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Optical Lithography: Here is Why.pdf
Optical
Lithography
Here Is Why
Optical
Lithography
Here Is Why
Burn J. Lin
Bellingham, Washington USA
Library of Congress Cataloging-in-Publication Data
Lin, Burn Jeng, 1942-
Optical lithography : here is why / Burn J. Lin.
p. cm.
Includes index.
ISBN 978-0-8194-7560-2
1. Microlithography. 2. Semiconductors--Etching. 3. Lasers--Industrial
applications. I. Title.
TK7872.M3.L56 2009
621.3815'31--dc22
2009049350
Published by
SPIE
P.O. Box 10
Bellingham, Washington 98227-0010 USA
Phone: +1 360.676.3290
Fax: +1 360.647.1445
Email: Books@spie.org
Web: http://spie.org
Copyright © 2010 Society of Photo-Optical Instrumentation Engineers (SPIE)
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.
Contents
Preface .................................................................................................... xiii
Chapter 1 Introducing Optical Lithography / 1
1.1 The Role of Lithography in Integrated Circuit Fabrication ................... 2
1.2 The Goal of Lithography ...................................................................... 4
1.3 The Metrics of Lithography .................................................................. 4
1.4 The Contents of This Book .................................................................. 6
Chapter 2 Exposure Systems / 7
2.1 Proximity Printing ................................................................................ 7
2.2 Projection Printing and a Comparison to Proximity Printing ................ 9
2.3 Full-Wafer Field ................................................................................. 11
2.4 Step and Repeat ............................................................................... 14
2.5 Step and Scan ................................................................................... 17
2.6 Reduction and 1X Systems ............................................................... 21
2.7 1X Mask Fabricated with a Reduction System .................................. 24
2.8 Summary ........................................................................................... 25
References .............................................................................................. 25
Chapter 3 Image Formation / 29
3.1 The Aerial Image ............................................................................... 29
3.1.1 Effects of a spherical wavefront and deviations from it ............... 30
3.1.2 Spherical wavefront .................................................................... 30
3.1.3 The effect of a finite numerical aperture on the spherical
wavefront ............................................................................................. 31
3.1.4 Deviation from a spherical wavefront .......................................... 36
3.1.4.1 The Seidel aberration coefficients ....................................... 36
3.1.4.2 The Zernike aberration coefficients ..................................... 38
3.1.5 Imaging from a mask pattern ...................................................... 40
3.1.5.1 Coherent imaging from a mask pattern ............................... 41
3.1.5.2 Incoherent imaging from a mask pattern ............................. 44
3.1.5.3 Partial coherent imaging from a mask pattern ..................... 45
3.1.6 Spatial frequencies ..................................................................... 47
3.1.6.1 Spatial frequencies of an isolated line opening ................... 48
3.1.6.2 Spatial frequencies of line-space pairs ................................ 49
3.1.6.3 Angular spectrum ................................................................ 56
3.1.7 Imaging results ............................................................................ 56
3.2 Reflected and Refracted Images ....................................................... 60
v
vi
Contents
3.2.1 Methods to evaluate the reflected and refracted image from a
mask .................................................................................................... 62
3.2.2 Impact of multiple reflections on DOF ......................................... 63
3.3 The Latent Image .............................................................................. 64
3.4 The Resist Image .............................................................................. 65
3.4.1 The A, B, C coefficients .............................................................. 68
3.4.2 The lumped parameters .............................................................. 71
3.4.3 β and η ........................................................................................ 83
3.5 From Aerial Image to Resist Image ................................................... 85
3.6 The Transferred Image ...................................................................... 87
3.6.1 Isotropic etching .......................................................................... 87
3.6.2 Anisotropic etching ...................................................................... 88
3.6.3 Lift off .......................................................................................... 89
3.6.4 Ion implantation ........................................................................... 90
3.6.5 Electroplating .............................................................................. 91
References .............................................................................................. 92
Chapter 4 The Metrics of Lithography / 95
4.1 The Resolution and DOF Scaling Equations ..................................... 95
4.2 Determination of k1 and k3 Based on Microscopy ............................. 97
4.3 Determination of k1, k2, and k3 Based on Lithography ....................... 99
4.3.1 E-D branches, trees, and regions ............................................... 99
4.3.2 E-D window, DOF, and exposure latitude ................................. 101
4.3.3 Determination of k1, k2, and k3 using E-D windows ................... 103
4.4 k1, k2, and k3 as Normalized Lateral and Longitudinal Units of
Dimension ............................................................................................. 106
4.5 The E-D Tools ................................................................................. 106
4.5.1 Construction of E-D trees .......................................................... 106
4.5.1.1 E-D tree construction from E-D matrix linewidth data ........ 106
4.5.1.2 E-D tree construction from E-D matrix edge data .............. 107
4.5.1.3 E-D tree construction from intensity distribution ................ 109
4.5.2 Importance of log scale in the exposure axis ............................ 109
4.5.3 Elliptical E-D window ................................................................. 110
4.5.4 EL-versus-DOF tradeoff ............................................................ 111
4.5.5 Incorrect elliptical E-D window .................................................. 112
4.5.6 CD-centered versus full-CD-range E-D windows ...................... 113
4.5.7 E-D window and CD control ...................................................... 115
4.5.8 Application of E-D tools ............................................................. 115
4.5.8.1 Combination of feature types ............................................. 116
4.5.8.2 Combination of feature sizes ............................................. 116
4.5.8.3 Combination of cuts for 2D features .................................. 119
4.5.8.4 Combination of CD tolerances ........................................... 120
4.5.8.5 Combination of resist processing tolerances ..................... 122
4.5.8.6 Combination of image field positions ................................. 124
4.5.8.7 Setting the mask-making tolerance ................................... 127
Contents
vii
4.5.8.8 Effects of phase-shifting mask errors ................................ 130
4.5.8.9 Comparison of experiment and theory .............................. 130
References ............................................................................................ 132
Chapter 5 Components in Optical Lithography / 133
5.1 Light Source .................................................................................... 133
5.1.1 Mercury arc lamp ...................................................................... 133
5.1.2 Excimer laser ............................................................................ 136
5.1.2.1 Operation principle ............................................................ 136
5.1.2.2 Bandwidth narrowing ......................................................... 137
5.1.2.3 Spatial coherence .............................................................. 139
5.1.2.4 Maintenance, safety, and lifetime of excimer lasers .......... 140
5.2 Illuminator ........................................................................................ 141
5.2.1 Köhler illumination system ........................................................ 142
5.2.2 Off-axis illumination ................................................................... 142
5.3 Masks .............................................................................................. 142
5.3.1 Mask substrate and absorber ................................................... 144
5.3.2 Pellicles ..................................................................................... 145
5.3.3 Critical parameters for masks ................................................... 146
5.3.3.1 CD control.......................................................................... 146
5.3.3.2 Placement accuracy .......................................................... 146
5.3.3.3 Mask transmission and thermal expansion ....................... 147
5.3.3.4 Mask reflectivity ................................................................. 147
5.3.3.5 Mask flatness ..................................................................... 149
5.3.3.6 Physical size ...................................................................... 149
5.3.3.7 Defect level ........................................................................ 149
5.3.4 Phase-shifting masks ................................................................ 150
5.3.4.1 Operating principle ............................................................ 150
5.3.4.2 Un-flat BIM is not a PSM ................................................... 151
5.3.4.3 PSM types and mechanisms of imaging improvement ...... 151
5.3.4.4 PSM configurations ........................................................... 156
5.4 Imaging Lens ................................................................................... 159
5.4.1 Typical lens parameters ............................................................ 159
5.4.1.1 Numerical aperture ............................................................ 159
5.4.1.2 Field size ........................................................................... 160
5.4.1.3 Reduction ratio .................................................................. 161
5.4.1.4 Working distance ............................................................... 161
5.4.1.5 Telecentricity ..................................................................... 161
5.4.2 Lens configurations ................................................................... 161
5.4.2.1 Dioptric systems ................................................................ 162
5.4.2.2 Reflective systems ............................................................. 162
5.4.2.3 Catadioptric systems ......................................................... 163
5.4.3 Lens aberrations ....................................................................... 165
5.4.4 Lens fabrication ......................................................................... 165
5.5 Lens Maintenance ........................................................................... 166
viii
Contents
5.6 Photoresists ..................................................................................... 167
5.6.1 Classifications ........................................................................... 168
5.6.1.1 Polarity ............................................................................... 168
5.6.1.2 Working principle ............................................................... 170
5.6.1.3 Imaging configurations ...................................................... 175
5.6.2 Light interactions with a photoresist .......................................... 181
5.6.2.1 Wavelength compression .................................................. 181
5.6.2.2 Light absorption ................................................................. 181
5.6.2.3 Resist bleaching or dyeing ................................................ 181
5.6.2.4 Resist outgassing .............................................................. 182
5.6.2.5 Multiple reflections ............................................................. 182
5.6.3 Profile of resist images .............................................................. 187
5.7 Antireflection Coatings .................................................................... 189
5.8 Wafer ............................................................................................... 202
5.9 Wafer Stage .................................................................................... 202
5.10 Alignment System ......................................................................... 204
5.10.1 Off-axis alignment and through-the-lens alignment ................ 205
5.10.2 Field-by-field, global, and enhanced global alignment ............ 206
5.10.3 Bright-field and dark-field alignments ...................................... 207
References ............................................................................................ 208
Chapter 6 Processing and Optimization / 213
6.1 Optimization of the Exposure Tool .................................................. 213
6.1.1 Optimization of NA .................................................................... 213
6.1.2 Optimization of illumination ....................................................... 217
6.1.3 Exposure and focus .................................................................. 220
6.1.4 DOF budget .............................................................................. 222
6.1.4.1 Components in DOFrequired ................................................. 222
6.1.4.2 Focus monitoring ............................................................... 225
6.1.5 Exposure tool throughput management .................................... 231
6.2 Resist Processing ............................................................................ 238
6.2.1 Resist coating ........................................................................... 238
6.2.1.1 Defects .............................................................................. 239
6.2.1.2 Resist adhesion ................................................................. 239
6.2.1.3 Resist thickness ................................................................. 239
6.2.1.4 Resist uniformity ................................................................ 240
6.2.1.5 Saving resist material ........................................................ 241
6.2.2 Resist baking ............................................................................ 243
6.2.2.1 Precoating bake................................................................. 244
6.2.2.2 Postapply bake (pre-exposure bake) ................................. 244
6.2.2.3 Postexposure bake ............................................................ 245
6.2.2.4 Hard bake .......................................................................... 247
6.2.3 Resist developing ...................................................................... 248
6.2.4 Aspect ratio of resist image ....................................................... 251
6.2.5 Environmental contamination .................................................... 252
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