Patent Publication Number: US-8126167-B2

Title: Condenser microphone

Description:
The present invention claims priority based on five Japanese patent applications, i.e., Japanese Patent Application No. 2006-92039 (filing date: Mar. 29, 2006), Japanese Patent Application No. 2006-92063 (filing date: Mar. 29, 2006), Japanese Patent Application No. 2006-92076 (filing date: Mar. 29, 2006), Japanese Patent Application No. 2006-278246 (filing date: Oct. 12, 2006), and Japanese Patent Application No. 2006-281902 (filing date: Oct. 16, 2006), the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to condenser microphones, which are manufactured by way of semiconductor device manufacturing processes and are adapted to MEMS (micro-electromechanical system), and in particular to condenser microphones in which diaphragm vibrating due to sound waves are arranged opposite to plates so as to generate electric signals in response to variations of electrostatic capacitance therebetween. 
     2. Background Art 
     Conventionally, various types of condenser microphones manufactured by way of semiconductor device manufacturing processes have been developed. A conventionally-known condenser microphone is constituted in such a way that a diaphragm having a moving electrode, which vibrates due to sound waves, is arranged opposite to a plate having a fixed electrode, wherein the diaphragm and the plate are distanced from each other and are supported via an insulating spacer. That is, a condenser (i.e., electrostatic capacitance) is formed by means of the diaphragm and the plate, which are arranged opposite to each other. 
     In the aforementioned condenser microphone, when the diaphragm vibrates due to sound waves, the electrostatic capacitance varies due to the displacement thereof, so that variations of the electrostatic capacitance are converted into electric signals. The sensitivity of the condenser microphone increases when the ratio of the displacement of the diaphragm to the distance between the oppositely arranged electrodes is increased, i.e., by improving the vibration characteristics of the diaphragm. In addition, the sensitivity of the condenser microphone increases when the parasitic capacitance that does not contribute to variations of the electrostatic capacitance is decreased. 
     The paper issued by the Japanese Institute of Electrical Engineers and entitled “Mechanical Properties of Capacitive Silicon Microphone” teaches a condenser microphone in which a diaphragm and a plate are formed using conductive thin films. Herein, a spacer is fixed to the overall periphery of the diaphragm; hence, when sound waves are transmitted to the diaphragm, a relatively large displacement occurs in the center portion of the diaphragm, while a very small displacement occurs in the periphery of the diaphragm. As a result, vibration at the center portion of the diaphragm is efficiently detected as capacitance variations, while only the parasitic capacitance occurs in the periphery of the diaphragm. The parasitic capacitance reduces the sensitivity of the condenser microphone. 
     Japanese Patent Application Publication No. H09-508777 and U.S. Pat. No. 4,776,019 teach condenser microphones in which vibration characteristics of the diaphragm are improved by use of spring structures for supporting diaphragms so as to improve sensitivities. Specifically, slits are formed in the diaphragm, and spring functions are applied to regions defined by the slits. However, since the plate is arranged to entirely correspond to the diaphragm having the spring function, a parasitic capacitance occurs in a region causing small displacement due to vibration of the diaphragm, whereby the sensitivity of the condenser microphone decreases. 
     Japanese Patent Application Publication No. 2004-506394 teaches a condenser microphone, in which a plate arranged opposite to a diaphragm having a moving electrode is formed using an insulating material, and a rear electrode is arranged only in the prescribed portion of the plate positioned opposite to the center portion of the diaphragm, so that variations of electrostatic capacitance are efficiently detected in correspondence with the center portion of the diaphragm, thus reducing the parasitic capacitance at the periphery of the diaphragm and thus improving the sensitivity. However, since the rear electrode is arranged only in the prescribed portion of the plate positioned opposite to the center portion of the diaphragm, the manufacturing process becomes complex and the manufacturing yield decreases, thus increasing the manufacturing cost. When a gap is formed by removing a sacrifice layer intervened between the diaphragm and the plate by way of etching, the insulating material for fixing the plate and the rear electrode should be slightly etched. The countermeasure coping with this problem must be incorporated into the manufacturing process, which further increases the manufacturing cost. 
     The sensitivity of the condenser microphone depends upon the vibration characteristics of the diaphragm, the parasitic capacitance formed between the diaphragm and the back plate, and the rigidity of the back plate: hence, the prior-art technology for improving the sensitivity of the condenser microphone has problems in that structural complexity and operational instability occur, and the manufacturing yield becomes low due to the complex manufacturing process. 
     For example, it is possible to adopt a countermeasure in which, in order to reduce the parasitic capacitance, a plurality of small holes are formed in the region of the back plate positioned opposite to the periphery of the diaphragm so as to reduce the substantially opposite area therebetween; however, this reduces the mechanical strength of the back plate and increases the unwanted deformation of the back plate. In addition, it is possible to form projections so as to control excessive vibration of the diaphragm, whereby even when excessive sound pressure is applied to the diaphragm, or a mechanical impact is applied to the condenser microphone from the exterior, it is possible to prevent the diaphragm from coming in contact with the rear electrode arranged in the prescribed portion of the back plate. However, this requires a complex process for forming the rear electrode on the back plate composed of the insulating material, which reduces the manufacturing yield and increases the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a condenser microphone, which improves vibration characteristics of a diaphragm without making the manufacturing process complex and which reduces a parasitic capacitance between the diaphragm and a plate, thus increasing the sensitivity. 
     According to a first aspect of the present invention, in a condenser microphone including a diaphragm having a conductivity, which includes a center portion and a plurality of arms extended externally in a radial manner and which vibrates due to sound waves, a back plate having a conductivity, which is positioned opposite to the diaphragm, a substrate, which is positioned opposite to the back plate so as to face the diaphragm and which has a cavity for relaxing pressure applied to the diaphragm, and a support member, which supports the diaphragm above the substrate while insulating the tip ends of the arms of the diaphragm from the external periphery of the diaphragm, thus forming a gap between the center portion of the diaphragm and the back plate, a high acoustic resistance, which is higher than an acoustic resistance formed between the plurality of arms, is formed between the substrate surrounding the cavity and the diaphragm. 
     In the aforementioned constitution, the diaphragm having a gear-like shape is improved in vibration characteristics, and the external circumference of the back plate is not positioned oppositely at the cutouts formed between the arms of the diaphragm; hence, it is possible to avoid the occurrence of a parasitic capacitance. In addition, the diaphragm and the back plate can be easily manufactured using conductive materials. Furthermore, a high acoustic resistance is formed between the substrate surrounding the cavity and the diaphragm, it is possible to prevent sound waves reaching the diaphragm from being transmitted between the arms. That is, with a simple manufacturing process, it is possible to improve the vibration characteristics of the diaphragm, and it is possible to reduce the unwanted parasitic capacitance between the diaphragm and the back plate; hence, it is possible to improve the sensitivity of the condenser microphone. 
     It is preferable that the distance from the center to the external end of the back plate be shorter than the distance from the center of the center portion of the diaphragm to the tip end of the arm. Thus, it is possible to further reduce the parasitic capacitance. Since the size of the back plate is reduced in comparison with the diaphragm, it is possible to increase the rigidity of the back plate; hence, it is possible to enlarge the size of the diaphragm without degrading the operation stability of the condenser microphone. 
     It is preferable that cutout be formed in the back plate at the positions opposite to the arms of the diaphragm. Thus, no parasitic capacitance occurs between the back plate and the arms of the diaphragm so that the electrostatic capacitance is formed between the back plate and the center portion of the diaphragm; hence, it is possible to reduce the ratio of the parasitic capacitance. 
     It is preferable that the support member be constituted of a first support for supporting the tip ends of the arms of the diaphragm and a second support, which is positioned between the arms of the diaphragm so as to support the back plate. Since only the tip ends of the arms of the diaphragm are supported by the first support, it is possible to improve the vibration characteristics of the diaphragm in comparison with the prior-art technology in which the overall periphery of the diaphragm is fixed. Since the second support for supporting the external periphery of the back plate is positioned to match the cutouts formed between the arms of the diaphragm, it is possible to reduce the size of the back plate compared with the diaphragm; hence, it is possible to increase the rigidity of the back plate. Furthermore, since the diaphragm and the back plate are directly supported above the substrate, it is possible to manufacture the condenser microphone with a simple manufacturing process. 
     It is preferable that the cavity has an opening formed along the inside of the center portion of the diaphragm. That is, the opening of the cavity is formed to substantially match the center portion of the diaphragm, so that the cavity has a sufficiently large volume. As a result, the spring constant of the air inside of the cavity becomes adequately small; hence, it is possible to maintain good vibration characteristics of the diaphragm. Due to the formation of a passage having a high acoustic resistance, which is higher than the acoustic resistance between the arms of the diaphragm, it is possible to prevent sound waves reaching the diaphragm from being transmitted between the arms. 
     It is possible for the cavity to have an opening formed along and inwardly of the external end of the diaphragm. In this case, the opening of the cavity is formed to entirely match the diaphragm; hence, the cavity has a sufficiently large volume; thus, it is possible to maintain good vibration characteristics of the diaphragm. 
     It is preferable that a plurality of holes be formed in the arms of the diaphragm. Thus, it is possible to reduce the rigidity of the arms of the diaphragm; this makes it easy for the arms to be deformed in a vibration mode of the diaphragm, and this increases the displacement of the center portion. Thus, it is possible to further improve the vibration characteristics of the diaphragm. In the manufacturing process, an etching solution is infiltrated via the holes of the arms of the diaphragm so as to remove a sacrifice layer intervened between the arms of the diaphragm and the substrate by way of etching, thus forming a gap. That is, by forming the holes in the arms of the diaphragm, it is possible to simplify the manufacturing process, and it is possible to further improve the vibration characteristics of the diaphragm; thus, it is possible to improve the sensitivity of the condenser microphone. 
     Alternatively, the condenser microphone can be constituted of a back plate having a conductivity, which includes a center portion and a plurality of arms extended externally in a radial manner, a diaphragm having a conductivity, which is positioned opposite to the back plate so as to vibrate due to sound waves, a substrate, which is positioned opposite to the back plate so as to face the diaphragm and which has a cavity for relaxing pressure applied to the diaphragm, and a support member, which supports the diaphragm above the substrate while insulating the external periphery of the diaphragm from the tip ends of the arms of the back plate, thus forming a gap between the diaphragm and the center portion of the back plate. In this case, it is preferable that cutouts be formed in the diaphragm at the positions opposite to the arms of the back plate. 
     According to a second aspect of the present invention, the support member adapted to the aforementioned condenser microphone is constituted of a spacer whose lower surface joins the tip ends of the plurality of arms of the diaphragm, a bridge whose inner end joins the upper surface of the spacer, a first support having an insulating property, which supports the outer end of the bridge above the support, and a second support having an insulating property, which supports the external periphery of the back plate above the substrate, wherein a gap is formed between the center portion of the diaphragm and the back plate. 
     As described above, due to the structure in which the diaphragm joins the bridge supported above the substrate by means of the first support via the spacer, it is possible to relax the stress of the diaphragm, and it is possible to further improve the vibration characteristics. 
     It is preferable that the second support be positioned between the plurality of arms of the diaphragm. That is, since the second support for supporting the external periphery of the back plate is positioned to match the cutouts formed between the plurality of arms of the diaphragm, it is possible to reduce the size of the back plate compared with the diaphragm. This makes it possible to increase the rigidity of the back plate; hence, it is possible to enlarge the diaphragm without damaging the operation stability of the condenser microphone. Due to the structure in which the diaphragm and the back plate are independently supported above the substrate, it is possible to produce the condenser microphone with a simple manufacturing process. 
     It is preferable that the bridge be composed of the same material as the back plate and be formed simultaneously with the back plate. Thus, without the necessity of a special process for the formation of the bridge, it is possible to simplify the manufacturing process of the condenser microphone. It is preferable that a plurality of holes be formed in the bridge. This reduces the rigidity of the bridge; this makes it easy for the bridge to be deformed in a vibration mode of the diaphragm; and this increases the displacement of the center portion of the diaphragm; hence, it is possible to further improve the vibration characteristics of the diaphragm. Furthermore, an etching solution is infiltrated via the holes of the bridge so as to remove a sacrifice layer intervened between the back plate and the diaphragm by way of etching, thus forming a gap therebetween. 
     It is preferable that a high acoustic resistance, which is higher than the acoustic resistance between the plurality of arms of the diaphragm, be formed between the substrate surrounding the cavity and the diaphragm. Thus, it is possible to prevent sound waves reaching the diaphragm from being transmitted between the plurality of arms; hence, it is possible to further improve the sensitivity of the condenser microphone. 
     According to a third aspect of the present invention, the support member adapted to the aforementioned condenser microphone, is constituted of a first support having an insulating property, which supports a peripheral portion of the diaphragm, and a plurality of second supports, which are inserted into a plurality of holes formed in the center portion of the diaphragm so as to support the back plate above the substrate. This limits the size of the back plate to match only the size of the center portion of the diaphragm; hence, it is possible to downsize the condenser microphone. Due to the increase of the mechanical strength of the back plate, even when a voltage applied between the diaphragm and the back plate is increased for the purpose of the improvement of the sensitivity of the condenser microphone, it is possible to avoid the deformation of the back plate due to the electrostatic attraction occurring between the opposite electrodes, and it is possible to avoid the deformation of the back plate due to an impact from the exterior; hence, it is possible to improve the vibration characteristics of the diaphragm. In addition, it is possible to secure the operation stability of the condenser microphone. Since the back plate is directly supported above the substrate by means of the plurality of second supports, the back plate can be held in a stable manner. Since the peripheral portion of the diaphragm is not positioned opposite to the back plate, no parasitic capacitance occurs between them. 
     In the above, it is preferable that a stopper layer having an insulating property be arranged in the gap formed between the diaphragm and the back plate. Thus, even when excessive sound pressure is applied to the diaphragm, or even when an impact is applied to the condenser microphone from the exterior, it is possible to avoid excessive deformation of the diaphragm due to intervention of the stopper layer; hence, it is possible to prevent the diaphragm from coming in contact with the back plate. It is preferable that the stopper layer be fixed to the second supports. That is, the stopper layer is directly supported above the substrate in a stable manner by means of the second supports; hence, it is possible to reliably prevent the diaphragm from coming in contact with the back plate. 
     It is preferable that a plurality of small holes be formed respectively in a plurality of regions of the peripheral portion of the diaphragm positioned opposite to the substrate. This reduces the rigidity of the diaphragm so that the diaphragm is easily deformed and the displacement of the center portion increases in a vibration mode; hence, it is possible to improve the vibration characteristics of the diaphragm. Incidentally, the plurality of holes are formed only in the plurality of regions positioned opposite to the substrate but they are not formed in other regions positioned opposite to the cavity; hence, sound waves reaching the diaphragm are not transmitted through the plurality of holes without contributing to vibration. 
     The present invention demonstrates effects, in which, with a simple manufacturing process, the vibration characteristics of the diaphragm are improved, and the unwanted parasitic capacitance between the diaphragm and the back plate is reduced, so that the sensitivity of the condenser microphone is improved. Specifically, it is possible to improve the vibration characteristics of the diaphragm having a gear-like shape; and it is possible to avoid the occurrence of parasitic capacitance because the external periphery of the back plate is not positioned oppositely at the cutouts formed between the arms of the diaphragm. Since a high acoustic resistance is formed between the substrate surrounding the cavity and the diaphragm, it is possible to prevent sound waves reaching the diaphragm from being transmitted between the arms. Since the size of the back plate is reduced in comparison with the diaphragm, it is possible to increase the rigidity of the back plate; hence, it is possible to increase the size of the diaphragm without degrading the operation stability of the condenser microphone. Since the cavity has an opening formed inwardly of the external periphery of the diaphragm, it has a sufficiently large volume; hence, it is possible to maintain good vibration characteristics of the diaphragm. Due to the formation of the plurality of holes in the arms of the diaphragm, the rigidity of the arms of the diaphragm decreases so that the arms can be easily deformed in a vibration mode of the diaphragm, thus increasing the displacement of the center portion. An etching solution is infiltrated via the holes of the arms of the diaphragm so as to remove a sacrifice layer intervened between the arms of the diaphragm and the substrate by way of etching, thus forming the gap. Thus, it is possible to further improve the vibration characteristics of the diaphragm. 
     When the support member adapted to the condenser microphone of the present invention is constituted of the spacer, the bridge, the first support, and the second support, the diaphragm joins the bridge supported above the substrate by means of the first support via the spacer; hence, it is possible to relieve the stress of the diaphragm, and it is possible to further improve the vibration characteristics. In addition, a plurality of holes are formed in the bridge so as to reduce the rigidity, wherein an etching solution is infiltrated via the holes so as to remove a sacrifice layer intervened between the back plate and the diaphragm, thus forming a gap between them. Thus, it is possible to further improve the vibration characteristics of the diaphragm. 
     When the support member adapted to the condenser microphone of the present invention is constituted of the first support having an insulating property, which supports the peripheral portion of the diaphragm and a plurality of second supports having insulating property, which are inserted into the plurality of holes formed in the center portion of the diaphragm so as to support the back plate above the substrate, it is possible to improve the vibration characteristics of the diaphragm, and it is possible to increase the mechanical strength of the back plate. That is, even when a voltage applied between the diaphragm and the back plate is increased for the purpose of the improvement of the sensitivity of the condenser microphone, it is possible to avoid deformation of the back plate due to the electrostatic attraction occurring between the opposite electrodes, and it is possible to avoid deformation of the back plate due to an impact from the exterior; hence, it is possible to improve the vibration characteristics of the diaphragm, and it is possible to secure the operation stability of the condenser microphone. Furthermore, since the stopper layer having an insulating property is arranged in the gap formed between the diaphragm and the back plate, even when excessive sound pressure is applied to the diaphragm, or even when a mechanical impact is applied to the condenser microphone from the exterior, it is possible to avoid the excessive deformation of the diaphragm due to the intervention of the stopper layer; hence, it is possible to prevent the diaphragm from coming in contact with the back plate. 
     Furthermore, a plurality of small holes are formed in a plurality of regions of the peripheral portion of the diaphragm positioned opposite to the substrate so as to reduce the rigidity of the diaphragm; this makes it easy for the diaphragm to be easily deformed in a vibration mode, and this increases the displacement of the center portion; hence, it is possible to improve the vibration characteristics of the diaphragm. Incidentally, the plurality of holes are formed only in the plurality of regions positioned opposite to the substrate and are not formed in other regions; hence, sound waves reaching the diaphragm are not transmitted through the plurality of holes without contributing to vibration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view showing the constitution of a condenser microphone in accordance with a first embodiment of the present invention. 
         FIG. 1B  is a cross-sectional view taken along line A-A in  FIG. 1A . 
         FIG. 1C  is a fragmentary enlarged view of  FIG. 1B . 
         FIG. 2A  is a plan view showing a condenser microphone having a conventionally-known structure. 
         FIG. 2B  is a cross-sectional view of  FIG. 2A . 
         FIG. 3A  is a plan view showing a condenser microphone that is prepared for use in an experiment. 
         FIG. 3B  is a cross-sectional view of  FIG. 3A . 
         FIG. 4  is a cross-sectional view showing a first step of a manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 5  is a cross-sectional view showing a second step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 6  is a cross-sectional view showing a third step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 7  is a cross-sectional view showing a fourth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 8  is a cross-sectional view showing a fifth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 9  is a cross-sectional view showing a sixth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 10  is a cross-sectional view showing a seventh step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 11  is a cross-sectional view showing an eighth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 12  is a cross-sectional view showing a ninth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 13  is a cross-sectional view showing a tenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 14  is a cross-sectional view showing an eleventh step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 15  is a cross-sectional view showing a twelfth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 16  is a cross-sectional view showing a thirteenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 17  is a cross-sectional view showing a fourteenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 18  is a cross-sectional view showing a fifteenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 19  is a cross-sectional view showing a sixteenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 20  is a cross-sectional view showing a seventeenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 21  is a cross-sectional view showing an eighteenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 22  is a cross-sectional view showing a nineteenth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 23  is a cross-sectional view showing a twentieth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 24  is a cross-sectional view showing a twenty-first step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 25  is a cross-sectional view showing a twenty-second step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 26  is a cross-sectional view showing a twenty-third step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 27  is a cross-sectional view showing a twenty-fourth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 28  is a cross-sectional view showing a twenty-fifth step of the manufacturing method of the condenser microphone according to the first embodiment. 
         FIG. 29A  is a circuit diagram showing the constitution of a detection circuit that converts variations of electrostatic capacitance formed between a diaphragm and a back plate into electric signals. 
         FIG. 29B  is a circuit diagram showing the constitution of a detection circuit arranging a conductive film. 
         FIG. 30A  is a plan view showing the constitution of a condenser microphone in accordance with a second embodiment of the present invention. 
         FIG. 30B  is a cross-sectional view taken along line A-A in  FIG. 30A . 
         FIG. 30C  is a cross-sectional view taken along line B-B in  FIG. 30A . 
         FIG. 31A  is a circuit diagram showing the constitution of a detection circuit that converts variations of electrostatic capacitance formed between a diaphragm and a back plate into electric signals. 
         FIG. 31B  is a circuit diagram showing the constitution of a detection circuit arranging a conductive film. 
         FIG. 32A  is a plan view showing the constitution of a condenser microphone in accordance with a third embodiment of the present invention. 
         FIG. 32B  is a plan view showing the constitution in which the back plate is removed from the constitution shown in  FIG. 32A . 
         FIG. 32C  is a cross-sectional view taken along line A-A in  FIG. 32A . 
         FIG. 32D  is a cross-sectional view taken along line B-B in  FIG. 32A . 
         FIG. 33A  is a plan view showing the constitution of a condenser microphone in accordance with a first variation of the third embodiment. 
         FIG. 33B  is a plan view showing the constitution in which the back plate is removed from the constitution shown in  FIG. 33A . 
         FIG. 33C  is a cross-sectional view taken along line A-A in  FIG. 33A . 
         FIG. 33D  is a cross-sectional view taken along line B-B in  FIG. 33A . 
         FIG. 34A  is a plan view showing the constitution of a condenser microphone in accordance with a second variation of the third embodiment. 
         FIG. 34B  is a plan view showing the constitution in which the back plate is removed from the constitution shown in  FIG. 34A . 
         FIG. 34C  is a cross-sectional view taken along line A-A in  FIG. 34A . 
         FIG. 34D  is a cross-sectional view taken along line B-B in  FIG. 34A . 
         FIG. 35A  is a plan view showing the constitution of a condenser microphone in accordance with a fourth variation of the first embodiment of the present invention. 
         FIG. 35B  is a cross-sectional view taken along line A-A in  FIG. 35A . 
         FIG. 35C  is a fragmentary enlarged view of  FIG. 35B . 
     
    
    
     PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Incidentally, the same constituent elements are designated by the same reference numerals in the embodiments. 
     First Embodiment 
     The constitution of a condenser microphone according to a first embodiment of the present invention will be described with reference to  FIG. 1 .  FIG. 1A  is a plan view showing the constitution of the condenser microphone according to the first embodiment;  FIG. 1B  is a cross-sectional view taken along line A-A in  FIG. 1A ; and  FIG. 1C  is an enlarged view of a portion denoted by B in the cross-sectional view of FIG. B. The condenser microphone shown in  FIGS. 1A to 1C  is constituted of a diaphragm  10 , a back plate  20 , and a substrate  30  having a support member having an insulating property. The diaphragm  10  and the back plate  20  each have electrodes, wherein they are positioned opposite to each other and are supported by means of the support member having the insulating property. 
     The diaphragm  10  is a thin film having a conductivity, which is composed of polysilicon added with phosphorus (P) as impurities, wherein it is constituted of a disk-like center portion  12  and six arms  14  expanded externally in a radial manner, so that it collectively has a gear-like shape. A plurality of holes  16  are formed in the six arms respectively. The thickness of the diaphragm  10  is approximately 0.5 μm; the radius of the center portion  12  is approximately 0.35 mm; and the length of the arm  14  is approximately 0.15 mm. 
     The back plate  20  is arranged in parallel with the diaphragm  10  via a gap  40  of approximately 4 μm, for example. The back plate  20  is a thin film having a conductivity, which is composed of polysilicon added with phosphorus, wherein it is constituted of a disk-like center portion  22  and six arms  24  expanded externally in a radial manner, so that it collectively has a gear-like shape. A plurality of holes  26  are formed in the center portion  22  and the arms  24  of the back plate  20 . The holes  26  of the back plate  20  function as sound holes that pass sound waves emitted from the exterior therethrough so as to transmit them toward the diaphragm  10 . The thickness of the back plate  20  is approximately 1.5 μm; the radius of the center portion  22  is approximately 0.3 mm; and the length of the arm  24  is approximately 0.1 mm. 
     The center portion  22  of the back plate  20  is arranged concentrically with the center portion  12  of the diaphragm  10 , wherein the radius of the center portion  22  of the back plate  20  is smaller than the radius of the center portion  12  of the diaphragm  10 . In addition, the six arms  24  of the back plate  20  are arranged alternately with the arms  14  of the diaphragm  10 , wherein each of the arms  24  is positioned between the adjacent arms  14 . In other words, each of the arms  14  is positioned between the adjacent arms  24 . The distance between the center of the center portion  22  of the back plate  20  and the tip end of the arm  24  is longer than the radius of the center portion  12  of the diaphragm  10  but is smaller than the distance between the center of the center portion  12  of the diaphragm  10  and the tip end of the arm  14 . 
     The tip end of the arm  14  of the diaphragm  10  is supported above the substrate  30  by means of a first support  50  having an insulating property. The tip end of the arm  24  of the back plate  20  is supported by means of a second support  54  having an insulating property. The second support  54  is arranged at a position defined between the arms  14  of the diaphragm  10 . Incidentally, it is possible to form a plurality of cutouts in the diaphragm  10  so that the arms  14  are formed between the cutouts. 
     The first support  50  is composed of a silicon oxide film, for example. The second support  54  is constituted of insulating films  541  and  543  and a conductive film  542 . The insulating films  541  and  543  are composed of silicon oxide films, for example. It is preferable that the conductive film  542  be formed simultaneously with the formation of the diaphragm  10  having a conductivity, wherein it is composed of polysilicon added with phosphorus impurities. The conductive film  542  is placed at the same potential with the back plate  20  and the substrate  30 , so that it functions as a guard electrode for reducing the parasitic capacitance of the condenser microphone. Incidentally, it is possible to omit the conductive film  542 . 
     The substrate is constituted of a silicon substrate whose thickness ranges from 500 μm to 600 μm, for example, wherein a cavity  32  runs through the substrate in conformity with the center portion  12  of the diaphragm  10 , so that the diaphragm  10  is exposed. The cavity  32  is formed along the inside of the center portion  12  of the diaphragm  10 , so that it function as a pressure relaxation room for relaxing pressure that is applied to the diaphragm  10  oppositely to the back plate  20 . In addition, a passage  34  is a space formed between the substrate  30  existing in the vicinity of the cavity  32  and the diaphragm  10 , wherein it has a high acoustic resistance that is higher than an acoustic resistance between the arms  14  of the diaphragm  10 . As shown in  FIG. 1C , the acoustic resistance is controlled based on a height H (i.e., the distance between the diaphragm  10  and the substrate  30 ) and a length L (i.e., the distance from an innermost hole  16  within the plurality of holes  16  formed in the arm  14  of the diaphragm  10  to the end portion of the cavity  32 , or the distance from the end portion of the center portion  12  of the diaphragm  10  to the end portion of the cavity  32 ) of the passage  34 , thus realizing a high acoustic resistance that is higher than the acoustic resistance between the arms  14  of the diaphragm  10 . Thus, it is possible to prevent sound waves, which are transmitted to the diaphragm  10 , from propagating between and leaking between the arms  14 . For example, the height H of the passage  34  is 2 μm, and the length L is 15 μm. 
       FIG. 29A  is a circuit diagram showing the constitution of a detection circuit that converts variations of electrostatic capacitance formed between the diaphragm  10  and the back plate  20  into electric signals. A stable bias voltage is applied to the diaphragm  10  by means of a charge pump CP. Variations of electrostatic capacitance between the back plate  20  and the diaphragm  10  are input into a pre-amplifier A in the form of voltage variations. Since the substrate  30  and the diaphragm  10  are short-circuited, a parasitic capacitance occurs between the back plate  20  and the substrate  30  without the intervention of the conductive film  542 . 
       FIG. 29B  shows the constitution of a detection circuit arranging the conductive film  542 . Herein, the pre-amplifier A forms a voltage-follower circuit so as to make the conductive film  542  function as a guard electrode. That is, since the back plate  20  and the conductive film  542  are controlled to be placed at the same potential by means of the voltage-follower circuit, it is possible to remove the parasitic capacitance occurring between the back plate  20  and the conductive film  542 . In addition, since the substrate  30  and the diaphragm  10  are short-circuited, the capacitance between the conductive film  542  and the substrate  30  becomes irrelevant to the output of the pre-amplifier A. As described above, since a guard electrode is formed by way of the provision of the conductive film  542 , it is possible to further reduce the parasitic capacitance of the condenser microphone. 
     As described above, in the condenser microphone according to the first embodiment, the diaphragm  10  and the back plate  20  both have gear-like shapes, wherein the center portion  12  of the diaphragm  10  and the center portion  22  of the back plate  20  are mutually positioned opposite to each other. In a plan view, the arms  14  of the diaphragm  10  and the arms  24  of the back plate are alternately arranged with each other, wherein they are not arranged oppositely. Thus, it is possible to avoid the occurrence of an unwanted parasitic capacitance. That is, an electrostatic capacitance is formed between the center portion  12  of the diaphragm  10  and the center portion  22  of the back plate  20 , whereby electric signals are produced in response to variations of the electrostatic capacitance; hence, it is possible to remarkably reduce the parasitic capacitance in the other portions of the condenser microphone, thus remarkably improving the sensitivity. 
     The tip ends of the arms  14  of the diaphragm  10  are supported by the first support  50 . In addition, the distance from the center of the center portion  12  of the diaphragm  10  to the first support  50  is longer than the distance from the center of the center portion  22  of the back plate  20  to the second support  54  for supporting the tip ends of the arms  24 . That is, it is possible to improve the vibration characteristics of the diaphragm  10  in the condenser microphone of the first embodiment compared with the conventionally-known condenser microphone, in which the overall periphery of the diaphragm is fixed, and the conventionally-known condenser microphone, in which both of the diaphragm and back plate have substantially the same shape in plan view. 
     In addition, the radius of the center portion  22  of the back plate  20  is smaller than the radius of the center portion  12  of the diaphragm  10 , and the distance from the center of the center portion  22  to the second support  54  is shorter than the distance from the center of the center portion  12  to the first support  50 . That is, it is possible to increase the rigidity of the back plate  20  in the condenser microphone of the first embodiment compared with the conventionally-known condenser microphone, in which both of the diaphragm and back plate have substantially the same shape in plan view; hence, it is possible to enlarge the diaphragm  10  without damaging the operation stability, thus improving the vibration characteristics of the diaphragm  10 . 
     Due to the formation of the plurality of holes  16  in the arms  14  of the diaphragm  10 , it is possible to reduce the rigidity of the arms  14 , and this makes it possible for the arms  14  of the diaphragm  10  to be easily deformed. Thus, it is possible to further improve the vibration characteristics of the diaphragm  10 . 
     In order to confirm the effect of the condenser microphone of the first embodiment, the inventor of the present application produces a condenser microphone having the conventionally-known structure and a condenser microphone for use in experiments, thus performing the following experiments. Specifically,  FIGS. 2A and 2B  are a plan view and a cross-sectional view showing the condenser microphone having the conventionally-known structure, and  FIGS. 3A and 3B  are a plan view and a cross-sectional view showing the condenser microphone for use in experiments. 
     In the condenser microphone having the conventionally-known structure shown in  FIGS. 2A and 2B , the overall periphery of a disk-like diaphragm  100  is supported above a substrate  300  by means of a first support  500 . The radius of the diaphragm  100  is set identical to the distance from the center of the center portion  12  of the diaphragm  10  to the tip end of the arm  14  in the condenser microphone of the first embodiment. In addition, a disk-like back plate  200  is arranged to cover the upper surface of the diaphragm  100 , wherein the overall periphery of the back plate  200  is supported above the substrate  300  by means of a second support  540 . 
     The condenser microphone for use in experiments shown in  FIGS. 3A and 3B  has substantially the same structure as the condenser microphone shown in  FIGS. 2A and 2B , wherein six cutouts  700  are formed in the periphery of the back plate  200  in order to reduce the parasitic capacitance, and wherein the cutouts  700  are positioned in proximity to the external circumference supported by the first support  500  of the diaphragm  100 . 
     Measurements are performed on the condenser microphone having the conventionally-known structure shown in  FIGS. 2A and 2B , the condenser microphone for use in experiments shown in  FIGS. 3A and 3B , and the condenser microphone of the first embodiment shown in  FIGS. 1A ,  1 B, and  1 C with respect to the electrode pressure resistance, vibration displacement value, and sensitivity, thus producing results shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Electrode 
                 Vibration 
                   
               
               
                   
                 pressure 
                 displacement 
               
               
                   
                 resistance 
                 value 
                 Sensitivity 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Conventionally-known 
                 1.0 
                 1.0 
                 1.0 
               
               
                 structure 
               
               
                 Experimental structure 
                 0.8 
                 — 
                 — 
               
               
                 First embodiment 
                 1.2 
                 2.0 
                 3.0 
               
               
                   
               
            
           
         
       
     
     The electrode pressure resistance is equivalent to a value of a voltage, which is applied between the diaphragm and the back plate so that the back plate being deformed due to electrostatic attraction comes in contact with the diaphragm in the condition that a sacrifice oxide film is intervened between the diaphragm and the substrate, i.e., in the condition that the diaphragm is entirely fixed to the substrate, wherein it may define a target of the strength of the back plate. 
     The vibration displacement value is a value of displacement of the center portion of the diaphragm when the prescribed sound pressure is applied to the diaphragm. The sensitivity is represented by the output voltage of the condenser microphone when the prescribed sound pressure is applied to the diaphragm, wherein it is represented by the following equation.
 
Sensitivity•Vibration displacement value×Voltage applied between electrodes×[Electrostatic capacitance/(Electrostatic capacitance+Parasitic capacitance)]
 
     In Table 1, numerical values are expressed as relative values calculated on the basis of the values (i.e., “1.0”) representing the electrode pressure resistance, vibration displacement value, and sensitivity of the condenser microphone having the conventionally-known structure. 
     In the condenser microphone for use in experiments, the electrode pressure resistance is reduced to 0.8 in comparison with the condenser microphone having the conventionally-known structure. This is because reduction of the strength is caused by the formation of the cutouts  700  in the back plate  200  which reduces the parasitic capacitance. The reduction of the electrode pressure resistance makes the operation of the condenser microphone unstable. 
     On the other hand, in the condenser microphone of the first embodiment, even though the back plate  20  has a gear-like shape, and the cutouts are formed between the arms  24  arranged in the external circumference of the center portion  22 , the electrode pressure resistance is increased 1.2 times higher in comparison with the condenser microphone having the conventionally-known structure. This is because the second support  54  for supporting the tip ends of the arms  24  of the back plate  20  is positioned at the cutouts formed between the arms  14  of the diaphragm  10 , and the distance from the center of the center portion  22  of the back plate  20  to the second support  54  is shorter than the distance from the center of the diaphragm  100  to the first support  500  in the condenser microphone having the conventionally-known structure. Thus, it is possible to relatively increase the rigidity of the back plate  20 , thus increasing the electrode pressure resistance. By increasing the electrode pressure resistance, it is possible to stabilize the operation of the condenser microphone of the first embodiment. 
     In the condenser microphone of the first embodiment, the vibration displacement value of the diaphragm  10  is increased 2.0 times higher than that of the condenser microphone having the conventionally-known structure. This is because the diaphragm  10  has a gear-like shape, and the tip ends of the arms  14  are supported by the first support  50 . That is, in the condenser microphone of the first embodiment compared with the condenser microphone having the conventionally-known structure, in which the periphery of the diaphragm  100  is entirely fixed, it is possible to improve the vibration characteristics of the diaphragm  10 , wherein the plurality of holes  16  formed in the arms  14  contribute to the increase of the vibration displacement value. 
     Furthermore, in the condenser microphone of the first embodiment, the sensitivity is increased 3.0 times higher than that of the condenser microphone having the conventionally-known structure. This is because the vibration displacement value of the diaphragm  10  is increased to be higher than that of the diaphragm  100  of the condenser microphone having the conventionally-known structure. In addition, the electrostatic capacitance is mainly formed between the center portion  12  of the diaphragm  10  and the center portion  22  of the back plate  20 , and the arms  14  and the arms  24  are positionally shifted from each other so as not to cause the parasitic capacitance therebetween. That is, in the condenser microphone of the first embodiment compared with the condenser microphone having the conventionally-known structure, it is possible to remarkably reduce the parasitic capacitance. 
     The condenser microphone of the first embodiment is a silicon capacitor microphone, which is manufactured by way of the semiconductor device manufacturing process. Hereinafter, a manufacturing method of the condenser microphone of the first embodiment will be described with reference to  FIGS. 4 to 28 . 
     First, as shown in  FIG. 4 , a first insulating film  50   a  of 2 μm thickness composed of a silicon oxide film is formed on the substrate  30 , which is formed using a semiconductor substrate composed of monocrystal silicon, for example, by way of plasma CVD (Plasma Chemical Vapor Deposition). The first insulating film  50   a  is removed in the after-treatment, wherein it serves as a sacrifice layer that is used to form the cavity  32  in the substrate  30  below the diaphragm  10  and that is used to form the passage  34  realizing a desired acoustic resistance between the substrate  30  surrounding the cavity and the diaphragm  10 . In addition, the first insulating film  50   a  is used to form the first support  50  for supporting the diaphragm  10  above the substrate  30 . 
     Next, as shown in  FIG. 5 , a first conductive layer  10   a  of 0.5 μm thickness composed of phosphorus-doped polysilicon is formed on the first insulating film  50   a  by way of decompression CVD (Decompression Chemical Vapor Deposition). The first conductive layer  10   a  is formed on the backside of the substrate  30  as well. Next, as shown in  FIG. 6 , a photoresist film is applied to the entire surface of the first conductive layer  10   a  formed on the first insulating film  50   a ; then, exposure and development are performed by way of the photolithography technique using a resist mask having a prescribed shape, thus forming a photoresist pattern P 1 . Next, as shown in  FIG. 7 , anisotropic etching such as RIE (Reactive Ion Etching) is performed by use of the photoresist pattern P 1  serving as a mask so as to electively remove the first conductive layer  10   a , which is thus processed in a prescribed shape, thus forming the diaphragm  10  of 0.5 μm thickness and the wiring  18  connected thereto as well as the plurality of holes  16  of the arms  14  of the diaphragm  10 . 
     Next, as shown in  FIG. 8 , incineration (ashing) using oxygen plasma (O 2  plasma) and dissolution for soaking into a mixed solution composed of sulfuric acid and hydrogen peroxide are performed so as to remove the photoresist pattern P 1 . Thus, the diaphragm  10  is formed by way of the patterning of the first conductive layer  10   a , wherein, as shown in  FIG. 1A , the diaphragm  10  has a gear-like shape constituted of the center portion  12  having a disk-like shape in plan view and the six arms  14  expanded externally in a radial manner. A plurality of holes  16  are formed in the six arms  14  respectively. 
     Next, as shown in  FIG. 9 , a second insulating film  52   a  of 4 μm thickness composed of a silicon oxide film is formed on the diaphragm  10 , the extension wire  18 , and the first insulating film  50   a  by way of plasma CVD. The second insulating film  52   a  is deposited on the first insulating film  50   a  so as to form a laminated insulating film  54   a . The second insulating film  52   a  serves as a sacrifice film for use in the formation of the gap  40  between the diaphragm  10  and the back plate  20 , which is removed in the after-treatment. In the after-treatment, the laminated insulating film  54   a  is used to form the second support  54  for supporting the back plate  20  above the substrate  30 . 
     Next, as shown in  FIG. 10 , a second conductive layer  20   a  of 1.5 μm thickness composed of phosphorus-doped polysilicon is formed on the second insulating film  52   a  by way of decompression CVD. The second conductive layer  20   a  is formed on the first conductive layer  10   a  at the backside of the substrate  30  as well. Next, as shown in  FIG. 11 , a photoresist film is applied to the entire surface of the second conductive layer  20   a  on the second insulating film  52   a ; then, a photoresist pattern P 2  is formed by way of the photolithography technique. Next, as shown in  FIG. 12 , anisotropic etching such as RIE is performed by use of the photoresist pattern P 2  serving as a mask so as to selectively remove the second conductive layer  20   a  and to process it into a prescribed shape, thus forming the back plate  20  of 1.5 μm thickness and an extension wire  28  connected thereto and thus forming a plurality of holes  26  in the center portion  22  of the back plate  20 . 
     Next, as shown in  FIG. 13 , incineration and dissolution using a mixed solution composed of sulfuric acid and hydrogen peroxide are performed so as to remove the photoresist pattern P 2 ; then, heat treatment is performed for the purpose of quenching. As described above, as shown in  FIG. 1A , the back plate  20  formed by way of the patterning of the second conductive layer  20   a  has a gear-like shape including the center portion  22  having a disk-like shape in plan view and the six arms  24  extended externally in a radial manner, wherein a plurality of holes  26  are formed in the center portion  22  and the six arms  24  respectively. 
     As shown in  FIG. 1A , the center portion  22  of the back plate  20  is arranged concentrically with the center portion  12  of the diaphragm  10 , wherein the radius of the center portion  22  of the back plate  20  is smaller than the radius of the center portion  12  of the diaphragm  10 . In addition, the six arms  24  of the back plate  20  are positioned at the cutouts formed between the six arms  14  of the diaphragm  10 . In other words, the six arms  14  of the diaphragm  10  are positioned at the cutouts formed between the six arms  24  of the back plate  20 . Furthermore, the distance from the center of the center portion  22  of the back plate  20  to the tip end of the arm  24  is longer than the radius of the center portion  12  of the diaphragm  10  but is shorter than the distance from the center of the center portion  12  of the diaphragm  10  to the tip end of the arm  14 . 
     Next, as shown in  FIG. 14 , a third insulating film  56  of 0.3 μm thickness composed of a silicon oxide film is formed on the back plate  20  and its extension wire  28  as well as the second insulating film  52   a  by way of plasma CVD. Next, as shown in  FIG. 15 , a photoresist is applied to the entire surface of the third insulating film  56 ; then, a photoresist pattern P 3  is formed by way of the photolithography technique. The photoresist pattern P 3  has openings above the extension wire  18  connected to the diaphragm  10  and the extension wire  28  connected to the back plate  20 . 
     Next, as shown in  FIG. 16 , one or both of wet etching and dry etching is performed by use of the photoresist pattern P 3  serving as a mask so as to selectively remove the third insulating film  56  and the second insulating film  52   a , thus forming electrode exposing holes  58   a  and  58   b  for exposing the extension wires  18  and  28 . Next, as shown in  FIG. 17 , incineration and dissolution using a mixed solution composed of sulfuric acid and hydrogen peroxide are performed so as to remove the photoresist pattern P 3 . 
     Next, as shown in  FIG. 18 , a metal layer  60  composed of Al—Si is deposited on the entire surface of the third insulating film  56  including the extension wires  18  and  28  exposed in the electrode exposing holes  58   a  and  58   b . Next, as shown in  FIG. 19 , a photoresist film is applied to the entire surface of the metal layer  60 ; then, a photoresist pattern P 4  covering the electrode exposing holes  58   a  and  58   b  is formed by way of the photolithography technique. Next, as shown in  FIG. 20 , wet etching using a mixed acid is performed by use of the photoresist pattern P 4  serving as a mask so as to selectively remove the metal layer  60  and to process it into a prescribed shape, thus forming a first electrode  60   a  and a second electrode  60   b , which are connected to the extension wires  18  and  28  via the electrode exposing holes  58   a  and  58   b  respectively. 
     Next, as shown in  FIG. 21 , incineration using O 2  plasma and dissolution for soaking into an organic peeling solution are performed so as to remove the photoresist pattern P 4 . Thus, the first electrode  60   a  is connected to the diaphragm  10  via the extension wire  18 , and the second electrode  60   b  is connected to the back plate  20  via the extension wire  28 . 
     Next, as shown in  FIG. 22 , the second conductive layer  20   a  and the first conductive layer  10   a  positioned at the backside of the substrate  30  are polished and removed by use of a grinder; furthermore, the backside of the substrate  30  is polished so as to adjust the thickness of the substrate  30  within the range of 500 μm to 600 μm. Next, as shown in  FIG. 23 , a photoresist pattern P 5  is formed on the backside of the substrate  30  by way of the photolithography technique. The photoresist pattern P 5  has an opening in conformity with the center portion  12  of the diaphragm  10 . 
     Next, as shown in  FIG. 24 , anisotropic etching such as Deep RIE is performed by use of the photoresist pattern P 5  serving as a mask so as to selectively remove the substrate  30 , thus forming an opening  32   a  reaching the first insulating film  50   a . The opening  32   a  is positioned along the inside of the center portion  12  of the diaphragm  10 . Next, as shown in  FIG. 25 , incineration and dissolution using an organic peeling solution are performed so as to remove the photoresist pattern P 5 . 
     Next, as shown in  FIG. 26 , a photoresist film is applied to the first electrode  60   a  and the second electrode  60   b  as well as the entire surface of the third insulating film  56 ; then, a photoresist pattern P 6  is formed by way of the photolithography technique. The photoresist pattern P 6  covers the first electrode  60   a  and the second electrode  60   b  as well as the third insulating film  56  above the extension wires  18  and  28 . 
     Next, as shown in  FIG. 27 , wet etching using buffered hydrofluoric acid (Buffered HF) is performed by use of the photoresist pattern P 6  serving as a mask so as to selectively remove the third insulating film  56 , the second insulating film  52   a , and the first insulating film  50   a . At this time, a plurality of holes  26  formed in the arms  24  and the center portion  22  of the back plate  20  serve as guide holes for introducing an etching solution when the second insulating film  52   a  intervened between the back plate  20  and the diaphragm  10  is removed. In addition, the buffered hydrofluoric acid is introduced into the opening  32   a  of the substrate  30  so as to selectively remove the first insulating film  50   a  by way of etching. 
     As described above, the gap  40  is formed by removing the second insulating film  52   a  intervened between the back plate  20  and the diaphragm  10 . In addition, by removing the first insulating film  50   a , the opening  32   a  of the substrate  30  is expanded to reach the diaphragm  10  so as to form the cavity  32 , and the passage  34  having a desired acoustic resistance is formed between the substrate  30  surrounding the cavity  32  and the diaphragm  10 . 
     At the same time, the first insulating film  50   a  is intentionally left between the tip ends of the six arms  14  of the diaphragm  10  and the substrate  30 , thus forming the first support  50 . In addition, the laminated insulating film  54   a  is intentionally left between the tip ends of the six arms  24  of the back plate  20  and the substrate  30 , thus forming the second support  54 . 
     Next, as shown in  FIG. 28 , incineration and dissolution using an organic peeling solution are performed so as to remove the photoresist pattern P 6 . Thus, it is possible to produce the condenser microphone of the first embodiment having the structure shown in  FIGS. 1A ,  1 B, and  1 C. 
     In the manufacturing method of the condenser microphone of the first embodiment, resist masks having different patterns are used to perform the photolithography multiple times; hence, it is possible to directly adopt the conventionally-known semiconductor manufacturing process. In addition, it does not need the complex process, which is taught in the prior-art technology and in which the rear electrode is arranged on the prescribed portion of the surface of the plate composed of an insulating material positioned opposite to the diaphragm so as to reduce the manufacturing yield; hence, it is possible not to increase the manufacturing cost. 
     The first embodiment of the present invention is not necessarily limited to the condenser microphone having the structure as shown in  FIGS. 1A ,  1 B, and  1 C; hence, it is possible to realize a variety of modifications. Hereinafter, variations will be explained. 
     (First Variation) 
     The condenser microphone of the first embodiment is modified such that the back plate  20  is entirely shaped in a disk-like shape, in which the radius thereof is longer than the radius of the center portion  12  of the diaphragm  10  but is shorter than the distance from the center of the center portion  12  of the diaphragm  10  to the tip end of the arm  14 . 
     In the first variation, the diaphragm  10  has a gear-like shape including the center portion  12  and the six arms  14 ; hence, the back plate  20  does not exist at the positions corresponding to the cutouts formed between the arms  14 , so that no parasitic capacitance occurs therebetween. In addition, the arms  14  of the diaphragm  10  are positioned externally of the external periphery of the back plate  20 ; hence, no parasitic capacitance occur therebetween. Therefore, in the condenser microphone of the first variation compared with the condenser microphone having the conventionally-known structure shown in  FIGS. 2A and 2B , it is possible to remarkably reduce the parasitic capacitance. 
     However, since the inner portions of the arms  14  of the diaphragm  10  are positioned to match the external circumference of the back plate  20  having a disk-like shape, some parasitic capacitance may occur therebetween. That is, the first variation is simple in structure in comparison with the first embodiment, whereas the parasitic capacitance may slightly increase. 
     (Second Variation) 
     The condenser microphone of the first embodiment is modified such that the diaphragm  10  is entirely shaped in a disk-like shape. In this case, since the back plate  20  has a gear-like shape including the center portion  22  and the six arms  24 , the diaphragm  10  does not exist at the positions corresponding to the cutouts formed between the arms  24 ; hence, no parasitic capacitance occurs therebetween. Therefore, in the condenser microphone of the second variation compared with the condenser microphone having the conventionally-known structure as shown in  FIGS. 2A and 2B , it is possible to reduce the parasitic capacitance. However, since the inner portions of the arms  24  of the back plate  20  are positioned to match the external circumference of the diaphragm  10  having a disk-like shape, some parasitic capacitance may occur therebetween. That is, in the second variation compared with the first embodiment, the parasitic capacitance may slightly increase. 
     (Third Variation) 
     The condenser microphone of the first embodiment is modified such that the holes  16  are not formed in the arms  14  of the diaphragm  10 , and the cavity  32  is formed along the exterior periphery of the diaphragm  10  having a gear-like shape constituted of the center portion  12  and the arms  14 . In this case, the opening of the cavity  32  is formed entirely in conformity with the diaphragm having a gear-like shape except for the tip ends of the arms  14 ; hence, the volume of the cavity  32  according to the third variation becomes larger than the volume of the cavity  32  according to the first embodiment. Thus, it is possible to further improve the vibration characteristics of the diaphragm  10 . 
     (Fourth Variation) 
     A condenser microphone according to a fourth variation of the first embodiment will be described with reference to  FIGS. 35A to 35C .  FIG. 35A  is a plan view showing the constitution of the condenser microphone of the fourth variation;  FIG. 35B  is a cross-sectional view taken along line A-A in  FIG. 35A ; and  FIG. 35C  is a fragmentary enlarged view of  FIG. 35B . As shown in  FIGS. 35A and 35B , first projections  60  and second projections  70  are formed in the diaphragm  10  in the condenser microphone of the fourth variation. The first projections form step-difference shapes with respect to the arms  14 , wherein they are directed toward the substrate  30  so as to further reduce the space corresponding to the passage  34  formed between the diaphragm  10  and the substrate  30  surrounding the cavity  32 . The second projections  70  form step-difference shapes at the positions opposite to the arms  24  of the back plate  20 , i.e., at the cutouts of the diaphragm  10 . The second projections  70  are directed toward the substrate  30  so as to further reduce the space corresponding to the passage  34  formed between the cutout of the diaphragm  10  and the substrate  30  surrounding the cavity  32 . By means of the first projections  60  and the second projections  70 , it is possible to further reduce the space of the passage  34 , wherein since the space forms an acoustic resistance, it is possible to prevent sound waves transmitted to the diaphragm  10  from propagating between the arms  14  and from leaking therefrom. Due to the formation of the first projections  60  and the second projections  70  in the diaphragm  10 , it is possible to reduce the rigidity of the diaphragm  10 , which makes it possible for the diaphragm  10  to be easily deformed due to sound pressure. Thus, it is possible to further improve the vibration characteristics of the diaphragm  10 . Incidentally, the first projections  60  and the second projections  70  form step-difference shapes in the fourth variation; but this is not a restriction; hence, it is possible to form dimples or corrugations projecting toward the substrate  30 . Furthermore, the second projections  70  are formed at the positions opposite to the arms  24  of the back plate  20 ; but this is not a restriction; hence, it is possible to continuously form the second projections  70 , i.e., it is possible to form a second projection  70  having a ring shape. In addition, the portions of the first projections  60  and the second projections  70  positioned opposite to the substrate  30  can be formed using insulating materials. 
     Second Embodiment 
     Next, a condenser microphone according to a second embodiment of the present invention will be described with reference to  FIGS. 30A ,  30 B, and  30 C.  FIG. 30A  is a plan view showing the constitution of the condenser microphone of the second embodiment;  FIG. 30B  is a cross-sectional view taken along line A-A in  FIG. 30A ; and  FIG. 30C  is a cross-sectional view taken along line B-B in  FIG. 30A . 
     The condenser microphone of the second embodiment is constituted of a diaphragm  1010 , a back plate  1020 , and a substrate  1030  having a support member for supporting the diaphragm  1010  and the back plate  1020 . 
     The diaphragm  1010  is a thin film having a conductivity composed of polysilicon, which is added with phosphorus as impurities, wherein it has a gear-like shape including a center portion  1012  having a disk-like shape and six arms  1014  extended externally in a radial manner. The thickness of the diaphragm  1010  is approximately 0.5 μm; the radius of the center portion  1012  is approximately 0.35 mm; and the length of the arm  1014  is approximately 0.15 mm. 
     The back plate  1020  is arranged in parallel with the diaphragm  1010  with a prescribed distance therebetween, e.g., via a gap  1040  of 0.4 μm therebetween. Similar to the diaphragm  1010 , the back plate  1020  is a thin film having a conductivity composed of phosphorus-doped polysilicon, wherein it has a gear-like shape including a center portion  1022  having a disk-like shape and six arms  1024  extended externally in a radial manner. A plurality of holes  1026  are formed in the center portion  1022  and the arms  1024  of the back plate  1020 . The holes  1024  of the back plate  1020  function as sound holes by which sound waves from the exterior pass through and are then transmitted to the diaphragm  1010 . The thickness of the back plate  1020  is approximately 1.5 μm; the radius of the center portion  1022  is approximately 0.3 mm; and the length of the arm  1024  is approximately 0.1 mm. 
     The center portion  1022  of the back plate  1020  is arranged concentrically with the diaphragm  1010 , wherein the radius of the center portion  1022  of the back plate  1020  is smaller than the radius of the center portion  1012  of the diaphragm  1010 . In addition, the six arms  1024  of the back plate  1020  are positioned at the six cutouts formed between the six arms  1014  of the diaphragm  1010 . In other words, the six arms  1014  of the diaphragm  1010  are positioned at the six cutouts formed between the six arms  1024  of the back plate  1020 . The distance from the center of the center portion  1022  of the back plate  1020  to the tip end of the arm  1024  is longer than the radius of the center portion  1012  of the diaphragm  1010  but is shorter than the distance from the center of the center portion  1012  of the diaphragm  1010  to the tip end of the arm  1014 . 
     The tip ends of the arms  1014  of the diaphragm  1010  join lower surfaces of spacers  1052  having an insulating property. Upper surfaces of the spacers  1052  join the inner end of a bridge  1020   b . The bridge  1020   b  is a thin film composed of the same material of the back plate  1020 , i.e., polysilicon having a conductivity, and is formed simultaneously with the back plate  1020 . The outer end of the bridge  1020   b  has a circumferential shape surrounding the external periphery of the diaphragm  1010  having a gear-like shape, wherein it is supported above the substrate  1030  by means of a first support  1054   b  having an insulating property. In the bridge  1020   b , a plurality of holes  1026   a  are formed in regions defined between the spacers  1052  and the first support  1054   b . The tip ends of the arms  1024  of the back plate  1020  are supported above the substrate  1030  by means of second supports  1054 , each having an insulating property, which are positioned at the cutouts formed between the arms  1014  of the diaphragm  1010 . The spacers  1052 , the first support  1054   b , and the second supports  1054  are composed of silicon oxide films, for example. 
     The second support  1054  for supporting the back plate  1020  is formed using insulating films  1541  and  1543  and a conductive film  1542 . The insulating films  1541  and  1543  are composed of silicon oxide films, for example. It is preferable that the conductive film  1542  be formed simultaneously with the diaphragm  1010  and is composed of polysilicon, which is added with phosphorus as impurities. The conductive film  1542  is placed at the same potential as the back plate  1020  or the substrate  1030 , wherein it functions as a guard electrode for reducing the parasitic capacitance of the condenser microphone. Incidentally, it is possible to omit the conductive film  1542 . 
     The substrate  1030  is constituted of a silicon substrate whose thickness ranges from 500 μm to 600 μm, wherein a cavity  1032  having an opening reaching the diaphragm  1010  runs through the substrate  1030  in conformity with the diaphragm  1010  having a gear-like shape. The cavity  1032  is formed along the inside of the external periphery of the diaphragm  1010 , wherein it functions as pressure relaxation space for relaxing pressure applied to the diaphragm  1010  opposite to the back plate  1020 . In addition, a passage  1034  having an acoustic resistance that is higher than the acoustic resistance between the arms  1014  of the diaphragm  1010  is formed between the substrate  1030  surrounding the cavity  1032  and the diaphragm  1010 . The acoustic resistance is controlled in response to a height H (i.e., the distance between the diaphragm  1010  and the substrate  1030 ) and a length L (i.e., the distance from the external periphery of the diaphragm  1010  having a gear-like shape to the end portion of the cavity  1032 ) of the passage  1034 , thus realizing an acoustic resistance that is higher than the acoustic resistance between the arms  1014  of the diaphragm  1010 . The passage  1034  having a high acoustic resistance prevents sound waves reaching the diaphragm  1010  from passing between the arms  1014  and from leaking therefrom. Incidentally, the height H of the passage  1034  is approximately 2 and the length L is approximately 15 mm. 
       FIG. 31A  is a circuit diagram showing the constitution of a detection circuit for converting variations of electrostatic capacitance between the diaphragm  1010  and the back plate  1020  into electric signals. A stable bias voltage is applied to the diaphragm  1010  by means of a charge pump CP. Variations of electrostatic capacitance between the back plate  1020  and the diaphragm  1010  are input into a pre-amplifier A in the form of voltage variations. Since the substrate  1030  and the diaphragm  1010  are short-circuited, no parasitic capacitance may occur between the back plate  1020  and the substrate  1030  with the intervention of the conductive film  1542  shown in  FIG. 30C . 
       FIG. 31B  shows the constitution of a detection circuit arranging the conductive film  1542 . Herein, an output terminal of the pre-amplifier A is connected to the conductive film  1542  so that a voltage-follower circuit is formed using the pre-amplifier A; this makes it possible for the conductive film  1542  to function as a guard electrode. The back plate  1020  and the conductive film  1542  are controlled at the same potential by means of the voltage-follower circuit, whereby it is possible to eliminate the parasitic capacitance occurring between the back plate  1020  and the conductive film  1542 . In addition, the diaphragm  1010  and the substrate  1030  are short-circuited, so that the capacitance between the conductive film  1542  and the substrate  1030  becomes irrelevant to the output of the pre-amplifier A. As described above, the guard electrode is formed using the conductive film  1542  so as to further reduce the parasitic capacitance of the condenser microphone. 
     In the condenser microphone of the second embodiment, the diaphragm  1010  and the back plate both have the gear-like shapes, wherein the center portion  1012  of the diaphragm  1010  is arranged opposite to the center portion  1022  of the back plate  1020 . The six arms  1024  of the back plate  1020  are positioned at the six cutouts formed between the six arms  1014  of the diaphragm  1010 ; in other words, the six arms  1014  of the diaphragm  1010  are positioned at the cutouts formed between the six arms  1024  of the back plate  1020 . For this reason, the arms  1014  of the diaphragm  1010  and the arms  1024  of the back plate  1020  are positionally shifted from each other and are not arranged opposite to each other; therefore, no parasitic capacitance may occur between them. That is, electrostatic capacitance is formed between the center portion  1012  of the diaphragm  1010  and the center portion  1022  of the back plate  1020 , whereby electric signals are generated in response to variations of electrostatic capacitance. Since the parasitic capacitance between the diaphragm  1010  and the back plate  1020  is remarkably reduced, it is possible to remarkably increase the sensitivity of the condense microphone. 
     The tip ends of the arms  1014  of the diaphragm  1010  are supported by means of the spacers  1052 , the bridge  1020   b , and the first support  1054   b , wherein the distance from the center of the center portion  1012  of the diaphragm  1010  to the spacers  1052  is longer than the distance from the center of the center portion  1022  of the back plate  1020  to the second supports  1054  for supporting the tip ends of the arms  1024 . For this reason, in comparison with the structure, in which the external periphery of the diaphragm  1010  is directly supported above the substrate  1030 , and the structure, in which the diaphragm  1010  and the back plate  1020  both have the same shape in plan view, the structure of the second embodiment can further improve the vibration characteristics of the diaphragm  1010 . 
     In addition, the radius of the center portion  1022  of the back plate  1020  is smaller than the radius of the center portion  1012  of the diaphragm  1010 , and the distance from the center of the center portion  1022  to the second support  1054  is shorter than the distance from the center of the center portion  1012  to the spacer  1054 . For this reason, in comparison with the structure, in which both of the diaphragm  1010  and the back plate  1020  have the same shape in plan view, it is possible to increase the rigidity of the back plate  1020 ; therefore, it is possible to increase the size of the diaphragm  1010  without damaging the operation stability of the condenser microphone, and it is possible to improve the vibration characteristics of the diaphragm  1010 . 
     Since a plurality of holes  1026   a  are formed in the bridge  1020   b , the rigidity of the bridge  1020   b  joining the arms  1014  of the diaphragm  1010  decreases; this makes it easy for the bridge  1020   b  to be deformed at a vibration mode of the diaphragm  1010 ; hence, it is possible to further improve the vibration characteristics of the diaphragm  1010 . 
     In order to confirm the effect of the condenser microphone of the second embodiment, the inventor of the present application produced a condenser microphone having the conventionally-known structure shown in  FIGS. 2A and 2B  and a condenser microphone as shown in  FIGS. 3A and 3B  for use in experiments, and thus performed experiments. Results of experiments are shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Electrode 
                 Vibration 
                   
               
               
                   
                 pressure 
                 displacement 
               
               
                   
                 resistance 
                 value 
                 Sensitivity 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Conventionally-known 
                 1.0 
                 1.0 
                 1.0 
               
               
                 structure 
               
               
                 Experimental structure 
                 0.8 
                 — 
                 — 
               
               
                 Second embodiment 
                 1.2 
                 8.0 
                 12.0 
               
               
                   
               
            
           
         
       
     
     In the results of experiments regarding the second embodiment shown in Table 2 compared with the first embodiment shown in Table 1, the electrode pressure resistance is increased 1.2 times higher than that of the conventionally-known structure. This is because the second supports  1054  for supporting the tip ends of the arms  1022  of the back plate  1020  are positioned at the cutouts formed between the arms  1014  of the diaphragm  1010 ; the distance from the center of the center portion  1022  of the back plate  1020  to the second support  1054  is shorter than the distance from the center of the diaphragm  100  to the first support  500  in the conventionally-known structure; and the rigidity of the back plate  1020  is relatively high. Due to the increase of the electrode pressure resistance, it is possible to improve the operation stability of the condenser microphone of the second embodiment. 
     In the second embodiment, the vibration displacement value of the diaphragm  1010  is increased 8.0 times higher than that of the conventionally-known structure. This is because the tip ends of the arms  1014  of the diaphragm  1010  having a gear-like shape are supported by the spacers  1052  and the bridge  1020   b . That is, in comparison with the conventionally-known structure in which the overall periphery of the diaphragm  100  is fixed, it is possible to remarkably improve the vibration characteristics of the diaphragm  1010 . 
     In the second embodiment, the sensitivity of the condenser microphone is increased 12.0 times higher than that of the conventionally-known structure. This is because the vibration displacement value of the diaphragm  1010  is remarkably higher than that of the diaphragm  100  of the conventionally-known structure, wherein electrostatic capacitance is formed between the center portion  1012  of the diaphragm  1010  and the center portion  1022  of the back plate  1020 , and wherein the arms  1014  and the arms  1024  are not positioned opposite to each other so that no parasitic capacitance occurs therebetween. That is, in the condenser microphone of the second embodiment, it is possible to remarkably reduce the parasitic capacitance. 
     Next, a manufacturing method of the condenser microphone of the second embodiment will be described. This condenser microphone is a silicon microphone (or a silicon capacitor microphone), which can be manufactured using the semiconductor manufacturing process. 
     First, a first conductive layer composed of phosphorus-doped polysilicon is formed on the substrate  1030 , which is a semiconductor substrate such as a monocrystal silicon substrate, via a first insulating film (or a first sacrifice film) composed of a silicon oxide film. The first conductive layer is subjected to etching and is thus processed into a prescribed shape, thus forming the diaphragm  1010 . As shown in  FIG. 30A , the diaphragm  1010  has a gear-like shape including the center portion  1012  having a disk-like shape and the six arms  1014  extended externally in a radial manner. 
     Next, a second conductive layer composed of phosphorus-doped polysilicon is formed on the diaphragm  1010  and the first insulating film via a second insulating film (or a second sacrifice film). The second conductive layer is subjected to etching and is thus processed into a prescribed shape, thus forming the back plate  1020  and the bridge  1020   b . As shown in  FIG. 30A , the back plate  1020  has a gear-like shape including the center portion  1022  having a disk-like shape and the six arms  1024  extended externally in a radial manner, wherein a plurality of holes  1026   a  are formed in the bridge  1020   b.    
     As shown in  FIG. 30A , the center portion  1022  of the back plate  1020  is arranged concentrically with the center portion  1012  of the diaphragm  1010 , wherein the radius of the center portion  1022  of the back plate  1020  is shorter than the radius of the center portion  1012  of the diaphragm  1010 . In addition, the six arms  1025  of the back plate  1020  are positioned at the six cutouts formed between the six arms  1014  of the diaphragm  1010 . In other words, the six arms  1014  of the diaphragm  1010  are positioned at the six cutouts formed between the six arms  1024  of the back plate  1020 . Furthermore, the distance from the center of the center portion  1022  of the back plate  1020  to the tip end of the arm  1024  is shorter than the distance from the center of the center portion  1012  of the diaphragm  1010  to the tip end of the arm  1014 . 
     As shown in  FIG. 30A , the inner end of the bridge  1020   b  is positioned to overlap with the tip ends of the arms  1014  of the diaphragm  1010  in plan view, wherein the external end of the bridge  1020   b  has a circumferential shape surrounding the external periphery of the diaphragm  1010  having a gear-like shape. 
     Next, a third insulating film composed of a silicon oxide film is formed on the back plate  1020 , the bridge  1020   b , and the second insulating film  1052   a ; then, the backside of the substrate  1030  is polished so as to adjust the thickness thereof. Next, anisotropic etching such as Deep RIE is performed so as to selectively remove the substrate  1030 , thus forming an opening reaching the first insulating film. This opening is positioned along the inside of the external periphery of the diaphragm  1010  having a gear-like shape. 
     Next, wet etching using buffered hydrofluoric acid (Buffered HF) is performed by use of a prescribed photoresist pattern serving as a mask, thus selectively remove the third insulating film, the second insulating film, and the first insulting film. At this time, an etching solution is introduced via the holes  1026  formed in the center portion  1022  and the arms  1024  of the back plate  1020  as well as the holes  1026   a  formed in the bridge  1020   b , thus removing the second insulating film intervened between the back plate  1020  and the diaphragm  1010 . In addition, buffered hydrofluoric acid is introduced into the opening of the substrate  1030  so as to selectively remove the first insulating film by way of etching. 
     As described above, the second insulating film between the back plate  1020  and the diaphragm  1010  is removed so as to form the gap  1040 . In addition, the opening of the substrate  1030  is enlarged to reach the diaphragm  1010  by removing the first insulating film, thus forming the cavity  1032 . Furthermore, the passage  1034  realizing a desired acoustic resistance is formed between the substrate  1030  surrounding the cavity  1032  and the diaphragm  1010 . 
     At the same time, the second insulating film is intentionally left between the tip ends of the arms  1014  of the diaphragm  1010  and the bridge  1020   b , thus forming the spacers  1052 . In addition, a laminated insulating film composed of the first insulating film and the second insulating film is intentionally left between the bridge  1020   b  and the substrate  1030 , thus forming the first support  1054   b . Furthermore, the laminated insulating film is intentionally left between the tip ends of the arms  1024  of the back plate  1020  and the substrate  1030 , thus forming the second supports  1054 . 
     According to the aforementioned manufacturing method, it is possible to produce the condenser microphone of the second embodiment shown in  FIGS. 30A ,  30 B, and  30 C. In this manufacturing method, resist masks having different patterns are used in the photolithography, whereas it is possible to directly use the conventionally-known semiconductor manufacturing process. 
     Incidentally, the structure of the condenser microphone of the second embodiment is not necessarily limited to the structure shown in  FIGS. 30A ,  30 B, and  30 C; hence, it is possible to realize a variety of modifications. For example, the back plate  1020  is entirely formed in a disk-like shape, in which the radius thereof is longer than the radius of the center portion  1012  of the diaphragm  1010  but is shorter than the distance from the center of the center portion  1012  of the diaphragm  1010  to the inner end of the bridge  1020   b.    
     In the aforementioned variation in which the diaphragm  1010  has a gear-like shape including the center portion  1012  and the six arms  1014 , the diaphragm  1010  is not positioned opposite to the external periphery of the back plate  1020  at the cutouts formed between the arms  1014 ; hence, no parasitic capacitance occurs therebetween. No parasitic capacitance occurs with respect to the outer portions of the arms  1014  of the diaphragm  1010 , which are positioned externally of the external periphery of the back plate  1020 , as well. That is, it is possible to reduce the parasitic capacitance in the variation compared with the conventionally-known structure shown in  FIGS. 2A  and  2 B. 
     However, since the inner portions of the arms  1014  of the diaphragm  1010  are positioned opposite to the external periphery of the back plate  1020  having a disk-like shape, some parasitic capacitance may occur therebetween. For this reason, the parasitic capacitance may be slightly increased in the variation compared with the second embodiment. 
     Third Embodiment 
     Next, the constitution of a condenser microphone according to a third embodiment of the present invention will be described with reference to  FIGS. 32A ,  32 B, and  32 C.  FIG. 32A  is a cross-sectional view showing the constitution of the condenser microphone of the third embodiment;  FIG. 32B  is a plan view showing the constitution excluding the back plate from the constitution shown in  FIG. 32A ;  FIG. 32C  is a cross-sectional view taken along line A-A in  FIG. 32A ; and  FIG. 32D  is a cross-sectional view taken along line B-B in  FIG. 32A . 
     As shown in  FIGS. 32A to 32D , the condenser microphone of the third embodiment is constituted of a diaphragm  2010  and a back plate  2020 , which are positioned opposite to each other, as well as a substrate  2030  having a support member for supporting the diaphragm  2010  and the back plate  2020  to be insulated from each other. 
     The diaphragm  2010  is a conductive thin film composed of polysilicon that is added with phosphorus as impurities, wherein it is constituted of a center portion  2012  having a disk-like shape and a peripheral portion  2014  surrounding it. In the center portion  2012  of the diaphragm  2010 , four circular holes  2016  are formed in a circumferential direction with equal spacing therebetween in a region adjoining the peripheral portion  2014  (hereinafter, referred to as “an intermediate region”), and a plurality of small holes  2018  are formed therein. In addition, a plurality of small holes  2018  are formed in four regions, which are formed in a circumferential direction with equal spacing therebetween in conformity with the four holes  2016  in the peripheral portion  2014  of the diaphragm  2010 . The regions in which the four holes  2016  and the plurality of small holes  2018  are formed in the diaphragm  2010  are arranged in correspondence with the substrate  2030 . The thickness of the diaphragm  2010  is approximately 0.5 μm; the radius of the center portion  2012  is approximately 0.35 mm; the overall radius of the diaphragm  2010  including the peripheral portion  2014  is approximately 0.5 mm; and the radius of each hole  2016  is approximately 25 μm. 
     The back plate  2020  is arranged in parallel with the diaphragm  2010  with a prescribed distance, e.g., a gap  2040  of 4 μm, therebetween. The back plate  2020  is a conductive thin film composed of phosphorus-doped polysilicon, wherein it has a disk-like shape of approximately 2 μm thickness. The back plate  2020  is arranged concentrically with the diaphragm  2010 , wherein the radius of the back plate  2020  is substantially identical to the radius of the diaphragm  2010 . For this reason, the back plate  2020  is arranged opposite to the diaphragm  2010 , while the peripheral portion  2014  is extended outside of the back plate  2020  in plan view. A plurality of small holes  2022  serving as sound holes for transmitting sound waves from the exterior therethrough and for making them reach the diaphragm  2010  are formed in the back plate  2020 . Herein, the plurality of small holes  2022  of the back plate  2020  are aligned not to overlap with the plurality of small holes  2018  of the diaphragm  2010  in plan view. In addition, an extension wire  2024  connected to an electrode (not shown) is extended from the external periphery of the back plate  2020 . 
     The external periphery of the peripheral portion  2014  of the diaphragm  2010  is supported in a circumferential manner above the substrate  2030  by means of a first support  2050  having an insulating property. The back plate  2020  is supported above the substrate  2030  by means of four cylindrical second supports  2052  having insulating properties, which are inserted into the four holes  2016  of the diaphragm  2010 . The first support and the second supports are composed of silicon oxide films, for example. 
     The substrate  2030  is a silicon substrate whose thickness ranges from 500 μm to 600 μm, wherein it has an opening running through the substrate  2030  to reach the diaphragm  2010  at a position corresponding to a region (hereinafter, referred to as “a central region”) surrounding by the intermediate region in the center portion  2012  of the diaphragm  2010 . It also has an opening running through the substrate  2030  to reach the diaphragm  2010  at a position at which none of the small holes  2018  is formed in the peripheral portion  2014  of the diaphragm  2010 . A cavity  2032  is formed by means of the aforementioned openings. The cavity  2032  functions as a pressure relaxation room for relaxing pressure applied to the diaphragm  2010  oppositely to the back plate  2020 . 
     A passage  2034  realizing a prescribed acoustic resistance is formed between the substrate  2030  surrounding the cavity  2032  and the diaphragm  2010 . The acoustic resistance is controlled by way of a height H (i.e., the distance between the diaphragm  2010  and the substrate  2030 ) and a length L (i.e., the shortest distance among distances from the four holes  2016  and the plurality of small holes  2018  of the diaphragm  2010  to the end portion of the cavity  2032 ) of the passage  2034 , thus making the center portion  2012  efficiently vibrate due to sound waves reaching the diaphragm  2010 . Incidentally, the height of the passage  2034  is 2 μm, and the length is 15 μm. 
     Other than the aforementioned constituent members, the condenser microphone of the third embodiment includes an extension wire extended from the external periphery of the diaphragm  2010 , an electrode connected to the extension wire, an electrode connected to the extension wire  2024  of the back plate  2020 , a bias voltage circuit for applying a prescribed voltage between the diaphragm  2020  and the back plate  2020  via these electrodes, and a detection circuit for converting variations of electrostatic capacitance formed between the diaphragm  2010  and the back plate  2020 , which are applied with the prescribed voltage, into electric signals. For the sake of convenience, their illustrations and explanations are omitted. 
     In the condenser microphone of the third embodiment, the back plate  2020  is downsized to match the size of the center portion  2012  of the diaphragm  2010 ; hence, it is possible to increase the mechanical strength of the back plate  2020  in comparison with the conventionally-known structure in which both of the back plate and diaphragm have substantially the same size. Therefore, even when a voltage applied between the diaphragm  2010  and the back plate  2020  is increased for the purpose of the improvement of the sensitivity of the condenser microphone, it is possible to suppress the deformation of the back plate  2020  due to the electrostatic attraction between the opposite electrodes, and it is possible to prevent the back plate  2020  from being deformed due to an impact from the exterior. That is, it is possible to improve the vibration characteristics of the diaphragm  2010 , and it is possible to secure the operation stability of the condenser microphone. 
     Since the back plate  2020  is directly supported above the substrate  2030  by means of the four second supports  2052 , it is possible to maintain the stability of the back plate  2020 . That is, it is possible to suppress the deformation of the back plate  2020 ; it is possible to improve the vibration characteristics; thus, it is possible to secure the operation stability of the condenser microphone. 
     Although the back plate  2020  is positioned opposite to the center portion  2012  of the diaphragm  2010 , it is not positioned opposite to the peripheral portion  2014  of the diaphragm  2010  existing externally of the back plate  2020  in plan view. For this reason, no parasitic capacitance occurs between the peripheral portion  2014  of the diaphragm  2010  and the back plate  2020 . That is, compared with the conventionally-known structure in which the back plate and the diaphragm are entirely positioned opposite to each other, the condenser microphone of the third embodiment can remarkably reduce the parasitic capacitance, thus improving the sensitivity. 
     The four holes  2016  are formed in the intermediate region of the center portion  2012  of the diaphragm  2010 , and a plurality of holes  2018  are formed in the periphery. This reduces the rigidity of the diaphragm  2010  so as to realize deformation in a vibration mode with ease, whereby it is possible to increase the displacement of the diaphragm  2010 . Thus, it is possible to improve the vibration characteristics of the diaphragm  2010 , thus improving the sensitivity of the condenser microphone. 
     The passage  2034  is formed between the substrate  2030  surrounding the cavity  2032  and the diaphragm  2010 , whereby the acoustic resistance is controlled by appropriately setting the height H and the length L of the passage  2034 . This makes it possible for the center portion  2012  to efficiently vibrate due to sound waves transmitted to the diaphragm  2010  via a desired acoustic resistance; hence, it is possible to remarkably improve the vibration characteristics of the diaphragm  2010 , thus improving the sensitivity of the condenser microphone. Incidentally, the four holes  2016  and the plurality of small holes  2018  are limitedly formed in the regions of the diaphragm  2010  directly facing the substrate  2030 , wherein they are not formed in the region directly facing the cavity  2032 . For this reason, sound waves reaching the diaphragm  2010  do not cause vibration energy; hence, it is possible to prevent sound waves from passing through the holes  2016  or the small holes  2018 . 
     Since both of the diaphragm  2010  and the back plate  2020  are formed using conductive materials, it is not necessary to perform a complex manufacturing process, in which, as similar to the prior-art technology, a rear electrode facing the diaphragm is formed in the prescribed portion of the back plate composed of an insulating material; this makes it possible to simplify the manufacturing process of the condenser microphone. 
     In addition, an etching solution is transmitted through the plurality of small holes  2018  formed in the diaphragm  2010  so as to remove the sacrifice layer intervened between the diaphragm  2010  and the substrate  2030  by way of etching, thus forming a gap therebetween. Furthermore, the etching solution is transmitted through the plurality of small holes  2022  formed in the back plate  2020  so as to remove the sacrifice layer intervened between the back plate  2020  and the diaphragm  2010  by way of etching, thus forming an air gap therebetween. Thus, it is possible to simplify the manufacturing process. 
     In the condenser microphone of the third embodiment, the back plate  2020  is supported above the substrate  2030  by means of the four second supports  2052 , whereas the number of the second supports  2052  is not necessarily limited to four. For example, it is possible to support the back plate  2020  in a stable manner by means of three supports  2052 . In this case, it is necessary to form three circular holes  2016  in the diaphragm  2010 . 
     The condenser microphone of the third embodiment employs the structure in which the external periphery of the peripheral portion  2014  of the diaphragm  2010  is supported in a circumferential manner above the substrate  2030  by means of the first support  2050 ; however, the support structure of the diaphragm  2010  is not necessarily limited to this structure; hence, it is possible to employ a variety of support structures. For example, the external periphery of the peripheral portion  2014  of the diaphragm  2010  is not supported continuously in a circumferential manner, but it is supported locally at a plurality of positions above the substrate  2030 . Alternatively, the diaphragm  2010  can be supported by means of a bridge supported by the substrate  2030  via a spacer; furthermore, the diaphragm  2010  can be supported by means of arms extended externally from the external periphery of the back plate  2020  via a spacer. That is, within the range not disturbing the structure in which the back plate  2020  is supported above the substrate  2030  by means of the second supports  2052  inserted into a plurality of holes  2016  formed in the diaphragm  2010 , it is possible to realize a variety of modifications for the purpose of stress relaxation and for the purpose of the improvement of vibration characteristics with respect to the support structure of the diaphragm  2010 . 
     Next, a manufacturing method of the condenser microphone of the third embodiment will be described. Incidentally, the condenser microphone of the third embodiment is a silicon microphone that is manufactured by way of the semiconductor manufacturing process. 
     First, a first conductive layer composed of phosphorus-doped polysilicon is formed on the substrate  2030 , which is constituted of a monocrystal silicon substrate, via a first insulating film (or a first sacrifice film) composed of a silicon oxide film. The first conductive layer is processed into a prescribed shape by way of etching, thus forming the diaphragm  2010  and its extension wire. As shown in  FIG. 30(B) , the diaphragm  2010  has the center portion  2012  having a disk-like shape and the peripheral portion  2014  formed in its surrounding. The four circular holes  2016  are formed in a circumferential manner with equal spacing therebetween in the intermediate region of the center portion  2012  of the diaphragm  2010 , in which a plurality of small holes  2018  are formed as well. A plurality of small holes  2018  are formed in the four regions in correspondence with the four holes  2016  within the peripheral portion  2014  of the diaphragm  2010 . In addition, an extension wire connected to an electrode (not shown) is extended from the external periphery of the diaphragm  2010 . 
     Next, a second conductive layer composed of phosphorus-doped polysilicon is formed on the diaphragm  2010  and the first insulating film via a second insulating film (or a second sacrifice film) composed of a silicon oxide film. The second conductive layer is processed into a prescribed shape by way of etching, thus forming the back plate  2020  and the extension wire  2024 . As shown in  FIG. 32A , the back plate  2020  has a disk-like shape and is arranged concentrically with the diaphragm  2010 , wherein the radius thereof is substantially identical to the radius of the center portion  2012  of the diaphragm  2010 . A plurality of small holes  2022  serving as sound holes, which transmit sound waves from the exterior therethrough so that sound waves reach the diaphragm  2010 , are formed in the back plate  2020 . Furthermore, the extension wire  2024  connected to an electrode (not shown) is extended from the external periphery of the back plate  2020 . 
     Next, a third insulating film composed of a silicon oxide film is formed on the back plate  2020  and the second insulating film; then, the backside of the substrate  2030  is polished so as to adjust the thickness thereof. Subsequently, anisotropic etching such as Deep RIE is performed so as to selectively remove the substrate  2030 , thus forming an opening reaching the first insulating film. The opening is formed in conformity with the central region of the center portion  2012  of the diaphragm  2010  and the region in which none of the small holes  2018  is formed in the peripheral portion  2014 . 
     Next, wet etching using buffered hydrofluoric acid (Buffered HF) is performed by use of a prescribed photoresist pattern serving as a mask, thus selectively removing the third insulating film, the second insulating film, and the first insulating film. In addition, an etching solution is infiltrated into a plurality of small holes  2022  formed in the back plate  2020 , thus removing the second insulating film intervened between the back plate  2020  and the diaphragm  2010 . The etching solution is infiltrated into the four holes  2016  and a plurality of small holes  2018  formed in the diaphragm  2010 , thus removing the first insulating film intervened between the diaphragm  2010  and the substrate  2030 . Furthermore, buffered hydrofluoric acid is infiltrated into the opening of the substrate  2030 , thus selectively removing the first insulating film. 
     As described above, the second insulating film intervened between the back plate  2020  and the diaphragm  2010  is removed so as to form the gap  2040 . Due to the removal of the first insulating film, the opening of the substrate  2030  is enlarged to reach the diaphragm  2010  so as to form the cavity  2032  and to form the passage  2034  realizing a desired acoustic resistance between the substrate  2030  surrounding the cavity  2032  and the diaphragm  2010 . 
     At the same time, the first insulating film is intentionally left between the diaphragm  2010  and the substrate  2030  so as to form the first support  2050 . In addition, a laminated insulating film composed of the first insulating film and the second insulating film is left between the back plate  2020  and the substrate  2030 , thus forming the second supports  2052  inserted into the four holes  2016  of the diaphragm  2010 . 
     By way of the aforementioned process, it is possible to produce the condenser microphone of the third embodiment shown in  FIGS. 32A to 32D . 
     As described above, it is possible for the manufacturing method of the condenser microphone of the third embodiment to directly use the conventionally-known semiconductor manufacturing process except for the use of resist masks having different patterns in the photolithography. 
     The third embodiment of the present invention is not necessarily limited to the constitution shown in  FIGS. 32A to 32D ; hence, it is possible to realize a variety of modifications. Hereinafter, variations will be described. 
     (First Variation) 
     A first variation of the third embodiment will be described with reference to  FIGS. 33A to 33D .  FIG. 33A  is a plan view showing the constitution of a condenser microphone according to a first variation;  FIG. 33B  is a plan view showing the constitution in which a back plate is excluded from the constitution shown in  FIG. 33A ;  FIG. 33C  is a cross-sectional view taken along line A-A in  FIG. 33A ; and  FIG. 33D  is a cross-sectional view taken along line B-B in  FIG. 33A . The structure of the condenser microphone shown in  FIGS. 33A to 33D  is substantially identical to the constitution of the condenser microphone shown in  FIGS. 32A to 32D ; hence, the following description explains only the difference between them. 
     The condenser microphone of the first variation provides a diaphragm  2110 , which does not have a disk-like shape but is entirely formed in a rectangular shape in plan view and which is constituted of a center portion  2112  having a rectangular shape and peripheral portions  2114 . Three circular holes  2116  are arranged with equal spacing therebetween and are formed in each of two regions, which lie along opposite long sides and adjoin the peripheral portions, in the center portion  2112  of the diaphragm  2110 , wherein a plurality of small holes  2118  are formed as well. In addition, a plurality of small holes  2118  are formed in four regions, which lie along opposite short sides and adjoin the holes  116 , in the peripheral portions  2114  of the diaphragm  2110  as well. The regions, in which in total six holes  2116  and plural small holes  2118  are formed, are positioned opposite to the substrate  2130 . 
     Aback plate  2120  is arranged in parallel with the diaphragm  2110  with a gap  2140  therebetween. Similar to the diaphragm  2110 , the back plate  2120  has a rectangular shape in plan view, wherein it is positioned opposite to the center portion  2112  of the diaphragm  2110 . In plan view, the peripheral portions  2114  of the diaphragm  2110  extend externally of the back plate  2120 . A plurality of small holes  2122  serving as sound holes are formed in the back plate  2120 . An extension wire  2124  connected to an electrode (not shown) is extended from the external periphery of the back plate  2120 . 
     External peripheries along the opposite long sides in the peripheral portions  2114  of the diaphragm  2110  are supported above the substrate  2130  by means of first supports  2150  having insulating property. In addition, the back plate  2120  is supported above the substrate  2130  by means of six cylindrical second supports  2152  having insulating property, which are inserted into the six holes  2116  of the diaphragm  2110 . 
     An opening, which runs through the substrate  2130  to reach the diaphragm  2110 , is formed in conformity with the center portion  2112  of the diaphragm and the regions of the peripheral portions  2114 , in which none of the six holes  2116  and none of the small holes  2118  are formed, thus forming a cavity  2132 . A passage  2134  realizing a desired acoustic resistance is formed between the substrate  2130  surrounding the cavity  2132  and the diaphragm  2110 . 
     The manufacturing method of the condenser microphone of the first variation is substantially identical to the aforementioned manufacturing method except for the use of resist masks having different patterns in the photolithography; hence, the description thereof will be omitted. 
     In the condenser microphone of the first variation, the back plate  2120  is supported above the substrate  2130  by means of the second supports  2152  inserted into the holes  2116  of the diaphragm  2110  and is positioned opposite to the center portion  2112  of the diaphragm  2110 ; however, it is not positioned opposite to the peripheral portions  2114 . That is, the condenser microphone of the first variation shown in  FIG. 31  is similar to the condenser microphone shown in  FIG. 32  in terms of the basic features thereof except that the diaphragm  2110  and the back plate  2120  each have rectangular shapes; hence, it demonstrates similar effects. 
     In the condenser microphone of the first variation, the back plate  2120  is supported above the substrate  2130  by means of the six second supports  2152 ; hence, in comparison with the condenser microphone shown in  FIG. 32 , the back plate  2120  is held in a more stable manner and is more difficult to be deformed. Thus, it is possible to further improve the operation stability of the condenser microphone. That is, it is possible for the condenser microphone of the first variation to further improve the sensitivity by increasing the dimensions thereof. 
     Furthermore, the external peripheries lying along the long sides of the peripheral portions  2114  of the diaphragm  2110  are supported above the substrate  2130  by means of the first supports  2150 . That is, compared with the condenser microphone shown in  FIG. 30  in which the external periphery of the peripheral portion  2014  of the diaphragm  2010  is supported in a circumferential manner above the substrate  2030  by means of the first support  2150 , the condenser microphone shown in  FIG. 33  is further improved in terms of the vibration characteristics of the diaphragm  2110 ; hence, it is possible to further improve the sensitivity. 
     In the condenser microphone of the first variation, the back plate  2120  is supported above the substrate  2130  by means of a plurality of second supports  2152 , wherein the number of the second supports  2152  is not necessarily limited to six. For example, it is possible to further add two second supports  2152  lying along the opposite short sides of the back plate  2120 , so that in total eight second supports  2152  are arranged. In this case, it is necessary to increase the number of the holes  2116  formed in the diaphragm  2110  to eight, and it is necessary to modify the position of the opening forming the cavity  2132  in the substrate  2130 . By increasing the number of the second supports  2152 , it is possible to hold the back plate  2120  in a stable manner, thus suppressing the deformation thereof. Thus, it is possible to improve the sensitivity by increasing the dimensions of the condenser microphone. 
     (Second Variation) 
     A condenser microphone according to a second variation of the third embodiment will be described with reference to  FIGS. 34A to 34D .  FIG. 34A  is a plan view showing the constitution of the condenser microphone of the second variation;  FIG. 34B  is a plan view showing the constitution in which a back plate is removed from the constitution shown in  FIG. 34A ;  FIG. 34C  is a cross-sectional view taken along line A-A in  FIG. 34 ; and  FIG. 34D  is a cross-sectional view taken along line B-B in  FIG. 34A . 
     As shown in  FIGS. 34A to 34D , the condenser microphone of the second variation has the constitution substantially similar to the constitution of the condenser microphone of the first variation shown in  FIGS. 33A to 33D ; hence, only the difference between them will be described. 
     The condenser microphone of the second variation is characterized by providing a stopper layer  2160  having insulating properties fixed to each of the intermediate portions of the six second supports  2152 , which support the back plate  2120  above the substrate  2130  in the gap  2140  formed between the diaphragm  2110  and the back plate  2120 . The stopper layer  2160  is a thin film composed of polysilicon, which is not added with impurities, wherein it has a disk-like shape in which the thickness thereof is approximately 0.5 μm, and the radius thereof is approximately 30 μm. Incidentally, the distance between the stopper layer  2160  and the diaphragm  2110  is approximately 3 μm. 
     The manufacturing method of the condenser microphone of the second variation shown in  FIG. 34  additionally introduces the following step in comparison with the manufacturing method of the condenser microphone of the first variation. 
     Similar to the manufacturing method of the first variation, after the formation of the diaphragm  2110 , a polysilicon layer not added with impurities is formed above the diaphragm  2110  and the first insulating film via an additional insulating film (or an additional sacrifice film) composed of a silicon oxide film of approximately 3 μm thickness, wherein it is processed into a prescribed shape by way of etching, thus forming the stopper layer  2160 . 
     Thereafter, a second conductive film is formed above the stopper layer  2160  and the additional insulating film via a second insulating film (or a second sacrifice film); then, the second conductive layer is processed into a prescribed shape by way of etching, thus forming the back plate  2120 . Furthermore, a third insulating film is formed above the back plate  2120  and the second insulating film; then, the backside of the substrate  2130  is polished so that the substrate  2130  is selectively removed, thus forming an opening. 
     Thereafter, the third insulating film, the second insulating film, the additional insulating film, and the first insulating film are selectively removed by way of etching, thus forming the gap  2140  between the back plate  2120  and the diaphragm  2110 . The cavity  2132  is formed in the substrate  2130 ; the passage  2134  having a desired acoustic resistance is formed; and the first support  2152  is formed between the diaphragm  2110  and the substrate  2130 . At this time, a laminated insulating film, which is composed of the second insulating film between the back plate  2120  and the stopper layer  2160  as well as the additional insulating film and the first insulating film, which are intervened between the stopper layer  2160  and the substrate  2130 , is intentionally left, thus forming the second supports  2152  in which the stopper layer  2160  is fixed to each of the intermediate portions thereof and which support the back plate  2120  above the substrate  2130 . 
     As described above, it is possible to produce the condenser microphone of the second variation shown in  FIGS. 34A to 34D . 
     The condenser microphone of the second variation shown in  FIGS. 34A to 34D  demonstrates an effect, in which, by arranging the stopper layer  2160  having insulating property in the gap  2140  between the diaphragm  2110  and the back plate  2120 , it is possible to prevent the diaphragm  2110  and the back plate  2120  from coming in contact with each other even when excessive sound pressure is applied to the diaphragm  2110  and even when mechanical impact is applied from the exterior, in addition to the effect realized by the condenser microphone shown in  FIGS. 32A to 32D . Thus, it is possible to further stabilize the operation of the condenser microphone. 
     The present invention is adapted to condenser microphones incorporated into electronic devices such as portable telephones, information terminals, and personal computers as well as audio devices.