Abstract:
An infrared gas sensor includes: an infrared light source having a resistor for emitting an infrared light by heating the resistor; an infrared light sensor having a detection device for generating an electric signal in accordance with a temperature change of the detection device corresponding to the infrared light in a case where the sensor receives the infrared light; a reflection member for reflecting the infrared light emitted from the light source to introduce the infrared light to the sensor; a casing for accommodating the light source, the light sensor, and the reflection member; and a substrate. The reflection member faces the light source. The resistor and the detection device are disposed on the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on Japanese Patent Application No. 2004-17427 filed on Jan. 26, 2004, the disclosure of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to an infrared gas sensor.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventionally, for example, there is known the infrared gas sensor as disclosed in Japanese Patent Application Publication No. H9-184803. This infrared gas sensor comprises an infrared source, an infrared sensor to detect infrared light, and a reflection member disposed opposite to the infrared source to apply the reflected infrared light to the infrared sensor, all contained in the same case.  
         [0004]     The infrared gas sensor (hereafter referred to as the gas sensor) provides a light source (infrared source) opposite to a concave reflecting mirror (reflection member). A light receiver (infrared sensor) is provided at or near a position to converge a flux of reflected infrared light radiated from the light source. Gas containing gas under test is filled in spaces between the light source, the light receiver, and the concave reflecting mirror to measure ratios of absorbing the infrared light by means of the gas.  
         [0005]     However, the gas sensor in Japanese Patent Application Publication No. H9-1874803 is provided with the light source and the light receiver separately (on different chips). It is difficult to miniaturize the gas sensor size.  
         [0006]     In such gas sensor, increasing the amount of infrared light energy applied to the infrared sensor also increases changes in output from the infrared sensor. Thus, the gas sensor sensitivity improves. However, the gas sensor needs to position the light source and the light receiver with reference to the concave reflecting mirror. The installation positions are easily subject to errors. Accordingly, variations in the installation positions change the infrared light energy amount to be applied to the light receiver. The sensor sensitivity may vary.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the above-described problem, it is an object of the present invention to provide an infrared gas sensor having a small size and stable sensitivity.  
         [0008]     An infrared gas sensor includes: an infrared light source having a resistor for emitting an infrared light by heating the resistor; an infrared light sensor having a detection device for generating an electric signal in accordance with a temperature change of the detection device corresponding to the infrared light in a case where the sensor receives the infrared light; a reflection member for reflecting the infrared light emitted from the light source to introduce the infrared light to the sensor; a casing for accommodating the light source, the light sensor, and the reflection member; and a substrate. The reflection member faces the light source. The resistor and the detection device are disposed on the substrate.  
         [0009]     In the above sensor, the resistor and the detection device are disposed on the same substrate, i.e., they are integrated on the same substrate. Accordingly, the arrangement of the resistor, i.e., the light source and the detection device, i.e., the light sensor can be compact. Thus, the dimensions of the gas sensor become smaller.  
         [0010]     Further, since the resistor and the detection device are disposed on the same substrate so that their positioning relationship is predetermined, the positioning accuracy between the light source and the light sensor can be improved, compared with a sensor having the light source and the sensor chip individually disposed on different substrates. Thus, the deviation of the sensor sensitivity is reduced.  
         [0011]     Preferably, the reflection member is a concave mirror. In this case, amount of the infrared light reaching the light sensor, i.e., a coefficient of a received infrared light becomes larger with using the concave mirror so that the sensor sensitivity is increased. Further, the deviation of the sensor sensitivity is improved.  
         [0012]     Preferably, the substrate includes a plurality of membranes as a thin portion of the substrate. The resistor and the detection device are disposed on different membranes, respectively. In this case, the resistor and the detection device are thermally isolated from the substrate. Therefore, the infrared light source can emit the infrared light effectively, and further, the infrared light sensor has a large sensor output.  
         [0013]     Preferably, the detection device is a thermocouple including a measurement junction and a reference junction. The measurement junction is disposed on one membrane, and the reference junction is disposed on the substrate except for the membrane.  
         [0014]     Preferably, the detection device has a part made of the same material as the resistor. Further, the detection device has a part, which is disposed on the same plane as the resistor. In this case, the manufacturing process can be simplified. Specifically, when the detection device and the resistor are formed of the same material to be disposed on the same plane, both the resistor and the detection device are formed in the same process at the same time so that the manufacturing process is simplified. Thus, the manufacturing cost of the sensor is reduced.  
         [0015]     Preferably, the substrate is a semiconductor substrate, and the resistor and the detection device are disposed on the semiconductor substrate through an insulation film. In this case, the resistor and the detection device are formed with high positioning accuracy by a conventional semiconductor process method. Thus, the gas sensor with high sensor sensitivity can be formed with low cost.  
         [0016]     Preferably, the sensor further includes a circuit chip. The substrate having the resistor and the detection device is mounted on the circuit chip so that the circuit chip with the substrate is disposed inside the casing. Specifically, when the resistor and the detection device are formed on the same substrate, the arrange areas of the infrared light source and the infrared light sensor becomes smaller. Therefore, the circuit chip for operating the infrared light source and the infrared light sensor can be accommodated in a space of the casing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
         [0018]      FIG. 1  is a schematic view showing a gas sensor according to a preferred embodiment of the present invention;  
         [0019]      FIG. 2A  is a plan view showing a sensor chip, and  FIG. 2B  is a cross sectional view showing the sensor chip taken along line IIB-IIB in  FIG. 2A , according to the preferred embodiment;  
         [0020]      FIG. 3  is a cross sectional view showing a sensor chip of a gas sensor according to a modification of the preferred embodiment; and  
         [0021]      FIG. 4  is a schematic view showing a gas sensor according to another modification of the preferred embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Embodiments of the present invention will be described in further detail with reference to the accompanying drawings. The present invention is applied to infrared gas sensors having a so-called reflective structure. In such infrared gas sensor, an infrared source radiates infrared light. A reflection member is disposed opposite to the infrared source and reflects the infrared light. An infrared sensor detects the reflected light.  
         [0023]      FIG. 1  schematically shows the configuration of an infrared gas sensor (hereafter referred to as a gas sensor) according to a preferred embodiment of the present invention.  
         [0024]     As shown in  FIG. 1 , a gas sensor  100  has a reflection member to reflect infrared light and comprises a case  10 , a cap  20 , and a sensor chip  30 . The case  10  is provided so that gas under test can enter. The cap  20  is disposed in the case  10  and limits the infrared light. The sensor chip  30  is disposed in the case  10 . The sensor chip  30  is configured to be an integration of an infrared source to radiate infrared light and an infrared sensor to detect infrared light.  
         [0025]     The case  10  comprises a pedestal  11  as a base and a cylindrical container  12  attached to the pedestal  11 .  
         [0026]     The container  12  has a plurality of gas entry/exits  12   a  (two in  FIG. 1 ) on the side. The gas entry/exit  12   a  enables gas containing the gas under test to flow into the case  10 . The case  10  contains a concave mirror  12   b  on the inside top surface opposite to the pedestal  11 . The concave mirror  12   b  functions as a reflection member to reflect infrared light. The concave mirror  12   b  is shaped to have a specified radius. This aims at reflecting infrared light radiated from the infrared source of the sensor chip  30  and applying the infrared light to the infrared sensor of the sensor chip  30 . The infrared source and the infrared sensor will be described later.  
         [0027]     The cap  20  limits directions of infrared light radiated from the infrared source. In addition, the cap  20  limits an incident region on the sensor chip  30  for the infrared light reflected by the concave mirror  12   b . The cap  20  is configured to shield infrared light except a radiation window  21  and an incident window  22 . The radiation window  21  is positioned correspondingly to the infrared source. The incident window  22  is positioned correspondingly to the infrared sensor. The radiation window  21  is provided with an infrared light transmission filter  21   a . The incident window  22  is provided with a band-pass filter  22   a  to selectively transmit the infrared light having a specific wavelength only. The cap  20  has a partition wall  23  extending form the top toward the surface of the sensor chip  30 . When the infrared source isotropically radiates the infrared light, the partition wall  23  prevents the radiated infrared light from directly entering the infrared sensor inside the cap  20 .  
         [0028]     The sensor chip  30  is fixed on the pedestal  11  in the case  10  and has a light source section  31  and a light receiving section  32  on a single chip. The light source section  31  works as an infrared source that radiates infrared light. The light receiving section  32  works as an infrared sensor to receive the infrared light that is radiated from the light source section  31  and is reflected on the concave mirror  12   b . That is, the light source section  31  and the light receiving section  32  are integrated on the sensor chip  30  as a single chip. This makes it possible to reduce the space for mounting the light source section  31  and the light receiving section  32  in the case  10 . The size of the gas sensor  100  can be minimized.  
         [0029]     As mentioned above, the light source section  31  and the light receiving section  32  are integrated on the sensor chip  30  as a single chip. This predetermines positional relationship between the light source section  31  and the light receiving section  32 . Accordingly, the light source section  31  and the light receiving section  32  can be disposed on the pedestal  11  of the case  10  just by positioning the sensor chip  30  against the concave mirror  12   b . This improves the accuracy of positioning the light source section  31  and the light receiving section  32  against the concave mirror  12   b . That is, this decreases variations of the infrared light energy applied to the light receiving section  32 . Consequently, it is possible to decrease variations of the sensor sensitivity for each gas sensor  100 .  
         [0030]     In particular, as a reflection member, the concave mirror  12   b  having a specified radius may be used to increase the infrared light energy amount (i.e., the infrared light receiving efficiency) applied to the light receiving section  32 . The positional accuracy for the light source section  31  and the light receiving section  32  greatly affects variations of the sensor sensitivity. According to the construction presented in this embodiment, the use of the concave mirror  12   b  can increase the infrared light receiving efficiency (i.e., the sensor sensitivity) and decrease variations of the sensor sensitivity. The sensor chip  30  will be described later in more detail.  
         [0031]     The sensor chip  30  is electrically connected to a terminal  34  via a bonding wire  33 . The terminal  34  works as a fixed external output terminal that pierces through the pedestal  11 .  
         [0032]     In this manner, the gas sensor  100  according to the embodiment is provided with the concave mirror  12   b  on the top inside surface of the case  10 . The sensor chip  30  is provided with the light source section  31  and the light receiving section  32 . The sensor chip  30  is disposed on the pedestal  11  for the case  10  with high positional precision against the concave mirror  12   b . The infrared light is radiated from the light source section  31 , passes through the infrared light transmission filter  21   a  attached to the radiation window  21 , and is reflected on the concave mirror  12   b . The band-pass filter  22   a  is attached to the incident window  22  of the cap  20  and transmits only the infrared light having a specified wavelength out of the reflected light. The transmitted infrared light efficiently reaches the light receiving section  32 .  
         [0033]     The infrared light goes back and forth in the gas under test that flows into the case  10  (except the inside of the cap  20 ) through the gas entry/exit  12   a . Meantime, the infrared light having the specified wavelength is absorbed and the remaining infrared light reaches the light receiving section  32 . At this time, the density of the gas under test changes the intensity of the infrared light that reaches the light receiving section  32 . An output from the light receiving section  32  changes accordingly to measure the gas undertest. Since this reflective construction extends the optical path length of the infrared light, the sensor sensitivity can be improved.  
         [0034]     The construction of the sensor chip  30  will be described with reference to  FIGS. 2A and 2B .  FIGS. 2A and 2B  show enlarged details of the sensor chip  30  in  FIG. 1 .  FIG. 2A  is a plan view.  FIG. 2B  is a cross sectional view taken along line IIB-IIB of  FIG. 2A . For convenience,  FIG. 2A  shows a resistor  60 , a wiring section to connect the resistor  60  with an electrode, a detection element  70 , and a wiring section to connect the detection element  70  with the electrode. In  FIG. 2A , two rectangular regions enclosed in broken lines indicate regions where cavities  41   a ,  41   b  are formed on a top surface of the substrate  40 . A rectangular region enclosed in a dot-dash line indicates a region where an infrared light absorbing layer  80  is formed.  
         [0035]     As shown in  FIG. 2B , the sensor chip  30  comprises a substrate  40 , a membrane  50 , a resistor  60 , a detection element  70 , and an infrared light absorbing layer  80 . A plurality of membranes  50  are provided as thin portions on the substrate  40 . The resistor  60  is electrified to generate heat. The detection element  70  detects infrared light. According to the embodiment, the substrate  40  is provided with a membrane  50   a  and a membrane  50   b  as the membranes  50 . The membrane  50   a  includes the resistor  60 . The membrane  50   b  includes the detection element  70  and the infrared light absorbing layer  80 .  
         [0036]     The substrate  40  is a silicon semiconductor substrate. The substrate  40  has cavities  41   a  and  41   b  corresponding to regions for forming the membranes  50   a  and  50   b , respectively. According to the embodiment, the cavities  41   a  and  41   b  are opened with rectangular regions. The opening areas are gradually reduced toward the top of the substrate  40 . On the top surface of the substrate  40 , the rectangular regions are formed as indicated by the broken lines in  FIG. 2A . The membrane  50   a  includes the resistor  60 . The membrane  50   b  includes the detection element  70 . The membranes  50   a  and  50   b  are formed so as to float above the substrate  40 . The membranes are thinner than the other parts on the sensor chip  30 . In this manner, the resistor  60  is heat-separated from the substrate  40 . When the resistor  60  is electrified to generate heat, the light source section  31  can efficiently radiate infrared light. The rectangular regions  41   a  and  41   b  indicated by the broken lines in  FIG. 2A  correspond to regions to form the membranes  50   a  and  50   b  in the light source section  31  and the light receiving section  32 , respectively.  
         [0037]     A silicon nitride layer  42  is provided under the substrate  40 . An insulating layer  43  (e.g., silicon nitride layer) is provided on the substrate  40 . A silicon oxide layer  44  is provided on the insulating layer  43 .  
         [0038]     A polysilicon layer  45  is provided on the silicon oxide layer  44 . The polysilicon layer  45  comprises a polysilicon layer  45   a  for the light source section and a polysilicon layer  45   b  for the light receiving section. The polysilicon layer  45   a  is provided in the region for forming the membrane  50   a . The polysilicon layer  45   b  is provided from the membrane  50   b  to a specified range of a thick portion of the substrate  40  outside the membrane  50   b . The polysilicon layers  45   a  and  45   b  are patterned to specified shapes. of the polysilicon layer  45 , the polysilicon layer  45   a  for the light source section is the resistor  60  constituting the light source section  31 . The polysilicon layer  45   b  for the light receiving section is part of the detection element  70  constituting the receiving section  32 . Since the resistor  60  and at least part of the detection element  70  are formed of the same material on the same plane, they can be simultaneously formed in the same process.  
         [0039]     The polysilicon layer  45  connects with an aluminum wiring section  47  via an interlayer insulating layer  46  made of BPSG (Boron-doped Phospho-Silicate Glass). The wiring section  47  also comprises a wiring section  47   a  for the light source section and a wiring section  47   b  for the light receiving section. The wiring section  47   a  is connected to the polysilicon layer  45   a  for the light source section. The wiring section  47   b  is connected to the polysilicon layer  45   b  for the light receiving section. The wiring section  47   a  for the light source section connects the resistor  60  (the polysilicon layer  45   a  for the light source section) with the electrode. The wiring section  47   b  for the light receiving section connects between edges of the polysilicon layer  45   b  for the light receiving section via a contact hole formed in the interlayer insulating layer  46 . Along with the polysilicon layer  45   b  for the light receiving section, the wiring section  47   b  constitutes a thermocouple functioning as the detection element  70 . The wiring section  47   b  connects the detection element  70  with the electrode.  
         [0040]     As shown in  FIG. 2A , the thermocouple as the detection element  70  comprises different materials of the polysilicon layer  45   b  for the light receiving section and the wiring section  47   b  for the light receiving section. A plurality of sets of the polysilicon layer  45   b  and the wiring section  47   b  are alternately and serially disposed (thermopile) to constitute the thermocouple. A hot junction and a cold junction are alternately provided. The hot junction is formed on the membrane  50   b  having a small thermal capacity. The cold junction is formed on the substrate  40  having a large thermal capacity outside the membrane  50   b . Accordingly, the substrate  40  works as a heat sink.  
         [0041]     The applicable detection element  70  is constructed as follows. At least part of the detection element  70  is formed on the membrane  50   b . The infrared light absorbing layer  80  at least partially covers parts formed on the membrane  50   b . The detection element  70  generates electric signals based on thermal changes caused when receiving infrared light. In addition to the above-mentioned thermocouple, the detection element  70  may be a bolometric detection element having a resistor or a pyroelectric detection element having pyroelectrics.  
         [0042]     The wiring section  47  has a pad  48  as the electrode at its end. A protective layer  49  (e.g., silicon nitride layer) is provided on the wiring section  47  except the pad  48 . Of the pad  48  in  FIGS. 2A and 2B , the reference numeral  48   a  denotes a light source section pad connected to the wiring section  47   a  for the light source section  31 . The reference numeral  48   b  denotes a light receiving section pad connected to the wiring section  47   b  for the light receiving section.  
         [0043]     The infrared light absorbing layer  80  is formed on the protective layer  49  in the membrane  50   b  formation region so as to cover at least part of the detection element  70 . The infrared light absorbing layer  80  according to the embodiment is produced by sintering the polyester resin containing carbon. The infrared light absorbing layer  80  is formed on the membrane  50   b  by covering the hot junctions so as to absorb infrared light and efficiently increase the temperature of the hot junctions for the detection element  70 . The infrared light absorbing layer  80  is formed with a specified gap with reference to the end of the region for forming the membrane  50   b . The applicant discloses this gap (a ratio between the width of the infrared light absorbing layer  80  and the width of the membrane  50   b ) in Japanese Patent Application Publication No. 2002-365140. Further description is omitted in this embodiment.  
         [0044]     The sensor chip  30  having the above-mentioned construction is placed in the case  10 . The resistor  60  of the light source section  31  is electrified and is heated to radiate infrared light. The concave mirror  12   b  reflects the infrared light. The reflected light reaches the light receiving section  32 . The infrared light absorbing layer  80  absorbs the infrared light to increase the temperature. As a result, the temperature rises at the hot junction for the deletion  70  disposed under the infrared light absorbing layer  80 . By contrast, the cold junction indicates a smaller temperature rise than the hot junction because the substrate  40  works as the heat sink. When the detection element  70  receives the infrared light, a temperature difference occurs between the hot junction and the cold junction. According to this temperature difference, an electromotive force for the detection element  70  changes (Seebeck effect). Based on the changed electromotive force, the detection element  70  detects the infrared light intensity, i.e., the gas density. The thermocouple in  FIG. 2A  constitutes a thermopile. Output Vout from the detection element  70  is equivalent to the sum of electromotive forces generated from the set of the polysilicon layer  45   b  for the light receiving section and the wiring section  47   b  for the light receiving section.  
         [0045]     The method of manufacturing the gas sensor  100  will be described with reference to  FIGS. 1 and 2 B.  
         [0046]     First, the method of manufacturing the sensor chip  30  will be described with reference to  FIG. 2B .  
         [0047]     The silicon nitride insulating layer  43  is formed on all over the silicon substrate  40  by means of the CVD, for example. The insulating layer  43  becomes an etching stopper for etching on the substrate  40  to be described later. The insulating layer  43  is the constituent element of the membranes  50   a  and  50   b . Accordingly, it is important to form the insulating layer  43  by controlling the membrane stress. For this reason, it may be preferable to form the insulating layer  43  as a composite layer comprising the silicon nitride layer and the silicon oxide layer.  
         [0048]     For example, the CVD is used to form the silicon oxide layer  44  so as to cover the insulating layer  43 . The silicon oxide layer  44  increases the adhesiveness between the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section formed immediately on the silicon oxide layer  44 . The silicon oxide layer  44  is used as an etching stopper when forming the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section by means of etching.  
         [0049]     A polysilicon layer is formed on the silicon oxide layer  44  by means of the CVD, for example. Impurities such as phosphorus are implanted for adjustment to obtain a specified resistance value. A photo lithography process is performed for patterning to form the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section into specified shapes. At this time, though not shown, thermal oxidation is used to form a silicon oxide layer on the surfaces of the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section. The polysilicon layer  45   a  for the light source section becomes the resistor  60  constituting the light source section  31 . The polysilicon layer  45   b  for the light receiving section becomes part of the detection element  70  constituting the light receiving section  32 . Accordingly, the same process can be used to simultaneously form the resistor  60  and at least part of the detection element  70 . This makes it possible to simplify the manufacturing process of the sensor chip  30  and improve the positional accuracy of the resistor  60  and the detection element  70 . Polysilicon is not the only construction material for the resistor  60  and the detection element  70 . The other construction materials are available such as monocrystal silicon implanted with impurities and metal materials such as gold and platinum for forming the resistor  60  and the detection element  70 . It is not necessarily use the same process to simultaneously form the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section. Different processes may be used to form these polysilicon layers so as to provide corresponding impurity densities.  
         [0050]     After formation of the polysilicon layer  45   a  for the light source section  31  and the polysilicon layer  45   b  for the light receiving section  32 , the CVD method is used to form a BPSG layer on the silicon oxide layer  44  containing these polysilicon layers. The BPSG layer works as the interlayer insulating layer  46 . The BPSG layer is then heat-treated at 900 to 1000° C., for example. Heat-treating the BPSG layer as the interlayer insulating layer  46  at a high temperature smoothes steps at the edges of the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section. The stepping shape can be gently sloped. Consequently, it is possible to solve a problem of insufficient coverage of the wiring section  47 . After the heat treatment, the photolithography is applied to the interlayer insulating layer  46 . A contact hole for connection is formed in the regions for forming the membranes  50   a  and  50   b  at a position where the polysilicon layers  45   a  and  45   b  overlap with the wiring sections  47   a  and  47   b  in the lamination direction. As mentioned above, the polysilicon layer  45   a  is used for the light source section. The polysilicon layer  45   b  is used for the light receiving section. The wiring section  47   a  is used for the light source section. The wiring section  47   b  is used for the light receiving section. The interlayer insulating layer  46  is not limited to the BPSG layer. The interlayer insulating layer  46  may be a silicon nitride layer, a silicon oxide layer, or a composite layer of the silicon oxide layer and the silicon nitride layer.  
         [0051]     As a low-resistance metal material, an aluminum layer is formed in the contact hole and on the interlayer insulating layer  46 . The photolithography is applied for patterning. This process forms the wiring section  47   a  for the light source section and the wiring section  47   b  for the light receiving section. The wiring sections  47   a  and  47   b  are electrically connected with the polysilicon layer  45   a  for the light source section and the polysilicon layer  45   b  for the light receiving section. Pads are formed as electrodes along with the formation of the wiring section  47   a  for the light source section and the wiring section  47   b  for the light receiving section. That is, pads  48   a  and  48   b  are formed at the edges of the wiring sections  47   a  and  47   b . The pad  48   a  is used for the light source section. The pad  48   b  is used for the light receiving section. In addition to aluminum, the other low-resistance metals such as gold and copper can be used as materials for constructing the wiring section  47   a  for the light source section and the wiring section  47   b  for the light receiving section.  
         [0052]     The wiring section  47   a  for the light source section is used as connection between the resistor  60  (the polysilicon layer  45   a  for the light source section) and the pad  48   a  for the light source section. The wiring section  47   b  for the light receiving section makes connection between edges of the polysilicon layer  45   b  for the light receiving section via the contact hole formed in the interlayer insulating layer  46 . Together with the polysilicon layer  45   b  for the light receiving section, the wiring section  47   b  constructs the detection element  70  (thermocouple) of the light receiving section  32 . The wiring section  47   b  connects the detection element  70  with the pad  48   b.    
         [0053]     For example, the CVD method is used to form the protective layer  49  made of silicon nitride. The photolithography is applied for patterning to form apertures for forming the pad  48   a  for the light source section and the pad  48   b  for the light receiving section. The apertures expose the pads  48   a  and  48   b  from the protective layer  49 . The pad  48   a  for the light source section and the pad  48   b  for the light receiving section are provided at the edges of the wiring section  47   a  for the light source section and the wiring section  47   b  for the light receiving section.  
         [0054]     After formation of the protective layer  49 , paste is screen-printed on the protective layer  49  in the formation region for the membrane  50   b  so as to cover the hot junction of the detection element  70 . The paste is made of polyester resin containing carbon. The formed layer is sintered to form the infrared light absorbing layer  80 .  
         [0055]     Finally, for example, plasma CVD method is used to form the silicon nitride layer  42  for an etching mask entirely on the undersurface of the substrate  40 . The photolithography is applied to form cavities corresponding to the regions for forming the membranes  50   a  and  50   b  on the silicon nitride layer  42 . Using potassium hydroxide water solution, for example, anisotropic etching is performed to etch the silicon substrate  40 . The etching is performed until exposing the insulating layer  43  provided on the top surface of the substrate  40 . The membranes  50   a  and  50   b  are formed on the cavities  41   a  and  41   b  etched on the substrate  40 .  
         [0056]     The above-mentioned process forms the sensor chip  30  comprising the light source section  31  and the light receiving section  32 . The light source section  31  has the resistor  60  on the membrane  50   a  for the substrate  40 . The light receiving section  32  has at least part of the detection element  70  on the membrane  50   b  for the substrate  40 . The manufacturing method according to the embodiment can use the same process to simultaneously form all elements except the infrared light absorbing layer  80  of the light receiving section  32 . Accordingly, the manufacturing process can be simplified. Further, it is possible to improve the accuracy of positions between the light source section  31  and the light receiving section  32 .  
         [0057]     The general semiconductor process can be used to form the sensor chip  30  according to the embodiment, making it possible to reduce manufacturing costs. The infrared light absorbing layer  80  may be formed after formation of the cavity  11 , instead of after formation of the protective layer  49 . The above-mentioned manufacturing process may include formation of moisture-absorbent layers such as the silicon oxide layer  44 . In this case, the heat treatment may be performed as needed after the layer formation to prevent membrane stress variations due to moisture absorption.  
         [0058]     As shown in  FIG. 1 , the formed sensor chip  30  is bonded to a specified position on the pedestal  11  so that the concave mirror  12   b  faces the top surface of the substrate  40  where the resistor  60  and the detection element  70  are formed. The specified position should be capable of allowing a large amount of infrared light energy to reach the light receiving section  32 . The specified position is determined by the distance between the sensor chip  30  and a reflecting portion of the concave mirror  12   b , the reflecting shape (radius) of the concave mirror  12   b , and positional relationship between the light source section  31  (resistor  60 ) and the light receiving section  32  (detection element  70 ). According to the embodiment, the light source section  31  and the light receiving section  32  are integrated into the sensor chip  30  as a single chip. This determines the positional relationship between the resistor  60  and the detection element  70 . The sensor chip  30  can be accurately aligned to the specified position. Consequently, it is possible to decrease variations of the sensor sensitivity.  
         [0059]     With the sensor chip  30  fixed to the pedestal  11 , the bonding wire  33  is used to electrically connect the pads  48   a  and  48   b , and the terminal  34 . The pads  48   a  and  48   b  are used for the light source section and the light receiving section on the sensor chip  30 , respectively. Using laser welding, for example, the cap  20  is mounted on the pedestal  11  so that the sensor chip  30  is contained in the cap. The cap is previously equipped with the infrared light transmission filter  21   a , the band-pass filter  22   a , and the partition wall  23 . After the cap  20  is mounted, the container  12  is mounted on the pedestal  11 . The concave mirror  12   b  is provided on the inside top of the container  12 . In this manner,the gas sensor  100  is formed with the case  10  containing the sensor chip  30 .  
         [0060]     The substrate  40  has a thick portion (defined to be an intermediate thick portion) between the cavities  41   a  and  41   b , i.e., between the light source section  31  and the light receiving section  32 . When the resistor  60  of the light source section  31  generates heat, the intermediate thick portion can suppress (i.e., weaken) transmission of the generated heat directly to the detection element  70  of the light receiving section  32  via the substrate  40  itself or various layers on its surface. That is, heat generated by the resistor  60  can be dissipated to the air or the pedestal  11  via the intermediate thick portion.  
         [0061]     While there have been described specific preferred embodiments of the present invention, the present invention is not limited thereto but may be otherwise variously modified to be embodied.  
         [0062]     According to the embodiment, the concave mirror  12   b  exemplifies the reflection member that is disposed opposite to the light source section  31  and reflects infrared light to the light receiving section  32 . However, the reflection member is not limited to the concave mirror  12   b  having a specified radius. The reflection member may be otherwise embodied as a flat mirror, for example.  
         [0063]     The position to form the concave mirror  12   b  is not limited to the top inside of the container  12  constituting the case  10 . The concave mirror  12   b  can be formed at any position which can reflect the infrared light radiated from the light source section  31  to the light receiving section  32  in the case  10  (except the space in the cap  20 ).  
         [0064]     In the example of the embodiment, the sensor chip  30  has cavities  41   a  and  41   b  opening on the undersurface of the substrate  40  below the membranes  50   a  and  50   b  on the substrate  40 . As shown in  FIG. 3 , however, the sensor chip  30  may be structured to have the cavities  41   a  and  41   b  as closed spaces on the undersurface of the substrate  40  below the membranes  50   a  and  50   b  on the substrate  40 . In this case, the photolithography is first applied to form etching holes (not shown) for etching in the insulating layer  43 , the silicon oxide layer  44 , the interlayer insulating layer  46 , and the protective layer  49 . The protective layer  49  is used as an etching mask to selectively etch the substrate  40  below the membranes  50   a  and  50   b  through the etching holes. In this manner, the closed cavities  41   a  and  41   b  can be formed on the undersurface of the substrate  40 . In this case, however, the etching holes for etching are formed in the regions for forming the membranes  50   a  and  50   b . This method causes more restrictions on shapes and areas (along the plane direction) of the resistor  60 , the detection element  70 , and the infrared light absorbing layer  80  than those on formation of the cavities  41   a  and  41   b  by means of selective etching from the undersurface of the substrate  40 .  FIG. 3  is a sectional view showing a modification of the sensor chip  30  according to the embodiment.  
         [0065]     According to the embodiment, two membranes  50   a  and  50   b  are formed on one substrate  40 . However, the present invention is not limited to the above-mentioned number of membranes formed on the substrate  40 . For example, no membrane may be formed on the substrate  40 . The light source section  31  and the light receiving section  32  may be formed on a single membrane. There may be provided a plurality of light source sections  31  and light receiving sections  32  and the corresponding number of membranes  50   a  and  50   b.    
         [0066]     The embodiment has shown the example of bonding the sensor chip  30  on the pedestal  11 . On the other hand, the light source section  31  and the light receiving section  32  are integrated into the sensor chip  30  as a single chip. Compared to the prior art (other chips), the sensor chip  30  can reduce the installation space for the light source section  31  and the light receiving section  32  in the case  10 . As shown in  FIG. 4 , it is possible to dispose a circuit chip  90  for the light source section  31  and the light receiving section  32  in a free space in the case  10  without increasing the size of the case  10 . The circuit chip  90  can be integrated with the gas sensor  100 . The circuit chip  90  contains a constant current circuit to supply current to the resistor  60  of the light source section  31 , a processing circuit to process output from the light receiving section  31 , and the like. Specifically, the circuit chip  90  is fixed to the pedestal  11  as shown in  FIG. 4 . The sensor chip  30  is stacked on the circuit chip  90 . The bonding wire  33  may then be used to make electrical connection between the sensor chip  30  and the circuit chip  90  as a circuit substrate and between the circuit chip  90  as the circuit substrate and the terminal  34 .  FIG. 4  illustrates a modification of the gas sensor  100  according to the embodiment and shows only parts of the bonding wire  33  for convenience.  
         [0067]     The embodiment has shown the example of using the semiconductor substrate made of silicon as the substrate  40  constituting the sensor chip  30 . However, the substrate  40  is not limited to semiconductor substrates. Further, for example, a glass substrate and the like may be used for the substrate  40 .  
         [0068]     Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.