Capacitive absolute pressure sensor and method

• US 5,585,311 A
• Filed: 04/10/1996
• Issued: 12/17/1996
• Est. Priority Date: 11/22/1994
• Status: Expired due to Fees

• Current Assignee: Case Western Reserve University
• Original Assignee: Case Western Reserve University
• Inventors: Ko, Wen H.
• Primary Examiner(s): Picard, Leo P.
• Assistant Examiner(s): Vu, Phuong T.
• Application Number:
  US08/630,519
• Time in Patent Office: 251 Days
• Field of Search: 361/283.1, 361/283.2, 361/283.4, 73/715, 73/718, 73/729, 29/25.41, 29/25.42, 437/225-228, 437/919, 437/921, 437/901, 437/7
• US Class Current: 438/53
• CPC Class Codes: G01L 9/0073 using a semiconductive diaphragm

Abstract

A capacitive absolute pressure sensor. The sensor includes a substrate having an electrode deposited thereon and a diaphragm assembly disposed on the substrate. As pressure increases, the diaphragm deflects, touches the electrode (in the touch mode), and changes the capacitance of the sensor. The changed capacitance is sensed to thus sense pressure changes. A buried feedthrough is used to sense the change in a capacitance in a chamber under the diaphragm and thus determine the pressure sensed. A vacuum in the chamber is maintained by proper selection of a thickness of a sensing electrode and an insulating layer, exposition thereof to a thermal cycle, and the hermetic bonding of the diaphragm assembly to the substrate.

Claims

1. A method for forming a capacitive pressure sensor comprising steps of:

etching a first and second gap on a first side of a silicon wafer, the first and second gaps being separated by a dividing wall;

performing a diffusion process to form a p+ layer in the gaps;

depositing metal in the form of an electrode and a pad on a surface of a substrate, the substrate having a first portion and a second portion, the electrode spanning the first and second portions and the pad being dispersed in the second portion;

sputtering glass on the surface and the electrode;

deforming the glass around the electrode;

bonding the silicon wafer to the substrate so the first gap is aligned with the first portion of the substrate and the second gap is aligned with the second portion of the substrate, the first gap and the first portion defining a chamber;

etching a second side, opposite the first side, of the wafer to form (1) a diaphragm aligned with the first gap and the first portion, and (2) a cap aligned with the second gap and the second portion; and

,

removing the cap and corresponding portions of the glass on the electrode to expose the electrode,

whereby the chamber is sealed as a result of the deforming of the glass around the electrode and the bonding.

2. The method as set forth in claim 1, further comprising selecting a thickness of the glass such that the glass deforms around the electrode.

3. The method as set forth in claim 2 wherein the selecting comprises selecting the thickness to be 0.3-3 microns.

4. The method as set forth in claim 1 wherein the etching to form the first and second gaps, the diaphragm and the cap defines a frame structure dimensioned such that the sensor withstands large external forces and temperature extremes thereby maintaining stability.

5. The method as set forth in claim 1 further comprising evacuating the first gap while the wafer and the substrate are bonded.

6. The method as set forth in claim 1 wherein the deforming and bonding comprise exposing the substrate and silicon wafer to a selected thermal cycle.

7. The method as set forth in claim 6 wherein the selected thermal cycle comprises:

exposing the glass substrate, deposited metal electrode, and sputtered glass to a first temperature for approximately one-half hour;

exposing the glass substrate, deposited metal electrode, and sputtered glass and silicon wafer to a second temperature for approximately one half hour to bond the silicon wafer to the substrate; and

cooling the substrate, deposited metal electrode, sputtered glass, and bonded silicon wafer slowly for approximately one hour.

8. The method as set forth in claim 7 wherein the first temperature is approximately 500°

C.

9. The method as set forth in claim 7 wherein the second temperature is in the range of 350°

C. to 400°

C. to reduce residual stress the sensor.