System and method for a comb-drive MEMS device

1. A method of forming a microelectromechanical systems (MEMS) transducer, the method comprising:

forming a transducer frame in a layer of monocrystalline silicon, wherein forming the transducer frame comprises:

forming a support portion adjacent to a cavity,

forming a membrane layer on the support portion and overlying the cavity, and

forming a first set of comb-fingers extending from the support portion;

forming a spring support from an anchor to the support portion, wherein forming the spring support comprises patterning a torsional spring support between the anchor and the support portion adjacent to a first side of the membrane layer opposite the first set of comb-fingers; and

forming a second set of comb-fingers in the layer of monocrystalline silicon, the second set of comb-fingers being interdigitated with the first set of comb-fingers.

2. The method of forming a MEMS transducer of claim 1 , wherein the first set of comb-fingers and the second set of comb-fingers are formed with an inter-finger spacing distance between each comb-finger of the first set of comb-fingers and each corresponding adjacent comb-finger of the second set of comb-fingers between 100 nm and 300 nm.

3. The method of forming a MEMS transducer of claim 1 , wherein the first set of comb-fingers and the second set of comb-fingers are formed with a comb-finger aspect ratio equal to or exceeding 1:50, the comb-finger aspect ratio being a ratio of an inter-finger spacing distance between each comb-finger of the first set of comb-fingers and each corresponding adjacent comb-finger of the second set of comb-fingers to a layer thickness of the first set of comb-fingers and the second set of comb-fingers.

4. The method of forming a MEMS transducer of claim 1 , further comprising forming a stress layer mechanically coupled to the spring support and configured to deflect the transducer frame at rest.

5. The method of forming a MEMS transducer of claim 1 , further comprising forming the MEMS transducer with a chip thickness that is less than or equal to 300 μm.

6. The method of forming a MEMS transducer of claim 1 , further comprising forming a stress decoupling gap extending from the anchor and surrounding the transducer frame, the spring support, and the second set of comb-fingers, wherein the stress decoupling gap is configured to reduce a transfer of layer stress from a substrate of the MEMS transducer to the transducer frame, the spring support, and the second set of comb-fingers.

7. A method of forming a microelectromechanical systems (MEMS) transducer, the method comprising:

forming a silicon-on-nothing structure in a monocrystalline silicon substrate;

etching high aspect ratio trenches through the silicon-on-nothing structure;

forming a membrane on the silicon-on-nothing structure;

forming a cavity in the silicon-on-nothing structure adjacent the high aspect ratio trenches and beneath the membrane; and

exposing the cavity by removing a backside portion of the monocrystalline silicon substrate.

8. The method of forming a MEMS transducer of claim 7 , wherein etching high aspect ratio trenches comprises etching trenches with an aspect ratio equal to or exceeding 1:50.

9. The method of forming a MEMS transducer of claim 7 , wherein exposing the cavity by removing the backside portion of the monocrystalline silicon substrate comprises performing a backside etch through the monocrystalline silicon substrate.

10. The method of forming a MEMS transducer of claim 7 , wherein exposing the cavity by removing the backside portion of the monocrystalline silicon substrate comprises:

etching a trench surrounding the silicon-on-nothing structure; and

performing a lift-off step to remove the silicon-on-nothing structure from the monocrystalline silicon substrate.

11. The method of forming a MEMS transducer of claim 10 , wherein, after performing the lift-off step, the silicon-on-nothing structure has a layer thickness less than or equal to 300 μm.

12. The method of forming a MEMS transducer of claim 7 , wherein

forming the membrane on the silicon-on-nothing structure comprises forming a framework and forming the membrane on the framework; and

forming the cavity in the silicon-on-nothing structure comprises forming the cavity in the framework such that the framework surrounds the cavity.

13. The method of forming a MEMS transducer of claim 12 , further comprising forming a torsional spring in the silicon-on-nothing structure, wherein the torsional spring is coupled to and supports the framework.

14. The method of forming a MEMS transducer of claim 13 , further comprising forming a stress layer mechanically coupled to the torsional spring.

15. The method of forming a MEMS transducer of claim 13 , further comprising forming a stress decoupling gap extending from an anchor coupled to the torsional spring and surrounding the silicon-on-nothing structure, wherein the stress decoupling gap is configured to reduce a transfer of layer stress from the monocrystalline silicon substrate to the silicon-on-nothing structure.

16. A method of forming a microelectromechanical systems (MEMS) device, the method comprising:

etching a plurality of trenches in a semiconductor substrate;

reflowing the plurality of trenches to:

form a substrate-on-nothing structure from top portions of the plurality of trenches, and

form an empty space in the semiconductor substrate from bottom portions of the plurality of trenches;

forming an etch stop liner in the empty space;

forming a plurality of comb-fingers in the substrate-on-nothing structure;

forming a membrane on the substrate-on-nothing structure;

forming a cavity in the substrate-on-nothing structure beneath the membrane, the cavity extending from a top surface of the semiconductor substrate to the etch stop liner;

removing a backside portion of the semiconductor substrate from a bottom surface of the semiconductor substrate to the empty space; and

releasing the membrane and a first portion of the plurality of comb-fingers to deflect.

17. The method of forming a MEMS device of claim 16 , wherein the plurality of comb-fingers are formed with a comb-finger aspect ratio equal to or exceeding 1:50, the comb-finger aspect ratio being a ratio of an inter-finger spacing distance between adjacent comb-fingers of the plurality of comb-fingers to a layer thickness of the plurality of comb-fingers.

18. The method of forming a MEMS device of claim 16 , further comprising forming a support spring, wherein

forming the membrane on the substrate-on-nothing structure comprises:

forming a framework coupled to the support spring, and

forming the membrane on the framework; and

forming the cavity in the substrate-on-nothing structure comprises forming the cavity in the framework such that the framework surrounds the cavity.

19. The method of forming a MEMS device of claim 18 , further comprising forming a stress layer mechanically coupled to the support spring.

20. The method of forming a MEMS device of claim 18 , further comprising forming a stress decoupling gap extending from an anchor coupled to the support spring and surrounding the substrate-on-nothing structure, wherein the stress decoupling gap is configured to reduce a transfer of layer stress from the semiconductor substrate to the substrate-on-nothing structure.

21. The method of forming a MEMS device of claim 16 , wherein removing the backside portion of the semiconductor substrate from the bottom surface of the semiconductor substrate to the empty space comprises performing a backside etch through the semiconductor substrate.

22. The method of forming a MEMS device of claim 16 , wherein removing the backside portion of the semiconductor substrate from the bottom surface of the semiconductor substrate to the empty space comprises:

etching a trench surrounding the substrate-on-nothing structure; and

performing a lift-off step to remove the substrate-on-nothing structure from the semiconductor substrate.

23. A microelectromechanical systems (MEMS) transducer comprising:

a transducer frame formed in a layer of monocrystalline silicon, the transducer frame comprising:

a support portion adjacent to a cavity,

a membrane layer on the support portion and overlying the cavity, and

a first set of comb-fingers extending from the support portion;

a spring support coupled from an anchor to the support portion, wherein the spring support comprises a torsional spring support connected between the anchor and the support portion adjacent to a first side of the membrane layer opposite the first set of comb-fingers; and

a second set of comb-fingers formed in the layer of monocrystalline silicon and being interdigitated with the first set of comb-fingers.

24. The MEMS transducer of claim 23 , wherein an inter-finger spacing distance between each comb-finger of the first set of comb-fingers and each corresponding adjacent comb-finger of the second set of comb-fingers is between 100 nm and 300nm.

25. The MEMS transducer of claim 23 , wherein a comb-finger aspect ratio is equal to or exceeding 1:50, the comb-finger aspect ratio being a ratio of an inter-finger spacing distance between each comb-finger of the first set of comb-fingers and each corresponding adjacent comb-finger of the second set of comb-fingers to a layer thickness of the first set of comb-fingers and the second set of comb-fingers.

26. The MEMS transducer of claim 23 , further comprising a stress layer mechanically coupled to the spring support and configured to deflect the transducer frame at rest.

27. The MEMS transducer of claim 23 , wherein the MEMS transducer has a chip thickness that is less than or equal to 300 μm.

28. The MEMS transducer of claim 23 , further comprising a stress decoupling gap extending from the anchor and surrounding the transducer frame, the spring support, and the second set of comb-fingers, wherein the stress decoupling gap is configured to reduce a transfer of layer stress from a substrate of the MEMS transducer to the transducer frame, the spring support, and the second set of comb-fingers.

29. The method of forming a MEMS transducer of claim 1 , wherein the method does not include forming a stress layer mechanically coupled to the spring support, and does not include forming a stress decoupling gap.