Surface activated bonding of Ti/Au and Ti/Pt/Au films after vacuum annealing for MEMS packaging

For micro electro mechanical system (MEMS) packaging that can control the internal atmosphere for a long period, we demonstrated a low-temperature bonding process for Au films after vacuum annealing for degassing. Ti (5 nm)/Au (12 nm) and Ti (5 nm)/Pt (10 nm)/Au (12 nm) (from bottom to top) films on Si substrates were bonded by a sequence consisting of surface activation by Ar plasma, vacuum annealing at 200 degrees C, and bonding at 200 degrees C or 40 degrees C. The Ti/Au films failed to form bonding at low temperatures in the case of bonding after the vacuum annealing. However, the Ti/Pt/Au films were successfully bonded even after the vacuum annealing. After the vacuum annealing step, TiO2 formed on the Ti/Au film surface due to thermal diffusion of Ti atoms through the thin Au film. Neither Pt nor Ti atoms were detected on the Ti/Pt/Au film surface after the vacuum annealing step. It is assumed that the thin Pt layer blocked the thermal diffusion of Ti atoms.