JP4811035B2 - Acoustic sensor - Google Patents

Description

  The present invention relates to a capacitive acoustic sensor that detects sound such as audible sound and ultrasonic waves transmitted through air or the like as a medium.

  Conventionally, there has been known an acoustic sensor that performs acoustic detection by vibrating one electrode of a counter electrode capacitor, for example, a parallel plate capacitor, by acoustically, and converting a change in acoustic pressure into an electrical change in the capacitance of the capacitor. (For example, refer to Patent Document 1).

  Such a capacitive acoustic sensor uses, for example, a substrate such as a silicon semiconductor, a diaphragm formed by thinning a part of the substrate, one or more connecting portions formed on the substrate, and connecting portions. Connected to each of the back plate having a plurality of minute through-holes (acoustic holes, acoustic holes) formed to face the diaphragm with a certain narrow gap, and the diaphragm and back plate forming the capacitor. The connection pad for taking out an electric signal is used as a component.

  At least a part of each of the diaphragm and the back plate is a conductor, and constitutes a counter electrode that forms a parallel plate capacitor. The connecting portion integrates both of the diaphragm and the back plate so as not to disturb the vibration of the diaphragm. Such an acoustic sensor can convert an electric signal into sound in addition to converting sound into an electric signal, and functions as a so-called bidirectional acoustoelectric transducer.

  In addition, small and high-accuracy capacitive acoustic sensors are used for portable devices and hearing aids, but with the miniaturization, on the other hand, the capacitance for acoustic detection can be lowered to about 1 pF level. Inevitably, the S / N ratio decreases, and on the other hand, the mutual distances of the diaphragm, the back plate, the substrate, and the like are close to each other, and a parasitic capacitance is generated. This parasitic capacitance increases the capacitance of the entire acoustic sensor, and becomes a background capacitance or background that does not contribute to signal detection. Therefore, the parasitic capacitance pushes down the ratio of the minute capacitance change component (signal component) generated between the diaphragm and the back plate to the total capacitance. As a result, the sensitivity of sound detection decreases due to the parasitic capacitance.

In view of this, there is known a technique in which the parasitic capacity is reduced by processing the area of the back plate to be smaller than the area of the diaphragm (see, for example, Patent Document 2). In addition, there is known a technique in which a metal film serving as a counter electrode is formed only on a necessary portion on the diaphragm (see, for example, Patent Document 3).
JP-A-6-217396 Japanese Patent Publication No. 4-081868 Japanese Unexamined Patent Publication No. 7-050899

  However, in the acoustic sensor as shown in Patent Document 2 described above, there is a problem in that the diaphragm is held by arms extending from the four corners of the diaphragm, and the parasitic capacitance generated at the arm portion cannot be reduced. is there. Moreover, in the acoustic sensor as shown in Patent Document 3 described above, there is a problem that the groove construction of the acoustic sensor and the manufacturing method thereof are complicated.

  The present invention solves the above-described problems, and reduces the parasitic capacitance by an easy manufacturing method, and realizes a capacitance type that achieves miniaturization, improved noise resistance, high sensitivity, and low cost. An object is to provide an acoustic sensor.

In order to achieve the above object, the invention of claim 1 includes a diaphragm that is formed on a silicon substrate and vibrates by sound, and a back plate that faces the diaphragm, and detects a change in capacitance between the two. In the acoustic sensor for detecting the sound, an oxide film formed on the silicon substrate and supporting the diaphragm is provided, and the silicon layer on which the diaphragm is formed and the oxide film below the inner side are separated by separation grooves, respectively. The oxide film in the inner region is separated into a region and an outer region, and the oxide film in the inner region is a ring-shaped oxide film, and forms an elastic hinge-like support structure that supports the diaphragm in the inner region at its outer peripheral portion. The plate includes a conductor portion serving as a counter electrode for forming the capacitance between the diaphragm and the back plate in a range smaller than the diaphragm and the back plate, and the conductor portion is the vibration plate. Are those formed by doping impurities on.

A second aspect of the present invention, in the acoustic sensor according to claim 1, wherein the impurity is boron or phosphorus.

According to a third aspect of the present invention, in the acoustic sensor according to the second aspect , the silicon substrate is an SOI substrate.

According to a fourth aspect of the present invention, in the acoustic sensor according to the second aspect , the diaphragm is polysilicon.

  According to the first aspect of the present invention, since the conductor portion serving as the counter electrode is provided not in the entire surface of the diaphragm but in a portion that is smaller than the diaphragm, the minimum capacitance necessary for the acoustic sensor is ensured, Generation of capacitance can be suppressed, and therefore, downsizing, improved noise resistance, and higher sensitivity can be realized. In addition, since the conductor portion to be the counter electrode is formed on the diaphragm made of a silicon substrate by doping, which is an aged technique, manufacturing is easy and cost reduction can be realized.

In addition, since the conductor part that serves as the counter electrode is provided not in the entire surface of the diaphragm but in a part that is smaller than the back plate, it is possible to secure the minimum capacitance necessary for the acoustic sensor and suppress the occurrence of parasitic capacitance. Therefore, downsizing, improvement in noise resistance, and high sensitivity can be realized. In addition, since the conductor portion to be the counter electrode is formed on the vibration plate made of a silicon substrate by doping technology, which is an aged technology, manufacturing is easy and cost reduction can be realized. Moreover, since the diaphragm can be more effectively electrically insulated and held, parasitic capacitance and noise can be further reduced. Furthermore, a structure for supporting the diaphragm like an elastic hinge can be realized, and the degree of freedom of vibration of the diaphragm can be increased, so that a larger capacity change can be obtained and higher sensitivity can be realized.

According to the invention of claim 2 , doping of boron (element symbol B) and phosphorus (element symbol P) is a well-known and matured technique that has good compatibility with semiconductor processes and can obtain desired conductivity. Therefore, manufacturing is easy and cost reduction can be realized in terms of quality yield.

According to the third aspect of the present invention, for example, using an SOI substrate having a structure of silicon / silicon oxide film / silicon, a diaphragm having a predetermined thickness can be easily formed with silicon having a good thickness accuracy on the oxide film. Cost reduction can be realized. Further, since the silicon oxide film can be used as an etching stopper, the diaphragm can be easily formed.

According to the fourth aspect of the present invention, a polysilicon layer having a desired film thickness can be easily formed on a silicon or silicon oxide film by using a film forming method such as chemical vapor deposition (CVD). Can be controlled with high accuracy, product characteristics can be stabilized, and cost can be reduced.

  Hereinafter, a capacitive acoustic sensor according to an embodiment of the present invention will be described with reference to the drawings.

(First embodiment)
FIGS. 1A and 1B show a capacitive acoustic sensor 1 according to the first embodiment of the present invention, and FIGS. 2A and 2B show the acoustic sensor 1 with the back plate 4 removed. 3 and 4 show the appearance of the acoustic sensor 1. The acoustic sensor 1 of the present embodiment includes a diaphragm 3 that is formed on a silicon substrate 2 and vibrates by sound, and a back plate 4 that faces the diaphragm 3, and detects a change in capacitance between the two. The diaphragm 3 is a sensor that detects a sound, and the diaphragm 3 is a conductor portion that serves as a counter electrode for forming a capacitance between the diaphragm 3 and the back plate 4 in a range smaller than both the diaphragm 3 and the back plate 4. 31 and the conductor portion 31 is formed with a low resistance by doping impurities into the high resistance diaphragm 3.

  Next, each component of the acoustic sensor 1 will be described. The manufacturing process of the acoustic sensor 1 will be described later (FIG. 5). The silicon substrate 2 (abbreviated as substrate 2) is formed by forming a recess 21 and a diaphragm on a substantially rectangular frame (frame) in a plan view, and the frame retains the shape of the acoustic sensor 1; The recess 21 guides sound to the diaphragm 3. The bottom surface of the recess 21 is the diaphragm 3. That is, the structure is such that one surface of the diaphragm 3 faces the concave portion 21 and the periphery of the diaphragm 3 is held by the substrate 2. The diaphragm 3 facing the recess 21 vibrates due to the sound propagating from the opening side of the recess 21.

  The recess 21 is formed, for example, by etching a silicon wafer by a semiconductor process. The recess 21 of the present embodiment has a quadrangular pyramid shape and is formed by etching utilizing the crystal anisotropy of silicon. In addition, when crystal anisotropy is not used, for example, when a processing method such as a dry process is used, the shape of the recess 21 is not limited to a square, but may be a desired shape such as a circular shape by a circular mask shape. be able to. The substrate 2 is made of silicon, has an outer size of about 1 mm to 2 mm square, and a thickness of about 0.5 mm.

The diaphragm 3 is a member that vibrates due to a minute change in sound pressure of sound that reaches from the outside, and is a member that forms a parallel plate capacitor together with the back plate 4. The diaphragm 3 has a substantially rectangular shape with a side of about 1.0 mm in a plan view, is sufficiently thinner than the frame body of the substrate 2 in a cross-sectional view, and has a minute change in sound pressure of sound reaching from the outside. For example, it is formed to have a thickness of about 1 to 2 μm. The substrate 2 on which the diaphragm 3 is formed is, for example, a high-resistance silicon substrate having a resistivity of 1 × 10 ⁺³ (Ω · cm) or more. The diaphragm 3 formed of such a high resistance material is formed into a conductor at the center thereof, and a conductor portion 31 is formed as a counter electrode of the parallel plate capacitor.

The conductor portion 31 described above is made into a conductor by doping an approximately square region of about 0.5 mm in the center portion of the diaphragm 3 with an impurity element such as phosphorus by a semiconductor diffusion process. The resistivity of the conductor part 31 is desirably 1 × 10 ⁻³ (Ω · cm) or less. Thus the difference in resistivity between the semiconductor substrate of high resistivity becomes 10 ⁶ times or more, it can be effectively limit the area of the conductive portion 31.

  Further, the above-described conductor portion 31 has a characteristic dimension d smaller than the characteristic dimension D1 of the back plate 4 and the characteristic dimension D2 of the diaphragm 3 (d <D1, d <D2). Is formed.

  In order to output an electrical signal to the outside from the capacitor electrode on the diaphragm 3 side, that is, the conductor portion 31, a circuit 32 by a doping portion extending from the conductor portion 31 and a connection pad 33 disposed at the terminal portion of the circuit 32 are formed. Has been. For example, the connection pad 33 is used as a bonding pad by the wire W when the acoustic sensor is mounted on the mounting substrate PCB.

  The main part of the back plate 4 has a substantially rectangular shape in plan view, is formed of a conductive material, and serves as a fixed electrode for the vibration plate 3 that vibrates by sound, that is, the vibration electrode. In addition, a parallel plate capacitor is formed so that a change in sound pressure due to sound can be detected as a change in capacitance. Further, the back plate 4 has a minimum external dimension so that air can freely flow in or out from its peripheral portion and further suppress the generation of parasitic capacitance.

  Further, the back plate 4 releases air between the diaphragm 3 and the back plate 4 so as not to vibrate due to air pressure, and so that the diaphragm 3 can freely vibrate without delay according to sound. A plurality of through holes for air circulation to reduce resistance, so-called acoustic holes 41, are provided. The acoustic hole 41 has a substantially square shape with a side of about 10 μm, and is formed by a semiconductor process such as etching. The acoustic hole 41 allows the diaphragm 3 to be freely displaced with respect to the sound pressure. Furthermore, by appropriately designing the shape and number of the acoustic holes 41, it is possible to suppress the excessive resonance of the diaphragm 3 and realize the acoustic sensor 1 having a broadband and flat sensitivity characteristic.

  The material of the back plate 4 is polysilicon formed by a method such as CVD, for example, and impurities are doped to impart conductivity to the high resistance polysilicon. In addition, in order to take out an electric signal from the back plate 4, connection pads 42 used for wire bonding or the like are provided.

  Further, the back plate 4 is configured on the substrate 2 via one or more insulating connecting portions 5. The connecting portion 5 is located between the diaphragm 3 and the back plate 4 and is kept at a constant distance in a state where both are electrically insulated. For example, a silicon oxide film, a silicon nitride film, or the like can be used as the material of the connection portion 5 in a semiconductor process. The gap between the back plate 4 and the diaphragm 3 is adjusted according to the height of the connecting portion 5. The gap is, for example, about 1 to 10 μm or less depending on the characteristic design of the acoustic sensor 1. In the present embodiment, the connection portion 5 has a minimum of two locations so as not to hinder the flow of air that enters and exits from the gap.

  The structure of the diaphragm 3, the conductor portion 31, the connection portion 5, and the back plate 4 formed on the substrate 2 is formed by stacking thin films and reducing the resistance of the high resistor (conducting) by a semiconductor process. The acoustic sensor 1 having a final three-dimensional shape is formed through processes such as shape formation by etching.

  Next, with reference to FIGS. 5A to 5G, a manufacturing process of the acoustic sensor 1 of the first embodiment will be described. The acoustic sensor 1 according to the first embodiment is roughly divided into two stages: a film forming process shown in FIGS. 5A to 5D and a subsequent etching process shown in FIGS. 5E to 5G. Manufactured. In addition, many acoustic sensors 1 are manufactured collectively on a single wide silicon substrate, and finally, a large number of acoustic sensors 1 can be obtained by dicing the silicon substrate to obtain a large number of acoustic sensors 1. Manufactured by the method.

First, as shown in FIGS. 5A and 5B, a region to be the conductor portion 31 of the silicon substrate 2 is doped with an impurity so that the region is made a conductor. As described above, the substrate 2 made of silicon has a high resistivity of 1 × 10 ⁺³ (Ω · cm) or more, and the conductor portion 31 made conductive by doping has a resistivity of 1 × 10 ^−3. (Ω · cm) or less. For doping, for example, boron (element symbol B) or phosphorus (element symbol P) can be used as an impurity.

  Next, as shown in FIGS. 5C and 5D, a silicon oxide film 50 is formed on the entire surface of the substrate 2 including the region of the conductor portion 31 by a method such as CVD, and the back plate 4 is formed thereon. A polysilicon layer 40 is formed, and the polysilicon layer 40 is made conductive by doping. A part of the silicon oxide film 50 is left as the connection portion 5, but most of the silicon oxide film 50 is a so-called sacrificial layer that is removed by etching to form the upper polysilicon layer 40 (back plate 4) three-dimensionally. is there.

  As shown in FIG. 5E, unnecessary polysilicon portions are removed by etching from the above-described conductive polysilicon layer 40 to form acoustic holes 41 that are through holes and to form an outer shape. Thus, the back plate 4 is formed. The back plate 4 is a conductor as a whole and serves as one electrode of a capacitor for detecting sound.

  Next, as shown in FIG. 5F, a recess 21 is formed on the lower surface of the substrate 2 by etching. By this etching, the bottom surface of the recess 21 that is the upper surface of the substrate 2 is formed as the diaphragm 3.

  Next, as shown in FIG. 5G, a predetermined gap is formed between the back plate 4 and the diaphragm 3 by isotropically etching the silicon oxide film 50 using an aqueous solution of hydrofluoric acid or the like. A three-dimensional structure that secures and leaves the connecting portion 5 is formed (so-called sacrificial layer etching). In isotropic etching, etching of a portion to be etched that is in contact with an aqueous solution proceeds substantially uniformly. Therefore, the through hole of the acoustic hole 41 functions as an etching solution supply hole when the silicon oxide film 50 (sacrificial layer) under the back plate 4 is removed by etching. After this sacrificial layer etching, post-processing such as formation of the connection pads 42 and the like and separation into pieces by dicing is performed to complete the acoustic sensor 1.

  As described above, according to the acoustic sensor 1 of the first embodiment, not the entire surface of the diaphragm 3 but a range that is smaller than the diaphragm 3 and that is smaller than the back plate 4. Since the conductor portion 31 serving as an electrode is provided, the minimum capacitance necessary for the acoustic sensor 1 can be ensured, the generation of parasitic capacitance can be suppressed, and the overall capacitance can be suppressed to be small. Improve noise and increase sensitivity. Further, since the center portion of the diaphragm 3 is usually displaced with the vibration surface being parallel to the surface of the back plate 4, and is a maximum amplitude portion, the place where the conductor portion 31 is formed vibrates. By setting it as the center part of the board 3, high sensitivity can be achieved.

  Further, the method of forming the conductor portion 31 to be the counter electrode by doping the diaphragm 3 made of the silicon substrate 2 with impurities such as boron (element symbol B) and phosphorus (element symbol P) is compatible with the semiconductor process. This is a well-known and matured technology that can easily obtain the desired conductivity in the desired region, so that the acoustic sensor 1 can be easily manufactured, and the cost can be reduced in terms of quality yield. it can.

(Second Embodiment)
6A to 6G show the manufacturing process of the acoustic sensor 1 of the second embodiment in the order of main processes. As shown in FIG. 6A, the acoustic sensor 1 is formed using a so-called SOI substrate 2 having a structure of a silicon substrate 20 / a silicon oxide film 22 / a silicon layer 23 from the lower layer. In this acoustic sensor 1, as shown in FIG. 6G, a silicon oxide film 22 is interposed between the silicon layer 23 that forms the diaphragm 3 and the silicon substrate 20 that forms the frame of the acoustic sensor 1. Unlike the acoustic sensor 1 of the first embodiment shown in FIG. 1B described above, the other points are the same as the acoustic sensor of FIG. 1B.

  The formation of the back plate 4 shown in FIGS. 6A to 6E by etching is the same as the formation of the back plate 4 shown in FIGS. 5A to 5E. 6 (f), the silicon substrate 20 of the SOI substrate 2 is etched, and the silicon oxide film 22 remains without being etched. That is, the silicon oxide film 22 of the OSI substrate 2 functions as an etching stop.

  In the sacrificial layer etching of FIG. 6G, the etching of the silicon oxide film 50 proceeds in the same manner as the sacrificial layer etching of FIG. 5G, and the etching of the silicon oxide film 22 functioning as the above-described etching stopper is also performed. Progress at the same time. Therefore, the diaphragm 3 having a predetermined film thickness is accurately formed by this sacrifice layer etching. That is, the film thickness of the diaphragm 3 is the thickness of the silicon layer 23 formed on the SOI substrate 2.

  In this way, by using the SOI substrate 2 having the structure of the silicon substrate 20 / silicon oxide film 22 / silicon layer 23, the diaphragm 3 having a predetermined thickness can be easily formed with silicon having a good thickness accuracy on the oxide film. The cost of the acoustic sensor can be reduced. Further, since the silicon oxide film can be used as an etching stopper, the film thickness variation of the diaphragm 3 can be suppressed, and the acoustic sensor 1 having uniform characteristics can be easily obtained.

  Instead of the SOI substrate 2 described above, a silicon oxide film 22 is formed on a silicon substrate by a thermal oxidation process, CVD, or the like, and then a polysilicon layer is formed by CVD or the like instead of the silicon layer 23. Thus, the diaphragm 3 may be formed. Alternatively, a polysilicon layer may be formed directly on the silicon substrate, and the diaphragm 3 may be formed from this polysilicon layer.

  Since the polysilicon layer can be easily formed on the silicon substrate or on the silicon oxide film formed on the silicon substrate by using a film forming method such as chemical vapor deposition (CVD), the vibration plate 3 can be controlled with high accuracy, the product characteristics can be stabilized, and the cost can be reduced.

(Third embodiment)
FIGS. 7A and 7B show a capacitive acoustic sensor 1 according to the third embodiment of the present invention, and FIGS. 8 and 9 show the appearance of the acoustic sensor 1. FIG. The acoustic sensor 1 according to the third embodiment is similar to the acoustic sensor 1 according to the second embodiment described above in that the silicon layer 23 on which the diaphragm 3 is formed and the silicon oxide film 22 below the inner layer and the outer region are respectively formed. The other structure is the same as that of the acoustic sensor 1 of the second embodiment. That is, the silicon layer 23 is formed in a portion other than the portion of the circuit 32 extending from the conductor portion 31 to the connection pad 33, and is separated from the diaphragm 3 in the inner region by a separation groove 34 that substantially goes around the recess 21. It is roughly separated into a portion and a portion of the outer peripheral layer 30 in the outer region. According to such a separation structure, since the diaphragm 3 can be more effectively electrically insulated and held, parasitic capacitance and noise can be further reduced.

  Further, the silicon oxide film 22 is separated by the separation groove 11 into a portion of the ring-shaped oxide film 10 which is a support portion that supports the diaphragm 3 at its outer peripheral portion and a portion of the outer peripheral layer 12 in the outer region. Yes. As shown in FIG. 7B, the ring-shaped oxide film 10 that supports the diaphragm 3 is formed so that its width w is as narrow as its thickness h. Since the support portion made of the ring-shaped oxide film 10 forms an elastic hinge-like support structure, the rigidity of the support portion around the diaphragm 3 can be reduced, and the diaphragm 3 can vibrate more freely and with a large displacement. . Therefore, the acoustic sensor 1 of the third embodiment can achieve higher sensitivity than the acoustic sensor 1 of the second embodiment described above.

  Next, with reference to FIGS. 10A to 10G, the manufacturing process of the acoustic sensor 1 of the third embodiment will be described. As shown in FIG. 10A, the acoustic sensor 1 is formed by using an SOI substrate 2 having a structure of a silicon substrate 20 / a silicon oxide film 22 / a silicon layer 23 from the lower layer. This is the same as the acoustic sensor of the second embodiment. In the acoustic sensor 1 of the present embodiment, first, as shown in FIG. 10B, the silicon layer 23 of the SOI substrate 2 is etched to form a separation groove 34 in a portion other than the portion to be the circuit 32, and silicon The layer 23 is separated into a part of the diaphragm 3 and a part of the outer peripheral layer 30.

  After the above-mentioned separation, as shown in FIGS. 10C to 10F, the conductor by doping is performed by the same process as that shown in FIGS. 6B to 6F of the second embodiment. Formation of the portion 31 (FIG. 10C), formation of the silicon oxide film 50 as a sacrificial layer, formation of the polysilicon layer 40 for the back plate 4 and formation of a conductor by doping (FIG. 10D), polysilicon The back plate 4 is formed by etching the layer 40 (FIG. 10E), and the recess 21 is formed by etching the silicon substrate 20 (FIG. 10F).

  Then, as shown in FIG. 10G, the sacrificial silicon oxide film 50 and the silicon oxide film 22 functioning as an etching stopper are etched. By this sacrificial layer etching process, the gap between the diaphragm 3 and the back plate 4 is formed, the connection portion 5 is formed, the silicon oxide film 22 on the back surface of the diaphragm 3 is removed, and the isolation groove 11 is formed in the silicon oxide film 22. Is done. The separation groove 11 is formed by the etching solution entering from the separation groove 34 formed in the silicon layer 23 in order to separate the diaphragm 3.

  As described above, the capacitive acoustic sensor 1 of the present invention that detects sound such as audible sound and ultrasonic waves transmitted through air or the like as a medium is based on a semiconductor material such as silicon and has an IC ( It is manufactured in a very small size using a semiconductor process similar to that of an integrated circuit), and can be used for a microphone, an ultrasonic sensor, a hearing aid, and the like for portable devices. The present invention is not limited to the above-described configuration, and various modifications can be made.

(A) is a top view of the capacitive acoustic sensor which concerns on the 1st Embodiment of this invention, (b) is the sectional view on the AA line of (a). (A) is a top view of the state except the back plate of an acoustic sensor same as the above, (b) is the BB sectional drawing of (a). The virtual disassembled perspective view of an acoustic sensor same as the above. The perspective view of an acoustic sensor same as the above. (A)-(g) is sectional drawing which shows the manufacturing process of an acoustic sensor same as the above in order of main processes. (A)-(g) is sectional drawing which shows the manufacturing process of the capacitive acoustic sensor which concerns on the 2nd Embodiment of this invention in order of main processes. (A) is a top view of the capacitive acoustic sensor which concerns on the 3rd Embodiment of this invention, (b) is CC sectional view taken on the line of (a). The perspective view of an acoustic sensor same as the above. The virtual disassembled perspective view of an acoustic sensor same as the above. (A)-(g) is sectional drawing which shows the manufacturing process of an acoustic sensor same as the above in order of main processes.

Explanation of symbols

1 Acoustic sensor 2 Substrate (silicon substrate, SOI substrate)
DESCRIPTION OF SYMBOLS 3 Diaphragm 4 Back plate 10 Support part 22 Oxide film layer 23 Polysilicon layer 31 Conductor part

Claims (4)

In an acoustic sensor that has a diaphragm that is formed on a silicon substrate and vibrates by sound, and a back plate that faces the diaphragm, and detects the sound by detecting a change in capacitance between the two,
An oxide film formed on the silicon substrate and supporting the diaphragm;
The silicon layer forming the diaphragm and the underlying oxide film are separated into an inner region and an outer region by a separation groove, respectively.
The oxide film in the inner region is a ring-shaped oxide film, and forms an elastic hinge-like support structure that supports the diaphragm in the inner region at its outer peripheral portion,
The diaphragm includes a conductor portion serving as a counter electrode for forming the capacitance between the diaphragm and the back plate in a range smaller than the diaphragm and the back plate, and the conductor portion is doped with impurities in the diaphragm. It is formed by doing, and the acoustic sensor characterized by the above-mentioned.   The acoustic sensor according to claim 1, wherein the impurity is boron or phosphorus.   The acoustic sensor according to claim 2, wherein the silicon substrate is an SOI substrate.   The acoustic sensor according to claim 2, wherein the diaphragm is polysilicon.

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