Force feedback electrodes in mems accelerometer

Abstract

A microelectromechanical system (MEMS) accelerometer having separate sense and force-feedback electrodes is disclosed. The use of separate electrodes may in some embodiments increase the dynamic range of such devices. Other possible advantages include, for example, better sensitivity, better noise suppression, and better signal-to-noise ratio. In one embodiment, the accelerometer includes three silicon wafers, fabricated with sensing electrodes forming capacitors in a fully differential capacitive architecture, and with separate force feedback electrodes forming capacitors for force feedback. These electrodes may be isolated on a layer of silicon dioxide. In some embodiments, the accelerometer also includes silicon dioxide layers, piezoelectric structures, getter layers, bonding pads, bonding spacers, and force feedback electrodes, which may apply a restoring force to the proof mass region. MEMS accelerometers with force-feedback electrodes may be used in geophysical surveys, e.g., for seismic sensing or acoustic positioning.

What is claimed is: 
     
         1 . A method, comprising:
 detecting, by at least one sense capacitor within an apparatus, an acceleration of a proof mass within the apparatus;   applying a feedback force to the proof mass via at least one feedback capacitor, wherein the feedback force is based on the acceleration of the proof mass; and   determining an acceleration of the apparatus based on the feedback force.   
     
     
         2 . The method of  claim 1 , wherein the detecting the acceleration includes detecting a change in capacitance of the at least one sense capacitor. 
     
     
         3 . The method of  claim 2 , wherein the detecting a change in the capacitance includes:
 detecting a change in the capacitance of a fully differential set of at least four sense capacitors.   
     
     
         4 . The method of  claim 1 , wherein determining the acceleration includes determining the Z-axis acceleration. 
     
     
         5 . The method of  claim 1 , wherein determining the acceleration includes using front-end readout circuitry connected to the at least one feedback capacitor. 
     
     
         6 . An apparatus, comprising:
 a MEMS accelerometer configured to measure Z-axis acceleration, wherein the MEMS accelerometer includes:
 a sense electrode configured to detect changes in position of a proof mass in the MEMS accelerometer; and 
 a force feedback electrode configured to provide a restoring force to the proof mass, wherein the restoring force is based on the detected changes in the position of the proof mass. 
   
     
     
         7 . The apparatus of  claim 6 , wherein the proof mass is adjacent to at least two vacuum-sealed cavities. 
     
     
         8 . The apparatus of  claim 6 , further comprising a plurality of piezoelectric elements configured to dampen vibrations of the proof mass. 
     
     
         9 . The apparatus of  claim 6 , wherein the MEMS accelerometer includes two sense electrodes arranged in a differential orientation. 
     
     
         10 . The apparatus of  claim 6 , wherein the MEMS accelerometer includes four sense electrodes arranged in a fully differential orientation. 
     
     
         11 . The apparatus of  claim 6 , further comprising closed-loop circuitry configured to measure the detected changes in the position of the proof mass and configured to apply a voltage to the force feedback electrode. 
     
     
         12 . The apparatus of  claim 11 , wherein the apparatus is further configured to output an acceleration value based on the voltage applied to the force feedback electrode. 
     
     
         13 . The apparatus of  claim 12 , wherein the acceleration value output by the apparatus is the voltage applied to the force feedback electrode. 
     
     
         14 . The apparatus of  claim 6 , wherein the force feedback electrode forms a capacitor with an electrode on the proof mass. 
     
     
         15 . An apparatus, comprising:
 a central substrate region;   a first bonded substrate opposing a first surface of the central substrate region;   a second bonded substrate opposing a second surface of the central substrate region;   a first sense capacitor and a first force feedback capacitor formed between the first bonded substrate and the central substrate region; and   a second sense capacitor and a second force feedback capacitor formed between the second bonded substrate and the central substrate region.   
     
     
         16 . The apparatus of  claim 15 , wherein the central substrate region further includes:
 a proof mass region bounded by a first spring structure, a second spring structure, a first protection structure, and a second protection structure.   
     
     
         17 . The apparatus of  claim 16 , further comprising:
 a vacuum-sealed cavity bounded in part by the first and second bonded substrates, the first and second protection structures, a third protection structure, and a fourth protection structure.   
     
     
         18 . The apparatus of  claim 17 , wherein the first and second vacuum-sealed cavities are disposed laterally on either side of the proof mass region, and wherein the first and second bonded substrates are disposed vertically on either side of the central substrate region. 
     
     
         19 . The apparatus of  claim 17 , wherein the first, second, third, and fourth protection structures include silicon dioxide. 
     
     
         20 . The apparatus of  claim 16 , further comprising feedback circuitry configured to apply a continuous restoring force to the proof mass region based on measured values of the first and second sense capacitors.