JPH0348643B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a humidity sensor made of a zinc oxide thin film containing lead and lithium.

Conventional electrical resistance humidity sensors include humidity sensors made of an organic film made by cross-linking and polymerizing a solid electrolyte with a polymeric material on a comb-shaped electrode, and humidity sensors using porous metal oxide porcelain. It is being

However, the former humidity sensor made of an organic film has drawbacks such as not being able to be used at high temperatures and the electrolyte leaching out in an environment with long-term dew condensation.

Humidity sensors using porous metal oxide porcelain have an excellent humidity response in the initial stage, but when used for a long time, water molecules penetrate deep into the pores and become chemically adsorbed on the surface. Resistance increases.

In order to regenerate this, cleaning by heating is required, and a complicated mechanism called a heater control circuit is required.

On the other hand, humidity sensors using zinc oxide have also been well known. This consists of forming a thin film of zinc oxide on an insulating substrate such as ceramic or glass, and further forming a pair of detection electrodes on this.

This humidity sensor has a characteristic that its resistance value changes when the humidity of the atmosphere changes, and is characterized by quick response.

However, although it is important that this resistance value be within a practical measurement range, in the case of zinc oxide, the elements used to control the resistance value within this practical measurement range are not well known.

Moreover, the humidity sensor using this zinc oxide thin film is mainly an n-type semiconductor with a metal excess of zinc oxide.

In order to form this n-type semiconductor by a sputtering method, a method of sputtering with a reduced amount of oxygen can be considered, but when the amount of oxygen is reduced, a sputtered film of zinc metal suddenly forms at a certain limit. However, it was difficult to control this condition.

In view of the above, the present inventors conducted studies to obtain a humidity sensor using a zinc oxide thin film that has a resistance value within the practical measurement range.If the humidity sensor is made of a zinc oxide thin film containing lead, They discovered that the resistance value could be easily reduced and that humidity could be measured continuously without cleaning by heating, and they filed a patent application.

However, it has been found that although lead alone has the effect of lowering the resistance, it has drawbacks in its responsiveness to humidity, particularly in its long recovery time.

Therefore, as a result of continuing studies to improve this response, we found that adding lithium along with lead would have the effect of lowering the resistance value, and would also shorten the response to humidity, especially the recovery time. It has been discovered that a humidity sensor can be obtained.

That is, in the humidity sensor of the present invention, the humidity sensing element is made of a zinc oxide thin film containing lead and lithium, and as a result, (1) it is self-resetting and does not require cleaning; 2) It has a fast response, (3) It has a resistance value within a range that is easy to carry out practical measurements, and (4) It has excellent durability.

The humidity sensor of the present invention will be explained in detail below.

FIG. 1 is a schematic plan view showing a row of humidity sensors according to the present invention, in which 1 is an insulating substrate made of ceramic, glass, etc., and a pair of comb-shaped detection electrodes 2, 3 are mounted on this substrate 1. is formed,
Furthermore, a zinc oxide thin film 4, which is a moisture-sensitive element, is formed to cover the comb tooth portion.

5 and 6 are terminals connected to the detection electrodes 2 and 3, respectively. Although not shown, it may be a porous detection electrode formed on the zinc oxide thin film 4 in a mesh or comb shape, or a bulk type with upper and lower parallel electrodes, and the electrodes may be made of Au, Ni,
A conductive material such as Cr, Ti, Cu, or Fe may be formed by a mask evaporation method or the like.

Methods for forming the zinc oxide thin film 4 include, for example, a sputtering method, a vacuum evaporation method, an ion plating method, etc., and a reactive sputtering method is particularly suitable because of its ease of formation.

In this invention, the usage range of lead and lithium to be contained in the zinc oxide thin film, which is a moisture-sensitive element, is 0.1 to 20 at% for lead, preferably 0.5 to 5 at%, and 0.1 to 20 at% for lithium. Preferably it is 0.5 to 2 atomic %.

The reason for this is that when lead is less than 0.1 atomic%, the resistance value is
10 ⁸ Ω or more, exceeding the practical measurable range, and 20 Ω or more.
If lithium is used in an amount higher than 0.1 atomic percent, the resistance change with humidity will become small, which is undesirable.
Below atomic %, there is no effect of addition, and above 20 atomic %, the resistance value increases and no effect can be obtained when used in combination with lead, and both are inappropriate as a humidity sensor.

Here, the combination of lead and lithium, which is the moisture-sensitive element of the present invention, will be explained. Adding lead lowers the resistance value, but the response and reproducibility are unfavorable. However, when both lead and lithium are added as levers, the response and reproducibility are improved. This is because when lead is added to zinc oxide, the resistance value decreases, but when lithium is further added to this, when the lithium combined with zinc oxide comes into contact with water,
Because it immediately dissociates into mobile ions, it is thought that in the presence of water, resistance decreases based on ion conduction, resulting in better current flow and quick response characteristics.
Furthermore, it is presumed that the small ionic radius combined with the high ion mobility of lithium enhances the property of moving quickly with respect to water, resulting in improved reproducibility.

However, since the resistance value increases with the addition of lithium, it is first necessary to adjust the addition ratio of lead and lithium so that the practical resistance value is 1 MΩ or less at 40% humidity. Therefore, the amounts of both used are 3 to 4 atomic percent of lead and 0.5 to 1 atomic percent of lithium in the zinc oxide thin film.
The most preferable amount is about atomic percent.

In addition to lithium, there are other alkali metals such as sodium and potassium, but they are difficult to use as humidity sensors for the following reasons.
In other words, lithium (the ionic radius of lithium is
0.70), the ionic radius is larger (the ionic radius of sodium is 1.00, the ionic radius of potassium is 1.33), and its mobility is not as good as that of lithium. Therefore, the responsiveness and reproducibility are poor, and the humidity sensor of the present invention cannot be put to practical use.

The present invention will be explained in detail below using one example.

Example A comb-shaped detection electrode made of gold with a comb-teeth spacing of 0.5 mm and a facing length of 65 mm was formed on an alumina substrate.

Furthermore, a zinc oxide thin film containing lead and lithium was formed on top of this by a sputtering method so as to cover the comb tooth portions.

This zinc oxide thin film was formed using metallic zinc as a target in the following manner.

An alumina substrate on which a comb-shaped detection electrode was formed was placed on the anode side of the sputtering device. On the other hand, on the cathode side, metal zinc is mixed with 3 atomic percent lead in advance, and a press-molded carbon lithium (Li ₂ CO ₃ ) containing 1 atomic percent in terms of lithium is placed on the alloy obtained by melting the mixture. The object was placed as a target.

Then, the vacuum pressure in the sputtering chamber was increased to 5×
A mixed gas consisting of oxygen (O ₂ ) and argon (Ar) in a ratio of 1:1 was introduced into the sputtering chamber, and the pressure in the chamber was maintained at ³ ×10 ⁻⁴ Torr.

After that, the exhaust valve was adjusted so that the sputtering chamber pressure was 1.5 × 10 ^-1 Torr, and a 300W high-frequency power source (13.56MHz) was supplied to deposit lead and lithium on an alumina substrate with a comb-shaped detection electrode. A 1 μm thick zinc oxide thin film containing .

In the above embodiment, metal zinc was used as the target and a zinc oxide thin film was formed by the reactive sputtering method, but it is also possible to use a sintered body of zinc oxide as the target. In this case, the zinc oxide sintered body is mainly made of zinc oxide powder, and contains 3 at.% in terms of lead.
Pb ₃ O ₄ and lithium carbonate (Li ₂ CO ₃ ) of 1 atomic % in terms of lithium were added, the mixture was thoroughly ground and mixed, and then calcined at 500 to 600°C, and then press-formed at 700 to 1300°C. It can be obtained by firing.

Further, it is preferable to use a mixed gas of Ar:O ₂ =9:1 in the sputtering chamber in reactive sputtering when this zinc oxide sintered body is targeted.

In the above embodiment, a pair of comb-shaped detection electrodes were formed on an insulating substrate such as alumina, and a zinc oxide thin film was formed thereon by sputtering. In addition, the structure is not limited to this structure, but there is also a structure in which a zinc oxide thin film is formed by sputtering on an insulating substrate such as alumina, and a pair of comb-like detection electrodes are formed on it, or a conductive structure. A zinc oxide thin film is formed by sputtering on a substrate such as a stainless steel or aluminum plate, and a comb-like detection electrode, an island-like vapor-deposited film of gold, or a porous silver paste electrode is formed on top of the zinc oxide thin film, and the gap between the upper and lower electrodes is It may also be of a structure for measuring resistance values.

The thus obtained zinc oxide thin film based on the sputtering method contains many defective structures on the surface when it is formed, and gases etc. are easily adsorbed.
Time annealing was performed. This annealing makes the resistance value of the zinc oxide thin film somewhat higher than when the film was formed, but the change in resistance value over time becomes small.

The humidity sensor of the present invention functions as a humidity sensor at low temperatures, but functions as a gas sensor at high temperatures of 300 to 500 degrees Celsius. Therefore, when measuring gas concentration, the sensing element is heated, and when measuring relative humidity, it is heated. It is also possible to use it at room temperature.

The response characteristics of the humidity sensor of the present invention obtained in the above example are shown in FIG. ₃ Expressing the change in the relative humidity reading (%) calculated from the relative humidity-electrical resistance curve when the saturated salt solution is taken out into the atmosphere with a relative humidity of 55%, it is found that the humidity sensor A of the present invention contains only lead. It was observed that the response was faster than humidity sensor B, which uses the added zinc oxide thin film as a humidity-sensitive element.

In addition, when sodium and potassium are contained in zinc oxide with lead instead of lithium,
Humidity sensor A of this invention and humidity sensor B of comparative example
It showed intermediate characteristics.

Furthermore, when the self-resetting property of the humidity sensor of the present invention was measured by the change in electrical resistance with respect to relative humidity, the results shown in FIG. 3 were obtained, and it was recognized that the reproducibility was good.

Note that the solid line in the figure is that of the humidity sensor A of the present invention, and the broken line is that of the humidity sensor B of the comparative example.

[Brief explanation of drawings]

FIG. 1 is a schematic plan view showing an example of a humidity sensor according to the present invention, FIG. 2 is a graph showing response characteristics, and FIG. 3 is a relative humidity-electrical resistance characteristic diagram. 1... Insulating substrate, 2, 3... Detection electrode, 4...
Zinc oxide thin film.

Claims (1)

[Claims] 1. A humidity sensor characterized in that the humidity sensing element is made of a zinc oxide thin film containing lead and lithium. 2. Claim 1, characterized in that the amount of lead contained in the zinc oxide thin film is 0.1 to 20 atomic %.
Humidity sensor as described in section. 3 The amount of lithium contained in the zinc oxide thin film is 0.1
The humidity sensor according to claim 1, wherein the humidity is 20 atomic %.