METHOD FOR PRODUCING A FREESTANDING POLYCRYSTALLINE DIAMOND SUBSTRATE

Abstract

Method for producing a freestanding polycrystalline diamond substrate (100), the method comprising: a step of applying a solution containing negatively charged diamond particles (14) to a compound semiconductor substrate (10) on which a positively charged natural oxide film is formed and which is not on any other substrate, and subsequently performing a heat treatment on the compound semiconductor substrate (10) at a temperature of 30 °C or more and less than 100 °C, whereby the diamond particles (14) are bonded to the compound semiconductor substrate (10); a step of growing a polycrystalline diamond layer (16) to a thickness of 100 µm or more on the compound semiconductor substrate (10) by chemical vapor deposition using the diamond particles (14) as seeds; and a step of subsequently reducing the thickness of the compound semiconductor substrate (10) to obtain a compound semiconductor layer (18), wherein the polycrystalline diamond layer (16) serves as a support substrate for the compound semiconductor layer (18) in the freestanding polycrystalline diamond substrate (100).

Description

TECHNICAL AREA

[0001] This disclosure relates to a process for producing a freestanding polycrystalline diamond substrate in which a compound semiconductor layer is formed on a polycrystalline diamond layer serving as a support substrate. BACKGROUND

[0002] In high-voltage semiconductor devices, such as high-frequency and power devices, self-heating is problematic. One known technique to address this problem involves placing a material with high thermal conductivity beneath the device formation area.

[0003] For example, a method is known in which a highly heat-dissipating diamond layer is placed directly beneath a compound semiconductor layer, such as a gallium nitride (GaN) layer, which serves as the device layer, to fabricate a semiconductor device. JP 2015-509479 A (PTL 1) discloses a method for fabricating a gallium nitride-on-diamond wafer.The process produces a wafer in which a gallium nitride layer is formed on diamond, and the process comprises: after forming a thin silicon nitride film of 60 nm or less on a GaN layer located on a support substrate, a step of embedding and fixing diamond particles in a surface of the silicon nitride film by dry scratching; a step of growing a diamond layer on the GaN layer with the silicon nitride film in between by chemical vapor deposition using the diamond particles fixed in the surface as seeds; and a step of removing the support substrate. Further processes and techniques for the deposition of polycrystalline diamond layers are disclosed in US 4,925,701 A, JP H05-58,784 A, US 6,159,604 A, and EP 3,656,896 A1. CITATION LIST Patent literature

[0004] PTL1: JP 2015-509479 A SUMMARY (Technical Problem)

[0005] However, according to the inventor's investigations, the method disclosed in PTL 1 results in the formation of cracks in the GaN layer due to the embedding process. These cracks propagate through the subsequent long-term high-temperature heat treatment by chemical vapor deposition, thereby forming dislocations. If a semiconductor device is fabricated on such a GaN layer, the leakage current is high, which would degrade the device properties.

[0006] It could therefore be helpful to provide a method for producing a freestanding polycrystalline diamond substrate that makes it possible to produce a freestanding polycrystalline diamond substrate in which a high-quality compound semiconductor layer is formed. (Solution to the problem)

[0007] The inventor conducted careful investigations to overcome the aforementioned challenges and found the following. First, the inventor of this disclosure considered growing a diamond layer on a previously prepared compound semiconductor substrate instead of growing a diamond layer on a compound semiconductor layer located on a support substrate, as in PTL 1. However, when a diamond layer is grown by chemical deposition from the vapor phase, using diamond particles as seeds embedded and fixed in the surface of a compound semiconductor substrate, as in PTL 1, it was found that the compound semiconductor substrate was fractured. This fracture was thought to originate from cracks in the surface of the compound semiconductor substrate caused by the embedding process.

[0008] Further investigations by the inventor on this topic show that the application of diamond particles to a compound semiconductor substrate by a process in which a solution containing diamond particles is applied to the compound semiconductor substrate and the solvent is subsequently evaporated by heat treatment enables the growth of a polycrystalline diamond layer without causing fracture of the compound semiconductor substrate. Furthermore, no dislocations are formed in a compound semiconductor layer obtained by reducing the thickness of the compound semiconductor substrate thus obtained.

[0009] This revelation, made on the basis of the above findings, includes above all the following characteristics. (1) A method for producing a freestanding polycrystalline diamond substrate, the method comprising: a step of applying a solution containing negatively charged diamond particles to a compound semiconductor substrate on which a positively charged natural oxide film is formed and which is not located on any other substrate, and subsequently performing a heat treatment on the Compound semiconductor substrate at a temperature of 30 °C or more and less than 100 °C, causing the diamond particles to bind to the compound semiconductor substrate; a step of growing a polycrystalline diamond layer to a thickness of 100 µm or more on the compound semiconductor substrate by chemical vapor deposition using the diamond particles as seeds; and a step of subsequently reducing the thickness of the compound semiconductor substrate to obtain a compound semiconductor layer, wherein the polycrystalline diamond layer serves as a support substrate for the compound semiconductor layer in the freestanding polycrystalline diamond substrate. (2) Method for producing a freestanding polycrystalline diamond substrate according to (1) above, wherein the average particle diameter of the diamond particles in the solution is 50 nm or less. (3) Method for producing a freestanding polycrystalline diamond substrate according to (1) or (2) above, wherein during heat treatment a temperature of the compound semiconductor substrate is maintained at less than 100 °C for 1 min or more and 30 min or less. (4) Method for producing a freestanding polycrystalline diamond substrate according to one of (1) to (3) above, further comprising a step for planarizing a surface of the polycrystalline diamond layer. (5) Method for producing a freestanding polycrystalline diamond substrate according to one of (1) to (4) above, wherein the compound semiconductor substrate is made from one of GaN, AlN, InN, SiC, Al2O3, Ga2O3, MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb and SiGe. (6) Method for producing a freestanding polycrystalline diamond substrate according to one of (1) to (5) above, wherein the thickness of the compound semiconductor layer is 1 µm or more and 500 µm or less. (Beneficial effect)

[0010] The disclosed method for producing a freestanding polycrystalline diamond substrate according to this disclosure can produce a freestanding polycrystalline diamond substrate in which a high-quality compound semiconductor layer is formed. BRIEF DESCRIPTION OF THE DRAWINGS The attached drawing shows Fig. 1A to 1F Cross-sectional views showing a method for producing a freestanding polycrystalline diamond substrate 100 according to an embodiment of this disclosure. DETAILED DESCRIPTION

[0011] (Method for producing a freestanding polycrystalline diamond substrate)

[0012] With reference to the Fig. Sections 1A to 1F comprise a process for producing a freestanding polycrystalline diamond substrate 100 according to an embodiment of this disclosure, comprising the following steps. First, as in Fig. Figures 1A and 1B show a solution containing diamond particles being applied to a compound semiconductor substrate 10. In this way, a liquid film 12 containing diamond particles is formed on the compound semiconductor substrate 10. Then, as shown in Fig. 1B and Fig. As shown in Figure 1C, by heat treatment of the compound semiconductor substrate 10, a solvent in the diamond particle-containing liquid film 12 is evaporated, and the bond strength between a surface of the compound semiconductor substrate 10 and the diamond particles 14 is increased, thereby binding the diamond particles 14 to the compound semiconductor substrate 10. Subsequently, as shown in Fig. 1C and Fig. As shown in 1D, a polycrystalline diamond layer 16 with a thickness of 100 µm or more is formed on the compound semiconductor substrate 10 by chemical vapor deposition (CVD) using the diamond particles 14 as nuclei. Then, as shown in Fig. 1D and Fig. Figure 1E shows that a surface of the polycrystalline diamond layer 16 can optionally be planarized. Subsequently, as shown in the Fig. 1E and Fig. 1F shows the thickness of the compound semiconductor substrate 10 reduced to obtain a compound semiconductor layer 18.

[0013] In this embodiment, a freestanding polycrystalline diamond substrate 100 can be produced by the steps described above, in which the polycrystalline diamond layer 16 serves as a support substrate for the compound semiconductor layer 18. The compound semiconductor layer 18 serves as a component layer in which a semiconductor device is to be formed. The steps of this embodiment are now described in detail. [Production of a compound semiconductor substrate]

[0014] Referring to Fig. In step 1A, the compound semiconductor substrate 10 is first prepared. The compound semiconductor forming the compound semiconductor substrate 10 is not particularly limited and can be selected appropriately, e.g., depending on the type of semiconductor device to be formed in the compound semiconductor layer 18. For example, the compound semiconductor preferably consists of GaN, AlN, InN, SiC, Al₂O₃, Ga₂O₃, MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb, or SiGe. Furthermore, the thickness of the compound semiconductor substrate 10 is preferably 200 µm or more and 3 mm or less. If the thickness is less than 200 µm, the compound semiconductor substrate would warp, which would lead to separation of the polycrystalline diamond or fracture of the compound semiconductor substrate.Furthermore, a thickness of more than 3 mm is not preferred with regard to the process time and material costs in the step of reducing the thickness of the compound semiconductor substrate 10 to be described. [Application of the diamond particle-containing solution]

[0015] Next, as in Fig. 1A and Fig. Figure 1B shows a solution containing diamond particles being applied to the compound semiconductor substrate 10, thereby forming the liquid film 12 containing the diamond particles on the compound semiconductor substrate 10. Examples of the application method are spin coating, spray coating, and dip coating; spin coating is particularly preferred. In spin coating, the diamond particle-containing solution is applied uniformly to only one of the surfaces of the compound semiconductor substrate 10, on which the diamond particles 14 are to be bonded.

[0016] The average particle size of the diamond particles contained in the diamond particle-containing solution is preferably 1 nm or more and 50 nm or less, particularly preferably 10 nm or less. If the average particle size is 1 nm or more, the diamond particles 14 can be prevented from being ejected from the surface of the compound semiconductor substrate 10 by sputtering in early stages of the growth of the diamond layer 16. If the average particle size is 50 nm or less, a dense polycrystalline diamond can be grown without abnormal growth, and the planarization (polishing) process on the polycrystalline diamond surface can be easily carried out. Diamond particles of such a size can be suitably produced from graphite by a known method such as detonation, implosion, or pulverization.It should be noted that “the average particle size of the diamond particles contained in the diamond particle-containing solution” is calculated on the basis of JIS 8819-2 and means the average particle size calculated under the assumption that the size distribution found with a known laser diffraction particle size analyzer corresponds to a normal distribution.

[0017] Here, the compound semiconductor substrate 10 is typically etched, e.g., with hydrofluoric acid, before being coated with the diamond particle-containing solution to remove metal impurities deposited on its surface. Since the surface of the etched compound semiconductor substrate 10 is an active, repellent surface, the particles readily adhere to it. Therefore, the etched compound semiconductor substrate 10 is preferably washed with pure water or a similar solution to transform its surface into a hydrophilic surface on which a natural oxide film forms. Alternatively, the etched compound semiconductor substrate 10 is preferably left in a cleanroom for an extended period to allow a natural oxide film to form on its surface. This prevents the particles from adhering to the surface of the compound semiconductor substrate 10.At this point, positive fixed charges are generated in the natural oxide film. When the diamond particle-containing solution, which contains negatively charged diamond particles, is applied to the positively charged natural oxide film, the compound semiconductor substrate 10 and the diamond particles 14 are accordingly firmly bonded to one another due to Coulomb attraction. This improves the adhesion between the polycrystalline diamond layer 16 and the compound semiconductor substrate 10. Such negatively charged diamond particles can be obtained by oxidizing the diamond particles to terminate them with carboxyl or ketone groups. Examples of the oxidation process include methods in which the diamond particles are thermally oxidized and methods in which the diamond particles are immersed in an ozone solution, a nitric acid solution, an aqueous hydrogen peroxide solution, or a perchloric acid solution.

[0018] Examples of solvents for the diamond particle solution include water, as well as organic solvents such as methanol, ethanol, 2-propanol, and toluene. One of these solvents can be used alone, or two or more can be used in combination.

[0019] The concentration of diamond particles in the diamond particle-containing solution is preferably 0.03 wt% or more and 10 wt% or less, based on the total diamond particle-containing solution. At a concentration of 0.03 wt% or more, the diamond particles 14 can be uniformly bound to the compound semiconductor substrate 10, and a concentration of 10 wt% or less can prevent the bound diamond particles 14 from growing abnormally during the growth process of the diamond layer 16.

[0020] To improve adhesion between the diamond particles 14 and the compound semiconductor substrate 10, the diamond particle-containing solution is preferably in gel form. Alternatively, a thickening agent may be included in the diamond particle-containing solution. Examples of thickening agents include agar, carrageenan, xanthan gum, gellan gum, guar gum, polyvinyl alcohol, polyacrylate-based thickeners, water-soluble celluloses, and polyethylene oxide. One or more of these may be used. When the thickening agent is included, the pH of the diamond particle-containing solution is preferably in the range of 6 or higher and 8 or lower.

[0021] The diamond particle-containing solution can be prepared by mixing and stirring diamond particles in the aforementioned solvent, such that the diamond particles are dispersed in the solvent. The stirring speed is preferably 500 rpm or more and 3000 rpm or less, and the stirring time is preferably 10 minutes or more and 1 hour or less. [Heat treatment]

[0022] Next, as in Fig. 1B and Fig. As shown in Figure 1C, a heat treatment is performed on the compound semiconductor substrate 10. This evaporates the solvent in the diamond particle-containing liquid film 12 and increases the bond strength between the surface of the compound semiconductor substrate 10 and the diamond particles 14, thereby binding the diamond particles 14 to the compound semiconductor substrate 10. The temperature of the compound semiconductor substrate 10 during the heat treatment is less than 100 °C, 30 °C or more, and preferably 80 °C or less. A temperature of less than 100 °C can prevent bubbles from forming during the boiling of the diamond particle-containing solution, thus avoiding the formation of areas where the diamond particles 14 are not present on the compound semiconductor substrate 10, and also preventing detachment of the polycrystalline diamond layer 16 from these areas.When the temperature is 30 °C or higher, the compound semiconductor substrate 10 and the diamond particles 14 are sufficiently bonded to one another; this prevents the diamond particles 14 from being ejected due to sputtering during the growth of the polycrystalline diamond layer 16 by CVD, and the polycrystalline diamond layer 16 can grow uniformly. The heat treatment time is preferably 1 minute or more and 30 minutes or less. A known heat treatment apparatus can be used; for example, the heat treatment can be carried out by placing the compound semiconductor substrate 10 on a heated hot plate.

[0023] In this embodiment, it is important to use a method in which a solution containing diamond particles is applied to a compound semiconductor substrate and then a heat treatment is performed as described above. This method allows the polycrystalline diamond layer 16 to grow without fracturing the surface of the compound semiconductor substrate 10 and thus without causing a fracture of the compound semiconductor substrate 10. Furthermore, no dislocations are formed in the compound semiconductor layer 18, which is obtained by reducing the thickness of the compound semiconductor substrate 10. [Growth of the polycrystalline diamond layer]

[0024] Next, as in Fig. 1C and Fig. Figure 1D shows the polycrystalline diamond layer 16 with a thickness of 100 µm or more formed on the compound semiconductor substrate 10 by CVD using the diamond particles 14 as nuclei. Plasma-assisted CVD, hot filament CVD, etc., can be used for the CVD process as appropriate.

[0025] When plasma-assisted CVD is used, the polycrystalline diamond layer 16 is grown, for example, by introducing a source gas such as methane into a chamber with hydrogen as the carrier gas, whereby the temperature of the compound semiconductor substrate 10 is controlled to 700 °C or more and 1300 °C or less. To improve the uniformity of the thickness of the polycrystalline diamond layer 16, microwave plasma-assisted CVD is preferably used. Microwave plasma-assisted CVD is a process in which a source gas, such as methane, is digested by microwaves to form a plasma in a plasma chamber, and the plasma-forming source gas is supplied to the compound semiconductor substrate 10, thereby growing the polycrystalline diamond layer 16. The pressure in the plasma chamber, the microwave power, and the temperature of the compound semiconductor substrate 10 are preferably set as follows.The pressure in the plasma chamber is preferably 1.3 × 10. 3 Pa or more and 1.3 × 10 5 Pa or less, especially preferably 1.1 × 10 4 Pa or more and 4.0 × 10 4 Pa or less. The microwave power is preferably 0.1 kW or more and 100 kW or less, more preferably 1 kW or more and 10 kW or less. The temperature of the compound semiconductor substrate 10 is preferably 700 °C or more and 1300 °C or less, more preferably 900 °C or more and 1200 °C or less.

[0026] In hot filament CVD, carbon radicals are generated from a hydrocarbon-based source gas such as methane using a filament of tungsten, tantalum, rhenium, molybdenum, iridium, or the like at a filament temperature of approximately 1900 °C or higher and 2300 °C or lower. The carbon radicals are directed onto the heated compound semiconductor substrate 10, thereby growing the polycrystalline diamond layer 16. Hot filament CVD can be readily applied to larger diameter wafers. The pressure in the chamber, the distance between the filament and the compound semiconductor substrate 10, and the temperature of the compound semiconductor substrate 10 are preferably set as follows. The pressure in the chamber is preferably 1.3 × 10⁻⁶ 3 Pa or more and 1.3 × 10 5Pa or less. The distance between the filament and the compound semiconductor substrate 10 is preferably 5 mm or more and 20 mm or less. The temperature of the compound semiconductor substrate 10 is preferably 700 °C or more and 1300 °C or less.

[0027] Since the polycrystalline diamond layer 16 serves as a support substrate for the compound semiconductor layer 18, its thickness is 100 µm or more, preferably 500 µm or more. Although there is no particular upper limit for the thickness of the polycrystalline diamond layer 16, the thickness is preferably 3 mm or less to prevent the growth process from being carried out by CVD for an excessively long time. [Planarization of the polycrystalline diamond layer]

[0028] Next, as in Fig. 1D and Fig. Figure 1E shows that the surface of the polycrystalline diamond layer 16 is planarized. Undesired irregularities are found on the surface of the formed polycrystalline diamond layer 16. Planarizing the surface of the polycrystalline diamond layer 16 makes it possible to securely place and fix the resulting free-standing polycrystalline diamond substrate 100 on a sample holder in a semiconductor processing device. Although the planarization method is not particularly restricted, a known chemical-mechanical polishing (CMP) process is preferably used. It should be noted that even after planarization, the thickness of the polycrystalline diamond layer 16 is 100 µm or more, preferably 500 µm or more. [Thickness reduction of the compound semiconductor substrate]

[0029] Next, as in Fig. 1E and Fig. As shown in Figure 1F, the thickness of the compound semiconductor substrate 10 is reduced to obtain the compound semiconductor layer 18. In particular, the thickness of the compound semiconductor substrate 10 is reduced by grinding and polishing the surface of the compound semiconductor substrate 10 opposite the interface with the polycrystalline diamond layer 16. In this way, the freestanding polycrystalline diamond substrate 100 can be obtained, in which the compound semiconductor layer 18 is formed with a desired thickness on the polycrystalline diamond layer 16, which serves as the support substrate. The thickness of the compound semiconductor layer 18 can be suitably determined depending on the type or structure of the semiconductor device to be formed in the compound semiconductor layer and is preferably 1 µm or more and 500 µm or less.It should be noted that any known or specified grinding and polishing method should preferably be used for grinding and polishing. Specific examples include surface grinding and high-gloss polishing. EXAMPLES (Example 1) [Invention Example 1]

[0030] Through the in Fig. In the steps shown in 1A to 1F, a freestanding polycrystalline diamond substrate was produced according to Invention Example 1.

[0031] First, a GaN substrate with a diameter of 2 in and a thickness of 600 µm was produced by cutting and machining a gallium nitride (GaN) single crystal formed by hydride vapor phase epitaxy (HVPE).

[0032] Next, diamond particles with an average particle diameter of 4 nm were produced by detonation. The diamond particles were immersed in an aqueous hydrogen peroxide solution, resulting in carboxyl group (COOH) termination and thus a negative charge. The diamond particles were then mixed and stirred in a solvent (H₂O) to produce a diamond particle solution with a diamond particle content of 6 wt%. The stirring speed was 1100 rpm, the stirring time was 50 min, and the temperature of the diamond particle solution during stirring was 25 °C. Subsequently, the GaN substrate was purified with pure water, and after a natural oxide film had formed on its surface, the diamond particle solution was centrifugally coated onto a surface of the GaN substrate, thus forming a liquid film containing diamond particles.

[0033] Next, the GaN substrate was placed on a hot plate heated to 80 °C for 5 minutes to perform a heat treatment to strengthen the bond between the GaN substrate and the diamond particles; this bonded the diamond particles to the surface of the GaN substrate.

[0034] Next, a 300 µm thick polycrystalline diamond layer was grown using the microwave plasma-assisted CVD described above, with hydrogen as the carrier gas, methane as the source gas, and the diamond particles bound to the GaN substrate as seeds. It should be noted that the pressure in a plasma chamber was 1.5 × 10 4 Pa was the power of the microwaves 5 kW and the temperature of the GaN substrate 1050 °C.

[0035] Next, the surface of each polycrystalline diamond layer was planarized by CMP. The thickness of the planarized polycrystalline diamond layer was 290 µm.

[0036] Next, the GaN substrate was ground and polished to obtain a GaN layer 10 µm thick. This resulted in a freestanding polycrystalline diamond substrate where the 10 µm thick GaN layer was formed on top of the 290 µm thick polycrystalline diamond layer.

[0037] In this example of the invention, the polycrystalline diamond layer was grown without causing fracture of the GaN substrate. No dislocations were found when examining a cross-section of the GaN layer under TEM. [Comparison example 1-1]

[0038] An attempt was made to produce a freestanding polycrystalline diamond substrate using the same method as in Invention Example 1, except that the method for attaching the diamond particles was changed.

[0039] A GaN substrate similar to that in Invention Example 1 was prepared. Next, diamond particles were embedded into a surface of the GaN substrate using a known scratching method. In particular, the GaN substrate was subjected to ultrasonic cleaning in a solution containing the diamond particles with an average particle size of 1 µm, thereby embedding the diamond particles into the surface of the GaN substrate. Next, an attempt was made to form a polycrystalline diamond layer with a thickness of 300 µm by microwave plasma-assisted CVD under the same conditions as in Invention Example 1, using the diamond particles embedded in the surface of the GaN substrate as nuclei.

[0040] In comparative example 1-1, the GaN substrate fractured during the attempt to form the polycrystalline diamond layer. This is likely due to a crack forming on the surface of the GaN substrate during the embedding process at a high temperature of 1050 °C, which then propagated within the substrate. TEM observation of the fractured area confirmed the presence of the crack at the fracture initiation point. [Comparison example 1-2]

[0041] An attempt was made to produce a freestanding polycrystalline diamond substrate using the same method as in Invention Example 1, except that the thickness of the polycrystalline diamond layer was changed to 5 µm.

[0042] In comparative example 1-2, the GaN substrate, along with the polycrystalline diamond layer, was fractured during grinding of the GaN substrate. It was thus determined that the 5 µm thick polycrystalline diamond layer is not capable of serving as a freestanding substrate. (Example 2)[Invention Example 2]

[0043] A freestanding polycrystalline diamond substrate was produced according to the same method as in Invention Example 1, except that the type of compound semiconductor substrate was changed from the GaN substage to a SiC substrate.

[0044] First, a 4H-SiC substrate with a diameter of 2 in and a thickness of 600 µm was produced by cutting and processing a silicon carbide (SiC) single crystal obtained by a sublimation recrystallization process (physical vapor transport).

[0045] The remainder of the process was carried out in the same steps and under the same conditions as in Invention Example 1, so that a freestanding polycrystalline diamond substrate was obtained in which a SiC layer with a thickness of 10 µm was formed on a polycrystalline diamond layer with a thickness of 460 µm.

[0046] In this example of the invention, the polycrystalline diamond layer was grown without causing fracture of the SiC substrate. No dislocations were found when examining a cross-section of the GaN layer under TEM. [Comparison example 2-1]

[0047] An attempt was made to produce a freestanding polycrystalline diamond substrate using the same method as in Invention Example 2, except that the method of attaching diamond particles was changed.

[0048] A SiC substrate similar to that in Invention Example 2 was prepared. Next, diamond particles were embedded into the surface of the SiC substrate using a known scratching method. In particular, the SiC substrate was subjected to ultrasonic cleaning in a solution containing diamond particles with an average particle size of 1 µm, thereby embedding the diamond particles into the surface of the SiC substrate. Next, an attempt was made to form a polycrystalline diamond layer with a thickness of 460 µm using microwave plasma-assisted CVD under the same conditions as in Invention Example 2, using the diamond particles embedded in the surface of the SiC substrate as nuclei.

[0049] In comparative example 2-1, the SiC substrate fractured during the attempt to form the polycrystalline diamond layer. This is likely due to a crack forming on the surface of the SiC substrate during the embedding process at a high temperature of 1050 °C, which then propagated within the substrate. TEM observation of the fractured area confirmed the presence of the crack at the fracture initiation point. [Comparison example 2-2]

[0050] An attempt was made to produce a freestanding polycrystalline diamond substrate using the same method as in Invention Example 2, except that the thickness of the polycrystalline diamond layer was changed to 5 µm.

[0051] In comparative example 2-2, the SiC substrate, along with the polycrystalline diamond layer, was fractured during grinding of the SiC substrate. It was thus determined that the 5 µm thick polycrystalline diamond layer is not capable of serving as a freestanding substrate. COMMERCIAL APPLICABILITY

[0052] The disclosed method for producing a freestanding polycrystalline diamond substrate according to this embodiment can produce a freestanding polycrystalline diamond substrate in which a high-quality compound semiconductor layer is formed. REFERENCE MARK LIST 100 Freestanding polycrystalline diamond substrate 10 Compound semiconductor substrate Liquid film containing 12 diamond particles 14 diamond particles 16 Polycrystalline diamond layer 18 Compound semiconductor layer

Claims

[1] Method for producing a freestanding polycrystalline diamond substrate (100), the method comprising: a step of applying a solution containing negatively charged diamond particles (14) to a compound semiconductor substrate (10) on which a positively charged natural oxide film is formed and which is not on any other substrate, and subsequently performing a heat treatment on the compound semiconductor substrate (10) at a temperature of 30 °C or more and less than 100 °C, whereby the diamond particles (14) are bonded to the compound semiconductor substrate (10); a step of growing a polycrystalline diamond layer (16) to a thickness of 100 µm or more on the compound semiconductor substrate (10) by chemical vapor deposition using the diamond particles (14) as seeds; and a step of the subsequent reduction of the thickness of the compound semiconductor substrate (10) to obtain a compound semiconductor layer (18), wherein the polycrystalline diamond layer (16) serves as a support substrate for the compound semiconductor layer (18) in the freestanding polycrystalline diamond substrate (100). [2] Method for producing a freestanding polycrystalline diamond substrate (100) according to claim 1, wherein the average particle diameter of the diamond particles (14) in the solution is 50 nm or less. [3] Method for producing a freestanding polycrystalline diamond substrate (100) according to claim 1 or 2, wherein during the heat treatment a temperature of the compound semiconductor substrate (10) is maintained at less than 100 °C for 1 min or more and 30 min or less. [4] Method for producing a freestanding polycrystalline diamond substrate according to any one of claims 1 to 3, further comprising a step for planarizing a surface of the polycrystalline diamond layer (16). [5] Method for producing a freestanding polycrystalline diamond substrate (100) according to any one of claims 1 to 4, wherein the compound semiconductor substrate (10) is made from one of GaN, AlN, InN, SiC, Al2O3, Ga2O3, MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb and SiGe. [6] Method for producing a freestanding polycrystalline diamond substrate (100) according to any one of claims 1 to 5, wherein the thickness of the compound semiconductor layer (18) is 1 µm or more and 500 µm or less.

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