Capacitive Lamé mode resonator with gap closing mechanism for motional resistance reduction

This work presents a Lamé mode resonator featuring a novel gap closing mechanism which employs electrostatic force to reduce the capacitive transduction gaps to sub-micron values to overcome fabrication technology limitations. This leads to significant resonator loss and motional resistance reduction, which is highly advantageous as it simplifies the design of oscillation sustaining circuitry. Prototypes were fabricated in a commercial silicon-on-insulator technology, PiezoMUMPs from MEMSCAP. Upon the application of a DC voltage of 55 V between the resonator structure and electrodes, the gaps are reduced from 2.5 μm to 0.5 μm. A resonance frequency of 18 MHz with a quality factor of 120,000 was observed under 1 mTorr vacuum. A loss of 55 dB was measured at 55 V, which corresponds to a motional resistance of 56 kΩ, 5 times lower than that of a similar design without gap closers at the same voltage. The frequency tuning range is also increased significantly as a result of the gap reduction, which can be very useful for overcoming ambient conditions and fabrication variations.