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Library of Congress Cataloging-in-Publication Data Mack, Chris A. Field guide to optical lithography / Chris A. Mack. p. cm. -- (SPIE field guides; FG06) Includes bibliographical references and index. 1. Integrated circuits--Design and construction. 2. Microlithography. I. Title. II. Series. TK7874.M195 2006 621.3815'31--dc22 2005034584 Published by SPIE—The International Society for Optical Engineering P.O. Box 10 Bellingham, Washington 98227-0010 USA Phone: +1 360 676 3290 Fax: +1 360 647 1445 Email: spie@spie.org Web: http://spie.org The content of this book reflects the work and thought of the author. 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.
Introduction to the SeriesWelcome to the SPIE Field Guides—a series of publications written directly for the practicing engineer or scientist. Many textbooks and professional reference books cover optical principles and techniques in depth. The aim of the SPIE Field Guides is to distill this information, providing readers with a handy desk or briefcase reference that provides basic, essential information about optical principles, techniques, or phenomena, including definitions and descriptions, key equations, illustrations, application examples, design considerations, and additional resources. A significant effort will be made to provide a consistent notation and style between volumes in the series. Each SPIE Field Guide addresses a major field of optical science and technology. The concept of these Field Guides is a format-intensive presentation based on figures and equations supplemented by concise explanations. In most cases, this modular approach places a single topic on a page, and provides full coverage of that topic on that page. Highlights, insights and rules of thumb are displayed in sidebars to the main text. The appendices at the end of each Field Guide provide additional information such as related material outside the main scope of the volume, key mathematical relationships and alternative methods. While complete in their coverage, the concise presentation may not be appropriate for those new to the field. The SPIE Field Guides are intended to be living documents. The modular page-based presentation format allows them to be easily updated and expanded. We are interested in your suggestions for new Field Guide topics as well as what material should be added to an individual volume to make these Field Guides more useful to you. Please contact us at fieldguides@SPIE.org. John E. Greivenkamp, Series Editor Optical Sciences Center The University of Arizona The Field Guide SeriesKeep information at your fingertips with all of the titles in the Field Guide Series: Field Guide to Geometrical Optics, John E. Greivenkamp (FG01) Field Guide to Atmospheric Optics, Larry C. Andrews (FG02) Field Guide to Adaptive Optics, Robert K. Tyson & Benjamin W. Frazier (FG03) Field Guide to Visual and Ophthalmic Optics, Jim Schwiegerling (FG04) Field Guide to Polarization, Edward Collett (FG05) Field Guide to Optical Lithography, Chris A. Mack (FG06) Field Guide to Optical Thin Films, Ronald R. Willey (FG07) Field Guide to Optical LithographyThe material in this Field Guide to Optical Lithography is a distillation of material I have been putting together for the last 20 years or so. I have been subjecting students in my graduate-level lithography course at the University of Texas at Austin to my disorganized notes for 14 years, and have published some similar material in my first book Inside PROLITH and my column in Microlithography World called The Lithography Expert. However, the challenge here was not in creating the material for the book but rather deciding what material to leave out and how to make what remained as condensed as possible. As people who know me can attest, I am rarely lacking for words and brevity is not my strong suit (I am a lousy poet). I hope, however, that the kind reader will forgive me when one page on a topic of interest does not satisfy—it is an unavoidable consequence of the Field Guide format, and my own limitations as an overly verbose writer. I thank Jeff Byers, William Howard, and Rob Jones for their help in reviewing the draft manuscript of this Field Guide. My many mistakes kept them quite busy. This Field Guide is dedicated to my wife Susan and our daughter Sarah, who have taught me that there is indeed something more fun in this world than lithography. Chris Mack Table of ContentsSymbol Glossary x The Lithography Process 1 Definition: Semiconductor Lithography 1 Overview of the Lithography Process 2 Processing: Substrate Preparation 3 Processing: Photoresist Spin Coating 4 Processing: Post-Apply Bake 5 Processing: Alignment and Exposure 6 Processing: Post-Exposure Bake 7 Processing: Development 8 Processing: Pattern Transfer 9 Image Formation 11 Maxwells Equations: The Mathematics of Light 11 The Plane Wave and the Phasor 12 Basic Imaging Theory 13 Diffraction 14 Fraunhofer Diffraction: Examples 15 The Numerical Aperture 16 Fourier Optics 17 Spatial Coherence and Oblique Illumination 18 Partial Coherence 19 Aberrations 20 Aberrations: The Zernike Polynomial 21 Aberrations: Zernike Examples 22 Chromatic Aberration 23 Horizontal-Vertical (H-V) Bias 24 Defocus 25 Flare 26 Vector Nature of Light 27 Polarization 29 The Optical Invariant 30 Immersion Lithography: Resolution 31 Immersion Lithography: Depth of Focus 32 Imaging into a Photoresist 33 Standing Waves: Definition 33 Standing Waves: Mathematics 34 Fresnel Reflectivity 35 Swing Curves 36 Top Antireflective Coatings (TARC) 37 Bottom Antireflective Coatings (BARC) 38 Photoresist Chemistry 39 Novolak/DNQ Resists 39 Chemically Amplified Resists 40 Absorption of Light 41 Photoresist Bleaching and the Dill Parameters 42 Exposure Kinetics 43 Measuring the Dill ABC Parameters 44 Chemically Amplified Resist Kinetics 45 Diffusion in Chemically Amplified Resists 46 Acid Loss Mechanisms 47 Post-Apply Bake Effects 48 Photoresist Development Kinetics 49 Surface Inhibition 50 Developer Temperature and Concentration 51 The Development Path 52 Lithography Control and Optimization 53 NILS: The Normalized Image Log-Slope 53 NILS: The Log-Slope Defocus Curve 54 NILS: Image Optimization 55 NILS: Exposure Optimization 56 NILS: PEB Optimization 57 NILS: Development Optimization 59 NILS: Total Process Optimization 60 Defining Photoresist Linewidth 61 Critical Dimension Control 62 Critical Dimension Control: Effect on Devices 64 Overlay Control 65 Line Edge Roughness 66 Metrology: Critical Dimension 67 Metrology: Overlay 68 The Process Window 69 Depth of Focus 71 Resolution 73 Rayleigh Criteria: Resolution 74 Rayleigh Criteria: Depth of Focus 75 Mask Error Enhancement Factor (MEEF) 76 Resolution Enhancement Technologies 77 Phase-Shift Masks 78 Phase-Shift Masks: Alternating 79 Phase-Shift Masks: Attenuated 80 Optical Proximity Effects 81 Optical Proximity Correction (OPC) 82 Off-Axis Illumination 83 Lithography Simulation 85 Moores Law 86 Next-Generation Lithography (NGL) 87 Equation Summary 88 Glossary 92 Index 117 Glossarya Dose-dependent curvature of the CD-through-focus curve; molar absorption coefficient; constant in the Mack 4-parameter dissolution rate model A Electric field amplitude; bleachable absorption co-efficient Ar Arrhenius coefficient B Magnetic induction, non-bleachable absorption co-efficient c Speed of light; concentration C Photoresist exposure rate constant CD Critical dimension d Shifter thickness for a phase-shift mask D Electric displacement, photoresist thickness; ARC thickness DH Diffusivity of acid in photoresist DOF Depth of focus E Electric field, incident exposure dose Ea Activation energy E0 Dose to clear Ez Exposure dose at depth z in the resist fx Spatial frequency G0 Initial PAG concentration h Planck’s constant; normalized acid concentration in a chemically amplified resist H Magnetic field; acid concentration in a chemically amplified resist I Intensity of light, aerial image J Electric current density k Propagation constant, wavenumber; chemical reaction rate constant k1 Normalized Rayleigh resolution k2 Normalized Rayleigh depth of focus Leff Effective gate length m Magnification; normalized unreacted site concentration in conventional or chemically amplified resists mTH Threshold inhibitor concentration M Photoactive compound concentration, unreacted site concentration M0 Initial PAC concentration n Index of refraction; dissolution selectivity parameter; diffraction order number nj Complex index of refraction of layer j NA Avogadro’s number NA Numerical aperture NILS Normalized image log-slope OPD Optical path difference p Pitch P Pupil function; photoresist exposure products; a point in x-y-z space r Photoresist dissolution rate rmax Dissolution rate of fully exposed positive resist rmin Dissolution rate of unexposed positive resist R Resin concentration; resolution; relative pupil radius position; intensity reflectivity; universal gas constant; photoresist dissolution rate S Solvent concentration t Time, exposure time t′ Bake time tm Mask transmittance function T Transmittance; absolute temperature Tm Fourier transform of the mask transmittance function (diffraction pattern amplitude) U Phasor representation of the sinusoidal e-field v Process variable w Slit width, mask feature width, nominal linewidth x Normalized concentration of reacted sites in a chemically amplified resist; horizontal position X Concentration of reacted sites in a chemically amplified resist Z Zernike polynomial coefficient α Maximum angle of diffraction captured by a lens; absorption coefficient δ Dirac delta function; defocus distance ε Dielectric constant θ Angle; polar angle of pupil position; photoresist sidewall angle γ Photoresist constrast κj Imaginary portion of complex refractive index λ Wavelength (in vacuum) μ Magnetic permeability ρ Electric charge density ρij Reflection coefficient between films i and j σ Conductivity; partial coherence factor; diffusion length τij Transmission coefficient between films i and j Φ Phase of an electric field; fraction of absorbed photons producing a chemical change (quantum yield) ω Frequency of monochromatic light; photoresist spin coat speed |
CITATIONS
Image processing
Photoresist materials
Lithography
Chemically amplified resists
Nanoimprint lithography
Optical lithography
Optimization (mathematics)
