RF Microwave Hybrids: Basics, Materials and Processes

RF/MICROWAVE HYBRIDS Basics, Materials and Processes

RF/MICROWAVE HYBRIDS Basics, Materials and Processes

by

Richard Brown Richard Brown Associates, Inc. Shelton, CT

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

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0-306-48153-7 1-4020-7233-3

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DEDICATION

TO JUDY

TABLE OF CONTENTS Preface Acknowledgements

CHAPTER 1:

Hybrids vs MMICs

CHAPTER 2: Basic Concepts 2.1 Introduction 2.2 Maxwell's Laws 2.3 Permittivity and Permeability 2.4 Free Space Wavelength 2.5 Propagation velocity 2.6 Decibel Scale (dB) 2.7 Q Measurements 2.8 Small Signal (S-Parameters)

xiii xv

1 5 5 5 6 7 8 9 9 10

CHAPTER 3: Planar Waveguides 3.1 Impedance 3.2 Microstrip 3.2.1 Guide Wavelength 3.3 Coplanar 3.4 Stripline

13 15 18 21 24

CHAPTER 4: Current Flow and Loss Considerations 4.1 Dielectric losses 4.1.1 Tan 4.1.2 Anisotropy 4.2 Conductor losses 4.2.1 Guide length losses 4.2.2 Attenuation 4.2.3 Return Loss 4.2.4 VSWR, Voltage Standing Wave Ratio 4.2.5 Skin Depth 4.2.6 Adhesion layers 4.2.7 Surface roughness

29 29 29 32 35 35 35 35 37 39 44 49

CHAPTER 5:

55

Substrates

13

viii

5.1 Glass 5.2 Single crystals 5.3 Polycrystalline ceramics 5.3.1 Fabrication 5.3.1.1 Powder pressing 5.3.1.2 Tape casting 5.3.1.3 Roll compaction 5.3.1.4 Lamination 5.3.1.5 Glazing 5.3.2 Substrate characteristics 5.4 Low temperature cofired (LTCC) 5.5 Clad Materials 5.5.1 Glass transition temperature 5.5.2 Material properties 5.5.3 Fabrication 5.5.4 Mechaical patterning 5.6 Cleaning 5.6.1 Wet processes 5.6.2 Dry processes 5.7 Safety

55 57 57 57 57 59 60 61 62 63 70 73 73 73 81 85 87 88 88 90

CHAPTER 6: Thick Film 6.1 Screen printing 6.2 Metal foil screens 6.3 Lithographically defined thick film 6.3.1 Photoengravable thick film 6.3.2 Photoimagable thick film 6.4 Additive techniques 6.4.1 Metal-organics 6.4.2 Direct write 6.4.3 Direct bond

93 93 98 101 102 103 105 105 107 109

CHAPTER 7: Thin Film 7.1 Physical vapor deposition 7.1.1 Evaporation 7.1.1.1 Filament 7.1.1.2 Electron beam 7.1.2 Sputtering 7.1.2.1 DC 7.1.2.2 RF

113 113 113 113 114 115 116 117

ix

7.1.2.3 Magnetron 7.1.2.4 Reactive CHAPTER 8: Dielectric Deposition 8.1 PELPCVD 8.2 Anodization

118 119 123 123 124

CHAPTER 9: Polymers 9.1 Material Properties 128 9.1.1 Moisture absorption 9.1.2 Mechanical properties 9.1.3 Glass transition temperature 9.1.4 Planarization 9.2 Deposition 9.2.1 Spin coating 9.2.2 Spray coating 9.2.3 Screen printing 9.2.4 Other deposition methods 9.3 Patterning 9.3.1 Wet etching 9.3.2 Dry etching 9.3 Photosensitive polymers 138

129

CHAPTER 10

141

Processing Strategies

CHAPTER 11: Photolithography 11.1 Photoresist 11.1.1 Spin-on 11.1.2 Spraying 11.1.3 Roller coating 11.1.4 Meniscus coating 11.1.5 Electrodeposited 11.1.6 Dry Film 11.1.7 Dip coating 11.2 Artwork and masks 11.3 Exposure 11.3.1 Non-colllimated 11.3.2 Large flood 11.3.3 Short flood 11.3.4 Collimated 11.3.5 Laser exposure

130 131 131 131 132 132 133 134 134 136 136 136

143 143 146 148 148 149 150 151 153 156 160 161 161 161 162 163

x

CHAPTER 12:

Electroplating

General Inorganic additives Organic additives Waveforms 12.4.1 Asymmetric dc 12.4.2 Pulse 12.5 Field density 12.6 Electroless

12.1 12.2 12.3 12.4

169 169 171 172 174 174 175 180 182

CHAPTER 13: Etching 13.1 Wet etching 13.2 Dry etching 13.2.1 Sputtering 13.2.2 Ion beam milling 13.2.3 Reactive techniques 13.3 Etching effects on imedance

185 185 186 186 187 190 191

CHAPTER 14 Components 14.1 Passive components 14.1.1 Resistors 14.1.2 Attenuators 14.1.3 Capacitors 14.1.3.1 Parallel plate 14.1.3.2 Interdigitated 14.1.4 Inductors 14.2 Transmission line components 14.2.1 Reciprocal dividers/combiners 14.2.2 Filters

195 195 195 202 205 211 217 218 220 220 224

CHAPTER 15

229

Packaging

15.1 Level of Integration 15.2 Interconnects 15.2.1 Round wire 15.2.2 Strip ribbon 15.2.3 Modified TAB 15.2.4 Integrated wiring 15.2.5 Enclosures

230 231 232 234 237 239 239

xi

15.2.6 Thermal expansion 15.2.7 Substrate attachment 15.2.8 Grounding 15.2.9 Vias 15.2.10 Platability 15.2.11 Time domain reflectometry (TDR)

240 241 243 243 249 251

CHAPTER 16: Superconductivity 16.1 Properties of High-Tc materials 16.2 Materials considerations 16.3 Substrate materials 16.4 Expansion coefficient 16.5 Buffer (barrier) layers 16.6 Film formation 16.6.1 Off-axis sputtering 16.6.2 Pulsed laser deposition 16.6.3 Evaporation 16.6.4 Metalorganic 16.7 Patterning 16.7.1 Wet etching 16.7.2 Dry etching

257 259 261 262 263 263 263 263 264 265 265 266 266 267

CHAPTER 17:

269

MEMS

APPENDIX A: Definition of symbols APPENDIX B: Company directory APPENDIX C: Conversion table APPENDIX D: Graphic evaluation of w/h and SUBJECT INDEX

271 273 275 for microstrip 277 279

PREFACE

xiii

In 1991 this author published a monograph[l] based on his experience teaching microwave hybrid materials and processing technology at the annual ISHM (now the International Microelectronics and Packaging Society, IMAPS) symposia. Since that time, the course has been presented at that venue and on-site at a number of industrial and government organizations. The course has been continually revised to reflect the many evolutionary changes in materials and processes. Microwave technology has existed for almost 175 years. It was only after the invention of the klystron, just before World War II, that microwave design and manufacture moved from a few visionaries to the growth the industry sees today. Over the last decade alone there have been exploding applications for all types of high frequency electronics in the miltary, automotive, wireless, computer, telecommunications and medical industries. These have placed demands, unimaginable a decade ago, on designs, materials, processes and equipment to meet the ever expanding requirements for increasingly reliable, smaller, faster and lower cost circuits. Microwave electronics is realized by monolithic microwave integrated circuits (MMICs), or hybrid microwave integrated circuits (HMICs). Growth in the computer and wireless industries in particular, has spurred the volume manufacture of both products. Mass fabrication of 300mm silicon (Si) and gallium arsenide (GaAs) wafers is being introduced. Additionally, efforts are ongoing to perfect Si and GaAs, moving toward the creation of defect-free crystals , leading to new levels of performance. Hybrid technologists have responded as well to compliment the MMIC efforts. The past decade has witnessed innovative advances in many areas, leading to a variety of new materials and processes. Among these are new powder technologies for photoengravable and photo-definable thick film inks, allowing the use of thick films at frequencies once reserved for thin films. New generation liquid, dry and electrophoretic resists with improved application and sensitivity have appeared on the market. New organic-based substrate composites, organic encapsulants, via technology, planar and buried passives and technology (low temperature co-fired and multi-chip modules) for advanced packaging and interconnects are being exploited to take advantage of advancements in monolithic technology. This text is directed to acquaint technical managers, engineers and technicians, either with experience, or just enetering the field, with the capabilities and limitations of the materials and processes used for fabricating high frequency circuits. It is essentially introductory in nature. Where possible, equations have been kept simple and to a minimum. Unfortunately, there is little consistency with measurement units and notation. In many of the figures and tables originally published by other authors, I have reproduced their data "as is". As such, the conversion table, Appendix E, may be of some help.

xiv

This text begins with an introduction to hybrid technology and basic high frequency principles. Following these, the major forms of transmission waveguide are discussed, and then current flow and loss considerations. Substrates, thick and thin film deposition, polymers, artwork, masks, photolithography, subtractive, additive and semi-additive methods, electro- and electroless plating and etching are covered. Passive and transmission line components are then treated within the confines of process requirements. With this background established, the text is directed toward the effects of processing and materials on passive and transmission line-based components. Packaging is discussed with emphasis on inductance considerations. Materials and processes for superconductive components are briefly highlighted. 1. R. Brown, Materials and Processes for Microwave Hybrids, International Microelectronics and Packaging Society, ISHM, Reston, VA., (1991)

ACKNOWLEDGEMENTS

xv

This book is really the product of contributions from many people. It is impossible to recognize the advice and suggestions of everyone. However, many colleagues from industry freely gave their time on a variety of topics. A special thank you is extended to the editorial staff at Kluwer for their Jobian patience. I also want to acknowledge the many vendors who sent catalogues and answered innumerable questions, and I want to express my appreciation to all those who gave me permission to use their material and graphics for this text. As such, certain commercial materials, equipment and processes are identified in the text for illustration. Their use neither implies endorsement nor recommendation by the author. Also no implication is implied or expressed that any of the said materials, equipment or processes are the best available or suitable for the purpose. If anyone has been omitted, it was inadvertent. The author assumes sole responsibility for all errors and omissions.

CHAPTER 1

HYBRIDS vs. MIMICS

Microwave electronics is realized by monolithic microwave integrated circuits (MMICs), hybrid microwave integrated circuits (HMICs) or radio frequency integrated circuits (RFICs). The latter two offer economic, design, reliability, reparability and flexibility advantages. Obviously, all three technologies must coexist to satisfy the requirements of a wide variety of applications. Materials and process requirements for RF and microwave hybrids are far more stringent than those for low frequency circuits. As the available microwave spectrum becomes congested at the lower end, there will be a shift into the higher frequencies in the millimeter range. Optoelectronic interconnections will place even higher demands on board properties. Conductor and substrate properties are of particular importance since they substantially affect circuit design and performance. Many of the advantages of solid-state devices can only be achieved if the hybrid circuitry is of comparable size.

2

Toward this end, hybrid technologists have steadily been improving their ability to batch produce sophisticated, smaller, low-cost circuits, while simultaneously improving reliability. Some examples of these circuits are shown in Figure 1.1. The goals of a successful high frequency hybrid circuit design are shown in Table 1-1.

What distinguishes RF/Microwave packages from low frequency or digital packages? Typically RF/Microwave packages require low lead High isolation is required for high sensitivity, noise immunity and wide dynamic range Controlled transmission line impedance is required at RF leads The package exerts greater influence, hence importance, in maintaining overall performance despite reflections and insertion loss The majority of the temperature gradient is in the semiconductor chip Package dimensions are comparable to, or larger than the wavelength. As a result, multiple resonances may arise within the package. As opposed to monolithic devices, where all the circuit elements are planar, hybrid circuits may employ discrete surface (add-on) components. These add-ons may include: Semiconductor IC chips - MMICS, power conditioning chips Discrete semiconductor devices - pin diodes, temperature sensors Passive components - Large inductors, large capacitors, resistors High Q components - Dielectric resonators Non-reciprocal (Ferrite) components - Isolators Transmission line components - Directional couplers, power dividers/combiners Interconnections - Bond wires, connectors

3

In addition multilayer modules may include embedded passives such as resistors, capacitors, inductors and splitters/couplers.

Table 1-2 compares hybrid and monolithic MICs with respect to their major features. It is apparent from Table 1-2 that each of the technologies have both plusses and minuses. In general, HMICs are more versatile, but at the expense of poorer reproducibility and higher

4

parasitics. Table 1-3 summarizes the advantages and disadvantages of HMICs.

The following section is devoted to understanding some of the underlying concepts and terminology used in high frequency circuits

References 1.1. 1.2 1.3

With permission, DAV Technology, Tynsborough, MA With permission, DAV Technology, Tynsborough, MA With permission, DuPont, Wilmington, DE

CHAPTER 2

BASIC CONCEPTS

2.1 Introduction. The electronics industry continues to increase function per unit area, reduce overall packaging size and cost, all with a concomitant improvement in reliability. These factors are spawning a wider effort by the electronics industry to exploit these advances by identifying additional applications. As an example, the demand for bandwidth is increasing. Cellular will operate in the 800 MHz to 6 GHz (“Bluetooth”) range. Broadband access for wireless will operate in the 2 to 30 GHz spectrum. Automotive radar operates in the 70 GHz area. At these frequencies, one can no longer consider a device package attached to a substrate as simply meeting the needs of the designer. What makes design life relatively easy at the lower frequencies, but more difficult at higher frequencies, is the wavelength of the signal relative to interconnect length. Wavelength proportionately decreases with increasing frequency with the result that as interconnect lengths decrease, signal propagation time between circuit parts becomes comparable to the period of the sinusoidal currents. Conductor traces become transmission lines; mechanical design and electrical performance of these circuits become interrelated. It is important for those interested in the application of high frequency circuits to have an understanding of the concepts and manufacturing processes necessary for their production. It is the intent of this text to first distill some basic design information necessary to appreciate the material and processing requirements of high frequency circuits. A variety of fabrication techniques used to realize circuits are then compared and discussed in light of their applicability for microwave circuitry. 2.2 Maxwell's Laws. Maxwell discovered that, by its very nature, light is electromagnetic and shortly developed a mathematical theory of electrostatics and magnetostatics in terms of fields, extending it to include Faraday’s laws of emf. The essence of the classical Maxwellian equations is the theoretical prediction of the existence of electromagnetic waves, and was the starting point for the concept of an electromagnetic spectrum that extends from dc to rays. The electromagnetic wave is a type of time-varying field, combining electric and magnetic components , which carries energy away from its source. Essentially Maxwell’s four laws may be qualitatively described as follows: Law #1 Electric Field (E): Naturally occurring electric charges are monopolar, i.e., they may be either positive or negative, the choice of polarity, a matter of convention. (Coulomb’s Law): As such, the overall electric field pattern, strength and direction are determined by the geometric distribution of the electrical charges producing the field.

6

Law #2 Magnetic Field (H): Conductors carrying current produce magnetic fields. Magnets, as opposed to electric charges are dipolar, and as such, the magnetic field must describe a closed loop Law #3 If the rate of change of an electric field is not constant, e.g., it changes in a sinusoidal pattern, then the generated magnetic fields are also sinusoidal. The magnetic field changes concomitantly as the electric field. Law #4 (Faraday’s Law) A changing magnetic field applied to a system results in a changing electric field in within the system in opposition to the change. Law #4 completes the relationship between electric and magnetic fields. These laws, impact for the purposes of this text, on substrate and conductor design, effects of conductor and dielectric properties, impedance, and the import of high frequency on loss. In 1888 Hertz confirmed Maxwell’s theories of electromagnetic propagation. Indeed, Hertz may be considered the first microwave engineer, and possibly the only one for at least a generation. In 1893 Hertz[2.1] conducted a series of experiments proving beyond doubt the existence of wave motion. Although the propagation of EM waves through metal pipe was soon used by German investigators before the turn of the century, and scientific interest continued in this new field, it was close to 40 years before microwaves were commercially applied. This was primarily due to a lack of high frequency sources and, as is usually the case, need. 2.3 Permittivity and Permeability. The way an electric or magnetic field propagates inside a medium primarily depends on the dielectric and magnetic properties of the individual materials. In dielectric materials where charges are rigidly fixed and cannot move under an applied field, the permittivity, is a quantitative measure of the ease with which the dielectric can be polarized to form dipoles which cancel the applied electric field (voltage), in effect a measure of the dielectric to store energy.

where, in free space (F/m), in air and vacuum, The permittivities of other media are generally expressed as the relative dielectric constant, and equation (3) may be rearranged to:

Similarly, permeability and relative permeability are defined by:

7

where

and:

is typically 1 for non-magnetic materials.

2.4 Free Space Wavelength. The term frequencies, as used here, commonly refers to electromagnetic radiation from about 400 Megahertz (MHz) to 100 gigahertz (GHz). The free space wave length may be readily calculated from the basic relationship between the propagation velocity of light and frequency, Equation (2-5), where:

where: c = velocity of light in free space f = frequency in Hertz and:

In Figure 2-1, the free space wavelength, is plotted as function of frequency, with selected applications annotated at various frequencies. What makes life relatively easy at the lower frequencies, but difficult at microwave frequencies, is the size of the signal wavelength relative to the signal processing circuitry. From Figure 2.1 note there are 2 orders of magnitude in wavelength between 1 and 100 GHz. The free space wavelength at 950 MHz, used by cellular frequencies, for example, is 31.5 cm. As frequency increases, the wavelengths shorten. At 16 GHz is only 1.9 centimeters, (slightly over 0.7 inches), while at 70 Ghz, in the automotive sensor area, the wavelength shrinks even further to 0.3cm, (slightly over 0.1 inches). It will be shown in later chapters how increasing the dielectric constant of the transmission line media further reduces the signal wavelength. The free space wavelength is replaced by the guide wavelength, discussed in Chapter 3 further complicates phase differences between nearby points since its length is shorted than In addition, high frequency effects such as radiation and dielectric loss and inductive and capacitive interactions make high frequency design a rigorous challenge. We shall see later how the realization of MICs can be just as arduous as the design.

8

2.5 Propagation Velocity. Inside transmission line media inductive and capacitive parasitics along the transmission line combine to slow the propagating electromagnetic wave down. If the inductance and capacitance are known, the propagation velocity or phase velocity, may be expressed by:

9

Alternatively, the propagation velocity in the medium may be expressed by:

typically

so that (2-7) reduces to:

2.6 Decibel Scale (dB). Originally used for comparison of sound intensities, the decibel scale is generally used to compare power or voltage levels. By definition, the decibel, abbreviated dB, is:

Thus, if and we end up with 3dB. Similarly, and has a value of -3dB. Table 2-1 lists selected power ratios and their corresponding decibel values.

2.7 Q Measurements. Unfortunately, predicted ohmic losses can not take into account line cross-section, roughness or variations in sheet resistivity due to processing. Q measurements of resonators offer a convenient and simple method of determining losses due to processing, dielectric and other system contributions. A general definition of the Q of a resonant circuit is:

10

A useful expression from Collin[2-2] for determining unloaded Q is:

where:

For thick, high quality films theoretical conductor Q values may be as high as 500. As a rule of thumb many workers in the field prefer to use 60% to 75% of the theoretical unloaded Q to determine actual conductor losses Conductor loss per unit guide length in dB may be derived from the following expression:

Simply stated, Q is inversely proportional to For maximum Q, then, conductors should have as highly conductive as possible.

2.8 Small Signal (S-Parameters). S-parameters are reflection coefficients directly related to VSWR. Impedance and transmission coefficients usually referred to as gain or attenuation. S-parameters describe circuit inputs and outputs in terms of power, thus are a measure of the power transmitted and reflected from a circuit component along a S-parameters also are measured with all circuits terminated in an actual line impedance of the system, S-parameters being vector quantities, contain both magnitude and phase information. and are ratios of the reflected and incident power, identical to the reflection coefficient commonly used on the Smith Chart. As such, for any two port device the input and output parameters of any two port device, as well as its corresponding characteristic impedance, may be extracted from

11

the polar display. A summary of the more commonly encountered Sparameters, usually reported in dB, appears in Table 2-2. References

2.1. 2.2

H. Hertz, "Electric Waves", MacMillan, London (1893) R.E. Collin, Foundation for Microwave Engineering, McGrawHill, New York, (1966) p 314

CHAPTER 3

PLANAR WAVEGUIDES

Three parameters are of utmost importance to the transmission line are: characteristic impedance (Zo), line, or guide wavelength and attenuation constant The following section discusses these parameters and how they are affected by conductor shape, conductivity, field distribution, discontinuities and the nature of the dielectric. 3.1 Impedance. In free space a propagating sinusoidal electric field generates a magnetic field of equal amplitude so their ratios are always the same. While free space has no mechanical resistance to motion, it will impede any electromagnetic wave propagation. The ratio of the electric field strength E to the magnetic field strength H is known as the free space impedance (or wave impedance), In Standard International (SI) units, the electric field E is in volt/meter, and the magnetic field H in ampere/meters. the characteristic impedance of devices and transmission lines, for example, is expressed in ohms. For our purposes, impedance is the measure of total opposition to current flow in an alternating current or ultra high speed dc system which approaches the behavior of an ac system. This opposition is due to the linear resistance, and inductive and capacitive reactances. Impedance mismatches degrade circuit performance through unwanted power reflections within the circuit. Optimum circuit performance is obtained when design impedance values are maintained. Unwanted reflections result, as we have seen, result in diminished element performance.

The electric and magnetic field strengths are respectively related to the permittivity and permeability of the medium. The characteristic impedance of free space, may be expressed as their ratio:

14

However, transmission lines are comprised of metals and operate in or on dielectric media with properties very different than those in free space. An electromagnetic wave may travel unimpeded in free space, inside a substrate this is not the case. The impedance of these lines is dependent on their geometry and properties of the media in which they are located. A schematic of the distributed parameters of a transmission line is shown in Figure. 3-1. Lumped elements represent infinitesimal lengths of the physical transmission line. Inductance (L), Resistance (R), Capacitance (C) and Conductance (G), are expressed per unit length. In a planar transmission line configuration, L and R represent the series inductance and resistance of the metal conductor, while C represents the shunt capacitance to ground from the metal conductor and G represents the dielectric leakage resistance. The impedance for the structure shown in Figure. 3.1 is given by the ratio of the voltage to the current at point y, and time t. If the series resistance of the transmission line, R and dielectric losses are not small, the impedance is expressed as:

If the series resistance of the transmission line, R and dielectric losses, G are very small they may be ignored. Equation 3-2 then reduces to:

where L and C are respectively inductance and capacitance per unit length. A constant ratio of these two parameters maintains a constant characteristic impedance. This basic relationship explains why as the substrate dielectric constant changes, and thus C, the inductance L, of the conductor must also change if the characteristic impedance is to remain constant. In the early 50’s new forms of waveguide were developed that would allow the microwave industry to catch up with the general process of miniaturization taking place in the electronics industry. Heretofore, development of innovative and complex circuits using waveguide, coaxial cable and two-wire transmission systems was severely hindered by the high cost and other constraints of these systems. These new types of transmission lines were planar and employed conventional printed circuit techniques. These milestone developments provided the microwave

15

designer with new degrees of design freedom. The planar, transmission systems upon which the microwave printed-circuit technique is based, can be conceived as a progressive evolution of the coaxial and parallel line transmission systems. Also, monolithic integrated circuits are not well suited for high power applications. This is particularly true when high performance with high design flexibility is required As such, the use of the microwave integrated circuit, MIC, based on the planar waveguide technologies described below, are more cost effective when used in conjunction with active devices. Even with advances in design and yield improvements in active devices, the controversy still continues. The three major planar waveguide transmission configurations are microstrip, coplanar and stripline. In each of these there are a number of variations, but we shall concern ourselves in the next section with the basic structures.

3.2 Microstrip. Microstrip is one of the most widely used transmission media at microwave frequencies. Its open-ended configuration facilitates the integration and mounting of discrete monolithic components for ease in production, assembly, tuning and repair. A basic microstrip circuit, Figure 3.2, consists of a dielectric substrate of thickness h and relative dielectric constant coated on one side with a patterned line of width w and thickness t, and a metal ground plane on the other.

The microstrip design problem is one of finding the proper w/h ratio to satisfy impedance requirements, and the effective dielectric constant all of which are interdependent. From Figure. 3.2, it is apparent some of the electric field is in air, and the remaining fraction (q) in the substrate, As such the effective dielectric

16

constant, is some value in-between. The effective dielectric constant for microstrip is made up of the dielectric constant of air plus the dielectric constant of the substrate in excess of that of air times the filling fraction, q. This expression holds true for all microstrip media to be discussed.

The filling fraction, q, a measure of the amount of field in the substrate is a quantity vital to the determination of Graphical methods were once employed[3.1], (see appendix A), but CAD is now routinely used to determine w, h and Microstrip impedance, incorporating values of w, h and w/h is given by:

17

For a given substrate thickness, h, line width becomes particularly important in determining impedance, as inductance decreases with increasing cross-section, affecting field distribution. For a line, this effect is shown in Figure. 3.3, which plots the ratio of line width (w) to substrate thickness (h) as a function of substrate dielectric constant, Various substrate materials are annotated at appropriate values. A line on fused silica, is twice the line width on comparably thick One obtains a family of useful curves when, for example, line widths on 0.025” substrates with different dielectric constants are plotted against characteristic impedance, Figure 3.4. For a line with the line width is 0.025 inches. For the same line, when

the line broadens to .050 inches. Although more easily processed, space may not permit such wide lines, negating the use of lower dielectric constant materials. Further, a 0.025” substrate with an requires only a 0.0025 inch wide line. Widths of this magnitude demand high substrate surface quality, and special processing may be needed, particularly for some ferrites and titanates characterized by more porous bodies and rougher surfaces. It is readily apparent from Figures. 3.3 and

18

3.4 that for a given characteristic impedance, the w/h ratio decreases with increasing substrate dielectric constant, Accordingly, where minimum substrate area is the dominant factor, the line width w can be reduced by either using thinner substrates or higher dielectric constant substrates. Unfortunately, as we shall see later, other substrate and practical processing considerations may negate either approach. Conversely, where wide lines are permissible or desirable, lower dielectric constant materials provided by polymer based substrates can be used. Guide Wavelength When an electromagnetic field travels through free space, the environment does not interact with the electric or magnetic fields. However, inside other media, interactions occur between either the electric or magnetic field and the atoms within the medium. The wavelength inside a medium is related to the phase velocity by:

3.2.1

Again, for non-magnetic materials,

and 3-6 reduces to:

f = frequency in Hz = effective dielectric constant

Figure 3.6 plots the effective dielectric constant, against impedance, with varying dielectric constant for. This figure illustrates the effect of line widening on with lower dielectric constant substrate showing less percentage change with increasing impedance. The curves for the higher dielectric constants were based on the assumption that the substrates were “pure” dielectrics. When using ferrites and garnets, the effective permeability must be included in the effective dielectric calculation. The effect is most pronounced in high k dielectrics as the fields are more constrained than in dielectrics of lower However, the line width changes with low materials are minimal with changes in impedance, since traces are wide to begin with. Figure 3.6 illustrates how using higher dielectric constant materials also reduces the guide length of a line for a given frequency of operation, achieving additional size reduction. Figure 3.5 shows that as the frequency increases for a circuit on a substrate of given dielectric constant, more of the field, a situation akin to widening the conductor line, penetrates the substrate. As a consequence, the resultant increase in

19

the effective dielectric constant also serves to reduce the guide length. However, since is used, at a given frequency the guide length changes less with increasing

Whenever possible, it is desirable to have complete ground coverage. Reductions in ground coverage reduce line shunt capacitance, and without simultaneously altering line inductance, reduce line impedance. Graphically, this is shown in Figure 3.6 where root capacitance per unit length and impedance of a 0.025 cm wide line on a 0.5mm substrate is plotted against percent ground coverage. The effect of changing the inductance-capacitance ratio on impedance is greatest at lower values, since from Eq. 3.3 the impedance is proportional to the reciprocal of the square root of the capacitance.

20

21

3.3 Coplanar Waveguide. Coplanar waveguide (CPW) configuration is shown in Figure 3.8. Visualize the ground plane in microstrip moving to the top surface and splitting, so the resultant pieces straddle the signal track. In contrast to microstrip, where the field is predominantly in the substrate, in CPW , the field is more evenly distributed between the substrate and air and confined, for the most part, between the slots (s) separating the signal trace (w) and ground planes (b). As a uniplanar technology, CPW possesses many well known advantages over microstrip These include, low dispersion, insensitivity to substrate thickness, easy connection to both shunt and series elements and reduction in the number of via holes. An attractive feature of CPW that is not often considered is the fact that the characteristic impedance is governed by the ground plane separation and signal conductor crosssection. The characteristic impedance will remain constant if the signal track inductance is changed while at the same time appropriately compensating the incremental capacitance by adjusting the gap width. As a result, variable geometry, constant impedance transmission lines can be easily fabricated, reducing discontinuities, and allowing low parasitic interconnections and transitions.

A comparison the field patterns between microstrip and CPW shows that the electric field in CPW is comprised almost entirely of fringing fields. In contrast, microstrip may exhibit a considerable amount of parallel plate capacitor electric field flux, depending on the signal trace width, w. CPW does have a small, but important, amount of parallel plate capacitance in the gap (s) between the signal trace and adjacent ground. This extra capacitance is attributed to the metallization sidewalls on each side of the gap As such, failure to consider the metal thickness

22

(t) when calculating impedance will lead to errors in the design value of Using 0.7 mil thick metallization and a substrate with an of 10.5, Bachert[3.3], Figure 3.9, plotted the calculated impedance of a circuit, with t = 0 against s & w values. The diminished values for Zo are attributed to the lower overall capacitance due to field distribution between the conductors. Nevertheless, at very low a/b ratios, significant impedance errors may arise. Giani and Naldi [3.4] showed that very small h/w + 2s ratios result in sharply higher impedance values than those calculated for Since fields are distributed more or less equally between substrate and air, the effective dielectric constant for circuits with thin conductors and thick substrates can be approximated as:

If the second medium is not air, then the effective dielectric constant can be determined from:

The basic problem in coplanar design is relating impedance to the w/s ratio and the conductor thickness, t. Impedance in coplanar waveguide is primarily a function ofw and s, as defined in Equations 1 and 2. Over the frequency range for which CPW is non-dispersive, the characteristic impedance is given by:

where K’(k) is the complete elliptical integral of the first kind, and K(k) is its complementary function, defined by:

For computation of ratio of the complete elliptical integrals, K/K’, the reader is directed to Hilberg[3.5]

23

For 0< k < 0.7, K/K’ may be very closely approximated by:

For 0.7< k < 1.0, K/K’ may be very closely approximated by:

Using Equations 3-14 to 3-18, Stegens[3-6] plotted impedance as a function of w/w + 2s, Figure 3-10, using as a variable, assuming metal thickness (t) = 0 and the normalized substrate thickness

24

3.4 Stripline. A typical centered stripline configuration is shown in Figure 3-11 Here the buried metal trace is equidistant from the two ground planes. As with microstrip and coplanar waveguide, there are variations to this geometry, but this section will concern itself only with the centered structure. The fields in stripline structures are confined entirely within the dielectric. As such, Characteristic impedance for structures where the strip width, w, is wide enough so fringing fields do not interact, i.e.; w/b greater to or equal to 0.35, is given by:

25

This approximation[3.7] is valid for t/h2.5 and w/h>0.1, yielding Zo values, according to the authors, to be accurate to within 0.5%

26

From Figure 3.12, for a given w/b ratio the appropriate t/h ratio may be determined. Figure 3.12 also demonstrates that as w/h increases, the trace thickness effect on the impedance diminishes.

References 3.1 3.2

3.3 3.4 3.5 3.6 3.7

A. Presser, Microwaves, vol. 53, no. 5, March (1968) D. C. Howe and G. A. Senf, Proc. 1969 Electron. Comp Conf, Washington, DC, April-May 2, (1969) P S. Bachert, "A Coplanar Waveguide Primer" RF Design, vol 52, July (1988) G. Gione and C. Naldi, Analytical Formulas for Coplanar Lines in Hybrid and Monolithic MICs", Electron Lett, vol. 20, no 4, (1984) p 178 W. Hilberg, "From Approximations to Exact Relations for Characteristic Impedances", IEEE Trans. on Microwave Theory and Techn, MTT-17, no 12, (1969) p1087 Personal Comunication, R. E. Stegens, Veritech Microwave, South Plainfield, NJ B. Nauwelaers and A. van Capelle, “Characteristic Impedance of Stripline”, Electron Lett vol. 23, no. 18, August 27, (1987) pp. 930-1

27

3.8

H. Howe, Jr., Stripline Circuit Design, Artech House, Dedham, MA (1974)

CHAPTER 4

CURRENT FLOW AND LOSS CONSIDERATIONS

The current distribution in the various circuits determine the required conductor thickness. From a consideration of the RF magnetic fields tangential to the conductors, we see that most of the fields in the circuits shown in Figure 2.1 are in the dielectric and the currents lie on the conductor periphery. Only up to three skin depths of metal are needed. Because of non-uniform current distribution inductors typically require three times the thickness of ordinary transmission lines. Losses in MICs are generally divided into two parts: (1) that caused by current flow in lossy conductors, the conducting loss and (2) that contributed by the dielectric material through which the fields propagate, the dielectric loss The total loss is defined as:

Interestingly, for lumped elements, the relative contribution of each vary little with the integration figure but are very frequency dependent as will be described later. The total loss, or 1/Q does vary with for distributed circuits. We will direct our attention first to the dielectric and see how material selection and processing can affect circuit performance.

4.1 Dielectric Losses. Conductor losses greatly exceed dielectric losses in most circuits; nonetheless for good performance the dielectric should have the following characteristics: Low dissipation factor Be isotropic Be homogeneous Be constant over frequency and temperature ranges involved Be chemically stable Be dimensionally stable and easily handled For microstrip, in particular, high density alumina meets most of these requirements. Yet there are applications which successfully use, and in some cases, require other materials such as sapphire, beryllia, aluminum nitride, fused silica, clad substrates and selected high dielectrics.

4.1.1 Tan In an alternating electric field the total potential obtainable depends on the ability of the ions and electrons to reorient themselves as the polarity of the electric field changes. In a loss free dielectric, if the voltage changes sinusoidally, the current leads the voltage by 90°. For real materials there invariably is a delay in the

30

polarization, and the current leads the voltage by 90° - d', where d' is defined as the dielectric loss angle, and refers only to losses directly due to the dielectric. The dissipation factor (loss tangent) is defined as A schematic showing the vector diagram of current, voltage and loss factor is shown in Figure 4.1. For illustrative clarity a scalar diagram is presented. For high density aluminum oxide the dielectric loss angle is about 0.01°, and its tangent is

In general the dielectric constant and loss tangent of a given material do not vary much over the microwave range. The effect of dielectric loss, is basically the tan of the substrate media weighted by the effective dielectric constant. Dielectric loss is usually much less than, as will be shown, conductor loss. An approximation for the dielectric loss is given by:

while a more accurate expression, by Hammerstad and Bakkudal[3.1] is:

derived by making the assumption that On alumina with a 12µ” surface, is almost an order of magnitude less than the conductor loss

31

When considering a planar trace on a silicon substrate, the propagation characteristics of the Si-SiO2 base must be considered. The planar conductor is above a double layer substrate consisting of an oxide over bulk silicon. Due to the finite conductivity of the bulk, typically in the order of 0.01 to both conduction and displacement currents exist. As such, substrate resistive losses from these currents are generally significant. To illustrate the effect of substrate losses on the conductor losses, one has only to look at the effect of substituting air for silicon under coplanar traces. Milanovic [4.2] demonstrated how electrically isolating the trace from the lossy silicon resulted in significantly lower attenuation losses. They were able to reduce line attenuation at 40 GHz from 40dB to about 5dB by etching the silicon from under the transmission lines. The 5 dB loss was attributed to resistive loss of the aluminum conductor. In practice, hybrid substrates exhibit far less loss than does silicon, and usually gold or silver conductors are used, which also are even less lossy. Nevertheless, Figure 4.2 graphically illustrates the effect of high substrate electrical losses.

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4.1.2 Anisotropy Anisotropy in the dielectric arises from three major sources: 1. Crystallographic orientation in single crystals such as sapphire. 2. Matrix contributions in polycrystalline materials due to preferred orientation in processing. Ferrites and tape produced ceramics are prime examples. 3. Fillers in organic dielectrics used for clad substrates. An example of this is woven glass mat in clad PTFE substrates. This is important because in structures such as microstrip, anisotropy produces additional fringing capacitance. This largely affects narrow lines and in structures using edge coupling such as filters and couplers. Excessive anisotropy, if not corrected for, can alter circuit performance. In sapphire the anisotropy is well defined, for a-axis and 11.55 for c-axis. By orienting the crystal during slicing either axis may be obtained with no batch-to-batch variation. High density polycrystalline alumina is composed of many grains of sapphire, which if perfectly randomly oriented, would yield a fired body with an of 10.08[4.3]. However, tape process substrates exhibit non-random, or preferred orientation. Values of 10.22 to 10.80 have been reported to 99.96% alumina suggesting preferred orientation with the C-axis perpendicular to the substrate surface. This condition appears to develop during the casting operation and is accentuated during the firing operation. Again, proper control during slurry preparation and processing is important for batch to batch consistency. Filled clad materials show the largest batch-to-batch variation in anisotropy. Also, the type of filler has an effect. Table 4-4 compares various filled and unfilled dielectrics showing how the filler changes other properties as well. Figure 4..3 plots the effect of increased dielectric constant (more filler) on the anisotropy ratio for both woven and random fiber filled PTFE. Increasing filler percentage increases the anisotropy, the random fiber filling exhibiting, as would be expected, lower anisotropy. Szentkuti[4.4] plotted the effects of dielectric anisotropy on the effective dielectric constant on sapphire, Epsilam-10* and isotropic materials of comparable dielectric properties as a function of w/h, Figure 4.4. With narrow lines there was a 3% increase with sapphire over the isotropic materials and with the filled PTFE a 9% increase in There is, however, very little difference with large w/h ratios. This is explained by the fact that the fringing fields are larger for narrow strips, and it is only the horizontal

33

component, of the fringing field that is affected by the dielectric constant in the x direction.

Variations in dielectric constant are minimized to less than 1% in clad substrates. In ceramic materials, however, the higher substrate firing temperatures and differences in fabrication methods account for a wider range of dielectric constants even within the same part. Variations of as much as 1.5% were reported for a single 2” x 2” substrate. Improvements in recent years in ceramic fabrication have reduced the spread. Nevertheless, the user should be aware that variations do occur, and that critical designs based on “one-value,” may be off the target. For LTCC substrates, variation of 3% is not uncommon, particularly among suppliers and sometimes between lot to lot from the same vendor. One may expect a 1% impedance error from the substrate alone.

34

35

4.2. Conductor losses. Conductor losses are caused by the series resistance of the conducting medium. For minimum loss the ideal conductor should have: High conductivity, as near to bulk as possible. Smooth upper and lower surfaces Good adherence to the substrate, and should be: Easily definable Readily processable Readily bondable 4.2.1 Guide Length Losses. As discussed earlier, the guide length is inversely proportional to the frequency and root effective dielectric constant. As such, guide length losses at lower frequencies and dielectric constants are higher than unit loss because of the longer guide wave length. Increasing the frequency and dielectric constant reduces the guide length and the guide losses. For a trace with a unit loss of 0.04 dB/cm, at 2 GHz the guide length for a trace on a substrate with a dielectric constant of 2 is 11.4 cm (4.5 inches), and the total guide loss is 0.456 dB. However, for the same unit loss and substrate dielectric constant, the guide length is 0.841 cm (0.33 inch), reducing the guide loss to 0.034 dB. This is illustrated in Figure 4.6, where losses per guide length are calculated for a microstrip conductor with a unit loss of 0.04 dB/cm (0.1 dB/inch) at various frequencies and dielectric constants. 4.2.2

Attenuation. Conductor losses,

arise from primarily from:

Current flow distribution Conductor resistance Surface roughness Attenuation describes the difference between the power transmitted and the power received. In any signal path, conductor, radiation, dielectric and reflective losses combine to use energy, which saps signal power. The signal is thus attenuated. Losses measured in dB are usually normalized to per unit length. Figure 4.7 compares the attenuation per unit length of several transmission line structures. It is clear that stripline and microstrip exhibit higher losses per unit length. However, with relatively short conductor lengths overall losses are lower. 4.2.3 Return loss. When a signal traverses a conductor geometric or electrical discontinuities part of the signal is reflected. The reflected signal in principle "returns" down its original path and sums to the existing incident wave, where it may distort the wave form and intensity,and is a negative number (-30dB is better than -15dB) However,

36

37

losses are usually expressed as absolute values Return loss has two detrimental parts: a) attenuation, which reduces the magnitude of the incident signal degrading the signal. Return loss, as is insertion loss, is measured in dB and is a negative number (-30dB is better than -15dB) However, losses are usually expressed as absolute values Return loss has two detrimental parts: a) attenuation, which reduces the magnitude of the incident signal (energy), and (b) energy which may distort or cancel the inbound signal to an extent that it may be unrecognizable. This is expressed as VSWR in the next section.

4.2.4

VSWR, Voltage Standing Wave Ratio Two waves, traveling in opposite directions and interfering with each other, form a standing wave. In a transmission line, voltage and current standing waves may be formed. A useful expression for characterizing these standing waves is Standing Wave Ratio (SWR). Since voltage is usually the most important parameter which is measured, voltage standing wave ratio (VSWR) is generally used. In a circuit the voltage of the forward wave combines either constructively or unconstructively with the reflected wave. A detector that can measure

38

rms voltage, will detect a maximum rms value at some locations, a minimum at others and intermediate values at the remaining locations. The ratio of maximum rms voltage to minimum rms voltage is VSWR.

VSWR may be also be defined in terms of the absolute reflection coefficient, by

Power is proportional to rms voltage by:

A component with no reflections has a VSWR of 1. Reflections are manifested as positive values of VSWR greater than 1. An abbreviated comparison of VSWR, return loss and other conversions is shown in Table 4-1. Generally a return loss of -20dB, equivalent to a VSWR of 1.22 is considered acceptable.

39

4.2.5 Skin Depth. The formation of eddy currents is one consequence of Maxwell's 4th Law.:

where: E = Electric field intensity B = Magnetic flux intensity t = metal thickness Magnetic fields penetrate most materials, so when alternating current flows, the changing magnetic field generated by the oscillating current creates an opposing current. This inductive current itself creates an opposing magnetic field to cancel any changes. The resistivity and permeability of the metal determine the degree of opposition. Since most metals have a permeability of one, only the resistivity usually needs consideration. Also as the frequency increases, so does the rate of change of the magnetic field, and correspondingly, the magnitude of opposition. With increasing frequency, the field and thus the current, is concentrated closer to the conductor surface as shown schematically in Figure 4.9. Skin depth is defined as the distance from the metal surface beyond which the current density falls below 1/e (about 37%) of its original magnitude.

40

The thickness of the layer where the current density drops to 1/e of its value at the surface is defined as its skin depth calculated from:

= specific resistivity of the conductor in = permeability of free space, = 1 for non-ferromagnetic materials = operating frequency in hertz The skin depth for various conductors as a function of frequency is shown in Figure 4.9. Three skin depths handle about 98% of the total signal energy. It follows that to eliminate current crowding, the trace thickness be at least three skin depths. The effect of skin depth on the attenuation of sputtered copper films with various ratios is shown in Figure 4.10[4.5]. The two curves represent theoretical curves at 8 GHz for w/h = 0.5 and 2.0. Horton’s experimental results are in line with those of Levy's results with electroplated gold[4.6]. The empirical sputtered copper results are in good agreement with his theoretical plots. The attenuation of the electroplated gold plate is higher than expected for the 2 GHz difference. This data is consistent with the model that electroplated films are not as always as pure or as dense as vapor deposited. Thus one would expect higher losses. Note the decreasing minimum value of t/d value and the lower attenuation with increasing line widths. This suggests that to minimize conductor losses, narrow lines should be avoided. Wider lines of only 3-4 skin depths are preferable since thinner lines enjoy a processing advantage. Figure 4.10 also illustrates that additional line thickness is not only unnecessary but slightly detrimental from a performance point of view as well as impractical from process and cost. As skin depth is proportional to the conductor resistivity, thinner conductors may be used when higher conductivity materials are used since their skin depth is smaller.. The skin depth is also inversely proportional to the frequency so thinner layers may be used at higher frequencies as well. Table 4-2 lists the specific resistivities of various materials from which Figure 4.9, a plot of skin depth as a function of frequency was derived. Nickel exhibits pronounced ferromagnetic properties, and as such its relative permeability, , is greater than one. This accounts for its small skin depth relative to other metals shown in Figure 4.9, even though its resistivity may be higher. It should be noted that the exact value of permeability varies as a function of frequency. As such the location of the line denoting nickel is arbitrary. It becomes readily

41

apparent how conductor conductivity, surfaces and interfaces are critical in determining circuit performance.

Caution must be exercised when calculating skin depth from bulk resistivity values or assume resistivity is independent of process method. Evaporated films and quality electroplated films are nearer bulk than sputtered and higher resistivities are characteristic of older thick screened films. Earlier fired thick film conductors were generally characterized by a continuous array of metal particles through which ran a secondary network of glass[47]. This glass tended to concentrate, as shown in Figure 4.12, near the substrate, often to the extent of forming a continuous layer. In such a heterogeneous system, there is certain to be a resistivity gradient, with the lowest resistivity at the surface.

42

43

Developments in reactive (“fritless”) inks have produced more homogeneous and lower loss materials than either the frit or mixed bonded conductor inks[4.8]. The skin depth vs frequency of one such thick copper film is shown in Figure 4.9. The actual resistance of the material will vary depending on material, processing and surface. Similarly, electroplated metals can also have wide variations in conductivity, again depending on such factors as both composition and plating bath parameters. Electroplated copper films frequently exhibit columnar grain growth which results in undesirable porosity and surface texture. Values of resistivity as much as twice that of bulk have been measured[4.9]. The adherence of thick films relies on both a mechanical bond and the reaction between ink and substrate which is why lower purity, rougher ceramics are preferred[4.10]. Most thin film conductors on the other hand are deposited onto smooth surfaces, requiring a chemical bond. Low resistivity materials, with the exception of aluminum, do not readily react with the substrate, necessitating a thin glue layer, an additional source of conductor loss. Table 4-3 characterizes the prevalent thin film conductors for conductivity and substrate adhesion.

44

Table 4-4 lists some of the thin film multilayer systems. Many of the "glue" or adhesion , seed and plating layers are summarized. This list is by no means exhaustive but covers many of the more popular schemes. The effects of the the adhesion layer on conductor attenuation is discussed in the next section.

4.2.6 Adhesion layers. Since much of the dc current in microstrip and coplanar waveguide lies on the dielectric side of the conductor, the resistance of not only the conductor, but of the adhesion layer, barrier and in some cases a resistive layer directly affect the attenuation of the transmission line as well. For a two layer system with a Cr adhesion layer and Au conductor, Sobol and Caulton[4.11] showed that as the Cr thickness increased the line loss increased. At 2 GHz the loss increased by 3% as shown in Figure. 4.12, while at 50 GHz they predicted a 60% increase in loss with only half the chromium. Figure 4.13 shows the losses using a tri-layer Ti-Pd-Au system[4.11], where a few hundred angstroms of titanium are used as an adhesion layer and 0.2 - 0.4

45

46

µm of palladium as a angstroms of titanium are used as an adhesion layer and 0.2 - 0.4 µm of barrier layer. Up to X-band, this system will introduce losses as high as 10% more than monometallic gold, and 3050% in the millimeter range. For millimeter wave systems, if miniaturization is unnecessary, high conductivity copper conductors bonded directly to soft substrates may be an attractive alternative.

47

It is important that high resistivity adherence layers be as thin as possible since the relative RF loss is directly related to both layer thickness and frequency. Since the typical adhesion layer is between 10 and 50 nm thick only negligible loss at frequencies well into the millimeter bands is expected. The adherence layer should be carefully chosen to minimize diffusion into the conductor, since excessive diffusion serves to increase the over-all conductor resistance and thus RF loss. A summary curve[4.12], Figure 4.14 illustrates the importance of

48

minimizing the number of conductor layers particularly at higher frequencies. Ramy et al [4.13] investigated the losses of a variety of etched material on several substrates. They first calculated the losses without considering the substrate contribution, and then allowed for substrate effects and compared the measured with the calculated values. Calculated and measured data for films on 99.5% at 10 GHz are shown in Figure 4.15. They found excellent agreement between calculated and measured values. They also found substrate surface smoothness and high conductivity of ground layer were important for high Q conductors. From Figure 4.15 it is apparent that the etched fritless golds closely approach thin film performance. Also, with increasing conductor resistivity, not only does the Q decrease but the substrate has less effect on the overall Q. For this reason, where high Q circuits are not required, thick film conductors on the rougher 96% alumina suffice.

49

4.2.7 Surface Roughness. Surface roughness is major source of conductor loss. Because current penetration into the trace surface is shallow, currents must follow all surface imperfections, effectively lengthening the current path and increasing the actual resistance. This effect was treated by Sobol[4.14] who applied Morgan’s theory[4.15] to microstrip. Sobol’s calculations for copper films are shown for various roughness values in Figure 4.16, where the increase in conductive loss against surface roughness is plotted, with frequency as a parameter. The agreement between the two approaches is excellent as shown by the superposition of empirical results at 4 GHz from Sobol[4.14].

50

A simplified curve-fitted formula for the effect of surface roughness on line attenuation by Hammerstad and Bekkadal[4.17] is :

where:

= = =

r.m.s surface roughness in µ inches skin depth at appropriate frequency uncorrected conductive loss

A plot of Equation 4-9 is shown in Figure 4.17 where the increase in surface resistivity due to roughness is plotted against substrate roughness normalized to skin depth.. The results of Bhasin et al[4.16], who investigated the effects of surface roughness of glass filled teflon on conductor loss, are included in Figure 4.17. Although the copper layer is between 12.5 and 25 µm thick, the roughness of the substrate, and to some extent the granularity of the copper, is reflected in higher losses than typically found with thin films on smother materials. Others who investigated the effect of roughness on increases in conductor resistance include Lending[4.17] and Benson[4.18]. Figure 4.18 summarizes the plotted data from all five investigators. The significant difference between Lending and Benson and the others results from differnces in roughness models

51

52

Screened thick film conductors with conventional particle distribution, shown in Figure 4.18, are characterized by irregular edges and textured surfaces. When deposited onto rough surfaces, such as 96% alumina with its inherent granular surface, typically such films may exhibit even higher losses, approximately 20% higher, than corresponding films on smoother substrates. This shows the conductor surface is strongly influenced by substrate texture in spite of the film thicknesses.

A summary set of curves for and specific substrate thicknesses developed by comparing conductor and dielectric losses up to 20 GHz is shown in Figure 4.19[4.21].

References 4.1.

Cited in T. C. Edwards, “Foundations for Microstrip Circuit Design,” John Wiley N.Y., (1981) p 91

53

4.2. 4.3 4.4. 4.5. 4.6

4.7.

4.8. 4.9. 4.10. 4.11. 4.12.

4.13

4.14. 4.15

4.16 4.17. 4.18. 4.19. 4.20.

M. Milanovic et. al., "Micro-machined Microwave Transmission Lines in CMOS Technology", IEEE Trans. on Microwave. Theory and Techn., vol. 45, #5, (May 1997) pp.630-35 V. Borase, Microwaves and R. F., (February 1983) pp. 83-7 B.T. Szentuki, Electron Lett. 12, (1976) pp. 672-3 R. Horton et. al., Electron Lett, Vol. 7, No. 17, (August 26, 1971).pp. 490-1 A. Levy., Manufacturing Technology for Microwave Integrated Circuits. Final Report May 1984, AFWAL-TR-84-4030 Contract #FO 8635-80-C-0243 Air Force Materials Laboratory, Wright Patterson AFB, OH 45433. J. Savage, Chap 5, “Handbook of Thick Film Technology”,: P J. Holmes and R. G. Loasby editors. Electrochemical Publications Limited, Ayr. Scotland , (1976) P. Sayers, Solid State Technol. (September 1974) pp. 66-9 F. Z. Keister, IEEE Trans. on Microwave Theory and Techniques, MTT-16, #7 (July 1968) pp. 469-75 M. V. Coleman and G. E. Gurnett, Electrocomponent Science and Technol. vol. 5 (1978) pp.55-9 H. Sobol and M. Caulton, "Technology and Design of Hybrid Microwave Integrated Circuits" in Advances in Microwaves vol. 8, Academic Press, NY (1974) pp. 12-64 B. F. Gunshinam et. al., “MIC Technology Short Course” ISHM Technical Monograph Series ISHM Reston, VA (1985). J. P. Ramy et. al., "Experimental and Theoretical Characterization of Thick and Thin Films for Microwave Uses on 99.% Alumina Substrates" Electrocomp Sci. and Technol., vol 10 (1983) pp l57-62 H. Sobol, Proc. IEEE, Vol. 59, No. 8, pp. 1200-11, August 1971. S. P. Morgan, "Effect of Surface Roughness on Eddy Current Losses at Microwave Frequencies" J. Appl. Phys., Vol. 20, No. 4 (April 1949) pp. 352-362 K. B. Bhasin et. al., "Interfacial Roughness in High Frequency Microelectronic Interconnections and Packaging" J. Vac. Sci. and Technol. A3(3) May/June (1985) pp778-9 E. O. Hammerstad et. al., ELAB Report, STF 44A 74169, University of Trondheim, Norway (1975) pp. 98-110 R. D. Lending, New Criteria for Microwave Component Surfaces", Proc. National Electronics Conf. vol. 11 (1955) pp391-401 F. A. Benson, "Waveguide Attenuation and its Correlation with Surface Roughness" Proc. Inst. Elect. Engrs. (London) Part 3, vol. 100 (1953) pp85-90 Cited in R. C. Gupta et. al., Micros trip Lines and Slotlines, Artech House, Dedham, MA (1979) p 70

CHAPTER 5 SUBSTRATES Substrate properties, such as surface finish and the fabrication processes such as metallization and definition determine the accuracy of line width and gap width, and ultimately circuit performance. For optimum performance, microwave substrates should have: 1. Low loss tangent to reduce dielectric loss. A uniform, isotropic dielectric constant to minimize, for 2. example, impedance changes within the circuit. Constancy of dielectric constant within a manufacturing batch 3. and from batch to batch to allow for circuit production without compensation of circuit design. 4. A smooth surface finish to minimize conductor ohmic losses. 5. High thermal conductivity for power circuits. 6. Good thermal expansion compatibility with component and package requirements. 7. High chemical resistance. Moreover, the correct choice of a particular substrate material depends on factors such as: 1. Is the substrate cost justifiable for the application? 2. What technology is to be used? 3. What frequency and temperature ranges are involved? 4. Are the substrates available with sufficient area to realize the circuit design? 5. If necessary, can tight physical dimensions be maintained either initially or through secondary processing? The overwhelming number of substrates used for microwave circuits are “hard”, usually a polycrystalline ceramic, or “semi-rigid”, a metal clad, filled organic. Other substrate types are available and will be briefly mentioned, but first the “hard” substrates will be discussed. Their important characteristics are listed in Table 5-1

5.1

GLASS Uncrystallized glasses are only used for thin film work. They are generally produced by melting crushed glass (cullet) in a tank type furnace and withdrawing the molten glass in sheet form at the other end. Glass is the lowest in cost of all materials normally used for thin film substrates. Glasses, on the other hand, suffer from low mechanical strength and poor thermal conductivity, creating problems in handling and thermalcompression bonding. For microwave applications where superior chemical and electrical properties are necessary, fused silica has emerged virtually the only practical glass substrate. An extraordinary smooth surface, low dielectric constant and low loss tangent make fused silica an ideal candidate for microwave hybrid integrated circuits. Thin film circuits defined on fused silica are

56

characterized by precisely measured fine lines and the almost total absence of yield problems due to surface defects. Thus this substrate finds widespread application for millimeter wave circuits. Fused silica may be produced two ways. In the first, ground Brazilian quartz is first melted by flame fusion. A long, thick disc of silica is made by slowly pulling, while rotating, on the molten pool of This boule, typically 72 inches in diameter and 26 inches thick is later sliced and polished to thickness. This technique is similar to the thermal Flame Fusion process for making silicon.

The water content, which directly affects the adherence of deposited films and other properties, ranges in the 100-200 ppm range. Using an oxyhydrogen flame, may also be made by hydrolyzing silicon tetrachloride;

A disc, 5” to 6” thick and 30” to 50” in diameter is grown. Again, subsequent cutting and polishing creates substrates .010” to .020” thick. However, the water content typically 5 to 10 times higher, ~1000 ppm, than the material made by flame fusion, can adversely affect adherence

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of deposited thin films by reducing the interfacial oxides necessary for good adhesion. 5.2.

Single crystals One single crystal substrate is uniquely desirable for microwave applications. From Table 5-1, note that sapphire has an excellent surface, low tan and high thermal conductivity. Sapphire substrates, are cut and polished from larger boules as are most single crystal materials. Both Verneuil (flame fusion) and Czochralski methods are used to fabricate the larger single crystals. As a single crystal, it suffers from somewhat lower mechanical strength than its polycrystalline counterparts and its properties are anisotropic. Further, as a single crystal material it is inherently expensive with substrate areas greater than are prohibitively expensive. As a result, sapphire’s polycrystalline fine grain analogs are used whenever possible. 5.3

Polycrystaline ceramics

Fabrication. Polycrystalline substrates possess attributes of superior strength, high thermal conductivity, resistance to ion migration, chemical and thermal shock resistance. Before discussing the various substrate materials, let us review their fabrication methods. Usually the raw materials for ceramic substrates are available as purified oxide powders. The oxides are mixed, reground and then mixed with organic compounds functioning as plasticizers, binders, or lubricants. A flow chart indicating three established methods to form ceramic substrates is shown in (Figure 5.1) These techniques, their advantages and limitations, are briefly reviewed in the following paragraphs. 5.3.1

5.3.1.1 Powder Pressing. In powder pressing, dry or slightly dampened powder is packed into an abrasion resistant die under sufficiently high pressure (8,000 to 20,000 psi), to form a dense body. This process allows rapid or automatic production of parts with reasonably controlled tolerances since the shrinkage during the sintering process is lower than other ceramic substrate forming techniques. There are, however, limitations. Pressure variations from uneven filling of long or complex dies lead to defects with inhomogeneous properties. As a consequence, these materials are limited to those applications where lack of uniform properties does not adversely affect circuit performance. Holes cannot be located too close to an outside edge and the process limits the size of the substrate. These substrates are normally more porous, with higher surface roughness and lower mechanical strength than other ceramic forming methods. Nevertheless, substrates made in the manner are commonly used in the thick film industry and have limited

58

applications for thin films. When used for thin films, such substrates are usually mechanically polished, but in doing so, lose their cost advantage over other forming techniques, and still suffer from inhomogeneity problems. As a result, substrates made by powder pressing rarely meet the requirements for high frequency applications

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5.3.1.2 Tape Casting. The most widely used method for producing substrates for thin film hybrid applications is tape casting. The manufacturing process consists of the following steps: Milling. Fine grained, high purity reactive oxides are the prime starting material. Wet grinding of the powder is carried out in ball mills using grinding media. The grinding mill is also charged with deflocculents, binders, plasticizers, lubricants, grain growth inhibitors, and mixtures of organic solvents. The selection of the solvent system and deflocculents is critical to the process. Complete dispersion is essential since any cluster of small particles has the same effect on surface roughness as equivalent large particles. Casting. At present, the doctor blade method of sheet or tape formulation offers the best potential for a smooth surface substrate which is relatively free of surface defects. The slurry is spread onto a suitable carrier film, (polyester or cellulose acetate), which moves at constant speed under a metal knife blade, (frequently called a doctor blade). This blade is positioned a short distance above the film. As the film and slurry move under the blade, a thin sheet of wet ceramic forms, Figure 5.2. The thickness of this sheet is controlled by adjusting the height of the blade over the carrier film.

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5.3.1.3 Roll Compaction. Continuous thin sheets of ceramic material may be formed by roll compaction. In this method a mixture of spray dried powder, binders and plasticizers are fed into a vibrating sieve to break up any agglomerates. The mixture is then fed between two rollers, whose rotational speed and separation determine the thickness of the resultant green ceramic. A polyester carrier tape, similar to that used for tape casting, engages the green ceramic as it leaves the rollers. The edges of the tape are trimmed and the ceramic surface is cleaned before being rolled onto a storage reel. Surfaces with between 2 and 4 µinch finishes, adequate for many microwave microcircuit applications are obtained with this process. Using roll compaction, shown schematically in Figure 4.3, claimed significantly improved surface quality, with a concomitant reduction in polishing costs.

The resultant ceramic sheet, usually referred to as “green tape” or “in the green state”, is air dried to remove the solvents. Typically, a 30 to 50% reduction in green tape thickness occurs during solvent evaporation. Shrinkage in the X-Y direction also occurs. Unfortunately, it not always predictable and uniform. As a result, shrinkage must be measured before holes and other artifacts are punched into the green tape. Frequently, elliptical holes are formed to compensate for tape

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shrinkage during firing. In tape cast ceramics, the substrate surface on one side approaches the characteristics of the carrier material, and the other surface is determined by the doctor blade, the temperature of the air, and the amount of air flow used in the tape casting machine. By choosing an ultrasmooth carrier, a fine surface finish is facilitated, and by careful attention to process, a similar, but not identical, finish is produced on the air side. Controlled raw materials, slurry properties, and properly designed drying or curing conditions help produce substrates with minimum warp and waviness. This process has been successfully applied for the manufacture of substrates for several decades. More recently, production runs of BeO and AlN have appeared. While the casting technology of these two materials is not as mature as that of substrates so formed show improved surface, mechanical and electrical characteristics compared to dry pressed parts. Tape cast BeO substrates are available up to 4.50” x 4.25” and 0.35” thick. AlN is available in substrate sizes up to 3.75 “ x 4.50” and 0.40” thick. 5.3.1.4 Lamination. Conventional uniaxial press lamination shown in Figure 5.4 is accomplished by aligning and stacking parts between two parallel plates. The two plates apply heat and pressure to accomplish fusion. Very uniform heating and pressure are difficult to maintain, especially in very large systems. As a result, the build-up may be slightly distorted after the lamination cycle, occasionaly requiring the scaling of new phototools for proper pattern generation.

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In isostatic pressing, Figure 5.5, parts to be laminated are enclosed in an elastic container which is inserted into a sealable cavity. The air in the cavity is first evacuated and the cavity then filled with a liquid, typically water, which surrounds the elastic container. The pressure and temperature applied to the cavity is uniformly distributed over the surface of the container yielding a relatively uniform compact piece. An important advantage of isostatic pressing is that it permits fabrication of pieces with relatively large length to width ratios. Over the past few years refinements in the isostatic process have led to its widespread adoption for the volume manufacture of LTCC and multilevel, semirigid substrate assemblies.

5.3.1.5 Glazing. During the machining operations to obtain substrate parallelism and flatness, surface grains from polycrystalline ceramics are physically pulled out. The size and frequency of these defects, Figure 5.6, which interrupt otherwise flat, smooth surface depends upon other things, substrate grain size distribution, secondary phase and machining techniques. For thin film capacitors and components requiring fine lines such surface disruptions can significantly reduce yield. An appropriately smooth surface can be restored by glazing the surface. Glazes are glass

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frit in an organic binder. After application by screening or spinning onto the ceramic substrate, the parts are fired in air to typically between 750° and 1000°C. The organic binders burn out at 500°-600°C and as the temperature rises above the flow point of the glass, the particles of glass fuse together forming a continuous glassy layer with a smooth, albeit wavy surface. Figure 5.7 clearly demonstrates the glass-like smoothness attainable with this method. In addition to providing a smooth surface, the glaze must be chemical resistant, have good electrical properties and closely match the expansion of the parent ceramic.

5.3.2 Substrate characteristics. For the bulk of applications where low or moderate power dissipation is required, alumina is the preferred substrate. Currently, alumina represents about 90% of the ceramic substrate sales. For thick films, 96% alumina is generally used. These substrates are generally characterized by as-fired surfaces, typically 20-30 µinches average roughness. Such surfaces are ideal for good adhesion, but as will be seen later, limit the useful operating frequency of the circuits made on them. Note also from Table 5-1 that the dielectric losses, which also affect circuit performance, are as much as 3 times as lossy and slightly mechanically weaker than the higher purity alumina. Unpolished 96% alumina substrates find application where substrate surfaces and uniformity has little, if any, effect on yield or performance. For microwave work, where reproducible dimensions of conductors are important, an improved surface can be furnished by lapping from 10 to 50 µinches and decreasing the camber to between 0.3-

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0.5%. Such surfaces provide improved fine printing, better etch-back characteristics and firing distribution.

For thin film microwave hybrids where the substrate, as with thick film microwave hybrids, actively participates in the microwave circuit, the surface is even more critical. Thinner films are more sensitive to surface irregularities and consequently, most substrates for thin film microwave circuits are lapped and polished to about a l µinch finish. The large number of surface pullouts exhibited by polished 96% alumina requires that a higher density, smaller grain size alumina be used. Polished 99.6% alumina provides the surface amenable for precision photolithography (fine line and spaces), and the reproducible fabrication of circuit components such as couplers, interdigitated capacitors, coils and precision resistors. SEMs of the surfaces of as-fired 96% polished 99.6% and polished 99% BeO, which is discussed in the next paragraph, are compared in Figure 5-8a-c. The reason for the

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improved surface quality of the 99.6% alumina over 96% alumina is due primarily to the reduction in surface defect number and size. Fewer pullouts in the higher density material is attributed its intrinsic smaller average grain size. Even with polishing, pullouts of the larger beryllia grains preclude surfaces with the quality exhibited even by the 96% aluminas. The use of zirconia as a grain growth inhibitor in beryllia has been reported to reduce the average grain size by about 50% and improve electrical and mechanical properties of the body, at a slight expense in thermal conductivity however[5.2]. For power applications, beryllia (BeO) substrates have been extensively used. Its high thermal conductivity, about 8 times that of alumina, and low dielectric loss would appear to make it the ideal substrate. However, beryllia is a large grained material and cannot be machined to nearly as good a surface finish as alumina and generally the substrate surface is more porous, as shown earlier, than alumina. Whereas, such surfaces are good for thick films, they may cause processing problems when dealing with thin films. Unfortunately, the reproducibility on BeO is not as good as on particularly on dry pressed bodies. While the newer tape process offers improvement, the larger grains and porosity are still present. Of greater concern, but very manageable is the potential toxicity of BeO dust[5.3]. While the fired ceramic poses no health hazard, the powder can, in some individuals, cause severe lung disorders. Care must be exercised when handling this material to eliminate or minimize dust formation. At room temperature its thermal conductivity of aluminum nitride is about 75% that of BeO with the high purity version of the nitride approaching BeO at around 150°C. Its lower thermal expansion closely matches silicon making this material attractive for VHSIC packaging. Significant improvements have been made in reducing its dielectric losses, rough surfaces and film adhesion problems. Unprocessed, polished AlN is characterized by a relatively smooth surface with few pull-outs, as shown in Figure 5.9a. Small amounts are yttria are frequently used as sintering aids. The light areas in Figures 5.9a and b are yttria containing phases, exposed by the polishing operation. In boiling water, AlN decomposes forming a porous surface film of hydrated alumina. The surface etching as a result of exposing the surface to boiling DI water for 10 minutes is shown in Figure 5.9b. A1N is also affected by solution Chanchani[5.4] observed decomposition in the range of 4.4 to 5.5, but not at 3.0 and below, nor at 8.2. He also found the rougher surface of the nitride compared to alumina severely affected the definition of 76µm wide, thin resistor films creating opens. NiCr films[5.5] deposited onto AlN adhered well providing the existing surface oxide is continuous. Substantial improvements in surface quality have resulted from advances in the reduction of average body grain size and polishing techniques.

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Rapid oxidation of AlN occurs only above 1000°C. Excessive oxide, (2u), results in poor thick film adhesion by inhibiting chemical reaction between the paste and substrate[4.6]. The expansion of aluminum oxide is almost double that of the nitride, and in rapidly cooled substrates, thermal stresses induce cracks in the oxide causing flaking and poor adhesion in thin films. Polishing of AlN for microwave applications removes the surface oxide so that some sort of postoxidation treatment is necessary prior to deposition for good thin film adhesion. Improved adhesion may be obtained by the use of very thin, oxidized layers of metals typically used as adhesion layers. Part of the processing inconsistency for both thick and thin film circuits on AlN lies in differences in chemical composition of the respective bodies. In contrast to alumina, where there are only slight variations in chemical composition, the sintering aids used by some of the AlN manufacturers contribute to broader differences in substrate composition. Dettmar and Charles[5.7] surveyed the available nitride substrates and discussed their chemical and surface morphology differences. Differences in laser drilling of aluminum nitride were

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attributed to differences in substrate chemical composition[5.8]. The composition of the additive powder affected the amount of free aluminum formed during the high temperature decomposition of the aluminum nitride. The free aluminum can be removed by chemical etching, during which the substrate surface may also be attacked[5.5]. Film adhesion problems arise from the poor chemical resistance and in the case of thick film, by differences in CTE between the fired paste and substrate. Some resistor problems [5.9] arise from the fact that glasses containing lead oxide (PbO) can react with AlN during the firing process:

Ruthenium oxide used as the conductive phase in the resistor paste, may also react with AlN:

Polishing after laser processing will restore the substrate surface. Maintaining a nitrogen ambient during drilling insures the formation of aluminum nitride in the drilled area[5.10]. Theoretically, the thermal conductivity of aluminum nitride is about 340 W/cm-K, however, it is improbable that polycrystalline aluminum nitride will ever reach this value...The maximum thermal conductivity obtained for single crystal aluminum nitride by Harris et al was 275m-K[5.11]. They attributed the lower value to oxygen related lattice parameter changes and oxygen induced defects. The effect of oxygen concentration in both the powder and sintered polycrystalline aluminum nitride on its thermal conductivity is shown in Figure 5.10[5.12, 5.13, 5.14]. The rapid rise in thermal conductivity at oxygen concentrations below 0.85 weight percent is in close agreement with reported lattice changes[5.11]. Norton[5.15] correlated differences in thermal conductivity with the presence of lower thermal conductivity second phases between grains, crystal defects, oxygen impurities and poor sintering behavior. Feil[5.16] showed that differences in the choice of sintering aids and sintering process affected thermal conductivity and thick film adhesion. Conductor pastes show adhesive strengths within an order of magnitude of results on alumina. However, resistive pastes require adjustment in thermal coefficient of expansion for good adhesion. Here the chemical composition of the paste as well has to be compatible with the nitride. Considerable work is also underway to develop compatible thick film conductors and dielectrics since its nitride surface inhibits chemical to bonding. The increasing demand for smaller, lighter weight circuits has spurred renewed interest in high dielectric constant substrate

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materials. In the past, high dielectric losses, low density, poor surfaces and high temperature coefficient of dielectric constant have restricted the commercialization of these materials. Materials such as barium nanotitanate zirconium tin titanate (ZrSn) and barium magnesium tantalate have been produced by dry pressing with nearly bulk density. Polishing these substrates yields surfaces with between 2 and 5 µinch finishes, more than adequate for most microcircuit applications. However, the alkaline earth additives limit the use of process reagents such as mineral acids. Cleaning and pattern etching must be accomplished using precautions not usually practiced with more chemically inert substrates. Care must also be taken when heating many of the titanate based materials, since they are much more prone to thermal shock than, for example, alumina or beryllia. Figure 5-9 is an SEM of a polished zirconium tin titanate surface. It is characterized by a low incidence of pull-outs , but by slightly deeper polishing scratches than found on alumina or beryllia.

Silicon nitride, is a relatively new substrate material that occupies a specialty niche in the microelectronics arena. Its structure and some of its properties are similar to AlM. Its lower thermal conductivity, typically 90 W/m-K is about one-half that of AlN. is stiffer, with higher fracture toughness, which permits the use of thinner material in some cases, reducing thermal resistance, It is used, as of now, in applications which require rapid, high power thermal cycling, or where rapid power surges are encountered, such as radar. As a material, which

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has widely used in anti-friction bearing and spindle applications, it is apparent the material can be polished to a fine finish, although no actual values are available. Silicon Carbide (SiC) with 0.1 to 3.5 wt% BeO as a sintering aid is mentioned here for completeness only. It has the highest thermal conductivity of all substrate materials, slightly higher than Beryllia (270 w/m-K to 250 w/m-K) and its expansion value of 3.7 ppm/°C is even closer to silicon than aluminum nitride. Thus SiC will no doubt find widespread application in silicon power device packaging. However, its very high dielectric loss, compared with for high density alumina precludes its use as a microwave substrate. It would be imprudent not to mention the potential toxicity of silicon carbide whiskers. While the granular powder used for substrates appears safe, whiskers used for reinforcement of ceramics, in the absence of contradictory evidence from more relevant testing, indicate a potential carcinogenic hazard[5.17]. 5.4 Low Temperature Cofired Ceramic (LTCC). In the barely two decades since its introduction, [5.18] Low Temperature Cofired Ceramic (LTCC) has enjoyed considerable interest and explosive growth. Gupta [5.19] listed 26 different LTCC tape systems produced by a variety of manufacturers. As propagation delay is directly proportional to the square root of the dielectric constant, LTCC manufacturers have centered their efforts on developing materials with low dielectric constants. Rapid refinements in ceramic technology, via fabrication, compatible highly conductive inks and lamination have brought LTCC to a maturity level enabling high volume manufacturing. With conventional screen printing, paste transfer and via formation occur simultaneously. LTCC, in contrast, requires separate operations, enabling higher yields. Further, conventional screen printed multilayer circuits mainly involve sequential processing, so that yield and quality are only as good as the layer with the poorest yield or quality. Again, each individual LTCC tape layer can be fully inspected prior to collating and firing. LTCC substrates, as do conventional high temperature ceramics, begin with a dried, blanked and punched tape layer composed of a refractory combined with an organic vehicle as illustrated in Figure 5.1, The LTCC process uses multiple layers of thin ceramic patterned to create conductive, resistive and dielectric features. Interconnecting vias are filled with conductive paste. Prior to firing, the ceramic layer is flexible enough to be punched and formed to precise configurations, allowing for firing shrinkage. The individual layers are inspected, registered and then fused at sufficiently low temperature to slightly melt the organic binder. The laminate is then trimmed and fired into a rigid assembly. With all the conductors and dielectrics co-fired, fewer firing

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steps are employed compared to conventional screened thick film. The sintered product may be post-processed with additional metallization on the top and/or bottom faces. At this point any additional discreet passive and/or solid state components may be appliquéed. LTCC is a multilayer electronic manufacturing technology utilizing glass-ceramic composites. The vast majority of these materials have dielectric constants ranging from 4 to 9[5.20] although some development work has been reported on ceramics with dielectric constants in the 17 to 70 range[5.21]. An excellent review by Jones et. al., [5.22] compared the chemical. structural and mechanical properties of green and fired LTCC from various suppliers. There are two general classes of substrate materials: a composite refractory and glass system, or a system in which the glass crystallizes during high temperature processing. The degree of crystallization of the latter directly impacts the dielectric and mechanical properties of the substrate. This fact may used to tailor substrate properties for specific applications. A typical surface roughness value of about 10 µinches will increase the resistance compared to a smooth surface, of a silver conductor approximately 3% at 2 GHz, while at 10 GHz. the relative resistance increases about 14%. To post-process with thin film, a surface roughness of this magnitude requires a planarizing layer, usually using one of the polymers listed in Table 9-1. After polymer application, vias would be necessary to interconnect the top layers. Photodefinition of the uncured material, or in the case of cured polymer, RIE or laser etching would be employed. A comparison of some LTCC tapes is found in Table 5-2 An LTCC-M technology is described by Kumar et. al., [5.23] the M denoting a metal layer. The LTCC-M process is identical to the traditional LTCC process up to and including the lamination step to obtain the green ceramic. At this step the laminate is pressed onto a metal core specially prepared to promote good adhesion during the firing step This process was developed to overcome three shortcomings in the conventional LTCC process: "1) large shrinkage and distortion during sintering; 2) significantly lower thermal conductivity of compared to alumina ceramics, and 3) low rupture strengths that give rise to breakage during processing, assembly and use."

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Clad Materials. High frequency applications place many of the same demands on clad substrates as high and low temperature ceramics. These include such critical parameters as uniform dielectric thickness, reproducible dielectric constant and low loss tangent within the desired temperature and frequency range. Maximum moisture absorption should also be specified since absorbed moisture will change substrate electrical properties. A parameter, not a concern when dealing with hard substrates, critical to the performance of clad materials is their glass transition temperature. It would be appropriate at this point to briefly review this important property. 5.5

5.5.1 Glass Transition Temperature, Tg. The glass transition temperature is a fundamental thermodynamic phase transition, characteristic of an amorphous polymer, and depends on the both the starting material and its manufacturing process. The addition of fillers to neat resins generally increase moduli, permittivity and anisotropism, but do not impact Tg. Similarly, ionic contamination from the manufacturing process increases the substrate dissipation factor without affecting the transition temperature. Tg is the temperature at which a relatively brittle material with a small free molecular volume transforms into a material with a higher molecular free volume. With this transformation, individual molecules within the material have increased freedom to move. At Tg the specific volume continuously changes with temperature. One important consequence of this is a significant increase in the coefficient of thermal expansion (CTE), shown schematically in Figure 5.11. Tg essentially defines the upper operating and process range of polymer based materials, such as epoxy and cyanate-esters, since the changes experienced by the polymer base are non-reversible. PTFE, on the other hand, is best described as viscoelastic. Variations in electrical properties are related to density changes in the substrate during heating 5.5.2 Material properties. For most communications systems operating at RF and low microwave frequencies, conventional printed wiring boards (PWB) are adequate. The mainstay laminate material for these applications is FR-4. These conventional epoxy based systems have historically been based on a dicyandiamide curing system. This is relatively stable, easy to process and available at reasonable cost. However, the cross-linked network obtained with this system readily degrades at higher temperatures. As such the low Tg of 170°C resulted in poorer thermal performance. For such systems, the PWB, with its relatively poor dielectric properties, acts merely as a mechanical support. Early substrates were often non-uniform and characterized by relatively high dielectric losses, poor dimensional stability and dielectric uniformity. These factors frequently contributed to excessive variations in line

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impedance and poor circuit performance. Since the early 1990s, when these materials were developed, thermal and electrical performance requirements have become more demanding. As circuits move into higher frequencies, the PWB, as do other substrate materials, requires higher performance materials. Newer, emerging laminate technologies are better suited for the demands of higher speed, interconnect density, better CTE matching in the z direction for via reliability and improved thermal performance. Other considerations for clad materials include dimensional stability, Tg, chemical resistance and low thermal conductivity.

Some of the many improvements include tightening the dimensional thickness of dielectric and cladding, providing uniform, low loss dielectrics with variations of dielectric constants of the order of in the range from 3 GHz to millimeter frequencies. The low loss of PTFE at microwave frequencies had led to its widespread use as a core material. Nevertheless, a number of improvements have been made in the epoxy based materials by reformulating the resin systems and curing agents to meet the increased thermal demands made by the use of higher temperature solders.. Changes that are made must be "drop-ins" for existing manufacturing process and equipment. Further the use of thicker boards requires materials with CTE's that more closely match that of copper to prevent via failure. Pure Teflon* and irradiated polyolefins are preferred as dielectrics since they exhibit isotropic dielectric and expansion properties and low loss over a wide frequency range. However, these materials suffer from two major, although not insurmountable, problems. As pure thermoplastics they suffer from cold flow, i.e., change in dimension or

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distortion caused by application of pressure and a higher coefficient of expansion than rigid or semi-rigid materials. Some manufacturers add fillers to these core materials to increase rigidity. Some of these additives are woven glass, glass fibers, fiberglass or ceramics, which affect the isotropic nature of the original material. This is shown in Figure 5.12, which illustrates the effect of adding about 40% filler to pure PTFE on substrate CTE in the Z direction. This is important for the manufacture of vias, where the expansion match is critical. The composite dielectric constant can be theoretically predicted using the relationship shown in equation 5.1

Woven fiberglass laminates begin with either fine, medium or coarse weaves of fiberglass cloth. The cloth, typically 0.002” thick, is impregnated with PTFE through immersion in a stable, aqueous dispersion of PTFE. Silanes are used, either by prior fabric coating or added to the dispersion to provide the necessary moisture resistance. The concentration of Teflon particles in the bath helps determine the dielectric constant of the final laminate. The thin layers are fused together to achieve the desired thickness. A copper layer is then fused to one or both surfaces. Non-woven fiberglass laminates are made in a process similar to paper making. Particles of E-glass (about 5µm in diameter and a 100X aspect ratio) are mixed with PTFE in a slurry. A screen raises a layer of the materials and the liquid is sucked away. The dried and pressed material is layered as with the woven glass process. Even with this process the glass fibers still exhibit considerable alignment. This process yields substrates with higher PTFE content than the woven. PTFE has a lower dielectric constant than the glass, and the result is a lower substrate dielectric constant. During this process, some of the glass fibers align. Consequently, some degree of anisotropy follows. Anisotropy is more important at high microwave frequencies than at the usual RF frequencies since the dielectric and thermal properties of the substrate will vary according to the anisotropy extent Improved forms of laminate combine layers of coated fiberglass interspersed with layers of PTFE to make a stronger and less expensive product. These products have more predictable dielectric constants and lower loss tangent than other laminate types. Mechanical stability is improved, however, at the expense of electrical performance and other properties such as machinability. The dissipation factor can increase by as much as a factor of 10, depending on the type and amount of additive. This is shown in Table 5-3 which

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compares the properties of some of the clad substrates. Table 5-3 also shows that moisture absorption can increase by as much as two orders of magnitude. Extreme care must be exercised when processing these materials to insure the moisture is baked out, particularly for hermetic packages. This is aided by that fact that the temperature limit for the PTFE/glass core increases from 170°C to about 260°C for the filled materials. However, copper etched circuit lines cannot withstand continuous exposure at these temperatures without bonded side oxidation. It is recommended that continuous operation of unprotected copper conductors should be less that 50°C.

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While mechanical stability is improved, there still remains a significant difference in thermal expansion between the plastic and cladding. This is most evident when copper is removed from one side of the substrate, causing warping. The degree of mismatch is maximized for pure, unfilled materials and minimized for woven fabric materials. Anisotropy in thermal expansion between the xy and z axis is caused by fiber and filler alignment in the core material. High Z expansion causes two major problems. First, the effective dielectric thickness increases with temperature, changing the impedance value. Secondly, and perhaps more importantly, the reliability of plated through holes (PTH) is directly related to excessive expansion differences in the Z direction. Mismatch between the copper in the barrel and the core material creates opens and flaking of the conductor. Recently, a ceramic filled PTFE was introduced which more closely matches the expansion in the Z direction to the expansion of copper, 17 ppm/°C. Table 5-4 summarizes properties of some of the dielectrics used for clad substrates.

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5.5.3 Fabrication. Another important consideration is the copper foil cladding. It is available in two basic forms: electrodeposited (ED) and rolled (sometimes called wrought or calendered). Schematically the two process are shown in Figures 5.13 and 5.14. The thickness of copper foil for cladding is designated in terms of ounces of copper per square foot. One ounce copper is equivalent to a thickness of approximately 1.4 mils. Half-ounce copper is equivalent to 0.7 mils thick, and so on. The ED foil is produced in continuous lengths by deposition onto a rotating, non-reactive, stainless steel drum. The drum is partially immersed in an electrolytic copper bath. After plating, the copper foil is continuously peeled from the drum and coiled. On one side the resultant foil replicates the relatively smooth surface of the drum. The other side, where copper growth occurs, typically has a rougher, nodular surface. This effect of having two disparate surfaces is similar to that found in tape cast ceramics, where the carrier surface is also smoother. The degree of roughness of the growing surface depends to a large extent on the foil manufacturers who may either roughen or smooth it to optimize subsequent bonding to the core material. Rolled copper is formed in a process similar to one used for aluminum foil. A raw ingot is hot rolled (calendered) to an intermediate gauge. It is alternately cold rolled and annealed to a thin foil. This process results in a tight grain structure with aligned, elongated copper particles, as opposed to the dendritic structure of the ED copper. Both surfaces are smoother than the ED copper. As a result, the bonding surface must be roughened for good bond adhesion. Rolled copper is more ductile. The manufacturing process limits rolled copper to 1/2 ounce thickness. ED copper is available down to 1/8 ounce. The initial bond strength of ED copper is higher than rolled, but degrades more quickly at about 260°C, to end up about the same. The copper foil is now laminated to the core resin. The copper may be bonded directly onto the pure core materials. In this process, as with the filled, the copper should be oxide free and a passivation layer may be employed. Immersion in dilute or benzotriole has been successful. During lamination, heat and pressure are applied. The core material, being thermoplastic, softens sufficiently to “wet” the roughened copper, providing mechanical anchoring of the copper. This is the simplest of the bonding processes. For reinforced core materials, the method used most is to employ a thin (0.001” - 0.002”) film of pure PTFE between the copper and core as a bonding layer. Alternatively, a thin layer of PFA or unsintered PTFE adhesive can be used as a bonding layer. The pure PTFE film beneath the foil, besides its use for bonding, serves other functions. It provides a smooth surface which enhances etching acuity. It also provides a moisture resistant barrier and protection for the glassy fillers

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against the hot caustics used for producing subsequent copper oxide bonding treatment.

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During lamination, the bonded copper surface is replicated in the melted bonding resin. Photographs of these resin surfaces after etching, Figure 5.16, clearly show the effects of copper processing on the corecopper interface quality.

The roughness of the ED Copper on both the top and substrate increases with increasing deposit thickness due to copper grain growth. Typically the interface roughness at the substrate for 1oz copper can reach 95 µinch (2.4 µm). The substrate surface, on the other hand, of the rolled copper, is fairly constant at about 55 µinches (about 1.4 µm). The high surface roughness will be shown in a later section to add to the RF losses, particularly as the frequency increases. It is generally preferable to use the rolled copper for microwave applications, where low losses demand smoother, more consistent surfaces. The large substrate areas available with soft substrate materials is illustrated by the microwave circuit shown in Figure 5-17 The panel size of this circuit, operating at about 1-2 GHz, is 11 by 5.6 inches. The 0 060” thick substrate is clad with 1 ounce copper on both sides, and the circuitry defined by etch back

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techniques, discussed in Sections 12 and 13. This commercial avionics circuit finds application in a cockpit environment.

A proprietary process developed by Holl Technologies for substrate manufacture and copper lamination results in forming a mirror finish on both. The substrate is formed by filling a proprietary polymer with "nano-particles" of dielectric. Another advantage of this substrate is that it's properties are isotropic. Depending on the filler materials and filler-polymer ratios, the company is able to tailor the dielectric constant from 25 up to 200, the thermal conductivity from 1 to 50 W/mK and CTE from 0 to +20 ppm/°C. Neither the substrate nor copper is roughened to enhance adhesion, and both feature very smooth, mirror surafces. Figure 5.16 graphically illustrates the mirror finish. These very smooth surfaces allow for improved pattern generation as well as lower signal losses and improved circuit performance. Substrate sizes up to 3 in. by 3 in. are presently available.

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5.5.4 Mechanical patterning. Most clad substrates are patterned using the standard lithographic methods of applying photoresist, exposing, and etching. The next section describe alternative techniques for obtaining circuits. Clad substrates may be patterned by selectively machining the copper with a computer driven router. This method is ideally suited not only for prototypes, but low volume applications and for circuit tuning. Boards up to 21.3" x 15" can be processed with larger features, typically 4 mil lines and 4 mil spaces. Once these larger features have been milled, a laser attached to the unit is capable of ablating the copper cladding for higher density features, forming 2.4 mil traces with 1.0 mil gaps. To prevent the organic substrate from carbonizing, requiring an additional step, the ablation process is prematurely stopped, leaving a thin layer of copper. This residual thin layer is removed by conventional etching, with minimal undercutting. SEMs of the routed traces are shown . These techniques enjoy the environmental advantages of using minimal etchant,

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avoiding significant pre-treatment and disposal into the waste stream or atmosphere. The machined copper dust, however, should be vacuumed during the process. Conductor losses using this approach are in the neighborhood of 0.01dB/mil. An SEM of a filter fabricated by the router method is shown in Figure 5.18. For example, Reinhardt et. al., automatically tuned LC circuits operating up to 12.4 GHz, by removing material from the shunt capacitor pads to achieve the desired performance[5.24].

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5.6 Cleaning. Nowhere is the old adage" cleanliness is next to godliness" more appropriate than in the microelectronics industry. Today's electronic components have lower tolerance for impurities. Good adhesion, the handmaiden to reliability, is interwoven with mandatory clean surfaces for metal, dielectric and photoresist deposition Yet, cleaning is often considered a "non-value added" process. Only disaster awaits those who embrace this oversimplification. Higher interconnection densities have created new challenges for manufacturing personnel. The choice of cleaning techniques depends upon the nature of the substrate, the type of contaminant and desired degree of cleanliness. When handling substrates some general rules include: (i) hold in non-critical areas such as edges, with little abrasion as possible; (ii) store in clean containers; (iii) operator handling should be done with tooling or "low lint" or "low extractable" gloves, finger cots and wiping materials. Low extractable means that solutions used in the cleaning should not extract material from the material being used. Vinyl gloves, for example, should never be used with alcohol, as alcohol extracts phthalates from vinyl. Latex products should always be ordered without powder. Even if latex gloves are being worn, if contaminated surfaces are being touched, polyethylene gloves should be worn over the latex and then discarded so as to protect the integrity of critical parts. Residues from manufacturing, polishing, fingerprints, oil and airborne particulate matter are examples of frequently encountered contaminants. Too often cleaning procedures are an amalgam of band-aids. Effective cleaning can only be accomplished after empirical identification of the problems. Once the

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cleaning requirements have been established, a process should be established and rigorously maintained. The following sections briefly reviews some of the basic techniques. The reader is directed to an early, but still appropriate, primer by this author on this subject[5.25].

5.6.1 Wet Processes. Wet etching is probably the most versatile all cleaning processes. It may be very selective, permitting the removal of only certain materials. Equipment varies from beaker size containers to elaborate , computerized tanks. Some substrate materials require special cleaning procedures due to undesirable side reactions with susceptible areas. As aluminum nitride is readily attacked by alkali and even boiling DI water, dilute acidic solutions such as phosphoric acid are usually used. Hot sulfuric acid and potassium dichromate has been used but has a tendency to leave residues. Chromerge®, a solution of chromium trioxide and sulfuric acid available from Fischer Chemicals, has also been successful. Disposal of these chromate based solutions is a problem. Titanates, readily attacked by strong acids should be cleaned in a benign alkaline solution. A cleaning standby for ceramic substrates such as alumina and beryllia is a modification of the "RCA Clean". This procedure, with its many variants, uses the following steps: at a ratio of 4:1 at 60°C Hot DI water rinse : DI water 1 : 1 : 5 at 60°C : DI water 1 : 1 . 5 at 60°C Hot DI rinse This cleaning is followed by a high temperature (900°C) bake. The substrates should be used as quickly as possible. Copper surfaces on clad substrates may be cleaned by mechanical scrubbing and/or slight etching Clad substrates should be rigorously rinsed with hot DI water to remove etching residues, and then scrupulously dried. 5.6.2 Dry Processes. In some cases, dry cleaning is preferable to confine the reaction to surface impurities. For example, a technique originally reported for cleaning silicon wafer surfaces may be applicable to hybrid substrates. In this technique, high purity liquid or gaseous is expanded in a special nozzle to form a high speed jet[5.26]. The jet contains numerous small diameter particles of solid which strike the surface. Upon impingement at the surface, this “snow” is reported to remove even sub-micron adherent particles, hydrocarbon stains such as fingerprints, and silicone greases. It has been this author’s experience that, under certain conditions, particles trapped in substrate surface pores can be removed also. One of the major advantages of this technique is its inertness. It is rare that will react with the substrate or circuitry.

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Another advantage is its “dry” nature, that is, the represents none of the environmental threats and occupational hazards associated with conventional chemical reagents and solvents. It spontaneously sublimes, leaving no disposable waste. Ozone used for cleaning, can be generated at atmospheric pressures and ambient temperatures by short wavelength ultraviolet radiation. The UV is produced by a mercury vapor lamp in a fused silica envelope at 185 and 254 nm. The short wavelength radiation breaks the hydrocarbon bonds and then generates ozone, which react with the fractured hydrocarbon to form volatile CO and Plasma cleaning or plasma treatment can be used as a separate cleaning procedure (ex vacuo cleaning) or performed in the vacuum chamber prior to the deposition process (in situ cleaning). Plasma cleaning is defined as when a partially ionized gas is used to remove loosely held, low-molecular weight material from the surface. Systems range from 2 inch diameter chamber to ten-foot walk in systems. The plasma may vary in charged particle density and particle energies, depending on the method used to form the plasma. Plasma cleaning is a high energy process, and the reader should be aware that the substrate surface may be chemically and/or physically altered. This is particularly true when dealing with polymer based and polymer materials. Here, when a plasma gas interacts with a polymer surface, four primary effects can occur: removal of organic materials, cross-linking, ablation or surface restructuring. Care must be taken in the selection of process parameters to ensure organics are completely removed. It is possible to modify the contamination instead of removing it, creating a surface barrier which will cause poor or no adhesion. The specific chemistry imparted to the new surfaces is determined by a number of plasma operating parameters, such as gas composition, flow rate, energy flux, electrode geometry and exposure time. Martin and Wong[5.27], showed that adhesion of metals deposited on epoxy after plasma cleaning in oxygen exhibited higher peel strengths than the same metals deposited after plasma cleaning in argon. They attributed this to the chemical adhesion mechanism of metal oxidation. It should be noted that their results are specific for the particular cleaning conditions and metal deposition system used. Their results may not necessarily translate directly to any other facility If the reaction product is volatile, it may be desorbed from the surface. This called reactive plasma etching. An example is the removal of hydrocarbon contamination by an oxygen plasma where the reaction products are CO and These gases may be monitored with a mass spectrometer to indicate the state of cleaning and determine the reaction end point. Plasma cleaning can be used to remove a variety of contaminants from substrate surfaces. It can also be used to condition

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vacuum surfaces by removing contaminants such as hydrocarbon pump oils water vapor which are difficult to remove by pumping alone. 5.7 Safety. This section, essentially common sense, is included primarily as a reminder. Many chemicals used for cleaning are hazardous and toxic. Even boiling water, for example, poses a threat. It is always prudent to use safety clothing, glasses, and when practical, safety shields and hoods when handling chemicals. Operators should be familiar with the threshold limit values (TLV), Permissible Exposure Limits (PEL) and Short Term Exposure Limits (STEL) of the chemicals they are in contact with. The Material Safety Data Sheet (MSDS), available from the supplier, should be available at each work station. Besides supplier information, the reader is directed to two, of many good sources [5.28, 5.29]. Of special note is hydrofluoric acid (HF). This is a particularly insidious reagent It causes burns which continue to attack the protein in the skin unless the chemical is neutralized or removed. Peroxide is generally a non-stabilized, commercial 30% solution. Compare this to the 2% stabilized peroxide available at the local drug store. When boiling, this strong oxidizing agent can cause severe burns When not in use, it should be refrigerated to slow its decomposition. Use with of hot sulfuric acid or hot ammonium hydroxide poses a substantial threat. When mixing concentrated acids with water, the rule is to always slowly add the acid to the water since the mixing reaction is very exothermic. Adding the water to acid may cause the water to boil and splatter. If the ozone concentration is low enough you can smell it, (it's the odor after a lightning storm), but in excess of l0ppm/volume it will damage the olfactory nerve inhibiting the sense of smell. Over l0ppm/v ozone becomes toxic. Exposure to UV radiation, particularly short wave, can be harmful to the eyes, and as everyone is aware from suntan ads, can promote skin cancer. Proper eye protection and protective clothing are essential. Pumping pure oxygen from a vacuum system presents its own safety problem. Explosions have occurred when pure oxygen is compressed with hydrocarbon pump oil. Using air as the plasma gas can negate this safety problem, but more often than not, nitrogen is bled into the exhausted oxygen before entering the oil-filled pump The properties of the more popular substrate materials have been discussed. Conductor and dielectric deposition methods will be reviewed to see how the deposited films are affected by various substrate properties. References

5.1.

Tolino, D, Personal Communication, Trans-Tech Corporation, Adamstown, MD

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5.2. 5.3. 5.4. 5.5. 5.6. 5.7. 5.8. 5.9.

55.10. 5.11. 5.12. 5.13. 5.14. 5.15. 5.16. 5.17. 5.18. 5.19. 5.20. 5.21.

Dawe, A. J., et al., Proc. 1990 Intern. Symp Hybrid Microelectron., Chicago Oct. 15-17 (1990) p316 Foley E. and Rees G., Prog. 1980 Intern. Microelectron. Symp, October 20-22, (1980) New York.pp. 99-103, Chanchani, R., IEEE Trans. Comp Hybrids and Manufact. Tech., vol. 11, #4 Dec. (1988) p427 Bostelaar, L. J. et al., Hybrid Circuits, #21, 14 Jan. (1990) Kuromitsu, Y., et al., Proc. 7th European Hybrid Microelectronics Conf., Hamburg, (paper 4.3), (1989) Dettmar, E. S. and Charles, H. K. Jr., "Hybrid Design and Processing Using Aluminum Nitride Substrates" Intern. J. Hybrid Microelectron.,10, #2, 9 (1987) Kurihara, Y., et al., IEEE Trans. Comp Hybrids, and Manuf. Techn., 14,#1, March (1991) p204 C. Kretzschmar et. al., "A New Paste System for A1N", Proc. 2001 Intern. Symp on Microelectron. Oct. 7-9, IMAPS, Baltimore (2001) pp672-5 Lodge, K. J., et al., IEEE Trans. Comp, Hybrids and Manuf. Techn., 13, #4, Dec. (1960) p633 Harris, J. H. et al.,, "On the Nature of Oxygen-Related Defect in Aluminum Nitride" J. Mater. Res., 5, #8, Aug. (1990) p1763 Konsowski, S. G., et al., "Evaluation of Advanced Ceramics for High Power and Microwave Circuitry-Part II" Intern. J. Hybrid qMicroelectron., 10, #3, (1987) p13 Anzai K., et al, "Development of High Thermal Conductivity Aluminum Nitride Substrate Material by Pressureless Sintering" Proc. 1st IEEE CHMT SYMP,CHMT ., Tokyo (1984) Kuramoto, N., et al., "Translucent A1N Substrate" Proc. 36th Electron. Comp Conf., Seattle, WA May 5-7 (1986) p424 Norton, M. G., "Characterization of Aluminum Nitride Ceramic Substrates" Hybrid Circuits, 18, #20, Sept. (1989) Feil, M., Hybrid Circuits, 29, #18, Jan. (1989) Birchall, J. D., et al., J. Materials Sci. Lett., 1, 350 (1988) W.A. Vitriol and J. L. Steinberg, “Development of a Low Temperature Cofired Ceramic Technology”, Intern. J. Microelectron. and Electron. Pkg., (1983) pp593-8 T.K. Gupta, "In Search of Low Dielectric Constant Materials for Electronic Packages” Intern. J. Microelectron. and Electron. Pkg., vol. 17, #1, (1994) pp80-7 A. J. Piloto et. al., "Low Loss, Low Temperature Cofired Ceramic for Microwave Multichip Modules" Proc. 1994 Intern. Symp on Microelectron., ISHM, Boston MA, Nov. 15-17 (1994) pp318-23 G. Kniajer et. al., "Low Loss, Low Temperature Cofired Ceramics with Medium Dielectric Constants", Intern. J. Microcircuits and Electron. Pkg, vol. 20, #3, (1997) pp246-53.

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5.22. 5.23. 5.24. 5.25. 5.26. 5.27. 5.28. 5.29.

W. Kinzy Jones, et al., "Chemical, Structural and Mechanical Properies of LTCC Tapes" Intern. J. Microelectrn. and Electron. Pkg., vol. 23, #4, (2000) pp469-73 A. H. Kumar et. al., "Versatile, Low Cost, Multilayer Ceramic Board on Metal Core" Advancing Microelectronics, July/Aug (1995)pp30-37 Reinhardt, J. E., et. al., "Automatic Process Cuts Filter Tuning From Hours to Minutes", Microwaves and RF, June (2001) ppl03-4 Brown, R., in chapter 6, “Handbook of Thin Film Technology”, edited by Leon I. Maissel and Reinhard Glang, McGraw-Hill, New York (1970) Sherman, R. and Whitlock, W., J. Vac. Sci. Techn., B. 8 (3), May/June (1990) p563 L. J. Martin and C. P. Wong, "Chemical and Mechanical Adhesion Mechanisms of Sputter Deposited Metal on Epoxy Dielectric for HDI PCBs" PC Fab, Jan. (2002) pp18-34 "Sax's Dangerous Properties of Industrial Materials" 10th Edition, R. J. Lewis and N. Irving, eds., John Wiley and Sons, (1999) "CRC Handbook of Laboratory Safety", 5th Edition, A. K. Furr ed., CRC Press (2000)

CHAPTER 6

THICK FILM

6.1. Screen Printing. Screen printing, as a graphic art, has been practiced for almost two millennia. Adaptation of this technique has been a staple for forming thick film patterns for electronic applications for almost 70 years. Nevertheless, in just the past five decades screen printing has been transformed from, essentially a graphic art, to one capable of handling high density interconnects on a variety of substrate materials. The basis of screen printing lies in depositing a viscous paste through a mesh reinforced stencil. Because the pastes will, by themselves, not flow, they must be forced through the openings in the mesh by pressure, and onto the substrate surface. While the concept appears quite simple, over 50 printing variables have been identified[6.1] grouped into 5 specific categories: Ink Printer Squeegee Post print treatment Substrate In practice, a squeegee is traversed across the surface of the screen completely filling each aperture. The action of the squeegee on the screen also serves to bring the screen in close contact with the substrate, permitting the paste to wet the substrate surface. This contact permits accurate thickness control of the ink. Finally when the squeegee has passed over the screen, the mesh rapidly pulls away, leaving the ink behind, wetting the substrate in the form of small globules, which then coalesce to form a continuous pattern. This sequence is shown in Figure. 6.1 a-c[6.2]. All thick film inks have certain general characteristics in common: They are viscous paste like materials with a rheology suitable for screen printing. They are composed of two distinct phases; a functional phase that imparts the electrical properties to the finished film and a vehicle phase used to impart proper rheology. They are very processing sensitive and frequently designed for specific substrates. Thus, an ink suitable for might not be appropriate for Be0. Screen printed inks experience a wide range of shear during application to a substrate. In rheological terms, the ideal ink should be both shear thinning and thixotropic with a yield point. More simply,

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after being forced through the screen, the ink structure should be fast, but not instantaneous. For minimum ohmic loss it has been stated that the conductor should exhibit near bulk conductivity, be rectangular in cross-section and have smooth surfaces. Screened thick film conductors tend to have: Rougher, sintered surfaces, 5-10µ” Rounded edges from ink slump Mesh impressions on the edges

Shrinking circuits and interconnect density require a higher degree of resolution. To meet this criteria, the rheological properties of the ink must be tightly controlled[6.3]. Unfortunately, the advances in ink rheology have not always produced sufficient printing improvements to make a real impact in a production environment. Except for some isolated cases, 3 mil lines and spaces are still the best that can be routinely achieved in production with printing using conventional screens. By optimizing and combining methods, Bacher[6.4] was able to resolve 2 and 3 mil lines and spaces. He concluded: 1. Screens with 400 mesh, 19 µm (0.75 mil) wires and 10 µm (0.4 mil) emulsion can produce high resolution prints of 50 µm and 754 µm (2 and 3 mils) lines and spaces. The screens have a large open area, allow high shear rates to be used and deposit a low wet

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2.

3.

print thickness 28 cm long (11 in.) with 50 and 75 µm (2 and 3 mils) wide lines. To obtain high resolution, emulsion gasketing must occur before the hydraulic pressure applied to the composition by the squeegee is allowed to build up Emulsion thickness should be sufficient to provide proper gasketing, yet be limited to 8 to 15 µm (0.3 to 0.6 mils) to provide thin wet prints for minimal paste spreading. Bringing the point of maximum hydraulic pressure closer to the squeegee tip improves line and via resolution. This can be accomplished by using an appropriate pseudoplastic composition, printing at high speed with a high mesh screen and limiting the amount of composition in front of the squeegee tip

McCormick and Ruwe[6.5] claimed that using screens with greater than 40% open area, and using a 0.3-0.5 mil thick direct/indirect emulsion, they were routinely able to generate 2 mil lines and spaces. They tried a number of gold pastes and found that thinning the pastes was necessary to prevent clogging the screens.

Gaglani[6.6] also investigated various techniques to obtain thick film fine line geometries. He obtained 3 mil lines and spaces with gold ink. He tried several inks and he also concluded that a 400 mesh screen, 45° angle and 1 mil wire will produce fine lines. He also noted that in addition to the ink rheology, the mono-sphericity of the gold particles

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plays a major role in fine line printing. A number of thick film suppliers have developed mono-spherical inks for fine line printing. The resolution of fine lines is limited, however, not by the ink or screening parameters, but by the very nature of the screen itself. The problem is graphically demonstrated in Figure 6.2, a reproduction of a 400 mesh screen adapted from Stalnecker[6.7]. Here the same pattern is developed on four different mesh orientations. Note that at 90° mesh orientation, perfect alignment of the emulsion to the mesh is required for optimum line acuity in the vertical and horizontal directions. Very narrow spaces between and mesh will not develop out, restricting the deliverable pattern dimensions to certain multiples. The 45° mesh orientation is commonly used to eliminate the alignment requirement, being more forgiving of partial openings. There is a tendency to develop a saw tooth edge on the vertical and horizontal directions. Lines at 45° must now be parallel, otherwise a wash out problem arises and patterns such as those shown in Figure 6.3 occur. For fine line fabrication, then, secondary processing is required.

Improvements are continually being implemented to obtain better screen life and print quality. One of the more significant ones within the past decade has been the introduction of new wire alloys. The wire is still stainless steel, but the carbon composition has been increased

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along with retaining additional nitrogen within the metal lattice while reducing the nickel content at the same time. With higher tensile strength, smaller diameter wire can be fabricated with improved strength allowing increases in screen opening from 40% to near 60%. This enlarged opening, shown in Figure 6.4, permits more material passage through the screen and better line definition. Another innovation being used today to enhance fine line screening is calendering. About 20 years ago, the wire cloth weaving industry adopted the calendering process shown in Figure 6.5 as a means of creating a more uniform mesh thickness. The process relies on passing the wire cloth between two steel rollers to reduce its over-all thickness. With advances in mesh manufacturing, the uniformity of present day product far exceeds earlier product which required the calendering process. Today calendering is used to only to thin the mesh, further enhancing print resolution

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6.2 Metal foil screens. This technique uses etched foil masks, which do not possess the flexibility of a fabric screen. In this system, the mask remains in contact with the substrate during the printing operation and then is mechanically separated at the end of the squeegee stroke. Fine lines down to 50 µm (2 mils) are possible. Because the screens lack resiliency they are very susceptible to accidental damage. One improvement in metal foil screen technology was the epoxy mounting of the foil onto a stainless steel screen. This reduced the incidence of accidental damage, but the poor stability of the epoxies used resulted in low life expectancy[6.8]. The elimination of the epoxy greatly enhanced the screen life. In this technology, nickel foil is formed in a process very similar to that used to make the ED copper for the clad substrates. This foil is then clamped to a stainless steel mesh, and fused to the mesh by additional nickel plating. The nickel foil is then patterned and etched. SEMs of a conventional and metal foil screen are shown in Figures. 6.6 A and B. The metal foil is attached only the tops of the overlapping weave, while the emulsion penetrates into the stainless mesh. The emulsion is textured and shows surface undulations due to the stainless weave. Nevertheless, the defined edge is reasonably sharp The gasket surface of the nickel foil is flat and the pattern edges

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sharp The “bald” spots are where the nickel plating has been etched during the patterning exposing the stainless wire. Note the thickening of the wire diameter by the nickel plate. Control during this plating cycle is extremely important to maintain uniform hole-to-wire ratio over the entire pattern. This poses a particular problem for bends in conductors and for coils[6.9] Nevertheless, these screens do have some advantages such as: long life, robustness and immunity to most solvents. However, they do require more processing steps and are not as facile as conventional screens. As such they are more costly. For short runs, which are typical of most microwave hybrid circuits, they may not be cost effective. Alternate methods of defining narrow lines and spaces include laser and abrasive trimming. Rickard[6.10], for example, used laser trimming and etching to fabricate parallel line coupled filters. He found no differences in performance. The gap width tolerance was ±10%. He did find he could make 5-6 dB proximity couplers by this technique at 16 GHz, whereas with screened only patterns he could realize only 8 dB or higher performance. In the 4-6 GHz band, attempts were made to laser and abrasive trim the edges of printed microstrip lines[6.11]. The author concluded that more work is necessary before these become practical.

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Up to the mid-80s the base technology for the manufacture of thick film inks employed finely divided metal particles dispersed in a glass matrix. To produce finer lines, Gaglini attempted pattern etching blanket screened gold films[6.6] He was able to fabricate 2.5 mil lines, but the etch rate varied among the inks used. He attributed this to the various amounts of glass and non-gold constituents in the ink. One may infer that longer etch rates result in patterns with more ragged edges. Problems with glass at the interfaces of multi-layer printings were discussed by Anderson and Olesen[6.12]. They found that their gold etch did not penetrate the glass layers concentrated in the interface between successively printed layers. As a result of these investigation, other investigators looked at changing the particle shape and distribution and glass composition. In many cases they were able to improve on earlier works. Hsu[6.13] reported on etching a gold film made from a specially prepared gold powder containing both sphericals and flakes. He claimed better density, surface characteristics and conductivity. Etching a single layer, 10 µm thick, Hsu reported 2 mil lines and 1 mil spaces. Recent advances in powder metallurgy and developments in ink technology has led to the development of improved, finely dispersed and uniformly spherical powders. Figure 6.7 clearly illustrates the advances made in the powder metallurgy over a relatively short time period.

Early work by Kani and Endo[6.14] demonstrated production applications for etched silver thick films. They showed that thick film conductors could be routinely etched to 1 and 2 mil widths. They pattern

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printed their substrates with either gold or silver, using uniformly dispersed spherical powders. After drying and roller coating their substrates with resist, exposing and developing, they spray etched to define their narrow lines. An SEM of a 0.001” finger on a thermal print is shown in Figure. 6.8. Thickness is approximately 3-4 µm on 96% The film surface is slightly rough as one would expect from films on 96% Improved edge definition and surface smoothness would have been realized on 99.5% Eason[6.15] carefully evaluated etched silver films on 99.6% and 96% alumina substrates and showed that microwave losses at 18 GHz on 99.6% were less than 0.01 dB/cm, slightly less than for comparably etched gold thin films. Some of his results are shown in Figure 6.10. He also pointed out that the substrate surface roughness plays a critical role in determining conductor attenuation, as etched films on 96% showed higher losses.

6.3 Lithographically defined thick film. The advances in powder metallurgy described earlier, not only improve the quality of screened films, but allow for secondary processing, using photolithography, a

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technology once restricted to thin film circuits. Two techniques, both subtractive have emerged as major manufacturing candidates. They are described in the next two sections. 6.3.1. Photoengraveable. In this process, Figure 6.9, the substrate is either blanket coated, or selectively screened wider than the target dimensions The paste is dried and fired. A positive acting photoresist is applied to the fired pattern which is then photolithographically defined. Either positive or negative acting resists (cf Chapter 11) may be used, although positive acting resists are generally preferred. 25 to 30 µm lines are spaces are routinely obtainable by wet etching Figure 6.10. Losses have been measured as low as 0.03dB/cm at 60 GHz[6.16]. This technique is ideal for top layer metallization. It can not be used for cofired LTCC modules, where all the substrates, conductors and dielectrics are simultaneously fired. As photosensitve materials are sensitive to UV light, all processes should be carried out in a "yellow" or safe light room. Short exposures to normal or low-level room lighting will not affect the pre-exposed films. However, exposure to high intensity light, such as sunlight, will expose the photoresist, and prevent proper exposure.

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6.3.2. Photoimagable thick film. A process, Figure 6.11, well adapted for multi-level substrates is the use of photoimagable thick film. Metal particles, primarily silver and gold, are incorporated with a negative acting photosensitive resin, creating a photoprintable paste. The dried, screened film is patterned using conventional ultraviolet(UV) photolithography[6.17]. The screened area, when exposed to ultraviolet light polymerizes and is retained when the unexposed material is removed by aqueous washing. As such this is a subtractive technique. The same exposure precautions used for the photoengraveable paste above should be exercized for this process.

Standard thick film firng processes complete the fabrication cycle. For multilayer ceramics, vias may be incorporated. The process flow is shown schematically in Figure 6.11. On 96% line resolution of 2 mil (50µ) are attainable, with improved edge definition over screened patterns. Better resolution is expected on 99.6% bodies, providing adhesion is not a problem. Figure 6.12 shows the line quality. The conductor is characterized by a smooth top surface and good edge acuity, both necessary for low high-frequency losses. In summary, etching, either of blanket or patterned screened films, appears to be the most promising way to substantially increase thick film resolution. The lower cost of the printer compared to vapor deposition equipment is offset by the large number of variables in the printing process and inks. Tredinnick et. al.,[6.18] compared the

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advantages and disadvantages of the three major methods of thick film deposition. They concluded that the "cost per length of interconnect" is greatest for direct print and least for photoengravable films, while the unit area cost is inverse. They also concluded the photoengravable technique provided the highest quality features, but for less precise, 2 mil lines, photoimagable conductors provide an "excellent solution". Again, photoengravable conductors are limited to surface applications.

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6.4

Additive techniques

6.4.1 Metal-Organics. Metal-Organics provide “thin film” capability without the need for vacuum systems. In essence an organic-bound metal (ligand) in liquid form is applied to a substrate by screening, brushing, spinning, spraying or dipping. Base metals, also in ligand form, are frequently added as fluxes or glass formers and serve the same purposes as in conventional thick film technology, particularly in noble metal applications. In addition to coating the surface, the low viscosity liquid penetrates and coats any via openings. After drying to remove the solvent the organic portion is vaporized by heating, leaving a very thin, layer (~ 0.1-0.2µm) on the substrate. The substrate material determines the upper temperature at which the film is processed. Soft, organic based substrates are typically processed up to about 400°C, while 800°C is not uncommon for materials such as alumina. For conductors, photoresist is first applied to the metal coated substrate. Openings are then defined exposing the base metal and permitting electroplating of thick conductors. The electroplating solution must be compatible with the fired film as unwanted reactions may occur between the electrolyte and any base metal additive. After electroplating the unwanted seed metal is removed leaving thick conductors in the desired pattern. The number of suitable organic compounds is almost limitless, and ligand formulations are usually proprietary. A schematic of the basic spin-on spray pyrolysis system is shown in Figure 6.13. The ligand is deposited onto a substrate

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held down by vacuum on a spinning, heated substrate. The deposit is sufficiently dry after deposition to allow for handling. Copper and gold pyrolysis processes have been have been developed as alternatives to both screening and vapor deposition. Copper has the advantage of higher conductivity but it must be plated further with nickel and gold for bonding. An additive method reported by ICI has been applied to various substrates including polished, high density alumina[6.18]. After applying the seed layer the top surface is pattern plated with copper along with through holes and ground planes. These copper traces are then overplated for the appropriate bonding process. The seed layer between the patterns is then removed. Resolution of 2 mil lines and spaces is routine on 96% alumina. Better resolution is expected on polished, high density Air-fired resistors may be incorporated on 96% alumina substrates. For lower frequencies, requiring terminations, this extends the applications of this technology. The resistor is applied first, a technique similar to thin film resistor technology, and as such, better control can be exercised over dimension and value. An SEM of a plated line on 96% is shown in Figure. 6.14. As one would expect from a plated line, the cross-section is rectangular. The smooth uninterrupted, walls are typical of fine grained, monometallic systems. Such conductors are expected to have the low losses necessary for good microwave performance.

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6.4.2 Direct Write. Additive direct writing is an emerging technology which is expected to have a major impact on the generation of thick film circuits. A typical system has three key components: Internal pattern generator to interpret designs through a link to an external CAD system. Some systems can be taught by digitizing. Precision positive displacement delivery system. Precision X-Y table, or movable dispensing head. An intriguing feature of one system is a contour following and sensing system that permits fabrication of multilevel circuits. Figure 6.12 is a functional block diagram of a direct writing system. A CAD system work station interfaces with the writing system controller. This controller synchronously drives the ink delivery system and substrate table. The advantages offered by the direct writing systems include: Elimination of artwork, screens and associated hardware Elimination of screen inventory and storage Rapid prototype turn around Rapid design change capability Efficient small volume production Disadvantages include: Only one substrate at a time can be processed - slower than printing

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Highly trained personnel are required.

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A gold microwave circuit made by direct writing is shown in Figure. 6.16. Note the absence of screen marks, providing a smoother surface and edge. The ability to make bends and curves without concern about screen alignment makes this technology very attractive. 6.4.3 Direct Bonded Copper (DBCu). Where high power and thermal management issues are a major concern and where wide lines, in the 15 mil wide range are tolerable, DBCu may be used. In the conventional process, oxygen-free, high conductivity copper (OFHC), electronic, tough pitch (ETP) or silver bearing copper foil, nominally 5 to 20 mils, direct is used. The foil is carefully oxidized and then placed against an oxide ceramic, where it is heated to about 1065°C, about 50°C below the melting point of copper, in a nitrogen furnace. At this temperature a Cu-O eutectic is formed, and the copper bonds to the cermic. The Cirqon Corporation has adapted this concept to form semiadditive, fine line, thick copper circuits. The ceramic substrate is first plated with a thin 25 µinch layer of electroless copper. Next the coated substrate is coated with photoresist, and patterned using conventional photolithography. The coated substrate is then electoplated with copper until the plated thickness reaches 3 to 5 mils. The remaining photoresist is the stripped, exposing the underlying electroless copper. The substrate is given a light etch to remove only the unwanted electroless copper. Fonally the board is fired ina controlled atmosphere over 1000°C to create the bond between the copper and substrate. Figure 6.17 shows the two mil line and space capability of this process

110

We have discussed the importance of accurately defining high quality conductors on low loss substrates. Some of the thin film deposition and definition techniques will be reviewed to see how they relate to fabricating microwave integrated circuits.

References 6.1 6.2

6.3. 6.4. 6.5

6.6. 6.7. 6.8. 6.9. 6.10. 6.11.

6.12. 6.13.

C. Brown, Circuits Manufacturing, 72-4, Oct. (1984) J. Savage Chap 3, “Handbook of Thick Film Technology,” P J. Holmes and R. G. Loasby eds., Electrochemical Publications Ltd., Ayr, Scotland (1976). H Baudry and F. Franconville, "Rheology and Printing of High Definition Thick-Film Inks", Intern. J. Hybrid Microelectr., 6, #2, December (1983) pp. 15-23 R. Bacher, "High Resolution Thick Film Printing" Proc. 1986 Intern. Symp on Microelectr., October 6-8, Atlanta (1986) pp576-81 P. McCormick and V. Ruwe, "Fine Line Thick Film Hybrids for High Reliability Applications" Intern. J. Hybrid Microelectr., 4, #2, Oct. 1981) pp428 -9 J. A. Gaglani, "Obtaining Fine-Line Geometries in Today's Hybrids" Proc. 1986 Intern. Symp on Microelectr., Oct. 6-8, Atlanta (1986) pp819-25 S. G. Stalnecker Jr., "Stencil Screens for Fine Line Printing" Electrocomp S. and Technol., 7, (1980) p.47 R. A. Vogel, "Fine Line Printing for Consumer Electronics" Solid State Technology, vol. 51, May (1972) p.51-4 Funk, W., and Schilz, W., "Thick Film Technique for Hybrid Integrated Microwave Circuits", Radio and Electronic Engineering 44, #9, Sept. (197) pp 129-38 Rickard, D.C., "Thick Film MIC Components in the Range 10-20 GHz" 6th European Microwave Conf., Sept. 14-17, Rome, Italy, (1976) pp687-91 Gondek, J., "The Possibilities of Producing Thick Film Microwave Stripline Resonators with DuPont/USA/Compositions Operating Within A Band of 4-8GHz /Design and Manufacture/" Proc. 29th Electron. Comp Conf., May 14-16, Cherry Hill, NJ, (1979) pp407-20. Anderson, E. and Olesen, S. T., "An Evaluation of Materials and Processes for Thick Film Striplines at Microwave Frequencies," Danish Research Centre for Applied Electronics February (1972) Hsu, K. F., "Thin Print Etchable Gold - An Alternative to Thick and Thin Film" Proc. 1986 Intern. Symp on Microelectr., 149 Oct. 6-8, Atlanta (1986) pp149-53

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6.14.

6.15 6.16 6.17

6.18. 6.19.

Kani, Y. and Endo, H., "Thick-Film Photoetching Technology : Materials, Process, Applications" Proc. 1988 Intern. Symp on Microelectr.,Oct. 17 19, Seattle (1988) pp 269-73 Easson, R. M., "Thick-Film Technology for Microwave Integrated Circuits" 7th European Microwave Conf., May (1989) M. P. O'Neill and P.Barnwell, "High Density MCM-C Utilizing Tape Dielectric and Photopatterning Processes" Proc. 1988 IMAPS MCM Conf., Denver, April (1998) M. Ehlert and P. Barnwell, "High Density LTCC Technology, Using Photo Patterned Conductors" Proc. 2001 Intern. Confp on High Density Intercon. and Systems Pkg., Apr. 17-20, Santa Clara (2001) pp 358-63 M. Tredinnick et. al.,."Thick Film Line Patterning - A Definitive Discussion of the Alternatives" 2001 International Symp on Microelectron., IMAPS, Baltimore Oct. 7-9 (2001) pp676-81 D. B. James and T. W Dekleva, "Novel High Density Copper-onCeramic Interconnects" 1988 8th Annual IEPS Conference, Dallas, TX Nov. (1988)

CHAPTER 7 THIN FILM 7.1 Physical vapor deposition. Physical vapor deposition (PVD) processes are atomistic deposition processes in which material from a source, is transported in vapor form through a vacuum or low-pressure gaseous environment to the substrate. Here condensation takes place and subsequent film growth occurs. PVD processes are used to deposit unalloyed or compound materials. Compound materials may be deposited by either co-deposition or by reacting the depositing material with an ambient gas environment. There are five basic PVD techniques: evaporation in vacuum, sputter deposition, arc vapor deposition, laser ablation and ion plating. Only evaporation and sputter deposition will be considered here. 7.1.1 Evaporation. The process of evaporation is a phase change as familiar to us as melting and freezing. Evaporation in a vacuum is, however, not as familiar as the two preceding samples. Two major types of evaporation techniques will be covered here, filament and electron beam

7.1.1.1 Filament. The first, filament, is the simpler form of evaporation. As a pot is used to boil water, so too, a variety of containers, shown in Figure 6.1, are used to boil materials such as aluminum by the simple expedient of passing current through a container

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to heat it. The parts to be coated are placed in line-of-sight of the heated charge. The boat is then resistively heated until the material in the container (charge) evaporates, or in cases such as chromium, silicon dioxide or carbon, sublimes. The resultant vapor condenses onto the substrate. The deposit thickness varies from a few nanometers to a few microns Figure 6.1 shows some of the filaments used in this type of evaporation. Dielectric films, because of their high melting points are difficult to evaporate. As a result, this technique is usually restricted to metals such as copper, gold, chromium and silver. Compound metals may be achieved by simultaneously or sequentially evaporating from multiple sources.

7.1.1.2 Electron beam. The basic electron beam evaporation principle is the same as filament, only the system of heating the material differs. A schematic of an electron gun is shown in Figure. 6.2. A stream of electrons, from a thermionic filament (usually a bare tungsten wire), is accelerated by a positively polarized anode and impinge on the surface of the charge, stored in a water coo;ed cavity. Heat is essentially

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constrained to the metal surface permitting more power to be applied before spitting occurs. This higher allowable power allows for the evaporation of very high boiling point materials. It should be realized that when electrons are directed through the vapor above the evaporant, some will be positively ionized and will follow a path back to the electron gun. By deflecting the electron beam 270°, the number of metal ions hitting the electron gun is minimized. A further advantage of this design, is that it will keep the gun, with its closely toleranced parts, hidden from any falling debris, which cause shorts. Many metals can be converted into their refractory analogs by introducing a reactive gas, such as oxygen into the system during the evaporation cycle. The gas reacts with the evaporant converting it to, in the case of oxygen, its oxide. Caution must be exercised, however, to insure the thermionic filament does not react with the gas forming an oxide or other refractory and fail.

A comparison of the two types of evaporation is shown in Table 7-1. 7.1.2 Sputtering This is probably the most versatile vapor deposition technique. Numerous variations exist, each with their own peculiar assets and liabilities, and the configuration is usually product specific. Some of the more common sputtering techniques are discussed in the following sections.

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7.1.2.1 DC A schematic of the process is shown in Figure. 6.3. The target is brought to a negative voltage of 3 to 4 KV. A second electrode, kept at ground, or biased to collect electrons, acts as the substrate holder. A gas, usually argon, is bled into the system and ionized by the impressed electric field.

This ionization takes the form of a luminescent cloud located between the two electrodes, much like a neon tube. An electric current is established between the two electrodes since part of the gas is now a conductor. The positive ions are attracted to the negatively biased target and the faster electrons are attracted to the anode. The positive ions impinge on the target and, like the cue ball hitting a racked set of billiard balls, dislodges atoms from the target. These atoms migrate toward the substrate holder gradually coating the substrates. As a result of the mechanical nature of sputtering, refractory substances such as Ta, W, Mo and TiC can just as easily be sputtered as those with lower melting points. Alloys are sputtered with a composition often equal to that of the target material. NiCr films are readily deposited with predictable stoichiometry, in contrast to evaporation methods where the chromium first sublimes forming Cr-rich layers followed by Ni-rich layers. To overcome this problem, dielectric materials may be sputtered using an RF technique, discussed in 7.1.2.2.

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7.1.2.2

RF DC deposition is limited primarily to targets of essentially metallic composition. The deposition of insulating materials by conventional dc presents obvious issues. Insulating targets cannot quickly drain the positive charge and as a result, build up a surface charge of positive ions on the front surface of the target. This positive sheath repels most of the positive ions, preventing further bombardment and deposition. A high frequency (13.56 GHz) actuating voltage is applied across the two electrodes. The polarization of the target is reversed so as to attract, alternatively, the positive ions which do the sputtering and the electrons which neutralize the positive surface charge. The physical appearance of an RF sputtering system is similar to that of a dc system. One major difference between systems is that RF sputtering requires impedance matching between power supply and discharge chamber. This is similar to the impedance matching requirements described earlier for ac components.

Dielectric targets have been used for direct sputtering of the dielectric. These materials are, for the most part, relatively porous, with target density sometimes only 50% of bulk[7.1]. This porosity leads to entrapped gases which are released slowly as the target ablates, in essence, a virtual leak. These gases, mostly water and air, contaminate the films, usually degrading them. Often, to insure stoichiometry, reactive sputtering is used to insure the total conversion of the target material to

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its oxide or nitride. RF sputtering is used to deposit metals as well as dielectrics. Thus it is possible in a single pump down to deposit both conductive and dielectric films. Dielectric targets have been used for direct sputtering of the dielectric. These materials are, for the most part, relatively porous, with target density sometimes only 50% of bulk[7.1]. This porosity leads to entrapped gases which are released slowly as the target ablates, in essence, a virtual leak. These gases, mostly water and air, contaminate the films, usually degrading them. Often, to insure stoichiometry, reactive sputtering is used to insure the total conversion of the target material to its oxide or nitride. RF sputtering is used to deposit metals as well as dielectrics. Thus it is possible in a single pump down to deposit both conductive and dielectric films. However, both DC and RF diode methods are limited to thermally stable substrates because of substrate heating due to electron bombardment. Also, where high throughput is required, their low deposition rates are limiting. Magnetron sputtering eliminates both these limitations. 7.1.2.3 Magnetron sputtering. It is well known that if an ionized gas is subjected to a magnetic field, the ionic density is increased throughput the area covered by the field. The Helmholtz coil, an external magnet surrounding the sputtering system, uses this principal to enhance ionization of the sputtering gas. This is due to the fact that the electronic paths wind themselves around the field lines, increasing the probability of collision, ionization. This phenomenon is shown schematically in Figure. 6.4. If the applied field, DC or RF, is concentrated near the target, and oriented so that the field lines are parallel to the target surface, secondary electrons formed during ionic collision concentrate in front of the target and enhance the ion density. For a given voltage then, the number of ions available for target bombardment increases, resulting in more atoms being ejected from the target per unit time, another way of saying the deposition rate increases. Secondary electrons formed during diode sputtering are accelerated to the anode and bombard the substrate. In the magnetron system, these electrons are diverted to a nearby end shield or anode. An important consequence of the effect of containing the electrons in the neighborhood of the target during magnetron sputtering is the considerable reduction of substrate heating. Table 7-2 compares the deposition rates of some representative metals sputtered by both conventional and magnetron methods, and Table 7-3 summarizes the advantages of sputtering.

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7.1.2.4 Reactive sputtering. Changing the bulk properties of a material is frequent difficult. However, one of the unique properties of thin films is that no matter how they are deposited, their properties can be readily changed or modified. For many applications it is necessary to tailor the electrical, mechanical and optical properties of the film. As thin film deposition is essentially non-equilibrium in nature, the film's composition and microstructure may be widely varied.

Reactive sputtering, evaporation and ion plating are the more common methods of film property modification. In these processes, a reactive gas is introduced into the vacuum chamber, where it chemically combines with the depositing film to alter the material's composition or structure. Using this technique, it is possible to deposit, for example,

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stoichiometric oxides, carbides and nitrides. In addition to changing compostion, film structure can be modfied, as in the case of the structural change of from anatase to rutile, by varying the partial pressure of oxygen in the sputtering gas[7.1].

As a rule, any impurity will increase the resistivity of a pure metal. Bombardment of metallic targets in a reactive gas such as oxygen, nitrogen or methane will generally increase resistivity. Tantalum nitride is probably the prime illustration of the ability to tailor film properties. By altering the deposition conditions, either resistor or capacitor films may be routinely deposited. The effect of nitrogen on the and TCR of tantalum films, shown in Figure 7.5 is widely known[7.2]. The resistivity curve in Figure 7. 5 includes a plateau and levels out at about with a TCR in order of -75 ppm/°C. The exact shape and displacement of both TCR and resistivity will shift depending on a number of deposition factors, such as substrate temperature, pumping speed, gas ratios and internal sputtering configuration. Reactive sputtering has also been used to deposit ruthenium dioxide resistors, which are usually screened. Jia et. al.,[7.3] deposited resistive films with controlled resistivity and TCR by varying deposition temperature and oxygen partial pressure.

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References 7.1 7.2 7.3

W. T. Pawlewicz et. al., "Recent Developments in ReactivelySputtered Optical Thin Films", SPIE Proceedings 325, Optical Thin Films (1982) ppl05-l16 Gerstenberg, D. and C. J. Calbick, J. Appl. Phys.,. 35, 402, (1964) Jia, Q. X., K. L. Jiao, W. A. Anderson and F. M. Collins, “Development and Fabrication of RuO2 Thin Film Resistors”, Materials Science and Engineering, B18 (1970) pp. 220-5

CHAPTER 8

DIELECTRIC DEPOSITION

In addition to sputtering and evaporation, listed in Table 8-1 other methods of depositing dielectric materials are tabulated.

8.1 PE LPCVD . Temperature considerations are the major reasons for using plasma promoted CVD. Unlike thermally activated CVD, highly reactive species are generated from gaseous reactants by an rf glow discharge. Because of this, processing temperatures are 200-400°, well below the 700-1000° range normally encountered in CVD. The basic reactant gases Silane Ammonia and Nitrogen combine to form Silicon Nitride, viz; The stoichiometric considerations in these films are the Si/N ratio (0.75 theoretical) and the amount of hydrogen in the film. With

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PE CVD the stoichiometry can vary from .8 to 1.0. The hydrogen content, very much a concern is dependent on a) the reaction temperature and b) the reactant gases. Most nitride is deposited at between 350°C and 380°C, since at 400° aluminum metallization, used in silicon device technology, upon which the nitride is deposited, is degraded. At the above temperatures, the film contains about 10-18% Eliminating the ammonia helps to reduce the hydrogen to about 2%[7.1]. Kern and Tracy[7.2], studied films deposited at 275°C to 300°C. They found they were stable to ~400°C. Heating beyond this temperature may cause local blistering characteristic of stress relief of films under compression. Further heating may cause openings of these blisters resulting in the formation of small circular holes. They concluded that stress relief with outgassing of hydrogen was responsible. While it is unlikely that most MICs will see the temperatures reported, the potential for reliability problems remain. Silicon dioxide films are formed at low temperature through the following established reactions:

However, such films are inferior as capacitor dielectrics because of particle entrapment and pin hole formation. Their primary function, by themselves, or in concert with silicon nitride, is for passivation. Nevertheless, PECVD plasma is widely used and as processing technology progresses toward lower temperatures and with better understanding of the mechanisms, additional improvements in film quality will be forthcoming. One such method, photochemically enhanced CVD films shows promise [7.3]. 8.2 Anodization. Anodization is the electrochemically conversion of the parent metal to its oxide This electrochemical method as the name implies, involves making the parent metal the anode in an electrolytic cell, as opposed to being the cathode in conventional electroplating systems. The dielectric is a thin film of oxide, formed on the surface of a certain class of metals by electrolytic oxidation in a suitable electrolyte. The only metals which demonstrate this and are commercially important are aluminum, tantalum and – to a slight extent at present – niobium, but this property is also exhibited by zirconium, titanium, bismuth, and silicon, among others. Of these, titanium offers some promise as a capacitor material. When one of these metals is made the anode in the

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proper electrolyte and a constant current is applied to the electrochemical cell, an insulating layer of oxide forms on the surface, and the applied potential must therefore be increased to keep the current constant. If the entire charge passed is used in forming oxide, the rise in voltage is linear. With reasonably close electrode spacing and moderately high electrolyte conductivity there is only a small, usually negligible, voltage drop in the cell, and the measured cell voltage may be considered to be entirely across

the anodic film. The voltage at which oxidation is terminated is the socalled formation voltage ( in Figure. 7.1a). Application of a constant voltage beyond the time required to attain causes the current flowing through the cell to decay, and before terminating the process the current is usually allowed to reach a value which is very low compared to the initial anodizing current. A typical current-time plot is shown in Figure. 7.1b. The particular current value observed under a given set of conditions is called the leakage current; it may vary widely depending on the temperature, voltage, time of formation, anode material and electrolyte.

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The basic equations that govern the process may be written as At anode: At cathode:

As the oxide grows on the metal hydrogen generates at the cathode. Anodization usually employs an aqueous electrolyte, thus implied in these equations is the presence of water. There is a limit to the number of elements which can be usefully converted to their oxide by anodization. Many elements which will anodize may not form electrically useful layers. The metals that can be anodized to give useful films are listed below[7.5]. The parent metal is usually a rectifying type and is characterized by a relatively high resistance. The thinned electrode will exhibit an even higher series resistance and resistor lengths should be kept to a minimum. Circuits employing anodized tantalum films have been widely used in both monolithic[7.6] and hybrid[7.7] structures. A major advantage of metals such as tantalum is that when reactively sputtered, for example in oxygen or nitrogen, they may also be used as resistive elements. The anodic

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layer serves to trim the resistor by decreasing the cross-section of the resistive layer while simultaneously sealing it. The anodizing properties of selected metals is shown in Table 8-3

The pH of the electrolyte is critical since parts of the oxide will dissolve in very alkaline solutions (