Design, process integration and characterization of wafer level vacuum packaging for MEMS resonator

This paper discusses wafer level vacuum sealing technology with evaporated AuSn solder for a microelectromechanical systems (MEMS) resonator without getter material. The MEMS resonator is fabricated and characterized in a vacuum chamber. Relationship between the Q-factor of the MEMS resonator and the vacuum level is established and used as a reference for later vacuum level calibration. Wafer bonding using evaporated AuSn solder is performed in an EVG wafer bonder. With optimized bonding conditions, the achieved shear strength is higher than 59 MPa and uniform cross-section of the bonding ring has been achieved. The calculated He leakage rate is between 10⁻¹³ atm cc/s and 10⁻¹⁴ atm cc/s. By comparing the measured Q-factor of packaged resonator with the reference curve, the corresponding vacuum level is 0.2 Torr. Reliability tests results show that shear strength decreases for 7% and still high enough for real application. The vacuum level after reliability tests is comparable to that of long term vacuum level.