III-V Integrated Circuit Fabrication Technology

Transcription

1 III-V Integrated Circuit Fabrication Technology Shiban Tiku Dhrubes Biswas

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3 III-V Integrated Circuit Fabrication Technology

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5 III-V Integrated Circuit Fabrication Technology Shiban Tiku Dhrubes Biswas

6 Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore editorial@panstanford.com Web: British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. III V Integrated Circuit Fabrication Technology Copyright 2016 by Pan Stanford Publishing 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 or any 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., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN (Hardcover) ISBN (ebook) Printed in the USA

7 Contents Preface Acknowledgments xxiii xxvii 1. Semiconductor Basics Introduction GaAs Device Applications GaAs Crystal Structure Bonding in III V Semiconductors Bonding in a Doped Crystal Energy Band Structure Band Structure and Mobility Free Carrier Concentration and Fermi Level Energy Levels in Doped Semiconductors Impurities in GaAs Specific impurities Crystal Defects Point Defects Dislocations Other Defects Other Properties GaAs Devices p n and Metal Semiconductor Junctions p n Junction Physics I V characteristics Space charge and junction capacitance Metal Semiconductor Junctions Junction physics Junction characteristics MESFETs Basic MESFETs Low-Noise FETs FETs for Digital Logic Circuits 51

8 vi Contents 2.3 HEMTs and PHEMTs Device Operation Bipolar Junction Transistors Phenomenological Description of the BJT Current Voltage Characteristics of a BJT HBT Principles of Operation Basic Transport Equations Current Gain and Injection Efficiency Figures of Merit for HBTs PIN Diodes IMPATT Read-Type IMPATT Gunn Diodes MOSFET Metal Insulator Semiconductor Devices I V Characteristics Remarks on Applications Phase Diagrams and Crystal Growth of Compound Semiconductors Phase Diagrams Introduction Phase Diagram Types Isomorphous phase diagram Eutectic diagrams Peritectic diagrams Congruent Transformation Crystal Growth Starting Materials and Compounding Method Single-Crystal Growth Bridgman/gradient freeze technique Liquid-encapsulated Czochralski method Vertical boat and vertical gradient freeze methods 89

9 Contents vii Vapor pressure controlled Czochralski method InP Crystal Growth Doping and Resistivity Control n- and p-type Crystals Epitaxy Liquid-Phase Epitaxy Vapor-Phase Epitaxy System Configuration VPE Chemistries for GaAs Substrate orientation Halide process Ga AsCl 3 H Hydride process Ga AsH 3 HCl H MOCVD Process control and mechanisms MOCVD sources Doping HBT growth Volume production Specific materials Selective epitaxy In situ monitoring of epigrowth Molecular Beam Epitaxy System Description MBE Sources RHEED intensity oscillation Specific Materials AlGaAs InGaAs InGaAlAs GaN and related alloys Doping HBT Growth AlGaAs HBT InGaP HBT 127

10 viii Contents InP HBT GaN HBT PHEMTs Atomic Layer Epitaxy GaAs on Silicon Substrates Epilayer Characterization Concluding Remarks Photolithography Introduction Mask Making Basics of Printing/Imaging Typical Etch Photoresist Process Lift-Off Photoresist Process Photoresist Resolution and Contrast Sensitivity Optical Photoresist Reaction Mechanism Image Reversal of a Positive Photoresist Negative Resists Resolution Improvement Physics of Photolithograpy Diffraction Step and Repeat Projection Aligner Pattern Registration Resist Processing Prebake Dehydration Adhesion Promoter Resist Coating Soft Bake Exposure Standing Waves and Other Interference Effects Developing De-scum Postbake Stripping Electron Beam Lithography X-Ray Lithography Process Monitoring 166

11 Contents ix Optical Systems SEM Advanced Photolithography Techniques Wet Etching, Cleaning, and Passivation Introduction Wet Etch Advantages Wet Etch Disadvantages GaAs Etching Basics Mechanism GaAs Etch Chemical Systems Hydrogen Peroxide Based Etches H 2 SO 4 :H 2 O 2 :H 2 O system H 3 PO 4 :H 2 O 2 :H 2 O system Citric acid system (C 3 H 4 (OH) (COOH) 3 H 2 O:H 2 O 2 :H 2 O) Ammonia peroxide system (NH 4 OH:H 2 O 2 :H 2 O) HCl-based systems Special Etches Polishing etches Crystallographic etches Wet Etches InP InGaP InGaAs Wet Etching of GaN/AlN Etching of Other Materials Wet Etching in Production Wet Etch Application Examples Ion damage avoidance Wet etching of multilayer III V compounds Cleaning Plasma Cleaning Surface Passivation Wet-Chemical Passivation Chalcogenide Passivation Dielectric Passivation 188

12 x Contents 7. Plasma Processing and Dry Etching Plasma Processing Plasma Basics Glow Discharge Plasma Voltage Distribution Interaction of Ions with a Surface/Sputter Yield Dry Etching Problems with Wet Etching Advantages of Dry Etching Plasma Etch Systems Reaction Basics Rate Equation Process Parameters Plasma Etch System Types Barrel reactor Parallel-plate planar reactor Downstream reactor High-density plasma reactor ECR ICP Ion milling Etch Processes Etch Rate and Selectivity Loading Selectivity Uniformity Microuniformity CD and Etch Profile Plasma Etching of Materials Used in III V IC Processing Selective Etches Silicon Nitride and Oxide Etching Metal Etching Refractory metals Aluminum Gold/copper Organic films High-Aspect-Ratio Etching Through-Wafer Via Etching 225

13 Contents xi Etch chemistry for profile control Wet Etching Aspect-Ratio-Dependent Etching Plasma Damage Particle and Veil Generation Etch Process Monitoring Film Monitoring Gas-Phase Monitoring Optical Emission Deposition Processes Physical Vapor Deposition: Introduction Vacuum Basics Flow Regimes Pumping Systems for Semiconductor Processing Cryogenic Pumps Turbomolecular Pumps Pressure Measurement Evaporation Evaporation Sources Electron beam sources Deposition Rate Vapor pressure Evaporation rate Film thickness variation Deposition Rate Monitors Alloy Deposition Film Growth Mechanism Sputter Deposition Advantages of Sputter Deposition Deposition System Types Planar diode Triode Magnetron sputtering RF Sputtering Reactive Sputtering Bias Sputtering System selection 260

14 xii Contents Mechanism and Rates Plasma-Enhanced Chemical Vapor Deposition Film Requirements CVD Systems CVD reactor types Plasma-Enhanced CVD Production Multistation System High-density plasma systems Atomic Layer Deposition ALD Principles ALD Reactors Ion Implantation and Device Isolation Introduction Advantages Disadvantages Ion Implantation: Theory Theory of Ion Stopping Channeling Transverse Effects Implant Damage Ion Implantation Systems Implantation System Parts Ion source Ion extraction and analyzing device Accelerator tube Beam scanning system System end station Ion Implanter Types System/Process Issues Masking Considerations Doubly Ionized Species Common Ion Implant Species for GaAs n-type Dopants p-type Dopants Implants for Isolation Ion Implant Characterization Sheet Resistivity Monitoring Optical Dosimetry 296

15 Contents xiii C V Method Implant Activation Annealing Encapsulation for Annealing Rapid Thermal Annealing History of development System description Temperature control Process Description Activation of Dopants n-type Dopants p-type Dopant Activation Device Isolation Introduction Isolation by Etching Ion Implant Isolation Mechanism Isolation-Related Reliability Issues for HBT Diffusion in III V Compound Semiconductors Introduction Rate Equations Diffusion Basics Basic Mechanisms Interstitial mechanism Substitutional mechanism Kick-out mechanism Interstitial-substitutional mechanism Impurity Diffusion Rates in GaAs Diffusion Equations for III V Semiconductor Processing Constant Diffusion Coefficient Thin-film solution Diffusion from a constant source Diffusion from a limited source 324

16 xiv Contents Concentration-dependent diffusion coefficient Interstitial-Substitutional Diffusion Measurement of Diffused Layers Diffusion in GaAs Diffusion by Periodic Table Groups Zn Diffusion in GaAs Sulfur Diffusion in GaAs Diffusion Systems Rapid Thermal Diffusion Ohmic Contacts Introduction History Theory of Metal Semiconductor Ohmic Contacts Contact Resistance Contact Resistance Measurement by TLM Ohmic Contact Technology for n-type Contacts Epigrown Contacts Contacts with heavy donor doping Contacts with lower barrier height Alloyed Ohmic Contacts Gold:germanium contacts Silicon:tin contacts Indium-based contacts Ohmic Contact Deposition Alloy Process and Alloying Systems Alloying systems Mechanism of Contact Formation Refractory Contacts Ohmic Contacts to p-type GaAs Ohmic Contacts to InP Devices Ohmic Contacts to GaN Mechanism Ohmic Contact Corrosion 357

17 Contents xv 12. Schottky Diodes and FET Processing Schottky Diodes Depletion Width Schottky Diode Metallization Reverse Breakdown FET Gate Fabrication Gate Metallization and Fabrication Gate Recess Process Gate Formation T-gate Digital FETs Gate Fabrication Self-Aligned n+-technique Substitutional Gate Processes Mixed-Signal Process Heterojunction and Insulated Gate FETs HMESFET SAG FET Technology: Remarks Pregate Surface Preparation and Passivation Current Developments HEMT Process Introduction Device Fabrication InGaP HEMT Low-Noise Process Power Amplifier Process Switch Process InP HEMT Processing Issues Gate Walk Gate Sinking Breakdown Voltage Improvement HBT Processing Introduction Review of HBT Process Evolution Basic HBT Fabrication Process Self-Alignment Base Emitter Self-Alignment 407

18 xvi Contents Other Self-Alignment Schemes Collector-Up HBT Common HBT Epimaterials 411 I4.6.1 InGaP/GaAs HBTs InP HBTs HBT Contacts HBT Geometry Base Width Layout Comparison HBT Fabrication Issues Junction Considerations HBT Epilayer Design Emitter Layer Design Collector Layer Subcollector layer Base Layer Design Other HBT Structure Improvements Graded-Base HBTs Double-Heterojunction Bipolar Transistor BiFET and BiHEMT Processing BiFET Process Stacked Devices Merged Devices Guidelines for Extra Layers Epitaxial Layer Screening Fabrication Process BiFET Gate Process BiHEMT Process BiHEMT Process and Yield Improvement MOSFET Processing Introduction Oxidation Wet Oxidation Liquid-Phase Oxidation Dielectric Passivation Atomic Layer Deposition 453

19 Contents xvii 16.5 p-type Devices Concluding Remarks Passive Components Resistors Semiconductor/GaAs Resistors Thin-Film Resistors Common TFR Materials Capacitors MIM Capacitors Silicon Nitride for MIM Stacked capacitors Inductors Interconnect Technology Introduction Interconnect Requirements Electrical Requirements Adhesion and Barrier Requirements Diffusion and Electromigration Effects Interlevel Dielectric Layer Requirements Production Interconnect Processes Baseline Gold Interconnect Process Plated Metal Interconnect Process Air Bridge Process Digital GaAs Interconnect Process Future of Interconnect Technology Copper Interconnects Backend Processing and Through-Wafer Vias 497 Section I: Through-Wafer Via Process Introduction Wafer Bonding Wafer Thinning TWV Photolithography TWV Etch Backside Metallization Backside Plating 504

20 xviii Contents 19.7 Backside Street Etching Wafer Demounting/Debonding Wafer Dicing Scribe and Break Laser Dicing 509 Section II: Wafer-Bumping Process Introduction Advantages of the Wafer-Level Bump Process Requirements of Components of the Solder Ball or Pillar Process Underbump Metallurgy Solder Ball and Pillar Fabrication Process Solder Ball Process Copper Pillar Process Electroplating Electroplating History Electroplating Fundamentals Electroplating Bath Types Electroplating Deposition Process Pulse Plating Metal Deposition Mechanisms Polarization Diffusion and Mass Transport Microthrowing Power Brightening Process Monitoring Hull Cell Electroless Plating Copper Electroplating Large-Volume Production Measurements and Characterization Introduction Sheet Resistance Four-Point Probe Method Van der Pauw Method Contactless Resistivity Measurement 539

21 Contents xix 21.4 Carrier Mobility Hall Mobility Drift Mobility Doping Profile by C V Method Schottky Diode Parameter Measurement Current Voltage Method Activation Energy Method Capacitance Voltage Method for Schottky Diode Barrier Height Measurement FET Characteristics FET Transconductance FET Source Resistance Measurement HBT Parameter Extraction Output I V characteristics Gummel Plot Emitter Resistance V CE Offset R on RF Characterization Introduction S-Parameter Measurements RF Figures of Merit VSWR Load pull test PAE Linearity Noise figure Smith Chart Film Thickness and Refractive Index Ellipsometry Interferometry Film Stress Measurement Reliability Introduction Basic Reliability Testing Test Procedure Step Stress Test Temperature measurement 577

22 xx Contents Simulation of operation FET/HEMT Failure Modes and Mechanisms Gate Sinking Ion-Induced Failure Mechanisms Effect of Hydrogen Mobile Ion Contamination Hot Electron Trapping Surface State Effects HBT Degradation and Reliability HBT Reliability Issues Related to Base Doping Hydrogen and Ion Implant Isolation Ohmic Contact Degradation Other III V IC Failure Mechanisms Electromigration Moisture Ingression and Corrosion Stress-Induced Burnout GaN Device Reliability GaN Devices 593 Section I: GaN Electronic Devices Introduction Bulk Crystal Growth Hydride Vapor-Phase Epitaxy MOCVD templates High-Pressure Solution Growth Ammonothermal Growth Epitaxial Growth MBE OMVPE Doping HVPE Device Physics and Device Types Process Technology Etching and Surface Passivation of GaN Ohmic Contacts Schottky Contacts Implant Isolation 613

23 Contents xxi 23.6 Device Fabrication AlN/GaN HEMT Device Performance Optimization Surface passivation Normally Off GaN Devices MMIC Fabrication Reliability General Reliability Concerns Gate sinking Ohmic contacts Current Collapse III N HBT Devices GaN HBT Device Challenges Current State-of-the-Art Performance Other Devices 628 Section II: GaN Optical Devices Introduction to LEDs Junction Luminescence Device Behavior: Electrical Optical Characteristics LED Processing Visible LEDs UV LEDs Epitaxial Growth Current Challenges Current Performance Introduction to III N Lasers Basic Principles Diode Laser Fabrication Fabry Perot Semiconductor Diode Laser VCSEL RF MEMS Introduction Differences with Silicon Basics of MEMS Ohmic Contact Switches Capacitive Switches 653

24 xxii Contents Electrostatic or Capacitive Excitation Actuation Voltage Piezoelectric Excitation Process Technology OMMIC Process University of Illinois Process Examples of Applications Waveguide Switch Fabry Perot Filter MEMS on MMIC Hybrid Circuits 663 Appendix 667 Acronyms 673 Index 677

25 Contents xxiii Preface In this Internet age, practicing engineers still need a book that they can keep on their desk. This book is aimed for them and also graduate students and engineers new to the field of III V semiconductor integrated circuit (IC) processing. This book specifically addresses the needs of students who know semiconductor theory but lack detailed processing knowledge. The content is chosen on the basis of the needs of students as seen by a teacher and the needs of practicing engineers dealing with processing issues as seen by an experienced process engineer. GaAs processing has reached a mature stage, a long way from a few decades ago, when it was more of an art than a science. New semiconductor compounds are emerging that will dominate future materials and device research; however, the processing techniques used for GaAs will still remain relevant. This book covers all aspects of the current state of the art of III V processing, with emphasis on heterojunction bipolar transistors (HBTs), the volume leader technology, having grown due to the explosive growth of wireless technology. The book s primary purpose is to discuss processing; only necessary equations are derived and device behavior is discussed for the purpose of understanding device figures of merit and electrical parameters that engineers need to understand and control. All aspects of processing of active and passive devices, from crystal growth to backside processing, including lithography, etching, and film deposition, are covered. New material systems based on GaN are playing a larger role on the development side; although the etching chemistries, deposition materials, and temperature regimes are different, similar principles apply. The most promising structures of these material systems and devices are covered in the book. The book covers semiconductor material basics, physics of devices used in semiconductor IC processing, and all the processing technologies used in III V semiconductor fabrication. In the discussion, differences with silicon IC processes are emphasized. Crystal growth and particularly epitaxy are discussed in depth because of the special role played by them and device structures

26 xxiv Preface made possible by them. Photolithography, ion implantation, wet and plasma etching, and deposition of films are covered in detail. Thermal processes and diffusion are discussed to the level needed for III V processing. Schottky and ohmic contact physics and processing are discussed from a practical point of view for controlling these in high-volume production. All the device technologies currently in use in the III V semiconductor marketplace are discussed in depth, including recently introduced bipolar field-effect transistor (BiFET) and bipolar high-electron-mobility transistor (BiHEMT) technologies. Device types that are emerging and expected to be important in the near future, like metal oxide semiconductor field-effect transistors (MOSFETs), are also introduced. Passive devices and interconnects are covered, being integral to monolithic microwave integrated circuit (MMIC) fabrication. Also, backside processing, which is absolutely necessary for high efficiency, is described in detail and wafer-scale bumping is introduced, being critical to future higher-frequency needs. Characterization of films and semiconductor layers, as well as device parameter measurement, is covered in detail. Reliability issues relevant to III V semiconductors are discussed. Finally, emerging GaN devices and microelectromechanical systems (MEMS) are briefly described. Most published books on the market emphasize III V device physics. No new processing book has been published in a decade. Published books are old and cover mostly FET processing. Ralph Williams s book, Modern GaAs Processing, is over 20 years old and does not cover processing technologies in detail. S. K. Ghandhi s book, VLSI Fabrication Principles: Silicon and Gallium Arsenide, covers processing techniques in detail but IC processing very briefly. This book is also old, published in the 1980s. Fazal Ali s book, HEMTs and HBTs: Devices, Fabrication and Circuits, covers fabrication very broadly and was also published in 1991, over 20 years ago. Baca and Ashby s book, Fabrication of GaAs Devices, has a narrow focus, specializing in cleaning and passivation; basic IC processing techniques are not covered. It was published in 2005, a decade old now. This (present) book covers all aspects of processing, from crystal growth to backside processing. It covers the current volume production device types, HBTs, HEMTs, etc. The book is not restricted to GaAs; other emerging III V materials are covered, too.

27 Preface xxv Epigrowth, device structure, and processing discussions are connected together through different chapters. Processing techniques relevant to III V IC fabrication are described as they are used in III V processing facilities in high-volume production. Process flows are illustrated by step-by-step block diagrams. Scanning electron microscopy (SEM) pictures of actual devices are included, where needed. This is one book to find any topic relevant to III V processing. Practical process problems and ways to handle these are described. The current understanding of III V processing has come a long way from the era when GaAs processing was based on practical knowledge and company trade secrets. This book attempts to connect practice on the fabrication floor to current scientific understanding. Shiban Tiku Dhrubes Biswas

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29 Acknowledgments The earlier books on GaAs and III V semiconductor materials processing were written in an era when GaAs was considered the technology of the future. This present work, although inspired by those, is aimed at fulfilling the needs of this era in which the technology is well established and perhaps becoming the technology of the past, while paving the way for future technologies. Many minds and hands have contributed to this work. I am indebted to all my teachers over the years, who left an indelible mark on my life. I also thank the people of India for the almost free education I received. This book would not have been possible without support from Skyworks Solutions management, Ravi Ramanathan, Nercy Ebrahimi, and Andy Hunt, and IP council Donald Bollella, who weighed the benefits of contributing to the III V industry worldwide over the risk of disclosing trade secrets. A lot of data came from my fellow engineers at Skyworks and the CS MANTECH community in general. Early feedback from Martin Brophy (Avago) and Peter Asbeck (UCSD) encouraged me. In particular, help from the following Skyworks colleagues is acknowledged: Heather Knoedler, Jens Riege, Dave Crawford, Ravi Ramanathan, Mike Sun, Jiro Yota, Pete Zampardi, Lance Rushing, Cristian Cismaru, Sam Mony, Lam Luu, and Manjeet Singh. Constant support from my wife, Sushma, and son, Vikram, helped me during difficult times. I am also thankful to Archana Ziradkar of Pan Stanford Publishing for systematic editorial help and to Barron Miller for drafting of many of the figures. Shiban Tiku

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