JPH0620003B2 - 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] Up to now, as a humidity sensor for a humidity sensor, one that utilizes expansion due to moisture absorption of hair or the like, an alternating current due to moisture absorption such as a porous ceramic sintered body or a polymer thin film. There are known ones that use impedance (hereinafter abbreviated as impedance) changes and capacitance changes.

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 ceramic sintered body is excellent in environmental resistance due to the characteristics of the material called ceramics, it has a slow response speed and is deteriorated due to an increase in the impedance of the element due to the chemical adsorption of adsorbed water. There are problems such as miniaturization combined with a sensor or the inability to perform various functions.

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 drawbacks 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] As a result of intensive studies conducted by the present inventors in view of such circumstances, the tin oxide thin film containing an alkali metal undergoes a sufficient impedance change with respect to a change in relative humidity. We have found that it can be a high-performance moisture sensitive thin film for humidity sensors.

That is, when a thin film is formed by adding an alkali metal to tin oxide, the impedance of the thin film rises, the impedance change due to water adsorption increases, and it becomes easier to detect. The inventors have found that the humidity sensor is much faster than the humidity sensor described above and has excellent environmental resistance, and thus completed the present invention.

The present invention provides a moisture sensitive thin film made of an alkali metal-containing tin oxide provided on a substrate and a solution containing an alkali metal compound and a tin compound, applied on a substrate, dried, and then heat-treated to obtain the moisture sensitive material. A method of manufacturing a thin film.

[Operation] The humidity sensitive material that utilizes the impedance change of the moisture sensitive material thin film due to the adsorption of water changes the proton amount with the generation of protons due to the dissociation of adsorbed water molecules, and has a conductivity that does not change with the proton amount. Because of the change, the impedance of the moisture sensitive thin film changes in response to the change in relative humidity.

However, when a tin oxide thin film is used as the moisture sensitive thin film, the tin oxide alone has a low resistivity, so that the impedance drop due to the adsorption of water molecules, that is, the change in conductivity due to the change in the amount of protons is difficult to detect. turn into.

Therefore, in order to use the tin oxide thin film as a moisture sensitive thin film, it is necessary to increase its electrical resistivity, which is realized by adding an alkali metal.

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

When water molecules are adsorbed on the tin oxide thin film containing the alkali metal, the water molecules are partially dissociated 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 wet body.

[Example] The moisture sensitive thin film of the present invention is composed of a tin oxide thin film containing an alkali metal. The inclusion in the tin oxide containing an alkali metal means that tin oxide and an alkali metal are mutually diffused, dispersed, or solid-dissolved. Or, a state in which tin oxide particles or alkali metal oxide particles are segregated as alkali metal oxides or tin oxide at the grain boundary part, and further, a general formula: xM ₂ O ・ ySnO ₂・ zH ₂ O
(M is an alkali metal, x is an integer of 1-2, y is an integer of 1-9, and z is an integer of 0-8) (for example, Na ₂ O.SnO ₂ .3H ₂ O, Na ₂ O ・ 5SnO ₂・ 8H ₂ O, Na ₂ O ・ 9SnO ₂・ 8H ₂ O, K ₂ O ・ 3SnO ₂・ 3H ₂ O, K ₂ O ・ 5SnO ₂・ 4H ₂ O, 2Na ₂ O ・ SnO ₂ etc.) Is a concept that means a state of being formed wholly or partially, and as long as the thin film finally formed on the substrate is in such a state,
The tin oxide containing an alkali metal used in the present invention can be used regardless of the production method.

As the alkali metal, any of alkali metals shown in the periodic table, that is, lithium, sodium, potassium, rubidium and cesium can be used. These may be used alone or in combination of two or more. Of these alkali metals, lithium, sodium, potassium, or 2 or more of these alkali metals is used because it is easy to form a uniform thin film and has good moisture sensitivity.
It is preferable to use one or more kinds in combination.

The content of the alkali metal is preferably 0.1 mol% or more with respect to tin because it is necessary to increase the resistivity of the tin oxide thin film to some extent or more. Further, in order to obtain a uniform moisture sensitive thin film, it is preferably 500 mol% or less with respect to tin, though it depends on the alkali metal to be added. 1 to 200 mol%, more preferably 5 to 200 mol%
Is preferred.

The purity of the alkali metal and tin oxide constituting the alkali metal-containing tin oxide is not particularly limited, and may be as long as it is a commercially available product.

The alkali metal-containing tin oxide moisture sensitive thin film of the present invention contains other metal in a range of 10 mol% or less with respect to tin as long as it does not adversely affect the performance as a moisture sensitive substance. May be.

Specific examples of the other metal include, for example, Mg, Ca, Sr,
Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Si, G
Examples thereof include e, P, Pb, Zr, Nb, Mo and In.

Next, regarding the film thickness of the alkali metal-containing tin oxide moisture sensitive thin film, the thicker it is, the lower the impedance of the moisture sensitive substance, so the measured humidity range is widened, which is advantageous, but an alkali with a membrane pressure of more than about 10 μm is used. When a metal-containing tin oxide thin film is formed, cracks and peeling easily occur, and it becomes difficult to form a uniform thin film. Therefore, 0.02-5
It is preferable that the thickness is about μm, and more preferably about 0.05 to 2 μm.

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

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

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

The method for obtaining the alkali metal-containing tin oxide moisture sensitive thin film of the present invention is not particularly limited, and it can be obtained by a method similar to the method for obtaining a normal tin oxide thin film, but a tin oxide thin film containing an alkali metal is more preferable. CV is an easy way to get
Alkaline rather than thin film forming technology using vacuum technology such as D method (chemical vapor deposition method), sputtering method, vacuum deposition method or spray method in which a metal compound solution is sprayed on a high temperature substrate and pyrolyzed. Application of a solution containing a metal compound and a tin compound The thermal decomposition method is relatively easier to add an alkali metal, and is preferable as a humidity sensor thin film for a humidity sensor having surface porosity necessary for adsorbing water molecules. It is excellent in that it is easy to obtain a thin film.

In addition, a tin oxide thin film that does not contain an alkali metal is
Method, sputtering method, vacuum deposition method, coating heating decomposition method,
By a film forming technique such as a spray method, a film is formed on a substrate containing an alkali metal such as soda glass, and then by heat treatment, the alkali metal is diffused into the tin oxide thin film by thermal diffusion from the substrate, and then contained. The method of making it possible 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.

Examples of the organotin compound which is one of the tin compounds used in the coating thermal decomposition method of the solution containing the alkali metal compound and the tin compound include, for example, the general formula (I): Sn (OR ¹ ) ₂ (I) (wherein R ¹ is a divalent tin alkoxide represented by a C 1-20 carbon-hydrogen group, a general formula (II): Sn (OR ¹ ) ₄ (II) (in the formula, R ¹ is as described above). Same as the above, tetravalent tin alkoxides represented by the general formula (III): Sn (OR ¹ ) _4-a Y _a (III) (wherein R ¹ is the same as above, Y is a chelating functional group). Group or halogen atom, a is an integer of 1 to 3), a tetravalent tin partial alkoxide represented by formula (I), general formula (II), condensation of a compound represented by general formula (III) multimers, the general formula ^{(IV): Sn (OCOR 2} ) 2 (IV) ( wherein, R ² is a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms) mosquito divalent tin represented by Bonn acid salts of the general formula ^{(V): Sn (OCOR 2} ) 4 (V) ( wherein, R ² is as defined above) carboxylic acid salts of tetravalent tin represented by a divalent or tetravalent tin Β-diketone complexes that are reaction products with acetylacetone, benzoylacetone, etc., tinoxy β-diketone complexes, alkyltins such as tetramethyltin and tetraethyltin, organotin compounds such as tetraphenyltin, general formula (VI) R _a SnX _4-a (VI) (wherein R is a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, X is a halogen atom such as a chlorine atom or a fluorine atom, an alkoxy group, a carboxylic acid residue, a Are the same as those described above), compounds represented by general formula (VII): R ₂ SnO (VII) (wherein R is the same as above), and the like, but are not limited to these. .

Specific examples of the compound represented by the general formula (I) include diethoxytin, dipropoxytin, di-2-ethylhexoxytin, and distearoxtin, as specific examples of the compound represented by the general formula (II). Is, for example, tetraethoxytin, tetrapropoxytin, tetrabutoxyzides, tetrakis (2-ethylhexoxy) tin, tetrastearoxytin, tin butoxydichloride, tristearoxytin. Specific examples of the compound represented by the general formula (IV) such as monochloride include stannous acetate, stannous oxalate, stannous tartrate, stannous octylate, stannous oleate, stannous linoleate. Specific examples of the compound represented by the general formula (V) such as stannous acid and stannous stearate include stannous acetate, stannous lactate and stannous acetate. Tin, octylic acid second
Specific examples of the compound represented by the general formula (VI) such as tin and stannous linoleate include dioctyl tin diacetate, diethyl tin oxide, dibutyl tin maleate, diphenyl tin dichloride, dibenzyl tin dihydroxide, tributyl tin laurate, Examples of the compound represented by the general formula (VII), such as dibutyltin laurate, dibutyltin propoxide, and divinyltin dichloride, include oxidized di-n-
Butyl tin and the like. In addition to these organic tin compounds, inorganic tin compounds such as tin tetrachloride may be used.
These compounds may be used alone or as a mixture of two or more kinds, but those which are soluble in an organic solvent and decompose into tin oxide by heating at 350 ° C. or more 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 acid 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; glycol ethers such as methyl cellosolve and ethyl cellosolve; N-methyl-2- Examples thereof include, but are not limited to, nitrogen-containing organic solvents such as pyrrolidone, dimethylformamide, and dimethylacetamide. These organic solvents may be used alone or in combination of two or more.

As the compound used in the case of containing an alkali metal, any of organic compounds containing an alkali metal and inorganic compounds that can be dissolved in the above organic solvent may be used, for example, a hydroxide of an alkali metal or the like. Organic compounds such as inorganic compounds, alkali metal alkoxides such as sodium methylate and sodium ethylate, and alkali metal carboxylates such as sodium octylate, lithium octylate and potassium octylate are preferred because they are easy to use and easy to contain.

Further, as the organotin compound, a compound represented by the general formula (VIII): M [Sn (OR ¹ ) ₅ ] (VIII) or the general formula (IX): M [Sn ₂ (OR ¹ ) ₉ ] (IX) (wherein M is Alkali metal may be added directly using a composite alkoxide of tin represented by ( ¹ ) is an alkali metal and R ¹ is the same as above.

Specific examples of the complex alkoxides represented by the general formulas (VIII) and (IX) include, for example, lithium tin isopropoxide: Li [Sn (OCH (CH ₃ ) ₂ ) ₅ ], sodium tin isopropoxide: Na [ Sn ₂ (OCH (CH ₃ ) ₂ ) ₉ ] and the like.

The metal content in the solution may be appropriately selected so that the hydrolysis rate at the time of coating 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 or organic metal salt, a thickener, a stabilizer and the like which do not impair the object of the present invention may be added.

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

There is no particular limitation on the temperature at which the solvent is applied and dried, as long as it is a temperature at which the solvent volatilizes. 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 a temperature at which the organotin compound is converted to tin oxide by thermal decomposition, and is usually 3
Although it requires about 50 ° C or higher, it is desirable to bake at about 400 ° C or higher in order to obtain a moisture sensitive thin film having excellent reproducibility.
However, if the firing temperature is too high, the densification of the thin film is promoted,
Since it becomes difficult to obtain a good moisture-sensitive property, it is preferably 1000 ° C. or lower, more preferably 800 ° C. or lower, though it depends on the substrate material.

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 10 g of tin tetrabutoxide was dissolved in 100 g of n-propyl alcohol, 3 g of sodium 2-ethylhexanoate (Na content 6.3% by weight) was added, and the mixture was thoroughly stirred to give a transparent, homogeneous mixture. A solution was obtained (Na / Sn ratio about 34 mol%). The obtained solution is dip-coated on an alumina substrate at a pulling rate of 15 cm / min, dried in air at 80 ° C., and then dried in a muffle furnace for 10 minutes.
The temperature was raised to 500 ° C. at a temperature rising rate of ° C./min, and the temperature was maintained for 1 hour and then allowed to cool to obtain a homogeneous sodium-containing tin oxide film.

The obtained sodium-containing tin oxide film has transparency, and as a result of observation with a scanning electron microscope, the film thickness was about 0.3 μm.
It was m.

Moisture-sensitive elements were manufactured by forming gold comb-shaped electrodes (electrode spacing 0.2 mm, total electrode length 200 mm) on the surface of this film by a vacuum vapor deposition method. When the humidity-sensitive property of this humidity-sensitive element at room temperature, that is, the impedance change with respect to the relative humidity change was measured, the impedance changed by about 3 digits between 10 and 95% RH (see FIG. 1).

In addition, by examining the response at the time of humidification from 50% RH to 90% RH at 25 ° C and the response at the time of dehumidification from 90% RH to 50% RH, we compared it with a commercially available porous ceramic fired body wet and dry element. By comparison, the response speed was fast and the moisture sensitive element was excellent (see FIG. 2).

Furthermore, when the change in impedance with time was examined at 25 ° C. and 70% RH, almost no change was observed (see FIG. 3).

Example 2 10 g of tin tetraisopropoxide was dissolved in 100 g of anhydrous isopropyl alcohol, and about 10 g of an ethanol solution of potassium hydroxide in which 1 g of potassium hydroxide was dissolved in 100 g of ethanol was added, and the mixture was sufficiently stirred to be transparent. A homogeneous solution was obtained (K / Sn ratio about 6 mol%).

The obtained solution was dip-coated at a pulling rate of 15 cm / min on a soda glass substrate on which a gold comb-shaped electrode similar to that in Example 1 was formed by a vacuum evaporation method, and then applied in a muffle furnace in the same manner as in Example 1. By baking, a transparent tin oxide film containing potassium was obtained. The thickness of this tin oxide film was about 0.2 μm as a result of observation with a scanning electron microscope.

The humidity-sensitive property of the moisture-sensitive element having this potassium-containing tin oxide thin film was measured in the same manner as in Example 1. As a result, it was almost the same as that of the sodium-containing tin oxide thin-film thin film element obtained in Example 1. Similarly, the impedance changed about 3 orders of magnitude with respect to a change in relative humidity of 10 to 95% RH, and the humidity sensor showed sufficient humidity sensitivity. The responsiveness and temporal stability of this moisture sensitive element were almost the same as in Example 1.

Example 3 A solution of 4 g of tin (II) octylate in petroleum ether 20
g was dissolved in 100 g of n-butyl alcohol, and 20 g of 10 wt% ethanol solution of lithium octylate (Li content 4.7 wt%) was added, and stirred well while heating to obtain a uniform transparent solution (Li / Sn ratio is about 130 mol%).

This solution was applied onto a quartz glass substrate having a gold comb-shaped electrode formed thereon in the same manner as in Example 1, dried in air at 120 ° C., and then heated in a muffle furnace at 900 ° C. for 1 hour, Transparent tin oxide film containing lithium (film thickness 0.
A moisture sensitive element having a thickness of 4 μm) was obtained. When the humidity-sensitive characteristics of this humidity-sensitive element were measured, it showed almost the same humidity-sensitive characteristics as in Example 1 except that the impedance changed by about 4 digits with respect to the relative humidity change of 10 to 95% RH. Moreover, the responsiveness and temporal stability of this moisture-sensitive element 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 multi-functionalization by combining with 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 by using a humidity sensitive element manufactured using the alkali metal-containing tin oxide thin film of the present invention obtained in Example 1 as a humidity sensor, FIG. 2 shows the response when using the humidity sensitive element prepared by using the alkali metal-containing tin oxide thin film of the present invention obtained in Example 1 as a humidity sensor and using the conventional ceramics humidity sensor. FIG. 3 is a graph showing the responsiveness to the humidity, and the humidity-sensitive element manufactured using the alkali metal-containing tin oxide thin film of the present invention obtained in Example 1 was used as a humidity sensor at 25 ° C. and 70% RH. It is a graph which shows the time-dependent change of the case.

Claims (6)

[Claims] 1. A moisture sensitive thin film comprising an alkali 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 alkali metal is at least one selected from lithium, sodium and potassium. 4. The moisture sensitive thin film according to claim 1, wherein the alkali metal content is 0.1 to 500 mol% with respect to tin. 5. A solution containing an alkali metal compound and a tin compound is applied onto a substrate, dried and then heat-treated.
A method for producing the moisture-sensitive thin film described. 6. The ratio of the alkali metal compound to the tin compound in the solution is such that the alkali metal is 0.1 to 500 relative to tin.
6. The method according to claim 5, which is mol%.