Abstract:
Provided is a pressure measuring device that can stably bond a strain detection element even to a diaphragm made of metal having a large coefficient of thermal expansion. In order to achieve the above object, the pressure measuring device of the present invention includes: a metal housing including a pressure introduction unit and a diaphragm deformed by a pressure introduced via the pressure introduction unit; and a strain detection element for detecting strain generated in the diaphragm, wherein a base made of a first brittle material is provided on the metal housing, and the strain detection element is bonded to the base via a second brittle material having a melting point lower than a melting point of the base.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a pressure measuring device attached to various measurement target apparatuses to detect pressures thereof. 
       BACKGROUND ART 
       [0002]    A pressure measuring device is formed as, for example, a high-pressure sensor mounted on a vehicle and is used to measure a fuel pressure of an engine, a brake hydraulic pressure, various kinds of gas pressures, and the like. 
         [0003]    For example, PTLs 1 and 2 propose conventional pressure measuring devices. 
         [0004]    PTL 1 discloses “a pressure detector formed by bonding plate glass a diaphragm surface of pressure-receiving metal diaphragm visa low-melting glass layer, placing a strain gauge semiconductor chip on the plate glass, and performing anodic bonding on the plate glass and the strain gauge semiconductor chip”. 
         [0005]    PTL 2 discloses “a pressure sensor in which a sensor element is bonded to a diaphragm by a bonding member having a first bonding surface bonded to the sensor element and a second bonding surface bonded to the diaphragm formed in a metal stem, wherein the bonding member is formed so that a coefficient of thermal expansion of the first bonding surface is closer to a coefficient of thermal expansion of the sensor element than to a coefficient of thermal expansion of the metal stem, a coefficient of thermal expansion of the second bonding surface is closer to the coefficient of thermal expansion of the metal stem than to the coefficient of thermal expansion of the sensor element, and the coefficient of thermal expansion is continuously changed from the first bonding surface the second bonding surface”. 
       CITATION LIST 
     Patent Literatures 
       [0006]    PTL 1: JP-A-62-291533 
         [0007]    PTL 2: JP-A-2013-36935 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In the conventional pressure measuring devices including the device in PTL 1, a strain detection element made of silicon is bonded to a diaphragm via low-melting glass, and, in order to prevent breakage of the strain detection element and a bonding layer due to stress generated in a cooling step in bonding, the diaphragm is made of an Fe—Ni—Co-based alloy having a coefficient of thermal expansion close to coefficients of thermal expansion of silicon and glass. However, the Fe—Ni—Co-based alloy has comparatively low proof stress and therefore is not suitably used at a high pressure and is corroded in a high temperature high environment. 
         [0009]    Thus, it is considered that the diaphragm is made of stainless steel having high proof stress and high corrosion resistance. However, coefficients of thermal expansion of stainless steel and the strain detection element are largely different, and therefore large stress may be generated in the bonding layer in the cooling step in bonding. This may result in breakage of the strain detection element and the bonding layer. 
         [0010]    Although PTL 2 discloses the bonding member formed by mixing a plurality of bonding materials so that a coefficient of thermal expansion thereof is continuously changed, it is difficult to control the coefficient of thermal expansion of the bonding member because, in general, the coefficient of thermal expansion thereof is abruptly changed around a melting point. Further, in the case where mixing is not sufficiently controlled, a bonding state may vary because the coefficient of thermal expansion is ununiform when mixing is ununiform. Thus, stability of bonding is problematic. 
         [0011]    In the inventions disclosed in PTLs 1 and 2, connection reliability obtained when the strain detection element is connected on the diaphragm having a large coefficient of thermal expansion can still be improved. 
         [0012]    The present invention has been made to solve the above problems, and an object thereof is to provide a pressure measuring device having high bonding reliability between a strain detection element and a diaphragm made of a metal material whose coefficient of thermal expansion is larger than coefficients of thermal expansion of silicon and glass. 
       Solution to Problem 
       [0013]    In order to solve the problem, a pressure detection device according to the present invention,: includes: a metal housing including a pressure introduction unit and a diaphragm deformed by a pressure introduced via the pressure introduction unit; and a strain detection element for detecting strain generated in the diaphragm, wherein a base made of a first brittle material is provided on the metal housing, and the strain detection element is bonded to the base via a second brittle material having a melting point lower than a melting point of the base. 
       Advantageous Effects of Invention 
       [0014]    According to the present invention, it is possible to provide a pressure detection element having high bonding reliability between a strain detection element and a diaphragm made of a metal material whose coefficient of thermal expansion is larger than coefficients of thermal expansion of silicon and glass 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a schematic cross-sectional view of the whole pressure measuring device in Example 1 of the present invention. 
           [0016]      FIG. 2  is a circuit diagram of the pressure measuring device in Example 1 of the present invention. 
           [0017]      FIG. 3  is an enlarged cross-sectional view of a bonding portion in Example 1 of the present invention. 
           [0018]      FIG. 4  is an enlarged cross-sectional view of a bonding portion in Example 2 of the present invention. 
           [0019]      FIG. 5  is an enlarged cross-sectional view of a bonding portion in Example 3 of the present in 
           [0020]      FIG. 6  is a schematic top view of a bonding portion in Example 4 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Example 1 of the present invention is hereinafter described with reference to  FIG. 1  to  FIG. 3 . 
         [0022]    A pressure measuring device  100  in Example 1 of the present invention is hereinafter described with reference to  FIG. 1 . 
         [0023]    The pressure measuring device  100  includes a metal housing  10  in which a pressure port  12 , a diaphragm  14 , and a flange  13  are formed, a strain detection element  15  for measuring a pressure introduced into a pressure port  11 , a substrate  16  electrically connected to the strain detection element  15 , a cover  18 , and a connector  19  electrically connected to an external device. 
         [0024]    The pressure port  11  includes a hollow tubular pressure introduction unit  12   ha  in which a pressure introduction port  12   a  is formed on one-end side (lower side) in an axial direction and the cylindrical flange  13  formed on the other end side (upper side) in the axial direction of the pressure introduction unit  12   ha.  The diaphragm  14  deformed by a pressure to cause strain is provided in a central portion of the flange  13 . 
         [0025]    The diaphragm  14  has a pressure-receiving surface for receiving a pressure introduced via the pressure introduction port  12   a  and a sensor mounting surface that is a surface opposite to the pressure-receiving surface. 
         [0026]    A tip end portion  12   hat  facing the strain detection element  15  on the diaphragm  14  side in the pressure introduction unit  12   ha  of the pressure port  11  has a rectangular shape and is continuously provided to perforate the central portion of the flange  13  toward a portion having a height slightly lower than that of an upper surface of the diaphragm  14 . The rectangular shape of this tip end portion  12   hat  causes a strain difference in an x direction and a −y direction in the diaphragm  14 . 
         [0027]    The strain detection element  15  is bonded to a substantially central portion of the sensor mounting surface of the diaphragm  14 . The strain detection element  15  is formed on a silicon chip as a semiconductor chip including one or more strain resistance bridges  30   a  to  30   c  that output electrical signals according to deformation (strain) of the diaphragm  14 . 
         [0028]    On the substrate  16 , an amplifier for amplifying each detection signal output from the strain detection element  15 , an A-D converter for converting an analog output signal of the amplifier to a digital signal, a digital signal calculation processing circuit for performing correction calculation described below on the basis of the digital signal, a memory in which various kinds of data are stored, a capacitor  17 , and the like are mounted. 
         [0029]    A predetermined diameter range around the center of a blocking plate  18   a  for blocking the other end in axial direction of the cover  18  is cut out, and the connector  19  that is made of, for example, resin and is for outputting a detection pressure value detected in the pressure measuring device  100  to an external device is inserted into the cut-out portion. 
         [0030]    One end of the connector  19  is fixed to the cover  18  in the cover  18 , and the other end of the connector  19  is exposed to the outside from the cover  18 . 
         [0031]    Rod-like terminals  20  inserted by, for example, insert molding are provided inside the connector  19 . Those terminals  20  are, for example, three terminals for supplying power, for grounding, and for outputting signals and are electrically connected to an ECU or the like of an automobile via wiring members by connecting one end of each terminal  20  to the substrate  16  and connecting the other end thereof to an external connector (not illustrated). 
         [0032]    A circuit configuration of the plurality of strain resistance bridges of the strain detection element  15  and the circuit members mounted on the substrate  16  will be described with reference to  FIG. 2 . The strain resistance bridges  30   a  to  30   c  are formed by bridge-connecting resistance gauges whose resistance values are changed when the respective strain resistance bridges are strained in accordance with deformation of the diaphragm  14 . 
         [0033]    Output signals (bridge signals corresponding to pressure) of the strain resistance bridges  30   a  to  30   c  are amplified by amplifiers  31   a  to  31   c,  and the amplified output signals are converted into digital signals by A-D (analog-digital) converters  32   a  to  32   c.    
         [0034]    A digital signal calculation processing circuit  33  performs calculation processing for correcting, for example, a pressure value detected by the single strain resistance bridge  30   a  with the use of detection pressure values of the other strain resistance bridges  30   b  and  30   c  on the basis of output signals of the A-D converters  32   a  to  32   c  and outputs the corrected pressure value as a detection value of the pressure measuring device. 
         [0035]    This digital signal calculation processing circuit performs not only the correction calculation processing but also, for example, processing for comparing the detection pressure values of the plurality of strain resistance bridges or comparing the detection pressure values of the strain resistance bridges with prescribed pressure values stored in advance in a nonvolatile memory  34  to thereby determine deterioration of a measurement target apparatus and deterioration of the strain detection element  16  and output a failure signal at the time of such determination. 
         [0036]    Note that supply of power to the strain resistance bridges  30   a  to  30   c  from a voltage source  35  and output of each signal from the digital signal calculation processing circuit  33  are performed via the terminals  21  illustrated in  FIG. 1  and  FIG. 2 . 
         [0037]    The nonvolatile memory  34  may be mounted on a circuit chip different from a chip on which the other circuit members are mounted Further, the correction calculation may be performed by an analog circuit instead of the digital signal calculation processing circuit  33 . 
         [0038]    A bonding portion of the strain detection element  15  and the diaphragm  14  in Example 1 will be described with reference to  FIG. 3 . 
         [0039]    A material of the diaphragm  14  is required to have corrosion resistance and high proof stress so as to withstand a high voltage. Therefore, a material containing chrome and having corrosion resistance is subjected to precipitation hardening so as to have high proof stress, and the resultant material is used. Specifically, SUS630 is employed. 
         [0040]    A brittle material base  21  is formed by mounting a brittle material that is broken in an elastic region, such as glass, ceramics, or concrete, on the sensor mounting surface of the diaphragm  14 . The brittle material base  21  is formed on the diaphragm  14  by applying glass paste having a melting point equal to or higher than 800° C. to the diaphragm  14  and then burning the glass paste at a temperature equal to or higher than the melting point of the glass paste. The glass paste is crystallized glass and has a coefficient of thermal expansion smaller than 11 ppm. 
         [0041]    The strain detection element  15  is bonded to the brittle material base  21  via a low-melting brittle material  22  having a melting point lower than that of the brittle material from which the brittle material base  21  is made. As the low-melting brittle material  22 , glass paste containing, as a main component, glass containing vanadium whose melting point is equal to or lower than 400° C. is employed. Because the melting point of the low-melting brittle material.  22  is lower than the melting point of the brittle material base  21 , a physical property of the brittle material base  21  is not changed before and after bonding. Further, in the case where a bonding temperature is equal to or lower than 400° C., it is possible to suppress deterioration of the strain detection element  15  at the time of bonding. 
         [0042]    In Example 1 of the present invention, an object bonded to the strain detection element  15  is not the diaphragm  14  but is the brittle material base  21  in which a difference between coefficients of thermal expansion of the brittle material base  21  and the strain detection element  15  is smaller than a difference between coefficients of thermal expansion of the brittle material base  21  and the diaphragm  14 . This makes it possible to reduce a difference between coefficients of thermal expansion of the strain detection element  15  and the object bonded thereto, and therefore it is possible to reduce stress generated in a cooling step in bonding. Further, because the brittle material base  21  and the strain detection element  15  are bonded by the low-melting brittle material  22 , it is possible to reduce a heating temperature when the strain detection element  15  is bonded. This makes it possible to reduce internal stress generated in the strain detection element  15 . In particular, when the low-melting brittle material  22  is made of a material having a melting point equal to or lower than 400° C. and the bonding temperature of the strain detection element  15  is equal to or lower than 400° C., it is possible to suppress deterioration of a terminal and wiring of the strain detection element  15 . This is particularly effective. In addition, the low-melting brittle material  22  is deformed in the elastic region also in a high-temperature environment in which the pressure measuring device  100  is mounted and therefore is not plastically deformed in the high-temperature environment, unlike a ductile material. This makes it possible to suppress deterioration of detection accuracy caused by plastic deformation of the bonding portion. Thus, it is possible to provide an accurate pressure detection device according to Example 1 of the present invention. 
         [0043]    Example 2 of the present invention will be described with reference to  FIG. 4 . Note that, regarding a configuration similar to the configuration in Example 1, the description thereof is omitted 
         [0044]    In the pressure measuring device  100  in Example 2, the strain detection element  15  and a glass plate  23  are subjected to anodic bonding, and the glass plate  23  and the brittle material base  21  are bonded via the low-melting brittle material  22 . When the glass plate  23  is made of a material that keeps a solid shape even at the melting point of the low-melting brittle material  22 , a thickness thereof can be easily managed, as compared with the case of a paste material. Therefore, according to Example 2 of the present invention, sensitivity of the strain detection element  15  can be easily controlled to have a desired value by designing the thickness of the glass plate  23  having an inverse correlation with the sensitivity of the strain detection element  15 . 
         [0045]    Example 3 of the present invention will be described with reference to  FIG. 5 . Note that, regarding a configuration similar to the configuration in Example 1, the description thereof is omitted. 
         [0046]    In the pressure measuring device  100  in Example 3, a base  24  and a top base  25  are formed by stacking a plurality of brittle materials having melting points equal to or higher than the melting point of the low-melting brittle material  22 , and at least one brittle material of the stacked brittle materials is a brittle material having a solid shape even at the melting point of the low-melting brittle material  22 . Each of the stacked brittle materials has a coefficient of thermal expansion smaller than 11 ppm. As illustrated in  FIG. 5 , a base including two layers of brittle materials can be formed by applying glass paste having a melting point equal to or higher than that of the low-melting brittle material  22  to the diaphragm  14  as the base  24 , mounting a glass plate on the base  24  as the top base  25 , and burning the glass paste and the glass plate. According to Example 3 of the present invention, because solid brittle materials are used, the thickness of the bonding layer is easily managed, and the sensitivity of the strain detection element  15  can be set to have a desired value. 
         [0047]    Example 4 of the present invention will be described with reference to  FIG. 5 . Note that, regarding a configuration similar to the configuration in Example 1, the description thereof is omitted. 
         [0048]    In Example 4, as illustrated in  FIG. 6 , the brittle material base  21  and the low-melting brittle material  22  are formed to have a circular shape. When the brittle material base  21  has a circular shape, it is possible to reduce stress concentration upon corner portions of the strain detection element  15 . Similarly, because the low-melting brittle material  22  has a circular shape, it is possible to reduce the stress concentration upon the corner portions of the strain detection element  15 . Therefore, according to Example 4 of the present invention, it is possible to suppress breakage of members caused by stress generated in the cooling step in bonding, i.e., breakage of the strain detection element  15 , the brittle material base  21 , and the low-melting brittle material  22 .  0   
         [0049]    Note that, as in the case of a circular shape, the stress concentration upon the corner portions of the strain detection element  15  generated in the cooling step in bonding can also be reduced when the brittle material base  21  has an octagonal or higher-order polygonal shape. This makes it possible to suppress breakage of members in the cooling step. Similarly, when the low-melting brittle material  22  has an octagonal or higher-order polygonal shape, it is also possible to obtain an effect similar to the effect obtained when the low-melting brittle material  22  has a circular shape. 
         [0050]    It is also possible to obtain the similar effect by combining Example 4 with Example 2 or Example 3. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  . . . metal housing 
           11  . . . pressure port 
           12  . . . pressure introduction unit 
           12   a  . . . pressure introduction port 
           12   ha  . . . pressure introduction hole 
           12   hat  . . . tip end portion 
           13  . . . flange 
           14  . . . diaphragm 
           15  . . . strain detection element 
           16  . . . substrate 
           17  . . . capacitor 
           18  . . . cover 
           18   a  . . . blocking plate 
           19  . . . connector 
           20  . . . terminal 
           21  . . . brittle material base 
           22  . . . low-melting brittle material 
           23  . . . glass plate 
           24  . . . base 
           25  . . . top base 
           30   a  to  30   c  . . strain resistance bridge 
           31   a  to  31   c  . . . amplifier 
           32   a  to  32   c  . . . A-D converter 
           33  . . . digital signal calculation processing circuit 
           34  . . . nonvolatile memory 
           35  . . . voltage source 
           100  . . . pressure measuring device