Patent Publication Number: US-2022221416-A1

Title: Mems gas sensor and method for manufacturing mems gas sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a gas sensor, in particular, a MEMS gas sensor and a method for manufacturing the same. 
     BACKGROUND ART 
     A gas sensitive material of a semiconductor type gas sensor is made of a metal oxide semiconductor (such as tin oxide). When reducing gas contacts high-temperature tin oxide, oxygen on the surface reacts with the reducing gas and is removed. As a result, electrons in the tin oxide become free (i.e., the resistance of the tin oxide decreases). Gas is detected in the semiconductor type gas sensor in accordance with the principles described above. 
     A MEMS (Micro Electro Mechanical Systems) gas sensor that is a type of semiconductor type gas sensor mainly includes a semiconductor chip and a package containing the semiconductor chip. 
     A cavity is formed in the semiconductor chip. An insulating film is formed in an opening of the cavity, and a gas sensitive portion is provided on the insulating film. The gas sensitive portion includes a gas sensitive material and a thin film heater. The gas sensitive portion further includes wiring. The wiring is drawn from the gas sensitive material and the thin film heater to the outside of the cavity and is connected to an electrode pad (see, for example, Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2012-98234A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In general, Pt is used in a heater layer of the MEMS gas sensor. However, since the life of the Pt heater is short, a NiCr heater has been considered. 
     In the development of the NiCr heater, the inventors have investigated the use of a SiN film as an interlayer insulating film. However, since the deposition rate of the SiN film is low, productivity is low. 
     Therefore, the inventors have investigated the use of a SiO 2  film as the interlayer insulating film in order to increase the deposition rate. However, it has been revealed that in a life test of the heater, a change in a resistance value of the SiO 2  film is large and thus the lifespan is short. 
     An object of the present invention is to extend the life of a MEMS gas sensor. 
     Solution to Problem 
     Some aspects will be described below as means to solve the problems. These aspects can be combined arbitrarily as necessary. 
     A MEMS gas sensor according to one aspect of the present invention includes an insulator, a gas sensitive material, a first protective film and a second protective film, heater wiring, and a gas barrier layer. 
     The insulator includes a cavity. 
     The gas sensitive material is provided corresponding to the cavity. 
     The first protective film and the second protective film are provided on the insulator and are disposed to overlap in a plan view. 
     The heater wiring serves to heat the gas sensitive material and is disposed between the first protective film and the second protective film. 
     The gas barrier layer covers, in direct contact, both surfaces and a side surface of the heater wiring. 
     In this sensor, both the surfaces and the side surface of the heater wiring are covered by the gas barrier layer, and thus a change in a resistance value of a heater can be reduced. Therefore, the life can be increased. The reason for this is that, even when gas barrier properties of the first protective film and the second protective film are low or even when gas components such as hydrogen or oxygen inside the first protective film and the second protective film move to the outside, the gas barrier layer restricts the movement of gas and thus the heater wiring is not influenced by the gas. 
     At least a portion of the side surface of the heater wiring may extend obliquely in a side view. 
     In this sensor, the side surface of the heater wiring is inclined, and thus it is easier to form the gas barrier layer on the side surface of the heater wiring. Therefore, the adhesion of the gas barrier layer increases. 
     The first protective film and the second protective film may be made of SiO 2 . 
     In this sensor, the deposition rates of the first protective film and the second protective film are increased, and a thick film can be easily formed. 
     The heater wiring may be made of NiCr. 
     In this sensor, the lifespan of the heater is extended. 
     The gas barrier layer may be a metal oxide film. 
     In this sensor, the gas barrier layer can be formed by sputter deposition, and the gas barrier layer has an insulating property or a resistance value significantly high compared with that of the heater wiring. 
     The gas barrier layer may be made of Ta 2 O 5 . 
     In this sensor, the adhesion of the gas barrier layer is high. 
     The heater wiring may be formed in an annular shape in a plan view at a location corresponding to the gas sensitive material. 
     In this sensor, the center portion of the heater is not formed. Therefore, the difference in temperature between the center side and the outer circumferential side of the heater is reduced. As a result, the heater lifespan is extended, and sensor characteristics also become stable. 
     Conventionally, the patterns are densely disposed in the center portion of the heater; therefore, the temperature of the center portion rises. Consequently, the temperature distribution is deteriorated. 
     A method for manufacturing a MEMS gas sensor according to another aspect of the present invention includes the following steps. Note that the order of execution of the steps is not particularly limited.
         Forming a first protective film on an insulator including a cavity.   Forming a first gas barrier layer on the first protective film.   Forming heater wiring on the first gas barrier layer, the heater wiring being configured to heat a gas sensitive material.   Forming a second gas barrier layer configured to cover an upper surface and a side surface of the heater wiring.   Forming a second protective film to sandwich the heater wiring between the first protective film and the second protective film.   Forming the gas sensitive material to be provided corresponding to the cavity of the insulator.       

     In this method, both the surfaces and the side surface of the heater wiring are covered by the first and second gas barrier layers, and thus a change in a resistance value of the heater can be reduced. Therefore, the life can be increased. The reason for this is that, even when gas barrier properties of the first protective film and the second protective film are low or even when gas components such as hydrogen or oxygen inside the first protective film and the second protective film move to the outside, the first and second gas barrier layers restrict the movement of gas and thus the heater wiring is not influenced by the gas. 
     The method for manufacturing a MEMS gas sensor may further include the following steps.
         Processing at least a portion of the side surface of the heater wiring such that the at least a portion of the side surface extends obliquely in a side view.
 
In this method, the side surface of the heater wiring is inclined, and thus it is easier to form the second gas barrier layer on the side surface of the heater wiring. Therefore, the adhesion of the second gas barrier layer increases.
       

     Advantageous Effects of Invention 
     A MEMS gas sensor according to the present invention results in a longer lifespan. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of a MEMS gas sensor as a first embodiment of the present invention. 
         FIG. 2  is a plan view of a transverse cross-section in a portion of the MEMS gas sensor. 
         FIG. 3  is a schematic cross-sectional view of the MEMS gas sensor. 
         FIG. 4  is a cross-sectional view of heater wiring of the MEMS gas sensor. 
         FIG. 5  is a cross-sectional photograph of the heater wiring of the MEMS gas sensor. 
         FIG. 6  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 7  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 8  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 9  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 10  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 11  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 12  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 13  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 14  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 15  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 16  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 17  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 18  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 19  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 20  is a cross-sectional view illustrating a manufacturing process of the MEMS gas sensor. 
         FIG. 21  is a schematic view illustrating the principles of milling with Ar ions. 
         FIG. 22  is a plan view of a transverse cross-section in a portion of the MEMS gas sensor as a second embodiment. 
         FIG. 23  is a plan view of a heater wiring pattern. 
         FIG. 24  is a plan view of a heater wiring pattern according to a third embodiment. 
         FIG. 25  is a plan view of a heater wiring pattern according to a fourth embodiment. 
         FIG. 26  is a plan view of a heater wiring pattern according to a fifth embodiment. 
         FIG. 27  is a plan view of a heater wiring pattern according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     1. First Embodiment 
     (1) MEMS Gas Sensor 
     Using  FIGS. 1 to 3 , a MEMS gas sensor  1  (hereinafter referred to as a gas sensor  1 ) as an embodiment of the present invention will be described.  FIG. 1  is a plan view of a MEMS gas sensor as a first embodiment of the present invention.  FIG. 2  is a plan view of a transverse cross-section in a portion of the MEMS gas sensor.  FIG. 3  is a schematic cross-sectional view of the MEMS gas sensor. 
     As illustrated in  FIG. 3 , the gas sensor  1  includes a base  3  (an example of an insulator). The base  3  includes a first main surface  3   a  and a second main surface  3   b  that face each other in a thickness direction. The material of the base  3  is, for example, silicon, sapphire glass, quartz glass, ceramic wafer, or SiC. The thickness of the base  3  is 100 to 800 μm. 
     The base  3  includes a cavity  3   c  (an example of a cavity). The cavity  3   c  includes an opening  5  opened to the first main surface  3   a . The depth of the cavity  3   c  is 100 to 800 μm. The cavity  3   c  has a quadrangular pyramid shape having a transverse cross-sectional area that increases from the bottom to the opening. Note that the shape of the cavity may be a vertical hole and that the planar shape may be a square, a rectangle, or a circle. 
     In addition, a first oxide film  6  (an example of a first protective film) is formed on the first main surface  3   a  of the base  3 . A second oxide film  8  is formed on the second main surface  3   b  of the base  3 . The thickness of each of the first oxide film  6  and the second oxide film  8  is from 0.05 to 2 μm. 
     The gas sensor  1  includes a base insulating layer  7 . The base insulating layer  7  is formed on the first main surface  3   a  of the base  3 . The base insulating layer  7  includes an interlayer insulating film  13  (an example of a second protective film). As described above, the interlayer insulating film  13  is disposed as the base insulating layer  7  so as to overlap with the first oxide film  6  in a plan view. 
     The thickness of the interlayer insulating film  13  is 1 to 5 μm. 
     The material of the interlayer insulating film  13  is, for example, SiO 2 , SiON, SiOC, or SiOCN. As an example, when the interlayer insulating film  13  is made of SiO 2 , the deposition rate of the interlayer insulating film  13  is increased, and a thick film can be easily formed. 
     As illustrated in  FIG. 2 , the base insulating layer  7  includes a fixing portion  15  fixed to the first main surface  3   a  of the base  3 , and a thin plate-shaped bridge portion  17  integrally provided with the fixing portion  15  and positioned corresponding to the opening  5  of the base  3 . The bridge portion  17  is a thin film-shaped support film formed on the base  3  so as to close the opening  5  of the cavity  3   c . As illustrated in  FIG. 2 , the bridge portion  17  includes a central portion  19  and four connecting portions  21  connecting the central portion  19  and the fixing portion  15 . Cutouts  21   a  are formed between the connecting portions  21 . The cutouts  21   a  are portions allowing the opening  5  of the cavity  3   c  to connect to the outside. In the present embodiment, as illustrated in  FIG. 2 , the bridge shape of the four connecting portions  21  is generally an X shape and is exactly a type of four cross rounded corners. This is preferable from push strength and temperature distribution results. 
     The number of connecting portions is, for example, two to five connecting portions, and the connecting portions are formed in a swastika shape, an X shape, a plus shape, or the like. In addition, instead of the bridge portion, the thin plate-shaped portion may be a membrane portion that has no cutout. 
     As illustrated in  FIGS. 2 and 3 , the gas sensor  1  includes a heater wiring pattern  23  (an example of heater wiring). The heater wiring pattern  23  serves to heat a gas sensitive material  33  (described below). The heater wiring pattern  23  is disposed between the first oxide film  6  and the interlayer insulating film  13 . 
     As illustrated in  FIG. 3 , the layer structure of the heater wiring pattern  23  includes a heater layer  23   a . The thickness of the heater layer  23   a  is 0.1 to 1 μm. The material of the heater layer  23   a  is, for example, NiCr, Pt, Mo, Ta, W, NiCrFe, NiCrFeMo, NiCrAl, FeCrAl, or NiFeCrNbMo. As an example, when the heater layer  23   a  is made of NiCr, the lifespan of a heater is extended. 
     Note that when the heater layer  23   a  is made of a material other than NiCr, a heater layer bonding film may be provided. The material of the heater layer bonding film is, for example, Ti, Ta, Ta 2 O 5 , or Al 2 O 3 . The thickness of the heater layer bonding film is from 0.01 to 0.5 μm. 
     As illustrated in  FIGS. 4 and 5 , the heater wiring pattern  23  is covered by a lower protective film  11  (an example of a gas barrier layer, a first gas barrier layer) and an upper protective film  20  (an example of a gas barrier layer, a second gas barrier layer).  FIG. 4  is a cross-sectional view of heater wiring of the MEMS gas sensor.  FIG. 5  is a cross-sectional photograph of the heater wiring of the MEMS gas sensor. The heater wiring pattern  23  includes an upper surface  23   c , a lower surface  23   d , and a side surface  23   e . The lower surface  23   d  is covered by the lower protective film  11 , and the upper surface  23   c  and the side surface  23   e  are covered by the upper protective film  20 . 
     NiCr in  FIG. 5( b )  corresponds to the heater wiring pattern  23 , TEOS-SiO 2  corresponds to the first oxide film  6  and the interlayer insulating film  13 , and Ta 2 O 5  corresponds to the lower protective film  11  and the upper protective film  20 . 
     The lower protective film  11  and the upper protective film  20  are made of, for example, Ta 2 O 5 , Al 2 O 3 , SiN, SiO, SiC, SiCN, TiN, TiC, TiB 2 , Cr 2 O 3 , HfO 2 , Nb 2 O 5 , ZrO 2 , CrN, or AlN. The thickness of the lower protective film  11  and the upper protective film  20  ranges from 0.05 to 0.20 μm. 
     The entire surface of the heater wiring pattern  23  is covered by the upper protective film  20  and the lower protective film  11  which are gas barrier layers, and thus a change in a resistance value of the heater wiring pattern  23  can be reduced. Therefore, the life can be increased. This is because, even when gas barrier properties of the first oxide film  6  and the interlayer insulating film  13  are low or even when gas components such as hydrogen or oxygen inside the first oxide film  6  and the interlayer insulating film  13  move to the outside, the upper protective film  20  and the lower protective film  11  which are gas barrier layers restrict the movement of gas and thus the heater wiring pattern  23  is not influenced by the gas. 
     The side surface  23   e  extends obliquely in a side view, that is, is an inclined surface. Accordingly, the upper protective film  20  is easily formed at the side surface  23   e  of the heater wiring pattern  23 , and thus the adhesion of the upper protective film  20  is increased. Note that the inclination angle of the side surface  23   e  is, for example, 30 to 80 degrees. 
     When the upper protective film  20  is made of, for example, a metal oxide film, the upper protective film  20  can be formed by sputter deposition, and the upper protective film  20  has an insulating property or a resistance value significantly high compared with that of the heater wiring pattern  23 . 
     The upper protective film  20  is preferably made mainly of Ta 2 O 5 . In this case, the adhesion of the upper protective film  20  to the heater wiring pattern  23  is high. 
     As illustrated in  FIGS. 1 to 3 , the heater wiring pattern  23  includes an electric heater unit  25  in the central portion  19  of the bridge portion  17 . The electric heater unit  25  is connected to a pair of heater electrode pads  27 ,  27 . The electric heater unit  25  serves to heat the gas sensitive material  33  (described below) and functions to facilitate reaction of the measurement gas and the gas sensitive material  33  and to rapidly diffuse the absorbed gas and moisture after the reaction. 
     The electric heater unit  25  corresponds to the center of the central portion  19  of the bridge portion  17  and includes an annular portion  52 . Specifically, the annular portion  52  is configured such that connecting portions  54  (described below) respectively branch at the central portion to be connected in an annular shape. As just described, the center portion of the electric heater unit  25  is not formed. Therefore, the difference in temperature between the center side and the outer circumferential side of the electric heater unit  25  is reduced. As a result, the heater lifespan is extended, and sensor characteristics also become stable. 
     The electric heater unit  25  includes the connecting portions  54  extending circumferentially about 270 degrees in the central portion  19  of the bridge portion  17 . One end of each connecting portion  54  is connected to the annular portion  52 . 
     The gas sensor  1  includes an electrode wiring pattern  29 . The layer structure of the electrode wiring pattern  29  includes a sense layer  29   a  and a sense layer bonding film  29   b  (see  FIG. 20 ). The thickness of the sense layer  29   a  is 0.1 to 1 μm. The thickness of the sense layer bonding film  29   b  is 0.01 to 0.5 μm. The material of the sense layer  29   a  is, for example, Pt, W, or Ti. The material of the sense layer bonding film  29   b  is, for example, Ti, Ta, Ta 2 O 5 , or Al 2 O 3 . 
     As illustrated in  FIGS. 2 and 3 , the electrode wiring pattern  29  configures a detection electrode portion  31  in the central portion  19  of the bridge portion  17 . The detection electrode portion  31  is formed on the surface of the interlayer insulating film  13 . The detection electrode portion  31  is connected to a pair of detection electrode pads  28 ,  28 . The detection electrode portion  31  functions to detect a change in a resistance value within the gas sensor  1  when gas to be detected adheres to the gas sensitive material  33  (described below). 
     The gas sensor  1  includes the gas sensitive material  33 . The gas sensitive material  33  includes a property sensitive (reacting) to gas to be measured. Specifically, a resistance value of the gas sensitive material  33  changes in accordance with a change in concentration of the gas to be measured. The gas sensitive material  33  is formed on the central portion  19  of the bridge portion  17  so as to cover the detection electrode portion  31 . In other words, the gas sensitive material  33  is provided corresponding to the cavity  3   c.    
     The thickness of the gas sensitive material  33  is 3 to 50 μm. The material of the gas sensitive material  33  is, for example, SnO 2 , WO 3 , ZnO, NiO, CuO, FeO, or In 2 O 3 . A method for forming the gas sensitive material  33  is, for example, screen printing, dispenser application, ink jet application, or sputtering. 
     In addition, a surface protective film  30  is formed on the surface of the base insulating layer  7 . The surface protective film  30  is made of a known material. 
     (2) Method for Manufacturing Gas Sensor 
     A method for manufacturing the gas sensor  1  will be described with use of  FIGS. 6 to 20 .  FIGS. 6 to 20  are cross-sectional views illustrating a manufacturing process of the MEMS gas sensor. Note that even during the manufacturing process, the same reference signs may be assigned to components corresponding to components of a finished product. 
     As illustrated in  FIG. 6 , a large area wafer  3 A made of, for example, a silicon single crystal substrate, is loaded as a material of the base  3 . The wafer  3 A includes the first main surface  3   a  and the second main surface  3   b.    
     Furthermore, the first oxide film  6  and the second oxide film  8  are respectively formed on the first main surface  3   a  and the second main surface  3   b  of the wafer  3 A. The oxide film is formed by, for example, a thermal oxidation method. 
     Next, steps of forming the heater wiring pattern  23  on the wafer  3 A will be described with use of  FIGS. 7 to 9 . 
     In  FIG. 7 , the lower protective film  11  is further formed by sputtering. However, in  FIG. 7 , the lower protective film  11  is not illustrated. 
     Furthermore, in  FIG. 7 , a heater solid layer  23 A is formed on the lower protective film  11 . 
     As illustrated in  FIG. 8 , a resist  48  in a predetermined pattern is formed on the heater solid layer  23 A. The predetermined pattern is formed by resist coating, exposure, and development processes. 
     As illustrated in  FIG. 9 , the heater solid layer  23 A is dry-etched. Further, the resist  48  is removed. As a result, the heater wiring pattern  23  is obtained. 
     Next, as illustrated in  FIG. 21 , an ion milling device  41  is used to process the side surface  23   e  of the heater wiring pattern  23  into an inclined surface shape.  FIG. 21  is a schematic diagram illustrating the principles of milling with Ar ions. 
     The ion milling device  41  is a device that performs etching by irradiating a surface of an object with a weak argon ion beam. The ion milling device  41  includes a chamber  46 , an Ar ion source  45 , and a wafer holding unit  47 . The Ar ion source  45  and the wafer holding unit  47  are disposed in the chamber  46 . The wafer holding unit  47  is provided to face the Ar ion source  45  and carries a plurality of the wafers  3 A. The wafer holding unit  47  rotates while being tilted with respect to an irradiation direction of the Ar ions. 
     As a result, as illustrated in  FIG. 4 , the side surface  23   e  of the heater wiring pattern  23  is formed into an inclined surface. 
     Further, the upper protective film  20  is formed on the heater wiring pattern  23  by sputtering. However, in  FIG. 9 , the upper protective film  20  is not illustrated. 
     As illustrated in  FIG. 4 , the upper protective film  20  is formed on the upper surface  23   c  and the side surface  23   e  of the heater wiring pattern  23 . At this time, since the side surface  23   e  is an inclined surface, the adhesion of the upper protective film  20  is favorable. 
     Hereinafter, steps of forming the electrode wiring pattern  29  on the wafer  3 A will be described with use of  FIGS. 10 to 12 . 
     As illustrated in  FIG. 10 , the interlayer insulating film  13  is formed on the heater wiring pattern  23  by TEOS. 
     Furthermore, an electrode wiring solid layer  29 A is formed on the interlayer insulating film  13 . 
     As illustrated in  FIG. 11 , a resist  49  in a predetermined pattern is formed on the electrode wiring solid layer  29 A. The predetermined pattern is formed by resist coating, exposure, and development processes. 
     As illustrated in  FIG. 12 , the electrode wiring solid layer  29 A is dry-etched. Dry etching is, for example, plasma etching. Furthermore, the resist  49  is removed. As a result of the above steps, the electrode wiring pattern  29  is obtained. 
     As illustrated in  FIG. 13 , the surface protective film  30  is formed on the electrode wiring pattern  29 . 
     As illustrated in  FIG. 14 , a resist  50  in a predetermined pattern is formed on portions other than of a sense pad opening  43  of the surface protective film  30  and a gas sensitive forming portion opening. Thereafter, exposed portions of the surface protective film  30  are removed. The resist  50  is also removed. 
     As illustrated in  FIG. 15 , a new resist  50  covers portions other than a heater pad opening  42  and dicing line openings  44 , and the heater pad opening  42  and the dicing line openings  44  are formed by etching. 
     As illustrated in  FIG. 16 , the resist  50  is filled in the dicing line openings  44 . Additionally, the sense pad opening  43  is formed. 
     As illustrated in  FIG. 17 , the heater electrode pads  27  and the detection electrode pads  28  are formed by lift-off. Thereafter, the resist  50  is removed. 
     As illustrated in  FIG. 18 , a resist  51  is formed. Further, an insulating film opening of the cavity  3   c  is formed. In other words, the cutouts  21   a  between the connecting portions  21  are formed. As a result, the central portion  19  is also formed. 
     As illustrated in  FIG. 19 , the resist  51  is removed. 
     In addition, the cavity  3   c  is formed in the wafer  3 A. Specifically, anisotropic etching is performed to form the cavity  3   c  having the opening  5 . 
     As illustrated in  FIG. 20 , dicing is performed on the wafer  3 A to obtain the base  3 . 
     Finally, as illustrated in  FIG. 3 , the gas sensitive material  33  is formed. The gas sensitive material  33  is formed on the detection electrode portion  31  of the central portion  19 . In other words, the gas sensitive material  33  is formed on the surface of the central portion  19  so as to cover the detection electrode portion  31 . As an example, the gas sensitive material  33  is formed by applying a metal compound semiconductor mainly containing In 2 O 3  and made into a paste to the surface of the central portion  19  and baking the surface at 650° C. or higher. 
     As a result, the gas sensor  1  is obtained. 
     Note that the gas sensitive material  33  may be formed before dicing. 
     2. Second Embodiment 
     In the first embodiment, the bridge shape of the four connecting portions  21  is an X-shape but may be another shape. 
     Such an embodiment will be described with use of  FIGS. 22 and 23 .  FIG. 22  is a plan view of a transverse cross-section in a portion of the MEMS gas sensor as a second embodiment.  FIG. 23  is a plan view of a heater wiring pattern. Note that the basic configuration is the same as that of the first embodiment and thus differences will be mainly described below. 
     The number of connecting portions  21  is three connecting portions. The three connecting portions  21  radially extend in a straight line and are precisely three straight types. 
     In addition, the electric heater unit  25  is in a zigzag pattern. 
     3. Third to Sixth Embodiments 
     In the first embodiment and the second embodiment, the electric heater unit  25  of the heater wiring pattern  23  is in a zigzag pattern but may have another shape. Embodiments in which the electric heater unit  25  according to third to sixth embodiments has another shape will be described below. Note that the basic configuration is the same as that of the first embodiment and thus differences will be mainly described below. 
     Third Embodiment 
     The third embodiment will be described with use of  FIG. 24 .  FIG. 24  is a plan view of a heater wiring pattern according to the third embodiment. 
     An electric heater unit  25 A corresponds to the central portion  19  of the bridge portion  17  and includes the annular portion  52 . Specifically, the annular portion  52  is configured such that connecting portions  53  (described below) respectively branch at the central portion to be connected in an annular shape. As just described, the center portion of the electric heater unit  25 A is not formed. Therefore, the difference in temperature between the center side and the outer circumferential side of the electric heater unit  25 A is reduced. As a result, the heater lifespan is extended, and sensor characteristics also become stable. 
     The electric heater unit  25 A includes a pair of the connecting portions  53  extending circumferentially, for example, about 250 degrees in the central portion  19  of the bridge portion  17 . One end of each of the connecting portions  53  is connected to the annular portion  52 . The pair of connecting portions  53  are arranged so as to have a triple circle around the annular portion  52 . 
     Note that the electric heater unit  25 A is made of, for example, NiCr and has a line width larger than that of an electric heater unit made of, for example, Pt. 
     Fourth Embodiment 
     The fourth embodiment will be described with use of  FIG. 25 .  FIG. 25  is a plan view of a heater wiring pattern according to the fourth embodiment. 
     An electric heater unit  25 B corresponds to the center of the central portion  19  of the bridge portion  17  and includes the annular portion  52 . Specifically, the annular portion  52  is formed in a continuous annular shape of a pair of parallel lines respectively extending from connecting portions  55  (described below). As just described, the center portion of the electric heater unit  25 B is not formed. Therefore, the difference in temperature between the center side and the outer circumferential side of the electric heater unit  25 B is reduced. As a result, the heater lifespan is extended, and sensor characteristics also become stable. 
     The electric heater unit  25 B includes a pair of the connecting portions  55  extending circumferentially in the central portion  19  of the bridge portion  17  to be folded back and further extend. As just described, the connecting portions  55  are folded back and thus are densely disposed at the outer circumferential side of the electric heater unit  25 B. One end of each of the connecting portions  55  is connected to the annular portion  52 . 
     Note that the electric heater unit  25 B is made of, for example, NiCr and has a line width larger than that of an electric heater unit made of, for example, Pt. 
     Fifth Embodiment 
     The fifth embodiment will be described with use of  FIG. 26 .  FIG. 26  is a plan view of a heater wiring pattern according to the fifth embodiment. 
     An electric heater unit  25 C corresponds to the center of the central portion  19  of the bridge portion  17  and includes the annular portion  52  in a plan view. Specifically, the annular portion  52  is formed in a continuous annular shape of a pair of parallel lines respectively extending from connecting portions  57  (described below). As just described, the center portion of the electric heater unit  25 C is not formed. Therefore, the difference in temperature between the center side and the outer circumferential side of the electric heater unit  25 C is reduced. As a result, the heater lifespan is extended, and sensor characteristics also become stable. 
     The electric heater unit  25 C includes a pair of the connecting portions  57  extending circumferentially, for example, about 290 degrees in the central portion  19  of the bridge portion  17 . One end of each of the connecting portions  57  is connected to the annular portion  52 . 
     Note that the electric heater unit  25 C is made of, for example, Pt and has a line width smaller than that of an electric heater unit made of, for example, NiCr. 
     Sixth Embodiment 
     The sixth embodiment will be described with use of  FIG. 27 .  FIG. 27  is a plan view of a heater wiring pattern according to the sixth embodiment. 
     An electric heater unit  25 D corresponds to the center of the central portion  19  of the bridge portion  17  and includes the annular portion  52  in a plan view. Specifically, the annular portion  52  is formed in a continuous annular shape of a pair of parallel lines respectively extending from connecting portions  59  (described below). As just described, the center portion of the electric heater unit  25 D is not formed. Therefore, the difference in temperature between the center side and the outer circumferential side of the electric heater unit  25 D is reduced. As a result, the heater life extends, and sensor characteristics also become stable. 
     The electric heater unit  25 D includes a pair of the connecting portions  59  extending circumferentially in the central portion  19  of the bridge portion  17  to be folded back and further extend. As just described, the connecting portions  59  are folded back and thus are densely disposed at the outer circumferential side of the electric heater unit  25 D. One end of each of the connecting portions  59  is connected to the annular portion  52 . 
     Note that the electric heater unit  25 D is made of, for example, Pt and has a line width smaller than that of an electric heater unit made of, for example, NiCr. 
     4. Common Matters of Embodiments 
     The MEMS gas sensor  1  includes an insulator (for example, the base  3 ), a gas sensitive material (for example, the gas sensitive material  33 ), a first protective film (for example, the first oxide film  6 ) and a second protective film (for example, the interlayer insulating film  13 ), heater wiring (for example, the heater wiring pattern  23 ), and a gas barrier layer (for example, the lower protective film  11 , the upper protective film  20 ). 
     The insulator includes a cavity (for example, the cavity  3   c ). 
     The gas sensitive material is provided corresponding to the cavity. 
     The first protective film and the second protective film are provided on the insulator and are disposed so as to overlap in a plan view. 
     The heater wiring serves to heat the gas sensitive material and is disposed between the first protective film and the second protective film. 
     The gas barrier layer covers, in direct contact, both surfaces (for example, the upper surface  23   c , the lower surface  23   d ) and a side surface (for example, the side surface  23   e ) of the heater wiring. 
     In such a sensor, both the surfaces and the side surface of the heater wiring are covered by the gas barrier layer, and thus a change in a resistance value of a heater can be reduced. Therefore, the lifespan can be increased. The reason for this is that, even when gas barrier properties of the first protective film and the second protective film are low or even when gas components such as hydrogen or oxygen inside the first protective film and the second protective film move to the outside, the gas barrier layer restricts the movement of gas and thus the heater wiring is not influenced by the gas. 
     5. Other Embodiments 
     Although the plurality of embodiments of the present invention have been described as above, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the gist of the invention. In particular, the plurality of embodiments and modifications described herein can be combined arbitrarily with one another as necessary. 
     In the third to sixth embodiments, the annular portion of the electric heater unit is formed in an end portion-less annular shape of a pair of parallel lines extending respectively from the connecting portions. However, the annular portion may include both end portions located close to each other, and may be formed such that one of the end portions extends from one of the connecting portions and the other of the end portions extends from the other of the connecting portions. 
     The cavity may have an opening at the lower side. 
     The gas sensitive material, the heater wiring pattern, and the like may be provided on the second main surface of the insulating material. 
     INDUSTRIAL APPLICABILITY 
     The present invention is broadly applicable to a MEMS gas sensor and a method for manufacturing the same. 
     REFERENCE SIGNS LIST 
       1 : MEMS gas sensor 
       3 : Base 
       3   c : Cavity 
       5 : Opening 
       11 : Lower protective film 
       13 : Interlayer insulating film 
       20 : Upper protective film 
       23 : Heater wiring pattern 
       23   a : Heater layer 
       23   c : Upper surface 
       23   d : Lower surface 
       23   e : Side surface 
       25 : Electric heater unit 
       27 : Heater electrode pad 
       28 : Detection electrode pad 
       29 : Electrode wiring pattern 
       31 : Detection electrode portion 
       33 : Gas sensitive material