Micromachined force-balance feedback accelerometer with optical displacement detection
An accelerometer includes a proof mass and a frame that are formed in a handle layer of a silicon-on-an-insulator (SOI). The proof mass is separated from the frame by a back-side trench that defines a boundary of the proof mass. The accelerometer also includes a reflector coupled to a top surface of the proof mass. An optical detector is located above the reflector at the device side. The accelerometer further includes at least one suspension spring. The suspension spring has a handle anchor that extends downwards from the device side to the handle layer to mechanically support upward and downward movement of the proof mass relative to a top surface of the proof mass.
1. A method of fabricating a micro-machined accelerometer, comprising:
forming first device-side trenches that extend from a device side of a silicon-on-an-insulator (SOI) through a buried oxide layer to reach a handle layer of the SOI, the first device-side trenches filled with one or more materials different from oxide;
forming second device-side trenches that extend from the device side to reach the buried oxide layer, the second device-side trenches filled with oxide;
forming accelerometer components at the device side, the accelerometer components comprising an optical detector and at least one suspension spring for a proof mass, the at least one suspension spring having a handle anchor formed from one of the filled first device-side trenches;
forming a back-side trench that extends from a bottom surface of the handle layer to reach the buried oxide layer, the back-side trench define a boundary of the proof mass; and
removing a portion of the buried oxide layer through the second device-side trenches and the back-side trenches to allow upward and downward movement of the proof mass relative to a top surface of the proof mass.
2. The method of claim 1 , wherein the accelerometer components further comprise at least one up actuation electrode and at least one down actuation electrode.
3. The method of claim 1 , further comprising:
forming a plurality of release holes that extend from the device side to reach the buried oxide layer;
filling the release holes with oxide; and
removing the portion of the buried oxide layer through the second device-side trenches, the back-side trenches and the release holes.
4. The method of claim 1 , further comprising:
before forming the back-side trenches, filling the accelerometer components that are formed at the device side with oxide to form a solid oxide-filled block.
5. The method of claim 1 , wherein the material different from oxide comprises doped polysilicon.
6. The method of claim 1 , further comprising:
forming third device-side trenches that extend from the device side down to the handle layer; and
filling the third device-side trenches with silicon nitride to form part of a reflector structure of the accelerometer.
7. An apparatus comprising:
a proof mass and a frame in a handle layer of a silicon-on-an-insulator (SOI), the proof mass separated from the frame by a back-side trench that defines a boundary of the proof mass;
a reflector at a device side of the SOI and coupled to a top surface of the proof mass;
an optical detector located above the reflector at the device side; and
at least one suspension spring having a handle anchor that extends downwards from the device side to the handle layer to mechanically support upward and downward movement of the proof mass relative to a top surface of the proof mass.
8. The apparatus of claim 7 , further comprising:
at least one up actuation electrode coupled to a first electrical connection that supplies a first voltage to control upward movement of the proof mass; and
at least one down actuation electrode coupled to a second electrical connection that supplies a second voltage to control downward movement of the proof mass.
9. The apparatus of claim 8 , wherein at least one up actuation electrode is formed in a device layer of the SOI and has a boundary defined by silicon nitride.
10. The apparatus of claim 8 , wherein at least one down actuation electrode has an electrode element coupled to the second electrical connection, and a ground-voltage layer that is above the electrode element and is maintained at a ground voltage.
11. The apparatus of claim 7 , wherein the optical detector further comprises:
a grating maintained at a ground voltage; and
a support structure that mechanically supports the grating, at least part of the support structure coupled to an electrical connection to receive a voltage that controls the position of the grating.
12. The apparatus of claim 7 , further comprising:
three suspension springs, three up actuation electrodes and three down actuation electrodes, all of which are symmetrically arranged around the optical detector.
13. The apparatus of claim 7 , further comprising:
a plurality of device-side trenches that reach downwards through a buried oxide layer of the SOI to the handle layer, wherein at least one of the device-side trenches defines a boundary of the at least one suspension spring.
14. The apparatus of claim 13 , further comprising:
a plurality of release holes that reach downwards through a buried oxide layer of the SOI to the handle layer, wherein each of the release holes is smaller in size than each of the device-side trenches.
15. A system comprising:
a handle layer, which comprises:
a proof mass and a frame, the proof mass separated from the frame by a back-side trench that defines a boundary of the proof mass; and
a device layer coupled to the handle layer via a buried oxide layer, the device layer comprising:
a reflector;
an optical detector;
a plurality of suspension springs, each of the suspension springs having a handle anchor that extends downwards to the handle layer to mechanically support upward and downward movement of the proof mass relative to a top surface of the proof mass;
a plurality of up actuation electrodes to receive a first voltage that controls the upward movement of the proof mass; and
a plurality of down actuation electrodes to receive a second voltage that controls the downward movement of the proof mass.
16. The system of claim 15 , wherein each of the up actuation electrodes has a boundary defined by silicon nitride.
17. The system of claim 15 , wherein each of the down actuation electrodes has an electrode element coupled to the second electrical connection, and a ground-voltage layer that is above the electrode element and is maintained at a ground voltage.
18. The system of claim 15 , wherein the optical detector further comprises:
a grating maintained at a ground voltage; and
a support structure that mechanically supports the grating, at least part of the support structure coupled to an electrical connection to receive a voltage that controls the position of the grating.
19. The system of claim 15 , wherein a boundary of each of the suspension springs is defined by a device-side trench that reaches downwards through the buried oxide layer to the handle layer.
20. The system of claim 15 , wherein the system comprises a laser configured to shine through the optical detector.
21. A method of operating the system of claim 15 , the method comprising:
vibrating the optical detector at a frequency signal and
measuring a position of the reflector in step with the frequency signal.
22. A method of operating the system of claim 15 , the method comprising:
applying a voltage to the optical detector, the plurality of up actuation electrodes, or the plurality of down actuation electrodes, wherein the applied voltage is proportional to the motion of the proof mass, and
measuring the applied voltage to measure the acceleration or motion of the proof mass.