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Etched V-grooves along the [1 1 0] direction on (0 0 1) surface of Si wafers have been used to achieve high precision alignment between fibers in opto-electronic devices by epoxy-bonding. Nevertheless, the positioning of fibers using epoxy-bonding lacks long-term stability. We developed a novel technology of using metallic solder bonding instead of the organic epoxy-bonding.The challenge is because of the fact that solder does not wet SiO2 surface. To circumvent this problem, electron-beam evaporation was used to deposit a multi-layered metallic coating on the surfaces of fibers and V-grooved chips. Three types of coatings were investigated, and they are Ti/Au, Ti/Cu/Au, and Ti/Ni/Au. These metallic film coatings have good adhesion on the fiber and the V-groove. We utilized a low melting point Pb-free solder, eutectic 43Sn57Bi (in wt.%), with a melting point of 139 °C to bond an array of fibers to V-grooved chips. The mechanical and optical tests illustrated that we can achieve precision alignment of fibers by the soldering method and the bonded structure is stable at room temperature. The metallic solder bonding can be hermetic and it can isolate the optical device from ambient environment. [Copyright &y& Elsevier]

Electromigration (EM) has been the most persistent interconnect reliability issue over the decades. In general, EM damages tend to occur at atomic flux divergence sites. The EM failure rate can be further accelerated by current crowding, which occurs when current flows between inter-level wires. In this work, we used two different via etch schemes to study the effect of contact resistance on current crowding and EM. We found that the etch stop structures show longer EM lifetimes than the over etch. The contact resistance of the etch stop is higher than that of the over etch. Two-dimensional simulation results show that the higher contact resistance in the etch stop can suppress current crowding and improve EM lifetimes. Differences in void morphology between the over etch and etch stop as a result of current crowding are discussed. [Copyright &y& Elsevier]

• Head-shaped Cu-Sn solder joints with diameters about 10 μm, 20 μm, and 50 μm are reflowed. • Grain growth in the small size bumps has the faster growth rate. • In the 10 μm bumps, there is only one Cu 6 Sn 5 grain after reflow. • The classic scallop-type grain growth model of Cu 6 Sn 5 does not apply. • A surface diffusion-controlled model is proposed to explain the new kinetics. We made head-shaped Cu-Sn solder joints with diameters about 10 μm, 20 μm, and 50 μm and reflowed the samples at 240 °C, 260 °C, and 280 °C for 60 s to 600 s. In the 10 μm bumps, there is only one Cu 6 Sn 5 grain after reflow, thus the classic model of scallop-type grain growth of Cu 6 Sn 5 does not apply. Also, the grain growth in the small size bumps has the faster growth rate. We proposed a surface diffusion-controlled model to explain the new kinetics. According to our model, the diffusion activation energy is calculated to be Q S = 0.18 ± 0.02 eV/atom and diffusion frequency factor to be D S0 = 5.65 × 10−3 cm2/s. [ABSTRACT FROM AUTHOR]

A significant grain rotation in < 111 >-oriented nanotwinned Cu film under tensile test at room temperature is reported. After rotation, the dislocation network in low angle tilt-type boundaries increases the strain energy, which provides the driving force for recrystallization in a subsequent annealing, resulting in the formation of millimeter size grains. [ABSTRACT FROM AUTHOR]

We investigated the microstructure changes in abnormal grain growth of nanotwinned Cu (nt-Cu) upon annealing by electron backscattered diffraction (EBSD) and focused ion beam (FIB). The nt-Cu films were produced by electrodeposition on a Si wafer with a barrier/seed layer of TiW/Cu. When a 3 μm thick nt-Cu film was annealed at 350 °C for 45 min, its microstructure was found to have changed from the (111) oriented twins to the (200) oriented grains with an average grain size larger than 100 μm. When the nt-Cu film is thicker than 5 μm, the change can occur at a lower temperature of 300 °C. We observed that there is a critical abnormal grain growth temperature, below which no microstructure change can occur. We propose that it is due to nucleation limitation. After nucleation, the growth is highly anisotropic, where the lateral growth is at the least one order of magnitude faster than the normal growth. We have attempted to use JMAK theory of phase transformation to analyze the abnormal grain growth kinetics, yet we obtained very little success. • When a 3-μm-thick nanotwinned Cu film was annealed at 350 ̊C for 45 min, it changed from the (111) oriented twins to the (200) oriented grains. • When the nanotwinned Cu film is thicker than 5 μm, the change can occur at a lower temperature of 300 ̊C. • We observed that there is a critical abnormal grain growth temperature, below which no microstructure change can occur. • These results are useful to design nt-Cu films for various applications. [ABSTRACT FROM AUTHOR]

• The effect of Ag addition to SnBiIn medium entropy alloy was investigated. • The activation energy of the reaction between SnBiIn(Ag) and Cu was measured. • The interfacial IMC growth in SnBiIn solder will be suppressed by Ag addition. Wetting reactions of the medium entropy alloy of SnBiIn and SnBiInAg on Cu substrate have been investigated. The melting points of both SnBiIn and SnBiInAg are about 80 °C. The reactions were performed at 120 °C, 140 °C, and 160 °C. The kinetics of interfacial intermetallic compound (IMC) growth of Cu 6 Sn 5 was studied to be ripening-controlled with activation energy about 11.1 kJ/mol for SnBiIn and 10.4 kJ/mol for SnBiInAg. However, the addition of Ag has effectively reduced the IMC growth by 20–30%. We propose that it is because Ag has reduced the pre-factor of diffusivity by increasing the entropy of the alloy. [ABSTRACT FROM AUTHOR]

Abstract Thermomigration (TM) has become a critical reliability issue in advanced electronic packaging because of Joule heating. A temperature gradient is required to conduct heat away, and only 1 °C of temperature difference across a 10 μm thick microbump produces a temperature gradient of 1000 °C/cm, which can cause TM, especially in low melting eutectic Sn-Bi solder interconnects. We report here that Bi atoms moving from the hot end to the cold end of the temperature gradient, are the dominant diffusing species. Under the assumption of constant volume, the Sn atoms are squeezed by the Bi atoms at the cold end and have to accommodate for the Bi atoms, which makes them move to the hot end. Consequently, the opposing fluxes of Bi and Sn are found to be about the same. Moreover, the growth of Cu-Sn intermetallic compound (IMC) layers at the cold and the hot end were symmetrical, and were unaffected by TM. Additionally, finite-element-method (FEM) simulations showed that the phase separation of Bi and Sn reduced the current crowding regions which affect the electromigration of the eutectic Sn-Bi solder interconnects. Graphical abstract Unlabelled Image Highlights • The thermomigration behavior in Cu/Sn58Bi/Cu joints is hardly seen, not to mention the process and details. • Bi accumulation at the cold end by thermomigration induces a significant Sn-Bi phase separation in the Sn-58Bi solder joints. • The growth of Cu-Sn intermetallic compound layers at the cold and hot end was symmetrical, unaffected by thermomigration. • The phase separation of Bi and Sn reduces the current crowding regions. [ABSTRACT FROM AUTHOR]