Method for forming crystalline metal oxide film and laminate

Abstract

To provide a method for forming a crystalline metal oxide film which is low in material cost and production cost and excellent in productivity by directly forming a crystalline metal oxide film such as ε - phase gallium oxide on a substrate, and to provide a laminate.SOLUTION: A film forming method for forming a crystalline metal oxide film containing gallium as a main component as a metal on a substrate by thermally reacting a misted raw material solution, the method comprising: A method of forming a crystalline metal oxide film, the method comprising: forming a mist of a raw material solution; conveying the mist to a film forming unit by a carrier gas; and forming the crystalline metal oxide film on a substrate by a thermal reaction of the mist in the film forming unit, wherein an annealed sapphire substrate is used as the substrate, and the crystalline metal oxide film of orthorhombic crystal or hexagonal crystal is formed on the annealed sapphire substrate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for forming a crystalline metal oxide film and a laminate. [Background technology]

[0002] As a method for forming a crystalline thin film at low temperature and under atmospheric pressure, a film formation technique using water particles, such as mist CVD, is known. This method has particularly attracted attention for its ability to form a metastable phase gallium oxide thin film suitable for power semiconductor devices, and recently, the development of the ε (or κ) phase applicable to high-frequency devices has been active. In this case, much research has been done on the crystals that serve as the base for the gallium oxide layer, and for example, Non-Patent Document 1 discloses an example in which ε-GaFeO3 is used. In addition, a manufacturing method using an inexpensive sapphire substrate has also been studied, but it is not possible to form a film of ε-phase gallium oxide only directly on a sapphire substrate, and in Patent Document 1 and other documents, an α-(Al x Ga 1-x )2O3, an example has been disclosed in which only ε-phase gallium oxide was produced. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] ACS Omega 2020, 5, 29585-29592 [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-46984 Summary of the Invention [Problem to be solved by the invention]

[0005] However, these conventional methods have the problem of high production costs because the substrate (hereinafter also referred to as base) itself is expensive and the process of forming a crystalline metal oxide film such as ε-phase gallium oxide on the substrate is complicated.

[0006] The present invention has been made to solve the above problems, and aims to provide a method for forming a crystalline metal oxide film and a laminate thereof, which can reduce material costs and production costs and have excellent productivity by directly forming a crystalline metal oxide film such as ε-phase gallium oxide on a substrate. [Means for solving the problem]

[0007] The present invention has been made to achieve the above-mentioned object, and provides a film formation method for forming a crystalline metal oxide film containing gallium as the metal main component on a substrate by thermally reacting a mist of a raw material solution, the method comprising: a mist-forming step of misting the raw material solution; a mist transport step of transporting the mist obtained in the mist-forming step to a film formation section by a carrier gas; and a film formation step of thermally reacting the mist in the film formation section to form a crystalline metal oxide film on the substrate, wherein an annealed sapphire substrate is used as the substrate, and an orthorhombic or hexagonal crystalline metal oxide film is formed on the annealed sapphire substrate.

[0008] This method for forming a crystalline metal oxide film allows a gallium-based orthorhombic or hexagonal crystalline metal oxide film to be formed directly on an inexpensive annealed sapphire substrate, thereby reducing the material and production costs required for film formation and increasing productivity.

[0009] It is also preferable to use a sapphire substrate that has been annealed at a temperature of 1000° C. or higher as the base.

[0010] Annealing a sapphire substrate at a temperature of 1000°C or higher rearranges the surface atoms to form a sapphire substrate with a step-terrace structure, and a crystalline metal oxide film with an orthorhombic or hexagonal crystal structure can be easily formed on this step-terrace structure.

[0011] It is also preferable to use a sapphire substrate that has been annealed at a temperature of 1200° C. or higher as the base.

[0012] Annealing a sapphire substrate at a temperature of 1200°C or higher reliably rearranges the surface atoms to form a sapphire substrate with a step-terrace structure. In particular, the terrace width is expanded to form a flatter substrate, making it extremely easy to form a crystalline metal oxide film of an orthorhombic or hexagonal crystal structure on this step-terrace structure.

[0013] It is also preferable that the thermal reaction in the film formation step is carried out at a temperature of 450°C or higher and 650°C or lower.

[0014] By setting the temperature at 450°C or higher and 650°C or lower, the mist can be reliably thermally reacted, and a cubic or hexagonal crystalline metal oxide film containing gallium as the main component can be more reliably formed directly on an annealed inexpensive sapphire substrate.

[0015] The substrate may be 4 inches (100 mm) or larger in diameter or 75 cm 2 It is preferable to use a substrate having an area of ​​at least this.

[0016] By using a substrate of this shape, it is possible to reliably form a gallium-based orthorhombic or hexagonal crystalline metal oxide film directly on an annealed, inexpensive sapphire substrate, and since the diameter is particularly large, it is possible to achieve higher quality and higher productivity.

[0017] The present invention also provides a laminate comprising a sapphire substrate having a step-terrace structure on its surface, and an orthorhombic or hexagonal crystalline metal oxide film containing gallium as a metal component and in contact with the surface of the sapphire substrate.

[0018] Such a laminate has low material costs, good quality, and is industrially useful. When such a laminate is applied to a semiconductor device, it has excellent semiconductor properties and can be used in high-frequency devices. [Effects of the Invention]

[0019] As described above, the method for forming a crystalline metal oxide film of the present invention allows a gallium-based orthorhombic or hexagonal crystalline metal oxide film to be formed directly on an inexpensive annealed sapphire substrate, thereby reducing the material and production costs required for film formation and increasing productivity.

[0020] Furthermore, the laminate of the present invention has low material costs, good quality, and is industrially useful. When such a laminate is applied to a semiconductor device, it exhibits excellent semiconductor properties and can be applied to high-frequency devices. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic diagram showing an example of a film formation apparatus that can be used in the film formation method of the present invention. [Figure 2] 1 is a schematic diagram showing an example of a mist-forming unit that can be used in the film-forming method of the present invention. [Figure 3] 1 is a schematic diagram showing an example of a nozzle that can be used in the film forming method of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing another example of a nozzle that can be used in the film forming method of the present invention. [Figure 5] 1 is a schematic diagram showing an example of a reciprocating movement mechanism that can be used in the film forming method of the present invention. [Figure 6]1 is a schematic diagram showing an example of a rotary movement mechanism that can be used in the film formation method of the present invention. [Figure 7] FIG. 2 is a schematic diagram illustrating a step-terrace structure. [Figure 8] 1 is a schematic diagram illustrating an example of a laminate of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described in detail below, but the present invention is not limited thereto.

[0023] As described above, there has been a demand for a method for forming a crystalline metal oxide film and a laminate that are inexpensive in material cost and production cost and have excellent productivity.

[0024] As a result of extensive research into the above-mentioned problems, the inventors discovered that a crystalline metal oxide film of orthorhombic or hexagonal crystal structure containing gallium as a main component can be directly formed on an inexpensive sapphire substrate that has been annealed in advance using the mist CVD method, thereby completing the present invention.

[0025] That is, the present invention is a film formation method for forming a crystalline metal oxide film containing gallium as the main metal component on a substrate by thermally reacting a mist of a raw material solution, the method comprising: a mist-forming step of misting the raw material solution; a mist transport step of transporting the mist obtained in the mist-forming step to a film formation section by a carrier gas; and a film formation step of thermally reacting the mist in the film formation section to form a crystalline metal oxide film on the substrate, wherein an annealed sapphire substrate is used as the substrate, and an orthorhombic or hexagonal crystalline metal oxide film is formed on the annealed sapphire substrate.

[0026] The present invention also provides a laminate comprising a sapphire substrate having a step-terrace structure on its surface, and an orthorhombic or hexagonal crystalline metal oxide film containing gallium as a metal component and in contact with the surface of the sapphire substrate.

[0027] Hereinafter, a detailed description will be given with reference to the drawings.

[0028] (crystalline metal oxide film) The crystalline metal oxide film according to the present invention is gallium oxide having a primarily orthorhombic or hexagonal crystal structure. Generally, crystalline metal oxide films are composed of metal and oxygen, but in the crystalline metal oxide film according to the present invention, the metal is primarily gallium. The term "primarily primary component" as used herein means that 50 to 100% (atomic percent) of the metal components is gallium. Furthermore, the metal components other than gallium may include, for example, one or more metals selected from iron, indium, aluminum, vanadium, titanium, chromium, rhodium, iridium, nickel, and cobalt.

[0029] The method for determining the content of each metal is not particularly limited, and any known analytical method can be used, such as a non-destructive analysis such as EDX, or an analytical method in which a crystalline metal oxide film is dissolved in a solvent, such as ICP.

[0030] Furthermore, the gallium oxide in the present invention is not particularly limited, but is preferably an oxide in which the number of oxygen atoms / the number of gallium atoms is 1.2 or more and 2 or less, more preferably 1.35 or more and 1.65 or less. The method for determining the atomic ratio is not particularly limited. Analysis can be performed by a known method such as EDX.

[0031] Furthermore, the crystalline metal oxide film of the present invention (for example, but not limited to, gallium oxide) has a hexagonal or orthorhombic crystal structure. The method for analyzing the crystal structure is not particularly limited, and it can be analyzed by known methods such as electron diffraction and X-ray diffraction.

[0032] Although not particularly limited, it is preferable that the gallium oxide film has a peak at least near 2θ=38.9° when CuKα rays are used as the radiation source in X-ray diffraction measurement such as XRD.

[0033] Although not particularly limited, the crystalline metal oxide film according to the present invention is preferably a single-oriented film in which the entire film is oriented along one crystal axis, or a single crystal.

[0034] Although not particularly limited, a dopant can be contained depending on the intended use of the crystalline metal oxide film. The dopant is not particularly limited, but examples thereof include n-type dopants such as tin, germanium, silicon, titanium, zirconium, vanadium, or niobium, and p-type dopants such as copper, silver, tin, iridium, or rhodium. The dopant concentration is not particularly limited, but examples thereof include about 1.0 × 10 16 ~1.0×10 22 / cm 3 may be about 1.0 x 10 17 / cm 3 Even at low concentrations below this, the 20 / cm 3 A concentration higher than this may be used.

[0035] Furthermore, the thickness of the crystalline metal oxide film according to the present invention is not particularly limited, and may be, for example, 0.05 to 100 μm, preferably 0.1 to 50 μm, and more preferably 0.5 to 20 μm.

[0036] Although not particularly limited, the area of ​​the oxide semiconductor film is 4 inches (100 mm) or more in diameter or 75 cm 2 The upper limit of the area is not particularly limited, but it is preferably 750 cm or more. 2 If circular, it can be no larger than 12 inches (300 mm) in diameter.

[0037] Furthermore, although not particularly limited, the crystalline metal oxide film according to the present invention can be used in semiconductor devices by appropriately designing the structure. For example, 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. The crystalline metal oxide film according to the present invention is suitable for both horizontal and vertical devices, but is particularly suitable for lateral devices. Specifically, it can be used in, for example, high electron mobility transistors (HEMTs).

[0038] (Film forming equipment) 1 shows an example of a film formation apparatus 100 that can be used in the film formation method according to the present invention, although it is not particularly limited thereto. The film formation apparatus 100 includes a mist-generating unit 120 that generates mist by misting a raw material solution, a carrier gas supply unit 130 that supplies a carrier gas for transporting the mist, a film formation unit 140 that thermally reacts the mist to form a crystalline metal oxide film on a base (crystalline substrate) 110, a transport unit 109 that connects the mist-generating unit 120 and the film formation unit 140 and transports the mist by the carrier gas, and a nozzle 150 that supplies the rectified mist onto the base (crystalline substrate) 110. The operation of the film formation apparatus 100 may also be controlled by including a control unit (not shown) that controls all or part of the film formation apparatus 100.

[0039] (Mist generating section) In the mist generating section 120, the raw material solution is turned into mist to generate mist. The mist generating means is not particularly limited as long as it can turn the raw material solution into mist, and any known mist generating means may be used, but it is preferable to use a mist generating means that uses ultrasonic vibrations, as this allows for more stable mist generation.

[0040] An example of such a mist-generating unit 120 is shown in FIG. 2. For example, the mist-generating unit 120 may include a mist source 104 that contains a raw solution (aqueous solution) 104a, a container 105 that contains a medium capable of transmitting ultrasonic vibrations, such as water 105a, and an ultrasonic vibrator 106 attached to the bottom of the container 105. More specifically, the mist source 104, which is a container that contains the raw solution 104a, is housed in the container 105 that contains the water 105a using a support (not shown). The bottom of the container 105 is equipped with an ultrasonic vibrator 106, which is connected to an oscillator 116. When the oscillator 116 is activated, the ultrasonic vibrator 106 vibrates, and ultrasonic waves propagate through the water 105a into the mist source 104, turning the raw solution 104a into mist.

[0041] Although not particularly limited, a crystalline metal oxide film consisting of two or more metals (for example, α-(Al X Ga 1-X When forming a )2O3 film (0≦X<1), raw material solution 104a, which is a mixture of metal raw material solutions, may be contained in mist source 104 of one mist-forming unit 120 and turned into mist, or multiple mist-forming units may be provided and each metal raw material solution may be turned into mist in a different mist-forming unit. When multiple mist-forming units are provided, a mist mixer (not shown) may be provided to mix the mist-formed raw material solutions, or the raw material solutions may be supplied separately to the film formation chamber without providing a mist mixer.

[0042] (Carrier gas supply unit) 1 may have a carrier gas source 102a that supplies a carrier gas, and may also have a flow rate control valve 103a that controls the flow rate of the carrier gas sent from the carrier gas source 102a. In addition, the carrier gas supply unit 130 may also have a dilution carrier gas source 102b that supplies a dilution carrier gas as needed, and a flow rate control valve 103b that controls the flow rate of the dilution carrier gas sent from the dilution carrier gas source 102b.

[0043] The type of carrier gas is not particularly limited and can be selected appropriately depending on the film to be formed. Examples include inert gases such as oxygen, ozone, nitrogen, and argon, and reducing gases such as hydrogen gas and forming gas. The type of carrier gas may be one type or two or more types. For example, a diluted gas obtained by diluting the same gas as the first carrier gas with another gas (e.g., diluted 10 times) may be further used as the second carrier gas, or air may be used.

[0044] Furthermore, the number of points at which the carrier gas is supplied is not limited to one, but may be two or more.

[0045] The flow rate of the carrier gas is not particularly limited. For example, when forming a film on a substrate with a diameter of 4 inches (100 mm), the flow rate is preferably 1 to 80 L / min, and more preferably 4 to 40 L / min. Note that this carrier gas flow rate is a value measured at 20°C and atmospheric pressure. However, when measured at other temperatures and pressures or when a different type of flow rate (mass flow rate, etc.) is measured, it can be converted to a volumetric flow rate at 20°C and atmospheric pressure using the gas state equation.

[0046] (Film forming section) In the film formation unit 140, the misted raw material solution is heated to cause a thermal reaction, thereby forming a film on a part or all of the surface of the base (crystalline substrate) 110. The film formation unit 140 is not particularly limited, but may include, for example, a film formation chamber 107, a base (crystalline substrate) 110 placed in the film formation chamber 107, and a hot plate 108 for heating the base (crystalline substrate) 110. The hot plate 108 may be provided outside the film formation chamber 107 as shown in FIG. 1, or may be provided inside the film formation chamber 107. Furthermore, the film formation chamber 107 may be provided with an exhaust gas outlet 112 at a position that does not affect the supply of mist to the base (crystalline substrate) 110. Furthermore, the film formation unit 140 may include a nozzle 150 for rectifying the mist.

[0047] In addition, the base (crystalline substrate) 110 may be placed face down, for example, on the top surface of the film formation chamber 107, or may be placed face up, as shown in Figure 1, on the bottom surface of the film formation chamber 107.

[0048] The thermal reaction may be carried out under any conditions, as long as the raw material solution is misted by heating. The reaction conditions are not particularly limited. They can be appropriately set depending on the raw material and the film to be formed. For example, the heating temperature may be in the range of 120 to 700°C, preferably in the range of 200 to 700°C, and more preferably in the range of 450 to 650°C. If the temperature is too low, the crystallinity may be poor or the film may become amorphous. Conversely, if the temperature is too high, the most stable β phase may be mixed in. Therefore, although not particularly limited, in order to minimize such possibilities, it is particularly preferable to carry out the thermal reaction in the film formation process at a temperature between 450°C and 650°C.

[0049] The thermal reaction may be carried out under any of the following atmospheres: vacuum, non-oxygen atmosphere, reducing gas atmosphere, air atmosphere, and oxygen atmosphere, and may be appropriately set depending on the film to be formed. The reaction pressure may be atmospheric pressure, elevated pressure, or reduced pressure, but film formation under atmospheric pressure is preferred because it simplifies the device configuration.

[0050] (nozzle) 1, the film forming unit 140 is equipped with a nozzle 150 for supplying mist to the substrate 110. The nozzle 150 straightens the mist that flows into the nozzle 150 from the supply pipe 109a and supplies it onto the substrate 110. Although not particularly limited, an example of such a nozzle is shown in FIG.

[0051] The nozzle 150 is a box-shaped member having a connection part 151 that connects to the transport part 109, an internal space (not shown) for rectifying the mist, and a discharge surface 152 that discharges the mist toward the substrate 110. Here, rectification means that the flow directions of the mist and carrier gas discharged at the nozzle discharge surface are aligned.

[0052] The installation position of the nozzle is not particularly limited. As shown in Fig. 1, the substrate 110 may be installed on the lower surface of the film formation chamber 107 and the nozzle 150 may be installed vertically above the substrate 110, thereby forming a face-up arrangement, or the substrate 110 may be installed on the upper surface of the film formation chamber 107 and the nozzle 150 may be installed vertically below the substrate, thereby forming a face-down arrangement.

[0053] The number of nozzles and the number of nozzle ejection surfaces are not particularly limited as long as they are at least 1. A plurality of nozzles may be provided, and as shown in Figure 4, the nozzle 150a may have a plurality of ejection surfaces (152a, 152b).

[0054] Furthermore, there are no particular limitations on the angle formed between the plane including the nozzle discharge surface 152 (or 152a, 152b) and the plane including the substrate 110. A nozzle (not shown) may be provided in which part or all of the nozzle discharge surface is inclined so that the mist flows more easily in a specific direction, but it is preferable that the substrate 110 and the nozzle discharge surface 152 are arranged parallel to each other, as shown in Figure 1. This is because a uniform film can be formed even on a large-diameter substrate with a simpler structure.

[0055] The nozzle may also be equipped with a nozzle position adjustment mechanism (not shown) that can appropriately adjust the longest distance between a point on the nozzle discharge surface and the surface of the substrate, although this is not particularly limited. In the examples of Figures 1 and 3, the longest distance between a point on the nozzle discharge surface 152 and the surface of the substrate 110 is not particularly limited, but is preferably 0.1 cm or more and less than 10 cm, more preferably 0.1 cm or more and less than 6 cm, and even more preferably 0.1 cm or more and less than 3 cm. By setting the distance within this range, a uniform film can be formed even on a large-diameter substrate.

[0056] The shape of the ejection surface of the nozzle is not particularly limited, and may be polygonal, circular, elliptical, or the like, but is preferably square, and more preferably rectangular.

[0057] Furthermore, the representative length L [cm] of the nozzle discharge surface 152 in the example of FIG. 3 is not particularly limited, but is preferably greater than 3 cm and can be determined appropriately depending on the size of the substrate 110. Furthermore, although not particularly limited, the representative length L [cm] of the nozzle discharge surface 152 is preferably longer than the representative length R [cm] (not shown) of the substrate 110. Note that, in the present invention, the representative length refers to the diameter in the case of a circle, the major axis length in the case of an ellipse, and the length of the longest side in the case of a polygon. For example, if the substrate 110 is a circle with a diameter of 4 inches, R is approximately 10 cm, and if the nozzle discharge surface is a rectangle with a major axis of 15 cm and a minor axis of 2 cm, L is 15 cm. By setting L and R such that (L>R), the mist supplied from the nozzle discharge surface 152 can cover the entire longitudinal direction of the substrate 110 at once, making it possible to form a uniform film even on a large-diameter substrate.

[0058] Furthermore, although not particularly limited, the film forming section 140 may be provided with a movement mechanism that moves the substrate 110 below the nozzle 150. In this case, the direction in which the substrate is moved is not particularly limited.

[0059] As examples of the movement mechanism, film formation units equipped with a reciprocating movement mechanism 160a and a rotary movement mechanism 160b are shown in FIGS. 5 and 6. FIGS. 5 and 6 are views of the substrate 110 viewed from vertically above to below (the substrate 110 side). The method shown in FIG. 5 is a method in which a movement stage 161a on which the substrate 110 and the hot plate 108 are placed is provided, and the substrate 110 and the hot plate 108 move back and forth below the nozzle 150. The method shown in FIG. 6 is a method in which the substrate 110 and the hot plate 108 are rotated and moved below the nozzle 150 by a movement stage 161b on which the substrate 110 and the hot plate 108 are placed.

[0060] As another method, although not shown, a mechanism for rotating the substrate may be provided and the substrate may be rotated on its axis.

[0061] By employing the various techniques described above, the mist supplied from the nozzle discharge surface 152 can be supplied to the entire substrate, and a uniform film can be formed on the substrate.

[0062] (Transportation section) The transfer unit 109 connects the mist generation unit 120 and the film formation unit 140. Mist is transferred by a carrier gas from the mist generation source 104 of the mist generation unit 120 to the film formation chamber 107 of the film formation unit 140 via the transfer unit 109. The transfer unit 109 is not particularly limited, but can be, for example, a supply pipe 109a. The supply pipe 109a is not particularly limited, but can be, for example, a quartz pipe or a resin tube.

[0063] (Raw material solution) The raw material solution (aqueous solution) 104a is not particularly limited as long as it contains gallium (Ga), and may be an inorganic or organic material. A solution of a metal or metal compound is preferably used as the raw material solution, and one or more metals selected from iron, indium, aluminum, vanadium, titanium, chromium, rhodium, nickel, and cobalt can be used.

[0064] The raw material solution is not particularly limited as long as it is a metal solution that can be turned into a mist. However, a solution in which the metal is dissolved or dispersed in an organic solvent or water in the form of a complex or salt can be suitably used as the raw material solution. Examples of the complex include an acetylacetonate complex, a carbonyl complex, an ammine complex, and a hydride complex. Examples of the salt include a metal chloride salt, a metal bromide salt, and a metal iodide salt. Furthermore, a solution in which the above metal is dissolved in hydrobromic acid, hydrochloric acid, hydroiodic acid, or the like can also be used as an aqueous salt solution.

[0065] The solute concentration is not particularly limited, but is preferably 0.01 to 1 mol / L, more preferably 0.05 to 0.5 mol / L, and even more preferably 0.08 to 0.30 mol / L. If the concentration is too low, the film formation rate may decrease. Conversely, if the concentration is too high, excess raw materials may cause side reactions, precipitation, foreign matter contamination, abnormal growth, etc., making it difficult to form a uniform film. Therefore, although not particularly limited, in order to eliminate such possibilities as much as possible, it is particularly preferable to set the solute concentration to 0.08 to 0.30 mol / L.

[0066] The source solution may also contain additives such as, but not limited to, halogen-containing compounds (e.g., hydrohalic acid) and oxidizing agents. Examples of hydrohalic acids include hydrobromic acid, hydrochloric acid, and hydroiodic acid, with hydrobromic acid and hydroiodic acid being preferred. Examples of oxidizing agents include peroxides such as hydrogen peroxide (HO), sodium peroxide (NaO), barium peroxide (BaO), and benzoyl peroxide (CHCO)O, as well as hypochlorous acid (HClO), perchloric acid, nitric acid, ozone water, and organic peroxides such as peracetic acid and nitrobenzene.

[0067] Furthermore, the raw material solution may contain a dopant, although the type of dopant is not particularly limited. Examples of the dopant include n-type dopants such as tin, germanium, silicon, titanium, zirconium, vanadium, and niobium, and p-type dopants such as copper, silver, tin, iridium, and rhodium.

[0068] The concentration of the dopant is not particularly limited, but is, for example, about 1.0×10 -9 It may be up to 1.0 mol / L, i.e., about 1.0×10 -7 The concentration may be as low as 0.01 mol / L or less, or as high as about 0.01 mol / L or more.

[0069] (Base) The substrate 110 is an annealed sapphire substrate. A sapphire substrate refers to a crystalline aluminum oxide substrate having a corundum structure. It may be single crystal or polycrystalline, but single crystal is preferable. Note that aluminum oxide here refers to an oxide in which 90% (atomic percent) or more of the metal components is aluminum. There are no particular limitations on the method for analyzing the components, and known techniques can be used. For example, analysis can be performed using EDX, ICP, or the like.

[0070] Furthermore, although not particularly limited, a dopant may be added to the sapphire substrate in order to adjust the electrical properties. Known dopants can be used as such dopants.

[0071] The shape of the substrate 110 may be any shape as long as it has a surface to be deposited and a back surface, and may be, for example, a plate-like shape such as a flat plate or a disc, a rod-like shape, a cylindrical shape, a prism-like shape, a tube-like shape, a ring-like shape, etc., but a plate-like shape is particularly preferable.

[0072] Furthermore, the sapphire substrate may be polished to an optically mirror-finished surface, or may be subjected to a surface roughening process, although this is not particularly limited. However, it is preferable that both the surface to be film-formed (the surface on which the crystalline metal oxide film is formed) and the back surface thereof are polished to a mirror-finished surface.

[0073] More specifically, although not particularly limited, the surface roughness Ra of the film-forming surface and the back surface is preferably 1 μm or less, and the waviness Warp (hereinafter referred to as Wa) is preferably 50 μm or less.

[0074] The smaller the surface roughness Ra, the better, and the lower limit is not particularly limited, but it can be, for example, 0.1 nm or more. The surface roughness Ra may be measured at one or more arbitrary locations on the deposition surface, with a measurement length of, for example, 10 μm or more.

[0075] The smaller the waviness Wa, the more preferable, and the lower limit is not particularly limited, but can be, for example, 0.5 μm or more. The waviness Wa may be measured on one or more arbitrary straight lines on the deposition surface, which are determined appropriately depending on the shape of the substrate 110.

[0076] For example, in the case of a disk-shaped substrate having a diameter of 4 inches (100 mm), any length on two straight lines intersecting at right angles at the center of the substrate can be used as the measurement length.

[0077] The surface roughness Ra and waviness Wa refer to values ​​calculated in accordance with JIS B 0601 using the results of surface shape measurement using a non-contact measurement method using a laser microscope or confocal microscope, such as the stylus method, atomic force microscope (AFM) method, or optical interference method, confocal method, or focal shift image synthesis method.

[0078] The surface roughness Ra of 1 μm or less and the waviness Wa of 50 μm or less as described above are of high quality and allow for processing of the laminate by irradiating light from the backside of the substrate 110, thereby increasing the design flexibility of the semiconductor device. Furthermore, by combining this with a hot plate 108 having a smooth substrate mounting surface, a laminate having a high-quality semiconductor film with excellent crystal orientation can be obtained even when a thick film is formed. Such smoothness of the substrate surface can be easily obtained, for example, in the case of sapphire, by lapping the surface of the substrate obtained by processing the crystal with diamond abrasive grains and then mirror-finishing it by chemical mechanical polishing (CMP) using colloidal silica.

[0079] The annealing temperature of the substrate is not particularly limited as long as it is within the stable temperature range of the crystal structure, but a high temperature is preferable. Specifically, a temperature of 1000°C or higher is preferable, and 1200°C or higher is more preferable. Annealing the sapphire substrate at such a temperature rearranges the surface atoms, resulting in a sapphire substrate with a step-terrace structure. A step-terrace structure 170 is schematically shown in FIG. 7, in which steps 172, such as monoatomic or monomolecular steps, form an atomically or molecularly flat terrace 171 surface. Reference numeral 173 in FIG. 7 denotes a kink, which is formed at the intersection of two steps.

[0080] The step-terrace structure shown in Figure 7 can be observed using an atomic force microscope (AFM), etc. When the annealing temperature is 1200°C or higher, it can be observed that the width of the terrace 171 in particular increases further, resulting in a flatter substrate.

[0081] The upper limit of the annealing temperature is not particularly limited, but is preferably 1600° C. or less, and more preferably 1400° C. or less. If the temperature is too high, more power than necessary will be consumed, which may increase the manufacturing cost.

[0082] The annealed sapphire substrate has a small surface roughness Ra and waviness Wa due to the formation of the step-terrace structure 170. The surface roughness Ra of the annealed sapphire substrate is not particularly limited, but can be, for example, 0.1 nm or more and 1 μm or less.

[0083] The heating method for the annealing treatment is not particularly limited. A heater or a heating furnace such as a horizontal furnace may be used. The use of a horizontal furnace or the like is preferred because it allows stable heating.

[0084] The annealing time is not particularly limited. It is preferably at least 1 minute, more preferably at least 1 hour, and even more preferably at least 5 hours. By performing annealing for such a time, a substrate 110 having a sufficiently formed step-terrace structure can be obtained. There is no particular upper limit to the annealing time, but it is preferably 24 hours or less. If the annealing time exceeds 24 hours, more power than necessary will be consumed, which may increase the manufacturing cost.

[0085] The atmosphere in which the annealing treatment is performed is not particularly limited, and may be an inert gas such as nitrogen or argon, a reducing gas such as hydrogen, or air.

[0086] As described above, by using an annealed sapphire substrate as the base 110, it becomes possible to directly form an orthorhombic or hexagonal crystalline metal oxide film containing gallium as a main component on the annealed sapphire substrate. Although the detailed mechanism by which orthorhombic or hexagonal gallium oxide is formed is still being elucidated, a hypothesis is presented below.

[0087] It is generally believed that crystalline metal oxide films grow by inheriting the crystalline structure of the substrate. However, because the terraces (171 in Figure 7) on the crystal surface are less affected by the crystalline structure of the substrate than the steps 172 and kinks 173, and because the lattice mismatch between gallium oxide with a corundum structure (trigonal) and gallium oxide with an orthorhombic or hexagonal crystal structure is not very large, it is assumed that a thermally stable orthorhombic or hexagonal crystalline metal oxide film will form.

[0088] The plane orientation of the main surface of the substrate is not particularly limited, and major planes such as the c-plane, m-plane, and a-plane can be used. The main surface may also have an off-angle relative to the just-plane. The off-angle is not particularly limited, but is preferably 0° to 15°, more preferably 0° to 1°, and more preferably 0° to 0.3°. Generally, the relationship between the off-angle (θ), the width (D) of the terrace 171, and the height (H) of the step 172 is D=H / tan θ, so the smaller the off-angle θ, the wider the terrace 171.

[0089] The thickness of the base 110 is not particularly limited, but is preferably about 200 to 800 μm from the viewpoint of cost.

[0090] Although not particularly limited, the base 110 may have a diameter of 4 inches (100 mm) or more, or a diameter of 75 cm 2 It is preferable that the substrate has an area of ​​6 inches (150 mm) or more, and more preferably a diameter of 6 inches (150 mm) or more. By using a substrate of this shape, it is possible to reliably deposit a gallium-based orthorhombic or hexagonal crystalline metal oxide film directly on an annealed inexpensive sapphire substrate, and the large diameter in particular results in higher quality and higher productivity.

[0091] The shape of the base 110 is not particularly limited.

[0092] Although not particularly limited, an annealing treatment may be performed after film formation. The temperature of the annealing treatment is not particularly limited, but is preferably 600°C or less, and more preferably 550°C or less. This is to avoid impairing the crystallinity of the film. The treatment time of the annealing treatment is not particularly limited, but is preferably 10 seconds to 10 hours, and more preferably 10 seconds to 1 hour.

[0093] (Film forming method) An example of a film forming method according to the present invention will be described below with reference to Fig. 1. First, raw material solution 104a is placed in mist source 104 of mist generating unit 120, and base (crystalline substrate) 110 is placed on hot plate 108, which is then operated. The base used here is a sapphire substrate.

[0094] Next, the flow rate control valves 103a and 103b are opened to supply carrier gas from the carrier gas source 102a (main carrier gas) and the dilution carrier gas source 102b (dilution carrier gas) into the film formation chamber 107, and the atmosphere in the film formation chamber 107 is thoroughly replaced with the carrier gas, while the flow rates of the main carrier gas and the dilution carrier gas are adjusted and controlled, respectively.

[0095] [Misting process] In the mist-forming step, the ultrasonic vibrator 106 is vibrated, and the vibration is propagated to the raw material solution 104a through the water 105a, thereby turning the raw material solution 104a into mist and generating mist.

[0096] [Mist transport process] Next, in the mist transport step of transporting the mist by a carrier gas, the mist is transported by the carrier gas from the mist-forming unit 120 to the film-forming unit 140 via the transport unit 109 and introduced into the film-forming chamber 107 .

[0097] [Film forming process] In the film formation process, the mist is supplied onto the substrate 110 placed on the hot plate 108, and a thermal reaction occurs due to the heat of the hot plate 108 in the film formation chamber 107, forming a film on the substrate 110. In this case, the reaction temperature of the thermal reaction is not particularly limited, but is preferably 450°C or higher and 650°C or lower.

[0098] The annealing treatment of the substrate 110 may be performed using the hot plate 108, or may be performed using another heater or a heating furnace, without any particular limitation. In this case, the substrate 110 may be cooled to room temperature once, or may be set directly from the annealing temperature to the temperature at which the film formation step will be performed.

[0099] Furthermore, although not particularly limited, annealing may be performed after film formation. In this case, the annealing temperature is not particularly limited, but is preferably 600°C or lower, more preferably 550°C or lower. This is to avoid impairing the crystallinity of the film. Furthermore, although not particularly limited, the annealing time is preferably 10 seconds to 10 hours, more preferably 10 seconds to 1 hour.

[0100] Here, the features and effects of the above-mentioned film formation method are summarized. The above-mentioned film formation method is a film formation method in which a mist-formed raw material solution 104a is thermally reacted to form a crystalline metal oxide film containing gallium as the main metal component on a substrate 110, and includes a mist-forming step of forming the raw material solution 104a into a mist, a mist transport step of transporting the mist obtained in the mist-forming step to a film formation unit 140 by a carrier gas, and a film formation step of thermally reacting the mist in the film formation unit 140 to form a crystalline metal oxide film on the substrate 110. An annealed sapphire substrate is used as the substrate 110, and an orthorhombic or hexagonal crystalline metal oxide film is formed on the annealed sapphire substrate.

[0101] This method for forming a crystalline metal oxide film allows a gallium-based orthorhombic or hexagonal crystalline metal oxide film to be formed directly on an inexpensive annealed sapphire substrate, thereby reducing the material and production costs required for film formation and increasing productivity.

[0102] Although not particularly limited, it is preferable to use a sapphire substrate that has been annealed at a temperature of 1000° C. or higher as the base 110 .

[0103] Annealing a sapphire substrate at a temperature of 1000°C or higher rearranges the surface atoms to form a sapphire substrate with a step-terrace structure 170, and a crystalline metal oxide film of an orthorhombic or hexagonal crystal structure can be easily formed on this step-terrace structure 170.

[0104] Although not particularly limited, it is more preferable to use a sapphire substrate that has been annealed at a temperature of 1200° C. or higher as the base 110.

[0105] Annealing a sapphire substrate at a temperature of 1200°C or higher reliably rearranges the surface atoms to form a sapphire substrate having a step-terrace structure 170. In particular, the terrace width is expanded to form a flatter substrate, which makes it extremely easy to form a crystalline metal oxide film of an orthorhombic or hexagonal crystal structure on the step-terrace structure 170.

[0106] Although not particularly limited, it is preferable that the thermal reaction in the film formation step be carried out at a temperature of 450°C or higher and 650°C or lower.

[0107] By setting the temperature at 450°C or higher and 650°C or lower, the mist can be reliably thermally reacted, and a cubic or hexagonal crystalline metal oxide film containing gallium as the main component can be more reliably formed directly on an annealed inexpensive sapphire substrate.

[0108] Although not particularly limited, the base 110 may have a diameter of 4 inches (100 mm) or more, or a diameter of 75 cm 2 It is preferable to use a substrate having an area of ​​at least this.

[0109] By using a substrate 110 of this shape, it is possible to reliably deposit a gallium-based orthorhombic or hexagonal crystalline metal oxide film directly on an annealed inexpensive sapphire substrate, and since the diameter is particularly large, it is possible to achieve higher quality and higher productivity.

[0110] (Laminate) 8 can be fabricated by the above-described film formation method. The laminate 180 includes a sapphire substrate (base) 181 having a step-terrace structure 170 (terraces 171, steps 172) on its surface, and an orthorhombic or hexagonal crystalline metal oxide film 182 containing gallium as a metal as a main component and in contact with the surface of the sapphire substrate 181.

[0111] Such a laminate 180 is industrially useful because it has low material costs and good quality. When such a laminate 180 is applied to a semiconductor device, it has excellent semiconductor properties and can also be used in high-frequency devices.

[0112] (peeling) Although not particularly limited, the substrate 110 may be peeled off from the crystalline metal oxide film after film formation. The peeling means is not particularly limited and may be any known means. Examples include a means of peeling off by applying mechanical impact, a means of peeling off by applying heat and utilizing thermal stress, a means of peeling off by applying vibration such as ultrasonic waves, a means of peeling off by etching, and laser lift-off. By peeling off the substrate 110, the crystalline metal oxide film can be obtained as a free-standing film.

[0113] (electrode) To apply the crystalline metal oxide film formed by the above method to a semiconductor device, electrodes must be formed. Conventional methods can be used for this purpose. These methods include vapor deposition, sputtering, CVD, plating, and printing, which involves bonding with a resin or other material. Electrode materials include metals such as Al, Ag, Ti, Pd, Au, Cu, Cr, Fe, W, Ta, Nb, Mn, Mo, Hf, Co, Zr, Sn, Pt, V, Ni, Ir, Zn, In, and Nd; conductive metal oxide films such as tin oxide, zinc oxide, indium oxide, indium tin oxide (ITO), and indium zinc oxide (IZO); and organic conductive compounds such as polyaniline, polythiophene, and polypyrrole. These materials can also be alloys or mixtures of two or more of these. The thickness of the electrode is preferably 1 to 1,000 nm, more preferably 10 to 500 nm. [Example]

[0114] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0115] Example 1 The method for forming a crystalline metal oxide film in this Example 1 will be described with reference to FIGS.

[0116] [1-1. Preparation of raw material solution] Gallium iodide was added to water to prepare a 0.1 mol / L aqueous solution, which was used as raw material solution 104a and placed in mist generating source 104. The temperature of the solution at this time was 25°C.

[0117] [1-2. Annealing of the substrate] Next, a c-plane sapphire substrate with a diameter of 4 inches (100 mm) was used as the base 110. The base 110 was annealed in a horizontal furnace (not shown) under atmospheric pressure at 1000°C for 1 hour, and after returning to room temperature, it was placed on a hot plate 108 in the film formation chamber 107, and the hot plate 108 was operated to raise the temperature to 500°C.

[0118] [1-3. Carrier gas supply] Next, the flow control valves 103a and 103b were opened to supply nitrogen gas as a carrier gas from the carrier gas source 102a (main carrier gas) and the dilution carrier gas supply source 102b (dilution carrier gas) into the film formation chamber 107, and the atmosphere in the film formation chamber 107 was thoroughly replaced with these carrier gases, while the flow rate of the main carrier gas was adjusted to 12 L / min and the flow rate of the dilution carrier gas was adjusted to 12 L / min, respectively.

[0119] [1-4. Mist generation] Next, the ultrasonic vibrator 106 was vibrated at 2.4 MHz, and the vibration was propagated to the raw material solution 104a through the water 105a, thereby turning the raw material solution 104a into mist and generating mist.

[0120] [1-5. Mist transport to film formation] Then, under conditions of atmospheric pressure and 500°C, while gas was being exhausted from exhaust port 112, a substrate was reciprocated under nozzle 150 in film formation chamber 107 by reciprocating movement mechanism 160a, causing a thermal reaction of the mist, thereby forming a film of orthorhombic or hexagonal Ga2O3 on base 110. The film formation time was set to one hour, and the substrate was moved so that it passed under nozzle 150 once per minute. Furthermore, during the film formation process, raw material solution 104a was appropriately replenished into mist generation source 104 using a liquid replenishment mechanism (not shown) so that the water level of raw material solution 104a in mist generation source 104 during film formation was kept constant.

[0121] (Comparative Example 1) No annealing treatment The same procedure as in Example 1 was carried out except that the annealing treatment of the base 110 was not carried out in the above [1-2. Annealing treatment of base].

[0122] Example 2 The same procedures as in Example 1 were carried out except that in the above [1-2. Annealing of the substrate], a c-plane sapphire substrate with a diameter of 6 inches (150 mm) was used as the substrate 110, the annealing of the substrate 110 was carried out at 1000°C for 5 hours, and in the above [1-3. Supply of carrier gas], the flow rates of the main carrier gas and the dilution carrier gas were both doubled.

[0123] Example 3 The same procedures as in Example 1 were carried out except that in the above [1-2. Annealing of the substrate], a c-plane sapphire substrate with a diameter of 8 inches (200 mm) was used as the substrate 110, the annealing of the substrate 110 was carried out at 1200°C for 10 hours, and in the above [1-3. Supply of carrier gas], the flow rates of the main carrier gas and the dilution carrier gas were both increased by four times.

[0124] (Evaluation 1: Crystal structure, crystallinity evaluation) The crystalline structure of the films prepared in Examples 1-3 and Comparative Example 1 was evaluated by 2θ-ω scanning using CuKα radiation as the radiation source in XRD. The measurement points were 25 points expressed by the combinations of (r=0, R / 4, R / 2, 3R / 4), (θ=0, π / 4, π / 2, 3π / 4, π, 5π / 4, 3π / 2, 7π / 4) in a polar coordinate system (r, θ) with the center of the substrate as the origin, where R is the radius of the substrate.

[0125] First, the films prepared in Examples 1-3 only showed a diffraction peak of the sapphire substrate at 38.9° at all 25 points. When CuKα radiation is used as the radiation source in X-ray diffraction measurements such as XRD, it is known that orthorhombic or hexagonal Ga2O3 single-phase films have a peak near 2θ = 38.9°. Therefore, it was confirmed that all of the films prepared in Examples 1-3 were orthorhombic or hexagonal Ga2O3 single-phase films.

[0126] On the other hand, in the film produced in Comparative Example 1, a diffraction peak at 40.3° was observed at more than half of the measurement points, in addition to the diffraction peaks at 38.9° and sapphire substrate. Since this diffraction peak at 40.3° is known to be observed in corundum structure (α phase) Ga2O3, it was determined that a mixture of corundum structure (α phase) Ga2O3 and orthorhombic or hexagonal Ga2O3 had grown in the film produced in Comparative Example 1.

[0127] (Evaluation 2: In-plane distribution of film thickness) The film thickness of the films prepared in Examples 1-3 and Comparative Example 1 was measured using an optical interference film thickness meter F50. The measurement points were 25 points expressed by the combinations of (r = 0, R / 4, R / 2, 3R / 4), (θ = 0, π / 4, π / 2, 3π / 4, π, 5π / 4, 3π / 2, 7π / 4) in a polar coordinate system (r, θ) with the center of the substrate as the origin, where R is the radius of the substrate. The film thickness measurements [nm] at the 25 measurement points of the crystalline metal oxide film are shown in Table 1.

[0128] [Table 1]

[0129] In addition, from the film thickness measurements at each measurement point, the in-plane film thickness distribution [±%] (= (maximum film thickness - minimum film thickness) / 2 / average film thickness × 100), standard deviation [nm], and coefficient of variation [-] were calculated and are shown in Table 2.

[0130] [Table 2]

[0131] The coefficient of variation was calculated using the following formula.

[0132]

number

[0133] As described above, a comparison between Examples 1 to 3 and Comparative Example 1 has shown that in a film formation method in which a mist-formed raw material solution is thermally reacted on a substrate to form a crystalline metal oxide film, by using an annealed sapphire substrate as in Examples 1 to 3, a uniform film of orthorhombic or hexagonal gallium oxide can be formed directly on the sapphire substrate.

[0134] The present specification includes the following aspects. [1]: A film formation method for forming a crystalline metal oxide film containing gallium as the main component on a substrate by thermally reacting a mist of a raw material solution, the method comprising: a mist-forming step of misting the raw material solution; a mist transport step of transporting the mist obtained in the mist-forming step to a film formation unit by a carrier gas; and a film formation step of thermally reacting the mist in the film formation unit to form a crystalline metal oxide film on the substrate, wherein an annealed sapphire substrate is used as the substrate, and an orthorhombic or hexagonal crystalline metal oxide film is formed on the annealed sapphire substrate. [2]: The method for forming a crystalline metal oxide film according to [1] above, wherein the base is a sapphire substrate that has been annealed at a temperature of 1000° C. or higher. [3]: The method for forming a crystalline metal oxide film according to [1] or [2] above, wherein the base is a sapphire substrate that has been annealed at a temperature of 1200° C. or higher. [4]: The method for forming a crystalline metal oxide film according to any one of [1] to [3] above, wherein the thermal reaction in the film formation step is carried out at a temperature of 450°C or higher and 650°C or lower. [5]: The base is 4 inches (100 mm) or more in diameter or 75 cm 2 The method for forming a crystalline metal oxide film according to any one of [1] to [4] above, wherein a substrate having an area of ​​at least 100 nm is used. [6]: A laminate comprising a sapphire substrate having a step-terrace structure on its surface and a cubic or hexagonal crystalline metal oxide film containing gallium as the main component as a metal, in contact with the surface of the sapphire substrate.

[0135] 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. [Explanation of symbols]

[0136] 100...film forming apparatus, 102a...carrier gas source, 102b...dilution carrier gas source, 103a...flow rate control valve, 103b...flow rate control valve, 104...mist generating source, 104a...raw material solution (aqueous solution), 105...container, 105a...water, 106...ultrasonic vibrator, 107...film formation chamber, 108...hot plate, 109...transport unit, 109a...supply pipe, 110... base (crystalline substrate), 112... exhaust port, 116... oscillator, 120... mist generating section, 130... carrier gas supply section, 140... film forming section, 150, 150a... nozzle, 151... connection part, 152, 152a, 152b...Discharge surface, 160a...Reciprocating moving mechanism (moving mechanism), 160b...rotary moving mechanism (moving mechanism), 161a, 161b...moving stage, 170...Step terrace structure, 171...Terrace, 172...Step, 173...kink, 180...laminated body, 181...sapphire substrate (base), 182...Crystalline metal oxide film. L…Representative length.

Claims

1. A film formation method for forming a crystalline metal oxide film containing gallium as a metal as a main component on a substrate by thermally reacting a mist of raw material solution, comprising: a mist-forming step of forming a raw material solution into a mist; a mist transport step of transporting the mist obtained in the mist-forming step to a film-forming unit by a carrier gas; a film-forming step of thermally reacting the mist in the film-forming unit to form a crystalline metal oxide film on the substrate; Including, A method for forming a crystalline metal oxide film, comprising using an annealed sapphire substrate as the substrate and forming an orthorhombic or hexagonal crystalline metal oxide film on the annealed sapphire substrate.

2. 2. The method for forming a crystalline metal oxide film according to claim 1, wherein the substrate is a sapphire substrate that has been annealed at a temperature of 1000[deg.] C. or higher.

3. 3. The method for forming a crystalline metal oxide film according to claim 2, wherein the substrate is a sapphire substrate that has been annealed at a temperature of 1200° C. or higher.

4. 2. The method for forming a crystalline metal oxide film according to claim 1, wherein the thermal reaction in the film forming step is carried out at a temperature of 450° C. or higher and 650° C. or lower.

5. The substrate may be 4 inches (100 mm) or larger in diameter or 75 cm ２ 5. The method for forming a crystalline metal oxide film according to claim 1, wherein the surface area of ​​the substrate is greater than or equal to 100 nm.

6. A laminate comprising a sapphire substrate having a step-terrace structure on its surface, and a cubic or hexagonal crystalline metal oxide film containing gallium as a main component as a metal, in contact with the surface of the sapphire substrate.

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