Method for manufacturing hexagonal boron nitride thin film, and layered body

Abstract

This invention provides a method for manufacturing a hexagonal boron nitride thin film and a laminate. The method for manufacturing the hexagonal boron nitride thin film includes: Step A, which involves preparing a substrate with an iron-coated thin film, the substrate having a substrate and an iron thin film disposed on the substrate with a thickness in the range of 200 nm to 1800 nm; Step B, which involves forming a Fe-containing gas on the surface of the iron thin film on the substrate by supplying a gas containing a boron compound to the substrate with the iron-coated thin film. 2B is an iron boride layer; and step C is to nitride the iron boride layer by supplying at least one gas selected from the group consisting of nitrogen and gases containing nitrogen compounds to a substrate on which the iron-attached thin film of the iron boride layer is formed.

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing a hexagonal boron nitride thin film and a laminate. [Previous Technology]

[0002] Regarding hexagonal boron nitride thin films, research has been conducted in many application fields based on their characteristics, and they are expected to be applied in the following fields: light-emitting diodes and semiconductor lasers that maximize light emission in the ultraviolet region, broadband light-receiving elements, insulating films with the characteristics of two-dimensional semiconductors represented by graphene, and release layers used in the manufacture of self-supporting substrates such as gallium nitride.

[0003] Regarding methods for manufacturing hexagonal boron nitride thin films, there are many proposals, such as the following: high-temperature and high-pressure (HTHP) method, which melts the raw material in a flux under high temperature and high pressure conditions and precipitates boron nitride from the flux; and chemical vapor deposition (CVD) method, which deposits boron nitride by contacting the raw material gas with the substrate surface. Although HTHP can produce hexagonal boron nitride thin films that dramatically improve the field-effect mobility of graphene transistors, it is difficult to achieve the large-area required for applications when using HTHP for crystal growth. In this respect, CVD is a suitable method for obtaining large-area thin films, and therefore, in recent years, new manufacturing methods for hexagonal boron nitride thin films using CVD have been actively developed. Methods for manufacturing hexagonal boron nitride thin films using CVD can be broadly divided into two types. One method involves using ammonia borane, boron yne, etc., as raw materials, and subjecting these raw materials to a dehydrogenation reaction on a metal catalyst disposed on a substrate, thereby growing hexagonal boron nitride on the substrate. Another method involves using metal-organic chemical vapor deposition (MOCVD) to supply boron-containing raw materials such as triethylborane and nitrogen-containing raw materials such as ammonia, allowing them to react on the substrate, thereby growing hexagonal boron nitride on the substrate (for example, see Japanese Patent No. 4358143). Furthermore, in recent years, as a novel method, a Vapor-Liquid-solid (VLS) thin film growth method has also been reported, which utilizes the three states of matter—gas, solid, and liquid—to grow boron nitride crystals (see, 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-area multilayer hexagonal boron nitride on dielectric substrates", NATURE COMMUNICATIONS, volume 11, Article number. 849(2020), pp. 1-8.). [Summary of the Invention]

[0004] [The problem the invention aims to solve]

[0005] In recent years, expectations for the application of hexagonal boron nitride thin films as electronic materials have been increasing, leading to a demand for methods to manufacture higher-quality hexagonal boron nitride thin films. While previous CVD methods utilizing metal catalysts have yielded hexagonal boron nitride thin films with relatively high crystallinity, the catalytic effect is reduced because the metal catalyst is covered by the precipitated hexagonal boron nitride, making it difficult to obtain hexagonal boron nitride thin films with multiple layers. If the hexagonal boron nitride thin film is thin, it is a major cause of mobility degradation, for example, when applied to graphene transistors, as it cannot adequately prevent phonon scattering and interface trapping of charged impurities. On the other hand, previous MOCVD methods that do not require metal catalysts can obtain hexagonal boron nitride thin films with multiple layers, increasing the film thickness. However, hexagonal boron nitride lacks dangling bonds that enable regular crystal growth. Therefore, if hexagonal boron nitride layers are stacked using the previous MOCVD method, the crystallinity of the hexagonal boron nitride will gradually deteriorate, losing its ordered layer structure and making it difficult to utilize the properties of the hexagonal boron nitride film (such as its ability to improve the mobility of graphene transistors). Therefore, a method is needed to manufacture highly crystallinity hexagonal boron nitride films suitable for applications in electronic materials. Furthermore, when forming metal films, metal agglomeration can sometimes occur, leading to impaired film flatness; this phenomenon can also occur when forming hexagonal boron nitride films. Therefore, a method for manufacturing hexagonal boron nitride films is also required to produce hexagonal boron nitride films with high uniformity. However, previous methods for manufacturing hexagonal boron nitride films have not focused on improving both the crystallinity and uniformity of the hexagonal boron nitride film.

[0006] This invention was made in view of the above circumstances. One embodiment of the invention aims to solve the problem of providing a method for manufacturing a hexagonal boron nitride thin film capable of producing a highly crystalline hexagonal boron nitride thin film with high uniformity. Another embodiment of the invention aims to solve the problem of providing a laminate of a hexagonal boron nitride thin film possessing both high crystallinity and high film uniformity. [Technical Means for Solving the Problem]

[0007] The specific method for solving the above problems includes the following form. [1] A method for manufacturing a hexagonal boron nitride thin film, comprising: Step A, which is to prepare a substrate with an iron-bonded thin film, the substrate having a substrate and an iron thin film disposed on the substrate and having a film thickness in the range of 200 nm to 1800 nm; Step B, which is to form an iron boride layer containing Fe2B on the surface of the iron thin film of the substrate by supplying a gas containing a boron compound to the substrate with the iron-bonded thin film; and Step C, which is to nitride the boron in the iron-bonded layer by supplying at least one gas selected from the group consisting of nitrogen gas and a gas containing a nitrogen compound to the substrate with the iron-bonded thin film on which the iron-bonded thin film has formed. [2] The method for manufacturing a hexagonal boron nitride thin film as described in [1], wherein in step C above, the temperature of the substrate on which the iron-bonded thin film with the boron layer is formed is in the range of 900°C to 1200°C. [3] The method for manufacturing a hexagonal boron nitride thin film as described in [1] or [2], wherein the substrate is a sapphire substrate. [4] The method for manufacturing a hexagonal boron nitride thin film as described in any one of [1] to [3], wherein the boron compound is diborane. [5] The method for manufacturing a hexagonal boron nitride thin film as described in any one of [1] to [4], wherein the nitrogen compound is ammonia. [6] A laminate comprising: a substrate, a thin film containing Fe2B and iron, and a plurality of hexagonal boron nitride thin films having multiple layers, wherein the total thickness of the thin film containing Fe2B and iron and the hexagonal boron nitride thin film is in the range of 300 nm to 2100 nm. [7] The laminate as described in [6], wherein the substrate is a sapphire substrate. [Effects of the Invention]

[0008] According to one embodiment of the present invention, a method for manufacturing a hexagonal boron nitride thin film capable of producing a highly crystalline hexagonal boron nitride thin film with high uniformity is provided. According to another embodiment of the present invention, a laminate of a hexagonal boron nitride thin film having high crystallinity of hexagonal boron nitride and high film uniformity is provided.

Implementation Method

[0010] Hereinafter, the manufacturing method of the hexagonal boron nitride thin film and the laminate of the present invention will be described in detail. The description of the necessary conditions described below is sometimes based on a representative embodiment of the present invention. The present invention is not limited to this embodiment, and modifications may be appropriately made to implement it within the scope of the purpose of the present invention.

[0011] In this invention, the numerical range represented by "~" means a range including the values ​​recorded before and after "~" as the lower and upper limits, respectively. Regarding the numerical ranges recorded in stages in this invention, the upper or lower limit recorded within a certain numerical range can be replaced by the upper or lower limit of another numerical range recorded in stages. Furthermore, regarding the numerical ranges recorded in this invention, the upper or lower limit recorded within a certain numerical range can be replaced by the values ​​shown in the embodiments.

[0012] In this invention, a combination of two or more preferred forms is a more preferred form.

[0013] The term "step" in this invention is not only an independent step, but also includes a step as long as the intended purpose of the step is achieved, even when it cannot be clearly distinguished from other steps.

[0014] [Method for Manufacturing Hexagonal Boron Nitride Thin Film] The method for manufacturing a hexagonal boron nitride thin film of the present invention (hereinafter also referred to as "the manufacturing method of the present invention") includes: Step A, which involves preparing a substrate with an iron-coated thin film, the substrate having a substrate and an iron thin film disposed on the substrate with a film thickness in the range of 200 nm to 1800 nm; Step B, which involves forming an iron boride layer containing Fe₂B on the surface of the iron thin film of the substrate by supplying a gas containing a boron compound to the substrate with the iron-coated thin film; and Step C, which involves nitriding the boron in the iron boride layer by supplying at least one gas selected from the group consisting of nitrogen and a gas containing a nitrogen compound to the substrate with the iron-coated thin film on which the iron boride layer is formed. According to the manufacturing method of the present invention, a highly crystalline hexagonal boron nitride thin film can be manufactured with high uniformity.

[0015] The details of the mechanism by which the manufacturing method of the present invention generates hexagonal boron nitride thin films are not clear, and the inventors speculate as follows. However, the following speculation is not intended to limit the manufacturing method of the present invention, but rather to illustrate an example.

[0016] In the manufacturing method of the present invention, a boron compound-containing gas is supplied to a prepared iron-attached thin film substrate, thereby forming an iron boride layer containing Fe₂B on the surface of the iron thin film of the iron-attached thin film substrate. The boron compound is decomposed on the surface of the iron thin film by supplying the boron compound gas to the iron-attached thin film substrate. The boron generated by the decomposition slowly penetrates from the surface of the iron thin film into the interior and reacts with the iron, thereby generating iron boride. The boron compound can be decomposed, for example, by heating the iron-attached thin film substrate. The type of iron boride generated can be controlled, for example, by the amount of boron supplied, the temperature of the iron-attached thin film substrate, and the thickness of the iron thin film. Types of iron boride include, for example, Fe₃B, Fe₂B, and FeB. In the manufacturing method of the present invention, by including Fe₂B in the iron boride layer, there is a tendency to ultimately obtain a highly crystalline hexagonal boron nitride thin film. Furthermore, in the manufacturing method of the present invention, at least one gas selected from the group consisting of nitrogen and gases containing nitrogen compounds (hereinafter also referred to as "nitrogen feed gas") is supplied to a substrate with an iron-bonded thin film having a boronized iron layer, thereby causing boron nitriding in the iron-bonded iron layer. By supplying the nitrogen feed gas to the substrate with the iron-bonded thin film having the boronized iron layer, the nitrogen feed gas decomposes on the surface of the iron-bonded iron layer. The nitrogen feed gas can be decomposed, for example, by heating the substrate with the iron-bonded thin film having the boronized iron layer. The nitrogen generated by the decomposition slowly penetrates from the surface of the iron-bonded iron layer into the interior, causing boron nitriding in the iron-bonded iron layer, thereby generating hexagonal boron nitride. That is, the manufacturing method of the present invention differs from previous methods such as MOCVD that stack layers of hexagonal boron nitride.

[0017] In the manufacturing method of the present invention, the location where the hexagonal boron nitride thin film is formed can be controlled, for example, by the substrate temperature (in other words, the film formation temperature) in step C, the raw material supply method, etc. For example, if the film formation temperature is lowered, the hexagonal boron nitride thin film will be formed on the side of the iron boride layer opposite to the substrate. That is, the hexagonal boron nitride thin film is formed on the side of the iron boride layer opposite to the substrate by means of boron and nitrogen in the iron boride layer, and boron and nitrogen are supplied from the iron boride layer to the interface between the formed hexagonal boron nitride thin film and the iron boride layer, thereby continuously forming the hexagonal boron nitride thin film. Furthermore, this formation mechanism is different from the previous mechanism of growing new hexagonal boron nitride on hexagonal boron nitride, in which the hexagonal boron nitride does not undergo polycrystalline formation, but forms an ordered layer structure. For example, if the film-forming temperature is increased, boron and nitrogen permeate the interior of the iron boride layer. Therefore, a hexagonal boron nitride film is formed not only on the side of the iron boride layer opposite to the substrate, but also on the substrate-side side of the iron boride layer (i.e., between the substrate and the iron boride layer). Furthermore, when the substrate temperature rises and the iron boride melts, a hexagonal boron nitride film is also formed on both the side of the iron boride layer opposite to the substrate and the substrate-side side of the iron boride layer. In this case, since the iron boride layer has melted, the boron nitride is less likely to interact with the iron boride, resulting in a flatter hexagonal boron nitride film.

[0018] [Step A] Step A is the step of preparing a substrate with an iron-coated thin film. The substrate with an iron-coated thin film has a substrate and an iron thin film disposed on the substrate with a film thickness in the range of 200 nm to 1800 nm. "Preparing a substrate with an iron-coated thin film" means making the substrate with an iron-coated thin film in a usable state, including: obtaining a substrate with an iron-coated thin film to be pre-manufactured, and manufacturing the substrate with an iron-coated thin film. That is, the substrate with an iron-coated thin film used in the manufacturing method of the present invention can be a pre-manufactured substrate with an iron-coated thin film, or it can be the substrate with an iron-coated thin film manufactured in Step A.

[0019] The iron-coated thin film substrate of the present invention can be manufactured by forming an iron thin film with a thickness in the range of 200 nm to 1800 nm on the substrate. Hereinafter, a preferred embodiment of the manufacturing method of the iron-coated thin film substrate of the present invention will be described.

[0020] Regarding the substrate material, it is preferable to select one that takes into account factors such as melting point, thermal decomposition, and stability under a reducing atmosphere. Examples of substrate materials include: sapphire (Al₂O₃), silicon (Si), silicon dioxide (SiO₂), aluminum nitride (AlN), silicon carbide (SiC), spinel (MgAl₂O₄), magnesium oxide (MgO), gallium arsenide (GaAs), gallium phosphide (GaP), and neodymium gallium oxide (NdGaO₃). From the viewpoint of ensuring consistent orientation of the iron layers, a single-crystal substrate is preferred. Furthermore, considering the compatibility (e.g., lattice matching) with the iron layers and the generated hexagonal boron nitride, the substrate is more preferably a sapphire substrate, an aluminum nitride substrate, or a silicon carbide substrate, and more preferably a sapphire substrate.

[0021] The thickness of the substrate is not particularly limited, but is usually 200 μm to 1000 μm, preferably 300 μm to 800 μm.

[0022] The iron thin film may be formed on only one side of the substrate or on both sides of the substrate.

[0023] The temperature of the substrate during the formation of the iron thin film is not particularly limited, but is preferably 25°C to 300°C. The mechanism for heating the substrate is not particularly limited, such as a heater.

[0024] The thickness of the iron thin film formed on the substrate is in the range of 200 nm to 1800 nm. If the thickness of the iron thin film is within the above range, the 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 to 1750 nm, more preferably in the range of 500 nm to 1750 nm, further preferably in the range of 500 nm to 1500 nm, and even more preferably in the range of 500 nm to 1000 nm.

[0025] In this invention, "film thickness of the iron thin film" refers to the average film thickness of the iron thin film. The average film thickness of the iron thin film is a value obtained by forming the iron thin film using a separately prepared substrate and by the following method. A substrate (hereinafter also referred to as "test substrate") with a mask (e.g., photoresist, polyimide tape, etc.) is prepared. After forming the iron thin film on the prepared test substrate, the mask is peeled off. The step difference between the surface of the iron thin film and the surface of the substrate after the mask is peeled off is measured using a profilometer. The arithmetic mean of the measured values ​​of the step difference at 5 randomly selected locations is calculated, and the obtained value is taken as the film thickness of the iron thin film.

[0026] Regarding the thickness of the iron thin film, for example, in the case of forming the iron thin film by sputtering, it can be controlled by the applied power, film formation time, etc. For example, if the applied power is increased, the thickness of the iron thin film becomes thicker; if the applied power is decreased, the thickness of the iron thin film becomes thinner. Also, for example, if the film formation time is prolonged, the iron thin film becomes thicker; if the film formation time is shortened, the iron thin film becomes thinner. Furthermore, regarding the applied voltage, considering that it may sometimes affect the surface roughness of the iron thin film, it is better to appropriately select the optimal condition.

[0027] Regarding the method for forming the iron thin film, sputtering and vacuum evaporation methods can be used, for example. From the viewpoint of being able to form a highly uniform iron thin film on a large-area substrate, sputtering is a preferred method for forming the iron thin film. Furthermore, considering the purity of the formed iron thin film, magnetron sputtering is a more preferred method. The magnetron sputtering method can be either RF (Radio Frequency) magnetron sputtering or DC (Direct Current) magnetron sputtering, with DC magnetron sputtering being more preferred.

[0028] When the iron thin film is formed by magnetron sputtering, the sputtering conditions are appropriately set, for example, according to the desired thickness of the iron thin film. The film-forming atmosphere is preferably argon. The film-forming pressure is preferably 0.1 Pa to 10 Pa. The film-forming substrate temperature is preferably 25°C to 300°C. The applied voltage is preferably 500 W to 6000 W.

[0029] [Step B] Step B is a step of forming an iron boride layer containing Fe₂B on the surface of the iron film on the iron film substrate prepared in Step A by supplying a gas containing a boron compound to the iron film substrate prepared in Step A. If a gas containing a boron compound is supplied to the iron film substrate, the iron in the iron film will be boronized, and an iron boride layer containing Fe₂B will be formed on the surface of the iron film on the iron film substrate.

[0030] The gas supplied to the substrate with the iron-coated thin film contains a boron compound. Examples of boron compounds include organoboron compounds such as trimethylborane (C3H9B) and triethylborane (C6H15B); boron halides such as boron trichloride (BCl3); and boron hydrides such as monoborane (BH3) and diborane (B2H6). Preferably, the boron compound is a boron halide or a boron hydride, and more preferably diborane. These boron compounds do not generate carbon upon decomposition, thus eliminating concerns about carbon contamination into the final hexagonal boron nitride, making them more suitable in this respect.

[0031] Regarding the gas supplied to the substrate with the iron-coated thin film, when the boron compound is a gas, it can be the boron compound itself in gaseous form; when the boron compound is a liquid, it can be obtained by vaporizing a liquid boron compound. In this invention, both the boron compound itself in gaseous form and the boron compound obtained by vaporizing a liquid boron compound are sometimes collectively referred to as "boron compound gas". When the boron compound is a liquid, the method for vaporizing the liquid boron compound is not particularly limited; for example, a foaming method is suitable. In the foaming method, a carrier gas is used to foam the liquid boron compound in the tank, thereby vaporizing it.

[0032] Regarding the gas supplied to the substrate to which the iron-coated thin film is attached, from an operational point of view, it is preferable to include a carrier gas in addition to a boron compound gas. The carrier gas is preferably hydrogen, or a mixture of hydrogen and an inert gas. If the carrier gas contains hydrogen, boron particles are less likely to form, thus there is a tendency to easily obtain a film of good quality. The mixture of hydrogen and an inert gas is preferably a mixture of hydrogen and argon.

[0033] The supply amount of boron compound gas is not particularly limited, and can be appropriately set according to, for example, the composition of iron boride in the desired iron boride layer and the desired thickness of the hexagonal boron nitride thin film. The supply amount of boron compound gas can be controlled, for example, by the flow rate and supply time of the boron compound gas. The flow rate of the boron compound gas is preferably 0.05 cm³ / min to 1 cm³ / min, more preferably 0.1 cm³ / min to 0.7 cm³ / min. The supply time of the boron compound gas is preferably 10 minutes to 80 minutes, more preferably 20 minutes to 60 minutes.

[0034] The temperature of the substrate with the iron-coated thin film when the gas containing the boron compound is supplied is not particularly limited, and may be appropriately set according to the composition of the iron boride in the desired iron boride layer. The temperature of the substrate with the iron-coated thin film when the gas containing the boron compound is supplied is preferably, for example, 1000°C to 1100°C. The mechanism for heating the substrate with the iron-coated thin film is not particularly limited, and a heater may be cited as an example.

[0035] The iron boride layer formed by step B contains Fe2B. If the iron boride layer contains Fe2B, a thin film of highly crystalline hexagonal boron nitride can be manufactured. In addition to Fe2B, the iron boride layer may also contain at least one type selected from the group consisting of Fe, FeB, and Fe3B. The type and ratio of iron boride contained in the iron boride layer can be controlled, for example, by adjusting the supply amount of boron compound, the temperature of the substrate on which the iron thin film is attached, and the thickness of the iron thin film. For example, if the supply amount of boron compound is increased, boron is introduced into the iron, and iron boride with a higher proportion of boron is gradually obtained. The proportion of boron increases in the order of Fe < Fe3B < Fe2B < FeB. Fe3B is called a metastable phase and is relatively difficult to obtain. The conditions for obtaining Fe3B are not clearly defined. In this invention, it is considered that the thinner the iron thin film, the easier it is to obtain. Furthermore, a phase diagram of the iron-boron system is disclosed in the literature (Intermetallics 11 (2003) 1293-1299). By increasing or decreasing the supply of boron compounds according to this phase diagram, it is relatively easy to control the type and ratio of iron boride. The contents of the aforementioned literature are incorporated into this specification by reference. In addition, the rate at which boron is introduced into the iron can also be adjusted by controlling the temperature of the substrate with the iron thin film, thereby obtaining iron boride with the desired composition.

[0036] The composition of the iron boride contained in the iron boride layer was confirmed by X-ray diffraction (XRD). Furthermore, the composition ratio of the iron boride contained in the iron boride layer was determined by X-ray diffraction. Specifically, a substrate with an iron-coated thin film having the iron boride layer formed was irradiated with CuKα rays (characteristic X-rays), and the ratio of the intensities (in cps) of the diffraction peaks of FeB (130), Fe 2B (022), and Fe 3B (330) was determined based on the obtained X-ray diffraction pattern. Regarding the X-ray conditions, the voltage was set to 45 kV and the current to 40 mA. For example, the X'Pert Pro MRD manufactured by PANalytical is suitable for use with X-ray diffraction. However, the apparatus is not limited to this.

[0037] When the iron boride contained in the iron boride layer is Fe 2B and FeB, the ratio of their composition, that is, the ratio of the intensity of the diffraction peak of Fe 2B (022) to the intensity of the diffraction peak of FeB (130) in the X-ray diffraction pattern of the iron boride layer [intensity of diffraction peak of Fe 2B (022) / intensity of diffraction peak of FeB (130)], is preferably 0.38 or more, and more preferably 0.40 or more. If the ratio of the intensity of the diffraction peak of Fe 2B (022) to the intensity of the diffraction peak of FeB (130) is 0.38 or more, there is a tendency to better suppress the agglomeration of hexagonal boron nitride and to produce a hexagonal boron nitride film with higher uniformity.

[0038] When the iron boride contained in the iron boride layer is Fe 2B and Fe 3B, the ratio of their composition, that is, the ratio of the intensity of the diffraction peak of Fe 3B (330) to the intensity of the diffraction peak of Fe 2B (022) in the X-ray diffraction pattern of the iron boride layer [intensity of diffraction peak of Fe 3B (330) / intensity of diffraction peak of Fe 2B (022)], is preferably 0.32 or less, and more preferably 0.30 or less. If the ratio of the intensity of the diffraction peak of Fe 3B (330) to the intensity of the diffraction peak of Fe 2B (022) is 0.32 or less, there is a tendency to manufacture a thin film of hexagonal boron nitride with higher crystallinity.

[0039] [Step C] Step C is a step of nitriding the boron in the iron-bonded thin film formed in Step B by supplying at least one gas selected from the group consisting of nitrogen and gases containing nitrogen compounds (i.e., nitrogen raw material gas) to the substrate on which the iron-bonded thin film with the iron-bonded layer is formed. If the nitrogen raw material gas is supplied to the substrate on which the iron-bonded thin film with the iron-bonded layer is formed, the boron in the iron-bonded layer will be nitrided to generate hexagonal boron nitride, forming a hexagonal boron nitride thin film containing multiple layers.

[0040] There is no particular limitation on the types of nitrogen compounds. Examples of nitrogen compounds include ammonia and hydrazine. As a nitrogen feed gas, from the point of view of reactivity, it is preferable to select at least one gas selected from nitrogen and ammonia, and more preferably ammonia.

[0041] The supply amount of nitrogen feedstock gas is not particularly limited, and can be appropriately set according to the required thickness of the hexagonal boron nitride thin film. The supply amount of nitrogen feedstock gas can be controlled, for example, by the flow rate and supply time of the nitrogen feedstock gas. The flow rate of the nitrogen feedstock gas is preferably 100 cm³ / min to 2000 cm³ / min, more preferably 150 cm³ / min to 1000 cm³ / min. The supply time of the nitrogen feedstock gas is preferably 5 minutes to 300 minutes, more preferably 10 minutes to 200 minutes.

[0042] When supplying a nitrogen-based gas to a substrate on which an iron-bonded thin film with an iron boride layer is formed, from an operational point of view, it is preferable to supply the nitrogen-based gas together with a carrier gas. The carrier gas is preferably hydrogen, or a mixture of hydrogen and an inert gas, and more preferably a mixture of hydrogen and an inert gas. If the carrier gas contains hydrogen, on the one hand, the reaction between boron and nitrogen contained in the iron boride is moderated, resulting in a good film; however, on the other hand, the hexagonal boron nitride thin film formed may sometimes be etched by hydrogen. In this regard, if the carrier gas contains an inert gas in addition to hydrogen, the etching of the hexagonal boron nitride thin film caused by hydrogen becomes more stable.

[0043] In step C, the temperature of the substrate on which the iron-bonded thin film with the iron boride layer is formed is preferably set to a range of 850°C to 1200°C, more preferably to a range of 900°C to 1200°C, and even more preferably to a range of 930°C to 1200°C. If the temperature of the substrate on which the iron-bonded thin film with the iron boride layer is formed is set to 850°C or higher, there is a tendency to produce a thin film of hexagonal boron nitride with higher crystallinity with better uniformity. If the temperature of the substrate on which the iron-bonded thin film with the iron boride layer is formed is set to 1200°C or lower, there is a tendency to better suppress the agglomeration of hexagonal boron nitride and produce a thin film of hexagonal boron nitride with higher uniformity. The mechanism for heating the substrate on which the iron-bonded thin film with the iron boride layer is formed is not particularly limited; for example, a heater can be used.

[0044] The thickness of the hexagonal boron nitride thin film formed in step C is not particularly limited and can be appropriately set according to the purpose, for example, preferably 5 nm to 100 nm. The "thickness of the hexagonal boron nitride thin film" in this invention is determined by the following method: Using a scanning electron microscope (SEM), the cross-section of the hexagonal boron nitride thin film is observed, and the thickness of the cross-section of the hexagonal boron nitride thin film at five randomly selected locations is measured. The arithmetic mean of the measured values ​​is calculated, and the obtained value is taken as the thickness of the hexagonal boron nitride thin film.

[0045] Regarding the thickness of the hexagonal boron nitride thin film, it can be controlled, for example, by adjusting the thickness of the iron thin film, the supply amount of boron compound, and the supply amount of nitrogen feed gas. For example, if the thickness of the iron thin film is increased, and the supply amount of boron compound and nitrogen feed gas is increased, the thickness of the hexagonal boron nitride thin film will become thicker. On the other hand, if the supply amount of boron compound and nitrogen feed gas is reduced regardless of the thickness of the iron thin film, the thickness of the hexagonal boron nitride thin film will become thinner.

[0046] [Other Steps] Within the scope of not impairing the effects of the present invention, the manufacturing method of the present invention may include steps other than steps A, B, and C (so-called other steps) as needed. The manufacturing method of the present invention preferably includes the following step as an other step: after forming an iron thin film on the substrate and before generating hexagonal boron nitride, the iron thin film is annealed using hydrogen. Annealing the iron thin film can remove trace amounts of oxygen that may be present in the iron thin film, and can also improve the adhesion between the substrate and the iron thin film, as well as the orientation of the iron thin film. From the viewpoint of removing trace amounts of oxygen that may be present in the iron thin film, the annealing of the iron thin film using hydrogen is preferably performed just before the generation of hexagonal boron nitride, for example, in a reactor for generating hexagonal boron nitride. The annealing temperature is preferably, for example, 950°C to 1100°C, more preferably 1000°C to 1050°C.

[0047] FIG1 is a schematic configuration diagram showing an example of an apparatus suitable for use in the manufacturing method of the present invention. In FIG1, symbol 1 indicates an exhaust line for directly venting the gas containing each raw material without supplying it to the substrate, symbol 2 indicates a carrier gas line for supplying a boron compound-containing gas to the substrate with an iron-coated thin film, and symbol 3 indicates a carrier gas line for supplying nitrogen raw material gas to the substrate with an iron-coated thin film having a boronized iron layer formed thereon. In the apparatus shown in FIG1, 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, thus preventing the mixing of boron raw material and nitrogen raw material and suppressing the formation of by-reactants called adducts.

[0048] Symbols 4, 5, and 6 indicate valves used to switch between supplying gases containing various raw materials to the carrier gas line or allowing them to flow into the exhaust line. Symbols 7, 8, 9, and 10 indicate gas lines used to supply carrier gas A, carrier gas B, carrier gas C, and gas D to the corresponding raw materials, respectively. Gas D supplied from gas line 10 functions not only as a carrier gas but also as a foaming gas for vaporizing liquid boron compounds (e.g., triethylborane).

[0049] Symbol 11 indicates a container for storing liquid boron compounds. Liquid boron compounds such as triethylborane are stored in container 11, and one end of gas pipeline 10 is immersed in the liquid to enable bubbling. Symbol 12 indicates a container for storing gaseous boron compounds (e.g., diborane), and symbol 13 indicates a container for storing nitrogen feedstock gases (e.g., ammonia).

[0050] Symbol 14 represents a reactor, symbol 15 represents a substrate with an iron film attached, symbol 16 represents a base supporting the substrate 15 with an iron film attached, and symbol 17 represents a heater used to heat the substrate 15 with an iron film attached.

[0051] An example of the specific steps of the manufacturing method of the present invention using the apparatus shown in FIG. 1 will be described. First, in the reactor 14, the substrate 15 with an iron-coated thin film disposed on the base 16 is heated to a specific temperature (e.g., 1000°C to 1100°C) using the heater 17. After the substrate 15 with the iron-coated thin film is heated to the specific temperature, carrier gas A, carrier gas B, carrier gas C, and gas (carrier gas) D are supplied to gas lines 7, 8, 9, and 10, respectively. Carrier gas (e.g., hydrogen) is continuously supplied to gas lines 7, 8, 9, and 10 until the formation of the hexagonal boron nitride thin film is completed. Then, only hydrogen, which is used as a carrier gas, is supplied to the substrate 15 with the iron-coated thin film heated to the specific temperature to anneal the iron thin film. Subsequently, a specific amount of gas containing a boron compound (e.g., diborane) is supplied to the substrate 15 with the iron-coated thin film inside the reactor 14 via carrier gas line 2, causing the iron film to be boronized, thereby forming an iron boride layer containing Fe₂B on the surface of the iron film on the substrate 15. Next, the set temperature of the heater 17 is switched to a specific temperature (e.g., 900°C to 1200°C) for the substrate with the iron-coated thin film containing the boride layer. After the temperature stabilizes, a specific amount of nitrogen feedstock gas (e.g., ammonia) is supplied to the substrate with the iron-coated thin film containing the boride layer via carrier gas line 3, causing the boron in the iron-coated layer to be nitrided. Hexagonal boron nitride is generated through this boron nitriding, forming a hexagonal boron nitride thin film. After the supply of nitrogen feedstock gas ends, the temperature of the heater 17 is slowly reduced to room temperature (e.g., 25°C) while carrier gas flows in, and then the substrate with the hexagonal boron nitride thin film is removed. The removed substrate has a structure consisting of a substrate, a thin film containing iron boron and iron, and multiple layers of hexagonal boron nitride thin film.

[0052] [Laminated Structure] The laminate of the present invention comprises a substrate, a thin film containing Fe₂B and iron, and a hexagonal boron nitride thin film having multiple layers, wherein the total thickness of the Fe₂B and iron thin film and the hexagonal boron nitride thin film is in the range of 300 nm to 2100 nm. The laminated system of the present invention possesses a hexagonal boron nitride thin film with high crystallinity and film uniformity, and is suitable for use as an electronic material, and particularly suitable as a substrate for graphene.

[0053] The laminate of the present invention has a substrate. Specific examples of the material of the substrate in the laminate of the present invention are the same as those of the material of the substrate in the manufacturing method of the present invention. The substrate in the laminate of the present invention 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 invention is not particularly limited, and is typically 200 μm to 1000 μm, preferably 300 μm to 800 μm.

[0054] The laminate of the present invention has a thin film comprising Fe₂B and iron. For example, X-ray diffraction can be used to confirm that the laminate of the present invention has a thin film comprising Fe₂B and iron. The thin film may contain iron boride other than Fe₂B. Examples of iron boride other than Fe₂B include FeB and Fe₃B.

[0055] The laminate of the present invention comprises a hexagonal boron nitride thin film having a plurality of layers. The hexagonal boron nitride thin film may have two or more layers, preferably 30 or more, more preferably 60 or more, and even more preferably 100 or more layers. There is no particular limitation on the upper limit of the number of layers in the hexagonal boron nitride thin film.

[0056] In the laminate of the present invention, the total thickness of the film containing Fe 2B and iron and the hexagonal boron nitride film is in the range of 300 nm to 2100 nm, preferably in the range of 600 nm to 2100 nm, more preferably in the range of 600 nm to 1750 nm, and even more preferably in the range of 600 nm to 1500 nm.

[0057] The "total film thickness of the film containing Fe 2B and iron and the hexagonal boron nitride film" in this invention is obtained by the following method: Using a scanning electron microscope (SEM), the cross-sections of the film containing Fe 2B and iron and the hexagonal boron nitride film are observed, and the thicknesses of the cross-sections of the film containing Fe 2B and iron and the hexagonal boron nitride film are measured at five randomly selected locations. The arithmetic mean of the measured values ​​is calculated, and the obtained value is taken as the total film thickness of the film containing Fe 2B and iron and the hexagonal boron nitride film.

[0058] The laminate of the present invention is suitably manufactured, for example, by the manufacturing method of the present invention as described above. [Example]

[0059] The following examples illustrate the manufacturing method and laminate of the present invention in further detail. The manufacturing method and laminate of the present invention are not limited to the following examples as long as they do not depart from its spirit.

[0060] [Manufacturing of Hexagonal Boron Nitride Thin Film] <Example 1> An iron thin film was formed on a sapphire substrate by magnetron sputtering, and a substrate with an iron thin film was prepared [Step A]. The sputtering conditions are as follows.

[0061] -Sputtering Conditions- Target: Fe Film Deposition Pressure: 0.4 Pa Film Deposition Substrate Temperature: 250℃ Film Deposition Atmosphere: Ar Gas Applied Power: 2000 W Film Deposition Time: 10 minutes

[0062] The thickness of the iron thin film in the prepared iron thin film substrate was measured by the method described above, and the result was 500 nm.

[0063] Next, using the apparatus having the configuration shown in FIG. 1, the following operations are performed. The prepared iron-coated film substrate is placed on the base 16 provided inside the reactor 14. Heating is performed using heater 17 to bring the temperature of the iron-coated film substrate to 1033°C, and then hydrogen gas, as a carrier gas, is supplied to gas lines 7, 8, and 9. Subsequently, the heated iron-coated film substrate is annealed by supplying only hydrogen gas, which is used as a carrier gas.

[0064] Next, by switching valve 5, diborane, a boron compound, in container 12 is supplied to gas line 8, which supplies hydrogen as a carrier gas. This allows a mixture of diborane and hydrogen (i.e., a gas containing boron compounds) to be supplied from carrier gas line 2 to the iron-coated thin film substrate heated to 1033°C within reactor 14. Diborane is supplied at a flow rate of 0.33 cm³ / min for 20 minutes. The internal pressure of the reactor is set to 30 mbar. By supplying the gas containing boron compounds, an iron boride layer containing Fe₂B is formed on the surface of the iron thin film on the iron-coated thin film substrate [Step B].

[0065] The composition of the iron boride in the iron boride layer was confirmed by the X-ray diffraction method described above, and the result was Fe 3B and Fe 2B. Furthermore, the intensity of the diffraction peak of Fe 3B (330) in the X-ray diffraction pattern of the iron boride layer was 21 cps, and the intensity of the diffraction peak of Fe 2B (022) was 68 cps. Furthermore, the ratio of the intensities of these diffraction peaks [intensity of diffraction peak of Fe 3B (330) / intensity of diffraction peak of Fe 2B (022)] was 21 / 68 = 0.308... ≒0.31.

[0066] Next, the temperature of the substrate with the iron-coated thin film is maintained at 1033°C, and ammonia gas, which serves as the nitrogen feedstock gas, is supplied from container 13 to gas line 9, which serves as the carrier gas, via switching valve 6. This supplies nitrogen feedstock gas to the substrate with the iron-coated thin film to which the iron-coated layer is formed, via carrier gas line 3. By supplying nitrogen feedstock gas, the boron in the iron-coated layer is boronized [Step C]. Ammonia gas is supplied at a flow rate of 250 cm³ / min for 100 minutes. The internal pressure of the reactor is set to 30 mbar.

[0067] After the nitrogen feed gas supply is completed, hydrogen gas, used as a carrier gas, flows into the reactor 14 while the temperature of the heater 17 is slowly reduced to room temperature (25°C). The substrate inside the reactor 14 is then removed. The removed substrate is a laminate containing a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin film and the hexagonal boron nitride thin film in this laminate is 650 nm. The presence of Fe₂B and iron in the thin film was confirmed by X-ray diffraction. The formation of multiple layers of hexagonal boron nitride thin films was confirmed by X-ray diffraction measurement and cross-sectional observation using a transmission electron microscope (TEM). The total thickness of the Fe₂B and iron thin film and the hexagonal boron nitride thin film was measured using the methods described above. The following examples and comparative examples are similarly performed.

[0068] <Example 2> A substrate with an iron thin film thickness of 500 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron thin film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.1 cm³ / min for 60 minutes, a boronized iron layer containing Fe₂B was formed on the surface of the iron thin film on the substrate in the same manner as in Example 1 [Step B]. Then, except that ammonia was supplied at a flow rate of 500 cm³ / min for 60 minutes, the boronized iron layer was boronized in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate was a laminate having a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The combined thickness of the Fe 2B and iron thin film and the hexagonal boron nitride thin film in the laminate is 661 nm.

[0069] <Example 3> A substrate with an iron film thickness of 500 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Next, except that diborane was supplied at a flow rate of 0.66 cm³ / min for 20 minutes, an iron boride layer containing Fe₂B was formed on the surface of the iron film on the substrate in the same manner as in Example 1 [Step B]. Next, the substrate with the iron film containing the iron boride layer was heated to 1200°C, and ammonia was supplied at a flow rate of 970 cm³ / min for 60 minutes without using hydrogen as a carrier gas. The internal pressure of the reactor was set to 1013 mbar. Otherwise, the boron nitriding in the iron boride layer was performed in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 665 nm.

[0070] <Example 4> Except that the film formation time was set to 5 minutes, a substrate with an iron film thickness of 250 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.1 cm³ / min for 20 minutes, a boronized iron layer containing Fe₂B was formed on the surface of the iron film on the substrate in the same manner as in Example 1 [Step B]. Then, except that ammonia was supplied at a flow rate of 150 cm³ / min for 50 minutes, the boronized iron layer was boronized in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate was a laminate having a sapphire substrate, a film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride film. The combined thickness of the Fe 2B and iron thin film and the hexagonal boron nitride thin film in the laminate is 328 nm.

[0071] <Example 5> Except that the film formation time was set to 20 minutes, a substrate with an iron film thickness of 1000 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.66 cm³ / min for 20 minutes, a boronized layer containing Fe₂B was formed on the surface of the iron film on the substrate with the iron film attached in the same manner as in Example 1 [Step B]. Then, except that the substrate with the iron film attached and the boronized layer was heated to 1130°C and ammonia was supplied at a flow rate of 500 cm³ / min for 100 minutes, the boronized layer was boronized in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 1301 nm.

[0072] <Example 6> Except that the film formation time was set to 30 minutes, a substrate with an iron film thickness of 1500 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.33 cm³ / min for 40 minutes, a boronized iron layer containing Fe₂B was formed on the surface of the iron film on the substrate in the same manner as in Example 1 [Step B]. Then, except that ammonia was supplied at a flow rate of 500 cm³ / min for 100 minutes, the boronized iron layer was boronized in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate was a laminate having a sapphire substrate, a film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride film. The combined thickness of the Fe 2B and iron thin film and the hexagonal boron nitride thin film in the laminate is 1711 nm.

[0073] <Example 7> Except that the film formation time was set to 35 minutes, a substrate with an iron film thickness of 1750 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.66 cm³ / min for 20 minutes, a boronized iron layer containing Fe₂B was formed on the surface of the iron film on the substrate with the iron film attached in the same manner as in Example 1 [Step B]. Then, except that the substrate with the iron film attached and the boronized iron layer was heated to 1130°C and ammonia was supplied at a flow rate of 500 cm³ / min for 100 minutes, the boronized iron layer was boronized in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 2041 nm.

[0074] <Example 8> Except that the film formation time was set to 20 minutes, a substrate with an iron film thickness of 1000 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.1 cm³ / min for 60 minutes, a boronized layer containing Fe₂B was formed on the surface of the iron film on the substrate with the iron film attached in the same manner as in Example 1 [Step B]. Then, except that the substrate with the iron film attached and the boronized layer was heated to 922°C and ammonia was supplied at a flow rate of 500 cm³ / min for 60 minutes, the boronized layer was boronized in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 1150 nm.

[0075] <Example 9> Except that the film formation time was set to 20 minutes, a substrate with an iron film thickness of 1000 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.33 cm³ / min for 40 minutes, an iron boride layer containing Fe₂B was formed on the surface of the iron film on the substrate with the iron film attached in the same manner as in Example 1 [Step B]. Then, except that the substrate with the iron film attached and the iron boride layer formed was heated to 943°C and ammonia was supplied at a flow rate of 250 cm³ / min for 200 minutes, the boron nitriding in the iron boride layer was carried out in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 1168 nm.

[0076] <Example 10> Except that the film formation time was set to 20 minutes, a substrate with an iron film thickness of 1000 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, except that diborane was supplied at a flow rate of 0.66 cm³ / min for 20 minutes, an iron boride layer containing Fe₂B was formed on the surface of the iron film on the substrate with the iron film attached in the same manner as in Example 1 [Step B]. Then, except that the substrate with the iron film attached and the iron boride layer was heated to 1072°C and ammonia was supplied at a flow rate of 500 cm³ / min for 100 minutes, the boron nitriding in the iron boride layer was carried out in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 1207 nm.

[0077] <Example 11> Except that the film formation time was set to 20 minutes, a substrate with an iron film thickness of 1000 nm was prepared in the same manner as in Example 1 [Step A]. Then, the iron film was annealed in the same manner as in Example 1. Then, an iron boride layer containing Fe2B was formed on the surface of the iron film on the substrate with the iron film in the same manner as in Example 1 [Step B]. Then, the substrate with the iron film having the iron boride layer was heated to 1200°C, and ammonia gas was supplied at a flow rate of 970 cm3 / min for 10 minutes without using hydrogen as a carrier gas, and the internal pressure of the reactor was set to 1013 mbar. Otherwise, the boron nitriding in the iron boride layer was carried out in the same manner as in Example 1 [Step C]. Then, the substrate was removed in the same manner as in Example 1. The removed substrate is a laminate comprising a sapphire substrate, a thin film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride thin films. The total thickness of the Fe₂B and iron thin films and the hexagonal boron nitride thin films in the laminate is 1341 nm.

[0078] <Comparative Example 1> A substrate with an iron thin film thickness of 500 nm was prepared in the same manner as in Example 1. Then, the iron thin film was annealed in the same manner as in Example 1. Next, except that diborane was supplied at a flow rate of 0.1 cm³ / min for 20 minutes, an iron boride layer without Fe₂B as iron boride was formed on the surface of the iron thin film on the substrate in the same manner as in Example 1. Next, except that ammonia was supplied at a flow rate of 150 cm³ / min for 50 minutes, the boron nitride in the iron boride layer was boronized in the same manner as in Example 1. Then, the substrate was removed in the same manner as in Example 1. The removed substrate was a laminate having a sapphire substrate, a thin film containing Fe₃B and iron, and multiple layers of hexagonal boron nitride thin films. The combined thickness of the Fe3B and iron thin film and the hexagonal boron nitride thin film in this laminate is 580 nm. Furthermore, the Fe3B and iron thin film is a thin film that does not contain Fe2B as iron boride.

[0079] <Comparative Example 2> Except that the film formation time was set to 3 minutes, a substrate with an iron film thickness of 150 nm was prepared in the same manner as in Example 1. Then, the iron film was annealed in the same manner as in Example 1. Next, except that diborane was supplied at a flow rate of 0.1 cm³ / min for 20 minutes, an iron boride layer containing Fe₂B was formed on the surface of the iron film on the substrate with the iron film attached, in the same manner as in Example 1. Next, except that the substrate with the iron film attached and the iron boride layer formed was heated to 1130°C and ammonia was supplied at a flow rate of 150 cm³ / min for 50 minutes, the boron nitriding in the iron boride layer was performed in the same manner as in Example 1. Then, the substrate was removed in the same manner as in Example 1. The removed substrate was a laminate having a sapphire substrate, a film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride film. The combined thickness of the Fe 2B and iron thin film and the hexagonal boron nitride thin film in the laminate is 282 nm.

[0080] <Comparative Example 3> Except that the film formation time was set to 40 minutes, a substrate with an iron film thickness of 2000 nm was prepared in the same manner as in Example 1. Then, the iron film was annealed in the same manner as in Example 1. Next, except that diborane was supplied at a flow rate of 0.66 cm³ / min for 20 minutes, a boronized iron layer containing Fe₂B was formed on the surface of the iron film on the substrate in the same manner as in Example 1. Next, except that ammonia was supplied at a flow rate of 500 cm³ / min for 100 minutes, the boronized iron layer was boronized in the same manner as in Example 1. Then, the substrate was removed in the same manner as in Example 1. The removed substrate was a laminate having a sapphire substrate, a film containing Fe₂B and iron, and multiple layers of hexagonal boron nitride film. The combined thickness of the Fe 2B and iron thin film and the hexagonal boron nitride thin film in the laminate is 2249 nm.

[0081] [Measurement and Evaluation] 1. Crystallinity of Hexagonal Boron Nitride The crystallinity of hexagonal boron nitride is determined based on the half-width at half-maximum (WHM) (in °) of the diffraction peak of hexagonal boron nitride in the X-ray diffraction pattern of the hexagonal boron nitride thin film. For the hexagonal boron nitride thin film obtained above, an X'Pert Pro MRD manufactured by PANalytical was used as the measuring device, and CuKα rays (characteristic X-rays) were irradiated under the conditions of 45 kV and 40 mA to obtain an X-ray diffraction pattern. The WHM of the diffraction peak of hexagonal boron nitride (002) in the obtained X-ray diffraction pattern was used as the distance between the inflection points on both sides of the curve reaching the maximum peak value, and was calculated using the spreadsheet software "Excel (registered trademark)". Then, based on the obtained WHM 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. An "A" rating represents a practically acceptable level.

[0082] -Evaluation Criteria- A: The half-width at half-maximum (WWHM) of the diffraction peak of hexagonal boron nitride (002) is less than 0.4°. B: The half-width at half-maximum (WWHM) of the diffraction peak of hexagonal boron nitride (002) exceeds 0.4°.

[0083] The rationale for setting the half-width of the diffraction peak below 0.4° as a practically permissible range is as follows: When observing the cross-section of a hexagonal boron nitride thin film with a half-width of the diffraction peak of about 0.4° using a transmission electron microscope (TEM), if the half-width of the diffraction peak is below 0.4°, good stacking in the C direction is confirmed. In contrast, if the half-width of the diffraction peak exceeds 0.4°, the stacking structure is confirmed to be disordered and the stacking in the C direction cannot be maintained.

[0084] 2. Uniformity of Hexagonal Boron Nitride Thin Film The uniformity of the hexagonal boron nitride thin film is evaluated by using the area ratio of the exposed portion of the substrate caused by the agglomeration of iron boride, iron, and hexagonal boron nitride contained in the iron boride layer as an indicator. This is based on the following: if the iron boride, iron, and hexagonal boron nitride contained in the iron boride layer agglomerate, the uniformity of the hexagonal boron nitride thin film will be impaired. A transmission image of the aforementioned laminate (i.e., the substrate on which the hexagonal boron nitride thin film is formed) is captured using an optical microscope. The shooting conditions are set to a shutter speed of 1 / 25 second, ISO (International Organization for Standardization) sensitivity of 800, and magnification of 200x. After image processing (binarization) of the exposed and unexposed portions of the substrate in the transmission image, the ratio of the area of ​​the exposed portion of the substrate to the area of ​​the photographed portion (also called the "exposed portion ratio") is calculated based on the following formula. Round the obtained value to two decimal places for evaluation. Exposure ratio (%) = [Area of ​​exposed substrate / Area of ​​photographed portion] × 100... Formula

[0085] Image processing and the calculation of the exposed portion ratio were performed using ImageJ, an image processing software developed by the National Institutes of Health (NIH). Binarization was performed by converting the transmitted image to grayscale using 8 bits, with the Threshold set to Auto. Then, based on the exposed portion ratio of the substrate, the uniformity of the hexagonal boron nitride film was evaluated according to the following evaluation criteria. The results are shown in Tables 1, 3, and 4. An evaluation of "A" represents a practically acceptable level.

[0086] - Evaluation Criteria- A: The percentage of exposed area is less than 2.0%. B: The percentage of exposed area exceeds 2.0%.

[0087] An example of a transmission image of an optical microscope of a laminate with an evaluation result of A is shown in Figure 2, and an example of a transmission image of an optical microscope of a laminate with an evaluation result of B is shown in Figure 3 for reference. The white parts in Figures 2 and 3 are the parts of the substrate exposed due to the agglomeration of iron boride, iron, and hexagonal boron nitride contained in the iron boride layer (the so-called exposed parts).

[0088] [Table 1] Composition of iron boride in the iron boride layer Iron film thickness (nm) Temperature at which an iron-bonded thin film is formed on a substrate with an iron boride layer (°C) Crystallinity of hexagonal boron nitride Uniformity of hexagonal boron nitride thin films Half-width (°) Evaluate Ratio of exposed area (%) Evaluate Example 1 Fe3B + Fe2B 500 1033 0.239 A 1.4 A Example 2 Fe2B 500 1033 0.179 A 0.0 A Example 3 Fe2B+FeB 500 1200 0.132 A 1.6 A Comparative Example 1 Fe3B 500 1033 0.414 B 0.0 A

[0089] [Table 2] Composition of iron boride in the iron boride layer Intensity of diffraction peak [cps] intensity ratio of diffraction peak Fe3B (330) Fe2B (022) FeB (130) Example 1 Fe3B + Fe2B twenty one 68 - Fe3B(330) / Fe2B(022) 0.31 Example 3 Fe2B+FeB - 13 33 Fe2B(022) / FeB(130) 0.39

[0090] As shown in Table 1, it can be seen that according to the manufacturing methods of Examples 1 to 3, which include step B of forming an iron boron layer containing Fe 2B, a thin film of highly crystalline hexagonal boron nitride can be manufactured with high uniformity. Furthermore, as shown in Tables 1 and 2, it can be seen that when the iron boron layer formed in step B contains Fe 3B and Fe 2B, the lower the proportion of Fe 3B, the more uniform the hexagonal boron nitride thin film can be manufactured with higher crystallinity. Also, it can be seen that when the iron boron layer formed in step B contains Fe 2B and FeB, the lower the proportion of FeB, the more uniform the hexagonal boron nitride thin film can be manufactured with higher crystallinity. On the other hand, it can be seen that in the manufacturing method of Comparative Example 1, which includes the step of forming an iron boron layer but the iron boron layer does not contain Fe 2B, a thin film of highly crystalline hexagonal boron nitride cannot be manufactured.

[0091] [Table 3] Composition of iron boride in the iron boride layer Iron film thickness (nm) Temperature at which an iron-bonded thin film is formed on a substrate with an iron boride layer (°C) Crystallinity of hexagonal boron nitride Uniformity of hexagonal boron nitride thin films Half-width (°) Evaluate Ratio of exposed area (%) Evaluate Comparative Example 2 Fe2B 150 1033 0.164 A 35.4 B Example 4 Fe2B 250 1033 0.261 A 1.0 A Example 2 Fe2B 500 1033 0.179 A 0.0 A Example 5 Fe2B 1000 1130 0.190 A 0.1 A Example 6 Fe2B 1500 1033 0.204 A 0.0 A Example 7 Fe2B 1750 1130 0.190 A 0.0 A Comparative Example 3 Fe2B 2000 1130 0.140 A 15.7 B

[0092] The Example 2 described in Table 3 is described for comparison with other examples and comparative examples, and is the same as the Example 2 described in Table 1.

[0093] As shown in Table 3 and Figure 4, it can be seen that according to the manufacturing methods of Examples 2 and 4 to 7, where the thickness of the iron thin film on the substrate with the iron thin film attached is in the range of 200 nm to 1800 nm, a highly crystalline hexagonal boron nitride thin film can be manufactured with high uniformity. On the other hand, it can be seen that in the manufacturing method of Comparative Example 2, where the thickness of the iron thin film on the substrate with the iron thin film attached is less than 200 nm, and in the manufacturing method of Comparative Example 3, where the thickness of the iron thin film on the substrate with the iron thin film attached exceeds 1800 nm, a highly uniform hexagonal boron nitride thin film cannot be formed.

[0094] [Table 4] Composition of iron boride in the iron boride layer Iron film thickness (nm) Temperature at which an iron-bonded thin film is formed on a substrate with an iron boride layer (°C) Crystallinity of hexagonal boron nitride Uniformity of hexagonal boron nitride thin films Half-width (°) Evaluate Ratio of exposed area (%) Evaluate Example 8 Fe2B 1000 922 0.218 A 1.9 A Example 9 Fe2B 1000 943 0.161 A 0.1 A Example 10 Fe2B 1000 1072 0.168 A 1.2 A Example 11 Fe2B 1000 1200 0.165 A 1.1 A

[0095] As shown in Table 4, it was confirmed that the hexagonal boron nitride thin films manufactured by the manufacturing methods of Examples 8 to 11 exhibited higher crystallinity and film uniformity. Based on these results, it is speculated that the crystallinity and film uniformity of the hexagonal boron nitride thin films are less affected by the temperature of the substrate with the iron-bonded thin film formed in step C. 1: Exhaust line 2: Carrier gas line 3: Carrier gas line 4: Valve 5: Valve 6: Valve 7: Gas line 8: Gas line 9: Gas line 10: Gas line 11: Container 12: Container 13: Container 14: Reactor 15: Substrate with iron-bonded thin film 16: Base 17: Heater A: Carrier gas B: Carrier gas C: Carrier gas D: Gas [Simplified Explanation of the Diagram]

[0009] Figure 1 is a schematic diagram showing an example of an apparatus suitable for use in the manufacturing method of the present invention. Figure 2 is an example of a transmission image from an optical microscope of a laminate for which the uniformity evaluation result of the hexagonal boron nitride thin film is A. Figure 3 is an example of a transmission image from an optical microscope of a laminate for which the uniformity evaluation result of the hexagonal boron nitride thin film is B. Figure 4 is a graph showing the relationship between the thickness of the iron thin film in the substrate with the attached iron thin film and the ratio of the exposed portion of the substrate in the laminate.

Claims

1. A method for manufacturing a hexagonal boron nitride thin film, comprising: Step A involves preparing a substrate with an iron-coated thin film, the substrate having a substrate and an iron thin film disposed on the substrate with a thickness in the range of 200 nm to 1800 nm; Step B involves forming an iron boride layer containing Fe₂B on the surface of the iron thin film of the substrate by supplying a gas containing a boron compound to the substrate with the iron-coated thin film; and Step C involves nitriding the boron in the iron boride layer by supplying at least one gas selected from the group consisting of nitrogen and a gas containing a nitrogen compound to the substrate with the iron-coated thin film on which the iron boride layer is formed.

2. The method for manufacturing a hexagonal boron nitride thin film as claimed in claim 1, wherein in step C above, the temperature of the substrate on which the iron-attached thin film on which the above-mentioned iron boride layer is formed is in the range of 900°C to 1200°C.

3. A method for manufacturing a hexagonal boron nitride thin film as claimed in claim 1 or 2, wherein the substrate is a sapphire substrate.

4. The method for manufacturing a hexagonal boron nitride thin film as claimed in claim 1 or 2, wherein the boron compound is diborane.

5. A method for manufacturing a hexagonal boron nitride thin film as claimed in claim 1 or 2, wherein the nitrogen compound is ammonia.

6. A laminate comprising: a substrate, a thin film containing Fe₂B and iron, and a plurality of hexagonal boron nitride thin films, wherein the total thickness of the Fe₂B and iron thin film and the hexagonal boron nitride thin film is in the range of 300 nm to 2100 nm.

7. The laminate as claimed in claim 6, wherein the substrate is a sapphire substrate.

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