|Professor; B.S., National Taiwan University;
M.S., Brown University; Ph.D. in Applied Physics, Harvard
University(1968) ; Senior Manager of Materials Science Department at
IBM T.J. Watson Research Center; Science Research Council Senior
Research Fellow and The Royal Society Guest Research Fellow at
Cavendish Laboratory, UK; Fellow of American Physical Society; Fellow
of the Metallurgical Society; Overseas Fellow of Churchill College;
Application to Practice Award of the Metallurgical Society; Alexander
von Humboldt Research Award for senior US scientists; President of the
Materials Research Society in 1981, Member of Academia Sinica,
Republic of China.|
As of April 2007, the total number of citation N = 10186.
h-factor = 56.
a-factor = N/(h x h) = 3.25.
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
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
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.
Electronic Thin Film Lab
A. M. Gusak, G. V. Lutsenko, and K. N.
Tu, “Ostwald ripening with non-equilibrium vacancies,” Acta Mat.,
54, 785-791 (2006).
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).
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).
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).
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).
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).
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 8th
Workshop on Stress-induced Phenomena in metallization, Dresden,
Germany, vol. 817, p.34-42 (2006).
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 8th Workshop
on Stress-induced Phenomena in metallization, Dresden, Germany, vol.
817, p. 211-216 (2006).
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).
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 8th Workshop
on Stress-induced Phenomena in metallization, Dresden, Germany, vol.
817, p. 327-338 (2006)
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
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,
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,
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,
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).
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).
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).
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).
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).
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
and K. N. Tu, “Preparation of hierarchically porous nickel from
macroporous silicon,” J. of Am. Chem. Soc., Communication, 128 (47),
15306-15307, Nov. 2006.
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.
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).
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
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
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).
K. N. Tu,
Chin Chen, and Albert T. Wu, “Stress analysis of spontaneous Sn
whisker growth,” J. Mater. Sci: Mater. Electron., 18, 269-281
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).
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).
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,
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).
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
List of thesis students in
the last five years
Dr. Harqkyun Kim, 1996
Dissertation title: "Instability at wetting tip and wetting interface
of Sn-based solders on Cu substrate." Now at BMR, Orange
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,
Dr. Lowen Chow, 1999
Dissertation title: "Structure and mechanical propertes of low
dielectric constant xerogel thin films" Now at Intel, Santa
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,
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,
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
row: Fei Ren,
Annie Huang, Prof. King-Ning Tu, Prof. Andrei
Gusak, Shengquan Ou
2nd row: Bob Yu-Huan Xu, Minyu Yan, Jong-ook Suh,
Albert Wu, Xi Zhang, Masayoshi Date