High sensitivity integrated micromechanical electrostatic potential sensor

• US 5,517,123 A
• Filed: 08/26/1994
• Issued: 05/14/1996
• Est. Priority Date: 08/26/1994
• Status: Expired due to Term

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Zhao, Yang, Payne, Richard S.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): DO, DIEP N
• Application Number:
  US08/296,529
• Time in Patent Office: 627 Days
• Field of Search: 324/72, 324/72.5, 324/109, 324/457, 324/458, 73/517 B, 73/517 R, 361/280
• US Class Current: 324/458
• CPC Class Codes: G01R 29/12 Measuring electrostatic fields or voltage-potential

Abstract

The invention is a method and apparatus for electrostatic potential sensing by using electromechanical microstructures. The mechanical microstructure may be fabricated by either surface micromachining or bulk-micromachining. The sensor comprises a movable plate which is movable in the lateral dimension and which is suspended from and above a substrate bearing an isolated electrically conductive layer. The layer acts as the sensor electrode and is brought in contact with or near the object the electrostatic potential of which is to be sensed. The movable plate is suspended between two fixed, suspended and cantilevered plates. The three suspended plates collectively form two capacitors with the two fixed plates comprising the first plate of first and second capacitors and the movable plate comprising the second plate of both capacitors. A potential differential between the movable plate and the conductive layer causes lateral movement of the movable plate, resulting in a change in the capacitance of the two capacitors. Resolving circuitry converts the change in capacitance into an output signal indicative of the electrostatic difference between the movable plate and the conductive layer.

Claims

1. An apparatus for sensing electrical potential of an object, the apparatus comprising:

a semiconductor substrate,

a dielectric layer over said substrate,

a conductive layer over said dielectric layer,

first and second electrically conductive and substantially stationary plates suspended above said conductive layer, said first and second plates being substantially coplanar in a plane parallel to said conductive layer and charged to first and second bias voltages, respectively,

a third electrically conductive plate suspended above said conductive layer in said plane between said first and second plates such that said first, second, and third plates form first and second capacitors, said first plate forming a first electrode of the first capacitor, said second plate forming a first electrode of the second capacitor, and said third plate forming a second electrode of both of said first and second capacitors with said third plate being resiliently suspended such that said third plate is movable in said plane in response to electrostatic force and is biased to a third voltage,

a remote electrode, electrically coupled to said conductive layer, for placement at least electrically proximal to the object whose electrical potential is being sensed, and

resolving circuitry coupled to said first and second capacitors for detecting movement of said third plate relative to said first and second plates.

2. An apparatus as set forth in claim 1 wherein said third electrode is substantially immovable perpendicularly to said plane.

3. An apparatus as set forth in claim 2 wherein said third voltage is between said first and second voltages.

4. An apparatus as set forth in claim 3 further comprising:

a voltage source which supplies a voltage equivalent to said third voltage,

a switch coupling said voltage source to said conductive layer, and

a controller for periodically closing said switch for a predetermined period such that said conductive layer is coupled to said voltage source.

5. An apparatus as set forth in claim 4 wherein said third voltage is half way between said first and second voltages.

6. An apparatus as set forth in claim 3 wherein said resolving circuit comprises:

circuitry for generating first and second sinusoidal signals of equal amplitude and opposite phase and supplying said first and second sinusoidal signals to said first and second plates, respectively, and

a demodulator coupled to receive the voltage on said third plate.

7. An apparatus as set forth in claim 6 wherein said resolving circuitry further comprises an amplifier coupled between said third plate and said demodulator.

8. An apparatus as set forth in claim 7 wherein said resolving circuitry further comprises a force feedback loop for forcing said third plate back to a neutral position when it is displaced.

9. An apparatus as set forth in claim 8 wherein

said third plate comprises a polysilicon bridge suspended above said surface by a plurality of anchors extending from said surface, said bridge comprising a pair of resilient members coupled to said anchors, a central beam coupled at opposite ends thereof to said first and second resilient members, respectively, and a plurality of generally parallel movable fingers extending transversely from said beam, and

said first and second plates comprises a plurality of generally parallel electrically conductive substantially stationary fingers, each of said stationary fingers corresponding to one of said movable fingers and positioned relative to said corresponding movable finger such that said corresponding movable finger and stationary finger form a capacitor, wherein a first set of said stationary fingers comprises said first plate, each of said first set of stationary fingers positioned on one side of said corresponding movable finger, and wherein a second set of said stationary fingers comprises said second plate, each of said second set of stationary fingers positioned on an opposite side of said corresponding movable finger, and wherein said first set of stationary fingers are electrically coupled to a first node and said second set of stationary fingers are electrically coupled to a second node.

10. An apparatus as set forth in claim 1 wherein said first, second and third plates are formed of polysilicon suspended from anchors extending upwardly from said substrate.

11. An apparatus as set forth in claim 1 wherein said conductive layer comprises a metal formed over said substrate and said dielectric.

23. A method comprising using the apparatus of claim 1 to sense the electrical potential of the object.

12. An apparatus for sensing electric potential of an object, the apparatus comprising:

a semiconductor substrate,

a dielectric layer over said substrate,

a conductive layer over said dielectric layer,

a movable plate that includes a polysilicon bridge suspended above said conductive layer by a plurality of anchors extending from said substrate, said bridge including first and second resilient members coupled to said anchors, a central beam coupled at opposite ends thereof to said first and second resilient members, respectively, and a plurality of substantially parallel movable fingers extending transversely from said beam, said bridge being substantially planar in a plane above and parallel to said conductive layer and substantially immovable perpendicular to said plane, said movable plate being biased to a third voltage,

first and second substantially stationary plates, each including a plurality of substantially parallel, electrically conductive polysilicon fingers suspended above said conductive layer, each of said stationary fingers corresponding to at least one of said movable fingers and positioned relative to said corresponding movable finger such that each pair of corresponding movable and stationary fingers forms a capacitor, with a first set of said movable fingers including said first plate and are positioned on one side of said corresponding stationary fingers and are coupled to a first node, and a second set of stationary fingers including said second plate and are positioned on an opposite side of said corresponding movable fingers and are coupled to a second node, and with said first and second stationary plates and said movable plate forming a differential pair of capacitors including first and second capacitors,

said first and second stationary plates being biased to first and second voltages, respectively,

a remote electrode, electrically coupled to said conductive layer, for placement at least electrically proximal to the object whose electrical potential is being sensed, and

resolving circuitry coupled to said first and second capacitors for detecting movement of said movable plate relative to said first and second plates.

13. An apparatus as set forth in claim 12 further comprising:

a voltage source which supplies a voltage equivalent to said third voltage,

a switch coupling said voltage source to said conductive layer, and

a controller for periodically closing said switch for a predetermined period such that said conductive layer is coupled to said voltage source.

14. An apparatus as set forth in claim 13 wherein said third voltage is 1.8 volts, said first voltage is 0.2 volts and said second voltage is 3.4 volts.

15. An apparatus as set forth in claim 12 wherein said resolving circuit comprises:

circuitry for generating first and second signals of equal amplitude and opposite phase and supplying said first and second sinusoidal signals to said first and second plates, respectively, and

a demodulator coupled to receive the voltage on said third plate.

16. An apparatus as set forth in claim 15 wherein said resolving circuitry further comprises an amplifier coupled between said third plate and said demodulator.

17. An apparatus as set forth in claim 16 wherein said resolving circuitry further comprises a force feedback loop for forcing said third plate back to a neutral position when it is displaced.

24. A method comprising using the apparatus of claim 12 to sense the electrical potential of the object.

18. A method for sensing electrical potential of an object with a sensor having a conductive layer, first and second electrically conductive plates suspended above and substantially stationary relative to the conductive layer, and a third electrically conductive plate suspended above the conductive layer between the first and second plates, the first, second, and third plates lying in a plane parallel to the conductive layer and forming a differential capacitor, the method comprising the steps of:

positioning a remote electrode at least electrically proximal to the object, the electrode being electrically coupled to the conductive layer to provide a voltage to the conductive layer to cause the third plate to move in the plane relative to the first and second plates in response to a change in the voltage on the conductive layer;

sensing movement of the third plate relative to the first and second plates; and

using the sensed movement to determine the electrical potential of the object.

19. The method of claim 18, wherein the positioning step includes positioning the electrode near the object to perform non-contact sensing.

20. The method of claim 18, wherein the positioning step includes positioning the electrode on the object to perform contact sensing.

21. The method of claim 18, further comprising a step of generating first and second signals of equal amplitude and opposite phase and applying the first and second signals to the respective first and second plates.

22. The method of claim 18, wherein the sensing step includes determining the electrical potential with a force feedback loop.