Mems process and device

• US 20070284682A1
• Filed: 03/20/2007
• Published: 12/13/2007
• Est. Priority Date: 03/20/2006
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Wolfson Microelectronics plc
• Inventors: Begbie, Mark, Traynor, Anthony, Laming, Richard
• Granted Patent: US 7,781,249 B2
• Time in Patent Office: Days
• US Class Current: 257/416
• CPC Class Codes: B81B 2201/0257 Microphones or microspeakers
• CPC Class Codes: B81B 2203/0127 Diaphragms, i.e. structures separating two media that can control the passage from one medium to another; Membranes, i.e. diaphragms with filtering function
• CPC Class Codes: B81C 1/00158 Diaphragms, membranes manufacture process for semi-permeable inorganic membranes B01D67/0039
• CPC Class Codes: B81C 2201/0109 Sacrificial layers not provided for in B81C2201/0107 - B81C2201/0108
• CPC Class Codes: B81C 99/004 during manufacturing
• CPC Class Codes: H04R 19/005 using semiconductor materials
• CPC Class Codes: H04R 2307/207 Shape aspects of the outer suspension of loudspeaker diaphragms
• CPC Class Codes: H04R 7/10 comprising superposed layers in contact
• CPC Class Codes: H04R 7/18 at the periphery
• CPC Class Codes: Y10S 977/733 Nanodiaphragm

Abstract

A MEMS device, for example a capacitive microphone, comprises a flexible membrane 11 that is free to move in response to pressure differences generated by sound waves. A first electrode 13 is mechanically coupled to the flexible membrane 11, and together form a first capacitive plate of the capacitive microphone device. A second electrode 23 is mechanically coupled to a generally rigid structural layer or back-plate 14, which together form a second capacitive plate of the capacitive microphone device. The capacitive microphone is formed on a substrate 1, for example a silicon wafer. A back-volume 33 is provided below the membrane 11, and is formed using a “back-etch” through the substrate 1. A first cavity 9 is located directly below the membrane 11, and is formed using a first sacrificial layer during the fabrication process. Interposed between the first and second electrodes 13 and 23 is a second cavity 17, which is formed using a second sacrificial layer during the fabrication process. A plurality of bleed holes 15 connect the first cavity 9 and the second cavity 17. Acoustic holes 31 are arranged in the back-plate 14 so as to allow free movement of air molecules, such that the sound waves can enter the second cavity 17. The first and second cavities 9 and 17 in association with the back-volume 33 allow the membrane 11 to move in response to the sound waves entering via the acoustic holes 31 in the back-plate 14. The provision of first and second sacrificial layers has the advantage of protecting the membrane during manufacture, and disassociating the back etch process from the definition of the membrane. The bleed holes 15 aid with the removal of the first and second sacrificial layers. The bleed holes 15 also contribute to the operating characteristics of the microphone.

Claims

1. A method of fabricating a micro-electrical-mechanical system (MEMS) transducer on a substrate, the method comprising:

depositing a first sacrificial layer with respect to a first side of a membrane;

depositing a second sacrificial layer with respect to a second side of the membrane; and

removing the first and second sacrificial layers so as to form a MEMS transducer in which the membrane is moveable.

2. A method as claimed in claim 1, wherein the membrane is configured to be moveable in response to pressure.

3. A method as claimed in claim 2, wherein the membrane is configured to be moveable in response to absolute pressure.

4. A method as claimed in claim 2, wherein the membrane is configured to be moveable in response to differential pressure.

5. A method as claimed in claim 4, wherein the membrane is configured to be moveable in response to pressure differences generated by sound waves.

6. A method as claimed in claim 4, wherein the transducer is a microphone.

43. A method as claimed in claim 1, wherein at least one of the sacrificial layers comprises an organic material.

44. A method as claimed in claim 43, wherein at least one of the sacrificial layers is a polyacrylate, a polyimide, a polyamide, a polycarbonate, or a polyethylene terapthalate.

45. A method as claimed in claim 1, wherein the steps of removing the first and/or second sacrificial layers comprises using a dry etch process.

46. A method as claimed in claim 45, wherein the step of removing the first and/or second sacrificial layers comprises using a plasma oxygen etch process.

47. A method as claimed in claim 46, wherein the plasma oxygen is mixed with one or more of the following gases:

SF6, CF4, CH2F2, H2, Ar, C4F8, C2F6, CFH3.

7. A method of fabricating a micro-electrical-mechanical system (MEMS) microphone on a substrate, the method comprising:

depositing first and second electrodes;

depositing a membrane, the membrane being mechanically coupled to the first electrode; and

depositing a back plate, the back plate being mechanically coupled to the second electrode;

wherein the step of depositing the membrane further comprises the step of depositing the membrane on a first sacrificial layer; and

wherein the method further comprises the steps of;

depositing a second sacrificial layer in an area between the first and second electrodes; and

removing the first and second sacrificial layers so as to form a MEMS microphone having a first cavity beneath the membrane, and a second cavity between the first and second electrodes, such that the membrane and the first electrode are able to move relative to the second electrode.

8. A method as claimed in claim 7 further comprising the step of forming a third cavity, the third cavity connecting with the first cavity to form a back volume.

9. A method as claimed in claim 7, further comprising the step of forming the first sacrificial layer on at least one structural layer, the at least one structural layer being positioned between the membrane and the substrate.

10. A method as claimed in claim 9, wherein the step of forming the third cavity comprises the step of etching the substrate and the at least one structural layer, and wherein the first sacrificial layer acts as an etch stop for the third cavity.

11. A method as claimed in claim 9, wherein the third cavity is formed such that the area of the third cavity at a plane where the third cavity meets the first sacrificial layer is smaller than the area of the first sacrificial layer, such that the area of the third cavity in said plane falls substantially within the perimeter of the first sacrificial layer.

12. A method as claimed in claims 8, wherein the third cavity is formed using a dry etch process.

13. A method as claimed in claim 8, wherein the third cavity is formed using a wet etch process.

14. A method as claimed in any one of claim 7, further comprising the step of forming a plurality of openings through the membrane material between the first sacrificial layer and the second sacrificial layer.

15. A method as claimed in claim 14, wherein the step of forming the plurality of openings comprises the steps of:

etching a plurality of holes in the material of the membrane; and

depositing the second sacrificial layer over the membrane and the holes etched in the material of the membrane, such that the second sacrificial layer fills the holes etched in the material of the membrane.

16. A method as claimed in claim 14, wherein said openings are positioned in an outer area of the first sacrificial layer, the outer area of the first sacrificial layer being outside the area defined by the third cavity at the plane where the third cavity meets the first sacrificial layer.

17. A method as claimed in any one of claim 14, wherein the plurality of openings are arranged such that an area etched through a first opening substantially overlaps an area etched through an adjacent opening.

18. A method as claimed in claim 17, wherein the openings having a spacing of less than 100 um.

19. A method as claimed in claim 16, wherein said openings are formed on one or more concentric circles in the outer area of the first sacrificial layer.

23. A method as claimed in claim 7, wherein the step of removing the first and second sacrificial layers comprises the steps of:

removing the first sacrificial layer; and

subsequently removing the second sacrificial layer.

24. A method as claimed in claim 23 wherein the first and second sacrificial layers are removed by etching in a first direction, the first direction being from the substrate side towards the first sacrificial layer.

25. A method as claimed in any one of claim 7, wherein the step of removing the first and second sacrificial layers comprises the steps of:

removing the second sacrificial layer; and

subsequently removing the first sacrificial layer.

26. A method as claimed in claim 25 wherein the first and second sacrificial layers are removed by etching in a second direction, the second direction being from the back plate side towards the second sacrificial layer.

27. A method as claimed in any one of claim 7, comprising forming a plurality of MEMS microphones on a wafer.

28. A method as claimed in claim 27 further comprising the steps of:

placing the wafer on a carrier between the step of removing the inner portion of the first sacrificial layer and the step of removing the second sacrificial layer and the outer area of the first sacrificial layer; and

singulating the wafer prior to the step of removing the second sacrificial layer and the outer area of the first sacrificial layer.

29. A method as claimed in claim 27, further comprising the steps of:

placing the wafer on a carrier prior to the step of removing the inner portion of the first sacrificial layer; and

singulating the wafer prior to the step of removing the second sacrificial layer and the outer area of the first sacrificial layer.

30. A method as claimed in claim 29, further comprising the step of perforating the carrier prior to the step of removing the inner portion of the first sacrificial layer.

31. A method as claimed in claim 29, wherein the carrier is configured to allow the inner portion of the first sacrificial layer to be etched via the back volume while located on the carrier.

32. A method as claimed in claim 31, wherein the carrier comprises one or more channels or trenches or protrusions for allowing the inner portion of the first sacrificial layer to be etched via the back volume while located on the carrier.

33. A method as claimed in claim 29, wherein the substrate is configured to allow the inner portion of the first sacrificial layer to be etched via the back volume while located on the carrier.

34. A method as claimed in claim 33, wherein the substrate comprises one or more channels or trenches or protrusions for allowing the inner portion of the first sacrificial layer to be etched via the back volume while located on the carrier.

35. A method as claimed in claim 28, wherein the step of singulating comprises cutting the substrate using a saw.

36. A method as claimed in claim 28, wherein the step of singulating comprises scribing and cleaving the substrate.

37. A method as claimed in claim 28, wherein the step of singulating comprises cutting the substrate using a laser.

38. A method as claimed in claim 28, wherein the carrier is a dicing tape.

39. A method as claimed in claim 38, wherein the dicing tape is able to sustain temperatures that are higher than the temperature of the process for removing the sacrificial layers.

40. A method as claimed in claim 31, wherein the carrier is made from glass or silicon wafer.

20. A method as claimed in 7, wherein the step of removing the first and second sacrificial layers comprises:

removing an inner area of the first sacrificial layer;

removing the second sacrificial layer; and

removing an outer area of the first sacrificial layer.

21. A method as claimed in claim 20, wherein the step of removing the second sacrificial layer and the step of removing the outer area of the first sacrificial layer are performed separately.

22. A method as claimed in claim 20 comprising:

removing the inner area of the first sacrificial layer by etching in a first direction, the first direction being from the substrate side towards the first sacrificial layer; and

removing the second sacrificial layer and the outer area of the first sacrificial layer by etching in a second direction, the second direction being from the back plate side towards the second sacrificial layer.

41. A method as claimed in 7, further comprising the step of forming a plurality of openings in the back plate.

42. A method as claimed in claim 41, further comprising the step of depositing a protective layer on the back plate.

48. A micro-electrical-mechanical system (MEMS) capacitive microphone comprising:

first and second electrodes;

a membrane that it is mechanically coupled to the first electrode; and

a back plate that it is mechanically coupled to the second electrode;

wherein the first and second electrodes each have a diameter that is different to the diameter of the membrane.

49. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48, wherein the diameters of the first and second electrodes are smaller than the diameter of the membrane.

50. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 49, wherein the diameters of the first and second electrodes are less than 90% of the diameter of the membrane.

51. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 49, wherein the diameters of the first and second electrodes are less than 80% of the diameter of the membrane.

52. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 49, wherein the diameters of the first and second electrodes are between 50-70% of the diameter of the membrane.

53. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48, wherein the diameters of the first and second electrodes are substantially equal.

85. An electronic device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

86. A communications device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

87. A portable telephone device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

88. An audio device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

89. A computer device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

90. A vehicle comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

91. A medical device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

92. An industrial device comprising a micro-electrical-mechanical system (MEMS) microphone as claimed in claim 48.

54. A micro-electrical-mechanical system (MEMS) capacitive microphone comprising:

first and second electrodes;

a membrane that it is mechanically coupled to the first electrode; and

a back plate that it is mechanically coupled to the second electrode;

wherein the second electrode comprises one or more openings.

55. A micro-electrical-mechanical system (MEMS) capacitive microphone as claimed in claim 54, wherein the one or more openings correspond to one or more openings formed in the back plate.

56. A micro-electrical-mechanical system (MEMS) capacitive microphone as claimed in claim 55, wherein at least one of the openings in the second electrode is larger than a corresponding opening in the back plate.

57. A micro-electrical-mechanical system (MEMS) capacitive microphone as claimed in claim 54, wherein the membrane comprises one or more openings corresponding to one or more openings in the first electrode.

58. A method as claimed in claim 57, wherein at least one of the openings in the first electrode is larger than a corresponding opening in the membrane.

59. A method of fabricating a micro-electrical-mechanical system (MEMS) microphone, the method comprising the steps of:

depositing first and second electrodes;

depositing a membrane that it is mechanically coupled to the first electrode; and

depositing a back plate that it is mechanically coupled to the second electrode;

wherein the step of depositing the second electrode comprises the step of forming a predetermined pattern in the second electrode, and wherein the predetermined pattern comprises one or more openings.

60. A method as claimed in claim 59, wherein the one or more openings correspond to one or more openings formed in the back plate.

61. A method as claimed in claim 60, further comprising the step of forming at least one of the openings in the second electrode to be larger than a corresponding opening in the back plate.

62. A method of testing a plurality of micro-electrical-mechanical system (MEMS) microphones formed on a wafer, each MEMS microphone comprising a membrane and at least one sacrificial layer, the method comprising the steps of:

attaching the wafer to a carrier;

singulating the wafer to form two or more MEMS microphones;

removing the at least one sacrificial layer; and

testing the MEMS microphones while attached to the carrier.

63. A method as claimed in claim 62, wherein the carrier comprises a dicing tape.

64. A micro-electrical-mechanical system (MEMS) microphone comprising:

a substrate;

first and second electrodes;

a membrane that it is mechanically coupled to the first electrode; and

a back plate that it is mechanically coupled to the second electrode;

and further comprising;

a first cavity beneath the membrane, the first cavity formed using a first sacrificial layer; and

a second cavity between the first and second electrodes, the second cavity formed using a second sacrificial layer.

65. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, further comprising a third cavity, the third cavity connecting with the first cavity to form a back volume.

66. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 65, wherein the third cavity is formed such that the area of the third cavity at a plane where the third cavity meets the first cavity is smaller than the area of the first cavity, such that the area of the third cavity in said plane falls substantially within the perimeter of the first cavity.

67. A micro-electrical-mechanical system (MEMS) microphone as claimed in any one of claims 64, further comprising a plurality of openings in the material of the membrane, the plurality of openings connecting the first cavity and the second cavity.

68. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 67, wherein the plurality of openings are located in an outer area of the first cavity, the outer area of the first cavity being outside the area defined by the third cavity at the plane where the third cavity meets the first cavity.

69. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 68, wherein said openings are formed on one or more concentric circles in the outer area of the first cavity.

70. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 67, wherein the plurality of openings are arranged such that an area etched through a first opening substantially overlaps an area etched through an adjacent opening.

71. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 70, wherein the openings having a spacing of less than 100 um.

72. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, wherein the first and second electrodes each have a diameter that is different to the diameter of the membrane.

73. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 72, wherein the diameters of the first and second electrodes are smaller than the diameter of the membrane.

74. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 73, wherein the diameters of the first and second electrodes are less than 90% of the diameter of the membrane.

75. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 73, wherein the diameters of the first and second electrodes are less than 80% of the diameter of the membrane.

76. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 73, wherein the diameters of the first and second electrodes are between 50-70% of the diameter of the membrane.

77. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 72, wherein the diameters of the first and second electrodes are substantially equal.

78. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, further comprising a plurality of openings in the back plate.

79. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 78, wherein at least some of the openings in the back plate pass through the second electrode.

80. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, further comprising one or more dimples on an inner surface of the back plate.

81. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, further comprising one or more dimples on a surface of the membrane.

82. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 81, wherein the one or more dimples are located on a surface of the membrane facing the first cavity, and on the outer area of the membrane, the outer area of the membrane being outside the area defined by the third cavity at the plane where the third cavity meets the first cavity

83. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, further comprising electronic circuitry on the same substrate as the MEMS microphone.

84. A micro-electrical-mechanical system (MEMS) microphone as claimed in claim 64, further comprising one or more electrical interface pads for connection to external electronic circuitry.