MEMS catalytic combustion gas sensor

Abstract

To provide a MEMS type contact combustion type gas sensor that, even when water is generated by the reaction of a gas to be detected, can prevent a reduction in the accuracy of detecting the gas to be detected.SOLUTION: A MEMS type contact combustion type gas sensor (1) comprises a gas reaction part (13) and a water scattering prevention layer (14) provided on the gas reaction part (13). The water scattering prevention layer (14) transmits a gas to be detected, and has a lower activity for the gas to be detected than the gas reaction part (13).SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a MEMS catalytic combustion type gas sensor. [Background technology]

[0002] A catalytic combustion gas sensor has a gas reaction unit that comes into contact with a target gas such as hydrogen, and detects the target gas by measuring the change in resistance value of the gas reaction unit due to the heat of combustion of the target gas (see, for example, Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-002888 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned catalytic combustion type gas sensor, there is a demand for miniaturization and power saving. In order to satisfy such demand, MEMS (Micro Electro Mechanical System) type catalytic combustion type gas sensors that utilize MEMS technology that enables microfabrication are being developed and put into practical use.

[0005] However, in the above-mentioned MEMS catalytic combustion gas sensor, there has been a problem in that the detection accuracy of the target gas is reduced due to water produced by the combustion (reaction) of the target gas.

[0006] Specifically, when a MEMS-type catalytic combustion gas sensor is constructed, the heat generating part becomes smaller as a result of miniaturization compared to conventional catalytic combustion gas sensors, and water vapor generated by hydrogen combustion can easily condense in the part close to room temperature near the tiny heat generating part. For this reason, in conventional MEMS-type catalytic combustion gas sensors, the temperature of the gas reaction part can drop due to water adhesion, causing the resistance value of the gas reaction part to fluctuate. As a result, in conventional MEMS-type catalytic combustion gas sensors, the sensor output can be unstable, leading to a decrease in the detection accuracy of the target gas.

[0007] An object of the present disclosure is to provide a MEMS catalytic combustion gas sensor that can suppress a decrease in detection accuracy of a target gas even when water is produced by a reaction of the target gas with the target gas. [Means for solving the problem]

[0008] In order to solve the above problems, a MEMS-type catalytic combustion gas sensor according to one aspect of the present disclosure is a MEMS-type catalytic combustion gas sensor comprising a substrate, a detection electrode provided on the substrate, a gas reaction section provided on the substrate so as to contact the detection electrode, and a water scattering suppression layer provided on the gas reaction section, wherein a gas to be detected includes at least one gas that generates water by reaction, and the water scattering suppression layer is permeable to the gas to be detected and has a lower activity against the gas to be detected than the gas reaction section.

[0009] According to the above configuration, since the water-scattering suppression layer has a lower activity with respect to the target gas than the gas reaction section, the water-scattering suppression layer is less likely to produce water as a result of the reaction of the target gas with the water-scattering suppression layer than the gas reaction section, and the water scattering can be suppressed. As a result, even if water is produced as a result of the reaction of the target gas with the water-scattering suppression layer with the gas reaction section, the produced water is not evaporated all at once, and the amount of condensation near the gas reaction section can be suppressed, thereby making it possible to configure a MEMS catalytic combustion gas sensor that can suppress a decrease in the detection accuracy of the target gas.

[0010] The gas to be detected may be hydrogen. In this case, hydrogen can be detected to a higher concentration.

Advantages of the Invention

[0011] According to one aspect of the present disclosure, it is possible to provide a MEMS type catalytic combustion gas sensor that can suppress a decrease in the detection accuracy of the gas to be detected even when water is generated by the reaction of the gas to be detected.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a plan view showing a MEMS type catalytic combustion gas sensor according to Embodiment 1 of the present disclosure. [Diagram 2] FIG. 2 is a sectional view taken along line II-II of FIG. 1. [Diagram 3] FIG. 3 is a circuit diagram showing a configuration example of a detection circuit included in the MEMS type catalytic combustion gas sensor. [Figure 4] FIG. 4 is a graph showing an example of a detection result in a comparative example. [Diagram 5] FIG. 5 is a graph showing an example of a detection result of Embodiment 1. [Figure 6] FIG. 6 is a sectional view showing a main part configuration of a MEMS type catalytic combustion gas sensor according to Embodiment 2 of the present disclosure.

Modes for Carrying Out the Invention

[0013] 〔Embodiment 1〕 <Schematic Configuration of MEMS Type Catalytic Combustion Gas Sensor 1> Hereinafter, the MEMS type catalytic combustion gas sensor 1 according to Embodiment 1 of the present disclosure will be described in detail with reference to FIGS. 1 to 3. FIG. 1 is a plan view showing the MEMS type catalytic combustion gas sensor 1 according to Embodiment 1 of the present disclosure. FIG. 2 is a sectional view taken along line II-II of FIG. 1. FIG. 3 is a circuit diagram showing a configuration example of a detection circuit included in the MEMS type catalytic combustion gas sensor 1.

[0014] 1 and 2, the MEMS catalytic combustion gas sensor 1 of this embodiment includes a detection element 10 that detects a target gas, and a compensation element 20 that compensates for the detection result of the detection element 10. The target gas contains at least one gas that can generate water by reaction (combustion) in a gas reaction section 13 included in the detection element 10. Specifically, the target gas is a flammable gas that contains hydrogen atoms, such as hydrogen, methane, isobutane, ethane, and propane.

[0015] <Detection circuit configuration> The MEMS catalytic combustion type gas sensor 1 of this embodiment also includes a detection circuit including, for example, a bridge circuit. That is, as shown in Fig. 3, the detection circuit of this embodiment includes a bridge circuit having a detection element 10, a compensation element 20, resistors 71 and 72, a power source 73 that supplies power to the bridge circuit, and a voltmeter 74 that detects the voltage of the bridge circuit and outputs it as the sensing result of the MEMS catalytic combustion type gas sensor 1.

[0016] In this embodiment, power is intermittently supplied to the detection element 10 from the power source 73, and the detection element 10 receiving the power is maintained at a predetermined combustion temperature (e.g., 300°C to 700°C) at which the target gas is easily combusted. The compensating element 20 compensates for changes in the resistance value of the detection element 10 based on temperature changes other than those due to the combustion of the target gas (e.g., changes in the ambient temperature).

[0017] In this embodiment, the detection element 10 and the compensation element 20 are set to the same resistance value. Therefore, when no target gas is present around the MEMS catalytic combustion type gas sensor 1, the bridge circuit is in a balanced state, and the output of the voltmeter 74 (i.e., the sensing result) becomes a value of 0, which indicates that no target gas is present.

[0018] On the one hand, when the gas to be detected exists, the temperature of the detection element 10 rises due to the combustion, and the resistance value of the detection element 10 also becomes a large value. Therefore, in the detection circuit, the balance of the bridge circuit is disrupted, and the output of the voltmeter 74 becomes larger than the value of 0 in proportion to the concentration of the gas to be detected. Thereby, in the MEMS type catalytic combustion gas sensor 1 of the present embodiment, the concentration of the gas to be detected can be detected.

[0019] <Main Component Configuration of MEMS Type Catalytic Combustion Gas Sensor 1> As shown in FIGS. 1 and 2, the MEMS type catalytic combustion gas sensor 1 of the present embodiment includes, for example, a substrate 30 made of silicon, an insulating film 40 formed on the substrate 30, a detection element 10 supported on a recess 31 formed in the substrate 30, and a compensation element 20 supported on a recess 32 formed in the substrate 30.

[0020] The insulating film 40 is formed, for example, using silicon oxide, silicon nitride, tantalum oxide, or a laminated film thereof, on the side of the exposure surface of the substrate 30 to the gas to be detected (the upper surface in FIG. 2) excluding the surfaces of the recess 31 and the recess 32.

[0021] <Configuration of Detection Element 10> The detection element 10 includes a first support portion 11 that supports the detection element 10, a heater wiring 12 provided on the first support portion 11, the gas reaction portion 13 provided above the heater wiring 12 so as to contact the heater wiring 12, and a water splash suppression layer 14 provided on the gas reaction portion 13.

[0022] 1 and 2, the first support 11 is attached to the substrate 30 via a plurality of, for example, four, bridging portions 15 in a state of being suspended in midair above the recess 31. That is, one end of each of the four bridging portions 15 is connected to the first support 11, and the other end of each of the four bridging portions 15 is connected to the substrate 30. By separating the first support 11 from the substrate 30 in this manner, it is possible to suppress the heat generated in the heater wiring 12 from being dissipated to the substrate 30, and the temperature drop in the gas reaction portion 13 is suppressed, thereby enabling the MEMS catalytic combustion type gas sensor 1 to achieve power saving.

[0023] Like the insulating film 40, the first support portion 11 and the bridge portion 15 are made of, for example, silicon oxide, silicon nitride, tantalum oxide, or a laminated film of these materials.

[0024] Heater wire 12 is made of a precious metal wire such as platinum, and also serves as a detection electrode in the bridge circuit (FIG. 3). In other words, heater wire 12 heats gas reaction unit 13 in response to the power from power source 73, and the resistance value changes in response to a temperature rise in gas reaction unit 13 due to heating by heater wire 12 and combustion with the target gas. The change in resistance value of heater wire 12 is used as detection data for detecting the concentration of the target gas.

[0025] Specifically, one end and the other end of the heater wire 12 are connected to thin film wires 51 and 52, respectively, provided on the bridge portion 15. The thin film wire 51 is connected to an electrode pad 61 connected to a connection point between the sensing element 10 and a resistor 72 in the bridge circuit (FIG. 3). The thin film wire 52 is connected to an electrode pad 62 connected to a connection point between the sensing element 10 and a compensation element 20 in the bridge circuit.

[0026] The electrode pads 61 and 62 are connected to one end of a power source 73 and one end of a voltmeter 74 via lead wires (not shown), respectively. This allows the detection element 10 to receive power from the power source 73 and detect the concentration of the target gas.

[0027] The gas reaction unit 13 is configured by supporting a precious metal catalyst, such as palladium or platinum, which has activity against a detection target gas such as hydrogen, on a carrier using, for example, alumina or silica-alumina. In this manner, the gas reaction unit 13 contains a precious metal catalyst that reacts with the detection target gas and burns. In addition, the gas reaction unit 13 is formed on the first support 11 by, for example, providing a carrier supporting the precious metal catalyst on the first support 11 by a coating method, and then firing the carrier in, for example, an electric furnace.

[0028] The water scattering suppression layer 14 is made of an inactive material that is inactive to the target gas, such as alumina, tin oxide, or silica-alumina. In other words, unlike the gas reaction section 13, the water scattering suppression layer 14 does not contain a precious metal catalyst that is active to the target gas, and is less active to the target gas than the gas reaction section 13. The water scattering suppression layer 14 is formed on the gas reaction section 13 by providing the inactive material on the gas reaction section 13 by, for example, a coating method, and then firing the material in, for example, an electric furnace.

[0029] The water-scattering suppression layer 14 is configured to allow the target gas to pass therethrough so as not to impede the reaction with the target gas in the gas reaction section 13. Specifically, the water-scattering suppression layer 14 has a thickness of, for example, about 10 to 40 μm. The water-scattering suppression layer 14 also has a small specific surface area (for example, 5 m 2 / g or less), which allows the gas to be detected to pass through and prevents the water (water droplets) generated in gas reaction section 13 from scattering.

[0030] In addition, in the detection element 10, components other than the gas reaction section 13 and the water scattering suppression layer 14 are formed using MEMS (Micro Electro Mechanical System) technology.

[0031] <Configuration of compensation element 20> The compensating element 20 includes a second support portion 21 that supports the compensating element 20, a heater wire 22 provided on the second support portion 21, and a compensating portion 23 provided above the heater wire 22 so as to be in contact with the heater wire 22.

[0032] 1 and 2, the second support 21 is attached to the substrate 30 via a plurality of, for example, four, bridging portions 25 in a state of being suspended in midair above the recess 32. That is, one end of each of the four bridging portions 25 is connected to the second support 21, and the other end of each of the four bridging portions 25 is connected to the substrate 30. By separating the second support 21 from the substrate 30 in this manner, it is possible to suppress the heat generated in the heater wiring 22 from being dissipated to the substrate 30, and the temperature drop in the compensation portion 23 is suppressed, thereby enabling the MEMS catalytic combustion type gas sensor 1 to achieve power saving.

[0033] Like the insulating film 40, the second support portion 21 and the bridge portion 25 are made of, for example, silicon oxide, silicon nitride, tantalum oxide, or a laminated film of these materials.

[0034] The heater wire 22 is made of a precious metal wire such as platinum, and also serves as a compensation detection electrode in the bridge circuit (FIG. 3). That is, the heater wire 22 heats the compensation unit 23 in response to the power from the power source 73, and the resistance value changes in response to a temperature rise caused by heating by the heater wire 22 in the compensation unit 23. The change in the resistance value of the heater wire 22 is used as compensation data when detecting the concentration of the target gas.

[0035] Specifically, one end and the other end of the heater wire 22 are connected to thin film wires 53 and 54, respectively, provided on the bridge portion 25. The thin film wire 53 is connected to an electrode pad 62 connected to a connection point between the detection element 10 and the compensation element 20 in the bridge circuit (FIG. 3). The thin film wire 54 is connected to an electrode pad 63 connected to a connection point between the compensation element 20 and a resistor 71 in the bridge circuit.

[0036] The electrode pads 62 and 63 are connected to one end of a voltmeter 74 and the other end of a power source 73 via lead wires (not shown), respectively. This allows the compensating element 20 to receive power from the power source 73 and to perform compensation in detecting the concentration of the target gas.

[0037] The compensation section 23 is made of an inactive material that is inactive to the detection target gas, such as alumina or silica alumina, and does not include a precious metal catalyst that is active to the detection target gas, such as hydrogen, such as palladium or platinum. The compensation section 23 is formed on the second support section 21 by providing the inactive material on the second support section 21, for example, by a coating method, and then firing the material in an electric furnace, for example.

[0038] Moreover, in the compensation element 20, the components other than the compensation section 23 are formed using MEMS (Micro Electro Mechanical System) technology.

[0039] In the above description, the heater wires 12 and 22 are exposed and provided on the first support 11 and the second support 21 so as to be in direct contact with the gas reaction unit 13 and the compensation unit 23, respectively, as shown in Fig. 2. However, the present embodiment is not limited to this, and for example, the heater wires 12 and 22 may be embedded inside the first support 11 and the second support 21, respectively.

[0040] In the MEMS catalytic combustion gas sensor 1 of this embodiment configured as described above, the detection element 10 includes the gas reaction section 13 and the water scattering suppression layer 14 provided on the gas reaction section 13. Furthermore, since the water scattering suppression layer 14 has a lower activity with respect to the target gas than the gas reaction section 13, the water scattering suppression layer 14 is less likely to generate water due to the reaction of the target gas than the gas reaction section 13, and can suppress the scattering of water. As a result, in this embodiment, even if water is generated due to the reaction of the target gas, the generated water is not evaporated all at once, and the amount of condensation near the gas reaction section 13 can be suppressed, so that the MEMS catalytic combustion gas sensor 1 can be configured that can suppress a decrease in the detection accuracy of the target gas.

[0041] In addition, in this embodiment, the water-scattering-suppression layer 14 suppresses the scattering of water (water droplets) generated in the gas reaction section 13, and therefore it is possible to significantly suppress the scattering of water droplets into the peripheral area of ​​the detection element 10. As a result, in this embodiment, it is possible to reliably suppress fluctuations in the sensing result (sensor output) of the MEMS catalytic combustion type gas sensor 1 caused by water droplets scattered into the peripheral area.

[0042] In this embodiment, hydrogen is detected as the detection target gas, which allows the detection of hydrogen at a higher concentration.

[0043] Here, the effect of the MEMS catalytic combustion type gas sensor 1 of this embodiment will be specifically described with reference to Fig. 4 and Fig. 5. Fig. 4 is a graph showing an example of a detection result in a comparative example. Fig. 5 is a graph showing an example of a detection result in this embodiment 1.

[0044] First, in the comparative example, unlike the product of this embodiment, only the gas reaction section is formed in the detection element, and the water scattering suppression layer is not formed. Therefore, in the comparative example, it is not possible to prevent the water generated by the reaction (combustion) with hydrogen (detection target gas) in the gas reaction section from scattering in the peripheral area of ​​the detection element. As a result, in the comparative example, the sensor output is unstable and fluctuates greatly, as shown in the graph of FIG.

[0045] On the other hand, in the product of this embodiment, the water-scattering-suppression layer 14 suppresses water from scattering into the surrounding area of ​​the detection element 10, so the sensor output did not fluctuate and was stable at a constant value, as shown in the graph of Fig. 5. That is, in the product of this embodiment, for example, as shown at 100% LEL, when the hydrogen concentration was 4%, the sensor output was maintained at a value of about 80. In this way, it was demonstrated that the product of this embodiment can detect hydrogen with high accuracy.

[0046] [Embodiment 2] The second embodiment of the present disclosure will be specifically described with reference to Fig. 6. Fig. 6 is a cross-sectional view showing a main configuration of a MEMS catalytic combustion type gas sensor according to the second embodiment of the present disclosure. For convenience of explanation, the same reference numerals are given to members having the same functions as those described in the above embodiment, and the explanation thereof will not be repeated.

[0047] The main difference between the second embodiment and the first embodiment is that a dike window 85 is provided around the gas reaction section 83.

[0048] In the MEMS-type catalytic combustion gas sensor 1 of this embodiment, as shown in Figure 6, the detection element 80 comprises a first support part 81, a heater wiring 82 provided on the first support part 81, a gas reaction part 83 provided above the heater wiring 82 so as to be in contact with the heater wiring 82, and a water scattering suppression layer 84 provided on the gas reaction part 83.

[0049] In this embodiment, embankment window 85 having a substantially circular window (not shown) is provided on first support 81 so as to abut against the outer periphery of gas reaction section 83 and surround gas reaction section 83. This embankment window 85 is configured, for example, from the same material as first support 81 using MEMS (Micro Electro Mechanical System) technology.

[0050] Furthermore, in this embodiment, the provision of the embankment window 85 allows the water-scattering suppression layer 84 to be formed with a more uniform thickness. That is, the embankment window 85 surrounds the outer periphery of the gas reaction section 83, and this makes it possible to easily form the end portions of the water-scattering suppression layer 84 on the embankment window 85 so that the thickness of the end portions is uniform with the thickness of the central portion of the water-scattering suppression layer 84.

[0051] With the above-described configuration, this embodiment achieves the same effects as those of the above-described embodiment 1. In addition, since the water-scattering suppression layer 84 can be formed with a more uniform thickness, it is possible to more reliably suppress water scattering into the peripheral area of ​​the detection element 80, and it is possible to more reliably suppress fluctuations in the sensor output of the MEMS catalytic combustion type gas sensor 1.

[0052] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0053] 1. MEMS catalytic combustion gas sensor 12, 82 Heater wiring (detection electrode) 13, 83 Gas reaction section 14, 84 Water scattering suppression layer 30 Substrate

Claims

1. A MEMS type catalytic combustion gas sensor comprising: a substrate; a detection electrode provided on the substrate; a gas reaction section provided on the substrate so as to be in contact with the detection electrode; and a water scattering suppression layer provided on the gas reaction section, The detection target gas includes at least one gas that generates water by reaction; The water scattering suppression layer is permeable to the gas to be detected and has a lower activity with respect to the gas to be detected than the gas reaction portion.

2. 2. The MEMS catalytic combustion gas sensor according to claim 1, wherein the gas to be detected is hydrogen.

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