Apparatus and method for minimizing drift of a piezo-resistive pressure sensors due to progressive release of mechanical stress over time

• US 20110209553A1
• Filed: 02/27/2010
• Published: 09/01/2011
• Est. Priority Date: 02/27/2010
• Status: Active Grant

• Current Assignee: DePuy Synthes Products LLC (Johnson & Johnson)
• Original Assignee: Codman Neuro Sciences Sarl
• Inventors: Crivelli, Rocco
• Granted Patent: US 8,393,222 B2
• Time in Patent Office: Days
• US Class Current: 73/721
• CPC Class Codes: G01L 19/0092 Pressure sensor associated with other sensors, e.g. for measuring acceleration or temperature G01L9/025, G01L9/045, G01L9/065, G01L9/085, G01L9/105, G01L9/125, G01L19/02, G01L19/04 take precedence; measuring two or more variable G01D21/02; temperature sensors with pressure compensation G01K1/26
• CPC Class Codes: G01L 27/002 Calibrating, i.e. establishing true relation between transducer output value and value to be measured, zeroing, linearising or span error determination
• CPC Class Codes: G01L 9/0052 of piezoresistive elements circuits therefor G01L9/06
• CPC Class Codes: G01L 9/065 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

An absolute piezo-resistive pressure sensor system and method employing multiple pressure sensing elements operating simultaneously to detect pressure. Both pressure sensing elements being subject to a common reference pressure within a sealed cavity. The first pressure sensing element detecting an offset voltage resulting from the progressive release of mechanical stress at an assembly interface between the sensing element and a base plate on which the sensing elements are assembled. Electronic circuitry compensates the pressure measured by the second pressure sensing element based on the offset voltage detected by the first pressure sensing element.

Claims

1. An absolute piezo-resistive pressure sensor system comprising:

a single wafer having respective first and second cavities etched therein forming respective first and second deflectable membranes serving as respective first and second pressure sensing elements;

the first deflectable membrane having an aperture defined therethrough, while the second deflectable membrane does not;

a first surface of each of the first and second deflectable membranes being exposed to a pressure within the respective first and second cavities;

an opposing second surface of each of the first and second deflectable membranes being enclosed in a common hermetically sealed chamber formed by a cover assembled to the single wafer, wherein a pressure within the chamber is substantially constant;

a piezo-resistive material is assembled to the opposing second surface of each of the first and second deflectable membranes subject to the same pressure within the sealed chamber; and

a base plate having a single hole define therein;

the wafer being assembled to the base plate with the hole substantially aligned with the second pressure sensing element.

2. The system in accordance with claim 1, wherein the wafer is made of silicon and the base plate is made of glass.

3. The system in accordance with claim 1, the aperture defined therethrough the first deflectable membrane is between approximately 0.05 mm-approximately 0.1 mm.

4. The system in accordance with claim 1, further comprising a cover assembled to the base plate to form a packaging that encapsulates the wafer forming the two pressure sensing elements in a hermetically sealed chamber at a substantially constant pressure.

5. The system in accordance with claim 1, further comprising electronic circuitry for:

(i) simultaneously while detecting using the second pressure sensing element a pressure in the second cavity, calibrating a measured offset voltage detected by the first pressure sensing element due to mechanical stress release at an assembly interface between the wafer and the base plate; and

(ii) compensating the pressure in the second cavity detected by the second pressure sensing element based on the offset voltage as calibrated by the first pressure sensing element.

6. The system in accordance with claim 1, wherein a material used to fabricate the single wafer has a different coefficient of thermal expansion than a material used to fabricate the base plate.

7. An absolute piezo-resistive pressure sensor system comprising:

a first wafer, having a single cavity etched therein forming a first deflectable membrane serving as a first pressure sensing element;

the first deflectable membrane having an aperture defined therethrough;

the first deflectable membrane having a first surface exposed to a pressure within the single cavity etched in the first wafer and an opposing second surface;

a second wafer having a single cavity etched therein forming a second deflectable membrane serving as a second pressure sensing element;

the second deflectable membrane not having an aperture defined therein;

the second deflectable membrane having a first surface exposed to a pressure within the single cavity etched in the second wafer and an opposing second surface;

the opposing second surfaces of the first and second deflectable membranes being enclosed in a common hermetically sealed chamber formed by a cover enclosing the two silicon wafers, wherein a pressure within the chamber is substantially constant;

a common piezo-resistive material is assembled to each of the opposing second surfaces of the first and second deflectable membranes subject to the same pressure within the sealed chamber; and

a base plate having a single hole define therein;

the first and second wafers being assembled to the base plate with the hole substantially aligned with the second pressure sensing element;

wherein the first and second wafers are fabricated using identical manufacturing steps, and the first and second pressure sensing elements are assembled using identical mounting steps.

8. The system in accordance with claim 7, wherein the first and second wafers are made of silicon, and the base plate is made of glass.

9. The system in accordance with claim 7, the aperture defined therethrough the first deflectable membrane is between approximately 0.05 mm-approximately 0.1 mm.

10. The system in accordance with claim 7, further comprising a cover assembled to the base plate to form a packaging that encapsulates the two wafers forming the two pressure sensing elements in a hermetically sealed chamber at a substantially constant pressure.

11. The system in accordance with claim 7, further comprising electronic circuitry for:

(i) simultaneously while detecting using the second pressure sensing element a pressure in the second cavity, calibrating a measured offset voltage detected by the first pressure sensing element due to mechanical stress release at an assembly interface between the two wafers and the base plate; and

(ii) compensating the pressure in the second cavity detected by the second pressure sensing element based on the offset voltage as calibrated by the first pressure sensing element.

12. The system in accordance with claim 7, wherein a material used to fabricate the two wafers has a different coefficient of thermal expansion than a material used to fabricate the base plate.

13. A method for operating an absolute piezo-resistive pressure sensor system including a single wafer having respective first and second cavities etched therein forming respective first and second deflectable membranes serving as respective first and second pressure sensing element;

the first deflectable membrane having an aperture defined therethrough, while the second deflectable membrane does not;

a first surface of each of the first and second deflectable membranes being exposed to a pressure within the respective first and second cavities;

an opposing second surface of each of the first and second deflectable membranes being enclosed in a common hermetically sealed chamber formed by a cover assembled to the single wafer, wherein a pressure within the chamber is substantially constant;

a piezo-resistive material is assembled to the opposing second surface of each of the first and second deflectable membranes subject to the same pressure within the sealed chamber;

a base plate having a single hole define therein;

the silicon wafer being assembled to the base plate with the hole substantially aligned with the second pressure sensing element, the method comprising the steps of;

simultaneously while detecting using the second pressure sensing element a pressure in the second cavity, calibrating a measured offset voltage detected by the first pressure sensing element due to mechanical stress release at an assembly interface between the wafer and the base plate; and

compensating using electronic circuitry the pressure in the second cavity detected by the second pressure sensing element based on the offset voltage as calibrated by the first pressure sensing element.

14. The method in accordance with claim 13, wherein the single wafer is made of silicon and the base plate is made of glass.

15. The method in accordance with claim 13, the aperture defined therethrough the first deflectable membrane is between approximately 0.05 mm-approximately 0.1 mm.

16. The method in accordance with claim 13, further comprising a cover assembled to the base plate to form a packaging that encapsulates the single wafer forming the two pressure sensing elements in a hermetically sealed chamber at a substantially constant pressure.

17. The method in accordance with claim 13, wherein a material used to fabricate the single wafer has a different coefficient of thermal expansion than a material used to fabricate the base plate.

18. A method for operating an absolute piezo-resistive pressure sensor system including a first wafer, having a single cavity etched therein forming a first deflectable membrane serving as a first pressure sensing element;

the first deflectable membrane having an aperture defined therethrough;

the first deflectable membrane having a first surface exposed to a pressure within the single cavity etched in the first wafer and an opposing second surface;

a second wafer having a single cavity etched therein forming a second deflectable membrane serving as a second pressure sensing element;

the second deflectable membrane not having an aperture defined therein;

the second deflectable membrane having a first surface exposed to a pressure within the single cavity etched in the second wafer and an opposing second surface;

the opposing second surfaces of the first and second deflectable membranes being enclosed in a common hermetically sealed chamber formed by a cover enclosing the two wafers, wherein a pressure within the chamber is substantially constant;

a common piezo-resistive material is assembled to each of the opposing second surfaces of the first and second deflectable membranes subject to the same pressure within the sealed chamber; and

a base plate having a single hole define therein;

the first and second wafers being assembled to the base plate with the hole substantially aligned with the second pressure sensing element;

the first and second wafers being fabricated using identical manufacturing steps, and the first and second pressure sensing elements being assembled using identical mounting steps;

the method comprising the steps of;

simultaneously while detecting using the second pressure sensing element a pressure in the second cavity, calibrating a measured offset voltage detected by the first pressure sensing element due to mechanical stress release at an assembly interface between the two wafers and the base plate; and

compensating using electronic circuitry the pressure in the second cavity detected by the second pressure sensing element based on the offset voltage as calibrated by the first pressure sensing element.

19. The method in accordance with claim 18, wherein the first and second wafers are made of silicon, and the base plate is made of glass.

20. The method in accordance with claim 18, the aperture defined therethrough the first deflectable membrane is between approximately 0.05 mm-approximately 0.1 mm.

21. The method in accordance with claim 18, further comprising a cover assembled to the base plate to form a packaging that encapsulates the two wafers forming the two pressure sensing elements in a hermetically sealed chamber at a substantially constant pressure.

22. The method in accordance with claim 18, wherein a material used to fabricate the two wafers has a different coefficient of thermal expansion than a material used to fabricate the base plate.

23. The method in accordance with claim 18, wherein a material used to fabricate the two wafers has a different coefficient of thermal expansion than a material used to fabricate the base plate.