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Spie Press Book

Optical and EUV Lithography: A Modeling Perspective
Author(s): Andreas Erdmann
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Book Description

State-of-the-art semiconductor lithography combines the most advanced optical systems of our world with cleverly designed and highly optimized photochemical materials and processes to fabricate micro- and nanostructures that enable our modern information society. The precise fabrication and characterization of nanopatterns requires an in-depth understanding of all involved physical and chemical effects. This book supports such an understanding from a model-driven perspective, but without a heavy mathematical emphasis. The material for the book was compiled during many years of lecturing on optical lithography technology, physical effects, and modeling at the Friedrich-Alexander-University Erlangen-Nuremberg and in preparation for dedicated courses on special aspects of lithography. The book is intended to introduce interested students with backgrounds in physics, optics, computational engineering, mathematics, chemistry, material science, nanotechnology, and other areas to the fascinating field of lithographic techniques for nanofabrication. It should also help senior engineers and managers expand their knowledge of alternative methods and applications.

"This work is indispensable for researchers in photolithography modeling fields due to the wealth of succinctly and precisely presented information. It is also essential for the lithographers and Reticle Enhancement Technology and Correction Engineers working alongside modelers, as the author clearly explains lithography modeling concepts, making them accessible to all lithographers."
Larry Melvin, Ph.D., Technical Program Manager, Mask Solutions, Synopsys, Inc.

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Book Details

Date Published: 3 March 2021
Pages: 374
ISBN: 9781510639010
Volume: PM323

Table of Contents
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Table of Contents

Preface
Abbreviations and Acronyms
Frequently Used Symbols

1 Overview of Lithographic Processing
1.1 From Miniaturization in Microelectronics towards Nanotechnology
1.2 Historical Development
1.3 Aerial Image Formation in Projection Scanners
1.4 Photoresist Processing
1.5 Process Characteristics
1.6 Summary
References

2 Image Formation in Projection Lithography
2.1 Projection Scanners
2.2 Theory of Image Formation
      2.2.1 Fourier optical description
      2.2.2 Oblique illumination and partially coherent imaging
      2.2.3 Alternative image simulation methods
2.3 Abbe−Rayleigh Criteria and Consequences
2.4 Summary
References

3 Photoresists
3.1 Overview, General Reaction Schemes, and Phenomenological Description
      3.1.1 Classification of photoresists
      3.1.2 Diazonaphthoquinone (DNQ)-based photoresists
      3.1.3 State-of-the-art positive-tone chemically amplified resists (CARs)
      3.1.4 Phenomenological model
3.2 Photoresist Processing Steps and Modeling Approaches
      3.2.1 Selected technical aspects
      3.2.2 Exposure
      3.2.3 Post-exposure bake
      3.2.4 Chemical development
3.3 General Remarks on Modeling Approaches and Compact Resist Models
3.4 Negative- versus Positive-Tone Materials and Processes
3.5 Summary
References

4 Optical Resolution Enhancements
4.1 Off-Axis Illumination
      4.1.1 Optimum off-axis illumination for line–space patterns
      4.1.2 Off-axis illumination for arrays of contact holes
      4.1.3 From conventional and parametric source shapes to free-form illumination
4.2 Optical Proximity Correction
      4.2.1 Compensation of the iso-dense bias
      4.2.2 Compensation of line-end shortening
      4.2.3 From rule-based to model-based OPC and inverse lithography
      4.2.4 OPC models and process flows
4.3 Phase Shift Masks
      4.3.1 Strong phase shift masks: Alternating PSMs
      4.3.2 Attenuated or weak PSMs
4.4 Pupil Filters
4.5 Source and Mask Optimization
4.6 Multiple-Exposure Techniques
4.7 Summary
References

5 Material-Driven Resolution Enhancements
5.1 The Resolution Limit Revisited
5.2 Nonlinear Double Exposure
      5.2.1 Two-photon absorption materials
      5.2.2 Optical threshold materials
      5.2.3 Reversible contrast-enhancement materials
5.3 Double and Multiple Patterning
      5.3.1 Litho-etch-litho-etch (LELE)
      5.3.2 Litho-freeze-litho-etch (LFLE)
      5.3.3 Self-aligned double patterning (SADP)
      5.3.4 Dual-tone development (DTD)
      5.3.5 Selection of options for double and multiple patterning
5.4 Directed Self-Assembly (DSA)
5.5 Thin-Film-Imaging Technologies
5.6 Summary
References

6 Lithography with Extreme-Ultraviolet Light
6.1 Light Sources
6.2 Optical Material Properties in the EUV and Multilayer Coatings
6.3 Masks
6.4 Exposure Tools and Image Formation
6.5 Resists
6.6 Mask Defects
6.7 Optical Resolution Limits of EUV Lithography
      6.7.1 Beyond EUV (BEUV) lithography at 6.x nm wavelength
      6.7.2 Towards high-NA lithography
      6.7.3 Towards smaller k1: Optical resolution enhancements for EUV lithography
6.8 Summary
References

7 Optical Lithography Beyond Projection Imaging
7.1 Optical Lithography without a Projection Lens: Contact and Proximity Lithography
      7.1.1 Image formation and resolution limit
      7.1.2 Technical realization
      7.1.3 Advanced mask aligner lithography
7.2 Optical Lithography without a Mask
      7.2.1 Interference lithography
      7.2.2 Laser direct write lithography (LDWL)
7.3 Optical Lithography without a Diffraction Limit
      7.3.1 Near-field lithography
      7.3.2 Employing optical nonlinearities
7.4 Optical Lithography in Three Dimensions
      7.4.1 Grayscale lithography
      7.4.2 3D interference lithography
      7.4.3 Stereolithography and 3D microprinting
7.5 A Few Remarks on Lithography without Light
7.6 Summary
References

8 Lithographic Projection Systems: Advanced Topics
8.1 Wave Aberrations in Real Projection Systems
      8.1.1 Zernike representation of wave aberrations
      8.1.2 Wavefront tilt
      8.1.3 Power aberration
      8.1.4 Astigmatism
      8.1.5 Coma
      8.1.6 Spherical aberration
      8.1.7 Trefoil aberration
      8.1.8 Concluding remarks on Zernike-type wave aberrations
8.2 Flare
      8.2.1 Constant flare model
      8.2.2 Modeling of flare with power spectral densities
8.3 Polarization Effects in High-NA Projection Lithography
      8.3.1 Mask polarization effects
      8.3.2 Polarization effects in image formation
      8.3.3 Polarization effects resulting from the resist and wafer stack interfaces
      8.3.4 Polarization effects in the projector and the vector model for image formation
      8.3.5 Polarized illumination
8.4 Other Imaging Effects in Projection Scanners
8.5 Summary
References

9 Mask and Wafer Topography Effects in Lithography
9.1 Methods for Rigorous Electromagnetic Field Simulation
      9.1.1 Finite-difference time-domain (FDTD) method
      9.1.2 Waveguide method
9.2 Mask Topography Effects
      9.2.1 Mask diffraction analysis
      9.2.2 Oblique incidence effects
      9.2.3 Mask-induced imaging effects
      9.2.4 Mask topography effects in EUV lithography and mitigation strategies
      9.2.5 Varieties of 3D mask models
9.3 Wafer Topography Effects
      9.3.1 BARC deposition strategies
      9.3.2 Resist footing close to poly-lines
      9.3.3 Linewidth variation in double patterning
References

10 Stochastic Effects in Advanced Lithography
6.1 Random Variables and Processes
6.2 Phenomena
6.3 Modeling Approaches
6.4 Dependencies and Consequences
6.5 Summary
References

Index

Preface

State-of-the-art semiconductor lithography combines the most advanced optical systems of our world with cleverly designed and highly optimized photochemical materials and processes to fabricate micro- and nanostructures that enable our modern information society. The unique combination of applied optics, chemistry, and material science provides an ideal playground for scientists and engineers with an interest in applied natural sciences and technology. For many years the development of lithographic patterning techniques was almost exclusively scaling driven and focused on the improvement of resolution to support Gordon Moore's vision of "cramming more components onto integrated circuits." Although this scaling has still not reached its ultimate limits, it gets increasingly difficult and expensive to generate even more and smaller patterns on semiconductor chips with the required uniformity and without defects. Future lithographic techniques for emerging novel applications will have to emphasize different requirements, including three-dimensional (3D) shape control, integration of novel (functional) materials, patterning over non-planar surfaces, flexible adaptation of the target patterns to the final application, etc. The knowledge and experience of semiconductor lithographers, which were gained during more than 50 years of technology development, provide an important key to the development of novel micro- and nanotechnology-driven applications.

The material for this book was compiled over many years of giving lectures on Optical Lithography: Technology, Physical Effects, and Modeling at the Friedrich-Alexander-University Erlangen-Nuremberg and in preparation for dedicated courses on special aspects of lithography in companies and as side events of conferences. The book is intended to help interested students with backgrounds in physics, optics, computational engineering, mathematics, chemistry, material science, nanotechnology, and other areas to get started in the fascinating field of lithographic techniques for nanofabrication. It should also help senior engineers and managers to widen their view on alternative methods and applications.

It is not the intention of this book to provide a complete description of all aspects of lithographic patterning techniques. Instead, the book focuses on the explanation of the fundamental principles of image and pattern formation. These fundamental principles are demonstrated by simple, hopefully easy to understand, examples. The pros and cons of certain approaches and technology options are discussed. Extensive lists of references direct the reader to articles and books for further reading on special topics. To limit both the volume of this book and the time needed to write it, several important aspects of lithographic patterning technologies are not or are only rarely addressed in this book: Metrology and process control becomes increasingly important for high-volume lithographic fabrication. Advanced DUV and EUV projection lithographies require flexible fabrication, inspection, tuning, and repair of high-quality masks. Modern semiconductor fabrication involves a close interaction between the designers of electronic circuits and lithography process technology experts to provide a lithography-friendly design. Finally, there are many non-optical lithography techniques. These aspects are covered in several other books and review articles.

There are already several excellent books on semiconductor lithography. Why do we need another book on this topic? Most importantly, because lithography is one of the most dynamic fields of technology. It evolves due to the integration of new ideas and technologies with very different backgrounds. Research and development for modern lithography is highly multidisciplinary. The precise fabrication and characterization of nanopatterns requires an in-depth understanding of all involved physical and chemical effects. This book tries to support such understanding from a modeling-driven perspective, but without relying on heavy mathematics. The contents of this book reflects my special interest and background in applied optics, diffractive optics, rigorous modeling, and optimization of the interaction of light with micro- and nanostructures. Consequently, mask- and wafer-topography effects and related light-scattering effects are more extensively discussed than in other books on lithography. Finally, this book aims to bridge the gap between highly specialized engineers in semiconductor fabrication and scientists and other engineers exploring novel applications of lithographic patterning techniques for alternative applications.

Optical (projection) lithography combines the imaging of a mask or template onto a photosensitive material (photoresist) with the processing of the photoresist to transfer the optical image into a 3D pattern. The first chapter of the book provides an introduction to aerial image formation and photoresist processing. Typical metrics for the quantitative evaluation of images, of photoresist profiles, and of lithographic process variations are explained. Analysis of these metrics helps one to understand the impact of image and process enhancements that are discussed in the following parts of the book.

Chapter 2 describes the image formation by superposition of diffracted light that is transmitted through the opening (numerical aperture) of a projection lens and focused onto the photoresist. The resolution limit of projection systems is governed by the Abbe–Rayleigh equation. The fundamentals of photoresist chemistry and processing are explained in Chapter 3. The next two chapters provide an overview of resolution enhancements that are employed to print smaller features with a given wavelength and numerical aperture of the optical system. Optical resolution enhancements include off-axis illumination (OAI), optical proximity correction (OPC), phase shift mask (PSM), and source mask optimization (SMO). Multiple patterning and directed self-assembly (DSA) employ special materials and processing techniques to fabricate smaller features. Extreme ultraviolet (EUV) lithography with a wavelength of 13.5 nm extends optical projection lithography into the spectral range of soft x-rays. There are no materials that transmit light at these small wavelengths. As explained in Chapter 6, extreme ultraviolet (EUV) lithography has to employ reflective optics and mask, but also novel light sources and photoresist materials. Chapter 7 provides an overview of alternative optical lithography methods, including approaches to 3D lithography.

The remaining chapters of the book are dedicated to the description of important physical and chemical effects in advanced optical and EUV lithography. Chapter 8 discusses the impact of wave aberrations, polarization effects, and randomly scattered light on the intensity distribution inside the photoresist. Mask- and wafer-topography effects, which are caused by the scattering of light from small features on the mask and on the wafer, are described in Chapter 9. The last chapter of the book is devoted to stochastic effects that are responsible for non-smooth photoresist profiles with a line edge roughness (LER) on the order of a few nanometers and for the occurrence of fatal patterning defects such as microbridging and the incomplete opening of contact holes.

The order of the chapters follows the sequence of my lecture at the Friedrich-Alexander University Erlangen-Nuremberg. It is intended to provide an interesting mixture of theoretical background and application of optics and chemistry, and a description of various technology options. Chapters 1–5 describe the general background of optics and photoresist chemistry and should be read in this sequence. The reading order of Chapters 6–10 can be adapted to the special interests of the reader. Chapter 7 provides a general overview of alternative (optical) lithography methods that are more interesting for various applications of micro- and nanofabrication beyond nanoelectronics. People with exclusive interest in lithography for (advanced) semiconductor fabrication can skip this chapter.

Joint research work and fruitful discussions with many colleagues and project partners provided invaluable input for the material in this book. I am most grateful for suggestions from experts on special sections of this book, particularly the following: Antony Yen from ASML, Hans-Jürgen Stock from Synopsys, John Sturtevant from Mentor Graphics, Marcus Müller from the University of Göttingen, Michael Mundt from Zeiss SMT, Uzodinma Okoroanyanwu from Enx Labs, and Raluca Tiron from CEA-Leti.

Many thanks to all present and former members and students of the Fraunhofer IISB Computational Lithography and Optics group, especially to Peter Evanschitzky, Zelalem Belete, Hazem Mesilhy, Sean D'Silva, Abdalaziz Awad, Tim Fühner, Alexandre Vial, Balint Meliorisz, Bernd Tollkühn, Christian Motzek, Daniela Matiut, David Reibold, Dongbo Xu, Feng Shao, Guiseppe Citarella, Przemislaw Michalak, Shijie Liu, Temitope Onanuga, Thomas Graf, Thomas Schnattinger, Viviana Agudelo Moreno, and Zhabis Rahimi. All of these people contributed to our Fraunhofer IISB Development and Research LiTHOgraphy simulator Dr.LiTHO, which was used to generate to most of the figures in this book. Many useful remarks and tips from members of the Fraunhofer Lithography group and from students of my lithography lecture at the Erlangen University helped me to improve the material for this book.

Special thanks to Dara Burrows and Tim Lamkins from SPIE Press for their many useful tips and editorial assistance.

Andreas Erdmann
Erlangen, December 2020


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