Excimer Laser Technology

D. Basting · G. Marowsky (Eds.)

Excimer LaserTechnology

With 257 Figures

ABC

Editors

Dr. Dirk Basting2200 South Ocean LaneFort Lauderdale, FL 33316-3831USA

Professor Dr. Gerd MarowskyLaser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 Göttingen, Germany

Copy editor

Dr. Uwe Brinkmann, Göttingen-Bovenden, Germany

Library of Congress Control Number: 2005923161

ISBN -10 3-540-20056-8 Springer Berlin Heidelberg New YorkISBN -13 978-3-540-20056-7 Springer Berlin Heidelberg New York

This work is subject to copyright. All rights are reserved, whether the whole or part of the material isconcerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting,reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publicationor parts thereof is permitted only under the provisions of the German Copyright Law of September 9,1965, in its current version, and permission for use must always be obtained from Springer. Violations areliable for prosecution under the German Copyright Law.

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Preface

The aim of this book is to provide a practical guide on the fundamentals,status of present technology and applications of excimer lasers. Theoreticaldetails have been intentionally omitted. The book has emerged from “Excimerlaser technology: Laser sources, optics, systems and applications” issued ear-lier by a manufacturer of excimer lasers. The scope of the book has beenconsiderably enlarged and the knowledge deepened by the participation ofseveral renowned international researchers. Thus, it may form not only thebasis for training programs for service and maintenance personnel on excimerlasers, but serves also to address researchers, technologists and newcomers inthe field of UV lasers and applications who require a thorough introductionto the field.

The book is divided into two main chapters: Fundamentals and Appli-cations, along with a short introductory chapter, briefly covering a varietyof subjects and some historical remarks and a final outlook chapter. Thechapter ‘Fundamentals’ provides a short introduction to laser physics andsome descriptions of optical components needed for successful operation andapplication of lasers operating in the UV spectral range. This includes thespecial demands concerning optical materials, beam shaping optics, and beamdiagnostics. The chapter ‘Applications’ opens with an overview of the variousfields of actual excimer laser applications, then goes into more detail in impor-tant fields like ablative micro-fabrication, nano-structuring with femtosecondexcimer laser pulses, material modification including microlithography, de-position of thin films, combustion analysis, and medical applications. At theleading edge of current research, the chapter on ultra-high intensity applica-tions deals with the generation of “hollow atoms”. Micro-fabrication usingF2-laser radiation at 157nm completes the range of UV-laser applications.

The last chapter focuses on next-generation lithography using 13.5 nmradiation. Radiation sources in the extreme UV (EUV), realized by bothlaser and discharge pumping, are expected to transform micro-lithographyinto nano-lithography. The editors and authors hope that this publicationwill be a valuable source of information for students, engineers, and scientistswishing to work in the field of excimer lasers.

VI Preface

Acknowledgement. The editors wish to thank Jay Jethwa, Charles K. Rhodesand Petra Tregel for critical reading of the manuscripts and many practicalhints and fruitful discussions. Valuable coordination and constant technicalassistance of Dirk Born, Jutta Steckel, Thomas Müller, Karsten Roetmann,and Sebastian Kranzusch is gratefully acknowledged. Sparkasse Göttingen,represented by Rainer Hald, made the financial backing of this project possi-ble. With the help of the Otto-Hahn-Bibliothek, Bernhard Reuse and Rein-hard Harbaum, we have been able to identify additional references from ear-lier times.

The editors thank R. Srinivasan for various contributions to chapter 1.2,Historical Review of Excimer Laser Development.

We owe particular thanks to our copy editor Uwe Brinkmann, who pre-pared editable manuscripts from the submitted material and attempted toachieve correct printing together with Thomas Eggers, who coped with thedemanding desk-top page making work.

The editors took liberty to make appropriate changes in some of themanuscripts for further improvement of the book.

Göttingen, Dirk BastingDecember 2004 Gerd Marowsky

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 The Unique Microstructuring Capabilitiesof Excimer Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Commercial Impact of Excimer Lasers . . . . . . . . . . . . . . 31.1.3 Could Excimers Have Been the First Media

with Optical Gain? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1.4 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2 Historical Review of Excimer Laser Development . . . . . . . . . . . 8References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.3 Trends in Worldwide Excimer Laser Sales . . . . . . . . . . . . . . . . . 22

1.3.1 The World’s Laser Markets at the End of 2004 . . . . . . . 221.3.2 Excimer Laser Markets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231.3.3 Excimer Laser Markets: Industrial . . . . . . . . . . . . . . . . . . 251.3.4 Excimer Laser Markets: Medical Therapy . . . . . . . . . . . . 271.3.5 Excimer Laser Markets: Research . . . . . . . . . . . . . . . . . . . 29

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Part I Fundamentals

2 Some Fundamentals of Laser Physics . . . . . . . . . . . . . . . . . . . . . 332.1 The Wave-Particle Duality of Light . . . . . . . . . . . . . . . . . . . . . . . 332.2 Electromagnetic Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3 Spontaneous and Stimulated Emission, Population Inversion . 342.4 Design Principle of a Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.5 Types of Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3 Principles of Excimer Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

VIII Contents

4 Design and Technology of Excimer Lasers . . . . . . . . . . . . . . . . 474.1 Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.2 Preionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.3 Discharge Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514.4 Discharge Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524.5 Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.6 Circulation, Laser Gas Cooling, and Cleaning . . . . . . . . . . . . . . 624.7 Safety Standards for Industrial Excimer Lasers . . . . . . . . . . . . . 64

4.7.1 CE Mark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.7.2

Directive 89/366/ECC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.7.3 Design Rules for Electromagnetic Compatibility . . . . . . 664.7.4 Low Voltage Directive 73/23/ECC . . . . . . . . . . . . . . . . . . 684.7.5 Pressure Equipment Directive 97/23/EC . . . . . . . . . . . . 684.7.6 Machinery Directive 89/392/EEC

(Amended 98/37/EEC) . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.7.7 Laser Radiation Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.7.8 SEMI Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5 Specially Designed Excimer Lasers . . . . . . . . . . . . . . . . . . . . . . . . 755.1 High-Repetition-Rate and High-Power Lasers . . . . . . . . . . . . . . 75References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795.2 High-Energy Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.2.1 KrF and ArF Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825.2.2 XeCl Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

6 Excimer Lasers for Microlithography . . . . . . . . . . . . . . . . . . . . . 896.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 896.2 Wavelengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 926.3 Spectral Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946.4 Line Narrowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946.5 Wavelength Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956.6 Laser Power and Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986.7 Energy Dose Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986.8 Dual-Chamber Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996.9 Pulse Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016.10 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

7 Laser Beam Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057.2 Recording of Spatial Beam Profiles . . . . . . . . . . . . . . . . . . . . . . . 1057.3 Evaluation of Beam Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Electromagnetic Compatibility (EMC)

Contents IX

7.3.1 Beam Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1077.3.2 Beam Divergence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1107.3.3 Beam Positional and Directional Stability . . . . . . . . . . . 1107.3.4 Homogenized Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

7.4 Wavefront Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

8 Optical Coatings for Excimer Laser Applications . . . . . . . . . 1198.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1198.2 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1198.3 Interaction Mechanisms of UV Photons

with Coated Optical Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . 1208.4 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1218.5 Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

8.5.1 Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1228.5.2 Dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

8.6 XUV Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1248.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

9 Small Structures with Large Excimer Lasers . . . . . . . . . . . . . . 1279.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1279.2 Nozzle Plates for Ink Jet Printers . . . . . . . . . . . . . . . . . . . . . . . . . 1279.3 Attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1289.4 Homogenizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1309.5 Projection Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1329.6 Mechanically and Thermally Stable Support System . . . . . . . . 1339.7 Beam Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339.8 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Part II Applications

10 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

11 Ablative Micro-Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14911.1 Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15311.2 Micro-Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

11.2.1 Processing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15611.2.2 Removal of Thin Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . 16011.2.3 Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16211.2.4 Fabrication of Micro Parts . . . . . . . . . . . . . . . . . . . . . . . . . 174

X Contents

11.2.5 Indirect Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17911.2.6 Micro Optics Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 18111.2.7 MEMS and Nanofabrication . . . . . . . . . . . . . . . . . . . . . . . 183

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18411.3 Via Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

11.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18711.3.2 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18711.3.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

12 Micro-Processing of Borosilicate Glass and Polymers . . . . . 20112.1 Borosilicate Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

12.1.1 Drilling of Holes in Pyrex Glass . . . . . . . . . . . . . . . . . . . . 20312.1.2 Generation of Channels in Borosilicate Glass . . . . . . . . . 20612.1.3 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

12.2 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21212.2.1 Generation of Channels in PMMA Using Scanner . . . . . 21212.2.2 Generation of Three Dimensional Structures

in PMMA Using Scanner . . . . . . . . . . . . . . . . . . . . . . . . . . 21312.2.3 Microstructuring of Polymer Tubes . . . . . . . . . . . . . . . . . 21612.2.4 Writing of Mixer Structures in PMMA . . . . . . . . . . . . . . 21712.2.5 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

13 F2-Laser Microfabrication for Photonicsand Biophotonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22113.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22113.2 Vacuum-Ultraviolet Optical Tools . . . . . . . . . . . . . . . . . . . . . . . . 222

13.2.1 High-Fluence F2-Laser Optical Tool . . . . . . . . . . . . . . . . 22213.2.2 Optical Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

13.3 Micromachining of Silica Glasses . . . . . . . . . . . . . . . . . . . . . . . . . 22813.3.1 Ablation Rates and Surface Morphology . . . . . . . . . . . . . 22913.3.2 Heat-Affected Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23113.3.3 Ablation Debris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

13.4 Micro-Optics Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23413.4.1 Microchannels and Holes . . . . . . . . . . . . . . . . . . . . . . . . . . 23513.4.2 Fiber and Rib Waveguides . . . . . . . . . . . . . . . . . . . . . . . . . 23713.4.3 Mask Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23813.4.4 Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24313.4.5 Diffractive Optical Elements . . . . . . . . . . . . . . . . . . . . . . . 245

13.5 Controlling Refractive Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25313.5.1 F2-Laser Photosensitivity: Silica Glasses . . . . . . . . . . . . . 25413.5.2 F2-Laser Photosensitivity: Optical Fiber . . . . . . . . . . . . . 25813.5.3 Trimming Planar Lightwave Circuits . . . . . . . . . . . . . . . . 26113.5.4 Writing Buried Optical Waveguides . . . . . . . . . . . . . . . . . 264

Contents XI

13.6 Biophotonics on a Chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

14 Nano-Structuringwith Femtosecond Excimer Laser Pulses . . . . . . . . . . . . . . . . . . 27914.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27914.2 The Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28014.3 Mask Projection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28114.4 Interference Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

15 Physical Aspects of Ultra-Fast UV Laser Transfer . . . . . . . . 28515.1 Introduction to the Laser Transfer Process . . . . . . . . . . . . . . . . . 28515.2 Visualization of the Laser Materials Transfer . . . . . . . . . . . . . . . 287

15.2.1 Time Resolved Schlieren Imaging Method . . . . . . . . . . . 28715.2.2 Dynamics of the sub-ps- and ns-Process –

A Comparative Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28715.2.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

16 Material Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29516.1 Microlithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

16.1.1 The Patterning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . 29516.1.2 Optical Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29616.1.3 Optical Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29816.1.4 Technology Roadmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30416.2 TFT Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

16.2.1 Electron Mobility Increases with Grain Size . . . . . . . . . . 30816.2.2 Layer Stratification,

Near and Complete Melt-Through . . . . . . . . . . . . . . . . . . 30916.2.3 Excimer-Laser Based Annealing System . . . . . . . . . . . . . 31116.2.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31216.3 Fiber Bragg Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

16.3.1 Introduction to Fiber Bragg Gratings (FBGs) . . . . . . . . 31316.3.2 Manufacturing of Fiber Bragg Gratings . . . . . . . . . . . . . 31416.3.3 Applications for Fiber Bragg Gratings . . . . . . . . . . . . . . 318

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32016.4 Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33016.5 Activation and Metallization of Dielectrics . . . . . . . . . . . . . . . . . 331References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

XII Contents

17 Excimer-Laser Assisted Deposition of Carbon and BoronNitride-Based High-Temperature Superconducting Films . 33517.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33517.2 Characteristics of Excimer Laser Ablation . . . . . . . . . . . . . . . . . 33617.3 Deposition Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34017.4 Deposition and Properties

of Tetrahedral Amorphous Carbon (ta-C) Films . . . . . . . . . . . . 34117.5 Deposition and Properties

of Cubic Boron Nitride (c-BN) Films . . . . . . . . . . . . . . . . . . . . . . 34617.6 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350

18 Combustion Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35118.1 Scattering Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35218.2 Fluorescence Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

19 Medical Applications of Excimer Lasers . . . . . . . . . . . . . . . . . . 36119.1 Refractive Laser Surgery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

19.1.1 Photo Refractive Keratectomy (PRK) . . . . . . . . . . . . . . . 36119.1.2 Photo Therapeutic Keratectomy (PTK) . . . . . . . . . . . . . 36319.1.3 Laser In-situ Keratomileusis (LASIK) . . . . . . . . . . . . . . . 36419.1.4 Customized Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

19.2 Glaucoma Surgery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36719.3 Laser Angioplasty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36919.4 XeCl Excimer Laser Treatment of Psoriasis . . . . . . . . . . . . . . . . 370References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

20 High-Intensity Applications of Excimer Lasers . . . . . . . . . . . . 37320.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37320.2 Précis of Experimental Findings . . . . . . . . . . . . . . . . . . . . . . . . . . 373

20.2.1 Anomalous Nonlinear Coupling to Atoms at 193nm . . 37320.2.2 Direct Multiquantum Inner-Orbital Excitation

of N2 at 248 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37520.2.3 Hollow Atoms and the Cluster Concept . . . . . . . . . . . . . 37820.2.4 Multikilovolt X-ray Amplification with Clusters

in Self-Trapped Plasma Channels . . . . . . . . . . . . . . . . . . . 38020.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

21 High-Repetition-Rate Applications of Excimer Lasers . . . . . 38521.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38521.2 Technological Advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

21.2.1 Solid-State Pulsed High-Voltage Switch . . . . . . . . . . . . . 38621.2.2 Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . 387

Contents XIII

21.3 High-Repetition-Rate Applications . . . . . . . . . . . . . . . . . . . . . . . . 39021.3.1 Flexible Mask Patterning . . . . . . . . . . . . . . . . . . . . . . . . . . 39021.3.2 Long-Term Testing of Optical Material and Coatings . . 39421.3.3 Advanced Medical Excimer Laser Systems . . . . . . . . . . . 396

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397

22 New Frontiers: Extreme-Ultraviolet (EUV) Technologyat 13.5 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39922.1 Prospects of EUV Lithography Drive the Development

of High-Power EUV Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39922.2 Challenges of EUV Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . 40122.3 Basic Technology and Requirements for EUV Sources . . . . . . . 40422.4 Gas-Discharge-Produced

and Laser-Produced Plasma EUV Sources . . . . . . . . . . . . . . . . . 40622.4.1 Gas-Discharge-Produced Plasma EUV Sources . . . . . . . 40622.4.2 Laser-Produced Plasma EUV Sources . . . . . . . . . . . . . . . 40822.4.3 How to Characterize EUV Sources . . . . . . . . . . . . . . . . . . 409

22.5 GDPP and LPP EUV Sources – State of the Art . . . . . . . . . . . 41222.5.1 GDPP EUV Source XTS 13–35 for Integration

into EUV Microstepper . . . . . . . . . . . . . . . . . . . . . . . . . . . 41222.5.2 High-Power Gas-Discharge-Produced Plasma EUV

Sources at XTREME Technologies . . . . . . . . . . . . . . . . . . 41322.5.3 Laser-Produced Plasma EUV Sources

at XTREME technologies . . . . . . . . . . . . . . . . . . . . . . . . . 41722.6 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

List of Contributors

Stephen G. AndersonSect. 1.3Laser Focus WorldAssociate Publisher, Editor-in-chiefPennwell98 Spit Brook Rd.Nashua, NH 03062Phone: +1-603-891-9320Fax: +1-603-891-0574stevega@pennwell.com

Dr. D. BastingSects. 1.1, 1.2Lambda Physik AGNow:2200 South Ocean LaneFort LauderdaleFL 33316-3831 USADirk@Basting.net

Dr. J. BekesiSect. 14Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503528Fax: +49-551-503599jbekesi@llg.gwdg.de

Prof. H. von BergmannSects. 3, 4, 5Laser Research InstituteDept. PhysicsUniversity of StellenboschSouth Africa

Private Bag X1, Matieland, 7602Phone: +21-808-3376Fax: +21-808-3385hmvb@sun.ac.za

Dr. H. BernitzkiSect. 8Laser-Zentrum Hannover e.V.Now:Jenoptik Laser Optik SystemeGmbHGöschwitzer Str. 2507746 JenaPhone: +49-3641-65 3256Fax: +49-3641-65 3850Helmut.Bernitzki@Jenoptik.com

Dr. V. BeushausenSect. 18Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503523Fax: +49-551-503599vbeushau@llg.gwdg.de

Prof. A. B. BorisovSect. 20University of Illinois at ChicagoDepartment of PhysicsCollege of Liberal Arts and Sciences845 West Taylor StreetChicago, Illinois 60607-7059USAPhone: +1-312-996-4868Fax: +1-312-996-8824alexbor@uic.edu

XVI List of Contributors

Prof. V. M. BorisovSect. 5.2State Research Centre RussianFederation TRINITIPushkov Street142092 TroitskPhone: +7-095-334-0666Fax: +7-095-334-5775borisov@triniti.ru

Dr. K. BoyerSect. 20250 East Alameda (Apt. 812)Santa Fe, New Mexico 87501Keithxb@comcast.net

Dr. I. BraginSect. 5.2Lambda Physik AGHans-Böckler-Str 1237079 GöttingenPhone: +49-551-6938204Fax: +49-551-68691ibragin@lambdaphysik.com

J. DavisSect. 20davisj@ppdmail.nrl.navy.mil

Prof. N. DjeuSect. 1.2University of South FloridaTampa FL 33620ndjeu@chuma.cas.usf.edu

Dr. R.F. DelmdahlSect. 21Lambda Physik AGHans-Böckler-Str 1237079 GöttingenPhone: +49-551-6938397Fax: +49-551-68691rdelmdahl@lambdaphysik.com

Prof. H. ExnerSect. 12Laserinstitut Mittelsachsen e.V.Hochschule MittweidaUniversity of Applied SciencesTechnikumplatz 1709648 MittweidaPhone: +49-3727-580Fax: +49-3727-58-1379exner@laserinstitut-mittelsachsen.de

Dr. M. FiebigSect. 16.2Lambda Physik AGNow:Max-Born-Institut BerlinMax-Born-Str. 2a12489 BerlinPhone: +49-30-6392-1404Fax: +49-30-6392-1404fiebig@mbi-berlin.de

Prof. C. FotakisSect. 15Foundation for Research andTechnology-Hellas (FORTH)Institute of Electronic Structureand LaserP.B. 1527, Vasilika VoutonHeraklion, GR-711 10Crete, HellasPhone: +30-2810-391273Fax: +30-2810-391305fotakis@iesl.forth.gr

Dr. L. HerbstSect. 21Lambda Physik AGHans-Böckler-Str 1237079 GöttingenPhone: +49-551-6938404Fax: +49-551-68691lherbst@lambdaphysik.com

List of Contributors XVII

Prof. P.R. HermanSect. 13Electrical and ComputerEngineeringUniversity of Toronto10 King’s College RoadToronto, Canada M5S 3G4Phone: +1-416-978-7722Fax: +1-416-971-3020p.herman@utoronto.ca

Dr. M. HesslingSect. 11.3LINOS Photonics GmbH & Co. KGBU Bio PhotonicsIsartalstr. 4380469 MünchenPhone: +49-89-7202-539Fax: +49-89-7202-141Martin.hessling@linos.de

Dr. J. IhlemannSects. 11.2, 11.2.5, 14Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503544Fax: +49-551-503599jihle@llg.gwdg.de

Dr. K. JainSect. 1.2Anvik Corporation6 Skyline Dr.HawthorneNY 10532kjain@anvik.com

Dr. N. KaiserSect. 8Fraunhofer Institut AngewandteOptik und FeinmechanikAlbert-Einstein-Straße 707745 JenaPhone: +49-3641-807-321Fax: +49-3641-807-601norbert.kaiser@iof.fraunhofer.de

Dr. B. KeiperSect. 123D-Micromac AGAnnaberger Str. 24009125 ChemnitzPhone: +49-371-400-4334Fax: +49-371-400-4340keiper@3d-micromac.com

J.-H. Klein-WieleSect. 14Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503529Fax: +49-551-503599jhkleinw@llg.gwdg.de

Prof. A. KochSect. 9HAWK FachhochschuleHildesheim/Holzminden/GöttingenVon-Ossietzky-Str. 9937085 GöttingenPhone: +49-551-3705-100Fax: +49-551-3705-101Andrea.koch@pmf.fh-goettingen.de

Dr.-Ing. K. KörberSects. 2, 16.3, 16.5Oberer Stephansberg 38 E96049 BambergPhone: +49-951-5098493klaus.koerber@web.de

Dr. C. KulikSects. 2, 16.3Laser-Zentrum Hannover e.V.Hollerithallee 830419 HannoverPhone: +49-511-2788-270Fax: +49-511-2788-100kk@lzh.de

XVIII List of Contributors

Dr. H. LauthSect. 8Jenoptik Laser OptikSysteme GmbHGöschwitzer Str. 2507746 JenaPhone: +49-3641-65 3188Fax: +49-3641-65 3764Hans.Lauth@Jenoptik.com

Dr. H. LubatschowskiSect. 19Laser-Zentrum Hannover e.V.Hollerithallee 830419 HannoverPhone: +49-511-2788-279Fax: +49-511-2788-100hl@lzh.de

Prof. G. MarowskySect. 1.1Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503530Fax: +49-551-503599gmarows@gwdg.de

Dr. K. MannSect. 7Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503541Fax: +49-551-503599kmann@llg.gwdg.de

Dr. B. NikolausSect. 21Lambda Physik AGHans-Böckler-Str 1237079 GöttingenPhone: +49-551-6938157Fax: +49-551-68691bnikolaus@lambdaphysik.com

Dr. P. OesterlinSect. 9Innovavent GmbHBertha-von-Suttner-Str. 537085 GöttingenPhone: +49-551-90047-10Fax: +49-551-90047-19oesterlin@innovavent.com

R. PätzelSect. 6Lambda Physik AGHans-Böckler-Str 1237079 GöttingenPhone: +49-551-6938155Fax: +49-551-68691rpaetzel@lambdaphysik.com

Dr. D. G. PapazoglouSect. 15Foundation for Research andTechnology-Hellas (FORTH)Institute of Electronic Structureand LaserP.B. 1527, Vasilika VoutonHeraklion, GR-711 10Crete, HellasPhone: +30-2810-391273Fax: +30-2810-391305dpapa@iesl.forth.gr

Dr. T. PetschSect. 123D-Micromac AGAnnaberger Str. 24009125 ChemnitzPhone: +49-371-400-430Fax: +49-371-400-4340petsch@3dmicromac.com

Dr. U. RebhanSect. 4Lambda Physik AGHans-Böckler-Str 1237079 Göttingen

List of Contributors XIX

Phone: +49-551-6938-221Fax: +49-551-68691urebhan@lambdaphysik.com

Prof. G. ReißeSect. 17Hochschule MittweidaUniversity of Applied SciencesTechnikumplatz 1709648 MittweidaPhone: +49-3727-580Fax: +49-3727-58-1379greisse@htwm.de

Prof. Ch.K. RhodesSect. 20University of Illinois at ChicagoDepartment of PhysicsCollege of Liberal Arts and Sciences845 West Taylor StreetChicago, Illinois 60607-7059USAPhone: +1-312-996-4868Fax: +1-312-996-8824rhodes@uic.edu

Dr. P. SimonSect. 14Laser-Laboratorium Göttingen e.V.Hans-Adolf-Krebs-Weg 137077 GöttingenPhone: +49-551-503521Fax: +49-551-503599psimon@llg.gwdg.de

Dr. U. StammSects. 3, 4, 6, 22Xtreme technologies GmbHHans-Adolf-Krebs-Weg 1

37077 GöttingenPhone: +49-551-82073-101Fax: +49-551-82073-110ustamm@xtremetec.de

Dr. M. WehnerSects. 10, 11.1, 11.2, 11.2.1–11.2.4,11.2.6–11.2.7, 16.1, 16.4Fraunhofer-Institut LasertechnikSteinbachstr. 1552074 AachenPhone: +49-241-8906-202Fax: +49-241-8906-121wehner@ilt.fhg.de

Dr. S. WeißmantelSect. 17Laserinstitut Mittelsachsen e.V.Technikumplatz 1709648 MittweidaPhone: +49-3727-581396Fax: +49-3727-581449steffen@htwm.de

Dr. I. ZergiotiSect. 15Foundation for Research andTechnology-Hellas (FORTH)Now:National Technical Universityof AthensPhysics DepartmentIroon Polytehneiou 915780 ZografouAthens, GreecePhone: +30-210-772-3345Fax: +30-210-772-2928zergioti@central.ntua.gr

1 Introduction

1.1 Introductory Remarks

D. Basting and G. Marowsky

In 1984, Ch. K. Rhodes stated in the introduction to the world’s first-ever book on excimer lasers [1]: “The development of excimer laser systemsmarked a significant turning point in the development of coherent sources.The progress of the last years has been largely predicated upon the combinedknowledge of several disciplines including atomic and molecular physics, op-tical technology, and pulsed-power technology”. This early statement wasprimarily associated with electron beam excitation of laser transitions of raregases and rare-gas halogen mixtures. Excimer lasers of today, however, arebased upon precisely controlled electrical discharges, and rely on detailedknowledge of material chemistry to provide long lifetimes of the dischargetubes. Hence, cost-effective operation has become possible, a prerequisite forindustrial applications. Together with electronic laser beam monitoring andcontrol, the multidisciplinary approach has made excimer lasers importanttools for a great variety of industrial and medical applications.

1.1.1 The Unique Microstructuring Capabilitiesof Excimer Lasers

The properties relevant for microstructural applications of excimer lasers aretheir short wavelengths in the UV and deep UV, high pulse energy, and highaverage power. Each feature enables specific applications. Common to all ap-plications: They benefit from (i) the short wavelengths that allow high pre-cision imaging, (ii) the inherent high quantum energy that provides a strongradiation-matter interaction, and (iii) from both simultaneously. With thehigh peak intensity available, strong non-linear interactions may be achievedand utilized for highly precise material processing, like for example, semicon-ductor exposure, material microstructuring, and modeling of the refractivepower of the human eye to compensate for myopia. An outstanding exam-ple of materials processing is the formation of grating structures possessingsub-micrometer dimensions. Figure 1.1 shows the morphology of a periodicstructure created by superposition of two plane waves from a KrF excimer

2 1 Introduction

laser at 248 nm wavelength on the surface of a LiNbO3 crystal, a result takenfrom Chen et al. [2]. The authors report on a surface modulation of 80nmand a grating period of 360nm achieved with a single ablation pulse. Theablation depth could be increased in 10nm steps upon application of multiplepulses. Figure 1.2 shows for comparison a very recent result of a more com-plicated nanostructure of similar dimensions obtained by phase-controlledsuperposition of two pairs of plane waves – details in Ref. [3].

Fig. 1.1. Periodic nanostructure on a LiNbO3-crystal obtained by superpositionof two plane waves from a KrF excimer laser according to Ref. [2]

Fig. 1.2. Periodic nanostructure by phase-controlled multiple-beam interferenceaccording to Ref. [3]

1.1 Introductory Remarks 3

1.1.2 Commercial Impact of Excimer Lasers

Before getting into the technical and physical details in this book, let usbriefly address some aspects of the economics of excimer lasers. Due totheir potential for various applications, excimer lasers have gained consider-able commercial importance. The position of excimer lasers within the totallaser market may be assessed by inspecting two diagrams: Figure 1.3 dis-plays worldwide revenues obtained with all lasers, split into diode lasers andnon-diode lasers. The overwhelming part of the diode category shown in thediagram belongs to optical telecommunications and, hence, reflects the well-known uncertainties of this market. In contrast, the non-diode sectors expe-rienced considerably smaller fluctuations because they share many differentmarkets. Compared to the total world laser market with multi-billion dollarrevenues, the world-wide excimer laser market (Fig. 1.4) occupies a scale ofactivity about one order-of-magnitude smaller. A more detailed analysis in-corporating the three main application areas namely industrial, medical, andscientific reveals that the industrial and the medical markets were responsi-ble for the ups and downs, whereas the scientific market shows a persistentdecline. The latter is not surprising because the above mentioned features ofexcimer laser radiation are no longer of major importance to scientific ap-plications like spectroscopy or have been replaced by alternative UV sourcessuch as frequency-converted solid state lasers. However, in the earlier years,the scientific segment made up a considerable percentage of the total excimerlaser market.

Fig. 1.3. World laser market 1996 to 2003: Diode and non-diode laser sales

4 1 Introduction

Market fluctuations stand out primarily in the industrial sector, which ismainly governed by (a) the semiconductor market, and (b) the medical partwhich is dominated by fluctuations in the sales of lasers for ophthalmologicalapplications.

Fig. 1.4. Excimer laser sales between 1993 and 2003 and their distribution accord-ing to their main fields of application – industry, medicine, and research

It is interesting to monitor the market position of excimer lasers as com-pared to all other non-diode lasers, as shown in Fig. 1.5. In 2002, excimerlasers made up 21.7% of all laser sources used in materials processing and26.5% of all laser sources used for medical therapeutic applications [4]. Mate-rials processing includes all activities with lasers for processing such as weld-ing, cutting, annealing, drilling, semiconductor, and microelectronics manu-facturing, and the marking of all sorts of materials.

Medical therapeutics includes all laser applications such as ophthalmol-ogy; examples are refractive surgery, where excimer lasers play a major role,and photocoagulation. In addition a strong role is played in general surgery,therapeutics, imaging, and cosmetic applications.

An overview of all laser applications, with the inclusion of these two majorapplications of excimer lasers, is shown in the following table. Although thistable was already presented in the former edition of this book [5], it remainsinformative because it contains relative numbers only, which are not subjectto great changes. This table summarizes laser systems (second column) andthe relative laser sources (third column) sales within the various applicationswhich have been collected into two groups. It turns out that Group I, whichactually contains materials processing and medical applications, covers only12% of the worldwide sales activity of laser systems as compared to the ma-jority contribution of 88% for those applications shown in Group II. It should,therefore, be pointed out that materials processing and medical therapeuticapplications are actually minor, but very important activities in the large listof worldwide laser applications.

1.1 Introductory Remarks 5

Fig. 1.5. Market position of excimer lasers compared to all other non-diode lasersaccording to Ref. [4]

The fourth column presents, for completeness, the ratio of the sales pricesof the built-in laser source versus total system price for the various appli-cations. The percent values in this column indicate that the laser sourcesplay a much bigger role in Group I than in Group II, so the applicationsmaterials processing with 32% and medical therapeutics with 33% are play-ing the most important role for the light source – and, in fact, this is thepart where excimer lasers come in. On average per piece, a laser system istypically a factor 20 more expensive than its source. The two main fields ofexcimer laser applications (materials processing and medical therapeutics)

Table 1.1. Global distribution (in percent of the actual total sales price) of lasersand laser systems according to applications. The averages in the ratio column referto weighted averages. Details see text.

6 1 Introduction

are the exception: Here the price of the laser source is typically nearly onethird of the system price. Hence, cost effectiveness of excimer lasers becomesan important issue for applications in industry.

In addition to the use of excimer lasers in materials processing, there existsa large variety of other applications like chemical surface alteration, heating,annealing, vaporization, laser-assisted chemistry, surface cleaning, semicon-ductor processing, and communications; applications that will be discussedin detail in subsequent chapters of section 3. As to the excimer laser impacton markets: semiconductor processing is considered as the largest-revenueand fastest growing market for excimer laser applications. In optical commu-nications, the excimer laser is of particular importance in the production ofoptical fiber Bragg gratings, obviously a special case of nondestructive andnonintrusive materials processing.

1.1.3 Could Excimers Have Been the First Mediawith Optical Gain?

It is tempting to speculate that excimers might have been the first mediashowing optical gain. As early as 1926, F. G. Houtermans considered in [6, 7]the idea of energy storage and deactivation by photons in experiments withmercury dimers – work done in his thesis with James Franck as advisorin Göttingen, Germany. He considered discharge excitation of mercury andpostulated the existence of metastable dimers from spectroscopic studies.Metastable gaseous compounds are in fact excellent candidates for laser op-eration. In 1960, after the maser had been discovered, Houtermans proposedexcimers again as candidates for light amplification, just before the laser wasfirst realized by Maiman in a solid state material – ruby. At that time –i.e. 34 years later – he was still convinced by his early thesis studies andproposed mercury dimers in the famous Helvetica Physica Acta paper [8]as potential candidates. He formulated the idea of avalanche amplificationas a result of continuous stimulated emission in a mercury discharge systemin this paper. Mercury had no future compared to the reasonable amplifica-tion by metastable gaseous compounds. Rare-gas dimers and rare-gas-halideexciplexes were the winners as gaseous laser media.

1.1.4 Literature

The literature on excimer lasers is widespread: Since the 2nd edition of Ch. K.Rhodes’ book on this subject [1], no other comprehensive textbook has ap-peared. Lots of books exist on excimer-laser-related applications such as mi-crostructuring or ablation. We have listed these books in chronological orderat the end of this introductory chapter [9, 10, 11, 12, 13, 14, 15, 16, 17].The relevant references to each sub-chapter are added at the end of eachcontribution in this book.

References 7

References

1. C.K. Rhodes (Ed.): Excimer Lasers – Topics in Applied Physics, Vol. 30, 2ndedn. (Springer-Verlag, Berlin, Heidelberg, New York, Tokyo, 1984)

2. K. Chen, J. Ihlemann, P. Simon, I. Baumann, W. Sohler: Appl. Phys. A 65,517 (1997)

3. J. Klein-Wiele, P. Simon: Appl. Phys. Lett. 83, 4707–4709 (2003)4. S.G. Anderson et al.: Laser Focus World (January issues) (1994–2004)5. D. Basting (Ed.): Excimer Laser Technology: Laser Sources, Optics, Systems

and Applications (Lambda Physik AG, Göttingen, 2001)6. F. Houtermans: Zeitschrift für Physik 41, 140–154 (1927). Julius Springer-

Verlag, Berlin7. F. Houtermans: Zeitschrift für Physik 41, 619–635 (1927). Julius Springer-

Verlag, Berlin8. F. Houtermans: Helv. Phys. Acta 33, 933–940 (1960)9. H. Stafast, G. Herziger, H. Weber (Eds.): Angewandte Laserchemie – Verfahren

und Anwendungen (Springer-Verlag, Berlin, Heidelberg, New York, 1993)10. R. Crafer, P. Oakley (Eds.): Laser Processing in Manufacturing (Chapman &

Hall, London, Glasgow, New York, Tokyo, Melbourne, Madras, 1993)11. J. Miller (Ed.): Laser Ablation – Springer Series in Materials Science, Vol. 28

(Springer-Verlag, Berlin, Heidelberg, New York, 1994)12. D. Bäuerle: Laser Processing and Chemistry , 2nd edn. (Springer-Verlag, Berlin,

Heidelberg, New York, 1995)13. D. Elliott: Ultraviolet Laser Technology and Applications (Academic Press,

London, 1995)14. W. Duley: UV Lasers - Effects and Applications in Materials Science (Cam-

bridge University Press, Cambridge, 1996)15. R. Russo, D. Geohegan, R.H. (Jr.), K. Murakami (Eds.): Laser Ablation –

Proc. of Fourth Int. Conference on Laser Ablation, Monterey 1997 (Elsevier,Amsterdam, 1998)

16. J. Horwitz, H. Krebs, K. Murakami, M. Stuke: “Laser ablation”, in Proc. ofthe 5th International Conference on Laser Ablation (Applied Physics A (1999),Materials, Science & Processing, Göttingen, 1999)

17. U. Stamm: J. Phys. D: Appl. Phys. 37, 3244–3253 (2004)

8 1 Introduction

1.2 Historical Review of Excimer Laser Development

D. Basting, N. Djeu, and K. Jain

When J. W. Ritter discovered the ultraviolet light in the spectrum of the sunin Jena, Germany in 1801 [1], he could not expect that 200 years later ultra-violet (UV) light has found a wide variety of applications in photochemistryand material processing. The main reason for this development is that at thevery short UV wavelengths almost all substances show significant absorp-tion. Historically, this made photochemists ask for powerful UV sources toinvestigate molecules and compounds under strong UV excitation. Soon afterthe invention of the laser in 1960, nitrogen lasers were capable to generatelaser pulses with megawatts peak power, thus opening new photochemical re-search areas. In 1970, another revolution in UV photochemistry started withthe first experimental demonstration of excimer lasers [2].

The name excimer comes from excited dimer. The first experimentalevidence of excimer lasing was obtained by N. G. Basov et. al in 1970. Theyused a high-current electron beam to excite liquid Xe [2]. With Xe2∗ emittingaround 172nm excimer lasers were invented as a new class of lasers. It isinteresting to note that excimer lasers of today (“exciplex” lasers, excitedcomplexes) were discovered basically within the short time frame of one yearafter the investigation of fluorescence spectra of their active molecules, therare-gas monohalides. Having actively taken part in the further development,we present here our view of the rapid growth of excimer laser technology.

Fluorescence spectra of rare-gas halides were investigated almost at thesame time, i.e. in 1974, by several groups at University of Cambridge, Cam-bridge, UK [3], at Kansas State University, Kansas, USA [4], and at the AvcoEverett Research Laboratory, Everett, Massachusetts, USA [5]. The first laseraction of exciplexes was then reported in 1975, again almost simultaneouslyby several research teams at Naval Research Laboratory, Washington, USA[6], Northrop Research and Technology Center, Hawthorne, USA [7], at AvcoEverett Research Laboratory, Everett, Massachusetts, USA [8, 9, 10], and atSandia Laboratories, Albuquerque, USA [11]. At the end of 1975, excimerlasers with all important UV wavelengths had been experimentally demon-strated.

One of the authors of this chapter, N. Djeu, at U.S. Naval Research Lab-oratory in Washington D.C, was among the first scientists to work in thefield since the mid’s 70’s. The Navy had an interest in developing efficient,powerful blue-green lasers for a variety of underwater applications. When therare-gas halide (RGH) lasers were discovered, it appeared they would fit thebill, since they were both efficient and energetic, and it was suspected thatdown-conversion from the UV to the blue-green would not be a serious chal-lenge. However, those early RGH excimer lasers were pumped by electronbeams. These machines were bulky and had excruciatingly slow pulse repe-tition rates. Worse yet, the foil that separated the laser gas from the e-beam

1.2 Historical Review of Excimer Laser Development 9

generator ruptured with great regularity. At that time, the transverse-electric-discharge-pumped atmospheric (TEA) CO2 laser was already an establishedtechnology, suggesting discharge excitation also for excimer lasers. However,halogen-containing gases, with their tendency for electron attachment, ap-peared to be a stumbling block at that time. Djeu enlisted the help of RalphBurnham (an NRL contractor) and decided to attempt discharge pumpingof RGH excimers. For the initial try they used a pair of band saw blades aselectrodes. To the great delight of the researchers, lasing in XeF was obtainedwithout much effort. Although the discharge was erratic, on the best shotsan overall efficiency of 0.1% was realized [12].

Encouraged by those results, they went on to build a more elaborate(though still primitive) device. This second laser made use of polished nickel-plated electrodes, and, more importantly, incorporated an UV preionizationsource to initialize the electric discharge. The resulting improvements in thelaser’s performance were dramatic. An output of 100 mJ with an overallefficiency of 1% was obtained for XeF. Also, discharge pumping of the KrFlaser was demonstrated for the first time [13].

Their work on excimer lasers might have ended, if Charlie Chase fromTachisto had not visited them shortly afterwards. Sensing his interest in de-veloping a commercial excimer laser, Djeu persuaded him to provide themwith a Tachisto CO2 laser to see how it would work with the excimers. Realiz-ing the importance of preionization, they modified the Tachisto laser to havea variable delay between the preionizer and the main discharge. With thisdevice, efficiencies of better than 1% were achieved in XeF and KrF as wellas ArF [14]. Within a short time, Tachisto introduced the excimer version ofits TEA laser. However, as is often the case, being the first to market didnot guarantee long-term success. Tachisto eventually went out of business asmore agile companies came on the scene.

The main difficulty in developing commercial excimer lasers arose fromthe lack of combined technologies, for example the handling of the stronglyaggressive gas in the laser. Pretty fast, the German start-up companyLambda Physik developed and manufactured its first commercial excimerlaser – the EMG 500, the first commercial multigas laser – which was in-troduced into the market in 1977. Figure 1.6 shows a photograph of theEMG 500. That laser could be operated at wavelengths 193 nm, 222 nm,248 nm, 282nm, 308nm, 337nm (as nitrogen laser), 351nm, 427nm (as ni-trogen laser), and 713nm, with repetition rates selectable between 0.05 Hz(i.e. 1 pulse in 20 seconds) and 20Hz. The laser allowed applications in basicresearch, with an until that time unachievable peak power up to 10MW (!).Its pulse energy at 248nm exceeded 220 mJ , almost two orders of magnitudehigher than the energy obtained from the UV nitrogen laser. In 1977 about5 lasers were shipped to the first customers, some of them still being in usetoday.

10 1 Introduction

Fig. 1.6. The first commercial multigas excimer laser EMG 500. Pulse energy of220 mJ @ 248 nm, repetition rate between 0.05 Hz and 20 Hz.

Fig. 1.7. Excimer laser EMG 103 MSC without cover showing the main compo-nents including high-voltage circuit, pulsed power modules with thyratron, laserchamber, and cooling circuit

To illustrate the main modules and technologies involved in excimer lasers,Fig. 1.7 shows the basic components of the model EMG 103 MSC, introducedin 1983. The following technologies and components were necessary:

1.2 Historical Review of Excimer Laser Development 11

i as the heart of the laser, a fast high-voltage and pulsed-power technologyincluding thyratron switching, main gas discharge, and pre-ionization,

ii fluorine and/or chlorine resistive materials and their mounting technologyfor the vacuum tight and high pressure resistant excimer laser chamber,

iii optical components like sealing UV transparent windows or resonatormirrors used in contact with the corrosive laser gases,

iv cooling technology for the powerful high-voltage excitation module andexcimer laser tube,

v gas fill circuit technology for the fluorine/chlorine gases and the prepa-ration and handling of high purity laser gases.

At Lambda Physik, pulsed transverse-excitation nitrogen lasers had beendeveloped earlier for UV generation. This technology was adapted to therequirements of excimer lasers. In addition pre-ionization was developed toensure a uniform large-volume discharge. Since both company founders, DirkBasting and Bernd Steyer, were chemists they were used to work also withaggressive chemicals. Located in Göttingen, Germany, they had close con-tact to scientists both at the Max-Planck-Institutes and the University ofGöttingen. For example, fluorine was not available on the market with therequired purity and in sufficient quantities. So the support by Prof. OskarGlemser from University of Göttingen, one of the worldwide experts in fluo-rine chemistry, did accelerate the development of excimer lasers significantly.Lambda Physik had started its business with nitrogen lasers for dye laserpumping as spin-off from Prof. Fritz Schäfer’s laboratory at the Max-PlanckInstitute for Biophysical Chemistry. Thus, the multi-disciplinary know-howavailable at Lambda Physik and in the area of Göttingen facilitated the fastdevelopment of the excimer laser, providing an advantage over the Tachistoapproach based on CO2 laser technology.

With respect to possible applications, there were different requirementsfor the ideal ultraviolet laser source. Some applications need high pulse en-ergy or variable pulse energy, and good pulse-to-pulse energy stability. Otherapplications demand for high repetition rate and/or high average power. Ho-mogeneous near field and/or far field beam profile and beam profile stabilityare always necessary. Other applications require high brightness, i.e. low-divergence short laser pulses of nanosecond duration or less, down to thefemtosecond time scale. Some applications need narrow spectral bandwidths.Common to all applications, the lasers should ideally be maintenance freewith a long lifetime of all components. Also, buying cost and cost of opera-tion should be low and installation requirements simple – for example watercooling should be avoided.

Soon after the introduction of the EMG 500 fast progress improved ex-cimer laser performance. The automatic synchronization of pre-ionizationand main gas discharge led to better output energy stability. The develop-ment of gas processors that purified the laser gas resulted in increased lasergas lifetime. This resulted in reduced operating cost and increased mainte-

12 1 Introduction

nance intervals. The incorporation of magnetic assist (Questek trademark)and switch control (Lambda Physik trademark) increased the thyratron life-time, discharge electrode lifetime, and gas lifetime by an order of magni-tude. With the magnetic switch control technique (MSC) the excimer lasersEMG 100/200 MSC could be run under typical operation conditions in re-search environment for several years without change of thyratrons or the laserchamber. For the first time, the available UV laser power exceeded the 100Wlevel. The gas lifetime was increased by more than an order of magnitude; onSeptember 16, 1983, scientists switched off an EMG 103 MSC after 110 mil-lion shots of continuous operation with a single gas fill of XeCl - a worldrecord of gas lifetime (a gas lifetime of 100 million shots is comparable tothe gas lifetime requirements for today’s lithography lasers in semiconductorchip manufacturing).

In addition to the direct use of the excimer lasers in photochemistry,material research, and micromachining of materials, a large application ofexcimer laser arose in pumping dye lasers. Early work by F. P. Schäfer, firstat University of Marburg and then at Max-Planck Institute of BiophysicalChemistry in Göttingen [15, 16, 17, 18], and P. P. Sorokin at IBM [19, 20, 21]had demonstrated lasing of organic dyes. Until 1976, using nitrogen laser ex-citation, hundreds of organic dyes had been investigated and demonstratedlasing at almost any wavelength across the visible and near UV spectrum[22, 23, 24]. With orders-of-magnitude higher power and energy, XeCl ex-cimer lasers at 308 nm wavelength were the ideal excitation sources for dyelasers. In 1978 the first excimer-pumped tunable narrow-bandwidth dye laser– the “Farbstoff-Laser” FL 1000 was developed at Lambda Physik. With theexcimer-laser-pumped dye laser FL 2000 (Fig. 1.8) and frequency doublingcrystals any laser wavelength between 200 nm and 1000nm was available forhigh-resolution spectroscopy [25, 26], for photochemists a dream of experi-mental equipment. Thus dye laser pumping became one of the driving forcesfor the development of the excimer laser market.

In 1984 an oscillator-amplifier configuration of excimer lasers with unsta-ble resonator was commercialized. The principle of the EMG 150 was adaptedfrom solid-state lasers, which had been operated with amplifiers over manyyears. The resonator of the excimer laser oscillator contained apertures thatreduced the beam divergence from several mrad to about 100µrad. The beamfrom the oscillator was injected into the excimer amplifier equipped withan unstable resonator. The injection seeding of the high-power amplifier bythe low divergence beam from the oscillator resulted in orders-of-magnitudehigher brightness, lowest beam divergence, and extremely good focusabilityof the laser pulses at energy levels in the several 100mJ range. In a specialoscillator configuration with bandwidth-narrowing resonator, the EMG 150delivered high-power UV pulses with less than 3 pm spectral bandwidth –later used to investigate the ozone hole [27, 28], and an early predecessor ofthe lasers for deep-UV photolithography.

1.2 Historical Review of Excimer Laser Development 13

Fig. 1.8. Excimer-laser-pumped dye laser FL 2000 from Lambda Physik in 1981

The availability of oscillator-amplifier excimer laser systems led to the suc-cessful demonstration of ultraviolet laser systems operating in the picosecondand femtosecond time scale. In 1982, by amplification of ps light pulses from asynchronously pumped cavity-dumped dye laser and their frequency conver-sion into the ultraviolet, seed pulses for excimer amplifiers became available.After amplification in XeCl excimer modules [29] and ArF excimer modules[30], ultraviolet pulses with 40mJ energy and several picoseconds pulse dura-tion could be generated. For the first time intense ultraviolet pulses with peakpowers reaching above 10 GW could be used for fundamental studies of theinteraction with matter. While these new UV ps light sources were extremelycomplicated and very expensive laser systems, in 1983 Szatmári and Schäfersucceeded in a considerably simplified approach based on the combination ofan EMG 150 oscillator-amplifier with quenched and distributed feedback dyelasers as well as dye amplifiers [31]. A few years later the continued researchefforts resulted in the availability of 150 fs laser pulses with 45 mJ , yield-ing 300GW peak power [32] and new ideas for the experimental approachto develop X-ray laser sources with pulse durations in the attosecond range[33].

During the 80’s, in order to achieve highest energies from excimer lasers,various approaches for pre-ionizing and exciting the laser gas mixtures wereinvestigated as X-ray pre-ionization [34, 35], creeping discharge pre-ionization[36] and electron beam pumping [37]. With those methods pulse energiesof several 10 J up to kJ could be obtained. The intense UV pulses havebeen applied in fusion research, plasma generation, and material processing.However, while SOPRA and Lambda Physik were trying to commercialize