Principles of Lithography, Second Edition

435 Pages·2005·9.639 MB·English

by  Levinson, Harry J

Principles of
Lithography
Second Edition
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Principles of
Lithography
Second Edition
Harry J. Levinson
Bellingham, Washington USA
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Library of Congress Cataloging-in-Publication Data

Levinson, Harry J.
Principles of lithography / Harry J. Levinson.—2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 0-8194-5660-8
1. Integrated circuits—Design and construction. 2. Microlithography. I. Title.

TK7874.L397 2004
621.3815'31—dc22 2004025977

Published by

SPIE—The International Society for Optical Engineering
P.O. Box 10
Bellingham, Washington 98227-0010 USA
Phone: [phone]
Fax: [phone]
Email: [email protected]
Web: http://spie.org

Copyright © 2005 The Society of Photo-Optical Instrumentation Engineers

All rights reserved. No part of this publication may be reproduced or distributed
in any form or by any means without written permission of the publisher.

The content of this book reflects the work and thought of the author(s).
Every effort has been made to publish reliable and accurate information herein,
but the publisher is not responsible for the validity of the information or for any
outcomes resulting from reliance thereon.

Printed in the United States of America.

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TABLE OF CONTENTS

Preface to the second edition / ix

Preface / xi

1 Overview of lithography / 1
Problems / 6

2 Optical pattern formation / 7
2.1 The problem of imaging / 7
2.2 Aerial images / 10
2.3 The contribution of physics and chemistry / 24
2.4 Focus / 33
Problems / 49
References / 49

3 Photoresists / 53
3.1 Positive and negative resists / 53
3.2 Adhesion promotion / 57
3.3 Resist spin coating, softbake, and hardbake / 60
3.4 Photochemistry of novolak/DNQ g- and i-line resists / 70
3.5 Acid-catalyzed DUV resists / 72
3.6 Development and post-exposure bakes / 79
3.7 Operational characterization / 83
3.8 Line edge roughness / 84
3.9 Multilayer resist processes / 86
Problems / 89
References / 90

4 Modeling and thin film effects / 99
4.1 Models of optical imaging / 99
4.2 Aberrations / 105
4.3 Modeling photochemical reactions / 114
4.4 Thin film optical effects / 117
4.5 Post-exposure bakes / 123
4.6 Methods for addressing the problems
of reflective substrates / 125
4.7 Development / 133
Problems / 133
References / 134

5 Wafer steppers / 139
5.1 Overview / 139
5.2 Light sources / 142
5.3 Illumination systems / 154
5.4 Reduction lenses / 159
v
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vi PRINCIPLES OF LITHOGRAPHY
5.5 Autofocus systems / 176
5.6 The wafer stage / 179
5.7 Scanning / 183
5.8 Dual-stage exposure tools / 185
Problems / 186
References / 187

6 Overlay / 199
6.1 Alignment systems / 201
6.1.1 Classification of alignment systems / 205
6.1.2 Optical methods for alignment and
wafer-to-reticle referencing / 206
6.1.3 Number of alignment marks / 212
6.2 Overlay models / 213
6.3 Matching / 222
6.4 Process-dependent overlay effects / 233
Problems / 235
References / 236

7 Masks and reticles / 243
7.1 Overview / 243
7.2 Mask blanks / 245
7.3 Mechanical optical-pattern generators / 248
7.4 Electron beam lithography and mask writers / 248
7.5 Optical mask writers / 258
7.6 Resists for mask making / 259
7.7 Etching / 261
7.8 Pellicles / 262
Problems / 268
References / 268

8 Confronting the diffraction limit / 273
8.1 Off-axis illumination / 273
8.2 Optical proximity effects / 282
8.3 The mask error factor / 292
8.4 Phase-shifting masks / 295
Problems / 307
References / 308

9 Metrology / 313
9.1 Linewidth measurement / 313
9.1.1 Linewidth measurement using
scanning electron microscope / 313
9.1.2 Scatterometry / 320
9.1.3 Electrical linewidth measurement / 322
9.2 Measurement of overlay / 324
References / 327

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CONTENTS vii
10 The limits of optical lithography / 331
10.1 The diffraction limit / 331
10.2 Improvements in optics / 333
10.3 The shortest wavelength / 335
10.4 Improved photoresists / 336
10.5 Flatter wafers / 337
10.6 How low can k go? / 338
1
10.7 Immersion lithograpy and maximum numerical aperture / 339
10.8 How far can optical lithography be extended? / 345
10.9 Resist limits / 350
10.10 Interferometric lithography / 351
Problems / 352
References / 352

11 Lithography costs / 355
11.1 Cost of ownership / 355
11.1.1 Capital costs / 357
11.1.2 Consumables / 365
11.1.3 Mask costs / 366
11.1.4 Rework / 367
11.1.5 Metrology / 368
11.1.6 Maintenance costs / 369
11.1.7 Labor costs / 369
11.1.8 Facilities costs / 370
11.2 Mix and match strategies / 370
Problems / 373
References / 373

12 Alternative lithography techniques / 375
12.1 Proximity x-ray lithography / 375
12.2 Extreme ultraviolet lithography––EUV / 382
12.3 Electron-beam direct-write lithography / 392
12.4 Electron-projection lithography––EPL / 395
12.4.1 Small-field EPL systems / 395
12.4.2 Large-field EPL systems / 396
12.5 Ion-projection lithography—IPL / 398
12.6 Imprint lithography / 400
12.7 The ultimate future of lithography / 402
Problems / 403
References / 404

Appendix A: Coherence / 413
Problems / 417
References / 417

Index / 419

Color Plates Section follows page 210

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PREFACE TO THE SECOND EDITION

This book was written to address several needs, and the revisions for the second
edition were made with those original objectives in mind. First, and foremost,
this book is intended to serve as an introduction to the science of
microlithography for people who are unfamiliar with the subject. Most papers
written for journals or conference proceedings assume that the reader is familiar
with pre-existing knowledge. The same can be said for compilations of chapters
written by experts who are providing their colleagues and peers with useful
summaries of the current state of the art and of the existing literature. Such
papers and books, while quite useful to experienced lithographers, are not
intended to address the needs of students, who first need to understand the
foundations on which the latest advances rest. It is the intention of this book to
fill that need.
For the experienced lithographer, there are many excellent books written on
specialized topics, such as photoresist and resolution enhancement techniques,
and I have referenced many of those fine works. However, I have often felt that
several topics have not been well addressed in the past; most notably those
subjects directly related to the tools we use to manufacture integrated circuits.
Consequently, this book goes into a few subjects in depth. These include such
topics as overlay, the stages of exposure tools, and light sources. Finally, this text
contains numerous references. These are resources for students who want to
investigate particular topics in more detail, and they provide the experienced
lithography with lists of references by topic.
A wise leader once told me that one of the most challenging tasks is to
transform complexity to simplicity; in other words, to make apparent the forest
obscured by all of the trees. I hope that I have succeeded adequately on the
subjects covered in this book. Of course, simplicity should never be confused
with easiness or completeness. To assist the student in recognizing these
distinctions, more problems have been added to the end of each chapter. It is
expected that the reader of this book will have a foundation in basic physics and
chemistry. No topics will require knowledge of mathematics beyond elementary
calculus.
Lithography is a field in which advances proceed at a swift pace, and many
new topics have been included in this second edition, commensurate with the
learning that has taken place during the past few years, and several subjects are
discussed in more detail. Optical proximity corrections and next-generation
lithography are examples where the landscape looks quite different than it did
just a few years ago. Other topics, such as immersion lithography, were ideas that
few took seriously just a few years ago, yet today are considered quite
mainstream.
It has been my good fortune to work with a number of outstanding
lithographers. In addition to the people acknowledged in the preface to the first
ix
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x PREFACE TO THE SECOND EDITION
edition, I would like to thank several people who contributed to this update.
These include Tim Brunner of IBM, Wolfgang Henke of Infineon, Margaret
Conkling of Nikon Precision, Nigel Farrar, Vladmir Fleurov, Palash Das and
Charles Hindes of Cymer, Andreas Erdmann of Fraunhofer-Institut für Integrierte
Schaltungen, Doug Resnick and John Algair of Motorola, Wilhelm Maurer of
Mentor Graphics, Christian Wagner and Robert Socha of ASML, Paul Graeupner
of Carl Zeiss, Johannes Nieder of Leica, John Ricardi and Harry Rieger of
JMAR, Ray Morgan of Canon, USA, Walter Gibson of XOS, and Sandy Burgan
of DNS. Merry Schnell and Sharon Streams of the publications staff of SPIE
have been very helpful and supportive. I apologize if I have failed to mention
anyone who has helped me with this update.
It has also been a privilege and joy to work on a more frequent basis with
some exceptionally outstanding lithographers in my own department, as well as
other lithography departments, at AMD. In particular, this includes
manufacturing organizations, where the principles discussed in this book have
been put skillfully applied and expertly enhanced to produce high performance
non-volatile memory and the world’s most powerful Windows-compatible
microprocessors. From AMD, I would like to thank Bruno La Fontaine,
Jongwook Kye, Ivan Lalovic, Adam Pawloski, Uzodinma Okoroanyanwu, Rolf
Seltmann, Wolfram Grundke, and Rick Edwards for useful and informative
discussions on lithography.
I would like to thank my wife, Dr. Laurie Lauchlan, and my daughters, Sam
and Sarah, who continued to exhibit amazing patience while I worked on the
second edition of this book. On September 11, 2001, the world witnessed the
destructive power of the irrational mind. I hope that this book will be a small
reminder of the tremendous capacity of the rational human mind to improve the
world around us.

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