Abstract:
The disclosed substrate ( 12 ) has an electro-acoustic conversion element ( 11 ), which converts sound signals into electric signals, mounted thereon. Furthermore, the substrate is provided with: a mounting surface ( 12   a ) in which an opening ( 121 ) covered by the electro-acoustic conversion element ( 11 ) is formed; an intra-substrate space ( 122 ) connected to the opening ( 121 ); and a coating layer (CL) that covers at least part of the wall surface ( 122   a ) of the intra-substrate space ( 122 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an electro-acoustic conversion device mount substrate that is mounted with an electro-acoustic conversion device that converts a sound signal into an electric signal and to a microphone unit that includes the electro acoustic conversion device mount substrate. Besides, the present invention relates to a manufacturing method for the electro-acoustic conversion device mount substrate and microphone unit. 
       BACKGROUND ART 
       [0002]    Conventionally, a microphone, which has a function to convert an input sound into an electric signal and output it, is applied to various types of voice input apparatuses (e.g., voice communication apparatuses such as a mobile phone, a transceiver and the like, information process apparatuses such as a voice identification system and the like that use a technology for analyzing an input voice, recording apparatuses and the like). 
         [0003]    A microphone unit includes an electro-acoustic conversion device that converts a sound signal into an electric signal. The electro-acoustic conversion device is mounted on a substrate (electro-acoustic conversion device mount substrate) on which a wiring pattern is formed as shown in patent documents 1 and 2, for example. There is a case where an electro-acoustic conversion device, as shown in the patent document 1, is mounted on a substrate to cover an opening that connects to an intra-substrate space (which functions as a rear chamber in some cases and functions as a sound hole (sound path) in other cases) which is formed in the substrate. 
         [0004]    Here, an “intra-substrate space” in the present specification is a space that is formed at a more inner portion with respect to a substrate outer circumferential surface (assuming that an opening surface forms the outer circumferential surface at a portion where the opening is formed) as a reference surface. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PLT1: JP-A-2008-510427 
         PLT2: JP-A-2010-41565 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    In the meantime, there is a case where as an electro-acoustic conversion device included in a microphone unit, a MEMS (Micro Electro Mechanical System) chip formed by using a semiconductor manufacturing technology is used because of reasons that size reduction is possible and the like. The MEMS chip includes a diaphragm and a fixed electrode that is disposed to oppose the diaphragm with a gap therebetween and forms a capacitor together with the diaphragm. 
         [0008]    In the MEMS chip, the gap formed between the diaphragm and the fixed electrode is, for example, 1 μm, which is narrow. Because of this, if dust invades the gap, defective operation of the MEMS chip is caused. 
         [0009]    As for a substrate such as an FR-4 substrate (glass epoxy substrate) and the like that includes a resin fiber, a fiber garbage (an example of dust) easily occurs from a surface that is scraped to form a through-hole, a groove and the like, for example. Because of this, in a microphone unit (e.g., shown in the patent document 1) having a structure which is mounted with a MEMS chip to cover an opening that connects to an intra-substrate space (which has a surface to which machining such as scraping and the like is applied), there is a problem that if a substrate such as an FR-4 substrate that easily emits dust is employed as the substrate, defective operation of the MEMS chip easily occurs. 
         [0010]    In light of the above points, it is an object of the present invention to provide an electro-acoustic conversion device mount substrate that is able to reduce the likelihood that an electro-acoustic conversion device malfunctions because of dust. Besides, it is another object of the present invention to provide a small and high-quality microphone unit for which anti-dust measures are taken by including the electro-acoustic conversion device mount substrate. Further, it is another object of the present invention to provide a preferred method for manufacturing the electro-acoustic conversion device mount substrate and the microphone unit. 
       Solution to Problem 
       [0011]    To achieve the above objects, an electro-acoustic conversion device mount substrate according to the present invention is an electro-acoustic conversion device mount substrate that is mounted with an electro-acoustic conversion device which converts a sound signal into an electric signal, the electro-acoustic conversion device mount substrate includes: a mount surface on which the electro-acoustic conversion device is mounted and which is provided with an opening that is covered by the electro-acoustic conversion device; an intra-substrate space that connects to the opening; and a coating layer that covers at least a portion of a wall surface of the intra-substrate space. 
         [0012]    According to this structure, it is possible to obtain a state in which dust is unlikely to occur by covering a surface, to which machining such as severing, scraping and the like is applied, by means of the coating layer. Because of this, it is easy to prevent malfunction of the electro-acoustic conversion device by using the electro-acoustic conversion device mount substrate that has the present structure. 
         [0013]    In the electro-acoustic conversion device mount substrate having the above structure, the coating layer may be a plated layer. According to this structure, it is easy to form the coating layer as well for anti-dust measures concurrently when forming, for example, a through-wiring through the electro-acoustic conversion device mount substrate, which is convenient. 
         [0014]    Besides, in the electro-acoustic conversion device mount substrate having the above structure, a glass epoxy material may be used as a substrate material. As described above, the glass epoxy substrate easily emits dust from the surface to which machining such as severing, scraping and the like is applied. Because of this, in the case of this structure, the effect of the anti-dust measures due to the coating process becomes great. 
         [0015]    Besides, in the electro-acoustic conversion device mount substrate having the above structure, the intra-substrate space may or may not connect to an opening other than the opening that is covered by the electro-acoustic conversion device. Further, in a case where the intra-substrate space connects to an opening other than the opening that is covered by the electro-acoustic conversion device, the other opening may be disposed through a rear surface which is opposite to the mount surface, or may be disposed through the mount surface. A microphone unit is produced into various forms depending on the purpose, and the electro-acoustic conversion device mount substrate according to the present invention is widely applicable to the various forms. 
         [0016]    To achieve the above objects, a microphone unit according to the present invention includes: the electro-acoustic conversion device mount substrate having the above structure; the electro-acoustic conversion device that is mounted on the mount surface to cover the opening; and a cover portion that collaborates with the electro-acoustic conversion device mount substrate to form a housing space for housing the electro-acoustic conversion device. 
         [0017]    The microphone unit having the above structure is unlikely to emit dust in the intra-substrate space, accordingly, malfunction of the electro-acoustic conversion device is unlikely to occur. In other words, according to this structure, it is possible to provide the microphone unit that has a high quality. 
         [0018]    In the microphone unit having the above structure, the electro-acoustic conversion device may be a MEMS chip that includes: a diaphragm and a fixed electrode that is disposed to oppose the diaphragm with a gap therebetween and forms a capacitor together with the diaphragm. It is possible to form the MEMS chip to be small, because of this, according to this structure, it is possible to provide the microphone unit that is small and has a high quality. 
         [0019]    To achieve the above objects, a method for manufacturing the electro-acoustic conversion device mount substrate according to the present invention includes: a first step for preparing a substrate that is provided with an opening covered by the electro-acoustic conversion device, an intra-substrate space that connects to the opening, and a through-hole for a through-wiring; a second step for applying a plating process to the intra-substrate space and the through-hole for the through-wiring; and a third step for forming a wiring pattern on a substrate outer surface by performing an etching process after the plating process. 
         [0020]    According to this structure, concurrently with the forming of the through-wiring, it is possible to cover, by means of the plated layer (a form of the coating layer), a wall surface of the intra-substrate space that connects to the opening covered by the electro-acoustic conversion device, whereby it is easy to perform the forming of the electro-acoustic conversion device mount substrate to which the anti-dust measures are applied. 
         [0021]    The method for manufacturing the electro-acoustic conversion device mount substrate having the above structure may further include: a fourth step for attaching another substrate to a rear surface opposite to a surface of the substrate, on which the wiring pattern is formed in the third process, through which the opening is formed; a fifth step for mounting a protection cover to cover an entire surface of the substrate, on which the wiring pattern is formed in the third process, through which the opening is formed; a sixth step for forming a through-hole for a through-wiring through the other substrate; a seventh step for applying a plating process to the through-hole for the through-wiring that is formed in the sixth step after completion of the fourth step, the fifth step and the sixth step that are performed in random order; an eighth step for forming a wiring pattern on the other substrate by means of an etching process after the seventh step is completed; and a ninth step for separating the protection cover after the wiring pattern is formed on the other substrate. 
         [0022]    For example, in a case where it is impossible to form the intra-substrate space by only digging in a substrate thickness direction, there is a case where it is convenient to form the electro-acoustic conversion device mount substrate by using a plurality of substrates. This structure envisions the case where the electro-acoustic conversion device mount substrate having the intra-substrate space is formed by using a plurality of substrates. And, in the case where the electro-acoustic conversion device mount substrate is formed by using a plurality of substrates, there is a worry over that a plating process liquid, an etching process liquid and the like invade the intra-substrate space, residues of them remain to the end and a contaminated electro-acoustic conversion device mount substrate is produced. In this point, according to this structure, in expectation of the likelihood that the plating liquid and the like remain in the intra-substrate space in the later steps, the protection cover is mounted beforehand to cover the intra-substrate space, thereafter, the plating process and the etching process are performed. Because of this, it is possible to reduce the likelihood of providing the above contaminated electro-acoustic conversion device mount substrate. 
         [0023]    To achieve the above objects, a method for manufacturing a microphone unit according to the present invention includes: a step for manufacturing an electro-acoustic conversion device mount substrate by means of the manufacturing method having the above structure; a step for mounting the electro-acoustic conversion device onto the electro-acoustic conversion device mount substrate to cover the opening; and a step for placing a cover portion onto the electro-acoustic conversion device mount substrate to cover the electro-acoustic conversion device. 
         [0024]    According to this structure, the electro-acoustic conversion device mount substrate to which the anti-dust measures are applied and which has a low contamination likelihood is manufactured and the microphone unit is assembled by using the electro-acoustic conversion device mount substrate, accordingly, it is possible to provide the microphone unit that has a high quality. 
       Advantageous Effects of Invention 
       [0025]    According to the present invention, it is possible to provide an electro-acoustic conversion device mount substrate that is able to reduce the likelihood that an electro-acoustic conversion device malfunctions thanks to dust. Besides, according to the present invention, by including the electro-acoustic conversion device mount substrate, it is possible to provide a small and high-quality microphone to which anti-dust measures are applied. Further, according to the present invention, it is possible to provide a preferred manufacturing method for the electro-acoustic conversion device mount substrate and the microphone unit. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1  is a schematic sectional view showing a structure of a microphone unit according to a first embodiment to which the present invention is applied. 
           [0027]      FIG. 2  is a schematic sectional view showing a structure of a MEMS chip included in the microphone unit according to the first embodiment. 
           [0028]      FIG. 3  is a sectional view for describing a manufacturing method for a mike substrate included in the microphone unit according to the first embodiment. 
           [0029]      FIG. 4  is a schematic sectional view showing a structure of a microphone unit according to a second embodiment to which the present invention is applied. 
           [0030]      FIG. 5  is a sectional view for describing a manufacturing method for a mike substrate included in the microphone unit according to the second embodiment. 
           [0031]      FIG. 6  is a schematic sectional view showing a structure of a microphone unit according to a third embodiment to which the present invention is applied. 
           [0032]      FIG. 7  is a sectional view for describing a manufacturing method for a mike substrate included in the microphone unit according to the third embodiment. 
           [0033]      FIG. 8  is a schematic sectional view showing a structure of a microphone unit according to a fourth embodiment to which the present invention is applied. 
           [0034]      FIG. 9  is a sectional view for describing a manufacturing method for a mike substrate included in the microphone unit according to the fourth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    Hereinafter, an electro-acoustic conversion device mount substrate, a microphone unit, and manufacturing methods of them are described in detail with reference to the drawings. 
       First Embodiment 
       [0036]      FIG. 1  is a schematic sectional view showing a structure of a microphone unit according to a first embodiment to which the present invention is applied. As shown in  FIG. 1 , a microphone unit  1  according to the first embodiment includes: a MEMS chip  11 ; a mike substrate  12  on which the MEMS chip  11  is mounted; and a cover  13 . The microphone unit  1  according to the first embodiment functions as an omnidirectional mike. 
         [0037]    The MEMS chip  11  including a silicon chip is an embodiment of the electro-acoustic conversion device according to the present invention, and a small capacitor type microphone unit that is manufactured by using a semiconductor manufacturing technology.  FIG. 2  is a schematic sectional view showing a structure of the MEMS chip which the microphone unit according to the first embodiment includes. The MEMS chip  11  has a substantially rectangular parallelepiped shape in outer shape, and as shown in  FIG. 2 , includes: an insulating base substrate  111 ; a fixed electrode  112 ; an insulating intermediate substrate  113 ; and a diaphragm  114 . 
         [0038]    The base substrate  111  is provided with a through-hole  111   a  having a substantially circular shape when viewed from top through its central portion. The plate-shaped fixed electrode  112  is disposed on the base substrate  111  and provided with a plurality of small-diameter (about 10 μm in diameter) through-holes  112   a . The intermediate substrate  113  is disposed on the fixed electrode  112  and, like the base substrate  111 , is provided with a through-hole  113   a  having a substantially circular shape when viewed from top through its central portion. The diaphragm  114  disposed on the intermediate substrate  113  is a thin film which receives a sound pressure to vibrate (i.e., vibrate vertically in  FIG. 2 . Besides, in the present embodiment, the substantially circular portion vibrates), has electrical conductivity and forms an end of the electrode. The fixed electrode  112  and the diaphragm  114 , which are disposed to be in a substantially parallel relationship with each other across a gap Gp thanks to the presence of the intermediate substrate  113 , form a capacitor. 
         [0039]    In the MEMS chip  11 , when a sound wave reaches and the diaphragm  114  vibrates, an inter-electrode distance between the diaphragm  114  and the fixed electrode  112  changes, accordingly, electrostatic capacity changes. As a result of this, it is possible to fetch the sound wave (sound signal) entering the MEMS chip  11  as an electric signal. Here, in the MEMS chip  11 , thanks to the presence of the through-hole  111   a  formed through the base substrate  111 , the plurality of through-holes  112   a  formed through the fixed electrode  112  and the through-hole  113   a  formed through the intermediate substrate  113 , a lower surface of the diaphragm  114  also is able to communicate with an outside space (outside the MEMS chip  11 ). 
         [0040]    The mike substrate  12 , which is formed to have a substantially rectangular shape when viewed from top, is an embodiment of the electro-acoustic conversion device mount substrate according to the present invention, and on an upper surface  12   a  of which the MEMS chip  11  is mounted. Although skipped in  FIG. 1 , the mike substrate  12  is provided with a wiring pattern (inclusive of a through-wiring) that is necessary to apply a voltage to the MEMS chip  11  and to fetch an electric signal from the MEMS chip  11 . 
         [0041]    Besides, the mike substrate  12  is provided with an opening  121  through the mount surface (the upper surface)  12   a  on which the MEMS chip  11  is mounted, and the MEMS chip  11  is disposed to cover the opening  121 . The opening  121  connects to an intra-substrate space  122  that has a substantially cylindrical shape. The intra-substrate space  122  connects to only the opening  121  but does not connect to another opening. In other words, the mike substrate  12  is provided with a recess by means of the opening  121  and the intra-substrate space  122 . The intra-substrate space  122  is disposed with intention of increasing a volume of a rear chamber (a tightly closed space that faces a lower surface of the diaphragm  114 ). If the rear chamber volume increases, the diaphragm  114  is easily displaced, and the mike sensitivity of the MEMS chip  11  improves. 
         [0042]    Here, the mike substrate  12  may be, for example, an FR-4 (glass epoxy substrate) substrate, however, may be another kind of substrate. 
         [0043]    The cover  13 , which is formed to have a substantially rectangular-parallelepiped shape in outer shape, is placed over the mike substrate  12 , thereby collaborating with the mike substrate  12  to form a housing space  14  that houses the MEMS chip  11 . The cover  13  is provided with a sound hole  131  that guides a sound occurring outside the microphone unit  1  to the diaphragm  114  of the MEMS chip  11 . Here, the cover  13  is an embodiment of a cover portion of the present invention. 
         [0044]    When a sound wave input into the housing space  14  via the sound hole  131  reaches the diaphragm  114 , the diaphragm  114  vibrates, whereby as described above, a change in the electrostatic capacity occurs. The microphone unit  1  is structured to fetch the change in the electrostatic capacity as an electric signal and to output the electric signal. Here, it is preferable that an electric circuit portion for fetching the change in the electrostatic capacity as an electric signal is disposed in the housing space  14 ; however, the electric circuit portion may be disposed outside the housing space  14 . Besides, the electric circuit portion may be monolithically formed on a silicon substrate that forms the MEMS chip  11 . 
         [0045]    In the meantime, in the microphone unit  1  according to the first embodiment, a wall surface  122   a  (in the present embodiment, the entire wall surface of the intra-substrate space  122 ) of the intra-substrate  122  formed in the mike substrate  12  is covered by a coating layer CL. The covering by the coating layer CL is obtainable by, for example, a plating process, and the coating layer CL may be, for example, a metal plated layer such as a Cu plated layer and the like. Thanks to the covering by the coating layer CL, it is possible to reduce a likelihood that dust occurs in the intra-substrate space  122  of the mike substrate  12 . 
         [0046]    In a case where the mike substrate  12  is composed of, for example, a glass epoxy substrate (FR-4 substrate), a fiber-like dust easily occurs from a machined surface (a surface to which machining such as severing, scraping or the like is applied) of the mike substrate  12 . In a case where the wall surface  122   a  of the intra-substrate space  122  is not covered by means of the coating layer CL (in a case different from the present embodiment), dust easily enters the MEMS chip  11  that is disposed to cover the opening  121  which connects to the intra-substrate space  122 . The invasion of dust into the MEMS chip  11  causes malfunction of the MEMS chip  11 . As an example, there is a situation in which dust enters from the through-hole  112   a  disposed through the fixed electrode  112  and clogs the gap Gp (see  FIG. 2 ) between the fixed electrode  112  and the diaphragm  114 . Regarding this point, in the microphone unit  1  according to the first embodiment, thanks to the presence of the coating layer CL, dust is unlikely to occur from the intra-substrate space  122 , and it is possible to reduce the likelihood that the MEMS chip  11  malfunctions. 
         [0047]    Next, methods for manufacturing the mike substrate  12  and the microphone unit  1  described above are described with chief reference to  FIG. 3 .  FIG. 3  is a sectional view for describing a manufacturing method for the mike substrate that the microphone unit according to the first embodiment includes, of which (a) to (f) show states during the manufacturing, and (g) shows a state in which the mike substrate is completed. 
         [0048]    When manufacturing the mike substrate  12 , first, a substrate  12 ′ (flat-plated shape), whose upper surface and lower surface are covered by a metal material (electro-conductive material)  101  such as Cu or the like, is prepared (step a; see  FIG. 3  ( a )). The thickness of the substrate  12 ′ is 1.0 mm for example, and the thickness of the electro-conductive material  101  is 0.15 μm. 
         [0049]    At a substantially central position of the prepared substrate  12 ′, the substrate  12 ′ is dug from the upper surface to a position in a thickness direction (the vertical direction of  FIG. 3 ). In this way, as shown in  FIG. 3  ( b ), the opening  121  having a substantially circular shape when viewed from top and the substantially cylindrical-shaped intra-substrate space  122  (which connects to the opening  121  only but does not connect to another opening) connecting to the opening  121  are formed (step b). The digging into the substrate  12 ′ is performed by using, for example, an NC (Numerical Control) apparatus that is able to perform the scrape machining of a 3D object controlling coordinate positions. The size of the intra-substrate space  122  is, for example, 0.6 mm in diameter and 0.5 mm in depth. 
         [0050]    In the meantime, here, the substrate (a substrate provided with a recess) which is provided with the opening  121  and the intra-substrate space  122  is obtained by using the NC apparatus; however, this is not limiting. In other words, a first substrate (flat-plated shape) provided with a through-hole (formed by a drill or a laser, for example) and a second substrate with no through-hole are attached to each other, whereby one substrate provided with the opening  121  and the intra-substrate space  122  may be obtained. 
         [0051]    Next, in the substrate  12 ′ where the opening  121  and the intra-substrate space  122  are formed, a through-hole  103  (e.g., 0.3 mm in diameter) is formed through a portion where it is necessary to electrically connect the upper surface and the lower surface to each other as shown in  FIG. 3  ( c ) (step c). For the forming of the through-hole  103 , for example, a drill, a laser, an NC apparatus or the like is used. The portion where it is necessary to electrically connect the upper surface and the lower surface of the substrate  12 ′ to each other is suitably decided by how a circuit structure of the microphone unit is designed. In  FIG. 3  ( c ), three places are shown as the places where to form the through-hole  103 ; however, this is not limiting. Besides, the step b and the step c may be changed with each other in order. 
         [0052]    When the through-hole  103  is formed through the substrate  12 ′, next, a plating process (e.g., electroless copper plating process) is applied to the through-hole  103  to form a through-wiring  104  as shown in  FIG. 3  ( d ) (step d). At this time, the plating process is applied to the wall surface of the intra-substrate space  122  as well. Because of this, at the same time of the forming of the through-wiring  104 , the entire wall surface of the intra-substrate space  122  is covered by a metal (e.g., Cu) plated layer CL (coating layer CL). 
         [0053]    Here, the forming of the through-wiring  104  and the process of covering the wall surface of the intra-substrate space  122  by means of the coating layer CL may be performed with a method other than the plating process, for example, may be performed with a method (burying, applying and the like) that uses electro-conductive paste and the like. 
         [0054]    Next, a portion of the upper surface and the lower surface of the substrate  12 ′ where the wiring pattern is necessary is masked by means of an etching resist  105  as shown in  FIG. 3  ( e ) (step e). At this time, also, the coating layer CL (e.g., a Cu plated layer) applied to the wall surface of the intra-substrate space  122  is masked by means of the etching resist  105 . 
         [0055]    When the masking by means of the etching resist  105  is completed, the substrate  12 ′ is dipped into an etching liquid (step f). In this way, of the electro-conductive material (e.g., Cu) disposed on the upper surface and the lower surface of the substrate  12 ′, a portion which is not covered by the etching resist  105  is removed as shown in  FIG. 3  ( f ). 
         [0056]    In the meantime, here, the unnecessary electro-conductive material is removed by the etching; however, this is not limiting, and the unnecessary electro-conductive material may be removed by, for example, laser machining and scrape machining. 
         [0057]    When the etching is completed, the washing of the substrate  12 ′ and the removal of the etching resist  105  are performed (step g). In this way, as shown in  FIG. 3  ( g ), the mike substrate  12  is obtained, which includes the opening  121  and the intra-substrate space  122  whose wall surface is covered by the coating layer CL, and is provided with the wiring pattern (inclusive of the through-wiring). 
         [0058]    By disposing the MEMS chip  11  onto the upper surface  12   a  of the mike substrate  12  to cover the opening  121  and further by placing the cover  13  to cover the MEMS chip  11 , the microphone unit  1  shown in  FIG. 1  is obtained. Here, the MEMS chip  11  is connected to the mike substrate  12  by means of a die bonding material (e.g., an epoxy resin adhesive, a silicone resin adhesive or the like) such that a sound leak does not occur and a gap is not formed between the bottom surface and the upper surface of the mike substrate  12 . 
         [0059]    Besides, the cover  13  also is connected to the upper surface of the mike substrate  12  by using, for example, an adhesive or an adhesive sheet for air-tight sealing. In a case where the electric circuit portion is mounted onto the mike substrate  12 , the MEMS chip  11  and the electric circuit portion are connected to the mike substrate  12 , thereafter, the cover  13  is connected to the upper surface (a mount surface of the MEMS chip  11  and the like) of the mike substrate  12 . The wiring pattern formed on the lower surface of the mike substrate  12  is used as an external electrode. 
         [0060]    In the above description, the structure is described, in which the wiring pattern disposed on the mike substrate  12  is formed by means of a subtraction method that uses the etching method; however this is not limiting. In other words, the wiring pattern disposed on the mike substrate  12  may be formed by means of an addition method that uses printing, burying and the like. 
       Second Embodiment 
       [0061]      FIG. 4  is a schematic sectional view showing a structure of a microphone unit according to a second embodiment to which the present invention is applied. As shown in  FIG. 4 , a microphone unit  2  according to the second embodiment includes: a MEMS chip  21 ; a mike substrate  22  on which the MEMS chip  21  is mounted; and a cover  23 . The microphone unit  2  according to the second embodiment functions as an omnidirectional mike. 
         [0062]    The structure of the MEMS chip  21  (an embodiment of the electro-acoustic conversion device according to the present invention), which has a fixed electrode  212  (which has a plurality of through-holes  212   a ) and a diaphragm  214 , is the same as the structure of the MEMS chip  11  in the first embodiment, accordingly, a detailed description is skipped. 
         [0063]    The structure of the mike substrate  22  (an embodiment of the electro-acoustic conversion device mount substrate according to the present invention) is substantially the same as the structure of the mike substrate  12  in the first embodiment, but is different from the structure of the first embodiment in that an intra-substrate space  222  connecting to a first opening  221  formed through a mount surface (upper surface) of the mike substrate  22  connects to a second opening  223  that is formed through a rear surface (lower surface)  22   b  opposite to the mount surface of the mike substrate  22 . In other words, the mike substrate  22  is not provided with a recess, unlike the first embodiment, but provided with a through-hole, by means of the first opening  121 , the intra-substrate space  122  and the second opening  223 , that penetrates the mike substrate  22  in a thickness direction. Besides, the cover  23  (an embodiment of the cover portion according to the present invention) also has substantially the same structure as the cover  13  in the first embodiment, but is different from the structure of the first embodiment in that a sound hole is not disposed. 
         [0064]    Here, the mike substrate  22  may be, for example, an FR-4 (glass epoxy substrate) substrate, however, may be another kind of substrate. 
         [0065]    In the microphone unit  2  according to the second embodiment, the MEMS chip  21  is disposed to cover the first opening  221  that is formed through the mount surface  22   a  of the mike substrate  22 . The through-hole formed of the first opening  221 , the intra-substrate space  222  and the second opening  223  functions as a sound hole. In other words, a sound wave occurring outside the microphone unit  2  reaches a lower surface of the diaphragm  214  via the second opening  223 , the intra-substrate space  222  and the first opening  221 . 
         [0066]    In this way, the diaphragm  214  vibrates, whereby a change in the electrostatic capacity occurs. The microphone unit  2  is structured to fetch the change in the electrostatic capacity as an electric signal and to output the electric signal. Here, a caution item regarding the disposition of the electric circuit portion for fetching the change in the electrostatic capacity of the MEMS chip  21  as an electric signal is the same as the case of the first embodiment. 
         [0067]    The microphone unit  2  according to the second embodiment is structured to use a tightly closed space  24  (a housing space for housing the MEMS chip  21 ), which is formed by the mike substrate  22  and the cover  23 , as the rear chamber; accordingly, it is easy to enlarge the rear chamber volume. Because of this, it is easy to improve the mike sensitivity. 
         [0068]    Here, also in the microphone unit  2  according to the second embodiment, a wall surface  222   a  (in the present embodiment, the entire wall surface of the intra-substrate space  222 ) of the intra-substrate space  222  formed in the mike substrate  22  is covered by the coating layer CL. The covering by the coating layer CL is obtainable by, for example, a plating process, and the coating layer CL may be, for example, a metal plated layer such as a Cu plated layer and the like. The effect of the covering by the coating layer CL is the same as the case of the first embodiment, and also in the microphone unit  2  according to the second embodiment, it is possible to prevent the occurrence of dust in the intra-substrate space  222  and reduce malfunction of the MEMS chip  21 . 
         [0069]    Next, methods for manufacturing the mike substrate  22  and the microphone unit  2  described above are described with chief reference to  FIG. 5 .  FIG. 5  is a sectional view for describing a manufacturing method for the mike substrate that the microphone unit according to the second embodiment includes, of which (a) to (f) show states during the manufacturing, and (g) shows a state in which the mike substrate is completed. 
         [0070]    When manufacturing the mike substrate  22 , first, a substrate  22 ′ (flat-plated shape), whose upper surface and lower surface are covered by a metal material (electro-conductive material)  201  such as Cu or the like, is prepared (step a; see  FIG. 5  ( a )). The thicknesses of the substrate  22 ′ and the electro-conductive material  201  may be the same as the first embodiment. 
         [0071]    At a substantially central position of the prepared substrate  22 ′, along a thickness direction (the vertical direction of  FIG. 5 ) of the substrate  22 ′, a hole (e.g., 0.6 mm in diameter), which penetrates from the upper surface to the lower surface, is opened by using, for example, a drill, a laser, an NC apparatus or the like. In this way, the first opening  221  having a substantially circular shape is formed through the upper surface of the substrate  22 ′, the intra-substrate space  222 , which has a substantially cylindrical shape and connects to the first opening  221 , is formed, and the second opening  223 , which is disposed through the lower surface of the substrate  22 ′ and connects to the intra-substrate space  222 , is formed (step b; see  FIG. 5  ( b )). 
         [0072]    Thereafter, the same processes as the first embodiment are successively performed. 
         [0073]    First, a through-hole  203  is formed through a portion where it is necessary to electrically connect the upper surface and the lower surface to each other as shown in  FIG. 5  ( c ) (step c). Here, the step b and the step c may be changed with each other in order. And, a plating process is performed to form a through-wiring  204  as shown in  FIG. 5  ( d ) (step d). At this time, the plating process is applied to a wall surface of the intra-substrate space  222  as well, and the entire wall surface of the intra-substrate space  222  is covered by the metal (e.g., Cu) plated layer CL (coating layer CL). Here, the forming of the through-wiring  204  and the process of covering the wall surface of the intra-substrate space  222  by means of the coating layer CL may be performed with other methods, which is the same as the first embodiment. 
         [0074]    Next, portions of the upper surface and the lower surface of the substrate  22 ′ where a wiring pattern is necessary are masked by means of an etching resist  205  (step e). At this time, the coating layer CL (e.g., a Cu plated layer) applied to the wall surface of the intra-substrate space  222  is also masked by means of the etching resist  105 . 
         [0075]    When the masking by means of the etching resist  205  is completed, the substrate  22 ′ is dipped into an etching liquid to remove an unnecessary electro-conductive material (e.g., Cu) as shown in  FIG. 5  ( f ) (step f), thereafter, the washing and the removal of the etching resist  205  are performed (step g). In this way, as shown in  FIG. 5  ( g ), the mike substrate  22  is obtained, which includes the first opening  221 , the intra-substrate space  222  covered by the coating layer CL and the second opening  223 , and is provided with the wiring pattern (inclusive of the through-wiring). 
         [0076]    By disposing the MEMS chip  21  onto the upper surface  22   a  of the mike substrate  22  to cover the opening  221  and further by placing the cover  23  to cover the MEMS chip  21 , the microphone unit  2  shown in  FIG. 4  is obtained. The connection methods for the MEMS chip  21  and the cover  23  and the caution item in the case of mounting the electric circuit portion onto the mike substrate  22  are the same as the case of the first embodiment. Besides, the wiring pattern disposed on the mike substrate  22  may be formed by means of the addition method instead of the subtraction method, which is also the same as the case of the first embodiment. 
       Third Embodiment 
       [0077]      FIG. 6  is a schematic sectional view showing a structure of a microphone unit according to a third embodiment to which the present invention is applied. As shown in  FIG. 6 , a microphone unit  3  according to the third embodiment includes: a MEMS chip  31 ; a mike substrate  32  on which the MEMS chip  31  is mounted; and a cover  33 . The microphone unit  3  according to the third embodiment functions as an omnidirectional mike. 
         [0078]    The structure of the MEMS chip  31  (an embodiment of the electro-acoustic conversion device according to the present invention), which has a fixed electrode  312  (which has a plurality of through-holes  312   a ) and a diaphragm  314 , is the same as the structure of the MEMS chip  11  in the first embodiment, accordingly, a detailed description is skipped. Besides, the structure of the cover  33  (an embodiment of the cover portion of the present invention) provided with a sound hole  331  is also the same as the structure of the cover  13  in the first embodiment, accordingly, a detailed description is skipped. 
         [0079]    The structure of the mike substrate  32  (an embodiment of the electro-acoustic conversion device according to the present invention) is different from the structure of the first embodiment. Because of this, in the microphone unit  3  according to the third embodiment, the rear chamber has a structure different from the microphone unit  1  according to the first embodiment. 
         [0080]    The mike substrate  32  formed to have a substantially rectangular shape when viewed from top is composed by attaching three substrates  32   a ,  32   b  and  32   c  to one another as shown in  FIG. 6 . Although skipped in  FIG. 6 , the mike substrate  32  is provided with a wiring pattern (inclusive of a through-wiring) that is necessary to apply a voltage to the MEMS chip  31  mounted on the upper surface  32   d  and to fetch an electric signal from the MEMS chip  31 . 
         [0081]    Besides, the mike substrate  32  is provided with an opening  321  through the mount surface (upper surface)  32   d  on which the MEMS chip  31  is mounted, and the MEMS chip  31  is disposed to cover the opening  321 . The opening  321  connects to an intra-substrate space  322  that has a substantially L shape in section. The intra-substrate space  322  connects to only the opening  321  but does not connect to another opening. As described above, the mike substrate  32  has the structure obtained by attaching the plurality of substrates, accordingly, it is easy to obtain the intra-substrate space  322  that has the substantially L shape in section. Here, the mike substrate  32  may be, for example, an FR-4 (glass epoxy substrate) substrate, however, may be another kind of substrate. 
         [0082]    The intra-substrate space  322  is disposed with intention of increasing a volume of the rear chamber (a tightly closed space that faces a lower surface of the diaphragm  314 ). Because of the shape (substantially L shape in section), it is possible to enlarge the volume of the intra-substrate space  322  in the present embodiment compared with the intra-substrate space  122  in the first embodiment. Because of this, the microphone unit  3  according to the third embodiment is expected to be improved in mike sensitivity compared with the microphone unit  1  according to the first embodiment. Here, to make it possible to enlarge the rear chamber volume, the intra-substrate space  322  may be structured to have a hollow space that connects to the digging in the substrate thickness direction, or is not limited to the structure of the present embodiment: for example, a substantially inverse T shape in section and the like may be used. 
         [0083]    In the microphone unit  3  according to the third embodiment, when a sound wave input into a housing space  34  (formed by the mike substrate  32  and the cover  33 ) via the sound hole  331  of the MEMS chip  31  reaches the diaphragm  314 , the diaphragm  314  vibrates, whereby a change in the electrostatic capacity occurs. The microphone unit  3  is structured to fetch the change in the electrostatic capacity as an electric signal and to output the electric signal. Here, the caution item regarding the disposition of the electric circuit portion for fetching the change in the electrostatic capacity of the MEMS chip  31  as an electric signal is the same as the case of the first embodiment. 
         [0084]    In the meantime, in the microphone unit  3  according to the third embodiment, a portion of a wall surface  322   a  (a portion except for a bottom wall of the intra-substrate space  322 ) of the intra-substrate space  322  formed in the mike substrate  32  is covered by the coating layer CL. The covering by the coating layer CL is obtainable by, for example, a plating process, and the coating layer CL may be, for example, a metal plated layer such as a Cu plated layer and the like. The effect of the covering by the coating layer CL is the same as the case of the first embodiment, and also in the microphone unit  3  according to the third embodiment, it is possible to prevent occurrence of dust in the intra-substrate space  322  and reduce malfunction of the MEMS chip  31 . 
         [0085]    Here, of course, a structure may be employed, in which also the bottom wall of the intra-substrate space  322  is covered by the coating layer CL. In the present embodiment, the structure is employed, in which the mike substrate  32  is formed by attaching the plurality of substrates  32   a  to  32   c  to one another, and the bottom wall of the intra-substrate space  32  is formed of an upper surface of the substrate  32   c . The upper surface of the substrate  32   c  is not a surface to which machining such as severing, scraping and the like is applied, accordingly, dust is unlikely to occur. Because of this, in the present embodiment, the structure is employed, in which the bottom wall of the intra-substrate space  322  is not covered by means of the coating layer CL. 
         [0086]    Next, methods for manufacturing the mike substrate  32  and the microphone unit  3  described above are described with chief reference to  FIG. 7 .  FIG. 7  is a sectional view for describing a manufacturing method for the mike substrate that the microphone unit according to the third embodiment includes, of which (a) to (o) show states during the manufacturing, and (p) shows a state in which the mike substrate is completed. 
         [0087]    When manufacturing the mike substrate  32 , first, a first substrate  32   a  (flat-plated shape), whose upper surface is covered by, for example, a metal material (electro-conductive material)  301  such as Cu or the like, is prepared. And, along a thickness direction (the vertical direction of  FIG. 7 ) of the first substrate  32   a , a first through-hole  302  having a substantially circular shape when viewed from top, which penetrates from the upper surface to the lower surface, is opened by using, for example, a drill, a laser, an NC apparatus or the like (step a; see  FIG. 7  ( a )). The forming position of the first through-hole  302  is a substantially central position of the first substrate  32   a . Here, the thickness of the first substrate  32   a  is 0.3 mm for example, and the thickness of the electro-conductive material  301  is 0.15 μm. Besides, the diameter of the first through-hole  302  is 0.6 mm. 
         [0088]    Besides, a second substrate  32   b  (flat-plated shape), whose lower surface is covered by the metal material (electro-conductive material)  301  such as Cu or the like, is prepared. The thicknesses of the second substrate  32   b  and the electro-conductive material  301  are the same as the case of the first substrate  32   a . And, along a thickness direction (the vertical direction of  FIG. 7 ) of the second substrate  32   b , a second through-hole  303  having a substantially circular shape when viewed from top, which penetrates from the upper surface to the lower surface, is opened by using, for example, a drill, a laser, an NC apparatus or the like (step b; see  FIG. 7  ( b )). The second through-hole  303  is disposed at a position that overlaps the first through-hole  302 , and is disposed larger than the first through-hole  302  in diameter. Here, of course, the order of step a and step b may be reversed. 
         [0089]    Next, an adhesive is applied onto at least one of the lower surface of the first substrate  32   a  and the upper surface of the second substrate  32   b , and the first substrate  32   a  and the second substrate  32   b  are attached to each other by pressing (step c; see  FIG. 7  ( c )). In this way, the opening  321  of the mount surface on which the MEMS chip  31  is mounted is obtained, and the intra-substrate space  322  (substantially L shape in section) connecting to the opening  321  is obtained. Here, instead of the adhesive, an adhesive sheet (e.g., a thermoplastic sheet having a thickness of about 50 μm) may be used, or the first substrate  32   a  and the second substrate  32   b  may be attached by means of thermocompression. 
         [0090]    Besides, the substrate formed by attaching the first substrate  32   a  and the second substrate  32   b  as shown in  FIG. 7  ( c ) may be formed of one substrate. In this case, a substrate whose upper surface and lower surface are provided with an electro-conductive material is prepared. And, a digging is formed onto the substrate from both of the upper surface and the lower surface by using an NC apparatus. If the area of the digging formed from the upper surface and the area of the digging formed from the lower surface are made different, the same substrate as shown in  FIG. 7  ( c ) is obtained. 
         [0091]    Next, a third through-hole  304  (e.g., 0.3 mm in diameter) is formed through a portion where electric connection is necessary between the upper surface of the first substrate  32   a  and the lower surface of the second substrate  32   b  (step d; see  FIG. 7  ( d )). For the forming of the through-hole  304 , for example, a drill, a laser, an NC apparatus or the like is used. 
         [0092]    Next, a plating process (e.g., electroless copper plating process) is applied to the third through-hole  304  to form a first through-wiring  305  as shown in  FIG. 7  ( e ) (step e). At this time, the plating process is applied to a wall surface as well of the intra-substrate space  322 , and the entire wall surface of the intra-substrate space  322  is covered by the metal (e.g., Cu) plated layer CL (coating layer CL). Here, the forming of the first through-wiring  305  and the process of covering the wall surface of the intra-substrate space  322  by means of the coating layer CL may be performed with a method other than the plating process, for example, may be performed with a method (burying, applying and the like) that uses electro-conductive paste and the like. 
         [0093]    Next, portions of the upper surface of the first substrate  32   a  and the lower surface of the second substrate  32   b  where a wiring pattern is necessary are masked by means of an etching resist  306  (step f; see  FIG. 7  ( f )). At this time, the coating layer CL (e.g., a Cu plated layer) applied to the wall surface of the intra-substrate space  322  is also masked by means of the etching resist  306 . 
         [0094]    Next, the first substrate  32   a  and the second substrate  32   b  which are in a relationship of being attached to each other, are dipped into an etching liquid. In this way, of the electro-conductive material (e.g., Cu) disposed on the substrate, a portion which is not covered by the etching resist  306  is removed (step g;  FIG. 7  ( g )). In the meantime, here, the unnecessary electro-conductive material is removed by the etching; however, this is not limiting, and the unnecessary electro-conductive material may be removed by, for example, laser machining and scrape machining. 
         [0095]    Next, the substrate dipped in the etching liquid is washed, thereafter, the removal of the etching resist  306  is performed (step h; see  FIG. 7  ( h )). And, the third substrate  32   c  (an embodiment of another substrate according to the present invention) whose lower surface is covered by the electro-conductive material  301  is attached onto the lower surface of the second substrate  32   b  (step i; see  FIG. 7  ( i )). The thicknesses of the third substrate  32   c  and the electro-conductive material are the same as the cases of the first substrate  32   a  and the second substrate  32   b . The attachment of the third substrate  32   c  onto the second substrate  32   b  may be performed by means of the same method as the attachment of the first substrate  32   a  and the second substrate  32   b.    
         [0096]    Next, a protection cover  307  is mounted to cover and close tightly the entire upper surface of the first substrate  32   a  (step j; see  FIG. 7  ( j )). In the present embodiment, the protection cover  307  has a box shape, and an outer edge portion  307   a  is bonded and fixed onto the first substrate  32   a  with an opening of the box facing downward. At a position other than the outer edge portion  307   a , a gap is formed between the first substrate  32   a  and the protection cover  307 . Here, the shape of the protection cover  307  is not limited to this, and may be a flat-plated shape. In a case where the protection cover  307  has a flat-plated shape, the entire surface may be bonded to the upper surface of the first substrate  32   a.    
         [0097]    The step of mounting the protection cover  307  is disposed for the purpose of preventing a substrate treatment liquid from invading the intra-substrate space  322  during later substrate manufacturing processes and the finally obtained electro-acoustic conversion device mount substrate  32  from being contaminated. In detail, in a case where the protection cover  307  is not present, there is a likelihood that the plating liquid and the etching liquid invade the intra-substrate space  322  during the plating, etching and washing steps and residues remain to contaminate the substrate. In this point, as in the present embodiment, by mounting the protection cover  307 , it is possible to prevent the contamination of the substrate. 
         [0098]    Next, a fourth through-hole  308  having a substantially circular shape when viewed from top is opened, which extends from the lower surface of the third substrate  32   c  to the lower surface of the second substrate  32   b  (step k; see  FIG. 7  ( k )). The fourth through-hole  308  is formed by means of, for example, a laser, an NC apparatus and the like, and it is possible to form the diameter to be about 0.5 mm. Here, the order of the step i to the step k may be changed suitably. 
         [0099]    Next, a plating process (e.g., electroless copper plating process) is applied to the fourth through-hole  308  to form a second through-wiring  309  as shown in  FIG. 7  ( l ) (step l). In this way, electric connection between the wiring pattern on the lower surface of the second substrate  32   b  and the electro-conductive material  301  on the lower surface of the third substrate  32   c  is performed. When performing the plating process, the etching liquid does not invade the intra-substrate space  322  thank to the presence of the protection cover  307 . Here, the forming of the second through-wiring  309  may be performed by means of a method other than the plating process, for example, may be performed by means of a method (burying, applying and the like) that uses electro-conductive paste and the like. 
         [0100]    Next, a portion of the lower surface of the third substrate  32   c  where a wiring pattern is necessary is masked by means of the etching resist  306  (step m; see  FIG. 7  ( m )). Next, the substrate (which is formed by attaching the first substrate  32   a , the second substrate  32   b  and the third substrate  32   c  to one another) is dipped into the etching liquid to remove an unnecessary electro-conductive material (e.g., Cu) on the lower surface of the third substrate  32   c  (step n; see  FIG. 7  ( n )). At this time, the etching liquid does not invade the intra-substrate space  322  thanks to the presence of the protection cover  307 . 
         [0101]    In the meantime, here, the unnecessary electro-conductive material is removed by the etching; however, this is not limiting, and the unnecessary electro-conductive material may be removed by, for example, laser machining and scrape machining. 
         [0102]    When the etching is completed, the substrate washing is performed, and further, the removal of the etching resist  306  is performed (step o; see  FIG. 7  ( o )). And, finally, as shown in  FIG. 7  ( p ), the bonded portion of the protection cover  307  is demounted to separate the protection cover  307  (step p). In this way, the mike substrate  32  is obtained, which includes the opening  321  and the intra-substrate space  322  whose wall surface is partially covered by the coating layer CL, and is provided with the wiring pattern (inclusive of the through-wiring). 
         [0103]    By disposing the MEMS chip  31  onto the upper surface  32   d  of the mike substrate  32  to cover the opening  321  and further by placing the cover  33  to cover the MEMS chip  31 , the microphone unit  3  shown in  FIG. 6  is obtained. The connection methods of the MEMS chip  31  and the cover  33  and the caution item in the case of mounting the electric circuit portion onto the mike substrate  32  are the same as the case of the first embodiment. Besides, the wiring pattern disposed on the mike substrate  32  may be formed by means of the addition method instead of the subtraction method, which is also the same as the case of the first embodiment. 
       Fourth Embodiment 
       [0104]      FIG. 8  is a schematic sectional view showing a structure of a microphone unit according to a fourth embodiment to which the present invention is applied. As shown in  FIG. 8 , a microphone unit  4  according to the fourth embodiment includes: a MEMS chip  41 ; a mike substrate  42  on which the MEMS chip  41  is mounted; and a cover  43 . The microphone unit  4  according to the fourth embodiment functions as a bidirectional differential mike. 
         [0105]    The structure of the MEMS chip  41  (an embodiment of the electro-acoustic conversion device according to the present invention), which has a fixed electrode  412  (which has a plurality of through-holes  412   a ) and a diaphragm  414 , is the same as the structure of the MEMS chip  11  according to the first embodiment, accordingly, detailed description is skipped. 
         [0106]    The structure of the mike substrate  42  (an embodiment of the electro-acoustic conversion device according to the present invention) is different from the structures of the first, second and third embodiments. The mike substrate  42  formed to have a substantially rectangular shape when viewed from top is composed by attaching three substrates  42   a ,  42   b  and  42   c  to one another as shown in  FIG. 8 . Although skipped in  FIG. 8 , the mike substrate  42  is provided with a wiring pattern (inclusive of a through-wiring) that is necessary to apply a voltage to the MEMS chip  41  mounted on the upper surface  42   d  and to fetch an electric signal from the MEMS chip  41 . 
         [0107]    Besides, the mike substrate  42  is provided with a first opening  421  near a center of the mount surface (upper surface)  42   d  on which the MEMS chip  11  is mounted, and the MEMS chip  41  is disposed to cover the first opening  421 . The first opening  421  connects to an intra-substrate space  422  that has a substantially U shape in section. The intra-substrate space  422  connects to not only the first opening  421  but also to a second opening  423  that is formed through the mount surface  42   d  of the mike substrate  42 . As described above, the mike substrate  42  has the structure obtained by attaching the plurality of substrates, accordingly, it is easy to obtain the intra-substrate space  422  that has the first opening  421 , the intra-substrate space  422  and the second opening  423 . Here, the mike substrate  42  may be, for example, an FR-4 (glass epoxy substrate) substrate, however, may be another kind of substrate. 
         [0108]    The cover  43 , which is formed to have a substantially rectangular-parallelepiped shape, is placed over the mike substrate  42 , thereby collaborating with the mike substrate  42  to form a housing space  44  that houses the MEMS chip  41 . The cover  43  is provided with a first sound hole  431  that communicates with the housing space  44 . Besides, the cover  33  is provided with a second sound hole  432  that communicates with the intra-substrate space  422  via the second opening  423 . Here, the cover  43  is an embodiment of the cover portion of the present invention. 
         [0109]    In the microphone unit  4  according to the fourth embodiment, a sound wave input into the housing space  44  via the first sound hole  431  reaches an upper surface of the diaphragm  414 . Besides, a sound wave input into the intra-substrate space  422  via the second sound hole  432  reaches a lower surface of the diaphragm  414 . Because of this, when a sound occurs outside the microphone unit  4 , the diaphragm  414  vibrates thanks to a difference between a sound pressure acting on the upper surface and a sound pressure acting on the lower surface. 
         [0110]    A sound pressure (amplitude of a sound wave) of a sound wave is inversely proportional to the distance from a sound source. And, the sound pressure attenuates sharply at a position near the sound source, and attenuate more slowly at a position that is more distant from the sound source. Because of this, in a case where a distance from the sound source to the upper surface of the diaphragm  414  and a distance from the sound source to the lower surface of the diaphragm  414  are different from each other, a user voice, which occurs near the microphone unit  4  and enters from the upper surface and the lower surface of the diaphragm  414 , generates a large sound pressure difference between the upper surface and the lower surface of the diaphragm  414  to vibrate the diaphragm. On the other hand, noises from distant places, which enter from the upper surface and the lower surface of the diaphragm  414  have substantially the same sound pressures, accordingly, they cancel out each other and hardly vibrate the diaphragm. 
         [0111]    Accordingly, the electric signal fetched by means of the vibration of the diaphragm  414  is regardable as an electric signal from which the noise is removed and which indicates the user voice. In other words, the microphone unit  4  according to the present embodiment is suitable for a close-talking mike that is required to alleviate a distant noise and collect a near sound. 
         [0112]    Here, the electric circuit portion for fetching the change in the electrostatic capacity of the MEMS chip  41  as an electric signal may be disposed, for example, in the housing space  44 , or outside the microphone unit. Besides, the electric circuit portion may be monolithically formed on the silicon substrate that forms the MEMS chip  41 . 
         [0113]    In the meantime, in the microphone unit  4  according to the fourth embodiment, a portion of a wall surface  422   a  of the intra-substrate space  422  formed in the mike substrate  42  is covered by the coating layer CL. The covering by the coating layer CL is obtainable by, for example, a plating process, and the coating layer CL may be, for example, a metal plated layer such as a Cu plated layer and the like. The effect of the covering by the coating layer CL is the same as the case of the first embodiment, and also in the microphone unit  4  according to the fourth embodiment, it is possible to prevent the occurrence of dust in the intra-substrate space  422  and reduce malfunction of the MEMS chip  41 . 
         [0114]    Here, of course, a structure may be employed, in which the entire wall surface that forms the intra-substrate space  422  is covered by the coating layer CL. In the present embodiment, the structure is employed, in which the mike substrate  42  is formed by attaching the plurality of substrates  42   a  to  42   c  to one another. A portion (wall surface) where the coating layer CL of the intra-substrate space  422  is not disposed is formed of an upper surface of the third substrate  42   c . This portion is not a surface to which machining such as severing, scraping and the like is applied, accordingly, dust is unlikely to occur. Because of this, in the present embodiment, the structure is employed, in which a portion of the wall surface of the intra-substrate space  422  is not covered by means of the coating layer CL. 
         [0115]    Next, methods for manufacturing the mike substrate  42  and the microphone unit  4  described above are described with chief reference to  FIG. 9 .  FIG. 9  is a sectional view for describing a manufacturing method for the mike substrate that the microphone unit according to the fourth embodiment includes, of which (a) to (o) show states during the manufacturing, and (p) shows a state in which the mike substrate is completed. 
         [0116]    When manufacturing the mike substrate  42 , first, a first substrate  42   a  (flat-plated shape), whose upper surface is covered by, for example, a metal material (electro-conductive material)  401  such as Cu or the like, is prepared. And, along a thickness direction (the vertical direction of  FIG. 9 ) of the first substrate  42   a , a first through-hole  402  and a second through-hole  403  having a substantially circular shape, which penetrate from the upper surface to the lower surface, are opened by using, for example, a drill, a laser, an NC apparatus or the like (step a; see  FIG. 9  ( a )). Here, the thickness of the first substrate  42   a  is 0.3 mm for example, and the thickness of the electro-conductive material  401  is 0.15 μm. Besides, the diameters of the first through-hole  402  and the second through-hole  403  are 0.6 mm. Here, the shapes of the first through-hole  420  and the second through-hole  403  are the same as each other, however, may have different shapes. 
         [0117]    Besides, a second substrate  42   b  (flat-plated shape), whose lower surface is covered by the metal material (electro-conductive material)  401  such as Cu or the like, is prepared. The thicknesses of the second substrate  42   b  and the electro-conductive material  401  are the same as the case of the first substrate  42   a . And, along a thickness direction (the vertical direction of  FIG. 9 ) of the second substrate  42   b , a third through-hole  404  having a substantially rectangular shape when viewed from top, which penetrates from the upper surface to the lower surface, is opened by using, for example, a drill, a laser, an NC apparatus or the like (step b; see  FIG. 9  ( b )). The third through-hole  404  is disposed to overlap the first through-hole  402  and the second through-hole  403 . 
         [0118]    Here, in the present embodiment, a right end of the third through-hole  404  is formed to be situated at the same position of a right end of the first through-hole  402 , while a left end of the third through-hole  404  is formed to be situated at the same position of a left end of the second through-hole  403 ; however, this structure is not limiting. For example, a structure may be employed, in which the left and right ends of the third through-hole  404  are more widened to the left and right than the present embodiment. Besides, also the shape of the third through-hole  404  is not limited to the shape (substantially rectangular shape when viewed from top) of the present embodiment, and is suitably modifiable. Here, of course, the order of the step a and the step b may be reversed. 
         [0119]    Next, the lower surface of the first substrate  42   a  and the upper surface of the second substrate  42   b  are attached to each other (step c; see  FIG. 9  ( c )). In this way, the first opening  421  of the mount surface on which the MEMS chip  41  is mounted is obtained, the intra-substrate space  422  (a substantially U shape in section) connecting to the first opening  421  is obtained, and the second opening  423  is obtained which is disposed, independent of the first opening  421 , on the mount surface on which the MEMS chip  41  is mounted and connects to the intra-substrate space  422 . Here, the attachment of the first substrate  42   a  and the second substrate  42   b  may be performed in the same way as the attachment of the first substrate  32   a  and the second substrate  32   b  in the third embodiment. Besides, in the same way as the case of the third embodiment, the substrate (the board formed by attaching the first substrate  42   a  and the second substrate  42   b ) having the structure shown in  FIG. 9  ( c ) may be formed of one substrate. 
         [0120]    Hereinafter, although there is a difference in the substrate shape, the manufacturing of the mike substrate  42  is performed in a procedure similar to the case of the third embodiment. Points overlapping the third embodiment are skipped or described briefly. 
         [0121]    A fourth through-hole  405  (e.g., 0.3 mm in diameter) is formed through a portion where electric connection is necessary between the upper surface of the first substrate  42   a  and the lower surface of the second substrate  42   b  by using, for example, a drill, a laser, an NC apparatus or the like (step d; see  FIG. 9  ( d )). Next, by applying a plating process (e.g., electroless copper plating process) onto the fourth through-hole  405 , a first through-wiring  406  shown in  FIG. 9  ( e ) is formed (step e). At this time, the plating process is also applied to a wall surface of the intra-substrate space  422 , and the entire wall surface of the intra-substrate space  422  is covered by the metal plated layer CL (coating layer CL). 
         [0122]    Here, the forming of the through-wiring  406  and the process of covering the wall surface of the intra-substrate space  422  by means of the coating layer CL may be performed with a method other than the plating process, which is the same as the case of the third embodiment. 
         [0123]    Next, portions of the upper surface of the first substrate  42   a  and the lower surface of the second substrate  42   b  where a wiring pattern is necessary to be formed are masked by means of an etching resist  407  (step f; see  FIG. 9  ( f )). At this time, the coating layer CL applied to the wall surface of the intra-substrate space  422  is also masked by means of the etching resist  407 . And, the removal of the unnecessary electro-conductive material  401  is performed by means of the etching liquid (step g; see  FIG. 9  ( g )), and after the etching, the washing and the removal of the etching resist  407  are performed (step h; see  FIG. 9  ( h )). 
         [0124]    In the meantime, here, the unnecessary electro-conductive material is removed by the etching; however, this is not limiting, and the unnecessary electro-conductive material may be removed by, for example, laser machining and scrape machining. 
         [0125]    Next, the third substrate  42   c  (an embodiment of another substrate according to the present invention) whose lower surface is covered by the electro-conductive material  401  is attached onto the lower surface of the second substrate  42   b  (step i; see  FIG. 9  ( i )). Next, a protection cover  408  is mounted to cover and close tightly the entire upper surface of the first substrate  42   a  (step j; see  FIG. 9  ( j )). The shape and the mounting method of the protection cover  408  and the reason for using the protection cover  408  are the same as the case of the third embodiment. Next, a fifth through-hole  409  having a substantially circular shape when viewed from top is opened by using, for example, a laser, an NC apparatus or the like, which extends from the lower surface of the third substrate  42   c  to the lower surface of the second substrate  42   b  (step k; see  FIG. 9  ( k )). Here, the order of the step i to the step k may be changed suitably. 
         [0126]    Next, a plating process (e.g., electroless copper plating process) is applied to the fourth through-hole  409  to form a second through-wiring  410  as shown in  FIG. 9  ( l ) (step  l ). In this way, electric connection between the wiring pattern on the lower surface of the second substrate  42   b  and the electro-conductive material  401  on the lower surface of the third substrate  42   c  is performed. When performing the plating process, the etching liquid does not invade the intra-substrate space  422  thank to the presence of the protection cover  408 . Here, the forming of the second through-wiring  410  may be performed by means of a method other than the plating process, which is the same as the third embodiment. 
         [0127]    Next, a portion of the lower surface of the third substrate  42   c  where a wiring pattern is necessary is masked by means of an etching resist  407  (step m; see  FIG. 9  ( m )); the substrate (which is formed by attaching three substrates of the first substrate  42   a  to the third substrate  42   c  to one another) is dipped into the etching liquid to remove an unnecessary electro-conductive material (e.g., Cu) on the lower surface of the third substrate  42   c  (step n; see  FIG. 9  ( n )). At this time, the etching liquid does not invade the intra-substrate space  422  thank to the presence of the protection cover  408 . 
         [0128]    In the meantime, here, the unnecessary electro-conductive material is removed by the etching; however, this is not limiting, and the unnecessary electro-conductive material may be removed by, for example, laser machining and scrape machining. 
         [0129]    When the etching is completed, the substrate washing is performed, and further, the removal of the etching resist  407  is performed (step o; see  FIG. 9  ( o )). And, finally, as shown in  FIG. 9  ( p ), the bonded portion of the protection cover  408  is demounted to separate the protection cover  408  (step p). In this way, the mike substrate  42  is obtained, which includes the first opening  421 , the second opening  423 , and the intra-substrate space  422  whose wall surface is partially covered by the coating layer CL, and is provided with the wiring pattern (inclusive of the through-wiring). 
         [0130]    By disposing the MEMS chip  41  onto the upper surface  42   d  of the mike substrate  42  to cover the first opening  421  and further by placing the cover  43  such that the second sound hole  432  overlies the second opening  423 , the microphone unit  4  shown in  FIG. 8  is obtained. The connection methods of the MEMS chip  41  and cover  43  and the caution item in the case of mounting the electric circuit portion onto the mike substrate  42  are the same as the case of the first embodiment. Besides, the wiring pattern disposed on the mike substrate  42  may be formed by means of the addition method instead of the subtraction method, which is also the same as the case of the first embodiment. 
         [0131]    (Others) 
         [0132]    The microphone units  1  to  4 , the electro-acoustic conversion device mount substrates (e.g., the mike substrates)  12 ,  22 ,  32 ,  42 , and the manufacturing methods of them according to the embodiments described above are mere examples of the present invention, and the application scope of the present invention is not limited to the embodiments described above. In other words, it is possible to add various modifications to the embodiments described above without departing the object of the present invention. 
         [0133]    For example, in the embodiments described above, the structure is employed, in which the electro-acoustic conversion device is the MEMS chip that is formed by using a semiconductor manufacturing technology; however, the structure is not limiting. The electro-acoustic conversion device formed of the MEMS chip is especially weak for dust, accordingly, the present invention is preferably applied; however, the present invention is applicable to a case where an electro-acoustic conversion device other than the MEMS chip is used. 
         [0134]    Besides, in the above embodiments, the case is described, in which the electro-acoustic conversion device is a so-called capacitor type microphone; however, the present invention is applicable to a case where the electro-acoustic conversion device is a microphone (e.g., a moving conductor (dynamic) microphone, an electromagnetic (magnetic) microphone, a piezo-electric microphone and the like) which has a structure other than the capacitor type microphone. 
         [0135]    Besides, in the above embodiments, the case is described, in which the coating layer disposed in the intra-substrate space of the electro-acoustic conversion device mount substrate is a metal layer such as a plated layer and the like; however, this is not limiting. In short, the coating layer disposed in the intra-substrate space may be a layer other than the metal layer if the layer has the function to alleviate dust that has a likelihood of occurring in the intra-substrate space. 
         [0136]    In addition, the shapes of the electro-acoustic conversion device and the microphone unit (inclusive of the opening, the intra-substrate space and the like that are disposed in them) are not limited to the shapes according to the embodiments, and of course, modifiable into various shapes. 
       INDUSTRIAL APPLICABILITY 
       [0137]    The present invention is suitable for a microphone unit that is included in voice input apparatuses such as, for example, a mobile phone and the like. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 ,  2 ,  3 ,  4  microphone units 
               11 ,  21 ,  31 ,  41  MEMS chips (electro-acoustic conversion devices) 
               12 ,  22 ,  32 ,  42  mike substrates (electro-acoustic conversion device mount substrates) 
               12   a ,  22   a ,  32   d ,  42   d  mount surfaces 
               13 ,  23 ,  33 ,  43  covers (cover portions) 
               14 ,  24 ,  34 ,  44  housing spaces 
               22   b  rear surface opposite to mount surface 
               31   c ,  41   c  third substrates (other substrates) 
               103 ,  203 ,  304 ,  308 ,  405 ,  409  through-holes for through-wiring 
               112 ,  212 ,  312 ,  412  fixed electrodes 
               114 ,  214 ,  314 ,  414  diaphragms 
               121 ,  221 ,  312 ,  322  openings or first openings 
               122 ,  222 ,  322 ,  422  intra-substrate spaces 
               122   a ,  222   a ,  322   a ,  422   a  walls surfaces of intra-substrate spaces 
               223 ,  423  second openings (other openings) 
               307 ,  408  protection covers 
             CL coating layer