Low-pressure-encapsulated resonant structures with integrated electrodes for electrostatic excitation and capacitive detection

Abstract

Low-pressure-encapsulated resonant structures with lateral electrical feedthrough conductors for electrostatic excitation and capacitive detection are presented. The encapsulated device consists of a triple-stack wafer sandwich. The middle wafer is the silicon substrate with the resonant structure. The top and the bottom substrates are micromachined Pyrex 7740 glass wafers with metal electrodes. The resulting pressure inside the hermetically sealed cavity is 1 mbar, obtained by low-pressure anodic bonding, starting from 10 −5 mbar, without using any getter material or gas-evacuation procedure after the bonding. A special electrode design is presented, making it possible to have electrodes on both glass lids using only standard fabrication steps. Low power consumption can be achieved and voltages of only 5 to 10 V r.m.s. are sufficient for the excitation. A long-term stability test for low-pressure-encapsulated structures shows that after storage for one year (without integrated electrodes) and three months (with integrated electrodes) no leakage has been observed. Finally, a new fabrication technique is investigated to improve the quality factor of the resonator. It consists of using the anodically bonded frame of the silicon structure as a mask for deep glass etching to increase the gap between the electrode wall and the resonator, thus yielding a high Q-factor.