THIN FILM PHENOMENA KASTURI L. CHOPRA Staff Scientist, Ledgemont Laboratory, Kennecott Copper Corporation Lexington, Massachusetts

Adjunct Professor of Mechanical Engineering Northeastern University Boston, Massachusetts

McGRAW-HILL BOOK COMPANY New York St. Louis

San Francisco London Sydney Toronto Mexico Panama

CONTENTS

Preface vii

1 INTRODUCTION 1

П THIN FILM DEPOSITION TECHNOLOGY 10

1. Introduction 10 2. Thermal Evaporation 11

2.1 General Considerations 11 2.2 Evaporation Methods 14

Resistive Heating; Flash Evaporation; Arc Evaporation; Ex-ploding-wire Technique; Laser Evaporation; RF Heating; Electron-bombardment Heating

3. Cathodic Sputtering 23 3.1 Sputtering Process 23 3.2 Glow-discharge Sputtering 29

Pressure; Deposit Distribution; Current and Voltage Depend­ence; Cathode; Contamination Problem; Deposition Control

3.3 Sputtering Variants 34

xi

XÜ Contents

3.4 Low-pressure Sputtering 36 Magnetic Field; Assisted (Triode) Sputtering; RF Sputtering; Ion-beam Sputtering

3.5 Reactive Sputtering 41 3.6 Sputtering of Multicomponent Materials 42

4. Chemical Methods 43 4.1 Introduction 43 4.2 Electrodeposition 44

Electrolytic Deposition; Electroless Deposition; Anodic Oxi­dation

4.3 Chemical Vapor Deposition (CVD) 46 Pyrolysis (Thermal Decomposition); Hydrogen Reduction; Halide Disproportionation; Transfer Reactions; Polymeri­zation

4.4 Miscellaneous Methods 51 5. Vacuum-deposition Apparatus . 55

5.1 Vacuum Systems 55 5.2 Substrate-deposition Technology 61

Substrate Materials; Substrate Cleaning; Uniform and Non­uniform Deposits; Masks and Connections; Multiple-film Deposition

6. Conclusions 67 7. Appendix 69

References 76

I I I THICKNESS MEASUREMENT AND ANALYTICAL TECHNIQUES . . . 83

1. Thickness Measurement 83 1.1 Electrical Methods 84

Film Resistance; Capacitance Monitors; Ionization Monitors

1.2 Microbalance Monitors 90 Microbalances; Quartz-crystal Monitor

1.3 Mechanical Method (Stylus) 96 1.4 Radiation-absorption and Radiation-emission Methods . . 97 1.5 Optical-interference Methods 99

Photometric Method; Spectrophotometry Method; Inter­ference Fringes; X-ray Interference Fringes

1.6 Summary of Methods 105 2. Analytical Techniques 108

2.1 Chemical Analysis. 108 2.2 Structural Analysis 110 2.3 Surface Structure 110

Optical Methods; Low-energy Electron Diffraction (LEED); Auger-electron Spectroscopy; Field-emission, Field-ion, and

Contents xiii

Sputter-ion Microscopy; Reflection Electron Diffrac­tion; Replica Electron Microscopy

2.4 Volume Structure 121 X-ray Diffraction; X-ray Microscopy (Topographic Methods); Transmission Electron-diffraction and Electron-microscope Methods

3. Conclusions 129 References 130

I V NUCLEATION, GROWTH, AND STRUCTURE OF FILMS 137

1. Nucleation 137 1.1 Condensation Process 138 1.2 Langmuir-Frenkel Theory of Condensation 140 1.3 Theories of Nucleation 142

Capillarity Theory; Statistical or Atomistic Theory; Miscel­laneous Models; Further Deductions of the Nucleation Theories

1.4 Experimental Results 149 Sticking Coefficient; Observations on Nucleation; Condensa­tion Centers; Condensate Temperature

2. Growth Processes 163 2.1 General Description 163 2.2 Liquid-like Coalescence 166

Experimental Observations; Coalescence Model 2.3 Influence of Deposition Parameters 171

General Aspects; Kinetic-energy Effect; Oblique Deposition; Electrostatic Effects

3. Some Aspects of the Physical Structure of Films 182 Crystallite Size; Surface Roughness; Density of Thin Films

4. Crystaflographic Structure of Films 189 4.1 Lattice Constant of Thin Films 191

Size Effect; Surface Pseudomorphism 4.2 Disordered and Amorphous Structures 195

Impurity Stabilization; Vapor Quenching (VQ): Codeposit Quenching

4.3 Abnormal Metastable Crystalline Structures 199 Amorphous-Crystalline Transformation; Codeposit Quenching of Metastable Alloys; Deposition-parameter-controlled and Nucleated Metastable Structures; Pseu-domorphs and Superstructures; Conclusions; Meta-stabilization Mechanisms

4.4 Two-dimensional Superstructures 214 4.5 Fiber Texture (Oriented Overgrowth) 220 4.6 Alloy Superlattices 223

xiv Contents

5. Epitaxial-growth Phenomenon 224 5.1 Influence of Substrate and Deposition Conditions . . . . 225

Substrate; Substrate Temperature; Deposition Rate; Con­tamination; Film Thickness; Electrostatic Effects; Depo­sition Methods; Summary

5.2 Theories of Epitaxy 238 Royer Hypothesis; van der Merwe Theory; Brück-Engel Theory; Nucleation Theories; Summary

6. Structural Defects in Thin Films 244 7. Concluding Remarks 252

References 254

V MECHANICAL EFFECTS IN THIN FILMS 266

1. Introduction 266 2. Internal Stresses 267

2.1 Experimental Techniques 267 Bending-plate or-beam Methods; X-ray and Electron-diffraction Methods; Other Techniques

2.2 Experimental Results 271 Thermal Stress; Intrinsic Stress; Substrate Temperature Dependence; Thickness Dependence; Deposition Rate and Angle-of-incidence Dependence; Annealing Effects; Anisotropic Stresses; Stresses in Chemically Prepared Films

2.3 Origin of Intrinsic Stress 287 Thermal Effect; Volume Changes; Surface Layer; Surface Tension; Electrostatic Effects; Lattice-misfit Accommo­dation Model; Structural-defect Hypothesis; Crystallite-boundary Mismatch Model; Anisotropic Growth

3. Mechanical Properties 295 3.1 Experimental Techniques 296

High-speed Rotor; Bulge Test; Tensile Test; Electron-microscope Devices; Direct Measurement of Strain

3.2 Experimental Results 298 Stress-Strain Curves; Tensile Strength; Microhardness

3.3 Origin of the Tensile-strength Effects 306 Structural-defect Hypothesis; Surface Effects; Volume Effects; Phenomenological Approach

3.4 Films vs. Whiskers 310 3.5 Stress Relief 311

4. Adhesion of Films 313 4.1 Measurement of Adhesion 314 4.2 Experimental Results 316 4.3 Origin of Adhesion 321

Contents XV

5. Concluding Remarks 322 References 323

V I ELECTRON-TRANSPORT PHENOMENA IN METAL FILMS 328

1. Introduction 328 2. Electrical Conduction in Discontinuous Films 329

2.1 Conduction Mechanisms 329 Thermionic (Schottky) Emission; Quantum-mechanical Tunneling; Activated Tunneling; Tunneling between Allowed States; Tunneling via Substrate and Traps

2.2 Experimental Results 335 2.3 Temperature Coefficient of Resistivity (TCR) 340 2.4 Network (Porous) Films 342 2.5 Elastoresistance 344

3. Electrical Conduction in Continuous Films 344 3.1 Theories of Size Effect 345 3.2 Experimental Results 355

Thickness Dependence; Low-temperature Results; Size-effect Anisotropy; Magnetic Boundary Scattering; Superimposed Films

3.3 Temperature Coefficient of Resistivity (TCR) 365 Theory; Experimental Results

3.4 Specular Scattering 368 3.5 Field Effect 375 3.6 Influence of Absorption and Adsorption on Con­

ductivity 378 3.7 Conductivity Changes Due to Annealing 381 3.8 High-resistivity Films 387

4. Galvanomagnetic Size Effects in Thin Films 390 4.1 Introduction 390 4.2 Longitudinal Magnetoresistance 392 4.3 Transverse Magnetoresistance (Field Perpendicular to the

Film Surface-Hl) 396 4.4 Transverse Magnetoresistance (Field Parallel to the Film

Surface-H||) 398 4.5 Hall Effect in Thin Films 401 4.6 The Anomalous Skin Effect 405 4.7 Eddy-current Size Effects 409

5. Transport of Hot Electrons 411 6. Thermal Transport 417

6.1 Thermal Conductivity 417 6.2 Thermoelectric Power 419 6.3 Heat Transport across Film - Insulator Interface 423

Contents

7. Concluding Remarks 425 References 425

TRANSPORT PHENOMENA IN SEMICONDUCTING FILMS 434

1. Introduction 434 2. Theoretical Considerations 435

Mobility; Galvanomagnetic Surface Effects; Anisotropy Effects; Quantum Size Effects

3. Experimental Results 440 3.1 Size Effects 440 3.2 Transport Properties of Thick Films 444

4. Photoconduction in Semiconductor Films 452 4.1 Activation Process 452 4.2 Photoconductivity Mechanisms 454 4.3 High-voltage Photovoltaic Effect 455

5. Field Effect - Thin-film Transistor (TFT) 456 6. Concluding Remarks 459

References 460

TRANSPORT PHENOMENA IN INSULATOR FILMS 465

1. Introduction 465 2. Dielectric Properties 466

2.1 Thin Films 466 2.2 Thick Films 470 2.3 Dielectric Losses 471

3. Piezoelectric Films 477 4. Electrical Conduction in Insulator Films 479

4.1 Conduction Mechanisms 479 4.2 Thermionic (Schottky) Emission 481 4.3 Quantum-mechanical Tunneling 483

Theories; Image-force Correction; Temperature-field (TF) Emission; Temperature Dependence; Experimental Re­sults; Conclusions

4.4 Bulk-limited Conduction 499 Space-charge-limited Current (SCLC) Flow; Trap and Im­purity Effects

4.5 Voltage-controlled Negative Resistance (VCNR) . . . . 503 4.6 Current-controlled Negative Resistance (CCNR) 506 4.7 Tunnel Emission (Hot-electron Transport) 508 4.8 Tunnel Spectroscopy 514

5. Photoeffects in Tunnel Structures 516 5.1 Electroluminescence 516

Contents xvii

5.2 Photoconduction and Photoemission 518 6. Concluding Remarks 522

References 522

I X SUPERCONDUCTIVITY IN THIN FILMS 529

1. Introduction 529 2. Basic Concepts 530 3. Transition Temperature of Thin-film Superconductors . . . . 535

3.1 Introduction 535 3.2 Thickness Dependence 537 3.3 Superconductivity-enhancement Phenomenon 539 3.4 Mechanisms of Enhanced Superconductivity 544 3.5 Influence of Stress 547 3.6 Influence of Impurities- 548 3.7 Electrostatic-charge (Field) Effect 549 3.8 Proximity Effects in Superimposed Films 550

4. Critical Magnetic Field 555 4.1 Type I Films 556 4.2 Type II Films 560 4.3 Mean-free-path Dependence of Я р 562 4.4 Transverse Critical Field 563

5. Critical Current 566 6. Propagation of Normal and Superconducting Phases . . . . 572 7. Superconductive Tunneling 573

7.1 Experimental Results 576 General; Tunneling in Superimposed Films; Gapless Superconductivity

7.2 Supercurrent (Josephson) Tunneling 582 8. Infrared Transmission through Thin Films 586 9. Superconductive Thin-film Devices 588

9.1 Cryotrons 588 Wirewound Cryotron; Crossed-film Cryotron (CFC); The Shielded CFC; In-line Cryotron; Multicrossover Cryotron (MCC); Ferromagnetic Cryotron Configuration

9.2 Computer Memory Devices 593 Crowe Cell; Persistor, Persistatron, and Continuous-film Memory (CFM)

9.3 Superconducting Magnets 595 9.4 Low-frequency Devices 596 9.5 Bolometers — Radiation Detectors 598 9.6 Tunnel Devices 598

10. Concluding Remarks 599 References 600

XVÜi Contents

X FERROMAGNETISM IN FILMS (by M. H.Cohen) 608

1. Introduction 608 2. Magnetization vs. Thickness 610 3. Stoner-Wohlfarth Model (A First Approximation) 614 4. Magnetic Anisotropy 619

4.1 Shape Anisotropy 619 4.2 Magnetocrystalline Anisotropy 620 4.3 Strain-Magnetostriction Anisotropy 621 4.4 M-induced Uniaxial Anisotropy 622 4.5 Oblique-incidence Uniaxial Anisotropy 629 4.6 Unidirectional Anisotropy 633 4.7 Perpendicular Anisotropy 633

5. Magnetization Ripple 635 5.1 Anisotropy-dispersion Model 638 5.2 Micromagnetic Model 639

Hoffmann's Theory; Harte's Theory 6. Quasi-static Manifestations of Ripple 649

6.1 Rotational Hysteresis 649 6.2 HA Domain Splitting (HA Fallback) 652 6.3 Initial Susceptibility 654 6.4 Effects of High KD/KU 658

7. Domain Walls 659 7.1 Structure of Walls 659

Bloch Walls; Ne'el Walls; Crosstie Walls; Other Wall Cal­culations; Experimental Results

7.2 Wall Motion 666 Domain Nucleation and Growth; Coercive Force; Wall Velocity; Magnetic Viscosity; Creep; Wall Streaming

8. Resonance 677 8.1 Ferromagnetic Resonance 677 8.2 Spin-wave Resonance 681 8.3 Energy-loss Mechanisms 685

9. Switching 685 9.1 Coherent Rotation Theory 685 9.2 Experimental Results 687 9.3 Interpretation 691

10. Complex Magnetization Configurations 692 10.1 M Normal to Film Plane 693

MnBi Films (К]_ < 0); Stripe Domains (Ki > 0) 10.2 Multilayer Films 696

Interaction Mechanisms; Domain Wall Interactions 10.3 Cylindrical Films 698

Contents XIX

11. Measurement Techniques 699 11.1 Microscopic Observations 699

Bitter Technique; Kerr and Faraday Magneto-optical Effects; Lorentz Electron Microscopy

11.2 Macroscopic Measurements 702 Mechanical Detection (Torque Magnetometer); Flux-pickup Detection; Magneto-optic Detection; Magneto-resistive or Hall-effect Detection; Ripple-independent H„ Determinations

12. Applications 706 12.1 Flat-film Memory 707 12.2 Cylindrical-film Memory 709 12.3 Future Developments 710 References 710

X I OPTICAL PROPERTIES OF THIN FILMS 721

1. Introduction 721 2. Thin Film Optics 722

2.1 Reflection and Transmission at an Interface 723 2.2 Reflection and Transmission by a Single Film . . . . 724 2.3 Anisotropic and Inhomogeneous Films 728 2.4 Multilayer Films 728 2.5 Optical Absorption 729

3. Optical Constants of Thin Films 732 3.1 Experimental Methods 732

Reflection Methods; Reflectance and Transmittance Methods; Interferometric Methods; Spectrophoto-metric Methods; Critical-angle Method; Polarimetrie Methods (Ellipsometry); Summary

3.2 Results on Optical Constants 741 General Remarks; Metal Films; Abnormal Absorp­tion Phenomenon; Maxwell-Garnett Theory of Abnormal Absorption; Dielectric and Semicon­ducting Films

3.3 Size Effects in Optical Properties 757 Semiconductor and Dielectric Films; Metal Films

4. Thin Film Absorption and Photoemission Phenomena . . . . 761 4.1 Infrared Absorption 761

Optical Modes; Surface Modes 4.2 Magneto-optical Absorption 763 4.3 Plasma-resonance Absorption 764 4.4 Ultraviolet Absorption 767 4.5 Photoemission from Metal Films 767

XX Contents

5. Multilayer Optical Systems 769 5.1 Antireflection Coatings 770

Inhomogeneous Films; Homogeneous Single Films; Multi­layer Films; Infrared Antireflection Coatings

5.2 Reflection Coatings 777 Metal Mirrors; All-dielectric System

5.3 Interference Filters 781 Reflection Filters; Transmission Interference Filters; Frustrated Total-re flection Filter

5.4 Absorptance and Thermal Emittance of Coatings . . . 785 5.5 Thin-film Polarizers 786

6. Concluding Remarks 786 References 787

Author Index 795

Subject Index 827