Patent Publication Number: US-2017366104-A1

Title: Capacitive electromechanical transducer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of U.S. patent application Ser. No. 14/562,464 filed Dec. 5, 2014, which is a Continuation of U.S. patent application Ser. No. 13/377,925 filed Dec. 13, 2011, which now becomes U.S. Pat. No. 8,928,203 issued Jan. 6, 2015; which is a National Phase application of International Application PCT/JP2010/003974, filed Jun. 15, 2010, which claims the benefit of Japanese Patent Application No. 2009-146936, filed Jun. 19, 2009. The disclosures of the above-named applications are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a capacitive electromechanical transducer, such as a capacitive micromachined ultrasonic transducer. 
     BACKGROUND ART 
     Capacitive micromachined ultrasonic transducers (CMUTs) are proposed as electromechanical transducers that perform at least either of transmission and reception of ultrasonic waves (for example, refer to PCT Japanese Translation Patent Publication No. 2003-527947). The CMUTs are manufactured by using a Micro Electro Mechanical Systems (MEMS) process to which a semiconductor process is applied. 
       FIG. 7  is a schematic cross-sectional view of a typical CMUT. Referring to  FIG. 7 , a set of a first electrode  102  and a second electrode  105  is called a cell. The first electrode  102  is opposed to the second electrode  105  with a vibrating membrane  101  and a gap  104  sandwiched therebetween. The vibrating membrane  101  is supported by a supporter  103  formed on a substrate  106 . All the first electrodes  102  are electrically connected to each other in the CMUT. A certain level of direct-current (DC) voltage is uniformly applied to the first electrode  102  so that a desired potential difference is generated between the first electrode  102  and the second electrode  105 . The second electrodes  105  are electrically separated for every element (a collection of cells). An alternating-current (AC) drive voltage is applied to the second electrode  105  to produce an AC electrostatic attraction between the first and second electrodes, which vibrates the vibrating membrane  101  at a certain frequency to transmit ultrasonic waves. In addition, the vibrating membrane  101  vibrates in response to the ultrasonic waves that are received to generate a minute electric current caused by electrostatic induction at the second electrode  105 . The value of the electric current can be measured to acquire a reception signal for every element. 
     The characteristics in the transmission and reception of ultrasonic waves are determined by the amount of deflection of the vibrating membrane  101  when the DC voltage is applied to the first electrodes  102 . The pressure in the gap  104  of each cell is normally lower than the atmospheric pressure, and the vibrating membrane  101  is deflected toward the substrate  106  due to the difference between the atmospheric pressure and the pressure in the gap  104 . The amount of deflection of the vibrating membrane  101  is determined by mechanical characteristics based on parameters including the size, shape, thickness, and material of the vibrating membrane  101 . When the CMUT is operated, in order to increase the efficiency of the transmission and reception of ultrasonic waves, a certain potential difference is applied between the two electrodes to cause an electrostatic attraction between the electrodes. The vibrating membrane  101  is further deflected toward the substrate  106  due to this electrostatic attraction. In the transmission of ultrasonic waves, the efficiency of the transmission and reception is increased with the decreasing distance between the electrodes because the electrostatic attraction is in inverse proportion to the square of the distance. In contrast, in the reception of ultrasonic waves, the efficiency of the transmission and reception is also increased with the decreasing distance between the electrodes because the magnitude of the detected minute electric current is in inverse proportion to the distance between the electrodes and is in proportion to the potential difference between the electrodes. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     PCT Japanese Translation Patent Publication No. 2003-527947 
     SUMMARY OF INVENTION 
     However, increasing the potential difference between the two electrodes causes a force caused by the electrostatic force and the difference in pressure between the electrodes to exceed the restoration force of the mechanical characteristics of the vibrating membrane. As a result, the vibrating membrane is in contact with the electrode on the substrate to make a collapse state and, thus, the characteristics of the CMUT are greatly varied. Accordingly, when the CMUT is normally operated (is not driven in the collapse state), the potential difference between the electrodes is set so that a high efficiency of the transmission and reception is achieved and the vibrating membrane has an amount of deflection that does not cause the collapse state. 
     All the first electrodes are electrically connected to each other in the CMUT in related art. Accordingly, a uniform voltage is applied to the first electrodes during the operation of the CMUT. Since the above-mentioned parameters of the vibrating membranes are varied due to various factors in manufacture in the CMUT, a variation in the amount of deflection of the vibrating membrane is caused even with no potential difference between the electrodes. In addition, the amount of deflection of the vibrating membrane during the operation of the CMUT is also varied. 
     As a result, the efficiency of the transmission and reception is shifted from an expected value and/or the collapse state is sometimes caused in some of the cells to greatly vary the transmission and reception characteristics for every element. In order to resolve the above problems, the present invention provides a capacitive electromechanical transducer capable of arbitrarily varying the amount of deflection of the vibrating membrane for every element. 
     According to an embodiment of the present invention, an electromechanical transducer includes a plurality of elements including at least one cell that includes a first electrode and a second electrode opposed to the first electrode with a gap sandwiched therebetween and a direct-current voltage applying unit configured to be provided for each element and to separately apply a direct-current voltage to the first electrodes in each element. The first electrodes and the second electrodes are electrically separated for every element. 
     According to the present invention, it is possible to arbitrarily adjust the amount of deflection of the vibrating membrane in the electromechanical transducer for every element. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       [ FIG. 1 ] 
         FIG. 1  is a schematic cross-sectional view of an electromechanical transducer according to a first embodiment of the present invention. 
       [ FIG. 2 ] 
         FIG. 2  illustrates an example of the configuration of the electromechanical transducer according to the first embodiment of the present invention. 
       [ FIG. 3 ] 
         FIG. 3  is a cross-sectional view of an electromechanical transducer according to a second embodiment of the present invention. 
       [ FIG. 4 ] 
         FIG. 4  illustrates an example of the configuration of an electromechanical transducer according to a third embodiment of the present invention. 
       [ FIG. 5 ] 
         FIG. 5  illustrates an example of the configuration of an electromechanical transducer according to a fourth embodiment of the present invention. 
       [ FIG. 6A ] 
         FIG. 6A  illustrates an example of the configuration of an electromechanical transducer according to a fifth embodiment of the present invention. 
       [ FIG. 6B ] 
         FIG. 6B  illustrates another example of the configuration of the electromechanical transducer according to the fifth embodiment of the present invention. 
       [ FIG. 7 ] 
         FIG. 7  is a schematic cross-sectional view of a capacitive electromechanical transducer in related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Capacitive electromechanical transducers according to embodiments of the present invention will herein be described in detail with reference to the attached drawings. 
     First Embodiment 
       FIG. 1  is a schematic cross-sectional view of an electromechanical transducer according to a first embodiment of the present invention. The first electrode is called an upper electrode and the second electrode is called a lower electrode in the embodiments of the present invention. A vibrating membrane  101  on which an upper electrode  102  is formed is supported by a supporter  103  formed on a substrate  106 . The vibrating membrane  101  vibrates along with the upper electrode  102 . A lower electrode  105  is formed on the substrate  106  at a position that is opposed to the upper electrode  102  on the vibrating membrane  101  with a gap  104  sandwiched therebetween. A composition including the upper electrode and the lower electrode, which are opposed to each other with the gap  104  sandwiched therebetween, is called a cell  107  in the embodiments of the present invention. An element  108  includes at least one cell  107 . Specifically, the element  108  is a composition including one cell  107  or including multiple (at least two) cells that are electrically connected (connected in parallel) to each other. Although two cells compose one element in  FIG. 1 , the present invention is not limited to this configuration. Multiple cells may be connected to each other in a two-dimensional array pattern. Multiple (two or more) elements are formed in the electromechanical transducer. 
     The upper electrode used in the embodiments of the present invention may be made of at least one kind of metals including aluminum (Al), chromium (Cr), titanium (Ti), gold (Au), platinum (Pt), copper (Cu), silver (Ag), tungsten (W), molybdenum (Mo), tantalum (Ta), and Nickel (Ni) and alloys including AlSi, AlCu, AlTi, MoW, and AlCr. The upper electrode may be provided on at least one of on the upper face of the vibrating membrane, on the rear face of the vibrating membrane, and inside the vibrating membrane or, when the vibrating membrane is made of a conductive material or a semiconductor material, the vibrating membrane itself may serve as the upper electrode. The lower electrode used in the embodiments of the present invention may be made of the same metal or alloy as that of the upper electrode. When the substrate is made of a semiconductor material, such as silicon, the substrate may serve as the lower electrode. 
       FIG. 2  illustrates an example of the configuration of the electromechanical transducer according to the first embodiment of the present invention. The electromechanical transducer according to the first embodiment is characterized in that not only the lower electrodes  105  but also the upper electrodes  102  are electrically separated for every element. In each element, the multiple upper electrodes  102  are electrically connected to each other and the multiple lower electrodes  105  are electrically connected to each other. Although the upper electrodes in each element are separately formed for every cell and are electrically connected to each other via wiring lines (not shown) formed on the vibrating membrane in the first embodiment, one upper electrode may be formed for every element. The lower electrodes may also be separately formed for every cell, as in the first embodiment, or one lower electrode may be formed for every element. 
     A DC voltage applying unit  201  is connected to the upper electrodes  102  in each element. The DC voltage applying unit  201  applies a desired level of voltage to the upper electrodes for every element to cause a potential difference from the voltage of the lower electrodes between the upper and lower electrodes. The amount of deflection of the vibrating membranes  101  is determined by this potential difference. A drive detecting unit  202  is connected to the lower electrodes  105  in each element. The drive detecting unit  202  includes an AC voltage generating unit  203 , a current detecting unit  205 , and a protection switch  204 . 
     When the electromechanical transducer includes N-number elements, the DC voltage applying units  201  of the N-number are included in the configuration of the first embodiment. The drive detecting units  202  of the same number as that of the elements are also included in the configuration of the first embodiment. 
     An operation of each drive detecting unit  202  in transmission of ultrasonic waves and an operation thereof in reception of ultrasonic waves will now be described. In the transmission of ultrasonic waves, an AC voltage is applied by the AC voltage generating unit  203  connected to the lower electrodes  105 . The application of the AC voltage causes an AC potential difference between the upper electrodes  102  and the lower electrodes  105  to produce an AC electrostatic attraction on the vibrating membrane  101 . The electrostatic attraction causes the vibrating membrane  101  to vibrate to transmit the ultrasonic waves. Since the protection switch  204  connected to the lower electrodes  105  is turned off in the transmission of the ultrasonic waves, an input part of the current detecting unit  205  is protected from the voltage generated by the AC voltage generating unit  203 . 
     In the reception of ultrasonic waves, the AC voltage generating unit  203  is in a high-impedance state and has no effect on the electric potential of the lower electrodes  105 . The protection switch  204  is turned on to connect the lower electrodes  105  to an input part of the AC voltage generating unit  203 . The ultrasonic waves that are externally applied cause the vibrating membrane  101  to vibrate to vary the electrostatic capacitance between the upper and lower electrodes. Since the upper electrodes are fixed to a certain electric potential, the induction charge occurring at the lower electrodes  105  causes a minute electric current to pass through the wiring line for the lower electrodes  105 . A variation in the minute electric current can be detected by the current detecting unit  205  to detect the magnitude of the ultrasonic wave causing the variation in the capacitance. The electric potential of the lower electrodes is fixed to a certain value by the drive detecting unit  202  except when ultrasonic waves are transmitted. 
     Since not only the lower electrodes but also the upper electrodes are electrically separated for every element and the DC voltage applying unit is connected to the upper electrodes for every element in the configuration of the first embodiment, it is possible to separately apply the DC voltage to the upper electrodes in each element. Accordingly, different electrostatic attractions can be applied to different elements to adjust the amount of deflection of the vibrating membranes. Consequently, it is possible to reduce the variation in the transmission and reception characteristics of the ultrasonic waves. 
     Although the DC voltage applying unit  201  is connected to the upper electrodes  102  and the current detecting unit  205  is connected to the lower electrodes  105  in the first embodiment, the DC voltage applying unit may be connected to the lower electrodes and the current detecting unit may be connected to the upper electrodes. In addition, the configuration of the drive detecting unit is not limited to the one described in this description, another configuration, only the configuration used for the transmission, or only the configuration used for the reception may be used. 
     Second Embodiment 
     A second embodiment of the present invention will now be described with reference to  FIG. 3 . The second embodiment concerns the configuration of wiring lines from the upper electrodes and the lower electrodes. The remaining configuration in the second embodiment is the same as in the first embodiment. 
       FIG. 3  is a cross-sectional view of an electromechanical transducer according to the second embodiment of the present invention. A through wiring line substrate  303  includes two kinds of wiring lines passing through the substrate: a lower-electrode through wiring line  301  (second-electrode through wiring line) and an upper-electrode through wiring line  302  (first-electrode through wiring line). All the lower electrodes  105  in each element are connected to one lower-electrode through wiring line  301 . The lower-electrode through wiring line  301  extends from the face of the through wiring line substrate  303  toward the lower electrodes to the face thereof toward a printed circuit board to be connected to the corresponding bump electrode  304 . The upper electrodes  102  in each element are also connected to one upper-electrode through wiring line  302 , which is connected to the corresponding bump electrode  304 . 
     A bump  305  is formed on each of the bump electrodes  304  formed on the face of the through wiring line substrate  303  toward the printed circuit board. The lower-electrode through wiring line  301  and the upper-electrode through wiring line  302  are connected to wiring lines on a printed circuit board  306  via the bumps  305 . An electrical signal from each of the lower electrodes  105  is supplied to the drive detecting unit  202  through the wiring line on the printed circuit board  306  electrically connected to the lower-electrode through wiring line  301 . An electrical signal from each of the upper electrode  102  is supplied to the DC voltage applying unit  201  through the wiring line on the printed circuit board  306  electrically connected to the upper-electrode through wiring line  302 . 
     The configuration in the second embodiment is characterized by the presence of the lower-electrode through wiring lines  301  and the upper-electrode through wiring lines  302  of the same number as that of elements. The presence of the lower-electrode through wiring lines  301  and the upper-electrode through wiring lines  302  of the same number as that of elements allows the wiring lines for the upper electrodes to be led to the rear face of the through wiring line substrate with the wiring lines for the upper electrodes  102  separated for every element even when the multiple elements are provided. Accordingly, it is possible to connect the wiring lines for the upper electrodes to the multiple DC voltage applying units  201  with little reduction in the area of the elements used for the transmission and reception of ultrasonic waves (with little reduction in the efficiency of the transmission and reception). 
     Third Embodiment 
     A third embodiment of the present invention will now be described with reference to  FIG. 4 . The third embodiment is characterized in that a control-signal generating unit is provided to instruct a DC voltage to be applied to the DC voltage applying unit  201 . The remaining configuration in the third embodiment is the same as in either of the first and second embodiments. 
       FIG. 4  illustrates an example of the configuration of an electromechanical transducer according to the third embodiment of the present invention. A vibrating-membrane state detecting unit  401  detects the amount of deflection of the vibrating membranes  101  (this is equivalent to detection of the distance between the upper electrodes and the corresponding lower electrodes). Since the magnitude of a current detected at each lower electrode is in reverse proportion to the square of the distance between the upper electrode and the lower electrode (hereinafter simply referred to as an electrode distance) and is in proportion to the potential difference between the electrodes, the current detecting unit that is connected to the lower electrodes serves as the vibrating-membrane state detecting unit in the third embodiment. 
     The use of the vibrating-membrane state detecting unit  401  of the third embodiment allows a difference in the amount of deflection of the vibrating membranes between the elements to be detected by, for example, externally transmitting ultrasonic waves of a single frequency and detecting the current output from the lower electrodes for every element. Alternatively, an AC voltage may be superimposed on the DC voltage to be applied to the upper electrodes to detect the current generated by the superimposed AC voltage (described below as a fourth embodiment). A unit other than the unit for detecting the current may be used as the vibrating-membrane state detecting unit to, for example, directly measure the amount of deflection of the vibrating membranes. Specifically, a method of detecting the deflection of the vibrating membrane by using a piezoresistive effect or a method of optically detecting the amount of deflection may be used. 
     A signal detected by each vibrating-membrane state detecting unit  401  is supplied to the corresponding control-signal generating unit  402 . The control-signal generating unit  402  supplies a signal to instruct a DC voltage to be applied to the DC voltage applying unit  201  on the basis of the detected signal so that the vibrating membrane  101  in the CMUT has a desired amount of deflection. The DC voltage applying unit  201  generates a DC voltage on the basis of the signal instructed by the control-signal generating unit  402  and applies the generated DC voltage to the upper electrodes  102 . The DC voltage applying unit  201  performing the above operation can be easily formed by the use of, for example, a voltage-control-signal transmitting circuit and a capacitor. 
     According to the third embodiment, it is possible to detect the amount of deflection of the vibrating membranes  101  for every element. In addition, since the DC voltage can be applied to the upper electrodes in each element so as to reduce a variation in the amount of deflection between the elements, the difference in the amount of deflection between the vibrating membranes can be further reduced. Furthermore, even if the parameters affecting the vibrating membranes  101  are varied due to the variation with time and/or the change in the environment, it is possible to adjust the state of the vibrating membranes  101  for every element. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will now be described with reference to  FIG. 5 . The fourth embodiment is characterized in that an AC-voltage superimposing unit  403  is provided to superimpose an AC voltage on the DC voltage to be applied to the upper electrodes. The remaining configuration in the fourth embodiment is the same as in the third embodiment.  FIG. 5  illustrates an example of the configuration of an electromechanical transducer according to the fourth embodiment of the present invention. 
     One AC-voltage superimposing unit  403  is connected to the upper electrodes  102  in each element. The AC-voltage superimposing unit  403  superimposes an AC voltage on the DC voltage to be applied to the upper electrodes  102  by the DC voltage applying unit  201  only during a period in which the amount of deflection of the vibrating membranes is detected (the variation in the amount of deflection is measured). The AC voltage superimposed on the DC voltage to be applied to the upper electrodes  102  induces an electric charge on each of the lower electrodes  105  even if the vibrating membrane  101  does not vibrate to cause a current from the lower electrode  105 . The current has a value of the magnitude corresponding to the electrode distance between the upper electrode  102  and the lower electrode  105  if the AC voltage to be superimposed has a constant level. Accordingly, the detection of the current by the current detecting unit  205  allows the amount of deflection of the vibrating membranes  101  to be detected as the electrode distance. 
     The AC voltage to be superimposed may have a frequency that is not equal to the frequency at which the vibrating membrane  101  vibrates. This allows only the electrode distance to be detected without vibrating the vibrating membrane  101  with the AC voltage that is superimposed. 
     A signal switching unit  404  is connected to an output part of each current detecting unit  205 . The signal switching unit  404  supplies an output signal to the control-signal generating unit  402  during a period in which the variation in the amount of deflection is measured. In contrast, the signal switching unit  404  supplies an output signal to, for example, an external image processing apparatus as the output from the sensor during a period in which the variation in the amount of deflection is not measured (the ultrasonic wave is measured on the basis of the vibration of the vibrating membrane). The provision of the signal switching unit  404  allows the current detecting unit  205  to be used both in the measurement of the variation in the amount of deflection and in the measurement of ultrasonic waves. 
     The control-signal generating unit  402  supplies a signal instructing the DC voltage to the DC voltage applying unit  201  on the basis of the received signal so that the vibrating membranes  101  have a desired amount of deflection. 
     With the configuration of the fourth embodiment, the superimposition of the AC voltage and the detection of the current generated on the lower electrodes by the current detecting unit allow the amount of deflection of the vibrating membranes to be detected for every element. In addition, setting the frequency of the AC voltage to be superimposed to a value that is not equal to the value of the frequency at which the vibrating membrane vibrates allows the amount of deflection of the vibrating membranes (the electrode distance between the upper and lower electrodes) to be detected without vibrating the vibrating membrane. Accordingly, mechanical vibration characteristics of the vibrating membrane can be removed to achieve the measurement with a higher accuracy. Since the DC voltage can be applied to the upper electrodes for every element so as to reduce the variation in the amount of deflection between the elements, it is possible to reduce the difference in the amounts of deflection between the vibrating membranes. In addition, it is possible to adjust the amount of deflection of the vibrating membranes  101  for every element even if the parameters affecting the vibrating membranes  101  are varied due to the variation with time and/or the change in the environment. Furthermore, since the current detecting unit  205  can be used both in the measurement of the variation in the amount of deflection and in the measurement of ultrasonic waves, it is possible to realize the electromechanical transducer with a simple configuration. 
     Fifth Embodiment 
     A fifth embodiment of the present invention will now be described with reference to  FIGS. 6A and 6B . The fifth embodiment is characterized in that the current detecting unit  205  switches circuit parameters between in the measurement of the variation in the amount of deflection and in the measurement of ultrasonic waves. The remaining configuration in the fifth embodiment is the same as in the fourth embodiment. 
     A transimpedance circuit, which is a current-voltage conversion circuit that converts a change in a minute electric current into a voltage, is used to describe the fifth embodiment.  FIGS. 6A and 6B  illustrate examples of the configuration of the transimpedance circuit, which is the current detecting unit  205  according to the fifth embodiment. Referring to  FIGS. 6A and 6B , reference numeral  601  denotes an operational amplifier (op-amp), reference numerals  602 ,  604 , and  606  denote resistors, reference numerals  603 ,  605 , and  607  denote capacitors, and reference numeral  608  denotes a circuit-element switching unit. 
     In the examples in  FIGS. 6A and 6B , the op-amp  601  is connected to positive and negative power supplies. An operation in the measurement of ultrasonic waves will now be described with reference to  FIG. 6A . An inverting input terminal −IN of the op-amp  601  is connected to the upper electrodes  102  via the protection switch  204 . An output terminal OUT of the op-amp  601  is connected to the inverting input terminal −IN via the resistor  602  and the capacitor  603 , which are connected in parallel to each other, and the circuit-element switching unit  608  to feed back an output signal. A non-inverting input terminal +IN of the op-amp  601  is connected to a ground terminal via the resistor  604  and the capacitor  605 , which are connected in parallel to each other. The voltage of the ground terminal is equal to an intermediate value between the voltage value of the positive power supply and the voltage value of the negative power supply. The resistor  602  has the same resistance value as that of the resistor  604  and the capacitor  603  has the same capacitance value as that of the capacitor  605 . The resistance values of the resistors  602  and  604  and the capacitance values of the capacitors  603  and  605  are parameters matched with the specifications in the measurement of ultrasonic waves. 
     An operation in the measurement of the variation in the amount of deflection will now be described with reference to  FIG. 6B . In the measurement of the variation in the amount of deflection, the circuit-element switching unit  608  is switched and the feedback of an output signal is performed by the resistor  606  and the capacitor  607 , which are connected in parallel to each other. The resistance value of the resistor  606  and the capacitance value of the capacitor  607  are parameters matched with the specifications in the measurement of the variation in the amount of deflection. 
     According to the fifth embodiment, it is possible to perform the current detection in accordance with the specifications including the frequency and the magnitude of the current used in the measurement of the variation in the amount of deflection and the specifications including the frequencies of the ultrasonic waves and the magnitude of the current used in the measurement of ultrasonic waves. Although the circuit-element switching unit  608  is used only in the feedback part of the op-amp  601  in the examples in  FIGS. 6A and 6B , the same configuration may be used between the non-inverting input terminal +IN of the op-amp and the ground terminal to switch element constants in accordance with the switch between in the measurement of the variation in the amount of deflection and in the measurement of the ultrasonic waves. 
     Although the vibrating membrane  101  is described to be operated in a conventional mode in which the gap between the vibrating membrane  101  and the lower electrode  105  constantly exists during the transmission and reception operation of the electromechanical transducer in the above embodiments, the present invention is not limited to the above operation. The present invention is applicable to the operation in another state, such as a collapse mode in which part of the gap between the vibrating membrane  101  and the lower electrode  105  is eliminated. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     REFERENCE SIGNS LIST 
       101  Vibrating membrane 
       102  Upper electrode (first electrode) 
       103  Supporter 
       104  Gap 
       105  Lower electrode (second electrode) 
       106  Substrate 
       201  DC voltage applying unit 
       202  Drive detecting unit 
       203  AC voltage generating unit 
       204  Protection switch 
       205  Current detecting unit 
       301  Lower-electrode through wiring line 
       302  Upper-electrode through wiring line 
       303  Through wiring line substrate 
       401  Vibrating-membrane state detecting unit 
       402  Control-signal generating unit 
       403  AC-voltage superimposing unit 
       404  Signal switching unit 
       608  Circuit-element switching unit