JPS5847021B2 - Manufacturing method of gas detection element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a gas sensing element mainly composed of a metal oxide semiconductor thin film whose conductivity changes upon contact with a reducing gas.

It has been well known that metal oxides, such as 5n021 ZnO, have the property of increasing conductivity when they come into contact with and adsorb reducing gases, etc., and this property can be used to detect gases of metal oxides. The element is used as a gas leak detector such as propane gas.

However, these elements have shortcomings such as low sensitivity to carbon monoxide and slow response speed, and are completely useless for early detection of carbon monoxide generated by incomplete combustion of fuel, etc. Ta.

In view of the above, the present inventors conducted intensive research and found that when creating a metal oxide semiconductor thin film by gas introduction vapor deposition method, the above oxide was created by controlling the vapor deposition rate, and an appropriate thickness was deposited on the oxide. By providing a Pt layer or a Pd layer, it was possible to obtain an excellent gas detection element that has the same high sensitivity to carbon monoxide as to other combustible gases and has a fast response speed. .

In the manufacturing method according to the present invention, when metals such as Sn and Zn are vapor-deposited on electrically insulating substrates, a relative humidity of 70% is used in addition to a mixed gas of oxygen and an inert gas such as a rare gas such as argon or krypton. In an atmosphere containing a large amount of water vapor so that is 2~4λ/s
5n02. Forming a film of a reduced metal oxide semiconductor such as ZnO, and further providing a catalyst layer of a film of Pt, Pd, etc. of 2 to 50 people, preferably 4 to 10 people, on the metal oxide semiconductor film. It is a feature.

In the present invention, the reason why the atmospheric pressure is limited to 1 O-1 Torr or less is that if it exceeds 10'' Torr, the deposition efficiency decreases and the heating boat is likely to be damaged, especially when vapor deposition is performed using a resistance heating method. This is because the adhesion between the substrate and the oxide film decreases, and the resistance value changes over time.

The reason why it is preferably 10-3 to 1 O-1 Torr is that if the pressure is less than 10'' Torr, the oxidation of the vapor-deposited metal during vapor deposition tends to be insufficient.

In the present invention, the reason why the relative humidity is limited to 70% or more is that if the relative humidity is 70% or less, cracks tend to occur in the deposited film.

In the present invention, the reason why the oxygen capacity is limited to 10 to 30% is that if the oxygen content is less than 10%, the oxidation of the evaporated metal during evaporation becomes insufficient and the gas detection ability decreases.
If the oxygen content is 30% or more, oxidation progresses too much during vapor deposition, resulting in excessive film resistance and a decrease in gas detection ability.
This is because cracks are likely to occur in the film when heated at high temperatures.

In the present invention, the thickness of the catalyst layer of Pt, Pd, etc. is 2 to 5.
The reason why the number is limited to 0 is that if the film thickness is outside the range of 2 to 50, the sensitivity to reducing gases, especially the sensitivity to carbon monoxide, will be significantly reduced.

In the present invention, the Sn vapor deposition rate is 1.4 to 7 persons/se.
The reason why the range of c is preferable is that at 1.4 A/sec or less, cracks tend to occur in the 5n02 film due to the manufacturing process.
The film resistance becomes unstable, and conversely, if it exceeds 7λ/sec, the sensitivity to reducing gases, especially the sensing function for carbon monoxide gas, will drop significantly, and the response speed will also become slow, making it unsuitable for gas detection elements. It is.

Next, an embodiment of the manufacturing method of the present invention will be described in detail.

First, after evacuating the inside of the vacuum container to 1 to 5 X 10-5 Torr, 02 with a capacity of about 21 and Argon with a capacity of about 79
A mixed gas of water vapor and water vapor with a relative humidity of 90% or more was introduced, the pressure inside the vacuum container was set to 2X10-2 Torr, and metal Sn was evaporated at a rate of 1.4 by resistance heating in a tungsten boat.
It was deposited at ~7 people/sec, l.

At this time, the atomic Sn particles combine with oxygen while reaching the element substrate, become an unsaturated oxide SnO, and adhere to the heat-resistant electrically insulating substrate, forming a tin oxide film mainly composed of SnO.

Next, Sn is completely oxidized by heat treatment at 500 to 600° C. in air to form a SnO2 film.

Next, reduce the pressure inside the vacuum container to 1 to 5 x 10'Torr.
Then, metal Pt was deposited on the 8n02 film to a thickness of 2 to 50 mm by resistive heating of a tungsten filament.

A schematic cross-sectional view of the gas sensing element thus obtained is shown in FIG.

In the figure, 1 is a heat-resistant and electrically insulating substrate, such as an alumina silicate glass substrate (Corning #70
59,0.7mrrbt X 1iiX 5mm),
2 is a SnO2 thin film (thin film thickness: 0.1 μm), 3 is a catalyst (
Pt ) layer, 4 and 5 are lead wires such as platinum wires provided at both ends of the surface of the catalyst layer, and 6 and 7 are baking gold paste for fixing the lead wires.

In addition, on the back surface of the substrate 1, in order to raise the temperature of the substrate 1 to an appropriate temperature, a resistive film of, for example, S n 02 ((film thickness approximately 0
．． 1 μm) is provided, and current-carrying lead wires 9, 10 are attached to both ends of the heater 8 with baked metal pastes N1, 1.
Connected by 2.

As the resistive thin film 8, a semiconductor thin film such as ZnO or a platinum thin film can also be used.

Next, the gas detection characteristics of the element shown in FIG. 1 will be explained.

The relationship between the thickness of the deposited Pt film and the value (RN 1 /RO) representing the change in conductivity was investigated for gas sensing elements fabricated by the method of the present invention by varying the thickness of the deposited layer of metal PT.

Here, RNl is the carbon monoxide gas concentration in the air, N=
The electrical resistance value between the electrodes 4 and 5 of the device when N=Oppm is 500 ppm, and Ro is the electrical resistance value between the device electrodes 4 and 5 when N=Oppm.

In this test, the deposition rate of metal Sn was 3 people/3 people/
sec, and the element temperature was set at about 350°C.

From this result, it was found that the case without a Pt layer showed almost no sensitivity to carbon monoxide, and the thickness of the Pt evaporated film was 2 to 50 mm.
It has been found that preferably about 4 to 10 people exhibit high sensitivity to carbon monoxide.

Furthermore, according to the present invention, for the case (a) with a Pt deposition film thickness of about 5 to 8 layers, and the case (b) without a Pt layer,
The relationship between the Sn deposition rate V and the (RNI/RO) value (7) was investigated.

In this test, carbon monoxide gas concentration N = 500
ppm, and the element temperature was set at about 350°C.

From this result, in the case of the conventional method without providing a Pt layer, Sn
Although the gas detection sensitivity hardly increases even if the deposition rate is changed, in the present invention in which a Pt layer is provided, the Sn deposition rate is 1.4 to 7 people/sec, preferably 2 to 4 people/sec.
It was found that in the range of /sec, high sensitivity to carbon monoxide gas is exhibited.

Next, test results of the relationship between the (RNlRo)-1 characteristic and the gas concentration N for two types of elements a' and b' will be shown.

Here, element a' has a Sn evaporation rate v = 2.5 people/s
ec and P have a deposited film thickness of 5 to 8 A, and element b' has no pt deposited film.

Further, RN is the inter-electrode resistance value of the element at a gas concentration of Nppm, and Ro is the resistance value between the element electrodes in air.

Regarding the type of gas, the second one is for carbon monoxide gas.
Fig. 3 shows the case for city gas, and Fig. 4 shows the case for ethanol gas.

In either case, the element temperature is set at about 350°C.

What can be seen from Figure 2 is that element b
' exhibits almost no sensitivity, whereas element a' exhibits high sensitivity, for example, when carbon monoxide gas concentration N = 500
ppm, it reached about 15 times that of element b'.

Moreover, element a' has gas concentration N=50 ppm (RN/
RO)'' = 2.2, indicating that it has sufficient detection ability even for low concentration carbon monoxide gas.

It can be seen from FIG. 3 that element b' has sufficient sensitivity to city gas, but has lower sensitivity than element a'. For example, when the city gas concentration N = 500 ppm, (RN/
RO)-1, element b' is 4.5, while element a
' is 25.0, and the ratio is 5.6 times higher.

It can be seen from FIG. 4 that although element b' clearly has sensitivity to ethanol gas, it is low compared to element a', for example, when the ethanol gas concentration N=500.
The (RN/RO) -1 value at ppm is 2 for element b', while element a' is 9, and has a sensitivity about 4.5 times higher.

Next, the response speeds of the above elements a' and b' were measured.

Here, as a method of expressing response speed, when an element is placed in a gas atmosphere with a constant concentration, the change from the element resistance R8 in air to the saturation value Ra of the element resistance in gas, as shown in Figure 5. The resistance value when the resistance value changes by 90% of minutes (Ro Ra) is defined as Rb, and the response time τ is the time taken from when the element is placed in the gas atmosphere until the resistance value reaches Rb.

Closed. Table 1 shows the response time τ9 when elements a' and b' are placed in a carbon monoxide gas concentration N = 500 ppm.
Indicates o.

From this table, it can be seen that the element a' according to the method of the present invention is different from the conventional element b'
It can be seen that the response time is about 1/4 faster than that of .

The relationship between the Sn deposition rate v (A15ec) and the response time τ, 0 is determined by the thickness of the Pt layer of 5 to 8 A, and the carbon monoxide gas concentration of 500
ppm, and the device temperature was 350° C. The results show that the response time strongly depends on the deposition rate, and the response time is faster as the Sn deposition rate is lower.

Generally, in a gas detection element, it is important that the gas sensitivity and gas response speed be fast.

In order to increase the response speed, it is usually considered to increase the element temperature, but increasing the element temperature tends to cause changes and deterioration of element components such as thin films, electrodes, and substrates over time, which may reduce reliability. Problems that are disadvantageous in practice, such as reduction in power consumption and power consumption for heating, become serious.

Next, metal Sn for forming a metal oxide semiconductor thin film
Value (R1/Ro) for vapor deposition rate (A15ec)
-' relationship was actually measured.

Table 2 shows the 1j data.

The manufacturing conditions are carbon monoxide gas concentration N = 500 ppm, P
The thickness of the t-layer was 5 to 8, and the element temperature was 350°C.

As is clear from this data, the Sn deposition rate is 1.4~
It shows high sensitivity to carbon monoxide gas at 7λ/sec.

Next, while keeping the Sn deposition rate at V=3λ/sec,
Carbon monoxide gas concentration N = 500 ppm, element temperature 3
Under manufacturing conditions of 50°C, we actually measured the value (RN/R□) -1 when the thickness of the Pt film formed on the semiconductor thin film was varied in the range of 0 to 100. The results are shown in Table 3. I got it.

Based on this data, Pt thin film 2 to 50A1 preferably 4
It can be seen that high sensitivity to carbon monoxide gas is exhibited when the number of people is 10.

As another embodiment of the present invention, metal Z is used instead of metal Sn.
Similar effects were obtained using n.

Furthermore, the same effect was obtained even when Pd was used instead of Pi.

As is clear from the above test results, according to the present invention, Sn is deposited on an insulating substrate in an atmosphere of 10' Torr or less containing a mixed gas of inert gas and oxygen and water vapor with a relative humidity of 70% or more. , metal such as Zn at a deposition rate of 1.4 to 7
A metal oxide semiconductor thin film is formed by vapor deposition with SeC,
Furthermore, by providing an extremely thin soot layer of Pi or Pd on top of this, the sensitivity to reducing guns, especially carbon monoxide gas, can be increased and the gas response speed can be increased compared to conventional methods. The sensing element can be easily manufactured.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a device manufactured according to the present invention. 2 to 4 are characteristic diagrams showing the test results of the device according to the present invention. FIG. 5 shows the response time τ. FIG. 1... Board, 2...
S n O2 thin film, 3... Soot layer, 4.5...
····electrode.

Claims (1)

[Claims] 1. A wet gas containing water vapor with a relative humidity of 70% or more is introduced into a mixed gas of 70 to 90 volumes of inert gas and 10 to 30 volumes of oxygen, and the atmospheric pressure is 10 to 1 Torr or less. In a method for manufacturing a gas sensing element in which a semiconductor thin film is formed by vapor-depositing a main metal on the surface of an electrically insulating substrate in an atmosphere, the metal is vapor-deposited at a vapor deposition rate of 1.4 to 7 [person 15 ec]. 1. A method for manufacturing a gas sensing element, comprising: forming a metal oxide semiconductor thin film, and then providing a Pt or Pd catalyst layer of 2 to 50 A on the metal oxide semiconductor thin film.