FPD Manufacturing

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DK543X_half 10/21/05 4:05 PM Page 1

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing

DK543X_title 10/21/05 4:05 PM Page 1

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing edited by

Tadahiro Ohmi

Boca Raton London New York

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

DK543X_Discl.fm Page 1 Friday, September 2, 2005 11:41 AM

Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 0-8493-3543-4 (Hardcover) International Standard Book Number-13: 978-0-8493-3543-3 (Hardcover) Library of Congress Card Number 2005024414 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Scientific wet process technology for innovative LSI/FPD manufacturing / editor, Tadahiro Ohmi. p. cm. Includes bibliographical references and index. ISBN 0-8493-3543-4 (978-0-8493-3543-3 : alk. paper) 1. Semiconductors--Design and construction. 2. Semiconductors--Cleaning. 3. Integrated circuits-Design and construction. I. Ohmi, Tadahiro, 1939TK7871.85.S3965 2006 621.3815'2--dc22

2005024414

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Preface In 1965, Gordon E. Moore foretold that the number of transistors in integrated circuits would increase by a factor of 4 every 3 years. Over the past few decades, semiconductor technologies have steadily developed and have advanced the integration density of LSI devices in accordance with Moore’s law. This revolutionary achievement was made possible by improving device miniaturization technologies, such as photolithography technologies, various plasma processes, and so on. However, the current molecule reaction-based semiconductor manufacturing technologies are now facing great difficulty in achieving further device miniaturization. The present standstill in semiconductor technologies resulting from this has caused severe stagnation in industries all over the world. Microprocessors brought to the market in 1971 triggered a drastic change in industrial and social structure by bringing the so-called microelectronics revolution to all industries and social systems. Semiconductor technologies have supported the continuous progress of all industries in subsequent decades. The continuous development of semiconductor technologies must not be suspended if the healthy advancement of the world economy is to be maintained. Miniaturization in the critical dimensions of integrated circuits is accompanied by a decrease in thickness of the gate insulator films of MOS transistors. The biggest reason for the standstill in current semiconductor technologies is very large leakage current, and a significant increase in standby electric power consumptions of LSI devices up to a few tenths of watts. Large leakage currents have the following two constituents: (1) leakage current through the very thin gate insulator film (approaching 1 nm thickness) and (2) drain leakage current. More than 1 103 A/cm2 of leakage current is generated when a voltage of 1 V is applied to a current thermal-oxidationbased gate insulator film having a thickness of about 1 nm. Therefore, the current thermaloxidation-based gate insulator films can no longer serve as insulator films when their thickness decreases to around 1 nm. Meanwhile, in an attempt to improve the mobility of electrons and holes to enhance the speed performance of LSI devices, the semiconductor industry has introduced SiGe to MOS transistors, where the germanium concentration in SiGe is gradually increased to improve the mobility of the electrons and holes. This attempt is theoretically incorrect. It is very well known that the bandgaps of Si and Ge are 1.12 and 0.69 eV, respectively, resulting in a huge difference in the leakage currents in the reverse direction of the pn diodes of Si and Ge by a factor of 2.8 104 at room temperature. Thus, the increase in germanium concentration in the SiGe of MOS transistors is inevitably accompanied by a drastic increase in the drain leakage current of 2 to 3 orders of magnitude. The author has developed radical-reaction-based semiconductor manufacturing using microwave-excited very low-electron-temperature high-density plasma equipment instead of the current molecule reaction-based semiconductor manufacturing in order to overcome the difficulties that limit the progress of semiconductor technologies. Current plasma equipment cannot be used for transistor fabrication, but it can be applied to interconnect fabrications. This is because of the major disadvantages of using the current plasma equipment, such as severe metallic contamination on the substrate surface by high-energy ion bombardment at the inner surface of the process chamber, severe damage to the substrate surface by high-energy ion bombardment, severe charge-up damage due to the residual electric charges on the substrate surface just after turning off the plasma, the charges induced by secondary electron emission caused by high-energy ion bombardment, and so on.

It has been established that the newly developed microwave-excited high-density plasma having very low electron temperatures overcomes all these disadvantages of the current plasmaprocessing equipment, and can be applied to transistor fabrication, including gate insulator formation. High-density plasma is excited by circularly polarized microwaves with a frequency of 2.45 GHz in the region of around 10 to 20 mm under the ceramic shower plate. This is carried out in order to introduce the flow of plasma excitation gas and radical generation gas onto the substrate surface in a uniform manner. In the newly developed high-density plasma, there exists a plasma diffusion region just under this plasma excitation region. This region is characterized by very low electron temperatures, for example, 1.0 eV for Ar gas, 0.7 eV for Kr gas, and 0.5 eV for Xe gas. Thus the bombarding ion energies are less than the critical values that would cause metal sputtering and substrate surface damage. Si substrates are set in this diffusion plasma region where an electron current and an ion current flowing onto the substrate surface are equal with each other at any instant, so that a surface electric charge does not remain even after the plasma is turned off. This means that the most severe disadvantages, that is, charge-up damage and high-energy ion bombardment-induced damage, are essentially eliminated. The introduction of this new plasma processing, free of charge-up damage, brings about a drastic change in circuit layout pattern regulation, that is, a very limited antenna ratio, which is defined as the ratio of the area of an interconnect of the gate electrode of a MOS transistor with the area of the gate electrode. At present, the antenna ratio of circuit layout patterns of LSI devices is strictly limited to less than 100 to 200, in order to obtain reasonable manufacturing yields. When circuit layout patterns having very high antenna ratios such as 106 are designed in LSI chips, all the chips on the Si substrates suffer fatal charge-up damage when using the current plasma processing, resulting in zero percent yield. The number of plasma processing stages throughout the entire LSI manufacturing process is up to several tens, so the antenna ratio in the circuit layout pattern is strictly limited to less than 100 to 200 at present. The newly developed plasma processing, free of charge-up damage, is the microwave-excited high-density plasma, which has very low electron temperatures and enables realization of LSI of any circuit layout pattern having arbitrary antenna ratios. Various reactive radicals (such as oxygen radicals O† and NH† radicals) generated in the plasma excitation region diffuse to the Si substrate surface and form very high-integrity SiO2 and Si3N4 films on any crystal orientation Si substrate surface at low temperatures (such as 400 to 6008C). The leakage current through this radical-reaction-based gate insulator film has been confirmed to be smaller by a factor of 3 orders of magnitude compared with that of the current molecule reaction-based gate insulator film. The stagnation of current Si technologies resulting from the existence of very large leakage currents through the gate insulator has thus been completely overcome by introducing these radical-reaction-based gate insulators. Moreover, the introduction of a radical-reaction-based Si3N4 film to the gate insulator improves the mobility of electrons and holes by a factor of at least 2 compared with that of the current thermal oxide MOS transistor. It is therefore not necessary to introduce a SiGe region to MOS transistors in order to improve the speed performance of LSI. Any difficulties arising from very large drain leakage currents are completely overcome by introducing this radical-reaction-based Si3N4 gate insulator. The flicker noise component, that is, 1/f noise, is also decreased by the introduction of radical-reaction-based gate insulators, by a factor of 2 orders of magnitude. This is very important to the future progress of LSI device miniaturization, while accompanied by a gradual decrease in supply and signal voltages and without introduction of operation errors of the devices. Whereas the current molecule reaction-based thermal oxidation can produce relatively high-integrity SiO2 films only on (1 0 0) Si surface orientations, the radical-reaction-based direct oxidation and direct nitridation have been proven to produce very high-integrity insulator films

on any crystal orientation of Si substrate surface. Thus, LSI device fabrication by the current molecule reaction-based semiconductor manufacturing process is limited only to the (1 0 0) Si surface and the structure of MOS transistors is limited to the two-dimensional planar structure. As a result, only a small part of the capabilities of the Si material has been used for practical applications so far. On the other hand, the radical-reaction-based semiconductor manufacturing process has been shown to utilize almost all of the Si material capabilities, for example, by LSI device fabrication any crystal orientation of the Si substrate surface and in 3-dimensional MOS transistors. The electric current drivability of pMOS transistors fabricated on a (1 1 0) Si substrate surface with k1 1 0l direction is 3 times larger than that of a pMOS transistor on a (1 0 0) Si surface, resulting in an improvement of CMOS speed performance by a factor of 2. Dual-shower plate microwave-excited high-density plasma processing has been developed simultaneously for application to various plasma CVD film formations and various material pattern etchings. In this method, the lower shower plate, supplying source gases for film formation and pattern etchings, is introduced to the diffusion plasma region described previously. The plasma potential in the plasma diffusion region may be limited to less than 10 V, at most, due to the very low electron temperatures. This completely eliminates the sputtering of the surface of the lower shower plate due to ion bombardment. Source material gases are supplied to the plasma diffusion region, which has very low electron temperatures, and not to the plasma excitation region, which has relatively high electron temperatures. This arrangement ensures that the source material gas molecules are not decomposed so much. In other words, the source material gas molecules can be supplied even to the bottom of the contact and the through holes, however narrow the diameter and however deep, resulting in the realization of processing of any narrow contact, through holes etchings, and film depositions at a constant process speed. This process, which is free of the microloading effect, indicates that the speed of processes such as etching rate and film-deposition rate are independent of the pattern size (for example, from 10 mm to 25 nm). Current semiconductor equipment commonly exhibits very severe microloading effects, that is, the equipment can be applied to 100-nm generation LSI fabrication, but not to 65-nm generation LSI fabrication. Thus, a huge amount of investment is continuously required for new generation LSI fabrication. The new radical-reaction-based semiconductor equipment, such as the microwave-excited high-density plasma with single and dual-shower plate structures, can be used continuously for all LSI generations, to 25-nm. This is a revolutionary change in semiconductor manufacturing. It is well known that current plasma equipment for plasma CVD and RIE cannot maintain plasma uniformity on the entire Si substrate surface for different gas pressures, different gas combinations and concentrations, different substrate electrode self-biases, and different Si substrate surfaces (different surface materials and different surface patterns). In such equipment, plasma uniformity on the entire Si substrate surface has to be realized by tuning various equipment parameters, which is quite tedious and time-consuming. Therefore, the current semiconductormanufacturing technologies work very effectively only for very large volume production of a small variety of LSI devices. The key business area in the field of electronics is rapidly becoming digital consumer electronics rather than of conventional personal-computer-oriented business, which requires only very large-volume production of microprocessors and DRAMs. Digital electronics, however, requires a very wide variety of LSIs with very small volume production. The new microwaveexcited high-density plasma with single- and dual-shower plate structures maintains plasma uniformity on the entire substrate surface even under widely varying operation parameters such as gas working pressure, gas combinations and concentrations, self-biases of the substrate electrode, and substrate surface patterns and materials. Thus, Si substrates of any kind and structure can be continuously processed at optimum conditions for each individual Si substrate without tuning the various equipment parameters and structures.

The new radical-reaction-based semiconductor manufacturing using microwave-excited high-density plasma will realize the production of a wide variety of LSI devices in very small volumes, as is required in the digital consumer electronics era, in keeping with its very high productivity. The semiconductor industry is now facing a real revolution. The present Radical-Reaction-Based Semiconductor-Manufacturing Series describes all these newly developed technologies in a very academic manner. Tadahiro Ohmi

About the Editor Tadahiro Ohmi was born in Tokyo, Japan, in 1939. He earned his B.S., M.S., and Ph.D. degrees in electrical engineering from Tokyo Institute of Technology, Tokyo, in 1961, 1963, and 1966, respectively. Prior to 1972, he served as a research associate in the Department of Electronics of Tokyo Institute of Technology, where he worked on Gunn diodes such as velocity overshoot phenomena, multi-valley diffusion and frequency limitation of negative differential mobility due to an electron transfer in the multi-valleys, high-field transport in semiconductor such as unified theory of space – charge dynamics in negative differential mobility materials, Blochoscillation-induced negative mobility and Bloch oscillators, and dynamics in injection lasers. In 1972, he moved to Tohoku University and is now a professor at the New Industry Creation Hatchery Center (NICHe), Tohoku University. He is currently engaged in research on highperformance ULSI free from gate and drain leakage currents, threshold voltage fluctuations of MOS transistors and 1/f noises such as ultra-high-speed ULSI by introducing directly nitrided Si3N4 gate insulation as a high-K gate dielectric, nonporous fluorocarbon film having dielectric constant less than 1.90 as an inter-metallic dielectric and Si (110) surface, and metal substrate SOI, ULSI and large size flat-panel display, and advanced semiconductor and FPD process technologies by developing radical-reaction-based semiconductor and FPD manufacturing due to microwave-excited very low electron temperature high-density plasma free from charge-up damages and bombarding ion-induced damages. Dr. Ohmi’s research activities include 2000 original papers and 1800 patent applications. He received the Ichimura Award in 1979, the Inoue Harushige Award in 1989, the best paper award of IEEE Transactions on Semiconductor Manufacturing in 1989, the Ichimura Prizes in Industry-Meritorious Achievement Prize in 1990, the Okouchi Memorial Technology Prize in 1991, the Minister of State for Science and Technology Award for the promotion of invention in 1993, the Invention Prize and 4th International Conference on Soft Computing (IIZKA’96) Best Paper Award in 1996, the IEICE Achievement Award in 1997, the Werner Kerm Award in 2001, ECS Electronics Division Award in 2003, the Medal with Purple Ribbon from the government of Japan in 2003, and the Best Collaboration Award (the Prime Minister’s Award) in 2003. Dr. Ohmi is a member of the Institute of Electronics, Information and Communication Engineers of Japan (fellow), the Japan Society of Applied Physics, the ECS, and the IEEE (fellow).

Contributors Nobukazu Ikeda Fujikin Incorporated Osaka, Japan

Masaaki Nagase Fujikin Incorporated Osaka, Japan

Takashi Imaoka ORGANO Corporation Tokyo, Japan

Koji Nishino Fujikin Incorporated Osaka, Japan

Nobuhiko Inoue Oki Electric Industry Corporation Tokyo, Japan

Tadahiro Ohmi New Industry Creation Hatchery Center Tohoku University Sendai, Japan

Hiroto Izumi Stella Chemifa Corporation Osaka, Japan Hirohisa Kikuyama Stella Chemifa Corporation Osaka, Japan Masafumi Kitano New Industry Creation Hatchery Center Tohoku University Sendai, Japan Kenichi Mitsumori Alps Electric Co., Ltd. Sendai, Japan

Senri Ojima Nomura Micro Science Co., Ltd. Kanagawa, Japan Hiroshi Sugawara ORGANO Corporation Tokyo, Japan Jun Takano Stella Chemifa Corporation Osaka, Japan

Masayuki Miyashita Stella Chemifa Corporation Osaka, Japan

Akinobu Teramoto New Industry Creation Hatchery Center Tohoku University Sendai, Japan

Hitoshi Morinaga New Industry Creation Hatchery Center Tohoku University Sendai, Japan

Tatsuhiro Yabune Stella Chemifa Corporation Osaka, Japan

Hiroshi Morita Kurita Water Industries Ltd. Tokyo, Japan

Ikunori Yokoi Kurita Water Industries Ltd. Tokyo, Japan

Contents Chapter 1

Surface Chemical Electronics at the Semiconductor Surface

... 1

Tadahiro Ohmi Chapter 2

Principles of Semiconductor Device Wet Cleaning

. . . . . . . . . 35

Hitoshi Morinaga Chapter 3

High-Performance Wet Cleaning Technology . . . . . . . . . . . . . 61

Hiroshi Morita, Akinobu Teramoto, Hitoshi Morinaga, Senri Ojima, and Kenichi Mitsumori Chapter 4

Etching of Various SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

153

Tatsuhiro Yabune, Masayuki Miyashita, Hirohisa Kikuyama, Jun Takano, and Akinobu Teramoto Chapter 5

Silicon Etching

..................................

251

Kenichi Mitsumori and Nobuhiko Inoue Chapter 6

Chemical Composition Control Technology

.............

271

Tatsuhiro Yabune, Masayuki Miyashita, Hirohisa Kikuyama, and Jun Takano Chapter 7

Wet Vapor Resist Stripping Technology . . . . . . . . . . . . . . . .

285

Senri Ojima and Tadahiro Ohmi Chapter 8

Antistatic Technology

.............................

293

Kenichi Mitsumori and Takashi Imaoka Chapter 9

Chemical Waste Reclamation Technology

..............

315

Advanced Ultrapure Water and Liquid Chemical Supply System and Materials for Fluctuation-Free Facility . . . . . . .

331

Hiroshi Sugawara and Takashi Imaoka Chapter 10

Ikunori Yokoi, Masaaki Nagase, Koji Nishino, Nobukazu Ikeda, Masafumi Kitano, Hiroto Izumi, and Tadahiro Ohmi Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

379

1

Surface Chemical Electronics at the Semiconductor Surface Tadahiro Ohmi New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan

CONTENTS I. Hydrogen Atomic Model and Molecular Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Behavior of Molecules Adsorbed on the Si Surface: Taking a SiH4-Type Molecule as an Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Electronegativity and Energy Level: Change in State of Electrons on Surface due to Terminating Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Oxidation of Si Surface at Room Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Selective W Film Deposition by Means of (SiH4 þ WF6) . . . . . . . . . . . . . . . . . . . . . VI. Redox Potential and Energy Level of Liquid Solution — Native Oxide Formation on the Si Surface and Metal Contaminant Adhesion and Removal with Solution . . VII. New Era of Si Technology — (1 1 0) Si Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 4 9 15 17 18 23 31 33

I. HYDROGEN ATOMIC MODEL AND MOLECULAR BOND The author describes molecular behavior in many aspects by means of the hydrogen atomic model, as there is no general theoretical equation of molecular bond energy. The energy of an electron (1n) and the Bohr radius of the first orbit (aB) in this model are expressed, when the principal quantum number of n is used, as 1n ¼

me4 1 13:6 ¼ 2 (eV) n 2(4p10 )2 h2 n2

(1:1)

aB ¼

(4p10 )2 h2 ¼ 0:053 nm me2

(1:2)

where m, e, 10 and h ¼ 2ph are, respectively, the mass of the electron, unit charge, the dielectric constant of vacuum, and Planck’s constant. The energy level is determined by the Coulomb force between positive and negative charges. This energy is inversely proportional to the squared dielectric constant and directly proportional to the mass. Si is a tetravalent monatomic crystal. When phosphor (P) or arsenic (As), either of which is a pentavalent atom, is doped into Si, one electron of P or As does not contribute to the covalent bond of Si. The energy (1n ) to excite this electron to the conduction band, which is called impurity level 1


2

(energy level from the bottom of the conduction band), is given by the hydrogen atomic model as follows: 1n ¼ 13:6

m (eV) m12Si

(1:3)

where m and 1Si are the effective mass of the electron in the conduction band and the relative dielectric constant of Si, respectively. When 0.19m and 11.8 are substituted for m and 1Si , respectively, 1n , is calculated to be 217 meV. This indicates that electrons at impurity level can be easily excited to the conduction band with the thermal energy of 26 meV at room temperature. A radius of circular orbit, which the electron at impurity level forms around the doped impurity atom in Si (Bohr radius), aB, is also changed from 0.053 to 3.3 nm in accordance with the following equation: aB ¼ 0:053

m1Si (nm) m

(1:4)

As the doping density of an impurity atom such as As, P, and B exceeds 1 1018 cm23, the average interatomic distance of neighboring impurity atoms gets smaller than 10 nm. Accordingly, the wave function of electrons circulating around the impurity atoms comes to overlap each other. As a result, the impurity level is changed to the impurity band. This is a typical example that the energy level and the bond energy of atoms or molecules show a drastic change due to the dielectric constant of the environment where they exist. HF, HCl, and HBr are often used in the semiconductor-manufacturing process both in gaseous and liquid phases. They are hydrides of halogen-type elements of fluorine, chlorine, and bromine. Table 1.1 shows their bond energy and typical properties. Table 1.1 also shows properties of H2O, hydride of oxygen. HF and H2O featuring high dipole moment are in the liquid state in the wide range of temperature up to 1008C, and so, they feature extremely high dielectric constants. The electronegativity of fluorine and oxygen is extremely high: 4.0 and 3.5, respectively, compared with that of hydrogen (2.1). HF and H2O are electrically neutral as a molecule, therefore, electrons gather on the side of fluorine and oxygen atoms, respectively. This raises a dipole moment of HF and H2O, which lets them easily form clustered molecules. This is why they stay in liquid phase during a wide temperature range. Table 1.2 shows the electronegativity and the energy level of H, O, N, F, Cl, Br, and Si, calculated on the basis of their vacuum level. The relationship between

TABLE 1.1 Bond Dissociation Energy (Homolysis and Heterolysis) and Typical Properties of Hydrogen Halides (HF, HCl, HBr) and H2O Bond Energy Bond Energy Melting Point Boiling Dipole Ionization Dielectric (Homolysis) (eV) (Heterolysis) (eV) (8C) Point (8C) Moment (DU) Potential (eV) Constant HF HCl HBr H2 O

5.8 4.4 3.8 4.8

16.0 14.4 14.0 16.9

283 2114 289 0

19.5 285 267 100

1.83 1.11 0.83 1.94

16.1 12.8 11.7 12.6

83 12 7 81

Note: Dipole moment: 1 DU ¼ 3.3356 10230 cm) Homolysis bond dissociation: HF ! H þ F Heterolysis bond dissociation: HF ! Hþ þ F .


3

TABLE 1.2 Electronegativity (Energy Level) and Typical Properties of Various Atoms such as H, O, N, F, Cl, Br, and Si which are Widely Used in Semiconductor Manufacturing

Atomic number Mass number Group Electronegativity Energy level (eV) Ionization potential (eV)

H

O

N

F

Cl

Br

Si

1 1.01 Ia 2.1 25.11 13.6

8 16.01 VIb 3.5 28.29 13.6

7 14.10 Vb 3.0 27.15 14.5

9 19.00 VIIb 4.0 29.42 17.4

17 35.45 VIIb 3.0 27.15 13.0

35 79.90 VIIb 2.8 26.70 11.8

14 28.09 IVb 1.8 24.43 8.15

electronegativity (X) and energy level (1) is expressed as [1]: 1 ¼ (2:27X þ 0:34) (eV)

(1:5)

Table 1.2 also shows the ionization energy of each atom which corresponds to the energy level of electron at the ground state. As shown in Table 1.1, HF, HCl, and HBr feature extremely high bond energies of 5.8, 4.4, and 3.8 eV, respectively. They, therefore, never get decomposed with thermal energy of 0.026 eV at room temperature in gaseous phase. In other words, HF, HCl, and HBr are extremely stable gas molecules in gaseous phase. As shown in Table 1.2, the electronegativity of F, Cl, and Br is 4.0, 3.0, and 2.8, respectively, whereas that of H is 2.1. When converted from electronegativity by means of Equation (1.5), the energy level based on the vacuum level of F, Cl, and Br is 29.42, 27.15, and 26.70 eV, respectively, which is much lower than that of H of 25.11 eV. When a molecular bond is formed between H and F, H and Cl, or H and Br, the molecule is electrically neutral as a whole, but electrons get locally concentrated on the side of F, Cl, or Br. So in practice, H is positively charged that makes the bond like ionic bond. HF features a high dipole moment, as the electronegativity gap of its component atoms is the biggest and the local concentration of electrons is remarkable. When two atoms form a molecule in an ionic-bond-like way, the electric force lines to connect positive and negative charges are distributed widely in adjacent region, and the molecule is affected more by the dielectric constant of the surrounding medium. If it is assumed, as shown in Equation (1.1), that the bond energy of this molecule is inversely proportional to the squared dielectric constant, then the bond energy of the molecule, which is dissolved in water, is lowered by the high dielectric constant of water (relative dielectric constant of 81). Specifically, the bond energy of HF, HCl, and HBr, when dissolved in water, decreases to 2.5, 2.25, and 2.19 meV, respectively. So all these three molecules are most likely to be well decomposed and dissociated to ions such as Hþ, F2, Cl2, and Br2 even with the thermal energy of 26 meV at room temperature. HCl and HBr are completely dissociated in water. In contrast, HF gets only partially dissociated when its concentration is fairly high (over 1023 mol/kg). Some F2 ions generated in HF dissociation (HF ! Hþ þ F ) get bound with neutral HF molecules to form HF22 ions which have been revealed as dominant ions to etch SiO2 film [2,3]. This fact suggests that the bond energy of molecule is actually not simply inversely proportional to the squared dielectric constant. H2, N2, O2, F2, Cl2, and Br2 comprise two exactly identical atoms, which make them different from those molecules that comprise two atoms featuring different electronegativity. In this case, therefore, localization of electron concentration to result in polarity does not occur. In other words, these molecules feature a covalent bond. External extension of their electric force lines is limited, and they are hardly affected by dielectric constant of the surrounding medium. Table 1.3 shows the bond energy and ionization energy of H2, O2, N2, F2, Cl2, and Br2, in which negatively charged

4


TABLE 1.3 Bond Dissociation Energy (Homolysis) and Ionization Potential of Molecules such as H2, O2, N2, F2, Cl2, and Br2

Bond energy (eV) Ionization potential (eV)

H2

O2

N2

F2

Cl2

Br2

4.49 15.4

5.13 12.1

9.51 15.6

1.61 15.7

2.49 11.6

1.98 10.5

electrons attract positively charged atoms by means of Coulomb force. Even when H2, N2, or O2 molecule get dissolved in water, they are not however decomposed or dissociated. The ionization energy of the H2 molecule is about 15.4 eV in gaseous phase. A redox potential of 0 V is defined as the energy required to ionize H2 into 2Hþ and 2e2 in ultrapure water. It is equal to an energy level of 24.44 eV. Even in the case of molecules with covalent bond, bond energy and ionization energy get lower when being in media with high dielectric constants. Various active species, which are never generated in gaseous phase, can be easily generated at room temperature in liquid solutions having very high dielectric constant. This is why various reactions can take place in water solutions at room temperature as relative dielectric constant of water is extremely high at 81. The wet process, therefore, will remain remarkably important for the Si technology even in the future.

II. BEHAVIOR OF MOLECULES ADSORBED ON THE Si SURFACE: TAKING A SiH4-TYPE MOLECULE AS AN EXAMPLE If it is assumed that the bond energy of a molecule is inversely proportional to the squared dielectric constant of the medium where the molecule exists, then the binding state of the molecule and the energy state of electrons must differ greatly between when the molecule is isolated in gaseous phase and when the molecule is adsorbed onto solid surface. Let us take the Si surface as an example, where the relative dielectric constant of Si is 11.8. Let us assume, as shown in Figure 1.1, that molecule AB comprising atoms A and B gets adsorbed on the solid surface (the Si surface) from the gaseous phase. Owing to interaction

A

Gas Phase

A

B

B

Solid Si

ε

Si

FIGURE 1.1 Schematic sketch of a molecule (AB) comprising atoms A and B adsorbed on the solid surface (Si), where the bond of the molecule is affected by the solid dielectric constant in addition to the gas-phase dielectric constant.


5

between the wave function of electrons in molecule AB and that of electrons in the vicinity of the Si surface, the state of electrons must become very different when molecule AB is adsorbed from when it is isolated in the gaseous phase. Let us limit our discussion to difference in dielectric constant. Molecule AB in the gaseous phase is subjected to relative dielectric constant of 1. When it is adsorbed onto the Si surface, it is subjected to apparent relative dielectric constant of 1eff which is expressed as 1eff ¼

1 þ g1Si 1þg

(1:6)

where g is an influence coefficient of Si on adsorbed molecule and 1Si the relative dielectric constant of Si. As a result, the bond energy of molecule AB and energy level of electrons must be decreased by a factor of 1 1þg 2 ¼ (1:7) 1 þ g1Si 12eff Meanwhile, interaction between various Si surfaces and SiH4-type gas molecules is studied [4,5]. A perfectly closed reaction system is prepared for this experiment by using a Hastelloy tube reactor of 1/2 in. in diameter and 40 cm in length. SiH4 gas is introduced into this reactor. SiH4 gas contains residual impurities of moisture of several parts per billion (ppb) [6] and siloxane of 5 ppb. The gas-delivery system is made up of all-metal Cr2O3-passivated stainless steel pipes which are characterized by complete chemical stability for various specialty gases [7]. No contamination is introduced from this gas-delivery piping system. Three types of Si surfaces, that is nondoped surface, nþ surface, and pþ surface, are prepared on the inner surface of the tube reactor by means of the thermal CVD method with gases SiH4, SiH4 þ PH3, and SiH4 þ B2H6. Every deposited Si thin-film features (1 1 1) orientation. A predetermined gas is continuously introduced into the tube reactor with its inner surface covered with a predetermined silicon thin film, whereas the inner surface is not at all exposed to the air. Gas components produced as a result of interaction between the three different silicon surfaces such as nondoped, nþ, and pþ and the introduced gases are evaluated with the infrared Fourier transform spectrometer (FT-IR) or with the gas chromatography when they come out from the outlet of the reactor tube. The decomposition reaction induced by the interaction between SiH4 gas and various Si surfaces is assumed to be first-order reaction for the purpose of simplifying discussion. The reaction formula is as follows: b SiH4 ¼ Si(sol) þ aH(ad) þ H2 (gas) (1:8) 2 d½SiH4 ¼ K ½SiH4 (1:9) dt where a þ b ¼ 4 and K is the rate constant. The rate constant (K) is usually expressed as K ¼ A eEa =kT

(1:10)

where A is an oscillation factor and Ea an activation energy. The SiH4 concentration at the outlet of the tube reactor ([SiH4]t0) can be expressed as ½SiH4 t0 ¼ ½SiH4 0 ekt0 where t0 is the residence time of SiH4 in the tube reactor.

(1:11)

6


[SiH4]0 stands for the initial concentration of SiH4 which is introduced into the tube reactor. SiH4 concentration decreases in an exponential manner against the residence time t0. Experimental results demonstrate that the first-order reaction (Equation (1.8) and Equation (1.9)) is approximately correct [8,9]. The rate constant K of the SiH4 decomposition obtained from the experiments depends on the SiH4 concentration in Ar or N2 carrier gas: it decreases as SiH4 concentration rises. This is because hydrogen atoms generated in SiH4 decomposition terminate the Si surface. Figure 1.2 shows the rate constant K as a function of an inverse of temperature when an initial SiH4 concentration in Ar gas is set at 100 ppm. The surface to be interacted with SiH4 gas are hydrogen-terminated nondoped Si, that is nþ Si, which is scarcely terminated with hydrogen, and pþ Si featuring a boron concentration of 1 1020 cm23. The pþ Si film is annealed at 6008C in an Ar gas ambience for a long time, whereas the nþ Si film is treated at 8508C in an Ar gas ambience. Terminating hydrogen, therefore, is almost removed from these two surfaces. The SiH4 decomposition on the nondoped Si surface and nþ Si surface features almost the same rate constant. On the pþ Si surface, SiH4 gas starts to decompose even at extremely low temperatures of 90 – 1008C. On the nondoped Si surface and the nþ Si surface, however, the SiH4 decomposition is detected only when the temperature exceeds 330 – 3408C. The reaction product of SiH4 decomposition on the pþ Si surface is only H2, which is also applied to the SiH4 decomposition on the nondoped Si surface and the nþ Si surface. The activation energy is found to be 0.3 eV which is obtained from the Arrhenius plot to show the rate constant of SiH4 decomposition induced by interaction with the pþ Si surface in the temperature range 90– 2008C. Around 4008C, the rate constant of SiH4 decomposition on the pþ Si surface gradually gets closer to that

Activation Energy T (°C) 500 400 300

200

100 p+ Si

10 0

Rate Constant, k [10−3/sec]

non-dope Si n+ Si 10

1

Ea=0.3 eV

Ea=1.45 eV 0.1 Ea=2.0 eV

0 .0 1 1

1.5

2

2 .5

3

1000/T [1/K ]

FIGURE 1.2 Rate constant of SiH4 decomposition at the surface of nondoped Si, nþ Si, and pþ Si is plotted as a function of an inverse temperature, where the rate constant is measured by using a tube reactor of 1/2 in. in diameter and 40 cm of length.


7

on the nondoped surface and on the nþ Si surface. The following two facts suggest that the SiH4 decomposition process on the pþ Si surface is not induced by thermal energy but by a kind of catalytic effect of the pþ Si surface: 1. SiH4 gets decomposed at extremely low temperature only on the pþ Si surface. 2. Its activation energy is extremely low at 0.3 eV. SiH4 decomposition on the pþ Si surface at low temperature can be explained in the following way. The electron to bind Si and H atoms in a SiH4 molecule, isolated in gaseous phase, has an ionization energy of 212.8 eV, but this electronic energy level changes to 25.45 eV when the SiH4 molecule is adsorbed on the Si surface. The electron is recombined with a hole on the top of the valence band (25.15 eV) of the pþ Si surface when it gains thermal energy of 0.3 eV. As the electron which contributes to Si22H bond in the SiH4 molecule disappears due to recombination with the hole of the pþ Si surface, the Si22H bond can be dissociated at low temperature: SiH4 molecule decomposition hardly requires thermal energy in the process. The energy level of an electron to bind Si and H is different by as much as 6.2 eV between when the SiH4 molecule is isolated in gaseous phase (212.8 eV) and when SiH4 molecule is absorbed on the Si surface (25.45 eV) (Figure 1.3). In every gas molecule, its bond energy and the energy of its electron to contribute to the bond must get lower when it is absorbed onto the solid surface, due to the effect of dielectric constant of the solid surface, than when it is isolated in gaseous phase. It is essential to reveal not only the behavior of molecules of specialty gases in gaseous phase, but also the behavior of those adsorbed on the substrate surface to scientifically understand various processes such as chemical vapor deposition (CVD) and reactive ion etching (RIE). This type of investigation is indispensable to develop a perfect computer simulation of the semiconductor-manufacturing process. The SiH4 molecule decomposition characteristics are plotted as a function of the tube reactor temperature, as shown in Figure 1.4, where the gas flow rate is maintained at 20 cm3/min and the inner surface of the tube reactor is covered by nondoped Si, pþ Si, hydrogen-terminated Si, and SiO2. If low-temperature SiH4 decomposition on the pþ Si surface is attributed to effective dielectric constant as speculated from Equation (1.7), the experimental results give the following values for

=

FIGURE 1.3 Surface reaction model of adsorbed SiH4 molecule and pþ Si surface, where the energy level of the electron in SiH4 is changed from 212.8 to 25.45 eV by receiving an influence of the Si dielectric constant.


8 150 SiH4 Concentration [ppm]

non-dope Si p+ Si

Si-H

SiO2

100

50

0 0

100

200

300

400

500

Temp [°C]

FIGURE 1.4 Temperature dependence of the SiH4 concentration in Ar gas at the outlet of the tube reactor of 1/2 in. in diameter and 40 cm in length where the gas flow rate is 20 cm3/min and the temperature is increased with 18C/min. The inner surface of the tube reactor is covered by nondoped Si, hydrogen-terminated Si, pþ Si, and SiO2.

1eff and g of Equation (1.7):

1þg 2 12:8 ¼ 5:45 ) 1eff ¼ 1:53, 1 þ g1Si

g ¼ 0:052

(1:12)

It is found that the molecule adsorbed on the Si surface is subjected to an effective dielectric constant of 1.53. Figure 1.5 and Figure 1.6 show the Arrhenius plot of the rate constant when Si2H6 and Si3H8 interact with the nondoped Si surface and the pþ Si surface. Concentration of both Si2H6 and Si3H8 in Ar gas is set at 100 ppm in these experiments. In the reaction with the nondoped Si surface around 4008C, as shown in Figure 1.7, the rate constant gets higher in the order of SiH4, Si2H6, and Si3H8. In the low-temperature decomposition triggered by interaction with the pþ Si surface, however, the rate constant and the activation energy of SiH4, Si2H6, and Si3H8 are almost the same. This indicates that a portion of SiH3 in Si2H6 and Si3H8 is absorbed on the Si surface, as shown in Figure 1.8. Interaction between various metal surfaces and SiH4 molecule is also studied. In this experiment, SiH4 at a concentration of 100 ppm is introduced at flow rate of 26.3 cm3/min into various metal tube reactors featuring a diameter of 1/4 in. and length of 1 m. Four different metal tube reactors such as 100% Ni, Hastelloy (Ni: 50%), SUS316L-EP (Electro-polished) (Ni: 12%), and Cr2O3-passivated stainless steel are used. Ar gas is used as a carrier gas. When SiH4 is introduced, the temperature of the tube reactors is raised at a rate of 0.338C/ min. Figure 1.9 shows the SiH4 concentration detected at an outlet of the tube reactors. The SiH4 decomposition characteristics of SiH4 totally depend on the metal surface which it interacts with, when the SiH4 concentration is set at the same level. The temperature to trigger SiH4 molecule decomposition gets lower as the mixing ratio of Ni is raised. This indicates that Ni features very strong catalytic effects toward SiH4 molecule decomposition. This is considered because the metal surface containing more Ni is harder to be oxidized even when it is exposed to the air.


9


200

100 p+ Si

10 0


non-dope Si

10

1 Ea=0.20 eV

0.1

Ea=2.15 eV

0 .0 1 1

1.5

2

2 .5

3

1000/T [1/K]

FIGURE 1.5 The temperature dependence of the rate constant of Si2H6 molecule decomposition at nondoped Si and pþ Si surface obtained in the same procedure as in Figure 1.2.

III. ELECTRONEGATIVITY AND ENERGY LEVEL: CHANGE IN STATE OF ELECTRONS ON SURFACE DUE TO TERMINATING ATOMS The concept of electronegativity is often used when atoms are studied in chemistry. Study of metal contamination on the Si surface in liquid solutions has revealed that those metal ions which feature higher electronegativity than Si, such as Cu, Pd, Hg, Ag, Pt, and Au directly take electrons from Si surface to chemically bind with Si surface, and that it is extremely hard to remove them [10,11]. In contrast, other metals which feature lower electronegativity than Si, such as Fe, Ni, Cr, Na, Ca, and K, do not directly bind with Si surface, but they are included in native oxide or chemical oxide formed on the Si surface because they are easier to be oxidized than Si. These metals featuring lower electronegativity than Si can be easily removed with the diluted HF (DHF) treatment for oxide removal [10,11]. CuCl2, FeCl2, and NiCl2 of 3 ppb each are injected into 0.5% HF for contaminating Si surface with Cu, Fe, and Ni. Cu features higher electronegativity than Si whereas Fe and Ni feature lower electronegativity than Si. This DHF solution of 25 cm3 each is spread on the entire surface of 6-in. Si wafer surface which is displaced a little bit to concave shape by vacuum chuck. Then the DHF solution is evaporated with a halogen lamp irradiation to observe segregation of Cu, Fe, and Ni to the bare Si surface. Figure 1.10 shows Cu, Fe, and Ni contamination concentration profile on the entire Si wafer surface, where zero on the horizontal axis stands for the center of the wafer. As the DHF solution gets evaporated, its residual amount decreases, and its droplet gets smaller in diameter during shrinking toward the center of the Si wafer. Eventually droplet of DHF remaining at the center of the Si wafer is evaporated, and the entire Si wafer surface gets dried. No oxide exists

10

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing Activation Energy T (°C) 400 300

200

100 p+ Si

10 0


non-dope Si

10

1 Ea=0.13 eV Ea=2.11 eV

0.1

0 .0 1 1

1.5

2

2 .5

3

1000/T [1/K]

FIGURE 1.6 Temperature dependence of rate constant of the Si3H8 molecule decomposition at nondoped Si and pþ Si surface obtained in the same procedure as in Figure 1.2.

300


300

200

*

Rate Constant, k [10-3/sec]

100

2.11 eV

* 50

* 2.0 eV

2.15 eV

Si3H8

* *

SiH4 10 5

1 1.3

Si2H6

1.4

1.5

1.6

1.7

1.8

103/ T [K]

FIGURE 1.7 Temperature dependence of rate constants of SiH4, Si2H6, and Si3H8 molecules decomposition interacting with the nondoped Si surface.


11

Si2H6

Si3H8

H H Si H H Si Si H H H H

H

H H Si Si H H H

H H

H

H

Si

Si H

H

Si

H

H

H

H

Si surface

Si surface

H

H

Si

Si H

H

FIGURE 1.8 Si2H6 and Si3H8 absorption model on the Si surface.

on the Si surface which is in contact with diluted HF, where hydrogen-terminated bare Si surface is maintained all the time. If metallic ions are more stable when they are absorbed on the bare Si surface than when they are dissolved in diluted HF, they must get absorbed onto the Si surface from the very beginning of this procedure. These metals must be found absorbed in an almost uniform way on the entire Si surface. In contrast, if they are more stable when they are dissolved in diluted HF, they must remain in DHF solution until the end of the evaporation process. In this case, they must be found absorbed around at the center of the Si wafer. Figure 1.10 shows that Cu featuring higher electronegativity than Si is absorbed almost on the entire Si wafer surface, and that Fe and Ni featuring lower electronegativity than Si are detected only around at the center of the Si wafer. When the same experiment is performed by using Si wafer surface covered with thermal oxide (SiO2), selective adsorption of metals is not detected. Unlike the experimental result on the bare Si surface, in this case, Cu as well as Fe and Ni are found adsorbed just at the center of the wafer.

120

SiH4 Concentration [ppm]

100 Cr2O3

80 60 Hastelloy-EP

40

Pure Ni SUS316L-EP

20 0 50

100

150

200

250

300

350

400

450

500

Temperature [°C]

FIGURE 1.9 Temperature dependence of SiH4 concentration in Ar at the outlet of the tube reactor having a diameter of 1/4 in. and a length of 1 m whose inner surface is pure Ni, electropolished hastelloy, electropolished SUS 316, and Cr2O3, where the gas flow rate is 26.3 cm3/min and the temperature of the tube reactor is raised to 0.338C/min.


12 1.E+15

Metal Adhesion (atm/cm2)

1.E+14

Bare Si(N) Cu 3ppb/DHF (0.5%) AIR

Cu

Bare Si(N) Fe 3ppb/DHF (0.5%) AIR

Bare Si(N) Ni 3ppb/DHF (0.5%) AIR

Ni

Fe

1.E+13

1.E+12

1.E+11

1.E+10 −10 0 10 20 30 40 50 60 70 −10 0 10 20 30 40 50 60 70 −10 0 10 20 30 40 50 60 70 Distance from Center (mm)

Distance from Center (mm)


FIGURE 1.10 Metallic impurities adhesion concentration profile to the bare Si[p-Si(1 0 0)] surface from contaminated DHF (0.5%) in air where the DHF of 25 cm3 contaminated with 3 ppb Cu, Fe, and Ni by CuCl2, FeCl2, and NiCl2 is spread on the entire 6-in. wafer and evaporated by a halogen lamp. The surface concentrations of metals are measured by total x-ray reflection fluorescence. The horizontal axis is the distance in radial direction from the wafers center and the vertical axis is the surface metallic concentration. Fe and Ni each adhere only to the center of the wafer where they especially do not adhere to the peripheral area of the wafer.

In another test, HCl:H2O2:H2O (1:1:6) (HPM) solution to form chemical oxide on the Si wafer surface is spread on the entire Si wafer surface so as to contaminate the Si surface with Cu, Fe, and Ni. As shown in Figure 1.11, these three metals are found distributed on the Si wafer surface in almost the same manner. On this contaminated wafer surface, when high-purity 0.5% HF solution (25 cm3 having no metallic contamination) is spread, and evaporated just in the same way as described earlier, a new pattern emerges. Figure 1.12 shows this pattern in which the metallic impurity concentration

1.E+15 Bare Si(N) Cu,Fe,Ni 3ppB/HPM (1:1:6) AIR

Bare Si(N) Cu,Fe,Ni 3ppb/HPM (1:1:6) AIR

Bare Si(N) Cu,Fe,Ni 3ppb/HPM (1:1:6) AIR


1.E+14

Cu

Fe

Ni

1.E+13

1.E+12

1.E+11




FIGURE 1.11 Metallic impurities adhesion to the bare Si[n-Si(100)] surface from HPM solution (HCl:H2O2:H2O ¼ 1:1:6) contaminated with 3 ppb Cu, Fe, and Ni in air. The adhesion profile on the entire wafer surface is almost similar for Cu, Fe and Ni. Fe, and Ni adhere even to the peripheral area in this case which are included in the chemical oxide formed in the HPM solution.


13

profile in radial direction pertains to the effect of evaporation of ultrapure DHF, whereas the dashed lines indicate initial Cu, Fe, and Ni contamination included in chemical oxide. It has been revealed from these results that Fe and Ni included in chemical oxide are removed with HF solution together with the chemical oxide, because they are detected only around at the center of the wafer surface. In contrast, concentration distribution of absorbed Cu remains unchanged. This indicates that metals featuring lower electronegativity than Si, even when they are included in oxide and remain on the Si surface, can be completely removed in the DHF treatment for oxide removal. It is also proved that metals featuring higher electronegativity than Si which form direct chemical bond with the Si surface cannot be removed in the DHF treatment. Cu included in oxide is dissolved once when the diluted HF solution etches the oxide, but it gets reabsorbed immediately from the DHF solution when the bare Si surface is exposed. These experiments demonstrate the following: .

.

Metallic atoms having higher electronegativity than Si, such as Cu, Hg, Pd, Ag, Pt, and Au, chemically bind with Si by directly taking electrons from Si. Metallic atoms having lower electronegativity than Si, such as Fe, Ni, Cr, Ca, K, and Na, are not directly absorbed onto the bare Si surface, but they are included in native oxide or chemical oxide formed on the Si surface as they are easier to be oxidized than Si.

Researchers and engineers whose background is either the semiconductor or the solid-state physics field are familiar with the concept of energy level. As shown in Table 1.2, the energy level of H and F elements is 25.11 and 29.42 eV, respectively. If the Si surface is terminated by these elements, a surface-band bending is generated. The surface-band bending patterns of hydrogen- and fluorine-terminated p-type Si are illustrated in Figure 1.13 and Figure 1.14. The energy level of H is almost the same as the energy at the top of the Si valence band, whereas the energy level of F is deeper than that of the top of the Si valence band by 4.27 eV. When H or F terminates the dangling bonds of the Si surface, the terminating, F element captures, many electrons in the valence band (contributing to the Si22Si covalent bonds) but the terminating-H does not capture them. In other words, electron transfer in the valence band is not triggered by

1.E+15

Cu


1.E+14

1.E+13

Bare Si(N) DHF (0.5%) After HPM (1:1:6) Cu/Fe/Ni 3ppb AIR

Fe Bare Si(N) DHF (0.5%) After HPM (1:1:6) Cu/Fe/Ni 3ppb AIR

Ni Bare Si(N) DHF (0.5%) After HPM (1:1:6) Cu/Fe/Ni 3ppb AIR

1.E+12

1.E+11




FIGURE 1.12 Adhesion profile of metallic impurities of Cu, Fe, and Ni before and after the pure DHF, is spread on the entire wafer surface and evaporated by halogen lamp irradiation, where the wafer surface is initially contaminated by HPM solution in Figure 1.11. Fe and Ni are removed by the DHF along with the oxide film removal but Cu cannot be removed by the DHF. The dashed line indicates an initial contamination profile and the solid line indicates ultrapure DHF-treated profile.

14

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing p-Si Vacuum Level

Conduction Band −4.05 eV H− H −5.15 eV

H H 5.11 eV

Valence Band

FIGURE 1.13 Band structure of hydrogen-terminated p-type Si surface.

hydrogen termination. On a surface where electrons exist in large volume in the conduction band, such as the nþ Si surface, the conduction band electrons make a shift toward terminating hydrogen. Figure 1.15(a) shows the hydrogen-terminated Si surface. It is clearly evident that features the same state of electrons as bulk Si crystal for valence electrons. In the case of the fluorine-terminated

p-Si Vacuum Level

Conduction Band −4.05 eV F− −5.15 eV Valence Band

F 9.42 eV

F

FIGURE 1.14 Band structure of fluorine-terminated p-type Si surface.


15

(a) H

H

H

Si

Si

Si

Si

-

-

-

F

-

-

-

F

Si

F

-

-

F

Si

Si

Si

Si

Si

-

F

H

Si

Si

(b)

H

H

F Si

Si

Si

Si

FIGURE 1.15 Electron state in the valence band of (a) hydrogen- and (b) fluorine-terminated Si(1 0 0) surfaces.

surface, as electrons are locally concentrated on terminating fluorine elements, the Si surface runs very much short of electrons (contributing to the covalent bond of Si crystal) as shown in Figure 1.15(b). This makes Si22Si covalent bonds in the vicinity of the surface very weak. The author next describes how this difference in state of electrons on the surface caused by terminating elements affects various surface reactions.

IV. OXIDATION OF Si SURFACE AT ROOM TEMPERATURE Even when 50 –200 Si wafers are treated altogether in a batch process to form thin oxide film having a thickness less than 3 nm, it is required to maintain uniformity of film thickness on the entire surface of every individual wafer as well as among all the wafers. To satisfy this requirement, it is essential to perform (i) wafer loading to oxidation furnace (to raise wafer temperature); (ii) wafer unloading (to decrease wafer temperature) in inert gas ambience, such as Ar or N2 gas, completely excluding O2 and H2O molecules; and (iii) to start the oxidation process by introducing O2 or H2O gas for a predetermined time maintaining a constant temperature. If the bare Si surface completely free from oxides is exposed to elevated temperature of over 6008C in Ar gas or other inert gases, however, hydrogens terminating the surface are removed and surface microroughness is extremely increased [12,13]. But once a monoatomic layer oxide is formed with O2 gas on the Si surface (for instance at 3008C at which terminating hydrogens are stable), increasing the wafer temperature even to as high as 9008C in the inert atmosphere does not produce any increase in microroughness. Further, extremely thin and uniform high-quality oxide layer can be


16

formed on all Si wafer surfaces [12]. Oxidation at 3008C allows the terminating hydrogen to exist stable, and oxidation proceeds as oxygen molecules penetrate into back bonds of Si. Monoatomiclayer oxide (0.4 nm in thickness) is formed on the hydrogen-terminated Si surface with 20-min oxidation in O2 gas ambience at 3008C whose thickness was evaluated by XPS [12,13] as shown in Figure 1.16. In contrast, when the Si surface is terminated with fluorine, valence bond electrons contributing to the covalent bond of Si crystal are taken from just around the surface by the terminating fluorine elements, and the binding state of the Si elements gets weakened. This should mean that oxygen atoms at surface can easily penetrate into back bonds of Si surfaces at lower temperatures. Figure 1.16 shows the time dependence of oxide film formation on the hydrogen-terminated Si surface at room temperature, 2008C, 3008C, and 4008C as well as on the fluorine-terminated Si surface at room temperature. It is seen in Figure 1.16 that the hydrogen-terminated Si surface is not oxidized even when it is exposed to the O2 gas ambience at room temperature for 104 min. The fluorine-terminated Si surface is easily oxidized in the O2 gas ambience at room temperature. This is because Si22Si covalent bond of the fluorine-terminated Si surface is weakened as shown in Figure 1.14 and Figure 1.15, which allows oxygen molecules to easily penetrate into back bonds of Si and to react with Si atoms even at room temperature. The fluorine-terminated surface is obtained by removing SiO2 film on the Si surface with anhydrous HF gas at room temperature [14], where the terminated fluorine is not removed even with thermal treatment at 9308C in an inert gas ambience.

10 25°C 25°C

5

200°C 300°C

400°C

H terminated

F terminated

3

Oxide thickness (nm)

2

1

0.5

H terminated

0.3

F terminated 0.2

0.1 2

5

10

20

50

100

200

Time (min)

FIGURE 1.16 Oxide film thickness as a function of treatment time in dry O2 for various temperatures for hydrogen- and fluorine-terminated Si(1 0 0) surfaces.


17

V. SELECTIVE W FILM DEPOSITION BY MEANS OF (SiH4 1 WF6) The tungsten (W) film deposition by means of SiH4 molecular reduction of WF6 molecules can be selectively performed only on the Si surface or a metal surface at low temperature of 100 – 2008C. What is the mechanism of this selective film deposition? Let us discuss the mechanism of the W film deposition on the hydrogen-terminated Si surface. As described in Section II, SiH4 molecules are not decomposed at these temperatures on the hydrogen-terminated Si surfaces. As SiH4 molecules are not decomposed at low temperature of 100 –2008C, W film deposition which is actually observed must be attributed to decomposition of WF6 molecules triggered by their interaction with the hydrogen-terminated Si surface. WF6 molecules have been demonstrated to decompose due to interaction with the hydrogen-terminated Si surface even at room temperature although they feature very high bond energy. When WF6 molecules are introduced into the tube reactor at room temperature whose inner surface is covered by nondoped hydrogen-terminated Si surface, WF6 molecules are not detected at the outlet of this reactor, but SiF4 molecules and SiHF3 molecules are detected as shown in Figure 1.17. This means that WF6 molecules are completely decomposed even at room temperature on the hydrogen-terminated Si surface [15]. WF6 molecules start to be detected at the outlet of the tube reactor only when W film of about 50 atomic layers is deposited on the hydrogen-terminated Si surface. This means that WF6 molecules also get decomposed on the W surface which is terminated with a mixture of hydrogen and fluorine, and that WF6 molecular decomposition would stop when the W film surface is completely covered by fluorine elements. These results clearly indicate that fluorinated molecules such as WF6 can easily react with hydrogen radicals such as surface-terminating hydrogens on a Si surface. It has been found that SiH4 molecules are decomposed even around room temperature on the fluorine-terminated W surface [16]. As shown in Figure 1.18, SiH4 molecules are completely decomposed on the fluorine-terminated W surface, where the reaction product is SiF4. The W film deposited by means of H2 reduction reaction of WF6 at the Si surface was not adopted in actual manufacturing processes due to the encroachment difficulty. This method of W film deposition process was found to consume Si at the surface at random, that is, there is an origin of the appearance of encroachment. When WF6 and SiH4 are used together [17,18], however, Si in the substrate surface is only consumed until the first W monoatomic layer is deposited. Afterward, Si in SiH4 is consumed in the reaction. Equation (1.13) and Equation (1.14) express the basic

Absorbance

2316 SiHF3

2188 SiH4

1029 SiF4

992 SiHF3

712 WF6

50 0.05

0

WF6 Concentration (ppm)

0.1

0 0

100

200

300

400

Time (min)

FIGURE 1.17 Time variation of WF6, SiF4, and SiHF3 concentration at the outlet of the tube reactor having a diameter of 1/2 in. and a length of 45 cm at room temperature whose inner surface is covered by hydrogenterminated nondoped Si. The flow rate of Ar including 100 ppm WF6 is 10 cm3/min. WF6 molecules are confirmed to completely decompose at room temperature by interacting with the hydrogen-terminated Si surface, whereas WF6 molecules are detected at the outlet of the tube reactor after about 50 atomic layers deposition of tungsten on the Si surface.

0.1

2316 2188 SiHF3 SiH4

1029 992 SiF4 SiHF3

712 WF6

Absorbance

200 0.05 100

0

0

100

200

300

400

500

600

SiH4 Concentration (ppm)


18

0

Time (min)

FIGURE 1.18 Time variation of SiH4 and SiF4 concentration at the outlet of the tube reactor whose inner surface is covered by fluorine-terminated tungsten. The flow rate of Ar, including 100 ppm SiH4 is 100 cm3/min. SiH4 molecules are confirmed to completely decompose at room temperature by interacting with fluorine-terminated tungsten surface. SiH4 react with fluorine-terminated tungsten surface completely at room temperature. So intermediates such as SiHF3, SiHF3 and WE6 are not detected.

reaction mechanism: 3 3 WF6 þ SiH4 ) W þ SiF4 þ 3H2 2 2

(1:13)

WF6 þ 2SiH4 ) W þ 2SiHF3 þ 3H2

(1:14)

SiHF3 is sometimes included in reaction byproduct. In this case, however, SiHF3 is decomposed on SiO2 to induce W deposition, which puts an end to selective film deposition on the Si and metal surfaces. It is not clearly understood why the fluorine-terminated surface decomposes SiH4 at room temperature. Current speculation on this issue is as follows: Regardless of the underlying substrate material, a fluorine atom terminating the surface attracts a large number of electrons due to its high electronegativity. Its surface status, therefore, is just like an oxide-free platinum surface having a strong catalytic behavior. Existence of a large number of electrons induces a shielding effect to make Coulomb force of adsorbed molecules extremely low, which makes it possible to decompose adsorbed molecules by means of thermal energy at room temperature. Figure 1.19 shows the decomposition and dissociation process of adsorbed molecules, which is induced by catalytic effects of fluorine-terminated surface and the metal surface to weaken the bond energy of molecules. The mirror image of positive and negative charges in adsorbed molecules is generated on the surface, which makes a part of electric force lines in the molecules to be terminated on the surface. This is how the bond energy of adsorbed molecules on the metal surface or on the fluorine-terminated surface is weakened.

VI. REDOX POTENTIAL AND ENERGY LEVEL OF LIQUID SOLUTION — NATIVE OXIDE FORMATION ON THE Si SURFACE AND METAL CONTAMINANT ADHESION AND REMOVAL WITH SOLUTION The redox (reduction and oxidation) potential is a well-known parameter in the field of solution chemistry. A redox potential of 0 V is defined as the potential of the reduction –oxidation reaction of a H2 molecule in solution. Solutions which feature stronger oxidizing force are expressed with positive redox potential, whereas those which feature stronger reducing forces are expressed with


19

(a)

A

B

A

B

(b)

Metal FIGURE 1.19 Catalytic effect of metal surface and fluorine-terminated surface for adsorbed molecules (AB). Electric force line distribution in a molecule (AB) in gaseous phase (a) and adsorbed on metal or fluorineterminated surface (b).

negative redox potentials. The “redox potential – pH diagram” is popular in physical inorganic chemistry, particularly in the field of metal surface corrosion, where this diagram has been used to study two-phase systems of metals and water solutions in detail. The redox potential –pH diagram is effective in investigating metallic contamination on the Si surface. The diagram cannot be applied as it is, because this investigation deals with the three-phase system of metal, water solution, and the Si surface. Figure 1.20 shows the redox potential –pH diagram for Cu. In acid solutions featuring pH 0– 6 and redox potential of over þ0.25 V, Cu is dissolved in an ionic form such as Cu2þ. When the Si surface coexists, however, Cu is not dissolved in solutions in an ionic form but adsorbed on the Si surface unless the redox potential is higher than a certain level. To be more specific, as shown in Figure 1.21, the redox potential needs to be higher than þ0.85 V in HF solution and higher than þ0.75 V in other acid solutions to keep Cu dissolved in an ionic form [19,20]. As no oxide is formed on the Si surface in HF solution, the redox potential should be kept higher by 0.1 V than the other acid solutions where the Si surface is covered by chemical oxides. In other words, Cu adsorption is primarily suppressed due to oxidation of the Si surface in the other acid solutions [19,20]. It has been recently revealed that a redox potential of 0 V is equivalent to an energy level of 24.44 eV [21]. A redox potential of þ0.75 V is equivalent to an energy level of 25.19 eV, which is equal to the energy level at the top of Si valence band. This triggers oxidation of the Si surface as solutions take electrons contributing to Si covalent bond from the Si surface. The solution featuring the redox potential of E features the energy level (1) given by the following equation: 1 ¼ (4:44 þ E) (eV)

(1:15)

which can be considered as the Fermi level of solution. Figure 1.22 shows “energy level – pH diagram,” where the energy level of various cleaning solutions and ultrapure water is plotted. Dotted horizontal lines stand for an energy level at the bottom of the Si conduction band and an energy level at the top of the Si valence band. It is seen from Figure 1.22 that the energy level of acid solutions featuring a E higher than þ0.75 V

20


FIGURE 1.20 Redox potential – pH diagram for Cu. Cu is dissolved as a Cu2þ ion into the solution having a redox potential larger than þ0.25 V and a pH less than 6. Cu, 1 ppm in water.

is deeper than that at the top of the Si valence band. Electrons in the valence band contributing to the Si covalent bonds, therefore, are taken by these acid solutions, that is, the Si22Si bond at the surface gets weakened, and the Si surface is easily oxidized. This is how the chemical oxide is rapidly formed in acid solutions having a redox potential larger than þ0.75 V and relative dielectric constant of 81. In ultrapure water which comes in contact with the air and contains dissolved oxygen with its saturation solubility of 8.4 ppm, where its redox potential is þ0.6 V, chemical oxide is gradually formed even at room temperature. In ultrapure water from which dissolved oxygen is removed, where its redox potential is þ0.4 V, in contrast, chemical oxide is hard to be formed [22]. These facts have something to do with the oxidizing force (force to take electrons) of these solutions against the Si surface. It is revealed in Figure 1.22 that the energy level of ultrapure water containing dissolved oxygen of 8.4 and of 0 ppm is 25.14 eV, same is the status of valence band energy level with 24.84 eV, higher than the valence band energy. The redox potential of the ultrapure water including four different dissolved oxygen concentrations such as 0.01, 3, 10, and 18 mg/l is plotted as a function of dissolved hydrogen concentrations as shown in Figure 1.23. It is seen from the curve patterns of Figure 1.23 that chemical characteristics of the ultrapure water, namely, the redox potential, are strongly controlled by dissolved oxygen concentration and dissolved hydrogen concentration with ppm order concentrations. The redox potential gradually decreases down to less than þ200 mV with an increase of dissolved hydrogen concentration and suddenly changes down to around 2400 mV at the dissolved hydrogen concentration of equimolar concentration of dissolved oxygen. The redox potential of the ultrapure water maintains at a level of 2400 mV for dissolved hydrogen concentration greater than equimolar concentration of dissolved oxygen. The activation energy of normal high-temperature oxidation by means of dry oxygen is around 2.0 eV. From extrapolation on the basis of this activation energy, room-temperature oxidation can


21

2.5

Cu Deposition (atoms/cm2)

>1E+14 1E+13~14

2

1E+12~13

1E+11~12

1E+11>

1.5

E (V vs. NHE)

HCuO2−

Cu(OH)2

Cu2+

−

1

0.5

CuO22 Cu-Si-Wafer Cu Cu O 2

Cu-Water

0

−0.5

Cu

−1

−1.5

0

2

4

6

8

10

12

14

pH

FIGURE 1.21 Redox potential– pH diagram for Cu when the Si surface is introduced. The critical potential for Cu dissolution into acid solution as Cu2þ ion is shifted to þ0.75 from þ0.25 V due to the existence of the Si surface. Cu, 1 ppm (using Cucl2), dipping time 3 min.

form oxide film, less than 10215 nm in thickness in a few hours. Growth of chemical or native oxide features a totally different mechanism from that of high-temperature oxidation. The Si surface is oxidized around 0.5 nm in thickness at room temperature in the air [22]. This native oxide growth features the following mechanism: When being exposed to air, the Si surface is covered with moisture molecules of several tens of molecule layers due to the moisture contained in the air [23]. This means that the Si surface is just in the same state as contacting with water having dissolved oxygen of 8.4 ppm. Owing to the very high relative dielectric constant of water of 81, Si22Si bonds on the surface get weakened. Besides, electrons contributing to the covalent bond are taken away by the absorbed water layer having a redox potential of þ600 mV, which further weakens Si22Si bonds on the surface. So, oxygen can easily penetrate into the back bond of Si. This is how room-temperature oxidation of Si surface proceeds in the air. In the Si technology, metallic contaminations such as Cu on the Si surface have been removed by SPM (H2SO4/H2O, 4:1, 120 – 1508C). This is accompanied with too much chemical vapor generation and too much liquid chemical consumption. This high-concentration chemical solution has been proved to be required to obtain a higher redox potential or lower energy level to take electrons from the surface-contaminated metals to dissolve them into the solution as metal ions. Ozonated ultrapure water having a ppm level of O3 concentration has been confirmed to exhibit very high redox potentials such as þ1.30 V, which corresponds to the energy level of 25.74 eV, as shown in Figure 1.22. Thus, ozonated ultrapure water has been speculated to have metal contamination removal capability from the Si surface instead of the SPM solution. This speculation has been demonstrated experimentally as shown in Figure 1.24, where an initial Cu contamination of up


22

FIGURE 1.22 Energy level 1 of solution converted from redox potential PRED by 1 ¼ 2(PRED þ 4.44) (eV) is plotted as a function of pH. The horizontal dashed line is the conduction band energy (24.05 eV) and the valence band energy (25.15 eV) of Si. The energy level of the acid solution is deeper than the valence band energy.

Oxidation-Reduction Potential (mV vs NHE)

to 1015 atoms/cm2 is reduced down to the order of 1010 atoms/cm2 by ozonated ultrapure water cleaning and sulfuric acid-hydrogen peroxide mixture (SPM) (H2SO4/H2O2) cleaning. The remaining Cu of 1010 atoms/cm2 is included in the chemical oxide formed in ozonated ultrapure and SPM solution, so that this remaining Cu contamination is completely removed by fluoric acid-hydrogen 700 600 500 400 300 200 100 0 −100 −200 −300 −400 −500

0.01mg/L=0.31x10−6 mol/L 3mg/L=0.09x10−3mol/L 10mg/L=0.31x10−3mol/L 18mg/L=0.56x10−3mol/L

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 (× 10−3mol/L)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 (mg/L)

Dissolved Hydrogen Concentration

FIGURE 1.23 Redox potential of ultrapure water having four different dissolved oxygen concentrations are plotted as a function of dissolved hydrogen concentration.


UPW

23

UPW

UPW

UPW

FIGURE 1.24 Cu cleaning capability of ozonated ultrapure water from the Si surface is illustrated as well as the SPM (H2SO4/H2O2) cleaning capability. Initial Cu contamination (1015 atoms/cm2) is decreased to 1010 atoms/cm2 by ozonated ultrapure water and SPM cleaning. The remaining 1010 atoms/cm2 Cu contamination is included in chemical oxide, which can be removed by FPM (HF/H2O2/H2O) cleaning.

peroxide mixture (FPM) cleaning (HF/H2O2/H2O, 0.05 –0.5%:0.1– 1%, room temperature) as shown in Figure 1.24. It is very important to note that ozonated HF solution cannot remove Cu contamination from the Si surface. Figure 1.24 indicates the Cu-cleaning capability of ozonated ultrapure water having a pH value of 6.5 for just contaminated Cu on Si wafer surfaces form Cucontaminated DHF solutions. Ozonated ultrapure water gradually looses its Cu-cleaning capability with an increase of exposure times of Cu-contaminated Si wafer surfaces to the clean room air as shown in Figure 1.25, where Cu-contaminated Si wafer surfaces have been exposed to the clean room air for 2 days. Cu removal capability of ozonated ultrapure water has been confirmed by controlling pH value down to 4.0 whose redox potential is shown in Figure 1.26.

VII. NEW ERA OF Si TECHNOLOGY — (1 1 0) Si SURFACE The author is now developing a very new semiconductor-manufacturing technologies, that is, radical-reaction-based semiconductor manufacturing by using microwave-excited high-density plasma with very low electron temperatures instead of current molecule-reaction-based semiconductor manufacturing [25 –32]. The newly developed microwave-excited high-density plasma equipment is completely free from surface damages due to high-energy ion bombardment, charge-up damages, and metallic contaminations coming from high-energy ion bombardment onto the inner surface of the process chamber. These characteristics have been obtained by two remarkable features of this plasma equipment: (i) very low electron energy around 1.0 eV or less and (ii) complete separation of plasma excitation region and process region. In this new plasma equipment, silicon wafers are set in the diffusion plasma region where the ion current and the electron current flowing onto the silicon wafer surface compensate with each other at an instant, so that electric charges do not remain on the silicon wafer surface in principle even if the plasma is turned off at any time. Thus, the newly developed high-density plasma equipment can be applied to transistor fabrications, whereas current plasma equipment is not used for transistor fabrications but is used only for interconnect fabrications.


Cu Concentration (atoms/cm2)

24

O3-UPW Cleaning conditions 1. O3 : 5 mg/I 2. Batch Cleaning : 5 mg/I 3. Exposing time : 2 days

1.30V (vs. NHE)

1015 1014 1013

1.36V (vs. NHE)

1012

1.42V (vs. NHE)

1011 1010 109

as-contamination

pH 5.0

pH 6.5 UPW only

pH 4.0

CO2 concentration Ozonated Ultrapure Water

FIGURE 1.25 Cu removal capability of ozonated ultrapure water having different pH value such as 6.5, 5.0, and 4.0 by injecting CO2 gases for Cu-contaminated Si wafers after two days exposure to the clean room air.

1mg/I Cu in Water 2.5 Dissolution

Dissolution

2 CO2 + O3-UPW and HCI + O3-UPW with pH value of 5

−

HCuO2

1.5 Cu(OH)2

CO2 + O3-UPW and HCI + O3-UPW

E (V vs. NHE)

1

O3-UPW only

−

with pH value of 4

Cu2+

0.5

CuO22

O2 + 4H + + 4e − 2H 2O

UPW only

Cu O 2

0

2H +

−0.5

+ 2e − H2

Cu

−1

−1.5 0

2

4

6

8

10

12

14

pH

FIGURE 1.26 Redox potential of ozonated ultrapure water is plotted as a function of pH value.


25

SiO2 films are essential key materials for large-scale integration (LSI) devices, where SiO2 films are currently fabricated by thermal oxidations using O2 on H2O molecules. Thermal oxidations require very high temperature treatment from 8008C to 11008C. High-integrity SiO2 films have been confirmed to be obtained only on Si (1 0 0) surfaces by the thermal-oxidation-based semiconductor-manufacturing technologies. Thus, we can produce LSI devices only on Si (1 0 0) surfaces, where transistor structures are limited to two-dimensional planar structures. The newly developed microwave-excited high-density plasma with very low electron temperatures has been confirmed to generate various radicals such as oxygen radicals O by Kr/O2 and NHþ radicals by Xe/NH3 very effectively. Generated O and NH radicals have been proved to produce very-high-integrity SiO2 and Si3N4 films, respectively, not only on Si (1 0 0) surfaces at very low temperatures of 200 –6008C but also on any crystal orientation Si surface. Leakage current densities through these radical-reaction-based SiO2 and Si3N4 films obtained at 4008C and molecule-reaction-based thermal oxide SiO2 obtained at 9008C for the voltage application of 1 V across these films are plotted as a function of equivalent oxide thickness (EOT) in Figure 1.27. It has been confirmed from Figure 1.27 that the leakage current through radicalreaction-based SiO2 and Si3N4 films is smaller than that through molecule-reaction-based thermal oxide film by three orders of magnitude. To obtain these very-high-integrity radicalreaction-based insulator films, it is very crucial to remove terminated hydrogen atoms from base Si surface by very-low-energy Krþ and Xeþ ion bombardment just before starting the radical reaction oxidation and nitridation. Figure 1.28 indicates interface-state density for radical-reaction-based SiO2 film and moleculereaction-based thermal oxide film fabricated on Si (1 0 0), (1 1 0), and (1 1 1) surfaces. The interface-state density of radical-reaction-based SiO2 films obtained at 4008C is maintained at just around 1 1010 cm22 for all Si crystal orientation surfaces, whereas the interface-state density rapidly increases to different surfaces such as (1 1 0) and (1 1 1) from (1 0 0) surface. In Figure 1.29, the oxide film thickness is plotted as a function of the oxidation time for radicalreaction-based oxidation at 4008C and molecule-reaction-based oxidation at 9008C on four different Si surfaces such as single-crystal (1 0 0), (1 1 0), (1 1 1) surfaces and highly doped nþ polycrystalline

FIGURE 1.27 Leakage current densities through the radical-reaction-based SiO2 and Si3N4 films obtained at 4008C, and the molecule-reaction-based thermal oxide films obtained at 9008C vs. (equivalent oxide thickness EOT) where 1 V is applied across these insulator films.

26


Dit at midgap (eV−1cm−2)

Interface Trap Density 1012 1011

AI Gate MOS Tox=10nm

1010 109 (100) (110) (111) (100) (110) (111) O* oxidation (400°C)

Dry oxidation (conventional)

Kr/O2 Plasma (Radical Oxidation)

Dry 900°C (Molecular Oxidation)

FIGURE 1.28 Interface-state densities of the radical-reaction-based SiO2 films obtained at 4008C and the molecule-reaction-based SiO2 films obtained at 9008C for three different crystal orientation surfaces such as (1 0 0), (1 1 0), and (1 1 1).

Si surface. It is clearly seen from Figure 1.29 that the oxidation speed is maintained at the same level for these four different Si surfaces in the radical-reaction-based oxidation, whereas the oxidation speed is completely different for single-crystal (1 0 0), (1 1 0), and (1 1 1) surfaces, and highly doped nþ polycrystalline surface. These results clearly indicate that identical samequality SiO2 films have been fabricated by the radical-reaction-based oxidation even for the different Si surfaces whereas completely oxide films of different quality have been fabricated by the molecule-reaction-based high-temperature thermal oxidation for the different Si surfaces.

FIGURE 1.29 Oxidation speeds of the radical-reaction-based oxidation at 4008C and the molecule-reactionbased oxidation at 9008C for four different Si surfaces such as single-crystal (1 0 0), (1 1 0), (1 1 1) and highly doped nþ polycrystalline Si surface.


27

The radical-reaction-based SiO2 and Si3N4 films have been confirmed to decrease flicker noise on 1/f noise component by at least one order of magnitude compared with the current moleculereaction-based thermal oxide films. The radical-reaction-based gate insulator films have been proved very crucial for coming LSI device manufacturing. New era of Si technologies are going to start right now by introducing the radical-reactionbased semiconductor-manufacturing technologies, when three-dimensional transistors will become available on any crystal orientation Si surface instead of two-dimensional planar structure transistors on (1 0 0) Si surface. Currently, most of LSI devices consist of Complementary Metal Oxide Semiconductor (CMOS) circuit to establish very low power consumption operation where nMOS and pMOS transistors are connected in series manner. The current drivability of pMOS transistor is smaller than that of nMOS transistor by a factor of about 1/3 on (1 0 0) Si surface, so that the size on the gate width of pMOS transistor must be designed larger than that of nMOS transistor by a factor of about 3 in CMOS circuit configuration (1 0 0) Si surface, that is, unbalanced CMOS. This poor current drivability of pMOS limits performances of the CMOS circuits such as highspeed operation, wide dynamic range analog operation, and so on. Current–voltage characteristics of pMOS having the same size fabricated on (1 0 0) and (1 1 0) Si surfaces are shown in Figure 1.30. From Figure 1.30, an introduction of (1 1 0) surface has been confirmed to improve the current drivability of pMOS by a factor of about 3, that is, completely similar current drivability of nMOS on (1 0 0) Si surface, resulting in a balanced CMOS circuit configuration. The current drivability of MOS transistors is very sensitive to the surface microroughness particularly on (1 1 0) Si surface, so that it is very essential to establish very smooth (1 1 0) Si surfaces having an atomic order flatness [33,34]. The surface microroughness of Si wafer surfaces strongly depends on various wet chemical processing including ultrapure water treatment [24,35]. Particularly, the surface microroughness has been confirmed to rapidly increase with the ultrapure water treatment as shown in Figure 1.31, wherein AFM images of (1 1 0) silicon surfaces are shown for 1-h and 12-h treatment in the ultrapure water having dissolved oxygen less than 1 ppb and whose redox potential is þ400 mV (i.e., 24.84 eV in energy levels). The average surface microroughness increases up to 0.30 and 1.53 nm from the initial value of 0.14 nm by these ultrapure water treatments, whereas the peak to valley of the microroughness increases up to 3.85 and 10.8 nm from the initial value of 1.64 nm. pMOS (110) 10

8

8

6 4 2

ID/COX [102A/F]

ID/COX [102A/F]

pMOS (100) 10

VG−Vth=−2.5V VG−Vth=−2.0V VG−Vth=−1.5V VG−Vth=−1.0V VG−Vth=−0.5V

VG−Vth=−2.5V VG−Vth=−2.0V

6 3

4

VG−Vth=−1.5V VG−Vth=−1.0V

2

VG−Vth=−0.5V 0 0

−1

−2 VDS [V]

−3

0 0

−1

−2 VDS [V] TOX=4.5 nm

−3

L=100um, W=100 um P-channel MOSFET

Clock rate of LSI is improved by a factor of three merely by building a transistor on (110) surface.

FIGURE 1.30 Current – voltage characteristics of pMOS transistors having a channel length of 100 mm, a gate width of 100 mm and a gate oxide thickness of 4.5 nm fabricated on (1 0 0) and (1 1 0) Si surfaces.

28

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing Before dipping

Ra: 0.14 nm Rmax: 1.64 nm

1 hour

12 hours

Ra: 0.30 nm Rmax: 3.85 nm Recipe; cleaning

Ra: 1.53 nm Rmax: 10.8 nm

dipping into solution

FIGURE 1.31 AFM images of p(1 1 0) Si wafer surface after treatment with ultrapure water for 1 and 12 h in nitrogen atmosphere.

To suppress the increase of the surface microroughness of (1 1 0) Si surface, it is essentially required to introduce a new ultrapure water treatment technology, that is, an iso-propylalcohol (2-propylalcohol)-added ultrapure water treatment without dissolved oxygen (IPA-UPW treatment, ultrapure water to which IPA is added at 30 wt%). In Figure 1.33, the average surface microroughness of (1 0 0) and (1 1 0) Si surfaces is shown after 24-h ultrapure water treatment having two different dissolved oxygen concentrations of less than 1 ppb and 8.4 ppm, that is, an equilibrium oxygen concentration of ultrapure water contacting the air. This surface microroughness comes from the dissolution of Si surface into the ultrapure water as shown in Figure 1.32, where the dissolved Si atom volume is plotted for (1 0 0) and (1 1 0) Si surfaces after 24-h treatment in various ultrapure water including dissolved oxygen concentrations of 0 ppm, 8.4, and 42 ppm. Si surface dissolutions into the ultrapure water are very different between (1 0 0) and (1 1 0) Si surfaces. The number of dissolved Si atomic layers is plotted in Figure 1.32. The redox potential of the ultrapure water including dissolved hydrogen of 1.6 ppm without dissolved oxygen is 2400 mV, that is, corresponding to 24.04 eV in the energy level from Figure 1.22. It has been confirmed from Figure 1.32 that hydrogenated ultrapure water does not suppress the Si surface dissolution, resulting in an increase of the surface microroughness. The average surface microroughness is simultaneously plotted for 24-h treatment of (1 0 0) and (1 1 0) Si surfaces in 30% IPA-UPW in Figure 1.33. It is seen from Figure 1.33 that the average surface microroughness of (1 1 0) Si surface decrease from 4.8 to 2.5 nm by 24-h ultrapure water treatment with an increase of dissolved oxygen concentration from 0 to 8.4 ppm, that is, the redox potential of þ400 and þ600 mV. The surface microroughness of (1 0 0) Si surface also decreases from 0.56 to 0.29 nm with this increase of dissolved oxygen concentration. It is very important to note from Figure 1.33 that the 30% IPA-added ultrapure water treatment maintains the surface microroughness at its initial level even after 24-h treatment. Si surface dissolutions of (1 0 0) and (1 1 0) Si surfaces into the IPA-added ultrapure water are plotted as a function of IPA concentrations after 24-h treatment in Figure 1.34.


29

FIGURE 1.32 The dissolved gas (oxygen or hydrogen) and oxygen concentration dependency of dissolved Si amount for 24 h treatment.

Figure 1.35 and Figure 1.36 show that the Si surface dissolutions drastically decreased in IPAadded ultrapure waters with an increase of IPA concentration up to 30%. AFM images of (1 0 0) and (1 1 0) Si surfaces after 24-h treatment are shown in Figure 1.35 and Figure 1.36. Addition of 30% IPA to the ultrapure water has been confirmed to maintain the Si surface smoothness at its initial level even for 24-h treatment. Figure 1.37 shows the molecular structure of IPA, where an a-site hydrogen is easily dissociated even at room temperature in the ultrapure water having very high dielectric constant. Huge amount of hydrogen radicals generated are immediately combined with OH† radicals that

FIGURE 1.33 Effect of IPA addition for the suppression of surface microroughness increase of p(1 1 0) Si wafer during 24 h treatment in different dissolved oxygen conditions which is less than 1 ppb and 8.4 ppm.

30


FIGURE 1.34 IPA concentration dependence of dissolved p(1 1 0) and p(1 0 0) Si amount for 24 h treatment in nitrogen atmosphere.

are generated in the ultrapure water. This drastic reduction of OH† radicals in the ultrapure water has been understood to decrease the Si surface dissolution and to maintain the Si surface smoothness. Thus, the new wet cleaning processes of the (1 1 0) Si surface and Si surfaces for the threedimensional structure transistor must be replaced by the cleaning steps shown in Figure 1.38. IPA molecules chemically absorbed on bare Si surface cannot be removed by high-temperature annealing such as 10008C [36], but can be easily removed by very low energy Kþ ion or Xeþ ion bombardments onto the bare Si surface before starting O† radical oxidation or NH† radical nitridation.

FIGURE 1.35 AFM images of p(1 0 0) Si wafer surface after treatment with ultrapure water or IPA-added ultrapure water (30%) for 24 h in nitrogen atmosphere.


31

FIGURE 1.36 AFM images of p(1 1 0) Si wafer surface after treatment with ultrapure water or IPA-added ultrapure water (30%) for 24 hr in nitrogen atmosphere.

Our introduction of IPA-added ultrapure water cleaning has been confirmed not to degrade the very thin gate insulation integrity.

VIII. CONCLUSION This chapter discusses the energy of a solid, electronegativity of surface-terminating atoms, and redox potential of solutions in an integrated way in relation with dielectric constants of relating materials to reveal the possibility of electron transfer through the surface with absorbed molecules. Now that technologies have progressed highly enough to realize a surface completely free from contamination both in gaseous and liquid phases, this type of integrated approach makes sufficient sense. This study presents a way to discuss issues of the surface exposed to gaseous and liquid phases in the context of two factors: (1) dielectric constant of the solid itself and of the media contacting the surface and (2) transfer of electrons contributing to the Si22Si covalent bond to the energy level of atoms terminating the surface, the surface-absorbed molecules, and the redox potential (energy level, Fermi level) of solutions contacting the surface.

α -site hydrogen

hydrogen

oxygen

carbon

FIGURE 1.37 Structure of IPA and the position of a-site hydrogen.

32


pH controlled O3-UPW Removal of Organic Carbon and Metal FPM (HF/H2O2) / Surfactant+H2+Megasonic Removal of Particle and Metal Removal of Chemical Oxide pH controlled O3-UPW +Megasonic Shower Cleaning Removal of Surfactant FPM (HF/H2O2/H2O) Removal of Chemical Oxide H-Termination UPW/IPA+Megasonic Rinse

FIGURE 1.38 The procedure of new wet cleaning process.

The author has also made it possible to understand an interaction between molecules and the solid surface by considering the change of bond energy of molecules and the energy level of electrons in molecules absorbed on the solid surface. Ozonated ultrapure water features a redox potential of þ1.30 V, that is, an energy level of 25.74 eV. This means that the ozonated ultrapure water including a few ppm to several tenth ppm order O3 concentrations features a stronger oxidizing force than that of H2SO4, HNO3, and HCl, all of which feature a redox potential of about 1.0 V. Ozonated ultrapure water with pH values of 4.0 – 5.0 cannot only remove organic impurities in a short time, but also easily dissolve and remove metals featuring higher electronegativity than Si from the Si surface such as Cu. The wet cleaning of the Si surface will face a drastic change [24]. By making these phenomena much clearer, it will be possible to simulate the behavior of the semiconductor surface in gaseous and liquid phases. Eventually extremely effective semiconductor production will be realized by using a virtual factory. The Si technology and the semiconductor technology are now changing and progressing to a new scientific stage wherein semiconductor-manufacturing technologies are provided with a scientific understanding of mechanisms of all processes. By introducing the newly developed radical-reaction-based semiconductor-manufacturing technologies, most disadvantages of the current molecule-reaction-based semiconductor-manufacturing have been overcome such as low-temperature transistor fabrications around 400–6008C (1000– 12008C at present), very-high-integrity gate insulators Si3N4 having very low leakage current by a factor of three orders of magnitude, very small flicker noise component by a factor of two orders of magnitude, and very high current drivability transistors by a factor of around four. At present, LSI device fabrications are limited only on (1 0 0) Si surfaces where high-integrity SiO2 films are obtained by thermal oxidations. But LSI device fabrications become available on any crystal orientation Si surface such as (1 1 0) Si surface by the radical-reaction-based semiconductor manufacturing, where the current drivability of pMOS transistors is enhanced by a factor of about 3 compared with that of pMOS transistors on (1 0 0) Si surface. The behavior of (1 1 0) Si surface in the ultrapure water is completely different from that of (1 0 0) Si surface, so that the very new wet chemical processings must be introduced to the (1 1 0) Si surface LSI manufacturing or the three-dimensional structure transistor integrations.


33

REFERENCES 1. N.D. Lang and W. Kohn, Phys. Rev. B 3 (1971) 1215. 2. H. Kikuyama, M. Waki, I. Kawanabe, M. Miyashita, T. Yabune, N. Miki, J. Takano, and T. Ohmi, J. Electrochem. Soc. 139 (1992) 2239. 3. H. Kikuyama, M. Waki, M. Miyashita, T. Yabune, N. Miki, J. Takano, and T. Ohmi, J. Electrochem. Soc. 141 (1994) 366. 4. T. Ohmi, M. Nakamura, A. Ohki, K. Kawada, K. Hirao, and T. Watanabe, in: 23rd Annual Meeting of the Fine Particle Society, Proceeding Abstracts, Las Vegas, Vol. 2, 4th revised ed., 1992, p. 103. 5. T. Watanabe, M. Nakamura, A. Ohki, K. Kawada, S. Miyoshi, S. Takahashi, M.S.K. Chen, and T. Ohmi, In: 1992 International Conference on Solid State Devices and Materials, Extended Abstracts, 1992, p. 132. 6. T. Ohmi, M. Nakamura, and A. Ohki, J. Electrochem. Soc. 139 (1992) 2654. 7. Y. Shirai, M. Narazaki, and T. Ohmi, IEICE Trans. Electron. E79-C (1996) 385. 8. S. Takahashi, T. Watanabe, S. Miyoshi, A. Ohki, K. Kawada, M. Nakamura, M.S.K. Chen, and T. Ohmi, in: 182nd Electrochemical Society Meeting, Toronto, Extended Abstracts, 4th revised ed., Abstract No. 414, 1992, p. 612. 9. M.S.K. Chen, K. Kawada, S. Miyoshi, M. Nakamura, A. Ohki, S. Takahashi, T. Watanabe, and T. Ohmi, in: 182nd Electrochemical Society Meeting, Toronto, Extended Abstracts, 4th revised ed., Abstract No. 415, 1992, p. 614. 10. T. Ohmi, T. Imaoka, I. Sugiyama, and T. Kezuka, J. Electrochem. Soc. 139 (1992) 3317. 11. T. Ohmi, T. Imaoka, T. Kezuka, J. Takano, and M. Kogure, J. Electrochem. Soc. 140 (1993) 811. 12. T. Ohmi, M. Morita, A. Teramoto, K. Makihara, and K.S. Tseng, Appl. Phys. Lett. 60 (1992) 2126. 13. T. Ohmi, K. Matsumoto, K. Nakamura, K. Makihara, J. Takano, and K. Yamamoto, J. Appl. Phys. 77 (1995) 1159. 14. N. Miki, H. Kikuyama, I. Kawanabe, M. Miyashita, and T. Ohmi, IEEE Trans. Electron Devices 37 (1990) 107. 15. K. Kawada, H. Suzuki, Y. Maeda, and T. Ohmi, in: Proceedings 21st Symposium on ULSI Ultra Clean Technology, 1994, p. 352. 16. T. Watanabe, M. Nakamura, A. Ohki, K. Kawada, S. Miyoshi, S. Takahashi, M.S.K. Chen, and T. Ohmi, in: 1992 International Conference on Solid State Devices and Materials, Extended Abstracts, 1992, p. 132. 17. T. Ohba, S. Inoue, and M. Maeda, IEEE IEDM Tech. Dig., Washington, DC, Dec. 6 – 9, 1987, p. 213. 18. T. Ohba and W. Dig, in: Papers Presented at the 1988 1st Microprocess Conference, 1988, p. 122. 19. H. Morinaga, M. Suyama, and T. Ohmi, J. Electrochem. Soc. 141 (1994), 2834. 20. H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, IEICE Trans. Electron. E79-C (1996) 343. 21. N. Sato, Denkyokukagaku, Nittetsu Joho Center, Tokyo, 1993 (in Japanese). 22. M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, and M. Ohwada, J. Appl. Phys. 68 (1990), 1272. 23. T. Ohmi, Y. Nakagawa, H. Aomi, J. Takano, Rev. Sci. Instrum. 64 (1993), 2683. 24. T. Ohmi, J. Electrochem. Soc. 143 (1996) 2957. 25. Y. Saito, K. Sekine, M. Hirayama, and T. Ohmi, Jpn. J. Appl. Phys. 38 (1999) 2329. 26. Y. Saito, K. Sekine, N. Ueda, M. Hirayama, S. Sugawa, and T. Ohmi, in: 2000 Symposium on VLSI Tech. Dig. of Technical Papers, Honolulu, 2000, p. 176. 27. K. Sekine, Y. Saito, M. Hirayama, and T. Ohmi, IEEE Trans. Electron Devices 47 (2000) 1370. 28. K. Ryoo, W. Shindo, M. Hirayama, and T. Ohmi, J. Electrochem. Soc. 147 (2000) 3859. 29. T. Ohmi, S. Sugawa, K. Kotani, M. Hirayama, and A. Morimoto, Proc. IEEE 89 (2001) 394. 30. K. Sekine, Y. Saito, M. Hirayama, and T. Ohmi, IEEE Trans. Electron Devices 48 (2001) 1550. 31. S. Sugawa, I. Ohshima, H. Ishino, Y. Saito, M. Hirayama, and T. Ohmi, in: 2001 IEEE International Electron Devices Meeting, 2001, p. 37.3.1. 32. T. Goto, H. Yamauchi, T. Kato, M. Terasaki, A. Teramoto, M. Hirayama, S. Sugawa, and T. Ohmi, Jpn. J. Appl. Phys. 43 (2004) 1784. 33. T. Ohmi, K. Kotani, A. Teramoto, and M. Miyashita, IEEE Electron Device Lett. 12 (1991) 652. 34. T. Ohmi, M. Miyashita, M. Itano, T. Imaoka, and I. Kawanabe, IEEE Trans. Electron Devices 39 (1992) 537. 35. H. Mishima, T. Yasui, T. Mizuniwa, M. Abe, and T. Ohmi, IEEE Trans. Semicond. Manuf. 2 (1989) 69. 36. H. Mishima, T. Ohmi, T. Mizuniwa, and M. Abe, IEEE Trans. Semicond. Manuf. 2 (1989) 121.

2

Principles of Semiconductor Device Wet Cleaning Hitoshi Morinaga Tohoku University, Sendai, Japan

CONTENTS I. II. III. IV.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outline of Current Wet Cleaning and Roles of Major Chemicals . . . . . . . . . . . . . . . . Functions Essential for Cleaning Trace Contaminants . . . . . . . . . . . . . . . . . . . . . . . . Cleaning Mechanism of Metallic Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Desorption of Metallic Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Significance to Prevent Contaminants in the Cleaning Solutions from Redepositing on the Substrate Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Mechanism of Metallic Contaminant Adsorption . . . . . . . . . . . . . . . . . . . . . . . . 1. Electrochemical Adsorption of Noble Metals . . . . . . . . . . . . . . . . . . . . . . . 2. Metal Adsorption in Alkali Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Technology to Prevent Metallic Contaminants from Depositing on Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Prevention of Electrochemical Adsorption of Noble Metals . . . . . . . . . . . . 2. Prevention of Metallic Contaminant Adsorption in Alkali Solution . . . . . . E. Removal of Metallic Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Cleaning Mechanism of Particulate Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Mechanism of Particle Deposition in Solution . . . . . . . . . . . . . . . . . . . . . . . . . . B. Technology to Prevent Particle Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Removal of Particulate Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Cleaning Mechanism of Organic Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Improvement of Cleaning Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII. Optimization of Cleaning Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IX. Cleaning Issues to Note in Each Stage of ULSI Manufacturing Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Cleaning Steps in FEOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Cleaning Steps in BEOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 36 37 38 38 39 40 40 43 44 45 48 48 50 50 51 52 54 55 56 56 56 57 57 57

I. INTRODUCTION The advanced manufacturing process of ultra large-scale integration (ULSI) and flat panel display (FPD), in which circuits are fabricated in the nanometer order, is constantly engaged in battles against contamination. Even in the most advanced manufacturing process armed with cleanroom, 35


36

process tools, and materials all of which feature uppermost cleanliness, the surface of substrates and devices being processed is always exposed to contamination introduced through various sources. Potential contaminants include particle and metallic impurity derived from chemical mechanical polishing (CMP) process, photoresist residues, particles generated by sliding parts of process tools, and organic impurity derived from outgassing components coming out of cleanroom interiors. These contaminants need to be removed before being carried over to subsequent process steps as they potentially deteriorate the yield and reliability of final devices. Wet cleaning or dry cleaning is employed as a means to remove these contaminants. In ULSI-manufacturing line, more than 100 cleaning steps are performed, which account for about 25% of the entire process. At a temperature of about 1008C (the highest), wet-cleaning is capable of dissolving or dispersing most substances into the liquid solution and it causes no damages to the substrate surface. As it features such outstanding characteristics, wet-cleaning grows in importance and value even in the current device-manufacturing process which has significantly shifted toward the dry process. Cleaning process is required to achieve (i) ultraclean surface (ii) without any adverse side effects (iii) within a short period of time (iv) with high reproducibility (v) at low cost. The requirement by the industry is becoming more severe along with further integration of ULSI. Cleaning technology, therefore, must be innovated to meet the ever-increasing requirements. Technology innovation is always supported and facilitated by correct understanding of its underlying mechanism. This chapter will discuss adsorption/desorption mechanisms of trace contaminants, principles of semiconductor cleaning based on the mechanisms, the cleaning issues to note, and the strategies to improve cleaning effectiveness. Also, the author will focus on ULSI cleaning because ULSI is expected to feature extremely high cleanliness.

II. OUTLINE OF CURRENT WET CLEANING AND ROLES OF MAJOR CHEMICALS Contaminants are morphologically categorized into four groups: particles, metallic contaminants, organic contaminant, and unintended native oxide. Cleaning process is required to thoroughly remove these contaminants without inducing any adverse side effects such as excessive etching, increase of surface microroughness, and pattern damage. The current ULSI wet-cleaning technology is based on the so-called RCA cleaning published by Kern and Puotinen of RCA, Inc. in 1970 [1]. Table 2.1 shows a process flow of a typical current ULSI wet-cleaning based on the RCA cleaning. It also shows characteristics of each cleaning solution. These cleaning steps use ultrahigh-purity chemicals and ultrapure water in which metallic contaminant is reduced to the ppt order (1/1,000,000 of ppm) or less. H2O2 used in SPM (H2SO4/ H2O2), HPM (HCl/H2O2/H2O), and APM (NH4OH/H2O2/H2O) is a strong oxidizing agent. H2O2 oxidizes and decomposes contaminants to raise the removal efficiency, and oxidizes the top surface of Si substrate to form a passivation (SiO2) film (thickness 1 nm). HCl, a typical strong acid,

TABLE 2.1 Typical Wet-Cleaning Sequence Based on the RCA Cleaning Method Cleaning Target and Adverse Effect of Each Cleaning Cleaning Solutions H2SO4/H2O2 (SPM) NH4OH/H2O2/H2O (APM) HCl/H2O2/H2O (HPM) HF/H2O (DHF)

Cleaning Targets

Adverse Effects (Re-adsorption)

Organic (photoresist), Metals Particles Metals (on surface) Oxide, Metals (in oxide)

Particles Metals Particles Noble metals (Cu, etc.), Particles


37

dissolves oxidized metals (oxides, hydroxides, and ions). H2SO4 decomposes organic contaminant and HF dissolves SiO2, whereas these two acids can also dissolve oxidized metals. NH4OH, the only alkali component, not only dissolves organic contaminants but also removes insoluble particle (the mechanism of action will be discussed later). The biggest problem of this RCA-based wet-cleaning method is its side effects shown in Table 2.1: a contaminant is being removed, whereas another contaminant gets redeposited on substrate surface. For example, APM cleaning is extremely effective in removing particle and organic contaminant, but it allows metallic contaminant to redeposit on substrate surface wherever the APM solution contains traces of metal ions. As each cleaning step has specific side effects, the only way to overcome this problem is to perform a multistep cleaning by using multiple cleaning solutions. It is also necessary to frequently replace cleaning solutions to reduce redeposition of contaminants as much as possible. Owing to these constraints, the RCA-based wet-cleaning method (i) has a number of cleaning steps, (ii) consumes large amount of chemicals or ultrapure water, (iii) needs large-sized process tools, and (iv) is hard to improve for higher cleanliness due to contaminant redeposition. In addition, this method (v) cannot be applied to metal surface as it uses H2O2, strong acids, and strong alkali which corrode metal materials such as Al, W, and Cu. Although out organic solvent or ultrapure water can be used for post-metalization cleaning, their cleaning performance is not sufficient. Reduction of pattern size and increase of number of interconnect layers are important in these devices, in addition, it becomes increasingly important to improve the cleaning performance. At the same time, to decrease resistance and increase speed, the gate electrode needs to be made from metal materials, to which the RCA-based wet cleaning cannot be applied. Ever since 1990 when these intrinsic problems of the RCA cleaning started be reported, indepth studies to reveal the mechanisms of trace contaminant cleaning have been performed in an attempt to address contaminant redeposition and to improve wet-cleaning performance [2 – 36]. Section III to Section VI present essential points of the mechanisms of trace contaminant cleaning.

III. FUNCTIONS ESSENTIAL FOR CLEANING TRACE CONTAMINANTS [11,26] Figure 2.1 shows three essential functions for removing trace contaminants. First, the contaminant needs to get desorbed from the substrate surface (Function 1). To efficiently achieve an ultraclean surface, it is necessary to prevent the contaminant in the cleaning solution from redepositing on the substrate surface (Function 2). To remove the contaminant included in the film or to remove the

FIGURE 2.1 Three essential functions for cleaning trace contaminants.


38

contaminant which forms a strong chemical bond with film surface, the underlying film needs to be slightly etched (Function 3). Taking metallic contamination as an example, the author will describe significance and effectiveness of the three functions first. Then the cleaning mechanism of particles and organic contaminants will be discussed.

IV. CLEANING MECHANISM OF METALLIC CONTAMINATION [8–12] A. DESORPTION

OF

METALLIC CONTAMINANTS

For soluble metallic contaminants, its desorption from substrate surface (Function 1) can be achieved by making the metallic contaminant dissolved onto the solution. Parameters which are important for dissolution of metallic contaminant include pH and redox (reduction – oxidation) potential of the cleaning solution and the complexing agent, these are the basic parameters. Redox potential is well known, in the field of solution chemistry, as a parameter to indicate oxidizing capability and reducing capability of solution. Potential of Hþ redox reaction is set at 0 V as a reference. Those with stronger oxidizing capability (to take electron) are to feature positive redox potential, and those with stronger reducing capability (to give electron) are to feature negative potential. Usually V versus NHE is used as unit of redox potential. (It means potential against Hþ potential as a reference.) Potential –pH diagram (or also referred to as Pourbaix diagram after a scientist who proposed it) is a matrix of pH and redox potential, showing the state of metals (atom, ion, hydroxide, oxide, etc.) in thermodynamic equilibrium calculated with equilibrium constant. Potential –pH diagram is widely used in inorganic chemistry, particularly in the field of metal corrosion [37]. This diagram is also useful when the dissolution of metallic contaminants in solution is studied. Figure 2.2 shows potential –pH diagrams for Cu – H2O, Fe – H2O, and Al –H2O systems. For a diphasic system comprising a metal and an aqueous solution, the metal gets dissolved when, the pH and redox potential of the solution are controlled in such a way as to fit into the dissolution region (shadowed region) in the diagram. In a solution featuring low pH and high redox potential, the metal is dissolved in the form of its cation. In a weak alkali solution, the metal gets precipitated as hydroxide. In a solution with no oxidizing capability, metal gets precipitated in a metallic state (metallic atom). These phenomena are very general, commonly observed for a number of other metals. Redox potential boundary in Figure 2.2 where the metallic atom turns into a metallic ion is varied, depending on the type of Cu : 1ppm in Water

2

Fe : 1ppm in Water

2

Al : 100ppm in Water

2

Dissolution

1.5

1.5

E (V vs. NHE)

1

2+

HCu(OH) 4

Cu

Cu(OH)2

Cu(OH)

1

FeOH

2+

Fe(OH)

0

(a)

Cu

(b)

Fe(OH)3 Fe

Fe(OH) 4

2+

−0.5

−1

0.5

+

2

(b)

−0.5

−1.5

1

Fe3+

0.5

0

HFe(OH) 4

Fe(O

Al(OH)4

-

(b)

HFe(OH)4-

−1 −1.5

Al(OH)3

−0.5

H)

2

Al3+

0

-

(a)

Fe

−1

−2

−2

−2

−2.5

−2.5

0 2 4 6 8 10 12 14

(a)

−1.5

−2.5

0 2 4 6 8 10 12 14

Dissolution

Dissolution

2-

4

0.5 Cu O 2

1.5

Dissolution

Al

0 2 4 6 8 10 12 14

pH

pH

pH

(a) Cu-H2O system

(b) Fe-H2O system

(c) Al-H2O system

FIGURE 2.2 Potential-pH diagram (Pourbaix diagram) for three typical metals (at 258C).


39

TABLE 2.2 Redox Potential where Matallic Atoms are Ionized (Standard Reduction Potential: E 0) Standard Reduction Potential E 0 (V vs. NHE)

Element Auþ/Au Pt2þ/Pt Agþ/Ag Cu2þ/Cu

1.692 1.188 0.799 0.337

Hþ/H2

0.000

Pb2þ/Pb Ni2þ/Ni Fe2þ/Fe SiO2/Si Al3þ/Al Ca2þ/Ca Naþ/Na Kþ/K

–0.126 –0.250 –0.440 –0.857 –1.663 –2.866 –2.714 –2.924

metal as shown in Table 2.2 [37]. (Precisely speaking, potential shown in Table 2.2 slightly changes, depending on the metallic ion concentration and temperature. This is why redox potential of Cu is slightly different between Table 2.2 and Figure 2.2: Cu of 1 ppm.) When the list shown in Table 2.2 is rearranged to start from the one with the lowest redox potential, it will show the ionization tendency of metallic atoms. Au is harder to ionize than Fe and Al. An oxidizing agent with a redox potential of 1.69 V or more versus NHE is required to ionize Au. Depending on the type of metal, pH boundary where the metallic ion turns into hydroxide changes as well. In particular, Fe3þ is a typical metal which turns to hydroxide even at low pH. It gets precipitated at pH of 2 or higher. To dissolve the metals containing Au or Fe, a solution with a strong acidic oxidizing agent having a redox potential higher than 1.69 V versus NHE and a pH lower than 2 are needed. Figure 2.3 shows pH and redox potential of various solutions (both theoretical and experimental values). By adding acid, alkali, oxidizing agent, and reducing agent, it is possible to control the pH and the redox potential of the solution. Using Figure 2.2 and Figure 2.3, we can estimate whether the metals are dissolved or precipitated in various solutions. HPM and SPM solutions, both feature extremely low pH and high redox potential as they are mixtures of a strong acid and a strong oxidizing agent, and are capable of dissolving most metals. This means that these solutions have the first essential functional quality as the cleaning solution for metallic contaminants (Function 1).

B. SIGNIFICANCE TO PREVENT CONTAMINANTS REDEPOSITING ON THE SUBSTRATE SURFACE

IN THE

CLEANING SOLUTIONS

FROM

To achieve an ultraclean substrate surface with high reproducibility, it is essential to prevent contaminants in cleaning solution from redepositing on substrate surface (Function 2). Figure 2.4(a) shows remaining contaminant concentration on a substrate surface which is intentionally contaminated with metallic contaminants and then cleaned with various cleaning solutions [10]. Figure 2.4(a) clearly indicates that APM cleaning is not effective in removing Al or Fe and that diluted HF (DHF) cleaning hardly removes Cu. Figure 2.4(b) shows how much metallic contaminants redeposit on Si substrate surfaces from the various cleaning solutions contaminated with metals at 100 ppb [10]. A comparison between (a) and (b) of Figure 2.4 indicates that the


40

2

E (V vs. NHE)

1.5

UPW

HCI

H2SO4

HNO3

KOH

Oxidizing Agent (+ Acid)

Reducing Agent

DHF

SPM HPM(SC-2) H2SO4/O3 (10%/10ppm) H2SO4/O3 (100ppm/10ppm) HCI/KCIO 03 UPW HIO3

1

(b) O

DHF

2

0.5

APM(SC-1)

+ 4H +

UPW

Hig h Acid C

Low entr atio n

onc

+ 4e −

2H

2O

Low O2 UPW KOH

0 High H2 UPW Na2S2O4

−0.5

(a) 2 + H

+ 2e −

−1 0

2

4

6 pH

8

10

12

H2

14

: E of SPM, HPM and APM is calculated by using E° of H2O2

FIGURE 2.3 The pH level and redox potential (E) of various solutions (E of SPM, HPM, and APM are calculated. Others are measured (From H. Morinaga, T. Hoshino, Y. Omura, M. Kitagawa, and M. Aoki, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 585 – 592, 2000.)

dissolution and redeposition behaviors are very similar, suggesting that capability to prevent a contaminant in a cleaning solution from redepositing (on substrate surface) should be a dominant factor of contaminant removal performance. Another important issue to note is that Cu and Al redeposit on substrate surface although Cu is dissolved in DHF as Cu2þ and Al is dissolved in APM as Al(OH)2 4 as shown in Figure 2.2 and Figure 2.3. This means that a simple metal dissolution function is not sufficient to prevent Cu and Al from redepositing on substrate surface. These metallic ions, which though dissolved in the cleaning solution, do redeposit on substrate surface because they chemically react with substrate surface. To overcome this problem, therefore, the chemical reaction responsible for the deposition must be suppressed. It is important to precisely understand the reaction mechanism.

C. MECHANISM

OF

METALLIC CONTAMINANT ADSORPTION

Electrochemical adsorption of noble metals and metal adsorption in alkali solution are the two primarily important adsorptions in wet-cleaning process, as shown in Table 2.3. The mechanism of the two adsorptions will be discussed below. 1. Electrochemical Adsorption of Noble Metals In acidic solutions, metals are unlikely to deposit onto the substrate surface in general because the metal is dissolved in the form of the cations. Cu ion in DHF, however, is highly likely to deposit on


Metal Remaining (atoms/cm2)

(a)

Metal Adsorption (atoms/cm2)

(b)

41

1015 1014 1013 1012

Dipping Time: 10min

Cu Fe

Cu

Al

1011 1010 109

Initial

1015 1014

Al

APM

HPM

SPM

Al Fe DHF

Dipping Time: 10min Metal : Fe, Al,Cu,Ca,Na,Cr, Zn, Ni, and Mg of 100ppb each

1013 Fe 1012

Cu

Cu

1011 1010

Al Fe

109 APM

APM : NH4OH/H2O2/H2O= 1/1/30, 60ºC, SPM : H2SO4/H2O2= 4/1, 100ºC

HPM

SPM

DHF

HPM : HCl/H2O2/H2O = 1/1/6, 60ºC DHF : 0.5%HF, RT

FIGURE 2.4 The effect of various cleaning solutions on the metallic contaminations. (a) Removal of metallic contamination with various cleaning solutions. Initial contamination was deposited from APM with metals. (b) Metal deposition onto Si surfaces from various cleaning solutions containing metallic impurities. (From H. Morinaga, T. Hoshino, Y. Omura, M. Kitagawa, and M. Aoki, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 585– 592, 2000.)

Si surface (Figure 2.4(b)). This tendency is common among noble metals such as Au, Ag, and Cu which have redox potential higher than hydrogen. Noble metal deposition to Si surface takes place not only in HF cleaning step but also in other steps in which bare Si surface (oxide-free Si surface) is exposed to cleaning solution. Figure 2.5 shows the mechanism of noble metal adsorption to bare Si surface that has been revealed as a result of the investigations [8,9]. In Figure 2.5, Cu is taken as an example. Cu2þ features far higher redox potential (capability to take electron) than Si. Cu2þ, therefore, pulls out electron from Si surface to get reduced, and becomes Cu atom to adsorb onto Si surface. The first Cu atom that adsorbs on the surface functions as a nucleus, and Cu2þ in solution takes electron from Si through the nucleus one after another to get precipitated. In this way, Cu grains grow in size as shown in Figure 2.6. In the meantime, Si underneath the Cu grains, giving as much electrons as

TABLE 2.3 Adsorption Mechanisms of the Typical Metallic Contamination in Solutions [10] Adsorption Mechanisms Electrochemical adsorption ¼ oxidation–reduction reaction Hydroxide adsorption ¼ dehydration reaction

Metals

Solutions

Surfaces

Cu, Ag, Au, . . . (noble metals)

DHF, Water Alkali Polishing slurry (without oxidizer)

Al, Fe, Zn, . . . (metal hydroxides)

Alkali, Water

Bare Si SiO2 (including p-Si, a-Si); Generally: base metals SiO2 . Bare Si; Generally: OH-terminated surfaces

42


FIGURE 2.5 Mechanism of Cu adsorption onto Si surfaces in solutions. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343 – 362, 1996. With permission.)

required for Cu grain growth, gets oxidized to turn to SiO2. In HF solution, SiO2 is etched right after being formed to generate pits (MIP, metal-induced pit) [12] (Figure 2.7). As described above, a noble metal is highly likely to deposit onto a bare Si surface because the noble metal with high redox potential directly takes electron from Si and gets electrochemically deposited onto Si surface. Cu unlikely deposits on SiO2 surface although it is extremely apt to get deposited on bare Si surface in HF solution (Figure 2.8). This is because even Cu2þ with high redox potential cannot take electron from SiO2 surface. As shown in Figure 2.5, deposition starts from the moment when one Cu2þ takes electron from Si to form a nucleus. It is speculated that the nucleation takes place in a region of Si surface which is electrically active in particular. Based on the study of the Cu behavior, it is reported that Si surface area where hydrogen termination is not complete is used as a nucleus to trigger Cu deposition from DHF or ultrapure water to bare Si surface [8].

Initial

30 sec

90 sec

180 sec

0.3 µm

FIGURE 2.6 SEM images of the Cu particle growth process on the Si surfaces. (From 0.5% HF solution containing 1 ppm Cu using CuCl2). (H. Morinaga, M. Suyama, and T. Ohmi, J. Electrochem. Soc., 141, 2834– 2841, 1994. With permission.)


43

FIGURE 2.7 AFM image of the MIPs on the Si surface made by the Cu adsorption process from DHF(0.5%) solution. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343– 362, 1996; H. Morinaga, M. Suyama, and T. Ohmi, J. Electrochem. Soc., 141, 2834– 2841, 1994. With permission.)

2. Metal Adsorption in Alkali Solution [10] In alkali solution such as APM and neutral ultrapure water, not only noble metals but also other metals such as Fe and Al are likely to deposit on substrate surface (see Figure 2.4(b)). In alkali solution with pH 10 – 11 (like APM), most of Fe and Al exist in a form of Fe(OH)3 and Al(OH)2 4, respectively (Figure 2.2). What should be noted is that Al exists as Al(OH)2 4 , a water-soluble hydroxide ion in the solution. In other words, it is not possible to rely on such a simple speculation as insoluble hydroxide which is commonly formed in alkali solution gets deposited on substrate surface. Morinaga et al. considered that surface chemical reaction between Si surface and metal induces this adsorption, and investigated the influences of Si surface conditions on metal adsorption. Figure 2.9 indicates that Fe, Al, and Zn are more likely to adsorb on oxide surface than on bare Si surface in both neutral and alkali solutions, that is, in the region where metal hydroxide is formed. Especially, this tendency is obvious in the case of Al adsorption. (Cu is the only exception. More Cu adsorption is detected on bare Si surface. This is because another mechanism described earlier above, that is, electrochemical adsorption mechanism, works for Cu adsorption. In the region with pH 10.5 where copper hydroxide is formed, however, Cu adsorption to oxide surface increases.)

Si

SiO2

FIGURE 2.8 SEM images of selective Cu deposition on the surfaces with Si and SiO2 from a DHF (0.5%) solution (with 100 ppm Cu using CuCl2). The dipping time is 60 min. (From H. Morinaga, M. Suyama, and T. Ohmi, J. Electrochem. Soc., 141, 2834– 2841, 1994. With permission.)


1014 1013

Ultrapure Water Metal : 10ppb each RT, 10min pH= 6

Bare Oxide

1012 1011 1010 Fe

Al

Cu

(1) Ultrapure Water [Pre-treatment]

Zn



44

1014 1013

Bare = Si-H Oxide = Si-OH

Alkali (KOH:0.12%) Metal : 10ppb each RT, 10min, pH= 10.5

1012 1011 1010 Fe

Al

Cu

Zn

(2) Alkali (pH 10.5)

Bare : SPM=> HF=> UPW Rinse=> (within 30min) Adhesion Experiment Oxide: SPM=> HF=> SPM=> UPW Rinse=> Adhesion Experiment

FIGURE 2.9 Influence of Si surface conditions (bare or chemical oxide) on metal adsorption from ultrapure water and alkali solution with metals (Fe, Al, Cu, Ca, Na, Cr, Zn, Ni, and Mg of 10 ppb each are added). (From H. Morinaga, T. Hoshino, Y. Omura, M. Kitagawa, and M. Aoki, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 585– 592, 2000. With permission.)

In the neutral-to-alkali region, the oxide surface is covered with OH group bound with Si (silanol group). In contrast, HF-pretreated bare Si surface is terminated with hydrogen. The results shown in Figure 2.9 suggest that the OH termination of Si surface play a dominant role in the adsorption of metal hydroxide. Taking Al(OH)2 4 adsorption onto OH-terminated Si surface (Si –OH group) as an example, Morinaga et al. have built a reaction model based on the assumption that a dehydration reaction takes place between Si –OH group and the OH group bound with metal (Figure 2.10(a)), and they calculated the change of potential energy during the adsorption reaction by using quantum chemical calculation (with density functional theory). Figure 2.10(a) indicates that first adsorption state is energetically very stable and activation energy of adsorption reaction is low enough to pass at room temperature. Generally in a chemical equilibrium, the state after adsorption is more stable than that before adsorption. (The potential energy decreases due to adsorption.) For comparison, the change of potential energy was also calculated for adsorption onto the hydrogen-terminated Si surface. This calculation has revealed that the activation energy is extremely high and that state after adsorption is unstable in terms of equilibrium (Figure 2.10(b)). (The potential energy after the adsorption is higher than that before adsorption.) The reaction models and the calculation results indicate that metal hydroxide is by far more likely to adsorb on OH-terminated surface than hydrogen-terminated surface. In Figure 2.9, more Al adsorption onto bare Si surface is observed in alkali solution (pH 10.5) than in ultrapure water. This is considered because hydrogen termination of Si surface is partially replaced by OH termination in the alkali solution. Fe and Zn adsorption on oxide surface in the alkali solution is much more than that in the ultrapure water. When pH level of the solution is neutral, iron hydroxide and zinc hydroxide are neutral: Fe(OH)3 and Zn(OH)2. When pH level goes up to around 10.5, however, iron hydroxide and zinc hydroxide partially or mostly turn to 2 2 ionic hydroxide: Fe(OH)2 4 , Zn(OH)3 , and Zn(OH)2 . The fact that more Fe and Zn adsorbed on the surface in the alkali solution with pH of 10.5 implies that dehydration reaction is more likely to take place between ionic hydroxide and the OH-terminated surface.

D. TECHNOLOGY

TO

PREVENT METALLIC CONTAMINANTS FROM DEPOSITING

ON

SURFACE

Once the adsorption mechanism is revealed, it becomes possible to develop a technology to prevent deposition. For example, noble metal adsorption on bare Si surface takes place due to redox


45

FIGURE 2.10 A model for the metal adsorption in alkali solution. (a) Change of atomic configurations and 2 potential energies when Al(OH)2 4 adsorbs onto OH-terminated Si surface. (b) In the case of when Al(OH)4 adsorbs onto H-terminated Si surface. (Results of quantum chemical calculation with density functional theory.) (From H. Morinaga, T. Hoshino, Y. Omura, M. Kitagawa, and M. Aoki, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 585 – 592, 2000. With permission.)

reaction between the noble metal ion and Si surface. This means that the adsorption can be suppressed by preventing this redox reaction by suitable means. Technologies to prevent metallic contaminant adsorption which is designed on the basis of the adsorption mechanisms will be described in the following sections. 1. Prevention of Electrochemical Adsorption of Noble Metals When a chemical reaction between a metallic ion and Si does not contribute to metallic contaminant adsorption, by using a diphasic potential – pH diagram shown in Figure 2.2, it is possible to obtain potential – pH conditions of a solution that can prevent metallic contaminant adsorption. For metallic contaminant adsorption induced by a chemical reaction between a metallic ion and Si surface, however, the diphasic potential –pH diagram cannot be applied as it is. Instead it is necessary to study a new potential –pH diagram of the three-phase system (metal – water –Si surface) [8]. Figure 2.11 experimentally analyzes which potential –pH region actually prevents noble metal adsorption onto Si surface. It is revealed that Cu adsorption is suppressed in the region of pH 7 or less and a redox potential of 0.75 V or more versus NHE. To prevent noble metal adsorption, it is essential not to allow redox reaction between noble metal ion and Si to take place. For this purpose, it is effective to cover Si surface with oxide or to spike an acid or an oxidizing agent which is stronger than noble metal ion in terms of activity to take electron (i.e., which features higher redox potential than noble metal ion). The results shown in Figure 2.11 indicate that Cu adsorption is suppressed as redox potential exceeds 0.75 V versus NHE due to addition of acid or oxidizing agent. This is because Si, reducing agent, raises the Cu/Cu2þ boundary line. In Cu –aqueous solution system, Cu is dissolved in it form of Cu2þ when the solution has pH 7 or less and a redox potential of 0.2 V versus NHE or more. When Si enters into this system, Cu2þ is reduced by Si, and accordingly Cu gets deposited on Si wafer surface. This phenomenon, however, is strongly affected by redox potential. To be more

46

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing Cu: 1ppm (using CuCI2), Dipping Time: 3min 2.5

2

Cu Deposition (atoms/cm2)

≥1E+14

1E+13-14

1E12-13

1E+11-12

1E+11>

1.5

E (V vs. NHE)

Cu2+ 1

0.5

HCuO2-

Cu(OH)2

CuO22-

Cu-Si-Water Cu Cu

2O

Cu-water

0 −0.5

Cu

−1 −1.5

0

2

4

6

8

10

12

14

pH

FIGURE 2.11 The effect of pH level and redox potential of solutions (ultrapure water with various chemicals added) on the Cu adsorption onto the Si surface. The amount of adsorbed Cu is described with the size of solid square. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343 –362, 1996; H. Morinaga and T. Ohmi, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 257 – 268, 1996. With permission.)

specific, this phenomenon is affected when a strong oxidizing agent is injected to the solution and redox potential exceeds 0.75 V versus NHE. Figure 2.12 shows the mechanism of preventing Cu deposition by raising redox potential with O3 (ozone) as an example. Without ozone, Cu2þ ion receives electrons from Si and gets deposited. When O3 is injected into the Cu – water system, it is O3 that takes electrons from Si with a higher thermodynamic driving force than Cu2þ. Therefore, Si surface is oxidized by O3, and copper remains in the solution in the form of Cu2þ ions. Per chance, even if Cu gets deposited on Si surface, its electron is taken by O3, when copper becomes ionized (dissolved) again. Furthermore, when Si surface is oxidized and covered with oxide, electron exchange between Cu2þ ion and Si is suppressed, which makes it more difficult for Cu to deposit. In HF solution, no oxide grows on Si surface. To prevent Cu deposition, therefore, redox potential in HF solution needs to be 0.85 V, which is 0.1 V higher than in the other solutions [8,9]. Figure 2.13 demonstrates that injection of O3, H2O2, high-concentration H2SO4, HCl, or HNO3 to DHF makes its redox potential rise to 0.85 V or more and thereby suppresses Cu deposition on bare Si surface [8,9]. Fe and Al in DHF hardly deposit on bare Si surface (Figure 2.4). This is because redox potential of these metals is lower than that of Hþ in DHF (Figure 2.12) and therefore they are ionized by oxidizing capability of Hþ even if no oxidizing agent is spiked. Relationship between redox potential E (V versus NHE) and energy level of electron based on infinity electron in vacuum (real potential), ae (eV), can be expressed as [38]

ae ¼ (E þ 4:44)


47

FIGURE 2.12 Mechanism of preventing Cu deposition by oxidizing agent (with ozone as an example). E 0 (standard redox potential) and ae (real potential) of typical metals and oxidizing agents are also shown for reference. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343 – 362, 1996; H. Morinaga and T. Ohmi, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 257 – 268, 1996. With permission.)

Cu Deposition ( atoms / cm2)

1E+16

1E+15

Bare - Si , N (100) Cu : 1 ppm ( using /CuCI2) Dipping Time : 3 min

Redox Potential (V vs. NHE) 0.83

1E+14

1E+13

1E+12

0.91

0.89

1.53

0.90 (1.67)

1E+11

1E+10 DHF (0.5%)

HF/HCI HF/H2SO4 HF/HNO3 HF/H2O2 HF/O3 (0.5% / 10%) (0.5% / 10%) (0.5% / 10%) (0.5% / 10%) (0.5% / 10ppm)

Dipping Solutions

FIGURE 2.13 The Effect of adding various acids and oxidizing agents in preventing Cu deposition onto the Si surface from DHF solution. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343 – 362, 1996; H. Morinaga and T. Ohmi, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 257 – 268, 1996. With permission.)

48


Figure 2.12 also shows ae values converted from this equation. The ae value can be regarded as the Fermi level of solution. 2. Prevention of Metallic Contaminant Adsorption in Alkali Solution Alkali solution cleaning such as APM cleaning is widely adopted in the cleaning processes of ULSI and FPD, as it can effectively remove particulate and organic contaminants. As described before, however, alkali solutions allow metallic contaminants to adsorb on substrate surface. Metal adsorption during APM cleaning is one of the most serious metallic contaminations in ULSI-manufacturing process, and a number of papers have reported its negative impacts on device performance. For instance, if APM cleaning is adopted as a final cleaning, breakdown voltage [39] and thickness [40] of gate oxide get deteriorated due to metallic contamination. It is also reported that Fe deposition during APM cleaning causes local microroughness increase on Si surface [14,15] and that once metal gets deposited on the bottom of contact hole with diameter of 0.2 mm, or less, it cannot be removed even in a subsequent acid cleaning [17]. Technology to effectively prevent metal deposition during alkali cleaning becomes increasingly important. As described before, the metal adsorption in the alkali solution is caused by the dehydration reaction between metal hydroxide and OH-terminated surface. This means that the metal adsorption can be eliminated by preventing metal hydroxide from being formed. Injection of chelating agent is effective to prevent metal hydroxide formation. It is proposed to spike a chelating agent to APM and other alkali cleaning solutions [10,26,41–44]. “Chelate” is a Greek word meaning crab claws. Just like a crab catching a food, the chelating agent surrounds a metallic ion to form a metal complex to make it soluble. Traditionally chemical industry has been widely using chelating agents such as EDTA to prevent formation of metal hydroxide. Their application, however, has been limited to a few metals such as Fe and Ca. For example, EDTA is not effective in preventing Al hydroxide formation in alkali solution, which does not a matter at all as far as the traditional applications in the chemical industry are concerned. In ULSI wet-cleaning process, however, the chelating agent is expected to prevent deposition of various metals such as Al, Fe, Zn, Cu, Co, and Ni. It is also important to assess the purity of chelating agent itself and its impacts on manufacturing process. Recently, new chelating agents meeting these specific needs of ULSI wet process have been designed and adopted [10,26]. Figure 2.14 shows how effectively a chelating agent injected to APM with 1 ppb metallic contamination can prevent metal adsorption to the substrate surface. In APM cleaning process using the conventional NH4OH solution, metals such as Fe, Al, Cu, and Zn adsorb on the substrate surface in large amount. However, when NH4OH solution with spiked a chelating agent is used, Al adsorption, which is usually detected in large amount, is reduced to less than 1/100, and adsorption of other metals decreases below the detection limit.

E. REMOVAL OF METALLIC CONTAMINANTS With the function of metal dissolution (Function 1) and the function of redeposition prevention (Function 2) described earlier, it is possible to remove metallic contamination on substrate surface, provided that each reaction reaches the chemical equilibrium. The remaining matter is about how to reduce reaction time or cleaning time. To overcome this problem, it is necessary to take appropriate actions such as to raise temperature to activate the reactant chemicals, to increase concentration of chemicals thus raising the probability for chemical species and contaminant to come in contact with each other, to fluidize the solution, or to rotate substrate at a high speed. When these actions fail to get metallic contaminants completely removed, it is speculated that the contaminant might form a strong chemical bond with the film or that it might have been included in the film. Cu and Al easily get diffused into SiO2 film, and invade into the film even during room-temperature wet-cleaning process [8,18]. To remove this kind of a metallic contaminant, it is necessary to etch the film (Function 3). For example, DHF and APM are effective in etching the top surface of SiO2 film. Here again, the above-mentioned Functions 2 and 3 are


H − +

Fe3

OH

−

−

1012

OH +

Al3

1011

OH−

1010 109 Fe

Al

Cu

Zn

Cr

Ni

Mg

Na

H O

–


1013

APM (NH4OH) APM (MC1)

Alkali/H2O2/H2O (APM) = 1/1/30 45ºC, 10min, Metal: 1ppb

O

1014

49

–Si–

MC-1 = NH4OH 29% + Chelating agent

FIGURE 2.14 The effectiveness of adding chelating agent in preventing metal adsorption from APM with metals of 1 ppb added. Comparison of the conventional APM (NH4OH/H2O2/H2O) and chelate-added APM (MC1/H2O2/H2O) (MC1, chelate-added NH4OH; Mitsubishi Chemical Corporation, Japan).

very important. The film etching cannot be effective enough unless the metal contaminant in the film is thoroughly dissolved and it is prevented from redepositing on, the surface during etching [8 – 10]. Figure 2.15 compares various cleaning methods in terms of effectiveness of removal of Cu contamination. Generally, Cu contaminant is included in the oxide film. SPM cleaning cannot remove Cu contamination perfectly even if it is repeated for four times. For DHF cleaning, although the oxide itself can be removed with DHF, Cu contamination still remains on surface because dissolved Cu2þ ion redeposits on bare Si surface. When DHF or H2O2 solution is used to etch the oxide, Cu contamination can be reduced almost to its background level. As discussed above, it is important to select a cleaning solution that provides the three functions, namely desorption, prevention of re-adsorption, and slight etching. Recently proposed solutions such as DHF/H2O2 [45], DHF/HCl [8,9,20,46], DHF/O3 [8,9], and APM with chelating

Cleaning Solution

Blank Initial SPM Bare-Si, N(100) [Blank]

SPM×4

Cu Deposition [Initial]

SPM+DHF SPM+HF/H2O2 (0.5% /10%)

Cu:1 pm in UPW Dip:3 min

Cleaning (10 min) 1E+10 1E+11 1E+12 1E+13 1E+14 1E+15 1E+16 Cu Remaining (atoms/cm2)

FIGURE 2.15 The effect of various cleaning methods on the Cu removal from the Si surface [8,9]. Some of the Cu is included in an oxide on the Si surface. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343 – 362, 1996; H. Morinaga and T. Ohmi, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 257 – 268, 1996. With permission.)


50

agent injected feature these three essential functions. However, their removal efficiency varies, depending on the concentration of the additives.

V. CLEANING MECHANISM OF PARTICULATE CONTAMINATION In Section IV, three essential functions for trace contaminant cleaning are described by taking metallic contamination as an example. This basic concept is applicable to any type of contaminant in gernal. What is different from one contaminant to other is a means to desorb it from substrate and to prevent its redeposition.

A. MECHANISM

OF

PARTICLE DEPOSITION

IN

SOLUTION

For particulate contaminations, it is also important to prevent particles in solution from redepositing on a substrate surface. Herein the mechanism of particle deposition will be described first. There are two major drivers to make a particle in the solution to deposit onto a substrate surface: van der Waals force (intermolecular force) and electrostatic force induced by electrical double layers. Van der Waals force works strongly in near distance, whereas electrostatic force works even in relatively long distance. Also, the electrostatic force acts either as an attractive force or a repulsive force, depending on charge-up status of the particle and the substrate. When a substance comes in contact with polar solvent (such as aqueous solution), its surface is charged up, and electrical double layers are formed around the charged-up interface by ions in the solution. The electrical double layers are defined by a number of potentials. What is measurable among them is the zeta potential, which is an important parameter in surface chemistry and colloidal chemistry, to control dispersion and aggregation of particles in solution. Zeta potential is also useful when adsorption and desorption of particle on substrate are studied. In general, when zeta potential of two substance surfaces is high and of the same in polarity, repulsive force works between the two surfaces. In contrast, when zeta potential of two substance surfaces is different (positive and negative), and of opposite polarity attractive force works. Figure 2.16 shows an example of calculation of potential energy between particle in solution and substrate surface. Total potential energy is expressed as a sum of electrostatic potential energy due to electrical double layers (double-layer potential) and potential energy due to

FIGURE 2.16 Calculation examples of total potential energy curve between a substrate surface and a particle in a solution. (From M. Itano, T. Kezuka, and M. Kubo, in: SPWCC Conference Proceeding, Santa Clara, pp. 257 – 287, 1996. With permission.)


51

van der Waals force (van der Waals potential). Slope of the curves represents the force between the particle and substrate surface at a point. When zeta potential of the particle and substrate surface is the same in polarity, total potential energy curve has a maximum value, which constitutes a barrier to hinder particle deposition. In contrast, when zeta potential of particle and substrate surface is different, only attractive force is applied, as no electrostatic repulsive force due to electrical double layers exists. Under this circumstance, the particle deposits on substrate surface when it moves by means of Brownian movement and approaches to less than 50 nm from substrate [30]. Wettability of the particle and substrate surface is another important parameter. In aqueous solution, a hydrophobic particle cannot exist in a stable manner, and it thereby tends to get gathered around gas-to-liquid interface or solid-to-liquid interface (i.e., substrate surface). In particular, it should be noted that a hydrophobic particle easily gets gathered onto hydrophobic substrate surfaces (e.g., bare Si surface after HF cleaning step and carbon-doped low-k film surface). In general, surface wettability can be characterized by measuring contact angle of a droplet dropped onto the surface.

B. TECHNOLOGY

TO

PREVENT PARTICLE DEPOSITION

To prevent deposition of particle in solution, it is important to control the zeta potential of the substrate surface and the particle to the same polarity. Zeta potential can be controlled by adjusting pH of solution or by adding additives such as surfactant. In alkali solution with pH 10 or more, many substrate surfaces feature negative and high zeta potential due to OH2 ion (Figure 2.17) [31]. Even if Si3N4 particle or SiO2 particle is in the vicinity of an Si substrate surface, the particle hardly deposits on the surface due to strong repulsive force generated in-between. In acid solution with pH 3, Si surface is charged up negatively, whereas zeta potential of SiO2 particle is around 0 and Si3N4 particle is charged up positively, which is highly likely to lead to particle deposition due to nonexistence of repulsive force. This is how particle becomes apt to redeposit during acid cleaning step (Table 2.1). APM, which aims at removing particles, features pH 10 – 11 as NH4OH (alkali species) is injected, and thereby it does not allow the particles to redeposit. Surfactant addition is also effective in controlling the zeta potential. A surfactant is composed of bolt hydrophobic and hydrophilic groups. It is categorized into anion type, cation type, and nonion type, depending on how the hydrophilic group is ionized. Table 2.4 demonstrates that, by

FIGURE 2.17 Zeta potentials of various substrates and particles as a function of solution pH. (From M. Itano, T. Kezuka, M. Ishi, T. Unemoto, M. Kubo, and T. Ohmi, J. Electrochem. Soc., 142, 971 – 978, 1995. With permission)


52

TABLE 2.4 The Effect of Surfactant Addition on Zeta Potential of Various Material Surfaces in the Acidic Solution (pH 3.3; Dilute HCl; Unit, mV) [31]

Si Si3N4 SiO2 PSL particles

Without Surfactant

Anionic A

Cationic A

223 43 7 39

232 252 27 267

63 45 55 78

injecting a surfactant, zeta potential of substances can be controlled to the same polarity even in acid solution; anion-type surfactant makes everything negatively charged up, whereas cationtype surfactant makes everything positively charged up. This indicates that surfactant injection enables us to hinder particle deposition even in acid solution. It is reported that particle deposition is significantly reduced by means of an anion-type surfactant injection in DHF solution [31]. Improvement of wettability of particle and substrate surface is also important to prevent particle deposition. A surfactant changes a hydrophobic surface into a hydrophilic one. Once the particles become hydrophilic, they get dispersed into solution in a stable manner, and hardly deposit on solid-to-liquid interface (substrate surface).

C. REMOVAL

OF

PARTICULATE CONTAMINANTS

The above-mentioned technique is not enough to eliminate particle contamination on the substrate surface. In addition, the particle needs to be get desorbed from the substrate surface (Function 1). When the particle and the substrate surface form a strong chemical bond, it is also necessary to use etching function (Function 3). For insoluble or refractory particles, mechanical force is applied to get the particle desorbed from substrate surface. Forces to get particles released from substrate in solution include (i) liquid molecule collision to the particle, (ii) dissolved gas bubble generation by means of ultrasonic wave irradiation (so-called cavitation), and (iii) scrubbing with brush. Liquid molecule collision to particles is driven by Brownian movement, which gets more active as temperature is raised. The higher the temperature is, the more easily the particle can be removed. To remove the particle at low temperature, ultrasonic wave irradiation is effective. Ultrasonic wave with higher frequency causes less damage on the device and removes finer particles. Recently megasonic irradiation with frequency of about 1 MHz or higher has been adopted. Figure 2.18 demonstrates that a combination of megasonic irradiation and low-temperature APM cleaning (about 408C) is sufficiently effective in removing particles. Brush scrubbing cleaning is capable of efficiently removing particles which persistently deposits onto the substrate surface as the brush directly contacts with the particles. However, when substrate features three-dimensional morphology such as a hole, brush scrubbing is not able to remove contaminant inside of the vertical structure. Brush scrubbing cleaning, therefore, is often used as a post-CMP cleaning as the substrate surface is planarized but seriously contaminated in CMP process. A particle often forms a strong chemical bond with the substrate surface. This type of particle contamination needs to be removed by etching the substrate surface. For APM cleaning without megasonic irradiation, Figure 2.19 shows particle (TiOx) removal efficiency as a function of etching depth on Si surface [32]. Figure 2.19 indicates that particle removal efficiency does not depend on the etching rate (which is determined by the composition ratio of APM), and that a


53

FIGURE 2.18 The Effectiveness of megasonic irradiation during low temperature APM cleaning on the removal of various particles (particles: Fe2O3, Si3N4, PFA). (From H. Morinaga, M. Aoki, T. Maeda, M. Fujisue, H. Tanaka, and M. Toyoda, Materials Research Society Symposium Proceedings, 477, pp. 35 – 46, 1997. With permission).

certain level of etching depth needs to be achieved to remove the particle. Even without etching, it is possible to remove the particles that form a strong chemical bond with the substrate if strong external forces such as megasonic irradiation and brush scrubbing are used. It is necessary, however, to pay sufficient attention to potential damage any of the device. Alkali solution and HF are capable of etching the surfaces of Si and SiO2, which are dominant as substrate surfaces in ULSI-manufacturing process. Ammonia used in APM cleaning is equipped not only with a function to etch the substrate surface as an alkali but also with a function to keep the zeta potential of the substrate surface negative. The APM cleaning is widely adopted as a means to remove particles in ULSI-manufacturing process. For surface etching by means of alkali, surface microroughness needs to be noted especially on bare Si surface. When bare Si surface is directly etched with alkali solution, surface microroughness will increase (because

FIGURE 2.19 Particle removal efficiency as a function of etch amount of Si substrate. (From H. Kawahara, K. Yoneda, I. Murozono, and Y. Todokoro, IEICE Trans. Electron. E77-C, Tokyo, No. 3, 492, 1994. With permission.)

54


etching rate is varied depending on crystal orientation). To address this problem, H2O2 needs to be spiked to APM to oxidize the Si surface to be covered with thin SiO2 passivation film. HF is capable of etching SiO2, but it hardly removes the particles by itself because of the above-mentioned action of zeta potential. When DHF with H2O2 and the surfactant (injected) is combined with megasonic irradiation, all functions which are theoretically considered essential for particle removal become available. This combination is experimentally proven to remove particles [36]. Particle removal from substrate surface can be achieved by combining the three functions that is: particle desorption by applying mechanical driving force (Function 1), prevention of particle redeposition by controlling zeta potential and wettability of solid-to-liquid interface (Function 2), and slight etching of underlying film (Function 3). However, with the ever-increasing sizereduction of ULSI devices, it is becoming more and more difficult to apply powerful megasonic irradiation or to etch underlying film by as much as several nanometers. For ULSI devices, beyond the 100 nm node, patterns collapse because of megasonic irradiation and dimensional fluctuation because of etching will become critical. In order to maintain an ultraclean surface, with reduced physical and chemical force, it is important to improve the technology to capture a particle released slightly from substrate surface to prevent it from redepositing (Function 2). For this, new alkali cleaning technologies using surfactants are proposed [26].

VI. CLEANING MECHANISM OF ORGANIC CONTAMINATION Although there are various types of organic contaminants, it is possible to remove most of them by dissolving or decomposing them in liquid. The chemicals used for organic contaminant removal include (i) an oxidizing agent to decompose organic contaminant by taking electrons from the organic molecule, (ii) an organic solvent to dissolve organic contaminant, and (iii) a surfactant to emulsify the oil contaminant. Wet-cleaning process of ULSI/FPD adopts such a technique as cleaning with oxidizing agents including SPM and ozonated ultrapure water, cleaning with alkali solutions combined with oxidizing agent such as APM, and cleaning with organic solvent (for photoresist dissolution). H2O2 in SPM and APM decomposes (oxidizes) organic contaminants by taking electrons from its molecule, and turns it to a soluble organic acid with low molecular weight (e.g., formic acid and acetic acid) or to CO2 to be removed. In particular, SPM is extremely effective in decomposing the organic contaminants as it features very high redox potential (Figure 2.3). SPM cleaning, therefore, efficiently removes photoresist as well as other organic molecules adsorbed on the substrate surface. O3 gas is dissolved in ozonated UPW, and thereby ozone concentration is kept low (around 10 ppm at room temperature). However, ozonated UPW is highly effective in removing adsorbed organic molecules as it features high redox potential (Figure 2.3) and high reaction rate [35,36]. In general, an organic contaminant is easily dissolved into alkali solution as well. Ammonia injected to APM, TMAH used as resist developer, and amines used for resist removal are capable of dissolving organic contaminants. DHF is also capable of removing organic molecules adsorbed on oxide. It is considered suitable because organic contaminant is lifted off as DHF etches underlying oxide. Through oxidation and decomposition, an organic contaminant finally turns into CO2 and H2O. As these two feature low boiling point, gaseous-phase dry cleaning, which is not effective for other contaminants, is capable of removing the organic contaminants. Oxygen excited with plasma and ozone dry cleaning are used to remove the photoresist and adsorbed organic contaminants. Particulate contamination derived from post-dry-ashing photoresist residue contains polymerized or graphitized contaminants, which are difficult to be dissolved or oxidized. This type of contaminant is to be removed through the particle contaminant removal mechanism described in the previous section.


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VII. IMPROVEMENT OF CLEANING EFFICIENCY A contaminant can be removed when the three functions (in Figure 2.1) are made available, provided that each reaction reaches the chemical equilibrium. In order to efficiently perform the cleaning within a short period of time, it is necessary to study reaction rates. Cleaning efficiency can be improved by activating a reaction and by increasing the probability for the chemical species and contaminants to come in contact with each other. Temperature rise (heating) is an effective way to activate reactions. In general, rate of a chemical reaction is doubled as temperature is raised by 108C. (More strictly speaking, this relationship is applicable when the activation energy for the reaction is around 50 kJ/mol.) To increase the probability for chemical species and contaminants to meet each other, it is effective to raise concentration of chemical species. Figure 2.20 shows Cu removal efficiency as a function of concentration of the oxidizing agent. Both HF/O3 and HF/H2O2 solutions feature all the three essential functions (dissolution, prevention of redeposition, and slight etching). However, as the concentration of dissolved ozone in HF/O3 is about 10 ppm at the highest, HF/H2O2 solution with 1000 ppm H2O2 shows better Cu removal efficiency per unit time. Even if the concentration of the chemical species is limited, probability for their reaction with contaminants can be raised by fluidizing or by rotating the substrate at a high speed. A single-wafer spinning cleaning system into which a substrate is loaded one by one and rotated at a high speed with chemical solution being applied is capable of constantly feeding fresh chemical solution in the vicinity of substrate surface and maintaining highly efficient reaction. Compared with cleaning

FIGURE 2.20 The amount of remaining Cu as a function of concentration of the oxidizing agent when the contaminated Si surface is cleaned with DHF solution with various oxidizing agents added. (From H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343– 362, 1996.; H. Morinaga and T. Ohmi, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 257– 268, 1996. With permission.)

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system where substrate is dipped into cleaning bath (so-called wet station), the single-wafer spinning cleaning system can reduce cleaning duration and chemical concentration.

VIII. OPTIMIZATION OF CLEANING SEQUENCE So far the author discussed the cleaning mechanism of each contaminant. What is essential in the actual cleaning process, however, is to efficiently remove all kinds of contaminants with the minimum cleaning steps. It is critical to understand advantages and disadvantages of each cleaning solution, to combine different cleaning techniques in a way to compensate for each other’s, merits and demerits and to select appropriate sequence of the techniques. For example, DHF features high etching capability and effectively removes Al and many other metals adhering to the substrate surface and into film in large amount, but it can not remove Cu or particles. APM with a chelating agent spiked is highly effective in removing organic contaminants, particles, and Cu and in preventing metallic contaminants from redepositing although it is not effective in removing Al that adheres in large amount [10]. DHF cleaning and APM cleaning, when combined each other, capitalize their advantages and make up for their shortcomings. This two-step cleaning process, therefore, is capable of efficiently removing various contaminants [10]. Sequence of cleaning steps is also important. When an organic contaminant is extensively adsorbed on a surface, the organic contaminant must be removed first of all. If a cleaning agent incapable of removing and organic contaminant is used first, contaminants hidden below organic contamination cannot be removed. This leads to uneven performance of cleaning and etching. When no native oxide is allowed on Si surface (e.g., contact hole cleaning), DHF cleaning must be performed as a final cleaning step to completely remove native oxide and the cleaned wafers must be transferred immediately to subsequent process steps.

IX. CLEANING ISSUES TO NOTE IN EACH STAGE OF ULSI MANUFACTURING PROCESS ULSI-manufacturing process is divided into (i) front end of the line (FEOL) in which transistors are built on wafer surface and (ii) back end of the line (BEOL) where metal interconnects are fabricated.

A. CLEANING STEPS IN FEOL Cleaning steps in FEOL include Si substrate cleaning, pregate cleaning, postgate etch cleaning, and cleaning inside of contact hole. In particular, cleaning prior to gate dielectric formation (pregate cleaning) and Si substrate cleaning require the highest cleanliness. In these cleaning steps, it is necessary to minimize not only metallic contaminant, particle and organic contaminants but also surface microroughness. Various cleaning steps after gate dielectric formation, such as postgate etch cleaning, need to give considerations to minimize Si and oxide film loss. Along with shrink of ULSI device, it is increasingly required to reduce film loss. Cleaning steps featuring etching function, such as APM cleaning and DHF cleaning, need to be precisely controlled to reduce the extent of etching by taking such a measure as diluting cleaning solution, reducing cleaning time, and lowering process temperature. Collapse of fine pattern (pattern damage) must be prevented as well. Traditionally, cleaning step combined with megasonic irradiation has been adopted to remove particle. However, highpower megasonic irradiation causes damage to the sub-100-nm device pattern. Other cleaning methods that are combined with strong mechanical driving force also have the same problem. It is critical to find a way to reduce mechanical force in these cleaning steps [26]. To clean the inside of a contact hole, it is important to completely remove the native oxide at the bottom of hole and to prevent excessive etching of dielectric film on the sidewall of hole. In the case of multilevel dielectric films, it is also critical to maintain uniform etching at each film.


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When cleaning step is completed with bare Si surface exposed, watermark generation needs to be checked out because the bare Si surface is hydrophobic. Watermark is generated when water droplet partially remains on a hydrophobic surface. More precisely watermark is generated as HSiO2 3 dissolved in UPW gets precipitated during drying process. There are several ways to hinder watermark generation. To dry the wafer surface, for example, IPA is used to replace ultrapure water on wafer surface. Or the drying process is performed in N2 ambience to prevent Si from eluting into UPW. Conventional substrate surfaces comprise Si or Si compounds such as p-Si, SiO2, and silicide in FEOL. As they are chemically stable, aggressive cleaning methods to dissolve all materials other than Si and oxides, such as RCA cleaning, can be applied. Recently various metallic materials have been introduced as gate materials to raise the density and speed of ULSI device. Metals such as tungsten are adopted for the gate electrode. For gate dielectric and capacitor dielectric, high-k materials such as HfO2, Al2O3, and BST are introduced. As APM cleaning dissolves tungsten and other metals, APM cleaning cannot be used for particle removal. Hf and Al redeposit on substrate once they get dissolved into solution. To remove particles from metal surface, surfactant technology should be applied [26]. A chelating agent can be adopted to prevent dissolved metals from redepositing on substrate surface [26].

B. CLEANING STEPS

IN

BEOL

Cleaning steps in BEOL include post-dry-etch cleaning of Al interconnect, post-dry-etch cleaning of low-k film, and post-CMP cleaning of Cu interconnect fabrication. RCA cleaning cannot be used in BEOL as it dissolves the metal interconnect exposed on surface. Cleaning steps in BEOL, therefore, use an organic solvent, UPW, an organic alkali solution, an organic acid, and low-concentration an inorganic acid. A number of films are being proposed as low-k films. If carbon-doped SiO2 (referred to as SiOC or CDO), fluorocarbon, or organic dielectric film is adopted as the low-k film, cleaning with aqueous solution is extremely ineffective as the surface is hydrophobic. Surfactant addition is effective in changing hydrophobic surface to hydrophilic [26]. Prior to post-CuCMP cleaning, the substrate surface is contaminated with large amount of CMP slurry and Cu fragment. To selectively remove the slurry and Cu fragments without dissolving the partially exposed Cu interconnect, it is effective to use surfactant technology and chelating agent technology [26].

X. SUMMARY This chapter discussed the adsorption and desorption mechanism of trace contaminants, principles of ULSI wet cleaning, and cleaning issues of importance. As ULSI device continuously gets more integrated and less expensive, the industry will keep introducing new materials and new processes. As described in the beginning of this chapter, wet-cleaning is expected to evolve furthermore in the future. What is important is to precisely understand underlying process science and to develop efficient and fluctuation-free technologies based on the science. It will be a great pleasure if the cleaning principles presented in this chapter are found useful among engineers working on improvement of cleaning technology.

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Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing 3. T. Ohmi, T. Imaoka, I. Sugiyama, and T. Kezuka, Metallic impurities segregation at the interface between Si wafer and liquid during wet cleaning, J. Electrochem. Soc., 139, 3317– 3335, 1992. 4. T. Imaoka, T. Kezuka, J. Takano, I. Sugiyama, and T. Ohmi, The segregation and removal of metallic impurities at the interface of silicon wafer surface and liquid chemicals, in: IEICE Transactions on Electronics E75-C, Tokyo, No. 7, 816– 828, 1992. 5. F. W. Kern Jr., M. Itano, I. Kawanabe, M. Miyashita, R. W. Rosenberg, and T. Ohmi, in: Proceedings of 11th Workshop on ULSI Ultra Clean Technology, UCS, Tokyo, p. 23, 1991. 6. H. Hiratsuka, M. Tanaka, I. Tada, R. Yoshimura, and Y. Matsushita, Ultra Clean Technol., 3-3, 258, 1991. 7. Y. Mori, K. Uemura, K. Shimanoe, and T. Sakon, Adsorption species of transition metal ions on silicon wafer in SC-1 solution, J. Electrochem Soc., 142, 3104, 1995. 8. H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, A model for the electrochemical deposition and removal of metallic impurities on Si surfaces, in: IEICE Transactions on Electronics E79-C, Tokyo, No. 3, 343– 362, 1996. 9. H. Morinaga and T. Ohmi, Electrochemical deposition and removal of metallic impurities on si surfaces, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 257 – 268, 1996. 10. H. Morinaga, T. Hoshino, Y. Omura, M. Kitagawa, and M. Aoki, Mechanism of metal hydroxide adsorption onto Si surfaces in alkali solutions and its prevention, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 585 – 592, 2000. 11. H. Morinaga, Physical chemistry of semiconductor wet cleaning, Oyobutsuri Jpn. Soc. Appl. Phys., 69, 568– 574, 2000. 12. H. Morinaga, M. Suyama, and T. Ohmi, Mechanism of metallic particle growth and Metal Induced Pitting (MIP) on Si wafer surface in wet chemical processing, J. Electrochem. Soc., 141, 2834– 2841, 1994. 13. L. Mouche, F. Tardif, and J. Derrien, Mechanism of metallic impurity deposition on silicon substrates dipped in cleaning solution, J. Electrochem. Soc., 142, 2395– 2401, 1995. 14. S. De Gendt, D. M. Knotter, K. Kenis, P. W. Mertens, and M. M. Heyns, Impact of iron contamination and roughness generated in ammonia hydrogen peroxide mixtures (SC1) on 5 nm gate oxide, J. Electrochem. Soc., 145, 2589– 2594, 1998. 15. D. M. Knotter, S. De Gendt, P. W. Mertens, and M. M. Heyns, Silicon surface roughning mechanisms in ammonia hydrogen peroxide mixtures. J. Electrochem. Soc., 147 (2), 736 – 740, 2000. 16. I. Teerlinck, H. F. Schmidt, A. L. P. Rotondaro, T. Q. Hurd, L. Mouche, P. W. Mertens, M. Meuris, M. M. Heyns, D. Vanhaeren, and W. Vandervorst, Impact of the electrochemical properties of silicon wafer surfaces on copper outplating from HF solutions, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 284 – 291, 1996. 17. H. Aoki, S. Yamasaki, and N. Aoto, A new technique for Quantitative analysis of metallic contamination inside deep-submicron-diameter holes, in: Extended Abstracts of SSDM’96, Yokohama, pp. 154– 156, 1996. 18. M. Tsuji, Y. Muramatsu, and N. Aoto, Depth profile of metallic contamination deposited on Si and SiO2 surfaces in APM solution, in: Electrochemical Society Proceedings Series PV.95-20, Pennington, NJ, pp. 316– 322, 1996. 19. G. M. Choi, H. Morita, J. S. Kim, and T. Ohmi, The nature of metallic contamination on various silicon substrates, IEICE Trans. Electron. E82-C, Tokyo, No. 10, 1839–1845, 1994. 20. M. C. Chung, G. A. Marshall, C. W. Pearce, and K. P. Yanders. The prevention of Si pitting in hydrofluoric acid cleaning by additions of hydrochloric acid, J. Electrochem. Soc., 144, 652 – 657, 1997. 21. J. S. Kim, H. Morita, J. D. Joo, and T. Ohmi, The role of metal induced oxidation for copper deposition on silicon surface, J. Electrochem. Soc., 144, 3275– 3283, 1997. 22. J. S. Kim, H. Morita, G. M. Choi, and T. Ohmi, Cleaning efficiency of various chemical solutions for noble metals such as Cu, Ag, and Au on Si wafer surfaces, J. Electrochem. Soc., 146, 4281–4289, 1999. 23. G. M. Choi, H. Morita, H. Morinaga, J. S. Kim, and T. Ohmi, Dependence of Cu impurity removal efficiency on substrates etching rate for various substrates in hydrofluoric solutions, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 272 – 279, 2000. 24. L. M. Loewenstein and P. W. Mertens, Competitive adsorption of cations onto the silicon surface: the role of the ammonium ion in ammonia-peroxide solution, in: Electrochemical Society Proceedings Series PV.99-36, Pennington, NJ, pp. 512 – 519, 2000.


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High-Performance Wet Cleaning Technology Hiroshi Morita Kurita Water Industries Ltd., Tokyo, Japan

Akinobu Teramoto Tohoku University, Sendai, Japan

Hitoshi Morinaga Tohoku University, Sendai, Japan

Senri Ojima Nomura Micro Science Co., Ltd., Kanagawa, Japan

Kenichi Mitsumori Alps Electric Co., Ltd., Sendai, Japan

CONTENTS I. Roles of Wet Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Improvement of RCA Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Current Status of Wet Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Improvement of RCA Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Surface Microroughness Caused by Wet Process . . . . . . . . . . . . . . . . . . . 2. Removal Efficiency of Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Effect of Surface Microroughness on Electrical Characteristics of Thin Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Effect of Megasonic Irradiation in UPW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. MS-Induced Chemical Reactions in UPW . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. MS-Induced Radical Formation in UPW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Specific Gas Dissolved Functional Water and its Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Definition of Functional Water for Wet Cleaning Process . . . . . . . . . . . . . . . . B. Idea of H2-UPW Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Effects of Dissolved Gas in MS Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . 2. Particle Removal by Means of Electrolytic Cathode Water . . . . . . . . . . . 3. Idea of Gas-Dissolving-Type H2-UPW . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Adoption of Module with Built-in Gas Permeating Membrane . . . . . . . . . . . . D. Control of Dissolved Gases (Application of Henry’s Law) . . . . . . . . . . . . . . . E. Deaeration Using Water Vapor Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

64 64 64 64 65 65 68 69 71 71 71 71 73 74 74 74 75 75 75 75 76 76 78 78 61

62


V. O2-Free Wafer-Cleaning Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Microroughness of Silicon Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Hydrogen Termination of Silicon Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Total Room-Temperature Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Total Room-Temperature 4-Step Cleaning Process . . . . . . . . . . . . . . . . . . . . . 1. O3-UPW for Wet Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Cleaning Efficiency of O3-UPW . . . . . . . . . . . . . . . . . . . . . . . . . . . . i. Removal of Organic Impurities . . . . . . . . . . . . . . . . . . . . . . . . ii. Removal of Metallic Contaminants . . . . . . . . . . . . . . . . . . . . . 2. H2-UPW for Wet Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Relation between Dissolved Hydrogen Concentration and Particle Removal Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Removal of Alumina Particle and Silica Particle . . . . . . . . . . . . . . . d. Comparison between APM Cleaning and H2-UPW Cleaning . . . . . e. Effects of H2-UPW Cleaning on Wafer Surface Microroughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f. Cleaning by UPW with Other Gases than Hydrogen Gas Dissolved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g. Particle Removal Mechanism of H2-UPW Cleaning . . . . . . . . . . . i. Effects of NH3 Spike . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii. Effects of MS Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii. Effects of Dissolved Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . iv. Model of Correlation between Dissolved Hydrogen and MS Irradiation (Surplus Radical Model) . . . . . . . . . . . . . v. Verification of Surplus Radical Model . . . . . . . . . . . . . . . . . . h. Applications of H2-UPW Other than Particle Removal . . . . . . . . . . i. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Total Room-Temperature 4-Step Cleaning Process . . . . . . . . . . . . . . . . . 4. Purity of Gas-Dissolved Functional Water . . . . . . . . . . . . . . . . . . . . . . . B. Total Room-Temperature 5-Step Cleaning Process . . . . . . . . . . . . . . . . . . . . 1. Design of 5-Step Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Cleaning by HF þ O3-UPW (FOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Prevention of Decomposition of Surfactant . . . . . . . . . . . . . . . . . . . . . . . C. Strategy in Cleaning Process in BEOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Surfactant and Chelating Agent Technologies for Innovating Semiconductor Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Challenges in Cleaning Technology to Address Higher-Density ULSI Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Solutions to the Challenges from the Viewpoint of Cleaning Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Preventing Metal Cross Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Accelerating Fine Particle Removal without Device Pattern Damage . . . . . . E. Single-Wafer Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Cleaning of New Material Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Challenges of New Material Surface Cleaning . . . . . . . . . . . . . . . . . . . . . 2. Preventing both Material Corrosion and Contaminant Redeposition . . . .

79 79 79 82 86 86 86 86 86 87 87 88 90 90 90 91 91 92 94 95 95 96 97 97 99 100 100 101 101 102 102 104 106 107 109 110 110 111 112 112 115 115 115 116

High-Performance Wet Cleaning Technology

63

3. Use of Surfactant as Corrosion Inhibiter . . . . . . . . . . . . . . . . . . . . . . . . . . 117 4. Improvement of Wettability of Hydrophobic Surface . . . . . . . . . . . . . . . . 118 G. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 VIII. FTIR-ATR Calibration Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 A. FTIR-ATR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 B. Preparation of Reference Block with Organic Film Adsorbed to Obtain Calibration Curve for Quantification of Organic Compound Adsorption onto Si Surface: Langmuir – Blodgett Technique . . . . . . . . . . . . . . 119 C. Preparation of Calibration Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 D. Adsorption of Organic Compound to Si Surface in Cleanroom Ambience . . . 123 E. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 IX. Balanced Push Pull Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 B. Ultrasonic Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 1. Necessity of Ultrasonic Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 2. Fine Particle Elimination Mechanism of Ultrasonic Cleaning . . . . . . . . . 127 a. Elimination Principle of Adhesive Particles from Substrates . . . . . . 127 b. Possibility of Occurrence of Megasonic and Cavitation . . . . . . . . . 127 c. Noise Generation Research at the Time of Megasonic Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 d. Estimation of Elimination Mechanism of Adhesive Particles from Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 C. BPP Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 1. Concept of BPP Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 2. Ultrasonic Oscillator Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 a. Oscillator Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 b. Oscillator Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 c. Enclosure Materials and Surface Processing . . . . . . . . . . . . . . . . . . . 135 d. Board Thickness of Enclosures (Ultrasonic Radiant Boards) . . . . . 136 e. Oscillator and Enclosure (Radiant Board) Bonding . . . . . . . . . . . . . 136 f. Enclosure (Radiant Board) Structure . . . . . . . . . . . . . . . . . . . . . . . . . 136 3. Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 4. Cleaning Fluid Supply/Discharge Section (Rectification Mechanism) . . 137 a. Optimization of the Cleaning Fluid Supply/Discharge Section . . . . 139 b. Fluid Flow Calculation Method for the Current Plate of the Drilling Tubule Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 i. Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 ii. Calculation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 c. Fluid Flow Calculation Method for the Current Plate Made by Porous Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 i. Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 ii. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 iii. Calculation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 d. Validation of Calculation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 e. Structure of the Cleaning Fluid Supply/Discharge Section of the BPP Nozzle for Cleaning of 720 mm Substrate . . . . . . . . . . . 142 5. Optimum BPP Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 D. Cleaning Capacity of BPP Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144


64

1. Cleaning Power Evaluation using 6-in. Glass Substrates and Silicon Wafers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Cleaning Evaluation using the 550 mm 650 mm Glass Substrate . . . . 3. Cause of Performance Differences with Traditional Ultrasonic Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Comparison of Acoustic Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Comparison of Ultrasonic Applied Areas . . . . . . . . . . . . . . . . . . . . 4. Ideal Situation of Ultrasonic Cleaning Equipment . . . . . . . . . . . . . . . . . . E. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

144 145 146 147 147 148 149 150

I. ROLES OF WET CLEANING PROCESS HIROSHI MORITA

In order to improve the quality and yield in semiconductor manufacturing process, it is essential to realize “ultraclean Si wafer surface” on which ultra-fine patterns are fabricated. The ultraclean Si wafer surface is defined as a surface which satisfies the following eight requirements [1]: 1. 2. 3. 4. 5. 6. 7. 8.

Free from particles Free from metallic impurities Free from organic impurities Free from unintended native oxide Maintain atomic-order “microroughness” With top surface completely terminated with hydrogen Free from moisture adsorption Free from charge-up

Si wafer goes through wet cleaning process before and after each cycle composed of film deposition, photolithography, and etching steps. The number of cleaning steps is about twice as many as the number of masks used in semiconductor manufacturing process. In addition, chemical mechanical polishing (CMP) process which has been rapidly adopted to planarize wafer surface requires cleaning step right after its completion. Needs for cleaning step keep increasing. Needless to say, the main players of semiconductor device production process are process technologies such as film deposition, photolithography, and etching. Without the wet process which supports the three essential steps, semiconductor devices cannot however be manufactured.

REFERENCE 1. T. Ohmi, Proposal of advanced wet cleaning of silicon surface, in: Extended Abstracts of 188th Electrochemical Society Meeting, Chicago, No. 429, pp. 680 –681, 1995.

II. IMPROVEMENT OF RCA CLEANING PROCESS HIROSHI MORITA

A. CURRENT STATUS OF WET CLEANING PROCESS Wet process in mass production factories still uses RCA cleaning which was published in 1970 [1] while modifying it to some extent to meet their specific needs. The RCA cleaning is usually combined with SPM (H2SO4 þ H2O2) and DHF (diluted hydrofluoric acid). For example, the RCA


65

cleaning is performed as follows: SPM ! DHF ! APM (NH4 OH þ H2 O2 þ UPW) ! hot UPW ! HPM (HCl þ H2 O2 þ UPW) ! DHF Between cleaning steps and after the entire RCA process, ultra pure water (UPW) rinsing needs to be repeated several times to rinse off high-concentration chemicals from wafer surface. The RCA cleaning consumes chemicals and UPW in large volume, and it requires heat source to heat chemicals and UPW. As a result, the RCA cleaning process discharges chemical vapor as well as chemical wastes in large volume. It also requires huge energy consumption for air-conditioning system which processes the outdoor air into high-purity clean air in order to replace air contaminated with chemical vapor. Because of the active vapor generation, control of chemical concentration is not at all easy in the RCA process. In order to maintain acceptable reproducibility in cleaning performance, special concentration monitors need to be mounted on chemical delivery system, and chemicals need to be replenished regularly to compensate for the reduction due to evaporation and decomposition. The conventional cleaning process which repeatedly treats Si wafer with high-concentration chemicals at high temperature is too aggressive and wasteful as contamination of Si wafer is extremely marginal. As described earlier, the RCA cleaning is found problematic in terms of environmental conservation and manufacturing cost. At present, however, most mass-production facilities of semiconductor devices keep using the RCA cleaning and modified RCA cleaning, although they are aware of its problems, as the RCA cleaning enables them to maintain some device yield.

B. IMPROVEMENT OF RCA CLEANING PROCESS Prof. Ohmi’s Laboratory of Tohoku University made ahead starting to improve the RCA cleaning process to overcome the above-mentioned problems. The laboratory revealed that microroughness as well as impurities on Si wafer surface had a lot to do with electrical characteristics of semiconductor device. To meet the dual objectives, namely to remove impurities and to reduce microroughness on Si wafer, the Laboratory succeeded in reducing chemical concentration of NH4OH in APM solution. 1. Surface Microroughness Caused by Wet Process Such surface contaminants as particles, organic materials, metallic impurities, and native oxide, degrade the device performance and yield. Before moving on to a following process, these impurities have been usually removed in the RCA cleaning in factory. However, the concept of RCA cleaning does not place the emphasis on the surface microroughness. This is because the microroughness did not affect the device characteristics when Kern proposed the RCA cleaning, since the device geometry at that time was at the 10-mm level, and the thickness of gate oxide was several hundred nanometers. First of all in this section, the studies on surface microroughness of the wafer treated with the conventional RCA cleaning evaluated with Scanning Tunneling Microscope (STM) are presented. The RCA cleaning employed in this experiment used the cleaning solutions with the conventional composition as follows: SPM ¼ H2SO4 (98%): H2O2(30%) ¼ 4:1, APM ¼ NH4OH(28%):H2O2(30%):H2O ¼ 1:1:5, HPM ¼ HCl(36%): H2O2(30%):H2O ¼ 1:1:6. There is a clear difference of the surface microroughness between the wafer before cleaning and the wafer after cleaning. This result indicates that the reason for the increase in surface microroughness can be found in the wet process. The next investigation is to identify which process of the RCA cleaning affects the surface microroughness. Figure 3.1 presents the surface microroughness of the n-type Cz wafer when it was treated with the SPM cleaning and the HPM cleaning. Figure 3.2 shows the surface

66

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing 1.0 Cz n type

Ra [nm]

0.8

0.6

0.4

0.2

0.0 BLANK

SPM

HPM

Surface–Microroughness of Various cleaning Methods SPM:H2SO4–H2O2 Cleaning HPM:HCI–H2O2–H2O Cleaning

FIGURE 3.1 Surface microroughness for HPM cleaning and fourth-time cycle SPM cleaning.

microroughness of n-type Cz wafer, p-type Cz wafer, and n-type FZ wafer which were treated with APM cleaning with the mixing ratio varying from 1:1:5 to 0.05:1:5. The temperature during the APM cleaning was kept at 80 –908C. Figure 3.1 indicates that neither the SPM cleaning nor the HPM cleaning increases the surface microroughness. On the other hand, as shown in Figure 3.2, the surface microroughness on the Cz wafer gradually increases as the NH4OH mixing ratio in the APM cleaning gets higher. The increase and the deviation of the surface microroughness on the Cz wafer are found to be suppressed at the initial level when the NH4OH mixing ratio in the APM cleaning is reduced to 0.05. In the case of the FZ wafer, however, the surface microroughness shows little increase.

1.0

Ra [nm]

0.8

Cz n type Cz p type Fz n type

0.6

0.4

0.2

0.0 BLANK 0.05

0.10

0.25

1.0

Mixing Ratio of NH4OH in NH4OH–H2O2–H2O Solution (a:1:5)

FIGURE 3.2 Relationship for surface microroughness of Cz and FZ wafer having various NH4OH mixing ratio in APM solution.


67

Besides the dependence on the NH4OH mixing ratio is not so obvious for the FZ wafer as that for the Cz wafer: the surface microroughness slightly increases only when the FZ wafer is treated with the NH4 OH22H2 O222H2 O solution with the mixing ratio of 1:1:5. The difference between the Cz and FZ wafers is considered to be caused by difference of vacancy concentration in the wafer substrate which depends on the time when the temperature is kept close to 14208C (the melting point of silicon), during the crystal growth. It is considered that the surface microroughness is increased on the Cz wafer because the Si vacancy clusters are scattered in a non-uniform manner. Meanwhile, no difference of the surface microroughness level is indicated between the p-type and the n-type wafer in these experimental results. Figure 3.3 shows the relationship between the Si etching rate and NH4OH mixing ratio in APM cleaning at 808C, using Cz and FZ wafers. The Si etching rate of APM solution gets lower as the NH4OH mixing ratio decreases. It is clear that the etching rate completely depends on the NH4OH concentration. There is no difference between the Cz and FZ wafers in terms of the etching rate. These results as shown in Figure 3.2 and Figure 3.3 indicate that the APM solution with the reduced NH4OH mixing ratio of 0.05:1:5 can be used for the Si surface cleaning without degrading the surface microroughness. The entire process of RCA cleaning employing the APM cleaning with lower NH4OH mixing ratio was evaluated. As shown in Figure 3.4, the surface microroughness increases after going through the entire RCA cleaning process even if the level of the mixing ratio was 0.05:1:5 [2,3,5]. In order to further investigate this problem, two different procedures were evaluated for the process following the APM cleaning as shown in Figure 3.5: Process 1 (conventional procedure) and Process 2 (a new procedure). Even when the mixing ratio of APM solution is changed to 0.05:1:5, the surface microroughness increases in the hot UPW rinsing of the conventional RCA cleaning. When the room temperature UPW rinsing is introduced right after the APM cleaning, however, there is no degradation of the surface microroughness. In the conventional RCA process, ammonium ions adhering on the wafers and wafer cassettes are carried over to the hot UPW bath to raise the pH level of the UPW to about 8.0. This is regarded as the main reason for the surface microroughness increase. Therefore, in order to maintain the surface smoothness, the room temperature UPW rinsing is definitely required right after the APM cleaning.

1.0

ETCHING RATE [nm/min]

Cz n (100) 0.8

FZ n (100)

0.6

0.4

0.2 ER = 0.36LogCNH

4OH

0.0 10−3

10−2

10−1

10−0

+ 0.60 101

Mixing Ratio of NH4OH in Mixture Solution (a:1:5)

FIGURE 3.3 Relationship between the Si etching rate and the NH4OH mixing ratio in APM cleaning using Cz and FZ wafers.

68

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing RCA Cleaning 1.0 Cz n type

Ra [nm]

0.8

0.6

0.4

0.2

0.0 BLANK 0.05

0.10

0.25

1.0

Mixing Ratio of NH4OH in NH4OH–H2O2–H2O solution (a:1:5)

FIGURE 3.4 Surface microroughness of wafers treated in an entire RCA cleaning process having four different NH4OH mixing ratios in APM solution.

Further, it has been revealed that this combination of room temperature UPW rinsing and hot UPW rinsing makes it easier to remove organic materials.

2. Removal Efficiency of Particles The previous section described that the surface smoothness can be maintained by reducing the NH4OH mixing ratio and by introducing the room temperature UPW rinsing. 1. NH4OH/H2O2 0.05

HOT DI Water

RT DIW

2. NH4OH/H2O2 0.05

RT DI Water

HOT DIW

1.0 Cz n type

Ra [nm]

0.8

0.6

0.4

0.2

0.0 BLANK

Hot DI RT DI Water Water 1 2 Various Rinsing Methods

After NH4OH–H2O2–H2O Cleaning

FIGURE 3.5 Influence of succeeding step of APM cleaning on surface microroughness.


69

FIGURE 3.6 Particle removal efficiency as a function of NH4OH mixing ratio in APM cleaning for four different particles such as PSL, silica, particles from city water, and particles from outside air.

The major purpose of APM cleaning is to remove particles from the wafer surface. The particle removal efficiency of APM cleaning with various NH4OH mixing ratios was evaluated, using polystyrene latex (PSL), silica, particle in city water, and particle in the air. The result is shown in Figure 3.6, which indicates that the highest particle removal efficiency can be obtained when the NH4OH mixing ratio is reduced to 0.05 –0.01 from the conventional mixing ratio, [2,3,5]. In this experiment, the particle number was measured after the room temperature UPW rinsing following the APM cleaning. In Figure 3.6, the efficiency to remove PSL and particles in the air plummets because particles are organic materials. As the NH4OH mixing ratio in the APM cleaning goes up, the viscosity of the organic particle surface gets higher in the solution. As a result, these organic particles adhere more easily onto the surface. The hot UPW rinsing is required after the room temperature UPW rinsing to remove organic materials.

3. Effect of Surface Microroughness on Electrical Characteristics of Thin Oxide As the feature size of the ultra large-scale integration (ULSI) devices get smaller, the gate oxide and oxide used as the storage capacitor are getting thinner and thinner. Therefore, in order to maintain the performance, the reliability, and yield of devices, it is extremely important to improve the film quality of thin oxide. This experiment has confirmed that the electrical characteristics of thin oxide such as the dielectric breakdown field intensity (EBD) have a close relationship with the surface microroughness. Metal oxide semiconductor (MOS) diodes were prepared by depositing 9.5 and 10 nm oxide film on the n- and p-type substrates with various surface microroughness level followed by the aluminum metalization. Figure 3.7 shows the relationship between the surface microroughness and the EBD. In this experiment, EBD means the average value of the electric field intensity of the intrinsic breakdown when the positive voltage is applied to the Al electrode to form the electron accumulation layer on the n-type substrate surface and when the negative voltage is applied to the

70


BREAKDOWN FIELD EBD [MV/cm]

17 15

Epi p Cz p Cz n FZ n

13

tox = 10 nm

11 9 0.0

0.2

0.4 0.6 Ra [nm]

0.8

1.0

FIGURE 3.7 Dielectric breakdown field intensity (EBD) as a function of average surface microroughness having oxide thickness of 10 nm formed on n-type Cz and FZ wafers, and having oxide thickness of 9.5 nm formed on p-type Cz and EPI wafer.

Al electrode to form the electron accumulation layer on the p-type substrate surface. The measure˚. ment area was 1.6 1024 cm2 and the judgment electric current was 1 1024 A It is shown that EBD gets higher as the surface microroughness decreases. Moreover, no difference is observed between the n-type Cz wafer and n-type FZ wafer provided that the surface microroughness is at the same level. And, furthermore, no difference is observed between the p-type Cz wafer and the p-type EPI wafer provided the surface microroughness level is the same for both the wafers. Succeedingly, the charge to breakdown QBD has been evaluated at a field intensity of 9.5 MV/cm for these MOS diodes having an area of 1.6 1024 cm2 as shown in Figure 3.8, where the breakdown is defined by the diode current increasing up to 1 1025 A. It is clearly seen from Figure 3.8 that the value of QBD rapidly increases with a decrease of surface microroughness. The QBD characteristics strictly relate to the reliability of the ULSI device having very thin oxide films. Results obtained in Figure 3.7 and Figure 3.8 clearly indicate the importance of substrate surface smoothness in future advanced ULSI devices particularly for the reliability. Besides it has been found the surface microroughness on Si – SiO2 interface affects not only the breakdown voltage of oxide and the QBD but also the channel mobility of MOSFET: the channel mobility gets bigger as the surface microroughness decreases [4,5].

103

QBD [C/cm2]

tox = 9.8 nm E = 9.5 MV/cm

102

101 0.0

0.2

0.4

0.6

0.8

1.0

Ra [nm]

FIGURE 3.8 Surface microroughness dependence of QBD (C/cm2) under a constant field of 9.5 MV/cm.


71

REFERENCES 1. W. Kern and D. A. Puotinen, Cleaning solution based on hydrogen peroxide for use in silicon semiconductor technology, RCA Rev., 31, 187 – 205, 1970. 2. M. Miyashita, M. Itano, T. Imaoka, I. Kawanabe, and T. Ohmi, Optimized NH4OH/H2O2 cleaning process for ultraclean wafer surface preparation, in: Extended Abstracts of the 179th Electrochemical Society Meeting, Washington, DC, No. 463, pp. 709 – 710, 1991. 3. M. Miyashita, M. Itano, T. Imaoka, I. Kawanabe, and T. Ohmi, Dependence of thin oxide films quality on surface microroughness. in: 1991 Symposium on VLSI Technology, Oiso, pp. 45–46, 1991. 4. T. Ohmi, M. Miyashita. M. Itano, T. Imaoka, and I. Kawanabe, Dependence of thin oxide films quality on surface microroughness, IEEE Trans. Electron Devices, 39, 537 – 545, 1992. 5. M. Miyashita, T. Tsuga, K. Makihara, and T. Ohmi, Dependence of surface microroughness of CZ, FZ and EPI wafers on wet chemical process, J. Electrochem. Soc., 139, 2137– 2146, 1992.

III. EFFECT OF MEGASONIC IRRADIATION IN UPW [1] HIROSHI MORITA

A. INTRODUCTION Recently, megasonic (MS, ultrasonic featuring frequency of about 1 MHz) irradiation becomes to be often adopted in wet cleaning process. In this section, the effect of MS in cleaning solution is discussed.

B. MS-INDUCED CHEMICAL REACTIONS

IN

UPW

When UPW is irradiated by MS, the properties of the water change. Figure 3.9 shows the change in resistivity of UPW [2 – 4]. It was reduced drastically by MS irradiation. However, when thoroughly “deaerated UPW” was suffered by MS irradiation, the resistivity of the water did not change. In this figure, a lag time from beginning of MS irradiation to resistivity reduction was found. It corresponds to the holding time taken for the MS-irradiated water to cover the distance to the resistivity sensor.

FIGURE 3.9 Variation of resistivity of UPW with MS irradiation time.

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2 FIGURE 3.10 Effect of dissolved air on formation of NO2 2 and NO3 in MS-irradiated UPW.

2 2 Figure 3.10 shows NO2 x (NO2 and NO3 ) generation in UPW irradiated by MS. When the dissolved air concentration in UPW was the saturated level (around 24 ppm), NO2 x concentration was increased with MS irradiation time. On the other hand, when the dissolved air concentration was at a low level (0.14 ppm), NO2 x generation could not be observed. This suggests that a critical concentration of “dissolved gas” exists for NO2 x generation by MS irradiation. From Figure 3.9 and Figure 3.10, it can be understood that resistivity reduction in UPW is due to NO2 x and other ionic substances generated by MS irradiation. Figure 3.11 shows reaction paths which will occur during MS irradiation in UPW containing nitrogen gas. In the case of NHþ 4 formation, H radical formed from UPW by MS irradiation reacts with N2 to produce NH3. In the case of NO2 2 formation, two reaction courses can be assumed. In the first reaction course, H2O2 is generated by recombination of †OH radicals formed from H2O. H2O2 decomposes to give þ oxygen atom O and H2O. The oxygen atom O and H† radical react with N2 to produce NO2 2 and H . † † In the second reaction course, OH radical and H radical which were formed from H2O and þ oxygen atom (formed from H2O2) react with NH3 to produce NO2 2 , H , and H2O.

FIGURE 3.11 Ionic substance generation process in UPW with MS irradiation.


73

20

Concentration of H2O2 [ppm]

Dissolved air : 0.14 [ppm] Dissolved air : 24.0 [ppm] 15 Megasonic 0.95 MHz Supplied power 270 W 10

5

0 3

10 20 Megasonic irradiation time [min]

40

FIGURE 3.12 Effect of dissolved air on H2O2 formation in MS-irradiated UPW.

In the case of NO2 3 formation, three reaction courses can be assumed. In the first reaction course, H2O2 decomposes to give oxygen atom O and H2O. The oxygen atom formed from þ H2O2 reacts with H† radical and N2 to produce NO2 3 and H . In the second reaction course, OH þ radical, H radical and oxygen atom O react with NH3 to produce NO2 3 , H , and H2O. In the 2 2 third reaction course, oxygen atom O reacts on NO2 to produce NO3 [5]. When dissolved gas was removed sufficiently, H2O2 could not be found in the solution irradiated by MS. Figure 3.12 shows the test result. It can be understood that OH radical recombination cannot happen in the thoroughly deaerated water because no OH radicals are generated in this water [6].

C. MS-INDUCED RADICAL FORMATION IN UPW From the previous section, it is confirmed that some chemical species are generated in UPW by MS irradiation. Furthermore, it was suggested that the chemical species generation is because of radical formation by MS irradiation. Some factors which influence radicals formation were investigated. It is essential to control dissolved gas concentration in order to achieve high radical generation efficiency. It is necessary, therefore, to clarify the reaction between dissolved gas concentration and radical formation. Generated radicals in sample solutions were measured by electron spin resonance (ESR) analysis.

FIGURE 3.13 Effect of dissolved gas concentration on OH radical formation in MS-irradiated UPW.


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Before MS irradiation

After MS irradiation Deaerated UPW

After MS irradiation O2 bubbling: 3 min

FIGURE 3.14 Radical formation in MS-irradiated UPW: deaerated water versus O2 dissolved water.

Figure 3.13 shows the effect of dissolved gas concentration and its species in MS-irradiated water. Concerning the enhancement of †OH radical formation, Figure 3.14 shows the ESR spectrum of UPW in various conditions. Signal of †OH radical did not occur to increase in thoroughly deaerated UPW both with and without MS irradiation. It seems that it was difficult to generate OH radical in deaerated UPW, even when MS was irradiated. On the other hand, the signal of OH radical was detected therefore, when O2 gas was dissolved into UPW.

REFERENCES 1. T. Ohmi, M. Toda, M. Kato, K. Kawada, and H. Morita, Megasonic irradiation induced chemical reaction in the solution for silicon wafer cleaning, in: Proceedings of Material Research Society Symposium, Vol. 477, pp. 3 – 14, 1997. 2. M. Toda, M. Kato, K. Kubo, S. Ojima, and T. Ohmi, Radical activation of DI water and cleaning solution by megasonic. in: Extended Abstracts of the 188th Electrochemical Society Meeting, Chicago, No. 444, pp. 708– 709, 1995. 3. K. Kubo, S. Ojima, M. Toda, and T. Ohmi, Study on megasonic in advanced wet cleaning process, in: Extended Abstracts of the 188th Electrochemical Society Meeting, Chicago, No. 445, pp. 710 – 711, 1995. 4. M. Kato, M. Toda, K. Kubo, S. Ojima, Y. Sakata, and T. Ohmi, The effect of sonication on ultra pure water, in: Proceedings of 1996 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, pp. 179– 195, 1996. 5. M. Toda, M. Kato, and T. Ohmi, The effect of megasonic irradiation on ultra pure water, in: Abstract of Third International Symposium on Ultra Clean Processing of Silicon Surfaces (UCPSS’96), Antwerp, p. 2, 1996. 6. K. Kubo, S. Ojima, Y. Sakata, M. Kato, M. Toda, and T. Ohmi, The impact of radical activated ultra pure water, in: Proceedings of 1996 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, pp. 196– 214, 1996.

IV. SPECIFIC GAS DISSOLVED FUNCTIONAL WATER AND ITS PRODUCTION HIROSHI MORITA

A. DEFINITION

OF

FUNCTIONAL WATER FOR WET CLEANING PROCESS

Recently, the word “functional water” is used often. The definition, however, is uncertain. Ohmi of Tohoku University defined the functional water for wet process as follows: . . . .

Available solution having high efficiency in wet cleaning process Based on UPW Added very small amount of specific gas and chemical (ppm order) Sometime, combined with some physical energy (e.g., ultrasonic)


75

Ozonated ultrapure water (O3-UPW), hydrogenated ultrapure water (H2-UPW), and electrolytic ionized water are given examples.

B. IDEA OF H2-UPW CLEANING [1] 1. Effects of Dissolved Gas in MS Cleaning A simple cleaning test using MS irradiation combined with “surfactant” added UPW was performed, and two cases were compared with each other. In the first case, surfactant-added UPW was deaerated, and in the second case, it was not. Sound pressure measurement of cleaning solution revealed that MS propagation was improved as concentration of dissolved gases was decreased by enhancing “deaeration” process [2]. Figure 3.15 shows the concentration of dissolved gas as a function of sound pressure. It was expected that cleaning efficiency improved as sound pressure was raised, however, particle removal efficiency was found to be lowered in highly deaerated solution. It was recognized that gases should be fully dissolved when UPW is used as the primary component in cleaning solution with MS irradiation. 2. Particle Removal by Means of Electrolytic Cathode Water Around the same time, a new wet cleaning technology was established which was based on electrolytic ionized water prepared by a special electrolysis system [3 – 5]. In general, electrolytic ionized water includes acidic ionized water (anode water) and alkaline ionized water (cathode water) both of which are generated by electrolyzing city water with much salt dissolved. For wet process, however, electrolytic ionized water was often produced by using salt-free UPW as a source water. Reducing cathode water was reported to feature high particle removal efficiency. 3. Idea of Gas-Dissolving-Type H2-UPW Electrolytic cathode water is defined, in a simplified manner, as hydrogen-dissolved water: hydrogen gas generated by electrolyzing water is dissolved in UPW. Meanwhile, as discussed earlier, particle removal efficiency is improved as the gas is fully dissolved in cleaning solution by MS irradiation combined with a cleaning solution having UPW as the primary component. On the basis of these two facts, a hypothesis can be formulated that water with high-concentration hydrogen gas dissolved in it should feature excellent particle removal efficiency when combined with MS irradiation.

FIGURE 3.15 Variation of sound pressure of UPW with MS irradiation: effect of deaeration.


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C. ADOPTION

OF

MODULE

WITH

BUILT-IN GAS PERMEATING MEMBRANE

It was explored that a simple method to generate H2-UPW which was expected to feature high cleaning efficiency based on the idea and the assumption described earlier. Four requirements were identified: 1. 2. 3. 4.

To dissolve hydrogen at high concentration Not to sacrifice purity of UPW To minimize hydrogen gas consumption To keep water free from bubbles

For requirement 1 (hydrogen concentration), a specific target was set to increase hydrogen concentration to 1.6 ppm which is the saturation concentration of hydrogen in UPW at room temperature under atmospheric pressure. For requirement 2 (purity), maintaining purity of H2-UPW at the same level as source water (UPW) was aimed. For requirement 3, a specific target was to have the whole delivered hydrogen gas dissolved or not to allow any hydrogen gas to be excessive. For requirement 4, it was noted not to allow any bubbles to generate in H2-UPW delivery piping system and at the point of use (POU). It is because bubbles if any are present in the cleaning solution, would deteriorate the cleaning uniformity as they adhere to substrate surface during cleaning step. In order to make an efficient and safe use of hydrogen gas without allowing it to evaporate into the cleanroom, a module with a built-in gas-permeating membrane which would selectively allow gas to permeate but not allow liquid to permeate through was tried to apply.

D. CONTROL

OF

DISSOLVED GASES (APPLICATION OF HENRY’S LAW)

Initially it was tried to dissolve hydrogen gas into UPW merely by feeding UPW into the module and introducing hydrogen gas to gaseous phase in the module, UPW used as source water was not treated with the preliminary deaeration step. By measurement with a polarograph-type dissolved hydrogen concentration meter, hydrogen concentration in the treated water prepared with this method was found as low as 0.6 ppm. When more hydrogen gas was introduced in a bid to raise hydrogen concentration, excessive hydrogen gas which did not dissolve appeared as bubbles in treated water. This method was found not effective in increasing dissolved hydrogen concentration to a level of saturation concentration. It was speculated because partial pressure of hydrogen was not sufficiently raised due to much amount of nitrogen which was originally dissolved in UPW. Concentration of dissolved gas is determined by partial pressure of the gas and its specific saturation concentration (Henry’s law). For air-saturated water, for instance, it is reported that concentration of dissolved oxygen and nitrogen is about 9 ppm and about 16 ppm, respectively, at room temperature and under atmospheric pressure. These figures are derived from the following equations: Saturation concentration of oxygen (44 ppm) partial pressure ð0:2 atmÞ ¼ 9 Saturation concentration of nitrogen ð20 ppmÞ partial pressure ð0:8 atmÞ ¼ 16 In order to produce high-concentration H2-UPW, partial pressure of hydrogen should be raised. There are two methods to raise hydrogen partial pressure: . .

To feed large amount of hydrogen into UPW to force hydrogen proportion to rise To remove dissolved nitrogen and other gases from UPW to raise hydrogen proportion

It was decided to adopt the latter method and try a series treatment. Two gas-permeating membrane modules were connected in series. At the first module, the gaseous phase was connected to


H2 Gas UPW

77

Degas

H2 Gas

UPW

Saturated Conc. 1.6 Dissolved H2 (ppm)

1.4 1.2

Dissolved H2 Increase by pre-Degassing

1.0 0.8 0.6 0.4 0.2 0 Without Deaeration

After Deaeration

FIGURE 3.16 Dissolved hydrogen concentration of two kinds of treated water: only dissolving versus predeaeration and dissolving.

vacuum pump to treat the fed UPW with reduced-pressure membrane deaeration. At the second module, hydrogen gas is introduced to the UPW. Figure 3.16 shows concentration of dissolved hydrogen. This method was proven to be effective in easily raising concentration of dissolved hydrogen to a level of saturation concentration. It was also revealed that injected hydrogen gas was completely dissolved in a short time unless the amount of injected hydrogen gas was more than that of dissolved gases which were removed with deaeration treatment. Figure 3.17 shows concentration of dissolved hydrogen in treated UPW at various flow rates of hydrogen gas introduced. In this test, UPW which was used as source water went through membrane module first for preliminary deaeration. Then, the test was performed by changing flow rate of deaerated UPW to three different levels. At any flow rate, measured concentration of hydrogen gas shows

FIGURE 3.17 Relationship between supplied hydrogen gas and dissolved hydrogen concentration.


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FIGURE 3.18 Reduction of dissolved oxygen using water vapor sweeping in deaeration membrane module.

a good agreement with theoretical concentration of when the entire hydrogen gas fed to the module is completely dissolved. This test result demonstrates that the above-described series treatment of “deaeration ! gas injection” is effective in dissolving the whole hydrogen gas introduced, which makes it possible to minimize hydrogen gas consumption and prevent excessive hydrogen gas from remaining not dissolved. Thus, a simple production system of high-concentration H2-UPW to satisfy the four requirements was established [6 –9]. The cleaning efficiency of H2-UPW is described later.

E. DEAERATION USING WATER VAPOR SWEEP [10] Concerning deaeration with membrane module, an effective method was developed. Figure 3.18 shows the relation dissolved oxygen to vacuum level when cleaning solution deaerated by evacuating and introducing sweep gas of 100% water vapor in evacuation area. As to dissolved oxygen (DO2) and nitrogen (DN2) at 7 Torr, although it indicated 3– 5 ppb of DO2 and 5– 6 ppm of DN2 when solution was deaerated by normal evacuating, it indicated 0.5– 0.7 ppb of DO2 and under detection limit of DN2 (1 ppm) by using water vapor sweeping. Although DO2 and DN2 increased clearly at 7 torr under deaeration by normal evacuation, under deaeration using water vapor, DO2 and DN2 did not increase even upto a vacuum level of 30 torr. It is believed that the conventional method could not deaerate efficiently whereas the new method which employs a sweep gas of 100% water vapor in evacuation area decreases the dissolved gases efficiently and hence effective deaeration.

REFERENCES 1. H. Morita, New Si Substrate Cleaning Technology Using Functional Water, Ph.D. Dissertation, Tohoku University, Sendai, Japan, 2000. 2. H. Morita, K. Kawada, S. Okano, T. Nitta, and T. Ohmi, Development of an advanced wet cleaning equipment with megasonic enhancing system, in: Proceedings of the Fifth International Symposium on Semiconductor Manufacturing (ISSM’96), Tokyo, pp. 337 – 340, 1996. 3. H. Aoki, M. Nakamori, N. Aoto, and E. Ikawa, Ecologically-safe ionized water treatment for wafer processing, in: Symposium of the VLSI Technical, Digest, Kyoto, pp. 107 – 108, 1993. 4. H. Aoki, T. Nakajima, K. Kikuta, and Y. Hayashi. Novel electrolysis-ionized-water cleaning technique for the chemical-mechanical polishing (CMP) process, in: Symposium of the VLSI Technical, Digest, Kyoto, pp. 79– 80, 1994.


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5. K. Yamanaka, T. Imaoka, T. Futatsuki, T. Iwamori, Y. Yamashita, H. Aoki, and A. Yamazaki, Improved electrolytic ionized water produced by a novel electrolytic cell for wet cleaning process, in: Proceedings of 1995 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, pp. 1 – 22, 1995. 6. H. Morita, J. Ida, T. Mizuniwa, and T. Ohmi, Hydrogenated ultrapure water production system for future wet cleaning process, in: Abstract of Fourth International Symposium on Ultra Clean Processing of Silicon Surfaces (UCPSS’98), Ostende, pp. 3 – 4, 1998. 7. H. Morita, J. Ida, T. Mizuniwa, and T. Ohmi, Hydrogenated ultrapure water production system for future wet cleaning process, in: Proceedings of UCPSS’98, pp. 7 – 10, 1998. 8. H. Morita, J. Ida, T. Mizuniwa, N. Yabumoto, S. Kushibe, and T. Ohmi, Dissolved gas controlled ultrapure water production system for wet cleaning processes, in: Proceedings of the Seventh International Symposium on Semiconductor Manufacturing (ISSM’98), Tokyo, pp. 428 – 431, 1998. 9. J. Ida, H. Morita, and T. Ohmi, Particle removal effect of hydrogenated ultrapure water with megasonic irradiation, in: Proceedings of Precision Science and Technology for Perfect Surfaces (ICPE’99), Osaka, pp. 428– 431, 1999. 10. H. Kanetaka, T. Kujime, H. Yazaki, T. Kezuka, and T. Ohmi, Influence of the dissolved gas in cleaning solution on Si wafer cleaning efficiency, in: Proceedings of UCPSS’98, pp. 43 – 48, 1998.

V. O2-FREE WAFER-CLEANING TECHNOLOGY AKINOBU TERAMOTO

A. INTRODUCTION In Section IV, the room temperature 4-step cleaning technology and the room temperature 5-step cleaning technology which does not use alkali solutions are described. The room temperature 4-step cleaning technology and the room temperature 5-step cleaning are sufficient for manufacturing of current large-scale integration (LSI) devices fabricated on Si(1 0 0) surface. However, when cleaning of Si(1 1 0) surface, higher performance cleaning technology is indispensable. On Si(1 1 0) surface, the hole mobility is two to three times larger than that on current Si(1 0 0) surface [1 – 4], as a result, the current drivability of p-MOSFETs and n-MOSFETs can be the same and p- and n-MOSFETs can be balanced in complementary metal oxide semiconductor structures (balanced CMOS) [4]. In this section, new cleaning technology in which the silicon surface is not exposed to OH2 ions and OH* radicals by isolating the atmosphere of cleaning area from oxygen is described.

B. MICROROUGHNESS

OF

SILICON SURFACE

Figure 3.19 shows (a) AFM image and (b) ultra-high vacuum STM image of Si(1 1 0) surface treated by modified RCA clean (shown in Section IIB) [5]. The lines like trough to the k21 1 0l direction are observed. This is caused when the silicon surface is etched by OH2 ion in NH4OH/H2O2 solution during the RCA cleaning. It is described that the modified RCA clean can remove the particles on Si(1 0 0) surface without generating the surface microroughness by optimizing the NH4OH concentration in the NH4OH/H2O2 solution. However, the surface microroughness is generated on Si(1 1 0) surface even if the same cleaning technology is used. Then, the 5-step cleaning technology which does not use alkali solutions is applied to cleaning of Si(1 1 0) surface. The OH* radicals generated by the megasonic (MS) irradiation decompose the surfactant. Then, in MS-irradiated FPM (HF/H2O2)/surfactant/H2-UPW which is the second step of the 5-step roomtemperature cleaning, H2-UPW is introduced instead of UPW as hydrogen in H2-UPW reacts with OH* to form water in an instant. Figure 3.20 shows the quantity of OH* radicals in the water with and without MS irradiation [6,7]. Even in the acid solution, larger quantity of OH* radicals are generated by the MS irradiation, 10 times that in alkali solutions. Figure 3.21 shows the AFM images of Si(1 1 0) surfaces before treatment (a), treated with MS-irradiated H2-UPW (b), and treated with ms-irradiated UPW (c). Although microroughness of Si(1 1 0) surface in the case of


80

(a) AFM image (1 µm × 1 µm)

(b) STM image (40 nm × 40 nm)

FIGURE 3.19 (a) AFM image and (b) Ultra-high vacuum STM image of Si(1 1 0) surface treated by modified RCA clean.

MS-irradiated UPW increases compared with that before treatement, that treated in MS-irradiated UPW does not increase and is the same as that before treatement. These indicate that the OH* radicals generated by MS irradiation to UPW are suppressed by hydrogen addition. Microroughness of the silicon surface increases by dissolution of silicon atoms with growth of native oxide in the oxygen-dissolved water [8,9]. Figure 3.22 shows (a) the native oxide thickness, (b) quantity of the dissolved silicon, and (c) average microroughness of silicon surface (Ra) of Si(1 0 0) and Si(1 1 0) wafers in the 8 ppm dissolved-oxygen UPW for 24 h. Although growth of both the oxides are the same, the quantity of silicon atoms from Si(1 1 0) surface is about five times larger than that from Si(1 0 0) surface, as the result, the Ra value of Si(1 1 0) surface is much larger than that of Si(100) surface. Figure 3.23 shows (a) the native oxide thickness, (b) quantity of the dissolved silicon, and (c) average microroughness of silicon surface (Ra) of Si(1 0 0) and Si(1 1 0) wafers in the 0, 8, and 42 ppm dissolved-oxygen UPW for 24 h. Also, 42 and 8 ppm in the

Number of pH on radicals [1/l]

4.000E + 19

3.000E + 19

2.000E + 19

1.000E + 19

0.000E + 00 0.0

2.0

4.0

6.0

8.0

10.0

12.0

pH [−]

FIGURE 3.20 Quantity of OH* radicals in the water with and without MS irradiation [6,7].


(a) Before treatment (1 µm × 1 µm)

(b) After H2UPW + MS rinse (1 µm × 1 µm)

81

(c) After UPW + MS (1 µm × 1 µm)

FIGURE 3.21 AFM images of Si(1 1 0) surfaces (a) before treatment, (b) treated in MS irradiated H2-UPW, and (c) treated in MS-irradiated UPW.

UPW is the saturation concentration of 100% of ambient oxygen and the air, respectively, and 0 ppm is realized by isolating the O2 in the N2 ambient. The dissolved Si concentration from Si(1 1 0) surface and the average microroughness of the Si(1 1 0) surface have the largest values in 8 ppm dissolved-oxygen UPW compared with other dissolved oxygen concentrations. This is considered as follows. 1. When dissolved oxygen concentration is sufficiently high, the stable SiO2 film is formed on the silicon surface and the silicon atoms are hard to dissolve into UPW from silicon surface, as a result, the microroughness does not increase.

FIGURE 3.22 (a) The native oxide thickness, (b) quantity of the dissolved silicon, and (c) average microroughness of silicon surface (Ra) of Si(1 0 0) and Si(1 1 0) wafers in the 8 ppm dissolved-oxygen UPW for 24 hr.

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FIGURE 3.23 (a) The native oxide thickness, (b) quantity of the dissolved silicon, and (c) average microroughness of silicon surface (Ra) of Si(1 0 0) and Si(1 1 0) wafers in the 0, 8, and 42 ppm dissolvedoxygen UPW for 24 h. In this, 42 and 8 ppm in the UPW is the saturation concentration of 100% of oxygen ambient and the air, respectively, and 0 ppm are realized by isolating from O2 in the N2 ambient.

2. When dissolved oxygen concentration is sufficiently low, the oxidation of silicon surface and dissolution of silicon atoms cannot occur in UPW, as result, the microroughness does not increase. 3. Except for the above conditions, the microroughness of the silicon surface increases by dissolution of silicon atoms with the growth of native oxide in the oxygen-dissolved water. In the high dissolved-oxygen concentration case, the growth of native oxide cannot be controlled during the cleaning and the rinsing periods. Then, in order not to make the microroughness increase, it is important to remove the dissolved oxygen of UPW thoroughly. Moreover, reduction of rinsing time is also very effective for microroughness reduction. Figure 3.24 shows (a) AFM image and (b) UHV-STM image of Si(1 1 0) surface after 5-step room temperature clean including the above measures. In STM image, the portion which forms the same terrace is shown by the same contrast. Compared with Figure 3.19, the lines like trough to the k21 1 0l direction are not observed and the wide area terrace is observed on Si(1 1 0) surface treated by the 5-step room temperature clean. Thus, the cleaning technology which dose not generate the microroughness of Si(1 1 0) surface can be realized as the suitable 5-step room temperature clean in which the silicon surface is not exposed to OH2 ions or the OH* radicals.

C. HYDROGEN TERMINATION

OF

SILICON SURFACE

On the Si(1 0 0) surface, the energy level of the silicon surface is the same as a bulk of the silicon, as a result, the silicon surface terminated by hydrogen is chemically stable [10]. Figure 3.25 shows the


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(b) STM image (40 nm × 40 nm)

(a) AFM image (1 µm × 1 µm)

FIGURE 3.24 (a) AFM image and (b) UHV-STM image of Si(1 1 0) surface after 5-step room temperature clean including above measures.

Relative Ion Intensity [%]

2.5

2.0 1.5

1.0 0.5

0.0

0

200

400 600 Wafer temperature [°C]

800

FIGURE 3.25 Thermal desorption characteristics of the hydrogen from Si(1 0 0) surface treated by the diluted HF and the UPW rinsing.

FIGURE 3.26 Schematic molecule model of the hydrogen-terminated Si(1 0 0) surface.


84


2.5 2.0 1.5 1.0 0.5 0.0 0

200

400

600

800

Wafer temperature [°C]

FIGURE 3.27 TDS characteristics of the hydrogen from Si(1 1 0) surface treated by the diluted HF and the UPW rinsing.

thermal desorption characteristics of hydrogen from Si(1 0 0) surface treated by the diluted HF and the UPW rinsing. Two peaks at 3808C and 5208C are observed in this graph [11]. A silicon atom in the Si(1 0 0) surface is terminated by two hydrogen atoms (shown in Figure 3.26) [12]. A dangling bond of a silicon atom which is generated by desorption of one of two hydrogen atoms bonds with the next one at 3808C and remaining hydrogen is desorbed from Si(1 0 0) surface at 5208C [11]. Hydrogen is desorbed from the Si(1 1 0) surface only at 5308C (shown in Figure 3.27). Figure 3.28 shows the characteristics of the thermal desorption spectroscopy (TDS) of (a) Si(1 0 0) and (b) Si(1 1 0) surfaces just after diluted HF treatment and UPW rinsing and after exposure to air for 2 and 12 h after diluted HF treatment and UPW rinsing. Only the 5308C peak decreases after exposure to air for 2 hours, and then, both 5308C and 3808C peaks decrease after exposure to air for 12 hours. Figure 3.29 shows the quantity of Si22O bond evaluated by an attenuated total reflectance Fourier transform infrared (FTIR/ATR) spectrometry. Although the hydrogen terminations decreased, the Si22O bonds on the Si(1 0 0) surface increased. This means the Si(1 0 0) surface is oxidized with desorption of the surface hydrogen terminations. Figure 3.30 shows the schematic structure of (a) a silicon atom terminated by two hydrogens on Si(1 0 0) (a)

(b) 2.5



2.5 2.0 1.5 1.0 0.5 0.0

0

200 400 600 Wafer temperature [°C]

800

2.0 1.5 1.0 0.5 0.0 0

200

400

600

800

Wafer temperature [°C]

FIGURE 3.28 TDS characteristics of (a) Si(1 0 0) and (b) Si(1 1 0) surfaces just after diluted HF treatment and UPW rinsing and after exposure to the air for 2 and 12 hr after diluted HF treatment and UPW rinsing.


85

0.05

0.05 12 h

0.03 2h 0.02 0.01 0.00

12 h

0.04 Absorbance

Absorbance

0.04

0.03 2h 0.02 0.01 0.00

0h − 0.01

1300

1200 1100 1000 Wave number [cm−1] (a) Si(100)

0h −0.01

1300

1200 1100 1000 Wave number [cm−1] (b) Si(110)

FIGURE 3.29 Quantity of Si2 2O bond evaluated by an ATR/FT-IR spectrometry.

surface, (b) Si(1 0 0) surface terminated by hydrogen after desorption at 3808C, and (c) Si(1 1 0) surface terminated by hydrogen atoms. In order to oxidize the surface (a), the oxygen atom must replace the terminated hydrogen atoms of the surface, or oxygen must enter into the back bond of the surface silicon atoms. Since the Si22H bond of the Si(1 0 0) surface is stable and the back bond of silicon cannot be exposed easily to the O2 or H2O molecules, when the hydrogen termination of the Si(1 0 0) surface is carried out completely, it is very hard to oxidize in the air atmosphere. In the case of (b) and (c) surfaces, as a combination of adjacent two silicon atoms appears in the surface, this silicon atoms are easily oxidized by the oxygen in the atmosphere. The electrons of Si22H’s covalent bond of oxidized silicon can be attracted to the direction of the oxygen atoms having large electronegativity (oxygen: 3.0). The bonds of the silicon atoms are therefore polarized by the opposite side of oxygen (They are hydrogen atoms and silicon atoms in the case of Si(1 0 0) surface and Si(1 1 0) surface respectively), and these bonds become weaker, and oxidation of the

FIGURE 3.30 Schematic structure of (a) Si(1 0 0) surface terminated by two hydrogens a silicon atom, (b) Si(1 0 0) surface terminated by hydrogen after desorption at 3808C, and (c) Si(1 1 0) surface terminated by hydrogen.


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silicon surface is accelerated further. In the case of Si(1 1 0) surface, the oxidation of the surface is not avoided in essence. Because it is difficult to make hydrogen termination of silicon surface 100% on Si(1 0 0) surface, the oxidation of the surface cannot be avoided too. Thus, the native oxide growth cannot be suppressed in air. It is concluded that the wet processes and transfer atmosphere before next process at which the silicon surface must not oxidize (as the gate oxidation), and so on must be carried out in an atmosphere without oxygen and moisture.

REFERENCES 1. T. Sato, Y. Takeishi, H. Hara, and Y. Okamoto, Mobility anisotropy of electrons in inversion layers on oxidized silicon surfaces, Phys. Rev. B, 4, 6, pp. 1950– 1960, 1971. 2. S. Sugawa, I. Ohshima, H. Ishino, Y. Saito, M. Hirayama, and T. Ohmi, Advantage of silicon nitride gate insulator transistor by using microwave-excited high-density plasma for dappling 100 nm technology node, in: Proceedings of the IEDM Technical Digest 2001, pp. 817 –820, 2001. 3. T. Mizuno, N. Sugiyama, T. Tezuka, Y. Moriyama, S. Nakaharai, and S. Takagi, [1 1 0]-surface strained-SOI CMOS devices with higher carrier mobility, in: Proceedings of the VLSI Technical Digest 2003, pp. 97– 98, 2003. 4. A. Teramoto, T. Hamada, H. Akahori, K. Nii, T. Suwa, K. Kotani, M. Hirayama, S. Sugawa, and T. Ohmi, Low noise balanced-CMOS on Si(1 1 0) surface for analog/digital mixed signal circuits, in: Proceedings of the IEDM Technical Digest 2003, pp. 801 – 804, 2003. 5. T. Ohmi, M. Miyashita, M. Itano, T. Imaoka, and I. Kawanabe, Dependence of thin-oxide films quality on surface microroughness, IEEE Trans. Electron Devices, 39, 537 – 545, 1992. 6. M. Toda, M. Kato, K. Kubo, S. Ojima, and T. Ohmi, Radical activation of DI water and cleaning solution by megasonic, R. E. Novak and J. Ruzyllo, Eds., Cleaning Technology in Semiconductor Device Manufacturing, PV95-20, The Electrochemical Society, Pennington, NJ, pp. 99 – 106, 1996. 7. S. Ojima, K. Kubo, M. Kato, M. Toda, and T. Ohmi, Megasonic excited ozonized water for the cleaning of silicon surface, J. Electrochem. Soc., 144, 1482 –1487, 1997. 8. M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, and M. Ohwada, Growth of native oxide on a silicon surface, J. Appl. Phys., 68, 1272– 1281, 1990. 9. M. Morita, T. Ohmi, E. Hasegawa, and A. Teramoto, Native oxide growth on silicon surface in wet ambient, in: Extended Abstracts of the 22nd (1990 International) Conference on Solid State Devices and Materials, Part II, Sendai, pp. 1063– 1066, 1990. 10. T. Ohmi, Surface chemical electronics at the semiconductor surface, Appl. Surf. Sci., 121/122, 44– 62, 1997. 11. N. Yabumoto, K. Minegishi, K. Saito, M. Morita, and T. Ohmi, An analysis for cleaned silicon surface with thermal desorption spectroscopy, J. Ruzyllo and R.E. Novak, Eds., Semiconductor Cleaning Technology/1989, PU90-5, The Electrochemical Society, Pennington, NJ, pp. 265–272, 1990. 12. Y. J. Chabal, G. S. Higashi, and K. Raghavachari, Infrared spectroscopy of Si(1 1 1) and Si(1 0 0) surfaces after HF treatment: hydrogen termination and surface morphology, J. Vac. Sci. Technol., A7(3), 2104– 2109, 1989.

VI. TOTAL ROOM-TEMPERATURE CLEANING PROCESS [1–4] HIROSHI MORITA

A. TOTAL ROOM-TEMPERATURE 4-STEP CLEANING PROCESS 1. O3-UPW for Wet Cleaning Process a. Introduction First, the history of the wet-cleaning process using O3-UPW is reviewed. Ozonated water prepared by dissolving O3 gas in water has extremely high redox potential. Both ozone gas and ozonated water have been used for a long time for sterilization and bleaching because of their strong


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oxidizing action. Prof. Ohmi’s laboratory at Tohoku University first noted the outstanding sterilizing effect of dissolved ozone, and applied ozone to continuous sterilization of UPW circulation system. The system is composed of two parts: a subsystem to increase cleanliness of pure water and a piping system to deliver UPW to POU and send excess UPW back to a tank at the inlet of the subsystem. O3 gas dissolved UPW was found to feature sufficiently high sterilization effect even when ozone concentration was as low as 50 ppb [5]. It was also learned that the inner surface of the piping system, which came in contact with O3 gas dissolved UPW, deteriorated due to the oxidizing action of ozone. In other words, it was found necessary to select an appropriate piping material having high oxidation resistance. Based on these findings, Prof. Ohmi’s laboratory studied the application of O3-UPW to wet-cleaning processes to use its strong oxidizing effect to decompose and remove organic impurities from Si wafer surfaces [6,7]. As a result, O3-UPW was found to have high removal efficiency against some metallic impurities as well as against organic impurities [8 – 11]. Ozone dissolved in water features a characteristic to convert itself to oxygen through selfdecomposition. This means that effluent from O3-UPW cleaning processes does not adversely affect the environment as ozone in effluent gets self-decomposed to oxygen. O3-UPW, therefore, was studied actively, being suitable for application in numerous areas. In spite of its high-cleaning efficiency and advantage in terms of environment conservation, the introduction of O3-UPW to mass production struck a snag because it was found extremely difficult to precisely control ozone concentration.

b. Cleaning Efficiency of O3-UPW i. Removal of Organic Impurities

Semiconductor devices are manufactured in a cleanroom. ULPA and HEPA filters are installed in the cleanroom to considerably reduce particles in the air that circulates in it. A common air filtering system does not have a function to remove organic impurities but a large amount of plastic materials such as a wafer box are used in cleanroom. Organic impurity concentration of cleanroom air is therefore as high as or higher than that of outdoor air. Consequently, a Si wafer surface is subject to organic impurity contamination during its manufacturing and storage processes. O3-UPW is highly effective in cleaning the wafer surface contaminated with organic impurities in the cleanroom air. In this study, the contact angle of wafer surface was measured to evaluate the cleaning efficiency of O3-UPW [2,3]. Clean and oxidized Si wafer surface is hydrophilic. When contaminated with organic impurities, the surface gradually gets hydrophobic and the contact angle of water droplet on the surface gets higher. Contact angle has been used as a rough measure to evaluate organic impurities contamination. Before measuring contact angle, Si wafers were treated with 10-min-long SPM cleaning and 10-min long UPW overflow rinsing to remove fully the organic impurities. Si wafer surface treated with SPM cleaning is highly hydrophilic as it is free from organic contamination and covered with chemical oxide. Contact angle of water droplet on Si wafer right after SPM cleaning and UPW rinsing was 2– 38. These wafers were left in a cleanroom (class 100) for 5 days, and contact angle of water droplet was measured again. That was found to increase to 23– 248. This is considered because Si wafer surface becomes hydrophobic due to adhesion of organic impurities. These wafers contaminated with organic impurities were cleaned with O3-UPW by spin-cleaning equipment. Figure 3.31 shows the contact angle as a function of cleaning time. The contact angle was restored to its original level within 1 min by adjusting ozone level at 0.6 ppm or higher. The contact angle decreased more rapidly with rise in ozone concentration. When ozone


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Pre-Cleaning : After SPM cleaning, wafer was exposed itself to clean room for 5 days. Result

Procedure

Pre-Cleaning Exposure O3-UPW Cleaning Measurement Hydrophobic (Contaminated) Water Drop

Contact Angle (°)

25 0 ppm

20 15

0.6 ppm

10

3 ppm 5

Hydrophilic 0 (Clean) Contact Angle 0

5 ppm 10

20

30

40

50

60

Cleaning Time (sec)

FIGURE 3.31 Organic contaminants removal efficiency of O3-UPW.

concentration was set at 5 ppm, contact angle was brought back to its original level in about 10 sec. When being cleaned with UPW, contact angle was decreased only by 58 in 1 min. ii. Removal of Metallic Contaminants Removal of Cu contamination When the cleaning or rinsing solution contains Cu, Cu2þ gets

easily precipitated on bare Si surface as metallic copper (Cu0) [9,12,13]. Unlike the base metals such as Fe and Al, on Si surface Cu is very hard to be removed in DHF cleaning, and it diffuses into bulk Si to generate crystalline defects and deep levels which trigger increase of junction leakage. Because Cu features high electronegativity, Cu2þ strongly tends to bind with electron. Taking electrons from Si substrate, Cu2þ turns into Cu0 and adheres electrochemically to bare Si surface. O3-UPW features much higher electronegativity than Cu. Therefore, O3-UPW, is capable of taking electrons from Cu on wafer surface to oxidize (ionize) Cu. Figure 3.32 shows Cu adhesion after various cleanings. Prior to this test, sample wafers were immersed into DHF with a very small amount of copper sulfate to be contaminated with Cu. Then, the contaminated sample wafers were treated with O3-UPW (ozone concentration: 5 ppm) at room temperature. In a an attempt to enhance the cleaning efficiency, small amounts of HCl (3.6 ppm)

Procedure

Cleaning 5 ppm O3-UPW 25°C HPM (1:1:6) 40°C

Measurement TXRF

Cu Remaining (atom/cm2)

Initial in CuCl2 spiked DHF

Result 1.0E + 12

Initial 5 ppm O3-UPW 25°C HPM (1:1:6) 40°C

1.0E + 11

1.0E + 10

1.0E + 09 Initial

1

2 Cleaning Time (min)

FIGURE 3.32 Cu contaminants removal efficiency of O3-UPW.

15


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was injected to O3-UPW to adjust its pH at 4.0. For comparison, HPM cleaning was conducted at 408C (though it is usually performed at 708C). This experiment reveals that room-temperature O3UPW with trace HCl injected is more effective than plain HPM solution, in removing copper contamination. The surface of copper metal on Si wafer is oxidized in the cleanroom air just like Si surface. Copper oxide or copper hydroxide layer cannot be dissolved in neutral O3-UPW. However, the layer is easy to be dissolved in acidic solution. Therefore, dosing of acid in O3-UPW is very important to remove Cu contamination. Even without any acidic chemical dosing, low-pH O3-UPW can be produced by CO2 gas dissolution. CO2 added O3-UPW does not need rinsing after cleaning because it is a chemicalfree solution. Figure 3.33 shows Cu removal efficiency of CO2-added O3-UPW [14].

Removal of other metallic impurities: Characteristics are greatly varied from one metal to another

[15]. From the cleaning viewpoint, metals are divided into two categories: those which are easily dissolved into oxidizing solution and those which are easily dissolved into acidic solution. Cu is a representative of the first group whereas Fe and Al are constitute the second group. In order to have high cleaning efficiency for all these different metals, the cleaning solution needs to be strongly oxidizing and acidic. The conventional cleaning solutions such as HPM and SPM are strongly oxidizing as well as acidic, and therefore they are widely applicable. As O3-UPW without any chemical injection or with CO2 addition is neutral or weakly acidic, it is not capable of removing those metallic impurities that are to be dissolved in strong acid solutions. With large amount of acid addition, O3-UPW becomes effective against most of metallic impurities just like HPM solution. Injection of acid, however, makes O3-UPW environmentally hazardous, which in essence cancels one of the major advantages of O3-UPW, namely environmentally benign cleaning solution. It is true that O3-UPW is not universally effective against various metals. Those metals that are easily dissolved into strong acid solution can be readily removed by means of oxide etching in HFbased cleaning though they are hard to remove with O3-UPW. Combined with HF-based cleaning, O3-UPW is regarded as a highly efficient functional water because (i) it can remove not only metals such as Cu and Ag which cannot be removed in DHF cleaning and (ii) also organic impurities and also removed simultaneously.

Cu concentration (atoms/cm2)

1015

1.30 V (vs NHE)

1014 1013 1.36 V (vs NHE)

1012

1.42 V (vs NHE)

1011 1010 109

as-contamination

pH 6.5 UPW only

pH 5.0

pH 4.0

CO2 concentration

FIGURE 3.33 Dependence of Cu removal efficiency on pH and ORP value in CO2 added O3-UPW.


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2. H2-UPW for Wet Cleaning Process [16] a. Introduction This section describes H2-UPW in detail. H2-UPW is nothing but a reducing water prepared by dissolving hydrogen of high concentration (more than 1 ppm) into UPW. It has been revealed, however, that H2-UPW becomes a functional water for the wet process featuring extremely high-cleaning efficiency when it is combined with megasonic irradiation. In some cases, its cleaning efficiency is found much higher than that of the conventional high-concentration chemical cleaning process. Why does UPW with hydrogen dissolved feature outstanding cleaning efficiency? Its particle removal mechanism is studied next by making a model. Results of verification of the model are also reported. Other benefits of H2-UPW cleaning than particle removal are to be reported at the end of this Section. b. Relation between Dissolved Hydrogen Concentration and Particle Removal Efficiency [17,18] H2-UPW produced by the gas-dissolving method was tested for its cleaning efficiency. Si wafers of 6- and 8-in. diameter were used in these tests. The test procedure is as follows: Sample wafers contaminated intentionally ! Measure particle count on the wafers ! Clean the contaminated wafers by spin-type cleaning equipment with MS nozzle ! Measure particle count again At the outset of this study, particle removal efficiency of H2-UPW was examined first to prove the relevant assumption. When the series treatment of “deaeration ! gas injection” is used to dissolve hydrogen gas into UPW fed at a constant flow rate, concentration of dissolved hydrogen is determined solely by the amount of introduced hydrogen gas. H2-UPW with various hydrogen concentrations were prepared for this experiment, by varying the amount of introduced hydrogen gas. Figure 3.34 shows particle removal efficiency as a function of concentration of dissolved hydrogen when cleaning time is fixed at 1 min. It is revealed that these two parameters have positive

FIGURE 3.34 Relationship between dissolved hydrogen concentration of UPW and alumina particle removal efficiency.


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correlation. Merely by raising the concentration of dissolved hydrogen above 1 ppm, alumina abrasive, which is hard to remove, can be removed by 95% or more. c. Removal of Alumina Particle and Silica Particle Figure 3.35 and Figure 3.36 show respectively the alumina and silica particle removal efficiency of H2-UPW. Particles larger than 0.12 mm are measured. In these tests, H2-UPW and UPW were used with and without injection of 1 ppm NH3. As shown in Figure 3.35, H2-UPW with NH3 of 1 ppm is effective in removing minute alumina particles: removal efficiency of 100% is achieved by 30-sec long cleaning. H2-UPW without NH3 injection is also found to feature alumina particle removal efficiency of over 90%, but it is not able to completely remove the particles. The silica particle used in the test (Figure 3.36) are commonly present in the slurry of CMP process, increasingly adopted as surface planarization technology presently. CMP process was not performed in this test. Instead, similar to the test with alumina abrasive, sample wafers were immersed in UPW with diluted silica slurry and then they were dried in order to prepare sample wafers contaminated with silica particle. The slurry residue is extremely hard to be removed once it dries on the wafer surface. It is a common practice in current CMP, therefore, to send wet wafers to a single-wafer cleaning system on completion of the CMP process. In our study, however, sample wafers were dried after being immersed in silica slurry solution, and then sent to cleaning. H2-UPW, even without NH3 spike, was found effective in completely removing silica particles within as short as ten seconds. Both UPW and UPW with 1 ppm NH3 were found totally ineffective in removing silica particles of CPM slurry, which demonstrates how difficult it is to remove dried silica particles. These results have proven that high-concentration H2-UPW is extremely outstanding as a functional water with far higher cleaning efficiency than UPW. d. Comparison between APM Cleaning and H2-UPW Cleaning The above-described test results demonstrate that H2-UPW cleaning can be used as a substitute not only for UPW cleaning but also for high-concentration chemical cleaning. In an attempt to come up with further supporting data, cleaning using H2-UPW was compared with cleaning using APM solution which has been widely adopted throughout the world to remove particles.

FIGURE 3.35 Removal efficiency of alumina particle by spin-type cleaning equipment with MS nozzle using various cleaning solution.


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FIGURE 3.36 Removal efficiency of silica particle by spin-type cleaning equipment with MS nozzle using various cleaning solution.

Spin-type equipment is not suitable for cleaning using APM solution as it uses high-concentration chemicals at high temperature. In this comparison test, therefore, batch-type cleaning was performed, instead of spin cleaning, using MS bath (0.75 MHz and 650 W) for 6 in. wafer. Conventional APM . . .

NH4OH:H2O2:H2O ¼ 1:1:5 Heated to 708C Without MS irradiation H2-UPW

. . . . .

NH3 of 1 ppm spiked Dissolved hydrogen concentration of 1.3 ppm Overflow at flow rate of 5 l/min Room temperature With MS irradiation

After each cleaning step, sample wafers were treated with 5-min overflow rinsing in UPW bath, and then with spin drying. Figure 3.37 shows the results. The H2-UPW cleaning is found more effective in removing alumina particles from wafer surface. As cleaning time is shorter, gap in particle removal efficiency between the two recipes is found to grow bigger. This implies that H2-UPW cleaning features a different particle removal mechanism from that of APM cleaning. The conventional APM cleaning requires several minutes to remove particles. It is considered because alumina particle removal requires a certain etching depth. e. Effects of H2-UPW Cleaning on Wafer Surface Microroughness [16] Outstandingly high particle removal efficiency of H2-UPW has been discussed so far. Information on particle removal efficiency alone, however, is not sufficient to apply this cleaning technology to mass production process.


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Particle Removal Rate (%)

100

80

H2-UPW + NH3 (1 ppm) Room Temp. with MS

60 40 SC-1(NH4OH:H2O2:UPW = 1:1:5) 70°C without MS

20 0

0

50

100

150

200

Cleaning Time (sec)

FIGURE 3.37 Comparison of cleaning efficiency between NH3 added H2-UPW þ MS and APM in batch-type cleaning equipment.

To closely simulate the practical device manufacturing process, four different wafer surfaces were prepared in this study. . . . .

Si wafer surface with native oxide (not intentional) Si wafer surface with chemical oxide grown in O3-UPW cleaning Bare Si wafer surface right after DHF cleaning ˚ thick Si wafer surface with thermal oxide of 1000 A

Just like the particle removal tests, sample wafers were cleaned with a spin cleaner equipped with MS nozzle (1.6 MHz, 48 W). Cleaning time was set at 600 sec, 20 times as long as an ordinary cleaning time (30 sec at the longest) as the primary purpose of this study was to find out whether Si wafer surface was damaged during the cleaning step. Sample wafers were fragmented before and after cleaning step. Angstrom-order surface microroughness of these fragments were evaluated with atomic force microscope. Table 3.1 shows representative values. The followings have been revealed: 1. For Si surfaces with native oxide and with chemical oxide, no changes are detected in terms of angstrom-order microroughness both after H2-UPW cleaning and after H2-UPW þ NH3 (1 – 5 ppm) cleaning. 2. For bare Si surface right after DHF cleaning, microroughness does not increase in H2-UPW cleaning, but considerable increase of microroughness is detected after H2-UPW þ NH3 cleaning. 3. For Si surface with thermal oxide, MS cleaning using both H2-UPW and H2UPW þ 1 ppm NH3 considerably reduce microroughness. These results suggest that Si wafer should not be treated with H2-UPW with even trace amount of NH3 injected immediately after DHF cleaning. In the other cases, H2-UPW cleaning does not affect the surface microroughness deterioration of Si wafer. This is a clear advantage of H2-UPW cleaning over APM cleaning. Deterioration of surface microroughness of Si wafer is intrinsically inevitable in APM cleaning, which etches wafer surface to lift off particles to be removed. These test results have demonstrated that H2-UPW cleaning is fully feasible in mass production line.

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TABLE 3.1 Influence of NH3 Added H2-UPW 1 MS Cleaning on Surface Microroughness of Si Wafer ˚) z-Range (A

˚) SD (A

Chemical oxide surface (after O3-UPW) Nontreatment Nonchemical 1 ppm-NH3 added 5 ppm-NH3 added

15.6 13.3 16.8 12.3

2.2 2.3 3.2 1.6

Bare Si surface (right after DHF) Nontreatment Nonchemical 1 ppm-NH3 added

26.9 27.3 77.5

5.0 2.9 3.9

˚ ) surface Thermal oxide (1000 A Nontreatment Nonchemical 1 ppm-NH3 added 5 ppm-NH3 added

24.5 9.1 7.8 21.8

3.2 1.3 1.0 2.6

Note: Dissolved H2 concentration ¼ 1.2 ppm-H2-UPW; Megasonic condition ¼ 1.6 MHz, 13.5 W/cm2; Cleaning time ¼ 600 sec; SD, standard deviation.

In addition, an unexpected result has been obtained in this experiment: H2-UPW cleaning is found to improve surface smoothness of thermal oxide. It would be possible to apply this technology in a process which requires angstrom-order rigorous surface smoothness.

f. Cleaning by UPW with Other Gases than Hydrogen Gas Dissolved [16,19,20] In order to find out whether extremely high particle removal efficiency of H2-UPW is attributable to a unique characteristic of hydrogen gas or other gases, namely oxygen, nitrogen, argon, and helium, the gas-dissolving method was adopted. There is no simple means available to measure concentration of dissolved nitrogen, argon, and helium. In this study, UPW is almost fully deaerated in advance. Then, each one of the three gases was introduced to the gas-dissolving module while limiting the amount of gas below theoretical maximum to be dissolved. Each gas species features unique saturation concentration. In order to evaluate the effects of every gas-dissolved solution on an equal basis, the degree of saturation (percentage against saturation concentration) was aligned among the five gases as much as possible. Figure 3.38 shows the results. Alumina particle removal efficiency is higher in the following order: hydrogen . oxygen . argon . helium . nitrogen (. deaerated UPW) This means H2-UPW is particularly effective in removing particles. UPW is usually purged with nitrogen in the reserving tank, therefore it is close to an uncontrollable nitrogenated UPW (N2-UPW). N2-UPW is found to feature lower particle removal efficiency than UPW with dissolved oxygen or inert gases (Ar or He). However, the particle removal efficiency of nitrogen is higher than degassed UPW.


95

Al2O3 Particle Removal Rate (%)

100 90 80 70 60 50 40 30 20 10 0 H2-UPW 1.2ppm

He-UPW 1.5ppm

N2-UPW 20ppm

O2-UPW 30ppm

Spin Cleaning: (500 rpm) × 60 sec Megasonic 13.5 W/cm2

Ar-UPW 36ppm saturation rate = 60–100%

FIGURE 3.38 Effect of dissolved gas species for particle removal rate in spin-type cleaning with MS nozzle.

g. Particle Removal Mechanism of H2-UPW Cleaning [16] Why does UPW with hydrogen gas and trace NH3 dissolved feature higher particle removal efficiency than high-concentration chemical solutions? This Section discusses particle removal mechanism of H2-UPW, setting up and verifying its particle removal model. There are three controllable parameters in H2-UPW cleaning: (1) concentration of dissolved hydrogen, (2) NH3 concentration (pH), and (3) MS power. Temperature of H2-UPW could be another parameter, but temperature was fixed at room temperature in this study. This is because one of the important practical advantages of H2-UPW cleaning is that it features high particle removal efficiency even at room temperature. In order to completely remove the particles from Si wafer surface, it is essential to prepare an environment which does not allow particles in cleaning solution to re-adhere to Si wafer surface and then lifts off particles from Si surface by means of some force. Three controllable parameters were investigated in detail to find out which one contributes to which mechanism, namely prevention of particle re-adhesion or release of particle from Si surface, in what way. i. Effects of NH3 Spike [2,3]

Information on “zeta potential” of various material surfaces is extremely important to come up with rough idea whether or not a particle easily adheres to various substrate surfaces such as Si surface. Figure 3.39 shows the data. In acidic solutions, materials other than Si are charged positively and Si is charged negatively. Si surface, therefore, is prone to attract electrically particles of other materials in acid solutions. In alkaline solutions, on the other hand, zeta potential is always negative regardless of the material. Once particles are lifted off from Si surface, therefore, they hardly re-adhere to Si surface. Particle removal mechanism of APM cleaning is explained in two ways. Particle-lift off action is attributed to oxidation of Si surface because of the oxidizing force of H2O2 and etching of oxidized surface with high-temperature ammonia (alkali). Particle re-adhesion is prevented as both particle surface and Si wafer surface feature negative zeta potential in alkaline region. H2-UPW, when NH3 is not injected, is neutral just like UPW. In the neutral region, most materials are charged negatively whereas alumina and Si3N4 are charged positively. This is why these impurities are prone to adhere to oxide and bare Si surface in neutral solution. When NH3 of 1 ppm is spiked, pH goes up to 9.4. In this region, the zeta potential of alumina and Si3N4 also turns negative. This is why the removal rate of alumina particles are improved when NH3


96 80

PSL

Zeta Potential (mV)

60 40

Si

SiO2

PSL

Si3N4

a-Al2O3

Si3N4 a-Al2O3

20 0

SiO2

−20

Si

−40 −60 −80

2

4

6

8

10

12

pH

FIGURE 3.39 Relationship between pH and zeta potential of some materials in aqueous solution.

of as little as 1 ppm is injected. Even in neutral solution, silica particles as well as Si wafer surface covered with oxide layer feature zeta potential of the same polarity. Silica particles are hard to readhere to Si wafer surface once they are lifted off. This is why even neutral H2-UPW features as high a removal efficiency as NH3-injected H2-UPW. Figure 3.40 schematically shows these mechanisms. Increase of NH3 injection makes the cleaning solution not only etches the Si wafer surface it as well charges every material negatively. Unlike APM cleaning, H2-UPW þ NH3 cleaning does not have the ability to oxidize Si wafer surface. As a result, Si surface is not oxidized but just etched, which is not preferable in terms of prevention of microroughness deterioration. NH3 concentration needs to be high enough to keep all materials charged negatively but low enough to prevent unwanted etching. Optimum level of NH3 concentration should be 1 –5 ppm.

ii. Effects of MS Irradiation [2,3]

Development of H2-UPW cleaning was triggered by the idea of “effects of deaeration in MS cleaning” as described before. H2-UPW cleaning, therefore, was always combined with MS irradiation. It was studied here whether or not it was essential to perform MS irradiation in order to obtain high particle removal efficiency. MS power was varied widely in alumina particle removal tests to see how removal efficiency was affected. It has been revealed that particle removal efficiency drops dramatically when MS power is reduced below a certain level. In other words, there seems to be a threshold level Al2O3 + + ++ Adhere -

-

Al2O3

SiO2

- - Repulse

-

Repulse -

-

-

Wafer Surface (Si, SiO2) pH7 (non Chemical)

-

-

SiO2 -

Repulse -

-

-

Wafer Surface (Si, SiO2) pH9.4 (1ppm-NH3 Dosed)

FIGURE 3.40 Model of particle adhesion and repulse on wafer surface in aqueous solution.


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of MS power in terms of particle removal efficiency. Without MS power above a certain level or some equivalent physical force, H2-UPW remains ineffective in removing the particles. iii. Effects of Dissolved Hydrogen [16,19,20]

This section discusses whether hydrogen gas dissolved in UPW is effective in preventing re-adhesion of particles to Si wafer. For this purpose, zeta potential fluctuation was studied as hydrogen gas was increasingly dissolved into UPW. Then, H2-UPW and UPW were compared in terms of capability to prevent particles in solution from re-adhering to Si surface. Zeta potential fluctuation [2,3]: Dissolution of hydrogen gas in UPW does not change the pH and

resistivity. Redox potential, however, changes drastically due to dissolution of hydrogen gas. UPW becomes reducing as hydrogen gas is dissolved. Does this shift in redox potential change zeta potential drastically enough to affect impurity adhesion to Si surface? Noting this question, zeta potential of particles made of various materials which were dispersed in UPW and in H2-UPW were measured. Zeta potential of various materials in H2-UPW was found slightly different from that in UPW. Dissolving hydrogen gas in UPW, however, did not change the zeta potential so dramatically to turn polarity of various materials. Degree of particle adhesion [16]: Alumina and silica particles are dispersed in UPW and in H2-UPW

separately. Then, clean sample wafers were immersed into these solutions to be contaminated. Particles in H2-UPW were found to adhere to Si wafer in the same way as those in UPW. These test results demonstrate that H2-UPW itself does not have any outstanding effects to prevent particles from adhering to Si surface. iv. Model of Correlation between Dissolved Hydrogen and MS Irradiation (Surplus Radical Model) [19,20]

It has been found that neither MS alone nor H2-UPW alone were effective enough to sufficiently remove particles. Figure 3.41 shows the data. When these two are combined with each other,

FIGURE 3.41 Cleaning data of silica particle: combination effect of H2-UPW and MS irradiation.

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however, particle removal efficiency is sharply improved. It was speculated that some factor triggered by the combination of dissolved hydrogen and MS irradiation would dramatically accelerate lift-off of particles. It could be considered that this factor has something to do with microcavitation which is closely tied with MS power. There is a possible factor generated by microcavitation, that is radical. A model was worked out as shown in Figure 3.42 to describe their particle removal mechanism. It is known that when water is exposed to MS or ultrasonic (US) irradiation having power of more than the threshold, some water molecules get decomposed into H† and †OH radicals. This decomposition mechanism is explained as pyrolysis of water molecules triggered by local hot spots generated when some cavities are crushed in an expansion or shrink cycle. The number of H† radicals and that of †OH radicals are equal when they are generated. So, needless to say, a primary reaction takes place between H† and †OH radicals, which makes them reverse back to H2O state. It is also observed, however, that they react with some kinds of dissolved gases. For H2-UPW, some parts of †OH radicals seem to react with dissolved hydrogen to make the UPW having “surplus H radical.” H radicals easily react with top surface of Si wafer and particles. Highly reactive H radicals bind with dangling bonds (Si22 and Si22O22), which are active sites on top surface of Si wafer and particles, to inactivate them. It could be also expected that H radicals displace end groups of the surface to accelerate particles lift off. Although there is little change macroscopically, physical effects of vibration and scrubbing by microbubbles are combined with repulsive force between wafer surface and particles. This is due to inactivation of top surface and pseudo-etching because of atom displacement on top surface. In oxygenated UPW (O2-UPW), specifically, H radicals react with dissolved oxygen to turn to water, which makes UPW having surplus OH radical. Table 3.2 shows the relation between dissolved gas species and radical formation and behavior. There is a case in which radicals find few counterparts and appropriate dissolved gases featuring opposite properties. In such case, the “surplus radicals” are considered to be prone to react with the surface of wafer and particles. It is also speculated that H† radical, which is smaller than †OH radical, is more effective in reacting with surface of wafer and particles. There are a number of surplus H† radicals in H2-UPW exposed to MS irradiation whereas a large amount of surplus †OH radicals are found in O2-UPW. When inert gases are dissolved in UPW, some portion of H† and †OH radicals remain. Argon-dissolved UPW generates more radicals than helium-dissolved UPW. In nitrogenated UPW (N2-UPW), H† radicals and †OH radicals react with dissolved nitrogen to generate ammonia, nitric acid, and nitrous acid. N2-UPW, therefore, has fewer surplus radicals, which react with wafers and particles, than UPW with inert gases dissolved. Deaerated UPW by its nature scarcely generates radicals. The above explanation reasonably justifies the order of particle removal efficiency, namely hydrogen . oxygen . argon . helium . nitrogen . deaerated UPW, although it looked totally random in the beginning.

FIGURE 3.42 Model of surplus radicals: formation and reaction.


99

TABLE 3.2 Affect of Dissolved Gas Species for Radical Formation in MS Irradiated UPW Dissolved Gas H2 O2 Ar He N2

Main Reaction H† þ †OH ! H2O H†2 þ †OH ! H2O H† þ †OH ! H2O O2 þ H† ! H2O † H þ †OH ! H2O H†þ †OH ! H2O H† þ †OH ! H2O N2 þ H† ! NHþ 4 N2 þ †OH ! NO2 3 ! NO2 2

Degassed

Result Surplus H radicals Surplus OH radicals

Nitric compounds

No generation

v. Verification of Surplus Radical Model [19,20]

In an attempt to verify the effects of surplus radical on the cleaning mechanism, a cleaning test with hydrogen- and oxygen-dissolved UPW was conducted. UPW with oxygen and hydrogen dissolved was prepared by injecting hydrogen of 0.4 ppm (degree of saturation: 25%) to O2-UPW featuring dissolved oxygen concentration of 33 ppm (degree of saturation: 75%). Using this solution, Si wafer contaminated with alumina particles was cleaned. The above-mentioned “surplus radical model” was applied to explain the particle removal mechanism. When hydrogen is spiked to O2UPW which generates surplus †OH radicals was exposed to MS irradiation, surplus †OH radicals decrease in number due to reaction with dissolved hydrogen. This is why UPW with oxygen and hydrogen dissolved in it features lower particle removal efficiency than simple O2-UPW although hydrogen injection usually enhances particle removal. Figure 3.43 shows results of the cleaning test, comparing O2-UPW and O2 þ H2-UPW. O2 þ H2-UPW is found to feature lower particle removal efficiency than O2-UPW. This test results is in good agreement with the assumption.

FIGURE 3.43 Comparison of MS cleaning efficiency between normal O2-UPW and H2 added O2-UPW: verification of surplus radical reaction.


100

h. Applications of H2-UPW Other than Particle Removal It has been discussed that H2-UPW combined with MS irradiation works as an extremely effective cleaning solution. There are, however, other applications of H2-UPW in the wet process than particle removal. This section describes other benefits of H2-UPW. Suppression of native oxide growth [2,3]: Rinsing step of the wet-cleaning process usually adopts

UPW from which oxygen is thoroughly removed. Based on the report that Si surface gets oxidized as water and oxygen coexist, it is encouraged to remove dissolved oxygen (one of the two affecting factors), to suppress native oxide growth. For H2-UPW, dissolved gases including oxygen are removed, and furthermore reducing hydrogen gas is dissolved. H2-UPW, therefore, is more effective in suppressing native oxide growth than UPW from which dissolved oxygen is removed. Acceleration of hydrogen termination [2,3]: H2-UPW cleaning is also proven to improve ratio of

hydrogen termination on bare Si wafer surface. Figure 3.44 shows FTIR spectrum to represent hydrogen termination, comparing the following two cases: . .

FPM (0.5% HF þ 0.5% H2O2) [21] ! UPW rinsing FPM ! H2-UPW cleaning ! spin-drying combined with purge with nitrogen gas containing H radicals

Clearly the degree of hydrogen termination is higher in the latter case. The test results demonstrate that H2-UPW cleaning is also effective as a final rinsing step in the precleaning of gate oxidation process as bare Si surface is exposed at this stage. i. Summary This section reported a gas-dissolving method; namely, H2-UPW, describing its extremely high particle removal efficiency and the mechanism. Here we summarize the important points. 1. It is proven that room-temperature cleaning by means of H2-UPW with trace NH3 injected features higher particle removal efficiency than APM cleaning when it is combined with MS irradiation.

0.04 FPM cleaning followed by H2-UPW rinse and H∗ radical dry

Absorbance

0.03 after FPM cleaning followed by UPW rinse (FPM; HF: 0.5%, H2O2: 0.2%)

0.02

0.01

0.00 2200

2180

2160

2140

2120

2100

Wave number

2080

2060

2040

2020

(cm−1)

FIGURE 3.44 Increase of hydrogen termination by H2-UPW þ MS rinse and hydrogen radical drying.


101

2. Surplus H† radicals seem to be generated by combining dissolved hydrogen and MS irradiation. A surplus radical is defined as a radical which has limited number of its counterpart radicals to react with in solution. A model has been established that surplus radical is a major factor to accelerate lift-off of particles from wafer surface. Particle removal efficiency of O2-UPW combined with MS irradiation which intrinsically contains a large amount of surplus OH radicals is deteriorated when H2 molecules of the gas phase are injected. This fact indirectly verifies the model. 3. H2-UPW is found to be also effective in suppressing native oxide growth on bare Si surface and in accelerating hydrogen termination. Hydrogen dissolved in water is completely stable. It is possible, therefore, to produce H2-UPW just like UPW in large volume and deliver it to a number of cleaning equipments through piping system. 3. Total Room-Temperature 4-Step Cleaning Process [2 –4] Previous sections reported O3-UPW and H2-UPW. Each technology by itself is expected to have a remarkable contribution to device manufacturing process. Combined in an appropriate manner, however, these technologies will enable us to design a novel total cleaning process which will outperform the conventional RCA cleaning. This section reports a total room-temperature cleaning process. Figure 3.45 shows a flow chart of the four-step cleaning process based on previously described new technology. In the first step, O3-UPW cleaning removes organic impurities from wafer surface, enhances wafer wettability, and removes Cu. Even if the wafer is contaminated with Ag which is hard to be removed in FPM cleaning, O3-UPW cleaning is capable of removing Ag easily. A clean chemical oxide is also formed during this process step. In the second step, a combination of H2-UPW with 1 ppm NH3 spiked and MS irradiation effectively removes particles while keeping the surface cleanliness. Precisely speaking, this step uses chemicals, but no rinsing step is required to follow especially in the case of spin-type cleaning. This is because the amount of chemicals is extremely marginal. In the third step, FPM cleaning is performed to completely remove any residual metallic impurities as well as chemical oxide. The fourth step is the final rinsing step, and it uses H2-UPW þ MS again to suppress native oxide growth and enhance hydrogen termination. This final step also features high particle removal efficiency. Its particle removal efficiency is lower than that of the second step, but the conventional cleaning process does not have any final rinsing step featuring such a function. 4. Purity of Gas-Dissolved Functional Water [22] Minimal requirement for chemicals and UPW used in wet-cleaning process is sufficiently high purity so as not to contaminate the wafer surface. Purity of chemicals and of UPW are raised to

↓ ↓

O3-UPW (O3:5 ppm)

Organic, Noble Metal

H2-UPW (H2:1.2~1.6 ppm, pH9.3)

Particle

FPM

Metal, Chemical Oxide

H2-UPW Rinse

Hydrogen Termination

↓

FIGURE 3.45 4-Step cleaning process conducting at room temperature.


102

TABLE 3.3 Purity of UPW-Based Functional Water: Data of Trace Metal Analysis

Sample

UPW H2-UPW O3-UPW

Concentration of Dissolved H2 or O3 (ppm)

— 1.2 2.0

ORP (mV)

250 2600 1000

Ti

Cr

Ni

,1 ,1 ,1

,1 ,1 ,1

,1 ,1 ,1

Concentration (ppt) Cu Zn Fe ,1 ,1 ,1

,1 ,1 ,1

,1 ,1 ,1

Mg

Al

,1 ,1 ,1

,1 ,1 ,1

meet rigorous requirements. Gas-dissolved functional water, which replaces chemicals and UPW, is also expected to feature as high a purity as chemicals and UPW. A system for the production of functional water by the gas-dissolution method can be composed only of high-purity materials which are used in UPW and chemical delivery lines. Purity of source water, therefore, is not to be deteriorated in the production system. Table 3.3 shows results of trace metal analysis, comparing H2-UPW, O3-UPW, and source water (UPW). Metal concentration is less than 1 ppt (ng/l) in every case, which demonstrates outstanding purity of these functional waters. They can be used in mass production line without any concern about introduction of unwanted contamination.

B. TOTAL ROOM-TEMPERATURE 5-STEP CLEANING PROCESS [23] Sometimes a cleaning recipe using only neutral or acidic solutions is required. In this section, another total cleaning process without any alkaline solution is reported. 1. Design of 5-Step Cleaning Process There are two well-known important indicators about wet cleaning: 1. To remove particles, it is essential to satisfy the two requirements in parallel: to prepare conditions to prevent particles from re-adhering to wafer surface and to lift off particles from wafer surface. 2. To remove metallic impurities, it is important to etch oxide with embedded metals and to keep the redox potential high enough to prevent metallic impurities from re-adhering to the bare Si surface. FPM is a solution that etches Si surface and has a very high redox potential. Figure 3.46 shows Si surface etching with FPM as a function of time, and it also shows effects of surfactant addition. The initial etching rate is 0.3 nm/min at 258C. When a surfactant is injected, the etching rate is lowered to less than one tenth. Etching rate of surfactant-added FPM (FPMS) is not much enough to lift off particles. Therefore, assistance of some physical power is required to remove particles. It is speculated that particles as well as oxides and metals must be removed simultaneously if MS radiation is applied to the FPMS solution. The experiment demonstrates that this speculation is correct. Figure 3.47 shows particle removal efficiency of various solutions. The Si wafer was intentionally contaminated with 3000 – 4000 particles of PSL with size of 0.3 mm or more. DHF, FPM, FPMS, and FPM þ MS are not found to be effective in removing particles. Particle removal efficiency of high-temperature APM cleaning is exceeded at room temperature only when the MS irradiation is applied to FPMS (FPMS þ MS) at room temperature.


Etching Depth (nm)

2,000

103

P(100) Sol.: 0.1%HF/10%H2O2/H2O Surfactant:nonionic Temp.: 25°C Megasonic: off

1,500

Surfactant Conc. 0 ppm 50 ppm

1,000 100 ppm

Etching Rate 0.3 nm/min

200 ppm

500

0

0

10 20 30 40 50 60 70 80 90 100 110 120 Etching time (h)

FIGURE 3.46 Si surface etching depth with FPM as a function of time and effect of surfactant addition.

The APM cleaning process uses a large amount of ammonia which is extremely volatile even at room temperature as well as at elevated temperature. Ammonia is evaporated in large volume in conjunction with water evaporation, which makes it impossible to control the chemical composition accurately in this process. Figure 3.48 shows microroughness of n-type Cz wafers before and after cleaning by FPMS þ MS. It indicates that FPMS þ MS does not damage Si surface. Figure 3.49(a) shows the newly proposed Si surface cleaning process in which every step is performed at room temperature using neutral or acidic solutions only. First step: O3-UPW for removal of organic impurities and noble metals (Cu and Ag) Second step: FPMS þ MS for removal of particles, metallic impurities and Si oxide layer Third step: O3-UPW for removal of residual surfactant on the surface Fourth step: DHF or FPM for removal of Si oxide layer Fifth step: UPW for final rinse

Particle Number (pcs/4 inch wafer)

Figure 3.49(b) shows the conventional RCA cleaning process. Chemicals used in the new process are just HF and H2O2 when chemical concentration is maintained at less than 1%. By 10,000 Particle Diameter 8,000 6,000

0.3–0.5 µm 0.5–1.0 µm >1.0 µm

DHF : 0.5%(HF) FPM : 0.5%(HF) + 10% (H2O2) FPMS : FPM + surfactant APM : (NH4OH:H2O2:UPW)=(0.05:1:5) MS : Megasonic (950 KHz)

4,000 2,000 0

Initial FPMS + MS APM

DHF

FPM FPM + MS FPMS

Sample Condition

FIGURE 3.47 Particle removal efficiency of several kinds of HF based solution (effect of FPMS þ MS scientifically developed based on mechanism of contaminants adhesion and removal).

104

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing Cz N(100) Cleaning : 10 min at R.T. FPMS : 0.5%HF/10%H2O2/H2O + Surfactant(50 ppm) MS : Megasonic (950 kHz)

before cleaning Rms = 0.12 nm

after cleaning Rms = 0.11 nm

FIGURE 3.48 AFM image of Si surface before and after cleaning by FPMS þ MS after O3-UPW cleaning.

keeping the humidity of the chemical baths higher than 80%, evaporation of UPW can be suppressed to a very low level, and consequently chemical composition can be controlled to an extremely accurate level. Accurate control of chemical composition and temperature enables us to realize the wet process exhibits extremely high accuracy and perfect reproducibility. In particular, the second step, FPMS þ MS (0.5% HF þ 0.5% H2O2 þ 50 ppm surfactant þ MS irradiation), is an extremely versatile technology designed scientifically on the basis of cleaning mechanisms to remove particulate and metallic impurities. This 5-step cleaning has been confirmed to exhibit same excellent contamination removal capability by evaluating the gate oxide integrity having a thickness of 12 nm, as shown in Figure 3.50, where the current density is plotted as a function of the voltage for the conventional RCA cleaning and the 5-step cleaning. 2. Cleaning by HF 1 O3-UPW (FOM) [4,24] As described above, FPM is a very useful cleaning solution having the ability both for oxidation of Si surface and etching of the Si oxide layer. In this respect, adoption of O3 instead of H2O2 in HFbased solution was examined.

FIGURE 3.49 5-Step cleaning process conducting at room temperature compared with conventional RCA cleaning.


105

FIGURE 3.50 Current density– voltage characteristic of Al/n-Si diodes. Comparison between conventional RCA cleaning and new 5-step cleaning.

Figure 3.51 shows Cu contamination removal efficiency by FPM and FOM. O3 in water shows a very strong oxidation power, however the concentration of dissolved ozone can not be raised up to percent order. Normally, it is used less than 100 ppm (0.01%). On the other hand, concentration of H2O2 in FPM solution is 0.1% or more. Figure 3.52 shows a key reaction of Cu removal in FOM solution with 5 ppm dissolved O3 as a function of HF concentration. At less than 200 ppm of HF concentration in FOM, the main reaction on contaminated Cu particle surface is the the formation of copper oxide or copper hydroxide. At HF concentration higher than 600 ppm, the amount of electron tunneling from silicon substrate suppresses the dissolution reaction of Cu particle. The optimum HF concentration of FOM seems to be in the range of 200 –600 ppm. However, FOM cleaning cannot remove the copper contaminant to as low a detection limit as compared with FPM cleaning with high concentration oxidant. It is clear that O3 of ppm order cannot be used instead of percent order of H2O2 in FPM solution even though O3 has higher ORP value than H2O2.

Cu concentration (atoms/cm2)

1015

as-contamination : 2 x 1014 atoms/cm2

1014 1013 1012 1011 1010 FPM FOM

109 1

10

100

1000

Cleaning time (sec)

FIGURE 3.51 Comparison of Cu contaminants removal efficiency between FPM and FOM as a function of cleaning time.


106

FIGURE 3.52 Affect of HF concentration in FOM for Cu removal.

3. Prevention of Decomposition of Surfactant [25] When a surfactant which consists of some kind of organic carbon was used in FPMS solution, there was a problem due to decomposition of the surfactant. Figure 3.53 shows that dependence of MS irradiation time on particle removal efficiency using surfactant added UPW. The result represents that particle removal efficiency is decreased when gas species such as O2 and air is dissolved in cleaning solution. When gas species such as Ar, He, and H2 is dissolved, however, it was found that a high particle removal efficiency has been maintained. Finally, particle removal efficiency using H2 added FPMS maintained for 10 h. Surfactant contributes having same polarity of zeta potential between wafer surface and particles. Particle removal efficiency falls off when surfactant is decomposed by reaction with free †OH radicals in MS-irradiated solution. Free †OH radicals are generated much in MS-irradiated UPW-dissolving gases such as O2 and air, but they are not generated in in all the gases used in gas dissolution except for oxygen. The lifetime of FPMS solution can be prolonged by using dissolved gas species such as Ar, He, and H2, conclusively.

Ar, He, H2 and deaerated

100 90 Removal rate (%)

80

Air

70 60

CZ-P type 5 inch Si wafer Particle = alumina Surfactant = 50 ppm (Nonion) MS = 1 MHz, 2.0 W/cm2 Cleaning Time = 10 min Particle Size > 0.3 µm

50 40 30 20

O2

10 0

0

100

200 300 400 MS irradiation time (min)

500

600

FIGURE 3.53 Dependence of MS irradiation time on particle removal efficiency using surfactant added UPW. Effect of H2, Ar, or He gas dissolving.


107

Device level : 1G/4G DRAM

26 steps

9 steps

BEOL (26) 55% Si3N4

FEOL(9) 28% 33%

Si3N4

7% Silicon

Metal

Silicon

SiO2

SiO2 28%

55% 44%

FIGURE 3.54 Occurrence of materials in wet process during fabrication of DRAM.

C. STRATEGY IN CLEANING PROCESS

IN

BEOL [26,27]

In the wet process, cleaning before gate oxidation is the most important. On the other hand, cleaning time in back end of line (BEOL) is more than that in the front end of line. Figure 3.54 shows the occurrence of materials during fabrication of 1 or 4 G DRAM. It indicates the most often exposed surface is SiO2 and the second is the metal. It is, therefore, mandatory to clean simultaneously the metals as well as oxides. Figure 3.55 shows particle removal efficiency from Si oxide (TEOS) surface by surfactantadded DHF (DHFS). The solution can etch the SiO2 surface and prevent particle re-adhesion on the surface. It is recognized that DHFS having more than 600 ppm HF can remove particles on Si oxide well even without MS. Figure 3.56 shows etching rate of DHF þ H2O2 solution for various films as a function of H2O2 concentration. It indicates that etching rate of various metallic films can be controlled by H2O2 concentration. Figure 3.57 shows the influence of surfactant addition in FPM on the etch rate of various films. Two solutions, namely FPM and FPMS were used to study the etch rate of films. It has been clearly found that the etch rates of various metals as well as oxide films do not strongly depend on adding

Residual particles > 0.2 micron

2000 as-contaminated > 2000 LPDs on 6 inches wafer Cleaning and rinse time : 5 min 1500 Al2O3 SiO2

Thickness

1000

Si3N4

500

0 0

200 400 600 800 1000 1200 0 HF in dHFS (ppm)

20

40

60

80

100

Etched thickness (Å)

FIGURE 3.55 Particle removal with HF concentration on TEOS in surfactant added DHF (DHFS).


108

105

Etch rate (Å/min)

104

Aluminium (AI) Titanium (Ti) Copper (Cu)

Cobalt (Co) Tungsten (W) Platinum (Pt)

103 102 101 100 10-1 1

10 100 H2O2 concentration (ppm)

1000

10000

FIGURE 3.56 Film etch rate with H2O2 concentration in DHF þ H2O2 (FPM).

the surfactant to the solution by which a meaningful influence is obtained on silicon surfaces such as crystalline-, poly-, and a-Si films. FPMS only without MS does not have particle removal efficiency on silicon surface, as silicon etch rate is drastically decreased by adding the surfactant. In contrast, in the situation in which both oxide and metal layers have to be cleaned at the same time, FPMS or surfactant-added DHF (DHFS) without MS are good candidates. In fact, as shown in Figure 3.57, both oxides and metals have nearly the same etch rate irrespective of surfactant addition into FPM or DHF, whereas a surfactant can establish the same polarity of zeta potential between particles and substrate. Therefore, HF-based solutions with etch capability of substrate and same zeta potential polarity between particles and substrate by surfactant can have excellent particle removal efficiency on metals and the oxide-exposed surface. It is suggested that FPMS having various H2O2 concentrations can be applied to oxides and metal films. It seems to be available for damageless cleaning process in BEOL.

FIGURE 3.57 Etch rate versus surfactant effect in FPM solution.


109

REFERENCES 1. H. Morita, K. Kawada, S. Okano, T. Nita, and T. Ohmi, Development of an advanced wet cleaning equipment with megasonic enhancing system, in: Proceedings of the Fifth International Symposium on Semiconduct Manufacturing (ISSM’96), Tokyo, pp. 337 – 340, 1996. 2. H. Morita, J. Ida, T. Ii, and T. Ohmi, Advanced UCT cleaning process based on specific gases dissolved ultrapure water, in: Proceedings of the Eighth International Symposium on Semiconductor Manufacturing (ISSM’99), Santa Clara, CA, pp. 453 – 456, 1999. 3. H. Morita, T. Ii, J. Ida, and T. Ohmi, Total room temperature wet cleaning process based on specific gas dissolved ultrapure water. Electrochem. Soc. Proceed., vol. 99 – 36, 45 – 50, 2000. 4. T. Ohmi, Advances in the use of UPW in wafer cleaning, in: Proceedings of 2000 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, pp. 1 – 13, 2000. 5. T. Ohmi, T. Isagawa, T. Imaoka, and I. Sugiyama, Ozone decomposition in ultrapure water and continuous ozone sterilization for a semiconductor ultrapure water system, J. Electrochem. Soc., 139, 3336– 3345, 1992. 6. T. Isagawa, M. Kogure, T. Imaoka, and T. Ohmi, Ozone added ultrapure water application for ULSI advanced processing, in: Chemical Proceedings of 1992 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, pp. 224– 247, 1992. 7. T. Isagawa, T. Imaoka, M. Kogure, H. Shimada, and T. Ohmi, Ozone application to wet cleaning processes for ultra clean surface preparation, in: 1992 Proceedings of the 38th Annual Technical Meeting, Institute of Environmental Sciences, Vol. 1, Nashville, pp. 466 – 474, 1992. 8. H. Morita, J. S. Kim, and T. Ohmi, Cleaning of noble metals on silicon wafer surface by ozonized ultra pure water, in: Proceedings of 1996 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, pp. 215– 229, 1996. 9. J. S. Kim, H. Morita, J. D. Joo, and T. Ohmi, The role of metal induced oxidation for copper deposition on silicon surface, J. Electrochem. Soc., 144, 3275–3283, 1997. 10. J. D. Joo, J. S. Kim, H. Morita, and T. Ohmi, Comparison of cleaning efficiency of noble metals on Si surface between O3-UPW and SPM., Cleaning Technology in Semiconductor Device Manufacturing, PV97-35, The Electrochemical Society, pp. 280 – 288, 1998. 11. J. S. Kim, H. Morita, G. M. Choi, and T. Ohmi, Cleaning efficiency of various chemical solution for noble metals such as Cu, Ag, and Au on Si wafer surfaces. J. Electrochem. Soc., 146, 4281– 4289, 1999. 12. H. Morinaga, M. Suyama, and T. Ohmi, Mechanism of metallic particle growth and metal induced pitting (MIP) on Si wafer surface in wet chemical processing, J. Electrochem. Soc., 141, 2834– 2841, 1994. 13. H. Morinaga, M. Suyama, M. Nose, S. Verhaverbeke, and T. Ohmi, A model for the electrochemical deposition and removal of metallic impurities on Si surfaces, IEICE Trans. Electron., E79-C, 343– 362, 1996. 14. I. Yokoi, G. M. Choi, and T. Ohmi, Effect of pH values in ozonized ultrapure water on cleaning efficiency, Electrochemical Society Proceedings, vol. 2001-26, 69 – 76, 2001. 15. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966. 16. H. Morita, New Si Substance Cleaning Technology using Functional Water, Ph.D. Disseration, Tohoku University, Sendai, Japan, 2000. 17. H. Morita, J. Ida, T. Mizuniwa, and T. Ohmi, Hydrogenated ultrapure water production system for future wet cleaning process, in: Abstract of Fourth International Symposium on ultra clean processing of silicon surfaces (UCPSS’98), Ostende, pp. 3,4, 1998. 18. H. Morita, J. Ida, T. Mizuniwa, and T. Ohmi, Hydrogenated ultrapure water production system for future wet cleaning process, in: Proceedings of UCPSS’98, pp. 7 – 10, 1998. 19. H. Morita, J. Ida, O. Ota, K. Tsukamoto, and T. Ohmi, Particle removal mechanism of hydrogenated ultrapure water with megasonic irradiation, in: Fifth International Symposium on Ultra Clean Processing of Silicon Surfaces (UCPSS 2000), Abstract Book, Ostende, pp.160 – 161, 2000. 20. H. Morita, J. Ida, O. Ota, K. Tsukamoto, and T. Ohmi, Particle removal mechanism of hydrogenated ultrapure water with megasonic irradiation, Solid State Phenomena, 76 – 77, 245 – 250, 2001. 21. T. Shimono and M. Tsuji, in: The Electrochemical Society Meetings Abstract, 91-1, Washington, DC, pp. 278, 1991.


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22. H. Morita, J. Ida, T. Mizumiwa, N. Yabumoto, S. Kushire, and T. Ohmi, Dissolved gas controlled ultrapure water production system for wet cleaning process, in: Proceedings of the Seventh International Symposim on Semiconductor Manufacturing (ISSM’98), Tokyo, pp. 428 – 431, 1998. 23. T. Ohmi, Total room temperature wet cleaning for Si substrate surface, J. Electrochem. Soc., 143, 2957– 2964, 1996. 24. G. M. Choi, I. Yokoi, and T. Ohmi. The role of oxidant in HF-based solution for noble metal removal from substrate, Solid State Phenomena, vol. 76 – 77, 267 – 270, 2001. 25. H. Kanetaka, T. Kujime, H. Yazaki, T. Kezuka, and T. Ohmi, Influence of the dissolved gas in cleaning solution on Si wafer cleaning efficiency, in: Proceedings of UCPSS’98, pp. 43 –48, 1998. 26. G.M. Choi, F. Pipia, and T. Ohmi, Strategy in cleaning processes for future materials, in: Extended Abstracts of the 2000 International Conference on Solid State Devices and Materials, Sendai, pp.174 – 175, 2000. 27. F. Pipia, G. M. Choi, and T. Ohmi, Metal wet cleaning with no corrosion: a novel approach, Solid State Phenomena, 76– 77, 35– 38, 2001.

VII. SURFACTANT AND CHELATING AGENT TECHNOLOGIES FOR INNOVATING SEMICONDUCTOR CLEANING HITOSHI MORINAGA

A. CHALLENGES

IN

CLEANING TECHNOLOGY

TO

ADDRESS HIGHER-DENSITY ULSI DEVICE

Density of ULSI device keeps increasing without showing any sign of leveling off. Feature size of ULSI device is going down below 100 nm, which was considered next to impossible a decade ago. Cleaning process is required to achieve: (i) ultraclean surface, (ii) without any side effects, (iii) within a short period of time, (iv) with high reproducibility, and (v) at low cost. The requirements get increasingly demanding year by year as the device density goes up and device price goes down. At the same time, challenges to be overcome get diversified more than ever before along with rapid diversification of device materials, manufacturing processes and production systems (e.g., small-volume

FIGURE 3.58 Technology roadmap of semiconductor and challenges of the cleaning technology.


111

production of multiple types of products) (Figure 3.58). Introduction of metallic materials also brings about a new challenge. Specifically when metallic materials are dissolved even in trace amounts when they redeposit into Si or dielectric film, a serious problem of cross-contamination will take place. Along with ULSI device shrink, finer particles also adversely affects the device yield. Also, higherdensity ULSI device is more susceptible to damages induced by mechanical action such as megasonic irradiation as well as chemical action of cleaning solutions. This tendency is more outstanding in ULSI devices beyond the 100 nm node [1,2]. Cleaning technology in the 100 nm node and beyond is required to effectively remove ultrafine particle without damaging fragile patterns on ULSI device. Wafer fab engaged in small-volume production of multiple types of products is expected to manufacture small-lot products within the shortest possible time. To meet this requirement, the conventional batch-type cleaning in which a 50-wafer batch is cleaned in an hour must be replaced with a single-wafer cleaning system which cleans each wafer within 2 min [1,3].

B. SOLUTIONS

TO THE

CHALLENGES FROM

THE

VIEWPOINT OF CLEANING MECHANISM [1]

Functions essential for cleaning trace contaminants are to get the contaminant desorbed (Function 1), to prevent redeposition (Function 2), and to etch the underlying film (when contaminant is buried into underlying film or when contaminant forms a strong chemical bond with underlying film) (Function 3). For example, APM (NH4OH/H2O2/H2O) cleaning, which is widely used for particle removal, is equipped with the three functions. Function 1 is achieved by megasonic irradiation or other mechanical forces. Function 2 is achieved by repulsive force between particle and substrate surface induced by zeta potential which is controlled by alkali solution. Function 3 is achieved as alkali solution etches Si and SiO2 film. To an ULSI device of ultrahigh density, the conventional methods of intensive megasonic irradiation for contaminant desorption and etching of the underlying film by as much as several nanometers can be no longer applied. It is because (i) pattern collapses because of megasonic irradiation and (ii) dimensional fluctuation because of etching will become critical. In order to remove the contaminant while minimizing the contributions of Functions 1 and 3, it is important to enhance Function 2 (prevention of contaminant redeposition). In other words, it is critical to develop a redeposition prevention technology that ensures to capture a trace contaminant lifted off from the surface even slightly (Figure 3.59).

FIGURE 3.59 Significance to prevent contaminant redeposition in advanced semiconductor device cleaning.

112


In the conventional RCA cleaning [4], a means of redeposition prevention is very much limited: control of pH and redox potential to prevent redeposition of metallic contaminants and control of zeta potential through pH adjustment to prevent particle redeposition. Meanwhile trace additives such as a chelating agent and a surfactant are highly effective in preventing redeposition of contaminant as discussed in Chapter 2. This section will present advanced cleaning technologies using these additives to address the needs of the 100 nm node and beyond.

C. PREVENTING METAL CROSS CONTAMINATION [1] The primary drawback of the RCA-based wet-cleaning methods [4] is their side effect, that is, redeposition of the contaminant from the cleaning on solution to the wafer surface. APM cleaning is extremely effective in removing particle and organic contaminants, but it is hardly effective in removing metallic contaminants. In other words, APM cleaning allows metallic contaminant to adsorb to wafer surface in large amounts when the metallic impurity is allowed at lease in trace amounts in solution. In alkali solution, a metallic contaminant forms the hydroxide which adsorbs to substrate surface by dehydration reaction with Si – OH group there at [5,6]. This problem can be overcome by using the chelating agent technology presented in Chapter 2. The chelating agent captures a metallic contaminant in APM solution and prevents its redeposition onto substrate surface. It is necessary to select chelating agents which are capable of reliably capturing various metals such as Fe, Zn, and Cu that are likely to re-adsorb to surface in APM solution. The chelating agent technology becomes increasingly important in addressing metal crosscontamination induced by various new metallic materials being introduced to FEOL (Figure 3.58). Co and Hf brought into APM solution from Co salicide gate and high-k dielectric films are larger in amount than the conventional trace metal contaminants, exceeding the ppb order [7]. Figure 3.60 demonstrates that the chelating agent is sufficiently effective in preventing these metals from re-adsorbing onto the Si surface.

D. ACCELERATING FINE PARTICLE REMOVAL WITHOUT DEVICE PATTERN DAMAGE [1] Needless to say, ultrafine particle (100 nm or less) must be removed in the 100 nm node and beyond. Generally, the smaller the particle size, the more difficult to remove and more likely to deposit it [8,9]. This is because even if both the particle and the substrate surface have negative

FIGURE 3.60 Effectiveness of adding chelating agent in preventing Co and Hf adsorption from APM. Comparison of the conventional APM (NH4OH/H2O2/H2O) and chelate-added APM (MC1/H2O2/H2O) (MC1 chelate-added NH4OH; Mitsubishi Chemical Corporation, Japan). (From H. Morinaga, A. Itou, H. Mochizuki, and M. Ikemoto, in: Electrochemical Society Proceedings Series, PV.2003-26, Pennington, NJ, pp. 370 – 377, 2004. With permission.)

INTERACTION ENERGIES (×10−18 J)


113

1 µm

2

0.5 µm

1

0.1 µm 0 10 nm −1 −2 0

0.5

1

1.5

2

2.5

DISTANCE BETWEEN PARTICLE AND SURFACE (nm)

FIGURE 3.61 Potential energy changes when the particles with various diameters deposit onto the substrate surfaces (in the liquid medium. when the both particles and substrates are negatively charged). (From H. Morinaga, A. Itou, H. Mochizuki, and M. Ikemoto, in: Electrochemical Society Proceedings Series, PV.2003-26, Pennington, NJ, pp. 370– 377, 2004. With permission.).

zeta potential in alkali solution, repulsive force generated between the two gets smaller as the particle size becomes smaller (Figure 3.61). To remove such ultrafine particles, high-temperature APM cleaning or APM cleaning combined with megasonic irradiation is effective. The conventional 10-min APM cleaning at high temperature (808C), however, cannot be applied to the device of the 100 nm node and beyond because it etches substrate surface by several nanometers and increase the surface microroughness. Also, cleaning combined with strong mechanical force such as megasonic irradiation may lead to pattern collapse. This is another problem in growing evidence in the 100 nm node and beyond. Surfactant changes zeta potential of the surface and improves wettability. As discussed in Chapter 2, the surfactant technology is effective in preventing particle from redepositing on the substrate surface. It is being proven that the surfactant technology facilitates removal of ultrafine particle even at low temperature and with mild megasonic irradiation. Figure 3.62 shows

FIGURE 3.62 The effect of temperature in removing ultrafine particles. The surfactant-added APM can effectively remove the particles even with lower temperature or lower Si and Oxide loss (MC1-SP surfactant and chelate-added NH4OH; Mitsubishi Chemical Corporation, Japan). (From H. Morinaga, A. Itou, H. Mochizuki, and M. Ikemoto, in: Electrochemical Society Proceedings Series, PV.2003-26, Pennington, NJ, pp. 370– 377, 2004. With permission.)


114

FIGURE 3.63 The number of remaining particles as a function of etch amount when the contaminated Si surface is cleaned with APM. Oxide loss needed to remove 60 nm particles could be reduced by adding surfactant.

temperature dependence of SiO2 particle removal efficiency in APM cleaning combined with megasonic irradiation, comparing the conventional APM cleaning and APM cleaning with surfactant spiked. When a surfactant is injected, the ultrafine particles can be removed even at low temperature and with marginal etching. Figure 3.63 shows residual particle count as a function of film loss after the cleaning presented in Figure 3.62. With the conventional APM cleaning, surface etching of about 3 nm is essential to reduce the particle count below the background. When the surfactant is spiked to APM solution, surface etching of 0.3 nm is found sufficient. Also megasonic power can be reduced when surfactant is spiked to APM solution while maintaining particle removal efficiency (Figure 3.64 and Figure 3.65). In other words, surfactant addition facilitates particle removal even with lower mechanical force. It is considered because the surfactant ensures to prevent redeposition of particle which is lifted off from substrate surface slightly by mild mechanical driving force. Unlike chelating agent, a surfactant works as it gets adsorbed onto the surface of a substrate or particles. This means that a thorough rinsing is essential, otherwise the residual surfactant may cause some problems. It is important to adopt surfactants without side effects such as residue, metallic contamination, and excessive bubbling. Dip cleaning, 70°C, 10 min Particle: Si3N4Particles (0.05–3 µm) Surfactant-added APM: MC1-SP/H2O2/H2O = 1/2/40 APM: NH4OH/H2O2/H2O = 1/2/40 Megasonic: KAIJO Hi Megasonic 600, 950 kHz, 136 × 163 mm2

Particle Remaining (pcls/wafer)@>0.06 µm

6000 Initial Counts

5000 4000 3000 APM

2000 1000

Surfactant-added APM

0 0

0.2

0.4 Megasonic Power

0.6 (W/cm2)

0.8 (200 W/plate)

FIGURE 3.64 Dependence of megasonic power in removing ultrafine particles with 708C–10 min–APM (oxide loss: 1.3 nm) (MC1-SP surfactant and chelate-added NH4OH; Mitsubishi Chemical Corporation, Japan). (From H. Morinaga, A. Itou, H. Mochizuki, and M. Ikemoto, in: Electrochemical Society Proceedings Series, PV.2003-26, Pennington, NJ, pp. 370–377, 2004. With permission.)


115

Particle Remaining (pcls/wafer)@>0.06 µm

6000 Initial Counts

5000 4000 3000 APM

2000 1000

Surfactant-added APM 0

0

0.2

0.4

0.6

Megasonic Power (W/cm2)

0.8 (200 W/plate)

FIGURE 3.65 Dependence of megasonic power in removing ultrafine particles with 508C –10 min – APM (oxide loss: 0.4 nm). (From H. Morinaga, A. Itou, H. Mochizuki, and M. Ikemoto, in: Electrochemical Society Proceedings Series, PV.2003-26, Pennington, NJ, pp. 370 – 377, 2004. With permission.)

E. SINGLE-WAFER PROCESSING [1] Single-wafer processing has significant advantages compared with traditional batch systems to dramatically reduce the production cycle time. Single-wafer processing speeds up processing time and therefore time to market. It makes possible to timely supply of various devices to meet diversified customer needs. However, long processing time in RCA cleaning has been an obstacle for single-wafer cleaning to be commercially adopted. In the conventional batch-type RCA cleaning, several cleaning steps each of which is followed by rinsing and drying steps taking more than 1 h in total. To introduce the single-wafer processing method, it is necessary to reduce the total cleaning time to 2 min or less. In the 100 nm node and beyond, it is also essential for the cleaning process to meet the above-mentioned three additional requirements: remove ultrafine particle, lower film loss, and prevent pattern damage. To overcome these challenges, it is necessary to make a full use of the chelating agent technology and the surfactant technology. Surfactant and chelating agent which do not remain on substrate surface after cleaning step must be selected. First of all, low-power megasonic irradiation is applied to get particle on substrate surface slightly lifted off. Then, the lifted-off particle is captured by surfactant in a way not to allow it to redeposit to surface. In the meantime, chelating agent added to cleaning solution captures metallic contaminant [1,10]. Basically, the number of cleaning steps can be reduced down to a single step with the surfactant- or the chelate-added (modified) APM. To remove specific contaminants such as Al included in the oxide film, a two-step process of DHFmodified APM (,0.3 nm etch) can be used. Even in this case, however, DHF cleaning can be limited to its minimum as the modified APM is capable of removing most contaminants. In other words, the DHF cleaning in the advanced wet-cleaning process is not expected to etch the substrate surface by as much as 1 nm as it does in the conventional cleaning process [10]. Introduction of these new technologies will enable cleaning process of 2 min or less to achieve ultraclean surface without excessive etching or pattern damage in the 100 nm node and beyond (Figure 3.66 and Figure 3.67).

F. CLEANING

OF

NEW MATERIAL SURFACE [1]

1. Challenges of New Material Surface Cleaning Along with increasing ULSI device shrink, various new materials are being introduced. What needs to be noted from the viewpoint of cleaning includes: (i) metallic materials (e.g., W, SiGe, high-k materials) are adopted for gate which is sensitive to contamination, (ii) Cu is adopted for interconnect though Cu is easy to be corroded and is regarded as the most problematic contaminant,


116

300-600

Particle Remaining (pcls / Wafer) @ >0.1µm

400 300

300-700 Initial Conventional APM Surfactant/Chelateadded APM

200-400

200 100 skin depth)

Wafer splitting (100)Epi-Si AlN

SiO2(BOX)

Cu

Ni-silicide-Layer

p+ or n+-Si (Handle-Wafer) p+ or n+Si (Device-Wafer) Metal-substrate SOI wafer

FIGURE 5.16 Metal-substrate SOI wafer.

degasifying the membrane. By anodic reaction porous silicon is formed. For a representative value, the etching rate of silicon is 1.2 mm/min and 150-mm wafer uniformity is 8.6%. Figure 5.18 shows a double-layered porous silicon formed by the anodic reaction in two sequential conditions. ELTRAN process uses two phenomena: 1. Mechanical stress is focused at the interface of a double-layered porous silicon and the wafer can be split at the interface. 2. Crystalline silicon grows epitaxially on porous silicon. Figure 5.19 shows x-ray diffraction of porous silicon formed under each condition, which indicates that porous silicon is crystalline and its lattice constant depends on the anodic reaction

FIGURE 5.17 Schematic diagram of anodic reaction system.


266

ELTRAN…double-layered porous Silicon p+Silicon (100), resistivity:0.02 Ω cm, 1st porous Si layer…36% HF, 10 mA/cm2, 5 min 2nd porous Si layer…5% HF, 3.3 mA/cm2, 5 min

3000 Å

10 µm 1st layer 5.0 µm

p+Silicon

3000 Å 1st layer

2nd layer 2nd layer

p+Silicon

FIGURE 5.18 Double-layered porous silicon.

condition. Next, the pores of porous silicon surface are closed by H2 annealed subsequently and the crystalline silicon grows epitaxitially. The silicon is oxidized to form BOX and the wafer is bonded to another handle wafer. The wafers are split at the interface of a double-layered porous silicon, and then SOI wafer is finished (Figure 5.16).

C. PRECISE CONTROL

OF

ANODIC REACTION

OF

SILICON

1. Introduction Wet process can be controlled precisely by some parameters such as the oxidation –reduction potential (ORP) of liquid phase, potential of silicon surface, and wetness of liquid phase to silicon surface. Anodic reaction of silicon is a kind of wet process. Conventional wet process is mainly controlled by ORP of liquid phase. The anodic reaction is characterized by the control of silicon surface potential. 2. Formation Mechanism of Porous Silicon and Improvement on Surface Morphology Figure 5.20 shows porous silicon formed by anodic reaction of (1 0 0) oriented nþ silicon (phosphorus concentration Nd is 1E þ 18 cm23), which has periodic pores perpendicular to silicon surface with a pitch width of 200 nm. The silicon surface potential is þ6 V, and space charge density is Nd ¼ 1E þ 18 cm23. By solving Poisson equation on depletion approximation, the width of the depletion layer is calculated to be about 100 nm, and pore pitch of porous silicon is equal to two times of the width. Figure 5.21 shows porous silicon by the anodic reaction of non-polished surface in the same condition. The direction of straight pores is not dependent on the surface wave, but on the direction of electric field. By means of controlling the applied voltage of electrolyte with high wetness to

Silicon Etching

267

counts/s 10M L

S

1M 100K 10K

Porous Silicon

Si substrate (5.43 Å)

1K 100 10 1 0.1 34.35

34.40

34.45

34.50

34.55

34.60

34.65

34.70 34.75 34.80 Omega/2Theta(degrees)

1st layer-Porous Silicon counts/s 10M S 1M

100K

Porous Silicon

Si substrate (5.43 Å) L

10K

1K

100

10 34.40

34.45

34.50

34.55

34.60

34.65 34.70 Omega/2Theta(degrees)

2nd layer-Porous Silicon

˚ ). FIGURE 5.19 X-ray diffraction of two-type porous silicon (Cu Ka: 1.5443 A

silicon, precise control of the depletion layer and various selective etching of silicon can be realized. Kleimann et al. [4] demonstrated interesting figures in this regard. As mentioned earlier, the next ELTRAN step of anodic reaction is closing the pores of porous silicon surface by H2 annealing, and epitaxial growth of crystalline silicon [5]. The crystalline silicon becomes an active layer and the interface between porous silicon and the epitaxial silicon becomes the surface of SOI wafer when finished. In order to make sub-micron full-depletion SOI devices, active silicon layer width is to be thinner than 50 nm. To produce such extremely thin SOI wafer, flatness of porous silicon


268

Cross-section

5.9 µm Pitch-width = 200 nm

1 µm n-type Si substrate (Nd=1 E+18 cm−3)

Pitch-width = W × 2 Anodic condition The width of depletion layer W=

2εs εoV ~100 nm qNd

@ V=6V, Nd =1 E+18 cm−3

Constant current: 10 mA/cm2 Electrolite: 5% HF + 65% HF + 30% IPA Reaction time: 5 min

FIGURE 5.20 Porous silicon formed by depletion layer of silicon surface (1).

surface is essential not only to epitaxial silicon crystal growth, but also to wafer uniformity of silicon layer. Uniformity is one of the important quality of SOI wafer, which has direct influence on yield of integrated circuit. This is because highly controlled dry etching technology is necessary to form contact hole in thin silicon layer. When SOI wafer has poor uniformity of silicon layer, dry etching cannot stop at silicon layer. Figure 5.22 shows surface morphology of porous silicon by anodic reaction of constant current 10 mA/cm2. Figure 5.23 shows surface morphology of constant voltage 6 V-porous silicon. When porous silicon is formed under constant voltage, only micropits, (which have a pitch length two times) that of the depletion layer, grows pores and porous silicon has smoother surface than under constant current. Constant voltage anodic reaction has advantage to produce high-quality SOI wafer.

6.0 µm 5.4 µm Porous Silicon

1 µm n+Silicon n+Silicon of no-polished surface (0.04 Ω cm) Constant current: 10 mA/cm2, Electrolite: 5% HF + 56% H2O + 30% IPA 5 min

FIGURE 5.21 Porous silicon formed by depletion layer of silicon surface (2).

Silicon Etching

269

3 µm

300 nm

Porous Silicon surface

n-type Silicon (Nd=1 E+18 cm−3), Constant current : 10 mA/cm2, Electrolite : 5% HF + 65% H2O + 30% IPA

Voltage

Cross-section

Anodic reaction time : 5 min

FIGURE 5.22 Surface morphology of porous silicon (1). 3 µm

300 nm

Porous Silicon surface

n-type Silicon (Nd=1 E+18 cm−3), Constant voltage : 6 V, Electrolite : 5% HF + 65% H2O + 30% IPA Anodic reaction time : 5 min

Current

Cross-section

Reaction time

FIGURE 5.23 Surface morphology of porous silicon (2).

D. DISCUSSION

ON

ANODIC REACTION

OF

SILICON

In the case of conventional wet process where no voltage is applied on silicon substrate, HF22 ions react SiO2 which has polarity, but do not react silicon having no polarity. When a negative bias is applied on to the silicon substrate by electrolyte and potential, the silicon surface becomes positive with respect to the electrolyte, silicon surface can receive electrons from HF22 ions, and silicon can be dissolved by fluoric acid.


270

(cathode)

Electrolite (HF+IPA+H2O)

(cathode)

Depletion layer with high resistance

(cathode)

W W

N-type Si substrate (anode) Micro pits formation

(anode)

H H H H H H H H H H

H H H H H H H H H H

Current path (anode)

Current path is confined at the top of pores, and periodic porous silicon is formed.

FIGURE 5.24 Mechanism of periodic porous silicon formation.

Figure 5.24 shows the formation mechanism of periodic porous silicon. Negative bias is applied on silicon surface from electrolyte, and depletion layer is formed at n-type silicon surface. Electrolyte has high wettness by mixture of IPA, which penetrates into micro-pits on silicon surface, and a depletion layer is formed conformal to the silicon surface. When the pitch of pores becomes two times that of the depletion layer width, current path is confined to the tips of pores, and a periodic porous silicon is formed.

E. SUMMARY In this chapter our research on anodic reaction of silicon is introduced. Our target is to make gas-isolated metal-substrate SOI wafer, which can realize ultimate integrated circuits using silicon. To sum up, our research on anodic reaction of silicon was started as an element of production process of SOI wafer. The anodic reaction of silicon is nevertheless thought to be a new wet-process technology, because silicon surface potential is a new control parameter in addition to ORP of liquid phase.

REFERENCES 1. A. Morimoto, K. Kotani, S. Sugawa, and T. Ohmi, Interconnect and substrate structure for gigascale integration, Jpn. J. Appl. Phys., 40, 3038–3043, 2001. 2. T. Yonehara, K. Sakaguchi, and N. Sato, Epitaxial layer transfer by bond and etch back of porous Si, Appl. Phys. Lett., 64, 2108– 2110, 1994. 3. T. Ushiki, H. Ishino, and T. Ohmi, Effect of starting SOI material quality on low-frequency noise characteristics in partially depleted floating-body SOI MOSFETs, IEEE Electron Device Lett., 21, 610– 612, 2000. 4. P. Kleimann, J. Linnros, and R. Juhasz, Formation of three-dimensional microstructures by electrochemical etching of silicon, Appl. Phys. Lett., 79, 1727– 1729, 2001. 5. N. Sato, S. Ishii, S. Matsumura, M. Ito, J. Nakayama, and T. Yonehara, Defect engineering in epitaxial layers over porous silicon for ELTRAN SOI wafers, Extended Abstracts of SSDM, 302 – 303, 1998.

6

Chemical Composition Control Technology Tatsuhiro Yabune Stella Chemifa Corporation, Osaka, Japan

Masayuki Miyashita Stella Chemifa Corporation, Osaka, Japan

Hirohisa Kikuyama Stella Chemifa Corporation, Osaka, Japan

Jun Takano Stella Chemifa Corporation, Osaka, Japan

CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Current Status and Existing Problems of Cleaning Solution Control . . . . . . . . . . . . A. Problems of High-Temperature Wet Process . . . . . . . . . . . . . . . . . . . . . . . . . . B. Effects of Decomposition of Etchant Components . . . . . . . . . . . . . . . . . . . . . . III. Parameters to Change of Etchant Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Evaporation Amount Depended on Humidity and Temperature (Ambience Condition): Relationship between Humidity and Amount of Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Effects of Chemical Species Consumption through Etching . . . . . . . . . . . . . . C. Carry-In of Chemicals Used at Just before Step and Carry-Over of Etchant . IV. BHF Composition Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Control of HF22H2O2 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

271 272 273 274 275

275 276 278 278 281 281 282

I. INTRODUCTION As ultra large-scale integration (ULSI) device is constantly innovated in terms of its density and functionality, wet-process technology becomes increasingly important in ULSI device manufacturing line. Along with the ongoing improvement of purity and functionality of chemicals, wet process technology is making remarkable progress. In the traditional cleaning technology, three major cleaning techniques are combined to remove different contaminants. SPM (H2SO422H2O2) cleaning (120 –1508C) is adopted to remove organic and metallic impurities. APM (NH4OH22 H2O2) cleaning (80 – 908C) is used to remove particles and organic impurities. DHF (e.g., 0.5% HF) cleaning (at room temperature) is used to remove chemical or native oxide. Also, the conventional etching process is sometimes performed at high temperature of 308C or more in order to achieve high etch rate. These cleaning and etching techniques at high temperature, however, generate chemical vapor in a huge amount to be exhausted. 271

272


A perfectly controlled wet-cleaning process based on the ultraclean technology has been proposed as a new cleaning technology to overcome these challenges [1,2]. It is essential in the new cleaning technology to reduce the number of process steps as much as possible, to minimize chemical vapor generation by performing every process step at room temperature, and to perfectly control the chemical composition. In the case of the cleaning technology, for instance, a new cleaning process has been established that uses cleaning solutions comprising HF and H2O2. Relevant chemical waste-recovery technologies have also been established [3]. Focusing buffered hydrofluoric acid (BHF), HF, and HF22H2O2, all of which are typical solutions used in wet process, the present authors identified the parameters that affect process ambience and chemical composition, and studied the chemical composition control technology by controlling these parameters. On the basis of this study, the authors have developed a stable wet process with optimum accuracy and reproducibility.

II. CURRENT STATUS AND EXISTING PROBLEMS OF CLEANING SOLUTION CONTROL BHF has been commonly used in ULSI device manufacturing process. As shown in Table 6.1, BHF solution has been usually prepared by mixing 50% HF and 40% NH4F with arbitrary mixing ratio. The mixing ratio has been determined in a way to achieve a desirable etch rate on target oxide. Along with increase of wafer diameter and device density as well as diversification of types of oxide (dielectric film) to be etched, chemicals and gases used in the process have been increased in type. Consequently, utilities to be controlled have been considerably increased both in type and in volume. For example, it becomes difficult for the wet process to use numerous types of chemicals as only limited space is available for chemical tank installations. All etching processes need to be fulfilled by only using one type of chemicals or two. The conventional approach to control etch rate by using multiple chemicals with different mixing ratio must be replaced by a new approach in which the types of chemical are strictly limited and etching progress is controlled by changing the etching time and etching temperature. Table 6.2 shows the crystal precipitation temperature of BHFs with different chemical compositions. BHFs with some chemical compositions are highly likely to allow crystal to get precipitated when temperatures go down in winter. These BHFs need to be carefully handled.

TABLE 6.1 Relationship between Composition of Conventional BHF and Etch Rate of Thermal SiO2 Film at 2588 C Mixed Ratio of BHF (wt. ratio) 40% NH4F:50% HF 5:1 6:1 9:1 10:1 20:1 30:1 100:1 400:1

NH4F (wt.%)

(mol/l)

33.3 34.3 36.0 36.4 38.1 38.7 39.6 39.9

9.00 9.27 9.73 9.84 10.30 10.46 10.70 10.78

Composition HF (wt.%) (mol/l) 8.3 7.1 5.0 4.5 2.4 1.6 0.5 0.1

4.17 3.55 2.50 2.25 1.20 0.80 0.25 0.05

NH4F/HF mol Ratio

Etch Rate Thermal SiO2 at 258C (nm/min)

2.16 2.61 3.89 4.37 8.58 13.07 42.81 215.60

141 115 78 70 37 26 9 2.7


273

TABLE 6.2 Relationship between Composition of BHF and Segregation Temperature Chemical Composition HF (%) NH4F (%)

BHF 30:1 20:1 15:1 10:1 9:1 7:1 6:1 5:1

1.61 2.38 3.13 4.55 5.00 6.25 7.14 8.33

Etch Rate of Thermal SiO2 at 258C (nm/min)

Segregation Temperature (8C)

26 37 49 70 78 102 115 141

220 213 27 2 4 10 14 18

38.71 38.10 37.50 36.36 36.00 35.00 34.28 33.33

As HF concentration becomes higher and etch rate goes up, crystals get precipitated at higher temperature too. On the other hand, crystal precipitation temperature goes down as HF concentration in BHF gets lower. The traditional ULSI manufacturing processes use these phenomena in handling etchants. For example, it is easy to handle 20:1 BHF even in winter as its crystal precipitation temperature is low. Etch rate of 20:1 BHF, is low due to its low HF concentration. Hence, in order to achieve high etch rate, 20:1 BHF is used at high temperature. Low etch rate of 20:1 BHF is compensated by raising the etching temperature. No effective measures, however, are taken against increase of chemical vapor generation from the 20:1 BHF, induced by high etching temperature. It has become increasingly necessary to use etchants without replacement for extended period of time, and to improve stability and reproducibility of etching process.

A. PROBLEMS

OF

HIGH-TEMPERATURE WET PROCESS

Figure 6.1 and Table 6.3 show change in chemical composition of 20:1 BHF over time when this etchant is left at 368C Both HF concentration and NH4F concentration remain stable for the first 49 Segregation Temp. +18°C

NH4F Concentration (%)

47 45 43 41 39 ?

37

20 : 1BHF

35 0

5

10 15 20 25 30 35 40 45 50 Standing Time (hrs) Standing Temp. :36°C

FIGURE 6.1 Relationship between standing time and chemical composition.


274

TABLE 6.3 Relationship between Standing Time and Chemical Composition Standing Time (h)

Chemical Composition (%) HF NH4F

0 24 48

2.38 2.61 2.89

Segregation Temperature (8C) 212 29 þ18

38.1 41.9 45.6

Note: Chemical Composition: 20:1 BHF (HF: 2.38%, NH4F: 38.1%); etching temperature: 368C.

24 h, and start increasing afterward. Crystal precipitation temperature also goes up from 212 to 98C in the first 24 h, and to þ188C in the next 24 h. It is concerned that crystal precipitation takes place when etchant temperature is reduced from 368C to room temperature after the above-mentioned change in etchant composition. Such crystal precipitation triggers increase of particle count in the etchant and further changes in etchant composition. They will change the etch rate and make etching nonuniform, which eventually leads to yield drop.

B. EFFECTS

OF

DECOMPOSITION

OF

ETCHANT COMPONENTS

Traditionally mixing ratio between 50% HF and 40% NH4F was adjusted to achieve target etch rate. To get lower etch rate, for example, 40% NH4F and 50% HF were mixed with ratio of 400:1 (400:1 BHF) as shown in Table 6.1. Figure 6.2 shows the relationship between etch rate of 400:1 BHF on thermal SiO2 film and standing time at 238C. Table 6.4 shows relationship between composition change and etch rate of thermal SiO2 film of 400:1 BHF, and standing time. 4.5 4.0

Etch Rate (nm/min)

3.5 3.0 2.5 2.0 1.5 ?

1.0

400 : 1BHF

0.5 0.0 0

10

20

30 40 50 60 Standing Time (hrs) Etching Temp. : 23°C Humidity : 53%

FIGURE 6.2 Relationship between standing time and etch rate.

70

80


275

TABLE 6.4 Relationship between Standing Time and Etch Rate of Thermal SiO2 Standing Time (h)

Chemical Composition (%) HF NH4F

0 24 48 72

0.13 0.17 0.22 0.26

Etch Rate of Thermal SiO2 at 238C (nm/min)

39.69 39.97 40.30 40.52

2.07 2.88 3.35 3.94

Note: Chemical Composition: HF: 0.13%, NH4F: 39.69%; etching temperature: 238C; humidity: 53%.

In the case of 400:1 BHF, very low etch rate is achieved by decreasing HF concentration and increasing NH4F concentration, HF concentration rises over time whereas NH4F concentration drops. Initially etch rate of this etchant on thermal SiO2 film was 2.07 nm/min. Forty-eight hours later, the etch rate increased by about 1.6 times. 400:1 BHF which features very low etch rate was selected to precisely control etch rate. In reality, however, its etch rate showed a significant change within a short period of time, and it is not possible to realize etching process with high reproducibility. This change in 400:1 BHF composition is considered to be attributed to NH4F decomposition to HF and NH3 gas. NH4 F ! HF þ NH3 "

(6:1)

It has been found extremely difficult to control BHF composition when wet process is performed at high temperature or when NH4F concentration in BHF is high. The authors also identified parameters to control chemical composition of etchant.

III. PARAMETERS TO CHANGE OF ETCHANT COMPOSITION Three parameters are considered to change etchant composition. Figure 6.3 indicates the parameters that affect the entire etching process. 1. Evaporation amount depended on humidity and temperature (ambience condition) 2. Consumption of chemical species through etching reaction 3. Carry-in of chemicals used at just before step and carry-over of etchant Several experiments were conducted on these about the parameters.

A. EVAPORATION AMOUNT DEPENDED ON HUMIDITY AND TEMPERATURE (AMBIENCE CONDITION): RELATIONSHIP BETWEEN HUMIDITY AND AMOUNT OF EVAPORATION The etchant is usually used at a wet station or at a clean bench. Humidity is controlled at around 40%. It is common that a local ventilation system is installed in order not to release chemical vapor into the clean-room ambience. When etching process is performed at high temperature, the local ventilation system needs to feature a high pumping capability of 30 –40 m3/min. Figure 6.4 shows the evaporation rate as a function of NH4F concentration at 69% humidity. Evaporation


276

humidity Down flow and Horizontal air seal

Evaporation Chemical carry over

Chemical carry in

Chemical decomposition Chemical concentrated Change of etch rate Chemical consumption

FIGURE 6.3 Perfect-controlled wet-cleaning process parameters.

rate is found to be dependent on NH4F concentration. Figure 6.5 shows the relationship between the amount of evaporation and humidity. As shown in Figure 6.4 and Figure 6.5, the higher the NH4F concentration and humidity are, the lesser is the evaporation amount.

B. EFFECTS

OF

CHEMICAL SPECIES CONSUMPTION

THROUGH

ETCHING

As BHF and HF etch SiO2 film, their composition change due to chemical species consumed by the etching reaction. Etching reaction of BHF and SiO2 is commonly expressed as: SiO2 þ 4HF þ 2NH4 F ! (NH4 )2 SiF6 þ 2H2 O

65

Evaporation Rate (g/[hr • m2])

Rh : 69% 55 45 35 25 15 5 -5 0

5

10

15

20

25

30

NH4F concentration (%)

FIGURE 6.4 Relationship between NH4F concentration and evaporation.

35

40

(6:2)


277

FIGURE 6.5 Relationship between standing time and environmental humidity.

Consumption of chemical species through etching reaction can be calculated as follows: HF consumption:

2:68 108 DE (mol)=wafer

NH4 F consumption:


H2 O generation:


where wafer size is equal to 6 in., D is the amount of etching (nm); E is the rate of etched oxide surface area to entire wafer surface area (%). Figure 6.6 shows the relationship between chemical composition and the number of treated wafers. In this experiment, 6-in. wafer with 80% of its surface covered with the photoresist was etched by 100

Chemical Composition (%)

HF (%) NH4F (%)

10

1 10

100

1000

The number of treated wafer

FIGURE 6.6 Influence of consumption with etching species.

10000


278

100 nm. This figure indicates that very little HF and NH4F are consumed during this etching reaction. Even after etching several thousands of wafers, chemical composition was kept stable enough not to affect etching result. This means etchant evaporation, rather than consumption of chemical species during etching, is a dominant factor to affect chemical composition change of etchant.

C. CARRY-IN

OF

CHEMICALS USED AT JUST BEFORE STEP AND CARRY-OVER

OF

ETCHANT

In wet process, particularly in etching process where BHF is used, wafer surface is mostly covered with the photoresist. BHF is used in wet etching process or in the combined process of dry etching and wet etching. A photoresist surface, just like Si surface, is water repellent. The conventional BHF cannot penetrate into minute contact hole, which results in etching failure [4 – 7]. Traditionally, in order to overcome this problem, Si wafer was preliminary treated with a surfactant-added ultrapure water (UPW) to get its surface covered with the surfactant-added UPW before it was etched with BHF. In this traditional procedure, however, the pretreatment solution was carried over together with wafer and wafer cassette to the etching bath, eventually to dilute BHF composition. As a result, etch rate was lowered. One of the solutions to this problem is to simultaneously perform the pretreatment and the wet etching. In other words, the surfactant is spiked not to UPW but to BHF to combine the two process steps together. With this procedure, no carry-over of pretreatment solution takes place, and accordingly etch rate does not drop because of BHF dilution does not happen (Figure 6.7).

IV. BHF COMPOSITION CONTROL As a result of studies to identify the parameters essential for changing the etchant composition, the authors have identified the following: 1. Etchant evaporation can be suppressed by increasing humidity. For BHF, the higher the NH4F concentration is, the lesser the evaporation. NH4F decomposition is, however, accelerated when its concentration is high. 2. Consumption of chemical species during etching reaction has little impact on the change in etchant composition. 3. The traditional two-stage etching process has a problem in that BHF is diluted due to carry-in of pretreatment solution. This problem is overcome by eliminating the pretreatment and using the surfactant-added BHF.

FIGURE 6.7 Chemical carry-over.


279

Increasing amount of HF (g/m2)

70 : : : :

60 50

HF=0.1% HF=0.2% HF=0.3% HF=1.0%

40 30 20 10 0 0

5

10

15

20

25

30

35

40

NH4F concentration (%)

FIGURE 6.8 Relationship between NH4F concentration and HF increasing amount.

It was very hard to completely stop etchant evaporation and change of etchant composition. Instead of finding a way to eliminate change of etchant composition, the authors decided to explore a means to minimize composition change by controlling relevant parameters. The authors performed a 70-h evaporation test, using BHFs with varied NH4F concentration. HF concentration in the BHFs was varied to 0.1, 0.2, 0.3, and 1.0%. In the evaporation test, containers with evaporation surface area of 78.5 cm2 were filled with the different BHFs of 500 g, and left in a cleanroom with humidity kept at 50%. Figure 6.8 shows rise of absolute HF amount (per unit area) as a function of initial NH4F concentration in BHF. These experimental results indicate that keeping the initial HF concentration the same, the rise of absolute HF amount because of evaporation is more suppressed when the initial NH4F concentration is lower. It is also revealed that rise of absolute HF amount as a result of evaporation gets to a certain level as NH4F concentration is lower, which suggests that only H2O is evaporated from BHF. Table 6.5 shows the etchant constituents that evaporated during the 70-hr evaporation test. In this experiment, containers with an evaporation surface area of 24 cm2 were filled with 80 –100 g BHFs with composition varied. Temperature of BHFs was controlled at 258C. After the containers were left in cleanroom with 40% humidity, absolute amount of BHF components was measured. According to

TABLE 6.5 Time Dependence of Evaporated Chemical Composition Chemicals HF–NH4F (%) DIW 0.1–20 0.1–40 2.5–20 2.5–38

Weight Variation (%) 27.2 24.7 20.2 24.0 20.3

Chemical Composition HF(g) NH4F(g) — 0.11 ! 0.11 0.08 ! 0.15 2.84 ! 2.84 2.58 ! 2.58

— 22.1 ! 22.1 40.7 ! 40.4 22.4 ! 22.4 42.4 ! 42.3

Note: Standing time: 70 h; humidity: 40%; sample weight: 80– 100 g evaporation area: 24 cm2. a

NH4F ! HF þ NH3".

Evaporated Chemical Composition H2O H2O H2O þ NHa3 H2O H2O


280

Table 6.5, when NH4F concentration is 40% and HF concentration is 0.1%, rise of absolute HF amount as a result of NH4F decomposition is detected as well as H2O evaporation. This experiment demonstrates that along with increase of NH4F concentration and decrease of HF concentration, HF generation due to NH4F decomposition is more facilitated. By reducing NH4F concentration in BHF, it is possible to limit the evaporating component only to H2O though total evaporation amount will increase. As it is only H2O that gets evaporated, it is possible to recover initial BHF composition merely by adding UPW of equivalent amount. For example, initial BHF composition can be maintained by monitoring liquid level of etching bath and by adding UPW as necessary to maintain the liquid level constant. This enables etching process to be stable and to keep etch rate constant. Low NH4F concentration also improves the overall etching performance because it helps to increase solubility of reaction products to BHF [4 – 7]. Succeedingly, based on the above-mentioned findings, the present authors studied the logical formula to control etchant composition. Table 6.6 shows an example.

TABLE 6.6 Wet Process Parameters (BHF) Parameters Initial HF concentration Initial NH4F concentration Initial chemical weight Etching bath area SiO2 exposure ratio Etching depth Wafer batch (1 batch/25 pieces) Chemical carry over weight Rate of evaporation Standing time

a (%) b (%) W (g) S (m2) E (%) D (nm) B (batch) C (g/batch) V (g/min.m2) T (min)

Chemical composition after T times HF concentration (%):

(W a) (1:34 103 B D E) (a B C) (%) W þ (1:01 105 B D E) (S T V) (B C)

NH4F concentration (%):

(W b) (1:24 103 B D E) (b B C) (%) W þ (1:01 105 B D E) (S T V) (B C)

Example: HF concentration: 0.7% NH4F concentration: 17% Initial chemical weight: 20 kg Bath area: 0.1 m2 SiO2 area: 10% Etching depth: 500 nm Etching batch: 100 batch (2500 pieces 6-in. wafer) Standing time: 1440 min Environmental humidity: 40% Rate of evaporation: V ¼ 7.0146 0.290 e(20.0389T) (g/min m2) HF conc. after 24 h: 0.68% NH4F conc. after 24 h: 17.45% Chemical weight after 24 h: 17.98 kg Supply chemical composition: HF: 0.87%, NH4F: 12.99%


281

The following are found important to control etchant composition. 1. Limit evaporating chemical species to only H2O: Select etchant composition so that does not allow decomposition of chemical species. Decrease NH4F concentration in BHF. 2. Simplify the wet process: Keep the etchant composition stable by eliminating carry-over of pretreatment solution to the etching bath, wafers and wafer cassettes. Use the surfactantadded etchant to eliminate pretreatment. 3. Improve process tools: Set up a supplying system of replenishment solution which adjusts its composition in accordance with the recovering (changed) etchant composition to its initial composition besides etchant delivery system. This composition-recovery solution should be composed only of the same chemical components and equivalent amount of each components as that evaporated and consumed chemical components. As this system is, introduced the amount of chemical wastes and frequency of etchant replacement decreases. 4. By introducing controlled ambience, etchant, process, and process tools, it will become possible to control etchant composition and realize a wet process with accuracy and reproducibility.

V. CONTROL OF HF22H2O2 COMPOSITION Recently a new cleaning technology proposed by Ohmi et al. of Tohoku University has been drawing attention as it is to replace the RCA cleaning. The new technology is based on a HF22H2O222surfactant solution and megasonic irradiation [8,9]. Table 6.7 and Figure 6.9 show the impact of evaporation on this chemical composition. The evaporation test was performed in cleanroom with humidity of 40%. Containers with evaporation surface area of 24 cm2 were filled with 258C etchants of 90– 100 g, and left in the cleanroom for 70 h. As shown in Table 6.7, only H2O evaporated from the etchants even after they were left for 70 h. Etch rate is found to increase because of evaporation. It is found, however, possible to recover the initial etch rate and keep etch rate constant merely by adding UPW of equivalent amount.

VI. CONCLUSION The present authors studied parameters to change etchant composition and ways to control them. Although maintaining initial composition of etchant constantly is extremely important, it is not

TABLE 6.7 Time Dependence of Evaporated Chemical Composition Chemicals HF2 2H2O2 (%) 0.1–0 0.1–1 0.1–3 0.5–0 0.5–1 0.5–3

Weight Variation (%) 26.24 26.48 27.33 27.70 27.16 27.17

Chemical Composition HF(g) H2O2(g) 0.11 ! 0.11 0.11 ! 0.11 0.12 ! 0.12 0.55 ! 0.55 0.55 ! 0.54 0.55 ! 0.54

1.21 ! 1.20 3.20 ! 3.23 1.17 ! 1.12 3.10 ! 3.13

Note: Standing time: 70 h; humidity: 40%; sample weight: 90– 110 g; evaporation area: 24 cm2.

Evaporated Chemical Composition H2O H2O H2O H2O H2O H2O

282


3.4

Etch Rate (nm/min)

3.2 3.0 ? 2.8 2.6 2.4

?

0.5%HF-3%H2O2

?

delivery H2O

2.2 0

10

20

30

40

50

60

70

Standing time (hrs.) Etching Temp : 25°C Humidity : 40 %

FIGURE 6.9 Relationship between standing time and etch rate.

sufficient in order to meet the ever-demanding requirements for reliability, stability, and yield in future ULSI device manufacturing process. Along with introduction of larger diameter wafer, etching bath will become larger in size. It is also necessary to give consideration to the evaporation of chemicals adsorbed on large-sized wafer during its transportation process. These potential problems need to be identified and addressed, or it will become necessary to explore the possibility of replacing batch-type etching process with single-wafer etching process. These are key issues to determine whether or not to use the wet process in the future ULSI manufacturing process.

REFERENCES 1. T. Ohmi, Ultraclean technology reform further manufacturing technology of highly efficient semiconductor and LCD, in: Proceedings of the 23rd Symposium on ULSI Ultra Clean Technology, part I, Tokyo, pp. 26–31, 1994. 2. T. Ohmi, Electronic chemistry of semiconductor surface. in: 27th Workshop on ULSI Ultra Clean Technology Advanced Wet Chemical Proceeding V, Tokyo, pp. 3 – 27, 1995. 3. N. Miki, T. Fukudome, M. Maeno, and T. Ohmi, Advanced Recovery Technology of HF, BHF and H3PO4, in: Proceedings of the 23rd Symposium on ULSI Ultra Clean Technology, part II, Tokyo, pp. 264– 274, 1994. 4. H. Kikuyama, N. Miki, K. Saka, J. Takano, I. Kawanabe, M. Miyashita, and T. Ohmi, Surface active buffered hydrogen fluoride having excellent wettability for ULSI processing, IEEE, Trans. Semicond. Manuf. 3, 99– 108, 1990. 5. H. Kikuyama, N. Miki, K. Saka, J. Takano, I. Kawanabe, M. Miyashita, and T. Ohmi, Principles of wet chemical processing in ULSI microfabrication, IEEE Trans. Semicond. Manuf. 4, 26 – 35, 1991. 6. H. Kikuyama, I. Kawanabe, K. Saka, N. Miki, M. Miyashita, J. Takano, and T. Ohmi, Buffered hydrogen fluoride, Fluor. Chem. Ultra Clean Technol Series, 11, 31 – 61, 1991.


283

7. H. Kikuyama, J. Takano, M. Miyashita, T. Yabune, H. Izumi, and T. Ohmi, Optimization of composition of buffered hydrogen fluoride for ULSI processing, in: 1995 Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, 237 – 259, 1995. 8. K. Kubo, S. Ojima, H. Hiroto, M. Nose, and T. Ohmi, Advanced Wet Treatment System, in: Proceedings of the ULSI Ultraclean Technology Symposium, 24, pp. 83 –91, 1995. 9. T. Kujime, Y. Nishiyama, and T. Ohmi. The cleaning of particles from Si wafer surface by fluorine solution excited by megasonic, in: 1996 Semiconductor Pure and Chemicals Conference, UPW and Chemical Proceedings, Santa Clara, CA, pp. 245 – 256, 1996.

7

Wet Vapor Resist Stripping Technology Senri Ojima Nomura Micro Science Co., Ltd., Kanagawa, Japan

Tadahiro Ohmi Tohoku University, Sendai, Japan

CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 II. Wet Vapor Resist Stripping Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 III. Resist Removal Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

I. INTRODUCTION At present, wet-cleaning using ultrapure water (UPW) and chemicals is a mainstream technology to clean substrate surfaces of ultra large-scale integration (ULSI) devices. RCA cleaning developed by Kern and Puotinen of Radio Corporation of America in 1970 is the most widely used wet-cleaning technique [1,2]. The RCA cleaning is composed of NH4OH/H2O2 cleaning and HCl/H2O2 cleaning. Organic films, such as photoresist films, are hard to remove by the RCA cleaning method. O2 plasma ashing is commonly used to remove them. The resist is burnt and ashed by oxygen radical in O2 plasma. After such steps like photoresist coating, exposure, development, etching, and ion implantation, the residual resist waste needs to be removed. A mixture of 98% H2SO4 and 30% H2O2 in the ratio of 4:1 (SPM, i.e., sulfuric acid and hydrogen peroxide mixing) is used to remove residual resist in chemical cleaning. The SPM cleaning is performed for 3 – 15 min at 95– 1208C [3]. It is also being studied to get residual resist dissolved into ozonated UPW to be removed [4]. Removing the residual resist to which ion is implanted is difficult. The ashing rate tends to decline as the resist with ion gets hardened. A combination of O2 plasma ashing and SPM cleaning is often employed to tackle this problem. The RCA cleaning is also used as a post-SPM cleaning step. Resist with high-concentration ion implanted is particularly hard to remove. Inert polymer layer with low hydrogen concentration is formed on its top surface [5], and the resist film is strongly hardened. Usually O2 plasma ashing is employed to remove this kind of resist, but it always leaves some residue on the substrate surface. Even SPM cleaning cannot totally remove the residue [6]. If metal atoms remain in resist film, they are pushed into Si wafer due to ion collision during O2 plasma ashing. This may induce crystalline defects and affect device characteristics [7]. This chapter will present a technology to use water vapor to remove resist film with highconcentration ion implanted.

II. WET VAPOR RESIST STRIPPING TECHNOLOGY Figure 7.1 shows optical microscopic images of Si wafer surface after resist with high-concentration ion implanted was removed by UPW vapor jet. Even merely by UPW vapor jet, the resist 285

286

Scientific Wet Process Technology for Innovative LSI/FPD Manufacturing Resist

5 min

10 min

FIGURE 7.1 Wet vapor cleaning using the ultrapure water of implanted resist.

hardened due to ion implantation is partially removed. In this experiment, a test sample was ˚ in thickness was grown. This wafer was prepared by using Si wafer on which SiO2 of 3000 A coated with positive-type resist and baked before phosphorus of 6 1015 atoms/cm2 was implanted. UPW vapor is blasted onto this test sample with a-1.5 kW vapor generator which is capable of blasting a vapor of 80 –1008C continuously. Figure 7.2 shows a conceptual diagram of this wetvapor cleaning mechanism. Gap between Si wafer and vapor nozzle was set at 10 mm. Vapor pressure of UPW jet was 0.1 –0.2 MPa. Figure 7.3 shows optical microscopic images of Si wafer surface treated with wet vapor cleaning using alkali solution. Specifically, KOH of 0.08 wt% was injected to UPW. The resist was completely removed in as short a time as 2 min. Figure 7.4 shows optical microscopic images of Si wafer surface treated with 0.3 wt% UPW to which nonionic surfactant was injected. The cleaning solution featured pH 6. Initially the resist was slightly removed in some areas on the surface and more resist removed as time went by. Resistremoval efficiency of this cleaning solution was found much lower than that of the alkali solution.

Wet vapor supply equipment

0.1~0.2MPa

Steam

75~85°C

10 mm

Resist SiO2 Si

FIGURE 7.2 Wet vapor supply schematic drawings.

Wet Vapor Resist Stripping Technology

287

Resist

1 min

2 min

FIGURE 7.3 Wet vapor cleaning using the alkali aqueous of implanted resist.

Figure 7.5 shows pH dependence of resist-removal efficiency. The sample wafer was coated with i-line positive-type resist to which arsenic of 5 1015 atoms/cm2 was implanted. This figure indicates that rise of OH2 concentration accelerates the process of dissolution and removal of the resist. When KOH concentration was raised to 10 wt% (pH 13.8), the resist of 10 10 mm could be completely removed within 10 sec.

III. RESIST REMOVAL MECHANISM The SPM cleaning removes the resist as strong acid and oxidant in the SPM solution dissolve it. What is the mechanism for wet vapor cleaning to remove the resist? UPW vapor jet can remove resist though it takes some time. Resist-removal efficiency was found to decline when surfactant was added to UPW. This is considered because the surfactant gets adsorbed onto the resist surface and blocks resist removal. This observation suggests that H2O molecule should play a significant

Resist

1 min

3 min

FIGURE 7.4 Wet vapor cleaning using the nonionic detergent of implanted resist.


288

FIGURE 7.5 Relationship between removal hour and KOH concentration of implanted positive resist (As: 5 1015 atoms/cm2).

role in resist removal. It is also true that gas and liquid coexist in the wet vapor cleaning. Because of the the gas –liquid coexistence, it is speculated that water cluster gets smaller. Authors have considered that the resist gets swollen by H2O molecule which passes through it, and accordingly the resist is removed by vapor flow. In an attempt to prove this speculation, permeation of H2O and several gases through Teflon were studied first. Table 7.1 shows the results obtained. As shown in Table 7.1 a large amount of H2O molecule permeates through Teflon. Since the resist is also made from organic resin, it is speculated that H2O molecule should easily permeate through the resist as well. As described in Chapter 10, oxygen permeation through PFA pipe was also studied. UPW with dissolved oxygen of 5 ppb (mg/l) or Ar gas was fed into PFA pipe. This PFA pipe was placed in an ambience with O2 gas flow at 0.5 MPa. Under these conditions, oxygen permeation through the wall of the PFA pipe was measured. When UPW was fed into the PFA pipe, oxygen of 50 ppb (mg/l) was found dissolved in UPW. When Ar gas was fed into the pipe, no oxygen was detected. Giving considerations to detect the limit of gas concentration measurement, the authors have concluded that oxygen permeation through the PFA pipe is several thousand times higher when UPW was fed into the pipe.

TABLE 7.1 Teflon Permeability of Any Gas (cm3 mm/m2 day atm)

O2 N2 CO2 H2O

PTFE [8]

FEP [9]

147 50 1469 2333 [9]

389 141 112 3283

Wet Vapor Resist Stripping Technology

289

This experimental result indicates that water contributes to increased oxygen permeation into PFA resin, which suggests that gas permeation into an organic film should also be facilitated in an ambience where water exists. The authors studied a model in which the PFA resin was replaced by a resist resin. When water comes in contact with the surface of the resist resin, penetration of H2O molecule in vapor into the resist resin is facilitated. The resist resin gets swollen as H2O molecule permeates through it, and the swollen resist is easily removed by vapor-jet flow. In other words, water featuring high relative dielectric constant weakens the covalent bond between resist carbon atoms (or reduces binding energy of resist carbon atoms), which makes it easier for H2O molecule cluster to get smaller because of the vapor that permeates through resist resin. After penetrating into the resist, gaseous vapor stops at Si –resist interface and turns into liquid, and the resist swells. Because of the relative dielectric constant of water (81), the binding energy of carbon atoms gets weakened, and eventually, the resist is lifted up from the Si substrate by vapor-jet force. As described in detail in Chapter 1, binding energy of a molecule is inversely proportional to square of the dielectric constant. When carbon molecules are dissolved into water, their binding energy drops due to high relative dielectric constant of water (1r ¼ 81). Covalent bonds such as H2, N2, O2, F2, Cl2, and Br2 are composed of identical atoms with the same electronegativity, and thereby they do not allow electrons to be localized somewhere away from the center to feature any polarity. External leakage of electron current lines is very limited. These covalent bonds are hardly affected by the surrounding medium. It is true that the electron with negative charge attracts polarized terminal of molecules with positive charge (via Coulomb force), but none of the molecules mentioned above get decomposed or dissociated even when they are dissolved in water, because they are nonpolar. However, ionization energy of H2 molecule in gaseous ambience is about 15.4 eV, whereas in UPW it is about 4 eV. In solvents featuring high relative dielectric constant, even covalent-bond molecules feature lower binding energy and ionization energy. Figure 7.6 shows time dependence of thickness of the oxide layer grown on Si surface exposed to the air at room temperature. In the air, Si surface is oxidized and accordingly, thickness of the oxide increases over time. When H2O concentration in the air is controlled to be low (H2O , 0.1 ppm), Si surface is scarcely oxidized even after a 7-day exposure [10]. 1 In Air(H2O:~1.2%) n (10E15cm−3)

Oxide Thickness [nm]

0.8

In O2:N2=1.4(H2O:

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