Browsing by Author "Tu, K. N."

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A unified model of mean-time-to-failure for electromigration, thermomigration, and stress-migration based on entropy production
(2019-08-21)
Tu, K. N.
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Gusak, A. M.
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國際半導體學院
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International College of Semiconductor Technology
We have revisited Black's equation of mean-time-to-failure (MTTF) for electromigration from the viewpoint that in irreversible processes, entropy production is the controlling behavior. We justify that the power factor on current density is n = 2, as given in the original Black's equation. Furthermore, on the basis of entropy production, we provide a unified model of MTTF for thermomigration and stress-migration. We note that up to now, no MTTF for thermomigration and stress-migration is given. Published under license by AIP Publishing.
Anisotropic grain growth and crack propagation in eutectic microstructure under cyclic temperature annealing in flip-chip SnPb composite solder joints
(2013-07-01)
Liang, Y. C.
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Lin, H. W.
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Chen, H. P.
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Chen, C.
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Tu, K. N.
The evolution of eutectic structure under temperature cycling tests (TCTs) in SnPb composite solder joints has been investigated. After 500 cycles of TCT, the Sn grains coarsened and developed anisotropic stripes close to the necking site in the solder joint because of stress-induced atomic migration. Then, cracks triggered by thermal stress were observed to propagate along the Sn stripe interfaces. After a prolonged 14,410 cycles of TCT, the cracks expanded across the entire solder joint and led to electrical open failure. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Asymmetrical growth of Cu6Sn5 intermetallic compounds due to rapid thermomigration of Cu in molten SnAg solder joints
(2012-10-01)
Guo, Ming-Yung
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Lin, C. K.
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Chen, Chih
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Tu, K. N.
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材料科學與工程學系
We observed asymmetrical growth of Cu6Sn5 intermetallic compounds (IMCs) on the two interfaces of Cu/SnAg/Cu solder joints during reflow at 260 degrees C on a hot plate. The IMCs grew to 12.3 mu m on the cold end and 3.5 mu m on the hot end after reflow for 40 min. However, the consumption of Cu on the cold end is less than that on the hot end. We propose that rapid thermomigration of Cu is responsible for the asymmetrical growth of the IMCs. With the simulated thermal gradient of 51 degrees C/cm across the liquid solder, the heat of transport of Cu is calculated as 20 kJ/mol. (C) 2012 Elsevier Ltd. All rights reserved.
Blocking hillock and whisker growth by intermetallic compound formation in Sn-0.7Cu flip chip solder joints under electromigration
(2010-05-01)
Liang, S. W.
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Chen, Chih
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Han, J. K.
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Xu, Luhua
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Tu, K. N.
Mass extrusion occurs in electromigration at the anode in cross-sectioned Sn-0.7Cu flip-chip solder joints. In a pair of joints, the hillock squeezed out at the anode on the board side is more serious than the whisker grown at the anode on the chip side. The difference of mass extrusion has been found to be affected by the amount of intermetallic compound (IMC) formation in the solder bump. It is found that when a large amount of Cu-Sn IMCs form in the grain boundaries of Sn grains, small hillocks are extruded on the anode end. It is proposed that the excessive IMC formation may be able to block the diffusion path of Sn atoms, so the growth of both the Sn whiskers and hillocks are retarded. (C) 2010 American Institute of Physics. [doi:10.1063/1.3410796]
Chip-to-chip copper direct bonding in no-vacuum ambient using (111) oriented nano-twinned copper
(2018-01-01)
Juang, Jing Ye
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Shie, Kai Cheng
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Li, Yu Jin
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Lin, Benson
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Chang, Chia Cheng
In this study, we demonstrate the chip-to-chip copper direct bonding by using (111) oriented nano-twinned copper in N-2 ambient, without vacuum. A well bonded interface in the Cu-to-Cu joint was identified by the focus ion beam (FIB) microstructure observation. The secondary electron images (SEI) showed a void-less bonding interface within the bonded Cu joint. In addition, a die shear test was conducted. The test result revealed that the Cu joint has a robust bonded Cu structure due to its high value. The shear strength is more than 100 MPa. There is nearly twice strength value higher than the traditional SnAg solder joint (64 MPa). In addition, the scanning electron microscope (SEM) image showed the joint fractured in a ductile manner. Besides, we also performed the resistance measurement by using Kelvin probes on the bonded chip-to-chip test vehicle. The result showed 4.12 m Omega in single joint resistance and 4.26 x 10(-8) Omega.cm(2) in contact resistivity. More than 30% resistance reduction has been confirmed as compared to the traditional SnAg solder joint (6.32 m Omega). Moreover, for further post annealing process on the bonded test vehicle, we can further reduce the joint resistance by the value of 2.9 m Omega. It approaches the value of bulk Cu. The evidences revealed that the Cu-to-Cu joint is superior to the traditional SnAg solder. In last, the chip-to-chip copper direct bonding by using (111) oriented nano-twinned copper in N-2 ambient was achieved.
Communication-Formation of Porous Cu3Sn by High-Temperature Current Stressing
(2016)
Lin, C. K.
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Chen, Chih
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Chu, David T.
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Tu, K. N.
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材料科學與工程學系
Cu3Sn is a compound commonly formed in solder joints on Cu. In this study, two different Cu3Sn structures were observed at different stages of current stressing. A normal layer-type and pore-less Cu3Sn structure was obtained at the current density of 2.27 x 10(4) A/cm(2) at 222 degrees C after 12 h. However, a porous Cu3Sn structure was formed under the same current stressing conditions after 22 h. The formation of porous Cu3Sn is explained by the mechanisms of phase transformation and reaction. The porous Cu3Sn structure is frequently observed in Cu/solder/Cu microbumps with a reduced solder height after electromigration tests. (C) 2016 The Electrochemical Society. All rights reserved.
Comparison of oxidation in uni-directionally and randomly oriented Cu films for low temperature Cu-to-Cu direct bonding
(2018-07-13)
Tseng, Chih-Han
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Tu, K. N.
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Chen, Chih
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材料科學與工程學系
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國際半導體學院
Cu-to-Cu direct bonding has attracted attention because it has been implemented in CMOS image sensors. Prior to the bonding, the oxides on the Cu surface needs to be removed, yet the surface may oxidize right after cleaning. Thus, oxidation is an inherent issue in the application of Cu direct bonding. Our previous study reported that Cu direct bonding can be achieved below 250 degrees C by using (111)-oriented nanotwinned Cu because it has the fastest surface diffusivity. However, the oxidation behavior of the nanotwinned Cu is unclear. Here, we examined the oxidation behavior of highly (111) and (200) oriented, and randomly-oriented Cu films at temperatures ranging from 120 to 250 degrees C. Transmission electron microscopy was used to measure the oxide thickness. The results show that the oxidation rate of (111)-oriented nanotwinned Cu has the lowest oxidation rate among them. Together, it is unique to possess the combination of the fastest surface diffusivity and the lowest oxidation rate.
Copper-to-copper direct bonding on highly (111)-oriented nanotwinned copper in no-vacuum ambient
(2018-09-17)
Juang, Jing-Ye
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Lu, Chia-Ling
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Chen, Kuan-Ju
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Chen, Chao-Chang A.
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Hsu, Po-Ning
A vacuum-free Cu-to-Cu direct bonding by using (111)-oriented and nanotwinned Cu has been achieved. A fast bonding process occurs in 5 min under a temperature gradient between 450 and 100 degrees C. It is verified by grain growth across the bonded interface. To investigate the grain growth behavior, further annealing in the temperature gradient, as well as in a reversed temperature gradient, was performed. They showed similar recrystallization behavior with de-twinning. To analyze the de-twinning, we recall the classic model of annealing twin formation by Fullman and Fisher as comparison. Our case is opposite to the model of Fullman and Fisher. A mechanism of direct bonding by surface diffusion creep is proposed.
Correlation between the Microstructures of Bonding Interfaces and the Shear Strength of Cu-to-Cu Joints Using (111)-Oriented and Nanotwinned Cu
(2018-12-01)
Juang, Jing-Ye
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Lu, Chia-Ling
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Li, Yu-Jin
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Tu, K. N.
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Chen, Chih
Highly (111)-oriented Cu pillar-bumps were bonded to highly (111)-oriented Cu films at temperatures ranging from 200 degrees C/100 degrees C to 350 degrees C/100 degrees C in N-2 ambient conditions. The microstructures of the bonded interfaces affected the shear strength performance of the bonded Cu joints. The bonded interfaces at 300 degrees C/100 degrees C and 350 degrees C/100 degrees C had far fewer voids than interfaces bonded at 200 degrees C/100 degrees C and 250 degrees C/100 degrees C. In addition, grain growth took place across the bonding interfaces at temperatures above 300 degrees C/100 degrees C. The corresponding orientation map (OIM) showed the preferred orientation of large grown grains to be <100>. Shear tests revealed that the fracture mode was brittle for joints bonded at 200 degrees C/100 degrees C, but became ductile after bonded above 300 degrees C/100 degrees C. Based on the results, we found that voids and grain growth behavior play import roles in the shear strength performance of bonded Cu joints.
Effect of Elastic Strain Fluctuation on Atomic Layer Growth of Epitaxial Silicide in Si Nanowires by Point Contact Reactions
(2015-06-01)
Chou, Yi-Chia
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Tang, Wei
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Chiou, Chien-Jyun
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Chen, Kai
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Minor, Andrew M.
Effects of strain impact a range of applications involving mobility change in field-effect-transistors. We report the effect of strain fluctuation on epitaxial growth of NiSi2 in a nanowire via point contact and atomic layer reactions, and we discuss the thermodynamic, kinetic, and mechanical implications. The generation and relaxation of strain shown by in situ TEM is periodic and in synchronization with the atomic layer reaction. The Si lattice at the epitaxial interface is under tensile strain, which enables a high solubility of supersaturated interstitial Ni atoms for homogeneous nucleation of an epitaxial atomic layer of the disilicide phase. The tensile strain is reduced locally during the incubation period of nucleation by the dissolution of supersaturated Ni atoms in the Si lattice but the strained-Si state returns once the atomic layer epitaxial growth of NiSi2 occurs by consuming the supersaturated Ni.
Effect of migration and condensation of pre-existing voids on increase in bump resistance of flip chips on flexible substrates during electromigration
(2008-07-01)
Liang, S. W.
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Chang, Y. W.
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Chen, Chih
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Preciado, Jackie
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Tu, K. N.
In Pb-free solder joints formed by reflowing a bump of solder paste, voids are formed within the solder due to the residue of flux in the reflow process. These voids migrate toward the cathode contact during electromigration under current stressing. Accompanying the electromigration, resistance jumps of a few 100 m Omega were observed. It was postulated that a jump occurs when a void touches the cathode contact. This study investigated the effect of the void migration and condensation on the change in bump resistance using three-dimensional (3D) simulations and finite element analysis. It was found that there was negligible change in bump resistance during void migration towards the high-current-density region before touching the cathode contact opening. When a small void condensed on the contact opening and depleted 18.4% of the area, the bump resistance increased only 0.4 m Omega. Even when a large void depleted 81.6% of the opening, the increase in bump resistance was 3.3 m Omega. These values are approximately two orders of magnitude smaller than those reported in the literature for the change in resistance due to void migration in flip chips on flexible substrates. We conclude that the reported change in resistance was most likely that of the Al or Cu interconnection in the flip-chip samples.
Effect of three-dimensional current and temperature distributions on void formation and propagation in flip-chip solder joints during electromigration
(2006-07-10)
Liang, S. W.
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Chang, Y. W.
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Shao, T. L.
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Chen, Chih
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Tu, K. N.
Effect of three-dimensional current distribution on void formation in flip-chip solder joints during electromigration was investigated using thermoelectrical coupled modeling, in which the current and temperature redistributions were coupled and simulated at different stages of void growth. Simulation results show that a thin underbump metallization of low resistance in the periphery of the solder joint can serve as a conducting path, leading to void propagation in the periphery of the low current density region. In addition, the temperature of the solder did not rise significantly until 95% of the contact opening was eclipsed by the propagating void. (c) 2006 American Institute of Physics.
Electromigration and Thermomigration in Pb-Free Flip-Chip Solder Joints
(2010)
Chen, Chih
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Tong, H. M.
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Tu, K. N.
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材料科學與工程學系
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Department of Materials Science and Engineering
Pb-free solders have replaced Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Both electromigration (EM) and thermomigration (TM) have serious reliability issues for fine-pitch Pb-free solder bumps in the flip-chip technology used in consumer electronic products. We review the unique features of EM and TM in flip-chip solder bumps, emphasizing the effects of current crowding and joule heating. In addition, the challenges to a better understanding of EM and TM in Pb-free solders are discussed. For example, the anisotropic nature of Sn microstructure in Pb-free solders can enhance the dissolution rates of Ni and Cu in solders driven by EM and TM.
Electromigration in Pb-free solder bumps with Cu column as flip chip joints
(2006)
Nah, Jae-Woong
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Suh, J. O.
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Tu, K. N.
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Yoon, Seung Wook
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Chong, Chai Tai
The electromigration in flip chip joints of Cu column bump with two different kinds of solder bumps; eutectic SnPb or eutectic SnAg was studied. The Cu columns were 50 mu m thick and 50 mu m diameter for eutectic SnPb solders and 80 mu m thick and 85 mu m diameter for eutectic SnAg solders. We observed that these flip chip joints did not fail after 1 month at 100 degrees C with a current density of 1 x 10(4) A/cm(2). In these flip chip joints of Cu column bumps with solder bumps, the current distribution in the solder region was uniform and they had strong resistance against electromigration induced failure. The effect of Cu column bump on current distribution in the solder joint has been confirmed by simulation. When we compared the difference in microstructural change between the Cu column with eutectic SnPb and the Cu column with eutectic SnAg, we found that electromigration enhanced the formation of Kirkendall voids at the Cu/Cu3Sn interface in the eutectic SnPb case due to the smaller amount of Sri in the solder bump. Therefore, the flip chip joints of Cu column bumps with eutectic SnPb solder bumps would be weakened at the interface after high current stressing.
Electromigration-induced Pb and Sn whisker growth in SnPb solder stripes
(2008-07-01)
Wei, C. C.
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Liu, P. C.
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Chen, Chih
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Tu, K. N.
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材料科學與工程學系
Electromigration at 5 x 10(4) A/cm(2) and 100 degrees C was conducted to grow composite Pb/Sn whiskers from SnPb solders, in which a Pb whisker grows first and then a whisker of Sn grows. In some cases, small Sn islands are embedded in Pb whiskers. The diameter of a composite whisker is < 1 mu m, which is much smaller than that of spontaneous Sn whisker growth on leadframes. The growth orientation of Pb whiskers was in the [110], [111], and [112] directions. This investigation proposes that compressive stress generated by electromigration at the anode provides the force driving whisker growth. Therefore, accelerated tests of whisker growth at higher temperatures using electromigration are feasible.
Element Effects on High-Entropy Alloy Vacancy and Heterogeneous Lattice Distortion Subjected to Quasi-equilibrium Heating
(2019-10-15)
Huang, E-Wen
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Chou, Hung-Sheng
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Tu, K. N.
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Hung, Wei-Song
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Tu-Ngoc Lam
We applied Simmons-Balluffi methods, positron measurements, and neutron diffraction to estimate the vacancy of CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs) using Cu as a benchmark. The corresponding formation enthalpies and associated entropies of the HEAs and Cu were calculated. The vacancy-dependent effective free volumes in both CoCrFeNi and CoCrFeMnNi alloys are greater than those in Cu, implying the easier formation of vacancies by lattice structure relaxation of HEAs at elevated temperatures. Spatially resolved synchrotron X-ray measurements revealed different characteristics of CoCrFeNi and CoCrFeMnNi HEAs subjected to quasi-equilibrium conditions at high temperatures. Element-dependent behavior revealed by X-ray fluorescence (XRF) mapping indicates the effect of Mn on the Cantor Alloy.
Extremely rapid grain growth in scallop-type Cu6Sn5 during solid-liquid interdiffusion reactions in micro-bump solder joints
(2020-04-01)
Gusak, A. M.
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Tu, K. N.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
In 3D IC technology, when the size of the micro-bump solder joint is reduced to 10 mu m, the scallop-type Cu6Sn5 grains on opposite sides of the joint can interact directly by contacting each other during the solid-liquid interdiffusion reaction. Upon contact, an extremely rapid grain growth of the scallops occurs. We propose that the liquid solder wets the grain boundary between scallops and provides an extremely fast kinetic path of liquid channel for grain growth. The width of the liquid channel has been estimated to be about 1.5 nm. This is a unique phenomenon observed in micro-bump solid joints. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Fabrication of (111) nanotwinned Cu and its applications in interconnects of microelectronic devices
(2016)
Chen, Chih
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Liu, Chien-Min
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Lu, Tien-Lin
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Lin, Han-wen
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Chu, Yi-Cheng
Flux-driven cellular precipitation in open system to form porous Cu3Sn
(2016)
Gusak, Andriy M.
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Chen, Chih
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Tu, K. N.
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材料科學與工程學系
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Department of Materials Science and Engineering
Recently, a new morphology of porous Cu3Sn with lamellar structure is observed. Several possible explanations of the formation are proposed and compared. The most reasonable one seems to be the one based on a theory of flux-driven cellular precipitation in open system. Outflux of Sn from Cu6Sn5 generates simultaneously supersaturation with vacancies and with copper leading to the eutectoid-like transformation + (where is void). The transformation is complete due to a complete outflux of Sn from the Cu6Sn5 phase. Simple formulae for prediction of the lamellar structure parameters and the propagation velocity are obtained and compared reasonably with experimental data. The suggested model can be interpreted as one more case of the flux-driven phase transformations in open systems.
Growth competition between layer-type and porous-type Cu3Sn in microbumps
(2017-12-01)
Chu, David T.
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Chu, Yi-Cheng
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Lin, Jie-An
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Chen, Yi-Ting
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Wang, Chun-Chieh
Experimental study of growth competition between the co-existing layer-type and porous-type Cu3Sn in solder microbumps of Cu/SnAg/Cu is reported. The thickness of the SnAg solder is about 14 pm and the Cu column on both sides is 20 pm. Upon wetting-reflow, the solder is reacted completely to form Cu Sn iptermetallic compounds in a multi-layered structure of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu. Upon further annealing at 220 degrees C and 260 C, we obtain Cu/Cu3Sn/porous Cu3Sn/Cu3Sn/Cu, in which both types of Cu3Sn co-exist and form an interface. In the layer-type growth, we assume Cu to be the dominant diffusing species, coming from the Cu column. The Cu reacts with Cu6Sn5 to grow the Cu3Sn layer. In the porous-type growth, we assume Sn to be the dominant diffusing species, coming from the depletion of Sn in Cu6Sn5. The depleted Cu6Sn5 transforms to the porous-type Cu3Sn. At the same time, the Sn diffuses to the side-wall of Cu column to form a coating of Cu3Sn. Experimental observations of 3-dimensional distribution of voids in the porous-type Cu3Sn are performed by synchrotron radiation tomography; the voids are interconnected for the out-diffusion of Sn. The competing growth between the layer-type and the porous-type Cu3Sn is analyzed. (C) 2017 Elsevier Ltd. All rights reserved.