JP2010185774A - Membrane gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane gas sensor not largely fluctuated in its characteristics even if used as a battery driving type gas sensor. <P>SOLUTION: In the membrane gas sensor constituted so that a pair of electrodes are formed on an electric insulating layer, a gas sensing layer 7 comprising a semiconductor membrane is formed on the electrodes and a selective combustion layer 8 is formed on the uppermost surface of the gas sensing layer 7, a catalyst diffusion preventing layer 9 is provided between the gas sensing layer 7 and the selective combustion layer 8 so as not to diffuse the noble metal catalyst of the selective combustion layer 8 to the gas sensing layer 7 even under a high-humidity environment to prevent the sensor characteristics of the membrane gas sensor from being largely changed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a low power consumption thin film gas sensor with battery driving in mind, and more particularly to an improvement thereof.

In general, a gas sensor is used for applications such as a gas leak alarm, and is a device that selectively responds to a specific gas, such as CO, CH ₄ , C ₃ H ₈ , and CH ₃ OH. In addition, high sensitivity, high selectivity, high response, high reliability, and low power consumption are indispensable. By the way, the gas leak alarms that are widely used for household use include those for the purpose of detecting flammable gases for city gas and propane gas, and those for the purpose of detecting incomplete combustion gases in combustion equipment, or There are some that have both functions, but the penetration rate is not so high due to cost and installation problems.

  Under such circumstances, in order to improve the penetration rate, it is desired to improve the installation property, specifically, to be battery-driven and cordless. Low power consumption is the most important for realizing battery drive, but in a catalytic combustion type or semiconductor type gas sensor, it is necessary to detect it by heating it to a high temperature of 200 ° C to 500 ° C. Therefore, the appearance of a thin film gas sensor having a high heat insulation and low heat capacity structure such as a diaphragm structure by a microfabrication process is awaited.

  However, when a semiconductor thin film is used for the sensing film or sensing layer (referred to as the sensing layer), the sensing layer alone is sensitive to a plurality of reducing gas species, and may be selectively sensitive to a specific gas. Can not. Therefore, it is effective to provide a selective combustion layer made of a noble metal catalyst such as Pd or Pt on the sensing layer and burn a gas having a stronger oxidation activity than the detection gas. From such a viewpoint, for example, a thin film gas sensor having a selective combustion layer as shown in Patent Documents 1 and 2 has appeared.

JP 2000-298108 A Japanese Patent No. 3898999

  By the way, in order to realize low power consumption with a battery-driven gas sensor, an intermittent operation with a duty ratio (ratio of time during which the heater is turned on) of about 1/300 to 1/100 is required. Therefore, in a high-humidity environment, the moisture adsorbed on the sensing layer and the selective combustion layer during the OFF time that is sufficiently longer than the ON time remains without being separated by short-time heating, and the moisture remains in the selective combustion layer. The noble metal catalyst such as Pd or Pt supported on the surface of the catalyst is liberated, the noble metal catalyst diffuses to the surface of the gas sensing layer through moisture, and the characteristics of the sensor greatly vary.

  Accordingly, an object of the present invention is to provide a gas sensor whose characteristics do not vary greatly even when used as a battery-driven gas sensor.

According to the first aspect of the present invention, a pair of electrodes are formed on the electrical insulating layer, a gas sensing layer made of a semiconductor thin film is formed thereon, and selective combustion comprising a porous metal oxide and a noble metal catalyst is further formed on the outermost surface. In a thin film gas sensor with a layer formed,
A catalyst diffusion prevention layer is provided between the selective combustion layer and the gas sensing layer.
In the first aspect of the present invention, a porous layer having an electric resistivity higher than that of the gas sensing layer can be used as the catalyst diffusion preventing layer (the second aspect of the present invention).

In the invention of claim 1 or 2, the porous metal oxide is a metal such as Al ₂ O ₃ , Cr ₂ O ₃ , Fe ₂ O ₃ , Ni ₂ O ₃ , ZnO, or SiO ₂ that does not contain a noble metal catalyst. An oxide is used and Pd or Pt can be used as a noble metal catalyst (the invention of claim 3), and in the inventions of claims 1 to 3, the semiconductor thin film is SnO ₂ , In ₂ O _3. Metal oxides such as ZnO and TiO ₂ can be used (Invention of Claim 4).

  According to the present invention, since the porous catalyst diffusion preventing layer having a higher electrical resistivity than the gas sensing layer is provided between the selective combustion layer and the gas sensing layer, it is stable and reliable even in a high humidity environment. It is possible to obtain a thin film gas sensor with high performance.

FIG. 1 shows a general sectional structure of a thin film gas sensor to which the present invention is applied.
As shown in the figure, a Si ₃ N ₄ and SiO ₂ film is sequentially applied to the Si substrate (Si wafer in the figure) 1 with a thermal oxide film on both sides as a support layer of a diaphragm structure and a thermal insulating layer 2 by plasma CVD. Form. Next, the Pt—W heater layer 3 and the SiO ₂ insulating layer 4 are formed in this order by sputtering. A bonding layer 5, a sensing layer electrode 6, and a sensing layer 7 are formed thereon. Film formation is performed by an ordinary sputtering method using an RF magnetron sputtering apparatus. The film formation conditions are the same for the bonding layer (Ta or Ti) 5 and the sensing layer electrode (Pt or Au) 6, Ar gas pressure 1 Pa, substrate temperature 300 degrees, RF power 2 W / cm ² , film thickness is bonding layer 5 / Sensing layer electrode 6 = 500 mm / 2000 mm.

Next, SnO ₂ which is the sensing layer 7 is formed. Film formation is performed by a reactive sputtering method using an RF magnetron sputtering apparatus. For the target, SnO ₂ having 0.1 weight percent (wt%) of Sb is used. The film formation conditions are Ar + O ₂ gas pressure 2 Pa, substrate temperature 150 to 300 ° C., RF power 2 W / cm ² , and film thickness 0.4 μm. The size of the sensing layer 7 is preferably about 50 to 200 μm square and the thickness is preferably about 0.2 to 1.6 μm.
<Example 1>
As shown in Fig. 2A, the same weight of ethylene glycol monoethyl ether was added to γ-alumina (average particle size 2 to 3 µm) to form a paste, and 5 µm thick catalyst diffusion prevention just above SnO ₂ as the sensing layer 7 The layer 9 is formed by screen printing, and the solvent component is further dried by a drier kept at 100 ° C. After the catalyst diffusion preventing layer 9 is sufficiently dried, the selective combustion layer 8 is formed.

  The selective combustion layer 8 has a thickness obtained by adding the same weight of ethylene glycol monoethyl ether to γ-alumina (average particle diameter of 2 to 3 μm) to which 7.0 wt% of Pd is added, and further adding 5 to 20 wt% of a silica sol binder. 30 μm is formed by screen printing and then baked at 500 ° C. for 1 hour. However, the selective combustion layer 8 has a diameter larger than that of the catalyst diffusion preventing layer 9 so as to sufficiently cover the catalyst diffusion preventing layer 9.

<Example 2>
As the catalyst diffusion barrier layer 9, followed after gas sensing layer 7 formed under the same conditions as SnO ₂ is sensitive membrane of metal oxides _as Al ₂ O ₃ and SiO ₂ (SiO ₂ in FIG. 2B), The same effect can be obtained by using an RF magnetron sputtering apparatus with a thickness of 0.2 μm as shown in FIG. 2B. Further, in both Examples 1 and 2, the catalyst diffusion preventing layer is a porous layer having a higher electrical resistivity than the gas sensing layer, so that the noble metal catalyst layer of the selective combustion layer can be used even in a high humidity environment. It is desirable not to diffuse into the gas sensing layer.

In addition, after forming the catalyst diffusion prevention layer 9 and the selective combustion layer 8 in Examples 1 and 2, Si is removed from the back surface of the substrate, and the diaphragm structure as shown in FIG. is there. In addition to Al ₂ O ₃ and SiO ₂ , porous metal oxides such as Cr ₂ O ₃ , Fe ₂ O ₃ , Ni ₂ O ₃ and ZnO are used as the porous layer of the catalyst diffusion preventing layer 9. The same effect can be obtained.

FIG. 3 shows an example of changes in sensor resistance in 3000 ppm of methane before and after energization for 10 days at high humidity (40 ° C. and 80% RH). Here, the change in sensor resistance after energization is logarithmically expressed as a ratio based on the sensor resistance value before energization.
In the comparative example of FIG. 2C in which the selective combustion layer uniformly contains Pd 7.0 wt%, the sensor resistance greatly increases before and after the high-humidity energization as shown by “x”, whereas the catalyst diffusion prevention layer is provided. In Examples 1 and 2 shown in FIGS. 2A and 2B provided, it can be seen that the sensor resistance hardly changes (see “◯” mark and “●” mark). That is, Examples 1 and 2 show that stable sensor characteristics can be obtained even when energized in high humidity. In high humidity, only the comparative example of FIG. 2C shows an increase in sensor resistance because the catalyst that has diffused in the vicinity of SnO ₂ has an adverse effect on the gas sensing layer.

Sectional drawing which shows the gas sensor used by this invention Sectional drawing which shows 1st Example of this invention Sectional drawing which shows 2nd Example of this invention Sectional view showing a conventional example having no catalyst diffusion preventing layer Characteristics chart for explaining the gas sensor according to the present invention in comparison with the conventional example

  DESCRIPTION OF SYMBOLS 1 ... Si substrate (Si wafer), 2 ... Support layer and thermal insulation layer, 3 ... Heater layer, 4 ... Insulation layer, 5 ... Bonding layer, 6 ... Sensing membrane electrode, 7 ... Sensing layer, 8 ... Selective combustion layer, 9: Catalyst diffusion prevention layer.

Claims (4)

In a thin film gas sensor in which a pair of electrodes are formed on an electrically insulating layer, a gas sensing layer made of a semiconductor thin film is formed thereon, and a selective combustion layer made of a porous metal oxide and a noble metal catalyst is formed on the outermost surface. ,
A thin film gas sensor, wherein a catalyst diffusion preventing layer is provided between the selective combustion layer and the gas sensing layer. The thin film gas sensor according to claim 1, wherein a porous layer having a higher electrical resistivity than the gas sensing layer is used as the catalyst diffusion preventing layer. As the porous metal oxide, metal oxides such as Al ₂ O ₃ , Cr ₂ O ₃ , Fe ₂ O ₃ , Ni ₂ O ₃ , ZnO, and SiO ₂ that do not contain a noble metal catalyst are used, and Pd is used as the noble metal catalyst. Or it has Pt, The thin film gas sensor of Claim 1 or 2 characterized by the above-mentioned. The thin film gas sensor according to claim 1, wherein a metal oxide such as SnO ₂ , In ₂ O ₃ , ZnO, or TiO ₂ is used as the semiconductor thin film.

Images