Laser-Plasma Sources for Extreme-Ultraviolet Lithography BJÖRN A. M. HANSSON Doctoral Thesis Stockholm, Sweden 2003 TRITA FYS 2003-56 ISSN 0280-316X KTH ISRN KTH/FYS/--03:56--SE SE-100 44 Stockholm ISBN 91-7283-658-X SWEDEN Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexamen fredagen den 19 december 2003 kl. 10.00 i Kollegiesalen, Administrationsbyggnaden, Kungl Tekniska högskolan, Valhallavägen 79, Stockholm. °c Björn A. M. Hansson, november 2003 Tryck: Universitetsservice US AB iii Abstract This thesis describes the development and characterization of a liquid- xenon-jet laser-plasma source for extreme-ultraviolet (EUV) radiation. It is shown how this source may be suitable for production-scale EUV lithography (EUVL). EUVL is one of the main candidates to succeed deep-ultraviolet (DUV) lithography for large-scale manufacturing of integrated circuits (IC). However, a major obstacle towards the realization of EUVL is the current unavailability of a source meeting the tough requirements on especially power and clean- liness for operation in an EUVL stepper. The liquid-xenon-jet laser-plasma concept has key advantages that may make it suitable for EUVL since, e.g., its plasma consists only of the inert noble gas xenon and since the liquid- jet target technology enables plasma operation at large distances from the source-hardware thereby reducing sputtering and to allowing for high-power operation. At the beginning of the work described in this thesis, a spatial insta- bility of the liquid-xenon-jet made stable operation of a plasma at practical distances from the nozzle orifice difficult. However, an invention of a stabiliza- tion method based on applying localized heating to the tip of the jet-forming nozzle, resulted in stable jet operation. The longitudinal droplet stability of a liquid-droplet laser-plasma source has also been investigated and improved. Continuous improvements of especially the laser-power to EUV-radiation conversion efficiency (CE) and the stability of laser-plasma operation at large distances (several centimeter) from the nozzle are reported for the liquid- xenon-jet laser plasma source. Furthermore, this source is characterized re- garding many parameters relevant for EUVL operation including, ion emission from the plasma and related sputtering of nearby components, source size and shape, the repetition-rate limit of the source and non-EUV emission from the plasma. Although the main focus of the thesis has been the development and characterization of a liquid-xenon-jet laser-plasma source for production-scale EUVL, the source may also be suitable for small field applications that ben- efit from the high potential brightness of the source. A method to scan the plasma and thus minimize the photon losses while maintaining the object plane uniformity was developed. Furthermore, the first operation of a liquid- tin-jet laser plasma is reported. Quantitative EUV flux measurements yield record CE, but quantitative contamination measurements also indicate that a liquid-tin-jet laser plasma is not likely to be applicable as a source for EUVL. iv Contents Contents v List of papers vii Other publications ix List of acronyms xi 1 Introduction 1 1.1 Background . 1 1.2 Extreme-ultraviolet radiation . 2 1.3 Semiconductor technology . 3 2 Microlithography 5 2.1 Overview of microlithography . 5 2.2 The limitations of present microlithography . 7 2.3 Next-generation lithography . 8 3 EUV lithography 11 3.1 Introduction . 11 3.2 Multilayer optics . 11 3.3 A brief history of EUV lithography . 13 3.4 Overview of an EUV-lithograpy stepper . 14 3.5 Main challenges for EUV lithography . 15 3.6 The wafer level - the resist . 15 3.7 The projection optics . 16 3.8 The mask . 16 3.9 The collector/illuminator . 17 3.10 Contamination issues . 21 4 EUV sources 23 4.1 Introduction . 23 4.2 Gas-discharge plasma . 24 v vi CONTENTS 4.3 Laser plasma . 25 4.4 Synchrotron radiation . 25 4.5 Other sources . 28 4.6 Source-generated contamination . 28 4.7 Debris mitigation . 29 5 Laser plasma 31 5.1 Plasma physics . 31 5.2 Choice of target material . 32 5.3 Target geometry . 33 6 Liquid-xenon-jet laser plasma 37 6.1 Introduction . 37 6.2 Liquid-xenon-jet operation . 39 6.3 Laser-plasma operation . 40 6.4 A suitable source for EUV lithography? . 41 7 Summary of the papers 43 Acknowledgments 45 Bibliography 47 List of papers Paper 1 B. A. M. Hansson, L. Rymell, M. Berglund, and H. M. Hertz, “A Liquid- Xenon-Jet Laser-Plasma X-Ray and EUV Source”, Microel. Engin. 53, 667– 670 (2000). Paper 2 O. Hemberg, B. A. M. Hansson, M. Berglund and H. M. Hertz, “Stability of droplet-target laser-plasma soft x-ray sources”, J. Appl. Phys. 88, 5421–5425 (2000). Paper 3 B. A. M. Hansson, L. Rymell, M. Berglund, O. Hemberg, E. Janin, J. Thore- sen, and H. M. Hertz, “Liquid-Xenon-Jet Laser-Plasma Source for EUV Lithog- raphy”, SPIE 4506, 1–8 (2001). Paper 4 B. A. M. Hansson, M. Berglund, O. Hemberg, and H. M. Hertz, “Stabi- lization of liquefied-inert-gas jets for laser-plasma generation”, submitted to J. Appl. Phys. Paper 5 B. A. M. Hansson, S. Mosesson, and H. M. Hertz, “Improved emission unifor- mity from a liquid-jet laser-plasma EUV source”, submitted to Appl. Opt. Paper 6 P. A. C. Jansson, B. A. M. Hansson, O. Hemberg, M. Otendal, A. Holm- berg, J. de Groot, and H. M. Hertz, “Liquid-metal-jet laser-plasma extreme ultraviolet generation”, submitted to Appl. Phys. Lett. Paper 7 B. A. M. Hansson, O. Hemberg, H. M. Hertz, M. Berglund, B. Jacobsson, E. Janin, S. Mosesson, L. Rymell, J. Thoresen, and M. Wilner, “Characteriza- tion of a liquid-xenon-jet laser-plasma EUV source”, submitted to Rev. Sci. In- strum. vii Other publications The following papers and patents are related to the work in the thesis but have not been included in this thesis. Papers ² L. Rymell, M. Berglund, B. A. M. Hansson, and H. M. Hertz,"X-ray and EUV laser-plasma sources based on cryogenic liquid-jettarget", Proc. SPIE 3676, 421 (1999). ² H. M. Hertz, B. A. M. Hansson, M. Berglund, and L. Rymell,"Liquid-jet tar- get laser-plasma sources for EUV and x-ray lithography", Proc. SPIE 3767, 2 (1999) (invited). ² B. A. M. Hansson, M. Berglund, O. Hemberg, and H. M. Hertz, “Xenon liquid-jet laser-plasma source for EUV lithography”, Proc. SPIE 3997, 729 (2000). ² O. Hemberg, B. A. M. Hansson, M. Berglund, and H. M. Hertz, "Drift analysis and stabilization of laser-plasma droplet-target system", Proc. SPIE 4144, 38 (2000). ² H. M. Hertz, M. Berglund, B. A. M. Hansson, O. Hemberg, and G. A. Jo- hansson, "Liquid-jet laser-plasma source for microscopy and lithography", J. Phys. IV France 11, 389 (2001) (invited). ² B. A. M. Hansson, L. Rymell, M. Berglund, O. Hemberg, E. Janin, S. Moses- son, J. Thoresen, and H. M. Hertz, "Status of the Liquid-Xenon-Jet Laser- Plasma Source for EUV Lithography", Proc. SPIE 4688, 102 (2002) Patent and patent application ² Hans M. Hertz, Oscar Hemberg, Lars Rymell, Magnus Berglund ,Björn A. M. Hansson, "Method and apparatus for generating x-ray or EUV radiation", ix x OTHER PUBLICATIONS Swedish patent SE 520 087 (2003). Published international patent application WO 02/32197. ² European Patent Application No. EP 1 365 635 List of acronyms BW Bandwidth CoO Cost of Ownership CD Critical Dimension CE Conversion Efficiency CW continuous wave DUV Deep Ultraviolet EPL electron projection lithography ETS Engineering Test Stand EUV Extreme Ultraviolet EUV LLC Extreme Ultraviolet Limited Liability Company EUVL Extreme Ultraviolet Lithography FCII Flying Circus II FEL Free Electron Laser HCT Hollow-Cathode Triggered gas discharge IC Integrated Circuit IF Intermediate Focus ITRS International Technology Roadmap for Semiconductors IR Infrared LWR Line Width Roughness MET Micro Exposure Tool NA Numerical Aperture NGL Next Generation Lithography OPC Optical-Proximity Correction PSM Phase-Shifting Masks PXL Proximity X-ray Lithography SCALPEL Scattering with Angular Limitation in Projection Electron Lithography sr steradian TBD To Be Determined UV Ultraviolet VNL Virtual National Laboratory VUV Vacuum ultraviolet XPS X-ray Photoelectron Spectroscopy xi One Introduction 1.1 Background The work presented in this thesis is part of the development of laser-plasma sources for extreme-ultraviolet (EUV) radiation that takes place at the Royal Institute of Technology and also took place at the former company Innolite AB. The purpose of the work has been to develop laser-plasma sources and to investigate their ap- plicability for EUV lithography (EUVL). EUVL may be the main lithographic choice for semiconductor production to- wards the end of this decade and the beginning of next. A major problem, though, is that no EUV source is available today that meets the requirements for a production- scale EUVL stepper. In particular, no source has been able to sustain the high average powers needed or prove capability to do so. Furthermore, no source has shown its ability to operate at the required power for long times without degrading the fragile optical components in its vicinity. However, laser-plasma sources based on liquid jets or liquid droplet targets have certain features such as the ability to use a liquefied inert gas as target material, and the ability to generate the plasma far from any source hardware, that indicates the possibility to meet the requirements of EUVL. Therefore, the work described in this thesis has mainly been to develop liquid jet/droplet laser plasma sources and to investigate and improve their performance from a system’s point of view for applicability in EUVL.