Electrostatic silicon accelerometer

• US 5,115,291 A
• Filed: 07/12/1991
• Issued: 05/19/1992
• Est. Priority Date: 07/27/1989
• Status: Expired due to Fees

• Current Assignee: Hughes Aircraft Company (Rtx Corporation)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Stokes, John F.
• Primary Examiner(s): James, Andrew J.
• Assistant Examiner(s): Nguyen, Viet Q.
• Application Number:
  US07/730,037
• Time in Patent Office: 312 Days
• Field of Search: 357/26, 357/74, 73/517 R, 73/517 AV
• US Class Current: 257/438
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends

Abstract

A solid state accelerometer having an all silicon sensor for measuring accelerational and gravitational forces. The accelerometer measuring system also has associated electronics that include an analog rebalance loop and a digitizer loop.

Claims

1. A solid state accelerometer, comprising:

a first conductive silicon plate;

a conductive silicon frame bonded to said first silicon plate;

a first thin insulating oxidized silicon layer located between said first silicon plate and said silicon frame;

a second conductive silicon plate bonded to said silicon frame;

a second thin insulating oxidized silicon layer located between said second silicon plate and said silicon frame;

wherein the bonding of the first and second silicon plates to the silicon frame results in an enclosure sealed from an external environment;

a silicon conductive pendulum, situated within the enclosure, said pendulum comprising an epitaxial layer of silicon deposited between two silicon wafers and having at least one conductive silicon flexure connecting said pendulum to said frame; and

feedback loop electronics electrically connected to said first and second plates and to said frame; and

wherein;

said pendulum is located between and approximately equidistant from said first and second plates;

said pendulum and said first plate have a first capacitance;

said pendulum and said second plate have a second capacitance;

the first and second capacitances have first and second original values, respectively, that are nearly equal to each other in absence of any gravitational or acceleration force upon said accelerometer;

said pendulum moves slightly upon an occurrence of any gravitational or acceleration force in a direction of a normal axis that intersects said first and second plates and said frame, thereby causing a slight change in the first and second capacitances;

any movement of said pendulum from being approximately equidistant between said first and second plates is counteracted by electrostatic forces between said pendulum and said first and second plates, said electrostatic forces being generated by said feedback loop electronics which operate in response to slight changes in the first and second capacitances to provide voltages across said pendulum and said first silicon plate and across said pendulum and said second silicon plate sufficient to move the pendulum to a position approximately equidistant from said silicon plates; and

magnitudes of the voltages provided indicate a magnitude of accelerational or gravitational force acting on said accelerometer in a direction approximately parallel to the normal axis.

2. Apparatus of claim 1 wherein said pendulum, said frame, and said first and second plates have approximately equal coefficients of thermal expansion.

3. Apparatus of claim 2 wherein each of said first and second insulating oxidized silicon layers has a thickness less than 3 microns.

4. Apparatus of claim 3 wherein said pendulum is a diaphragm.

5. A solid state accelerometer, comprising:

a sensor capsule, comprising;

a first doped silicon wafer;

a doped layer of epitaxial silicon deposited on said first doped silicon wafer and patterned with dry etch to define a proof mass and one or more flexures;

a second doped silicon wafer bonded to said epitaxial silicon layer;

a first dielectric interlayer deposited opposite said epitaxial silicon layer on said first doped silicon wafer and etched to form a first recessed area;

a second dielectric interlayer deposited opposite said epitaxial silicon layer on said second doped silicon wafer and etched to form a second recessed area; and

third and fourth doped silicon wafers bonded to said first and second dielectric interlayers, respectively, to seal said capsule and to form a first gap over said first recessed area and a second gap over said second recessed area, said gaps defining first and second capacitors, respectively;

wherein said proof mass and flexures are formed by patterning and etching said first and second wafers and said epitaxial silicon layer; and

feed back loop electronics electrically connected to said first capacitor and said second capacitor, said electronics operating to provide voltages as a function of gravitational or accelerational force applied to said proof mass by generating electrostatic forces required to force said proof mass and flexure to a null position approximately equidistant from said first and second wafers.

6. The solid state accelerometer according to claim 5 wherein said first and second wafers are p^- doped, said third and fourth wafers are p⁺ doped and said epitaxial silicon layer is p⁺ doped.

7. The solid state accelerometer according to claim 5 wherein said proof mass and flexures form a pendulum.

8. The solid state accelerometer according to claim 5 wherein said proof mass and flexures form a diaphragm.

9. The solid state accelerometer according to claim 5 wherein said feedback loop electronics comprises:

an analog rebalance loop connected to said first and second capacitors; and

a digitizer loop connected to said analog rebalance loop circuit.

10. The solid state accelerometer according to claim 5 wherein said first and second dielectric interlayers comprise SiO₂ and SiN₃.

11. The solid state accelerometer according to claim 10 wherein said first and second dielectric interlayers are approximately 2 micrometers thick.

12. The solid state accelerometer according to claim 10 wherein said third and fourth wafers are fusion bonded to said epitaxial silicon layer.