Electromigration in Thin Films of Aluminium"
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Co / Y A thesis entitled "ELECTROMIGRATION IN THIN FILMS OF ALUMINIUM" by Stephen Roberts B.Sc., A.R.C.S. Submitted for the Degree of Doctor of Philosophy of the University of London Imperial College of Science and Technology London 1982 2 ABSTRACT Electromigration is a common cause of failure of the aluminium thin film metallizations used in many microelectronic devices and circuits. The objective of the present work was to draw correlations between processing parameters, micro- structure and susceptibility to electromigration failure of vacuum-evaporated aluminium films on amorphous Si O^. The chemical composition of the Al - Si C>2 interface was examined using x-ray photoelectron spectroscopy, Auger electron spectroscopy and transmission electron microscopy and diffraction. The effects of substrate surface preparation and temperature on Al - Si O^ interfacial reaction were investigated. Reduction of the Si02 by the aluminium, resulting in the production of polycrystalline silicon and Tj - alumina, was found to occur under the conditions prevalent in many industrial metallization processes. The implications of Al-Si O2 reaction for device reliability are discussed. A range of techniques, including transmission and scanning electron microscopy and grazing incidence x-ray diffraction, were used to characterize the grain size, preferred orientation and surface topography of aluminium films deposited onto Si C>2 under various conditions. 1 yum thick aluminium films were found to have a ^111) fibre texture and the mean grain size ranged from less than 0.5yjm for deposition at 300 K, to ** 5 yum, for deposition at 675 K. Some thinner films were also examined, and trends in grain size and orientation with thickness were established. "Growth hillocks", "annealing hillocks" and other surface features were observed in certain films. Photolithography was used to prepare 10 j.im wide tracks from 1 yjm thick aluminium films, and these were tested for electromigration failure under accelerated conditions. The median time to failure was found to increase with grain size in the range 2yum to 5 fJm, and an "activation energy" for electromigration failure of 3 0.6 + .05 eV was measured in the 433 K to 473 K temperature range. Films having a 1 fjm mean grain size, prepared at lower temperature, gave an "activation energy" of 0.38 eV, suggesting a different failure mode. Examination of failed films using optical and scanning electron microscopy revealed the presence of the voids, hillocks and whiskers known to be associated with electromigration in polycrystalline thin films. The results are interpreted in terms of the various models for electromigration damage proposed in the literature, and the possible importance of the Al - Si O inter- face is also discussed. CONTENTS Page ABSTRACT 2 CONTENTS 4 CHAPTER 1 INTRODUCTION 11 CHAPTER 2 REVIEW OF EXPERIMENTAL WORK ON THE 13 A|-Si02 INTERFACE 2.1 Introduction 13 2.2 Indirect Evidence for Reaction at the A| - Si O^ 15 Interface 2.3 Direct Observations of Chemical Reaction at the 20 A| - Si 02 Interface CHAPTER 3 A STUDY OF THE Al-Si02 INTERFACE USING 33 X-RAY PHOTOELECTRON SPECTROSCOPY 3.1 Introduction 33 3.2 XPS and AES as Techniques for Surface and Interface 34 Analysis 3.2.1 Basic Principles of XPS 34 3.2.2 Chemical Shifts in XPS 35 3.2.3 Surface Sensitivity and Quantitative XPS 37 3.2.4 Basic Principles of AES 41 3.3 Apparatus 44 3.4 Operating Principles of the Spectrometer 47 3.5 Experimental Method 49 3.6 Results 52 3.6.1 Al uminium on Argon-Ion Bombarded SiO. 52 3.6.2 Aluminium on "As-Prepared" Si 00 57 5 Page 3.6.3 Al uminium on Ion Bombarded Silicon 61 3.7 Discussion 64 3.8 Subsidiary Experiment : XPS and AES Sputter-Profiling 66 of Thick Aluminium Films 3.8.1 Introduction 66 3.8.2 Experimental Method 66 3.8.3 Results and Discussion 66 CHAPTER 4 A STUDY OF THE A|-Si02 INTERFACE USING /4 TRANSMISSION ELECTRON MICROSCOPY 4.1 Introduction 74 4.2 The Evaporator Experiments 75 4.2.1 The Substrate Material 75 4.2.2 The Evaporation Plant 75 4.2.3 The Jet-Thinning Apparatus 80 4.2.4 The Transmission Electron Microscope 80 4.2.5 Experimental Method 83 4.2.6 Results 85 4.2.6.1 The Substrates 85 4.2.6.2 Aluminium Deposited onto Si O^ at Room Temperature 85 4.2.6.3 Aluminium Deposited onto Si O^ at Elevated 88 Temperatures 4.2.6.4 The Effects of Annealing Films Deposited at Room 90 Temperature 4.2.6.5 The Effect of Heating the Si O^ Prior to Aluminium ^q Deposition 4.2.6.6 Al uminium Films on Amorphous Carbon 90 4.2.7 Discussion 90 4.3 The in-situ TEM Experiments 92 4.3.1 Modifications to the Electron Microscope 92 6 Page 4.3.2 Substrate Preparation 97 4.3.3 Experimental Method 99 4.3.4 Results 100 4.3.4.1 Aluminium on Steam-Grown Si 02 100 4.3.4.2 Aluminium on Evaporated Si O 103 4.3.5 Discussion 103 CHAPTER 5 THE A| - Si 02 INTERFACE IN SILICON DEVICES 109 5.1 The A|- Si 02 Interface - Summary of Results 109 5.2 The A|-Si02 Interface in Devices 109 5.2.1 The "Sintering" Process 110 5.2.2 Al uminium-Silicon Dioxide Adhesion 112 5.2.3 Mechanical Stress in Aluminium Films on Si 02 114 Substrates 5.2.4 Surface and Interface Electromigration 115 5.2.5 Penetration of Thin Si 02 Insulators by Aluminium 117 5.2.6 The Electronic Properties of the A| - Si 02 Interface H8 CHAPTER 6 A REVIEW OF THE STRUCTURE AND PROPERTIES 119 OF ALUMINIUM THIN FILMS 6.1 Introduction 119 6.2 The Mechanism of Film Formation 119 6.2.1 Condensation and Nucleation 120 6.2.2 Island Growth 122 6.2.3 Coalescence 123 6.2.4 Final Film Structure and Annealing Effects 123 6.2.5 Preferred Orientations in Films on Amorphous 124 Substrates 6.3 The Properties of Aluminium Thin Films 128 7 Page 6.3.1 The General Growth Characteristics of Aluminium 128 on Amorphous Si O^ 6.3.2 The Effects of Residual Gases on the Properties of 132 Aluminium Films on Amorphous Si O^ 6.3.3 The Resistivity of Aluminium Thin Films 134 6.3.4 The Surface Topography of Aluminium Thin Films 137 6.4 Aluminium as a Contact Metallization 148 6.4.1 Deposition Methods 6.4.2 Electrical Resistivity 149 6.4.3 Step Coverage 150 6.4.4, Etching 150 6.4.5 Oxidation and Corrosion 151 6.4.6 Bonding 151 6.4.7 Aluminium-Silicon Contact Stability 151 6.4.8 Electromigration Failure 152 6.4.9 Multilevel Metallization 152 6.4.10 A|-Si02 Reaction 153 CHAPTER 7 THE MICROSTRUCTURE OF ALUMINIUM THIN 154 FILMS ON AMORPHOUS Si 02 7.1 Introduction 154 7.2 Structural Analysis Techniques 155 7.2.1 Transmission Electron Microscopy 155 7.2.2 Transmission High Energy Electron Diffraction 155 7.2.3 Grazing Incidence X-ray Diffraction 158 7.2.4 Replica Technique 150 7.2.5 Scanning Electron Microscopy 162 7.2.6 Optical Microscopy 152 7.3. Experimental Method 163 8 Page 7.3.1 Film Deposition 163 7.3.2 Examination of the Films I53 7.4 Results 164 7.4.1 The Growth of Evaporated Aluminium Films on 164 Si 02 at Room Temperature 7.4.2 The Growth of Evaporated Aluminium Films on Si O^ ]jq at Elevated Temperatures 7.4.3 The Effect of Annealing 174 7.4.4 The Effect of Residual Gases Present During 174 Deposition 7.4.5 The Microstructure of 1 pm-Thick Tungsten Filament 178 Evaporated Aluminium Films 7.4.6 The Microstructure of 1 jum-Thick Electron-Beam ^g^ Deposited Films 7.4.7 The Microstructure of Sputter-Deposited Aluminium ]89 and A| - Si Films 7.5 Summary and Discussion 193 7.5.1 Grain Growth 194 7.5.2 Surface Topography 198 7.5.3 Preferred Orientations 200 7.5.4 Electron-Beam and Sputter-Deposited Films 201 7.6 Conclusions 202 CHAPTER 8 A REVIEW OF ELECTROMIGRATION 203 8.1 Introduction 203 8.2 Theory 205 8.3 Experiments on Single Crystals 210 8.4 Experiments on Polycrystalline Thin Films 211 9 Page 8.4.1 Measurement Techniques 212 8.4.2 Results and Discussion 217 8.5 Electromigration Failure Modes 220 8.5.1 Temperature Effects 220 8.5.2 Grain Boundary Effects 223 8.5.3 Surface Effects 226 8.5.4 Contact Effects 228 8.6 Accelerated Life Testing and the Assessment of 229 Electromigration Failure 8.6.1 General Observations 229 8.6.2 Failure Models 231 8.6.3 Anomalous Effects in Narrow Tracks 238 8.6.4 Annealing Effects during Testing 241 8.7 Methods of Improving Aluminium Metallizations 243 8.7.1 Glass Passivation 243 8.7.2 Alloy Additions 244 CHAPTER 9 AN EXPERIMENTAL STUDY OF ELECTROMIGRATION 246 IN THIN FILMS OF ALUMINIUM 9.1 Introduction 246 9.2 Specimen Preparation 246 9.2.1 The Substrates 246 9.2.2 Metallization 247 9.2.3 Pattern Definition 247 9.2.4 Bonding and Encapsulation 250 9.3 Apparatus for Accelerated Life Testing 250 9.3.1 The Heater Block 250 10 Page 9. 3.2 Current and Voltage Measurement 250 9.4 Experimental Method 253 9.5 Test Conditions 254 9.6 Results 255 9.6.1 Observations on the Failed Specimens 255 9.6.2 Thermal Resistance 260 9.6.3 Resistance Changes during Testing 260 9.6.4 Life Test Results 262 9.6.5 Life Test Results from 50 nm-Oxide 267 9.7 Discussion 272 9.8 Cone I us ions 275 CHAPTER 10 SUMMARY 276 APPENDIX A THE PREPARATION OF THE Si 02 SUBSTRATES 280 APPENDIX B CALIBRATION OF THE QUARTZ CRYSTAL 281 MICRO BALANCE APPENDIX C CALIBRATION OF THE THERMOCOUPLES 285 APPENDIX D THE CRYSTAL STRUCTURE OF THE ALUMINAS 287 APPENDIX E STATISTICAL NOTES 288 E.l The Log-Normal Distribution 288 E.2 The Log-Extreme Value Distribution 288 ACKNOWLEDGEMENTS 291 REFERENCES 292 11 CHAPTER 1 INTRODUCTION Electromigration (the motion of atoms in a conductor due to the flow of an electric current) was discovered in 1861 by Gerardin.