JPH063308A - Gas sensor - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a gas sensor having a gas selectivity, in which a noble metal catalyst layer is provided on a metal oxide semiconductor layer, the selectivity and sensitivity of which are further improved. [Structure] An overhanging portion made of an electrically insulating material and extending in the air, a plurality of gas-detecting metal oxide semiconductor layers provided on the overhanging portion, electrode leads in contact with the metal oxide semiconductor layer, and the metal oxide. In a gas sensor having a noble metal catalyst layer provided on a semiconductor layer and a heater lead provided substantially side by side with the electrode lead,
(I) The noble metal catalyst layers provided on the metal oxide semiconductor layers have different thicknesses, or (ii) without contacting the noble metal catalyst layers provided on the metal oxide semiconductor layers. A gas-selective gas sensor, which is provided with the heater for heating the noble metal catalyst layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a gas sensor, and more specifically, by providing a noble metal catalyst layer on a metal oxide semiconductor layer, hydrogen (H ₂ ), ethanol (C ₂ H ₅ OH) and methane are provided. (CH ₄ ), isobutane (is
It relates o-C ₄ H ₁₀₎ and selectively structure of the detection to the gas sensor.

[0002]

2. Description of the Related Art A metal oxide semiconductor layer is used as a gas sensitive material, and (i) a heater film is provided on the back surface of the metal oxide semiconductor via an electrode and an insulating film, or (ii) inside the metal oxide semiconductor layer. A gas sensor is known in which a heater coil also serving as an electrode and an electrode is provided, and the resistance value of the heater film and / or the metal oxide semiconductor heated by the heater coil changes due to gas adsorption on the surface. A typical one of the gas sensors is a thin film gas sensor, and the outline thereof is, as shown in FIGS. 1A and 1B, a gas sensitive film 51 on one side of the heat resistant substrate 1.
And electrodes 41 and 42 for detecting a change in resistance value thereof are formed, and a heater film 20 is formed on the opposite side. When the heat resistant substrate 1 is electrically conductive, it is necessary to form an insulating film between it and the heater film. In addition, (a)
Is a sectional view and (b) is a perspective view. Reference numerals 61 and 62 represent power supply lines to the heater film 20, and 71 and 72 represent signal extraction lines of the gas sensitive film 51. On the other hand, FIG. 2 shows an outline of a typical one of other gas sensors. Here, a metal oxide semiconductor sintered body (gas-sensitive) of 2 to 3 mm square is provided between the heater coils 43 and 44 which also serve as a pair of electrodes. It holds the substance 52). This type of gas sensor is devised so that the signal extraction line of the gas sensitive substance 52 can be drawn from one of the pair of electrodes (for example, the electrode 43) and the other (for example, the electrode 44), and the heater coil 43, 44 is made to exist in the state of being embedded in the layer of the gas sensitive material 52. In the figure, 40 is a base and 30 is an electrode pin. However, the one shown in FIG. 2 has a large power consumption and a large heat capacity, which causes a problem in response. On the other hand, the gas sensor of the type shown in FIG. 1 has good power consumption and responsiveness because the gas-sensitive substance is a thin film, but generally, the sensitivity of the gas sensor is small only with a metal oxide semiconductor, and the gas selection Since it cannot be said that the properties are sufficient, it has been attempted to enhance the sensitivity of the device and improve the Kazu selectivity by using a noble metal such as platinum (Pt) or palladium (Pd) as a catalyst.
Generally, there are a method of directly mixing the catalyst with the material of the metal oxide semiconductor layer and a method of providing the catalyst on the metal oxide semiconductor layer as a catalyst layer. Currently, by using these catalysts, it has been possible to improve the selectivity for various gases, but it is difficult to study the optimization of the type, loading amount, and loading method of the catalyst. Various studies have been conducted, but satisfactory results have not yet been obtained.

[0003]

[Object] The present invention provides a gas sensor having a gas selectivity, in which a noble metal catalyst layer is provided on a metal oxide semiconductor layer,
The purpose is to further improve its selectivity and sensitivity.

[0004]

According to a first aspect of the present invention, an overhanging portion made of an electrically insulating material is provided so as to extend in the air, an electrode lead in contact with a metal oxide semiconductor layer for gas detection provided on the overhanging portion, A gas sensor having a noble metal catalyst layer provided on the metal oxide semiconductor layer and a heater lead provided substantially side by side with the electrode lead.
The present invention relates to a gas-selective gas sensor comprising at least two overhangs, wherein the noble metal catalyst layers provided on the metal oxide semiconductor layer on the at least two overhangs have different film thicknesses. The second of the present invention is an overhanging portion made of an electrically insulating material provided in the air,
A plurality of metal oxide semiconductor layers for gas detection provided on the projecting portion, electrode leads in contact with the metal oxide semiconductor layer, a noble metal catalyst layer provided on the metal oxide semiconductor layer, and the electrode leads provided substantially in parallel with each other. A gas sensor having the above heater lead, wherein the noble metal catalyst layer is provided on the metal oxide semiconductor layer without making contact with the metal oxide semiconductor layer, and the noble metal catalyst layer heating heater is provided. This gas-selective gas sensor has gas selectivity by operating the metal oxide semiconductor layer and the noble metal catalyst layer at different temperatures. In this case, as one specific configuration, a structure having one or more overhanging portions and separately providing the metal oxide semiconductor layer and the noble metal catalyst layer on the one or more overhanging portions is adopted. You can

The noble metal catalyst layer provided on the metal oxide semiconductor layer requires the requirements of the first invention when provided in contact with the metal oxide semiconductor layer, and is formed without contact. In some cases, the requirements of the second invention are necessary. When provided in a state where they do not contact each other, it is preferable to provide a heater for heating in contact with the noble metal catalyst layer and operate the metal oxide semiconductor layer and the noble metal catalyst layer at different heating temperatures. The heater for heating the noble metal catalyst layer can be provided above and / or below the noble metal catalyst layer, or inside. The precious metal catalyst layer is C
VD, metal salt impregnation method, coprecipitation method, or other vapor phase chemical reaction method or chemical method, or sputtering,
It can also be formed by a physical method such as vapor deposition. Further, the noble metal catalyst layer may be formed by a noble metal supported on a carrier. The film thickness of the noble metal catalyst layer can be selected, for example, in the range of several 10Å to several 10 μm. For example, when two noble metal catalyst layers are provided, a relatively thin film of several 10Å to several 1000Å and several 1000Å. It may be a combination with a relatively thick film of several tens of μm. Examples of the noble metal include Pd, Pt, Rh, Ir, Ag and Au.

The gas sensor of the present invention will be described in more detail below with reference to FIGS. The substrate is easy to undercut and is a material that does not deform even at high temperatures, such as S
i, Al, Cu, Ni, Cr or the like is used, and preferably the Si (100) plane is used. The reason why it is preferable to use the (100) plane is that a known anisotropic etching solution can be used for undercut etching. The outer dimension of the substrate 1 is about 1 to 4 mm square, and its thickness is suitably 0.1 to 1 mm. Materials for the metal oxide semiconductor layer include tin, zinc, iron, titanium, indium,
Examples thereof include oxides of nickel, tungsten, cadmium, vanadium and the like, and among these, use of tin oxide is most preferable. The metal oxide semiconductor layer may be formed by a physical method such as sputtering or vapor deposition, or C
There are various methods such as VD, impregnation method, coprecipitation method, vapor phase chemical reaction method, chemical method, etc., but any method may be used. Next, the present invention will be described in two cases.

(I) When the catalyst layer is provided in contact with the surface of the metal oxide semiconductor layer. FIG. 3 and FIG. 4 are examples of gas sensors according to the present invention that employ a cross-linked structure. Here, two protruding portions 2 and 3 made of an electrically insulating material are provided as a bridge structure in the air on a substrate 1 having a substantially square shape, but the number of protruding portions is 2 or more. Any number can be used, and it can be set arbitrarily according to the conditions. Further, FIG. 4 is a sectional view taken along the line of FIGS. 3 (A)-(A '). The structure shown in FIG. 4B is a partially enlarged view of FIG. 4A in which the film thicknesses of the noble metal catalyst layers 11 and 12 are changed. The thickness of the noble metal catalyst layer is from several tens of tens to several tens.
μm, for example, when two films are used,
(A) and (b) a comparatively thin film 12 of several 10Å to several 1000Å and several 1000Å to 1 shown in FIGS.
A relatively thick film 11 of 0 μm is used in combination. Specific methods for providing the noble metal catalyst layer include physical methods such as sputtering and vapor deposition, or vapor phase chemical reaction methods such as CVD, metal salt impregnation and coprecipitation, and chemical methods. For example, the relatively thick film 11 in FIG.
It is also possible to form each of them by combining them with each other, for example, by providing a metal salt impregnation method and forming a relatively thin film 12 by a sputtering method. Further, as the noble metal catalyst layer, a carrier in which a noble metal is supported on a carrier such as alumina or silica may be used. Generally, in order to prepare a catalyst of this structure, H ₂ PtCl ₆ .6H ₂ O, PdCl ₂ , RhC
l ₃ · 3H ₂ precious metal chlorides O such as solution or (NH _{4) 2} PtC
_{l 6, (NH 4) 2} PdCl 6, carried out by a wet method such as using a noble metal ammonium salt solution such as (NH _{4) 3} RhCl _6.

FIGS. 5 and 6 show an example of a gas sensor according to the present invention having a cantilever structure. The number of overhanging portions is not particularly limited as long as it is two or more. It can be set arbitrarily. Figure 6
FIG. 6 is a sectional view taken along the line (B)-(B ′) of FIG. Figure 6 (b)
FIG. 7 is a partially enlarged view of FIG. 7 and 8 show
In another specific example of the gas sensor of the present invention, two metal oxide semiconductor layers 6 and 7 are formed on one bridge structure type overhang 2.
8A is provided, and FIG. 8B is a partially enlarged view of FIG. 8A. 9 and 10 show another specific example of the gas sensor of the present invention, in which two metal oxide semiconductor layers 6 and 7 are provided on one cantilever structure 2. 10B is a partially enlarged view of FIG. 10A.

(Ii) When the catalyst layer is provided separately on the surface of the metal oxide semiconductor layer. 11 and 12 show an example of the gas sensor according to the present invention of claim 2 in which the metal oxide semiconductor layers 6 and 7 on the substrate 1 and the catalyst layer 13 are separated via a cavity 16. 11A is a sectional view, FIG. 11B is an enlarged view of a dotted line portion of FIG. 11A, FIG. 11C is a partially enlarged view of the dotted line portion of FIG. 12B, and FIG. And (b) is (a)
It is an upper surface enlarged view of a figure. Here, the catalyst layer 13 has a filter effect, and the thickness of the cavity 16 is 1
It can be widely selected from mm to 50 mm, and as shown in FIG. 11, heaters 14 and 15 for heating provided in contact with the catalyst layer 13 and heaters 4 and 5 for heating the metal oxide semiconductor layer are provided, and the catalyst The layer 13 and the metal oxide semiconductor layer 6,
Both of 7 and 7 can be heated separately. The catalyst layer heating heaters 14 and 15 in FIG.
The shape may be provided on the membrane surface of the catalyst layer 13 as shown in FIGS. 11 and 12, or may be provided inside the membrane.

[0010]

【Example】

Example 1 For example, in a micro gas sensor having two protruding portions, first, a SnO ₂ thin film having a film thickness of 3000 Å is provided on the two protruding portions. Of SnO ₂ thin film formed, on one of SnO ₂ thin film provided in a thickness of 1000Å of Pt by sputtering, the other SnO ₂ thin film, the metal salt impregnation, the Pt with thickness 10μm Set up. In the metal salt impregnation method, first, H ₂ PtCl ₆ · 6H
_{The 2} O chloride solution is applied to the surface of the SnO ₂ thin film to a predetermined thickness. After coating, dry under reduced pressure for 1.5 hours at 350 ° C
Bake at. In the gas detector thus provided, the heater is heated to 350 ° C. H ₂ , CO, C _{2
H 5 OH, C 6 H 6} , C 7 H 8 , etc., making it very burned, P
The combustion start temperature on the t catalyst is from room temperature to about 130 ° C and CH ₄
Of about 370 ° C, lower than about 200 ° C. Therefore,
The gas detector is heated to 350 ° C. with a heater. As a result, the Pt catalyst layer on the surface of the gas detector also has a thickness of about 35.
It will be heated to 0 ° C. FIG. 13 and FIG. 14 show the gas sensitivity characteristics of various gas detectors for various gases when the operating temperature of the sensor without a catalyst is changed. First, Fig. 1
3, from FIG. 14, when iso-C ₄ H ₁₀ or CH ₄ is used as a detection target gas, H is higher than the gas sensitivity to the gas.
_The gas sensitivity to ₂ and C ₂ H ₅ OH is higher. Or, since they are equivalent, H ₂ , C ₂ H ₅ OH and iso
-C ₄ H _10, separation detection CH ₄ is can be seen that impossible. In the case of coating with a relatively thick Pt catalyst layer having a film thickness of 10 μm, H ₂ and C ₂ H ₅ OH are burned and consumed before reaching the gas detection section due to the combustion reaction on the catalyst layer surface. This is because, as described above, the combustion start temperature of the gas is relatively low at room temperature to 130 ° C., so at 350 ° C.
This is because the catalyst layer is burned almost completely. Therefore, if the gas detection part is coated with a relatively thick catalyst layer,
As shown in FIG. 15, it was found that the gas sensitivity to H ₂ and C ₂ H ₅ OH was not exhibited. On the other hand, film thickness 10
A film coated with a relatively thin Pt catalyst layer of 00Å has gas sensitivity characteristics as shown in FIG. From this figure,
It can be seen that H ₂ , C ₂ H ₅ OH, etc. reach the gas detection portion without being completely burned and consumed in the catalyst layer, and thus have gas sensitivity to some extent. As a result, if the catalyst layers that coat the surfaces of the two SnO ₂ thin films are provided with different film thicknesses, as shown in the above results, the reactivity of the catalyst surface reaction with various gases has a gas selectivity with respect to a certain specific gas. It is possible to Furthermore, if combined with changing the heating conditions (heating temperature, heating pattern) of the heater, a wider range and accurate gas selectivity can be obtained. Further, by analyzing the waveform of the gas output response to various gases by various signal processing methods, more reliable gas selectivity can be obtained.

Embodiment 2 For example, as shown in FIGS. 11 and 12, an isolated catalyst layer is provided via a gas detecting portion having two protruding portions and a cavity. A SnO ₂ thin film with a film thickness of 3000 Å formed by a vapor deposition method is used for the gas detection part, and the catalyst layer is Al ₂ O ₃
A Pt thick film catalyst layer (10 μm) having W (tungsten) provided as a heating heater on the surface is formed on the substrate. Here, the heater for heating the gas detector and the heater for heating the catalyst layer can be operated separately and can be operated at different temperatures. To form the catalyst layer, an H ₂ PtCl ₆ .6H ₂ O solution was prepared to a predetermined concentration, applied on an Al ₂ O ₃ substrate, dried (100 ° C., 1.5 hours), and baked (350 ° C.). To do. The gas detector and the catalyst layer thus produced are assembled as shown in FIGS. Here, the cavity between the gas detector and the catalyst layer is about 5 mm. And, for example, the gas detector is 2
The catalyst layer is heated at 00 ° C and 500 ° C. Then, general flammable and combustible gases (H ₂ , CO, C ₂ H ₅ OH, C
_{6 H 6, C 7 H 8} , iso-C 4 H 10, CH 4 , etc.) are burned off the catalyst layer which is heated by the heating conditions. However, for example, a flame-retardant gas such as Freon does not burn in the catalyst layer and reaches the gas detection unit. in this way,
It can be used as a means to separate flammable gas, flammable gas and flame-retardant gas, and by selecting a wide range of heating temperatures for the gas detection part and catalyst layer, selective detection for a wider variety of gases is possible. It becomes possible to do. Further, as in Example 1, the difference between the combustion start temperatures, such as the case where the gas detection unit and the catalyst layer are heated at the same temperature, is used, and the gas detection unit is heated at the temperature having the highest sensitivity for various gases. When combined with the ones described above, it becomes possible to separate more kinds of gases accurately and almost completely.

[0012]

[Effect] According to the gas sensor of the present invention, by forming the noble metal catalyst layer on the surfaces of the plurality of metal oxide semiconductor layers or by separating the cavities, it is possible to obtain a higher sensitivity than the conventional gas sensor based on the difference in reactivity of gas reaction. Much more accurate gas selective sensing is possible.

[Brief description of drawings]

FIG. 1 shows a conventional gas sensor having a thin gas-sensitive film.
It is an example, (a) is sectional drawing, (b) is a perspective view.

FIG. 2 is a perspective view showing an example of another conventional gas sensor.

FIG. 3 is a plan view showing an example of the gas sensor of the present invention in which a catalyst layer is provided in contact with a metal oxide semiconductor layer.

4A is a sectional view taken along line (A)-(A ′) of FIG. 3, and FIG. 4B is a partially enlarged view thereof.

FIG. 5 is a plan view showing another example of the gas sensor of the present invention provided with a catalyst layer in contact with the metal oxide semiconductor layer.

FIG. 6 is a sectional view taken along line (B)-(B ′) of FIG.
(B) is the elements on larger scale.

FIG. 7 is a plan view showing another example of the gas sensor of the present invention in which a catalyst layer is provided in contact with the metal oxide semiconductor layer.

FIG. 8 is a sectional view taken along line (C)-(C ′) of FIG.
(B) is the elements on larger scale.

FIG. 9 is a plan view showing another example of the gas sensor of the present invention in which the catalyst layer is provided in contact with the metal oxide semiconductor layer.

FIG. 10 is a sectional view taken along line (D)-(D ′) of FIG.
(B) is the elements on larger scale.

11A is a cross-sectional view showing an example of the gas sensor of the present invention in which a catalyst layer is separately provided on a metal oxide semiconductor layer, and FIG. 11B is an enlarged cross-sectional view of a dotted line portion in FIG. Figure, (c)
[Fig. 3] is an enlarged cross-sectional view of a dotted line portion of the (b) diagram.

FIG. 12 (a) is a perspective view of FIG. 11 (a),
(B) is an enlarged top view of the dotted line part of (a) figure.

FIG. 13 is a bar graph showing the sensitivities of various gases measured at a sensor temperature of 350 ° C. using a conventional sensor having no catalyst layer.

FIG. 14 is a bar graph showing the sensitivities of various gases measured at a sensor temperature of 450 ° C. using a conventional sensor having no catalyst layer.

FIG. 15 is a bar graph showing the sensitivities of various gases measured at a sensor temperature of 350 ° C. using the sensor of the present invention using a thick catalyst layer.

FIG. 16 is a bar graph showing the sensitivities of various gases measured at a sensor temperature of 350 ° C. using the sensor of the present invention using a thin catalyst layer.

[Explanation of symbols]

0 Cavity 1 Heat resistant substrate 2 Overhang part 3 Overhang part 4 Heater lead (heater for heating metal oxide semiconductor layer) 5 Heater lead (heater for heating metal oxide semiconductor layer) 6 Metal oxide semiconductor layer 7 Metal oxide semiconductor layer 8 Electrode Lead 9 Electrode Lead 10 Electrical Insulating Film 11 Noble Metal Catalyst Layer 12 Noble Metal Catalyst Layer 13 Noble Metal Catalyst Layer 14 Heater for Heating Catalyst Layer 15 Heater for Heating Catalyst Layer 16 Cavity 20 Heater Film 30 Electrode Pin 40 Base 41 Electrode 42 Electrode 43 Coiled electrode 44 Coiled electrode 51 Gas sensitive film 52 Gas sensitive substance 61 Electric power supply line to heater film 62 Electric power supply line to heater film 71 Signal extraction line of gas sensitive film 72 Signal extraction line of gas sensitive film

Claims (3)

[Claims] 1. A projecting portion made of an electrically insulating material, which is projecting in the air, a plurality of metal oxide semiconductor layers for gas detection provided on the projecting portion, electrode leads in contact with the metal oxide semiconductor layer, and the metal. In a gas sensor having a noble metal catalyst layer provided on an oxide semiconductor layer and a heater lead provided substantially side by side with the electrode lead,
A gas-selective gas sensor, wherein the noble metal catalyst layers provided on the metal oxide semiconductor layer have different film thicknesses. 2. An overhanging portion made of an electrically insulating material provided in the air, a plurality of metal oxide semiconductor layers for gas detection provided on the overhanging portion, electrode leads in contact therewith, and the metal. In a gas sensor having a noble metal catalyst layer provided on an oxide semiconductor layer and a heater lead provided substantially side by side with the electrode lead,
A gas-selective gas sensor, wherein the noble metal catalyst layer is provided on the metal oxide semiconductor layer without being in contact with the metal oxide semiconductor layer, and a heater for heating the noble metal catalyst layer is provided. 3. The gas sensor according to claim 1, wherein the noble metal catalyst layer is a catalyst layer formed by supporting a noble metal on a carrier.