WO2023189258A1

WO2023189258A1 - Production method for hexagonal boron nitride thin film, laminate

Info

Publication number: WO2023189258A1
Application number: PCT/JP2023/008427
Authority: WO
Inventors: 健司小廣; 崇之岡地; 永山田
Original assignee: 住友化学株式会社; 国立研究開発法人産業技術総合研究所
Priority date: 2022-03-31
Filing date: 2023-03-06
Publication date: 2023-10-05
Also published as: TW202348828A

Abstract

The present invention provides a production method for a hexagonal boron nitride thin film that involves a step A for preparing an iron thin film–coated substrate that includes a substrate and an iron thin film that is provided on the substrate and has a film thickness of 200–1800 nm, a step B for supplying a gas that includes a boron compound to the iron thin film–coated substrate to form an iron boride layer that includes Fe₂B on the surface of the iron thin film of the iron thin film–coated substrate, and a step C for supplying at least one type of gas selected from the group that consists of nitrogen gas and gases that include a nitrogen compound to the iron thin film–coated substrate on which the iron boride layer has been formed to nitride the boron in the iron boride layer. The present invention also provides a laminate.

Description

Manufacturing method and laminate of hexagonal boron nitride thin film

The present disclosure relates to a method for manufacturing a hexagonal boron nitride thin film and a laminate.

Due to its properties, research on hexagonal boron nitride thin films has progressed in many application fields, and has been used to maximize the properties of light-emitting diodes and semiconductor lasers that emit light in the ultraviolet region, broadband light-receiving devices, and two-dimensional semiconductors such as graphene. It is expected to be applied to pull-out insulating films and release layers used in the production of free-standing substrates such as gallium nitride.

Many proposals have been made regarding methods for producing hexagonal boron nitride thin films, including a high-temperature, high-pressure method in which boron nitride is precipitated from a flux in which raw materials are dissolved under high-temperature, high-pressure conditions, and a method in which boron nitride is deposited by bringing a raw material gas into contact with the substrate surface. There is a chemical vapor deposition (CVD) method for depositing boron nitride. Although high-temperature, high-pressure methods can produce, for example, hexagonal boron nitride thin films that dramatically improve the field-effect mobility of graphene transistors, crystal growth using high-temperature, high-pressure methods makes it difficult to grow the area necessary for application. In this respect, since the CVD method is suitable for obtaining a thin film with a large area, new methods for manufacturing hexagonal boron nitride thin films using the CVD method have been actively developed in recent years. There are roughly two types of methods for manufacturing hexagonal boron nitride thin films using the CVD method. One is a method of growing hexagonal boron nitride on the substrate by using ammonia borane, borazine, etc. as a raw material and dehydrogenating the raw material on a metal catalyst placed on the substrate. First, as in the metal organic chemical vapor deposition (MOCVD) method, a raw material containing boron such as triethylborane and a raw material containing nitrogen such as ammonia are separately supplied and reacted on the substrate. This is a method of growing hexagonal boron nitride on the substrate (see, for example, Japanese Patent No. 4358143). Additionally, in recent years, a new method, Vapor-Liquid-Solid Thin Film Growth (VLS), has been reported in which boron nitride crystals are grown using three states of matter: gas phase, solid phase, and liquid phase. (For example, Zhiyuan Shi, Xiujun Wang, Qingtian Li, Peng Yang, Guangyuan Lu, Ren Jiang, Huishan Wang, Chao Zhang, Chunxiao Cong, Ahi Liu, Tianru Wu, Haomin Wang, Qingkai Yu, and Xiaoming Xie, “Vapor -liquid -solid growth of large-areamultilayer hexagonal boron nitride on dielectric substrates”, NATURE COMMUNICATIONS, volume 11, Article number. 849(2020), pp. 1-8.).

In recent years, expectations have been increasing for the application of hexagonal boron nitride thin films as electronic materials, and there is a need to develop a method that can produce higher quality hexagonal boron nitride thin films. According to the conventional CVD method that utilizes the action of a metal catalyst, a thin film of hexagonal boron nitride with relatively high crystallinity can be obtained, but the metal catalyst is covered with the precipitated hexagonal boron nitride, which reduces the catalytic action. This makes it difficult to obtain hexagonal boron nitride thin films with multiple layers. If the hexagonal boron nitride thin film is thin, for example, it cannot sufficiently prevent phonon scattering, interface trapping of charged impurities, etc., which are factors that degrade mobility when applied to a graphene transistor. On the other hand, according to the conventional MOCVD method which does not require the action of a metal catalyst, a hexagonal boron nitride thin film having a plurality of layers can be obtained, and the thickness of the hexagonal boron nitride thin film can be increased. However, hexagonal boron nitride does not have dangling bonds that allow crystals to grow regularly, so when layers of hexagonal boron nitride are stacked as in the conventional MOCVD method, the hexagonal boron nitride crystals gradually grow. The properties of the hexagonal boron nitride thin film deteriorate, and the regular layer structure is lost, making it difficult to bring out the properties of the hexagonal boron nitride thin film (for example, properties that improve the mobility of graphene transistors). Therefore, there is a need for a method that can produce a highly crystalline hexagonal boron nitride thin film suitable for application to electronic materials and the like. Furthermore, when a thin film of metal is formed, the metal may aggregate and the flatness of the thin film may be impaired, and such a phenomenon may also occur in the formation of a thin film of hexagonal boron nitride. Therefore, the method for producing a hexagonal boron nitride thin film is also required to be able to produce a hexagonal boron nitride thin film with high uniformity. However, conventional methods for producing hexagonal boron nitride thin films do not focus on improving both the crystallinity of hexagonal boron nitride and the uniformity of the hexagonal boron nitride thin film.

The present disclosure has been made in view of the above circumstances.
An object of an embodiment of the present disclosure is to provide a method for producing a hexagonal boron nitride thin film that can produce a highly crystalline hexagonal boron nitride thin film with high uniformity.
A problem to be solved by other embodiments of the present disclosure is to provide a laminate including a hexagonal boron nitride thin film with high crystallinity and film uniformity.

Specific means for solving the above problems include the following aspects.
[1] Step A of preparing a substrate with an iron thin film, which includes a substrate and an iron thin film provided on the substrate and having a film thickness in a range of 200 nm or more and 1800 nm or less;
Step B of forming an iron boride layer containing Fe ₂ B on the surface of the iron thin film of the iron thin film-coated substrate by supplying a gas containing a boron compound to the iron thin film-coated substrate;
Boron in the iron boride layer is removed by supplying at least one gas selected from the group consisting of nitrogen gas and a gas containing a nitrogen compound to the iron thin film-coated substrate on which the iron boride layer is formed. nitriding step C;
A method for producing a hexagonal boron nitride thin film, comprising:
[2] The method for producing a hexagonal boron nitride thin film according to [1], wherein in the step C, the temperature of the iron thin film-coated substrate on which the iron boride layer is formed is in a range of 900° C. or more and 1200° C. or less.
[3] The method for producing a hexagonal boron nitride thin film according to [1] or [2], wherein the substrate is a sapphire substrate.
[4] The method for producing a hexagonal boron nitride thin film according to any one of [1] to [3], wherein the boron compound is diborane.
[5] The method for producing a hexagonal boron nitride thin film according to any one of [1] to [4], wherein the nitrogen compound is ammonia.
[6] A substrate,
A thin film containing Fe ₂ B and iron,
a hexagonal boron nitride thin film having multiple layers;
has
A laminate, wherein the total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film is in the range of 300 nm or more and 2100 nm or less.
[7] The laminate according to [6], wherein the substrate is a sapphire substrate.

According to an embodiment of the present disclosure, there is provided a method for manufacturing a hexagonal boron nitride thin film that can manufacture a highly crystalline hexagonal boron nitride thin film with high uniformity.
According to another embodiment of the present disclosure, a laminate including a hexagonal boron nitride thin film with high hexagonal boron nitride crystallinity and film uniformity is provided.

FIG. 1 is a schematic configuration diagram showing an example of an apparatus suitably used in the manufacturing method of the present disclosure. This is an example of an optical microscope transmission image of a laminate in which the uniformity evaluation result of the hexagonal boron nitride thin film is A. This is an example of an optical microscope transmission image of a laminate whose uniformity evaluation result of the hexagonal boron nitride thin film is B. It is a graph showing the relationship between the film thickness of the iron thin film in a substrate with an iron thin film and the ratio of the exposed portion of the substrate in the laminate.

Hereinafter, a method for manufacturing a hexagonal boron nitride thin film and a laminate according to the present disclosure will be described in detail. Although the description of the requirements set forth below may be based on representative implementations of this disclosure, this disclosure is not limited to such implementations and is within the scope of this disclosure. can be implemented with appropriate changes.

In the present disclosure, a numerical range indicated using "~" means a range that includes the numerical values written before and after "~" as the lower limit and upper limit, respectively.
In the numerical ranges described step by step in the present disclosure, the upper limit or lower limit described in a certain numerical range may be replaced with the upper limit or lower limit of another numerical range described step by step. Furthermore, in the numerical ranges described in this disclosure, the upper limit or lower limit described in a certain numerical range may be replaced with the value shown in the Examples.

In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

In this disclosure, the term "step" is used not only to refer to an independent process, but also to include a process that is not clearly distinguishable from other processes, as long as the intended purpose of the process is achieved. .

[Method for manufacturing hexagonal boron nitride thin film]
The method for producing a hexagonal boron nitride thin film of the present disclosure (hereinafter also simply referred to as "the production method of the present disclosure") includes a substrate, an iron film provided on the substrate, and having a film thickness of 200 nm or more and 1800 nm or less. a step A of preparing an iron thin film-coated substrate having a thin film; and supplying a gas containing a boron compound to the iron thin film-coated substrate, so that Fe is formed on the surface of the iron thin film of the iron thin film-coated substrate. Step B of forming an iron boride layer containing ₂ B, and at least one gas selected from the group consisting of nitrogen gas and a gas containing a nitrogen compound on the iron thin film-coated substrate on which the iron boride layer is formed. and a step C of nitriding boron in the iron boride layer by supplying.
According to the manufacturing method of the present disclosure, a highly crystalline hexagonal boron nitride thin film can be manufactured with high uniformity.

Although the details of the production mechanism of the hexagonal boron nitride thin film by the manufacturing method of the present disclosure are not clear, the present inventors speculate as follows. However, the following speculations are not intended to limit the manufacturing method of the present disclosure, but are explained as an example.

In the manufacturing method of the present disclosure, an iron boride layer containing Fe ₂ B is formed on the surface of the iron thin film of the iron thin film-coated substrate by supplying a gas containing a boron compound to the prepared iron thin film-coated substrate. do. By supplying a gas containing a boron compound to a substrate with an iron thin film, the boron compound is decomposed on the surface of the iron thin film. The boron produced by decomposition gradually enters the interior of the iron thin film from the surface and reacts with iron to produce iron boride. A boron compound can be decomposed, for example, by heating a substrate with an iron thin film. The type of iron boride produced can be controlled by conditions such as the amount of boron supplied, the temperature of the substrate with the iron thin film, and the thickness of the iron thin film. Types of iron borides include, for example, Fe ₃ B, Fe ₂ B, and FeB, but in the manufacturing method of the present disclosure, the iron boride layer contains Fe ₂ B, so that the final layer has high crystallinity. Thin films of hexagonal boron nitride tend to be obtained. Further, in the manufacturing method of the present disclosure, at least one gas selected from the group consisting of nitrogen gas and a gas containing a nitrogen compound (hereinafter referred to as "nitrogen source gas ), the boron in the iron boride layer is nitrided. By supplying nitrogen source gas to the iron thin film-coated substrate on which the iron boride layer is formed, the nitrogen source gas is decomposed on the surface of the iron boride layer. The nitrogen source gas can be decomposed, for example, by heating a substrate with an iron thin film on which an iron boride layer is formed. Nitrogen generated by decomposition gradually enters the interior of the iron boride layer from the surface and nitrides boron in the iron boride layer, thereby generating hexagonal boron nitride. That is, the manufacturing method of the present disclosure is distinguished from methods of stacking layers of hexagonal boron nitride, such as conventional MOCVD methods.

In the manufacturing method of the present disclosure, where the hexagonal boron nitride thin film is generated can be controlled by, for example, the substrate temperature in step C (in other words, the film formation temperature), the raw material supply method, etc. For example, when the deposition temperature is lowered, a hexagonal boron nitride thin film is generated on the side of the iron boride layer opposite to the substrate. That is, due to the boron and nitrogen in the iron boride layer, a hexagonal boron nitride thin film is formed on the surface of the iron boride layer opposite to the substrate, and the formed hexagonal boron nitride thin film and iron boride By supplying boron and nitrogen from the iron boride layer to the interface with the iron boron layer, a hexagonal boron nitride thin film is continuously generated. Note that this generation mechanism differs from the conventional mechanism in which new hexagonal boron nitride is grown on hexagonal boron nitride, and hexagonal boron nitride does not become polycrystalline but forms a regular layer structure. For example, when the film-forming temperature is raised, boron and nitrogen spread to the inside of the iron boride layer, so not only the side of the iron boride layer opposite to the substrate, but also the side of the iron boride layer facing the substrate ( That is, a hexagonal boron nitride thin film is also formed between the substrate and the iron boride layer. Furthermore, when the temperature of the substrate rises and the iron boride melts, a hexagonal boron nitride thin film is formed on the side of the iron boride layer opposite to the substrate and on the side of the iron boride layer facing the substrate. However, in this case, since the iron boride layer is melted, boron nitride is less likely to interact with iron boride, and a flatter hexagonal boron nitride thin film can be obtained.

[Process A]
Step A is a step of preparing a substrate with an iron thin film, which includes a substrate and an iron thin film provided on the substrate and having a film thickness in a range of 200 nm or more and 1800 nm or less.
"Preparing a substrate with an iron thin film" means to prepare a substrate with an iron thin film in a usable state, and includes preparing a substrate with an iron thin film manufactured in advance by obtaining it, and preparing a substrate with an iron thin film. It includes manufacturing. That is, the substrate with an iron thin film used in the manufacturing method of the present disclosure may be a substrate with an iron thin film manufactured in advance, or may be a substrate with an iron thin film manufactured in Step A.

The substrate with an iron thin film in the present disclosure can be manufactured by forming an iron thin film having a thickness in the range of 200 nm or more and 1800 nm or less on the substrate. Preferred embodiments of the method for manufacturing a substrate with an iron thin film according to the present disclosure will be described below.

It is preferable to select the material of the substrate as appropriate, taking into consideration, for example, melting point, thermal decomposition properties, stability under a reducing atmosphere, and the like.
Examples of substrate materials include sapphire (Al ₂ O ₃ ), silicon (Si), silicon dioxide (SiO ₂ ), aluminum nitride (AlN), silicon carbide (SiC), spinel (MgAl ₂ O ₄ ), and magnesium oxide. (MgO), gallium arsenide (GaAs), gallium phosphide (GaP), and neodymium gallate (NdGaO ₃ ).
From the viewpoint of aligning the plane orientations of the laminated iron, the substrate is preferably a single crystal substrate. Furthermore, in consideration of the compatibility (for example, lattice matching) between the iron to be laminated and the hexagonal boron nitride to be produced, the substrate is more preferably a sapphire substrate, an aluminum nitride substrate, or a silicon carbide substrate. More preferred.

The thickness of the substrate is not particularly limited, but is generally 200 μm to 1000 μm, preferably 300 μm to 800 μm.

The iron thin film may be formed on only one side of the substrate, or may be formed on both sides of the substrate.

The temperature of the substrate when forming the iron thin film is not particularly limited, but is preferably 25° C. to 300° C., for example.
The means for heating the substrate is not particularly limited, and examples thereof include a heater.

The thickness of the iron thin film formed on the substrate is in the range of 200 nm or more and 1800 nm or less.
When the thickness of the iron thin film is within the above range, agglomeration of hexagonal boron nitride is suppressed, and a highly uniform hexagonal boron nitride thin film can be manufactured.
The thickness of the iron thin film is preferably in the range of 250 nm or more and 1750 nm or less, more preferably 500 nm or more and 1750 nm or less, even more preferably 500 nm or more and 1500 nm or less, and 500 nm or more and 1000 nm or less. It is particularly preferable that the range is within the range.

The "thickness of the iron thin film" in the present disclosure means the average thickness of the iron thin film.
The average film thickness of the iron thin film is a value obtained by forming an iron thin film using a separately prepared substrate and using the following method.
A substrate (hereinafter also referred to as a "measurement substrate") equipped with a mask (eg, resist, polyimide tape, etc.) is prepared. After forming an iron thin film on the prepared measurement substrate, the mask is peeled off. The level difference between the surface of the iron thin film and the surface of the substrate from which the mask has been removed is measured using a level difference meter. The arithmetic mean value of the measured values of the step difference at five randomly selected locations is determined, and the obtained value is taken as the thickness of the iron thin film.

The thickness of the iron thin film can be controlled by the applied power, film formation time, etc. when forming the iron thin film by sputtering, for example. For example, when the applied power is increased, the thickness of the iron thin film becomes thicker, and when the applied power is lowered, the thickness of the iron thin film becomes thinner. Further, for example, when the film formation time is increased, the iron thin film becomes thicker, and when the film formation time is shortened, the iron thin film becomes thinner.
Regarding the applied voltage, it is preferable to appropriately select optimal conditions, taking into account that it may affect the surface roughness of the iron thin film.

As a method for forming the iron thin film, for example, a sputtering method and a vacuum evaporation method can be applied.
From the viewpoint of being able to form a highly uniform iron thin film on a wide-area substrate, the method for forming the iron thin film is preferably a sputtering method. Further, in consideration of the purity of the formed iron thin film, it is more preferable that the method for forming the iron thin film is a magnetron sputtering method.
The magnetron sputtering method may be an RF (Radio Frequency) magnetron sputtering method or a DC (Direct Current) magnetron sputtering method, but a DC magnetron sputtering method is preferable.

When the method for forming the iron thin film is magnetron sputtering, the sputtering conditions are appropriately set, for example, depending on the desired thickness of the iron thin film.
The film-forming atmosphere is preferably argon gas, for example.
The film forming pressure is preferably 0.1 Pa to 10 Pa, for example.
The temperature of the film-forming substrate is preferably 25° C. to 300° C., for example.
The applied voltage is preferably 500W to 6000W, for example.

[Process B]
In step B, iron boride containing Fe ₂ B is added onto the surface of the iron thin film of the iron thin film-coated substrate by supplying a gas containing a boron compound to the iron thin film-coated substrate prepared in step A. This is the process of forming layers.
When a gas containing a boron compound is supplied to a substrate with an iron thin film, the iron in the iron thin film is borated, and an iron boride layer containing Fe ₂ B is formed on the surface of the iron thin film of the iron thin film on the substrate. .

The gas supplied to the substrate with the iron thin film contains a boron compound.
Examples of boron compounds include organic boron compounds such as trimethylborane (C ₃ H ₉ B) and triethyl borane (C ₆ H ₁₅ B); boron halides such as boron trichloride (BCl ₃ ); and monoborane (BH ₃ ), and boron hydrides such as diborane (B ₂ H ₆ ).
Among these, as the boron compound, a boron halide or a boron hydride is preferable, and diborane is more preferable. These boron compounds are suitable because they do not produce carbon during decomposition, and therefore there is no concern that carbon will be mixed into the hexagonal boron nitride finally obtained.

When the boron compound is a gas, the gas supplied to the substrate with the iron thin film may be the gaseous boron compound itself, and when the boron compound is a liquid, the gas may be a gas obtained by vaporizing the liquid boron compound. There may be. In the present disclosure, the gaseous boron compound itself and the vaporized liquid boron compound may be collectively referred to as "boron compound gas."
When the boron compound is a liquid, the method of vaporizing the liquid boron compound is not particularly limited, but, for example, a bubbling method is suitable. In the bubbling method, a liquid boron compound in a tank is bubbled using a carrier gas to vaporize it.

The gas supplied to the iron thin film-coated substrate preferably contains a carrier gas in addition to the boron compound gas, for example, from the viewpoint of operability.
As the carrier gas, hydrogen gas or a mixed gas of hydrogen gas and an inert gas is preferable.
When hydrogen gas is included in the carrier gas, boron particles are less likely to be generated, so a film of good quality tends to be obtained.
The mixed gas of hydrogen gas and inert gas is preferably a mixed gas of hydrogen gas and argon gas.

The supply amount of the boron compound gas is not particularly limited, and is set appropriately depending on, for example, the desired composition of iron boride in the iron boride layer and the desired thickness of the hexagonal boron nitride thin film. .
The amount of boron compound gas supplied can be controlled by, for example, the flow rate and supply time of boron compound gas.
The flow rate of the boron compound gas is, for example, preferably 0.05 cm ³ /min to 1 cm ³ /min, more preferably 0.1 cm ³ /min to 0.7 cm ³ /min.
The boron compound gas supply time is, for example, preferably 10 minutes to 80 minutes, more preferably 20 minutes to 60 minutes.

The temperature of the iron thin film-coated substrate when supplying the gas containing the boron compound is not particularly limited, and is appropriately set, for example, depending on the desired composition of iron boride in the iron boride layer.
The temperature of the iron thin film-coated substrate when supplying the gas containing the boron compound is preferably 1000° C. to 1100° C., for example.
The means for heating the substrate with an iron thin film is not particularly limited, and examples thereof include a heater.

The iron boride layer formed in step B contains Fe ₂ B.
When the iron boride layer contains Fe ₂ B, a highly crystalline hexagonal boron nitride thin film can be produced.
The iron boride layer may contain, in addition to Fe ₂ B, at least one member selected from the group consisting of Fe, FeB, and Fe ₃ B.
The type and proportion of iron boride contained in the iron boride layer can be controlled, for example, by adjusting the supply amount of the boron compound, the temperature of the substrate with the iron thin film, the thickness of the iron thin film, and the like.
For example, when the amount of boron compound supplied is increased, boron is incorporated into iron, and iron boride with a gradually increasing proportion of boron is obtained. The proportion of boron increases in the order of Fe<Fe ₃ B<Fe ₂ B<FeB, but Fe ₃ B is said to be a metastable phase and is relatively difficult to obtain. Although the conditions for obtaining Fe ₃ B have not been clarified, the present disclosure believes that it is easier to obtain Fe 3 B when the iron thin film is thinner. Note that the literature (Intermetallics 11 (2003) 1293-1299) discloses a phase diagram of iron-boron system, and according to this phase diagram, if the amount of boron compound supplied is increased or decreased, the amount of iron boride is relatively reduced. Easy to control types and proportions. The contents of the above documents are incorporated herein by reference.
Further, by controlling the temperature of the substrate with the iron thin film, the speed at which boron is incorporated into iron can be adjusted, and iron boride with a desired composition can be obtained.

The composition of iron boride contained in the iron boride layer is confirmed by an X-ray diffraction (XRD) method.
Further, the composition ratio of iron boride contained in the iron boride layer is measured by an X-ray diffraction method. Specifically, a substrate with an iron thin film on which an iron boride layer is formed is irradiated with CuKα rays (characteristic X-rays), and based on the obtained X-ray diffraction pattern, the diffraction peak of FeB (130) is determined. It is determined from the ratio of the intensity (unit: cps), the intensity of the diffraction peak of Fe ₂ B (022), and the intensity of the diffraction peak of Fe ₃ B (330).
The X-ray conditions are a voltage of 45 kV and a current of 40 mA.
As a device used for the X-ray diffraction method, for example, X'Pert Pro MRD manufactured by PANalytical is suitable. However, the device is not limited to this.

When the iron borides contained in the iron boride layer are Fe ₂ B and FeB, the composition ratio of these, that is, the Fe ₂ B relative to the intensity of the diffraction peak of FeB (130) in the X-ray diffraction pattern of the iron boride layer. The ratio of the intensity of the diffraction peak of (022) [Intensity of the diffraction peak of Fe ₂ B (022)/Intensity of the diffraction peak of FeB (130)] is preferably 0.38 or more, for example, 0.40 It is more preferable that it is above.
When "Intensity of the diffraction peak of Fe ₂ B (022) / Intensity of the diffraction peak of FeB (130)" is 0.38 or more, the aggregation of hexagonal boron nitride is better suppressed and the uniformity is higher. There is a tendency to produce hexagonal boron nitride thin films.

When the iron borides contained in the iron boride layer are Fe ₂ B and Fe ₃ B, the composition ratio of these, that is, the intensity of the diffraction peak of Fe ₂ B (022) in the X-ray diffraction pattern of the iron boride layer The ratio of the intensity of the diffraction peak of Fe ₃ B (330) to the intensity of the diffraction peak of Fe ₃ B (330)/the intensity of the diffraction peak of Fe ₂ B (022) is, for example, 0.32 or less. is preferable, and more preferably 0.30 or less.
When "Intensity of diffraction peak of Fe ₃ B (330)/Intensity of diffraction peak of Fe ₂ B (022)" is 0.32 or less, it tends to be possible to produce a thin film of hexagonal boron nitride with higher crystallinity. be.

[Process C]
In step C, at least one gas selected from the group consisting of nitrogen gas and a gas containing a nitrogen compound (i.e., nitrogen source gas) is applied to the iron thin film-coated substrate on which the iron boride layer has been formed in step B. This is a step of nitriding boron in the iron boride layer by supplying the iron boride layer.
When a nitrogen raw material gas is supplied to a substrate with an iron thin film on which an iron boride layer has been formed, the boron in the iron boride layer is nitrided and hexagonal boron nitride is generated, resulting in hexagonal nitride consisting of multiple layers. A thin boron film is formed.

The type of nitrogen compound is not particularly limited.
Examples of nitrogen compounds include ammonia and hydrazine.
As the nitrogen raw material gas, for example, from the viewpoint of reactivity, at least one gas selected from nitrogen gas and ammonia gas is preferable, and ammonia gas is more preferable.

The supply amount of the nitrogen raw material gas is not particularly limited, and is appropriately set, for example, depending on the desired thickness of the hexagonal boron nitride thin film.
The supply amount of the nitrogen source gas can be controlled by, for example, the flow rate and supply time of the nitrogen source gas.
The flow rate of the nitrogen source gas is, for example, preferably 100 cm ³ /min to 2000 cm ³ /min, more preferably 150 cm ³ /min to 1000 cm ³ /min.
The supply time of the nitrogen source gas is, for example, preferably 5 minutes to 300 minutes, more preferably 10 minutes to 200 minutes.

When supplying the nitrogen raw material gas to the iron thin film-coated substrate on which the iron boride layer is formed, it is preferable to supply the nitrogen raw material gas together with the carrier gas, for example, from the viewpoint of operability.
As the carrier gas, for example, hydrogen gas or a mixed gas of hydrogen gas and inert gas is preferable, and a mixed gas of hydrogen gas and inert gas is more preferable.
When hydrogen gas is included in the carrier gas, the reaction between the boron contained in iron boride and nitrogen is slowed down, resulting in a good film. However, the formed hexagonal boron nitride thin film is etched by hydrogen. may be done. On the other hand, if the carrier gas contains an inert gas in addition to hydrogen gas, the hexagonal boron nitride thin film is etched gently by the hydrogen gas.

In step C, the temperature of the iron thin film-coated substrate on which the iron boride layer is formed is preferably in the range of 850°C or more and 1200°C or less, and more preferably in the range of 900°C or more and 1200°C or less. , more preferably in the range of 930°C or higher and 1200°C or lower.
When the temperature of the iron thin film-coated substrate on which the iron boride layer is formed is set to 850° C. or higher, a thin film of hexagonal boron nitride with higher crystallinity tends to be produced with more uniformity.
When the temperature of the iron thin film-coated substrate on which the iron boride layer is formed is kept below 1200°C, agglomeration of hexagonal boron nitride is better suppressed, and a more uniform hexagonal boron nitride thin film tends to be produced. .
The means for heating the iron thin film-coated substrate on which the iron boride layer is formed is not particularly limited, and includes, for example, a heater.

The thickness of the hexagonal boron nitride thin film formed in step C is not particularly limited and can be set as appropriate depending on the purpose, but is preferably 5 nm to 100 nm, for example.
The "thickness of a hexagonal boron nitride thin film" in the present disclosure is a value determined by the following method. A cross section of the hexagonal boron nitride thin film is observed using a scanning electron microscope (SEM), and the thickness of the cross section of the hexagonal boron nitride thin film at five randomly selected locations is measured. The arithmetic mean value of the measured values is determined, and the obtained value is taken as the thickness of the hexagonal boron nitride thin film.

The thickness of the hexagonal boron nitride thin film can be controlled, for example, by adjusting the thickness of the iron thin film, the amount of boron compound supplied, the amount of nitrogen source gas supplied, and the like. For example, if the thickness of the iron thin film is increased and the amount of boron compound and nitrogen raw material gas supplied is increased, the thickness of the hexagonal boron nitride thin film becomes thicker. On the other hand, regardless of the thickness of the iron thin film, if the supply amount of the boron compound and the nitrogen source gas are reduced, the thickness of the hexagonal boron nitride thin film becomes thinner.

[Other processes]
The manufacturing method of the present disclosure may include steps other than Step A, Step B, and Step C (so-called other steps) as necessary within a range that does not impair the effects of the present disclosure.
The manufacturing method of the present disclosure preferably includes, as another step, a step of annealing the iron thin film with hydrogen gas after forming the iron thin film on the substrate and before producing hexagonal boron nitride. .
By annealing the iron thin film, not only can trace amounts of oxygen that may be contained in the iron thin film be removed, but also the adhesion between the substrate and the iron thin film and the orientation of the iron thin film can be improved. From the viewpoint of removing trace amounts of oxygen that may be contained in the iron thin film, it is preferable that the annealing of the iron thin film with hydrogen gas be performed immediately before hexagonal boron nitride is produced, for example, in a reactor that produces hexagonal boron nitride. .
The annealing temperature is, for example, preferably 950°C to 1100°C, more preferably 1000°C to 1050°C.

FIG. 1 is a schematic configuration diagram showing an example of an apparatus suitably used in the manufacturing method of the present disclosure.
In FIG. 1, reference numeral 1 indicates a vent gas line for exhausting gas containing each raw material without supplying it onto the substrate, and reference numeral 2 indicates a vent gas line for supplying a gas containing a boron compound to the substrate with an iron thin film. Reference numeral 3 indicates a carrier gas line for supplying nitrogen raw material gas to the iron thin film coated substrate on which the iron boride layer is formed.
In the apparatus shown in FIG. 1, the carrier gas line is divided into a carrier gas line 2 for supplying a gas containing a boron compound and a carrier gas line 3 for supplying a nitrogen raw material gas. Mixing with the nitrogen raw material is prevented, and the formation of side reactants called adducts can be suppressed.

Reference numerals 4, 5, and 6 indicate valves for switching between supplying the gas containing each raw material to the carrier gas line or flowing it to the vent gas line. Reference numerals 7, 8, 9 and 10 indicate gas lines for supplying the corresponding raw materials carrier gas A, carrier gas B, carrier gas C and gas D, respectively. The gas D supplied from the gas line 10 functions not only as a carrier gas but also as a bubbling gas for vaporizing a liquid boron compound (for example, triethylborane).

Reference numeral 11 indicates a container for storing a liquid boron compound. A liquid boron compound such as triethylborane is stored in the container 11, and one end of the gas line 10 is immersed in the liquid so that bubbling can be performed. Reference numeral 12 indicates a container for storing a gaseous boron compound (for example, diborane), and reference numeral 13 indicates a container for storing a nitrogen source gas (for example, ammonia).

Reference numeral 14 indicates a reactor, reference numeral 15 indicates a substrate with an iron thin film, reference numeral 16 indicates a susceptor that supports the substrate 15 with the iron thin film, and reference numeral 17 indicates a heater for heating the substrate 15 with the iron thin film. shows.

An example of a specific procedure of the manufacturing method of the present disclosure using the apparatus shown in FIG. 1 will be described.
First, in the reactor 14, the iron thin film coated substrate 15 placed on the susceptor 16 is heated using the heater 17 to a predetermined temperature (for example, 1000° C. to 1100° C.). After heating the iron thin film coated substrate 15 to a predetermined temperature, carrier gas A, carrier gas B, carrier gas C, and gas (carrier gas) D are supplied to gas line 7, gas line 8, gas line 9, and gas line 10, respectively. supply A carrier gas (for example, hydrogen gas) is constantly supplied to the gas line 7, gas line 8, gas line 9, and gas line 10 until the formation of the hexagonal boron nitride thin film is completed.
Next, only hydrogen gas used as a carrier gas is supplied to the iron thin film coated substrate 15 heated to a predetermined temperature to anneal the iron thin film.
Next, a predetermined amount of gas containing a boron compound (for example, diborane) is supplied from the carrier gas line 2 to the substrate 15 with the iron thin film in the reactor 14 to boride the iron in the iron thin film. An iron boride layer containing Fe ₂ B is formed on the surface of the iron thin film of the attached substrate 15 .
Next, the set temperature of the heater 17 is changed so that the iron thin film coated substrate on which the iron boride layer is formed reaches a predetermined temperature (for example, 900° C. to 1200° C.). After the temperature has stabilized, a predetermined amount of nitrogen raw material gas (for example, ammonia gas) is supplied from the carrier gas line 3 to the iron thin film-coated substrate on which the iron boride layer has been formed, and the boron in the iron boride layer is nitrided. let By nitriding this boron, hexagonal boron nitride is generated, and a hexagonal boron nitride thin film is formed.
After the supply of the nitrogen raw material gas is completed, the temperature of the heater 17 is gradually lowered to room temperature (for example, 25° C.) while flowing the carrier gas, and then the substrate on which the hexagonal boron nitride thin film is formed is taken out. The taken out substrate has a structure in which the substrate, a thin film containing iron boride and iron, and a hexagonal boron nitride thin film having a plurality of layers are stacked.

[Laminated body]
The laminate of the present disclosure includes a substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having a plurality of layers, the thin film containing Fe ₂ B and iron and the hexagonal boron nitride The total thickness of the boron thin film is in the range of 300 nm or more and 2100 nm or less.
The laminate of the present disclosure is a laminate including a hexagonal boron nitride thin film with high crystallinity and film uniformity, and is suitable for use as an electronic material, for example, and particularly suitable as a graphene substrate. It is.

The laminate of the present disclosure includes a substrate.
A specific example of the material of the substrate in the laminate of the present disclosure is the same as a specific example of the material of the substrate in the manufacturing method of the present disclosure.
The substrate in the laminate of the present disclosure is preferably a single crystal substrate, more preferably a sapphire substrate, an aluminum nitride substrate, or a silicon carbide substrate, and even more preferably a sapphire substrate.
The thickness of the substrate in the laminate of the present disclosure is not particularly limited, but is generally 200 μm to 1000 μm, preferably 300 μm to 800 μm.

The laminate of the present disclosure has a thin film containing Fe ₂ B and iron.
It can be confirmed, for example, by X-ray diffraction that the laminate of the present disclosure has a thin film containing Fe ₂ B and iron.
The thin film may contain iron borides other than Fe ₂ B.
Examples of iron borides other than Fe ₂ B include FeB and Fe ₃ B.

The laminate of the present disclosure has a hexagonal boron nitride thin film having multiple layers.
The hexagonal boron nitride thin film may have two or more layers, preferably 30 or more layers, more preferably 60 or more layers, and even more preferably 100 or more layers. The upper limit of the number of layers included in the hexagonal boron nitride thin film is not particularly limited.

In the laminate of the present disclosure, the total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film is in the range of 300 nm or more and 2100 nm or less, preferably 600 nm or more and 2100 nm or less, and 600 nm or more. The range is more preferably 1750 nm or more, and even more preferably 600 nm or more and 1500 nm or less.

In the present disclosure, "the total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film" is a value determined by the following method.
Using a scanning electron microscope (SEM), the cross sections of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film were observed, and the thin film containing Fe ₂ B and iron was observed at five randomly selected locations. Measure the cross-sectional thickness as well as the cross-sectional thickness of the hexagonal boron nitride thin film. The arithmetic mean value of the measured values is determined, and the obtained value is taken as the total film thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film.

The laminate of the present disclosure is suitably manufactured, for example, by the manufacturing method of the present disclosure described above.

The manufacturing method and laminate of the present disclosure will be explained in more detail by giving examples below. The manufacturing method and laminate of the present disclosure are not limited to the following examples unless the gist thereof is exceeded.

[Manufacture of hexagonal boron nitride thin film]
<Example 1>
An iron thin film was formed on a sapphire substrate by magnetron sputtering to prepare a substrate with an iron thin film [Step A]. The sputtering conditions are shown below.

-Sputtering conditions-
Target: Fe
Film forming pressure: 0.4Pa
Temperature of film-forming substrate: 250℃
Film-forming atmosphere: Ar gas Applied power: 2000W
Film forming time: 10 minutes

The thickness of the iron thin film on the prepared substrate with iron thin film was 500 nm when measured by the method described above.

Next, using an apparatus having the configuration shown in FIG. 1, the following operations were performed.
The prepared substrate with the iron thin film was placed on the susceptor 16 installed in the reactor 14. After heating the substrate with the iron thin film using the heater 17 so that the temperature thereof reached 1033° C., hydrogen gas was supplied as a carrier gas to the gas line 7, the gas line 8, and the gas line 9. Next, only hydrogen gas used as a carrier gas was supplied to the heated substrate with the iron thin film to anneal the iron thin film.

Next, by switching the valve 5, diborane, which is a boron compound in the container 12, is supplied to the gas line 8 through which hydrogen gas, which is a carrier gas, flows. A mixed gas of diborane and hydrogen gas (ie, a gas containing a boron compound) was supplied to the heated substrate with the iron thin film. Diborane was supplied for 20 minutes at a flow rate of 0.33 cm ³ /min. The internal pressure of the reactor was set at 30 mbar. By supplying a gas containing a boron compound, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate [Step B].

The composition of iron boride in the iron boride layer was confirmed by the aforementioned X-ray diffraction method, and was found to be Fe ₃ B and Fe ₂ B. Further, in the X-ray diffraction pattern of the iron boride layer, the intensity of the diffraction peak of Fe ₃ B (330) was 21 cps, and the intensity of the diffraction peak of Fe ₂ B (022) was 68 cps. Further, the ratio of the intensities of these diffraction peaks [intensity of the diffraction peak of Fe ₃ B (330)/intensity of the diffraction peak of Fe ₂ B (022)] is 21/68=0.308...≒0. It was 31.

Next, while maintaining the temperature of the substrate with the iron thin film at 1033° C., by switching the valve 6, ammonia gas, which is the nitrogen raw material gas, in the container 13 is supplied to the gas line 9 through which hydrogen gas, which is the carrier gas, flows. In this way, nitrogen raw material gas was supplied from the carrier gas line 3 to the iron thin film coated substrate on which the iron boride layer was formed. By supplying nitrogen source gas, boron in the iron boride layer was nitrided [Step C]. Ammonia gas was supplied for 100 minutes at a flow rate of 250 cm ³ /min. The internal pressure of the reactor was set at 30 mbar.

After the supply of nitrogen raw material gas is completed, the temperature of the heater 17 is gradually lowered while flowing hydrogen gas as a carrier gas into the reactor 14, and after reaching room temperature (25° C.), the substrate in the reactor 14 is heated. I took it out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 650 nm.
It was confirmed by X-ray diffraction that the thin film contained Fe ₂ B and iron. The formation of a hexagonal boron nitride thin film having a plurality of layers was confirmed by measurement using an X-ray diffraction method and cross-sectional observation using a transmission electron microscope (TEM). The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film was measured by the method described above. The same applies to the following Examples and Comparative Examples.

<Example 2>
A substrate with an iron thin film having a thickness of 500 nm was prepared in the same manner as in Example 1 [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.1 cm ³ /min for 60 minutes. was formed [Step B]. Next, boron in the iron boride layer was nitrided in the same manner as in Example 1, except that ammonia gas was supplied at a flow rate of 500 cm ³ /min for 60 minutes [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 661 nm.

<Example 3>
A substrate with an iron thin film having a thickness of 500 nm was prepared in the same manner as in Example 1 [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.66 cm ³ /min for 20 minutes. was formed [Step B]. Next, the iron thin film-coated substrate on which the iron boride layer was formed was heated to 1200° C., and ammonia gas was supplied at a flow rate of 970 cm ³ /min for 60 minutes without using hydrogen gas as a carrier gas, and Boron in the iron boride layer was nitrided in the same manner as in Example 1, except that the internal pressure of the reactor was set to 1013 mbar [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 665 nm.

<Example 4>
A substrate with an iron thin film having a thickness of 250 nm was prepared in the same manner as in Example 1, except that the film formation time was 5 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the iron thin film surface of the iron thin film coated substrate in the same manner as in Example 1 except that diborane was supplied at a flow rate of 0.1 cm ³ /min for 20 minutes. was formed [Step B]. Next, boron in the iron boride layer was nitrided in the same manner as in Example 1, except that ammonia gas was supplied at a flow rate of 150 cm ³ /min for 50 minutes [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 328 nm.

<Example 5>
A substrate with an iron thin film having a thickness of 1000 nm was prepared in the same manner as in Example 1, except that the film formation time was 20 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.66 cm ³ /min for 20 minutes. was formed [Step B]. Next, the substrate with the iron thin film on which the iron boride layer was formed was heated to 1130° C., and ammonia gas was supplied for 100 minutes at a flow rate of 500 cm ³ /min. Boron in the iron oxide layer was nitrided [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 1301 nm.

<Example 6>
A substrate with an iron thin film having a thickness of 1500 nm was prepared in the same manner as in Example 1, except that the film formation time was 30 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.33 cm ³ /min for 40 minutes. was formed [Step B]. Next, boron in the iron boride layer was nitrided in the same manner as in Example 1, except that ammonia gas was supplied at a flow rate of 500 cm ³ /min for 100 minutes [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 1711 nm.

<Example 7>
A substrate with an iron thin film having a thickness of 1750 nm was prepared in the same manner as in Example 1, except that the film formation time was 35 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.66 cm ³ /min for 20 minutes. was formed [Step B]. Next, the substrate with the iron thin film on which the iron boride layer was formed was heated to 1130° C., and ammonia gas was supplied for 100 minutes at a flow rate of 500 cm ³ /min. Boron in the iron oxide layer was nitrided [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 2041 nm.

<Example 8>
A substrate with an iron thin film having a thickness of 1000 nm was prepared in the same manner as in Example 1, except that the film formation time was 20 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.1 cm ³ /min for 60 minutes. was formed [Step B]. Next, the iron boride film substrate on which the iron boride layer was formed was heated to 922° C., and ammonia gas was supplied at a flow rate of 500 cm ³ /min for 60 minutes. Boron in the iron oxide layer was nitrided [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 1150 nm.

<Example 9>
A substrate with an iron thin film having a thickness of 1000 nm was prepared in the same manner as in Example 1, except that the film formation time was 20 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.33 cm ³ /min for 40 minutes. was formed [Step B]. Next, the iron boride layer was heated to 943° C., and ammonia gas was supplied at a flow rate of 250 cm ³ /min for 200 minutes. Boron in the iron oxide layer was nitrided [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 1168 nm.

<Example 10>
A substrate with an iron thin film having a thickness of 1000 nm was prepared in the same manner as in Example 1, except that the film formation time was 20 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.66 cm ³ /min for 20 minutes. was formed [Step B]. Next, the substrate with the iron thin film on which the iron boride layer was formed was heated to 1072° C., and ammonia gas was supplied for 100 minutes at a flow rate of 500 cm ³ /min. Boron in the iron oxide layer was nitrided [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 1207 nm.

<Example 11>
A substrate with an iron thin film having a thickness of 1000 nm was prepared in the same manner as in Example 1, except that the film formation time was 20 minutes [Step A]. Next, the iron thin film was annealed in the same manner as in Example 1. Next, in the same manner as in Example 1, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate [Step B]. Next, the iron thin film-coated substrate on which the iron boride layer was formed was heated to 1200° C., and ammonia gas was supplied at a flow rate of 970 cm ³ /min for 10 minutes without using hydrogen gas as a carrier gas, and Boron in the iron boride layer was nitrided in the same manner as in Example 1, except that the internal pressure of the reactor was set to 1013 mbar [Step C]. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 1341 nm.

<Comparative example 1>
In the same manner as in Example 1, a substrate with an iron thin film having a thickness of 500 nm was prepared. Next, the iron thin film was annealed in the same manner as in Example 1. Next, in the same manner as in Example 1 except that diborane was supplied at a flow rate of 0.1 cm ³ /min for 20 minutes, Fe ₂ B was contained as iron boride on the surface of the iron thin film of the iron thin film-coated substrate. No iron boride layer was formed. Next, boron in the iron boride layer was nitrided in the same manner as in Example 1 except that ammonia gas was supplied at a flow rate of 150 cm ³ /min for 50 minutes. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₃ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₃ B and iron and the hexagonal boron nitride thin film in this laminate was 580 nm. Note that the thin film containing Fe ₃ B and iron was a thin film that did not contain Fe ₂ B as iron boride.

<Comparative example 2>
A substrate with an iron thin film having a thickness of 150 nm was prepared in the same manner as in Example 1, except that the film formation time was 3 minutes. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the iron thin film surface of the iron thin film coated substrate in the same manner as in Example 1 except that diborane was supplied at a flow rate of 0.1 cm ³ /min for 20 minutes. was formed. Next, the substrate with the iron thin film on which the iron boride layer was formed was heated to 1130° C., and ammonia gas was supplied for 50 minutes at a flow rate of 150 cm ³ /min. The boron in the iron oxide layer was nitrided. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 282 nm.

<Comparative example 3>
A substrate with an iron thin film having a thickness of 2000 nm was prepared in the same manner as in Example 1 except that the film formation time was 40 minutes. Next, the iron thin film was annealed in the same manner as in Example 1. Next, an iron boride layer containing Fe ₂ B was formed on the surface of the iron thin film of the iron thin film-coated substrate in the same manner as in Example 1, except that diborane was supplied at a flow rate of 0.66 cm ³ /min for 20 minutes. was formed. Next, boron in the iron boride layer was nitrided in the same manner as in Example 1 except that ammonia gas was supplied at a flow rate of 500 cm ³ /min for 100 minutes. Next, in the same manner as in Example 1, the substrate was taken out. The substrate taken out was a laminate including a sapphire substrate, a thin film containing Fe ₂ B and iron, and a hexagonal boron nitride thin film having multiple layers. The total thickness of the thin film containing Fe ₂ B and iron and the hexagonal boron nitride thin film in this laminate was 2249 nm.

[Measurement and evaluation]
1. Crystallinity of hexagonal boron nitride The crystallinity of hexagonal boron nitride was determined based on the half-width (unit: °) of the diffraction peak of hexagonal boron nitride in the X-ray diffraction pattern of the hexagonal boron nitride thin film.
The hexagonal boron nitride thin film obtained above was irradiated with CuKα rays (characteristic A line diffraction pattern was obtained. The half width of the diffraction peak of hexagonal boron nitride (002) in the obtained X-ray diffraction pattern was calculated as the distance between the inflection points on both sides of the curve leading to the peak maximum value using the spreadsheet software "Excel (registered trademark)". ” was used. Then, based on the obtained half-width value, the crystallinity of hexagonal boron nitride was evaluated according to the following evaluation criteria. The results are shown in Tables 1, 3 and 4.
Evaluation "A" is a practically acceptable level.

-Evaluation criteria-
A: The half width of the diffraction peak of hexagonal boron nitride (002) was 0.4° or less.
B: The half width of the diffraction peak of hexagonal boron nitride (002) exceeded 0.4°.

The reason why we set the half-width of the diffraction peak to be 0.4° or less as a practical allowable range is that the cross-section of a hexagonal boron nitride thin film whose half-width of the diffraction peak is around 0.4° was examined using a transmission electron microscope (TEM). When the half-width of the diffraction peak was 0.4° or less, good lamination in the C direction was confirmed, whereas when the half-width of the diffraction peak exceeded 0.4°, the laminated structure This is based on the fact that it was confirmed that the stacking structure in the C direction was not maintained.

2. Uniformity of a hexagonal boron nitride thin film The uniformity of a hexagonal boron nitride thin film is determined by the area ratio of the exposed portion of the substrate caused by agglomeration of iron boride, iron, etc., and hexagonal boron nitride contained in the iron boride layer. was evaluated as an index. This is based on the fact that when iron boride, iron, etc., hexagonal boron nitride, etc. contained in the iron boride layer aggregate, the uniformity of the hexagonal boron nitride thin film is impaired.
A transmitted image of the laminate obtained above (ie, the substrate on which the hexagonal boron nitride thin film was formed) was photographed using an optical microscope. The photographing conditions were a shutter speed of 1/25 second, ISO sensitivity of 800, and magnification of 200 times. After performing image processing (binarization) on the exposed and unexposed areas of the substrate in the transmission image, the ratio of the area of the exposed area of the substrate to the area of the photographed area (also known as the "exposed area ratio") is calculated. ) was calculated based on the following formula. The obtained value was rounded to the second decimal place and used for evaluation.
Percentage of exposed area (%)
= [Area of the exposed part of the board/Area of the photographed part] x 100...Formula

Image processing and calculation of the proportion of the exposed area were performed using "ImageJ", which is image processing software developed by the National Institutes of Health (NIH). Binarization was performed by converting the transmitted image into 8-bit grayscale, and then setting Threshold to Auto. The uniformity of the hexagonal boron nitride thin film was then evaluated based on the proportion of the exposed portion of the substrate and according to the following evaluation criteria. The results are shown in Tables 1, 3 and 4.
Evaluation "A" is a practically acceptable level.

-Evaluation criteria-
A: The ratio of exposed parts was 2.0% or less.
B: The ratio of exposed parts exceeded 2.0%.

For reference, an example of an optical microscope transmission image of a laminate with an evaluation result of A is shown in FIG. 2, and an example of an optical microscope transmission image of a laminate with an evaluation result of B is shown in FIG. The white portions in FIGS. 2 and 3 are portions where the substrate is exposed due to aggregation of iron boride, iron, etc., hexagonal boron nitride, etc. contained in the iron boride layer (so-called exposed portions).

As shown in Table 1, according to each of the manufacturing methods of Examples 1 to 3 including step B of forming an iron boride layer containing Fe ₂ B, a thin film of hexagonal boron nitride with high crystallinity was It has become clear that it can be manufactured with uniformity. Moreover, as shown in Tables 1 and 2, when the iron borides contained in the iron boride layer formed in step B are Fe ₃ B and Fe ₂ B, the smaller the proportion of Fe ₃ B, the more It has become clear that thin films of more highly crystalline hexagonal boron nitride can be produced with higher uniformity. In addition, when the iron borides contained in the iron boride layer formed in step B are Fe ₂ B and FeB, the smaller the proportion of FeB, the more uniform the highly crystalline hexagonal boron nitride thin film. It has become clear that it can be manufactured using
On the other hand, it is clear that the manufacturing method of Comparative Example 1, which includes the step of forming an iron boride layer but in which the iron boride layer does not contain Fe ₂ B, cannot manufacture a thin film of hexagonal boron nitride with high crystallinity. became.

Example 2 shown in Table 3 is described for comparison with other Examples and Comparative Examples, and is the same as Example 2 shown in Table 1 already described.

As shown in Table 3 and FIG. 4, according to each of the manufacturing methods of Example 2 and Examples 4 to 7, in which the thickness of the iron thin film of the iron thin film-coated substrate is in the range of 200 nm or more and 1800 nm or less, crystallinity It has become clear that thin films of hexagonal boron nitride with high chromatography can be produced with high uniformity.
On the other hand, in the manufacturing method of Comparative Example 2, in which the thickness of the iron thin film on the substrate with the iron thin film is less than 200 nm, and in the manufacturing method of Comparative Example 3, in which the film thickness of the iron thin film on the substrate with the iron thin film exceeds 1800 nm, the uniformity It has become clear that it is not possible to form a hexagonal boron nitride thin film with a high crystallinity.

As shown in Table 4, it was confirmed that the hexagonal boron nitride thin films manufactured by each of the manufacturing methods of Examples 8 to 11 had high crystallinity of hexagonal boron nitride and high film uniformity.
From these results, it is assumed that the crystallinity of hexagonal boron nitride and film uniformity in the hexagonal boron nitride thin film are not easily affected by the temperature of the iron thin film-coated substrate on which the iron boride layer is formed in step C. Ru.

The disclosure of Japanese Patent Application No. 2022-061216 filed on March 31, 2022 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards mentioned herein are specifically and exclusively incorporated by reference as if each individual document, patent application, and technical standard were specifically and individually indicated to be incorporated by reference. Incorporated herein by reference.

Claims

Step A of preparing a substrate with an iron thin film, which has a substrate and an iron thin film provided on the substrate and having a film thickness in a range of 200 nm or more and 1800 nm or less;
Step B of forming an iron boride layer containing Fe 2 B on the surface of the iron thin film of the iron thin film coated substrate by supplying a gas containing a boron compound to the iron thin film coated substrate;
Boron in the iron boride layer is removed by supplying at least one gas selected from the group consisting of nitrogen gas and a gas containing a nitrogen compound to the iron thin film-coated substrate on which the iron boride layer is formed. nitriding step C;
A method for producing a hexagonal boron nitride thin film, comprising:
The method for producing a hexagonal boron nitride thin film according to claim 1, wherein in the step C, the temperature of the iron thin film-coated substrate on which the iron boride layer is formed is set in a range of 900°C or more and 1200°C or less.
The method for manufacturing a hexagonal boron nitride thin film according to claim 1 or 2, wherein the substrate is a sapphire substrate.
The method for producing a hexagonal boron nitride thin film according to claim 1 or 2, wherein the boron compound is diborane.
The method for producing a hexagonal boron nitride thin film according to claim 1 or 2, wherein the nitrogen compound is ammonia.
A substrate and
A thin film containing Fe 2 B and iron,
a hexagonal boron nitride thin film having multiple layers;
has
A laminate, wherein the total thickness of the thin film containing Fe 2 B and iron and the hexagonal boron nitride thin film is in the range of 300 nm or more and 2100 nm or less.
The laminate according to claim 6, wherein the substrate is a sapphire substrate.