JPH04202018A - Production of thin lithium niobate film - Google Patents

Abstract

PURPOSE:To enhance crystallinity by hydrolyzing a multiple ethoxide consisting of ethoxy lithium and pentaethoxy niobium, adding polyhydric alcohol or its deriv., concentrating them, coating a substrate with the resulting precursor, calcining this precursor and repeating the coating and firing. CONSTITUTION:Ethoxy lithium is mixed with pentaethoxy niobium in 1:1 molar ratio in an N2 atmosphere and the mixture is dissolved in alcohol such as ethanol to prepare multiple ethoxide. This ethoxide is hydrolyzed by adding water, the prescribed amt. of polyhydric alcohol having a low vapor pressure and not forming an azeotrope with alcohol, e.g. ethylene glycol is added and they are concentrated to obtain the soln. of a precursor of lithium niobate. This soln. is dropped on a substrate of Al2O3, etc., dried by rotation at a constant speed, heated in a gaseous O2-steam mixed atmosphere and further heated to >=400 deg.C in dry air. These coating, drying and heating process are repeated to obtain a thin crystalline lithium niobate film subjected to C-axis orientation.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a lithium niobate thin film, and in particular to a method for producing a lithium niobate thin film with high purity, composition controlled to a stoichiometric ratio, and sinterable at low temperatures. This relates to a manufacturing method.

[Prior Art] Lithium niobate has piezoelectricity and pyroelectricity, so its single crystals are used as SAW devices and infrared sensors. Lithium niobate has a solid solution region near the stoichiometric composition, and the harmonic melt composition differs from the stoichiometric composition.
It has been difficult to grow a homogeneous lithium niobate single crystal with a controlled stoichiometric composition. Furthermore, since single crystal manufacturing requires high temperatures of 1000° C. or higher, the manufacturing equipment is large.

Therefore, attempts have been made to create thin films of highly crystalline sintered bodies of lithium niobate that are dense and free of impurities, and to use them as SAW devices and infrared sensors instead of single crystals by polarizing them during sintering. It's here. One method is to apply a solution in which powdered lithium niobate is uniformly dispersed onto the substrate and sinter it.
It has been difficult to create a dense film without cracks or pinholes. Furthermore, since sintering requires a high temperature of 1000° C. or higher, the materials for the substrate are limited and the manufacturing equipment becomes large.

Other methods for producing lithium niobate films, such as sputtering, vacuum evaporation, and gas phase reaction methods, are being considered, but with these methods, it is difficult to obtain a dense film and it is extremely difficult to control the stoichiometric ratio. Due to the difficulty, excellent lithium niobate thin films have not been obtained.

On the other hand, it is already known that a stoichiometric lithium niobate thin film can be obtained by the following method.

That is, a composite alkoxide is prepared by mixing ethoxylithium and pentaethoxyniobium at a molar ratio of 1:1, the resulting solution is concentrated, a substrate is immersed in the precursor solution, and it is dried. A lithium niobate thin film can be created on the substrate by repeating the step of heating and baking at a temperature of 400° C. or more at least once (Patent Application No. 1983).
-102422).

[Problem to be solved by the invention] In the conventional method, a coating process called dip coating was performed in which the substrate was immersed in a precursor solution, but the amount of solution required was large and it was difficult to coat both sides of the substrate. There was a problem with it being put away. On the other hand, there is a method called spin coating in which the required amount of the precursor solution is dropped onto one side and a uniform film is created on the substrate by rotating the substrate. It has the advantage that it can be applied only to However, in the conventional method, the solvent for the precursor solution is alcohol, and the vapor pressure at room temperature is high, so the solution dries immediately after dropping, making it difficult to obtain a high-quality thin film.

The purpose of the present invention is to solve the above-mentioned problems, and to produce a lithium niobate thin film that can be performed by spin coating in the coating process, has high purity, and can be fired at low temperatures with a composition controlled to a stoichiometric ratio. Our goal is to provide the following.

[Means for Solving the Problems] The present invention involves preparing a composite alkoxide by mixing and dissolving ethoxylithium and pentaethoxyniobium in alcohol at a molar ratio of 1:1, and then preparing a composite alkoxide, which is then dissolved in a diluted solution or its hydrated solution. A polyhydric alcohol or its derivative which has a low saturated vapor pressure and does not form an azeotrope with the alcohol is added to the decomposition solution, concentrated, and the precursor solution is applied onto a substrate and heated at 400°C or higher. This method of producing a lithium niobate thin film is characterized in that the process of heating and baking at a temperature is repeated at least once or more to form a lithium niobate thin film on the substrate.

The method of the present invention will be detailed as follows. Lithium alkoxide and niobium alkoxide are used as starting materials, and are mixed and dissolved in dehydrated and purified alcohol so that the molar ratio thereof is 1:1. The alcohol only needs to be liquid at room temperature, and ethanol is preferably used.

A composite alkoxide is produced by reacting this solution while stirring and refluxing for 22 hours or more. These operations are performed in a dry nitrogen atmosphere because metal alkoxides are easily hydrolyzed by moisture in the air.

The solution prepared in this way can be directly or added to water.
- After decomposition, use it as a precursor solution and drop it onto the substrate.

When hydrolysis is carried out, water in an amount equal to or greater than the number of moles required to hydrolyze the composite metal alkoxide is added dropwise to the prepared solution in the form of decarbonated water, preferably diluted with alcohol as a solvent. After that, stirring and refluxing are continued to complete the reaction, and a LiNbO3 precursor is obtained. Obtained LiN
After adding an amount of polyhydric alcohol or its derivative solvent to the desired concentration to the bO3 precursor solution, all the alcohol solvent is evaporated and concentrated to obtain a precursor solution.

This solution is dropped onto the substrate and the substrate is rotated at a constant speed to form a coating film on the surface of the substrate.

After drying for several minutes, a single-phase lithium niobate thin film can be obtained by heat treatment in a mixed stream of oxygen and water vapor and then in a stream of dry oxygen. After this,
1 rotation of application.

By repeating the drying and heat treatment steps several times, a lithium niobate thin film with a desired thickness can be obtained. The thickness of the lithium niobate thin film can be varied in the range of several hundred angstroms to several micrometers.

In the present invention, the polyhydric alcohol or its derivative may be one that is liquid at room temperature and has a low saturated vapor pressure and does not form an azeotrope with the alcohol used to form the complex alkoxide, such as ethylene glycol, Examples include cellosolve-based, carbide-based polyhydric alcohols, and derivatives thereof.

U Effect J In the present invention, by adding a polyhydric alcohol or its derivative solvent during concentration, the saturated vapor pressure of the precursor solution can be lowered, and when the solution is dropped onto the substrate during spin coating, By rotating the substrate in a drying rack, a coating film of uniform thickness can be obtained. 1 Furthermore, since the polyhydric alcohol or its derivative used in the present invention hardly forms an azeotrope with the alcohol solvent, only the alcohol solvent can be evaporated during concentration, and the calculated amount of the polyhydric alcohol or its derivative can be evaporated during concentration. By adding , the concentration of the precursor solution can be accurately controlled. Furthermore, since the solubility of the precursor is higher in a polyhydric alcohol or its derivative solvent than in an alcohol solvent, a highly concentrated precursor solution can be obtained, and in the case of dip coating and spin coating Dorara. Even thicker films can be formed in one step.

[Example] Hereinafter, the present invention will be explained in more detail with reference to Examples. However, the scope of the present invention is not limited in any way by the following examples.

A composite alkoxide solution of niobium and lithium, ethoxylithium: LiO02H5 and pentaethoxyniobium: Nb (OC9H5) 5, were weighed out at a Li:Nb ratio of 1:1, and placed in dehydrated and purified ethanol. mixed and dissolved.

This solution was stirred and refluxed for 24 hours. These operations were performed in a dry nitrogen atmosphere because each raw material alkoxide was easily hydrolyzed by moisture in the air.

Preparation of Niobium Acid Tsum Precursor Decarbonated water diluted with ethanol was added dropwise to the obtained composite alkoxide to perform hydrolysis. Further, stirring and reflux were continued for 24 hours to complete the reaction. After this, a predetermined amount of ethylene glycol is added, ethanol is evaporated,
Precursor concentration of lithium niobate: 1.0 mol/l
A solution was created.

Coating a lithium dibutate precursor onto a substrate and heating the resulting lithium niobate precursor concentration to 1.0m01
/l) layer line solution a -A I 203 (001
) was dropped onto the substrate, rotated at a constant rotation speed, and dried. Thereafter, the coating film of the lithium niobate precursor obtained was heated at 350°C and 400°C.

held in a mixed atmosphere of oxygen and water vapor for 1 hour and then in dry air for 1 hour under three different temperature conditions of 450 ° C;
Heat treatment was performed. Apply this twice.

By repeating the process of drying and heat treatment (350°C, 400°C, 450°C), crystalline LiNb with dense C-axis orientation is obtained.
We were able to synthesize an O3 thin film. As a result of examining these obtained thin films by powder X-ray diffraction, it was found that although no crystallization was observed in the 350°C heat treatment, (006)-oriented crystallization was observed in the 400°C heat treatment. It was confirmed that the film had an almost perfect orientation.

Ta.

FIG. 1 shows X-ray diffraction patterns of thin films finally obtained by changing the heat treatment temperature in Examples. In the diagram of Fig. 1, the parts marked with a circle are (0) of the 1iNbo3 thin film.
06) The peak of the diffraction line marked with Δ is the α of the substrate.
-AI203 (006) diffraction line peaks are shown.

[Effects of the Invention] As described above, by using the method of the present invention, it is possible to produce a thin film of oriented sodium niobate with high crystallinity by the spin coating method, which is difficult to do with conventional methods. Therefore, this invention is expected to be utilized in various applications such as piezoelectric elements and pyroelectric elements using lithium niobate.

[Brief explanation of the drawing]

Figure 1 shows the 1-1Nb obtained in one embodiment of the present invention.
○3 It is an X-ray diffraction diagram of the thin film.

Claims (1)

[Claims] (1) Create a composite alkoxide by mixing and dissolving ethoxylithium and pentaethoxyniobium in alcohol at a molar ratio of 1:1, and the resulting solution or its hydrolyzed solution has a low saturated vapor pressure. and at least one step of adding and concentrating a polyhydric alcohol or a derivative thereof that does not form an azeotrope with the alcohol, applying the precursor solution on a substrate, and heating and baking it at a temperature of 400° C. or higher.
A method for producing a lithium niobate thin film, comprising forming a lithium niobate thin film on the substrate by repeating the process several times or more.