Chih Chen

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

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Publications (166)372.06 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Direct Cu-to-Cu bonding was achieved at temperatures of 150-250 °C using a compressive stress of 100 psi (0.69 MPa) held for 10-60 min at 10(-3) torr. The key controlling parameter for direct bonding is rapid surface diffusion on (111) surface of Cu. Instead of using (111) oriented single crystal of Cu, oriented (111) texture of extremely high degree, exceeding 90%, was fabricated using the oriented nano-twin Cu. The bonded interface between two (111) surfaces forms a twist-type grain boundary. If the grain boundary has a low angle, it has a hexagonal network of screw dislocations. Such network image was obtained by plan-view transmission electron microscopy. A simple kinetic model of surface creep is presented; and the calculated and measured time of bonding is in reasonable agreement.
    Scientific Reports 05/2015; 5:9734. DOI:10.1038/srep09734 · 5.58 Impact Factor
  • Chih Chen, Doug Yu, Kuan-Neng Chen
    MRS Bulletin 03/2015; 40(03):257-263. DOI:10.1557/mrs.2015.29 · 5.07 Impact Factor
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    ABSTRACT: The electromigration behavior was investigated in 18-μm SnAg microbumps at 150 °C. The monitored resistance increase abruptly soon after the current stressing of 4.6×104 A/cm2 was applied but the resistance rose much slower after a certain period of time. The formation of the high-resistivity Ni3Sn4 intermetallic compounds (IMCs) was responsible for the abrupt resistance increase in the beginning of the current stressing. The whole solder joint was transformed into an IMC joint. Because IMC has a higher electromigration resistance than the solder, a much slower increase in resistance was resulted after the solder joint was transformed into IMCs.
    Materials Letters 12/2014; 137:136–138. DOI:10.1016/j.matlet.2014.08.156 · 2.27 Impact Factor
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    ABSTRACT: By electroplating of nearly unidirectionally o1114-oriented nanotwinned and fine-grained Cu on a Si wafer surface followed by annealing at 400–500 °C for up to 1 h, we grew many extremely large o1004-oriented single crystals of Cu with sizes ranging from 200 to 400 μm. By patterning and annealing the nanotwinned Cu films, we grew an array of o1004-oriented single crystals of Cu with sizes ranging from 25 to 100 μm on Si. In comparison, single-crystal nano-wire growth is a one-dimensional anisotropic growth process, in which the growth along the axial direction is much faster than in the radial direction. We report here a bulk-type two-dimensional crystal growth of an array of numerous o1004-oriented single crystals of Cu on Si. This growth process has the potential for microbump applications in three-dimensional integrated circuit-packaging technology for hand-held consumer electronic products.
    NPG Asia Material 10/2014; 6:e135. DOI:10.1038/am.2014.90 · 9.90 Impact Factor
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    ABSTRACT: The grain growth of pulse-plated (1 1 1)-oriented nanotwinned Cu (nt-Cu) was investigated at 200–350 °C. The results indicate that after the annealing, the nt-Cu exhibits good thermal stability, and columnar grains with a (1 1 1)-oriented nt-Cu structure are maintained up to 300 °C. The columnar grains consume the original fine-grained region at the bottom of the sample. In addition, these fine grains were converted into nanotwinned columnar grains with a (1 1 1) orientation. The electroplated Cu film possessed extremely high (1 1 1) preferred orientation after the annealing.
    Scripta Materialia 10/2014; 89:5–8. DOI:10.1016/j.scriptamat.2014.06.008 · 2.97 Impact Factor
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    ABSTRACT: We investigate the growth of Cu films on two different Cu seed layers: one with regular <111>-oriented grains and the other with very strong <111>-preferred orientation. It is found that densely-packed nanotwinned Cu (nt-Cu) can be grown by pulsed electroplating on the strong <111>-oriented Cu seed layer without a randomly-oriented transition layer between the nt-Cu and the Cu seed layer. The electroplated nt-Cu grow almost epitaxially on the seed layer and formed <111>-oriented columnar structures. However, with the regular <111>-oriented Cu seed, there is a randomly-oriented transition layer between the nt-Cu and the regular <111>-oriented Cu seed. The results indicate that the seed layer plays a crucial role on the regularity of <111>-oriented nanotwinned Cu.
    Scientific Reports 08/2014; 4:6123. DOI:10.1038/srep06123 · 5.58 Impact Factor
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    ABSTRACT: This study examined the formation of Sn-rich phases in the matrix of Cu-Sn-Ni intermetallic compounds (IMCs) after current stressing of 1.2 X 104 A/cm(2) at 160 degrees C. The Sn-rich phases were formed at the cathode end of the solder joints with Cu metallization, and this formation was attributed to the decomposition of Cu6Sn5 IMCs. When the Cu6Sn5 IMCs were transformed into Cu3Sn during current stressing, Sn atoms were released. When the supply of Cu atoms became deficient, Sn atoms accumulated to form Sn-rich phases among the Cu-Sn-Ni IMCs.
    Materials Letters 06/2014; 124:261-263. DOI:10.1016/j.matlet.2014.03.071 · 2.27 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 degrees C at a stress of 114 psi for 30 min at 10(-3) torr. The temperature is lower than the reflow temperature of 250 degrees 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 05/2014; s 78–79:65–68. DOI:10.1016/j.scriptamat.2014.01.040 · 2.97 Impact Factor
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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 04/2014; 104(17):171902-171902-4. DOI:10.1063/1.4874608 · 3.52 Impact Factor
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    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.
    Journal of Applied Physics 01/2014; 115(8). DOI:10.1063/1.4867048 · 2.19 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 09/2013; 114(11):113711-113711-7. DOI:10.1063/1.4821427 · 2.19 Impact Factor
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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. DOI:10.1016/j.actamat.2013.04.056 · 3.94 Impact Factor
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    ABSTRACT: An immortal solder micro-bump (μbump) electromigration (EM) lifetime has been demonstrated for 3D IC integration. This ultimate goal was achieved under strictly controlled conditions, including the optimal design of bump metallurgy, geometry, optimized processes, along with well-defined stressing conditions and manufacturing window. The current carrying capability and EM lifetime of μbump have been investigated as functions of stressing conditions which are correlated with the degradation mechanisms. When stressed under the appropriate g conditions, all μbump test samples survived prolonged stressing, some over 13,000 hours, without a failure. Cross-sectional analyses revealed that the entire solder joint almost all transformed into intermetallic compounds (IMCs) with very minor or no voids. The resistance plots showed an initial fast rise in resistance due to IMC formation, then gradually leveled off and eventually reached a steady state. The observed degradation mechanism is dominated by IMC formation, which is the same as that of the user conditions. On the other hand, void formation that eventually led to open failure was the dominant degradation mechanism when samples were aggressively stressed. In other words, when all other conditions were the same, the stressing conditions make a huge difference in determining between an almost immortal EM lifetime vs. a short lifetime using the same high quality μbumps. The boundary that separates the stressing conditions is roughly defined and will be discussed. In addition, since full IMC μbump will become inevitable in the future miniaturized solder interconnect structure, the EM behavior of IMC dominated μbump has also been evaluated in this study. Under highly accelerated stressing conditions of 174°C, at 1.6×105 A/cm2 current density, the IMC dominated μbumps were able to survive more than 600 hours and are still going stron- . In comparison, solder μbump failed quickly after just 107 hours when stressed under the same condition. This comparison study clearly demonstrated that IMC dominated joint has significantly higher current carrying capability than that of the solder joint. After reviewing all the data, we have concluded that the failure criteria for solder μbump should be raised significantly higher than the 20% criteria traditionally used for the much larger C4 bumps.
    2013 IEEE 63rd Electronic Components and Technology Conference (ECTC); 05/2013
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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(9). DOI:10.1021/am302219n · 5.90 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. DOI:10.1016/j.scriptamat.2012.10.024 · 2.97 Impact Factor
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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. DOI:10.1016/j.microrel.2012.08.021 · 1.21 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. DOI:10.1016/j.microrel.2012.07.029 · 1.21 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 proposed a novel technique developed from focused ion beam (FIB) polishing for sample preparation of electron backscatter diffraction (EBSD) measurement. A low-angle incident gallium ion beam with a high acceleration voltage of 30 kV was used to eliminate the surface roughness of cross-sectioned microbumps resulting from mechanical polishing. This work demonstrates the application of the FIB polishing technique to solders for a high-quality sample preparation for EBSD measurement after mechanical polishing.
    Materials Characterization 12/2012; 74:42–48. DOI:10.1016/j.matchar.2012.09.002 · 1.93 Impact Factor
  • T.-C. Liu, Y.-S. Huang, Chih Chen
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    ABSTRACT: We investigate the cross-interaction in Cu/SnAg/Ni microbumps with a reduced solder thickness of 30 and 10 mu m. The concentration of Ni atoms at the opposite site increased with the decrease in solder-height. A considerable concentration gradient of Ni was detected in 10-mu m microbumps sample, which strongly triggers the diffusion of Ni atoms to the Cu side. The diffused Ni atoms at Cu side form the ternary intermetallic compounds of (Cu,Ni)(6)Sn-5, which possesses a lower free energy than Cu6Sn5 does. Eventually, the growth of the Cu3Sn was inhibited due to the formation of the thermodynamically stable (Cu, Ni) 6Sn5.
    11/2012; 2(2):P15-P18. DOI:10.1149/2.009302ssl

Publication Stats

2k Citations
372.06 Total Impact Points

Institutions

  • 2001–2015
    • National Chiao Tung University
      • • Department of Material Science and Engineering
      • • Department of Mechanical Engineering
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2010
    • National Tsing Hua University
      • Department of Materials Science and Engineering
      Hsin-chu-hsien, Taiwan, Taiwan
  • 1999–2000
    • University of California, Los Angeles
      • Department of Materials Science and Engineering
      Los Angeles, CA, United States