Micro-electro-mechanical transducer having an insulation extension

• US 8,796,901 B2
• Filed: 06/16/2006
• Issued: 08/05/2014
• Est. Priority Date: 06/17/2005
• Status: Active Grant

• Current Assignee: Kolo Medical, Ltd.
• Original Assignee: Kolo Technologies, Inc.
• Inventors: Huang, Yongli
• Primary Examiner(s): Mullins, Burton
• Assistant Examiner(s): Johnson, Eric
• Application Number:
  US11/917,666
• Publication Number:
  US 20080290756A1
• Time in Patent Office: 2,972 Days
• Field of Search: 438 50- 53, 438/117, 310/309, 310/300, 600/459, 381/174, 381/191, 381/181, 367/163, 367/181, 735/143.2
• US Class Current: 310/309
• CPC Class Codes: B06B 1/0292 Electrostatic transducers, e.g. electret-type
• CPC Class Codes: B81B 3/0021 Transducers for transforming electrical into mechanical energy or vice versa dynamo-electric machines H02K99/00; electrostatic machines H02N1/00; piezoelectric devices H10N30/00
• CPC Class Codes: G01N 29/2406 Electrostatic or capacitive probes, e.g. electret or cMUT-probes
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
• CPC Class Codes: H02N 1/006 of the gap-closing type H02N1/004 takes precedence
• CPC Class Codes: H10N 30/2047 Membrane type
• CPC Class Codes: Y10T 29/49005 Acoustic transducer

Abstract

A micro-electro-mechanical transducer (such as a cMUT) having two electrodes separated by an insulator with an insulation extension is disclosed. The two electrodes define a transducing gap therebetween. The insulator has an insulating support disposed generally between the two electrodes and an insulation extension extending into at least one of two electrodes to increase the effective insulation without having to increase the transducing gap. Methods for fabricating the micro-electro-mechanical transducer are also disclosed. The methods may be used in both conventional membrane-based cMUTs and cMUTs having embedded springs transporting a rigid top plate.

Claims

1. An electrostatic transducer comprising:

a first conductive layer having a first major surface;

a second conductive layer having a second major surface opposing the first major surface of the first conductive layer, the first major surface and the second major surface defining an electrode separation gap therebetween; and

an insulating support including;

a main portion disposed within the electrode separation gap between the first conductive layer and the second conductive layer; and

an insulation extension portion extending into a cavity formed in the first conductive layer, wherein;

the cavity is directly under the main portion of the insulating support;

the first conductive layer includes a first conductive surface that is directly under the main portion, and the second conductive layer includes a second conductive surface that is directly over the main portion; and

the first conductive surface that is directly under the main portion of the insulating support, and that is nearest to the second conductive surface that is directly over the main portion of the insulating support, is separated from the second conductive surface that is directly over the main portion of the insulating support by a distance greater than the electrode separation gap.

2. The electrostatic transducer of claim 1 wherein the first conductive layer includes a semiconductor layer and a supplemental conductive layer having a conductivity significantly higher than that of the semiconductor layer.

3. The electrostatic transducer of claim 1 wherein:

the first conductive layer includes a semiconductor layer; and

the semiconductor layer of the first conductive layer is thicker than the insulation extension portion such that the insulation extension portion is contained therein.

4. The electrostatic transducer of claim 3 wherein the semiconductor layer of the first conductive layer includes a silicon/polysilicon layer.

5. The electrostatic transducer of claim 4 wherein the insulation extension portion extends into the silicon/polysilicon layer.

6. The electrostatic transducer of claim 1 wherein the distance greater than the electrode separation gap is at least 25% greater than the electrode separation gap.

7. The electrostatic transducer of claim 1 wherein the main portion of the insulating support comprises a first insulating material, and the insulation extension portion comprises a second insulating material which is different from the first insulating material.

8. The electrostatic transducer of claim 1 wherein the main portion of the insulating support is connected to the insulation extension portion.

9. The electrostatic transducer of claim 1 wherein the cavity and the insulation extension portion have a cross-sectional size greater than that of the main portion of the insulating support.

10. The electrostatic transducer of claim 1 wherein the insulation extension portion comprises a solid material completely filling the cavity.

11. The electrostatic transducer of claim 1 wherein the insulation extension portion comprises a solid material partially filling the cavity leaving a partial void therein.

12. The electrostatic transducer of claim 1 wherein the insulation extension portion is a first insulation extension portion extending into the first conductive layer and the insulating support includes a second insulation extension portion extending into a cavity in the second conductive layer.

13. The electrostatic transducer of claim 12 wherein the first insulation extension portion comprises a first insulating material and the second insulation extension portion comprises a second insulating material which is different from the first insulating material.

14. The electrostatic transducer of claim 1 wherein an additional cavity is formed in the first conductive layer with an additional insulation extension portion extending into the additional cavity, the additional cavity and the additional insulation extension portion are located at a position where the first conductive layer and the second conductive layer are most likely to contact or come close to contacting each other during operation of the electrostatic transducer—

since the insulation extension portion (842, 942, FIGS. 8-10) are not located directly under the main portion (830, 930) like the insulation extension portion (840, 940) of claim 1.

15. The electrostatic transducer of claim 1 further comprising a motion stopper extending partially across the electrode separation gap.

16. The electrostatic transducer of claim 1 which is a capacitive micromachined ultrasonic transducer, wherein the first conductive layer serves as a bottom electrode and the second conductive layer serves as a movable top electrode.

17. The electrostatic transducer of claim 16 wherein the second conductive layer comprises a resilient membrane supported by the insulating support.

18. The electrostatic transducer of claim 1 wherein the cavity is formed in the semiconductor layer by oxidation.

19. The electrostatic transducer of claim 1 wherein the insulating support and the insulation extension are oxide grown at the cavity.

20. A capacitive micromachined ultrasonic transducer comprising:

a lower layer including a substrate and serving as a bottom electrode;

a top layer including a membrane and serving as a top electrode, the membrane being adapted for vibrating in relation to the substrate upon a transducing excitation, the lower layer having a first major surface, and the top layer having a second major surface defining an electrode separation gap therebetween; and

an insulator having a main portion and an insulation extension portion, the main portion being disposed within the electrode separation gap between and supporting the lower layer and the top layer, and the insulation extension portion extending beyond the electrode separation gap and into at least one cavity formed in at least one of the substrate or the membrane, respectively, the at least one cavity formed respectively directly under or directly above the main portion of the insulating support, wherein;

the at least one cavity includes a first conductive surface respectively directly under or directly above the main portion of the insulator; and

an opposing one of the first major surface or the second major surface includes a second conductive surface respectively directly under or directly above the main portion and separated from a nearest portion of the first conductive surface by a distance greater than the electrode separation gap.

21. The capacitive micromachined ultrasonic transducer of claim 20 wherein:

the at least one cavity comprises a first cavity in the substrate and a second cavity in the membrane; and

the insulation extension portion comprises a first insulation extension portion extending into the first cavity in the substrate and a second insulation extension portion extending into the second cavity in the membrane.

22. The capacitive micromachined ultrasonic transducer of claim 20 wherein the substrate of the lower layer comprises a silicon/polysilicon layer.

23. The capacitive micromachined ultrasonic transducer of claim 20 wherein the cavity is formed in the at least one of the substrate or the membrane by oxidation.

24. The capacitive micromachined ultrasonic transducer of claim 20 wherein the main portion and the insulation extension of the insulator are oxide grown at the cavity.

25. The capacitive micromachined ultrasonic transducer of claim 20, wherein the distance greater than the electrode separation gap is at least 25% greater than the electrode separation gap.

26. A method for fabricating a micro-electro-mechanical transducer, the method comprising:

forming a recess below a major surface of a first conductive layer;

forming a standing feature of an insulating material over the recess, the standing feature having an insulation extension portion extending from within the recess to the major surface, and a main portion extending from the major surface to a free end above the major surface; and

placing a second conductive layer over the free end of the standing feature, wherein;

the major surface of the first conductive layer and an opposing surface of the second conductive layer define an electrode separation gap therebetween,

the main portion is a portion of the standing feature directly above the recess and within the electrode separation gap, and

a first conductive surface directly under the main portion of the standing feature is separated, at a nearest point, from a second conductive surface, directly over the free end of the standing feature, by a distance greater than the electrode separation gap.

27. The method of claim 26 wherein the first conductive layer comprises a silicon/polysilicon layer.

28. The method of claim 26 wherein the forming the standing feature comprises:

growing an insulation layer over the recess; and

patterning and etching the insulation layer.

29. The method of claim 26 wherein the placing the second conductive layer over the free end of the standing feature comprises:

bonding a composite wafer having a functional layer to the free end of the standing feature; and

etching back the composite wafer to leave the functional layer on the standing feature.

30. The method of claim 29 wherein the placing the second conductive layer further comprises depositing a metal layer over the functional layer.

31. The method of claim 29 wherein the functional layer on the standing feature comprises a silicon/polysilicon layer.

32. The method of claim 29 wherein the functional layer on the standing feature is a membrane layer adapted for vibrating in relation to the first conductive layer upon a suitable transducing excitation.

33. The method of claim 26 wherein the placing the second conductive layer over the free end of the standing feature comprises:

depositing a sacrificial layer over the first conductive layer and the standing feature;

depositing a functional layer over the sacrificial layer; and

removing the sacrificial layer to leave the functional layer over the free end of the standing feature.

34. The method of claim 33 wherein the placing the second conductive layer further comprises depositing a metal layer over the functional layer.

35. The method of claim 33 wherein the functional layer comprises a silicon, polysilicon, or nitride layer.

36. The method of claim 33 wherein the functional layer on the standing feature is a membrane layer adapted for vibrating in relation to the first conductive layer upon a suitable transducing excitation.

37. The method of claim 26 wherein the forming the recess on the major surface of the first conductive layer comprises:

growing and patterning a first oxide layer over the major surface of the first conductive layer, the first oxide layer having an opening leaving a corresponding part of the major surface of the first conductive layer uncovered;

growing a second oxide layer over the first oxide layer including the opening such that the second oxide layer has a first depth into the first conductive layer at where the opening is located and a second depth into the first conductive layer at positions covered by the first oxide layer, the first depth being greater than the second depth; and

removing the first oxide layer and the second oxide layer.

38. The method of claim 26 wherein the forming the recess on the major surface of the first conductive layer comprises:

growing a first oxide layer over the major surface of the first conductive layer;

growing a nitride layer over the first oxide layer;

patterning the first oxide layer to form an opening leaving a corresponding part of the major surface of the first conductive layer uncovered;

patterning the nitride layer to form an opening coincide with the opening of the first oxide layer;

growing a second oxide layer over the opening of the first oxide layer and the nitride layer such that the second oxide layer reaches a desired depth into the first conductive layer at where the opening is located; and

removing the nitride layer, the first oxide layer and the second oxide layer.

39. The method of claim 26 in which the forming the recess below the major surface of the first conductive layer further comprises forming the recess by oxidation.

40. The method of claim 26 in which the forming the standing feature of the insulating material further comprises using oxide growth to form the standing feature.

41. The method of claim 26, wherein the distance greater than the electrode separation gap is at least 25% greater than the electrode separation gap.

42. A method for fabricating a micro-electro-mechanical transducer having two electrodes separated by an insulator with an insulation extension, the method comprising:

forming a patterned trench over a major surface of a substrate by removing material of the substrate, wherein the patterned trench comprises thin lines of unremoved material of the substrate;

oxidizing the thin lines of unremoved material of the substrate in the patterned trench such that the patterned trench constitutes an insulator;

patterning and etching the major surface of the substrate such that the insulator has a top end standing above the substrate; and

placing a top conductive layer over the top end of the insulator.

43. A method for fabricating a micro-electro-mechanical transducer, the method comprising:

forming a trench over a major surface of a substrate by removing material of the substrate;

filling the trench with an insulating material;

patterning and etching the major surface of the substrate such that the insulating material in the trench has a standing feature with a top end standing above the substrate; and

placing a top conductive layer over the top end of the standing feature of the insulating material, wherein;

the major surface of the substrate and an opposing surface of the top conductive layer define an electrode separation gap therebetween, and

the standing feature of the insulating material includes an insulation extension portion that extends into the trench beyond the electrode separation gap, and a main portion, directly above the trench, and that extends from the major surface to the opposing surface, wherein;

a first conductive surface of the trench directly under the main portion is separated from a second conductive surface of the opposing surface directly above the main portion, and

a separation between a nearest portion of the first conductive surface and the second conductive surface is by a distance greater than the electrode separation gap.

44. The method of claim 43 in which the filling the trench with the insulating material further comprises using oxide growth to form the standing feature.

45. The method of claim 43, wherein the distance greater than the electrode separation gap is at least 25% greater than the electrode separation gap.