Characterization of AuSn Solder in Laser Die Attachment for Photonic Packaging Applications

80 wt.% Au, 20 wt.% Sn solder is used in optoelectronics because it allows soldering in fluxless processes. Au-Sn solder is also applied in the field of RF and MEMS packaging. Normally the deposited solders are re-flowed after electroplating but it is also possible to use the bumps in the as-deposited condition. Au-Sn solder which has been reflowed consists of an Au pedestal with a eutectic solder cap on top. Reflow soldering is a cost effective approach for the flip chip technology or die attachment process. Using pick & place equipment the substrate is populated with the components. Soldering is performed afterwards in an oven. During soldering it is possible to utilize the self-alignment mechanism. In this study, Au-Sn besides 80 wt.% Au, 20 wt.% Sn solder composition is evaluated and characterized in evaporation deposition process for SiOB (silicon optical bench) applications. Multiple layers of thin Sn and Au solders are deposited one-by-one. The layer thickness is calculated and optimized to meet the desired solder composition. The deposited solder is evaluated using FIB (Focused Ion Beam) to observe the layer thickness uniformity. Micro-structural observations for deposited solder samples are conducted before and after solder reflow for melting. The composition is identified by chemical composition analysis (EDS - energy dispersive spectroscopy) and thermal analysis (DSC - differential scanning calorimetry). The interfacial reaction and morphology with Ti/Pt/Au UBM (under bump metallurgy) layers are also observed with different reflow conditions. Results of die shear for photonic diode attachment are shown.