Thin film electret microphone

• US 20010033670A1
• Filed: 05/15/2001
• Published: 10/25/2001
• Est. Priority Date: 04/18/1996
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Hsu, Tseng-Yang, Hsieh, Wen H., Tai, Yu-Chong
• Granted Patent: US 6,806,593 B2
• Time in Patent Office: Days
• US Class Current: 381/174
• CPC Class Codes: H04R 19/016 for microphones
• CPC Class Codes: H04R 25/604 of acoustic or vibrational transducers
• CPC Class Codes: Y10T 29/42 Piezoelectric device making
• CPC Class Codes: Y10T 29/49005 Acoustic transducer
• CPC Class Codes: Y10T 29/49226 Electret making

Abstract

An electret formed by micro-machining technology on a support surface, including a self-powered electret sound transducer, preferably in the form of a microphone, formed by micro-machining technology. Each microphone is manufactured as a two-piece unit, comprising a microphone membrane unit and a microphone back plate, at least one of which includes an electret formed by micro-machining technology. When juxtaposed, the two units form a microphone that can produce a signal without the need for external biasing, thereby reducing system volume and complexity. The electret material used is a thin film of spin-on polytetrafluoroethylene (PTFE). An electron gun preferably is used for charge implantation. The electret has a saturated charged density in the range of about 2×10⁻⁵ C/m² to about 8×10⁻⁴ C/m². Thermal annealing is used to stabilize the implanted charge. An open circuit sensitivity of about 0.5 mV/Pa has been achieved for a hybrid microphone package.

Claims

1. A method of fabricating an electret by the step of forming, by micro-machining techniques, an electret layer on a support structure.

2. The method of

claim 1

, further including the step of thermally annealing the electret layer to stabilize charge therein.

3. The method of

claim 2

, wherein the step of thermally annealing comprises heating the electret layer to about 100°

C. for about 3 hours.

4. The method of

claim 1

, wherein the support structure is formed from an electrically insulating or semiconducting glass, ceramic, crystalline, or polycrystalline material.

5. The method of

claim 1

, wherein the support structure comprises a membrane fabricated to a thickness of about 1 μ

m.

6. The method of

claim 1

, wherein the electret layer is formed by the steps of:

(a) applying a dielectric film on the support structure;

(b) implanting electrons into the dielectric film.

7. The method of

claim 6

, further including the step of implanting electrons into the dielectric film by means of a pseudo-spark electron gun.

8. The method of

claim 6

, wherein the dielectric film is formed from one of Mylar, FEP, a PTFE fluoropolymer, Teflon®

AF, a silicone, or Parylene.

9. The method of

claim 1

, wherein the electret has a saturated charged density from about 2×

10<sup>−

5</sup> C/m² to about 8×

10<sup>−

4</sup> C/m².

10. A method of fabricating an electret sound transducer by the steps of:

(a) forming, by micro-machining techniques, a transducer membrane having a first electrode;

(b) forming, by micro-machining techniques, a transducer back plate having a second electrode;

(c) forming an electret layer on at least one of the transducer membrane or the transducer back plate;

(d) positioning the transducer membrane adjacent to the transducer back plate to form an electret sound transducer.

11. The method of

claim 10

, further including the step of thermally annealing the electret layer to stabilize charge therein.

12. The method of

claim 11

, wherein the step of thermally annealing comprises heating the electret layer to about 100°

C. for about 3 hours.

13. The method of

claim 10

, wherein the transducer membrane is formed on a support structure formed from an electrically insulating or semiconducting glass, ceramic, crystalline, or polycrystalline material.

14. The method of

claim 10

, wherein the transducer back plate is formed from an electrically insulating or semiconducting glass, ceramic, crystalline, or polycrystalline material.

15. The method of

claim 10

, wherein the transducer membrane is fabricated to a thickness of about 1 μ

m.

16. The method of

claim 10

, wherein the electret layer is formed by the steps of:

(a) applying a dielectric film on at least one of the transducer membrane or the transducer back plate;

(b) implanting electrons into the dielectric film.

17. The method of

claim 16

, further including the step of implanting electrons into the dielectric film by means of a thyratron.

18. The method of

claim 16

, wherein the dielectric film is formed from one of Mylar, FEP, a PTFE fluoropolymer, Teflon®

AF, a silicone, or Parylene.

19. The method of

claim 10

, wherein the electret has a saturated charged density from about 2×

10<sup>−

5</sup> C/m² to about 8×

10<sup>−

4</sup> C/m².

20. The method of

claim 10

, further including the step of:

(a) operating the electret sound transducer as a microphone, whereby ambient sounds are transformed by the electret sound transducer into electrical signals on the first electrode and the second electrode.

21. The method of

claim 20

, wherein the microphone has an open circuit sensitivity of about 0.5 mV/Pa.

22. The method of

claim 10

, further including the steps of:

(a) operating the electret sound transducer as a speaker by applying electrical signals through the first electrode and the second electrode so as to induce physical motion of the membrane under the influence of the electret layer, thereby generating sound waves.

23. An electret comprising:

(a) a support structure;

(b) an electret layer formed on the support structure by micro-machining techniques.

24. The electret membrane of

claim 23

, wherein the electret layer is thermally annealed to stabilize charge therein.

25. The electret membrane of

claim 23

, wherein the electret layer is heated to about 100°

C. for about 3 hours for thermal annealing.

26. The electret membrane of

claim 23

, wherein the support structure is formed from an electrically insulating or semiconducting glass, ceramic, crystalline, or polycrystalline material.

27. The electret membrane of

claim 23

, wherein the support structure comprises a membrane about 1 μ

m thick.

28. The electret membrane of

claim 23

, wherein the electret layer comprises a charged dielectric film formed on the support structure.

29. The electret membrane of

claim 28

, wherein the dielectric film is charged by implanting electrons into the dielectric film by means of a thyratron.

30. The electret membrane of

claim 28

, wherein the dielectric film is formed from one of Mylar, FEP, a PTFE fluoropolymer, Teflon®

AF, a silicone, or Parylene.

31. The electret membrane of

claim 23

, wherein the electret has a saturated charged density from about 2×

10<sup>−

5</sup> C/m² to about 8×

10<sup>−

4</sup> C/m².