JP2001099801A - Contact combustion type gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive contact combustion type gas sensor in a simple constitution which can obtain larger sensor outputs to combustible gases and therefore can improve a gas detect sensitivity to combustible gases of a low concentration or combustible gases of a low sensitivity. SOLUTION: The contact combustion type gas sensor calibrates a combustible gas by detecting a heat of combustion generated when the combustible gas is burnt. The sensor has a micro heater 4 formed on an Si substrate 2 for promoting burning the combustible gas, a catalyst layer 29 of palladium or the like formed on the micro heater 4 which heats in accordance with a calorific power of the micro heater 4 and acts as a catalyst to burning the combustible gas, and a first and a second thermopiles 5 and 8 set to the vicinity of the catalyst layer 29 and micro heater 4 for detecting the heat of combustion of the combustible gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact combustion type gas sensor for measuring a combustible gas by detecting combustion heat generated when a combustible gas is combusted by a heater and a gas detecting element, and more particularly to a combustible gas sensor. The present invention relates to a catalytic combustion type gas sensor capable of obtaining a larger sensor output for a reactive gas.

[0002]

2. Description of the Related Art Conventional contact combustion type gas sensors include:
For example, a contact combustion type gas sensor described in JP-A-11-6811 is known. FIG. 10 shows a configuration diagram of a conventional catalytic combustion type gas sensor described in this publication.

This contact combustion type gas sensor is shown in FIG.
As shown in (a), a gas detection element 130 and compensation elements 321, 322, 323 are provided adjacently on a diaphragm 123 formed on a substrate 112 with a predetermined thickness,
Gas detection element 130 and compensation elements 321, 322, 323
Then, the combustible gas is calibrated by detecting the combustion heat generated when the combustible gas is burned.

As shown in FIGS. 10 (a) and 10 (b), a gas detecting element 130 is formed on a diaphragm 123 and made of platinum 118 for promoting the combustion of a combustible gas. Alumina 122 serving as a heat conductive layer provided in thermal contact with this alumina 122
And a palladium catalyst layer 124 that generates heat in accordance with the amount of heat generated by the heater 118 and that acts as a catalyst for combustion of the combustible gas.

A heater 118 is laminated on the oxide film 114 and the tantalum pentoxide 116 in a state of being in contact with the oxide film 114 and the tantalum pentoxide 116, and an alumina 122 is laminated on the heater 118 in a state of being in thermal contact with the heater 118. A palladium catalyst layer 124 is laminated on the alumina 122 in thermal contact.

By providing such a gas detecting element 130 and providing a heater 118 on a diaphragm 123 having a smaller heat capacity than the substrate 112, the amount of heat generated by the heater 118 is thermally diffused into the substrate 112. Since the phenomenon can be avoided, the calorific value generated by the heater 118 can be efficiently transmitted to the palladium catalyst layer 124 in a short time, so that the combustible gas can be burned with high sensitivity and high speed.

Further, by providing the alumina 122, the calorific value generated by the heater 118 can be efficiently transmitted to the palladium catalyst layer 124 in a short period of time. Can be burned. Further, by providing the palladium catalyst layer 124 acting as a catalyst for the combustion of the combustible gas, sufficient gas detection sensitivity can be realized.

FIG. 11 is a circuit diagram of a gas detection circuit when the conventional catalytic combustion type gas sensor shown in FIG. 10 is incorporated in a bridge circuit. As shown in FIG. 10, the gas detection circuit 400 includes three compensating elements 321, 322, 32
The power supply 136 and the current detection means 138 are incorporated in a Wheatstone bridge configured using the gas detection element 130 and the gas detection element 130.

In the gas detection circuit 400, the resistance change of the gas detection element 130 caused by the combustion heat generated when the combustible gas is combusted by the gas detection element 130 and the compensation elements 321, 322, 323. , And the compensating element 32
The combustible gas can be calibrated by detecting a change in the resistance value of 1,322,323 by the Wheatstone bridge and the current detecting means 138 connected thereto.

As described above, in the conventional contact combustion type gas sensor, the catalyst layer such as the palladium catalyst layer 124 for burning the combustible gas is formed on the heater 118 and generated by the combustion heat of the combustible gas. The change in the temperature of the catalyst layer 124 was detected by the change in the resistance value of the heater 118.

[0011]

However, FIG.
In the conventional contact combustion type gas sensor shown in (1), when the combustible gas is measured, the lower the concentration of the combustible gas, the smaller the combustion heat. For this reason, the resistance value of the heater becomes small, and for example, it is difficult to detect a low-concentration combustible gas of about several ppm to several tens ppm.

The output voltage obtained from the change in the resistance value of the heater is about several mV to several tens mV even for a flammable gas having a high concentration of several thousand ppm. Therefore, FIG.
As shown in (1), the sensor output was increased by using a bridge circuit incorporating a reference (a comparison element, which corresponds to a compensation element in FIG. 11).
The configuration of the sensor was complicated. Furthermore, by taking the potential difference between both ends of the bridge circuit, temperature characteristics, humidity characteristics,
In addition, the influence of the resistance value fluctuation due to the deterioration of the heater is eliminated.

According to the present invention, it is possible to obtain a larger sensor output for combustible gas, thereby improving the gas detection sensitivity for low-concentration combustible gas or low-sensitivity combustible gas. An object of the present invention is to provide an inexpensive catalytic combustion type gas sensor which can be manufactured and has a simple structure.

[0014]

The present invention has the following arrangement to solve the above problems. The invention according to claim 1 is a contact combustion type gas sensor for measuring a combustible gas by detecting combustion heat generated when combusting a combustible gas, wherein the contact combustion type gas sensor is formed on a substrate and detects combustion of the combustible gas. A heater for encouraging the fuel, a catalyst layer formed on the heater and acting as a catalyst for the combustion of the combustible gas by generating heat in accordance with the calorific value of the heater, and a catalyst layer in the vicinity of the catalyst layer and the heater. A thermopile disposed to detect heat of combustion of the combustible gas.

According to the first aspect of the present invention, when the heater formed on the substrate generates heat, the catalyst layer formed on the heater generates heat in accordance with the calorific value of the heater to act as a catalyst. Flammable gas burns. Therefore, the thermopile disposed near the catalyst layer and the heater detects the heat of combustion of the combustible gas, so that a larger sensor output can be obtained for the combustible gas. It is possible to improve the gas detection sensitivity for gas and flammable gas with low sensitivity. Further, since the sensor output is large, a bridge circuit using a reference (compensation element) as in the related art is not required, and the configuration is simpler and less expensive. Also, a heater and a thermopile are separately provided,
Since the thermopile is not affected by the aging of the heater, the fluctuation of the sensor output becomes smaller.

According to a second aspect of the present invention, in the contact combustion type gas sensor according to the first aspect, the thermopile has a plurality of thermocouples having a hot junction and a cold junction, and the thermocouples are connected in series. It is characterized by being done.

According to the second aspect of the invention, in addition to the effect of the first aspect, since the thermopile is formed by connecting a plurality of thermocouples in series, a larger sensor output is proportional to the number of thermocouples. Can be obtained.

According to a third aspect of the present invention, in the catalytic combustion type gas sensor according to the first or second aspect, the catalyst layer is formed so as to cover a hot junction of a thermocouple constituting the thermopile and the heater. It is characterized by having.

According to the third aspect of the present invention, in addition to the effects of the first or second aspect, the catalyst layer is formed so as to cover the hot junction of the thermocouple and the heater constituting the thermopile. The heat generated by the heater is efficiently transmitted to the catalyst layer in a short time and the combustible gas burns, and the heat of combustion is transmitted to the hot junction of the thermocouple, thereby increasing the difference in heat capacity between the cold junction and the hot junction. can do. Thus, the electromotive force of the thermocouple can be increased, so that a larger sensor output can be obtained.

The invention according to claim 4 is the invention according to claims 1 to 3.
The catalytic combustion gas sensor according to any one of the above,
The substrate has a diaphragm having a predetermined thickness, and a portion of the thermopile except a cold junction of the thermocouple and the heater are formed on the diaphragm.

According to a fourth aspect of the present invention, in addition to the effects of any one of the first to third aspects, the substrate has a diaphragm having a predetermined thickness, and the thermocouple of the thermopile constituting the thermopile is provided. Since the portion excluding the cold junction and the heater are formed on the diaphragm, the heat capacity of the thermopile and the heater can be reduced. As a result, it is possible to avoid the phenomenon that the calorific value of the heater is thermally diffused into the substrate, so that the calorific value of the heater can be efficiently transmitted to the catalyst layer in a short period of time, thereby burning the gas with high sensitivity and high speed. And a large sensor output can be obtained from the thermopile.

According to a fifth aspect of the present invention, in the contact combustion type gas sensor according to the fourth aspect, the cold junction of the thermopile is formed in a region excluding the diaphragm.

According to the fifth aspect of the invention, in addition to the effect of the fourth aspect, since the cold junction of the thermopile is formed in a region other than the diaphragm, the heat capacity of the thermocouple between the cold junction and the hot junction is reduced. The difference can be increased. As a result, the electromotive force of the thermocouple can be increased,
A larger sensor output can be obtained.

The invention of claim 6 is the invention of claims 1 to 5
The catalytic combustion gas sensor according to any one of the above,
A plurality of the thermopiles are provided near the heater and the catalyst layer.

According to the sixth aspect of the present invention, in addition to the effect of any one of the first to fifth aspects, a plurality of thermopiles are provided near the heater and the catalyst layer. A sensor output can be obtained from each of them, and if these sensor outputs are added, a larger sensor output can be obtained.

The invention according to claim 7 is the invention according to claims 1 to 6
The catalytic combustion gas sensor according to any one of the above,
An insulating film formed on the diaphragm in contact with the diaphragm, and an adhesive film formed on the insulating film in contact with the insulating film and the heater and for bringing the insulating film and the heater into close contact with each other It is characterized by the following.

According to the seventh aspect of the present invention, in addition to the effects of any one of the first to sixth aspects, the adhesion film for forming the insulation film and the heater in close contact with each other is formed. The adhesion to the film is improved, and peeling of the heater is eliminated.

The invention of claim 8 is the first to seventh aspects of the present invention.
The catalytic combustion gas sensor according to any one of the above,
The adhesion film is formed using hafnium oxide.

According to the eighth aspect of the present invention, in addition to the effect of any one of the first to seventh aspects, since the adhesion film is formed using hafnium oxide, the heater, the insulating film and Is improved, and peeling of the heater is eliminated.

According to the ninth aspect of the present invention, there are provided the first to eighth aspects.
The catalytic combustion gas sensor according to any one of the above,
The heater is formed using platinum.

According to the ninth aspect of the invention, in addition to the effect of any one of the first to eighth aspects, since the heater is formed using platinum, chemically stable platinum can be used. By using the heater, it is possible to realize a contact combustion type gas sensor having high stability, reproducibility, and reliability over a long period of time.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the catalytic combustion type gas sensor of the present invention will be described below in detail with reference to the drawings.

(First Embodiment) FIG. 1 is a top view of a catalytic combustion type gas sensor according to a first embodiment. FIG. 2 is a sectional view of the catalytic combustion type gas sensor according to the first embodiment. FIG. 3 is a cross-sectional view and a top view of a thermopile provided in the contact combustion type gas sensor according to the first embodiment.

As shown in FIG. 1, the contact combustion type gas sensor 1 has a Si substrate 2 made of silicon single crystal, and a diaphragm 3 formed by anisotropic etching is formed on the back surface of the Si substrate 2. ing.

A micro-heater 4 made of platinum or the like for promoting the combustion of the combustible gas is provided on the diaphragm 3.
A first thermopile 5 and a second thermopile 8 which are arranged near both sides of the heater pattern in the longitudinal direction of the microheater 4 and are provided with portions other than the cold junction of the thermocouple and which detect the combustion heat of the combustible gas; Are formed.
As shown in FIG. 1, the micro heater 4 has a plurality of heater patterns arranged substantially in parallel and in a zigzag shape.

As shown in FIG. 1, the micro heater 4 is connected to platinum pads 6A and 6B to which a drive current is supplied from a power source (not shown). The first thermopile 5 is connected with platinum pads 7A and 7B for outputting the first temperature detection signal from the first thermopile 5 to the outside,
The second thermopile 8 includes a platinum pad 9A that outputs the second temperature detection signal from the second thermopile 8 to the outside,
9B is connected. Further, platinum pads 17A and 17B for outputting the ambient temperature signals from the resistors 15 and 16 to the outside are connected to the resistors 15 and 16 for measuring the ambient temperature.

An insulating film 23 is formed on the diaphragm 3 in contact with the diaphragm 3, and a micro heater 4 is formed on the insulating film 23 in contact with the insulating film 23. A catalyst layer 29 of palladium or the like is formed in contact with the micro heater 4.

The catalyst layer 29 generates heat in accordance with the amount of heat generated by the microheater 4 and acts as a catalyst for combustible gas combustion. Further, as described later, this catalyst layer 29
The first thermopile 5 and the second thermopile 5 are formed so as to cover the hot junction 31 and the microheater 4 respectively.

The insulating film 23 functions to efficiently transmit the heat generated by the micro heater 4 to the catalyst layer 29 in a short time. As the insulating film 23, an oxide film such as silicon oxide formed on the diaphragm 3 in contact with the diaphragm 3, a nitride film such as silicon nitride formed on the oxide film in contact with the oxide film, or the like. Can be used.

The microheater 4 may be made of a metal or a compound which has a large temperature coefficient of resistance and is thermally stable up to a high temperature, in addition to platinum. For example, tungsten may be used.

On the other hand, as shown in FIGS. 2 and 3, each of the first thermopile 5 and the second thermopile 8 is formed on the surface of the Si substrate 2 and on the diaphragm 3 in contact with p ^{+. +} -Si21, this p ⁺⁺ -Si2
An insulating film 2 formed on the substrate 1 in contact with the p ^++- Si 21
3. a metal film 25 such as platinum or aluminum formed on the insulating film 23 in contact with the insulating film 23;
It has a protective film 27 made of silicon oxide or the like formed thereon in contact with the metal film 25. A concave portion is formed in the protective film 27, and the concave portion corresponds to a hot junction 31 and a cold junction 33 described later.

Each of the first and second thermopiles 5 and 8 is arranged slightly apart from the micro heater 4 via the insulating film 23. First thermopile 5 and second thermopile
Each of the thermopiles 8 is composed of a thermocouple having a hot junction 31 and a cold junction 33, as shown in FIG. 3, and detects the heat of combustion of the combustible gas burned by the heat generated by the microheater 4, and detects the temperature. The first thermopile 5 outputs a first temperature detection signal, and the second thermopile 8 outputs a second temperature detection signal by generating a thermoelectromotive force from the temperature difference between the contact 31 and the cold junction 33. It is supposed to. The cold junction 33 is provided on a portion of the Si substrate 2 (thickness of about 400 μm) where the diaphragm 3 is not formed, and the hot junction 31 is formed on the diaphragm 3.

Further, each of the first thermopile 5 and the second thermopile 8 is, as shown in FIG.
It has a plurality of thermocouples. In two adjacent thermocouples, the hot junction 31 of one thermocouple and the cold junction 33 of the other thermocouple are connected by a metal film 25 such as aluminum or platinum. That is, the first thermopile 5 and the second thermopile 5
Of the thermopile 8 are connected in series.

The thermopile thermocouple may be made of a material other than the above-mentioned platinum-p ^++- Si.
Copper / Constantan (Operating temperature is -200 ℃ ~ + 350
℃), iron and constantan (operating temperature -200 ℃ ~ +
800 ° C), Chromel Constantan (operating temperature is-
200 ° C. to + 800 ° C.), chromel alumel (operating temperature of −200 ° C. to + 1200 ° C.), and platinum rhodium platinum (operating temperature of 0 ° C. to + 1600 ° C.) can also be used.

Here, constantan is an alloy of copper (Cu) and iron (Fe), and chromel is chromium (C
r) and nickel (Ni).
An alloy of aluminum (Al), nickel (Ni), manganese (Mn), and silicon (Si).

According to the contact combustion type gas sensor 1 configured as described above, when the microheater 4 starts heating with an external drive current, the heat generated from the microheater 4 is transferred to the catalyst through the insulating film 23. Conducted to layer 29,
Since the catalyst layer 29 generates heat according to the calorific value of the microheater 4 and acts as a catalyst, the combustible gas burns.

Then, the heat of combustion of the combustible gas is transmitted to the respective hot junctions 31 of the first thermopile 5 and the second thermopile 8. Each thermopile 5, 8
Is located on the Si substrate 2 and is at the substrate temperature.

On the other hand, since each hot junction 31 is on the diaphragm 3, it is heated by the transmitted heat,
The temperature rises higher than the temperature of the i-substrate. Each of the thermopiles 5 and 8 generates a thermoelectromotive force from the temperature difference between the hot junction 31 and the cold junction 33, and the first thermopile 5 generates the first thermopile.
And the second thermopile 8 outputs the second temperature detection signal.
Output the temperature detection signal.

After that, the first temperature detection signal (first sensor output) from the first thermopile 5 and the second temperature detection signal (second sensor output) are added by an adder (not shown). Then, a third temperature detection signal (third sensor output) can be obtained.

As a result, a larger sensor output can be obtained for a flammable gas. Detection sensitivity can be improved.

FIG. 4 shows sensor outputs of the catalytic combustion type gas sensor according to the first embodiment for various combustible gases.
In FIG. 4, for example, when the combustible gas is 100 ppm, the sensor output of carbon monoxide (CO) is about 200 mV, the sensor output of methane (CH ₄ ) is about 400 mV, and hydrogen (H ₂ ) Sensor output is about 60
It is 0 mV, the sensor output of isobutane (i-C ₄ H ₁₀₎ of about 800 mV, a large sensor output for various combustible gas is obtained.

Also, the first and second thermopiles 5, 8
Are formed by connecting a plurality of thermocouples in series, so that a larger sensor output can be obtained in proportion to the number of thermocouples.

FIG. 5 shows the relationship between the number of thermocouples of the thermopile provided in the catalytic combustion type gas sensor of the embodiment and the sensor output. The number of thermopiles shown in FIG. 5 corresponds to the number of thermocouples.

In FIG. 5, for example, the combustible gas is carbon monoxide (CO), and the concentration of carbon monoxide is 100.
ppm, in 16 thermopiles,
The sensor output is about 50 mV. With 32 thermopiles, the sensor output is about 100 mV.
With four, the sensor output is about 220 mV. As can be seen from FIG. 5, a larger sensor output can be obtained in proportion to the number of thermopiles of the thermopile.

Further, since the sensor output is large, a bridge circuit using a reference (compensation element) as in the related art is not required, and the configuration is simpler and less expensive. Further, the micro heater 4 and the first and second thermopiles 5 and 8 are separately provided, and the first and second thermopiles 5 and 8 are provided.
Generates a thermoelectromotive force due to the temperature difference between the hot junction 31 and the cold junction 33. Therefore, even if the resistance value fluctuates due to the aging of the micro-heater 4, it is affected by the fluctuation of the resistance value due to the aging of the micro-heater 4. Disappears. Thereby, the sensor output fluctuation becomes smaller.

Further, since the catalyst layer 29 is formed so as to cover the hot junction 31 of the thermocouple of the first and second thermopiles 5 and 8 and the microheater 4, the heat generation of the microheater 4 is efficiently performed. The combustible gas is burned by being transmitted to the catalyst layer 29 in a short time, and the heat of combustion is transferred to the hot junction 31 of the thermocouple.
, The difference in heat capacity between the cold junction 33 and the hot junction 31 can be increased. Thus, the electromotive force of the thermocouple can be increased, so that a larger sensor output can be obtained.

Also, the first and second thermopiles 5, 8
Of the thermocouple except the cold junction 33 and the micro heater 4
Is formed on the diaphragm 3, so that the heat capacities of the first and second thermopiles 5, 8 and the microheater 4 can be reduced. As a result, it is possible to avoid a phenomenon in which the heat generated by the micro heater 4 is thermally diffused into the substrate, so that the heat generated by the micro heater 4 can be efficiently transmitted to the catalyst layer 29 in a short time. In addition to burning the gas, a large sensor output can be obtained from the first and second thermopiles 5 and 8.

Also, the first and second thermopiles 5, 8
Are formed in a region excluding the diaphragm 3, the cold junction 33 and the hot junction 3 of the thermocouple are formed.
The difference in heat capacity from the heat capacity of No. 1 can be increased. As a result, the electromotive force of the thermocouple can be increased,
A larger sensor output can be obtained.

Further, since the insulating film 23 is formed between the microheater 4 and the first and second thermopiles 5 and 8, heat generation of the microheater 4 can be efficiently performed in a short time through the insulating film 23. Since the power is transmitted to the first and second thermopiles 5 and 8, a larger sensor output can be obtained.

Further, since the micro heater 4 is formed using platinum, by using chemically stable platinum for the micro heater 4, a contact having high stability, reproducibility and reliability for a long time can be obtained. A combustion type gas sensor can be realized.

(Second Embodiment) Next, a second embodiment of the present invention will be described.
The contact combustion type gas sensor according to the embodiment will be described. FIG. 6 is a sectional view of a contact combustion type gas sensor according to the second embodiment. FIG. 7 is a top view of the contact combustion type gas sensor of the second embodiment shown in FIG. FIG. 8 is a diagram showing the sensor output with respect to the gas concentration of the contact combustion type gas sensor of the second embodiment shown in FIG.

The contact combustion type gas sensor according to the second embodiment provides an excellent sensor that operates stably for a long period of time by improving the adhesion between the platinum heater and the oxide film as the base film. This was created prior to the creation of the catalytic combustion type gas sensor of the first embodiment.

As shown in FIG. 6, the contact combustion type gas sensor has a silicon substrate 51, an oxide film 55 formed in contact with the surface of the silicon substrate 51, and an oxide film 55 on the oxide film 55. Nitride film 57, a hafnium oxide film 59 formed on the nitride film 57 in contact with the nitride film 57, a platinum heater 61 formed on the hafnium oxide film 59 and in contact with the hafnium oxide film 59 Is formed on the platinum heater 61 in a state of being in contact with the platinum heater 61 and a catalyst layer (for example,
It comprises a gas sensitive film 63 acting as palladium) and a diaphragm 53 formed on the back surface of the silicon substrate 51 by anisotropic etching. The platinum heater 61 is formed on the diaphragm 53, and
1 is connected to a platinum pad 65.

The oxide film 55 is a silicon oxide obtained by subjecting the surface of the silicon substrate 51 to a thermal oxidation treatment, and has a thickness of, for example, about 6000 °. The nitride film 57
The hafnium oxide film 5 having a thickness of, for example, about 2500 °
9 has a thickness of, for example, about 500 °.

According to the contact combustion type gas sensor having such a configuration, since the hafnium oxide film 59 is formed between the platinum heater 61 and the nitride film 57, the platinum heater 61 at a high temperature and the nitride film 57 serving as a base film are formed. Adhesion with
The separation of platinum from the platinum heater 61 can be eliminated, and the aging deterioration characteristics of the sensor and the breakdown durability characteristics of the sensor can be improved.

Further, since the coefficient of thermal expansion of the hafnium oxide film 59 is between the platinum heater 61 and the nitride film 57 as the base film, the thermal stress generated by the heat cycle of the platinum heater 61 is relaxed. be able to. Further, since the film stress of the hafnium oxide 59 is small, the residual stress of the diaphragm made of a thin film can be reduced, so that the yield when manufacturing the sensor can be improved.

The thermal conductivity of hafnium oxide 59 is as follows:
Since it is very small, thermal diffusion can be suppressed and the power consumption of the platinum heater 61 can be reduced. Further, since the hafnium oxide 59 hardly dissolves in water, a strong acid, or a strong alkali, it can exhibit strong resistance to a wet etching process of another film or a Si substrate in a manufacturing process. Hafnium oxide 59 is 10 ⁻¹⁴ (Ω ⁻¹ · cm ⁻¹ ).
As described above, since the conductivity is small, almost no current leaks from the platinum heater 61.

FIG. 8 shows sensor outputs for various combustible gases of the contact combustion type gas sensor according to the second embodiment shown in FIG. In FIG. 8, for example, when the combustible gas is 1000 ppm, carbon monoxide (C
O) The sensor output is about 2 mV, and methane (CH ₄ )
Is about 3 mV, the sensor output of hydrogen (H ₂ ) is about 6 mV, the sensor output of isobutane (i-C ₄ H ₁₀ ) is about 8 mV, and the flammable gas is 100 pp.
In the case of m, the sensor output is about 0.5 mV for all of these combustible gases.

The sensor output of the catalytic combustion type gas sensor of the second embodiment shown in FIG. 8 is compared with the sensor output of the catalytic combustion type gas sensor of the first embodiment shown in FIG. It can be seen that the sensor output of the contact combustion type gas sensor is very large. That is, in the contact combustion type gas sensor of the first embodiment, since the first and second thermopiles 5 and 8 are used, a large sensor output can be obtained for various combustible gases.

(Third Embodiment) Next, a third embodiment of the present invention will be described.
The contact combustion type gas sensor according to the embodiment will be described. FIG. 9 is a top view of the contact combustion type gas sensor according to the third embodiment.

The contact combustion type gas sensor according to the third embodiment has the structure of the contact combustion type gas sensor according to the first embodiment, and is formed on an insulating film 23 as shown in FIG. And a hafnium oxide film 35 disposed in contact with the microheater 4. Here, the insulating film 23 includes an oxide film such as silicon oxide and a nitride film such as silicon nitride formed on the oxide film in contact with the oxide film.

The other structure of the contact combustion type gas sensor of the third embodiment is the same as that of the contact combustion type gas sensor of the first embodiment, and the same parts are denoted by the same reference numerals. , And a detailed description thereof will be omitted.

As described above, according to the contact combustion type gas sensor of the third embodiment, in addition to the effect of the contact combustion type gas sensor of the first embodiment, it is formed on the insulating film 23 and Since the hafnium oxide film 35 disposed in contact with the microheater 4 is formed, the adhesion between the microheater 4 made of platinum and the insulating film 23 as a base film at high temperature is improved, and the platinum Peeling can be eliminated, and the aging degradation characteristics of the sensor and the destruction durability characteristics of the sensor can be improved.

Since the coefficient of thermal expansion of the hafnium oxide film 35 is intermediate between the micro-heater 4 made of platinum and the insulating film 23 as a base film, the thermal stress generated by the heat cycle of the micro-heater 4 and the like. Can be alleviated.

Further, since the film stress of the hafnium oxide 35 is small, the residual stress of the thin-film diaphragm can be reduced, so that the yield when manufacturing the sensor can be improved.

The thermal conductivity of hafnium oxide 35 is as follows:
Since it is very small, it suppresses heat diffusion and
Power consumption can be reduced. Further, since the hafnium oxide 35 is hardly dissolved in water, strong acid, or strong alkali, it can exhibit strong resistance to a wet etching process of another film or a Si substrate in a manufacturing process.
The hafnium oxide 35 is 10 ⁻¹⁴ (Ω ⁻¹ · c).
Since the conductivity is as low as m ^-1 ) or more, current leakage from the micro heater 4 is almost eliminated.

The present invention is not limited to the contact combustion type gas sensors of the first to third embodiments described above. In the first and third embodiments, the first and second thermopiles 5 and 8 are arranged in the vicinity of the microheater 4, but the present invention is not limited to this. For example, an insulating film may be formed on the microheater 4. For example, even if one thermopile is arranged via 23, the same effect as the effect of the contact combustion type gas sensor of the first and third embodiments can be obtained.

In the first and third embodiments, the first thermopile 5 and the second thermopile 8 are provided on both sides of the microheater 4. For example, one thermopile is provided for the microheater 4. It may be provided on one side.

Further, in the first and third embodiments,
The first thermopile 5 and the second thermopile 8 are provided on both sides of the microheater 4 and sensor outputs are taken out from the first thermopile 5 and the second thermopile 8, respectively. One end of the second thermopile 8 is connected in series to one end of
The sensor output may be extracted from the other end of the thermopile 5 and the other end of the second thermopile 8. In this way, sensor outputs of two thermopiles can be extracted. In addition, it goes without saying that various modifications can be made without departing from the technical idea of the present invention.

[0080]

According to the first aspect of the present invention, when the heater formed on the substrate generates heat, the catalyst layer formed on the heater generates heat according to the calorific value of the heater to act as a catalyst. Combustible gas burns. Therefore, the thermopile disposed near the catalyst layer and the heater detects the heat of combustion of the combustible gas, so that a larger sensor output can be obtained for the combustible gas. It is possible to improve the gas detection sensitivity for gas and flammable gas with low sensitivity. Further, since the sensor output is large, a bridge circuit using a reference (compensation element) as in the related art is not required, and the configuration is simpler and less expensive. Further, since the heater and the thermopile are separately provided, and the thermopile is not affected by the aging of the heater, the fluctuation of the sensor output becomes smaller.

According to the second aspect of the present invention, in addition to the effect of the first aspect, since the thermopile is formed by connecting a plurality of thermocouples in series, a larger sensor output is provided in proportion to the number of thermocouples. Can be obtained.

According to the third aspect of the invention, in addition to the effects of the first or second aspect, the catalyst layer is formed so as to cover the hot junction of the thermocouple and the heater constituting the thermopile. The heat generated by the heater is efficiently transmitted to the catalyst layer in a short time and the combustible gas burns, and the heat of combustion is transmitted to the hot junction of the thermocouple, thereby increasing the difference in heat capacity between the cold junction and the hot junction. can do. Thus, the electromotive force of the thermocouple can be increased, so that a larger sensor output can be obtained.

According to the fourth aspect of the present invention, in addition to the effects of any one of the first to third aspects, the substrate has a diaphragm of a predetermined thickness, and the thermocouple of the thermopile constituting the thermopile is provided. Since the portion excluding the cold junction and the heater are formed on the diaphragm, the heat capacity of the thermopile and the heater can be reduced. As a result, it is possible to avoid the phenomenon that the calorific value of the heater is thermally diffused into the substrate, so that the calorific value of the heater can be efficiently transmitted to the catalyst layer in a short period of time, thereby burning the gas with high sensitivity and high speed. And a large sensor output can be obtained from the thermopile.

According to the fifth aspect of the present invention, in addition to the effect of the fourth aspect, since the cold junction of the thermopile is formed in a region other than the diaphragm, the heat capacity between the cold junction and the hot junction of the thermocouple is reduced. The difference can be increased. As a result, the electromotive force of the thermocouple can be increased,
A larger sensor output can be obtained.

According to the sixth aspect of the present invention, in addition to the effects of any one of the first to fifth aspects, a plurality of thermopiles are provided near the heater and the catalyst layer. A sensor output can be obtained from each of them, and if these sensor outputs are added, a larger sensor output can be obtained.

According to the seventh aspect of the present invention, in addition to the effect of any one of the first to sixth aspects, the adhesion film for forming the insulation film and the heater in close contact with each other is formed. The adhesion to the film is improved, and peeling of the heater is eliminated.

According to the eighth aspect of the present invention, in addition to the effect of any one of the first to seventh aspects, since the adhesion film is formed using hafnium oxide, the heater, the insulating film and Is improved, and peeling of the heater is eliminated.

According to the ninth aspect of the present invention, in addition to the effect of any one of the first to eighth aspects, since the heater is formed using platinum, chemically stable platinum can be used. By using the heater, it is possible to realize a contact combustion type gas sensor having high stability, reproducibility, and reliability over a long period of time.

[Brief description of the drawings]

FIG. 1 is a top view of a contact combustion type gas sensor according to a first embodiment.

FIG. 2 is a cross-sectional view of the contact combustion type gas sensor according to the first embodiment.

FIG. 3 is a cross-sectional view and a top view of a thermopile provided in the catalytic combustion type gas sensor according to the first embodiment.

FIG. 4 is a diagram showing sensor outputs for various combustible gases of the contact combustion type gas sensor according to the first embodiment.

FIG. 5 is a diagram illustrating a relationship between the number of thermocouples of a thermopile provided in the catalytic combustion type gas sensor according to the first embodiment and a sensor output.

FIG. 6 is a sectional view of a catalytic combustion type gas sensor according to a second embodiment.

FIG. 7 is a top view of the catalytic combustion type gas sensor according to the second embodiment shown in FIG.

8 is a diagram showing sensor outputs for various combustible gases of the contact combustion type gas sensor according to the second embodiment shown in FIG.

FIG. 9 is a top view of a contact combustion type gas sensor according to a third embodiment.

FIG. 10 is a configuration diagram of a conventional catalytic combustion type gas sensor.

11 is a circuit configuration diagram of a gas detection circuit when the conventional catalytic combustion type gas sensor shown in FIG. 10 is incorporated in a bridge circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Contact-combustion type gas sensor 2 Si substrate 3 Diaphragm 4 Micro heater 5 First thermopile 8 Second thermopile 21 P ^++- Si 23 Insulating film 25 Metal film 27 Protective film 29 Catalyst layer 31 Hot contact 33 Cold contact 35 Hafnium oxide Reference Signs List 51 silicon substrate 53 diaphragm 55 oxide film 57 nitride film 59 hafnium oxide 61 platinum heater 63 gas sensitive film

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 2G060 AA02 AB03 AB08 AB17 AB18 AE19 AF02 AF07 AG06 AG10 BA03 BB02 BB15 HA01 HB05 HB06 HC02 HE10 JA01 KA01

Claims (9)

[Claims] 1. A contact combustion type gas sensor for measuring a combustible gas by detecting a combustion heat generated when the combustible gas is burned, wherein the contact combustion type gas sensor is formed on a substrate to promote the combustion of the combustible gas. A catalyst layer formed on the heater and generating heat in accordance with the calorific value of the heater to act as a catalyst for the combustion of the combustible gas; and a catalyst layer disposed near the catalyst layer and the heater. And a thermopile for detecting heat of combustion of the combustible gas. 2. The catalytic combustion type according to claim 1, wherein the thermopile has a plurality of thermocouples having a hot junction and a cold junction, and the thermocouples are connected in series. Gas sensor. 3. The catalytic combustion type gas sensor according to claim 1, wherein the catalyst layer is formed so as to cover a hot junction of a thermocouple constituting the thermopile and the heater. . 4. The substrate has a diaphragm having a predetermined thickness, and a portion excluding a cold junction of a thermocouple constituting the thermopile and the heater are formed on the diaphragm. The catalytic combustion type gas sensor according to any one of claims 1 to 3. 5. The catalytic combustion type gas sensor according to claim 4, wherein the cold junction of the thermopile is formed in a region excluding the diaphragm. 6. The thermopile according to claim 1, wherein a plurality of thermopiles are provided near the heater and the catalyst layer.
The catalytic combustion type gas sensor according to any one of claims 1 to 5. 7. An insulating film formed on the diaphragm in contact with the diaphragm, and an insulating film formed on the insulating film in contact with the insulating film and the heater, and bringing the insulating film into close contact with the heater. The contact combustion type gas sensor according to claim 1, further comprising: an adhesion film. 8. The method according to claim 1, wherein the adhesion film is formed using hafnium oxide.
The contact combustion type gas sensor according to any one of the above items. 9. The catalytic combustion type gas sensor according to claim 1, wherein the heater is formed using platinum.