Capacitance pressure sensor

• US 4,730,496 A
• Filed: 06/23/1986
• Issued: 03/15/1988
• Est. Priority Date: 06/23/1986
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Knecht, Thomas A., Frick, Roger L.
• Primary Examiner(s): Woodiel, Donald O.
• Application Number:
  US06/877,281
• Time in Patent Office: 631 Days
• Field of Search: 73/724, 73/718, 73/708, 73/706, 73/756, 73/DIG. 4, 361/283
• US Class Current: 73/724
• CPC Class Codes: G01L 9/0073 using a semiconductive diaphragm

Abstract

A capacitive pressure sensor that is fabricated by a batch process affords isolation for the sensing element and leads from the pressure media and provides stress isolation as well. The pressure sensor is made up of a sandwich construction including a silicon wafer which is etched from one side to make cavities in a plurality of desired locations to form deflecting diaphragms, one surface of which acts as a capacitor plate. A glass layer is metalized on both sides and has holes drilled in locations that align with the diaphragms formed on the silicon wafer. The glass layer is anodically bonded to the wafer to form capacitance gap of a few microns relative to the one surface of each diaphragm. The assembly of the metalized glass layer and the silicon wafer is in a preferred form sandwiched between two additional layers, and bonded together in a vacuum atmosphere. The four layer sandwich is then cut up into individual sensors. The initial assembly can be formed to provide dampening of the diaphragm response times and to minimize the likelihood of false signals at high frequency inputs.

Claims

1. A capacitive pressure sensor for connection to a capacitance sensing circuit providing an output representative of a sensed pressure comprising:

a first layer formed of a dielectric material having at least one substantially planar surface spaced from a second opposite surface thereof and a first aperture extending through the first layer from the first planar surface to the second surface;

electrical conductive means deposited on the first layer and extending over a first portion of the first planar surface for forming a first capacitor plate and extending over a portion of the second surface to form an electrical contact layer spaced away from the aperture and extending through the aperture to the first capacitor plate for connecting the first capacitor plate to a sensing circuit through the contact layer;

a diaphragm layer formed of a brittle material having a rim surrounding a diaphragm sealingly bonded to the first surface of the first layer to surround the first capacitor plate, said diaphragm facing and being spaced from the first capacitor plate, said diaphragm forming a second capacitor plate in the brittle material responsive to a pressure applied to the diaphragm; and

a sealing layer having a peripheral rim overlying the rim of the diaphragm layer to form a peripheral support, the peripheral rim having a sealing surface thereon bonded to the electrical conductive means on the second surface of the first layer for sealing the aperture to provide a reference pressure to the side of the diaphragm facing the first capacitor plate.

2. The capacitive pressure sensor of claim 1 wherein the diaphragm is formed in a layer of semi-conductor material.

3. The capacitive pressure sensor of claim 2 wherein the layer of semi-conductor material has a metal layer in ohmic contact with the semi-conductor material.

4. The capacitive pressure sensor of claim 3 and a fourth layer of material bonded to the semi-conductor layer on the second surface of the semi-conductor layer and spanning the diaphragm region, said fourth layer having an aperture for permitting fluid under pressure to be sensed to act on the diaphragm.

5. The capacitive sensor of claim 4 wherein the fourth layer includes a neck portion of substantially smaller dimension than the transverse width of the sensor, the aperture through the fourth layer passing through the neck.

6. The capacitive sensor of claim 5 wherein said neck has a mounting surface for mounting the sensor to said neck providing stress isolation means for isolating external stresses from the diaphragm.

7. The capacitive sensor of claim 4 wherein the fourth layer defines a chamber in combination with the diaphragm, and restricted aperture means defined between the chamber and the exterior of the sensor to restrict flow of fluids therethrough as the diaphragm deflects from a reference position.

8. A batch fabricated capacitive sensor comprising:

a dielectric layer of rigid material having first and second surfaces;

a diaphragm assembly comprising a second layer comprising an edge supported diaphragm sealingly bonded to the first side of the dielectric layer and forming a capacitor plate;

said dielectric layer having an opening therethrough in registry with the diaphragm and a metal layer formed on both sides of the dielectric layer and through the opening; and

a third layer having a peripheral rim overlying the diaphragm assembly'"'"'s edge to form a peripheral support sealingly bonded to the metal layer on the second surface of the dielectric layer to overlie and seal the opening, a portion of said third layer being removed to expose the metal layer on the second side of the dielectric layer to provide an exposed metal surface for attachment of leads to such metal layer on the second side of the dielectric layer.

9. A pressure sensor for sensing the magnitude of a pressure comprising:

a support layer having a opening therethrough and having a metal layer formed on both sides of the layer and through the opening;

a layer of material including a diaphragm member surrounded by a diaphragm rim mounted on said support layer, said diaphragm and support layer having facing surfaces forming capacitive sensing means, and being spaced apart to provide capacitive spacing, said diaphragm being deflected toward said support layer when subjected to a pressure;

a sealing layer disposed on the support layer having a peripheral rim overlying the diaphragm rim to form a peripheral support, the peripheral rim having a sealing surface thereon bonded to the metal layer for sealing the opening;

means forming a dampening chamber on a side of said diaphragm opposite from said support layer, said means comprising a cover member overlying said diaphragm and being fixed with respect to the edges of the diaphragm to form a small volume dampening chamber disposed directly over the diaphragm, and

opening means comprising a control orifice leading from said dampening chamber to the exterior, a filling of fluid provided within said dampening chamber, the pressure applied to said diaphragm causing flow of the filling of fluid in the dampening chamber through the control orifice such that the response of the diaphgragm is dampened.

10. The sensor as specified in claim 9 said diaphragm has a rim surrounding a deflecting central portion, said rim being fixed to said support layer, and said cover member being fixed to said rim, said control orifice being formed to extend across the diaphragm rim.

11. The sensor as specified in claim 9 wherein said diaphragm member has a rim surrounding a central deflecting portion, and said cover member is supported on said rim, said cover member having a boss that extends toward the diaphragm member within the space defined by the rim to restrict the volume in said dampening chamber, and said control orifice being defined in said cover member.

12. the sensor as specified in claim 9 wherein said sensor is mounted in an external housing, said external housing having an isolation diaphragm, and a fluid filled chamber in the housing surrounding said sensor.

13. The sensor as specified in claim 9 wherein said diaphragm is made of a brittle material.

14. The sensor of claim 12 wherein the diaphragm is made of silicon.

15. A capacitive sensor for sensing the magnitude of a pressure, comprising:

a rigid support;

a brittle diaphragm member mounted on said support, said diaphragm member and said support having facing surfaces closely spaced from each other;

sensing means for sensing diaphragm deflection, said diaphragm member being deflected toward said support when subjected to a pressure;

means forming a small volume dampening chamber on a side of said diaphragm opposite from said support comprising a cover member overlying said diaphragm and being fixed with respect to the edges thereof to form said small volume chamber;

opening means comprising a passageway leading from said dampening chamber to the exterior;

a filling of oil provided in said chamber, the pressure applied to said diaphragm acting only through said chamber and the filling of oil in the chamber to cause deflection of the diaphragm whereby movement of the diaphragm is directly related to fluid flow through the passageway; and

circuit means for sampling the signal of the capacitive sensor at a desired frequency, the control orifice restricting the dynamic response of the diaphragm to a frequency not substantially greater than one-half of the sampling frequency.

16. A capacitive pressure sensor for connection to a capacitance sensing circuit providing an output representative of a sensed pressure comprising:

a dielectric layer having an aperture extending therethrough between a substantially planar surface and an opposite second surface thereon;

electrical conductive means for forming a first capacitor plate on the planar surface and forming an electrical contact on the second surface spaced away from the aperture and connecting through the aperture to the first capacitor plate;

a semiconductor layer having a rim surrounding a diaphragm responsive to the pressure sealingly bonded to the planar surface around the first capacitor plate, said diaphragm forming a second capacitor plate facing and spaced from the first capacitor plate, the semiconductor layer also having a metal ohmic contact thereon;

a sealing layer having a sealing surface thereon bonded to the electrical contact on the second surface of the dielectric layer for sealing the aperture to provide a reference pressure to the diaphragm;

a fourth layer bonded to the semiconductor layer opposite the dielectric layer, said fourth layer having a neck substantially smaller than the width of the sensor with an aperture therethrough permitting the sensed pressure to act on the diaphragm; and

a reference capacitor disposed in the sensor comprising a first reference capacitor plate disposed on the substantially planar surface and being coupled capacitively to a portion of the rim.

17. A capacitive pressure sensor for connection to a capacitance sensing circuit providing an output representative of a sensed pressure comprising:

a dielectric layer having an aperture extending therethrough between a substantially planar surface and an opposite second surface thereon;

electrical conductive means for forming a first capacitor plate on the planar surface and forming an electrical contact on the second surface spaced away from the aperture and connecting through the aperture to the first capacitor plate;

a semiconductor layer having a rim surrounding a diaphragm responsive to the pressure sealingly bonded to the planar surface around the first capacitor plate, said diaphragm forming a second capacitor plate facing and spaced from the first capacitor plate, the semiconductor layer also having a metal ohmic contact thereon;

a sealing layer having a sealing surface thereon bonded to the electrical contact and overlying the aperture for sealing the aperture to provide a reference pressure to the diaphragm;

a fourth layer bonded to the semiconductor layer opposite the dielectric layer, said fourth layer having a neck substantially smaller than the width of the sensor with an aperture therethrough permitting the sensed pressure to act on the diaphragm;

a base member bonded to the neck; and

a reference capacitor bonded to the base member and spaced away from the fourth layer and coupled to the sensing circuit such that vibration sensitivity of the output is reduced.