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Characterization and Requirements for Cu-Cu Bonds for Three-Dimensional Integrated Circuits by Rajappa Tadepalli

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Characterization and Requirements for Cu-Cu Bonds for Three-Dimensional Integrated Circuits by Rajappa Tadepalli Characterization and Requirements for Cu-Cu Bonds for Three-Dimensional Integrated Circuits by Rajappa Tadepalli B.Tech., Indian Institute of Technology - Madras, India (2000) S.M., Massachusetts Institute of Technology (2002) Submitted to the Department of Materials Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February 2007 © Massachusetts Institute of Technology 2007. All rights reserved. Author ................................................................................................................ Department of Materials Science and Engineering 6 November 2006 Certified by ........................................................................................................ Carl V. Thompson Stavros Salapatas Professor of Materials Science and Engineering Thesis Supervisor Accepted by ....................................................................................................... Samuel M. Allen POSCO Professor of Physical Metallurgy Chair, Departmental Committee on Graduate Students 2 Characterization and Requirements for Cu-Cu Bonds for Three-Dimensional Integrated Circuits by Rajappa Tadepalli Submitted to the Department of Materials Science and Engineering on 6 November 2006, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Three-dimensional integrated circuit (3D IC) technology enables heterogeneous integration of devices fabricated from different technologies, and reduces global RC delay by increasing the device density per unit chip area. Wafer-level Cu-Cu thermocompression bonding provides an attractive route to 3D IC fabrication, with Cu serving as both the electrical and mechanical interconnection between adjacent device layers. While the bonding process is currently employed for such applications, the lack of quantitative understanding of the bond quality and reliability has made developing robust processes extremely challenging. The current work addresses this problem through the development and implementation of bond toughness measurement techniques that investigate the effects of thin film patterning, surface chemistry and process parameters on the Cu-Cu bond quality under a range of loading conditions. The four-point bend test was used to quantify Cu-Cu bond toughness, Gc, under mixed- mode loading and to develop an optimized process flow that enabled the creation of high- toughness bonds (> 5 J/m2) at a bonding temperature of 300 oC. Mixed-mode loading induces significant plastic energy dissipation in ductile layers, resulting in an overestimation of the true adhesive strength of the interface. The chevron test method has been developed to allow bond toughness measurements under mode I loading, thereby probing the ‘true’ work of adhesion of the bonded interface. Furthermore, analysis of the bonded chevron specimen with different layer thicknesses was performed to allow the specimen to be used to characterize the bonded interface under mixed-mode loading conditions. Chevron tests reveal that the toughness of patterned Cu films is a strong function of the feature size and orientation. For debond propagation across periodic bonded and unbonded regions, a pronounced increase in Gc was observed, compared to debond propagation along a continuous bonded interface. Effects of patterning were significantly different in ductile thermocompression and brittle fusion bonded systems, with the latter showing a reduction in toughness due to patterning. The ultimate limit of low temperature Cu-Cu adhesion was investigated using pull-off force measurements in Atomic Force Microscope (AFM) under ultra-high vacuum (UHV) conditions. These measurements show that the work of adhesion of Cu bonds created at room 2 o temperature is ~ 3 J/m , similar to Gc for wafer-level bonds created at 300 C and measured -6 using the chevron test. Deliberate pre-adhesion exposure of the Cu surfaces to 10 Torr O2 leads to a dramatic reduction in adhesion (to 0.1 J/m2), suggesting the formation of a Cu oxide that is detrimental to the Cu-Cu bonding process. The UHV-AFM measurements suggest that strong Cu- Cu bonds can be created by bonding clean Cu surfaces at room temperature, thereby eliminating several thermal stability issues in the thermocompression bonding process. The thermal management problem in 3D ICs containing multiple device layers was examined using an analytical model of forced liquid cooling via Cu-sealed integrated microchannels. Integration of microchannels requires a reduction in the area available for interconnects and adhesion, causing a trade-off between the inter-layer bonded area and the size and density of the channels that can be included. The optimum channel density is a function of the achievable local Cu-Cu bond strength. Thesis Supervisor: Carl V. Thompson Title: Professor of Materials Science and Engineering 3 4 In loving memory of my father 5 6 Acknowledgements First and foremost, I am grateful to God Almighty for blessing me with the strength and energy to pursue my dreams. Without His grace, none of this work would have been possible. During my stint at MIT, I had the privilege of working with some brilliant minds. I would first like to thank my thesis advisor, Professor Carl Thompson, for accepting this novice researcher in his group six years ago and for providing me with all the tools and the intellectual freedom to pursue the projects of my interest. Essentially, Carl taught me how to perform research. Above all, I will always remember Carl for his passion for all things science, and for his sincere and ethical approach towards research. Several other faculty members at MIT have contributed to my thesis work. In particular, I am grateful to my thesis committee members, Prof. Samuel Allen and Prof. Duane Boning, for the stimulating discussions and their valuable comments on the dissertation. I am also indebted to Prof. Donald Troxel and Prof. Rafael Reif, who have provided several key inputs to this project as part of the MIT 3D IC team. I acknowledge the contributions from all my collaborators, especially Prof. Kevin Turner, who has been a constant source of information on fracture mechanics and mechanical testing. Some of the key results in this thesis have been a result of our joint efforts. I am also grateful to Ajay Somani and Prof. Duane Boning for collaborating on the fusion bonding work, which has added a useful new dimension to my research. I am appreciative of the discussions by Prof. Chee Lip Gan and Leong Hoi Liong of NTU- Singapore during our numerous early morning (and late night in Singapore!) meetings. I wish to thank Dr. Andy Fan, Dr. Ramkumar Krishnan, Dr. Syed Alam, Dr. Chuan Seng Tan and Dr. Christine Tsau for the various brainstorming discussions that have helped shape this project. Several research staffs have helped me with my experiments over the course of this work. I am indebted to Kurt Broderick for educating me on microfabrication and cleanroom operation. I also acknowledge the help from 7 all the other MTL and CMSE staffs and students. I am grateful to Dr. Richard Schalek of Harvard University for all the long hours and the useful discussions on the UHV-AFM project. I am thankful to the CVT group members, with whom I have shared many scientific and philosophical conversations over the years. In particular, I will fondly remember the association with Ramkumar Krishnan, Harsh Verma, Zung-Sun Choi, Reiner Mönig, Robert Bernstein, Cody Friesen, Amanda Giermann, Frank Wei, Jihun Oh and Jeff Leib. I count myself lucky to have made several friendships during my stay at MIT, which will hopefully be life long. Though numerous to list, I would like to thank Chaitanya Ullal, Ramkumar Krishnan, Ajay Somani, Harsh Verma, Sajan Saini, Ramachandran Balakrishna and Srinivasan Sundaram for their support during some testing periods in my graduate student life. Mere words do not suffice to express my gratitude to my family that has been chiefly responsible for whatever little I have achieved. My parents, Late Shri. Gopalakrishna and Smt. Vijayalakshmi have toiled and sacrificed a lot in their lives to provide the best education for my brother and myself. This dissertation is just a small token of gratitude for their efforts. My only regret is that my father is not alive to witness this moment. I am indebted to my brother Hari for being there for me whenever it counts. My better half, Padmashree, has been my best friend and the strongest source of support throughout my graduate years. Despite all the hurdles that we faced, Padma has been with me all the way. Moreover, she has shelved many of her ambitions to help me pursue this work. More than anything, Padma has given me the greatest gift of all, our baby Aniket, whose mere presence fills our lives with so much joy. I can only hope that over this lifetime, I can return the love that Padma has given me. 8 Contents 1 Introduction 17 1.1 Vertical Integration of Integrated Circuits……………………………….. 17 1.2 Motivation – Challenges in 3D IC Technology…………………………. 19 1.2.1 Wafer Bonding Characterization………………………………... 19 1.2.2 Thermal Management ………………………………................... 23 1.3 Thesis Objectives………………………………...................................... 23 1.4 Thesis Scope………………………………............................................. 24 2 Wafer Bonding Characterization – Background and Motivation 27 2.1 Three-Dimensional Integrated Circuits................................................. 27 2.1.1 Interconnect
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