JP2015227822A - Heat conduction type gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat conduction type gas sensor capable of performing a highly accurate measurement even when the temperature of sensor itself is constantly fluctuated due to the change of an environmental temperature and a disturbance.SOLUTION: A heat conduction type gas sensor includes: a detection element for detecting an object gas concentration; and a reference element for detecting an environmental temperature. The detection element and the reference element are disposed in the same space exposed to a measurement environment, and includes: heating means for heating the detection element on the basis of an output of the reference element; power control means for controlling a power to the heating means; and correction means for correcting the output of the detection element on the basis of the output of the reference element. The correction means obtains in advance an approximation formula with the output of the detection element measured under the environment of which target gas concentration is known, and determines a correction reference value by the approximation formula on the basis of the output of the reference element, and obtains the target gas concentration by obtaining a difference between the output of the detection element containing the target gas and the correction reference value.

Description

  The present invention relates to a heat conduction type gas sensor that detects a gas concentration from a change in heat conduction of a gas.

  The thermal conductivity gas sensor uses the fact that the thermal conductivity differs depending on the type of gas. It detects the change in thermal conductivity as a temperature change and electrically detects this temperature change as a resistance change in the thermal element. To do.

  For example, in Patent Document 1 (Japanese Patent No. 2889909), based on a change in the resistance value of a resistor heated in the atmosphere, a low temperature at which the resistance change is affected only by the ambient temperature, and a resistance change in the atmosphere. There is disclosed a humidity sensor that detects humidity by comparing a voltage at a low temperature with a voltage at a high temperature generated at both ends of the resistor at a high temperature that is sensitive to temperature and humidity.

Japanese Patent No. 2889909

  However, in an environment where the sensor is actually used, the sensor itself fluctuates due to the influence of environmental temperature change or disturbance. In such an environment, the sensor heating temperature is not stable even if a constant current or a constant voltage is applied in order to heat to a certain low temperature or high temperature. If the correction coefficient K is not adjusted in accordance with the change in the environmental temperature, the calculation accuracy is poor and the sensor accuracy is low.

  In another embodiment in the prior art document, two resistors are used to simultaneously heat one at a low temperature and the other at a high temperature, thereby eliminating the time difference between the low temperature and high temperature measurement due to switch switching. The correction accuracy by the ambient temperature is being improved. However, it is difficult to make the characteristics between the two resistors completely uniform, and this characteristic variation itself becomes the main factor that reduces the sensor accuracy.

  The object of the present invention has been made in consideration of the above points, and provides a heat conduction type gas sensor that can measure with high accuracy even if the temperature of the sensor itself constantly fluctuates due to the change of environmental temperature or the influence of disturbance. There is to do.

  In order to solve the above-mentioned problems, the invention according to claim 1 of the present invention provides a sensing element for detecting a target gas concentration, a reference element for detecting an environmental temperature, the sensing element and the reference element in a measurement environment. Located in the same exposed space, based on the output of the reference element, heating means for heating the sensing element, power control means for controlling power to the heating means, and based on the output of the reference element, A correction unit that corrects the output of the detection element; the correction unit obtains an approximate expression in advance using the output of the detection element measured in an environment in which the target gas concentration is known as a temperature characteristic; A heat conduction equation for determining a target gas concentration by determining a correction reference value by the approximate expression based on the output of the element and taking a difference between the output of the sensing element including the target gas and the correction reference value Ga It is a sensor.

  The invention according to claim 2 of the present invention is characterized in that the detection element has a membrane structure, and includes the heating means and a heat sensitive body that detects a temperature that changes according to the concentration of the target gas.

  The invention according to claim 3 of the present invention is characterized in that the heating means is heated by a pulse voltage, and the reference element detects an environmental temperature when the pulse voltage is not applied.

  The invention according to claim 4 of the present invention is characterized in that the sensing element and the reference element are formed on the same substrate.

  The invention according to claim 5 of the present invention is characterized in that the sensing element and the reference element are thin film thermistors.

  According to the present invention, even if the temperature of the sensor itself fluctuates due to environmental temperature changes or disturbances, a heat conduction type gas sensor that can be measured with high accuracy can be obtained.

It is a section construction diagram for explaining an embodiment in the present invention. BRIEF DESCRIPTION OF THE DRAWINGS It is the circuit block diagram outline for describing embodiment in this invention. It is a detection processing flowchart for demonstrating embodiment in this invention. It is a timing chart for demonstrating embodiment in this invention. It is a figure for demonstrating the correction | amendment means in this invention. It is a figure which shows the relationship between the environmental temperature in this invention, and the applied voltage for element heating. It is an output waveform diagram in the present invention. It is a relationship diagram of absolute humidity and output voltage.

  Hereinafter, embodiments of the present invention will be described. In addition, although the case where the heat conduction type gas sensor of the present invention is applied to a humidity sensor will be described as an example, as described above, the present invention is based on a detection principle utilizing the property that the thermal conductivity differs depending on the type of gas. It is based on and is not limited to humidity sensors.

  FIG. 1 is a cross-sectional structure diagram for explaining the humidity sensor of the present embodiment. The humidity sensor 1 according to the present embodiment includes a sensing element 2 that detects a target gas concentration and a reference element 3 that detects an environmental temperature, and is disposed in the same space exposed to the measurement environment. In the present embodiment, the sensing element 2 and the reference element 3 are arranged in the ceramic package 4, and the humidity sensor 1 is formed by the lid 5 having the vent 6 in order to be exposed to the measurement environment.

  The sensing element 2 includes a substrate 7, an insulating film 8, a microheater 9, a microheater protective film 10, a thin film thermistor electrode 11, a thin film thermistor 12, and a thin film thermistor protective film 13.

  The reference element 3 is the same as the detection element 2 except that the reference element 3 does not include the micro heater 9 that is a means for heating the detection element. With this configuration, the element characteristics of the detection element 2 and the reference element 3 can be made the same. That is, there is no difference in response time due to a difference in heat capacity, and the same behavior can always be obtained with respect to changes in environmental temperature.

  The sensing element 2 and the reference element 3 are formed on the same substrate. Thereby, since the ambient temperature is measured using the reference element 3 formed on the same substrate, there is no temperature difference between the detection element 2 and the reference element 3, and the output of the detection element 2 can be accurately corrected.

  Furthermore, by forming the detection element 2 and the reference element 3 adjacent to each other at the same time, variations in the manufacturing process are the same, and elements with uniform characteristics can be produced. As a result, it is possible to eliminate a selection step of combining elements having the same characteristics between elements.

  The substrate 7 is not particularly limited as long as it has a suitable mechanical strength and is suitable for fine processing such as etching. For example, a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, a glass substrate, or the like is suitable. An insulating film 8 such as a silicon oxide film or a silicon nitride film is formed on the front and back surfaces of the substrate. For example, in order to form a silicon oxide film as the insulating film 8, a thermal oxidation method or a film formation method by CVD (Chemical Vapor Deposition) may be applied. The film thickness may be such that the film formed on the insulating film 8 can be insulated from the substrate and function as an etching stop layer when the cavity 14 is formed. Usually, about 0.1-1.0 micrometer is suitable.

  The substrate 7 has a cavity 14 in which a part of the substrate is thinned corresponding to the position of the microheater 9 in order to suppress heat conduction to the substrate when the microheater 9 is operated at a high temperature. Yes. A portion where the substrate is removed by the cavity 14 is called a membrane 15. Since the heat capacity of the membrane 15 is reduced by the thickness of the substrate, the microheater 9 can be heated to a high temperature with very little power consumption. Further, since the conduction path to the substrate 7 is a heat insulating structure formed only by a thin film portion of several μm, the heat conduction to the substrate 7 is small, and the microheater 9 can be efficiently heated to a high temperature.

  The material of the microheater 9 is a metal layer made of a material having a relatively high melting point, which is a conductive substance that can withstand processes such as a film forming process and a heat treatment process of the thin film thermistor 12, for example, molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing any two or more thereof is preferable. Further, a conductive material capable of high-precision dry etching such as ion milling is preferable, and Pt or the like having higher corrosion resistance is more preferable. In order to improve the adhesion with the insulating film 8, it is preferable to form an adhesion layer such as titanium (Ti) under the Pt.

  In FIG. 1, a thin film thermistor 12 is formed as a heat sensitive body for detecting the temperature of the microheater 9 by gas. The thin film thermistor 12 includes a thin film thermistor electrode 11 and is formed so as to cover the microheater 9. Thereby, the temperature of the micro heater 9 can be directly detected.

  As a material of the thermistor for forming the thin film thermistor 12, a material having a negative temperature resistance coefficient such as a composite metal oxide, amorphous silicon, polysilicon, germanium or the like is formed using a thin film process such as sputtering or CVD. The film thickness may be adjusted according to the target thermistor resistance value. For example, if the resistance value (R25) at room temperature is set to about 140 kΩ using MnNiCo-based oxide, the distance between the electrodes of the element is set. However, it may be set to a film thickness of about 0.2 to 1 μm.

  A thin film thermistor 12 is suitable for detecting the temperature of the microheater 9. First, because of the laminated structure of thin films, the heat generated by the microheater 9 can be directly detected. Moreover, since the resistance temperature coefficient is larger than that of a platinum temperature detector, the detection sensitivity can be increased.

  In order to take out an electric signal from the thin film thermistor 12, a thin film thermistor electrode 11 is formed. The material of the thin film thermistor electrode 11 is a conductive material that can withstand processes such as a film forming process and a heat treatment process of the thin film thermistor 12, and has a relatively high melting point, such as molybdenum (Mo), platinum (Pt), gold ( Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or an alloy containing any two or more thereof is preferable.

  A microheater protective film 10 is formed so as to cover the microheater 9 and the insulating film 8. The microheater protective film 10 is preferably made of the same material as the insulating film 8. The microheater 9 is repeatedly subjected to thermal stress that rises to several hundred degrees and then drops to room temperature. Continuously receiving this thermal stress leads to destruction such as delamination and cracks. The same material has the same material characteristics, strong adhesion, and high mechanical strength compared to the case where different materials are laminated. For this reason, destruction can be prevented even against thermal stress of the microheater 9. In order to form, for example, a silicon oxide film as the microheater protective film 10, a thermal oxidation method or a film formation method by CVD may be applied. The film thickness is good enough to reliably cover the microheater 9 and to provide interlayer insulation. Usually, about 0.1-3.0 micrometers is suitable.

  In the case of using a composite metal oxide or the like for the thin film thermistor 12, the microheater protective film 10 is preferably an insulating oxide film, for example, a silicon oxide film. A thin film thermistor 12 and a thin film thermistor electrode 11 are formed on the microheater protective film 10. The microheater protective film 10 is a protective film for the microheater 9 and is also an underlayer for the thin film thermistor 12 and is in direct contact with the thin film thermistor 12.

  In general, a thermistor using a composite metal oxide has a reduction deterioration at a high temperature. Therefore, a method of coating the whole thermistor with a reduction resistant material is known. That is, when the thermistor is brought into contact with a reducing material and brought to a high temperature state, oxygen is taken from the thermistor to cause reduction, which affects the thermistor characteristics. Therefore, the thin film thermistor protective film 13 is also preferably an insulating oxide film such as a silicon oxide film.

  For the same reason, it is desirable that the thin film thermistor electrode 11 is formed on the substrate side of the thin film thermistor 12. That is, the thin film thermistor electrode 11 and the thin film thermistor 12 are laminated on the microheater 9 in this order via the microheater protective film 10 that is an insulating layer. That is, the thin film thermistor 12 is formed on the thin film thermistor electrode 11. In general, in the thin film electrode, an adhesion layer is formed in order to increase the adhesion between the electrode material and the base. For example, chromium (Cr), titanium (Ti), or the like is formed with a film thickness of about several nm. When the thin film thermistor electrode 11 is formed on the thin film thermistor 12, the adhesion layer is in direct contact with the thin film thermistor and is oxidized by depriving oxygen from the thermistor, thereby increasing the interface resistance and the detection characteristics of the thin film thermistor 7. Fluctuates and is not preferable.

  The thin film thermistor electrode 11 and the micro heater 9 are connected to the electrode pad 16 outside the membrane 15. The electrode pad 16 is electrically connected to an external circuit by a ceramic package electrode 18 or the like by a wire bond 17 or the like, and is formed of a material such as aluminum (Al) or gold (Au), for example, and may be laminated as necessary. Good.

  After the element is cut from the wafer state into individual pieces, the element is fixed to the package 4 using a die paste (not shown), and then the electrode pad 16 and the package electrode 18 are wire-bonded using a wire bonding apparatus. Connect with 17. The wire 17 is preferably a metal wire having a low resistance such as Au, Al, or Cu.

  Finally, the lid 5 provided with the vent 6 for the package 4 and the outside air is fixed using a resin (not shown). At this time, when the resin (not shown) is cured and heated, a substance contained in the resin is generated as a gas. However, since it is easily released from the package through the vent 6, the element itself is adversely affected. There is no. Thus, the humidity sensor 1 can be obtained.

  Subsequently, the gas detection operation will be described with reference to FIGS. 2 is a schematic circuit configuration diagram, FIG. 3 is a flow chart of detection processing, and FIG. 4 is a timing chart.

  First, as shown in FIG. 4, the humidity sensor 1 operates the micro heater 9 by intermittent operation, that is, so-called pulse drive. As for the measurement timing, the output Vc of the reference element 3 is detected when the microheater 9 is OFF, the membrane 15 is heated when the microheater 9 is ON, and the output Vd of the detection element 2 is detected.

  The output Vc is a value that is output only affected by the environmental temperature without being affected by humidity, and the output Vd is a value that is output including the influence of humidity and environmental temperature. The microheater 9 is generally heated to a temperature of 150 ° C. or higher in the case of humidity although it depends on the target gas to be detected. This is because if the temperature of the air is low, the change in the thermal conductivity of the air due to humidity is small and the sensitivity is lowered.

  Since the membrane 15 has a very small heat capacity, it immediately reaches a desired temperature when the micro heater 9 is turned on, and immediately returns to the environmental temperature when it is turned off. In the humidity measurement, the output Vd of the sensing element 2 and the output Vc of the reference element 3 measured immediately before that are one cycle of the measurement flow. For example, a combination of 27 Vc and Vd for pulse 24, 28 Vc and Vd for pulse 25, and 29 Vc and Vd for pulse 26 is one cycle.

  Here, the output Vc of the reference element 3 is detected at a timing when the membrane 9 is not heated. This is because the sensing elements 2 are formed adjacent to each other, so when the membrane 15 is heated, the heat is instantaneously transferred to the reference element 3 through the substrate or space (air) as a main path, and the accurate environment This is because the temperature cannot be measured. That is, the heat transmitted through the space (air) is influenced by the thermal conductivity of the gas present in the space (air), so it is not the same every time. It does not mean that the ambient temperature is being measured.

  Next, a description will be given with reference to FIG. First, as shown in FIG. 3A, the humidity sensor 1 controls the voltage applied to the micro heater 9 of the detection element 2 from the output Vc based on the environmental temperature obtained from the reference element 3. That is, since the temperature at which the membrane 15 is heated fluctuates due to the influence of the environmental temperature, the membrane heating temperature is not stable when heated at a constant voltage or constant current. On the other hand, desired stable heating can always be realized by controlling the voltage applied to the micro heater 9 on the basis of the environmental temperature. FIG. 6 shows the relationship. A relational expression between the environmental temperature and the applied voltage is made in advance, and the applied voltage of the microheater 9 is determined based on this.

  In the present embodiment, the microheater 9 is driven so as to be heated with constant power consumption. That is, when the microheater 9 is made of, for example, platinum (Pt) having a positive temperature coefficient, the resistance of the microheater 9 rises following the environmental temperature as the environmental temperature increases, so that the power consumption is constant. Therefore, the applied voltage is adjusted high. Conversely, when the environmental temperature decreases, the resistance of the microheater 9 also decreases following the environmental temperature. Therefore, the applied voltage is adjusted to be low in order to achieve a certain constant power consumption.

  For example, in the pulses 24, 25, and 26 shown in FIG. 4 and subsequent pulses (not shown), the voltage applied to the micro heater 9 is adjusted according to the environmental temperature at that time. In the microheater 9 according to the present embodiment, the applied voltage is adjusted to a high value when the environmental temperature is high at the timing of the pulse 25 with respect to the environmental temperature at the timing of the pulse 24, and at the timing of the pulse 24. On the other hand, when the environmental temperature is low at the timing of the pulse 26, the applied voltage is adjusted to a low value. Thus, according to the present invention, the problem that occurs when heating at a constant current or a constant voltage, the heating temperature fluctuates due to the influence of the fluctuation of the environmental temperature, and cannot be stably controlled to a desired temperature. Solved.

  Next, as shown in FIG. 3B, the output Vd of the detection element 2 is detected while being heated by the microheater 9. This output Vd is a value that is output including the influence of humidity and environmental temperature, and it is necessary to correct the environmental temperature from this output Vd.

  In parallel with this, as shown in FIG. 3 (e), a correction reference value Vref for correcting the output Vd detected in FIG. 3 (b) is calculated from the output Vc based on the ambient temperature obtained from the reference element 3. . Here, the correction reference value Vref will be described in detail. Here, an example in which measurement is performed in an environment where the target gas concentration is 0% will be described.

  The correction reference value Vref is an output Vd (0) that does not include the influence of humidity at a certain environmental temperature. Here, in order to distinguish the output Vd including the influence of humidity from the convenience of explanation, it is expressed as Vd (0). Vd (0) means an output Vd obtained by measurement in a humidity 0% RH or humidity 0 g / m 3 environment. The humidity sensor 1 is measured in advance in an environment with a humidity of 0% RH or a humidity of 0 g / m3, and the relationship between the environmental temperature and the output Vd (0) is obtained by an approximate expression. (A) in FIG.

  The output Vc at the ambient temperature obtained from the reference element 3 is substituted into the approximate expression for the previously obtained output Vd (0) to determine the correction reference value Vref at a certain ambient temperature t0. When the humidity sensor 1 is measured in an environment including humidity, an output Vd is obtained with an output change due to a difference in heat conduction of air due to the humidity. (B) in FIG. The difference ΔV between (a) and (b) in FIG. 5 is an output change due to humidity, and the environmental temperature is corrected by taking the difference between the output Vd (t0) at a certain temperature t0 and the correction reference value Vref. Thus, the temperature change ΔV due to humidity alone can be obtained. FIG. 3 (c). By calculating using the difference calculation result and the output Vc based on the environmental temperature obtained from the reference element 3, the relative humidity and the absolute humidity can be calculated. FIG. 3 (d).

  Also at this time, the calculation processing is performed based on the temperature information of the adjacently formed reference element 3. FIG. 3 (f). That is, the temperature sensor formed on the same substrate has no temperature distribution with the detection element 2 and shows an accurate temperature at that time. By using this temperature as all the references in the arithmetic processing, it is possible to calculate the applied voltage of the microheater 9, the correction reference value Vref, and the humidity more accurately.

  It is also important to obtain an approximate expression of the output Vd (0) in advance using the humidity sensor 1 that is actually used. That is, when the output Vd (0) is obtained and used using a humidity sensor of the same form as a representative value, the humidity cannot be accurately calculated due to variations in individual sensor characteristics. Such a problem does not occur.

  Next, description will be made with reference to the schematic circuit configuration diagram of FIG. The sensing element 2 and the reference element 3 constituting the humidity sensor 1 are connected to external fixed resistors R2 and R1 via the ceramic package electrode 18 to constitute a bridge circuit.

  The fixed resistors R2 and R1 do not have to have the same resistance value, and the resistance value Rc of the reference element 3 and the resistance value Rd of the detection element 2 constituting the half-bridge circuit are values that allow the output Vc and the output Vd to be large, respectively. Can be adjusted individually. As a result, high-sensitivity sensors can be obtained because highly sensitive measurements are possible. For example, the reference element 3 may have a resistance value that is easy to handle in the ambient temperature range in which it is used, and is optimally from several kiloohms to several hundred kiloohms. This is because if the resistance value is too small, the value of the current flowing through the element increases, resulting in an increase in power consumption. On the other hand, if it is too large, the signal tends to contain noise. On the other hand, in the detection element 2, the resistance value when heated is adjusted to an easy-to-handle value, and for example, when heated to 200 ° C., it is about several kiloΩ to several hundred kiloΩ at 200 ° C. It is best to design.

  The output Vc of the reference element 3 is input to the microprocessor 23 via the voltage follower 19 and the analog / digital converter 20. The microprocessor 23 stores an approximate expression of Vd (0) in advance, and the applied voltage to the microheater 9 and the correction reference value Vref are calculated according to the output Vc of the reference element 3. The calculated value is output to the micro heater 9 and the amplifier circuit 21 via the digital / analog converter 22.

  A voltage is applied to the microheater 9 to heat the membrane 15 of the detection element 2. At this timing, the output Vd is detected. Thereafter, the output Vd is input to the amplifier circuit 21 and is processed with the correction reference value Vref. Here, the influence due to the environmental temperature is corrected, and the output change ΔV only due to the influence of the humidity is obtained. Further, this signal is input to the microprocessor 23, the environmental temperature is calculated from the previous output Vc, and the relative humidity and absolute humidity are calculated based on the environmental temperature.

(Example)
The output characteristic of the humidity sensor based on embodiment of this invention is disclosed. FIG. 7 shows output waveforms at certain times ta, tb, and tc in the circuit of FIG. 2, and Table 1 shows output values in FIG.
When ta, 35 ° C 30% RH (11.88 g / m3), when tb, 25 ° C 50% RH (11.50 g / m3), and when tc, 15 ° C 90% RH (11.52 g / m3) The humidity sensor 1 was put in the environment set to Measured. The output Vd (0) for determining the correction reference value Vref is measured in advance in an environment with a humidity of 0%, and the relationship with temperature is obtained as an approximate expression of a quadratic function (Formula 1). b and c are shown in Table 2.

Note that the output ΔV based only on the influence of humidity, which is calculated by the output Vd and the correction reference value Vref, has temperature characteristics and is expressed by an approximate expression of a quadratic function (Expression 1) as shown in FIG. The coefficients a, b, and c in the approximate expression at 15 ° C., 25 ° C., and 35 ° C. are shown together in Table 2.
The ambient temperature was calculated from the output Vc by using the B constant of the thin film thermistor 12, and the humidity was calculated by using a quadratic function at each temperature.

  The output at the time of ta was 11.76 g / m3 with respect to the set value of 11.88 g / m3. The output at tb was 11.43 g / m3 with respect to the set value of 11.50 g / m3. The output at tc was 11.55 g / m3 with respect to the set value of 11.52 g / m3. As described above, it is possible to provide a heat conduction type gas sensor that can measure with high accuracy even when the temperature of the sensor itself fluctuates due to the change of environmental temperature or the influence of disturbance.

  The present invention is suitable for a heat conduction type gas sensor that detects a gas concentration from a change in gas heat conduction.

DESCRIPTION OF SYMBOLS 1 Humidity sensor 2 Sensing element 3 Reference element 4 Ceramic package 5 Lid 6 Vent 7 Substrate 8 Insulating film 9 Micro heater 10 Micro heater protective film 11 Thin film thermistor electrode 12 Thin film thermistor 13 Thin film thermistor protective film 14 Cavity 15 Membrane 16 Electrode pad 17 Wire 18 Ceramic package electrode 19 Voltage follower 20 Analog-digital converter 21 Amplifier circuit 22 Digital-analog converter 23 Microprocessor 24, 25, 26 Pulse

Claims (5)

  The sensing element for detecting the target gas concentration, the reference element for detecting the environmental temperature, the sensing element and the reference element are arranged in the same space exposed to the measurement environment, and the sensing element is based on the output of the reference element. Heating means for heating the element; power control means for controlling power to the heating means; and correction means for correcting the output of the detection element based on the output of the reference element. Determining an approximate expression using the output of the sensing element measured in an environment where the target gas concentration is known as a temperature characteristic, determining a correction reference value by the approximate expression based on the output of the reference element, and A heat conduction type gas sensor characterized in that a target gas concentration is obtained by taking a difference between an output of the sensing element containing gas and the correction reference value.   2. The heat conduction type gas sensor according to claim 1, wherein the detection element has a membrane structure, and includes the heating unit and a heat sensitive body that detects a temperature that changes according to the concentration of the target gas.   The heat conduction type gas sensor according to claim 1 or 2, wherein the heating means is heated by a pulse voltage, and the reference element detects an environmental temperature when the pulse voltage is not applied.   4. The heat conduction type gas sensor according to claim 1, wherein the detection element and the reference element are formed on the same substrate. The heat conduction type gas sensor according to any one of claims 1 to 4, wherein the detection element and the reference element are thin film thermistors.

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