SEMICONDUCTOR FABRICATION
`
`
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`TSMC 1025
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`US. Patent No. 7,265,450
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`TSMC 1025
`U.S. Patent No. 7,265,450
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`

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`Emu—22:2
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`«mg—325:3:
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`Fundamentals of
`Semiconductor Fabrication
`
`

`

`

`

`Fundamentals of
`Semiconductor Fabrication
`
`GARY S. MAY
`Motorola Foundation Professor
`School of Electrical and Computer Engineering
`Georgia Institute of Technology
`Atlanta, Georgia
`
`SIMON M. SZE
`UMC Chair Professor
`National Chiao Tung University
`National Nano Device Laboratories
`Hsinchu, Taiwan
`
`ffi WILEY
`
`JOHN WILEY & SONS, INC.
`
`

`

`Senior Acquisitions Editor Bill Zob!ist
`Production Editor Sandra Dumas
`Senior Marketing Manager Kathe!ine Hepburn
`Senior Designer Kevin Murphy
`Production Management Services Argosy
`
`Photo Credit: Nicole Capello/Georgia Institute of Technology
`Cover Desc!iption: An eight-inch silicon wafer containing Intel Pentium processors.
`
`This book was typeset in 10/12 New Caledonia (NC) by Argosy and printed and bound by
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`
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`sustained yield harvesting of its timberlands. Sustained yield harvesting principles ensure that
`the number of trees cut each year does not exceed the amount of new growth.
`
`This book is printed on acid-free paper. §
`
`Copyright© 2004 by John Wiley & Sons, Inc. All rights reserved.
`No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by
`any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted
`under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permis(cid:173)
`sion of the Publisher or authorization through payment of the appropriate per-copy fee to the Copyright
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`to the Publisher for pemlission should be addressed to the Permissions Department, John Wiley & Sons,
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`
`May, Gary, S., Simon M. Sze
`Fundamentals of Semiconductor Fabrication
`
`ISBN 0-471-23279-3
`Wiley International Edition ISBN 0-471-45238-6
`Printed in the United States of America
`
`10987654321
`
`

`

`To LeShelle and Therese,
`
`who enable, uplift, and sustain us.
`
`

`

`

`

`Preface
`
`This book provides an introduction to semiconductor fabrication technology, from crys(cid:173)
`tal growth to integrated devices and circuits. It covers theoretical and practical aspects
`of all major steps in the fabrication sequence. It is intended as a textbook for senior under(cid:173)
`graduates or fust-year graduate students in physics, chemistry, electrical engineering, chem(cid:173)
`ical engineering, and materials science. The book can be used conveniently in a
`semester-length course on integrated circuit fabrication. Such a course may or 1pay not
`be accompanied by a corequisite laboratory. The text can also serve as a reference. for
`practicing engineers and scientists in the semiconductor industry.
`Chapter I gives a brief historical overview of major semiconductor devices and key
`technology developments, as well as an introduction to basic fabrication steps. Chapter
`2 deals with crystal growth techniques. The next several chapters are organized accord(cid:173)
`ing to a typical fabrication sequence. Chapter 3 presents silicon oxidation. Photolithography
`and etching are discussed in Chapters 4 and 5, respectively. Chapters 6 and 7 present
`the primary techniques for the introduction of dopants: diffusion and ion implantation.
`The final chapter on individual process steps, Chapter 8, covers various methods of tl1in
`fUm deposition. The ftnal three chapters focus on broad, summative topics. Chapter 9
`ties the individual process steps together by presenting the process flows for Ciitical pro(cid:173)
`cess technologies, integrated devices, and microelectrical mechanical systems (MEMS).
`Chapter 10 introduces high-level integrated circuit manufacturing issues, including elec(cid:173)
`trical testing, packaging, process control, and yield. Finally, Chapter 11 discusses tl1e future
`outlook and challenges for the semiconductor industry.
`Each chapter begins with an introduction and a list of learning goals and concludes
`with a summary of important concepts. Solved example problems are provided through(cid:173)
`out, and suggested homework problems appear at the end of the chapter. The concept
`of process simulation is presented in several chapters, using the popular SUPREM and
`PRO LITH software packages as application vehicles. Mastery of this software is intended
`to supplement, but not replace, learning the fundamental concepts associated witl! micro(cid:173)
`electronics processing.
`A complete set of detailed solutions to all end-of-chapter problems has been pre(cid:173)
`pared. This instructor's manual is available to all adopting faculty. The figures in tl!e text
`are also available, in electronic format, from tl!e publisher at the following website:
`http://www.wiley.com/college/may.
`
`vii
`
`

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`

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`Acknowledgments
`
`The authors wish to thank Drs. T. C. Chang, T. S. Chao, M. C. Chiang, F. H. Ko, M. C.
`Liaw, and S.C. Wu of the National Nano Device Laboratories, and Prof. T. L. Li of the
`National Chia-Yi University for their helpful suggestions and discussions. We are further
`indebted to Mr. N. Erdos for technical editing of the manuscript, Ms. Iris Lin for typ(cid:173)
`ing the many revisions of the draft, and Ms. Y. G. Yang of the Semiconductor Laboratory,
`National Chiao Tung University who furnished the hundreds of technical illustrations used
`in the book.
`At John Wiley and Sons, we wish to thank Mr. W. Zobrist, who encouraged us to under(cid:173)
`take the project. S. M. Sze wishes also to acknowledge the Spring Foundation of the
`National Chiao Tung University for its financial support, and the United Microelectronic
`Corporation for the UMC Chair Professorship grant that provided the environment to
`work on this book. G. S. May would like to similarly acknowledge the Motorola Foundation
`Professorship.
`
`ix
`
`

`

`

`

`Contents
`
`CHAPTER 1
`Introduction 1
`1.1 Semiconductor Materials 2
`1.2 Semiconductor Devices 2
`1.3 Semiconductor Process Technology 5
`1.3.1 Key Semiconductor
`Technologies 5
`1.3.2 Technology Trends 8
`1.4 Basic Fabrication Steps 11
`1.4.1 Oxidation 11
`1.4.2 Photolithography and
`Etching 13
`1.4.3 Diffusion and Ion
`Implantation 14
`1.4.4 Metallization 14
`1.5 Summary 14
`References 15
`
`CHAPTER 2
`Crystal Growth 17
`2.1 Silicon Crystal Growth from the
`Melt 18
`2.1.1 Starting Material 18
`2.1.2 The Czochralski Technique 18
`2.1.3
`r>istribution of Dopant 19
`2.1.4 Effective Segregation
`Coefficient 22
`2.2 Silicon Float-Zone Process 24
`2.3 GaAs Crystal Growth Techniques 26
`2.3.1 Starting Materials 26
`2.3.2 Crystal Growth Techniques 30
`2.4 Material Characterization 31
`2.4.1 Wafer Shaping 31
`2.4.2 Crystal Characterization 33
`2.5 Summary 38
`References 39
`Problems 39
`
`CHAPTER 3
`Silicon Oxidation 41
`
`3.1 Thermal Oxidation Process 42
`3.1.1 Kinetics of Growth 42
`3.1.2 Thin Oxide Growth 49
`
`3.2
`
`Impurity Redistribution During
`Oxidation 50
`3.3 Masking Properties of Silicon
`Dioxide 51
`3.4 Oxide Quality 53
`3.5 Oxide Thickness Characterization 54
`3.6 Oxidation Simulation 54
`3.7 Summary 57
`References 58
`Problems 59
`
`CHAPTER 4
`Photolithography 60
`4.1 Optical Lithography 60
`4.1.1 The Clean Room 60
`4.1.2 Exposure Tools 62
`4.1.3 Masks 65
`4.1.4 Photoresist 67
`4.1.5 Pattern Transfer 70
`4.1.6 Resolution Enhancement
`Techniques 72
`4.2 Next-Generation Lithographic
`Methods 73
`4.2.1 Electron Beam Lithography 73
`4.2.2 Extreme Ultraviolet
`Lithography 76
`4.2.3 X-Ray Lithography 78
`4.2.4
`Ion Beam Lithography 79
`4.2.5 Comparison of Various
`Lithographic Methods 80
`4.3 Photolithography Simulation 81
`4.4 Summary 83
`References 83
`Problems 84
`
`CHAPTER 5
`Etching 85
`5.1 Wet Chemical Etching 85
`5.1.1 Silicon Etching 86
`5.1.2 Silicon Dioxide Etching 87
`5.1.3 Silicon Nitride and Polysilicon
`Etching 88
`5.1.4 Aluminum Etching 88
`5.1.5 Gallium Arsenide Etching 88
`
`xi
`
`

`

`xii I> Contents
`
`5.2 Dry Etching 89
`5.2.1 Plasma Fundamentals 90
`5.2.2 Etch Mechanism, Plasma
`Diagnostics, and End-Point
`Control 91
`5.2.3 Reactive Plasma Etching
`Techniques and
`Equipment 93
`5.2.4 Reactive Plasma Etching
`Applications 97
`5.3 Etch Simulation 101
`5.4 Summary 102
`References 103
`Problems 103
`
`CHAPTER 6
`Diffusion 105
`6.1 Basic Diffusion Process 106.
`6.1.1 Diffusion Equation 107
`6.1.2 Diffusion Profiles 109
`6.1.3 Evaluation of Diffused
`Layers 113
`6.2 Extrinsic Diffusion 114
`6.2.1 Concentration-Dependent
`Diffusivity 115
`6.2.2 Diffusion Profiles 117
`6.3 Lateral Diffusion 118
`6.4 Diffusion Simulation 120
`6.5 Summary 121
`References 122
`Problems 122
`
`7.2
`
`CHAPTER 7
`lon Implantation 124
`7.1 Range of Implanted Ions 125
`Ion Distribution 125
`7.1.1
`Ion Stopping 127
`7.1.2
`Ion Channeling 130
`7.1.3
`Implant Damage and Annealing 131
`Implant Damage 131
`7.2.1
`7.2.2 Annealing 134
`Implantation-Related Processes 136
`7.3.1 Multiple Implantation and
`Masking 136
`7.3.2 Tilt-Angle Ion Implantation 138
`7.3.3 High-Energy and High-Current
`Implantation 139
`Ion Implantation Simulation 140
`7.4
`7.5 Summary 141
`References 142
`Problems 142
`
`7.3
`
`CHAPTER 8
`Film Deposition 144
`8.1 Epitaxial Growth Techniques 144
`8.1.1 Chemical Vapor Deposition 145
`8.1.2 Molecular Beam Epitaxy 148
`8.2 Structures and Defects in Epitaxial
`Layers 152
`8.2.1 Lattice-Matched and Strained(cid:173)
`Layer Epitaxy 152
`8.2.2 Defects in Epitaxial Layers 153
`8.3 Dielectric Deposition 155
`8.3.1 Silicon Dioxide 156
`8.3.2 Silicon Nitride 160
`8.3.3 Low-Dielectric-Constant
`Materials 162
`8.3.4 High-Dielectric-Constant
`Materials 164
`8.4 Polysilicon Deposition 165
`8.5 Metallization 167
`·
`8.5.1 Physical Vapor Deposition 167
`8.5.2 Chemical Vapor Deposition 168
`8.5.3 Aluminum Metallization 169
`8.5.4 Copper Metallization 173
`8.5.5 Silicide 175
`8.6 Deposition Simulation 177
`8.7 Summary 177
`References 179
`Problems 180
`
`CHAPTER 9
`Process Integration 182
`9.1 Passive Components 184
`9.1.1 The Integrated Circuit
`Resistor 184
`9.1.2 The Integrated Circuit
`Capacitor 185
`9.1.3 The Integrated Circuit
`Inductor 187
`9.2 Bipolar Technology 188
`9.2.1 The Basic Fabrication
`Process 189
`9.2.2 Dielectric Isolation 192
`9.2.3 Self-Aligned Double-Polysilicon
`Bipolar Structures 193
`9.3 MOSFEt Technology 196 ·
`9.3.1 The Basic Fabrication
`Process 196
`9.3.2 Memory Devices 199
`9.3.3 CMOS Technology 203
`9.3.4 BiCMOS Technology 210
`9.4 MESFET Technology 212
`
`

`

`Contents <I xiii
`
`I> APPENDIX A
`List of Symbols 265
`
`I> APPENDIX B
`International System of Units lSI Units) 267
`
`I> APPENDIX C
`Unit Prefixes 269
`
`I> APPENDIX D
`Greek Alphabet 271
`
`I> APPENDIX E
`Physical Constants 273
`
`I> APPENDIX F
`Properties of Si and GaAs at 300 K 275
`
`I> APPENDIX G
`Some Properties of the Error Function 277
`
`I> APPENDIX H
`Basic Kinetic Theory of Gases 281
`
`I> APPENDIX I
`SUPREM Commands 283
`
`I> APPENDIX .J
`Running PROLITH 287
`
`I> APPENDIX K
`Percentage Points of the t Distribution 289
`
`I> APPENDIX L
`Percentage Points of the FDistribution 291
`
`Index 297
`
`9.5 MEMS Technology 212
`9.5.1 Bulk Micromachining 215
`9.5.2 Surface Micromachining 215
`9.5.3 LIGA Process 215
`9.6 Process Simulation 218
`9.7 Summary 223
`References 223
`Problems 224
`
`CHAPTER 10
`IC Manufacturing 226
`
`10.1 Electrical Testing 227
`10.1.1 Test Structures 227
`10.1.2 Final Test 228
`10.2 Packaging 228
`10.2:1 Die Separation 230
`10.2.2 Package Types 230
`10.2.3 Attachment
`Methodologies 232
`10.3 Statistical Process Control 237
`10.3.1 Control Charts_ for
`Attributes 237
`10.3.2 Control Charts for Variables 239
`.Statistical Experimental Design 242
`10.4.1 Comparing Distributions 242
`10.4.2 Analysis of Variance 243
`10.4.3 Factorial Designs 246
`10.5 Yield 250
`10.5.1 Functional Yield 250
`10.5.2 Parametric Yield 254
`10.6 Computer-Integrated
`Manufacturing 256
`10.7 Summary 257
`References 257
`Problems 258
`
`10.4
`
`CHAPTER 11
`Future Trends and Challenges 259
`11.1 Challenges for Integration 259
`11.1.1 Ultrashallow Junction
`Formation 261
`11.1.2 Ultrathin Oxide 261
`11.1.3 Silicide Formation 261
`11.1.4 New Materials for
`Interconnection 261
`11.1.5 Power Limitations 261
`11.1.6 SOl Integration 262
`11.2 System-on-a-Chip 262
`11.3 Summary 264
`References 264
`Problems 264
`
`

`

`

`

`1
`Introduction
`
`Semiconductor devices are the foundation of the electronics industry, which is the largest
`industry in the world, with global sales of over one trillion dollars since 1998. Figure 1.1
`shows the sales volume of the semiconductor device-based electronics industry in the
`past 20 years and projects sales to the year 2010. Also shown are the gross world prod(cid:173)
`2
`uct ( GWP) and the sales volumes of the automobile, steel, and semiconductor industries. 1
`•
`Note that the electronics industry surpassed the automobile industry in 1998. If current
`trends continue, the sales volume of the electronics industry will reach three trillion dol(cid:173)
`lars and will constitute about 10% of GWP by 2010. The semiconductor industry, a sub(cid:173)
`set of the electronics industry, will grow at an even higher rate to surpass the steel industry
`in the early twenty-first century and to constitute 25% of the electronics industry in 2010.
`The multitrillion dollar electronics industry is fundamentally dependent on the man(cid:173)
`ufacture of semiconductor integrated circuits (ICs). The solid-state computing, telecom(cid:173)
`munications, aerospace, automotive, and consumer electronics industries all rely heavily
`on these devices. A basic lmowledge of semiconductor materials, devices, and processes
`is thus essential to the understanding of modem electronics. Although this text deals pri(cid:173)
`marilywith the basic processes involved in IC fabrication, a brief historical review of each
`of these three topics is warranted.
`
`lOS
`
`104
`
`;g
`0
`.:.::l
`-:::l cq
`.,
`~ loJ
`"'
`~
`-a
`..c _g
`0
`
`102
`
`GWP
`
`Semiconductors
`
`l0 1 L---------L---------~---------L--------~--------~
`1980
`1990
`2000
`2010
`
`Year
`Figure 1.1 Gross world product (GWP) and sales volumes of the electronics, automobile,
`semiconductor, and steel industries from 1980 to 2000 and projected to 2010I·2
`
`1
`
`

`

`2 II- Chapter 1. Introduction
`
`I> 1.1 SEMICONDUCTOR MATERIALS
`Germanium was one of the first materials used in semiconductor device fabrication. In
`fact, the first transistor, developed by Bardeen, Brattain, and Shockley in 1947, was made
`of this element. However, germanium was rapidly replaced by silicon in the early 1960s.
`Silicon became the dominant material because it has several advantages. First, silicon
`can be easily oxidized to form a high-quality silicon dioxide (Si02) insulator, and Si02 is
`an excellent barrier layer for the selective diffusion steps needed in IC fabrication. Silicon
`also has a wider bandgap than germanium, which means that silicon devices can oper(cid:173)
`ate at a higher temperature than their germanium counterparts. Finally, and perhaps most
`important, as a primary constituent in ordinary sand, silicon is a very inexpensive and abun(cid:173)
`dant element in nature. Thus, in addition to its processing advantages, silicon provides
`a very low-cost source material.
`The next most popular material for IC fabrication is gallium arsenide (GaAs). Although
`GaAs has a higher electron mobility than silicon, it also possesses severe processing lim(cid:173)
`itations, including less stability during thermal processing, a poor native oxide, high cost,
`and much higher defect densities. Silicon is therefore the material of choice in ICs and
`is emphasized more thoroughly in this text. GaAs is used for circuits that operate at very
`high speeds (in excess of 1 GHz) but with low to moderate levels of integration.
`
`I> 1.2 SEMICONDUCTOR DEVICES
`The unique properties of semiconductor materials have enabled the development of a
`wide variety of ingenious devices that have literally changed our world. These devices
`have been studied for over 125 years. 3 To date, there are about 60 major devices, with
`over 100 device variations related to them. 4 Some major semiconductor devices are listed
`in Table 1.1 in chronological order.
`The earliest systematic study of semiconductor devices (metal-semiconductor con(cid:173)
`tacts) is generally attributed to Braun,5 who in 1874 discovered that the resistance of
`contacts between metals and metal sulfides (e.g., copper pyrite) depended on the mag(cid:173)
`nitude and polarity of the applied voltage. The electroluminescence phenomenon (for
`the light-emitting diode) was discovered by Round6 in 1907. He observed the genera(cid:173)
`tion of yellowish light from a crystal of carborundom when he applied a potential of 10
`V between two points on the crystals.
`In 1947, the point-contact transistor was invented by Bardeen and Brattain? This
`was followed by Shockley's8 classic paper on p-n junctions and bipolar transistors in 1949.
`Figure 1.2 shows the first transistor. The two point contacts at the bottom of the trian(cid:173)
`gular quartz crystal were made from two stripes of gold foil separated by about 50 Jl.m
`(1Jl.m = 10-4 em) and pressed onto a semiconductor surface. The material used was ger(cid:173)
`manium. With one gold contact forward biased (i.e., positive voltage with respect to the
`third terminal) and the other reverse biased, transistor action was observed; that is, the
`input signal was amplified. The bipolar transistor is a key semiconductor device and has
`ushered in the modem electronic era.
`In 1952; Ebers9 developed the basic model for the thyristor, which is an extremely
`versatile switching device. The solar cell was developed by Chapin et al. 10 in 1954 using
`a silicon p-n junction. The solar cell is a major candidate for obtaining energy from the
`sun because it can convert sunlight directly to electricity-and is environmentally benign.
`In 1957, Kroemer11 proposed the heterojunction bipolar transistor to improve transistor
`performance. This device is potentially one of the fastest semiconductor devices. In 1958,
`Esaki 12 observed negative resistance characteristics in a heavily doped p-n junction, which
`led to the discovery of the tunnel diode. The tunnel diode and its associated tunneling
`phenomenon are important for ohmic contacts arid carrier transport through thin layers.
`
`

`

`1.2 Semiconductor Devices -<1 3
`
`TABLE 1.1 Major Semiconductor Devices
`
`Year
`
`Semiconductor Device
`
`Author(s)!Inventor(s)
`
`1874
`1907
`1947
`1949
`1952
`1954
`1957
`1958
`1960
`1962
`1963
`1963
`1965
`1966
`1967
`1970
`1974
`1980
`1994
`
`2001
`
`Metal-semiconductor contact"
`Light emitting diode•
`Bipolar transistor
`p-n junction•
`Thyristor
`Solar cell"
`Heterojunction bipolar transistor
`Tunnel diode'
`MOSFET
`Laser'
`Heterostructure laser'
`Transferred-electron diode"
`IMPATI diode•
`MESFET
`Nonvolatile semiconductor memory
`Charge-coupled device
`Resonant tunneling diode•
`MODFET
`Room-tempyrature single-electron
`memory cell
`15-nm MOSFET
`
`Braun
`Round
`Bardeen and Brattain; Shockley
`Shockley
`Ebers
`Chapin, Fuller, and Pearson
`Kroemer
`Esaki
`Kahng and Atalla
`Hallet al.
`Kroemer; Alferov and Kazarinov
`Gunn
`Johnston, DeLoach, and Cohen
`Mead
`Kahng and Sze
`Boyle and Smith
`Chang, Esaki, and Tsu
`Mimura et al.
`Yano eta!.
`
`Yu eta!.
`
`Ref.
`
`5
`6
`7,8
`8
`9
`10
`11
`12
`13
`15
`16, 17
`18
`19
`20
`21
`23
`24
`25
`22
`
`14
`
`MOSFET, metal-oxide-semiconductor field-effect transistor; MESFET, metal-semiconductor field-effect
`transistor; MODFET, modulation-doped field-effect transistor.
`'Denotes a two-terminal device; otherwise, it is a three- or four-terminal device.
`
`Figure 1.2 The first transistor. 7 (Photograph courtesy of Bell Laboratories.)
`
`3
`
`

`

`4 ~ Chapter 1. Introduction
`
`The most important device for advanced integrated circuits is the MOSFET (metal(cid:173)
`oxide-semiconductor field-effect transistor), which was reported by Kahng and Atalla 13
`in 1960. Figure 1.3 shows the first device using a thermally oxidized silicon substrate. The
`device has a gate length of20 ~m and a gate oxide thickness of 100 nm (1 nm = 10-7 em).
`The two keyholes are the source and drain contacts, and the top elongated area is the
`aluminum gate evaporated through a metal mask. Although present-day MOSFETs have
`been scaled down to the deep-submicron regime, the choice of silicon and thermally grown
`silicon dioxide used in the first MOSFET remains the most important combination of
`materials. The MOSFET and related integrated circuits now constitute about 90% of the
`semiconductor device market. An ultrasmall MOSFET with a channel length of 15 nm
`has been demonstrated recently. 14 This device can serve as the basis for the most advanced
`integrated circuit chips containing over one trillion (>10 12
`) devices.
`In 1962, Hallet al. 15 first achieved lasing in semiconductors. In 1963, Kroemer16 and
`Alferov and Kazarinov17 proposed tl1e heterostructure laser. These proposals laid the foun(cid:173)
`dation for modem laser diodes, which can be operated continuously at room tempera(cid:173)
`ture. Laser diodes are the key components for a wide range of applications, including
`digital video disks, optical-fiber communication, laser printing, and atmospheric pollu(cid:173)
`tion monitoring.
`
`Figure 1.3 The first metal-oxide-semiconductor field-effect transistorY (Photograph courtesy of
`Bell Laboratories.)
`
`

`

`1.3 Semiconductor Process Technology -<1 5
`
`Three important microwave devices were invented or realized in the next three years.
`The first device was the transferred-elecb-on diode (TED; also called Gunn diode), invented
`by Gunn 18 in 1963. The TED is used extensively in such millimeter-wave applications as
`detection systems, remote controls, and microwave test instruments. The second device
`is the IMPATT diode; its operation was first obse1ved by Johnston et al. 19 in 1965. IMPATT
`diodes can generate the highest continuous-wave ( CW) power at millimeter-wave fre(cid:173)
`quencies of all semiconductor devices. They are used in radar systems and alarm systems.
`The third device is the MESFET (metal-semiconductor field-effecttransitor), invented
`by Mead20 in 1966. It is a key device for monolithic microwave integrated circuits (MMIC).
`An important semiconductor memory device was invented by Kahng and Sze21 in
`1967. This is the nonvolatile semiconductor memory (NVSM), which can retain its stored
`information when the power supply is switched off. Although it is similar to a conven(cid:173)
`tional MOSFET, the major difference is the addition of a "floating gate" in which semiper(cid:173)
`manent charge storage is possible. Because of its attributes of nonvolatility, high device
`density, low power consumption, and electrical rewritability (i.e., the stored charge can
`be removed by applying voltage to the control gate), the NVSM has become the domi(cid:173)
`nant memory for portable electronic systems such as the cellular phone, notebook com(cid:173)
`puter, digital camera, and smart card.
`A limiting case of the floating-gate nonvolatile memory is the single-electron mem(cid:173)
`ory cell (SEMC), which is obtained by reducing the length of the floating gate to ultra(cid:173)
`small dimensions (e.g., 10 nm). At this dimension, when an electron moves into the floating
`gate, the potential of the gate will be altered so that it will prevent the entrance of another
`electron. The SEMC is the ultimate floating-gate memory cell, since we need only one
`electron for information storage. The operation of a SEMC at room temperature was first
`demonstrated by Yano et al. 22 in 1994. The SEMC can serve as the basis for the most
`advanced semiconductor memories, which can contain over one trillion bits.
`The charge-coupled device (CCD) was invented by Boyle and Smith23 in 1970. CCD
`is used extensively in video cameras and in optical sensing applications. The resonant tun(cid:173)
`neling diode (RTD) was first studied by Chang et al. 24 in 1974. RTD is the basis for most
`quantum-effect devices, which offer extremely high density, ultrahigh speed, and enhanced
`functionality, because it permits a greatly reduced number of devices to perform a given
`circuit function. In 1980, Mimura et al. 25 developed the MODFET (modulation-doped field(cid:173)
`effect transistor). Witl1 the proper selection ofheterojunction materials, the MODFET is
`the fastest field-effect transistor.
`Since the invention of the bipolar transistor in 1947, the number and variety of semi(cid:173)
`conductor devices have increased tremendously as advanced technology, new materials,
`and broadened comprehension have been applied to the creation of new devices. However,
`one compelling question remains: What processes are required to construct these won(cid:173)
`drous devices from basic semiconductor materials?
`
`I> 1.3 SEMICONDUCTOR PROCESS TECHNOLOGY
`1.3.1 Key Semiconductor Technologies
`
`Many important semiconductor technologies have been derived from processes invented
`centuries ago. For example, the growth of metallic crystals in a furnace was pioneered by
`Africans living on the western shores of Lake Victoria more than 2000 years ago. 26 This
`process was used to produce carbon steel in preheated forced-draft furnaces. Another
`example is the lithography process, which was invented in 1798. In this original process,
`the pattern, or image, was transferred from a stone plate (litho). 27 This section considers
`
`

`

`6 Ill- Chapter 1. Introduction
`
`the milestones of technologies that were applied for the first time to semiconductor pro(cid:173)
`cessing or developed specifically for semiconductor device fabrication.
`Some key semiconductor technologies are listed in Table 1.2 in chronological order.
`In 1918, Czochralski28 developed a liquid-solid monocomponent growth technique.
`Czochralski growth is the process used to grow most of the crystals from which silicon
`wafers are produced. Another growth technique was developed by Bridgman29 in 1925.
`The Bridgman technique has been used extensively for the growth of gallium arsenide
`and related compound semiconductor crystals. Although the semiconductor properties
`of silicon have been widely studied since early 1940, the study of semiconductor com(cid:173)
`pounds was neglected for a long time. In 1952, Welkef0 noted that gallium arsenide and
`related III-V compounds were semiconductors. He was able to predict their character(cid:173)
`istics and to prove them experimentally. Technology and devices using these compounds
`have since been actively studied.
`The diffusion of impurity atoms in semiconductors is important for device process(cid:173)
`ing. Basic diffusion theory was considered by Fick31 in 1855. The idea of using diffusion
`techniques to alter the type of conductivity in silicon was disclosed in a patent in 1952
`by Pfann.32 In 1957, the ancient lithography process was applied to semiconductor device
`fabrication by Andrus.33 He used photosensitive, etch-resistant polymers (photoresist) for
`pattern transfer. Lithography is a key teclmology for the semiconductor industry. The con(cid:173)
`tinued growth of the industry has been the direct result of improved lithographic tech(cid:173)
`nology. Lithography is also a significant economic factor, currently representing over 35%
`of IC manufacturing costs.
`
`TABLE 1.2 Key Semiconductor Technologies
`
`Year
`
`1918
`1925
`1952
`1952
`1957
`1957
`1957
`1958
`1959
`1959
`1960
`1963
`1967
`1969
`1969
`1971
`1971
`1971
`1982
`1989
`1993
`
`Technology
`
`Author(s)!Inventor(s)
`
`Ref.
`
`Czochralski crystal growth
`Bridgman crystal g~owth
`III-V compounds
`Diffusion
`Lithographic photoresist
`Oxide masking
`Epitaxial CVD growth
`Ion implantation
`Hybrid integrated circuit
`Monolithic integrated circuit
`Planar process
`CMOS
`DRAM
`Polysilicon self-aligned gate
`MOCVD
`Dry etching
`Molecular beam epitaxy
`Microprocessor (4004)
`Trench isolation
`Chemical mechanical polishing
`Copper interconnect
`
`Czochralski
`Bridgman
`Welker
`rfann
`Andrus
`Frosch and Derrick
`Sheftal, Kokorish, and Krasilov
`Shockley
`Kilby
`Noyce
`Hoemi
`Wanlass and Sah
`Dennard
`Kerwin, Klein, and Sarace
`Manasevit and Simpson
`Irving, Lemons, and Bobos
`Cho
`Hoff et al.
`Rung, Momose, and Nagakubo
`Davari et al.
`Paraszczak et al.
`
`28
`29
`30
`32
`33
`34
`35
`36
`37
`38
`39
`40
`41
`42
`43
`44
`45
`46
`47
`48
`49
`
`CVD, chemical vapor deposition; CMOS, complementary metal-oxide-semiconductor field-effect transistor;
`DRAM, dynamic random access memory; MOCVD, metalorganic CVD.
`
`

`

`1.3 Semiconductor Process Technology ~ 7
`
`The oxide masking method was developed by Frosch and Derrick34 in 1957. They
`found that an oxide layer can prevent most impurity atoms from diffusing through it. In
`the same year, the epitaxial growth process based on the chemical vapor deposition tech(cid:173)
`nique was developed by Sheftal et al. 35 Epitaxy, derived from the Greek words epi, mean(cid:173)
`ing "on," and taxis, meaning "arrangement," describes a technique of crystal growth to
`form a thin layer of semiconductor materials on the surface of a crystal that has a lattice
`structure identical to that of the crystal. This method is important for the improvement
`of device performance and the creation of novel device structures. In 1959, a rudimen(cid:173)
`tary integrated circuit was made by Kilby. 37 It contained one bipolar transistor, three resis(cid:173)
`tors, and one capacitor, all made in germanium and connected by wire bonding-a hybrid
`circuit. Also in 1959, Noyce38 proposed the monolithic IC by fabricating all devices in a
`single semiconductor substrate (monolith means "single stone") and connecting the devices
`by aluminum metallization. Figure 1.4 shows the first monolithic IC of a flip-flop circuit
`containing six devices. The aluminum interconnection lines were obtained by etching evap(cid:173)
`orated aluminum layer over the entire oxide surface using the lithographic technique. These
`inventions laid the foundation for the rapid growth of the microelectronics industry.
`The planar process was developed by Hoerni39iilr96e. In this process, an oxide layer
`is formed on a semiconductor surface. With the help of a lithography process, portions
`of the oxide can be removed and windows cut in the oxide. Impurity atoms will diffuse
`only through the exposed semiconductor surface, and p-n junctions will form in the oxide
`window areas.
`
`Figure 1.4 The first monolithic integ~ated circuit.37 (Photograph courtesy of Dr. G. Moore.)
`
`

`

`8 ~ Chapter 1. Introduction
`
`As the complexity of the IC increased, technology has moved from NMOS (n-channel
`MOSFET) to CMOS (complementary MOSFET), which employs both NMOS and PMOS
`(p-channel MOSFET) to form the logic elements. The CMOS concept was proposed by
`Wanlass and Sah40 in 1963. The advantage of CMOS technology is that logic elements draw
`significant current only during the transition from one state to another (e.g., from 0 to 1)
`and draw very little current between transitions, allowing power consumption to be min(cid:173)
`imized. CMOS technology is the dominant technology for advanced ICs.
`In 1967, an important two-element circuit, the dynamic random access memory
`(DRAM), was invented by Dennard.41 The memory cell contains one MOSFET and one
`charge-storage capacitor. The MOSFET serves as a switch to charge or discharge the capac(cid:173)
`itor. Although a DRAM is volatile and consumes relatively high power, we expect that
`DRAMs will continue to be the first choice among various semiconductor memories for
`nonportable electronic systems in the foreseeable future.
`To improve device performance, the polysilicon self-aligned gate process was pro(cid:173)
`posed by Kerwin et al. 42 in 1969. This process not only improved device reliability but
`also reduced parasitic capacitances. Also in 1969, the metalorganic chemical vapor depo(cid:173)
`sition (MOCVD) method was developed by Manasevit and Simpson.43 This is a very impor(cid:173)
`tant epitaxial growth technique for compound semiconductors such as GaAs.
`As the device dimensions were reduced, the dry etchin