Characterization of plasma-activated, thermally-annealed Si-SiO2 direct bond strength for vapor HF etching

This paper presents the bond strength of Si-SiO2 direct-bonded wafers characterized by vapor HF etching and discusses the effects of several bonding conditions. Vapor HF etching is used for the release of free-standing MEMS structures, and its controllability is dependent on Si-SiO2 bond strength. However, the bond strength characterized by mechanical methods such as crack opening method is not valid for this purpose. A comparative study of the effects of bonding conditions, including the type of thermal annealing, annealing temperature, and gas for plasma activation, was conducted using vapor HF. The results reveal that the thermal annealing temperature is the most critical parameter affecting bond strength, with bond strength increasing as the annealing temperature increases. However, the bond strength did not reach saturation at 1000 °C, which is different from the results obtained by the crack opening method. Although there are some differences in the effects of different thermal annealing types, the differences are not significant. Furthermore, N2 plasma activation shows a slightly better effect than O2 plasma activation. Additionally, the bond strength was found to be consistent across the entire wafer.