Our research interest is in wafer-based and flux-driven materials science.  Modern microelectronic, opto-electronic, bio-sensor, and MEMS devices are built on wafers, involving the growth or removal of mono-layers of atoms from the wafer surface or an interface. They are open systems, in which the initial wafer surface area is constant and the flux-driven processes can come from atoms, molecules, or energy beams.  Specifically, we concentrate on interfacial reactions, including metal-Si reaction, Cu-Sn reaction in solder joints, nanoscale interdiffusion and reaction, polarity effect of electromigation on interfacial reaction, and kinetic theories of interfacial reaction.

Our major research areas are (1) Cu-Sn reactions in Pb-free solder metallurgy for electronic packaging technology, (2) Advanced materials reliability problems of microelectronic devices, especially the interaction among electromigration, chemical reaction, and mechanical stress in flip chip technology, and (3) Nanoscale interdiffusion and reactions. In addition, we also conduct exploratory research on (4) Interaction of implanted metallic atoms with dislocations and grain boundaries in Si, and (5) Kinetic theories of interdiffusion and reactions.

On Pb-free solder metallurgy, we study the applications of eutectic SnAg, SnAgCu, SnCu, SnZn as solder bumps to flip chip technology. The wetting reaction and solid state aging of these Pb-free alloys with thin film under-bump-metallization are of interest. Due to the large difference in thermal expansion coefficients between the Si chip and its packaging substrate, the solder joints are stressed. In turn, the stress affects chip-packaging interaction and the integrity of Cu/ultra low k multi-layered interconnect structure on the chip.  The diameter of the solder balls is approaching 50 mm, so electromigration becomes a reliability issue. The advanced materials reliability problems due to a combined action from chemical, electrical, and mechanical forces in flip chip technology will be studied systematically. Electromigration induced microstructure evolution and grain rotation in Pb-free solder alloys requires investigation. A unique nature of most Pb-free solders is that they are very rich in Sn, hence the old topics of Sn whisker, Sn pest, and Sn cry are of interest again. We shall combine micro-diffraction in synchrotron radiation, focused ion beam imaging, and cross-sectional transmission electron microscopy to study these issues. In optical packaging, we interest in how to wet an optical fiber by molten solder and how to achieve high precision alignment by solder joints.

On interconnect technology, our research emphasizes the effect of current crowding on vacancy and solute diffusion in electromigration. The nature of the electromigration force along the direction of current density gradient, i.e., normal to the current flow, will be explored. Why actual failures tend to initiate in the low current density regions will be studied.  The effect of current crowding on joule heating as well as on stress concentration will be analyzed.  The nature of back stress induced by electromigration and whether or not there is back stress in Cu interconnect will be investigated.

On nanoscale interdiffusion and reactions, we study the reaction of ultra thin metal films on nano Si wires and vice versa, and the nanoscale explosion in multi-layered nano-thickness thin films.  Hollow nanostructure formation based on the Kirkendall effect will be investigated.

On extended defects in Si, we study the nucleation, growth, and ripening of dislocation loops formed by ion implantation and post-implantation annealing. The interaction of these loops with metallic atoms such as Ni and Co will be investigated. We also investigate the nano-grid of screw dislocation network, or very small angle twist-type grain boundary in Si bicrystals formed by wafer bonding. Again we examine the interaction of implanted metallic atoms with these dislocation networks.

Kinetic theory of phase transformations in open systems under the constraint of a constant surface area applies to phase changes on a wafer or on a given area of surface or interface will be developed. For example, the constraint of constant area is fundamental to the ripening of hemispherical scallops during the reaction between molten solder and Cu. The existence of nano channels between the scallops will be studied.  The constraint of constant area also applies to the linear rate of grain growth in thin film deposition.

Website: Electronic Thin Film Lab

1. A. M. Gusak, G. V. Lutsenko, and K. N. Tu, “Ostwald ripening with non-equilibrium vacancies,” Acta Mat., 54, 785-791 (2006).
   
   

2. Lingyun Zhang, Shengquan Ou, Joanne Huang, K. N. Tu, Stephen Gee, and Luu Nguyen, “Effect of current crowding on void propagation at the interface between intermetallic compound and solder in flip chip solder joints, “ Appl. Phys. Lett., 88, 012106 (2006).
   
   

3. S. W. Liang, T. L. Shao, Chih Chen, Everett C. C. Yeh, and K. N. Tu, “Relieving the current crowding effect in flip-chip solder joints during current stressing, “ J. Mater. Res., 21, 137-146 (2006).
   
   

4. M. O. Alam, B. Y. Wu, Y. C. Chan, and K. N. Tu, "High electric current density induced interfacial reactions in the micro Ball Grid Array (µBGA) solder joint" Acta Mat., 54, 613-621 (2006).
   
   

5. J. W. Nah, Fei Ren, K. N. Tu, Sridharan Venk, and Gabe Camara, “Electromigration in Pb-free flip chip solder joints on flexible substrates,“ J. Appl. Phys., 99, 023520 (2006).
   
   

6. Young-Woo Okm Tae-Yeon Seong, Chel-Jong Choi, and K. N. Tu, “Field emission form Ni-disilicide nanorods formed by using implantation of Ni an Si couples with laser annealing,” Appl. Phys. Lett., 88, 043106 (2006).
   
   

7. Z. H. Gan, W. Shao, M. Y. Yan, A. V. Vairagar, T. Zaporozhets, M. A. Meyer, A. Krishnamoorthy, , K. N. Tu, A. Gusak, E. Zschech, and S. G. Mhailkar, “Understanding the impact of surface engineering, structure, and design on electromigration through Monte Carlo simulation and in-situ SEM studies,” in “Stress-induced phenomena in metallization,” AIP Proceedings of 8^th Workshop on Stress-induced Phenomena in metallization, Dresden, Germany, vol. 817, p.34-42 (2006).
   
   

8. M. Y. Yan, K. N. Tu, A. V. Vairagar, M. A. Meyer, H. Geisler, A. Preusse, and E. Zschech, “Effect of overburden thicknss on the copper microstructure of dual-inlaid interconnect structures,” AIP Proceedings of 8^th Workshop on Stress-induced Phenomena in metallization, Dresden, Germany, vol. 817, p. 211-216 (2006).
   
   

9. M. Y. Yan, K. N. Tu, A. V. Vairagar, S. G. Mhaisalkar, and Ahila Krishnamoorthy, “A direct measurement of electromigration induced drift velocity in Cu dual damascene interconnects,” Microelectronics Reliability, 46, 1392-1395 (2006).
   
   

10. K. N. Tu, M. Y. Yan, Fei Ren, Joannne Huang, Emily Ou, L. Y. Zhang, and J. W. Nah, “Electromigration in flip chip solder joints,” AIP Proceedings of 8^th Workshop on Stress-induced Phenomena in metallization, Dresden, Germany, vol. 817, p. 327-338 (2006)
    
    

11. Jae-Woong Nah, Fei Ren, Kyung-Wook Paik, and K. N. Tu, “Effect of electromigration on mechanical shear behavior of flip chip solder joints,” J. Mater. Res., 21, 698-702 (2006).
    
    

12. Annie T. Huang, A. M. Gusak, K. N. Tu, and Yi-Shao Lai, “Thermomigration in SnPb composite flip chip solder joints,” Appl. Phys. Lett., 88, 141911 (2006).
    
    

13. S. W. Liang, Y. W. Chang, T. L. Shao, Chih Chen, and K. N. Tu, “Effect of three-dimensional current and temperature distribution on void formation and propagation in flip chip solder joints during electromigration,” Appl. Phys. Lett., 89, 022117 (2006).
    
    

14. Xi Zhang, K. N. Tu, Y. H. Xie, C. H. Tung, and S. Y. Xu, “Single-step fabrication of Ni films with arrayed macropores and nanostructured skeletons,” Adv. Mater., 18, 1905-1909 (2006).
    
    

15. Xi Zhang, K. N. Tu, Y. H. Xie, and C. H. Tung, “High aspect ration Ni structure fabricated by electrochemical replication of hydrofluoric acid etched Si,” Electrochemical and Solid State Letters, 9, C150-C152 (2006).
    
    

16. R. Agarwal, Shengquan E. Ou, and K. N. Tu, “Electromigration and critical product in eutectic SnPb solder lines at 100 C,” J. Appl. Phys., 100, 024909 (2006).
    
    

17. Annie T. Huang, K. N. Tu and Yi-Shao Lai, “Effect of the combination of electromigration and thermomigration on phase migration and partial melting in flip chip composite SnPb solder joints,” J. Appl. Phys., 100, 033512 (2006).
    
    

18. Fei Ren, Jae-Woong Nah, K. N. Tu, Bingshou Xiong, Luhua Xu, and John H. L. Pang, “Electromigration induced ductile-to-brittle transition in lead-free solder joints,” Appl. Phys. Lett., 89, 141914 (2006).
    
    

19. Luhua Xu, John H. L. Pang and K. N. Tu, “Effect of electromigration-induced back stress gradient on nano-indentation marker movement in SnAgCu solder joints,” Appl.  Phys. Lett., 89, 221909 (2006).
    
    

20. Fan-Yi Ouyang, K. N. Tu, Yi-Shao Lai, and Andriy M. Gusak, “Effect of entropy production on microstructure change in eutectic SnPb flip chip solder joints by thermomigration.” Appl. Phys. Lett., 89, 221906 (2006).
    
    

21. Xi Zhang and K. N. Tu, “Preparation of hierarchically porous nickel from macroporous silicon,” J. of Am. Chem. Soc., Communication, 128 (47), 15306-15307, Nov. 2006.  
    
    

22. Xi Zhang, F. Ren, M. S. Goorsky, and K. N. Tu, “Study of the initial stage of electroless nickel deposition on Si (100) substrates in aqueous alkaline solution”, Surface and Coatings Technology, 201 (6), 2724 -2732, Dec. 2006.
    
    

23. Jae-Woong Nah, J. O. Suh, K. N. Tu, Seung Wook Yoon, Vempati Srinivasa Rao, Vaidyanathan Kripesh, and Fay Hua, “Electromigration in flip chip solder joints having a thick Cu column bump and a shallow solder interconnect,” J. Appl. Phys., 100, 123513 (2006).
    
    

24. Zhenghao Gan, A. M. Gusak, W. Shao, Zhong Chen, S. G. Mhaisalkar, T. Zaporozhets, and K. N. Tu, “Analytical modeling of reservoir effect on electomigration in Cu interconnects,” J. Mater. Res., 22, 152-156 (2007).
    
    

25. Luhua Xu, Pradeep Dixit, Jianmin Miao, John H. L. Pang, Xi Zhang, and K. N. Tu, “Through-wafer electroplated copper interconnect with ultrafine grains and high density of nanotwins,” Appl. Phys. Lett., 90, 033111 (2007).
    
    

26. Chengkun Xu, Xi Zhang, K. N. Tu, and Y. H. Xie, “Nickel displacement deposition of porous silicon with ultrahigh aspect ratio,” J. of Electrochemical Society, 154(3), D170-D174 (2007).
    
    

27. K. N. Tu, Chin Chen, and Albert T. Wu, “Stress analysis of spontaneous Sn whisker growth,” J. Mater. Sci: Mater. Electron., 18, 269-281 (2007).
    
    

28. W. Shao, S. G. Mhaisalkar, T. Sritharan, A. V. Vairagar, H. J. Engelmann, O. Aubel, E. Zschech, A. M. Gusak, and K. N. Tu, „Direct evidence of Cu/cap/liner edge being the dominant electromigration path in dual damascene Cu interconnects,“ Appl. Phys. Lett., 90, 052106 (2007).
    
    

29. Xi Zhang, Zhong Chen, and K. N. Tu, "Immersion nickel deposition on blank silicon in aqueous solution containing ammonium fluoride", Thin Solid Films, 515, 4696-4701 (2007).
    
    

30. Jae-Woong Nah, Kai Chen, K. N. Tu, Bor-Rung Su, and Chih Chen, “Mechanism of electromigration-induced failure in flip chip solder joints with a 10 micron thick Cu under-bump-metallization”, J. Mater. Res., 22, 763-769 (2007).
    
    

31. Chengkun Xu, Mingheng Li, Xi Zhang, K. N. Tu, and Y. H. Xie, “Theoretical studies of displacement deposition of Ni into porous Si with ultrahigh aspect ratio”,  Electrochimica Acta, 52, 3901-3909 (2007).
    
    

32. J. W. Jang, J. K. Lin, D. R. Frear, T. Y. Lee, and K. N. Tu, “Ripening-assisted void formation in the matrix of Pb-free solder joints during solid-state aging,” J. Mater. Res., 22, 826-830 (2007).
    
    

Ph.D. students:

Dr. Harqkyun Kim, 1996 Dissertation title: "Instability at wetting tip and wetting interface of Sn-based solders on Cu substrate."  Now at BMR, Orange County, CA.

Dr. Jia-Sheng Huang, 1997 Dissertation title: "Polarity effect on failure of Ni and Ni₂Si contacts on p+-Si and n+-Si under high current densities." Now at Agere, Alhambra, CA.

Dr. Patrick G. Kim, 1998 Dissertation title: "Wetting behaviors of Pb-based and Pb-free solders on Au, Pd, and Ni substrates."  Now at Amkor Technology, Chandler, AZ.

Dr. Lowen Chow, 1999 Dissertation title: "Structure and mechanical propertes of low dielectric constant xerogel thin films"  Now at Intel, Santa Clara, CA.

Dr. Chien-Neng Liao, 1999 Dissertation title: "Thermoelectric characterization of Si thin films in SOI wafers and thermal conductivity of low dielectric constant thin films."  Now assistant professor at National Tsing Hua University, Hsinchu, Taiwan, ROC.

Dr. Chih Chen, 1999 Dissertation title: "Grain boundary structure in twist-type Si bicrystals made from SOI and dopant activation in SOI by high density currents."  Now assistant professor at National Chiao Tung Univeristy, Hsinchu, Taiwan, ROC (starting 08/01/00).

Dr. Dawei Zheng, 1999 Dissertation title: “Measurement of local stress for microelectronics applications,” Now at Lightcrosss, Los Angeles, CA.

Dr. Chengyi Liu, 3/00, Ph.D. Dissertation on "Wetting behavior and electromigration of SnPb solders as a function of alloy composition." Now assistant professor at National Central University, Chungli, Taiwan, ROC.

Dr. Taek Yeong Lee, 2001 Dissertation title: "Electromigration and solid state aging of Pb-free flip chip solder joints and synchrotron radiation study of Sn whisker growth," Now at AT&T Bell Lab., Lucent Technologies, Murray Hill, NJ.

Dr. Peter Sangwoo Nam, 11/01, Dissertation title: "GaAs backside through chip via hole integration using inductively coupled plasma etching." Now at TRW, Redondo Beach, CA.

Dr. Woojin Choi, 2002 Dissertation title "Reliability of Pb-free solders in electronic packaging technology." Now at Intel, Chandra, AZ.

Dr. Hua Gan, 2004 Dissertation Title, “Polarity effect of electromigration on intermetallic compound formation in Pb-free solder V-groove samples.”  Now at IBM T. J. Watson Research Center, Yorktown Heights, NY.

Albert T. C, Wu, 12/04, Ph.D. dissertation on "Electromigration and Microstructure Evolution in Anisotropic Conducting Tin Studied by Synchrotron X-ray Microdiffraction." Now at Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA.

Dr. Emily Shengquan Ou, 2005 Dissertation Tiltle, "The polarity effect of electromigration on intermetallic compound formation and back stress in V-groove solder lines." Now at Intel, Chandler, AZ.

M.Sc. students:

Wang Yang, 1995 Thesis title: "Ultra-fast soldering reaction of eutectic SnPb and eutectic SnBi on Pd surfaces." Now at Vitesse Semiconductor Corp., Camarillo, CA.

Ann A Liu, 1996 Thesis title: "Spalling of Cu-Sn compounds in the soldering reaction between eutectic SnPb and Au/Cu/Cr thin films." Now at TRW, Redondo Beach, CA.

Sharon S. Y. Huang, 1997 Thesis title: "Ultra-thin TaN as diffusion barrier for Cu interconnects." Now at Applied Materials, Santa Clara, CA.

Jessica P. Almaraz, 1998 Thesis title: "Morphology of the wetting reaction of Pb-free solder (eutectic SnAg and eutectic SnBi) on Ni substrates." Now at Northrop, Palmdale, CA.

Ben Zhengyi Jia, 1997 Thesis title: “Stress of Ni thin films on Si wafers,” Now a graduate student in Dept. EE, UCLA.

Yi-Pin Tsai, 1999 thesis title: “Microstructure and properties of low dielectric constant porous polymer PAE thin films,” Now at Intel, Los Angeles, CA.

Judy Pei-Yao Liu, 2000 Thesis title "Microstructure and property of Organically Modified Silicate Film Used for Interlayer Dielectric with Low dielectric Constant." Now at Applied Materials, Hsinchu, Taiwan, ROC.

Quyen Tang Huynh, 2000 Thesis title "Electromigration Study in PbSn Solder Lines." Now at Intel, Los Angeles, CA.

Gu Xu, 11/01, Thesis title "Effect of electromigration in V-shaped solder lines." Now at Dept. MSE, UCLA.

Cindy Wan-Ying Ma, 2002 Thesis title "The synthesis and characterization of porous low-k methylsilsesquioxane films for interlayer dielectric applications."

Jongsung Kim, 2002 Thesis title "Flow kinetics of molten Pb-free solders along V-groove etched on (001) Si surface." Now at UCLA.

Seung-Yub Lee, 2003 Thesis Title "Synthesis and Characterization of Organically Modified Silicates Thin Film for Low Dielectric Constant Materials." Now at Caltech.

Xi Zhang, 2004 Thesis Title, “Electroless Ni metallization of macro-porous silicon for the application to cross-talk isolation in mixed signal integrated circuits.”  Now a Ph. D. candidate in Dept. of MSE, UCLA.

Minyu Yan, 2005 thesis on "The effect of immersion and evaporated Sn coating on the electromigration failure mechanism and lifetime of Cu damascene interconnects." Now a Ph. D. student in UCLA.

Mr. Rajat Agarwal, 2005 Thesis Title, "Electromigration in eutectic SnPb V-groove solder lines at 100 C." Now at Intel, Santa Clara, CA.

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