# Patent

## Title
Robust diaphragm for an acoustic device

## Abstract

A rigid, flat plate diaphragm for an acoustic device is illustrated. The internal supporting structure of the diaphragm provides a combination of torsional and translational stiffeners, which resemble a number of crossbars. These stiffeners brace and support the diaphragm motion, thus causing its response to not be adversely affected by fabrication stresses and causing it to be very similar in dynamic response to an ideal flat plate operating in a frequency range that extends well beyond the audible.

## Background
STATEMENT OF GOVERNMENT INTEREST
This invention was made with Government support under DAA D17-00-C-0149 from DARPA. The Government has certain rights in the invention.


FIELD OF THE INVENTION
The present invention relates to acoustic devices such as microphones and hearing aids and, more particularly, to an improved diaphragm for a microphone having a robust dynamic response in a frequency range extending well past the audible.
BACKGROUND OF THE INVENTION
Fabrication of substantially flat, compliant diaphragms is essential to the success of sensitive microphones. A significant obstacle to achieving this goal is the inevitable residual stresses induced during the process of manufacturing miniature microphone diaphragms. The thickness of miniature microphone diaphragms is typically on the order of microns. Stresses in such thin films can result in warpage or buckling, or can lead to breakage. Much effort has been put into controlling the flatness and dynamic performance of thin film diaphragms.
One common method to prevent the aforementioned warpage is to clamp all four edges or all four corners of a thin diaphragm and utilize tensile stress to control the flatness. The tension, however, increases the stiffness of the diaphragm and consequently decreases the sensitivity of the microphone. The inability to accurately control the tensile stress during fabrication also leads to unpredictable dynamic characteristics for the microphone.
To achieve an acceptable sensitivity, a microphone diaphragm needs to be very compliant. The cantilever structure described in this invention is an alternative to conventional four-edge (or four-corner) clamped devices. The new cantilever design seeks to achieve a sensitive microphone, since cantilever diaphragms are much more compliant than tensioned diaphragms.
One of the objects of the present invention is to provide a robust microphone diaphragm design that maintains good dimensional control under the influences of residual stresses, either compressive or tensile, while having its dynamic response dominated only by a single mode of vibration. The response of the diaphragm is predicted to be extremely close to that of an ideal rigid plate over a frequency range extending well beyond the audible range.
The internal supporting structure of this diaphragm provides a combination of torsional and translational stiffeners that resemble a number of crossbars. These stiffeners brace and support the diaphragm motion, thus causing it to be very similar in dynamic response to an ideal flat plate operating in a frequency range extending well beyond the audible. The diaphragm is essentially constrained to pivot about an edge upon which it is supported. The supported end has an overlapping T-section whose length and cross-sectional dimensions can be adjusted to tune the resonant frequency.
DISCUSSION OF RELATED ART
In U.S. Pat. No. 5,633,552, issued to Lee et al, a method is disclosed for fabricating a micro-machined pressure transducer having a multilayer silicon nitride thin film cantilever diaphragm. The technique relies on the symmetry of the stress gradient in the two outer layers, and a larger tensile stress (250 MPa) in the second layer to maintain diaphragm flatness.
The diaphragm of the present invention relies on the use of stiffeners to maintain flatness rather than, as the prior art teaches, attempting to balance existing stresses in the various layers of the diaphragm. The patent shows static deflections due to stress of more than 15 microns. Predictable maximum deflection of the diaphragm of the current invention will be approximately 0.5 microns. This is an improvement over the related art by a factor of 30.
In U.S. Pat. No. 5,870,482, issued to Loeppert et al, a cantilever center support diaphragm is illustrated. This patent uses a corrugated structure and a sandwich of two quilted films separated by a thin 2-3 micron sacrificial layer, in order to match the diaphragm compliance to the desired pressure range. It is also desired to counter any curling tendency of the diaphragm. In the current invention the design provides better control over the flatness.
In U.S. Pat. No. 5,146,435, issued to Bernstein, a structure consisting of a single crystal silicon diaphragm supported on its corners by patterned silicon springs is shown. By supporting the diaphragm only at the corners as suggested by Bernstein, it is possible to increase the diaphragm compliance and subsequently, the sensitivity to sound.
While this approach permits a design that is more compliant than the usual approach where the diaphragm is supported entirely around its perimeter, it does not ensure that the stresses in the structure will not result in warpage (if the stress is tensile) and it is quite possible that compressive stresses will result in buckling.
By incorporating stiffeners in the present inventive diaphragm, improved flatness is achieved. The current inventive diaphragm is supported on specially designed torsional springs that have very high stiffness in the transverse direction, but which have well-controlled stiffness in torsion.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an improved diaphragm for a microphone, acoustic sensor, or hearing aid that is not adversely affected by fabrication stresses. It is robust in the sense that it is not affected by fabrication stresses. The diaphragm comprises a rigid flat plate of polysilicon or similar material. The internal supporting structure provides a combination of torsional and translational stiffeners that resemble a number of crossbars. These stiffeners brace and support the diaphragm motion, thus causing it to be very similar in dynamic response to an ideal flat plate operating in a frequency range that extends well beyond the audible. The diaphragm is essentially constrained to pivot about an edge upon which it is supported. The supported end has an overlapping T-section, whose length and cross-sectional dimensions can be adjusted to tune the resonant frequency.
It is an object of this invention to provide an improved diaphragm for a microphone, hearing aid, or acoustic device.
It is another object of the invention to provide a diaphragm for a microphone, hearing aid, or acoustic sensor that is not affected by fabrication stresses.



BRIEF DESCRIPTION OF THE DRAWINGS
A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which:
FIG. 1 illustrates a schematic perspective view of the diaphragm with internal support structure, in accordance with this invention;
FIG. 2 depicts a schematic, perspective, enlarged top view of a fixed end “T” section of the diaphragm shown in FIG. 1;
FIG. 3 shows the predicted deformation of the diaphragm due to 40 MPa of compressive stress along four lines across the diaphragm at z=0 and y=0 μm, y=500 μm, x=0 μm, and x=1000 μm.



DESCRIPTION OF THE PREFERRED EMBODIMENT
Generally speaking, the invention features an internally stiffened, rigid, flat plate diaphragm for an acoustic device. The internal supporting structure of the diaphragm provides a combination of torsional and translational stiffeners, which resemble a number of crossbars. These stiffeners brace and support the diaphragm motion, thus causing it to be very similar in dynamic response to an ideal flat plate operating in a frequency range that extends well beyond the audible.
Now referring to FIG. 1, a schematic view of a stiffened diaphragm 10 for use in an acoustic device in accordance with the present invention is illustrated. The diaphragm 10 is shaped like a flat rectangular box having internal stiffeners 11 and 12, respectively, forming crossbar bracing members. The crossbar bracing members cause the motion of the diaphragm 10 to approach that of an ideal flat plate. The crossbar members provide the diaphragm 10 with torsional and translational stability. Diaphragm 10 is supported and pivots about a fixed end, “T” section 14, as shown in FIG. 2.
The diaphragm 10 can be used in a microphone, and can be fabricated from polycrystalline silicon or similar material in a microfabrication process. In the microfabrication process, the diaphragm is highly robust and tolerant of fabrication defects. The diaphragm 10 maintains exceptional flatness under the influence of either compressive or tensile stresses that may occur during manufacture. The dynamic response of the diaphragm conforms to an ideal flat plate over a frequency range extending well beyond the audible range. The dynamic characteristics of the diaphragm 10 can be readily tuned without adversely influencing the flatness or ruggedness thereof.
The “T” section 14 can be adjusted in length and cross-section for tuning the resonant frequency. The overall dimensions of the diaphragm 10 are 1 mm by 1 mm. The stiffening crossbars 11 and 12, respectively, can be 4 microns thick and 40 microns tall.
A first mode of vibration is predictably at 24 kHz, and a second mode is at 84 kHz. The second mode is well above the audible frequency, and therefore will not influence the response. Utilization of stiffeners 11 and 12 pushes the unwanted modes of diaphragm 10 into the ultrasonic frequency range so that the response is very similar to an ideal flat plate structure.
The diaphragm 10 has high bending rigidity, as shown in FIG. 3. The diaphragm is not prone to buckling when subjected to 40 Mpa of isotropic compressive stress. The identical result, with opposite sign, is obtained with a tensile stress loading.
Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.
Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.

## Claims

1. An acoustic diaphragm having a dynamic response extending throughout the audible range, comprising a rigid plate-shaped member supported upon a stiffened edge of a side thereof which pivots on torsional springs, said rigid plate-shaped member having torsional and translational stiffeners, a resonant frequency of the rigid plate-shaped member being substantially dependent on at least a set of physical characteristics of the stiffened edge.

2. The acoustic diaphragm in accordance with claim 1, wherein said torsional and translational stiffeners comprise cross members traversing said rigid plate-shaped member.

3. The acoustic diaphragm in accordance with claim 1, wherein the stiffened edge of the side that supports the diaphragm comprises a “T”-shaped cross section whose length and cross-section are adapted to tune said acoustic diaphragm so that its lowest resonant frequency is higher than the audible range.

4. The acoustic diaphragm in accordance with claim 1, wherein said rigid plate-shaped member is fabricated of polycrystalline silicon.

5. The acoustic diaphragm in accordance with claim 1, wherein said rigid plate-shaped member comprises a substantially flat shape.

6. The acoustic diaphragm in accordance with claim 1, wherein said rigid plate-shaped member comprises a shape substantially corresponding to a box.

7. The acoustic diaphragm in accordance with claim 1, wherein said rigid plate-shaped member is approximately 2 microns thick.

8. The acoustic diaphragm in accordance with claim 1, wherein said torsional and translational stiffeners comprise cross rectangular members extending from a flat surface of the rigid plate-like structure which are approximately 4 microns thick and 40 microns tall.

9. The acoustic diaphragm in accordance with claim 1, having a first resonance frequency of approximately 24 kHz.

10. The acoustic diaphragm in accordance with claim 1, having a second resonance frequency of approximately 84 kHz.

11. An acoustic diaphragm having a robust dynamic response extending throughout an audible range, comprising a rigid plate-shaped member supported upon a “T”-shaped cross section disposed on a side thereof which is pivotally suspended by torsional springs, said rigid plate-shaped member having torsional and translational crossbar stiffeners.

12. The acoustic diaphragm in accordance with claim 11, wherein said rigid plate-shaped member is fabricated of polycrystalline silicon.

13. The acoustic diaphragm in accordance with claim 11, wherein said rigid plate-shaped member comprises a substantially flat shape.

14. The acoustic diaphragm in accordance with claim 11, wherein said rigid plate-shaped member comprises a shape substantially corresponding to a box.

15. The acoustic diaphragm in accordance with claim 11, wherein said plate-shaped member is approximately 2 microns thick.

16. The acoustic diaphragm in accordance with claim 11, wherein said torsional and translational stiffeners comprise rectangular structures approximately 4 microns thick and 40 microns tall extending from a flat surface of said plate-shaped member.

17. The acoustic diaphragm in accordance with claim 11, having a first frequency mode of approximately 24 kHz.

18. The acoustic diaphragm in accordance with claim 11, having a second frequency mode of approximately 84 kHz.

19. An acoustic diaphragm having a dynamic response extending throughout the audible range, comprising a plate-shaped member cantilevered about one side thereof from a stiff edge structure, said stiff edge structure being pivotally supported by torsional springs, said plate-shaped member having torsional and translational stiffeners to provide a robust dynamic response to acoustic waves by displacement about said torsional springs extending throughout the audible range, having a dynamic response dominated by a single mode of vibration outside of the audible range which is substantially dependent on said stiff edge structure.

20. The acoustic diaphragm in accordance with claim 19, wherein said torsional and translational stiffeners comprise continuous cross members.

21. The acoustic diaphragm in accordance with claim 19, wherein the stiff edge comprises a “T”-shaped cross section whose length and cross-section are adapted to tune said acoustic diaphragm so that its lowest resonant frequency is higher than the audible range.

22. The acoustic diaphragm in accordance with claim 19, wherein said plate-shaped member is fabricated of polycrystalline silicon.

23. The acoustic diaphragm in accordance with claim 19, wherein said plate-shaped member comprises a substantially flat shape.

24. The acoustic diaphragm in accordance with claim 19, wherein said plate-shaped member comprises a shape substantially corresponding to a box.

25. The acoustic diaphragm in accordance with claim 19, wherein said plate-shaped member is approximately 2 microns thick.

26. The acoustic diaphragm in accordance with claim 25, wherein said torsional and translational stiffeners comprise rectangular structures approximately 4 microns thick and 40 microns tall extending from a flat surface of said plate-shaped member.

27. The acoustic diaphragm in accordance with claim 26, wherein said dominating single mode of vibration outside of the audible range comprises a first frequency mode of approximately 24 kHz.

28. The acoustic diaphragm in accordance with claim 27, having a second frequency mode of approximately 84 kHz.