JP2006258422A - Thin film gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film gas sensor enhanced in response, selectivity and environmental resistance by forming voids having a larger volume than before to a first gas selective combustion layer. <P>SOLUTION: The gas sensing layer 5 of the thin film gas sensor 10 includes a joint layer 5a, a sensing electrode layer 5b, a sensing layer 5c and the first gas selective combustion layer 5d in detail. The sensing layer 5c is an Sb-doped SnO<SB>2</SB>layer and the first gas selective combustion layer 5d comprises a Pd supported Al<SB>2</SB>O<SB>3</SB>sintered material. The first gas selective combustion layer 5d among those layers is obtained by employing a layer comprising a porous material formed by sintering the sintering material containing a catalyst powder which is obtained supporting a first catalyst on a mixture composed of a small-diameter powder of alumina (Al<SB>2</SB>O<SB>3</SB>) and a large-diameter powder of alumina. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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 200 ° C. to 500 ° C., and this heating is a factor that consumes electric power. The gas sensor according to the gas sensing film produced by sintering the powder, such as SnO _2, and using methods such as screen printing and thin as possible the thickness of the gas sensing film by reducing the heat capacity of the gas sensing film However, reduction of heat capacity other than the gas sensing film has not been considered. For this reason, in the sensor structure of the prior art, the heat capacity is still too large to be driven by a battery, and a large amount of power is required to heat it to a high temperature, resulting in a large consumption of the battery. It is impossible to guarantee the service life, and it is difficult to put a gas sensor using a battery-driven gas sensing film into practical use with the conventional structure.

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. The prior art of such a thin film gas sensor is disclosed in Patent Document 1, for example. The thin film gas sensor described in Patent Document 1 is a filter layer for gas selective combustion that covers a gas sensing layer, in particular, Al ₂ O ₃ , Cr ₂ O ₃ , Fe ₂ O ₃ , primary particle size of 1.0 μm or less, By using porous metal oxides such as Ni ₂ O ₃ , ZnO, and SiO ₂ and making the film thickness up to 100 μm from the primary particle diameter of the oxide, it has sufficient responsiveness, excellent gas selectivity, and interference The thin film gas sensor operates stably even in a poor environment where substances coexist.

JP 2001-221760 A (paragraph number 0014, FIG. 1)

However, as in the thin film gas sensor described in Patent Document 1, there is a case where a problem arises by making the particle diameter substantially uniform. This point will be described with reference to the drawings. FIG. 8 is a cross-sectional SEM photograph of a conventional thin film gas sensor. The thin film gas sensor shown in FIG. 8 includes a heat insulating support layer 100 having a diaphragm structure, a heater layer 101, a SiO ₂ insulating layer 102, a sensing layer 103, and a first gas selective combustion layer 104. The first gas selective combustion layer 104 corresponds to the filter layer of Patent Document 1, and is provided so as to cover the sensing layer 103, and further, holes 105 (black portions in the SEM photograph) are formed in various places. It is a porous body.

The porous formation of the first gas selective combustion layer 104 will be described. FIG. 9 is a particle size distribution of the catalyst powder used in the first gas selective combustion layer 104 of the prior art. The horizontal axis in FIG. 9 represents the average diameter as the particle diameter, and the vertical axis represents the occurrence probability. In addition, although there are various definitions for the average diameter of the particles, the average diameter in the present specification is, for example, the major axis diameter of a single particle of alumina (Al ₂ O ₃ ) powder before supporting the catalyst. These are the biaxial average diameter, which is the average value of the short axis diameter, the triaxial average diameter, which is the average value of the long axis diameter, the short axis diameter, and the intermediate axis. As shown in the particle size distribution of FIG. 9, the first gas selective combustion layer 104 shown in the cross-sectional SEM photograph of FIG. 8 has alumina (Al) with an average value of particle diameter (average diameter of particles) = 4 μm and standard deviation = 2 μm. ₂ O ₃ ) powder is used as catalyst powder. Now, although the alumina powder constituting the first gas selective combustion layer 104 has a small variation in particle size, the holes 105 formed between the alumina particles have a small variation as shown in FIG. It is narrow and small.

Due to the narrowness and smallness of the individual holes 105 as described above, even if the concentration of the detection gas diffuses from the outside, it takes a complicated path to reach the sensing layer 103, and it takes time to reach the sensing layer 103. As a result, the responsiveness of the gas sensor may not be improved.
Further, due to the narrowness and smallness of the holes 105, there is a possibility that the gas selectivity cannot be improved because the chance of the interfering gas contacting the catalyst is reduced and the combustion efficiency is low.
Furthermore, when the gas sensor is operated by energizing the heater layer 101 in a high-temperature and high-humidity environment of 40 ° C. and 80% RH, for example, the moisture is easily condensed in the holes 105 between the alumina particles. Once the moisture is condensed, the condensed moisture becomes a shielding substance for gas concentration diffusion, and the OH group decomposed by H ₂ O is adsorbed on the adsorption site of the SnO ₂ surface constituting the sensing layer 103, and the sensor sensitivity is lowered. There is also a risk.

  Therefore, the present invention has been made to solve the above-mentioned problems, and the object thereof is to form a void having a larger volume than the conventional one in the first gas selective combustion layer, thereby improving responsiveness, selectivity and environmental resistance. An object of the present invention is to provide an improved thin film gas sensor.

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 gas sensing layer having a pair of sensing electrode layers provided on the electrically insulating layer, a sensing layer provided to pass the pair of sensing electrode layers, and a first gas selective combustion layer provided on a surface of the sensing layer When,
A thin film gas sensor comprising:
The first gas selective combustion layer is a porous body obtained by sintering a sintered material containing a catalyst powder obtained by supporting a first catalyst on a mixture of alumina (Al ₂ O ₃ ) small diameter powder and alumina large diameter powder. It is characterized by being a layer by.

A thin film gas sensor according to claim 2 of the present invention is
The thin film gas sensor according to claim 1,
The mixture is a mixture of an alumina small-diameter powder whose average diameter of small-diameter alumina particles is less than 5 μm, and an alumina large-diameter powder whose average diameter of large-diameter alumina particles is 5 μm or more and 15 μm or less. It is characterized by that.

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 said mixture is a mixture mix | blended so that the occurrence probability of the average value of the average diameter of a small diameter alumina particle may become larger than the occurrence probability of the average value of the average diameter of a large diameter alumina particle.

A thin film gas sensor according to claim 4 of the present invention is
In the thin film gas sensor according to any one of claims 1 to 3,
The first catalyst is palladium (Pd).

A thin film gas sensor according to claim 5 of the present invention is
In the thin film gas sensor according to any one of claims 1 to 4,
A second gas selective combustion layer of a thin film provided between the sensing layer and the first gas selective combustion layer and carrying a second catalyst;
It is characterized by providing.

A thin film gas sensor according to claim 6 of the present invention is
The thin film gas sensor according to claim 5,
The second gas selective combustion layer is a thin film layer made of SnO ₂ supporting platinum (Pt) as a second catalyst.

  According to the present invention as described above, it is possible to provide a thin film gas sensor in which a void having a volume larger than that of the conventional gas is formed in the first gas selective combustion layer, and the responsiveness, selectivity and environmental resistance are improved.

The best mode (first mode) thin film gas sensor for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a sectional structural view of a thin film gas sensor of this embodiment. FIG. 2 is a cross-sectional SEM photograph of the thin film gas sensor of this embodiment. The thin film gas sensor includes a Si substrate 1, a heat 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. The gas sensing layer 5 includes a bonding layer 5a, a sensing electrode layer 5b, a sensing layer 5c, and a first gas selective combustion layer 5d in detail. The sensing layer 5c is an Sb-doped SnO ₂ layer, and the first gas selective combustion layer 5d is a Pd-supported Al ₂ O ₃ sintered material.

In the SEM photograph shown in FIG. 2, the thermal insulation support layer 2, the heater layer 3, the electrical insulation layer 4, the sensing layer 5c, and the first gas selective combustion layer 5d are shown to be distinguishable (other configurations are display magnifications). Can be seen by increasing.
The thin film gas sensor 10 is configured to cover the surface of the sensing layer 5c of the Sb-doped SnO ₂ layer so as to cover the first gas selective combustion layer 5d in which Pd is supported on Al ₂ O ₃ as a catalyst. The gas sensitivity is improved by burning a strong gas and causing only a specific gas (for example, CH ₄ or CO) to reach the sensing layer 5c.

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 is formed on the upper side of the CVD-Si ₃ N ₄ layer 2b to improve the adhesion with the heater layer 3 and ensure 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.
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 electrode layer 5b, and the electrical insulation layer 4 improves adhesion with 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 electrode layer 5b and the electrical insulating layer 4 and has a function of increasing the bonding strength.
The sensing electrode layer 5b is made of, for example, a Pt film (platinum film) or an Au film (gold film), and is provided in a pair on the left and right sides so as to be a sensing electrode of the sensing layer 5c.
The sensing layer 5c is composed of an Sb-doped SnO ₂ layer, and is formed on the electrically insulating layer 4 so as to pass the pair of sensing electrode layers 5b and 5b.

The first gas selective combustion layer 5d is a sintered body supporting palladium (Pd) as the 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, a large number of voids (described later) are formed, and the detection gas passing through the large number of voids increases the chance of contact with Pd to promote the combustion reaction. The first gas selective combustion layer 5d is provided so as to cover the surfaces of the electrical insulating layer 4, the bonding layer 5a, the pair of sensing electrode layers 5b and 5b, and the sensing layer 5c.
Such a thin film gas sensor has a structure of high heat insulation and low heat capacity by a diaphragm structure. The configuration of the thin film gas sensor is such.

Subsequently, characteristic portions of the present invention will be described in detail.
The function of the first gas selective combustion layer 5d is to allow the detection gas (CH _{4 in} the case of the CH ₄ sensor and CO in the case of the CO sensor) to pass through to reach the sensing layer 5c, and to prevent the main interfering gas. (In the case of the CH ₄ sensor, H ₂ and CO, and in the case of the CO sensor, H ₂ and CH ₄ ) are heated to a high temperature with a heater and burned by the oxidation catalytic action of palladium (Pd), thereby sensing layer. It is to prevent reaching 5c. Therefore, the first gas selective combustion layer 5d comes into contact with palladium (Pd) so that a sufficient amount and size of voids exist inside when sintering a sintered material containing the catalyst powder and the binder. It is necessary to facilitate the combustion and make the structure easy to diffuse the concentration of gas from the outside.
Further, in order to prevent the binder from entering between the particles of the catalyst powder and filling the voids between the particles, the amount of the binder needs to be as small as possible to fix the catalyst powder.

  Therefore, the alumina powder used as the catalyst powder is composed of small-diameter alumina powder with small-diameter alumina particles having a small average diameter of single particles, and large-diameter alumina powder with large-diameter alumina particles having a large average diameter of single particles. The mixture is an alumina powder that is a mixture, and a Pd catalyst is supported on the alumina powder to form a catalyst powder. Subsequently, the preferred size of the large-diameter alumina particles constituting such a large-diameter alumina powder and the preferred size of the small-diameter alumina particles constituting the small-diameter alumina powder will be described. FIG. 3 is a characteristic diagram showing the particle size distribution of the large diameter alumina powder and the small diameter alumina powder contained in the mixture, and FIG. 4 is the particle size distribution of the alumina powder as the mixture.

  As shown in FIG. 3, the particle size distribution of the small-diameter alumina powder is such that the average value of the average diameter (particle diameter) of a single small-diameter alumina particle is less than 5 μm. Specifically, the average value of particle diameter = 4 μm, standard deviation = It is a normal distribution of about 2 μm, and the particle size distribution of the large-diameter alumina powder is that the average value (particle diameter) of a single large-diameter alumina particle is 5 μm or more and 15 μm or less. It is a normal distribution with a value = 10 μm and a standard deviation = 2 μm.

  When these large-diameter alumina powder and small-diameter alumina powder are mixed, an alumina powder that follows the particle size distribution of FIG. 4 is obtained. Specifically, it is a particle size distribution of a powder obtained by mixing a large-diameter alumina powder and a small-diameter alumina powder, and the horizontal axis in FIG. 4 represents the average diameter of the small-diameter alumina powder and the large-diameter alumina powder before carrying Pd as the particle diameter. Represents the probability of occurrence. That is, it is an alumina powder having a particle size distribution that has two peaks at 4 μm and 10 μm.

  The actual structure of the first gas selective combustion layer 5d using such alumina powder is as shown in FIG. In particular, in this embodiment, it is composed of large-diameter alumina particles and small-diameter alumina particles, and a large number of small-diameter alumina particles are present at the portion where the large-diameter alumina particles are in contact with each other. There are no particles, and there are gaps 5e that facilitate gas concentration diffusion.

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, the heater layer 3 is formed by depositing and patterning a Ni—Cr film on the upper surface of the CVD-SiO ₂ layer 2c by sputtering. 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 electrode layer 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 electrode layer (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 electrode layer 5b = 500/2000.

A Sb-doped SnO ₂ film is deposited by sputtering on the electrical insulating layer 4 so as to be passed to the pair of sensing electrode layers 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.

  Subsequently, the first gas selective combustion layer 5d is formed so as to cover the electrical insulating layer 4, the bonding layer 5a, the pair of sensing electrode layers 5b and 5b, and the sensing layer 5c. The first gas selective combustion layer 5d is composed of a catalyst powder, which is a powder in which a Pd catalyst is supported on an alumina powder that is a mixture of a small-diameter alumina powder and a large-diameter alumina powder, a binder such as a silica sol binder or an aluminum sol binder, and a viscosity. A viscosity adjusting material added for adjustment is mixed with an organic solvent or water to produce a printing paste that is viscous upon stirring. Here, when an organic solvent is used, a resin such as methylcellulose can be employed as a viscosity modifier that dissolves in the organic solvent and disappears upon heating. Further, when water is used, a resin such as ethyl cellulose can be employed as a viscosity modifier that is water-soluble and disappears by heating. These are appropriately selected. Using this printing paste, printing is performed by screen printing with a screen printer having a metal mask. The size of the printing paste that becomes the first gas selective combustion layer 5d after firing is set to sufficiently cover the sensing layer 5c. Thus, the heat capacity is reduced by reducing the thickness by screen printing.

  The printing paste is dried at room temperature and then baked at, for example, 500 ° C. for 1 hour. The printing paste before firing contains catalyst powder and a binder together with bubbles mixed during stirring, and is a paste body in which large-diameter alumina particles occupy a large volume, and has a high viscosity and a desired shape. Such a printing paste is present in a state where large-diameter alumina particles exist around the inner bubbles and small-diameter alumina particles and a binder enter a gap formed between adjacent large-diameter alumina particles. When this printing paste is baked, the solvent or water disappears due to evaporation, the viscosity modifier disappears due to burning, and the bubble portion remains as a gap 5e, and the first gas selective combustion layer 5d is sintered.

  The fired first gas selective combustion layer 5d is sintered in a state in which the small-diameter alumina particles enter the voids between the large-diameter alumina particles, and can enhance the adhesion between the alumina particles. Accordingly, it is possible to reduce the amount of binder added compared to the conventional one, and the main component can be “catalyst powder”. Moreover, the location where bubbles existed can be formed as a large gap 5e.

  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 manufacturing method of the thin film gas sensor is as follows.

In such a thin film gas sensor of this embodiment, the interfering gas (H ₂ and CO in the case of the CH ₄ sensor, and H ₂ and CH ₄ in the case of the CO sensor) is selected by the first gas selective combustion layer 5d. Then, it is burned and the detection gas (CH _{4 in} the case of a CH ₄ sensor and CO in the case of a CO sensor) is preferentially passed. At this time, the detection gas from the outside diffuses in concentration using the large gap 5e and easily reaches the sensing layer 5c, so that the responsiveness of the gas sensor can be improved. Moreover, since the opportunity to contact palladium which is a 1st catalyst is increased, the combustion efficiency of interference gas can be improved and selectivity can be improved.

Further, in the thin film gas sensor of this embodiment, the moisture is difficult to condense as the wide gap 5e. For example, even if the heater is turned on / off in a high temperature and high humidity environment of 40 ° C. and 80% RH, A situation in which the condensed moisture becomes a shielding substance for gas concentration diffusion as in the prior art, or OH groups decomposed by H ₂ O are adsorbed on the adsorption sites on the SnO ₂ surface constituting the sensing layer 5c. The occurrence of such a situation can be suppressed as compared with the conventional case, and a decrease in sensor sensitivity can be suppressed.

Next, an improved embodiment (second embodiment) of the present invention will be described with reference to the drawings. FIG. 5 is a sectional structural view of another embodiment of a thin film gas sensor. The thin film gas sensor 20 includes a 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 this embodiment, the gas sensing layer 5 is different from the previously described embodiment. Specifically, in addition to the bonding layer 5a, the sensing electrode layer 5b, the sensing layer 5c, and the first gas selective combustion layer 5d, the second gas selection layer is selected. A combustion layer 5f is provided. The sensing layer 5c is an Sb-doped SnO ₂ layer, the first gas selective combustion layer 5d is a Pd-supported Al ₂ O ₃ sintered material, and the second gas selective combustion layer 5f is a Pt-doped SnO ₂ layer. . The first gas selective combustion layer 5d also includes a gap 5e as shown in the SEM photograph of FIG.

Subsequently, only the second gas selective combustion layer 5f, which is a difference, will be described, and the other components have the same configuration and function as those of the previous embodiment, and the same reference numerals are given and redundant description is omitted.
The second gas selective combustion layer 5f is a thin film containing platinum (Pt) as the second catalyst, and is a Pt-doped SnO ₂ layer as described above. The second gas selective combustion layer 5f is formed by depositing a Pt-doped SnO ₂ film on the surface of the sensing layer 5c by a sputtering method. Further, the surface of the second gas selective combustion layer 5f is formed so as to cover the first gas selective combustion layer 5d. The first gas selective combustion layer 5d is provided so as to cover the surfaces of the electrical insulating layer 4, the bonding layer 5a, the pair of sensing electrode layers 5b and 5b, and the sensing layer 5c.
Even if comprised in this way, the effect of this invention can be show | played.

The first and second embodiments have been described above. However, further modifications are possible in these forms. This point will be described. FIG. 6 is a particle size distribution of the catalyst powder used in the first gas selective combustion layer of the thin film gas sensor in the modified embodiment.
In the first gas selective combustion layer 5d described so far, the large-diameter alumina powder and the small-diameter alumina powder are mixed in equal amounts so that the occurrence probabilities are the same, but they need not be mixed in equal amounts. In particular, in order to sufficiently collect the small-diameter alumina particles in the gap formed at the portion where the large-diameter alumina particles are in contact with each other and ensure the fixing force as a binder, the amount of the small-diameter alumina powder is set to be larger than that of the large-diameter alumina powder. It is effective to increase it. Therefore, it is preferable to use a mixture in which the occurrence probability of the average value of the average diameter of the small-diameter alumina particles is larger than the occurrence probability of the average value of the average diameter of the large-diameter alumina particles. For example, it is a mixture as shown by the particle size distribution of the catalyst powder mixed by increasing the amount of the small-diameter alumina powder compared to the large-diameter alumina powder of FIG. The respective occurrence probabilities in the particle size distribution are distributions having occurrence probabilities that make the total volume of the large-diameter alumina powder and the small-diameter alumina powder substantially the same. By having such a particle size distribution, the small-diameter alumina particles can be sufficiently gathered in the gaps formed in the portions where the large-diameter alumina particles are in contact with each other, and the fixing force can be further strengthened.

  In the present specification, the particle size distribution of the alumina powder has been described on the assumption that the horizontal axis represents the particle size and is a normal distribution. However, the particle size distribution as shown in FIG. You may employ | adopt a particle size distribution that the occurrence probability with respect to the logarithm of a particle size becomes a normal distribution on a horizontal axis. Even in such a case, the essence of the discussion so far has not changed.

1 is a cross-sectional structure diagram of a thin film gas sensor of the best mode for carrying out the present invention. It is a cross-sectional SEM photograph of the thin film gas sensor of the best form for implementing this invention. It is a characteristic view which shows each particle size distribution of the large diameter alumina powder and small diameter alumina powder which are contained in a mixture. It is a particle size distribution of the alumina powder which is a mixture. It is a cross-section figure of the thin film gas sensor of other forms. It is a particle size distribution of the catalyst powder used in the 1st gas selective combustion layer of the thin film gas sensor in a modification. It is a particle size distribution of the catalyst powder showing the occurrence establishment with respect to the logarithm of the particle size. It is a cross-sectional SEM photograph of the thin film gas sensor of a prior art. It is a particle size distribution of the catalyst powder used in the prior art first gas selective combustion layer.

Explanation of symbols

10, 20: Thin film gas sensor 1: Si substrate 2: Thermal insulation support layer 2a: Thermal oxidation SiO ₂ layer 2b: CVD-Si ₃ N ₄ layer 2c: CVD-SiO ₂ layer 3: Heater layer 4: Electrical insulation layer 5: Gas sensing layer 5a: bonding layer 5b: sensing electrode layer 5c: sensing layer (Sb-doped SnO ₂ layer)
5d: first gas selective combustion layer (Pd-supported _Al 2 _{O 3} sintered material)
5e: gap 5f: second gas selective combustion layer (Pt-doped SnO ₂ layer)

Claims (6)

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 gas sensing layer having a pair of sensing electrode layers provided on the electrically insulating layer, a sensing layer provided to pass the pair of sensing electrode layers, and a first gas selective combustion layer provided on a surface of the sensing layer When,
A thin film gas sensor comprising:
The first gas selective combustion layer is a porous body obtained by sintering a sintered material containing a catalyst powder obtained by supporting a first catalyst on a mixture of alumina (Al ₂ O ₃ ) small diameter powder and alumina large diameter powder. A thin film gas sensor, characterized by comprising The thin film gas sensor according to claim 1,
The mixture is a mixture of an alumina small-diameter powder whose average diameter of small-diameter alumina particles is less than 5 μm, and an alumina large-diameter powder whose average diameter of large-diameter alumina particles is 5 μm or more and 15 μm or less. A thin film gas sensor characterized by that. The thin film gas sensor according to claim 1 or 2,
The thin film gas sensor is characterized in that the mixture is a mixture blended so that the occurrence probability of the average value of the average diameter of the small-diameter alumina particles is larger than the occurrence probability of the average value of the average diameter of the large-diameter alumina particles. In the thin film gas sensor according to any one of claims 1 to 3,
The thin film gas sensor, wherein the first catalyst is palladium (Pd). In the thin film gas sensor according to any one of claims 1 to 4,
A second gas selective combustion layer of a thin film provided between the sensing layer and the first gas selective combustion layer and carrying a second catalyst;
A thin film gas sensor comprising: The thin film gas sensor according to claim 5,
The second gas selective combustion layer is a thin film layer made of SnO ₂ supporting platinum (Pt) as a second catalyst.