Optimization of Ag-Ag Direct Bonding for Wafer-Level Power Electronics Packaging via Design of Experiments

In this study, Ag stress migration bonding (SMB) is demonstrated with various Ag film materials and bonding conditions. The main effects and interactions of various processing parameters such as bonding temperature, process time and applied pressure on the interfacial shear strength of the DUTs are firstly investigated via the design of experiments (DoE) method. The hillock and grain growth process in Ag films deposited on a Si substrate depending on process temperature and time has been investigated. Hillock formation is clearly observed on all film surfaces at 300 °C. Furthermore, various direct bonding tests are carried out with the optimal parameters using two different metal-stacks, Cr/Ni/Ag and Ti/Ag. Compared to the Cr/Ni/Ag metallized samples, a highly increased shear strength of 73 MPa is achieved with Ti/Ag film. In addition, the lifetime of direct bonded Ag joints was examined by passive thermal cycling tests. The results show no significant change in the shear strength after 700 thermal cycles.