Microstructure of Ag-Sn Bonding for MEMS Packaging

Different metallization systems and bonding designs of Ag-Sn bonding were investigated to achieve good bonding. The bonding strength was evaluated by shear force. The microstructure of bonding interface was inspected by scanning electronic microscopy and ED AX. Shear force test was performed for as-bonded dice. The test results indicate differences among different metallization systems. The bonding pair with Ti/Au as the UBM has a quite low shear strength because of the bad adhesion on the silicon substrate. The bonding pair of Ti/Ni/Sn/Au and Ti/Ni/Au/Ag obviously has higher shear strength than that of Ti/Ni/Sn/Au and Ti/Ni/Au/Ag/Au. The former is 55.17 MPa on average while the later is 36.05 MPa. The shear strength of the pair of Ti/Ni/Sn/Au and Ti/Ni/Au/Ag is similar to that of Ti/Ni/Sn/Au and Ti/Ag which has the shear strength of 55.32 MPa on average. The Ni and Au in the Ag-Sn bonding system have significant effect on the microstructure of the bonding interface. The diffusion of Au into Sn is quicker than both Ag and Ni. The diffusion between Au and Sn would induce the obstacle of the inter-diffusion between Sn and Ag. Ni will also diffuse quickly into Sn and form Ni₃Sn₄. The existence of Ni in Sn will also influence the diffusion of Ag into Sn and make the bad wettability during bonding. After several metallization systems have been investigated, finally a uniform bonding layer has been achieved by excluding Ni and Au in the bonding system. The bonding interface is Ag₃Sn layer dispersed with some pure Ag.