Room-temperature pressureless wafer-scale hermetic sealing in air and vacuum using surface activated bonding with ultrathin Au films

Wafer-scale hermetic sealing in air and vacuum was achieved by using Au–Au surface activated bonding (SAB) and sputtered ultrathin Au films (thickness: 15 nm). Because such films with small grains have smooth surfaces (root mean square surface roughness: around 0.3 nm), pressureless sealing at room temperature is feasible. For air sealing, the leakage rate in an He leak test was less than the lower limit of the tester (<1.6 × 10 −10 Pa m 3 s −1 ). For vacuum sealing, thin glass caps (thickness: 50 μ m) over vacuum-sealed cavities exhibited deflection after bonding due to the pressure difference between the cavities and ambient atmosphere. The deflection was unchanged even 900 d after sealing. The maximum air leakage rate is thus estimated to be less than 1.3 × 10 −14 Pa m 3 s −1 . These results suggest that Au–Au SAB with ultrathin Au films is useful for low-temperature wafer-level packaging of MEMS and various electronic devices.