MEMS sensor with dynamically variable reference capacitance

• US 9,410,981 B2
• Filed: 06/05/2013
• Issued: 08/09/2016
• Est. Priority Date: 06/05/2012
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Zhang, Xin, Judy, Michael W.
• Primary Examiner(s): Caputo, Lisa
• Assistant Examiner(s): Woodward, Nathaniel T
• Application Number:
  US13/910,755
• Publication Number:
  US 20150355222A1
• Time in Patent Office: 1,161 Days
• Field of Search: 735/143.2, 735/141.6, 735/140.1, 734/88
• US Class Current: 1/1
• CPC Class Codes: B81B 2201/0221 Variable capacitors
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 2203/0118 Cantilevers
• CPC Class Codes: B81B 2203/0307 Anchors
• CPC Class Codes: B81B 2203/04 Electrodes
• CPC Class Codes: B81B 3/0086 Electrical characteristics, e.g. reducing driving voltage, improving resistance to peak voltage
• CPC Class Codes: B81B 3/0094 Constitution or structural means for improving or controlling physical properties not provided for in B81B3/0067 - B81B3/0091
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/0837 the mass being suspended so as to only allow movement perpendicular to the plane of the substrate, i.e. z-axis sensor

Abstract

An MEMS device has a dynamically variable reference capacitor that provides a reference to a sense capacitance. In some embodiments, a 3-axis accelerometer includes a proof mass suspended above a substrate from an anchor, and a cantilevered Z-axis reference capacitor arm suspended above the substrate from the same anchor. In some embodiments, the proof mass is suspended from a plurality of anchors, and each anchor also supports one or more cantilevered arms, the cantilevered arms forming a dynamically variable reference capacitance.

Claims

1. A MEMS device comprising:

a substrate having a surface, the surface defining a Z-axis normal to the surface;

an anchor extending from the surface in the direction of the Z-axis;

a beam suspended from the anchor such that the beam extends over the surface, the beam forming a sense capacitance with the substrate; and

a dynamically variable reference capacitance comprising a first cantilevered arm suspended from the anchor, the anchor being the same anchor from which the beam is suspended, the first cantilevered arm forming the reference capacitance with the substrate, the reference capacitance providing a reference for the sense capacitance.

2. The MEMS device of claim 1, wherein the dynamically variable reference capacitance is matched to the sense capacitance such that a ratio of the sense capacitance to the reference capacitance remains substantially the same in response to deformation of the substrate.

3. The MEMS device of claim 1, the cantilevered arm further comprising an electrode, the electrode forming the reference capacitance with the substrate.

4. The MEMS device of claim 1, the substrate further comprising a reference electrode, the reference electrode forming the reference capacitance with the cantilevered arm.

5. The MEMS device of claim 1, wherein the beam has an edge length, and the cantilevered arm has a length in a direction extending from the anchor, wherein the length of the cantilevered arm is substantially smaller than the edge length of the beam.

6. The MEMS device of claim 1, wherein the sense capacitance and the dynamically variable reference capacitance are electrically coupled to form a node at the anchor.

7. The MEMS device of claim 1, wherein the anchor defines a cross-section in a plane parallel to the substrate, the cross-section having a first branch and a second branch, the first branch and the second branch meeting at a right angle to form an intersection.

8. The MEMS device of claim 1, the anchor further comprising a chamfered outside corner at the intersection of the first branch and the second branch.

9. The MEMS device of claim 1, wherein the beam has a periphery and the anchor is within the periphery of the beam.

10. The MEMS device of claim 1 wherein the first cantilevered arm comprises a T-shaped electrode.

11. The MEMS device of claim 1 wherein the first cantilevered arm comprises an offset-T electrode.

12. The MEMS device of claim 1, further comprising a second cantilevered arm suspended from the anchor parallel to the substrate and extending from the anchor in a direction orthogonal to the first cantilevered arm.

13. The MEMS device of claim 1, further comprising a sensing circuit electrically coupled to the sense capacitance and the dynamically variable reference capacitance, the sensing circuit configured to assess a difference between a charge on the sense capacitance and a charge on the reference capacitance.

14. A capacitive accelerometer comprising:

a conductive substrate having a first surface, the first surface defining a plane, the plane defining a Z-axis normal to the plane;

a first quadrant structure comprising;

(a) a chamfered L-shaped anchor having a first branch and a second branch;

(b) a first cantilevered reference electrode rigidly suspended from the first branch;

(c) a second cantilevered reference electrode rigidly suspended from the second branch, such that the first cantilevered reference electrode extends from the anchor in a direction orthogonal to the direction of the second cantilevered reference electrode;

a second quadrant structure identical to the first quadrant structure, the second quadrant structure adjacent to the first quadrant structure and rotated in plane by 90 degrees relative to the first quadrant structure;

a third quadrant structure identical to the first quadrant structure, the second quadrant structure adjacent to the second quadrant structure and rotated in plane by 90 degrees relative to the second quadrant structure and by 180 degrees relative to the first quadrant structure;

a fourth quadrant structure identical to the first quadrant structure, the fourth quadrant structure adjacent to the third quadrant structure and rotated in plane by 270 degrees relative to the first quadrant structure, such that the accelerometer has a plurality of anchors, one anchor in each quadrant, and a corresponding plurality of cantilevered reference electrodes; and

a beam suspended from the plurality of chamfered L-shaped anchors, the beam having an outer periphery that surrounds the plurality of chamfered L-shaped anchors and the plurality of cantilevered reference electrodes.

15. The capacitive accelerometer of claim 14, wherein each of the cantilevered reference electrodes comprises a T-shaped electrode.

16. The capacitive accelerometer of claim 14, wherein each of the cantilevered reference electrodes comprises an offset-T-shaped electrode.

17. A method of sensing Z-axis acceleration, the method comprising:

providing a substrate having a surface, the surface defining a Z-axis normal to the surface, and an anchor extending from the surface in the direction of the Z-axis;

providing a sense capacitance comprising a beam suspended from the anchor such that the beam is extends over the surface, the beam forming a sense capacitance with the substrate, the substrate forming one electrode of the sense capacitance; and

providing a dynamically variable reference capacitance, the dynamically variable reference capacitance comprising a cantilevered reference electrode suspended from the anchor, the anchor being the same anchor from which the beam is suspended, the reference capacitance providing a reference for the sense capacitance.

18. The method of claim 17, further comprising dynamically varying the reference capacitance such that a ratio of the sense capacitance to the reference capacitance remains substantially the same in response to deformation of the substrate.

19. The method of claim 17, wherein the sense capacitance has a nominal sense value, and the dynamically variable reference capacitance has a nominal reference value, and the method further comprises packaging the substrate, such that the reference capacitance adapts automatically and contemporaneously to a distortion of the sense capacitance.

20. The method of claim 17, further comprising providing a sensing circuit electrically coupled to the sense capacitance and to the reference capacitance, the sensing circuit configured to assess the difference between a charge on the sense capacitance and a charge on the reference capacitance.