阅读说明：本技术 多晶金刚石自立基板的制造方法 (Method for manufacturing polycrystalline diamond self-supporting substrate ) 是由 古贺祥泰 于 2019-10-10 设计创作，主要内容包括：本发明提供一种能够制造层叠有高品质的化合物半导体层的多晶金刚石自立基板的多晶金刚石自立基板的制造方法。将含有金刚石粒子的溶液涂布于化合物半导体基板(10)上,然后,对化合物半导体基板(10)实施热处理,由此使金刚石粒子(14)附着于化合物半导体基板(10)上。以金刚石粒子(14)为核心,通过化学气相沉积法,在化合物半导体基板(10)上使厚度为100μm以上的多晶金刚石层(16)生长。然后,对化合物半导体基板(10)进行减厚而形成化合物半导体层(18)。经这些工序,获得多晶金刚石层(16)作为化合物半导体层(18)的支撑基板而发挥作用的多晶金刚石自立基板(100)。(Provided is a method for manufacturing a polycrystalline diamond self-supporting substrate, wherein a polycrystalline diamond self-supporting substrate on which high-quality compound semiconductor layers are laminated can be manufactured. A solution containing diamond particles is applied to a compound semiconductor substrate (10), and then the compound semiconductor substrate (10) is subjected to a heat treatment, whereby the diamond particles (14) are attached to the compound semiconductor substrate (10). A polycrystalline diamond layer (16) having a thickness of 100 [ mu ] m or more is grown on a compound semiconductor substrate (10) by a chemical vapor deposition method using diamond particles (14) as cores. Then, the compound semiconductor substrate (10) is reduced in thickness to form a compound semiconductor layer (18). Through these steps, a polycrystalline diamond free-standing substrate (100) is obtained in which the polycrystalline diamond layer (16) functions as a support substrate for the compound semiconductor layer (18).)

1. A method for manufacturing a polycrystalline diamond self-supporting substrate, comprising:

a step of applying a solution containing diamond particles on a compound semiconductor substrate, and then performing heat treatment on the compound semiconductor substrate to thereby attach the diamond particles to the compound semiconductor substrate;

a step of growing a polycrystalline diamond layer having a thickness of 100 μm or more on the compound semiconductor substrate by a chemical vapor deposition method using the diamond particles as cores; and

then, a step of forming a compound semiconductor layer by reducing the thickness of the compound semiconductor substrate,

a polycrystalline diamond self-supporting substrate in which the polycrystalline diamond layer functions as a support substrate for the compound semiconductor layer is obtained.

2. The method of manufacturing a polycrystalline diamond free-standing substrate according to claim 1,

the diamond particles in the solution have an average particle diameter of 50nm or less.

3. The method of manufacturing a polycrystalline diamond free-standing substrate according to claim 1 or 2,

the diamond particles in the solution are negatively charged.

4. The method of manufacturing a polycrystalline diamond free-standing substrate according to any one of claims 1 to 3,

in the heat treatment, the temperature of the compound semiconductor substrate is maintained at a temperature of less than 100 ℃ for 1 minute or more and 30 minutes or less.

5. A method of fabricating a polycrystalline diamond free-standing substrate according to any one of claims 1 to 4, further comprising:

and a step of planarizing the surface of the polycrystalline diamond layer.

6. The method of manufacturing a polycrystalline diamond free-standing substrate according to any one of claims 1 to 5,

the compound semiconductor substrate is made of GaN, AlN, InN, SiC and Al₂O₃、Ga₂O₃MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb or SiGe.

7. The method of manufacturing a polycrystalline diamond free-standing substrate according to any one of claims 1 to 6,

the thickness of the compound semiconductor layer is set to be 1 [ mu ] m or more and 500 [ mu ] m or less.

Technical Field

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

Background

In a high-voltage semiconductor device such as a high-frequency device or a power device, self-heating of the device becomes a problem. As a countermeasure, a technique is known in which a material having a large thermal conductivity is disposed below the device formation region.

For example, a technique is known in which a diamond layer having high heat dissipation properties is disposed directly below a compound semiconductor layer such as a gallium nitride (GaN) layer to be a device layer for forming a semiconductor device. Patent document 1 discloses a method for producing a gallium nitride-on-diamond wafer. The method comprises the following steps: forming a thin silicon nitride film of 60nm or less on a GaN layer on a support substrate, and then embedding and fixing diamond particles on the surface of the silicon nitride film by dry scraping; a step of growing a diamond layer on the GaN layer through the silicon nitride film by a chemical vapor deposition method using the diamond particles fixed to the surface as cores; and a step of removing the support substrate to produce a wafer having a gallium nitride layer formed on diamond.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 2015-509479

Disclosure of Invention

Technical problem to be solved by the invention

However, according to the study of the present inventors, it is judged that in the method described in patent document 1, cracks appear in the GaN layer due to the burying, and the cracks propagate in the GaN layer during the high-temperature long-time heat treatment by the chemical vapor deposition method thereafter, and dislocations occur. When a semiconductor device is formed on such a GaN layer, leakage current increases, which may deteriorate device characteristics.

In view of the above-described problems, an object of the present invention is to provide a method for manufacturing a polycrystalline diamond self-supporting substrate, which is capable of manufacturing a polycrystalline diamond self-supporting substrate in which high-quality compound semiconductor layers are stacked.

Means for solving the technical problem

The present inventors have conducted extensive studies to solve the above-mentioned problems and have obtained the following findings. First, the present inventors conceived a method of growing a diamond layer on a compound semiconductor substrate prepared in advance, instead of growing a diamond layer on a compound semiconductor layer located on a support substrate as in patent document 1. However, it was found that, when diamond particles are embedded and fixed in the surface of a compound semiconductor substrate and a diamond layer is grown by a chemical vapor deposition method with the diamond particles as cores, the compound semiconductor substrate is broken, as in patent document 1. It is presumed that this also causes a crack introduced into the surface of the compound semiconductor substrate due to the embedding to serve as a starting point.

As a result of further studies, the present inventors have found that, in a method in which a solution containing diamond particles is applied to a compound semiconductor substrate and then heat-treated to evaporate the solvent, the compound semiconductor substrate can be grown without cracking the compound semiconductor substrate by attaching the diamond particles to the compound semiconductor substrate. After that, it was found that dislocations did not occur in the compound semiconductor layer obtained by thinning the compound semiconductor substrate.

The gist of the present invention achieved based on the above findings is as follows.

(1) A method for manufacturing a polycrystalline diamond self-supporting substrate, comprising:

a step of applying a solution containing diamond particles on a compound semiconductor substrate, and then performing heat treatment on the compound semiconductor substrate to thereby attach the diamond particles to the compound semiconductor substrate;

a step of growing a polycrystalline diamond layer having a thickness of 100 μm or more on the compound semiconductor substrate by a chemical vapor deposition method using the diamond particles as cores; and

then, a step of forming a compound semiconductor layer by reducing the thickness of the compound semiconductor substrate,

a polycrystalline diamond self-supporting substrate in which the polycrystalline diamond layer functions as a support substrate for the compound semiconductor layer is obtained.

(2) The method for producing a polycrystalline diamond self-supporting substrate according to the item (1), wherein an average particle diameter of the diamond particles in the solution is 50nm or less.

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

(4) The method for producing a polycrystalline diamond self-supporting substrate according to any one of the above (1) to (3), wherein in the heat treatment, the temperature of the compound semiconductor substrate is maintained at a temperature of less than 100 ℃ for 1 minute or more and 30 minutes or less.

(5) The method of producing a polycrystalline diamond self-supporting substrate according to any one of the above (1) to (4), further comprising a step of planarizing the surface of the polycrystalline diamond layer.

(6) The method for producing a polycrystalline diamond self-supporting substrate according to any one of the above (1) to (5), wherein the compound semiconductor substrate is made of GaN, AlN, InN, SiC, Al₂O₃、Ga₂O₃MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb or SiGe.

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

Effects of the invention

According to the method for producing a polycrystalline diamond self-supporting substrate of the present invention, a polycrystalline diamond self-supporting substrate in which high-quality compound semiconductor layers are stacked can be produced.

Drawings

Fig. 1(a) to (F) are schematic cross-sectional views illustrating a method for manufacturing a polycrystalline diamond self-supporting substrate 100 according to an embodiment of the present invention.

Detailed Description

(method of manufacturing polycrystalline Diamond self-supporting substrate)

Referring to fig. 1, a method of manufacturing a polycrystalline diamond self-supporting substrate 100 according to an embodiment of the present invention includes the following steps. First, as shown in fig. 1(a) and (B), a solution containing diamond particles is applied to a compound semiconductor substrate 10. Thereby, the liquid film 12 containing diamond particles is formed on the compound semiconductor substrate 10. Then, as shown in fig. 1(B) and (C), the compound semiconductor substrate 10 is subjected to heat treatment, thereby evaporating the solvent in the liquid film 12 containing the diamond particles, and strengthening the bonding force between the surface of the compound semiconductor substrate 10 and the diamond particles 14, thereby adhering the diamond particles 14 to the compound semiconductor substrate 10. Then, as shown in FIGS. 1C and D, a polycrystalline diamond layer 16 having a thickness of 100 μm or more is grown on the compound semiconductor substrate 10 by a Chemical Vapor Deposition (CVD) method using the diamond particles 14 as cores. Then, as shown in fig. 1(D), (E), the surface of the polycrystalline diamond layer 16 may be optionally planarized. Then, as shown in fig. 1(E) and (F), the compound semiconductor substrate 10 is reduced in thickness to form a compound semiconductor layer 18.

In the present embodiment, through the above steps, a polycrystalline diamond self-supporting substrate 100 in which the polycrystalline diamond layer 16 functions as a support substrate for the compound semiconductor layer 18 can be manufactured. Here, the compound semiconductor layer 18 becomes a device layer for forming a semiconductor device. Hereinafter, each step in the present embodiment will be described in detail.

[ preparation of Compound semiconductor substrate ]

Referring to fig. 1(a), first, a compound semiconductor substrate 10 is prepared. The compound semiconductor constituting the compound semiconductor substrate 10 is not particularly limited, and may be appropriately selected according to the type of semiconductor device formed in the compound semiconductor layer 18, and for example, is preferably made of GaN, A1N, InN, SiC, or Al₂O₃、Ga₂O₃MgO, ZnO, CdO, GaAs, GaP, GaSb, InP, InAs, InSb or SiGe. The thickness of the compound semiconductor substrate 10 is preferably 200 μm or more and 3mm or less. When less than 200 μm, peeling of polycrystalline diamond occurs due to warping of the compound semiconductor substrate, or cracking of the compound semiconductor substrate occurs. When the average particle diameter exceeds 3mm, the average particle diameter is determined from the compounds described laterThis is not preferable from the viewpoint of processing time and material cost in the thickness reducing step of the semiconductor substrate 10.

[ coating of the solution containing Diamond particles ]

Next, as shown in fig. 1(a) and (B), a solution containing diamond particles is applied to the compound semiconductor substrate 10, and a liquid film 12 containing diamond particles is formed on the compound semiconductor substrate 10. The coating method includes a spin coating method, a spray coating method, and a dipping method, and particularly, a spin coating method is preferable. According to the spin coating method, the solution containing diamond particles can be uniformly applied only to the surface on which the diamond particles 14 are desired to be attached, of the two surfaces of the compound semiconductor substrate 10.

The average particle diameter of the diamond particles contained in the diamond particle-containing solution is preferably 1nm or more and 50nm or less, and more preferably 10nm or less. This is because, if the thickness is 1nm or more, the phenomenon in which the diamond particles 14 fly off from the surface of the compound semiconductor substrate 10 by sputtering can be suppressed in the initial stage of growing the polycrystalline diamond layer 16, and if the thickness is 50nm or less, the polycrystalline diamond layer can be formed densely without abnormal growth, and the planarization (polishing) process of the polycrystalline diamond surface can be easily performed. Diamond particles having such a size can be suitably produced from graphite by a known detonation method, implosion method, or pulverization method. Further, "the average particle diameter of diamond particles contained in a diamond particle-containing solution" is an average particle diameter calculated according to JIS 8819-2, and represents an average particle diameter calculated assuming that a particle size distribution measured by a well-known laser diffraction type particle size distribution measuring apparatus follows a normal distribution.

Here, the compound semiconductor substrate 10 before being coated with the solution containing diamond particles is usually subjected to acid cleaning using hydrofluoric acid or the like in order to remove metal impurities adhering to the surface thereof. Since the surface of the pickled compound semiconductor substrate 10 is an active hydrophobic surface, fine particles are likely to adhere to the surface. Therefore, it is preferable to wash the compound semiconductor substrate 10 after the acid washing with pure water or the like to make the surface of the compound semiconductor substrate 10 a hydrophilic surface on which a natural oxide film is formed. Alternatively, it is preferable that the compound semiconductor substrate 10 after the acid cleaning is placed 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 natural oxide film. Therefore, when a solution containing diamond particles containing negatively charged diamond particles is applied to 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. In this manner, the diamond particles are subjected to oxidation treatment, and the diamond particles are terminated with a carboxyl group or a ketone group, thereby obtaining negatively charged diamond particles. For example, the oxidation treatment may be a method of thermally oxidizing diamond particles, a method of immersing diamond particles in an ozone solution, a nitric acid solution, a hydrogen peroxide solution, or a perchloric acid solution, or the like.

The solvent of the diamond particle-containing solution may be an organic solvent such as methanol, ethanol, 2-propanol, or toluene, in addition to water, and these solvents may be used alone or in combination of two or more.

The content of diamond particles in the diamond particle-containing solution is preferably 0.03 mass% to 10 mass% with respect to the total amount of the diamond particle-containing solution. This is because, if 0.03 mass% or more, the diamond particles 14 can be uniformly adhered to the compound semiconductor substrate 10, and if 10 mass% or less, abnormal growth of the adhered diamond particles 14 during the growth of the diamond layer 16 can be suppressed.

From the viewpoint of improving the adhesion between the diamond particles 14 and the compound semiconductor substrate 10, the solution containing diamond particles is preferably in a gel state, and a thickening agent may be contained in the solution containing diamond particles. Examples of the thickener include agar, carrageenan, xanthan gum, gellan gum, guar gum, polyvinyl alcohol, a polyacrylate thickener, water-soluble celluloses, and polyethylene oxide, and one or more of them can be used. When the thickener is contained, the pH of the solution containing diamond particles is preferably set to a range of 6 to 8.

The solution containing diamond particles may be prepared by mixing diamond particles in the above solvent and stirring the mixture to disperse the diamond particles in the solvent. The stirring speed is preferably 500rpm to 3000rpm, and the stirring time is preferably 10 minutes to 1 hour.

[ Heat treatment ]

Next, as shown in fig. 1(B) and (C), the compound semiconductor substrate 10 is subjected to heat treatment. Thereby, the solvent in the liquid film 12 containing diamond particles evaporates, and the bonding force between the surface of the compound semiconductor substrate 10 and the diamond particles 14 is strengthened, and 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 ℃, and more preferably 30 ℃ to 80 ℃. If the temperature is less than 100 ℃, the generation of bubbles accompanying the boiling of the solution containing diamond particles can be suppressed, and therefore, a portion where the diamond particles 14 are not locally present is not generated on the compound semiconductor substrate 10, and the polycrystalline diamond layer 16 is not peeled off from this portion as a starting point. When the temperature is 30 ℃ or higher, the compound semiconductor substrate 10 and the diamond particles 14 are sufficiently bonded, and therefore, the diamond particles 14 can be suppressed from being flicked by the sputtering action in the process of growing the polycrystalline diamond layer 16 by the CVD method, and the polycrystalline diamond layer 16 can be uniformly grown. The heat treatment time is preferably 1 minute to 30 minutes. The heat treatment apparatus may be a known heat treatment apparatus, and may be performed by placing the compound semiconductor substrate 10 on a heated hot plate, for example.

In this embodiment, as described above, it is essential to adopt a method of applying a solution containing diamond particles on a compound semiconductor substrate and then performing heat treatment. With this method, cracks are not introduced into the surface of the compound semiconductor substrate 10, and therefore the compound semiconductor substrate 10 can grow the polycrystalline diamond layer 16 without breaking. Further, the compound semiconductor layer 18 obtained by thinning the compound semiconductor substrate 10 does not have dislocations.

[ growth of polycrystalline Diamond layer ]

Next, as shown in fig. 1(C) and (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 particles 14 as cores. As the CVD method, a plasma CVD method, a hot wire CVD method, or the like can be suitably used.

When the plasma CVD method is used, for example, hydrogen is used as a carrier gas, a source gas such as methane is introduced into the chamber, and the compound semiconductor substrate 10 is set to a temperature of 700 ℃ to 1300 ℃ to grow the polycrystalline diamond layer 16. From the viewpoint of improving the uniformity of the thickness of the polycrystalline diamond layer 16, it is preferable to use a microwave plasma CVD method. The microwave plasma CVD method is a method in which a source gas such as methane is decomposed by microwaves in a plasma chamber to be converted into plasma, and the source gas converted into plasma is guided onto the heated compound semiconductor substrate 10, thereby growing the polycrystalline diamond layer 16. Here, the pressure in the plasma chamber, the output of the microwave, and the temperature of the compound semiconductor substrate 10 are preferably set as follows. The pressure in the plasma chamber is preferably set to 1.3X 10³Pa or more and 1.3X 10⁵Pa or less, more preferably 1.1X 10⁴Pa or more and 4.0X 10⁴Pa or less. The output power of the microwaves is preferably 0.1kW or more and 100kW or less, and more preferably 1kW or more and 10kW or less. The temperature of the compound semiconductor substrate 10 is preferably 700 ℃ to 1300 ℃, and more preferably 900 ℃ to 1200 ℃.

When the hot filament CVD method is used, carbon radicals are generated from a hydrocarbon source gas such as methane by using a filament composed of tungsten, tantalum, rhenium, molybdenum, iridium, or the like, and setting the filament temperature to about 1900 ℃ or higher and about 2300 ℃ or lower. By guiding the carbon radicals onto the heated compound semiconductor substrate 10, the polycrystalline diamond layer 16 is grown. According to the hot filament CVD method, the increase in the diameter of the wafer can be easily coped with. Here, the pressure in the chamber, the distance between the filament and the compound semiconductor substrate 10, and the compound semiconductor baseThe temperature of the plate 10 is preferably set in the following manner. The pressure in the chamber is preferably set to 1.3X 10³Pa or more and 1.3X 10⁵Pa or less. The distance between the filament and the compound semiconductor substrate 10 is preferably set to 5mm or more and 20mm or less. The temperature of the compound semiconductor substrate 10 is preferably set to 700 ℃ or higher and 1300 ℃ or lower.

The thickness of the polycrystalline diamond layer 16 is set to 100 μm or more, and more preferably 500 μm or more, because it functions as a support substrate for the compound semiconductor layer 18. The upper limit of the thickness of the polycrystalline diamond layer 16 is not particularly limited, but is preferably 3mm or less from the viewpoint that the treatment time in the case of growth by the CVD method is not too long.

[ planarization of the polycrystalline diamond layer ]

Next, as shown in fig. 1(D) and (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 the film formation. By flattening the surface of the polycrystalline diamond layer 16, the polycrystalline diamond self-supporting substrate 100 obtained later can be reliably mounted (held) on a sample stage of a semiconductor processing apparatus. The planarization method is not particularly limited, and a known Chemical Mechanical Polishing (CMP) method can be suitably used. After the planarization, the thickness of the polycrystalline diamond layer 16 is also set to 100 μm or more, and more preferably 500 μm or more.

[ thickness reduction of Compound semiconductor substrate ]

Next, as shown in fig. 1(E) and (F), the compound semiconductor substrate 10 is reduced in thickness to form a compound semiconductor layer 18. Specifically, the compound semiconductor substrate 10 is ground and polished from the surface on the opposite side to the interface between the polycrystalline diamond layers 16 to be reduced in thickness. Thereby, the polycrystalline diamond self-supporting substrate 100 in which the compound semiconductor layer 18 having a desired thickness is laminated on the polycrystalline diamond layer 16 as the supporting substrate can be obtained. The thickness of the compound semiconductor layer 18 can be appropriately determined depending on the type and structure of the semiconductor device to be formed therein, and is preferably 1 μm or more and 500 μm or less. In addition, known or arbitrary grinding methods and polishing methods can be appropriately used for the grinding and polishing, and specifically, a plane grinding method and a mirror polishing method can be used.

Examples

(example 1)

[ inventive example 1]

The polycrystalline diamond self-supporting substrate according to example 1-1 of the present invention was produced through the steps shown in fig. 1(a) to (F).

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

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

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

Next, a polycrystalline diamond layer having a thickness of 300 μm was grown using diamond particles adhered to the GaN substrate as cores by the microwave plasma CVD method described above, using hydrogen as a carrier gas and methane as a source gas. Further, the pressure in the plasma chamber was set to 1.5 × 10⁴Pa, the microwave output was 5kW, and the GaN substrate temperature was 1050 ℃.

Next, the surface of the polycrystalline diamond layer was planarized by the CMP method. The thickness of the planarized polycrystalline diamond layer was set to 290 μm.

Subsequently, the GaN substrate was ground and polished to form a GaN layer having a thickness of 10 μm. Thus, a polycrystalline diamond self-supporting substrate was obtained in which a GaN layer having a thickness of 10 μm was laminated on a polycrystalline diamond layer having a thickness of 290 μm.

In the present example, the GaN substrate was not broken and the polycrystalline diamond layer was able to grow. As a result of observing the cross section of the GaN layer by TEM, no dislocation was observed.

Comparative examples 1 to 1

Production of a polycrystalline diamond free-standing substrate was attempted in the same manner as in invention example 1, except that the method of adhering diamond particles was changed.

A GaN substrate similar to inventive example 1 was prepared. Next, diamond particles were embedded in the surface of the GaN substrate by a known scratching method. That is, the GaN substrate was ultrasonically cleaned in a solution containing diamond particles having an average particle size of 1 μm, thereby embedding the diamond particles in the surface of the GaN substrate. Next, under the same conditions as in inventive example 1, the formation of a polycrystalline diamond layer having a thickness of 300 μm was attempted using diamond particles embedded in a GaN substrate as cores by a microwave plasma CVD method.

In comparative example 1-1, the GaN substrate was broken during polycrystalline diamond film formation. This is considered to be because cracks introduced into the surface of the GaN substrate due to embedding become starting points, and the cracks propagate in the GaN substrate during polycrystalline diamond film formation at a high temperature of 1050 ℃. As a result of TEM observation of the fractured site, it was found that there was a crack at the starting point of fracture.

Comparative examples 1 and 2

The production of a polycrystalline diamond free-standing substrate was attempted 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 was broken together with the polycrystalline diamond layer during grinding of the GaN substrate. That is, it was found that the polycrystalline diamond layer did not function as a self-supporting substrate when the thickness was 5 μm.

(example 2)

[ inventive example 2]

A polycrystalline diamond self-supporting substrate was produced in the same manner as in invention example 1, except that the type of the 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, which was cut out from a silicon carbide (SiC) single crystal produced by a sublimation recrystallization method, was prepared.

Thereafter, a polycrystalline diamond free-standing substrate in which a SiC layer having a thickness of 10 μm was laminated on a polycrystalline diamond layer having a thickness of 460 μm was obtained by the same procedure and conditions as in inventive example 1.

In the present example, the SiC substrate was not broken and the polycrystalline diamond layer could be grown. As a result of observing the cross section of the SiC layer by TEM, no dislocation was observed.

Comparative example 2-1

Production of a polycrystalline diamond free-standing substrate was attempted in the same manner as in invention example 2, except that the method of adhering diamond particles was changed.

The same SiC substrate as in invention example 2 was prepared. Next, diamond particles are embedded in the surface of the SiC substrate by a known scratching method. That is, the SiC substrate is ultrasonically cleaned in a solution containing diamond particles having an average particle diameter of 1 μm, thereby embedding the diamond particles in the surface of the SiC substrate. Next, under the same conditions as in inventive example 2, the formation of a polycrystalline diamond layer having a thickness of 460 μm was attempted using diamond particles embedded in a SiC substrate as cores by a microwave plasma CVD method.

In comparative example 2-1, the SiC substrate was broken in the polycrystalline diamond film formation. This is considered to be because cracks introduced into the surface of the SiC substrate due to embedding become starting points, and the cracks propagate in the SiC substrate during the polycrystalline diamond film formation at a high temperature of 1050 ℃. As a result of TEM observation of the fractured site, it was found that there was a crack at the starting point of fracture.

Comparative examples 2 and 2

The production of a polycrystalline diamond free-standing substrate was attempted 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 was broken together with the polycrystalline diamond layer during grinding of the SiC substrate. That is, it was found that the polycrystalline diamond layer did not function as a self-supporting substrate when the thickness was 5 μm.

Industrial applicability

According to the method for producing a polycrystalline diamond self-supporting substrate of the present invention, a polycrystalline diamond self-supporting substrate in which high-quality compound semiconductor layers are stacked can be produced.

Description of the reference numerals

100-polycrystalline diamond free-standing substrate, 10-compound semiconductor substrate, 12-liquid film containing diamond particles, 14-diamond particles, 16-polycrystalline diamond layer, 18-compound semiconductor layer.