A method for preparing a high thermal conductivity diamond polycrystalline substrate

Abstract

The present application discloses a method for preparing a high thermal conductivity diamond polycrystalline substrate, which belongs to the technical field of diamond polycrystalline substrates. The preparation method includes the following steps: (1) preparation before growth: etching the carbon surface of the silicon carbide substrate; (2) growth of the diamond polycrystalline substrate: using methane as the reaction gas, hydrogen as the carrier gas, and oxygen as the auxiliary gas, a diamond polycrystalline substrate is grown on the carbon surface of the etched silicon carbide substrate by chemical vapor deposition; (3) in-situ annealing self-stripping: after the growth is completed, in-situ annealing is performed to separate the diamond polycrystalline substrate and the silicon carbide substrate in situ, thereby obtaining a high thermal conductivity diamond polycrystalline substrate. The method can improve the thermal conductivity and uniformity of the diamond polycrystalline substrate by growing the diamond polycrystalline substrate on the carbon surface of the etched silicon carbide substrate, thereby improving the commercial application prospects of the diamond polycrystalline substrate.

Description

Technical Field

[0001] The present application relates to a method for preparing a high thermal conductivity diamond polycrystalline substrate, belonging to the technical field of diamond polycrystalline substrates. Background Art

[0002] As power electronic devices and optical devices develop towards high power, integration, and miniaturization, the devices will generate a lot of heat during operation. If the heat cannot be effectively dissipated, the device temperature will rise rapidly, leading to aging of materials and devices, performance degradation, and even failure. The development of heat dissipation substrates with high thermal conductivity can effectively remove the heat generated by the device, thereby ensuring the effective operation of the device.

[0003] The development of polycrystalline diamond substrates as heat sink substrates is of great significance, mainly reflected in their excellent thermal conductivity and thermal management capabilities. Diamond has the highest thermal conductivity in nature, which can effectively dissipate heat and significantly improve the reliability and performance of high-power electronic devices. In addition, diamond also shows excellent stability in extreme environments such as high temperature, radiation resistance and corrosion resistance, and is suitable for aerospace, national defense and deep-sea exploration. With the advancement of technology and the reduction of costs, the development of polycrystalline diamond heat sink substrates will help improve the power density and heat dissipation performance of devices, and promote the widespread application of high-performance electronic and optoelectronic devices.

[0004] Existing polycrystalline diamond substrates are generally grown heteroepitaxially on silicon substrates. Due to the low thermal conductivity of silicon and poor compatibility with the diamond lattice, the grains are not uniform and cracks are easily generated during large-scale heteroepitaxial growth. In addition, when obtaining diamond from a silicon-based diamond substrate, the silicon base is often removed through thinning or chemical etching, which further increases the difficulty of preparing a self-supporting polycrystalline diamond substrate. In addition, energy waste and environmental pollution will occur during the silicon base removal process. Summary of the invention

[0005] In order to solve the above problems, a method for preparing a high thermal conductivity diamond polycrystalline substrate is provided. The method can improve the thermal conductivity and thermal conductivity uniformity of the diamond polycrystalline substrate and improve the commercial application prospects of the diamond polycrystalline substrate by growing the diamond polycrystalline substrate on the carbon surface of the etched silicon carbide substrate.

[0006] The present application provides a method for preparing a high thermal conductivity diamond polycrystalline substrate, which comprises the steps of:

[0007] (1) Preparation before growth: Etching the carbon surface of the silicon carbide substrate;

[0008] (2) Diamond polycrystalline substrate growth: Methane is used as the reaction gas, hydrogen is used as the carrier gas, and oxygen is used as the auxiliary gas. The diamond polycrystalline substrate is grown on the carbon surface of the etched silicon carbide substrate in stages I to III using chemical vapor deposition.

[0009] Stage I: hydrogen and methane are introduced at 1000-1100°C for 4-5 hours.

[0010] Stage II: hydrogen, methane and oxygen are introduced at 600-800°C for 20-30 hours of growth.

[0011] Stage III: introducing hydrogen, methane and oxygen at 950-1050°C for 150-300 hours of growth;

[0012] (3) In-situ annealing self-stripping: After the growth is completed, the temperature is lowered to separate the diamond polycrystalline substrate and the silicon carbide substrate in situ, thus obtaining a high thermal conductivity diamond polycrystalline substrate.

[0013] The preparation method is to grow a diamond polycrystalline substrate on the carbon surface of a silicon carbide substrate with a specific surface state. The silicon carbide substrate serves as a growth substrate and provides a favorable environment for the deposition of diamond polycrystalline, so that the diamond grains formed in step (2) are all crystallized along the (001) plane, and the grains are evenly arranged, and the axial deviation angle of the grains is small. The microstructure of the grains can effectively reduce the transmission loss of phonons and photons, and realize the improvement and uniform distribution of the thermal conductivity and transmittance of the polycrystalline substrate. In step (3), in-situ annealing self-stripping is adopted, which can make the silicon carbide substrate and the diamond polycrystalline substrate peel off under the difference of thermal expansion coefficient, and obtain a diamond polycrystalline substrate with uniform morphology.

[0014] The three steps of the above-mentioned preparation method work together, which is beneficial to improving the uniformity of the microscopic grains of the diamond polycrystalline substrate, avoiding damage to the outer surface of the diamond polycrystalline substrate and introducing other defects such as stress, thereby obtaining a diamond polycrystalline substrate with high thermal conductivity and transmittance, and a thermal expansion coefficient that is more compatible with the chip.

[0015] Optionally, in stage I, the amount of hydrogen used is 300-1000 sccm, and the amount of methane used is 5%-7% of the amount of hydrogen used.

[0016] Optionally, in stage II, the amount of hydrogen used is 300-1000 sccm, the amount of methane used is 3%-4% of the amount of hydrogen used, and the amount of oxygen used is 0.1-0.2% of the amount of hydrogen used.

[0017] Optionally, in stage III, the amount of hydrogen used is 300-1000 sccm, the amount of methane used is 4%-7% of the amount of hydrogen used, and the amount of oxygen used is 0.1-0.2% of the amount of hydrogen used.

[0018] The diamond polycrystalline substrate grows in three stages. In stage I, the growth temperature is high and the methane content is high, which can quickly form nuclei and increase the nucleation density, facilitating the subsequent in-situ annealing and stripping. Oxygen is not introduced because the etching of the silicon carbide substrate is reduced to prevent the newly formed diamond and silicon carbide from forming a mortise and tenon structure. In stage II, the growth temperature is reduced and the amount of methane is reduced, which can obtain small and uniform diamond grains, improve the transmittance, thermal conductivity and uniformity of the diamond polycrystalline substrate, and lay a good foundation for the subsequent growth of diamond polycrystalline in stage III. In stage III, the temperature is increased and the amount of methane is increased, which can uniformly deposit diamond polycrystalline on the basis of stage II to obtain a diamond polycrystalline substrate with a more uniform grain arrangement and a more uniform grain size, improve the thermal conductivity and transmittance of the diamond polycrystalline substrate, and achieve a uniform distribution of thermal conductivity and transmittance.

[0019] If the growth temperature of stage III is too high, the growing diamond polycrystalline grains will be unevenly arranged, so the uniformity of the thermal conductivity, transmittance and thermal expansion coefficient of the diamond polycrystalline substrate will decrease, and amorphous carbon will be easily generated, resulting in black spots in the diamond polycrystalline substrate.

[0020] Among the above three gases, hydrogen is used as a carrier gas to drive methane gas to deposit and form grains and to drive borane to dope, so as to obtain a high-quality diamond polycrystalline substrate. In the three stages: if the amount of hydrogen is increased, the quality of the diamond polycrystalline substrate will not be improved much, but it will cause an increase in cost. If the amount of hydrogen is reduced, amorphous carbon will be generated, so that black spot defects will be generated in the diamond polycrystalline substrate, affecting the use of the substrate. The addition of oxygen in stage II and stage III can also inhibit the formation of amorphous carbon, thereby inhibiting the generation of black spot defects. If the amount of oxygen is increased, the etching rate of the diamond surface will be enhanced, and the growth rate of the diamond polycrystalline substrate will be reduced. If the amount of oxygen is reduced, black spot defects will appear. In addition, the amount of hydrogen and oxygen mentioned above can not only improve the uniformity of the distribution of methane during growth, so that a uniform diamond polycrystalline substrate can be uniformly deposited on the silicon carbide substrate, but also improve the consistency of the growth temperature field in step (2), reduce the temperature difference between the edge and the center, and thus obtain a diamond polycrystalline substrate with a high degree of consistency.

[0021] Optionally, step (3) is divided into two stages, wherein the first stage: after the growth is stopped, the introduction of methane is stopped and the temperature is first lowered to 600-750° C.;

[0022] Stage 2: Stop the oxygen supply and continue to cool to room temperature.

[0023] The present application adopts two-step cooling to perform in-situ annealing, which can realize the separation of the diamond polycrystalline substrate from the silicon carbide substrate, reduce the internal stress of the diamond polycrystalline substrate, and improve the self-supporting property of the diamond polycrystalline substrate.

[0024] In the first stage, the introduction of methane is stopped, while hydrogen and oxygen are introduced normally. This setting can remove non-sp3 carbon on the surface of the diamond substrate and reduce surface defects.

[0025] Optionally, the cooling rate in the first stage is 5-20°C / min.

[0026] Optionally, the cooling rate in the second stage is 50-100° C. / min.

[0027] The cooling rate in the first stage is lower than that in the second stage because the stress in the diamond polycrystalline substrate is released in the first stage, the shrinkage consistency of the diamond polycrystalline substrate is improved, and the foundation is laid for the in-situ stripping after the second stage cooling. The faster cooling rate in the second stage is to increase the shrinkage difference between the diamond polycrystalline substrate and the silicon carbide substrate, which is convenient for the rapid stripping of the two. At the same time, the stripping consistency of the contact interface between the diamond polycrystalline substrate and the silicon carbide substrate is high, which avoids damage to the surface of the diamond polycrystalline substrate and improves the surface quality of the diamond polycrystalline substrate. In addition, the cooling rates in the above two stages can also cooperate with each other to reduce defects and warpage in the diamond polycrystalline substrate to obtain a high-quality diamond polycrystalline substrate.

[0028] Optionally, in step (1), the carbon surface roughness of silicon carbide is Ra≤1 nm.

[0029] The above etching process makes the carbon surface of the silicon carbide substrate have a specific surface state. The surface roughness is convenient for the diamond polycrystalline substrate to form nuclei for growth, and is beneficial to the crystallization and uniform arrangement of the diamond grains (001) plane, thereby obtaining a diamond polycrystalline substrate with high thermal conductivity, transmittance and better uniformity.

[0030] Optionally, the thickness of the diamond polycrystalline substrate grown in step (2) is ≥ 400 μm.

[0031] The thickness of the diamond polycrystalline substrate can ensure that the diamond polycrystalline substrate has self-supporting performance, thereby improving the performance of the diamond polycrystalline substrate.

[0032] Optionally, the cooling rate of step (3) is 50 to 100° C. / min.

[0033] Optionally, the etching in step (1) is performed in a hydrogen-oxygen plasma atmosphere, the surface temperature of the silicon carbide substrate is 950-1050° C., and the etching time is 0.5-2 h.

[0034] Optionally, in the hydrogen-oxygen plasma atmosphere, 1%≤O2:H2≤4%.

[0035] Specifically, hydrogen and oxygen are used as reaction gases, and the hydrogen and oxygen gas molecules are ionized into a plasma state, generating a large number of high-energy electrons, ions and free radicals. The active free radicals undergo oxidation and reduction reactions with the surface of silicon carbide, and the high-energy ions physically bombard the surface of silicon carbide. By limiting the ratio of hydrogen and oxygen, the etching time, and the surface temperature of silicon carbide, efficient and controllable etching can be achieved.

[0036] The beneficial effects of this application include but are not limited to:

[0037] 1. The method for preparing a high thermal conductivity diamond polycrystalline substrate of the present application can grow diamond polycrystalline with small and uniform grain size on a silicon carbide substrate, and obtain a diamond polycrystalline substrate with self-supporting properties, which meets the application performance of the diamond polycrystalline substrate such as heat sink and optical window.

[0038] 2. The method for preparing a high thermal conductivity diamond polycrystalline substrate of the present application uses methane as a reaction gas. By controlling the gas flow rate and process mode, the thermal conductivity and transmittance of the substrate can be improved, and the decrease in the thermal expansion coefficient can be suppressed as much as possible, so as to obtain a diamond polycrystalline substrate that is more compatible with the chip and has better heat dissipation.

[0039] 3. The method for preparing a high thermal conductivity diamond polycrystalline substrate of the present application, in step (2), during the growth of the diamond polycrystalline substrate, the uniformity of grain size and arrangement can be achieved during the growth process, thereby improving the distribution uniformity of thermal conductivity, light transmittance and thermal expansion coefficient. The difference between the edge and the center of the diamond polycrystalline substrate is reduced, and there is no need to remove the edge portion when using the diamond gold substrate, thereby effectively increasing the usable area of ​​the diamond polycrystalline substrate.

[0040] 4. The method for preparing a high thermal conductivity diamond polycrystalline substrate of the present application has no black spots formed in the prepared diamond polycrystalline substrate, thereby improving the quality of the diamond polycrystalline substrate.

[0041] 5. The method for preparing a high thermal conductivity diamond polycrystalline substrate of the present application realizes separation of the silicon carbide substrate and the grown diamond polycrystalline substrate by in-situ annealing self-stripping, which can not only improve the operational convenience of the preparation method, but also reduce the impact of annealing on the diamond polycrystalline substrate, and avoid introducing thermal stress or other defects into the diamond polycrystalline substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0043] Figure 1 This is a picture of the diamond polycrystalline substrate prepared in Example 3 of the present application.

[0044] Figure 2 This is the XRD pattern of the diamond polycrystalline substrate prepared in Example 3 of the present application. DETAILED DESCRIPTION

[0045] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0046] Unless otherwise specified, the raw materials in the examples and comparative examples of the present application were purchased through commercial channels.

[0047] Unless otherwise specified, the methods used in the examples and comparative examples of the present application are conventional methods in the prior art.

[0048] Example 1

[0049] This embodiment relates to a method for preparing a high thermal conductivity diamond polycrystalline substrate, which comprises the following steps:

[0050] (1) Preparation before growth: The carbon surface of the silicon carbide substrate is etched in a hydrogen-oxygen plasma atmosphere (1% ≤ O2:H2 ≤ 4%), the surface temperature of the silicon carbide substrate is 950°C, the etching time is 2 h, and the surface roughness of the silicon carbide substrate is Ra ≤ 1 nm;

[0051] (2) Diamond polycrystalline substrate growth: Stage I, hydrogen and methane were introduced at 1000°C, the hydrogen dosage was 300 sccm, the methane dosage was 7% of the hydrogen dosage, and the growth time was 5 h;

[0052] Stage II: hydrogen, methane and oxygen were introduced at 600°C, with a hydrogen dosage of 300 sccm, a methane dosage of 4% of the hydrogen dosage, and an oxygen dosage of 1.0% of the hydrogen dosage, and the growth time was 30 h.

[0053] Stage III: hydrogen, methane and oxygen are introduced at 950°C, with a hydrogen dosage of 300 sccm, a methane dosage of 7% of the hydrogen dosage, and an oxygen dosage of 0.1% of the hydrogen dosage, and the growth is carried out for 300 hours to obtain a SiC / diamond composite substrate;

[0054] (3) In-situ annealing self-stripping: In the first stage, after the growth is completed, the introduction of methane is stopped and the temperature is first cooled to 750°C at a cooling rate of 5°C / min. In the second stage, the introduction of oxygen is stopped and the temperature is cooled to room temperature at a cooling rate of 50°C / min, so that the diamond polycrystalline substrate and the silicon carbide substrate are separated in situ, and a high thermal conductivity diamond polycrystalline substrate is obtained.

[0055] Example 2

[0056] This embodiment relates to a method for preparing a high thermal conductivity diamond polycrystalline substrate, which comprises the following steps:

[0057] (1) Preparation before growth: The carbon surface of the silicon carbide substrate is etched in a hydrogen-oxygen plasma atmosphere (1% ≤ O2:H2 ≤ 4%), the surface temperature of the silicon carbide substrate is 1050°C, the etching time is 0.5 h, and the surface roughness of the silicon carbide substrate is Ra ≤ 1 nm;

[0058] (2) Diamond polycrystalline substrate growth: Stage I, hydrogen and methane were introduced at 1100°C, the hydrogen dosage was 1000 sccm, the methane dosage was 5% of the hydrogen dosage, and the growth time was 4 h;

[0059] Stage II: hydrogen, methane and oxygen were introduced at 800°C, with a hydrogen dosage of 1000 sccm, a methane dosage of 3% of the hydrogen dosage, and an oxygen dosage of 1.5% of the hydrogen dosage, for 20 h of growth;

[0060] Stage III: hydrogen, methane and oxygen were introduced at 1050°C, with a hydrogen dosage of 1000 sccm, a methane dosage of 4% of the hydrogen dosage, and an oxygen dosage of 0.2% of the hydrogen dosage, and the growth was continued for 150 h.

[0061] (3) In-situ annealing self-stripping: In the first stage, after the growth is completed, the introduction of methane is stopped and the temperature is first cooled to 600°C at a cooling rate of 20°C / min. In the second stage, the introduction of oxygen is stopped and the temperature is cooled to room temperature at a cooling rate of 100°C / min, so that the diamond polycrystalline substrate and the silicon carbide substrate are separated in situ, and a high thermal conductivity diamond polycrystalline substrate is obtained.

[0062] Example 3

[0063] This embodiment relates to a method for preparing a high thermal conductivity diamond polycrystalline substrate, which comprises the following steps:

[0064] (1) Preparation before growth: The carbon surface of the silicon carbide substrate is etched in a hydrogen-oxygen plasma atmosphere (1% ≤ O2:H2 ≤ 4%), the surface temperature of the silicon carbide substrate is 1000°C, the etching time is 1 h, and the surface roughness of the silicon carbide substrate is Ra ≤ 1 nm;

[0065] (2) Diamond polycrystalline substrate growth: Stage I, hydrogen and methane were introduced at 1050°C, the hydrogen dosage was 800 sccm, the methane dosage was 6% of the hydrogen dosage, and the growth was 5 h;

[0066] Stage II: hydrogen, methane and oxygen were introduced at 700°C, with a hydrogen dosage of 800 sccm, a methane dosage of 4% of the hydrogen dosage, and an oxygen dosage of 1.5% of the hydrogen dosage, for 3 h of growth;

[0067] Stage III: hydrogen, methane and oxygen are introduced at 1000°C, with a hydrogen dosage of 800 sccm, a methane dosage of 4.5% of the hydrogen dosage, and an oxygen dosage of 0.2% of the hydrogen dosage, and the growth time is 250 h.

[0068] (3) In-situ annealing self-stripping: In the first stage, after the growth is completed, the introduction of methane is stopped and the temperature is first cooled to 650°C at a cooling rate of 15°C / min. In the second stage, the introduction of oxygen is stopped and the temperature is cooled to room temperature at a cooling rate of 60°C / min, so that the diamond polycrystalline substrate and the silicon carbide substrate are separated in situ, and a high thermal conductivity diamond polycrystalline substrate is obtained.

[0069] according to Figure 1 It can be seen that the font on the back of the diamond polycrystalline substrate is clearly visible, indicating that it has good light transmittance. This is because its surface grains are mainly composed of (001) planes (reference Figure 2 ), resulting in higher thermal conductivity.

[0070] Example 4

[0071] The difference between this embodiment and embodiment 3 is that the amount of methane used in step (2) stage III is 8% of the amount of hydrogen used, and the rest is the same as embodiment 3.

[0072] Example 5

[0073] The difference between this embodiment and embodiment 3 is that the amount of oxygen used in step (2) stage III is 0.05% of the amount of hydrogen used, and the rest is the same as embodiment 3.

[0074] Example 6

[0075] The difference between this embodiment and embodiment 3 is that the growth temperature of step (2) stage I is 900° C., and the rest is the same as embodiment 3.

[0076] Example 7

[0077] The difference between this embodiment and embodiment 3 is that the cooling rate in the first stage of step (3) is 30°C / min, and the rest is the same as embodiment 3.

[0078] Example 8

[0079] The difference between this embodiment and embodiment 3 is that the cooling rate in the second stage of step (3) is 120°C / min, and the rest is the same as embodiment 3.

[0080] Comparative Example 1

[0081] The difference between this comparative example and Example 3 is that the silicon carbide substrate is not etched in step (1), and the rest is the same as Example 3.

[0082] Comparative Example 2

[0083] The difference between this comparative example and Example 3 is that step (2) does not include stage I, and the rest is the same as Example 3.

[0084] Comparative Example 3

[0085] The difference between this comparative example and Example 3 is that step (2) does not include stage II, and the rest is the same as Example 3.

[0086] Experimental Example 1

[0087] The thermal conductivity of the diamond polycrystalline substrate prepared as above was tested by harmonic method. The test results are shown in Table 1.

[0088] The thermal conductivity in the second column of Table 1 refers to the minimum thermal conductivity value obtained from all test points in the diamond polycrystalline substrate, which means that the thermal conductivity at any position of the diamond polycrystalline substrate is greater than this value; the diamond polycrystalline substrate is divided into a number of square areas with a side length of 3 mm × 3 mm, and each square area obtains a thermal conductivity value of the area. The thermal conductivity standard deviation coefficient refers to the ratio of the thermal conductivity standard deviation calculated from all square areas to the thermal conductivity mean, K1 represents the maximum thermal conductivity of all square areas, and K2 represents the minimum thermal conductivity of all square areas.

[0089] Table 1 Thermal conductivity measurement results

[0090]

[0091]

[0092] Experimental Example 2

[0093] The transmittance of the diamond polycrystalline substrate prepared as described above was tested using the haze method. For the diamond polycrystalline substrate, the higher the haze, the lower the transmittance. The test results are shown in Table 2.

[0094] The transmittance in the second column of Table 2 refers to the minimum transmittance value obtained by testing all test points in the diamond polycrystalline substrate, which means that the transmittance at any position of the diamond polycrystalline substrate is greater than this value; the diamond polycrystalline substrate is divided into a number of square areas with a side length of 10 mm × 10 mm, and each square area obtains a transmittance value of the area. The transmittance standard deviation coefficient refers to the ratio of the transmittance standard deviation calculated for all square areas to the transmittance mean, T1 represents the maximum transmittance of all square areas, and T2 represents the minimum transmittance of all square areas.

[0095] Table 2 Transmittance measurement results

[0096]

[0097]

[0098] Experimental Example 3

[0099] The diamond polycrystalline substrate prepared as above was subjected to a grain arrangement test. The specific test method was a microscope combined with an X-ray diffraction test. The test results are shown in Table 3.

[0100] Table 3 Grain arrangement measurement results

[0101]

[0102] According to the above tests, the amount of methane and oxygen used in stage III will affect the size and axial deviation of the grains; the in-situ annealing cooling rate in step (3) will not change the grain size, but may affect the orientation of the grains, which mainly affects the stress of the diamond polycrystalline substrate, and then affects the thermal conductivity and transmittance of the substrate.

[0103] The above is only the embodiment of the present application, and the protection scope of the present application is not limited by these specific embodiments, but is determined by the claims of the present application. For those skilled in the art, the present application can have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the technical ideas and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing a high thermal conductivity diamond polycrystalline substrate, characterized in that: It includes the steps of: (1) Preparation before growth: Etching the carbon surface of the silicon carbide substrate; (2) Diamond polycrystalline substrate growth: Methane is used as the reaction gas, hydrogen is used as the carrier gas, and oxygen is used as the auxiliary gas. The diamond polycrystalline substrate is grown on the carbon surface of the etched silicon carbide substrate in stages I to III using chemical vapor deposition. Stage I: hydrogen and methane are introduced at 1000-1100°C for 4-5 hours. Stage II: hydrogen, methane and oxygen are introduced at 600-800°C for 20-30 hours of growth. Stage III: introducing hydrogen, methane and oxygen at 950-1050°C for 150-300 hours of growth; (3) In-situ annealing self-stripping: After the growth is completed, in-situ annealing is performed to separate the diamond polycrystalline substrate and the silicon carbide substrate in situ, thereby obtaining a high thermal conductivity diamond polycrystalline substrate.

2. The preparation method according to claim 1, characterized in that: In stage I, the amount of hydrogen used is 300-1000 sccm, and the amount of methane used is 5%-7% of the amount of hydrogen used.

3. The preparation method according to claim 1, characterized in that: In stage II, the amount of hydrogen used is 300-1000 sccm, the amount of methane used is 3%-4% of the amount of hydrogen used, and the amount of oxygen used is 0.1-0.2% of the amount of hydrogen used; and / or In stage III, the amount of hydrogen used is 300-1000 sccm, the amount of methane used is 4%-7% of the amount of hydrogen used, and the amount of oxygen used is 0.1-0.2% of the amount of hydrogen used.

4. The preparation method according to claim 1, characterized in that: Step (3) is divided into two stages, wherein the first stage: after the growth is completed, the introduction of methane is stopped, the temperature is first lowered to 600-750°C, and the temperature is kept for 30 minutes; Stage 2: Stop the oxygen supply and continue to cool to room temperature.

5. The preparation method according to claim 4, characterized in that: The cooling rate in the first stage is 5-20℃ / min.

6. The preparation method according to claim 4, characterized in that: The cooling rate in the second stage is 50-100°C / min.

7. The preparation method according to claim 1, characterized in that: In step (1), the carbon surface roughness of silicon carbide is Ra≤1nm.

8. The preparation method according to claim 1, characterized in that: The thickness of the diamond polycrystalline substrate grown in step (2) is ≥400 μm.

9. The preparation method according to claim 1, characterized in that: In step (1), etching is performed in a hydrogen-oxygen plasma atmosphere, the surface temperature of the silicon carbide substrate is 950-1050° C., and the etching time is 0.5-2 h.

10. The preparation method according to claim 9, characterized in that: In the hydrogen-oxygen plasma atmosphere, 1%≤O2:H2≤4%.

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