JP2007024508A - Membrane gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane gas sensor constituted so as to suppress the increase in powder consumption to the utmost, excluding the situation where a gas-sensing layer is affected due to temperature, made responsive to a target gas so as to be not affected by the ambient environment. <P>SOLUTION: The membrane gas sensor is constituted so that a heater layer is driven over a predetermined period so that the gas-sensing layer becomes a targeted gas-detecting temperature to calculate the resistance value of the gas sensing layer, after the temperature has stabilized to calculate the concentration of the target gas, the heater layer is driven over a predetermined period so that the gas-sensing layer becomes a carbon monoxide detecting temperature; and the resistance value of the gas sensing layer is calculated, after the temperature has stabilized, and the concentration of a carbon monoxide gas is calculated and effect dues to temperature are excluded. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a low power consumption thin film gas sensor with battery driving in mind.

Generally, a gas sensor is used for applications such as a gas leak alarm, and a specific gas such as carbon monoxide (CO), methane gas (CH ₄ ), propane gas (C ₃ H ₈ ), methanol vapor ( It is a device that selectively responds to (CH ₃ OH) and the like, and high sensitivity, high selectivity, high response, high reliability, and low power consumption are indispensable due to its nature.

  By the way, the gas leak alarms that are widely used for household use are for the purpose of detecting flammable gas for city gas and propane gas, for the purpose of detecting incomplete combustion gas of combustion equipment, or There are things that have both functions, etc., but the spread rate is not so high due to the problem of cost and installation (gas detection is necessary but power supply is impossible). Therefore, in order to improve the penetration rate, it is desired to improve the installation property, specifically, to be cordless as a battery-driven gas leak alarm.

Low power consumption of the gas sensor is the most important for realizing the battery drive of the gas leak alarm. However, in order to operate a catalytic combustion type or semiconductor type gas sensor, it is necessary to heat the gas sensing film of the gas sensor to a high temperature of 400 ° C. to 500 ° C., and this heating is a factor that consumes electric power. In a gas sensor using a gas sensing film produced by sintering powder such as SnO _2, the thickness of the gas sensing film is made as thin as possible by using a method such as screen printing to reduce the heat capacity of the gas sensing film. However, there is a limit to thinning the film and it cannot be made thin enough. For this reason, the heat capacity of the gas sensing film is too large for battery operation, and a large amount of power is required to heat the gas sensing film to a high temperature, resulting in increased battery consumption. It was difficult to put it into practical use.

Thus, a thin film gas sensor with low power consumption that is practically acceptable has been developed and put into practical use as a diaphragm structure with high heat insulation and low heat capacity by a microfabrication process. Next, such a thin film gas sensor will be described.
FIG. 12 is a longitudinal sectional view schematically showing a conventional thin film gas sensor. FIG. 13 is a circuit block diagram of the thin film gas sensor.
This conventional thin film gas sensor includes a silicon substrate (hereinafter referred to as Si substrate) 1, a thermal insulating support layer 2, a heater layer 3, an electrical insulating layer 4, and a gas sensing layer 5. Specifically, the heat insulating support layer 2 has a three-layer structure of a thermally oxidized SiO ₂ layer 2a, a CVD-Si ₃ N ₄ layer 2b, and a CVD-SiO ₂ layer 2c. In detail, the gas sensing layer 5 includes a bonding layer 5a, a sensing layer electrode 5b, a sensing layer 5c, and a first gas selective combustion layer 5d. The sensing layer 5c is a tin dioxide layer to which antimony is added (hereinafter referred to as Sb-doped SnO ₂ layer), and the first gas selective combustion layer 5d is an alumina burner that supports palladium (Pd) or platinum (Pt) as a catalyst. It is a binder (hereinafter referred to as a catalyst-supported Al ₂ O ₃ sintered material). As shown in FIG. 13, the heater layer 3 and the gas sensing layer 5 (specifically, the sensing layer 5 c via the sensing layer electrode 5 b) are connected to the driving / processing unit 6. The inventors have confirmed that similar prior art has been filed.

  This conventional thin film gas sensor utilizes a phenomenon in which the electrical resistance (sensing layer resistance) of the sensing layer 5c, which is an oxide semiconductor, is changed by contact with various gas components. Metal oxide semiconductors heated to about 300-400 ° C have the property that the electrical conductivity varies depending on the gas concentration. In the air, they absorb oxygen and increase resistance, but in combustible gases, adsorb combustible gases. To lower the resistance.

Specifically, the sensing layer 5c, which is an n-type metal oxide semiconductor such as an Sb-doped SnO ₂ layer and heated to about 300 to 400 ° C., activates and adsorbs oxygen and the like on the particle surface in the air. Has a strong electron accepting property and adsorbs negative charges, so that a space charge layer is formed on the surface of the oxide semiconductor particles, resulting in a decrease in conductivity and high resistance. In addition, an electron-donating reducing gas such as a flammable gas When adsorbed and a combustion reaction occurs, oxygen adsorbed on the surface is consumed, electrons trapped in the oxygen are returned to the semiconductor, electron density increases, conductivity increases, and resistance decreases. .

The sensing layer 5c can detect various gases, but it is difficult to selectively detect a specific gas.
Therefore, the gas sensing layer 5 is formed by sintering the surface of the sensing layer 5c, which is an electrically insulating layer 4, a bonding layer 5a, a pair of sensing layer electrodes 5b and 5b, and an Sb-doped SnO ₂ layer, with catalyst-supported Al ₂ O ₃ sintered. The first gas selective combustion layer 5d made of a material is covered.
As described above, the gas sensing layer 5 is configured so as to cover the entire sensing layer 5c with the first gas selective combustion layer 5d made of the sintered material supporting the catalyst, and thus has a higher oxidation activity than the target gas to be detected. In addition to improving the sensitivity of only the target gas (especially methane or propane) that is detected by burning the gas, the gas of the target gas that you want to detect is devised in terms of the size, film thickness, and ratio of the diaphragm diameter. Selectivity is increased and power consumption can be reduced.

In such a thin film type gas sensor, the driving method of the heater layer 3 is devised in order to realize low power consumption even when detecting a combustible gas such as CH ₄ or C ₃ H ₈ as a target gas. . This point will be described with reference to the drawings. FIG. 14 is an explanatory diagram for explaining the temporal characteristics of the heater layer temperature by the High + Off method, and FIG. 15 is an explanatory diagram for explaining the temporal characteristics of the heater layer temperature by the High + Off + Low + Off method.

The High + Off method is used for detecting the concentration of a combustible gas such as CH ₄ or C ₃ H ₈ as a target gas. The heater layer 3 is supplied with a drive signal based on a current as shown in FIG. The heater temperature is maintained in a high temperature state (High state: 400 to 500 ° C.) for a certain period (for example, 0.05 to 0.5 s), and then the drive signal is not supplied to the heater layer 3 for a certain period (Off state) ) To suppress unnecessary power consumption except during detection. And the driving by such High + Off is repeated with a predetermined period (for example, 30 second period), and the heater layer 3 is intermittently driven.

In this method, gas detection is performed in a high state, and in gas detection, the sensing layer resistance of the sensing layer 5c is measured via the sensing layer electrode 5b, and a combustible gas such as CH ₄ or C ₃ H ₈ is determined from the change. Detect concentration. This is because, when the heater temperature is high, in the first gas selective combustion layer 5d, a reducing gas such as CO and H _{2 and} other miscellaneous gases are burned, and an inflammable gas such as inert CH ₄ and C ₃ H _{8 is used.} Gas diffuses through the first gas selective combustion layer 5d, reaches the sensing layer 5c, reacts with SnO _{2 of the} sensing layer 5c, and changes the resistance value of SnO ₂ to gas such as gas equipment It detects the concentration of combustible gas such as CH ₄ , C ₃ H _8, etc. generated at the time of leakage.

The High + Off + Low + Off method is used for detecting the concentration of carbon monoxide (CO) generated at the time of incomplete combustion instead of the previous combustible gas concentration, and the heater layer 3 has a current as shown in FIG. The sensing layer 5c is caused by flowing the drive signal and maintaining the heater temperature of the heater layer 3 in a high temperature state (High state: 400 to 500 ° C.) for a certain period (for example, 0.05 to 0.5 s) by the oxidizing action of the catalyst. The gas adhering to the surface of the sensing layer 5 is once burned and cleaned, and then the surface of the sensing layer 5c is CO on the surface of the sensing layer 5c while the sensor temperature is at a normal temperature so that the drive signal does not flow to the heater layer 3 for a certain period of time (Off state). Next, the reaction between CO adsorbed at room temperature and SnO _{2 in the} sensing layer 5c while maintaining a low temperature state (Low state: 50 to 150 ° C.) By using the change in the resistance value of SnO ₂ , the concentration of CO generated during incomplete combustion of a gas appliance or the like is detected. Thereafter, in a state where the drive signal is not supplied to the heater layer 3 for a certain period (Off state), unnecessary power consumption is suppressed except during detection. The driving by the High + Off + Low + Off method is repeated at a predetermined cycle (for example, a cycle of 150 seconds) to drive the heater layer 3 intermittently.
A similar prior art is disclosed in Japanese Patent Application Laid-Open No. 2003-270185.

  When both the detection of combustible gas concentration by the High + Off method of FIG. 14 and the detection of CO gas concentration at the time of incomplete combustion by the High + Off + Low + Off method of FIG. 15, the sensor for detecting the combustible gas concentration and the time of incomplete combustion are used. The CO gas concentration detection sensors are separately prepared and the heaters are driven separately.

On the other hand, there is also a demand to be able to detect the combustible gas concentration and the CO gas concentration during incomplete combustion with a single sensor. In this case, it is possible to cope with the problem by devising the heater drive format. This point will be described with reference to the drawings. FIG. 16 is an explanatory diagram for explaining the temporal characteristics of the heater layer temperature by the High + Low method, and FIG. 17 is an explanatory diagram for explaining the temporal characteristics of the heater layer temperature by the High + Low + Off method.
The High + Low method shown in FIG. 16 has a function of detecting the concentration of combustible gas and the concentration of CO generated during incomplete combustion, and a drive signal based on the current shown in FIG. The heater temperature of the heater layer 3 is kept at a high temperature (High state: 400 to 500 ° C.) for a certain period (for example, 0.05 to 0.5 s) and adheres to the surface of the sensing layer 5c by the oxidation action of the catalyst. The inflammable gas such as inactive CH ₄ , C ₃ H _8, etc. is detected while burning the cleaned gas once to clean it, and then in a low temperature state (Low state: 50 to 150 ° C.) for a certain period (several seconds to several tens of seconds). ) And sufficiently adhere CO to the surface of the sensing layer 5c, and the resistance value of SnO ₂ changes due to the reaction between this CO and SnO _{2 of the} sensing layer 5c. Detects the concentration of CO that is generated during incomplete combustion, such as in a gas appliance.
A similar prior art is disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2000-193623).

The High + Low + Off method shown in FIG. 17 has a function of performing both the detection of the combustible gas concentration and the concentration of CO generated during incomplete combustion, as in the High + Low method shown in FIG. Is driven by a battery such as a dry cell, in response to a strong demand to minimize the average power consumption of the sensor, and as shown in FIG. The period (for example, 0.05 s to 0.5 s) is short, but since the thin film gas sensor is small and has a low heat capacity and excellent heat insulation, the temperature time constant is as short as 50 ms or less, and the high state is shortened to 50 to 500 ms. The temperature of the sensing layer 5c can be raised to 400 to 500 ° C.) and kept at a high temperature (High state: 400 to 500 ° C.). To the oxidation of the catalyst detects a flammable gas, such as sensing layer while cleaning once by burning gas adhering to the surface of the 5c inert CH _4, C ₃ H _8, a low temperature state (Low state: The temperature is lowered to about 100 ° C., CO gas is detected, and after that, the drive signal is not supplied to the heater layer 3 for a certain period (OFF state), and unnecessary power consumption is suppressed except during detection. Then, such driving by High + Low + Off is repeated at a predetermined cycle (for example, a cycle of 30 seconds), and the heater layer 3 is intermittently driven. Similar prior art is disclosed in Patent Document 3 (Japanese Patent Laid-Open No. 2005-83840).

Further, although not particularly shown in Patent Document 4 (Japanese Patent Laid-Open No. 2005-134311), the temperature of the heater is repeatedly set to High + Medium + Low + Off at a predetermined cycle (for example, a cycle of 30 seconds) using the temperature characteristics of the thin film gas sensor. This is a proposal for an algorithm for detecting CO, H ₂ , and CH ₄ . However, in this method, the selectivity of CH ₄ is good when the sensor low temperature is set to a high temperature (450 ° C.) (in the atmosphere, in 100 ppm of CO ₂ , compared to the resistance value in 1000 ppm of H ₂ , the sensing in CH ₄ 4000 ppm. Compared with the case where the layer resistance value is lower by one digit or more), the selectivity is low when the sensor low temperature is set to a medium temperature (240 ° C.) and a low temperature (120 ° C.). That is, when the sensor low temperature is set to an intermediate temperature, both the sensing layer resistance values in 1000 ppm of H _{2 and} 4000 ppm of CH ₄ are lower by one digit or more than the resistance values in CO 100 ppm in the atmosphere.

In other words, CH ₄ and H ₂ cannot be distinguished only from the change in the sensing layer resistance value. Further, when the sensor low temperature is lowered, the sensing layer resistance values in CO at 100 ppm, H ₂ in 1000 ppm, and CH ₄ at 4000 ppm are both one or more orders of magnitude lower than the resistance value in the atmosphere. That is, CO, CH ₄ , and H ₂ cannot be distinguished only from the change in the sensing layer resistance value.
For this reason, in Patent Document 4, an algorithm for recognizing which gas is present among CO, CH ₄ , and H ₂ from the measurement result of the sensing layer resistance value when the sensor low temperature is high, medium, and low. However, it is said that the concentration of the detected gas cannot be estimated as it is.
The driving method of the thin film gas sensor is as described above.

JP 2003-270185 A JP 2000-193623 A JP 2005-83840 A Japanese Patent Laying-Open No. 2005-134311

  In the thin film gas sensor as described above, it has been found in the course of research and testing by the present inventors that the sensing layer resistance is affected by the environment. Next, the resistance of the sensing layer that changes depending on the environment will be discussed. FIG. 18 is a temperature-sensing layer resistance characteristic diagram of the sensing layer according to the High + Low + Off method. The characteristic diagram shown in FIG. 18 shows that the thin film gas sensor shown in FIG. 12 is driven at a cycle of 30 seconds using the High + Low + Off method as shown in FIG. It is held in the Low state for a while. When the low state temperature is changed from 50 ° C. to 450 ° C. for driving, the value of the sensing layer resistance at the end of the low state is plotted against the low state temperature. The external ambient temperature during measurement is constant at 25 ° C.

  As shown in the sensor characteristics of FIG. 18, it can be seen that the sensing layer resistance value in CO 100 ppm has temperature dependence in the section of 50 to 120 ° C. during the low state (50 to 150 ° C.). With such temperature dependence, when the external ambient temperature rises from 25 ° C to 50 ° C, the temperature of the thin film gas sensor rises by 25 ° C even when the same power is applied to the heater layer, and the CO concentration is constant at 100 ppm. Even so, the output of the sensing layer may change and the CO concentration may not be detected accurately.

  Therefore, the present invention has been made to solve the above problems, and its purpose is to suppress an increase in power consumption as much as possible, eliminate the situation where the gas sensing layer is affected by temperature, and respond to the target gas. Thus, an object of the present invention is to provide a thin film gas sensor which is not affected by the surrounding environment.

Such a thin film gas sensor according to claim 1 of the present invention includes:
A Si substrate having a through hole;
A diaphragm-like heat insulating support layer stretched on the opening of the through hole;
A heater layer provided on the heat insulating support layer;
An electrical insulation layer provided to cover the thermal insulation support layer and the heater layer;
A pair of sensing layer electrodes provided on the electrically insulating layer, a sensing layer provided so as to be passed through the pair of sensing layer electrodes, and a sintered material provided on the surface of the sensing layer and carrying the first catalyst. A gas sensing layer comprising: a gas selective combustion layer; and a thin film semiconductor second gas selective combustion layer provided between the sensing layer and the first gas selective combustion layer and including a second catalyst;
A drive connected to the heater layer;
A processing unit connected to the gas sensing layer;
The drive unit comprises
A target gas detection driving means for driving the heater layer over a predetermined period so that the gas detection layer has a target gas detection temperature;
Carbon monoxide gas detection driving means for driving the heater layer over a predetermined period so that the gas detection layer has a carbon monoxide gas detection temperature;
Function as
The processing unit
A target gas concentration calculating means for calculating a target gas concentration by calculating a value of a sensing layer resistance of the gas sensing layer after the temperature is stabilized by driving the target gas detection temperature;
A carbon monoxide gas concentration calculating means for calculating the carbon monoxide gas concentration by calculating the value of the sensing layer resistance of the gas sensing layer after the temperature is stabilized by the carbon monoxide gas detection temperature driving;
It functions as.

A thin film gas sensor according to claim 2 of the present invention is
The thin film gas sensor according to claim 1,
The drive unit functions as a carbon monoxide gas detection drive unit after functioning n times continuously as a target gas detection drive unit.

A thin film gas sensor according to claim 3 of the present invention is
The thin film gas sensor according to claim 1 or 2,
The target gas detection drive means is driven High + Off, and the carbon monoxide gas detection drive means is driven Low + Off, so that the target gas detection drive means and the carbon monoxide gas detection drive means are continuously set to High + Off + Low + Off drive,
The heater layer is driven for a predetermined period so that the gas sensing layer reaches the combustible gas detection temperature when driving High, and the heater layer is driven for a predetermined period so that the gas sensing layer becomes the carbon monoxide gas detection temperature when driving Low. It is a means.

A thin film gas sensor according to claim 4 of the present invention is
The thin film gas sensor according to claim 1 or 2,
The target gas detection drive means is driven High + Medium + Off (room temperature), and the carbon monoxide gas detection drive means is driven Low + Off, so that the target gas detection drive means and the carbon monoxide gas detection drive means are continuously high + medium + off + low + off. Drive and
The heater layer is driven for a predetermined period so that the gas sensing layer becomes a combustible gas detection temperature when driving High, and the heater layer is driven for a predetermined period so that the gas detection layer becomes the hydrogen gas detection temperature when Medium is driven. It is a means for driving the heater layer over a predetermined period so that the gas sensing layer reaches the carbon monoxide gas detection temperature during driving.

A thin film gas sensor according to claim 5 of the present invention is
The thin film gas sensor according to claim 1 or 2,
The target gas detection drive means is driven to High + Medium1 + Medium2 + Off (normal temperature), and then the carbon monoxide gas detection drive means is driven to Low + Off, so that the target gas detection drive means and the carbon monoxide gas detection drive means are continuously High + Medium1 + Medium2 + Off + Low + Off + Drive and
The heater layer is driven for a predetermined period so that the gas sensing layer becomes the cleaning temperature when driving High, the heater layer is driven for the predetermined period so that the gas sensing layer becomes the combustible gas detection temperature when driving Medium 1, and the medium 2 is driven when Medium 2 is driven The heater layer is driven for a predetermined period so that the gas sensing layer is at the hydrogen gas detection temperature, and the heater layer is driven for a predetermined period so that the gas detection layer is at the carbon monoxide gas detection temperature during low driving. It is characterized by.

A thin film gas sensor according to claim 6 of the present invention is
In the thin film gas sensor according to claim 1,
The carbon monoxide gas detection drive means is a means for driving the heater layer over a predetermined period so as to achieve a multi-step carbon monoxide gas detection temperature.
The carbon monoxide gas concentration calculating means is a means for calculating the carbon monoxide gas concentration by calculating the value of the sensing layer resistance of the gas sensing layer after the temperature is stabilized by the carbon monoxide gas detection temperature driving. And

A thin film gas sensor according to claim 7 of the present invention is
In the thin film gas sensor according to claim 1,
The carbon monoxide gas detection driving means is means for driving the heater layer over a predetermined period so as to be a lamp-like carbon monoxide gas detection temperature,
The carbon monoxide gas concentration calculating means is a means for calculating a carbon monoxide gas concentration by calculating a value of a sensing layer resistance of the gas sensing layer every predetermined period by driving a carbon monoxide gas detection temperature. .

A thin film gas sensor according to claim 8 of the present invention is
In the thin film gas sensor according to any one of claims 1 to 7,
The sensing layer is a layer made of SnO ₂ to which Sb (antimony) is added.

A thin film gas sensor according to claim 9 of the present invention is
In the thin film gas sensor according to any one of claims 1 to 8,
The first gas selective combustion layer is a layer made of an Al ₂ O ₃ sintered material supporting Pd (palladium) as a catalyst.

A thin film gas sensor according to claim 10 of the present invention is
In the thin film gas sensor according to any one of claims 1 to 9,
The second gas selective combustion layer is a layer of SnO ₂ to which Pt (platinum) is added.

  According to the present invention as described above, the increase in power consumption is suppressed as much as possible, the situation where the gas sensing layer is affected by the temperature is eliminated and the target gas is sensitive so as not to be affected by the surrounding environment. A thin film gas sensor can be provided.

The present invention seeks to detect both CO (carbon monoxide gas) and CH ₄ (target gas) with low power consumption by using one thin film gas sensor by devising a heater driving method. It is. Note that CH ₄ generated due to gas leakage needs to be frequently monitored and detected, but the detection interval of CO generated due to incomplete combustion may be longer than that of CH ₄ . That is, CH ₄ must be detected, for example, every 30 seconds, but CO may be detected every 150 seconds.

Hereinafter, a thin film gas sensor of the best mode for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view schematically showing a thin film gas sensor of this embodiment. FIG. 2 is a circuit block diagram of the thin film gas sensor.
The thin film gas sensor of this embodiment includes a silicon substrate (hereinafter referred to as Si substrate) 1, a heat insulating support layer 2, a heater layer 3, an electric insulating layer 4, and a gas sensing layer 5. Specifically, the heat insulating support layer 2 has a three-layer structure of a thermally oxidized SiO ₂ layer 2a, a CVD-Si ₃ N ₄ layer 2b, and a CVD-SiO ₂ layer 2c. In detail, the gas sensing layer 5 includes a bonding layer 5a, a sensing layer electrode 5b, a sensing layer 5c, a first gas selective combustion layer 5d, and a second gas selective combustion layer 5e. This sensing layer 5c is a tin dioxide layer to which antimony is added (hereinafter referred to as Sb-doped SnO ₂ layer), and the first gas selective combustion layer 5d is an alumina sintered material (hereinafter referred to as “palladium (Pd)” supported as a catalyst). a Pd-loaded _Al 2 _{O 3} sintered material), the second gas selective combustion layer 5e tin dioxide layer of platinum is added (hereinafter, a Pt-doped SnO ₂ layers). As shown in FIG. 2, the heater layer 3 and the gas sensing layer 5 (specifically, the sensing layer 5 c via the sensing layer electrode 5 b) are connected to the driving / processing unit 6. The driving / processing unit 6 is configured by integrating the driving unit and the processing unit of the present invention, and is, for example, a CPU (Central Processing Unit).

In the gas sensing layer 5, the entire surface of the sensing layer 5c, which is an Sb-doped SnO ₂ layer, is covered with the second gas selective combustion layer 5e, and further the electric insulating layer 4, the bonding layer 5a, and the pair of sensing layer electrodes 5b and 5b. The surfaces of the sensing layer 5c and the second gas selective combustion layer 5e are covered with a first gas selective combustion layer 5d made of a Pd-supported Al ₂ O ₃ sintered material.

  In this embodiment, the temperature dependence and sensitivity of carbon monoxide concentration detection (CO concentration detection) are improved by driving the thin film gas sensor of FIG. 1 by a special driving method. This point will be described below. This special driving method is to drive High + Off + Low + Off particularly when the sensor Low temperature is approximately 200 ° C. or less, and to drive High + Low + Off when the sensor Low temperature exceeds approximately 200 ° C. Among these, it has been found in the course of the present inventor's research and test that when the gas concentration is detected in the low state by the High + Off + Low + Off method at about 200 ° C. or lower, the selectivity of CO is improved and the temperature dependency is improved. . This point will be described with reference to the drawings. 3, 4 and 5 are temperature-sensing layer resistance characteristics of the sensing layer according to a special driving method.

  Specifically, the temperature characteristic of the temperature-sensing layer of this special driving method shows that the sensor low temperature is approximately 200 ° C. or less, and the High + Off + Low + Off driving (the High state is held at 450 ° C. for a constant 200 ms and then the Off state is set). Then, when the Low state is maintained at 200 ° C. for 500 ms and then the Off state is performed), the resistance of the sensing layer at the end of the Low state is determined with respect to the sensor Low temperature. In the case of the temperature characteristics where the sensor Low temperature exceeds 200 ° C., the High state is maintained at High + Low + Off (the High state is held at 450 ° C. for a constant time of 200 ms, and the Low state is changed from 450 ° C. to 200 ° C.). Low state when driving at a constant temperature of 500 ° C for a period of 500 ms) with a period of 30 seconds The value of the sensing layer resistance of Ryoji, is plotted against the sensor Low temperature.

  In such a special drive method, as shown in FIG. 3, in the region where the sensor low temperature is 100 ° C. or less, the resistance value in CO 100 ppm is almost constant, that is, the temperature dependency of the sensing layer resistance is not noticeable. It can be seen that the dependency is improved. This is clear even when compared with the region where the sensor low temperature in the temperature-sensing layer resistance characteristic diagram shown in FIG.

Furthermore, in this special drive system, as shown in FIG. 3, the selectivity of CO is improved in the region where the sensor low temperature is 100 ° C. or lower, that is, in the atmosphere, in H ₂ 1000 ppm, in CH ₄ 4000 ppm. Compared to the sensing layer resistance, the sensing layer resistance value of CO 100 ppm is lower by one digit or more. This is also clear compared with the fact that the value of the sensing layer resistance in the region where the sensor low temperature in the temperature-sensing layer resistance characteristic diagram shown in FIG.

The following phenomena can be cited as reasons for such behavior. That is, when the thin-film gas sensor whose cross-sectional structure is shown in FIG. Further, the adsorption of CO in the subsequent Off state is promoted, and as a result of the CO being sufficiently adsorbed on the SnO ₂ surface, the resistance value change of SnO ₂ due to the reaction between CO and SnO _{2 of the} sensing layer 5c is large in the subsequent Low state. It is thought to be.

Furthermore, the experimental results when the CO concentration is changed are shown in FIG. When the thin film gas sensor is driven by a special driving method (when the sensor Low temperature is approximately 200 ° C. or lower, High + Off + Low + Off is driven, and when the sensor Low temperature exceeds approximately 200 ° C., High + Low + Off is driven), the difference in CO concentration causes Off. As a result, when the sensor low temperature is 150 ° C. or lower, the resistance value of SnO ₂ at the time of sensor low differs depending on the CO concentration.

When the sensor low temperature exceeds 150 ° C., CO, which is a reducing gas adsorbed when it is turned off, starts to burn by the action of the first gas selective combustion layer 5d and the second gas selective combustion layer 5e. Concentration dependence is lost, and at a temperature where the sensor low temperature exceeds 230 ° C., CO, which is a reducing gas, is completely burned by the action of the first gas selective combustion layer 5d and the second gas selective combustion layer 5e, and SnO ₂ The sensing layer resistance of is almost equal to the value in the atmosphere.

Next, the experimental results when the CH ₄ concentration is changed are shown in FIG. When the thin film gas sensor is driven by a special driving method (when the sensor Low temperature is approximately 200 ° C. or less, High + Off + Low + Off is driven, and when the sensor Low temperature exceeds approximately 200 ° C., the sensor Low temperature is 350 ° C.). From the vicinity of exceeding Sn, the sensing layer resistance of SnO ₂ varies depending on the CH ₄ concentration. This is because when the sensor low temperature exceeds 350 ° C., H ₂ starts to burn in the first gas selective combustion layer 5d and the second gas selective combustion layer 5e (CO starts to burn from a lower temperature), but inert gas. CH ₄ is reacted with the SnO ₂ reaches the sensitive layer 5c without even beyond the sensor Low temperature 350 ° C. complete combustion partitions, presumably because the resistance value change of SnO ₂ increases.

In the case the sensor Low temperature is 450 ° C., High Temperature sensor Low temperature equal results in the case of High + Off drive giving High temperature in a pulse form, CH ₄ at High + Off driven by thin-film gas sensor having the characteristics of FIG. 5 The concentration can be detected reliably.
The present invention intends to provide a thin film gas sensor employing a heater driving method that uses the thin film gas sensor of FIG. 1 described above to simultaneously detect gas leakage and incomplete combustion with a single thin film gas sensor. .

Next, the configuration of each part will be described.
The Si substrate 1 is formed of silicon (Si) and has a through hole.
The heat insulating support layer 2 is stretched over the opening of the through hole and formed in a diaphragm shape, and is provided on the Si substrate 1.

Specifically, the heat insulating support layer 2 has a three-layer structure of a thermally oxidized SiO ₂ layer 2a, a CVD-Si ₃ N ₄ layer 2b, and a CVD-SiO ₂ layer 2c.
The thermally oxidized SiO ₂ layer 2a is formed as a heat insulating layer and has a function of reducing the heat capacity by preventing heat generated in the heater layer 3 from being conducted to the Si substrate 1 side. The thermally oxidized SiO ₂ layer 2a exhibits high resistance to plasma etching and facilitates formation of a through hole in the Si substrate 1 by plasma etching, which will be described later.
The CVD-Si ₃ N ₄ layer 2b is formed above the thermally oxidized SiO ₂ layer 2a.
The CVD-SiO ₂ layer 2c improves the adhesion with the heater layer 3 and ensures electrical insulation. The SiO ₂ layer formed by CVD (chemical vapor deposition) has a small internal stress.

The heater layer 3 is a thin-film Ni—Cr film (nickel-chromium film), and is provided on the upper surface at substantially the center of the heat insulating support layer 2. A power supply line (not shown) is also formed. This power supply line is connected to the driving / processing unit 6.
The electrical insulating layer 4 is formed of a sputtered SiO ₂ layer that ensures electrical insulation, and is provided so as to cover the heat insulating support layer 2 and the heater layer 3. Electrical insulation is ensured between the heater layer 3 and the sensing layer electrode 5b, and the electrical insulating layer 4 improves adhesion to the sensing layer 5c.

The bonding layer 5 a is made of, for example, a Ta film (tantalum film) or a Ti film (titanium film), and is provided on the electrical insulating layer 4. The bonding layer 5a is interposed between the sensing layer electrode 5b and the electric insulating layer 4 and has a function of increasing the bonding strength.
The sensing layer electrodes 5b are made of, for example, a Pt film (platinum film) or an Au film (gold film), and are provided in a pair on the left and right sides so as to be sensing electrodes of the sensing layer 5c.
The gas sensing layer 5c is composed of an Sb-doped SnO ₂ layer, and is formed on the electrical insulating layer 4 so as to pass the pair of sensing layer electrodes 5b and 5b.

The first gas selective combustion layer 5d is a sintered body supporting palladium as a first catalyst, and is a Pd-supported Al ₂ O ₃ sintered material as described above. Since Al ₂ O ₃ is a porous body, it increases the chance that the detection gas passing through the holes comes into contact with Pd and promotes the combustion reaction.
The second gas selective combustion layer 5e is a thin film semiconductor containing platinum as a second catalyst, and is a Pt-doped SnO ₂ layer as described above.
The second gas selective combustion layer 5e is provided on the surface of the sensing layer 5c, and the first gas selective combustion layer 5d further includes the electrical insulating layer 4, the bonding layer 5a, the pair of sensing layer electrodes 5b and 5b, and the sensing layer. 5c and the second gas selective combustion layer 5e are provided so as to cover the surface.
Such a thin film gas sensor has a structure of high heat insulation and low heat capacity by a diaphragm structure.

The driving / processing unit 6 is configured by integrating the driving unit and the processing unit of the present invention, is electrically connected to the heater layer 3 so as to be electrically energized, and has a sensing layer via a gas sensing layer electrode 5b. 5c is electrically communicably connected.
The configuration of the thin film gas sensor is such.

Then, the manufacturing method of the thin film gas sensor of this form is demonstrated roughly.
First, a plate-like silicon wafer (not shown) is thermally oxidized on one side (or both sides) by a thermal oxidation method to form a thermally oxidized SiO ₂ layer 2a as a thermally oxidized SiO ₂ film.
Then, a CVD-Si ₃ N ₄ film is deposited on the surface on which the thermally oxidized SiO ₂ layer 2a is formed by a plasma CVD method to form a CVD-Si ₃ N ₄ layer 2b. Then, a CVD-SiO ₂ film is deposited on the upper surface of the CVD-Si ₃ N ₄ layer 2b by a plasma CVD method to form a CVD-SiO ₂ layer 2c.

Further, a Ni—Cr film is deposited on the upper surface of the CVD-SiO ₂ layer 2 c by a sputtering method to form the heater layer 3. Then, a sputtered SiO ₂ film is deposited on the upper surfaces of the CVD-SiO ₂ layer 2c and the heater layer 3 by a sputtering method to form an electrical insulating layer 4 that is a sputtered SiO ₂ layer.

A bonding layer 5a and a sensing layer electrode 5b are formed on the electrical insulating layer 4. Film formation is performed by an ordinary sputtering method using an RF magnetron sputtering apparatus. The film formation conditions are the same for the bonding layer (Ta or Ti) 5a and the sensing layer electrode (Pt or Au) 5b, Ar gas (argon gas) pressure 1 Pa, substrate temperature 300 ° C., RF power 2 W / cm ² , film thickness Bonding layer 5a / sensing layer electrode 5b = 500/2000.

A Sb-doped SnO ₂ film is deposited by sputtering between the electrical insulating layers 4 so as to be passed to the pair of sensing layer electrodes 5b, 5b, thereby forming the sensing layer 5c.
Film formation is performed by a reactive sputtering method using an RF magnetron sputtering apparatus. SnO ₂ containing 0.5 wt% Sb is used as the target. The film forming conditions are Ar + O ₂ gas pressure 2 Pa, substrate temperature 150 to 300 ° C., RF power 2 W / cm ² . The size of the sensing layer 5c is preferably about 50 to 200 μm square, and the thickness is preferably about 0.2 to 1.6 μm.

On the surface of the sensing layer 5c, a Pt-doped SnO ₂ film is deposited by sputtering to form a second gas selective combustion layer 5e.
A first gas selective combustion layer 5d is formed so as to cover the insulating layer 4, the bonding layer 5a, the pair of sensing layer electrodes 5b and 5b, the sensing layer 5c, and the second gas selective combustion layer 5e. This first gas selective combustion layer 5d is printed by screen printing a printing paste prepared by mixing and preparing an alumina powder (Pd / alumina) supporting a Pd catalyst, either a silica sol binder or an aluminum sol binder, and an organic solvent. After drying, it is formed by baking at 500 ° C. for 1 hour. The size of the first gas selective combustion layer 5d is sufficient to cover the sensing layer 5c and the second gas selective combustion layer 5e. In this way, the thickness is reduced by screen printing.

Finally, silicon is removed from the back surface of the silicon wafer (not shown) by etching as a microfabrication process to form a through hole to form the Si substrate 1, thereby forming a thin film gas sensor having a diaphragm structure. The heater layer 3 and the sensing layer electrode 5b are electrically connected to the driving / processing unit 6.
The manufacturing method of the thin film gas sensor is as follows.

Next, a special driving method by the driving / processing unit 6 of the thin film gas sensor configured as described above will be described. FIG. 6 is a driving pattern diagram for explaining a heater layer driving method.
The driving / processing unit 6 shown in FIG. 2 supplies a driving signal as shown in FIG. 6 to drive the heater layer 3. Then, the heater temperature of the heater layer 3 follows and becomes a heater temperature as shown in FIG.

The driving / processing unit 6 functions as target gas detection driving means for driving the heater layer 3 high for a predetermined period so that the gas detection layer 5 becomes the target gas detection temperature (CH ₄ concentration detection temperature).
The driving / processing unit 6 is a target gas concentration calculating means for detecting the CH ₄ concentration by measuring the sensing layer resistance of the sensing layer 5c at the rear end (white circle in FIG. 6) of the period of high temperature in the form of pulses. Function. Note that the measurement at the rear end of the period is performed after the temperature is stabilized, and in the following description, the measurement is performed at the rear end of the period in order to stabilize the measurement. .
The driving / processing unit 6 performs the Off driving, which is a fixed interval, and then sets the heater layer 3 to Low for a predetermined period so that the gas sensing layer 5 becomes the carbon monoxide gas detection temperature (CO concentration detection temperature). It functions as carbon monoxide gas detection driving means for driving.
The driving / processing unit 6 also calculates the carbon monoxide gas concentration for detecting the CO concentration by measuring the resistance value of the sensing layer at the rear end portion (white circle in FIG. 6) of the period lower than the above temperature. Functions as a means.
The driving / processing unit 6 functions continuously n times (5 times in the case of FIG. 6) as target gas detection driving means and target gas concentration calculation means. And it functions as the above-mentioned carbon monoxide gas detection drive means and carbon monoxide gas concentration calculation means, and repeats such a cycle thereafter.

  In this case, High + Off driving is performed in the target gas detection drive, but when performing carbon monoxide gas detection driving, the entire target gas detection driving and carbon monoxide gas detection driving are continuously performed as High + Off + Low + Off driving. Yes. From the above-described action, the temperature dependency and sensitivity of the sensing layer resistance to CO are improved.

In such a High + Off + Low + Off drive, it is desirable that a period from when the heater temperature becomes the CH ₄ concentration detection (High state) to the next CH ₄ concentration detection (High state) is, for example, 30 seconds apart. Further, the CH ₄ concentration detection drive (High state) is desirably an interval of 50 to 500 ms and a temperature of 400 ° C. to 500 ° C. Furthermore, it is desirable that the Off state, which is a rest period from the CH ₄ concentration detection to the CO concentration detection, be 5 to 20 sec. The CO concentration detection driving (Low state) is preferably performed at an interval of 300 to 1000 ms and at a temperature of 50 ° C. to 120 ° C. Thus, the detection of CH _{4 is performed} , for example, every 30 seconds, and the detection of CO is performed, for example, every 150 seconds, with one thin film gas sensor.
Needless to say, these values are determined depending on the characteristics of the thin film gas sensor used. This value is particularly suitable for the thin film gas sensor having the structure of FIG.

In the detection method described above, the target gas detection (CH ₄ detection) is equivalent to that detected by driving with the above-described High + Off method, and as described in the case of the sensor low temperature = 450 ° C. in FIG. It is possible to reliably detect _four concentrations.
Further, carbon monoxide detection (CO detection) is the same as that detected by driving with the above-described High + Off + Low + Off method, and the CO concentration can be reliably detected in the vicinity of the sensor Low temperature = 90 ° C. in FIG.

Next, another driving method by the driving / processing unit of another thin film gas sensor will be described. FIG. 7 is a driving pattern diagram for explaining another driving method of the heater layer. In this drive pattern, as shown in FIG. 6, the target gas detection drive means is continuously functioned n times, and thereafter the carbon monoxide gas detection drive means is functioned once. Only the driving pattern in which the carbon monoxide gas detection driving is continuous is shown, and the repeated pattern is not shown.
The driving / processing unit 6 shown in FIG. 2 supplies a driving signal as shown in FIG. 7 to drive the heater layer 3. Then, the heater temperature of the heater layer 3 follows and becomes a heater temperature as shown in FIG.

In this drive pattern, the target gas detection drive is driven by High + Medium + Off (room temperature), and the carbon monoxide gas detection drive is driven by Low + Off, so that the target gas detection drive and the carbon monoxide gas detection drive are continuously performed. High + Medium + Off + Low + Off driving.
The driving / processing unit 6 functions as a combustible gas detection driving unit that drives the heater layer 3 high for a predetermined period so that the gas detection layer 5 reaches the combustible gas detection temperature (CH ₄ concentration detection temperature).
The driving / processing unit 6 functions as a combustible gas concentration calculating means for calculating the CH ₄ concentration at the rear end (white circle in FIG. 7) of the period in which the heater temperature is stable and in a high state.
The driving / processing unit 6 functions as a hydrogen gas detection driving unit that drives the heater layer 3 to a medium for a predetermined period so that the gas detection layer 5 becomes a hydrogen gas detection temperature (H ₂ detection temperature).
The driving / processing unit 6 functions as a hydrogen gas detection unit that detects the presence of H ₂ at the rear end (white circle in FIG. 7) of the period in which the heater temperature is stable.
The driving / processing unit 6 is turned off for a certain period, and further, the carbon monoxide gas that drives the heater layer 3 low for a predetermined period so that the gas sensing layer 5 becomes the carbon monoxide gas detection temperature (CO concentration detection temperature). It functions as detection drive means.
The driving / processing unit 6 functions as a carbon monoxide gas concentration calculating means for calculating the CO concentration at the rear end portion (white circle ◯ in FIG. 7) of the period when the heater temperature is stable and in the low state.

Such high + medium + off (room temperature) + low + off driving is repeated, for example, every 30 seconds. Here, High = 450 ° C./0.2 s, Medium = 250 ° C./0.5 s, Off = normal temperature / 9.3 s, and Low = 90 ° C./0.5 s. Further, CH ₄ · H ₂ detection is performed, for example, every 30 seconds, and CO detection is performed, for example, every 150 seconds.
Needless to say, the temperature and the duration are selected so that the power consumption is minimized and the detection sensitivity is maximized according to the characteristics of the thin film gas sensor.

In the detection method described above, combustible gas detection (CH ₄ detection) detects the CH ₄ concentration at the final point of the High period after the Off of the previous cycle, and is thus detected by the driving of the aforementioned High + Off method. As described in the case of the sensor low temperature = 450 ° C. in FIG. 5, the CH ₄ concentration can be reliably detected.

In addition, since the presence of H ₂ is detected at the final point of the period when the heater temperature is Medium, the characteristics when the sensor Low temperature in FIG. 5 is 250 ° C. can be obtained by the driving of the above High + Low + Off method. it is obvious.

  Further, incomplete combustion detection (CO detection) detects the CO concentration at the final point of the Low period after sufficient CO adsorption time has elapsed, and therefore the sensor Low temperature in FIG. 4 is driven by the driving of the above High + Off + Low + Off method. = CO concentration can be reliably detected at around 90 ° C.

Next, another driving method by the driving / processing unit of another thin film gas sensor will be described. FIG. 8 is a driving pattern diagram for explaining another driving method of the heater layer. In this drive pattern, as shown in FIG. 6, the target gas detection drive means is continuously functioned n times, and thereafter the carbon monoxide gas detection drive means is functioned once. Only the driving pattern in which the carbon monoxide gas detection driving is continuous is shown, and the repeated pattern is not shown.
The driving / processing unit 6 shown in FIG. 2 supplies a driving signal as shown in FIG. 8 to drive the heater layer 3. Then, the heater temperature of the heater layer 3 follows and becomes a heater temperature as shown in FIG.

In this drive pattern, High + Medium1 + Medium2 + Off (normal temperature) drive is performed, and subsequently, Low + Off drive is performed in the carbon monoxide gas detection drive, so that the target gas detection drive and the carbon monoxide gas detection drive are continuously performed as High + Medium1 + Medium2 + Off + Low + Off drive.
The driving / processing unit 6 functions as a cleaning driving unit that drives the heater layer 3 high for a predetermined period so that the gas sensing layer 5 reaches the cleaning temperature.
The driving / processing unit 6 functions as a combustible gas detection driving means for driving the heater layer 3 to Medium 1 for a predetermined period so that the gas detection layer 5 becomes the combustible gas detection temperature (CH4 concentration detection temperature).
The driving / processing unit 6 functions as a combustible gas concentration calculating unit that calculates the CH ₄ concentration at the rear end portion (white circle in FIG. 8) in the medium 1 state in which the heater temperature is stable.
The driving / processing unit 6 functions as a hydrogen gas detection driving unit that drives the heater layer 3 to Medium 2 for a predetermined period so that the gas detection layer 5 reaches the hydrogen gas detection temperature (H2 detection temperature).
The driving / processing unit 6 functions as a hydrogen gas detection unit that detects the presence of H ₂ at the rear end (white circle in FIG. 8) of the period in which the heater 2 is in the medium 2 state where the heater temperature is stable.
The driving / processing unit 6 is turned off for a certain period, and further, the carbon monoxide gas that drives the heater layer 3 low for a predetermined period so that the gas sensing layer 5 becomes the carbon monoxide gas detection temperature (CO concentration detection temperature). It functions as detection drive means.
The driving / processing unit 6 functions as a carbon monoxide gas concentration calculating unit that detects the CO concentration at the rear end portion (white circle ◯ in FIG. 8) of the period in which the heater temperature is in a low state.

Such driving of High + Medium1 + Medium2 + Off (room temperature) + Low + Off is repeated, for example, every 30 seconds. Here, High = 450 ° C./0.2 s, Medium 1 = 400 ° C./0.5 s, Medium 2 = 250 ° C./0.5 s, Off = normal temperature / 8.8 s, and Low = 90 ° C./0.5 s. Further, CH ₄ is detected every 30 seconds, for example, and CO is detected every 150 seconds, for example.
Needless to say, the temperature and the duration are selected so that the power consumption is minimized and the detection sensitivity is maximized according to the characteristics of the thin film gas sensor.

In the detection method described above, the first gas selective combustion layer 5d, the second gas selective combustion layer 5e, and the sensing layer 5c are sufficiently cleaned while the heater temperature is high.
Combustible gas detection (CH ₄ detection) detects CH ₄ concentration at the final point of the medium 1 period after Off in the previous cycle, and is equivalent to that detected by the High + Low + Off driving method described above, as shown in FIG. As described in the case of the sensor low temperature = 400 ° C., the CH ₄ concentration can be reliably detected.

In addition, since the presence of H ₂ is detected at the end of the period when the heater temperature is Medium ₂ , it is clear that the characteristics when the sensor low temperature in FIG. 5 is 250 ° C. can be obtained by the above-described high + low + off driving. is there.

  Further, incomplete combustion detection (CO detection) detects the CO concentration at the final point of the Low period after sufficient CO adsorption time has elapsed, and therefore the sensor Low temperature in FIG. 4 is driven by the driving of the above High + Off + Low + Off method. = CO concentration can be reliably detected at around 90 ° C.

As described above, in the drive patterns described with reference to FIGS. 7 and 8, the difference in sensitivity between H ₂ and CH ₄ is not large, and it is difficult to distinguish them. However, advance because it detects the CH ₄ concentration at the last time point of the period of the High by the above method (or Medium1), if either CH ₄ is not present CH ₄ concentration is low, the period of Medium1 (or Medium2) Although it is not possible to accurately detect the concentration of H ₂ at the final point of time, it is possible to detect the presence of H ₂ .

Next, another driving method by the driving / processing unit of another thin film gas sensor will be described.
As shown in FIG. 3, the thin film gas sensor has a small temperature characteristic as shown in FIG. 3, but the temperature dependence cannot be completely eliminated. That is, even if the CO concentration is constant, the sensor low temperature changes. When the sensing layer resistance value changes, even if the same electric power is applied to the heater layer 3, the heater temperature changes due to the influence of the external ambient temperature. As a result, even if the CO concentration is constant, the sensing layer resistance value changes. Cannot be measured accurately. This also applies to the sensor low temperature-sensing layer resistance characteristic of the prior art shown in FIG. Therefore, the external ambient temperature is detected, and correction is performed based on the external ambient temperature. This is a mode in which only the above-described carbon monoxide gas detection drive (CO concentration detection drive) is improved.

Hereinafter, description will be given with reference to the drawings. FIG. 9 is a circuit block diagram of a thin film gas sensor, and FIG. 10 is a drive pattern diagram for explaining a heater layer drive system. In addition, as shown in FIG. 7, the part which repeats the pattern which heater temperature becomes high in a pulse shape is abbreviate | omitted.
The circuit block of this embodiment shown in FIG. 9 is provided with a temperature sensor 7 for detecting the external ambient temperature in addition to the circuit block shown in FIG. Then, in order to increase the CO concentration detection accuracy, the heater temperature is controlled to change to a plurality of steps (three stages in the example of FIG. 10) in the low state, and when the temperature becomes constant at each step. The resistance value of the gas sensing layer is measured.

First, the driving / processing unit 6 functions as a registration unit that detects and registers the external ambient temperature.
Subsequently, when driving the heater layer 3, the driving / processing unit 6 functions as a driving unit that supplies and drives a driving signal as shown in FIG. 10. Then, the heater temperature of the heater layer 3 follows and becomes a heater temperature as shown in FIG.

  Specifically, the driving / processing unit 6 reduces the power applied to the heater layer 3 in the first step, and subsequently detects the resistance of the sensing layer 5 of the gas sensing layer 5 at the timing of the rear end of the step when the temperature becomes constant. It functions as a means for measuring CO concentration detection (1). Then, in the second step, the electric power to be applied to the heater layer 3 is turned on, and then the sensing layer resistance of the gas sensing layer 5 is measured at the end timing of the step when the temperature becomes constant to detect the CO concentration ( It functions as a means for performing 2). Then, the power applied to the heater layer 3 is increased in the third step, and then the sensing layer resistance of the gas sensing layer 5 is measured at the rear end of the step when the temperature becomes constant to detect the CO concentration (3 ).

  The drive / processing unit 6 uses the result of the CO concentration detection (2) at the rear end of the second step in which the electric power applied to the heater layer 3 is normal when the external ambient temperature detected in advance is normal. When the temperature is high, the electric power given to the heater layer 3 is small, and the result of the CO concentration detection (1) at the rear end of the first step is used. It functions as a determination means that uses the result of CO detection concentration detection (3) at the rear end of the three steps.

  At this time, when the external ambient temperature is normal (for example, 25 ° C.), the result of the CO concentration detection (2) when the power applied to the heater layer 3 is medium is set in advance accurately. When the external ambient temperature is high, the power is lowered and the increase is subtracted. Conversely, when the external ambient temperature is low, the power is increased and the decrease is added. Thereby, even if the CO concentration of the thin film gas sensor is constant, it is possible to correct the phenomenon that the sensing layer resistance value changes with the change of the sensor Low temperature.

By the operation method of the thin film gas sensor as described above, it is possible to detect the sensing layer resistance when the heater temperature is substantially constant when detecting low-temperature CO that is easily affected by the external ambient temperature. In order to control the heater temperature more precisely, the number of steps of the heater temperature within the period of low temperature may be increased.
In this case, a difference between the external ambient temperature and the detected temperature may be taken and a CO detection value at a step corresponding to the difference may be adopted.

Next, another driving method by the driving / processing unit of another thin film gas sensor will be described. As in the previous embodiment, when the temperature dependence on the CO of the thin film gas sensor cannot be completely removed, that is, when the sensing layer resistance changes with changes in the sensor low temperature even if the CO concentration is constant, the external atmosphere The temperature is detected, and correction is performed based on the external ambient temperature. This is a mode in which only the above-described carbon monoxide gas detection drive (CO concentration detection drive) is improved.
In order to increase the CO concentration detection accuracy, the heater temperature is controlled to rise in a linear ramp as a low state, and the resistance value of the gas sensing layer is measured every time a predetermined period elapses.

  Hereinafter, it demonstrates, referring a figure. FIG. 11 is a drive pattern diagram for explaining the heater layer drive system. Also in this embodiment, the circuit block of the thin film gas sensor of FIG. 9 is used. Further, as shown in FIG. 6, a portion of repeating a pattern in which the heater temperature becomes high in pulses is omitted.

First, the driving / processing unit 6 functions as a registration unit that detects and registers the external ambient temperature.
Subsequently, when driving the heater layer 3, the driving / processing unit 6 functions as a driving unit that supplies and drives a driving signal as shown in FIG. 11. Then, the heater temperature of the heater layer 3 follows and becomes a heater temperature as shown in FIG.
Specifically, the driving / processing unit 6 controls the heater temperature of the heater layer 3 to change in a ramp shape, and measures the sensing layer resistance of the gas sensing layer 5 when a predetermined period elapses to detect the CO concentration (1). Acts as a means to do. And it continues to increase the electric power given to the heater layer 3, and it functions as a means which measures the sensing layer resistance of the gas sensing layer 5 when a predetermined period passes and performs CO concentration detection (2). And it continues to increase the electric power given to the heater layer 3, and it functions as a means to measure the sensing layer resistance of the gas sensing layer 5 and to detect the CO concentration (3) when a predetermined period has passed.

  The driving / processing unit 6 uses the result of the CO concentration detection (2) in which the electric power given to the heater layer 3 is normal when the external atmospheric temperature detected in advance is normal, but the electric power given to the heater layer 3 when the external atmospheric temperature is high Is used as a determination means that uses the result of CO concentration detection (1) in which the electric power applied to the heater layer 3 is large when the external ambient temperature is low.

  At this time, when the external ambient temperature is normal (for example, 25 ° C.), the result of the CO concentration (2) when the power applied to the heater layer 3 is medium is set in advance. When the ambient temperature is high, the power is reduced and the increase is subtracted. Conversely, when the external atmosphere is low, the power is increased and the decrease is added. Thereby, even if the CO concentration of the thin film gas sensor is constant, it is possible to correct the phenomenon that the sensing layer resistance changes with the change of the sensor Low temperature.

The thin film gas sensor of the present invention has been described above.
According to the present invention, by driving the heater layer 3 by the heater driving pattern sufficiently utilizing the temperature characteristics of the gas sensing layer 5 of the thin-film gas sensor with the structure shown in FIG. 1, and SnO ₂ of the detection gas and the gas sensing layer 5 Since the resistance value of SnO ₂ was measured after the reaction of (2) was sufficiently stable, the concentration of CO and CH ₄ can be detected with one thin film gas sensor, and even if H ₂ is present, It became possible to detect.

Further, by controlling the electric power with the driving pattern of the heater layer 3 as shown in FIG. 6, a pattern in which the heater temperature is increased in a pulse shape at a predetermined time interval is repeated, and is constant every n times of the pulse. A period in which the heater is at a temperature lower than the above temperature is inserted at intervals of time pause, so that power consumption can be reduced.
Further, the heater layer 3 detects the CH ₄ concentration by measuring the resistance value of the sensing layer at the rear end of the period when the temperature becomes high in a pulsed manner, and senses at the rear end during the period when the temperature becomes lower than the above temperature. The resistance value of the layer is measured to detect the CO concentration. For this reason, it becomes possible to measure the concentration of CH ₄ and CO simultaneously with one sensor.

Further, as shown in FIG. 7, the heater layer 3 is driven by High + Medium + Off (room temperature) + Low + Off, or as shown in FIG. 8, the heater layer 3 is driven by High + Medium1 + Medium2 + Off (room temperature) + Low + Off. Thus, it is possible to calculate the CH ₄ concentration and CO concentration and detect the presence or absence of H ₂ .

  Further, the heater temperature is controlled to change in a plurality of steps as shown in FIG. 10, or the heater temperature is changed to a ramp shape as shown in FIG. Measure the resistance of the sensing layer. Therefore, by detecting the temperature of the external atmosphere using the temperature sensor 7 and selecting the CO detection result from the result, the CO detection accuracy of the thin film gas sensor can be improved.

1 is a longitudinal sectional view schematically showing a thin film gas sensor of the best mode for carrying out the present invention. It is a circuit block diagram of a thin film gas sensor. FIG. 6 is a temperature-sensing layer resistance characteristic diagram of a sensing layer according to a special driving method. FIG. 6 is a temperature-sensing layer resistance characteristic diagram of a sensing layer according to a special driving method. FIG. 6 is a temperature-sensing layer resistance characteristic diagram of a sensing layer according to a special driving method. It is a drive pattern figure explaining the other drive system of a heater layer. It is a drive pattern figure explaining the other drive system of a heater layer. It is a drive pattern figure explaining the other drive system of a heater layer. It is a circuit block diagram of a thin film gas sensor. It is a drive pattern figure explaining the drive system of a heater layer. It is a drive pattern figure explaining the other drive system of a heater layer. It is a longitudinal cross-sectional view which shows the thin film gas sensor of a prior art schematically. It is a circuit block diagram of a thin film gas sensor. It is explanatory drawing explaining the time characteristic of the heater layer temperature by a High + Off system. It is explanatory drawing explaining the time characteristic of the heater layer temperature by a High + Off + Low + Off system. It is explanatory drawing explaining the time characteristic of the heater layer temperature by a High + Low system. It is explanatory drawing explaining the time characteristic of the heater layer temperature by a High + Low + Off system. FIG. 6 is a temperature-sensing layer resistance characteristic diagram of a sensing layer according to a High + Low + Off method.

Explanation of symbols

1: Si substrate 2: insulating support layer 2a: thermally oxidized SiO ₂ layer 2b: CVD-Si ₃ N ₄ layer 2c: CVD-SiO ₂ layer 3: heater layer 4: electrical insulating layer 5: gas sensing layer 5a: bonding layer 5b: Sensing layer electrode 5c: Sensing layer (Sb-doped SnO ₂ layer)
5d: 1st gas selective combustion layer (Sb-doped SnO ₂ layer)
5e: second gas selective combustion layer (Pd-supported Al ₂ O ₃ sintered material)
6: Drive / Processing unit 7: Temperature sensor

Claims (10)

A Si substrate having a through hole;
A diaphragm-like heat insulating support layer stretched on the opening of the through hole;
A heater layer provided on the heat insulating support layer;
An electrical insulation layer provided to cover the thermal insulation support layer and the heater layer;
A pair of sensing layer electrodes provided on the electrically insulating layer, a sensing layer provided so as to be passed through the pair of sensing layer electrodes, and a sintered material provided on the surface of the sensing layer and carrying the first catalyst. A gas sensing layer comprising: a gas selective combustion layer; and a thin film semiconductor second gas selective combustion layer provided between the sensing layer and the first gas selective combustion layer and including a second catalyst;
A drive connected to the heater layer;
A processing unit connected to the gas sensing layer;
The drive unit comprises
A target gas detection driving means for driving the heater layer over a predetermined period so that the gas detection layer has a target gas detection temperature;
Carbon monoxide gas detection driving means for driving the heater layer over a predetermined period so that the gas detection layer has a carbon monoxide gas detection temperature;
Function as
The processing unit
A target gas concentration calculating means for calculating a target gas concentration by calculating a value of a sensing layer resistance of the gas sensing layer after the temperature is stabilized by driving the target gas detection temperature;
A carbon monoxide gas concentration calculating means for calculating the carbon monoxide gas concentration by calculating the value of the sensing layer resistance of the gas sensing layer after the temperature is stabilized by the carbon monoxide gas detection temperature driving;
A thin film gas sensor characterized by functioning as: The thin film gas sensor according to claim 1,
The driving unit functions as a carbon monoxide gas detection driving unit after functioning n times continuously as a target gas detection driving unit. The thin film gas sensor according to claim 1 or 2,
The target gas detection drive means is driven High + Off, and the carbon monoxide gas detection drive means is driven Low + Off, so that the target gas detection drive means and the carbon monoxide gas detection drive means are continuously set to High + Off + Low + Off drive,
The heater layer is driven for a predetermined period so that the gas sensing layer reaches the combustible gas detection temperature when driving High, and the heater layer is driven for a predetermined period so that the gas sensing layer becomes the carbon monoxide gas detection temperature when driving Low. A thin film gas sensor, characterized in that it is means. The thin film gas sensor according to claim 1 or 2,
The target gas detection drive means is driven High + Medium + Off (room temperature), and the carbon monoxide gas detection drive means is driven Low + Off, so that the target gas detection drive means and the carbon monoxide gas detection drive means are continuously high + medium + off + low + off. Drive and
The heater layer is driven for a predetermined period so that the gas sensing layer becomes a combustible gas detection temperature when driving High, and the heater layer is driven for a predetermined period so that the gas detection layer becomes the hydrogen gas detection temperature when Medium is driven. A thin film gas sensor, characterized in that the heater layer is driven for a predetermined period so that the gas sensing layer is at a carbon monoxide gas detection temperature when driven. The thin film gas sensor according to claim 1 or 2,
The target gas detection drive means is driven to High + Medium1 + Medium2 + Off (normal temperature), and then the carbon monoxide gas detection drive means is driven to Low + Off, so that the target gas detection drive means and the carbon monoxide gas detection drive means are continuously High + Medium1 + Medium2 + Off + Low + Off + Drive and
The heater layer is driven for a predetermined period so that the gas sensing layer becomes the cleaning temperature when driving High, the heater layer is driven for the predetermined period so that the gas sensing layer becomes the combustible gas detection temperature when driving Medium 1, and the medium 2 is driven when Medium 2 is driven The heater layer is driven for a predetermined period so that the gas sensing layer is at the hydrogen gas detection temperature, and the heater layer is driven for a predetermined period so that the gas detection layer is at the carbon monoxide gas detection temperature during low driving. A thin film gas sensor. In the thin film gas sensor according to claim 1,
The carbon monoxide gas detection drive means is a means for driving the heater layer over a predetermined period so as to achieve a multi-step carbon monoxide gas detection temperature.
The carbon monoxide gas concentration calculating means is a means for calculating the carbon monoxide gas concentration by calculating the value of the sensing layer resistance of the gas sensing layer after the temperature is stabilized by the carbon monoxide gas detection temperature driving. A thin film gas sensor. In the thin film gas sensor according to claim 1,
The carbon monoxide gas detection driving means is means for driving the heater layer over a predetermined period so as to be a lamp-like carbon monoxide gas detection temperature,
The carbon monoxide gas concentration calculating means is a means for calculating a carbon monoxide gas concentration by calculating a value of a sensing layer resistance of the gas sensing layer every predetermined period by driving a carbon monoxide gas detection temperature. Thin film gas sensor. In the thin film gas sensor according to any one of claims 1 to 7,
The thin film gas sensor, wherein the sensing layer is a layer made of SnO ₂ to which Sb (antimony) is added. In the thin film gas sensor according to any one of claims 1 to 8,
The thin film gas sensor according to claim 1, wherein the first gas selective combustion layer is a layer made of an Al ₂ O ₃ sintered material supporting Pd (palladium) as a catalyst. In the thin film gas sensor according to any one of claims 1 to 9,
The second gas selective combustion layer is a layer of SnO ₂ to which Pt (platinum) is added.

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