Abstract:
A MEMS pressure sensor includes a diaphragm portion that becomes displaced according to a pressure, and a resonator arranged on a main surface of the diaphragm portion. The resonator includes: a first fixed electrode provided on the main surface; and a drive electrode having a second fixed electrode provided on the main surface, a movable electrode spaced apart from the first fixed electrode, overlapping with the first fixed electrode, as viewed in a plan view seen from a normal direction to the main surface, and driven in a direction that intersects the main surface, and a supporting electrode supporting the movable electrode and connected to the second fixed electrode.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a MEMS pressure sensor, an electronic device, an altimeter, an electronic apparatus, and a moving object. 
       BACKGROUND ART 
       [0002]    Traditionally, as a device for detecting a pressure, a semiconductor pressure sensor as disclosed in JP-A-2001-332746 is known. In the semiconductor pressure sensor disclosed in JP-A-2001-332746, a strain sensing element is formed on a silicon wafer, the surface of the silicon wafer that is opposite to the surface where the strain sensing element is formed is ground to reduce the thickness and thus form a diaphragm portion, a strain generated in the diaphragm portion displaced by a pressure is detected by the strain sensing element, and the result of the detection is converted into electrical signal. 
       Technical Problem 
       [0003]    However, in the pressure sensor having the strain sensing element disclosed in JP-A-2001-332746, the silicon wafer needs to be thin, making it difficult to integrate the pressure sensor with a semiconductor device (IC) serving as an arithmetic unit that processes a signal from the pressure sensor. 
         [0004]    Meanwhile, a so-called MEMS (micro electro mechanical systems) element, that is, a micro mechanical system manufactured by a semiconductor device manufacturing method and device, is attracting attention. The use of a MEMS element enables provision of various types of very small sensors or oscillators or the like. In these sensors or oscillators, a micro oscillating element can be formed on a substrate by the MEMS technique, and an element that carries out detection of acceleration, generation of a reference signal and the like, using the oscillation characteristic of the oscillating element, can be provided. 
         [0005]    By forming an oscillating element using this MEMS technique, and forming a pressure sensor that detects pressure based on a change in the oscillation frequency of the MEMS oscillating element, it is possible to realize a pressure sensor integrated with an IC. Moreover, a thin diaphragm portion can be formed in a substrate and can be deformed even by a low pressure. Thus, a MEMS pressure sensor that can form a pressure sensor capable of accurately measuring a very small pressure is provided. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention is made to solve at least a part of the foregoing problems and can be realized in the following forms or application examples. 
       Application Example 1 
       [0007]    A MEMS pressure sensor according to this application example includes a diaphragm portion that becomes displaced according to a pressure, and a resonator arranged on a main surface of the diaphragm portion. The resonator includes: a first fixed electrode provided on the main surface; and a drive electrode having a second fixed electrode provided on the main surface, a movable electrode spaced apart from the first fixed electrode, overlapping with the first fixed electrode, as viewed in a plan view seen from a normal direction to the main surface, and driven in a direction that intersects the main surface, and a supporting electrode supporting the movable electrode and connected to the second fixed electrode. 
         [0008]    According to the MEMS pressure sensor of this application example, as an external pressure is applied to the diaphragm portion, the diaphragm portion flexes, causing a change in the oscillation characteristic of the resonator, that is, in resonance frequency. By deriving the relation between this external pressure and the change in the frequency characteristic of the resonator, a MEMS pressure sensor that detects the external pressure from the change in the frequency characteristic of the resonator can be provided. 
       Application Example 2 
       [0009]    In the above application example, the diaphragm portion has a recessed portion arranged on a back side of the main surface, and a thin portion made up of a bottom surface of the recessed portion and the main surface. If a distance between opposite ends of the first fixed electrode and the second fixed electrode is a, and a diameter of an inscribed circle in a planar shape as viewed in a plan view seen from a normal direction to the main surface, of the recessed portion of the diaphragm portion, is b B , 
         [0000]      0 &lt;a≦ 0.3 b   B    
         [0000]    holds. 
         [0010]    According to the above application example, a MEMS pressure sensor that has the resonator capable of efficiently converting a deformation of the diaphragm portion due to a pressure applied thereto into a change in the gap between the first fixed electrode and the movable electrode without lowering a signal intensity and thus securely detecting a change in the resonance frequency due to a change in the gap, can be provided. 
       Application Example 3 
       [0011]    In the above application example, the first fixed electrode is arranged in an area that is concentric with the inscribed circle, having a diameter c in a planar shape as viewed in a plan view seen from a normal direction to the main surface, of the recessed portion of the diaphragm portion. The diameter c is 
         [0000]      0 &lt;c≦ 0.93 b   B . 
         [0012]    According to this application example, a MEMS pressure sensor that can generate a large gap between the first fixed electrode and the movable electrode even if the pressure applied to the diaphragm portion is small and the amount of displacement, that is, the amount of flexure of the diaphragm portion is small, and thus can detect a very small pressure, can be provided. 
       Application Example 4 
       [0013]    In the above application example, if a diameter of a bottom-surface inscribed circle in a planar shape in the bottom surface of the recessed portion is b B , and a diameter of an opening inscribed circle in a planar shape in an opening of the recessed portion is b W , as viewed in a plan view seen from a normal direction to the main surface, 
         [0000]    
       
      
       b 
       B 
       &lt;b 
       W  
      
     
         [0000]    holds. 
         [0014]    According to the above application example, a corner portion formed by the bottom surface of the recessed portion and the sidewall of the recessed portion does not have an acute angle. Even if the flexing deformation of the diaphragm portion is repeated, damage to the wafer forming the substrate due to stress concentration in the corner portion can be restrained. Moreover, etching performance in shaping the recessed portion can be improved and productivity can be improved. 
       Application Example 5 
       [0015]    An electronic device according to this application example includes the MEMS pressure sensor described in the above application example, and a holding unit that holds the opening and the bottom surface of the recessed portion on the back side of the substrate, in a state of being exposed to a pressure changing area. 
         [0016]    According to the electronic device of this application example, as an external pressure is applied to the diaphragm portion, the diaphragm portion flexes, causing a change in the oscillation characteristic of the resonator, that is, in resonance frequency. By deriving the relation between this external pressure and the change in the frequency characteristic of the resonator, a pressure sensor as an electronic device that detects the external pressure from the change in the frequency characteristic of the resonator can be provided. 
       Application Example 6 
       [0017]    An altimeter according to this application example includes the MEMS pressure sensor described in the above application example, a holding unit that holds the opening and the bottom surface of the recessed portion on the back side, in a state of being exposed to a pressure changing area, and a data processing unit that processes measurement data from the MEMS pressure sensor. 
         [0018]    According to the altimeter of this application example, as an external pressure is applied to the diaphragm portion, the diaphragm portion flexes, causing a change in the oscillation characteristic of the resonator, that is, in resonance frequency. By deriving the relation between this external pressure and the change in the frequency characteristic of the resonator, an altimeter that detects the external pressure from the change in the frequency characteristic of the resonator and then calculates the altitude based on the pressure value can be provided. 
       Application Example 7 
       [0019]    An electronic apparatus according to this application example includes the MEMS pressure sensor, the electronic device or the altimeter described in the above application example. 
         [0020]    According to the electronic apparatus of this application example, an electronic apparatus that obtains the pressure value of an extremely low pressure and operates based on the pressure value can be provided. 
       Application Example 8 
       [0021]    A moving object according to this application example includes the MEMS pressure sensor, the electronic device, the altimeter or the electronic apparatus described in the above application example. 
         [0022]    According to the moving object of this application example, a moving object that obtains the pressure value of an extremely low pressure and operates based on the pressure value can be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  shows a MEMS pressure sensor according to a first embodiment.  FIG. 1(   a ) is a plan view.  FIG. 1(   b ) is a cross-sectional view taken along A-A′ shown in  FIG. 1(   a ).  FIG. 1(   c ) is a cross-sectional view taken along B-B′ shown in  FIG. 1(   a ). 
           [0024]      FIG. 2  shows the MEMS pressure sensor according to the first embodiment.  FIG. 2(   a ) shows the configuration of a MEMS oscillator portion for explaining operation in a static state.  FIG. 2(   b ) is a view of the configuration of the MEMS oscillator portion for explaining operation in a pressurized state. 
           [0025]      FIG. 3  shows the MEMS pressure sensor according to the first embodiment.  FIG. 3(   a ) is a plan view in which the diaphragm portion has a circular planar shape.  FIG. 3(   b ) is a plan view in which the diaphragm portion has a hexagonal planar shape.  FIG. 3(   c ) is a cross-sectional view showing the pressurized state. 
           [0026]      FIGS. 4(   a ) and  4 ( b ) are plan views for explaining the arrangement of a first fixed electrode in the MEMS pressure sensor according to the first embodiment. 
           [0027]      FIG. 5  shows another form of the MEMS pressure sensor according to the first embodiment.  FIG. 5(   a ) is a plan view.  FIG. 5(   b ) is a cross-sectional view taken along C-C′ shown in  FIG. 5(   a ).  FIG. 5(   c ) is an enlarged cross-sectional view of a MEMS oscillator portion. 
           [0028]      FIG. 6  is a cross-sectional view showing another form of the MEMS pressure sensor according to the first embodiment. 
           [0029]      FIG. 7  shows an altimeter according to a second embodiment.  FIG. 7(   a ) is a view of the configuration.  FIG. 7(   b ) is an enlarged view of a D portion shown in  FIG. 7(   a ). 
           [0030]      FIG. 8  is a partial cross-sectional view showing an altimeter according to another embodiment. 
           [0031]      FIG. 9  is a view of appearance showing a moving object according to a third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Hereinafter, embodiments of the invention will be described with reference to the drawings. 
       First Embodiment 
       [0033]      FIG. 1  shows a MEMS pressure sensor according to a first embodiment.  FIG. 1(   a ) is a plan view, seen by penetrating a covering layer, described later.  FIG. 1(   b ) is a cross-sectional view taken along A-A′ shown in  FIG. 1(   a ). FIG.  1 ( c ) is a cross-sectional view taken along B-B′ shown in  FIG. 1(   a ). As shown in  FIG. 1(   b ), a MEMS pressure sensor  100  according to this embodiment includes a substrate  10  made up of a wafer substrate  11 , a first oxide film  12  formed on a main surface  11   a  of the wafer substrate  11 , and a nitride film  13  formed on the first oxide film  12 . The wafer substrate  11  is a silicon substrate and is also used as a wafer substrate  11  forming a semiconductor device or so-called IC, described later. 
         [0034]    A MEMS oscillator  20  as a resonator is formed on a main surface  10   a  as a first surface of the substrate  10 , that is, on a surface  13   a  of the nitride film  13 . The MEMS oscillator  20  includes a first fixed electrode  21   a  provided in a first conductor layer  21  shown in  FIG. 1(   b ), and a movable electrode  22   a  provided in a second conductive layer  22  as a drive electrode. As shown in  FIG. 1(   b ), the first conductive layer  21  has a first wiring portion  21   b  connected to the first fixed electrode  21   a  and an external wire, not shown. Also, the second conductive layer  22  has the movable electrode  22   a , a second fixed electrode  22   c  formed on the main surface  10   a , and a supporting electrode  22   b  supporting the movable electrode  22   a  and connected to the second fixed electrode  22   c , and has a second wiring portion  22   d  that connects the second fixed electrode  22   c  to an external wire, not shown. The first conductive layer  21  and the second conductive layer  22  are formed by photolithographic patterning of conductive polysilicon. It should be noted that, while an example where polysilicon is used for the first conductive layer  21  and the second conductive layer  22  is given in this embodiment, the first conductive layer  21  and the second conductive layer  22  are not limited to this example. 
         [0035]    In the MEMS oscillator  20 , a gap portion G as a space where the movable electrode  22   a  can move is formed between the first fixed electrode  21   a  and the movable electrode  22   a . Also, the MEMS oscillator  20  is formed to be accommodated in a space S formed on the main surface  10   a  of the substrate  10 . The space S is formed as follows. After the first conductive layer  21  and the second conductive layer  22  are formed, a second oxide film  40  is formed. In the second oxide film  40 , to allow connection to a bottom layer  30  made of polysilicon of a space wall portion  30 , described later, a hole where the bottom layer  33  is exposed is formed simultaneously with the formation of the second conductive layer  22 , and a first wiring layer  31  is formed by photolithographic patterning. 
         [0036]    Moreover, a third oxide film  50  is formed on the second oxide film  40 . In the third oxide film  50 , a hole where the first wiring layer  31  is exposed is formed, and a second wiring layer  32  is formed by photolithographic patterning. The second wiring layer  32  includes a wall portion  32   a  forming a top layer of the space wall portion  30 , described later, and a lid portion  32   b  forming the space S housing the MEMS oscillator  20 . Moreover, in the lid portion  32   b  of the second wiring layer  32 , an opening  32   c  is provided for release etching of the second oxide film  40  and the third oxide film  50  in the area of the space S that are formed in the manufacturing process, in order to form the space S. 
         [0037]    Next, a protection film  60  is formed to expose the opening  32   c  of the second wiring layer  32 . An etching solution for etching the second oxide film  40  and the third oxide film  50  is introduced from the opening  32   c , thus forming the space S by release etching. The space S is an area surrounded by the space wall portion  30  formed by the bottom layer  33 , the first wiring layer  31  and the second wiring layer  32 . 
         [0038]    The gap portion G provided in the MEMS oscillator  20  is formed by the above release etching at the time of forming the space S. That is, after the first conductive layer  21  is formed, a fourth oxide film, not shown, is formed on the first fixed electrode  21   a , and the movable electrode  22   a  is formed on the fourth oxide film. Then, the fourth oxide film is eliminated together with the second oxide film  40  and the third oxide film  50  by release etching, thus forming the gap portion G. The second oxide film  40 , the third oxide film  50  and the fourth oxide film in the area corresponding to the space S, which are eliminated by the above release etching, are called sacrifice layers. 
         [0039]    As the release etching ends and the space S is formed, a covering layer  70  is formed, covering the lid portion  32   b  of the second wiring layer  32  that is not covered by the protection film  60 , and thus sealing the opening  32   c . The space S is thus sealed airtightly. 
         [0040]    The MEMS pressure sensor  100  is formed in this way. In the MEMS pressure sensor  100  according to this embodiment, a recessed portion  11   b  is formed on the wafer substrate  11  from aback side  10   c  of the substrate  10  as a second surface opposite to the main surface  10   a  of the substrate  10  corresponding to the MEMS oscillator  20 . As the recessed portion  11   b  is formed, a thin portion  11   c  is formed in the area on the main surface  10   a  where the MEMS oscillator  20  is formed. This thin portion  11   c , the first oxide film  12  formed on the thin portion  11   c , and the nitride film  13 , form a diaphragm portion  10   b . In other words, the MEMS oscillator  20  is formed on the main surface  10   a  in the area of the diaphragm portion  10   b.    
         [0041]      FIG. 2  is a view of the configuration for explaining the operation of the MEMS pressure sensor  100 . The operation state of the MEMS pressure sensor  100  shown in  FIG. 2(   a ) is the operation of the MEMS oscillator  20  in a so-called static state where an external pressure as an external force is not applied to the diaphragm portion  10   b . As shown in  FIG. 2(   a ), in the MEMS oscillator  20  in the static state, the movable electrode  22   a  is spaced apart by the gap portion G from the first fixed electrode  21   a . The movable electrode  22   a  has a cantilever structure in which the movable electrode  22   a  is fixed to the substrate  10  by the second fixed electrode  22   c , at a junction point Pf between the main surface  10   a  of the substrate  10  and the supporting electrode  22   b  as a fixing point. An electrostatic force generated by electric charges applied to the first fixed electrode  21   a  and the movable electrode  22   a  causes the movable electrode  22   a  to oscillate in an F-direction. Also, by detecting a change in the electrostatic capacitance of the gap portion G, oscillation characteristics such as oscillation frequency of the MEMS oscillator  20  can be acquired. 
         [0042]    In the MEMS pressure sensor  100  having the MEMS oscillator  20  that can be made to oscillate as described above, a pressure p is applied as an external force to the diaphragm portion  10   b  of the substrate  10 , as shown in  FIG. 2(   b ), and the pressure p applied to a bottom surface  10   d  of the diaphragm portion  10   b  causes a stress to be applied to the diaphragm portion  10   b . The main surface  10   a  of the substrate  10  is deformed into a main surface  10   a ′ having a flexure  8 . At this time, the direction of a tangent Lt to the deformed main surface  10   a ′ of a diaphragm portion  10   b ′ deformed at the junction point Pf is inclined at an angle  8  to the main surface  10   a  of the substrate  10  where the diaphragm portion  10   b  is not formed. 
         [0043]    With the angle of inclination  8  of the deformed main surface  10   a ′ relative to the main surface  10   a , the movable electrode  22   a  is also inclined relative to the main surface  10   a . As a result, a gap portion G′ following the deformation is enlarged from the gap portion G in the MEMS oscillator  20  in the static state. The electrostatic force between the first fixed electrode  21   a  and the movable electrode  22   a  changes, and the resonance frequency changes. By finding the relation between this change in the resonance frequency and the pressure p applied to the diaphragm portion  10   b , the MEMS pressure sensor  100  can be provided. 
         [0044]    As described above, as the diaphragm portion  10   b  is deformed by the pressure p, the gap portion G changes into the gap portion G′ and this is detected as a change in the resonance frequency. Therefore, it is preferable that the first fixed electrode  21   a  and the movable electrode  22   a  are arranged in such a way as to increase the amount of change into the gap portion G′ following the change. The arrangement of the first fixed electrode  21   a  and the movable electrode  22   a  is described with reference to  FIG. 3 .  FIG. 3(   a ) is a plan view of the MEMS pressure sensor  100 , and  FIG. 3(   b ) is a plan view of a MEMS pressure sensor  110 . In the case of the MEMS pressure sensor  100  shown in  FIG. 3(   a ), the diaphragm portion  10   b  has a circular shape as viewed in a plan view, and this is the same as the form shown in  FIG. 1(   a ). In the case of the MEMS pressure sensor  110  shown in  FIG. 3(   b ), a diaphragm portion  10   e  has a hexagonal shape as viewed in a plan view, as an example of a polygonal shape.  FIG. 3(   c ) is a schematic cross-sectional view showing the MEMS oscillator  20  in the state where the pressure p is applied to the diaphragm portions  10   b ,  10   e.    
         [0045]    In the MEMS pressure sensor  100  shown in  FIG. 3(   a ), the diaphragm portion  10   b  is formed in a circular shape as viewed in a plan view. As the positional relation between the first fixed electrode  21   a  and the movable electrode  22   a , as shown in  FIG. 3(   a ), there is a distance between a first fixed electrode end portion  21   c  of the first fixed electrode  21   a  facing the second fixed electrode  22   c  and a second fixed electrode end portion  22   e  of the second fixed electrode  22   c  facing the first fixed electrode  21   a . That is, the first fixed electrode end portion  21   c  and the second fixed electrode end portion  22   e  are end portions facing each other, and the first fixed electrode end portion  21   c  and the second fixed electrode end portion  22   e  are spaced apart from each other by a distance a. 
         [0046]    Also, the circular shape of the diaphragm portion  10   b  as viewed in a plan view is formed with a diameter φb B . In this case, it is preferable that the distance a between the first fixed electrode end portion  21   c  and the second fixed electrode end portion  22   e  is set to meet the following condition. 
         [0000]      0 &lt;a&lt; 0.3 b   B   (1)
 
         [0047]    As shown in  FIG. 3(   c ), with the angle of inclination O relative to the main surface  10   a  of the substrate  10  where the diaphragm portion  10   b  is not formed, which is in the direction of the tangent Lt to the deformed main surface  10   a ′ of the diaphragm portion  10   b ′ deformed at the junction point Pf, the movable electrode  22   a  is spaced apart from the first fixed electrode  21   a  and the gap portion G′ is generated due to the application of the pressure p. Therefore, by setting the distance a under the condition expressed by the formula (1), the MEMS pressure sensor  100  having the MEMS oscillator  20  which can efficiently convert the deformation of the diaphragm portion  10   b  due to the applied pressure p into the change into the gap portion G′ and can securely detect the change in the resonance frequency due to the change of the gap portion G into the gap portion G′, while continuing oscillation drive of the movable electrode  22   a , can be provided. 
         [0048]    Meanwhile, in the case where the diaphragm portion  10   e  has a hexagonal shape as viewed in a plan view, as in the MEMS pressure sensor  110  shown in  FIG. 3(   b ), the diameter of the inscribed circle  10   f  of an imaginary shape inscribed in the hexagonal planar shape may be regarded as the diameter b B , and the distance a between the first fixed electrode end portion  21   c  and the second fixed electrode end portion  22   e  may be set to meet the condition of the formula (1). 
         [0049]      FIG. 4  is a plan view showing another arrangement of the MEMS oscillator  20  shown in  FIGS. 3(   a ) and  3 ( b ).  FIG. 4(   a ) shows the case where the diaphragm portion  10   b  provided in the MEMS pressure sensor  100  has a circular shape as viewed in a plan view.  FIG. 4  ( b ) shows the case where the diaphragm portion  10   e  provided in the MEMS pressure sensor  110  has a hexagonal shape as viewed in a plan view, as an example of a polygonal shape. 
         [0050]    As shown in  FIG. 4(   a ), the center C B  of the shape as viewed in a plan view (shaded portion as illustrated) of the first fixed electrode  21   a  is arranged to fall within a circular area having a diameter c that is concentric with the circle having the diameter b B  as viewed in a plan view of the diaphragm portion  10   b . It is preferable that the diameter c of the circular area where the center C E  is arranged has the following relation. 
         [0000]      0 &lt;c&lt; 0.93 b   B   (2)
 
         [0051]    As the first fixed electrode  21   a  is arranged in such a way that the planar shape center C E  of the first fixed electrode  21   a  is arranged within the area set by the condition expressed by the formula (2), and the distance a between the first fixed electrode end portion  21   c  and the second fixed electrode end portion  22   e  is set according to the condition expressed by the formula (1), the gap portion G′ can be made large even if the pressure p applied to the diaphragm portion  10   b  is small and therefore the flexure  8  is small. 
         [0052]    As shown in  FIG. 3(   c ), in the recessed portion  11   b  forming the diaphragm portion  10   b , the diameter b W  of the opening of the recessed portion  11   b  on the back side  10   c  of the substrate  10  has the relation of 
         [0000]    
       
      
       b 
       B 
       &lt;b 
       W  
      
     
         [0000]    relative to the diameter b B  of the shape as viewed in a plan view on the bottom surface  10   d . With this configuration, a corner portion  11   f  formed by a recessed portion bottom surface  11   d  of the wafer substrate  11  that corresponds to the bottom surface  10   d  and a recessed portion wall surface  11   e , of the recessed portion  11   b , does not have an acute angle, and damage to the wafer substrate  11  due to stress concentration or the like at the corner portion  11   f  can be restrained even if flexure and deformation of the diaphragm portion  10   b  is repeated. Moreover, etching performance for shaping the recessed portion  11   b  can be improved. 
         [0053]    In the case of the MEMS pressure sensor  110  shown in  FIGS. 4  ( b ), the center C E  of the shape as viewed in a plan view (shaded portion as illustrated) of the first fixed electrode  21   a  is arranged to fall within a circular area having a diameter c that is concentric with the inscribed circle  10   f  of an imaginary shape as viewed in a plan view of the diaphragm portion  10   e . It is preferable that the diameter c of the circular area where the center C E  is arranged has the condition expressed by the formula (2). 
         [0054]      FIG. 5  shows another form of the MEMS pressure sensor.  FIG. 5  shows a MEMS pressure sensor  200 .  FIG. 5(   a ) is a plan view, seen by penetrating the covering layer  70 .  FIG. 5(   b ) is a cross-sectional view taken along C-C′ shown in  FIG. 5(   a ). The MEMS pressure sensor  200  is different from the above MEMS pressure sensors  100 ,  110  only in the configuration of the second conductive layer  22 , and the same in the other configurations. Therefore, the same configurations as the MEMS pressure sensors  100 ,  110  are denoted by the same reference numerals and explanation thereof is omitted. 
         [0055]    As shown in  FIG. 5(   b ), in the MEMS pressure sensor  200 , a MEMS oscillator  20  as a resonator is formed on a main surface  10   a  as a first surface of a substrate  10 , that is, on a surface  13   a  of a nitride film  13 . The MEMS oscillator  20  includes a first fixed electrode  21   a  provided in a first conductive layer  21 , and a movable electrode  24   a  provided in a third conductive layer  24 . In the third conductive layer  24 , a supporting electrode  24   b  is provided extending from the movable electrode  24   a . Then, a connection electrode  24   c  as a second fixed electrode is provided extending from the supporting electrode  24   b . Also, a second conductive layer  23  is provided on the main surface  10   a  of the substrate  10 . The second conductive layer  23  has a substrate electrode  23   a . As the connection electrode  24   c  provided in the third conductive layer  24  is connected to the substrate electrode  23   a , the third conductive layer  24  is fixed to the substrate  10  via the substrate electrode  23   a . Also, the first conductive layer  21  has a first wiring portion  21   b  connected to the first fixed electrode  21   a  and an external wire, not shown. Moreover, the second conductive layer  23  has a second wiring portion  23   b  connected to the substrate electrode  23   a  and an external wire, not shown. 
         [0056]    In the first conductive layer  21  and the second conductive layer  23 , the first fixed electrode  21   a  and the substrate electrode  23   a  are formed by photolithographic patterning of a conductive polysilicon on the main surface  10   a  of the substrate  10 . A fourth oxide film, not shown, is formed on the first fixed electrode  21   a  and the substrate electrode  23   a  thus formed. In the fourth oxide film on the substrate electrode  23   a , an opening for forming the connection electrode  24   c  of the third conductive layer  24  on the substrate electrode  23   a  is provided. Then, the third conductive layer  24  is formed on the fourth oxide film. Then, the fourth oxide film is eliminated together with the second oxide film  40  and the third oxide film  50  by release etching. The gap portion G is formed as the gap between the first fixed electrode  21   a  and the movable electrode  24   a.    
         [0057]    In the MEMS pressure sensor  200  shown in  FIG. 5 , the diaphragm portion  10   b  is formed with a circular shape as viewed in a plan view. As the positional relation between the first fixed electrode  21   a  and the movable electrode  24   a , as shown in  FIG. 5(   c ), which is an enlarged view of the part of the MEMS oscillator  20 , there is a distance between a first fixed electrode end portion  21   c  facing the substrate electrode  23   a , of the first fixed electrode  21   a , and a connection electrode end portion  24   d  facing the first fixed electrode  21   a , of the connection electrode  24   c  of the third conductive layer  24 . That is, the first fixed electrode end portion  21   c  and the connection electrode end portion  24   d  are end portions facing each other, and the first fixed electrode end portion  21   c  and the connection electrode end portion  24   d  are spaced apart from each other by a distance d. 
         [0058]    If the circular shape of the diaphragm portion  10   b  as viewed in a plan view is formed with a diameter φb B , it is preferable that the distance d between the first fixed electrode end portion  21   c  and the connection electrode end portion  24   d  is set to meet the following condition. 
         [0000]      0 &lt;d&lt; 0.3 b   B   (3)
 
         [0059]    That is, the distance d is equivalent to the distance a in the formula (1) in the cases of the above MEMS pressure sensors  100 ,  110 . Also, even in the case where a diaphragm that is the same as the diaphragm portion  10   e  having a hexagonal planar shape in the MEMS pressure sensor  110  is formed, the diameter of the inscribed circle  10   f  of an imaginary shape inscribed in the hexagonal planar shape may be regarded as the diameter b B  (see  FIG. 3 ), and the distance d between the first fixed electrode end portion  21   c  and the connection electrode end portion  24   d  may be set to meet the condition of the formula (3). 
         [0060]    Also, as shown in  FIG. 5(   a ), the center C E  of the shape as viewed in a plan view (shaded portion as illustrated) of the first fixed electrode  21   a  is arranged to fall within a circular area having a diameter c that is concentric with the circle having the diameter b B  as viewed in a plan view of the diaphragm portion  10   e . It is preferable that the diameter c of the circular area where the center C E  is arranged is set under the condition of the formula (2) also in the case of the MEMS pressure sensor  200 . 
         [0061]    According to the above MEMS pressure sensors  100 ,  110 ,  200 , since the MEMS oscillator  20  is formed on the part of the main surface  10   a  of the diaphragm portion  10   b  that is flexed and deformed by an external pressure, even a slight flexure and deformation of the diaphragm  10   b , that is, even a very small external pressure causes a change in the resonance frequency of the MEMS oscillator  20 . Therefore, a pressure sensor capable of detecting such a change can be provided. Moreover, a small-sized pressure sensor that can be formed in the same process as a semiconductor process can be provided. 
         [0062]    As described above, the MEMS pressure sensors  100 ,  110 ,  200  according to this embodiment are manufactured using a semiconductor process. Therefore, these MEMS pressure sensors can be integrated with a semiconductor device or so-called IC.  FIG. 6  shows a configuration in which the above MEMS pressure sensor  100  and a semiconductor device are formed in one chip. A MEMS pressure sensor  300  shown in  FIG. 6  has a configuration in which the MEMS pressure sensor  100  and a semiconductor device  310  are formed in one chip. The MEMS pressure sensor  100  is a micro device that can be manufactured by using a semiconductor manufacturing device and by a semiconductor manufacturing method. Therefore, the semiconductor device  310  can be easily formed on the same wafer substrate  11  as the MEMS pressure sensor  100 . The semiconductor device  310  is provided with an oscillation circuit which drives the MEMS pressure sensor  100 , and an arithmetic circuit which calculates a frequency change in the MEMS pressure sensor  100 , and the like. As shown in the MEMS pressure sensor  300 , by forming the semiconductor device  310  with the MEMS pressure sensor  100  in one chip, a MEMS pressure sensor as a small-sized sensor device can be provided. 
       Second Embodiment 
       [0063]    As a second embodiment, an altimeter will be described with reference to the drawings. The altimeter according to the second embodiment is a form of an electronic apparatus having a pressure sensor as an electronic device having the MEMS pressure sensor  100 ,  110 ,  200 ,  300  according to the first embodiment. 
         [0064]    As shown in  FIG. 7(   a ), an altimeter  1000  according to the second embodiment has, in a casing  1100 , the MEMS pressure sensor  300  according to the first embodiment, a sensor fixture frame  1200  as a holding unit which holds the MEMS pressure sensor  300  and is installed in the casing  1100 , and an arithmetic unit  1300  as a data processing unit which calculates a data signal obtained from the MEMS pressure sensor  300  into altitude data. In the casing  1100 , an opening  1100   a  is provided through which air can circulate between the diaphragm portion  10   b  (see  FIG. 1)  of the MEMS pressure sensor  100  provided in the MEMS pressure sensor  300  and the atmosphere. 
         [0065]    Details of a D portion shown in  FIG. 7(   a ), that is, a cross-section of an installing portion of the MEMS pressure sensor  300 , is shown in  FIG. 7(   b ). As shown in  FIG. 7(   b ), the diaphragm portion  10  of the MEMS pressure sensor  100  is arranged to be exposed to the side of the opening  1100   a . Also, the sensor fixture frame  1200  has a through-hole  1200   a , and the through-hole  1200   a , too, is arranged in such a way that the diaphragm portion  10   b  of the MEMS pressure sensor  100  is exposed thereto. The sensor fixture frame  1200  and the MEMS pressure sensor  300  are bonded together by such measures as adhering to a bonding surface  1200   b  of the sensor fixture frame  1200 . The sensor fixture frame  1200  with the MEMS pressure sensor  300  bonded thereto is installed in the casing  1100  with a screw  1400 . The method for fixing the sensor fixture frame  1200  to the casing is not limited to the screw  1400  and may be such fixture measures as adhering. 
         [0066]    In the altimeter  1000 , air is circulated to and from the atmosphere in a pressure changing area applied to the diaphragm portion  10   b  of the MEMS pressure sensor  100  which air is allowed to circulate to and from via the opening  1100   a  in the casing  1100  and the through-hole  1200   a  in the sensor fixture frame  1200 , and the altimeter  1000  detects the pressure in the atmosphere (hereinafter referred to as atmospheric pressure) and outputs altitude data. The outputted altitude data is transmitted to a personal computer  2000  (hereinafter referred to as PC  2000 ) having a display unit  2100  shown in  FIG. 7(   a ) and is displayed on the display unit  2100  of the PC  2000 . In this case, with processing software provided in the PC  2000 , various kinds of data processing such as storing the altitude data, graph representation, and display on map data can be carried out. Also, a data processing device, a display unit, an external operation unit and the like can be provided in the altimeter  1000 , instead of the PC  2000 . 
         [0067]      FIG. 8  shows another form of the MEMS pressure sensor  300  provided in the altimeter  1000  according to the second embodiment.  FIG. 8  shows the D portion in  FIG. 7(   a ), of the altimeter  1000  shown in  FIG. 7(   a ). As shown in  FIG. 8 , a flexible film  400  having flexibility and airtightness is fixed to the MEMS pressure sensor  300 . As the flexible film  400 , for example, a material having elasticity and low gas permeability such as fluorine resin or synthetic rubber, or a metal thin film is preferable. 
         [0068]    The flexible film  400  is arranged to cover the diaphragm portion  10   b  of the MEMS pressure sensor  100  and fixed to the substrate  10  at a flange portion  400   a . In this case, a space Q (dot-hatched portion as illustrated) formed by the substrate  10  and the flexible film  400  is filled with gases, for example, air and inert gas, and formed as a pressure changing area. The MEMS pressure sensor  300  having the flexible film  400  is fixed to the sensor fixture frame  1200  and installed in the casing  1100 . 
         [0069]    Since the MEMS pressure sensor  300  has the flexible film  400 , foreign bodies, dust and the like from outside can be prevented from attaching to the MEMS pressure sensor  100  and the MEMS pressure sensor  100  can be kept clean. Therefore, stable altimeter performance can be provided. Also, even if the external environment of the flexible film  400  has a liquid, corrosive gas or the like, damage to the MEMS pressure sensor  300  can be restrained. 
       Third Embodiment 
       [0070]    A navigation system as an electronic apparatus having the MEMS pressure sensor  100 ,  110 ,  200 ,  300  according to the first embodiment or the altimeter  1000  according to the second embodiment, and an automobile as an example of a moving object equipped with the navigation system, will be described. 
         [0071]      FIG. 9  is a view of the appearance of an automobile  4000  as a moving object having a navigation system  3000  as an electronic apparatus. The navigation system  3000  includes map information, not shown, a position information acquisition unit based on the GPS (global positioning system), an autonomous navigation unit using a gyro sensor, an acceleration sensor and vehicle speed data, and the altimeter  1000  according to the second embodiment, and displays predetermined position information or traveling route information on a display unit  3100  arranged at a position that can be visually recognized by the driver. 
         [0072]    In the automobile  4000  shown in  FIG. 9 , since the altimeter  1000  is provided in the navigation system  3000 , altitude information can be acquired in addition to the acquired position information. For example, in the case of traveling on an elevated road which represents substantially the same position as an ordinary road in terms of position information, if having no altitude information, the navigation system cannot determine whether the automobile is traveling on an ordinary road or an elevated road and consequently provides information about an ordinary road to the driver as priority information. Thus, in the navigation system  3000  according to this embodiment, altitude information can be acquired by the altimeter  1000 , and a change in altitude due to entry into an elevated road from an ordinary road can be detected. Thus, navigation information of the traveling state on the elevated road can be provided to the driver. 
         [0073]    Also, with the MEMS pressure sensor  100 ,  110 ,  200 ,  300  according to the first embodiment, a small-sized pressure detection device can be formed, and a hydraulic or air-pressure drive system can be easily incorporated in the automobile  4000 . Thus, monitoring and control data of the pressure in the device can be easily acquired. 
         [0074]    The entire disclosure of Japanese Patent Application No. 2013-089119, filed Apr. 22, 2013 is expressly incorporated by reference herein.