Microelectromechanical resonators with passive frequency tuning using built-in piezoelectric-based varactors

• US 8,575,819 B1
• Filed: 07/18/2011
• Issued: 11/05/2013
• Est. Priority Date: 07/18/2011
• Status: Expired due to Fees

• Current Assignee: Integrated Device Technology Incorporated (Renesas Electronics Corporation)
• Original Assignee: Integrated Device Technology Incorporated (Renesas Electronics Corporation)
• Inventors: Bhugra, Harmeet, Samarao, Ashwin
• Primary Examiner(s): DOUGHERTY, THOMAS M
• Application Number:
  US13/184,970
• Time in Patent Office: 841 Days
• Field of Search: 333/186, 331/16, 331/18, 331/154, 331/175, 331/176, 310314-319
• US Class Current: 310/318
• CPC Class Codes: H03H 2009/02204 operating on an additional circuit element, e.g. applying a tuning DC voltage to a passive circuit element connected to the resonator
• CPC Class Codes: H03H 2009/155 using MEMS techniques
• CPC Class Codes: H03H 9/173 Air-gaps

Abstract

Microelectromechanical resonators include a resonator body with a built-in piezoelectric-based varactor diode. This built-in varactor diode supports passive frequency tuning by enabling low-power manipulation of the stiffness of a piezoelectric layer, in response to controlling charge build-up therein at resonance. A resonator may include a composite stack of a bottom electrode, a piezoelectric layer on the bottom electrode and at least one top electrode on the piezoelectric layer. The piezoelectric layer includes a built-in varactor diode, which is defined by at least two regions having different concentrations of electrically active dopants therein.

Claims

1. A microelectromechanical resonator, comprising:

a resonator body comprising a composite stack of a bottom electrode, a piezoelectric layer on said bottom electrode and at least one top electrode on said piezoelectric layer, said piezoelectric layer comprising a built-in varactor diode defined by at least two regions in said piezoelectric layer having different concentrations of electrically active dopants therein.

2. The resonator of claim 1, wherein the electrically active dopants in a first of the at least two regions are N-type dopants.

3. The resonator of claim 1, wherein the at least two regions comprise an N-type piezoelectric layer on an undoped piezoelectric layer.

4. The resonator of claim 1, wherein the built-in varactor diode comprises a first piezoelectric region therein doped with N-type impurities selected from a group consisting of silicon and chromium.

5. The resonator of claim 4, wherein said piezoelectric layer comprises aluminum nitride.

6. The resonator of claim 5, wherein said bottom electrode and said at least one top electrode comprise molybdenum.

7. The resonator of claim 1, wherein said resonator body comprises a suspended semiconductor layer anchored to a surrounding substrate by a pair of tethers;

and wherein said bottom electrode extends between the semiconductor layer and said piezoelectric layer.

8. The resonator of claim 7, wherein the built-in varactor comprises an N-type piezoelectric layer on an undoped piezoelectric layer;

and wherein the N-type piezoelectric layer defines an interface with said bottom electrode or said at least one top electrode.

9. The resonator of claim 7, wherein the N-type piezoelectric layer defines a doped/undoped piezoelectric junction with the undoped piezoelectric layer.

10. The resonator of claim 9, wherein the N-type piezoelectric layer comprises an N-type aluminum nitride layer having impurities therein selected from a group consisting of silicon and chromium.

11. The resonator of claim 1, wherein the at least two regions comprise an N-type piezoelectric region and a P-type piezoelectric region that defines a P-N junction with the N-type piezoelectric region.

12. The resonator of claim 11, wherein the P-type piezoelectric region is doped with P-type impurities selected from a group consisting of zinc and magnesium.

13. The resonator of claim 1, wherein the at least two regions comprise a N-type aluminum nitride region and at least one of an undoped aluminum nitride region and a P-type aluminum nitride region.

14. The resonator of claim 1, wherein the at least two regions comprise a P-type aluminum nitride region and at least one of an undoped aluminum nitride region and an N-type aluminum nitride region.

15. A periodic signal generator, comprising:

a resonator body including a composite stack of a semiconductor layer anchored by at least a pair of tethers to a surrounding substrate, a bottom electrode on the semiconductor layer, a piezoelectric layer on the bottom electrode and at least one sense electrode and at least one drive electrode on the piezoelectric layer, said piezoelectric layer comprising a built-in varactor diode defined by at least two regions in said piezoelectric layer having different concentrations of electrically active dopants therein; and

a DC reference voltage generator electrically coupled to one of the sense electrode and drive electrode.

16. The generator of claim 15, wherein the at least two regions are of opposite conductivity type.

17. The generator of claim 15, wherein the at least two regions are of net first conductivity type.

18. The generator of claim 15, wherein one of the at least two regions is undoped and another of the at least two regions is of net N-type conductivity or net P-type conductivity.

19. The generator of claim 15, wherein one of the at least two regions is doped with dopants selected from a group consisting of silicon, chromium, zinc and magnesium.

20. The generator of claim 15, wherein a width of a depletion region in the varactor diode changes in response to changes in a magnitude of a voltage generated by said DC reference voltage generator.