Patent Description:
A gas sensor detects the concentration of a measurement target gas contained in the atmosphere, but a measurement error may occur due to the influence of a gas contained in the atmosphere that is different from the detection target gas. Patent Document <NUM> discloses a method of correcting a signal obtained from a hydrogen sensor unit that detects a hydrogen gas as the detection target, based on oxygen concentration and/or humidity obtained respectively from an oxygen concentration measurement part and/or a humidity measurement part which are provided separately from the hydrogen sensor unit. Further, from <CIT>, from <CIT>, from <CIT> and from <CIT> gas sensor devices comprising signal evaluation circuitry are known.

However, with the method disclosed in Patent Document <NUM>, there is a necessity of performing computation processing for calculation of the gas concentration. In addition, the oxygen concentration measurement part and/or a humidity measurement part are provided outside the hydrogen sensor unit, so that not only the size of the entire sensor is increased, but also it is difficult to perform accurate correction.

It is therefore an object of the present invention is to provide a gas sensor capable of highly accurately eliminating the influence of a gas different from a detection target gas without performing computation processing.

A gas sensor according to the independent claim is provided. Dependent claims provide preferred embodiments.

Thus, according to an example, it is possible to highly accurately eliminate the influence of a gas different from a detection target gas without performing computation processing, thus allowing the concentration of the detection target gas to be highly accurately measured.

Preferred embodiments of an example will be explained below in detail with reference to the accompanying drawings.

<FIG> is a circuit diagram illustrating the configuration of a gas sensor 10A according to the first example (not in accordance with the present invention).

As illustrated in <FIG>, the gas sensor 10A has a sensor part S and a signal processing circuit <NUM>. Although not particularly limited, the gas sensor 10A detects the concentration of a CO<NUM> gas in the atmosphere and can cancel a measurement error due to humidity by hardware as will be described later. In the present specification, a detection target gas and a gas to become noise are sometimes referred to as "first gas" and "second gas", respectively. In the present example, the first gas is a CO<NUM> gas, and the second gas is vapor.

The sensor part S is a heat conduction type gas sensor for detecting the concentration of a CO<NUM> gas to be detected and has a first sensor part S1 and a second sensor part S2. The first sensor part S1 includes a first thermistor Rd1 and a first heater resistor MH1 that heats the first thermistor Rd1. Similarly, the second sensor part S2 includes a second thermistor Rd2 and a second heater resistor MH2 that heats the second thermistor Rd2. As illustrated in <FIG>, the first and second thermistors Rd1 and Rd2 are connected in series to each other between a wiring supplied with a power supply potential Vcc and a wiring supplied with a ground potential GND. The first and second thermistors Rd1 and Rd2 are each made of a material having a negative resistance temperature coefficient, such as a composite metal oxide, amorphous silicon, polysilicon, or germanium. The first and second thermistors Rd1 and Rd2 both detect the concentration of the CO<NUM> gas but differ in operating temperature as will be described later.

The first thermistor Rd1 is heated by the first heater resistor MH1. The heating temperature of the first thermistor Rd1 by the first heater resistor MH1 is, e.g., <NUM>. When the CO<NUM> gas is present in the measurement atmosphere in a state where the first thermistor Rd1 is heated, heat dissipation characteristics of the first thermistor Rd1 changes according to the concentration of the CO<NUM> gas. This change appears as a change in the resistance value of the first thermistor Rd1. When the heating temperature of the first thermistor Rd1 is <NUM>, the resistance value of the first thermistor Rd1 changes according to the CO<NUM> gas concentration with a first sensitivity. The first sensitivity has a sensitivity that can sufficiently change the potential of a detection signal Vout1 appearing at a connection point between the first and second thermistors Rd1 and Rd2.

When vapor is present in the measurement atmosphere in a state where the first thermistor Rd1 is heated, heat dissipation characteristics of the first thermistor Rd1 changes according to the concentration of the vapor. When the heating temperature of the first thermistor Rd1 is <NUM>, the resistance value of the first thermistor Rd1 changes according to humidity with a second sensitivity.

The second thermistor Rd2 is heated by the second heater resistor MH2. The heating temperature of the second thermistor Rd2 by the second heater resistor MH2 is, e.g. , <NUM>. Even when the CO<NUM> gas is present in the measurement atmosphere in a state where the second thermistor Rd2 is heated, the resistance value of the second thermistor Rd2 hardly changes. That is, when the heating temperature of the second thermistor Rd2 is <NUM>, the resistance value of the second thermistor Rd2 changes according to the CO<NUM> gas concentration with a third sensitivity; however, the third sensitivity is significantly lower than the first sensitivity and is preferably <NUM>/<NUM> or less of the first sensitivity, and more preferably, substantially <NUM>. Thus, even when the CO<NUM> gas concentration changes, the resistance value of the second thermistor Rd2 hardly changes.

When vapor is present in the measurement atmosphere in a state where the second thermistor Rd2 is heated, heat dissipation characteristics of the second thermistor Rd2 changes according to the vapor concentration. When the heating temperature of the second thermistor Rd2 is <NUM>, the resistance value of the second thermistor Rd2 changes according to humidity with a fourth sensitivity. The fourth sensitivity is higher than the second sensitivity.

Further, the gas sensor 10A according to the present example has a correction resistor R1 connected parallel to the second thermistor Rd2. As described later, the correction resistor R1 is provided to cancel a difference between a sensitivity (second sensitivity) of the first thermistor Rd1 with respect to humidity and a sensitivity (fourth sensitivity) of the second thermistor Rd2 with respect to humidity.

As described above, the first and second thermistors Rd1 and Rd2 are connected in series to each other, and the detection signal Vout1 is output from the connection point therebetween. The detection signal Vout1 is input to the signal processing circuit <NUM>.

The signal processing circuit <NUM> has differential amplifiers <NUM> to <NUM>, an AD converter (ADC) <NUM>, a DA converter (DAC) <NUM>, a control part <NUM>, and resistors R2 to R4. The differential amplifier <NUM> compares the detection signal Vout1 and a reference voltage Vref and amplifies the detected difference. The gain of the differential amplifier <NUM> is arbitrarily adjusted by the resistors R2 to R4. An amplified signal Vamp output from the differential amplifier <NUM> is input to the AD converter <NUM>.

The AD converter <NUM> converts the amplified signal Vamp into a digital signal and supplies the obtained value to the control part <NUM>. On the other hand, the DA converter converts a reference signal supplied from the control part <NUM> to an analog signal to generate the reference voltage Vref and generates control voltages Vmh1 and Vmh2 to be supplied respectively to the first and second heater resistors MH1 and MH2. The control voltage Vmh1 is applied to the first heater resistor MH1 through a differential amplifier <NUM> which is a voltage follower. Similarly, the control voltage Vmh2 is applied to the second heater resistor MH2 through a differential amplifier <NUM> which is a voltage follower.

<FIG> is a top view for explaining the configuration of the sensor part S. <FIG> is a cross-sectional view taken along line A-A in <FIG>. The drawings are schematic, and for explanatory convenience, the relation between thickness and plane dimension, ratio between the thicknesses of devices, and the like may be different from those in the actual structure within a range in which the effect of the present example can be obtained.

The sensor part S is a heat conduction type gas sensor that detects the concentration of a gas based on a change in heat dissipation characteristics according to the CO<NUM> gas concentration and has, as illustrated in <FIG> and <FIG>, two sensor parts S1 and S2 and a ceramic package <NUM> housing the sensor parts S1 and S2.

The ceramic package <NUM> is a box-shaped case having an opened upper part, and a lid <NUM> is provided at the upper part. The lid <NUM> has a plurality of vent holes <NUM>, through which CO<NUM> gas in the atmosphere can flow into the ceramic package <NUM>. In <FIG>, the lid <NUM> is omitted for ease of viewing.

The first sensor part S1 includes insulating films <NUM> and <NUM> formed respectively on the lower and upper surfaces of a substrate <NUM>, a first heater resistor MH1 provided on the insulating film <NUM>, a heater protective film <NUM> covering the first heater resistor MH1, a first thermistor Rd1 and a thermistor electrode <NUM> which are provided on the heater protective film <NUM>, a thermistor protective film <NUM> covering the first thermistor Rd1 and thermistor electrode <NUM>.

There is no particular restriction on the material of the substrate <NUM> as long as it has an adequate mechanical strength and is suitable for fine processing such as etching, and, examples thereof include a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, a glass substrate, and the like. A cavity 31a is provided at a position overlapping the first heater resistor MH1 in a plan view so as to suppress conduction of heat due to the first heater resistor MH1 to the substrate <NUM>. A part where the substrate <NUM> is removed by the cavity 31a is called a membrane. The presence of the membrane reduces heat capacity by the thinning of the substrate <NUM>, allowing heating to be achieved with less power consumption.

The insulating films <NUM> and <NUM> are each made of an insulating material such as silicon oxide or silicon nitride. When silicon oxide is used as the insulating films <NUM> and <NUM>, a film deposition method such as a thermal oxidation method or a CVD (Chemical Vapor Deposition) method may be used. There is no particular restriction on the thickness of the insulating films <NUM> and <NUM> as long as the insulating property thereof is ensured and may be, e.g., about <NUM> to <NUM>. Particularly, the insulating film <NUM> is used also as an etching stop layer when the cavity 31a is formed in the substrate <NUM>, so that the thickness thereof is preferably set to a value suitable for fulfilling the function as the etching stop layer.

The first heater resistor MH1 is made of a conductive substance whose resistivity changes depending on temperature and is preferably made of a metal material having a comparatively 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 them. Among them, a conductive material that can be subjected to high accuracy dry etching such as ion milling is preferable, and more preferably, it contains platinum (Pt) having high corrosion resistance as a main component. Further, an adhesion layer such as a titanium (Ti) layer is preferably formed as a base of Pt so as to improve adhesion with respect to the insulating film <NUM>.

The heater protective film <NUM> is formed above the first heater resistor MH1. The heater protective film <NUM> is preferably made of the same material as the insulating film <NUM>. The first heater resistor MH1 generates violent thermal changes (repetition of temperature rises between room temperature to <NUM> and then a drop to room temperature again), so that strong thermal stress is applied to the insulating film <NUM> and heater protective film <NUM>. When being continuously subject to the thermal stress, the insulating film <NUM> and heater protective film <NUM> may suffer damage such as interlayer peeling or crack. However, when the insulating film <NUM> and the heater protective film <NUM> are made of the same material, material characteristics thereof are the same, and adhesion strength therebetween is high, so that the damage such as interlayer peeling or crack is less likely to occur as compared to when the insulating film <NUM> and the heater protective film <NUM> are made of mutually different materials. When silicon oxide is used as the material of the heater protective film <NUM>, film deposition may be performed by a thermal oxidation method or a CVD method. The film thickness of the heater protective film <NUM> is not particularly restricted as long as insulation between the first thermistor Rd1 and the thermistor electrode <NUM> can be ensured and may be, e.g., <NUM> to <NUM>.

The first thermistor Rd1 is made of a material having a negative resistance-temperature coefficient, such as a composite metal oxide, amorphous silicon, polysilicon, or germanium and can be formed by using a thin-film process such as a sputtering method or a CVD method. The film thickness of the first thermistor Rd1 may be adjusted according to a target resistance value. For example, when the resistance value (R25) at room temperature is set to about <NUM> MΩ using MnNiCo based oxide, the film thickness may be set to about <NUM> to <NUM> although it depends on the distance between a pair of thermistor electrodes <NUM>. The reason that the thermistor is used as a temperature-sensitive resistive element is that the thermistor is larger in resistance temperature coefficient than a platinum temperature detector and can thus obtain high detection sensitivity. Further, heat generation of the first heater resistor MH1 can efficiently be detected because of the thin-film structure.

The thermistor electrode <NUM> is configured of a pair of electrodes arranged spaced apart from each other at a predetermined interval, and the first thermistor Rd1 is provided between the pair of thermistor electrodes <NUM>. With this configuration, the resistance value between the pair of thermistor electrodes <NUM> is determined by the resistance value of the first thermistor Rd1. The thermistor electrode <NUM> may be made of a conductive substance that can endure a process such as a film deposition step and a heat treatment process for the first thermistor Rd1 and is preferably made of a material having a comparatively 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 them.

The first thermistor Rd1 and thermistor electrode <NUM> are covered with the thermistor protective film <NUM>. When the first thermistor Rd1 is brought into contact with a material having reducibility so as to make it turn into a high-temperature state, the material deprives the thermistor of oxygen to cause a reduction, thus affecting thermistor characteristics. To prevent this, an insulating oxide film having no reducibility, such as silicon oxide film, is preferably used as the material of the thermistor protective film <NUM>.

As illustrated in <FIG>, both ends of the first heater resistor MH1 are connected respectively to electrode pads 37a and 37b provided on the surface of the thermistor protective film <NUM>. Further, both ends of the thermistor electrode <NUM> are connected respectively to electrode pads 37c and 37d provided on the surface of the thermistor protective film <NUM>. The electrode pads 37a to 37d are connected to a package electrode <NUM> installed to the ceramic package <NUM> through a bonding wire <NUM>. The package electrode <NUM> is connected to the signal processing circuit <NUM> illustrated in <FIG> through an external terminal <NUM> provided on the back surface of the ceramic package <NUM>.

As described above, the first sensor part S1 has a configuration in which the first heater resistor MH1 and first thermistor Rd1 are laminated on the substrate <NUM>, so that heat generated by the first heater resistor MH1 is efficiently conducted to the first thermistor Rd1.

Similarly, the second sensor S2 includes insulating films <NUM> and <NUM> formed respectively on the lower and upper surfaces of a substrate <NUM>, a second heater resistor MH2 provided on the insulating film <NUM>, a heater protective film <NUM> covering the second heater resistor MH2, a second thermistor Rd2 and a thermistor electrode <NUM> which are provided on the heater protective film <NUM>, and a thermistor protective film <NUM> covering the second thermistor Rd2 and thermistor electrode <NUM>.

The substrate <NUM> is made of the same material as the substrate <NUM> used for the first sensor part S1 and has the same configuration as the substrate <NUM>. That is, a cavity 41a is provided at a position overlapping the second heater resistor MH2 in a plan view so as to suppress heat due to the second heater resistor MH2 from conducting to the substrate <NUM>. The insulating films <NUM> and <NUM> are made of the same material (insulating material such as silicon oxide or silicon nitride) as the insulating films <NUM> and <NUM>. The insulating films <NUM> and <NUM> have the same thickness as the insulating films <NUM> and <NUM>.

The second heater resistor MH2, heater protective film <NUM>, second thermistor Rd2, thermistor electrode <NUM>, and thermistor protective film <NUM> have the same configurations as the first heater resistor MH1, the heater protective film <NUM>, the first thermistor Rd1, the thermistor electrode <NUM>, and the thermistor protective film <NUM>, respectively, used for the first sensor part S1. Both ends of the second heater resistor MH2 are connected respectively to electrode pads 47a and 47b provided on the surface of the thermistor protective film <NUM>. Further, the both ends of the thermistor electrode <NUM> are connected respectively to electrode pads 47c and 47d provided on the surface of the thermistor protective film <NUM>. The electrode pads 47a to 47d are connected to the package electrode <NUM> fitted in the ceramic package <NUM> through the bonding wire <NUM>.

The thus configured first and second sensor parts S1 and S2 are each produced in multiple numbers in a wafer state at a time, followed by dicing into individual pieces, and then fixed to the ceramic package <NUM> using a die paste (not illustrated). Thereafter, electrode pads 37a to 37d and 47a to 47d are connected to their corresponding package electrodes <NUM> through the bonding wires <NUM> using a wire bonding machine. As the material of the bonding wire <NUM>, a metal having low resistance, such as Au, Al, or Cu is preferably used.

Finally, adhesive resin (not illustrated) or the like is used to fix the lid <NUM> having the outside air vent holes <NUM> to the ceramic package <NUM>. Although a substance contained in the adhesive resin is turned into gas during heating/curing of the adhesive resin (not shown), the gas is easily discharged outside the package through the vent holes <NUM>, so that the first and second sensor parts S1 and S2 are hardly affected.

The thus accomplished the sensor part S is connected to the signal processing circuit <NUM> or a power supply through the external terminal <NUM>. The correction resistor R1 may be incorporated in the signal processing circuit <NUM>, housed in the ceramic package <NUM>, or provided on a circuit board on which the signal processing circuit <NUM> is mounted.

The configuration of the gas sensor 10A according to the first example has been described. Next, the operation of the gas sensor 10A according to the first example will be described.

The gas sensor 10A utilizes a significant difference between the heat conductivity of the CO<NUM> gas and that of air to take out a change in the heat dissipation characteristics of the thermistors Rd1 and Rd2 according to the CO<NUM> gas concentration as the detection signal Vout1. However, the heat conductivity of the measurement atmosphere changes according not only to the CO<NUM> gas concentration but also to humidity, i.e., the vapor concentration, so that the influence of humidity may cause a measurement error. Thus, the gas sensor 10A adjusts the resistance value of the correction resistor R1 so as to make an error component of the first thermistor Rd1 due to humidity and an error component of the second thermistor Rd2 due to humidity coincide with each other to cancel a change in the detection signal Vout1 based on humidity.

<FIG> is a graph illustrating the relationship between the heating temperature and sensitivity of the thermistors Rd1 and Rd2.

As illustrated in <FIG>, when the heating temperature of the thermistors Rd1 and Rd2 is <NUM> or lower, it is possible to obtain a sufficiently high sensitivity with respect to the CO<NUM> gas concentration; while when the heating temperature exceeds <NUM>, the sensitivity with respect to the CO<NUM> gas concentration decreases, and when the heating temperature reaches <NUM>, the sensitivity with respect to the CO<NUM> gas concentration becomes substantially <NUM>. Actually, even when the heating temperature is <NUM>, there is a slight sensitivity with respect to the CO<NUM> gas concentration; however, it is significantly lower than (about <NUM>/<NUM> of) that when the heating temperature is <NUM> and can thus be substantially ignored.

Taking the above into consideration, in the gas sensor 10A, the first thermistor Rd1 is heated to <NUM> to sufficiently increase the sensitivity (first sensitivity) with respect to the CO<NUM> gas concentration, and the second thermistor Rd2 is heated to <NUM> to reduce the sensitivity (third sensitivity) with respect to the CO<NUM> gas concentration to substantially <NUM>. Since the first and second thermistors Rd1 and Rd2 are connected in series to each other, the level of the detection signal Vout1 represents the CO<NUM> gas concentration when there is no influence of humidity.

On the other hand, the sensitivity (second sensitivity) with respect to the humidity when the heating temperature of the first thermistor Rd1 is <NUM> and the sensitivity (fourth sensitivity) with respect to the humidity when the heating temperature of the second thermistor Rd2 is <NUM> differ from each other. Specifically, the second sensitivity is about <NUM>µV/%RH, while the fourth sensitivity is about <NUM>µV/%RH. Therefore, the influence of humidity is reflected on the detection signal Vout1 when the first and second thermistors Rd1 and Rd2 are simply connected in series.

Thus, in the gas sensor 10A according to the first example, the correction resistor R1 is connected in parallel to the second thermistor Rd2 so as to cancel a change in the detection signal Vout1 according to humidity. Assuming that the second sensitivity is a, the fourth sensitivity is b, and the resistance value of the second thermistor Rd2 heated to <NUM> is Rd2, the resistance value of the correction resistor R1 is set to: R1 = (b/a) × Rd2, whereby it is possible to substantially cancel a change in the detection signal Vout1 according to humidity. When this expression is applied to the above-described example, R1 may be set to (<NUM>/<NUM>) × Rd2 = (<NUM>/<NUM>) × Rd2.

Thus, the influence that humidity has on the first thermistor Rd1 and the influence that humidity has on the second thermistor Rd2 effectively coincide with each other, so that the detection signal Vout1 does not change even with a change in humidity. That is, the level of the detection signal Vout1 is determined by the CO<NUM> gas concentration.

<FIG> is a timing chart illustrating an example of the waveforms of the control voltages Vmh1 and Vmh2. As illustrated in <FIG>, the control voltage Vmh1 and control voltage Vmh2 are simultaneously brought to an active level to simultaneously heat the first heater resistor MH1 and second heater resistor MH2. Then, the detection signal Vout1 is sampled at the timing when the control voltages Vmh1 and Vmh2 are activated, whereby it is possible to measure the CO<NUM> gas concentration without necessity of computation processing for canceling the influence of humidity.

<FIG> are graphs illustrating actual measurement values. <FIG> illustrates a change in the CO<NUM> gas and a change in humidity, and <FIG> illustrates a change in the detection signal Vout1. As illustrated in <FIG>, it can be found that when the correction resistor R1 is not employed, the level of the detection signal Vout1 significantly changes depending on humidity, while when the correction resistor R1 is employed, the influence of humidity is substantially completely canceled from the detection signal Vout1.

As described above, in the gas sensor 10A according to the first example, the two thermistors Rd1 and Rd2 different in heating temperature are connected in series to each other, and the correction resistor R1 is connected parallel to the second thermistor Rd2, so that the level of the detection signal Vout1 appearing at the connection point between the first and second thermistors Rd1 and Rd2 accurately represents the CO<NUM> gas concentration without being influenced by humidity. Thus, it is possible to directly measure the CO<NUM> gas concentration without through computation processing for canceling the influence of humidity.

<FIG> is a circuit diagram illustrating the configuration of a gas sensor 10B according to a first embodiment of the present invention.

As illustrated in <FIG>, the gas sensor 10B according to the present embodiment differs from the gas sensor 10A illustrated in <FIG> in that a common control voltage Vmh is supplied in common to the differential amplifiers <NUM> and <NUM> and that the differential amplifier <NUM> does not serve as a voltage follower and is gain-adjusted using resistors R5 to R7. Other configurations are the same as those of the gas sensor 10A according to the first example, so the same reference numerals are given to the same elements, and overlapping description will be omitted.

The resistors R5 to R7 are elements for adjusting the gain of the differential amplifier <NUM>. For example, when the resistance values of the resistors R5 to R7 are set such that R5 = R6 = R7, Vmh2 = <NUM> × Vmh1 can be satisfied. That is, it is possible to generate two mutually different control voltages Vmh1 and Vmh2 using the common control voltage Vmh.

As a result, even when the level of the common control voltage Vmh temporarily changes due to, e.g., fluctuation of a power supply potential, both the control voltages Vmh1 and Vmh2 fluctuate simultaneously in conjunction with the common control voltage Vmh, thereby canceling the influence due to the fluctuations of the control voltages Vmh1 and Vmh2. Therefore, the level of the detection signal Vout1 does not substantially change even with a fluctuation of the common control voltage Vmh. Thus, according to the present embodiment, it is possible to measure the CO<NUM> gas concentration more stably.

<FIG> is a circuit diagram illustrating the configuration of a gas sensor 10C according to a second embodiment of the present invention.

As illustrated in <FIG>, the gas sensor 10C according to the present embodiment differs from the gas sensor 10B according to the first embodiment illustrated in <FIG> in that the correction resistor R1 is connected in parallel to the first thermistor Rd1. Other configurations are the same as those of the gas sensor 10B according to the first embodiment, so the same reference numerals are given to the same elements, and overlapping description will be omitted.

As exemplified in the present embodiment, when the sensitivity (second sensitivity) of the first thermistor Rd1 with respect to humidity is higher than the sensitivity (fourth sensitivity) of the second thermistor Rd2 with respect to humidity, the correction resistor R1 may be connected parallel to the first thermistor Rd1 so as to effectively reduce the second sensitivity.

<FIG> is a circuit diagram illustrating the configuration of a gas sensor 10D according to a second example (not in accordance with the present invention).

As illustrated in <FIG>, the gas sensor 10D differs from the gas sensor 10A according to the first example illustrated in <FIG> in that a third sensor part S3 as a temperature sensor and a resistor R8 are additionally provided in the sensor part S. The third sensor part S3 includes a third thermistor Rd3, and the third thermistor Rd3 and the resistor R8 are connected in series to each other between a wiring supplied with a power supply potential Vcc and a wiring supplied with a ground potential GND. A temperature signal Vout2 is output from a connection point between the third thermistor Rd3 and the resistor R8. The temperature signal Vout2 is supplied to the AD converter <NUM>. Other circuit configurations are the same as those of the gas sensor 10A according to the first example, so the same reference numerals are given to the same elements, and overlapping description will be omitted.

<FIG> is a top view for explaining the configuration of the sensor part S in the present example. <FIG> is a cross-sectional view taken along line B-B in <FIG>. The drawings are schematic, and the relation between thickness and plane dimension, the ratio between the thicknesses of devices, and the like may be different from those in the actual structure within a range in which the effect of the present example can be obtained.

As illustrated in <FIG> and <FIG>, the third sensor part S3 is disposed between the first sensor part S1 and the second sensor part S2. Although not particularly limited, the three sensor parts S1 to S3 are integrated on a single substrate <NUM>. The substrate <NUM> has formed therein three cavities 61a to 61c corresponding to the three sensor parts S1 to S3.

The substrate <NUM> has insulating films <NUM> and <NUM>, a heater protective film <NUM>, the third thermistor Rd3 and a thermistor electrode <NUM> which are provided on the heater protective film <NUM> at a position overlapping the cavity 61c, and a thermistor protective film <NUM> that covers the first thermistors Rd1 to Rd3 and thermistor electrodes <NUM>, <NUM>, and <NUM>. As illustrated in <FIG>, both ends of the thermistor electrode <NUM> constituting the third thermistor Rd3 are connected respectively to electrode pads 67a and 67b provided on the surface of the thermistor protective film <NUM>. The electrode pads 67a and 67b are each connected to a package electrode <NUM> attached to a ceramic package <NUM> through a bonding wire <NUM>. Other basic configurations are the same as those illustrated in <FIG> and <FIG>, so the same reference numerals are given to the same elements, and overlapping description will be omitted.

The above is the configuration of the gas sensor 10D according to the present example. As described above, in the gas sensor 10D according to the present example, the three sensor parts S1 to S3 are integrated on the single substrate <NUM>, so that it is possible to additionally provide the third sensor part S3 serving as a temperature sensor without unnecessarily increasing the number of parts. In addition, by disposing the third sensor part S3 in the center of the gas sensor 10D, the distance between the first sensor part S1 and the second sensor part S2 can be increased, making it possible to reduce mutual thermal interference. That is, the first sensor part S1 and the second sensor part S2 differ in heating temperature and are heated simultaneously, so that thermal interference may occur when the distance therebetween is small. However, in the present example, the sensor part S3 is disposed between the first sensor part S1 and the second sensor part S2, so that thermal interference between the first sensor part S1 and the second sensor part S2 is reduced, allowing more accurate measurement.

<FIG> is a timing chart for explaining a timing at which the temperature signal Vout2 is sampled. As illustrated in <FIG>, also in the present example, the control voltage Vmh1 and the control voltage Vmh2 are simultaneously brought to an active level to simultaneously heat the first heater resistor MH1 and second heater resistor MH2. Then, the detection signal Vout1 is sampled at the timing when the control voltages Vmh1 and Vmh2 are activated, and the temperature signal Vout2 is sampled at a timing before the activation of the control voltages Vmh1 and Vmh2. This makes it possible to accurately measure an environmental temperature using the third sensor part S3 without being influenced by heating by the first and second heater resistors MH1 and MH2.

The temperature signal Vout2 is supplied to the AD converter <NUM> illustrated in <FIG>. The temperature signal Vout2 supplied to the AD converter <NUM> is converted into a digital signal and is supplied to the control part <NUM>. The control part <NUM> stores therein mathematical expressions or tables representing the relationship between an environmental temperature and the control voltages Vmh1, Vmh2, and the control voltages Vmh1 and Vmh2 are corrected thereby. <FIG> is a graph illustrating the relationship between an environmental temperature and control voltages Vmh1, Vmh2. As illustrated in <FIG>, the control part <NUM> corrects the control voltages Vmh1 and Vmh2 so that the levels of the control voltages Vmh1 and Vmh2 decrease as the environmental temperature rises. Thus, by changing the levels of the control voltages Vmh1 and Vmh2 according to the current environmental temperature obtained as the temperature signal Vout2, it is possible to set heating temperatures by the first and second heater resistors MH1 and MH2 to values substantially the same as designed temperatures irrespective of the current environmental temperature.

As described above, in the gas sensor 10D according to the present example, not only the control voltages Vmh1 and Vmh2 are corrected based on the temperature signal Vout2, but also the sensor part S3 is disposed between the first sensor part S1 and the second sensor part S2, so that thermal interference between the first sensor part S1 and the second sensor part S2 is reduced. This allows the CO<NUM> gas concentration to be measured more accurately.

In addition, the third sensor part S3 is disposed on the same chip as the first and second sensor parts S1 and S2, so that the third sensor part S3 can measure substantially the same environmental temperature as an environmental temperature to which the first sensor part S1 and second sensor part S2 are subject. This allows very accurate temperature measurement, making it possible to set heating temperatures by the first and second heater resistors MH1 and MH2 to values substantially the same as designed temperatures.

It is apparent that the gas sensor is not limited to the above embodiments, but may be modified and changed.

Claim 1:
A gas sensor (10B) comprising:
a first thermistor (Rd1) having a resistance value that changes according to a concentration of a first gas with a first sensitivity and changes according to a concentration of a second gas with a second sensitivity;
a second thermistor (Rd2) connected in series to the first thermistor (Rd1), the second thermistor (Rd2) having a resistance value that changes according to a concentration of the first gas with a third sensitivity that is lower than the first sensitivity and changes according to a concentration of the second gas with a fourth sensitivity that is different from the second sensitivity;
a correction resistor (R1) connected in parallel with the first (Rd1) or second (Rd2) thermistor, wherein a resistance value of the correction resistor (R1) is selected based on a ratio of the second sensitivity and the fourth sensitivity so as to cancel a change in potential at a connection point between the first (Rd1) and second (Rd2) thermistors according to the concentration of the second gas;
a first heater (MH1) configured to heat the first thermistor (Rd1) to a first temperature based on a first control voltage (Vmh1);
a second heater (MH2) configured to heat the second thermistor (Rd2) to a second temperature different from the first temperature based on a second control voltage (Vmh2);
a first amplifier (<NUM>) configured to receive a common control voltage (Vmh) and apply the first control voltage (Vmh1) to the first heater (MH1); and
a second amplifier (<NUM>) configured to receive the common control voltage (Vmh) and apply the second control voltage (Vmh2) to the second heater (MH2),
wherein a gain of the first amplifier (<NUM>) and a gain of the second amplifier (<NUM>) are different from each other.