Fundamental Principles of Optical Lithography: The Science of Microfabrication

by Chris Mack

Overview

Microlithography is the main technical driving force behind one of the most important phenomenon in the history of technology - microelectronics and the incredible shrinking transistor. These dramatic increases in electronic functionality per unit cost each year for early five decades, have transformed society. The gating piece of technology in this marvel of manufacturing progress has always been the process of lithography - the photochemical printing of circuit patterns onto semiconductor wafers.

This text attempts a difficult task - to capture the fundamental principles of the incredibly fast-changing field of semiconductor microlithography in such a sway that these principles may be effectively applied to past, present and future microfabrication technology generations. Its focus is on the underlying scientific principles of optical lithography, rather than its practice. It will serve equally well as a university textbook (each chapter has an extensive set of problems) and as an industry resource.

Much of the material contained in this book is, of course, a tutorial review of the published literature on lithography and related sciences, but a significant portion is new work, never before having been published. there is no other single book that covers the wide breadth of scientific disciplines needed in the practice of optical microlithography. The major topics covered within this text are optics (imaging and thin film interference effects), photoresist chemistry (chemical reactions, diffusion, and development phenomenon), lithography as a manufacturing process (process control, critical dimension control, and overlay), and resolution enhancement technologies.

Product Details

ISBN-13:	9781119965077
Publisher:	Wiley
Publication date:	08/10/2011
Sold by:	JOHN WILEY & SONS
Format:	eBook
Pages:	544
File size:	16 MB
Note:	This product may take a few minutes to download.

About the Author

Chris A. Mack, Vice-President of Lithography Technology, Lithoguru.com.

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Table of Contents

Preface     xv
Introduction to Semiconductor Lithography     1
Basics of IC Fabrication     2
Patterning     2
Etching     3
Ion Implantation     5
Process Integration     6
Moore's Law and the Semiconductor Industry     7
Lithography Processing     12
Substrate Preparation     14
Photoresist Coating     15
Post-Apply Bake     18
Alignment and Exposure     19
Post-exposure Bake     23
Development     24
Postbake     25
Measure and Inspect     25
Pattern Transfer     25
Strip     26
Problems     26
Aerial Image Formation - The Basics     29
Mathematical Description of Light     29
Maxwell's Equations and the Wave Equation     30
General Harmonic Fields and the Plane Wave in a Nonabsorbing Medium     32
Phasors and Wave Propagation in an Absorbing Medium     33
Intensity and the Poynting Vector     36
Intensity and Absorbed Electromagnetic Energy     37
Basic Imaging Theory     38
Diffraction     39
Fourier Transform Pairs     43
Imaging Lens     45
Forming an Image     47
Imaging Example: Dense Array of Lines and Spaces     48
Imaging Example: Isolated Space     50
The Point Spread Function     51
Reduction Imaging     53
Partial Coherence     56
Oblique Illumination     57
Partially Coherent Illumination     58
Hopkins Approach to Partial Coherence     62
Sum of Coherent Sources Approach     63
Off-Axis Illumination     65
Imaging Example: Dense Array of Lines and Spaces Under Annular Illumination     66
Kohler Illumination     66
Incoherent Illumination     69
Some Imaging Examples     70
Problems     71
Aerial Image Formation - The Details     75
Aberrations     75
The Causes of Aberrations     75
Describing Aberrations: the Zernike Polynomial     78
Aberration Example - Tilt     81
Aberration Example - Defocus, Spherical and Astigmatism     83
Aberration Example - Coma     84
Chromatic Aberrations     85
Strehl Ratio     90
Pupil Filters and Lens Apodization     90
Flare     91
Measuring Flare     92
Modeling Flare     94
Defocus     95
Defocus as an Aberration     95
Defocus Example: Dense Lines and Spaces and Three-Beam Imaging     98
Defocus Example: Dense Lines and Spaces and Two-Beam Imaging     100
Image Isofocal Point     102
Focus Averaging     103
Reticle Defocus     104
Rayleigh Depth of Focus     105
Imaging with Scanners Versus Steppers     106
Vector Nature of Light     108
Describing Polarization     111
Polarization Example: TE Versus TM Image of Lines and Spaces     113
Polarization Example: The Vector PSF     114
Polarization Aberrations and the Jones Pupil     114
Immersion Lithography     117
The Optical Invariant and Hyper-NA Lithography     118
Immersion Lithography and the Depth of Focus     120
Image Quality     121
Image CD     121
Image Placement Error (Distortion)     123
Normalized Image Log-Slope (NILS)     123
Focus Dependence of Image Quality     125
Problems      126
Imaging in Resist: Standing Waves and Swing Curves     129
Standing Waves     130
The Nature of Standing Waves     130
Standing Waves for Normally Incident Light in a Single Film     131
Standing Waves in a Multiple-Layer Film Stack     135
Oblique Incidence and the Vector Nature of Light     137
Broadband Illumination     141
Swing Curves     144
Reflectivity Swing Curve     144
Dose-to-Clear and CD Swing Curves     148
Swing Curves for Partially Coherent Illumination     149
Swing Ratio     151
Effective Absorption     154
Bottom Antireflection Coatings     156
BARC on an Absorbing Substrate     157
BARCs at High Numerical Apertures     160
BARC on a Transparent Substrate     164
BARC Performance     165
Top Antireflection Coatings     167
Contrast Enhancement Layer     170
Impact of the Phase of the Substrate Reflectance     170
Imaging in Resist     173
Image in Resist Contrast     173
Calculating the Image in Resist     177
Resist-Induced Spherical Aberrations     179
Standing Wave Amplitude Ratio     181
Defining Intensity     183
Intensity at Oblique Incidence     183
Refraction into an Absorbing Material     184
Intensity and Absorbed Energy     187
Problems     188
Conventional Resists: Exposure and Bake Chemistry     191
Exposure     191
Absorption     191
Exposure Kinetics     194
Post-Apply Bake     199
Sensitizer Decomposition     200
Solvent Diffusion and Evaporation     205
Solvent Effects in Lithography     209
Post-exposure Bake Diffusion     210
Detailed Bake Temperature Behavior     214
Measuring the ABC Parameters     217
Problems     219
Chemically Amplified Resists: Exposure and Bake Chemistry     223
Exposure Reaction     223
Chemical Amplification     224
Amplification Reaction     225
Diffusion     227
Acid Loss     230
Base Quencher     232
Reaction-Diffusion Systems     233
Measuring Chemically Amplified Resist Parameters     235
Stochastic Modeling of Resist Chemistry     237
Photon Shot Noise     237
Chemical Concentration     239
Some Mathematics of Binary Random Variables     241
Photon Absorption and Exposure     242
Acid Diffusion, Conventional Resist     246
Acid-Catalyzed Reaction-Diffusion     247
Reaction-Diffusion and Polymer Deblocking     251
Acid-Base Quenching     253
Problems     254
Photoresist Development     257
Kinetics of Development     257
A Simple Kinetic Development Model     258
Other Development Models     261
Molecular Weight Distributions and the Critical Ionization Model     264
Surface Inhibition     265
Extension to Negative Resists     267
Developer Temperature     267
Developer Normality     268
The Development Contrast     270
Defining Photoresist Contrast     270
Comparing Definitions of Contrast     274
The Practical Contrast     276
Relationship between [gamma] and r[subscript max]/r[subscript min]     277
The Development Path     278
The Euler-Lagrange Equation     279
The Case of No z-Dependence     280
The Case of a Separable Development Rate Function     282
Resist Sidewall Angle     283
The Case of Constant Development Gradients     284
Segmented Development and the Lumped Parameter Model (LPM)     286
LPM Example - Gaussian Image     287
Measuring Development Rates     292
Problems     293
Lithographic Control in Semiconductor Manufacturing     297
Defining Lithographic Quality     297
Critical Dimension Control     299
Impact of CD Control     299
Improving CD Control     303
Sources of Focus and Dose Errors     305
Defining Critical Dimension     307
How to Characterize Critical Dimension Variations     309
Spatial Variations     309
Temporal Variations and Random Variations     311
Characterizing and Separating Sources of CD Variations     312
Overlay Control     314
Measuring and Expressing Overlay     315
Analysis and Modeling of Overlay Data     317
Improving Overlay Data Analysis     320
Using Overlay Data     323
Overlay Versus Pattern Placement Error     326
The Process Window     326
The Focus-Exposure Matrix      326
Defining the Process Window and DOF     332
The Isofocal Point     336
Overlapping Process Windows     338
Dose and Focus Control     339
H-V Bias     343
Astigmatism and H-V Bias     343
Source Shape Asymmetry     345
Mask Error Enhancement Factor (MEEF)     348
Linearity     348
Defining MEEF     349
Aerial Image MEEF     350
Contact Hole MEEF     352
Mask Errors as Effective Dose Errors     353
Resist Impact on MEEF     355
Line-End Shortening     356
Measuring LES     357
Characterizing LES Process Effects     359
Critical Shape and Edge Placement Errors     361
Pattern Collapse     362
Problems     366
Gradient-Based Lithographic Optimization: Using the Normalized Image Log-Slope     369
Lithography as Information Transfer     369
Aerial Image     370
Image in Resist     377
Exposure     378
Post-exposure Bake     381
Diffusion in Conventional Resists     381
Chemically Amplified Resists - Reaction Only      383
Chemically Amplified Resists - Reaction-Diffusion     384
Chemically Amplified Resists - Reaction-Diffusion with Quencher     391
Develop     393
Conventional Resist     397
Chemically Amplified Resist     399
Resist Profile Formation     400
The Case of a Separable Development Rate Function     400
Lumped Parameter Model     401
Line Edge Roughness     404
Summary     406
Problems     408
Resolution Enhancement Technologies     411
Resolution     412
Defining Resolution     413
Pitch Resolution     416
Natural Resolutions     418
Improving Resolution     418
Optical Proximity Correction (OPC)     419
Proximity Effects     419
Proximity Correction - Rule Based     422
Proximity Correction - Model Based     425
Subresolution Assist Features (SRAFs)     427
Off-Axis Illumination (OAI)     429
Phase-Shifting Masks (PSM)     434
Alternating PSM     435
Phase Conflicts     438
Phase and Intensity Imbalance     439
Attenuated PSM      441
Impact of Phase Errors     445
Natural Resolutions     450
Contact Holes and the Point Spread Function     450
The Coherent Line Spread Function (LSF)     452
The Isolated Phase Edge     453
Problems     454
Glossary of Microlithographic Terms     457
Curl, Divergence, Gradient, Laplacian     491
The Dirac Delta Function     495
Index     501