A zero-level vacuum encapsulation technique with less parasitic effect for VHF MEMS resonators

This paper presents a zero-level vacuum encapsulation technique with the development of effective parasitic optimization methods for VHF MEMS resonator. Sn-rich Au-Sn solder bonding is used to achieve vacuum hermeticity packaging with high shear strength. The measured average leak rate and shear strength are (2.2 ± 0.9) × 10−8 atm-cc s−1 and 40.65 ± 8.25 MPa among 12 packaged chips, respectively. An equivalent circuit model of the packaged resonator is established to analyze the parasitic effect caused by encapsulation. A significant reduction of the parasitic effect is achieved via grounding sealing ring, and decreasing parasitic capacitances between the sealing ring and signal feedthroughs. The measured frequency spectrum of a packaged resonator is not distorted by parasitic effect, which is well consistent with the simulation result. Besides, the Q value of the resonator increases from 6400 to 10,300, as well as the transmission gain at harmonic peak from −73.92 dB to −70.36 dB due to an excellent electrical performance of the encapsulation structure and a high vacuum in this packaging cavity.