JPH07294861A - Oxide dielectric thin film and manufacturing method thereof - Google Patents

Abstract

(57) [Summary] [Object] To provide an oxide dielectric thin film which has good composition control and crystal orientation, can prevent loss of raw materials, and can be processed at a lower temperature than before, and a method for producing the same. Offer. An oxide dielectric thin film comprising: a step of applying a precursor solution to a substrate; and a step of heating the substrate in a plasma atmosphere of a gas containing at least an oxygen element as a component, and the same. Production method.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an optical waveguide, an optical switch,
The present invention relates to a method for manufacturing an oxide dielectric thin film used for a modulator, a light deflector, and the like.

[0002]

Oxide dielectric materials are piezoelectric, pyroelectric,
Since many of them have various characteristics such as electro-optic effect, they are expected to be applied to non-volatile memories, capacitors, optical waveguides, optical switches, modulators, optical deflectors, etc., and many studies have been conducted. . In particular, for application to electronic devices, which are becoming shorter, smaller, lighter and thinner, thinning of oxide dielectric materials is essential. Furthermore, in order to efficiently develop the various properties of the oxide dielectric material, the composition ratio of the metal element and oxygen is controlled to a predetermined value, and the crystal axis directions are well aligned and the orientation is good. Is desired.

Conventional sputtering, vapor deposition or C
In the method of forming an oxide dielectric thin film such as the VD method, the composition ratio of the metal element in the formed film varies from the composition ratio of the metal element in the supplied raw material, and the oxygen composition ratio is set to a predetermined value. Since it tends to be smaller than the value, it is difficult to obtain a film having a desired composition ratio. Further, according to these methods, the cost of the device is high.

In Japanese Patent Laid-Open No. 1-260780, a solution of a metal alkoxide or a metal organic acid salt is applied onto a substrate in order to form an oxide dielectric thin film whose composition is well controlled and whose crystal axes are aligned. A method for producing an oxide dielectric thin film, which is called a sol-gel method, in which heating, drying, oxidation and crystallization are performed is disclosed. According to this method, the ratio of the metal elements constituting the thin film after coating can be made the same as the ratio in the supplied raw material, and since the heating can be performed under the atmospheric pressure with a high oxygen partial pressure, the composition of oxygen is Almost stoichiometrically stable. In this way, it was possible to obtain an oxide dielectric thin film whose composition ratio was well controlled without passing through a high-cost vacuum process.

[0005]

However, even with the above method, when a part of the metal is vaporized during the drying after coating and the metal composition ratio changes, or the temperature required for crystallization is high. However, since the diffusion and the reaction occur between the substrate and the oxide dielectric thin film, it was not possible to completely prevent the composition change and the mixing of impurities. Moreover, dozens at a time
Since the solution applied to a thickness of nm to several μm is dried and then crystallized, it is possible to improve the crystal orientation and create a single crystal thin film, unlike the case where a thin film is gradually grown from the substrate. The drawback of being difficult still remained.

In order to solve such a problem, various methods have been proposed as a method of applying a solution containing a metal to a substrate and then drying and heating. That is, a method using lamp heating (JP-A-5-229827), a method using excimer laser (JP-A-5-200277), and the like. However, even by these methods, the problem that the raw material such as metal alkoxide, which is the most preferable because it is easily crystallized, is vaporized and lost during heating has not been solved.

Therefore, the present invention provides a method for producing an oxide dielectric thin film, which has good composition control and crystal orientation, which prevents loss of raw materials, particularly metal alkoxide, and which can be processed at a lower temperature than before. The purpose is to provide.

[0008]

A method for producing an oxide dielectric thin film according to the present invention comprises a step of applying a precursor solution containing a metal constituting the thin film onto a substrate, and a gas containing at least an oxygen element. Heating the substrate in a plasma of an atmosphere containing the substrate. In a preferred embodiment of the present invention, a solution prepared by dissolving a metal alkoxide in an organic solvent is used as the precursor solution. In a preferred aspect of the present invention, plasma is used in which the gas containing oxygen element is water vapor and the water vapor partial pressure is 10% or more of the total pressure of the atmosphere. In a preferred embodiment of the invention Li ₂
B ₄ O ₇ or LiNbO ₃ is formed as the oxide dielectric.

In the present invention, the substrate is glass,
An amorphous substance such as fused quartz can also be used. More preferably, a single crystal substrate is used because the crystal orientation of the oxide dielectric thin film can be uniformly aligned with the orientation of the single crystal substrate when a single crystal substrate is used as the substrate. Particularly preferred single crystal substrates include, for example, silicon, sapphire, quartz, magnesium oxide, strontium titanate and the like.

The oxide dielectric thin film which can be formed by the method of the present invention includes lithium niobate (LiNbO ₃ ), lithium borate (Li ₂ B ₄ O ₇ ), PbZrTiO ₃ , BaTiO ₃ and
SrTiO ₃ , PbLaZrTiO ₃ , LiTaO ₃ , ZnO, Ta ₂
It is almost all oxide dielectric thin films such as O ₅ . The film thickness is not particularly limited, but is usually 5 nm to 50 μm,
It is preferably 30 nm to 5 μm.

In the present invention, the precursor solution containing a metal forming the oxide dielectric thin film means, for example, Li, Nb, B, Pb, Zr, Ti, depending on the oxide dielectric to be formed.
Metals such as Ba and Sr and, for example, OCH ₃ , OC
₂ H ₅ , OC ₃ H ₇ , OC ₄ H ₉ , OC ₂ H ₄ OCH _{3, and} other metal alkoxides consisting of alkoxyl groups, or metal organic acid salts such as acetates and oxalates of the above metal elements, or these Is a solution containing an organic solvent capable of dissolving these as a solute, for example, alcohol such as methanol, ethanol, propanol, ethylene glycol, methoxyethanol, or acetic acid, butyric acid, lactic acid, ethyl acetate or the like. The precursor solution can optionally be heated to a temperature in the range up to the reflux temperature before coating.

Examples of the metal alkoxide that can be used in the method of the present invention include LiOCH ₃ , LiOC ₂ H ₅ and Nb.
(OC ₂ H ₅ ) ₅ , B (OCH ₃ ) ₃ , Pb (OC ₄ H ₉ ) ₄ , Zr (O
_{CH 3) 4, Zr (OC} 2 H 5) 4, Zr (OC 3 H 7) 4, Zr (OC 4
H ₉ ) ₄ , Ti (OCH ₃ ) ₄ , Ti (OC ₂ H ₅ ) ₄ , Ti (iso-OC
_{3 H 7) 4, Ti (} OC 4 H 9) 4, Ba (OC 2 H 5) 2, Sr (OC 2
H ₅ ) ₂ , LiOC ₂ H ₄ OCH ₃ or Nb (OC ₂ H ₄ OC
One or more metal alkoxides arbitrarily selected from H ₃ ) _{5 and the} like can be used.

As the metal organic acid salt, LiOCOCH ₃ ,
At least one of Nb (OCOCH ₃ ) ₅ and B (OCOCH ₃ ) ₃ can be used.

Gases containing at least oxygen element for use in plasma treatment include water vapor and oxygen (O 2).
₂ ), carbon dioxide, carbon monoxide, nitrogen dioxide, nitric oxide, air and combinations thereof such as mixtures. However, if the oxidizing property of the gas is too strong, the reaction tends to proceed rapidly and the orientation tends to be lost. Therefore, a gas containing water vapor having both oxidizing property and reducing property is most preferable. The atmosphere for forming plasma may include a gas such as Ar, N ₂ or H ₂ in addition to the gas containing the oxygen element. In that case, the ratio of the gas containing oxygen element is 5
% Or more is preferable.

As a plasma generation condition for the heat treatment, the pressure is 0.1 to 760 Torr, more preferably 10 to
It is preferably in the range of 200 Torr. This is because if the total pressure of the atmosphere for generating plasma is too low, the oxidizing action will be insufficient, and if it is too high, it will be difficult to generate plasma. The temperature range for heating the substrate by plasma or another heating means can be appropriately selected depending on the precursor solution used and the oxide dielectric to be formed, but in general,
The temperature is 100 to 1200 ° C, preferably 300 to 800 ° C.

In order to generate plasma in the above pressure range, it is necessary to apply an electromagnetic field. In that case, an alternating electromagnetic field is preferable to a direct current. This is because the alternating electromagnetic field can generate plasma in a wider pressure range. More preferable electromagnetic field is 30 kHz to 30
AC electromagnetic field of GHz, that is, high frequency.

When plasma is generated in an atmosphere containing water vapor, the water vapor partial pressure is 10% or more of the total pressure,
It is preferably 20% to 50%, but since H ₂ O is composed of hydrogen and oxygen and has a weak oxidizing power,
This is because the partial pressure is preferably 20% or more, and plasma is less likely to be generated when the partial pressure exceeds 50%. In this case, Ar, N ₂ , H ₂ or the like can be used as the other gas component.

The method of the present invention is effective for use in the production of any oxide dielectric by the sol-gel method using metal alkoxides and metal organic acid salts, but in particular, oxidation containing an element having a small atomic weight such as Li or B is performed. The effect is remarkable when a dielectric substance is formed. This is because alkoxides and organic acid salts, which are the raw materials for these light elements, also have a small molecular weight and a high vapor pressure, and are liable to vaporize during drying and heating to change the composition. Accordingly, Li ₂ B is an oxide dielectric material that can be advantageously formed by applying the method of the present invention.
₄ O ₇ or LiNbO ₃ and the like.

The sol-gel method applicable in the present invention is a method in which a metal is dissolved or dispersed in the form of an alkoxide or a salt in a solvent, and a solid oxide is obtained through a sol or gel form by heat treatment or the like. When this method is applied to the present invention, for example, one or more metal alkoxides, for example alcoholic solutions such as LiOC ₂ H ₅ and Nb (OC ₂ H ₅ ) ₅ , optionally water and hydrochloric acid, nitric acid, amines, etc. Is added, and if necessary, a heat treatment operation such as heating in a temperature range up to the reflux temperature is performed to form a sol of the precursor solution, and the sol is applied to the substrate by a method such as dip coating or spin coating. Then, it is heated at a temperature of 300 to 700 ° C. for 0.5 to 10 hours to be dried, oxidized and crystallized to form an intended oxide dielectric thin film.

[0020]

It is considered that the following steps are taken as the temperature rises as a route for the decomposition of metal alkoxide or metal organic acid salt to produce metal oxide crystals. That is, (1) a step in which a solvent such as alcohol is evaporated, (2) a step in which an alkoxyl group or an organic acid group is separated from a raw material molecule, (3)
The step of forming a metal-oxygen-metal bond (gelation), (4)
The stage where crystallization occurs due to rearrangement of metal-oxygen-metal chains.

In the above process, the temperature condition for transition from (2) to (3) depends on the amount of water in the system and the partial pressure of the gas containing oxygen element in the atmosphere.
It was also known that the higher the partial pressure of the gas containing the oxygen element, the lower the temperature of the reaction (3). (4)
Since the stage of does not start until the process of (3) is completed,
If crystallization is to be carried out at a low temperature, it is important to carry out step (3) at a low temperature.

Therefore, according to the present invention, by applying plasma in an atmosphere containing a gas containing oxygen element or containing water vapor, the reactions (2) to (4) can be carried out at a lower temperature than before, The reactions of (2) and (3) proceed with oxidation of metal alkoxides and metal organic acid salts through the presence of water and oxygen, but the activity of oxygen is greatly increased in plasma, and the oxidation reaction occurs even at low temperatures. Since it proceeds rapidly, it is possible to accelerate the decomposition reaction of the metal alkoxide and the metal organic acid salt. Therefore, an oxide dielectric thin film having good crystallinity and orientation and having a desired composition can be easily prepared.

[0023]

【Example】

Example 1 A lithium niobate thin film was formed on a sapphire single crystal (001) substrate by a sol-gel method using a metal alkoxide as a raw material and evaluated. First, follow the steps below
A precursor solution was prepared. Pentaethoxyniobium (Nb (C
₂ H ₅ O) ₅ ) and ethoxylithium (LiOC ₂ H ₅ ) each having a concentration of 0.05 mol / l are mixed,
Refluxed at 78.5 ° C for 24 hours. Then, concentrate each concentration of niobium and lithium to 0.5 mol / l under reduced pressure,
The obtained solution was used as a precursor solution. As the substrate, a (001) plane single crystal substrate of sapphire which was mirror-polished was used. After ultrasonically cleaning this substrate with acetone,
Pretreatment was performed by sequentially performing treatment with hydrochloric acid (20% by volume), washing with pure water, and drying.

After spin coating the above precursor solution once on the substrate, it was placed in a vacuum vessel under a nitrogen atmosphere at atmospheric pressure.
Dry at 80 ℃ for 30 minutes, and once in the container 100mT
After exhausting to orr or lower, supply a mixed gas of steam (partial pressure 20%) and argon (residual pressure) to adjust the pressure to 5To.
After stabilizing to rr, an atmosphere having a composition of 20% steam and 80% argon was formed. 13.5 between parallel plate electrodes
The atmosphere temperature was raised to 400 ° C. at 10 ° C./min while applying a high-frequency electromagnetic field of 6 MHz and 150 W to generate plasma, and the temperature was kept at 400 ° C. for 2 hours. As a result, a 0.15 μm thick lithium niobate thin film could be formed on the sapphire single crystal substrate. The crystallinity of the oxide dielectric thin film was evaluated by the half-value width of the X-ray diffraction peak by each orientation plane obtained by Cu-Kα ray. The lithium niobate thin film on the sapphire (001) single crystal substrate produced by the above method has a (001) axis orientation, and (00)
The half width of the X-ray diffraction peak of the 6) plane was 0.75 degree.

Comparative Example 1 In the same manner as in Example 1, preparation of the precursor solution, pretreatment of the substrate, spin coating of the precursor solution on the substrate once, and drying were performed. Then, the sample is treated under atmospheric pressure with oxygen (partial pressure 20
%) + Steam (partial pressure 2%) + argon (residual pressure) mixed gas while the ambient temperature is 10 ° C./min.
When the temperature was raised to 0 ° C. and kept at 400 ° C. for 2 hours, a thin film having a thickness of about 0.2 μm could be formed on the sapphire single crystal substrate. The result of X-ray diffraction of this thin film is
It was found that the diffraction peak was extremely weak and the crystallization of lithium niobate did not proceed. The full width at half maximum of the X-ray diffraction peak of the (006) plane was extremely large at 8.0 degrees.

Comparative Example 2 In the heating step, the temperature was raised to 600 ° C. at 10 ° C./min to 60
The same operation as in Comparative Example 1 was performed except that the temperature was kept at 0 ° C. for 2 hours. As a result, a thin film having a thickness of about 0.15 μm could be formed on the sapphire single crystal substrate. As a result of X-ray diffraction of this thin film, a (001) oriented lithium niobate crystal film was obtained, but the half width of the X-ray diffraction peak of the (006) plane was as large as 1.4 degrees, The crystallinity was inferior to the case of using plasma together.

Example 2 A lithium borate thin film was formed on a sapphire single crystal (012) substrate by a sol-gel method using a metal alkoxide as a raw material and evaluated. A precursor solution used in the sol-gel method was prepared by the following procedure. Trib Toxibolone (B
(C ₄ H ₉ O) ₃ ) and lithium ethoxide (LiOC
₂ H ₅ ) ethanol solutions each having a concentration of 0.05 mol / l were mixed in a ratio of 2: 1 and stirred at 0 ° C. for 24 hours. Then, under reduced pressure, B was 0.5 mol / l and Li was 0.25 mol.
It concentrated so that it might become / l, and the obtained solution was used as a precursor solution.

As the substrate, a (012) plane single crystal substrate of sapphire which was mirror-polished was used. This substrate was ultrasonically cleaned with acetone, and then subjected to hydrochloric acid (20 vol%) treatment, pure water cleaning and drying in order to perform pretreatment. After spin-coating the above-mentioned precursor solution once on the substrate, it was dried in a vacuum vessel under a reduced pressure nitrogen atmosphere at 40 ° C. for 30 minutes, and then the vessel was evacuated to 10 mTorr or less and oxygen (partial pressure 10 %) + Steam (partial pressure 10%) + argon (residual pressure) mixed gas, and pressure is 60 Torr
Stabilized to form an atmosphere of 10% oxygen, 10% water vapor and 80% argon.

While the plasma is generated by the microwave of 2.45 GHz and 200 W by the microwave plasma generator that guides the quartz tube into the microwave waveguide, the ambient temperature is raised to 550 ° C. at 10 ° C./min and 550 ° C. Hold at 2 ° C for 2 hours. The sample was placed at a distance of 25 cm from the center of the waveguide. As a result, the thickness on the sapphire single crystal substrate is about 0.
A 1 μm lithium borate thin film could be formed.

The crystallinity of the oxide dielectric thin film was evaluated by Cu.
The half-value width of the X-ray diffraction peak by each orientation plane obtained by -Kα ray was evaluated. The lithium borate thin film on the sapphire (012) plane single crystal substrate manufactured by the above method is (12
2) It was axially oriented, and the half-value width of the X-ray diffraction peak of the (244) plane was 1.1 degrees. Further, the abundance ratio of Li and B in this lithium borate thin film was approximately 1: 2 when evaluated by secondary ion mass spectrometry.

Comparative Example 3 In the heating step, the temperature was raised to 550 ° C. at 10 ° C./min, and 55
The same operation as in Comparative Example 1 was performed except that the temperature was kept at 0 ° C. for 2 hours. As a result, a thin film having a thickness of about 0.1 μm could be formed on the sapphire single crystal substrate. X of this thin film
The result of line diffraction revealed that the diffraction peak was extremely weak and that the crystallization of lithium borate did not proceed.
The full width at half maximum of the X-ray diffraction peak of the (006) plane was extremely large at 8.0 degrees.

Comparative Example 4 In the heating step, the temperature was raised to 800 ° C. at 10 ° C./min to 80
The same operation as in Comparative Example 1 was performed except that the temperature was kept at 0 ° C. for 2 hours. As a result, a thin film having a thickness of about 0.1 μm could be formed on the sapphire single crystal substrate. X of this thin film
From the results of line diffraction, crystalline lithium borate (Li ₂ B
It was confirmed that a ₄ O ₇ ) thin film had been formed, but the obtained thin film had almost no orientation and was a polycrystal with no uniform orientation. The half width of the X-ray diffraction peak of the (244) plane was 2.0 degrees. Li and B in this lithium borate thin film
When the abundance ratio of was evaluated by secondary ion mass spectrometry,
It was 1: 1.7.

[0033]

As described above, according to the present invention, the substrate,
Preferably, when forming an oxide dielectric thin film using a metal alkoxide as a raw material on a single crystal substrate, a precursor solution is applied to the substrate and an atmosphere containing a gas containing at least an oxygen element is contained in the components. By heating the substrate in plasma, it is possible to produce an oxide dielectric thin film having a better crystallinity and a better orientation than ever before at a low temperature.

Claims (6)

[Claims] 1. An oxide dielectric thin film comprising: a step of applying a precursor solution to a substrate; and a step of heating the substrate in a plasma atmosphere of a gas containing at least oxygen element as a component. Manufacturing method. 2. The method according to claim 1, wherein the precursor solution is a solution of a metal alkoxide or a metal organic acid salt in an organic solvent. 3. The method according to claim 1, wherein the gas containing oxygen element is water vapor, and plasma in an atmosphere having a water vapor partial pressure of 10% or more of the total pressure is used. 4. The oxide dielectric is Li ₂ B ₄ O ₇ or Li.
The method according to claim 1, which is NbO ₃ . 5. The substrate is silicon, sapphire, quartz, magnesium oxide, strontium titanate.
The method according to any one of to 4. 6. An oxide dielectric thin film produced by the method according to claim 1.