gas sensor

Abstract

In a gas sensor using a heater, the change over time of temperature-sensing elements such as thermistors is suppressed. [Solution] The gas sensor 100 comprises a base material 110 having a cavity 111, a heater MH1 supported on the base material 110 so as to overlap with the cavity 111, a thermistor Rd1 supported on the base material 110 so as not to overlap with the heater MH1, and a signal processing circuit 20 that heats the heater MH1 when measuring the gas concentration. The temperature of the thermistor Rd1 when measuring the gas concentration changes in accordance with the change in the temperature of the heater MH1 due to heat transmitted from the heater MH1 mainly through the base material 110.

Description

Technical Field

[0001] The present disclosure relates to a gas sensor, and particularly to a gas sensor using a heater.

Background Art

[0002] Patent Document 1 discloses a gas sensor that measures the concentration of a target gas in a measurement atmosphere by heating a detection thermistor to 100 to 200 °C with a heater and heating a reference thermistor to 250 to 350 °C with the heater, and referring to the output voltage that appears at the connection point between the detection thermistor and the reference thermistor in this state.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the gas sensor described in Patent Document 1, since both the detection thermistor and the reference thermistor are disposed directly above the heater, the detection thermistor and the reference thermistor themselves are heated to a high temperature. As a result, there is a problem that the change over time of the detection thermistor and the reference thermistor is accelerated.

[0005] In the present disclosure, a technique for suppressing the change over time of temperature-sensitive elements such as thermistors in a gas sensor using a heater is described.

Means for Solving the Problems

[0006] A gas sensor according to one aspect of this disclosure comprises a substrate having a first cavity, a first heater supported on the substrate so as to overlap with the first cavity, a first temperature-sensing element supported on the substrate so as not to overlap with the first heater, and a signal processing circuit that heats the first heater when measuring gas concentration, wherein the temperature of the first temperature-sensing element when measuring gas concentration changes in accordance with the change in the temperature of the first heater due to heat transmitted from the first heater mainly through the substrate. [Effects of the Invention]

[0007] According to this disclosure, a technology is provided for suppressing changes in the temperature sensing element over time in a gas sensor using a heater. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a circuit diagram showing the configuration of a gas sensor 100 according to a first embodiment of the technology described herein. [Figure 2] Figure 2 is a schematic plan view showing the configuration of the sensor unit 10. [Figure 3] Figure 3(a) is a schematic cross-sectional view along the line A-A' shown in Figure 2, and Figure 3(b) is a schematic cross-sectional view along the line B-B' shown in Figure 2. [Figure 4] Figure 4 is a flowchart illustrating the operation of the gas sensor 100 during gas concentration measurement. [Figure 5] Figure 5 is a timing diagram illustrating the operation of the gas sensor 100. [Figure 6] Figure 6 is a schematic plan view showing the configuration of the sensor unit 10A according to the first modified example. [Figure 7] Figure 7(a) is a schematic cross-sectional view along the line A-A' shown in Figure 6, and Figure 7(b) is a schematic cross-sectional view along the line B-B' shown in Figure 6. [Figure 8] Figure 8 is a schematic plan view showing the configuration of the sensor unit 10B according to the second modified example. [Figure 9] Figure 9 is a schematic cross-sectional view along the line C-C' shown in Figure 8. [Figure 10]FIG. 10 is a circuit diagram showing the configuration of the gas sensor 200 according to the second embodiment of the technology according to the present disclosure. [Figure 11] FIG. 11 is a table for explaining the function of the multiplexer 27. [Figure 12] FIG. 12 is a flowchart for explaining the operation of the gas sensor 200 when measuring the gas concentration. [Figure 13] FIG. 13 is a timing diagram for explaining the operation of the gas sensor 200. [Figure 14] FIG. 14 is a schematic plan view showing the configuration of the sensor unit 30. [Figure 15] FIG. 15 is a schematic plan view showing the configuration of the sensor unit 30A according to the first modification. [Figure 16] FIG. 16 is a schematic plan view showing the configuration of the sensor unit 30B according to the second modification. [Figure 17] FIG. 17 is a schematic cross-sectional view taken along the line A-A' shown in FIG. 16. [Figure 18] FIG. 18 is a schematic plan view showing the configuration of the sensor unit 30C according to the third modification. [Figure 19] FIG. 19 is a circuit diagram showing the configuration of the gas sensor 300 according to the third embodiment of the technology according to the present disclosure. [Figure 20] FIG. 20 is a table for explaining the function of the multiplexer 28. [Figure 21] FIG. 21 is a circuit diagram showing the configuration of the gas sensor 400 according to the fourth embodiment of the technology according to the present disclosure. [Figure 22] FIG. 22 is a schematic plan view showing the configuration of the sensor unit 50. [Figure 23] FIG. 23 is a schematic cross-sectional view taken along the line D-D' shown in FIG. 22. [Figure 24] FIG. 24 is a circuit diagram showing the configuration of the gas sensor 500 according to the fifth embodiment of the technology according to the present disclosure. [Figure 25] FIG. 25 is a schematic plan view showing the configurations of the sensor unit 70 and the temperature sensor 80.

MODE FOR CARRYING OUT THE INVENTION

[0009] Hereinafter, embodiments of the technology according to the present disclosure will be described in detail while referring to the accompanying drawings.

[0010] <First Embodiment>

[0011] FIG. 1 is a circuit diagram showing the configuration of a gas sensor 100 according to a first embodiment of the technology according to the present disclosure.

[0012] As shown in FIG. 1, the gas sensor 100 according to the first embodiment includes a sensor unit 10 and a signal processing circuit 20. Although not particularly limited, the gas sensor 100 according to the first embodiment is a thermal conductivity type gas sensor for detecting the concentration of CO2 gas in the measurement atmosphere.

[0013] The sensor unit 10 includes a thermistor Rd1, a fixed resistor R1, and a heater MH1 that are connected in series in this order between a power supply Vcc and a ground GND. The temperature of the thermistor Rd1 changes in response to a change in the temperature of the heater MH1. The thermistor Rd1 is a temperature-sensitive element whose resistance value changes with temperature. As will be described later, a temperature detection signal Vtemp appears at the connection point N1 between the thermistor Rd1 and the fixed resistor R1 during the temperature detection period, and a gas detection signal Vgas appears during the gas detection period.

[0014] During the gas detection period, the heater MH1 is heated to a first temperature range. The first temperature range is, for example, a predetermined temperature range included within a range of 100°C or higher and 230°C or lower, for example, a temperature range around 170°C. The "temperature range" in this specification has a temperature width of, for example, within 1°C. For example, the temperature range around 170°C may be a region of 169.5°C or higher and 170.5°C or lower.

[0015] In the first temperature range, the thermal conductivity of CO2 gas is lower than that of air. Therefore, when a constant power is applied to heater MH1 to heat it to the first temperature range and CO2 gas is present in the measurement atmosphere, the higher the CO2 gas concentration, the higher the temperature of heater MH1 rises, and as a result, the temperature of thermistor Rd1 also rises. For this reason, if heater MH1 is heated to 170°C when the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm), if the CO2 gas concentration in the measurement atmosphere exceeds the normal concentration of CO2 gas in the atmosphere, the temperature of heater MH1 will rise above 170°C according to the concentration, and the temperature of thermistor Rd1 will also be higher than when the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm). As a result, for example, if thermistor Rd1 has a negative temperature coefficient of resistance (i.e., if thermistor Rd1 is an NTC thermistor), the higher the CO2 gas concentration in the measurement atmosphere, the lower the resistance value of thermistor Rd1 becomes. In this way, when heater MH1 is heated to a first temperature range and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of heater MH1 change according to its concentration, and this change manifests as a change in the temperature of thermistor Rd1, that is, a change in the resistance value of thermistor Rd1.

[0016] Furthermore, since thermistor Rd1 and fixed resistor R1 are connected in series between the power supply Vcc and ground GND, if thermistor Rd1 has a negative temperature coefficient of resistance, the level of the gas detection signal Vgas appearing at connection point N1 will be higher as the CO2 gas concentration in the measurement atmosphere increases.

[0017] The signal processing circuit 20 includes a multiplexer (MUX) 21, differential amplifiers 22 and 23, an AD converter (ADC) 24, a DA converter (DAC) 25, and a control circuit 26.

[0018] The multiplexer 21 is a circuit that connects connection point N1 included in the sensor unit 10 to either selection node S1 or S2, and its selection operation is controlled by the control circuit 26. The differential amplifier 22 compares the gas detection signal Vgas appearing at selection node S1 with the reference potential Vref1 to generate an amplified signal Vamp1, which is the level difference between the gas detection signal Vgas and the reference potential Vref1 (=Vgas-Vref1). The differential amplifier 23 compares the temperature detection signal Vtemp appearing at selection node S2 with the reference potential Vref2 to generate an amplified signal Vamp2, which is the level difference between the temperature detection signal Vtemp and the reference potential Vref2 (=Vtemp-Vref2). The amplified signals Vamp1 and Vamp2 are input to the AD converter 24. The AD converter 24 generates digital values ​​by performing AD conversion on the amplified signals Vamp1 and Vamp2 and supplies these to the control circuit 26.

[0019] The control circuit 26 calculates the concentration of the CO2 gas to be measured based on the AD-converted amplified signal Vamp1 and generates an output signal Vout indicating the CO2 gas concentration. The output signal Vout is output to the outside of the gas sensor 100. The CO2 gas concentration can also be calculated using a calculation formula set within the control circuit 26. Furthermore, the control circuit 26 supplies the digital values ​​of various control parameters to the DA converter 25. The DA converter 25 generates the heater voltage Vmh1 and reference potentials Vref1 and Vref2 by converting the digital values ​​of the various control parameters into analog. The heater voltage Vmh1 is applied to the heater MH1, thereby heating the heater MH1. The reference potentials Vref1 and Vref2 are supplied to the differential amplifiers 22 and 23, respectively. The levels of the reference potentials Vref1 and Vref2 may be the same.

[0020] Figure 2 is a schematic plan view showing the configuration of the sensor unit 10. Figure 3(a) is a schematic cross-sectional view along the line A-A' shown in Figure 2, and Figure 3(b) is a schematic cross-sectional view along the line B-B' shown in Figure 2.

[0021] As shown in Figures 2 and 3, the sensor unit 10 comprises a base material 110 consisting of a main body 120 and insulating films 121 and 122 formed on its lower and upper surfaces, respectively; a heater MH1 and a pair of thermistor electrodes 131 and 132 provided on the insulating film 122; a thermistor resistor 130 covering the pair of thermistor electrodes 131 and 132; and an insulating film 123 covering the heater MH1 and thermistor resistor 130. The thermistor resistor 130 and the pair of thermistor electrodes 131 and 132 constitute the thermistor Rd1 shown in Figure 1.

[0022] The substrate 110 is a support that supports the heater MH1 and thermistor Rd1. The main body 120 of the substrate 110 is not particularly limited as long as it has appropriate mechanical strength and is made of a material suitable for microfabrication such as etching, and can be a silicon substrate, sapphire substrate, ceramic substrate, quartz substrate, glass substrate, etc. In order to improve the thermal efficiency of the heater MH1, a cavity 111 is provided in the substrate 110 at a position that overlaps with the heater MH1 in a plan view from the Z direction. In the region where the cavity 111 is provided, the insulating film 121 of the substrate 110 is removed, and the thickness of the main body 120 of the substrate 110 in the Z direction is locally reduced, or the main body 120 and the insulating film 121 of the substrate 110 are removed. In the example shown in Figure 3(a), the main body portion 120 and the insulating film 121 of the substrate 110 are removed in the region where the cavity 111 is provided, so that the heater MH1 is supported by the substrate 110 via insulating films 122 and 123.

[0023] The insulating films 121-123 may be made of inorganic insulating materials such as silicon oxide or silicon nitride. The heater MH1 has a configuration in which wiring made of a metallic material with 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 two or more of these, is arranged in a meandering pattern. One end of the heater MH1 is connected to a terminal electrode 143 to which the heater voltage Vmh1 is supplied. The other end of the heater MH1 is connected to a terminal electrode 144 to which the ground potential GND is supplied.

[0024] The thermistor resistor 130 is made of a material whose resistance changes with temperature, such as vanadium oxide, amorphous silicon, polycrystalline silicon, a spinel-type crystalline oxide containing manganese, titanium oxide, or yttrium-barium-copper oxide. A pair of thermistor electrodes 131 and 132 are in contact with the thermistor resistor 130. Thus, the resistance between the pair of thermistor electrodes 131 and 132 is determined by the resistance of the thermistor resistor 130 located between these electrodes. The distance W1 between thermistor electrodes 131 and 132 is, for example, 5 to 8 μm and can be selected according to the desired resistance value of the thermistor Rd1. Thermistor electrode 131 is connected to terminal electrode 141 to which the power supply potential Vcc is supplied. Thermistor electrode 132 is connected to terminal electrode 142 which constitutes connection point N1.

[0025] As shown in Figure 2, in this embodiment, the thermistor Rd1 is positioned so as not to overlap with the heater MH1. In other words, in this embodiment, the planar positions of the heater MH1 and thermistor Rd1 are different. In the example shown in Figure 2, the positions of the heater MH1 and thermistor Rd1 in the X direction are different. Therefore, thermistor Rd1 is not directly heated by the heater MH1, but is mainly heated by heat conduction through the base material 110. In this embodiment, the heater MH1 and thermistor Rd1 are separated by a part of the base material 110 (first part 118). The thickness of the first part 118 of the base material 110 in the Z direction is greater than the thickness of the part of the base material 110 that overlaps with the cavity 111 in the Z direction. Also, in this embodiment, there is no cavity in the base material 110 at a position that overlaps with thermistor Rd1.

[0026] With this configuration, if the CO2 gas concentration in the measurement atmosphere is the same as the normal atmospheric CO2 gas concentration (e.g., 400 ppm), and the heater MH1 is heated to, for example, 170°C, then if the CO2 gas concentration in the measurement atmosphere exceeds the normal atmospheric CO2 gas concentration, the temperature of the heater MH1 will rise above 170°C, depending on the concentration. The heat from the heater MH1 is mainly conducted to the thermistor Rd1 via the substrate 110. As a result, the temperature of thermistor Rd1 will also be higher than when the CO2 gas concentration in the measurement atmosphere is the same as the normal atmospheric CO2 gas concentration (e.g., 400 ppm). Consequently, if thermistor Rd1 has a negative temperature coefficient of resistance (i.e., if thermistor Rd1 is an NTC thermistor), the higher the CO2 gas concentration in the measurement atmosphere, the lower the resistance value of thermistor Rd1 will be.

[0027] Thus, in this embodiment, the temperature of thermistor Rd1 changes in accordance with the change in the temperature of heater MH1 due to heat mainly transmitted from heater MH1 through the substrate 110. Compared to the case where thermistor Rd1 is placed directly above or below heater MH1 so as to overlap with heater MH1 when viewed from the Z direction, the rise in the temperature of thermistor Rd1 is significantly suppressed. For example, when heater MH1 is heated from room temperature to 170°C, the rise in the temperature of thermistor Rd1 is limited to a rise of a few degrees from room temperature. The temperature of thermistor Rd1 when heater MH1 is heated can be adjusted by the distance D1 between the thermistor resistor 130 and heater MH1.

[0028] Next, the operation of the gas sensor 100 according to the first embodiment during gas concentration measurement will be described.

[0029] Figure 4 is a flowchart illustrating the operation of the gas sensor 100 during gas concentration measurement. Figure 5 is a timing diagram illustrating the operation of the gas sensor 100. Gas concentration measurement is performed during the period T shown in Figure 5.

[0030] First, the control circuit 26 included in the signal processing circuit 20 selects the selection node S2 by controlling the multiplexer 21 and supplies a reference potential Vref2 to the differential amplifier 23 via the DA converter (step 151). This generates a temperature detection signal Vtemp indicating the current temperature of the thermistor Rd1. At this time, by not supplying a heater voltage Vmh1 to the heater MH1, the temperatures of thermistor Rd1 and heater MH1 are maintained at the ambient temperature, so the temperature detection signal Vtemp can be considered a signal indicating the ambient temperature. Ambient temperature is the temperature of the measurement atmosphere. Therefore, the temperature detection signal Vtemp appearing at the connection point N1 in this state will be at a level corresponding to the current ambient temperature. The temperature detection signal Vtemp is converted into an amplified signal Vamp2 by the differential amplifier 23 and then digitally converted by the AD converter 24. Next, the control circuit 26 calculates the current ambient temperature based on the digitally converted amplified signal Vamp2 (step 152). These operations are performed during the temperature detection period T1 shown in Figure 5. The temperature detection period T1 is the first half of the gas concentration measurement period T.

[0031] Next, the control circuit 26 calculates the heater voltage Vmh1 based on the ambient temperature calculated based on the amplified signal Vamp2 (step 153). For example, regardless of the current ambient temperature, if the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm), the level of the heater voltage Vmh1 is set so that the heater MH1 is heated to 170°C. Step 152 may be omitted, and the heater voltage Vmh1 may be calculated directly based on the amplified signal Vamp2 in step 153. The heater voltage Vmh1 thus calculated is supplied to the heater MH1 (step 154).

[0032] In this state, the control circuit 26 selects the selection node S1 by controlling the multiplexer 21 and supplies a reference potential Vref1 to the differential amplifier 22 via the DA converter (step 155). At connection point N1, a gas detection signal Vgas appears at a level obtained by dividing the power supply potential Vcc by the resistance value of thermistor Rd1 and the resistance value of fixed resistor R1. The resistance value of thermistor Rd1 is affected not only by the heat of heater MH1 transmitted through the substrate 110 but also by the ambient temperature. For this reason, the control circuit 26 changes the reference potential Vref1 according to the ambient temperature (amplified signal Vamp2) so that the amplified signal Vamp1 does not change due to the ambient temperature. The reference potential Vref1 according to the ambient temperature can be calculated, for example, by the following method. First, the gas detection signal Vgas is measured while changing the ambient temperature, with the CO2 gas concentration maintained at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). The heater voltage Vmh1 is set to a level that heats heater MH1 to, for example, 170°C, regardless of the ambient temperature. This allows us to determine the relationship between the ambient temperature (amplified signal Vamp2) and the gas detection signal Vgas when the CO2 gas concentration is maintained at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). The level of the gas detection signal Vgas corresponding to each ambient temperature (each amplified signal Vamp2) is then determined as the level of the reference potential Vref1 at that ambient temperature (each amplified signal Vamp2). The relationship between the ambient temperature (amplified signal Vamp2) and the reference potential Vref1 may be stored in the control circuit 26 in the form of an approximate formula, for example.

[0033] The gas detection signal Vgas is converted into an amplified signal Vamp1 by the differential amplifier 22. The control circuit 26 then calculates the CO2 gas concentration based on the A / D converted amplified signal Vamp1 and outputs an output signal Vout indicating the CO2 gas concentration (step 156). The operation from step 153 to step 156 is performed during the gas detection period T2 shown in Figure 5. The gas detection period T2 is the latter half of the gas concentration measurement period T.

[0034] By periodically performing such gas concentration measurements, it becomes possible to periodically detect changes in the concentration of CO2 gas in the measurement atmosphere.

[0035] As described above, in this embodiment, the gas sensor 100 is arranged so that the thermistor Rd1 and heater MH1 do not overlap, by positioning them at different locations on the base material 110. As a result, the temperature of the thermistor Rd1 during gas concentration measurement is sufficiently lower than the heating temperature of the heater MH1. This makes it possible to suppress changes over time caused by the thermistor Rd1 being heated to a high temperature.

[0036] Furthermore, in this embodiment, the temperature rise of the thermistor Rd1 during heating of the heater MH1 is significantly suppressed, resulting in a small difference between the temperature of thermistor Rd1 during the temperature detection period T1 and the temperature of thermistor Rd1 during the gas detection period T2. Therefore, thermistor Rd1 can be used not only for gas concentration detection but also for temperature detection, eliminating the need to provide a separate thermistor for temperature detection, as is common with typical gas sensors.

[0037] Furthermore, in this embodiment, since there is no cavity in the substrate 110 in the area directly below the thermistor Rd1, the heat capacity near the thermistor Rd1 is large. Therefore, by setting the heating time of the heater MH1 to a relatively long time, the temperature of thermistor Rd1 stabilizes, making it possible to measure the CO2 gas concentration more accurately.

[0038] Figure 6 is a schematic plan view showing the configuration of the sensor unit 10A according to the first modified example. Figure 7(a) is a schematic cross-sectional view along the line A-A' shown in Figure 6, and Figure 7(b) is a schematic cross-sectional view along the line B-B' shown in Figure 6.

[0039] As shown in Figures 6 and 7, the sensor unit 10A according to the first modification differs from the sensor unit 10 shown in Figures 2 and 3 in that another cavity 112 is provided in the base material 110. Since the other basic configurations are the same as those of the sensor unit 10 shown in Figures 2 and 3, the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0040] In the region where the cavity 112 is provided, the insulating film 121 of the substrate 110 is removed, and the thickness of the main body portion 120 of the substrate 110 in the Z direction is locally reduced, or the main body portion 120 and the insulating film 121 of the substrate 110 are removed. In the first modified example, the thermistor Rd1 is provided in a position that overlaps with the cavity 112 in a plan view from the Z direction. In the example shown in Figure 7(b), the main body portion 120 and the insulating film 121 of the substrate 110 are removed in the region where the cavity 112 is provided, and the thermistor Rd1 is supported by the substrate 110 via insulating films 122 and 123. The thickness of the substrate 110 in the Z direction in the first portion 118 located between the cavities 111 and 112 is greater than the thickness of the substrate 110 in the Z direction at the position that overlaps with the cavity 111 or cavity 112. The heat from the heated heater MH1 is mainly conducted to the thermistor Rd1 by conducting through the first portion 118 of the substrate 110 located between the cavities 111 and 112.

[0041] As in the sensor section 10A of the first modification, if another cavity 112 is provided in the base material 110 not only at a position overlapping with the heater MH1 but also at a position overlapping with the thermistor Rd1, the heat capacity near the thermistor Rd1 is reduced, thereby improving the temperature response of the thermistor Rd1 to heating by the heater MH1. This makes it possible to measure the concentration of CO2 gas in a shorter time. The temperature of thermistor Rd1 when the heater MH1 is heated can be adjusted by the distance D2 between the cavities 111 and 112.

[0042] Figure 8 is a schematic plan view showing the configuration of the sensor unit 10B according to the second modified example. Figure 9 is a schematic cross-sectional view along the line C-C' shown in Figure 8.

[0043] As shown in Figures 8 and 9, in the sensor section 10B according to the second modification, a cavity 113 is provided in the substrate 110, and the heater MH1 and thermistor Rd1 are arranged so as to overlap with the cavity 113. In the position overlapping with the cavity 113, slits SL1 are provided in the insulating films 122 and 123, except for the portion overlapping with the heater MH1 and thermistor Rd1 and the portion located around the heater MH1 and thermistor Rd1. As a result, the heater MH1 and the insulating films 122 and 123 supporting it constitute a membrane 161 that overlaps with the cavity 113, and thermistor Rd1 and the insulating films 122 and 123 supporting it constitute a membrane 162 that overlaps with the cavity 113. In this embodiment, the heater MH1 and thermistor Rd1 are separated by space.

[0044] The membranes 161 and 162 may have a portion of the main body 120 of the substrate 110 remaining, or the main body 120 and insulating film 121 of the substrate 110 may be completely removed, leaving the insulating film 122 exposed on the back surface. In the example shown in Figure 9, the main body 120 and insulating film 121 of the substrate 110 have been removed in the region where the cavity 113 is provided, and as a result, the insulating film 122 is exposed on the back surface of the membranes 161 and 162.

[0045] The membrane 161 is supported by the substrate 110 by bridges consisting of wiring 171, 172 connecting the heater MH1 and terminal electrodes 143, 144, wiring 173, 174 connected to dummy electrodes 145, 146, and insulating films 122, 123 located around these wirings 171-174. The membrane 162 is supported by the substrate 110 by bridges consisting of wiring 181 connecting the thermistor electrode 131 and terminal electrode 141, wiring 182 connecting thermistor electrode 132 and terminal electrode 142, wiring 183, 184 connected to dummy electrodes 147, 148, and insulating films 122, 123 located around these wirings 181-184. The bridges supporting the membranes 161 and 162 may include a portion of the substrate 110.

[0046] Since the other basic components of the sensor unit 10B are the same as those of the sensor unit 10 shown in Figures 2 and 3, the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0047] As in the sensor unit 10B of the second modification, the heater MH1 and thermistor Rd1 may be placed in the same cavity 113. In this case, the heat generated by the heater MH1 is mainly transferred to thermistor Rd1 via the bridge supporting the membrane 161, the substrate 110, and the bridge supporting the membrane 162. By placing the heater MH1 and thermistor Rd1 in the same cavity 113 in this way, it is possible to miniaturize the substrate 110. The temperature of thermistor Rd1 when the heater MH1 is heated can be adjusted by the distance D3 between the membranes 161 and 162.

[0048] <Second Embodiment>

[0049] Figure 10 is a circuit diagram showing the configuration of a gas sensor 200 according to a second embodiment of the technology described herein.

[0050] As shown in Figure 10, the gas sensor 200 according to the second embodiment differs from the gas sensor 100 according to the first embodiment in that the sensor unit 10 is replaced by the sensor unit 30, the multiplexer 21 included in the signal processing circuit 20 is replaced by the multiplexer 27, and a fixed resistor R2 is added. Since the other basic configurations are the same as those of the gas sensor 100 according to the first embodiment, the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0051] The sensor unit 30 includes a thermistor Rd1 connected between the power supply Vcc and node N2, a thermistor Rd2 connected between nodes N3 and N4, and heaters MH1 and MH2. Thermistor Rd1 changes temperature in response to changes in the temperature of heater MH1. Thermistor Rd2 changes temperature in response to changes in the temperature of heater MH2. Thermistor Rd2 is a temperature-sensing element whose resistance changes with temperature.

[0052] During the gas detection period, heater MH1 is heated to a first temperature range, and heater MH2 is heated to a second temperature range. The first temperature range is a predetermined temperature range, for example, a range of 100°C or more and 230°C or less, for example, a temperature range around 170°C. The second temperature range is a predetermined temperature range, for example, a range of 250°C or more and 450°C or less, for example, a temperature range around 340°C. For example, the temperature range around 340°C may be the range of 339.5°C or more and 340.5°C or less. The temperature in the second temperature range is higher than that in the first temperature range, and the signal processing circuit 20 heats heater MH2 to a higher temperature than heater MH1 when measuring the gas concentration.

[0053] In the second temperature range, the ratio of the thermal conductivity of CO2 gas to that of air is closer to 1 than the ratio in the first temperature range. Therefore, even when a constant power is applied to the heater MH2 to heat it to the second temperature range, and CO2 gas is present in the measurement atmosphere, the change in the temperature of the heater MH2 due to the CO2 gas concentration is minimal. Consequently, when the CO2 gas concentration in the measurement atmosphere is the same as the normal atmospheric CO2 gas concentration (e.g., 400 ppm), and the heater MH2 is heated to 340°C, even if the CO2 gas concentration in the measurement atmosphere exceeds the normal atmospheric CO2 gas concentration, the change in the temperature of the heater MH2 in response to that concentration is minimal and it is maintained at approximately 340°C. Thus, even when heater MH2 is heated to the second temperature range and CO2 gas is present in the measurement atmosphere, the change in the heat dissipation characteristics of heater MH2 in relation to the CO2 gas concentration is slight. For this reason, the change in the temperature of thermistor Rd2 in relation to the CO2 gas concentration, and therefore the change in the resistance value of thermistor Rd2, is also slight.

[0054] The multiplexer 27 includes a first selector MUX1, a second selector MUX2, and a third selector MUX3. The first selector MUX1 is a circuit that connects either of the selection nodes S11 or S12 to the output node of the second selector MUX2. The selection nodes S11 and S12 are connected to the differential amplifiers 22 and 23, respectively. The second selector MUX2 is a circuit that short-circuits two selected selection nodes from S21 to S24 and connects them to the input node of the first selector MUX1. The selection node S22 is connected to thermistor Rd1 via node N2. The selection node S23 is connected to thermistor Rd2 via node N3. The selection nodes S21 and S24 are dummy nodes and are floating. The third selector MUX3 is a circuit that connects either of the selection nodes S31 or S32 to thermistor Rd2 via node N4. The selection nodes S31 and S32 are connected to the power supply Vcc and ground GND, respectively. The selection operation of these nodes by the multiplexer 27 is controlled by the control circuit 26.

[0055] The fixed resistor R2 is connected between the selection node S12 of the first selection unit MUX1 and ground GND.

[0056] Figure 11 is a table illustrating the functions of the multiplexer 27. In Figure 11, ○ marks indicate selected nodes, and × marks indicate nodes that are not selected.

[0057] When measuring ambient temperature using thermistor Rd1, the selection node S12 of the first selection unit MUX1 is selected, the selection nodes S21 and S22 of the second selection unit MUX2 are selected, and the selection node S32 of the third selection unit MUX3 is selected. As a result, node N2 is connected to selection node S12. Consequently, thermistor Rd1 and fixed resistor R2 are connected in series between the power supply Vcc and ground GND, and the temperature detection signal Vtemp1 appearing at the selection node S12, which is the connection point, is supplied to the differential amplifier 23. The temperature detection signal Vtemp1 is the output value caused by thermistor Rd1, which indicates the ambient temperature. The differential amplifier 23 compares the temperature detection signal Vtemp1 with the reference potential Vref2 to generate an amplified signal Vamp21, which is an amplified version of the level difference between the temperature detection signal Vtemp1 and the reference potential Vref2 (=Vtemp1-Vref2).

[0058] When measuring ambient temperature using thermistor Rd2, selection node S12 of the first selection unit MUX1 is selected, selection nodes S23 and S24 of the second selection unit MUX2 are selected, and selection node S31 of the third selection unit MUX3 is selected. As a result, node N3 is connected to selection node S12, and node N4 is connected to the power supply Vcc. Consequently, thermistor Rd2 and fixed resistor R2 are connected in series between the power supply Vcc and ground GND, and the temperature detection signal Vtemp2 appearing at the selection node S12, which is the connection point, is supplied to the differential amplifier 23. The temperature detection signal Vtemp2 is the output value caused by thermistor Rd2, which indicates the ambient temperature. The differential amplifier 23 compares the temperature detection signal Vtemp2 with the reference potential Vref2 to generate an amplified signal Vamp22, which is an amplified version of the level difference between the temperature detection signal Vtemp2 and the reference potential Vref2 (=Vtemp2-Vref2).

[0059] The reference potential Vref2 used when measuring ambient temperature with thermistor Rd1 and the reference potential Vref2 used when measuring ambient temperature with thermistor Rd2 may be the same or different.

[0060] When measuring gas concentration, the selection node S11 of the first selection unit MUX1 is selected, the selection nodes S22 and S23 of the second selection unit MUX2 are selected, and the selection node S32 of the third selection unit MUX3 is selected. As a result, thermistors Rd1 and Rd2 are connected in series between the power supply Vcc and ground GND, and the gas detection signal Vgas that appears at the connection point, selection node S11, is supplied to the differential amplifier 22.

[0061] Next, the operation of the gas sensor 200 during gas concentration measurement according to the second embodiment will be described.

[0062] Figure 12 is a flowchart illustrating the operation of the gas sensor 200 during gas concentration measurement. Figure 13 is a timing diagram illustrating the operation of the gas sensor 200. Gas concentration measurement is performed during the period T shown in Figure 13.

[0063] First, the control circuit 26 included in the signal processing circuit 20 selects selection nodes S12, S21, S22, and S32 by controlling the multiplexer 27, and also supplies a reference potential Vref2 to the differential amplifier 23 via the DA converter (step 201). This generates an amplified signal Vamp21 indicating the current ambient temperature measured using thermistor Rd1. At this time, heater voltages Vmh1 and Vmh2 are not supplied to heaters MH1 and MH2, and thermistor Rd1 is kept at the ambient temperature. Therefore, the temperature detection signal Vtemp1 appearing at selection node S12 in this state will be at a level corresponding to the current ambient temperature. The temperature detection signal Vtemp1 is converted into an amplified signal Vamp21 by the differential amplifier 23, and then digitally converted by the AD converter 24.

[0064] Next, the control circuit 26 included in the signal processing circuit 20 selects selection nodes S12, S23, S24, and S31 by controlling the multiplexer 27, and also supplies a reference potential Vref2 to the differential amplifier 23 via the DA converter (step 202). This generates an amplified signal Vamp22 indicating the current ambient temperature measured using thermistor Rd2. At this time, heater voltages Vmh1 and Vmh2 are not supplied to the heaters MH1 and MH2, and thermistor Rd2 is kept at the ambient temperature. Therefore, the temperature detection signal Vtemp2 appearing at selection node S12 in this state will be at a level corresponding to the current ambient temperature. The temperature detection signal Vtemp2 is converted into an amplified signal Vamp22 by the differential amplifier 23, and then digitally converted by the AD converter 24.

[0065] Next, the control circuit 26 calculates the current ambient temperature based on the digitally converted amplified signals Vamp21 and Vamp22 (step 203). However, it is not mandatory to perform both steps 201 and 202; the current ambient temperature can be calculated by performing only one of either step 201 or step 202. In other words, the current ambient temperature can be calculated based on only one of the amplified signals Vamp21 or Vamp22. This makes it possible to shorten the time required to measure the ambient temperature. The above operations are performed during the temperature detection period T1 shown in Figure 13. The temperature detection period T1 is the first half of the gas concentration measurement period T.

[0066] Next, the control circuit 26 calculates the heater voltage Vmh1 based on the ambient temperature calculated based on the amplified signal Vamp21, and calculates the heater voltage Vmh2 based on the ambient temperature calculated based on the amplified signal Vamp22 (step 204). For example, regardless of the current ambient temperature, if the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm), the level of heater voltage Vmh1 is set so that heater MH1 is heated to 170°C, and the level of heater voltage Vmh2 is set so that heater MH2 is heated to 340°C. Step 203 may be omitted, and in step 204, the heater voltage Vmh1 may be calculated directly based on the amplified signal Vamp21, and the heater voltage Vmh2 may be calculated directly based on the amplified signal Vamp22. The heater voltages Vmh1 and Vmh2 calculated in this way are supplied to heaters MH1 and MH2, respectively (step 205). In this way, by calculating the heater voltage Vmh1 to be applied to heater MH1 located near thermistor Rd1 based on the amplified signal Vamp21, which is the output value caused by thermistor Rd1, and calculating the heater voltage Vmh2 to be applied to heater MH2 located near thermistor Rd2 based on the amplified signal Vamp22, which is the output value caused by thermistor Rd2, the heating temperatures of heaters MH1 and MH2 can be controlled with higher precision. Alternatively, in step 204, the heater voltages Vmh1 and Vmh2 may be calculated based on only one of the amplified signals Vamp21 or Vamp22. Alternatively, if the temperature calculated based on the amplified signal Vamp21 and the temperature calculated based on the amplified signal Vamp22 are different, their average value may be used as the current ambient temperature.

[0067] In this state, the control circuit 26 selects selection nodes S11, S22, S23, and S32 by controlling the multiplexer 27, and also supplies a reference potential Vref1 to the differential amplifier 22 via the DA converter (step 206). As a result, a gas detection signal Vgas appears at selection node S11 at a level obtained by dividing the power supply potential Vcc by the resistance values ​​of thermistor Rd1 and thermistor Rd2.

[0068] As described above, when heater MH1 is heated to the first temperature range and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of heater MH1 change according to the concentration of CO2 gas. This change manifests as a change in the temperature of thermistor Rd1, i.e., a change in the resistance value of thermistor Rd1. In contrast, even when heater MH2 is heated to the second temperature range and CO2 gas is present in the measurement atmosphere, the change in the heat dissipation characteristics of heater MH2 according to the concentration of CO2 gas is small. Therefore, the change in the temperature of thermistor Rd2, i.e., the change in the resistance value of thermistor Rd2 according to the concentration of CO2 gas, is also small. As a result, when heaters MH1 and MH2 are heated to the first and second temperature ranges, respectively, a gas detection signal Vgas corresponding to the concentration of CO2 gas in the measurement atmosphere appears at the selection node S11.

[0069] On the other hand, even if other gases are present in the measurement atmosphere that do not have a significant difference in heat dissipation characteristics when heater MH1 is heated to the first temperature range and when heater MH2 is heated to the second temperature range, the concentration of those gases will have little effect on the level of the gas detection signal Vgas. As a result, the sensor unit 30 can selectively detect the concentration of CO2 gas.

[0070] The gas detection signal Vgas is converted into an amplified signal Vamp1 by the differential amplifier 22. The control circuit 26 then calculates the CO2 gas concentration based on the A / D converted amplified signal Vamp1 and outputs an output signal Vout indicating the CO2 gas concentration (step 207). The operation from step 204 to step 207 is performed during the gas detection period T2 shown in Figure 13. The gas detection period T2 is the latter half of the gas concentration measurement period T.

[0071] By periodically performing such gas concentration measurements, it becomes possible to periodically detect changes in the concentration of CO2 gas in the measurement atmosphere.

[0072] Thus, the gas sensor 200 according to this embodiment acquires a gas detection signal Vgas from the connection point of thermistors Rd1 and Rd2 connected in series. Furthermore, the temperature of thermistor Rd1 depends on the heating temperature of heater MH1, which changes according to the concentration of CO2 gas, while the temperature of thermistor Rd2 depends on the heating temperature of heater MH2, which changes only slightly according to the concentration of CO2 gas. As a result, it is possible to selectively detect the concentration of CO2 gas while reducing the influence of other gases.

[0073] Figure 14 is a schematic plan view showing the configuration of the sensor unit 30. The cross-sections along line A-A' and line B-B' shown in Figure 14 are as shown in Figures 3(a) and 3(b), respectively.

[0074] As shown in Figure 14, the sensor unit 30 comprises two base materials 110 and 210. Heater MH1 and thermistor Rd1 are supported on base material 110, and heater MH2 and thermistor Rd2 are supported on base material 210. Base materials 110 and 210 are separate components and are arranged so that a space SP is formed between them. The configuration of base material 110 and the heater MH1 and thermistor Rd1 supported thereon is as described with reference to Figure 2.

[0075] The configuration of the substrate 210 is the same as that of the substrate 110. The configuration of the heater MH2 is the same as that of the heater MH1. The configuration of the thermistor Rd2 is the same as that of thermistor Rd1. The configuration of the heater MH2 and thermistor Rd2 supported by the substrate 210 is the same as that of the heater MH1 and thermistor Rd1 supported by the substrate 110. In other words, the substrate 210 has a cavity 211 at a position that overlaps with the heater MH2 in a plan view from the Z direction. One end of the heater MH2 is connected to a terminal electrode 243 to which the heater voltage Vmh2 is supplied. The other end of the heater MH2 is connected to a terminal electrode 244 to which the ground potential GND is supplied. The thermistor Rd2 is composed of a pair of thermistor electrodes 231 and 232 and a thermistor resistor 230 that is in contact with the pair of thermistor electrodes 231 and 232. The thermistor electrodes 231 and 232 are connected to terminal electrodes 241 and 242, respectively, which constitute connection points N3 and N4. The distance W2 between thermistor electrode 231 and thermistor electrode 232 is, for example, 5 to 8 μm, and can be selected according to the desired resistance value of the thermistor Rd2.

[0076] The ratio of the resistance between thermistor electrodes 231 and 232 of thermistor Rd2 to the resistance between thermistor electrodes 131 and 132 of thermistor Rd1 at the same temperature (e.g., room temperature) may be between 0.9 and 1.1.

[0077] The thermistor Rd2 is positioned so as not to overlap with the heater MH2. Therefore, thermistor Rd2 is not directly heated by the heater MH2, but is mainly heated by heat conduction through the base material 210. In this embodiment, the heater MH2 and thermistor Rd2 are separated by a part of the base material 210 (second part 218). The thickness of the second part 218 of the base material 210 in the Z direction is greater than the thickness of the part of the base material 218 that overlaps with the cavity 211 in the Z direction. Furthermore, in this embodiment, there is no cavity in the base material 210 in the position that overlaps with thermistor Rd2.

[0078] Thus, in this embodiment, the temperature of thermistor Rd1 changes in accordance with the change in the temperature of heater MH1 due to heat transmitted mainly from heater MH1 through the substrate 110, and the temperature of thermistor Rd2 changes in accordance with the change in the temperature of heater MH2 due to heat transmitted mainly from heater MH2 through the substrate 210. As a result, the temperature rise of thermistors Rd1 and Rd2 is significantly suppressed. For example, when heater MH2 is heated from room temperature to 340°C, the temperature rise of thermistor Rd2 is limited to an increase of only a few degrees from room temperature.

[0079] As described above, in this embodiment, the gas sensor 200 is configured such that the thermistor Rd1 and heater MH1 are positioned at different locations on the base material 110 so as not to overlap, and thermistor Rd2 and heater MH2 are positioned at different locations on the base material 210 so as not to overlap. As a result, the temperature of thermistor Rd1 during gas concentration measurement is sufficiently lower than the heating temperature of heater MH1, and the temperature of thermistor Rd2 during gas concentration measurement is sufficiently lower than the heating temperature of heater MH2. This makes it possible to suppress changes over time caused by thermistors Rd1 and Rd2 being heated to high temperatures.

[0080] Furthermore, by providing a space SP between the base material 110 and the base material 210, the heat from the heater MH1 is hardly transferred to the thermistor Rd2, and the heat from the heater MH2 is hardly transferred to the thermistor Rd1, thereby reducing measurement errors due to thermal interference. However, it is not essential that the base material 110 supporting the heater MH1 and thermistor Rd1 and the base material 210 supporting the heater MH2 and thermistor Rd2 are separate components; they may be formed on a single base material.

[0081] Furthermore, if the resistance between the electrodes of thermistor Rd1 and thermistor Rd2 are designed to be approximately the same at the same temperature (e.g., room temperature), for example, by designing the ratio of the resistance between the electrodes of thermistor Rd2 to the resistance between the electrodes of thermistor Rd1 to be between 0.9 and 1.1, the level of the gas detection signal Vgas can be set to a level close to Vcc / 2, thereby enabling a wide dynamic range.

[0082] Figure 15 is a schematic plan view showing the configuration of the sensor unit 30A according to the first modified example. The cross-sections along the line A-A' and the line B-B' shown in Figure 15 are as shown in Figures 7(a) and 7(b), respectively.

[0083] As shown in Figure 15, the sensor unit 30A according to the first modification differs from the sensor unit 30 shown in Figure 14 in that another cavity 112 is provided in the base material 110 and another cavity 212 is provided in the base material 210. The other basic configurations are the same as those of the sensor unit 30 shown in Figure 14, so the same elements are denoted by the same reference numerals and redundant explanations are omitted.

[0084] In the region where cavity 112 is provided, the insulating film 121 of the substrate 110 is removed, and the thickness of the main body portion 120 of the substrate 110 in the Z direction is locally reduced, or the main body portion 120 and the insulating film 121 of the substrate 110 are removed. Similarly, in the region where cavity 212 is provided, the thickness of the substrate 210 in the Z direction is locally reduced. In the first modified example, in a plan view from the Z direction, thermistor Rd1 is provided at a position overlapping cavity 112, and thermistor Rd2 is provided at a position overlapping cavity 212. The thickness of the substrate 110 in the Z direction in the first portion 118 located between cavity 111 and cavity 1112 is greater than the thickness of the substrate 110 in the Z direction at a position overlapping cavity 111 or cavity 112. Similarly, the thickness of the substrate 210 in the Z-direction in the second portion 218 located between cavities 211 and 212 is greater than the thickness of the substrate 210 in the Z-direction in the position overlapping with cavity 211 or cavity 212.

[0085] As in the sensor section 30A of the first modification, if a cavity 112 is provided in the base material 110 not only at a position overlapping with the heater MH1 but also at a position overlapping with thermistor Rd1, and if a cavity 212 is provided in the base material 210 not only at a position overlapping with the heater MH2 but also at a position overlapping with thermistor Rd2, the heat capacity near thermistors Rd1 and Rd2 is reduced, thereby improving the temperature responsiveness of thermistor Rd1 when the heater MH1 is heated, and improving the temperature responsiveness of thermistor Rd2 when the heater MH2 is heated. This makes it possible to measure the concentration of CO2 gas in a shorter time. The temperature of thermistor Rd1 when the heater MH1 is heated can be adjusted by the distance D2 between the cavities 111 and 112, and the temperature of thermistor Rd2 when the heater MH2 is heated can be adjusted by the distance D4 between the cavities 211 and 212.

[0086] Distance D4 may be greater than distance D2. This means that the distance between heater MH2 and thermistor Rd2 is greater than the distance between heater MH1 and thermistor Rd1. As a result, the difference between the temperature of thermistor Rd1 when heater MH1 is heated to the first temperature range and the temperature of thermistor Rd2 when heater MH2 is heated to the second temperature range is reduced. This allows, for example, if the ratio of the resistance between the electrodes of thermistor Rd2 to the resistance between the electrodes of thermistor Rd1 at the same temperature is designed to be between 0.9 and 1.1, the level of the gas detection signal Vgas can be brought closer to Vcc / 2, thereby enabling a wider dynamic range.

[0087] Figure 16 is a schematic plan view showing the configuration of the sensor unit 30B according to the second modified example. Figure 17 is a schematic cross-sectional view along the line A-A' shown in Figure 16.

[0088] As shown in Figures 16 and 17, in the sensor section 30B according to the second modification, cavities 114 and 214 are provided in the substrates 110 and 210, respectively. Heater MH1 is positioned to overlap with cavity 114, and heater MH2 is positioned to overlap with cavity 214. A slit SL2 is provided at the position overlapping with cavity 114, and heater MH1 is supported by membrane 163. Similarly, a slit SL3 is provided at the position overlapping with cavity 214, and heater MH2 is supported by membrane 263.

[0089] The membrane 163 is supported on the substrate 110 by a bridge consisting of wiring 191, 192 connecting the heater MH1 to terminal electrodes 143, 144, wiring 193, 194 connected to dummy electrodes 145, 146, and an insulating film located around these wirings 191 to 194. The bridge supporting the membrane 163 may include a portion of the substrate 110. The membrane 263 is supported on the substrate 210 by a bridge consisting of wiring 221, 222 connecting the heater MH2 to terminal electrodes 243, 244, wiring 223, 224 connected to dummy electrodes 225, 226, and an insulating film located around these wirings 221 to 224. The bridge supporting the membrane 263 may include a portion of the substrate 210.

[0090] Since the other basic components of the sensor unit 30B are the same as those of the sensor unit 30 shown in Figure 14, the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0091] As shown in the sensor unit 30B of the second modification, the heaters MH1 and MH2 may be placed on the membranes 163 and 263, respectively.

[0092] Figure 18 is a schematic plan view showing the configuration of the sensor unit 30C according to the third modified example. The cross-section along the line C-C' shown in Figure 18 is as shown in Figure 9.

[0093] As shown in Figure 18, in the sensor section 30C according to the third modification, cavities 113 and 213 are provided in the base materials 110 and 210, respectively. A heater MH1 and thermistor Rd1 are arranged so as to overlap with cavity 113, and a heater MH2 and thermistor Rd2 are arranged so as to overlap with cavity 213. A slit SL1 is provided at the position overlapping with cavity 113, with heater MH1 supported by membrane 161 and thermistor Rd1 supported by membrane 162. Similarly, a slit SL4 is provided at the position overlapping with cavity 213, with heater MH2 supported by membrane 261 and thermistor Rd2 supported by membrane 262. In this modification, heater MH1 and thermistor Rd1 are separated by space, and heater MH2 and thermistor Rd2 are separated by space.

[0094] The membrane 261 is supported by the substrate 210 by a bridge consisting of wiring 271, 272 connecting the heater MH2 to terminal electrodes 243, 244, wiring 273, 274 connected to dummy electrodes 245, 246, and an insulating film located around these wirings 271-274. The membrane 262 is supported by the substrate 210 by a bridge consisting of wiring 281 connecting the thermistor electrode 231 to terminal electrode 241, wiring 282 connecting thermistor electrode 232 to terminal electrode 242, wiring 283, 284 connected to dummy electrodes 247, 248, and an insulating film located around these wirings 281-284. The bridge supporting the membranes 261 and 262 may include a portion of the substrate 210.

[0095] Since the other basic components of the sensor unit 30B are the same as those of the sensor unit 10 shown in Figure 8 and the sensor unit 30 shown in Figure 14, the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0096] As in the sensor section 10C of the third modified example, the heater MH1 and thermistor Rd1 may be placed in the same cavity 113, and the heater MH2 and thermistor Rd2 may be placed in the same cavity 213. The temperature of thermistor Rd2 when the heater MH2 is heated can be adjusted by the distance D5 between membranes 261 and 262.

[0097] <Third Embodiment>

[0098] Figure 19 is a circuit diagram showing the configuration of a gas sensor 300 according to a third embodiment of the technology described herein.

[0099] As shown in Figure 19, the gas sensor 300 according to the third embodiment differs from the gas sensor 200 according to the second embodiment in that the sensor unit 30 is replaced by a sensor unit 40, the multiplexer 27 included in the signal processing circuit 20 is replaced by a multiplexer 28, a differential amplifier 29 is added between the multiplexer 28 and the differential amplifier 22, and the fixed resistor R2 is removed. The other basic configurations are the same as those of the gas sensor 200 according to the second embodiment, so the same reference numerals are used for the same elements and redundant explanations are omitted.

[0100] The sensor unit 40 includes a thermistor Rd1 and a fixed resistor R3 connected in series in that order between the power supply Vcc and the ground GND, a thermistor Rd2 and a fixed resistor R4 connected in series in that order between the power supply Vcc and the ground GND, and heaters MH1 and MH2. The temperature of thermistor Rd1 changes in response to the temperature change of heater MH1, and the temperature of thermistor Rd2 changes in response to the temperature change of heater MH2.

[0101] The multiplexer 28 includes a fourth selection unit MUX4 and a fifth selection unit MUX5. The fourth selection unit MUX4 is a circuit that connects either selection node S41 or S42 to node N5, which is the connection point between thermistor Rd1 and fixed resistor R3. A gas detection signal Vgas1 appears at selection node S41, and a temperature detection signal Vtemp1 appears at selection node S42. The fifth selection unit MUX5 is a circuit that connects either selection node S51 or S52 to node N6, which is the connection point between thermistor Rd2 and fixed resistor R4. A gas detection signal Vgas2 appears at selection node S51, and a temperature detection signal Vtemp2 appears at selection node S52. These selection operations by the multiplexer 28 are controlled by the control circuit 26.

[0102] Figure 20 is a table illustrating the functions of the multiplexer 28. In Figure 20, ○ indicates a node that is selected, and × indicates a node that is not selected.

[0103] When measuring ambient temperature using thermistor Rd1, selection node S42 of the fourth selection unit MUX4 is selected, and selection node S51 of the fifth selection unit MUX5 is selected. As a result, the temperature detection signal Vtemp1 appearing at node N5 is supplied to the differential amplifier 23. The differential amplifier 23 compares the temperature detection signal Vtemp1 with the reference potential Vref2 to generate an amplified signal Vamp21, which is an amplified version of the level difference (=Vtemp1-Vref2) between the temperature detection signal Vtemp1 and the reference potential Vref2.

[0104] When measuring ambient temperature using thermistor Rd2, selection node S41 of the fourth selection unit MUX4 is selected, and selection node S52 of the fifth selection unit MUX5 is selected. As a result, the temperature detection signal Vtemp2 appearing at node N6 is supplied to the differential amplifier 23. The differential amplifier 23 compares the temperature detection signal Vtemp2 with the reference potential Vref2 to generate an amplified signal Vamp22, which is an amplified version of the level difference between the temperature detection signal Vtemp2 and the reference potential Vref2 (=Vtemp2-Vref2).

[0105] When measuring gas concentration, the selection node S41 of the fourth selection unit MUX4 is selected, and the selection node S51 of the fifth selection unit MUX5 is selected. As a result, the gas detection signal Vgas1 appearing at node N5 and the gas detection signal Vgas2 appearing at node N6 are supplied to the differential amplifier 29. The differential amplifier 29 compares the gas detection signals Vgas1 and Vgas2 to generate an amplified signal Vamp0, which is an amplified version of the level difference between the gas detection signals Vgas1 and Vgas2 (=Vgas1-Vgas2). The amplified signal Vamp0 is supplied to the differential amplifier 22. The differential amplifier 22 compares the amplified signal Vamp0 with the reference potential Vref1 to generate an amplified signal Vamp1, which is an amplified version of the level difference between the amplified signal Vamp0 and the reference potential Vref1 (=Vamp0-Vref1).

[0106] The mechanical configuration of the sensor unit 40 may be the same as that of the sensor unit 30 shown in Figure 14, the sensor unit 30A shown in Figure 15, the sensor unit 30B shown in Figures 16 and 17, or the sensor unit 30C shown in Figure 18.

[0107] As illustrated by the gas sensor 300 according to the third embodiment, it is not essential to connect the thermistor Rd1 and thermistor Rd2 in series. Thermistor Rd1 and thermistor Rd2 can be connected in parallel between the power supply Vcc and the ground GND, and the concentration of the gas to be measured can be calculated based on the difference between the output voltage due to thermistor Rd1 (gas detection signal Vgas1) and the output voltage due to thermistor Rd2 (gas detection signal Vgas2).

[0108] <Fourth Embodiment>

[0109] Figure 21 is a circuit diagram showing the configuration of a gas sensor 400 according to a fourth embodiment of the technology described herein.

[0110] As shown in Figure 21, the gas sensor 400 according to the fourth embodiment differs from the gas sensor 200 according to the second embodiment in that the sensor unit 30 is replaced by a sensor unit 50, and a differential amplifier 29 is added between the multiplexer 27 and the differential amplifier 22. The other basic configurations are the same as those of the gas sensor 200 according to the second embodiment, so the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0111] The sensor unit 50 further includes thermistors Rd3 and Rd4 connected in series in that order between the power supply Vcc and ground GND. Thermistors Rd3 and Rd4 are temperature-sensitive elements whose resistance changes with temperature. Thermistor Rd4 changes temperature in response to changes in the temperature of heater MH1, and thermistor Rd3 changes temperature in response to changes in the temperature of heater MH2. The connection point between thermistors Rd4 and Rd3 constitutes node N7. The gas detection signal Vgas2 appears at node N7. In addition, the gas detection signal Vgas1 appears at selection node S11 of the first selection unit MUX1 of the multiplexer 27.

[0112] The differential amplifier 29 generates an amplified signal Vamp0 by comparing the gas detection signal Vgas1 output from the multiplexer 27 with the gas detection signal Vgas2 appearing at node N7, thereby amplifying the level difference between the gas detection signal Vgas1 and the gas detection signal Vgas2 (=Vgas1-Vgas2). The amplified signal Vamp0 is supplied to the differential amplifier 22. The differential amplifier 22 generates an amplified signal Vamp1 by comparing the amplified signal Vamp0 with the reference potential Vref1, thereby amplifying the level difference between the amplified signal Vamp0 and the reference potential Vref1 (=Vamp0-Vref1).

[0113] Figure 22 is a schematic plan view showing the configuration of the sensor unit 50. Figure 23 is a schematic cross-sectional view along the line D-D' shown in Figure 22.

[0114] As shown in Figure 22, the sensor unit 50 has a pair of thermistor electrodes 133 and 134 added to contact the thermistor resistor 130 provided on the base material 110, and a pair of thermistor electrodes 233 and 234 added to contact the thermistor resistor 230 provided on the base material 210. The thermistor resistor 130 and the pair of thermistor electrodes 133 and 134 constitute thermistor Rd4 shown in Figure 21. The thermistor resistor 230 and the pair of thermistor electrodes 233 and 234 constitute thermistor Rd3 shown in Figure 21.

[0115] The distance W4 between thermistor electrodes 133 and 134 is, for example, 5 to 8 μm, and can be selected according to the desired resistance value of thermistor Rd4. The resistance value of thermistor Rd4 may be approximately the same as that of thermistor Rd1. Thermistor electrode 133 is connected to terminal electrode 401 to which ground potential GND is supplied. Thermistor electrode 134 is connected to terminal electrode 402 which constitutes connection point N7.

[0116] The distance W3 between thermistor electrodes 233 and 234 is, for example, 5 to 8 μm, and can be selected according to the desired resistance value of thermistor Rd3. The resistance value of thermistor Rd3 may be approximately the same as that of thermistor Rd2. Thermistor electrode 233 is connected to terminal electrode 403, which constitutes connection point N7. Thermistor electrode 234 is connected to terminal electrode 404, to which the power supply potential Vcc is supplied.

[0117] Thermistors Rd1 and Rd4 are positioned on the substrate 110 so as not to overlap with the heater MH1. Therefore, thermistors Rd1 and Rd4 are not directly heated by the heater MH1, but are mainly heated by heat conduction through the substrate 110. Here, the distance between the heater MH1 and thermistor Rd1 is approximately the same as the distance between the heater MH1 and thermistor Rd4. Therefore, when the heater MH1 is heated, thermistors Rd1 and Rd4 are heated to approximately the same temperature.

[0118] Thermistors Rd2 and Rd3 are positioned on the substrate 210 so as not to overlap with the heater MH2. Therefore, thermistors Rd2 and Rd3 are not directly heated by the heater MH2, but are mainly heated by heat conduction through the substrate 210. Here, the distance between the heater MH2 and thermistor Rd2 is approximately the same as the distance between the heater MH2 and thermistor Rd3. Therefore, when the heater MH2 is heated, thermistors Rd2 and Rd3 are heated to approximately the same temperature.

[0119] As shown in Figure 21, in this embodiment, the gas detection signal Vgas1 appearing at the selected node S11, which is the connection point of thermistors Rd1 and Rd2 connected in series in that order between the power supply Vcc and ground GND, is supplied to the non-inverting input terminal (+) of the differential amplifier 29, and the gas detection signal Vgas2 appearing at node N7, which is the connection point of thermistors Rd4 and Rd3 connected in series in that order between the power supply Vcc and ground GND, is supplied to the inverting input terminal (-) of the differential amplifier 29. Therefore, the differential amplifier 29 compares the gas detection signals Vgas1 and Vgas2 to generate an amplified signal Vamp0, which is an amplified version of the level difference between the gas detection signals Vgas1 and Vgas2 (=Vgas1-Vgas2).

[0120] Thus, in this embodiment, since thermistors Rd1 to Rd4 are connected in a full bridge configuration, the amount of change in the amplified signal Vamp1 in response to the CO2 gas concentration becomes larger. This makes it possible to further improve the detection sensitivity of the CO2 gas concentration.

[0121] <Fifth Embodiment>

[0122] Figure 24 is a circuit diagram showing the configuration of a gas sensor 500 according to a fifth embodiment of the technology described herein.

[0123] As shown in Figure 24, the gas sensor 500 according to the fifth embodiment differs from the gas sensor 300 according to the third embodiment in that the sensor unit 40 is replaced by a sensor unit 70, a temperature sensor 80 is added, and the multiplexer 28 is removed from the signal processing circuit 20, and differential amplifiers 291 and 292 are provided instead. The other basic configurations are the same as those of the gas sensor 300 according to the third embodiment, so the same reference numerals are used for the same elements, and redundant explanations are omitted.

[0124] The sensor unit 70 includes thermopile elements TP1 and TP2, and heaters MH1 and MH2. The temperature of the hot junction of thermopile element TP1 changes in accordance with the temperature change of heater MH1, and the temperature of the hot junction of thermopile element TP2 changes in accordance with the temperature change of heater MH2. Thermopile elements TP1 and TP2 are temperature-sensitive elements whose potential difference across their ends changes with temperature. The potential difference across the ends of thermopile element TP1 is used as the output signal Vtp1, and the potential difference across the ends of thermopile element TP2 is used as the output signal Vtp2. The reference potential Vref3 is generated by fixed resistors R6 and R7. Fixed resistors R6 and R7 are connected in series between the power supply Vcc and ground GND, and the reference potential Vref3 appears at their connection point N9. The reference potential Vref3 is supplied in common to the inverting input terminal (-) of the differential amplifiers 291 and 292 included in the signal processing circuit 20. The output signal Vtp1 is supplied to the non-inverting input terminal (+) of the differential amplifier 291 included in the signal processing circuit 20, and the output signal Vtp2 is supplied to the non-inverting input terminal (+) of the differential amplifier 292 included in the signal processing circuit 20.

[0125] The potential supplied to the non-inverting input terminal (+) of differential amplifier 291 is the level obtained by superimposing the output signal Vtp1, which corresponds to the electromotive force of the thermopile element TP1 depending on the temperature, onto the reference potential Vref3. The potential supplied to the non-inverting input terminal (+) of differential amplifier 292 is the level obtained by superimposing the output signal Vtp2, which corresponds to the electromotive force of the thermopile element TP2 depending on the temperature, onto the reference potential Vref3.

[0126] The output signal Vtp1 is amplified by the differential amplifier 291 included in the signal processing circuit 20 to generate the gas detection signal Vgas1. The differential amplifier 291 compares the levels of the reference potential Vref3 supplied to the inverting input terminal (-) and Vref3 + Vtp1 supplied to the non-inverting input terminal (+), and generates the gas detection signal Vgas1 by amplifying the difference (=Vtp1).

[0127] The output signal Vtp2 is amplified by the differential amplifier 292 included in the signal processing circuit 20 to generate the gas detection signal Vgas2. The differential amplifier 292 compares the levels of the reference potential Vref3 supplied to the inverting input terminal (-) and Vref3 + Vtp2 supplied to the non-inverting input terminal (+), and generates the gas detection signal Vgas2 by amplifying the difference (=Vtp2).

[0128] Similar to the third embodiment, the differential amplifier 29 generates an amplified signal Vamp0 by comparing the gas detection signal Vgas1 and the gas detection signal Vgas2, thereby amplifying the level difference between the gas detection signal Vgas1 and the gas detection signal Vgas2 (=Vgas1-Vgas2). The amplified signal Vamp0 is supplied to the differential amplifier 22. The differential amplifier 22 generates an amplified signal Vamp1 by comparing the amplified signal Vamp0 with the reference potential Vref1, thereby amplifying the level difference between the amplified signal Vamp0 and the reference potential Vref1 (=Vamp0-Vref1).

[0129] The temperature sensor 80 is a circuit for detecting ambient temperature and includes a thermistor Rd5 and a fixed resistor R5 connected in series between the power supply Vcc and ground GND. The temperature detection signal Vtemp output from the temperature sensor 80 appears at the connection point N8 between the thermistor Rd5 and the fixed resistor R5. The temperature sensor 80 may be designed to be unaffected by, or less affected by, heating by, for example, heaters MH1 and MH2. The temperature detection signal Vtemp is supplied to the differential amplifier 23 included in the signal processing circuit 20.

[0130] Even with this circuit configuration, when heater MH1 is heated to a first temperature range, for example, approximately 170°C, as the CO2 gas concentration in the measurement atmosphere increases, the temperature of heater MH1 rises, and accordingly, the temperature of the hot junction of thermopile element TP1 rises, causing the level of the gas detection signal Vgas1, which corresponds to the thermoelectric voltage of thermopile element TP1, to rise. On the other hand, when heater MH2 is heated to a second temperature range, for example, approximately 340°C, even if the CO2 gas concentration in the measurement atmosphere increases, the temperature of heater MH2 and the temperature of the hot junction of thermopile element TP2 hardly rise, and the level of the gas detection signal Vgas2, which corresponds to the thermoelectric voltage of thermopile element TP2, hardly rises. The level difference between these gas detection signals Vgas1 and Vgas2 is amplified by the differential amplifier 29, and the amplified signal Vamp0 is generated.

[0131] Figure 25 is a schematic plan view showing the configuration of the sensor unit 70 and the temperature sensor 80.

[0132] In the example shown in Figure 25, the heater MH1, thermopile element TP1, and temperature sensor 80 are supported on the substrate 110, and the heater MH2 and thermopile element TP2 are supported on the substrate 210.

[0133] The substrate 110 is provided with three cavities 115 to 117. A slit SL5 is provided in a position overlapping with cavity 115, and the heater MH1 is supported by the membrane 164. A slit SL6 is provided in a position overlapping with cavity 116, and a part of the thermopile element TP1 is supported by the membrane 165. The heater MH1 and the thermopile element TP1 are arranged in the X direction. The end of the thermopile element TP1 in the X direction closest to the heater MH1 constitutes a hot junction TP1A. One end of the thermopile element TP1 is connected to a terminal electrode 501 that constitutes node N10. The other end of the thermopile element TP1 is connected to a terminal electrode 502 that constitutes node N9.

[0134] A slit SL9 is provided in a position overlapping with the cavity 117, and the thermistor resistor 330 and a pair of thermistor electrodes 331 and 332 that are in contact with the thermistor resistor 330 are supported by the membrane 340. The thermistor resistor 330 and the pair of thermistor electrodes 331 and 332 constitute the temperature sensor 80 shown in Figure 24. Thermistor electrode 331 is connected to terminal electrode 511 to which the power supply potential Vcc is supplied. Thermistor electrode 332 is connected to terminal electrode 512 that constitutes node N8.

[0135] In the example shown in Figure 25, a thermopile element TP1 is placed between the heater MH1 and the temperature sensor 80. As a result, heat from the heater MH1 is less likely to be transferred to the temperature sensor 80 compared to the thermopile element TP1.

[0136] The substrate 210 is provided with two cavities 215 and 216. A slit SL7 is provided in a position overlapping with cavity 215, and the heater MH2 is supported by the membrane 264. A slit SL8 is provided in a position overlapping with cavity 216, and a portion of the thermopile element TP2 is supported by the membrane 265. The heater MH2 and the thermopile element TP2 are arranged in the X direction. The end of the thermopile element TP2 in the X direction closest to the heater MH2 constitutes a hot junction TP2A. One end of the thermopile element TP2 is connected to a terminal electrode 503 that constitutes node N11. The other end of the thermopile element TP2 is connected to a terminal electrode 504 that constitutes node N9.

[0137] Thus, in this embodiment, the temperature of the thermopile element TP1 changes in accordance with the change in the temperature of the heater MH1 due to heat transmitted from the heater MH1 mainly through the substrate 110, and the temperature of the thermopile element TP2 changes in accordance with the change in the temperature of the heater MH2 due to heat transmitted from the heater MH2 mainly through the substrate 210.

[0138] As illustrated by the gas sensor 500 according to the fifth embodiment, it is not essential to use a thermistor as the temperature sensing element; other types of temperature sensing elements, such as thermopile elements, may be used. Furthermore, as illustrated by the gas sensor 500 according to the fifth embodiment, the concentration of the gas to be measured may be calculated based on the difference between the output voltage (gas detection signal Vgas1) caused by thermopile element TP1 and the output voltage (gas detection signal Vgas2) caused by thermopile element TP2.

[0139] While embodiments of the technology described herein have been explained above, it goes without saying that the technology described herein is not limited to the embodiments described above, and various modifications are possible without departing from its spirit, and these modifications are also included within the scope of the technology described herein.

[0140] The technology relating to this disclosure includes, but is not limited to, the following configuration examples.

[0141] A gas sensor according to one aspect of this disclosure comprises a substrate having a first cavity, a first heater supported by the substrate so as to overlap with the first cavity, a first temperature-sensing element supported by the substrate so as not to overlap with the first heater, and a signal processing circuit that heats the first heater when measuring gas concentration. The temperature of the first temperature-sensing element during gas concentration measurement changes in accordance with the change in the temperature of the first heater due to heat transmitted from the first heater mainly through the substrate. As a result, the temperature rise of the first temperature-sensing element during gas concentration measurement is suppressed, making it possible to suppress changes in the first temperature-sensing element over time.

[0142] In the gas sensor described above, the substrate further has a second cavity provided at a planar position different from the first cavity, and the first temperature sensing element may be supported by the substrate so as to overlap with the second cavity. This improves the temperature responsiveness of the first temperature sensing element.

[0143] In the gas sensor described above, the first temperature-sensing element may be supported on the substrate so as to overlap with the first cavity. This improves the temperature responsiveness of the first temperature-sensing element and makes it possible to miniaturize the substrate.

[0144] In the gas sensor described above, the signal processing circuit may heat the first heater based on the output value from the first temperature sensing element before heating the first heater. In this case, since the first temperature sensing element also functions as a temperature sensor, it becomes unnecessary to provide a separate temperature sensor.

[0145] The gas sensor described above further comprises a second heater and a second temperature-sensing element. The substrate further has a second cavity located in a planar position different from the first cavity. The second heater is supported by the substrate so as to overlap with the second cavity, and the second temperature-sensing element is supported by the substrate so as not to overlap with the first and second heaters. The signal processing circuit heats the second heater to a different temperature than the first heater when measuring the gas concentration. The temperature of the second temperature-sensing element during gas concentration measurement may change in accordance with the change in the temperature of the second heater due to heat mainly transmitted from the second heater through the substrate. This makes it possible to selectively detect the concentration of the target gas by reducing the influence of other miscellaneous gases, and also to suppress the temperature rise of the second temperature-sensing element during gas concentration measurement, thereby suppressing the change in the second temperature-sensing element over time.

[0146] In the gas sensor described above, the first temperature-sensing element and the second temperature-sensing element are connected in series, and the signal processing circuit may calculate the concentration of the target gas based on the detection voltage that appears at the connection point between the first and second temperature-sensing elements when measuring the gas concentration. This makes it possible to obtain a gas detection signal using a half-bridge circuit including the first and second temperature-sensing elements.

[0147] In the gas sensor described above, the signal processing circuit may calculate the concentration of the gas to be measured based on the voltage corresponding to the difference between the output voltage from the first temperature sensing element and the output voltage from the second temperature sensing element when measuring the gas concentration. The level of the second output voltage can be adjusted independently of the first temperature sensing element, and the level of the first output voltage can be adjusted independently of the second temperature sensing element.

[0148] In the gas sensor described above, the substrate includes a first substrate having a first cavity and supporting a first heater and a first temperature-sensing element, and a second substrate having a second cavity and supporting a second heater and a second temperature-sensing element, and a space may exist between the first substrate and the second substrate. This makes it possible to prevent thermal interference between the first heater and the second temperature-sensing element, and between the second heater and the first temperature-sensing element.

[0149] In the gas sensor described above, the substrate includes a first substrate having a first cavity and supporting a first heater and a first temperature-sensing element, and a second substrate having a second cavity and supporting a second heater and a second temperature-sensing element, wherein the first substrate further has a third cavity provided at a planar position different from the first cavity, and the second substrate further has a fourth cavity provided at a planar position different from the second cavity, the first temperature-sensing element may be supported on the first substrate so as to overlap with the third cavity, and the second temperature-sensing element may be supported on the second substrate so as to overlap with the fourth cavity. This improves the temperature responsiveness of the first and second temperature-sensing elements.

[0150] In the gas sensor described above, the first temperature-sensing element may be supported on the substrate so as to overlap with the first cavity, and the second temperature-sensing element may be supported on the substrate so as to overlap with the second cavity. This improves the temperature responsiveness of the first and second temperature-sensing elements and makes it possible to miniaturize the substrate.

[0151] In the gas sensor described above, the signal processing circuit may heat the first heater and the second heater based on at least one of the output value from the first temperature-sensing element before heating the first heater and the output value from the second temperature-sensing element before heating the second heater. In this case, since the first temperature-sensing element or the second temperature-sensing element also functions as a temperature sensor, it becomes unnecessary to provide a separate temperature sensor.

[0152] In the gas sensor described above, the signal processing circuit may heat the first heater based on the output value from the first temperature sensing element before heating the first heater, and heat the second heater based on the output value from the second temperature sensing element before heating the second heater. This eliminates the need to provide a separate temperature sensor and allows for more accurate measurement of the ambient temperature.

[0153] In the gas sensor described above, the distance between the second temperature-sensing element and the second heater is greater than the distance between the first temperature-sensing element and the first heater, and the signal processing circuit may heat the second heater to a higher temperature than the first heater when measuring gas concentration. This makes it possible to reduce the difference between the temperature of the first temperature-sensing element when the first heater is heated and the temperature of the second temperature-sensing element when the second heater is heated.

[0154] In the gas sensor described above, each of the first and second temperature-sensing elements has a resistor and a pair of electrodes connected to the resistor, and the ratio of the resistance between the electrodes of the second temperature-sensing element to the resistance between the electrodes of the first temperature-sensing element at the same temperature may be between 0.9 and 1.1. This makes it possible to obtain a wide dynamic range. [Explanation of Symbols]

[0155] 10, 10A~10C, 30, 30A~30C, 40, 50, 70 Sensor section 20 Signal Processing Circuits 21 Multiplexer 22, 23, 29 Differential Amplifier 24 AD converters 25 DA Converters 26 Control circuits 27,28 Multiplexer 80 Temperature Sensor 100 Gas Sensors 110,210 Base material 111-117, 211-216 Cavities 118 Part 1 120 Main body 121-123 Insulating film 130 Thermistor Resistor 131-134 Thermistor electrodes 141~144 Terminal electrode 145-148 Dummy electrodes Steps 151-156 161-165 Membrane Wiring for 171-174, 181-184, and 191-194. 200 Gas Sensors Steps 201-207 218 Part 2 221~224 Wiring 225,226 dummy electrodes 230 Thermistor Resistor 231-234 Thermistor electrodes 241~244 Terminal electrode 245-248 Dummy electrodes 261-265 Membrane Wiring 271-274, 281-284 291,292 Differential Amplifier 300 Gas Sensors 330 Thermistor Resistor 331,332 Thermistor electrodes 400 Gas Sensor 401~404 Terminal electrode 500 Gas Sensors 501~504,511,512 Terminal electrode D1~D5 distance MH1, MH2 Heater MUX1~MUX5 Selection Section N1~N11 Nodes (connection points) R1~R7 Fixed resistance Rd1~Rd4 Thermistors S1, S2, S11, S12, S21~S24, S31, S32, S41, S42, S51, S52 Selected Nodes SL1~SL9 Slit SP space T Gas concentration measurement period T1 Temperature detection period T2 gas detection period TP1, TP2 Thermopile Element TP1A, TP2A hot junction Vamp1, Vamp2, Vamp21, Vamp22 Amplified signals Vgas, Vgas1, Vgas2 gas detection signals Vmh1, Vmh2 Heater voltage VOUT output signal Vref1~Vref3 Reference Potentials Vtemp, Vtemp1, Vtemp2 Temperature detection signals Vtp1, Vtp2 output signals W1~W4 distance

Claims

1. A substrate having a first cavity, A first heater supported on the substrate is positioned so as to overlap with the first cavity, A first temperature-sensing element is supported on the substrate so as not to overlap with the first heater, A signal processing circuit that heats the first heater when measuring the gas concentration, Equipped with, The temperature of the first temperature sensing element during the gas concentration measurement changes in accordance with the change in the temperature of the first heater due to heat transmitted from the first heater mainly through the substrate. Gas sensor.

2. The substrate further has a second cavity provided at a planar position different from the first cavity, The first temperature sensing element is supported on the substrate so as to overlap with the second cavity. The gas sensor according to claim 1.

3. The first temperature sensing element is supported on the substrate so as to overlap with the first cavity. The gas sensor according to claim 1.

4. The signal processing circuit heats the first heater based on the output value from the first temperature sensing element before heating the first heater. The gas sensor according to any one of claims 1 to 3.

5. The second heater, The second temperature sensing element, Furthermore, The substrate further has a second cavity provided at a planar position different from the first cavity, The second heater is supported on the substrate so as to overlap with the second cavity. The second temperature sensing element is supported on the substrate so as not to overlap with the first and second heaters. The signal processing circuit heats the second heater to a different temperature than the first heater when measuring the gas concentration. The temperature of the second temperature sensing element during the gas concentration measurement changes in accordance with the change in the temperature of the second heater due to heat transmitted from the second heater mainly through the substrate. The gas sensor according to claim 1.

6. The first temperature sensing element and the second temperature sensing element are connected in series. The signal processing circuit calculates the concentration of the gas to be measured based on the detection voltage that appears at the connection point between the first temperature sensing element and the second temperature sensing element when measuring the gas concentration. The gas sensor according to claim 5.

7. The signal processing circuit calculates the concentration of the gas to be measured based on the voltage corresponding to the difference between the first output voltage generated by the first temperature sensing element and the second output voltage generated by the second temperature sensing element when measuring the gas concentration. The gas sensor according to claim 5.

8. The substrate includes a first substrate having the first cavity and supporting the first heater and the first temperature sensing element, and a second substrate having the second cavity and supporting the second heater and the second temperature sensing element, A space exists between the first substrate and the second substrate. The gas sensor according to any one of claims 5 to 7.

9. The substrate includes a first substrate having the first cavity and supporting the first heater and the first temperature sensing element, and a second substrate having the second cavity and supporting the second heater and the second temperature sensing element, The first substrate further has a third cavity provided at a planar position different from the first cavity, The second substrate further has a fourth cavity provided at a planar position different from the second cavity, The first temperature sensing element is supported on the first substrate so as to overlap with the third cavity. The second temperature sensing element is supported on the second substrate so as to overlap with the fourth cavity. The gas sensor according to any one of claims 5 to 7.

10. The first temperature sensing element is supported on the substrate so as to overlap with the first cavity, The second temperature-sensing element is supported on the substrate so as to overlap with the second cavity. The gas sensor according to any one of claims 5 to 7.

11. The signal processing circuit heats the first heater and the second heater based on at least one of the output value due to the first temperature sensing element before heating the first heater and the output value due to the second temperature sensing element before heating the second heater. The gas sensor according to any one of claims 5 to 7.

12. The signal processing circuit heats the first heater based on the output value of the first temperature sensing element before heating the first heater, and heats the second heater based on the output value of the second temperature sensing element before heating the second heater. The gas sensor according to claim 11.

13. The distance between the second temperature sensing element and the second heater is greater than the distance between the first temperature sensing element and the first heater. The signal processing circuit heats the second heater to a higher temperature than the first heater when measuring the gas concentration. The gas sensor according to any one of claims 5 to 7.

14. Each of the first temperature sensing element and the second temperature sensing element has a resistor and a pair of electrodes connected to the resistor. The ratio of the resistance between the electrodes of the second temperature-sensing element to the resistance between the electrodes of the first temperature-sensing element at the same temperature is between 0.9 and 1.

1. The gas sensor according to any one of claims 5 to 7.

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