Study of frequency tuning methods for vibrating MEMS devices

Abstract

Maintaining an accurate resonant frequency among fabricated devices is a key issue during fabrication of vibrating MEMS devices to achieve high performance and uniformity. In order to fabricate devices with accurate resonant frequencies, fabrication errors during the MEMS processes should be prevented, and the resonant frequency trimming method becomes necessary for the vibrating MEMS devices. This thesis provides two methods regarding the fabrication of devices with accurate resonance frequency. One is the frequency-trimming method of MEMS resonator using CNT rope synthesis, and the other is the gyroscope fabrication method for decreasing the deviation among the devices in a single wafer. A novel passive frequency-trimming method for microelectromechanical system (MEMS) resonators using carbon nanotube (CNT) rope synthesis on the side of the MEMS resonator is described in first part. The method has a number of advantages over conventional methods, including low cost, reduced processing time, and greater applicability. First, the method requires only an AC voltage source and a dispersed CNT suspension to synthesize CNT ropes using dielectrophoresis. The method can be implemented in 3 min at room temperature and atmospheric pressure. The resonant frequency of the MEMS resonator can be shifted by 0.5%–24%. In addition, the method can restore the original frequency by cutting the CNT ropes by passing a current through them, so that it will be possible that trimming can be carried out repeatedly by connecting and disconnecting the CNT ropes until the desired frequency is achieved. The performance of MEMS gyroscopes depends on the displacement sensitivity of the capacitors. In second part, I describe the fabrication of 300-µm-thick gyroscopes that can provide high displacement sensitivity and are robust to fabrication tolerances; i.e., deep reactive ion etch (DRIE) rate uniformity. When thick structures are perforated using DRIE to achieve high-aspect-ratio features, footing is commonly observed; however, I describe a fabrication method that circumvents problems associated with footing and side-wall etching, so that the gyroscopes can have uniform dimensions and small variations across the wafer. It leads that the each gyroscopes in a wafer have similar resonant frequencies. Using a post-fabrication translation approach, the position of capacitors is modified following DRIE, and the gap in the gyroscopes can be reduced to 3 μm, which leads to an aspect ratio of 100. Using this method, I fabricated MEMS gyroscopes that can overcome the DRIE aspect ratio limit and have capacitors with higher sensitivities than those of other gyroscopes, which typically employ substrates that are less than 100 µm thick.