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`University of Wisconsin-Madison
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`U.S.A. A
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`Fundamentals of
`MICROFABRICATION
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`Fundamentals of
`MICROFABRICATION ‘
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`Marc Madou
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`mattress: fl
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`2'13 L
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`Development Editor: Marleen Madou
`Publisher: Ron Powers
`Project Editor: Paul Gottehrer
`Prepress: Gary Bennett, Kevin Luong, Carlos Esser, Walt Cerny, Greg Cuciak
`Cover design: Denise Craig
`
`Library of Congress Cataloging-iu—Publication Data
`
`Madou, Marc J.
`Fundamentals of microfabrication / Marc Madou.
`p.
`cm.
`Includes bibliographical references and index.
`_ ISBN O-8493»9451—1 (alk. paper)
`2. Machining.
`1. Microelectronics—Design and construction.
`3. Microelectronic packaging.
`4. Lasers—Industrial applications.
`I. Title.
`1997
`TK7836.M33
`621.3815’2—-dc20
`
`96-43344
`CIP
`
`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.
`Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying,
`microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher.
`The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission
`must be obtained in writing from CRC Press LLC for such copying.
`Direct all inquiries to CRC Press LLC, 2000 Corporate Blvd., NW, Boca Raton, Florida 33431.
`
`© 1997 by CRC Press LLC
`
`No claim to original US. Government works
`International Standard Book Number 0-8493—9451-1
`Library of Congress Card Number 96-43344
`Printed in the United States of America 1 2 3 4 5 6 7 8 9 0
`Printed on acid-free paper
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`WWI?
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`“2;;
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`Dedication 1‘
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`A LL6- 0 i ”I l
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`For my wife, Marleen Madou, thanks for urging me to write a second book and for all your love and support. For my children,
`Ramses and Maura, sorry for all the time I could have been with you but worked on the book instead. For my late parents to whom
`I wish I had been able to show this second book.
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`Preface
`
`Fundamentals of Microfabrication explores the science of miniaturization. Miniaturization methods and materials surveyed include
`micromachining in single crystal and polycrystalline Si and other micromachining methods and materials based on lithography as
`well as more traditional non—lithography miniaturization options and materials. In dealing with micromachining techniques borrowed
`from the electronics industry, we emphasize the processes that differ the most from the standard processes encountered in regular
`IC and hybrid manufacturing.
`New developments in lithography largely determine which direction the IC industry and microfabrication will take in the coming
`years. Therefore, Chapter 1 appropriately introduces the book by discussing different lithography techniques and tries to define the
`optimum lithography for the future which may differ for micromachining and for IC technology; Whereas finer linewidths and
`standarized materials are the main quest in the IC industry, microfabrication seeks high features and high aspect ratios and the
`introduction of new materials. Resist patterns created by lithography on a substrate can be transferred to the substrate by subtractive
`(etching) or by additive (deposition) techniques. Dry etching, discussed in Chapter 2, is an important subtractive pattern transfer
`method in IC fabrication. Recent progress in deep directional etching as well as environmental concerns helped push dry etching to
`the foreground in micromachining applications as well. Chapter 3 covers all types of additive pattern transfer techniques. A limited
`description of thin film deposition technologies and doping methods sufficed as these techniques are, in most cases, the same for
`both [C and microfabrication. Thick film deposition technologies proved important in the manufacture of all types of new chemical
`and biological sensors. More effort is dedicated to this subject as many newly pioneered thick film materials and processes are foreign
`to IC production. Chapter 4 is dedicated to wet bulk micromachining, a key process in sensor fabrication but less common in IC
`manufacturing. Surface micromachining, a method involving thin film additive techniques as well as wet and dry etching, is covered
`in Chapter 5. The rapid commercial acceptance of surface micromachining is explained in terms of its compatibility with existing
`IC equipment and processes. LIGA, the newest microfabrication tool based on deep etch x—ray lithography, electrodeposition, and
`molding gets extensive coverage in Chapter 6.
`Chapter 7 compares traditional and non-traditional microfabrication tools. The arsenal of microfabrication tools has increased
`dramatically over the last 20 years and different applications require different fabrication means. Several micromachining methods,
`especially the more traditional precision engineering grounded methods, bear little relevance on today’s IC Industry but they are
`viable microfabrication tools. It is one of the objectives of the book to broaden the perspective of the reader on all of the different
`options available to manufacture small things. Applying microfabrication tools correctly to the problem at hand might generate a
`lot more commercial successes. Chapter 7 also speculates on the future of micromachining. We believe in the merging of microma—
`chining and nanomachining. Micromachining often provides the tools (e.g. the scanning tunneling microscope) to enable nanoma-
`chining. In nanomachining we distinguish between nanofabrication: heir to micromachining, using the same subtractive and additive
`processes to build devices in the sub 0.1 pm regime, and molecular engineering where one mimics nature’s way to build nanoma-
`chinery. Nature has a different way of ‘building’ things than micromachinists. The elemental building blocks of everything we know
`are atoms and molecules. In molecular engineering one attempts to build functional structures by building up from the atomic or
`molecular level in the hope of constructing the same diversity in shapes, functions and memory size offered by nature. In comparison,
`the micromachining world represents a crude construction site with Si wafers as building blocks as thick as 500 pm, insulating layers
`of up to a micron thick, Al and Au metal layers between a few tens to thousands of angstroms, and in general a very limited choice
`of other materials. In micromachining we are building down towards smaller and smaller structures whereas in molecular engineering
`we are building with a plethora of different atoms and molecules towards bigger and bigger molecular entities. The synergy between
`nanofabrication, molecular engineering and microfabrication may prove the most fruitful research domain for decades to come.
`Once all currently available microfabrication tools have been explored, we turn our attention to new device development and
`packaging in Chapter 8. Given that no standard design rules yet permeate micromachining and because of the dilemma of partitioning
`sensing and electronics functions correctly, early attention to design and packaging of micromachines is even more important than
`in the IC industry. We cannot stress enough the importance of starting the micromachine design from a good understanding of the
`application and from the application specific package and real world interface and only then applying the preferred micromachine
`inside the package. Merging of IC design software with micromechanical design code is helping a slow but certain introduction of
`micromachining with even the most conservative manufacturing firms. We also will see how micromachining itself provides many
`excellent solutions for future packaging strategies.
`After fabrication, design and packaging technologies are exposed, Chapter 9 explores the importance of miniaturization in general
`with an emphasis on the most difficult to miniaturize components, i.e., actuators and power sources. A good understanding of scaling
`laws will help the reader develop ‘micro-intuition’ and assist him/her in making mature decisions about the optimum micromachining
`approach and design. We will see the emergence of the most exciting opportunities in micromachining in those areas where the
`macro continuum models break down in the microdomain.
`
`In Chapter 10 on ‘Microfabrication Applications’ we present a list of current and potential applications and discuss market
`opportunities.
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`Because of the rapidly changing nature of the micromachining field a homepage (http://Www.crcpress.com/microfab) dedicated
`to ‘Fundamentals of Micromachining’ was set up to transform this book into a living, hyperlinked document with frequent updates,
`questions and input from readers all around the world Finally, a few Appendices give useful information for aspiring micromachinists-
`ranging from a list of metrology methods (Appendix A), further reading on the internet (Appendix B), detailed Si and SiO2 KOH
`etching data (Appendix C), important further reading (Appendix D) and a glossary (Appendix E).
`The goal of this book15 to familiarize the reader with the micromachinists tools, directions and jargon in order to facilitate a
`confident choice of fabrication method for a particular miniaturization problem.
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`Acknowledgments
`
`I would like to thank Mr. Iohn Hines, manager of‘ Sensors 2000! at NASA Ames, for his support and for sharing my vision on
`micromachines in biomedical applications. From the Center for Advanced Microdevices (CAMD) in Baton Rouge, I acknowledge
`Dr. Volker Saile, Director of CAMD, for giving me the opportunity to participate in LIGA development work. My thanks also go to
`the Miller Institute for Basic Research in Science for the Visiting Miller Professorship awarded for my stay at the Berkeley Sensors
`and Actuators Center (BSAC) at UC Berkeley; my gratitude to Professor Richard White for many stimulating discussions and his
`open attitude to cross disciplinary work.
`My sincere thanks go to all of the colleagues who helped review different chapters of the book. In alphabetical order they are: Mr.
`Rashid Bashir (National Semiconductor), Dr. Barry Block (consultant), Dr. Luc Bousse (Caliper), Mr. Jim Bustillo (UCB), Mr. Sean
`Cahill (MicroScape), Mr. Michael Cohn (UCB), Dr. Ben Costello (Berkeley Microinstruments), Mr. Peter Hillen (Congruity), Dr.
`Keith Iackson (LBNL), Mr. Jack Iudy (UCB), Mr. Chris Keller (UCB), Professor Chantal Khan Malek (CAMD/LSU), Dr. Kim Kinoshita
`(LBNL), Dr. Peter Krulevitch (LLNL), Dr. Adolfo Lopez-Otero (Stanford University), Professor Roy Morrison (Simon Fraser Uni-
`versity), Professor Richard Muller (UCB), Professor Michael Murphy (LSU), Dr. Armand Neukermans (Adagio), Dr. Seajin Oh (SRI
`International), Dr. Tony Ricco (Sandia), Dr Angel Sanjurjo (SRI International), Mr. Tim Slater (consultant), Dr. Michael Thierny
`(Cygnus), Dr. Volker Saile (CAMD), Dr. Stuart Wenzel (Berkeley Microinstruments), Professor Richard White (UCB)
`Dr. Adolfo Lopez-Otero (Stanford University) assisted with reference research, the internet, Appendix A and the index. My son
`Ramses helped with some of the artwork. Finally, I would like to thank my wife, Marleen for spending countless hours converting
`my Flemish—English into‘her Flemish—English.
`L
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`Table of Contents
`
`,
`Chapter 1
`JLithography
`Introduction‘ ...................................................................................................... '. ........................................................................................ 1
`Historical Note: Lithography’s Origins .................................................................................................................................................... 1
`Photolitho graphy . . ..
`........................................................................................................................................................ 2
`
`Introduction...
`..... 2
`
`Masks .................................................. 2
`
`Spinning Resist and Soft Baking .................... 2
`Exposure and Post—Exposure Treatment.......... 3
`
`..... 4
`Development...
`
`De-Scumming andPost—Baldng.......................................................................................................................................................... 5
`Glass Transition Temperature of a Resist (Tg) .................................................................................................................................. 5
`Resist Tone ............................................................................................................................................................................................ 5
`
`Wafer Cleaning and Contaminants ..................................................................................................................................................... 9
`Critical Dimension, Overall Resolution,Line—Width 11
`Lithographic Sensitivity and Intrinsic Resist Sensitivity (Photochemical Quantum Efficiency) ................................................. 11
`Radiation and Resist Profile .............................................................................................................................................................. 13
`
`Contrast and Experimental Determination of Lithographic Sensitivity........................................................................................ 15
`Photolithography Resolution ........................................................................................................................................... .................. 16
`Mathematical Expression for ResistProfiles................... 24
`25
`Improving Resist Sensitivity-Chemical Amplification.
`
`26
`Image Reversal.............
`
`27
`Multilayer Resist ........................
`28
`Thin Film Interference Effects .........
`
`29
`New Types of Resists ........................
`Resist Stripping ................................................................................................................................................................................... 33
`Expanding the Limits of Photolithography Through Improved Mask Technology...................................................................... 34
`Alternative Lithographies ........................................................................................................................................................................ 37
`Introduction ........................................................................................................................................................................................ 37
`
`X—Ray Lithography ............................................................................................................................................................................. 37
`Charged Particle Beam Lithography ................................................................................................................................................. 41
`Emerging Lithography Technologies ...................................................................................................................................................... 45
`Proximal Probe Lithography ............................................................................................................................................................. 45
`
`Holographic Lithography............................................................................................................................................... 47
`Stereolithography/Micro-Photoforming Process ............................................................................................................ 47
`Lithography on Nonplanar Substrates ................
`48
`References ................................................................................................................................................................................................. 50
`
`
`
`Chapter 2
`\/Pattern Transfer with Dry Etching Techniques
`Introduction ............................................................................................................................................................................................. 53
`
`Dry Etching: Definitions and Jargon...................................................................................................................................................... 53
`Plasmas or Discharges .....................................................................................................
`54
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`Physics of DC Plasmas...
`55
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`Physics of RF Plasmas .....................................................................
`59
`Physical Etching: Ion Etching or Sputtering and Ion—Beam Milling...
`62
`
`Introduction .........................................................._...........................
`62
`
`
`Sputtering or Ion—Etching.
`63
`63
`Ion—Beam Etching or Ion-Beam Milling"
`
`Etching Profiles1n Physical Etching..........
`.......................................................... 64
`
`Physical Etching Summary .................
`.......................................................... 66
`Dry Chemical Etching ............................................................................................................................................................................. 66
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`Introduction ..................
`..... 66
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`Reactor Configurations
`..... 67
`Reaction Mechanism
`................... 67
`
`[on Energy vs. Pressure Relationship in a Plasma.......... 69
`
`Gas Phase Etching without Plasma (Xer) ............................................................................................... 70
`Plasma Jet Etching .............................................................................................................................................................................. 70
`Physical-Chemical Etching ...................................................................................................................................................................... 71
`Introduction ........................................................................................................................................................................................ 71
`
`Energy-Driven Anisotropy ................................................................................................................................................................. 71
`Reactive Ion-Beam Etching and Chemically Assisted Ion-Beam Etching Compared ................................................................... 72
`Inhibitor-Driven Anisotropy ............................................................................................................................................................. 73
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`Gas Composition
`......................................................................... 74
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`Simplifying Rules........... 75
`New Plasma Sources............................................ 77
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`Dry Etching Models — In Situ Monitoring.
`..... 77
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`Comparing Wet and Dry Etching............................ 78
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`Dry Etching Micromachining Examples ........................................................... 79
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`Example 1. Silicon Single Crystal Reactive Etching and Metallization ........................................................ 79
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`Example 2. GaAs—SCREAM ............................................................................................................................. 82
`Example 3. Dry Etching of Polymeric Materials .............................................................................................................................. 83
`Example 4. Combination Wet and Dry Etching .............................................................................................................................. 84
`Example 5. Bulk Dry Etching Applications In IC Technology ....................................................................................................... 84
`References ................................................................................................................................................................................................. 87
`
`\/Chapter 3
`Pattern Transfer with Additive Techniques
`Introduction ............................................................................................................................................................................................. 89
`Growth ...................................................................................................................................................................................................... 91
`Silicon Growth .................................................. 91
`
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`Oxidation of Silicon ............................................ 92
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`Physical Vapor Deposition
`..... 96
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`Introduction ..................................................................................... 96
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`Thermal Evaporation ...................................................................... 96
`Sputtering................................................................................................................................................................. 99
`
`.. 102
`Molecule Beam Epitaxy......................................................................................................................................
`Laser Sputtering Deposition or Ablation Deposition .................................................................................................................... 103
`Ion Plating......................................................................................................................................................................................... 104
`Cluster-Beam Technology ................................................................................................................................................................ 104
`Chemical Vapor Deposition .................................................................................................................................................................. 105
`Introduction ..............'. ........................................................................................................................i............................................... 1 05
`Reaction Mechanisms ....................................................................................................................................................................... 105
`Energy Sources for the CVD Processes...
`................................................................... 109
`
`................................................................... 109
`Overview of CVD Process Types...
`
`Eptiaxy .................................................................. 1 13
`................ 113
`Introduction...................
`
`Liquid and Solid Phase Epitaxy.
`.. 114
`Selective Epitaxy
`........................................................................................................
`.. 114
`
`Epi-Layer Thickness .............................................................................................................
`.. 115
`Recent Trends in Vacuum Equipment ................................................................................................................................................. 115
`Electrochemical Deposition .................................................................................................................................................................. 115
`Introduction ..................... 115
`Electroless Metal Deposition ........................................................................................................................................................... 115
`Electrodeposition — Electroplating and Anodization ..
`I
`.................................................. 119
`
`Anodization.......................................................................................... 125
`
`
`..... 126
`Silk-Screening or Screen Printing
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`Introduction .......................................................................................................................... 126
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`How itWorks ..........................................‘. ......................................................... 126
`
`127
`Types of Inks....
`
`128
`Comparison of Thin— vs. Thick-Film Deposition...
`
`Deposition Methods of Organic Layers .......................................................................................................................................... 128
`Planarization .......................................................................................................................................................................................... l 3 1
`Introduction ...................................................................................................................................................................................... 13 1
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`Planarization Strategies .................................................................................................................................................................... 132
`Plasma Spraying ........_............................................................................................................................................................................. 133
`Selection Criteria for Deposition Method ........................................................................................................................................... 134
`Doping .................................................................................................................................................................................................... 135
`Two Main Doping Techniques ........................................................................................................................................................ 135
`
`Incorporation by Diffusion .......
`136
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`Ion Implantation ..........................................................
`138
`Examples of Pattern Transfer with Additive Techniques.
`139
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`Example 1. Spray Pyrolysis ..........................................
`139
`
`.. 140
`Example 2. Nickel Micromechanical Structures ..
`
`References ............................................................................................................................................................................................... 14 1
`
`Chapter 4
`Wet Bulk Micromachining
`Introduction ........................................................................................................................................................................................... 145
`Historical Note ....................................................................................................................................................................................... 145
`
`SiliconCrystallography ............ 148
`Miller Indices .................................................................................................................................................................................... 148
`
`Crystal Structure of Silicon ............................................................................................................................................................. 148
`Geometric Relationships Between Some Important Planes in the Silicon Lattice...
`149
`
`...... 154
`Silicon as a Substance and Structural Material ..
`Silicon as Substrate..........................................................
`154
`
`Silicon as a Structural Element in Mechanical Sensors.
`155
`
`
`163
`Wet Isotropic and Anisotropic Etching...................................
`163
`Wet Isotropic and Anisotropic: Empirical Observations .
`
`177
`Chemical Etching Models .......................................................
`Etching with Bias and/or Illumination of the Semiconductor .......................................................................................................... 187
`Electropolishing and Microporous Silicon ..................................................................................................................................... 189
`Etch-Stop Techniques ............................................................................................................................................................................ 193 ‘
`Introduction ...................................................................................................................................................................................... 193
`
`194
`Boron Etch Stop
`Electrochemical Etch-Stop Technique ............................................................................................................................................ 195
`
`Photo-Assisted Electrochemical Etch Stop (for n—Type Silicon) ......
`197
`
`Photo-Induced Preferential Anodization, PiPA (for p-Type Silicon)...
`198
`
`Etch Stop at Thin Films—Silicon on Insulator ....................................
`198
`
`Problems with Wet Bulk Micromachining ............
`199
`
`Introduction ..............................................
`199
`
`Extensive Real Estate Consumption ......................................................................................................................................... 199
`Corner Compensation ........................................................................................................................................................... 200
`Wet Bulk Micromachining Examples ................................................................................................................................................... 205
`Example 1. Dissolved Wafer Process ............................................................................................................................................... 205
`Example 2. An Electrochemical Sensor Array Measuring pH, C02, and O2 in a Dual Lumen Catheter .................................. 206
`References ............................................................................................................................................................................................... 209
`
`
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`Chapter 5
`Surface Micromachining
`Introduction ........................................................................................................................................................................................... 217
`Historical Note ....................................................................................................................................................................................... 219
`
`Mechanical Properties of Thin Films ................................................................................................................................................... 219
`Introduction...................................................................................................................................................................................... 219
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`Adhesion ........................................................................................................................................................................................... 220
`Stress in Thin Films
`..... 229
`
`
`Strength of Thin Films...................... 230
`
`Surface Micromachining Processes ..
`..... 231
`
`Basic Process Sequence ..................... 231
`
`Fabrication Step Details ........
`........................................................ 232
`
`Control of Film Stres