Anti-stiction bottom cavity surface for micromachined ultrasonic transducer devices

1. A method of forming an ultrasonic transducer device, the method comprising:

depositing a first layer on a substrate;

depositing a second layer on the first layer;

patterning the second layer at a region corresponding to a location of a transducer cavity;

depositing a third layer that refills regions created by patterning the second layer;

planarizing the third layer to a top surface of the second layer;

removing the second layer;

conformally depositing a fourth layer over the first layer and the third layer;

defining the transducer cavity in a support layer formed over the fourth layer; and

bonding a membrane to the support layer.

2. The method of claim 1 , wherein depositing the first layer on the substrate comprises depositing the first layer on a transducer lower electrode layer incorporated into an upper portion of the substrate.

3. The method of claim 1 , wherein the first layer comprises silicon oxide.

4. The method of claim 1 , wherein the first layer is formed at a thickness of about 10-30 nanometers.

5. The method of claim 1 , wherein the second layer comprises silicon nitride.

6. The method of claim 1 , wherein the second layer is formed at a thickness of about 30-70 nanometers.

7. The method of claim 1 , wherein the third layer is a same material as the first layer.

8. The method of claim 1 , wherein the third layer comprises silicon oxide.

9. The method of claim 1 , wherein the third layer is formed at a thickness of about 400-700 nanometers.

10. The method of claim 1 , wherein planarizing the third layer comprises using chemical mechanical polishing.

11. A method of forming an ultrasonic transducer device, the method comprising:

depositing a first layer on a substrate;

patterning the first layer at a region corresponding to a location of a transducer cavity;

conformally depositing an additional layer of a same type as the first layer on the first layer and exposed portions of the substrate;

conformally depositing a second layer on the additional layer of the same type as the first layer;

conformally depositing a third layer on the second layer;

conformally depositing a fourth layer on the third layer;

planarizing the fourth layer to a top surface of the third layer;

depositing a membrane support layer;

defining the transducer cavity in the support layer, wherein exposed portions of the third layer and the fourth layer are removed;

bonding a membrane to the support layer.

12. The method of claim 11 , wherein depositing the first layer on the substrate comprises depositing the first layer on a transducer lower electrode layer incorporated into an upper portion of the substrate.

13. The method of claim 11 , wherein the first layer comprises silicon oxide.

14. The method of claim 11 , wherein the first layer is formed at a thickness of about 10-30 nanometers.

15. The method of claim 11 , wherein the additional layer of the same type as the first layer is formed at a thickness of about 10-30 nanometers.

16. The method of claim 11 , wherein the additional layer of the same type as the first layer is formed at a first thickness and then etched down to a second thickness.

17. The method of claim 11 , wherein the third layer comprises silicon nitride.

18. The method of claim 11 , wherein the third layer is formed at a thickness of about 20-50 nanometers.

19. The method of claim 11 , wherein the fourth layer comprises silicon oxide.

20. The method of claim 11 , wherein the fourth layer is formed at a thickness of about 400-700 nanometers.