JPH0642401B2 - Moisture-sensitive thin film and method for manufacturing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a humidity sensitive thin film for a humidity sensor and a method for manufacturing the same.

[Problems to be solved by the prior art / invention] Hitherto, as a humidity sensor for a humidity sensor, one that utilizes expansion due to moisture absorption by hair or the like, or a porous ceramic sintered body or a polymer thin film Known are ones that utilize changes in AC impedance (hereinafter referred to as impedance) and changes in capacitance.

However, there is a problem in that the response speed is slow and the reliability is low in accuracy in the case of utilizing expansion of hair or the like.

In addition, although the porous ceramics sintered body has excellent environmental resistance due to the characteristics of the material called ceramics, it has a slow response speed and is chemically adsorbed by adsorbed water, resulting in deterioration caused by an increase in the impedance of the element. However, there is a problem in that it is impossible to make a small composite or to have multiple functions in combination with other sensors.

In addition, the response speed of polymer thin films is relatively fast, and moisture sensitive materials with excellent sensitivity are beginning to be marketed, but the disadvantages of being an organic substance are that the environment resistance is not good and that the operating temperature range is narrow. There's a problem.

[Means for Solving the Problems] In view of such circumstances, the inventors of the present invention have conducted diligent research, and as a result, have found that an indium oxide thin film containing an alkali metal and / or an alkaline earth metal is resistant to changes in relative humidity. It has been found that the film shows a sufficient impedance change and becomes a high-performance humidity sensor thin film for humidity sensors.

That is, when a thin film is formed by adding an alkali metal and / or an alkaline earth metal to indium oxide, the impedance of the thin film increases and the impedance change due to water adsorption becomes large, which facilitates detection.
The inventors have found that the response speed when used as a humidity sensor is much faster than that of a conventional humidity sensor, and that the humidity sensor is excellent in environmental resistance, and has completed the present invention.

The present invention provides a moisture sensitive thin film made of indium oxide containing an alkali metal and / or an alkaline earth metal (hereinafter also referred to as an alkali metal) provided on a substrate, an alkali metal compound and / or an alkaline earth. The present invention relates to a method for producing the moisture sensitive thin film by applying a solution containing a metal compound (hereinafter also referred to as a compound such as an alkali metal) and an indium compound on a substrate, drying and then heat treating the solution.

[Function] In the humidity sensitive material utilizing the impedance change of the moisture sensitive material thin film due to the adsorption of water, the amount of protons changes with the generation of protons due to the dissociation of adsorbed water molecules, and the conductivity changes with the change of the proton amount. Therefore, the impedance of the moisture sensitive thin film changes in accordance with the change in relative humidity.

However, when an indium oxide thin film is used as the moisture sensitive thin film, the resistivity of indium oxide alone is low,
It becomes difficult to detect a decrease in impedance due to adsorption of water molecules, that is, a change in conductivity due to a change in the amount of protons.

Therefore, in order to use the indium oxide thin film as the moisture sensitive thin film, it is necessary to increase its electric resistivity, which is realized by adding an alkali metal or the like.

According to the experimental results of the present inventors, the resistivity of an indium oxide thin film that actually contains an alkali metal or the like rapidly increases.

When water molecules are adsorbed on the indium oxide thin film containing the alkali metal or the like, the water molecules are partially exfoliated to generate protons. Since the adsorption amount of water molecules changes according to the change in relative humidity, the proton concentration also changes accordingly, and a sufficient impedance change occurs according to the change in relative humidity. It can be used as a moisture sensitive material.

[Example] The moisture sensitive thin film of the present invention comprises an indium oxide thin film containing an alkali metal and / or an alkaline earth metal (such as an alkali metal).

The alkali metal or the like in the indium oxide containing the alkali metal or the like includes not only those existing in a metal state but also those existing in an oxide state, a hydroxide state or a state such as a hydrate thereof. It is a concept. Further, the inclusion means that indium oxide and alkali metal or the like are mutually diffused, dispersed, or in solid solution, a state in which the alkali metal or the like is segregated in the grain boundary portion of the indium oxide particle, a grain boundary portion of the alkali metal or the like particle. Indium oxide is segregated on the surface, and the general formula: M ₃ InO ₃ (M is an alkali metal), M′In ₂ O
₄ (M 'is an alkaline earth metal) is a concept that means a state in which an indium salt (eg, K ₃ InO ₃ , MgIn ₂ O ₄ etc.) is formed wholly or partially, and finally As long as the thin film formed on the substrate is in such a state, it can be used as indium oxide containing an alkali metal or the like used in the present invention without being limited to the production method and the like.

The alkali metal and alkaline earth metal are alkali metal and alkaline earth metal shown in the periodic table, namely lithium, sodium, potassium, rubidium,
Any of cesium, beryllium, magnesium, calcium, strontium, and barium can be used. These may be used alone or in combination of two or more. Among these alkali metals and alkaline earth metals, lithium, sodium, potassium, magnesium, calcium, strontium, barium, and these two kinds are preferable because a uniform thin film can be easily formed and they have good moisture sensitivity. The above combination is preferable.

Regarding the content of the alkali metal or the like, it is necessary to increase the resistivity of the indium oxide thin film to a certain level or more, for example, about 10 ⁴ Ω · cm or more.
It is preferably at least mol%. Also, in order to obtain a uniform moisture sensitive thin film, it depends on the alkali metal to be added,
It is preferably 500 mol% or less with respect to indium, and in order to obtain a moisture sensitive body having good moisture sensitive properties with good reproducibility, it is 5 to 200 mol%, and further 10 to 200 mol%. preferable.

There is no particular limitation on the purity of the alkali metal or the like or the indium oxide constituting the indium oxide containing the alkali metal or the like, and any one can be used as long as it is a commercially available product.

Incidentally, the moisture sensitive thin film made of indium oxide containing an alkali metal or the like of the present invention, within a range of 10 mol% or less with respect to indium as long as it does not adversely affect the performance as a moisture sensitive material. May be included.

Specific examples of the other metal include, for example, Ti, V, Cr, Mn and F.
Examples thereof include e, Co, Ni, Cu, Zn, B, Al, Si, Ge, P, Pb, Zr, Nb, Mo and Sn.

Next, regarding the indium oxide moisture sensitive substance thin film containing an alkali metal or the like, the thicker the lower the impedance of the moisture sensitive substance, the wider the measurement temperature range, which is advantageous.
When the indium oxide thin film containing an alkali metal or the like having a film thickness exceeding the range, cracks or peeling easily occur, and it becomes difficult to form a uniform thin film. therefore,
A film having a thickness of 0.02 to 5 μm, more preferably 0.05 to 2 μm is preferable.

In the present invention, the indium oxide thin film containing an alkali metal or the like as described above is provided on the substrate.

The substrate is not particularly limited as long as it can form and hold an indium oxide thin film containing an alkali metal or the like, but it is not limited, but a glass substrate such as soda glass or quartz glass, a ceramic substrate such as alumina, stainless steel, silicon, a polyimide film. A substrate made of, for example, is preferable in terms of heat resistance and surface smoothness.

Next, a method for manufacturing the moisture sensitive thin film of the present invention will be described.

There is no particular limitation on the method for obtaining the indium oxide moisture sensitive thin film containing an alkali metal or the like of the present invention, and it can be obtained by a method similar to the method for obtaining a normal indium oxide thin film,
As a method for easily obtaining an indium oxide thin film containing an alkali metal or the like, a CVD method (chemical vapor deposition method),
Alkali metal compounds and / or alkaline earth metal compounds rather than thin film forming technology using vacuum technology such as sputtering method or vacuum deposition method or spray method in which a metal compound solution is sprayed on a high temperature substrate and thermally decomposed. Application of a solution containing (a compound such as an alkali metal) and an indium compound is relatively easy to add an alkali metal by the thermal decomposition method, and a sensor for a humidity sensor having surface porosity necessary for adsorbing water molecules. It is excellent in that it is easy to obtain a preferable thin film as a wet body thin film.

Also, an indium oxide thin film containing no alkali metal or the like is formed on a substrate containing alkali metal such as soda glass by a film forming technique such as a CVD method, a sputtering method, a vacuum deposition method, a coating heating decomposition method, or a spray method. However, a method in which an alkali metal or the like is diffused and contained in the indium oxide thin film by thermal diffusion from the substrate by heat treatment after that can also be adopted. However, the coating heat decomposition method is excellent as a method for easily obtaining a moisture-sensitive thin film having good and reproducible moisture-sensitive characteristics.

As the organic indium compound which is one of the indium compounds used in the coating thermal decomposition method of the solution containing the compound such as the alkali metal and the indium compound, for example, a compound represented by the general formula:
(1): In (OR ¹ ) ₃ (I) (wherein R ¹ is a hydrocarbon group having 1 to 20 carbon atoms), an alkoxide of indium represented by the general formula (II): In (OR ¹ ) _3-a Y _a (II) (wherein R ¹ is the same as above, Y is a functional group having chelating ability or a halogen base paper, and a is an integer of 1 to 2) and is a partial alkoxide of indium. , General formula (I), condensed multimers of compounds represented by general formula (II), general formula (III): In (OCOR ² ) ₃ (III) (wherein R ² is hydrogen raw paper or a carbon number of 1 to 1). Carboxylic acid salts of indium represented by 30 hydrocarbon groups), β-diketone complexes that are the reaction products of indium with acetylacetone, benzoylacetone, etc., alkylindium such as trimethylindium, and organics such as triphenylindium. Examples include indium compounds, but are not limited to these. Not to.

Specific examples of the compound represented by the general formula (I) include triethoxyindium and tripropoxyindium, and specific examples of the compound represented by the general formula (II) include indium dibutoxycyclolide and the general formula (III Specific examples of the compound represented by), indium acetate, indium oxalate, indium tartrate,
Examples thereof include indium octylate, indium oleate, indium linoleate, indium stearate and indium naphthenate. In addition to these organic indium compounds, inorganic indium compounds such as indium nitrate may be used. These compounds may be used alone or as a mixture of two or more kinds, but those soluble in an organic solvent and decomposed into indium oxide by heating at 350 ° C. or higher are particularly preferable.

Examples of the organic solvent include monohydric alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, butyl alcohol and pentanol; polyhydric alcohols such as ethylene glycol, glycerin and 1,4-butanediol; ethyl acetate and acetic acid. Carboxylic esters such as propyl, isoamyl acetate and propyl formate; Ketones such as acetone, acetylacetone, diethyl ketone and methyl ethyl ketone; Aromatic solvents such as benzene, toluene and xylene; Ethers such as dioxane and tetrahydrofuran; Methyl cellosolve, Glycol ethers such as ethyl cellosolve: N-methyl
Examples thereof include, but are not limited to, nitrogen-containing organic solvents such as -2-pyrrolidone, dimethylformamide, and dimethylacetamide. These organic solvents may be used alone or in combination of two or more.

The compound used when containing an alkali metal or the like may be any organic compound or an inorganic compound containing an alkali metal, as long as it is soluble in the above organic solvent, for example, a hydroxide of an alkali metal or the like. Compounds such as inorganic compounds, sodium methoxide, sodium ethylate, alkali metal or alkaline earth metal alkoxides such as magnesium diethoxide, alkali metal or alkaline earth metal such as sodium octylate, lithium octylate, magnesium octylate. Organic compounds such as carboxylic acid salts are preferable because they are easy to use and easy to contain.

The metal content in the solution may be appropriately selected so that the hydrolysis rate at the time of application is appropriate and for adjusting the film thickness, but is usually 0.5 to 20% by weight, preferably 3 to 10% by weight. .

Further, in the present invention, an inorganic metal salt, an organic metal salt, a thickener, a stabilizer or the like which does not impair the object of the present invention may be added.

The method of applying the solution containing the compound such as an alkali metal and the indium compound onto the substrate is not particularly limited, and it may be performed by a commonly used method such as a dip coating method, a spray method, and a spin coating method.

There is no particular limitation on the temperature at which the solvent is applied and then dried, as long as it is a temperature at which the solvent exhibits. Therefore, it is usually 50 to 300 ° C., though it varies depending on the solvent used.

The heat treatment temperature after drying may be at least the temperature at which the organic indium compound is converted to indium oxide by thermal decomposition, and usually requires about 350 ° C. or higher, but in order to obtain a moisture-sensitive thin film having excellent reproducibility. It is desirable to bake above 400 ° C. However, if the baking temperature is too high, the densification of the thin film is promoted, and it becomes difficult to obtain good moisture-sensitive properties. Therefore, depending on the substrate material, it is 1000 ° C or less, and even about 800 or less. preferable.

As the atmosphere during firing, an atmosphere containing an inert gas such as nitrogen or oxygen can be used.

The moisture-sensitive thin film of the present invention thus obtained has an increased impedance of the thin film itself and a large impedance change due to water adsorption, which facilitates detection, and has a very high response speed when used as a humidity sensor. It is fast, has excellent environmental resistance, and has long-term stability sufficient for use as a humidity sensor. Further, when a moisture sensitive element is manufactured using this moisture sensitive thin film, it can be made compact and can be made intelligent.

Next, the present invention will be described based on examples, but the present invention is not limited to the examples.

Example 1 and Comparative Example 1 Indium 2-ethylhexanoate (In content 8.6% by weight)
Dissolve 5g in 100g benzene, and then add 5g linoleic acid
After adding 20 g, 20 g of 10 wt% ethanol solution of sodium 2-ethylhexanoate (Na content 6.3 wt%) was added and sufficiently stirred while heating to obtain a uniform transparent solution (Na to In was About 146 mol%).

Apply this solution to a gold comb-shaped electrode (electrode spacing 0.2 mm, total electrode length 200
mm) coated on the formed alumina substrate, in air, 120
After being dried at ℃, it was heated at 800 ℃ in a muffle furnace for 1 hour to obtain a moisture sensitive element having a transparent indium oxide thin film containing sodium.

The obtained sodium-containing indium oxide thin film had transparency, and as a result of observation with a scanning electron microscope, the thin film was about 0.4 μm.

When the humidity sensitivity characteristic of this humidity sensitive element at room temperature, that is, the impedance change with respect to the relative humidity change is measured, the impedance is about 3 at a relative humidity of 10 to 95% RH.
Digits changed (see Figure 1).

In addition, the responsiveness when humidified from 50% RH to 90% RH at 25 ° C and the responsiveness when dehumidified from 90% RH to 50% RH were investigated, and a commercially available porous ceramic sintered body moisture-sensitive element Compared with, the response speed was fast and it was a good moisture sensitive element (see FIG. 2).

Example 2 20 g of indium acetylacetonate was dissolved in 100 g of a solution of ethanol and acetone 1: 1, and then 5 g of magnesium acetylatetonate was added and well stirred to obtain a uniform transparent solution (Mg was about 46 mol with respect to In). %).

The obtained solution is dip-coated on a soda glass substrate at a pulling rate of 30 cm / min, dried in air at 100 ° C, and then heated in a muffle furnace to 500 ° C at a rate of 10 ° C / min. After holding for 1 hour, the mixture was allowed to cool to obtain a homogeneous magnesium-containing indium oxide thin film (film thickness: about 0.1 μm).

A gold comb-shaped electrode similar to that of Example 1 was formed on this thin film by a vacuum vapor deposition method to fabricate a moisture sensitive element.

When the humidity-sensitive characteristics and the responsiveness of the obtained moisture-sensitive element were measured, the characteristics were almost the same as in Example 1.

[Advantages of the Invention] A moisture-sensitive element using the moisture-sensitive thin film of the present invention has high-speed responsiveness as compared with a conventional moisture-sensitive element, and is small because it is an element using a thin film. It is possible to realize a multi-functionalization by combining other sensors. Moreover, it can be easily produced by the method of the present invention.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the relative humidity and the impedance value measured using the humidity sensitive element having the indium oxide thin film containing an alkali metal etc. of the present invention obtained in Example 1 as a humidity sensor. The figure shows the response in the case of using as a humidity sensor of the humidity sensitive element having the indium oxide thin film containing alkali metal etc. of the present invention obtained in Example 1 and the response in the case of using the conventional ceramics humidity sensor. It is a graph shown.

Claims (5)

[Claims] 1. An alkali metal provided on a substrate and /
Alternatively, a moisture sensitive thin film made of indium oxide containing an alkaline earth metal. 2. The moisture sensitive thin film according to claim 1, wherein the thin film has a thickness of 0.02 to 5 μm. 3. The moisture sensitive thin film according to claim 1, wherein the content of the alkali metal and / or the alkaline earth metal is 5 to 500 mol% with respect to indium. 4. A method for producing the moisture-sensitive thin film according to claim 1, wherein a solution containing an alkali metal compound and / or an alkaline earth metal compound and an indium compound is applied onto a substrate, dried and then heat-treated. . 5. The ratio of the alkali metal compound and / or alkaline earth metal compound to the indium compound in the solution is 5 to 500 mol% of alkali metal and / or alkaline earth metal with respect to indium. 4. The method described in 4.