Chih Chen

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

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Publications (172)390.12 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Transient liquid reactions in Cu/Sn/Cu sandwich structures during various stages were conducted at 340 °C. In the early stages of the reaction, Cu atoms dissolved into the molten Sn through the valleys between the scallop-like Cu6Sn5 intermetallic compounds (IMCs). When the Sn layer was almost consumed, formation of plate-like Sn channels in Cu6Sn5 IMCs was observed at later stages of the reaction. In addition, plate-like Cu6Sn5 channels formed in Cu3Sn IMCs because Cu atoms diffused faster in Cu6Sn5 than in Cu3Sn IMCs. These channels serve as fast diffusion paths for Cu to react with the remaining Sn. Hence, the growth kinetics at later stages are quite different from those at the beginning stages.
    No preview · Article · Feb 2016 · Materials Letters
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    ABSTRACT: Electromigration tests of SnAg solder bump samples with 15 μm bump height and Cu under-bump-metallization (UBM) were performed. The test conditions were 1.45 × 104 A/cm2 at 185 ºC and 1.20 × 104 A/cm2 at 0 ºC. A porous Cu3Sn intermetallic compound (IMC) structure was observed to form within the bumps after several hundred hours of current stressing. In direct comparison, annealing alone at 185 ºC will take more than 1000 h for porous Cu3Sn to form, and it will not form at 170 ºC even after 2000 h. Here we propose a mechanism to explain the formation of this porous structure assisted by electromigration. The results show that the SnAg bump with low bump height will become porous-type Cu3Sn when stressing with high current density and high temperature. Polarity effects on porous Cu3Sn formation is discussed.
    Preview · Article · Jan 2016 · Crystals
  • M.-Y. Chen · Y.C. Liang · Chih Chen
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    ABSTRACT: This paper investigates the current-enhanced surface diffusion of solder along the side walls of Cu pillars. Two solder joints, Cu pillar/10-µm-solder/Ni and Cu pillar/Ni/10-µm-solder/Ni, were subjected to a 1.2 × 104 Å/cm2 current density at 125 °C for various amounts of time. The Sn atoms may diffuse to the Cu pillars side walls and form Cu6Sn5 intermetallic compounds (IMCs). Because the Sn atoms are drawn from the solder joints, serious voids are formed in these joints. Side diffusion is more serious for the Cu pillar/Ni/10-µm-solder/Ni joints than the Cu pillar/10-µm-solder/Ni joints because, instead of forming Ni3Sn4 IMCs, the Sn atoms prefer diffusing into the Cu side walls to form Cu6Sn5 IMCs. This Sn side diffusion accelerates the depletion of solder in the joints and thus may be a serious reliability issue for solder joints with a reduced solder height.
    No preview · Article · Dec 2015 · Journal of Materials Science Materials in Electronics
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    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.
    Preview · Article · May 2015 · Scientific Reports
  • Chih Chen · Doug Yu · Kuan-Neng Chen
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    ABSTRACT: With the electronics packaging industry shifting increasingly to three-dimensional packaging, microbumps have been adopted as the vertical interconnects between chips. Consequently, solder volumes have decreased dramatically, and the solder thickness has reduced to a range between a few and 10 microns. The solder volume of a microbump is approximately two orders of magnitude smaller than a traditional flip-chip joint. In contrast, the thickness of the under-bump metallization (UBM) remains almost the same as that in flip-chip solder joints. Therefore, many issues concerning materials and reliability of microbumps arise. This article reviews the challenges related to microbump materials for vertical interconnects, including transformation of solder joints into intermetallic (IMC) joints, necking or voiding induced by side wetting/diffusion on the circumference of the UBM, formation of porous Cu3Sn IMCs, early electromigration failures caused by specific orientations of Sn grains, and precipitation of plate-like Ag3Sn IMCs. An alternative way of fabricating vertical interconnects using direct Cu-to-Cu bonding is also discussed.
    No preview · Article · Mar 2015 · MRS Bulletin
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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.
    Full-text · Article · Dec 2014 · Materials Letters
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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.
    Full-text · Article · Oct 2014 · NPG Asia Material
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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.
    No preview · Article · Oct 2014 · Scripta Materialia
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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.
    Preview · Article · Aug 2014 · Scientific Reports
  • C.K. Lin · Chien-Ming Liu · Chih Chen
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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.
    No preview · Article · Jun 2014 · Materials Letters
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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.
    No preview · Article · May 2014 · Scripta Materialia
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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.
    No preview · Article · Apr 2014 · Applied Physics Letters
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    ABSTRACT: Recently, three dimensional integration circuits technology has received much attention since the demands of functionality and performance in microelectronic packaging for electronic products are rapidly increasing. For high-performance 3D chip stacking, high density interconnections are required. In the current types of interconnects, solder micro bumps have been widely adopted. For fine pitch solder micro bump joints, selections of bump height and UBM structure are the important issues that would show the significant effects on the reliability performances of solder micro bump interconnection. In this study, effects of bump height and UBM structure on the reliability properties of lead-free solder micro interconnections with a pitch of 60μm were discussed. The chip-to-chip test vehicle having more than 4290 solder micro bump interconnections with a bump pitch of 60μm was used in this study. To evaluate the effects of bump height and UBM structure on the reliability performance of micro joints, two groups of solder joint were made. The first group of micro joints had a total bump height of 29μm. In this group, Cu/Sn/Cu joint with a thickness of 7μm/15μm/7um, Cu/Sn/Ni/Cu joint having a thickness of 7μm/15μm/2μm/5μm and Cu/Ni/Sn/Ni/Cu joint with a thickness of 5μm/2μm/15μm/2um/5μm were selected. The second group of micro joints had a total bump height of 24μm. In this group, Cu/Sn/Cu joint having a thickness of 7μm/10μm/7um, Cu/Sn/Ni/Cu joint with a thickness of 7μm/10μm/2μm/5μm and Cu/Ni/Sn/Ni/Cu joint having a thickness of 5μm/2μm/10μm/2um/5μm were chosen. We used the fluxless thermocompression bonding process to form these two groups of micro joints. After bonding process, the chip stack was assembled by capillary-type underfill. Reliability tests of temperature cycling test- (TCT), high temperature storage (HTS) and electromigration test (EM) were selected to assess the effect of bump height and UBM structure on the reliability properties of those two groups of solder micro bump interconnections.
    No preview · Conference Paper · Apr 2014
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    C. K. Lin · Yuan Wei Chang · Chih Chen
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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.
    Full-text · Article · Jan 2014 · Journal of Applied Physics
  • C.K. Lin · Wei An Tsao · Y.C. Liang · Chih Chen
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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.
    No preview · Article · Sep 2013 · Journal of Applied Physics
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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.
    Full-text · Article · Aug 2013 · Acta Materialia
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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.
    No preview · Conference Paper · May 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.
    No preview · Article · Apr 2013 · ACS Applied Materials & Interfaces
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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.
    No preview · Article · Mar 2013 · Scripta Materialia
  • K. N. Tu · Hsiang-Yao Hsiao · Chih Chen
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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.
    No preview · Article · Jan 2013 · Microelectronics Reliability

Publication Stats

2k Citations
390.12 Total Impact Points

Institutions

  • 2000-2016
    • 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