Chih Chen

National Chiao Tung University, Hsin-chu-hsien, Taiwan, Taiwan

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Publications (135)258.24 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Resistance curves play a crucial role in detecting damage of solder joints during electromigration. In general, resistance increases slowly in the beginning, and then rises abruptly in the very late stage; i.e., the resistance curve behaves concave-up. However, several recent studies have reported concave-down resistance curves in solder joints with no satisfactory explanation for the discrepancy. In this study, electromigration failure mode in Sn2.5Ag solder joints was experimentally investigated. The bump resistance curve exhibited concave-down behavior due to formation of intermetallic compounds (IMCs). In contrast, the curve was concave-up when void formation dominated the failure mechanism. Finite element simulation was carried out to simulate resistance curves due to formation of IMCs and voids, respectively. The simulation results indicate that the main reason causing the concave-down curve is rapid formation of resistive Cu6Sn5 IMCs in the current-crowding region, which are 9 times larger than Cu IMCs. Therefore, when Cu reacted with Sn to form Cu6Sn5 IMCs, resistance increased abruptly, resulting in the concave-down resistance curve.
    01/2014; 115(8).
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    ABSTRACT: Cu3Sn intermetallic compounds (IMCs) are more resistant to fracture than solders. In addition, the Cu3Sn IMCs are more conductive than the solders. In this study, we manufactured Cu3Sn IMCs to serve as a joint using electroplated nanotwinned Cu as a metallization layer to react with pure Sn at 260 °C and 340 °C. The results show that there were almost no Kirkendall voids generated inside the Cu3Sn layer. In addition, the kinetics of the Cu3Sn growth was analyzed to predict the time needed to form the Cu3Sn joint.
    Applied Physics Letters 01/2014; 104(17):171902-171902-4. · 3.79 Impact Factor
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    ABSTRACT: We achieve low-temperature Cu-to-Cu direct bonding using highly (1 1 1)-orientated Cu films. The bonding temperature can be lowered to 200 °C at a stress of 114 psi for 30 min at 10−3 torr. The temperature is lower than the reflow temperature of 250 °C for Pb-free solders. Our breakthrough is based on the finding that the Cu (1 1 1) surface diffusivity is the fastest among all the planes of Cu and the bonding process can occur through surface diffusion creep on the (1 1 1) surfaces.
    Scripta Materialia. 01/2014; s 78–79:65–68.
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    ABSTRACT: Anisotropic microstructure is becoming a critical issue in microbumps used in 3-D integrated circuit packaging. We report here an experimental approach for controlling the microstructure of η-Cu6Sn5 intermetallic compound in microbumps by using 〈1 1 1〉 oriented and nanotwinned Cu pads as the under-bump-metallization. By electroplating arrays of large numbers of 〈1 1 1〉 oriented and nanotwinned Cu pads and by electroplating the Sn2.3Ag solder on the pads, we form η-Cu6Sn5 in the reflow at 260 °C for 1 min. The η-Cu6Sn5 showed a highly preferential growth along the 〈0 0 0 1〉 direction. As reflow time is extended, the preferred texture of η-Cu6Sn5 changed to {21¯1¯3}. The results indicate that we can control the uniform microstructure of η-Cu6Sn5 intermetallic by controlling the microstructure of the Cu under-bump-metallization.
    Acta Materialia. 08/2013; 61(13):4910–4919.
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    ABSTRACT: 2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 °C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.
    ACS Applied Materials & Interfaces 04/2013; · 5.01 Impact Factor
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    ABSTRACT: Electroplating was used to fabricate a high density of nanotwins that exhibited the preferred (1 1 1) orientation in Cu. We found no formation of Kirkendall voids in solder reactions on the nanotwinned Cu. This was due to the high density of steps and kinks on the nanotwin boundaries, which serve as vacancy sinks. Thus the vacancy concentration cannot reach supersaturation and nucleate voids. The finding is a significant advance in the problem of solder joint reliability in microelectronic three-dimensional integrated circuit devices.
    Scripta Materialia. 03/2013; 68(5):241–244.
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    ABSTRACT: To keep up with the demand of continuous increase in device densities, the integration of three-dimensional integrated circuits (3D-IC) has become the most probable solution, and the utilization of ultra-fine-pitch microbump has emerged as an essential component of 3D-IC technology. In this study, a Kelvin bump structure was fabricated and resistances measured at different angles on a 20.0 μm microbump were investigated. The microbump resistance at 0°, 60°, 120°, and 180° are 74.7, 45.9, 14.6, and 13.7 mΩ, respectively. These high resistances in microbumps may result in high interconnect resistance and cause resistance/capacitance (RC) delay, and thus lower the electrical performance of 3D-IC. A series of finite-element-model (FEM) was built to analyze the distribution of electric field in microbump. The FEM results have shown that the current is distributed uniformly in the thin solder joint, but current crowding still occurs in the Cu under-bump-metallization (UBM). The finding of the current crowding in the Cu UBM is the main cause of high resistances in the microbump. Thickening the Al trace, for example, from 0.4 μm to 1.5 μm, is a direct solution to reduce the unexpected high microbump resistance. A numerical model which treated solder joints as a resistance network was also performed in this study. For comparison, both FEM and the numerical model show the same trend and agree with the measurement results from Kelvin bump structure. The results all point to one thing: thickening the Al trace turn out to be the most effective approach to reduce high microbump resistance. When the Al trace thickness is increased from 0.8 to 3.0 μm, the microbump resistance is decreased to half of the original value, resulted from the alleviation of current crowding in the Cu UBM.
    Microelectronics Reliability 01/2013; 53(1):41–46. · 1.14 Impact Factor
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    ABSTRACT: As microelectronic industry develops 3D IC on the basis of through-Si-vias (TSV) technology, the processing and reliability of microbumps, which are used to interconnect the stacking chips, is being actively investigated. Due to the reduction in size of microbumps, the diameter is about one order of magnitude smaller than that of flip chip solder joints, and the volume is 1000 times smaller. Its microstructure and in turn its properties will be anisotropic because the number of grains in a microbump becomes very small. Its statistical failure will have a wide distribution because of anisotropy, including early failure. This review addresses this issue and the remedy.
    Microelectronics Reliability 01/2013; 53(1):2–6. · 1.14 Impact Factor
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    ABSTRACT: Temperature-dependent electromigration failure was investigated in solder joints with Cu metallization at 126 °C, 136 °C, 158 °C, 172 °C, and 185 °C. At 126 °C and 136 °C, voids formed at the interface of Cu6Sn5 intermetallic compounds and the solder layer. However, at temperature 158 °C and above, extensive Cu dissolution and thickening of Cu6Sn5 occurred, and few voids were observed. We proposed a model considering the flux divergency at the interface. At temperatures below 131 °C, the electromigration flux leaving the interface is larger than the in-coming flux. Yet, the in-coming Cu electromigration flux surpasses the out-going flux at temperatures above 131 °C. This model successfully explains the experimental results.
    Journal of Applied Physics 01/2013; 114(11):113711-113711-7. · 2.21 Impact Factor
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    ABSTRACT: We report a necking reliability issue in low-bump-height solder induced by reactive wetting of solder on Au finish in print circuit board. During reflow and solid state aging, the solders diffused to the Au finishes on the print circuit boards and reacted with Au to from Au-Sn compounds. Because the solder volume is very limited, this reaction took a lot of percentage of solders and caused necking of the solder joints. On the other hand, the joints without the Au layers have less necking problem. The necking will have detrimental effect on the mechanical properties of the solder joints. Therefore, it will be a serious reliability issue for flip-chip solder joints with low bump heights and microbumps in 3D IC.
    Microsystems, Packaging, Assembly and Circuits Technology Conference (IMPACT), 2013 8th International; 01/2013
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    ABSTRACT: We report an approach to fabricating (111)-oriented and nanotwinned Cu (nt-Cu) by dc electroplating. A 200 nm thick Cu with (111) preferred orientation is required as a seed layer. Highly oriented Cu films with densely packed nanotwins can be grown to exceed 20 μm thick at high current and high stirring speeds. X-ray diffraction indicates that the intensity ratio of (111) to (220) is as high as 506, which is the highest among the reported electroplated Cu films. The spacing of twins ranges from 10 to 100 nm, which reveals a high hardness value of 2.23 GPa. The (111)-oriented nt-Cu will have many potential applications in interconnects and 3D IC packaging.
    Crystal Growth & Design 09/2012; 12(10):5012–5016. · 4.69 Impact Factor
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    ABSTRACT: Highly oriented [111] Cu grains with densely packed nanotwins have been fabricated by direct-current electroplating with a high stirring rate. The [111]-oriented and nanotwinned Cu (nt-Cu) allow for the unidirectional growth of Cu(6)Sn(5) intermetallics in the microbumps of three-dimensional integrated-circuit packaging; a uniform microstructure in a large number of microbumps of controlled orientation can be obtained. The high-density twin boundaries in the nt-Cu serve as vacancy sinks during the solid-state reaction between Pb-free solder and Cu and greatly reduce the formation of Kirkendall (or Frenkel) voids.
    Science 05/2012; 336(6084):1007-10. · 31.20 Impact Factor
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    ABSTRACT: Microbumps have been adopted as interconnects between Si chips in 3D integrated-circuit packaging. The solder volume of a microbump decreases dramatically due to fine-pitch requirement and it is approximately two orders smaller in magnitude than that of a traditional flip-chip solder joint. The metallurgical reactions in the microbumps may behave quite differently to those in flip-ship bumps. Liquid-state metallurgical reactions were examined in SnAg2.5 microbumps with Ni metallization. The results indicate that large particles of Ag3Sn intermetallic compounds (IMCs) precipitate after a 10-min reflow on microbumps with 4.0-μm-thick solder, which does not occur with flip-chip solder bumps. It is proposed that the Ag concentration in the remaining solder may increase as Sn reacts with Ni. The increase in the Ag concentration is mainly responsible for the occurrence of the large Ag3Sn precipitates. The formation of these Ag3Sn IMCs would be detrimental to the mechanical properties of the microbumps.
    Materials Chemistry and Physics. 05/2012; 134(1):340–344.
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    ABSTRACT: Titanium has a very large heat of transport value of −768 kJ mol–1. However, thermomigration of Ti in solder joints has not been examined. In this study, void formation was observed in non-current-stressed Al traces during electromigration and thermomigration tests of SnAg solder joints. Voids formed inside the traces located above the solder bumps with no current flow. We found that Ti thermomigration occurred in the joint, resulting in the reaction of Al and Cu, leaving voids behind in the Al trace.
    Scripta Materialia. 05/2012; 66(9):694–697.
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    ABSTRACT: We report new findings in Ni/SnAg–solder/Cu microbumps having a reduced solder thickness in the range of 40–10 μm; the growth rate of the intermetallic compounds depends strongly on the solder thickness. In the Ni/40 μm solder/Cu samples, the compound which grew on the Ni side grew slightly faster than that on the Cu side. However, the growth reverses as the solder thickness decreases below 20 μm due to the change in Cu and Ni concentration gradients in the solder.
    Scripta Materialia. 05/2012; 66(10):741–744.
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    ABSTRACT: The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.
    Nanoscale Research Letters 04/2012; 7(1):231. · 2.52 Impact Factor
  • Hsiao-Yun Chen, Chih Chen
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    ABSTRACT: Ni-based under-bump metallization (UBM) has attracted wide attention due to its low reaction rate with Sn, compared with Cu and Cu alloy. In this study, the interfacial reactions between eutectic Sn–3.5Ag solder and Ni-based UBM, including electroplated Ni (EP-Ni) and electroless Ni (EL-Ni) are investigated. Morphology and growth kinetics of Ni3Sn4 intermetallic compounds are studied at different reflow temperatures and durations. The growth rate and the growth activation energy of Ni3Sn4 were measured for the two sets of samples. The activation energies are measured to be 25 kJ/mol and 38 kJ/mol for the Ni3Sn4 growth on EP-Ni and EL-Ni, respectively. The Ni3Sn4 on EP-Ni UBMs shows a slower growth rate and the Ni3Sn4/solder interface is void free even after 20-min reflow at 240 °C. On the other hand, the interface of Ni3Sn4/EL-Ni has a lot of microvoids after reflowing at 240 °C for 20 min.
    Journal of Materials Research 04/2012; 27(08):1169-1177. · 1.71 Impact Factor
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    ABSTRACT: The electromigration behavior of SnAg solder bumps with and without Cu column under-bump-metallizations (UBMs) has been investigated under a current density of 2.16 × 104 A/cm2 at 150 °C. Different failure modes were observed for the two types of samples. In those without Cu column UBMs, when SnAg solder bumps that had implemented 2 μm Ni UBMs were current stressed at 2.16 × 104 A/cm2, open failure occurred in the bump that had an electron flow direction from the chip side to the substrate side. However, in those with Cu column UBMs, cracks formed along the interface of Cu6Sn5 intermetallic compounds and the solder on the substrate side in the Sn-3.0Ag–0.5Cu solder bump that had an electron flow direction from the substrate side to the chip side. A three-dimensional simulation of the current density distribution was performed in order to obtain a better understanding of the current crowding behavior in solder bumps. The current crowding effect was found to account for the void formation on both the chip and the substrate side for the two kinds of solder bumps. One more important finding, as confirmed by infrared microscopy, is that the alleviation of current crowding by Cu column UBMs also helped decrease the Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2 μm Ni UBMs.
    Journal of Applied Physics 02/2012; 111(4). · 2.21 Impact Factor
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    ABSTRACT: In this paper, we produce a layer of (111) uni-directional Cu by electroplating. The SnAg 2.3 was used as solder materials. The orientations relationship between uni-directional Cu and Cu6Sn5 were examined. Also, the effect of quality of uni-directional Cu on the orientation of Cu6Sn5 was discussed.
    Microsystems, Packaging, Assembly and Circuits Technology Conference (IMPACT), 2012 7th International; 01/2012
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    ABSTRACT: Highly oriented [111] Cu grains with densely packed nanotwins have been fabricated by direct-current electroplating with a high stirring rate. The [111]-oriented and nanotwinned Cu (nt-Cu) allow for the unidirectional growth of Cu₆Sn₅ intermetallics in the microbumps of three-dimensional integrated-circuit packaging; a uniform microstructure in a large number of microbumps of controlled orientation can be obtained. The high-density twin boundaries in the nt-Cu serve as vacancy sinks during the solid-state reaction between Pb-free solder and Cu and greatly reduce the formation of Kirkendall (or Frenkel) voids.
    Science 01/2012; 336(6084):1007-1010. · 31.03 Impact Factor

Publication Stats

609 Citations
258.24 Total Impact Points


  • 1–2014
    • National Chiao Tung University
      • Department of Material Science and Engineering
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2008
    • National Tsing Hua University
      • Institute of NanoEngineering and MicroSystems
      Hsinchu, Taiwan, Taiwan
  • 2000–2006
    • University of California, Los Angeles
      • Department of Materials Science and Engineering
      Los Angeles, CA, United States