JP3000711B2 - Gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor.
More specifically, the present invention relates to a gas sensor used for city gas and the like.

[0002]

2. Description of the Related Art A gas sensor used for city gas or the like needs to selectively detect methane or hydrogen gas. However, a conventional sensor using a metal oxide semiconductor cannot provide essential selectivity in view of the detection principle, and the addition of a catalyst or the provision of a gas selective permeable membrane mainly requires a gas before the gas to be detected. Processing is in progress. That is, it is necessary to remove the interfering gas by such a method and to provide selectivity.

[0003] However, there is a limit to the improvement of selectivity by such a method, and it is impossible to discriminate between similar gases such as methane and ethane, and methanol and ethanol.

On the other hand, in order to perform such gas identification,
There is also an attempt to use a plurality of gas sensors having different gas detection capabilities and use their output patterns. However, in this case, since a plurality of sensors having different properties are required, the device or the data processing circuit becomes complicated, and it is inevitable that the cost becomes extremely high.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a gas sensor provided with an improved gas selectivity and a gas discriminating ability formed by a single gas sensor.

[0006]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
This is achieved by a gas sensor in which a gas diffusion limiting film having a film thickness gradient is laminated on a gas-sensitive film covering a plurality of pairs of electrodes formed on an insulating substrate.

Next, the present invention will be described with reference to the drawing whose plan view is shown in FIG. 1 and whose sectional view is shown in FIG.

On an insulating substrate 1 in which an inorganic material such as quartz, alumina, glass or the like is used in the form of a plate, a sheet, or the like, a plurality of sets, generally 3 to 5 sets of electrode pairs are provided. The three electrode pairs 2,2,3,3,4,4
Approximately 5 to 5 by paste printing and firing from gold, platinum, etc.
It is formed with a thickness of 0 μm, preferably about 20 to 50 μm.

[0009] These electrode pairs are covered with a gas-sensitive film 5. As the gas-sensitive film, a thin film of an oxide semiconductor such as SnO ₂ , ZnO, or Fe ₂ O ₃ having a thickness of about 0.1 to 1 μm formed by a CVD method, a sputtering method, or the like is generally used.

On this gas sensitive film, a gas diffusion limiting film 6 having a film thickness gradient is laminated. Gas diffusion limiting membrane also serves as an insulating _{film, SiO 2, AlN, Si 3} N 4, Al 2 O 3 paste printing or the like, about the like baking or CVD 0.1~2μm
It is formed with a film thickness of about.

At this time, the difference in thickness of the gas diffusion limiting film on the electrode pair (2, 2) on one end and the electrode pair (4, 4) on the other end is about
It must be set to be at least 00 nm. That is, when the difference in film thickness is about 500 nm or less, no significant difference is observed in the electric conductivity between each pair of electrodes at the time of gas detection.

A method of forming such a film thickness gradient is generally performed by a method as shown in FIG. That is, a susceptor 13 having a wall portion 12 is installed in a chamber 11 for a CVD method, and insulating substrates 14 and 15 on which a gas-sensitive film-coated electrode pair group is formed are mounted thereon. If, for example, an SiO ₂ gas diffusion limiting film is to be laminated, SiH ₄ -O ₂
When the CVD method is applied using the raw material gas of the system, the CV gradually increases in thickness from the center of the susceptor to the wall.
A D film is formed. Specific examples of the film thickness gradient are as shown in 14 'and 15'.

The outer diameter of the susceptor used here is, for example,
When the thickness is 150 mm, the height of the susceptor wall is preferably about 3 to 10 mm. In this case, when the thickness at the center is 1200 nm, the thickness at the outer periphery is 1800 nm [see FIG. ]. Further, when a plurality of substrates are actually set on the susceptor, for example, a 10 mm square substrate is radially arranged to form an SiO ₂ film having a film thickness gradient.

[0014]

According to the gas sensor of the present invention, a plurality of pairs, generally three to five pairs of electrodes are formed in one sensor, the conductivity between each pair of electrodes is measured, and the data is processed to perform detection. The type of gas is determined and the concentration is measured.On the gas-sensitive film covering the electrode pairs, a gas diffusion limiting film with a film thickness gradient is laminated, As a result, the conductivity between the electrode pairs differs even for a gas having a constant concentration. By comparing the conductivity pattern at this time with the conductivity patterns of various gases stored in advance, the type and concentration of the detection gas can be known, and this can be performed between similar gases. .

[0015]

The gas sensor according to the present invention has a higher selectivity than the conventional method because the concept of pattern recognition is introduced. In addition, since a plurality of sensors are not required and the same gas discriminating ability is provided on one gas sensitive body, it is possible to reduce the size and cost of the sensors.

[0016]

EXAMPLE On an alumina substrate, paste printing was used.
3 sets of platinum electrode pairs 20μm facing each other
A SnO ₂ gas-sensitive film having a uniform thickness of 200 nm was formed by a CVD method so as to cover them.
This was mounted on a susceptor (outer diameter 150 mm, wall height 5 mm) installed in a chamber for the CVD method, and the CVD method was applied using a SiH ₄ —O ₂ source gas. As a result, an SiO ₂ gas diffusion limiting film having a thickness difference of 1.4 μm was formed between the electrode pair on one end and the electrode pair on the other end.

Using the gas sensor manufactured as described above, the thickness of the SiO ₂ gas diffusion limiting film was 0.1 μm and 0.25 μm, respectively.
When the device resistance of the electrode pair covered with the μm and 1.5 μm SiO ₂ gas diffusion limiting films was measured, the results shown in FIG. 3 were obtained for ethanol and methanol.

[Brief description of the drawings]

FIG. 1 is a plan view of one embodiment of a gas sensor according to the present invention.

FIG. 2 is a cross-sectional view of one embodiment of a gas sensor according to the present invention.

FIG. 3 is a graph showing a relationship between a film thickness of an SiO ₂ gas diffusion limiting film of a gas sensor and an element resistance.

FIG. 4 is a schematic view showing a method for forming a gas diffusion limiting film having a film thickness gradient.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Insulating substrate 2 Electrode pair 3 Electrode pair 4 Electrode pair 5 Gas sensitive film 6 Gas diffusion limiting film

Claims (2)

(57) [Claims] 1. A gas sensor comprising a gas-sensitive film covering a plurality of pairs of electrodes formed on an insulating substrate and a gas diffusion limiting film having a film thickness gradient laminated thereon. 2. The gas sensor according to claim 1, wherein the difference between the thickness of the gas diffusion limiting film on the electrode pair on one end side and the electrode pair on the other end side is set to 500 nm or more.