Abstract:
In an acceleration sensor, a sensor unit includes a weight portion having a recess section with one open surface and a solid section one-piece formed with the recess section, beam portions for rotatably supporting the weight portion such that the recess section and the solid section are arranged along a rotation direction, a movable electrode, fixed electrodes, detection electrodes electrically connected to the fixed electrodes to detect a capacitance between the movable electrode and the fixed electrodes. A fixed plate is arranged in a spaced-apart relationship with a surface of the weight portion on which the movable electrode is provided, and embedment electrodes are embedded in the fixed plate to extend along a thickness direction of the fixed plate, the embedment electrodes having one end portions facing the movable electrode to serve as the fixed electrodes and the other end portions configured to serve as the detection electrodes.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a capacitive acceleration sensor. 
       BACKGROUND OF THE INVENTION 
       [0002]    There is conventionally known an acceleration sensor that includes a rectangular parallelepiped weight portion having a movable electrode; a pair of beam portions for rotatably supporting the weight portion substantially at a center in the longitudinal direction of the weight portion; and a pair of fixed electrodes arranged in a spaced-apart opposing relationship with respect to one side and the other side of the surface of the weight portion demarcated by a straight line interconnecting the beam portions (see, e.g., Japanese Patent Application Publication No. 2008-544243). 
         [0003]    One prior art example of the acceleration sensor will now be described with reference to  FIGS. 4 and 5 . In the following description, upper and lower sides in  FIG. 4  will be defined as an up-down direction, the direction parallel to the width direction of a sensor chip  1  as an x-direction, the direction parallel to the longitudinal direction of the sensor chip  1  as a y-direction and the direction orthogonal to the x-direction and the y-direction as a z-direction. As shown in  FIGS. 4 and 5 , the acceleration sensor of the prior art example includes the sensor chip  1  having a rectangular plate-like outward appearance; an upper fixed plate  2   a  fixed to the upper surface of the sensor chip  1 ; and a lower fixed plate  2   b  fixed to the lower surface of the sensor chip  1 . 
         [0004]    The sensor chip  1  includes a frame portion  3  having two rims  3   a  and  3   b  formed into a rectangular shape when seen in the up-down direction and arranged side by side along the longitudinal direction; rectangular parallelepiped weight portions  4  and  5  arranged adjacent to each other inside the rims  3   a  and  3   b  in a spaced-apart relationship with respect to the inner circumferential surfaces of the rims  3   a  and  3   b ; two pairs of beam portions  6   a  and  6   b , and  7   a  and  7   b  for respectively interconnecting the inner circumferential surfaces of the rims  3   a  and  3   b  and the side surfaces of the weight portions  4  and  5  to rotatably support the weight portions  4  and  5  with respect to the frame portion  3 ; and movable electrodes  4   a  and  5   a  formed on the upper surfaces of the weight portions  4  and  5 . 
         [0005]    Each of the weight portions  4  and  5  includes a recess section  41  or  51  opened in one surface (the lower surface) thereof and a solid section  40  or  50  one-piece formed with the recess section  41  or  51 . The recess section  41  or  51  is formed to have a rectangular plan-view shape when seen in the direction normal to the open surface (in the up-down direction). Reinforcing walls  42  and  52  for bisecting the inside of the recess sections  41  and  51  are respectively one-piece formed with the weight portions  4  and  5 . 
         [0006]    One pair of the beam portions  6   a  and  6   b  interconnects the rim  3   a  and the substantially central sections in the x-direction of the side surfaces of the weight portion  4  facing the rim  3   a . Similarly, another pair of the beam portions  7   a  and  7   b  interconnects the rim  3   b  and the substantially central sections in the x-direction of the side surfaces of the weight portion  5  facing the rim  3   b . Accordingly, the straight line interconnecting the beam portions  6   a  and  6   b  and the straight line interconnecting the beam portions  7   a  and  7   b  respectively become rotation axes about which the weight portions  4  and  5  rotate. 
         [0007]    The sensor chip  1  is formed by processing a SOI (Silicon-On-Insulator) substrate by a semiconductor fine processing technology. The sections including the upper surfaces of the weight portions  4  and  5  respectively become the movable electrodes  4   a  and  5   a . Protrusions  43   a  and  43   b , and  53   a  and  53   b  for preventing the weight portions  4  and  5  from directly colliding with the upper fixed plate  2   a  and the lower fixed plate  2   b  are respectively provided to protrude from the upper and lower surfaces of the weight portions  4  and  5 . 
         [0008]    The upper fixed plate  2   a  is made of an insulating material, e.g., glass, and is provided at the side of the movable electrodes  4   a  and  5   a , i.e., above the sensor chip  1  in the illustrated example. On the lower surface of the upper fixed plate  2   a , first and second fixed electrodes  20   a  and  20   b  are arranged side by side in the x-direction in such positions as to face the weight portion  4  (the movable electrode  4   a ) of the sensor chip  1  along the up-down direction. First and second fixed electrodes  21   a  and  21   b  are arranged side by side in the x-direction in such positions as to face the weight portion  5  (the movable electrode  5   a ) of the sensor chip  1  along the up-down direction. At one x-direction end side of the upper fixed plate  2   a , five through-holes  22   a  through  22   e  are formed to extend through the upper fixed plate  2   a  in the y-direction to penetrate through the upper fixed plate  2   a . On the lower surface of the upper fixed plate  2   a , there is formed a plurality of conductive patterns (not shown) electrically connected to the respective fixed electrodes  20   a ,  20   b ,  21   a  and  21   b.    
         [0009]    On the other hand, four electrode units  8   a ,  8   b ,  9   a  and  9   b  spaced apart from the frame portion  3  are arranged side by side at one x-direction end side of the sensor chip  1 . Detection electrodes  80   a ,  80   b ,  90   a  and  90   b  made of metal films are formed substantially at the centers of the upper surfaces of the four electrode portions  8   a ,  8   b ,  9   a  and  9   b , respectively. Pressure contact electrodes  81   a ,  81   b ,  91   a  and  91   b  made of metal films are formed on the upper surfaces of the end sections of the four electrode units  8   a ,  8   b ,  9   a  and  9   b  facing the rims  3   a  and  3   b . The detection electrode  80   a  ( 80   b ) and the pressure contact electrode  81   a  ( 81   b ) are connected to each other. 
         [0010]    An earth electrode  10  is formed on the upper surface of the frame portion  3  between the electrode units  8   b  and  9   a . The earth electrode  10  is electrically connected to the movable electrode  4   a  through the beam portions  6   a  and  6   b  and to the movable electrode  5   a  through the beam portions  7   a  and  7   b . If the upper fixed plate  2   a  is bonded to the upper surface of the sensor chip  1 , the conductive patterns formed on the lower surface of the upper fixed plate  2   a  are connected, by pressure contact, to the pressure contact electrodes  81   a ,  81   b ,  91   a  and  91   b . Thus the respective detection electrodes  80   a ,  80   b ,  90   a  and  90   b  are electrically connected to the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  and are exposed to the outside through the through-holes  22   a  through  22   d  of the upper fixed plate  2   a . The earth electrode  10  is also exposed to the outside through the through-hole  22   e.    
         [0011]    Similar to the upper fixed plate  2   a , the lower fixed plate  2   b  is made of an insulating material such as glass or the like. The lower fixed plate  2   b  is provided at the opposite side of the sensor chip  1  from the upper fixed plate  2   a , i.e., below the sensor chip  1 . Adherence-preventing films  23   a  and  23   b  are respectively formed on the upper surface of the lower fixed plate  2   b  in positions corresponding to the weight portions  4  and  5  of the sensor chip  1  along the up-down direction. The adherence-preventing films  23   a  and  23   b  are made of the same material as the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b , e.g., aluminum-based alloy. The adherence-preventing films  23   a  and  23   b  serve to prevent the lower surfaces of the rotated weight portions  4  and  5  from adhering to the lower fixed plate  2   b.    
         [0012]    In the conventional example stated above, the rim  3   a , the weight portion  4 , the beam portions  6   a  and  6   b , the movable electrode  4   a , the first and second fixed electrodes  20   a  and  20   b  and the detection electrodes  80   a  and  80   b  make up one sensor unit. The rim  3   b , the weight portion  5 , the beam portions  7   a  and  7   b , the movable electrode  5   a , the first and second fixed electrodes  21   a  and  21   b  and the detection electrodes  90   a  and  90   b  make up another sensor unit. Two sensor units are one-piece formed with each other in a state that the orientations of the weight portions  4  and  5  (the arrangements of the solid sections  40  and  50  and the recess sections  41  and  51 ) are 180 degrees inverted on the same plane. 
         [0013]    Description will now be made on the detection operation of the prior art example. First, it is assumed that acceleration is applied to the weight portion  4  in the x-direction. If acceleration is applied in the x-direction, the weight portion  4  rotates about the rotation axis thereof, thereby changing the distances between the movable electrode  4   a  and the first and second fixed electrodes  20   a  and  20   b . As a result, capacitances C 1  and C 2  between the movable electrode  4   a  and the respective fixed electrodes  20   a  and  20   b  are also changed. In the regard, the capacitances C 1  and C 2  at the time of application of acceleration in the x-direction can be represented by the following equations: 
         [0000]        C 1= C 0−Δ C   Eq. 1
 
         [0000]      and 
         [0000]        C 2= C 0+Δ C   Eq. 2,
 
         [0014]    where C 0  denotes the capacitance between the movable electrode  4   a  and the respective fixed electrodes  20   a  and  20   b  when acceleration is not applied in the x-direction and ΔC denotes the capacitance change generated by the application of acceleration. 
         [0015]    Similarly, the capacitances C 3  and C 4  between the movable electrode  5   a  and the respective fixed electrodes  21   a  and  21   b  at the time of application of acceleration to the weight portion  5  in the x-direction can be represented by the following equations: 
         [0000]        C 3= C 0−Δ C   Eq. 3
 
         [0000]      and 
         [0000]        C 4= C 0+Δ C   Eq. 4.
 
         [0016]    In this connection, the values of the capacitances C 1  through C 4  can be detected by arithmetically processing the voltage signals extracted from the detection electrodes  80   a ,  80   b ,  90   a  and  90   b . Then, the sum (±4ΔC) of a differential value CA (=C 1 −C 2 ) between the capacitances C 1  and C 2  acquired from one of the sensor units and a differential value CB (=C 3 −C 4 ) between the capacitances C 3  and C 4  acquired from the other sensor unit is calculated. Based on the sum of the differential values CA and CB, it is possible to calculate the direction and magnitude of the acceleration applied in the x-direction. 
         [0017]    Next, it is assumed that acceleration is applied to the weight portion  4  in the z-direction. If acceleration is applied in the z-direction, the weight portion  4  rotates about the rotation axis thereof, thereby changing the distances between the movable electrode  4   a  and the first and second fixed electrodes  20   a  and  20   b . As a result, capacitances C 1  and C 2  between the movable electrode  4   a  and the respective fixed electrodes  20   a  and  20   b  are also changed. In the regard, the capacitances C 1 ′ and C 2 ′ at the time of application of acceleration in the z-direction can be represented by the following equations: 
         [0000]        C 1′= C 0′−Δ C′   Eq. 5
 
         [0000]      and 
         [0000]        C 2′= C 0′+Δ C′   Eq. 6,
 
         [0018]    where CO′ denotes the capacitance between the movable electrode  4   a  and the respective fixed electrodes  20   a  and  20   b  when acceleration is not applied in the z-direction and ΔC′ denotes the capacitance change generated by the application of acceleration. 
         [0019]    Similarly, the capacitances C 3 ′ and C 4 ′ between the movable electrode  5   a  and the respective fixed electrodes  21   a  and  21   b  at the time of application of acceleration to the weight portion  5  in the z-direction can be represented by the following equations: 
         [0000]        C 3= C 0−Δ C   Eq. 7
 
         [0000]      and 
         [0000]        C 4= C 0+Δ C   Eq. 8.
 
         [0020]    Then, the difference (±4ΔC) of a differential value CA′ (=C 1 ′−C 2 ′) between the capacitances C 1 ′ and C 2 ′ acquired from one of the sensor units and a differential value CB′ (=C 3 ′−C 4 ′) between the capacitances C 3 ′ and C 4 ′ acquired from the other sensor unit is calculated. Based on the difference of the differential values CA′ and CB′, it is possible to calculate the direction and magnitude of the acceleration applied in the z-direction. The arithmetic processing for finding the direction and magnitude of the acceleration applied in the x-direction and the z-direction using the sum of the differential values CA and CB and the difference of the differential values CA′ and CB′ is well-known in the art and, therefore, will not described in detail herein. 
         [0021]    In the meantime, in the conventional acceleration sensor stated above, the respective electrode units  8   a ,  8   b ,  9   a  and  9   b  spaced apart from the weight portions  4  and  5  and the frame portion  3  and electrically insulated from the sensor chip  1  are connected to the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  by way of the pressure contact electrodes  81   a ,  81   b ,  91   a  and  91   b . The capacitances C 1  through C 4  are detected through the detection electrodes  80   a ,  80   b ,  81   a  and  81   b  provided in the respective electrode units  8   a ,  8   b ,  9   a  and  9   b . However, the spaces required to provide the respective electrode units  8   a ,  8   b ,  9   a  and  9   b  occupy about 30% to 40% of the area of the sensor chip  1 . This poses a problem in that it becomes difficult to reduce the size of the acceleration sensor. Even if the size of the respective weight portions  4  and  5  is reduced, it is still necessary to provide the spaces for installation of the respective electrode units  8   a ,  8   b ,  9   a  and  9   b . Thus difficulties are involved in reducing the size of the acceleration sensor. 
       SUMMARY OF THE INVENTION 
       [0022]    In view of the above, the present invention provides an acceleration sensor in which no electrode units are required and the size thereof is reduced. 
         [0023]    In accordance with an aspect of the present invention, there is provided an acceleration sensor. The acceleration sensor includes a sensor unit including a weight portion having a recess section with one open surface and a solid section one-piece formed with the recess section, a pair of beam portions configured to rotatably support the weight portion in such a state that the recess section and the solid section are arranged along a rotation direction, a movable electrode provided on the opposite surface of the weight portion from the open surface of the recess section to extend across the recess section and the solid section, a first fixed electrode arranged at the opposite side of the movable electrode from the recess section, a second fixed electrode arranged at the opposite side of the movable electrode from the solid section, detection electrodes electrically connected to the first fixed electrode and the second fixed electrode to detect a capacitance between the movable electrode and the first fixed electrode and the second fixed electrode, the acceleration sensor being configured to detect acceleration based on a change in the capacitance between the movable electrode and the first fixed electrode and the second fixed electrode generated when the weight portion rotates about a rotation axis defined by a line interconnecting the beam portions; a first fixed plate arranged in a spaced-apart relationship with a surface of the weight portion on which the movable electrode is provided; and embedment electrodes embedded in the first fixed plate to extend along a thickness direction of the first fixed plate, the embedment electrodes having one end portions facing the movable electrode to serve as the first fixed electrode and the second fixed electrode and the other end portions configured to serve as the detection electrodes. 
         [0024]    The embedment electrodes may be formed of a P+ type semiconductor. 
         [0025]    Acceleration applied to the weight portion in a first direction and acceleration applied to the weight portion in a second direction orthogonal to the first direction may be detected. 
         [0026]    The sensor unit may include a plurality of sensor units formed in a single chip. 
         [0027]    The sensor unit may include two sensor units formed in the single chip, one of the two sensor units rotated by 180 degrees on a single plane with respect to the other sensor unit. 
         [0028]    The two sensor units may be arranged adjacent to each other. 
         [0029]    The sensor unit includes three sensor units formed in the single chip, two of the three sensor chips respectively rotated by 90 degrees and 180 degrees on a single plane with respect to the remaining sensor unit. 
         [0030]    Protrusions may be formed on the surfaces of the first fixed electrode and the second fixed electrode facing the movable electrode or on the surface of the movable electrode facing the first fixed electrode and the second fixed electrode. 
         [0031]    The protrusions may be made of silicon or silicon oxide films. 
         [0032]    The protrusions may have surface layers made of a carbon material. 
         [0033]    The carbon material may be a carbon nano tube. 
         [0034]    The sensor may further include a second fixed plate arranged in a spaced-apart relationship with the opposite surface of the weight portion from the first fixed electrode and the second fixed electrode, where the second fixed plate includes an adherence-preventing film for preventing the weight portion from adhering to the second fixed plate, the adherence-preventing film formed on the surface of the second fixed plate facing the weight portion. 
         [0035]    The adherence-preventing film may be made of the same material as the first fixed electrode and the second fixed electrode. 
         [0036]    The adherence-preventing film may be formed together with the first fixed electrode and the second fixed electrode. 
         [0037]    The adherence-preventing film may be formed by a semiconductor manufacturing process. 
         [0038]    The adherence-preventing film may be made of aluminum-based alloy. 
         [0039]    The change in the capacitance between the movable electrode and the first fixed electrode and the second fixed electrode may be detected by generating an attraction force between the movable electrode and the first fixed electrode and the second fixed electrode. 
         [0040]    A thin film made of an organic material may be formed on a surface of the adherence-preventing film. 
         [0041]    The thin film may be a polyimide thin film. 
         [0042]    The beam portions may be shifted toward the recess section such that an angle between a perpendicular line extending from a central position of the weight portion to the rotation axis and a surface of the movable electrode becomes substantially equal to 45 degrees. 
         [0043]    In accordance with the aspect of the present invention, the embedment electrodes can serve as the fixed electrodes and the detection electrodes. This makes it possible to arrange the fixed electrodes and the detection electrodes along the thickness direction of the first fixed plate. Accordingly, electrode units for electrically connecting the fixed electrodes and the detection electrodes to the sensor become unnecessary, which makes it unnecessary to provide the spaces for installation of the electrode units. It is therefore possible to reduce the size of the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  is an exploded perspective view showing an acceleration sensor in accordance with a first embodiment of the present invention. 
           [0045]      FIG. 2  is a cross sectional view of the acceleration sensor. 
           [0046]      FIG. 3A  is a plan view showing an acceleration sensor in accordance with a second embodiment of the present invention and  FIG. 3B  is a cross sectional view taken along line A-A′ in  FIG. 3A . 
           [0047]      FIG. 4  is an exploded perspective view showing a conventional acceleration sensor. 
           [0048]      FIG. 5  is a section view of the conventional acceleration sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0049]    Embodiments of an acceleration sensor in accordance with the present invention will now be described in detail with reference to the accompanying drawings. The basic configuration of the present invention is common to the prior art example set forth earlier. Common portions will be designated by like reference symbols with no description made thereon. In the following description, upper and lower sides in  FIG. 1  will be defined as an up-down direction, the direction parallel to the width direction of a sensor chip  1  as an x-direction, the direction parallel to the longitudinal direction of the sensor chip  1  as a y-direction and the direction orthogonal to the x-direction and the y-direction as a z-direction. 
       First Embodiment 
       [0050]    An acceleration sensor in accordance with a first embodiment is characterized in that, as shown in  FIGS. 1 and 2 , a plurality of (five in the illustrated example) silicon-made embedment electrodes A 1  through A 5  is embedded in the upper fixed plate  2   a  to extend along the thickness direction thereof. The embedment electrodes A 1  through A 4  are made of, e.g., low-resistance silicon (having a resistance of 0.2 Ω·cm or less) doped with impurities at a high concentration. The lower end portions of the embedment electrodes A 1  through A 4  are exposed from the lower surface of the upper fixed plate  2   a  in such a fashion as to face the respective movable electrodes  4   a  and  5   a . The upper end portions of the embedment electrodes A 1  through A 4  are exposed from the upper surface of the upper fixed plate  2   a . Wiring electrodes B for wire bonding are attached to the exposed upper end portions of the embedment electrodes A 1  through A 4 , the wiring electrodes B being made of a metallic material (e.g., aluminum). 
         [0051]    Accordingly, the lower end portions of the embedment electrodes A 1  through A 4  are used as fixed electrodes (corresponding to the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  shown in  FIG. 4 ), and the upper end portions of the embedment electrodes A 1  through A 4  are used as detection electrodes (corresponding to the detection electrodes  80   a ,  80   b ,  90   a  and  90   b  shown in  FIG. 4 ). An earth electrode  10  is provided in the upper end portion of the embedment electrode A 5 . The lower end portion of embedment electrode A 5  is brought into pressure contact with and electrically connected to a pressure contact electrode  11  embedded in the sensor chip  1 , the pressure contact electrode  11  being made of a metallic material (e.g., aluminum). The movable electrode  4   a  ( 5   a ) is electrically connected to the earth electrode  10  via the pressure contact electrode  11  and the embedment electrode A 5 . 
         [0052]    As set forth above, the embedment electrodes A 1  through A 4  can serve as the fixed electrodes and the detection electrodes. This makes it possible to arrange the fixed electrodes and the detection electrodes along the thickness direction of the upper fixed plate  2   a . Therefore, the electrode units  8   a ,  8   b ,  9   a  and  9   b  of the conventional example (see  FIG. 4 ) for electrically interconnecting the fixed electrodes and the detection electrodes need not be provided in the acceleration sensor of the present embodiment. This makes it unnecessary to provide the spaces for installation of the electrode units  8   a ,  8   b ,  9   a  and  9   b , which assists in reducing the size of the acceleration sensor. 
         [0053]    In addition, unlike the acceleration sensor of the prior art example, there is no need to interconnect the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  and the electrode units  8   a ,  8   b ,  9   a  and  9   b  through the use of the pressure contact electrodes  81   a ,  81   b ,  91   a  and  91   b . In the manufacturing process of the acceleration sensor, it is therefore possible to partially simplify the step for anodic bonding of the sensor chip  1  and the upper fixed plate  2   a.    
         [0054]    In order to reduce noises, it is preferred that the wiring resistances of the embedment electrodes A 1  through A 4  serving as the fixed electrodes and the detection electrodes be as small as possible. In the present embodiment, the wiring resistances are reduced by forming the embedment electrodes A 1  through A 4  into a mass shape. It is also preferred that the embedment electrodes A 1  through A 4  be formed of a P+ type semiconductor. This makes it possible to secure an ohmic property with respect to the wiring electrodes B. Accordingly, a simplified process can be used in bonding the wiring electrodes B and the earth electrode  10  to the embedment electrodes A 1  through A 5 . The material of the embedment electrodes A 1  through A 5  is not limited to the P+ type semiconductor but may be, e.g., an N+ type semiconductor. Moreover, the material of the embedment electrodes A 1  through A 5  is not limited to the semiconductor but may be a metallic material. 
       Second Embodiment 
       [0055]    An acceleration sensor in accordance with a second embodiment of the present embodiment will now be described with reference to the drawings. The basic configuration of the present embodiment is common to the first embodiment. Common portions will be designated by like reference symbols with no description made thereon. The acceleration sensor of the present embodiment is characterized in that, as shown in  FIGS. 3A and 3B , substantially rectangular extension portions  24   a ,  24   b ,  25   a  and  25   b  extending out of alignment with the weight portions  4  and  5  are one-piece formed with the respective fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  and further that substantially cylindrical columnar embedment electrodes A 1  through A 4  are embedded in the upper fixed plate  2   a , the embedment electrodes A 1  through A 4  having lower end portions bonded to the extension portions  24   a ,  24   b ,  25   a  and  25   b  and upper end portions to which wiring electrodes B are attached. A through-hole  22   e  is formed substantially at the center of the upper fixed plate  2   a . An earth electrode  10  is provided in the portion of the sensor chip  1  exposed to the outside through the through-hole  22   e.    
         [0056]    As stated above, the embedment electrodes A 1  through A 4  are arranged out of alignment with the weight portions  4  and  5 . This eliminates the possibility that a foreign material falls on the weight portions  4  and  5  through between the embedment electrodes A 1  through A 4  and the upper fixed plate  2   a . Accordingly, it is possible to prevent the operations of the weight portions  4  and  5  from being hindered by a falling foreign material. In the present embodiment, the earth electrode  10  is exposed to the outside through the through-hole  22   e . Therefore, unlike the first embodiment, there is no need to employ the embedment electrode AS and the pressure contact electrode  11 . This makes it possible to further simplify the step for anodic bonding of the sensor chip  1  and the upper fixed plate  2   a.    
         [0057]    In the respective embodiments described above, protrusions  43   a ,  43   b ,  53   a  and  53   b  are provided as is the case in the acceleration sensor of the prior art example. If the protrusions  43   a ,  43   b ,  53   a  and  53   b  are made of the same material, e.g., silicon or silicon oxide films, as the sensor chip, it becomes easy to manufacture the protrusions  43   a ,  43   b ,  53   a  and  53   b . The surface layers of the protrusions  43   a ,  43   b ,  53   a  and  53   b  may be coated with a carbon material. In this case, the mechanical strength of the protrusions  43   a ,  43   b ,  53   a  and  53   b  grows higher. It is also possible to prevent the protrusions  43   a ,  43   b ,  53   a  and  53   b  from being damaged by the collision of the upper fixed plate  2   a  and the lower fixed plate  2   b  with the protrusions  43   a ,  43   b ,  53   a  and  53   b.    
         [0058]    If a carbon nano tube is used as the carbon material, it becomes possible to reduce the thickness of a coating and to easily adjust the height of the protrusions  43   a ,  43   b ,  53   a  and  53   b  to a desired value. The protrusions  43   a  and  43   b , and  53   a  and  53   b  may be respectively provided on the surfaces of the movable electrodes  4   a  and  5   a  facing the fixed electrodes  20   a  and  20   b , and  21   a  and  21   b  as is the case in the prior art example shown in  FIG. 5 , or the protrusions  43   a ,  43   b ,  53   a  and  53   b  may be provided on the surfaces of the respective fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  facing the movable electrodes  4   a  or  5   a.    
         [0059]    In the respective embodiments described above, adherence-preventing films  23   a  and  23   b  are provided as is the case in the acceleration sensor of the prior art example. If the adherence-preventing films  23   a  and  23   b  are made of the same material as the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b , it becomes possible to easily form the adherence-preventing films  23   a  and  23   b . At this time, if the adherence-preventing films  23   a  and  23   b  are formed together with the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b , it is possible to increase the accuracy of the distance between the weight portions  4  and  5  and the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b  and the distance between the weight portions  4  and  5  and the lower fixed plate  2   b.    
         [0060]    If the adherence-preventing films  23   a  and  23   b  are formed through a semiconductor manufacturing process, fine asperities are left on the surfaces of the adherence-preventing films  23   a  and  23   b . This makes it possible to more reliably prevent the weight portions  4  and  5  from adhering to the lower fixed plate  2   b . In this regard, if the adherence-preventing films  23   a  and  23   b  are made of aluminum-based alloy, it becomes easy to perform etching. Short-circuit between the adherence-preventing films  23   a  and  23   b  and the weight portions  4  and  5  may be prevented by forming an organic thin film, e.g., a polyimide thin film, on the surfaces of the adherence-preventing films  23   a  and  23   b , to be highly compatible with a semiconductor manufacturing process and easy to process. 
         [0061]    In the respective embodiments described above, the beam portions  6   a  and  6   b , and  7   a  and  7   b  are respectively shifted toward the recess sections  41  and  51  from the longitudinal centers of the weight portions  4  and  5 . Thus the angle θ between the perpendicular line extending from the central position of each of the weight portions  4  and  5  to the rotation axis and the surface of each of the movable electrodes  4   a  and  5   a  becomes substantially equal to 45 degrees. Accordingly, the angle θ can be kept substantially at 45 degrees by merely shifting the beam portions  6   a ,  6   b ,  7   a  and  7   b . This makes it possible to enhance the detection sensitivity without having to increase the thickness of the weight portions  4  and  5  or to make the weight portions  4  and  5  light. 
         [0062]    In the respective embodiments described above, the operation of the acceleration sensor can be confirmed in the below-mentioned order. Specifically, the weight portion  4  or  5  is rotated by generating an attraction force between the embedment electrode A 1  or A 2  (the first fixed electrode  20   a  or the second fixed electrode  20   b ) and the movable electrode  4   a  or between the embedment electrode A 3  or A 4  (the first fixed electrode  21   a  or the second fixed electrode  21   b ) and the movable electrode  5   a . The normal operation of the acceleration sensor can be confirmed by detecting the change in the capacitance between the embedment electrodes A 1  through A 4  (the fixed electrodes  20   a ,  20   b ,  21   a  and  21   b ) and the weight portion  4  or  5  generated by the rotation of the weight portion  4  or  5 . Alternatively, the operation of the acceleration sensor may be confirmed by generating an attraction force between the adherence-preventing films  23   a  and  23   b  and the movable electrodes  4   a  and  5   a.    
         [0063]    In the respective embodiments described above, the acceleration sensor for detecting accelerations in two directions, i.e., in the x-direction and the z-direction, has been described by way of example. However, if one of the sensor units is rotated 90 degrees within the x-y plane and arranged in a symmetrical relationship with respect to the other sensor unit, it is possible to realize an acceleration sensor capable of detecting accelerations in three directions including the y-direction. As an alternative example, if three sensor units are arranged in a same single chip and if two of the sensor units are rotated by 90 degrees and 180 degrees within the x-y plane and arranged in a symmetrical relationship with respect to the remaining sensor unit, it is possible to realize an acceleration sensor capable of detecting accelerations in three directions. 
         [0064]    While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.