Method for temperature compensation in MEMS resonators with isolated regions of distinct material

1. A method of forming a MEMS resonator device, comprising:

forming a first structural material on a substrate;

forming a trench in the first structural material;

forming in the trench, a second structural material having a different Young's modulus temperature coefficient than the first structural material;

patterning a resonator comprising both the first and second structural materials; and

anchoring the patterned resonator to an anchor;

where the second material is confined to a region of the resonator having a longest dimension that is shorter than a distance between the anchor and a point of the resonator furthest from the anchor.

2. The method of claim 1 , wherein forming the first and second structural materials on the substrate further comprises depositing films at a temperature below 500° C.

3. The method of claim 1 , wherein forming the first structural material comprises depositing a polycrystalline silicon germanium alloy and wherein forming the second structural material comprises depositing silicon dioxide.

4. The method of claim 1 , wherein forming the first structural material comprises depositing an silicon dioxide and wherein forming the second structural material comprises depositing a polycrystalline silicon germanium alloy.

5. The method of claim 1 , wherein the trench in the first structural material is defined through lithography and etch to have a dimension required to compensate the temperature coefficient of frequency of the resonator.

6. The method of claim 1 , wherein forming the second structural material in the trench further comprises:

filling the trench with the second structural material to form a void-free structure; and

removing the second structural material from the top surface of the first structural material with at least one of an etch back and chemical-mechanical polish.

7. The method of claim 1 , wherein the second structural material in the trench forms a spacer on the sidewalls of the trench and patterning the resonator removes the first structural material from a portion of the spacer to form a spacer ring attached to a sidewall of the resonator.

8. The method of claim 7 , further comprising isotropically etching the second structural material to remove the portions of the ring not attached to the sidewall of the resonator.

9. The method of claim 1 , where the first material has a negative Young's modulus temperature coefficient and the second material has a positive Young's modulus temperature coefficient.

10. The method of claim 1 , where the first material is a semiconductor and the second material is a dielectric.

11. The method of claim 1 , where the top surface of the first material is planar with the top surface of the second material.

12. The method of claim 1 , where the resonator is a beam; and where the region of the resonator to which the second material is confined includes a point of maximum flexural stress within the resonator when the resonator is made to resonate.

13. The method of claim 1 , where the resonator is a beam; and where the region of the resonator to which the second material is confined includes a point of maximum stress within the resonator and excludes a point of maximum displacement when the resonator is made to resonate.

14. The method of claim 1 , where the resonator is a bulk mode resonator; and where the region of the resonator to which the second material is confined is an isolated block completely surrounded by the first material.

15. The method of claim 1 , where the second material stiffens the resonator region to which the second material is confined as a function of temperature to tune a temperature coefficient of frequency of the resonator in a manner at least partially decoupled from the resonator temperature coefficient.

16. The method of claim 1 , where an edge of the second material farthest from the anchor is disposed a first distance from the anchor, the first distance being less than a distance between the anchor and a point of maximum displacement during resonance.

17. The method of claim 16 , further comprising forming the plurality of trenches in the first structural material such that the plurality of trenches are arranged in a radial array about the anchor.

18. The method of claim 16 , where the first material has a negative Young's modulus temperature coefficient and the second material has a positive Young's modulus temperature coefficient.

19. The method of claim 16 , where the top surface of the first material is planar with the top surface of the second material.

20. A method of forming a MEMS resonator, comprising:

forming a first structural material on a substrate;

forming a trench in the first structural material;

forming in the trench, a second structural material having a different Young's modulus temperature coefficient than the first structural material; and

patterning a resonator comprising both the first and second structural materials;

where the second material is isolated to a region of the resonator proximate to a point of maximum stress within the resonator during operation.

21. A method of forming a MEMS resonator, comprising:

forming a first structural material on a substrate;

forming a trench in the first structural material;

forming in the trench, a second structural material having a different Young's modulus temperature coefficient than the first structural material; and

patterning a resonator comprising both the first and second structural materials;

where the resonator is a bulk-mode resonator, and where the method further comprises:

forming a plurality of trenches in the first structural material, the plurality of trenches being arranged in a radial array; and

forming the second structural material in the plurality of trenches.