Microelectromechanical capacitive accelerometer and method of making same

• US 6,035,714 A
• Filed: 09/08/1997
• Issued: 03/14/2000
• Est. Priority Date: 09/08/1997
• Status: Expired due to Fees

• Current Assignee: Board of Regents of the University of Michigan (University of Michigan)
• Original Assignee: Board of Regents of the University of Michigan (University of Michigan)
• Inventors: Yazdi, Navid, Najafi, Khalil
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): Kwok, Helen C.
• Application Number:
  US08/925,257
• Time in Patent Office: 918 Days
• Field of Search: 73/514.32, 73/514.17, 73/514.18, 73/514.38, 361/283.3, 361/280, 361/281
• US Class Current: 73/514.32
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up

Abstract

A high sensitivity, Z-axis capacitive microaccelerometer having stiff sense/feedback electrodes and a method of its manufacture are provided. The microaccelerometer is manufactured out of a single silicon wafer and has a silicon-wafer-thick proofmass, small and controllable damping, large capacitance variation and can be operated in a force-rebalanced control loop. The multiple stiffened electrodes have embedded therein damping holes to facilitate both force-rebalanced operation of the device and controlling of the damping factor. Using the whole silicon wafer to form the thick large proofmass and using the thin sacrificial layer to form a narrow uniform capacitor air gap over a large area provide large capacitance sensitivity. The structure of the microaccelerometer is symmetric and thus results in low cross-axis sensitivity. In one embodiment, because of its all-silicon structure, the accelerometer exhibits very low temperature sensitivity and good long term stability. The manufacturing process is simple and thus results in low cost and high yield manufacturing. In the one embodiment, the electrodes are formed by thin polysilicon deposition with embedded vertical stiffeners. The vertical stiffeners are formed by refilling vertical trenches in the proofmass and are used to make the electrodes stiff in the sense direction (i.e. Z or input axis). In a second embodiment, the electrodes are metallized and dimensioned so as to be stiff in the sense direction.

Claims

1. A microelectromechanical capacitive accelerometer having an input axis, the accelerometer comprising:

at least one conductive electrode including a planar layer which is relatively thin along the input axis, the at least one conductive electrode being stiff so as to resist bending movement along the input axis;

a proofmass which is thicker than the planar layer by at least one order of magnitude along the input axis; and

a support structure for supporting the proofmass in spaced relationship from the at least one conductive electrode, the at least one conductive electrode and the proofmass having a substantially uniform narrow air gap therebetween wherein the conductive electrode and the proofmass form an acceleration-sensitive capacitor and wherein the proofmass is formed from a single silicon wafer having a predetermined thickness and wherein the thickness of the proofmass is substantially equal to the predetermined thickness.

2. The accelerometer as claimed in claim 1 wherein the at least one electrode is sufficiently stiff to force-balance proof-mass displacement due to acceleration along the input axis without substantial bending of the at least one conductive electrode along the input axis.

3. The accelerometer as claimed in claim 1 wherein the planar layer is dimensioned and is formed of a material so that the at least one conductive electrode is stiff along the input axis.

4. The accelerometer as claimed in claim 1 wherein the planar layer is a metallized planar layer.

5. The accelerometer as claimed in claim 1 wherein the at least one conductive electrode includes a plurality of stiffeners extending from the planar layer along the input axis to stiffen the at least one conductive electrode.

6. The accelerometer as claimed in claim 1 wherein the at least one conductive electrode comprises a plurality of electrically isolated conductive electrodes.

7. The accelerometer as claimed in claim 1 wherein the planar layer of the at least one conductive electrode has a plurality of damping holes formed completely therethrough.

8. The accelerometer as claimed in claim 5 wherein the stiffeners extend towards the proofmass from the planar layer and wherein the proofmass includes a plurality of cavities, the stiffeners being received within the cavities and wherein the stiffeners and the proofmass have the substantially uniform narrow air gap therebetween.

9. The accelerometer as claimed in claim 5 wherein the stiffeners extend away from the proofmass from the planar layer.

10. The accelerometer as claimed in claim 5 wherein the planar layer and the stiffeners are formed of different forms of the same material.

11. The accelerometer as claimed in claim 10 wherein the material is a semiconductor material.

12. The accelerometer as claimed in claim 11 wherein the semiconductor material is a silicon material.

13. The accelerometer as claimed in claim 1 wherein the planar layer and the proofmass are formed of different forms of the same material.

14. A microelectromechanical capacitive accelerometer having an input axis, the accelerometer comprising:

a pair of spaced conductive electrodes, each of the conductive electrodes including a planar layer which is relatively thin along the input axis but is stiff to resist bending movement along the input axis;

a proofmass which is thicker than each of the planar layers by at least one order of magnitude along the input axis; and

a support structure for supporting the proofmass between the conductive electrodes wherein the conductive electrodes and the proofmass form a pair of substantially uniform narrow air gaps on opposite sides of the proofmass and wherein the pair of conductive electrodes and the proofmass form a pair of acceleration-sensitive capacitors and wherein the proofmass is formed from a single silicon wafer having a predetermined thickness and wherein the thickness of the proofmass is substantially equal to the predetermined thickness.

15. The accelerometer as claimed in claim 14 wherein both of the conductive electrodes are sufficiently stiff to force-balance proof-mass displacement due to acceleration along the input axis without substantial bending of the conductive electrodes along the input axis.

16. The accelerometer as claimed in claim 14 wherein each of the planar layers is dimensioned and is formed of a material so that the conductive electrodes are stiff along the input axis.

17. The accelerometer as claimed in claim 14 wherein each of the planar layers is a metallized planar layer.

18. The accelerometer as claimed in claim 14 wherein each conductive electrode includes a plurality of stiffeners extending from the planar layer of the conduction electrode along the input axis to stiffen the respective conductive electrode.

19. The accelerometer as claimed in claim 14 wherein the support structure includes a plurality of conductive beams for suspending the proofmass between the conductive electrodes.

20. The accelerometer as claimed in claim 14 wherein each of the planar layers of the conductive electrodes has a plurality of damping holes formed completely therethrough.

21. The accelerometer as claimed in claim 18 wherein the stiffeners extend away from the proofmass from the respective planar layers.

22. The accelerometer as claimed in claim 18 wherein the stiffeners extend towards the proofmass from the respective planar layers and wherein the proofmass includes a plurality of cavities on opposite sides thereof, the stiffeners being received within the cavities and wherein the stiffeners and the proofmass have the substantially uniform narrow air gaps therebetween.

23. The accelerometer as claimed in claim 18 wherein the planar layers and the stiffeners are formed of different forms of the same material.

24. The accelerometer as claimed in claim 23 wherein the material is a semiconductor material.

25. The accelerometer as claimed in claim 24 wherein the semiconductor material is a silicon material.

26. The accelerometer as claimed in claim 14 wherein the planar layers and the proofmass are formed of different forms of the same material.