Low-temperature hermetic packaging for microsystems using Au–Au surface-activated bonding at atmospheric pressure

Low-temperature hermetic bonding based on surface activation is useful for optical microsystem packaging because high bonding temperatures may degrade microsystem performance and sensitivity. However, surface-activated bonding (SAB) is usually performed under ultra-high-vacuum conditions, and the bonding environment cannot be chosen freely. In this study, thin Au sealing rings (300–500 nm thick, and 100 μm wide) were used as bonding layers for SAB at atmospheric pressure. A sufficiently high die-shear strength was achieved via surface activation using an argon radio-frequency plasma treatment. On examination of the fracture surfaces of the broken seal after the die-shear test, we observed that the fractures typically occurred at the deposited interface or partially inside the bulk substrates. Hermeticity was evaluated by measuring the resonance characteristics of photothermally excited microcantilevers inside the cavities. The samples bonded at the low temperature of 150 °C under the application of a bonding pressure of 313 MPa for 30 s showed leakage rates of less than 5.0 × 10−9 Pa·m3 s−1, which is the rejection limit defined by the MIL-STD-883G specification.