Gas Sensor

Abstract

To provide a gas sensor that can reduce a measurement error associated with heat deformation of a device.SOLUTION: A gas sensor has a substrate that has a cavity part, and a detection unit that is supported by two or more beam parts extending from the substrate above the cavity part. The detection unit has a laminated structure including a heater, a sensitive membrane, and a pair of counter electrodes in contact with the sensitive membrane. The pair of counter electrodes extend along a direction orthogonal to the lamination direction, and face each other at a predetermined interval in an in-plane direction of the sensitive membrane. The maximum width Wy0 of the detection unit in the extension direction of the counter electrodes is larger than the maximum width Wx0 of the detection unit in the facing direction of the counter electrodes.SELECTED DRAWING: Figure 1B

Description

[Technical Field]

[0001] The present invention relates to a gas sensor having an air bridge structure. [Background technology]

[0002] Gas sensors with an air-bridge structure, in which a membrane-shaped sensing element is supported above a cavity in a substrate, are known. In these gas sensors, the sensing element is heated to its operating temperature by a heater resistor to measure the concentration of a target gas present in the atmosphere. If the membrane-shaped sensing element deforms during measurement due to heating, the inter-electrode distance (the distance between a pair of opposing electrodes) fluctuates with the element deformation, resulting in measurement errors. Therefore, it is necessary to suppress the fluctuation of the inter-electrode distance due to thermal stress.

[0003] For example, Patent Document 1 discloses suppressing warpage of the detection unit by controlling the internal stress of the detection unit. Specifically, Patent Document 1 offsets the internal stress of the detection unit by stacking a silicon oxide film with compressive stress and a silicon nitride film with tensile stress on the detection unit. However, with the technology of Patent Document 1, the balance of internal stresses is easily disrupted due to manufacturing errors, and sufficient deformation suppression effects may not be achieved. In addition, changing the design of the detection unit, such as dimensions such as film thickness, the material of the stacked film, or the electrode pattern, also changes the balance of internal stresses, making it difficult to make design changes.

[0004] Furthermore, Patent Document 2 discloses that deformation of the detection unit is suppressed by forming holes or slits that penetrate the front and back surfaces of the detection unit. However, the technology of Patent Document 2 requires that heater resistors and electrode patterns be stacked while avoiding the holes or slits, which results in a problem of low freedom of element design. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-264844 [Patent Document 2] Patent No. 3374498 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a gas sensor capable of reducing measurement errors caused by thermal deformation of the element. [Means for solving the problem]

[0007] In order to achieve the above object, the gas sensor according to the present invention comprises: a substrate having a cavity; and a detection unit supported by two or more beams extending from the substrate above the cavity; the detection unit has a laminated structure including a heater, a sensitive film, and a pair of opposing electrodes in contact with the sensitive film, the pair of opposing electrodes each extend in a direction perpendicular to the stacking direction and face each other at a predetermined interval in the in-plane direction of the sensitive film, A maximum width Wy0 of the detection portion in the extension direction of the counter electrode is wider than a maximum width Wx0 of the detection portion in the opposing direction of the counter electrodes.

[0008] In a gas sensor having the above characteristics, even if the detection part is deformed by heating from a heater, the variation in the inter-electrode distance (the distance between a pair of opposing electrodes) can be suppressed, thereby reducing measurement errors due to element deformation.

[0009] Preferably, the detection unit has a narrow portion in which the width of the detection unit in the opposing direction of the opposing electrodes is partially narrowed. By having the narrow portion in the detection unit, deformation of the detection unit can be alleviated, and fluctuations in the inter-electrode distance can be more effectively suppressed.

[0010] Furthermore, when the detection unit has a narrow width portion, the narrow width portion is preferably located at the center of the detection unit in the extension direction. Since stress generated by heating the detection unit tends to concentrate at the center of the detection unit, forming the narrow width portion at the center of the detection unit in the longitudinal direction can more effectively suppress fluctuations in the inter-electrode distance.

[0011] Preferably, the detection unit has a cutout portion in a part of the edge along the longitudinal direction of the detection unit. By having the cutout portion in the detection unit, deformation of the detection unit can be alleviated and fluctuations in the inter-electrode distance can be more effectively suppressed. Furthermore, when the detection unit has a cutout portion, the cutout portion is preferably located in the center of the edge. Since stress generated by heating the detection unit tends to concentrate in the center of the edge of the detection unit, forming the cutout portion in the center of the edge along the longitudinal direction can more effectively suppress fluctuations in the inter-electrode distance.

[0012] The gas sensor according to the present invention can be used as a thermal conduction type, catalytic combustion type, semiconductor type, or solid electrolyte type gas sensor. When the thermal conduction type or catalytic combustion type is adopted as the driving method of the gas sensor, the sensitive film is preferably a thermistor film. [Brief explanation of the drawings]

[0013] [Figure 1A] FIG. 1A is an exploded perspective view showing a gas sensor 1a according to one embodiment of the present invention. [Figure 1B] FIG. 1B is a plan view of the gas sensor 1a shown in FIG. 1A. [Figure 1C] FIG. 1C is a cross-sectional view taken along line IC-IC in FIG. 1B. [Figure 1D] FIG. 1D is a cross-sectional view taken along line ID-ID in FIG. 1B. [Figure 2] FIG. 2 is a plan view showing a conventional gas sensor 1b. [Figure 3] FIG. 3 is a plan view showing a gas sensor 1c according to another embodiment of the present invention. [Figure 4A]FIG. 4A is a plan view showing a modified example of the gas sensor of the present invention. [Figure 4B] FIG. 4B is a cross-sectional view taken along line IVB-IVB in FIG. 4A. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below based on the embodiments shown in the drawings.

[0015] First embodiment In the first embodiment, a thermal conduction type gas sensor 1a shown in FIGS. 1A to 1D will be described as an example of a gas sensor according to the present invention.

[0016] 1A, the gas sensor 1a includes a substrate 50 and a film stack 10 formed on the substrate 50. The substrate 50 of the gas sensor 1a has a cavity 55 penetrating the front and back surfaces of the substrate 50, and has an overall shape of a hollow quadrangular prism. The substrate 50 only needs to have a structure that allows for the formation of an air bridge structure, which will be described later, and the shape and dimensions of the substrate 50 are not limited to those shown in FIG. 1A.

[0017] On the other hand, the film stack 10 includes a first insulating film 21, a heater 3, a second insulating film 23, a pair of detection electrodes 4, a sensitive film 6, and a third insulating film 25. In the film stack 10, the above-described multiple constituent films are stacked in the order shown along the Z axis. Four pad electrodes 8 are formed on the upper surface of the film stack 10, and an external circuit (not shown) can be electrically connected to the heater 3 or the detection electrodes 4 via the pad electrodes 8. In this embodiment, the pair of detection electrodes 4 may be referred to as a first detection electrode 4a and a second detection electrode 4b. When simply referred to as a "detection electrode 4," this means that the feature is common to the first detection electrode 4a and the second detection electrode 4b.

[0018] 1B, the film stack 10 of the gas sensor 1a includes a detection section 12 that covers an upper portion of the cavity 55, a peripheral edge 14 that is in direct contact with the surface of the substrate 50, and four beams 16 that integrally connect the detection section 12 and the peripheral edge 14. That is, the gas sensor 1a has an air bridge structure in which the detection section 12 is supported above the cavity 55 by the four beams 16 that extend from the peripheral edge 14 on the surface of the substrate. Note that the four beams 16 may be referred to as first to fourth beams 16a to 16d.

[0019] In this embodiment, the detection unit 12 has four edges parallel to the X-axis or Y-axis and has a substantially rectangular shape in a plan view. The four beams 16 are all connected to the corners of the detection unit 12 and extend along diagonal lines of the detection unit 12. The first beam 16a and the third beam 16c form a pair and are arranged on the same diagonal line. The second beam 16b and the fourth beam 16d form a pair and are arranged on the same diagonal line.

[0020] The number and positions of the beams 16 are not limited to those shown in Fig. 1B. For example, at least two beams 16 are required, and the greater the number of beams 16, the greater the mechanical strength of the detection unit 12. However, if the number of beams 16 is large, heat from the detection unit 12 is more likely to be transmitted to the peripheral portion 14 via the beams 16. Therefore, the number of beams 16 is preferably two to four, and more preferably four.

[0021] Furthermore, the beam portions 16 may be connected to the center of the edge of the detection unit 12, and more preferably extend along the extending direction of the opposing portion 41 of the detection electrode 4, which will be described later. Furthermore, the beam portions 16 are preferably arranged in a balanced manner, taking into consideration the mechanical strength of the detection unit 12. For example, the multiple beam portions 16 are preferably arranged so as to be point-symmetric with respect to the center point of the detection unit 12 on the XY plane. Alternatively, the multiple beam portions 16 are preferably arranged at positions that are line-symmetric with respect to the center line of the detection unit 12 along the X-axis or Y-axis.

[0022] The cavity 55 in the gas sensor 1a has a square shape in plan view that is larger than the detection unit 12. However, the shape of the cavity 55 is not limited to the shape shown in Figures 1A and 1B as long as it can form an air bridge structure. The cavity 55 may have a rectangular, polygonal, circular, elliptical, or other shape in plan view.

[0023] 1B is an internal perspective view, and dashed lines indicate the heater 3 and the pair of detection electrodes 4 stacked on the detection unit 12. Specifically, the heater 3 is indicated by a thin dashed line, and the pair of detection electrodes 4 is indicated by a thick dashed line.

[0024] 1B, the heater 3 preferably has a meander pattern 31 in which wiring is folded back multiple times and arranged in parallel at predetermined intervals. The meander pattern 31 of the heater 3 is layered on the detection unit 12 and serves to heat the detection unit 12 to an operating temperature when detecting a target gas. By forming the wiring pattern of the heater 3 layered on the detection unit 12 as a meander pattern, the detection unit 12 can be heated efficiently and uniformly. The end wiring 32 of the heater 3 is drawn from the end of the meander pattern 31 to the peripheral edge 14, passing through the second beam 16b or the fourth beam 16d.

[0025] The pair of detection electrodes 4 are stacked on the same plane and each have an opposing portion 41 (opposing electrode) and an extraction portion 42. The opposing portion 41 of the first detection electrode 4a and the opposing portion 41 of the second detection electrode 4b both extend in a direction parallel to the Y axis. The opposing portion 41 of the first detection electrode 4a and the opposing portion 41 of the second detection electrode 4b face each other in the X axis direction with a predetermined inter-electrode distance D0. The inter-electrode distance D0 is not particularly limited and may be appropriately set depending on the desired characteristics. For example, the ratio of the inter-electrode distance D0 to the width of the detection unit 12 in the X axis direction can be 0.02 to 0.8, preferably 0.02 to 0.3. Note that in FIGS. 1A to 1D of the first embodiment, as described above, the X axis is the opposing direction of the opposing portions 41, the Y axis is the extension direction of the opposing portions 41, and the Z axis is the stacking direction of each film in the film stack unit 10. The X-axis, Y-axis, and Z-axis are substantially perpendicular to each other.

[0026] The lead-out portion 42 of the first detection electrode 4a is led out from one end of the facing portion 41 through the first beam portion 16a to the peripheral portion 14. Similar to the first detection electrode 4a, the lead-out portion 42 of the second detection electrode 4b is led out from one end of the facing portion 41 through the third beam portion 16c to the peripheral portion 14.

[0027] In the cross section shown in FIG. 1C , the detection unit 12 is not connected to the peripheral portion 14, but is spaced apart from the peripheral portion 14 and the substrate 50. By spaced apart from the peripheral portion 14 and above the cavity 55, the heat capacity of the detection unit 12 can be made smaller than the heat capacity of the peripheral portion 14, which is in contact with the substrate 50. Furthermore, the thermal insulation between the detection unit 12 and the peripheral portion 14 can be improved. Therefore, in the gas sensor 1a having the air bridge structure, when the detection unit 12 is heated by the heater 3, the detection unit 12 can be heated efficiently with little power consumption.

[0028] 1C, the heater 3 is laminated between the first insulating film 21 and the second insulating film 23 in the detection unit 12. That is, the heater 3 is covered by the second insulating film 23 so as not to be exposed to the external atmosphere. Furthermore, by laminating the second insulating film 23 between the heater 3 and the pair of detection electrodes 4, short-circuiting between the heater 3 and the pair of detection electrodes 4 is suppressed.

[0029] 1C, each of the pair of detection electrodes 4 is laminated on the second insulating film 23. A sensitive film 6 is laminated on the pair of detection electrodes 4, and the facing portions 41 are covered by the sensitive film 6. That is, the facing portions 41 of the first detection electrode 4a and the second detection electrode 4b are both on the same plane and directly contact the sensitive film 6. A third insulating film 25 is laminated on the top layer of the detection unit 12. That is, the sensitive film 6 is covered by the third insulating film 25 so that the pair of detection electrodes 4 and the sensitive film 6 are not exposed to the external atmosphere.

[0030] As shown in the cross section of FIG. 1D, a first insulating film 21, a second insulating film 23, and a third insulating film 25 are stacked in the peripheral portion 14. A pad electrode 8 is formed on the surface of the third insulating film 25 in the peripheral portion 14. A via-hole electrode 9a penetrating the third insulating film 25 is formed below the pad electrode 8 adjacent to the first beam portion 16a. An end of the lead-out portion 42 of the first detection electrode 4a is electrically connected to the pad electrode 8 through the via-hole electrode 9a. Meanwhile, a via-hole electrode 9b penetrating the second insulating film 23 and the third insulating film 25 is formed below the pad electrode 8 adjacent to the second beam portion 16b. An end of the end wiring 32 of the heater 3 is electrically connected to the pad electrode 8 through the via-hole electrode 9b.

[0031] In the first beam portion 16a, as shown in FIG. 1D, the first insulating film 21 to the third insulating film 25, the lead-out portion 42 of the first detection electrode 4a, and the sensitive film 6 are laminated. On the other hand, in the second beam portion 16c, the first insulating film 21 to the third insulating film 25, the end wiring 32 of the heater 3, and the sensitive film 6 are laminated. As described above, the sensitive film 6 exists across the respective beam portions 16 from the detection portion 12, but the sensitive film 6 only needs to be laminated at least on the detection portion 12, and the beam portions 16 may not include the sensitive film 6. Note that cross-sections passing through the third beam portion 16c and the fourth beam portion 16d are not shown, but are the same as the cross-section shown in FIG. 1D, and the description thereof is omitted.

[0032] Dimensions such as the width and length L of the beam portion 16 are not particularly limited, and the dimensions of the beam portion 16 may be determined so that an air bridge structure can be formed and the mechanical strength of the detection portion 12 can be ensured.

[0033] In the gas sensor 1a of the present embodiment, the longitudinal direction of the detection portion 12 is defined based on the extending direction of the detection electrode 4. Specifically, the maximum width of the detection portion 12 in the facing direction of the facing portion 41 is defined as Wx0, and the maximum width of the detection portion 12 in the extending direction of the facing portion 41 is defined as Wy0, and Wy0 is wider than Wx0, satisfying Wx0 < Wy0. That is, it is preferable that the longitudinal direction of the detection portion 12 is substantially parallel to the extending direction of the facing portion 41.

[0034] Here, "substantially parallel" means that the angle β formed by the extending direction of the facing portion 41 and the longitudinal direction of the detection portion 12 is ±10° or less. The angle β is preferably ±5° or less, and more preferably​​​​​​​The substrate 50 is not particularly limited as long as it has a mechanical strength sufficient to support the film stack portion 10 and is made of a material suitable for microfabrication such as etching. For example, the substrate 50 may be a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, a glass substrate, or the like.

[0037] The first insulating film 21, the second insulating film 23, and the third insulating film 25 may all be made of insulating materials, and are not particularly limited. For example, the first insulating film 21 to the third insulating film 25 may be made of silicon oxide or silicon nitride, with silicon oxide being preferred. The first insulating film 21 to the third insulating film 25 may be made of different materials, but are preferably made of the same material. By making the first insulating film 21 to the third insulating film 25 of the same material, adhesion between the layers is improved, and the mechanical strength of the detection unit 12 can be ensured.

[0038] The thickness of the first insulating film 21 is not particularly limited as long as it can sufficiently ensure insulation between the substrate 50 and the heater 3 and function as an etching stop layer when forming the cavity 55. For example, the thickness of the first insulating film 21 is preferably about 0.1 to 1.0 μm.

[0039] The thickness of the second insulating film 23 is not particularly limited as long as it is thick enough to reliably cover the heater 3 and ensure sufficient interlayer insulation. For example, the second insulating film 23 preferably has a thickness of about 0.06 to 1.2 μm. The thickness of the third insulating film 25 is not particularly limited as long as it is thick enough to reliably cover the sensitive film 6 and ensure sufficient interlayer insulation. For example, the third insulating film 25 preferably has a thickness of about 0.06 to 1.2 μm.

[0040] The heater 3 is preferably made of a material that is conductive and has a relatively high melting point. Examples of such materials include molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing one or more of the above elements. Among the above materials, platinum is suitable for high-precision dry etching such as ion milling and has high corrosion resistance, so it is preferable to make the heater 3 from platinum. When the heater 3 is made of platinum, it is preferable to form an adhesion layer such as titanium (Ti) between the first insulating film 21 and the platinum material to improve adhesion of the heater 3 to the first insulating film 21.

[0041] Preferably, the pair of detection electrodes 4 are both made of a material that is conductive and has a relatively high melting point. Like the heater 3, the pair of detection electrodes 4 can be made of molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing one or more of the above elements, and are preferably made of platinum.

[0042] The sensitive film 6 may be made of a material whose resistance changes with temperature. For example, the sensitive film 6 may be a thermistor film or a platinum film, with a thermistor film being preferred. Examples of materials for thermistor films include composite metal oxides, amorphous silicon, polysilicon, and germanium. The material and thickness of the sensitive film 6 are both factors that affect the initial resistance of the detection unit 12. Therefore, the material and thickness of the sensitive film 6 may be appropriately selected to achieve a desired initial resistance, taking into account the type of gas to be detected and the intended use of the gas sensor 1a.

[0043] Next, an example of a method for manufacturing the gas sensor 1a will be described.

[0044] First, a substrate 50 is prepared, and a first insulating film 21 is formed on one main surface of the substrate 50. The first insulating film 21 may be formed by thermal oxidation, CVD, or the like. Then, a thin film of the conductive material (conductive thin film) contained in the heater 3 is formed on the first insulating film 21. When the heater 3 has a layered structure made of multiple conductive materials (for example, a Ti / Pt layered structure), multiple conductive thin films may be stacked. The conductive thin film may be formed using a known film formation method. Examples of known film formation methods include sputtering, CVD, and PVD. After the conductive thin film is formed, the conductive thin film is patterned by etching to form the heater 3 having a meander pattern.

[0045] Next, the second insulating film 23 is formed so as to cover the heater 3. The second insulating film 23 may be formed by the same method as the first insulating film 21. Then, a thin film of a conductive material that will form the detection electrodes 4 is formed on the second insulating film 23. The conductive thin film is patterned by etching to form a pair of detection electrodes 4 having the pattern shown in FIG. 1B.

[0046] Next, the sensitive film 6 is formed so as to cover the pair of detection electrodes 4. The sensitive film 6 may be formed by a known film formation method depending on the material of the sensitive film 6. For example, if the sensitive film 6 is a thermistor film of a complex oxide, the sensitive film 6 may be formed by a sputtering method so as to have the composition of the complex oxide. After the sensitive film 6 is formed, it is subjected to a heat treatment at a predetermined temperature for a predetermined holding time, and is etched to have a predetermined shape. Thereafter, a third insulating layer 25 is formed so as to cover the sensitive film 6 by the same method as the first and second insulating films.

[0047] The first to third insulating films are also patterned by etching to form a shape having beam portions as shown in FIGS. 1A and 1B. After the film stack portion 10 is formed by the above steps, the pad electrode 8 and the via-hole electrode 9 are formed by a known method such as a lift-off method. Thereafter, a portion of the substrate 50 is removed by etching so that the substrate 50 remains only below the peripheral edge portion 14 in the Z-axis direction, thereby forming a cavity portion 55. The substrate 50 can be etched by dry etching such as Deep-RIE, anisotropic wet etching, or the like. By the above method, the gas sensor 1a shown in FIGS. 1A to 1D is obtained.

[0048] The gas sensor 1a of this embodiment can be used as a thermal conduction gas sensor. The thermal conduction gas sensor 1a detects gases such as CO2, H2, He, and CO, which have thermal conductivities different from those of air. To measure the concentration of a target gas in a given atmosphere, the detection unit 12 is heated to its operating temperature by the heater 3. When the target gas comes into contact with the detection unit 12, the heat dissipation characteristics of the detection unit 12 change in accordance with the concentration of the target gas in the atmosphere. When the temperature of the detection unit 12 changes due to the change in heat dissipation characteristics, the resistance value of the sensitive film 6, which is formed of a thermistor film or the like, changes. This change in resistance is converted into an electrical signal by the pair of detection electrodes 4, allowing the concentration of the target gas to be determined.

[0049] (Summary of the first embodiment) The gas sensor 1a of this embodiment has an air bridge structure in which the detection section 12 is supported above the cavity 55 by four beams 16 extending from the surface of the substrate 50. The detection section 12 includes a heater 3, a sensitive membrane 6, and a pair of opposing electrodes (opposing portions 41) in contact with the sensitive membrane. The opposing portions 41 each extend along the Y-axis direction and oppose each other in the X-axis direction with an inter-electrode distance D0. In the gas sensor 1a having such an air bridge structure, the maximum width Wy0 of the detection section 12 in the extension direction of the opposing portions 41 is wider than the maximum width Wx0 of the detection section 12 in the opposing direction of the opposing portions 41.

[0050] The gas sensor 1a has the above-described characteristics, which can suppress measurement errors caused by deformation of the detection part 12. This effect is thought to be related to the variation in the inter-electrode distance D0.

[0051] Conventionally, gas sensors with an air bridge structure have generally had a structure as shown in FIG. 2. Specifically, in the gas sensor 1b of FIG. 2, which corresponds to a comparative example, the detection section 12′ has a square shape in a plan view, satisfying Wx0 = Wy0. When the detection section 12′ is heated by the heater 3′, the thermal expansion of the detection section 12′ is blocked by the beams 16′. When the thermal expansion is suppressed, stress is applied between the beams 16′ so that the detection section 12′ is compressed, and the detection section 12′ is deformed. As the detection section 12 deforms, the inter-electrode distance D0 increases, which changes the resistance value between the electrodes (between a pair of opposing portions), resulting in an error in the gas concentration measurement result.

[0052] In conventional gas sensors, as shown in gas sensor 1b in Fig. 2, the shape of the detection section 12' has generally been square (or circular) to achieve a well-balanced arrangement of the heater pattern and beam portion 16', taking into consideration the homogeneity and mechanical strength of the detection section 12'. However, according to the results of verification by the present inventors, when the detection section 12' has a square shape in plan view, the displacement in the X-axis direction and the displacement in the Y-axis direction become equal, and the detection section 12' also deforms significantly in the opposing direction of the opposing portion 41'. Furthermore, the inter-electrode distance D0 in gas sensor 1b tends to be wider at the end side of the opposing portion 41' than at the center side, resulting in large variations in the inter-electrode distance D0.

[0053] On the other hand, in the gas sensor 1a of the present embodiment, the deformation of the detection unit 12 depends on the longitudinal direction of the detection unit 12 and becomes anisotropic. Specifically, in the Y-axis direction which is the longitudinal direction of the detection unit 12, the detection unit 12 is likely to deform, but in the X-axis direction which is the short-side direction of the detection unit 12, the detection unit 12 is difficult to deform. That is, in the gas sensor 1a of the present embodiment, by dominating the deformation of the detection unit 12 in the extending direction of the facing portion 41, the deformation of the detection unit 12 is suppressed in the facing direction of the facing portion 41. As a result, even if the detection unit 12 is deformed by thermal stress, it is possible to reduce the change and variation in the electrode-to-electrode distance D0 and the resulting change in the resistance value, and to suppress the occurrence of an error in the measurement result of the gas concentration.

[0054] In particular, by setting Wy0 / Wx0 in the detection unit 12 to preferably exceed 1.0 and be 2.0 or less, more preferably 1.2 or more and 1.8 or less, it is possible to more effectively suppress the fluctuation and variation of the electrode-to-electrode distance D0 while sufficiently ensuring the mechanical strength of the detection unit 12.

[0055] Second embodiment Hereinafter, the gas sensor 1c according to the second embodiment will be described based on FIG. 3. Regarding the configuration common to the first embodiment in the second embodiment, the description will be omitted and the same reference numerals will be used.

[0056] As shown in FIG. 3, the gas sensor 1c has an air bridge structure in which the detection unit 12 is supported by four beam portions 16. And, similar to the gas sensor 1a of the first embodiment, in the gas sensor 1c as well, the detection unit 12 has a shape satisfying Wx0 < Wy0, and the longitudinal direction of the detection unit 12 is substantially parallel to the extending direction of the facing portion 41.

[0057] In the second embodiment, the four edges of the detection unit 12 are distinguished and described as follows. Specifically, the edge between the first beam portion 16a and the second beam portion 16b is referred to as the first edge 12a, the edge between the third beam portion 16c and the fourth beam portion 16d is referred to as the second edge 12b, the edge between the second beam portion 16b and the third beam portion 16c is referred to as the third edge 12c, and the edge between the first beam portion 16a and the fourth beam portion 16d is referred to as the fourth edge 12d. The first edge 12a and the second edge 12b are parallel to the extension direction (Y-axis direction) of the facing portion 41, and the third edge 12c and the fourth edge 12d are parallel to the facing direction (X-axis direction) of the facing portion 41. Note that the first edge 12a and the second edge 12b are also edges that intersect with the facing direction of the facing portion 41.

[0058] The detection section 12 of the gas sensor 1c has a notch 71 in part of the edge (12a, 12b) along the extension direction of the facing section 41. Specifically, the notch 71 is formed in the center of the first edge 12a, and the notch 71 is also formed in the center of the second edge 12b. The notch 71 is a region where part of the detection section 12 is cut out and missing.

[0059] Although the cutout 71 may be formed at a position offset from the center of the edge, it is preferable that it be located at the center of the edge as shown in Fig. 3. The cutout 71 may be formed on only one of the first edge 12a and the second edge 12b. However, as shown in Fig. 3, it is preferable to form cutouts on both the first edge 12a and the second edge 12b, and it is preferable that the cutout 71 on the first edge side and the cutout 71 on the second edge side are arranged in pairs on the same straight line in the X-axis direction.

[0060] The detection section 12 of the gas sensor 1c has a narrow width section 70 formed by a cutout portion 71. The narrow width section 70 is a region in the detection section 12 where the width in the X-axis direction is partially narrowed, and is shown by gray shading in FIG. 3. In other words, the narrow width section 70 is a region in which the detection section 12 is partially constricted toward the inside in the X-axis direction. The narrow width section 70 is preferably located at the center of the detection section 12 in the extension direction (Y-axis direction) of the facing section 41.

[0061] Forming the cutout portion 71 and the narrow width portion 70 in the detection unit 12 can alleviate stress caused by thermal expansion of the detection unit 12, thereby more effectively suppressing fluctuations in the inter-electrode distance D0. Note that stress applied to the detection unit 12 tends to concentrate in the central portion between the beam portions (i.e., the central portion of the edge). Therefore, forming the cutout portion 71 in the center of the edge can more effectively suppress deformation of the detection unit 12 in the X-axis direction. Similarly, forming the narrow width portion 70 in the center in the Y-axis direction can more effectively suppress deformation of the detection unit 12 in the X-axis direction.

[0062] Furthermore, it is preferable that the cutout portion 71 has a predetermined size rather than being a linear slit. This is because a linear slit may warp and cause stress to concentrate in the slit portion. The maximum width Wx2 of the cutout portion 71 in the X-axis direction may be such that it does not reach the opposing portion 41 of the detection electrode 4. For example, the ratio Wx2 / Wx0 of Wx2 to the maximum width Wx0 of the detection portion 12 is preferably 0.05 to 0.15, and more preferably 0.05 to 0.1.

[0063] Furthermore, if the maximum width of cutout portion 71 in the Y-axis direction is Wy1, the ratio Wy1 / Wy0 of Wy1 to the maximum width Wy0 of detection unit 12 is preferably 0.05 to 0.5, and more preferably 0.15 to 0.2. Note that, although cutout portion 71 shown in FIG. 4 has a triangular, wedge-like shape in plan view, the shape of cutout portion 71 is not particularly limited. For example, cutout portion 71 may have a semicircular or semi-elliptical shape in plan view, with the edges of the cutout portion being arc-shaped.

[0064] The minimum width Wx1 of narrow portion 70 in the X-axis direction may be determined according to the set value of inter-electrode distance D0. For example, the ratio Wx1 / Wx0 of Wx1 to the maximum width Wx0 of detection unit 12 is preferably 0.85 to 0.95, and more preferably 0.9 to 0.95. The width of narrow portion 70 in the Y-axis direction may be set to the same value as the maximum width Wy1 of notch 71 in the Y-axis direction.

[0065] The cutout 71 in the first edge 12a and the cutout 71 in the second edge 12b may have different shapes and dimensions, but preferably have similar shapes and dimensions. Cutouts may also be formed in the third edge 12c and the fourth edge 12d.

[0066] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified in various ways within the scope of the present invention.

[0067] (Variation) For example, in the above embodiment, a thermal conduction type gas sensor has been described, but the gas sensor according to the present invention may be a catalytic combustion type, a semiconductor type, or a solid electrolyte type gas sensor.

[0068] 4A and 4B is an example of a catalytic combustion gas sensor. The gas sensor 1d has a film laminate 10 with an air bridge structure, and the configuration of each film (3, 4, 6, 21 to 23) included in the film laminate 10 can be the same as that of the gas sensor 1a of the first embodiment. In the gas sensor 1d, a catalyst section 80 is formed on the detection section 12 of the film laminate 10.

[0069] The catalyst part 80 can be composed of a porous carrier material carrying a catalyst material. As the carrier material, for example, oxide materials such as aluminum oxide (γ-alumina, etc.), titanium oxide, silicon oxide, and cerium oxide can be used. As the catalyst material, noble metals such as platinum (Pt), gold (Au), and palladium (Pd), or metal oxides such as rare earth element oxides and bismuth oxides can be used. It is preferable to control the size of the catalyst part 80 such that the volume ratio of the catalyst part 80 to the detection part 12 is 10 or more and 40 or less. Also, the catalyst part 80 can be formed by applying a raw material paste onto the detection part 12 by means such as screen printing or discharging using a dispenser, and then heat-treating at a predetermined temperature.

[0070] In the catalytic combustion type gas sensor 1d, the concentration of a combustible gas such as CO can be measured. At the time of measurement, the detection part 12 is heated to a predetermined temperature by the heater 3. At this time, if a combustible gas to be detected exists in the space where the gas sensor 1d is disposed, the combustible gas and oxygen, etc. combine and burn on the catalyst part 80 according to the existence ratio. At this time, the catalyst part 80 has the role of promoting the combination of the combustible gas and oxygen. When the combustion heat (reaction heat) generated by the combustion of the combustible gas is transmitted to the sensitive film 6 composed of a thermistor film, etc., the resistance value of the sensitive film 6 changes. By taking out this resistance value change as an electrical signal by a pair of detection electrodes 4, the concentration of the combustible gas can be determined.

[0071] Even in such a catalytic combustion type gas sensor 1d, by having the detection part 12 have a shape satisfying Wx0 < Wy0, the variation in the electrode distance can be suppressed, and the same effect as in the first embodiment can be obtained.

[0072] In addition, when applying the present invention to a semiconductor-type gas sensor, the sensing film 6 may be a semiconductor film of a metal oxide such as SnO2, ZnO, or In2O3. When applying the present invention to a solid electrolyte-type gas sensor, the sensing film 6 may be a solid electrolyte film composed of yttria-stabilized zirconia (YSZ), a lithium ion conductive solid electrolyte, or the like. In the case of a solid electrolyte-type gas sensor, one of the pair of detection electrodes 4 may be used as the working electrode and the other as the reference electrode. The working electrode contains an electron conduction material such as platinum, gold, palladium, or silver, and an auxiliary electrode substance such as an alkali metal carbonate or an alkaline earth metal carbonate. The reference electrode may be composed of an electron conduction material such as platinum, gold, palladium, or silver.

[0073] In the first and second embodiments, the detection unit 12 had a substantially rectangular shape in plan view. However, the detection unit 12 may have a shape satisfying Wx0 < Wy0, and the plan view shape of the detection unit 12 may be an elliptical shape or other polygonal shapes.

Explanation of Reference Numerals

[0074] 1a~1d... Gas sensor 10... Film stacking portion 12... Detection unit {12a~12d}... (Edges of the detection unit) 14... Peripheral portion 16, {16a~16d}... Beam portion 70... Narrow portion 71... Notch portion 3... Heater 31... Meander pattern 32... End wiring 4, {4a, 4b}... Detection electrode 41... Opposing portion (opposing electrode) 42... Lead-out portion 6... Sensing film 8... Pad electrode {9a, 9b}... Via hole electrode {21, 23, 25}... Insulating film 80... Catalyst portion 50... Substrate <000029l>55... Cavity portion

Claims

1. a substrate having a cavity; and a detection unit supported by two or more beams extending from the substrate above the cavity; the detection unit has a laminated structure including a heater, a sensitive film, and a pair of opposing electrodes in contact with the sensitive film, the pair of opposing electrodes each extend in a direction perpendicular to the stacking direction and face each other at a predetermined interval in the in-plane direction of the sensitive film, a maximum width Wy0 of the detection portion in the extension direction of the counter electrode is wider than a maximum width Wx0 of the detection portion in the opposing direction of the counter electrodes; The gas sensor wherein the detection portion has a narrow portion where the width of the detection portion in the facing direction is partially narrowed.

2. 2. The gas sensor according to claim 1, wherein the narrow width portion is located at a center of the detection portion in the extending direction.

3. A gas sensor as described in claim 1, wherein the detection portion has a cutout portion in a part of the edge along the longitudinal direction of the detection portion.

4. A device comprising: a substrate having a hollow portion; and a detection unit supported by two or more beam portions extending from the substrate above the hollow portion; the detection unit has a laminated structure including a heater, a sensitive film, and a pair of opposing electrodes in contact with the sensitive film, the pair of opposing electrodes each extend in a direction perpendicular to the stacking direction and face each other at a predetermined interval in the in-plane direction of the sensitive film, a maximum width Wy0 of the detection portion in the extension direction of the counter electrode is wider than a maximum width Wx0 of the detection portion in the opposing direction of the counter electrodes; A gas sensor in which the detection section has a notch in a part of an edge along a longitudinal direction of the detection section.

5. 5. The gas sensor according to claim 4, wherein the notch is located at the center of the edge.

6. 6. The gas sensor according to claim 1, wherein the sensitive film is a thermistor film.

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