LOW-PRESSURE-ENCAPSULATED RESONANT STRUCTURES WITH INTEGRATED ELECTRODES FOR ELECTROSTATIC EXCITATION AND CAPACITIVE DETECTION

Low-pressure-encapsulated resonant structures with lateral electrical feedthrough conductors for electrostatic excitation and capacitive det ection are presented. The encapsulated device consists of a triple-sta ck wafer sandwich. The middle wafer is the silicon substrate with the resonant structure. The top and the bottom substrates are micromachine d pyrex 7740 glass wafers with metal electrodes. The resulting pressur e inside the hermetically sealed cavity is 1 mbar, obtained by low-pre ssure anodic bonding, starting from 10(-5) mbar, without using any get ter material or gas-evacuation procedure after the bonding. A special electrode design is presented, making it possible to have electrodes o n both glass lids using only standard fabrication steps. Low power con sumption can be achieved and voltages of only 5 to 10 V-r.m.s. an suff icient for the excitation. A long-term stability test for low-pressure -encapsulated structures shows that after storage for one year (withou t 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 consi sts 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. (C) 1998 Elsev ier Science S.A. All rights reserved.