JP2002328108A - Hydrogen gas detecting element and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen gas detecting element excellent in sensitivity to hydrogen gas, having sufficient selectivity and excellent in responsiveness, and a manufacturing method therefor. SOLUTION: This hydrogen gas detecting element is provided with a diaphragm type silicon substrate 1, a gas sensitive membrane 5 comprising an oxide semiconductor such as SnO2 , a pair of comb-toothed electrodes 4 comprising Pt or the like, a catalyst cluster 6 comprising Pd, Pt or the like, and a patially poisoned body 7 comprising SiO2 which is formed by oxidizing a Si film without heating. A portion corresponding to the gas sensitive membrane of the silicon substrate is etched anisotropically, and formed with a cavity 11. The gas sensitive membrane may comprise SnO2 or the like formed by oxidizing a metal tin film formed by vacuum deposition under an oxidizing atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a hydrogen gas detecting element and a method for manufacturing the same. More specifically, the present invention relates to a hydrogen gas detection element having high sensitivity to hydrogen gas, sufficient selectivity, and excellent responsiveness, and a method for manufacturing the same.

[0002]

A gas sensor having a gas sensitive film mainly composed of an oxide semiconductor of the Related Art like SnO _2, In ₂ O ₃ is conventionally known, has been practically used as such a gas sensor for detecting the combustible gas . And the gas sensitive film itself,
Alternatively, many methods have been proposed for improving gas selectivity depending on the type of catalyst layer or the like formed on the surface.
Among them, a method of performing heat treatment in the final step of element formation is known as one having improved selectivity to hydrogen gas. Thereby, the sensitivity to hydrogen gas is improved, and the sensitivity to other gases is reduced, so that selectivity is improved. Further, there has been proposed a gas sensor in which a thin film having a selectivity for hydrogen gas made of an oxide such as alumina or silica is formed on the surface of a gas-sensitive film to reduce the influence of a gas other than hydrogen gas and increase sensitivity.

[0003]

However, conventionally, the gas-sensitive film is often formed by a printing method or the like, and in this case, there is a problem that the sensitivity, selectivity and the like are varied. Further, in the method of performing heat treatment in the final step of element formation, a considerable time is required for heat treatment, resulting in poor productivity.
In the method of forming a thin film made of an oxide on the surface of a gas-sensitive film, the hydrogen gas comes into contact with the gas-sensitive film after passing through the thin film, and thus the sensitivity and responsiveness are reduced. is there.

The present invention solves the above-mentioned problems of the prior art, and provides a hydrogen gas detecting element having high sensitivity to hydrogen gas, sufficient selectivity, and excellent responsiveness, and a method of manufacturing the same. The purpose is to do.

[0005]

According to the present invention, there is provided a hydrogen gas detecting element comprising: a substrate having at least a surface layer having an insulating property; a gas-sensitive film formed of an oxide semiconductor formed on the substrate; A pair of electrodes formed in contact with each other, a catalyst cluster made of a noble metal in contact with the gas-sensitive film, and a partial poisoning body made of SiO _{2 in} contact with at least the catalyst cluster, and the partial poisoning body is made of Si. It is characterized by being formed by oxidizing without heating.

[0006] The substrate may be entirely made of an insulating material, or may have an insulating surface layer having a thickness enough to be electrically insulated from other members such as electrodes. Good. If the substrate is made of ceramic such as alumina, glass, or the like, the entire substrate becomes an insulating substrate. In addition, if the surface of a substrate made of a semiconductor such as silicon is oxidized, the surface of the substrate becomes an insulating substrate. The use of this semiconductor substrate for the detection element is particularly preferable because the detection element can be easily reduced in size by applying a general semiconductor manufacturing technique.

A semiconductor substrate made of silicon can be formed by forming silicon into a substrate shape and then thermally oxidizing to form an insulating layer made of SiO _{2 on} the entire surface. The thickness of the insulating layer can be 5 to 1000 nm, particularly 30 to 500 nm, whereby a substrate having sufficient insulating properties can be obtained. The thickness of the substrate is not particularly limited, and is 100 to 1000 μm, particularly 20 μm.
It can be about 0 to 800 μm.

As an oxide semiconductor for forming a gas-sensitive film, SnO ₂ , In ₂ O ₃ , TiO ₂ , ZnO, Cu
₂ O, NiO, FeO, WO _{3 and the} like can be used. Among these oxide semiconductors, SnO ₂ , In ₂
O _3, an oxide semiconductor which the SnO ₂ as a main component, and In ₂
An oxide semiconductor containing O ₃ as a main component is preferable. When these oxide semiconductors are used, a hydrogen gas detection element having high sensitivity and excellent selectivity for hydrogen gas can be obtained. Note that this “main component” means that the oxide semiconductor contains 85% by mass or more, particularly 95% by mass or more of SnO ₂ or In ₂ O ₃ .

Although the thickness of the gas-sensitive film is not particularly limited,
The thickness is preferably 25 nm or less, particularly preferably 15 nm or less. Gas sensitive membranes are thus relatively thin, in particular 15 nm.
By performing the following, when exposed to an atmosphere containing hydrogen gas, the proportion of the region where the resistance value greatly changes can be increased. Thereby, the sensitivity of the detection element can be greatly improved.

The material, shape, etc., of the pair of electrodes for detecting hydrogen gas are not particularly limited, and may be the general materials, shapes, etc., of this type of gas detection element. Examples of the material include Pt, Au, Ni, and the like.
It can be a parallel electrode, a comb electrode or the like. The pair of electrodes can be formed on either the surface of the gas-sensitive film that is in contact with the substrate or the opposite surface. In addition, it can be formed by burying it in a gas-sensitive film, but the process is slightly more complicated than when it is formed on the surface. This electrode can be formed by a printing method such as a sputtering method or a screen printing method, and its thickness can be generally about 10 to 200 nm by a sputtering method and about 5 to 30 μm by a printing method. In addition, the thickness of the gas-sensitive film and the electrode can be measured by an optical film thickness meter, a surface step meter or the like.

The noble metal forming the catalyst cluster is Au,
It may be any of Ag, Ru, Rh, Pd, Os, Ir, and Pt, but is preferably Pd or Pt, which greatly improves the selectivity to hydrogen gas. The average thickness of the catalyst cluster is preferably 1 to 5 nm, particularly preferably 2 to 4 nm. If the average thickness of the catalyst cluster is less than 1 nm, the sensitivity tends to decrease over time, which is not preferable. On the other hand, if the thickness exceeds 5 nm, the adjacent clusters come into contact with each other and a current flows on the element surface, thereby deteriorating the element performance.

The catalyst cluster may be formed on the substrate or on the surface of the gas-sensitive film. However, in order to function as a detecting element, the gas-sensitive film, the catalyst cluster, and the catalyst cluster And the partially poisoned body must be in contact at least in part. Therefore,
When the catalyst cluster is formed on the substrate, the gas-sensitive film is formed after forming the catalyst cluster and the partial poisoning body on the substrate. When the catalyst cluster is formed on the surface of the gas-sensitive film, the catalyst cluster and the partially poisoned body are formed on the surface of the gas-sensitive film after forming the gas-sensitive film on the substrate. The formation order of the catalyst cluster and the partial poisoning substance is not limited, and whichever one is formed can be formed in a state where the gas-sensitive film and the catalyst cluster and the catalyst cluster and the partial poisoning substance are in contact with each other. The method for forming the catalyst cluster is not particularly limited, and may be formed by a general film forming method such as a vacuum evaporation method, a sputtering method, or a CVD method, or may be formed by a printing method such as screen printing.

By contacting the surface of the catalyst cluster with a partially poisoned substance made of SiO _2, selectivity to hydrogen gas can be improved without any loss of sensitivity and responsiveness. This partially poisoned body is formed by depositing Si on the surface of a catalyst cluster or the like by a vacuum deposition method, a sputtering method, a CVD method, or the like to form a Si film.
The film is formed by oxidizing the film without heating and converting Si into SiO ₂ . The oxidation is easily performed by opening to the atmosphere, and particularly the surface layer of the Si film is oxidized. If the Si film is thick, much of the inside will not be oxidized, but it will still function sufficiently as a partially poisoned body, and there is no problem. When the Si film is heated and oxidized, the performance of the device deteriorates due to the diffusion of the catalyst cluster into the gas-sensitive film, the diaphragm-type substrate described later is broken, the heater and the insulating layer are separated, and the heater electrode and the like are removed. However, problems such as a decrease in bonding properties due to interdiffusion of constituent elements between different metals at the bonding pad portion occur.

The average thickness of the partial poisoning body is preferably at least 2 nm, more preferably 2 to 50 nm, particularly preferably 2 to 20 nm. The average thickness of the partially poisoned body is 2n
If it is less than m, or if it exceeds 50 nm, the poisoning effect decreases, and the selectivity to hydrogen gas cannot be sufficiently improved. The partially poisoned body is formed in contact with the catalyst cluster formed on the substrate or the surface of the gas-sensitive film,
In a normal manufacturing method, it comes into contact with the substrate and the gas-sensitive film. The partial poisoning body formed in contact with the substrate, the gas-sensitive film and the like does not impair the element performance and does not need to be particularly removed.

The catalyst cluster forms an island-like structure such as particles and discontinuous films. The Si film and the partial poisoning body formed by oxidizing Si similarly form an island-like structure. In some cases, the film may be more continuous. Therefore, their average thickness is a value converted as a continuous film having a uniform thickness. This average thickness can be calculated from the film formation time on the detection element when the film is formed under the same conditions, based on the film formation speed obtained from the thickness of the film formed for the film thickness calculation. The thickness of the film formed for calculating the film thickness can be measured by a surface step meter for the catalyst cluster and by an ellipsometer for the partially poisoned substance. Alternatively, the mass of a cluster or a film per unit area is equal to a value calculated in advance from a density (known or assumed) and a desired film thickness based on a change in a resonance frequency of a crystal resonator installed in the film forming apparatus. The film formation time may be controlled so as to be as follows.

In this hydrogen gas detecting element, it is preferable to use a diaphragm type substrate made of a semiconductor and having a cavity formed at least at a portion corresponding to the gas sensitive film. The cavity can be easily formed by applying a technique such as etching. When the cavity is provided in this manner, the gas-sensitive film and the like can be thermally insulated from the surroundings. This makes it possible to provide a detection element that starts in a short time and has better responsiveness.

It is preferable that an element is provided in which an insulating layer is provided on one surface of the substrate including the portion where the cavity is formed, and a heater for activating the gas-sensitive film is embedded in the insulating layer. . Even when the heater is attached in this way, because it is thermally insulated from the surroundings,
The temperature can be raised and lowered in a short time, and the power consumption of the heater can be reduced. The heater may be any one that can easily raise the temperature of the gas-sensitive film to a predetermined temperature, and a heater made of a noble metal such as Pt, Au, Ru, or a silicon heater can be used. Although the material of the insulating layer is not particularly limited, it can be formed of at least one of Si oxide film, Si nitride film, Si oxynitride film, Ta ₂ O ₅ , alumina and the like.

The hydrogen gas detecting element of the present invention is manufactured by forming a gas sensitive film by oxidizing a metal tin film or a metal indium film formed by a vacuum deposition method in an atmosphere containing at least one of oxygen and water vapor. Is preferred. The atmosphere containing at least one of oxygen and water vapor,
It can be an air atmosphere. The oxidation of the metal tin film or the metal indium film is performed at 300 to 700 ° C., particularly 400 to 6 ° C.
In the temperature range of 00 ° C, the required time, usually 5 to 20
It is preferable to carry out by heating for about a minute.

The gas-sensitive film can be formed by forming a metal tin film or a metal indium film having a desired thickness and then oxidizing the film. If the film is an extremely thin film having a thickness of less than 15 nm, a uniform oxide film is used. It is not easy. In such a case, after forming a metal tin film or a metal indium film having a thickness of about の of a predetermined thickness, the film is opened to the air or heated in an atmosphere containing oxygen, water vapor, etc., and oxidized.
By forming a metal tin film or a metal indium film again and oxidizing it, a more uniform gas-sensitive film can be obtained.

According to the present invention, the thickness of the gas-sensitive film is 7 to
In an atmosphere having a relative humidity of 40% containing hydrogen gas of 400 to 4000 ppm and having a relative humidity of 20 nm, the sensitivity (Ra / Rg) evaluated by the method in Examples described later is about 2
It is possible to obtain a hydrogen gas detecting element that is up to 750 times.

The hydrogen gas detecting element of the present invention can be incorporated into an appropriate container or the like to form a hydrogen gas sensor. This sensor has high sensitivity and excellent selectivity for hydrogen gas. In equipment using hydrogen gas, it is extremely difficult to completely prevent hydrogen gas from leaking. In addition, hydrogen gas itself is nontoxic to living organisms. Therefore, it is not particularly necessary for the hydrogen gas sensor to detect the presence or absence of hydrogen gas, as long as it can detect the risk of explosion. Therefore, if the output for the hydrogen gas having a concentration at which there is a danger of explosion is sufficiently large as compared with the output for the interfering gas other than the hydrogen gas having the actual concentration, it can be put to practical use.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the hydrogen gas detecting element of the present invention will be described in more detail with reference to examples. [1] Production of Hydrogen Gas Detector Using Alumina Substrate After washing and drying an alumina substrate having a thickness of 0.6 mm,
A 20 μm-thick comb-shaped electrode made of Pt was formed at a predetermined position by a screen printing method. Next, a Sn film having a thickness of 5, 10 and 15 nm is deposited on the substrate including the surface of the comb-teeth electrode at room temperature by a vacuum evaporation method, and the Sn film is heated at 400 ° C. for 10 minutes in an air atmosphere. ,
Sn was oxidized to form three types of gas-sensitive films of SnO ₂ having different thicknesses. The thickness of this SnO ₂ film was 7, 13 and 20 nm, respectively, and the thickness was about 1.3 times the Sn film before oxidation, respectively.

Thereafter, a catalyst cluster made of Pd was formed on the surface of the gas-sensitive film at room temperature by a vacuum evaporation method. Further, on the surface of the gas-sensitive film including the surface of the catalyst cluster, a Si film is formed at room temperature by a vacuum deposition method, and is opened to the atmosphere to be oxidized.
A partial poison that became O ₂ was formed. In this way,
Comb electrodes made of Pt, a gas-sensitive film made of SnO ₂ , a catalyst cluster made of Pd, and Si on an alumina substrate
A hydrogen gas detection element in which partial poisoning bodies made of O ₂ were formed in this order was manufactured.

[2] Fabrication of a hydrogen gas detecting element provided with a heater using a diaphragm type silicon substrate (1) Structure of the hydrogen gas detecting element FIG. 1 is a longitudinal sectional view of the hydrogen gas detecting element. FIG. 2 is a longitudinal sectional view showing, in an enlarged manner, a portion above the heater in FIG. 1, particularly, a catalyst cluster and a partial poisoning body. An insulating layer (not shown) made of a thermal oxide film is formed on the surface of the silicon substrate 1, and an insulating layer 21 made of silicon nitride is provided thereon. A cavity 11 is formed on the back surface of the silicon substrate 1. The heater 3 is formed in a portion of the surface of the insulating layer 21 corresponding to the cavity 11, and the heater 3 is covered with an insulating layer 22 made of silicon nitride. Heater terminals (not shown) are formed at both ends of the heater 3.
Is supplied with power.

A comb electrode 4 is formed on the surface of the insulating layer 22, and electrode terminals (not shown) for supplying electricity to the comb electrode 4 are formed at both ends. The gas-sensitive film 5 is formed on the surface of the insulating layer 22 including the comb electrode 4. Further, a catalyst cluster 6 is formed on the surface of the gas-sensitive film 5.
Is formed, and the gas-sensitive film 5 including the surface of the catalyst cluster 6 is formed.
Is partially poisoned on the surface.

(2) Manufacture of hydrogen gas detecting element Formation of heater The silicon substrate 1 having a thickness of 400 μm was washed and dried, and a 100 nm-thick insulating layer made of SiO ₂ was formed on the surface of the silicon substrate by thermal oxidation. Thereafter, an insulating layer 21 made of silicon nitride and having a thickness of 400 nm was formed on one surface thereof, and an insulating layer 23 having a thickness of 200 nm was formed on the other surface thereof by PCVD. Next, a 25-nm-thick Ti film and a 250-nm-thick Pt film were stacked on the surface of the insulating layer 21 by a sputtering method. The Ti film was used to improve the adhesion strength of the Pt film to the substrate. Then, a heater pattern was formed by etching the Pt / Ti film. After that, a 1000-nm-thick insulating layer 22 made of silicon nitride was further deposited on the insulating layer 21 on which the heater 3 was formed by PCVD. Next, in order to connect the heater 3 to an external circuit, a portion of the insulating layer 22 corresponding to a bonding pad (not shown) of the heater 3 was etched.

A 25 nm-thick Ti film and a 40 nm-thick Pt film are laminated on one surface of the insulating layer 22 formed in the formation of the comb electrode and the cavity of the silicon substrate by sputtering, and the comb electrode 4 is formed by etching. Was formed. Next, a cavity pattern was formed on the surface of the insulating layer 23 formed on the other surface of the silicon substrate 1 by photolithography. Thereafter, a portion of the insulating layer 23 corresponding to the cavity was removed by dry etching. Then 22% by volume heated to 85 ° C.
Cavity 11 (diaphragm) by anisotropic etching of silicon
Was formed. Thereafter, the substrate was washed with ultrapure water and dried naturally.

Formation of Gas Sensitive Film, Formation of Catalyst Cluster and Partially Poisoned Body On the surface of the insulating layer 22 on which the comb-teeth electrodes 4 are formed, a thickness of SnO ₂ is formed in the same manner as in [1]. Gas-sensitive film 5 of thickness 7, 13 and 20 nm, average thickness of 2 nm made of Pd
And a Si film having an average thickness of 2, 20 and 50 nm were formed in this order. The partial poisoning body 7 was formed by opening the Si film to the atmosphere and oxidizing the Si. Further, the gas-sensitive film 5 was formed using a metal mask such that the gas-sensitive film 5 was formed at a portion corresponding to the cavity 11 of the silicon substrate 1. Thus, a hydrogen gas detecting element having the same configuration as the element in [1] except that the diaphragm type silicon substrate and the heater embedded in the insulating layer were provided.

(3) Performance Evaluation Using the hydrogen gas detecting element prepared in (2), the performance was evaluated by a circuit in which this detecting element, a power supply and a capacitor were connected in series. The measurement conditions are as follows. Element temperature: 220 ° C. Applied voltage: 5 V Measurement pipe inner diameter: 80 mm Test gas flow rate: 8 L / min Test gas flow rate: about 0.03 m / sec Base gas: Relative humidity is 40%, 20.9 mass Nitrogen gas containing% oxygen gas Test gas and base gas temperature; 30 ° C. Test gas type; Base gas containing a predetermined amount of each of H ₂ , CO and NO ₂ gas Each test gas Measurement time; 5 minutes

The sensitivity (Ra / Rg) is calculated by the following equation. The larger the value, the higher the sensitivity.
Ra / Rg = resistance value of gas-sensitive film when device is exposed to base gas / resistance value of gas-sensitive film when device is exposed to each test gas

(A) Correlation between thickness of gas-sensitive film and element resistance or sensitivity when hydrogen gas concentration is changed.
In various measurement atmospheres containing 000 ppm and 4000 ppm of hydrogen gas, the element resistance was measured when the thickness of the gas-sensitive film made of SnO ₂ was changed to 7 nm, 13 nm, and 20 nm. The results are shown in FIG. FIG. 4 shows the sensitivity of the gas-sensitive film of each thickness at each hydrogen gas concentration.

According to FIG. 3, the element resistance is greatly reduced in the base gas atmosphere as the gas-sensitive film becomes thicker. On the other hand, in an atmosphere containing hydrogen gas, the element resistance is low irrespective of the film thickness as compared with the case of the base gas atmosphere,
In addition, the change in resistance depending on the film thickness is small. Therefore, it can be said that the sensitivity of the device to hydrogen gas is mainly determined by the initial resistance value of the gas-sensitive film. According to FIG. 4, the sensitivity increases as the film thickness decreases and as the hydrogen gas concentration increases. It is understood from FIG. 4 that the sensitivity sharply increases when the thickness of the gas-sensitive film is 13 nm or less.

(B) Correlation between the average thickness of the Si film that becomes a partially poisoned body by oxidation with respect to various gases and the sensitivity, and the base gas and 100 ppm of C
The measurement atmosphere was made to contain O gas or 2 ppm NO ₂ gas, and the average thickness of the Si film before oxidation was 2 nm, 20 n
m, and the sensitivity when changing to 50 nm was measured. FIG. 5 shows the results. Comparing FIG. 4 with FIG. 5, even when considering that the respective thicknesses of the Sn film and the Si film before oxidation are not always the same, the concentration of CO gas or NO The sensitivity in the case of the hydrogen gas atmosphere is much higher than that in the atmosphere containing the _two gases, and it is presumed that this element has sufficient selectivity for the hydrogen gas.

[0034]

According to the hydrogen gas detecting element of the present invention, hydrogen gas can be measured selectively and efficiently.
This detection element is useful in various fields such as measurement of hydrogen gas concentration in the environment.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a hydrogen gas detection element having a diaphragm type substrate.

FIG. 2 is a longitudinal sectional view showing, in an enlarged manner, a portion of the hydrogen gas detecting element shown in FIG. 1 above a heater, particularly, a catalyst cluster and a partial poisoning body.

FIG. 3 is a graph showing a correlation between a thickness of a gas-sensitive film and an element resistance when a hydrogen gas concentration is changed.

FIG. 4 is a graph showing the correlation between the thickness of a gas-sensitive film and the sensitivity when the hydrogen gas concentration is changed.

FIG. 5 is a graph showing the results of evaluating the sensitivity to various gases by changing the thickness of a Si film before oxidation.

[Explanation of symbols]

1; silicon substrate, 11; cavity, 21, 22, 2
3; insulating layer, 3; heater, 4; comb electrode, 5; gas-sensitive film, 6; catalyst cluster, 7;

Continuation of the front page (72) Inventor Yokoi et al. 14-18, Takatsuji-cho, Mizuho-ku, Nagoya-shi Inside Japan Specialty Ceramics Co., Ltd. (72) Inventor Yoshikazu Yasukawa 4830 Takatsuka-cho, Hamamatsu-shi, Shizuoka Pref. (72) Inventor Kyo Matsumura 4830 Takatsuka-cho, Hamamatsu-shi, Shizuoka Pref. BA06 BA09 BB02 BB04 BC03 BE03 BF01 DB05 EA04 EA08 FB02 FE00 FE02 FE03 FE11 FE12 FE15 FE16 FE25 FE26 FE29 FE31 FE34 FE35 FE38 FE39 FE41 FE44 FE46 FE48

Claims (9)

[Claims] 1. A substrate having at least a surface layer having insulating properties, a gas-sensitive film formed of an oxide semiconductor formed on the substrate, a pair of electrodes formed in contact with the gas-sensitive film, A catalyst cluster made of a noble metal in contact with the film, and a partial poisoning body made of at least SiO _{2 in} contact with the catalyst cluster, the partial poisoning body being formed by oxidizing Si without heating. A hydrogen gas detection element characterized by the above-mentioned. 2. The oxide semiconductor according to claim 1, wherein (1) SnO ₂ ,
2. The hydrogen gas detection element according to claim 1, wherein (2) In ₂ O ₃ , (3) an oxide semiconductor mainly containing SnO ₂ , or (4) an oxide semiconductor mainly containing In ₂ O ₃ . 3. The hydrogen gas detecting element according to claim 1, wherein the thickness of the gas-sensitive film is 25 nm or less. 4. The hydrogen gas detecting element according to claim 1, wherein the noble metal is Pd or Pt. 5. The catalyst cluster has an average thickness of 1 to 5 n.
The hydrogen gas detection element according to any one of claims 1 to 4, wherein m is m. 6. The hydrogen gas detecting element according to claim 1, wherein an average thickness of the partial poisoning body is 2 nm or more. 7. The hydrogen gas detecting element according to claim 1, wherein the substrate is made of a semiconductor material, and a cavity is formed in a portion of the substrate corresponding to the gas-sensitive film. 8. The method for manufacturing a hydrogen gas detecting element according to claim 1, wherein the gas-sensitive film is a metal tin film or a metal indium film formed by a vacuum deposition method. A method for manufacturing a hydrogen gas detection element, wherein the method is formed by oxidizing in an atmosphere containing at least one of oxygen and water vapor. 9. The method according to claim 8, wherein the substrate is made of a semiconductor material, and a portion of the substrate corresponding to the gas-sensitive film is etched to form a cavity.