High-Q Single Crystal Silicon HARPSS Capacitive Beam Resonators With Self-Aligned Sub-100-nm Transduction Gaps

This paper reports on the fabrication and characterization of high-quality factor (Q) single crystal silicon (SCS) in-plane capacitive beam resonators with sub-100 nm to submicron transduction gaps using the HARPSS process. The resonating element is made of single crystal silicon while the drive and sense electrodes are made of trench-refilled polysilicon, yielding an allsilicon capacitive microresonator. The fabricated SCS resonators are 20–40 m thick and have self-aligned capacitive gaps. Vertical gaps as small as 80 nm in between 20 m thick silicon structures have been demonstrated in this work. A large number of clamped-free and clamped–clamped beam resonators were fabricated. Quality factors as high as 177 000 for a 19 kHz clamped-free beam and 74 000 for an 80 kHz clamped–clamped beam were measured under 1 mtorr vacuum. Clamped–clamped beam resonators were operated at their higher resonance modes (up to the fifth mode); a resonance frequency of 12 MHz was observed for the fifth mode of a clamped–clamped beam with the fundamental mode frequency of 0.91 MHz. Electrostatic tuning characteristics of the resonators have been measured and compared to the theoretical values. The measured Q values of the clamped–clamped beam resonators are within 20% of the fundamental thermoelastic damping limits ( ) obtained from finite element analysis. [950]