Patent Publication Number: US-2013247677-A1

Title: Pressure sensor

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a pressure sensor. 
     2. Description of the Related Art 
     In the related art, for example, a differential pressure sensor (pressure sensor) including two pressure sensor elements of both-side pressure receiving type arranged in the proximity of two symmetrical positions on a pressure introducing route so as to have reverse polarities from each other, and configured to amplify differential of outputs from both of the pressure sensor elements, thereby obtaining an output from which a detection error due to temperature characteristics of the pressure sensor elements and vibrations caused by disturbance are compensated is known (for example, see JP-A-4-29027). 
     Incidentally, if the pressure sensor of the related art described above has, for example, a gentle or no frequency dependency of sensitivity with respect to the pressure in accordance with the shape and material of the sensing portion, and has a sensitivity substantially equivalent to a wide range of frequency band, a noise (sound) caused by signals in other frequency bands may increase with respect to signals in a desired frequency band, so that there is a risk of saturation of the output from the pressure sensor due to the signals other than the desired frequency band. 
     SUMMARY 
     In view of such circumstances, it is an object of the invention to provide a pressure sensor configured to be capable of obtaining desired frequency characteristics while reducing detection errors and vibrations due to disturbance. 
     In order to solve the problem described above and achieve the object described above, there is provided a pressure sensor includes: two pressure variation sensors (for example, a first pressure variation sensor (P 1 )  11   a  and a second pressure variation sensor (P 2 )  11   b  of the embodiment), and a detecting unit configured to detect the difference between outputs from the two pressure variation sensors (for example, a detection circuit  12  of the embodiment), the pressure variation sensors each includes: an opening cavity (for example, a cavity  21  of the embodiment); a cantilever (for example, a cantilever  22  of the embodiment) formed into a plate shape extending from a proximal side toward a distal side, including a proximal end portion (for example, a proximal end portion  22   a  of the embodiment) supported in a cantilevered state at an opening end (for example, an opening end  21   a  of the embodiment) of the cavity and a distal end portion (for example, a distal end portion  22   b  of the embodiment) as a free end and configured to be subject to a flexural deformation in accordance with the pressure difference between the interior and the exterior of the cavity; a gap (for example, a gap  23  of the embodiment) provided between the distal end portion of the cantilever and the opening end of the cavity and configured to communicate the interior and the exterior of the cavity; and a deformation detecting unit (for example, a piezoresistance of the embodiment) configured to detect a flexural deformation of the cantilever and output a signal of a result of detection, wherein the two pressure variation sensors have frequency characteristics different from each other in accordance at least with the capacities of the cavities or the distance of the gaps. 
     In addition, according to the pressure sensor of the invention, the frequency characteristic is a lower limit frequency which provides the sensitivities of the pressure variation sensors equal to or higher than a predetermined value. 
     Furthermore, according to the pressure sensor of the invention, the two pressure variation sensors are arranged so as to be adjacent with the distal end portion of one of the cantilevers and the proximal end portion of the other cantilever faced each other in the direction of extension of the cantilevers. 
     Still further, according to the pressure sensor of the invention, the two pressure variation sensors are arranged so as to be adjacent with the distal end portions of the cantilevers faced each other in the direction of extension of the cantilevers. 
     In addition, according to the pressure sensor of the invention, the deformation detecting unit includes a piezoresistance (for example, a piezoresistance  24  of the embodiment) formed by doping impurity at the proximal end portion of the cantilever formed of a semiconductor material. 
     According to the pressure sensor of the invention, by detecting the difference of the output from the two pressure variation sensors having frequency characteristics different from each other, only the pressure variation having desired frequency characteristics corresponding to the difference in different frequency characteristics may be detected. 
     Accordingly, increase in noise (sound) with respect to the pressure variations in the desired frequency characteristics due to the pressure variations in other frequency characteristics other than the desired frequency characteristics is prevented, and saturation of the signal in the amplifying circuit of the first step is prevented. 
     In addition, the detection error due to the temperature characteristic or vibrations due to the disturbance generated in the pressure variation sensors may be compensated by the difference in output from the two pressure variation sensors, so that the detection accuracy of the pressure variations may be improved. 
     Furthermore, only the pressure variations in the desired frequency band corresponding to the difference between different lower limit frequencies may be detected by setting the frequency characteristics of the two pressure variation sensors different from each other to, for example, lower limit frequencies which provide the sensitivities of the pressure variation sensor equal to or higher than the predetermined value such as the cutoff frequency. 
     In other words, the pressure variations at higher frequencies and lower frequencies with respect to the desired frequency band between one of the lower limit frequencies and the other lower limit frequency may be compensated by detecting the difference between the outputs from the two pressure variation sensors. 
     Accordingly, the pressure sensor may be operated so as to have the sensitivity only for the pressure variations in so-called a desired frequency band. 
     Furthermore, the two pressure variation sensors are arranged so that the cantilevers extend in the same direction from the proximal ends to the distal ends of the respective cantilevers in the direction of extension of the cantilevers, whereby the cantilevers are subject to the action of the vibrations caused by the disturbance such as wind or light evenly. Therefore, the vibrations caused by the disturbance generated in the two pressure variation sensors respectively may be compensated adequately by the difference in outputs from the two pressure variation sensors. 
     Furthermore, the two pressure variation sensors are arranged so that the cantilevers extend in the opposite directions from the proximal ends to the distal ends of the respective cantilevers in the direction of extension of the cantilevers, whereby generation of phase difference in sensitivities of the cantilevers may be inhibited with respect to vibrations of a high frequency band such as a sound. Therefore, the vibrations in the high frequency band such as the sound generated in the respective pressure variation sensors may be compensated by the difference between the outputs from the two pressure variation sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a plan view and a cross-sectional view of a pressure variation sensor of a pressure sensor according to an embodiment of the invention; 
         FIGS. 2A and 2B  are graphs showing an example of an output from the pressure variation sensor of the pressure sensor according to the embodiment of the invention; 
         FIGS. 3A to 3C  are drawings illustrating an example of an action of the pressure variation sensor of the pressure sensor according to the embodiment of the invention; 
         FIG. 4  is a configuration diagram of the pressure sensor according to the embodiment of the invention; 
         FIG. 5  is a configuration diagram of the pressure sensor according to the embodiment of the invention; 
         FIGS. 6A and 6B  are graphs showing an example of an output from the pressure sensor according to the embodiment of the invention; 
         FIG. 7  is a configuration diagram of the pressure sensor according to a first modification of the invention; 
         FIG. 8  is a configuration diagram of the pressure sensor according to a second modification of the invention; 
         FIGS. 9A and 9B  are a plan view and a cross-sectional view of the pressure sensor according to a third modification of the invention; and 
         FIGS. 10A and 10B  are a plan view and a cross-sectional view of the pressure sensor according to a fourth modification of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, a pressure sensor according to an embodiment of the invention will be described. 
     A pressure sensor  10  of the embodiment includes two pressure variation sensor  11  (for example, a first pressure variation sensor (P 1 )  11   a  and a second pressure variation sensor (P 2 )  11   b ) having frequency characteristics different from each other, and a detection circuit  12  configured to detect the difference between outputs from the two pressure variation sensors  11 . The pressure sensor  10  outputs a signal according to variations in pressure (for example, atmospheric pressure or the like). 
     The pressure variation sensor  11  of the pressure sensor  10  is formed of an SOI substrate obtained by thermally sticking a silicon supporting layer, an oxidized layer formed of SiO 2 , and a silicon active layer. For example, as illustrated in  FIGS. 1A and 1B , the pressure variation sensor  11  of the pressure sensor  10  includes a cavity  21 , a cantilever  22 , a gap  23 , and piezoresistances  24 . 
     The cavity  21  is formed into a bottomed cylindrical shape with an opening using, for example, the silicon supporting layer of the SOI substrate. 
     The cantilever  22  is formed into a plate shape extending in the direction from a proximal end side toward a distal end side (longitudinal direction) using the silicon active layer of the SOI substrate, includes a proximal end portion  22   a  supported at an opening end  21   a  of the cavity  21  in a cantilevered manner and a distal end portion  22   b  having a free end, and is subject to a flexural deformation in accordance with the pressure difference between the interior and the exterior of the cavity  21 . 
     The gap  23  is provided between the distal end portion  22   b  of the cantilever  22  and the opening end  21   a  of the cavity  21 , and communicates the interior and the exterior of the cavity  21 . 
     The piezoresistances  24  are formed by a doping agent (impurity) such as phosphorus doped on the proximal end portion  22   a  of the cantilever  22  by various methods such as an ion implantation method or a diffusion method, is provided so as to sandwich a through hole  22   c  penetrating the proximal end portion  22   a  of the cantilever  22  in the thickness direction from both sides in the short direction (the direction orthogonal to the longitudinal direction and the thickness direction of the cantilever  22 ), and varies the resistance value in accordance with the deformation amount of the flexural deformation of the cantilever  22  (that is, the magnitude of the stress). 
     The one and the other piezoresistances  24  provided on the both sides of the through hole  22   c  are connected to the detection circuit  12  described later, and a wiring portion  25  formed of a conductive material and provided at a position shifted toward the distal end side from the through hole  22   c  at the proximal end portion  22   a  of the cantilever  22 , and a general shape including the wiring portion  25  and the one and the other piezoresistances  24  is formed into a U-shape in plan view. 
     Accordingly, for example, when a predetermined voltage is applied to one of the piezoresistances  24 , a current caused by the voltage application run around the through hole  22   c  and flows by way of one of the piezoresistances  24  through the wiring portion  25  to the other piezoresistance  24 . This current corresponds to an output from the pressure variation sensor  11  varied in magnitude in accordance with the resistance value of the piezoresistance  24  varying in accordance with the amount of the flexural deformation of the cantilever  22 . 
     The pressure variation sensor  11  has specific frequency characteristics in accordance at least with the capacity V of the cavity  21  or the distance G of the gap  23 . 
     The frequency characteristics is a lower limit frequency having a sensitivity of the pressure variation sensor  11  equal to or higher than the predetermined value such as a cutoff frequency fc, for example, and the sensitivity has a decreasing tendency in association with the lowering of the frequency with respect to pressure variations in a frequency band lower than the lower limit frequency and the sensitivity is changed to have an increasing tendency from the predetermined value so as to be saturated to the upper limit value in association with the increase in frequency with respect to the pressure variations in the frequency band higher than the lower limit frequency. 
     An operation example of the pressure variation sensor  11  will be given below. 
     In the pressure variation sensor  11 , for example, when the pressure difference between a pressure Pout (first predetermined pressure Pa) on the exterior of the cavity  21  and a pressure Pin on the interior of the cavity  21  is zero as in a period A shown in  FIGS. 2A and 2B , the cantilever  22  is not subject to the flexural deformation and the output from the pressure variation sensor  11  (the sensor output) is zero, for example, as illustrated in  FIG. 3A . 
     In contrast, for example, as a period B from the time-of-day t 1  shown in  FIGS. 2A and 2B , when the outer pressure Pout of the cavity  21  is increased step by step (Pout←second predetermined pressure Pb&gt;Pa), the cantilever  22  starts the flexural deformation in accordance with the pressure difference between the exterior and the interior of the cavity  21 , for example, as illustrated in  FIG. 3B , and the output from the pressure variation sensor  11  is changed to the increasing tendency in association with the increase in this deformation amount. 
     Then, when a pressure transmission medium flows from the exterior to the interior of the cavity  21  via the gap  23  and the pressure Pin on the interior of the cavity  21  is increased gradually in a gentler response than the variations of the pressure Pout on the exterior thereof, the deformation amount of the cantilever  22  is changed to have a decreasing tendency in association with the decrease in the pressure difference between the exterior and the interior of the cavity  21 , and hence the output from the pressure variation sensor  11  is changed to have a decreasing tendency. 
     Then, for example, when the pressure Pin in the interior of the cavity  21  is equal to the pressure Pout on the exterior thereof as a period C from a time-of-day t 2  onward as shown in  FIGS. 2A and 2B  (Pin=Pout=Pb), the flexural deformation of the cantilever  22  is released as illustrated in  FIG. 3C , and the output from the pressure variation sensor  11  becomes zero. 
     The detection circuit  12  of the pressure sensor  10  includes a bridge circuit  31 , a reference voltage generating circuit  32 , a differential amplifying circuit  33 , and an output circuit  34  as illustrated in  FIG. 4  for example. 
     The bridge circuit  31  includes a branch portion including the piezoresistance  24  of the first pressure variation sensor (P 1 )  11   a  (first piezoresistance  24   a : resistance value RP 1 ) and the piezoresistance  24  (second piezoresistance  24   b : resistance value RP 2 ) of the second pressure variation sensor (P 2 )  11   b  connected in series and a branch portion including a fixed resistance  41  (resistance value R 1 ) and a fixed resistance  42  (resistance value R 2 ) connected in series, and these branches are connected in parallel to the reference voltage generating circuit  32 . 
     In the bridge circuit  31 , a connecting point between the first piezoresistance  24   a  and the second piezoresistance  24   b  is connected to an inverting input terminal of the differential amplifying circuit  33 , and a connecting point between the fixed resistances  41  and  42  is connected to a non-inverting input terminal of the differential amplifying circuit  33 . 
     The reference voltage generating circuit  32  applies a predetermined reference voltage Vcc to the bridge circuit  31 . 
     The differential amplifying circuit  33  detects a potential difference between connecting point between the fixed resistances  41  and  42  of the bridge circuit  31  and a connecting point between the first piezoresistance  24   a  and the second piezoresistance  24   b , and the potential difference is amplified in a predetermined gain before outputting therefrom. 
     The potential difference corresponds to the difference between the resistance value RP 1  of the first piezoresistance  24   a  and the resistance value RP 2  of the second piezoresistance  24   b  (RP 1 −RP 2 ), that is, a value in accordance with the difference between the output from the first pressure variation sensor ( 21 )  11   a  and the output from the second pressure variation sensor (P 2 )  11   b.    
     As illustrated in  FIG. 5 , the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  have the same distance G of the gap  23 , and have frequency characteristics different from each other, that is, cutoff frequencies fc 1  and fc 2  (&gt;fc 1 ) different from each other by setting the value of a capacity V 1  of the cavity  21  of the first pressure variation sensor (P 1 )  11   a  to be larger than the value of a capacity V 2  of the cavity  21  of the second pressure variation sensor (P 2 )  11   b.    
     Accordingly, as shown in  FIGS. 6A and 6B , the first pressure variation sensor (P 1 )  11   a  demonstrates a sensitivity equal to or higher than the predetermined value in the frequency band equal to or higher than the cutoff frequency fc 1  and the second pressure variation sensor (P 2 )  11   b  demonstrates a sensitivity equal to or higher than the predetermined value in the frequency band equal to or higher than the cutoff frequency fc 2  than that of the cutoff frequency fc 1 , so that the difference between the output from the first pressure variation sensor (P 1 )  11   a  and the output from the second pressure variation sensor (P 2 )  11   b  compensates the output in the frequency band other than the frequency band (fc 2 −fc 1 ) between the different cutoff frequencies fc 1  and f 2 . 
     Therefore, the pressure sensor  10  acts so as to have the sensitivity only for the pressure variations in so-called a desired frequency band (fc 2 −fc 1 ). 
     The output circuit  34  includes, for example, a low-pass filter, and performs a predetermined filtering process on a signal output from the differential amplifying circuit  33 , and outputs the signal after the process. 
     As described above, the pressure sensor  10  of the embodiment is capable of detecting only the pressure variations in the desired frequency band corresponding to the difference between the different lower limit frequencies by detecting the difference between the outputs from the two pressure variation sensors  11  (the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b ) having the lower limit frequencies which provides the sensitivity of the pressure variation sensor  11  equal to or higher than the predetermined value such as the cutoff frequency. 
     Accordingly, increase in noise (sound) with respect to the pressure variations in the desired frequency band due to the pressure variations in other frequency bands other than the desired frequency band is prevented, and saturation of the signal in the amplifying circuit of the first step is prevented. 
     In addition, the detection error due to the temperature characteristic or vibrations due to the disturbance generated in the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  may be compensated by the difference in output from the two pressure variation sensors  11 , so that the detection accuracy of the pressure variations may be improved. 
     In the embodiment described above, as in a first modification illustrated in  FIG. 7 , for example, the capacities V of the cavities  21  of the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  may have the frequency characteristics different from each other and the cutoff frequencies fc 1  and f 2  (&gt;fc 1 ) different from each other by setting the capacities V of the cavities  21  to be the same and setting a distance G 1  of the gap  23  of the first pressure variation sensor (P 1 )  11   a  to be smaller than a distance G 2  of the gap  23  of the second pressure variation sensor (P 2 )  11   b.    
     In the embodiment described above, as a second modification illustrated in  FIG. 8 , for example, the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  may have frequency characteristics different from each other, for example, the cutoff frequencies fc 1  and f 2  (&gt;fc 1 ) different from each other by setting the distance G 1  of the gap  23  of the first pressure variation sensor (P 1 )  11   a  to be smaller than the distance G 2  of the gap  23  of the second pressure variation sensor (P 2 )  11   b , and setting the capacity V 1  of the cavity  21  of the first pressure variation sensor (P 1 )  11   a  to be larger than the capacity V 2  of the cavity  21  of the second pressure variation sensor (P 2 )  11   b.    
     In the embodiments described above, the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  may be arranged adjacent to each other with the proximal end portion  22   a  of the cantilever  22  of the first pressure variation sensor (P 1 )  11   a  and the distal end portion  22   b  of the cantilever  22  of the second pressure variation sensor (P 2 )  11   b  faced each other in the direction of extension of the cantilevers  22  (longitudinal direction) for example, as a third modification illustrated in  FIGS. 9A and 9B . 
     According to the third modification, the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  are arranged so that the cantilevers  22  extend in the same direction from the proximal ends to the distal ends of the respective cantilevers  22  in the direction of extension of the cantilevers  22 , whereby the cantilevers  22  are subject to the action of the vibrations caused by the disturbance such as wind or light evenly. Therefore, the vibrations caused by the disturbance generated in the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  respectively may be compensated adequately by the difference in outputs from the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b.    
     In the embodiment described above, as in a fourth modification shown in  FIGS. 10A and 10B , the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  may be arranged adjacent to each other with the distal end portions  22   b  of the cantilevers  22  thereof faced to each other in the direction of extension (longitudinal direction) of the cantilevers  22 . 
     According to the fourth modification, the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  are arranged so that the cantilevers  22  extend in the opposite direction from the proximal ends to the distal ends of the respective cantilevers  22  in the direction of extension of the cantilevers  22 , whereby occurrence of phase difference in sensitivities of the cantilevers  22  due to the vibrations in a high frequency band such as sound may be inhibited. Therefore, the vibrations in the high frequency band such as sound generated in the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b  respectively may be compensated adequately by the difference in outputs from the first pressure variation sensor (P 1 )  11   a  and the second pressure variation sensor (P 2 )  11   b.    
     In the embodiment described above, the pressure sensor  10  includes the two pressure variation sensors  11  having frequency characteristics different from each other. However, the invention is not limited thereto, and a configuration in which at least a plurality of pressure variation sensors  11  are provided and the difference of the outputs from adequate two of the pressure variation sensors  11  therefrom may be detected. 
     In the embodiments described above, the pressure variation sensors  11  each have the specific frequency characteristics in accordance with the capacity V of the cavity  21  or the distance G of the gap  23 . However, the invention is not limited thereto, and may have the specific frequency characteristics in accordance with the other parameters, such as the shape of the cavity  21  and the shape and the position of the gap  23 , for example.