Temperature compensation for silicon MEMS resonator

• US 20040207489A1
• Filed: 04/16/2003
• Published: 10/21/2004
• Est. Priority Date: 04/16/2003
• Status: Active Grant

• Current Assignee: Robert Bosch GmbH
• Original Assignee: Robert Bosch GmbH
• Inventors: Partridge, Aaron, Lutz, Markus
• Granted Patent: US 6,987,432 B2
• Time in Patent Office: Days
• US Class Current: 333/186
• CPC Class Codes: B81B 2201/0271 Resonators; ultrasonic resonators
• CPC Class Codes: B81B 2203/0118 Cantilevers
• CPC Class Codes: B81B 3/0072 For controlling internal stress or strain in moving or flexible elements, e.g. stress compensating layers
• CPC Class Codes: H03H 2009/02496 Horizontal, i.e. parallel to the substrate plane
• CPC Class Codes: H03H 2009/02511 Vertical, i.e. perpendicular to the substrate plane
• CPC Class Codes: H03H 9/02259 Driving or detection means
• CPC Class Codes: H03H 9/02338 Suspension means
• CPC Class Codes: H03H 9/02417 involving adjustment of the transducing gap
• CPC Class Codes: H03H 9/02448 of temperature influence
• CPC Class Codes: H03H 9/2457 Clamped-free beam resonators
• CPC Class Codes: H03H 9/2463 Clamped-clamped beam resonators

Abstract

Thermally induced frequency variations in a micromechanical resonator are actively or passively mitigated by application of a compensating stiffness, or a compressive/tensile strain. Various composition materials may be selected according to their thermal expansion coefficient and used to form resonator components on a substrate. When exposed to temperature variations, the relative expansion of these composition materials creates a compensating stiffness, or a compressive/tensile strain.

Claims

1. A method of compensating for thermally induced frequency variations in a micromechanical resonator comprising an oscillating beam having a desired resonance frequency and an electrode, the method comprising:

determining an actual operating frequency for the resonator; and

,

applying a compensating stiffness to the beam in relation to the actual operating frequency and the desired resonance frequency.

2. The method of claim 1, wherein applying a compensating stiffness further comprises applying an electrostatic force to the beam via the electrode.

3. The method of claim 2, wherein applying the electrostatic force further comprises increasing a voltage applied to the electrode.

4. The method of claim 3, wherein the resonator further comprises a feedback circuit, and wherein determining the actual operating frequency further comprises:

measuring at least one of the resonator frequency and the operating temperature; and

producing an output signal indicative of the measurement, and

wherein the voltage applied to the electrode is varied in relation to feedback circuit output signal.

5. The method of claim 2, wherein the oscillating beam is separated from the electrode across of a working gap, and wherein applying the electrostatic force to the beam further comprises:

moving the electrode in relation to the oscillating beam to adjust the working gap in relation to the actual operating frequency of the resonator.

6. The method of claim 5, wherein moving the electrode further comprises:

physically moving the electrode in relation to the oscillating beam using a mechanical extension mechanism.

7. A method of fabricating a micromechanical resonator comprising an oscillating beam supported on a substrate by at least one support structure, and an electrode proximate to, but separated from the beam by a working gap, the method comprising:

forming at least one of the beam and the support structure from a first material having a first thermal expansion coefficient;

forming the electrode, at least in part, from a second material having a second thermal expansion coefficient different from the first thermal expansion coefficient;

whereby, the working gap is adjusted in accordance with temperature variations, such that resonator frequency remains substantially stable over the temperature variations.

8. The method of claim 7, wherein forming the electrode further comprises:

forming an electrode pedestal from a material other than the second material, the pedestal being fixed to the substrate; and

,

forming the electrode on the electrode pedestal.

9. The method of claim 7, wherein forming the electrode further comprises:

forming an electrode body from a material other than the second material, and thereafter;

vacating a portion of the electrode body; and

,

re-filling the vacated portion of the electrode body with the second material.

10. The method of claim 9, wherein the first material is silicon and the second material is silicon dioxide.

11. The method of claim 7, wherein the first material and second material comprise at least one selected from a group consisting of:

silicon, poly-silicon, epi-poly, LPCVD-poly, silicon dioxide, germanium, silicon-germanium compounds, silicon nitrides, and silicon carbide.

12. A micromechanical resonator, comprising:

a beam having a resonant frequency and being formed from a first material having a first thermal expansion coefficient;

an electrode proximate the beam across a working gap and adapted to exert an electrostatic force on the beam, the electrode being formed, at least in part, from a second material having a second thermal expansion coefficient different from the first thermal expansion coefficient;

wherein relative thermal expansion of the beam and the electrode over a temperature range adjusts the working gap, such that the resonant frequency remains substantially stable over the temperature range.

13. The resonator of claim 12, further comprising a substrate and an active layer of first material type deposited on the substrate;

wherein the beam is formed from the active layer and the electrode is formed, at least in part, from the active layer.

14. The resonator of claim 13, wherein the active layer comprises at least one selected from a group consisting of:

silicon, poly-silicon, epi-poly, LPCVD-poly, germanium, silicon-germanium compounds, silicon nitrides, and silicon carbide.

15. A micromechanical resonator, comprising:

a beam having a desired resonance frequency;

an electrode proximate the beam across a working gap, and adapted to exert an electrostatic force on the beam;

a lever arm supporting the electrode at a first end, and adapted to move the electrode to thereby adjust working gap.

16. The resonator of claim 15, further comprising:

a feedback circuit generating a feedback signal indicative of at least one of actual resonance frequency for the beam and resonator operating temperature.

17. The resonator of claim 16, further comprising:

an actuation driver motivating the lever arm in response to the feedback signal, such that the desired resonance frequency is maintained over an operating temperature range.

18. The resonator of claim 15, further comprising

a first support fixing the lever arm proximate a second end opposite the first end to a substrate, wherein the first support is formed from a first material having a first thermal expansion coefficient;

and,

a second support supporting the lever arm as a fulcrum between the first and second ends, wherein the second support is formed, at least in part, from a second material having a second thermal expansion coefficient;

wherein relative thermal expansion of the first and second supports over a temperature range adjusts the working gap, such that the resonant frequency remains substantially stable over the temperature range.

19. A micromechanical resonator, comprising:

a substrate formed from a first material having a first thermal expansion coefficient;

an oscillating beam formed from an active layer deposited over the substrate, wherein the active layer is formed from a second material having a second thermal expansion coefficient different from the first thermal expansion coefficient;

at least one support fixing the beam to the substrate via a first anchor;

an electrode proximate the beam across a working gap and fixed to the substrate via a second anchor;

wherein the first and second anchors are laterally disposed one from the other on the substrate in relation to the oscillating beam.

20. The resonator of claim 19, wherein the electrode is formed, at least is part, from the active layer.

21. The resonator of claim 20, wherein the at least one support is formed from the active layer.

22. The resonator of claim 19, wherein the first anchor is formed from the active layer:

and

the second anchor is formed, at least in part, from the active layer.

23. The resonator of claim 22, wherein the second anchor is formed in part from a material other than the second material.

24. A micromechanical resonator, comprising:

a substrate formed from a first material having a first thermal expansion coefficient;

an oscillating beam formed from an active layer deposited over the substrate, wherein the active layer is formed from a second material having a second thermal expansion coefficient different from the first thermal expansion coefficient and a third material having a third thermal expansion coefficient different from the second thermal expansion;

a first anchor structure and a second anchor structure;

wherein the first and second anchor structures are laterally disposed one from the other on the substrate in relation to the oscillating beam to support the oscillating beam above the substrate.

25. The resonator of claim 24, wherein the first anchor structure is formed from the active layer.

26. The resonator of claim 24, wherein the first anchor structure is formed from the active layer:

and

the second anchor structure is formed, at least in part, from the active layer.

27. The resonator of claim 26, wherein the second anchor structure is formed in part from a material other than the second material.

28. The resonator of claim 24, wherein the oscillating beam is formed in part from a material other than the second material and the third material.

29. A micromechanical resonator, comprising:

a substrate formed from a first material having a first thermal expansion coefficient;

a beam formed from a second material having a second thermal expansion coefficient different from the first thermal expansion coefficient, wherein the beam is suspended above the substrate by at least one anchor structure;

wherein the at least one anchor structure further comprises;

an anchor point fixing the anchor to the substrate; and

,

a composite anchor structure formed from the second material and a third material having a third thermal expansion coefficient different from the second thermal expansion coefficient.

30. A micromechanical resonator, comprising:

a substrate formed from a first material having a first thermal expansion coefficient;

a first lever arm fixed to the substrate at one end by a first anchor and supporting a first end of an oscillating beam at the other end;

a second lever arm fixed to the substrate at one end by a second anchor and supporting a second end of the oscillating beam at the other end;

a compression/expansion bar connected between the first and second lever arms, and laterally motivating the first and second lever arms to applied either a compressive strain or a tensile strain on the oscillating beam.

31. The resonator of claim 30, wherein the oscillating beam is formed from a first material having a first thermal expansion coefficient, and the compression/expansion bar is formed, at least in part, from a second material having a second thermal expansion coefficient different from the first thermal expansion coefficient.