Volker Lehmann Electrochemistry of Silicon Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) Cover illustrations Upper left: Electrolytic double cell for diffusion length mapping of 200 mm silicon wafers using the ELYMAT technique, as discussed in Section 10.3. After [21]. Upper right: Electroluminescence from a micro PS film anodized in an O-ring cell viewed from the top (10% acetic acid, 10 mA cm –2 , 2.6 cm 2 active area). Note that the luminescence appears or- ange in the center line, where the film has been formed under high current density (in 1:1 ethanoic HF at 200 mA cm –2 ), while it appears red for low formation current density (10 mA cm –2 ). After [Le3]. Lower left: Free-standing porous silicon samples mounted on top of a 20 lm thick bulk silicon grid (with grid bars of 7 lm width) and illuminated with white light from the back. Upper left square: 70 lm micro PS of 69% porosity (50 min at 30 mA cm –2 in 1:1 ethanoic HF, 1 X cm p- type), upper right square: 32 lm meso PS of 39% porosity (16 min at 30 mA cm –2 in 1:1 etha- noic HF, 0.03 X cm p-type), lower left square: 69 lm macro PS of 72% porosity (1.85 lm diame- ter pores in 2.3 lm trigonal pitch parallel to the light beam) and lower right square 7 lm bulk silicon. Note that porosity and thickness of all porous samples has been selected to correspond to 20 lm thick bulk silicon. After [Le27]. Lower right: First macroporous silicon-based chip capacitor (100 nF, 10 V) on a match for size comparison. Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) Volker Lehmann Electrochemistry of Silicon Instrumentation, Science, Materials and Applications Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) Author Dr. Volker Lehmann Infineon Technologies AG Corporate Research Otto-Hahn-Ring 6 81739 München Germany Library of Congress Card No. applied for British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek – CIP-Cataloguing-in-Publication Data A catalogue record for this book is available from Die Deutsche Bibliothek © WILEY-VCH Verlag GmbH D-69469 Weinheim, 2002 All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, mi- crofilm, or any other means – nor transmitted or translated into machine language without written permission from the publishers. Reg istered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. printed in the Federal Republic of Germany printed on acid-free paper Composition K+V Fotosatz GmbH, Beerfelden Printing Strauss Offsetdruck GmbH, Mörlenbach Bookbinding Großbuchbinderei J. Schäffer GmbH & Co. KG, Grünstadt ISBN 3-527-29321-3 n This book was carefully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) Dedicated to Hadley and other colleagues, with thanks for good advice Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) Silicon has been and will most probably continue to be the dominant material in semiconductor technology. Although the defect-free silicon single crystal is one of the best understood systems in materials science, its electrochemistry to many people is still a matter of alchemy. This view is partly a result of the interdisciplin- ary aspects of the topic: Physics meets chemistry at the silicon-electrolyte inter- face. So far, researchers interested in this topic have had to choose either mono- graphs that deal with the electrochemistry of semiconductors in general or recent editions that deal with special topics such as, for example, the luminescent prop- erties of microporous silicon. The lack of a book that specializes on silicon but which gives the whole spectrum of its electrochemical aspects was my motivation to write the Electrochemistry of Silicon. With this book I hope to address different groups in the scientific community. For beginners in the field a comprehensive overview of the topic is given in ten chapters, including a brief historical review and safety tips. The practitioner will find inspiration for instrumentation as well as examples of applications ranging from photonic crystals to biochips. For experts the book may serve as a quick reference with more than 150 technical tables, diagrams and micrographs, as well as ca. 1000 references cited for easy access to in-depth information. I did my best to eliminate mistakes and unclear descriptions, but I suspect that even writing is governed by the laws of thermodynamics. So, I welcome com- ments from readers and will attempt to correct any mistakes that they find. VII Preface Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) Preface VII 1 Introduction, Safety and Instrumentation 1 1.1 Early Studies of the Electrochemistry of Silicon 1 1.2 Safety First 3 1.3 The Basic Properties of Silicon 5 1.4 Common Electrolytes 7 1.5 The Electrodes 11 1.6 Cell Designs 15 2 The Chemical Dissolution of Silicon 23 2.1 The Basics of Wet Processing of Silicon 23 2.2 Silicon Surface Conditions and Cleaning Procedures 24 2.3 Chemical Etching in Alkaline Solutions 27 2.4 Chemical Etching in Acidic Solutions 30 2.5 Defect and Junction Delineation 33 2.6 Selective Etching of Common Thin Film Materials 36 3 The Semiconductor-Electrolyte Junction 39 3.1 Basics of the Semiconductor-Electrolyte Contact 39 3.2 The I–V Characteristics of Silicon Electrodes in Acidic Electrolytes 42 3.3 The I–V Characteristics of Silicon Electrodes in Alkaline Electrolytes 49 4 The Electrochemical Dissolution of Silicon 51 4.1 Electrochemical Reactions 51 4.2 The Dissolution Valence 57 4.3 The Characteristic Anodic Currents in HF 59 4.4 Reverse Currents, Electron and Hole Injection 63 4.5 Electrochemical Etch Stops 68 4.6 Photoelectrochemical Etching 72 IX Contents Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) 5 Anodic Oxidation 77 5.1 Silicon Oxidation Techniques 77 5.2 Native and Chemical Oxides 78 5.3 Anodic Oxide Formation and Ionic Transport 79 5.4 Oxide Morphology, Chemical Composition and Electrical Properties 82 5.5 Electrochemical Oscillations 89 5.6 Electropolishing 94 6 Electrochemical Pore Formation 97 6.1 Basics of Pore Formation 97 6.2 Porous Silicon Formation Models 99 6.3 Pore Size Regimes and Pore Growth Rates 104 6.4 Porosity, Pore Density and Specific Surface Area 108 6.5 Mechanical Properties and Drying Methods 114 6.6 Chemical Composition and Ageing Effects 117 6.7 Electrical Properties of Porous Silicon 120 7 Microporous Silicon 127 7.1 Micropore Formation Mechanism 127 7.2 Morphology of Microporous Silicon 128 7.3 Absorption, Reflection and Nonlinear Optical Effects 133 7.4 Luminescence Properties 138 7.5 Quantum Confinement and Models of the Luminescence Process 150 7.6 Oxidized Porous Silicon 159 7.7 Related Materials 162 8 Mesoporous Silicon 167 8.1 Mesopore Formation Mechanisms 167 8.2 Mesopores in Highly Doped p-Type Silicon 171 8.3 Mesopores in Highly Doped n-Type Silicon 174 8.4 Mesopore Formation and Spiking in Low-Doped n-Type Silicon 177 8.5 Etch Pit Formation by Avalanche Breakdown in Low-Doped n-Type Silicon 180 9 Macroporous Silicon 183 9.1 Macropore Formation Mechanisms 183 9.2 Macropores in p-Type Silicon 187 9.3 The Phenomenology of Macropore Formation in n-Type Silicon 190 9.4 Calculating Macropore Growth and Mass Transport 198 9.5 Design Rules and Limits of Macropore Array Fabrication 202 ContentsX 10 Applications 207 10.1 Overview 207 10.2 AC Properties of Silicon Electrodes and Carrier Concentration Profiling 208 10.3 Diffusion Length and Defect Mapping 211 10.4 Sensors and Biochips 219 10.5 Passive and Active Optical Devices 225 10.6 Porous Silicon-Based Electronic Devices 232 10.7 Sacrificial Layer Applications 236 Appendices 243 Supplier References 249 References 251 Subject Index 273 Contents XI absorption – chemical 220 – cross-section 137 – coefficient 136, 212 – optical 133, 145, 212 acceptor compensation 52 accumulation 39, 44 acidic etching 30 activation energy 11, 29, 61 active state 97, 186 aging 29, 117 alkaline etching 27, 49, 53, 193 alternating current properties 126, 208 ambipolar diffusion 124 amorphous silicon 131, 164 anisotropic etching 27, 50–54 annealing 88, 117 anodic oxide 77–96, 101, 149 – chemical composition 86 – defects 86, 87 – density 78, 85 – dissolution 67 – electrical properties 88 – etchrate 69, 83, 92 – formation mechanisms 52, 56, 79 – growth rate 81 – morphology 83, 92 – porous 90 – refractive index 86 anodic regimes 44–49 anti-reflective coating 227 anti-scatter grid 239 applications 207–241 atomic force microscopy 85 attenuated total reflection 20 Auger recombination 6, 136, 145, 156 autocatalytic 33, 163 avalanche breakdown 103, 180 backside photo current 212 band-structure 139, 144, 151, 229 bifluoride 55 biochips 223 Bragg-filter 130, 222, 226 breakdown electrical 88, 103, 168 breakdown passivity 101 Brunauer-Emmet-Teller method 112 buffered oxide etch 36 capacitance-voltage curve 209 capillary forces 115 carrier concentration profiling 208 cathodic regime 45, 51 cell designs 15–22, 214 – double 19, 214 – electrolyte circulation 21 – immersion 17 – internal 72, 75 – materials 15 – o-ring 16, 18 – windows 16 chemical – dissolution 23–38,53 – oxide 78 – polishing 31 – reactions 51–57 – vapor deposition 234 chemomechanical polishing 24, 64, 96 cleaning 24,57 cleaving 4, 14,17 cold cathode 232 collimator 239 colloidal silica 24 concentration 7, 201 conduction band 39–50, 128, 144 contact 14, 39, 120 contact angle 24 273 Subject Index Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29321-3 (Hardcover); 3-527-60027-2 (Electronic) [...]... 141 277 Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-5 2 7-2 932 1-3 (Hardcover); 3-5 2 7-6 002 7-2 (Electronic) 1 Introduction, Safety and Instrumentation 1.1 Early Studies of the Electrochemistry of Silicon This section briefly surveys the history of the electrochemistry of silicon Electrochemistry is a much... elemental silicon by chemical vapor deposition (CVD) The availability of dislocation-free silicon single crystals and the idea of an integrated silicon circuit, developed by Kilby in 1958, were the beginnings of what today is known as ‘the silicon age’ Silicon has long been the subject of numerous electrochemical investigations Early electrochemical studies on silicon dealt mainly with problems of anodic... etching [Da2] of n-type silicon were published In 1971, Watanabe and Sakai first reported the porous nature of electrochemically formed films on silicon electrodes [Wa7] 1.2 Safety First The number of publications dealing with the electrochemistry of silicon and PS has increased rapidly since 1971 The first model for pore formation in n-type silicon electrodes, based on a breakdown of the depletion... (a) Set-up for fast removal and rinsing of a strip-shaped electrode by fast rotation of the shaft (solid arrow) This set-up is useful for measurements of transient electrode processes like anodic oxide growth during electrochemical oscillations (b) PMMA immersion cell set-up for in situ determination of stress by optical measurement of the electrode curvature Stress is induced by the growth of anodic... order to avoid fracture An advantage of the set-ups shown in Fig 1.7 a–f is that 1.6 Cell Designs Fig 1.7 Cross-sectional views of various types of O-ring cells The O-ring can be pressed against the sample (a) by the weight of the upper part of the cell, (b) by screws, (c) by magnets, (d, e) by vacuum or (f) by pneumatic pistons These designs can be extended to double O-ring cells: this requires (g) a... product of the number of reflections by the penetration depth of the IR radiation in the electrolyte, which is typically a tenth of the wavelength The best compromise in terms of sensitivity often leads to about ten reflections [Oz2] 1.6 Cell Designs 1.6.5 The Rotating Disk Electrode None of the set-ups discussed so far provides stirring of the electrolyte for bubble removal or for enhancement of the... dependence of DHF From measurements of viscosity versus temperature, activation energies of 0.16 and 0.12 eV have been calculated for diffusion-controlled reactions in water and ethanol, respectively These results are supported by rotating disk electrode (RDE) measurements of JPS in ethanoic HF, which gave an activation energy of 0.125 eV for DHF [Me14] The product of the dissolution process of silicon. .. adjustable illumination conditions, etc This multitude of requirements is in stark contrast to the small number of cells specialized on silicon electrochemistry commercially available [8] Doing electrochemistry with silicon therefore commonly begins with designing a suitable cell, which is usually a source of frustration as a result of broken samples, leaky set-ups and corroded contacts The different cell designs... which is of lower transmission coefficient than PMMA, is therefore preferable for high HF concentrations Standard black O-rings made of an acrylonitrile-butadiene copolymer (such as Perbunan) have proved to be stable in HF at concentrations up to 50% If contamination of the silicon sample is an issue, the nitrile O-rings may be replaced by vinylidene fluoride-hexafluoropropylene (Viton) O-rings [9]... the cathodic reduction of an oxidizing agent and pointed out that the amount of collected holes depends on the diffusion length of these holes as well as on the thickness of the germanium disk An extension of this method was used by Harvey [Ha1] to measure the surface recombination velocity of the electrode A detailed study of the anodic dissolution mechanism of germanium and silicon was carried out . and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-5 2 7-2 932 1-3 (Hardcover); 3-5 2 7-6 002 7-2 (Electronic) Volker Lehmann Electrochemistry of Silicon Instrumentation,. Volker Lehmann Electrochemistry of Silicon Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-5 2 7-2 932 1-3 . Applications Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications. Volker Lehmann Copyright © 2002 Wiley-VCH Verlag GmbH ISBNs: 3-5 2 7-2 932 1-3 (Hardcover); 3-5 2 7-6 002 7-2 (Electronic) Author Dr.