JPS59231802A - Moisture sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

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

Conventional electrical resistance humidity sensors include a solid-state type jI? on a comb-shaped electrode. 2. Description of the Related Art Humidity sensors made of an organic film formed by cross-linking and polymerizing a polymeric material, or humidity sensors using porous metal oxide porcelain are known.

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.

Additionally, humidity sensors using porous gold-41 porcelain have an excellent humidity response initially, but when used for a long time, water molecules penetrate deep into the pores and become chemically adsorbed. 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 fast response.

However, it is important that this resistance value be within a practical measurement range, but in the case of zinc oxide, the elements used to control the resistance 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 mouth-shaped semiconductor by a sputtering method, a method of sputtering with a reduced amount of oxygen can be considered, but if the amount of oxygen is reduced, a sputtered film of zinc metal will suddenly be formed at a certain limit value. Therefore, it was difficult to control this condition.

In view of the above, the present inventors conducted tests to obtain a humidity sensor using a zinc oxide thin film with a resistance value within the practical measurement range. 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-containing materials have the effect of lowering resistance, they have drawbacks such as poor responsiveness to humidity, especially long recovery time.

Therefore, as a result of continuing studies to improve this response, we found that adding lithium along with lead has the effect of lowering the resistance value, and also reduces the response to humidity, especially the recovery time. I found it.

However, it is technically very difficult to obtain a zinc oxide thin film containing lead and lithium, especially adding lithium to zinc metal.
0) on a zinc alloy containing lead, but with this method it is difficult to stably add the same amount into a zinc oxide thin film.

Another idea was to use a zinc oxide sintered body sintered with lithium added in the form of L120 oxide, but lithium would scatter during sintering. Obtaining solid targets is difficult.

Therefore, the present inventors conducted various tests in order to obtain a humidity sensor that can be alloyed in place of lithium and has the same humidity response and shortened recovery time as when adding lithium. It has been discovered that a humidity sensor having the above characteristics 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 nickel, and as a result, (1) it has self-recovery properties even when left in high humidity, and does not require cleaning; (2) Fast response; (3) Resistance value within a range that makes practical measurement easy;
4) It has good durability; (5) It can be used as an alloy target, so it is inexpensive and has high sputtering efficiency, contributing to cost reduction.

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

FIG. 1 is a schematic plan view showing an example of a humidity sensor according to the present invention. Reference numeral 1 denotes an insulating substrate made of ceramic, glass, etc. On this substrate 1, a pair of comb-shaped detection electrodes 2 and 3 are arranged. is formed, and a zinc oxide thin film 4, which is a moisture-sensitive element, is formed so as to roughly cover the comb tooth portion.

5.6 are terminals respectively connected to the detection electrodes 2.3. 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. , NL, needle, Tj, Cu, Fe
The conductive material may be formed using a mask vapor deposition method or the like.

Methods for forming zinc oxide 1JIB14 include, for example, sputtering, vacuum evaporation, and ion blasting, but reactive sputtering is particularly suitable because of its ease of formation.

In this invention, the usage range of lead and nickel contained in the zinc oxide thin film, which is a moisture sensitive element, is 0.1 to 20 atomic %, preferably 3 to 4 atomic %, for lead 1.
Nickel is 0.1 to 20 at%, preferably 0.5 to 2
Atomic % is preferable.

The reason for this is that when lead is less than 0.1 atomic percent, the resistance value is 10
0 or more, which exceeds the practical measurable range; if 20 atom% or more is used, the change in resistance against humidity becomes small, which is undesirable; if nickel is less than 0.1 atom%, there is no effect of adding nickel, and if it is 20 atom% or more In this case, the resistance value increases and the effect of using it in combination with lead is not achieved, and both are inappropriate as humidity sensors.

Here, the combination of lead and nickel, which is a feature of this invention, will be explained. Adding lead lowers the resistance value, but the response and reproducibility are unfavorable.

Furthermore, when nickel is added, the resistance value increases.

However, when both lead and nickel are added to the lever, the response improves.
Reproducibility is improved.

However, the resistance value increases with the addition of nickel, so the addition ratio of lead and nickel should be adjusted so that the practical resistance value is 1M at 40% humidity.
It is necessary to adjust it so that it is Ω or less. Therefore, the most preferable amounts of both are 3 to 4 atom % of lead and 0.5 to 2 atom % of nickel in the zinc oxide thin film.

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

A comb-shaped detection electrode made of gold with a comb-tooth spacing of 0.5 mm and a facing length of 65 mm is formed on an alumina substrate. Furthermore, lead and nickel are contained on top of this to cover the comb-teeth portion. A zinc oxide thin film was formed using a sputtering method.

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, an alloy obtained by mixing 3 at. % lead and 1 at. % nickel in metal zinc and melting the mixture was placed in an array of 1 to 3 atomic percent.

Then, the vacuum pressure in the sputtering chamber was set to 5x10-2T.
A mixed gas consisting of oxygen (02) and argon (Ar) in a ratio of 1:1 was introduced into the sputtering chamber, and the pressure in the chamber was maintained at 3×1 OTorr.

After that, adjust the exhaust valve so that the sputtering chamber pressure l becomes 1.5X IOT Orr, and
A 1 μm thick zinc oxide thin film containing lead and nickel was formed on an alumina substrate having a comb-like detection electrode by supplying a W high frequency power source (13.56 M l-1z ).

Note that the above examples used metallic zinc as the target,
Although an example of forming a zinc oxide thin film using a reactive sputtering method has been shown, it is also possible to use a sintered body of zinc oxide as a target. The zinc oxide sintered body in this case is, for example, made mainly of zinc oxide powder, to which is added 3 atomic % of Pb30 in terms of lead and 1 atomic % of N=0 in terms of nickel. 500~
It can be obtained by calcining at 600°C, then press-forming into a disk shape, and sintering at 700-1300°C.

Furthermore, Ar is present in the sputtering chamber in reactive sputtering when this zinc oxide sintered body is used as a target.
It is preferable to use a mixed gas of: 02=9:1.

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, but the humidity sensor of this invention The structure is not limited to this one, but may also include 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-shaped detection electrodes are formed on the thin film, or A zinc oxide thin film is formed by sputtering on a flexible substrate, such as a stainless steel or aluminum plate, and a comb-shaped detection electrode, an island-shaped vapor-deposited film of gold, or a porous silver paste electrode is formed on the top and bottom electrodes. It may also be of a structure that measures the resistance value between the two.

The thus obtained zinc oxide thin film based on the sputtering method contained many defective structures on the surface when it was formed and gases were easily adsorbed, so it was annealed at 150° C. for 24 hours. Due to this annealing, the resistance value of the zinc oxide thin film becomes somewhat higher than that at the time of N9 formation, but the change in resistance value over time becomes smaller.

The humidity sensor 1) of the present invention functions as a humidity sensor at low temperatures, but at high temperatures of 300 to 500°C, it functions as a gas sensor. Therefore, when measuring gas concentration, the sensing element is heated and the relative humidity is measured. It is also possible to use it at room temperature when measuring.

The response characteristics of the humidity sensor of the present invention obtained in the above example are shown in FIG. When expressing the change in relative humidity reading (%) converted from the relative humidity-electrical resistance curve when the tJo 3 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 It was observed that the response was faster than Humidity Sensor 1-B, which uses a zinc oxide thin film containing lead and lithium as the humidity-sensing element, and Humidity Sensor C, which uses a zinc oxide thin film containing lead and lithium as the humidity-sensing element. .

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.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view showing an example of the moisture mark 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 patent applicant Murata Manufacturing Co., Ltd. Representative Patent attorney Kazu 1) Showa 1 Figure 2

Claims (3)

[Claims] (1) A humidity sensor characterized in that the humidity sensing element is made of a zinc oxide thin film containing lead and nickel. (2) The amount of lead contained in the zinc oxide thin film is 0.1 to 2.
The humidity sensor according to claim 1, characterized in that the humidity is 0 atomic %. (3) The amount of nickel contained in the zinc Mide thin film is 0.1~
Claim 1 characterized in that it is 20 atomic %.
Humidity sensor as described in section.