Patent Publication Number: US-9414139-B2

Title: Acoustic transducer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a National Stage application of PCT Application No. PCT/JP2013/071829, with an International filing date of Aug. 12, 2013, which claims priority of Japanese Patent Application No. 2012-199960 filed on Sep. 11, 2012, the entire contents of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an acoustic transducer that converts acoustic vibrations into electrical signals, or converts electrical signals into acoustic vibrations, and more particularly, to an acoustic transducer such as an acoustic sensor or a speaker manufactured using MEMS technology. 
     2. Related Art 
       FIG. 1  is a cross-sectional view showing a portion of a conventional acoustic sensor manufactured using MEMS technology. In an acoustic sensor  11 , a diaphragm  14  (vibration electrode film) having conductivity is provided above an upper surface of a silicon substrate  13 . The silicon substrate  13  has a back chamber  12  vertically penetrating therethrough. The top of the back chamber  12  is covered by the diaphragm  14 . Further, a dome-shaped protective film  15  is formed above the upper surface of the silicon substrate  13 , enclosing the diaphragm  14 . The protective film  15  is formed with a fixed electrode film  16  at a position facing the diaphragm  14 . The diaphragm  14  and the fixed electrode film  16  constitute a capacitor for converting acoustic vibrations into electrical signals. Multiple acoustic holes  17  are formed in the protective film  15  and the fixed electrode film  16  to allow acoustic vibrations (sound) to pass through them. 
     In the acoustic sensor  11  shown in  FIG. 1 , the diaphragm  14  is formed in parallel with the upper surface of the silicon substrate  13  in a region where the silicon substrate  13  and the diaphragm  14  face each other. In particular, in a direction parallel to the upper surface of the silicon substrate  13  and orthogonal to an edge of a top opening of the back chamber  12 , the height of a gap between the silicon substrate  13  and the diaphragm  14  (hereinafter, the gap is referred to as a vent hole  18 ) is uniform. Such an acoustic sensor is disclosed, for example, in Patent Document 1. 
     A vent hole of an acoustic sensor serves as an acoustic resistance to acoustic vibrations entering through acoustic holes and passing to a back chamber, and has an important function for ensuring sensitivity in the bass range. On the other hand, air in the vent hole has characteristics as a viscous fluid, and thus the vent hole also functions as a noise (thermal noise) source. 
     Noise in the vent hole is mainly caused by a mechanical resistance due to the viscosity of air present in the gap (vent hole) between an edge portion of a diaphragm and an upper surface of a silicon substrate (this is called a film damping effect.). Specifically, when the diaphragm tries to move in a direction to be taken off from the substrate (upward), the viscosity of air in the vent hole generates a resistance hindering the upward movement of the diaphragm. Conversely, when the diaphragm tries to move in a direction to be pressed against the substrate (downward), it generates a resistance hindering the downward movement of the diaphragm. Noise caused by a mechanical resistive component at this time constitutes noise in the vent hole. 
     In the acoustic sensor  11  shown in  FIG. 1 , in an attempt to reduce generation of noise in the vent hole  18 , the diaphragm  14  may be moved away from the upper surface of the silicon substrate  13  to increase the height H of the vent hole  18  like the diaphragm  14  shown in solid lines in  FIG. 2A . Alternatively, like the diaphragm  14  shown in solid lines in  FIG. 2B , the edge of the diaphragm  14  may be retracted toward the center to shorten the overlap length between the diaphragm  14  and the upper surface of the silicon substrate  13  (width W of the vent hole  18 ). 
     However, either when the height H of the vent hole  18  is increased or when the width W of the vent hole  18  is shortened, the acoustic resistance of the vent hole  18  is reduced. Therefore, acoustic vibrations are likely to leak into the back chamber  12  through the vent hole  18 , lowering the sensitivity of the acoustic sensor  11  in the bass range.  FIG. 3  is a graph showing the sensitivity of the acoustic sensor, with a horizontal axis representing the frequency of acoustic vibrations (vibration frequency), with a vertical axis representing the sensitivity. A curve shown in a dashed line in  FIG. 3  represents the sensitivity-frequency characteristics (hereinafter, referred to as frequency characteristics) when the diaphragm  14  is in a position shown in dashed lines in  FIG. 2A  or  FIG. 2B . When the height H of the vent hole  18  is increased as shown in solid lines in  FIG. 2A , the sensitivity of the acoustic sensor decreases in the bass range (low audio frequency range) like the frequency characteristics shown in a solid line in  FIG. 3 . When the width W of the vent hole  18  is shortened as shown in solid lines in  FIG. 2B , the sensitivity of the acoustic sensor decreases in the bass range like the frequency characteristics shown in the solid line in  FIG. 3 . That is, an attempt to reduce noise in the acoustic sensor causes a decrease in sensitivity in the bass range, narrowing a flat range in the frequency characteristics. 
     On the contrary, in order to provide excellent frequency characteristics of the acoustic sensor (that is, in order to widen the flat range in the frequency characteristics), the diaphragm  14  may be moved closer to the upper surface of the silicon substrate  13  to decrease the height H of the vent hole  18  to increase the acoustic resistance in the vent hole  18 . Alternatively, the width W of the vent hole  18  may be lengthened to increase the acoustic resistance. However, in these cases, noise generated in the vent hole  18  increases, degrading the S/N ratio of the acoustic sensor. 
     Thus, in the conventional acoustic sensor, achieving a high S/N ratio by reducing noise and achieving almost flat frequency characteristics also in the bass range are in a trade-off relationship. It has been difficult to achieve both of them.  FIG. 4  is a graph showing a relationship between the S/N ratio (vertical axis) and the roll-off frequency in an acoustic sensor as in  FIG. 1 . Generally, a roll-off frequency fr is a frequency at a point where the sensitivity decreases by −3 dB compared to the sensitivity at a frequency of 1 kHz. As the roll-off frequency fr becomes smaller, the flat range in sensitivity extends toward the bass range, providing excellent frequency characteristics.  FIG. 4  shows that when the roll-off frequency is decreased, the S/N ratio decreases, and when the S/N ratio is increased, the roll-off frequency increases, reducing the sensitivity in the bass range. 
     Next,  FIG. 5A  is a cross-sectional view showing a portion of another conventional acoustic sensor manufactured using MEMS technology.  FIG. 5B  is an enlarged perspective view showing a portion of a diaphragm used in the acoustic sensor in  FIG. 5A . In an acoustic sensor  21 , a plurality of stoppers  22  is provided on a lower surface of a diaphragm  14 . The stoppers  22  prevent an edge portion of the diaphragm  14  from sticking to an upper surface of a silicon substrate  13  and becoming immovable. Such an acoustic sensor is disclosed, for example, in Patent Document 2. 
     According to the acoustic sensor  21 , the distance between the stoppers  22  and the upper surface of the silicon substrate  13  is smaller than the distance between a lower surface of the edge portion of the diaphragm  14  and the upper surface of the silicon substrate  13 . Thus, it seems that the stoppers  22  can increase acoustic resistance to increase the sensitivity of the acoustic sensor  21  in the bass range. However, the stoppers  22  are intended to prevent the diaphragm  14  from sticking to the silicon substrate  13 , and are formed in a thin pillar shape and provided only sparsely at intervals. Therefore, the stoppers  22  do not have an effect of preventing acoustic vibrations from passing through the vent hole  18 . There is no effect of improving the sensitivity of the acoustic sensor  21  by increasing the acoustic resistance. 
     PATENT DOCUMENTS 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2010-056745 
     Patent Document 2: WO 2002/015636 A (JP 2004-506394 W) 
     SUMMARY 
     An acoustic transducer according to one or more embodiments of the present invention can reduce generation of noise in a vent hole and flatten frequency characteristics in the bass range more. 
     An acoustic transducer according to one or more embodiments of the present invention includes a substrate having a cavity opening at the top, a vibration electrode film provided above the substrate so as to cover the cavity, and a fixed electrode film provided above the vibration electrode film at a distance, in which a gap is formed between an upper surface of the substrate and a lower surface of the vibration electrode film around the cavity, and in the gap across which the upper surface of the substrate and the lower surface of the vibration electrode film face each other, one portion of the gap is narrower than the other portion of the gap, the narrower portion of the gap extending linearly. Here, the linearly extending portion is not limited to the portion extending in a straight line, and may be curved or bent. Further, it is not limited to the portion extending in one direction, and may be branched into two or more directions. 
     In the acoustic transducer in one or more embodiments of the present invention, since in the gap across which the upper surface of the substrate and the lower surface of the vibration electrode film face each other, a size of the gap in the linearly extending portion is smaller than that in the other portion of the gap, the portion having a smaller size of the gap can increase acoustic resistance, preventing a reduction in sensitivity in the bass range. Further, since the size of the gap in the other portion is larger, noise can be reduced to increase the S/N ratio. Thus, according to an acoustic transducer of one or more embodiments of the present invention, an acoustic transducer with a high S/N ratio and excellent frequency characteristics can be fabricated. 
     In an acoustic transducer according to one or more embodiments of the present invention, the narrower portion of the gap formed between the upper surface of the substrate and the lower surface of the vibration electrode film extends in a direction other than a direction orthogonal to an end edge of the vibration electrode film to increase acoustic resistance. In particular, when extending in a direction parallel to the end edge of the vibration electrode film, the narrower portion of the gap formed between the upper surface of the substrate and the lower surface of the vibration electrode film has a great effect of increasing acoustic resistance to provide excellent frequency characteristics. 
     In an acoustic transducer according to one or more embodiments of the present invention, a size of the gap at an end edge of the vibration electrode film is smaller than a size at an edge of the top opening of the cavity. One or more embodiments of the present invention may only require deformation of a portion of the vibration electrode film facing the substrate, thus facilitating processing of the vibration electrode film. 
     In an acoustic transducer of one or more embodiments of the present invention, a portion of the vibration electrode film facing the upper surface of the substrate is curved in cross section such that the end edge of the vibration electrode film comes closer to the upper surface of the substrate. One or more embodiments of the present invention may allow for easy deformation of the portion of the vibration electrode film facing the upper surface of the substrate by controlling the inner stress of the vibration electrode film, facilitating the manufacturing of the acoustic transducer. 
     Alternatively, a portion of the vibration electrode film facing the upper surface of the substrate may be bent in cross section such that the end edge of the vibration electrode film comes closer to the upper surface of the substrate. Alternatively, a size of the gap at an intermediate position between an edge of the top opening of the cavity and an end edge of the vibration electrode film may be smaller than a size of the gap at the edge of the top opening of the cavity and a size of the gap at the end edge of the vibration electrode film. 
     In an acoustic transducer according to one or more embodiments of the present invention, a stopper is projected from a lower surface of a portion of the vibration electrode film facing the upper surface of the substrate, the projection length of the stopper being greater than a height difference between a proximal end of the stopper and a lowermost end of the vibration electrode film. According to one or more embodiments of the present invention, the stopper can strike the substrate, preventing the substrate from contacting the vibration electrode film, and preventing the vibration electrode film from sticking to the substrate. 
     In an acoustic transducer according to one or more embodiments of the present invention, a projecting portion is provided on the upper surface of the substrate in a region of the upper surface of the substrate facing the vibration electrode film, the projecting portion reducing a size of the gap formed between the upper surface of the substrate and the lower surface of the vibration electrode film. One or more embodiments of the present invention may only require provision of the projecting portion on the upper surface of the substrate, thus increasing the degree of freedom in design and manufacturing. 
     Various combinations of the above-described components are within a scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a portion of a conventional acoustic sensor. 
         FIG. 2A  is a cross-sectional view showing a state where the position of a diaphragm is moved upward in the acoustic sensor shown in  FIG. 1 . 
         FIG. 2B  is a cross-sectional view showing a state where an end edge of the diaphragm is retracted toward the center in the acoustic sensor shown in  FIG. 1 . 
         FIG. 3  is a graph showing a relationship between the sensitivity of the acoustic sensor and frequencies (frequency characteristics). 
         FIG. 4  is a graph showing a relationship between the S/N ratio and the roll-off frequency in an acoustic sensor as in  FIG. 1 . 
         FIG. 5A  is a cross-sectional view showing a portion of another conventional acoustic sensor. 
         FIG. 5B  is a partially cross-sectional perspective view of a diaphragm used in the acoustic sensor in  FIG. 5A . 
         FIG. 6  is a plan view of an acoustic sensor according to Embodiment 1 of the present invention. 
         FIG. 7  is a cross-sectional view along line X-X in  FIG. 6 . 
         FIG. 8  is a plan view showing a diaphragm formed above an upper surface of a silicon substrate. 
         FIG. 9  is a partially cross-sectional perspective view showing a beam portion of the diaphragm formed above the upper surface of the silicon substrate and nearby portions. 
         FIG. 10  is an enlarged cross-sectional view showing a vent hole in  FIG. 7  and nearby portions. 
         FIG. 11  is a graph showing the frequency characteristics of the acoustic sensor. 
         FIG. 12  is a graph showing a relationship between the S/N ratio and the roll-off frequency in the acoustic sensor. 
         FIG. 13  is a graph showing a relationship between package internal volume and frequency characteristics. 
         FIG. 14  is a diagram for illustrating the definition of the package internal volume. 
         FIG. 15  is a cross-sectional view of a comparative example. 
         FIG. 16  is a cross-sectional view showing a portion of an acoustic sensor according to a modification of Embodiment 1 of the present invention. 
         FIG. 17  is a perspective view showing a portion of a diaphragm used in the modification shown in  FIG. 16 . 
         FIG. 18  is a cross-sectional view showing a portion of an acoustic sensor according to another modification of Embodiment 1 of the present invention. 
         FIG. 19  is a cross-sectional view showing a portion of an acoustic sensor according to still another modification of Embodiment 1 of the present invention. 
         FIG. 20A  is a cross-sectional view showing a portion of an acoustic sensor according to Embodiment 2 of the present invention. 
         FIG. 20B  is an enlarged cross-sectional view of an edge portion of a diaphragm of the acoustic sensor shown in  FIG. 20A . 
         FIG. 21  is a perspective view showing a portion of the diaphragm used in the acoustic sensor shown in  FIG. 20A . 
         FIG. 22  is a cross-sectional view showing a portion of an acoustic sensor according to Embodiment 3 of the present invention. 
         FIG. 23  is a cross-sectional view showing another form of Embodiment 3 of the present invention. 
         FIG. 24  is a plan view showing a diaphragm provided above an upper surface of a silicon substrate according to Embodiment 4 of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to the accompanying drawings, embodiments of the present invention will be described. In embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. 
     Although acoustic sensors will be illustrated as an example below, the present invention is not limited to acoustic sensors, and may be applied to speakers and others manufactured using MEMS technology. The present invention is not limited to the embodiments below, and various design changes may be made without departing from the scope of the present invention. 
     Embodiment 1 
     With reference to  FIGS. 6 and 7 , the configuration of an acoustic sensor  31  according to Embodiment 1 of the present invention will be described.  FIG. 6  is a plan view showing the acoustic sensor  31  in Embodiment 1 of the present invention.  FIG. 7  is a cross-sectional view along line X-X in  FIG. 6 .  FIG. 8  is a plan view showing the shape of a diaphragm  33  formed above an upper surface of a silicon substrate  32 .  FIG. 9  is a perspective view showing a portion of the diaphragm  33  formed above the upper surface of the silicon substrate  32 . 
     The acoustic sensor  31  is a capacitance type sensor fabricated using MEMS technology. As shown in  FIG. 7 , in the acoustic sensor  31 , the diaphragm  33  (vibration electrode film) is formed above an upper surface of the silicon substrate  32  (substrate), and a back plate  34  is provided above the diaphragm  33  via a minute air gap (gap). 
     A chamber  35  (cavity) is formed in the silicon substrate  32  made from single crystal silicon, penetrating therethrough from the front side to the back side. The chamber  35  constitutes a back chamber or a front chamber, depending on the usage pattern of the acoustic sensor  31 . The wall surface of the chamber  35  may be a vertical plane, or may be inclined in a tapered shape. 
     The diaphragm  33  is formed by a polysilicon thin film having conductivity. As shown in  FIG. 8 , the diaphragm  33  is formed in a substantially rectangular shape with beam portions  36  extending horizontally from the corners in diagonal directions. The diaphragm  33  is disposed above the upper surface of the silicon substrate  32  so as to cover the top of the chamber  35 . As shown in  FIG. 9 , lower surfaces of the beam portions  36  are supported by anchors  38 . Thus, the diaphragm  33  is disposed above the upper surface of the silicon substrate  32 , floated above the upper surface of the silicon substrate  32 . 
     Gaps narrow in a height direction to allow acoustic vibrations or air to pass through them, that is, vent holes  37  are formed between the lower surface of the diaphragm  33  and the upper surface of the silicon substrate  32  around the chamber  35 . The vent holes  37  are formed along portions where the diaphragm  33  faces the upper surface of the silicon substrate  32  (around the chamber  35 ) (hereinafter, these portions are each referred to as an edge portion of the diaphragm  33 ) between the beam portions  36 . The vent hole  37  below each edge portion of the diaphragm  33  is short in a width direction (direction orthogonal to an edge of the top opening of the chamber  35 ) and long in a length direction (direction parallel to an edge of the top opening of the chamber  35 ). 
     As shown in  FIGS. 7 and 9 , the edge portions of the diaphragm  33 , that is, the edge portions located between the beam portions  36  each have an edge (hereinafter, the outermost end of each edge portion of the diaphragm  33  is referred to as an end edge of the diaphragm  33 .) curved in an arc shape so as to come closer to the upper surface of the silicon substrate  32 . The curved portion constitutes a deformed portion  42 . Thus, the deformed portion  42  is formed along almost the entire length of the vent hole  37  at each side. 
       FIG. 10  is an enlarged view of a portion in which the vent hole  37  is formed in  FIG. 7 . Since the deformed portion  42  of the diaphragm  33  is curved to bulge on the upper surface side, the height of a portion of the gap between the silicon substrate  32  and the diaphragm  33  narrower than the other portion and extending linearly, that is, a gap  37   b  at an outer peripheral portion of the vent hole  37  located below the deformed portion  42  is smaller than the height of the other portion of the vent hole  37 , that is, a gap  37   a  at an inner peripheral portion of the vent hole  37  located below a flat portion of the diaphragm  33  other than the deformed portion  42 . In particular, as for the height of the vent hole  37 , that is, the gap between the lower surface of the diaphragm  33  and the upper surface of the silicon substrate  32 , a height h 1  of the vent hole  37  at the end edge of the diaphragm  33  is smaller than a height h 2  of the vent hole  37  at the edge of the top opening of the chamber  35 . A region of the vent hole  37  with a large height like the gap  37   a  at the inner peripheral portion located below the substantially flat region of the diaphragm  33  desirably has an area sufficiently greater than a region of the vent hole  37  with a small height like the gap  37   b  at the curved outer peripheral portion. 
     In order to curve the edge portion of the diaphragm  33  as described above, it is only required to control the stress gradient of the diaphragm  33  in a thickness direction. Specifically, in a conventional manufacturing process of acoustic sensors, a sacrificial layer (not shown) is formed on top of the silicon substrate  32 , the diaphragm  33  is formed thereon with polysilicon, and then ions such as phosphorous (P) or boron (B) are injected into the entire surface of the diaphragm  33 , followed by annealing. When the acoustic sensor  31  is fabricated by this manufacturing process, an inner stress gradient can be produced in the thickness direction of the diaphragm  33  by an ion implantation and annealing step, for example. At this time, when a stronger tension stress is generated on the lower surface side than on the upper surface side of the diaphragm  33 , the edge portions of the diaphragm  33  are curved to bulge on the upper surface side, forming the deformed portions  42 . Although an inner stress is generated also in a region other than the deformed portions  42  so as to curve the diaphragm  33 , the four corners of the diaphragm  33  are fixed to the anchors  38 , and thus the region other than the deformed portions  42  of the diaphragm  33  is strained and kept generally flat. 
     Inside the diaphragm  33 , it is desirable to produce a stress gradient of 10 MPa/μm or more in the thickness direction of the diaphragm  33  so that the diaphragm  33  has a stronger tension stress in the lower surface than in the upper surface. This is because a stress gradient smaller than this cannot cause the edge portions of the diaphragm  33  to be curved sufficiently. 
     The edge portions of the diaphragm  33  do not need to extend smoothly along the length of the vent holes  37  as shown in  FIGS. 8 and 9 . The edge portions of the diaphragm  33  may wave or warp regularly or irregularly along the length of the vent holes  37 . 
     The back plate  34  has a fixed electrode film  40  made from polysilicon provided on a lower surface of a protective film  39  made from SiN. As shown in  FIGS. 6 and 7 , the protective film  39  is formed in a substantially rectangular dome shape. It has a hollow portion below the protective film  39 , and covers the diaphragm  33  with the hollow portion. The fixed electrode film  40  is provided opposite to the diaphragm  33 . 
     A minute air gap (gap) is formed between the lower surface of the back plate  34  (that is, the lower surface of the fixed electrode film  40 ) and the upper surface of the diaphragm  33 . The fixed electrode film  40  and the diaphragm  33  face each other, constituting a capacitor to detect acoustic vibrations and convert them into electrical signals. 
     The back plate  34  is almost entirely perforated with multiple acoustic holes  41  penetrating therethrough from the upper surface to the lower surface, for allowing acoustic vibrations to pass through them. As shown in  FIG. 6 , the acoustic holes  41  are arranged with regularity. In the illustrated example, the acoustic holes  41  are arranged in a triangular shape along three directions forming an angle of 120° with each other. Alternatively, they may be arranged in a rectangular shape or concentrically. 
     As shown in  FIG. 7 , minute cylindrical stoppers  43  are projected from the lower surface of the back plate  34 . The stoppers  43  are provided to prevent the diaphragm  33  from sticking to the back plate  34 . They project integrally from the lower surface of the protective film  39 , pass through the fixed electrode film  40 , and project from the lower surface of the back plate  34 . The stoppers  43  are made from SiN like the protective film  39 , and thus have insulation. 
     As shown in  FIG. 6 , an electrode pad  44  electrically connected to the diaphragm  33  and an electrode pad  45  electrically connected to the fixed electrode film  40  are provided on the top of the acoustic sensor  31 . 
     In the acoustic sensor  31  configured as described above, when acoustic vibrations pass through the acoustic holes  41  and enter the air gap between the back plate  34  and the diaphragm  33 , the thin-film diaphragm  33  is vibrated by the acoustic vibrations. When the vibrations of the diaphragm  33  change the gap distance between the diaphragm  33  and the fixed electrode film  40 , the capacitance between the diaphragm  33  and the fixed electrode film  40  is changed. As a result, in the acoustic sensor  31 , the acoustic vibrations (change in sound pressure) sensed by the diaphragm  33  constitute a change in the capacitance between the diaphragm  33  and the fixed electrode film  40 , and are output as an electrical signal. 
     In the acoustic sensor  31 , as shown in  FIG. 10 , the height of the vent hole  37  is small at one portion of the vent hole  37 , specifically, at a portion on the outer peripheral side of the vent hole  37  in one or more embodiments of the present invention (hereinafter sometimes referred to as the gap  37   b  at the outer peripheral portion), and large at the other portion located on the inner peripheral side with respect to the gap  37   b  at the outer peripheral portion of the vent hole  37  (hereinafter sometimes referred to as the gap  37   a  at the inner peripheral portion). Therefore, the acoustic resistance is large in one region of the vent hole  37 , and the acoustic resistance is small in the other region of the vent hole  37 . The total acoustic resistance of the vent hole  37  equals to the acoustic resistance with a large resistance value in the one region connected in series to the acoustic resistance with a small resistance value in the other region. Thus, the total acoustic resistance of the vent hole  37  is determined by the acoustic resistance with the large resistance value. As a result, in the acoustic sensor  31 , by reducing the height of the vent hole  37  at the gap  37   b  of the outer peripheral portion, the total acoustic resistance can be increased, achieving flatter frequency characteristics in the bass range of the acoustic sensor  31 . 
     When the position of a diaphragm is moved upward to increase the height of a vent hole, with a flat diaphragm, while noise in the vent hole can be reduced to increase the S/N ratio, the sensitivity in the bass range decreases like the frequency characteristics shown in a solid line in  FIG. 11 , narrowing the flat range of the frequency characteristics in the bass range (see the above description of  FIG. 3 ). 
     By contrast, in the acoustic sensor  31  in Embodiment 1, when the position of the entire diaphragm  33  is moved upward, the height of the vent hole  37  becomes higher at the gap  37   a  of the inner peripheral portion. Thus, by reducing a film dumping effect and reducing noise of the acoustic sensor  31 , the S/N ratio can be increased. Furthermore, as a result of increasing the acoustic resistance at the gap  37   b  of the outer peripheral portion, the total acoustic resistance of the vent hole  37  is also increased, allowing for production of a sufficient sound pressure difference between the front and back of the diaphragm  33 . Therefore, the sensitivity in the bass range is improved as shown in a dashed line in  FIG. 11 , and the frequency characteristics can be flattened also in the bass range. Thus, according to Embodiment 1, the acoustic sensor  31  with low noise and excellent frequency characteristics can be fabricated. 
     This can also be explained using a graph of relationship between the S/N ratio and the roll-off frequency shown in  FIG. 12 . A curve a in a solid line shown in  FIG. 12  is a relationship between the S/N ratio and the roll-off frequency in a typical acoustic sensor having a flat diaphragm, which is the same as the curve shown in  FIG. 4 . When only an end edge of the diaphragm is curved downward without changing the vertical position thereof, the distance between the end edge of the diaphragm and the upper surface of a silicon substrate is reduced, thus increasing acoustic resistance in a vent hole. As a result, the relationship between the S/N ratio and the roll-off frequency becomes a curve b in a thin dashed line shown in  FIG. 12 . That is, the curve b at this time becomes close to a bass-range portion of the curve a in the solid line horizontally translated to the low frequency side, and the roll-off frequency decreases by 6. Further, when the diaphragm with the end edge curved downward is moved upward, noise is reduced and the S/N ratio is increased. That is, as for the relationship between the S/N ratio and the roll-off frequency, the curve b is translated upward to be a curve c in a thick dashed line shown in  FIG. 12 . Even when the roll-off frequency is increased more or less by moving the diaphragm upward, a reduction in the roll-off frequency caused by curving the end edge of the diaphragm exceeds. Thus, by moving the diaphragm upward and curving the end edge of the diaphragm downward, it becomes possible to increase the S/N ratio and at the same time make the frequency characteristics in the bass range equal to the original frequency characteristics or closer to it to be flatter, compared with the case where the original flat diaphragm is used. 
       FIG. 13  is a graph showing the relationship between package internal volume and frequency characteristics. Here, the package internal volume refers to the volume of a portion of space in a package not occupied by an acoustic sensor, a signal processing circuit, and others when the acoustic sensor is housed in the package together with the signal processing circuit and others. For example, in  FIG. 14 , the acoustic sensor  31  and a signal processing circuit  47  are housed in a package  46 , mounted on the bottom surface in the package  46 . The acoustic sensor  31  has the chamber  35  communicating with a sound introduction hole  48  in the package  46 . The chamber  35  constitutes a front chamber. A region outside the acoustic sensor  31  and the signal processing circuit  47  of the space in the package  46  (region depicted by a dotted pattern in  FIG. 14 ) constitutes a back chamber  49 . The volume of the region depicted by the dotted pattern is the package internal volume. As the package becomes larger, the package internal volume becomes larger. Even with the same package size, as the acoustic sensor and the signal processing circuit become larger, the package internal volume becomes smaller. 
       FIG. 13  shows frequency characteristics with package internal volumes of 0.6 mm 3 , 2.5 mm 3 , and 5 mm 3 . As can be seen from  FIG. 13 , when the acoustic sensor  31  is housed in the package, as the package internal volume becomes smaller, sensitivity reduction in the bass range becomes more marked. Therefore, as packages of acoustic sensors are becoming smaller, it becomes important to prevent degradation of frequency characteristics without increasing noise. 
       FIG. 15  shows a cross-sectional view of a comparative example. In this comparative example, an entire diaphragm  33  is made closer to an upper surface of a silicon substrate  32 , an edge portion of the diaphragm  33  is curved to bulge to the lower surface side so that an end edge of the diaphragm  33  is away from the upper surface of the silicon substrate  32 . In this comparative example, when the diaphragm  33  is moved closer to the upper surface of the silicon substrate  32  to increase acoustic resistance, the height of the vent hole  37  is reduced in most regions of the vent hole  37 , increasing noise. Thus, in the comparative example, it is difficult to achieve both noise reduction and excellent frequency characteristics. Thus, when a deformed portion  42  is formed by curving, it is important to curve the end edge of the diaphragm  33  toward the upper surface of the silicon substrate  32 . 
     Next, a configuration to partly narrow the distance between the edge portion of the diaphragm and the substrate upper surface in Embodiment 1 can be achieved in various forms other than curving the edge portion of the diaphragm in an arc shape as described above. 
     In a modification shown in  FIGS. 16 and 17 , a distal end portion of the edge portion of the diaphragm  33  is bent along the edge portion substantially at a right angle toward the substrate upper surface. In this modification, the distance between the distal end of a deformed portion  42  and the substrate upper surface is shortened at the deformed portion  42  bent substantially at a right angle. Specifically, a gap  37   c  between a lower surface of the deformed portion  42  and an upper surface of the silicon substrate  32  constitutes one portion of the gap between the silicon substrate  32  and the diaphragm  33  narrower than the other portion and extending linearly. A gap  37   d  below a flat region of the diaphragm  33  other than the deformed portion  42  constitutes the other portion with a relatively wide gap. This shape allows the height of the vent hole  37  to be increased in the most part of the vent hole  37  and to be decreased only in the narrow portion at the deformed portion  42 , thus providing a noticeable effect of reducing noise while reducing degradation of sensitivity in the bass range. 
     In a modification shown in  FIG. 18 , the end edge of the diaphragm  33  is bent in a stepped shape to form a deformed portion  42 . In this modification also, a gap  37   c  between a lower surface of the deformed portion  42  and an upper surface of the silicon substrate  32  constitutes one portion of the gap between the silicon substrate  32  and the diaphragm  33  narrower than the other portion and extending linearly. A gap  37   d  below a flat region of the diaphragm  33  other than the deformed portion  42  constitutes the other portion with a relatively wide gap. In this modification, acoustic resistance can be increased compared to the modification in  FIGS. 16 and 17 . 
     In  FIG. 19 , a portion near the end edge of the diaphragm  33  is bent in a bag shape to form a deformed portion  42 . In this modification also, a gap  37   c  between a lower surface of the deformed portion  42  and an upper surface of the silicon substrate  32  constitutes one portion of the gap between the silicon substrate  32  and the diaphragm  33  narrower than the other portion and extending linearly. A gap  37   d  below a flat region of the diaphragm  33  other than the deformed portion  42  constitutes the other portion with a relatively wide gap. In this modification, the height of the vent hole  37  at the edge of the top opening of the chamber  35  and the height of the vent hole  37  at the end edge of the diaphragm  33  are large, and the height of the vent hole  37  at an intermediate portion between the edge of the top opening of the chamber  35  and the end edge of the diaphragm  33  is small. 
     The above-described modifications can also provide functions and effects similar to those of the acoustic sensor  31  in Embodiment 1. 
     The above-described deformed portions  42  do not necessarily need to extend in parallel with the end edge of the diaphragm  33 , and may extend in an inclined direction with respect to the end edge of the diaphragm  33 . However, when the deformed portions  42  extend in a direction orthogonal to the end edge of the diaphragm  33 , acoustic resistance cannot be increased. Thus, the deformed portions  42  desirably extend in a direction not orthogonal to the end edge of the diaphragm  33 . 
     Further, the deformed portions  42  do not need to extend linearly, and may extend in a curve or extend while bending. The extending direction may be branched. 
     Embodiment 2 
       FIG. 20A  is a cross-sectional view showing a portion of an acoustic sensor  51  according to Embodiment 2 of the present invention.  FIG. 20B  is an enlarged cross-sectional view of a portion of a vent hole  37 .  FIG. 21  is an enlarged perspective view showing a corner portion of a diaphragm  33  formed above an upper surface of a silicon substrate  32 . In this acoustic sensor  51 , from a lower surface of an edge portion of the diaphragm  33 , stoppers  52  in a pillar shape for preventing the diaphragm  33  from sticking to and being fixed to an upper surface of the silicon substrate  32  are projected at appropriate intervals. The other components of the acoustic sensor  51  are almost identical to those of the acoustic sensor  31  in Embodiment 1, and thus identical components are denoted by the same reference numerals in Embodiment 1 and will not be described. 
     Among the stoppers  52  projected from the lower surface of the edge portion of the diaphragm  33 , the stopper  52  closest to an end edge of the diaphragm  33  has a projection length h 4  greater than a height difference h 3  between a proximal end of the stopper  52  and a lowermost end (end edge) of the diaphragm  33 . By forming the stopper  52  satisfying this condition, the lowermost end of the diaphragm  33  can be prevented from sticking to and being fixed to the upper surface of the silicon substrate  32 . 
     Embodiment 3 
       FIG. 22  is a cross-sectional view showing a portion of an acoustic sensor  61  according to Embodiment 3 of the present invention. The acoustic sensor  61  uses a diaphragm  33  having an entirely flat edge portion. On the other hand, a projecting portion  62  is formed on an upper surface of a silicon substrate  32  at a position facing an end edge of the diaphragm  33 . The projecting portion  62  extends along the length of a vent hole  37  or along a direction parallel to the end edge of the diaphragm  33 . In one or more embodiments of the present invention, the height of the vent hole  37  at the position where the projecting portion  62  is provided is smaller than the other. Specifically, a gap  37   e  between an upper surface of the projecting portion  62  and a lower surface of the diaphragm  33  constitutes one portion of the gap between the silicon substrate  32  and the diaphragm  33  narrower than the other portion and extending linearly. A gap  37   f  between a region of the upper surface of the silicon substrate  32  other than a region where the projecting portion  62  is formed and the diaphragm  33  constitutes the other portion with a relatively wide gap. Therefore, sensitivity degradation in the bass range is prevented by increasing acoustic resistance at the projecting portion  62 , and at the same time noise is reduced by increasing the height of the vent hole  37  at a portion where the projecting portion  62  is not provided. 
     The projecting portion  62  may be provided at an edge abutting a top opening of a chamber  35  as in an acoustic sensor  63  shown in  FIG. 23 . Alternatively, the projecting portion  62  may be provided midway between the edge at the end edge of the diaphragm  33  and the edge abutting the top opening of the chamber  35 . 
     Embodiment 4 
       FIG. 24  shows a diaphragm  33  provided above an upper surface of a silicon substrate  32  in Embodiment 4 of the present invention. In one or more embodiments of the present invention, deformed portions  42  with a cross-sectional shape as shown in  FIG. 19 , for example, extend in inclined directions with respect to end edges of the diaphragm  33 . 
     One or more embodiments of the present invention can also be applied to MEMS speakers. Speakers and acoustic sensors (microphones) are opposite in signal conversion direction. However, the basic configurations of speakers and acoustic sensors are substantially the same, and thus descriptions of speakers will not be provided. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE KEY 
     
         
         
           
               31 ,  51 ,  61 ,  63  acoustic sensor 
               32  silicon substrate 
               33  diaphragm 
               35  chamber 
               37  vent hole 
               37   a  gap at an inner peripheral portion (the other portion of a gap) 
               37   b  gap at an outer peripheral portion (one portion of the gap) 
               37   c ,  37   e  gap (one portion of a gap) 
               37   d ,  37   f  gap (the other portion of the gap) 
               40  fixed electrode film 
               42  deformed portion 
               52  stopper