Tunable MEMS resonator and method for tuning

• US 20030184412A1
• Filed: 03/27/2002
• Published: 10/02/2003
• Est. Priority Date: 03/27/2002
• Status: Active Grant

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Gorrell, Jonathan F.
• Granted Patent: US 6,707,351 B2
• Time in Patent Office: Days
• US Class Current: 333/186
• CPC Class Codes: H03H 3/0077 by tuning of resonance frequency

Abstract

MEMS resonators (100, 400, 500) include a source of material that is capable of sublimation (128, 130, 406, 408, 502, 504). Conductive pathways (132, 134, 402, 404, 502, 504) to the material are used to supply current of ohmically heat the material in order to cause the material to sublimate. The material may be located either on or in close proximity to a resonant member (114) of the resonator. By sublimating the material, the mass of the resonant member is either increased or decreased thereby altering the resonant frequency of the resonant member. The resonant member is preferably located in a recess that is capped by a cap (202) forming a vacuum enclosure, and the material capable of sublimation preferably comprises a material that serves to getter any residual gases in the vacuum enclosure.

Claims

1. A microelectromechanical resonator comprising:

a microelectromechanical resonant structure characterized by a mass;

a vaporizable material for altering the mass of the resonant structure.

2. The microelectromechanical resonator according to claim 1 wherein:

the vaporizable material is initially borne on the resonant structure.

3. The microelectromechanical resonator according to claim 2 wherein:

the resonant structure comprises;

a semiconductor structure including;

one or more selectively doped regions for establishing electric forces for driving a vibration mode of the semiconductor structure.

4. The microelectromechanical resonator according to claim 1 wherein:

the vaporizable material is initially located proximate to the resonant structure.

5. The microelectromechanical resonator according to claim 4 wherein:

the vaporizable material comprises an air bridge that extends over the resonant structure.

6. The microelectromechanical resonator according to claim 5 wherein:

the vaporizable material is electrically conductive and is adapted for coupling to an electrical power source of sufficient power to cause the vaporizable material to vaporize.

7. The microelectromechanical resonator according to claim 6 wherein:

the vaporizable material comprises Titanium.

8. The microelectromechanical resonator according to claim 7 further comprising:

a vacuum enclosure surrounding the resonant structure and the vaporizable material.

9. The microelectromechanical resonator according to claim 1 wherein the vacuum enclosure comprises:

a lid affixed over a recess in which the resonant structure and the vaporizable material are located.

10. The microelectromechanical resonator according to claim 1 further comprising:

a vacuum enclosure surrounding the resonant structure and the vaporizable material.

11. The microelectromechanical resonator according to claim 10 wherein:

the vaporizable material comprises Titanium.

12. The microelectromechanical resonator according to claim 1 wherein:

the resonant structure comprises;

a main beam; and

one or more torsional hinges supporting the main beam.

13. The microelectromechanical resonator according to claim 12 further comprising:

a base;

an oxide layer overlying the base, wherein the oxide layer includes;

an inner edge defining an opening in the oxide layer;

a semiconductor layer overlying the oxide layer,

wherein the semiconductor layer includes;

the main beam; and

the one or more torsional hinges supporting the main beam; and

the main beam and the one or more torsional hinges overlie the opening in the oxide layer.

14. The microelectromechanical resonator according to claim 13 wherein:

the vaporizable material includes a first patch that is located on the main beam.

15. The microelectromechanical resonator according to claim 13 wherein

the main beam includes:

a first end;

a second end; and

a longitudinal center;

the first patch is located proximate the first end of the main beam;

the vaporizable material further comprises a second patch that is located proximate the second end of the main beam;

the one or more torsional hinges include a first torsional hinge and a second torsional hinge; and

the resonator further comprises;

a first conductive pathway of a first dopant type that extends over the first torsional hinge, to the main beam, under the first patch and to the longitudinal center of the main beam; and

a second conductive pathway of a second dopant type, that extends over the second torsional hinge, under the second patch, and to the longitudinal center of the main beam, forming a PN junction with the first conductive pathway at the longitudinal center of the main beam.

16. The microelectromechanical resonator according to claim 13 wherein

the semiconductor layer includes a peripheral portion that is peripheral to the main beam and torsional hinges;

and

the vaporizable material includes a patch that is located on the peripheral portion.

17. The microelectromechanical resonator according to claim 16 further comprising:

a doped conductive pathway that includes;

a narrow segment proximate to the patch for ohmically heating the patch to cause the patch to sublimate.

18. The microelectromechanical resonator according to claim 13 further comprising:

one or more additional layers overlying the semiconductor layer; and

one or more inner peripheral edges of the one more additional defining an opening through the one or more additional layers surrounding main beam and torsional hinges; and

a cap over the opening.

19. The microelectromechanical resonator according to claim 17 wherein:

the vaporizable material comprises Titanium.

20. The microelectromechanical resonator according to claim 17 wherein:

the one or more additional layers includes;

a first layer; and

a second layer overlying the first layer; and

the resonator comprises;

an air bridge that extends from between the first layer and the second layer across the opening, wherein the air bridge includes the vaporizable material.

21. A method of tuning a microelectromechanical resonator that includes a microelectromechanical resonant structure, a vaporizable material, and a first conductive pathway for ohmically heating the vaporizable material, the method including the steps of:

driving the micromechanical resonator with a swept frequency signal;

monitoring a first current coupled to the microelectromechanical resonator to detect a resonant frequency;

supplying a second current to ohmically heat the vaporizable material to evaporate the vaporizable material and thereby alter the mass of the microelectromechanical resonant structure, and the resonant frequency;

stopping the second current when the resonant frequency is about equal to a predetermined value.