The present invention relates to an electret condenser microphone including a vibrating electrode, and particularly relates to a structure of an electret condenser microphone (ECM) carrying an electret condenser formed using a MEMS (Micro Electro Mechanical System) technology.

Conventionally, predominant condenser microphones are microphones in which components, such as a diaphragm, a fixed electrode, and the like separately manufactured by a mechanical processing or the like are incorporated, as disclosed in Patent Document 1. These microphones are called ECMs (Electret Condenser Microphones), and miniaturization thereof has been tackled. As a result, a product having a diameter of 6 mm and a thickness of 1 mm and a product having a diameter of 4 mm and a thickness of 1.5 mm, and the like have been on sale. However, an ECM of the type disclosed in Patent Document 1, which uses mechanically-processed components, encounters difficulty in miniaturization.

A processing technology applying a semiconductor integrated circuit fabrication method, which is different from the conventional mechanical processing, is called a MEMS technology or a micro machining technology and is effective in miniaturization of microphones. Under the circumstances, microphone manufacturing methods using the MEMS technology have been proposed. For example, Patent Document 2 discloses a method of manufacturing a microphone with no electret with the use of the MEMS technology.

• Patent Document 1: Japanese Patent Application Laid Open Publication No. 11-187494A
• Patent Document 2: Japanese Patent Application Laid Open Publication No. 2003-78981A

An electret condenser microphone having the structure disclosed in Patent Document 2, however, involves the following problems.

Namely, since the condenser and a drive circuit element, such as an 1C are connected to each other with the use of a bonding wire, electric loss occurs in the bonding wire.

Further, as the condenser is reduced in size, the back air chamber integrally formed with the condenser is also reduced in size, degrading high-frequency characteristic of the microphone with the reduced back air chamber. As a result, a microphone exhibiting excellent performance cannot be manufactured.

The present invention has been made in view of the foregoing and has its object of providing an electret condenser microphone having a structure excellent in high-frequency characteristic even when miniaturized.

To attain the above object, an electret condenser microphone according to the present invention includes: a substrate in which an opening is formed; an electret condenser connected to one face of the substrate so as to close the opening and having an acoustic hole and a cavity; a drive circuit element connected to the one face of the substrate; and a case mounted over the substrate so as to cover the electret condenser and the drive circuit element, wherein electric contact is established at a joint part between the electret condenser and the substrate, the acoustic hole communicates with an external space through the opening, and the cavity and an internal region of the case serve as a back air chamber for the electret condenser.

In the present invention, electric contact between the substrate and the electret condenser is established at a mechanical contact part therebetween, in other words, the substrate and the electret condenser are connected to each other electrically without using a wire bonder, suppressing parasitic capacitance and noise occurrence caused due to the presence of the bonding wire. This enhances the high-frequency characteristic of the electret condenser. Further, the substantial volume of the back air chamber of the electret condenser can be increased by utilizing the space (the internal region of the case) formed by the case that covers the electret condenser, so that the high-frequency characteristic of the condenser can be enhanced even when the electret condenser is miniaturized.

As described above, according to the present invention, a small-size electret condenser microphone excellent in high-frequency characteristic can be provided.

[FIG. 1] FIG. 1(a) is a sectional view of an electret condenser carried by an electret condenser microphone according to one embodiment of the present invention, and FIG. 1(b) is a plan view of the electret condenser.

• [FIG. 2] FIG. 2 is a sectional view of the electret condenser microphone according to the embodiment of the present invention.
• [FIG. 3] FIG. 3 is an enlarged sectional view of variation on a joint part between the electret condenser and a substrate in the electret condenser microphone according to the embodiment of the present invention.
• [FIG.4] FIG. 4(a) and FIG. 4(b) are sectional views showing variation on the electret condenser microphone according to the embodiment of the present invention.
• [FIG. 5] FIG. 5 is a sectional view showing a state where the electret condenser microphone according to the embodiment of the present invention is connected to another substrate.

1

silicon substrate

2

cavity

3

insulating film

4

lower electrode

5

extraction electrode

6

leak hole

7

electret film

8

insulating film

9

upper electrode

10

contact

11

insulating film

12

acoustic hole

13

substrate

14

wire

15

IC element

16

contact

17

case

18

protrusion

19

anisotropic conductive resin

20

contact

21

wire

22

substrate

25

opening

An electret condenser microphone according to one embodiment of the present invention will be described below with reference to the accompanying drawings.

Fist of all, a description will be given to an electret condenser (hereinafter referred to as an electret condenser of the present invention) carried by an electret condenser microphone according to the present embodiment. The electret condenser of the present invention is manufactured by a processing method called surface micro machining in the MEMS technology which uses only a single silicon substrate.

FIG. 1(a) and FIG. 1(b) are a sectional view and a plan view of an electret condenser 50 of the present invention, respectively.

As shown in FIG. 1(a) and FIG. 1(b), a cavity 2 is formed in a silicon substrate 1 by anisotropic etching, and an insulating film 3 formed of, for example, a silicon oxide film and a lower electrode 4 formed of, for example, a polysilicon film doped with phosphorus are formed on the substrate 1 in this order. Part of the insulting film 3 located above the cavity 2 is removed, and the lower electrode 4 is formed so as to cover the cavity 2. An electret film 7 formed of, for example, a silicon oxide film is provided on part of the lower electrode 4 located above the cavity 2. An insulating film 8 formed of, for example, a silicon oxide film is provided on part of the lower electrode 4 located above the external part of the cavity 2, and an upper electrode 9 is provided above the lower electrode 4 with the insulating film 8 interposed as a spacer therebetween. The surface of the upper electrode 9 is covered and protected with an insulating film 11 formed of, for example, a silicon nitride film.

An extraction electrode 5 electrically connecting to the lower electrode 4 is provided in the insulating film 8 and the insulating film 11. The extraction electrode 5 protrudes from the insulating film 11. A leak hole 6 communicating with the cavity 2 is formed in the lower electrode 4 and the electret film 7 so as not cause pressure difference between a region above and a region below the electret film 7. Further, a contact 10 electrically connecting to the upper electrode 9 is formed in the insulating film 11. The contact 10 protrudes from the insulating film 11.

Herein, in order to make the insulating film 11 of the electret condenser 50 of the present invention to close an opening 25 formed in a substrate 13 described later (see FIG. 2), the contact 10 is formed in an annular shape on the insulating film 11, as shown in FIG. 1(b). It should be noted, however, that the shape of the contact 10 is not limited particularly only if it has no break.

Moreover, a plurality of acoustic holes 12 are formed in the upper electrode 9 and the insulating film 11. The acoustic holes 11 are formed for allowing a space surrounded by the upper electrode 9, the lower electrode 4, and the insulating film 8 to communicate with the external space of the electret condenser 50 of the present invention. Namely, the electret condenser 50 of the present invention has a structure for causing the electret film 7 to vibrate upon receipt of sound pressure (see FIG. 2) through the acoustic holes 12. The cavity 2 serves as a back air chamber for the electret condenser 50 of the present invention.

One example of an electret condenser microphone according to the present embodiment will be described next.

FIG. 2 is a sectional view of an electret condenser microphone according to the present embodiment, that is, an electret condenser microphone carrying the electret condenser 50 of the present invention shown in FIG. 1(a) and FIG. 1(b).

As shown in FIG. 2, the above-described electret condenser 50 of the present invention is connected to one face of the substrate 13 in which the opening 25 is formed so as to close the opening 25. The opening 25 is formed in the substrate 13 so that the acoustic holes 12 of the electret condenser 50 of the present invention are exposed to the external space. In other words, the acoustic holes 12 communicate with the external space through the opening 25.

Specifically, the contact 10 of the electret condenser 50 of the present invention is mechanically and electrically connected to a contact 16A formed at the one face of the substrate 13. The contact 16A is electrically connected to a wiring 14A provided on the other face of and in the inside of the substrate 13. The electret condenser 50 of the present invention is electrically connected to another electric circuit on the substrate 13 through the contact 16A and the wiring 14A.

Further, an IC (Integrated Circuit) element 15 to be a drive circuit element for the microphone of the present embodiment is connected to the one face of the substrate 13. Specifically, the IC element 15 includes contacts 15a and 15b. The contact 15a is mechanically and electrically connected to one end of a wiring 14B provided at the one face of the substrate 13. The other end of the wiring 14B is mechanically and electrically connected to the extraction electrode 5 of the electret condenser 50 of the present invention. On the other hand, the contact 15b of the IC element 15 is mechanically and electrically connected to a contact 16B provided at the one face of the substrate 13. The contact 16B is electrically connected to a wiring 14C provided on the other face of and in the inside of the substrate 13. The IC element 15 is electrically connected to another electric circuit on the substrate 13 through the contact 16B and the wiring 14C.

In addition, a case 17 is mounted over the substrate 13 so as to cover the electret condenser 50 of the present invention and the IC element 15.

As described above, the contact 10 for the upper electrode 9 is formed in an annular shape on the insulating film 11 (see FIG. 1(b)). Further, the contact 16A for the substrate 13 is formed in an annular shape correspondingly so as to face the contact 10 for the upper electrode 9, whereby the following effects can be obtained. Namely, since the contact 10 and the contact 16A form an annular joint part for connecting the electret condenser 50 of the present invention and the substrate 13, sound pressure can be prevented from leaking between the substrate 13 and the electrode condenser 50 of the present invention.

Moreover, for joining the electret condenser 50 of the present invention and the substrate 13 in the present embodiment, for example, gold is used preferably as a metal material composing the contact 16A for the substrate 13 and the contact 10 for the upper electrode 9. In this case, the gold parts respectively forming the contacts 10 and 16A can be joined to each other by thermocompression bonding, facilitating joining of the contacts 10 and 16A. Even in the case where an alloy of, for example, gold and tin is used as a material of one of the contacts 10 and 16A, the contacts 10 and 16A can be joined by a similar thermocompression.

Furthermore, any of the following methods may be employed for joining the electret condenser 50 of the present invention and the substrate 13. FIG. 3 is an enlarged sectional view showing variation on the joint part of the electret condenser and the substrate in the electret condenser microphone according to the present embodiment. As shown in FIG. 3, the contact 16A for the substrate 13 may be connected mechanically and electrically to the contact 10 for the upper electrode 9 with the use of a metal protrusion 18 which is called bump and an anisotropic conductive resin 19. In this case, the joint part can be formed in an annular shape similarly, preventing sound pressure from leaking between the substrate 13 and the electret condenser 50 of the present invention. Further, with the use of the anisotropic conductive resin 19, the temperature for joining can be set lower than a case of direct joining of the contacts 10 and 16A made of, for example, gold, reducing the burden of a manufacturing device in the microphone manufacturing process.

As described above, in the electret condenser microphone of the present embodiment, the substrate 13 and the electret condenser 50 establish electric contact therebetween at the mechanically joined part thereof, in other words, the substrate 13 and the electret condenser 50 are connected to each other electrically without using a wire bonder, resulting in suppression of parasitic capacitance and noise occurrence caused due to the presence of the bonding wire. This enhances the high-frequency characteristic of the electret condenser 50. Further, not only the cavity 2 shown in FIG. 1(a) but also the space (the internal region of the case 17) surrounded by the substrate 13 and the case 17 that covers the electret condenser 50 can be utilized as a back air chamber necessary for the electret condenser 50, increasing the substantial volume of the back air chamber for the electret condenser 50. Hence, the high-frequency characteristic of the condenser can be enhanced even when the electret condenser 50 is miniaturized.

As described above, according to the present embodiment, a small-size electret condenser microphone excellent in high-frequency characteristic, such as high-frequency stability and the like can be manufactured.

A holder formed using a resin or the like has been necessary for joining a conventional ECM to another substrate. In contrast, in the ECM of the present embodiment, provision of a solder or the like at the wiring 14 shown in FIG. 2 leads to direct joint to another substrate without using the aforementioned holder. In other words, no component for mounting the microphone is necessary.

Furthermore, in the conventional ECM, a mesh-like porous cloth called a face cloth is arranged at part where sound pressure is received for preventing dust from entering. Wherein, the porous cloth has holes each having a diameter of approximately 3 µm or larger. In contrast, when the acoustic holes 12 (see FIG. 1) of the electret condenser 50 of the present invention is set to have a diameter smaller than the holes of the porous cloth, 3 µm or smaller, dust can be prevented from entering, similarly to the conventional ECM using the face cloth. In other words, the ECM of the present embodiment eliminates the need for providing the face cloth for preventing dust from entering, leading to reduction in the number of element components. It should be noted that the shape in plan of the acoustic holes 12 is not limited to a round shape in the present embodiment, but each maximum diameter of the acoustic holes 12 is desirably set to 3 µm or smaller in a case employing a shape other than the round shape.

Moreover, in the conventional microphone manufactured by the MEMS technology, a sound input hole (corresponding to the opening 25 of the present embodiment) for the microphone is arranged basically in the upper face portion of the sound pressure sensing section of the microphone, so that a constraint is imposed on a structure of a mobile phone or the like in which the microphone is mounted. As a result, in the conventional ECM, the position of the sound input hole should have been changed by devising the shape of an acoustic shield, such as a rubber holder for covering the ECM.

In contrast, in the present embodiment, as shown in FIG. 4(a) and FIG. 4(b), the opening 25 can be formed in the substrate 13 with part of the substrate 13 located above the acoustic holes 12 left so that the acoustic holes 12 do not overlap the opening 25 as viewed from above. In other words, the opening 25 can be formed so that the acoustic holes 12 cannot be seen from the external space. In this case, the electret condenser 50 of the present invention is not exposed to the external space directly, thereby being protected to attain a highly-reliable microphone.

Specifically, in the structure shown in FIG. 4(a) and FIG. 4(b), the substrate 13 is a ceramic multilayer substrate of, for example, three layers (a lower layer substrate 13a, an interlayer substrate 13b, and an upper layer substrate 13c) of which main component is, for example, alumina, borosilicate-based glass, or the like. This enables easy formation of a hollow to be the opening 25 in advance at the time when the substrate is a mere green sheet. Accordingly, the opening 25 for receiving sound pressure can be set in the upper face portion of the microphone as shown in FIG. 4(a) or in the side face portion of the microphone as shown in FIG. 4(b), enhancing design flexibility of a mobile phone or the like on which the microphone of the present embodiment is mounted.

Lastly, a method for joining the electret condenser microphone according to the present embodiment to another substrate will be described. FIG. 5 is a sectional view showing a state where the electret condenser microphone according to the present embodiment is joined to another substrate. As shown in FIG. 5, for joining the electret microphone of the present embodiment to a substrate 22, a contact 20 is formed on a surface other than the surface where the acoustic holes 12 are exposed (i.e., the obverse face of the substrate 13), for example, a surface of the case 17, and the electret condenser microphone of the present embodiment is connected to the substrate 22 electrically and mechanically by means of the contact 20. Specifically, the electret condenser 50 of the present invention is connected electrically to the substrate 22 through the contact 16A provided at the one face of the substrate 13, a wiring 21A provided inside the substrate 13 and the case 17, and a contact 20A provided at one face of the case 17. As well, the IC element 15 is connected electrically to the substrate 22 through the contact 16B provided at the one face of the substrate 13, a wiring 21B provided inside the substrate 13 and the case 17, and a contact 20B provided at the one face of the case 17.

The present invention relates to an electret condenser microphone. When the present invention is applied to an electret condenser microphone carrying an electret condenser formed by a MEMS technology, a small-size electret condenser microphone excellent in high-frequency characteristic can be provided to enhance reliability of a mobile phone or the like to which the microphone is mounted. Thus, the present invention is very useful.