Miniature condenser microphone and fabrication method therefor

• US 20030133588A1
• Filed: 11/25/2002
• Published: 07/17/2003
• Est. Priority Date: 11/27/2001
• Status: Active Grant

• Current Assignee: Corporation for National Research Initiatives
• Original Assignee: Corporation for National Research Initiatives
• Inventors: Pedersen, Michael
• Granted Patent: US 7,146,016 B2
• Time in Patent Office: Days
• US Class Current: 381/423
• 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: B81B 3/0072 For controlling internal stress or strain in moving or flexible elements, e.g. stress compensating layers
• CPC Class Codes: H01L 2224/16 of an individual bump connector
• CPC Class Codes: H01L 2924/15151 the die mounting substrate comprising an aperture, e.g. for underfilling, outgassing, window type wire connections
• CPC Class Codes: H01L 2924/16152 Cap comprising a cavity for hosting the device, e.g. U-shaped cap
• CPC Class Codes: H04R 19/005 using semiconductor materials
• CPC Class Codes: Y10T 29/49005 Acoustic transducer
• CPC Class Codes: Y10T 29/4902 Electromagnet, transformer or inductor
• CPC Class Codes: Y10T 29/4908 Acoustic transducer

Abstract

An acoustic pressure type sensor fabricated on a supporting substrate is disclosed. The acoustic sensor is fabricated by depositing and etching a number of thin films on the supporting substrate and by machining the supporting substrate. The resulting structure contains a pressure sensitive, electrically conductive diaphragm positioned at a distance from an electrically conductive fixed electrode. In operation, the diaphragm deflects in response to an acoustic pressure and the corresponding change of electrical capacitance between the diaphragm and the fixed electrode is detected using an electrical circuit. Two or more such acoustic sensors are combined on the same supporting substrate with an interaural flexible mechanical connection, to form a directional sensor with a small surface area.

Claims

1. An acoustic transducer comprising:

a supporting substrate;

an electrode formed on the substrate;

a diaphragm positioned over the electrode, said diaphragm including an indentation to maintain a selected distance between said diaphragm and said electrode when a bias voltage is applied between the electrode and the diaphragm; and

a suspension structure formed on the substrate and attached to the diaphragm to support the diaphragm, said suspension structure having a restoring force equal to or greater than surface contact forces between the diaphragm and supporting substrate;

whereby an electrostatic force associated with the bias voltage overcomes the restoring force of the suspension structure, causing the diaphragm to be pulled towards the supporting substrate; and

whereby said electrode includes a plurality of holes to allow air in a gap between the electrode and the diaphragm to escape, thereby reducing acoustical damping in the transducer.

2. The acoustic transducer according to claim 1 further comprising metal pads formed on said supporting substrate to provide an electrical connection to said diaphragm and said electrode.

3. The acoustic transducer according to claim 1, wherein the indentation formed on the diaphragm is an annular indentation.

4. The acoustic transducer according to claim 1, wherein the indentation formed on the diaphragm is of a shape that provides an acoustic seal between the front and back of the diaphragm when the electrostatic force causes the diaphragm to be pulled towards the supporting substrate to thereby enable a low frequency acoustical response of the diaphragm.

5. The acoustic transducer according to claim 3, wherein the diaphragm makes physical contact with the supporting substrate at the annular indentation to thereby set an operational air gap in the acoustic transducer.

6. The acoustic transducer according to claim 1, wherein when the electrostatic force causes the diaphragm to be pulled towards the supporting substrate an incident sound pressure wave produces a pressure differential over the diaphragm, causing it to deflect from its initial position and change an electrical capacitance between the diaphragm and the electrode.

7. The acoustic transducer according to claim 6 further comprising an electronic detection circuit for measuring the change in electrical capacitance between the diaphragm and electrode.

8. The acoustic transducer according to claim 7, wherein the electronic detection circuit is integrated onto the substrate.

9. The acoustic transducer according to claim 1, wherein diaphragm thickness, diaphragm size, diaphragm material and the selected distance between the diaphragm and electrode are selected to meet selected transducer specifications of resonance frequency, sensitivity, and bias voltage.

10. The acoustic transducer according to claim 1, wherein the bias voltage is a DC bias voltage.

11. The acoustic transducer according to claim 10, wherein the DC bias voltage is approximately 60% of the diaphragm'"'"'s collapse voltage.

12. The acoustic transducer according to claim 10, wherein the DC bias voltage is in the range of 1 V to 20V DC.

13. The acoustic transducer according to claim 1 in which said supporting substrate is silicon.

14. The acoustic transducer according to claim 1 in which said supporting substrate includes pre-processed electronic circuits.

15. The acoustic transducer according to claim 1 in which said electrode is made from one or more materials selected from the group consisting of silicon, silicon-germanium alloy, silicon nitride, and silicon dioxide.

16. The acoustic transducer according to claim 1 in which said diaphragm is made from a material selected from the group consisting of polycrystalline silicon or silicon-germanium alloy.

17. The acoustic transducer according to claim 2 in which said metal pads are made from one or more materials selected from the group consisting of gold, titanium, tungsten, chromium, tin, nickel, silver, copper, aluminum, iron, palladium, lead and zinc.

18. The acoustic transducer according to claim 2, wherein a solder ball is formed on each metal pad to allow the use of flip-chip bonding for the assembly of the acoustic transducer supporting substrate into a package.

19. The acoustic transducer according to claim 1, wherein the number of holes in the electrode and their location are selected to produce an upper comer frequency of the acoustic transducer that coincides with the diaphragm'"'"'s resonance frequency to produce a flat frequency response with maximum bandwidth of the acoustic transducer.

20. The acoustic transducer according to claim 3, wherein the diaphragm or the annular indentation include one or more openings to allow a controlled amount of air to bypass the diaphragm, whereby the lower comer frequency of the acoustic transducer is controlled to be in a range from at least 300 Hz to less than 1 Hz.

21. The acoustic transducer according to claim 1, wherein the electrode and the diaphragm are electrically conductive.

22. The acoustic transducer according to claim 1, wherein the electrode is fixed.

23. The directional acoustic transducer according to claim 2, wherein a solder ball is formed on each metal pad to allow the use of flip-chip bonding for the assembly of the directional acoustic transducer supporting substrate into a package.

24. The acoustic transducer according to claim 1 further comprising an assembly for mounting the acoustic transducer on a package carrier substrate, the assembly comprising:

a conductive layer deposited on the carrier substrate for electrical interconnection between the carrier substrate and the acoustic transducer;

an under filling material for providing encapsulation of the electrical interconnection and an acoustic seal between the front and back of the acoustic transducer; and

a hollow cap attached to the carrier substrate, whereby a back volume is formed within the hollow cap and a hermetic/acoustic seal of the acoustic transducer is achieved so that the only acoustic leakage path to the back volume is through an opening made in the diaphragm to thereby control of the lower roll-off frequency of the acoustic transducer.

25. The acoustic transducer according to claim 24, wherein the under filling material is a sealant.

26. The acoustic transducer according to claim 25, wherein capillary forces that cause the filling of a gap between the acoustic transducer and the carrier substrate are controlled by providing an opening in the carrier substrate which prevents the sealant from reaching the movable parts in the acoustic transducer.

27. An acoustic transducer comprising:

supporting means;

means fixedly formed on the supporting means for conducting an electrical field;

means positioned over the fixed conducting means for conducting the electrical field and sensing acoustical pressure;

said sensing means including means for maintaining a selected distance between said sensing means and said fixed conducting means when the electrical field is applied between the sensing means and the fixed conducting means;

means formed on the supporting means and attached to the sensing means to suspend and support the sensing means whereby intrinsic mechanical stress in said sensing means and imposed stress from said supporting means are eliminated, said suspension and support means having a restoring force equal to or greater than surface contact forces between the sensing means and supporting means; and

means to provide electrical connection to said sensing means and fixed conducting means;

whereby an electrostatic force associated with the electric field overcomes the restoring force of the suspension means, causing the sensing means to be pulled towards the supporting means; and

whereby said fixed conducting means includes a plurality of holes to allow air in a gap between the fixed conducting means and the sensing means to escape, thereby reducing acoustical damping in the transducer.

28. An acoustic transducer comprising:

a supporting substrate;

an electrically conductive, fixed counter electrode formed on the substrate;

an electrically conductive diaphragm positioned over the fixed counter electrode, said diaphragm including an annular indentation to maintain a selected distance between said diaphragm and said counter electrode when a bias voltage is applied between the counter electrode and the diaphragm;

a plurality of suspension structures formed on the substrate and attached to the diaphragm to support the diaphragm whereby intrinsic mechanical stress in said diaphragm and imposed stress from said supporting substrate are eliminated, said suspension structures having a restoring force equal to or greater than stiction forces between the diaphragm and supporting substrate; and

a plurality of metal pads formed on said supporting substrate to provide an electrical connection to said diaphragm and said counter electrode;

whereby an electrostatic force associated with the bias voltage overcomes the restoring force of the suspension structures, causing the diaphragm to be pulled towards the supporting substrate; and

whereby said counter electrode includes a plurality of holes to allow air in a gap between the counter electrode and the diaphragm to escape, thereby reducing acoustical damping in the transducer.

29. The acoustic transducer according to claim 28, wherein the annular indentation formed on the diaphragm provides an acoustic seal between the front and back of the diaphragm when the electrostatic force causes the diaphragm to be pulled towards the supporting substrate to thereby enable a low frequency acoustical response of the diaphragm.

30. The acoustic transducer according to claim 28, wherein the diaphragm makes physical contact with the supporting substrate at the annular indentation to thereby set an operational air gap in the acoustic transducer.

31. The acoustic transducer according to claim 28, wherein when the electrostatic force causes the diaphragm to be pulled towards the supporting substrate an incident sound pressure wave produces a pressure differential over the diaphragm, causing it to deflect from its initial position and change an electrical capacitance between the diaphragm and the counter electrode.

32. The acoustic transducer according to claim 31 further comprising an electronic detection circuit integrated onto the substrate for measuring the change in electrical capacitance between the diaphragm and counter electrode.

33. The acoustic transducer according to claim 28, wherein the bias voltage is a DC bias voltage in the range of 1 V to 20V DC.

34. The acoustic transducer according to claim 28 in which said supporting substrate is silicon.

35. The acoustic transducer according to claim 28 in which said counter electrode is made from one or more materials selected from the group consisting of silicon, silicon-germanium alloy, silicon nitride, and silicon dioxide.

36. The acoustic transducer according to claim 28 in which said diaphragm is made from a material selected from the group consisting of polycrystalline silicon or silicon-germanium alloy.

37. The acoustic transducer according to claim 28 in which said metal pads are made from one or more materials selected from the group consisting of gold, titanium, tungsten, chromium, tin, nickel, silver, copper, aluminum, iron, palladium, lead and zinc.

38. The acoustic transducer according to claim 28, wherein a solder ball is formed on each metal pad to allow the use of flip-chip bonding for the assembly of the acoustic transducer supporting substrate into a package.

39. A directional acoustic transducer comprising:

a supporting substrate;

at least two electrodes formed in said substrate;

at least two movable diaphragms, each diaphragm being positioned over a corresponding electrode and including an indentation to maintain a fixed distance between said diaphragm and said diaphragm'"'"'s corresponding electrode when a bias voltage is applied between said diaphragm and the corresponding electrode, each said electrode including a plurality of holes to allow air in a gap between the electrode and its corresponding diaphragm to escape, thereby reducing acoustical damping therein;

a plurality of suspension structures formed on the substrate and attached to the diaphragms to support the diaphragms, the suspension structures supporting each diaphragm having a restoring force equal to or greater than surface contact forces between said diaphragm supported by said suspension structures and the supporting substrate, whereby an electrostatic force associated with the bias voltage overcomes the restoring force of said suspension structures, causing the corresponding diaphragm to be pulled towards the supporting substrate; and

a support beam formed in the supporting substrate for providing a flexible mechanical coupling between said movable diaphragms, whereby the response of each diaphragm to incoming sound pressure waves not in phase with one another will be determined by the mechanical interaction between the diaphragms provided by the mechanical coupling beam.

40. The directional acoustic transducer according to claim 39 further comprising a plurality of metal pads formed on said supporting substrate to provide electrical connections to said diaphragms and said counter electrodes.

41. The directional acoustic transducer according to claim 39, wherein the indentations formed on the diaphragms are annular indentations.

42. The directional acoustic transducer according to claim 39, wherein the indentations fonned on the diaphragms are of a shape that provides an acoustic seal between the front and back of each diaphragm when the electrostatic force causes said diaphragm to be pulled towards the supporting substrate to thereby enable a low frequency acoustical response of said diaphragm.

43. The directional acoustic transducer according to claim 41, wherein each diaphragm makes physical contact with the supporting substrate at said diaphragm'"'"'s annular indentation to thereby set an operational air gap in the acoustic transducer.

44. The directional acoustic transducer according to claim 39, wherein when the electrostatic force causes each diaphragm to be pulled towards the supporting substrate an incident sound pressure wave produces a pressure differential over the diaphragm, causing it to deflect from its initial position and change an electrical capacitance between the diaphragm and the electrode.

45. The directional acoustic transducer according to claim 44 further comprising an electronic detection circuit for measuring the change in electrical capacitance between the diaphragm and electrode.

46. The directional acoustic transducer according to claim 45, wherein the electronic detection circuit is integrated onto the substrate.

47. The directional acoustic transducer according to claim 39, wherein thickness, diaphragm size, diaphragm material of each diaphragm and the selected distance between each diaphragm and its corresponding electrode are selected to meet selected transducer specifications of resonance frequency, sensitivity, bias voltage, and directivity.

48. The directional acoustic transducer according to claim 39, wherein the bias voltage is a DC bias voltage.

49. The directional acoustic transducer according to claim 48, wherein the DC bias voltage is approximately 60% of the diaphragm'"'"'s collapse voltage.

50. The directional acoustic transducer according to claim 48, wherein the DC bias voltage is in the range of 1 V to 20V DC.

51. The directional acoustic transducer according to claim 39 in which said supporting substrate is silicon.

52. The directional acoustic transducer according to claim 39 in which said supporting substrate includes pre-processed electronic circuits.

53. The directional acoustic transducer according to claim 39 in which each said electrode is made from one or more materials selected from the group consisting of silicon, silicon-germanium alloy, silicon nitride, and silicon dioxide.

54. The directional acoustic transducer according to claim 39 in which each said diaphragm is made from a material selected from the group consisting of polycrystalline silicon or silicon-germanium alloy.

55. The directional acoustic transducer according to claim 40 in which said metal pads are made from one or more materials selected from the group consisting of gold, titanium, tungsten, chromium, tin, nickel, silver, copper, aluminum, iron, palladium, lead and zinc.

56. The directional acoustic transducer according to claim 40, wherein a solder ball is formed on each metal pad to allow the use of flip-chip bonding for the assembly of the directional acoustic transducer supporting substrate into a package.

57. The directional acoustic transducer according to claim 39, wherein the number of holes in each electrode and their location are selected to produce an optimal directional response of the acoustic transducer in a selected frequency range.

58. The directional acoustic transducer according to claim 41, wherein each diaphragm or its corresponding annular indentation includes one or more openings to allow a controlled amount of air to bypass the diaphragm, whereby the lower corner frequency of the directional acoustic transducer is controlled to be in a range from at least 300 Hz to less than 1 Hz.

59. The directional acoustic transducer according to claim 39, wherein each of the electrodes and the diaphragms are electrically conductive.

60. The directional acoustic transducer according to claim 39, wherein each of the electrodes is fixed.

61. The directional acoustic transducer according to claim 39 further comprising an assembly for mounting the directional acoustic transducer on a package carrier substrate, the assembly comprising:

a conductive layer deposited on the carrier substrate for electrical interconnection between the carrier substrate and the directional acoustic transducer;

an under filling material for providing encapsulation of the electrical interconnection and an acoustic seal between the front and back of the directional acoustic transducer; and

a hollow cap attached to the carrier substrate, whereby a back volume is formed within the hollow cap and a hermetic/acoustic seal of the directional acoustic transducer is achieved so that the only acoustic leakage path to the back volume is through an opening made in the diaphragms to thereby control the lower roll-off frequency of the directional acoustic transducer.

62. The directional acoustic transducer according to claim 61, wherein the under filling material is a sealant.

63. The directional acoustic transducer according to claim 62, wherein capillary forces that cause the filling of a gap between the directional acoustic transducer and the carrier substrate are controlled by providing an opening in the carrier substrate which prevents the sealant from reaching the movable parts in the acoustic transducer.

64. A directional acoustic transducer comprising:

supporting means;

a plurality of means fixedly formed on the supporting means for conducting an electrical field;

a plurality of means positioned over the fixed conducting means for conducting the electrical field and sensing acoustical pressure;

each said sensing means including means for maintaining a selected distance between said sensing means and a corresponding said fixed conducting means when the electrical field is applied between the sensing means and the fixed conducting means;

a plurality of means formed on the supporting means and attached to the sensing means to suspend and support the sensing means whereby intrinsic mechanical stress in said sensing means and imposed stress from said supporting means are eliminated, each of said suspension and support means having a restoring force equal to or greater than surface contact forces between corresponding sensing means and supporting means;

means to provide electrical connections to said sensing means and fixed conducting means; and

means for providing a flexible mechanical coupling between said plurality of sensing means, whereby the response of each sensing means to incoming sound pressure waves not in phase is determined by the mechanical interaction between the plurality of sensing means and the mechanical coupling means, and

whereby an electrostatic force associated with the electric field overcomes the restoring force of each suspension means, causing the corresponding sensing means to be pulled towards the supporting means; and

each said fixed conducting means includes means to allow air in a gap between each fixed conducting means and corresponding sensing means to escape, thereby reducing acoustical damping therein;

65. A directional acoustic transducer comprising:

a supporting substrate;

at least two electrically conductive, fixed counter electrodes formed in said substrate;

at least two movable, electrically conductive diaphragms, each diaphragm being positioned over a corresponding counter electrode and including an annular indentation to maintain a fixed distance between said diaphragm and said diaphragm'"'"'s corresponding counter electrode when a bias voltage is applied between said diaphragm and the corresponding counter electrode, each said counter electrode including a plurality of holes to allow air in a gap between the electrode and its corresponding diaphragm to escape, thereby reducing acoustical damping therein;

a plurality of suspension structures formed on the substrate and attached to the diaphragms to support the diaphragms, the suspension structures supporting each diaphragm having a restoring force equal to or greater than surface contact forces between said supported diaphragm structures and the supporting substrate, whereby an electrostatic force associated with the bias voltage overcomes the restoring force of said suspension structures, causing the corresponding diaphragm to be pulled towards the supporting substrate; and

a support beam formed in the supporting substrate for providing a flexible mechanical coupling between said movable diaphragms, whereby the response of each diaphragm to incoming sound pressure waves not in phase with one another will be determined by the mechanical interaction between the diaphragms provided by the mechanical coupling beam.

66. A method of fabricating an acoustic transducer comprising the steps of:

forming a bulk layer on a supporting substrate;

forming a plurality of cavities in said bulk layer;

covering the supporting substrate with a first sacrificial material, whereby said plurality of cavities are filled with said first sacrificial material;

planarizing said first sacrificial layer until the bulk layer reappears;

depositing and patterning an electrically insulating layer on said substrate;

depositing and patterning an electrically conductive layer to form a fixed electrode;

depositing a second sacrificial layer and patterning the second sacrificial layer in first selected areas;

depositing a third sacrificial layer and patterning the third sacrificial layer in second selected areas;

depositing and patterning an electrically conductive layer to form a diaphragm with an annular indentation and a suspension structure;

depositing and patterning a metal to form contact pads for interconnection;

removing in bulk the substrate under the diaphragm; and

removing all sacrificial layers to complete formation of the acoustic transducer.

67. The method according to claim 66 wherein the formation of the bulk layer is achieved by implantation or diffusion of boron in a silicon substrate.

68. The method according to claim 66 wherein the formation of the bulk layer is achieved by epitaxial growth of a boron doped silicon layer on a silicon substrate.

69. The method according to claim 66 wherein the formation of the bulk layer is achieved by the use of silicon-on-insulator substrates.

70. The method according to claim 66 wherein the cavities are formed in said bulk layer using anisotropic reactive ion etching.

71. The method according to claim 66 wherein the electrically insulating layer, first, second, and third sacrificial layers, electrically conductive fixed electrode layer, and diaphragm, annular indentation, and suspension layer are formed using chemical vapor deposition (CVD).

72. The method according to claim 66 wherein the planarization is achieved by polishing the substrate.

73. The method according to claim 72 wherein the polishing is performed using chemical mechanical polishing (CMP).

74. The method according to claim 66 wherein the metal contact pads are formed using evaporation, sputter deposition or electrochemical deposition.

75. The method according to claim 66 wherein the bulk removing of the substrate is achieved by etching in a chemical solution containing potassium hydroxide (KOH) and water.

76. The method according to claim 66 wherein the bulk removing of the substrate is achieved by anisotropic reactive ion etching.

77. The method according to claim 66 wherein the removal of sacrificial layers is achieved by etching in a chemical solution containing hydrofluoric acid (HF) and water.

78. The method according to claim 66 wherein the removal of sacrificial layers is achieved by etching in a chemical solution of hydrogen peroxide and water.

79. A method of fabricating a directional acoustic transducer comprising the steps of:

forming a bulk layer on a supporting substrate;

forming a plurality of cavities, a coupling beam and torsional attachment points in said bulk layer;

covering the supporting substrate with a first sacrificial material, whereby said plurality of cavities are filled with said first sacrificial material;

planarizing said first sacrificial layer until the bulk layer reappears;

depositing and patterning an electrically insulating layer on said substrate;

depositing and patterning an electrically conductive layer to form a plurality of fixed electrodes;

depositing a second sacrificial layer and patterning the second sacrificial layer in first selected areas;

depositing a third sacrificial layer and patterning the third sacrificial layer in second selected areas;

depositing and patterning an electrically conductive layer to form a plurality of diaphragms, each with an annular indentation, and suspension structures;

forming metal contact pads for interconnection;

removing in bulk the substrate under each of the diaphragms;

removing all sacrificial layers to complete formation of the directional acoustic transducer.

80. The method according to claim 79 wherein the formation of the bulk layer is achieved by implantation or diffusion of boron in a silicon substrate.

81. The method according to claim 79 wherein the formation of the bulk layer is achieved by epitaxial growth of a boron doped silicon layer on a silicon substrate.

82. The method according to claim 79 wherein the formation of the bulk layer is achieved by the use of silicon-on-insulator substrates.

83. The method according to claim 79 wherein the cavities, coupling beam and torsional attachment points are formed in said bulk layer using anisotropic reactive ion etching.

84. The method according to claim 79 wherein the electrically insulating layer, first, second, and third sacrificial layers, electrically conductive fixed electrodes layer, and diaphragms, annular indentations, and suspension structure layer are formed using chemical vapor deposition (CVD).

85. The method according to claim 79 wherein the planarization is achieved by polishing of the substrate.

86. The method according to claim 85 wherein the polishing is performed using chemical mechanical polishing (CMP).

87. The method according to claim 79 wherein the metal contact pads are formed using evaporation, sputter deposition or electrochemical deposition.

88. The method according to claim 79 wherein the bulk removal of the substrate is achieved by etching in a chemical solution containing potassium hydroxide (KOH) and water.

89. The method according to claim 79 wherein the bulk removal of the substrate is achieved by anisotropic reactive ion etching.

90. The method according to claim 79 wherein the removal of sacrificial layers is achieved by etching in a chemical solution containing hydrofluoric acid (HF) and water.

91. The method according to claim 79 wherein the removal of sacrificial layers is achieved by etching in a chemical solution of hydrogen peroxide and water.