Electrostatic tuning in linear regime for 25.9-MHz micromechanical resonator with 120-nm capacitive gap

Abstract

In this paper, electrostatic tuning in the linear regime for a high-frequency single-crystal silicon resonator is investigated. An electrostatic actuated I-shaped bulk acoustic resonator (IBAR) is fabricated in our experiment owing to its advantage of a large electrode area. The single-crystal silicon resonator is actuated by electrostatic force. The electrostatic force is improved by reducing the capacitive gap to 120 nm. The fabrication of the capacitive gap is based on the process for a high-aspect-ratio self-aligned stepped polysilicon electrode for a single-crystal silicon microstructure. The effective stiffness of an IBAR is softened by increasing the bias voltage, which is applied between the polysilicon electrodes and the resonant structure. For the tuning frequency in the linear regime, the ac actuation voltage is small to avoid electrical nonlinearity. The resonant frequency of the IBAR is measured to be 25.9 MHz in a vacuum. The temperature coefficient of frequency (TCF) of our device is approximately −14 ppm/ °C, which is attained by boron doping. When the resonator operates in the linear regime, frequency tuning can achieve 1833 ppm over 130 °C by changing the bias voltage from 2 to 23 V at room temperature. The temperature drift of the full operating range of a high-frequency resonator is compensated by electrostatic tuning for the first time.