JPH07198651A - Thin film type gas sensor - Google Patents

Abstract

PURPOSE:To provide a thin film type gas sensor which can be easily manufactured, having an uniform quality, and which has not only an enhanced alcohol sensitivity but also an enhanced hydrogen sensitivity. CONSTITUTION:A thin film type gas sensor comprises an electrically insulative substrate 1, a pair of electrodes 2a, 2b formed on one surface of the substrate 1, a gas sensing SnO2 film 3 with which the electrodes 2a, 2b are covered, a heater 4 electrically insulated from the electrodes and the gas sensing film on the substrate 1, and more than one element of Sc, Y, La and Al and more than one element of Pt, Pd, Rh, Ir, Au and Ag, which are substantially uniformly dispersed in the gas sensing film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film gas sensor, and more particularly to a thin film gas sensor which has good sensitivity to alcohol and hydrogen.

[0002]

2. Description of the Related Art Metal oxides containing SnO ₂ which is an n-type oxide semiconductor as a main component are widely used as sensor materials for detecting reducing gas. Since SnO ₂ simple substance has almost all sensitivity to a general reducing gas, efforts have been made to improve the selectivity by adding a noble metal catalyst or surface modification. Conventionally, the main target of improvement in the selectivity is alcohol gas, and the focus has been on how to make a sensor that is insensitive to alcohol.

Alcohol gas is most often present in the general environment, is also emitted from humans and plants themselves, and can always be present as background gas. However, in recent years, awareness of air pollution has increased in the air conditioning field, and the detection of alcohol gas in a living room has been reviewed as an index of air pollution. This is because it has been found that the amount of alcohol emitted during human ecological activity has a strong correlation with the amount of air pollution, particularly the amount of carbon dioxide emitted by humans. In that sense, an alcohol sensor (having high sensitivity only to alcohol)
It is considered to be one of the leading air conditioning sensors.

Cigarettes are another major source of air pollution in the living space. Since hydrogen and carbon dioxide are the main components of smoke generated when smoking a cigarette, a hydrogen sensor can be used as a cigarette detection sensor. Therefore, in order to detect air pollution, it is necessary to detect tobacco by a hydrogen sensor and indirectly detect carbon dioxide gas (air pollution due to the presence of a person) by an alcohol sensor.

On the other hand, when another gas sensor, for example, a carbon monoxide or methane sensor alarm is used, it is usually affected by alcohol and hydrogen, so that it is necessary to detect hydrogen and alcohol as background with high sensitivity. is there.
In other words, if there is a sensor that can detect alcohol and hydrogen at the same time with high sensitivity, it is possible to detect air pollution and also to perform background detection for preventing malfunction of an alarm device for another gas.

Alcohol sensors having enhanced alcohol sensitivity have been developed so far. Among them, SnO ₂
It has been reported that a noble metal catalyst such as Pt or Pd is added to. In this manufacturing method, powder SnO ₂ and an organic or inorganic binder are mixed, and a so-called bulk type sensor is prepared by baking and solidifying the paste, which is immersed in an aqueous solution of chloroplatinic acid (H ₂ PtCl ₆ ) or the like, or It is generally mixed and baked at a pace and in a state.

On the other hand, a method has also been proposed in which a basic oxide such as La ₂ O ₃ or CaO is added to SnO ₂ to improve the sensitivity of ethanol. In the decomposition reaction of ethanol,
It is said that the dehydrogenation reaction is promoted by the basic substance, while the dehydration reaction is accelerated by the acidic oxide. Since it is the product of the dehydrogenation reaction that gives greater sensitivity to the SnO ₂ sensor, the addition of basic oxides such as La ₂ O ₃ and CaO is said to be effective in increasing the alcohol sensitivity. Generally, these basic oxides are La ₂ O.
₃ , Oxide such as CaO must be supported on the SnO ₂ powder. Further, the improvement of the alcohol sensitivity by the addition of the noble metal catalyst such as Pt and Pd or the basic oxide catalyst such as La ₂ O ₃ and CaO is premised on the use of powder SnO ₂ material.

In recent years, the gas-sensitive body (SnO ₂ ) of gas sensors
Research and development of thinning itself has been actively conducted. This is because the thinning of the device is expected to reduce the size of the device, improve mass production, improve the variation yield, improve gas sensitivity, and improve responsiveness. However, the above-described measures for improving the alcohol sensitivity make it difficult in principle to make the sensor thin.

That is, in the vapor phase film forming method, in order to support the noble metal element or the basic oxide on the surface of the SnO ₂ particle, the noble metal element or the basic oxide must be laminated on the SnO ₂ film. I won't. FIG. 6 shows a cross-sectional structure of a thin film gas sensor having a structure in which a catalyst layer is laminated on a gas sensitive film. Further, FIG. 7 shows a similar structure, in which the catalyst layers are dispersed and carried in the form of islands on the gas sensitive membrane. Many such catalyst addition methods have been proposed in the past. However, this laminated structure does not mean that it is only necessary to stack the layers, and it is difficult to control the catalytic effect unless it is formed extremely thin with an accuracy of several tens of angstroms. Further, it is considerably difficult to control this on the film forming apparatus, and even if it is possible, the yield of the product is significantly reduced. Further, even if the catalytic element is simply supported on the SnO ₂ surface, the supported state is unstable, and it is difficult to secure the long-term stability of the catalytic action.

The inventor of the present application has already proposed a hydrogen sensor that can be made into a thin film and found that the sensitivity to hydrogen can be increased by adding a noble metal element to the gas sensitive film (Japanese Patent Application No. 4-136270). However, an example of a sensor that simultaneously increases the sensitivity to alcohol has not been disclosed yet.

Therefore, an object of the present invention is to provide a gas sensor that solves such a problem, and is particularly easy to manufacture and has less variation in product characteristics.
It is an object of the present invention to provide a thin-film gas sensor that maintains and improves not only the alcohol sensitivity but also the hydrogen sensitivity.

[0012]

As a result of earnest research in view of the above object, the present inventor has found out the following.

(1) Hydrogen sensitivity can be increased by using a noble metal element as a catalyst element. Among them, in the case of a sensor using a Pd catalyst, alcohol sensitivity is slightly lowered, but poisoning by hydrogen sulfide is eliminated, and a gas sensor with stable output can be obtained.

(2) Group 3, Sc, Y, La, A elements
By adding 1 to SnO ₂ , the resistance of the sensitive film of the sensor is increased, and by raising the sensitivity of the sensor itself, a gas sensor with high alcohol sensitivity can be obtained.

(3) By using a mixed sintered body composed of the above-mentioned two kinds of elements and SnO ₂ as a target and forming a gas sensitive film on a gas sensor substrate by a sputtering method, it is possible to improve both hydrogen and alcohol. A sensitive thin film gas sensor is obtained. The present invention has been made based on the above findings.

That is, the thin film gas sensor of the present invention is
An electrically insulating substrate, a pair of electrodes provided on one surface of the substrate, and SnO formed so as to cover the electrodes.
_{2 a} gas sensitive film, a heater provided on the substrate and electrically insulated from the electrode and the gas sensitive film,
In the nO ₂ gas sensitive film, (a) at least one element of Sc, Y, La and Al, and (b) Pt, Pd, R
It is characterized in that at least one element selected from h, Ir, Au and Ag is added in a substantially uniformly dispersed manner.

Further, a method of manufacturing a thin film gas sensor of the present invention in which a SnO ₂ gas sensitive film is formed on a substrate having electrodes by a sputtering method is SnO ₂ and (a) S.
at least one element of c, Y, La, and Al,
(B) By using a mixed sintered body containing any one or more kinds of elements of Pt, Pd, Rh, Ir, Au, and Ag as a target, and vaporizing the respective components at the same time to deposit them on the substrate, It is characterized in that a SnO ₂ gas sensitive film is formed.

[0018]

EXAMPLE AND FUNCTION FIG. 1 shows an example of the thin film gas sensor of the present invention. The thin film gas sensor of the present invention comprises an insulating substrate 1, a pair of opposing electrodes 2a and 2b formed on one surface of the substrate 1, and a gas sensitive film formed to cover the electrodes 2a and 2b. 3 and a heater 4 formed on the opposite surface of the substrate 1.

The substrate 1 may be made of any material as long as it has electrical insulation, but it is preferably made of ceramics from the viewpoint of heat resistance and durability. Particularly preferable ceramics include Al ₂ O ₃ , SiO ₂ and mixed materials thereof. If there is no problem with other heat resistance, strength, stability, insulation, etc., it can be used. The thickness of the substrate 1 is preferably 0.1 to 0.6 mm. If it is thinner than 0.1 mm, the mechanical strength of the sensor is insufficient, and if it is thicker than 0.6 mm, the heating effect of the heater 4 is insufficient.

A pair of electrodes 2a and 2b facing each other is formed on one surface of the substrate 1. There are two types of thin-film gas sensor structures, a counter electrode type and a hot wire type, but in the thin-film gas sensor of the present invention, the resistance of the gas sensitive film is high, so the counter electrode type is suitable for detection output characteristics. ing. A thin film electrode is desirable as the structure of the electrode in consideration of downsizing of a thin film element and mass productivity. Further, electrodes such as screen printing are also possible. As the electrode material, Pt, Pd, Au, Ir,
Noble metals such as Rh and Ag, or alloy materials thereof are used. The sputtering method is suitable for forming the electrode from the viewpoints of adhesion and film thickness uniformity. However,
Other film forming methods are possible. After forming a noble metal film on the substrate, an electrode pattern is formed with a resist material, the noble metal film other than the electrode pattern is removed by plasma etching, and the resist film is removed to obtain a thin film electrode. When this is heat-treated, the strain in the electrode is removed and a stable electrode can be obtained. The heater 4 formed on the opposite surface of the substrate 1 is also subjected to the same method.

The gas sensitive film 3 formed between the pair of electrodes 2a and 2b facing each other is (a) a Group 3 element Sc,
At least one element selected from Y, La, and Al, and (b)
It is made of SnO ₂ in which at least one element selected from Pt, Pd, Rh, Ir, Au, and Ag, which is a noble metal, is uniformly dispersed. Addition of the noble metal element increases the sensitivity to hydrogen gas. On the other hand, by adding the Group 3 element as a solid solution, the resistance of the sensitive film is increased and the sensitivity to alcohol gas is increased. By adding both at the same time, high sensitivity can be obtained for both alcohol and hydrogen.

When the measurement gas contains hydrogen sulfide, it is preferable to use the element Pd among the above precious metals. When a noble metal element other than the element Pd is used, the sensitivity of the sensor to hydrogen gas decreases due to poisoning by hydrogen sulfide.

The Group 3 element (a) is contained in the SnO ₂ gas sensitive film.
Is dispersed and added so that the total amount is 0.5% to 10% by weight, and the noble metal element (b) is dispersed and added so that the total amount is 0.5% to 30% by weight. If the total amount of Group 3 elements is less than 0.5% by weight, the sensitivity to alcohol gas is insufficient. On the other hand, when the total amount of Group 3 elements exceeds 10% by weight, Group 3 elements are precipitated. A particularly preferable total amount of the Group 3 element is 0.5 to 5% by weight. If the total amount of noble metal elements is less than 0.5% by weight, the sensitivity to hydrogen gas is insufficient. On the other hand, when the total amount of noble metal elements exceeds 30% by weight, the properties of the SnO ₂ film as a semiconductor are lost, and the film impedance becomes too large to function as a hydrogen gas sensor. A particularly preferable total amount of noble metal elements is 2 to 30% by weight.

The gas sensitive film 3 must be formed to a thickness of 5000 angstroms or less. When the film thickness of the gas sensitive film 3 exceeds 5000 angstroms, the gas sensitivity decreases and the response time becomes long. The preferable film thickness of the gas sensitive film 3 is 500 to 5000 angstroms.

The thin-film gas sensor of the present invention is 300 to 4
It exhibits good detection sensitivity at an operating temperature of 50 ° C. The heater 4 is energized so as to fall within this temperature range.

Next, the method of the present invention for manufacturing a thin film gas sensor will be described. The gas sensitive film 3 in which the catalyst element is uniformly dispersed is directly formed on the insulating ceramic substrate 1 by a sputtering method using a SnO ₂ mixed sintered body in which the catalyst metal is substantially uniformly dispersed.

The mixed sintered body used as a target for the sputtering method is SnO ₂ powder, and (a) at least one element selected from the group III elements Sc, Y, La and Al, and (b) a noble metal. Pt, Pd, Rh, Ir, A
One obtained by uniformly mixing one or more elements of u and Ag and sintering the mixture is used.

The atmosphere gas in sputtering is preferably argon or a mixed gas of argon and oxygen. The sputtering temperature is preferably room temperature to 400 ° C. The sputtering time can be appropriately determined according to the film thickness of the gas sensitive film.

Although the thin film gas sensor of the present invention and the method for manufacturing the same have been described with reference to the drawings, the present invention is not limited thereto and various modifications can be made without departing from the spirit of the present invention. For example, a sensor element having a lead wire provided on an electrode is naturally included in the present invention.

The present invention will be described in more detail with reference to the following examples. Example 1 As a substrate of a sensor element, an alumina substrate having a thickness of 0.2 mm was used. A Pt thin film having a thickness of 2500 angstrom was formed on the surface of the substrate and patterned by a photolithography technique to form a counter electrode. Also, in the same manner, on the back surface of the substrate, a P having a thickness of 3200 angstrom
A thin film heater was formed. This was subjected to plasma etching treatment and then fired at about 600 ° C. in the atmosphere.

A SnO ₂ / Y ₂ O ₃ / Pd-based gas sensitive film was formed on the electrode surface of this sensor substrate by the RF magnetron sputtering method. As a target in the sputtering method, a mixed sintered body obtained by mixing and sintering the above-mentioned three component powders was used. The film thickness of the formed gas sensitive film was 1500 Å. Using the sensor element thus produced, an electric current was passed through the heater so that the operating temperature became 400 ° C., and 100 ppm hydrogen gas or 300 p
The gas sensitivity of the sensor to pm ethanol gas was measured.

FIG. 2 shows the sensitivity Pd and Y ₂ O of ethanol.
₃ Dependence of catalyst amount is shown. FIG. 3 shows the dependence of hydrogen sensitivity on the amounts of Pd and Y ₂ O ₃ catalysts. Here, the gas sensitivity is
The ratio (Ra / Rg) between the element resistance value (Ra) in clean air and the element resistance value (Rg) at the time of gas detection. The larger the sensitivity value, the larger the sensor output.

As is clear from these two figures, when only Pd is added, the hydrogen sensitivity gradually increases, but the ethanol sensitivity sharply decreases. However, it was found that when the amount of Y ₂ O ₃ added was increased at the same time, the sensitivity of hydrogen was almost unchanged while the sensitivity of ethanol was increased. Thus, by simultaneously containing Pd and Y ₂ O ₃ in the gas sensitive film, both the sensitivity to alcohol and hydrogen can be increased.

Example 2 A thin film hydrogen gas sensor was produced in the same manner as in Example 1, but the gas sensitive film of this example was SnO ₂ / Al ₂ O ₃ / P.
The thickness of the gas sensitive film was 2000 angstroms. 3. Set the Pt concentration in the gas sensitive film to 4.
It was fixed at 5% by weight and the amount of Al ₂ O ₃ added was varied to prepare 6 types of thin film gas sensors. Using the sensor element thus manufactured, a current was passed through the heater so that the operating temperature was 400 ° C., and the gas sensitivity of the sensor with respect to 100 ppm hydrogen gas or 300 ppm ethanol gas was measured. The results are shown in Fig. 4.

As is clear from FIG. 4, when the amount of Al ₂ O ₃ added was increased, the sensitivity of the sensor to hydrogen gas slightly increased, and the sensitivity to alcohol greatly increased.

Example 3 Using the same method as in Example 1, the gas sensitive film was made of Sn.
O ₂ / Al ₂ O ₃ (3.5% by weight) / Pd (5.2% by weight) system component and SnO ₂ / Al ₂ O ₃ (3.3% by weight) /
Two types of gas sensors composed of Pt (5.6% by weight) based components were manufactured. The thickness of the gas sensitive film was 2000 angstrom. The sensor element thus manufactured was left in a glass container having a hydrogen sulfide concentration of 5 ppm and a humidity of 80% and taken out every 50 hours at an operating temperature of 400 ° C.
A current was passed through the heater so that the gas sensitivity of the sensor with respect to 100 ppm hydrogen gas was measured. Results are shown in FIG.

As is clear from FIG. 5, P is added to the noble metal element.
The sensor using the element d showed stable sensitivity without being affected by hydrogen sulfide.

[0038]

The thin film gas sensor of the present invention having the above constitution has good alcohol gas sensitivity and hydrogen gas sensitivity, and can be obtained at a low cost with a small number of manufacturing steps.
Further, the thin film gas sensor of the present invention can be easily miniaturized, and has a large effect of mass production by a batch process. further,
The method for manufacturing a thin film gas sensor of the present invention can easily and reliably control the amount of the catalytic element added to the gas sensitive film.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a thin film gas sensor according to an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between ethanol sensitivity and the amounts of Y ₂ O ₃ and Pd added when the thin film gas sensor of Example 1 is used.

FIG. 3 is a graph showing the relationship between hydrogen gas sensitivity and the amounts of Y ₂ O ₃ and Pd added when the thin film gas sensor of Example 1 is used.

FIG. 4 shows the sensitivity to hydrogen gas and ethanol and Al ₂ when the thin film gas sensor of Example 2 is used.
It is a graph showing the relationship with the amount of O ₃ added.

FIG. 5 is a graph showing a change in sensitivity to hydrogen gas in a hydrogen sulfide poisoning test when the thin film gas sensor of Example 3 is used.

FIG. 6 is a schematic cross-sectional view showing an example of a conventional thin film gas sensor.

FIG. 7 is a schematic cross-sectional view showing another example of a conventional thin film gas sensor.

[Explanation of symbols]

 1 substrate 2a, 2b electrode 3 gas sensitive film 4 heater 6 catalyst layer

Claims (3)

[Claims] 1. An electrically insulating substrate, a pair of electrodes provided on one surface of the substrate, a SnO ₂ gas sensitive film formed so as to cover the electrodes, and an electrode and a gas on the substrate. In a thin film gas sensor having a sensitive film and a heater electrically insulated from the thin film gas sensor, (a) Sc, Y, La, or Al and one or more kinds of elements are contained in the SnO ₂ gas sensitive film. (B) Pt, Pd, Rh, Ir, A
A thin-film gas sensor, wherein at least one element of u and Ag is dispersed and added substantially uniformly. 2. The thin film gas sensor according to claim 1, wherein the SnO ₂ gas sensitive film of the gas sensor used in an environment where hydrogen sulfide gas is present, comprises (a) Sc,
At least one element selected from Y, La, and Al, and (b)
A thin film gas sensor, wherein Pd element is added to be dispersed substantially uniformly. 3. A method of manufacturing a thin film gas sensor, comprising forming a SnO ₂ gas sensitive film on a substrate having an electrode by a sputtering method, comprising: SnO ₂ and (a) Sc;
At least one element selected from Y, La, and Al, and (b)
The SnO ₂ gas is obtained by using a mixed sintered body composed of one or more elements selected from the group consisting of Pt, Pd, Rh, Ir, Au, and Ag as a target, and vaporizing the respective components at the same time to deposit them on the substrate. A method for manufacturing a thin film gas sensor, which comprises forming a sensitive film.