Tunable ultrasound transducers

1. A micromachined device comprising:

an ultrasonic transducer;

a tunable structure coupled to the ultrasonic transducer, the ultrasonic transducer and the tunable structure forming a resonator having a resonant frequency; and

a piezoelectric material within the tunable structure that changes shape according to an applied direct current, wherein the piezoelectric material is responsive to a change in the applied direct current to vary the resonant frequency of the resonator, and wherein the piezoelectric material forms an annular ring between an inside perimeter of a first substrate for the tunable structure and an outside perimeter of a second substrate for the ultrasonic transducer.

2. The device of claim 1 wherein the annular ring creates a tension about the ultrasonic transducer, wherein both the annular ring and the ultrasonic transducer are coupled to a surface of a substrate of the tunable structure.

3. The device of claim 2 wherein the substrate is formed of silicon.

4. The device of claim 1 wherein the piezoelectric material forms a planar surface coupled to the first substrate on a first side and coupled to the second substrate on a second side.

5. The device of claim 4 further comprising a support structure coupled to a perimeter of the tunable structure on a surface opposing the piezoelectric material, wherein the piezoelectric material overlaps the support structure.

6. The device of claim 5 wherein the substrate is formed of silicon.

7. The device of claim 1 wherein the first substrate and the second substrate are formed of a single layer of silicon.

8. The device of claim 7 further comprising a first electrical contact on the inside perimeter of the first substrate and a second electrical contact on the outside perimeter of the second substrate, the first electrical contact and the second electrical contact electrically coupled to the piezoelectric material for an application of direct current thereto.

9. The device of claim 1 wherein the ultrasonic transducer includes a piezoelectric micromachined ultrasonic transducer.

10. The device of claim 1 wherein the ultrasonic transducer includes a capacitive micromachined ultrasonic transducer.

11. A micromachined device comprising:

an ultrasonic transducer comprising a first piezoelectric material;

a tunable structure coupled to the ultrasonic transducer, the ultrasonic transducer and the tunable structure forming a resonator having a resonant frequency; and

a second piezoelectric material within the tunable structure that changes shape according to an applied direct current, the second piezoelectric material responsive to a change in the applied direct current to vary the resonant frequency of the resonator, wherein the ultrasonic transducer and the tunable structure form two layers of a piezoelectric bimorph.

12. The device of claim 1 further comprising a support structure coupled to a perimeter of the tunable structure so as to permit ultrasonic movement of ultrasonic transducer on the tunable structure relative to the support structure.

13. The device of claim 1 further comprising a direct current source coupled to the piezoelectric material.

14. The device of claim 1 further comprising an alternating current source coupled to the ultrasonic transducer.

15. The device of claim 1 wherein the piezoelectric material includes a lead-zirconate-titanate (PZT).

16. The device of claim 15 wherein the PZT is a Perovskite-phase PZT.

17. The device of claim 1 wherein the piezoelectric material includes a piezoelectric ceramic bulk material.

18. The device of claim 1 wherein the resonant frequency is tunable over a range from 1-18 MHz.

19. The device of claim 11 further comprising an array of ultrasonic transducers, each coupled to a support structure having a resonant frequency that varies according to an applied direct current.

20. The device of claim 19 wherein each one of the array of ultrasonic transducers has an independently controllable resonant frequency.

21. The device of claim 11 further comprising a support structure that suspends the micromachined device at one or more edges.