SURFACE MICROMACHINED DIFFERENTIAL MICROPHONE

• US 20090046883A1
• Filed: 01/25/2007
• Published: 02/19/2009
• Est. Priority Date: 01/31/2006
• Status: Active Grant

• Current Assignee: The Research Foundation for The State University of New York (State University of New York)
• Original Assignee: The Research Foundation for The State University of New York (State University of New York)
• Inventors: Ronald, Miles N.
• Granted Patent: US 8,276,254 B2
• Time in Patent Office: Days
• US Class Current: 381/369
• CPC Class Codes: H04R 1/38 in which sound waves act upon both sides of a diaphragm and incorporating acoustic phase-shifting means, e.g. pressure-gradient microphone
• CPC Class Codes: H04R 19/005 using semiconductor materials
• CPC Class Codes: H04R 19/04 Microphones H04R19/01 takes precedence
• CPC Class Codes: H04R 31/00 Apparatus or processes specially adapted for the manufacture of transducers or diaphragms therefor manufacture of microstructural arrangements of deformable or non-deformable structures in general B81C1/00182
• CPC Class Codes: Y10T 29/43 Electric condenser making
• CPC Class Codes: Y10T 29/49002 Electrical device making
• CPC Class Codes: Y10T 29/49005 Acoustic transducer
• CPC Class Codes: Y10T 29/4908 Acoustic transducer

Abstract

A differential microphone having a perimeter slit formed around the microphone diaphragm that replaces the backside hole previously required in conventional silicon, micromachined microphones. The differential microphone is formed using silicon fabrication techniques applied only wafer. The backside holes of prior art microphones typically require that a secondary machining operation be performed on the rear surface of the silicon wafer during fabrication. This secondary operation adds complexity and cost to the micromachined microphones so fabricated. Comb fingers forming a portion of a capacitive arrangement may be fabricated as part of the differential microphone diaphragm.

Claims

1. A method of forming a miniature, surface micromachined, differential microphone, the steps comprising:

a) depositing a sacrificial layer on a top surface of a silicon wafer;

b) depositing a diaphragm material on an upper surface of said sacrificial layer;

c) etching said diaphragm material layer to isolate a diaphragm therein; and

d) removing at least a portion of said sacrificial layer from a region beneath said defined diaphragm.

2. The method as recited in claim 1, wherein said etching step (c) further comprises the sub-step of forming comb sense fingers along at least a portion of a perimeter of said diaphragm.

3. The method as recited in claim 1, the steps further comprising:

e) forming a conductive layer intermediate said top surface of said silicon wafer and said sacrificial layer.

4. The method as recited in claim 1, wherein said depositing step (a) comprises-depositing a layer of at least one material from the group:

silicon dioxide, low temperature oxide (LTO), phosphosilicate glass (PSG), aluminum, photoresist material, a polymeric material.

5. The method as recited in claim 1, wherein said depositing step (b) comprises depositing a layer of at least one material from the group:

polysilicon, silicon nitride, gold, aluminum, and copper.

6. A miniature, surface micromachined, differential microphone, comprising:

a) a silicon substrate;

b) a sacrificial layer deposited upon an upper surface of said silicon wafer;

c) a diaphragm material layer deposited on an upper surface of said sacrificial layer;

d) a diaphragm formed in said diaphragm material layer supported by a hinge and otherwise isolated from a remaining portion of said diaphragm material layer by a slit adjacent a perimeter of said diaphragm; and

e) an enclosed back volume beneath said diaphragm having a depth defined by a thickness of said sacrificial layer, said back volume communicating with a region external thereto only via said slit.

7. The miniature, surface micromachined, differential microphone as recited in claim 6, further comprising:

f) a plurality of comb sense fingers disposed along at least a portion of a perimeter of said diaphragm.

8. The miniature, surface micromachined, differential microphone as recited in claim 6, further comprising:

f) a conductive layer intermediate said top surface of said silicon substrate and said sacrificial layer.

9. The miniature, surface micromachined, differential microphone as recited in claim 6, wherein said sacrificial layer comprises at least one material from the group:

silicon dioxide, low temperature oxide (LTO), phosphosilicate glass (PSG), aluminum, photoresist material, a polymeric material.

10. The miniature, surface micromachined, differential microphone as recited in claim 6, wherein said diaphragm material layer comprises at least one material from the group:

polysilicon, silicon nitride, gold, aluminum, and copper.

11. In a miniature, surface micromachined, differential microphone, comprising a diaphragm formed in a diaphragm material layer and supported by a hinge, and an enclosed back volume beneath said diaphragm and having a side surface and a bottom surface and having a hole in one of said side and said bottom surfaces allowing communication between the back volume and a region external thereto, the improvement comprising:

a) a slit disposed between a perimeter of said diaphragm and a diaphragm material layer from which said diaphragm is isolated by said slit; and

b) an enclosed back volume beneath said diaphragm and having a side surface and a bottom surface, each of said side and said bottom surfaces being isolated from a region external to said back volume except via said slit.

12. A microphone, comprising:

a substrate, having deposited on a surface thereof a sacrificial layer, and a diaphragm layer disposed on top of said sacrificial layer, an aperture being formed through said diaphragm layer, and at least a portion of said sacrificial layer beneath the diaphragm layer being removed, resulting in a floating diaphragm with a void between said diaphragm layer and said substrate, wherein said floating diaphragm has an axis of rotational movement in response to acoustic waves which is substantially parallel to a plane of said floating diaphragm; and

a transducer for producing an electrical signal responsive to a displacement of said floating diaphragm with respect to said substrate due to acoustic waves.

13. The microphone according to claim 12, wherein said axis is located such that a portion of said floating diaphragm moves in a direction along an axis normal to a plane of said floating diaphragm while another portion of said floating diaphragm moves in an opposite direction along an axis normal to a plane of said diaphragm, in response to an acoustic wave.

14. The microphone according to claim 13, wherein a volume behind said floating diaphragm is substantially constant with respect to movements in response to acoustic waves.

15. The microphone according to claim 12, wherein a void space behind said floating diaphragm has a depth approximately the same as a depth of said sacrificial layer.

16. The microphone according to claim 12, wherein said diaphragm has respectively differentially responsive regions, further comprising at least one acoustic barrier to isolate the respectively differentially responsive regions from different portions of an incident acoustic wave.

17. The microphone according to claim 12, wherein said aperture comprises a slit permitting air flow therethrough.

18. The microphone according to claim 17, wherein a moment M acting on one side of said floating diaphragm with respect to said axis, in response to acoustic waves of amplitude P and frequency ω

, having a wavelength larger than a maximum linear dimension of said void, said floating diaphragm having dimensions L_y along said axis and L_x perpendicular to, and measured from said axis, said acoustic waves deflecting said floating diaphragm over small angles, is approximately;

M=−

Pe<sup>î

ω

t</sup>L_y(k_xL_x³/12î

).

19. The microphone according to claim 12, wherein said transducer has an approximately first order directional response to acoustic waves.

20. The microphone according to claim 12, wherein said axis is located such that a portion of said floating diaphragm moves in a direction along an axis normal to a plane of said floating diaphragm while another portion of said floating diaphragm moves in an opposite direction along an axis normal to a plane of said diaphragm, in response to an acoustic wave, and wherein a void volume behind said floating diaphragm is substantially constant with respect to movements in response to acoustic waves, said aperture comprising a slit permitting air flow therethrough, and a moment M acting on one side of said floating diaphragm with respect to said axis, in response to acoustic waves of amplitude P and having a wavelength larger than a maximum linear dimension of said void and frequency ω

, said floating diaphragm having dimensions L_y along said axis and L_x perpendicular to, and measured from said axis, said acoustic waves deflecting said floating diaphragm over small angles, is approximately;

M=−

Pe<sup>î

ω

t</sup>L_y(k_xL_x³/12î

).