Abstract:
A capacitive acceleration sensor includes an acceleration sensor moving part and an acceleration sensor stationary part together forming a capacitor for detecting acceleration, a sealing structure hermetically enclosing but not contacting the acceleration sensor moving part, and at least one support pillar enclosed by but not directly contacted by the acceleration sensor moving part, both ends of the at least one support pillar being in contact with inside walls of the sealing structure. The acceleration sensor moving part is electrically connected to the at least one support pillar.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a capacitive acceleration sensor in which the moving part for detecting acceleration changes is disposed in a hermetically sealed air gap. 
         [0003]    2. Background Art 
         [0004]    Capacitive acceleration sensors measure acceleration by detecting a change in the capacitance of a capacitor. This capacitor includes a moving part (sometimes referred to as a “mass body”) and a stationary part. In the typical construction of a capacitive acceleration sensor, the moving part is supported by a beam so that the part is slightly displaced by acceleration and the resulting change in the capacitance of the capacitor is used to detect the acceleration. 
         [0005]    This moving part is hermetically enclosed in an air gap formed by a sealing structure so that the part is movable through a certain range of movement. Because of its being hermetically enclosed, the moving part is protected from foreign objects and moisture. The sealing structure is often made up of an upper glass and a lower glass between which is vertically sandwiched the silicon including the moving part and the stationary part. The sealing structure is desired to have an air gap for accommodating the moving part and to prevent the moving part from being affected by foreign objects and moisture. Patent Documents 1 to 4 describe sealing structures designed to meet the above requirements and acceleration sensors including such sealing structures. 
         [0006]    It is common that the sealing structure is sealed with a resin into a package by transfer molding in order to achieve smaller size, lower cost, and higher performance. In transfer molding, a molding resin is injected at high pressure by the injection molding machine. Therefore, when a capacitive acceleration sensor is subjected to injection molding, a high pressure is applied to the sensor. This high pressure has been found to break the sealing structure at the portion forming the air gap. Such breaking of the sealing structure has resulted in breakage of the moving part and stationary part for detecting acceleration and also resulted in a foreign object entering into the air gap, causing a variation in the characteristics of the acceleration sensor. 
         [0007]    In order to avoid breaking of the sealing structure, the injection molding pressure may be reduced. It has been found, however, that such reduction in the injection molding pressure slows down the flow of molding resin and causes formation of voids in the molding resin, thus degrading the internal portion-protecting capability and hence reliability of the sealing structure. Degradation of the reliability has also resulted from detachment of the capacitive acceleration sensor from the molding resin due to reduced adhesion between them. 
         [0008]    Another way to avoid breaking of the sealing structure may be to dispose a support pillar in the air gap formed by the sealing structure. Specifically, the support pillar may be disposed so that the top and bottom of the air gap are respectively supported on the opposite ends of the support pillar in order to prevent breaking of the sealing structure. It has been found, however, that if the support pillar is set in an electrically floating state, the moving part is displaced by the electrical action of the support pillar, thereby lowering the measuring accuracy of the acceleration sensor. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made to solve the above problems. It is, therefore, an object of the present invention to provide a capacitive acceleration sensor constructed so as to prevent breaking of its sealing structure when the structure is sealed by transfer molding and also prevent degradation of the acceleration measuring accuracy of the sensor. 
         [0010]    According to one aspect of the present invention, a capacitive acceleration sensor includes an acceleration sensor moving part and an acceleration sensor stationary part together forming a capacitor for detecting acceleration, a sealing structure hermetically enclosing but not contacting the acceleration sensor moving part, and at least one support pillar enclosed by but not directly contacted by the acceleration sensor moving part, both ends of the at least one support pillar being in contact with inside walls of the sealing structure. The acceleration sensor moving part is electrically connected to the at least one support pillar. 
         [0011]    According to another aspect of the present invention, a capacitive acceleration sensor includes an acceleration sensor moving part and an acceleration sensor stationary part together forming a capacitor for detecting acceleration, a sealing structure hermetically enclosing but not contacting the acceleration sensor moving part. The portions of the acceleration sensor moving part and the acceleration sensor stationary part that form the capacitor have a comb teeth shape. The comb-teeth-shaped portion of the acceleration sensor stationary part includes at least one support pillar for the sealing structure. The at least one support pillar is enclosed by but not directly contacted by the acceleration sensor moving part, and both ends of the at least one support pillar are in contact with inside walls of the sealing structure. 
         [0012]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a plan view illustrating a semiconductor portion of the first embodiment; 
           [0014]      FIG. 2  is a cross-sectional view corresponding to that taken along broken lines of  FIG. 1 , illustrating a capacitive acceleration sensor having a sealing structure; 
           [0015]      FIG. 3  is a plan view illustrating a semiconductor portion according to a variation of the first embodiment; 
           [0016]      FIG. 4  is a plan view of a semiconductor portion of the second embodiment; 
           [0017]      FIG. 5  is a perspective view of the stationary comb teeth portion; 
           [0018]      FIG. 6  is a cross-sectional view corresponding to that taken along the broken line of  FIG. 4 , illustrating a capacitive acceleration sensor having an upper glass and a lower glass such as those described in connection with the first embodiment; and 
           [0019]      FIG. 7  is a plan view illustrating a variation of the capacitive acceleration sensor (or semiconductor portion) of the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0020]    A first embodiment of the present invention will be described with reference to  FIGS. 1 to 3 . It should be noted that throughout the description of the first embodiment, like numerals represent like materials or like or corresponding components, and these materials and components may be described only once. This also applies to other embodiments of the invention subsequently described. 
         [0021]      FIG. 1  is a plan view illustrating a semiconductor portion  13  of the present embodiment. This semiconductor portion  13  is adapted to detect acceleration. The semiconductor portion  13  includes an acceleration sensor moving part  31  (hereinafter referred to as the “moving part  31 ”) and an acceleration sensor stationary part  32  (hereinafter referred to as the “stationary part  32 ”). The moving part  31  includes moving comb teeth portions  46  and  48  formed to have a comb teeth shape, and the stationary part  32  includes stationary comb teeth portions  42  and  44  also formed to have a comb teeth shape. The moving comb teeth portion  46  is interdigitated with the stationary comb teeth portion  42  to form a capacitor. Likewise, the moving comb teeth portion  48  is interdigitated with the stationary comb teeth portion  44  to form a capacitor. 
         [0022]    The moving part  31  is coupled to anchors  35  through beams  37 . The moving part  31  supported by the beams  37  is displaced by acceleration, resulting in a change in the capacitances of the above capacitors. Support pillars  34  are disposed such that they are enclosed by but not contacted by the moving part  31 . The function of the support pillars  34  will be described later. Each support pillar  34  is coupled to an anchor  35  through a conductive connection portion  36 . 
         [0023]      FIG. 2  is a cross-sectional view corresponding to that taken along broken lines of  FIG. 1 , illustrating a capacitive acceleration sensor  10  having a sealing structure  15 . The sealing structure  15  includes an upper glass  12  and a lower glass  14  between which the semiconductor portion  13  is sandwiched to hermetically enclose the moving part  31 . Specifically, the upper glass  12  and the lower glass  14  are in contact, respectively, with the opposite ends of the support pillars  34 . Further, the upper and lower glasses  12  and  14  are in contact with a coupling frame  39 . These glasses are also in contact with the stationary part  32 . It should be noted that the anchors are in contact with either the upper glass  12  or the lower glass  14 , or both. As a result, an air gas  33  is formed as shown in  FIG. 2 . 
         [0024]    The moving part  31  is mounted in the air gap  33  so that the part is not in contact with the sealing structure  15  including the upper glass  12  and the lower glass  14 . In this way, the moving part  31  is movable through a certain range of movement . Further, the moving part  31  is hermetically enclosed. 
         [0025]    The support pillars  34  are enclosed by the moving part  31 , but are not in contact with the moving part  31 , as described above. Further, the opposite ends of the support pillars  34  are in contact, respectively, with the upper glass  12  and the lower glass  14 . As a result, the air gap  33  around the moving part  31  is large, since the moving part  31  is not in contact with the sealing structure  15 . Further, in this construction, the support pillars  34  support the top and bottom of this large air gap  33  . This completes the description of the configuration of the capacitive acceleration sensor of the present embodiment. 
         [0026]    With the construction of the present embodiment, it is possible to prevent breaking of the upper glass  12  and the lower glass  14  when a high pressure is applied to these glasses in a transfer molding process, which is widely used to seal a capacitive acceleration sensor into a package. That is, both ends of the support pillars  34  of the present embodiment support inside walls of the air gap  33 , and in this way the support pillars  34  increase the strength of the sealing structure  15 . As a result, breaking of the upper glass  12  and the lower glass  14  can be prevented without reducing the injection molding pressure in the transfer molding process. Further, the air gap  33  can be maintained airtight, thereby protecting the moving part  31  and preventing foreign objects from entering the air gap. That is, the sealing structure  15  can protect the semiconductor portion  31 . Further, the injection molding can be performed at the desired pressure, resulting in the manufacture of a highly reliable package. It will be noted that the above construction of the present embodiment can prevent breaking of the upper and lower glasses  12  and  14  due to the application of external forces, as well as due to the pressure in the transfer molding process. 
         [0027]    Further, the support pillars  34  are coupled to the anchors  35  through the connection portions  36 , and the moving part  31  is also coupled to the anchors  35  through the beams  37 . Therefore, the support pillars  34  and the moving part  31  are at the same potential. This means that the support pillars  34  do not electrically affect the moving part  31 , thus preventing degradation of the function and accuracy of the acceleration sensor due to the use of the support pillars  34 . For example, if the support pillars  34  are in an electrically floating state, there may be a potential difference or a change in the potential difference between the moving part and the support pillars, which may affect the function and accuracy of the acceleration sensor. Particularly, if the moving part  31  is displaced by electrical action, not by acceleration, then degradation of the accuracy of the acceleration sensor will result. On the other hand, in the construction of the present embodiment, the support pillars  34  and the moving part  31  are at the same potential although the support pillars  34  are surrounded by and adjacent to the moving part  31 , thus avoiding the above problem. 
         [0028]    Although in the present embodiment the sealing structure  15  is made up of the upper glass  12  and the lower glass  14 , it is to be understood that the present invention is not limited to this particular structure. For example, the sealing structure  15  may be made of silicon. This allows the structure  15  to be formed by an ordinary semiconductor manufacturing line, which is advantageous in reducing manufacturing cost, etc. On the other hand, formation of a sealing structure using glass requires a special manufacturing line and special equipment, since the glass contains impurities. Further, the sealing structure may have any shape that allows an air gap to be formed by the structure alone or in combination with part of the semiconductor portion. 
         [0029]      FIG. 3  is a plan view illustrating a semiconductor portion according to a variation of the present embodiment. This semiconductor portion shown in  FIG. 3  differs in configuration from that shown in  FIG. 1  in that a support pillar  52  is coupled to a moving part  54  through connection portions  50 . This configuration is advantageous in that, although it is simpler than the configuration of  FIG. 1 , it can produce the same effect. 
       Second Embodiment 
       [0030]    A second embodiment of the present invention will be described with reference to  FIGS. 4 to 7 .  FIG. 4  is a plan view of a semiconductor portion of the second embodiment. A stationary comb teeth portion  64  and a moving comb teeth portion  46  together form a capacitor, and a stationary comb teeth portion  66  and a moving comb teeth portion  48  together form a capacitor. The stationary comb teeth portion  64  includes a comb tooth  68  having a support pillar. Likewise, the stationary comb teeth portion  66  includes a comb tooth  69  having a support pillar. 
         [0031]      FIG. 5  is a perspective view of the stationary comb teeth portion  64 . The comb tooth  68  of the stationary comb teeth portion  64  extends further than the other comb teeth and has a support pillar  70  at its tip, as shown in  FIG. 5 . The function of this support pillar  70  is the same as that of the support pillars  34  described in connection with the first embodiment. The comb tooth  69  with a support pillar shown in  FIG. 4  has the same configuration as the comb tooth  68 . 
         [0032]      FIG. 6  is a cross-sectional view corresponding to that taken along the broken line of  FIG. 4  , illustrating a capacitive acceleration sensor having an upper glass  12  and a lower glass  14  such as those described in connection with the first embodiment. Both ends of the support pillar  70  are in contact with inside walls of the sealing structure made up of the upper and lower glasses  12  and  14 , as can be seen from  FIG. 6 . Furthermore, both ends of the support pillar of the comb tooth  69  are also contact with inside walls of the sealing structure. 
         [0033]    Thus, the capacitive acceleration sensor of the second embodiment is characterized in that two comb teeth have support pillars that serve to increase the strength of the sealing structure. That is, portions of the two comb teeth are support pillars for the sealing structure, and the use of such support pillars simplifies the construction of the capacitive acceleration sensor while retaining the advantages described in connection with the first embodiment. It should will be noted that like the first embodiment, the support pillars of the present embodiment are enclosed by but not contacted by the moving part  64 . 
         [0034]      FIG. 7  is a plan view illustrating a variation of the capacitive acceleration sensor (or semiconductor portion) of the present embodiment. The configuration shown in  FIG. 7  is characterized in that support pillars are disposed so as to divide the length of a moving part  84  in the longitudinal direction into three substantially equal portions. More specifically, the support pillar of a comb tooth  80  and that of a comb tooth  82  are arranged and spaced so as to divide the length of the moving part  84  in the longitudinal direction into three substantially equal portions. In the example shown in  FIG. 4  described above, on the other hand, the support pillars are disposed at the center of the length of the moving part. However, breaking of the sealing structure can be more effectively prevented by distributing the support pillars, as in the configuration of  FIG. 7 . It will be noted that the number of support pillars is determined based on the set injection molding pressure in the transfer molding process. Therefore, the number of support pillars may be increased, and these support pillars may be disposed so as to divide the length of the moving part in the longitudinal direction into four or five substantially equal portions. Further, various other alterations may be made to the present embodiment without departing from the scope of the present invention. It should be noted that the present embodiment is susceptible of at least alterations which are the same as or correspond to those that can be made to the first embodiment. 
         [0035]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0036]    The entire disclosure of a Japanese Patent Application No. 2009-167637, filed on Jul. 16, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.