Patent Publication Number: US-2023146158-A1

Title: Pressure sensor structure, pressure sensor device, and method of manufacturing pressure sensor structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-124567 filed on Jul. 21, 2020 and is a Continuation application of PCT Application No. PCT/JP2021/026151 filed on Jul. 12, 2021. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pressure sensor structure for measuring pressure such as atmospheric pressure or water pressure and a pressure sensor device including the pressure sensor structure. Further, the present invention relates to a method of manufacturing a pressure sensor structure. 
     2. Description of the Related Art 
     Pressure sensors can be manufactured by using microelectromechanical systems (MEMS) technology, which is an applied technology of semiconductor manufacturing technology, and can be implemented as microsensors of approximately 0.5 mm square to approximately 2 mm square, for example. A typical pressure sensor has a capacitor structure including two electrodes and detects a change in electrostatic capacitance due to a change in ambient pressure, thereby being capable of measuring pressure. 
       FIG.  9 A  is a sectional view illustrating an example of a related-art pressure sensor structure, and  FIG.  9 B  is a plan view thereof. This pressure sensor structure is disclosed in International Publication No. 2015/107453, for example, and includes a diaphragm plate  87  that functions as a sense electrode, a base electrode  86  facing the diaphragm plate  87 , a sidewall layer  85 , and the like. The sidewall layer  85  includes a guard electrode layer  83  and electrically insulating layers  82  and  84  disposed on and under the guard electrode layer  83 . A base substrate  81  is made of a conductive material and conducted with the base electrode  86 . The guard electrode layer  83  is formed in the same layer as the base electrode  86  and sandwiched by the diaphragm plate  87 , which is located above the guard electrode layer  83 , and the base substrate  81 , which is located below the guard electrode layer  83 , to form a three-layer electrode structure. With this, stray capacitance unrelated to a pressure change can be canceled. 
     SUMMARY OF THE INVENTION 
     Such a pressure sensor structure is obtained as follows: a large number of pressure sensor structures are formed on a single semiconductor wafer by using MEMS technology, and the semiconductor wafer is then cut (singulated) into individual chips. The obtained chip is fixed on a circuit substrate  90  by using an adhesive  91  (chip bonding) and accommodated in a case together with an integrated circuit configured to perform signal processing. A pressure sensor device is complete in this way. 
     In this case, while the back surface of the base substrate  81  is in close contact with the circuit substrate  90 , the end surfaces of the base substrate  81  are exposed. Since the environment surrounding the pressure sensor structure interacts with the atmosphere, a liquid Q such as water may possibly adhere to the end surfaces of the base substrate  81  due to condensation, flooding, or the like. The liquid Q functions as a bridge between the diaphragm plate  87  and the base substrate  81  to change the stray capacitance, resulting in a shift in pressure output value in some cases. Further, the diaphragm plate  87  and the base electrode  86  are affected by electromagnetic noise coming from the outside, resulting in a shift in pressure output value in some cases. 
     Preferred embodiments of the present invention provide pressure sensor structures each capable of curbing the effects of disturbances and measuring pressure highly precisely, and pressure sensor devices including the pressure sensor structures. Further, other preferred embodiments of the present invention provide methods of manufacturing pressure sensor structures each capable of curbing the effects of disturbances and measuring pressure highly precisely. 
     An aspect of a preferred embodiment of the present invention is a pressure sensor structure to detect a change in interelectrode electrostatic capacitance, the pressure sensor structure including a sensor body including a diaphragm plate that functions as a sense electrode, a base electrode that faces the diaphragm plate, and a sidewall layer to maintain a gap between the diaphragm plate and the base electrode, and a conductive guard substrate to support the sensor body, wherein the sidewall layer includes a guard electrode layer and upper and lower guard electrode insulating layers to electrically insulate the guard electrode layer, and the guard substrate is electrically connected to the guard electrode layer to function as a guard electrode together with the guard electrode layer. 
     A pressure sensor device according to another aspect of a preferred embodiment of the present invention includes the above-described pressure sensor structure, an integrated circuit to process a signal from the pressure sensor structure, a circuit substrate on which the integrated circuit and the pressure sensor structure are mounted, and a case made of metal or a synthetic resin to accommodate the integrated circuit and the pressure sensor structure together with the circuit substrate. 
     Still another aspect of a preferred embodiment of the present invention provides a method of manufacturing a pressure sensor structure to detect a change in interelectrode electrostatic capacitance, the method including forming a lower substrate that includes a base electrode, forming an upper substrate that includes a diaphragm plate that functions as a sense electrode, joining the lower substrate and the upper substrate to each other, shaping the lower substrate and the upper substrate to form a stepped shape, and forming a passivation film on an outer surface of each of the lower substrate and the upper substrate, which have been shaped, wherein the forming the lower substrate includes sequentially forming a lower guard electrode insulating layer and a conductive layer on the lower substrate, which is conductive, and forming a base electrode and a guard electrode layer by etching the conductive layer, and the forming the upper substrate includes forming an electrically insulating layer on the upper substrate, which is conductive, and forming an upper guard electrode insulating layer by etching the electrically insulating layer. 
     According to preferred embodiments of the present invention, it is possible to curb the effects of disturbances and measure pressure highly precisely. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 1 of the present invention, and  FIG.  1 B  is a plan view thereof. 
         FIG.  2 A  is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 2 of the present invention, and  FIG.  2 B  is a plan view thereof. 
         FIG.  3    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 3 of the present invention. 
         FIG.  4    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 4 of the present invention. 
         FIG.  5    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 5 of the present invention. 
         FIGS.  6 A to  6 C  are sectional views illustrating various examples of a pressure sensor device according to Preferred Embodiment 6 of the present invention. 
         FIGS.  7 A to  7 F  are explanatory diagrams illustrating an example of a method of manufacturing a pressure sensor structure according to Preferred Embodiment 7 of the present invention. 
         FIGS.  8 A to  8 E  are explanatory diagrams illustrating the example of the method of manufacturing a pressure sensor structure according to Preferred Embodiment 7 of the present invention. 
         FIG.  9 A  is a sectional view illustrating an example of a related-art pressure sensor structure, and  FIG.  9 B  is a plan view thereof. 
         FIG.  10    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 8 of the present invention. 
         FIG.  11    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 9 of the present invention. 
         FIG.  12    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 10 of the present invention. 
         FIG.  13    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 11 of the present invention. 
         FIG.  14    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 12 of the present invention. 
         FIG.  15    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 13 of the present invention. 
         FIG.  16    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 14 of the present invention. 
         FIG.  17    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 15 of the present invention. 
         FIG.  18    is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 16 of the present invention. 
         FIG.  19 A  is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 17 of the present invention, and  FIG.  19 B  is a plan view thereof. 
         FIG.  20 A  is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 18 of the present invention, and  FIG.  20 B  is a plan view thereof. 
         FIG.  21 A  is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 19 of the present invention, and  FIG.  21 B  is a plan view thereof. 
         FIG.  22 A  is a sectional view illustrating an example of a pressure sensor structure according to Preferred Embodiment 20 of the present invention, and  FIG.  22 B  is a plan view thereof. 
         FIG.  23    is a circuit diagram illustrating an example of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. 
         FIG.  24    is a circuit diagram illustrating an example of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. 
         FIG.  25    is a circuit diagram illustrating an example of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. 
         FIG.  26    is a circuit diagram illustrating an example of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. 
         FIG.  27    is a circuit diagram illustrating an example of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. 
         FIG.  28    is a circuit diagram illustrating an example of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An aspect of a preferred embodiment of the present invention provides a pressure sensor structure to detect a change in interelectrode electrostatic capacitance, the pressure sensor structure including a sensor body including a diaphragm plate that functions as a sense electrode, a base electrode that faces the diaphragm plate, and a sidewall layer to maintain a gap between the diaphragm plate and the base electrode; and a conductive guard substrate to support the sensor body, wherein the sidewall layer includes a guard electrode layer and upper and lower guard electrode insulating layers to electrically insulate the guard electrode layer, and the guard substrate is electrically connected to the guard electrode layer to function as a guard electrode together with the guard electrode layer. 
     With this configuration, the guard substrate is electrically connected to the guard electrode layer to function as a guard electrode. Thus, even when a liquid such as water adheres to the substrate end surface, a shift in pressure output value due to a change in potential of the base electrode can be prevented. 
     Further, since the guard substrate, which is the bottom layer, functions as a guard electrode, the pressure sensor structure is less affected by electromagnetic noise coming from an electronic circuit under the pressure sensor structure. Thus, a shift in pressure output value due to disturbances can be prevented. 
     Further, in a state where a large number of pressure sensor structures are on a single wafer substrate, the diaphragm plate and the base electrode included in each of the individual pressure sensor structures are electrically isolated from each other. Thus, the characteristics of the individual chips can be tested before chip cutting. 
     Preferably, a substrate insulating layer to electrically insulate the guard substrate is provided between the guard substrate and the sensor body. 
     With this configuration, the guard substrate and the sensor body can be electrically insulated from each other. 
     Preferably, a passivation film is provided on an outer surface of each of the sensor body and the guard substrate. 
     With this configuration, even when a liquid such as water adheres to the outer surface of the pressure sensor structure due to condensation, flooding, or the like, the passivation film can curb the effect of the liquid. 
     Preferably, the base electrode is provided in a same layer as the guard electrode layer and electrically isolated from the guard electrode layer. 
     With this configuration, an error in the dimension of the gap between the diaphragm plate and the base electrode can be reduced. Further, since the base electrode and the guard electrode layer can be formed by using the same layer formation process, a simplified manufacturing process and reduced cost can be achieved. 
     Preferably, between the substrate insulating layer and the sensor body, a second base electrode and a second guard electrode layer are provided in a same layer and electrically isolated from each other. 
     With this configuration, the second guard electrode layer can function as a guard electrode together with the guard electrode layer. Thus, a further enhanced guard electrode function can be provided. 
     Preferably, at least one of the diaphragm plate, the upper guard electrode insulating layer, the guard electrode layer, the lower guard electrode insulating layer, and the guard substrate has formed therein a groove to relax stress. 
     With this configuration, stress to be applied to the sensor body and/or the guard substrate is relaxed so that the sensor structure with improved life and reliability can be provided. 
     Preferably, the lower guard electrode insulating layer, the guard electrode layer, and the upper guard electrode insulating layer have a frame shape along a periphery of the diaphragm plate, and a distance W1 between inner edges of the lower guard electrode insulating layer, a distance W6 between outer edges thereof, a distance W2 between inner edges of the guard electrode layer, a distance W5 between outer edges thereof, a distance W3 between inner edges of the upper guard electrode insulating layer, and a distance W4 between outer edges thereof satisfy W1&lt;W2&lt;W3 and W4&lt;W5&lt;W6. 
     With this configuration, a larger creepage distance is secured on the outer surface of the sensor body. Thus, the effects of external leakage current and stray capacitance between the diaphragm plate and the base electrode on a pressure output value can be reduced. 
     Preferably, the lower guard electrode insulating layer has a plate shape larger than a contour of the diaphragm plate, the guard electrode layer and the upper guard electrode insulating layer have a frame shape along a periphery of the diaphragm plate, and a distance W6 between outer edges of the lower guard electrode insulating layer, a distance W2 between inner edges of the guard electrode layer, a distance W5 between outer edges thereof, a distance W3 between inner edges of the upper guard electrode insulating layer, and a distance W4 between outer edges thereof satisfy W2&lt;W3 and W4&lt;W5&lt;W6. 
     With this configuration, a larger creepage distance is secured on the outer surface of the sensor body so that the effects of external leakage current and stray capacitance between the diaphragm plate and the base electrode on a pressure output value can be reduced. 
     Preferably, the upper and lower guard electrode insulating layers include silicon dioxide (SiO 2 ) at least in part. 
     With this configuration, silicon dioxide is obtained by a silicon oxidization process. Thus, a simplified manufacturing process and reduced cost can be achieved. 
     A pressure sensor device according to another aspect of a preferred embodiment of the present invention includes the above-described pressure sensor structure, an integrated circuit to process a signal from the pressure sensor structure, a circuit substrate on which the integrated circuit and the pressure sensor structure are mounted, and a case made of metal or a synthetic resin to accommodate the integrated circuit and the pressure sensor structure together with the circuit substrate. 
     With this configuration, the pressure sensor device capable of curbing the effects of disturbances such as condensation, flooding, and electromagnetic noise can be achieved. 
     Still another aspect of a preferred embodiment of the present invention provides a method of manufacturing a pressure sensor structure to detect a change in interelectrode electrostatic capacitance, the method including forming a lower substrate that includes a base electrode, forming an upper substrate that includes a diaphragm plate that functions as a sense electrode, joining the lower substrate and the upper substrate to each other, shaping the lower substrate and the upper substrate to form a stepped shape, and forming a passivation film on an outer surface of each of the lower substrate and the upper substrate, which have been shaped, wherein the forming the lower substrate includes sequentially forming a lower guard electrode insulating layer and a conductive layer on the lower substrate, which is conductive, and forming a base electrode and a guard electrode layer by etching the conductive layer, and the forming the upper substrate includes forming an electrically insulating layer on the upper substrate, which is conductive, and forming an upper guard electrode insulating layer by etching the electrically insulating layer. 
     With this configuration, the pressure sensor structure capable of curbing the effects of disturbances such as condensation, flooding, and electromagnetic noise can be achieved. 
     Preferred Embodiment 1 
       FIG.  1 A  is a sectional view illustrating an example of a pressure sensor structure  1  according to Preferred Embodiment 1 of the present invention, and  FIG.  1 B  is a plan view thereof. To promote a better understanding, the illustration of the passivation film is omitted in  FIG.  1 B . 
     This pressure sensor structure  1  includes a sensor body including a diaphragm plate  40 , a base electrode  22 , and a sidewall layer  30 , a guard substrate  10  to support the sensor body, and the like. 
     The diaphragm plate  40  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and functions as a sense electrode deformable depending on an ambient pressure difference. The diaphragm plate  40  can include one or a plurality of layers, and an electrically insulating layer may be provided on the upper surface of the diaphragm plate  40 , for example. The base electrode  22  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and disposed to face the diaphragm plate  40 . The sidewall layer  30  is provided to maintain a gap G between the diaphragm plate  40  and the base electrode  22 . The gap G is a space sealed from the outside and maintained at constant pressure with an inert gas filled into the gap G, for example. 
     The diaphragm plate  40  and the base electrode  22  define a parallel plate capacitor. An electrostatic capacitance C between the electrodes is represented by C=ε×S/d by using a dielectric constant of the gap G, an electrode area S, and an interelectrode distance d. When the diaphragm plate  40  is elastically deformed due to a pressure difference between the outside and the gap G, the interelectrode distance d is changed and the electrostatic capacitance C is accordingly changed. 
     The sidewall layer  30  is positioned to define a frame surrounding the gap G and includes at least three layers, that is, a guard electrode layer  32 , an electrically insulating layer  31  disposed under the guard electrode layer  32 , and an electrically insulating layer  33  disposed on the guard electrode layer  32 . Here, the three-layer configuration is exemplified, but the sidewall layer  30  may include four or more layers. 
     The guard substrate  10  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example. The guard substrate  10  can include one or a plurality of layers, and an electrically insulating layer may be provided on the lower surface of the guard substrate  10 , for example. An intermediate layer  20  is provided between the guard substrate  10  and the sidewall layer  30 . The intermediate layer  20  includes at least two layers, that is, a substrate-side electrically insulating layer  21  and the above-mentioned base electrode  22  located thereon. Here, the two-layer configuration is exemplified, but the intermediate layer  20  may include three or more layers. 
     The guard electrode layer  32  is interposed between the diaphragm plate  40 , which is located above the guard electrode layer  32 , and the base electrode  22 , which is located below the guard electrode layer  32 . With this, stray capacitance unrelated to a pressure change can be canceled. 
     Further, the guard substrate  10  is electrically connected to the guard electrode layer  32  to function as a guard electrode together with the guard electrode layer  32 . The guard substrate  10  and the guard electrode layer  32  may be conducted with each other by a via hole, a wire, or the like in this structure, or may be conducted with each other via an external electronic circuit. 
     With such a configuration, even when a liquid such as water adheres to the substrate end surface, a shift in pressure output value due to a change in potential of the base electrode can be prevented. Further, since the guard substrate  10 , which is the bottom layer, functions as a guard electrode (shield electrode), the pressure sensor structure is less affected by electromagnetic noise coming from an electronic circuit placed under the pressure sensor structure, with the result that a shift in pressure output value can be prevented. 
     A passivation film  45  is provided on the outer surfaces of the sensor body, the electrically insulating layer  21 , and the guard substrate  10 . The passivation film  45  is made of an electrically insulating material such as SiN x  or SiO 2 , for example, and protects the pressure sensor structure  1 . 
     Preferred Embodiment 2 
       FIG.  2 A  is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 2 of the present invention, and  FIG.  2 B  is a plan view thereof. To promote a better understanding, the illustration of the passivation film is omitted in  FIG.  2 B . In the present preferred embodiment, as compared to the configuration of  FIGS.  1 A and  1 B , a base electrode  34  is provided in the same layer as the guard electrode layer  32  instead of the base electrode  22 . 
     This pressure sensor structure  1  includes a sensor body including the diaphragm plate  40 , the base electrode  34 , and the sidewall layer  30 , and the guard substrate  10  configured to support the sensor body. 
     The diaphragm plate  40  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and functions as a sense electrode deformable depending on an ambient pressure difference. The diaphragm plate  40  can include one or a plurality of layers, and an electrically insulating layer may be provided on the upper surface of the diaphragm plate  40 , for example. The base electrode  34  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and disposed to face the diaphragm plate  40 . The sidewall layer  30  is provided to maintain a gap G between the diaphragm plate  40  and the base electrode  34 . The gap G is a space sealed from the outside and maintained at constant pressure with an inert gas filled into the gap G, for example. The diaphragm plate  40  and the base electrode  34  define a parallel plate capacitor. 
     The sidewall layer  30  is positioned to define a frame surrounding the gap G and includes at least three layers, that is, the guard electrode layer  32 , the electrically insulating layer  31  disposed under the guard electrode layer  32 , and the electrically insulating layer  33  disposed on the guard electrode layer  32 . Here, the three-layer configuration is exemplified, but the sidewall layer  30  may include four or more layers. 
     The base electrode  34  is provided on the electrically insulating layer  31  and in the same layer as the guard electrode layer  32  and electrically isolated from the guard electrode layer  32  with a trench  34   a . With such a configuration, the height dimension of the gap G only depends on an error in the thickness of the electrically insulating layer  33  so that the gap G with a greater accuracy than that in the configuration of  FIGS.  1 A and  1 B  are obtained. Further, since the base electrode  34  and the guard electrode layer  32  can be formed by using the same layer formation process, a simplified manufacturing process and reduced cost can be achieved. 
     The guard substrate  10  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example. The guard substrate  10  can include one or a plurality of layers, and an electrically insulating layer may be provided on the lower surface of the guard substrate  10 , for example. The intermediate layer  20  is provided between the guard substrate  10  and the sidewall layer  30 . The intermediate layer  20  includes at least two layers, that is, the substrate-side electrically insulating layer  21  and a base conductive layer  23  located thereon. The base conductive layer  23  is electrically connected to the base electrode  34 . Here, the two-layer configuration is exemplified, but the intermediate layer  20  may include three or more layers. 
     The guard electrode layer  32  is interposed between the diaphragm plate  40 , which is located above the guard electrode layer  32 , and the base conductive layer  23 , which is located below the guard electrode layer  32 . With this, stray capacitance unrelated to a pressure change can be canceled. 
     Further, the guard substrate  10  is electrically connected to the guard electrode layer  32  to function as a guard electrode together with the guard electrode layer  32 . The substrate  10  and the guard electrode layer  32  may be conducted with each other by a via hole, a wire, or the like in this structure, or may be conducted with each other via an external electronic circuit. 
     With such a configuration, even when a liquid such as water adheres to the substrate end surface, a shift in pressure output value due to a change in potential of the base electrode can be prevented. Further, since the guard substrate  10 , which is the bottom layer, functions as a guard electrode (shield electrode), the pressure sensor structure is less affected by electromagnetic noise coming from an electronic circuit placed under the pressure sensor structure, with the result that a shift in pressure output value can be prevented. 
     The passivation film  45  is provided on the outer surfaces of the sensor body, the base conductive layer  23 , the electrically insulating layer  21 , and the guard substrate  10 . The passivation film  45  is made of an electrically insulating material such as SiN x  or SiO 2 , for example, and protects the pressure sensor structure  1 . 
     Preferred Embodiment 3 
       FIG.  3    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 3 of the present invention. In the present preferred embodiment, as compared to the configuration of  FIGS.  2 A and  2 B , a second base electrode  25  and a second guard electrode layer  24  are provided in the same layer instead of the base conductive layer  23 . 
     This pressure sensor structure  1  includes a sensor body including the diaphragm plate  40 , the base electrode  34 , and the sidewall layer  30 , and the guard substrate  10  configured to support the sensor body. 
     The diaphragm plate  40  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and functions as a sense electrode deformable depending on an ambient pressure difference. The diaphragm plate  40  can include one or a plurality of layers, and an electrically insulating layer may be provided on the upper surface of the diaphragm plate  40 , for example. The base electrode  34  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and disposed to face the diaphragm plate  40 . The sidewall layer  30  is provided to maintain the gap G between the diaphragm plate  40  and the base electrode  34 . The gap G is a space sealed from the outside and maintained at constant pressure with an inert gas filled into the gap G, for example. The diaphragm plate  40  and the base electrode  34  form a parallel plate capacitor. 
     The sidewall layer  30  is positioned to define a frame surrounding the gap G and includes at least three layers, that is, the guard electrode layer  32 , the electrically insulating layer  31  disposed under the guard electrode layer  32 , and the electrically insulating layer  33  disposed on the guard electrode layer  32 . Here, the three-layer configuration is exemplified, but the sidewall layer  30  may include four or more layers. 
     The base electrode  34  is provided on the electrically insulating layer  31  and in the same layer as the guard electrode layer  32  and electrically isolated from the guard electrode layer  32  with the trench  34   a . With such a configuration, the height dimension of the gap G only depends on an error in the thickness of the electrically insulating layer  33  so that the gap G with a greater accuracy than that in the configuration of  FIGS.  1 A and  1 B  are obtained. Further, since the base electrode  34  and the guard electrode layer  32  can be formed by using the same layer formation process, a simplified manufacturing process and reduced cost can be achieved. 
     The guard substrate  10  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example. The guard substrate  10  can include one or a plurality of layers, and an electrically insulating layer may be provided on the lower surface of the guard substrate  10 , for example. The intermediate layer  20  is provided between the guard substrate  10  and the sidewall layer  30 . The intermediate layer  20  includes at least two layers, that is, the substrate-side electrically insulating layer  21  and the second base electrode  25  and the second guard electrode layer  24  that are located thereon. Here, the two-layer configuration is exemplified, but the intermediate layer  20  may include three or more layers. The second base electrode  25  and the second guard electrode layer  24  are provided in the same layer and electrically isolated from each other with a trench  25   a . The second base electrode  25  is electrically connected to the base electrode  34 . Further, the second guard electrode layer  24  is electrically connected to the guard electrode layer  32 . 
     The guard electrode layer  32  and the second guard electrode layer  24  are interposed between the diaphragm plate  40 , which is located above the guard electrode layer  32  and the second guard electrode layer  24 , and the guard substrate  10 , which is located below the guard electrode layer  32  and the second guard electrode layer  24 . With this, stray capacitance unrelated to a pressure change can be canceled. 
     Further, the guard substrate  10  is electrically connected to the guard electrode layer  32  to function as a guard electrode together with the guard electrode layer  32 . The substrate  10  and the guard electrode layer  32  may be conducted with each other by a via hole, a wire, or the like in this structure, or may be conducted with each other via an external electronic circuit. 
     With such a configuration, even when a liquid such as water adheres to the substrate end surface, a shift in pressure output value due to a change in potential of the base electrode can be prevented. Further, since the guard substrate  10 , which is the bottom layer, functions as a guard electrode (shield electrode), the pressure sensor structure is less affected by electromagnetic noise coming from an electronic circuit placed under the pressure sensor structure, with the result that a shift in pressure output value can be prevented. 
     Further, the second guard electrode layer  24  is electrically connected to the guard electrode layer  32  to function as a guard electrode together with the guard electrode layer  32 . Thus, a further enhanced guard electrode function can be provided. 
     The passivation film  45  is provided on the outer surfaces of the sensor body, the second guard electrode layer  24 , the electrically insulating layer  21 , and the guard substrate  10 . The passivation film  45  is made of an electrically insulating material such as SiN x  or SiO 2 , for example, and protects the pressure sensor structure  1 . 
     Preferred Embodiment 4 
       FIG.  4    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 4 of the present invention. In the present preferred embodiment, the optimization of the width dimension of the sidewall layer  30  is described. In the pressure sensor structure  1  illustrated in  FIGS.  1 A and  1 B , the electrically insulating layer  31 , which is the lower layer, the guard electrode layer  32 , and the electrically insulating layer  33 , which is the upper layer, of the sidewall layer  30  define a frame shape along the periphery of the diaphragm plate  40  and form the cross-sectional shape of a so-called step pyramid structure. 
     Specifically, a distance W1 between the inner edges of the electrically insulating layer  31 , a distance W6 between the outer edges thereof, a distance W2 between the inner edges of the guard electrode layer  32 , a distance W5 between the outer edges thereof, a distance W3 between the inner edges of the electrically insulating layer  33 , and a distance W4 between the outer edges thereof preferably satisfy the following expression: 
       W1&lt;W2&lt;W3 and W4&lt;W5&lt;W6. 
     As concrete numerical examples, W1, W2, and W3 may be within the range of from approximately 20 μm to approximately 500 μm, and W4, W5, and W6 may be within the range of from approximately 50 μm to approximately 1000 μm. The area of the diaphragm plate  40  may be within the range of from approximately 0.5 mm 2  to approximately 2 mm 2 . Further, the thickness of the diaphragm plate  40  may be within the range of from approximately 0.5 μm to approximately 10 μm. The thicknesses of the electrically insulating layers and the conductive layer may be within the range of from approximately 0.1 μm to approximately 1 μm. The thickness of the guard substrate  10  may be within the range of from approximately 100 μm to approximately 825 μm. 
     With such a configuration, a larger creepage distance is secured on the outer surface of the sensor body. Thus, the effects of external leakage current and stray capacitance between the diaphragm plate  40  and the base electrode  22  on a pressure output value can be reduced. 
     Preferred Embodiment 5 
       FIG.  5    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 5 of the present invention. In the present preferred embodiment, the optimization of the width dimension of the sidewall layer  30  is described. In the pressure sensor structure  1  illustrated in  FIGS.  2 A and  2 B , the electrically insulating layer  31 , which is the lower layer, of the sidewall layer  30  preferably has a plate shape larger than the contour of the diaphragm plate  40 . The guard electrode layer  32  and the electrically insulating layer  33 , which is the upper layer, of the sidewall layer  30  define a frame shape along the periphery of the diaphragm plate  40  and form the cross-sectional shape of a so-called step pyramid structure. 
     Specifically, a distance W6 between the outer edges of the electrically insulating layer  31 , a distance W2 between the inner edges of the guard electrode layer  32 , a distance W5 between the outer edges thereof, a distance W3 between the inner edges of the electrically insulating layer  33 , and a distance W4 between the outer edges thereof preferably satisfy the following expression: 
       W2&lt;W3 and W4&lt;W5&lt;W6. 
     With such a configuration, a larger creepage distance is secured on the outer surface of the sensor body. Thus, the effects of external leakage current and stray capacitance between the diaphragm plate  40  and the base electrode  34  on a pressure output value can be reduced. 
     Preferred Embodiment 6 
       FIG.  6 A  to  FIG.  6 C  are sectional views illustrating various examples of a pressure sensor device  100  according to Preferred Embodiment 6 of the present invention. 
     The pressure sensor device  100  includes the pressure sensor structure  1  described above, an integrated circuit  110 , a circuit substrate  120 , a case  130 , and the like. The integrated circuit  110  includes an ASIC, an FPGA, a PLD, a CPLD, or the like and has the function of processing a signal from the pressure sensor structure  1 . The integrated circuit  110  is electrically connected to the pressure sensor structure  1  through a terminal provided on the upper surface of the integrated circuit  110 . The integrated circuit  110  is electrically connected to a conductor of the circuit substrate  120  through a terminal provided on the lower surface of the integrated circuit  110 . The integrated circuit  110  may be electrically connected to the pressure sensor structure  1  with the circuit substrate  120  interposed therebetween. Further, the integrated circuit  110  may be electrically connected to the circuit substrate  120  through a terminal provided on the upper surface of the integrated circuit  110  by a wire or the like. 
     The circuit substrate  120  has the function of allowing the integrated circuit  110  and the pressure sensor structure  1  to be mounted thereon and includes the conductor for an electrical connection with an external circuit. 
     The case  130  is made of metal or a synthetic resin, for example, and accommodates the circuit substrate  120 , the integrated circuit  111 , and the pressure sensor structure  1 . 
     In the example illustrated in  FIG.  6 A , the case  130  preferably has a stepped cylindrical shape and is fixed to the circuit substrate  120 . An opening  131  for interaction with the atmosphere is provided in the upper portion of the case  130 . The space inside the case  130  may be filled with the air or a gel  132  as illustrated in the drawing. The gel  132  can enhance the waterproof property, water-resistant property, anticorrosion property, and the like of the pressure sensor structure  1 . 
     In the example illustrated in  FIG.  6 B , the case  130  preferably has a hollow cylindrical shape, a hollow prismatic shape, a hollow rectangular parallelepiped shape, or the like and mounted on the circuit substrate  120 . The opening  131  for interaction with the atmosphere is provided in the upper portion of the case  130 . 
     In the example illustrated in  FIG.  6 C , the case  130  preferably has a rectangular frame shape, a cylindrical frame shape, or the like and is resin-molded to be in close contact with the side surfaces of the pressure sensor structure  1  and the circuit substrate  120 . However, when the case  130  is made of a synthetic resin, the case  130  may possibly absorb water. In this case, the end portion of the pressure sensor structure  1  and the water may possibly be electrically conducted with each other to increase the parasitic capacitance between the sense potential in contact with the case  130 , which has absorbed the water, and the base potential, resulting in variations in characteristics. In this case, in the related-art pressure sensor structure, the characteristics may possibly vary since the base electrode is exposed. In a pressure sensor structure according to a preferred embodiment of the present invention, however, variations in characteristics can be prevented since the base electrode is not exposed. 
     Preferred Embodiment 7 
       FIGS.  7 A to  7 F  and  FIGS.  8 A to  8 E  are explanatory diagrams illustrating an example of a method of manufacturing a pressure sensor structure according to Preferred Embodiment 7 of the present invention. Here, the case of completing the pressure sensor structure  1  illustrated in  FIGS.  2 A and  2 B  is exemplified, but the same holds true for the pressure sensor structure  1  illustrated in  FIGS.  1 A and  1 B  or  FIG.  3   . 
     First, the lower substrate of the pressure sensor structure  1  is described. As illustrated in  FIG.  7 A , a conductive wafer substrate  50 , such as Si, is prepared, and an electrically insulating layer  51 , such as SiO 2 , is subsequently formed on the wafer substrate  50  by oxidation treatment. The wafer substrate  50  and the electrically insulating layer  51  correspond to the guard substrate  10  and the electrically insulating layer  21  illustrated in  FIGS.  2 A and  2 B , respectively. 
     Next, as illustrated in  FIG.  7 B , a conductive layer  52 , such as polycrystalline Si, is formed by using sputtering, and an electrically insulating layer  53 , such as SiO 2 , is subsequently formed on the conductive layer  52  by oxidation treatment. The conductive layer  52  and the electrically insulating layer  53  correspond to the base conductive layer  23  and the electrically insulating layer  31  illustrated in  FIGS.  2 A and  2 B , respectively. Subsequently, the electrically insulating layer  53  is processed into a desired pattern by using photolithography and etching, and at this time, for example, a via hole is formed. 
     Next, as illustrated in  FIG.  7 C , a conductive layer  54 , such as polycrystalline Si, is formed by using sputtering. At this time, the conductive layer  54  is in conduction with the conductive layer  52  through the via hole. Subsequently, as needed, the conductive layer  54  may be subjected to mirror polishing. 
     Next, as illustrated in  FIG.  7 D , the conductive layer  54  is processed into a desired pattern by using photolithography and etching to form the guard electrode layer  32  and the base electrode  34  illustrated in  FIGS.  2 A and  2 B . In this way, a lower substrate LS of the pressure sensor structure  1  is obtained. 
     Next, the upper substrate of the pressure sensor structure  1  is described. As illustrated in  FIG.  7 E , by using a silicon on insulator (SOI) substrate having a three-layer structure including Si, SiO 2 , and Si, for example, a three-layer structure including a conductive layer  62 , an electrically insulating layer  63 , and a conductive layer  64  is prepared. Subsequently, by oxidation treatment, an electrically insulating layer  61 , such as SiO 2 , is formed on the surface of the conductive layer  62  and an electrically insulating layer  65 , such as SiO 2 , is formed on the surface of the conductive layer  64 . 
     Next, as illustrated in  FIG.  7 F , the electrically insulating layer  65  is processed into a desired pattern by using photolithography and etching to form the electrically insulating layer  33  illustrated in  FIGS.  2 A and  2 B . In this way, an upper substrate US of the pressure sensor structure  1  is obtained. 
     Next, as illustrated in  FIG.  8 A , the upper substrate US is mounted on the lower substrate LS to be joined thereto by using an adhesive or the like. Next, as illustrated in  FIG.  8 B , the upper substrate US is thinned by polishing until the electrically insulating layer  61 , the conductive layer  62 , and the electrically insulating layer  63  are removed and the conductive layer  64  is exposed. The conductive layer  64  corresponds to the diaphragm plate  40  illustrated in  FIGS.  2 A and  2 B . 
     Next, as illustrated in  FIG.  8 C , by using photolithography and etching, the respective layers located above the wafer substrate  50  are shaped to form a step pyramid shape. In this way, the electrically insulating layer  21 , the base conductive layer  23 , the electrically insulating layer  31 , the guard electrode layer  32 , the electrically insulating layer  33 , and the diaphragm plate  40 , which are illustrated in  FIGS.  2 A and  2 B , are complete. Subsequently, by using sputtering, photolithography, and etching, a first electrode pattern is formed to be conducted with the diaphragm plate  40  and a second electrode pattern is formed to be conducted with the base electrode  34  and the base conductive layer  23 . The main components of the electrode patterns are Al or Au. 
     In this way, a large number of pressure sensor structures are formed on the single wafer substrate  50 . In this state, the diaphragm plate  40  and the base electrode  34  included in each of the individual pressure sensor structures are electrically isolated from each other. Thus, the characteristics of the individual chips can be tested by using a wafer prober before chip cutting. 
     Next, as illustrated in  FIG.  8 D , by using sputtering, for example, the passivation film  45 , such as SiN x  or SiO 2 , is formed on the surfaces and end surfaces of the layers from the diaphragm plate  40  to the electrically insulating layer  21  and the upper surface of the wafer substrate  50 . Subsequently, the passivation film  45  is processed into a desired pattern by using etching. 
     Next, as illustrated in  FIG.  8 E , the wafer substrate  50  is cut (singulated) into the individual chips with a dicing machine. The wafer substrate  50  corresponds to the guard substrate  10  illustrated in  FIGS.  2 A and  2 B . 
     In this way, the pressure sensor structure  1  as illustrated in  FIGS.  2 A and  2 B , which is capable of curbing the effects of disturbances such as condensation, flooding, and electromagnetic noise, can be achieved. 
     Preferred Embodiment 8 
       FIG.  10    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 8 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIGS.  9 A and  9 B  but different from the structure illustrated in  FIGS.  9 A and  9 B  in that the guard substrate  10  is the bottom layer. 
     This pressure sensor structure  1  includes a sensor body including the diaphragm plate  40 , the base electrode  34 , and the sidewall layer  30 , and the guard substrate  10  to support the sensor body. 
     The diaphragm plate  40  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and functions as a sense electrode deformable depending on an ambient pressure difference. The diaphragm plate  40  can include one or a plurality of layers, and an electrically insulating layer may be provided on the upper surface of the diaphragm plate  40 , for example. The base electrode  34  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example, and disposed to face the diaphragm plate  40 . The sidewall layer  30  is provided to maintain the gap G between the diaphragm plate  40  and the base electrode  34 . The gap G is a space sealed from the outside and maintained at constant pressure with an inert gas filled into the gap G, for example. The diaphragm plate  40  and the base electrode  34  define a parallel plate capacitor. 
     The sidewall layer  30  is positioned to define a frame surrounding the gap G and includes at least three layers, that is, the guard electrode layer  32 , the electrically insulating layer  31  disposed under the guard electrode layer  32 , and the electrically insulating layer  33  disposed on the guard electrode layer  32 . Here, the three-layer configuration is exemplified, but the sidewall layer  30  may include four or more layers. 
     The base electrode  34  is provided on the electrically insulating layer  31  and in the same layer as the guard electrode layer  32  and electrically isolated from the guard electrode layer  32  with the trench  34   a . With such a configuration, the height dimension of the gap G only depends on an error in the thickness of the electrically insulating layer  33  so that the gap G with a greater accuracy than that in the configuration of  FIG.  1    is obtained. Further, since the base electrode  34  and the guard electrode layer  32  can be formed by using the same layer formation process, a simplified manufacturing process and reduced cost can be achieved. 
     The guard substrate  10  is made of a conductive material such as polycrystalline Si, amorphous Si, or monocrystalline Si, for example. The guard substrate  10  can include one or a plurality of layers, and an electrically insulating layer may be provided on the lower surface of the guard substrate  10 , for example. The intermediate layer  20  is provided between the guard substrate  10  and the sidewall layer  30 . The intermediate layer  20  includes at least two layers, that is, the substrate-side electrically insulating layer  21  and the base conductive layer  23  located thereon. The base conductive layer  23  is electrically connected to the base electrode  34 . Here, the two-layer configuration is exemplified, but the intermediate layer  20  may include three or more layers. 
     Further, the guard substrate  10  is electrically connected to the guard electrode layer  32  to function as a guard electrode together with the guard electrode layer  32 . The substrate  10  and the guard electrode layer  32  may be conducted with each other by a via hole, a wire, or the like in this structure, or may be conducted with each other via an external electronic circuit. 
     With such a configuration, even when a liquid such as water adheres to the substrate end surface, a shift in pressure output value due to a change in potential of the base electrode can be prevented. Further, since the guard substrate  10 , which is the bottom layer, functions as a guard electrode (shield electrode), the pressure sensor structure is less affected by electromagnetic noise coming from an electronic circuit placed under the pressure sensor structure, with the result that a shift in pressure output value can be prevented. 
     The passivation film  45  is provided on the outer surfaces of the sensor body and the guard substrate  10 . The passivation film  45  is made of an electrically insulating material such as SiN x  or SiO 2 , for example, and protects the pressure sensor structure  1 . 
     Preferred Embodiment 9 
       FIG.  11    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 9 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIG.  1   . In present preferred embodiment, however, at a location on the inner side of the edges of the diaphragm plate  40 , for example, above the location of the sidewall layer  30 , a groove  41  with a depth equivalent to about half the thickness of the diaphragm plate  40  is formed. With this, stress to be applied to the diaphragm plate  40  can be relaxed. 
     Preferred Embodiment 10 
       FIG.  12    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 10 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIG.  1   . In the present preferred embodiment, however, at a location on the inner side of the edges of the guard substrate  10 , for example, under the location of the sidewall layer  30 , a groove  42  reaching the back surface of the guard substrate  10  is provided. With this, stress to be applied to the guard substrate  10  can be relaxed. 
     As an alternative example, the groove  42  may be provided in at least one of the guard substrate  10 , the electrically insulating layer  21 , the base electrode  22 , the electrically insulating layer  31 , the guard electrode layer  32 , the electrically insulating layer  33 , and the diaphragm plate  40 . The groove  42  may, but not necessarily, be exposed to the outside. 
     Preferred Embodiment 11 
       FIG.  13    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 11 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIGS.  2 A and  2 B . In present preferred embodiment, however, at a location on the inner side of the edges of the diaphragm plate  40 , for example, above the location of the sidewall layer  30 , the groove  41  with a depth equivalent to about half the thickness of the diaphragm plate  40  is formed. With this, stress to be applied to the diaphragm plate  40  can be relaxed. 
     Preferred Embodiment 12 
       FIG.  14    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 12 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIGS.  2 A and  2 B . In the present preferred embodiment, however, at a location on the inner side of the edges of the guard substrate  10 , for example, under the location of the sidewall layer  30 , the groove  42  reaching the back surface of the guard substrate  10  is formed. With this, stress to be applied to the guard substrate  10  can be relaxed. 
     As an alternative example, the groove  42  may be provided in at least one of the guard substrate  10 , the electrically insulating layer  21 , the base conductive layer  23 , the electrically insulating layer  31 , the guard electrode layer  32 , the electrically insulating layer  33 , and the diaphragm plate  40 . The groove  42  may, but not necessarily, be exposed to the outside. 
     Preferred Embodiment 13 
       FIG.  15    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 13 of the present invention. The present Preferred embodiment is similar to the structure illustrated in  FIG.  3   . In present preferred embodiment, however, at a location on the inner side of the edges of the diaphragm plate  40 , for example, above the location of the sidewall layer  30 , the groove  41  with a depth equivalent to about half the thickness of the diaphragm plate  40  is formed. With this, stress to be applied to the diaphragm plate  40  can be relaxed. 
     Preferred Embodiment 14 
       FIG.  16    is a sectional view illustrating an example of the pressure sensor structure  1  according to preferred Embodiment 14 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIG.  3   . In the present preferred embodiment, however, at a location on the inner side of the edges of the guard substrate  10 , for example, under the location of the sidewall layer  30 , the groove  42  reaching the back surface of the guard substrate  10  is provided. With this, stress to be applied to the guard substrate  10  can be relaxed. 
     As an alternative example, the groove  42  may be provided in at least one of the guard substrate  10 , the electrically insulating layer  21 , the second guard electrode layer  24 , the electrically insulating layer  31 , the guard electrode layer  32 , the electrically insulating layer  33 , and the diaphragm plate  40 . The groove  42  may, but not necessarily, be exposed to the outside. 
     Preferred Embodiment 15 
       FIG.  17    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 15 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIG.  10   . In present preferred embodiment, however, at a location on the inner side of the edges of the diaphragm plate  40 , for example, above the location of the sidewall layer  30 , the groove  41  with a depth equivalent to about half the thickness of the diaphragm plate  40  is provided. With this, stress to be applied to the diaphragm plate  40  can be relaxed. 
     Preferred Embodiment 16 
       FIG.  18    is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 16 of the present invention. The present preferred embodiment is similar to the structure illustrated in  FIG.  10   . In the present preferred embodiment, however, at a location on the inner side of the edges of the guard substrate  10 , for example, under the location of the sidewall layer  30 , the groove  42  reaching the back surface of the guard substrate  10  is provided. With this, stress to be applied to the guard substrate  10  can be relaxed. 
     As an alternative example, the groove  42  may be provided in at least one of the guard substrate  10 , the electrically insulating layer  31 , the guard electrode layer  32 , the electrically insulating layer  33 , and the diaphragm plate  40 . The groove  42  may, but not necessarily, be exposed to the outside. 
     Preferred Embodiment 17 
       FIG.  19 A  is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 17 of the present invention, and  FIG.  19 B  is a plan view thereof. To promote a better understanding, the illustration of the passivation film is omitted in  FIG.  19 B . The present preferred embodiment is similar to the structure illustrated in  FIG.  1   . In the present preferred embodiment, however, the edges of the electrically insulating layer  21  and the base electrode  22  are laterally extended, the edges of the guard substrate  10  are laterally extended, and cavities are provided in the passivation film  45  placed on the upper surfaces of the diaphragm plate  40 , the base electrode  22 , and the guard substrate  10  to provide respective pad electrodes PA, PB, and PC. With this, the pad electrodes PA, PB, and PC are bonded to lead wires easily. 
     Preferred Embodiment 18 
       FIG.  20 A  is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 18 of the present invention, and  FIG.  20 B  is a plan view thereof. To promote a better understanding, the illustration of the passivation film is omitted in  FIG.  20 B . The present preferred embodiment is similar to the structure illustrated in  FIGS.  2 A and  2 B . In the present preferred embodiment, however, the edges of the electrically insulating layer  21  and the base conductive layer  23  are laterally extended, the edges of the guard substrate  10  are laterally extended, and cavities are provided in the passivation film  45  placed on the upper surfaces of the diaphragm plate  40 , the base conductive layer  23 , and the guard substrate  10  to provide the respective pad electrodes PA, PB, and PC. With this, the pad electrodes PA, PB, and PC are bonded to lead wires easily. 
     Preferred Embodiment 19 
       FIG.  21 A  is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 19 of the present invention, and  FIG.  21 B  is a plan view thereof. To promote a better understanding, the illustration of the passivation film is omitted in  FIG.  21 B . The present preferred embodiment is similar to the structure illustrated in  FIG.  3   . In the present preferred embodiment, however, the edges of the electrically insulating layer  21  and the second guard electrode layer  24  are laterally extended, the edges of the guard substrate  10  are laterally extended, and cavities are provided in the passivation film  45  placed on the upper surfaces of the diaphragm plate  40 , the second guard electrode layer  24 , and the guard substrate  10  to provide the respective pad electrodes PA, PB, and PC. With this, the pad electrodes PA, PB, and PC are bonded to lead wires easily. 
     Preferred Embodiment 20 
       FIG.  22 A  is a sectional view illustrating an example of the pressure sensor structure  1  according to Preferred Embodiment 20 of the present invention, and  FIG.  22 B  is a plan view thereof. To promote a better understanding, the illustration of the passivation film is omitted in  FIG.  22 B . The present preferred embodiment is similar to the structure illustrated in  FIG.  10   . In the present preferred embodiment, however, a portion of the edge of the base electrode  34  and the edges of the guard electrode layer  32  are laterally extended, the edges of the guard substrate  10  are laterally extended, and cavities are provided in the passivation film  45  placed on the upper surfaces of the diaphragm plate  40 , the base electrode  34 , and the guard substrate  10  to provide the respective pad electrodes PA, PB, and PC. With this, the pad electrodes PA, PB, and PC are bonded to lead wires easily. 
     Next, the role of a guard electrode is described. In each preferred embodiment described above, the guard electrode layer  32  and the second guard electrode layer  24  preferably function as guard electrodes. 
     A guard electrode may be connected to a constant potential, and may preferably be connected to any of the ground of an electric circuit connected to the sensor, the virtual ground thereof, and another potential with which a current flowing between the diaphragm plate and the guard electrode and between the base electrode and the guard electrode and a current flowing between the diaphragm plate and the base electrode can be isolated from each other. In other words, a guard electrode may be used to prevent an electrical impedance between the diaphragm plate and the guard electrode and an electrical impedance between the base electrode and the guard electrode from affecting an electrostatic capacitance to be measured between the diaphragm plate and the base electrode. 
       FIG.  23    to  FIG.  28    are circuit diagrams illustrating various examples of a capacitance converter circuit connectable to the sensor structure according to a preferred embodiment of the present invention. The capacitance converter circuits each include an operational amplifier OP, a base terminal TB for the base electrode, a diaphragm terminal TD for the diaphragm plate electrode, a guard terminal TG for the guard electrode, a voltage source CV or a current source CC, and a reference impedance RA. With the use of those capacitance converter circuits, a voltage output indicating an electrostatic capacitance between the diaphragm plate and the base electrode is obtained while the effects of disturbances are curbed. 
       FIG.  23    to  FIG.  26    illustrate exemplary inverting operational amplifier circuits each including a positive non-inverting input connected to the common ground or a zero-voltage terminal. Since a closed-loop feedback circuit requires a differential input voltage of almost zero, the potential at the inverting input is equal or substantially equal to the potential at the non-inverting input so that a virtual ground point VG is generated. 
     In  FIG.  23   , the guard electrode is held at the same potential as the base electrode. As illustrated in  FIG.  23   , the base terminal TB is connected to the virtual ground point VG at the inverting input of the amplifier OP, and the guard terminal TG has the ground potential. Thus, voltage and current between the guard electrode and the base electrode are ignorable and thus have no substantial effect on a capacitance value to be measured between the base electrode and the diaphragm plate. The diaphragm terminal TD is connected to the voltage source CV to make a current between the guard electrode and the diaphragm plate ignorable and prevent the current from having a substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. An electrostatic capacitance between the guard electrode and the base electrode is connected between the ground and the virtual ground point VG and thus has no substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. 
     An electrostatic capacitance between the base terminal TB and the diaphragm terminal TD is denoted by C S , and an electrostatic capacitance between the base terminal TB and the guard terminal TG is denoted by C L . Further, the voltage source C V  is assumed to be an AC voltage source of an effective voltage Ui, the feedback circuit element RA is assumed to be a capacitor having an electrostatic capacitance equal to C F , and the open loop gain of the amplifier OP is assumed to be A. An output voltage Uo of the amplifier OP is represented as follows. 
     
       
         
           
             
               
                 
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     In this way, the effect of C L  is reduced depending on the amount of the open loop gain A of the amplifier. An electrostatic capacitance between the diaphragm terminal TD and the guard terminal TG is also connected to the voltage source Ui, which is an ideal voltage source capable of supplying a current to this electrostatic capacitance without a voltage change, in parallel and thus does not affect an output voltage either. 
     Next, in  FIG.  24   , the guard electrode is held at almost the same potential as the diaphragm plate. As illustrated in  FIG.  24   , the diaphragm terminal TD is connected to the virtual ground point VG at the inverting input of the amplifier, and the guard terminal TG has the ground potential. Thus, voltage and current between the guard electrode and the diaphragm plate are ignorable and thus have no substantial effect on a capacitance value to be measured between the base electrode and the diaphragm plate. The base terminal TB is connected to the voltage source C V  to prevent a current between the guard electrode and the base electrode from having a substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. An electrostatic capacitance between the guard electrode and the base electrode is connected between the ground and the voltage source C V  and thus has no substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. 
     Next, in  FIG.  25   , the guard electrode is held at almost the same potential as the diaphragm plate. As illustrated in  FIG.  25   , the diaphragm terminal TD is connected to the virtual ground point at the inverting input of the amplifier, and the guard terminal TG has the ground potential. Thus, voltage and current between the guard electrode and the diaphragm plate are ignorable and thus have no substantial effect on a capacitance value to be measured between the base electrode and the diaphragm plate. The base terminal TB is connected to the output of the amplifier OP to make a current between the guard electrode and the base electrode ignorable and prevent the current from having a substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. An electrostatic capacitance between the guard electrode and the base electrode is connected between the ground and the output of the amplifier OP and thus has no substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. 
     Next, in  FIG.  26   , the guard electrode is held at almost the same potential as the base electrode. As illustrated in  FIG.  26   , the base terminal TB is connected to the virtual ground point at the inverting input of the amplifier, and the guard terminal TG has the ground potential. Thus, voltage and current between the guard electrode and the base electrode are ignorable and thus have no substantial effect on a capacitance value to be measured between the base electrode and the diaphragm plate. The diaphragm terminal TD is connected to the output of the amplifier OP to make a current between the guard electrode and the diaphragm plate ignorable and prevent the current from having a substantial effect on a capacitance value to be measured between the diaphragm plate and the base electrode. An electrostatic capacitance between the guard electrode and the base electrode is connected between the ground and the virtual ground point and thus has no substantial effect on a capacitance value to be measured between the diaphragm plate and the planar base. 
     In  FIG.  23    to  FIG.  26   , the guard terminal TG is connected to the ground potential. The base terminal TB or the diaphragm terminal TD provides a sensed electrostatic capacitance, and one of the terminals of the sensor is connected to the virtual ground point of the amplifier circuit. With this, the guard terminal TG and one of the terminals of the sensor can be held at almost the same voltage while a current flowing through the guard terminal TG and a current flowing through the diaphragm terminal TD are isolated from each other. 
       FIG.  27    and  FIG.  28    illustrate exemplary non-inverting operational amplifier circuits. The operational amplifier OP is used as a voltage follower, and the output of the operational amplifier OP is directly connected to the inverting input of the operational amplifier OP. The potential at the inverting input is equal or substantially equal to the potential at the non-inverting input. 
     In  FIG.  27   , the guard electrode is held at almost the same potential as the diaphragm plate. As illustrated in  FIG.  27   , the diaphragm terminal TD is connected to the current source CC with a current determined in advance or known. Further, it is understood that the current source CC may include a combination of a current source, a voltage source, and an internal impedance. In this case, the current is not constant but is known by a measurement with a shunt resistor or another known current measurement method, for example. Since the diaphragm plate and the guard electrode herein have the same or almost the same potential, substantially no current flows between the diaphragm plate and the guard electrode. A leakage or capacitive current may possibly be generated between the guard electrode and the base electrode, but since this current is provided by the amplifier OP without a substantial effect on an output voltage, the current also has no substantial effect on a voltage or current between the diaphragm plate and the base electrode. 
     An electrostatic capacitance between the base terminal TB and the diaphragm terminal TD is denoted by C S , and an electrostatic capacitance between the inverting (−) input and non-inverting (+) input of the amplifier OP is denoted by C i . This electrostatic capacitance includes both an input electrostatic capacitance of the amplifier OP and an electrostatic capacitance between the diaphragm terminal TD and the guard terminal TG. Further, the current source CC is assumed to be an AC source of an effective current J i  at a frequency f, and the open loop gain of the amplifier OP is assumed to be A. An output voltage Uo of the amplifier is represented as follows. 
     
       
         
           
             
               
                 
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     In this way, the effect of C i  is reduced depending on the amount of the open loop gain A of the amplifier. An electrostatic capacitance between the base terminal TB and the guard terminal TG is also connected between the output terminal of the amplifier and the ground terminal and thus has almost no effect on an output voltage. The electrostatic capacitance therefore does not affect an output voltage either. 
     In  FIG.  28   , the guard electrode is held at almost the same potential as the base electrode. As illustrated in  FIG.  28   , the base terminal TB is connected to the current source CC with a current determined in advance or known. Since the base electrode and the guard electrode herein have almost the same potential, substantially no current flows between the base electrode and the guard electrode. A leakage or capacitive current may possibly be generated between the guard electrode and the diaphragm plate, but since this current is provided by the amplifier without a substantial effect on an output voltage, the current has no substantial effect on a voltage or current between the diaphragm plate and the base electrode. 
     In  FIG.  27    and  FIG.  28   , since the guard terminal TG is connected to the output of the amplifier OP to follow the potential at one of the terminals of the sensor, a current between the guard terminal TG and the terminal of the sensor is held at an ignorable value. 
     Although the present invention has been sufficiently described in terms of the preferred embodiments with reference to the accompanying drawings, various changes and modifications are apparent to those skilled in the art. It should be understood that such changes and modifications are included in the present invention without departing from the scope of the present invention according to the accompanying claims. 
     Preferred embodiments of the present invention can achieve pressure sensor structures capable of curbing the effects of disturbances and measuring pressure highly precisely and are thus industrially very useful. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.