Influence of Bonding Parameters on the Interaction Between Cu and Noneutectic Sn-In Solder Thin Films

Abstract

Thermocompression bonding of through-layer copper interconnects is of great interest for fabrication of three-dimensional (3D) integrated circuits. We have investigated interactions of Cu films with noneutectic Sn-In at length scales of 1 lm to 5 lm. The effects of bonding time, bonding temperature, and postbonding annealing temperature on intermetallic compound (IMC) formation, joint microstructure, and shear strength were investigated using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometry (XRD), and shear testing. It is shown that bonding temperature plays an important role in increasing the true contact area, while the postbonding annealing temperature affects the formation of a single IMC, the g-phase [Cu 6 (Sn,In) 5 ]. Both of these phenomena were found to contribute to the shear strength of the joints. It is shown that two-step bonding processes, involving short bonding times and longer postbonding annealing, can be used to optimize the bond formation for increased throughput.

Characterization of the Young’s modulus, residual stress and fracture strength of Cu–Sn–In thin films using combinatorial deposition and micro-cantilevers

“…Out of the many solder combinations, Sn-In is a promising candidate for lowering the bonding temperature due to its low melting point [7]. In a previous work, we have demonstrated that Sn 75 In 25 can bond to Cu at 200 °C with an acceptable shear strength [3,6]. This was a substantial temperature reduction compared to typical pure Sn solders, which bond at around 280 °C [8,9].…”

“…where k 1 , k 2 and k 3 are the spring constants of the cantilever at locations 1, 2 and 3, which are separated by a known separation distance (d). The cross sectional stiffness (S) can then be determined by taking the slope of k 1/3 versus L. Subsequently, Young's modulus (E) can be computed using equation (3).…”

“…Solder-based bonding has been widely used in microelectronics packaging technology, such as ball grid arrays (BGAs), flip-chip interconnects, 3D integrated circuits (3DICs) and hermetic sealing [1,2]. Bonding is typically achieved by reacting the solder with under-bump metallization (UBM) made of Cu, Ni and/or Au to form intermetallic compounds (IMCs) [3][4][5][6].…”

Liquid-phase diffusion bonding and the development of a Cu-Ni/Sn composite solder paste for high temperature lead-free electrical connections

“…With low temperature processing, this can lead to the precipitation of a Sn-In IMC that melts at 222°C, and therefore, does not offer any advantages over conventional Pb-free solder alloys. In contrast, the ternary Cu-Sn-In is a promising LPDB system because the Cu6(Sn, In)5 phase in equilibrium with neareutectic liquid compositions incorporates both components of the LTP, as seen in the calculated Cn-Sn-In isothermal section of the ternary phase diagram in Fig.10, and has been shown to solidify isothermally by Sasangka et al [81] . The ternary Ni-Sn-In exhibits similar characteristics to Cu-Sn-In, but the solubility of In in Ni3Sn4 is more limited.…”

Advances in Pb-free Solder Microstructure Control and Interconnect Design

“…With low temperature processing, this can lead to the precipitation of a Sn-In IMC that melts at 222°C, and therefore, does not offer any advantages over conventional Pb-free solder alloys. In contrast, the ternary Cu-Sn-In is a promising LPDB system because the Cu 6 (Sn, In) 5 phase in equilibrium with near-eutectic liquid compositions incorporates both components of the LTP, as seen in the calculated Cn-Sn-In isothermal section of the ternary phase diagram in Fig.10, and has been shown to solidify isothermally by Sasangka et al [81] The ternary Ni-Sn-In exhibits similar characteristics to Cu-Sn-In, but the solubility of In in Ni 3 Sn 4 is more limited. As with the Ag-In-Sn system, alloys of In-rich Ni-Sn-In are likely to form undesirable low-melting temperature In-Sn IMCs rather than Ni 3 (Sn, In) 4 .…”