A KINETICS STUDY OF THE BOND STRENGTH OF DIRECT-BONDED WAFERS

In order to determine whether low temperature wafer bonding is thermodynamically prohibited or simply a slow kinetic reaction, a systematic evaluation of the interface bond kinetics of silicon direct wafer bonding was performed over the temperature range of 200-1000-degrees-C for annealing times ranging from 15 min to 45 days. The tensile, shear, and torsion tests were developed to monitor the strength kinetics of silicon and silicon dioxide bonded wafers. The strength evolution is found to obey an Arrhenius relationship over the temperature range of 200-1000-degrees-C. Tensile strength of Si-Si bonded substrates varies from a minimum of 0.08 MPa at contact to a maximum of 4.25 MPa after high temperature thermal annealing. The failure of low temperature annealed samples to develop strong bonding in short periods of time has been correlated to microvoid formation at the interface as determined by TEM examination. Geometrical studies indicate that the microvoid formation is the result of trapped gases and can be reduced by appropriate choices of bonded geometries.