Extreme ultraviolet lithography (EUVL) is the leading patterning technology for the production of computer chips beyond the current 193-nm-based optical lithography. EUVL has been in development since the 1980s; today, alpha-level EUVL scanners are in the field, and beta-level scanners are expected to be deployed in a few years. EUVL uses photons of 13.5-nm wavelengths for patterning. Because photons are absorbed heavily by all materials at this wavelength, the entire EUVL scanner is in vacuum, and reflective opticsâmostly multilayer (ML) mirrorsâare used. To develop EUVL as a patterning technology for high-volume manufacturing (HVM) of computer chips, progress has been made on several fronts: EUV light sources, optics, optics metrology, contamination control, masks and mask handling, and resists. In addition, learning from microexposure tools (METs) has been invaluable in the development of EUV resists and full-field EUVL scanners.
This reference book contains 12 chapters contributed by leading researchers and suppliers in the EUVL field. It also contains a detailed appendix with EUVL reference data. The chapter topics are intended to cover the needs of practitioners of the technology as well as readers who want an introduction to EUVL.
This book starts with two chapters on the history of EUVL development. The first chapter covers pioneering work from the earliest days until the EUV Limited Liability Corporation (LLC) was formed to consider the commercial feasibility of EUVL. The contributions of the EUV LLC are covered in the second chapter. Chapter 3 presents an overview of EUV source technology (requirements, technology status, and metrology). Chapters 4aâ4d cover EUV optics (substrate, ML deposition, and optical design). Chapter 5 describes and compares different EUV wavefront measurement techniques for EUV optics testing. Chapters 6aâ6c explain contamination and its control in EUVL scanners. In today's EUVL scanners, the collector optics in EUV sources are subject to the most serious contamination. Chapter 6 addresses this issue as well as the contamination of scanner optics. Chapter 7 discusses EUV mask technology (substrate, blank fabrication, absorber stack and backside conductive coating, patterning, cleaning, and phase shift masks). Chapter 8 covers the fundamentals and development of EUV resist technology and addresses line edge roughness (LER). Chapter 9 describes the design and components of the first METs, which have enabled resist development. Two of these METs are in the field today. Chapter 10 presents the fundamental design considerations for an EUVL scanner and describes various components of a full-field scanner, including illumination and projection optics, stages, sensors, and handling systems. Chapter 11 covers the benefits of patterning with 13.5 nm and presents the results of patterning with MET optics; LER and flare in EUVL systems are also discussed in this chapter. Finally, Chapter 12 explains the cost of ownership (CoO) fundamentals and lithography cost trends for future lithography techniques. The appendix contains reference data for the EUV spectral region, information relevant for practitioners of EUVL.
As EUVL technology has continued to develop, I have worked with authors and referees to identify topics that could be covered by a reference book. For detailed information on EUV source technology, readers are encouraged to consult the previously published text, EUV Sources for Lithography. Also, because EUVL technology continually evolves, an overview of its status has been omitted from this volume. I expect many readers will be interested in one particular topic and thus go straight to that chapter; for this reason, we have tried to make each chapter as complete in itself as possible.
Over the last two decades, researchers, commercial suppliers, and end users around the world have published several hundred papers per year on EUVL, and the amount of technical data on EUVL technology just continues to increase. My effort as volume editor has been to produce an authoritative reference book on EUVL technology, which until now has not existed. Today, EUV source technology is the leading challenge for implementing EUVL in HVM. EUV mask technology has made significant progress; however, defect-free masks that satisfy defectivity requirements remains a challenge. EUV resist, which will simultaneously satisfy the requirements of resolution, sensitivity, and LER, is another leading challenge for EUVL. As we see continued progress in addressing these three critical technical challenges, through publications and technical conferences, this book will continue to provide the in-depth technical background information relevant to source, mask, resist, scanners, and other technology areas.
The primary strength of this book is that the contributions come from leading experts in each technical area. My initial call for contributions went to authors around the world (Asia, Europe, and the U.S.). Due mostly to time constraints, more U.S. authors are represented in this volume. I am happy to report, however, that each chapter was still reviewed by experts worldwide, who adjusted their busy schedules to review the work of their colleagues and suggest many changes. In this sense, the entire global EUVL community has participated in the generation of this volume. This project has been successful because of the authors' dedication and hard work. This book is the fruit of their labor. During the editing of this volume, I realized that EUVL is a reality today due to the labor of hundreds of technologists around the world. For this reason, I have dedicated this book to these technologists who have made EUVL a reality.
I would also like to acknowledge the contributions of my family, whose influence, encouragement, and support have allowed me to undertake such a project. First of all, my father, Mr. Om Prakash Bakshi, MA, set a very high standard for written communication and the pursuit of excellence, which still today I can only strive to meet. My mother, Mrs. Pushpa Bakshi, MA, retired lecturer of the Punjabi language, always set an example of hard work and taught me a pragmatic approach toward solving everyday problems, which still guides me. Without the support of my family, Laura and Emily, I would not have been able to complete this project. I very much appreciate their tolerance of my absences in the evenings, on weekends, and at vacation time, while I labored to complete this volume.
Finally, I would like to thank SPIE Press manager, Timothy Lamkins; SPIE editor, Scott Schrum; and copyeditor, Margaret Thayer. Their teamwork made this project possible. I very much appreciate their support and hard work for making this book project a reality.
Vivek Bakshi, PhD
President, EUV Litho, Inc.
© 2009 Society of Photo-Optical Instrumentation Engineers