PRINCIPLES OF PHYSICAL VAPOR

DEPOSITION OF THIN FILMS

K.S. SREE HARSHA San Jose State University, CA, USA

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Contents

Acknolowdgements xii

1. INTRODUCTION 1 1.1. Introduction 1

Problems 8 References 9

2. EVAPORATION 11 2.1. Process of Thin Film Formation by Evaporation 11 2.2. Thermodynamics of Evaporation 11

2.2.1. Equilibrium condition for phase transformation in one-component Systems 12

2.2.2. Clausius-Clapeyron equation 14 2.2.3. Source Container interaction 28 2.2.4. Changes in composition during alloy evaporation 40 2.2.5. Stability of Compounds and non-stoichiometry 49 2.2.6. Chemical reactions and equilibrium 53

2.3 Kinetic Theory of Gases 58 2.3.1. Calculation of pressure 59 2.3.2. Distribution of molecular velocities 67 2.3.3. Calculation of averages using the distribution function 73 2.3.4. Maxwell velocity distribution function in a force field 76 2.3.5. Number of molecules striking a surface 79 2.3.6. Evaporation 84 2.3.7. Effusion 88 2.3.8. Emission characteristics of vapor sources 91 2.3.9. Thermal transpiration 94

2.4. Elementary Description of Collision Processes 95 2.4.1 Mean free path 96 2.4.2. Mean time between collisions 98 2.4.3. Collision frequency 100 2.4.4. Collision cross section 101

2.5. Classical Theory of Binary Collisions 105 2.5.1. Kinematic theory of collisions 106 2.5.2. Center of mass and relative position coordinates 112

vi Contents

2.5.3. Motion of a particle in a central field of force 114 2.5.4. Equation of motion 115 2.5.5. Classification of trajectories 120 2.5.6. Impact parameter and angle of scattering 124 2.5.7. Relationship between differential scattering cross section

and impact parameter and scattering angle 129 2.5.8. Differential scattering cross section for columbic potential 132 2.5.9. Center of mass to laboratory transformation of coordinates 134 Problems 138 References 141

3. PLASMA STATE 145 3.1. Introduction 145 3.2. Plasma State 149 3.3. Thermal and Non-Thermal Plasma 156 3.4. Collisions in Plasma 160

3.4.1. Excitation collisions 170 3.4.2. Ionization collisions 178 3.4.3. Charge exchange collisions 183 3.4.4. Collisions involving metastables 186 3.4.5. Recombination and attachment 188 3.4.6. Dissociation 191 3.4.7. Heterogeneous reactions 193

3.5. Behavior of Plasma in Electric and Magnetic Fields 195 3.5.1. Constant and uniform electric field, no magnetic field 197 3.5.2. Constant and uniform magnetic field, no electric field 198 3.5.3. Uniform electric and magnetic fields 203 3.5.4. Non-uniform magnetic fields 207 3.5.5. Drift velocity due to other fields 212

3.6. Basic Equations in Cold Plasma 214 3.6.1. Boltzmann equation 215 3.6.2. Macroscopic averages 217 3.6.3. Flux of macroscopic quantities 218 3.6.4. Rate of change of macroscopic quantities 221

3.7. Macroscopic Characterization of Plasma 224 3.8. Collective Phenomena in Plasma 225

3.8.1 Shielding of a charge in a plasma 226 3.8.2. Plasma parameter 230 3.8.3. Diffusion of charges 233 3.8.4. Plasma oscillations 238 3.8.5 Collision frequencies 240

3.9. Waves in Cold Plasma 242 3.9.1. Electrostatic waves 249 3.9.2. Plasma waves in the presence of a magnetic field 251

Contents vii

3.9.3. Plasma waves at any angle to the imposed magnetic fiel'd 253 Problems 255 References 257

4. COLD PLASMA DISCHARGES ' 259 4.1. Introduction 259 4.2. The Plasma Sheath 260 4.3. Electron Emission from Surfaces 270 4.4. Glow Discharge Plasma 278

4.4.1. The positive column 290 4.4.2. Paschen's criterion 294 4.4.3. Cathode sheath 297 4.4.4. The negative glow and Faraday dark space 301 4.4.5. Characteristics of glow discharge 302

4.5. DC Sheaths 305 4.6. RF Discharges 310

4.6.1. Circuit model for RF discharges 317 4.6.2. Self-bias of RF electrodes 325 4.6.3. Matching network 332

4.7. Magnetrons 335 4.8. Inductive Plasma Discharges 342 4.9. Wave Heated Plasma Discharges 349

4.10. Plasma Process Modeling 355 Problems 360 References 362

5. THERMAL EVAPORATION SOURCES 367 5.1. Introduction 367 5.2. Electrically Heated Sources 368 5.3. Effusion and Free Jet Sources 381 5.4. Sources Utilizing Electron Beam 400

5.4.1. Emission of electrons from surfaces 414 5.5. Are Vapor Sources 425 5.6. Pulsed Laser Ablation Sources 431

Problems 446 References 449

6. GAS FLOW IN THIN FILM PROCESSING SYSTEMS 453 6.1. Introduction 453 6.2. Specific Heat of Gases 454 6.3. Transport Properties of Gases 458

6.3.1. Viscosity 459 6.3.2. Thermal conduetivity 462 6.3.3. Seif diffusion 464

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6.4. Gas Flow in Components 466 6.5. Flow of Gases 474

6.5.1. Gas flow regimes 475 6.5.2. Viscous flow 478 6.5.3. Slip flow 482 6.5.4. Molecular flow 484 6.5.5. Turbulent flow 489

6.6. Generation of Vacuum 490 6.6.1. Mechanisms for vacuum generation 491 6.6.2. Positive displacement pumps 494 6.6.3. Momentum transfer pumps 499 6.6.4. Capture pumps 504

6.7. Measurement of Pressure 512 6.7.1. Force measuring gauges 513 6.7.2. Energy transfer gauges 517 6.7.3. Momentum transfer gauges 519 6.7.4. lonization gauges 521 6.7.5. Partial pressure gages 523 Problems 530 References 532

7. SPECIAL SOURCES 535 7.1. Introduction 535 7.2. Molecular Beam Evaporation 536 7.3. Modified Evaporation Processes 548 7.4. Sputter Deposition Processes 552

7.4.1. Ion-solid Interactions 554 7.4.2. Ion neutralization 563 7.4.3. Sputtering Yields 566 7.4.4. Linear cascade theory of sputtering 576 7.4.5. Sputtering of multicomponent materials 582 7.4.6. DC Glow discharge sputtering 585 7.4.7. RF Sputtering 590 7.4.8. Magnetron sputtering 594 7.4.9. Reactive sputtering 597

7.5. Ion Beam Sputtering 601 7.6. Ionized Cluster Beam deposition 612

Problems 615 References 616

8. GAS SOLID INTERACTIONS 619 8.1. Introduction 619 8.2. Pumping to Low Pressures 621 8.3.' Leakage 627

Contents ix

8.4. Evaporation and condensation 632 8.5. Thermodynamics of surfaces 635 8.6. Adsorption 644

8.6.1. Langmuir's adsorption 650 8.6.2. Multilayer adsorption Model 655

8.7. Modulated Beam Growth Methods 661 8.8. Solubility of Gases in Solids 664 8.9. Diffusion 668

8.10. Permeation 673 8.11. Desorption 676 8.12. Stimulated Desorption 676 8.13. Degassing 678

Problems 681 References 681

9. NUCLEATION AND GROWTH OF FILMS 685 9.1. Introduction 685 9.2. Elastic Scattering 687 9.3. Thermal Accommodation Coefficient 697 9.4. Sticking Coefficient 701 9.5. Motion of Adsorbed Atoms on the Surface 711 9.6. Surface Energy of Solids 714 9.7. Vapor Pressure Above a Cluster 718 9.8. Mechanisms of Thin Film Formation 722 9.9. Growth Forms of Crystalline Solids 727

9.10. Atomic Structure of Surfaces 739 9.11. Three Dimensional Nucleation 761 9.12. Two Dimensional Nucleation 767 9.13. Rate of Nucleation 769 9.14. Atomistic Theory of Nucleation 773 9.15. Kinetic Nucleation Theory 777 9.16. Crystal Growth 780

9.16.1. Normal mechanism of crystal growth 781 9.16.2. Layer growth of faces 782 9.16.3. Spiral Growth 786 9.16.4. Growth by 2D Nucleation 787

9.17. Coalescence 788 9.18. Ripening 791 9.19. Film Growth and Microstructure 795 9.20. Grains In Films 804 9.21. Stresses in Thin Films 811 9.22. Ostwald's Step Rule 821

Problems 822 References 826

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10. EPITAXY 831 10.1. Introduction 831 10.2. Relationship Between Substrate and Epitaxial Layer 836 10.3. Growth Morphology 840 10.4. Structure and Energy of Epitaxial Interface 846 10.5. Doping 856 10.6. Ordering in Semiconductor Alloys 870 10.7. Strained Layer Epitaxy 875 10.8. Defects in Epitaxial Layers 882 10.9. Diffusions in Epitaxial Structures 887

10.10. Artificial Epitaxy 889 10.11. Solid phase epitaxy 892 10.12. Material Characterization 893

10.12.1. Optical methods of characterization 895 10.12.2. Photoluminescence spectroscopy 895 10.12.3. Reflection high energy electron diffraction 897 10.12.4 Ellipsometry 902 10.12.5. Transmission Electron Microscopy 904 Problems 907 References 908

11. SUBSTRATEPREPARATION 911 11.1. Introduction 911 11.2. Contamination 913 11.3. Cleaning Processes 921 11.4. Cleavage In Ultra High Vacuum 926 11.5. Ion Bombardment and Annealing 926 11.6. Evaporation and Condensation of Films 929 11.7. Solid/Solid Interfaces 930 11.8. Chemical and Mechanical Planarization 931 11.9. Substrate Surface Termination 936

11.10. Substrate Temperature 936 11.11. Characterization of Surface 939

11.11.1. Photoelectron spectroscopy 940 11.11.2. Auger electron spectroscopy 944 11.11.3. Secondary ion mass spectroscopy 950 11.11.4. Fourier Transform Infrared Spectroscopy 954 11.11.5. Surface profiling 956 Problems . , 958 References 959

12. STRUCTURE AND PROPERTIES OF FILMS 961 12.1, Introduction 961 12.2. Conducting Films 962

Contents xi

12.3. Semiconducting Films 971 12.4. Superconducting Films 982 12.5. Magnetic Films 993 12.6. Dielectric Films 1019 12.7. Ferroelectrics Films 1030 12.8. Mechanical Properties of Thin Films 1041 12.9. Optical Properties of Thin Films 1048

12.10. Diffusion in Thin Films 1064 12.11. Low dimensional structures 1067 Problems 1069 References 1071

13. DRY ETCHING 1073 13.1. Introduction 1073 13.2. Chemical Volatilization 1081 13.3. Loading effects 1087 13.4. Ion Assisted Gas Surface Chemistry 1090 13.5. Anisotropy In Etching 1095 13.6. Passivation Of Sidewalls 1100 13.7. Selectivity 1103 13.8. Ion Beam Based Etching 1105 13.9. Etching Reactor Configurations 1108

13.10. Plasma Diagnostic and Process Control 1121 13.11. Etching Induced Damage 1127 13.12. Modeling of Dry Etching 1130

Problems 1133 References 1133 Index 1135