JPH01302152A - Thin film moisture sensing element - Google Patents

Abstract

PURPOSE:To obtain a thin film moisture sensing element suitable for detection of a dew formation, by covering the surface of a conducting comb-shaped electrode formed on an insulation substrate with a plasma polymer film. CONSTITUTION:A plasma polymerization is performed under conditions of 300SCCM in zinc dimethyl flow rate, 0.01Torr in discharge pressure and 100W in high frequency power using a plasma reactor to cover a comb-shaped part of a comb-shaped electrode formed on a glass plate as insulating substrate with a plasma polymerized film. Then, the work is heated in the atmospheric air for one hour separately at 30 deg.C or 400 deg.C. A lead is connected to the electrode by silver pasting to form a moisture sensing element. When the element thus obtained is placed in a moisture sensitivity tester to evaluate a moisture sensing characteristic, in a relationship of a resistance value to a relative humidity, a sharp decrease in the resistance value occurs in a high humidity area with a relative humidity above 60%.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thin film moisture sensitive element. More specifically, the present invention relates to a thin film moisture sensitive element in which the surface of conductive comb-shaped electrodes formed on an insulating substrate is covered with a plasma polymerized film.

[Conventional technology]

Controlling the relative humidity in the air is extremely important in the precision industry, food industry, textile industry, building management, etc.

As a moisture sensing element for detecting this, one using a conventional material is known.

(1) Metals or semiconductors such as Se, Ge, SL (
2) Metal oxides such as Sn, Fe, Ti, etc. (3) Al
1.0. porous metal oxides such as (4) electrolyte salts such as LiCQ, (5) polymer membranes made of organic or inorganic materials.However, moisture sensing elements using these various materials are difficult to maintain, Or, there are problems with reliability or responsiveness, and the system is not in a satisfactory state.

For example, when using the metal oxide of (2) above, pressing and sintering are performed to shape it, but there are problems such as difficulty in homogeneous pressing or cracking during firing. In addition, even if the molding is completed without any problems during the manufacturing process, moisture-sensitive elements utilize changes in resistance caused by desorption of moisture, and as a result, fractures occur at grain boundaries due to the influence of moisture, resulting in poor durability.
In other words, there is also a problem with reliability.

Further, when the polymer membrane of (5) above is used, although it is inexpensive in terms of material, there are problems such as deterioration due to chemicals such as solvents and a decrease in reliability.

[Problem to be solved by the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a moisture-sensitive element in which the response speed is increased by forming a moisture-sensitive film as a thin film, and which also has excellent reproducibility of moisture-sensitive characteristics and heat resistance.

[Means to solve the problem]

The thin film moisture sensitive element of the present invention which achieves this object is constructed by covering the surface of conductive comb-shaped electrodes formed on an insulating substrate with an oxide film of an organic zinc compound plasma polymerized film.

Generally, glass, quartz, alumina, etc. are used as the insulating substrate, but in cases where even better temperature tracking is desired for the moisture-sensitive element, a silicon substrate, which has been proposed by the applicant, may be used. An insulating film formed by oxidizing the surface (Japanese Patent Application Laid-Open No. 61-281.958) can also be used.

When forming conductive comb-shaped electrodes on these insulating substrates, first, a corrosion-resistant metal such as stainless steel, Hastelloy C, Inconel, Monel, or gold, or an electrode forming material metal such as silver or aluminum is placed on the insulating substrate. A thin film having a thickness of about 0.1 to 0.5 μm is formed by sputtering, ion blating, etc., and then a photoresist pattern is formed thereon.

For example, in the case of aluminum, a photoresist pattern is formed on the metal thin film of the electrode forming material thus formed by applying a well-known photolithography process. That is, a photoresist coating is applied on the metal thin film.

A glass dry plate on which a negative or positive image of the comb-shaped electrode pattern has been printed is placed on top of this, and the process is carried out by printing with light irradiation and development. After this, wet chemical etching is performed, and the etching solution used is a mixed solution of phosphoric acid, sulfuric acid, chromic anhydride, and water (weight ratio 65:t5:5:15), B)
IF (hydrofluoric acid), ferric chloride aqueous solution, nitric acid, phosphoric acid
A nitric acid mixture is used.

The surface of the conductive comb-shaped electrode formed on the insulating substrate is further covered with an oxide film of an organic zinc compound plasma polymerized film.

Organic zinc compounds that form plasma polymerized films include:
Dilower alkylzincs such as dimethylzinc and diethylzinc are preferably used.

Plasma polymerization is carried out using, for example, an apparatus as shown in FIG. First, the pressure inside the plasma reaction vessel 1 is reduced to the order of 10'' Torr by the operation of the molecular turbo pump 3 connected to the oil rotary pump 2.The pressure inside the reduced pressure reaction vessel is set by adjusting the valve 4. The organic zinc compound is introduced to the order of 10-'' Torr while measuring the arbitrary flow rate of
A high frequency (13.56 MHz) of 00 W is applied to cause discharge. At this time, by adjusting the matching box, the reflected power is suppressed as much as possible so that the difference between the applied power and the reflected power becomes an arbitrary power. After discharging for a certain period of time, the application of high frequency and the supply of the organic zinc compound are stopped, the main valve (not shown) is closed, the reaction vessel 1 is leaked, and a plasma polymerized film is deposited on the surface of the substrate 9 mounted on the internal electrode 8. Take out the formed material.

The plasma polymerized film formed generally has a molecular weight of about 500 to 200
It has a film thickness of 0.00 mm, contains about 30% (element ratio) of carbon, and has a composite composition of metallic zinc and a polymerized organic zinc compound. As described above, since this plasma-polymerized film contains organic matter, moisture is trapped in the film, and it does not exhibit reproducible moisture-sensitive characteristics.

Therefore, an element on which such a plasma polymerized film is formed is heated in the atmosphere to about 300°C or higher, generally about 300 to 500°C.
When heated to 450°C, the organic components contained in the film are oxidized and burned, reducing the carbon content to about 10% at a combustion temperature of 300°C, or to about 2% at a combustion temperature of 450°C. If this oxidation is performed in the presence of oxygen gas, the oxidation reaction will proceed in a short time, which is efficient. In this oxidation reaction, unlike when ordinary metal zinc is oxidized, it is thought that the reaction occurs with heat, residual carbon is released as carbon dioxide, and external oxygen reacts with zinc.

The obtained oxide film is stable up to about 500° C. and exhibits a large negative resistance change with respect to humidity changes.

This is considered to be a characteristic obtained because the oxide film is composed of an amorphous substance and is not affected by grain boundaries.

〔Effect of the invention〕

A conductive comb-shaped electrode formed on an insulating substrate and covered with a plasma-polymerized film of an organic zinc compound has a reproducible humidity sensitivity because the plasma-polymerized film contains a considerable proportion of carbon. Although it does not show any characteristics, the plasma polymerized film is about 3
The oxide film obtained by heat oxidation treatment at 00°C or higher shows a rapid decrease in resistance value in a high humidity region of about 60% relative humidity or higher. It can be effectively used for detection purposes.

Furthermore, this thin film moisture sensitive element has good heat resistance because it is heated and oxidized at a temperature of approximately 300°C or higher, and the surface is free from stains due to oil, cigarette smoke, salt, dust, etc. However, such stains can be easily removed by heat treatment at a temperature lower than the temperature of the above-mentioned heating and oxidation treatment.

〔Example〕

Next, the present invention will be explained with reference to examples.

Examples 1 to 2 Using a plasma reactor as shown in Fig. 1, the dimethyl zinc flow rate was 3003 CCM, and the discharge pressure was 0.01 Tor.
Plasma polymerization is carried out under the conditions of 100°C of high-frequency power and the comb-shaped portions of the comb-shaped electrode formed on the glass plate serving as an insulating substrate are coated with a plasma polymerized film. Example 1) or 450℃ (
Example 2) was heated for 1 hour each.

Connect lead wires to the electrodes with silver paste to form a humidity sensing element, place it in a temperature/humidity tester, and set the frequency I.
Under the measurement conditions of KHz, voltage 1■, temperature 30℃, LCR
Moisture sensitivity characteristics were evaluated using a meter. The relationship between the resistance value and the relative humidity is shown in the graph in Figure 2. In the high humidity region where the relative humidity is about 60% or more, a rapid decrease in the resistance value is observed. It can also be seen that the higher the value, the higher the resistance value.

In addition, the same measurement was performed on the humidity sensor before heat treatment.
When repeated, the first time the results are O and the second time the results are shown as ., indicating that reproducibility is lacking.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a plasma polymerization apparatus used in the method of the present invention.6 Figure 2 is a graph showing the humidity-resistance characteristics of the humidity sensors produced in Examples 1 and 2 before and after heat treatment. . (Explanation of symbols) 1... Plasma reaction vessel 6... High frequency power source 7... Matching box 8... Internal electrode 9... Substrate

Claims (1)

[Claims] 1. A thin film moisture-sensitive element in which the surface of a conductive comb-shaped electrode formed on an insulating substrate is covered with an oxide film of an organic zinc compound plasma polymerized film.