APPARATUS AND METHODS FOR PHOTONIC INTEGRATED RESONANT ACCELEROMETER

• US 20160349283A1
• Filed: 05/27/2016
• Published: 12/01/2016
• Est. Priority Date: 05/29/2015
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Bramhavar, Suraj Deepak, Juodawlkis, Paul William
• Granted Patent: US 9,927,458 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01P 15/093 by photoelectric pick-up
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: H03H 2009/02346 Anchors for ring resonators
• CPC Class Codes: H03H 3/0073 Integration with other electronic structures
• CPC Class Codes: H03H 9/02338 Suspension means
• CPC Class Codes: H03H 9/2431 Ring resonators

Abstract

The accelerometers disclosed herein provide excellent sensitivity, long-term stability, and low SWaP-C through a combination of photonic integrated circuit technology with standard micro-electromechanical systems (MEMS) technology. Examples of these accelerometers use optical transduction to improve the scale factor of traditional MEMS resonant accelerometers by accurately measuring the resonant frequencies of very small (e.g., about 1 μm) tethers attached to a large (e.g., about 1 mm) proof mass. Some examples use ring resonators to measure the tether frequencies and some other examples use linear resonators to measure the tether frequencies. Potential commercial applications span a wide range from seismic measurement systems to automotive stability controls to inertial guidance to any other application where chip-scale accelerometers are currently deployed.

Claims

1. An accelerometer comprising:

a proof mass;

a first tether, mechanically coupled to a first side of the proof mass, to vibrate in response to acceleration of the proof mass;

a first ring resonator evanescently coupled to the first tether, wherein vibration of the first tether causes a change of a first resonance condition of the first ring resonator;

a detection system, operably coupled to the first ring resonator, to sense the change of the first resonance condition of the first ring resonator.

2. The accelerometer of claim 1, wherein the first tether comprises at least one of silicon or silicon nitride.

3. The accelerometer of claim 1, wherein the first tether has a width substantially equal to or less than 1 μ

m.

4. The accelerometer of claim 1, wherein the first tether has an internal tensile stress substantially equal to or greater than 500 MPa.

5. The accelerometer of claim 1, wherein the first vibration of the first tether has a vibration frequency of about 50 KHz to about 1 GHz.

6. The accelerometer of claim 1, wherein a distance between the first ring resonator and the first tether is about 100 nm to about 300 nm.

7. The accelerometer of claim 1, wherein the detection system comprises:

a waveguide, evanescently coupled to the first ring resonator, to guide light past the first ring resonator so as to couple a portion of the light into the first ring resonator; and

a detector, optically coupled to the waveguide, to detect a change in the portion of the light coupled into the first ring resonator caused by the change of the first resonance condition of the first ring resonator.

8. The accelerometer of claim 7, further comprising:

a substrate to support the first ring resonator and the detector; and

a semiconductor laser, fabricated in the substrate and optically coupled to the first ring resonator, to provide the light beam.

9. The accelerometer of claim 7, wherein the light beam has a power greater than 0.2 mW so as to cause an opto-mechanical oscillation of the first tether.

10. The accelerometer of claim 1, further comprising:

a second tether mechanically coupled to a second side, opposite the first side, of the proof mass, to vibrate in response to the acceleration of the proof mass; and

a second ring resonator, evanescently coupled to the second tether, wherein vibration of the second tether causes a change of a second resonance condition of the second ring resonator,

wherein the detection system senses the change of the second resonance condition of the second ring resonator.

11. The accelerometer of claim 1, further comprising:

an opto-mechanical oscillator, operably coupled to the detection system, to monitor a temperature fluctuation of the accelerometer.

12. The accelerometer of claim 1, further comprising:

a heater, thermally coupled to the first tether, to keep the first tether at a constant temperature so as to mitigate thermal drift.

13. A method of sensing acceleration with an accelerometer comprising a proof mass, a first tether mechanically coupled to a first side of the proof mass, and a first ring resonator evanescently coupled to the first tether, the method comprising:

detecting a change of a first resonance condition of the first ring resonator caused by vibration of the first tether; and

estimating the acceleration based at least in part on the change of the first resonance condition of the first tether in response to acceleration of the proof mass.

14. The method of claim 13, wherein detecting the change of the first resonance condition of the first ring resonator comprises:

detecting a change in an amount of light evanescently coupled into the first ring resonator.

15. The method of claim 14, wherein transmitting the light beam comprises transmitting the light beam having a power greater than 0.2 mW so as to cause opto-mechanical oscillation of the first tether in the accelerometer.

16. The method of claim 13, wherein the accelerometer further comprises a second tether mechanically coupled to a second side, opposite the first side, of the proof mass, a second ring resonator evanescently coupled to the second tether, the method further comprising:

detecting a change of a second resonance condition of the second ring resonator caused by vibration of the second tether in response to the acceleration of the proof mass; and

estimating the acceleration based at least in part on the change of the first resonance condition of the first ring resonator and the change of the second resonance condition of the second ring resonator.

17. The method of claim 13, further comprising:

monitoring a temperature fluctuation of the accelerometer using an opto-mechanical oscillator so as to mitigate thermal noise in sensing the acceleration.

18. The method of claim 13, further comprising:

maintaining the first tether at a constant temperature so as to mitigate thermal noise in sensing the acceleration.

19. A method of fabricating an accelerometer, the method comprising:

fabricating a first ring resonator in a first dielectric layer disposed on a substrate;

depositing a second dielectric layer on the first ring resonator;

fabricating a first tether on the second dielectric layer;

defining a proof mass mechanically coupled to the first tether by etching a back surface of the substrate; and

etching the second dielectric layer below the first tether so as to release the first tether from the first ring resonator.

20. The method of claim 19, wherein fabricating the first ring resonator comprises:

depositing an epitaxial layer on the substrate; and

etching the epitaxial layer so as to define the first ring resonator.

21. The method of claim 20, further comprising:

etching the epitaxial layer so as to define a waveguide optically coupled to the first ring resonator; and

forming a detector, optically coupled to the waveguide, on the substrate.

22. The method of claim 19, wherein fabricating the first tether comprises:

depositing a layer of stoichiometric silicon nitride on the second dielectric layer via low pressure chemical vapor deposition; and

etching the layer of stoichiometric silicon nitride so as to define the first tether.

23. The method of claim 19, wherein releasing the first tether comprises:

etching the second dielectric layer using buffered HF etching; and

point drying the first tether so as to reduce stiction between the first tether and the first ring resonator.

24. The method of claim 19, further comprising:

disposing another substrate on the first tether so as to enclose the first tether in vacuum.

25. The method of claim 19, further comprising:

growing an epitaxial layer on the back surface of the substrate to encapsulate the proof mass in vacuum.

26. The method of claim 19, further comprising:

fabricating a light source on the substrate to provide a light beam propagating in the first ring resonator.

27. An accelerometer comprising:

a semiconductor substrate;

a proof mass, suspended from the semiconductor substrate by a first tether and a second tether, to move in a first direction in response to a force applied to the accelerometer;

a first optical waveguide, optically coupled to the first tether, to guide a first optical beam in a second direction orthogonal to the first direction such that motion of the proof mass causes a change in optical coupling between the first optical waveguide and the first tether;

a second optical waveguide, optically coupled to the second tether, to guide a second optical beam in the second direction such that the motion of the proof mass causes a change in optical coupling between the second optical waveguide and the second tether;

a first photodetector, in optical communication with the first optical waveguide, to sense a change in frequency and/or amplitude of the first optical beam caused by the change in optical coupling between the first optical waveguide and the first tether; and

a second photodetector, in optical communication with the second optical waveguide, to sense a change in frequency and/or amplitude of the second optical beam caused by the change in optical coupling between the second optical waveguide and the second tether.

28. The accelerometer of claim 27, wherein the first optical waveguide is evanescently coupled to the first tether.

29. The accelerometer of claim 27, further comprising:

a first reflector disposed on the first tether in optical communication with the first optical waveguide; and

a second reflector disposed in the first optical waveguide so as to form an optical cavity having a length that changes in response to the force applied to the accelerometer.

30. The accelerometer of claim 27, further comprising:

differential detection circuitry, in electrical communication with the first photodetector and the second photodetector, to generate a difference signal based on the change in frequency and/or amplitude of the first optical beam and the change in frequency and/or amplitude of the second optical beam, the difference signal representing the force applied to the accelerometer.