RELIABLE FINE-PITCH CHIP-TO-SUBSTRATE COPPER INTERCONNECTIONS WITH HIGH-THROUGHPUT ASSEMBLY AND HIGH POWER-HANDLING A Dissertation Presented to The Academic Faculty by Ninad Makarand Shahane In Partial Fulfillment of the Requirements for the Degree Doctorate of Philosophy in the School of Material Science and Engineering Georgia Institute of Technology August 2018 COPYRIGHT © 2018 BY NINAD MAKARAND SHAHANE RELIABLE FINE-PITCH CHIP-TO-SUBSTRATE COPPER INTERCONNECTIONS WITH HIGH-THROUGHPUT ASSEMBLY AND HIGH POWER-HANDLING Approved by: Dr. Rao R. Tummala, Advisor Dr. Pulugurtha Raj Markondeya School of Electrical and Computer School of Electrical and Computer Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Antonia Antoniou, Co-Advisor Dr. Preet Singh School of Mechanical Engineering School of Material Science Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Naresh Thadhani Dr. Vanessa Smet School of Material Science Engineering School of Electrical and Computer Georgia Institute of Technology Engineering Georgia Institute of Technology Date Approved: June 18, 2018 Dedicated to my parents, Makarand and Seema And to my love, Damini ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my advisor, Professor Rao R. Tummala for his vision and ambition, his close guidance towards interdisciplinary research, and for providing extraordinary opportunities of close cooperation with industry partners. I would also like to thank my co-advisor Prof. Antonia Antoniou for exceptional guidance. Her constant push towards deep scientific inquiry encouraged and inspired me towards completing this Ph.D. thesis. I also thank my committee members, Professor Naresh Thadhani, Professor Preet Singh, Dr. Vanessa Smet and Dr. P.M. Raj for their willing and valuable inputs. I would especially like to thank my mentors, Dr. Vanessa Smet and Dr. Raj Pulugurtha for their knowledgeable inputs, painstaking mentoring, and the creative flexibilities they afforded me throughout the course of my research. I would like to extend my appreciation to all my research family at the Georgia Tech 3D System Research Center, especially Kashyap, Vidya, Siddharth, and Bhupender for their support in creating a happy and comfortable environment, through the ups and downs that we have shared together. I thank our visiting engineers, Satomi Kawamoto, Yutaka Takagi, and Hiroyuki Matsuura for their technical supports; and thank the interns, Laura Wambera, and Ramon Sosa who made direct contributions to this thesis. I would like to specially thank Scott McCann for ANSYS modeling and frequent discussions, and Chandrasekharan Nair for constructive discussions on materials processing. I would like to thank my family for their unconditional love and support in helping me achieving my ambitions. Finally, I also thank my love and wife to be, Damini Gandham for being a pillar of faith and support and for encouraging me to always better myself. iv TABLE OF CONTENTS ACKNOWLEDGEMENTS iv LIST OF TABLES viii LIST OF FIGURES ix LIST OF SYMBOLS AND ABBREVIATIONS xvii SUMMARY xx CHAPTER 1. INTRODUCTION 1 1.1 Research Motivation and Strategic Need 2 1.1.1 Pitch scaling for emerging systems 2 1.1.2 Evolution of interconnection and assembly technologies 7 1.1.3 State-of-the-art in all-Cu interconnections – the ‘holy grail’ 11 1.2 Research Objectives and Technical Challenges 14 1.2.1 High-throughput assembly 15 1.2.2 Reliability of chip-to-substrate system (C2S) with CTE mismatch 17 1.3 Proposed Unique Approach 18 1.4 Research Tasks 20 1.4.1 Research Task 1: Modeling, design, and demonstration of copper interconnection system for improved reactivity and accommodation of non- coplanarities 20 1.4.2 Research Task 2: Design and demonstration of high-speed assembly of Cu interconnections 21 1.4.3 Research Task 3: Design and demonstration for reliability at interconnection and IC level 22 1.5 Thesis Organization 23 CHAPTER 2. LITERATURE REVIEW 24 2.1 High-throughput Assembly 25 2.1.1 Improving interfacial reactivity 26 2.1.2 Achieving planar contact 30 2.1.3 High-speed assembly 35 2.1.4 Nano-sintering at low-temperature 38 2.2 Reliability in chip-package architecture 46 2.2.1 Failure modes in Cu pillar flip-chip packages 46 2.2.2 Reliability challenges in stiff-interconnections 47 CHAPTER 3. CU INTERCONNECTIONS WITH METALLIC COATINGS 50 3.1 Materials Design 50 3.1.1 Design Methodology 50 3.1.2 Design before assembly 52 3.1.3 Design for assembly 58 v 3.1.4 Design after assembly 61 3.2 Thermomechanical Process Modeling 65 3.2.1 Effect of surface finish 67 3.2.2 Effect of planarization 71 3.3 Test Vehicle Design 78 3.3.1 Test Vehicle 1 (TV1) at 100µm pitch 79 3.3.2 Test Vehicle 2 (TV2) at 40µm pitch 80 3.3.3 Test Vehicle 3 (TV3) at 50µm pitch 83 3.4 Assembly Demonstration and Characterization 85 3.4.1 Material Characterization 85 3.4.2 Validation of FEM models through assembly demonstration 88 3.4.3 Hi-speed assembly with TC-NCP 92 3.5 Thermal Ageing Test 100 3.6 Chapter Summary 104 CHAPTER 4. RELIABILITY OF CU-EPAG INTERCONNECTIONS 106 4.1 Reliability Modeling 106 4.1.1 Thermomechanical reliability of Cu-EPAG interconnection system 106 4.1.2 Low-K reliability at IC level 112 4.2 Thermomechanical Reliability 115 4.3 Electromigration Test 118 4.4 Summary of reliability characterization 121 CHAPTER 5. CU PILLAR INTERCONNECTIONS WITH NANOCOPPER FOAM CAP 123 5.1 Interconnection System Design 123 5.1.1 Motivation for low-modulus Cu interconnections 123 5.1.2 Design of precursor system for nano-Cu foams 128 5.1.3 Dealloying and pre-assembly characterization 130 5.1.4 Proof-of-Concept 138 5.1.5 Sintering kinetics of nanoporous copper 141 5.2 Manufacturable Synthesis with Co-electrodeposition 163 5.2.1 Choosing precursor alloy system 164 5.2.2 Stack-plating of Cu-Zn and dealloying 165 5.2.3 Co-electrodeposition of Cu-Zn and dealloying 166 5.2.4 Fine-pitch patternability 174 5.3 Fine-pitch Assembly Demonstration 176 5.4 Chapter Summary 180 CHAPTER 6. SUMMARY AND CONCLUSIONS 181 6.1 Research Summary 181 6.1.1 Modelling, design, and demonstration of copper interconnection system for improved reactivity and accommodation of non-coplanarities 182 6.1.2 Design and demonstration of high-speed assembly of Cu interconnections 185 6.1.3 Design and demonstration of reliability at interconnection and IC levels 186 6.1.4 Suggested future work 188 6.2 Conclusions 190 vi 6.3 Technical and Scientific Contributions 192 REFERENCES 194 vii LIST OF TABLES Table 1 Research objectives beyond prior art and associated technical 15 challenges Table 2 Analytical diffusion modeling data 61 Table 3 Isotropic elastic-plastic material properties for FEM model 68 Table 4 Properties of materials within the substrate stack-up 74 Table 6 Plain strain models for estimation of fatigure life 111 Table 7 Isotropic clastic-plastic material properties for low-modulus Cu 125 Table 8 Comparison of thermal conductivity (κ) and electrical 128 resistivity (ρ) between nanofoam and bulk system (at 300K). Table 9 Prior art on synthesis of nano-Cu foams 129 Table 10 Process parameters for Cu nanofoam-to-bulk Cu bonding 139 Table 10 Measured average ligament width (t) and junction dimensions 155 (j) for Electrolyte and F300 samples across as-prepared and 60 min coarsened conditions viii LIST OF FIGURES Figure 1.1 Gap between transistor and system scaling (Courtesy Dr. S. Iyer, IBM) 4 Figure 1.2 Comparison of advanced packaging technologies: I/O density v/s 5 routing density Figure 1.3 (Top) TSMC’s InFO package for processors and (bottom) TSMS 7 processor-memory stacking Figure 1.4 ITRS roadmap predictions (arbitrary units) vs I/O pitch for high- 8 performance interconnections Figure 1.5 Evolution of off-chip interconnection technologies for pitch and 10 performance scaling Figure 1.6 Comparison of the most advanced industry-wide compatible 13 technologies between the C2S and WLP applications Figure 1.7 Unique approaches beyond the current prior art for Cu interconnections 20 without solders Figure 2.1 Oxidation of copper at (a) ambient temperature [20] and (b) at 240oC 26 Figure 2.2 Bonding between two electroplated (111)-oriented Cu films at 200oC – 27 30min: (a) TEM cross-sectional image and (b) electron backscatter diffraction (EBSD) orientation image of the same Figure 2.3 (a) A schematic of SAM capped Cu surfaces with SEM images (b) prior 28 to desorption and (c) after bonding Figure 2.4 (a) Schematic and TEM cross-section of Cu-Cu bonding with 3nm Ti 29 passivation layers and (b) Cu interconnections with ENIG surface finish on glass substrates Figure 2.5 TEM image of Cu-Cu bonding by surface-activated bonding at room 30 temperature; inset figure represents HRTEM image of Cu-Cu bonded interface Figure 2.6 Schematic of non-thermocompression Cu/SiO2 hybrid bonding 32 Figure 2.7 TEM image of the Cu–Cu bonded interface obtained by CMP treatment 33 Figure 2.8 SEM cross-sectional image of Cu/BCB hybrid bonded structure 34 ix Figure 2.9 Cu-Cu insertion bonding: a) Schematic and b) X-section after 3D- 35 stacking with TSVs Figure 2.10 Gang bonding of (a) Cu pillar interconnections at 30um pitch [51] and 37 (b) Cu-Cu interconnections at 6um pitch for CoW 2.5D integration Figure 2.11 Illustration of the sintering mechanisms in a three particles array. The 40 numbers represent the different mechanisms and sources of material. (1) from surface by surface diffusion; (2) from surface by bulk diffusion; (3) from surface by evaporation/condensation; (4) from grain boundary by boundary diffusion; (5) from grain boundary by bulk diffusion; (6) from bulk by bulk-diffusion (through dislocations) Figure 2.12 SEM cross-section of all-Cu interconnection formed by capillary 41 bridging under TC bonding at 76MPa – 160oC with a high- magnification image of the bonded interface Figure 2.13 Nano-Cu paste systems from a) Hitachi Ltd.