GAS SENSOR

A gas sensor includes a gas detection part provided on an insulating film. The gas detection part includes a detector and an electrode in contact with the detector. The electrode includes an inner electrode portion and an outer electrode portion disposed so as to surround the inner electrode portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2023-056005 filed on Mar. 30, 2023, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a gas sensor.

Gas sensors can detect, for example, gas leakage and are installed in home appliances, industrial equipment, environmental monitoring equipment, etc. As described in Patent Document 1, a gas sensor includes: a detector whose physical characteristics change in accordance with the gas concentration in the atmosphere; and an electrode for outputting the change in physical characteristics as an electrical signal. When the physical characteristics of the detector change in accordance with a change in gas concentration, the electrical signal output from the electrode fluctuates. Based on the fluctuation value of this electrical signal, it is possible to determine the gas concentration, etc.

By the way, the gas concentration distribution or the gas flow direction in the atmosphere is constantly varying due to movement of people or objects, weather changes, etc. It is found that, in a conventional gas sensor as shown, for example, in Patent Document 1, noise is superimposed on the electrical signal output from the electrode due to variations in the gas concentration distribution or gas flow direction, which may result in a decrease in detection accuracy, responsiveness, etc. of the gas sensor.Patent Document 1: JP6917843 (B2)

SUMMARY

A gas sensor according to the present disclosure is a gas sensor comprising a gas detection part provided on an insulating film, whereinthe gas detection part includes:a detector; andan electrode in contact with the detector, andthe electrode includes:an inner electrode portion; andan outer electrode portion disposed so as to surround the inner electrode portion.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure are described with reference to figures. Note that, the illustrated contents are merely shown schematically and exemplarily for understanding the present disclosure and may be different from actual one in terms of appearance, dimensional ratio, etc. Moreover, the present disclosure is not limited to the following embodiments.

First Embodiment

A gas sensor1according to First Embodiment of the present disclosure shown inFIG.1andFIG.2is a device for detecting, for example, gas leakage and is installed in home appliances, industrial equipment, environmental monitoring equipment, etc. The gas sensor1is a contact-combustion-type gas sensor and detects gases such as CO, H2, CH4and C2H5OH. The gas sensor1may be a thermal-conduction-type gas sensor and may detect gases such as CO2, H2, H2O (water vapor) and CH4.

The gas sensor1includes, for example, a substrate2, a first insulating film3, a heating part4, a second insulating film5, an electrode6, a third insulating film7, a detector8, and a catalyst9, and terminals10ato10d. However, the structure of the gas sensor1is not limited to the structure shown inFIG.1,FIG.2, etc. and may be modified as appropriate without departing from the scope of the present disclosure.

InFIG.1,FIG.2, etc., the X-axis and the Y-axis are axes corresponding to two orthogonal sides of the substrate2, and the Z-axis is an axis orthogonal to the X-axis and the Y-axis. Hereinafter, for each of the X-axis, Y-axis, and Z-axis, the direction toward the center of the substrate2is defined as “inside”, and the direction away from the center of the substrate2is defined as “outside”. For the Z-axis, one side (positive side) is defined as “upper”, and the other side (negative side) is defined as “lower”.

As shown inFIG.1andFIG.2, the substrate2includes a cavity20formed by etching, etc. The substrate2is made of, for example, a material that has a mechanical strength capable of supporting the insulating films (the first insulating film3, etc.) and is favorable for microfabrication such as etching. The material constituting the substrate2is not limited and is, for example, a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, a glass substrate, or the like. The shape of the substrate2is square in plan view, but may be circular, oval, oblong, another polygon, or the like.

The cavity20is formed in a central part of the substrate2and penetrates the substrate2along the Z-axis. The shape of the cavity20is square in plan view, but may be circular, elliptical, oblong, another polygon, or the like. A plurality (four in the example shown inFIG.1) of curved portions is formed on the outer edge of the cavity20, but the curved portions may be omitted. The cavity20in the present embodiment is a through hole, but may also be a recess recessed from a top surface21toward a bottom surface22of the substrate2.

As shown inFIG.5A, the top surface21of the substrate2has a quadrangular ring shape and is formed along the outer edge of the cavity20. Although not illustrated in detail, the bottom surface22of the substrate2has a quadrangular ring shape and is formed along the outer edge of the cavity20.

As shown inFIG.2, the first insulating film3is made of an insulating material and is formed on the top surface21of the substrate2. As shown inFIG.5B, the first insulating film3has a film structure and is manufactured by a known film forming method (sputtering method, CVD method, thermal oxidation method, etc.). The material constituting the first insulating film3is not limited and is, for example, silicon oxide, silicon nitride, or the like. The first insulating film3may be a single-layer film or a multilayer film. The thickness of the first insulating film3is not limited and is, for example, 0.1 to 3.0 μm. The shape of the first insulating film3is obtained by, for example, etching (patterning). The first insulating film3includes a first base portion30, a first peripheral portion31, and first beam portions32ato32d.

The first base portion30is located inside an opening edge of the cavity20(an inner edge of the top surface21shown inFIG.5A) and is disposed along an opening portion23of the cavity20. A gap is locally formed between the first base portion30and the opening edge of the cavity20. The shape of the first base portion30is square in plan view, but may be circular, elliptical, oblong, another polygon, or the like.

The first peripheral portion31is disposed on the top surface21(FIG.5A) of the substrate2. The shape of the first peripheral portion31corresponds to the shape of the top surface21and has a quadrangular ring shape in plan view. The first peripheral portion31is disposed outside the first base portion30so as to surround the first base portion30.

The beam (bridge) portions32ato32dare located between the first base portion30and the first peripheral portion31and bridge the first base portion30and the first peripheral portion31. The beam portions32ato32dare arranged along the opening portion23of the cavity20. The beam portions32ato32dare connected to the four corners of the first base portion30and extend while being bent. However, the positions and shapes of the beam portions32ato32dare not limited to the positions and shapes shown inFIG.5B. The beam portions32ato32dsupport the first base portion30so that the first base portion30is disposed in the opening portion23. The number of beam portions32ato32dis four, but the number of beam portions32ato32dis not limited to this.

As shown inFIG.2, the heating part4is formed on the first insulating film3. The heating part4is for increasing the temperature of the detector8to a predetermined temperature (operating temperature). The heating part4has a film structure and is manufactured by a known film forming method. The shape of the heating part4is obtained by, for example, a lift-off method. Note that, in the lift-off method, a predetermined pattern is formed on a patterning surface as follows. First, a resist is applied to the patterning surface. Next, the resist is exposed and developed. Next, a film made of a pattern material is formed by sputtering, vapor deposition, etc. And then, an unnecessary part of the film and the resist are peeled off.

The heating part4is made of, for example, a conductive material having a comparatively high melting point. The material constituting the heating part4is not limited and is, for example, molybdenum, platinum, nickel, chromium, tungsten, tantalum, palladium, iridium, or an alloy containing one or more of these elements. Among them, platinum, which has a high corrosion resistance, is preferable. When the heating part4is made of platinum, an adhesion layer made of titanium, etc. may be formed between the first insulating film3and the heating part4. As shown inFIG.5C, the heating part4includes a heat generation portion40, heater leading portions41aand41b, and heater terminal portions42aand42b.

The heat generation portion40has a meander pattern and is disposed on the first base portion30(FIG.2) of the first insulating film3. Since the heat generation portion40has a meandering pattern, the detector8can be heated uniformly. Since the gas sensor1has an air bridge structure, the electric power consumption of the heating part4can be reduced at the time of heating the detector8.

The heater leading portion41ais connected to one end of the heat generation portion40, and the heater leading portion41bis connected to the other end of the heat generation portion40. The heater leading portion41ais led out to the first peripheral portion31via the beam portion32aon the first insulating film3(FIG.5B). The heater leading portion41bis led out to the first peripheral portion31via the beam portion32don the first insulating film3(FIG.5B).

The heater terminal portion42ais connected to the heater leading portion41a, and the heater terminal portion42bis connected to the heater leading portion41b. The heater terminal portions42aand42bare arranged on the first peripheral portion31(FIG.5B) of the first insulating film3. The shape of each of the heater terminal portions42aand42bis oblong in plan view, but may be circular, elliptical, square, another polygon, or the like. The heater terminal portions42aand42bare terminals for supplying electric power to the heat generation portion40.

As shown inFIG.2, the second insulating film5is laminated on the first insulating film3. For more detail, the second insulating film5is disposed on the first insulating film3so that the heating part4(FIG.5C) is interposed between the first insulating film3and the second insulating film5. The material and manufacturing method of the second insulating film5are the same as the material and manufacturing of the first insulating film3, but may be different from the material and manufacturing of the first insulating film3. Also, the second insulating film5may be a single-layer film or a multilayer film. As shown inFIG.5D, the second insulating film5includes a second base portion50, a second peripheral portion51, second beam portions52ato52d, and second hole portions53aand53b.

The second base portion50has the same structure as the first base portion30(FIG.5B) and is disposed on the first base portion30. The heat generation portion40(FIG.5C) is disposed between the first base portion30and the second base portion50. The second peripheral portion51has the same structure as the first peripheral portion31(FIG.5B) and is disposed on the first peripheral portion31.

The second beam portions52ato52dhave the same structure as the first beam portions32ato32d(FIG.5B) and are arranged on the first beam portions32ato32d. The heater leading portion41a(FIG.5C) is disposed between the first beam portion32aand the second beam portion52a, and the heater leading portion41bis disposed between the first beam portion32dand the second beam portion52d. The second hole portions53aand53bpenetrate the second peripheral portion51and are formed at positions corresponding with the heater terminal portions42aand42b(FIG.5C), respectively.

As shown inFIG.4, a lower electrode layer60is formed on the second insulating film5(second base portion50, etc.). The lower electrode layer60constitutes the electrode6together with an upper electrode layer61described below. The electrode6is for outputting a change in the physical characteristics (resistance value) of the detector8as an electrical signal. The lower electrode layer60has a film structure and is manufactured by a known film forming method.

The lower electrode layer60(the same applies to the upper electrode layer61) is made of, for example, a conductive material having a comparatively high melting point). The material constituting the lower electrode layer60(the same applies to the upper electrode layer61) is not limited and is, for example, molybdenum, platinum, gold, tungsten, tantalum, palladium, iridium, or an alloy containing one or more of these elements. As shown inFIG.5E, the lower electrode layer60includes an inner electrode portion62, an inner leading portion64, an inner terminal portion66, and lower conductive paths68aand68b.

The inner electrode portion62is disposed on the second base portion50(FIG.5D). The detailed structure of the inner electrode portion62is described below. The inner leading portion64is connected to the inner electrode portion62and led out from the second base portion50to the second peripheral portion51via the beam portion52b(FIG.5D).

The inner terminal portion66is connected to the inner leading portion64and is disposed on the second peripheral portion51(FIG.5D). The shape of the inner terminal portion66is oblong in plan view, but may be circular, elliptical, square, another polygon, or the like.

The lower conductive path68ahas the same shape as the heater terminal portion42a(FIG.5C) and is disposed on the heater terminal portion42avia the second hole portion53a(FIG.5D). The lower conductive path68ais electrically and physically connected to the heater terminal portion42a. The lower conductive path68bhas the same shape as the heater terminal portion42b(FIG.5C) and is disposed on the heater terminal portion42bvia the second hole portion53b(FIG.5D). The lower conductive path68bis electrically and physically connected to the heater terminal portion42b.

As shown inFIG.2, the third insulating film7is laminated on the second insulating film5. For more detail, the third insulating film7is disposed on the second insulating film5so that the lower electrode layer60is interposed between the second insulating film5and the third insulating film7. The material and manufacturing method of the third insulating film7are the same as the material and manufacturing method of the second insulating film5, but may be different from the material and manufacturing method of the second insulating film5. The third insulating film7may be a single-layer film or a multilayer film. When the first insulating film3, the second insulating film5, and the third insulating film7are made of the same material, the adhesion at each interface of the insulating films is improved, and the mechanical strength of each insulating film is ensured. When at least one of the first insulating film3, the second insulating film5, and the third insulating film7is made of a different material, the lamination of the different material reduces film stress and can form a stable laminated body of the insulating films. As shown inFIG.5F, the third insulating film7includes a third base portion70, a third peripheral portion71, third beam portions72ato72d, third hole portions73ato73c, and a base hole portion74.

The third base portion70has the same structure as the second base portion50(FIG.5D) and is disposed on the second base portion50. The inner leading portion64(FIG.5E) is disposed between the second base portion50and the third base portion70. The third peripheral portion71has the same structure as the second peripheral portion51(FIG.5D) and is disposed on the second peripheral portion51.

The third beam portions72ato72dhave the same structure as the second beam portions52ato52d(FIG.5D) and are arranged on the second beam portions52ato52d. The inner leading portion64(FIG.5E) is disposed between the second beam portion52band the third beam portion72b. The third hole portions73ato73cpenetrate the third peripheral portion71. The third hole portion73ais formed at a position corresponding to the lower conductive path68a(FIG.5E), the third hole portion73bis formed at a position corresponding with the lower conductive path68b(FIG.5E), and the third hole portion73cis formed at a position corresponding with the inner terminal portion66(FIG.5E).

The base hole portion74is formed in a central part of the third base portion70and penetrates the third base portion70. The base hole portion74has the same shape as the inner electrode portion62(FIG.5E) and is circular in plan view. The diameter of the base hole portion74is smaller than the diameter of the inner electrode portion62, but may be equal to the diameter of the inner electrode portion62. At least a part of the inner electrode portion62(FIG.5E) is in contact with the lower surface of the detector8(FIG.5G) via the base hole portion74.

The shape of the opening edge of the base hole portion74is circular in plan view. InFIG.5F, the opening edge of the base hole portion74is illustrated as a perfect circle in a plan view, but may have a shape slightly distorted from a perfect circle. The center position of the base hole portion74corresponds with the center position of the inner electrode portion62. However, the center position of the base hole portion74and the center position of the inner electrode portion62may be different from each other.

As shown inFIG.2, the detector8is formed on the third insulating film7(third base portion70). The detector8of the present embodiment is a thermistor film, which changes heat dissipation characteristics according to the gas concentration in the atmosphere, etc. and changes resistance values according to the change in the heat dissipation characteristics. The detector8is manufactured by a known film-forming method. The material constituting the thermistor film is not limited and is, for example, composite metal oxide, amorphous silicon, polysilicon, germanium, or the like.

As shown inFIG.5G, the detector8has a shape corresponding with the third base portion70(FIG.5F). The shape of the detector8is square in plan view, but may be circular, oval, oblong, or another polygon. A plurality (four in the example shown inFIG.5G) of curved portions is formed on the outer edge of the detector8, but the curved portions may be omitted. The size of the detector8is not limited as long as the outer edge of the detector8is located outside the outer electrode portion63(FIG.5H).

As shown inFIG.4, the upper electrode layer61is formed on the detector8, etc. The upper electrode layer61constitutes the electrode6together with the lower electrode layer60described above. The upper electrode layer61has a film structure and is manufactured by a known film forming method. As shown inFIG.5H, the upper electrode layer61includes an outer electrode portion63, an outer leading portion65, an outer terminal portion67, and upper conductive paths69a,69b, and69c.

The outer electrode portion63is disposed on the detector8(FIG.5G). The detailed structure of the outer electrode portion63is described below. The outer leading portion65is connected to the outer electrode portion63. The outer leading portion65is led out from the detector8(FIG.5G) and the third base portion70(FIG.5F) to the third peripheral portion71via the beam portion72c(FIG.5F).

The outer terminal portion67is connected to the outer leading portion65and is disposed on the third peripheral portion71. The shape of the outer terminal portion67is oblong in plan view, but may be circular, oval, square, another polygonal shape, or the like.

The upper conductive path69ahas the same shape as the lower conductive path68a(FIG.5E) and is disposed on the lower conductive path68avia the third hole73a(FIG.5F). The upper conductive path69ais electrically and physically connected to the lower conductive path68a. The upper conductive path69bhas the same shape as the lower conductive path68b(FIG.5E) and is disposed on the lower conductive path68bvia the third hole portion73b(FIG.5F). The upper conductive path69bis electrically and physically connected to the lower conductive path68b. The upper conductive path69chas the same shape as the inner terminal portion66(FIG.5E) and is disposed on the inner terminal portion66via the third hole portion73c(FIG.5F). The upper conductive path69cis electrically and physically connected to the inner terminal portion66.

The terminals10ato10dshown inFIG.5Iare formed by a method of, for example, plating, lift-off, metal paste printing, or the like. The material constituting the terminals10ato10dis not limited and is, for example, gold, silver, platinum, aluminum, or the like. The terminals10ato10chave the same shape as the upper conductive paths69ato69c(FIG.5H), respectively, and are arranged on the upper conductive paths69ato69c. The terminals10ato10care electrically and physically connected to the upper conductive paths69ato69c, respectively. The terminal10dhas the same shape as the outer terminal portion67(FIG.5H) and is disposed on the outer terminal portion67. The terminal10dis electrically and physically connected to the outer terminal portion67.

The terminals10ato10dare each electrically connected to an external circuit (not shown) by, for example, wire bonding. For example, the external circuit supplies electric power via the terminals10aand10b. This electric power is transmitted to the heat generation portion40(FIG.5C) via the upper conductive paths69aand69b(FIG.5H), the lower conductive paths68aand68b(FIG.5E), and the heater terminal portions42aand42b(FIG.5C). Also, the external circuit also supplies electric power via the terminals10cand10d. This electric power is supplied to the inner electrode portion62via the upper conductive path69c(FIG.5H) and the inner terminal portion66(FIG.5E) and is also supplied to the outer electrode portion63via the outer terminal section67(FIG.5H). Thus, the external circuit obtains an electrical signal reflecting a resistance change of the detector8.

As shown inFIG.4, the catalyst9is disposed in a central part of the detector8so as to be in contact with the detector8. The catalyst9covers the detector8and the upper electrode layer61(a part of the outer electrode portion63and the outer leading portion65). When a detection target gas (combustible gas) comes into contact with the catalyst9, a combustion heat is generated due to the catalytic reaction. The detector8detects a temperature change due to this combustion heat by its resistance change. The catalyst9has a convex shape (dome shape) projecting upward, but the shape of the catalyst9is not limited to this. The shape of the catalyst9is circular in plan view, but may be oval, quadrilateral, another polygon, or the like.

The catalyst9is formed by paste application, heat treatment, or the like. The material constituting the catalyst9is not limited and is, for example, a material in which noble metal particles are supported on a carrier. Examples of the carrier include oxide materials such as aluminum oxide (gamma alumina, etc.) and silicon oxide. Examples of the noble metal particles supported on the carrier include noble metal particles of platinum, palladium, ruthenium, rhodium, or the like.

Hereinafter, the structure of a gas detection part12is described while referring to the detailed structure of the inner electrode portion62and the outer electrode portion63described above. As shown inFIG.3, the gas detection part12is disposed on an insulating film (in the present embodiment, the second base portion50of the second insulating film5shown inFIG.4) and includes the detector8, the inner electrode portion62, and the outer electrode portion63. The inner electrode portion62and the outer electrode portion63are in contact with the detector8. As described above, the inner electrode portion62is disposed under the detector8, and the outer electrode portion63is disposed above the detector8. Also, the inner electrode portion62is in contact with the lower surface of the detector8, and the outer electrode portion63is in contact with the upper surface of the detector8.

The gas detection part12is formed in a central part of the second base portion50(FIG.4). InFIG.3, the region of the gas detection part12is indicated by dots. The outer edge (outer circumference) of the gas detection part12is along the inner edge (inner circumference) of the outer electrode portion63. That is, the gas detection part12is a region surrounded by the inner edge of the outer electrode portion63and is located inside the inner edge of the outer electrode portion63. However, the region of the gas detection part12may include a position on the inner edge of the outer electrode portion63. The detector8is exposed in the gas detection part12. In this case, however, “exposed” means that the detector8is not covered with the insulating film or the electrode6.

The gas detection part12is located above the heat generation portion40of the heating part4and located inside the outer edge of the heat generation portion40. Also, the gas detection part12is located under the catalyst9and is covered with the catalyst9. The catalyst9covers a wider area than the gas detection part12, and the gas detection part12is located inside the outer edge of the catalyst9in plan view. The catalyst9covers the detector8, the outer electrode portion63, etc. The shape of the outer edge of the gas detection part12is similar to the shape of the outer edge of the catalyst9and is circular in plan view. However, the shape of the outer edge of the gas detection part12may be different from the shape of the outer edge of the catalyst9.

In the gas detection part12, the detector8is sandwiched between the inner electrode portion62and the outer electrode portion63. The inner electrode portion62and the outer electrode portion63do not face each other along the Z-axis. Thus, the detector8is sandwiched between the inner electrode portion62and the outer electrode portion63in a direction inclined to the Z-axis. Instead, the detector8is sandwiched between the inner electrode portion62and the outer electrode portion63in a planar direction (a direction parallel to the XY plane) in plan view.

The shape of the outer electrode portion63is annular (circular or ring-shaped) in plan view. As shown inFIG.3andFIG.5H, both of the inner edge (inner circumference) and the outer edge (outer circumference) of the outer electrode portion63are circular in plan view, and the inner edge and the outer edge of the outer electrode portion63are arranged concentrically. Thus, a length L1along the radial direction between the inner edge and the outer edge of the outer electrode portion63is constant at any position along the circumferential direction of the outer electrode portion63. InFIG.3, both of the inner edge and the outer edge of the outer electrode portion63are illustrated as a perfect circle in plan view, but may have a shape slightly distorted from a perfect circle.

The length L1along the radial direction between the inner edge and the outer edge of the outer electrode portion63is smaller than a length L2of the outer leading portion65in a direction perpendicular to its extending direction, but may be equal to or larger than L2.

As described above, since the shape of the outer electrode portion63is annular in plan view, the shape of the outer edge of the gas detection part12defined by the inner edge of the outer electrode portion63is circular in plan view. The outer edge of the gas detection part12is illustrated as a perfect circle in plan view, but may have a shape slightly distorted from a perfect circle.

The shape of the inner electrode portion62is circular (disc-like) in plan view. InFIG.3, the inner electrode portion62is illustrated as a perfect circle in plan view, but may have a shape slightly distorted from a perfect circle. The inner electrode portion62is disposed inside the inner edge of the outer electrode portion63and is separated from the outer electrode portion63in the radial direction. The outer electrode portion63is disposed so as to surround the inner electrode portion62. As shown inFIG.5F, the base hole portion74is formed in a central part of the third base portion70. Thus, as shown inFIG.4, at least a part of the inner electrode portion62disposed under the third base portion70is exposed from the third base portion70via the base hole portion74. In this case, however, “exposed” means that at least a part of the inner electrode portion62disposed under the third base portion70is not covered with an insulating film. The distance between the outer edge of the part of the inner electrode portion62exposed from the third base portion70via the base hole portion74and the inner edge of the outer electrode portion63corresponds to an electrode width between the inner electrode portion62and the outer electrode portion63.

As shown inFIG.4, the inner electrode portion62includes an exposed portion62aexposed from the third base portion70via the base hole portion74and a non-exposed portion62bcovered with the third base portion70. The exposed portion62ais in contact with the detector8, whereas the non-exposed portion62bis not in contact with the detector8. The non-exposed portion62bis located outside the exposed portion62ain the radial direction (the outer edge of the inner electrode portion62). Both of the exposed portion62aand the non-exposed portion62bare surrounded by the outer electrode portion63from the outside in the radial direction. Note that, the outer edge of the inner electrode portion62does not need to be covered with the third base portion70. In this case, the inner electrode portion62is not provided with the non-exposed portion62band includes only the exposed portion62a.

As shown inFIG.3, a diameter D1of the inner electrode portion62is smaller than a diameter D2of the inner edge of the outer electrode portion63. However, the diameter D1of the inner electrode portion62may be larger than the diameter D2of the inner edge of the outer electrode portion63. Alternatively, the diameter D1of the inner electrode portion62may be larger than a diameter D3of the outer edge of the outer electrode portion63. The same applies to the size relation between the diameter of the exposed portion62aof the inner electrode portion62and the diameter D2of the inner edge of the outer electrode portion63. The length along the radial direction between the outer edge of the base hole portion74formed on the inner electrode portion62(the outer edge of the exposed portion62a) and the inner edge of the outer electrode portion63is constant at any position along the circumferential direction of the base hole portion74.

The diameter D1of the inner electrode portion62is larger than a length L3of the inner leading portion64in a direction perpendicular to its extending direction. The same applies to the diameter of the exposed portion62a. A distance (inter-electrode distance) LA along the radial direction between the outer edge of the exposed portion62aand the inner edge (inner circumference) of the outer electrode portion63is constant at any position along the circumferential direction of the inner electrode portion62. For example, as shown inFIG.4, in a cross section parallel to the XZ plane of the gas sensor1, L4aand L4bare substantially equal to each other. La is an inter-electrode distance along the X-axis between a position P1aof the outer edge of the exposed portion62aand a position P2aof the inner edge of the inner edge of the outer electrode portion63. L4bis an inter-electrode distance along the X-axis between a position P1bof the outer edge of the exposed portion62aand a position P2bof the inner edge of the outer electrode portion63. Note that, “substantially” means that an error of ±1% is allowed. Also, as shown inFIG.3, the distance LA along the radial direction between the outer edge of the exposed portion62aand the inner edge of the outer electrode portion63is not limited and is larger than the length L1along the radial direction between the inner edge and the outer edge of the outer electrode portion63.

In plan view, the inner electrode portion62and the outer electrode portion63are arranged so as not to overlap with each other. In particular, in plan view, the exposed portion62aand the outer electrode portion63are arranged so as not to overlap with each other. Also, a circle defined by the outer edge62cof the inner electrode portion62, a circle defined by the outer edge of the exposed portion62a, a circle defined by the inner edge of the outer electrode portion63, and a circle defined by the outer edge of the outer electrode portion63are arranged concentrically. That is, the center position of the inner electrode portion62, the center position of the exposed portion62a, and the center position of the outer electrode portion63correspond with each other. However, the center position of the inner electrode portion62, the center position of the exposed portion62a, and the center position of the outer electrode portion63may be different from each other.

In plan view, the outer edge62cof the inner electrode portion62is opposed to the inner edge of the outer electrode portion63in the radial direction. For more detail, the outer edge62cof the inner electrode portion62is opposed to the inner edge of the outer electrode portion63in the radial direction in all directions along the entire circumference of the inner electrode portion62. No matter which direction the electrode6is viewed along the entire circumference of the outer electrode portion63, the electrode pair consisting of the inner electrode portion62and the outer electrode portion63has the same shape.

In the present embodiment, the outer electrode portion63with an annular shape can surround the inner electrode portion62(exposed portion62a) in all directions. Thus, a resistance change of the detector8can be output as an electric signal in all directions by the inner electrode portion62(exposed portion62a) and the outer electrode portion63surrounding the inner electrode portion62. This makes it possible to effectively reduce the directional dependence of the detection sensitivity of the gas detection part12and significantly improve the detection accuracy and responsiveness of the gas sensor1.

As shown inFIG.5EtoFIG.5G, the third base portion70of the third insulating film7is formed on the inner electrode portion62, and the detector8is formed on the third base portion70. Here, the base hole portion74is formed in a central part of the third base portion70. Thus, as shown inFIG.4, the detector8is in direct contact with the inner electrode portion62(exposed portion62a) via the base hole portion74. The diameter of the base hole portion74is substantially equal to the diameter D1of the inner electrode portion62, and the substantially entire upper surface of the inner electrode portion62is in contact with the lower surface of the detector8. Note that, “substantially” means that an error of ±1% is allowed. The peripheral portion of the base hole portion74is stacked on the outer edge of the inner electrode portion62so as to cover the outer edge of the inner electrode portion62, and the non-exposed portion62bis formed. Thus, in the non-exposed portion62b, the outer edge of the inner electrode portion62and the peripheral portion of the base hole portion74overlap with each other. However, the outer edge of the inner electrode portion62and the peripheral portion of the base hole portion74may be arranged next to each other so as not to overlap with each other.

As shown inFIG.3andFIG.4, the detector8is exposed on the inner side (i.e., the gas detection part12) of the inner edge of the outer electrode portion63. In this case, however, “exposed” means that the detector8is not covered with the insulating film or the electrode6. As described above, the inner electrode portion62, which is circular in plan view, is disposed under the detector8. Thus, the upper surface of the detector8is not covered with the inner electrode portion62on the inner side of the inner edge of the outer electrode portion63. Since the surface (upper surface) of the detector8is exposed, it is possible to ensure a large detection range of gas concentration by the gas detection part12. Thus, it is possible to further improve the detection accuracy and responsiveness of the gas sensor1.

In the gas detection part12, the detector8includes an inner sensing portion80located above the exposed portion62aand an outer sensing portion81located between the outer edge of the exposed portion62aand the inner edge of the outer electrode portion63. As shown inFIG.4, the inner sensing portion80is disposed on the second insulating film5(second base portion50) via the exposed portion62a. On the other hand, the outer sensing portion81is disposed directly on the third insulating film7(third base portion70). Most of the inner sensing portion80is located above the outer sensing portion81. However, the positional relation between the inner sensing portion80and the outer sensing portion81is not limited to this. For example, when the thickness of the third base portion70is larger than the thickness of the inner electrode portion62, the upper surface of the outer sensing portion81may be disposed above the upper surface of the inner sensing portion80.

As shown inFIG.3andFIG.4, the shape of the inner sensing portion80is defined by the exposed portion62aand is circular in plan view. The shape of the outer sensing portion81is defined by the region between the exposed portion62aand the outer electrode portion63and is annular (circular or ring-shaped) in plan view.

As shown inFIG.4, a part of the third base portion70(the peripheral portion of the base hole portion74) extends from the second base portion50to the outer edge of the inner electrode portion62so as to cover the outer edge of the inner electrode portion62. Also, a part of the outer sensing portion81covers the peripheral portion of the base hole portion74. Thus, a step portion82is locally formed in the outer sensing portion81. Note that, the thickness of the insulating films (third insulating film7, etc.) or the detector8may be adjusted so that the step portion82is not formed in the outer sensing portion81.

The height position of the lower surface of the outer electrode portion63(the part that does not overlap with the inner leading portion64) is equal to the height position of the upper surface of the inner electrode portion62(substantially flush). However, the height position of the lower surface of the outer electrode portion63may be higher or lower than the height position of the upper surface of the inner electrode portion62.

As shown inFIG.3andFIG.4, a part of the outer electrode portion63intersects with the inner leading portion64. Thus, the height position at the intersection between the outer electrode portion63and the inner leading portion64is higher than the height position of other parts of the outer electrode portion63. At the intersection between the outer electrode portion63and the inner leading portion64, the outer electrode portion63and the inner leading portion64are insulated by the third base portion70.

A part of the inner leading portion64is led out from the inner electrode portion62to one side along the X-axis. Also, a part of the outer leading portion65is led out from the outer electrode portion63to the other side along the X-axis. That is, the leading direction of the inner leading portion64and the leading direction of the outer leading portion65are opposite to each other with respect to the X-axis direction. However, these leading directions may be changed as appropriate. Also, another part of the inner leading portion64and another part of the outer leading portion65are led out in the same direction along the Y-axis. However, these leading directions may be changed as appropriate.

An area S1of the upper surface or the lower surface of the outer electrode portion63is different from an area S2of the upper surface or the lower surface of the inner electrode portion62(moreover, the exposed portion62a). In the present embodiment, the area S1of the outer electrode portion63is smaller than the area S2of the inner electrode portion62(moreover, the exposed portion62a). However, the area S1of the outer electrode portion63may be equal to the area S2of the inner electrode portion62(moreover, the exposed portion62a). Alternatively, the area S1of the outer electrode portion63may be smaller than the area S2of the inner electrode portion62(moreover, the exposed portion62a).

Next, a method of manufacturing a gas sensor1is described. First, a base body (a substrate2in which a cavity20is not formed) as the basis of a base material2shown inFIG.5Ais prepared. Next, each film body shown inFIG.5BtoFIG.5Iis sequentially formed in this order on the upper surface of the base body by a known film forming method. The shape of each film body shown inFIG.5BtoFIG.5Iis formed by, for example, a lift-off method. Next, the bottom surface of the base body is etched until a first base portion30(FIG.5B) of a first insulating film3is exposed, and a cavity20is formed. As a result, a substrate2including the cavity20shown inFIG.5Ais formed. Next, as shown inFIG.3andFIG.4, a catalyst9shown inFIG.5Jis applied on a detector8and an outer electrode portion63so that at least a gas detection portion12is covered. Accordingly, the gas sensor1can be manufactured.

In the present embodiment, as shown inFIG.3andFIG.4, the electrode6includes the inner electrode portion62and the outer electrode portion63disposed so as to surround the inner electrode portion62(particularly, the exposed portion62a). Since the inner electrode portion62is surrounded by the outer electrode portion63, a physical characteristic change of the detector8(in the present embodiment, a resistance change of the detector8) can be uniformly output as an electrical signal from any direction along the circumference of the electrode6by the inner electrode portion62and the outer electrode portion63surrounding the inner electrode portion62. This makes it possible to reduce the directional dependence of the detection sensitivity of the gas detection part12and to detect the gas concentration with high accuracy regardless of the gas concentration distribution or the gas flow direction. In addition, when a thermal fluctuation (thermal distribution) is generated in the gas detection part12, the gas concentration can be detected with high accuracy without being affected by the thermal fluctuation. Thus, it is possible to obtain a gas sensor1with excellent detection accuracy and responsiveness.

Also, the inner electrode portion62(particularly, the exposed portion62a) has a circular shape in plan view. Since the inner electrode portion62with a circular shape is surrounded by the outer electrode portion63, it is possible to obtain a structure in which a change in the resistance of the detector8is uniformly output as an electrical signal from any direction along the circumference of the inner electrode portion62by the inner electrode portion62and the outer electrode portion63surrounding the inner electrode portion62. In particular, since the inner electrode portion62with a circular shape is surrounded by the outer electrode portion63with an annular shape, a resistance change of the detector8can be output as an electrical signal in all directions along the circumference of the inner electrode portion62by the inner electrode portion62and the outer electrode portion63surrounding the inner electrode portion62. This makes is possible to further reduce the directional dependence of the detection sensitivity of the gas detection part12and to further improve the detection accuracy and responsiveness of the gas sensor1.

Also, at least a part of the detector8is located between the inner electrode portion62(particularly, the exposed portion62a) and the outer electrode portion63in plan view. Also, the inner electrode portion62(particularly, the exposed portion62a) is in contact with the lower surface of the detector8, and the outer electrode portion63is in contact with the upper surface of the detector8. Thus, the inner electrode portion62and the outer electrode portion63are arranged so as to sandwich the detector8. This makes it possible to output a resistance change of the detector8as an electrical signal not only in the plane direction but also in the film thickness direction of the detector8(between the upper surface and the lower surface of the detector8) by the inner electrode portion62and the outer electrode portion63. Thus, it is possible to further reduce the directional dependence of the detection sensitivity of the gas detection part12and to further improve the detection accuracy and responsiveness of the gas sensor1.

Second Embodiment

Except for the following matters, a gas sensor1A of Second Embodiment shown inFIG.6AandFIG.6Bhas the same structures as the gas sensor1of First Embodiment. Portions overlapping with those of the gas sensor1of First Embodiment are provided with the same reference numerals and are not described in detail.

As shown inFIG.6AandFIG.6B, the gas sensor1A is not provided with the catalyst9, but a detector (gas sensitive body)8A is made of a semiconductor (metal oxide semiconductor) film. The material constituting the semiconductor film is not limited and is, for example, tin oxide, zirconium oxide, iron oxide, tungsten oxide, indium oxide, cobalt oxide, or the like. Other structures are similar to those of the gas sensor1of First Embodiment.

Also in the present embodiment, effects similar to those in First Embodiment can be obtained. Moreover, in the present embodiment, since the detector8A is made of a semiconductor film, the gas sensor1A with a high accuracy and a high reliability can be obtained, particularly in a low concentration region.

Third Embodiment

Except for the following matters, a gas sensor1B of Third Embodiment shown inFIG.7AandFIG.7Bhas the same structures as the gas sensor1A of Second Embodiment. Portions overlapping with those of the gas sensor1A of Second Embodiment are provided with the same reference numerals and are not described in detail.

As shown inFIG.7B, the gas sensor1B includes a detector (gas sensitive body)8B. The detector8B is made of the above-mentioned semiconductor (metal oxide semiconductor) material. The detector8B is applied on at least the inner electrode portion62, the outer electrode portion63, and the third base portion70so as to cover them. A part of the detector8B also extends to a region outside the outer electrode portion63in the radial direction, and a part of the outer leading portion65is also covered with the detector8B. However, the region to form the detector8B is not limited to the region shown inFIG.7AandFIG.7B.

The upper electrode layer61is formed on the third base portion70, and the upper electrode layer61and the third base portion70are in contact with each other. The detector8B has a convex shape (dome shape) projecting upward, but the shape of the detector8B is not limited to this. The shape of the detector8B is circular in plan view, but may be oval, quadrangular, another polygon, or the like.

Also in the present embodiment, effects similar to those in Second Embodiment can be obtained. That is, since the detector8B is made of an applied semiconductor material, the gas sensor1B with a high accuracy and a high reliability can be obtained, particularly in a low concentration region.

Fourth Embodiment

Except for the following matters, a gas sensor1C of Fourth Embodiment shown inFIG.8AandFIG.8Bhas the same structures as the gas sensor1of First Embodiment. Portions overlapping with those of the gas sensor1of First Embodiment are provided with the same reference numerals and are not described in detail.

As shown inFIG.8A, the gas sensor1C includes an electrode6C. The electrode6C includes an outer electrode portion63C. The shape of the outer electrode portion63C is an annular shape (C-shape) having a disconnected part in the circumferential direction in plan view. The outer electrode portion63C extends along the outer edge (outer circumference) of the inner electrode portion62and surrounds a part of the inner electrode portion62. The outer electrode portion63C is curved (rotated) by 270 degrees or more along the outer edge of the inner electrode portion62. As long as the outer electrode portion63C is curved (rotated) by at least 180 degrees along the outer edge of the inner electrode portion62, however, the degree of curvature (rotation) of the outer electrode portion63C is not limited.

The radius of curvature of the outer electrode portion63C is larger than the radius of curvature of the inner electrode portion62, and the center position of the outer electrode portion63C corresponds with the center position of the inner electrode portion62. However, the center of the outer electrode portion63C may be displaced from the center of the inner electrode portion62. A length L1along the radial direction between the inner edge and the outer edge of the outer electrode portion63C is constant at any position along the circumferential direction of the outer electrode portion63C.

The inner electrode portion62and the outer electrode portion63C are separated from each other along the radial direction. A distance LA along the radial direction between the inner electrode portion62and the outer electrode portion63C is constant at any position along the circumferential direction of the inner electrode portion62. In the present embodiment, L4>L1is satisfied, but similarly to First Embodiment, this size relation may be changed as appropriate.

One end and the other end of the outer electrode portion63C in its circumferential direction are separated from each other, and a gap G is formed between one end and the other end of the outer electrode portion63C in its circumferential direction. The inner leading portion64is led out from the inner electrode portion62along the X-axis so as to pass through the gap G. Thus, the outer electrode portion63C does not intersect with the inner leading portion64.

A distance L5aalong the Y-axis between the inner leading portion64and one end of the outer electrode portion63C in its extending direction (circumferential direction) is substantially equal to the distance LA along the radial direction between the outer edge of the inner electrode portion62and the inner edge of the outer electrode portion63C. Also, a distance L5balong the Y-axis between the inner leading portion64and the other end of the outer electrode portion63C in its extending direction (circumferential direction) is substantially equal to the distance LA along the radial direction between the outer edge of the inner electrode portion62and the inner edge of the outer electrode portion63C. Note that, “substantially” means that an error of ±1% is allowed.

As shown inFIG.8B, both of the inner electrode portion62and the outer electrode portion63C are formed on the second base portion50of the second insulating film5. That is, the inner electrode portion62and the outer electrode portion63C are located on the same plane.

Also, both of the inner electrode portion62and the outer electrode portion63C are covered with the detector8and are in contact with the lower surface of the detector8. That is, neither the inner electrode portion62nor the outer electrode portion63C is disposed on the upper surface of the detector8. Thus, the detector8is entirely exposed. In this case, however, “exposed” means that the detector8is not covered with the insulating film or the electrode6C.

A gas detection part12C is formed between the outer edge of the inner electrode portion62and the inner edge of the outer electrode portion63C and extends along the inner edge of the outer electrode portion63C. The shape of the gas detection part12C corresponds with the shape of the outer electrode portion63C and has an annular shape (C shape) having a disconnected part in the circumferential direction in plan view. The gas detection part12C is not formed at a position corresponding to the gap G.

Also in the present embodiment, effects similar to those in First Embodiment can be obtained. Moreover, in the present embodiment, the outer electrode portion63C has an annular shape having a disconnected part in the circumferential direction in plan view. That is, the inner leading portion64connected to the inner electrode portion62and the outer electrode portion63C can be arranged on the same plane (on the second base portion50) without crossing each other, and the manufacturing process of the electrode6C can be simplified.

Fifth Embodiment

Except for the following matters, a gas sensor1D of Fifth Embodiment shown inFIG.9AandFIG.9Bhas the same structures as the gas sensor1of First Embodiment. Portions overlapping with those of the gas sensor1of First Embodiment are provided with the same reference numerals and are not described in detail.

As shown inFIG.9A, the gas sensor1D includes an electrode6D. The electrode6D includes an inner electrode portion62D. As is clear from comparison betweenFIG.9AandFIG.3, the area of the inner electrode portion62D (moreover, the exposed portion62a) is larger than the area of the inner electrode portion62of First Embodiment. The outer edge (outer circumference) of the inner electrode portion62D is located outside the outer edge (outer circumference) of the outer electrode portion63. Thus, the outer electrode portion63is located inside the inner electrode portion62D, and a part of the inner electrode portion62D (the outer edge of the exposed portion62a) and at least a part of the outer electrode portion63overlap with each other in plan view.

As shown inFIG.9B, a part of the detector8is located between the outer electrode portion63and the exposed portion62aand is sandwiched between the outer electrode portion63and the exposed portion62ain the Z-axis direction (in the film thickness direction of the detector8). The gas detection part12D is formed by the outer electrode portion63, the outer edge of the exposed portion62a, and the detector8sandwiched between the outer electrode portion63and the outer edge of the exposed portion62a. That is, the gas detection part12D is located between the outer electrode portion63and the outer edge of the exposed portion62ain the Z-axis direction and is located at a portion where the outer electrode portion63and the exposed portion62aoverlap with each other in plan view.

In the present embodiment, a resistance change of the detector8in its film thickness direction (between the top surface and the bottom surface of the detector8) can be output as an electrical signal by the inner electrode portion62D (particularly, the exposed portion62a) and the outer electrode portion63. Thus, it is possible to further reduce the directional dependence of the detection sensitivity of the gas detection part12D and to improve the detection accuracy and responsiveness of the gas sensor1D.

Note that, the present disclosure is not limited to the above-mentioned embodiments and may variously be modified within the scope of the present disclosure. For example, in First Embodiment described above, the inner electrode portion62is circular in plan view as shown inFIG.3, but may be oval, quadrilateral, another polygon, or the like. The same applies to Second Embodiment to Fifth Embodiment described above.

In First Embodiment described above, as shown inFIG.3, the outer electrode portion63is preferably annular in plan view so that the electrode width between the outer electrode portion63and the inner electrode portion62is a constant distance from any direction, but may be elliptical ring, quadrangular ring, or another polygonal ring. The same applies to Second Embodiment, Third Embodiment, and Fifth Embodiment.

In First Embodiment described above, as shown inFIG.3, the outer electrode portion63extends continuously along its circumferential direction, but may extend intermittently. That is, a part of the outer electrode portion63in its circumferential direction may be disconnected at one location. Also, the outer electrode portion63may have a shape in which the width (the width along the radial direction between the inner edge and the outer edge of the outer electrode portion63) varies along the circumferential direction of the outer electrode portion63. Also, the outer edge and/or inner edge of the outer electrode portion63may have an undulating shape. The same applies to Second Embodiment to Fifth Embodiment described above.

In Fourth Embodiment described above, both of the inner electrode portion62and the outer electrode portion63C are arranged under the detector8. Similarly to First Embodiment, however, the outer electrode portion63C may be disposed on the detector8.

The technique shown in Fourth Embodiment (the technique in which the outer electrode portion63C is formed into an annular shape having a disconnected part in the circumferential direction in plan view) may be applied to Second Embodiment and Third Embodiment described above. Also, the technique shown in Fifth Embodiment (the technique in which the inner electrode portion62D and the outer electrode portion63overlap with each other in plan view) may be applied to Second Embodiment described above.

In First Embodiment described above, the detector8is made of a thermistor film or a semiconductor material, but may be made of a Pt wire or the like. The same applies to Second Embodiment to Fifth Embodiment described above.

Examples

Hereinafter, the present disclosure is described based on more detailed examples, but the present disclosure is not limited to these examples.

Example

Samples of a gas sensor1shown inFIG.1andFIG.2were manufactured using the following materials. Note that, the diameter of a catalyst9in plan view was 150 μm, and the thickness of the catalyst9was 20 μm.

For each of the above-mentioned samples, a response waveform in CO gas detection was obtained. The results are shown inFIG.10.FIG.10shows a concentration change of CO gas supplied to the gas sensor1and the response waveform. At a temperature of a gas detection part12of 300° C., a treatment was performed three times in which the concentration of CO gas was changed to 100 ppm, 300 ppm, and 500 ppm every 300 seconds.

In the first treatment, the concentration of CO gas was changed from 0 ppm to 100 ppm at an elapsed time of 1200 seconds, the concentration of CO gas was changed from 100 ppm to 300 ppm at an elapsed time of 1500 seconds, and the concentration of CO gas was changed from 300 ppm to 500 ppm at an elapsed time of 1800 seconds. In the second treatment, the concentration of CO gas was changed from 0 ppm to 100 ppm at an elapsed time of 2400 seconds, the concentration of CO gas was changed from 100 ppm to 300 ppm at an elapsed time of 2700 seconds, and the concentration of CO gas was changed from 300 ppm to 500 ppm at an elapsed time of 3000 seconds. In the third treatment, the concentration of CO gas was changed from 0 ppm to 100 ppm at an elapsed time of 3600 seconds, the concentration of CO gas was changed from 100 ppm to 300 ppm at an elapsed time of 3900 seconds, and the concentration of CO gas was changed from 300 ppm to 500 ppm at an elapsed time of 4200 seconds.

Comparative Example

Samples of Comparative Example were prepared using the same materials as in Example. In the samples of Comparative Example, for example, as shown inFIG.5of JP6917843 (B2), a pair of electrode portions having an oblong shape in plan view was prepared. Then, the electrode portions were arranged so as to face each other along a direction parallel to each short side of the pair of electrode portions. For each of the above-mentioned samples, a response waveform in CO gas detection was obtained in the same manner as in Example. The results are shown inFIG.11.

As is clear from comparison betweenFIG.10andFIG.11, it was confirmed that the variation in the baseline shown by the broken line inFIG.10was reduced in Example compared to the variation in the baseline shown by the broken line inFIG.11in Comparative Example. It was also confirmed that, in Example, the time required for the detection value to stabilize was shorter, and the saturation response was stable, compared to Comparative Example. It was also confirmed that, the waveform at low concentrations (e.g., 100 ppm) was stable in Examples compared to Comparative Example. These trends were confirmed in all three measurements, and a small variation and a high reproducibility of the response waveform were confirmed.

In Example, the directional dependence of the detection sensitivity of the gas detection part12was reduced, and the gas concentration was detected with high accuracy regardless of the gas concentration distribution or the gas flow direction. In addition, the gas detection part12was less susceptible to thermal fluctuations (heat distribution), and gas concentration was detected with high accuracy. It is clear from the results shown inFIG.10that the above-mentioned effects were obtained in Example.

DESCRIPTION OF THE REFERENCE NUMERICAL