Free mass MEMS accelerometer

• US 10,352,960 B1
• Filed: 10/28/2016
• Issued: 07/16/2019
• Est. Priority Date: 10/30/2015
• Status: Active Grant

• Current Assignee: Garmin International, Inc. (Garmin Limited)
• Original Assignee: Garmin International, Inc. (Garmin Limited)
• Inventors: Shcheglov, Kirill V., Cheng, Yingxiang, Maggipinto, Nolan F.
• Primary Examiner(s): Patel, Harshad R
• Assistant Examiner(s): Tran, Tran M.
• Application Number:
  US15/337,572
• Time in Patent Office: 991 Days
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 3/0072 For controlling internal stress or strain in moving or flexible elements, e.g. stress compensating layers
• CPC Class Codes: B81B 5/00 Devices comprising elements which are movable in relation to each other, e.g. comprising slidable or rotatable elements
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/13 by measuring the force required to restore a proofmass subjected to inertial forces to a null position
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 15/133 with piezoelectric counterbalancing means
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type
• CPC Class Codes: G01P 2015/082 for two degrees of freedom of movement of a single mass
• CPC Class Codes: G01P 2015/0854 using a particular shape of the mass, e.g. annular
• CPC Class Codes: G01P 2015/0871 using stopper structures for limiting the travel of the seismic mass

Abstract

A MEMS accelerometer incorporating a metrology element to directly measure minute changes in measurement baseline. In particular, the MEMS accelerometer incorporates a metrology bar (MB). Embodiments also relate to stress isolation into the sensor design to isolate the sensitive areas of the chip (i.e., the metrology baseline and the proof mass mounting points) from outside stress.

Claims

1. An accelerometer comprising:

a die package;

a planar resonator positioned within the package;

a plurality of slots within the resonator;

a metrology bar having a fixed location on the resonator, the metrology bar having at least one portion thereof positioned within at least one of the plurality of slots of the resonator;

a proof mass having a nominal position and configured to move from the nominal position upon excitation; and

at least one excitation electrode positioned proximate to the at least one portion of the metrology bar within the at least one of the plurality of slots of the resonator, the at least one excitation electrode configured to output a force causing movement of the proof mass to its nominal position;

wherein the force output by the at least one excitation electrode to move the proof mass to its nominal position corresponds to an acceleration of the die package.

2. The accelerometer of claim 1, wherein the metrology bar is centrally located in the resonator.

3. The accelerometer of claim 1, further comprising at least one sensing electrode positioned proximate to the at least one portion of the metrology bar within the at least one of the plurality of slots of the resonator;

wherein the at least one sensing electrode is separated from the proof mass by a separation distance, the separation distance changing based on the movement of the proof mass;

wherein the separation distance corresponds to an acceleration of the die package.

4. The accelerometer of claim 3, wherein the at least one excitation electrode and the at least one sensing electrode form a drive/sense electrode pair.

5. The accelerometer of claim 3, wherein a sensor structure is formed by the die package, the proof mass, the at least one excitation electrode, and the at least one sensing electrode.

6. The accelerometer of claim 1, wherein the at least one of the plurality of slots comprises spring flexures.

7. The accelerometer of claim 1, wherein the movement of the proof mass comprises in-plane lateral motion about a central mounting point of the metrology bar.

8. The accelerometer of claim 1, wherein the metrology bar comprises a T-shaped element.

9. The accelerometer of claim 1, further comprising a plurality of tuning electrodes configured to lower a natural frequency of the accelerometer.

10. The accelerometer of claim 1, wherein the accelerometer further comprises:

a mechanical wafer,

an electrical mounting wafer, and

a proof mass wafer mounted between the mechanical wafer and the electrical mounting wafer at a central attachment area of the die package.

11. The accelerometer of claim 10, further comprising a thermal and stress isolating interposer connected to the electrical mounting wafer.

12. The accelerometer of claim 10, wherein the electrical mounting wafer comprises a plurality of alternating regions of high and low stiffness to isolate the die package from external stress.

13. The accelerometer of claim 1, wherein the plurality of slots are arranged in a symmetrical pattern within the proof mass.

14. An accelerometer comprising:

a die package;

a planar resonator positioned within the die package;

a metrology bar having a central mounting point within the planar resonator;

a proof mass having a nominal position and configured to move from the nominal position upon excitation;

a plurality of slot openings in the proof mass, the plurality of slot openings arranged symmetrically in the proof mass;

an excitation electrode positioned in at least one of the plurality of slot openings, the excitation electrode configured to output a force causing movement of the proof mass to its nominal position; and

a sensing electrode positioned in at least one of the plurality of slot openings;

wherein the force output by the at least one excitation electrode to move the proof mass to its nominal position corresponds to an acceleration of the die package; and

wherein at least one portion of the metrology bar is positioned in at least one of the plurality of slot openings in which the excitation electrode and the sensing electrode are positioned.

15. The accelerometer of claim 14, wherein the excitation electrode and the sensing electrode together form a drive/sense electrode pair;

and wherein the metrology bar measures the relative movement of the drive/sense electrode pair.

16. The accelerometer of claim 14, wherein the metrology bar is a T-shaped element.

17. The accelerometer of claim 14, further comprising at least one tuning electrode in at least one of the plurality of slot openings.

18. The accelerometer of claim 14, wherein the plurality of slot openings comprise spring flexures.

19. The accelerometer of claim 14, wherein movement of the proof mass is measured using the sensing electrode and used to measure a compliance and damping of a support and utilized to compensate for measurement errors due to changes in compliance and damping, and wherein the metrology bar is used to measure an average distance change between the metrology bar and the excitation electrode and the sensing electrode.

20. The accelerometer of claim 14, further comprising a mechanical wafer and an electrical mounting wafer bonded to the planar resonator, and wherein the electrical mounting wafer supports the metrology bar at the central mounting point.

21. The accelerometer of claim 20, further comprising a thermal and stress isolating interposer, the mounting wafer supported by the thermal and stress isolating interposer.

22. The accelerometer of claim 20, wherein the mounting wafer comprises at least two concentric symmetric regions of high and low stiffness to isolate stresses from propagating into the die package around the central mounting point.

23. The accelerometer of claim 14, wherein a sensor structure is formed by the die package, the proof mass, the at least one excitation electrode, and the at least one sensing electrode.

24. The accelerometer of claim 14, wherein the sensing electrode is separated from the metrology bar by a separation distance between the sensing electrode and the proof mass, the separation distance changing based on the movement of the proof mass;

and wherein the separation distance corresponds to an acceleration of the die package.