Table of Contents

Preface

1 Introduction

1.1 The Role of Lithography in Integrated Circuit Fabrication

1.2 The Goal of Lithography

1.3 The Metrics of Lithography

1.4 Introduction to the Contents of this Book

2 Proximity Printing

2.1 Introduction

2.2 Proximity Imaging

2.3 Region of Validity for Various Approximations of Diffraction

2.4 Proximity Images

2.5 Exposure-Gap (E-G) Diagram

2.6 Conclusion

References

3 Exposure Systems

3.1 Projection Printing and a Comparison to Proximity Printing

3.2 Full-Wafer Field

3.3 Step and Repeat

3.4 Step and Scan

3.5 Reduction and 1X Systems

3.6 1X Mask Fabricated with a Reduction System

3.7 Summary

References

4 Image Formation

4.1 The Aerial Image

      4.1.1 Effects of a spherical wavefront and deviations from it

      4.1.2 Spherical wavefront

      4.1.3 The effect of a finite numerical aperture on the spherical wavefront

      4.1.4 Deviation from a spherical wavefront

      4.1.5 Imaging from a mask pattern

      4.1.6 Spatial frequencies

      4.1.7 Imaging results

4.2 Reflected and Refracted Images

      4.2.1 Methods to evaluate the reflected and refracted image from a mask

      4.2.2 Impact of multiple reflections on DOF

4.3 The Latent Image

4.4 Pupil Filters

      4.4.1 The A, B, C Coefficients

      4.4.2 The lumped parameters

      4.4.3 β and η

4.5 From Aerial Image to Resist Image

4.6 The Transferred Image

      4.6.1 Isotropic etching

      4.6.2 Anisotropic etching

      4.6.3 Lift off

      4.6.4 Ion implantation

      4.6.5 Electroplating

References

5 The Metrics of Lithography: Exposure-Defocus (E-D) Tools

5.1 The Resolution and DOF Scaling Equations

5.2 Determination of k₁ and k₃ Based on Microscopy

5.3 Determination of k₁, k₂, and k₃ Based on Lithography

      5.3.1 E-D branches, trees, and regions

      5.3.2 E-D window, DOF, and exposure latitude

      5.3.3 Determination of k₁, k₂, and k₃ using E-D windows

5.4 k₁, k₂, and k₃ as Normalized Lateral and Longitudinal Units of Dimension

5.5 The E-D Tools

      5.5.1 Construction of E-D trees

      5.5.2 The importance of using log scale in the exposure axis

      5.5.3 Elliptical E-D window

      5.5.4 CD-centered E-D windows versus full-CD-range E-D windows

      5.5.5 E-D windows and CD control

      5.5.6 Application of E-D tools

References

6 Hardware Components in Optical Lithography

6.1 Light Sources

      6.1.1 Mercury arc lamps

      6.1.2 Excimer lasers

6.2 Illuminator

      6.2.1 Köhler illumination system

      6.2.2 Off-axis illumination

      6.2.3 Arbitrary illumination

6.3 Masks

      6.3.1 Mask substrate and absorber

      6.3.2 Pellicles

      6.3.3 Critical parameters for masks

      6.3.4 Phase-shifting masks

6.4 Imaging Lens

      6.4.1 Typical lens parameters

      6.4.2 Lens configurations

      6.4.3 Lens aberrations

      6.4.4 Lens fabrication

      6.4.5 Lens maintenance

6.5 Photoresists

      6.5.1 Classifications

      6.5.2 Light interactions with a photoresist

      6.5.3 Developed resist images

      6.5.4 Antireflection coating (ARC) (by B.J. Lin and S.S. Yu)

6.6 Wafer

6.7 Wafer Stage

6.8 Alignment System

      6.8.1 Off-axis alignment and through-the-lens alignment

      6.8.2 Field-by-field, global, and enhanced global alignment

      6.8.3 Bright-field and dark-field alignments

6.9 Conclusion

References

7 Processing and Optimization

7.1 Optimization of the Exposure Tool

      7.1.1 Optimization of the NA

      7.1.2 Optimization of illumination

      7.1.3 Exposure and focus

      7.1.4 DOF budget

      7.1.5 Exposure tool throughput management

7.2 Resist Processing

      7.2.1 Resist coating

      7.2.2 Resist baking

      7.2.3 Resist developing

      7.2.4 Aspect ratio of the resist image

      7.2.5 Environmental contamination

7.3 k₁ reduction

      7.3.1 Phase-shifting masks

      7.3.2 Off-axis illumination

      7.3.3 Conceptual illustration

      7.3.4 Scattering bars

      7.3.5 Optical proximity correction

7.4 Polarized Illumination

7.5 Multiple Patterning

      7.5.1 Principle of the multiple-patterning technique (MPT)

      7.5.2 MPT processes

      7.5.3 MPT layouts

      7.5.4 G-rule for the double-patterning technique (DPT)

      7.5.5 Pack-unpack

      7.5.6 Resolution-doubling theory illustrated

      7.5.7 Overlay consideration of MPT

      7.5.8 Overcoming throughput penalty with double imaging

7.6 CD Uniformity (by S.S. Yu)

      7.6.1 CD nonuniformity (CDNU) analysis

      7.6.2 CDU improvement

7.7 Alignment and Overlay

      7.7.1 Alignment and overlay marks

      7.7.2 Using measured data for alignment

      7.7.3 Evaluation of interfield and intrafield overlay error components

References

8 Immersion Lithography

8.1 Introduction

8.2 Overview of Immersion Lithography

8.3 Resolution and DOF

      8.3.1 Wavelength reduction and spatial frequencies

      8.3.2 Resolution-scaling and DOF-scaling equations

      8.3.3 Improving resolution and DOF with an immersion system

      8.3.4 NA in immersion systems

8.4 DOF in Multilayered Media

      8.4.1 Transmission and reflection in multilayered media

      8.4.2 Effects of wafer defocus movements

      8.4.3 Diffraction DOF

      8.4.4 Required DOF

      8.4.5 Available DOF

      8.4.6 The preferred refractive index in the coupling medium

      8.4.7 Tradeoff between resolution and DOF_diffrac

8.5 Polarization in Optical Imaging

      8.5.1 Imaging with different polarizations

      8.5.2 Stray light

8.6 Immersion Systems and Components

      8.6.1 Configuration of an immersion system

      8.6.2 The immersion medium

      8.6.3 The immersion lens

      8.6.4 Bubbles in the immersion medium

      8.6.5 The mask

      8.6.6 Subwavelength 3D masks

      8.6.7 The photoresist

8.7 The Impact of Immersion Lithography on Technology

      8.7.1 Simulation of immersion lithography

      8.7.2 Poly layer

      8.7.3 Contact layer

      8.7.4 Metal layer

      8.7.5 Recommendations for the three technology nodes

8.8 Practicing Immersion Lithography

      8.8.1 Printing results

      8.8.2 Defect reduction

      8.8.3 Monitoring the immersion hood and special routing

      8.8.4 Other defect-reduction schemes

      8.8.5 Results

8.9 Extension of Immersion Lithography

      8.9.1 High-refractive-index materials

      8.9.2 Solid-immersion masks

      8.9.3 Polarized illumination

      8.9.4 Multiple patterning

8.10 Conclusion

References

9 EUV Lithography

9.1 Introduction

9.2 EUV Source

      9.2.1 Source power requirement

      9.2.2 The adopted LPP source

      9.2.3 Wall-power requirement of EUV systems

9.3 EUV Masks

      9.3.1 Configuration of EUV masks

      9.3.2 Effects of oblique incidence on mask

      9.3.3 EUV mask fabrication

      9.3.4 EUV pellicles

9.4 Resolution-Enhancement Techniques for EUVL

      9.4.1 EUV flexible illumination

      9.4.2 EUV proximity correction

      9.4.3 EUV multiple patterning

      9.4.4 EUV phase-shifting masks

9.5 EUV Projection Optics

9.6 EUV Resists

      9.6.1 Mechanism of EUV resist exposure

      9.6.2 CAR EUV resists

      9.6.3 Non-CAR EUV resists

9.7 Extendibility of EUVL

      9.7.1 Resist sensitivity, throughput, and power at each technology node

      9.7.2 Increasing NA

9.8 Summary of EUVL

9.9 Outlook of Lithography

References

Appendix: Methods to Evaluate the Region of Validity Based on Lithography Applications

by Yen Hui Hsieh, Mung Xiang Hsieh, and Burn J. Lin

A.1 Motivation

A.2 Similarity of the Approximation Methods According to the Pearson Correlation Coefficient

A.3 Critical Dimension

A.4 Log Slope–CD Control

A.5 Polychromatic Illumination

A.6 Summary and Conclusion

References

Index

Preface to the Second Edition

The first edition of this book was published in 2010. Eleven years is a long time in the field of lithography, even though much of the original content still withstands the passage of time and the advances in the technology nodes. Here are the reasons I decided to update this book:

1. Over these years, I am happy to have fulfilled my original goal of making this book suitable for (1) newcomers to the field who are interesting in growth into a technology career, (2) seasoned professionals who seek more depth into the technology, and (3) managers and executives who seek more breadth. People from each of these groups have told me that the book helped them and that they would recommend it to new readers. With this new edition, I hope to make the book even more useful.

2. For more than three years since my retirement from TSMC in Nov. 2015, I have taught the course Innovative Lithography, using the materials in this book. My students have given me the inspiration and enthusiasm to improve and update the book.

3. Lithography technology is fun to learn, fun to practice, and fun to teach. It is gratifying for me, once again, to preserve this knowledge in a book so that the torch can be passed.

This second edition features the following updates:

Chapter 2 Proximity Printing. This is a completely new chapter. In universities and many research labs, people need to make patterns with less-expensive equipment. Therefore, proximity printing is still very popular, even though there are not many publications on proximity imaging. I show rigorous and approximate ways to simulate the diffracted image at contact and in near, medium, and far fields, as well as the region of validity of these methods. The unintuitive positive- and negative-resist images from the proximity image are plotted. The E-G diagrams (from various authors) for quantifying proximity imaging are also covered. An appendix is provided to document our (two of my graduate students' and my) extensive research on methods to determine the region of validity of the approximation methods studied.

Chapter 3 Exposure Systems. To complement the coverage of historical and current exposure systems for replicating patterns described in the first edition, a carefully prepared step-by-step illustration of the mask and wafer movements in a step-and-scan system has been added. I also clarify that the projection-printed image is a mirror image, despite the common impression that only proximity printing produces a mirror image.

Chapter 4 Image Formation. In addition to adding derivations of the resolution-scaling and DOF-scaling equations and analyses of spatial frequency, light–resist interaction, and resist image development, I almost completely rewrote the section on Zernike polynomials, making their concepts easier for lithography engineers to grasp. The simulated partially coherent images in this chapter have been updated.

Chapter 5 E-D Methodology. This chapter is timeless. I provide a detailed explanation of how to construct E-D trees and emphasize why the log scale is preferred for intensity and exposure. The term apparent exposure—the reciprocal of intensity—is introduced, and its use in E-D diagrams is explained.

Chapter 6 Hardware Components. In this long chapter on the lithography components, I added examples of some enlightening resist development phenomena to help people visualize the resist development process. The coverage of chemically amplified resists is expanded. The coverage of wafers, the wafer stage, and alignment systems is also enriched.

Chapter 7 Processing and Optimization. This is another long chapter. I added new insightful derivations of off-axis illumination and a demonstration of extracting the overlay error components with a redundant number of data points to improve the accuracy. Multiple patterning is also extensively discussed, and the G-rule for double patterning is introduced.

Chapter 8 Immersion Lithography. This chapter continues the thorough coverage of this technology from the first edition with an outlook of its extendibility and its impact on the semiconductor technology. The best scaling equations for resolution and DOF are given, and the numerical aperture of the reduction immersion system is clarified.

Chapter 9 EUV Lithography. I almost completely rewrote this chapter, which is quite understandable because of EUVL's rapid growth during the last decade. Given that there are already other books on EUVL, I made sure that my contribution provides a valuable, unique perspective on the technology.

I omitted the material on MEB direct write to leave open the possibility of producing a single volume on this important topic for future development. Finally, I upgraded the figures with color for the eBook. Indeed, all of the figures in the second edition have been modernized.

I credited my wife Sue for her support of the first edition and of my life in general. Before starting my work on this revision, we celebrated our 50th anniversary in 2018. Sue continues to be an indispensable partner for her support during my writing of this second edition and her support in my professional life, family life, and spiritual life.

Burn J. Lin
June 2021