Laser Microfabrication. Thin Film Processes and Lithography PDF
Preview Laser Microfabrication. Thin Film Processes and Lithography
L A S ER M I C R O F A B R I C A T I ON Thin Film Processes and Lithography Edited by Daniel J. Ehrlich Lincoln Laboratory Massachusetts Institute of Technology Lexington, Massachusetts Jeffrey Y. Tsao Sandia National Laboratories Albuquerque, New Mexico ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers Boston San Diego New York Berkeley London Sydney Tokyo Toronto Copyright © 1989 by Academic Press, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permisssion in writing from the publisher. ACADEMIC PRESS, INC. 1250 Sixth Avenue, San Diego, CA 92101 United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. 24-28 Oval Road, London NW1 7DX Designed by Joni Hopkins Library of Congress Cataloging-in-Publication Data Laser microfabrication : thin film processes and lithography / edited by Daniel J. Ehrlich, Jeffrey Y. Tsao. p. cm. Includes bibliographies and index. ISBN 0-12-233430-2 1. Lasers—Industrial applications. 2. Thin film devices—Design and construction. 3. Microlithography. I. Ehrlich, Daniel J. II. Tsao, Jeffrey Y. TA1677.L366 1989 621.36'6-<ic 19 88-24215 CIP Printed in the United States of America 89 90 91 92 9 8 7 6 5 4 3 21 CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors' contributions begin. C.I.H. Ashby (231), Organization 1126, Sandia National Laboratories, PO Box 5800, Albuquerque, New Mexico 87185 M.R. Aylett (453), British Telecom Research Laboratories, Martlesham Heath, Ipswich, United Kingdom T.H. Baum (385), IBM-Almaden Research Center K91/802, 650 Harry Road, San Jose, California 95120-6099 Ian W. Boyd (539), Department of Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom T.J. Chuang (87), IBM-Almaden Research Center K33/801, 650 Harry Road, San Jose, California 95120-6099 P.B. Comita (385), IBM-Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 DJ. Ehrlich (285, 385), Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood Street, PO Box 73, Lexington, Massachusetts 02173 J. Haigh (453), British Telecom Research Laboratories, Martlesham Heath, Ipswich, United Kingdom S.J.C. Irvine (503), Royal Signals and Radar Establishment, St. Andrews Road, Great Malvern, Worcester WR14 3PS, United Kingdom R.L. Jackson (385), IBM-Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 ix χ CONTRIBUTORS T.T. Kodas (385), IBM-Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Y.S. Liu (1), General Electric Research and Development Center, Building KW-B1307, PO Box 8, Schenectady, New York 12301 John J. Ritsko (333), IBM-TJ. Watson Research Center, Mail Stop 39-144, PO Box 218, Yorktown Heights, New York 10598 M. Rothschild (163), Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood Street, PO Box 73, Lexington, Massachusetts 02173 J.Y. Tsao (231, 285), Organization 1141, Sandia National Laboratories, PO Box 5800, Albuquerque, New Mexico 87185 H.J. Zeiger (285), Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood Street, PO Box 73, Lexington, Massachusetts 02173 PREFACE Since the initial studies at the beginning of this decade, interest in laser microfabrication processes, especially those based on laser-stimulated thin-film chemistry, has undergone an explosive and sustained growth. An intriguing aspect of this growth is that it was (and continues to be) generated by scientists and engineers from a wide range of fields and disciplines, among them quantum electronics, surface science, materials science, chemical physics, and microelectronics engineering. Indeed, it is perhaps because of the interdisciplinary nature of laser microfabrication that it has grown so rapidly. From an applications point of view, selective and (in many cases) non-thermal chemistry, accompanied by micrometer spatial confinement, was seen to have wide potential for microelectronics. An entirely new set of technologies was spawned, based on laser direct-write "microchem- istry." From a basic science point of view, there has been interest in developing a fundamental understanding of, inter alia, energy transfer between adsorbates and surfaces, surface electromagnetism, surface photochemistry, catalysis, and chemical kinetics in small reaction zones. Our intent is that this volume will serve three distinct functions. First, we review in a convenient format the fundamental scientific knowledge on laser-stimulated surface chemistry, which has accumulated over the past few years. It can thus serve as a starting point for students and researchers to learn about the field. Second, we provide a capsule summary of the current state-of-the-art in the technology of these processes. It should thus be useful to applications engineers as well. Third, we provide up-to-date reference data (photodissociation cross- sections, thermochemical constants, etc.) compiled from many scattered sources. It should therefore be useful to the active researcher in the field. We are indebted to numerous colleagues throughout the years for discussions and collaborations. We especially thank Carol Ashby, Jerry Black, Steve Brueck, Tom Deutsch, Yung Liu, Alan McWhorter, Bob Mountain, Rick Osgood, Richard Reynolds, Mordy Rothschild, Jan Sedlacek, Don Silversmith and Howard Schlossberg. Many thanks also to the contributors to this volume! xi xii PREFACE Finally, we dedicate this volume to our (extended) families, with love and affection. J.Y. Tsao D.J. Ehrlich Result from the first day's experiments. Iodine, photodeposited from CFI using a 257-nm 3 laser beam that had passed around a vertical wire (note the fringed far-field diffraction pattern). Laser was on loan from H. Schlossberg (then at RADC) and CFI was on loan 3 from T.F. Deutsch, both deflected for the afternoon from fusion laser studies (D.J. Ehrlich, unpublished). CHAPTER 1 Sources, Optics, and Laser Microfabrication Systems for Direct Writing and Projection Lithography Y.S. LIU GE Research and Development Center Schenectady, New York 1. Introduction 4 2. Coherent and Incoherent Sources 5 2.1. Coherent Sources 5 2.2. Ion Lasers 8 2.3. Solid-State Lasers 9 2.4. Slab-Geometry Solid-State Lasers 11 2.5. Excimer Lasers 13 2.6. Nonlinear Optical Techniques 17 2.7. Incoherent UV Sources 24 3. Optical Considerations for Direct Writing 26 3.1. Resolution 27 3.2. Writing Speed 29 3.3. Gaussian Beam Propagation 32 3.4. Transformation of a Gaussian Beam 34 3.5. Beam Shaping 36 3.6. Beam Homogenization 42 3.7. Beam Profile Measurements 43 3.8. Beam Scanning 45 3.9. Laser Direct-Write Systems 51 4. Laser Projection Optics 58 4.1. Resolution and Depth of Focus 59 4.2. Linear Systems and Coherence 60 4.3. Modulation Transfer Function 62 4.4. Spatial Coherence 64 4.5. Spectral Bandwidth Considerations 67 4.6. Excimer Laser Projection Systems 69 4.7. Practical Constraints 73 5. Conclusion 74 Acknowledgements 76 References 76 Laser Microfabrication 3 Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved. 0-12-233430-2 4 Y.S. LIU 1. Introduction Since the first coherent laser radiation from ruby was demonstrated in 1960, coherent radiation has been observed from the UV to the far-IR from literally thousands of materials. During this period, lasers have been used in various material processing and device fabrication applications (Von Allmen, 1987, Bauerle, 1986). The recent increasing interest in laser processing has been driven by future needs for application specific integrated circuits designs, circuitry restructuring, and yield enhance ments, all of which can benefit from an adaptive processing technology such as laser processing for a higher degree of precision, resolution, and process automation, and for quick turnaround (Smart, et al., 1987). Similar to other particle-beam technologies, e.g., ion beam and electron beam, a laser is either used as a focused beam to deliver photon energy onto a small area or as a broad beam for larger area processing. Unlike other beam processing technologies, however, lasers are not required to operate in a vacuum, thus providing greater flexibility in beam delivery and transporting. In-situ process monitoring, diagnostics, and controlling are additional capabilities in laser processing (Boyd, 1987). The purpose of this chapter is to give readers the essential knowledge on optical considerations useful for laser microfabrication as well as to review several state-of-the art laser microfabrication systems and some instrumental design parameters. The organization of this chapter follows. In the next section, we review a selected number of coherent and incoherent sources useful for laser microprocessing. In addition, some useful nonlinear optical techniques such as harmonic generation and stimulated Raman scattering for visible and UV generation are discussed. In Section 3, we discuss optical considerations relevant to focused laser-beam technology, in which a scanning visible or UV laser, such as argon-ion and krypton-ion lasers, or high-repetition-rate, pulsed solid- state YAG lasers are frequently employed. Parameters such as spatial mode structures, Gaussian beam propagation, focusing, depth of focus, collimation and beam shaping, modulation, and monitoring are dis cussed. Several state-of-the-art laser direct-write systems for interconnec tion are reviewed. In Section 4, we discuss broad-beam laser projection technology in which an excimer laser source is typically used. In addition to the source (excimer laser) properties, we review important considerations such as image formation, coherent effects, resolution, modulation transfer func tion, depth of field, as well as other practical constraints such as source life, cost, and reliability. Several recently developed excimer-laser-based microfabrication systems are discussed. 1. MICROFABRICATION SYSTEMS 5 2. Coherent and Incoherent Sources 2.1. Coherent Sources A laser device generally consists of two components: an active medium that produces optical gain under proper pumping conditions and an optical resonator that consists of a pair of mirrors with specific radii of curvature and reflectivities to provide optical feedback and output coupling (Yariv, 1975; Svelto, 1982; Siegman, 1971). The laser medium can be pumped by various excitation mechanisms such as gas discharge, optical pumping (coherently or incoherently), or electrical current to produce a population inversion. Several selected laser sources useful for microfabrication are given in Table 1.1. The laser transitions in a gas laser involve electronic, vibrational, and rotational transitions in atoms, molecules, or ions (e.g., argon laser) and are commonly pumped with electrical discharges (DC, RF, or pulsed). Table 1.1 General Characteristics of a Selected Number of Laser Sources Useful for Laser Processing. Emission Pulse Pulse Beam Beam Wavelength Power Energy Width Diameter Divergence Laser (jam) (W) (J) (ns) (mm) (mrad) CW LASERS Argon (UV) 0.35-0.65 4 na na 1.5 0.4 (Visible) 0.45-0.53 20 na na 1.5 0.6 Krypton 0.33-0.8 4 na na 2 0.5 He-Ne 0.63 0.01 na na 1.5 0.5 GaAs 0.82 0.2 na na 0.1 x 0.001 100 x 350 Nd: YAG 1.06 400 na na 8 15 9-11 1000 na na 10 2 co2 PULSED LASERS Excimer Lasers ArF 0.193 50 0.4 20 20 x 10 2x3 KrF 0.248 100 1 20 20 x 10 2x3 XeCl 0.308 50 1 20 20 x 10 2x3 XeF 0.351 50 0.5 20 20 x 10 2x3 Argon 0.45-0.51 5 10"8 20 1.2 0.7 Copper 0.51-0.58 20 0.005 20 20 5 Nd: YAG 1.06 100 0.2 30 10 2 Nd: Glass 1.06 100 10 50 10 4 9-11 1000 1 200 10 2 co2 The numbers used here are representative and do not indicate the maximum values available.
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