JPH07104309B2 - Gas sensor manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a method for manufacturing a gas sensor.

(Prior Art) Many examples of gas sensors using a metal oxide semiconductor as a gas-sensitive material have been proposed. For example, when an n-type semiconductor such as zinc oxide, tin oxide, or indium oxide is used, the gas is detected by utilizing the fact that its resistance decreases due to contact with a reducing gas. On the contrary, when a p-type semiconductor is used, it is used that the resistance increases due to contact with a reducing gas. However, this resistance is unlikely to decrease or increase only with a semiconductor, and for example, a catalyst layer made of a porous metal oxide supporting a noble metal or a metal oxide is used by being stacked on the semiconductor. In this case, for example, a method is used in which a paste is prepared from alumina fine powder carrying a metal oxide and a basic aluminum chloride aqueous solution, and the paste is applied and fired.

2. Description of the Related Art Recently, a flat-plate gas sensor using a thick film technology has been attracting attention for the purpose of downsizing gas sensors, integration for improving selectivity and multi-functionality, automation of manufacturing process, and reduction of variations. The following technologies have been proposed as elemental technologies for flattening the gas sensor.

(1) A ceramic multi-layer substrate with a built-in heating element in which a heater and an insulating film are formed by thick film printing, the resistance change of a semiconductor due to contact with a reducing gas, even if the above contact layer or sensitizer is used, Since it is small and slow at room temperature, it is essential to heat the element with a heating element. In the case of the flat plate type, the heating element is often formed in a film shape on the back surface of the substrate to be integrated, but in order to enhance the thermal effect and improve mass productivity, the heating element, the insulating film, and the opposing surface are provided on the substrate surface. A structure in which an electrode and a gas sensitive film are laminated has been proposed.

(2) Forming a metal oxide semiconductor thin film sensitive to a reducing gas by screen printing.

Gas-sensitive metal oxide semiconductor thin film (film thickness 1 μm or less)
Gas sensors formed by thermal decomposition of organometallic compounds have been widely put into practical use. This is one in which a thin film of an organometallic compound is formed by the lift-off method, and then a metal oxide thin film is obtained by thermal decomposition. In order to facilitate the pattern formation of such a thin film, a screen-printable paste has been devised, and it has become possible to form a metal oxide thin film by screen printing.

In the flat-plate type gas sensor using the above technique, the contact layer is also required to be patterned by screen printing or the like. However, as described above, the system composed of the fine alumina powder and the basic aluminum chloride aqueous solution is not suitable as a paste for screen printing because of its high volatility of water. Further, when considering an organic system including an organic solvent, there are problems in the viscosity of the paste, thixotropy, film strength after firing, gas sensitivity and the like.

(Problems to be Solved by the Invention) The present invention has been made in view of the difficulty in print formation of a catalyst layer made of a porous metal oxide in the prior art, and rheological properties such as viscosity and thixotropy and It is an object of the present invention to obtain a method for producing a gas sensor, which can form a catalyst layer of a porous metal oxide having good characteristics by printing using an appropriate screen-printable paste such as solvent volatility.

[Configuration of the Invention] (Means for Solving the Problem) In the method for manufacturing a gas sensor according to the present invention, a step of attaching a heater material to the surface of a flat insulating substrate, and the heater material on the surface of the insulating substrate. A step of providing an insulating film on a region including a region where the counter electrode is provided, a step of providing a counter electrode on the insulating film, and a gas sensor on a predetermined region including a region where the counter electrode is provided on the insulating film. A step of providing a metal oxide semiconductor film as described above, and at least one kind of oxide fine powder selected from the group consisting of aluminum, silicon and zirconium in which a catalyst is supported on the metal oxide semiconductor film in the form of an oxide, A step of forming a porous catalyst layer by printing using a paste containing a solvent, a cellulosic polymer and at least one organometallic compound selected from the above group, and then firing the paste. It is characterized by comprising comprises a.

(Operation) According to the present invention, a film made of a porous metal oxide can be formed by printing by using a paste having good printability by the above-described means, and thus a miniaturized high-sensitivity / high-selectivity gas sensor. Can be provided with high mass productivity. That is, according to the means as described above, it is possible to obtain a printable paste having appropriate viscosity, rheological properties such as thixotropy, and volatility of the solvent, and using a paste containing the metal oxide fine powder and the organometallic compound. When an organic film is printed by using the above method, dried and then baked, the organometallic compound is thermally decomposed and converted into an oxide. At this time, a bond is formed with the fine metal oxide powder to act as a binder to form a strong porous metal oxide film. This porous metal oxide film functions as a catalyst that promotes gas adsorption / desorption at the interface with the underlying semiconductor film,
By making gas permeation into the semiconductor film have selectivity, this gas sensor is made to have high sensitivity and high selectivity.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is an explanatory view of the structure of an embodiment of the present invention. For example, a counter electrode 6, a counter electrode contact pad 9, and a heating element contact pad 4 are provided on the surface of an insulating substrate 1 such as an alumina substrate, and a tin oxide semiconductor film or the like is provided on the counter electrode 6 as an upper layer. The metal oxide semiconductor thin film 7 is formed, and the porous metal oxide film 8 is provided on the semiconductor thin film 7. A heating element lead wire 10 is attached to the heating element contact pad 4, and a counter electrode lead wire 11 is attached to the counter electrode contact pad 9.

Next, a method of manufacturing the gas sensor will be described with reference to FIGS. 2 and 3. FIG. 2 is a flow chart of the manufacturing process, and FIG. 3 is an explanatory diagram of the manufacturing process seen from the I-I cross section of FIG.

First, in step 201, a thick film heating element 2 made of 70% by weight of platinum and 30% by weight of tungsten is provided on the surface of the insulating substrate 1 (see FIG. 3 (a)).

Next, in step 202, the insulator 3 of alumina is formed on top of it.
Is provided in a film shape. At that time, in order to form the contact pads 4 on both ends of the heating element, through holes 5 are provided at the corresponding portions of the insulator 3 (see FIG. 3B).

In step 203, a pair of counter electrodes 6 made of a thick gold film are provided on the film insulator 3 (see FIG. 3C).

Subsequently, in step 204, a metal oxide semiconductor thin film 7 such as a tin oxide semiconductor thin film is formed on the counter electrode 6 (see FIG. 3D).

The semiconductor thin film 7 is formed by screen printing as follows. A homogeneous solution paste containing a mixture of organometallic compounds of hexoate tin and niobium resinate (100: 1 atomic ratio) and containing ethyl hydroxyethyl cellulose and turpentine oil was prepared and printed in a predetermined pattern at 120 ° C. After drying for 15 minutes at 600 ℃
Bake for 10 minutes on the electric path. The organometallic compound in the paste is thermally decomposed to obtain the metal oxide semiconductor thin film 7.

In step 205, at least one oxide fine powder selected from the group consisting of aluminum, silicon, and zirconium in which a catalyst is supported on the semiconductor thin film 7 in the form of an oxide, a solvent, a cellulosic polymer, and the above group are selected. After printing using a paste containing at least one kind of organometallic compound, the paste is fired to form a porous catalyst layer, that is, a porous metal oxide film 8 (see FIG. 3 (e)). .

This porous metal oxide film 8 is formed as follows. First, a predetermined amount of fine alumina powder is weighed, an aqueous solution of ammonium tungstate, for example, is added thereto to form a paste, and the mixture is stirred for a certain period of time. After this, dry
Bake. Next, an aqueous copper sulfate solution is added to this, and the same operation as above is repeated. As a result, a catalyst supporting oxides of tungsten and copper is obtained (hereinafter referred to as a catalyst).

The paste for printing the organic coating containing fine metal oxide powder is prepared as follows. First, the base polymer of the paste, ethyl hydroxyethyl cellulose, is added to turpentine oil heated to 60 to 80 ° C. and stirred. When it is sufficiently dissolved, aluminum resinate is added and mixed well, and further mixed with the catalyst in a mortar to obtain a paste. The composition of this paste is 30 wt% catalyst, 20 wt% aluminum resinate, 47 wt% turpentine oil, 3 wt% ethyl hydroxyethyl cellulose. This is printed in a predetermined pattern, dried at 120 ° C. for 15 minutes, and then baked at 500 ° C.

After forming the porous metal oxide film 8 as described above, in step 206, in the case of a large number of substrates, it is divided into each chip, and then the heat generating compact pad 4 and the counter electrode contact pad 9 are respectively divided. The leads 10 and 11 are attached and the chip is mounted (see FIG. 3 (f)).

Table 1 shows the sensitivities (values obtained by dividing the resistance value in the air by the resistance values in the gas) of the gas sensor manufactured as described above with respect to various gases of 500 ppm. The element temperature is set to 400 ° C. by adjusting the voltage applied to the heating element. In addition, as a conventional example, data of a sensor manufactured using a paste obtained by mixing the same catalyst as the above with a basic aluminum chloride aqueous solution is shown.

From Table 1, it is clear that the sensor according to the embodiment of the present invention has substantially the same sensitivity as the conventional sensor.

According to the conventional example in which a lead wire is bonded and then a low-viscosity paste containing a catalyst is applied, for example, since porous alumina and the lead wire are in contact with each other, acidic alumina is used in a high humidity atmosphere. May accelerate lead wire corrosion.

By patterning the porous metal oxide film by printing, this fear is reduced.

Regarding the amount of the organometallic compound when forming the porous metal oxide film, for example, when the amount of aluminum resinate is too small (when it is less than 0.5% by weight), the film strength of the porous alumina is too small for practical use. On the contrary, if the amount is too high (15% by weight or more), the alumina becomes dense, and the sensitivity and responsiveness as a sensor are too low. Therefore, neither case is suitable.

[Advantages of the Invention] As described above, according to the present invention, when forming a porous metal oxide film, the rheological properties such as viscosity and thixotropy are good by selecting the polymer and solvent of the paste used for printing, and the solvent It is possible to obtain a printable paste having low volatility, and since the paste contains an organometallic compound, a strong porous metal oxide film can be formed. Further, the porous metal oxide film formed as described above is made to have high sensitivity and high selectivity by using this gas sensor. Furthermore, not only that, but also because the thickness and pore distribution can be controlled by printing, variations in characteristics can be reduced. In addition, since all film forming processes can be performed by printing, mass productivity can be significantly improved.

[Brief description of drawings]

FIG. 1 is a plan view showing a gas sensor manufactured by a manufacturing method according to the present invention, FIG. 2 is a flow chart of structural steps of one embodiment of the present invention, and FIG. 3 is a sectional view taken along the line I-I of FIG. It is sectional drawing of each manufacturing process. 1 ... Insulating substrate, 2 ... Heating element, 3 ... Insulator, 4 ...
Heater contact pads, 5 ... through holes, 6 ...
... Counter electrode, 7 ... Metal oxide semiconductor thin film, 8 ... Porous metal oxide film, 9 ... Counter electrode contact pad,
10 …… Heating element lead wire, 11 …… Counter electrode lead wire.

Claims (1)

[Claims] 1. A step of mounting a heater material on a surface of a flat insulating substrate, a step of providing an insulating film on a surface of the insulating substrate including an area where the heater material is provided, and the insulating film. A step of providing a counter electrode thereon, a step of providing a metal oxide semiconductor film as a gas sensitizer on a predetermined region including a region where the counter electrode is provided on the insulating film; In addition, at least one kind of oxide fine powder selected from the group of aluminum, silicon and zirconium, which is supported in the form of an oxide, a solvent, a cellulosic polymer and at least one kind of organometallic compound selected from the above group And a step of forming a porous catalyst layer by firing the paste after printing using the paste containing the gas sensor.