JP3000726B2 - 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 using a tin oxide film as a gas-sensitive film.

[0002]

2. Description of the Related Art Conventionally, metal oxide sintered films such as tin oxide films and zinc oxide films have been actually used as gas-sensitive films for gas sensors. These gas-sensitive films are generally used by adding a noble metal catalyst such as platinum or palladium in order to improve the sensitivity. By the way, in the conventional metal oxide sintered body gas sensor, since the noble metal catalyst is added to the whole sintered body, not all the noble metal catalysts effectively act on gas detection.
The noble metal catalyst present on the surface of the metal oxide sintered body is
If left in the air under high temperature conditions, oxidation, aggregation and the like are caused to impair the stability over time, which also causes the output of the gas sensor to change over time.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a noble metal catalyst-added sintered metal oxide gas sensor which does not impair the stability over time even when left in the air under high temperature conditions. To provide.

[0004]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
In a gas sensor in which a tin oxide film is formed to cover a pair of electrodes formed on an insulating substrate, prior to the formation of the tin oxide film, Pd / (Pd + S
This is achieved by a gas sensor in which palladium is added in advance in an amount of 1 to 50% as a surface addition rate indicated by the atomic ratio of n).

After masking a set of electrode forming portions formed on the insulating substrate, palladium is added to the insulating substrate excluding the portions by a vacuum deposition method, a sputtering method, or the like. The amount of addition is controlled by a film thickness monitor, and is an amount that gives a ratio of 1 to 50% as a surface addition rate represented by an atomic ratio of Pd / (Pd + Sn).

That is, palladium on the insulating substrate is formed as a discontinuous layer, and a tin oxide film is subsequently formed on the discontinuous portion. At this time, palladium is added in advance in such an amount that the surface addition ratio becomes 1 to 50%, preferably 10 to 20%, and the addition of palladium at a higher ratio rather lowers the gas sensitivity.

Thereafter, a tin oxide film is formed so as to uniformly cover the insulating substrate so as to cover the electrode-formed portion from which the mask has been removed and the discontinuous palladium layer.

In general, a tin oxide film having a thickness of about 500 to 2000 ° is formed by a method of forming a metal tin film by a vacuum deposition method, a sputtering method and the like, followed by an oxidation treatment, and a reaction of the metal tin with oxygen. It is carried out by a method of sputtering, a method of oxidizing an organic tin compound plasma polymerized film having a metallic luster (Japanese Patent Application Laid-Open No. 63-261,148), or the like.

[0009]

As described above, in a gas sensor having a tin oxide film formed as a gas-sensitive film, palladium is added at a specific rate to the surface of the insulating substrate in contact with the tin oxide film, thereby stabilizing with time at high temperatures. A gas sensor with improved performance can be obtained. Further, it is easy to control the surface addition rate, in other words, to control the gas sensitivity. This gas sensor is effectively used for detecting gases such as ethylene and carbon monoxide. Further, all of the noble metal catalysts to be added are effectively used for gas detection.

[0010]

Next, the present invention will be described with reference to examples.

Comparative Example A tin oxide film having a thickness of about 1000 ° was formed by a sputtering method on an alumina substrate on which a pair of gold electrodes had been formed. Palladium was deposited on the tin oxide film by a vacuum deposition method, and Pd was deposited at a surface addition ratio of 20%. P
d The addition ratio was measured by ESCA, and the measured analysis depth of ESCA was as small as several tens of millimeters, and since Sn was detected by ESCA, it was confirmed that the underlying SnO ₂ was also exposed on the surface. , Indicating that the entire surface is not covered by Pd.

EXAMPLE After masking a gold electrode forming portion on an alumina substrate having a pair of gold electrodes formed thereon, palladium was vapor-deposited by vacuum vapor deposition so as to have a surface addition ratio of 20%. I let it. After removing the masking, a tin oxide film having a thickness of about 1000 ° was formed by a sputtering method.

As described above, the gas sensitivities of the gas sensors of Examples and Comparative Examples having a tin oxide film doped with Pd having a surface addition rate of 20% as measured by X-ray photoelectron spectroscopy were measured with respect to 1000 ppm of ethylene. . At the time of measurement, the sensor element was heated to 250 ° C, 300 ° C, 350 ° C or 400 ° C by a thin film heater provided on the back side of the insulating substrate, and the resistance value after 4 hours was taken as the resistance value Rair in air. The resistance was measured under the test gas as Rgas, and the ratio Rair / Rgas of the two was calculated as gas sensitivity.

The results obtained are shown in the following table.

Claims (1)

(57) [Claims] In a gas sensor in which a tin oxide film covering a pair of electrodes formed on an insulating substrate is formed, prior to forming the tin oxide film, Pd is formed on the insulating substrate other than the electrode forming portion. A gas sensor in which palladium is added in advance in an amount of 1 to 50% as a surface addition rate represented by an atomic ratio of / (Pd + Sn).