Micro-electro-mechanical transducer having an optimized non-flat surface

1. A method for a capacitive micromachined ultrasound transducer (cMUT), the method comprising:

growing and patterning a diffusion barrier layer over a substrate, the diffusion barrier layer having a first area and a second area, the diffusion barrier layer in the first area being patterned to allow a greater level of diffusion penetration of a gas therethrough to form a thermal oxide than a level of diffusion penetration of the gas to form the thermal oxide allowed by the patterning of the diffusion barrier layer at the second area;

performing a diffusion process over the diffusion barrier layer to form the thermal oxide to reach a first depth into the substrate below the first area;

forming an anchor using the thermal oxide, such that the anchor has a lower portion below a major surface of the substrate and an upper portion above the major surface of the substrate; and

forming a membrane layer over the anchor and the substrate, wherein the substrate has a first electrode, the membrane layer has a second electrode opposing the first electrode to define a gap therebetween, and at least one of the membrane layer and the substrate includes a flexible layer, such that the first electrode and the second electrode are movable relative to each other to cause a change of the gap.

2. The method of claim 1 , where the second area of the diffusion barrier layer allows substantially no diffusion penetration into the substrate.

3. The method of claim 1 , where the second area of the diffusion barrier layer allows partial diffusion penetration into the substrate, such that the formed thermal oxide reaches a second depth into the substrate below the second area, the second depth being smaller than the first depth.

4. The method of claim 3 , further comprising:

removing at least a part of the diffusion barrier layer and the thermal oxide at the second area to form a recess on the substrate.

5. The method of claim 1 , wherein the diffusion barrier layer has a third area allowing more diffusion penetration therethrough than the second area, and the thermal oxide reaches into the substrate below the third area, the method further comprising:

forming a motion stopper using the thermal oxide at the third area, such that the motion stopper has a lower portion below the major surface of the substrate and an upper portion above the major surface of the substrate, the upper portion of the motion stopper being shorter than the upper portion of the anchor.

6. The method of claim 1 wherein the substrate comprises a conductive substrate constituting at least a part of the first electrode.

7. The method of claim 1 wherein the forming the membrane layer comprises:

bonding an SOI layer carrying the membrane layer over the substrate; and

etching back the SOI layer to leave the membrane layer over the substrate.

8. The method of claim 1 wherein the diffusion barrier layer comprises at least an oxide layer.

9. The method of claim 1 wherein the diffusion barrier layer comprises a nitride layer.

10. The method of claim 1 wherein the gas contains oxygen or hydrogen.

11. The method of claim 1 further comprising removing at least a portion of the diffusion barrier layer and the thermal oxide prior to forming the membrane layer.

12. The method of claim 1 wherein the growing and patterning the diffusion barrier layer comprises:

growing and patterning a first diffusion barrier layer over the substrate; and

growing and patterning a second diffusion barrier layer over the first diffusion barrier layer.

13. A method for a capacitive micromachined ultrasound transducer (cMUT), the method comprising:

growing and patterning a diffusion barrier layer over a substrate, the patterned diffusion barrier layer having a first area and a second area, the first area being patterned to allow a greater level of diffusion penetration of a gas therethrough to form an oxide than a level of diffusion penetration of the gas to form the oxide allowed by the patterning of the diffusion barrier layer at the second area;

performing a diffusion process over the diffusion barrier layer to form the oxide to reach a first depth into the substrate below the first area, and to reach a second depth into the substrate below the second area, the first depth being greater than the second depth;

forming an anchor using the oxide, the anchor having a top portion higher than a surface of the substrate; and

forming a membrane layer over the anchor and the substrate, wherein the substrate has a first electrode, the membrane layer has a second electrode opposing the first electrode to define a gap therebetween, and the membrane layer includes a flexible layer, such that the first electrode and the second electrode are movable relative to each other to cause a change of the gap.

14. The method of claim 13 wherein the anchor is formed in the first area.

15. The method of claim 13 wherein the anchor is formed in the second area.

16. The method of claim 13 wherein the forming the anchor comprises at least partially patterning the oxide.

17. The method of claim 13 , wherein the diffusion barrier layer comprises at least one of: an oxide layer or a nitride layer.

18. The method of claim 13 wherein the diffusion process comprises thermal oxidation.

19. The method of claim 13 further comprising removing at least a portion of the diffusion barrier layer and the oxide prior to forming the membrane layer.

20. The method of claim 13 , wherein the gas contains oxygen or hydrogen.