Patent Publication Number: US-8542851-B2

Title: Acoustic sensor and microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     One or more embodiments of the present invention relate to acoustic sensors and microphones, and specifically to an MEMS (Micro Electro Mechanical Systems) type acoustic sensor manufactured by using the MEMS technique, and a microphone using such acoustic sensor. 
     2. Related Art 
     A capacitance type acoustic sensor is disclosed in Japanese Patent Publication No. 4338395 and Japanese Unexamined Patent Publication No. 2009-89097. In the capacitance type acoustic sensor, a diaphragm (movable electrode film) is arranged on a front surface of a silicon substrate, a back plate is fixed on the front surface of the silicon substrate so as to cover the diaphragm, and a capacitor is configured by the fixed electrode film and the diaphragm of the back plate. The diaphragm is vibrated with the acoustic vibration, and the change in electrostatic capacitance between the fixed electrode film and the diaphragm in such case is output. A great number of acoustic holes are opened in the back plate because the acoustic vibration needs to be introduced to an air gap between the fixed electrode film and the diaphragm in order to vibrate the diaphragm with the acoustic vibration. 
     In such acoustic sensor, the opening size of the acoustic hole needs to be made large to enhance the S/N ratio. However, the acoustic hole that is opened in the back plate is opened not only at the plate portion having a relatively thick film thickness but also at the fixed electrode film having a thin thickness. Therefore, if the opening size of the acoustic hole is made large, the extraction wiring portion of the fixed electrode film may easily break or the parasitic resistance may increase. 
     SUMMARY OF INVENTION 
     One or more embodiments of the present invention have been devised to provide an acoustic sensor in which the fixed electrode film is less likely to break and the parasitic resistance is less likely to increase even if the opening size of the acoustic hole (acoustic perforation) opened in the back plate is made large. 
     In accordance with one aspect of one or more embodiments of the present invention, there is provided an acoustic sensor including: a semiconductor substrate including a back chamber; a conductive diaphragm arranged on an upper side of the semiconductor substrate; an insulating fixed film fixed on an upper surface of the semiconductor substrate to cover the diaphragm with a gap; a conductive fixed electrode film arranged on the fixed film at a position facing the diaphragm; an extraction wiring extracted from the fixed electrode film; and an electrode pad, to which the extraction wiring is connected; the acoustic sensor converting an acoustic vibration to change in electrostatic capacitance between the diaphragm and the fixed electrode film; wherein a plurality of acoustic perforations is opened in a back plate including the fixed film and the fixed electrode film; and an opening rate of the acoustic perforation is smaller in the extraction wiring and a region in the vicinity thereof than in other regions. 
     The opening rate is the ratio of the total of the opening area of the acoustic perforations with respect to the area of the region in the relevant region of an extent including a plurality of acoustic perforations. In order to reduce the opening rate of the acoustic perforations arranged in the extraction wiring and the region in the vicinity of the extraction wiring, the opening area per one acoustic perforation is to be reduced compared to other regions in the extraction wiring and the region in the vicinity of the extraction wiring. Alternatively, the inter-center distance between the adjacent acoustic perforations is made longer than other regions in the extraction wiring and the region in the vicinity of the extraction wiring. 
     If the extraction wiring and the region in the vicinity of the extraction wiring includes an acoustic perforation having a smaller opening rate than the acoustic perforation in other regions, this includes a case where the extraction wiring or the region in the vicinity of the extraction wiring does not include the acoustic perforation (i.e., opening rate is zero). 
     The region in the vicinity of the extraction wiring refers to the region within six times and may more specifically refer to the region within substantially three times the average inter-center distance of the acoustic perforation measured from the basal end of the extraction wiring of the acoustic perforation formed region (excluding the region where the extraction wiring passes) of the back plate. 
     In a first acoustic sensor of one or more embodiments of the present invention, the acoustic perforation in the extraction wiring and in the region in the vicinity thereof has a relatively small opening rate, and hence the width of the electrode film between the acoustic perforations in the extraction wiring and in the region in the vicinity thereof is less likely to become narrow and the parasitic resistance in the extraction wiring and in the region in the vicinity thereof can be reduced. Therefore, the generation of noise in the extraction wiring and in the region in the vicinity thereof can be reduced and the S/N ratio of the acoustic sensor can be enhanced. Furthermore, the width of the electrode film between the acoustic perforations can be widened in the extraction wiring and in the region in the vicinity thereof, so that the lowering of the strength of the fixed electrode film by the acoustic perforation in the extraction wiring and in the region in the vicinity thereof can be reduced. Therefore, the mechanical strength of the extraction wiring and the fixed electrode film increase, so that disconnection and breakage are less likely to occur. 
     In the first acoustic sensor of one or more embodiments of the present invention, the opening rate of the acoustic perforation is relatively large in other regions excluding the extraction wiring and the vicinity thereof so that the acoustic vibration easily passes through the acoustic perforation, and the S/N ratio of the acoustic sensor is increased to enhance the sensitivity. 
     One embodiment of the first acoustic sensor according to the present invention has the acoustic perforation arranged in the extraction wiring and the region in the vicinity of the extraction wiring, and the acoustic perforation in other regions excluding the extraction wiring and the region in the vicinity thereof arrayed according to the same rule. When referring to being arrayed according to the same rule, this means that the form of array (e.g., square arrangement and concentric arrangement, honeycomb arrangement, zigzag arrangement etc.) and the array pitch (inter-center distance between the acoustic perforations) are the same. According to one or more embodiments, the sacrifice layer of the air gap can be uniformly etched in the manufacturing step. 
     In accordance with an aspect, a diameter of the acoustic perforation having a relatively small opening area arranged in the extraction wiring or the region in the vicinity of the extraction wiring is smaller than twice a spaced distance between the acoustic perforations. According to one or more embodiments, the strength of the fixed electrode film can be ensured and the parasitic resistance can be reduced. 
     In accordance with an aspect, a diameter of the acoustic perforation having a relatively large opening area arranged in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring is greater than a spaced distance between the acoustic perforations. According to one or more embodiments, the S/N ratio of the acoustic sensor can be enhanced because the opening rate of the acoustic perforation becomes large in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring. 
     In accordance with an aspect, a diameter of the acoustic perforation having a relatively large opening area arranged in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring is smaller than four times a spaced distance between the acoustic perforations. According to one or more embodiments, the strength of the back plate can be prevented from lacking by making the opening area of the acoustic perforation too large, and the electrode area of the fixed electrode film can be prevented from becoming too small. 
     In accordance with an aspect, an inter-center distance between the adjacent acoustic perforations of the acoustic perforations having a relatively small opening area arrayed in the extraction wiring and the region in the vicinity of the extracting wiring is equal to an inter-center distance between the adjacent acoustic perforations of the acoustic perforations having a relatively large opening area arranged in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring. According to one or more embodiments, the sacrifice layer of the air gap can be uniformly etched in the manufacturing step. 
     In accordance with an aspect, the acoustic perforation contained in five or less zones including the acoustic perforation of the extraction wiring when counting from the acoustic perforation arranged at the extraction wiring is the acoustic perforation having a relatively small opening area. When referring to zones, this refers to the virtual line passing through the centers of a plurality of acoustic perforations at substantially equal distance from the extraction wiring. According to one or more embodiments, the strength of the back plate can be ensured and the parasitic resistance can be reduced. 
     In accordance with another aspect of one or more embodiments of the present invention, there is provided an acoustic sensor including: a semiconductor substrate including a back chamber; a conductive diaphragm arranged on an upper side of the semiconductor substrate; an insulating fixed film fixed on an upper surface of the semiconductor substrate to cover the diaphragm with a gap; a conductive fixed electrode film arranged on the fixed film at a position facing the diaphragm; an extraction wiring extracted from the fixed electrode film; and an electrode pad, to which the extraction wiring is connected; the acoustic sensor converting an acoustic vibration to change in electrostatic capacitance between the diaphragm and the fixed electrode film; wherein a plurality of acoustic perforations is opened in a back plate including the fixed film and the fixed electrode film; and at least the extraction wiring, of the extraction wiring and a region in the vicinity of thereof, does not include the acoustic perforation or includes an acoustic perforation of small opening rate compared to an acoustic perforation arranged in other regions (excluding a case in which an acoustic perforation is not arranged in the extraction wiring, and an opening rate of an acoustic perforation arranged in the region in the vicinity of the extraction wiring and an opening rate of an acoustic perforation arranged in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring are equal). 
     The opening rate is the ratio of the total of the opening area of the acoustic perforations with respect to the area of the region in the relevant region of an extent including a plurality of acoustic perforations. In order to reduce the opening rate of the acoustic perforations arranged in at least the extraction wiring of the extraction wiring and the region in the vicinity of the extraction wiring, the opening area per one acoustic perforation is to be reduced compared to the acoustic perforation in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring. Alternatively, the inter-center distance between the adjacent acoustic perforations is made longer than the acoustic perforation in other regions excluding the extraction wiring and the region in the vicinity of the extraction wiring. 
     The region in the vicinity of the extraction wiring refers to the region within six times and may more specifically refer to the region within substantially three times the average inter-center distance of the acoustic perforation measured from the basal end of the extraction wiring of the acoustic perforation formed region (excluding the region where the extraction wiring passes) of the back plate. 
     In a second acoustic sensor of one or more embodiments of the present invention, the acoustic perforation in the extraction wiring and in the region in the vicinity thereof has a relatively small opening rate, or does the acoustic perforation is not provided, and hence the width of the electrode film between the acoustic perforations in the extraction wiring and in the region in the vicinity thereof is less likely to become narrow and the parasitic resistance in the extraction wiring and in the region in the vicinity thereof can be reduced. Therefore, the generation of noise in the extraction wiring and in the region in the vicinity thereof can be reduced and the S/N ratio of the acoustic sensor can be enhanced. Furthermore, the width of the electrode film between the acoustic perforations can be widened in the extraction wiring and in the region in the vicinity thereof, so that the lowering of the strength of the fixed electrode film by the acoustic perforation in the extraction wiring and in the region in the vicinity thereof can be reduced. Therefore, the mechanical strength of the extraction wiring and the fixed electrode film increase, so that disconnection and breakage are less likely to occur. 
     In the second acoustic sensor of one or more embodiments of the present invention, the opening rate of the acoustic perforation is relatively large in other regions excluding the extraction wiring and the vicinity thereof so that the acoustic vibration easily passes through the acoustic perforation, and the S/N ratio of the acoustic sensor is increased to enhance the sensitivity. 
     A first microphone according to one or more embodiments of the present invention has a first acoustic sensor according to one or more embodiments of the present invention and a signal processing circuit for processing an electrical signal output from the acoustic sensor accommodated in a housing. According to the microphone of one or more embodiments of the present invention, the generation of noise can be reduced and the S/N ratio of the microphone can be enhanced because the acoustic sensor of one or more embodiments of the present invention is used. Furthermore, the disconnection and breakage of the extraction wiring and the fixed electrode film in the acoustic sensor are less likely to occur, and the failure of the microphone is less likely to occur. 
     A second microphone according to one or more embodiments of the present invention has a second acoustic sensor according to one or more embodiments of the present invention and a signal processing circuit for processing an electrical signal output from the acoustic sensor accommodated in a housing. According to the microphone of one or more embodiments of the present invention, the generation of noise can be reduced and the S/N ratio of the microphone can be enhanced because the acoustic sensor of one or more embodiments of the present invention is used. Furthermore, the disconnection and breakage of the extraction wiring and the fixed electrode film in the acoustic sensor are less likely to occur, and the failure of the microphone is less likely to occur. 
     One or more embodiments of the present invention have a characteristic of appropriately combining the configuring elements described above, and one or more embodiments of the present invention enables a great number of variations by the combination of the configuring elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an acoustic sensor according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view taken along line X-X of  FIG. 1 ; 
         FIG. 3  is an operation explanatory view of the acoustic sensor of the first embodiment; 
         FIG. 4  is a view describing the arrangement of acoustic holes in the acoustic sensor of the first embodiment; 
         FIG. 5  is a plan view of an acoustic sensor according to a second embodiment of the present invention; 
         FIG. 6  is a plan view of a state in which the plate portion of the back plate is removed in the acoustic sensor of the second embodiment; 
         FIG. 7  is a plan view of an acoustic sensor according to a third embodiment of the present invention; 
         FIG. 8  is a plan view of an acoustic sensor according to a fourth embodiment of the present invention; and 
         FIG. 9  is a schematic cross-sectional view of a microphone according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanied drawings. It should be noted that the present invention is not limited to the following embodiments and that various design changes can be made within a scope not deviating from the present invention. In embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one with ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. 
     First Embodiment 
     First, a structure of an acoustic sensor  31  according to a first embodiment of the present invention will be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a plan view of the acoustic sensor  31 .  FIG. 2  is a cross-sectional view in a diagonal direction of the acoustic sensor  31  (cross-section taken along line X-X of  FIG. 1 ). 
     The acoustic sensor  31  is a capacitance type element manufactured by using the MEMS technique. As shown in  FIG. 2 , a diaphragm  33  is arranged on an upper surface of a silicon substrate  32  (semiconductor substrate) by way of an anchor  37 , and a back plate  34  is fixed thereon by way of a microscopic air gap. 
     The silicon substrate  32  made of monocrystalline silicon is formed with a back chamber  35  passed through from the front surface to the back surface. The inner peripheral surface of the back chamber  35  may be a perpendicular surface or may be inclined to a tapered shape. 
     A plurality of anchors  37  for supporting the lower surface of the outer peripheral part of the diaphragm  33  is arranged on the upper surface of the silicon substrate  32 , and a base part  41  of thick film is formed on the upper surface of the silicon substrate  32  to surround the diaphragm  33 . Furthermore, the region on the outer side than the base part  41  in the upper surface of the silicon substrate  32  is covered with an adhering layer  47  thinner than the base part  41 . The anchor  37  and the base part  41  are formed by SiO 2 . The adhering layer  47  is made by SiO 2  or polysilicon. 
     As shown in  FIG. 1 , the diaphragm  33  is formed by a substantially circular plate shaped polysilicon thin film, and has conductivity. A band plate shaped extraction wiring  43  is extended towards the outer side from the diaphragm  33 . 
     The diaphragm  33  is arranged on the silicon substrate  32  so as to cover the opening in the upper surface of the back chamber  35 . The entire periphery of the lower surface at the outer peripheral part of the diaphragm  33  is fixed to the upper surface of the silicon substrate  32  by the anchor  37 . Therefore, the diaphragm  33  floats in air at the upper side of the chamber  35 , and can film vibrate sympathized to the acoustic vibration (air vibration). 
     In the back plate  34 , a fixed electrode film  40  made of polysilicon is arranged at the lower surface of a plate portion  39  (fixed film) made of nitride film (SiN). The back plate  34  has a canopy shape, and covers the diaphragm  33  at the hollow portion underneath. The height of the hollow portion under the back plate  34  (height from the upper surface of the silicon substrate  32  to the lower surface of the fixed electrode film  40 ) is equal to the thickness of the base part  41  formed on the upper surface of the silicon substrate  32  due to manufacturing reasons. A microscopic air gap is formed between the lower surface of the back plate  34  (i.e., lower surface of the fixed electrode film  40 ) and the upper surface of the diaphragm  33 . The fixed electrode film  40  faces the diaphragm  33 , which is a movable electrode film, and configures a capacitor. 
     A great number of acoustic holes  38   a ,  38   b  (acoustic perforations) for passing the acoustic vibration is performed in the back plate  34  so as to pass from the upper surface to the lower surface. The acoustic hole  38   b  formed in the extraction wiring  44  of the fixed electrode film  40  and the region in the vicinity thereof in the back plate  34  has a smaller opening area than the acoustic hole  38   a  formed in other regions (i.e., majority of the region distant from the extraction wiring  44  of the acoustic perforation formed region of the back plate  34 ). The acoustic holes  38   a ,  38   b  pass from the plate portion  39  to the fixed electrode film  40 , where the acoustic hole of the plate portion  39  and the acoustic hole of the fixed electrode film  40  are denoted with the same reference number. 
     A small gap (passage of acoustic vibration) is formed between the lower surface of the outer peripheral part of the diaphragm  33  and the upper surface of the silicon substrate  32 . Therefore, the acoustic vibration that entered the back plate  34  through the acoustic holes  38   a ,  38   b  vibrates the diaphragm  33  and exits to the back chamber  35  through the gap between the outer peripheral part of the diaphragm  33  and the silicon substrate  32 . 
     A great number of microscopic stoppers  42  is projected at the inner surface of the back plate  34 , so that the diaphragm  33  is prevented from being adsorbed to the lower surface of the back plate  34  and not being able to return by the electrostatic attractive force of when excess voltage is applied between the diaphragm  33  and the fixed electrode film  40 . A phenomenon in which the diaphragm  33  fixes (sticks) to the back plate  34  and does not return due to the moisture that entered between the diaphragm  33  and the back plate  34  is also prevented by the stopper  42 . 
     A protective film  53  is continuously extended over the entire periphery from the outer peripheral edge of the canopy shaped plate portion  39 . Therefore, the protective film  53  is formed by a nitride film (SiN) same as the plate portion  39 , and has substantially the same film thickness as the plate portion  39 . The inner peripheral part of the protective film  53  is a base covering part  51  having a reverse groove shaped cross-section, and the outer peripheral part of the protective film  53  is a flat part  52 . 
     The back plate  34  is fixed to the upper surface of the silicon substrate  32 , and the protective film  53  covers the outer peripheral part of the upper surface of the silicon substrate  32  with the base part  41  and the adhering layer  47  interposed. The base covering part  51  of the protective film  53  covers the base part  41 , and the flat part  52  covers the upper surface of the adhering layer  47 . 
     The extraction wiring  43  of the diaphragm  33  is fixed to the base part  41 , and the extraction wiring  44  extended from the fixed electrode film  40  is also fixed to the upper surface of the base part  41 . An opening is formed in the base covering part  51 , a movable side electrode pad  46  (electrode terminal) is formed on the upper surface of the extraction wiring  43  through the opening, and the movable side electrode pad  46  is conducted to the extraction wiring  43  (therefore, to the diaphragm  33 ). A fixed side electrode pad  45  (electrode terminal) arranged on the upper surface of the plate portion  39  is conducted to the extraction wiring  44  (therefore, to the fixed electrode film  40 ) through a through hole and the like. 
     The acoustic hole  38   b  arranged in the extraction wiring  44  or the region of in the vicinity thereof has a relatively small opening area, and the acoustic hole  38   a  formed in other regions has a relatively large opening area. The acoustic holes  38   a ,  38   b  may be entirely arrayed to a triangular shape (or honeycomb shape) as shown in  FIG. 1 , or may be arrayed to a square shape or a circular ring shape, or may be arrayed at random. Therefore, the spaced distance between the acoustic holes  38   a  (i.e., shortest distance between the outer peripheries of the adjacent acoustic holes) is relatively small in the region where the acoustic hole  38   a  of a large opening area is formed, and the spaced distance between the acoustic holes  38   b  is relatively large in the region where the acoustic hole  38   b  of a small opening area is formed. 
     Furthermore, in the acoustic sensor  31 , when the acoustic vibration passes through the acoustic holes  38   a ,  38   b  and enters the space between the back plate  34  and the diaphragm  33 , the diaphragm  33  or the thin film resonates to the acoustic vibration and film vibrates. When the diaphragm  33  vibrates and the gap distance between the diaphragm  33  and the fixed electrode film  40  changes, the electrostatic capacitance between the diaphragm  33  and the fixed electrode film  40  changes. As a result, in the acoustic sensor  31 , the acoustic vibration (change in sound pressure) sensed by the diaphragm  33  becomes the change in the electrostatic capacitance between the diaphragm  33  and the fixed electrode film  40 , and is output from the electrodes pads  45 ,  46  as an electrical signal. 
     In the acoustic sensor  31 , the acoustic hole  38   a  having a relatively large opening area is formed in the region excluding the extraction wiring  44  and the region in the vicinity of the extraction wiring  44 , that is, the majority of the back plate  34 , and thus the acoustic vibration easily passes the acoustic holes  38   a ,  38   b , and the S/N ratio of the acoustic sensor  31  becomes large thus enhancing the sensitivity. 
     However, the current flows through the extraction wiring  44 , as shown with an arrow in  FIG. 3 , between the fixed side electrode pad  45  and the fixed electrode film  40  with the change in electrostatic capacity between the diaphragm  33  and the fixed electrode film  40 . Therefore, if the opening area of the acoustic hole  38   b  in the extraction wiring  44  and the region in the vicinity thereof is large, the cross-sectional area of the current passage becomes narrow and the parasitic resistance of the current passage becomes high. If the parasitic resistance becomes high, the electrical noise generated from the resistor body increases thus degrading the characteristics of the acoustic sensor. 
     In the acoustic sensor  31 , on the other hand, the opening area of the acoustic hole  38   b  formed in the extraction wiring  44  and in the region in the vicinity thereof is relatively small, and thus the cross-sectional area of the extraction wiring  44  and the current passage in the fixed electrode film  40  is less likely to become narrow by the acoustic hole  38   b , and the parasitic resistance of the current flowing as shown with an arrow in  FIG. 3  becomes smaller. Therefore, the parasitic resistance can be reduced and the noise can be reduced in the extraction wiring  44  and in the region of the vicinity thereof, and the S/N ratio of the sensor can be enhanced. As a result, the S/N ratio of the acoustic sensor  31  can be efficiently enhanced by the combination of the acoustic hole  38   a  having a relatively large opening area and the acoustic hole  38   b  having a relatively small opening area. 
     The portion of the extraction wiring  44  has a narrow width and thus has low strength, where the strength further lowers as the acoustic hole  38   b  is opened. The tip of the extraction wiring  44  is fixed to the base part  41 , so that stress easily concentrates at an area connected to the extraction wiring  44  of the fixed electrode film  40  and the strength also lowers by the acoustic hole  38   b . Therefore, if the acoustic hole of a large opening area is formed, the extraction wiring  44  or the fixed electrode film  40  may break or disconnect at the extraction wiring  44  or the region in the vicinity thereof, and the acoustic sensor  31  may stop its function. 
     In the acoustic sensor  31 , on the other hand, the opening area of the acoustic hole  38   b  is formed small in the extraction wiring  44  and in the region in the vicinity thereof, so that the lowering of strength caused by the acoustic hole  38   b  in the extraction wiring  44  and in the region in the vicinity thereof can be reduced. Therefore, in the acoustic sensor  31 , disconnection and breakage are less likely to occur in the extraction wiring  44  and the fixed electrode film  40 , and the mechanical strength of the acoustic sensor  31  enhances. 
     The relationship between the size and the pitch of the acoustic holes  38   a ,  38   b  for improving the characteristics of the acoustic sensor  31  will now be described. The acoustic holes  38   a ,  38   b  are substantially circular openings and are regularly arrayed. 
     The acoustic hole  38   a  having a relatively large opening area in the region distant from the extraction wiring  44  will be described. As shown in  FIG. 4 , the array pitch (inter-center distance) of the acoustic hole  38   a  is Wa+Da, where Wa is the diameter of the acoustic hole  38   a  having a large opening area and Da is the spaced distance between the acoustic holes  38   a . The spaced distance Da×the thickness of the fixed electrode film  40  represents the cross-sectional area of the current passage with respect to the current flowing between the acoustic holes  38   a.    
     First of all, the opening rate of the acoustic hole  38   a  may be set large in the region distant from the extraction wiring  44  to enhance the S/N ratio of the acoustic sensor  31 .
 
Diameter Wa&gt;Spaced distance Da
 
is desirable.
 
     The spaced distance Da between the acoustic holes  38   a  is desirably made as narrow as possible to efficiently escape the thermal noise generated in the air gap between the diaphragm  33  and the fixed electrode film  40 . However, if the spaced distance between the acoustic holes  38   a  is narrowed in excess, the strength of the back plate  34  may lack or the electrode area of the fixed electrode film  40  may reduce. Therefore,
 
Da&gt;0.25×Wa
 
is recommended. Therefore, the spaced distance Da between the acoustic holes  38   a  may be as narrow as possible, with a lower limit of 0.25Wa.
 
     Summarizing the above, the condition
 
0.25×Wa&lt;Da&lt;Wa is obtained.
 
     If the diameter Wa of the acoustic hole  38   a  is excessively small, the sacrifice layer present in the back plate  34  may not be etched in the manufacturing step or thermal noise may generate inside the acoustic hole  38   a . The diameter Wa of the acoustic hole  38   a  is desirably greater than or equal to 3 μm. For instance, if Wa=16 μm and Da=8 μm, the thermal noise in the air gap becomes small and high S/N ratio can be obtained. 
     The acoustic hole  38   b  having a relatively small opening area in the extraction wiring  44  and the region in the vicinity thereof will now be described. As shown in  FIG. 4 , the array pitch of the acoustic hole  38   b  is Wb+Db, where Wb is the diameter of the acoustic hole  38   b  having a small opening area and Db is the spaced distance between the acoustic holes  38   b . The spaced distance Db×the thickness of the fixed electrode film  40  represents the cross-sectional area of the current passage with respect to the current flowing between the acoustic holes  38   b.    
     The ensuring of the strength of the fixed electrode film  40  and the reduction of the parasitic resistance need to be prioritized in the vicinity of the extraction wiring  44 , and thus the spaced distance Db between the acoustic holes  38   b  is desirably wide. In particular, the spaced distance Db is to be greater than the radius of the acoustic hole  38   b  (e.g., Db&gt;0.5×Wb) to have the fixed electrode film  40  sufficiently strong. The array pitch Wb+Db of the acoustic holes  38   b  in the extraction wiring  44  and the region in the vicinity thereof may be equal to the array pitch Wa+Da of the acoustic holes  38   a  in the region distant from the extraction wiring  44  to uniformly etch the sacrifice layer of the air gap. That is, Da&lt;Db because Wa&gt;Wb when Wb+Db=Wa+Da. Therefore, if the arrangement of the acoustic hole  38   a  is such that Wa=16 μm and Da=8 μm, the fixed electrode film  40  can have sufficient strength and the sacrifice layer can be uniformly etched if the arrangement of the acoustic holes  38   b  is Wb=10 μm and Db=14 μm. 
     From the standpoint of ensuring the strength of the back plate  34  and reducing the parasitic resistance, the area for forming the acoustic hole  38   b  having a small opening area is desirably between one or more zones, also referred to as sections, and five or less zones, or sections. The thermal noise of the air gap cannot be reduced and the S/N ratio of the acoustic sensor  31  may be degraded if six or more zones, or sections. With regards to the zone or section of the acoustic hole  38   b , a set of acoustic holes  38   b  arranged continuously (in particular, lined in short interval and assumed as continuous) is assumed as one section. For instance, in the example of  FIG. 4 , three sections, or zones, are provided, where the first section/zone (I) includes one acoustic hole  38   b  including the extraction wiring  44 , the second section/zone (II) includes three acoustic holes  38   b  (e.g., acoustic hole  38   b  positioned on a hexagon in which the distance from the acoustic hole  38   b  of the first section (I) to the corner is Wb+Db), and the third section/zone (III) includes five acoustic holes  38   b  (acoustic hole  38   b  positioned on a hexagon in which the distance from the acoustic hole  38   b  of the first section (I) to the corner is 2Wb+2 Db). In the example of  FIG. 4 , the interval of the acoustic holes  38   b  is the same in all directions, and thus one section is defined in the direction the acoustic holes  38   b  are lined as much as possible. 
     Second Embodiment 
     An acoustic sensor according to a second embodiment of the present invention will be described.  FIG. 5  is a plan view of an acoustic sensor  61  according to a second embodiment of the present invention.  FIG. 6  is a plan view showing the acoustic sensor  61  in which the plate portion  39  is omitted, and also shows one part in an enlarged manner. 
     The acoustic sensor  61  of the second embodiment has a structure substantially similar to the acoustic sensor  31  of the first embodiment other than that the diaphragm  33  and the back plate  34  are formed to a substantially square shape, and hence the same reference numerals are denoted for the portions of the same structure as the first embodiment in the figures and the description thereof will be omitted. 
     As shown in  FIG. 6 , the diaphragm  33  is formed to a substantially square shape in the acoustic sensor  61 , where a beam  62  is extended in the diagonal direction from the four corners. The diaphragm  33  has the lower surface of each beam  62  supported by the anchor  37  arranged on the upper surface of the silicon substrate  32  by SiO 2 . 
     In the acoustic sensor  61  as well, the acoustic hole  38   b  having a small opening area is formed in the region in the vicinity of the extraction wiring  44  extended from the fixed electrode film  40 , and the acoustic hole  38   a  having a large opening area is formed in the region distant from the extraction wiring  44 . The acoustic holes  38   b  are arranged at the same array pitch as the acoustic holes  38   a  so as to form three sections/zones. 
     Third Embodiment 
     An acoustic sensor according to a third embodiment of the present invention will now be described.  FIG. 7  is a plan view of an acoustic sensor  71  according to the third embodiment of the present invention. 
     In the acoustic sensor  71  of the third embodiment, one or a plurality of acoustic holes  38   b  having a small opening area is formed in the extraction wiring  44 , and only the acoustic hole  38   b  at the extraction wiring  44  is assumed as the acoustic hole having a small opening area. The acoustic holes  38   a  having a large opening area is regularly arrayed in the region other than the region where the extraction wiring  44  is passed. However, some of the acoustic holes  38   a  are omitted from the acoustic holes  38   a  arrayed regularly in the region in the vicinity of the extraction wiring  44 , so that the number density of the acoustic holes  38   a  is reduced than the region distant from the extraction wiring  44  so that the array pitch of the acoustic holes  38   a  is reduced. 
     In the present embodiment, the opening rate of the acoustic hole is made small by reducing the opening area of the acoustic hole  38   b  in the extraction wiring  44 . Furthermore, the opening rate of the acoustic hole is made small by reducing the number density of the acoustic hole  38   a  in the region in the vicinity of the extraction wiring  44 . 
     The embodiment of  FIG. 7  can be assumed that the acoustic hole  38   b  having a small opening area is provided at the extraction wiring  44  and the acoustic hole is not provided (i.e., opening rate is zero) in the region in the vicinity of the extraction wiring  44 . 
     Furthermore, the opening rate may be made small only in the region where the extraction wiring  44  is passed as described next. In other words, the opening rate is made small by providing the acoustic hole  38   b  having a small opening area in the extraction wiring  44 . The acoustic hole  38   a  having a large opening area may be regularly arrayed in the region other than the region where the extraction wiring  44  is passed, so that the region in the vicinity of the extraction wiring  44  has an opening rate same as the region distant from the extraction wiring  44 . 
     Fourth Embodiment 
     An acoustic sensor according to a fourth embodiment of the present invention will be described.  FIG. 8  is a plan view of an acoustic sensor  81  according to a fourth embodiment of the present invention. 
     In the acoustic sensor  81  of the fourth embodiment, the acoustic hole  38   a  is formed in the region other than the region where the extraction wiring  44  is passed. The acoustic holes  38   a  all have an opening area (i.e., opening size) of the same size. The acoustic holes  38   a  are regularly arrayed so that the spaced distance between the adjacent acoustic holes  38   a  becomes relatively small in the acoustic hole  38   a  in the region distant from the extraction wiring  44 . The acoustic holes  38   a  are arranged irregularly or at random so that the spaced distance between the acoustic holes  38   a  becomes greater than the region distant from the extraction wiring  44  in the region in the vicinity of the extraction wiring  44 . 
     In the present embodiment, the opening rate is zero because the acoustic hole is not provided at the extraction wiring  44 , and the number density of the acoustic hole  38   a  is small and the opening rate thereof is smaller than the region distant from the extraction wiring  44  in the region in the vicinity of the extraction wiring  44 . 
     In the embodiment of  FIG. 8 , the small acoustic hole  38   b  may be provided at the extraction wiring  44 . 
     Fifth Embodiment 
       FIG. 9  is a schematic cross-sectional view showing a microphone  91  according to a fifth embodiment of the present invention. As shown in  FIG. 9 , an acoustic sensor  92  is mounted in a package  94  along with an IC circuit  93  (signal processing circuit), where an electrode pad  95  of the acoustic sensor  92  and the IC circuit  93  are wire connected with a bonding wire  96 , and the IC circuit  93  is wire connected to an electrode portion  98  of the package  94  with a bonding wire  97 . An acoustic vibration introducing hole  99  for introducing the acoustic vibration into the package  94  is opened at the upper surface of the package  94 . 
     Therefore, when the acoustic vibration enters the package  94  from the acoustic vibration introducing hole  99 , such acoustic vibration is detected by the acoustic sensor  92 . The electrostatic capacitance between the diaphragm  33  and the fixed electrode film  40  changes by the acoustic vibration, and such change in electrostatic capacitance is output to the IC circuit  93  as an electrical signal. The IC circuit  93  performs a predetermined signal processing on the electrical signal output from the acoustic sensor  92  so that it can be output to the outside from the electrode portion  98 . 
     LEGEND 
     
         
         Acoustic sensor  31   
         Silicon substrate  32  (semiconductor substrate) 
         Diaphragm  33   
         Back plate  34   
         Back chamber  35   
         Anchor  37   
         Acoustic holes  38   a ,  38   b  (acoustic perforations) 
         Plate portion  39  (fixed film) 
         Fixed electrode film  40   
         Base part  41   
         Microscopic stoppers  42   
         Band plate shaped extraction wiring  43   
         Extraction wiring  44   
         Fixed side electrode pad  45  (electrode terminal) 
         Movable side electrode pad  46  (electrode terminal) 
         Adhering layer  47   
         Base covering part  51   
         Flat part  52   
         Protective film  53   
         Acoustic sensor  61   
         Beam  62   
         Acoustic sensor  71   
         Acoustic sensor  81   
         Microphone  91   
         Acoustic sensor  92   
         IC circuit  93  (signal processing circuit) 
         Package  94   
         Electrode pad  95   
         Bonding wire  96   
         Bonding wire  97   
         Electrode portion  98   
         Acoustic vibration introducing hole  99   
       
    
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.