and Technology of Thin Films

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1 An Introduction to Physics and Technology of Thin Films

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3 An Introduction to Physics and Technology of Thin Films Alfred Wagendriste1 Institute of Applied and Technical Physics Technical University of Vienna YumingWang Department of Materials Science Jilin University b World Scientific II Singapore New Jersey London Hong Kong

4 Published fry World Scientific Publishing Co. Pte. Ltd. POBox 128, Farrer Road, Singapore 9128 USA office: Suite IB, 1060 Main Street, River Edge, NJ An Introduction to Physics and Technology of Thin Films Downloaded from UK office: 73 Lynton Mead, Totteridge, London N20 8DH Library of Congress Cataloging-in-Publication Data Wagendristel, Alfred. An introduction to physics and technology of thin films I Alfred Wagendristel, Yuming Wang. p. cm. Includes bibliographical references and index. ISBN Thin films. 2. Thin film devices. I. Wang, Yu-ming. II. Title. QCI76.83.W '175--dc ClP Copyright 1994 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means. electronic or mechanical. including photocopying. recording orany information storage and retrieval system now known or to be invented. without written permission from the Publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, USA. Printed in Singapore.

5 INTRODUCTION A thin film is a liquid or solid such that one of its linear dimensions is very small in comparison with the other two dimensions. Usually one classifies thin films (arbitrarily) into: thick films (D > 1 micrometer, D : film thickness) thin films (D < 1 micrometer). This book discusses mainly the systems of a (solid) film on a (solid) substrate (backed films) rather than unsupported films (foils). They (film + substrate) imply a production process in the form of film growth by sustaining an atomic or molecular flux to the surface of the substrate and subsequently by growing of the film. Film growth will either involve chemical reaction at the substrate such as discharge of ions, decomposition of a compound, reaction of a gas or liquid with substrate surface; or physical processes such as evaporation from a source and sputtering from a target, then condensation onto the substrate. The history of thin film technologies is as follows: 1650 Interference colors of thin liquid film on a liquid surface (oil on water) were observed by R. Boyle, R. Hooke, and I. Newton Electrodeposition (by M. Faraday), Chemical reduction deposition, Film formation during glow discharge (by W. Grove), and Evaporation of metallic wires by current (by T. A. Edison) were discovered.

6 Solid films produced by the first two methods received early recognition for their technical importance as anticorrosive films or films for mirrors, whereas those prepared by the latter methods lacked reproducibility for a long time. Only since the improvement of vacuum equipment for film preparation as well as for investigation (electron microscopy, LEED, other surface analytical techniques) were reproducible and useful films readily obtained. Since 1950 a vigorous development arose by production of 1940 Films for optical, electronic, mechanical, and protective applications Semiconductor electronics made use of thin film methods which reveal two major merits: mass fabrication by printing techniques, and miniaturisation by integration (integration density: 1966: 50 elements/mm2; 1974: 500 elements/mm2; 1980: 5000 elements/ mm2> Main fields of application: computer electronics, commercial electronics, medical electronics, space technology, and energy conversion (solar cells). Optical applications of metal and dielectric films: filters, reflection coatings, optical wave guides for opto-electronic communications of semiconducting films: IR-sensors and thin film laser diodes. Magnetic and superconducting films for memory and logical devices. Basic scientific interest is focussed on film formation processes in order to obtain an insight into the mechanisms leading to special structural properties: Island-, labyrinth-, continuous films (macrostructure) amorphous, polycrystalline, single crystal films (microstructure) on film stability (the mobility of atoms in the films enable relaxation and heterodiffusion). It is, however, not only the film structure but also the limited thickness which determines the physical properties. Thus the geometric anisotropy and size effects should also be studied. In the following chapters, firstly methods of film deposition with special emphasis on the basic physical phenomena, then the fundamental processes in film formation, and finally some thin film specific properties and their technical relevance will be discussed.

7 Introduction PART I CONTENTS Chapter 1. Thin Film Deposition Methods 1.1. Chemical Methods Electroplating Electroless plating Chemical vapor deposition Hydrophily Anodization Thermal growth Summary 1.2. Physical Methods Sputter deposition Physical fundamentals Technical aspects Sputtering yields Evaporation Physical fundamentals Technical aspects aansport of particles from source to substrate

8 viii Content. An Introduction to Physics and Technology of Thin Films Downloaded from Chapter 2. Theories of Nucleation and Film Growth Adsorption Film Growth Epitaxial Growth 38 Chapter 3. Control and Measurement of Film Thickness Rate and Thickness Monitors Measurement of vapor density Balance methods Vibrating quartz method Optical monitoring Other monitors Measurement of Film Thickness Optical film thickness determination X-ray interferometry Stylus methods 51 Chapter 4. Electrical Conduction in Thin Films Electron Transport in Metallic Films Conduction in discontinuous films Conduction in continuous films Practical aspects and applications Electron transport through insulating film 61 Chapter 5. Dielectric Properties of Thin Insulator Films Dielectric Constant Dielectric Loss Breakdown Voltage Thin Film Capacitors 66

9 Content. ix An Introduction to Physics and Technology of Thin Films Downloaded from Chapter 6. Supercollducting Films 6.1. General Features 6.2. Thin Film Effects 6.3. Applications, Thin Film Cryogenic Devices Switching elements, cryotrons Further applications Chapter 7. Semiconducting Films 7.1. Physical Fundamentals Electrical conduction ~n junctions Field effect 7.2. Devices for Integrated Circuits 7.3. The Thin Film Transistor 7.4. Photovoltaic Cells PART II Principle Practical aspects Future aspects Chapter 8. Thin Ferromagnetic Films 8.1. General 8.2. Fundamental Energy contributions External field Stray field, demagnetizing field Anisotropy Exchange energy Domain walls - Wall energies 8.3. Magnetization Reversal in Uniaxial Thin Films Switching by coherent rotation of M in single domain films

10 x Contenl6 An Introduction to Physics and Technology of Thin Films Downloaded from External field when H II EA External field when H II HA Arbitrary direction of H Reversal of M by domain growth Reversal by partial incoherent rotation and domain motion Fast switching 8.4. Applications Magnetic switching devices Preparation High speed random access memories Domain storage devices Bubble domains Domain wall motion devices Magnetic recording Chapter 9. Diffusion in Thin Films 9.1. General Fundamental Energy considerations Activation energy Energy barriers and jump frequencies in a driving force Average atomic displacements Atomic fluxes and diffusion coefficients Diffusion in Structural Inhomogeneous Systems Analytical approaches to grain boundary diffusion Application to Thin Films Whipple treatment of thin film depth profiles Short circuit diffusion Numerical approaches to grain-boundary-supported diffusion in thin films 121

11 Contents An Introduction to Physics and Technology of Thin Films Downloaded from Mean diffusion rates Diffusion into moving grain boundaries Diffusion in systems with miscibility gaps and intermediate phases 9.5. Selected Experimental Results Experimental techniques Chapter 10. Mechanical Properties of Thin Films General Micromorphology of Evaporated and Sputtered Deposits Evolution of the microstructure Internal stresses Model for growth-induced stress Further influence not accounted for in the model Determination of Mechanical Properties in Thin Films Internal stress References Index

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