ELECTROMIGRATION-INDUCED FAILURE CHARACTERISTICS OF GMR SPIN-VALVES AND MAGNETIC MULTILAYERS FOR THE ELECTRICAL RELIABILITY OF SPINTRONIC DEVICES JING JIANG (M. Eng., Hefei University of Technology, P. R. China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE January 2011 ACKNOWLEDGEMENT ACKNOWLEDGEMENT I would like to take this opportunity to thank all those who have helped and supported me in completing the work within this dissertation. First and foremost, I would like to give my utmost gratitude to my supervisor, Assistant Professor Seongtae Bae, for his kind and consistent concern, support and guidance in the project and also all the valuable discussion on the experimental results. He is a generous and caring mentor, always willing to offer a helping hand when I encountered difficulties over the past few years. Moreover, his active attitude and precise spirit of doing research have great influence on my personality. I do appreciate his precious advice and counseling. Without his encouragement and understanding, I would not have been able to achieve this research goal. I am also grateful to be in a caring, supportive and cooperative research team. I’d like to thank Dr Sunwook Kim, Dr Howan Joo, Mr. Minghong Jeun, Ms. Naganivetha Thiyagarajah, Ms. Lin Lin, and Ms. Ping Zhang for their help in carrying out the experiment. I would especially like to thank Mr. Dinggui Zeng working closely with me in BML, Mr. Bee Ling Tan in DSI helping me do the AES characterization and Dr Hojun Ryu from ETRI (Korea) helping me do the TEM analysis. Their valuable assistance and support have been indispensable for my research work. I would also like to express my heartfelt appreciation for all the staffs in BML and ISML for their efforts in maintaining the functionality of the equipments, caring for the welfare of the students, and making our life here safe and pleasant. In addition, deep appreciation also goes to my friends in Singapore and China for having faith in me and i ACKNOWLEDGEMENT encouraging me to pursue my research goal. Last but not least, I would not have survived the PhD process without the support and understanding from my parents. Equally noble and important is my beloved husband Yongshan Yuan, who accompanies me throughout the most severe time. Without his patience, continuous support and encouragement, all these things would have never been possible. ii TABLE OF CONTENTS TABLE OF CONTENTS ACKNOWLEDGEMENT ........................................................................................................... i TABLE OF CONTENTS ........................................................................................................... iii SUMMARY ................................................................................................................................ vi LIST OF FIGURES ................................................................................................................. viii LIST OF TABLES ................................................................................................................... xiv CHAPTER 1 INTRODUCTION ............................................................................................... 1 1.1 Background and Motivation ................................................................................................ 1 1.2 Objectives and Work Done ............................................................................................... 10 1.3 The Outline of this Thesis ................................................................................................. 12 References ............................................................................................................................... 14 CHAPTER 2 ELECTROMIGRATION AND GIANT MAGNETORESISTANCE - RELEVANT TOPICS ............................................................................................................... 20 2.1 General Aspects of Electromigration in Thin Films ......................................................... 20 2.1.1 Theoretical Development of Electromigration ........................................................... 20 2.1.2 Grain Boundary Diffusion and Atomic Flux Divergence .......................................... 24 2.1.3 Structural Factor ......................................................................................................... 28 2.1.3 Current Crowding and Thermal Gradient Effects ...................................................... 32 2.1.4 Self Healing Effect ..................................................................................................... 37 2.2 Inter-diffusion in Magnetic Multi-layers........................................................................... 38 2.3 Methods to Improve the EM Resistance ........................................................................... 42 2.3.1 Grain Size and Bamboo Structure .............................................................................. 43 2.3.2 Addition of Solutes .................................................................................................... 46 2.3.3 Diffusion Barrier ........................................................................................................ 47 2.4 Black Equation .................................................................................................................. 48 2.5 Giant Magnetoresistance (GMR) and Interlayer Coupling in Magnetic Multi-layers ...... 50 References ............................................................................................................................... 56 CHAPTER 3 EXPERINMENT AND CHARACTERIZATION TECHNIQUES .............. 68 3.1 Preparation of EM Test Samples ....................................................................................... 69 iii TABLE OF CONTENTS 3.1.1 Input/Output Electrode Pad Design ........................................................................... 69 3.1.2 EM Test Device Patterning and Fabrication .............................................................. 71 3.2 Lifetime Measurement and Failure Criterion .................................................................... 75 3.3 Fabrication and Characterization Techniques ................................................................... 78 3.3.1 Deposition Technique - AJA multi-target Sputtering System .................................... 78 3.3.2 Surface or Interface Characterization and Microstructure Analysis Techniques ....... 81 3.3.2.1 Field-Emission Scanning Electron Microscope (FE-SEM) ........................................ 81 3.3.2.2 Transmission Electron Microscopy (TEM) ................................................................ 83 3.3.2.3 Atomic Force Microscopy (AFM) .............................................................................. 85 3.3.2.4 Auger Electron Spectroscopy (AES) .......................................................................... 87 3.3.3Measurement of Magnetic Properties ......................................................................... 88 3.3.3.1 Vibrating Sample Magnetometer (VSM) .................................................................... 88 3.3.3.2 Four-point Probe CIP MR Measurement .................................................................... 90 References ............................................................................................................................... 91 CHAPTER 4 ELECTROMIGRATION-INDUCED FAILURE CHARACTERISTICS OF FM/Cu/FM BASED SPIN-VALVE MULTI-LAYERS ......................................................... 93 4.1 Effects of Cu Inter-diffusion on The Electromigration Failure of FM/Cu/FM Tri-layers for Spin-valve Read Sensors ......................................................................................................... 93 4.1.1 Introduction and Motivations ..................................................................................... 93 4.1.2 Experimental Works ................................................................................................... 94 4.1.3 Results and Discussion ............................................................................................... 96 4.1.3.1 EM-induced failure lifetime dependence on Cu spacer thickness .............................. 96 4.1.3.2 Effect of FM/Cu chemical interface on the EM lifetime ............................................ 98 4.1.3.3 Activation energy and current dependence factor, “n” values of NiFe(3)/Cu(2)/NiFe(3 nm) tri-layers ........................................................................................................................ 100 4.1.3.4 Typical EM-induced failure characteristics observed in NiFe(3)/Cu(2)/NiFe(3 nm) tri- layers .................................................................................................................................... 101 4.1.4 Summary and Conclusions ....................................................................................... 103 4.2 Electromigration-Induced Failure Characteristics of NiFe/(Co)/Cu/(Co)/NiFe Spin Valve Multi-layers ........................................................................................................................... 103 4.2.1 Introduction and Motivations ................................................................................... 103 4.2.2 Results and Discussions ..........................................................................................