Abstract:
A pressure sensor using two capacitors for measuring a pressure stimulus includes a substrate having a diaphragm positioned at a center portion thereof. The diaphragm has a reduced thickness so that the diaphragm displaces upward and downward in response to a pressure stimulus. A first capacitor is provided on the diaphragm and at least a second capacitor is provided on a bulk portion of the substrate so as to be adjacent to the first capacitor. The first and the second capacitor are connected to each other in series, wherein capacitance differs between the first and the second capacitor when the diaphragm moves up and down in response to the pressure stimulus.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a differential capacitive pressure sensor and a fabricating method therefor; and, more particularly, to a differential capacitive pressure sensor that implements a superior linearity and a simple fabricating method therefor. 
     BACKGROUND OF THE INVENTION 
     Various instruments, such as a measuring instrument or a controller, employ a pressure sensor to measure a pressure in a process or a system. The pressure sensor generally uses displacement, deflection, frequency of oscillation, or magnetic-thermal conductivity, each varying in response to a pressure stimulus. 
     Recently, a semiconductor pressure sensor, having a smaller size and being more combined, is getting attention for the development of a semiconductor technology and a micro-machining technology. Since creep rarely occurs in the semiconductor pressure sensor, a superior linearity can be obtained. Further, the semiconductor pressure sensor is small, lightweight, and very strong against vibration. Compared with a mechanical sensor, the semiconductor pressure sensor is more sensitive, more reliable, and presents a higher production yield. 
     In a typical semiconductor pressure sensor, a pressure stimulus causes distortion or strain of a diaphragm, which is usually formed of a monocrystalline silicon. Though a natural frequency change of a vibrator or a surface elastic wave occurring on a surface of the diaphragm can be used to convert the distortion or strain thereof into an electrical signal, the typical semiconductor pressure sensor is generally classified into a capacitive type and a piezoresistive type. 
     The piezoresistive pressure sensor is formed by diffusing impurities onto a semiconductor and has advantages such as easy fabrication and superior linearity. Though a simplified processing circuit can be applied in the piezoresistive pressure sensor, a correction circuit is usually added thereto to overcome a poor temperature characteristic thereof. 
     In the capacitive pressure sensor, an exterior stimulus, i.e., pressure or stress, causes a change in a gap interposed between opposing electrodes, so that capacitance therebetween is changed. The amount of the changed capacitance is then converted into an electrical signal, which involves with the magnitude of the stress or the pressure. Compared with the piezoresistive type, the capacitive pressure sensor has a smaller size as well as a better temperature characteristic. 
     The capacitive pressure sensor, however, has a relatively poor linearity, because the capacitance is inversely proportional to the interval between the opposing electrodes. The linearity thereof becomes more rapidly deteriorated as the capacitive pressure sensor is used for measuring a wider range of stimulus. 
     Mitsuhiro Yamada, et al., have disclosed a compensation method for improving linearity of a capacitive pressure sensor in a paper, “A Capacitive Pressure Sensor Interface Using Oversampling Δ−Σ Demodulation Techniques,” IEEE Transactions on Instrumentation and Measurement, Vol. 46, No. 1, February 1997. Since the above-mentioned method uses a look-up table, data is digitally stored or input to a circuit. Consequently, a continuous compensation is impossible, and therefore an irregular variance in the output of the sensor is a fatal drawback. 
     For achieving an improved linearity, a differential capacitive pressure sensor is further suggested. When a pressure acts on a typical differential capacitive pressure sensor, a first displacement (+)Δd and a second displacement (−)Δd are respectively involved with a first sensing capacitor “C1” and a second sensing capacitor “C2” thereof. The first and the second sensing capacitor “C1” and “C2” respectively present a first capacitance and a second capacitance, which are also respectively referred to as “C1” and “C2” for the sake of convenience. Because the absolute value “Δd” of the first and the second displacement is usually very small, a capacitance difference “ΔC” between the first and the second capacitance “C1” and “C2” (ΔC=C1−C2) is in proportion to the absolute value “Δd” thereof. Accordingly, the differential capacitive pressure sensor implements a superior linearity, and effects of a parasitic capacitance are almost excluded. 
     U.S. Pat. No. 5,925,824 by Shinichi Soma, et al., discloses a conventional differential capacitive pressure sensor. In the conventional differential capacitive pressure sensor, an insulator and a conductive plate, each having a concentric through hole, are sequentially assembled on opposing surfaces of a common conductive plate, such that two capacitors are respectively formed on both opposing surfaces of the common conductive plate. The above-explained structure is difficult to fabricate and therefore its production yield is low. Further, the structure is unsuitable for a small-sized sensor and presents a relatively low sensitivity. 
     ChuanChe Whang, et al., have disclosed another conventional differential capacitive pressure sensor in a paper, “Contamination-Insensitive Differential Capacitive Pressure Sensors”, Journal of MEMS, Vol. 9, No. 4, December 2000. The above-mentioned pressure sensor is fabricated by applying a micro-machining technology, so that two sensing capacitors are formed on a membrane that can elastically deflects in response to a pressure stimulus. 
     The above-mentioned differential capacitive pressure sensor includes a lower electrode, a center electrode, and an upper electrode. The lower electrode is a polysilicon membrane, and the center electrode is supported by a leg formed on a bulk silicon substrate, which presents no deflection. The upper electrode is disposed over the lower electrode and a supporter is interposed therebetween to support the upper electrode. When the lower electrode deflects in response to the pressure stimulus, the upper electrode also deflects as much as the lower electrode does. 
     Since the above-explained differential capacitive pressure sensor is hermetically sealed, contamination by particles is prevented. Further, because two capacitors thereof are respectively arranged on an upside and a downside, a large fill factor can be obtained, so that the temperature characteristic and the linearity thereof are superior. 
     In the above-explained differential capacitive pressure sensor, however, because two sacrificial layers are used during a fabrication process therefor, the fabrication process is very complicated. Further, unless gaps between the upper and the lower electrode are continuous, there occurs a difference in capacitances between the two capacitors. Because the thickness of the sacrificial layers determines the above-mentioned gaps, the sacrificial layers are required to have a same thickness. The two sacrificial layers, however, are very difficult to have the same thickness. 
     Further, because the support is made of an insulating material or a conductive material covered by an insulator, additional processes are required for forming the support or the insulator. Furthermore, because the support interposed between the upper and the lower electrode supports only the upper electrode, the deflection of the lower electrode differs from that of the upper electrode. In other words, the support causes a difference in deflection between the upper and the lower electrode, thereby deteriorating the accuracy of the sensor 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a differential capacitive pressure sensor that includes two capacitors arranged in such a way that a fabrication process therefor is relatively easy. 
     It is another object of the present invention to provide a differential capacitive pressure sensor that provides a superior linearity by maintaining an equal gap between an upper electrode and a lower electrode of each capacitor. 
     In accordance with one aspect of the invention, a preferred embodiment of the present invention provides a differential capacitive pressure sensor, which includes: a substrate including a diaphragm positioned at a center portion thereof, the diaphragm having a reduced thickness; a first to a third lower electrode sequentially arranged on the substrate, the second lower electrode being positioned corresponding to the diaphragm, the first lower electrode and the second lower electrode being electrically connected to each other; a first to a third upper electrode respectively floating over the first to the third lower electrode, the second upper electrode crossing over the second lower electrode; and a first to a third supporting member downwardly extending from the first to the third upper electrode, respectively, the first and the second supporting member contacting the second lower electrode. 
     Another preferred embodiment of the present invention provides a differential capacitive pressure sensor, which includes: a substrate including a diaphragm positioned at a center portion thereof, the diaphragm having a reduced thickness; a first to a third lower electrode sequentially arranged on the substrate, the second lower electrode being positioned corresponding to the diaphragm, the first lower electrode and the second lower electrode being electrically connected to each other; a first to a third upper electrode respectively floating over the first to the third lower electrode, the second upper electrode crossing over the second lower electrode; and a first to a third supporting member downwardly extending from the first to the third upper electrode, respectively, the first and the second supporting member contacting the second lower electrode. 
     In accordance with another aspect of the invention, there is provided a method for fabricating a differential capacitive pressure sensor, the method including the steps of: forming lower electrodes on a substrate; forming a sacrificial layer on the substrate to cover the lower electrodes; forming a plurality of via holes passing through the sacrificial layer to uncover portions of the lower electrodes; forming the upper electrodes by depositing a conductive material on the sacrificial layer, the conductive material filling the via holes so that the supporting members are formed; and removing the sacrificial layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a perspective view of a differential capacitive pressure sensor in accordance with a first preferred embodiment of the present invention; 
     FIGS. 2A and 2B show vertical sectional views of the differential capacitive pressure sensor illustrated in FIG. 1; 
     FIGS. 3A to  3 G provide sectional views of the differential capacitive pressure sensor of FIG. 1; 
     FIGS. 4A to  4 G provide plan views thereof, corresponding to a sequence of a fabrication process therefor; 
     FIG. 5 depicts a perspective view of a differential capacitive pressure sensor in accordance with a second preferred embodiment of the present invention; 
     FIG. 6 is a vertical sectional view of the differential capacitive pressure sensor depicted in FIG. 4; 
     FIGS. 7A to  7 G set forth sectional views of the differential capacitive pressure sensor of FIG. 4; and 
     FIGS. 8A to  8 G set forth plan views thereof, corresponding to a sequence of a fabrication process therefor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1 to  8 , a differential capacitive pressure sensor in accordance with preferred embodiments of the present invention will be described in detail. Like numerals represent like parts in the drawings. 
     In FIG. 1, a differential capacitive pressure sensor  1  in accordance with a first preferred embodiment includes an outer electrode  12 , a first lower electrode  14 , a second lower electrode  16 , and a semiconductor substrate  10 . The outer electrode  12 , the first lower electrode  14 , and the second lower electrode  16  are sequentially disposed on the semiconductor substrate  10  in that order. Between the adjacent electrodes, predetermined intervals are respectively interposed. 
     The pressure sensor  1  further includes a first upper electrode  22 , a second upper electrode  24 , a first supporting member  18 , and a second supporting member  20 . The first and the second upper electrode  22  and  24  are respectively disposed over the first and the second lower electrode  14  and  16  with an equal gap interposed therebetween. The first supporting member  18  connects the outer electrode  12  with the first upper electrode  22 , and the second supporting member  20  connects the first lower electrode  14  with the second upper electrode  24 . The first and the second supporting member  18  and  20  respectively support the first and the second upper electrode  22  and  24 , so that the first and the second upper electrode  22  and  24  are maintained parallel to the first and the second lower electrode  14  and  16 , respectively. 
     The differential capacitive pressure sensor  1  preferably further includes an upper insulating layer  11  disposed on a top surface of the semiconductor substrate  10 . The upper insulating layer  11  serves to electrically separate the semiconductor substrate  10  from the above-explained electrodes. 
     The first lower electrode  14  and the first upper electrode  22  form a first capacitor “C1” (in FIG.  2 A), and the second lower electrode  18  and the second upper electrode  24  form a second capacitor “C2” (in FIG.  2 A). When a stimulus, i.e., pressure is applied to the differential capacitive sensor  1 , the first lower electrode  14  and the second upper electrode  24  are simultaneously displaced by an equal amount because the second supporting member  20  is interposed between the first lower electrode  14  and the second upper electrode  24 . Consequently, gaps of the first and the second capacitor “C1” and “C2” are respectively displaced in different directions but by equal amounts. 
     As previously explained, the first and the second capacitor “C1” and “C2” initially have gaps of a same size interposed between the upper and the lower electrode thereof. Gap distances are preferably reduced below a few micrometers (μm), so that the first and the second capacitor “C1” and “C2” present a large capacitance. The larger capacitances can improve the sensitivity of the sensor. 
     In FIG. 2A, the semiconductor substrate  10  preferably has a diaphragm  9 , having a reduced thickness, under peripheries of the first lower electrode  14 . The diaphragm  9  may be formed of a trench shape by applying an etching. When a pressure is applied to the differential capacitive pressure sensor  1 , the diaphragm  9  is deflected in response thereto. Disposed on a bottom surface of the semiconductor substrate  10  is a lower insulating layer  8 . 
     One of the first and the second capacitor “C1” and “C2” is disposed on the diaphragm  9  of the semiconductor substrate  10 , and the other is disposed on bulk portions thereof. There occurs little deflection in the bulk portions of the semiconductor substrate  10 . 
     Capacitances “C1 0 ” and “C2 0 ” of the first and the second capacitor “C1” and “C2” can be expressed by Equations 1 and 2, respectively.                C                   1   0       =     ɛ          A     d   0       .               Equation                 1                 C                   2   0       =     ɛ          A     d   0       .               Equation                 2                                
     In Equations 1 and 2, “d 0 ” represents a vertical distance between each upper electrode and a corresponding lower electrode; “A” represents an overlapped area of the upper and the lower electrode; and “ε” represents a dielectric constant. As seen from the above-mentioned Equations 1 and 2, the first and the second capacitor “C1” and “C2” have same capacitances. 
     When the first and the second capacitor “C1” and “C2” are oppositely deflected by a displacement “δ”, a difference between the capacitances of the first and the second capacitor “C1” and “C2” is expressed by Equation 3.               Δ                 C     =         C                 1     -     C                 2       =         ɛ        A       d   0     -   δ         -     ɛ        A       d   0     +   δ           =       ɛ                 A          2      δ       d   0   2            1     1   -       (     δ     d   0       )     2           =     ɛ                 A          2      δ       d   0   2            (     1   +       (     δ     d   0       )     2     +       (     δ     d   0       )     4     +   …     )                     Equation                 3                                
     Because the vertical distance “d0” is usually much larger than the displacement “δ” (d 0 &gt;&gt;δ), “δ/d 0 ” can be neglected, so that Equation 3 is simplified into Equation 4.               Δ                 C     =     ɛ                 A            2      δ       d   0   2       .               Equation                 4                                
     In Equation 4, the capacitance difference “ΔC” is proportional to the displacement “δ”. Since the displacement “δ” is proportional to the pressure acted on the diaphragm  9 , it can be inferred that the capacitance difference “ΔC” between the first and the second capacitor “C1” and “C2” is proportional to the pressure. 
     As shown in FIG. 2B, the differential capacitive pressure sensor  1  preferably has a center boss  10   a  disposed at the diaphragm  9 . The center boss  10   a  is disposed under the first lower electrode  14 . Without the center boss  10   a  disposed thereunder, a center portion of the diaphragm  9  may protrude upward or downward in response to the pressure acted thereon, such that the displacement differs between the first lower electrode  14  of the first capacitor “C1” and the second upper electrode  24  of the second capacitor “C2”. That is to say, the center boss  10   a  serves to prevent an irregular deflection of the first lower electrode  14  of the first capacitor “C1”. In the above-explained structure, the first lower electrode  14  of the first capacitor “C1” is disposed within the range of the center boss  10   a.    
     Accordingly, when the diaphragm  9  is displaced in response to the pressure acted thereon, the capacitance difference occurs between the first and the second capacitor “C1” and “C2”, so that the differential capacitive pressure sensor  1  in accordance with the preferred embodiments of the present invention can measure the pressure. 
     In the preferred embodiments of the present invention, the pressure acted on the diaphragm  9  reduces a vertical distance “d 0 ” between the upper and the lower electrode of the first capacitor “C1” and simultaneously increases that of the second capacitor “C2”. Consequently, the first capacitance “C1 0 ” increases and the second capacitance “C2 0 ” decreases. The capacitance difference “ΔC” is then amplified by a C-V converter having a switch capacitor circuit, so that a voltage output “V out ” that is proportional to the capacitance difference “ΔC” can be obtained by Equation 5. 
     
       
           V   out   =A   g   ΔC.   Equation 5 
       
     
     In Equation 5, “A g ” represents a gain of the circuit. 
     FIGS. 3A to  3 G are cross-sectional views and FIGS. 4A to  4 G are plan views showing a sequence of a fabrication process for the differential capacitive pressure sensor  1  of FIG.  1 . FIGS. 4A to  4 G are specifically top plan views except FIG. 4F, which is a bottom plan view. 
     In FIGS. 3A and 4A, the upper insulating layer  11  and the lower insulating layer  8  are respectively formed on the top and bottom surface of the semiconductor substrate  10 . The upper and the lower insulating layer  11  and  8 , which are made of silicon oxide or silicon nitride, serve as a protecting layer when the substrate  10  is later etched. 
     In FIGS. 3B and 4B, the outer electrode  12 , the first lower electrode  14 , and the second lower electrode  16  are formed on the upper insulating layer  11  at a predetermined interval disposed between adjacent electrodes. The outer electrode  12 , the first lower electrode  14 , and the second lower electrode  16  are made of conductive material, e.g., polysilicon, metal, or metallic oxide. 
     In FIGS. 3C and 4C, a sacrificial layer  17  is formed on the upper insulating layer  11  to cover the above-mentioned electrodes. The thickness of the sacrificial layer  17  is determined on the basis of intervals disposed between the electrodes. Material of the sacrificial layer  17  depends on the electrodes, and an insulating thin film such as silicon oxide, a metallic thin film, or a polymer thin film can be selected therefor. 
     In FIGS. 3D and 4D, holes  19  are formed through the sacrificial layer  17 ,  50  that portions of the outer electrode  12 , the first lower electrode  14  are uncovered. 
     In FIGS. 3E and 4E, a conductive material is deposited and patterned on the sacrificial layer  17 , so that the first upper electrode  22  and the second upper electrode  24  are formed. Portions of the conductive material are embedded inside the holes  19 , thereby forming the first and the second supporting member  18  and  20 . The first upper electrode  22  is connected to the outer electrode  12  via the first supporting member  18 , and the second upper electrode  24  is connected to the first lower electrode  14  via the second supporting member  20 . The first upper electrode  22  is arranged parallel to the first lower electrode  14 , and the second upper electrode  24  is arranged parallel to the second lower electrode  16 . The upper electrodes and the supporting members may be made of polysilicon, metal, or metallic oxide. 
     In the step of forming the upper electrodes, it is important to minimize the residual stress of a conductive layer before the layer is patterned to form the upper electrodes. If the residual stress of the conductive layer is high, the upper electrodes may be deflected after being formed, so that there occurs a residual difference in capacitances between the first and the second capacitor “C1” and “C2” (FIG.  2 A). To prevent the above-explained problem, the electric plating may be used for forming a relatively thicker conductive layer, thereby increasing the strength of the upper electrodes 
     In FIGS. 3F and 4F, the thickness of the semiconductor substrate  10  is reduced to form the diaphragm  9 . In the step of forming the diaphragm  9 , after the lower insulating layer  8  is patterned to expose a portion of the semiconductor substrate  10 , the exposed portion thereof is etched away to be of a trench shape having a predetermined depth. The center boss  10   a  shown in FIG. 2B may be additionally formed in the above-explained step. 
     Finally, in FIGS. 3G and 4G, the sacrificial layer  17  is removed by employing a dry etching or a wet etching, so that the first and the second capacitor “C1” and “C2” (FIG. 2A) of the differential capacitive pressure sensor  1  are completed. To enhance the efficiency of etching the sacrificial layer  17 , a multiplicity of through holes may be formed through the first and the second upper electrode  22  and  24 . 
     FIG. 5 is a perspective view showing a differential capacitive pressure sensor  100  in accordance with a second preferred embodiment of the present invention. In the second preferred embodiment, one of two sensing capacitors is divided. 
     The differential capacitive pressure sensor  100  in accordance with the second preferred embodiment of the present invention includes a semiconductor substrate  10 , a first lower electrode  102 , a second lower electrode  104 , a third lower electrode  106 , and a center electrode  108 . The above-mentioned electrodes are formed on the semiconductor substrate  10  at a predetermined interval interposed between adjacent two electrodes. It is preferred that an insulating layer  11  is interposed between each of the above-mentioned electrodes and the semiconductor substrate  10 . The differential capacitive pressure sensor  100  further includes a first upper electrode  118 , a second upper electrode  120 , and a third upper electrode  122 , which are respectively disposed over the first to the third lower electrode  102 ,  104 , and  106 . Each upper electrode is parallel to a corresponding lower electrode. 
     The first upper electrode  118  is connected to a first end of the second lower electrode  104  via a first supporting member  112 , which is downwardly extended from the first upper electrode  118 . The second upper electrode  120  is connected to the center electrode  108  via a second supporting member  114 , which is downwardly extended from the second upper electrode  120 . The third upper electrode  122  is connected to a second end of the second lower electrode  104  via a third supporting member  116 , which is downwardly extended from the third upper electrode  122 . 
     The first lower electrode  102  and the third lower electrode  106  are electrically connected to a first bonding pad  105  disposed on the insulating layer  11 . Via the first bonding pad  105 , an electrical signal is applied to the first and the third lower electrode  102  and  106 . The second lower electrode  104 , the first upper electrode  118 , and the third upper electrode  122  are electrically connected to a second bonding pad  110  disposed on the insulating layer  11 . Via the second bonding pad  110 , an electrical signal is applied to the second lower electrode  104 , the first upper electrode  118 , and the third upper electrode  122 . Further, the center electrode  108  serves as a third bonding pad via which an electrical signal is applied to the second upper electrode  120 . 
     In the differential capacitive pressure sensor  100  in accordance with the second preferred embodiment of the present invention, the first to the third lower electrode  102  to  106  are preferably arranged in a straight line. The center electrode  108  is preferably positioned adjacent to the straight line, and, more particularly, to the second lower electrode  104 . The second upper electrode  120  crosses over the straight line, and, more particularly, over the second lower electrode  104 . 
     FIG. 6 is a sectional view of the differential capacitive pressure sensor  100  of FIG.  5 . As shown, the second lower electrode  104  and the second upper electrode  120  form a first capacitor “C1”. The first lower electrode  102  and the first upper electrode  118  form a first half of a second capacitor “C2”, and the third lower electrode  106  and the third upper electrode  122  form a second half thereof. Accordingly, if the second capacitor “C2” implements a second capacitance “C2”, each half of the second capacitor “C2” implements a half of the second capacitance “C2”, that is,          “       1   2        C2     ”     .                          
     In the above-explained structure, the second lower electrode  104  has a first width “W 1 ” while each of the first and the third lower electrode  102  and  106  has a second width “W 2 ”, which is about a half of the first width “W 1 ” (W 1 =2W 2 ). In that case, each of the first and the third upper electrode  118  and  122  has a smaller plan area than that of the first upper electrode  22  or the second upper electrode  24  (FIG. 2A) of the differential capacitive pressure sensor  1  in accordance with the first preferred embodiment. When an upper electrode floating over a corresponding lower electrode has a relatively smaller plan area, a more stable structure and a more easy fabrication process for the differential capacitive pressure sensor can be achieved. 
     In addition, because the second capacitor “C2” is divided, each area of the first to the third supporting member  112  to  116  is relatively large such that each supporting member can more stably support a corresponding upper electrode. Consequently, when a sacrificial layer is removed or in another step of the fabrication process, the upper electrode rarely contracts toward a corresponding lower electrode, so that the upper electrode maintains a predetermined gap with respect to a corresponding lower electrode. 
     The differential capacitive pressure sensor  100  in accordance with the second preferred embodiment of the present invention may further include the diaphragm  9 , which preferably has the center boss  10   a . Since the diaphragm  9  and the center boss  10   a  are previously explained in detail, additional description thereof will be omitted. 
     FIGS. 7A to  7 G are cross-sectional views and FIGS. 8A to  8 G are plan views showing a sequence of a fabrication process for the differential capacitive pressure sensor  100  of FIG.  5 . FIGS. 8A to  8 G are specifically top plan views except FIG. 8F, which is a bottom plan view. 
     In FIGS. 7A and 8A, the upper insulating layer  11  and the lower insulating layer  8  are respectively formed on the top and bottom surface of the semiconductor substrate  10 . The upper and the lower insulating layer  11  and  8 , which are silicon oxide or silicon nitride, serve as a protecting layer for protecting the substrate  10 , which is later etched. 
     In FIGS. 7B and 8B, the first lower electrode  102 , the second lower electrode  104 , the third lower electrode  106 , and the center electrode  108  (FIG. 5) are formed on the upper insulating layer  11  at a predetermined interval disposed between adjacent electrodes. The first lower electrode  102 , the second lower electrode  104 , the third lower electrode  106 , and the center electrode  108  are made of conductive material, e.g., polysilicon, metal, or metallic oxide. 
     In FIGS. 7C and 8C, a sacrificial layer  107  is formed on the upper insulating layer  11  to cover the above-mentioned electrodes. The sacrificial layer  107  is an insulating thin film such as silicon oxide, a metallic thin film, or a polymer thin film. 
     In FIGS. 7D and 8D, via holes  109  are formed through the sacrificial layer  107 , so that both end portions of the second lower electrode  104  as well as one end portion of the center electrode  108  are partially uncovered. 
     In FIGS. 7E and 8E, a conductive material is deposited and patterned on the sacrificial layer  107 ,  50  that the first upper electrode  118 , the second upper electrode  120 , and the third upper electrode  122  are formed. Portions of conductive material fill the holes  109 , thereby forming the first to the third supporting member  112  to  116 . The upper electrodes and the supporting members may be made of polysilicon, metal, or metallic oxide. 
     In FIGS. 7F and 8F, the thickness of the semiconductor substrate  10  is reduced to form the diaphragm  9 . In the step of forming the diaphragm  9 , after the lower insulating layer  8  is patterned to expose a portion of the semiconductor substrate  10 , the exposed portion thereof is etched away to obtain a trench shape having a predetermined depth. The center boss  10   a  shown in FIG. 2B may be additionally formed in the above-explained step. 
     Finally, as shown in FIGS. 7G and 8G, the sacrificial layer  107  is removed by applying a dry etching or a wet etching to complete the differential capacitive pressure sensor  100  in accordance with the second preferred embodiment of the present invention. 
     While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.