Patent Abstract:
A capacitive physical load sensor includes a substrate, which has fixed electrodes, and a diaphragm, which has movable electrodes. The diaphragm is located across a gap from the substrate, and retaining parts for the diaphragm are formed around the diaphragm. Protruding parts extend into the gap from the diaphragm or from the substrate. The protruding parts support the diaphragm at different levels of deformation to alter the characteristics of the diaphragm and extend its range.

Full Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application relates to and incorporates by reference Japanese patent application no. 2001-166350, which was filed on Jun. 1, 2001. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a capacitive physical load sensor and a capacitive physical load detection system. 
     An example of a capacitive physical load detection system having a conventional capacitive physical load sensor will first be described by referring to FIG.  14  through FIG.  18 . As shown in FIG. 14, the conventional capacitive pressure detection system  1  includes a capacitive pressure sensor  10  and capacitive detection circuits  64 . The capacitive pressure sensor  10  includes a pressure sensitive capacitor  20  with pressure capacitance C X  and a reference capacitor  30  with reference capacitance C R . The pressure sensitive capacitor  20  is connected to input  60  of a detection voltage V X . Reference capacitor  30  is connected to input  62  of a reference voltage V R . Pressure sensitive capacitor  20  and reference capacitor  30  are connected to the capacitance detection circuits  64 . The capacitance detection circuits  64  are connected to an output  78  of a voltage V OUT . 
     The capacitive pressure sensor  10  is manufactured by forming a diaphragm on a silicon substrate. More specifically, the capacitive pressure sensor  10  includes a silicon substrate  80 , a diaphragm  84 , which is formed across a gap  82  from the silicon substrate  80 , and a retaining part  86  for the diaphragm  84 , which is formed around the diaphragm  84 , as shown in FIGS. 16 to  18 . 
     Formed on a top surface of the silicon substrate  80  is a pressure sensitive capacitor lower electrode  22   b  and reference capacitor lower electrode  32   b . The pressure sensitive capacitor lower electrode  22   b  is connected to a pressure sensitive capacitor lower electrode pad  26   b  through a pressure sensitive capacitor lower electrode lead  24   b  (see FIG.  15  and FIG.  16 ), and the reference capacitor lower electrode  32   b  is connected to a reference capacitor lower electrode pad  36   b  through a reference capacitor lower electrode lead  34   b  (see FIG.  15  and FIG.  16 ). The surface of the silicon substrate  80  is covered by a substrate protective layer  88  (see FIG.  16  through FIG.  18 ). 
     The diaphragm  84  includes a semiconductor film  92 , which consists of a poly silicon film, and a protective film  96 , which consists of a silicon nitride film. A pressure sensitive capacitor upper electrode  22   a  and a reference capacitor upper electrode  32   a  are formed on top of the semiconductor film  92 . The pressure sensitive capacitor upper electrode  22   a  is connected to a pressure sensitive capacitor upper electrode pad  26   a  through a pressure sensitive capacitor upper electrode lead  24   a  (see FIG.  15  and FIG.  17 ), and the reference capacitor upper electrode  32   a  is connected to a reference capacitor upper electrode pad  36   a  through a reference capacitor upper electrode lead  34   a  (see FIG.  15  and FIG.  17 ). 
     A pressure capacitor  20  shown in FIG. 14 includes the pressure sensitive capacitor upper electrode  22   a  and the pressure sensitive capacitor lower electrode  22   b  shown in FIG.  16  through FIG.  18 . The reference capacitor  30  shown in FIG. 13 includes the reference capacitor upper electrode  32   a  and reference capacitor lower electrode  32   b  shown in FIGS. 16 to  18 . 
     When pressure is applied to the diaphragm  84 , the gap  82  acts as a pressure reference chamber that is sealed in a vacuum, and the diaphragm  84  stretches and changes shape in proportion to the applied pressure, as shown in FIGS. 16 to  18 . When the shape of the diaphragm  84  changes, the distance between the upper electrode  22   a  and the lower electrode  22   b  changes. When the distance between the two electrodes changes, the capacitance between the two electrodes also changes. The circuits shown in FIG. 14 detect a difference between a change in the pressure sensitive capacitance C X  of the pressure sensitive capacitor  20  and the reference capacitance C R  of the reference capacitor  30  and convert the results into an output voltage V OUT  using the capacitance detection circuits  64  in order to detect the magnitude of the pressure being applied on the diaphragm  84 . 
     The reference capacitor  30  makes up for changes in capacitance due to changes in temperature in the environment in which the sensor  10  is placed. As a result, the output voltage V OUT  of the sensor  10  is independent of temperature and dependent only on pressure. 
     In the conventional capacitive pressure sensor  1 , which was described above, the output voltage V OUT  is proportional to the applied pressure, until the applied pressure reaches a value P A , as shown in a graph in FIG.  19 . Once the applied pressure reaches the value P A , the diaphragm  84 , shown in FIG.  16  through FIG. 18, comes into contact with the silicon substrate  80 , starting at the center, where the diaphragm  84  deforms the most. Beyond this point, the output voltage V OUT  gradually becomes saturated and is no longer proportional to the applied pressure. When the applied pressure reaches a value P B , the center part of the diaphragm  84  comes into complete contact with the silicon substrate  80 . As a result, the output voltage V OUT  is completely saturated with respect to the applied pressure and can no longer represent the applied pressure. 
     When the diaphragm  84  is thicker, or the diameter of the diaphragm  84  is smaller, the shape of the diaphragm  84  would not change as much with respect to the applied pressure, and it would be possible detect a wider range of pressure levels. However, when the diaphragm  84  is thicker, or the diameter of the diaphragm  84  is smaller, sensor sensitivity suffers. That is, the resolution in detectable pressure is smaller. 
     An ideal pressure sensor is able to detect a wide range of physical loads (pressure, acceleration, vibration, sound pressure) and offer a high level of sensitivity to detect minute changes in the physical loads across their entire ranges. However, it is difficult to produce such a sensor. On the other hand, a normal application for a capacitive pressure sensor would require a measurement range over which the measurement results must be highly precise, as well as a range over which lower sensitivity is acceptable. In many cases, a lower detectible resolution would be acceptable when the magnitude of the physical load to be measured is large. 
     Therefore, it is the goal of this invention to provide a capacitive pressure sensor capable of both detecting small changes in pressure across a range over which a high sensitivity is required and of detecting a wide range of pressure levels across a range over which high sensitivity is not required. 
     SUMMARY OF THE INVENTION 
     This invention is essentially a capacitive physical load sensor including a substrate having a fixed electrode and a diaphragm having a movable electrode. The diaphragm is located across a gap from the substrate. A retaining part for the diaphragm is formed around the diaphragm a protruding part extends from a surface of the substrate or from a surface of the diaphragm into the gap. 
     The protruding part may be one of a plurality of protruding parts, and surfaces of the protruding parts support the diaphragm when certain physical loads are applied to the diaphragm, respectively. 
     In a further aspect, the invention may include a correction circuit for correcting a load detection value outputted by the diaphragm, so that the sensor correction circuit issues an output value that changes in a manner that is substantially proportional to changes in the physical load applied to the diaphragm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of one embodiment of a capacitive pressure detecting system of the present invention; 
     FIG. 2 is a plan view of the capacitive pressure sensor of FIG. 1; 
     FIG. 3 is a cross-sectional view along line  3 — 3  in FIG. 2; 
     FIG. 4 is a cross-sectional view along a  4 — 4  in FIG. 2; 
     FIG. 5 is a cross-sectional view along line  5 — 5  in FIG. 2; 
     FIG. 6 is a diagram showing a first part of a manufacturing process of the sensor of FIG. 2; 
     FIG. 7 is a diagram showing a second part of a manufacturing process of the sensor of FIG. 2; 
     FIG. 8 is a diagram showing a third part of a manufacturing process of the sensor of FIG. 2; 
     FIG. 9 is a diagram showing a fourth part of a manufacturing process for the sensor of FIG. 2; 
     FIG. 10 is diagram showing a fifth part of a manufacturing process of the sensor of FIG. 2; 
     FIG. 11 is a diagram showing a sixth part of a manufacturing process of the sensor of FIG.  2 . 
     FIG. 12 is a graph showing the applied pressure-output voltage characteristics of the capacitive pressure detection system of the sensor of FIG.  2 . 
     FIG. 13 is a diagram like to FIG. 4 for a capacitive pressure sensor of another embodiment; 
     FIG. 14 is a block diagram for a conventional capacitive pressure detection system; 
     FIG. 15 is a top view of a conventional capacitive pressure sensor; 
     FIG. 16 is a cross-sectional view along a line  16 — 16  in FIG. 15; 
     FIG. 17 a cross-sectional view along a  17 — 17  line in FIG. 15; 
     FIG. 18 is a cross-sectional view along an  18 — 18  in FIG. 15; and 
     FIG. 19 is a graph showing the applied pressure-output voltage characteristics of the conventional capacitive pressure detection system of FIGS.  14 - 18 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The structure of the capacitive pressure detection system of this embodiment will be explained by referring to FIG.  1  through FIG.  5 . 
     As shown in FIG. 1, a capacitive sensor detection system  101  includes a capacitive pressure sensor  110 , capacitive detection circuits  164 , a ROM  172 , and signal processing circuits  174 . The capacitive pressure sensor  110  includes a pressure sensitive capacitor  120  with pressure sensitive capacitance C X , reference capacitor  130  with reference capacitance C R , a first switch  140 , and a second switch  150 . The pressure sensitive capacitor  120  is connected to an input lead  160  of the detection voltage V X . The reference capacitor  130  is connected to an input lead  162  of a reference voltage V R . The pressure sensitive capacitor  120  and reference capacitor  130  are connected to the capacitive detection circuits  164 . 
     A first switch  140  is connected in series to a resistance  170   a , and a second switch  150  is connected in series to a resistance  170   b . A group including the first switch  140  and resistance  170   a  and a group including the second switch  150  and resistance  170   b  are connected in parallel with each other and to a power supply  168 . Wiring lines extend from a point between the first switch  140  and resistance  170   a  and from a point between the second switch  150  and resistance  170   b , respectively, to the ROM  172 . 
     Capacitance detection circuits  164  and ROM  172  are connected to the signal processing circuits  174 . Signal processing circuits  174  are connected to output lead  178  for a voltage V SEN . 
     Capacitive pressure sensor  110  is actually manufactured by forming a diaphragm on a silicon substrate. More specifically, the capacitive pressure sensor  110 , as shown in FIG. 3 to FIG. 5, includes a silicon substrate  180 , a diaphragm  184  formed across a gap  182  from the silicon substrate  180 , and a retaining part  186  for the diaphragm  184  formed around the diaphragm  184 . 
     A pressure sensitive capacitor lower electrode  122   b , reference capacitance lower electrode  132   b , first lower switch  142   b , and second lower switch  152   b  are formed on the silicon substrate  180 , with a highly concentrated impurity diffusion layer on the silicon substrate  180  for ensuring high conductance. A pressure sensitive capacitance lower electrode  122   b  is connected to a pressure sensitive capacitance lower electrode pad  126   b  through a pressure sensitive capacitance lower electrode lead  124   b  (see FIG.  2  and FIG.  3 ), and the reference capacitance lower electrode  132   b  is connected to a reference capacitor lower electrode pad  136   b  through a reference capacitance lower electrode lead  134   b  (see FIG.  2  and FIG.  3 ). The first lower switch  142   b  is connected to the first lower switch pad  146   b  through a first lower switch lead  144   b  (see FIG.  2 ), while the second lower switch  152   b  is connected to a second lower switch pad  156   b  through a second lower switch lead  154   b  (see FIG.  2 ). The surface of the silicon substrate  180  is protected with a substrate protective film  188  (see FIG.  3  through FIG.  5 ). 
     The diaphragm  184  includes a semiconductor film  192 , made of a polysilicon film, and a sealing film  196 , made of a silicon nitride film. A pressure sensitive capacitance upper electrode  122   a , reference capacitance upper electrode  132   a , first upper switch  142   a  (an example of a protruding part), and second upper switch  152   a  (an example of an protruding part) are formed on the semiconductor film  192  with a highly concentrated impurity diffusion layer in the semiconductor film  192  for ensuring high conductance. Pressure sensitive capacitance upper electrode  122   a  is connected to a pressure sensitive capacitance upper electrode pad  126   a  through a pressure sensitive capacitance upper electrode lead  124   a  (see FIG.  2  and FIG.  4 ), while the reference capacitance upper electrode  132   a  is connected to a reference capacitance upper electrode pad  136   a  through a reference capacitance upper electrode lead  134   a  (see FIG.  2  and FIG.  4 ). In addition, the first upper switch  142   a  is connected to a first upper switch pad  146   a  through a first upper switch lead  144   a  (see FIG.  2 ), while the second upper switch  152   a  is connected to a second upper switch pad  156   a  through a second upper switch lead  154   a  (see FIG.  2 ). 
     As shown in the top view in FIG. 2, the pressure sensitive capacitor upper electrode  122   a  is formed into a circular plate, and the pressure sensitive capacitor lower electrode  122   b , also formed into a similar circular plate, faces the pressure sensitive capacitor upper electrode  122   a  (shown in FIG.  3  through FIG.  5 ). 
     The second upper switch  152   a  is formed to surround the outer perimeter of the pressure sensitive capacitor upper electrode  122   a . The second upper switch  152   a  is formed into a ring along a topographical line along which the semiconductor film  192  changes shape. The second upper switch  152   a , as shown in FIG.  3  through FIG. 5, protrudes from the lower surface of the semiconductor film  192  into the gap  182 . The ring-shaped second lower switch  152   b  of the same size as the second upper switch  152   a  faces the second upper switch  152   a.    
     The first upper switch  142   a  is formed in such a way as to surround the outer perimeter of the second upper switch  152   a . The first upper switch  142   a  is formed into a ring shape along a topographical line along which the semiconductor film  192  changes shape. The first upper switch  142   a , as shown in FIG.  3  through FIG. 5, protrudes from the lower surface of the semiconductor film  192  into the gap  182 . The first upper switch  142   a  extends further than the second upper switch  152   a . The lengths over which the first upper switch  142   a  and the second upper switch  152   a  extend are adjusted so that the first upper switch  142   a  will first touch the first lower switch  142   b , and then the second upper switch  152   a  will touch the second lower switch  152   b , when a pressure is applied on the diaphragm  184 . 
     The pressure sensitive capacitor  120 , shown in FIG. 1, includes the pressure sensitive capacitor upper electrode  122   a  and pressure sensitive capacitor lower electrode  122   b , as shown in FIG.  3  through FIG.  5 . The reference capacitor  130  in FIG. 1 includes the reference capacitor upper electrode  132   a  and reference capacitor lower electrode  132   b , as shown in FIG.  3  through FIG.  5 . The first switch  140  shown in FIG. 1 includes the first upper switch  142   a  and the first lower switch  142   b , as shown in FIG.  3  through FIG.  5 . The second switch  150  shown in FIG. 1 includes the second upper switch  152   a  and the second lower switch  152   b , as shown in FIG.  3  through FIG.  5 . 
     The capacitance detection circuits  164  in FIG. 1 may be formed with switched capacitor circuits. Switched capacitor circuits can be easily formed with a normal semiconductor process and integrated into the same substrate as the capacitive pressure sensor  110 . The signal processing circuits  174 , shown in FIG. 1, may be formed with multiplier circuits that basically multiply the output voltage V OUT  from the capacitance detection circuits  164  with the correction parameters from the ROM  172 . The ROM  172 , shown in FIG. 1, may be implemented with battery backed RAM, flash memory, or non-volatile RAM. The signal processing circuits  174  and ROM  172  can also be integrated on the same substrate as the capacitive pressure sensor  110 . 
     Next, an example of a method of manufacturing the capacitive pressure sensor  110  in the capacitive pressure detection system  101  of this embodiment will be discussed by referring to FIG.  6  through FIG.  11 . The diaphragm structure and the electrode pair structure, mentioned above, are implemented using a manufacturing method that is described below. 
     As shown in FIG. 6, a diffusion layer (pressure sensitive capacitor lower electrode  122   b , reference capacitor lower electrode  122   b , first lower switch  142   b  and second lower switch  152   b ) is formed by adding impurities locally to the surface of the silicon substrate  180  by thermal diffusion or ion implanting. Then, a substrate protective layer  188 , which resists etching, is formed by depositing a silicon nitride film by, for example, a CVD method. A sacrificial layer  190  is formed by depositing a silicon oxide film by, for example, a CVD method. As shown in FIG. 7, dry etching is performed using a resist (not shown in the figure) as a mask to pattern the sacrificial layer  190 . This patterning step is performed in order to form parts that will later turn into the first upper switch  142   a  and the second upper switch  142   b . A part for the first upper switch  142   a  has a different depth from a part for the second upper switch  142   b  in these patterns. More specifically, the part where the first upper switch  142   a  is to be formed is initially etched to a prescribed depth. Next the part where the first upper switch  142   a  is to be formed and the part where the second upper switch  142   b  is to be formed are both etched simultaneously. As a result, the part where the first upper switch  142   a  is to be formed is etched more deeply than the part where the second upper switch  142   b  is to be formed. 
     As shown in FIG. 8, the semiconductor film  192 , which is etch-resistant, is next formed by depositing a polysilicon film by, for example, a CVD method. As shown in FIG. 9, a diffusion layer (pressure sensitive capacitor upper electrode  122   a , reference capacitor upper electrode  132   a , first upper switch  142   a  and second upper switch  152   a ) is formed by adding a small dose of p-type impurity, such as phosphorous, locally into the surface of the semiconductor film  192  by thermal diffusion or ion implanting. Because leakage currents may flow from the diffusion layer into the semiconductor film  192 , depending on the temperature, a small dose of n-type impurities should be added to the semiconductor film  192  to prevent leakage. As shown in FIG. 10, etching holes  194  are next formed in the semiconductor film  192 , and the sacrificial layer  190  is stripped by wet etching. An etching solution used for etching should preferably be able to etch well the silicon oxide film that makes up the sacrificial layer  190  but not the silicon nitride film that makes up the substrate protective layer  188  or the poly silicon film that makes up the semiconductor layer  192  (for example, HF acid solution). Besides the wet etching method mentioned so far, etching may also be accomplished by dry etching using a gas mixture with HF and water vapor or methyl alcohol. As shown in FIG. 11, the sealing film  196  is then formed to seal the etching holes  194 . As a result, the gap  182  turns into a vacuum and functions as a reference pressure chamber. Finally, the diaphragm  184  and the retaining part  186  for the diaphragm  184  are formed. 
     In the embodiment discussed above, the sacrificial layer  190  is formed by depositing a silicon oxide film with a CVD method. The sacrificial layer  190  may also be formed by depositing a silicon oxide film by thermal oxidation. Any material would work, as long as the material forms a stable deposit film on the silicon substrate  180  and would etch much more rapidly than the poly silicon film that makes up the semiconductor film  192 . 
     The following is a description of the operation of the capacitive pressure detection system  101  of this embodiment. When a prescribed level of pressure is applied on the diaphragm  184 , as shown in FIGS. 3 to  5 , the gap  182  acts as a reference pressure chamber, which is a sealed vacuum. The diaphragm  184  changes its shape in proportion to the pressure being applied. As the diaphragm  184  deforms, distance between the pressure sensitive capacitor upper electrode  122   a  and pressure sensitive capacitor lower electrode  122   b  changes. Capacitance between the two electrodes changes as the distance between the two electrodes changes. As shown in FIG. 1, capacitance detection circuits  164  sense the changes in the pressure sensitive capacitance C X  of the pressure sensitive capacitor  120  with respect to the reference capacitance C R  of the reference capacitor  130  and converts results into the output voltage V OUT . A solid line in FIG. 12 shows a relationship between the applied pressures and voltage value V OUT . 
     As shown in FIG. 12, once the applied pressure reaches a level P 1 , the first upper switch  142   a  comes into contact with the first lower switch  142   b  due to the diaphragm  184  changing shape, and the first switch  140  closes. From this point on, the first upper switch  142   a  and the first lower switch  142   b , which are in contact, determine the area of the diaphragm that changes shape under the applied pressure. In other words, the area inside the first upper switch  142   a  and the first lower switch  142   b  would be the area where the diaphragm  184  changes shape in proportion to pressure. Because the diameter of this area is smaller, the diaphragm  184  tends to change shape less, and the voltage value V OUT  tends to increase by a smaller increment in proportion to the increase in the pressure being applied. Once the applied pressure reaches a level P 2 , the second upper switch  152   a  comes into contact with the second lower switch  152   b , and the second switch  150  closes. From this point on, the second upper switch  152   a  and the second lower switch  152   b , which are in contact with each other, determine the area in which the diaphragm changes shape under pressure. In other words, only the area inside the second upper switch  152   a  and the second lower switch  152   b  is the area where the diaphragm  184  changes shape in proportion to the pressure. Because the diameter of the area in which the diaphragm  184  changes shape is even smaller, the diaphragm  184  is even less likely to change shape. The increments by which the output voltage V OUT  increases become even smaller with respect to the increase in applied pressure. 
     When each of the switches  140 ,  150  is closed, the conditions under which the diaphragm  184  is supported changes. When the first switch  140  closes, the pressure detecting area of the diaphragm  184  decreases in size from a circular area with a diameter L 0  supported by the retaining part  186  (shown in FIG. 5) to a circular area with a smaller diameter L 1  supported by the first upper switch  142   a . Furthermore, when the second switch  150  closes, the pressure detecting area of the diaphragm  184  decreases in size from a circular area of the diameter L 1  supported by the first upper switch  142   a  to a circular area of a diameter L 2  supported by the second upper switch  152   a . When the size of the pressure detecting area of the diaphragm  184  decreases, the amount by which the diaphragm  184  changes shape (amount of stretching) with respect to changes in the magnitude of the applied pressure decreases. As a result, the distance between the pressure sensitive capacitor electrodes  122   a  and  122   b  changes by smaller increments, and consequently the pressure sensitive capacitance between the pressure sensitive capacitor electrodes  122   a  and  122   b  changes by smaller increments (or changes in voltage value V OUT .) 
     As shown in FIG. 1, a voltage is applied by the power supply  168  on resistance  170   a , when the first switch  140  closes in the sensor. The ROM  172  provides a correction parameter signal output as a result of this voltage being transmitted to the ROM  172 . The correction parameter signals are sent to the signal processing circuits  174 . The signal processing circuits  174  also receive the output voltage V OUT  from the capacitance detection circuit  164  and output the value V SEN , which is a product of the voltage V OUT  and correction parameters from the correction parameter signals. 
     The steps described above provide a correction for ensuring that the rate at which the voltage V OUT  changes with respect to changes in the applied pressure before the first switch  140  closes are almost the same as the rate at which the voltage V SEN  changes with respect to changes in applied pressure after the first switch  140  closes. Similarly, these steps provide a correction to ensure that the rate at which the voltage V OUT  changes with respect to changes in the applied pressure before the second switch  150  closes is almost the same as the rate at which the voltage V SEN  changes with respect to the applied pressure after the second switch  150  closes. The relationship between the applied pressure and output voltage V OUT  before the correction is represented by the solid line in FIG. 12, while the relationship between applied pressure and output voltage V SEN  and after the correction is represented by the broken line, which has is almost linear. 
     In the embodiment described above, the correction parameters are applied to the voltage V OUT  while the first switch  140  or the second switch  150  is turned on. However, correction parameter values that are less than one might also be applied to the voltage V OUT  before the first switch  140  or the second switch  150  closes. It is also possible to apply two different sets of correction parameter values on the voltage V OUT  before and after the first switch  140  or the second switch  150  closes. 
     Furthermore, although the correction parameters are applied to the voltage V OUT  while the first switch  140  or the second switch  150  is turned on in the embodiment described above, the correction parameters may also be applied to the voltage V OUT  when the voltage V OUT , which is an output from the capacitance detection circuits  164 , shows values above prescribed voltage levels of V 1  or V 2 . It is also possible to apply correction parameter values that are less than one on the voltage V OUT  before the voltage V OUT  reaches V 1  or V 2 . Furthermore, it is also possible to apply different sets of correction parameter values on the voltage V OUT  before and after the voltage V OUT  reaches the voltage level V 1  or V 2 . 
     Although correction parameters are applied on the voltage V OUT  by multiplication in the embodiment described above, it is also possible to make corrections to ensure that the rate at which the voltage V OUT  changes with respect to the applied pressure before the switch closes would almost be the same as the rate at which the voltage V SEN  changes with respect to the applied pressure after the switch is turned on by applying the correction parameters on the voltage V OUT  by division, addition, or subtraction. 
     So far, one embodiment of the capacitive pressure sensor of this invention has been discussed. However, applications of this invention are not limited to the embodiment described. In other words, improvements and modifications to the embodiment of this invention are possible by those knowledgeable in the art. 
     Although in this embodiment, as shown in FIG.  3  through FIG. 5, it is the first upper switch  142   a  and the second upper switch  152   a  that protrude into the gap  182  from the surface of the semiconductor film  192  that faces the gap  182 , the applications of this invention are not so limited. For example, as shown in FIG. 13, it is also possible to have the first lower switch  142   b  and the second lower switch  152   b  protrude from the surface of the silicon substrate  180  into the gap  182 . These protruding parts might be formed by depositing a polysilicon film on the silicon substrate  180  by a CVD method, stripping unnecessary parts by etching, and adding impurities to the polysilicon film by thermal diffusion or ion implanting. It is also possible to have both the upper and lower switches protrude into the gap. 
     In this particular embodiment, capacitance changes as a result of the diaphragm  184  stretching under pressure. This invention, however, will also apply to capacitance changing as a result of the diaphragm  184  stretching under other types of physical loads, including acceleration, vibration, and sound pressure. 
     Furthermore, in this particular embodiment, two concentric rings of protruding parts  142   a  and  152   a  facilitate two stages of sensitivity levels. However, additional sensitivity levels are also possible with this invention. Furthermore, instead of the ring-shaped protruding parts  142   a  and  152   a , a multitude of column-shaped protruding parts may also be laid out in rings, if the diaphragm  184  is circular in shape when flat. If the diaphragm  184  is, for example, square-shaped when flat, a single protruding part or a multitude of protruding parts should preferably be laid out along the topographical line(s) along which the diaphragm  184  changes shape (lines along which the magnitude of stretching would be the same).

Technology Classification (CPC): 6