JPH04348267A - Gas sensor - Google Patents

Abstract

PURPOSE:To simply detect the leakage of specific gas by using gas responsive substance layers being metal oxide semiconductor films mutually different in particle size and/or density. CONSTITUTION:Detecting regions are provided to the overhangs 2, 3 composed of an electric insulating material on a substrate 1. At least a pair of detecting leads 8, 9, the heater leads 4, 5 arranged in parallel to said leads 8, 9 and metal oxide semiconductor films 6, 7 being the gas responsive substance layers coming into contact with the leads 8, 9 are provided to the detecting regions. The semiconductor films 6, 7 generate the contact reaction of gases and the change of the resistance values thereof is measured to detect the gases. At this time, the semiconductor films 6, 7 on the overhangs 2, 3 are mutually different in particle size and/or density and the gases sensed by said films are detected. By changing only the particle size and/or density of the semiconductor films 6, 7, gas selectivity can be simply provided.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor for detecting the presence of gas in an atmosphere, and more particularly to a gas sensor suitable for use as a gas leak alarm for LP gas or city gas.

0002

2. Description of the Related Art Gas sensors are sensitive to various gases (isobutane, propane, ethane, methane, ethanol, propylene, toluene, xylene, methanol, hydrogen, carbon monoxide, etc.) due to their nature. Therefore, when it is desired to detect a specific gas, noise caused by other gases becomes a big problem. For example, in the case of city gas, it is desirable for household gas sensors (gas leak alarms) to react only to methane gas, but since most gas leak alarms are installed near the ceiling, they react only to water vapor. It is easily affected by various types of steam and miscellaneous gases. Although alcohol is the miscellaneous gas that is most likely to be generated in the home, general gas sensors have high sensitivity to alcohol as well. For this reason, in order to give gas selectivity to a gas sensor, a filter is usually added or the operating temperature is changed, but this poses problems in terms of cost and reliability.

0003

[Object] An object of the present invention is to provide a gas sensor that can detect a specific gas leak without adding a conventional filter or taking troublesome measures such as changing the operating temperature.

0004

[Structure] The present invention provides a substrate, a protruding portion made of an electrically insulating material provided on the substrate to protrude into the air, and a detection area provided on the protruding portion separated from each other by a predetermined distance. at least one pair of detection leads, at least one heater lead provided on the overhang and substantially parallel to the detection leads, and at least one heater lead located in the detection area and connected to each of the at least one pair of detection leads. A gas sensor having a gas-sensitive material layer provided in contact with the gas and detecting a gas by measuring a change in resistance value when the gas-sensitive material layer reacts with the gas by contacting the gas sensor, wherein a plurality of the projecting portions are provided. and the gas-sensitive material layers on each overhang are comprised of metal oxide semiconductor films having different grain sizes and/or densities from each other, or at least one of them, from the others. or a metal oxide semiconductor, in which a plurality of gas sensitive material layers are present on one of the overhangs, at least one of which has a different particle size and/or density from the others. The present invention relates to a gas sensor having gas selectivity characterized by being made of a membrane. Incidentally, the present inventors formed a gas sensor with a plurality of gas-sensitive films, and also formed a gas sensor with a plurality of gas-sensitive films (metal oxide semiconductor films).
It was confirmed that by changing the particle size and/or density of the nanoparticles, selectivity in gas sensitivity can be achieved. The present invention has been made based on this. The present invention will be described in more detail below with reference to the accompanying drawings, but to put it simply, the gas sensor of the present invention comprises at least two gas-sensitive metal oxide semiconductor films having different particle sizes and/or densities. The gases sensed by the respective gas-sensitive material layers are detected.

FIG. 1 shows a gas sensor according to the present invention.
This is an example of a structure in which a crosslinked structure is adopted as an overhang. 1A is a plan view, and FIG. 1B is a sectional view taken along the line X-X'. Here, two projecting parts 2 and 3 made of an electrically insulating material are provided on a substantially square-shaped substrate 1 as a floating bridge structure. The number of overhangs may be any number as long as it is two or more. For the substrate 1, a material that can be easily undercut etched and does not deform even at high temperatures, such as Si, Al, Cu, Ni, Cr, etc., is used, and preferably a Si (100) plane is used. (100
) is used because a known anisotropic etching solution can be used when performing undercut etching. The outer dimensions of the substrate 1 are approximately 1 to 4 mm square, and the appropriate thickness is approximately 0.1 to 1 mm. On the overhanging parts 2 and 3 formed by undercut etching of the substrate 1, heater leads 4 and 5, metal oxide semiconductor films 6 and 7 which are gas sensitive material layers, and their detection leads 8 and 9 are disposed. It is formed. Preferable materials for the heater leads 4 and 5 and the detection leads 8 and 9 are Pt, Au, and the like. FIG. 2 shows an example in which two gas-sensitive material layers are present on one overhang, and the number of gas-sensitive material layers may be any number as long as it is two or more. The metal oxide semiconductor films 6 and 7 are made of the same metal oxide, but have different grain sizes or densities. Examples of the metal oxide semiconductor in the present invention include oxides of tin, zinc, iron, titanium, indium, nickel, tungsten, cadmium, vanadium, etc. Among them, it is most desirable to use tin oxide. The structure shown in FIG. 3 is similar to that shown in FIG. 1 except that the overhanging portions 2 and 3 have a cantilever structure. 4 is similar to the one in FIG. 2 except that the overhang portion has a cantilever structure.

Various methods can be considered for forming the metal oxide semiconductor films 6 and 7, such as sputtering and vacuum evaporation.
It is advantageous to use a thin film deposition apparatus (Japanese Patent No. 1,571,203). Now, the method for forming a tin oxide film as a gas-sensitive film using this thin film deposition apparatus will be described as follows. When forming a tin oxide film, one of Sn, SnO, and SnO2 is used as the evaporation substance. That is, for example, Sn is selected as the evaporation substance, it is held as an evaporation source, the degree of vacuum in the vacuum chamber is set in advance to the order of 1/104 Pa, oxygen gas is introduced into the vacuum chamber, and the pressure is set to, for example, 0.1 Pa. Keep it at a moderate level. In this state, for example, the counter electrode is brought to zero potential, a potential of 100 V is applied to the grid, and a power of 400 W is applied to the filament. When power is applied to the evaporation source in accordance with a desired film formation rate, a portion of the evaporated Sn is ionized and strongly combines with oxygen, forming a tin oxide film on the substrate. FIG. 5 shows the relationship between the gas sensitivity of the tin oxide film formed by this method to methane and ethyl alcohol and the film formation rate. The gas concentration is 3000 ppm and 10 ppm, respectively, relative to air.
00 ppm, and the temperature of the gas sensitive membrane is 450°C. In the figure, Ra is the in-air resistance value of the gas-sensitive membrane, and Rg is the in-gas resistance value of the gas-sensitive membrane. Next, a method for imparting selectivity to methane and ethyl alcohol will be described. The gas sensitive film 6 is formed at a deposition rate of 5 Å/sec, and the gas sensitive film 7 is formed at a deposition rate of 30 Å/sec. Thereafter, annealing is performed at 700° C. in an oxygen atmosphere. 5Å/sec
The gas sensitive film 6 formed at 30 Å/sec is sensitive to both methane and ethyl alcohol, but the gas sensitive film 7 formed at 30 Å/sec is sensitive only to ethyl alcohol. That is, if both the gas sensitive membranes 6 and 7 change in resistance value, ethyl alcohol is present, and if only the gas sensitive membrane 6 causes a change in resistance value, only methane exists. Electron micrographs of the cross section and surface of the tin oxide semiconductor films 6 and 7 are shown in FIGS. 6 and 7. The tin oxide film formed at a rate of 5 Å/sec is an aggregate of fine particles, the particle size is approximately several hundred Å, and the density thereof is relatively small. 30Å/s
The grain size of the tin oxide film formed by EC is as large as about 1000 Å, and its density is also relatively large. It can thus be seen that the particle size or density varies greatly depending on the film formation rate. This is because the reaction with oxygen can be controlled by changing the evaporation rate of Sn, which is the evaporation material. The method for forming the metal oxide semiconductor films 6 and 7 is not limited to changing the film formation rate. For example, using SnO2 as the evaporation material of the metal oxide semiconductor film 6,
The reaction between tin and oxygen in each of the metal oxide semiconductor films 6 and 7 may be controlled by using Sn as an evaporation material. Alternatively, the reaction may be controlled by varying the amount of oxygen introduced into the vacuum chamber.

[0007]

[Effects] According to the present invention, a gas sensor having gas selectivity can be easily obtained by simply changing the particle size and/or density of each of a plurality of gas-sensitive membranes.

[Brief explanation of drawings]

FIG. 1 shows a first example of a gas sensor of the present invention, (a)
is a plan view thereof, and (b) is a sectional view taken along line X-X' of (a).

FIG. 2 shows a second example of the gas sensor of the present invention, (a)
is a plan view thereof, and (b) is a sectional view taken along line X-X' of (a).

FIG. 3 shows a third example of the gas sensor of the present invention, (a)
is a plan view thereof, and (b) is a sectional view taken along line X-X' of (a).

FIG. 4 shows a fourth example of the gas sensor of the present invention, (a)
is a plan view thereof, and (b) is a sectional view taken along line X-X' of (a).

FIG. 5 is a graph showing the relationship between the gas sensitivity of the gas-sensitive film to methane and ethyl alcohol and the film formation rate.

FIG. 6 is an electron micrograph of a tin oxide film (after annealing) formed at 5 Å/sec.

FIG. 7 is an electron micrograph of a tin oxide film (after annealing) formed at 30 Å/sec.

[Explanation of symbols]

1 Board 2 Overhang part 3 Overhang part 4 Heater lead 5 Heater lead 6 Metal oxide semiconductor film 7 Metal oxide semiconductor film 8 Detection lead 9 Detection lead 10 Insulating film 0 Hollow

Claims (2)

[Claims] 1. A substrate, an overhang made of an electrically insulating material provided on the substrate to extend into the air, and at least a pair of overhangs provided at a detection area provided on the overhang and separated from each other by a predetermined distance. a detection lead, at least one heater lead located on the overhang and provided substantially parallel to the detection lead, and a heater lead located in the detection area and in contact with each of the at least one pair of detection leads. A gas sensor having a gas-sensitive material layer provided with a gas-sensitive material layer and detecting a gas by measuring a change in resistance value when the gas-sensitive material layer contacts and reacts with a gas,
A metal oxide semiconductor film in which a plurality of the overhangs exist, and the gas-sensitive material layers on each overhang have different particle sizes and/or densities from each other or at least one of them from the others. A gas sensor having gas selectivity characterized by the following. 2. A substrate, a protruding portion made of an electrically insulating material provided on the substrate and protruding into the air, and at least a pair of protruding portions provided on the protruding portion and separated from each other by a predetermined distance. a detection lead, at least one heater lead located on the overhang and provided substantially parallel to the detection lead, and a heater lead located in the detection area and in contact with each of the at least one pair of detection leads. A gas sensor having a gas-sensitive material layer provided with a gas-sensitive material layer and detecting a gas by measuring a change in resistance value when the gas-sensitive material layer contacts and reacts with a gas,
A plurality of gas-sensitive material layers are present on one of the overhangs, and at least one of them is made of a metal oxide semiconductor film having a different grain size and/or density from the others. Gas sensor with gas selectivity.