Patent Publication Number: US-8991252-B2

Title: Displacement amount monitoring electrode arrangement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application of International Application No. PCT/JP2011/069333, filed Aug. 26, 2011, the content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention is related to a displacement amount monitoring electrode arrangement. In particular, the present invention is related to a displacement amount monitoring electrode arrangement in which a fixed electrode fixed with respect to a substrate and a movable electrode configured to be movable in a predetermined axis direction parallel to the substrate, each of which has a comb shape formed by a base portion and plural electrode fingers extending from the base portion in the predetermined axis direction, are arranged such that the electrode fingers of the fixed electrode are engaged with the electrode fingers of the movable electrode, wherein the displacement amount monitoring electrode arrangement is configured to monitor a displacement amount of a detection mass which is to be driven to have target amplitude based on a change amount of a capacitance between the fixed electrode and the movable electrode. 
     BACKGROUND ART 
     JP 2008-170455 A discloses a displacement amount monitoring electrode arrangement for an angular velocity sensor, etc., which is configured to monitor a displacement amount of a detection mass so as to drive the detection mass with constant target amplitude. The displacement amount monitoring electrode includes a fixed electrode with a comb shape and a movable electrode with a comb shape. The fixed electrode is formed by a base portion and plural electrode fingers extending from the base portion in a predetermined axis direction, and is fixed with respect to a substrate. The movable electrode is formed by a base portion and plural electrode fingers extending from the base portion in the predetermined axis direction parallel to the substrate, and is movable in the predetermined axis direction with respect to the substrate. In such a displacement amount monitoring electrode, when the movable electrode is displaced in the predetermined axis direction with respect to the substrate, a capacitance between the fixed electrode and the movable electrode changes. At that time, a change amount of the capacitance depends on the displacement amount of the movable electrode. Then, the displacement amount (amplitude) of the detection mass is monitored based on the change amount of the capacitance, and the detection mass is controlled such that it is driven to have target amplitude. 
     In general, the change amount of the capacitance between the fixed electrode and the movable electrode changes linearly with the amplitude of the detection mass; however, it changes because of not only the amplitude of the detection mass but also a gap and an opposed area between the fixed electrode and the movable electrode. Specifically, if the gap and the opposed area change due to variations in stress or dimensions, etc., sensitivity (i.e., a gradient) of the change amount of the capacitance with respect to the displacement amount of the movable electrode changes accordingly. According to a configuration in which a target capacitance change amount of the capacitance when the detection mass is driven with the target amplitude (i.e., when the displacement of the movable electrode in the predetermined axis direction reaches a target displacement amount corresponding to the target amplitude) is set in the linear change region in which the change amount of the capacitance changes linearly with the displacement of the movable electrode, if the gap and the opposed area don&#39;t match with desired values due to the stress applied at the time of assembling the monitoring electrode, temperature change, variation in the dimensions, etc., there may be a case where the displacement amount of the movable electrode does not match the target displacement amount when the change amount of the capacitance reaches the target capacitance change amount, which makes it difficult to keep the amplitude of the detection mass at a constant target amplitude. 
     SUMMARY OF INVENTION 
     The present invention is made in consideration of the matters as described above, and it is an object of the present invention to provide a displacement amount monitoring electrode arrangement which can maintain amplitude of a detection mass at a constant target amplitude even if a relative relationship between a fixed electrode and a movable electrode changes. 
     The objects are achieved by a displacement amount monitoring electrode arrangement in which a fixed electrode fixed with respect to a substrate and a movable electrode configured to be movable in a predetermined axis direction parallel to a substrate, each of which has a comb shape formed by a base portion and plural electrode fingers extending from the base portion in the predetermined axis direction, are arranged such that the electrode fingers of the fixed electrode are engaged with the electrode fingers of the movable electrode, wherein the displacement amount monitoring electrode arrangement is configured to monitor a displacement amount of a detection mass which is to be driven to have a target amplitude based on a variation in a capacitance between the fixed electrode and the movable electrode, there are at least one linear change region in which the variation in the capacitance changes linearly with the displacement of the movable electrode in the predetermined axis direction, and at least one nonlinear change region in which the variation in the capacitance changes nonlinearly with the displacement of the movable electrode in the predetermined axis direction, the nonlinear change region includes a characteristic in which a change sensitivity of the variation in the capacitance with respect to the displacement amount of the movable electrode in the predetermined axis direction is greater than that in the linear change region, and a target capacitance change amount of the capacitance when the displacement of the movable electrode in the predetermined axis direction reaches a target displacement amount corresponding to the target amplitude is set in the nonlinear change region. 
     According to the present invention, it is possible to maintain amplitude of a detection mass at constant target amplitude even if a relative relationship between a fixed electrode and a movable electrode changes. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of a configuration of a sensor which adopts a displacement amount monitoring electrode arrangement according to a first embodiment of the present invention. 
         FIG. 2  is a plan view of a displacement amount monitoring electrode according to the embodiment. 
         FIG. 3  is a cross sectional view of the displacement amount monitoring electrode according to the embodiment. 
         FIG. 4  is a cross sectional view of the displacement amount monitoring electrode when it is deformed because of a change in stress. 
         FIG. 5  is a diagram for illustrating a relationship between a drive displacement amount x of a movable electrode and a capacitance change amount ΔC between a fixed electrode and the movable electrode, which relationship changes due to variations in dimensions of the displacement amount monitoring electrode. 
         FIG. 6  is a diagram for illustrating a relationship between a drive displacement amount x of a movable electrode and a capacitance change amount ΔC between a fixed electrode and the movable electrode in the displacement amount monitoring electrode according to the embodiment. 
         FIG. 7  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 8  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 9  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 10  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 11  is a perspective view of a displacement amount monitoring electrode according to a variant. 
         FIG. 12A  is a side view of the displacement amount monitoring electrode illustrated in  FIG. 11 . 
         FIG. 12B  is a plan view of the displacement amount monitoring electrode illustrated in  FIG. 11 . 
         FIG. 13  is a perspective view of a displacement amount monitoring electrode according to a variant. 
         FIG. 14A  is a side view of the displacement amount monitoring electrode illustrated in  FIG. 13 . 
         FIG. 14B  is a plan view of the displacement amount monitoring electrode illustrated in  FIG. 13 . 
         FIG. 15  is a perspective view of a displacement amount monitoring electrode according to a variant. 
         FIG. 16  is a perspective view of a displacement amount monitoring electrode according to a variant. 
         FIG. 17  is a perspective view of a displacement amount monitoring electrode according to a variant. 
         FIG. 18A  is a side view of the displacement amount monitoring electrode illustrated in  FIG. 17 . 
         FIG. 18B  is a plan view of the displacement amount monitoring electrode illustrated in  FIG. 17 . 
         FIG. 19  is a diagram for illustrating a relationship between a drive displacement amount x of a movable electrode and a capacitance change amount ΔC between a fixed electrode and a movable electrode in the displacement amount monitoring electrode illustrated in  FIG. 17 . 
         FIG. 20  is a plan view of a displacement amount monitoring electrode according to a second embodiment. 
         FIG. 21  is a diagram for illustrating a relationship between a drive displacement amount x of a movable electrode and a capacitance change amount ΔC between a fixed electrode and the movable electrode in the displacement amount monitoring electrode according to the embodiment. 
         FIG. 22  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 23  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 24  is a plan view of a displacement amount monitoring electrode according to a variant. 
         FIG. 25  is a plan view of a displacement amount monitoring electrode according to a variant. 
     
    
    
     EXPLANATION FOR REFERENCE NUMBERS 
     
         
         
           
               10 ,  28 - 1 ,  28 - 2 ,  30 - 1 ,  30 - 2 ,  100  displacement amount monitoring electrode 
               14  semiconductor substrate 
               16 ,  18  structure body 
               32  fixed electrode 
               34  movable electrode 
               32 - 1 ,  34 - 1  base portio 
               32 - 2 ,  34 - 2  electrode finger 
               102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  dielectric material 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following, concrete embodiments of a displacement amount monitoring electrode according the present invention will be described in detail by referring to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram of a configuration of a sensor  12  which adopts a displacement amount monitoring electrode arrangement  10  according to a first embodiment of the present invention. The sensor  12  according to the embodiment is an angular velocity sensor which is provided on a vehicle, for example, for detecting an angular velocity around a Z axis perpendicular to an X-Y plane. The sensor  12  is formed on a semiconductor substrate  14  such as silicon. The sensor  12  is formed by performing etching on a surface of the semiconductor substrate  14  with a micromachining technique. 
     The sensor  12  includes a pair of structure bodies  16 ,  18  having the same mass; drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2  for driving the structure bodies  16 ,  18  in an X axis direction on the semiconductor substrate  14 ; detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  for detecting vibrations (amplitudes) of the structure bodies  16 ,  18  in a Y axis direction on the semiconductor substrate  14 ; and displacement amount monitoring electrodes  28 - 1 ,  28 - 2 ,  30 - 1 ,  30 - 2  for monitoring drive displacement amounts of the structure bodies  16 ,  18  in the X axis direction. The structure bodies  16 ,  18  are disposed symmetrically with each other on the semiconductor substrate  14  such that the structure bodies  16 ,  18  are floated over the surface of the semiconductor substrate  14 , which forms an insulating layer, by a predetermined distance. Hereinafter, the displacement amount monitoring electrodes  28 - 1 ,  28 - 2 ,  30 - 1 ,  30 - 2  are also referred to as a displacement amount monitoring electrode  10  collectively. 
     Each of the drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2  and the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  includes a fixed electrode fixed with respect to the substrate  14  and a movable electrode configured to be movable in the X or Y axis direction parallel to the substrate  14 . The fixed electrodes are connected to a signal processing part via pads. Further, the movable electrodes form parts of the structure bodies  16 ,  18 . 
     The drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2  apply drive voltages to the fixed electrodes to generate electrostatic attraction between the fixed electrodes and the movable electrodes in order to drive the structure bodies  16 ,  18  with respect to the semiconductor substrate  14  in the X axis direction. Further, the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  detect changes in capacitances between the fixed electrodes and the movable electrodes due to the displacements of the structure bodies  16 ,  18  in the Y axis direction in order to detect vibrations of the structure bodies  16 ,  18  in the Y axis direction with respect to the semiconductor substrate  14 . 
     Specifically, the drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2  generate the electrostatic attraction between the fixed electrodes and the movable electrodes by applying the drive voltages whose frequencies are substantially the same as a resonance frequency of the structure bodies  16 ,  18  to the fixed electrodes in order to generate drive forces for driving the structure bodies  16 ,  18  with constant amplitudes in the X axis direction with frequencies which are substantially the same as the resonance frequency of the structure bodies  16 ,  18 . It is noted that the drive electrodes  20 - 1 ,  20 - 2  generate the drive forces in phase and the drive electrodes  22 - 1 ,  22 - 2  generate the drive forces in phase, while the drive electrodes  20 - 1 ,  20 - 2  generate the drive forces in opposite phase with respect to the drive electrodes  22 - 1 ,  22 - 2 . 
     Further, in the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2 , capacitance changes occur between the fixed electrodes and the movable electrodes according to the vibration displacements of the structure bodies  16 ,  18  in the Y axis direction with respect to the semiconductor substrate  14 . It is noted that the capacitance changes in the detection electrodes  24 - 1 ,  24 - 2  are in opposite phase with respect to the capacitance changes in the detection electrodes  26 - 1 ,  26 - 2 . The capacitance changes in the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  are substantially zero when the vibration displacement amounts of the structure bodies  16 ,  18  in the Y axis direction are zero. The greater the vibration displacement amounts of the structure bodies  16 ,  18  in the Y axis direction are, the greater the capacitance changes in the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  become. The detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  output the capacitance changes between the fixed electrodes and the movable electrodes as detection displacement signals to a signal processing circuit. The signal processing circuit processes the detection signals from the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  to detect the vibration displacement amounts of the structure bodies  16 ,  18  in the Y axis direction and detect the angular velocity around the Z axis based on the vibration displacement amounts. 
     In the following, an operation of the sensor  12  according to the embodiment will be described. 
     If the detection of the angular velocity around the Z axis which is perpendicular to the X and Y axes is performed, the drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2  in the sensor  12  are driven. Specifically, the drive voltages which have the same frequency as the resonance frequency of the structure bodies  16 ,  18  are applied to the fixed electrodes of the drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2 . If such drive voltages are applied to the drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2 , drive forces for exciting the structure bodies  16 ,  18  in the X axis direction are generated between the fixed electrodes and the movable electrodes of the respective drive electrodes  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2 . Thus, the structure bodies  16 ,  18  are excited with constant amplitudes in opposite phase in the X axis direction at a frequency which is substantially the same as the resonance frequency. 
     If the angular velocity around the Z axis is not generated under a status where the structure bodies  16 ,  18  are excited in the X axis direction as described above, a Coriolis force does not act on the structure bodies  16 ,  18 . In this case, the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  are not oscillated in the Y axis direction, and thus there is no change in capacitance between the fixed electrodes and the movable electrodes of the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2 . As a result of this, the detection displacement signals output from the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  represent that the amplitudes of the structure bodies  16 ,  18  in the Y axis direction is substantially zero. 
     On the other hand, if the angular velocity around the Z axis is generated under a status where the structure bodies  16 ,  18  are excited in the X axis direction as described above, a Coriolis force acts on the structure bodies  16 ,  18 . In this case, the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  are oscillated in the Y axis direction due to the Coriolis force, and thus there are changes in capacitances generated between the fixed electrodes and the movable electrodes of the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2 . When such changes in capacitances are generated, the detection displacement signals output from the detection electrodes  24 - 1 ,  24 - 2 ,  26 - 1 ,  26 - 2  represent the magnitude of the angular velocity which is related to the amplitudes of the structure bodies  16 ,  18  in the Y axis direction. The structure bodies  16 ,  18  are oscillated in opposite phase with respect to the Y axis direction when the Coriolis force acts. Thus, with the sensor  12 , it is possible to detect the angular velocity which is generated in an object around the Z axis. 
       FIG. 2  is a plan view of a displacement amount monitoring electrode  10  according to the embodiment.  FIG. 3  is a cross sectional view of the displacement amount monitoring electrode  10  according to the embodiment. 
     The displacement amount monitoring electrode  10  includes a fixed electrode  32  which is fixed with respect to the semiconductor substrate  14 ; and a movable electrode  34  which movable in the X axis direction with respect to the semiconductor substrate  14 . The fixed electrode  32  is connected to a signal processing part via a pad. Further, the movable electrode  34  forms a part of the structure bodies  16 ,  28 . The displacement amount monitoring electrode  10  is provided for detecting the change in the capacitance between the fixed electrode  32  and the movable electrode  34  due to the displacements of the structure bodies  16 ,  18  in the X axis direction so as to monitor the drive displacement amounts of the structure bodies  16 ,  18  in the X axis direction with respect to the semiconductor substrate  14 . 
     The fixed electrode  32  and the movable electrode  34  are formed to have comb shapes, and include base portions  32 - 1 ,  34 - 1  having relatively great width and electrode fingers  32 - 2 ,  34 - 2  extending from the base portions  32 - 1 ,  34 - 1  in the X axis direction parallel to the semiconductor substrate  14 , respectively. The electrode fingers  32 - 2 ,  34 - 2  are formed to have rectangular cross sectionals, and have equal cross sectional areas (i.e., constant widths w1, w2 in the Y axis direction and constant thickness T1, T2 in the Z axis direction) over a range from the connection portion with the base portions  32 - 1 ,  34 - 1  to the tip portions, respectively. More than two electrode fingers  32 - 2 ,  34 - 2  are provided for the base portions  32 - 1 ,  34 - 1 , respectively, and arranged side by side in the Y axis direction with equal spacing. The fixed electrode  32  and the movable electrode  34  are disposed such that the base portions  32 - 1 ,  34 - 1  are opposed in the X axis direction and the electrode fingers  32 - 2 ,  34 - 2  are opposed in the Y axis direction. 
     The electrode fingers  32 - 2  of the fixed electrode  32  are formed to have the same width w1 in the Y axis direction, the same thickness T1 in the Z axis direction, and the same length in the X axis direction. The electrode fingers  34 - 2  of the movable electrode  34  are formed to have the same width w2 in the Y axis direction, the same thickness T2 in the Z axis direction, and the same length in the X axis direction. 
     If the movable electrode  34  is not moved in the X axis direction, there is a gap d1 in the X axis direction between the tip portions of the electrode finger  32 - 2  of the fixed electrode  32  and the base portion  34 - 1  of the movable electrode  34 , and there is a gap d2 in the X axis direction between the base portion  32 - 1  of the fixed electrode  32  and the tip portions of the electrode finger  34 - 2  of the movable electrode  34 . 
     The electrode fingers  34 - 2  of the movable electrode  34  are inserted between corresponding two electrode fingers  32 - 2  of the fixed electrode  32 . There is a gap dm in the Y axis direction between the electrode finger  32 - 2  of the fixed electrode  32  and the electrode finger  34 - 2  of the movable electrode  34 . It is noted that the electrode fingers  34 - 2  may be disposed at midpoint between the corresponding two electrode fingers  32 - 2 , or may be biased to one of the corresponding two electrode fingers  32 - 2 . If the electrode fingers  34 - 2  are disposed such that they are biased to one of the corresponding two electrode fingers  32 - 2 , different gaps dm1, dm2 are formed on the opposite sides of the electrode fingers  34 - 2  in the Y axis direction. On the other hand, if the electrode fingers  34 - 2  are disposed at midpoint between the corresponding two electrode fingers  32 - 2 , the same gaps dm1, dm2 are formed on the opposite sides of the electrode fingers  34 - 2  in the Y axis direction. 
     Further, an overlapped thickness between the electrode fingers  32 - 2  and the electrode fingers  34 - 2  in the thickness direction, that is to say, the Z axis direction (i.e., a height of a part of the side wall of the electrode fingers  32 - 2  which are opposed to the side wall of the electrode fingers  34 - 2  in the Z axis direction) is Tm. It is noted that the thickness Tm on the side of the gap dm1 (i.e., Tm1) may be different from that on the side of the gap dm2 (i.e., Tm2). However, it is desirable that a relationship T1=T2=Tm (i.e., T1=T2=Tm1=Tm2) is met. 
     According to the displacement amount monitoring electrode  10  having the construction described above, when the structure bodies  16 ,  18  are excited in the X axis direction, the movable electrode  34  is displaced in the X axis direction due the excitation of the structure bodies  16 ,  18 . In this case, there is a capacitance change between the fixed electrode  32  and the movable electrode  34  according to the displacement of the movable electrode  34 . It is noted that the capacitance changes in the displacement amount monitoring electrodes  28 - 1 ,  28 - 2  are in opposite phase with respect to the capacitance changes in the displacement amount monitoring electrodes  30 - 1 ,  30 - 2 . The greater the drive displacement amounts of the structure bodies  16 ,  18  in the X axis direction are, the greater the capacitance change of the displacement amount monitoring electrode  10  becomes. 
     The fixed electrode of the displacement amount monitoring electrode  10  outputs the capacitance change between the fixed electrode  32  and the movable electrode  34  as a drive displacement amount monitor signal to the signal processing circuit. The signal processing circuit processes the drive displacement amount monitor signal from the displacement amount monitoring electrode  10  to monitor the drive displacement amounts of the structure bodies  16 ,  18  in the X axis direction. The drive voltages applied to the drive electrodes  20 - 1 ,  20 - 2  are controlled based on the drive displacement amounts such that the structure bodies  16 ,  18  are excited with constant amplitudes in the X axis direction. 
     In order to make the sensor  12  detect the angular velocity around the Z axis with high accuracy, it is necessary that the amplitudes of the structure bodies  16 ,  18  as detection masses excited in the X axis direction are always constant. In order to keep the driven amplitudes of the structure bodies  16 ,  18  in the X axis direction constant, it is general to maintain the capacitance change amount between the fixed electrode  32  and the movable electrode  34  of the displacement amount monitoring electrode  10 . Thus, it is appropriate to control the drive voltages applied to the drive electrodes  20 - 1 ,  20 - 2  such that the capacitance change amount is kept constant. Specifically, it is appropriate that the control is performed assuming that the drive displacement amounts of the structure bodies  16 ,  18  reach target displacement amounts (target amplitudes) when the capacitance change amount reaches a target capacitance change amount. 
       FIG. 4  is a cross sectional view of the displacement amount monitoring electrode  10  when it is deformed because of a change in stress.  FIG. 5  is a diagram for illustrating a relationship (gradient) between the drive displacement amount x of the structure bodies  16 ,  18 , that is to say, the movable electrode  34  and the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34 , which relationship changes due to variations in stress or dimensions of the displacement amount monitoring electrode  10 . “0” of the drive displacement amount x corresponds to the positions of the tips of the electrode fingers  34 - 2  in a case where the movable electrode  34  is not moved in the X axis direction. A side in which the tips of the electrode fingers  34 - 2  are close to the base portion  32 - 1  of the fixed electrode  32  corresponds to x&gt;0, and a side in which the tips of the electrode fingers  34 - 2  are away from the base portion  32 - 1  of the fixed electrode  32  corresponds to x&lt;0. 
     However, the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  changes due to not only the drive displacement amount x of the structure bodies  16 ,  18  but also a gap and a opposed area between the fixed electrode  32  and the movable electrode  34 . In other words, the relationship between the drive displacement amount x and the capacitance change amount ΔC changes according to the gap and the opposed area. 
     The electrode fingers  34 - 2  are inserted between corresponding two of the electrode fingers  32 - 2 ; however, if the inserted amount is not so great, the capacitance change between the fixed electrode  32  and the movable electrode  34  is generated only between the electrode fingers  32 - 2  and the electrode fingers  34 - 2 . In this case, the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  changes linearly with respect to the drive displacement amount x of the movable electrode  34  (linear change region). On the other hand, if the inserted amount of the electrode fingers  34 - 2  of the movable electrode  34  between corresponding two of the electrode fingers  32 - 2  of the fixed electrode  32  becomes greater, the capacitance change between the fixed electrode  32  and the movable electrode  34  is generated not only between the electrode fingers  32 - 2  and the electrode fingers  34 - 2  but also between the base portion  32 - 1  and the tips of the electrode fingers  34 - 2  and between the tips of the electrode fingers  32 - 2  and the base portion  34 - 1 . In this case, the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  changes nonlinearly with respect to the drive displacement amount x of the movable electrode  34  (nonlinear change region). 
     As illustrated in  FIG. 5 , in the case of a configuration in which a target capacitance change amount C0 between the fixed electrode  32  and the movable electrode  34  at a point where the movable electrode  34  reaches a target displacement amount A0 corresponding to the target amplitudes of the structure bodies  16 ,  18  is set in the linear change area, if the gap and the opposed area between the fixed electrode  32  and the movable electrode  34  greatly deviate from desired ones due to the stress change or dimensional variation of the displacement amount monitoring electrode  10 , the drive displacement amount x of the movable electrode  34  greatly deviates from the target displacement amount A0 when the capacitance change amount ΔC reaches target capacitance change amount C0, which makes it difficult to keep the amplitudes of the structure bodies  16 ,  18  constant. 
       FIG. 6  is a diagram for illustrating a relationship between the drive displacement amount x of the structure bodies  16 ,  18 , that is to say, the movable electrode  34  and the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  according to the displacement amount monitoring electrode  10  of the embodiment. 
     The capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  is calculated according to the following formula (1). ∈ indicates relative dielectric constant between the fixed electrode  32  and the movable electrode  34 , and N indicates the number of the opposed electrodes of the displacement amount monitoring electrode  10  (specifically, the number of the electrode fingers  34 - 2  of the movable electrode  34 ). It is noted that in the right-hand side of the formula (1) the first and second terms represent the linear change area, and the third and fourth terms represent the nonlinear change region. 
     
       
         
           
             
               
                 
                   
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     In the embodiment, the displacement amount monitoring electrode  10  is configured such that the following criteria are met. In the course of the driven displacement of the movable electrode  34  with the target displacement amount A0 corresponding to the target amplitudes of the structure bodies  16 ,  18 , the relationship between the drive displacement amount x of the movable electrode  34  and the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  includes the linear change area in which the capacitance change amount ΔC changes linearly with the driven displacement of the movable electrode  34  and the nonlinear change region in which the capacitance change amount ΔC changes nonlinearly. Further, a change sensitivity (gradient) of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the nonlinear change region includes a characteristic of a change sensitivity which is greater than that of the linear change region. Further, the target capacitance change amount C0 between the fixed electrode  32  and the movable electrode  34  at the point of the movable electrode  34  reaching the target displacement amount A0 is set in the nonlinear change region in which the change sensitivity is higher than that of the linear change region. 
     Specifically, the construction of the displacement amount monitoring electrode  10  is configured to have such dimensions that meet the following criterion indicated by a formula (2). However, if the thickness T1 of the electrode fingers  32 - 2  of the fixed electrode  32  and the thickness T2 of the electrode fingers  34 - 2  of the movable electrode  34  are constant regardless of the locations, and the thickness Tm1, Tm2 of the portions of the electrode fingers  32 - 2 ,  34 - 2  overlapped in the Z axis direction correspond to T1, T2, respectively, the relationship T1=T2=Tm1=Tm2 is established, and thus the dimensions that meet following formula (3) may be set. 
     
       
         
           
             
               
                 
                   
                     
                       
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     According to such a configuration of the displacement amount monitoring electrode  10 , the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  when the movable electrode reaches near the target displacement amount A0 becomes higher with respect to a configuration in which the target capacitance change amount C0 is set in the linear change area. Further, if the tips of the electrode fingers  34 - 2  of the movable electrode  34  are further displaced to be closer to the base portion  32 - 1  of the fixed electrode  32  after the movable electrode has reached the target displacement amount A0, the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  increases with the displacement. Therefore, even if the gap and the opposed area between the fixed electrode  32  and the movable electrode  34  change due to the stress change or dimensional variation of the displacement amount monitoring electrode  10 , the variation in the drive displacement amount x of the movable electrode  34  at the time when the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  reaches the target capacitance change amount C0 is reduced. 
     Thus, according to the configuration of the displacement amount monitoring electrode  10  of the embodiment, it is possible to prevent the drive displacement amount x of the movable electrode  34  at the time when the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  reaches the target capacitance change amount C0 from greatly deviating from the target displacement amount A0. Therefore, even if the relationship between the fixed electrode  32  and the movable electrode  34  changes, it is possible to keep the amplitudes of the structure bodies  16 ,  18  at constant target amplitudes as much as possible. Further, robustness for disturbance factors such as a stress generated at the time of assembling, a temperature change and manufacturing factors such as dimensional variations is improved, which enhances yield and efficiency percentage. 
     It is noted that in the first embodiment described above the structure bodies  16 ,  18  correspond to “detection mass” in claims, the X axis direction corresponds to “predetermined axis direction” in claims, the Y axis direction corresponds to “a direction perpendicular to a predetermined axis direction” in claims, w1*T1 corresponds to “an area of portions of the electrode fingers of the fixed electrode which face the base portion of the movable electrode” and “S1”, and w2*T2 corresponds to “an area of portions of the electrode fingers of the movable electrode which face the base portion of the fixed electrode” and “S2”. 
     In the first embodiment described above, the electrode fingers  32 - 2 ,  34 - 2  are formed to have rectangular cross sectionals, and have equal cross sectional areas over a range from the connection portion with the base portions  32 - 1 ,  34 - 1  to the tip portions, respectively; however, the present invention is not limited to this configuration. For example, the electrode fingers  32 - 2 ,  34 - 2  may be T-shaped when viewed in the Z axis direction as illustrated in  FIG. 7  or L-shaped when viewed in the Z axis direction as illustrated in  FIG. 8 . In other words, the widths w1, w2 of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  in the Y axis direction may be greater than those of other portions in the Y axis direction. In this case, at least one of the widths w1, w2 of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  may be increased. 
     According to such a configuration of the variant, areas of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  facing to the base portions  34 - 1 ,  32 - 1  of the movable electrode  34  and the fixed electrode  32  can be increased. Thus, the criteria indicated by the formulas (2) and (3) can be easily met, and the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the nonlinear change region can be increased. 
     Further, instead of the variant described above, the electrode fingers  32 - 2 ,  34 - 2  may be formed like a bar when viewed in the Z axis direction and have constant cross sectional areas over a range from the connection portion with the base portions  32 - 1 ,  34 - 1  to points near the tip portions, and the shapes of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  and the shapes of the base portions  32 - 1 ,  34 - 1  opposed to the tip portions may be inclined as illustrated in  FIG. 9  or rounded as illustrated in  FIG. 10 . 
     In other words, the areas of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  facing to the base portions  34 - 1 ,  32 - 1  (i.e., the whole length in the X-Y plane along which both face to each other) may be made greater than the cross sectional areas of other portions of the electrode fingers  32 - 2 ,  34 - 2  (i.e., the width in the Y axis direction). In this case, at least one of the opposed area of the tip portions of the electrode fingers  32 - 2  and the base portions  34 - 1  and the opposed area of the tip portions of the electrode fingers  34 - 2  and the base portions  32 - 1  may be made greater. Further, although the shapes of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  are convex and the shapes of base portions  34 - 1 ,  32 - 1  are concave, as illustrated in  FIG. 9 , the shapes of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  may be concave and the shapes of base portions  34 - 1 ,  32 - 1  may be convex. 
     According to such a configuration of the variant, areas of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  facing to the base portions  34 - 1 ,  32 - 1  of the movable electrode  34  and the fixed electrode  32  can be increased. Thus, the criteria indicated by the formulas (2) and (3) can be easily met, and the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the nonlinear change region can be increased. Further, it becomes possible to prevent the gap distance dm between the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  in the Y axis direction from becoming greater, and thus it becomes possible to prevent an area occupied by the displacement amount monitoring electrode  10  in the semiconductor substrate  14  from increasing. 
     Further, in the first embodiment described above, the thickness T1 of the electrode fingers  32 - 2  of the fixed electrode  32  in the Z axis direction and the thickness T2 of the electrode fingers  34 - 2  of the movable electrode  34  in the Z axis direction are constant from the connection portions with the base portions  32 - 1 ,  34 - 1  and the tip portions; however, the present invention is not limited to this configuration. For example, the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  may be T-shaped when viewed in the Y axis direction as illustrated in  FIG. 11 ,  FIG. 12A  and  FIG. 12B  or L-shaped when viewed in the Y axis direction as illustrated in  FIG. 13 ,  FIG. 14A  and  FIG. 14B . In other words, the thicknesses T1, T2 of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  in the Z axis direction may be greater than those of other portions in the Z axis direction, or the overlapped thickness Tm (i.e., Tm1 and Tm2) between the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  in the Z axis direction may be decreased. In this case, at least one of the thicknesses T1, T2 of the tip portions may be increased. 
     According to such a configuration of the variant, areas of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  facing the base portions  34 - 1 ,  32 - 1  of the movable electrode  34  and the fixed electrode  32  can be increased. Thus, the criteria indicated by the formulas (2) and (3) can be easily met, and the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the nonlinear change region can be increased. Further, since the opposed areas between the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  can be decreased, the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the linear change region can be decreased. Therefore, the variation in the drive displacement amount x of the movable electrode  34  at the time when the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  reaches the target capacitance change amount C0 can be reduced, and thus the robustness for disturbance factors such as a stress generated at the time of assembling, a temperature change and manufacturing factors such as dimensional variations is improved. 
     Further, instead of the variant described above, the electrode fingers  32 - 2 ,  34 - 2  may be formed like a bar when viewed in the Y axis direction and have constant cross sectional areas over a range from the connection portion with the base portions  32 - 1 ,  34 - 1  to points near the tip portions, and the shapes of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  and the shapes of the base portions  32 - 1 ,  34 - 1  opposed to the tip portions may be rounded as illustrated in  FIG. 15  or inclined as illustrated in  FIG. 16 . 
     In other words, the areas of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  facing to the base portions  34 - 1 ,  32 - 1  (i.e., the whole length in the X-Z plane along which both face to each other) may be made greater than the cross sectional areas of other portions of the electrode fingers  32 - 2 ,  34 - 2  (i.e., the width in the Z axis direction). In this case, at least one of the opposed area of the tip portions of the electrode fingers  32 - 2  and the base portions  34 - 1  and the opposed area of the tip portions of the electrode fingers  34 - 2  and the base portions  32 - 1  may be made greater. Further, although the shapes of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  are convex and the shapes of base portions  34 - 1 ,  32 - 1  are concave, as illustrated in  FIG. 15  and  FIG. 16 , the shapes of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  may be concave and the shapes of base portions  34 - 1 ,  32 - 1  may be convex. 
     According to such a configuration of the variant, areas of the tip portions of the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  facing to the base portions  34 - 1 ,  32 - 1  of the movable electrode  34  and the fixed electrode  32  can be increased. Thus, the criteria indicated by the formulas (2) and (3) can be easily met, and the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the nonlinear change region can be increased. Further, it becomes possible to prevent the maximum thicknesses of the fixed electrode  32  and the movable electrode  34  in the Z axis direction from becoming greater, and thus it becomes possible to prevent a volume occupied by the displacement amount monitoring electrode  10  in the semiconductor substrate  14  from increasing. 
     Further, in the embodiment described above, the overlapped thickness Tm (i.e., Tm1 and Tm2) between the electrode fingers  32 - 2 ,  34 - 2  of the fixed electrode  32  and the movable electrode  34  in the Z axis direction correspond to the thicknesses T1, T2 of the electrode fingers  32 - 2 ,  34 - 2  in the Z axis direction; however, the present invention is not limited to this configuration. For example, the thickness Tm (i.e., Tm1 and Tm2) may be less than the thicknesses T1, T2. Further, as illustrated in  FIG. 17 ,  FIG. 18A  and  FIG. 18B , the electrode fingers  32 - 2 ,  34 - 2  may be arranged such that portions other than the tip portions are not overlapped. It is noted that this variant may be applied to the first embodiment and the variants described above. 
     According to the configuration of the variant, since the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the linear change region can be made less, and the variation of the capacitance change amount ΔC with respect to the drive displacement amount of the movable electrode  34  is reduced, the variation in the drive displacement amount x of the movable electrode  34  at the time when the capacitance change amount ΔC reaches the target capacitance change amount C0 can be reduced. Further, robustness for disturbance factors such as a stress generated at the time of assembling, a temperature change and manufacturing factors such as dimensional variations is improved, which enhances yield and efficiency percentage. 
     In particular, according to the configuration in which the fixed electrode  32  and the movable electrode  34  are arranged such that the electrode fingers  32 - 2 ,  34 - 2  are not opposed to each other in the Y axis direction perpendicular to the X axis direction (i.e., the electrode fingers  32 - 2 ,  34 - 2  are arranged such that the portions other than the tip portions are not overlapped), the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the linear change region can be substantially zero, as illustrated in  FIG. 19 , and thus the effects described above can be obtained. It is noted that if the electrode fingers  32 - 2 ,  34 - 2  are arranged such that the portions other than the tip portions are not overlapped, the construction of the displacement amount monitoring electrode  10  may be set to the dimensions which meet a criterion indicated by the following formula (4). 
     
       
         
           
             
               
                 
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     Second Embodiment 
       FIG. 20  is a plan view of a displacement amount monitoring electrode  100  according to the second embodiment of the present invention. It is noted that elements of the configuration which are the same as those of the configuration illustrated in  FIG. 2  are given the same reference numerals and the explanation thereof is omitted or simplified. Further,  FIG. 21  is a diagram for illustrating a relationship between the drive displacement amount x of the structure bodies  16 ,  18 , that is to say, the movable electrode  34  and the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  according to the displacement amount monitoring electrode  100  of the embodiment. 
     The displacement amount monitoring electrode  100  of the embodiment includes the fixed electrode  32  and the movable electrode  34 . The fixed electrode  32  and the movable electrode  34  are formed to have comb shapes, and include base portions  32 - 1 ,  34 - 1  having relatively great width and electrode fingers  32 - 2 ,  34 - 2  extending from the base portions  32 - 1 ,  34 - 1  in the X axis direction parallel to the semiconductor substrate  14 , respectively. The electrode fingers  32 - 2 ,  34 - 2  are formed to have rectangular cross sectionals, and have equal cross sectional areas (i.e., constant widths w1, w2 in the Y axis direction and constant thickness T1, T2 in the Z axis direction) over a range from the connection portion with the base portions  32 - 1 ,  34 - 1  to the tip portions, respectively. 
     In the fixed electrode  32 , a dielectric material  102 - 1  is provided on a surface of the base portion  32 - 1  opposed to the tip portions of the electrode fingers  34 - 2  of the movable electrode  34  in the X axis direction, and a dielectric material  102 - 2  is provided on surfaces of the tip portions of the electrode fingers  32 - 2 . Further, in the movable electrode  34 , a dielectric material  104 - 1  is provided on a surface of the base portion  34 - 1  opposed to the tip portions of the electrode fingers  32 - 2  of the fixed electrode  32  in the X axis direction, and a dielectric material  104 - 2  is provided on surfaces of the tip portions of the electrode fingers  34 - 2 . 
     The dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  include materials with high insulation characteristics, low modulus of elasticity or viscosity. The dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  may be manufactured using a SiO 2  film (oxidized insulating film), which can be easily formed by thermal oxidation of silicon, for example, organic materials such as resist materials, etc. It is noted that the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  have a relative dielectric constant ∈r, which is higher than a relative dielectric constant of “1” of air (strictly speaking, a vacuum), and a predetermined thickness d∈ in the X axis direction. 
     If the movable electrode  34  is not moved in the X axis direction, there is a gap d1 in the X axis direction between the dielectric material  102 - 2  provided on the tip portions of the electrode finger  32 - 2  of the fixed electrode  32  and the dielectric material  104 - 1  provided on the base portion  34 - 1  of the movable electrode  34 , and there is a gap d2 in the X axis direction between the dielectric material  102 - 1  provided on the base portion  32 - 1  of the fixed electrode  32  and the dielectric material  104 - 2  provided on the tip portions of the electrode finger  34 - 2  of the movable electrode  34 . It is noted that there are gaps dm1, dm2 in the Y axis direction between the electrode finger  32 - 2  of the fixed electrode  32  and the electrode finger  34 - 2  of the movable electrode  34 . 
     Further, an overlapped thickness between the electrode fingers  32 - 2  and the electrode fingers  34 - 2  in the thickness direction, that is to say, the Z axis direction (i.e., a height of a part of the side wall of the electrode fingers  32 - 2  which are opposed to the side wall of the electrode fingers  34 - 2  in the Z axis direction) is Tm. It is noted that the thickness Tm on the side of the gap dm1 (i.e., Tm1) may be different from that on the side of the gap dm2 (i.e., Tm2). However, it is desirable that a relationship T1=T2=Tm (=Tm1=Tm2) is met. 
     In the embodiment, the displacement amount monitoring electrode  100  is configured such that the following criteria are met. In the course of the driven displacement of the movable electrode  34  with the target displacement amount A0 corresponding to the target amplitudes of the structure bodies  16 ,  18 , the relationship between the drive displacement amount x of the movable electrode  34  and the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  includes the linear change area in which the capacitance change amount ΔC changes linearly with the driven displacement of the movable electrode  34  and the nonlinear change region in which the capacitance change amount ΔC changes nonlinearly. Further, a change sensitivity (gradient) of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  in the nonlinear change region includes a characteristic of a change sensitivity which is greater than that of the linear change region. Further, the target capacitance change amount C0 between the fixed electrode  32  and the movable electrode  34  at the point of the movable electrode  34  reaching the target displacement amount A0 is set in the nonlinear change region in which the change sensitivity is higher than that of the linear change region. 
     Specifically, the construction of the displacement amount monitoring electrode  100  is configured to have such dimensions that meet following formula (5). However, if the thickness T1 of the electrode fingers  32 - 2  of the fixed electrode  32  and the thickness T2 of the electrode fingers  34 - 2  of the movable electrode  34  are constant regardless of the locations, and the thickness Tm1, Tm2 of the portions of the electrode fingers  32 - 2 ,  34 - 2  overlapped in the Z axis direction correspond to T1, T2, respectively, the relationship T1=T2=Tm1=Tm2 is established, and thus the dimensions that meet following formula (6) may be set. 
     
       
         
           
             
               
                 
                   
                     
                       
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     According to such a configuration of the displacement amount monitoring electrode  100 , the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  when the movable electrode  34  reaches near the target displacement amount A0 becomes higher with respect to a configuration in which the target capacitance change amount C0 is set in the linear change area. Further, if the tips of the electrode fingers  34 - 2  of the movable electrode  34  are further displaced to be closer to the base portion  32 - 1  of the fixed electrode  32  after the movable electrode has reached the target displacement amount A0, the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  increases with the displacement. Further, since the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  have a relative dielectric constant ∈r, which is higher than a relative dielectric constant of “1” of air (strictly speaking, a vacuum), the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x is increased due to the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2 . 
     Therefore, even if the gap and the opposed area between the fixed electrode  32  and the movable electrode  34  change due to the stress change or dimensional variation of the displacement amount monitoring electrode  100 , the variation in the drive displacement amount x of the movable electrode  34  at the time when the capacitance change amount ΔC between the fixed electrode  32  and the movable electrode  34  reaches the target capacitance change amount C0 is further reduced. Thus, according to the displacement amount monitoring electrode  100  of the embodiment, more advantageous effects can be obtained with respect to the displacement amount monitoring electrode  10  of the first embodiment described above. 
     Further, if the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  in the displacement amount monitoring electrode  100  have high insulation characteristics, electrical connection between the fixed electrode  32  and the movable electrode  34  can be prevented due to the existence of the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  even when the fixed electrode  32  comes into collision with the movable electrode  34  due to manufacturing variations, control variations, application of excessive stress from the outside, etc. Thus, it is possible to detect the change in the capacitance between the fixed electrode  32  and the movable electrode  34  with high accuracy, and erroneous operations can be prevented in exciting the structure bodies  16 ,  18  in the X axis direction with respect to the semiconductor substrate  14 . 
     Further, if the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  in the displacement amount monitoring electrode  100  have low modulus of elasticity or viscosity, impact which otherwise would be applied to the fixed electrode  32  and the movable electrode  34  can be absorbed due to the existence of the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  even when the fixed electrode  32  comes into collision with the movable electrode  34  due to manufacturing variations, control variations, application of excessive stress from the outside, etc., and thus breakages of the fixed electrode  32  and the movable electrode  34  can be prevented. 
     It is noted that in the second embodiment, the fixed electrode  32  and the movable electrode  34  are configured such that the dielectric materials  102 - 1 ,  102 - 2 ,  104 - 1 ,  104 - 2  are provided on the surfaces of the base portions  32 - 1 ,  34 - 1  and the surfaces of the tip portions of the electrode fingers  32 - 2 ,  34 - 2 . However, the present invention is not limited to this configuration. For example, as illustrated in  FIG. 22 , the fixed electrode  32  and the movable electrode  34  may be configured such that only the dielectric materials  102 - 2 ,  104 - 2  are provided on the surfaces of the tip portions of the electrode fingers  32 - 2 ,  34 - 2 . 
     The construction of the displacement amount monitoring electrode according to the variant is configured to have such dimensions that meet the following criterion indicated by a formula (7). However, if the thickness T1 of the electrode fingers  32 - 2  of the fixed electrode  32  and the thickness T2 of the electrode fingers  34 - 2  of the movable electrode  34  are constant regardless of the locations, and the thickness Tm1, Tm2 of the portions of the electrode fingers  32 - 2 ,  34 - 2  overlapped in the Z axis direction correspond to T1, T2, respectively, the relationship T1=T2=Tm1=Tm2 is established, and thus the dimensions that meet a criteria indicated by the following formula (8) may be set. 
     
       
         
           
             
               
                 
                   
                     
                       
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     According to such a configuration of the displacement amount monitoring electrode of the variant, the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  when the movable electrode reaches near the target displacement amount A0 becomes higher with respect to a configuration in which the target capacitance change amount C0 is set in the linear change area. Further, if the tips of the electrode fingers  34 - 2  of the movable electrode  34  are further displaced to be closer to the base portion  32 - 1  of the fixed electrode  32  after the movable electrode has reached the target displacement amount A0, the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x of the movable electrode  34  increases with the displacement. Further, since the dielectric materials  102 - 2 ,  104 - 2  have a relative dielectric constant ∈r, which is higher than a relative dielectric constant of “1” of air (strictly speaking, a vacuum), the change sensitivity of the capacitance change amount ΔC with respect to the drive displacement amount x is increased due to the dielectric materials  102 - 2 ,  104 - 2 . Thus, according to the displacement amount monitoring electrode  100  of the variant, more advantageous effects can be obtained with respect to the displacement amount monitoring electrode  10  of the first embodiment described above. 
     Further, as illustrated in  FIG. 23 , the fixed electrode  32  and the movable electrode  34  may be configured such that only the dielectric materials  102 - 1 ,  104 - 1  are provided on the surfaces of the base portions  32 - 1 ,  34 - 1 . The construction of the displacement amount monitoring electrode according to the variant may be configured to have such dimensions that meet the following criterion indicated by the formula (7) or (8). 
     Further, as illustrated in  FIG. 24 , only the movable electrode  34  may have the dielectric materials  104 - 1 ,  104 - 2  provided on the surface of the base portion  34 - 1  and the surfaces of the tip portions of the electrode fingers  34 - 2 . Further, as illustrated in  FIG. 25 , only the fixed electrode  32  may have the dielectric materials  102 - 1 ,  102 - 2  provided on the surface of the base portion  32 - 1  and the surfaces of the tip portions of the electrode fingers  32 - 2 . The constructions of the displacement amount monitoring electrodes according to these variants may be configured to have such dimensions that meet the following criterion indicated by the formula (7) or (8). 
     Further, in the second embodiment, the dielectric materials  102  of the fixed electrode  32  and the dielectric materials  104  of the movable electrode  34  have the same predetermined thickness d∈ in the X axis direction; however, the present invention is not limited to this configuration. For example, the dielectric materials  102 ,  104  may have different thickness d∈1, d∈2 in the X axis direction. Similarly, the dielectric materials  102  of the fixed electrode  32  and the dielectric materials  104  of the movable electrode  34  have the same relative dielectric constant ∈r; however, the present invention is not limited to this configuration. For example, the dielectric materials  102 ,  104  may have different relative dielectric constants ∈r1, ∈r2. 
     Further, the second embodiment is applied to the configuration described in the first embodiment; however, it can be applied to the configurations of the variants illustrated in  FIG. 7  through  FIG. 18 . 
     Further, in the first and second embodiments described above, the displacement amount monitoring electrodes  10 ,  100  are installed in the angular velocity sensor  12  for detecting the angular velocity; however, the present invention is not limited to this configuration. For example, it can be installed in other sensors which monitor the driven displacement amount of the structure body.