JPH0570281B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a moisture sensitive thin film for a humidity sensor and a method for producing the same.

[Conventional technology/issues to be solved by the invention] Until now, moisture sensors for humidity sensors that utilize expansion due to moisture absorption, such as hair, and porous ceramic sintered bodies and polymeric thin films have been used. There are known methods that utilize alternating current impedance (hereinafter abbreviated as impedance) changes and capacitance changes due to moisture absorption.

However, devices that utilize the expansion of hair or the like have problems such as slow response speed and low accuracy and reliability.

In addition, although porous ceramic sintered bodies have excellent environmental resistance due to the characteristics of the ceramic material, they have slow response times, deterioration due to increased element impedance due to chemical adsorption of adsorbed water, etc. There are problems such as the impossibility of miniaturization or multifunctionalization by combining with other sensors.

Furthermore, the response speed of polymeric thin films is relatively fast, and moisture-sensitive materials with excellent sensitivity are beginning to be commercially available, but they do not have the disadvantages of being organic, such as lack of environmental resistance and a narrow operating temperature range. There's a problem.

[Means for Solving the Problems] In view of these circumstances, the inventors of the present invention have conducted extensive research and have found that a tin oxide thin film containing an alkaline earth metal exhibits sufficient impedance changes in response to changes in relative humidity. It has been found that this results in a high-performance moisture-sensitive thin film for humidity sensors.

In other words, when a thin film is formed by incorporating an alkaline earth metal into tin oxide, the impedance of the thin film increases and the change in impedance due to moisture adsorption increases, making detection easier and increasing the response speed when used as a humidity sensor. They discovered that the sensor is much faster and has better environmental resistance than conventional humidity sensors, leading to the completion of the present invention.

In the present invention, a moisture-sensitive thin film made of alkaline earth metal-containing tin oxide provided on a substrate and a solution containing an alkaline earth metal compound and a tin compound are applied onto the substrate, dried, and then heat-treated. The present invention relates to a method of manufacturing the moisture sensitive thin film.

[Function] In a moisture sensitive element that utilizes the change in impedance of the thin membrane of the moisture sensitive body due to moisture adsorption, the amount of protons changes as protons are generated by dissociation of adsorbed water molecules, and the conductivity changes with the change in the amount of protons. Therefore, the impedance of the moisture-sensitive thin film changes in response to changes in relative humidity.

However, when a tin oxide thin film is used as a moisture-sensitive thin film, since tin oxide alone has low resistivity, the impedance decreases due to adsorption of water molecules.
In other words, it becomes difficult to detect changes in conductivity due to changes in the amount of protons.

Therefore, in order to use a tin oxide thin film as a moisture sensitive thin film, it is necessary to increase its electrical resistivity, which can be achieved by adding an alkaline earth metal.

According to the experimental results of the present inventors, the resistivity of a tin oxide thin film containing an alkaline earth metal increases rapidly.

When water molecules are adsorbed onto this alkaline earth metal-containing tin oxide thin film, the water molecules partially dissociate to generate protons. Since the amount of adsorbed water molecules changes in accordance with changes in relative humidity, the proton concentration also changes accordingly, and a sufficient change in impedance occurs in response to changes in relative humidity.Thin oxide thin films containing alkaline earth metals can be used as a moisture sensitive body.

[Example] The moisture sensitive thin film of the present invention consists of a tin oxide thin film containing an alkaline earth metal.

Containment in tin oxide containing an alkaline earth metal means that tin oxide and alkaline earth metal are mutually diffused, dispersed, or dissolved in solid solution, or that an alkali is present at the grain boundaries of tin oxide particles or alkaline earth metal oxide particles. In a segregated state as earth metal oxides or tin oxides, stannate salts (e.g. Mg ₂ SnO ₄ , Ca ₂ SnO ₄ , etc.) represented by the general formula: M ₂ SnO ₄ (M is an alkaline earth metal) This concept refers to a state in which the thin film is formed entirely or partially, and as long as the thin film finally formed on the substrate is in this state, the present invention is not limited to the manufacturing method etc. It can be used as tin oxide containing alkaline earth metals.

The alkaline earth metal may be any of the alkaline earth metals shown in the periodic table, ie, beryllium, magnesium, calcium, strontium, and barium. These may be used alone or in combination of two or more. Among these alkaline earth metals, magnesium,
It is preferable to use calcium, strontium, barium, or a combination of two or more thereof.

The content of the alkaline earth metal is preferably 0.1 mol % or more based on tin, since it is necessary to increase the resistivity of the tin oxide thin film to a certain level or higher. In addition, in order to obtain a homogeneous moisture-sensitive thin film, although it depends on the alkaline earth metal added, it is preferably 500 mol% or less based on tin.
In order to obtain a moisture sensitive material having good moisture sensitivity characteristics with good reproducibility, the content is preferably 1 to 200 mol%, more preferably 5 to 200 mol%.

There are no particular limitations on the purity of the alkaline earth metal and tin oxide constituting the alkaline earth metal-containing tin oxide, and any purity that can be produced from commercially available products may be used.

In addition, the alkaline earth metal-containing tin oxide moisture sensitive body thin film of the present invention may contain other metals in an amount of 10 mol% or less based on tin, as long as it does not adversely affect the performance as a moisture sensitive body. May be included.

Specific examples of the other metals include Li,
Na, K, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, B, Al, Si, Ge, P, Pb, Zr, Nb, Mo,
Examples include In.

Next, regarding the thickness of the thin film of the alkaline earth metal-containing tin oxide moisture sensitive body, the thicker the membrane, the lower the impedance of the moisture sensitive body, which is advantageous in broadening the measurement humidity range. When a thin film of tin oxide containing an alkaline earth metal is formed, cracks and peeling tend to occur, making it difficult to form a homogeneous thin film. Therefore, 0.02~5μm,
Furthermore, a film thickness of about 0.05 to 2 μm is preferable.

In the present invention, an alkaline earth metal-containing tin oxide thin film as described above is provided on a substrate.

The substrate is not particularly limited as long as it can form and hold an alkaline earth metal-containing tin oxide thin film, but examples include glass substrates such as soda glass, soda lime glass, and quartz glass; ceramic substrates such as alumina; Substrates made of stainless steel, silicon, polyimide film, etc. are preferable because of their heat resistance and surface smoothness.

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

The method for obtaining the alkaline earth metal-containing tin oxide moisture-sensitive thin film of the present invention is not particularly limited, and it can be obtained by the same method as for obtaining a normal tin oxide thin film. Easier methods to obtain a tin oxide thin film include thin film formation techniques using vacuum techniques such as CVD (chemical vapor deposition), sputtering, and vacuum evaporation, or by spraying a metal compound solution onto a high-temperature substrate. It is relatively easier to add alkaline earth metals by applying a solution containing an alkaline earth metal compound and a tin compound and thermally decomposing it than by a spray method in which alkaline earth metal compounds are added and thermally decomposed. This method is excellent in that it is easy to obtain a thin film suitable as a moisture-sensitive thin film for a humidity sensor that has the necessary surface porosity.

In addition, thin tin oxide films containing no alkaline earth metals can be applied to substrates containing alkaline earth metals such as soda lime glass using film forming techniques such as CVD, sputtering, vacuum evaporation, coating and thermal decomposition, and spraying. A method may also be adopted in which a film is formed on the tin oxide thin film and then a heat treatment is performed so that the alkaline earth metal is diffused and contained in the tin oxide thin film from the substrate by thermal diffusion. However, as a method for easily obtaining a moisture sensitive thin film having good and reproducible moisture sensitive characteristics, the above-mentioned coating thermal decomposition method is excellent.

As an organic tin compound which is a type of tin compound used in the coating thermal decomposition method of a solution containing an alkaline earth metal compound and a tin compound, for example, the general formula (): Sn(OR ¹ ) ₂ () (in the formula, R ¹ is a hydrocarbon group having 1 to 20 carbon atoms) divalent tin alkoxides;
Tetravalent tin alkoxides represented by the general formula (): Sn (OR ¹ ) ₄ () (in the formula, R ¹ is the same as above), General formula (): Sn (OR ¹ ) _4-x Y _x Tetravalent tin partial alkoxides represented by () (wherein R ¹ is the same as above, Y is a functional group having chelating ability or a halogen atom, and X is an integer of 1 to 3), general formula ( ), general formula (), condensation polymer of a compound represented by general formula (), general formula (): Sn(OCOR ² ) ₂ () (wherein R ² is a hydrogen atom or carbonized water with 1 to 30 carbon atoms) divalent tin carboxylates represented by the general formula (): Sn(OCOR ² ) ₄ () (in which R 2 is the same as above) (where R ² is the same as above); Acid salts, β-diketone complexes that are reaction products of divalent or tetravalent tin and acetylacetone, benzoylacetone, etc., tinoxy β-diketone complexes, alkyltins such as tetramethyltin, tetraethyltin,
Organotin compounds such as tetraphenyltin, general formula (): R _x SnX _4-x () (wherein R is a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, X is a chlorine atom, fluorine Compounds represented by the general formula (): R ₂ SnO () (wherein R is the same as above), etc. Examples include, but are not limited to.

Specific examples of the compound represented by the general formula () include diethoxytin, dipropoxytin, di-2-ethylhexoxytin, distearoxytin, etc. Specific examples of the compound represented by the general formula () include: Tetraethoxytin, tetraproxytin,
Specific examples of compounds represented by the general formula () such as tetrabutoxytin, tetrakis(2-ethylhexoxy)tin, and tetrastearoxytin include tin butoxydichloride, tristearoxytin monochloride, etc. Specific examples of compounds include stannous acetate, stannous oxalate, stannous tartrate, stannous octylate, stannous oleate, stannous linoleate, stannous stearate, etc. Specific examples of compounds represented by formula () include stannous acetate, stannous lactate, stannous butyrate, stannous octylate, stannous linoleate, etc. Examples include dioctyltin diacetate, diethyltin oxide, diptyltin maleate, diphenyltin dichloride, dibenzyltin dihydroxide, tributyltin laurate, dibutyltin laurate, dibutyltin propoxide, dibnyltin dichloride, etc. General formula Examples of the compound represented by () include di-n-butyltin oxide. In addition to these organic tin compounds, inorganic tin compounds such as tin tetrachloride may also be used. These compounds may be used alone or in combination of two or more, but those that are soluble in organic solvents and decompose into tin oxide compounds when heated to 350° C. or higher are particularly preferred.

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 acid esters such as propyl, isoamyl acetate, and propyl formate;
Acetone, acetylacetone, diethyl ketone,
Ketones such as methyl ethyl ketone; benzene,
Aromatic solvents such as toluene and xylene; ethers such as dioxane and tetrahydrofuran; glycol ethers such as methyl cellosolve and ethyl cellosolve; N-methyl-2-pyrrolidone,
Examples include nitrogen-containing organic solvents such as dimethylformamide and dimethylacetamide.
It is not limited to these. These organic solvents may be used alone or in combination of two or more.

The compound used in the case of containing an alkaline earth metal may be any organic compound or inorganic compound containing an alkaline earth metal as long as it dissolves in the above organic solvent. inorganic compounds such as hydroxides, nitrates, and sulfates, alkaline earth metal carboxylates such as acetates and octylates, alkaline earth metal alkoxides such as barium isopropoxide, and alkalis such as acetylacetonate. Organic compounds such as earth metal complexes are preferred because they are easy to use and easy to contain.

The metal content in the solution may be selected as appropriate to achieve an appropriate hydrolysis rate during coating and to adjust the film thickness, but is usually 0.5 to 20% by weight, preferably 3 to 10% by weight. .

In addition, in the present invention, inorganic or induced metal salts, thickeners, stabilizers, etc. may be added that do not interfere with the purpose of the present invention.

There are no particular limitations on the method for applying the solution containing the alkaline earth metal compound and the tin compound onto the substrate, and any commonly used methods such as dip coating, spraying, and spin coating may be used.

There is no particular limitation on the temperature at which the solvent is dried after being applied onto the substrate, as long as the temperature is such that the solvent evaporates. Therefore, although it varies depending on the solvent used, the temperature is usually 50 to 300°C.

The heat treatment temperature after drying should be at least the temperature at which the organic tin compound turns into tin oxide through thermal decomposition, and usually requires a temperature of about 350°C or higher, but in order to obtain a moisture-sensitive thin film with excellent reproducibility, It is desirable to fire at a temperature of about 400℃ or higher. However, if the firing temperature is too high, the densification of the thin film will be promoted and it will be difficult to obtain good moisture sensitivity characteristics, so depending on the substrate material, it is recommended to keep the firing temperature below about 1000℃, or even below about 800℃. is preferred.

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 obtained in this way increases the impedance of the thin film itself, and the impedance change due to moisture adsorption becomes large, making it easier to detect and exhibiting a very high response speed when used as a humidity sensor. It is fast, has excellent environmental resistance, and has sufficient long-term stability for use as a humidity sensor. Furthermore, if a moisture sensitive element is manufactured using this moisture sensitive thin film, it can be made smaller and more intelligent.

Next, the present invention will be explained based on Examples, but the present invention is not limited to these Examples.

Example 1 and Comparative Example 1 After dissolving 10 g of tin tetraptoxide in 100 g of n-propyl alcohol, 6 g of calcium 2-ethylhexanoate (Ca content 4.0% by weight) was added and stirred thoroughly to form a transparent solution. A homogeneous solution (Ca/Sn ratio approximately 25 mol%) was obtained. The resulting solution was dip coated onto an alumina substrate at a pulling speed of 15 cm/min, dried in air at 80°C, and then heated in a Matsufuru furnace for 10 min.
The temperature was raised to 800°C at a heating rate of °C/min, held for 1 hour, and then allowed to cool to obtain a homogeneous calcium-containing tin oxide film.

The obtained calcium-containing tin oxide film was transparent, and as a result of observation using a scanning electron microscope, the film thickness was about 0.3 μm.

Gold comb-shaped electrodes (electrode spacing 0.2 mm,
A moisture-sensitive element was fabricated by forming an electrode with a total length of 200 mm using a vacuum evaporation method. When we measured the humidity-sensing characteristics of this humidity-sensitive element at room temperature, that is, the impedance change in response to changes in relative humidity, we found that it was 10 to 95%.
It varied by about 3 orders of magnitude between RH (see Figure 1).

In addition, we investigated the response when humidifying from 50%RH to 90%RH and the response when dehumidifying from 90%RH to 50%RH at 25℃. When compared with other devices, the response speed was fast and it was a good moisture-sensitive device (see Fig. 2). Furthermore, there was almost no change in impedance over time.

Example 2 10g of tin tetraisopropoxide was dissolved in 100g of anhydrous isopropyl alcohol, and an additional 2g
10 g of a solution of magnesium sulfate dissolved in 100 g of ethanol was added and thoroughly stirred to obtain a transparent homogeneous solution (Mg/Sn ratio about 6 mol %).

The obtained solution was applied by dip coating at a pulling speed of 15 cm/min onto a soda lime glass substrate on which a gold comb-shaped electrode similar to that in Example 1 was formed by vacuum evaporation.
A transparent tin oxide film containing magnesium was obtained by firing at 500° C. in a Matsufuru furnace in the same manner as in Example 1. The thickness of this tin oxide film was approximately 0.4 μm as a result of scanning electron microscopy observation.

The moisture-sensitive characteristics of the moisture-sensitive element having this magnesium-containing tin oxide thin film were measured in the same manner as in Example 1, and the moisture-sensitive characteristics were almost the same as that of the calcium-containing tin oxide moisture-sensitive thin film element obtained in Example 1. similarly
The impedance changed by about three orders of magnitude in response to changes in relative humidity from 10 to 95% RH, demonstrating sufficient moisture sensitivity characteristics as a humidity sensor. Further, this moisture-sensitive element exhibited substantially the same responsiveness as the moisture-sensitive element of Example 1.

Example 3 20 g of a solution of 4 g of tin octylate dissolved in petroleum ether was dissolved in 100 g of n-butyl alcohol, and 3 g of strontium acetolacetonate (Sr content was 31% by weight) was added, and the mixture was heated while heating. Stir thoroughly to obtain a homogeneous transparent solution (Sr/Sn
The ratio was approximately 100 mol%).

The obtained solution was applied onto a quartz glass substrate on which gold comb-shaped electrodes were formed as in Example 1, and then heated in air.
After drying at 120℃, drying at 900℃ in a Matsufuru oven.
By heating for 20 minutes, a moisture-sensitive element having a transparent tin oxide film (thickness: 0.4 μm) containing strontium was obtained. When the moisture sensitivity characteristics of this moisture sensitive element were measured in the same manner as in Example 1, it was found that the moisture sensitivity was 10 to 95%.
It exhibited almost the same moisture sensitivity characteristics as Example 1, except that the impedance changed by about four orders of magnitude with respect to changes in RH relative humidity. Moreover, this moisture-sensitive element was used in Example 1.
The response was almost the same as that of the moisture-sensitive element.

Example 4 After dissolving 10 g of tin tetrabutoxide in 100 g of n-propyl alcohol, 4 g of barium 2-ethylhexanoate (Ba content 15% by weight) was added and thoroughly stirred to form a transparent homogeneous solution (Ba/ The Sn ratio was approximately 18 mol%). The obtained solution was prepared in Example 1.
A gold comb-shaped electrode similar to the above was dip-coated onto an alumina substrate formed by vacuum evaporation at a pulling rate of 15 cm/min, dried at 80°C in air, and then heated at a heating rate of 10°C/min in a Matsufuru furnace. The temperature was raised to 800℃ at 30℃.
After holding for a minute, the mixture was allowed to cool, yielding a moisture-sensitive element having a homogeneous barium-containing tin oxide film (thickness: 0.3 μm).

When the moisture-sensing characteristics of this moisture-sensitive element were measured in the same manner as in Example 1, the impedance changed by about three orders of magnitude with respect to relative humidity changes from 10 to 95% RH. The characteristics were shown. Further, this moisture-sensitive element exhibited substantially the same responsiveness as the moisture-sensitive element of Example 1.

[Effects of the Invention] A moisture-sensitive element using the moisture-sensitive thin film of the present invention has faster response than conventional moisture-sensitive elements, and because it is a thin film-based element, it can be made smaller. It has the characteristics that it can be combined with other sensors to make it multi-functional. Moreover, it can be easily produced by the method of the present invention.

[Brief explanation of drawings]

FIG. 1 shows the relationship between relative humidity and impedance values measured using a moisture-sensitive element produced using the alkaline earth metal-containing tin oxide thin film of the present invention obtained in Example 1 as a sensitivity sensor. graph,
FIG. 2 shows the response when the moisture-sensitive element produced using the alkaline earth metal-containing tin oxide thin film of the present invention obtained in Example 1 is used as a humidity sensor, and the response when a conventional ceramic humidity sensor is used. It is a graph showing responsiveness in an emergency.

Claims (1)

[Scope of Claims] 1. A moisture-sensitive thin film made of alkaline earth metal-containing tin oxide provided on a substrate. 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 alkaline earth metal is at least one selected from magnesium, calcium, strontium, and barium. 4. The moisture-sensitive thin film according to claim 1, wherein the alkaline earth metal content is 0.1 to 500 mol% based on tin. 5. A method for producing a moisture sensitive thin film according to claim 1, wherein a solution containing an alkaline earth metal compound and a tin compound is applied onto a substrate, dried, and then heat treated. 6 The ratio of the alkaline earth metal compound to the tin compound in the solution is such that the alkaline earth metal compound is the tin compound.
6. The method according to claim 5, wherein the amount is 0.1 to 500 mol%.