Optomechanical accelerometer

Patent metadata

Field	Value
Publication number	US-9618531-B2
Application number	US-201314379744-A
Country	US
Kind code	B2
Filing date	Mar 1, 2013
Priority date	Mar 2, 2012
Publication date	Apr 11, 2017
Grant date	Apr 11, 2017

Abstract

Technologies are generally described for operating and manufacturing optomechanical accelerometers. In some examples, an optomechanical accelerometer device is described that uses a cavity resonant displacement sensor based on a zipper photonic crystal nano-cavity to measure the displacement of an integrated test mass generated by acceleration applied to the chip. The cavity-resonant sensor may be fully integrated on-chip and exhibit an enhanced displacement resolution due to its strong optomechanical coupling. The accelerometer structure may be fabricated in a silicon nitride thin film and constitute a rectangular test mass flexibly suspended on high aspect ratio inorganic nitride nano-tethers under high tensile stress. By increasing the mechanical Q-factors through adjustment of tether width and tether length, the noise-equivalent acceleration (NEA) may be reduced, while maintaining a large operation bandwidth. The mechanical Q-factor may be improved with thinner (e.g., <1 micron) and longer tethers (e.g., 10-560 microns).

First claim

What is claimed is:

1. An optomechanical accelerometer device, comprising:
a frame;
a test mass;
a plurality of nano-tethers coupling the test mass to the frame; and
a zipper cavity structure formed by a portion of the test mass and an adjacent portion of the frame.

2. The optomechanical accelerometer device of claim 1 , wherein the zipper cavity structure is formed by a photonic crystal in the test mass and a second photonic crystal in the frame, separated by a slot.

3. The optomechanical accelerometer device of claim 1 , wherein the plurality of nano-tethers are configured to provide elongated mechanical support and have a thickness between approximately 100 nm and approximately 500 nm.

4. The optomechanical accelerometer device of claim 1 , further comprising a fiber taper waveguide anchored to the frame in a vicinity of the zipper cavity structure, wherein the waveguide is configured to couple a light beam to the zipper cavity structure to monitor photonic crystal nanobeams to detect a displacement of the test mass caused by an in-plane acceleration of the frame.

5. The optomechanical accelerometer device of claim 1 , wherein at least one of a size of the test mass and a number of the nano-tethers is selected such that a noise-equivalent acceleration (NEA) of the optomechanical accelerometer device is reduced and a mechanical quality factor of the optomechanical accelerometer device is increased, wherein the at least one of the size of the test mass and the number of the nano-tethers is selected such that an operational bandwidth of the optomechanical accelerometer device is substantially maintained.

6. The optomechanical accelerometer device of claim 1 , wherein the optomechanical accelerometer device is integrated into a microelectromechanical system (MEMS) device.

7. The optomechanical accelerometer device of claim 1 , wherein the test mass includes one or more cross-shaped cuts to facilitate undercutting the optomechanical accelerometer device.

8. The optomechanical accelerometer device of claim 1 , wherein a response characteristic of the optomechanical accelerometer device is damped through back-action cooling.

9. The optomechanical accelerometer device of claim 1 , wherein a sensor bandwidth is controlled by an optical spring effect, and an effective temperature of the sensor is controlled by one of passive damping and feedback cold-damping.

10. An optical microelectromechanical system (OMEMS) device to detect acceleration, the OMEMS device comprising:
an optomechanical accelerometer device comprising:
a frame,
a test mass,
a plurality of nano-tethers coupling the test mass to the frame, and
a zipper cavity structure formed by a portion of the test mass and an adjacent portion of the frame;

an optical beam source configured to direct a light beam onto the zipper cavity structure; and
an optical detector configured to detect a displacement of the test mass caused by an in-plane acceleration of the frame.

11. The OMEMS device of claim 10 , wherein the optical detector is configured to detect the displacement of the test mass based on a variation of one of light transmission through and light reflection from the zipper cavity structure.

12. The OMEMS device of claim 10 , wherein the zipper cavity structure includes two patterned photonic crystal nanobeams and a slot, one of the nanobeams formed as the portion of the test mass and another of the nanobeams formed as the portion of the frame, the nanobeams being separated by the slot.

13. The OMEMS device of claim 10 , wherein the optomechanical accelerometer device is fabricated in a silicon nitride thin film.

14. The OMEMS device of claim 10 , wherein the test mass includes one or more cuts to facilitate undercutting the optomechanical accelerometer device.

15. The OMEMS device of claim 10 , further comprising one or more electrostatic actuators for in-situ tuning of an optical resonance of the zipper cavity structure and damping of a mechanical mode of the zipper cavity structure to enable closed-loop operation.

16. The OMEMS device of claim 10 , further comprising a beam splitter configured to split the light beam, transmit a first portion of the split light beam through the zipper cavity structure, and transmit a second portion of the split light beam to the optical detector as a reference light beam.

17. The OMEMS device of claim 16 , wherein the optical beam source is a laser source providing a laser beam and the optical detector is a balanced photo detector (BPD).

18. The OMEMS device of claim 17 , further comprising a proportional-integral (PI) controller coupled to the optical detector, the PI controller configured to lock the laser beam half a line width red-detuned from an optical resonance of the zipper cavity structure.

19. A method to fabricate an optomechanical accelerometer device, the method comprising:
forming a mask for accelerometer structures comprising a test mass, support nano-tethers, and a zipper cavity structure; and
transferring the mask into a silicon nitride layer formed on a single-crystal silicon wafer.

20. The method of claim 19 , further comprising forming the accelerometer structures in a single electron-beam lithography step.

21. The method of claim 19 , further comprising growing the silicon nitride layer using low-pressure chemical vapor deposition (LPCVD) under conditions that enable large internal tensile stress in a range from approximately 0.5 to approximately 10 GPa.

22. The method of claim 19 , wherein transferring the mask into the silicon nitride layer includes dry etching by inductively coupled plasma/reactive-ion etching (ICP/RIE).

23. The method of claim 19 , further comprising undercutting the accelerometer structures employing anisotropic wet-etching.

24. The method of claim 19 , further comprising preventing a collapse of the zipper cavity structure employing critical point drying in CO 2 .

Assignees

• California Inst Of Techn
• Univ Rochester

Inventors

• Painter Oskar
• Winger Martin
• Lin Qiang
• Krause Alexander
• Blasius Tim D

Classifications

• G01N2201/063

  Illuminating optical parts · CPC title

• G01N2201/06113

  Coherent sources; lasers · CPC title

• G01N21/59

  Transmissivity (G01N21/25 takes precedence) · CPC title

• G01N21/55

  Specular reflectivity · CPC title

• B82Y99/00

  Subject matter not provided for in other groups of this subclass · CPC title