Characterization of Encapsulated Micromechanical Resonators Sealed and Coated With Polycrystalline SiC

This paper presents the characterization of sealed microelectromechanical devices and their packaging fabricated in a polycrystalline 3C silicon carbide (poly-SiC) thin-film encapsulation process. In this fabrication technique, devices are sealed with a nominally 2-¿m low-pressure chemical vapor deposition poly-SiC, and the device layer is simultaneously coated with a nominally 0.2- ¿m poly-SiC thin film. Device characterization includes measurement of the resonant frequency and the quality factor of double-ended tuning-fork micromechanical resonators, which have a Si-SiC composite beam structure. Experimental results show that the pressure inside the packaging can be controlled from 447 Pa to 15.5 kPa with a 400°C annealing process. The frequency drifts of the encapsulated resonators are less than the frequency noise level (±10.6 ppm) measured over 29 days at 84.6°C ±0.1°C, which suggests that the poly-SiC thin-film packaging technique can offer hermetic packaging for various applications in microelectromechanical systems including inertial sensors. In addition to the packaging performance, the temperature coefficient of Young's modulus for poly-SiC is derived from the resonant-frequency change of resonators with temperature. The reduction of the quality factor due to the poly-SiC coating, predicted in the theoretical model, is confirmed by measurements.