate motion induces a change of the gate capacitance that is detected by the corresponding current change of the MOS device. For a given actuation voltage, the maximum of the current modulation is detected for the mechanical resonance frequency of the cantilever (gate). All the concepts of microsystems are demonstrated in this resonance gate transistor, namely an actuator, a sensor and an electronic control in an integrated device. In spite of this early achievement, no significant progress immediately followed, mainly because the fabrication process (resist as sacrificial layer and aluminium for gate) was not directly compatible with silicon MOS technology.

Figure 2. Principle of surface micromachining and electrostatic actuation fully illustrated by the resonant gate transistor [NAT67]

An Overview of Surface Micromachining

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This limitation was overcome in 1983 by the work from Howe and Muller [HOW 83]who fabricated polusilicon beams using silicon dioxide as a sacrificial layer. Silicon dioxide is currently used in MOS devices for field isolation and gate oxide and polysilicon is the basic material for transistor gates. The two process steps and the realised clamped/free beams are shown in Figure 3.

Figure 3. Cantilever realised with polysilicon / oxide micromachining

Strongly motivated by the potentialities of surface micromachining and convinced by its process compatibility with microelectronic technologies, a very intensive research activity on microsystems immediately followed. This led to significant new results on micromachining and microactuators presented at Transducers 89, Montreux, Switzerland [TRA 89] including articulated micromechanisms, electrostatic grippers and micromotors, positioning system and sensors [TRI 97]. Starting from this date, many groups initiated work in that field, worldwide. Surface micromachining traditionally produced planar structures laying on the substrate plane, but in 1992, Pister et al. overcame this limitation by introducing microfabricated hinges [PIS 92]. These polysilicon tiny structures permit rotation of microplates out of the surface substrate. The fabrication of 3D structures such as lenses or mirrors became possible, pushing strongly the development of microopto electromechanical systems (MOEMS). In 1997, the automatic assembling of these 3D structures by the integration of microactuators were first demonstrated [FUK 97]. As micromachining and electrostatic actuators technologies became more and more stabilized, process standardisations were possible and, starting in 1998, foundries proposed surface micromachining services such as MUMPS [KOE 01] and SUMMIT [SAN 02]. Two years later, a significant number of start-up companies were created aiming to produce micromachined devices and systems either by proprietary processes or using external services or facilities. Following this historical introduction in which the technological breakthroughs have been pointed out, the following chapters will detail key principles and devices to illustrate surface micromachining. As it is not in the scope of this contribution to provide an extensive overview, the following chapters focus on 5 points. First, basic

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processing steps are presented with some comments on technology tolerances and limitations (Chapter 2) then, the polysilicon thin film-based process (the most popular one) will be detailed by looking to its multilayer processing capabilities (Chapter 3), its ability to build 3D devices (Chapter 4) and its standardisation through industrial foundries (Chapter 5). Finally, the last part will discuss some other material choices (Chapter 6).

2. Basic processes Surface micromachining, briefly introduced in Figure 1, acts to release surface microstructures and make them free to move by dissolving an underneath sacrificial layer acting as a mold. Two basic surface micromachining processes exist as shown in Figure 4. In the case of Figure 4(a), the sacrificial layer, the structural layer and photo-resist are subsequently deposited on the substrate. The microstructure geometry is transferred in the structural layer by standard lithography and etching. After removing the resist, the sacrificial layer is etched in an isotropic way. As the etching process progresses in all directions, the narrow structures (in the surface) are first released from the substrate while the wider ones remain attached as illustrated in Figure l(c). This process requires only one mask (for the definition of the microstructure topology) but the releasing steps have to be time-controlled.

Figure 4. The two basic processes for surface micromachining. a) the one mask process, b) the two mask process

In the two mask processes, Figure 4(b), a first mask opens contact zones through the sacrificial and defines the areas where the microstructures will be anchored to the substrate. The structural layer is then deposited and patterned, according to the second mask. Finally, the resist is stripped away and the sacrificial layer is completely dissolved.


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In these two basic processes, a good etching selectivity is mandatory between the structural and sacrificial layers to avoid the partial etching of the microstructures during the releasing step. The releasing step is generally performed by wet etching in hydrofluoric acid (HF) when silicon dioxide is the sacrificial material or by O2 plasma in the case of resist. With the two masks process, the final release can be performed in a longer time, for the realisation of wide structures. But, mask alignment is needed and, moreover, the deposition of the structural layer has to be conformal (same deposition rate in all directions) to avoid structural thinning around the step edges of the sacrificial layer. Special care is also required to obtain a clean contact surface just prior the structural layer, otherwise the anchor is inefficient and the microstructures can be easily separated from the substrate during the release. In a final stage, the microstructures have to be rinsed and dried. During the drying processes, capillarity forces produced by isolated droplets pull the released microstructures onto the substrate surface and can permanently stick them on the substrate [MAS 97]. To avoid this irreversible phenomenon, several protocols have been proposed such as resist embedding [HIR 93], dry releasing [LOB 88, LEE 97] or supercritical CO2 [MUL 93] drying. Internal mechanical stresses in the released structures have also to be controlled to avoid out-of-plane structure buckling or even structure breakage [FAN 90]. Low stress thin film material requires carefully monitoring of layer deposition parameters [FRE 96], thermal stress relief annealing [GUC 89] or stress compensation techniques in the structure design itself.

3. Multi-layer polysilicon processes When using polysilicon and silicon oxide as structural and sacrificial layers, respectively, the simple processes in Figure 4 can be easily enhanced thanks to the flexibility of the low pressure chemical deposition (LPCVD) of polysilicon and its excellent step coverage and also by the various ways to produce Si02 by either CVD deposition, sputtering or thermal oxidation. The introduction of multiple independent structural layers are possible, electrodes can be integrated for the electrostatic actuation and several layers of sacrificial layers can be patterned to form a mold and optimize the microstructure topologies. The surface micromachining technologies have quickly evolved to prove the feasibility of mobile microelements. The first demonstrations were of passive micromechanisms that were actuated by external means like micromanipulators or air flow.

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Figure 5. SEMphotograph of a microturbine, [MEH 88]

A pioneer device [MEH 88] is a microturbine based on a planar rotor inserted in an air flow channel. A SEM picture of this device is given in Figure 5 after the releasing step. The two inlet and the outlet ports are clearly shown, the rotor made of polysilicon has a diameter of 125 μm and is 4 μm in thickness. When an incoming air flow is applied, the turbine rotates and rotational speeds above 15,000 rpm have been measured. The fabrication process includes two structural levels of polysilicon, the first one for the rotor and the second one for the shaft and the cap. 5 mask levels are needed for the completion of the device. This process is detailed in Figure 6 along the AA' cross section in Figure 5.


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Figure 6. Process flow of the microturbine in Figure 5 [MEH88]. (a) Patterning of the first sacrificial layer. (b) Deposition and etching of the first structural layer. (c) Deposition of the second sacrificial layer. (d) Etching the second sacrificial layer. (d) Deposition and etching of the second structural layer. (f) Final release

In a first step, the silicon is thermally oxidized to produce a 4 μm thick SiO2. This first sacrificial layer is then patterned by two different etches as it is shown in Figure 4(a). With a first mask (M#l), the oxide is completely etched at the location of the flow channel, a second mask (M#2) determines the position of the inner shaft where the oxide is partially etched (2 μm). The depth of this etching step directly determines the height of the dimples under the rotor and so the vertical clearance between the mobile parts and the substrate. The first level of structural polysilicon is then deposited (4.5 μm) and patterned according to mask (M#3), Figure 6(b). A second sacrificial oxide layer is deposited by CVD, its thickness (1.2 μm) determines the lateral clearance between the rotor and the axe of the turbine. This

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oxide is then completely etched locally (M#4) to anchor the turbine axle and the channel vertical walls. The second layer of polysilicon is deposited and patterned (M#5) to form the shaft and cap as well as the channel walls, Figure 4(d). Finally, Figure 4(e), the rotor is released by HF immersion (40 min. in HF:H20 1:1 at room temperature). After drying, the turbine was free to rotate under the force of air jet only. With similar technologies, several other passive micromechanisms were demonstrated like gears, gripper, micorobot arms [FAN 88a] or sensor part on CMOS technology [PAR 88]. Soon after, electrodes were incorporated in the microstructures to actuate beams [PUT 89], micromotors [FAN 88b] or grippers [KIM 90] by electrostatic forces. 4. 3D folded structures In the above mentioned surface micromachined examples, the devices were planar, lying in the substrate plane. For multi-purpose MEMS and specially for optical applications, 3D structures are mandatory. A way to built out-of-plane components is to realize plate structures and to rotate them on a hinge to an upright position. This technique, demonstrated by Pister et al. [PIS 93], in the case of polysilicon micromachining is explained in Figure 7(a).

Figure 7. Hinge technology for 3D folded structures [PIS93J. (a) Simplified process. (b) SEM view of a fabricated hinge. (c) Example of assembled structure compared to the size of an ant


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After the deposition of the first sacrificial layer (2μm thick PSG) the first polysilicon layer is deposited and patterned (2μm thick polysilicon, poly 1) to form the desired structural elements and the hinge pin to rotate it. A second sacrificial layer is deposited to define a clearance for the rotation. Contact to the substrate is open through the two sacrificial layers on both side of the hinge pin. The second polysilicon layer (poly 2) is deposited and patterned, forming a staple to hold the structure to the substrate. Finally, the structures are released by dissolving the two sacrificial layers and are rotated to their final position by probe micromanipulation. A SEM view of this integrated hinge is displayed in Figure 7(b) while Figure 7(c) demonstrates that long structural polysilicon structures (> 1mm) can be assembled by this technique. A micro optical bench realized thanks to the hinge technology is displayed in Figure 8, [LIN 96]. This optical system includes a beam splitter, Fresnel lenses and mirrors and have been successfully applied for a free space optical fiber switch, optical disk pick-up heads and a scanning optical microscope, among others.

Figure 8. Micro optical bench realized thanks to integrated hinge technology [LIN96]

5. Standard processes For several years, polysilicon-based micromachining technologies have been implemented, improved and applied as laboratory owned processes. Except when very specific topologies or active material were mandatory for given applications, it turned out that a given level of standardisation appeared among all of these developments. So, polysilicon micromachining has matured and foundry services were proposed first in USA for research purposes and then with world wide availability on a more commercial basis. The MUMPs (multi-users micromachining processes) service proposes a three layer polysilicon process that was initially developed by MCNC (Microelectronic

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Center North Carolina, USA) from 1992 and derived from BSAC micromotor process [TAI 89].

Figure 9. MUMPS process for a rotating electrostatic micromotor. (a) Cross section at the end of the process. (b) Released structure. (c) SEM view. (d) Description of the mask levels

As shown in Figure 9 (a, b), the MUMPs process includes 2 structural polysilicon layers (poly 1 and poly 2), the first layer (poly 0) is a biasing electrode anchored on a nitride isolation layer. The sacrificial layers are made of phosphosilicate glass (PSG). A set of 8 masks levels with 2 μm rules is available for polysilicon and sacrificial layer patterning, contacts and metallization as detailed in Figure 9(d). An electrostatic micromotor realized with the MUMPs process is displayed in Figure 9(c). More details related to the design rules can be found in [KOE 01]. The MUMPS services have been proposed by Cronos, acquired by JDS Uniphase in 2000 and then distributed through MEMSCAP from 2002. A second well-known service is provided through Sandia National Lab. The Sandia Ultra-planar, Multi-level MEMS Technology (SUMMiT™) Fabrication Process is a four-layer polycrystalline silicon surface micromachining process (one ground plane/electrical interconnect and three mechanical layers). Thanks to the design flexibility provided by this technology, numerous examples of miniature gears and engines have been presented as the actuated mechanism in Figure 10.


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Complete information and many devices realized can be found in [SAN 02]. With 4th the SUMMit V process, a fifth layer of polysilicon ( mechanical layer) has even been added to increase the compactness of some integrated mechanisms. This multilayer process required specific development to minimize the internal mechanical stresses in all the released polysilicon layer. Moreover, chemical mechanical polishing (CMP) was introduced to recover flat surface reference during the processes and to ensure a global 1 μm design rule.

Figure 10. SUMMiT Process from Sandia [SAN02J. (a) Micro-engine and bug. (b) detail of gear mechanism

6. Alternative surface micromachining processes Even if the polysilicon based micromachining is the most widespread MEMS technology due to its high manufacturability, other materials can be successfully used to realize free standing microparts. Nevertheless, to investigate other technologies, at least four requirements have been to be fulfilled: - The couple of materials: sacrificial layer / structural layer have to present a very high etching selectivity to the etchant during the final releasing step, a factor 10 at least is required; - Both sacrificial and structural layers have to demonstrate low (< 10 Mpa) residual stresses along the process flow. Cracks in the sacrificial layer and distorted released structures must be avoided; - A very efficient adherence is mandatory between the structural layers and the substrate. Forces applied to the microstructures during the release such as the meniscus formation, and internal stresses act favourably to dissociate micromechanisms from their support substrate; - structural layer deposition temperature and annealing conditions have to be compatible with the melting point of the sacrificial layer.

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Following this requirement, alternative surface micromachining processes have been proposed. Metallic micromechanisms have been successfully processed and released with resist or polyimide as sacrificial layer. This technology is particularly suitable for relays [GRE 97] or RF switches [PAR 00]. An example is given in Figure 11 [MAJ 97].

Figure 11. Metallic switch realized with surface micromachining. (a) Process flow. (b) Operating mode. (c) SEM view. [MAJ97]

Micro-actuators made from titanium-nickel thin film shape memory alloys (SMA) were successfully fabricated by surface micromachining using chromium [NAK 97] or PMGI polyimide [ROC 01] as sacrificial layer. Thermal microactuator composed of nickel/gold embedded in polyimide were also fabricated, with aluminium as sacrificial material [ATA 93]. Silicon itself can be used as sacrificial layer, suspended structures composed of metallic and dielectric layers can be easily released using dry XeF2 isotropic etching [CHU 97]. By referring to data of etching rates of thin film materials by chemical etchants [WIL 96], more combinations of sacrificial/structural layers can be found, but numerous process tests will be needed to validate the other above mentioned structural requirements (internal stress, adherence, thermal compatibility), and finally the material choice will be drastically reduced.


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7. Summary A short overview has been presented of surface micromachining. It was intended to provide an exhaustive review of all the processes and devices that have been proposed during the last two decades, but to point out a main characteristic or a key issue of surface micromachining : an historical view point to fill the main stream of MEMS development, the basic micromachining steps and general process considerations, the capability to realize multi-layers structures, the way to realize out of plane devices, the now available standard processes and, finally some alternative process. Numerous examples can be found in the associated paper on electrostatic actuation [COL 02], the papers review [TRI 97] and textbooks [SEN 00]. Finally, surface micromachining processes is nowadays considered as a mature technology, numerous start-up companies in MEMS relying on this technology. Moreover, one of the most successful MEMS device, the digital micromirror device (DMD) from Texas Instruments [DMD 01, VAN 98], used as projection system from computer video signals, are fabricated thanks to the micromachining of metal pixels on CMOS integrated circuits. 8. References [ATA 93] ATAKA M., OMODAKA A., TAKESHIMA N., FUJITA H., "Fabrication and operation of polyimide actuators for a ciliary motion system", J. Microelectromech. Syst., Vol. 2, 1993, pp. 146-150. [CHU 97] CHU P.B., CHEN J.T., LIN G., HUANG J.C.P., WARNEKE B.A., PISTER K.S.J., "Controlled pul-etchinh with Xenon Difluride", Tech. Digest. 1997 Int. Conf. Solid-state Sensors and Actuators, Transducers 97, Chicago, 1997, pp. 665-665. [COL 2] COLLARD D., FUJITA H., TOSHIYOSHI H., LEGRAND B., "Electrostatic microactuation", Nano et Microtechnologie, No 1-2/2002, Hermes-Lavoisier. [DMD 01] http:/www.ti.com/dlp/resources/library/ [FAN 88a] FAN L.S., TAI Y.C., MULLER R.S., "Integrated movable micromechanical structure for sensors and actuators", IEEE Trans. Electron. Devices, Vol. ED-35, 1988, pp. 724730. [FAN 88b] FAN L.S. TAI Y.C., MULLER R.S., "IC-processed electrostatic micro-motor", Proceed. IEEE International Electronic Device Meeting, 1988, pp. 666-669. [FAN 90] FAN L.S., MULLER R.S., YUN W., HOWE R.T., HUANG J., "Spiral microstructures for the measurements of average stress gradients in thin films", Proceed. IEEE Micro Electro Mechanical Systems, 1990, pp. 177-181. [FRE 96] FRENCH P.J., VAN DRIEENHUISEN B.P., POENAR D., GOOSEN J.F.L., MALLEE R., SARRO P.M.; WOLFFENBUTTEL R.F., "The development of low stress polysilicon process

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Microsystems Technology compatible with standard device processing", J. Microelectromech. Syst., Vol. 5, 1996, pp. 187-196.

[FUK 97] FUKUTA Y., COLLARD D., AKIYAMA T., YANG E.H., FUJITA H., "Microactuated selfassembling of 3D polysilicon structures with reshaping technology", Proceed. IEEE Micro Electro Mechanical Systems, 1997, pp. 447-481. [GRE 97] GRETILLAT M.A., "Nonlinear electromechanical behavior of an electrostatic microrelay", Int. Conf. Solid-state sensors and Actuators, Transducers 97, Chicago, June 1997, pp. 1145-1148. [GUC 89] GUCKEL H., SNIEGOWSKI J.J., CHRISTENSON T.R., MOHNET S., KELLY T.F., "Fabrication of microelectromechanical devices from polysilicon films with smooth surface", Sensor and Actuator, Vol. 20, 1989, pp. 117-122. [HIR 93] HIRANO T., FURUHATA, FUJITA H., "Dry releasing of electroplated rotational and overhanging structures", Proceed. IEEE Micro Electro Mechanical Systems, 1993, pp. 278-283. [HOW 83] HOWE R.T., MULLER R.S., "Polycristalline silicon micromechanical beams", J. Electrochem.,Vol. 130, 1983, pp. 1420-1423. [KIM 90] KIM C.J., PISANO A.P., MULLER R.S., LIM M.G., "Polysilicon microgrippers", Proceed. IEEE Solid State Sensor and Actuator Workshop, 1990, pp. 48-51. [KOE 01] KOESTER D.A., MAHADEVAN R., HARDY B., MARKUS K.W., Mumps design handbook 7.0, http://www.memsrus.com/mumps.pdf/ [LEE 97] LEE Y.I., PARK K.H., LEE J., LEE C.S., Yoo H.J., KIM C.J., YOON Y.S., "Dry releasing for surface micromachining with HF vapor phase etching", J. Microelectromech. Syst., Vol. 6, 1997, pp. 226-232. [LIN 96] LIN L.Y., SHEN J.L., LEE S.S., Wu M.C., "Realization of novel monolithic free space optical disk pick-up heads by surface micromachining", Optical letters, Vol. 21, 1996, pp.155-157. [LOB 88] LOBER T.A., HOWE, "Surface micromachining for electrostatic microactuator fabrication", Proceed. Solid Workshop, Hilton Head state Sensor and Actuator, 1998. [MAJ 97] MAJUMDER S., MCGRUYER N.E., ZAVRACKY P.M., ADAMS G.G., MORRISON R.H., KRIM J., "Measurement and modeling of surface micromachined, electrostatically actuated microswitchs", Tech. Digest. 1997 Int. Conf. Solid-state Sensors and Actuators, Transducers 97, Chicago, June 1997, pp. 1145-1148. [MAS 97] MASTRANGELO, "surface force induced failures in microelectromechanical systems", Tribology Issues and Opportunities in MEMS, Kluwer Academic Publishers, 1997, pp. 367-396. [MEH 88] MEHREGANY M., GABRIEL K.J., TRIMMER W.S.N., "Integrated fabrication of polysilicon mechanisms", IEEE Trans. Electron. Devices, Vol. ED-35, 1988, pp. 719— 723. [MUL 93] MULHERN G.T., SOANE D.S., HOWE R.T., "Supercritical carbon dioxide drying for microstructures", Int. Conf. Solid-state sensors and Actuators, Transducers 93, Yokohama, 1993, pp. 296-299.


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[NAK 97] NAKAMURA Y., NAKAMURA S., BUCHAILLOT L., FUJITA H., "A three dimensional shape memory alloy loop actuator", Proceed. IEEE Micro Electro Mechanical Systems, 1997, pp. 262-266. [NAT 67] NATHANSON H.C., NEWELL W.E., WICKSTROM R.A., DAVIS J.R, "The resonant gate transistor", IEEE Trans. Electron. Devices, Vol. ED-14, 1967, pp. 117-133. [PAR 88] PARAMESWARAN M., Baltes H.P., Robinson A.M., "Polysilicon microbridge fabrication using standard CMOS technology", Proceed. IEEE Solid State Sensor and Actuator Workshop, 1988, pp. 148-150. [PAR 00] PARK J.Y., KIM G.H., CHUNG K.W., Bu J.U., "Electroplated RF MEMS capacitive switch", Proceed. IEEE Micro Electro Mechanical Systems, 2000, pp. 639-644. [PIS 93] PISTER K.S.J., JUDY M.W., BURGETT S.R., FEARING S.R., "Micro-fabricated hinges", Sensor and Actuator, Vol. A33, 1993, pp. 249-256. [PUT 89] PUTTY M.W., CHANG S.C., HOWE R.T., ROBINSON A.L., WISE K.D., "Process integration for active polysilicon resonant microstructures-fabricated", Sensor and Actuator, Vol. 20, 1989, pp. 143-151. [ROC 01] ROCH I., DELOBEL P., WALLART X., COLLARD D., BUCHAILLOT L., "Shape memory alloy thin film for microsystem", Proceed. 5th France-Japan Congress on Mechatronics, Besancon, France, Oct. 9-11, 2001, pp. 417-422. [SAN 02] Sandia National Laboratories: http://www.mdl.sandia.gov [SEN 00] SENTURIA S.D., Microsystem design, Kluwer Academic Publishers, Boston, 2000. [TAI 89] TAI Y.C., 1FAN L.S., MULLER R.S, "IC-processed micro-motor: design, technology and testing", Proceed. IEEE Micro Electro Mechanical Systems, 1989, pp. 1- 6. [TRA 89] Tech. Digest., 5th Int. Conf. Solid-state Sensors and Actuators Transducers 89 and Eurosensors HI, 1990. [TRI 97] TRIMMER W.S., Micromechanics and MEMS, classic and seminal papers to 1990, IEEE Press, New York, USA, 1997. [VAN 98] VAN KESSEL P.F., HORNBECK L.G., MEIER R.E., DOUGLAS M.R., "A MEMS based projection display", Proc. IEEE, Vol. 86, 1998, pp. 1687-1704. [WIL 96] WILLIAMS K.R., MULLER R.S., "Etch rates for micromachining processing", J. Microelectromech. Syst, Vol. 5, 1996, pp. 256-269.


Chapter 3

Bulk Micromachining and MEMS Packaging Masayoshi Esashi New Industry Creation Hatchery Center (NICHe), Tohoku University, Japan

1. Introduction MEMS (micro electromechanical systems) based on semiconductor microfabrication play important roles for example in the periphery of IT systems. They are value-added but not cost effective in many cases. The reason is that MEMS products are versatile and of small volume. Since the technology is interdisciplinary and needs semiconductor fabrication, open collaboration to reduce development costs and common usage of facilities are needed for cost effective MEMS production. It is said that packaging is 80% of the cost of MEMS devices. Wafer process packaging is effective for the small volume production of MEMS because assembly of each separate chip is not required [1]. Electrical interconnection from the MEMS device in the encapsulated cavity or between MEMS and an integrated circuit can be made using electrical feedthrough in a glass. This can simplify the MEMS fabrication process. In the following, array MEMS with nanostructures, sophisticated MEMS using electrostatic levitation and application of functional materials such as A1N, PZT, SiC, CNT (carbon nano tube) to microsensors, microactuators and micro energy sources will be described.

2. Packaged micro mechanical sensors 2.1. Electrostatically levitated spherical 3-axis accelerometer The spherical 3-axis accelerometer has been developed by Ball Semiconductor Inc, collaborating with Tokimec Inc. and Tohoku Univ.[2]. As shown in Figure 1, a 1mm diameter spherical proof mass (ball) made of silicon is electrostatically levitated inside a shell with closed-loop control. The principle of the levitation using a capacitive displacement sensing and an electrostatic actuation is shown in Figure 2 [3]. The picture in Figure 3 shows the ball before covering with the shell (after bump attachment). The three-dimensional patterning of the electrode was performed on the ball. A narrow (4μm) gap between the silicon ball and the electrodes was made by sacrificial etching of poly-Si through gas permeable shell, utilising XeF2 gas [4].

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Figure 1. Electrostatically levitated spherical 3-axis accelerometer

Figure 2. The principle of the electrostatic levitation

Figure 3. Ball before covering with shell

Bulk Micromachining and MEMS Packaging

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2.2. Electrostatically levitated rotational gyroscope A two-axis rotational gyroscope for navigation control system is shown in Figure 4 [5]. A silicon disk rotor is electrostatically levitated and therefore this can be used as three-axis accelerometer as described above. The levitation is actively controlled by force balancing in all directions. The rotation is based on the principle of a variable capacitance motor. The silicon rotor of diameter 5mm was fabricated using deep RIE and packaged in a vacuum cavity by incorporating a non-evaporable getter in order to prevent viscous dumping [6]. The gaps between the rotor and the electrodes on the glasses are 5μm. The levitated rotation (10,000 rpm) and the function as a gyroscope and an accelerometer were successfully demonstrated.

Figure 4. Electrostatically levitated rotational gyroscope (disk rotor type)

The electrostatically levitated gyroscope using the disk rotor requires a high voltage for suspending the disk laterally. A silicon ring rotor type as shown in Figure 5 has been developed to solve the problem [7]. A 4mm diameter ring rotor is

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electrostatically levitated and rotated at 10,000rpm. The lateral force to the ring is electrostatically force-balanced by the radial electrode (x,y control electrode in Figure 5) to reduce the control voltage. A 5μm radial gap between the ring rotor and electrode were formed using deep RIE (reactive ion etching) and the rotor is held between two glasses which have electrodes (z, 0, φ control electrode in Figure 5). The function as the high performance gyroscope and accelerometer were successfully demonstrated as shown in Figure 6.

3. High density electrical feedthrough for array MEMS 3.1. Electrical feedthrough in glass High density electrical feedthrough in glass is effective for array MEMS and encapsulated sensors. This provides electrical interconnections to the backside of the glass plate. The control circuit and MEMS devices can be fabricated separately and connected by flip chip bonding. The high density electrical feedthrough in the glass can be made by using deep RIE (reactive ion etching) of a Pyrex glass and an electroplating of nickel [8]. Holes of up to 200μm in the Pyrex glass can be made through the thickness. The holes made in a Pyrex glass are filled with nickel by electroplating.


Figure 5. Electrostatically levitated rotational gyroscope (ring rotor type)

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Figure 6. Measurement of angular rate acceleration using ring rotor type electrostatically levitated rotational gyroscope

The other fabrication method of the high density electrical feedthrough in the glass is shown in Figure 7. The glass holes are made by femtosecond laser drilling and the holes are filled by electroplating with copper. The leveling (polishing) process consists of removing the extra electroplated metal by following a 'throughhole auto-filling' technique. The end point of electroplating is detected by impedance monitoring and the electroplating is made by using a bipolar pulse current for dissolving and plating of copper alternately [9].


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Figure 7. High density electrical feedthrough made by femtosecond laser drilling of glass and Cu electroplating using through-hole auto-filling technique

3.2. Multiprobe data storage Multiprobe data storage has been developed using the high density electrical feedthrough. A nanoprobe which has a heater with a 30 nm metal junction was fabricated at the apex of a SiO2 tip as shown in Figure 8 [10]. Not only high spatial resolution but also quick response can be achieved owing to the extremely small tipsize. Thermal processing for writing and electrical resistance measurement for reading was adopted for the multiprobe data storage [11]. The structures of the multiprobe data storage system and the probe are shown in Figure 9. The electrical feedthrough is used to make an electrical connection from each probe to the

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backside IC chip. This thermal nanoprobe could be successfully used for recording data to the GeSbTe phase change media which is used for DVD RAM. The 32x32 probe array and the conductance image of the recorded pattern are shown in Figure 10. The conductance modification of approximately ten times was caused by the phase change. Owing to the small (30nm) tip-size the bit density can be Tbit/inch2 order which is about 100 times higher than the conventional data storage. The thermal response time of the nanoheater was 18us. Rewritable multiprobe data storage using the phase change media can performed by making the thermal response of the probe quicker. The recording could be made on the ferroelectric recording media as PZT as well using the nanoprobe [12].

Figure 8. Fabrication process and a photograph of nanoheater


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Figure 9. Structures of the multiprobe data storage system and a probe

Figure 10. 32 x 32 probe array and conductance image of the recorded bits on thin GeSbTe0

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To prevent wear of the contacting probes nanoprobe was fabricated using doped diamond [13]. The picture of the diamond nanoprobe and thermomechanically processed (recorded) pattern on a polymer (PMMA) are shown in Figure 11. The thermal response time of the probe was quick (0.2us) because of the large thermal conductivity of the diamond.

Figure 11. Diamond nanoprobe and thermomechanically recorded pattern on a polymer (PMMA) (2um x 2um)


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3.3. Multi-beam electron sources An electron field emitter array has been developed. Carbon nano tubes (CNT) are grown at the apex of each silicon tip as shown in Figure 12 [14]. The fabrication process is shown in Figure 13. After forming silicon tips 4 nm Fe film was deposited for catalyst and CNTs were grown using hot-filament CVD. A high electric field created by applying negative substrate bias was necessary for enhancing the growth of CNTs at the apex [15]. The emission current measured versus voltage is shown in Figure 14. A small threshold voltage for field emission was achieved and the CNT emitter has in principle a long life.

Figure 12. Electron field emitter array having CNT at the apex

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The field emission device provides non contact multiprobe data storage, high throughput multielectron beam lithography. A novel field emission device with integrated electrostatic lens array for electron extraction and focusing has been developed [16]. The structure and photographs are shown in Figure 15. Selective growth of CNTs at the emitter tip of this structure is under development. A result of focusing simulation and the concept of the multi electron beam lithography system are shown in Figure 16 and Figure 17 respectively.

Figure 13. Fabrication process of electron field emitter array

Figure 14. Electron field emission characteristics of a single electron emitter


Figure 15. Structure and photographs of lens integrated electron field emitter

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Figure 16. Focusing simulation of the lens integrated electron field emitter

Figure 17. Concept of multi electron beam lithography system

4. MEMS for LSI processing and testing 4.1. Contactor for LSI wafer prober The electrical feedthrough shown in Figure 7 was applied to a contactor for LSI wafer probing as shown in Figure 18 [17]. The silicon probes were fabricated by bonding a silicon wafer to a Pyrex glass and the silicon wafer was dissolved, except the p+ silicon probe.


Figure 18. Contactor for LSI wafer probing

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4.2. Micro relay for LSI tester A small size micro relay which has good high frequency characteristics are required for LSI tester. Figure 19 shows a micro relay which uses a bimetal actuator with a microheater for making contact with the multi-microspring [18]. The microsprings have electrical connections to the backside of the glass. This is also the application of the high density electrical feedthrough.

Figure 19. Thermal micro relay using electrical feedthrough in Pyrex glass


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4.3. Anti-corrosive integrated massflow controller An integrated massflow controller which has a control valve and thermal massflow sensor enables fine gas control owing to minimized dead volume and quick response [19]. Figure 20 shows an anti-corrosive integrated mass flow controller made of stainless steel and A1N. The operation as the anti-corrosive massflow controller in chlorine gas was successfully demonstrated [20]. Other MEMS devices such as a capacitive diaphragm vacuum sensor have been developed as well [21].

Figure 20. Anti-corrosive integrated massflow controller

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5. Fiber optic MEMS/NEMS 5.1. Fiber optic pressure sensor The small diameter (125umØ) fiber optic pressure sensor shown in Figure 21 has been developed to be used in a blood vessel [22]. A thin diaphragm is formed at the end of an optical fiber and its deformation by a pressure is detected interferometrically. Diaphragms are formed on a silicon wafer and the wafer is etched by the deep RIE to make silicon columns having diaphragms. The silicon column is bonded to the fiber end by heating in a glass tube. Finally the silicon is etched out using XeF2 gas. By measuring the shift of the interference curve in the wavelength axis the sensor is not influenced by the bending of the fiber.

Figure 21. Fiber optic pressure sensor


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5.2. Fiber end NSOMprobe An NSOM (near field scanning optical microscope) probe was fabricated at the end of an optical fiber as shown in Figure 22 [23]. A tiny aperture for the near field can be fabricated by the similar method to that shown in Figure 8 and the assembly of a cantilever which has the tiny aperture to the end of a fiber is carried out by applying the method shown in Figure 22. This configuration of the probe makes the instrument for the optical imaging simple and the microscopic optical imaging was successfully demonstrated by using this probe.

Figure 22. NSOM probe at the end of optical fiber

5.3. Integrated microlens at the core of the optical fiber A simple and effective technique for forming a microlens at the core of an optical fiber end was developed [24]. As shown in Figure 23, photoresist was patterned at the end by a light through the fiber core. After thermal melting of the resist to make a lens shape the shape was transferred to the fiber glass by dry etching.

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After photolithography (before melting)

Microlens at the fiber end (after dry etching) Figure 23. Fabrication process of microlens at the core of an optical fiber end and photographs

6. Microactuators 6.1. Distributed electrostatic microactuators Electrostatic microactuators can be used for levitation and rotation of the accelerometer and gyroscope as described above. High performance microactuators which generate large force with large displacement and quick motion are required for many different applications; however, there are no satisfactory microactuators yet.


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An electrostatic linear actuator with large force can be obtained using an electrode array with large electrode area and narrow gap. Figure 24 shows an electrostatic microactuator developed for disk head tracking [25]. Approximately 500 comb-drive structures having 5um gap and 100um depth are distributed in a 2mmx2mm chip. Serially connected distributed electrostatic displacement have been also developed [26] [27].

linear

actuators for large

Electrical discharge limits the maximum voltage that can applied to the electrostatic microactuators. It was found that the discharge occurs by secondary electron from the electrode and the electrical breakdown agrees approximately with Paschen's law if silicon is used as the electrode material [28].

Figure 24. Distributed electrostatic microactuator for tracking control of a hard disk

6.2. Multilayer piezoelectric actuator by groove cutting and electroplating A new planar fabrication method of multiplayer piezoelectric actuator was developed. Grooves were made in a PZT (lead zirconium titanate) plate by dicing and filled with metal using electroplating of nickel as shown in Figure 25 (a) [29]. The metal electrodes are connected alternately. The XY-stage shown in Figure 25 (b) is composed of two piezoelectric actuators and displacement enlarging mechanisms are fabricated on a PZT plate [30]. The fabrication process after making piezoelectric actuators is as follows. The PZT plate is cut by femtosecond laser and it is filled up with thick resist. After patterning the resist the XY-stage is made by

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metal electroplating using the resist as a mold. The XY-stage has been developed for the multiprobe data storage system shown in Figure 3.

Figure 25. Multilayer piezoelectric actuator by groove cutting and electroplating, (a) PZT plate which has grooves filled with nickel, (b) XY-stage composed of two piezoelectric actuators and displacement enlarging mechanisms

6.3. Electromagnetically actuated two axes optical switch with a thermal actuator for holding mechanism An electromagnetically driven optical switch of which the mirror can be held without electric power was developed [31]. The principle of the holding mechanism (ratchet system) is shown in Figure 26. This consists of holding arms and thermo-


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curable polyimide. The arms made of narrow and wide Si beams hold the mirror from both sides. As shown in Figure 26 (a) the thermo-curable polyimide is filled in a slot. By shrinkage of the cured polyimide the holding arms are bent inward to hold the mirror (Figure 26 (b)). A current in the holding arms heats the narrow beam which has higher electrical resistance than the wide beam, and therefore the arms are bent by thermal expansion to release the mirror as shown in Figure 26 (c). Once the mirror is released, it can be rotated electromagnetically. The operation was confirmed experimentally. A two axes electromagnetic optical switch was fabricated (Figure 27). The two axes optical beam steering is achieved by a gimbal structure [32].

Figure 26. The principle of holding mechanism

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Figure 27. Two axes electromagnetic optical switch with holding mechanism

6.4. Air driven microturbine Silicon micro air turbine for rotating a wire grid polarizer has been fabricated as a polarization modulator needed for surface infrared adsorption spectroscopy [33]. The structure is fabricated using deep RIE and shown in Figure 28. An air bearing is used and the rotational speed of 40,000 rpm was achieved.


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Figure 28. Micro air turbine

7. Micro energy sources 7.1. Si lost mold process for SiC microstructures Extending the research of the micro air turbine, micro gas turbine engine electrical power generators which have large output power are expected for application as self-moving robotic machines. For this purpose fabrication methods of the SiC microstructure have been studied. A micromachined silicon wafer was used as a mould for sintering ceramics. The process to fabricate a SiC microstructure using a silicon as a mold is shown in Figure 29 [34]. A silicon mould is formed by the deep RIE and is cast with a slurry of SiC and carbon powder. SiC ceramic microstructures were fabricated by hot isostatic pressing (HIP). The silicon mold melts at the HIP condition (1700°C, 100MPa) and reacts with the carbon powder to form SiC. Finally the silicon mold is etched out. The process is called the Si lost mold process.

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Similar processes which use silicon as a mold are called the silicon lost mold process and have been applied to fabricate PZT (lead zirconium titanate) [35][36]. SiC microstructures have been fabricated by CVD using silicon as a mold [37] and deep RIE [38]. Since SiC is a hard material which stands high temperature, the SiC microstructure can be used in a harsh environment as a mold for glass pressing.

Figure 29. Si lost mold process for SiC and a micro turbine made by this method

7.2. Silicon-based micro polymer electrolyte fuel cell Microfuel cells are being considered for portable energy sources because of the high energy density and the possibility of continuous operation by refueling. The structure of the micro polymer electrolyte fuel cell (u-PEFC) is shown in Figure 30 [39]. The structure consists of two silicon substrates with porous SiO2 layers, platinum-based catalytic electrodes and micro-gas channels, glass substrates with micro-gas channels and gas ports, and a PEM (polymer electrolyte membrane).


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Figure 30. MEMS fuel cell

7.3. Measurement of hydrogen storage capacity of carbon nanotube using resonant frequency change of thin silicon cantilever Thin silicon cantilever resonator can have high Q (~250,000) by heating in ultra high vacuum [40]. The hydrogen storage capacity of carbon nanotube was measured because hydrogen storage is needed for polymer electrolyte fuel cell systems. A carbon nanotube bundle was attached on a 170nm thick silicon cantilever (Figure 31) and the frequency changes of the cantilever after loading the carbon nanotube and hydrogen adsorption were measured as shown in Figure 32 [41]. Hydrogen storage capacity against the carbon nanotube weight was calculated from the experimental frequency change and it was 6 wt%.

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Figure 31. Carbon nanotube bundle attached on a 170nm thick silicon cantilever

Figure 32. Frequency change of a thin silicon cantilever on which carbon nanotube bundle is attached

8. Conclusions Micro-nano structures and electromechanical systems have been studied, based on bulk micromachining. High density electrical feedthrough in glass plays important roles for array MEMS as multi probe data storage. The high density electrical feedthrough in glass can separate an IC chip from array MEMS devices and therefore simplify the MEMS fabrication. Nanometric structures could be used for high density data storage and as electron field emitters. Sophisticated MEMS using electrostatic levitation and application of functional materials such as A1N,


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PZT, SiC, CNT (carbon nano tube) to microsensors, microactuators and micro energy sources were studied.

9. References [1] Esashi M., "Encapsulated Micro Mechanical Sensors", Microsystem Technologies, 1, 1994, p. 2. [2] Toda R., Takeda N., Murakoshi T., Nakamura S., Esashi M., "Electrostatically Levitated Spherical 3-axis Accelerometer", Technical Digest of MEMS '2002, Las Vegas, 2002, p. 710. [3] Jono K., Hashimoto M., Esashi M., "Electrostatic Servo System for Multi-axis Accelerometers", Proc. of MEMS'94, Oiso, 1994, p. 251. [4] Toda R., Minami K., Esashi M., "Thin Beam Bulk Micromachining Based on RIE and Xenon Difluoride Silicon Etching", Sensors & Actuators, A66, 1998, p. 268. [5] Fukatsu K., Murakoshi T., Esashi M., "Evaluation Experiment of Electrostatically Levitating Inertia Measurement System", Technical Digest of the 18th Sensor Symposium, Kawasaki, 2001, p. 285. [6] Henmi H., Shoji S., Shoji Y., Yoshimi K., Esashi M., "Vacuum Packaging for Microsensors by Glass-Silicon Anodic Bonding", Sensors and Actuators, A43, 1994, p. 243. [7] Fukatsu K., Murakoshi T., Nakamura S., Esashi M., "Electrostatically Levitated Rotational Ring-Shaped Gyro/Accelerometer for Inertia Measurement Systems", Symposium Gyro Technology, Stuttgart, 2002. [8] Li X., Abe T., Esashi M., "High Density Electrical Feedthrough Fabricated by Deep Reactive Ion Etching of Pyrex Glass", Technical Digest ofMEMS'2001, Interlaken, 2001, p. 98. [9] Li X., Abe T., Esashi M., "Endpoint Detectable Copper Through-hole Plating for the Fabrication of Glass with Electrical Feed-throughs", Pacific Rim Workshop on Transducers and Micro/Nano Technologies, Xiamen, China, 2002, p. 139. [10] Takimura N., Lee D.W., Phan M.N., Ono T., Esashi M., "Heater Integarated Micro Probe for High-Density Data Storage", Technical Digest of the 17th Sensor Symposium, Kawasaki, 2000, p. 423. [11] Lee D.W., Ono T., Esashi M., "Fabrication of Microprobe Array with Sub-100nm NanoHeater for Nanometric Thermal Imaging and Data Storage", Technical Digest of MEMS'2001, Interlaken, 2001, p. 20. [12] Lee D.W., Ono T., Esashi M., "Recording on PZT and AglnSbTe Thin Films for Probebased Data Storage", Technical Digest MEMS'2002, Las Vegas, 2002, p. 685. [13] Bae J.-H., Ono T., Kamiya S., Esashi M., "Scanning Diamond Probe for Nanoprocessing", Proceedings of the 19th Sensor Symposium, Kyoto, 2002, p. 315.

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[14] Minh P.N. et.al. "Fabrication and Characterization of Carbon Nanotube on a Si Tip for Electron Field Emitter", 15th International Vacuum Microelectronics Conference & 48th International Field Emission Symposium, Lyon PT.49, 2002. [15] Ono T., Miyashita H., Esashi M., "Electric-field-enhanced Growth of Carbon Nanotubes for Scanning Probe Microscopy", Nanotechnology, 13, 2002, p. 62. [16] Minh P.N. et.al., Microfabricatiom and Characterization of Field Emission Device with Integrated Electrostatic Lens Array, Pacific Rim MEMS Workshop, Xiamen, 2002, p. 561. [17] Hoshino T., Li S., Esashi M., "Basic Study of Micromachined Contactor for LSI" (in Japanese), Late news of 18th Sensor Symposium, Kawasaki, 2001, p. 63. [18] Liu Y., Li X., Abe T., Haga Y., Esashi M., "A Thermomechanical Relay with Microspring Contact Array", Technical Digest of MEMS'2001, Interlaken, 2001, p. 220. [19] Esashi M., Eoh S., Matsuo T., Choi S., "The Fabrication of Integrated Mass Flow Controller", Technical Digest of the Transducers'87, Tokyo, 1987, p. 830. [20] Hirata K., Sim D.Y., Esashi M., "Stainless Steel-Based Integrated Mass-Flow Controller for Reactive and Corrosive Gases", Technical Digest of the Transducers'01, Munchen, 2001, p. 962. [21] Miyashita H., Esashi M., "Wide Dynamic Range Silicon Diaphragm Vacuum Sensor by Electrostatic Servo System", J. Vac. Sci. Technology, B18, 2000, p. 2692. [22] Katsumata T., Haga Y., Minami K., M.Esashi, "Micromachined 125um Diameter Ultra Miniature Fiber-Optic Pressure Sensor for Catheter", Transactions of the IEE of Japan, 120-E, 2000, p. 58. [23] Minh P.-N. et.al., "Hybrid Optical Fiber-apertured Cantilever Near-field Probe", Applied Physics Letters, 79, 2001, p. 3020. [24] Minh P.N.et.al. "Integrated Microlens at the Cores of Optical Fiber Bundle for Optical Fiber Based Application", Proceedings of the 19th Sensor Symposium, Kyoto, 2002, p. 109. [25] Abe M., Esashi M., "Fabrication of Electrostatic Microactuator for Magnetic Head Tracking Control" (in Japanese), Late News, 16th Sensor Symposium, Kawasaki, 1998, p. 77. [26] Minami K., Kawamura S., Esashi M., "Fabrication of Distributed Electrostatic Micro Actuator", IEEE Journal of Micromechanical Systems, 2, 1993, p. 121. [27] Minami K., Morishita H., Esashi M., "A Bellows-shape Electrostatic Microactuator", Sensors & Actuators, A72, 1999, p. 269. [28] Ono T., Sim D.Y., Esashi M., "Micro-discharge and Electric Breakdown in a Microgap", J.Micromech. Microeng., 10, 2000, p. 445. [29] Suzuki G., Esashi M., "Planer Fabrication of Multilayer Piezoelectric Actuator by Groove Cutting and Electroplating", Proc. of the Micro Electro Mechanical Systems '2000, Miyazaki, 2000, p. 46.


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[30] Zhang D.-Y., Chang C., Ono T., Esashi M., "A Piezodriven XY-Microstage for Multiprobe Nano- recording", Pacific Rim Workshop on Transducers and Micro/Nano Technologies, Xiamen, China, 2002, p. 449. [31] Baba A. et.al., "Optical Switch with Holding Mechanism for Mirror Direction", Proceedings of the 19th Sensor Symposium, Kyoto, 2002, p. 359. [32] Asada N.et.al, "Silicon Micromachined Two-dimensional Galvano Optical Scanner", IEEE Trans. on Magnetics, 30, 1994, p. 46. [33] Tanaka S., Hara M., Esashi M., "Air-Turbine-Driven Micro-Polarization Modulator for Fourier Transform Infrared Spectroscopy", Technical Digest of the 17th Sensor Symposium, Kawasaki, 2000, p. 29. [34] Sugimoto S., Tanaka S., Li J.F., Watanabe R., Esashi M., "Silicon Carbide MicroReaction-Sintering Using Micromachined Silicon Molds", J. of Microelectro-mechanical Systems, 10, 2001, p. 55. [35] Wang S., Li J.F., Wakabayashi K., Esashi M., Watanabe R., "Lost Silicon Mold Process for PZT Microstructures", Advanced Materials, 11, 1999, p. 873. [36] Li J.-F., Wang S., Wakabayashi K., Esashi M., Watanabe R., "Properties of Modified Lead Zirconate Titanate Ceramics Prepared at Low Temperature (800°C) by Hot Isostatic Pressing", J. American Ceramic Soc., 83, 2000, p. 955. [37] Itoh T.et.al. "Micromachining of Silicon Carbide by Silicon Lost Molding, Chemical Vapor Deposition and Reactive-Sintering", Extended Abstracts of the 2002 International Conference on Solid State Device and Materials, Nagoya, 2002. [38] Tanaka S., Rajana K., Abe T., Esashi M., "Deep Reactive Ion Etching of Silicon Carbide",J. Vac. Sci. Technol., B 19, 2001, p. 2173. [39] Min K.-B., Tanaka S., Esashi M., "MEMS-Based Micro-Polymer Electrolyte Fuel Cell", Proceedings of the 19th Sensor Symposium, Kyoto, 2002, p. 491. [40] Yang J., Ono T., Esashi M., "Surface Effects and High Quality Factor in Ultrathin Single-crystal Silicon Cantilevers", Applied Physics Letters, 77, 2000, p. 3860. [41] Ono T., Li X., Lee D.-W., Miyashita H., Esashi M., "Nanometric Sensing and Processing with Micromachined Functional Probe", Technical Digest of the Transducers'01, Munchen, 2001, p. 1062.


Chapter 4

Electrostatic Micro-actuators Dominique Collard, Hiroyuki Fujita and Hiroshi Toshiyoshi CIRMM, Institute for Industrial Science, University of Tokyo, Japan

Bernard Legrand and Lionel Buchaillot Institut d'Electronique, de Microelectronique et de Nanotechnologie, Villeneuve d'Ascq, France

1. Introduction and principle Electrostatic forces are very well adapted to actuate elementary micro-structures or more complex micro-mechanisms thanks to their high intensity with short range interaction. A pioneering demonstrator, published in 1977, presented an array of micro-cantilevers, whose deflections were controlled by electrostatic forces [PET 77]. Since then, numerous electrostatic micro-actuators have been proposed and manufactured, among them mechanical oscillators, rotating micro-motors, linear displacement devices and micro-pumps. These micro-actuators are described according to their basic topology in this contribution, once the general principle of electrostatic forces and their equilibrium are introduced. An efficient adaptation of electrostatic force to micro-dimensions is easily explained by Coulomb's law:

This expresses the force exerted by the charge q' on the charge q, separated by a distance r. The unit vector u , is oriented from q' to q and ε0 is the vacuum permittivity. It is obvious that these forces become larger when the distances, r, are reduced (r-2). With the same distance variation, the gravity forces (r3) and elastic restoring forces (r2) decrease, and the effect of electrostatic forces therefore becomes significant and even dominant with the reduced sizes of the micro-structures. The electrostatic force intensity can be easily controlled by the applied voltages as the electromechanical structures behave as capacitances (the charges are proportional to the applied bias). So, referring to Equation 1 the forces will vary with the second power of the voltage (V2), the actuation voltage being in a useful 5 V-200 V range. Equation 1 alone does not permit the direct calculation of the electrostatic forces; the Poisson equation has also to be solved for the entire structure to determine the local present charges. A more suitable approach for simple geometries relies on the

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virtual work method that expresses the forces by the derivative of the stored energy in the system. The system considered is shown in Figure 1; it comprises a voltage source, V, a capacitor, C and an access resistance R.

Figure 1. Basic system for the calculation of electrostatic forces forces

Let us consider that the voltage V remains constant while the upper electrode of the capacitance is able to move in the x-axis direction under the action of the electrostatic force, F. Any given elementary work dW of the electrostatic force induces a change dEc of the energy stored in the capacitance C and an energy loss dEs spent by the source to maintain the voltage V at a constant value. The voltage V, remaining constant, dEc is only due to the capacity change dC. The energy provided by the source compensates for the work produced by the electrostatic forces and the change in the stored energy in the capacitance:

The energy provided by the source, dEs = dQ. V = dC. V2, is twice the energy gained by the capacitance, dEc = 1/2 C. V2 (during the capacity charge, an energy equal to dEc is dissipated by Joule heating in the resistance R, and so, whatever the value of the resistance R is) [AMZ 96, SEN 01]). It turns out that:

The general expression for the electrostatic force is:

This force acts to minimize the total energy stored in the system and tends to increase the capacititive value of the actuator.

Electrostatic Micro-actuators

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2. Parallel plates actuators 2.1. Principle The parallel plate actuator is a very commonly used device in pumps, deformable membranes and micro-switches. This actuator can be typically considered as a capacitor having a fixed electrode while a second one is connected to a mechanical spring and is free to move once a electrostatic force is applied, as shown in Figure 2.

Figure 2. Basic system for the calculation of electrostatic forces

Without any bias, g is the initial gap value between the electrodes; when a voltage V is applied, WO, the electrostatic attraction produces a displacement x of the upper electrode and the capacitance becomes:

The electrostatic force, FE, that acts on the upper electrode is given by:

By dividing Equation 6 by the electrode area, an equivalent electrostatic pressure can be expressed that acts in the gap on the electrodes. This tensile pressure can be written, in function of the electric field between the electrodes, E:

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Let us determine the order of magnitude of the forces in a real structure case: with a 100 x 100 urn2 electrode area and a gap of 1 urn, the electrostatic force created by an applied voltage V = 100V is FE = 442 uN and the corresponding electrostatic pressure is 4.42 104 Pa ~ 0,5 atm. This force seems rather weak at the macroscopic level but it proves to be intense enough to efficiently actuate microstructures. Equation 6 shows that the electrostatic force varies with the applied voltage but also with the displacement. The static characteristic of the actuator is therefore non-linear and its determination requires the following developments. The restoring mechanical force, FM, that is exerted by the spring on the mobile electrode is proportional to the spring stiffness, k, and to its deformation:

The equilibrium position is reached once the two forces compensate, FE+FM=0. A first approach to determine this equilibrium position consists in superimposing these forces variations versus the displacement, x, as in Figure 3. In the case of a moderate bias, V=V1, (see Figure 3(a)) for which FE(x) can be smaller than FM(X), the two curves have two intersection points. To find the correct solution, let us consider the variation of the total system energy. This latter one includes the energy changes in the source, in the capacitance and in the spring. As the energy provided by the source is twice the energy gained by the capacitance (see comments related to Equations 2 and 3), the total energy of the system is given by:

These variations versus the displacement, x, are plotted in Figure 3(a).


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Figure 3. Evolution of the electrostatic force, spring restoring force and total system energy versus the displacement x. (a) V=V,: FE(x)) < FM(x). (b)(X). (b) V=V2»E1: FE(x) ))

The extrema of the total energy correspond to the force equilibrium, but only the displacement x1, where the energy is minimal, gives the stable equilibrium condition, Figure 3(a). As the applied voltage, V, increases, the FE curve deforms to higher forces and the equilibrium condition point moves towards the larger displacement, corresponding to a larger spring expansion. For even larger applied voltages, such as V2»V}, the curve intersection does not exist anymore, the actuator behavior becomes unstable and the equilibrium condition, corresponding to the minimum total energy, is obtained for x=g, the upper electrode coming in contact with the lower one, Figure 3(b). To determine the extension of the stable actuation zone, the force equilibrium condition (Equations 6 and 8) can be rewritten as [TOS02]:

The graphic solution of Equation 10 determines the stability zone, indicated by the grey area in Figure 4. This zone ranges from x=0 to x=g/3, corresponding to one third of the initial gap between the electrodes. The stability limit is obtained for a pull-in voltage given by:

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Figure 4. Graphic solution of Equation 10. The stable zone, in grey, is determined by the maximum value of the expression h(x)

Figure 5 displays the voltage-displacement characteristic of the actuator with a schematic cross section view of its deformation. For a voltage lower than the pull in voltage, V pull-In , the actuator motion can be controlled and so, in one third of its initial gap. For V—V pull-In , (mark 1 in Figure 5), the upper electrode collapses on the lower one (mark 2). To release the contact, the voltage has to be reduced to a value VPull-Out (mark 3) and the actuator recovers a position in the stable zone (mark 4). The corresponding marks are also mentioned in Figure 4. The voltage Vpull-out mostly depends on the fabrication process of the actuator. The lower electrode has to be covered by a thin isolation layer (SiO2 or Si3N4) to prevent any short-circuit between the electrodes while they are in contact. Considering that the lower electrode is covered by a thin SiO2 layer (thickness tox), Vpull-out balances the electrostatic force at the contact position and the spring restoring force for an extension g:


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in which ε ox is the SiO2 permitivity.

Figure 5. Characteristic of the parallel plate electrostatic actuator

Equation 13 gives a theoretical expression: experimentally many effects tend to decrease the pull-out voltage such as striction effects, capillarity forces and residual electrical charge trapping [MAS 97]. It is even mandatory to control these effects to prevent any permanent sticking.

2.2. Examples of parallel plate actuators Various examples of parallel plate electrostatic micro-actuators have been published; they can be classified two types: — with analog mode actuation, when the command voltage V is controlled below VPull-In and the actuation range is limited to g/3; ~ with binary states, when the deflection is negligible for V V Pull-In . On analog mode devices, tiny stoppers are commonly inserted to avoid full plane contact between the electrodes. Among published analog mode devices, one can

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mention electrostatic pumps and valves for micro-fluidic applications [OHN 90], controlled deflection mirror for adaptive optics [VDO 97] or variable capacitances for the tunable frequency synthesis in local electronic oscillators [YON 96]. An example of an adaptive mirror is displayed in Figure 6 [VDO 97]. This device is intended to correct the optical aberrations in the spatial telescopes observations. In its simplest configuration, the mirror is composed of a thin flexible membrane maintained above an array of independent addressable electrodes. The membrane deflection results from the superimposition of individual deformations produced by each electrode as shown in Figure 6(a). In this device, the mirror is composed of a low stress thin silicon nitride membrane, 50 mm in diameter, coated with a thin aluminum layer to make it reflective and conductive. The actuation structure consists in a array of aluminum electrodes that are isolated from each other by a relatively thick deposited silicon oxide (1 urn). Depending upon the different configurations, 39 or 119 electrodes have been implemented, their individual bias and the resulting global membrane deformation is computer controlled in real time. Maximum local deflections of 33 um have been measured and the device response time is in the order of 1 ms. The variable capacitance, another family of analogue electrostatic actuator, is under intensive development as it can be successfully implemented for the frequency synthesis in low power mobile phone handsets [YAO 00]. A pioneering realization have been proposed in 1996, based on microsystems technology. The actuation principle is depicted in Figure 7 and exists to modulate the capacitance value by vertically moving the upper electrode above the fixed lower one. Figure 7 provides a top view of the realized component with a schematic cross section. The structural part of the variable capacitance are realized in aluminium to minimize the parasitic resistance and the entire device is electrically isolated from the substrate by a dielectric layer and an electrostatic shield to reduce high frequency losses in silicon and to obtain a suitable quality factor. The electrodes have a 200*200 um2 area and are separated by a 1.5 um gap. The capacitance value ranges from 2.11 pF for a zero bias to 2.46 pF for a controlled bias of 5.5 V, corresponding to a 16% tunability. The serial equivalent resistance at 1 Ghz is 1.2 Ω and provides a quality factor equal to 62. These characteristics are equivalent to and even outdo semiconductor varactor performances.


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Figure 6. Mirror for adaptive optics: (a) Actuation principle. (b) Interferometric visualisation of the membrane deformation from the initial state, on the left, to a deformed state, on the right, with 90 V uniformly applied to all electrodes, (c) Some implemented electrodes configurations, (d) Packaged device

Figure 7. Realized variable capacitance [YOU 96]. (a) Top view by SEM. (b) Cross section with different patterned layers

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The other type of parallel plate actuator works in binary mode. By looking back at Figure 5, when the actuation plate is grounded or with applied voltage less than VPull-out, the actuator is in open position, and, on the contrary, when the voltage is higher than V P u l l - I n the electrodes are in contact and the actuator is in closed position. This behavior is suitable for a commutation system such as a RF micro-switch, micro-relay, reflector for microwave application, optical switch and shutter as it will be shown in the following examples. For this actuation mode, the command is very simple, but the material and fabrication choices are crucial to obtain actuation voltage as low as possible while guaranteeing acceptable device reliability. After multiple actuation cycles, numerous phenomena contribute to degrade the actuator characteristics, plastic deformation of the mobile structure induced by high stress gradients, condensed water deposition on hydrophilic dielectric layers, degradation of contact resistances after multiple impacts, sticking between electrodes, among others. Intensive researches are nowadays performed to understand, model and overcome these effects [BHU 97]. An example of a micro-relay is displayed in Figure 8 [MAJ 97]. The operation principle is quite obvious and is equivalent to a commutation transistor one: as soon as a significant voltage is applied to the command electrode (gate), the cantilever is attracted downward and the relay is closed, the source and drain are directly contacted. When the gate is grounded, the cantilever recovers its initial position and the source and drain are isolated from each other. The cantilever in Figure 8 is composed of a 2 (um thick nickel layer on a thin gold layer to ensure a low resistance contact to the drain that is also locally coated by gold. The initial inter-electrodes gap is 2 um The pull-in voltages (threshold voltage) vary significantly among the devices and are distributed in the 30-300 V range for 65 um long and 30 um wide cantilevers. Interesting long life times (108109 cycles) have been obtained without significant degradation of the contact resistance (R h > d =', and t the forces usually range as:

The following realization examples will provide quantitative information concerning the above mentioned quantities: forces, displacements and actuation voltages.

3.2. Device examples The first example uses the comb drive actuation for the X/Y positioning of a AFM (atomic force microscope) tip [IND 95, SAM 93]. The device principle is explained in Figure 14(a). The micro-table containing the tip is linked to four actuators. They are coupled 2 by 2 for the positioning of the table and for the X and Y scanning of the surface to analyze. The sample surface topology changes are detected by an optical beam sensing in the back side of the table. A SEM micrograph of a realized device is given in Figure 14(b), the entire structure with the suspension beams and the anchor points are clearly visible. The suspension beams are 270 um in length, 0.6 um in width and 2 um in thickness that is also the thickness of the comb electrodes. The positioning and scanning system operates in static mode, and when a 40V voltage is applied, the table has a 5 (um displacement toward the biased actuator.

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Figure 14. Integrated AFMMicroscope with a X/Ypositioning system by comb drive actuators [IND 95]. (a) Device principle. (b) SEMview of a realized device

The second example presents an electromechanical resonator whose schematic principle is given in Figure 15. This basic resonator is one of the very first published devices using electrostatic comb drive as actuation mean [TAN 89]. The resonator is composed of a central shuttle (mass) that is free to laterally move in the substrate surface plane. The mass, M, is linked to the substrate by mechanical spring having a total stiffness, k. Comb drive actuators are located on both sides of the structure to produce the lateral motion of the mass. The mobile part remains grounded while voltage VL and VR are applied on the left and right fixed electrodes, respectively. These signals are composed of an excitation voltage vi superimposed on a polarization bias, VP. According to Equation 15, the total electrostatic force applied on the mobile shuttle is:

giving:


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where N is number of fingers on a fixed electrode.

Figure 15. Schematic principle of an electromechanical resonator with comb drives on both sides (top view)

For a unique comb drive actuator, the forces generated vary with the second power of the applied voltage between its electrodes, Equation 15. Nevertheless, with the polarization principle in Figure 15, the force are linearized with the excitation signal amplitude, vi, as mentioned in Equation 21. The generated force, and so the resulting displacement is proportional to the excitation voltage, v,, therefore, a better motion control can be obtain for precision system such the positioning device in Figure 14. This voltage/displacement linearity allows one to use the mobile shutter as a electromechanical resonator with an eigen frequency given by:

0

[22]

In that case, the operation principle is slightly modified: one of the comb structure acts as an actuator while the other one senses the motion by capacitive modulation, as shown in Figure 16. The suspension beams are here folded to improve the compactness of the device while assuming a correct guidance for the lateral motion. Moreover, in that folded configuration the effective spring stiffness is less sensitive to the internal stresses in the beams. The anchor pads maintain the central mass to the substrate and form a mechanical stopper that limits the shuttle lateral motion avoiding any short circuits with the electrodes. The shuttle is biased to

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a relatively high DC value, Vp, the same voltage is applied to a buried electrode that encompasses the mobile shuttle stoke. This electrode acts as an electrostatic shield that prevents the attraction of the biased mobile mass to the grounded substrate.

Figure 16. Electromechanical resonator with comb drive structures for both actuation and displacement capacitive sensing

When an ac signal, vi sin (cot), is applied to the input electrode, the lateral force acting on the shuttle, Fs, is expressed by:

by considering Vp»v i , and by neglecting the static displacement that does not generate any current in the output capacitance, the harmonic part of the force is:

The displacement is sensed by the output current, I0,, that reaches a peak value at the mechanical resonance frequency of the mobile part; as a consequence, this structure is an electromechanical filter. The reduced masses allow one to obtain resonance frequencies in the 10-100 kHz range with quality factor close to 50,000 [TAN 89] under vacuum. Oscillators have been realized from these electromechanical resonators, they have been integrated in vibrating initial sensors, such as accelerometers or gyroscope [YAZ98], or in time reference generation systems [NGU 99].


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A fabricated resonator is displayed in Figure 17; the overall structure is made from polysilicon, the fingers have a 40 m length, an initial overlap of 20 μm and a 2 2x2 μm cross section area. For a 5V actuation voltage, the generated forces are in the order of 2 nN, but thanks to a quality factor of 50,000, 20 μm vibration amplitudes have been measured at the resonance frequency.

Figure 17. SEMview of an electro-mechanical resonator based on comb drive structures [TAN 89]

Rotating oscillators are also possible, as shown in Figure 18(a), the mobile part being then linked to the substrate by a spring with a spiral shape [TAN 89]. This topology is particularly well adapted for the vibrating gyroscope [FUN 99, YAZ 98] or for the angular positioning micro-motor [PIS 97]. To increase the generated force and the resonator stability, micro-technologies aiming micro-structures fabrication with very high aspect ratio h/l (see Figure 13) have been developed. A first approach consists in reducing the lateral gap toward sub-micron dimension by cleaver design [HIR 92]. A second way is to increase the structure thickness while keeping small lateral design rules, using anisotropic deep etching step [CHO 98] or molded structures [KEL 97]. A realization example is displayed in Figure 18(b), the thickness of the structural epi-polysilicon (obtained by epitaxial growth) is 15 μm, the comb finger, as well as the spring beams are 3 μm in width.

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Figure 18. Other comb drive actuator topologies: (a) Rotating vibrating structures [TAN 89]. (b) Resonator realized with high aspect ratio technology [STM 01]

4. Torsion mode actuator Strongly motivated by the need for new miniature components for integrated optical systems, intensive research has focused on the development of torsion mode actuators. This device acts as rotational micro-mirror capable to reflect or deviate collimated light beams. They easily find practical applications as switch for optical fiber based communication networks or for image projection. The actuation principle is given in Figures 19 and 20. A plate is suspended above driving electrodes by torsion bars. Once a voltage difference is applied between the


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plate and the electrodes, the electrostatic forces create a torsion torque that produces an angular rotation of the mobile structure [TOS 02].

Figure 19. Structure and operation principle of the torsion mode actuator [TOS 02]

A typical characteristic of the actuator, rotation angle versus applied voltage, is plotted in Figure 21, in the case where only one electrode is biased. This characteristic is issued from analytical calculations based on the equilibrium between the torques created by the electrostatic forces and by the bars torsion [TOS 96, DEG 98]. The behavior obtained is similar to of the parallel plates actuator one, ie, the deflection angle variation are non-linear with the applied voltage and presents an hysteresis.

Figure 20. Cross section view of rotational mode actuator. (a) Controllable zone. (b) Plate in contact with the substrate

For voltage less than V Pull-In , the rotation angle is controllable, as shown in Figure 20(a). For actuation voltage larger or equal to VPull-In, the mirror abruptly comes in contact with the electrode, Figure 20(b). The voltage has to be reduced beyond V Pull _ out to return to the controllable zone.

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Figure 21. Calculated characteristic of a torsion mode actuator: rotation angle vs. actuation voltage. The d device dimensions correspond to the notation in Figure 19

Numerous technological parameters and design dimensions can be optimized to reduce the actuation voltages, or to expand the controllable angular zone: shapes and locations of the electrodes, dimensions and geometries of the torsion bars. This optimization can be efficiently performed using analytical calculations [TOS 96, DEG 98]. As for the comb drive actuators, the electrodes can be biased in differential modes (see Figure 15), in that case, a quasi-linear behavior (angle versus voltage) can be obtained within the controllable zone [TOS 02] even for two degrees of freedom rotation [TOSH 01].

Figure 22. SEMview of the torsion mode actuator for a DMD pixel [DMD 01]

A very representative example of operational torsion mode actuators is certainly the deformable mirror device (DMD) developed by Texas Instrument [VAN 98], see Figure 22. This system performs image projection by a reflecting surface composed of an array of pixels that can be individually addressed. Each pixel is composed on 3 structural levels as shown in Figure 23. The lower level integrates the electrodes that are directly connected to the underneath CMOS bistable gates. In active regime, one of the electrodes is biased while the other one remains grounded. The second level


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corresponds to the actuator itself (see SEM view in Figure 22), the torsion is produced by the attraction exerted by the biased electrode. The top level contains the micro-reflector.

Figure 23. Structure of a DMD pixel [DMD 01]

In the latest version of DMD, an elementary pixel occupies a 20*20 μm2 surface area. The actuation voltage is 5V for a response time of 12 us. In the SXGA version, the complete matrix includes 1280*1024 pixels and is a convincing illustration of the integration capabilities of microsystems technology. The second example is related to the optical fiber communication system, a very active research area. The increasing need for information exchange, especially for internet, requires the introduction of the optical fiber very close to the final user. Improved addressing capabilities and optical switching means are therefore mandatory for the propagated information multiplexing and for network reconfiguration. To achieve this challenge, a key component is located in the terminal devices: the n to m optical switching matrix with low insertion loss. An original solution based on micro-mechanism uses a torsion mode actuator, as depicted in Figure 24 [TOS 96]. Thanks to the mass production capabilities of microsystems technology, the foreseen price of this electromechanical solution is far below the price of classical optoelectronic solution.

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Figure 24. Switching matrix for optical communication network [TOS 96]. (a) operation principle of the 2X2 switches matrix. (b) SEMview of a micro-mirror, temporary support is still present. (c) Extension of the switching principle to a 5X5 matrix

The switching system includes a array of four micro-mirrors, Figure 24(a), located at the intersection of the optical paths of connected fibers. The 300*600 μm2 micro-mirrors are supported by two torsion bars (length: 320 μm, width: 16 μrn, thickness: 0.4 μm) above a bulk micro-machined through hole as shown in Figure 24(b). The mirror and the torsion bars are made of polysilicon covered by chromium and gold to improve both optical reflectivity and electrical conductivity. When the mirror is biased, it is attracted to the grounded counter-electrode and can rotate with an maximum angle of 90° precisely controlled by stoppers located in the support substrate. A 90° torsion is reached for an actuation voltage, V Pull-In = 120V, the initial horizontal mirror location is recovered for VPull-Out = 50V. This switching principle, extended to a larger number of mirrors, can lead to M X N switching matrix, as it is shown in Figure 24(c).

5. Rotating micro-motors The rotating micro-motor received much interest in early stages of microsystem development in 1985-1990. The research work was motivated to demonstrate new functionalities, a rotating micro-actuator, with standard micro-electronic technology [TRI 97].


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The first type of micro-motor is based on progressive capacitive overlap between the rotor and the stator that generates a step by step rotational motion [FAN 88]. Figure 25 displays both schematic top and cross views of this device.

Figure 25. Capacitive coupling rotating micro-motor [FAN 88]. (a) Top view. (b) Cross section view. (c) Temporal cycle of the actuation phases

The major concern for the design of this type of rotating micromotor is to estimate the initial torque produced by the electrostatic forces between the rotor and the stator. As the force is expressed as the spatial derivative of the stored energy in the capacitance, Equation 4, the torque, T, is determined by this energy derivative in respect to the rotation angle:

The rotor remaining grounded, when the phase Φ11 is solely activated, an electrostatic torque is created to maximize the capacitive coupling between the electrode polarized with Φ1. 1, and the closest rotor poles. This torque induces an incremental rotation in the anticlockwise to align the poles and the biased electrodes.

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The voltage cycle, shown in Figure 25(c), is then applied and repeated to produce the rotational movement. The incremental angular rotation 0 depends on the number of electrodes in the stator, Ns, and on the number of poles in the rotor, NR [FAN 88]:

For a rotor of 100 urn in diameter, the electrostatic torques are in the order of several pNm for actuation voltage of 100V. As illustrated by the cross section in Figure 25(a), the fabrication technology requires three levels of polysilicon (PolyO for the buried electrode, and two structural layers, one for the stator electrodes and the rotor, Polyl, and a second one for the rotor cap, Poly2). The rotor thickness and the lateral gap between the rotor pole extremities and the stator electrode are 1.5 urn and 4 μm, respectively. A fabricated device is displayed in Figure 26, in which the polysilicon layers stack is clearly visible.

Figure 26. Example of fabricated rotating micro-motor [LAA 01]

In the capacitance coupling micro-motors, large friction forces act on the rotor, mainly due to the contact with the axis, but also with the substrate. These friction effects are even amplified by the parasitic electrostatic forces existing between the rotor and the immobile parts, such as the substrate. Even with optimized fabrication processes and designs [MEH 89, TAI 89], it turns out that the basic operation principle of this motor cannot provide long term reliable behavior, friction effects overcoming driving forces at this microscale dimensions. To minimize the friction effects, a second type of motor, called the harmonic side drive micro-motor has been proposed by Mehregany et al. [MEH 90], its operation principle is explained in Figure 27.


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Figure 27. Operation principle of the harmonic side drive actuator. (a) structure without any polarization. Rotation induced by the cycling of the bias.. The rotor rotation and the polarization cycling turn in the same direction

The rotor is free to move in the substrate plane, but its displacement is limited by the axis that prevents any contact with the stator electrodes. Once an electrode is biased, the rotor is attracted to that electrode and enters in contact with the axis. When the bias moves to the neighboring electrode, the rotor also moves in the same direction in keeping the contact with the axis. The rotation progresses with a rolling of the inner circumference of the rotor on the axis external surface. The friction effects are drastically reduced as compared with the capacitive coupling mode motor, this is a clear advantage. A second advantage lies in the speed reduction that is naturally integrated with the wobbling behavior, no more reducer is needed to obtain significant torque. When, the polarization phase has achieved one turn, Figure 27(b), the corresponding rotor angular motion is given by:

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The motor torque is then multiplied by the speed reducer factor, n, expressed by:

With a rotor of 100 urn in diameter and a thickness of 2.5 μm (rotor and stator electrodes), a harmonic motor has produced a maximum torque of 10 pNm thanks to a reducer factor, n, egals to 70. This motor, actuated during 150 hours, has performed 100 million turns at a rotation speed of 10,000 rpm without showing any measurable fatigue or degradation [MEH 90]. A picture of a fabricated device, made in nickel is shown in Figure 29 [FUJ 01].

Figure 28. Harmonic wobble motor made of nickel. The rotor has a 100 μm diameter and is 7 μm thick. This motor has demonstrated a rotational speed of 10,000 rpm [FUJ 01]

A second example of a harmonic motor with a different design is presented in Figure 29 [DAN 95b]. In this device, the rotor is located outside the stator electrodes and roll on the tips of star shaped support that are grounded. The structure is realized with a 15 μm thick electroplated nickel. The inner diameter of the rotor is 800 μm. Rotational speed of 100 rpm has been measured with a 100 V pulsed actuation voltage. Even if the harmonic motor has demonstrated acceptable reliability, the motor torque is hardly exploitable because the rotor does not experience a pure rotation movement around a fixed axis. Until now (2002), neither the capacitive coupled motor nor the harmonic motor have found real industrial applications, the first being


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a reliability issue, the second one due to its complexity to be introduced in a functional mechanism.

Figure 29. Fabricated ROTEXwobble motor with outer rotor made of electroplated nickel [DAN 95b]

6. Other types of electrostatic micro-actuators The micro-actuators described so far have been easily categorized by their topology : parallel plates capacitance, comb-drive actuator, torsion mode mirror and finally the rotating micro-motor. Nevertheless, the flexibility of microsystem technologies combined with the ease of production of electrostatic actuation have led to actuation devices or systems that cleverly combine the above mentioned basic structures. Some actuators have very simple shape, such as the SDA (Scratch Drive Actuator), others have much more complex topology and are able to fulfil a complete function, specially for optical applications.

6.1. SDA actuator The SDA actuator proposed by Akiyama et al. [AKI 93], is a very simple structure, made of polysilicon, that is able to crawl on the substrate surface when it is cyclically biased. The structure and the operation principle of SDA are plotted in Figure 30.

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Figure 30. SDA actuator: (a) schematic representation and dimensions definition. (b) SEM view of a realized structure. c,d,e) step by step motion model showing the plate deflection in function of the bias cycle. [AKI93, AKI97]

The SDA is composed of a plate terminated by a bushing at one of its edge and is electrically isolated from the substrate by a thin dielectric layer, Figure 30(a). A SEM view is given in Figure 30(b) showing an individual SDA maintained to the anchor pad by mechanical links. Starting from the initial situation when the structure is grounded, Figure 30(c), a positive voltage pulse is applied to the SDA, Figure 30(b, c). The electrostatic forces attract the plate, and the SDA is deformed against the dielectric layer. This deformation slowly progresses, the contact zone between the plate and the isolator extends forward (toward the bushing) resulting in a sliding of the bushing edge on the surface. When the actuation voltage is removed, Figure 30(e), the stored mechanical energy in the deformed plate is relaxed and the SDA


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recovers its initial shape. During this relaxation, the bushing edge remains in contact with the surface, the SDA initial shape recovering operates with a forward sliding motion of the plate. The SDA have moved forward with an increment step, dx. The same motion also operated during the following negative pulse. Therefore, by cycling the polarisation pulses, SDA experiences a stepwise motion, the displacement stoke depending on the supporting spring stiffness or sliding support lengths. For an actuation signal frequency, f, the displacement velocity is simply expressed by:

For typical SDA dimensions (W = 70 m, L = 60 μm, h = 2 μm, t = 2 μm), average incremental motion steps of 80 nm have been measured for 150 V, 1 kHz actuation voltage [AKI 93]. The forward generated force by the stepping motion of an individual SDA have been estimated to be at least 50 μN by micro-beam buckling experiments [AKI 97].

Figure 31. Self assembly of 3D structures by SDA [FUK97, QUE 00]. (a) Initial plane structure. (b) Beam buckling induced by the SDA motion, subsequent plate rotation by electrostatic actuation

The forces generated by the SDA motion are large enough to self-assemble three-dimensional (3D) micro-structures as demonstrated in Figure 31 [FUK 97, QUE 00]. Starting from a initially plane surface micro-machined structure, including a plate maintained by two supporting beams, Figure 31 (a), the SDAs make the beam buckle and the structure is elevated out of the surface plane, Figure 31(b). During the actuation, the structure is automatically locked thanks to integrated mechanical latches and, therefore, the micro-structure 3D shape is permanently kept, even after removing the actuation voltages [QUE 00], a SEM view of such a fabricated structure is given in Figure 32.

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Figure 32. SEMview of a self-assembled 3D micro-structure [QUE 01]

Figure 33. Self-assembledXYZpositioning table including a Fresnel lens [FAN 97]. Right: complete structure; Left: look up view on the integrated hinge allowing the 3D folding

This self-assembling, applied to fabrication technology with multiple structural layers, allows the realization of 3D components for optical benches such as Fresnel lenses, mirrors, gratings with self-positioning capabilities, Figure 33 [FAN 97], or 2D optical scanners [TOS 01] for optical beams rerouting or image projection system.

6.2. Shuffle motor An electrostatic motor, the motion of which is based on successive beam deflection cycles have been proposed to produce rectilinear displacement [TAS 97]. The motion principle is detailed in Figure 34(a).


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Figure 34. Shuffle motor [TAS 97]. (a) Detail of the inchworm-like motion. (b) SEMview of a fabrication device. (c) Actuation voltages cycle

The device produces a step by step crawling motion, fully synchronized by the polarization cycles applied to three actuation zones. A top view of a fabricated motor is given in Figure 34(b). The linear motion is guided by a spring located on both sides of the mobile shuttle. The mobile part remains grounded while the actuation voltages are applied through parallel buried electrodes that determine the actuation stroke. The actuation voltages are close to 25V for the beam deflection and 40V for the electrostatic clamping. An average incremental step of 0.1 μm has been measured, therefore for a 1.16 kHz cycling frequency, the motion speed is close to lOOum/s. With these polarization conditions, and for a 0.5 μ,m thick beam (length: 200 μm; width 100 (μm), the forward pushing force have been estimated to be 43 +/- 13 μm, this value has been extracted from the maximum guiding spring deformation the motor is able to produce at the stroke end.

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6.3. Comb drive based vibromotor The comb drive actuators, that have been detailed in section 3, can only produce limited motion, typically in the range of several microns. Nevertheless their integration as basic actuator in a more complex mechanical design makes possible the realization of large stroke displacement system or 3D movements with large amplitudes.

Figure 35. A linear vibrometer with four combe-drive actuators and extended fingers that can contact the central slider. [DAN 95-a]

A linear vibromotor, presented in Figure 35, includes four comb-drive actuators with extended fingers that can contact a central slider. When two facing actuators are put in resonance, the resulting impacts laterally move the slider step by step. The symmetrical design of the vibromotor allows a two-directional motion. This actuator have been designed to accurately position micro-optical components [DAN95a]. With bias voltage of 12.5 V, a 0.27 μm positioning accuracy have been reached with displacement speed close to 1 mm/s. The force, produced by the moving slider have been estimated to be in the 2.5 μN range. Other long stroke displacement systems have been proposed and successfully tested, among them, some combined caption / displacement / releasing sequence to control slider motion step by step [BAL 97]. Comb drive actuators are also used to rotate micro-mirrors. Two realization examples are provided in Figure 36. The displacements are applied on the plate side and the torque produced with the supporting bar rotates the micro-mirror. These deflectors are implemented in bar code reading apparatus or projection system [KIA96,KIA98].


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Figure 36. Actuation of rotating mirror by comb-drive actuators. These reflectors are integrated in a XY projection system. (a) Mirror for vertical scanning. (b) Mirror for lateral scanning. [KIA 98]

7. Conclusion and perspectives This chapter has listed, in a quite exhaustive way, the different types of electrostatic actuators that have been realized so far, thanks to microsystems technology.

Figure 37. Linear translator actuated by comb-drive actuators. (a) View of the entire device showing the actuator and the 3D mirror. (b) Look up view of the gear that transforms the linear motion of the mobile shuttle of the comb-drive actuator into rotation movement. [SAN 01]

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Having determined the electrostatic force characteristic by analytical calculation, the micro-actuator have been sorted according to their topology or to the type of motion they are able to experiment: parallel plates capacitive actuator, comb-drive structure, torsion mode actuator and rotating micro-motor. In the last part has been presented more specific designs that synthesize more complex motion. As the implementation of electrostatic actuation is very simple, its utilization relies on fabrication technologies able to produce functional devices. When micromachining technology proposes multiple mobile structural layers, more and more complex mechanisms can be realized to cover specific applications. This is illustrated by the last example in Figure 37, realized with five layers of polysilicon (one electrical level and four mechanical ones) [SAN 01]. This figure shows a linear translator with a stepwise motion produced by combined comb-drive actuators. This system integrates a speed reducer to obtain forces intense enough to fold 3D structures. Despite the huge number of electrostatic actuators that have been demonstrated in laboratories, few of them have been introduced in industrial products. Most of the basic demonstrators suffer from too short life time due to a lack of reliability: stiction between mobile parts, cracks in the structures, contact wear, inefficient actuation due to trapped charges etc. Nowadays, intensive research is performed on material quality and on reliability issues to improve long time performances of electrostatic actuators and to make possible their introduction in innovative miniaturized industrial products.

8. References [AKI93] AKIYAMA T., SONO K., "Controlled Stepwise Motion in Polysilicon Microstructures", J. Microelectromech. Syst., Vol. 3, 1993, pp. 106-107. [AKI97] AKIYAMA T., COLLARD D., FUJITA H., "Scratch Drive Actuator with Mechanical Links for Self-assembly of Three Dimensional MEMS", J. Microelectromech. Syst., Vol. 6, 1997, pp. 10-17. [AMZ96] AMZALLAG E., BEN-AIM J., PICCIOLI N., Electrostatique, Rappel de cours et exercices corriges de physique, Edisciences International, 1996. [BAL 95] DANIAU W., BALLANDRAS S., KUBAT L., HARDIN J., MARTIN G., BASROUR S., "Fabrication of an electrostatic wobble motor using deep-etch UV lithography, nickel electroforming and a titanium sacrificial", J. of Micromechanics and Microengineering. Vol. 5, 1995, pp 270-275. [BAL 97] BALTZER M., KRAUS T., OBERMEIER E., "A Linear Stepping Actuator in Surface Micromachining Technology for Low Voltages and Large Displacements", Tech. Digest. 1997 Int. Conf. Solid-state Sensors and Actuators, Transducers 97, Chicago, June 1997, pp. 781-784.


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[BHU 97] BHUSHAN B., Tribology Issues and Opportunity in MEMS, Kluwer Academic Publishers, 1997, pp. 367-396. [CHE 99] CHEN T.C., NGUYEN H., Wu M.C., "A Low Voltage Micromachined Optical Switch by Stress Induced Bending", Proceed. IEEE Micro Electro Mechanical Systems, 1999, pp. 424-428. [CHO 98] CHOI J.J., TODA R., MANAMI K., ESASHI M, "Silicon angular resonance gyroscope by deep ICP RIE and XeF/sub2/gas etching", Proceed. IEEE Micro Electro Mechanical Systems, 1998, pp. 322-327. [COL 02] COLLARD D., FUJITA H., TOSHIYOSHI H., LEGRAND B., BuCHAiLLOT L., "Surface micro-machining - a review", Nano et Micro Technologies, this issue, 2002. [DAN 95] DANEMAN M.J., TIEN N.C., SOLGAARD, PISANO A.P., LAU K.Y., O., MULLER R.S., "Linear vibromotor for positioning optical components", Proceed. IEEE Micro Electro Mechanical Systems, 1995, pp. 55-50. [DEG 98] DEgANI O., SOCHER E., LIPSON A., LEITNER T., SETTER D.J., KALDOR S., NEMIROVSKY Y., "Pull-in study of an electrostatic torsion", J. Microelectromech. Syst., Vol.7, 1998, pp. 373-378. [DMD 01] http:/www.ti.com/dlp/resources/library/. [FAN 88] FAN L.S. TAI Y.C., MULLER R.S., "IC-Processed Electrostatic Micro-motor", Proceed. IEEE International Electronic Device Meeting, 1988, pp. 666-669. [FAN 97] FAN L., Wu M.C., "Self-assembled XYZ stage for optical scanning and alignment", Tech. Digest. 1997 Int. Conf. Solid-state Sensors and Actuators, Transducers 97, Chicago, 1997, pp. 319-322. [FUJ01] Image du laboratoire Fujita, Institute of Industrial Sciences, University of Tokyo: http://www.cirmm.iis.u-tokyo.ac.jp/. [FUK 97] FUKUTA Y., COLLARD D., AKIYAMA T., YANG E.H., FUJITA H., "Microactuated selfassembling of 3D polysilicon structures with reshaping technology", Proceed. IEEE Micro Electro Mechanical Systems, 1997, pp. 447-481. [FUN 99] FUNK K., EMERICH H., SCHILP A., OFFENBERG M., NEUL R., LARMER F., "A surface micromachined silicon gyroscope unsing a thick polysilicon layer", Proceed. IEEE Micro Electro Mechanical Systems, 1999, pp. 57-60. [GRE 97] GRETILLAT M.A, "Nonlinear Electromechanical Behavior of an Electrostatic Microrela/', 1997 Int. Conf. Solid-state sensors and Actuators, Transducers 97, Chicago, June 1997, pp. 1145-1148. [HIR92] HIRANO T., FURUHATA T., GABRIEL K.J., FUJITA H., "Design, Fabrication and Operation of Submicron Gap Comb-Drive Micro-Actuator", J. Microelectromech. Syst., Vol. 1,1992, pp. 52-59. [IEM 01] Dispositif realise a 1'IEMN: http://www.isen.fr. [IND 95] INDERMUHLE P.F. et Al, "AFM imaging with an xy-micropositioner with integrated tip", Sensors and Actuators A, 1995, pp. 562-565.

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[KEL 97] KELLER C.G., HOWE R.T., "Hexsil tweezers for teleoperated micro-assembly", Proceed. IEEE Micro Electro Mechanical Systems, 1997, pp. 72-77. [KIA 96] KIANG M.H., SOLGAARD O., MULLER R.S., LAU K.Y., "Surface micromachined electrostatic-comb driven scanning micrimirror for barcode scanners", Proceed. IEEE Micro Electro Mechanical Systems, 1996, pp. 192-197. [LAA 01] Dispositif realise au LAAS / CNRS: http://www.laas.fr. [LEG 95] LEGTENBERG R., BERENSCHOT E., ELWENSPOEK M., FLUITMAN J., "Electrostatic Curved Electrode Actuator", Proceed. IEEE Micro Electro Mechanical Systems, 1995, pp. 37-42. [MAJ 97] MAJUMDER S., MCGRUYER N.E., ZAVRACKY P.M., ADAMS G.G., MORRISON R.H., KRIM J., "Measurement and modeling of surface micromachined, electrostatically actuated microswitchs", Tech. Digest. 1997 Int. Conf. Solid-state Sensors and Actuators, Transducers 97, Chicago, June 1997, pp. 1145-1148. [MAS 97] MASTRANGELO, Surface Force Induced failures in Microelectromechanical Systems, Tribology Issues and opportunities in MEMS, Kluwer Academic Publishers, 1997, pp. 367-396. [MEH 89] MEHREGANY M., BART S.F., TAVROW L.S., LANG J.H., SENTURIA S.D., SCHLECHT M.F., "A study of three micro fabricated variable capacitance", Tech. Digest. 1989 Int. Conf. Solid-state Sensors and Actuators, Transducers 89 and Actuators and Eurosensors III, Vol. 2,1990, pp. 173-179. [MEH 90] MEHREGANY M., SENTURIA S.D., LANG J.H., "Friction and Wear in Microfabricated Harmonic Side-Drive", Tech. Digest. IEEE Solid-state Sensors and Actuators, Workshop, 1990, pp. 17-22. [NGU 99] NGUYEN C.T.C., HOWE R.T., "An integrated CMOS Micromechanical Resonator High Q Oscillator", IEEEJ. Solid. State Circuits, Vol. 34, 1999, pp. 440-455. [OHN90] OHNSTEIN T., FUKIURA, RIDLEY J., BONNE U., "Micromachined Silicon Microvalve", Proceed. IEEE Micro Electro Mechanical Systems, 1990, pp. 95-98. [PAR 00] PARK J.Y., KIM G.H., CHUNG K.W., Bu J.U., "Electroplated RF MEMS Capacitive Switch", Proceed. IEEE Micro Electro Mechanical Systems, 2000, pp. 639-644. [PET 77] PETERSEN K.E., Micromechanical light modulator array fabricated on silicon, Appl. Phys. Lett., Vol. 32, 1977, p.521 [PIS 97] PISANO A.P., "Angular Micropositioner for Disk Drive", Proceed. IEEE Micro Electro Mechanical Systems, 1997, pp. 454-459. [QUE 01] QUEVY E., BUCHAILLOT L., COLLARD D., "Realization and actuation of a continuous membrane by an array of 3D self-assembled micro-mirrors for adaptive optics", Proceed. IEEE Micro Electro Mechanical Systems, 2001, pp. 1145-1148. [SAM 93] SENSOR, ACTUATORS AND MICROSYSTEMS LABORATORY, Report on Research Activities, Institute of Microtechnology, University of Neuchatel, Suisse, 1993. [SAN 01] Sandia National Laboratories: http://www.mdl.sandia.gov.


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[SEN 00] SENTURIA S.D., Microsystem design, Kluwer Academic Publishers, Boston, ISBN 0-7923-7246-8, 2000. [STM 01] Procede THELMA, ST Microelectronics, Casteletto, Italy. [TAI 89] TAI Y.C., FAN L.S., MULLER R.S, "IC-processed Micro-motor: Design, Technology and Testing", Proceed. IEEE Micro Electro Mechanical Systems, 1989, pp. 1-6. [TAN 89] TAN W.C., NGUYEN T.C.H, HOWE R.T, "Laterally Driven Polysilicon Resonant", Proceed. IEEE Micro Electro Mechanical Systems, 1989, pp. 53-59. [TAN 90] TAN W.C., NGUYEN T.C.H, JUDY M.W., HOWE R.T., "Electrostatic Comb Drive of Lateral Polysilicon Resonators", Tech. Digest. 5th Int. Conf. Solid-state Sensors and Actuators Transducers 89 and Eurosensors III, Vol. 2, 1990, pp. 328-331. [TAS 97] TAS T., WISSING J., SANDER L., LAMMERINK T., ELWENSPOEK M., "The Shuffle Motor: A High Force, High Precision Linear Electrostatic Stepper", Tech. Digest. 1997 Int. Conf. Solid-state Sensors and Actuators, Transducers 97, Chicago, June 1997, pp. 777-780. [TOS 96] TOSHIYOSHI H., FUJITA H., "Electro Micro Torsion Mirrors for an Optical Switch Matrix", J. Microelectromech. Syst., Vol. 5, 1996, p. 231. [TOS 01] TOSHIYOSHI H., PIYAWATTANAMETHA W., CHENG-TA C;, Wu M.C., "Linearization of electrostatically actuated surface micromachined 2-D optical scanner", J. Microelectromech. Syst., Vol. 10, 2001, pp. 205-214. [TOS 02] http://toshi.fujita3.iis.u-tokyo.ac.jp. [TRI 97] TRIMMER W.S., Micromechanics and MEMS, Classic and Seminal Papers to 1990, IEEE Press, ISBN 0-7803-1085-3, New York, USA, 1997. [VAN 98] VAN KESSEL P.F., HORNBECK L.G., MEIER R.E., DOUGLAS M.R., "A MEMS based projection display", Proc. IEEE, Vol. 86, 1998, pp. 1687-1704. [VDO 97] VDOVIN G., SARRO P.M., MIDDELHOEK S., "Technology and Applications of Micromachined Silicon Adaptative Mirror", Optical Engineering, Vol. 36, 1997, pp. 5509-5513. [YAO 00] YAO J.J., "RF MEMS from a device perspective", J. Micromech. Microeng., Vol. 10, 2000, pp. R9-R38. [YAZ 98] YAZDI N., AYAZI F., NAJAFI K., "Micromachined Inertial", Proceed. IEEE, Vol. 86, 1998, pp. 1640-1659. [YOU 96] YONG D.J., BOSER B.E., "A micromachined variable capacitor for monolithic lownoise VCOs", Tech. Digest. Solid State Sensor and Actuator Workshop, 1996, pp. 86-89.


Chapter 5

The LIGA Microfabrication Technique Wolfgang Menz Institute for Microsystems Technology (IMTEK), Albert-Ludwigs-University Freiburg, Germany

Jurgen Mohr Institute for Microstructure Technology (IMT), Research Center Karlsruhe, Germany

1 Overview The production of microstructures by the LIGA process is schematically represented in Fig. 1 and 2. The essential process steps are X-ray lithography with synchrotron radiation, the electroplating of metals and the molding of plastics. This combination of process steps have been given the name LIGA; LI for X-ray lithography, G for galvanic or electroplating and A for Abformung (German word for molding). In X-ray lithography, as a first step, a plastic layer several hundreds of micrometers thick is applied to a metallic base plate or an isolated plate with an electrically conductive cover layer used as the substrate. The X-ray sensitive plastic is either polymerized in place directly on the base plate or glued to it. Until now PMMA (polymethyl methacrylate) has been used almost exclusively as X-ray resist because of its high contrast known from electron beam lithography. To form the micro structure an absorber pattern of a mask is transferred into the thick plastic layer with the aid of extremely parallel and high intensive synchrotron radiation with a characteris-

Fig. 1 The basic process steps of the LIGA Technology.

Fig. 2 Manufacturing of a molding tool for mass fabrication.

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tic wavelength between 0.2 and 0.6 nm. The X-ray radiation which passes through the mask is absorbed in the resist and leads to a chemical modification. In the case of PMMA the chemical resistance changes because of bond breaking in the long molecular chain, so that these regions can be dissolved with a suitable developer. Using micro-electroplating a complementary structure can be formed from the resultant resist structure after the development process. The metal e.g. copper, nickel or gold is deposited in the void spaces of the electrically non-conducting resists where the deposition of the metal starts on the electrically conducting base plate. Using these metal templates in injection molding, reaction resin casting or in hot embossing, an almost arbitrary number of highly detailed plastic copies can be fabricated at relatively low cost [NOKE92]. These plastic structures can again be filled by electrodeposition with metals or serve as 'lost forms' for the production of ceramic microstructures. In the following, the individual steps of the LIGA process will be described in detail. The first process step, X-ray lithography, which defines the structure quality for the following steps and thus represents the most critical process step, puts especially high requirements on the necessary X-ray masks. So first, the important steps of the production of suitable masks are discussed, before X-ray lithography, electroplating and molding techniques are dealt with. 2

Mask Production

2.1 The Principle Construction of a Mask A mask which can be used in the LIGA process consists of an absorber, the carrier foil and the frame of the mask [BACH91]. In contrast to masks for optical lithography as used in microelectronic fabrication, the absorption characteristics required are much higher while the thickness of the carrier foil is much lower. This requires a different mask fabrication process. 2.7.1 Absorber The information which is to be transferred into a thick resist is given by the structure of the absorber, that is the region of the resist which is shielded from synchrotron radiation by the absorber. Whilst in optical lithography using UV light, an approximately 0.1 μm thick chromium layer on the mask is sufficient, the absorber in X-ray lithography must consist of a material with a high absorbency for X-ray radiation in the particular wavelength region of interest. Materials are considered which have a high atomic weight and therefore a high absorption coefficient, such as gold, tantalum or tungsten. Gold is mostly used due to its ability to be deposited by electroplating. Tantalum or tungsten find relatively few applications and are structured by reactive ion etch processes. To achieve a low X-ray transmission, that is, to maximize the absorption of the appropriate synchrotron radiation necessary for structuring in the LIGA process, the gold absorber must have a thickness of more than 10 μm. The thickness required

The LIGA Microfabrication Technique

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depends not only on the characteristic wavelength of the synchrotron radiation, but also on the height of the irradiated resist. The necessary absorber thickness increases with increasing resist height and thus, decreasing wavelength. In the case of gold the absorber structure is built up by micro-electroplating. Thus, a resist layer is structured, whose thickness is somewhat greater than the required absorber thickness and subsequently gold is electro-deposited in the free space. With the exception of structuring with synchrotron radiation itself, there is no process at the moment which guarantees either the required precision or the structuring of this resist layer with a height of more than 10 μm and a precision in the submicrometer region. Therefore, initially an X-ray mask with an absorber height of about 3 μm has to be produced, a so-called intermediate mask. There are several processes for precise structuring of the resist layer of this height which will be described in more detail in the following. Using synchrotron radiation, the pattern of the intermediate mask is transferred into an approximately 20 μm thick resist layer, which after development serves as the mold for gold electro-deposition. The mask produced in this way which has sufficiently high absorber thickness, is called a process mask. The use of synchrotron radiation (which is "softer" than in the final process of exposure of the LIGA structure) leads next to no structure loss in this copy step because of the high parallelism and the small wavelengths. 2.1.2 Carrier Foil The absorber structures are fabricated on a suitable carrier [SCHO91]. In optical lithography approximately 2 mm thick polished glassor quartz plates are used. These materials cannot be used in X-ray lithography since at such thicknesses they are not transparent enough to the relevant X-rays. The carrier foil must have a low absorption coefficient and be thinner, in order to absorb less of the X-ray radiation. Therefore, materials with a low atomic weight like e.g. beryllium, carbon (diamond), silicon and its compounds, plastics or metals with a lower atomic number are chosen for membrane materials. When choosing materials an Fig. 3 Transmission of different optimum must be found between mechanical materials for mask carriers. rigidity, dimensional stability and transparency to the synchrotron radiation. Furthermore, the carrier material must be resistant to X-ray radiation. Among the metals, beryllium shows an ideal transmission (Fig. 3). With comparably thick carrier layers of several 100 μm, transparency is still high in the useful wavelength region. For reasons of high toxicity (dust from Be and its oxides can cause lung diseases) the handling of Be-layers represents a problem in laboratories which are not especially

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equipped for such work. Titanium is an alternative to beryllium as the carrier layer. Because of the high absorption coefficients of titanium compared to beryllium, the membrane thickness must be considerably smaller and should only be several micrometers. The thin carrier foils are stretched across a b e to provide the necessary mechanical robustness for the electroplated absorbers. This allows easier handling of the mask during fabrication and alignment.

2.2 Production of Carrier Foil The process steps for the production of an interMediate mask are schematicallyrepresented in Fig. 4. The starting point is a membrane which is freely stretched in a frame. In the case of metals it is typically produced by PVD processes onto a solid substrate. Known processes from the semiconductor technology are used to fabricate the membrane out of silicon and its compounds. After the manufacture of the membrane a window of a desired size of optical adjustment with a microscope is etched into the substrate. In the case of a titanium mask, the frame is made from invar (alloy of 18% Cu,28% Ni and 54% Fe) which best matches the thermal requirements. In the case of silicon a Pyrex ring, onto which the ~ j4 ~ hocess . steps for an wafer with its mask window is fastened, is used as intermediatemask. the frame.

2.3 Structuring of the Resist for X-ray ZntennediafeMasks In the case of an intermediate mask a resist of about 3-4 pm thick is spun onto a free structural foil. The resist is structured e.g., with the aid of an electron beam writer or by optical lithography. After developing, the free regions are filled with gold by electroforming. Finally the non-irradiatedresist is removed with a strong solvent or by applying an oxygen plasma. On duplicating the X-ray intermediate mask (gold absorber thickness < 3 pm) to the X-ray process mask (gold absorber thickness >10 pm) hardly any loss in quality results due to the advantageous properties of the synchrotronradiation. The quality of the microstructure is therefore largely determined by the quality of the intermediate mask. Therefore, on the intermediate mask the absorber structure should have the most perpendicular walls possible, in order to make sure that there is a sharp transition between the irradiated and non-irradiated regions on production of the process mask by X-ray lithography.


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Direct Electron Beam Lithography An electron beam writer with high acceleration voltage (e.g. 100 keV) can be used for direct structuring of an approximately 3 urn thick resist layer [HEIN92]. The 100 keV acceleration voltage keeps the necessary steepness of the edges because the electron lobe which results from scattered electrons in the material takes place primarily in the substrate. PMMA is used as the resist especially for high precision requirements. PMMA shows a very high resolution in electron beam lithography; however one of its disadvantages is a low sensitivity which results in long writing times. The 3 um thick PMMA layers which are required for the intermediate masks are produced by double coating with a spin coater with a very homogeneous thickness. A temperature annealing step is carried out after every coating step to reduce the susceptibility to stress cracking. In case of less demanding precision also negative resist materials based on the diazo systems are available, which represents a compromise between resolution and writing time. 2.4 Production of the Final Masks The final mask or process mask is produced by transferring the pattern of an intermediate mask into a resist using synchrotron radiation (Fig. 5). PMMA is used as the resist, which is applied to the mask carrier by direct polymerization with a thickness of about 20 um. In contrast to the spin coated layers of similar thicknesses polymer resist structures can be produced which are not susceptible to stress cracking. In the case of the process mask, beryllium instead of titanium can be used as mask membrane. Carrier foils of beryllium can be much thicker than titanium foils due to their lower absorption of X-ray radiation. They are produced by fine mechanical processing of metal sheets. At present this preparation process is limited to a thickness of 500 urn, so that the beryllium carrier can be used only for X-ray masks. This metal sheet is coated on both sides by a silicon nitride layer, in order to avoid corrosion during utilization. As the electroplating seeding layer, a PVD deposited gold layer is used. This may cause problems in resist adhesion which have to be overcome initially by adhesion promoters. As X-ray lithography involves pure shadow projection of the absorber structure an exact image of the absorber structure in the resist results in a sufficiently thick gold layer. Also lateral roughness of the absorber structure of the intermediate mask, which is only of some tens of nanometers, is completely transferred onto the process mask. For this copy step the wavelength of the synchrotron radiaFig. 5 Process steps of a tion must be considerably larger than for the strucworking mask. turing of resists with a thickness of several 100 um.

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With the small gold thickness of the absorber of the intermediate mask the necessary contrast between the illuminated and non-illuminated region can be achieved only by using soft X-ray radiation.

3

X-ray Lithography

By X-ray lithography, the pattern of the mask is transferred into a resist layer, which has a thickness of up to some millimeters, using synchrotron radiation. The region which is exposed to X-ray radiation undergoes a chemical modification. The degree of modification depends on the X-ray sensitivity of the material and on the energy of the absorbed radiation in the resist. The quality of the structure which can be achieved depends on the divergence of radiation, the degree of smearing of the radiation on the absorber edges and the range of the photoelectrons which are produced in the resist layer. In addition secondary effects must be considered, like fluorescence electrons produced in the mask membrane and photoelectrons which are released in the substrate. Also mask distortions resulting from thermal load of the mask membrane have to be taken into account. 3.1 Production of Thick Resist Layers In the LIGA process a resist layer is polymerized directly onto a base plate [MOHR88] or a polymerized plate is glued or welded onto the base plate. Polymethyl methacrylate (PMMA) is almost exclusively used as the resist material. In case of direct polymerization, the raw material is a viscous cast resin, which consists mainly of a low viscosity monomer, methyl methacrylate, and a solid component dissolved in it. After addition of a polymerization starter (e.g., peroxide), polymerization of MMA to PMMA results, either at a raised temperature or by addition of an initiator (e.g., anilin) at room temperature. The molecular structure of the monomer MMA and the polymer PMMA are represented in Fig. 6. In case of using solid PMMA plates the above described cast resin is applied as a thin adhesion layer (10 μm) onto the base plate and the PMMA plate is pressed onto this. The cast resin used assaaadhesive hardens at room temperature by using the same material as with direct polyFig. 6 Structure of the monomer MMA and the polymer PMMA.

merization.

The LIGA MicrofabricationTechnique

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3.2 Beam Induced Reactions and Development of Resists

The irradiation of resist leads to a scission of the polymer chain of PMMA i.e. to a radiation induced reduction of the molecular weight. From it can be assumed, as with PMMA, that by increasing the dosage of radiation the average molecular weight decreases from an initial value of 650,000 g/mol to a minimum limiting value between 2,500 g/mol and 3,000g/mol at very high dosages of radiation (Fig. 7). This is-initiated by an electronic excitation of the molecular bonds. The X-ray photons which hit the PMMA with energy in the keV region, are absorbed by single atoms by the photo effect and release high energy photo- and Auger-electrons. Their energy is transferred to other molecules or molecular building blocks thereby generating secondary electrons, which are available for ftuther excitation, until they are ultimately thermalized. Finally ionized and excited molecules as well as thermal elecFig. 7 Influence of the radiation dosis trons remain. D on the average molecular weight. In the case of PMMA a cleavage of the ester side chain results from the excitation (Fig. 8), so that a radical ester group and a radical C-atom are available in the polymer main chain. The radical C-atom generates the scission of the chain resulting in a radical part and a saturated part via a double bond, which originally forms from the free C-atom [SCHN83]. Thus, a short stable chain and two radicals remain, the radical polymer chain and the radical ester chain, which are free to react with one another or with other radicals to form stable molecules. A very good developer suitable for PMMA in X-ray lithography is a mixture of ethylene glycol-monobutyl ether, monoethanol amine, tetrahydro-1,4oxazine and water [EHRF88]. The solubility of PMMA in this developer at different temperatures is depende molecular weight. For example, at a developing temperature as low as 40 OC, PMMA with a molecular weight of over 60,000 can no longer be dissolved in the developer. With increasing temFig. 8 Mechanism of chain fiacture of perature the solubility increases, and PMh4A due to exposure of high energy radi- the solubility curve becomes much ation. flatter. Nevertheless, up to 50% of

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PMMA molecules with a molecular weight of 130,000 are dissolved in the developer at e.g. 80 °C. For low temperatures the solubility curve is much steeper which results in a higher contrast between non irradiated and irradiated parts. 3.3 Requirements on the Absorbed Radiation Dosage PMMA excels in that it gives very good image reproducibility i.e. a high resolution. However, it has the disadvantage that it is relatively insensitive, so that a high radiation dosage is necessary to realize a significant reduction in the molecular weight. A typical molecular weight distribution of PMMA prior to irradiation is shown in Fig. 9a. The distribution is bimodal with an average molecular weight of 600,000 [ELKH93]. At a temperature of 38 °C the developer dissolves 50% of the PMMA molecules with a molecular weight of up to about 20,000 (shaded area in Fig. 9a). As the fraction of such polymer chains in the non-irradiated PMMA is very small, the developer can in principle dissolve only a very small fraction of the nonirradiated resists. An impairment of the microstructure is not noticeable due to the presence of this small amount of low molecular weight components because it is integrated among the long chained molecules. Irradiation with a dosage of 4

Fig. 9 Distribution of the molecular weight before and after exposure with 4 and 20 kJ/cm3. The shaded area indicates the solubility of PMMA in GG developer by more than 50%.

kJ/cm3, results in a monomodal distribution with an average molecular weight of 5.700 (Fig. 9b). This distribution lies almost completely in the area in which the developer dissolves more than 50% of the PMMA, so that during the developing process it can be removed. With a radiation dosage of below 4 kJ/cm3 the molecular weight is not reduced enough, i.e. the fraction of insoluble polymer chains would be too large, so the irradiated region could not be completely dissolved, and therefore PMMA residues would remain. Therefore, 4 kJ/ cm3 represents a limiting value for the minimum dosage to be deposited. This limiting value depends on the temperature of


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the developer. It can be shifted down if the developing is supported by convective measures (e.g. stirring, ultrasonic). 3 With a radiation dosage of 20 kJ/cm a distribution results with an average molecular weight of 2.800, as shown in Fig. 9c. The whole of the PMMA is dissolved relatively quickly in the developer at such a high dosage. PMMA, however, should not be irradiated with a higher dosage, as it can then lead to damage such as formation of bubbles, which would hinder a defect free production of microstructures. Therefore, 20 kJ/cm3 corresponds to the limiting value, i.e. the maximum irradiation that PMMA can be subjected to. This value is dependent on the temperature of irradiation and decreases at higher temperatures in the resist. As a consequence there is a decrease in the value by 14 kJ/cm3, especially for tall samples (e.g. 1 mm), where the heat dissipation to the substrate is reduced due to the higher heat resistance of the thicker PMMA. Figure lOa is a sketch of a typical set-up for a X-ray exposure of a LIGA structure with synchrotron radiation. Figure lOb shows to what degree and in which regions the synchrotron radiation is absorbed in a typical experiment used to produce microstructures. One can now calculate how much dose would be deposited in a 1 urn thick PMMA sample placed at a particular location in the X-ray beam [BLEY91].

Fig. lOa Experimental set-up for exposure.

Fig. lOb Absorbed energy by a PMMA test piece of lμm thicknessat exposure to synchrotron radiation.

The curve is a typical representation for the electron-stretcher ring ELSA at Bonn University with a characteristic wavelength of 0.5 nm. The experimental conditions, especially the irradiation time and the pre-absorber, are chosen such that on the underside of a 500 μm thick resist layer, a value no less than 4 kJ/cm3 (limiting dose DG) and on the upper side a value no more than 20 kJ/cm3 (surface dose D0) is main-

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tained. As the long wavelength beam is absorbed in relatively thin layers near the surface, it does not contribute to the attainment of the lower limit dosage deep in the sample. In order not to exceed the upper limit, the long wavelength component of the synchrotron radiation is filtered out through a 200 μm thick pre-absorber, which is placed in the beam path in front of the mask. Polyimide (Kapton) is used as the preabsorber. In principle, beryllium or other materials can also be used. For these preabsorbers it is important that the absorption coefficient for soft X-ray radiation is as large as possible in contrast with that for hard X-ray radiation. The area of the resist which is shadowed by the absorber of the mask must not be attacked by the developer, therefore in this region the depos3

Attraction

ited doses must lie under 100 J/cm (damage dosage Ds). From this value the demands on the contrast of the mask are deduced as 200. In order to reach such a high value, the height of the gold absorber must amount to about 10 μm in this CXam

3.3.1

Ple.

Fresnel-Dijfraction, Photoelectrons

In Fig. 11 the effect, which results from the Fresnel diffraction in an absorber edge, is shown, as well as the influence of photoelectrons which Fig. 11 Resolution limiting are produced in the resist. Furthermore, the effect effects at exposure with synof the divergence of synchrotron radiation is chrotron radiation. shown. The X-ray radiation releases photoelectrons as well as Auger-electrons in the resist, whose reaction with the resist material brings about the desired chemical changes. These electrons collide with resist molecules and have an infinite range due to their energy. In transit the electrons gradually lose their energy. Thereby energy can also reach the shielded regions, which leads to a decrease in the sharpness of the edges. As the range of the released electrons increases approximately by a power of two with the energy, this effect increases with shorter wavelengths of the synchrotron radiation. For the wavelengths region of interest in X-ray lithography the range of interest lies only in the region of 0.1 μm. The total error, which is the result of adding both effects, is represented in Fig. 12. It can be recognized that the unsharp area amounts to about 0.1 μm. The minimum error is obtained with a wavelength of about 0.3 nm.

-


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4 Galvanic Deposition

The microstructures produced by Xray lithography from plastic, mostly PMMA, can be the end product in some cases, as with microoptical components. In many cases, however, metallic microstructures are manufactured in which the cavities of the plastic are filled with metal by electrofonning. To produc

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cation technique, robust and shape

Fig. 12 Influence of edge definition on therrretainingmoddinsertsfrommetalareareree characteristic wavelen& in 500 pm thick necessary. The fabrication Of these molds is carried out likewise by X-ray resist. lithography and finally electroforming. Likewise micro-electroforming is also applied, to produce the gold absorber structure of the X-ray mask. Thus, micro-electroforming plays an important role in the different manufacturing steps of the LIGA process. To produce the absorber on the mask, gold plating is used, whereas nickel plating is predominantly used to produce the structure. Up until now copper plating has been of lesser importance. Also alloy plating from a nickel-cobalt solution (hardness) or from a nickel-ion alloy (magnetic properties) is used for special structures. 4.I Galvanic Depositwn of Nickelfor the Production of Microstructures.

A nickel sulfamate electrolyte is used for the galvanic production of nickel microstructures and likewise for molds, which are required for plastic molding. It contains 75-90 g/1 of nickel in the form of nickel sulfamate, 40 g/l of boric acid as a buffer and approximately 4g/l of an anion active wetting agent. The pH of the bath is between 3.6 and 4 and the bath is running at a temperature between 50 and 60 "C. Electroplating is carried out on the substrate which has been structured by X-ray lithography, which generally is covered by an oxidized titanium adhesion layer. This is a compromise between the requirements for good resist adhesion and ability to initiate electroplating. Recent points of view are that gold or copper surfaces would be much more suitable, but in this case the resist adhesion is not sufficient. On oxidation of titanium under the influence of a hydrogen peroxide solution, an approximately 40 nm thick oxide layer is produced, which differs considerably fiom a natural oxide layer, which is only 3 nm thick. It was shown that the artificially produced oxide layer represents a very good initial seeding layer for the metal electrodeposition and provides adequate adhesion of the fabricated metal microstructures. This is because on oxidation TiO, is formed where n is somewhat less than 2. In contrast to crystalline TiOz, which is an excellent insulator, the amorphous TiO, is electrically conducting and can therefore be used as a primary electroplating seeding. Wet chemical

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oxidation of the titanium surface leads to microscopically roughened surfaces containing microchannels, in which the resist can be mechanically anchored. Using this method, good adhesion between plastic microstructures and metal can be achieved PACH921. The deposition is carried out with current densities of 1-10 A/dm2, which leads to a growth rate of 12-120 pm/h. The current density is the critical dimension for the intrinsic stress which Fig. 13 Metallic honeycomb ShlChUe. A results in the structure, and which human hairservessascomparisonn(600umMshouldldnotbeeegreaterthann100to200N/ diameter). mm2. These low intrinsic stresses are necessary so that a large structuree.g. a large honeycomb or a mold, does not bend after being detached from the base plate or does not detach from the base plate during the electroplating. Figure 13 shows a nickel honeycomb structure as an example of an electroformed structure. The height is 180 pm, the wall thickness 8 pm.One can see small non-uniformities of the edges. These are already present in the mask in the form of ridges. As can be seen from the picture the surface of the deposited nickel structure is rather smooth. This depends strongly on the roughness of the substrate and the layer thickness. In general, for layers with heights above 100 pm a roughness 0,of somewhat under 1 pm results. The roughness can be reduced by mechanical finishing (lapping, polishing, milling) if required. 2.22222MMolddddInserterterttttFabricartioncatcatca

The production of a LIGA mold insert is schematically represented in Fig. 14. In principle, it is processed similarly to the production of metallic microstructures. In contrast however, the metallic deposition is not stopped when the metal has reached the upper edge of the resist structure, but continues so that it is deposited laterally over the resist microstructure i.e. the structures are completely covered by the metal. The metal deposition over the top of the structure is continued, until an approximately 5 mm thick metal layer has formed. As the microstrucFig. 14 Principle of mold insert ture and the metal plate are produced in a continufabrication. ous deposition process, an excellent bonding


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between the microstructure and the metal plate results. Special care must be taken that no intrinsic stresses appear during the electroforming of the stable metal layers, which could lead to a bending of the mold insert. Thus, because of this risk the current density is limited. After surface treatment of the backside of the mold insert, it is removed from the base plate. In order to separate the two components by parallel stripping, the base plate is treated prior to electroforming, so that a bad adhesion between the mold insert and the base plate results. However, it is also possible to deposit metallic interfacial layers onto the base plate by electroplating. If these interfacial layers are selectively etched away from the nickel mold, it can be separated from the base plate without mechanical stress. The same is possible by fully removing the substrate by etching. 4.3 Fabrication of Microstructures by Injection Molding In the injection molding process polymerized plastics are processed as granulate, powder or as extruded profile material. The molding material is melted in the plasticizing unit of an injection molding machine. It is injected into the mold voids of an injection mold insert in this more or less viscous state. The solidification of the molding compound is carried out by cooling down the plastic melt in the injection mold insert. Classical materials for injection molding are polyvinyl chloride (PVC), polyacrylnitrile butadiene styrol (ABS) and also PMMA. Correspondingly, the equipment of an injection molding machine can process thermoplastics, duroplastics and elastomers. Thermoplastics become ductile and can be processed several times. Amorphous and semi crystalline thermoplastics are to be distinguished by their structure. Duroplastics react under the influence of heat and subsequently cross-link. Unlike thermoplastics they can not be melted again by heating. The technically important duroplastics are: phenolformaldehyde, melamine formaldehyde, epoxy resin, silicon resin and polyurethane. Elastomers are plastics, whose plastic-elastic behavior is similar to natural rubber, i.e. elastomers run mainly under the collective term of synthetic rubber. The typical operation of an injection molding machine is represented in Fig. 15. It can be divided in three main steps: •Plasticization, i.e. melting the raw material by heating the polymer in the region of the screw conveyer. •Injection of the melt in the normally cold mold insert under high pressure. This task is carried out by the injection unit. •Opening the tool and ejecting the hardened molded article. The retention capability of the tool is brought about by two mounting plates, where at least one is movable, in order to be able to open and close the tool. On the movable mounting plate a device is attached to apply and maintain a mold force. This part of the injection molding machine is called a closing unit. The hydraulics consist

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of a pump and a coil system with valve, washer and throttle for the production of pressure to control the machine movements. The granulate or powder molding compound is in a funnel, which is above the feed opening of the injection cylinders and can be opened or closed by a recorder. A Fig. 15 The functional principle of an injection moldscrew moves in the axial ing machine. direction inside the injection cylinder. By turning the screw, the molding compound reaches the feed opening and on further rotation reaches the screw tip. At the same time the molding compound is melted by these cylinders which are heated by electrical power or a temperature controlled oil bath. During initial feeding of the screw, the melt is compacted and injected under pressure through a die into a closed mold form. It solidifies and cools in the mold form and can be shaped into the finished product after a short time. The parameters, temperature, time and pressure must be very carefully regulated so that the functions, which include the molding cycles, plasticization, injection and cooling, can be carried out reproducibly. The material parameters which are influenced by the individual function cycles are listed for the case of macroscopic injection molding in Fig. 16 [HABE90]. Typical LIGA-structures with high aspect ratios and spaces of 10 μm width and a depth of more than 100 μm can be filled up with PMMA by microinjection molding. A defect free mold release is attained, however, only when starting with extremely smooth walls of the mold insert, as is attainable by the LIGA process, whilst with a spark erosion processing even in the polishing mode it is not possible to have mold release of structures with similar dimension because of the large surface roughness. 5 Plastic Molding in the LIGA Process

Fig. 16 Typical pressure-time sequence of an injection molding machine and the influence on material parameters.

The process for the production of a primary structure made of PMMA as well as using galvanic deposition leading to a metal microstructure, were outlined in the previous chapter.


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The process steps are work intensive and therefore expensive. For industrial commercialization, the LIGA process is especially interesting because of the possibility of mass fabrication by injection molding, reaction injection molding or hot embossing. Therefore, the processes of molding will be described next. In the context of micromolding, these processes are characterized less in the injection machine than in the tool retention fixture which is of special importance for microtechnology as well as in processes control and regulation. Other particular constraints are imposed by the mold insert. 5.1 Production of Microstructures by Reaction Injection Molding The materials used in reaction injection molding are low viscosity monomers, which are blended with a soluble initiator for polymerization in a mixing chamber shortly before injection into the mold. After injection into the mold, the molding compound hardens by polymerization. The classical material used for reaction injection molding is polyurethane. The scheme of a RIM machine is shown in Fig. 17. Two or more of the liquid reactants are injected into the mixing chamber under high pressure, typically 100-200 bar. The dose in the mixing chamber must be very precise, and the correct stoichiometric ratio should be maintained throughout the reaction. At higher pressures the low viscosity components attain a higher speed in the mixing chamber and are blended with each other. The mixture flows under a comparatively low pressure of 10 bar or less (in contrast to injection molding), and with low viscosity into the mold, because the polymerization reaction starts with a lag in time. The low viscosity and the resulting low injection pressure are the reasons for the growing success Fig. 17 Functional scheme of a reaction of the RIM-process. Molding parts of molding machine (RIM). up to 50 kg can already be prepared. Large molds for the reaction injection molding are relatively inexpensive to produce because of the low mechanical requirements due to the low pressure. 5.2 Fabrication of Microstructures by Hot Embossing In hot embossing an already polymerized plate of thermoplastic material is shaped by compression at high temperatures to form the microstructures.

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In the simplest case the plastic plate is put onto a solid base plate, which is made such that by hot embossing interlocking takes place with this base plate. Subsequently both plates are brought to a temperature, which lies above the glass transition temperature of the polymer, for PMMA e.g. about 160 °C. At this temperature the molding material is in the viscoelastic state, and the mold insert can be pressed relatively easily into the molding material. The mold release takes place after cooling below 80 °C. The pressure in hot embossing is about 107 Pa. Also in this case, to avoid bubbles, pre-evacuation of the mold insert and the space between the mold insert and the plastic plate is necessary. Although the plastic is in a viscoelastic state, and the pressure can be relatively high, the entire molding material between the base plate and the end of the mold is not displaced completely. A thin layer of several tens of micrometers remains on the base plate, depending on the pressure and the shaping temperature. In order to isolate the microstructures which are connected to this residual layer on the substrate, the film is removed by reactive ion etching (RIE) in an oxygenplasma. This process is suitable to produce microstructures on processed silicon wafers i.e. over microelectronic circuits. The process sequence is schematically represented in Fig. 18 [MICH92]. In the first step the molding compound is applied to the wafer, which is covered with protection- and metallization layers. This can be done e.g. by direct polymerization (a). After solidification the process described above is carried out (a, b). In this case after mold release the thin residual layer is located directly above the metallic layer of the wafer. Also, it is removed by reactive ion etching (RIE) in an oxygen plasma. Therefore, the RIE process is carried out such that the ions impact the substrate as perpendicularly as possible and thereby hardly any erosion occurs on the side walls of the plastic microstructure. In this way the metallizing layer between the microstructure is released and can be used as the electrode in a successive electroforming process (d). After metal deposition, the plastic structures on the wafer are removed by solvents. To electrically Fig. 18 Process sequence for isolate the metallic microstructures the metallizing microstructures on integrated layer is removed between the structures (e). A sputcircuits by hot embossing. ter etch process can be used with argon , but also wet chemical etch processes are possible, which are carried out in such a short period of time that the metallizing layer is not removed under the microstructure. In order to ensure no damage of the microelectronic circuit due to this process step, a metallization layer may still exist on the wafer as a protection layer, which was only opened by photolithographic processes at bonding pads to the integrated circuit.


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6 Variations and Additional Steps of the LIGA Technology In order to cover the widest possible spectrum of uses, the standard LIGA process is extended by numerous process variations. They will be introduced and discussed in the following chapters. 6.1 Sacrificial Layer Technology If micromechanical sensors or actuators are to be produced with microfabrication methods, in many cases stationary microstructure parts as well as moveable microstructures must be designed. Often movable and stationary microstructures are connected, so that usual assembly is not possible. Such assemblies are also often limited by the low dimensional tolerances which are required. These constraints apply to e.g. acceleration sensors, gyrometers, linear actuators, resonators and similar structures. Movable structures are produced in silicon micromechanics, in which e.g. a pit is made by anisotropic etching under a thin elastic cantilever. In surface micromechanics freely moving structures are produced, in which several thin, structured layers which are made of different materials are placed on top of each other. The so-called sacrificial layers are selectively etched from between the layers lying above and below. It is also possible with the LIGA process to produce moveable microstructures by introducing sacrificial layers [MOHR90]. Therefore, for optimum realization of sensors and actuators a large range of materials is available as well as the possibility of large structure height with no limits in the lateral shaping. As an example, the process steps for the production of moving LIGA microstructures is shown in Fig. 19 and 20 for an acceleration sensor. In this example microstructures have electrical functions, like most sensors and actuators, so that the individual parts of the microstructure must be electrically isolated from each other. The process is therefore based on an electrically non-conducting substrate, e.g. from a silicon wafer equipped with an insulation layer or a ceramic substrate. A metallization layer is applied onto this layer using a PVD process. Stringent requirements are made on this layer with respect to the adhesion to the substrate and also to the subsequently electroplated metal layer. These requirements cannot be readily fulfilled by a single layer. Therefore two different metal layers are used, one being the adhesion- and the other the primary electroplating initiating layer. The layer systems made of chromium and silver have proved to . exhibit good adhesion where chroFig. 19 An acceleration sensor in LIGA . , „ . , . , . mium possesses a good adhesion AtoAthe technique. . •* . ., , substrate and silver possesses a good

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adhesion to the electroplated layer. When necessary, pas-

sivationnnnlayers are intro--

duced, in order to avoid A h ,-1 problems with th etch processes. In order to isolate the microstructures fiom each other, produced later by the LIGA process, the layer system is produced Fig. 20 F’rocess steps for movable microstructures by the LIGA process and is structured by optical lithog(sacrificial layer technique). raphy and wet etching processes. Similarly, in the metallization layer, structured conducting paths are electrically connected to isolated areas of the system. The sacrificial layers are now applied using the PVD process on the pre-treated substrates.The following requirements are placed on these layers: -goad ability to patterning, -good adhesion of the resist used in X-ray lithography, -good initiation and good adhesion of the subsequent electroplating, *goodselective etchability compared with all materials which are used as substrate, metallization layer or sensor- i.e. actuator material, *quickremoval also underneath structures with large areas. In the LIGA process titanium has proven to be a good sacrificial layer, as it possesses both good adhesion to the resist and to the electroplating initiating layer and also can be etched with hydrofluoric acid, which does not attack standard materials used in the LIGA process (Cr, Ag, Ni, Cu). The thickness of the titanium layer should be large enough, so that the movable structures show a sufficiently large gap between them and the substrate. With too small a gap the risk may be that the movement of the microstrucftve will be impaired by contaminants. Also for etching of large areas under microstrucaues, it is favorable if the gaps which are formed are not too narrow. However, with increasing thickness of the sacrificial layers the precision with which these layers can be structured by simple photolithography and wet etch processes, decreases. In addition, with larger thicknesses, the inner stresses of the applied layers are too large so that a good adhesion onto the substrate is no longer guaranteed. A titanium thickness of 5 pm is a good compromise between these opposing requirements. This titanium layer is structured, using optical lithography and etch processes, so that on subsequent structuring with X-rayradiation the movable parts of the microstructure are constructed above the titanium layer, whilst the stationary parts are constructed directly on the electroplating initiating layer.


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The resist having a thickness of several hundred micrometers is applied to the substrate in the standard way over the structured metallization and sacrificial layers. This resist layer is finally irradiated with synchrotron radiation through a mask. The X-ray mask is adjusted with respect to the previously structured layer by alignment marks on the mask and on the metallization layer. After illumination with synchrotron radiation the illuminated area is dissolved and the spaces produced are filled with metal by an electroplating process. The electroforming takes place on the metallization, as well as on the sacrificial layer. After the metallic construction of the sensor i.e. the actuator structure, the non-illuminated resist is removed. Next the titanium sacrificial layer is selectively etched. Hydrofluoric acid (0.5%) has shown to be particularly suitable. Thus, the part of the microstructure which was on top of the sacrificial layer becomes freely movable using this process, whilst the other parts of the metallic microstructure on the metallization layer are well anchored to the substrate. The electric contacts can be made between the individual parts of the microstructure via bond pads and conducting lines, which were produced in the metallization layer. As a consequence electrical connection with separately manufactured integrated circuits is accomplished. The micromechanical and microelectrical components that are connected in such a way are then packaged in a common housing. However, direct structuring on wafers with integrated circuits is not possible using X-ray radiation since, for example, the gate-oxide in an electronic circuit is damaged by X-ray radiation. Direct integration of electric circuits and microstructure can be carried out using hot embossing. In this case the bond connections are avoided and a very high integration density is attained. At the same time, of course, moving parts can be produced. For this the substrate used in the injection-compression process is provided with a structured sacrificial layer and the molding, aligned with the pre-structured substrate, is carried out. Several examples for the movable microstructures are introduced in the following section. 6.2 3D-Structuring In principle, the standard LIGA process, whereby the structuring is carried out by a shadow projection, allows only the production of structures with a constant structure height and perpendicular walls. However, many structures require a variation in geometry in a third dimension. This can be achieved in simple forms by the structuring in different planes (stepped structures), by tilting of masks and substrates relative to the beam (suitable structures), by addition processes (structures with spherical surfaces) or by effective use of secondary radiation (conical structures). 6.2.1 Stepped Structures The production of stepped structures, of which some examples are shown in Fig. 21, are made possible by three different variations in the process.

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In the first process, microstructureswhich were structured in the first lithography

step are illuminated through a second mask. This mask, which must be aligned with

the structure of the fmt illumination, contains the structure details, with which the first structure should be overlaid. The radiation dose is chosen such that the lower limit dose is supplied not on the base, but at a specific height within the structure. During the subsequent development step, the resist is therefore not fully developed all the way to the base. This results in a step in the structure. Disadvantages of this relatively simple method are that this dose limit does not

represent an exact value and is especially dependent on the developmenttime. Therefore, the lack of definition, with which the step height can be achieved, is relatively large. Also the surface of the step is quite rough due to the developmentproperties of the resist. In principle, a similar effect can also be achieved if one makes the

absorber on the mask of two different materials. Whereas this leads to an expensive mask production, no second radiation is necessary during the production of the structure. In the second method, a structure is produced by X-ray lithography and electroplating during an earlier step. The desired step height of the finished structure is achieved by milling the surface. In a second Fig. 21 Stepped structures fabricated by a two- step, a second structure is produced fold LIGA process. on this substrate with its polymer/ metal compound. Although by using this method an exact step height can be achieved, the problem remains that the position of the two structure parts with respect to each other depends on the alignment precision during the irradiation. Variations in the vertical position of more than 1 pm

must be acceptable. Furthermore, the adhesion of both structure parts is not optimal. In the third method, a base plate is pre-structured such that a stepped substrate is present FlfjLL961. The structuring is carried out either with mechanical methods or via lithography and electroplating depending on the requirements of precision. The lateral precision of the steps can be set to less than 1 pm by sputter etching processes. These stepped plates are used as the substrate in the LIGA process and are coated with resist. The structuring with X-ray radiation is carried out all the way to the substrate base. The vertical position is determined exclusively by the second structuring process and does not depend on alignment precision. This method is particularly suitable for microoptical applications because the lateral precision of the location of the individual structures lies in the range of 0.1 pm.


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6.3 3-D Microfabrication by Moving Masks Another interesting process of fabricating 3-dimensional microstructures is by means of a moving mask [TABA02]. During exposure with X-radiation the mask is moved with respect to the substrate in a defined motion. The deposited dosis can thus be precisely controlled. The exposure stage is driven by two PZT actuators with a moving stroke of 80 μm in x and y each with 10 nm step resolution and 500 Hz resonant frequency at unloaded condition. Depending on the mask stage and the axes of movement, round, slanted, or wedge-shaped structure can be achieved, as can be seen in Figs. 22 and 23.

Fig. 22 Method of moving masks to pro- Fig. 23 Examples of 3-D structures made duce 3-D structures. by the method indicated in Fig. 22. 7 Examples of Applications The completely free lateral shaping with the LIGA process, the large variations of material, the structure height of several hundreds of micrometers, as well as the formation of mold details in the submicrometer range, enable the production of components in different areas, like micromechanics, microoptics, sensor and actuator technology and fluid technology. These components find uses in the automobile technology, process technology, general machine engineering, analytical techniques, communication technology or the chemistry, biology and medicine technology and many other fields. In this section examples of some applications are given. Predominantly, the section is about prototypes produced at the Research Center, Karlsruhe. Further examples from other groups can be taken from the literature.

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7.I Rigid Metall& M&rostructures

Simple structures, which are produced by the LIGA process, are either completely dissolved from the substrate or form one unit with the substrate. An example are simple honeycomb structures in polymer or metal, which can be used as particle or wavelength filters. Other examples are microcoils or microgears. 7.2 Moving M&rostructures,Microsensors and Microactuators

The specttum of producible sensors and actuators with the LIGA technology has increased substantiallywith the development of sacrificial layer technology. In addition to these actuators additional LIGA-structures are constructed resulting in more complex dcvices[LEHR96], [GUCK95]. 7.2.1 Acceleration Sensors

Figure 19 shows the principle construction of a capacitive acceleration sensor, which has been produced using sacrificial layers [BURB91]. A seismic mass hung from a leaf spring is located between two electrodes which are well attached to the substrate. On applying an acceleration, the distance between electrodes and seismic

mass changes, and thus the capacity change, which is related to the distance change, can be electronically detected. By exploiting the shaping capabilities of the LIGA process the seismic mass can be branch shaped (Fig. 24), so that disturbing changes in capacity due to temperature fluctuations of the structure are compensated. A fur-

ther increase of the precision is achieved by suspendingthe seismic mass on two elastic cantilevers, which leads to a linear deflection and therefore to a linear signal. Furthermore, the seismic mass can be split into several fork structures, which consei t h the distribution of opposite electrodes in single quently raises the basic capacity. W blocks, the air damping can be reduced [STROH95]. Figure 25a shows a sensor element produced by X-ray lithography and electroplating and Fig. 25b shows a detailed picture of the nickel structure. The distance between the opposite electrode and the seismic mass is only 4 pm, the width of the elastic spring about 20 pm.

Fig. 24 Design of a temperature! compensated

acceleration sensor.


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7.2.2 Microturbines, Flow Sensors In case of the acceleration sensors and the comb drive, besides the freely movable parts, there are still other parts fixed to the substrate. With rotating microstructures the rotors must, however, be fully detached from the substrate, whilst with integrated structures the axes must remain fixed to the substrate [WALL91]. Integrated microturbines, which are driven by gases or liquids, can in principle be used as flow sensors. Figure 26a shows a nickel microturbine, whose diameter of 130 μm is smaller than the height of 150 urn. The gap between the rotor and axis is about 5 μm Fig. 25 Realization of the in this particular case. Since conventional lubrication temperature compensated of me rotors would be difficult due to the very narrow acceleration sensor in LIGA gaps, air sliding bearings are used. To determine the technique. number of revolutions in the entire structure, a shaft is designed in which a glass fiber can be embedded, from which the number of rotations can be determined optically (Fig. 26b). The light emanating from the fiber is reflected on the front end of the turbine blades. The number of rotations can be ascertained with great precision by counting the number of light pulses. Turbine structures, which can be produced by the LIGA process, have turbine blades with dimensions of a few millimeters and are an essential component of micromillers used in minimally invasive medical instruments. The entire milling head is produced by a mounting technology, in which the fluid part is manufactured with a mold form, which was manufactured using mechanical micromanufacturing [WALL96]. With these turbines, flows of about 750 ml/min, torques of 10 μNm and 1000 to 2000 rotations per second were achieved. These values are over one order of magnitude higher than values that are achieved with electrostatically or electromagnetically driven micromotors. Fig. 26 A microturbine in LIGA technique. In b) an integrated 7.3 Fluidic Microstructures fiber allows the measurement of the rotational speed. The application of microfluidics is predicted to be a large market in the future. Application areas are dosing systems for medical purposes or

140


chemical analytical systems as well as microfluidic devices for use in biomedical analysis in the nanoliter regime, to name just a few examples. Extensive efforts have therefore been expended in order to realize micropumps or microvalves using different microtechnical methods [ZENG96].

7.3.1 Micropumps from the LIGA Process Micropumps, which work according to the thermopneumatic principle, are produced in a batch fabrication process (AMANDA process) [SCHO99] using molding technology of the LIGA process and adhesive assembling technique.

7.5.2 Microfluidic Switches To control fluid currents, bistable wall attachment elements, which are produced using the LIGA process, are suitable. These planar systems consist of a feed nozzle, two wall like structures and two control nozzles (Fig. 27). Because of the Coanda effect the fluid stream adheres to one of the two adhesive walls located directly beyond the feed nozzle The stream is switched from one position to the other by a short period control impulse, which is brought in above the steering nozzle, located behind the adhesive wall [VOLL93].

Fig. 27 A fluidic switch. The fluid input is in the middle of the lower margin, the two outputs at the upper right and left side.

Fig. 28 Fluidic switch with integrated heaters for genrating pressure pulses to switch the main jet.

As the necessary control pressure for switching is relatively low, it can be produced by warming up a gas volume. Thus, the elements are fabricated on a silicon wafer, on which a self supporting heat element is structured over an etch groove (Fig. 28). With this construction, the fluid currents can be switched with a power of about 500 mW, applied for about 1 msec. The switching time is approximately 40 μsec . In


141

addition the switching pulse can also be produced by feedback of a part of the fluid stream. In this case the fluidic switch works as an oscillator.

7.4 LIGA-Structures for Optical Uses The LIGA process is well suited to fabricate microoptical components and systems because of four reasons: •The resist used with the LIGA process, PMMA (polymethylmethacrylate), possesses good optical transmission properties in the visible and the near infrared wavelength range. Therefore, it is possible to use microstructures which are produced in the first step of the LIGA process for optical applications [G6tt92], [G6tt91]. Other optical materials such as polycarbonate, which possess a higher temperature stability (to about 150 °C) and a higher refractive index (n=1.6), are available by molding. This gives further possibilities for optical applications. • Side walls produced by X-ray lithography have a surface roughness of 30-40 nm, which provides the possibility to use these structures as reflecting elements. •The shadow printing process results in a high precision of the position of the structures fabricated either by X-ray lithography or in the molding process using an X-ray lithography fabricated mold insert. Thus, because of their height, the LIGA structures can be used as alignment elements for optical components. This results in a very precise micro optical bench. •By the process described in section 6.3 light guiding structures with very smooth surfaces can be patterned. Thus, beside the fabrication of simple optical elements like lenses and prisms two main concepts in micro optics are followed using the LIGA process: •Fabrication of precise micro optical benches with LIGA fabricated alignment structures to which optical components like fibers, lenses, diodes, wavelength selective filters, beam splitters, etc. can be added. These optical benches can also include mechanical structures like electrostatic linear actuators. •Fabrication of planar waveguides with sidewalls acting as optical elements. Light guiding will be achieved either in a three layer system by total internal reflection or in a free space covered by ground and top plate by Fresnel reflection.

7.4.1 Simple Optical Elements, Lenses, Prisms Simple examples of microoptical elements are cylindrical lenses and microprisms made from PMMA, which are produced by direct structuring. The forms can be arbitrarily chosen. Also the structures could be exactly positioned with respect to each other. Of particular advantage is the parallel production of microoptical components and of mechanical mount and guidance structures for glass fibers, as thereby the

142


alignment and mounting costs are considerably reduced. Figure 29 shows a beam splitter for multi-mode fibers. In this case a transmission- (fiber 1), a measurement (fiber 2) and a detector fiber (fiber 3) are accurately positioned to a microprism with the aid of fiber grooves. The arrangement enables use of the measurement fiber in a bidirectional mode. The light from a light source is coupled Fig. 29 Optical beam splitter and fiber guiding by the coupling element into the structure. ffiberabdtransmittedtoaafiberopt cal sensor head. The modulated sensor signal is transmitted back in the same fiber and coupled to the detector fiber by the coupling element. Any desired intensity splitting relation can be achieved by choosing the size and position of the prism. Structures with spherical lens geometry can be produced by the process described below. Cylindrical microstructures are fabricated using the regular LIGA process. Subsequently the whole array is exposed to X-radiation. The dosis is chosen so that only the upper part of the cylinders are reduced in their molecular weight. Together with the molecular weight the melting temperature is changed accordingly. After heating up the array, the upper part of the cylinders melt, and after resolidification a sherical surface is generated due to the surface tension of the molten PMMA (Fig. 30). in the upper part of the microcylindersa As the focal length corresponds to the diameter of the lens, lenses with high aperture ratio are produced in this way [GOTT95]. These lenses are either separated from substrates and integrated in an optical set-up or used as lens plates (Fig.31).

7.4.2 Light Guiding Structures, Microspectrometer Fig. 30 Process steps for fabricating micro lenses.

PMMA can be used as the core layer material for light guiding applications below 900 nm (Fig. 32). For wavelengths of about 900 nm and 1100-1250 nm PMMA shows high absorption because of resonance vibra-


143

tions of the hydrogen atoms in the polymer molecule. If the hydrogen is completely replaced by the heavier deuterium (PMMA-d8), the resonances are shifted to larger wavelengths. Thus, using PMMA-d8 as the core layer gives access to wavelength regions up to 1400 nm relevant for communication technology. An example for such a planar waveguide is a microspectrometer whose main features are shown in Fig. 31 Array of micro lenses. Fig. 33 [ANDE88, MULL95]. The relevant structures of the device are a self focussing reflection grid and fiber alignment structures for coupling and decoupling fibers. The light is coupled into the planar waveguide by an optical fiber which is precisely positioned to the reflecting grid. The light hits the reflection grid and is diffracted into its spectral components, which are focused on the focal plane. Aberration is minimized because of the design of the individual teeth of the grid. The grid possesses about 1200 teeth, which show an average step width of 1.8 μm and an average step height of only 0.2 μm. The reflectivity of the grid is achieved by sputtering with a thin silver layer after structuring. The spectrometer can be easily combined with a suitable diode array if at the place of the outcoupling fibers a 45° edge is patterned by inclined exposure which changes the light path by 90° (Fig. 34). The diode array is evaluated by 16 bit electronics which fit optimally on the array, resulting in a spectrometer with a 1200 1300 high dynamic range. The microspectrometer system includes all components of a Fig. 32 Optical prperties of PMMA. PMMA-d8 complete macroscopic spectromis the optical spectrum for deuteriated material. eter with the size of a cigarette box. With this construction, transmission values of 20% can be achieved, the resolution lies at about 7 nm and the dynamic range can be up to 20,000. Such microspectrometers find applications in color measurement systems, in on-line process photometers or flow-injection systems [MULL95] For building microspectrometers for the wavelength range above 1400 nm multilayer waveguides out of polymer materials can no longer be used because of the low transmission of the polymers. To stay with the same concept of a planar spectrometer

144


system the light has to be guided in a cavity by Fresnel reflection at the top and bottom cover. Such a cavity with the di-ctiontionngridd and the fiber €king groove can be easily fabricatedby molding techniques using a precise LIGA mold. The light guiding part is covered by a metallized cover which has an opening into which the InGaAs detector array fits. The detector array is mounted on Fig. 33 Design of an multiplexer-demultiplexer an electronic carrier and forms for optical data transmission. the second housing pact. Thus, the

whole set-up resuits in a very compact spectrometer system which has a size of half a cigarette box. The performance of the system was demonstrated for the wavelength range 0.95 pm to 1.75 pm. The resolution for a sensor element pitch of 52 pm is better than 20 nm, the sensitivity turned out to be better than 870 countslnw. The noise equivalent power is minimized to 2.65 pW at Fw 34 Array of microspectrometers fabricated I*% pm wavelength for an Operby LIGA technique and hot embossing. ating temperature of 42O which avoids a complex cooling system. With these values the system can easily be used in polymer characterizationas demonstrated IKRIp991. 8 Literature [ANDE88] PACH911

pACH92]

B. ANDERER, W. EHRFELD, D. MUENCHMEYER Proc. SPIE MicroOptics, SPIE Proc. Vol. 1014, 1988, p. 17.

W. BACHER, P. BLEY, H. HEIN, U. KLEPJ, J. MOHR, W.K. SCHOMBURG, R SCHWAFU, W. STARK: Herstellungvon Roentgenmasken her das LIGA-Vtrfahren; KfK-NachriChten,Vol. 23,1991, pp. 76-83. W. BACHER, R RUPRECHT, A. MICHAELIS, J.W. SCHULTZE, A. THIES; Dechmra-Monographienband 125, VCH Verlagsgesellschaft, 1992, p. 459.


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[BLEY91]

P. BLEY, W. MENZ, W. BACHER, K. FEIT, M. HARMENING, H. HEIN, J. MOHR, W.K. SCHOMBURG, W. STARK: Application of the LIGA process in fabrication of three-dimensional mechanical microstructures; Proc. MicroProcess Conference, Kanazawa, Japan, June 15-18, 1991; JSAP Cat. Nr. AP 911120, p. 18, and Jap. J. Appl. Physics, Series 5,1991, p. 547.

[BURB91]

C. BURBAUM, J. MOHR, P. BLEY, W. EHRFELD: Fabrication of capacitive acceleration sensors by the LIGA technique; Sens. Actuators, Vol. A27 Nos. 13, May 1991, pp.559-63.

[EHRF88]

W. EHRFELD, H.J. BAVING, D. BEETS, P. BLEY, F. GOTZ, J. MOHR, D. MUNCHMEYER, W. SCHELB; J. Vac. Sci. Technol., Vol. B6 No. 1, 1988, p. 178.

[ELKH93]

A. EL-KHOLI, P. BLEY, J. GOTTERT, J. MOHR: Examination of the solubility and the molecular weight distribution of PMMA in view of an optimised resist system in deep etch X-ray lithography; Microelectronic Engineering, Vol.21 Noa.1-4, April 1993, pp.271-4.

[GUCK95]

H. GUCKEL, T. EARLES, J. KLEIN, D. ZOOK, T. OHNSTEIN: Electromagnetic linear actuators with inductive position sensing for micro relay, micro valve and precision positioning applications; Proc. Transducers95/Eurosensors IX, Vol.1, Stockholm, Sweden, 1995, pp.324-327.

[GOTT95]

J. GOTTERT, M. FISCHER, A. MULLER; EOS Topical Meetings Digest Series Vol. 5,1995.

[HABE90]

E. HABERSTROH; SpritzgieBprozeB und Formteilqualitat. VDI-K-Buch 1990, VDI-Verlag, Dusseldorf, 1990, p. 87.

[HEIN92]

H. HEIN, P. BLEY, J. GOETTERT, U. KLEIN: Elektronenstrahllithographie und Simulationsrechnungen fuer die Herstellung von Roentgenmasken beim LIGA-Verfahren; Congress Geraetetechnik und Mikrosystemtechnik, Chemnitz, Germany, March 16-18, 1992, VDI-Verlag Dusseldorf, Report No. 960, 1992, pp. 75-86.

[KRIP99]

P. KRIPPNER, J. MOHR: Electromagnetically driven microchopper for integration in microspectrometers based on LIGA technology; SPIE Symp.on Micromachining and Microfabrication 99, Santa Clara, Calif, Sept. 20-22, 1999.

[LEHR96]

H. LEHR, W. EHRFELD, B. HAGEMANN, K.-P. KAMPER, F. MICHEL, CH. THURINGEN; VDI Report No. 1269, 1996, pp. 77-87.

[MICH92]

A. MICHEL, R.RUPRECHT, M. HARMENING, W. BACHER: Abformung von Mikrostrukturen auf prozessierten Wafern; KfK-5171 (March 93) and PhD thesis (A.Michel), University of Karlsruhe, 1992.

[MOHR90]

J. MOHR, C. BURBAUM, P. BLEY, W. MENZ, U. WALLRABE; in: Micro System Technologies '90, H. Reichl (ed.), Springer Verlag, 1990, p. 529.

[MULL95]

C. MULLER, J. MOHR: Miniaturisiertes Spektrometersystem in LIGA-Technik; Ph.D. Thesis (C.Muller), University of Karlsruhe, 1994, Scientific Report of the Forschungszentrum Karlsruhe, FZKA-5609, June 1995.

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[MULL96]

A. MULLER, J. GOETTERT, J. MOHR: Aufbau hybrider mikrooptischer Funktionsmodule fur die optische Nachrichtentechnik mit dem LIGA-Verfahren; Scientific Report of the Forschungszentrum Karlsruhe, FZKA-5786, May 1996, Ph.D. Thesis (A.Muller), University of Karlsruhe, 1996.

[NOKE92]

F. NOCKER, E. BEYER; Keramische Zeitschrift, Vol. 44, p. 1,1992.

[SCHN83]

W. SCHNABEL, H. SOTOBAYASHI: Polymers in electron beam and X-ray lithography; Progress in Polymer Science, Vol.9 No. 4, 1983, pp.297-365.

[SCHO91]

W.K. SCHOMBURG, H.J. BAVING, P. BLEY: TI- and BE-X-ray masks with alignment windows for the LIGA process; Microelectronic Engineering, Vol.13 Nos. 1-4, March 1991, pp.323-326.

[SCHO99]

W.K. SCHOMBURG, R. AHRENS, W. BACHER, J. MARTIN, Z. RUMMLER, V. SAILE: Microfluidic sensors and actuators from polymers fabricated by the AMANDA process; Proc. Transducers 99, Sendai, Japan, June 710, 1999.

[STRO95]

M. STROHRMANN, J. MOHR, J. SCHULZ: Intelligentes Mikrosystem zur Messung von Beschleunigungen basierend auf LIGA- Mikromechanik; Ph.D. Thesis (M. Strohrmann), University of Karlsruhe, 1994, Scientific Reports of the Forschungs-zentrum Karlsruhe, FZKA-5561, Feb. 1995.

[TABA02]

O. TABATA, H. YOU, N. MATSUZUKA, T. YAMAJI, S. UEMURA, I. DAMA; Moving mask deep X-ray lithography system with multi stage for 3-D microfabrication; Microsystem technologies 8 (2002), 93-98.

[VOLL93]

J. VOLLMER, H. HEIN, W. MENZ, F. WALTER; Proc. Transducers 93, Yokohama, Japan, June 7-10, 1993.

[WALL91]

U. WALLRABE, M. HIMMELHAUS, J. MOHR, P. BLEY, W. MENZ; VDI Report No. 933, VDI-Verlag, Dusseldorf, 1991, p. 327.

[WALL96]

U. WALLRABE, J. MOHR, I. TESARI, K. WULFF: Power characteristics of 3-D operated microturbines for minimally invasive therapy; Proc. IEEE Micro Electro Mechanical Systems MEMS 96 Workshop, San Diego, USA, 1996, pp.462-466.

[ZENG96]

R. ZENGERLE; FuM, Vol. 104 No. 4, 1996, p. 241.

Chapter 6

A Review of Wafer Bonding Stefan Bengtsson Solid State Electronics Laboratory, Department of Microelectronics and Microtechnology Centre at Chalmers, Chalmers University of Technology, Goteborg, Sweden

1. Introduction Over the last 15 years wafer bonding has emerged from a tricky laboratory experiment to an industrial process. Silicon wafer bonding is today used for the manufacture of silicon-on-insulator (SOI) materials and sensors [COL 97, HOL 92]. The possibility of using wafer bonding as a general method in integrating materials to circumvent problems related to epitaxial growth of dissimilar materials is under intensive discussion. The publication of scientific reports on wafer bonding are steadily increasing. However, the pre-history of wafer bonding started long ago. Back in 1734, Desaguliers observed how "The friction of surfaces decreases with decreasing surface roughness up to a point where the surfaces are so well polished that they stick together resulting in dramatically increased friction" [DAW 79]. In 1936 Lord Rayleigh made the first systematic study of sticking properties of polished glass pieces. In his work "A study of glass surfaces in optical contact" [RAL 36] he established the first scientific basis of wafer bonding. The modern history of wafer bonding started in the late 60's, when Wallis and Pomerantz reported the possibility of using a combination of elevated temperature and an electric field to fuse glass pieces to silicon [WAL 69]. The method is known as anodic bonding. Later on in 1985/1986 Lasky and co-workers reported the use of wafer bonding without electrostatic fields for the formation of SOI materials [LAS 86]. Shortly after that, in 1986, Shimbo et al. applied Lasky's method to the bonding of silicon wafers without any thermally grown oxide layers [SHI 86]. Since then a wide variety of bonding techniques have been developed. The fundamental principle of wafer bonding relies on the creation of smooth and clean surfaces covered by high concentration molecules being able to react with their counterparts on an opposing surface. The development has to a large extent been focused on new methods to increase the concentration of reactive molecules, while decreasing particulates and other contaminants without jeopardising the surface smoothness. Figure 1 shows an overview of the most important types of wafer bonding techniques. In the figure is also indicated the types of bonding forces that keep the wafers together at room temperature and the surface activation techniques used. In many cases the bonded samples need to undergo thermal treatment to achieve a mechanically strong interface. However, some of the methods shown in

148


Figure 1 show the potential of room temperature bonding techniques, as is indicated by the dashed areas of the figure. Today, wafer bonding has emerged into industrial applications. The present most important applications are the use of wafer bonding for manufacture of SOImaterials [COL 97] and sensors [HOL 92]. In the first type of application direct or fusion bonding is mainly used, while in the latter type the use of anodic bonding is widely spread. One may speculate that more application areas will be added in the future, for instance in use of wafer bonding for integration of dissimilar materials with the aim of forming alternative substrates with properties designed for niche applications. In this review, various bonding methods will be overviewed, the bonding mechanisms will be discussed and a few selected research projects critically dependant on successful bonding will be presented.

Figure 1. Overview of various wafer-bonding techniques, the bonding forces occurring after room temperature wafer bonding and various techniques used for surface preparation. The dashed area indicates surface preparation techniques potentially useful in room temperature bonding techniques

2. Bonding techniques, surface preparations and bonding mechanisms 2.1. Direct bonding The direct (fusion) bonding process can be divided into three steps [TON 99]. These are firstly the initial cleaning and surface activation step, secondly the mating

A Review of Wafer Bonding

149

of the surfaces into contact, and last the annealing step when the mechanical strength of the bonded interface increases and the interface stabilises. The first step, mostly conducted using wet chemicals, determines the maximum surface energy possible to achieve upon room temperature contacting. If the surfaces are treated in an oxidising mixture, i.e. SC1/RCA1 or Pirhana/SPM etch, the surfaces will be covered by a thin oxide terminated by hydroxyl and water molecules, while if HF-last processing is used the surfaces will consist of bare silicon terminated by mainly hydrogen [THO 89]. The first type of surface will be referred to as hydrophilic whereas the second type of surface will be referred to as hydrophobic. In the case of hydrophilic surfaces the static dipolar moment of the water molecules will give rise to an attractive force between the water films. This attractive force is caused by the hydrogen bonds [TON 99] between the water molecules. The hydrogen bond corresponds to an upper energy of approximately 30 kJ/mol. In the case of hydrophobic hydrogen terminated surfaces, no static dipoles occur due to the Si-H and Si-F groups. However, fluctuations in the shared electron cloud of these molecules give rise to an attractive but much weaker Van-der-Waals force [ISR 92]. Here the available energy is around 1 kJ/mol. These values should be compared to covalent bonds exhibiting a much stronger bonding, corresponding to 500 kJ/mol. For an overview of forces between surfaces, see [ISR 92]. Hydroxyl-terminated oxide surfaces (with hydrophilic properties) and hydrogen-terminated silicon surfaces (with hydrophobic properties) can both easily be obtained and as a consequence of the different attractive forces in action, they will result in different surface energies after room temperature contacting [TON 99]. The most common way of measuring the surface energy of a bonded interface is based on the crackopening method [MAS 88], where a metal blade is forced into the interface and the length of the resulting crack is measured in infrared transmission. In the case of mating two hydrophilic surfaces the formation of hydrogen bonds bridging the gap between the wafers will lead to a maximum surface energy being of the order of 0.2 J/m2. When hydrophobic surfaces are brought into contact the resulting interface exhibits a surface energy of the order of 0.02 J/m2. In the second step the wafers are brought into mechanical contact and adhere to each other. Here, surface microroughness and wafer bow are critical parameters determining the willingness of the surfaces to bond [BEN 96, BER 95, TON 99]. Gosele et al also reported that the ambient air pressure influences the contact wave velocity, that is the speed at which the surfaces adhere to each other [GOS 95A]. A reduced pressure increases the contact wave velocity. The crucial factors for a successful room temperature bonding are a high concentration of reactive molecules (e.g. -OH), water films on the surface helping bridging the gap between the wafers and a smooth enough surface ensuring that the gain in surface energy is larger than the energy associated with deforming the wafers microscopically (to overcome micro-roughness) and macroscopically (to overcome bow and warp). Storage of (directly) bonded wafers at room temperature has no major impact on the surface energy. The surface energy slowly increases, saturating around 0.2 J/m2 after a long time [TON 99] in the case of hydrophilic surfaces. For hydrophobic

150


surfaces not much happens at room temperature. The hydrogen-terminated surface is stable up to around 300°C. To reach higher surface energies than those obtained by hydrogen bonds or Van-der-Waals interactions the bonded samples need to be annealed. Tong et al have made thorough investigations of the dynamics upon annealing of bonded samples prepared using wet chemical activation [TON 99]. For hydrophilic and hydrophobic samples respectively, multi-step processes take place according to Tables 1 and 2 [TON 99]. Table 1. The four-step mechanism for the increase of the surface energy during annealing of bonded hydrophilic samples, according to Tong et al [TON 99] Temperature

Process

Surface energy

Slow rearrangement of water molecules and fracturing of Si-O-Si bonds to increase 0.2 ~J/m2 bridging hydroxyl groups Polymerisation of silanol groups across the interface according to the reaction Si-OH + ~1 J/m2 110-150 °C HO-Si Si-O-Si + HOH. Diffusion of water from the interface starts to take place The polymerisation of silanol groups across the interface and the diffusion of water from interface continue and the fraction of area 150- 800°C ~1.5 J/m2 where bonding occurs increases. The wafers deform more easily when the temperature increases Above 800°C Complete bonding as a result of oxide flow ~2.0 J/m2 20 -110°C

Activation energy ~0.044 eV

-0.47 eV

—

-0.57 eV

Table 2. The four-step mechanism for the increase of the surface energy during annealing of bonded hydrophobic samples, according to Tong et al [TON 99] Temperature 20-150°C

130- 300 °C

300 - 700 °C

Above 700°C

Process

Surface energy

Stable state, no relevant interface reactions, Van-der-Waals bonds. Additional Si-F bond formation. Rearrangement of HF molecules Hydrogen desorption and Si-Si covalent bond formation. Interface bubbles are generated The wafers deform more easily when the temperature increases. Complete bonding via surface diffusion of Si atoms at the bonded interface.

~0.02 J/m2

Activation energy

~0.15 J/m2

-0.21 eV

~1.5 J/m2

~0.36 eV

~2.0 J/m2

...


151

Hence, during annealing of the bonded samples the increase in surface energy will rely on transport of water or hydrogen away from the interface and diffusion of Si atoms and flow of surface oxides.

2.2. Plasma assisted wafer bonding Recently, the use of plasma exposure instead of wet chemistry for surface activation has been reported [FAR 95]. Surprisingly, when using activation in oxygen or argon plasma before wafer contacting, the annealing step is not necessary, and the surface energy increases upon storage at room temperature [AMI 00], [BEN OOA]. Immediately after room temperature contacting, both bonded samples prepared from wet chemically activated surfaces and plasma activated surfaces show roughly the same surface energy pointing to the plausible explanation that in both cases hydrogen bonding between water and hydroxyl groups present at the surfaces is responsible for the room temperature adhesion. As discussed above, the increase in surface energy upon storage at room temperature of bonded samples prepared using wet chemically activated surfaces is very modest. In contrast, the surface energy of bonded interfaces prepared from plasma activated surfaces increases dramatically during storage at room temperature. This is illustrated in Figure 2 [AMI 00].

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Storage time (min) Figure 2. The surface energy of bonded Si-Si interfaces prepared using Si surfaces pretreated in oxygen plasma vs. the storage time at room temperature. The times indicated for the different categories of symbols are the plasma exposure times [AMI 00]

Comparing the results obtained using plasma activated surfaces where a surface energy of roughly 1 to 1.5 J/m2 is obtained at room temperature with the data in Table 2, we can conclude that in plasma-activated samples the reaction of the first three steps are occurring at room temperature. There are two candidates for rate limiting factors. The first one is the activation energy for polymerisation of silanol groups at the bonded interface. For a bonded interface prepared using wet chemically activation, the activation energy for this process is roughly 0.47 eV [TON 99]. The second possible rate-limiting factor is the transport of water away from the interface by diffusion along the bonded interface or into the oxide. Let us for a moment consider the diffusion of water and hydroxyl groups in silicon dioxide. The diffusion constant of the species is given by equations of the form


153

where EA is the activation energy. The constants D0 and EA have the values shown in Table 3 for diffusion of water and hydroxyl groups in silicon dioxide [WOL 86]. Table 3. Pre-constant D0 and activation energy EA for the diffusion constant of water and hydroxyl groups in silicon dioxide in Equation (1) [WOL 86] Diffusing molecule

DO (cm2/s)

E A (eV)

Water

1.0 10-6

0.79

Hydroxyl

9.5 110

-4

0.68

The diffusion occurs from a limited number of molecules present at the bonded interface. Hence, the solution of Fick's second law gives a Gaussian profile as a function of depth x from the bonded interface according to Equation (2).

In Equation (2) Q is the surface concentration of molecules at the bonded interface at t=0. One can see that the surface concentration drops inversely proportional to the square root of time. Figure 3 shows the profiles of water molecules around the bonded interface as a function of time normalised to the case ten minutes after bonding (when we assumed nothing has yet happened). From Figure 3 it can be seen that if the water is free to move at the interface the water concentration at the bonded interface is reduced by more than a factor of 10 after 24 hours at room temperature. The diffusion of water may be even faster along the bonded interface or in the case of presence of pinholes or pores in the oxide. Indeed some results reported from analysis of plasma treated surfaces indicate that a porous structure is possible [AMI 00]. If the transport of water from the interface is limiting the formation of Si-O-Si bonds it will happen at room temperature with a rate, roughly corresponding to the increase of surface energy reported for bonding of plasma activated surfaces (see Figure 2).

154


Figure 3. Diffusion profiles of water molecules in silicon dioxide at room temperature from a delta shaped concentration at the bonded interface at t = 0

2.3. UHV bonding The use of ultra-high vacuum to obtain clean silicon surfaces have been reported by several groups [GOS 95B, HER 98, TAK98]. The method is based on wellestablished techniques of forming extremely clean and reconstructed silicon surfaces under UHV conditions. Surface contaminants and oxides are removed by thermal treatments at UHV base pressure. For instance, a silicon dioxide film can be evaporated from the silicon surface at roughly 900°C if the ambient pressure is low enough. The UHV conditions ensure that the surfaces stay clean until they are brought into contact in-situ. In the technique developed by Suga et al [TAK 98], Ar beams are used to mechanically clean the silicon surface under UHV conditions. Using all UHV techniques, covalent bonds form when the surfaces are brought into contact. This can be understood from the fact that it is energetically favourable for the interface to close, and since no contaminants prevent the silicon atoms from coming in contact with each other, bonding and covalent bond formation occurs spontaneously.


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2.4. Anodic bonding Anodic bonding was the first bonding technique to come into wide industrial use [HOL 92]. The method is based on the use of a combination of elevated temperature and an electric field to bond most commonly silicon wafers to borosilicate glasses [WAL69]. Many borosilicate glasses are developed to have thermal expansion coefficients being well matched to the thermal expansion coefficient of silicon. A crucial factor for the method is that the glass must contain mobile ions, such as sodium or potassium. In Figure 4 the bonding mechanism is schematically outlined.

Figure 4. Schematic description of the mechanisms involved in the anodic bonding process

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The stack of a silicon wafer and the glass is put on a electrically conducting hot plate with the silicon wafer in contact with the hot plate. A metal electrode makes contact to the backside of the glass. Positive polarity is connected to the silicon, while the glass is connected to the negative polarity. The temperature is increased to 400°-450° C while an electric field is applied across the structure. Typical voltages used for bulk glass wafers and bulk silicon wafers are around 400 V. The elevated temperature increases the mobility of the ions in the glass. Because of the polarity of the applied field the ions are drawn towards the backside of the glass wafer. This causes a depletion layer to form in the glass close to the silicon/glass interface. As a consequence, the electric field across this depleted region increases and a very large electrostatic pressure presses the wafers together. At high enough electric fields also the oxygen ions in the glass become mobile and the ions diffuse to the silicon wafer causing oxidation reactions, "glueing" the wafers together. Hence, the method relies on the movement of mobile ions, making it less applicable in cases where the silicon wafer will contain MOS-devices. However, because of the electrostatic force pressing the wafers together, the requirement on surface roughness is less demanding as compared with the case for direct bonding. As a consequence, anodic bonding is a very robust bonding technique, being excellent for microsystems applications. 2.5. Comparison of different bonding techniques The different bonding techniques have different surface requirements and give different control of the bonded interface. In addition, the potential for mass-volume production and CMOS integration are different. This is summarized in Table 4.

Table 4. A comparison of the different bonding techniques Bonding techniques

Surface requirements

Interface control

Production potential

CMOS compatibility

Fusion bonding

HIGH

HIGH

HIGH

Generally LOW

Plasma bonding

HIGH

LOW

HIGH

Potentially HIGH

Anodic bonding

LOW

LOW

HIGH

LOW

UHV bonding

HIGH

HIGH

LOW

LOW


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2.6. Mechanics of wafer bonding When clean and properly activated wafer surfaces are brought into contact, energy considerations will determine if they adhere or not. If the gain in surface energy upon closure of the interface is larger than the elastic energy needed for deforming the wafers and the kinetic energy needed to push out the air, the wafers will bond spontaneously. The fraction of bonded surface on a microscopic scale will determine the measured surface energy. The elastic energy stored in the wafers upon deformation can be divided in two parts, one that is related to the deformation of the wafers to overcome the surface microroughness and one that is related to deformation to comply with warp and bow of the wafers. The first of these terms will be independent of the wafer thickness while the second one will be dependent on the thickness of the wafers [TON 99, BEN 96]. Hence the following equation must be valid.

In Equation (3), Esurface is the gain in surface energy, Edeform, microrouchness and Edeform, macroscopic are the elastic energies related to deforming the wafers to overcome the surface microroughness and the macroscopic bow respectively. Egas is the energy used to push the air out from the gap of the wafers, that is the kinetic energy of the gas molecules. Assume a situation where all surface roughness occurs on one of the wafers. Assume also that the wafers show both a microscopical surface roughness and a macroscopical waviness, which can be approximated as sine functions. A schematic view of this assumption is shown in Figure 5.

Figure 5. Schematic view of the bonding of two wafers, in a situation where the total surface roughnesses occurring on both wafers are assumed to be present on only one of the wafers. Both the macroscopic surface waviness and the surface microroughness are assumed to be given by simple sine functions

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The gain in surface energy in Equation (3) is simply given by

where A is the area of the wafer and γs is the specific surface energy per unit area. If the macroscopical deformation, y(r) of the wafers needed for bonding can be expressed by a sine function it will have the form

In Equation (5), u is the amplitude of the waviness and R is a spatial wavelength of the deformation. The elastic energy stored in the wafer upon deformation to accommodate such a roughness is given by [TIM 40].

In Equation (6), a is the radius of the wafer and the factor D is given by

In Equations (6) and (7), r is the polar coordinate along the radius of the wafer (running from 0 to a), v is Poisson's ratio, E is Young's modulus and h is the thickness of the wafer. If a surface deformation according to Equation (5) is used in Equation (6) the energy term related to energy storage due to macroscopical deformation is found to be

2

In the last step of Equation (8) the wafer area A is substituting the factor πa . Considering the elastic energy stored in the wafer upon accommodation of surface microroughness assume that the microroughness can be expressed by a term similar to Equation (5):


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In analogy with Equation (5) and Figure 5; t is the amplitude of the microroughness and λ, is its spatial wavelength. Takagi et al [TAK 98] have shown that the elastic energy stored in the wafers during such a deformation can be expressed as

Again, E is Young's modulus, A is wafer area and v is Poisson's ratio. In a very simplified view, the air in the gap of the wafers can be considered as a mass, mgas, of air, which at a constant velocity, vgas, being equal to the contact wave velocity is pushed out from the gap. Under such an assumption the kinetic energy of the gas Egas can be expressed as

If all contributions to the energy balance equation are put into Equation (3) it can be rewritten as

As can be expected, the term related to macroscopical deformation of the wafer is dependent on the wafer thickness, h, while the term related to deformation to overcome microroughness is not. The equation predicts that the contact wave velocity should be roughly proportional to h3/2, and to the square root of the mass of the gas (being proportional to the air pressure), which have also been experimentally verified [BEN 96, GOS 95A]. The dependence of the contact wave velocity on the wafer thickness is shown in Figure 6 [BEN 96]. The equation obtained is valid for all types of bonding discussed here, i.e. hydrophilic and hydrophobic bonding using wet chemical activation and bonding using plasma activation. Since the available surface energy immediately after room temperature contacting is roughly the same for hydrophilic bonding using wet chemical activation and hydrophilic bonding using plasma activation, Equation (12) tells us that the requirements on surface roughness is about the same for the two techniques. Hence, plasma activation cannot be used to overcome a surface roughness too large for normal bonding using wet chemical activation. The equation

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also shows that in case of hydrophobic bonding the requirements on the surface roughness is larger than in the case of bonding hydrophilic surfaces.

Figure 6. The contact wave velocity during bonding of hydrophobic wafers as a function of wafer thickness. The wafers were of the same diameter and received from the same manufacturer [BEN 96]

To determine which of the deformation terms that is dominating it is interesting to form the relation

Assuming that the amplitude and wavelength of the macroscopic deformation is of the order of μm and mm respectively and the corresponding values for the microroughness are of the order of nm and μm respectively, Equation (13) gives

In such a case the energy storage related to macroscopic deformation is much larger than the energy storage to overcome surface micro-roughness. We see that if the macroscopic wafer deformation instead can be described by typical amplitude of


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1 μm over a length of 1 cm the relation between the terms are close to one. Hence, for different surfaces different terms related to storage of elastic energy in the wafer will dominate. Consequently, the micro-roughness and the macroscopic wafer form will not be equally important. However, the microroughness will influence the number of reactive molecules (i.e. hydroxyl groups) that will be able to reach each other and react. Hence, the microroughness will be important in determining the maximum available gain in surface energy at room temperature. Surface characterisation using atomic force microscopy and stylus profilometry must be used to map the roughness components of the surface to judge which term is the limiting one for a successful bonding process. In the limit when the contact wave velocity goes to zero (the bondability limit) Equation (12) simplifies to

From Equation (15) two simple criteria of bondability can be formed for the two branches where the elastic energy is dominated by the microroughness and the macroscopic properties of the wafer respectively. Hence, if the microroughness determines the bondability, the term related to macroscopical deformation can be neglected and Equation (15) can be expressed as a criterion for bonding as

For hydrophilic bonding of silicon (E=1.3 1011 Pa, v=0.27, γs=0.1 J/m2) this simplifies to

The amplitude of the surface micro-roughness is closely related to the rms value of the surface microroughness and can thus be measured using AFM [GUI 99]. In the other limit when the influence of surface micro-roughness can be neglected Equation (15) simplifies to another criterion for bonding

For hydrophilic bonding of silicon (E=1.3 1011 Pa, v=0.27, γs=0.1 J/m2) this simplifies to

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For a 100 mm wafer with just a simple sinusoidal form (a bow) with u equals 20 urn over the entire wafer (R=100 mm) Equation (19) gives that such a wafer should be bondable up to a thickness of roughly a 1.5 mm. Using hydrophobic 100 mm wafers (γs=O.02 J/m2) Bengtsson et al [BEN 96] (see Figure 6) showed experimentally that the wafers were bondable up to a thickness of approximately 3 mm. Using Equation (18) this implies that the wafers fulfilled a relation according to u2/R4=1.2 10-7.

3. Electrical properties of bonded interfaces Directly bonded interfaces, both in the cases when the surfaces are prepared using wet chemistry or plasma, are potentially interesting for electrical applications. Anodic bonding offers a robust method of materials integration with relatively relaxed requirements on surface smoothness and impurity control. When it comes to electrical properties of the bonded interfaces, whether it is conduction across the bonded interface or conduction in for instance a channel parallel but close to a bonded interface the question of impurity control and surface state reduction becomes of vital importance. The electrical properties of bonded interfaces formed from hydrophilic and hydrophobic surfaces are completely different. In contrast, at least for hydrophilic surfaces, it appears not to be important for the electrical properties whether the hydrophilic surface is formed by wet chemical means or in plasma [AMI 00]. The oxidising pre-treatments forming hydrophilic surfaces induce a high concentration of interface states in the chemical oxide, causing depletion layers to surround the bonded interface. Charge trapping occurring in these interface states, and the resulting depletion regions have a large impact on both current and capacitance of the interfaces formed. The simplest semiconductor interface imaginable is the interface between two equally doped pieces of semiconductors being of the same conduction type. Figure 7 shows the energy band diagram of such an n-type/n-type interface in the case when the interface is formed using hydrophilic surfaces. The presence of interface states at the bonded interface causes a barrier for electron transport across the interface. Hence, the current across the interface will be determined by thermionic emission above the barrier or possibly by tunnelling across the barrier in cases where the doping is high enough to ensure a very narrow space charge region [BEN 92]. Consequently, the formation of space charge regions will be seen in the capacitance vs. voltage curve (CV) as decreasing capacitance at increased bias across the interface. In the case where the wafers forming the bonded structure are equally doped we expect a symmetrical CV curve. Figure 8 shows typical IV and CV curves for bonded Si/Si interfaces formed using hydrophilic surfaces. For bonded interfaces formed from hydrophobic, oxide-free, silicon surfaces the density of interface states are considerably lower. As a consequence, the


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current across the structure is limited by the series resistance of the junction related to the resistance of the bulk silicon material. Hence, the IV curve is linear in this case. No space charge regions of any importance exist and minority carrier concentrations will not be enough to form a diffusion capacitance. Consequently, measurements of capacitance in such samples are not accurate and they do not deliver meaningful information. From the band diagrams in Figure 7 it is obvious that charge carrier trapping and emission from interface states at the bonded interface will affect the electrical properties of the bonded interface, both the IV and the CV characteristics will be affected under dc conditions where charge trapping at the bonded interface gives a very slow decrease of the barrier for electron transport as a function of applied bias. It appears that the formation of a defect-rich surface oxide is the reason for the restriction in current in hydrophilic samples and also for the characteristic CV curve of these samples. If the defect-rich oxide (and its interfaces to silicon) is the reason for the observed electrical characteristics, one may expect that the way these oxides are formed will only have a secondary impact on the properties as long as a defectrich oxide and interfaces are formed. Indeed, this is confirmed by the fact that plasma exposure and chemical treatments give roughly the same electrical properties of the bonded interfaces in cases when oxidizing treatments are used [AMI 00].

Distance (μm) Figure 7. The energy band diagram formed by bonding of two hydrophilic n-type silicon surfaces [BEN 89}

164


Figure 8. Typical current vs. voltage (upper graph) and capacitance vs. voltage characteristics (lower graph) of a bonded interface prepared using hydrophilic medium doped n-type silicon wafers. The current vs. voltage curve is shown for three different temperatures [BEN 92]. The capacitance vs. voltage curve is measured at room temperature. In this case the bonded interface was prepared using oxygen plasma treated silicon surfaces [AMI 00]

4. Examples of challenging bonding project In this section, short descriptions of a number of recent challenging bonding projects at Chalmers University of Technology will be given. The idea is to show how surface roughness problems (the silicon on diamond material) can be overcome to obtain materials integration and how interface barrier optimisation (the RF material and the heterostructure barrier varactor) can be pursued.


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4.1. Silicon-on-diamond materials Silicon-on-insulator materials with buried diamond films were manufactured by a wafer-bonding and etch-back procedure [SOD 95, BEN OOB]. A cross-sectional SEM-picture of the structure is shown in Figure 9. The polycrystalline layer shown in the figure was necessary to overcome surface roughness and diamond peaks appearing on the surface after diamond deposition. The poly-crystalline silicon film was polished to bondable conditions. These advanced silicon-on-insulator structures were made to address self-heating effects observed in conventional silicon-oninsulator materials with buried silicon dioxide layers. The compatibility of polycrystalline diamond films with silicon device manufacturing was evaluated by process experiments combined with Raman spectroscopy [EDH 96]. A method of encapsulating the diamond film during oxidation procedures in device manufacturing was developed. This procedure was necessary since the contact with oxygen at elevated temperatures detrimentally affects diamond, and may, at high enough temperatures, completely convert the polycrystalline diamond into carbon dioxide. Resistors, diodes and MOS-transistors were manufactures in silicon film and were used to characterise the diamond based silicon-on-insulator material. The results show a better heat dissipation capability of the material as compared with conventional silicon-on-insulator materials. Well-working diodes and MOStransistors were made in the active silicon film. It can be concluded that no fundamental obstacles exist for forming these materials and for transferring a silicon device process onto the material. The main problem during the project came from stress and surface roughness of the diamond films. In the future, when a more mature diamond deposition technology is available, it should be possible to overcome these problems.

Figure 9. Cross-section of the silicon-on-diamond material

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4.2. A novel RF substrate Wafer bonding was used to manufacture a silicon material intended as substrate for high frequency applications [JOH 00, JOH 01]. The space charge region surrounding the bonded silicon/silicon interface depletes the silicon, thereby causing a semi-insulating behaviour at high frequencies. The material formed was characterized using measurements on metal transmission lines and the results were compared to similar measurements on SIMOX and bulk silicon wafers. The interface charge of bonded silicon/silicon interfaces is determined by the cleaning procedure used prior to wafer bonding [BEN 89, BEN 92]. To compensate for the interface charge depletion regions will surround the interface to a depth determined by the doping level of the wafers. This is shown in the energy band diagram in Figure 7. The semi-insulating structure was made by bonding of medium doped "device layer" silicon wafers to "substrate" silicon wafers of different resistivities. Surface preparation using RCA cleaning and hot nitric acid (65%) was used before wafer bonding to form chemical oxides on the silicon surfaces. The bonded samples were annealed at 1100°C for 120 minutes in nitrogen. Following the bonding procedure, the device wafer was thinned to an approximate thickness of 0.3-0.5 μm by silicon etching. Co-planar transmission lines were formed by evaporating 1 μrn aluminium followed by metal patterning. Some samples were oxidized before metal deposition. For reference, transmission lines were made on SIMOX as well as on different bulk wafers. Data on the different samples are given in Table 5. Table 5. Description of substrate used for RF measurements Description

Sample

10 Ωcm/10Ωcm+oxide 4

4

Bulk wafer, N-type ~10 Ωcm without/with 1 μm of SiO2

10 Ωcm/10 Ωcm+oxide

Bulk wafer, N-type ~10000 Ωcm without/with 1 μrn of SiO2

SIMOX/SIMOX+oxide

SIMOX wafer, N-type ~10 Ωcm, Si film thickness: 400 nm without/ with 300 nm of SiO2

Si/Si/Si/Si+oxide

Bonded sample, film:N-type ~10 Ωcm, substrate :N-type 4 ~10 Ωcm without/with 300 nm SiO2

Si/Si(pn)/Si/Si(pn)+oxide

Bonded sample, film:N-type ~10 Ωcm, substrate:P-type ~10 Ωcm without/with 300 nm SiO2

Quartz

Quartz wafer

The attenuation coefficients of the transmission lines were measured in the frequency range 45 MHz-40 GHz using a HP851OC vector network analyzer with a test signal of about 1 mW power. In Figure 10 mean attenuation coefficients as a function of frequency are shown for the different samples in Table 5. The influence of the doping level of the reference samples is clearly seen and, as expected, the loss


167

decrease with increasing resistivity. The bonded structure consisting of a pn-junction (S13) show remarkable good characteristics; the losses are corresponding to what was achieved with SIMOX materials. It can also be seen that the bonded structures consisting of a medium doped device layer on top of a low-doped substrate caused small losses in the signal transmission in spite of the medium doped device layer. It can also be seen that the presence of a silicon dioxide layer between the transmission line and the silicon further decreases the losses. This influence is due to a changed barrier between the metal and the substrate.

Figure 10. Mean attenuation coeflcients of transmission lines fabricated on the substrates described in Table 5 [JOH 011

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4.3. A silicon/silicon dioxide heterostructure barrier varactor The possibilities and limitations of silicon technology for devices and subsystems operating at 1-100 GHz are of increasing importance. Since the available output power of direct generators generally drops when the frequency increases, the use of frequency multipliers may be necessary. Varactors are an important class of frequency multiplying devices and have been used for a long time in III-V technology. It would be a big advantage if the varactor device exhibited a symmetric CV curve. Such a device was proposed in 1989 by Kollberg et al [KOL 89]. The heterostructure barrier varactor (HBV) consists of a large bandgap barrier material sandwiched between two low bandgap materials. Such a device will exhibit a symmetrical CV curve and it will generate only odd harmonics. Heterostructure barrier varactors have been realized in a number of material systems, GaAs/AlGaAs, InGaAs/AlInAs and InAs/AlSb. We show how a heterostructure barrier varactor can be realized in silicon technology using wafer bonding. Some initial work has been presented during the last year [FUOO, MAM 01]. It is well known that the surface treatment before bonding largely influences the electrical properties of the bonded Si/Si interface [BEN 89]. For instance the interface exhibits a symmetrical CV curve due to the depletion regions in the two wafers [BEN 92, MAM 01]. We propose the use of this device for frequency multiplication. In many aspects the Si/SiO2 system is perfectly suited for this application. The silicon dioxide provides a very good barrier. The possible problems come from the low saturation velocity in silicon. In Figure 11 a schematic view of the varactor structure is shown. In our case the varactor is based on bonding of two SOI wafers, which after back-etch of one of the substrates will give two thin silicon films with a bonded interface positioned on the buried insulator of one of the substrates. At the bonded interface a thin film of silicon dioxide will restrict the leakage current. However, the thickness of the silicon dioxide film will influence the Cmax/Cmin ratio.

Figure 11. Schematic view of the varactor structure


169

To optimise the barrier initial experiments were made using Si/SiO2/Si structures with different silicon dioxide thickness ranging from 10 to 50 nm. The structures were manufactured using wafer bonding of n-type silicon wafers of 10 Ωcm resistivity. Capacitance vs. voltage curves for devices with various thickness of silicon dioxide at the bonded interface are shown in Figure 12. The figures are the thickness of the thermally grown silicon dioxide on each of the wafers before bonding. For 50 nm of silicon dioxide a ratio of Cmax and Cmin of more than 5 is achieved.

Voltage (V) Figure 12. Capacitance vs. voltage curves for Si/SiO2/Si structures made by wafer bonding. The varactor structures have oxide layers (on each of the bonded wafers) at the bonded interface as indicated by the figures

5. Conclusions Wafer bonding in the form of direct/fusion bonding and anodic bonding has been reviewed and various bonding techniques and their industrial applications have been overviewed. The importance of the bonding mechanisms, the influence of surface smoothness and surface chemistry, and the elasto-mechanics of wafer bonding have been discussed. It is shown how the surface properties of the wafers before bonding affect the electrical properties of the bonded interfaces. In the last part of the paper examples are given on the application of wafer bonding for solving specific problems, giving a flavour of the possibilities of manipulating the properties of the silicon surface as well as the bonded interface to tailor the structure for a specific application.

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Acknowledgements The author is indebted to the following co-workers for their contributions to the results presented in this review article: Mats Bergh, Per Ericsson, Anders Jauhiainen, Petra Amirfeiz, Anke Sanz-Velasco, Mikael Johansson, Gert I. Andersson, Mats O. Andersson, Olof Engstrom, Karin Hermansson, Stefan Tiensuu, Bengt Edholm, Anders Soderbarg, Martin Bring, Jan Vedde, Francois Grey, Jumana Boussey, Cindy Colinge, Ezzio Zanghellini, Ulf Jansson, Ulf Sodervall, and Valery V. Afanas'ev. Thanks are due to the cleanroom laboratory staffs of the Microtechnology Centre at Chalmers, Chalmers University of Technology, the Microelecronics Centre at the Technical University of Denmark and at CIME, Institut National Polytechnique de Grenoble. The work would not have been possible without the financially support, in at different times, by the Swedish National Board for Technical Development (NUTEK), the Swedish Strategic Research Foundation (SSF) and the European Commission, GROWTH program, contract GRD 1-1999-11045.

6. References [AMI 00] AMIRFEIZ P., BENGTSSON S., BERGH M., ZANGHELLINI E., BORJESSON L., "Formation of silicon structures by plasma activated wafer bonding", J. Electrochem. Soc.,Vol 147, 2000, p. 269. [BEN 89] BENGTSSON S., ENGSTROM O., "Interface charge control of directly bonded silicon structures", J. Appl. Phys., Vol. 66, 1989, p. 1231. [BEN 92] BENGTSSON S., ANDERSSON G.I., ANDERSSON M.O., ENGSTROM O., "The bonded uni-polar silicon-silicon junction", J. Appl. Phys., Vol. 72, 1992, p. 124. [BEN 96] BENGTSSON S., LJUNGBERG K., VEDDE J., "The influence of wafer dimensions on the contact wave velocity in silicon wafer bonding", Appl. Phys. Lett., Vol. 69, 1996, p. 3381. [BEN 00A] BENGTSSON S., AMIRFEIZ P., "Room temperature wafer bonding of silicon, oxidized silicon and crystalline quartz", J. Electron. Mat., Vol. 29, 2000, p. 909. [BEN OOB] BENGTSSON S., "Silicon on diamond structures and devices", Proc. NATO Advanced Research Workshop on Perspectives, Science and Technologies for Novel Silicon on Insulator Devices, Kiev (1998) Eds. P.L.F. Hemment, V.S. Lysenko and A.N. Nazarov (Kluwer Academic Press, Dordrecht, 2000, p. 97. [BER 951 BERGH M., BENGTSSON S., ANDERSSON M.O., "The influence of surface micrord roughness on bondability", Proc. of the 3 Int. Symp. on Semiconductor Wafer Bonding: Physics and Applications, Reno (1995) The Electrochem, Soc. Softbound Proc. Series Vol. 95-7, (The Electrochemical Soc., Pennington, 1995), p. 126. [COL 97] COLINGE J.P., in Silicon-on-Insulator Technology: Materials to VLSI, 2nd Edition, Kluwer Academic Publishers, 1997.


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[DAW 79] DAWSON D., in History of Tribology, Longman, 1979. [EDH 96] EDHOLM B., SODERBARG A., BENGTSSON S., "Reliability evaluation of manufacturing processes for bipolar and MOS devices on silicon-on diamond materials", J. Electrochem. Soc., Vol. 143, 1996, p. 1326. [FAR 95] FARRENS S.N., DEKKER J.R., SMITH J.K., ROBERDS B.E., "Chemical free room temperature wafer to wafer direct bonding", J. Electrochem. Soc., 142, 1995, p. 3949. [FU OO] Fu Y., MAMOR M., WILLANDER M., BENGTSSON S., DILLNER L., "Designing nSi/SiO2/Si heterostructure barrier varactor diode", Appl. Phys. Lett. Vol. 77, 2000, p. 103. [GOS 95A] GOSELE U., HOPFE S., MACK T., MARTINI M., REICHE M., SCHMIDT E., STENZEL H., TONG Q.-Y., "What determines the lateral bonding speed in silicon wafer bonding?", Appl. Phys. Lett., Vol. 67, 1995, p. 863. [GOS95B] GOSELE U., STENZEL H, MARTINI T., STEINKIRCHNER J., CONRAD D., SCHEERSCHMIDT K., "Self-propagating room-temperature silicon wafer bonding in ultrahigh vacuum", Appl. Phys. Lett., Vol. 67, 1995, p. 3614. [GUI 99] GUI C., ELWENSPOEK M., TAS N., GARDENIERS J.G.E., "The effect of surface roughness on direct wafer bonding", J. Appl. Phys, Vol. 85, 1999, p. 7448. [HER 98] LJUNGBERG K., GREY F., BENGTSSON S., SODERVALL U., "Ultra-clean Si/Si interface formation by surface preparation and direct bonding in ultra-high vacuum", J. Electrochem. Soc., Vol. 145, 1998, p. 1645. [HOL 92] HOLLINGUM J., "Silicon sensors microengineering", Sensor Review, Vol. 12, 1992, p. 16. [ISR92] ISRAELACHVILI J., in Intermolecular & Surface Forces, 2nd ed, Academic Press, London, 1992. [JOH 00] JOHANSSON M., BENGTSSON S., "Depleted semi-insulating silicon/silicon material formed by wafer bonding", J. Appl. Phys., Vol. 88, 2000, p. 1118. [JOH 01] JOHANSSON M., BERG J., BENGTSSON S., "High frequency properties of silicon-oninsulator and novel depleted silicon materials", Solid State Electronics, Vol. 45, 2002, p. 567. [KOL 89] KOLLBERG E.L., RYDBERG A., "Quantum-barrier-varactor diodes for highefficiency millimetre-wave multipliers", Electron. Lett., Vol. 25, 1989, p. 1696. [LAS 86] LASKY J.B., "Wafer bonding for silicon-on-insulator technologies", Appl. Phys. Lett., Vol. 48, 1986, p. 78. [MAM 01] MAMOR M., Fu Y., NUR O., WILLANDER M., BENGTSSON S., "Leakage current and capacitance characteristics of Si/SiO2/Si single barrier varactor", Applied Physics A, Materials Science & Processing, Vol. 72, 2001, p. 633. [MAS 88] MASZARA W.P., GOETZ G., CAVIGLIA A., MCKITTERICK J.B., "Bonding of silicon wafers for silicon-on-insulator", J. App. Phys., Vol. 64, 1988, p. 4943. [RAL 36] LORD RALEIGH, "A study of glass surfaces in optical contact", Proc. Phys. Soc., Vol. A156, 1936, p. 326.

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[SHI 86] SHIMBO M., FURUKAWA K., FUKUDA K., TANZAWA K., "Silicon-to-silicon direct bonding method", J. Appl. Phys., Vol. 60, 1986, p. 2987. [SOD 95] SODERBARG A., EDHOLM B., BENGTSSON S., "Evaluation of silicon device processes aimed for silicon-on-diamond material", in Proc. 1995 IEEE Int. SOI Conf., Tucson (1995) (IEEE, New York, 1995), p. 104. [TAK 98] TAKAGI H., MAEDA R., CHUNG T.R., HOSODA N., SUGA T., "Effect of surface roughness on room-temperature wafer bonding by Ar bea, surface activation", Jpn. J. Appl. Phys., Vol. 37, 1998, p. 4197. [THO 89] THORNTON J.M.C., WILLIAMS R.H., "A photoemission study of passivated silicon surfaces produced by etching in solutions of HF", Semicond. Sci. Technol., Vol. 4, 1989, p. 847. [TIM 40] TIMOSHENKO S., in Theory of Plates and Shells, McGraw-Hill, New York, 1940. [TON 99] TONG Q.-Y., GOSELE U., in Semiconductor Wafer Technology, John Wiley & Sons Inc., 1999.

Bonding: Science and

[WAL 69] WALLIS G., POMERANTZ D.T., "Field assisted glass-metal sealing" J. Appl. Phys., Vol.40, 1969, p. 3946. [WOL 86] WOLF S., TAUBER R.N., in Silicon processing for the VLSI era (Lattice Press, 1986).

Chapter 7

Single-crystal Silicon Micro-optoelectro-mechanical Devices Tarik Bourouina and Olivier Ecole Superieure d'Ingenieurs en Electrotechnique et Electronique, Noisy-leGrand, France

Philippe Helin MEMSCAP S.A., Crolles, France

1. Introduction Motivated by the need for manufacturing high quality building blocks for optical applications, we have developed some micro-optical components and the corresponding technological micro-fabrication processes, focusing our efforts on the fabrication of single crystalline silicon micro-structures. Among the most popular MEMS technologies, polycrystalline silicon and singlecrystalline silicon are the dominant structural materials. Although they have been widely used for optical applications, surface micro-machined polycrystalline silicon structures have shown a number of limitations, the most important being: - mechanical stress and stress gradient in the very thin structures (typically 1 micron) leads to relatively significant curvatures, which is undesirable in optics, in mirrors for instance; - surface micro-machining of large area structures requires the insertion of arrays of micro-holes for the release step, which also affects the optical characteristics. The use of SCS allows one to alleviate the above-mentioned drawbacks. Furthermore, there are other advantages in using this material: - it has superior mechanical properties as compared to polycrystalline silicon; - it has better surface quality (low roughness) and hence, better optical performance; - the crystalline nature can be used for alignment purposes; - sensing deformations by means of piezoresistive strain gages can be easily implemented. In this chapter, we first present two advanced micro-fabrication processes using SCS., providing at the same time illustrations of their applications. The first one allows the fabrication of three-dimensional (3D) micro-objects of nearly all shapes

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and was used to realize SCS micro-lenses. The second process relates to the fabrication of SCS self-aligned vertical mirrors and V-grooves for optical switching applications. Finally, we show examples of integrating piezoresistive strain gages in resonant SCS structures for providing auto-oscillation capabilities to (magneticallydriven) optical scanners.

2. Micro-lens fabrication using silicon 3D micro-machining 2.1. State of the art in three-dimensional micro-fabrication technologies Three-dimensional micro-structures with non-trivial shapes find a large number of applications, some of which being in the field of micro-optics. A large number of techniques dedicated to micro-fabrication of 3D structures were already reported in the literature. One can mention, for instance, the following processes: - Combination of multiple steps of lithography and etching leading to discrete levels of height [MOT 91]; - Reflow of photoresist [EIS 96]; - Electroplating with a special patterning and biasing of the seed layer [MAC 96]; - Scanning over the workpiece area and depth modulation of the different etched zones. This method was applied using electro-discharge machining [YU 96, MAS90], ultrasonic machining [EGA 97, SUN 96], laser ablation [MUL 96] and laser or electron beam exposure of photoresist [GAL 94, KLE 97]; - Stereolithography [ZIS 96, ZHA 99]. This is a layer-by-layer laser-induced polymerization technique in which, for each layer, a laser beam is focused on and scanned over different points of the open surface of a liquid to turn it solid; - Graytone lithography [OSH 95, NIC 98, DAS 97] and transfer to the underlying material. Graytone lithography uses a special mask with several graylevels, ranging from fully opaque to fully translucent. The different gray levels prevent the light from penetrating all regions of the photoresist with uniform intensity, leading to a 3D profile; - The hole area modulation (HAM) method [MAS 00]. This method is based on excimer laser micro-machining. The workpiece is radiated by the laser through a mask, while a movement is applied to the workpiece. The lateral diffusion of the radiated images through adjacent holes having different areas provides a way for spatial modulation of the exposure time. The three first methods are limited to a few possible shapes and/or materials. Because of the scanning process, the fourth and the fifth methods are limited in speed, when compared to the others, especially when considering a large number of units. Then, it appears that only the last two methods, namely, graytone lithography

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and the HAM method meet the requirements for getting nearly all 3D shapes in a batch-fabrication process. To sum up, it appears that all the above-listed techniques suffer at least from one of the following drawbacks: - Low manufacturing speed; - Restriction to a small number of materials or shapes; - The need for heavy/unconventional equipment or tools. The latter point applies for both graytone lithography and the HAM method: the fabrication of special masks is needed for graytone lithography [OSH 95, NIC 98, DAS 97], while the HAM method utilizes excimer laser and a dedicated experimental set up. We present in the following the MEMSNAS process as a simple alternative method, fully IC-compatible, using only equipment which is widespread in microelectronics industry and even laboratories.

2.2. Principle of the MEMSNAS process Micro-loading effect for micro-fabrication 3D structures of nearly all shapes (MEMSNAS) is a micro-fabrication process for manufacturing 3D micro-structures [BOU 01]. The MEMSNAS process is a 'two-step-RIE' and 'one-mask binary (conventional) lithography' fabrication process. It takes advantage of the microloading effect [HED 94]. For most applications, especially in microelectronics, micro-loading effect is usually seen as an unwanted phenomenon, which leads to different etching depths depending either on the opening areas exposed to the etchant or on the local density to be etched. In general, both wet and dry etching exhibit loading effects. In this paper, we concentrate only on a dry process by RIE. Contrary to conventional RIE processes, the etching parameters must be chosen preferably so as to obtain a noticeable loading effect as well as a nearly isotropic etching behavior. It is noteworthy that the term 'micro-loading effect' is nowadays largely misused to describe another phenomenon: the aspect-ratio dependent etching (ARDE), also called RIE-lag, which is considered in this paper [HED 94], Since different depths are obtainable from different opening areas, then it becomes possible to make a design for a given targeted 3D shape. The principle of the whole fabrication sequence, sketched in Figure 1, is the following: By using a mask layout consisting of an array of open circles having different diameters (Figure l(a)), the etching result after a first RIE step (Figure l(b)) is an array of uniformly distributed holes of different depths, as a consequence of the loading effect. Furthermore, due to the isotropic etching behavior, over-etching of the neighboring micro-holes occurs, which leads to their overlapping. This is a first step towards a continuous 3D profile. But at this stage, periodic 'defects', consisting in alternating peaks and valleys, still remain. They have a high frequency as compared with the desired shape and are representative of the array pitch. To

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remove these 'defects', spatial filtering is needed. This filtering is performed after the removal of the etching mask (Figure l(c)) by means of a second etching step (Figure l(d)), which is achieved by RIE. This second RIE step is also carried out in isotropic conditions, so that it produces surface smoothing. Both first and second etchings are performed in a classical RIE process using SF6. The corresponding mask material is a 400 nm-thick aluminum thin film. It is worth noting that the proposed process is applicable using other mask materials and other etchants, including wet chemicals.

Figure 1. Fabrication sequence of a 3D micro-object using the MEMSNAS process; (a) mask layout with an array of circles having different diameters, (b) after a first RIE step in nearly isotropic conditions with an aluminum thin film as a mask. Micro-loading effect and overetching lead to an array of overlapping micro-holes of different depths, (c) after removal of the aluminum mask and (d) after a second RIE step. The nearly isotropic etching conditions lead to a spatial filtering

2.3. Design of silicon optical micro-lenses In order to validate the proposed fabrication process, we have chosen as a case study the design and the fabrication of a set of micro-lenses. A first set of microlenses are circular with a radius of 375 µm. The second set of micro-lenses have an ellipsoidal shape. The small radius RS is 150 µm and the big radius RB is 750 µm. The micro-lens thickness t is about 30 µrn These dimensions correspond to a focal length of about 1 mm for the circular lens. The basis of the proposed process is that different etched depths result from different opening areas, according to the micro-loading effect. Because the removal of the etched species becomes more and more difficult for narrower and deeper holes, it is expected that the relation between the etched depth and corresponding


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hole diameter is highly non-linear. Thus, a calibration is needed for each process set of parameters, including the etching time. Such a calibration has not been performed yet. For this reason, we assumed that, for designing the masks, a simple linear relation between the opening area and the corresponding etched depth. An important design parameter is the pitch p of the circle array. It ranges from 10 µm to 40 µm. For a given pitch of the array, it is convenient to fix a diameter range [Φmin 'Φmax] of the circles, which fits with the available space. An example of mask layout (negative) for the fabrication of an ellipsoidal micro-lens is shown in Figure 2.

Figure 2. Mask layout (negative) for the fabrication of an ellipsoidal lens

2.4. Fabrication results of micro-lenses The SEM picture in Figure 3 shows an example of ellipsoidal silicon lens. The SEM pictures in Figure 4 show the intermediate fabrication steps of a circular lens. Figure 4(a) relates to the end of the first etching step, as depicted schematically in Figure l(b). As an effect of the residual compressive stress in the aluminum layer mask, one can notice a buckling of the 'grid' formed by the perforated membrane mask. Figure 4(b) relates to the fabrication stage after the removal of the aluminum mask, as schematically shown in Figure l(c). One can see the remaining periodic 'defects' mentioned above. Figure 4(c) relates to the end of the second and last etching step for high frequency spatial filtering, as sketched in Figure l(d). One can see the result of surface smoothing due to the isotropic etching behavior. For both first and second RIE steps, etching was performed using SF6 with a gas pressure of 20 Pa, a flow of 50 sccm and a power of 150 W. The choice of these parameters does not result from any optimization regarding to loading effect. We just wanted to use a process which has a rather high etching rate together with a nearly isotropic behavior. The etching time was typically 20 minutes for the first etching step and 10 minutes or 30 minutes for the second etching step.

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Figure 3. SEM pictures of an ellipsoidal micro-lens, made of silicon

Figure 4. SEM pictures showing fabrication results at intermediate stages: (a) after the first RIE step, under-etching of the circular openings in the aluminum mask layer leads to a perforated membrane with a buckling behavior due to compressive stress, (b) the removal of the aluminum mask reveals an array of overlapping micro-holes of different depths, which result from the micro-loading effect and the nearly isotropic behavior of RIE etching and (c) a second RIE step in nearly isotropic conditions leads to a surface smoothing

Several factors have an influence on surface quality. The duration of the second etching step has an obvious impact on the smoothing efficiency: the longer the isotropic etching, the better the smoothness. A satisfactory result was obtained with 30 minutes of smoothing in the same process. However, if this etching is longer, an additional low frequency spatial filtering occurs and it affects the desired shape. Indeed, the shape tends to be flattened. Thus, a trade-off must be found for determining the appropriate duration of this step. Furthermore, for a given shape and given process conditions, the chosen pitch value of the circle array has a strong influence on the final result. Indeed, if this pitch is too large, it nears the lens dimensions. It then becomes more difficult to perform an efficient spatial filtering without affecting the lens shape. The best surface quality was obtained with the smallest pitch of 10 µm. Evaluation of surface quality using optical surface profiler did not reveal any observable roughness (Figure 5). Characterization using an atomic force microscope (AFM) was also performed on the fabricated micro-lenses. Measurements on an area of 80x80 µm2 of a micro-lens did not reveal any observable residual periodic


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'defects' that may be introduced due to the pitch array. Scanning of a much smaller area of 1x1 µm2 revealed a roughness of about 25 nm, which is equivalent to about λ/60, A, being the reference infrared wavelength at 1.55 µm.

Figure 5. Evaluation of surface quality (a) using an optical surface profiler, (b) using atomic force microscopy (AFM)

3. Self-aligned vertical mirrors and V-grooves for optical crossconnects 3.1. Background Optical communication networks are being developed to fulfill the increasing demand of data flow rates. The high capacity of optical fibers, typically 40 Gb/s, combined with the concept of dense wavelength division multiplexing (DWDM), provide a way to further increase this capacity to 256 times more, corresponding to different carrying wavelengths, leading to a total capacity in the order of 10 Tb/s. However, in order to keep such transmission rates in networks, all functions, especially information routing have to be done without loss in speed, that is, without optical-electrical-optical conversion. In other words, in order to build 'all-optical networks', there is a need for components that are able to perform direct routing of

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optical signals. Because MEMS technology provides solutions in this field, intensive research and development have been conducted in the last few years to realize micro-opto-electro-mechanical devices for applications to optical communications, including optical switches and switch arrays for crossconnects. There are two main classes of optical crossconnects: 2D- and 3D-types. Figure 6 shows a configuration of a 2D optical switch array, which is considered in this paper. Input and output optical fibers are placed in a line. Micro-lenses are attached to fiber ends to collimate optical beams. Micro-mirrors which are driven by micro-actuators are put in a two-dimensional array. The micro-mirrors have typical sizes of hundreds of micrometers. Their surface should be a reflective material with excellent smoothness. The mirrors need to be vertical with respect to the substrate and at 45 degrees to the light beams, which are parallel to the substrate. By moving mirrors at appropriate places and reflecting beams, it is possible to couple any input signal to any output. This MOEMS component is called either digital, 2D or cross-bar switch array, sometimes also referred as a 2-dimensional MEMS switch.

Figure 6. Example of schematic representation of 8x8 2D optical crossconnect. Two sets of 8 fibers each are associated with the 8x8 movable vertical mirror array. Only mirrors in the ON position are shown in the figure

The advantage of this type of crossconnects is that the motion control of the mirrors is very easy and that self-latching mechanisms can be incorporated. However, the matrix size of 2D crossconnects is limited because (i) NxN crossconnects require rather large numbers (N2) of mirrors, that is as many as 256 mirrors for 16x16 crossconnects and (ii) the coupling loss becomes high for large matrix sizes due to increase in propagation length of beams. The maximum size is around a few tens of channels. This drawback is lessened by the fact that the market volume for small-scale switch arrays is relatively large as compared to largescale switch arrays. In a previous work [HEL 00-1, HEL 00-2], we have developed optical switches with self-aligned vertical mirrors and V-grooves. Anisotropic etching of (100) single


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crystalline silicon was used to obtain the device in a monolithic form. In that process, we take advantage of the crystalline nature of silicon. Indeed, the nearly perfect natural 45° angle between and directions is used for the selfalignment of the vertical mirror with respect to the V-grooves. Furthermore, because the mirror consists of a (100) sidewall (Figure 7), its surface is very smooth and it is strictly perpendicular to the optical axes. Using cantilever beams as mirror supports, we have shown the scalability of optical switch arrays to 1xN arrays.

Figure 7. Different views of a vertical silicon mirror obtained by KOH etching

Because of the limitation due to the fiber-to-fiber distance and corresponding optical loss, the mirror density fixes the maximum size of the array. Mirror density is limited by the fabrication process: because under-etching of (100) sidewalls occurs while the mirrors are formed, there is a lateral etching from both sides of the mirror which is equivalent to the etched depth perpendicularly to the wafer surface. This is nearly the whole wafer thickness. The result is that the minimum pitch between two adjacent mirrors is a little more than twice the mirror height. In the case of a silicon wafer of 225 urn, the best attainable mirror density is fixed to about 0.5x0.5 mm2/mirror. The extension of this process to N x M arrays necessitates however a new design of the mirror supports. Indeed, simple cantilever arrays are no longer suitable for such a purpose. Using this first fabrication process and using magnetic bi-stable actuation, the measured insertion loss was less than 0.5 dB and switching time was less than 300 µS.

3.2. Fabrication of N x N mirrors arrays with improved integration density Below we present the fabrication of new structures of vertical mirrors obtained in a process combining deep RIE and wet anisotropic etching using KOH [HEL 01-1, HEL 01-2]. The goal of this combination is to increase the mirror density using deep

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RIE while keeping the previous benefits of KOH (mirror smoothness and selfalignment). The optical structure is pre-defined in a first step by deep RIE, avoiding the excessive lateral under-etching which is introduced by KOH in the previous process. Then, KOH etching is performed in a second step, in order to give back a good smoothness to the mirrors and to recover the natural self-alignment according to the crystallographic directions. An example of the resulting structure is shown in Figure 8. The mirror shape shown in Figure 8(a) is no longer trapezoidal, as it was the case in the previous process (Figure 7), but its top is nearly rectangular. This is favorable for matching with the spot size. Moreover, we have used SOI wafers in order to realize independently the suspensions for the mirror supports from the wafer backside. These suspensions are arranged as folded structures linking the mirrors and a meshed frame as shown in Figure 8(b). Figure 9 shows a portion of a 16 x 16 mirror array.

Figure 8. a) Vertical silicon mirrors obtained by combining deep RIE and KOH etching and b) portion of a 1x4 optical matrix switch obtained with this process

Figure 9. Portion of a 16 x 16 optical switch array obtained with the process combining deep RIE and KOH etching


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4. Optical scanners with integrated piezoresistive strain gages 4.1. Background Optical scanners consist of devices that are able to control the spatial position of a light beam. All known devices operate by modifying the angular position of the incident beam rather than a translational displacement of the light source. For many applications, this action is performed continuously by sweeping the beam either inside an angle in the case of one-dimensional (1D) scanning, or inside a solid angle in the case of two-dimensional (2D) scanning. For other applications such as display and optical switching, one may also need to keep the light beam for a certain time in a prescribed angular position of space. This may be seen as a digital scanning. Most optical scanners use movable mirrors for their operation. The reflection of light on a mirror, which is tilted by an angle 6, leads to light deflection of an angle 20 (Figure 10). Therefore, by vibrating such a mirror, one can produce continuous optical scanning over an angle, whose magnitude is directly proportional to the mirror deflection amplitude. Vibration of the structure around two perpendicular axes leads to 2D scanning. This can be achieved for instance using either a double gimbaled (gyroscope-like) structure or using a cantilever beam working simultaneously in bending and torsion modes.

Figure 10. Operation principle of tilting mirror optical scanner

While optical scanners using tilting mirrors are based on reflection of light, there are many scanners that exploit different operating principles: mainly refraction or diffraction of light, using various structures. Beside the different optical phenomena used for their operation, there are several other characteristics that make the existing optical scanners different. One can propose for instance, the following classifications:

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- By the operation principle: as mentioned before, there are reflection-type, refraction-type and diffraction-type scanners [PET 80, TOS 00, YAS 95]; - By the structure that handles the light beam: micro-mirror, micro-lens, gratings; - By the actuation principle: electrostatic [CON 99], piezoelectric [KAW 97, IKE 97], electro-thermal [SCH 99], electromagnetic [ASA 94] and magnetostrictive [BOU 02] actuation amongst others were reported in the literature; - By the fabrication technology: it can be conventional or silicon-based, resulting from surface or bulk micro-machining, either fully IC-compatible or hybrid, resulting either in polycrystalline or monocrystalline silicon.

4.2. Scanners with magnetostrictive actuation and piezoresistive detection A possible actuation mechanism for optical scanners is magnetostriction. It is an emerging actuation mechanism in MEMS since the availability of the active material in thin films. In a first study on magnetostrictive actuators [GAR 00-1], it was shown that magnetostrictive materials with high anisotropy could be exploited to build bimorph resonators that are capable of simultaneous bending and torsional vibrations with only one excitation coil. It was demonstrated that this feature has promising applications for 2D-optical scanners. In order to evaluate magnetostrictivelyactuated scanners, a first device was built. It consists of a cantilever bimorph resonator. However, despite its rather high deflection angles (40°), this device has shown practical limitations in its original form. Indeed, since the deformable cantilever also acts as a mirror, this mirror does not remain flat when it is rotated. On the other hand, the mirror has poor reflectivity due to the roughness of the magnetostrictive film used, a sputter-deposited alloy of TbDyCoFe. A second generation of magnetostrictively-actuated resonators was then proposed [BOU 02] with improved characteristics for 2D-optical scanner applications. Its structure is schematically depicted in Figure 11. It is formed by a silicon micromachinned structure divided into two parts: i) a thin part that it is compliant enough to produce acceptable deflection and ii) a thick part used as a support for an aluminum mirror. It is so thick that the mirror does not deform when it is rotated. A picture of this device is shown in Figure 12. 2D piezoresistive detectors were integrated in the compliant part. These are able to measure bending and torsion vibrations separately and with low crosstalk (Figure 13). They also provide a way for feedback control of the angular positions in both directions and for self-sustained oscillations at resonance in both modes. The sensitivity of the piezoresistive detectors to rotation angles is in the range of 1030mV/deg without amplification. An example of a scan figure obtained with this device is shown in Figure 14. The magnetostrictive material is deposited by magnetron sputtering from the backside, in such a way that it does not interfere with the piezoresistors realized on


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the frontside as well as not affecting the optical quality of the mirror. Furthermore, the material is optimized for micro-actuators, with regard to dynamic operation at low magnetic excitation fields and to mechanical stress. This material consists of a TbFe/CoFe multilayer film [3], each layer having a thickness of about 10 nanometer, in order to ensure continuous properties at a macroscopic level. The total thickness of the film is 4.5 µm. This 'artificial' material takes advantage of the properties of both constituents. Combining giant magnetostrictive TbFe and soft magnetic high magnetization FeCo layers leads to materials with high magnetostriction and soft magnetic behavior [QUA 97, QUA 99].

Figure 11. Structure of the 2D-optical-scanner with magnetostrictive actuation and piezoresistive detection

Figure 12. Photograph of the optical scanner mounted on its packaging support, before assembly with electromagnetic excitation circuit

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Figure 13. Structure of the 2D-optical-scanner with magnetostrictive actuation and piezoresistive detection

Figure 14. Scan figures obtained with the magnetostrictively actuated 2D scanner


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4.3. Scanners with galvanic actuation using Laplace force and piezoresistive detection We present in what follows a two-degree-of-freedom (2DOF) resonant SCS. micro-structure with galvanic actuation and piezoresistive detection. We propose applications of this structure for 2D optical steering and for 2D magnetic field sensing. This research was inspired from two previous works in this area. On one hand, Bourouina et al. [BOU 01] developed magnetostrictively actuated 2D-opticalscanners integrating 2D piezoresistive detectors. Deflection angles of 8 degrees were demonstrated, which were further improved to angles up to 40 degrees. On the other hand, using either bulk micro-machining or surface micro-machining, Donzier et al. [DON 91] and Bourouina et al. [BOU 93] presented ID magnetic field sensors exploiting Laplace force. In its SCS. form, the magnetic field sensor also integrates piezoresistive detectors. Sensitivity to 10 nTesla was successfully demonstrated. Our first motivation comes from the need for a simple and widespread technology, which could be used instead of the magnetostrictive actuation of the previously reported magnetostrictive optical scanners (due to the low availability and maturation of the active materials). For this reason we studied galvanic actuation with Laplace force as an alternative actuation mechanism. As for the magnetostrictively-actuated optical scanner, integration of 2D piezoresistive detection provides a way for motion detection and self-excitation at resonance. These integrated sensors also offer the possible use of the same device as a 2D magnetic field sensor. The operation principles of these devices are presented below. 4.3.1. Operation principle of the optical steering device The device schematically depicted in Figure 15 is packaged with a permanent magnet, which produces a magnetic field whose orientation is 45° with respect to the cantilever axes. An AC current is applied to the coil. This current is the sum of two components: the first and the second are at the first bending and torsion resonance frequencies of the mechanical structure, respectively. Then, as a result of Laplace forces, bending and torsional vibrations are generated and amplified by the corresponding quality factors. Indeed, on one hand, combination of the y-component of the (DC) magnetic field and the (AC) current flowing along the x-axis leads to bending vibrations. On the other hand, the x-component of the magnetic field combines with the currents flowing along the y-axis. These currents have opposite directions on the right and left sides of the cantilever resulting in opposite alternating forces, which produce torsional vibrations. The two rotational degrees of freedom provide a way for 2D-optical-scanning of a light beam reflected by the vibrating structure.

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Figure 15. Structure of the device and operation principle as 2D optical steering device with integrated piezoresistive detectors or 2D magnetic field sensor

4.3.2. Operation principle of the 2D magnetic field sensor Similarly to the optical steering device, the operation of the 2D magnetic field sensor utilizes AC currents applied to the coil at both bending and torsional resonance frequencies. When the device is submitted to an external magnetic field to be measured (in the plane of the cantilever), Laplace forces are generated, leading to vibrations of the mechanical structure. Electrical signals are generated at the piezoresistive gages. Information about the measured component of the magnetic field is obtainable with low crosstalk using two selective piezoresistive detectors [1]. The same information is also included in the frequency of the detected electrical signal (bending or torsion resonance frequencies). Separation of both components is achieved by synchronous demodulation. 4.3.3. Fabrication The simplified fabrication process is as follows. Starting point is an SOI wafer of typically 20 µm Si / 1 (µm SiO2 / 500 (µm Si. The 20 (µm-thick silicon layer controls the thickness of the mechanical structure. The wafer is first oxidized and patterned. Then boron doping is achieved in a diffusion process, during 20 minutes at 1025 °C. The oxide mask is then removed and the wafer is oxidized and patterned again for contact openings. Then 0.3 [µm-thick aluminum is deposited. It is used not only to make the electrical connections but also to realize planar micro-coils for the electromagnetic actuation as well as optical mirrors. Annealing at 450 °C under nitrogen is then performed in order to achieve good electrical contact between


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aluminum and silicon. The final steps are double side lithography, pattering of the masking layers (oxide on the top-side and aluminum on the back-side) and etching by deep-RIE, in order to realize the mechanical structures. The fabrication result is shown in the picture in Figure 16.

Figure 16. Multifunctional two-degree-of-freedom resonant SCS micro-structure with galvanic actuation by Laplace force and piezoresistive detection. The same device can be used as 2D optical scanner or as 2D magnetic field sensor

5. Conclusion We presented a set of single crystal silicon micro-devices for various optical applications. Firstly, we proposed the MEMSNAS process as an alternative to existing methods for the micro-fabrication of 3D micro-objects. This process fulfills the requirements of batch-fabrication of SCS. devices in an IC-compatible technology and it utilizes only equipment and tools that are widely used in the microelectronics industry and even laboratories. The process was validated with the design and fabrication of optical micro-lenses, which have shown excellent surface quality, 25 nm rms in roughness, which is good enough for most optical applications. Secondly, we also proposed technological processes for the micro-fabrication of SCS. vertical mirrors monolithically integrated with V-grooves designed for supporting optical fibers. The mirrors are formed by (100) crystal planes at 45° with respect to the V-grooves lying along directions. This corresponds to natural self-alignment. Combining deep RIE with KOH etching allows an increase of mirror density while keeping the benefits of KOH: mirror smoothness and self-alignment.

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Finally, we proposed the integration of piezoresistive strain gages in SCS vibrating structures for application to optical scanners with auto-oscillation capability. Devices with either magnetostrictive actuation or galvanic actuation using Laplace force were proposed. Both devices have 2D scanning functionality and they integrate selective 2D strain sensors. All these technological processes allow one to manufacture robust, high yield SCS structures. For some of them, minor changes can lead to full compatibility with integrated electronics. Acknowledgements Part of the research work dealt with in this paper was launched in the framework of LIMMS and CIRMM. LIMMS is the Laboratory for Integrated MicroMechatronic Systems, located at The University of Tokyo (UT). It is a joint laboratory of the Centre National de la Recherche Scientifique (CNRS), France, and the Institute of Industrial Science (IIS) of UT. CIRMM is the Center for International Research on Micro-Mechatronics. It is a part of UT, extending its connections worldwide with many laboratories involved in the field of micromechatronics. Tarik Bourouina and Philippe Helin were research fellows of LIMMS from 1998 to 2001 and from 1998 to 2000, respectively. The authors would like to thank their co-workers for their valuable contribution to this work: Prof. Hiroyuki Fujita, Prof. Takahisa Masuzawa, Dr. Alexis Debray, Dr. Amalia Gamier, Dr. Lionel Houlet, Dr. Eric Lebrasseur, Dr. Makoto Mita and Dr. Gilbert Reyne from LIMMS; Dr. Jean-Claude Peuzin from Labor-atoire de Magnetisme Louis Neel, Grrenoble, France; Dr. Elisabeth Orsier, from LETI-CEA, Grenoble, France; Dr. Alfred Ludwig and Dr. Eckhard Quandt from CAESAR, Bonn, Germany; Dr. Hideo Muro, Mr Takahiko Oki and Mr. Akira Asaoka from NISSAN motors Corp, Yokosuka, Japan; Mr. Lionel Rousseau from E.S.I.E.E.; Dr. Eric Donzier from Schlumberger.

6. References [ASA 94] ASADA N., MATSUKi H., MINAMI K., ESASHI M., "Silicon micromachined galvano optical scanner", IEEE Trans. Magn., 30 (6), 1994, pp. 4647-4649. [BOU 93] BOUROUINA T., SPIRKOVITCH S., MARTY F., BAILLIEU F., DONZIER E., "Silicon etching techniques and application to mechanical devices", Appl. Surf. Science, 65/66, 1993, pp. 536-542. [BOU 01] BOUROUINA T., MASUZAWA T., FUJITA H., "The MEMSNAS process: microloading effect for micromachining of 3D structures with nearly arbitrary shape, application to micro-optics on silicon" , IEEE/LEOS International Conference on Optical MEMS, Okinawa, Japan, September 25-28, 2001, pp. 81-82.


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[BOU 02] BOUROUINA T., LEBRASSEUR E., REYNE G., DEBRAY A., FUJITA H., LUDWIG A., QUANDT E., MURO H., OKI T., ASAOKA A., "Integration of two degree-of-freedom magnetostrictive actuation and piezoresistive detection, application to a two-dimensional optical scanner", IEEE Journal of Microelectromechanical systems, N° 4, Vol. 11, August 2002, pp. 355-361. [CON 99] CONANT R.A., HAGELIN P.M., KRISHNAMOORTHY U., SLOGAARD O., LAU K.Y., MULLER R.S., "A raster-scanning full-motion video display using polysilicon micromachined mirors", Transducers'99, Sendai, Japan, 1999, pp. 376-379. [DAS 97] DASCHNER W., LONG P., STEIN R., Wu C., LEE S.H., "Cost-effective mass fabrication of multi-level diffractive optical elements by use of a single optical exposure by use of a gray-scale mask on high energy beam sensitive glass", Appl. Opt., 36, 1997, pp. 4676-4680. [DON 91] DONZIER E., LEFORT O., SPIRKOVITCH S., BAILLIEU F., "Integrated magnetic field sensor", Sensors and Actuators, 26, 1991, p. 113. [EGA 97] EGASHIRA K., MASUZAWA T., FUJINO M., SUN X.Q., "Application of USM to micromachining by on-the-machine tool fabrication", International Journal of Electrical Machining, No. 2, 1997, pp. 31-36. [EIS 96] EISNER M., SCHWIDER J., "Transferring resist microlenses into silicon by reactive ion etching", Opt. Eng., Vol. 35, N° 10, 1996, pp. 2979-2982. [GAL 94] GALE M.T., Rossi M., SCHUTZ H., "Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresist", Proc. SPIE, 2045, 1994, pp. 54-62. [HED 94] HEDLUND C., BLOM H-O., BERG S., "Microloading effect in reactive ion etching", J. Vac. Sci. Technol, A 12(4), 1994, pp. 1262-1965. [HEL 00-1] HELIN P., MITA M., FUJITA H., "Self-aligned mirror and v-grooves in free space micromachined optical switches" Electronics Letters, Issue 6, Vol. 36, March 16th, 2000. [HEL 00-2] HELIN P., BOUROUINA T., FUJITA H., MAEKOBA H., CUGAT O., REYNE G., "Selfaligned vertical mirrors and v-grooves for magnetic micro optical matrix switch, Si technology, modelling and optimisation of actuation", Nano et Micro-Technologies, Hermes, Vol. 1, N° 1, 2000, pp. 55-87. [HEL 01-1] HELIN P., MITA M., BOUROUINA T., REYNE G., FUJITA H., "Self-aligned micromachining process for large-scale, free-space optical cross-connects", Journal of Lightwave Technology, Vol. 18, No. 12, 2001, pp. 1785-1791. [HEL 01-2] HELIN P., BOUROUINA T., HOULET L., REYNE G., FUJITA H., "Monolithic, SingleCrystal Silicon, Vertical Mirror Arrays with Improved Integration Density", IEEE/LEOS International Conference on Optical MEMS, Okinawa, Japan, September 25-28 2001, 83-84. [IKE 97] IKEDA M., GOTO H., TOTANI H., SKATA M., YADA T., "Two-dimensional miniature optical-scanning sensor with silicon micromachined mirror" Proc. SPIE, Vol. 3008, 1997, pp. 111-122.

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[KAW 97] KAWABATA T., IKEDA M., GOTO H., MATSUMOTO M., YADA T., "The 2-D micro scanner integrated with PZT thin film actuator", Proc. Transducers'97, Chicago, 1997, pp. 339-342. [KLE 97] KLEY A.B., SCHNEIBEL B., ZEITNER U.D., "E-beam lithography, an efficient tool for fabrication of diffractive and refractive micro-optical elements" Proc. SP1E 3008, 1997, pp. 222-232. [MAC 96] MACIOSSEK A., "Electroplating of 3D microstructures without moulds", Proc. SPIE, Vol. 2879, 1996, pp. 275-279. [MAS 90] MASAKI T., KAWATA K., MASUZAWA T., "Micro-electro-discharge machining and its applications to MEMS", Proc. of IEEE MEMS'90, 1990, pp. 21-26. [MAS 00] MASUZAWA T., OLDE-BENNEKER J., EINDOVEN J.J.C., "A new method for threedimensional excimer laser micromachining, Hole Area Modulation (HAM)", Annals of the CIRP, Vol. 49. No. 1, 2000, pp. 139-142. [MOT 91] MOTAMEDI M.E., SOUTHWELL W.H., ANDERSON R.J., GUNNING W.J., Holtz M., "High speed binary micro-lens in GaAs", Proceedings of SPIE 1544, 1991, pp. 33-44. [MUL 96] MULLENBORN M., DIRAC H., PETERSEN J.W., BOUWSTRA S., "Fast threedimensional laser micromachining of silicon for microsystems", Sensors and Actuators A. Physical, 52 (1-3), 1996, pp. 121-125. [NIC 98] NICOLAS S., DUFOUR-GERGAM E., BOSSEBOEUF A., BOUROUINA T., GILLES J-P., GRANDCHAMP J-P., "Experimental study of gray-tone UV lithography of thick photoresists", J. Micromech. Microeng., Vol. 8, 1998, pp. 95-98. [OSH95] O'SHEA D.C., ROCKWARD D.C., "Gray-scale masks for diffractive-optics fabrication, Spatially filtered halftone screens", Appl. Opt 34, 1995, pp. 7518-7526. [PET 80] PETERSEN K.E., "Silicon torsional scanning mirror", IBM Res. Develop., Vol. 24, 1980, pp. 631-637. [SCH99] SCHWEIZER S., COUSSEAU P., LAMEL G., CALMES S., RENAUD Ph., "Twodimensional thermally actuated optical microscanner", Eurosensors XIII, The Hague, The Netherlands, pp. 29-32, 1999. [SUN 96] SUN X.Q., MASUZAWA T., FUJINO T., "Ultrasound micro-machining and its application to MEMS", Sensors and Actuators A. Physical, 57 (2), 1996, pp. 159-164. [TOS 00] TOSHIYOSHI H., SU G-D.J., LACOSSE J., WU M.C., "Surface micromachined 2D lens scanner array", Proc. IEEE/LEOS International Conference on Optical MEMS, Sheraton Kauai Resort, Kauai, Hawaii, 21-24 August 2000, late news session PD-1. [YAS 95] YASSEEN A.A., SMITH S., MEHREGANY M., MERAT F.L., "Diffraction grating scanners using polysilicon micromotors", Proceedings, IEEE Micro Electro Mechanical Systems Workshop (MEMS'95), Amsterdam, The Netherlands, January 29-February 2, 1995, pp. 175-180. [YU 96] YU Z., MASUZAWA T., FUJINO M., "3D Micro-EDM with simply shaped electrode", Proc. of 3rd France-Japan & Ist Europe-Asia congress on Mechatronics, Besancon, France 1996, pp. 519-523.


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[ZHA 99] ZHANG X., JIANG X.N., SUN C., "Micro-stereolithography of polymeric and ceramic microstructures", Sensors and Actuators A. Physical, 77 (2), 1999, pp. 149-156. [ZIS 96] ZISSI S., BERTSCH A., JEZEQUEL J-Y., CORBEL S., LOURGNOT D-J., ANDRE J-C., "Stereolithography and microtechniques", Microsystem Technology, 2, 1996, pp. 97-102.


Chapter 8

Permanent Magnets for MAGMAS (MAGnetic Micro-Actuators & Systems) Orphee Cugat Laboratoire d'Electrotechnique de Grenoble, France

1. Introduction MAGMAS (magnetic micro-actuators & systems) are an emerging family of MEMS (micro electromechanical systems) and MOEMS (Optical MEMS). They should experience an important growth within the next few years in the same way as MEMS and MOEMS did since the late 1980's [BUS 92]. Magnetically actuated miniature and micro-motors, -generators, -pumps, -switches, -relays, -scanners etc, offer potential applications within the IT, bio-medical, spatial and automotive fields. MAGMAS exploit magnetic interactions between magnets, coils and other magnetic materials. These materials and the elaboration techniques are currently developing very fast and thus MAGMAS now exhibit high power-to-mass performances as well as good efficiency, in comparison with electrostatic and piezoelectric counterparts. The key actors of such performances are chiefly the high energy product rareearth permanent magnets, combined with high current densities flowing through integrated micro-coils. Emphasis must be directed towards the elaboration of REPM thick films, and particularly on compatibility with the established MEMS microtechnology processes (thermal, chemical, substrate size etc). This contribution presents a review of the state of the art in miniature permanent magnets and their elaboration techniques, as well as a rapid panorama of MAGMAS and their potential applications. A book dedicated to MAGMAS is currently under press at Hermes/Lavoisier, within the EGEM series (in French) [CUG 02]. This book is coordinated by the author, and co-written by several partners. Its eleven chapters cover all topics about magnetic micro-actuators, starting with three chapters describing electro-magnetic interactions and their scale reduction laws. The following four chapters deal with micro-coils, micro-magnets, soft ferromagnetic materials, and magnetostrictive thin films. The final four chapters describe applications, including micro-motors, microswitches / relays, magnetic suspensions / bearings, and magnetostrictive devices.

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2. What are MAGMAS? Micro electromechanical systems (MEMS) have been developing since the mid 1980's. The first to be built were silicon micro-sensors, then came the electrostatic micro-actuators. After a few years, magnetic micro-actuators & systems (MAGMAS) were born and they are now rapidly developing. Similarly to their macroscopic counterparts, MAGMAS involve micro-coils and micro-magnets, as well as soft iron alloys and magnetostrictive films. Integrated micro-coils are well established, and founders are nowadays able to manufacture them collectively on silicon or glass substrates, be it in the form of planar spiral coils, multi-layered solenoids, or even out-of-plane articulated structures. The permanent magnets used in efficient MAGMAS so far have mainly been micro-machined form bulk REPMs, and individually handled and placed. While this method is acceptable for prototype fabrication and small scale production, it is not a viable technique for mass production of cheap actuators. Meanwhile, permanent magnet thin film deposition has been undertaken by many laboratories the world over, either for scientific characterisation or nano-scale applications. However, the techniques needed for the fabrication of thick films of functional, high-performance permanent magnets are not yet fully mastered. This is probably the major bottleneck which needs to be addressed in priority by the materials sciences experts. Lately, several teams have been reporting efforts towards the deposition of thick films of REPMs intended specifically for MAGMAS applications, using various techniques such as sputtering, pulsed laser deposition, bonding, screen-printing etc.

3. Applications of REPM in MAGMAS As in macroscopic systems, permanent magnets play various roles in MAGMAS: - Controlled forces and torques through Laplace interaction with currents; - Permanent latching forces against soft alloy or other magnets: stability; - Reactive forces and stiffness, when coupled to other magnets: bearing; suspension; - Generation of electrical energy when moving relative to a conductor: generator, sensor; - Polarisation of soft ferromagnetic circuits or magnetostrictive elements. All these functionalities can be applied to micro-actuators and sensors which find uses in bio-medical tools, telecoms, automotive and space applications. The most widespread by far are the miniature motors found in most electrical watches (over 600 million units sold every year!) [TAG 94]. Some high-tech sports watches boast up to 5 independent dials. Other watches also include electro-mechanical microgenerators used to recharge the battery [see Reference section about microgenerators] .

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In surgery, high speed 0 1 mm micro-motors are adapted onto non-invasive catheters for milling clots and cutting flesh [GOE 93], and implanted micro-pumps regularly deliver nanolitres of drugs [FEU 96, BEH 96]. Micro-pumps also handle fluids within bio-medical and pharmaceutical micro-reactors and "labs-on-chip". Some cardiac valves are equipped with 150 um thin micro-machined SmCo magnets which help the movement of the flaps against the blood flow [DEL 93]. Subminiature loudspeakers are now built for hearing aids implantable within the ear [CHE 01, LEE 01, REH 01]. Telecom applications offer a huge market for micro-actuators: miniaturised bistable commutators and micro-mirrors are needed in optical fibre networks [FIS 01]; micro-relays and RF micro-switches are found in mobile phones. Marginal applications also use micro-magnets to polarise magnetic sensors. Micro-robots use them in transmission "gears" [GAR 94, IKU 91], stick-to-the-wall wheels, and coupling latches [TAK 00].

4. Why REPMs? Permanent magnets in MAGMAS offer the same advantages as in macroscopic systems: continuous generation of fields and forces without expenditure of energy. Their great additional advantage in microsystems is that, in terms of magnetic moment, miniature magnets fare much better than coils as their size is reduced. Figure 1 shows the size-dependence of the equivalent current density needed in a coil in order to create the same magnetic moment as a 1 T magnet of the same dimensions.

Figure 1. Size dependence of the equivalent current density needed in a micro-coil compared to a micro-magnet of same dimensions

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For example, a 0 0,85 x 0,25 mm cylindrical SmCo magnet such as the one in Figure 5, with an induction of 0.9 T, is equivalent to a copper coil of same size wound without an iron core and carrying a current of 20 A. This is a current density of about 200 A/mm2. Although micro-coils do withstand huge current densities without burning (up to 10000 A/mm2 depending on conditions) they do nonetheless imply continuous power consumption as well as the associated thermal dissipation.

5. Problems It must be noted that given the small quantities involved, the high cost of REPMs is not a handicap for MAGMAS, especially since microsystems usually are high added-value products.

5.1. Thermal fluctuations The magnetic properties of permanent magnets are sensitive to temperature. Their magnetisation can fluctuate greatly when they are heated. This can become critical in the case of micro-magnets because their masses are extremely small and they are therefore more vulnerable to moderate heating. This is why some thermally driven micro-actuators can reach working frequencies of several 100 Hz.

5.2. Demagnetisation The geometry of micro-systems is often planar. Thus magnet films face strong demagnetising factors if they are magnetised perpendicular to the film. The materials should therefore exhibit very high coercivity and good texturation in order to survive.

5.3. Oxydation NdFeB are prone to oxidation, and so are SmCo to some extent. The surface grains of machined bulk magnets are generally magnetically destroyed over depths of about 50 um for NdFeB, less for SmCo. Magnets smaller than 100 um in size should therefore be made using different techniques.

6. Fabrication techniques Most current efficient MAGMAS make use of micro-machined bulk REPMs. Apart from the surface oxidation already mentioned, the individual handling of


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separate units hinders collective fabrication. More importantly, complex shapes are very difficult to machine and handle. To overcome these problems alternative techniques are developed which are more compatible with planar batch fabrication and micro-technologies such as deep lithography: - Bonded powders: moulded, screen-printed; - Sputtered thick films, pulsed laser deposition, low pressure plasma spray; - Electroplated CoPt (associated with lithographic technologies). Figure 2 shows three families of RE micro-magnets, around miniature rotors (0 10-18 mm).

Figure 2. Various magnet types a) bonded powder, b) micro-machine from bulk, c) sputtered thick films (LEG - MSA)

Figure 3. Bulk SmCoØ 8x0.5 mm multipolar rotor for planar micro-motor, Br = 0.9 T [GIL 02]

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Figure 4. Bulk SmCo Ø 1 x 0.2 mm rotor: multipolar magnetisation. Br = 0.8 T [YAN 01]

Figure 5. SmCo Ø 0.85x 0.25 mm magnet. Br = 0.9 T(COMADUR Switzerland)

As early as 1990 Wagner et al built simple miniature actuators using 1 mm3 micro-machined REPMs [WAG 91, 92]. Since then, such micro-magnets have been consistently used in MAGMAS. CTM-CETEHOR and MMT (Besan9on, France) built in 1996 a 0 2x6 mm stepper-motor in which the rotor houses sixty 100x150x500 um SmCo magnets micro-machined by EDM [CET 94, CTM].

6.1. Bonded powders Bonded REPM powders offer great advantages when it comes to shape, size and ease of fabrication. They face a good future in MAGMAS applications and many laboratories work on them: Goemans et al moulded 0 0.75 x 1.25 mm rotors for their micro-motors. They do not indicate the magnetic properties of the finished product [GOE 93].


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Pawlowski, Topfer, et al have developed the injection of bonded NdFeB powders for films 10 to 100 um thick, and screen-printing for 100 um to 1 mm films [PAW 98, KAL 00] (Figure 6). According to the type of powder used (MQP-B & MQP-Q, 10 um grains) the bonded magnets reach Br = 0.45 and 0.5 T.

Figure 6. Screen printing of NdFeB bonded powders [PAW 98, KAL 00]

Figure 7. Screen-printed SmCo magnets (EPFL, [DUT 99])

Dutoit et al (EPF, Switzerland) have developed screen-printed SmCo magnet arrays for Hall effect sensors (Figure 7). The powders are bonded in SU-8 resin and so far magnetic properties are moderate (Br = 0.34 T) despite grain sizes of about 10 um. The coercivity is relatively weak [DUT 99]. Christenson et al (Sandia National Lab, Albuquerque) use amorphous, isotropic Nd2Fe14B powders (grain size 3 - 20 urn) bonded into 200 um thick dies made using X-Ray lithography [CHR 98, 99]. Magnetic properties are good (Br = 0.63 T, He = 1.4 T, Ms = 1 T), and pressing under magnetic field should further enhance the remanence. Figure 8 show micro-magnets with a lateral precision of 5 um.

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Figure 8. Moulded NdFeB powders (Sandia National Labs). Right: rotor segments

Nienhaus et al (IMM Mainz & Univ. Hannover) have injected disk-rotors for planar micro-motors [NIE 99, KLE 00, MYM]. NdFeB powders are cold-injected into 400 um deep dies (Figure 9 left). Rotors 300 urn thick are obtained, with size tolerances of about 15 um (Figure 9 right).

Figure 9. Injected NdFeB powder rotors for planar micro-motors. Left: injection device and naked rotor disks. Right: Ø 12.8 mm Penny-motor [MYM]

Rodewald et al produced flexible sheets from bonded NdFeB and SmCo powders [ROD 98]. Sheets of thickness 100 to 200 um are spread, and the bonding material is then dissolved and evaporated. With Sm2(Co,Cu,Fe,Zr)17 powders field-oriented during the process, remanence reaches 0.45 T. With isotropic amorphous NdFeB grains (< 25 um), remanence is 0.41 T. Coercivity is ca. 1 T. Harris et al (Univ. of Birmingham) sinter screen-printed magnets. Strontium ferrites have been sintered successfully. Recent trials with NdFeB powders obtained by HDDR gave good results. Sintering offers the great advantage of eliminating the volume-wasting bonding resin [YUA 00].


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Fruchart et al (Laboratoire de Cristallographie & CRETA, CNRS Grenoble), produce coercive SmCo and NdFeB powders from bulk magnets, using HDDR. At LEG we have crushed and bonded amorphous ribbons of NdFeB (made by Laboratoire Louis Neel). Multipolar rotors including permanent magnet bearings have been made for planar miniature motors (Figure 2) [CUG 96]. Lagorce et al (Georgia Tech) develop screen-printed, patterned 100 urn thick and 250 um wide polymer-bonded ferrite powders (Br = 0.3 T, Hc = 0.4 T) using deep lithography [LAG 96]. Several other teams develop ferrite bonded micro-magnets, and are evolving towards bonded REPMs.

6.2.Electroplating-electrophoresis Many laboratories actively develop electroplated magnets. When associated with deep lithography techniques, electroplating is the ideal way to batch-produce complex-shaped parts with high accuracy while avoiding the usual problems of vacuum chambers, deposition temperature, compacting and sintering. Cavalotti et al (Univ. Cincinnati) were among the first to try electroplating semi-hard magnets [ZAN 96, CAL 96, CHO 00, EVA 00]. Liakopoulos et al electroplated magnetic dots with thickness of several 10 um [LIA 96]. Electroplating is currently limited to simple metals (Ni, but also Cu and Au for coils) as well as alloys of soft (FeNi, FeCo) or semi-hard (CoPt, CoNiMnP) materials, but not rare earth compounds. Magnetic properties are thus quite low (Br 0.2-0.35 T, Hc 0.2-0.4 T). A few teams work on the electrophoresis of rare earth compounds and alloys [SCH 98]. Present research focuses on obtaining alloys rich enough in RE, but the state of the art is still far from processing permanent magnets as such. However, these techniques - if successful - would achieve a major step towards the collective deposition of high quality micro-magnets for MAGMAS.

6.3. Low pressure plasma spraying Low pressure plasma spraying (LPPS) is an interesting technique for producing thick layers, in which magnet powder is melted and sprayed. The powders do not suffer from oxidation, and the high speed of particles ensures very little porosity and good adhesion of the film onto the substrate. Highly coercive SmCo5 thin films and Nd2Fe14B thick films have been produced with this technique [KUM 86, OVE 86, WYS 92]. In some experiments, coercivity seems to weaken when the film thickness exceeds a few 100 um [PAR 96]. However, Rieger et al have recently deposited NdFeB films with thickness in excess of 1 mm, using LPPS of 60 u.m powders. The very thick films obtained are isotropic and the magnetic properties are good (Br 0.45 T, Ms 0.8 T, Hc 1.9 T) [RIE 00].

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6.4. Nanostructured ribbons and flakes Most bonded magnets are made from coercive REPM powders obtained by crushing MQ ribbons of thickness 10-50 um obtained by cold wheel high speed quenching. An alternative way for the fabrication of nano-structured thin flakes is the splat cooling technique, where droplets of melted alloy are violently caught mild-fall between two flat, cold Cu plates [DAV 99]. The flakes offer interesting structural and magnetic properties. However, they do not form a continuous ribbons as is the case with other techniques, and this might be an obstacle to preparing large quantities of material. 6.5. Sputtered films One of the main techniques for the production of REPM films is sputtering. Many laboratories sputter thin and ultra-thin films for scientific purposes or for recording media, but only a few teams manage film thicknesses above 10 um for applications in MAGMAS. Homburg et al were among the first to deposit thick films of NdFeB. The 9 um thick films reached very good properties depending on the temperature: Br 0.6 T / Hc 2.1 T at 500° C, and Br 1 T / Hc 1 T at 700°C [HOM 90]. Around the same time Yamashita et al deposited 20 um films of NdFeB for the rotor of a planar miniature motor, with poor magnetic properties (Br 0.25 T, Hc 0.3 T) [YAM 91]. Kornilov et al (MISA Moscow Institute of Steel and Alloys) deposited the best and thickest NdFeB films reported so far in the literature, up to 300 um (Figure 10 left) by high-power triode magnetron sputtering with deposition speeds between 80 and 140 A/s. The perpendicularly textured films exhibit superb magnetic properties: Br up to 1.35 T, Hc up to 3 T (Figure 10 right) [KOR 93, LIN 95, KOR 99].

Figure 10. NdFeB magnets sputtered onto mica through a 1x1 mm2 grid - thickness 200 um (MISA) and magnetisation curves measured perpendicularly to the film (after correction for demag. factor) a: Ndl6Fe76B8 alloy sputtered at 575°C - b: Ndl6Fe76B8 alloy sputtered at 420°C (anneal 30 min, 700°C) c: (NdO,843TbO,094DyO,063)16(FeO,934CoO,066)76B8 alloy sputtered at 420°C (anneal 30 min, 700°C)


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Cadieu et al have been very active pioneers in sputtering Sm-based alloys, achieving thick films (up to 100 urn) with very good properties: Br 0.6-0.95 T, Hc 0.5-1.3 T [CAD 85-91-94, HEG 93]. Budde et al (Univ. Hannover) sputtered patterned films of SmCo of several um into 20 um thick Cu masks obtained by deep lithography [BUD 02]. The Cu mask wall angle is optimised so as to avoid lateral deposition of SmCo on the mask (Figure 11). Such patterning is well suited to MAGMAS.

Figure 11. 4 um SmCo film deposited in a Cu patterned Cu die [BUD 02]

6.6. Pulsed laser deposition (PLD) and magnetic confinement PLD achieves deposition speeds comparable to classical RF sputtering, about 1 A/s. In practice, the film thickness is limited to a few urn, which is a handicap for efficient micro-actuators. Moreover, droplets often disturb the film homogeneity and prevent good magnetic properties. Givord et al (Lab. Louis Neel, CNRS Grenoble) use magnetic confinement and deviation of the plasma in order to deposit dropletfree films [JUL 99]. This technique is currently being improved to achieve high deposition rates (20 um/h locally) under high magnetic fields (2 T). First trials in early 2001 have achieved 20 um thick films of NdFeB on Mo substrates, with very few droplets. After flash annealing, good magnetic properties are reached: Br 0.7 T, Bs 1.4 T, Hc 1.5-2 T [LLN 01].

7. Conclusions It is now clear that, at present, several techniques for thick-film deposition of REPMs of various compositions and structures are firmly or nearly established. Magnetic properties vary from moderate to excellent according to the materials, techniques and system configurations. Implementation of these magnets into

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magnetic microactuators & systems (MAGMAS) faces various obstacles owing to the fabrication techniques, but each application finds an appropriate solution: Bulk micro-machined magnets are good but not fully compatible with batch fabrication and extreme miniaturisation. Bonded powders are easily implemented and patterned into MAGMAS but their magnetic properties are not the best, due to the bonding resin. Sputtered and laser pulsed films exhibit the best properties, but the main problems stopping their large scale application to MAGMAS are chemical, thermal and mechanical compatibility between the material and the substrate. Electroplating, combined with deep lithography, is a fantastic tool but is presently limited to simple alloys; electrophoresis of REPM compounds is being studied but is not yet achievable. Acknowledgements The author wishes to thank Nikolai Kornilov (from MISA, currently at Lab. Louis Neel), for his help in compiling data on the sputtering of thick films.

8. Bibliography The author currently leads a 300 page book dedicated to MAGMAS which is due for publication in autumn 2002 [CUG 02]. This multiple-author book deals in detail with all aspects of MAGMAS, from the principles and reduction laws, to state of the art material fabrication and applications. In addition to the references listed hereunder, the reader is urged to refer to the following conference proceedings and journals which specialise in the MEMS culture. Conferences MEMS:

IEEE International Conference on Micro Electro Mechanical Systems, http://www.iqe.ethz.ch/mems2001/ EMSA: European conference on Magnetic Sensors and Actuators MECATRONICS: Europe-Asia conference on microtechnologies, http://www.mecatronics.org/ ACTUATOR: conference on actuators, in Bremen (Germany), http://www.messebremen.de/english/frame-veranstalt.htm TRANSDUCERS: conference on actuators, often coupled with Eurosensors, http://www.transducers01 .de/ MME: European Workshop on Micromachining, Micromechanics & Microsystems, http://www.mst.material.uu.se/mme00/


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Reviews and journals Journal of Micromechanics & Micro-engineering, http://www.iop.Org/EJ/S/0/30258/journal/0960-l 317 Journal of Micro Electro Mechanical Systems (J. MEMS), http://ieeexplore.ieee.org/lpdocs/epic03/RecentIssues.htm?punumber=84 Sensors & Actuators A, http://www.elsevier.n1/inca/publications/store/5/0/4/l/0/3/

References [BEH 96] BEHRENS J., MECKES A., GEBHARD M., BENECKE W., "Electromagnetic actuation for micropumps and microvalves", ACTUATOR '96, 1996, pp. 124—127. [BUS 92] BUSCH-VISHNIAC I., "The case for magnetically driven microactuators", Sensors & Actuators 33, 1992, pp. 207-220.

[CAL 96] CALLEGARO L., PUPPIN E., CAVALLOTTI P.L., ZANGARI G., "Electroplated, High Hc CoPt films: 8M magneto-optical measurements", J. Magn. Mag. Mat. 155, 1996, pp. 190-192. [CET 94] "Un moteur pas a pas de 2 mm de diametre!" CETEHOR (Besancon, France), Electronique Intal Hebdo 155, 1994, p. 23. [CHE 01] CHENG M.C, HUANG W.S., HUANG R.S., CHIN T.S., "A novel micro-machined electromagnetic loudspeaker for hearing aid", Proc. TRANSDUCERS '01, 2001. [CHO 00] CHO H.J., AHN C.H., "Electroplated Co-Ni-Mn-P-based hard magnetic arrays and their applications to micro-actuators", Electro-Chemical Soc. Proceeding ECS'00, 2000, p. 586. [CHO 00] CHO H.J., AHN C.H., "A novel bidirectional magnetic micro-actuator using electroplated permanent magnet arrays with vertical anisotropy", Proc. MEMS '00, 2000, pp. 686-691. [CHR 98] CHRISTENSON T., GARINO T., VENTURINI E., "Deep X-ray lithography based fabrication of rare-earth based permanent magnets and their applications to microactuators", Electrochemical Society Proceedings, Vol. 98-20, 1998, pp. 312-323. [CHR 99] CHRISTENSON T., GARINO T., VENTURINI E., BERRY D., "Application of deep X-ray lithography fabricated rare-earth permanent magnets to multipole magnetic microactuators", Proc. TRANSDUCERS '99, 1999, pp. 98-101. [CHR 99] CHRISTENSON T., GARINO T., VENTURINI E., "Microfabrication of fully-dense rareearth permanent magnets via deep X-ray lithography and hot forging", HARMST Workshop, Kisarazu, Japan, Book of Abstracts, 1999, pp. 82-83. [CTM] http://www.ctm-france.com/html_fr/frame.html.

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[CUG 96] CUGAT O., FERNANDEZ V., ROY D., REYNE G., DELAMARE J., "Miniature permanent magnet bearings: application to planar micromotors", Proc. 1st Europe-Asia Workshop MECATRONICS '96. [CUG 02] CUGAT O. et al, "Micro-actionneurs electro-magnetiques MAGMAS: Magnetic microactuators & systems", Hermes-Lavoisier Editors, December 2002, p. 320 in French. [DAV 99] David S., Optimisation, analyse et modelisation de la coercivite dans les materiaux nanocomposites R2MT14B/Fe3B pour aimants permanents, PhD, Lab. Louis Neel (Grenoble), 1999. [DEL 93] DELAMARE J., OLIVIER-RULLIERE E., YONNET J.P., "Etude d'une valve cardiaque a aimants permanents", Proc. FIRELEC '91,1991. [DUT99] DUTOIT B., BESSE P.A., BLANCHARD H., GUERIN L., POPOVIC R.S., "High performance micromachined Sm2Co17 polymer bonded magnets", Sens. & Act. 77, 1999, pp. 178-182. [EVA 00] EVANS P., ZANA I.., ZANGARI G. "Patterned electrodeposition of micromagnets", Electro-Chemical Society Proceeding ECS'00, 2000, p. 610. [FEU 96] FEUSTEL A., KRUSEMARK O., LEHMANN U., MULLER J., SPERLING T., "Electromagnetic membrane actuator with a compliant silicon suspension", Proc. ACTUATOR '96, 1996, p. 76-79. [FIS 01] FISHER K. et al, "A latching bistable optical fibre switch combining LIGA technology with micromachined permanent magnets", Proc. TRANSDUCERS '01, 2001. [GAR 94] GARSHELIS et al, "A torque transducer utilizing two oppositely polarised rings", IEEE Trans. Magn., 30 6, 1994, p. 4629. [GIL 02] GILLES P.-A., DELAMARE J., CUGAT O., "Rotor for a brushless micromotor", J. Mag. Mag. Mat., Vol. 242-245 P2, May 2002, pp. 1186-1189 (Proc. JEMS'Ol, Grenoble, France 2001). [GOE 93] GOEMANS P., KAMERBEEK E., KLIJN P., "Measurement of the pull-out torque of synchronous micromotors with permanent magnet rotor", 6th Int Conf on Elec. Mach. & Drives Oxford, pp. 4-8, 1993. [IKU 91] IKUTA et al, "Non-contact magnetic gear for micro transmission mechanism", Proc. MEMS '91, 1991, p. 125. [KAL 00] KALLENBACH E.et al, "Permanent magnetic polymer bonded material based on NdFeB and their application in mini and micro actuators", Proc. ACTUATOR'OO, 2000, p. 611. [KLE 00] KLEEN S., EHRFELD W., MICHEL F., NIENHAUS M., STOLING H.D., "Penny-motor: a family of novel ultraflat electromagnetic micromotors", Proc. ACTUATOR '00, 2000, pp. 193-196. [KUM 86] KUMAR K., DAS D., J. Appl. Phys. 60, 1986, p. 10. [LAG 96] LAGORCE L.K., ALLEN M.G., "Micromachined polymer magnets", Proc. MEMS '96 p. 85-90.


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[LEE 01] LEE K.C. et al, "The floating mass vibration transducer using polyimide elastic body for implantable hearing aid", Proc. TRANSDUCERS '01, 2001. [LIA96] LIAKOPOULOS T.M., ZHANG W., AHN C.H., "Micromachined thick permanent magnet arrays on silicon wafers", IEEE Trans Mag 32-5, 1996, pp. 5154-5156. [LIA96] LIAKOPOULOS T.M., ZHANG W., AHN C.H., "Electroplated thick CoNiMnP permanent magnet arrays for micromachined magnetic device applications", Proc. MEMS '96, 1996, pp. 79-84. [LIU 94] CHANG LIU et al, "Surface micromachined magnetic actuators", Proc. MEMS '94, 1994, p. 57. [MYM] Mymotors & Actuators GmbH, www.mymotors.de. [NIE 99] NIENHAUS M., EHRFELD W. et al, "Design and realization of a penny-shaped micromotor", Proc. SPIE 3680B-65, Paris, 1999, "Micromachining and Microfabrication". [OVE 86] OVERFELT R., ANDERSON D., FLAGNAN W., Appl. Phys. Lett., 49, 26, 1986. [PAR 96] PARHOFER S., GIERES G., WECKER J., SCHULTZ L., J. Mag. Mag. Mat. 163, 1996 p. 32. [PAW 98] PAWLOWSKI B., RAHMIG A., TOPFER J., "Preparation and properties of NdFeB thick films", Proceed REMXV (Dresden, Germany, September 1998), 1998, pp. 1045-1049. [REH01] REHDER J., ROMBACH P., HANSEN O., "Magnetic flux generator for balanced membrane loudspeaker", Proc. TRANSDUCERS '01, 2001. [RIE 00] RIEGER G., WECKER J., RODEWALD W. et al, "NdFeB permanent magnet (thick films) produced by a vacuum-plasma-spraying process", J. of Appl. Phys., 87-9, 2000, pp. 5329-5331. [ROD 98] RODEWALD W.et al, "Production of thin flexible R magnet foils", REM XV pp.1021-1027. [SCH 98] SCHWARTZ M., HE F., MYUNG N., KOBE K., "Thin film alloy electrodeposits of transition rare-earth metals from aqueous media", Electrochemical Society Proceedings, 98-20, 1998, pp. 646-659. [SHO 94] SHOJI, "Microflow devices and systems", J of Micromech Microeng 4, 1994, p. 157. [SMI 90] SMITH, "The design and fabrication of a magnetically actuated micromachined flow valve", Sensors & Actuators, 24, 1990, p. 47. [TAG 94] TAGHEZOUT, "2D and 3D finite analysis of watch stepping motor", Proc. ACTUATOR '94, 1994, p. 429, (Ed. AXON, Bremen, Germany, 1994). [TAK 00] TAKEDA M., NAMURA K., NAKAMURA K., SHIBAIKE N., HAGA T., TAKADA H., "Development of chain-type micromachine for inspection of outer tube surfaces (basic perfomance of the first prototype)", Proc. MEMS'00, Miyazaki, Japan, 2000, pp. 805810.

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[WAG 91] WAGNER B., BENECKE W., "Microfabricated actuator with moving permanent magnet", Proc. MEMS '91, 1991, pp. 27-32. [WAG 92] WAGNER B., "Microactuators with moving permanent magnets for linear, torsional or multiaxial motion", Sens &ActA, 32/1-3, 1992, p. 598. [WYS 92] WYSLOCKI J., J. Magn. Mag. Mat., 104-107, 1992, p. 363, & J. Mat. Sc. 27, 1992, p. 3777. [YAN01] YAN C., ZHAO X., DING G., ZHANG C., CAI B., "An axial flux electromagnetic micromotor", J. Micromech. Microeng., 11, 2001, pp. 113-117. [YUA 00] YUAN Z.C., WILLIAMS A.J., SHIELDS T.C., PONTON C.B., ABELL J.S., HARRIS I.R., "The production of Sr ferrite thick films by screen printing", Proceedings of the 8th International Conference on Ferrites, Kyoto, Japan, September 2000, p. 35. [ZAN 96] ZANGARI G., BUCHER P., LECIS N., CAVALLOTTI P.L., CALLEGARO L., PUPPIN E., "Magnetic properties of electroplated Co-Pt films", J. Mag. Mag. Mat., 157/158, 1996, pp. 256-257.

References (watch generators) [AUD 95] AUDEMARS S.A., "Module generateur multirotor", 63*me Congres de la SSC, 2021/10/1995. [AZZ 97] AZZAM N., MONDAINE Watch Ltd, "Electrical generator for electronic watch", Patent WO 97/39516, 23/10/1997. [CHE 97] CHEN S.C., ITRI Taiwan, "Swinging type power generator", Patent US 5684761 04/11/1997. [BOR97] BORN J.J., ASULAB, "Generateur electrique pour horlogerie", Pat. EP 0751445A1 02/01/1997. [HAR 95] HARA T., SEIKO EPSON, "Seiko Kinetic quartz", 63*me Congres de la SSC, 2021/10/1995. [JUF 97] JUFER M., KOECHLI C., EPFL, "Microgeneratrice", Journee d'etude de la SSC, 02/10/1997. [KEN 97] KENJI M., CITIZEN, "Small electronic apparatus equipped with generator", Patent WO 97/10534, 20/03/1997. [KNA 92] KNAPEN P. et al, KINETRON BV, "Generator", Patent WO 92/04662, 19/03/1992.

References (REPM film deposition) [BUD 02] BUDDE T., GATZEN H.H., "Patterned sputter deposited SmCo films for MEMS applications", J. Magn. Magn. Mater, 2002. [CAD 85] CADIEU F.J., CHEUNG T.D., WICKRAMASEKARA L., "Magnetic properties of Sm-TiFe and SmCo based films", J. Appl. Phys., Vol. 57(1), 1985, pp. 4161-4163.


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[CAD 91] CADIEU F.J., HEGDE H., NAVARATHNA A., RANI R., CHEN K., "High-energy product ThMn12 Sm-Fe-T and Sm-Fe permanent magnets synthesized as oriented sputtered films", Appl. Phys. Lett., Vol. 59(7), 1991, pp. 875-877. [CAD 94] CADIEU F.J, HEGDE H., SCHLOEMANN E., VAN HOOK H.J., "Dans le plan magnetized YIG substrates self-biased by SmCo based sputtered film coatings", J. Appl. Phys., Vol. 76, 1994, (10). [CAD 98] CADIEU F.J., RANI R., QIAN X.R., LICHEN, "High coercivity SmCo based films made by pulsed laser deposition", J. Appl. Phys., Vol. 83, No. 11, 1998, pp. 6247-6249. [CHE 86] CHEUNG T.D., WICKRAMASEKARA L., CADIEU F.J., "Magnetic properties of Ti stabilized Sm(Co,Fe)5 phases directly synthesized by selectively thermalized sputtering", J. Magn. Magn. Mater., Vol. 54-57, 1986, pp. 1641-1642. [CHO 98] CHOONG J.Y., SANG W.K., JONG S.K., "Magnetic properties of NdFeB thin films synthesized via laser ablation processing", J. Appl. Phys., Vol. 83, No. 1, 1998, pp. 66206622. [HEG 93] HEGDE H., JEN S.U., CHEN K., CADIEU F.J., "Film SmCo permanent magnets for the biasing of thin permalloy strips", J. Appl. Phys., Vol. 73(10), 1993, pp. 5926-5928. [HOM 90] HOMBURG H., SINNEMANN TH., METHFESSEL S., ROSENBERG M., Gu B.X., "Sputtered NdFeB-films of high coercivity", J. Magn. Mag. Mat., Vol. 83, 1990, pp. 231-233. [JUL 99] DE JULIAN FERNANDEZ C., VASSENT J.L., GIVORD D., "Thin film deposition by magnetic field-assisted pulsed laser assembly", Applied Surface Science, 138-139, 1999, pp. 150-154. [KAP 93] KAPITANOV B.A., KORNILOV N.V., LINETSKY YA. L., USSR Patent nI°1705892, 1993 (in Russian). [KOR 99] KORNILOV N.V., "Sputtered NdFeB thick films: technology, properties, texture", La Revue de Metallurgie - SF2M - JA 99, 1999, p. 85. [LEM 95] LEMKE H., LANG T., GODDENHENRICH T., HEIDEN C., "Micro patterning of thin NdFeB films", J. Magn. Magn. Mater., Vol. 148, 1995, pp. 426-432. [LIN 95] LINETSKY YA.L., KORNILOV N.V., "Structure and magnetic properties of sputtered NdFeB alloys", J. Mat. Engineering and Performance, Vol. 4(2), 1995, pp. 188-195. [LLN 01] LABORATOIRE Louis NEEL, private communications. [WAN 93] WANG D., Liou S.H., HE P., SELLMYER D.J., HADJIPANAYIS G.C., ZHANG Y., "SmFe12 and SmFe12Nx films fabricated by sputtering", J. Magn. Magn. Mater., Vol. 124, 1993, pp. 62-68. [YAM 91] YAMASHITA S., YAMASAKI J., IKEDA M., IWABUCHI N., "Anisotropic NdFeB thinfilm magnets for milli-size motor", J. Appl. Phys., Vol. 70(10), 1991, pp. 6627-6629.


Chapter 9

RF MEMS for the Mobile Communications Era: Present and Future Adrian M. lonescu Laboratoire d'Electronique Generale, Institut de Microelectronique et Microsystemes, Ecole Polytechnique Federale de Lausanne, Switzerland

1. Introduction The end of the 20th century experienced major evolutions in the radio-frequency (RF) technologies, [De Los Santos 1, Itoh, Katehi 1]. Exponentially growing interest in mobile communications and networking of information have brought new challenges, with particular emphasis on more affordable and integrable technologies able to provide superior RF functionalities per unit of volume. Immediate impact has been observed on the research and development of silicon-based RF integrated circuits (ICs) with special emphasis on sub-micron silicon CMOS, SOI and SiGe hetero-junction bipolar transistors. New technologies have also emerged, stimulated by the prospect of enormous markets. One successful example is micro-electromechanical-systems (MEMS) technology. Even if first developments in RF MEMS originated from the requirements of airborne systems [Brown], nowadays the telecom mass market [De Los Santos, Nguyen 1, Tilmans] is the main driver of this field. Even though first MEMS devices were demonstrated in the late 1960's [Petersen], major developments for applications had had to wait for more than 30 years because of non-adapted technologies. Generally, MEMS are considered as the merger of the IC world with the micronscale mechanical world. Conventional 1C fabrication techniques stand along with more exotic processes, in order to provide MEMS architectures. MEMS devices are extremely attractive for RF ICs (especially wireless) because of their major gains in terms of: (i) device and system miniaturization (resulting in lower costs), (ii) integration (same planar technology as to manufacture integrated circuits), (iii) power savings (ultra-low power operated devices), (iv) higher performance (such as high-Q for passive devices and more potential for wideband) and (v) new integrated functionalities (such as programmable or tunable passive devices and interconnects, resulting in programmable or tunable RF IC architectures). Particularly, wide-band performance requirements for RF MEMS, as necessitated by airborne applications, could positively impact the overall mobile communication applications in the near future. If one could accept that future mobile handsets should have multifunctions

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(being used as a classical mobile phone in two or three allowed bands and/or also as a LAN terminal and/or as a computer network terminal) and, moreover, offer access to different service providers emitting in different RF/microwave bands, the various frequencies associated with these broadcasts will require the use of wideband internal functions at the front end level of the transceiver. In this paper, a review of present and future RF MEMS is proposed. RF switches, high-Q passives (capacitors and inductors), resonators and various RF MEMS circuit applications are inspected. Generally, it is shown that RF MEMS impact at circuit level can be imagined under two fundamentally different approaches: (I) the first is to envision RF MEMS passives or switches as direct replacement components for off-chip passives and pin diodes (Figure 1), [Nguyenl, 2, 3], [Tilmans], and (II) the second is to project novel, more non-conventional RF circuit (transceiver/receiver) architectures and take full benefit of RF MEMS functionality such as for RF routing in radio front-end, suggested by [Brown], Figure 2; in this case the entire spectrum is divided into independent channels (C1, C2, ..., CN, in Figure 2), each of them with its own filter band-pass filer (a filter bank, achievable with RF MEMS, is then required).

Figure 1. Schematic of a super-heterodyne receiver (after [Nguyen I]); highlighted in grey are possible direct replacements using RF MEMS components (switches, passives and resonators)

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'AN

Figure 2. Schematic of a reconfigurable radio architecture working simultaneously with various operators at same physical site, after [Brown], and suggesting use of RF MEMS switches and band-pass filter (BPF) banks

2. RF MEMS switch: capacitive shunt and contact-series architectures The basic building block of any RF circuit is the RF MEMS switch, which plays a similar role to that of a MOS-type switch in a standard integrated RF ICs. In principle, the most successful switch categories using electrostatic actuation are [Pacheco, Rebeiz, Tilmans]: (i) the capacitive shunt switch (Figure 3), and, (ii) the contact series switch (Figure 4). The capacitive switch architecture (pioneered by [Goldsmith 1, Yao]) has two stable states with an associated capacitance variation of around two orders of magnitude between the two states (Con/C0ff > 100). It follows that the capacitive switch is essentially exploitable at high frequency where, when actuated in on state, it creates a shunt to the ground for the RF signal (see Figure 3 (b)). In contrast, the series contact switch shown in Figure 4 is usable from low (near zero) to high frequency and, when actuated, it behaves as a small series resistor (Ron100mW). The reliability problems, as it will be discussed later on this paper, are completely different for capacitive (charge build up in the thin insulator) and contact switch (degradation of the characteristics of the metal-metal contact). Alternative switch architectures, using liquid-metal contact [Simon, Kim] can result in some improved reliability but their technology and the need of encapsulation of liquids raise other concerns; - Packaging - because of their 3D mechanical nature [Katehi2], RF MEMS switches need more protection (hermetic sealing) in terms of ambient, compared to solid-state devices. The key problem with the present packages is their cost. One promising solution could be a wafer-scale package. From the technological point of view, because the most part of RF MEMS switch (contact or contact-less) fabrication requires surface micro-machining, integration with CMOS active devices is presently achievable: Use of materials such as metals (Al, Au, AlCu, Cu, Ni, Pt, W) or polysilicon for both suspended and fixed membranes/cantilevers and of sacrificial layers (l-4um) such as polymers (polyimide), SiO2 or silicon, are the most versatile solutions. A capacitive RF MEMS switch would also require a low-temperature (instances) THM=setof(X INAREA X->cellName=="FEMembraneT2") THB=setof(X INAREA X>cellName=="FEBeamT2") THM=sort(THM 'north) THB=sort(THE 'east) THB=sort(THB 'north) SEGl=dscGetInstSegsOnPins (THM list("t4")) SEG2 = dscGetInstSegsOnPins(THB list ("t2")) NET1=dscGetSegmentsNet(SEG1) NET2=dscGetSegmentsNet(SEG2) ;# List of all possible locations LOC=dscListInterlace(NETl NET2) ;# Specifying the current fault model dsc_CURRENT_FAULT=i±st("FAULT_DEMO" "thermalNetShortcut") !# Building the fault scenario LOCp1=dscLabelLocationsAs("concurrent" From(LOC 1 3)) LOCp2 = dscLabelLocationsAs ("concurrent" From(LOC 1 4)) LOCp3=dscLabelLocationsAs("concurrent" From(LOC 4 6)) LOCs = dscLabelLocationsAs ("sequential" List (LOCpl LOCp2 LOCp3)) dsc FAULT SCENARIO=dscBuildScenario(LOCs)

Figure 11. Test code for driving the injection of the fault model

Since the fault model implies the use of two nets that will be shorted, the tool function called dscListlnterlace will produce the list of all pairs of nets that can be shorted, corresponding to the list of all possible fault locations. Next the fault model is selected.

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Finally, from the list of all possible locations, three different fault scenarios are indicated. The first indicates that thermal shorts will occur in locations from 1 to 3 and that three thermal shorts will be injected concurrently. The result of this fault scenario is shown in Figure 12 (a). The faults are injected in the anchor region between the arm and the membrane. Similarly, the third fault scenario indicates that faults will occur in locations from 4 to 6 and the corresponding result is shown in Figure 12 (c). The faults are now injected away from the anchor region. The next program statement indicates that the three fault scenarios are scheduled to be simulated sequentially. Finally a pointer to the fault scenarios called dsc_FAULT_SCENARIO is kept. This pointer will be next used by the fault simulator to obtain the different faulty design representations required for simulation.

Figure 12. Fault injection results for two different fault scenarios of Figure 7(c): (a) scenario LOCpl and (b) scenario LOCp3

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7. Conclusions Current research efforts in MEMS development are trying to bring the analysis, design and test of microsystems in line with microelectronics. Intermediate levels of MEMS design abstraction are searched, keeping compatibility with IC design practice and with a cleaner separation between technological processing and design effort. This leverages research and development such as circuit simulation that makes also possible realistic fault modeling and design-for-test, and the use of AHDL's. In this way, test technology already developed for microelectronics can be used for addressing the test of MEMS parts. The design and validation of highly complex systems such as mixed-technology SoCs (system-on-a-chip) could well be addressed this way (Mir et al., 2000). Acknowledgements The research described in this paper stems from several years of research effort in the TIMA Laboratory. This research line has been established by B. Courtois. We would like to acknowledge the collaboration of A. Castillejo and F. Parrain in the analysis of new MEMS defects and failure mechanisms. F. Parrain participated in the design of the infrared imager and fingerprint sensor chips and the corresponding electronics interfaces. The new analog fault simulation tool and associated fault modeling and fault injection language has been developed by C. Roman. 8. References Kolpekwar A., Blanton R.D., "Development of a MEMs testing methodology", IEEE Int. Test Conference, Washington DC, USA, November 1997, pp. 923-931. Castillejo A., Veychard D., Mir S., Karam J.M., Courtois B., "Failure mechanisms and fault classes for CMOS-compatible microelectromechanical systems", IEEE International Test Conference, Washington DC, USA, October 1998, pp. 541-550. Mir S., Charlot B., "On the integration of design and test for chips embedding MEMS", IEEE, Design and Test of Computers, 16 (4): 28-38, October-December 1999. Chariot B., Mir S., Cota E.F., Lubaszewski M., Courtois B., "Fault simulation of MEMS using HDLs", SPIE Symposium on Design, Test and Microfabrication ofMEMS/MOEMS, Vol. 3680, Paris, France, April 1999, pp. 70-77. Lubaszewski M., Cota E.F., Courtois B., "Microsystems testing: an approach and open problems", IEEE Design, Automation and Test in Europe Conference, Paris, France, February 1998, pp. 23-26. Chariot B., Mir S., Parrain F., Courtois B., "Generation of electrically induced stimuli for MEMS self-test", Journal of Electronic Testing: Theory and Applications, 17 (6): 459470, December 2001, Kluwer Academic Publishers. Mir S., Chariot B., Nicolescu G., Coste P., Parrain F., Zergainoh N., Courtois B., Jerraya A., Rencz M., "Towards design and validation of mixed-technology SoCs", 10th Great Lakes Symp. on VLSI, Chicago, USA, March 2000, pp. 29-33.

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Courtois B., "Failure mechanisms, fault hypotheses and analytical testing of LSI-NMOS (HMOS) circuits", International Conference on Very Large Scale Integrated Systems VLSI'81, J.P. Gray Ed., pp. 341-350. Fantini F., Morandi C., "Failure modes and mechanisms for VLSI ICs - A review", IEEE Custom Integrated Circuits Conference, 1988, pp. 16.5.1-16.5.4. Hayes J.P., "Fault modeling - Tutorial", IEEE Design & Test of Computers, April 1985, pp. 88-95. Agrawal V.D, Cheng K.T, Agrawal P., "Contest: a concurrent test generator for sequential circuits", 25th Design Automation Conference, 1988, pp. 84-89. Eichelberger E.B., Williams T.W., "A logic design structure for LSI testability", 14th Design Automation Conference, June 1977, pp. 462—468. McCluskey E. J., "Built-in self-test structures", IEEE Design & Test of Computers, April 1985, pp. 29-36. Meixner A., Maly W., "Fault modeling for the testing of mixed integrated circuits", International Test Conference, 1991, pp. 564-572. Nagi N., Chatterjee A., Balivada A., Abraham J.A., "Fault-based automatic test generator for linear analogue circuits", Int. Conference on Computer-Aided Design, Santa Clara, California, November 1993. Mir S., Lubaszewski M., B. Courtois., "Fault-based ATPG for linear analogue circuits with minimal size multifrequency test sets", Journal of Electronic Testing, Theory and Applications, 9 (1/2): 43-57, 1996. Mir S., Lubaszewski M., Liberali V., Courtois B., "Built-in self-test approaches for analog and mixed-signal integrated circuits", IEEE 38th Midwest Symposium on Circuits and Systems, August 1995, pp. 1145-1150. Karam J.M., Courtois B., Boutamine H., Drake P., Poppe A., Szekely V., Rencz M., Hofmann K., Glesner M., "CAD and foundries for microsystems", Proc. of the Design Automation Conference, Anaheim, USA, 1997, pp. 674-679. Mukherjee T., Fedder G. K., "Structured design of microelectromechanical systems", Proc. of the Design Automation Conference, Anaheim, USA, 1997, pp. 680-685. Swart N.R., "A design flow for micromachined electromechanical systems", IEEE Design & Test of Computers, Vol. 16, No. 4, October-December 1999, pp. 39-47. Roman C., Mir S., Chariot B., "Building an analogue fault simulation tool and its application to MEMS", 8th IEEE International Mixed-Signal Testing Workshop, Montreux, Switzerland, June 2002. Zimmermann L., Ebersohl J.Ph., Le Hung F., Berry J.P., Baillieu F., Rey P., Diem B., Renard S., Caillat P., "Airbag application: a microsystem including a silicon capacitive accelerometer, CMOS switched capacitor electronics and true self-test capability", Sensors and Actuators A, 46-47, 1995, pp. 190-195. Puers R., De Bruyker D., Cozma A., "A novel redundant pressure sensor with self test function", Sensors and Actuators A, Vol. 60, 1997, pp. 68-71.


Chapter 11

Reliability and Failure Analysis Issues in MEMS Ingrid De Wolf IMECvzw, Leuven, Belgium 1. Introduction Micro electro mechanical systems (MEMS) are evolving from a challenging research subject into industrial products [1]. A very large diversity of new MEMS is being reported on in literature, going from systems for biomedical use to automotive and information technology systems. However, most of the reports on emerging MST-products do not mention anything on the reliability of the devices presented. There are various reasons for these missing data. Mostly reliability issues only become important at a later stage of the product development since the early phases are often dominated by considerations of design, functionality, feasibility and costs [2]. Also, often the system described in publications is still a lab prototype (and often will remain in this stage of the development) and the researcher, often a PhD student, is happy when already able to demonstrate its intended functionality. Or reliability studies have been performed, but are negative and cannot be published. Another important reason for missing reliability data is that the means and procedures to perform reliability tests and consequent failure analysis are not available, unknown or often even non-existent. This apparent lack of reliability tools and methods contrasts with the huge amount of know-how available on the reliability and failure analysis testing of ICdevices and standard IC-chip packaging. Most failure modes are well known and documented, and standard test procedures are available and described in standards such as the MIL-standards. Fortunately, some of this know-how can directly be used for the study of MEMS and MEMS packaging. On the other hand, other key issues are unique to MEMS, i.e. those that are mainly related to the mechanical part of the structure, such as creep, fatigue, fracture and stiction. To make things worse, the reliability specs depend highly on the intended application. For example an RF-MEMS switch which is intended for space applications might require only one switching event in its lifetime, but it should do this without failing. Thus in a 'space' environment, issues such as shock (during launch), vibrations, vacuum, and radiation hardness are of importance. The same RF-MEMS switch might be intended for switching in a mobile phone. In this case, the reliability specs demand at least 109 switching events without failure, and the

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environmental conditions include humidity, shock (ex. when the mobile phone drops), temperature variations, etc.

2. The reliability process Reliability is defined as the probability that an item will perform a required function under stated conditions for a stated period of time. As mentioned in the introduction, reliability is often the very last step that is considered in the development of new MEMS. However, reliability issues that are discovered at such a late phase may cause major delays in the product development going together with high costs. It is essential to consider reliability aspects from the very start of the development, beginning at the design phase. 'Design for reliability' [3] and the 'holistic approach to increase life time' [2], both based on the idea of considering reliability aspects in all phases of the product development, are essential. The goal of the reliability process is to understand the effect of design, processing and post-processing (e.g. packaging) on device lifetime. It can be described as follows: - get the specifications of the system (both electrical and environmental), - determine the MEMS specific failure mechanisms and the physics behind them. This is done through the: - design of specific test structures, - development of test methods, - determination of significant material and design parameters (experimental and simulation), - understand the acceleration factors for these failure mechanisms and perform adequate tests (electrical, mechanical, thermal, environmental), - develop predictive reliability models, using the correct data handling (statistics), - deduce design rules for increased reliability. This methodical approach of reliability is described in detail in [2]. It allows one to approach reliability issues more efficiently. The implementation of the methodology is illustrated in [2] with a real-life example: a hot film mass air-flow sensor.

3. MEMS issues In the following we will describe some typical MEMS failure modes and reliability issues, and analysis techniques that were used to study them.

Reliability and Failure Analysis Issues in MEMS

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3.1. Particle and obstructions: microscopy and SEM A wafer with MEMS cannot be handled in the same way as a wafer with standard ICs. Cleaning and die singulation after release of the structures is not possible, because the particles and contamination generated during this process can obstruct the movement of the MEMS or cause shorts. For this reason MEMS wafers are often diced before release, or use non-standard dicing techniques [4]. Another way to circumvent this problem is 'packaging' the MEMS before dicing, using either wafer to wafer bonding or local capping of the MEMS by for example a Si or glass cap [5-9]. This packaging, called wafer- or 0-level packaging, offers not only protection against particles during dicing of the wafer, but can also be used to keep the MEMS in a controlled environment during its operation. As will be discussed further on in this paper, hermetic packaging of MEMS can help to shift reliability issues from the MEMS to the package. And because one kind of 0-level package can be used for different kinds of MEMS, one good packaging solution will help circumvent several reliability issues for various MEMS. Failure of MEMS due to particles or other obstructions can in general be observed using an optical microscope or SEM, as for example shown in Figure 1 [10].

Figure 1. A particle wedging a MEMS sensor element out of plane, from [10]

Another critical issue in MEMS is the release of the structures. When the release process of MEMS is not optimal, the MEMS will show a 'sudden death' because parts intended to move are still partly stuck. Figure 2 shows an example of the optimization of the release etch of the sacrificial layer under a RF-MEMS switch. The metal beam of the switch is designed with long holes to facilitate the release etching. In this case, the release process could be optimized because the sacrificial layer can be observed through the backside of the wafer using an optical

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microscope, as shown in Figure 2. In Figure 2 a, the sacrificial layer is still nearly everywhere present under the metal beam of the switch. In Figure 2 b some remaining parts of the sacrificial layer can be observed at positions farthest away from etches, clearly showing that the etching process starts from the sides and holes. The beam is still stuck. In Figure 2 c the process was optimized and the beam is completely removed. In this example, the study was straightforward because the process could be monitored through the backside of the glass wafers. This is not the case for MEMS fabricated on a Si wafer. In that case destructive sample preparation is often used to investigate whether there are release problems due to incomplete sacrificial layer etch, such as embedding in epoxy and polishing, or removal of the top layer using tape.

Figure 2. Study of the etching of the sacrificial layer between a glass substrate and a metal bridge of a RF-MEMS switch. The pictures are taken using an optical microscope, through the back-side of a glass wafer. a. Incomplete removal, b. Some remaining parts at positions far from edges. c. Complete removal

However, for such structures a non-destructive method also exists, as was discussed in [11]. They used IR confocal laser microscopy to study the release process of SOI-based optical switches. Images of the silicon dioxide sacrificial layer were obtained with this technique through the front side of the 100 [um thick movable Si structure. Figure 3 a shows an example of an optical IR-image taken on


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a non-functioning mirror. Some non-etched SiO2 can be seen as a bright spot, confirming that the release process was not optimized. A SEM picture taken after removal of the moving Si part confirmed the observation (Figure 3 b) and the usefulness of the IR technique. This technique cannot be applied from the front side if a metal layer is present on top of the MEMS, such as in the RF-MEMS metal switch shown in Figure 2. However, it was shown in [11] that the technique also works when applied from the back side of a Si wafer. Unreleased structures could be detected through a 500um thick low-doped silicon substrate.

Figure 3. Left: IR confocal laser microscopy image of a defective optical switch showing nonetched sacrificial SiO2. The picture is taken from the top side through the Si structure. Right: SEM picture confirming the observation (from [11])

3.2. Stiction: SEM, AFM, ELT, motion analysis One of the most important failure modes for MEMS is stiction: the unwanted adhesion of two surfaces to each other. Stiction occurs if the forces attracting the surfaces are larger than the 'spring' forces that try to restore the surfaces to their original position. Attracting forces are capillary forces, electrostatic forces, hydrogen bridging, solid bridging and van der Waal's forces [12]. These surface forces are more pronounced in MEMS than they are in the macroscopic world because of the large surface over volume ratio in microsystems: surfaces scale with x2, volumes scale with x . The more one scales into the micro-domain, the more important surface effects become. Van der Waals forces between molecules will play a role in stiction when the surfaces of the MEMS are in a completely waterless environment, or when hydrophobic surfaces are used. If there is water between the surfaces, capillary forces will completely dominate the van der Waals forces, unless the surfaces are (even for MEMS) exceptionally smooth (like in wafer bonding) [12].

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When MEMS surfaces are covered with OH bonds, H-bridging may increase the surface interaction energy. H-bridging is a short-range force; for this reason it is much more sensitive to surface roughness than is capillary condensation. H-bridging is expected to play a role only in hydrophilic OH terminated surfaces in a waterless environment [12]. Capillary induced stiction may already occur during the release process [13,14]. The surface tension of droplets of rinse fluid which remain under free structures such as for example a bridge of an RF-MEMS switch can pull them down to the substrate causing stiction (see Figure 4). This can be avoided by freeze-drying or super-critical drying, or by using vapour HF. Figure 5 shows a SEM picture of a part of an RF-MEMS bridge which remained stuck after the release process.

Figure 4. Stiction due to capillary forces, caused by remaining rinse water. Top: the sacrificial layer is present under the bridge. Center: after removal of the sacrificial layer rinse water remains under the free standing bridge. Bottom: the bridge is stuck due to capillary forces during drying of the rinse water

Figure 5. SEM picture of a capacitive RF-MEMS switch with two bridge beams, showing stiction of the front beam


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Capillary induced stiction may of course also occur during use of the MEMS. The probability for stiction-failure due to capillary forces depends on the roughness of the contacting surfaces and on the magnitude of the restoring forces. Fig. 6 shows a model predicting stiction as a function of the restoring force of the bridge, the environmental humidity and the roughness of the surfaces [15]. This model shows clearly that the probability to have stiction due to capillary forces increases if the restoring force of the bridge decreases, if the surface roughness decreases and of course if the humidity of the environment increases. The first can be optimised in the design of the structures. The roughness can be controlled partly by the processing and studies using atomic force microscopy (AFM). However, one has to keep in mind that the roughness of the top surface of such a structure is not necessarily the same as the one of the bottom surface, as shown in the AFM measurements of an Al bridge of a RF-MEMS capacitive switch depicted in Figure 7. In order to measure the bottom side, the bridge was simply removed using scotch tape. The bottom surface was found to be much smoother than the top surface, increasing the risk for capillary induced stiction.

Figure 6. Model predicting stiction as a function of restoring force (a.u.), humidity and roughness [from 15]

Several solutions exist to avoid stiction due to capillary forces, such as using self-assembled molecular (SAM-) coatings and reactive getter materials in the package. However, the most straightforward solution, nearly independent of the MEMS, would be offered by a hermetic package. Another important cause of stiction in MEMS is charging. The main causes of charging induced stiction in MEMS are tribocharging of rubbing surfaces and charge trapping in oxide layers [16]. Charging can also be caused by radiation. This is a major concern in MEMS intended for space applications [17,18]. The charging due to the rubbing of surfaces against each other mostly disappears if the movement stops. For this reason tribocharging will not always result in permanent damage [19,20].

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Charge accumulation in a dielectric layer is of great concern in capacitive RFMEMS switches. RF-MEMS switches use mechanical movement to achieve a short circuit or a change in capacitance [21]. Figure 8 shows side and top views of a capacitive RF-MEMS switch.

Figure 7. Left: microscope picture of the bridge, Right: AFM measurement of a 5mmx5mm area. a) Top side of bridge, Ra = 8.1 nm, b) Bottom side of bride (removed using tape), Ra = 1.4 nm

Figure 8. Side and top view of a capacitive RF-MEMS switch


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A metal bridge is fixed at its ends to the ground lines and bridges the signal line. The signal line is locally covered by a dielectric, typically SiO2, SiN or Ta2O5. When the bridge is in the up position, the capacitance between the bridge and signal line is small and the RF signal can pass without much loss. When the bridge is pulled down, by applying a DC voltage between bridge and signal line, the capacitance increases and the RF signal will be shorted to ground. In that on-state, the metal bridge is in contact with the dielectric and a high electric field is present across the latter. This may result in trapping of charges in the dielectric, which in turn decreases the pull-in voltage of the switch, resulting in stiction. Figure 9 shows an electrical lifetime test of such a switch. Voltage pulses slightly higher than the pullin voltage (i.e. the voltage required to pull the metal bridge down to the dielectric) are applied with a certain frequency and duty cycle, and the capacitance change, AC, is monitored as a function of the number of pulses (i.e. pull-in events). A special electrical lifetime test (ELT) system was build for these experiments [22]. In this case, the Al switch failed after 2xl06 cycles. During this electrical testing, the outof-plane movement of the switches was also monitored using an optical analysis technique. The failure mode was found to be stiction of the bridge, due to charging of the dielectric. The addition of optical motion analysis to electrical testing of MEMS is often indispensable to find the failure mechanism and mode. There exist a variety of systems that can be used to monitor motion of MEMS: high-speed photomicrography and optical 3-D imaging methods based on laser interferometry, Mirau interferometry, speckle, micro-moire, scanning laser Doppler velocimetry etc. A comprehensive overview, showing results on ink-jet printheads, microrurbine, microrelays and micropumps is given in [23]. Examples of the use of different optical inspection tools for reliability studies and failure analysis of various MEMS can be found in [23-26]. A system allowing the monitoring of MEMS vibrations up to 15 MHz was presented in [26]. Figure 10 shows sequential images of the motion of a microrelay, indicating clearly the problem areas [26]. Such systems can also be used to investigate stiction in RF-MEMS switches.

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Figure 9. Reliability study showing the switching induced capacitance change as a function of switch cycles. Failure due to charging induced stiction occurred after 2 x 106 cycles

Figure 10. Sequential pictures taken from a moving defective micromachined relay vibrating at 1000 Hz [26]

Another type of RF-MEMS switch is the ohmic metal contact switch. In these devices there is no insulator between the top metal bridge and the signal line. In contact, the electrical resistance should be as low as possible and of course stiction should not occur. A nice alternative use of AFM to study such ohmic MEMS switches was presented in [36]. They used the tip of an AFM to apply a known force to the bridge of a switch while monitoring the contact resistance, as is shown in Figure 11. For small forces, contact is made only at local asperities, giving a high resistance. When the force increases, the contact improves and the resistance decreases. From such experiments they could deduce the force to initial contact (Ft), the force for stable resistance (Fs) and the corresponding resistance (Rs). Changes in


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Ft after reliability testing then give information on the bridge mechanics, while changes in Rs and Fs give information on the metal contact properties.

Figure 11. Application of AFM to measure contact resistance. Left: experimental set-up. Right: measured contact resistance versus applied force. From [36]

3.3. Mechanical stress: profilometry, uRS Another problem that plays a role in MEMS is the presence of mechanical stress or a stress gradient in the movable structure [27]. If the material is not optimized for stress, it may change shape after release. In addition, internal stresses may influence the sensitivity of the MEMS to creep, fatigue and corrosion. This leads to deformation and possible malfunctioning. Stresses should be already optimized during the processing stage. This can be done through the use of typical stress test structures with various dimensions, such as beams, cantilevers, diamond rings and Guckel rings [28]. The amount of deflection or buckling of such structures depends on the stress or stress gradient and on the dimensions of the structures, and from a study of different sizes information on the stress can be obtained. This can easily be studied using non-contact 3D optical surface metrology instruments which are commercially available. A result of such a measurement on a Guckel ring is shown in Figure 12. The center part of this ring is clearly buckled. The amount of buckling depends on the diameter of the ring.

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Figure 12. Optical metrology measurement of a Guckel ring. Left: 3D picture, the colors indicate the height. Right: Cross section along the ring's beam, showing a clear inward buckling

Local mechanical stresses in the silicon parts of MEMS, such as in membranes of pressure sensors, may also influence the functioning of the MEMS. These stresses can be studied using X-ray diffraction [34] or micro-Raman spectroscopy (uRS) [35]. The latter technique is not so common, but offers excellent possibilities for the study of MEMS. Figure 13 shows the mechanical stress measured in a Si membrane of a pressure sensor using uRS [15,35]. The technique can also provide information on crystallinity, temperature, composition, film thickness, etc. [37].

Figure 13. Mechanical stress in a Si membrane measured using uRS [15,35]


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3.4. Fatigue,fracture, creep and wear: FIB, pRS The movable parts of MEMS are sensitive to fracture, or the breaking of the parts. This can be due to external loads, such as drops or vibrations, or it can be a result of fatigue. Fracture can mostly easily be seen using an optical microscope or SEM. It can in general be avoided through a robust design and careful handling of the MEMS. Fatigue starts with the generation of a crack at an area of high stress concentration. This crack will grow during use of the device until failure due to fracture occurs. We refer to [10,29] and the references therein for examples of fractured MEMS and more details on fatigue. Fatigue is probably not an important failure mode for MEMS. It was up to now only tested on polysilicon MEMS test structures that were specially designed for that purpose [ 10, 291. Wear is of importance for MEMS with rubbing surfaces. It might lead to stiction, fracture, or the generation of debris. The latter can easily damage other devices in close proximity. Humidity was shown to have a large influence on wear [30,31]. This again stresses the importance of hermetic packaging of MEMS. Wear is typically of concern for micro-gears [40]. Adapting the design, or the application of a coating, can often reduce this problem. A very good, but destructive technique, that can be applied to study wear in gears, is focused ion beam (FIB). Figure 14 is a picture of a FIB cross-section of a pin joint area, showing clearly damage due to wear [29, 10, 401. FIB is of course not limited to wear problems. It is very useful to control design and to perform failure analysis of MEMS [ 10,41431.

Figure 14. FIB cross-section of apin joint area of a micro-gear, showing wear damage and debris [29,10,40]

In order to obtain a good performance, the coating should be uniformly applied, also in high-aspect-ratio areas such as between gear teeth. As mentioned above, pRS has a broad range of applications. It can sometimes be used to study the coating [38] but also to study its uniformity. For example, Ager et QI. [39] used the intensity of the Raman signal of an anti-friction carbon (DLC) coating to study its thickness variations on Ni-alloy gears. They showed that the coating was uniform on the top gear surface, but somewhat thinner at a certain depth inside the gear teeth (Figure 15).

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Figure 15. Left: SEM detail of area between gear teeth. The probed region is indicated by the white line. Right: DLC Raman intensity between gear teeth as a function of the distance from the top surface of the gear. The intensity observed from scans on the top of the gear is indicated by the solid straight line (from [39])

Creep is the plastic deformation of metal under mechanical stress. Creep in moving metal parts of MEMS was extensively studied by Texas Instruments (TI) on the digital micro-mirror device (DMD) [32, 44]. They showed that Al is very sensitive to creep. To stay compatible with the Al-etch processing which was already in use and well characterized, they developed Al compounds (patent in 1996 [33]) having fewer primary slip systems than aluminium and a much higher melting point (a high melting point metal often has low creep). In [44], an interesting and profound reliability study of the DMD is presented, with focus on the 'hinge memory effect'. They show that although the failure modes and mechanisms of these devices are quite different from the ones encountered throughout the semiconductor industry, the approach of identifying them, accelerating the failures and applying acceleration to estimate the lifetime may be the same. They also show that MEMS can meet the customer needs, i.e. that MEMS can be reliable.

4. Conclusions It is certainly as important for MEMS as for IC's to include reliability issues from the start of the development of a new MEMS. A correct design, taking into account non-typical semiconductor issues such as mechanical rigidity, stiction, creep, wear, etc., is essential. Test structures can help to provide information on material parameters (such as mechanical stresses) and failure modes. MEMS show a lot of failure modes that are not seen in IC's. Stiction, which can be caused by various mechanisms, is one of the most important reasons for failure. In addition, one has to take into account issues such as mechanical stress induced


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deformation, creep, fatigue, wear, fracture, particles, obstructions, variations in layer thickness, etc. Many commonly used instruments in the semiconductor industry, such as optical microscopy, AFM, SEM, FIB etc. can also be used to study MEMS. Some less well known techniques, such as Raman microscopy, also find their application to MEMS. However, new instrumentation has also to be developed, such as instruments for motion analysis or electrical lifetime testing, or the combination of both. Or existing instruments may be used in an alternative way, such as the AFM to study forcedisplacement or force-contact resistance relations. It is clear that the immersion of new MEMS will require the development of new test methods, methodology and instruments. Packaging of MEMS, although not discussed in this paper, is certainly an important issue for MEMS reliability. An hermetic package can shield the sensitive MEMS from the environment and in this way help to overcome problems such as particles and capillary induced stiction. Acknowledgements Merlijn van Spengen is thanked for providing information and data on MEMS reliability. Some results were obtained in the frame of the ESA project MEDINA 14627/00/NL/WK and the EC project MIPA, IST-2000-28276. Danielle Vanhaeren is thanked for AFM and metrology measurements. Joern Dechow, Markus Ulm and the RF-MEMS team of Robert Bosch (GmbH) are thanked for providing samples for the AFM analysis. The MEMS reliability team of CNES and the IMEC RF-MEMS team are acknowledged for providing samples and for many stimulating discussions.

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[6] Cho S.T., Erdmann F.M., "An on-chip hermetic package technology for micromechanical devices", Proc. Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, USA, 8-11 June, 1998, pp. 229-232. [7] Tilmans H.A.C., Fullin E., Ziad H., Van de Peer M., Kesters J., Van Geffen E., Bergqvist J., Pantus M., Beyne E., Baert K., Naso F., "A fully-packaged electromagnetic microrelay", Proc. MEMS '99, Orlando, Fl. USA, 17-21 Jan., 1999, pp. 25-30. [8] Gooch R., Schimert T., McCardel W., Ritchey B., Gilmour D., Koriarz W., "Wafer-level vacuum packaging for MEMS", J. Vac. Sci. Technol. A, 17, No. 4, Jul/Aug 1999, pp. 2295-2299. [9] Lin L., "MEMS post-packaging by localized heating and bonding", IEEE Transactions on Advanced Packaging, 23, No. 4, 2000, pp. 608-616. [10] Walraven J.A., Waterson B.A., De Wolf I., Failure Analysis of Microelectromechanical Systems (MEMS). "Failure analysis of MEMS", Chapter in Microelectronic Failure Analysis Desk Reference, published by ASM International, ISTFA 2002. [11] Lelluchi D., Beaudoin F., Le Touze C., Perdu P., Desplats R., "IR confocal laser microscopy for MEMS technological evaluation", Microelectronics Reliability 42, 2002, pp.1815-1817. [12] van Spengen W.M., Puers B., De Wolf I., " A physical model to predict stiction in MEMS", J. Micromech. Microeng., 12, 2002, pp. 702-713. [13] Mastrangelo C.H., H. Hsu C., "Mechanical stability and adhesion of microstructures under capillary forces - part I: basic theory", Journal of MEMS, Vol. 2, No. 1, 1993, p. 33. [14] Mastrangelo C.H., Hsu C.H., "Mechanical stability and adhesion of microstructures under capillary forces - part II: experiments", Journal of MEMS, Vol. 2, No. 1, 1993, p. 44. [15] Van Spengen W.M, De Wolf I., Puers B., "Materials and device characterization in micromachining III", Proceedings of the SPIE, Vol. 4175, 2000, p. 104. [16] Tas N., Sonnenberg T., Jansen H., Legtenberg R., Elwenspoek M., "Stiction in surface micromachining", J. Micromech. Microeng. Vol. 6, 1996, p. 385. [17] Knudson A.R., Buchner S., McDonald P., Stapor W.J., Campbell A.B., Grabowski K.S., Knies D.L., "The effect of radiation on MEMS accelerometers", IEEE Transactions on Nuclear Science, Vol. 43, No. 6, 1996, p. 3122. [18] Lee C.I., Johnston A.H., Tang W.C., Barnes C.E., "Total dose effects on microelectromechanical systems (MEMS): accelerometers", IEEE Transactions on Nuclear Science, Vol. 43, No. 6, 1996, p. 3127. [19] Peterson K.A., Tangyunyong P., Pimentel A.A., "Materials and Device Characterization" in Micromachining Symposium, Santa Clara, CA, September 21-22, Vol. 3512, 1998, pp. 190-200. [20] Maboudian R., Ashurst W.R., Carraro C., "Self-assembled monolayers as anti-stiction coatings for MEMS: characteristics and recent developments", Sensors & Actuators, Vol. 82, 2000, p. 219.


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[21] Tilmans H., "MEMS components for wireless communications", Proc. Eurosensors XVI, Prague, Czech Republic, Sept. 15-18, 2002. [22] van Spengen W.M., Mertens R., Puers B., De Wolf I., "Experimental characterization of stiction due to charging in RF MEMS", Proc. IEDM, 2002. [23] Krehl P., Engemann S., Rembe C., Hofer E.P., "High-speed visualization, a powerful diagnostic tool for microactuators - retrospect and prospect", Microsystem Technologies, 5, 1999, pp. 113-132. [24] Walraven J.A., Galambos P.C., Cole Jr. E.I., Pimentel A.A., Roller G., Gooray A. "Failure analysis of MEMS electrostatic drop ejectors", Proc. 27th ISTFA, 2001, pp. 365372. [25] http://umech.mit.edu/freeman/talks. [26] van Spengen W.M., De Wolf I., Puers R., Vikhagen E. "Optical imaging of highfrequency resonances and semi-static deformations in micro-electromechanical systems (MEMS)", Proc. 27th ISTFA, 2001, pp. 357-364. [27] Rigo S., Desmarres J.M., Masri T., Petit J.A., "Mechanical characterization of materials used in MEMS", Proc. 27th ISTFA, 2001, pp. 349-353. [28] Guckel H., Burns D., Rutigliana C., Lovell E, Choi B., "A simple technique for the determination of mechanical strain in thin films with applications to polysilicon", J. Micromech. Microeng. 2, USA, February 1992, pp. 86-95. [29] http://www.mems.sandia.gov. [30] Tanner D.M., Walraven J.A., Irwin L.W., Dugger M.T., Smith N.F., Eaton W.P., Miller W.M., Miller S.L., "The effect of humidity on the reliability of a surface micro-machined microengine", Proceedings of IRPS, San Diego CA, 1999, pp. 189-197. [31] Patton S.T., Cowan W.D., Zabinski J.S., "Performance and reliability of a new MEMS electrostatic lateral output motor", Proceedings of IRPS, San Diego CA, 1999, pp. 179188. [32] Douglass M.R., "Lifetime estimates and unique failure mechanisms of the digital micromirror device (DMD)", IEEE Proc. 36th IRPS, 1998, pp. 9-16. [33] Tregilgas J.H., Micromechanical device having an improved beam, Texas Instruments Inc., United States Patent 5,552,924, 3 Sept. 1996. [34] Enzler A., Herres N., Dommann A., "Analysis of etched cantilevers", Microelectronics Reliability 42, 2002, pp. 1807-1809. [35] De Wolf I., Jian C., Van Spengen W.M., "The investigation of microsystems using Raman spectroscopy", Optics and Lasers in Engineering, Vol. 36, 2001, pp. 213-223. [36] DeNatale J., Mihailovich R., Waldrop J., "Techniques for reliability analysis of MEMS RF switch", IEEE Proc. of the 40th IRPS, 2002, pp. 116-117. [37] De Wolf I., Analytical applications of Raman spectroscopy, Ed. M.J. Pelletier (Blackwell Science), Chapter 10, Semiconductors, pp. 435-472, 1999.

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[38] Maeda Y., Yamamoto H., Kitano H., "Self-assembled monolayers as novel biomembrane mimetics. 1. Characterization of cytochrome c bound to self-assembled monolayers on silver by surface-enhanced resonance Raman spectroscopy", J. Phys. Chem., Vol. 99, No. 13, pp. 4837-4841, 1995. [39] Ager III J.W., Monteiro O.R., Brown I.G., Follstaedt D.M., Knapp J.A., Dugger M.T., Christenson T.R., "Performance of ultra hard carbon wear coatings on microgears fabricated by LIGA", Proc. Materials Research Society, 1998. [40] Tanner D.M., Walraven J.A., Mani S.S., Swanson S.E., "Pin-joint design effect on the reliability of a polysilicon microengine", IEEE Proc. 40thIRPS, 2002, pp. 122-129. [41] Peterson K.A., Tangyunyong P., Pimentel A., "Failure analysis of surfacemicromachined microengines", Materials and Device Characterization in Micromachining Symposium, SPIE Proceedings, Santa Clara CA, 1998, Vol. 3512, pp.190-200. [42] Peterson K.A., Tangyunyong P., Barton D.L., "Failure analysis for Micro-ElectricalMechanical Systems (MEMS)", Proc. of ISTFA, Santa Clara CA, 1997, pp. 133-142. [43] Walraven J.A., Headley T.J., Campbell A.N., Tanner D.M., "Failure analysis of worn surface micromachined microengines", MEMS Reliability for Critical and Space Applications, Proc. of SPIE, Santa Clara CA, 1999, Vol. 3880, pp. 30-39. [44] Sontheimer A.B., "Digital micromirror device (DMD) hinge memory lifetime reliability modeling", IEEE Proc. 40th IRPS, 2002, pp. 118-121.

Chapter 12 TM CoventorWareTM MEMS Design Methodology

Christian Dupiller Coventor, Courtaboeuf, France

1. Introduction Coventor provides CoventorWare MEMS-specific software and a design methodology for MEMS device and system design. CoventorWare is effective at promoting first-pass manufacturing success, efficient for facilitating rapid convergence on an initial design, and provides an easy to use, seamless, integrated design environment that offers the most productive path to manufacturable MEMS. Its unique capabilities let designers explore MEMS design alternatives efficiently and converge on a device that has the highest probability of success without using computationally intensive FEM tools. Subsequently, CoventorWare can be used to perform FEM/BEM analysis on critical areas to fine-tune the MEMS device. The behavioural models, FEM/BEM analysis tools, and layout capabilities interface seamlessly. CoventorWare is useful for MEMS experts and newcomers alike. Using CoventorWare, designers are guided to create a MEMS device quickly, then evaluate the MEMS device within the surrounding system to understand its behaviour, environmental effects, and the effects of control circuitry.

2. CoventorWare products The four bundles that comprise CoventorWare are: - ARCHITECT™ - DESIGNER™ - ANALYZER™, and - SYSTEM BUILDER™ In the following subsections, we will present in more details these four tools.

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2.1. ARCHITECT™ ARCHITECT offers a schematic-driven system-level MEMS and microfluidics design environment plus the only simulator that includes fully coupled mechanical, optical, electronic, and fluidic simulation capabilities for evaluating the behaviour of a MEMS device within a surrounding system. Libraries of Coventor's proprietary parametric behavioural models of MEMS structures serve as building blocks to construct MEMS devices quickly in a schematic format, simulate them within a system, and rapidly converge on an effective initial design. The reduced order behavioural models are physically correct analytical equations, parameterised accurately in 6 DOF for geometric and material properties. Each model has a symbolic schematic representation that simplifies layout, visualisation, and rapid simulation.

Figure 1. ARCHITECT capabilities

Coventor's MEMS behavioural models simulate device performance rapidly and address manufacturing sensitivities in a system-level environment. Simulation results are comparable to those achieved using FEM, but take only a fraction of the time. Macromodels of custom MEMS structures created using SYSTEM BUILDER can be used to integrate custom MEMS devices into ARCHITECT'S system-level design environment. Coventor integrates parametric, statistical, and stress analyses with its parameterized behavioral libraries and a schematic capture and simulation engine to provide simulations that not only reveal traditional performance issues, but also examine manufacturability and quality assurance factors.

CoventorWare™ MEMS Design Methodology

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ARCHITECT includes manufacturing analysis capability that enables accurate assessment of the performance of a MEMS device, identifies the critical factors of a design, such as components that have the greatest influence on performance or that undergo the greatest stress, and highlights key sources of performance variation by evaluating part tolerances. These powerful virtual prototyping capabilities are valuable for identifying potential yield problems due to tolerance build-up. And, they offer cost reduction opportunities by pinpointing parts that have tolerances that greatly exceed necessary requirements. 2.1.1. Parametric analysis Sensitivity analysis capability is used to identify the critical components of a MEMS design by assessing the effect of change on critical operating characteristics. Minor modifications to an individual component and automatic assessment of the impact on the overall operation of a device are straightforward. Individual parameters can be altered one at a time, the resulting change measured and a relative change ratio computed and listed in a report.

Table 1. Coventor model libraries available in ARCHITECT ELECTROMECHANICAL - Generic mass - Rigid plate (triangular, rectangular, and pie-shaped) - Rectangular flexible plate - Beam - Beam w/electrode - Multi-layer beam - Longitudinal comb - Curved comb - Lateral comb - Electrode - Rectangular coils - Magnetic plate

ELECTRICAL - Transistors - Op amps - Converters - Comparators - Diodes - Regulators - Timers - Control electronics

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Microsystems Technology (Table 1 continued...) OPTICAL - Detector - 2D detector (semi-infinite) - Thin lens - Mirror - Polarizer - Beam splitter - VCSEL (Vertical Cavity Surface Emitting Laser) - Optical power source - Fiber emitter - Fiber receiver

2.1.2. Statistical analysis A complete statistical environment enables verification of manufacturability in the presence of real-world component variations. Access to tolerance specifications keeps production costs in check by providing information needed to select appropriately rated components. The Monte Carlo and 3 sigma statistical simulation method helps optimize the manufacturability of a design by giving valuable insight into the sources of manufacturing performance variation. It uses component tolerances and statistical distributions to randomly vary system parameters during successive simulations. Trends in the data flag the influence of particular parameters on performance to help identify where to change tolerances to improve yield and reduce cost. 2.1.3. Stress analysis An automated process is provided that alerts users to specific devices that exceed specified operating conditions. Users can then modify part ratings or use the flexible MAST hardware description language, to easily add new rating specifications, or modify ones, to suit requirements.


285

Figure 2. AC variation

2.2. DESIGNER™ DESIGN is a powerful design tool for creating layouts and 3D models of manufacturable MEMS and microfluidics devices. It provides a seamless environment to automatically generate 3D models from devices created in ARCHITECT, and includes a 2D layout editor, material properties database, process emulator, and 3D model builder. Models can be analyzed further with ANALYZER.

286


Figure 3. Resonant frequency variation

Figure 4. Stress analysis


287

2.3. ANALYZER™ After initial designs are complete, ANALYZER provides a seamless, configurable, and extensible environment for performing complex, fully coupled, multi-domain physics analyses on the initial design. Analysis modules can be used to reveal the interaction between domains using true-coupled electrostatic, mechanical, and thermal analyses; expose time-dependent behaviors of MEMS devices with transient mechanical and thermal analyses; and study other behaviors critical to the application. CoventorWare's FEM and BEM solvers are designed specifically for MEMS and microfluidics applications and include electrostatics, thermo-mechanics, optical scalar diffraction, gas film damping, frequency-dependent resistance and inductance, Joule heating deformation, 6 DOF spring behavior, CFD, electrokinetics, multiple reacting chemicals, droplet formation, and more.

2.4. SYSTEM BUILDER™ System Builder provides the ability to extract fixed-parameter reduced-order models of custom MEMS elements created in DESIGNER and simulated (FEM or BEM) using ANALYZER. The models can then be imported into ARCHITECT'S system-level simulation environment for integration with Coventor's behavioral models, thereby creating a complete representation of the MEMS device.

3. Design for manufacturability In addition to CoventorWare, Coventor offers test structures and a methodology for characterizing a process and integrating process information into the CoventorWare design environment to optimize yield using predictive simulations and process-centered designs. This design for manufacturability methodology combines CoventorWare and engineering expertise, either the customers or Coventor's, to develop a fully manufacturable MEMS capability. Coventor engineers can provide or use Coventor's test structures to evaluate fabrication and assembly variations resulting from misalignment or machine failures, determine process corners and centers, and identify the most suitable materials and minimal steps for successful fabrication. Relevant process variables are extracted to center the design for the process and predict future performance, then build material properties and process data sets to be used for simulating the design within the boundaries of the target process.

288

Microsystems Technology Table 2-a. Critical RF MEMS characteristics analyzed by CoventorWare

RF MEMS RF Performance - RF performance of moving parts - System specifications for high frequency RF design Dynamic Performance - Transient effects - Effects of gas damping on transient simulations and Quality Factor - Effects of applying a bias voltage on switching speed or frequency response Effect of Joule Heating - Effect of voltage potential or current distributions on thermal/mechanical performance Actuation Optimization - Benefits/detriments of various actuation methods Electro-Mechanical Design - Influence of device geometries on switch control voltage and contact force - Switching speed - Impact of manufacturing-induced residual stress on switch performance Manufacturability - Influence of process variations on switch performance - Key design parameters that drive reliability/performance

Table 2-b. Critical MEMS sensors characteristics analyzed by CoventorWare MEMS SENSORS High Sensitivity - Optimal comb finger design - Optimal flexure design - Reduction of damping effect while maintaining sensitivity Cross Sensitivity - Effects of operational frequency on cross-sensitivity and sensor dynamics - Maximum frequency level at which the plate remains rigid Quadrature Signals - Design variations that reduce device sensitivity to quadrature - Process development variations that minimize quadrature - Temperature Sensitivity - Performance over the operating temperature range Environmental Effects - Effects of packaging on performance Manufacturability - Best process and limits for process development - Expected yield with realistic fabrication tolerance


289

Table 2-c. Critical optical MEMS characteristics analyzed by CoventorWare OPTICAL MEMS Electro-mechanical Design - Mirror stroke - Influence of mirror geometry on controllability - Stress level of mirror tethers - Ideal switching speed - Influence of residual stress on mirror curvature Optical Performance - Gold layer thickness needed to optimize mirror optical performance - Effects of interaction between the optical and package layers - Optical quality of the mirror during operation Manufacturability - Best process and limits for process development - Expected yield with realistic fabrication tolerances

Coventor process engineers are also available to define process requirements, measure performance, and recommend a standard process that can be reused successfully and economically for an entire product family. The standard and custom test structures used to create Coventor Catalyst are built to exercise and provide data for evaluating critical aspects of the process. Many types of structures are valuable for evaluating the critical properties of CMOS and post-process (PP) layers used to integrate MEMS onto ICs. Process monitor structures are valuable to: - outline and examine process boundaries, uniformity, and repeatability, - assist evaluation of failed parts, yield, and reliability issues post-fabrication, and - help improve control of the process by establishing tolerances on the process design rules. Material property test structures: - are used to investigate the material properties of the structural materials and - can be placed on separate wafer or interspersed in the design to gain useful knowledge from the same fabrication run.

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Microsystems Technology Table 3. Partial list of test structures for post-processing MEMS onto CMOS

Layer stucture CMOS

Property of interest Impedance Electrical Resistivity Sheet Resistance Dielectric Permitivity Seebeck Coeff Thermal Conductivities Process Center & Corners Process

Post-processed

Line Width/Spacing Layer Thickness Side Wall Angle Alignment Impedance Electrical Resistivity Sheet Resistance Contact Resistance Dielectric Permitivity High Frequency Impedance Seebeck Coeff Thermal Conductivities TCE Residual Film Stress Young's Modulus, Poisson's Ratio Stress, Stress Gradient Stiction Frequency RF reliability Cyclic Fatigue Fracture Toughness

Test structures TRL Measurements Conductivity Lines Conductivity Lines Fixed PlateCapacitors Bridges & Beams Bridges & Beams Control Monitors (PCMs) Gate Oxide Monitors Interconnect Monitors Chevron Structures Layer Thickness Resonant Structures Alignment Structures TRL Measurements Conductivity Lines Conductivity Lines 4-Point Probe Structures Fixed Plate Capacitors TRL Measurements Bridges & Beams Bridges & Beams Cantilever Beams Doubly Supported Beams Arrays, Cantilever Beams, Bent Beam Multilayer Beams, M-Test Structures Cantilever Beams Resonators Resonators Torsion Beams Notch Structures

As more devices are manufactured in a process, the development team can perform ongoing process monitoring, data collection, and identification of key process variables. This information can be integrated into the design flow to improve new families of MEMS designs. Process test structures are measurable in situ using electrical, optical, or other non-contact methods according to the type of structure.


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4. Conclusion CoventorWare is a MEMS-specific design environment that offers semi-custom design efficiencies, powerful behavioral simulations, design for manufacturabiliry (DFM), layout and GDSII file export capabilities, and FEM/BEM capabilities for designers of MEMS and ICs integrating MEMS. More information is available on http://www.coventor.com.


Index 3D folded structures 34 ANALYZER 287 ARCHITECT 282 CMOS materials, long-term stability 9 CoventorWare™ MEMS design methodology 281 et seq deep silicon trench etching 5 DESIGNER 285 electrical feedthrough, high density for array MEMS 46 in glass 46 multibeam electron sources 53 multiprobe data storage 49 electrical properties, bonded surfaces 162 electrostatically levitated rotational gyroscope 45 spherical 3-axis accelerometer 43 fiber optic MEMS/NEMS 60 fiber end NSOM probe 61 integrated microlens at optical fiber core 61 pressure sensor 60 fracture strength 7 integrated microsystems testing, and microelectronics 241 et seq integration, monolithic vs hybrid 6 LIGA microfabrication technique 117 et seq 3-D structuring 135 applications 137 et seq

beam induced reactions 123 carrier foil 119 galvanic deposition 127 hot embossing 131 injection molding fabrication 129 mask production 118 mold insert fabrication 128 plastic molding 130 reaction injection molding 131 sacrificial layer technology 133 thick resist layers 122 X-ray intermediate masks, resist for 120 X-ray lithography 122 MAGMAS, permanent magnets for 195 et seq applications of REPM 196 fabrication techniques 198 bonded powders 200 electroplating-electrophoresis 203 low pressure plasma spraying 203 nanostructured ribbons and flakes 204 pulsed laser deposition and magnetic confinement 205 sputtered films 204 problems 198 MEMS application fields 11 automotive 11 auxiliary systems 13 chemical and biological 14 computer peripherals 14 defects and failure mechanisms 246 failure modes 264

294


fault modelling 248 fault simulation and test generation 254 mechanical stress 273 mobile communication 13 navigation and driver information 12 optical communication 14 reliability and failure analysis 263 et seq self test 250 MEMS for LSI processing and testing 56 anticorrosive integrated massflow controller 59 contactor for LSI wafer prober 56 microrelay for LSI tester 58 MEMS in production and development 15 airbag accelerometer 21 crash sensor 15 identification fingertip sensor 20 RF-MEMS switch 18 silicon microphone 17 MEMSNAS process 175 microactuators 62 air driven microturbine 66 distributed electrostatic 62 electromagnetically actuated optical switch 64 electrostatic 75 et seq comb drive actuator 88 vibromotor 110 parallel plate 77 et seq rotating micromotors 100 SDA actuator 105 shuffle motor 108 torsion mode 96 microelectronics and microtechnology 1 et seq processes used in 2 microenergy sources 67 SiC microstructures, Si lost mold process 67 silicon based electrolyte fuel cell 68

microlens fabrication 177 micromachining 3 bulk 3 and MEMS packaging 43 et seq surface 3 micromechanical resonators 231 micromechanical sensors, packaged 44 microsystems design and test flow 243 market 24 microtechnology materials used in 1 and microelectronics 1 et seq processes used in 2 MOFSET, suspended gate 222 multilayer polysilicon processes 31 multi-users micromachining processes (MUMP) 25 optical scanners with integrated piezoresistive strain gauges 183 resonant gate transistor 28 RF MEMS for mobile communications 213 et seq capacitors, etching process 224 circuit blocks 230 future develoments 235 switch 215 tunable capacitors 220 scanners 183 et seq self-aligned vertical mirrors 179 silicon optical microlens design 176 single-crystal silicon micro-optoelectro-mechanical devices 173 et seq microlens fabrication 174 surface micro machining, overview 27 et seq alternative 37 basic processes 30 suspended-gate SOI MOSFET 222 suspended high-Q inductors 227

Index

wafer bonding 5, 148 et seq anodic bonding 155 direct bonding 148 electrical properties 162 mechanics 157

plasma assisted 151 RF substrate 166 silicon on diamond 165 silicon technology 168

295

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