JPH04164243A - Gas sensor - Google Patents

Abstract

PURPOSE:To detect a specific gas leakage by providing a plurality of sensitive material layers on one and the same overhang part, and by forming at least one of the layers from a metal oxide semiconductor film having a different composition ratio of metal and oxide. CONSTITUTION:An overhang part 2 made of electrically insulating materials is laid on a substrate in a bridging structure. A heater lead 3, metal oxide semiconductor films 4, 5 and detecting leads 6, 7 are formed on the overhang part 2. In this arrangement, the semiconductor films 4, 5 are made of the same metal oxides such as tin oxide, zinc oxide or the like. With this arrangement, it is obtained a gas sensor having a gas selectively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a gas sensor that detects 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.

[Prior art]

Due to their nature, gas sensors can handle various gases (isobutane, propane, ethane, methane, ethanol, propylene, toluene, xylene, methanol, hydrogen, carbon oxide, etc.)
Sensitive to.

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 there are problems in terms of cost and reliability. In order to solve these problems, the applicant has already proposed a method of using multiple metal oxide semiconductor films with different composition ratios of metal and oxygen (
JP-A No. 2-1.34551). However, since they have only one metal oxide semiconductor film on one overhang, they require multiple overhangs, supplying current to all those heater leads, and metal oxide semiconductor film on each overhang. Since it is necessary to heat the semiconductor film, power consumption is relatively large.

[Objective] 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.

〔composition〕

The present invention relates to a circuit board, and an electric cable provided on the board extending into the air! an overhang made of edged material; at least a pair of detection leads located in a detection area provided on the overhang and separated from each other by a predetermined distance; and located on the overhang and substantially parallel to the detection leads; and a gas sensitive material layer provided in the detection region and in contact with each of the at least one pair of detection leads, the gas sensitive material layer In a gas sensor that detects gas by measuring the change in resistance value caused by contact reaction with gas, there are multiple gas-sensitive material layers on the same overhang, and at least one of them is made of other gas-sensitive material layers. The present invention relates to a gas sensor having gas selectivity characterized by being made of metal oxide semiconductor films having different composition ratios of metal and oxygen.

Incidentally, the present inventors formed a gas sensor with a plurality of gas-sensitive films, and by changing the composition ratio of metal and oxygen in the plurality of gas-sensitive films (metal oxide semiconductor films), the inventors achieved selectivity in gas sensitivity. I made sure that it was delivered. The present invention has been made based on this.

The present invention will be explained in more detail below with reference to the accompanying drawings, but to put it simply, the gas sensor of the present invention comprises metal oxide semiconductor films having different composition ratios of metal and oxygen to at least two gases. The gases are used as sensitive material layers, and the gases sensed by the respective gas sensitive material layers are detected. Moreover, since the structure has multiple gas-sensitive material layers on the same overhang, it is only necessary to heat one overhang, which makes it possible to reduce power consumption compared to the technology disclosed in JP-A-2-134551. Ta.

FIG. 1 shows an example of a gas sensor according to the present invention in which a crosslinked structure is adopted. Here, an overhanging portion 2 made of an electrically insulating material is placed on a substrate 1 having a substantially square shape.
It is set up as a bridge structure that looks like it is floating in the air.

The substrate 1 is made of a material that is easy to undercut and does not deform even at high temperatures, such as Si or Al.

Cu, Ni, Cr, etc. are used, preferably S(10
0) plane is used. The reason for using the (100) plane is
This is because a known anisotropic etching solution can be used when performing undercut etching.

The external dimensions of the substrate 1 are approximately 1 to 4 mm square, and its thickness is 0.
．． Approximately 1 to 1+ nm is appropriate.

On the overhang 2 formed by undercut etching of the substrate 1, there are formed heater leads 3, metal oxide semiconductor films 4, 5 which are gas sensitive material layers, and their detection leads 6.7. . The material of the heater lead 3 and the detection lead 6.7 is preferably PttAu or the like.

As mentioned earlier, the metal oxide semiconductor films 4.5 are made of the same metal oxide, but have different composition ratios of metal and elements. The metals used here are tin, zinc, iron, titanium, and indium.

Examples include nickel, tungsten, cadmium, vanadium, etc. Among them, it is most desirable to use tin.

The structure shown in FIG. 2 is similar to that shown in FIG. 1 except that the overhang 2 has a cantilever structure.

Various methods can be considered for forming the metal oxide semiconductor films 4 and 5, such as sputtering and vacuum evaporation. 53351).The method for forming a tin oxide film as a gas-sensitive film using this thin film deposition apparatus will now be described as follows.

] When forming a tin oxide film, Sn and S are used as evaporation substances.
Either n○ or SnO2 is used as the evaporative substance. For example, Sn is selected as the evaporation substance, held as an evaporation source, the degree of vacuum in the vacuum chamber is set in advance to the order of 10-' Pa, oxygen gas is introduced into the vacuum chamber, and the pressure is set to, for example, 0.
Maintain around IPa.

In this state, for example, set the counter electrode to zero potential, apply a potential of 100 V to the grid, and apply 400 W to the filament.
Gives power. When power is applied to the evaporation source according to 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.

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 was investigated in the third section.
As shown in the figure. Gas concentration is 3000PP each compared to air
m, 11000PP, and the temperature of the gas sensitive membrane is 45
It is 0°C. In the figure, Ra is the air resistance value of the gas-sensitive membrane;
Rg is the resistance value of the gas sensitive membrane in gas.

Next, we will discuss how to make it selective to methane and ethyl alcohol. In FIGS. 1 and 2, the gas-sensitive film 4 is formed at a deposition rate of 10 A/see.
The film formation rate is 20A/5ee=7-. The gas sensitive film 4 formed at 10 A/see is sensitive to both methane and ethyl alcohol, but 2
The gas sensitive film 5 formed at 0 A/see is sensitive only to ethyl alcohol. That is, if both the gas sensitive membranes 4 and 5 change in resistance value, ethyl alcohol is present, and if only the gas sensitive membrane 4 causes a change in resistance value, only methane exists.

In the above example, the tin oxide films that are the metal oxide semiconductor films 4 and 5 have different deposition rates, so the composition ratios of tin and oxygen are different (film formation The higher the speed, the higher the proportion of tin). The conditions for forming the metal oxide semiconductor films 4 and 5 are not limited to the difference in film formation speed; for example, S as the evaporation material for the metal oxide semiconductor film 4
The composition ratio of tin to oxygen in each of the metal oxide semiconductor films 4 and 5 may be changed by using nO2 and using Sn as the evaporation material for the metal oxide semiconductor film 5. do not have.

In addition, in Figures 1 and 2, gas-sensitive films with different composition ratios of metal and oxygen (however, the metals used here have different composition ratios of metal and oxygen and are of the same type). ) are two, but it goes without saying that there may be three or more.

Note that the number of overhangs may be one or more.

〔effect〕

According to the present invention, a gas sensor having gas selectivity can be easily obtained by simply changing the composition ratio of at least one other metal to oxygen among a plurality of gas-sensitive films, and moreover, they can be mounted on the same overhang. By forming it, power consumption can be reduced.

[Brief explanation of the drawing]

FIG. 1 shows an example of a gas sensor according to the present invention, in which (a) is a plan view thereof, and (b) is an end view taken along line xx' in FIG. 1 (a). FIG. 2 shows another example of the gas sensor according to the present invention, (a
) is a plan view thereof, and (b) is an end view taken along line X-x' in FIG. 2(a). FIG. 3 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. ■... Substrate 2... Overhang 3... Heater lead 6.7... Detection lead 8... Insulating film 0/Cavity Patent applicant Rico Co., Ltd. Figure 1 (C1) 1 Figure (b) X' Figure 2 (b) Film formation rate (A/sec)

Claims (1)

[Scope of Claims] 1. A substrate, an overhang made of an electrically insulating material provided on the substrate to overhang in the air, and a detection area provided on the overhang, 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 each of the at least one pair of detection leads located in the detection area. In a gas sensor that has a gas-sensitive material layer provided in contact with the gas, and detects gas by measuring a change in resistance value caused by the contact reaction of the gas-sensitive material layer with the gas, gas-selectivity characterized in that there are a plurality of gas-sensitive material layers, at least one of which is made of a metal oxide semiconductor film having a different composition ratio of metal and oxygen from the other layers. Gas sensor with. 2. The metal oxide semiconductor is tin, zinc, iron, titanium,
The gas sensor according to claim 1, wherein the gas sensor is an oxide of a metal selected from the group consisting of indium, nickel, tungsten, cadmium, and vanadium.