University of Central Florida STARS Electronic Theses and Dissertations, 2004-2019 2006 Debris Characterization And Mitigation Of Droplet Laser Plasma Sources For Euv Lithography Kazutoshi Takenoshita University of Central Florida Part of the Electrical and Electronics Commons Find similar works at: https://stars.library.ucf.edu/etd University of Central Florida Libraries http://library.ucf.edu This Doctoral Dissertation (Open Access) is brought to you for free and open access by STARS. It has been accepted for inclusion in Electronic Theses and Dissertations, 2004-2019 by an authorized administrator of STARS. For more information, please contact [email protected]. STARS Citation Takenoshita, Kazutoshi, "Debris Characterization And Mitigation Of Droplet Laser Plasma Sources For Euv Lithography" (2006). Electronic Theses and Dissertations, 2004-2019. 917. https://stars.library.ucf.edu/etd/917 DEBRIS CHARACTERIZATION AND MITIGATION OF DROPLET LASER PLASMA SOURCES FOR EUV LITHOGRAPHY by KAZUTOSHI TAKENOSHITA Bachelors degree in Electric and Electronic Engineering Niigata University, Niigata, Japan, 1994 Masters degree in Electric and Electronic Engineering Niigata University, Niigata, Japan, 1996 A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Electrical Engineering in the School of Electrical Engineering and Computer Science at the University of Central Florida Orlando, Florida Summer Term 2006 Major Professor: Martin C Richardson c 2006 Kazutoshi Takenoshita ii ABSTRACT Extreme ultraviolet lithography (EUVL) is a next generation lithographic techniques under development for fabricating semiconductor devices with feature sizes smaller than 32 nm. The optics to be used in the EUVL steppers is reflective optics with multilayer mirror coatings on each surface. The wavelength of choice is 13.5 nm determined by the optimum reflectivity of the mirror coatings. The light source required for this wavelength is derived from a hot-dense plasma produced by either a gas discharge or a laser. This study concentrate only on the laser produced plasma source because of its advantages of scalability to higher repetition rates. The design of a the laser plasma EUVL light source consists of a plasma produced from a high-intensity focused laser beam from a solid/liquid target, from which radiation is generated and collected by a large solid angle mirror or array of mirrors. The collector mirrors have the same reflectivity characteristics as the stepper mirrors. The EUVL light source is considered as the combination of both the hot-dense plasma and the collector mirrors. The EUVL light sources required by the stepper manufacturers must have sufficient EUV output power and long operational lifetimes to meet market-determined chip produc- tion rates. The most influential factor in achieving the required EUV output power is the conversion efficiency (CE) of laser input energy relative to the EUV radiation collected. A high CE is demonstrated in a separate research program by colleagues in the Laser Plasma laboratory at CREOL. Another important factor for the light source is the reflectivity life- time of the collection optics as mirror reflectivity can be degraded by deposition and ablation from the plasma debris. Realization of a high CE but low debris plasma source is possible by iii reducing the mass of the target, which is accomplished by using tin-doped droplet targets. These have sufficient numbers of tin atoms for high CE, but the debris generation is minimal. The first part of this study investigates debris emissions from tin-doped droplet tar- gets, in terms of aerosols and ions. Numerous tin aerosols can be created during a single laser-target interaction. The effects these interactions are observed and the depositions are investigated using SEM, AFM, AES, XPS, and RBS techniques. The generation of aerosols is found to be the result of incomplete ionization of the target material, corresponding to non-optimal laser coupling to the target for maximum CE. In order to determine the threats of the ion emission to the collector mirror coatings from an optimal, fully ionized target, the ion flux is measured at the mirror distance using various techniques. The ion kinetic energy distributions obtained for individual ion species are quantitatively analyzed. Incorporating these distributions with Monte-Carlo simulations provide lifetime estimation of the collec- tor mirror under the effect of ion sputtering. The current estimated lifetime the tin-doped droplet plasma source is only a factor of 500 less than the stepper manufacturer require- ments, without the use of any mitigation schemes to stop these ions interacting with the mirror. The second part of this investigation explores debris mitigation schemes. Two miti- gation schemes are applied to tin-doped droplet laser plasmas; electrostatic field mitigation, and a combination of a foil trap with a magnetic field. Both mitigation schemes demonstrate their effectiveness in suppressing aerosols and ion flux. A very small number of high-energy ions still pass through the combination of the two mitigation schemes but the sputtering caused by these ions is too small to offer a threat to mirror lifetime. It is estimated that the lifetime of the collector mirror, and hence the source lifetime, will be sufficient when tin-doped targets are used in combination with these mitigation schemes. iv ACKNOWLEDGMENTS I would like to take a moment to express great appreciations to those who have supported this study. Without them, this study would have never been completed as it is now. I would like to thank first, Dr. Martin Richardson for providing this opportunity, advising this study, and stimulating my scientific and engineering curiosity. I would also like to thank all the committee members for valuable contributions, Dr. Kalpathy Sundaram, Dr. Aravinda Kar, Dr. Donald Malocha for discussing the possibilities of SAW detectors, and Dr. William Silfvast for discussing on the fundamentals of laser plasmas. In addition, I would also like to thank Dr. David Attwood for encouraging me in the EUV research and contributing this thesis. I am very fortunate to join the Laser Plasma Laboratory where I meet fine scientists and students from all different countries. For the droplet laser plasma generation, Dr. Chris- tian Keyser, and Dr. Chiew-Seng Koay, because of their previous work, this study has its own meaning. I have gained precious help from, as well as a lot of knowledge through discussions with, Ms. Simi George, Mr. Tobias Schmid, Mr. Teddy Peponnet, Mr. Robert Bernath, Mr. Joshua Duncan, Mr. Somsak Teerawattanasook, Mr. Jose Cunado, Ms. Ji-Yeon Choi, and all the colleagues. I like to thank the people in CREOL who discussed with me in the research, laser plasmas, lasers and optics, especially Dr. Grag Shimkaveg, Dr. Etsuo Fujiwara, Dr. Nikolai Vorobiev, Mr. Isao Matsubara, Dr. Sebastian Gauza, Dr. Fumiyo Yoshino, Dr. Yung- Hsun Wu, and Dr. Yi-Hsin Lin. In addition, there are many scientist outside school gave me valuable inputs in different universities, organizations, and companies, especially Mr Yasuaki Fukuda and Dr.Kazuaki Hotta for explaining the whole picture of the lithography industry. v I have kept a strong desire in my mind towards the application of the droplet gener- ations. It is my turn to contribute to those who worked together in Silver Seiko Ltd. and Siemens-Elema AB. where I have gained all varieties of knowledge and techniques that I can apply throughout the study. I like especially to express my appreciation to Dr. Milan Pokorny for all the initiations and his friendship, Mr. Ulf Fahlstr¨omfor supporting and his friendship, Dr. Masayuki Muto, Mr. Shizuo Terashima, Mr. Takeshi Fujiwara, Mr. Kunio Takahashi, for giving me the opportunities for the SRjet printing head development. This study is also a result of all the support I have had from friends and family. Dr. Takeo Maruyama and Mrs. Makiko Maruyama, Mr. Masaki Uesugi, and all the friends in Niigata city and Kashiwazaki city have been supporting and encouraging me and my wife, Miyuki. Dr. Calvin Hayes and Mrs. Barbara Hayes have been encouraging me and believed in me for pursuing this study. My parents and parents-in-low have been very supportive and waiting for me to complete the study. Lastly I thank my wife, Miyuki, for supporting me in here in the states. She should receive my most appreciation. vi TABLE OF CONTENTS LIST OF FIGURES . xvii LIST OF TABLES . .xviii LIST OF ACRONYMS/ABBREVIATIONS . xix CHAPTER 1 INTRODUCTION . 1 1.1 EUV Lithography . 1 1.1.1 Overview of EUVL . 3 1.1.2 EUVL source requirement . 4 1.1.3 Conversion efficiency . 5 1.1.4 Source lifetime . 6 1.2 EUV - Soft X-ray sources . 7 1.2.1 Synchrotron radiation . 8 1.2.2 Gas discharge dense plasmas . 9 1.2.3 Laser plasmas . 10 1.3 Multilayer mirror coating and reflectivity . 12 1.3.1 Absorption of soft X-ray in materials . 12 1.3.2 Reflectivity of multi-layer mirror . 13 1.4 Summary of background and motivation . 15 CHAPTER 2 LASER PLASMAS AND DEBRIS . 17 2.1 Introduction . 17 vii 2.1.1 Laser Plasma generation . 18 2.1.2 Absorption of laser energy in fully ionized plasmas . 18 2.1.3 Ionization stages . 20 2.1.4 Fluid descriptions of laser plasmas . 23 2.1.5 Plasma expansion . 25 2.1.6 Recombination . 26 2.2 Debris generation . 26 2.2.1 Solid target and debris . 27 2.2.2 Hot rocks and aerosols . 28 2.2.3 Ions, electrons, and neutral atoms . 29 2.3 Mass-limited target . 30 2.3.1 Different target configurations . 30 2.3.2 Number of atoms in a target . 32 2.4 Summary of laser plasmas and debris generation . 34 CHAPTER 3 MULTILAYER MIRROR REFLECTIVITY DEGRADATION 35 3.1 Introduction . 35 3.2 Deposition . 35 3.2.1 EUV absorption of target materials .