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Spie Press Book

Optical Lithography: Here Is Why
Author(s): Burn J. Lin
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Book Description

This book is aimed at new and experienced engineers, technology managers, and senior technicians who want to enrich their understanding of the image formation physics of a lithographic system. Readers will gain knowledge of the basic equations and constants that drive optical lithography, learn the basics of exposure systems and image formation, and come away with a full understanding of system components, processing, and optimization. Readers will also get a primer on the outlook of optical lithography and the many next-generation technologies that may greatly enhance semiconductor manufacturing in the near future.

Burn Lin is editor-in-chief of the Journal of Micro/Nanolithography, MEMS, and MOEMS (JM3), past chair of the SPIE Advanced Lithography symposium, author of two book chapters and over 88 articles, and holder of 51 U.S. patents. He is a National Academy of Engineering member, SPIE fellow, and IEEE fellow.

Book Details

Date Published: 23 February 2010
Pages: 492
ISBN: 9780819475602
Volume: PM190

Table of Contents
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1 Introducing Optical Lithography
1.1 The Role of Lithography in Integrated Circuit Fabrication
1.2 The Goal of Lithography
1.3 The Meter of Lithography
1.5 Introduction to the Contents of This Book
2 Exposure Systems
2.1 Proximity Printing
2.2 Projection Printing and a Comparison to Proximity Printing
2.3 Full-Wafer Field
2.4 Step and Repeat
2.5 Step and Scan
2.6 Reduction and 1X Systems
2.7 1X Mask Fabricated with a Reduction System
2.8 Summary
3 Image Formation
3.1 The Aerial Image
3.1.1 Effects of a spherical wavefront and deviations from it
3.1.2 Spherical wavefront
3.1.3 The effect of a finite numerical aperture on the spherical wavefront
3.1.4 Deviation from a spherical wavefront The Seidel aberration coefficients The Zernike aberration coefficients
3.1.5 Imaging from a mask pattern Coherent imaging from a mask pattern Incoherent imaging from a mask pattern Partial coherent imaging from a mask pattern
3.1.6 Spatial frequencies Spatial frequencies of an isolated line opening Spatial frequencies of line-space pairs Angular spectrum
3.1.7 Imaging results
3.2 Reflected and Refracted Images
3.2.1 Methods to evaluate reflected and refracted image from a mask
3.2.2 Impact of multiple reflections on DOF
3.3 The Latent Image
3.4 The Resist Image
3.4.1 The A, B, C coefficients
3.4.2 The lumped parameters
3.4.3 β and η
3.5 From Aerial Image to Resist Image
3.6 The Transferred Image
3.6.1 Isotropic etch
3.6.2 Anisotropic etch
3.6.3 Lift off
3.6.4 Ion implantation
3.6.5 Electroplating
4 The Meter of Lithography
4.1 The Resolution and DOF Scaling Equations
4.2 Determination of k1 and k3 Based on Microscopy
4.3 Determination of k1, k2, and k3 Based on Lithography
4.3.1 E-D branches, trees, and regions
4.3.2 E-D window, DOF, and exposure latitude
4.3.3 Determination of k1, k2, and k3 using E-D windows
4.4 k1, k2, and k3 as Normalized Lateral and Longitudinal Units of Dimension
4.5 The E-D Tools
4.5.1 Construction of E-D trees E-D tree construction from E-D matrix linewidth data E-D tree construction from E-D matrix edge data E-D tree construction from intensity distribution
4.5.2 Importance of log scale in the exposure axis
4.5.3 Elliptical E-D window
4.5.4 EL-versus-DOF tradeoff
4.5.5 Incorrect elliptical E-D window
4.5.6 CD-centered versus full-CD-range E-D windows
4.5.7 E-D window and CD control
4.5.8 Application of E-D tools Combination of feature types Combination of feature sizes Combination of cuts for 2D features Combination of CD tolerances Combination of resist processing tolerances Combination of image field positions Setting the mask-making tolerance Effects of phase-shifting mask errors Comparison of experiment and theory
5 Components in Optical Lithography
5.1 Light Source
5.1.1 Mercury arc lamp
5.1.2 Excimer laser Operation principle Bandwidth narrowing Spatial coherence Maintenance, safety, and lifetime of excimer lasers
5.2 Illuminator
5.2.1 Kohler illumination system
5.2.2 Off-axis illumination
5.3 Masks
5.3.1 Mask substrate and absorber
5.3.2 Pellicles
5.3.3 Critical parameters for masks CD control Placement accuracy Mask transmission and thermal expansion Mask reflectivity Mask flatness Physical size Defect level
5.3.4 Phase-shifting masks Operating principle Un-flat BIM is not a PSM PSM types and mechanism of imaging improvement PSM configurations
5.4 Imaging Lens
5.4.1 Typical lens parameters Numerical aperture Field size Reduction ratio Working distance Telecentricity
5.4.2 Lens configurations Dioptric systems Reflective systems Catadioptric systems
5.4.3 Lens aberrations
5.4.4 Lens fabrication
5.5 Lens Maintenance
5.6 Photoresists
5.6.1 Classifications Polarity Working principle Imaging configurations
5.6.2 Light interactions with a photoresist Wavelength compression Light absorption Resist bleaching or dyeing Resist outgassing Multiple reflections
5.6.3 Profile of resist images
5.7 Antireflection Coatings
5.8 Wafer
5.9 Wafer Stage
5.10 Alignment System
5.10.1 Off-axis alignment and through-the-lens alignment
5.10.2 Field-by-field, global, and enhanced global alignment
5.10.3 Bright-field and dark-field alignments
6 Processing and Optimization
6.1 Optimization of the Exposure Tool
6.1.1 Optimization of NA
6.1.2 Optimization of illumination
6.1.3 Exposure and focus
6.1.4 DOF budget Components in DOFrequired Focus monitoring
6.1.5 Exposure tool throughput management
6.2 Resist Processing
6.2.1 Resist coating Defects Resist adhesion Resist thickness Resist uniformity Saving resist material
6.2.2 Resist baking Precoating bake Postapply bake (pre-exposure bake) Postexposure bake Hard bake
6.2.3 Resist developing
6.2.4 Aspect ratio of resist image
6.2.5 Environmental contamination
6.3 k1 Reduction
6.3.1 Phase-shifting masks Alternating phase-shifting mask (AltPSM) Attenuated phase-shifting mask (AttPSM)
6.3.2 Off-axis illumination Working principle of OAI Conceptual illustration 3D illumination on 2D geometry Simulation results OAI and AltPSM Comparison of imaging performance Other considerations Combination of OAI and AltPSM
6.3.3 Scattering bars Imaging improvement from scattering bars Complications Restricted pitch 2D features Mask-making concerns Full-size scattering bar Subresolution hollow scattering bars and subresolution assist PSM
6.3.4 Optical proximity correction The proximity effect Edge corrections Rule-based OPC Model-based OPC Local-dose OPC Full-depth OPC Correction to AEI Hot-spot checking
6.4 CD Uniformity
6.4.1 CDNU analysis Linear model for CDU contributions Geometrical decomposition Physical decomposition CDU summation
6.4.2 CDU improvement Active compensation with exposure tools Active compensation on tracks
7 Immersion Lithography
7.1 Introduction
7.2 Resolution and DOF
7.2.1 Wavelength reduction and spatial frequencies
7.2.2 Resolution and DOF scaling equations
7.2.3 Improving resolution and DOF with an immersion system
7.3 DOF in Multilayered Media
7.3.1 Transmission and reflection in multilayered media
7.3.2 Effects of wafer defocus movements
7.3.3 Diffraction DOF
7.3.4 Required DOF
7.3.5 Available DOF
7.3.6 Preferred refractive index in the coupling medium
7.3.7 Tradeoff between resolution and DOFdiffrac
7.4 Polarization-Dependent Stray Light
7.4.1 Imaging at different polarizations Recombination of spatial frequency vectors in the resist Polarized refraction and reflection at the resist surface Different effects of polarized illumination
7.4.2 Stray light System stray light Stray light from recombination of spatial frequency vectors inside the resist Stray light from reflection off the resist surface Incorporating polarization effects to E-D windows Simulation results with PDS
7.5 Immersion Systems and Components
7.5.1 Configuration of an immersion system
7.5.2 The immersion medium
7.5.3 The immersion lens
7.5.4 Bubbles in the immersion medium
7.5.5 The mask
7.5.6 Subwavelength 3D masks
7.5.7 The photoresist
7.6 Impact on Technology
7.6.1 Simulation for an immersion system
7.6.2 Poly layer
7.6.3 Contact layer
7.6.4 Metal layer
7.6.5 Recommendation for the three technology nodes
7.7 Practicing Immersion Lithography
7.7.1 Printing results
7.7.2 Defect reduction
7.7.3 Monitoring the immersion hood and special routing
7.7.4 Other defect-reduction schemes Wafer and equipment cleanliness Wafer seal ring
7.7.5 Results
7.8 Extension of Immersion Lithography
7.8.1 High-index materials
7.8.2 Solid-immersion mask
7.8.3 Polarized illumination
7.8.4 Double exposures and pitch splitting
7.8.5 Pack-unpack
7.8.6 Overcoming the throughput penalty with double imaging
7.9 Conclusion on Immersion Lithography
8 Outlook and Successors to Optical Lithography
8.1 Outlook of Optical Lithography
8.1.1 Optical lithography galaxy for logic gates
8.1.2 Optical lithography galaxy for contact holes
8.1.3 Optical lithography galaxy for equal lines and spaces
8.2 EUV Lithography
8.2.1 Introduction
8.2.2 EUV source Wall-power requirement of EUV systems
8.2.3 EUV mask Configuration of EUV mask Random phase shifting Effects of oblique incidence Overlay mismatch Pattern shadowing Overlay error from mask flatness EUV mask fabrication Absence of mask pellicle
8.2.4 EUV projection optics
8.2.5 Wall-power consumption
8.2.6 EUV resist
8.2.7 EUV OPC
8.2.8 Summary on EUVL
8.3 Massively Parallel E-beam Maskless Imaging
8.3.1 Introduction to e-beam imaging
8.3.2 MEB ML2 systems proposed MAPPER MEB ML2 system LV system with individual sources HV MEB ML2 system MBS MEB ML2 system Reflected e-beam lithography
8.3.3 Comparison of the different systems
8.3.4 Data volume and the rate of DW systems
8.3.5 Power consumption of MEB ML2
8.3.6 Extendibility of MEB ML2 systems
8.3.7 Comparison of 4X mask writing to 1X wafer writing
8.3.8 Applications for MEB ML2
8.3.9 Summary on MEB ML2
8.4 Outlook of lithography
8.4.1 Double patterning
8.4.2 EUV lithography
8.4.3 MEB ML2
8.4.4 Nanoimprint lithography
8.5 Conclusions


Optical lithography is a fascinating field. It requires knowledge in geometrical and wave optics, optical and mechanical systems, diffraction imaging, Fourier optics, resist systems and processing, quantification of the imaging performance, and the control of it. Even the history of its development helps to stimulate new ideas and weed out less promising ones. Practitioners of optical lithography may only have a vague idea of its theory, and likewise, theoreticians may not have the opportunity to practice the technology on the manufacturing floor. Although optical lithography seems to be easy to grasp on the surface, many executives or decisionmakers often muddle about in this important field without paying due respect to the experts. When it comes to lithography, everybody has an opinion. This book intends to cover the above fields in theory and practice to sufficiently bridge these gaps. Optical lithography need not remain "black magic." It is a multidiscipline science and treated as such in this book.

In the late 1980s I began teaching a course called "Here is Why in Optical Lithography" at the SPIE Microlithography Symposium. It was an overwhelming success. The course continued until 2005, when I could no longer afford to spend eight hours of my time at the symposium teaching the course. However, many people told me they missed it, especially those companies that routinely sent their new engineers to the course. Fortunately, Bruce Smith and his students (who once audited in my course) eagerly asked me for the course notes, explaining that they could use it in a training course they planned to offer. So, there is a good chance the course material has been put to good use. Quite a few people also asked me whether I had written a book based on the course. However, even though I planned for such a book long ago, my work schedule prevented me from spending the time its development deserved. At one time SPIE even video recorded an abbreviated version of the course, but these video tapes only reflect a part of the course, not complete documentation. In the last two years I expanded the course into a one-semester three-credit course at the National Taiwan University. There, I also felt the need of a book for the course.

Yet, as soon as I started writing, I realized that the book could not be just course notes. It had to stand by itself. The content had to be broadened, so that much more depth could be provided. Since time restrictions (as in a short course) and space constraints (as in a journal article) were no longer limiting conditions, I could cover the entire field of optical lithography and provide as much information as a lithographer ought to know.

This book spans from the very early phase of the technology up to this very moment, when the lithographic technique for 32-nm half pitch is still a looming question worldwide. It should be useful for people who use the technology in an academic way and for people who practice lithography in the industry.

Even though the subject has broadened and deepened, I am maintaining the original spirit of my short course, i.e., telling the readers the reasons and principles behind the techniques we use, so that even novices to the technology can gain insights from it. In addition, students in the field can use this text to delve to great depths. Veterans in the field might enjoy reviewing all phases of the technology that they spent their lives practicing and recalling how innovations have sprung up at the different phases of the technology's development.

I am indebted to my colleagues at TSMC who contributed figures and texts. Among them, Shinn Sheng Yu contributed Section, part of Section 3.4.3, Sections 5.7,, and 6.4. Ru Gun Liu contributed Figs. 6.79, 6.87-6.90, 6.94-6.105, and 6.111; Ching Yu Chang, Figs. 6.39, 6.40, and 7.56; Tsai Sheng Gau, Figs. 7.45 and 7.69-7.70; Chun Kuang Chen, Fig. 7.46; Shih Che Wang, Fig. 7.58; Fu Jye Liang, Figs. 7.59-7.66; Kuei Shun Chen, Figs. 7.71 and 7.77; John Chin Hsiang Lin, Fig. 7.74; and Da Chung Owe-Yang, Fig. 7.78. Some other SEM pictures were taken from the result of engineers of the Nanopatterning Division at TSMC. Figure credit from authors outside of TSMC is acknowledged where the figures appear.

Finally who writes a book without thanking their family for patience and understanding? Here, my wife Sue stands out as a great partner in my life to share my family responsibilities and even take them over in time of need. She is a brilliant professional who managed to have successful careers at IBM and AT&T, and yet has not shortchanged her support to my career and the upbringing of our children, Christiana and John.

Burn Lin
Hsin-Chu, Taiwan
January 2010

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