JP7172556B2 - Method for manufacturing polycrystalline diamond free-standing substrate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a polycrystalline diamond self-supporting substrate in which a compound semiconductor layer is formed on a polycrystalline diamond layer as a supporting substrate.

2. Description of the Related Art High-voltage semiconductor devices such as high-frequency devices and power devices have a problem of self-heating of the devices. As a countermeasure against this, a technique of arranging a material with high thermal conductivity under the device forming region is known.

For example, there is known a technique of arranging a diamond layer with high heat dissipation directly under a compound semiconductor layer such as a gallium nitride (GaN) layer, which is a device layer for forming a semiconductor device. Patent Document 1 discloses a method for producing a gallium nitride type wafer on diamond. This method includes the steps of: forming a thin silicon nitride film of 60 nm or less on a GaN layer positioned on a support substrate; growing a diamond layer on the GaN layer through the silicon nitride film by chemical vapor deposition using the obtained diamond particles as nuclei; and removing the supporting substrate, wherein the gallium nitride layer is formed on the diamond. is to manufacture a wafer on which is formed.

Japanese Patent Application Publication No. 2015-509479

However, according to the study of the present inventors, in the method described in Patent Document 1, cracks occur in the GaN layer due to the embedding, and the GaN layer cracks during the subsequent high-temperature, long-time heat treatment by the chemical vapor deposition method. It was found that cracks propagated inside and dislocations were generated. If a semiconductor device is formed on such a GaN layer, leakage current may increase and device characteristics may deteriorate.

In view of the above problems, an object of the present invention is to provide a method for manufacturing a free-standing polycrystalline diamond substrate that can manufacture a free-standing polycrystalline diamond substrate on which high-quality compound semiconductor layers are laminated.

In order to solve the above problems, the present inventors have made intensive studies and obtained the following findings. First, the inventor of the present invention conceived of growing a diamond layer on a compound semiconductor substrate prepared in advance, instead of growing a diamond layer on a compound semiconductor layer positioned on a support substrate as in Patent Document 1. . However, it has been found that when diamond particles are embedded and fixed on the surface of a compound semiconductor substrate in the same manner as in Patent Document 1 and a diamond layer is grown by chemical vapor deposition using the diamond particles as nuclei, the compound semiconductor substrate cracks. . It is presumed that this originated from cracks introduced into the surface of the compound semiconductor substrate due to the embedding.

Accordingly, the present inventor conducted further studies and found that a compound semiconductor substrate is coated with a solution containing diamond particles, and then heat-treated to evaporate the solvent. It was found that the polycrystalline diamond layer could be grown without cracking the compound semiconductor substrate. After that, no dislocation occurred in the compound semiconductor layer obtained by reducing the thickness of the compound semiconductor substrate.

The gist and configuration of the present invention completed based on the above findings are as follows.
(1) a step of applying a solution containing diamond particles onto a compound semiconductor substrate and then subjecting the compound semiconductor substrate to a heat treatment to adhere the diamond particles onto the compound semiconductor substrate;
growing a polycrystalline diamond layer having a thickness of 100 μm or more on the compound semiconductor substrate by chemical vapor deposition using the diamond grains as nuclei;
Thereafter, a step of reducing the thickness of the compound semiconductor substrate to form a compound semiconductor layer;
and wherein the polycrystalline diamond layer functions as a support substrate for the compound semiconductor layer.

(2) The method for producing a free-standing polycrystalline diamond substrate according to (1) above, wherein the diamond particles in the solution have an average particle size of 50 nm or less.

(3) The method for producing a free-standing polycrystalline diamond substrate according to (1) or (2) above, wherein the diamond particles in the solution are negatively charged.

(4) Manufacturing a free-standing polycrystalline diamond substrate according to any one of (1) to (3) above, wherein in the heat treatment, the temperature of the compound semiconductor substrate is kept below 100° C. for 1 minute or more and 30 minutes or less. Method.

(5) The method for producing a free-standing polycrystalline diamond substrate according to any one of (1) to (4) above, further comprising the step of flattening the surface of the polycrystalline diamond layer.

(6) The compound semiconductor substrate is made of GaN, AlN, InN, SiC, _{Al2O3 , Ga2O3 ,} MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb, or SiGe. A method for producing a free-standing polycrystalline diamond substrate according to any one of (1) to (5) above.

(7) The method for producing a free-standing polycrystalline diamond substrate according to any one of (1) to (7) above, wherein the compound semiconductor layer has a thickness of 1 μm or more and 500 μm or less.

According to the method for producing a free-standing polycrystalline diamond substrate of the present invention, it is possible to produce a free-standing polycrystalline diamond substrate on which high-quality compound semiconductor layers are laminated.

(A) to (F) are schematic cross-sectional views illustrating a method for manufacturing a free-standing polycrystalline diamond substrate 100 according to one embodiment of the present invention.

(Manufacturing method for polycrystalline diamond free-standing substrate)
Referring to FIG. 1, a method for manufacturing a free-standing polycrystalline diamond substrate 100 according to one embodiment of the present invention has the following steps. First, as shown in FIGS. 1A and 1B, a compound semiconductor substrate 10 is coated with a solution containing diamond particles. Thus, a diamond particle-containing liquid film 12 is formed on the compound semiconductor substrate 10 . Thereafter, as shown in FIGS. 1B and 1C, the compound semiconductor substrate 10 is subjected to a heat treatment to evaporate the solvent in the diamond particle-containing liquid film 12, and to separate the surface of the compound semiconductor substrate 10 from the diamond particles. 14 is strengthened to adhere the diamond particles 14 onto the compound semiconductor substrate 10 . After that, as shown in FIGS. 1(C) and 1(D), a diamond particle 14 is used as a nucleus to form a layer having a thickness of 100 μm or more on the compound semiconductor substrate 10 by chemical vapor deposition (CVD). A polycrystalline diamond layer 16 is grown. Optionally, the surface of polycrystalline diamond layer 16 may then be planarized, as shown in FIGS. Thereafter, as shown in FIGS. 1E and 1F, the thickness of the compound semiconductor substrate 10 is reduced to form a compound semiconductor layer 18. Next, as shown in FIG.

In this embodiment, the polycrystalline diamond free-standing substrate 100 in which the polycrystalline diamond layer 16 functions as a support substrate for the compound semiconductor layer 18 can be manufactured through the above steps. Here, the compound semiconductor layer 18 becomes a device layer for forming a semiconductor device. Each step in this embodiment will be described in detail below.

[Preparation of compound semiconductor substrate]
Referring to FIG. 1A, first, a compound semiconductor substrate 10 is prepared. The compound semiconductor forming the compound semiconductor substrate 10 is not particularly limited, and may be appropriately selected according to the type of semiconductor device to be formed on the compound semiconductor layer 18. Examples include GaN, AlN, InN, SiC, and _Al2O . ₃ , Ga ₂ O ₃ , MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb, or SiGe. Moreover, 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 is warped, resulting in peeling of the polycrystalline diamond or cracking of the compound semiconductor substrate. Further, if the thickness exceeds 3 mm, it is not preferable from the viewpoint of process time and material cost in the step of reducing the thickness of the compound semiconductor substrate 10, which will be described later.

[Application of solution containing diamond particles]
Next, as shown in FIGS. 1A and 1B, a solution containing diamond particles is applied onto the compound semiconductor substrate 10 to form a diamond particle-containing liquid film 12 on the compound semiconductor substrate 10 . Examples of the coating method include a spin coating method, a spray method, and an immersion method, with the spin coating method being particularly preferred. According to the spin coating method, the diamond particle-containing solution can be uniformly coated only on one surface of the compound semiconductor substrate 10 to which the diamond particles 14 are to be attached.

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, more preferably 10 nm or less. When the thickness is 1 nm or more, it is possible to suppress the phenomenon in which the diamond grains 14 are sputtered off the surface of the compound semiconductor substrate 10 in the initial stage of growing the polycrystalline diamond layer 16. When the thickness is 50 nm or less, many This is because the crystalline diamond layer can be densely formed without abnormal growth, and the surface of the polycrystalline diamond can be easily planarized (polished). Diamond particles of such a size can be suitably produced from graphite by a known detonation method, implosion method, or pulverization method. The "average particle diameter of the diamond particles contained in the diamond particle-containing solution" is calculated according to JIS 8819-2, and the particle size distribution measured by a known laser diffraction particle size distribution measuring device follows a normal distribution. Means the average particle size calculated on the assumption that

Here, the compound semiconductor substrate 10 before being coated with the diamond particle-containing solution is generally acid-cleaned with hydrofluoric acid or the like in order to remove metal impurities adhering to its surface. Since the surface of the acid-cleaned compound semiconductor substrate 10 is an active water-repellent surface, particles tend to adhere to the surface. Therefore, it is preferable to wash the acid-cleaned compound semiconductor substrate 10 with pure water or the like to make the surface of the compound semiconductor substrate 10 hydrophilic with a natural oxide film formed thereon. Alternatively, it is preferable to leave the acid-cleaned compound semiconductor substrate 10 in a clean room for a long time to form a natural oxide film on the surface of the compound semiconductor substrate 10 . This can prevent particles from adhering to the surface of the compound semiconductor substrate 10 . At this time, fixed charges having positive charges are generated in the native oxide film. Therefore, when a diamond particle-containing solution containing negatively charged diamond particles is applied onto the positively charged natural oxide film, the compound semiconductor substrate 10 and the diamond particles 14 are strongly bonded by Coulomb attraction. . As a result, the adhesion of the polycrystalline diamond layer 16 to the compound semiconductor substrate 10 is improved. Such negatively charged diamond particles can be obtained by subjecting the diamond particles to an oxidation treatment and terminating the diamond particles with carboxyl groups or ketone groups. For example, the oxidation treatment includes a method of oxidizing the diamond particles, a method of immersing the diamond particles in an ozone solution, a nitric acid solution, an aqueous hydrogen peroxide solution, or a perchloric acid solution.

Examples of the solvent for the diamond particle-containing solution include water and organic solvents such as methanol, ethanol, 2-propanol, and toluene. These solvents may be used alone or in combination of two or more. may

The content of diamond particles in the diamond particle-containing solution is preferably 0.03% by mass or more and 10% by mass or less with respect to the entire diamond particle-containing solution. If it is 0.03% by mass or more, the diamond particles 14 can be uniformly deposited on the compound semiconductor substrate 10, and if it is 10% by mass or less, the deposited diamond particles 14 are abnormal during the growth process of the diamond layer 16. This is because the growth can be suppressed.

From the viewpoint of improving the adhesion between the diamond particles 14 and the compound semiconductor substrate 10, the diamond particle-containing solution is preferably gel-like, and the diamond particle-containing solution may contain a thickening agent. Examples of thickeners 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. can be done. When a thickener is included, the pH of the diamond particle-containing solution is preferably in the range of 6 or more and 8 or less.

The diamond particle-containing solution may be prepared by mixing the diamond particles with the above solvent and stirring to disperse the diamond particles 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]
Next, as shown in FIGS. 1B and 1C, the compound semiconductor substrate 10 is heat-treated. As a result, the solvent in the diamond particle-containing liquid film 12 evaporates and the bonding force between the surface of the compound semiconductor substrate 10 and the diamond particles 14 is strengthened, so that the diamond particles 14 adhere to the compound semiconductor substrate 10 . The temperature of the compound semiconductor substrate 10 during the heat treatment is preferably less than 100.degree. C., more preferably 30.degree. C. or higher and 80.degree. If the temperature is less than 100° C., the generation of bubbles due to the boiling of the diamond particle-containing solution can be suppressed. There is no possibility that the polycrystalline diamond layer 16 will peel off starting from the site. If the temperature is 30° C. or higher, the compound semiconductor substrate 10 and the diamond grains 14 are sufficiently bonded, so that the diamond grains 14 are prevented from being sputtered off during the growth of the polycrystalline diamond layer 16 by the CVD method. and the polycrystalline diamond layer 16 can be grown uniformly. Moreover, the heat treatment time is preferably 1 minute or more and 30 minutes or less. As the heat treatment apparatus, a known heat treatment apparatus may be used. For example, the heat treatment can be performed by placing the compound semiconductor substrate 10 on a heated hot plate.

In this embodiment, as described above, it is essential to apply a solution containing diamond particles onto the compound semiconductor substrate and then apply a heat treatment. According to this method, cracks are not introduced into the surface of the compound semiconductor substrate 10, so that the polycrystalline diamond layer 16 can be grown without cracking the compound semiconductor substrate 10. FIG. Further, no dislocation occurs in the compound semiconductor layer 18 obtained by reducing the thickness of the compound semiconductor substrate 10 .

[Growth of polycrystalline diamond layer]
Next, as shown in FIGS. 1(C) and 1(D), a polycrystalline diamond layer 16 having a thickness of 100 μm or more is grown on the compound semiconductor substrate 10 by CVD using the diamond grains 14 as nuclei. As the CVD method, a plasma CVD method, a hot filament CVD method, or the like can be preferably used.

When the plasma CVD method is used, for example, a source gas such as methane is introduced into the chamber using hydrogen as a carrier gas, the temperature of the compound semiconductor substrate 10 is set to 700° C. or higher and 1300° C. or lower, and the polycrystalline diamond layer 16 is grown. Let From the viewpoint of improving the uniformity of the thickness of the polycrystalline diamond layer 16, it is preferable to use the microwave plasma CVD method. In the microwave plasma CVD method, a source gas such as methane is decomposed by microwaves in a plasma chamber to generate plasma, and the plasma generated source gas is led onto a heated compound semiconductor substrate 10 to form a polycrystalline diamond layer 16. is a way to grow Here, it is preferable to set the pressure in the plasma chamber, the output of the microwave, and the temperature of the compound semiconductor substrate 10 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, more preferably 1.1×10 ⁴ Pa or more and 4.0×10 ⁴ Pa or less. preferable. The output of the microwave 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 higher and 1300° C. or lower, more preferably 900° C. or higher and 1200° C. or lower.

When the hot filament CVD method is used, a filament made of tungsten, tantalum, rhenium, molybdenum, iridium, or the like is used, the filament temperature is set to about 1900° C. or higher and 2300° C. or lower, and carbon radicals are generated from a hydrocarbon-based source gas such as methane. Generate. A polycrystalline diamond layer 16 is grown by guiding the carbon radicals onto the heated compound semiconductor substrate 10 . According to the hot filament CVD method, it is possible to easily cope with an increase in the diameter of the wafer. Here, 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 ³ Pa or more and 1.3×10 ⁵ Pa 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 higher and 1300° C. or lower.

Since the polycrystalline diamond layer 16 functions as a support substrate for the compound semiconductor layer 18, its thickness is set to 100 μm or more, more preferably 500 μm or more. Although the upper limit of the thickness of the polycrystalline diamond layer 16 is not particularly limited, it is preferably 3 mm or less from the viewpoint of not excessively increasing the process time during growth by the CVD method.

[Planarization of polycrystalline diamond layer]
Next, as shown in FIGS. 1(D) and 1(E), the surface of the polycrystalline diamond layer 16 may be planarized. Excessive unevenness is formed on the surface of the polycrystalline diamond layer 16 after film formation. By flattening the surface of the polycrystalline diamond layer 16, the polycrystalline diamond free-standing substrate 100 to be obtained later can be reliably set (chucked) on the sample table of the semiconductor processing equipment. Although the planarization method is not particularly limited, for example, a known chemical mechanical polishing (CMP) method can be preferably used. Even after flattening, the thickness of the polycrystalline diamond layer 16 is 100 μm or more, and more preferably 500 μm or more.

[Thickness reduction of compound semiconductor substrate]
Next, as shown in FIGS. 1E and 1F, the thickness of the compound semiconductor substrate 10 is reduced to form a compound semiconductor layer 18 . Specifically, the thickness of the compound semiconductor substrate 10 is reduced by grinding and polishing the surface opposite to the interface with the polycrystalline diamond layer 16 . As a result, a polycrystalline diamond self-supporting substrate 100 can be obtained in which the compound semiconductor layer 18 having a desired thickness is laminated on the polycrystalline diamond layer 16 as a supporting substrate. The thickness of the compound semiconductor layer 18 can be appropriately determined according to the type and structure of the semiconductor device formed thereon, and is preferably 1 μm or more and 500 μm or less. For this grinding and polishing, a known or arbitrary grinding method and polishing method can be suitably used, and specifically, a surface grinding method and a mirror polishing method can be used.

(Example 1)
[Invention Example 1]
A free-standing polycrystalline diamond substrate according to Example 1-1 of the present invention was produced through the steps shown in FIGS.

First, a GaN substrate having a diameter of 2 inches and a thickness of 600 μm was prepared by cutting from a gallium nitride (GaN) single crystal produced by the HVPE (Hydride Vapor Phase Epitaxy) method.

Next, diamond particles having an average particle size of 4 nm were prepared by a detonation method. The diamond particles were immersed in an aqueous hydrogen peroxide solution to terminate with carboxyl groups (COOH) and to be negatively charged. Next, diamond particles were mixed with a solvent (H ₂ O) and stirred to prepare a diamond particle-containing solution having a diamond particle content of 6% by mass. The stirring speed was 1100 rpm, the stirring time was 50 minutes, and the temperature of the diamond particle-containing solution during stirring was 25°C. Subsequently, the GaN substrate was washed with pure water to form a natural oxide film on the surface, and then a diamond particle-containing solution was applied onto the GaN substrate by spin coating to form a diamond particle-containing liquid film.

Next, the GaN substrate was placed on a hot plate set at 80° C. for 5 minutes to perform a heat treatment for strengthening the bond between the GaN substrate and the diamond particles, thereby adhering the diamond particles to the GaN substrate.

Next, using hydrogen as a carrier gas and methane as a source gas, a polycrystalline diamond layer having a thickness of 300 μm was grown using the diamond particles adhering to the GaN substrate as nuclei using the above-described microwave plasma CVD method. . The pressure in the plasma chamber was 1.5×10 ⁴ Pa, the microwave output was 5 kW, and the temperature of the GaN substrate was 1050°C.

Next, the surface of the polycrystalline diamond layer was flattened by CMP. The thickness of the polycrystalline diamond layer after flattening was 290 μm.

Next, the GaN substrate was ground and polished to form a 10 μm thick GaN layer. Thus, a free-standing polycrystalline diamond substrate was obtained in which a 10 μm thick GaN layer was laminated on a 290 μm thick polycrystalline diamond layer.

In the example of the present invention, the polycrystalline diamond layer could be grown without cracking the GaN substrate. When the cross section of the GaN layer was observed with a TEM, no dislocations were observed.

[Comparative Example 1-1]
An attempt was made to fabricate a free-standing polycrystalline diamond substrate in the same manner as in Invention Example 1, except that the method of adhering diamond particles was changed.

A GaN substrate similar to that of Invention Example 1 was prepared. Next, diamond particles were embedded in the surface of the GaN substrate by a known scratching method. That is, by ultrasonically cleaning the GaN substrate in a solution containing diamond particles with an average particle size of 1 μm, diamond particles were embedded in the surface of the GaN substrate. Next, under the same conditions as in Inventive Example 1, a microwave plasma CVD method was used to form a polycrystalline diamond layer having a thickness of 300 μm using the diamond grains embedded in the GaN substrate as nuclei.

In Comparative Example 1-1, the GaN substrate cracked during polycrystalline diamond film formation. This is presumably because cracks introduced into the surface of the GaN substrate due to embedding started and propagated through the GaN substrate during polycrystalline diamond film formation at a high temperature of 1050°C. As a result of observing the cracked portion with a TEM, it was found that a crack was present at the starting point of the crack.

[Comparative Example 1-2]
An attempt was made to fabricate a free-standing polycrystalline diamond substrate in the same manner as in Invention Example 1, except that the thickness of the polycrystalline diamond layer was changed to 5 μm.

In Comparative Example 1-2, the GaN substrate cracked together with the polycrystalline diamond layer during the grinding process of the GaN substrate. That is, it was found that a polycrystalline diamond layer with a thickness of 5 μm does not function as a free-standing substrate.

(Example 2)
[Invention Example 2]
A free-standing polycrystalline diamond substrate was produced in the same manner as in Invention Example 1, except that the type of compound semiconductor substrate was changed from a GaN substrate to a SiC substrate.

First, a 4H—SiC substrate having a diameter of 2 inches and a thickness of 600 μm was prepared by cutting from a silicon carbide (SiC) single crystal produced by a sublimation recrystallization method.

Thereafter, in the same procedure and under the same conditions as in Invention Example 1, a polycrystalline diamond free-standing substrate was obtained in which a 10 μm thick SiC layer was laminated on a 460 μm thick polycrystalline diamond layer.

In the present invention example, the polycrystalline diamond layer could be grown without cracking the SiC substrate. When the cross section of the SiC layer was observed with a TEM, no dislocation was observed.

[Comparative Example 2-1]
An attempt was made to fabricate a free-standing polycrystalline diamond substrate in the same manner as in Invention Example 2, except that the method of adhering diamond particles was changed.

A SiC substrate similar to that of Invention Example 2 was prepared. Next, diamond particles were embedded in the surface of the SiC substrate by a known scratching method. That is, diamond particles were embedded in the surface of the SiC substrate by ultrasonically cleaning the SiC substrate in a solution containing diamond particles with an average particle diameter of 1 μm. Next, under the same conditions as in Example 2, an attempt was made to form a polycrystalline diamond layer having a thickness of 460 μm by microwave plasma CVD using the diamond grains embedded in the SiC substrate as nuclei.

In Comparative Example 2-1, the SiC substrate cracked during polycrystalline diamond film formation. This is probably because cracks introduced into the surface of the SiC substrate due to the embedding started and propagated through the SiC substrate during polycrystalline diamond film formation at a high temperature of 1050°C. As a result of observing the cracked portion with a TEM, it was found that a crack was present at the starting point of the crack.

[Comparative Example 2-2]
An attempt was made to fabricate a free-standing polycrystalline diamond substrate in the same manner as in Invention Example 2, except that the thickness of the polycrystalline diamond layer was changed to 5 μm.

In Comparative Example 2-2, the SiC substrate cracked together with the polycrystalline diamond layer during the grinding of the SiC substrate. That is, it was found that a polycrystalline diamond layer with a thickness of 5 μm does not function as a free-standing substrate.

According to the method for producing a free-standing polycrystalline diamond substrate of the present invention, it is possible to produce a free-standing polycrystalline diamond substrate on which high-quality compound semiconductor layers are laminated.

REFERENCE SIGNS LIST 100 polycrystalline diamond free-standing substrate 10 compound semiconductor substrate 12 diamond particle-containing liquid film 14 diamond particles 16 polycrystalline diamond layer 18 compound semiconductor layer

Claims (6)

A solution containing negatively charged diamond particles terminated with a carboxyl group or a ketone group is applied onto a compound semiconductor substrate on which a positively charged native oxide film is formed, and then the compound semiconductor substrate is coated. a step of attaching the diamond particles onto the compound semiconductor substrate by subjecting the substrate to a heat treatment;
growing a polycrystalline diamond layer having a thickness of 100 μm or more on the compound semiconductor substrate by chemical vapor deposition using the diamond grains as nuclei;
Thereafter, a step of reducing the thickness of the compound semiconductor substrate to form a compound semiconductor layer;
and wherein the polycrystalline diamond layer functions as a support substrate for the compound semiconductor layer. 2. The method for producing a free-standing polycrystalline diamond substrate according to claim 1, wherein said diamond particles in said solution have an average particle size of 50 nm or less. 3. The method for producing a free-standing polycrystalline diamond substrate according to claim 1, wherein in said heat treatment, the temperature of said compound semiconductor substrate is kept below 100[deg.] C. for 1 minute or more and 30 minutes or less. The method for producing a polycrystalline diamond free-standing substrate according to any one of claims 1 to 3, further comprising the step of flattening the surface of said polycrystalline diamond layer. 2. The compound semiconductor substrate of claim 1, comprising GaN, AlN, InN, SiC, _{Al2O3 , Ga2O3 ,} MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb, or SiGe. 5. A method for producing a free-standing polycrystalline diamond substrate according to any one of items 1 to 4. 6. The method for producing a free-standing polycrystalline diamond substrate according to claim 1, wherein the compound semiconductor layer has a thickness of 1 μm or more and 500 μm or less.

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