Film forming method, film forming device, susceptor, and α-gallium oxide film

Abstract

The present invention is a film forming method comprising: a step of forming a raw material mist by atomizing a raw material solution; a step of forming a gas mixture by mixing the raw material mist and a carrier gas; a step of placing a substrate on a placement portion of a susceptor; a step of forming a film due to a thermal reaction on the substrate, by supplying the gas mixture from an atomization means to the substrate; and a step of discharging the gas mixture by a discharging means after forming the film, the film forming method being characterized in that, in the step of forming the film due to a thermal reaction on the substrate by supplying the gas mixture from the atomization means to the substrate, at least a portion of the gas mixture is supplied from a flat portion that is adjacent to the placement portion and has a surface roughness of 200 μm or less to the surface of the substrate. Thereby, a film forming method is provided that makes it possible to manufacture a high-quality film on a large-diameter substrate surface in a uniform and stable manner.

Description

Film formation method, film formation apparatus, susceptor, and α-gallium oxide film

[0001] The present invention relates to a film formation method, a film formation apparatus, a susceptor, and an α-gallium oxide film.

[0002] Film formation techniques using water particles, such as mist CVD, are known as methods capable of forming epitaxial films at low temperatures and atmospheric pressure. Patent Document 1 discloses an apparatus for forming a film by tilting a substrate placed on a substantially semicircular susceptor in a horizontal furnace. Patent Document 2 discloses a film formation apparatus for forming a film by horizontally supplying a raw material mist to a substrate fixed in a flat flow path known as a fine channel. Patent Document 3 discloses a film formation apparatus in which a film formation nozzle equipped with a raw material solution ejection section and an exhaust section is positioned facing the film formation surface of the substrate and moved parallel to the substrate to form a film. Patent Document 4 discloses a film formation method in which a raw material mist is transported into a reaction vessel by a carrier gas and a swirling flow is generated to cause the mist to react with the substrate.

[0003] JP 2016-27636 A JP 2013-28480 A International Publication No. 2016 / 051559 JP 2016-146442 A

[0004] However, these conventional film forming apparatuses have had problems such as significant generation and adhesion of powder, which introduces a large number of defects into the film, and causing cloudiness due to abnormal film growth.

[0005] The present invention has been made to solve the above problems, and aims to provide a film formation method, a film formation apparatus, and a susceptor that can uniformly and stably produce high-quality films on the surfaces of large-diameter substrates. Another aim of the present invention is to provide an α-gallium oxide film of higher quality than conventional films.

[0006] The present invention has been made to achieve the above-mentioned object, and provides a film formation method including the steps of atomizing a raw material solution to form a raw material mist, mixing the raw material mist with a carrier gas to form a gas mixture, placing a substrate on a mounting portion of a susceptor, supplying the gas mixture from atomization means to the substrate and forming a film on the substrate by a thermal reaction, and exhausting the gas mixture after film formation by exhaust means, wherein in the step of supplying the gas mixture from the atomization means to the substrate and forming a film on the substrate by a thermal reaction, at least a part of the gas mixture is supplied to the surface of the substrate from a smooth portion adjacent to the mounting portion and having a surface roughness of 200 μm or less.

[0007] This film formation method allows a high-quality film to be uniformly and stably produced on the surface of a large-diameter substrate. Specifically, this film formation method allows a mixture containing the raw material mist to be supplied to the substrate while being maintained in a laminar flow, thereby suppressing the generation of powder and abnormal growth, thereby allowing a high-quality film to be stably produced on the surface of a large-diameter substrate.

[0008] In this case, the surface roughness can be set to 150 μm or less.

[0009] This allows for the production of higher quality films more uniformly and stably.

[0010] At this time, the difference in height between the surface of the flat portion and the surface of the substrate placed on the placement portion can be set to be not less than −1 mm and not more than +1 mm.

[0011] This allows the gas mixture to be maintained in a more stable laminar flow, and allows a high-quality film to be produced stably and uniformly.

[0012] In this case, the angle formed between the surface of the smooth portion and the direction in which the air-fuel mixture flows into the smooth portion can be set to be equal to or greater than 0° and equal to or less than 60°.

[0013] This effectively prevents the generation of powder, and allows for the more stable production of high-quality films.

[0014] The present invention has been made to achieve the above-mentioned object, and provides a film formation apparatus comprising: carrier gas supply means for supplying a carrier gas; atomization means for atomizing a raw material solution to generate a raw material mist; a film formation section for supplying a gas mixture of the raw material mist and the carrier gas to a substrate to form a film; and heating means for heating the substrate, wherein the film formation section has a susceptor with a mounting section on which the substrate is placed, and the susceptor is provided with a smooth section adjacent to the mounting section and through which at least a part of the gas mixture flows, and the surface roughness of the smooth section is 200 μm or less.

[0015] This film deposition apparatus allows for the consistent and stable production of high-quality films on the surfaces of large-diameter substrates. Specifically, this film deposition apparatus supplies the gas mixture containing the raw material mist to the substrate while maintaining a laminar flow, thereby suppressing the generation of powder and abnormal growth, thereby enabling the consistent production of high-quality films on the surfaces of large-diameter substrates.

[0016] In this case, the surface roughness can be set to 150 μm or less.

[0017] This allows for the production of higher quality films in a more uniform and stable manner.

[0018] At this time, the difference in height between the surface of the flat portion and the surface of the substrate placed on the placement portion can be set to be not less than −1 mm and not more than +1 mm.

[0019] This allows the gas mixture to be maintained in a more stable laminar flow, making it possible to stably and uniformly produce high-quality films.

[0020] In this case, the angle formed between the surface of the smooth portion and the direction in which the air-fuel mixture flows into the smooth portion can be set to be equal to or greater than 0° and equal to or less than 60°.

[0021] This effectively prevents the generation of powder, allowing for the stable production of higher quality films.

[0022] The present invention has been made to achieve the above-mentioned object, and provides a susceptor having a mounting portion on which a substrate is placed, in a film formation apparatus comprising: carrier gas supply means for supplying a carrier gas; atomization means for atomizing a raw material solution to generate a raw material mist; a film formation unit for supplying a gas mixture of the raw material mist and the carrier gas to a substrate to form a film; and heating means for heating the substrate, wherein the susceptor is characterized in that it is provided with a smooth portion adjacent to the mounting portion and through which at least a portion of the gas mixture flows, and the surface roughness of the smooth portion is 200 μm or less.

[0023] This susceptor allows for the consistent and stable production of high-quality films on the surfaces of large-diameter substrates. Specifically, this susceptor allows the gas mixture containing the raw material mist to be supplied to the substrate while maintaining a laminar flow, thereby suppressing the generation of powder and abnormal growth, thereby enabling the consistent production of high-quality films on the surfaces of large-diameter substrates.

[0024] In this case, the surface roughness can be set to 150 μm or less.

[0025] This allows for the production of higher quality films in a more uniform and stable manner.

[0026] In this case, the surface roughness of the flat portion can be 200 μm or less, and the difference in height between the surface of the flat portion and the surface of the substrate placed on the placement portion can be −1 mm or more and +1 mm or less.

[0027] This allows for the production of higher quality films in a more uniform and stable manner.

[0028] The present invention has been made to achieve the above-mentioned object, and provides an α-gallium oxide film formed on substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more, wherein the α-gallium oxide film has a rocking curve full width at half maximum of 9.5 seconds or less for the (0006) plane X-ray diffraction peak and a film thickness distribution of ±9% or less.

[0029] This results in a high-quality α-gallium oxide film with good crystal orientation and good film thickness distribution.

[0030] The present invention has been made to achieve the above object, and is directed to a method for forming a foreign particle on a c-plane sapphire substrate having a diameter of 4 inches or more over substantially the entire surface thereof, the method comprising: forming a foreign particle on a c-plane sapphire substrate having a diameter of 4 inches or more over a substantially entire surface thereof; －２ The present invention provides an α-gallium oxide film characterized by a thickness of 1000 nm or less and a film thickness distribution of ±9% or less.

[0031] This results in a high-quality α-gallium oxide film with a low foreign matter density and good film thickness distribution.

[0032] As described above, the film formation method of the present invention makes it possible to stably produce high-quality films, enabling highly productive film formation. The film formation apparatus and susceptor of the present invention make it possible to stably produce high-quality films, enabling highly productive film formation. As described above, the α-gallium oxide film of the present invention is of higher quality than conventional films.

[0033] FIG. 1 is a diagram showing one embodiment of a film formation apparatus according to the present invention; FIG. 2 is a diagram showing one embodiment of a susceptor of a film formation apparatus according to the present invention; FIG. 3 is a diagram showing another embodiment of a susceptor of a film formation apparatus according to the present invention (a) and a diagram showing yet another embodiment; FIG. 4 is a diagram showing yet another embodiment of a susceptor of a film formation apparatus according to the present invention; FIG. 5 is a diagram showing yet another embodiment of a susceptor of a film formation apparatus according to the present invention; FIG. 6 is a diagram showing one embodiment of a flow of a gas mixture toward a substrate placed on a susceptor of a film formation apparatus according to the present invention; FIG. 7 shows examples of various irregularities accompanied by abnormal growth on a film surface, which can be seen when cloudiness is observed microscopically; FIG. 8 shows other examples of various irregularities accompanied by abnormal growth on a film surface, which can be seen when cloudiness is observed microscopically.

[0034] The present invention will be described in detail below, but the present invention is not limited thereto.

[0035] As described above, there has been a demand for a film formation method, a film formation apparatus, and a susceptor that can uniformly and stably produce a high-quality film on the surface of a large-diameter substrate.

[0036] As a result of extensive research into the above-mentioned problems, the inventors have found that a film formation method including the steps of atomizing a raw material solution to form a raw material mist, mixing the raw material mist with a carrier gas to form a gas mixture, placing a substrate on a mounting portion of a susceptor, supplying the gas mixture from atomization means to the substrate and forming a film on the substrate by a thermal reaction, and exhausting the gas mixture after film formation by exhaust means, wherein in the step of supplying the gas mixture from the atomization means to the substrate and forming a film on the substrate by a thermal reaction, at least a part of the gas mixture is supplied to the surface of the substrate from a smooth portion adjacent to the mounting portion and having a surface roughness of 200 μm or less, makes it possible to stably produce high-quality films and to form films with high productivity; and a film formation apparatus comprising: carrier gas supply means for supplying a carrier gas; atomization means for atomizing a raw material solution to generate a raw material mist; a film formation section for supplying a mixture of the raw material mist and the carrier gas to a substrate to form a film; and heating means for heating the substrate, wherein the film formation section has a susceptor with a mounting section for mounting the substrate, the susceptor is provided with a smooth section adjacent to the mounting section and through which at least a part of the mixture flows, and the surface roughness of the smooth section is 200 μm or less, thereby enabling the stable production of high-quality films and enabling highly productive film formation; The inventors have found that a film formation apparatus comprising carrier gas supply means for supplying a carrier gas, atomization means for atomizing a raw material solution to generate a raw material mist, a film formation section for supplying a mixture of the raw material mist and the carrier gas to a substrate to form a film, and heating means for heating the substrate, has a susceptor having a mounting section for mounting the substrate, the susceptor being characterized in that it is provided with a smooth section adjacent to the mounting section through which at least a portion of the mixture flows, and the smooth section has a surface roughness of 200 μm or less, making it possible to stably produce high-quality films and to form films with high productivity, and have completed the present invention.

[0037] As mentioned above, there has been a demand for α-gallium oxide films of higher quality than ever before.

[0038] As a result of extensive research into the above-mentioned problems, the present inventors have discovered an α-gallium oxide film formed on substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more, characterized in that the half-width of the rocking curve of the (0006) plane X-ray diffraction peak of the α-gallium oxide film is 9.5 seconds or less and the film thickness distribution is ±9% or less; and an α-gallium oxide film formed on substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more and having a foreign matter density of 1.1 cm －２ The present inventors have found that an α-gallium oxide film having a thickness distribution of ±9% or less can provide an α-gallium oxide film of higher quality than conventional films, and have completed the present invention.

[0039] The present invention will be described in detail below with reference to the drawings, but the present invention is not limited thereto.

[0040] First Embodiment [Film Forming Apparatus] First, one embodiment of a film forming apparatus according to the present invention will be described. Fig. 1 shows one embodiment of a film forming apparatus according to the present invention. However, the film forming apparatus according to the present invention is not limited to the film forming apparatus shown in Fig. 1.

[0041] 1, a film forming apparatus 1 according to the present invention includes a gas-air mixture forming mechanism 50, a film forming unit 30, and an exhaust means 40. The gas-air mixture forming mechanism 50 includes a carrier gas supply means 10 and an atomization means 20.

[0042] The carrier gas supply means 10 and the atomization means 20 are connected by a pipe 101, and a carrier gas 111 is supplied from the carrier gas supply means 10 to the atomization means 20, and a gas mixture is formed in the atomization means 20. The atomization means 20 and the film formation chamber 31 of the film formation unit 30 are connected by a pipe 102, and a gas mixture 112 is supplied from the atomization means 20 to the film formation chamber 31 of the film formation unit 30. The film formation chamber 31 of the film formation unit 30 and the exhaust means 40 are connected by a pipe 103, and gas is exhausted from the film formation chamber 31 of the film formation unit 30 to the exhaust means 40.

[0043] The pipe 102 is not particularly limited as long as it is sufficiently stable against the temperature at the interface between the raw material solution 21 used and the film forming unit 30 and the pipe 102, and may be made of resin, metal, glass, ceramic, or a combination of these materials depending on the purpose. The pipes 101 and 103 are also not particularly limited as long as they are sufficiently stable.

[0044] (Carrier gas supply means 10) The carrier gas supply means 10 supplies a carrier gas 111 toward the atomization means 20. The carrier gas 111 is not particularly limited, and suitable examples include air, oxygen, ozone, inert gases such as nitrogen and argon, and reducing gases such as hydrogen gas and forming gas. The type of carrier gas 111 may be one type or two or more types. The flow rate of the carrier gas 111 may be set appropriately depending on the size of the substrate 33 and the size of the film formation unit 30, and may be, for example, about 0.01 to 100 L / min.

[0045] Although not shown, it is also possible to adjust the ratio of the raw material mist 22 to the carrier gas 111 by further adding a dilution gas. The flow rate of the dilution gas may be set appropriately, for example, to 0.1 to 10 times that of the carrier gas. The dilution gas may be supplied, for example, downstream of the atomization means 20. The dilution gas may be the same as or different from the carrier gas 111.

[0046] (Atomization means 20) A raw material solution 21 is stored in the atomization means 20 as a raw material. The raw material solution 21 is not particularly limited as long as it is a solution that can be atomized (also called "misted"), and aqueous solutions containing raw materials according to the purpose, or organic solvent solutions such as alcohols, ketones, esters, ethers, amides, halogenated solvents, and aromatic compounds can be used. The raw material solution 21 is atomized using known means to form raw material mist 22.

[0047] A carrier gas 111 is supplied to the atomizing means 20 and mixed with the raw material mist 22 to form a gas-air mixture 112 .

[0048] The atomization of the raw material solution 21 is not particularly limited as long as it can atomize or turn the raw material solution 21 into droplets, and any known means may be used, but in the present invention, ultrasonic waves are preferably used. The mist or droplets obtained using ultrasonic waves are preferable because they have an initial velocity of zero and float in the air. For example, rather than being sprayed like a spray, they are mist that floats in space and can be transported as a gas, which is highly suitable because they are not damaged by collision energy. The droplet size is not particularly limited, and may be droplets of about several millimeters, but is preferably 50 μm or less, and more preferably 0.1 to 10 μm.

[0049] The atomization means 20 may be equipped with a temperature control unit (not shown). The temperature control unit may directly or indirectly control the temperature of the raw material solution 21, and may be a unit that performs heat exchange using a liquid or gaseous heat transfer medium, or may utilize the Peltier effect. A wide variety of known heat transfer media can be used as the heat transfer medium, and suitable heat transfer media include liquids such as water, glycols, alcohols, and silicone oils, and gases such as air, helium, and fluorocarbons.

[0050] 1 , the film formation unit 30 includes a film formation chamber 31 and a heating means 34. A susceptor 32 having a triangular shape in a side view is disposed within the film formation chamber 31. A substrate 33 is placed on the oblique surface of the susceptor 32. A pipe 102 is connected to one wall of the film formation chamber 31, and a pipe 103 is connected to the other wall, and a gas mixture 112 is transported via the pipe 102 and supplied to the film formation chamber 31.

[0051] The film formation chamber 31 is not particularly limited as long as it is sufficiently stable against the raw material solution 21 used and the temperature at which it is used, and may be made of resin, metal, glass, ceramic, or a combination of these materials depending on the purpose. Film formation may be carried out under any of atmospheric pressure, pressurized, and reduced pressure conditions, but is preferably carried out under atmospheric pressure in terms of equipment cost and productivity.

[0052] Fig. 2 shows one embodiment of a susceptor for a film forming apparatus according to the present invention. Fig. 2 shows one embodiment of the susceptor in which the gas-air mixture 112 always flows in one direction (the y direction) on the susceptor 32. As shown in Fig. 2, the susceptor 32 has a circular mounting portion 32b at the center of its surface. A substrate 33 can be placed on the mounting portion 32b.

[0053] (Susceptor Structure and Material) The structure, material, etc. of the susceptor 32 are not particularly limited as long as they have sufficient stability depending on the characteristics and temperature conditions of the raw material solution 21 used. The structure of the susceptor 32 may include a mechanism for holding the substrate 33, although this is not shown in the figure. In this case, known substrate holding methods such as a vacuum chuck, a mechanical clamp, or an electrostatic chuck can be applied. The material of the susceptor 32 may be a metal such as aluminum or stainless steel. However, when film formation is performed at a higher temperature than the heat resistance temperature of these metals or when an acidic or alkaline raw material is used, an alloy such as Hastelloy (registered trademark), soda-lime glass, borosilicate glass, quartz, silicon carbide, or ceramics such as silicon nitride or aluminum nitride may also be used.

[0054] The susceptor 32 has a smooth portion 32a in a lower portion adjacent to the mounting portion 32b. The smooth portion 32a is installed adjacent to the mounting portion 32b on the upstream side of the mounting portion 32b, i.e., on the side where the gas-air mixture 112 flows into the susceptor 32. The smooth portion 32a has a smooth surface.

[0055] At least a part of the gas mixture 112 flows through the smooth portion 32a. That is, part of the gas mixture 112 supplied to the film formation chamber 31 flows directly in a straight line toward the surface of the substrate 33, and part of the gas mixture 112 is supplied along the smooth portion 32a to the surface of the substrate 33. In this process, the gas mixture 112 reacts on the substrate 33, and a film is formed on the substrate 33.

[0056] The smooth portion 32 a is preferably provided so as to include at least regions that contact both ends in the x direction of the mounting portion 32 b (or the substrate (not shown)). This allows the gas mixture 112 to be supplied to the substrate surface while always maintaining a laminar flow, thereby enabling stable and good crystal growth on the substrate surface.

[0057] The surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion 32a is set to 200 μm or less. This allows the gas mixture 112 supplied to the surface of the substrate 33 along the smooth portion 32a to flow with less turbulence, enabling the stable formation of a uniform, high-quality film. If the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion 32a exceeds 200 μm, the gas mixture becomes turbulent, resulting in significant particle adhesion and abnormal growth, causing the surface to become cloudy. Microscopically, cloudiness is observed as various irregularities accompanying abnormal growth on the film surface, as shown in FIGS. 7 and 8, for example. Such abnormal growth is often caused by abnormal nucleation due to foreign matter adhesion or uneven film composition. Macroscopically, the degree of cloudiness can usually be determined visually, but it can also be quantified by measuring the reflectance or transmittance of visible light or measuring the surface unevenness. These measurements can be performed using a wide range of known methods, with typical spectrophotometers, stylus surface profilers, and atomic force microscopes being suitable. In particular, when the thin film is transparent in the visible light range, the specular reflectance can be determined by measuring the specular reflectance of visible light incident on the film surface at a certain angle. Specifically, in the case of gallium oxide, for example, the specular reflectance can be 15% or less under conditions of an incident light wavelength of 633 nm and an incident angle of 45°. The specular reflectance is the ratio of the energy of incident light to the energy of reflected light when light from a light source is incident on the sample (film) surface and reflected, and the light is received by a light-receiving probe at a reflection angle equal to the incident angle. In this specification, the term "particles" refers to particles that are incorporated into the film and integrated with it, as well as particles that adhere to the film surface as foreign matter. These particles are observed as particles when the film surface is observed, and can be measured, for example, using light scattering. The surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion 32a is preferably 150 μm or less. The lower limit is not particularly limited, but can be set to, for example, 0.1 nm.

[0058] The surface roughness (arithmetic mean roughness) Ra refers to a value calculated in accordance with JIS B 0601 using the results of surface shape measurement by a stylus method, an atomic force microscope (AFM) method, or a non-contact measurement method using a laser microscope or a confocal microscope, such as an optical interference method, a confocal method, or a focus shift image synthesis method.

[0059] The smooth portion 32a may be made of the same material as the susceptor, or a different material. Specifically, alloys such as aluminum, stainless steel, or Hastelloy®, or ceramics such as soda-lime glass, borosilicate glass, quartz, silicon carbide, or silicon nitride or aluminum nitride, may be used. The smoothness of the smooth portion 32a can be easily achieved, for example, in the case of borosilicate glass, by lapping the surface with diamond abrasive grains and then mirror-finishing it by chemical mechanical polishing (CMP) using colloidal silica. The smooth portion 32a may also be maintained at a lower temperature than the substrate mounting portion 32b. This prevents the raw material mist from evaporating on the surface of the smooth portion 32a, thereby suppressing the generation of particles derived from the evaporated mist and enabling the smoothness of the smooth portion 32a to be maintained for a longer period of time. The temperature of the smooth portion 32a can be optimized depending on the type of raw material mist and various film formation conditions. However, when water is used as the solvent for the raw material mist, a temperature of approximately 30°C to 100°C is generally recommended. The method of cooling the smooth portion 32a is not particularly limited, and known temperature control means can be applied. For example, a refrigerant (not shown) may be brought into contact with the inside or back surface of the smooth portion 32a to exchange heat, or a Peltier element may be used for cooling.

[0060] (Substrate) The substrate 33 is capable of supporting the film to be formed. The material of the substrate 33 is not particularly limited and may be a known material, an organic compound, or an inorganic element or compound. Examples of the material include, but are not limited to, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyimide, polyetherimide, fluororesin, metals such as iron, aluminum, stainless steel, and gold, silicon, sapphire, quartz, glass, calcium carbonate, lithium tantalate, lithium niobate, gallium oxide, SiC, ZnO, and GaN. The size of the substrate 33 is 5 cm2. ２ More than 10 cm is preferable. ２ The above is more preferable, and a thickness of 50 to 2000 μm is more preferable, and 100 to 800 μm is even more preferable. As can be inferred from the shape of the mounting portion shown in FIG. 2, the shape of the substrate 33 is circular in plan view. However, this is not limited to this, and the substrate 33 may be rectangular or polygonal in plan view. Note that if the shape and size of the substrate 33 are changed, the shape and size of the mounting portion 32b are changed to match the shape and size of the substrate 33.

[0061] (Substrate Smoothness) The surface roughness (arithmetic mean roughness) Ra_sub of the surface of the substrate 33 is preferably equal to or less than the surface roughness (arithmetic mean roughness) Ra_cor of the surface of the smooth portion 32a. This allows the air-fuel mixture to maintain a laminar flow even on the substrate surface. If the surface roughness (arithmetic mean roughness) Ra_sub of the surface of the substrate 33 is greater than the surface roughness (arithmetic mean roughness) Ra_cor of the surface of the smooth portion 32a, abnormal growth on the substrate 33 is more likely to occur. The surface roughness (arithmetic mean roughness) Ra_sub of the surface of the substrate 33 is preferably, for example, 0.05 nm or more and 1 μm or less. Such surface smoothness of the substrate 33 can be easily achieved, for example, in the case of sapphire, by lapping the surface of the substrate 33 obtained by processing the crystal with diamond abrasive grains, and then further mirror-finishing it by chemical mechanical polishing (CMP) using colloidal silica.

[0062] Next, one form of the flow of the gas mixture toward the substrate placed on the susceptor of the film forming apparatus according to the present invention will be described with reference to FIG. 6 from the viewpoint of the inflow direction of the gas mixture and the height difference between the substrate and the susceptor.

[0063] (Direction of Inflow of Gas Mixture) In the film forming apparatus according to the present invention, the substrate 33 is placed on the substrate placement portion 32b of the susceptor 32, and a portion of the gas mixture 112 flows toward the substrate 33 along the smooth portion 32a. The angle θ between the inflow direction of the gas mixture 112 and the smooth portion 32a is not particularly limited. However, it is preferably set to 0° or more and 60° or less. If the angle exceeds 60°, the gas mixture 112 may collide with the upper surface of the susceptor 32 or the substrate 33, causing turbulence and resulting in the generation of powder and adhesion to the film surface. However, if the angle is set to 0° or more and 60° or less, such occurrences can be suppressed. An angle θ of 0° or more and 45° or less is more preferable. The angle θ in the embodiment of FIG. 1 can also be considered to be the inclination angle of the smooth portion 32a with respect to the horizontal plane.

[0064] (Elevation Difference) The elevation difference h between the surface of the substrate 33 and the surface of the smooth portion 32a is not particularly limited, but is preferably between -1 mm and 1 mm. Within this range, thickness uniformity at the substrate edge is improved and turbulence does not occur at the edge. A step of 1 mm or more may result in film thickness distribution, particularly near the edge of the substrate adjacent to the smooth portion 32a. The positive and negative values ​​here indicate the relative position of the substrate surface to the smooth portion surface. A negative value indicates that the substrate surface is recessed relative to the smooth portion surface, while a positive value indicates that the substrate surface is protruding relative to the smooth portion surface. If the substrate surface is higher than the smooth portion surface, the gas mixture will concentrate at the edge of the substrate, resulting in a thicker film thickness. Conversely, if the substrate surface is lower than the smooth portion surface, the gas mixture will skip over the edge of the substrate due to the step, resulting in a thinner film thickness. Furthermore, such a structure may cause turbulence in the flow of the gas mixture. The thickness of the film formed on the substrate can be measured by any known method, and for example, ellipsometry, optical interference spectroscopy, stylus method, or laser displacement method is preferably used.

[0065] (Exhaust means 40) The surplus gas-air mixture 112 that did not contribute to film formation and by-products generated during the reaction of the gas-air mixture 112 on the substrate 33 are sucked as a gas-air mixture after film formation and discharged to the outside of the system by exhaust means 40. The exhaust from exhaust means 40 may be treated, as necessary, by a detoxification device such as a particle collector, a combustion device, or a scrubber (not shown), or may be treated by a filter for recovering raw materials, or may further be treated by a combination of these.

[0066] 1 shows a configuration in which one atomization means 20 is used, but the film formation apparatus according to the present invention is not limited to this, and multiple atomization means 20 can be connected in series or in parallel. In this case, each atomization means 20 may contain a different raw material solution, or the same raw material solution. Furthermore, each raw material solution may be atomized individually and supplied to the film formation unit 30 independently, or multiple types of raw material mists may be mixed in advance and then supplied to the film formation unit 30.

[0067] 1 and 2 show a configuration in which the gas mixture 112 flows in one direction from the supply side to the exhaust side of the film formation chamber 31 relative to the substrate 33, but the present invention is not limited to this, and the configuration may be changed depending on the method of supplying the gas mixture 112 to the substrate.

[0068] [Film Forming Method] Next, a film forming method according to the present invention will be described with reference to Figures 1 and 2. The film forming method according to the present invention includes a raw material mist forming step, a gas mixture forming step, a substrate mounting step, a film forming step, and an exhaust step.

[0069] (Raw Material Mist Forming Step) The raw material mist forming step is a step of atomizing the raw material solution 21 to form the raw material mist 22 .

[0070] (Air-fuel mixture formation step) The air-fuel mixture formation step is a step of mixing the raw material mist 22 with the carrier gas 111 to form the air-fuel mixture 112 .

[0071] (Substrate Mounting Step) The substrate mounting step is a step of mounting the substrate 33 on the mounting portion 32 b of the susceptor 32 .

[0072] (Film Forming Step) The film forming step is a step in which the gas-air mixture 112 is supplied from the atomizing means 20 to the substrate 33, and a film is formed by a thermal reaction on the substrate 33. In this step, at least a part of the gas-air mixture 112 is supplied to the surface of the substrate 33 from the smooth portion 32a, which is adjacent to the mounting portion 32b and has a surface roughness (arithmetic mean roughness) Ra_cor of 200 μm or less.

[0073] (Exhaust Step) The exhaust step is a step in which the gas-air mixture 112 after film formation is exhausted by the exhaust means 40 .

[0074] With this film formation method, the gas mixture 112 containing the raw material mist 22 is supplied to the substrate 33 while being maintained in a laminar flow, thereby suppressing the generation of powder and abnormal growth, and making it possible to stably produce high-quality films on the surface of large-diameter substrates 33.

[0075] It is preferable that the surface roughness of the smooth portion 32a is 150 μm or less, which allows a higher quality film to be produced more stably.

[0076] It is preferable that the height difference h between the surface of the smooth portion 32a and the surface of the substrate 33 is set to be not less than −1 mm and not more than 1 mm. With such a film formation method, the gas mixture 112 can be maintained in a more stable laminar flow, and therefore a high-quality film can be produced stably and uniformly.

[0077] The angle θ formed by the surface of the smooth portion 32a and the flow direction of the gas-air mixture 112 into the smooth portion 32a is preferably set to be equal to or greater than 0° and equal to or less than 60°. With such a film formation method, the gas-air mixture 112 can be maintained in a more stable laminar flow, and therefore a high-quality film can be more stably produced.

[0078] Second Embodiment The second embodiment has the same configuration as the first embodiment, except that the susceptor 32 shown in Fig. 3(a) is used and the susceptor 32 is rotated intermittently by 180 degrees to form a film. In this case, as shown in Fig. 3(a), the gas mixture 112 is supplied to the mounting portion 32b from above and below the plane of the drawing, and accordingly, the smooth portions 32a are formed in the inflow direction of the gas mixture 112. This configuration also allows the gas mixture 112 to be supplied to the surface of the substrate in a laminar flow.

[0079] Third Embodiment The third embodiment has the same configuration as the first embodiment, except that a susceptor 32 shown in FIG. 3B was used, a nozzle (not shown) for discharging the gas mixture 112 was placed facing the susceptor 32, and the relative position of the nozzle and the susceptor 32 was changed in one axial direction to form a film. In this case, as shown in FIG. 3B, the gas mixture 112 was supplied to the mounting portion 32b from the top and bottom of the drawing. Accordingly, the smooth portion 32a was formed in the respective gas mixture inflow directions. However, the smooth portion 32a only needs to include at least the area surrounded by the nozzle outlet width W and movable length L of the nozzle, excluding the mounting portion 32b. In this case, the outlet width W need only be equal to or greater than the width of a substrate parallel to W. The outlet width W can be, for example, 1.0 to 2 times the width of the substrate. However, in order to more stably suppress deterioration of the film thickness distribution due to discontinuities in mist concentration that tend to occur at the nozzle end, it is more preferable to set the outlet width W to 1.1 to 1.5 times. Furthermore, although the movable length L depends on the shape and configuration of the nozzle used, it is generally sufficient if it is equal to or greater than the width of the substrate parallel to L, and can be set to, for example, 1.0 to 1.5 times the width of the substrate, but from the viewpoint of productivity, it is more preferable that it be 1.2 times or less. With this configuration, the gas mixture 112 can be supplied to the surface of the substrate in a laminar flow.

[0080] (Fourth Embodiment) The fourth embodiment has the same configuration as the first embodiment, except that a susceptor 32 having a smooth portion 32a surrounding a mounting portion 32b as shown in Fig. 4 is used, and film formation is performed by continuously rotating the susceptor 32 relative to the unidirectional gas mixture 112. As shown in Fig. 4, film formation is performed by continuously rotating the susceptor 32 relative to the unidirectional gas mixture 112. This configuration also makes it possible to supply the gas mixture 112 to the surface of the substrate in a laminar flow.

[0081] Fifth Embodiment The fifth embodiment has the same configuration as the first embodiment, except that a susceptor 32 having a smooth portion 32a surrounding a mounting portion 32b as shown in Fig. 5 is used, and a film is formed by supplying a gas-air mixture 112 in a swirling flow to the fixed susceptor 32. As shown in Fig. 5, a film is formed by supplying a gas-air mixture 112 in a swirling flow to the fixed susceptor 32. This configuration also makes it possible to supply the gas-air mixture 112 to the surface of the substrate in a laminar flow.

[0082] The susceptors 32 of the first to fifth embodiments are exemplified as having a circular mounting portion 32b corresponding to a circular substrate 33, but the susceptor of the film forming apparatus according to the present invention is not limited to this, and may have a mounting portion corresponding to a substrate of the shape described above.

[0083] In the first to fifth embodiments, the substrate 33 is installed so that the film formation surface faces upward, but this is not limited to this, and the substrate 33 can also be installed so that the film formation surface faces downward.

[0084] (Semiconductor Device) The film formation method according to the present invention can be applied to any film type, including metal films, semiconductor films, and insulator films, but is particularly suitable for use in the production of semiconductor films. Furthermore, semiconductor films and laminates formed by the film formation method according to the present invention have reduced defects such as particles and abnormal growth, and have excellent electrical properties, making them industrially useful.

[0085] Such a film and laminate can be suitably used in a semiconductor device, etc. Furthermore, the crystal layer formed as part of the laminate may be used as it is (in the state of the laminate), or may be peeled off from the crystal substrate, etc. by a known method, and then applied to a semiconductor device, etc.

[0086] Furthermore, semiconductor devices can be classified into horizontal elements (horizontal devices) in which an electrode is formed on one side of a semiconductor layer, and vertical elements (vertical devices) in which an electrode is formed on both the front and back sides of a semiconductor layer, and at least a part of the laminate according to the present invention can be suitably used in both horizontal and vertical devices, and is particularly preferably used in vertical devices.

[0087] Examples of the semiconductor device include a Schottky barrier diode (SBD), a metal semiconductor field effect transistor (MESFET), a high electron mobility transistor (HEMT), a metal oxide semiconductor field effect transistor (MOSFET), a junction field effect transistor (JFET), an insulated gate bipolar transistor (IGBT), and a light emitting diode (LED).

[0088] (α-Gallium Oxide Film) An α-gallium oxide film can be exemplified as a film formed by the film formation method according to the present invention. A specific example of an α-gallium oxide film is one formed on substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more, in which the half-width of the rocking curve of the (0006) plane X-ray diffraction peak of the α-gallium oxide film is 9.5 seconds or less and the film thickness distribution is ±9% or less. Note that "substantially the entire surface" as used herein refers to a region of 80% or more of the total area of ​​the substrate surface.

[0089] This results in a high-quality α-gallium oxide film with good crystal orientation and good film thickness distribution. The smaller the half-width of the rocking curve, the better, so the lower limit is, for example, more than 0 seconds. The film thickness distribution can be calculated by using a known method such as optical interference spectroscopy to calculate the film thickness at multiple points on the film surface, and then dividing the difference between the maximum and minimum values ​​by twice the average value. The upper limit of the diameter of the c-plane sapphire substrate is not particularly limited, but is, for example, about 12 inches (approximately 30 cm).

[0090] Alternatively, the particles are formed on almost the entire surface of a c-plane sapphire substrate with a diameter of 4 inches or more, and the foreign matter density is 1.1 cm －２ or less and the film thickness distribution is ±9% or less.

[0091] This results in a high-quality α-gallium oxide film with a low foreign matter density formed on almost the entire surface of the large-diameter substrate. The foreign matter here refers to a convex object present on the film surface, and particularly includes both particles and abnormally grown grains, and can be detected by a known wafer defect inspection device. Note that the lower the foreign matter density, the better, so the lower limit is, for example, 0 cm －２ The upper limit of the diameter of the c-plane sapphire substrate is not particularly limited, but is, for example, about 12 inches.

[0092] EXAMPLES The present invention will be specifically explained below using examples and comparative examples, but the present invention is not limited to these.

[0093] Example 1 An α-gallium oxide film was formed in the film formation apparatus shown in Figure 1 according to the following procedure. A gas cylinder filled with nitrogen gas was used to supply carrier gas to the film formation apparatus. The gas cylinder and atomization device were connected by a urethane resin tube, and the atomization device and film formation chamber were further connected by quartz piping.

[0094] First, a c-plane single crystal sapphire substrate having a thickness of 0.65 mm and a diameter of 4 inches (approximately 10 cm) was placed on a SiC susceptor having the surface structure shown in FIG. 2 and an inclination angle (θ) of 35°. The height difference h between the smooth surface and the substrate surface was 0 mm. The surface roughness (arithmetic mean roughness) Ra_cor of the smooth surface provided on the susceptor was measured at five points on the smooth surface using an AFM (Nanosurf Nanite AFM), yielding an average value of 0.55 μm. The surface roughness (arithmetic mean roughness) Ra_sub of the sapphire substrate was also measured at five points on a crosswise line passing through the center of the substrate using an AFM, yielding an average value of 0.08 nm.

[0095] Next, as a raw material solution, gallium acetylacetonate was dissolved at a ratio of 0.05 mol / L in a dilute hydrochloric acid solution to which 34% concentration hydrochloric acid was added at a volume ratio of 1%, and this was filled into an atomization device (atomization means). Next, ultrasonic vibration was propagated to the raw material solution in the atomization device through water using an ultrasonic vibration plate, and the raw material solution was atomized (misted). Next, nitrogen gas was added to the container of the raw material solution in the atomization device (atomization means) at a flow rate of 20 L / min, and a mixture of mist and nitrogen gas was formed.

[0096] Next, the substrate placed on the mounting portion of the susceptor was heated to a substrate temperature of 450° C. The gas mixture was then supplied to the film formation chamber, and film formation was carried out for 60 minutes. After this, the supply of nitrogen gas was stopped, and the supply of the gas mixture to the film formation chamber was also stopped.

[0097] The film thus produced was clear in appearance except for a 5 mm wide area around the film surface. X-ray diffraction measurement (Rigaku SmartLab) revealed that the film was composed of α-phase Ga. ２ O ３ It was confirmed that the film was an α-gallium oxide film.２ O ３ The rocking curve measurement was performed on the (0006) plane, and the half-width was evaluated. The rocking curve measurement was performed using a four-crystal monochromator combining two channel-cut crystals to increase the monochromaticity of the X-rays and achieve higher accuracy. Other measurement conditions were as follows. [Measurement conditions] Measurement device: Rigaku SmartLab Analysis conditions: Measurement method: Rocking curve measurement (ω scan) X-ray generator: Cu anticathode: Output 45 kV 200 mA Detector: Semiconductor detector Incident optical system: Ge (220) channel-cut monochromator Soller slit: Incident side -: Receiving side 5.0° Slit: Incident side IS = 1 (mm): Length limit 0.5 (mm): Receiving side RS1 = 1 RS2 = 1.1 (mm) Scanning conditions: Evaluation diffraction surface Ga ２ O ３ (006): Scan axis ω: Scanning mode: Step measurement: Scanning range: 18-22°: Step width: 0.005°: Accumulation time: 0.5 sec. / step. The density of foreign matter (diameter 0.3 μm or more) in the area excluding the 5 mm peripheral width of the film formation surface was evaluated using a substrate inspection machine (KLA Candela-CS10). Note that foreign matter here refers to convex objects present on the film surface, and specifically includes both particles and abnormally grown grains. The thickness of the produced film was then measured at 21 points on the film surface using optical interferometry (Filmetrics F50), and the difference between the maximum and minimum values ​​was divided by twice the average value to calculate the film thickness distribution.

[0098] (Example 2) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion in Example 1 was set to 183 μm. The film produced was clear in appearance except for a 5 mm outer periphery of the film-forming surface, and X-ray diffraction measurement revealed that α-phase Ga ２ O３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0099] (Comparative Example 1) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion was set to 220 μm. The appearance of the produced film showed some cloudiness from the outer periphery of the substrate adjacent to the smooth portion to the interior. X-ray diffraction measurement revealed the presence of α-phase Ga ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0100] (Example 3) Film formation was carried out in the same manner as in Example 1, except that the height difference h was set to 0.85 mm (the substrate surface was higher than the surface of the flat portion). The film formed was clear in appearance except for a 5 mm periphery of the film formation surface, and X-ray diffraction measurement revealed that the film was composed of α-phase Ga. ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0101] (Example 4) Film formation was carried out in the same manner as in Example 1, except that the height difference h was set to -0.85 mm (the substrate surface was lower than the surface of the flat portion). The film produced was clear in appearance except for a 5 mm periphery of the film formation surface, and X-ray diffraction measurement revealed that the film was composed of α-phase Ga. ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0102] (Comparative Example 2) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion was set to 220 μm and the height difference h was set to 1.2 mm. The appearance of the produced film showed widespread cloudiness from the outer periphery of the substrate adjacent to the smooth portion to the interior. X-ray diffraction measurement revealed α-phase Ga ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0103] (Comparative Example 3) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion was set to 220 μm and the height difference h was set to −1.2 mm. The appearance of the film formed showed widespread cloudiness from the outer periphery of the substrate adjacent to the smooth portion to the interior. X-ray diffraction measurement revealed α-phase Ga ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0104] (Example 5) Film formation was carried out in the same manner as in Example 1, except that the inclination angle (θ) was set to 55°. The film produced was clear in appearance except for a 5 mm outer periphery of the film-forming surface, and X-ray diffraction measurement revealed that the film was composed of α-phase Ga. ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0105] (Comparative Example 4) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion was set to 220 μm and the inclination angle (θ) was set to 80°. The appearance of the film formed showed widespread cloudiness from the outer periphery of the substrate adjacent to the smooth portion to the interior. X-ray diffraction measurement revealed α-phase Ga ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0106] (Example 6) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion in Example 1 was set to 200 μm. The film produced was clear in appearance except for a 5 mm outer periphery of the film-forming surface, and X-ray diffraction measurement revealed that the film was composed of α-phase Ga. ２ O ３ Thereafter, the film was evaluated in the same manner as in Example 1.

[0107] (Example 7) Film formation was carried out in the same manner as in Example 1, except that the surface roughness (arithmetic mean roughness) Ra_cor of the smooth portion in Example 1 was set to 150 μm. The film produced was clear in appearance except for a 5 mm outer periphery of the film-forming surface, and X-ray diffraction measurement revealed that the film was composed of α-phase Ga. ２ O ３It was confirmed that the film was then evaluated in the same manner as in Example 1. Table 1 shows the evaluation results of Examples 1, 2, 3, 4, 5, 6, and 7 and Comparative Examples 1, 2, 3, and 4.

[0108]

[0109] The film produced by the film-forming method of the present invention has a low full width at half maximum (good crystal orientation), stable film growth with good film thickness distribution, and low foreign matter density. In contrast, the film formed by the conventional method had poor crystal orientation accompanied by cloudiness due to abnormal growth, and the film thickness distribution and foreign matter density clearly increased. It is believed that the increased surface roughness of the smooth part caused turbulence in the mixed gas flow, which promoted abnormal growth due to abnormal nucleation and foreign matter adhesion due to precipitation of raw material solids.

[0110] From the above results, it was found that the present invention can provide a film forming apparatus capable of stably producing films of higher quality than conventional techniques.

[0111] This specification includes the following aspects. [1]: A film formation method including the steps of atomizing a raw material solution to form a raw material mist, mixing the raw material mist with a carrier gas to form a gas mixture, placing a substrate on a mounting portion of a susceptor, supplying the gas mixture from a misting means to the substrate and forming a film on the substrate by a thermal reaction, and exhausting the gas mixture after film formation by an exhaust means, wherein in the step of supplying the gas mixture from the misting means to the substrate and forming a film on the substrate by a thermal reaction, at least a portion of the gas mixture is supplied to the surface of the substrate from a smooth portion adjacent to the mounting portion and having a surface roughness of 200 μm or less. [2]: The film formation method according to [1] above, wherein the surface roughness is 150 μm or less. [3]: The film formation method according to [1] above, wherein the difference in elevation between the surface of the smooth portion and the surface of the substrate mounted on the mounting portion is -1 mm or more and +1 mm or less. [4]: The film formation method of any one of [1] to [3] above, wherein the angle between the surface of the smooth portion and the direction in which the gas mixture flows into the smooth portion is between 0° and 60°. [5]: A film formation apparatus comprising: a carrier gas supply means for supplying a carrier gas; an atomization means for atomizing a raw material solution to generate a raw material mist; a film formation unit for supplying the gas mixture, which is a mixture of the raw material mist and the carrier gas, to a substrate to form a film; and a heating means for heating the substrate, wherein the film formation unit has a susceptor with a mounting portion on which the substrate is mounted, the susceptor has a smooth portion adjacent to the mounting portion and through which at least a portion of the gas mixture flows, the smooth portion having a surface roughness of 200 μm or less. [6]: The film formation apparatus of [5] above, wherein the surface roughness is 150 μm or less. [7]: The film formation device of [5] or [6] above, characterized in that the difference in height between the surface of the flat and smooth portion and the surface of the substrate placed on the placement portion is -1 mm or more and +1 mm or less. [8]: The film formation device of any of [5] to [7] above, characterized in that the angle formed between the surface of the flat and smooth portion and the flow direction of the gas mixture into the flat and smooth portion is 0° or more and 60° or less.[9]: A susceptor having a mounting portion for mounting a substrate in a film formation apparatus including a carrier gas supply means for supplying a carrier gas, an atomization means for atomizing a raw material solution to generate a raw material mist, a film formation unit for supplying a mixture of the raw material mist and the carrier gas to a substrate to form a film, and a heating means for heating the substrate, the susceptor comprising: a smooth portion adjacent to the mounting portion through which at least a portion of the mixture flows, the smooth portion having a surface roughness of 200 μm or less;

[10] : The susceptor according to [9] above, wherein the surface roughness is 150 μm or less;

[11] : The susceptor according to [9] above, wherein the surface roughness of the smooth portion is 200 μm or less, and the difference in elevation between the surface of the smooth portion and the surface of the substrate mounted on the mounting portion is -1 mm or more and +1 mm or less.

[12] : An α-gallium oxide film formed on substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more, wherein the half-width of the rocking curve of the (0006) plane X-ray diffraction peak of the α-gallium oxide film is 9.5 seconds or less and the film thickness distribution is ±9% or less.

[13] : An α-gallium oxide film formed on substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more, and wherein the foreign matter density is 1.1 cm. －２ and a film thickness distribution of ±9% or less.

[0112] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits similar effects is included within the technical scope of the present invention.

Claims

1. A film formation method comprising the steps of: atomizing a raw material solution to form a raw material mist; mixing the raw material mist with a carrier gas to form a gas mixture; placing a substrate on a mounting section of a susceptor; supplying the gas mixture from atomization means to the substrate and forming a film on the substrate by a thermal reaction; and exhausting the gas mixture after film formation by exhaust means, wherein in the step of supplying the gas mixture from the atomization means to the substrate and forming a film on the substrate by a thermal reaction, at least a portion of the gas mixture is supplied to the surface of the substrate from a smooth section adjacent to the mounting section and having a surface roughness of 200 μm or less.

2. The film forming method according to claim 1, wherein the surface roughness is 150 μm or less.

3. The film forming method according to claim 1 or 2, wherein the difference in height between the surface of the flat portion and the surface of the substrate placed on the placement portion is between -1 mm and +1 mm.

4. A film forming method according to claim 1 or 2, characterized in that the angle formed between the surface of said smooth portion and the direction in which said mixture flows into said smooth portion is between 0° and 60°.

5. A film formation apparatus comprising: carrier gas supply means for supplying a carrier gas; atomization means for atomizing a raw material solution to generate a raw material mist; a film formation section for supplying a mixture of the raw material mist and the carrier gas to a substrate to form a film; and heating means for heating the substrate, wherein the film formation section has a susceptor with a mounting section on which the substrate is placed, and the susceptor is provided with a smooth section adjacent to the mounting section and through which at least a part of the mixture flows, and the surface roughness of the smooth section is 200 μm or less.

6. The film forming apparatus according to claim 5, wherein the surface roughness is 150 μm or less.

7. The film forming apparatus according to claim 5 or 6, wherein the difference in height between the surface of said flat portion and the surface of said substrate placed on said placement portion is between -1 mm and +1 mm.

8. The film forming apparatus according to claim 5 or 6, wherein the angle formed between the surface of said smooth portion and the direction in which said mixture flows into said smooth portion is between 0° and 60°.

9. A susceptor having a mounting section for mounting a substrate in a film formation apparatus comprising carrier gas supply means for supplying a carrier gas, atomization means for atomizing a raw material solution to generate a raw material mist, a film formation section for supplying a mixture of the raw material mist and the carrier gas to a substrate to form a film, and heating means for heating the substrate, wherein the susceptor has a smooth section adjacent to the mounting section through which at least a portion of the mixture flows, and the surface roughness of the smooth section is 200 μm or less.

10. The susceptor according to claim 9, wherein the surface roughness is 150 μm or less.

11. A susceptor according to claim 9, wherein the surface roughness of the flat and smooth portion is 200 μm or less, and the difference in height between the surface of the flat and smooth portion and the surface of the substrate placed on the placement portion is between -1 mm and +1 mm.

12. An α-gallium oxide film formed over substantially the entire surface of a c-plane sapphire substrate having a diameter of 4 inches or more, wherein the half-width of the rocking curve of the (0006) plane X-ray diffraction peak of the α-gallium oxide film is 9.5 seconds or less, and the film thickness distribution is ±9% or less.

13. A foreign particle is formed on almost the entire surface of a c-plane sapphire substrate with a diameter of 4 inches or more, and the foreign particle density is 1.1 cm －２ and a film thickness distribution of ±9% or less.