Patent Publication Number: US-2010119097-A1

Title: Microphone device and manufacturing method thereof

Description:
BACKGROUND OF THE INVENTION 
     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. 
       FIG. 10  shows an outline perspective view of a conventional MEMS microphone.  FIG. 11A  is a side view of the conventional MEMS microphone. 
       FIG. 11B  is a plan view of the conventional MEMS microphone.  FIG. 11C  is a cross-sectional view taken along line A-A in  FIG. 10 , showing the conventional MEMS microphone. 
     The conventional MEMS microphone  300  shown in  FIGS. 10 and 11A  to  11 C includes a substrate  301 , a MEMS chip  200  and a cover  303 . Here, the MEMS chip  200  is a chip constructing a microphone element for converting a sound signal into an electrical signal. 
     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  303   c  in a top portion  303   a  of the cover. Also, the MEMS microphone  300  is bonded to the substrate  301  through an adhesive  303   c  at the end  303   d  of a side portion  303   b  (for example, see JP-A-2000-165998). 
     SUMMARY OF THE INVENTION 
     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. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     f 
                     r 
                   
                   = 
                   
                     
                       
                         c 
                         
                           2 
                            
                           π 
                         
                       
                        
                       
                         
                           
                             π 
                              
                             
                                 
                             
                              
                             
                               d 
                               2 
                             
                           
                           
                             4 
                              
                             
                               V 
                                
                               
                                 ( 
                                 
                                   l 
                                   + 
                                   
                                     0.6 
                                      
                                     d 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     = 
                     
                       
                         c 
                         
                           2 
                            
                           π 
                         
                       
                        
                       
                         
                           s 
                           
                             V 
                              
                             
                                 
                             
                              
                             
                               d 
                               ′ 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1.1 
                   
                   ) 
                 
               
             
           
         
       
     
     where, f r  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&lt;&lt;0.6d, d′≈0.6d is satisfied, so that the following formula is derived from formula 1.1. 
       f r ∝√{square root over (d)}  (Formula 1.2) 
     Also, s∝d 2  is satisfied, so that the following formulas are derived from formula 1.1 and formula 1.2 in the case of l&lt;&lt;0.6d. 
     
       
         
           
             
               
                 
                   
                     f 
                     r 
                   
                   ∝ 
                   
                     s 
                     4 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1.3 
                   
                   ) 
                 
               
             
           
         
       
     
     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 mm 2 , 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 d 1  of an aperture for, for example, 20 kHz&lt;f r  is obtained. Next, a diameter d of an aperture of the present invention is set to be larger than d 1 . 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, f r  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 mm 2  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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing a microphone device of a first embodiment of the present invention. 
         FIG. 2  is a sectional view of a device for explaining a structure of a microphone element (MEMS microphone element) manufactured by a manufacturing process of silicon LSI shown in  FIG. 1 . 
         FIG. 3  shows a microphone device of a second embodiment of the present invention. 
         FIG. 4  shows a microphone device of a third embodiment of the present invention. 
         FIG. 5  shows a microphone device of a fourth embodiment of the present invention. 
         FIGS. 6A and 6B  show a manufacturing step of the microphone device of the fourth embodiment of the present invention. 
         FIGS. 7A and 7B  show a manufacturing step of the microphone device of the fourth embodiment of the present invention. 
         FIG. 8  shows a mobile telephone using a microphone device of a fifth embodiment of the present invention. 
         FIG. 9  is a sectional view taken on line A-A of  FIG. 8 . 
         FIG. 10  is a sectional view showing a structure of a conventional example. 
         FIGS. 11A to 11C  are sectional views showing the structure of the conventional example. 
         FIG. 12  shows frequency characteristics of microphone devices of a conventional example and embodiments of the present invention respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, embodiments of the present invention will be described with reference to the drawings. 
     First embodiment 
       FIG. 1  shows an outline perspective view of an MEMS microphone  100  of the first embodiment.  FIG. 2  shows a longitudinal sectional view (sectional view taken on line B-B of  FIG. 1 ) of the MEMS microphone  100 . As shown in  FIGS. 1 and 2 , the MEMS microphone  100  has a substrate  101 , a MEMS chip  102  and a cover  103 .  FIGS. 1 and 2  are sectional views showing an example of the cover comprising an acoustically-transmissive mesh structure, and  FIG. 2  is a sectional view showing a microphone element of a MEMS structure used herein. 
     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  FIG. 1 . Reference numeral  101  is a substrate on which the sound pickup element and the signal processor are mounted. 
     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  103   m ) 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  FIG. 2 . Concretely, the MEMS chip  102  has the vibrating film electrode  43  and an electret film  44  on a silicon substrate  41  (a first insulating layer  42  is interposed therebetween) and also has a fixed electrode  46 , in which apertures  47  are formed,and a second insulating layer  45  is formed between the fixed electrode and the substrate. Also, a back air chamber  55  formed by etching the silicon substrate  41  is formed at the side of a back surface of the vibrating film electrode  43 . The MEMS (Micro Electro Mechanical System) chip is an electromechanical element chip constructed by a minute component formed using a microfabrication technique of a semiconductor. 
     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. 
     Second Embodiment 
       FIG. 3  is a sectional view showing another example of a microphone device of the present invention. In  FIG. 3 , the same numerals are assigned to portions common to the diagram described in the first embodiment. 
     The whole surface of the cover of the first embodiment shown in  FIG. 2  is constructed by the mesh structure, but in the present embodiment, a mesh structure  103   m  is disposed corresponding to a MEMS chip  102  and the other region including a side surface is made of a metal substance as shown in  FIG. 3 . 
     The other portions than the cover are formed in a manner similar to the first embodiment. Here, the mesh structure  103   m  is disposed in an opening formed in the cover body  103   s,  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  103   m  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. 
     Third Embodiment 
       FIG. 4  is a sectional view showing another example of a microphone device of the present invention. In  FIG. 4 , the same numerals are assigned to portions common to the diagrams described in the first and second embodiments. 
     The whole surface of the cover of the first embodiment shown in  FIG. 2  is constructed by the mesh structure, but the embodiment is characterized in that a cover  103  has a punching metal (perforated structure) in which holes  103   h  are formed in a region opposed to a MEMS chip  102  as shown in  FIG. 4 . 
     The other portions than the cover are formed in a manner similar to the first embodiment. 
     The holes  103   h  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. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     f 
                     r 
                   
                   = 
                   
                     
                       
                         c 
                         
                           2 
                            
                           π 
                         
                       
                        
                       
                         
                           
                             π 
                              
                             
                                 
                             
                              
                             
                               d 
                               2 
                             
                           
                           
                             4 
                              
                             
                               V 
                                
                               
                                 ( 
                                 
                                   l 
                                   + 
                                   
                                     0.6 
                                      
                                     d 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     = 
                     
                       
                         c 
                         
                           2 
                            
                           π 
                         
                       
                        
                       
                         
                           s 
                           
                             V 
                              
                             
                                 
                             
                              
                             
                               d 
                               ′ 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1.1 
                   
                   ) 
                 
               
             
           
         
       
     
     where, f r  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&lt;&lt;0.6d, d′0.6d is satisfied, so that the following formula is derived from formula 1.1. 
       f r ∝√{square root over (d)}  (Formula 1.2) 
     Also, s∝d 2  is satisfied, so that the following formulas are derived from formula 1.1 and formula 1.2 in the case of l&lt;&lt;0.6d. 
     
       
         
           
             
               
                 
                   
                     f 
                     r 
                   
                   ∝ 
                   
                     s 
                     4 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1.3 
                   
                   ) 
                 
               
             
           
         
       
     
         l&lt;&lt; 1.2 √{square root over (s/π)}   (Formula 1.4) 
     For example, when d=2 mm is set in formula 1.1 described above, f r  becomes 24 kHz and a resonance point is outside an audible frequency range. 
     Also, when d=2 mm and an aperture area S=3 mm 2  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. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Unit 
                 Unit 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 c 
                 340 
                 m/sec 
                   
                   
               
               
                   
                 π 
                 3.141593 
               
               
                   
                 d 
                 2 
                 mm 
                 0.002 
                 M 
               
               
                   
                 V 
                 12 
                 mm 3   
                 0.000000012 
                 mm 3   
               
               
                   
                 I 
                 0.1 
                 mm 
                 0.0001 
                 M 
               
               
                   
                 f r   
                 24.28352 
                 kHz 
               
               
                   
                   
               
            
           
         
       
     
     On the other hand, it is shown in the following table 2 when using a conventional microphone device. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Unit 
                 Unit 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 c 
                 340 
                 m/sec 
                   
                   
               
               
                   
                 π 
                 3.141593 
               
               
                   
                 d 
                 0.6 
                 mm 
                 0.0006 
                 M 
               
               
                   
                 V 
                 12 
                 mm 3   
                 0.000000012 
                 mm 3   
               
               
                   
                 I 
                 0.1 
                 mm 
                 0.0001 
                 M 
               
               
                   
                 f r   
                 12.24689 
                 kHz 
               
               
                   
                   
               
            
           
         
       
     
     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. 
       FIG. 12  shows frequency characteristics of a conventional microphone device and the microphone device of the present invention respectively. The cover of the conventional microphone device does not have the mesh structure, thus in the conventional microphone device Helmholtz resonance occurs. Reference sign ‘a’ shows frequency characteristics of the conventional microphone device. On the other hand, the cover of the microphone device of the present invention has the acoustically-transmissive conductive structure, thus in the microphone device of the present invention Helmholtz resonance does not occur. Reference sign ‘b’ shows frequency characteristics of the present invention. By the cover comprising an acoustically-transmissive conductive structure as described in the present invention, as shown by the curve ‘b’, Helmholtz resonance does not occur at a usable frequency range in the cover, thus faithful sound pickup can be achieved. 
     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. 
     Fourth Embodiment 
       FIG. 5  is a sectional view showing a microphone device of a fourth embodiment of the present invention. In  FIG. 5 , the same numerals are assigned to portions common to the diagram described in the first embodiment. 
     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  FIG. 2 , and is formed in a manner similar to the first embodiment except that an electronic circuit such as an amplifier  48 S as a signal processing circuit is integrated into this chip, and the same numerals are assigned to the same portions. 
     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  48 S is also integrated is covered with the cover  103  constructed by the punching metal. 
     The microphone device is manufactured as below. 
     As shown in  FIGS. 6A and 6B , many element regions are formed in a silicon wafer  1 . In each of the element region a signal processing circuit such as the amplifier  48 S and a microphone element are integrated using a semiconductor manufacturing process.  FIG. 6B  is a cross-sectional view taken along line A-A in  FIG. 6A , showing the microphone element comprising the amplifier  48 S. As shown in  FIGS. 6A and 6B , a region  43  surrounded by dicing lines DL corresponds to the MEMS chip  102 . 
     On the other hand, as shown in  FIGS. 7A and 7B , punching holes (perforated structure)  103   h  are formed by punching a metal plate  103 W corresponding to the silicon wafer.  FIG. 7B  shows the cover  103  which is formed by processing the metal plate  103 W shown in  FIG. 7A . As shown in  FIGS. 7A and 7B , a region surrounded by lines corresponding to dicing lines DL shown in  FIG. 6A  is the cover  103 . Processing the metal plate  103 W comprises the step of forming protrusion portion corresponding to the MEMS chip  102 . 
     Next, the silicon wafer  1  is bonded to the metal plate  103 W with an adhesive. In this case, the dicing lines of the silicon wafer  1  is aligned and overlapped with those of the metal plate  103 W. 
     Next, it is divided into individual microphone devices along the dicing lines. As the result, the microphone device shown in  FIG. 5  is completed. 
     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. 
     Fifth Embodiment 
     An example of using a MEMS microphone  100  of the present invention in a mobile telephone will be described.  FIG. 8  is an outline perspective view of a mobile telephone  150  in which the MEMS microphone  100  is installed.  FIG. 9  is a main sectional view (sectional view taken on line E-E in  FIG. 8 ) of the vicinity of a microphone part of the mobile telephone  150 . 
     In a cabinet  151  of the mobile telephone  150  shown in  FIG. 8 , an aperture in the mobile telephone  152  for microphone is formed in a position in the vicinity of a mouth of a user. 
     A gasket  154  is sandwiched between an inside surface of the cabinet  151  and a top portion  103   a  of a cover of the MEMS microphone  100 . As shown in  FIG. 9 , a cover  103  ( 103   m ) of a metal mesh structure is positioned in the periphery of the aperture  152  in the cabinet  151 , so that 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. 
     Also, a hole  154   a  is formed in the gasket  154  with substantially the same shape as aperture formed in the mobile telephone  152 . Also, an acoustic resistance material  154   b  is formed in the end of the cabinet side of the hole  154   a.  This acoustic resistance material  154   b  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  103   a  and the gasket  154  is sandwiched in close contact from the cover  103  to the end of the top portion  103   a.    
     In other words, as a region in which the gasket  154  is sandwiched, a distance from an aperture  103   c  in the cover to each end of the top portion  103   a  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  103   a  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  154   b  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).