JPH04265849A - Humidity sensitive element and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a humidity sensitive element assuming small hysteresis in the using range from low temperatures to high temperatures and from low humidities to high humidities, and having quick sensor response. CONSTITUTION:A humidity sensitive film 3 is formed by using a polymer wherein polyimide oligomer containing fluorine is polymerized. In another method, a thin film is formed by using polyimide oligomer containing fluorine as a starting material and then treating with heat at 200 deg.C or higher in the atmosphere of nitrogen, thereby to obtain a humidity sensitive film 3. In this manner, it is possible to obtain a humidity sensitive element, providing a stable output value even if it is exposed to such conditions for a long term as high humidities, high temperatures and high humidities, low humidities, atmosphere of an organic solvent, dew drops or when it is dipped in water. Moreover, the humidity sensitive element has small temperature characteristics, so that the temperature correction is root necessary.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitive moisture sensing element whose moisture sensing film is made of an organic polymer resin material and a method for manufacturing the same.

0002

[Prior Art] Conventionally, this type of moisture sensing element has
Organic polymers such as cellulose acetate butyrate, cellulose acetate propionate, polyimide-imide, or linear polyamide polymerized with linear high-molecular-weight polyamic acid are used as moisture-sensitive materials. A moisture-sensitive capacitive element has been proposed that uses changes in the capacitance of a membrane for humidity detection (Japanese Patent Laid-Open No. 62-889).
Publication No. 51).

0003

[Problems to be Solved by the Invention] However, the moisture-sensitive element configured in this way has high hydrophilicity (the property of attracting water) and has a high water absorption rate, so it has a strong bond with polymers due to chemical adsorption. A lot of water remains. For this reason, when used for a long time in an atmosphere of high temperature and high humidity of about 40° C. and 90%, for example, there is a problem that the output value drifts and lacks long-term stability. Furthermore, there is a problem in that the difference in moisture sensitivity characteristics (hysteresis) between the moisture absorption process and the moisture removal process is small at low temperatures and large at high temperatures, resulting in slow sensor response. Furthermore, when used for a long period of time in a low humidity atmosphere, there is a problem in that hysteresis becomes large. Furthermore, there is a problem in that the output value drifts due to the occurrence of dew condensation or immersion in water. Furthermore, there is a problem in that the output value drifts when exposed to an organic solvent atmosphere. Furthermore, when the humidity sensing element has temperature characteristics, there is a problem in that it is necessary to perform temperature correction using the temperature sensing element (temperature sensor).

[0004] Accordingly, an object of the present invention is to provide a moisture sensing element that has a small hysteresis and a fast sensor response in the range of use from low to high temperatures and from low humidity to high humidity. In addition, our aim is to provide a humidity sensing element that can obtain stable output values even when exposed to conditions such as high humidity, high temperature and high humidity, low humidity, organic solvent atmosphere, dew condensation, or water immersion for a long period of time. There is. Another object of the present invention is to provide a moisture sensing element that has small temperature characteristics and does not require temperature correction.

[0005]

[Means for Solving the Problems] In order to achieve the above object, the moisture sensitive element according to the present invention comprises a moisture sensitive film using a polymer obtained by polymerizing a fluorine-containing polyimide oligomer. Further, in the method for manufacturing a humidity-sensitive element according to the present invention, a moisture-sensitive film is formed by forming a thin film using a fluorine-containing polyimide oligomer as a starting material, and then heat-treating the film at a temperature of 200° C. or higher in a nitrogen atmosphere.

[0006]

[Function] In the moisture-sensitive element of the present invention, by forming the moisture-sensitive film using a polymer obtained by polymerizing a fluorine-containing polyimide oligomer, the water absorption rate (absorbed water amount) is reduced and hysteresis is reduced. Furthermore, in the method for manufacturing a moisture-sensitive element according to the present invention, water absorption is suppressed to a low level and hydrophobicity is improved.

[0007]

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings. FIG. 1 is a diagram showing the structure of an embodiment of a moisture-sensitive element according to the present invention, in which FIG. 1A is a perspective view and FIG. 1B is a plan view thereof. In these figures, 1 is an insulating substrate made of, for example, an alumina substrate, a glass substrate, a thermally oxidized silicon substrate, etc., 2 is a lower electrode made of, for example, platinum, formed on the upper surface of the insulating substrate 1, and 3 is this lower part. This is a moisture-sensitive film that is laminated and applied across the electrode 2, and this moisture-sensitive film 3 is made of a moisture-sensitive material made by polymerizing a fluorine-containing polyimide oligomer. Reference numeral 4 denotes an upper electrode formed on the moisture sensitive film 3 and made of, for example, gold. That is, the humidity sensitive membrane 3 is sandwiched between the lower electrode 2 and the upper electrode 4, and lead wires 2a are connected to the lower electrode 2 and the upper electrode 4, respectively, in order to detect changes in the capacitance value of the moisture sensitive membrane 3 with respect to relative humidity. and 4a are connected.

Next, a method for manufacturing this moisture sensitive element will be explained. First, for example, 5 to 10 grams of fluorine-containing polyimide oligomer powder is prepared and dissolved in, for example, 10 to 50 ml of diethylene glycol dimethyl ether to obtain a fluorine-containing polyimide oligomer solution. Next, after applying this fluorine-containing polyimide oligomer solution onto the lower electrode 2 formed on the insulating substrate 1 by a spin coating method,
130-150°C in air or nitrogen atmosphere, 0.
It is dried for 5 to 1.0 hours to obtain a moisture sensitive film 3 having a thickness of 0.5 to 5 μm. At this time, the rotation speed of the spinner is 500 to 5
000 rpm. After drying in air or nitrogen atmosphere, dry at 180°C for 0.5 to 1 hour in nitrogen atmosphere for 2 hours.
1-2 hours at 30℃, finally 0.25-1 at 400℃
The polymerization reaction is completed by heat treatment for a period of time. In addition,
The fluorine-containing polyimide oligomer as a starting material has a structural formula as shown in Chemical Formula 1, and the average degree of polymerization is defined by the number of repeats n in [ ] in this formula. The average degree of polymerization n can be manufactured from 1 to 30, and any starting material may be used, or two or more types having different average degrees of polymerization n may be mixed.

[0009]

[Chemical formula 1]

In the structural formula shown in Chemical Formula 1, fluorine is introduced in order to improve hydrophobicity, and the introduction of fluorine makes it possible to suppress the water absorption rate to a low level. Next, on the insulating substrate 1 on which the moisture-sensitive film 3 is laminated and coated, gold is deposited, for example, by vapor deposition or sputtering to form an upper electrode 4 having a thickness of about 50 to 1000 Å. In addition to gold, any corrosion-resistant metal such as palladium, platinum, or chromium may be used as the deposited metal. Further, the lower electrode 2 on the insulating substrate 1 is formed by depositing platinum at a concentration of 1,000 to 100,000 by vapor deposition or sputtering.
It is obtained by forming a thin film with a thickness of 0.00 Å. Moreover, the same effect can be obtained by using a solvent such as tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, or a mixed solvent thereof in place of the above-mentioned diethylene glycol dimethyl ether.

[0011] As a result of measuring the relative humidity-capacitance characteristics of the humidity sensing element thus constructed, data as shown in FIG. 2 were obtained. In addition, an LCZ meter was used for this measurement, and the frequency was 100KHz and the temperature was 10℃ and 25℃, respectively.
℃, 40℃. As is clear from the figure, temperature dependence was small and good moisture sensitivity characteristics were obtained. Therefore, changes in detected values due to temperature (temperature characteristics) are reduced, so temperature correction by a circuit is not required. Furthermore, as is clear from the figure, the hysteresis was 1% RH or less when measured approximately 2 minutes after stabilization in the humidity chamber, which was extremely good.

FIG. 3 shows the humidity of 10% RH, 30% RH, and 5 at 25° C. after the humidity sensing element manufactured in the above-mentioned example was left in a high temperature and high humidity atmosphere of approximately 40° C. and 90% RH.
It shows the drift of each output at 0% RH, 70% RH, and 90% RH. In addition, FIG. 4 shows, as a comparative example, 20%RH, 40%RH, 60%RH, Each drift at 80%RH is shown. As is clear from these figures, the output drift of the humidity sensing element according to this example is stable despite the wider measurement humidity range compared to the conventional one (Fig. 4), that is, the humidity sensing characteristics with small drift are exhibited. It was obtained and was in good condition. In addition, the hysteresis is 1% RH or less when measured approximately 2 minutes after stabilization in the constant humidity chamber, which has better reproducibility than before, and allows the product to be left in the same atmosphere for a longer period of time in a high-temperature, high-humidity atmosphere. However, the capacitance ratio hardly changes and remains stable. Here, the capacity ratio is the output at 90% RH at 25°C (C90) and the 10%
It refers to the ratio C90/C10 of the RH output (C10). In addition, when the material was left in a high-temperature, high-humidity atmosphere and then returned to a room atmosphere, it was able to reversibly recover its initial characteristics.

[0013] In the above-mentioned embodiments, a sandwich-structured moisture-sensitive element was explained as an example, but the present invention is not limited thereto. It goes without saying that similar effects can be obtained even when applied to a moisture-sensitive element having a comb-shaped structure, which is obtained by forming a thin-film electrode and laminating a moisture-sensitive film to cover the comb-shaped thin-film electrode.

Furthermore, in the above-mentioned embodiment, humidity was detected by paying attention to the change in the capacitance value with respect to the relative humidity of the moisture-sensitive film. A detection method may also be adopted.

[0015] Furthermore, the moisture-sensitive membrane in the above-mentioned embodiments is
It is suitable as a moisture-sensitive film for a moisture-sensitive element that is formed on a crystal resonator and detects humidity from the shift in resonance frequency caused by adsorption of the moisture-sensitive film. It is also suitable as a moisture-sensitive film for a humidity-sensitive element that forms a wet film and detects humidity based on changes in the speed at which the film passes through the surface acoustic wave element.

[0016]

Effects of the Invention As explained above, according to the moisture-sensitive element according to the present invention, since the moisture-sensitive film is constructed using a polymer obtained by polymerizing a fluorine-containing polyimide oligomer, the water absorption rate is reduced and hysteresis is reduced. becomes smaller. Furthermore, the temperature characteristics become smaller, and temperature correction becomes unnecessary. Furthermore, it has extremely excellent effects such as stable output values even when exposed to conditions such as high temperature, high humidity, organic solvent atmosphere, and dew condensation for long periods of time. Further, according to the method for manufacturing a moisture-sensitive element according to the present invention, the water absorption rate is reduced by heat-treating the fluorine-containing polyimide oligomer as a starting material at a temperature of 200° C. or higher in a nitrogen atmosphere.
It is possible to obtain stable moisture sensitivity characteristics with less drift and no temperature dependence, less hysteresis, and better response. In addition, since the amount of adsorbed water is small and the hydrophilicity is low, extremely excellent effects such as conditioning in a constant temperature and constant humidity atmosphere and conditioning such as temperature and humidity cycles are unnecessary or easy after production can be obtained.

[Brief explanation of the drawing]

FIG. 1(a) is a perspective view illustrating the structure of an embodiment of a moisture-sensitive element according to the present invention, and FIG. 1(b) is a plan view of FIG.

FIG. 2 is a diagram showing relative humidity-capacitance characteristics of a humidity-sensitive element according to the present invention.

[Fig. 3] 10% RH and 30% R at 25°C after being left in a high temperature and high humidity atmosphere of the humidity sensing element according to the present invention.
FIG. 4 is a diagram showing the drift of each output at H, 50% RH, 70% RH, and 90% RH.

[Figure 4] 20% RH, 40% RH, 6 at 25°C after being left in a high temperature and high humidity atmosphere of a conventional humidity sensing element
It is a figure which shows the drift of each output at 0%RH and 80%RH.

[Explanation of symbols]

1 Insulating substrate 2 Lower electrode 3 Moisture sensitive membrane 4 Upper electrode

Claims (3)

[Claims] 1. A moisture-sensitive element comprising, as a moisture-sensitive film, a polymer obtained by polymerizing a fluorine-containing polyimide oligomer. 2. A method for producing a moisture-sensitive element, which comprises forming a thin film using a fluorine-containing polyimide oligomer as a starting material, and then heat-treating the film at a temperature of 200° C. or higher in a nitrogen atmosphere. 3. In claim 2, the starting material is an oligomer consisting of a single fluorine-containing polyimide oligomer or a mixture of a plurality of fluorine-containing polyimide oligomers having an average degree of polymerization n ranging from n=1 to n=30. A method for manufacturing a moisture-sensitive element.