MICROELECTROMECHANICAL RESONANT STRUCTURE HAVING IMPROVED ELECTRICAL CHARACTERISTICS

• US 20110128083A1
• Filed: 11/24/2010
• Published: 06/02/2011
• Est. Priority Date: 11/27/2009
• Status: Active Grant

• Current Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Flora, Pasquale, Morea, Annarita, Roselli, Giuditta, Pomarico, Anna
• Granted Patent: US 8,289,092 B2
• Time in Patent Office: Days
• US Class Current: 331/156
• CPC Class Codes: H03H 2009/02511 Vertical, i.e. perpendicular to the substrate plane
• CPC Class Codes: H03H 2009/2442 Square resonators
• CPC Class Codes: H03H 9/02259 Driving or detection means
• CPC Class Codes: H03H 9/02338 Suspension means
• CPC Class Codes: H03H 9/02409 by application of a DC-bias voltage H03H9/02417 takes precedence
• CPC Class Codes: H03H 9/02433 Means for compensation or elimination of undesired effects
• CPC Class Codes: H03H 9/2436 Disk resonators

Abstract

The present disclosure is directed to a MEMS resonant structure, provided with a substrate of semiconductor material; a mobile mass suspended above the substrate and anchored to the substrate by constraint elements to be free to oscillate at a resonance frequency; and a fixed-electrode structure capacitively coupled to the mobile mass to form a capacitor with a capacitance that varies as a function of the oscillation of the mobile mass; the fixed-electrode structure arranged on a top surface of the substrate, and the constraint elements being configured in such a way that the mobile mass oscillates, in use, in a vertical direction, transverse to the top surface of the substrate, keeping substantially parallel to the top surface.

Claims

1. A MEMS resonant structure, comprising:

a substrate of semiconductor material having a top surface;

a plurality of constraint elements;

a mobile mass suspended above said substrate and anchored to said substrate by the plurality of constraint elements, the mobile mass being configured to oscillate at a resonance frequency; and

a fixed-electrode structure capacitively coupled to said mobile mass to form a capacitor with a capacitance that varies as a function of the oscillation of said mobile mass, said fixed-electrode structure being positioned on the top surface of said substrate, and said constraint elements being configured to enable the mobile mass to oscillate in a vertical direction, transverse to said top surface of said substrate.

2. The structure according to claim 1, wherein said vertical direction is orthogonal to said top surface of said substrate.

3. The structure according to claim 1, wherein said mobile mass has a substantially planar extension in a horizontal plane;

and wherein said constraint elements are configured to enable the mobile mass to oscillate substantially parallel to said top surface of said substrate.

4. The structure according to claim 1, wherein said constraint elements comprise anchorage elements fixed to said substrate, and elastic elements extending from said mobile mass to said anchorage elements;

said elastic elements being compliant in said vertical direction.

5. The structure according to claim 4, wherein said mobile mass has a substantially planar extension in a horizontal plane defined by a first horizontal axis and a second horizontal axis;

and wherein said anchorage elements include a first anchorage element and one second anchorage element, which are set on opposite sides of said mobile mass with respect to said first horizontal axis or second horizontal axis.

6. The structure according to claim 5, wherein said first and second anchorage elements are set centrally, and externally, with respect to said mobile mass.

7. The structure according to claim 4, wherein said elastic elements are of a folded type.

8. The structure according to claim 4, wherein said elastic elements extend starting from respective vertices of said mobile mass and connect, in pairs, up to a respective one of said anchorage elements.

9. The structure according to claim 1, further comprising regulation electrodes arranged on said top surface of said substrate and capacitively coupled to said mobile mass;

said regulation electrodes being configured to receive a regulation voltage to vary an electrostatic force acting on said mobile mass, and consequently the value of said resonance frequency.

10. The structure according to claim 9, wherein said fixed-electrode structure comprises an actuation electrode set on said top surface of said substrate underneath said mobile mass, and said regulation electrodes are set laterally with respect to said actuation electrode, at least partially underneath said mobile mass;

and wherein between said actuation electrode and said mobile mass a biasing voltage of substantially constant amplitude is to be applied.

11. The structure according to claim 10, wherein the value of said regulation voltage is variable between a zero value and the value of said biasing voltage, as a function of a shift of the value of said resonance frequency due to process spread or temperature shifts.

12. The structure according to claim 10, wherein said fixed-electrode structure further comprises a first sensing electrode and a second sensing electrode, which are set on said top surface of said substrate underneath said mobile mass, laterally and on opposite sides of said actuation electrode;

said first and second sensing electrodes being configured to form with said mobile mass a capacitor with a capacitance that varies as a function of the oscillation of said mobile mass.

13. The structure according to claim 1, wherein said resonance frequency is a real-time-clock frequency.

14. The structure according to claim 1 wherein the plurality of constraint elements include a first constraint element and a second constraint element coupled to opposing sides of the mobile mass, each of the first and second constraint elements having an elongated first portion coupled to the mobile mass, an elongated second portion coupled to the substrate, and a third portion extends between the first and the second portions.

15. The structure according to claim 14 wherein the second portion is longer than the first portion.

16. The structure according to claim 14 wherein the third portion is transverse to the elongated first and second portions.

17. An oscillator circuit, comprising:

a MEMS resonant structure, including;

a substrate of semiconductor material having a top surface;

a plurality of constraint elements;

a mobile mass suspended above said substrate and anchored to said substrate by the plurality of constraint elements, the mobile mass being configured to oscillate at a resonance frequency; and

a fixed-electrode structure capacitively coupled to said mobile mass to form a capacitor with a capacitance that varies as a function of the oscillation of said mobile mass, said fixed-electrode structure being positioned on the top surface of said substrate, and said constraint elements being configured to enable the mobile mass to oscillate in a vertical direction, transverse to said top surface of said substrate; and

a biasing stage configured to bias said mobile mass and said fixed-electrode structure.

18. The circuit according to claim 17, further comprising a processing stage coupled to said MEMS resonant structure and configured to process said resonance frequency starting from the variation of said capacitance.

19. The circuit according to claim 17, further comprising regulation electrodes arranged on said top surface of said substrate and capacitively coupled to said mobile mass;

said regulation electrodes being configured to receive a regulation voltage to vary an electrostatic force acting on said mobile mass, and consequently the value of said resonance frequency.

20. The structure according to claim 19, wherein said fixed-electrode structure comprises an actuation electrode set on said top surface of said substrate underneath said mobile mass, and said regulation electrodes are set laterally with respect to said actuation electrode, at least partially underneath said mobile mass;

and wherein said biasing stage is configured to apply, between said actuation electrode and said mobile mass, a biasing voltage of substantially constant amplitude.

21. A method, comprising:

forming a MEMS resonant structure, the forming including;

forming a fixed-electrode structure on a top surface of a substrate;

forming a mobile mass capacitively coupled to the fixed-electrode structure, the mobile mass being configured to oscillate at a resonance frequency; and

forming a plurality of constraint elements coupled to the mobile mass, the constraint elements being configured to anchor the mobile mass to the substrate and enable the mobile mass to oscillate in a vertical direction transverse to the top surface of the substrate.

22. The method of claim 21, further comprising forming a biasing stage coupled to the mobile mass and the fixed-electrode structure, the biasing stage being configured to bias said mobile mass and said fixed-electrode structure.

23. The method of claim 21, further comprising:

forming a first regulation electrode on the top surface of the substrate, the first regulation electrode being adjacent a first side of the fixed-electrode structure and capacitively coupled to the mobile mass; and

forming a second regulation electrode on the top surface of the substrate, the second regulation electrode being adjacent a second side of the fixed-electrode structure and capacitively coupled to the mobile mass.

24. The method of claim 23, further comprising:

forming first and second sensing electrodes on the top surface of the substrate between the mobile mass and the first and the second regulation electrodes, respectively.