Optimization of contact metallizations for reliable wafer level Au[sbnd]Sn bonds

In this study, the focus was on the design for reliable wafer level hermetic Sn-Au based interconnection for MEMS devices by optimizing the contact metallization structures. The investigations were done in two parts: i) the formation and evolution of interconnection microstructures in AuSn | Cu, AuSn | Ni and AuSn | Pt systems were studied with bulk diffusion couples and ii) thin film structures (i.e. adhesion layers and diffusion barriers) for Pt metallization on AuSn SLID bonded wafers were investigated from manufacturability and reliability viewpoints. The failure analysis was carried out on as bonded as well as aged interconnections. Distinct thin film structures showed clear differences in shear strength and fracture mechanisms. The mechanical reliability of the interconnection was increased by i) introducing sputtered Ni metallization between TiW and AuSn bond, ii) increasing the Pt thickness from 100 nm to 200 nm and iii) using Mo diffusion barrier underneath the Pt layer. Based on the results obtained from diffusion couples the thermodynamic descriptions of the ternary systems were re-assessed and the thermodynamic data was utilized in rationalizing the observed interconnection microstructures, failure mechanisms in thin film samples and the effect of Pt metallization thickness on the re-melting temperature of the SLID bond.