Development of low temperature bonding using in-based solders

In-based solders were chosen for the low temperature bonding at lower than 180degC. Three kinds of bonding types on Au/Cu/Ti/SiO₂/Si dies, which were Sn/In and Au/In for Type 1, Au/In and Au/Sn for Type 2, and InSn alloy and InSn alloy for Type 3, were studied expecting that the whole In- solder layer is converted to the mixed intermetallic compound (IMC) phases of In-Cu and In-Au IMCs after bonding below 180degC and annealing at 100~120degC. The IMC in the joints were characterized in terms of the micro structure observations and the compositional analysis with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX), the phase identification with X-ray Diffraction (XRD) and the re-melting temperature with Differential Scanning Calorimetry (DSC). The phase equilibriums of the joints were examined by thermodynamic calculations to understand the re-melting behavior. As a result, complete bonding consisted of only high melting temperature IMCs, Cu₁₁ln₉, Cu₂In, eta-Cu₆Sn₅, and Auln₂, was successfully made at 120degC followed by annealing at 100degC in Type 3, and at 160degC with annealing for lOhrs or at 180degC without annealing for Type 1, which was confirmed by DSC measurements and explained through thermodynamic calculations.