Direct wafer bonded 2-D CUMT array

• US 20090122651A1
• Filed: 10/20/2008
• Published: 05/14/2009
• Est. Priority Date: 10/18/2007
• Status: Active Grant

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Khuri-Yakub, Butrus T., Kupnik, Mario
• Granted Patent: US 7,846,102 B2
• Time in Patent Office: Days
• US Class Current: 367/181
• CPC Class Codes: B06B 1/0292 Electrostatic transducers, e.g. electret-type
• CPC Class Codes: H02N 1/008 Laterally driven motors, e.g. of the comb-drive type
• CPC Class Codes: Y10T 29/49005 Acoustic transducer

Abstract

A capacitive micromachined ultrasonic transducer (CMUT) array connected to a separate electronic unit is provided. The CMUT array includes at least two active elements, a ground element at the array end, and a non-active element having isolation trenches disposed between the active and ground elements. The active element includes a doped first silicon layer, a doped second silicon layer, and a first insulating layer disposed there between. A cavity is in the first silicon layer having a cross section that includes vertical portions disposed at each end of a horizontal portion, and the vertical portion spans from the first insulating layer through the first silicon layer such that a portion of the first silicon layer is isolated by the first insulating layer and the cavity. A membrane layer on the first silicon layer spans the cavity. A bottom electrode is disposed on the bottom of the second silicon layer.

Claims

1. A capacitive micromachined ultrasonic transducer (CMUT) array comprising:

a. a silicon on insulator (SOI) substrate, wherein said SOI substrate comprises a doped first silicon layer and a first insulating layer;

b. a doped second silicon layer, wherein said first insulating layer is disposed between said first silicon layer and said second silicon layer;

c. at least two active elements, wherein each said active element is separated by an isolation trench, wherein said isolation trench is disposed through at least said SOI second silicon layer and surrounds said active element, wherein said first silicon layer provides mechanical support between said active elements;

d. at least one cell disposed in said active element, wherein said cell comprises;

i. a cavity in said first silicon layer, wherein a cross section of said cavity comprises a horizontal cavity portion on top of vertical cavity portions disposed at each end of said horizontal cavity portion, wherein said vertical cavity portion spans from said first insulating layer through said first silicon layer, wherein a portion of said first silicon layer is isolated by said first insulating layer and said cavity;

ii. a membrane layer, wherein said membrane layer is disposed on said first silicon layer top surface, wherein said membrane layer spans across at least one said cavity; and

iii. a bottom electrode, wherein said bottom electrode is disposed on a bottom surface of said second silicon layer;

e. at least one ground contact element, wherein said ground contact element is isolated from said active elements by at least one said trench surrounding said ground contact element, wherein said ground contact element comprises;

i. a ground electrode, wherein said ground electrode is disposed on a bottom surface of said doped second silicon layer;

ii. at least one ground conductive via, wherein said ground conductive via is disposed from said ground electrode to said SOI first silicon layer; and

iii. a conductive top layer, wherein said conductive top layer electrically conducts with said membrane layer, wherein said conductive top layer electrically conducts with said ground conductive via through said SOI first silicon layer, wherein said ground conductive via electrically conducts with said ground electrode, wherein said ground electrodes conduct with said membrane layer; and

f. a separate electronic unit, wherein said bottom electrodes and said ground electrode are conductively connected to said electronic unit, wherein said ground contact elements are disposed at an end of said array.

2. The CMUT array of claim 1, wherein said separate electronic unit is selected from a group consisting of a printed circuit board, an integrated circuit, a wafer, a flexible printed circuit board, connection pins and bonding wires.

3. The CMUT array of claim 2, wherein said conductive connection to said integrated circuit comprises connecting to different channels of said integrated circuit.

4. The CMUT array of claim 1, wherein said conductive connection is selected from a group consisting of solder bump bonding, wafer bonding, soldering, integrated circuit die bonding, wire bonding, connection pins, spring force loaded connection pins, and conductive gluing techniques.

5. The CMUT array of claim 1, wherein a non-active element is disposed between said active element and said ground contact element, wherein said non-active element comprises at least an isolated said second SOI layer, wherein said isolated second SOI layer is isolated by said buried oxide layer and at least one said isolation trench.

6. The CMUT array of claim 1, wherein said active element bottom electrodes are DC biased.

7. The CMUT array of claim 1, wherein said first insulating layer is an oxide layer.

8. The CMUT array of claim 1, wherein said CMUT array further comprises a top electrode, wherein said top electrode is disposed on a top surface of said membrane layer, wherein said membrane layer is a nonconductive layer or a conductive layer selected from a group consisting of undoped silicon, silicon nitride, undoped silicon carbide, nonconductive diamond, doped silicon, doped silicon carbide, and conductive diamond.

9. The CMUT array of claim 1, wherein said membrane layer is made from a conductive material selected from a group consisting of doped silicon, doped silicon from a second said SOI substrate, silicon nitride, doped silicon carbide, doped polycrystalline silicon, undoped polycrystalline and conductive diamond, wherein said membrane layer is also an electrode.

10. The CMUT array of claim 1, wherein said CMUT array further comprises a second insulating layer, wherein said second insulating layer is disposed on a top surface of said first silicon layer, on the walls of said vertical cavity portion and on a top surface of said isolated silicon layer portion of said first silicon layer, wherein said isolated silicon layer portion is enveloped by said first insulating layer and said second insulating layer.

11. The CMUT array of claim 10, wherein said second insulating layer is an insulating oxide layer, wherein said insulating oxide layer has a thickness in a range from 10 nm to 30 μ

m.

12. The CMUT array of claim 1, wherein said cavity comprises a vacuum or a gas, wherein said gas is selected from a group consisting of air, noble gas, nitrogen, oxygen, hydrogen and carbon dioxide.

13. The CMUT array of claim 1, wherein said cell further comprises at least one conductive via, wherein said conductive via is disposed through said second silicon layer and into said isolated silicon layer, wherein said conductive via is in contact with said bottom electrode layer.

14. The CMUT array of claim 13, wherein said first silicon layer of said SOI substrate is undoped, wherein said isolated silicon layer is doped, wherein said via is a conduit for doping said isolated silicon layer.

15. The CMUT array of claim 13, wherein said conducting via has a hole diameter in a range of 1 μ

m to 100 μ

m.

16. The CMUT array of claim 1, wherein said first silicon layer has a thickness in a range from 1 μ

m to 1,000 μ

m.

17. The CMUT array of claim 1, wherein said second silicon layer has a thickness in a range from 1 μ

m to 1,000 μ

m.

18. The CMUT array of claim 1, wherein said membrane layer has a thickness in a range from 0.1 μ

m to 500 μ

m.

19. The CMUT array of claim 1, wherein said buried oxide layer has a thickness in a range from 0.01 μ

m to 60 μ

m.

20. The CMUT array of claim 1, wherein said horizontal cavity portion has a thickness in a range from 10 nm to 500 μ

m.

21. The CMUT array of claim 1, wherein said isolated silicon layer has a thickness in a range from 1 μ

m to 1,000 μ

m.

22. The CMUT array of claim 1, wherein said first insulating layer disposed on said top surface of said second silicon layer is thicker than a second insulating layer disposed on said vertical cavity portion and on said top surface of said isolated silicon layer portion.

23. The CMUT array of claim 22, wherein said second insulating layer disposed on said vertical cavity portion and said top surface of said isolated silicon layer portion has a thickness in a range of 1 nm to 10 μ

m.

24. The CMUT array of claim 1, wherein said trenches are filled with an electrically insulating material, wherein said insulating material is selected from a group consisting of, air, epoxy, low temperature oxide, silicon nitride, polymer, PDMS, parylene, spin on glass, polyimide, TEOS, rubber, PMMA, and gel.

25. A method of fabricating a capacitive micromachined ultrasonic transducer (CMUT) array comprising:

a. providing a first silicon on insulator (SOI) substrate, wherein said first SOI substrate comprises a doped first silicon layer and a first insulating layer;

b. providing a doped second silicon layer, wherein said first insulating layer is disposed between said first silicon layer and said second silicon layer;

c. forming at least two active elements, wherein said active element is separated by an isolation trench surrounding said active element, wherein said trench is disposed through at least said SOI second silicon layer, wherein said first silicon layer provides mechanical support between said active elements;

d. forming at least one cell in said active element comprising;

i. forming at least one horizontal cavity portion in said first silicon layer;

ii. forming a vertical cavity portion at each end of said at least one horizontal cavity portion, wherein said vertical cavity portion spans from said first insulating layer through said first silicon layer;

iii. depositing a second insulating layer on said on a top surface of said first silicon layer, on the walls of said vertical cavity portion and on a top surface of said isolated silicon layer portion of said first silicon layer, wherein said isolated silicon layer portion is enveloped by said first insulating layer and said second insulating oxide layer;

iv. bonding a silicon substrate to said second insulating layer of said top surface of said first silicon layer, wherein a bottom region of said silicon substrate is a conductive membrane layer; and

v. removing a top region of said silicon substrate, wherein said silicon substrate bottom region forms said membrane layer across at least one said cavity;

e. forming at least one ground contact element, wherein said ground contact element is isolated from said active elements by at least one said trench surrounding said ground contact element, wherein said ground contact element comprises;

i. a ground electrode, wherein said ground electrode is disposed on a bottom surface of said doped second silicon layer;

ii. at least one ground conductive via, wherein said ground conductive via is disposed from said ground electrode to said SOI first silicon layer; and

iii. a conductive top layer, wherein said conductive top layer electrically conducts with said membrane layer, wherein said conductive top layer electrically conducts with said ground conductive via through said SOI first silicon layer, wherein said ground conductive via electrically conducts with said ground electrode, wherein said ground electrodes conduct with said membrane layer; and

f. providing a separate electronic unit, wherein said bottom electrodes and said ground electrodes are conductively connected to said electronic unit, wherein said ground contact elements are disposed at an end of said array.

26. The method of claim 25 further comprises depositing a top electrode on a top surface of said membrane layer, wherein said membrane layer is a nonconductive layer or a conductive layer selected from a group consisting of undoped silicon, silicon nitride, undoped silicon carbide, nonconductive diamond, doped silicon, doped silicon from a second said SOI substrate, silicon nitride, doped silicon carbide, and conductive diamond.

27. The method of claim 25, wherein said membrane layer is made from a conductive material selected from a group consisting of doped silicon, doped silicon from a second said SOI substrate, doped silicon carbide, and conductive diamond, wherein said membrane layer is also an electrode.

28. The method of claim 25, wherein before removing said top region of said silicon substrate, said method further comprises:

a. providing at least one contact hole disposed through said second silicon layer and into said isolated silicon layer; and

b. depositing a conductive layer in said contact hole, wherein said conductive layer in said contact hole provides a conductive via to said isolated silicon layer from said bottom electrode layer.

29. The method of claim 25, wherein said first SOI substrate comprises an undoped first silicon layer, wherein said isolated silicon layer is undoped, wherein before removing said top region of said silicon substrate, said method further comprises:

a. providing at least one contact hole disposed through said second silicon layer and into said isolated silicon layer;

b. doping said undoped isolated silicon layer through said contact hole; and

c. depositing a conductive layer in said contact hole, wherein said conductive layer in said contact hole provides a conductive via to said isolated silicon layer from said bottom electrode layer.

30. The method of claim 25, wherein said insulating layer disposed on said vertical cavity portion and said top surface of said isolated silicon layer portion has a thickness in a range of 1 nm to 10 μ

m.

31. The method of claim 25, wherein said bonding of said silicon substrate to said insulating layer is done in a vacuum or in a gas, wherein said cavity comprises said vacuum or said gas, wherein said gas is selected from a group consisting of air, noble gas, nitrogen, oxygen, hydrogen and carbon dioxide.

32. The method of claim 25, wherein said conductive bottom region of said silicon substrate is made from conductive material selected from a group consisting of doped silicon, doped silicon carbide, and conductive diamond.

33. The method of claim 25, wherein said separate electronic unit is selected from a group consisting of a printed circuit board, an integrated circuit, a wafer, a flexible printed circuit board, connection pins and bonding wires.

34. The method of claim 33, wherein said conductive connection to said integrated circuit comprises connecting to different channels of said integrated circuit.

35. The method of claim 25, wherein said conductive connection is selected from a group consisting of solder bump bonding, wafer bonding, soldering, integrated circuit die bonding, wire bonding, connection pins, spring force loaded connection pins, and conductive gluing techniques.

36. The method of claim 25, wherein a non-active element is disposed between said active element and said ground contact element, wherein said non-active element comprises at least an isolated said second SOI layer, wherein said isolated second SOI layer is isolated by said buried oxide layer and at least one said isolation trench.

37. The method of claim 25, wherein said active element bottom electrodes are DC biased.

38. The method of claim 25, wherein said trenches are filled with an electrically insulating material, wherein said insulating material is selected from a group consisting of, air, epoxy, low temperature oxide, silicon nitride, polymer, PDMS, parylene, spin on glass, polyimide, TEOS, rubber, PMMA, and gel.

39. A method of fabricating a capacitive micromachined ultrasonic transducer (CMUT) array comprising:

a. providing a first silicon on insulator (SOI) substrate, wherein said first SOI substrate comprises a doped first silicon layer and a first insulating layer;

b. providing a doped second silicon layer, wherein said first insulating layer is disposed between said first silicon layer and said second silicon layer;

c. forming at least two active elements, wherein said active element is separated by an isolation trench surrounding said active element, wherein said trench is disposed through at least said SOI second silicon layer, wherein said first silicon layer provides mechanical support between said active elements;

d. forming at least one cell in said active element comprising;

i. forming at least one horizontal cavity portion in said first silicon layer;

ii. forming a vertical cavity portion at each end of said at least one horizontal cavity portion, wherein said vertical cavity portion spans from said first insulating layer through said first silicon layer;

iii. depositing a second insulating layer on said on a top surface of said first silicon layer, on the walls of said vertical cavity portion and on a top surface of said isolated silicon layer portion of said first silicon layer, wherein said isolated silicon layer portion is enveloped by said first insulating layer and said second insulating oxide layer;

iv. bonding a silicon substrate to said second insulating layer of said top surface of said first silicon layer, wherein a bottom region of said silicon substrate is a conductive membrane layer; and

v. removing a top region of said silicon substrate, wherein said silicon substrate bottom region forms said membrane layer across at least one said cavity;

e. forming at least one ground contact element, wherein said ground contact element is isolated from said active elements by at least one said trench surrounding said ground contact element, wherein said ground contact element comprises;

i. a ground electrode, wherein said ground electrode is disposed on a bottom surface of said doped second silicon layer;

ii. at least one ground conductive via, wherein said ground conductive via is disposed from said ground electrode to said SOI first silicon layer; and

iii. a conductive top layer, wherein said conductive top layer electrically conducts with said membrane layer, wherein said conductive top layer electrically conducts with said ground conductive via through said SOI first silicon layer, wherein said ground conductive via electrically conducts with said ground electrode, wherein said ground electrodes conduct with said membrane layer; and

f. providing a separate electronic unit, wherein said bottom electrodes and said ground electrodes are conductively connected to said electronic unit, wherein said ground contact elements are disposed at an end of said array.

40. The method of claim 39 further comprises depositing a top electrode on a top surface of said membrane layer, wherein said membrane layer is a nonconductive layer or a conductive layer selected from a group consisting of undoped silicon, silicon nitride, undoped silicon carbide, nonconductive diamond, doped silicon, doped silicon from a second said SOI substrate, silicon nitride, doped silicon carbide, and conductive diamond.

41. The method of claim 39, wherein said membrane layer is made from a conductive material selected from a group consisting of doped silicon, doped silicon from a second said SOI substrate, doped silicon carbide, and conductive diamond, wherein said membrane layer is also an electrode.

42. The method of claim 39, wherein before removing said top region of said silicon substrate, said method further comprises:

a. providing at least one contact hole disposed through said second silicon layer and into said isolated silicon layer; and

b. depositing a conductive layer in said contact hole, wherein said conductive layer in said contact hole provides a conductive via to said isolated silicon layer from said bottom electrode layer.

43. The method of claim 39, wherein said first SOI substrate comprises an undoped first silicon layer, wherein said isolated silicon layer is undoped, wherein before removing said silicon substrate top layer, said method further comprises:

a. providing at least one contact hole disposed through said second silicon layer and into said isolated silicon layer;

b. doping said undoped isolated silicon layer through said contact hole; and

c. depositing a conductive layer in said contact hole, wherein said conductive layer in said contact hole provides a conductive via to said isolated silicon layer from said bottom electrode layer.

44. The method of claim 39, wherein said bonding of said silicon substrate to said top surface of said first silicon layer is done in a vacuum or in a gas wherein said cavity comprises said vacuum or said gas, wherein said gas is selected from a group consisting of air, noble gas, nitrogen, oxygen, hydrogen, and helium.

45. The method of claim 39, wherein said separate electronic unit is selected from a group consisting of a printed circuit board, an integrated circuit, a wafer, a flexible printed circuit board, connection pins and bonding wires.

46. The method of claim 45, wherein said conductive connection to said integrated circuit comprises connecting to different channels of said integrated circuit.

47. The method of claim 39, wherein said conductive connection is selected from a group consisting of solder bump bonding, wafer bonding, soldering, integrated circuit die bonding, wire bonding, connection pins, spring force loaded connection pins, and conductive gluing techniques.

48. The method of claim 39, wherein a non-active element is disposed between said active element and said ground contact element, wherein said non-active element comprises at least an isolated said second SOI layer, wherein said isolated second SOI layer is isolated by said buried oxide layer and at least one said isolation trench.

49. The method of claim 39, wherein said active element bottom electrodes are DC biased.

50. The method of claim 39, wherein said trenches are filled with an electrically insulating material, wherein said insulating material is selected from a group consisting of, air, epoxy, low temperature oxide, silicon nitride, polymer, PDMS, parylene, spin on glass, polyimide, TEOS, rubber, PMMA, and gel.

51. A capacitive micromachined ultrasonic transducer (CMUT) array comprising:

a. a doped first silicon layer;

b. a first insulating layer, wherein said doped first silicon layer is disposed on said first insulating layer;

c. at least two active elements;

d. at least one cell disposed in said active element, wherein said cell comprises;

i. a cavity in said first silicon layer, wherein a cross section of said cavity comprises a horizontal cavity portion on top of vertical cavity portions disposed at each end of said horizontal cavity portion, wherein said vertical cavity portion spans from said first insulating layer through said first silicon layer, wherein a portion of said first silicon layer is isolated by said first insulating layer and said cavity;

ii. a membrane layer, wherein said membrane layer is disposed on said first silicon layer top surface, wherein said membrane layer spans across at least one said cavity; and

iii. a bottom electrode, wherein said bottom electrode is disposed on a bottom surface of said second silicon layer;

e. at least one ground contact element, wherein said ground contact element is isolated from said active elements by a trench disposed through at least said SOI second silicon layer, wherein said ground contact element comprises;

i. a ground electrode, wherein said ground electrode is disposed on a bottom surface of said doped second silicon layer;

ii. at least one ground conductive via, wherein said ground conductive via is disposed from said ground electrode to said SOI first silicon layer; and

iii. a conductive top layer, wherein said conductive top layer electrically conducts with said membrane layer, wherein said conductive top layer electrically conducts with said ground conductive via through said SOI first silicon layer, wherein said ground conductive via electrically conducts with said ground electrode, wherein said ground electrodes conduct with said membrane layer; and

f. a separate electronic unit, wherein said bottom electrodes and said ground electrode is conductively connected to said electronic unit, wherein said ground contact elements are disposed at an end of said array.

52. The CMUT array of claim 51 further comprises at least one isolation trench, wherein each said active element is separated by said isolation trench, wherein said isolation trench is disposed through at least said first insulating layer and surrounds said active element, wherein said first silicon layer provides mechanical support between said active elements.

53. The CMUT array of claim 51, wherein said trenches are filled with an electrically insulating material, wherein said insulating material is selected from a group consisting of, air, epoxy, low temperature oxide, silicon nitride, polymer, PDMS, parylene, spin on glass, polyimide, TEOS, rubber, PMMA, and gel.

54. The CMUT array of claim 51, wherein said first insulating layer is selected from a group consisting of oxide, quartz, glass, pyrex, soda lime, borosilicate, borofloat glass, fused quartz, fused silica, and sapphire.

55. The CMUT array of claim 51, wherein said CMUT array further comprises a top electrode, wherein said top electrode is disposed on a top surface of said membrane layer, wherein said membrane layer is a nonconductive layer or a conductive layer selected from a group consisting of undoped silicon, silicon nitride, undoped silicon carbide, nonconductive diamond, doped silicon, doped silicon carbide, and conductive diamond.

56. The CMUT array of claim 51, wherein said membrane layer is made from a conductive material selected from a group consisting of doped silicon, doped silicon from a second said SOI substrate, silicon nitride, doped silicon carbide, doped polycrystalline silicon, undoped polycrystalline and conductive diamond, wherein said membrane layer is also an electrode.

57. The CMUT array of claim 51, wherein said CMUT array further comprises a second insulating layer, wherein said second insulating layer is disposed on a top surface of said first silicon layer, on the walls of said vertical cavity portion and on a top surface of said isolated silicon layer portion of said first silicon layer, wherein said isolated silicon layer portion is enveloped by said first insulating layer and said second insulating layer.

58. The CMUT array of claim 57, wherein said second insulating layer is an insulating oxide layer, wherein said insulating oxide layer.

59. The CMUT array of claim 57, wherein said second insulating layer has a thickness in a range of 1 nm to 30 μ

m.

60. The CMUT array of claim 51, wherein said cavity comprises a vacuum or a gas, wherein said gas is selected from a group consisting of air, noble gas, nitrogen, oxygen, hydrogen and carbon dioxide.

61. The CMUT array of claim 51, wherein said CMUT array further comprises at least one conductive via, wherein said conductive via is disposed through said first insulating layer and into said isolated silicon layer, wherein said conductive via is in contact with said bottom electrode layer.

62. The CMUT array of claim 61, wherein said conducting via has a hole diameter in a range of 1 μ

m to 100 μ

m.

63. The CMUT array of claim 51, wherein said first insulating layer has a thickness in a range from 1 μ

m to 1,000 μ

m.

64. The CMUT array of claim 51, wherein said first silicon layer has a thickness in a range from 1 μ

m to 1,000 μ

m.

65. The CMUT array of claim 51, wherein said horizontal cavity portion has a thickness in a range from 10 nm to 500 μ

m.

66. The CMUT array of claim 51, wherein said isolated silicon layer has a thickness in a range from 1 μ

m to 1,000 μ

m.

67. The CMUT array of claim 51, wherein said separate electronic unit is selected from a group consisting of a printed circuit board, an integrated circuit, a wafer, a flexible printed circuit board, connection pins and bonding wires.

68. The CMUT array of claim 67, wherein said conductive connection to said integrated circuit comprises connecting to different channels of said integrated circuit.

69. The CMUT array of claim 51, wherein said conductive connection is selected from a group consisting of solder bump bonding, wafer bonding, soldering, integrated circuit die bonding, wire bonding, connection pins, spring force loaded connection pins, and conductive gluing techniques.

70. The CMUT array of claim 51, wherein said active element bottom electrodes are DC biased.