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 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 detection unit has a first narrow width part where the width of the detection unit in the facing direction of the counter electrodes is partially reduced.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, a gas sensor according to a first aspect of the present invention comprises: a substrate having a cavity; and a detection unit supported by a beam 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, The detection section has a first narrow portion where the width of the detection section in the opposing direction of the opposing electrodes is partially narrowed.

[0008] In the gas sensor according to the first aspect having the above characteristics, even if the detection part is deformed by heating by the heater, the first narrow portion can relieve the stress, thereby suppressing fluctuations in the inter-electrode distance (the gap between the pair of opposing electrodes) and reducing measurement errors due to element deformation.

[0009] Preferably, the beam portions are integrally connected to the four corners of the detection unit, and the first narrow portion is located at the center between two adjacent beam portions in the extension direction of the counter electrode. By locating the first narrow portion at the center between the beam portions where stress is likely to concentrate, fluctuations in the inter-electrode distance can be more effectively suppressed.

[0010] Preferably, the detecting section has a second narrow portion where the width in the extension direction of the counter electrodes is partially narrowed. By forming the second narrow portion along the extension direction of the counter electrodes together with the first narrow portion along the extension direction of the counter electrodes, fluctuations in the inter-electrode distance can be more effectively suppressed.

[0011] When the detection unit has a first narrow portion and a second narrow portion, The maximum width of the detection unit in the facing direction is Wx0, The maximum width of the detection unit in the extension direction is Wy0, a minimum width of the first narrow portion in the opposing direction is Wx1; The minimum width of the second narrow portion in the extending direction is defined as Wy1. Preferably, a width reduction rate RD1 at the first narrow portion, expressed by (Wx0-Wx1) / Wx0, is greater than a width reduction rate RD2 at the second narrow portion, expressed by (Wy0-Wy1) / Wy0. By increasing the width reduction rate RD1 of the first narrow portion along the opposing direction of the opposing electrodes, it is possible to more effectively suppress fluctuations in the inter-electrode distance.

[0012] A gas sensor according to a second aspect of the present invention comprises: a substrate having a cavity; and a detection unit supported by a beam 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, The detection unit has a first notch in a part of a first edge along the extension direction of the counter electrode.

[0013] In the gas sensor according to the second aspect having the above features, even if the detection portion is deformed by heating by the heater, the first notch can relieve stress, thereby suppressing fluctuations in the inter-electrode distance (the gap between the pair of opposing electrodes) and reducing measurement errors due to element deformation.

[0014] Preferably, the beams are integrally connected to the four corners of the detection unit, and the first cutouts are located at the center between two adjacent beams across the first edge. By locating the first cutouts at the center between the beams where stress is likely to concentrate, fluctuations in the inter-electrode distance can be more effectively suppressed.

[0015] Preferably, the detection unit has a second notch in a part of a second edge along the opposing direction of the counter electrodes. By forming notches not only on the first edge along the extension direction of the counter electrodes but also on the second edge along the opposing direction of the counter electrodes, fluctuations in the inter-electrode distance can be more effectively suppressed.

[0016] When the detection unit has a first cutout portion and a second cutout portion, The opposing direction is a first axis, and the extending direction is a second axis, Preferably, a maximum width B1x of the first cutout portion in the direction of the first axis is greater than a maximum width B2y of the second cutout portion in the direction of the second axis. By making the dimensions of the first cutout formed on the first edge larger than those of the second cutout, fluctuations in the inter-electrode distance can be more effectively suppressed.

[0017] The gas sensors according to the first and second aspects of the present invention can be used as thermal conduction, catalytic combustion, semiconductor, or solid electrolyte gas sensors. When the thermal conduction 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]

[0018] [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. [Figure 5] FIG. 5 is a plan view showing a modified example of the gas sensor of the present invention. [Figure 6] FIG. 6 is a plan view showing a modified example of the gas sensor of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in detail below based on the embodiments shown in the drawings.

[0020] 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.

[0021] 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.

[0022] 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. Note that 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 referring to a "detection electrode 4," this refers to a feature common to the first detection electrode 4a and the second detection electrode 4b.

[0023] As shown in the plan view of FIG. 1B, the film stack 10 of the gas sensor 1a includes a detection section 12 that covers a portion of the upper part of the cavity 55, a peripheral section 14 that is in direct contact with the surface of the substrate 50, and four beam sections 16 that integrally connect the detection section 12 and the peripheral section 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 beam sections 16 that extend from the peripheral section 14 on the surface of the substrate. Note that the four beam sections 16 may be referred to separately as a first beam section 16a to a fourth beam section 16d. When simply referring to a "beam section 16," this refers to a feature common to the first beam section 16a to the fourth beam section 16d.

[0024] In this embodiment, the detection unit 12 has two first edges (12a, 12b) along the Y-axis direction and two second edges (12c, 12d) along the X-axis direction. The detection unit 12 has a partially constricted, approximately square shape in a plan view, and the maximum width Wx0 of the detection unit 12 in the X-axis direction is substantially equal to the maximum width Wy0 of the detection unit 12 in the Y-axis direction. In this embodiment, the first edge located between the first beam portion 16a and the second beam portion 16b is referred to as the firstA edge 12a, and the first edge located between the third beam portion 16c and the fourth beam portion 16d is referred to as the firstB edge 12b. In addition, the second edge located between the second beam portion 16b and the third beam portion 16c is referred to as the second C edge 12c, and the second edge located between the first beam portion 16a and the fourth beam portion 16d is referred to as the second D edge 12d.

[0025] All four beams 16 are connected to 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.

[0026] 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.

[0027] Furthermore, the beam portions 16 may be connected to the center of the edges (12a to 12d) of the detection unit 12, and more preferably extend along the extending direction of the opposing portions 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.

[0028] 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.

[0029] 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.

[0030] 1B, the heater 3 preferably has a meander pattern in which the wiring is folded back multiple times and arranged in parallel at predetermined intervals. The meander pattern of the heater 3 is layered on the detection unit 12 and serves to heat the detection unit 12 to its 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 to the peripheral edge 14, passing through the second beam 16b or the fourth beam 16d.

[0031] The pair of detection electrodes 4 are stacked on the same plane, and each has an opposing portion 41 (opposing electrode) and an extension 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 set appropriately depending on the desired characteristics. For example, the ratio (D0 / Wx0) of the inter-electrode distance D0 to the maximum width Wx0 of the detection unit 12 in the X axis direction can be 0.02 to 0.8, and preferably 0.02 to 0.3.

[0032] 1A to 1D of the first embodiment, as described above, the X axis is the opposing direction of the pair of 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 approximately perpendicular to each other. In the detection unit 12, the first edges (12a, 12b) are edges that extend along the extension direction of the opposing portions 41 and intersect with the opposing direction of the opposing portions 41. Meanwhile, the second edges (12c, 12d) are edges that extend along the opposing direction of the opposing portions 41 and intersect with the extension direction of the opposing portions 41.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 1D, the first beam portion 16a is laminated with the first insulating film 21 to the third insulating film 25, the lead portion 42 of the first detection electrode 4a, and the sensitive film 6. On the other hand, the second beam portion 16c is laminated with the first insulating film 21 to the third insulating film 25, the end wiring 32 of the heater 3, and the sensitive film 6. As described above, the sensitive film 6 is present across the detection portion 12 and each beam portion 16. However, it is sufficient that the sensitive film 6 is laminated at least on the detection portion 12, and the beam portion 16 does not necessarily include the sensitive film 6. Although a cross section passing through the third beam portion 16c and the fourth beam portion 16d is not shown, it is similar to the cross section shown in FIG. 1D, and therefore a description thereof will be omitted.

[0039] The dimensions of the beam portion 16, such as the width and length L, are not particularly limited, and the dimensions of the beam portion 16 may be determined so as to form an air bridge structure and ensure the mechanical strength of the detection unit 12.

[0040] In the gas sensor 1a of this embodiment, the detection section 12 has a first cutout 71 in a portion of the first edge (12a, 12b) along the extension direction of the facing section 41. This first cutout 71 is a region where a portion of the detection section 12 is cut away. In the plan view shown in FIG. 1B, the first cutout 71 has a roughly triangular, wedge-like shape in plan view. The shape of the first cutout 71 in plan view is not limited to the shape shown in FIG. 1B, and it may have a semicircular or semi-elliptical shape in plan view, with the edges of the cutout portion having an arc shape. However, it is preferable that the first cutout 71 has a predetermined size rather than a linear slit. This is because a linear slit may cause stress to concentrate in the slit portion.

[0041] Specifically, the first cutout portion 71 preferably has the following dimensions. First, the maximum width (maximum depth) B1x of the first cutout portion 71 in the X-axis direction may be any width that does not reach the opposing portion 41 of the detection electrode 4. For example, the ratio of B1x to the maximum width Wx0 of the detection portion 12 (B1x / Wx0) is preferably 0.05 to 0.15, and more preferably 0.05 to 0.1. Furthermore, if the maximum width of the first cutout portion 71 in the Y-axis direction is B1y, the ratio of B1y to the maximum width Wy0 of the detection portion 12 (B1y / Wy0) is preferably 0.05 to 0.5, and more preferably 0.15 to 0.2.

[0042] The first cutout 71 may be formed at a position offset from the center of the first edges (12a, 12b), but is preferably located at the center of the first edges (12a, 12b) as shown in Fig. 1B. In other words, the first cutout 71 of the first-A edge 12a is preferably located at the center between the first beam 16a and the second beam 16b, and the first cutout 71 of the first-B edge 12b is preferably located at the center between the third beam 16c and the fourth beam 16d.

[0043] The first cutout 71 may be formed on only one of the two first edges. However, as shown in FIG. 1B, it is preferable to form the first cutout 71 on both the first-A edge 12a and the first-B edge 12b, and it is preferable that the first cutout 71 on the first-A edge 12a side and the first cutout 71 on the first-B edge 12b side are arranged on the same straight line in the X-axis direction so as to form a pair. Furthermore, the first cutout 71 on the first-A edge 12a and the first cutout 71 on the first-B edge 12b may have different shapes and dimensions, but it is preferable that they have similar shapes and dimensions.

[0044] A first narrow width portion 70a is formed in the detection unit 12 by a first cutout portion 71. The first narrow width portion 70a is a region in the detection unit 12 where the width in the X-axis direction is partially narrowed, and is shown by gray shading in FIG. 1B. In other words, the first narrow width portion 70a is a region in which the detection unit 12 is partially constricted toward the inside in the X-axis direction. This first narrow width portion 70a is preferably located in the center of the detection unit 12 in the extension direction (Y-axis direction) of the facing portion 41, i.e., in the center between two beam portions 16 adjacent to each other in the extension direction of the facing portion 41.

[0045] The minimum width Wx1 of the first narrow portion 70a in the X-axis direction may be determined according to the set value of the inter-electrode distance D0. For example, the ratio (Wx1 / Wx0) of Wx1 to the maximum width Wx0 of the detection unit 12 is preferably 0.85 to 0.95, and more preferably 0.90 to 0.95. Meanwhile, the width of the first narrow portion 70a in the Y-axis direction corresponds to the maximum width B1y of the first cutout portion 71 in the Y-axis direction, and similarly to B1y, is preferably 0.05 to 0.5 times Wy0, and more preferably 0.15 to 0.2 times.

[0046] Next, the materials of the substrate 50 and the films included in the film stack portion 10 will be described in detail.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] Next, an example of a method for manufacturing the gas sensor 1a will be described.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] The first to third insulating films are also patterned by etching to form a shape having beam portions 16 and first notch portions 71 as shown in FIGS. 1A and 1B. After forming the film stack portion 10 through the above steps, the pad electrode 8 and the via-hole electrode 9 are formed by a known method such as lift-off. 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. Through the above method, the gas sensor 1a shown in FIGS. 1A to 1D is obtained.

[0059] 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.

[0060] (Summary of the first embodiment) The gas sensor 1a of this embodiment has an air bridge structure in which the detection unit 12 is supported above the cavity 55 by four beams 16 extending from the surface of the substrate 50. The detection unit 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 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 detection unit 12 has a first cutout portion 71 in part of a first edge (12a, 12b) along the extension direction of the opposing portions 41. The detection unit 12 also has a first narrow portion 70a where the width of the detection unit 12 in the opposing direction of the opposing portions 41 is partially narrowed.

[0061] In a conventional gas sensor 1b as shown in FIG. 2, when the detection section 12' is heated by a heater 3', the beams 16' block the thermal expansion of the detection section 12'. When the thermal expansion is suppressed, stress is applied between the beams 16' so that the detection section 12' is compressed, causing the detection section 12' to deform. This deformation of the detection section 12' increases the inter-electrode distance D0. In particular, in the gas sensor 1b, the inter-electrode distance D0 tends to increase more at the end portions of the opposing portions 41' than at the center, resulting in greater variation in the inter-electrode distance D0. The variation in the inter-electrode distance D0 due to the deformation of the detection section 12' changes the resistance between the electrodes (between the pair of opposing portions), resulting in errors in the gas concentration measurement results.

[0062] On the other hand, in the gas sensor 1a of this embodiment, by forming the first cutout portion 71 and / or the first narrow portion 70a in the detection part 12, it is possible to alleviate stress caused by thermal expansion of the detection part 12. As a result, it is possible to reduce changes in resistance value due to fluctuations or variations in the inter-electrode distance D0, and to suppress errors in the gas concentration measurement results.

[0063] Thermal stress acting on the detection unit 12 tends to concentrate in the central portion between the beam portions (i.e., the central portion of the edge). Therefore, by forming the first cutout portion 71 in the center of the first edge (12a, 12b), deformation of the detection unit 12 in the X-axis direction can be more effectively suppressed. Similarly, by forming the first narrow portion 70a in the center in the Y-axis direction, deformation of the detection unit 12 in the X-axis direction can be more effectively suppressed.

[0064] Second embodiment A gas sensor 1c according to the second embodiment will be described below with reference to Fig. 3. Note that the description of the components in the second embodiment that are common to those in the first embodiment will be omitted and the same reference numerals will be used.

[0065] 3, the detection section 12 of the gas sensor 1c has a first cutout portion 71 and a first narrow portion 70a, similar to the gas sensor 1a of the first embodiment. Therefore, the gas sensor 1c also has the same effects as the gas sensor 1a of the first embodiment.

[0066] The detection unit 12 of the gas sensor 1c has a second cutout 72 in a part of the second edge (12c, 12d) along the opposing direction of the opposing portion 41. Like the first cutout 71, the second cutout 72 is a region where a part of the detection unit 12 is cut out and missing, and can have a shape in a plan view such as a substantially triangular, semicircular, or semi-elliptical shape. By forming not only the cutout 71 in the first edge (12a, 12b) but also the cutout 72 in the second edge (12c, 12d), deformation of the detection unit 12 can be more effectively suppressed. In other words, fluctuations and variations in the inter-electrode distance D0 can be further reduced.

[0067] The second cutout 72 is preferably located at the center of the second edge (12c, 12d). That is, the second cutout 72 of the second C edge 12c is preferably located at the center between the second beam 16b and the third beam 16c, and the second cutout 72 of the second D edge 12d is preferably located at the center between the first beam 16a and the fourth beam 16d. By forming the second cutout 72 at the center of the second edge, deformation of the detection unit 12 can be more effectively suppressed.

[0068] If the maximum width (maximum depth) of the second cutout portion 72 in the Y-axis direction is B2y, then the ratio of B2y to the maximum width Wy0 of the detection portion 12 (B2y / Wy0) is preferably 0.03 to 0.12, and more preferably 0.03 to 0.06. On the other hand, if the maximum width of the second cutout portion 72 in the X-axis direction is B2x, then the ratio of B2x to the maximum width Wx0 of the detection portion 12 (B2x / Wx0) is preferably 0.05 to 0.5, and more preferably 0.15 to 0.2.

[0069] Furthermore, the second cutout portions 72 may have dimensions similar to those of the first cutout portions 71, but it is preferable that the first cutout portions 71 are larger than the second cutout portions 72. Specifically, the ratio of B1x to B2y (B1x / B2y) is preferably greater than 1.0 and less than or equal to 3.0, and more preferably greater than or equal to 2.0 and less than or equal to 3.0. Furthermore, the ratio of B1y to B2x (B1y / B2x) is preferably greater than 1.0 and less than or equal to 1.5, and more preferably greater than 1.0 and less than or equal to 1.2.

[0070] In this way, by making the dimensions of the first cutout portion 71 larger than those of the second cutout portion 72, deformation of the detection unit 12 in the X-axis direction tends to be particularly suppressed, and fluctuations and variations in the inter-electrode distance D0 can be more effectively suppressed.

[0071] The second cutout 72 may be formed on only one of the two second edges. However, as shown in Fig. 3, it is preferable to form the second cutout 72 on both the second C edge 12c and the second D edge 12d, and it is preferable that the second cutout 72 on the second C edge 12c side and the second cutout 72 on the second D edge 12d side are arranged on the same straight line in the Y-axis direction so as to form a pair. Furthermore, the second cutout 72 on the second C edge 12c and the second cutout 72 on the second D edge 12d may have different shapes and dimensions, but it is preferable that they have similar shapes and dimensions.

[0072] The detection unit 12 of the gas sensor 1c has a second narrow portion 70b formed by a second cutout 72. The second narrow portion 70b is a region in the detection unit 12 where the width in the Y-axis direction is partially narrowed. In FIG. 3, the gray shaded area extending along the Y-axis direction is the second narrow portion 70b. In other words, the second narrow portion 70b is a region in which the detection unit 12 is narrowed toward the inside in the Y-axis direction. By forming the second narrow portion 70b together with the first narrow portion 70a, deformation of the detection unit 12 can be more effectively suppressed. In other words, fluctuations and variations in the inter-electrode distance D0 can be further reduced.

[0073] The second narrow portion 70b is preferably located in the center of the detection unit 12 in the opposing direction (X-axis direction) of the opposing portion 41, that is, in the center between two beam portions 16 adjacent in the opposing direction of the opposing portion 41. By arranging the second narrow portion 70b in the center of the detection unit 12 in the X-axis direction, deformation of the detection unit 12 can be more effectively suppressed.

[0074] If the minimum width in the Y-axis direction of the second narrow portion 70b is Wy1, the ratio of Wy1 to the maximum width Wy0 of the detection unit 12 (Wy1 / Wy0) is preferably 0.88 to 0.97, and more preferably 0.94 to 0.97. On the other hand, the width in the X-axis direction of the second narrow portion 70b corresponds to the maximum width B2x in the X-axis direction of the second cutout portion 72, and, like B2x, is preferably 0.05 to 0.5 times Wx0, and more preferably 0.15 to 0.2 times.

[0075] Here, the width reduction rate at the first narrow portion 70a, expressed as (Wx0-Wx1) / Wx0, is defined as RD1, and the width reduction rate at the second narrow portion 70b, expressed as (Wy0-Wy1) / Wy0, is defined as RD2. When comparing the width reduction rates of the first narrow portion 70a and the second narrow portion 70b, it is preferable that RD1 of the first narrow portion 70a be greater than RD2 of the second narrow portion 70b. Specifically, the ratio of RD1 to RD2 (RD1 / RD2) is preferably greater than 1.0 and not greater than 8.1, and more preferably 3.0 or greater but not greater than 6.0.

[0076] In this way, by making the rate of width reduction in the first narrow portion 70a greater than that in the second narrow portion 70b, deformation of the detection unit 12 in the X-axis direction tends to be particularly suppressed, and fluctuations and variations in the inter-electrode distance D0 can be more effectively suppressed.

[0077] 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.

[0078] (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.

[0079] 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.

[0080] The catalyst section 80 can be formed from a porous carrier material that supports a catalyst material. Examples of the carrier material that can be used include oxide materials such as aluminum oxide (such as gamma alumina), titanium oxide, silicon oxide, and cerium oxide. Examples of the catalyst material that can be used include precious metals such as platinum (Pt), gold (Au), and palladium (Pd), as well as metal oxides such as rare earth element oxides and bismuth oxide. The size of the catalyst section 80 is preferably controlled so that the volume ratio of the catalyst section 80 to the detection section 12 is 10 or more and 40 or less. The catalyst section 80 can be formed by applying a raw material paste onto the detection section 12 by screen printing or dispensing using a dispenser, and then heat-treating the paste at a predetermined temperature.

[0081] The catalytic combustion gas sensor 1d can measure the concentration of combustible gases such as CO. During measurement, the heater 3 heats the detection unit 12 to a predetermined temperature. If combustible gas to be detected is present in the space in which the gas sensor 1d is placed, the combustible gas combines with oxygen and other gases on the catalyst unit 80, depending on the proportion of combustible gas present, and burns. The catalyst unit 80 promotes 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, which is composed of a thermistor film or the like, the resistance value of the sensitive film 6 changes. This change in resistance is converted into an electrical signal by the pair of detection electrodes 4, allowing the concentration of the combustible gas to be determined.

[0082] In the catalytic combustion type gas sensor 1d, the detection unit 12 has the first cutout portion 71 and / or the first narrow width portion 70a, which can suppress fluctuations in the inter-electrode distance and provide the same effects as in the first embodiment. Although not shown in Figures 4A and 4B, the catalytic combustion type gas sensor 1d may also have the second cutout portion 72 and the second narrow width portion 70b, which can more effectively suppress fluctuations in the inter-electrode distance and provide the same effects as in the second embodiment.

[0083] When the present invention is applied to a semiconductor gas sensor, the sensitive film 6 may be a semiconductor film of a metal oxide such as SnO2, ZnO, or In2O3. When the present invention is applied to a solid electrolyte gas sensor, the sensitive film 6 may be a solid electrolyte film made of yttria-stabilized zirconia (YSZ) or a lithium ion conductive solid electrolyte. In the case of a solid electrolyte gas sensor, one of the pair of sensing electrodes 4 may be a working electrode, and the other may be a reference electrode. The working electrode contains an electronically conductive material such as platinum, gold, palladium, or silver, and an auxiliary electrode material such as an alkali metal carbonate or an alkaline earth metal carbonate, and the reference electrode may be made of an electronically conductive material such as platinum, gold, palladium, or silver.

[0084] Furthermore, in the above-described embodiment, the detection unit 12 has a substantially square shape in plan view. However, the shape of the detection unit in plan view is not particularly limited and may be, for example, a substantially rectangular shape as shown in FIG. 5. In the gas sensor 1e shown in FIG. 5, the first cutout portion 71 is formed on an edge (first edge 12a, 12b) along the longitudinal direction of the detection unit 12. Furthermore, a portion of the detection unit 12 in the gas sensor 1e is narrowed toward the inside in the lateral direction, and a first narrow width portion 70a is present so that the width of the detection unit 12 in the lateral direction is partially narrowed. Even when the detection unit 12 has a rectangular shape in plan view, by forming the first cutout portion 71 and / or the first narrow width portion 70a, deformation of the detection unit 12 can be suppressed, and the same effect as in the first embodiment can be obtained.

[0085] 5, in the gas sensor 1e, second cutout portions 72 may be formed on the edges (second edges 12c, 12d) along the short direction of the detection section 12, and second narrow width portions 70b may be formed so as to partially narrow the longitudinal width of the detection section 12. By forming the second cutout portions 72 and / or the second narrow width portions 70b, deformation of the detection section 12 can be more effectively suppressed, and the same effect as in the second embodiment can be obtained.

[0086] Furthermore, when the detection unit 12 has a substantially rectangular shape in plan view, it is preferable that the extension direction of the facing portion 41 and the longitudinal direction of the detection unit 12 are substantially parallel to each other, as shown in gas sensor 1e in Fig. 5. In gas sensor 1e, deformation of the detection unit 12 is dominant in the Y-axis direction, and the detection unit 12 is less likely to deform in the X-axis direction. As a result, fluctuations and variations in the inter-electrode distance D0 can be more effectively suppressed, and measurement errors can be further reduced.

[0087] The planar shape of the detection unit may be circular, as shown in FIG. 6. In the gas sensor 1f of FIG. 6, the detection unit 13 has first cutout portions 71 at the first A edge 13a and the first B edge 13b, which are arc-shaped and extend along the direction in which the facing portion 41 extends. A portion of the detection unit 13 in the gas sensor 1f is constricted inward in the X-axis direction, and a first narrow portion 70a is present so that the radial width of the detection unit 13 is partially narrowed. Note that the "radial width" refers to the width passing through the center point of the circle, and the minimum radial width Wx1 of the first narrow portion 70a is smaller than the diameter or maximum width Wx0 of the detection unit 13. Even when the detection unit is circular, forming the first cutout portions 71 and / or the first narrow portion 70a can suppress deformation of the detection unit 13, thereby achieving the same effect as in the first embodiment.

[0088] 6, the detection section 13 of the gas sensor 1f may have second cutout portions 72 in the arc-shaped second C edge 13c and second D edge 13d along the opposing direction of the opposing section 41. The detection section 13 may also have second narrow width portions 70b formed along the extending direction of the opposing section 41 so that the radial width of the detection section 13 is partially narrowed. Even in the gas sensor 1f having a circular detection section 13, forming the second cutout portions 72 and / or the second narrow width portions 70b can more effectively suppress deformation of the detection section 13, thereby achieving the same effect as in the second embodiment.

[0089] The planar shape of the detection section may be an oval or other polygonal shape in addition to the square, rectangle, and circle mentioned above. [Explanation of symbols]

[0090] 1a~1f ... Gas sensors 10... Film stacking section 12, 13 ... Detection unit 12a,12b,13a,13b … 1st edge 12c, 12d, 13c, 13d ... Second edge 14...periphery 16,16a~16d … Beam part 70a … 1st narrow part 70b … 2nd narrow part 71 ... First notch 72 ... Second notch 3...Heater 32 … End wiring 4, 4a, 4b ... detection electrodes 41 ... opposing part (opposing electrode) 42 … Drawer part 6... Sensitive membrane 8... Pad electrode 9a, 9b ... via hole electrodes 21, 23, 25 ... insulating film 80... Catalyst section 50... PCB 55 … Cavity part

Claims

1. a substrate having a cavity; and a detection unit supported by a beam 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, The gas sensor includes a first narrow portion where the width of the detection portion in the opposing direction of the counter electrodes is partially narrowed.

2. The beam portion is integrally connected to each of the four corners of the detection portion, 2. The gas sensor according to claim 1, wherein the first narrow portion is located at a center between two of the beam portions adjacent to each other in the extending direction of the counter electrode.

3. 3. The gas sensor according to claim 1, wherein the detection portion has a second narrow portion where the width of the detection portion in the extending direction of the counter electrode is partially narrowed.

4. The maximum width of the detection unit in the facing direction is Wx0, The maximum width of the detection unit in the extension direction is Wy0, a minimum width of the first narrow portion in the opposing direction is Wx1; The minimum width of the second narrow portion in the extension direction is Wy1, 4. The gas sensor according to claim 3, wherein a width reduction rate RD1 at the first narrow portion, expressed by (Wx0-Wx1) / Wx0, is greater than a width reduction rate RD2 at the second narrow portion, expressed by (Wy0-Wy1) / Wy0.

5. a substrate having a cavity; and a detection unit supported by a beam 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, The gas sensor includes a first cutout portion in a part of a first edge of the detection portion along an extending direction of the counter electrode.

6. The beam portion is integrally connected to each of the four corners of the detection portion, 6. The gas sensor according to claim 5, wherein the first cutout portion is positioned at a center between two of the beam portions adjacent to each other across the first edge.

7. 7. The gas sensor according to claim 5, wherein the detection portion has a second notch portion in a part of a second edge along the opposing direction of the counter electrodes.

8. The opposing direction is a first axis, and the extending direction is a second axis, 8. The gas sensor according to claim 7, wherein a maximum width B1x of the first cutout portion in the direction of the first axis is larger than a maximum width B2y of the second cutout portion in the direction of the second axis.

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

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