Fully differential capacitive architecture for mems accelerometer

Abstract

A fully differential microelectromechanical system (MEMS) accelerometer configured to measure Z-axis acceleration is disclosed. This may avoid some of the disadvantages in traditional capacitive sensing architectures—for example, less sensitivity, low noise suppression, and low SNR, due to Brownian noise. In one embodiment, the accelerometer comprises three silicon wafers, fabricated with electrodes forming capacitors in a fully differential capacitive architecture. 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 force to the proof mass region. Fully differential MEMS accelerometers 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 two pairs of capacitors within an apparatus, a change in the acceleration of the apparatus;   determining an acceleration of the apparatus based at least in part on the detecting.   
     
     
         2 . The method of  claim 1 , wherein detecting the change in acceleration includes measuring an electrical current across the at least two pairs of capacitors. 
     
     
         3 . The method of  claim 2 , wherein measuring the electrical current includes:
 measuring a change in capacitance of the at least two pairs of capacitors; and   measuring a change in voltage of the at least two pairs of 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 two pairs of capacitors. 
     
     
         6 . An apparatus, comprising:
 a first substrate;   first and second spring layers respectively disposed on a first surface and a second, opposite surface of the first substrate;   first and second sets of electrodes respectively disposed on the first and second spring layers;   a second substrate spaced from the first spring layer, wherein a third set of electrodes are disposed on the second substrate at locations corresponding to those of the first set of electrodes; and   a third substrate spaced from the second spring layer, wherein a fourth set of electrodes are disposed on the third substrate at locations corresponding to those of the second set of electrodes.   
     
     
         7 . The apparatus of  claim 6 , wherein the first and third sets of electrodes and the second and fourth sets of electrodes respectively form capacitors operable to detect variations in a proof mass. 
     
     
         8 . The apparatus of  claim 6 , wherein electrodes in the first, second, third, and fourth sets of electrodes are operable at least in part to apply a force to the proof mass. 
     
     
         9 . The apparatus of  claim 6 ,
 wherein the second substrate is spaced from the first spring layer by a first vacuum-sealed cavity,   wherein the third substrate is spaced from the second spring layer by the vacuum-sealed cavity.   
     
     
         10 . The apparatus of  claim 9 ,
 wherein the vacuum-sealed cavity is bounded in part by bonding structures including metallic and silicon dioxide portions.   
     
     
         11 . The apparatus of  claim 7 , wherein each electrode in the first and second set of electrodes includes:
 an oxide portion disposed on the first spring layer; and   a metal contact disposed on the oxide portion.   
     
     
         12 . The apparatus of  claim 11 , wherein the first substrate further includes first and second sets of piezoelectric structures respectively disposed on the first and second spring layers, wherein each piezoelectric structure in the first and second sets of piezoelectric structures includes:
 a metallic portion disposed on the first spring layer;   a piezoelectric contact disposed on the metallic portion;   first and second oxide portions respectively disposed on opposite sides of the piezoelectric contact; and   a pair of electrodes disposed on the first and second oxide portions.   
     
     
         13 . 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 pair of capacitors formed between the first bonded substrate and the central substrate region; and   a second pair of capacitors formed between the second bonded substrate and the central substrate region.   
     
     
         14 . The apparatus of  claim 13 , wherein the central substrate region includes:
 a proof mass region bounded by a first spring structure, a second spring structure, a first protection structure, and a second protection structure.   
     
     
         15 . The apparatus of  claim 14 , 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.   
     
     
         16 . The apparatus of  claim 15 , wherein the first, second, third, and fourth protection structures 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. 
     
     
         17 . The apparatus of  claim 15 , wherein the first, second, third, and fourth protection structure include silicon dioxide. 
     
     
         18 . The apparatus of  claim 13 , wherein the first bonded substrate includes a first getter layer, wherein the second bonded substrate includes a second getter layer. 
     
     
         19 . An apparatus, comprising:
 a fully differential MEMS accelerometer configured to measure Z-axis acceleration of a proof mass.   
     
     
         20 . The apparatus of  claim 19 , wherein the apparatus is configured to measure Z-axis acceleration using at least four capacitors, wherein the apparatus includes a central substrate region including a proof mass, two anchor regions, and two portions of a vacuum-sealed cavity.