Patent Application
20100119097
1. Field of the Invention
The present invention relates to a microphone device and a manufacturing method thereof, and particularly to a microphone device with excellent frequency characteristics.
2. Description of the Background Art
A cover is conventionally used in order to protect an electronic component such as a chip mounted on a substrate from powder dust or electromagnetic-wave noise, etc. from the outside.
The conventional MEMS microphone 300 shown in
Such a MEMS microphone 300 is mounted on a main substrate of, for example, a mobile telephone. In this case, in order to ensure a passage of the sound signal, the microphone is mounted so that an aperture in the mobile telephone overlaps with an aperture 303c in a top portion 303a of the cover. Also, the MEMS microphone 300 is bonded to the substrate 301 through an adhesive 303c at the end 303d of a side portion 303b (for example, see JP-A-2000-165998).
In such a conventional MEMS microphone, it was found that frequency characteristics of the microphone have a disadvantage of having an output around a region of 12 kHz larger than one at 1 kHz by about 10 dB or more. In the conventional microphone, there is a peak (maximal point) of frequency characteristics around a region of 12 kHz.
Essentially, the microphone desires flat frequency characteristics in order to pick up sound faithfully, but the microphone having such a peak of frequency characteristics has a problem of being difficult to pick up sound faithfully because a high region (region of a high frequency) is pronounced.
This is probably because a sound pressure (pressure change according to the vibration of air by sound) applied to a vibrating plate becomes large at a resonance point since a chamber (front air chamber) formed between an aperture in the cover and the vibrating plate serves as a resonator.
The present invention has been implemented in view of the problem described above, and an object of the present invention is to provide a microphone device which has good frequency characteristics and can pick up sound faithfully.
In accordance with the present invention, there is provided a microphone device, comprising: a microphone element comprising a Si substrate, a vibrating film electrode formed on the substrate, a fixed electrode over the vibrating film electrode and a cavity between the vibrating film electrode and the fixed electrode, a signal processor, a printed circuit board, the microphone element and the signal processor disposed thereon; and a cover, the cover and the printed circuit board define an interior portion including the microphone element and the signal processor therein, wherein the cover including a mesh structure occupying 25% or more of at least one surface of the cover.
By this configuration, at least the part of the cover comprises an acoustically-transmissive conductive structure, so that the microphone device can be constructed so as not to construct a resonator causing the resonance described above. Also, in the case of being attached to a mobile telephone etc., there is no need the aperture in the cover is aligned with that of the mobile telephone and the microphone device is attached to the mobile telephone easily.
In a capacitor microphone element (MEMS microphone element) manufactured using a microfabrication technique (MEMS technique) of silicon LSI, processing accuracy is higher than that of a microphone element manufactured by assembly of mechanical components and accuracy of acoustoelectric conversion is high and stable. Using this advantage, a microphone element manufactured by a semiconductor manufacturing process is covered by the cover and a microphone device (microphone module) is constructed. However, the cover tends to construct a Helmholtz resonator. To solve the problem, the present invention provides the microphone device comprising the cover of which frequency characteristics are improved by constructing a structure in which a Helmholtz resonance frequency does not occur at an audible frequency range. Consequently, stable frequency characteristics with high accuracy can be achieved by covering the microphone element with the cover having an acoustically-transmissive conductive structure.
In addition, a signal processor may herein be constructed so as to make only impedance conversion.
That is, by this configuration, the present invention solves the disadvantage described above by adjusting frequency characteristics of a microphone and setting a resonance frequency which is a peak out of an audible frequency range (20 Hz to 20 kHz).
The resonance frequency is given by the following formula by a principle of Helmholtz resonance.
where, fr is a resonance frequency; c is a sound speed; π is the circular ratio; d is a diameter of an aperture in the cover; V is volume of a front air chamber; l is a length of the aperture in the cover (i.e., thickness of the cover); s is an area of the aperture in the cover; and d′ is l+0.6d.
In the case of l<<0.6d, d′≈0.6d is satisfied, so that the following formula is derived from formula 1.1.
fr∝√{square root over (d)} (Formula 1.2)
Also, s∝d2 is satisfied, so that the following formulas are derived from formula 1.1 and formula 1.2 in the case of l<<0.6d.
Also, when formula 1.4 is satisfied, formula 1.3 can also be applied to the cover where an aperture does not construct one hole. In the cover where the aperture constructs many holes, s is the total area of many holes.
When formula 1.4 is satisfied, formula 1.3 indicates that, in other words, a resonance frequency becomes high in proportion to a fourth root of the area of the aperture.
For example, there is a conventional microphone device which has a resonance frequency of 12 kHz. According to the present invention, s (the area of the aperture in the cover) of the present invention is 16 times as large as that of the conventional microphone device (now, formula 1.4 is satisfied). By this configuration, the resonance frequency of the present invention doubles and can be set at 24 kHz which is out of the audible range, thus the disadvantage described above can be solved.
Also, for example, there is a conventional microphone device for which a length of the aperture (i.e., thickness of a cover) is 0.1 mm, a diameter of the aperture is 0.6 mm, an area of a surface having the aperture formed in the cover is 12 mm2, and a resonance frequency is 12 kHz. According to the present invention, the aperture ratio of the surface having the aperture in the cover is set at 25% or more. In other words, a mesh structure occupies 25% or more of at least one surface (in particular, the surface having the aperture) of the cover. By this configuration, the resonance frequency can be set out of the audible range, thus the disadvantage described above can be solved.
A diameter of the aperture, in other words, the width of the aperture is determined according to the volume of a front air chamber so as to satisfy formula 1.
For example, Firstly, a diameter d1 of an aperture for, for example, 20 kHz<fr is obtained. Next, a diameter d of an aperture of the present invention is set to be larger than d1. As the result, a resonance point presents out of an audible frequency range, thus Helmholtz resonance can be avoided.
For example, when d=2 mm is set in formula 1 described above, fr becomes 24 kHz and a resonance point is out of the audible frequency range.
Also, when d=2 mm and an aperture area S=3 mm2 are set and the size of a surface having the aperture formed in a cover are set at substantially 3×4, the aperture ratio of the surface having the aperture formed in the cover could be about 25%.
That is, the aperture ratio of the surface having the aperture could be constructed so as to become 25% or more. An upper limit of the aperture ratio depends on a mechanical strength of a material. That is, the aperture ratio could be determined within a range capable of maintaining the mechanical strength.
The present invention includes the microphone device, wherein the shape of the cover is a rectangular parallelepiped shape, and at least a part of a surface of the cover opposed to the microphone element includes an acoustically-transmissive conductive structure
By this configuration, Helmholtz resonance can be avoided efficiently.
The present invention includes the microphone device, wherein the acoustically-transmissive conductive structure is formed by a conductive material having multiple holes.
By this configuration, occurrence of Helmholtz resonance can be suppressed by a space or a size of a hole, so that design is also easy.
The present invention includes the microphone device, wherein the acoustically-transmissive conductive structure comprises a mesh structure.
By this configuration, the microphone device is manufactured easily and it is easy to suppress occurrence of Helmholtz resonance by adjusting a size of a wire material which forms a mesh, so that design is also easy. Also, the mesh forms a part of the cover, so that it is desirable to have a shielding effect of electromagnetic-wave noise as well as guiding sound from a sound source to a microphone element. Hence, the mesh is formed by a conductive material (metal) and an electromagnetic shield effect is obtained.
The present invention includes the microphone device, wherein the acoustically-transmissive conductive structure comprises a punching metal (in other words, the perforated structure).
It is preferable that the present invention includes the microphone device comprising a microphone element comprising a Si substrate, a vibrating film electrode formed on the substrate, a fixed electrode over the vibrating film electrode and a cavity between the vibrating film electrode and the fixed electrode, a signal processor, a printed circuit board, the microphone element and the signal processor disposed thereon; and a cover, the cover and the printed circuit board define an interior portion including the microphone element and the signal processor therein, wherein the cover including a perforated structure occupying 25% or more of at least one surface of the cover.
By this configuration, occurrence of Helmholtz resonance can be suppressed efficiently by a space or a size of a hole while maintaining a mechanical strength by adjusting a punch for punching (in other words, the hole of the perforated structure), so that design is also easy.
The present invention includes the microphone device, wherein the acoustically-transmissive conductive structure comprises a sintered metal.
By this configuration, the microphone device is manufactured easily.
The present invention includes the microphone device, wherein the acoustically-transmissive conductive structure comprises a porous conductive material.
By this configuration, the microphone device is manufactured easily.
The present invention includes the microphone device, wherein the microphone and the signal processor are integrated inside the common substrate.
According to the configuration described above, miniaturization can be achieved while reducing transmission loss by integrating and forming a microphone element and a signal processor inside the common substrate. Desirably, LSI of the microphone element and the signal processor is performed and also its LSI is covered with a cover having multiple holes formed by a MEMS process and thereby, a very compact microphone device with excellent resonance frequency characteristics can be obtained. Also, further miniaturization can be achieved by this configuration.
It is preferable that the present invention includes the microphone device comprising: a microphone element, a signal processor; and a cover disposed over the microphone element and the signal processor, the cover including an aperture whose size is decided so that a resonant frequency presents out of audible frequency range.
The present invention includes the microphone device, wherein the substrate is disposed so as to be opposed to the acoustically-transmissive conductive material via a spacer, and the substrate and the conductive material have the same outer shape.
By this configuration, multiple microphone devices can be formed easily by a wafer level CSP. By using an acoustically-transmissive conductive material, in the case of being attached to a mobile telephone etc., there is no need the aperture in the cover is aligned with that of the mobile telephone and the microphone device is attached to the mobile telephone easily
The present invention includes the microphone device, wherein the cover is formed by processing a semiconductor substrate by an MEMS process.
By this configuration, using photolithography, an aperture having the desired diameter and aperture ratio can be formed in the cover easily, and a magnetic shield effect can also be maintained high. According to the configuration described above, further miniaturization and thinning can be achieved.
In accordance with the present invention, there is provided a method of manufacturing a microphone device, including the steps of: forming a microphone element using a semiconductor manufacturing process; forming a signal processor for performing predetermined arithmetic processing based on an output signal of the microphone element; forming a cover, at least a part of the cover having an acoustically-transmissive conductive structure; and disposing the cover over the microphone element and the signal processor.
The present invention includes the method of manufacturing the microphone device, wherein the step of forming the cover includes a step of forming multiple holes in a metal plate by punching (forming a perforated structure).
The present invention includes the method of manufacturing the microphone device, wherein the step of forming the cover includes a step of forming a mesh structure in the cover by a metal material.
The present invention includes the method of manufacturing the microphone device, including a step of integrating and forming the microphone element and the signal processor inside the common substrate.
The present invention includes the method of manufacturing the microphone device, including the steps of: forming plural sets of microphone elements and signal processors on a semiconductor wafer; aligning a metal plate having multiple holes with the semiconductor wafer, and bonding the metal plate to the semiconductor wafer via a spacer, so as to form a bonded body; and dividing the bonded body along a dicing line, wherein a microphone device including at least one of the microphone elements and at least one of the signal processors is formed.
The present invention includes the method of manufacturing the microphone device, wherein the step of forming the bonded body includes the steps of: forming multiple holes by performing punching process in a metal plate and forming a projection part used as a spacer by performing folding process; and bonding the projection part to the semiconductor wafer.
According to the present invention, by disposing a cover comprising an acoustically-transmissive conductive structure over a MEMS microphone element with high accuracy and excellent stability manufactured using a MEMS technique, Helmholtz resonance at an audible frequency range can be avoided and flat frequency characteristics can be obtained and faithful sound pickup can be achieved easily even at a high region.
In other words, Helmholtz resonance at an audible frequency range can be avoided by a mesh structure formed in a cover.
Also, a one-modularized microphone device capable of performing stable sound pickup with high accuracy can be obtained by receiving a signal processor in addition to the microphone element inside the cover.
Also, a shielding effect of electromagnetic-wave noise can be obtained by forming a conductive mesh.
Also, a microphone device in which attachment to a mobile telephone etc. is facilitated, and positioning is facilitated in the case of mounting is implemented.
Moreover, an extremely miniature microphone device with excellent frequency characteristics can be provided by mounting a cover by a wafer level CSP.
Next, embodiments of the present invention will be described with reference to the drawings.
This microphone device includes a microphone element manufactured using a semiconductor manufacturing process, a signal processor for performing predetermined arithmetic processing based on an output signal of the microphone element, and a cover 103 comprising an acoustically-transparent (acoustically-transmissive) mesh structure over the microphone element and the signal processor, and preventing Helmholtz resonance at an audible frequency range as shown in
As shown, the microphone device of the embodiment adopts a cover having an acoustically-transparent (acoustically-transmissive) mesh structure as the cover 103.
Sounds essentially go straight and a diffraction phenomenon does not occur unless path interference under a predetermined condition occurs. Hence, the whole cover comprises an acoustically-transparent (acoustically-transmissive) mesh structure (this mesh has a structure having multiple holes with diameters of the extent to which a bad influence by diffraction of sound is not caused) and sound arriving from a sound source go straight as they are and reach each of the microphone elements. In addition, the whole cover comprises the mesh structure (mesh structure 103m) herein, but the mesh structure is disposed corresponding to the microphone element. It is preferable that a mesh structure may be formed only in a region opposed to a microphone element. Also, the cover 103 may be constructed by a sintered body made of nitride etc. or oxide of metal or (sintered) metal such as titanium, nickel or chromium. In this case, the cover 103 may be constructed by a porous sintered conductive material having holes in a part or all of the cover.
Consequently, sounds from the sound source go straight as they are and reach microphone element without being blocked by the cover 130 of the microphone device. That is, sound can be picked up faithfully without a bad influence caused by Helmholtz resonance.
Also, a shielding effect of electromagnetic-wave noise can be obtained by a mesh structure formed by processing a material such as metal having conductivity.
The substrate 101 is a printed circuit board on which the MEMS chip 102 is mounted. The size of a mounting surface of the substrate 101, the microphone element mounting thereon, is substantially 3×4 mm (3 mm long and 4 mm wide).
The MEMS chip 102 is a chip for converting a sound signal captured by a vibrating film electrode 43 into an electrical signal as shown in
The vibrating film electrode 43 is formed by doped polysilicon having conductivity and the electret film 44 is formed by a silicon nitride film or a silicon oxide film and also, the fixed electrode 46 is constructed by doped polysilicon, a silicon oxide film and a silicon nitride film which are laminated.
Also, an amplifier 48 for amplifying an electrical signal from the MEMS chip 102 is electrically connected to the MEMS chip 102 by a wire 49. The MEMS chip 102 and the amplifier 48 are covered with the cover 103.
The microphone device is manufactured as below. Firstly, a semiconductor chip 48 as a signal processor for performing predetermined arithmetic processing based on an output signal of a microphone element is formed while forming the MEMS chip 102 as the microphone element using a semiconductor manufacturing process. Next, these chips are mounted on the substrate 101 and are connected electrically by wire, the cover 103 comprising a metal mesh structure is attached to the substrate 101
In a capacitor microphone element (MEMS microphone element) manufactured using a microfabrication technique (MEMS technique) of silicon LSI, processing accuracy is higher than that of a microphone element manufactured by assembly of mechanical components and accuracy of acoustoelectric conversion is high and stable. Using this advantage, a microphone element manufactured by a semiconductor manufacturing process is covered by the cover 103 and a microphone device (microphone module) is constructed. However, when this cover constructs a resonance chamber, frequency characteristics reduces and sound cannot be picked up faithfully, so that the cover having a mesh structure is adopted in the embodiment.
In the embodiment, Helmholtz resonance does not occur at an audible frequency range because the microphone device has a cover in which an acoustically-transmissive mesh structure is formed.
The whole surface of the cover of the first embodiment shown in
The other portions than the cover are formed in a manner similar to the first embodiment. Here, the mesh structure 103m is disposed in an opening formed in the cover body 103s, and is bonded using an adhesive. The opening is formed in the cover so as to make sounds arrive at a vibrating plate of the microphone element.
For example, the mesh structure 103m is formed using a coarse mesh sheet (cloth). As the coarse mesh sheet, a knit-shaped mesh comprising stitches in which a conductive stringy material is knitted or a punching mesh sheet in which fine small holes are bored in a thin metal sheet, etc. can be used and a width of one pitch of its mesh coarseness is suitably about 0.5 mm to 5.0 mm.
By forming at least a part of the cover 103 in an acoustically-transparent (acoustically-transmissive) mesh structure thus, a situation in which the inside of the cover is formed in a resonance chamber is avoided and faithful sound pickup characteristics can be obtained.
Also, a shielding effect of electromagnetic-wave noise can be obtained by forming a conductive mesh.
The whole surface of the cover of the first embodiment shown in
The other portions than the cover are formed in a manner similar to the first embodiment.
The holes 103h are formed so as to become, for example, an aperture ratio of 25% or more.
Here, in the case of being constructed so that an audible frequency is set at 20 hHz and a parameter such as an aperture width d is obtained so as to become larger than this audible frequency and a resonance point becomes larger than its aperture width d, Helmholtz resonance does not occur.
This resonance frequency is given by the following formula as described above.
where, fr is a resonance frequency; c is a sound speed; r is the circular ratio; d is a diameter of an aperture; V is volume of a front air chamber; l is a length of the aperture (i.e., thickness of the cover); s is an area of the aperture; and d′is l+0.6d.
In the case of l<<0.6d, d′0.6d is satisfied, so that the following formula is derived from formula 1.1.
fr∝√{square root over (d)} (Formula 1.2)
Also, s∝d2 is satisfied, so that the following formulas are derived from formula 1.1 and formula 1.2 in the case of l<<0.6d.
l<<1.2√{square root over (s/π)} (Formula 1.4)
For example, when d=2 mm is set in formula 1.1 described above, fr becomes 24 kHz and a resonance point is outside an audible frequency range.
Also, when d=2 mm and an aperture area S=3 mm2 are set and the size of a surface having the aperture is substantially 3×4 mm, an aperture ratio of a surface having the aperture could be about 25%.
That is, the aperture ratio of a surface having the aperture could be 25% or more. An upper limit of this aperture ratio depends on a mechanical strength of a material. That is, the aperture ratio could be determined within a range capable of maintaining the mechanical strength.
A resonance frequency is shown in the following table 1 when using the microphone device of the present invention.
On the other hand, it is shown in the following table 2 when using a conventional microphone device.
By forming a structure having an accoustically-transparent (acoustically-transmissive) opening in at least a part of the cover 103 thus, a situation in which a resonance chamber is formed in the inside of the cover is avoided and faithful sound pickup characteristics can be obtained.
Also, a shielding effect of electromagnetic-wave noise can be obtained by forming a hole in a conductive base substance.
Also, the cover 103 may be constructed so that a porous material is impregnated with a solvent including metal particles. Or, the cover may be constructed so that a material including conductive particles such as metal is molded and thus cover has porous.
In addition, in the embodiment described above, a microphone element chip and a signal processing circuit chip are formed by being mounted on a substrate, but LSI of MEMS microphone elements with high accuracy and excellent stability may be performed in a parallel arranged state. Moreover, a cover made of silicon in which fine holes are formed by a photolithography process in an MEMS process using the same silicon substrate as an LSI chip in which a microphone element and a signal processing circuit are installed as a start material may be adopted.
The present embodiment is characterized in that LSI of a microphone element chip and a signal processing circuit chip is performed and a MEMS chip formed on the same silicon substrate is accommodated in a cover 103 constructed by a punching metal.
A MEMS chip 102 is a chip for converting a sound signal captured by a vibrating film electrode 43 into an electrical signal in a manner similar to the MEMS chip 102 of the first embodiment shown in
Also, an amplifier 48 for amplifying an electrical signal of the MEMS chip 102 is electrically connected to a fixed electrode 46 through a through hole (not shown). Also, the MEMS chip 102 in which this amplifier 48S is also integrated is covered with the cover 103 constructed by the punching metal.
The microphone device is manufactured as below.
As shown in
On the other hand, as shown in
Next, the silicon wafer 1 is bonded to the metal plate 103W with an adhesive. In this case, the dicing lines of the silicon wafer 1 is aligned and overlapped with those of the metal plate 103W.
Next, it is divided into individual microphone devices along the dicing lines. As the result, the microphone device shown in
According to this configuration, the microphone device having faithful sound pickup characteristics can be obtained extremely easily. Also, the device is a microphone device of a chip size, so that an extremely fine outer shape can be obtained.
In addition, in the embodiment described above, the punching metal is used as the cover, but a mesh structure may be constructed by a metal material and be mounted in like manner.
Also, in the case of forming a body of bonding between a silicon wafer in which a microphone element and a signal processing circuit are formed and a metal plate of a wafer level in which shape processing of a punching metal is performed, the metal plate in which the protrusion part is formed using the metal mold is used, but a spacer may be formed by other member or a projection part used as a spacer may be formed by performing folding processing.
An example of using a MEMS microphone 100 of the present invention in a mobile telephone will be described.
In a cabinet 151 of the mobile telephone 150 shown in
A gasket 154 is sandwiched between an inside surface of the cabinet 151 and a top portion 103a of a cover of the MEMS microphone 100. As shown in
Also, a hole 154a is formed in the gasket 154 with substantially the same shape as aperture formed in the mobile telephone 152. Also, an acoustic resistance material 154b is formed in the end of the cabinet side of the hole 154a. This acoustic resistance material 154b reduces a propagation speed of a sound signal, and performs a function of adjusting acoustic characteristics of the MEMS microphone 100 herein.
A thickness of the gasket 154 is slightly thicker than a gap between the inside surface of the cabinet 151 and the top portion of the cover 103a and the gasket 154 is sandwiched in close contact from the cover 103 to the end of the top portion 103a.
In other words, as a region in which the gasket 154 is sandwiched, a distance from an aperture 103c in the cover to each end of the top portion 103a is designed to respectively have a spacing of 1 mm or more, so that airtightness after the gasket 154 is sandwiched is ensured.
Therefore, a sound signal entering from the aperture 152 in the cabinet does not leak in the gap between the inside surface of the cabinet 151 and the top portion 103a and acoustic characteristics of the MEMS microphone 100 are not damaged.
The sound entering from the aperture 152 in the cabinet passes through the acoustic resistance material 154b and passes through the cover 103 of the metal mesh structure and propagates to a vibrating film electrode 43 of an MEMS chip. Capacitance of a plate capacitor constructed by the vibrating film electrode 43 and a fixed electrode 46 varies and the sound is fetched as a change in voltage.
According to this configuration, the miniaturized MEMS microphone 100 can be installed in a mobile telephone, so that a shape of the whole mobile telephone 150 can be miniaturized and thinned.
Thus, without adding a special step and requiring high-accuracy alignment, mounting can be performed with extremely good workability and the miniature MEMS microphone device 100 with high reliability can be obtained.
The present invention can form a microphone device which has excellent sound pickup characteristics and avoids Helmholtz resonance at an audible frequency range by an extremely simple configuration, so that the present invention is useful as a microminiature microphone device (for example, a microminiature electret capacitor microphone array module).
| Number | Date | Country | Kind |
|---|---|---|---|
| P2007-209123 | Aug 2007 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2008/002181 | Aug 2008 | US |
| Child | 12610811 | US |
