JP3188541B2 - Method for producing single crystal thin film by liquid phase epitaxy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a single crystal thin film by liquid phase epitaxy (liquid phase epitaxial growth), and more particularly to the production of a single crystal thin film by liquid phase epitaxy to obtain a single crystal thin film free from macro defects. Here is a suggestion on how to do it.

[0002]

2. Description of the Related Art In recent years, with the development of optical IC technology, various optical thin films provided for forming a waveguide having higher optical characteristics have been actively studied. This optical thin film is manufactured by a method such as a sputtering method, a thermal diffusion method, a CVD method, an ion exchange method, or an epitaxial growth method. Particularly, in the case of liquid phase epitaxy, a thin film having high crystallinity is easily obtained. This is an advantageous method.

[0003] The liquid phase epitaxy is a method in which a component to be deposited as a thin film is melted in a flux, and this flux is brought into a supercooled state and brought into contact with a substrate having a similar crystal structure to deposit and grow an optical thin film. is there.

[0004]

However, in the case of this technique for growing a single crystal thin film by liquid phase epitaxy, even if a dry raw material is used, a small amount of crystallization water contained in the raw material is used. Is decomposed and pinhole-shaped macro defects are generated on the surface of the epitaxially grown thin film. That is, when a lithium niobate single crystal thin film is formed, gas (e.g., water of crystallization) contained in Li ₂ CO ₃ , Nb ₂ O _5, or V ₂ O ₅ used as a raw material is thermally decomposed to generate gas. This gas adheres to the surface of the thin film during the growth of a single crystal, resulting in pinhole-shaped macro defects as shown in FIG. Needless to say, the presence of such macro defects generated in the thin film means that the film cannot be used as an optical element, and therefore, the production yield is significantly reduced. An object of the present invention is to propose a technique for growing a single crystal thin film that does not cause such macro defects.

[0005]

Means for Solving the Problems In order to produce a single crystal thin film, the inventors mix single crystal thin film forming materials (hereinafter simply referred to as "raw materials"), and then heat and melt the mixed raw materials. Li ₂ C used as a raw material
In addition to O ₃ , Nb ₂ O ₅ , V ₂ O _5, etc., when they react with each other, they form hydrates with the passage of time and decompose to generate gas, which is taken into the thin film It has been found that these defects result in pinhole defects, that is, macro defects on the surface of the thin film. This finding suggested a solution to the above technical problem. In other words, it is sufficient to prevent the formation of hydrates, so that the raw materials having a particularly high melt composition ratio, ie, Li ₂ CO ₃ , Nb ₂ O ₅ , V ₂ O ₅
It is concluded that prior to mixing, it is effective to preheat and calcine each in an air atmosphere to remove adhering water and water of hydration (crystal water) contained in these raw materials. And completed the present invention.

That is, the present invention relates to a method of growing a single-crystal thin film by mixing a material for forming a single-crystal thin film, heating and melting the material, and then bringing the single-crystal substrate into contact with the melt. In the production method, a single-crystal thin film is produced by liquid phase epitaxy, wherein each of the materials is individually heated before being mixed before the respective materials are mixed.

In the present invention, the calcining is preferably carried out at a temperature not lower than the thermal decomposition temperature of the crystallization water contained in each material and below the melting point of each material.
The heating time is preferably 1 to 100 hours, more preferably 5 to 24 hours. Further, the present invention is characterized in that the material (raw material) for forming a single crystal thin film mainly comprises an oxide of lithium and a carbonate compound thereof, an oxide of niobium, and an oxide of vanadium.

[0008]

The single crystal thin film produced by the method of the present invention is a single crystal thin film of lithium niobate, which is produced by a liquid phase epitaxy method.
₂ CO ₃ (this is the same even if this Li ₂ CO ₃ is Li ₂ O), Nb
₂ O ₅ , V ₂ O _5, etc. have a high melt composition ratio and are preferably used. In general, these raw materials are substances that absorb moisture in the air with the passage of time even during storage, for example, and are liable to form pseudohydrates. And, once these powder raw materials have reached the water of crystallization, they are at the vaporization temperature of water.
Even when heating at 100 ° C., it is difficult to remove the water of hydration (water of crystallization) even if the attached water can be removed.

Further, Li ₂ CO ₃ or Li ₂ O and V ₂ O ₅ each react at around 300 ° C. to form lithium vanadate and start solidification, so that water of the powder raw material is not removed in advance. Then, water of crystallization is taken into lithium vanadate. This lithium vanadate is easily soluble in water,
It has the property of easily taking in water of crystallization. For these reasons, when using these raw materials, before mixing them, that is, before the main calcination, the temperature at which the crystallization water of the powder raw material can be removed in advance (≧ 380 ° C., the same applies to all raw materials)
To prevent gas formation.
In the present invention, it is preferable that the heating and burning of the raw material powder be performed under the conditions shown in FIG. 2 and Table 1.

[0010]

[Table 1]

[0011] In accordance with the holiday making method described above,
The water of crystallization of the powder raw material can be reliably removed, so that the generation of gas during the growth by liquid phase epitaxy can be suppressed reliably, and the pinhole-shaped macrodefects generated by the gas adhering to the thin film surface can be reduced. It is possible to obtain a single crystal thin film without any. As a result, the reduction in yield, which is a problem of the related art, can be effectively solved.

In the method of the present invention, the calcining temperature is desirably in a temperature range from the thermal decomposition temperature of the crystallization water to the melting point of the raw material. If the temperature is lower than the so-called thermal decomposition temperature of water of crystallization, the water of crystallization cannot be removed by thermal decomposition,
On the other hand, at a temperature higher than the melting point of the raw material, the raw material is melted and gas is difficult to escape.

[0013]

The present invention will be described in more detail with reference to the following examples. The conditions and results of the examples and comparative examples are collectively described later in Table 2.

[0014]

Embodiment 1 In this embodiment, a case where a lithium niobate single crystal is grown by a liquid phase epitaxial growth method will be described. (1) Before filling the following materials into a platinum crucible,
₂ CO ₃ : 100g at 450 ° C for 2 hours, Nb ₂ O ₅ : 100g at 600 ° C
For 2 hours, and heated at 450 ° C. for 5 hours with V ₂ O ₅ : 260 g and calcined. (2) Li ₂ CO ₃ : 42.2 mol% of the burned Nb ₂ O ₅ : 5.
8 mol%, V ₂ O ₅ : 52.0 mol%, and 57.4 mol% of the theoretical amount of LiNbO _{3 that} can precipitate Na ₂ CO ₃ from the melt composition.
Mol%, placed 5 mol% of the MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition, and becomes thus prepared mixture in a platinum crucible, heated in an electric furnace, lithium vanadate - niobium A lithium oxide-based melt was prepared. This melt was stirred and held in a liquid phase epitaxy apparatus using a propeller until the melt composition became uniform. (3) On the other hand, a single crystal of lithium tantalate having a diameter of 2 inches and a thickness of 1 mmt was optically polished on its (0001) plane, and then mounted on a substrate holder with its polished surface facing down. (4) Cool the melt at 930 ° C at a cooling rate of 60 ° C per hour.
After slowly cooling the substrate, the substrate was immersed in the melt for 8 minutes while rotating at 20 rpm. The growth rate of the lithium niobate single crystal thin film was 1 μm / min. (5) Next, the substrate material is pulled out of the melt, and the melt is shaken off by rotating the melt at 1,000 rpm for 3 minutes, and then gradually cooled to room temperature at a rate of 1 ° C./min. Thereby, a lithium niobate single crystal thin film of about 8 μm was formed on the substrate material. (6) Observation of the obtained lithium niobate single crystal thin film showed no pinhole-like macro defects on the thin film surface as shown in FIG.

[0015]

Example 2 (1) Before filling the following raw materials into a platinum crucible, first, Li ₂ C
O ₃ : 100 g at 600 ° C. for 2 hours, Nb ₂ O ₅ : 100 g at 900 ° C. for 2 hours
Time, V ₂ O ₅ : 260 g was heated at 600 ° C. for 5 hours and calcined. (2) Li ₂ CO ₃ : 42.2 mol% of the burned Nb ₂ O ₅ : 5.
8 mol%, the V ₂ O _5: 52.0 mol%, 57.4 mol% more of the same Na ₂ CO ₃ with respect to the theoretical amount of precipitation can be LiNbO ₃ from the melt composition, which can precipitate the MgO from the melt composition A mixture prepared to be 5 mol% with respect to the theoretical amount of LiNbO ₃ was placed in a platinum crucible and heated in an electric furnace to produce a lithium vanadate-lithium niobate-based melt. This melt was stirred and held in a liquid phase epitaxy apparatus using a propeller until the melt composition became uniform. (3) On the other hand, a single crystal of lithium tantalate having a diameter of 2 inches and a thickness of 1 mmt was optically polished on its (0001) plane, and then mounted on a substrate holder with its polished surface facing down. (4) Cool the melt at 930 ° C at a cooling rate of 60 ° C per hour.
After slowly cooling the substrate, the substrate was immersed in the melt for 8 minutes while rotating at 20 rpm. The growth rate of the lithium niobate single crystal thin film was 1 μm / min. (5) Next, the substrate material is pulled out of the melt, and the melt is shaken off by rotating the melt at 1,000 rpm for 3 minutes, and then gradually cooled to room temperature at a rate of 1 ° C./min. As a result, a lithium niobate single crystal thin film of about 8 μm was formed on the substrate material. (6) Observation of the obtained lithium niobate single crystal thin film revealed no pinhole-like macro defects on its surface.

[0016]

[Comparative Example 1] (1) In this comparative example, the material such as Li ₂ CO ₃ , Nb ₂ O ₅ , V ₂ O _5, etc. is not heated before mixing, but is commercially available. Was used as is. (2) Li ₂ CO ₃ : 42.2 mol%, Nb ₂ O ₅ : 5.8 mol%, V ₂ O ₅ :
52.0 mol%, Na ₂ CO ₃ can be precipitated from the melt composition
57.4 mol% based on the theoretical amount of LiNbO _3, put a mixture prepared as a 5 mol% of MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition in a platinum crucible, heated in an electric furnace Then, a lithium vanadate-lithium niobate-based melt was produced. The melt was stirred and held using a propeller in a liquid phase epitaxial apparatus until the melt composition became uniform. (3) On the other hand, a single crystal of lithium tantalate having a diameter of 2 inches and a thickness of 1 mmt was optically polished on its (0001) plane, and then mounted on a substrate holder with its polished surface facing down. (4) Cool the melt at 930 ° C at a cooling rate of 60 ° C per hour.
After slowly cooling the substrate, the substrate was immersed in the melt for 8 minutes while rotating at 20 rpm. The growth rate of the lithium niobate single crystal thin film was 1 μm / min. (5) Next, the substrate material is pulled out of the melt, and the melt is shaken off by rotating the melt at 1,000 rpm for 3 minutes, and then gradually cooled to room temperature at a rate of 1 ° C./min. As a result, a lithium niobate single crystal thin film of about 8 μm was formed on the substrate material. (6) Observation of the obtained lithium niobate single crystal thin film revealed that pinhole-shaped macro defects were found on the surface of the thin film. (See FIG. 1).

[0017]

Comparative Example 2 (1) Before filling the following raw materials into a platinum crucible, first add Li ₂ C
O ₃ : 100g at 450 ℃ for 0.5 hours, Nb ₂ O ₅ : 100g at 600 ℃
For 0.5 hours, 260 g of V ₂ O ₅ was heated at 450 ° C. for 0.5 hours. _{(2) Li 2 CO 3:} 42.2 mol%, Nb ₂ O _5: 5.8 mol%, V ₂ O _5:
52.0 mol%, Na ₂ CO ₃ can be precipitated from the melt composition
57.4 mol% based on the theoretical amount of LiNbO _3, put a mixture prepared as a 5 mol% of MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition in a platinum crucible, provisionally in an electric furnace Baking was performed to produce a lithium vanadate-lithium niobate-based melt. This melt was stirred and held in a liquid phase epitaxy apparatus using a propeller until the melt composition became uniform. (3) On the other hand, after a 2 inch φ, 1 mmt thick lithium tantalate single crystal is optically polished on its (0001) plane,
The crystal was mounted on a substrate holder with its polished surface facing down. (4) 930 ° C at a cooling rate of 60 ° C per hour
After slowly cooling the substrate, the substrate was immersed in the melt for 8 minutes while rotating at 20 rpm. The growth rate of the lithium niobate single crystal thin film was 1 μm / min. (5) Next, the substrate material is lifted out of the melt, and the melt is shaken off by rotating the melt at 1,000 rpm for 3 minutes, and then gradually cooled to room temperature at a rate of 1 ° C./min. As a result, a lithium niobate single crystal thin film of about 8 μm was formed on the substrate material. (6) Observation of the obtained lithium niobate single crystal thin film revealed that pinhole-shaped macrodefects were found on the surface of the thin film, and that both the temperature and time for the firing were insufficient. ５5 had occurred.

[0018]

[Comparative Example 3] (1) Before filling the raw materials in a platinum crucible, 100 g of Li ₂ CO _{3 was} added.
2 hours at 100 ° C, 100 g of Nb ₂ O ₅ at 100 ° C for 2 hours, V
260 g of ₂ O ₅ was heated and calcined at 100 ° C. for 5 hours. (2) Li ₂ CO ₃ : 42.2 mol%, Nb ₂ O ₅ : 5.8 mol%, V ₂ O ₅ :
52.0 mol%, Na ₂ CO ₃ can be precipitated from the melt composition
57.4 mol% based on the theoretical amount of LiNbO _3, put a mixture prepared as a 5 mol% of MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition in a platinum crucible, provisionally in an electric furnace Baking was performed to produce a lithium vanadate-lithium niobate-based melt. This melt was stirred and held in a liquid phase epitaxy apparatus using a propeller until the melt composition became uniform. (3) On the other hand, a 2 inch φ, 1 mmt thick lithium tantalate single crystal was optically polished on its (0001) plane.
This crystal was mounted on a substrate holder with its polished surface facing down. (4) Cool the melt at 930 ° C at a cooling rate of 60 ° C per hour.
After slowly cooling the substrate, the substrate was immersed in the melt for 8 minutes while rotating at 20 rpm. The growth rate of the lithium niobate single crystal thin film was 1 μm / min. (5) Next, the substrate material is pulled out of the melt, and the melt is shaken off by rotating the melt at 1,000 rpm for 3 minutes, and then gradually cooled to room temperature at a rate of 1 ° C./min. As a result, a lithium niobate single crystal thin film of about 8 μm was formed on the substrate material. (6) Observation of the obtained lithium niobate single crystal thin film revealed that pinhole-shaped macro defects were present on the surface of the thin film, and the drying temperature of the raw material was low.
Individuals had also occurred.

[0019]

[Comparative Example 4] (1) Before filling the raw materials in a platinum crucible, 100 g of Li ₂ CO _{3 was} added.
2 hours at 250 ° C, 100 g of Nb ₂ O ₅ at 250 ° C for 2 hours, V
₂ O _5: 260 g was heated for 5 hours firing at 250 ° C.. (2) Li ₂ CO ₃ : 42.2 mol%, Nb ₂ O ₅ : 5.8 mol%, V ₂ O ₅ :
52.0 mol%, Na ₂ CO ₃ can be precipitated from the melt composition
57.4 mol% based on the theoretical amount of LiNbO _3, put a mixture prepared as a 5 mol% of MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition in a platinum crucible, provisionally in an electric furnace Baking was performed to produce a lithium vanadate-lithium niobate-based melt. This melt was stirred and held in a liquid phase epitaxy apparatus using a propeller until the melt composition became uniform. (3) On the other hand, a 2 inch φ, 1 mmt thick lithium tantalate single crystal was optically polished on its (0001) plane.
This crystal was mounted on a substrate holder with its polished surface facing upward. (4) Cool the melt at 930 ° C at a cooling rate of 60 ° C per hour.
After slowly cooling the substrate, the substrate was immersed in the melt for 8 minutes while rotating at 20 rpm. The growth rate of the lithium niobate single crystal thin film was 1 μm / min. (5) Next, the substrate material is pulled out of the melt, and the melt is shaken off by rotating the melt at 1,000 rpm for 3 minutes, and then gradually cooled to room temperature at a rate of 1 ° C./min. As a result, a lithium niobate single crystal thin film of about 8 μm was formed on the substrate material. (6) Observation of the obtained lithium niobate single crystal thin film revealed that pinhole-shaped macrodefects were found on the surface of the thin film, and that the temperature was slightly insufficient.
~ 10 had occurred.

[0020]

[Table 2]

[0021]

As described above, in the liquid phase epitaxy of a lithium niobate single crystal thin film, according to the present invention, a raw material having a high melt composition ratio, that is, Li ₂ CO ₃ (Li ₂ O
), Nb ₂ O ₅ , and V ₂ O ₅ are heated and calcined in an air atmosphere before mixing to remove hydration water contained in the raw material, so that bubbles generated during growth can be suppressed. It is possible to obtain a single-crystal thin film free of pinhole-shaped macro defects generated by adhering to the surface of the thin film. This significantly improves the yield.

[Brief description of the drawings]

FIG. 1 is a micrograph of a crystal showing a pinhole defect found on the surface of a conventional single crystal thin film.

FIG. 2 is a graph showing a relationship between a heating temperature and a pinhole generation rate.

FIG. 3 is a photomicrograph of a crystal showing the surface of a single crystal thin film of the present invention.

Claims (3)

(57) [Claims] 1. A method for producing a single crystal thin film, comprising mixing a plurality of materials for forming a single crystal thin film, heating and melting the mixture to form a melt, and then bringing the single crystal substrate into contact with the melt to grow the single crystal thin film. In, before mixing the above materials,
A method for producing a single-crystal thin film by liquid phase epitaxy, characterized in that these materials are separately heated in advance by individually heating them. 2. The method according to claim 1, wherein the calcining is performed within a range of a temperature equal to or higher than a thermal decomposition temperature of crystal water contained in each material and a temperature lower than a melting point of each material. Production method. 3. The method according to claim 1, wherein the material for forming a single crystal thin film mainly comprises an oxide of lithium and a carbonate compound thereof, an oxide of niobium, and an oxide of vanadium.