Room-temperature wafer bonding using smooth gold thin films for wafer-level MEMS packaging

A room-temperature wafer bonding process using photolithographically patterned gold thin films has been demonstrated for MEMS packaging application. The smooth Au thin films with a root-mean-square surface roughness of less than 0.5 nm (thickness less than ~ 50 nm) were prepared by electron-beam evaporation. These films were fabricated into a square ring-shaped pattern (width: 100-200 μm) using wet etch chemistry for the purpose of hermetic sealing. The bonding process is based on Au surface activation by argon radio-frequency plasma. Without plasma treatment, bonding energy decreased with increasing exposure time to air or ethanol after Au deposition. On the other hand, with plasma treatment high bonding energy was obtained regardless of exposure time.