Gas sensor and gas sensor device

Abstract

To provide a high-sensitivity and long-life gas sensor on the basis of a configuration of an entire detection element, especially a relationship between a catalyst and a sensor element.SOLUTION: A gas sensor includes: an element part including a membrane part, a heater part including a heater, and a heat detection part including a heat detection material and a heat detection electrode; and a catalyst part formed on the element part. In the gas sensor, the heater is sandwiched between the membrane part and a first coating film, the heat detection material is sandwiched between a second coating film and a third coating film, the total thickness of the thickness of the first coating film and the thickness of the second coating film, and the thickness of the third coating film are each within a range of 0.6 to 1.2 when the thickness of the membrane part is set to 1, and the thickness of the membrane part, the total thickness of the thickness of the first coating film and the thickness of the second coating film, and the thickness of the third coating film are each larger than the thickness of the heater and the thickness of the heat detection material.SELECTED DRAWING: Figure 1B

Description

[Technical Field]

[0001] The present invention relates to a gas sensor and a gas sensor device. [Background technology]

[0002] A gas sensor is a device that detects gases present in the atmosphere and converts information such as their type and concentration into an electrical signal for output. Such gas sensors are installed in home appliances, industrial equipment, environmental monitoring equipment, etc., and are used to detect gas leaks that could have an impact on humans and the environment.

[0003] There are various types of gas sensors known, differing in the type of gas to be detected, concentration range, accuracy, operating principle, constituent materials, etc. When the gas to be detected is a combustible gas, catalytic combustion type, semiconductor type, thermal conduction type, etc. are known gas sensors.

[0004] In recent years, interest in gas sensors has been growing from the perspectives of environmental protection and creating a comfortable living environment. In particular, gas sensors that are small, highly sensitive, have a long life, and consume little power are being developed for mobile applications such as smartphones.

[0005] For example, Patent Document 1 discloses a catalytic combustion type gas sensor in which a catalyst is applied to a diaphragm-type element. The detection element of this catalytic combustion type gas sensor uses a catalyst in which palladium is supported on a porous gamma-alumina carrier. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6574472 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, to achieve high sensitivity and long life in combustion contact gas sensors intended for mobile use, it is necessary not only to pursue catalyst performance but also to optimize the structure of the entire sensing element, including the catalyst.

[0008] However, Patent Document 1 lacks the perspective of optimizing the overall configuration of the detection element, and there is a problem in that the gas sensor described in Patent Document 1 cannot exhibit sufficient performance from the perspectives of high sensitivity and long life.

[0009] The present invention has been made in view of the above circumstances, and has as its object to provide a gas sensor that has high sensitivity and a long life based on the overall configuration of the detection element, particularly the relationship between the catalyst and the sensor element. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention is embodied as follows. [1] An element unit having a membrane unit, a heater unit including a heater, and a heat detection unit including a heat detection material and a heat detection electrode; a catalyst portion formed on an element portion, The heater is sandwiched between the membrane portion and the first covering film, The thermal sensing material is sandwiched between the second coating film and the third coating film, When the thickness of the membrane portion is taken as 1, the total thickness of the first coating film and the second coating film and the thickness of the third coating film are each within a range of 0.6 to 1.2; In this gas sensor, the thickness of the membrane portion, the total thickness of the first coating film and the second coating film, and the thickness of the third coating film are greater than the thickness of the heater and the thickness of the thermal detection material, respectively. [2] The heater portion includes a heater coating film, and the heat detection portion includes a heat detection material coating film; the first coating film and the second coating film are heater coating films; The gas sensor according to [1], wherein the third coating film is a thermal sensitive material coating film. [3] The gas sensor according to [1] or [2], wherein the heat detection part is a thermistor part. [4] The gas sensor according to any one of [1] to [3], wherein the ratio of the volume of the catalyst portion to the volume of the element portion is 10 or more and 40 or less. [5] The gas sensor according to any one of [1] to [4], wherein the element portion has an air bridge structure supported by a beam. [6] The gas sensor according to any one of [1] to [5], wherein the catalytic portion includes an oxide support material on which a noble metal is supported. [7] The gas sensor according to any one of [1] to [6]; a drive circuit for driving the heater; The driving circuit is a gas sensor device configured to make the input signal to the heater a pulse signal. [8] The gas sensor device according to [7], wherein the drive circuit is configured to control the maximum temperature of the heater to within a range of 250°C to 350°C. [Effects of the Invention]

[0011] According to the present invention, a gas sensor having high sensitivity and a long life can be provided based on the overall configuration of the detection element, particularly the relationship between the catalyst and the sensor element. [Brief explanation of the drawings]

[0012] [Figure 1A] FIG. 1A is a schematic plan view of a gas sensor according to one embodiment of the present invention. [Figure 1B] FIG. 1B is a schematic cross-sectional view of the gas sensor taken along line IB-IB in FIG. 1A. [Figure 1C] FIG. 1C is a schematic cross-sectional view of the gas sensor taken along line IC-IC in FIG. 1A. [Figure 2] FIG. 2 is a graph showing the relationship between the time elapsed after the start of current application and the sensor sensitivity for the gas sensor samples fabricated in the examples of the present invention. [Figure 3]FIG. 3 is a graph showing the relationship between the ratio of the volume of the catalyst portion to the volume of the element portion and the sensor sensitivity immediately after the start of energization for the gas sensor samples fabricated in the examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below based on specific embodiments in the following order. 1. Gas sensors 1.1. Overall configuration of the gas sensor 1.2. Circuit board 1.3.Heater section 1.4.Heat detection unit 1.5. Catalyst section 1.6.Operating Principle of Gas Sensor 2. Gas sensor manufacturing method 3.Gas sensor device 4. Variations

[0014] (1. Gas sensor) The gas sensor according to this embodiment is a catalytic combustion type gas sensor for detecting a combustible gas. The gas sensor according to this embodiment may also include a sensor element for temperature compensation, another gas sensor element for detecting a different type of gas, or the like.

[0015] (1.1. Overall configuration of gas sensor) The gas sensor according to this embodiment includes an element portion and a catalyst portion formed on the element portion. The element portion includes a membrane portion, a heater portion, and a heat detection portion, with the heater portion and heat detection portion formed on the membrane portion. The membrane portion is a thin film-like portion that supports the heater portion, the heat detection portion, and the catalyst portion. The membrane portion is typically obtained by thinning a portion of a substrate. As a result, the heat capacity of the membrane portion is smaller than that of an unthinned substrate, and heat conduction from the membrane portion to the unthinned substrate is also small. Therefore, the heater portion formed on the membrane portion can be efficiently heated to a high temperature with little power consumption.

[0016] An example of a gas sensor having such a membrane portion is the gas sensor 1 shown in Fig. 1A. In Fig. 1A, an element portion 10 has an air bridge structure supported by four beam portions 70 connected to a peripheral portion 80. That is, the element portion 10 is connected to the peripheral portion 80 of the gas sensor 1 via the beam portions 70, and a cavity portion 53 is formed between the element portion 10 and the peripheral portion 80. A catalyst portion 20 is formed on the element portion 10.

[0017] FIG. 1B shows a schematic cross-sectional view of the gas sensor 1 taken along line IB-IB in FIG. 1A. As shown in FIG. 1B, the element section 10 is not connected to the peripheral section 80 along line IB-IB, but is spaced apart from the peripheral section 80. In the gas sensor 1 shown in FIG. 1B, the element section 10 has a structure in which a membrane section 52, a heater section 40, a heat detection section 30, and a catalyst section 20 are laminated in this order. The heat detection section 30 is composed of a heat detection material 31, a heat detection electrode 32, and a heat detection material coating film 33. The heater section 40 is composed of a heater 41 made of wiring having a predetermined resistance value and a heater coating film 42.

[0018] 1C is a schematic cross-sectional view of the gas sensor 1 taken along line IC-IC in FIG. 1A. As shown in FIG. 1C, the membrane portion 52 extends from the element portion 10 and is formed on the main surface of the support substrate 51 in the peripheral portion 80. The substrate portion 50 is composed of the support substrate 51 and the membrane portion 52. A cavity 53 is formed in the center of the support substrate 51 so as to correspond to the shape of the element portion 10. That is, the cavity 53 thins the portion of the substrate portion 50 that corresponds to the element portion 10.

[0019] 1C, the gas sensor 1 is formed with an extraction electrode 60 that is drawn out to the surface of the sensor so as to electrically connect an external circuit to the heat detection electrode 32 or the heater 41 embedded inside the element portion. Note that while Fig. 1C only shows the extraction electrode 60 that is electrically connected to the heat detection electrode 32 formed inside the heat detection portion and drawn out to the surface of the peripheral portion 80 of the gas sensor 1, in another cross section, another extraction electrode that is electrically connected to the heater 41 formed inside the heater portion and drawn out to the surface of the peripheral portion 80 is formed.

[0020] 1B, in this embodiment, the heater 41 is sandwiched between the membrane portion 52 and the heater coating film 42. As a result, the heater 41 is supported by the membrane portion 52, and the heater coating film 42 can prevent the heater 41 from being exposed to the external atmosphere. This reduces variations in thermal changes due to heat generation by the heater 41, and can suppress the occurrence of thermal stress in the element portion and the beam portion.

[0021] Moreover, the membrane part 52 is preferably an insulating film to prevent current leakage from the heater 41 to the support substrate 51. Furthermore, the heater coating film 42 is preferably an insulating film to prevent short-circuiting between the heater 41 and the heat detection part 30.

[0022] Similarly, the heat detecting material 31 is sandwiched between the heater coating film 42 and the heat detecting material coating film 33. As a result, the heat detecting material is not exposed to the external atmosphere, and chemical deterioration can be suppressed. Furthermore, since the heat detecting material coating film 33 is located between the heat detecting material 31 and the catalyst section 40, contact between the heat detecting material 31 and the catalyst section 40 can be prevented, thereby increasing the durability of the heat detecting material.

[0023] Furthermore, in this embodiment, in order to control the balance of thermal stresses occurring in the membrane, heater, and heat detection unit, the thickness of the membrane, heater coating, and heat detection material coating are controlled within predetermined ranges. Specifically, when the thickness of the membrane is taken as 1, the thickness of the heater coating and the heat detection material coating are each within the range of 0.6 to 1.2, and preferably within the range of 0.8 to 1.0.

[0024] Furthermore, the thicknesses of the membrane portion, the heater coating film, and the heat detecting material coating film are controlled to be greater than the thickness of the heater and the thickness of the heat detecting material.

[0025] This maintains a balance of thermal stresses generated in the element portion, reducing damage to the beam portion, which is susceptible to damage due to thermal stresses, thereby providing a gas sensor with excellent durability and a long life.

[0026] On the other hand, if the thickness of the heater coating film and the thermal detection material coating film are outside the above range when the thickness of the membrane part is taken as 1, and the thicknesses of the membrane part, heater coating film, and thermal detection material coating film are smaller than the thickness of the heater and the thickness of the thermal detection material, the balance of thermal stresses is likely to be disrupted, and thermal stresses that lead to damage to the beam part are likely to occur.

[0027] The above-mentioned effects tend to be more easily improved when the membrane portion, heater coating film and heat detecting material coating film are made of the same material, and when the heater and heat detecting electrode are made of the same material.

[0028] (1.2. Circuit board) 1A to 1C, the substrate section 50 is composed of a support substrate 51, a membrane section 52, and a cavity section 53. In this embodiment, the membrane section 52 only needs to have enough mechanical strength to support the heater section 40, the heat detection section 30, the catalyst section 20, etc. formed thereon. The membrane section 52 is formed by thinning (removing) a thin film formed on the support substrate 51 in a region of the support substrate 51 corresponding to the region where the element section 10 is formed, using the cavity section 53 to form a membrane.

[0029] The formation of the cavity 53 suppresses the conduction of generated heat to the support substrate 51 when the heater 41 is maintained at a high temperature. In other words, the heat capacity in the vicinity of the element unit 10 is reduced by the amount of the support substrate 51 removed, so the heater 41 can be heated to a high temperature with very little power consumption. Furthermore, the heat conduction path from the membrane unit 52 to the support substrate 51 is only through the thin film portion, so a structure is formed in which the element unit 10 including the membrane unit 52 and the peripheral portion 80 via the beam portion 70 are insulated. As a result, the heat generated by the heater 41 is less likely to be conducted to the support substrate 51, and the heater 41 can be efficiently heated to a high temperature.

[0030] The material constituting the membrane portion 52 is preferably a material that can sufficiently ensure insulation between the support substrate 51 and the heater 41 so as to prevent current leakage from the heater 41 to the support substrate 51. Examples of such a material include silicon oxide and silicon nitride, and in this embodiment, silicon oxide is preferred.

[0031] The thickness of the membrane part 52 may be any thickness as long as the relationship between the thickness of the heater coating film 42 and the thickness of the thermal detection material coating film 32 falls within the range described above. Furthermore, taking into consideration the mass load that the heater part formed on the membrane part 52 imposes on the membrane part 52, the thickness may be such that sufficient insulation between the support substrate 51 and the heater 41 is ensured and that the membrane part 52 functions as an etching stop layer when forming the cavity part 53. In this embodiment, the thickness of the membrane part 52 is approximately 0.1 to 1.0 μm.

[0032] The support substrate 51 is not particularly limited as long as it has a sufficient mechanical strength to support the members formed thereon and is made of a material suitable for microfabrication such as etching. In this embodiment, examples of the support substrate 51 include a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, and a glass substrate.

[0033] (1.3. Heater section) 1B and 1C, the heater section 40 is composed of a heater 41, which is a resistor that generates heat when current is applied, and a heater coating film 42. Wiring having a predetermined resistance value is arranged as the heater 41 so as to efficiently and reliably heat the heat detection material 31. In this embodiment, in the region corresponding to the formation region of the catalyst section 20 and the heat detection section 30, it is preferable that the wiring of the heater 41 has a pattern (meander pattern) in which it is folded back multiple times and arranged in parallel at predetermined intervals.

[0034] The material constituting the heater 41 is preferably a conductive material with a relatively high melting point that can withstand high-temperature processes such as the film formation process and heat treatment process of the thermal detection material 31. Examples of such materials include molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or alloys containing two or more of these. In this embodiment, platinum is particularly preferred because it allows for high-precision dry etching such as ion milling and has high corrosion resistance. When platinum is used as the material constituting the heater 41, it is preferable to form an adhesion layer such as titanium (Ti) between the platinum and the membrane portion 52 to improve adhesion with the membrane portion 52.

[0035] The heater coating film 42 is formed to cover the heater 41. The heater coating film 42 is formed by being layered on the membrane portion 52 except for the portion formed in contact with the heater 41, and therefore the material constituting the heater coating film 42 is preferably the same as the material of the membrane portion 52. This is because, compared to the case where different materials are layered, when the same materials are layered, the material properties are the same, and the adhesion at the interface between the membrane portion 52 and the heater coating film 42 is strong, resulting in sufficient mechanical strength.

[0036] In particular, during operation of the gas sensor according to this embodiment, the heater 41 formed between the heater coating film 42 and the membrane portion 52 is repeatedly subjected to thermal stress, rising to several tens to several hundred degrees and then cooling to room temperature. Therefore, if the material of the membrane portion 52 and the material of the heater coating film 42 are different, the continuous thermal stress may cause peeling or cracks between the membrane portion 52 and the heater coating film 42.

[0037] The thickness of the heater coating film 42 may be any thickness as long as the relationship between the thickness of the membrane portion 52 and the thickness of the thermal detection material coating film 33 is within the range described above. Also, the thickness may be any thickness that can reliably cover the heater 41 and ensure sufficient insulation between layers. In this embodiment, the thickness of the heater coating film 42 is approximately 0.06 to 1.2 μm.

[0038] (1.4. Heat detection unit) 1B and 1C, the heat detection unit 30 is composed of a heat detection material 31, a heat detection electrode 32, and a heat detection material coating film 33. The heat detection material 31 may be any material that can detect heat associated with the combustion of gas. Examples of such a material include a thermistor and platinum. In this embodiment, the heat detection material 31 is preferably a thermistor.

[0039] The thermistor has a negative temperature coefficient of resistance and detects heat by changing its resistance value due to a temperature change caused by the combustion of combustible gas in the catalyst section 20. There are no particular restrictions on the material that constitutes the thermistor as long as it is a material that can be used as a thermistor. In this embodiment, examples of the thermistor material include composite metal oxides, amorphous silicon, polysilicon, and germanium.

[0040] When the heat detection material 31 is a thermistor, the thickness thereof may be adjusted according to the target resistance value of the thermistor.

[0041] In this embodiment, a heat detection electrode 32 made of wiring having a predetermined resistance value is formed in order to detect changes in the resistance value of the heat detection material 31 due to heat detection and extract the changes as an electrical signal. The heat detection electrode 32 is covered by the heat detection material 31, and the wiring is arranged so that changes in the resistance value of the heat detection material 31 can be detected with high accuracy. In this embodiment, the wiring of the heat detection electrode 32 is preferably a pair of opposing electrodes or a pair of comb-shaped electrodes in the region corresponding to the formation region of the catalyst section 20 where gas is burned.

[0042] Like the heater 41, the material constituting the thermal detection electrode 32 is preferably a conductive material with a relatively high melting point that can withstand high-temperature processes such as the film formation process and heat treatment process of the thermal detection material 31. In this embodiment, examples of the material include molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing two or more of these. In this embodiment, like the heater 41, platinum is particularly preferred because it allows for high-precision dry etching such as ion milling and has high corrosion resistance.

[0043] If the heat detecting material 31 is exposed to the external atmosphere or in contact with the catalyst unit 20, chemical degradation due to oxidation, diffusion, etc. may occur. For example, if a thermistor made of a composite metal oxide is used as the heat detecting material 31, it is known that maintaining the thermistor at high temperatures deprives the composite metal oxide of oxygen, causing reduction and degradation of the composite metal oxide. Therefore, to prevent such chemical degradation from adversely affecting the heat detecting characteristics, it is preferable to cover the heat detecting material 31 with a heat detecting material coating film 33 to protect it. The material constituting the heat detecting material coating film 33 is preferably stable at high temperatures. In this embodiment, it is preferable to construct the heat detecting material coating film 33 from the same material as the membrane unit 52 and heater coating film 42 described above. Therefore, silicon oxide is a preferable material for such a material.

[0044] The thickness of the thermal detecting material coating film 33 may be any thickness as long as the relationship between the thickness of the membrane portion 52 and the thickness of the heater coating film 42 is within the range described above. Also, the thickness may be any thickness that can reliably cover the thermal detecting material 31 and ensure sufficient insulation between layers. In this embodiment, the thickness of the thermal detecting material coating film 33 is approximately 0.06 to 1.2 μm.

[0045] (1.5. Catalyst section) 1B and 1C, the catalyst section 20 is formed on the surface of the heat detection section 30 (in this embodiment, the heat detection material coating film 33). There are no particular limitations on the catalyst section 20, as long as it is made of a catalyst material supported on a carrier material, which is a porous material formed by the aggregation and integration of a plurality of particles.

[0046] The carrier material is not particularly limited as long as it is a material that is commonly used as a carrier. Specific examples include oxide materials such as aluminum oxide (such as gamma alumina), titanium oxide, silicon oxide, and cerium oxide. In this embodiment, the carrier material is preferably aluminum oxide.

[0047] Examples of the catalyst material supported on the support material include precious metals such as platinum (Pt), gold (Au), and palladium (Pd), and metal oxides such as rare earth element oxides and bismuth oxide. These may also be combined to support multiple catalyst materials on the support material. In this embodiment, the catalyst material is preferably a precious metal, and more preferably platinum.

[0048] The catalyst portion 20 is usually formed on the element portion by screen printing, dispensing with a dispenser, or the like, but there is variation in size.

[0049] On the other hand, in this embodiment, in order to optimize the relationship between the catalyst portion and the element portion, it is preferable to control the size of the catalyst portion 20. Specifically, it is preferable that the ratio of the volume of the catalyst portion 20 to the volume of the element portion 10 is 10 or more and 40 or less. It is more preferable that this ratio is 20 or more.

[0050] If this ratio is too small, the initial sensitivity will be low, and aging will tend to further reduce the sensitivity, whereas if this ratio is too large, the mass load on the membrane will be large, and the beam supporting the element unit 10 will be more likely to break due to slight vibrations.

[0051] In this embodiment, the volume of the catalyst portion is measured using a laser microscope. A laser microscope is a microscope that uses a confocal optical system with a laser as a light source. In a laser microscope, light that is focused and reflected from the sample surface is further focused on the detector side, passes through a pinhole, and is detected by the detector. Because unfocused light is prevented from reaching the detector by the pinhole, a high-resolution, high-contrast sample image can be obtained. Therefore, by scanning the sample or light source in the planar direction of the sample (XY direction), and then moving and scanning the sample or light source in the vertical direction of the sample (Z direction), an image showing the precise three-dimensional shape of the sample can be obtained.

[0052] By performing this operation from the surface of the element portion to the apex of the catalyst portion, a three-dimensional image of the catalyst portion can be obtained. The volume of the catalyst portion can be calculated from the information contained in the obtained three-dimensional image. The volume of the element portion can be calculated from the formation area and thickness of each part that makes up the element portion.

[0053] (1.6. Gas Sensor Operating Principle) In the gas sensor 1, the heater 41 and the heat detection electrode 32 are connected to an external circuit (not shown) via the extraction electrode 60. When the gas sensor 1 is activated, current is applied and a predetermined voltage is applied to the heater 41. A fixed resistor is connected in series to the heat detection electrode 32, and a bias voltage is applied to the resistor. The heat detection material 31 and the catalyst portion 20 located thereon are heated to a predetermined temperature in response to the voltage applied to the heater 41.

[0054] In this state, if a combustible gas to be detected, such as carbon monoxide, is present in the space in which the gas sensor 1 is placed, the combustible gas will combine with oxygen and other gases on the catalyst section 20 depending on the proportion of the combustible gas present, and combust. At this time, the combustion heat generated in the catalyst section 20 by the combustion of the combustible gas heats the heat detection material 31. A change in the resistance value of the heat detection material 31 caused by a temperature change due to this combustion heat is detected by the heat detection electrode 32 and output as an electrical signal to an external circuit. As a result, the concentration of the combustible gas can be detected.

[0055] (2. Gas Sensor Manufacturing Method) Next, an example of a method for manufacturing the gas sensor shown in FIGS. 1A to 1C will be described below.

[0056] First, a support substrate is prepared. An insulating film that will become a membrane portion is formed on one main surface of the prepared support substrate. The insulating film can be formed by a known film formation method such as thermal oxidation or CVD (Chemical Vapor Deposition).

[0057] Next, a heater portion is formed on the formed insulating film. First, a thin film of conductive material that constitutes the heater is formed using a known film formation method. If the heater is formed by laminating multiple conductive materials, multiple thin films can be formed and laminated. Next, in the region corresponding to the formation region of the catalyst portion, the thin film is etched so as to form a pattern (meander pattern) in which the heater wiring is folded back multiple times and arranged in parallel at predetermined intervals.

[0058] After forming the heater wiring pattern, a heater coating film is formed by a known film formation method to cover the heater, in the same manner as forming the insulating film, so that the heater wiring pattern is at least covered. This forms the heater portion. In this embodiment, the material of the heater coating film is the same as the material of the membrane portion (insulating film).

[0059] Next, a heat detection section is formed on the formed heater section. In this embodiment, a thermistor section is formed as the heat detection section. First, a thin film of a conductive material that constitutes the heat detection electrode is formed on the heater section, i.e., on the heater coating film, by a known film formation method. Next, in the same manner as in the formation of the heater wiring pattern, the thin film is etched in the area corresponding to the formation area of ​​the catalyst section so that the wiring of the heat detection electrode becomes a pair of opposing electrodes or a pair of comb-teeth electrodes. After forming the wiring pattern of the heat detection electrode, a thermistor is formed as the heat detection material so that the wiring pattern of the heat detection electrode is at least covered.

[0060] The thermistor may be formed by a known film formation method. For example, when the thermistor is made of the above-mentioned composite oxide, a film having the composition of the composite oxide is formed by a sputtering method. After that, the film is heat-treated at a predetermined temperature for a predetermined time, and then etched to obtain a predetermined shape.

[0061] Next, a thermal detection material coating film is formed to protect the thermistor so as to cover the thermal detection electrode and thermistor. Similar to the formation of the insulating film, the thermal detection material coating film is formed by a known film formation method. This forms the thermistor section. In this embodiment, the material of the thermal detection material coating film is the same as the material of the insulating film.

[0062] Through the above steps, a laminated structure is obtained in which an insulating film, a heater section, and a thermistor section serving as a heat detection section are laminated in this order on a support substrate. An extraction electrode is then formed on this laminated structure, for example, by a lift-off method. An etching mask is then applied to a predetermined region of the main surface of the support substrate on which no insulating film is formed, corresponding to the beam section, and the support substrate is etched until the insulating film formed on the other main surface is exposed, thereby forming a cavity in the region corresponding to the heater formation region. The insulating film corresponding to the region where the cavity is formed becomes the membrane section.

[0063] In this embodiment, a catalyst portion is formed on the surface of the laminated structure, i.e., on the heat-detecting material coating film of the thermistor portion, to obtain a gas sensor. Specifically, a coating body that becomes the catalyst portion is formed using a paste containing the raw materials for the catalyst portion, and this is then heat-treated at a predetermined temperature to form the catalyst portion.

[0064] First, the raw material of the material constituting the catalyst portion 20 is mixed with a solvent, a binder, and an additive to obtain a paste for the catalyst portion. The raw material of the material constituting the catalyst portion is a carrier material on which the catalyst material is supported. In this embodiment, a powdered carrier material (carrier material powder) is preferred. The average particle diameter of the carrier material powder is not particularly limited, but is preferably 0.1 to 5 μm.

[0065] The solvent is not particularly limited as long as it can dissolve the binder, and any known solvent can be used. The binder may also be any known binder. Examples of additives include dispersants.

[0066] First, the prepared catalyst paste is applied onto the thermal detection material coating film 33. The catalyst paste is applied to the area corresponding to the area where the heater is wired in a meander pattern. There are no particular limitations on the method for applying the catalyst paste, and examples include known methods such as screen printing and dispensing using a dispenser. At this time, it is preferable to control the amount of catalyst paste dispensed so that the volume of the catalyst portion relative to the volume of the element portion falls within the above-mentioned range.

[0067] The catalyst portion-coated body formed from the catalyst portion paste is a green body containing a solvent, a binder, etc. The catalyst portion is formed by heat-treating this catalyst portion-coated body. The heat treatment temperature when forming the catalyst portion is preferably 300°C or higher and 500°C or lower.

[0068] Through the above steps, a gas sensor having the configuration shown in FIGS. 1A to 1C can be obtained.

[0069] (3. Gas sensor device) The gas sensor device according to this embodiment includes the above-described gas sensor and a drive circuit for driving the gas sensor. One example of such a drive circuit is a drive circuit for driving a heater.

[0070] In this embodiment, the driving circuit for driving the heater is preferably configured to convert the input signal to the heater into a pulse signal. This allows the heater to operate intermittently, and the temperature of the catalyst section drops during periods when the heater is off. As a result, sensitivity degradation due to aggregation of the support material in the catalyst section is suppressed, resulting in a long-life gas sensor.

[0071] In this embodiment, the drive circuit for driving the heater preferably controls the heater so that the maximum temperature of the heater is within a range of 250° C. to 350° C. This suppresses sensitivity degradation due to aggregation of the support material in the catalyst portion, and also provides sufficient heat for the catalytic reaction, thereby providing a highly sensitive, durable, and long-life gas sensor.

[0072] (4. Modifications) In the above embodiment, the catalytic combustion type gas sensor element has been described, but any gas sensor element other than the catalytic combustion type may be used as long as it has a catalyst portion formed therein.

[0073] In the above-described embodiment, the heater is sandwiched between the membrane portion and the heater coating film, and the heat detecting material is sandwiched between the heater coating film and the heat detecting material coating film, but the heat detecting material may be sandwiched between a coating film different from the heater coating film and the heat detecting material coating film. Even with such a configuration, the above-described effects can be obtained.

[0074] In addition, in the above-described embodiment, the number of beams is four, but the number of beams is not particularly limited.

[0075] In the above-described embodiment, the catalyst portion is composed of a single layer. However, the catalyst portion may have a two-layer structure, for example, consisting of a support layer that does not support a catalyst material and a catalyst layer that supports a catalyst material. By forming the catalyst portion into a two-layer structure, the catalyst portion can be formed without containing materials unnecessary for the catalytic reaction. Furthermore, the adhesion between the catalyst portion and the surface on which the catalyst portion is formed can be improved.

[0076] In addition, in the above-described embodiment, as shown in FIG. 1A, the planar shape of the catalyst portion is circular, and the planar shape of the element portion is approximately rectangular, but the planar shapes of the catalyst portion and the element portion are not particularly limited and may be other shapes.

[0077] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and may be modified in various ways within the scope of the present invention. [Example]

[0078] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0079] The samples according to the examples were fabricated as follows. First, a silicon oxide film serving as an insulating film was formed by CVD on the main surface of a silicon single crystal substrate serving as a support substrate to a thickness of 0.5 μm. Next, a titanium thin film (5 nm thick) and a platinum thin film (100 nm thick) were sequentially deposited on the insulating film by sputtering to form a Pt / Ti film that would become the heater. The Pt / Ti film was then dry-etched to form a heater with a meander pattern. Next, a silicon oxide film serving as a heater coating was formed by CVD on the insulating film and heater to a thickness of 0.4 μm.

[0080] Next, a titanium thin film (5 nm thick) and a platinum thin film (100 nm thick) were deposited on the heater coating film by sputtering to form a Pt / Ti film that would become the thermal detection electrode.The Pt / Ti film was then dry-etched to form a thermal detection electrode with a pair of opposing electrodes.

[0081] Next, a thermistor (MnNiCo oxide) serving as a thermal detection material was deposited to a thickness of approximately 0.3 μm under sputtering conditions of a substrate temperature of 600°C, deposition pressure of 0.5 Pa, O2 / Ar flow ratio of 1%, and RF power of 400 W. The MnNiCo oxide film was then heat-treated in an air atmosphere at 650°C for 1 hour using a baking furnace, and then patterned into the desired shape by wet etching using a ferric chloride solution. Next, a silicon oxide film serving as a thermal detection material coating was deposited to a thickness of 0.4 μm on the thermal detection electrode and thermistor using the CVD method.

[0082] Next, a 1 μm thick aluminum pad was formed as an extraction electrode using the lift-off method. An etching mask was then applied to the main surface of the silicon single crystal substrate where the insulating film was not formed, and the silicon single crystal substrate was wet-etched using an alkaline solution until the insulating film was exposed, forming a cavity. This resulted in the formation of a membrane made of silicon oxide. The membrane was 0.5 μm thick.

[0083] Next, Al2O3 powder with an average particle size of 2 μm and Pt powder as the catalyst material were prepared as raw materials for the catalyst part. Pt powder was supported on this Al2O3 powder using a known method. The Al2O3 powder with Pt supported was the powder for the catalyst part.

[0084] 100 parts by weight of the above catalyst powder, 400 parts by weight of α-terpineol, 20 parts by weight of "ED-216" manufactured by Kusumoto Chemicals Co., Ltd. as a dispersant, and 40 parts by weight of ethyl cellulose as a binder were kneaded using a three-roll mill to form a paste, thereby obtaining a catalyst paste.

[0085] The obtained catalyst paste was applied onto the thermal detection material coating film while controlling the amount of the paste dispensed using a dispenser. The obtained catalyst-coated body was heat-treated at 400°C for 30 minutes to obtain multiple gas sensors with different ratios of the volume of the catalyst to the volume of the element.

[0086] The volume of the catalyst portion was measured using a laser microscope (Keyence VK-X250), and the volume of the element portion was calculated from the formation areas and thicknesses of the membrane portion, heater portion, and heat detection portion.

[0087] The gas sensors thus obtained were evaluated for changes in sensor sensitivity over time and initial sensitivity as follows.

[0088] The change in sensor sensitivity over time was evaluated by connecting the obtained gas sensor to an external circuit, applying current to the heat detection unit and heater unit, and measuring the sensitivity immediately after the start of power application and the sensitivity after 2000 hours. Figure 2 shows the measurement results for gas sensors with catalyst volume to element volume ratios of 2.1, 4.1, 10.6, 19.9, 22, and 40.

[0089] The initial sensitivity of the obtained gas sensor was evaluated by measuring the sensitivity immediately after the start of current application. The measurement results are shown in Figure 3.

[0090] 2 and 3, it was confirmed that samples in which the ratio of the volume of the catalyst portion to the volume of the element portion was within the above-mentioned range had high initial sensitivity, and that the sensitivity after 2000 hours exceeded the initial sensitivity.

[0091] On the other hand, it was confirmed that samples in which the ratio of the volume of the catalyst portion to the volume of the element portion was outside the above-mentioned range had low initial sensitivity, and furthermore, the sensitivity after 2000 hours was lower than the initial sensitivity. [Industrial Applicability]

[0092] The gas sensor according to the present invention is highly sensitive and has a long life because the performance of the catalyst portion and the overall configuration of the detection element including the catalyst portion are optimized, and therefore the gas sensor can be suitably used as a gas sensor to be mounted on mobile devices such as smartphones. [Explanation of symbols]

[0093] 1. Gas sensor 10... Element section 20... Catalyst section 30... Heat detection unit 31... Heat-sensing material 32... Heat detection electrode 33... Heat-sensitive coating film 40... Heater section 41... Heater 42... Heater coating film 50... Circuit board 51... Support substrate 52... Insulating film (membrane part) 53… Cavity part 60… Extraction electrode 70…Beam part 80... Periphery

Claims

1. an element section having a structure in which a membrane section, a heater section including a heater and a heater coating film, and a heat detection section including a heat detection material, a heat detection electrode, and a heat detection material coating film are laminated in this order; a catalyst portion formed on the heat detection portion, the heater is sandwiched between the membrane portion and the heater covering film, the thermal detection material is sandwiched between the heater coating film and the thermal detection material coating film, When the thickness of the membrane portion is taken as 1, the thickness of the heater coating film and the thickness of the thermal detection material coating film are each within a range of 0.6 to 1.2; A gas sensor in which the thickness of the membrane portion, the thickness of the heater coating film, and the thickness of the thermal detection material coating film are greater than the thickness of the heater and the thickness of the thermal detection material, respectively.

2. 2. The gas sensor according to claim 1, wherein the heat sensitive material is a thermistor.

3. 3. The gas sensor according to claim 1, wherein the ratio of the volume of the catalyst portion to the volume of the element portion is 10 or more and 40 or less.

4. 4. The gas sensor according to claim 1, wherein the element portion has an air bridge structure supported by a beam.

5. 5. The gas sensor according to claim 1, wherein the catalyst portion includes an oxide support material on which a noble metal is supported.

6. a gas sensor according to any one of claims 1 to 5; a drive circuit for driving the heater, The gas sensor device is configured such that the drive circuit converts an input signal to the heater into a pulse signal.

7. 7. The gas sensor device according to claim 6, wherein the drive circuit is configured to control the maximum temperature of the heater to be within a range of 250 to 350 degrees Celsius.

Images