Silicon carbide substrate having stepwise stress distribution

Abstract

The present application relates to the technical field of silicon carbide production and processing, and discloses a silicon carbide substrate having a stepwise stress distribution. The silicon carbide substrate comprises a first main surface and a second main surface; the first main surface is provided with a first gradient stress zone, a second gradient stress zone surrounding the first gradient stress zone, and a third gradient stress zone surrounding the second gradient stress zone, the width of the third gradient stress zone extending inwards from the edge of the substrate is 2.5-30 mm, and the width of the second gradient stress zone extending inwards from the edge of the third gradient stress zone is 10-50 mm; the relative internal stress of the first gradient stress zone is greater than or equal to the relative internal stress of the second gradient stress zone and smaller than the relative internal stress of the third gradient stress zone. The relative internal stresses of different zones of the silicon carbide substrate transition uniformly, and a stepwise distribution is realized, such that the quality of the silicon carbide substrate can be improved, and the use range of the silicon carbide substrate is expanded.

Description

A silicon carbide substrate with stepped stress distribution This application claims the priority of the Chinese patent application filed with the China Patent Office on July 7, 2023, with application number 202310837065.0 and invention name “A silicon carbide substrate with a stepped stress distribution”, the entire contents of which are incorporated by reference into this application. Technical Field The present application relates to the technical field of silicon carbide production and processing, and in particular to a silicon carbide substrate with a stepped stress distribution. Background Art Silicon carbide (SiC) single crystal substrates have become the preferred semiconductor material for the preparation of GaN-based high-frequency microwave devices due to their excellent physical properties such as large bandgap, high resistivity and thermal conductivity, and strong breakdown field. With the continuous development of 5G technology, the market demand for SiC single crystal substrates continues to expand. More importantly, mass commercial applications have also put forward higher requirements on the quality of SiC single crystal substrates. During the growth process of silicon carbide crystals, various defects such as polycrystalline, polymorphic, micropipes, dislocations, etc. are easily introduced into the crystal due to the imbalance of Si / C ratio, the introduction of impurities and temperature gradient changes. The defects will cause lattice distortion and more seriously, the crystal form will change, such as the generation of 4H, 6H, 3C, 15R and other polymorphic symbiosis. The lattice distortion releases stress by generating defects, that is, introducing internal stress in silicon carbide, thereby reducing the crystal quality of silicon carbide, limiting the scope of use of the crystal, and reducing the yield rate of the product; and the internal stress of the silicon carbide substrate is prone to uneven stress distribution due to the existence of temperature field or defects, thereby affecting the production and processing of downstream products. Summary of the invention In order to solve the above problems, a silicon carbide substrate with a stepped stress distribution is provided. The silicon carbide substrate is divided into a first gradient stress interval, a second gradient stress interval and a third gradient stress interval. The relative stress in each area is uniformly transitioned and a stepped distribution is achieved, which can improve the quality of the silicon carbide substrate and expand the scope of use of the silicon carbide substrate. The silicon carbide substrate can be used to process epitaxial wafers or as a seed crystal for growing crystals, and the stress of epitaxial wafers or crystals can be reduced to obtain high-quality epitaxial wafers or crystals. According to one aspect of the present application, there is provided a silicon carbide substrate with a stepped stress distribution, wherein the diameter of the silicon carbide substrate is greater than 150 mm, and the silicon carbide substrate comprises a first main surface and a second main surface; The first main surface has a first gradient stress interval, a second gradient stress interval surrounding the first gradient stress interval, and a third gradient stress interval surrounding the second gradient stress interval, wherein the width of the third gradient stress interval extending inward from the edge of the substrate is 2.5-30 mm, and the width of the second gradient stress interval extending inward from the edge of the third gradient stress interval is 10-50 mm; The internal relative stress in the first gradient stress interval is greater than or equal to the internal relative stress in the second gradient stress interval, and less than the internal relative stress in the third gradient stress interval. The internal relative stress is a stress value detected at a point extending vertically from the first main surface or the second main surface into the silicon carbide substrate by at least 3 μm. The internal relative stress in the second gradient stress interval is the smallest, and the internal relative stress in the third gradient stress interval is the largest. Therefore, the wider the width of the second gradient stress interval extending inward from the edge of the third gradient stress interval, the greater the proportion of the area with smaller internal relative stress in the silicon carbide substrate. Since the silicon carbide crystal grows in a crucible, the periphery of the silicon carbide crystal will contact the graphite crucible. In the high-temperature environment of crystal growth, the graphite crucible will expand due to heat, which will squeeze the edge of the silicon carbide crystal and cause edge stress, which is easy to cause lattice distortion and thus stress. When the silicon carbide crystal is processed to obtain a silicon carbide substrate, the stress in the third gradient stress interval of the substrate is the largest, and the first gradient stress interval is the central area of ​​the crystal. Since there is a radial temperature gradient in SiC crystal growth, and the radial temperature gradient increases from the center to the edge, the overall relative stress gradually increases from the center to the edge, so the stress shows a trend of gradually increasing from the center to the edge. The stress in the first gradient stress interval is greater than that in the second gradient stress interval. Optionally, the in-vivo absolute stress in the second gradient stress interval is greater than or equal to the in-vivo absolute stress in the first gradient stress interval, and less than the in-vivo absolute stress in the third gradient stress interval, and the in-vivo absolute stress is a stress value detected at a point extending vertically from the first main surface or the second main surface into the silicon carbide substrate by at least 3 μm; According to the different processing states of the SiC substrate, the internal absolute stress and the internal relative stress are stress values ​​detected at a point extending vertically from the first main surface or the second main surface to at least 3 μm into the silicon carbide substrate, which can ensure that the obtained stress value is the internal stress value of the SiC substrate and eliminate the stress introduced by the SiC substrate during cutting, grinding, polishing and other processes. Optionally, at any plane parallel to the first main surface and / or the second main surface, the radial internal absolute stress in the first gradient stress interval is -20 to 5 MPa, the radial internal absolute stress in the second gradient stress interval is 5 to 15 MPa, and the radial internal absolute stress in the third gradient stress interval is 15.2 to 50 MPa. Optionally, at any plane parallel to the first main surface and / or the second main surface, the radial internal absolute stress in the first gradient stress interval is -5 to 5 MPa, the radial internal absolute stress in the second gradient stress interval is 5 to 10 MPa, and the radial internal absolute stress in the third gradient stress interval is 15.2 to 20 MPa. Due to the tensile and compressive stress inside the silicon carbide substrate, the interplanar spacing d undergoes corresponding expansion and contraction changes. Therefore, when using Raman to test stress, the Raman peak intensity will move to low frequency or high frequency. When the substrate is subjected to tensile stress, the Raman peak shifts to low frequency, and the obtained stress value is positive. When the substrate is subjected to compressive stress, the Raman peak shifts to high frequency, and the obtained stress value is negative. Therefore, the positive and negative before the stress value in this application represent the force direction inside the substrate, and the absolute value of the value represents the magnitude of the stress. For example, the average value of the absolute stress in the body of the silicon carbide substrate in the first gradient is -3MPa, and the average value of the absolute stress in the body of the second gradient is 5MPa, which means that the absolute stress in the body of the first gradient stress interval is compressive stress, and the absolute stress in the body of the second gradient stress interval is tensile stress, and the radial absolute pressure in the body of the first gradient stress interval is less than the radial absolute pressure in the body of the second gradient stress interval. According to the radial internal absolute stress values ​​in the first gradient stress interval, the second gradient stress interval and the third gradient stress interval, the first gradient stress interval is a transition zone between compressive stress and tensile stress, and the second gradient stress interval and the third gradient stress interval are both tensile stress zones. The closer the value of the first gradient stress interval or the third gradient stress interval is to the second gradient stress interval, the more uniform the radial internal stress distribution of the silicon carbide substrate is. Optionally, an axial test is performed from any point of the first main surface vertically extending into the silicon carbide substrate, and the axial absolute stress in the first gradient stress interval is -30 to 10 MPa, the axial absolute stress in the second gradient stress interval is 10 to 30 MPa, and the axial absolute stress in the third gradient stress interval is 30.2 to 50 MPa. The reference value of absolute stress in this application is the reference stress calculated based on the standard Raman peak position obtained based on the perfect lattice parameters of SiC, and is marked as 0. Optionally, at any plane parallel to the first main surface and / or the second main surface, the radial internal relative stress in the first gradient stress interval is -15 to 15 MPa, the radial internal relative stress in the second gradient stress interval is -10 to 10 MPa, and the radial internal relative stress in the third gradient stress interval is -24.8 to 24.8 MPa. Optionally, the radial internal relative stress in the first gradient stress interval is -14.3 to 14.5 MPa, and the radial internal relative stress in the second gradient stress interval is -9.4 to 10.0 MPa. The radial in-body absolute stress can reflect the difference between the substrate and a substrate without residual stress in the body (ie, a perfect crystal), and the radial in-body relative stress can reflect the uniformity and distribution of the substrate in the radial direction. Optionally, at any plane parallel to the first major surface and / or the second major surface, S max1 represents the maximum absolute value of the relative stress in the radial body within the first gradient stress interval, S max2 represents the maximum absolute value of the relative stress in the radial body within the second gradient stress interval, S max3 represents the maximum absolute value of the relative stress in the radial body within the third gradient stress interval, △S1=S max1 -S max2 , △S2=S max3 -S max1 , △S1≤△S2; Optionally, 0MPa≤△S1≤10MPa, 0MPa≤△S2≤22MPa. Optionally, 0.2MPa≤△S1≤9.8MPa, 0.3MPa≤△S2≤21.4MPa. Optionally, at any plane parallel to the first major surface and / or the second major surface, S min1 represents the minimum absolute value of the relative stress in the radial body within the first gradient stress interval, S min2 Represents the second gradient The minimum absolute value of the relative stress in the radial body within the stress range, S min3 represents the minimum absolute value of the relative stress in the radial body within the third gradient stress interval, △S3=S min1 -S min2 , △S4=S min3 -S min1 , △S3≤△S4; Optionally, 0MPa≤△S3≤5MPa, 0MPa≤△S4≤20MPa. Optionally, 0.2MPa≤△S3≤4.7MPa, 0.3MPa≤△S4≤19.6MPa. The absolute value of the radial internal relative stress represents the relative stress magnitude in each region. △S1 and △S3 represent the difference in radial internal relative stress between the second gradient stress interval and the first gradient stress interval. The smaller the values ​​of △S1 and △S3 are, the closer the radial internal relative stress between the second gradient stress interval and the first gradient stress interval is. △S2 and △S4 represent the difference in radial internal relative stress between the third gradient stress interval and the first gradient stress interval. The smaller the values ​​of △S2 and △S4 are, the closer the radial internal relative stress between the third gradient stress interval and the first gradient stress interval is. △S1≤△S2 and △S3≤△S4 indicate that the difference in radial internal relative stress of the silicon carbide substrate between the second gradient stress interval and the first gradient stress interval is smaller than the difference between the third gradient stress interval and the first gradient stress interval. Optionally, at any plane parallel to the first main surface and / or the second main surface, S1 represents the average absolute value of the radial internal relative stress in the first gradient stress interval, S2 represents the average absolute value of the radial internal relative stress in the second gradient stress interval, S3 represents the average absolute value of the radial internal relative stress in the third gradient stress interval, 0MPa≤S1-S2≤14.5MPa, 0MPa≤S3-S1≤22.5MPa. Optionally, 0MPa≤S3-S1≤22.4MPa. The silicon carbide substrate is subjected to multi-point stress detection. The above-mentioned average absolute value refers to the value obtained by dividing the sum of the absolute values ​​of stress at all detection points in the area by the number of detection points. The multi-point detection can be performed by selecting multiple points at different positions in the area for detection, or by performing a mapping test on the substrate. Optionally, 0MPa≤S1-S2≤8MPa, 0MPa≤S3-S1≤10MPa. Optionally, an axial test is performed from any point of the first main surface vertically extending into the silicon carbide substrate, and the axial relative stress in the first gradient stress interval is -20 to 20 MPa, the axial relative stress in the second gradient stress interval is -16 to 16 MPa, and the axial relative stress in the third gradient stress interval is -25 to 25 MPa. Optionally, the relative axial stress in the first gradient stress interval is -19.3 to 19.7 MPa, the relative axial stress in the second gradient stress interval is -15.7 to 15.2 MPa, and the relative axial stress in the third gradient stress interval is -24.3 to 24.7 MPa. The axial absolute stress can reflect the difference between the substrate and the stress-free substrate in the body, and the axial relative stress can reflect the uniformity of the substrate in the axial direction. In the present application, the radial body relative stress in the first gradient stress interval is based on the first gradient region as a whole, a reference value is taken in it, and the value of each test point is related to the reference value by numerical calculation to obtain The reference value includes but is not limited to any one of the median, mean, mode and other statistical function calculation results of all stress values ​​in the entire test area. Similarly, the radial body relative stress in the second gradient stress interval and the third gradient stress interval is calculated in the same way. The reference values ​​of the radial body absolute stress in the first gradient stress interval, the second gradient stress interval, and the third gradient stress interval are reference stresses calculated based on the standard Raman peak position obtained from the perfect lattice parameters of SiC, and are marked as 0. Optionally, an axial test is performed by extending vertically from any point of the first main surface into the silicon carbide substrate, S max5 It is recorded as the maximum relative stress in the axial body in the first gradient stress interval, S min5 It is recorded as the minimum relative stress in the axial body in the first gradient stress interval, △S5 = S max5 -S min5 , 0MPa≤△S5≤10MPa. Optionally, 0MPa≤△S5≤9.6MPa. S max6 It is recorded as the maximum relative stress in the axial body in the second gradient stress interval, S min6 It is recorded as the minimum relative stress in the axial body in the second gradient stress interval, △S6 = S max6 -S min6 , 0MPa≤△S6≤10MPa. Optionally, 0MPa≤△S6≤9.2MPa. S max7 S is the maximum relative stress in the axial body in the third gradient stress interval. min7 It is recorded as the minimum relative stress in the axial body in the third gradient stress interval, △S7 = S max7 -S min7 , 0MPa≤△S7≤10MPa. Optionally, 0MPa≤△S7≤9.9MPa. The above-mentioned maximum value and minimum value of the axial internal relative stress are the true values ​​of the axial internal relative stress, and the compressive stress and the tensile stress are distinguished in the axial direction. For example, on any axis, the axial internal relative stress in the first gradient stress interval is -5 to 1 MPa, then the maximum value of the axial internal relative stress is 1 MPa, and the minimum value of the axial internal relative stress is -5 MPa, and △S5 is 6 MPa. △S5, △S6 and △S7 represent the degree of change in the axial crystal plane spacing of the substrate at the microscopic level. The smaller the above-mentioned △S5, △S6 and △S7 are, the smaller the change in the axial crystal plane spacing of the substrate is. Optionally, 0MPa≤△S5≤5.2MPa, 0MPa≤△S6≤3.9MPa, 0MPa≤△S7≤6.1MPa. Optionally, the diameter of the silicon carbide substrate is greater than 200 mm, the width of the third gradient stress interval extending inward from the edge of the substrate is 21-30 mm, and the width of the second gradient stress interval extending inward from the edge of the third gradient stress interval is 10-39 mm; and / or The thickness of the silicon carbide substrate is not less than 0.1 mm. Optionally, an off angle of the first major surface and / or the second major surface relative to the {0001} plane is less than 4°. Optionally, the curvature of the silicon carbide substrate is -50 μm to 50 μm. Optionally, the curvature of the silicon carbide substrate is -20 μm to 20 μm. Optionally, the curvature of the silicon carbide substrate is -10 μm to 10 μm. Optionally, the crystal form of the silicon carbide substrate is one of 2H-SiC, 4H-SiC, 6H-SiC, 3C-SiC, and 15R-SiC. Optionally, the crystal form of the silicon carbide substrate is 4H-SiC. Optionally, the silicon carbide substrate is a semi-insulating silicon carbide substrate or a conductive silicon carbide substrate. Optionally, the local thickness deviation of the first gradient stress interval is 0.1-0.3 μm, the local thickness deviation of the second gradient stress interval is 0.3-0.7 μm, and the local thickness deviation of the third gradient stress interval is 0.7-2 μm. The silicon carbide substrate described in any one of the above items is made by processing a silicon carbide single crystal. According to another aspect of the present application, a method for preparing a silicon carbide single crystal is provided, comprising the following steps: (1) Adding silicon carbide powder into a graphite crucible; (2) Evacuate the furnace to 10 -6mbar, and then introduce high-purity inert gas to 300-500mbar, repeat this process 2-3 times, and finally evacuate the furnace to 10 -6 Below mbar; (3) introducing high-purity inert gas into the furnace body, raising the pressure to 10-100 mbar within 1-3 hours, and continuously introducing high-purity inert gas and maintaining the pressure constant; (4) Crystal growth stage: While maintaining the pressure unchanged, the temperature inside the furnace is raised to the single crystal growth temperature of 2200K-2800K within 3-5 hours, and the growth time is 30-150 hours; (5) After the single crystal growth is completed, open the furnace body and take out the graphite crucible to obtain the silicon carbide single crystal. Optionally, the graphite crucible is coated with a composite coating, and the composite coating comprises: Ta x C y The permeation layer, the Ta x C y The infiltration layer is deposited on a graphite substrate, wherein 0 <x<1,0<y<1; TaC coating, the TaC coating is coated on the Ta x C y On the penetration layer. The Ta x C y The permeation layer is formed on the surface and inside of the graphite substrate. x C y The infiltration layer is evenly distributed on the surface of the graphite substrate and has good compactness, which improves the environmental stability of the silicon carbide crystal during the growth process and reduces the thermal expansion of the graphite substrate. Optionally, the Ta x C y Ta in the permeate layer x C y The grains grow anisotropically. The TaC grains in the TaC coating grow along

[0200] Oriented growth with

[0220] direction; the Ta x C y Ta in the permeate layer x C y The TaC grains and the TaC grains in the TaC coating are arranged in an alternating manner. x C y Ta in the permeate layer x C y The grains are arranged in a staggered manner. x C y The size of the grains is 5-30 μm; the TaC grains in the TaC coating are arranged in a stacking staggered manner, and the size of the TaC grains is 15-40 μm. Ta x C y The grain size can reduce the Ta x C y Stress in the penetration layer, avoid Ta x C y The formation of the infiltration layer forms a clear grain boundary and avoids Ta x C y The generation of cracks in the permeation layer prevents the graphite substrate from squeezing the crystal. The TaC grains in the TaC coating grow in an oriented manner along the

[0200] and

[0220] directions, which can improve the compactness of the TaC coating, thereby improving the bonding strength between the composite coating and the graphite substrate, as well as the temperature resistance and thermal chemical corrosion resistance of the composite coating, thereby reducing the interference of the graphite substrate in the crystal growth process, which is conducive to the preparation of silicon carbide crystals with uniform quality. Optionally, the Ta x C y The thickness of the permeation layer is 10-450 μm, the thickness of the TaC coating is 30-400 μm, and the Ta x C y The total thickness of the permeation layer and the TaC coating is 40-500 μm. The beneficial effects of this application include but are not limited to: 1. The stress introduced by temperature gradient changes or crystal growth environment changes during the growth of silicon carbide crystals is the internal stress of the silicon carbide substrate. The silicon carbide substrate of the present application is a compressive stress region in the first gradient stress interval, a stress transition region in the second gradient stress interval, and a tensile stress region in the third gradient stress interval, thereby obtaining a silicon carbide substrate with a stepped stress distribution. 2. The radial internal stress and axial internal stress of the silicon carbide substrate of the present application in the first gradient stress interval, the second gradient stress interval and the third gradient stress interval are all low, and the stress distribution in the above-mentioned areas is uniform, and a uniform transition of stress can be achieved at the junction of the areas, which proves that the silicon carbide substrate has good uniformity and can be used as an epitaxial substrate or seed crystal, which is beneficial to improving the production quality of downstream products. 3. According to the silicon carbide substrate of the present application, the changes in the lattice in different regions can be known based on the stress values ​​tested in different regions. The cell parameters of the substrate at the microscopic level can be analyzed from the stress values, thereby playing a certain feedback role and facilitating the mass production of high-quality silicon carbide substrates. 4. The present application utilizes a composite coating formed on the surface of the crucible to improve the tolerance of the graphite crucible and reduce the impact of the graphite crucible on the crystal growth process, thereby improving the stability of the crystal environment and the uniformity of the temperature field, and reducing the number of defects and impurity content in the crystal, thereby preparing high-quality silicon carbide crystals with a stepped stress distribution. 5. According to the stress distribution in various areas of the silicon carbide substrate of the present application, the parameters of each process in crystal growth can be adjusted to prepare a high-quality silicon carbide substrate, and through the quantitative testing method of the present application, the detected stress also includes the stress in the substrate that has not yet been released through defects due to lattice distortion, thereby improving the detection accuracy of stress in the silicon carbide substrate and reflecting the entire stress information of the entire silicon carbide substrate. BRIEF DESCRIPTION OF THE DRAWINGS In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. In the drawings: FIG. 1 is a schematic diagram of the Raman equidistant focusing test of the internal stress of a substrate according to Example 4 of the present application. FIG. 2 is a schematic diagram of the Raman isofocusing plane test of the internal stress of the substrate involved in Example 4 of the present application. FIG. 3 is a SEM image of the surface of crucible 1# involved in Example 5 of the present application. DETAILED DESCRIPTION The embodiment of the present application provides a silicon carbide substrate with a stepped stress distribution. In order to enable those skilled in the art to better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of this application. The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments. Unless otherwise specified, the raw materials in the examples of this application were purchased through commercial channels. The principle of using Raman to test the stress of the substrate in the following embodiments is as follows: the Raman spectrometer passes a monochromatic laser of a certain wavelength through a series of optical paths and then converges the laser through an objective lens to irradiate the substrate surface, and the laser photons interact with the substrate lattice to cause photon scattering. The occurrence of Raman scattering is related to the lattice vibration of the substrate itself. When there is residual stress in the sample, the tensile and compressive stresses will cause the atomic bond length to be elongated or shortened accordingly, thereby causing the atomic vibration frequency to decrease or increase, which is reflected in the Raman spectrum as the characteristic peak of the substrate moves to a low frequency or a high frequency, that is, a peak position offset occurs. Example 1 Quantitative Testing Method for Substrate Stress (1) Calculation of peak position shift and stress conversion coefficient First, a single crystal XRD diffractometer is used to test the silicon carbide substrate and the substrate is analyzed according to the reciprocal lattice space. The parameters of the standard 4H-silicon carbide substrate are calculated and compared with the standard parameters. Then, according to the Bragg diffraction equation: 2dsinθ=nλ, where d is the interplanar spacing, θ is the diffraction half-angle, n is the diffraction order, and λ is the wavelength, the θ and d of different substrates are tested. It can be seen that the tensile and compressive stress inside the crystal causes the interplanar spacing d to expand and contract accordingly. According to the different interplanar spacings d0 and d x , d x It is divided into d1 and d2, where d0 is the theoretical crystal plane spacing, d1 is the crystal plane spacing after tensile stress, and d1 is greater than d0, and d2 is the crystal plane spacing after compressive stress, and d2 is less than d0. The difference in crystal plane spacing △d at different positions is calculated, △d=d1-d0 or △d=d0-d2. The actual peak position of the substrate is obtained according to the Raman test, and the peak position offset △v at different test points is calculated compared with the standard peak position; the stress value σ of the silicon carbide substrate at different test points is calculated in combination with Hooke's law or stress-strain formula, and the ratio of the peak position offset to the stress value at different test points is the peak position offset and stress conversion coefficient μ, which ranges from -125 to -2500. The calculation formula of Hooke's law is σ / S=E×(△d / d0), where S is the area affected by σ, which is a fixed value. When Hooke's law is applicable to microscopic calculations, S is the microscopic area, which can be calculated as unit 1. E is the Young's modulus of the substrate, which is a fixed parameter of the substrate. △d is the difference in crystal plane spacing at different positions, d0 is the theoretical crystal plane spacing, and σ is the stress value that needs to be calculated. The calculation formula for stress and strain is σ=E×△d, where E is the Young's modulus of the substrate, which is a fixed parameter of the substrate, △d is the difference in crystal plane spacing at different positions, and σ is the stress value to be calculated. (2) Quantitative test method of substrate stress First, set the Raman test point. You can perform a single-point test or a whole substrate test. Therefore, the single-point test is a sporadic coordinate point setting, and the Mapping scan is 10x10 to 50x50 and more points. The appropriate test method can be selected according to the properties of the substrate to determine the Raman test coordinate diagram of the silicon carbide substrate. The test results are then output, including but not limited to editable peak position, peak intensity data and Mapping. The output data is then subjected to peak position fitting, where fitting functions include but are not limited to Gaussian, Lorentz, GaussLor, AGauss, Aloren and AGaussLor, to obtain precise peak position values ​​and peak position offsets, with the peak position accurate to more than one decimal place. Finally, according to the peak shift and stress conversion coefficient μ calculated in (1) and the peak shift △v obtained in the Raman test, the stress value of silicon carbide is calculated as follows: σ(MPa)=μ×△v(cm -1 ), and finally obtain the stress test image of the substrate. Example 2 Test of curvature of substrate surface Since SiC is subjected to a lot of external mechanical stress during the process of preparing wafers from crystals through a series of processes, it is difficult to ensure that the surface is completely level, that is, there is a certain degree of curvature. In order to eliminate the influence of substrate surface curvature on substrate surface stress and internal stress test, it is necessary to first perform a curvature test on the substrate. According to the curvature, the substrate surface stress is tested with equal spacing focus, and the substrate internal stress is tested with equal spacing focus or equal focus plane. After the curvature test, the tester needs to be calibrated using single crystal silicon. First, a Z-axis mapping test is performed on the single crystal Si to confirm that it is a Gaussian distribution. The surface of the single crystal Si substrate is taken as the zero point, and an axial mapping test is performed from -100μm inside the single crystal Si substrate to +20μm above the single crystal Si substrate to analyze the change in Raman signal intensity. The single crystal Si substrate is a non-transparent material, so its peak intensity presents a Gaussian distribution, with the surface zero point position as the symmetry axis; then a surface focusing single point test is performed on the single crystal Si, and the single crystal Si Raman peak position is made to be 520.70cm by the correction instrument. -1 , at this time, it means that the instrument optical path has been calibrated. The 4H-SiC substrate was tested in the Z direction using a calibrated tester. The Raman laser was emitted from -100μm inside the 4H-SiC substrate to +20μm above the 4H-SiC substrate. The 4H-SiC substrate and other silicon carbide substrates are transparent materials, so the Raman laser can penetrate into the silicon carbide substrate, and the feedback signal gradually weakens as the Raman laser penetration depth increases. Other silicon carbides include semi-insulating, conductive, and 3C, 4H, 15R, 6H and other silicon carbide substrates with different crystal forms. Therefore, the Raman peak intensity of the silicon carbide substrate gradually decreases as the Raman laser penetrates deeper into the silicon carbide substrate. The Raman device’s autofocus function can be used to automatically find the strongest peak of the test signal on the substrate surface, so that the curvature of the substrate surface can be measured with the help of this function. The specific method is as follows: S1: First, ensure that the test platform is level, select a fixed origin (0,0) on the test platform, and mark the point as the three-dimensional coordinate (0,0,0); S2: Raise the objective lens to a height that does not touch the substrate according to the thickness of the substrate, and automatically focus on any point on the substrate surface where the test signal is the strongest. Mark this position as (a, b, c), and then adjust the Raman laser Move down by a distance of H and focus to (a, b, cH); S3: Adjust the Raman laser test position to (0,0,cH) to perform an iso-focal plane test to obtain a two-dimensional peak intensity scan of the substrate surface. According to the peak intensity distribution relationship of the silicon carbide substrate and the two-dimensional peak intensity scan of the substrate surface, the distance between the focal plane of the Raman laser and the surface of the silicon carbide substrate can be obtained, thereby obtaining the in-plane curvature of the substrate and obtaining the in-plane curvature distribution map of the substrate. Example 3 Axial test distinguishes substrate surface stress from internal stress Since the substrate is affected by the temperature field and the introduction of inclusions during the crystal growth process, thermal stress and residual stress exist inside the crystal, and the substrate is obtained after a series of cutting, grinding and polishing. The above-mentioned processing process of the crystal to form the substrate will cause surface lattice distortion, thereby generating surface stress. In order to further distinguish the surface stress from the body stress of the substrate, the substrate is subjected to axial testing according to the light transmittance characteristics of silicon carbide, namely, Z-direction Mapping, with the substrate surface as the zero point. The test range of the axial test is from -150μm to -30μm inside the substrate to above the substrate surface. During the test, the laser spot gradually focuses upward from the substrate body to the damaged stress extension layer, further to the surface damaged layer, and then continues to focus upward into the air to determine the thickness of the substrate surface damaged layer and the damaged stress extension layer. With the substrate surface as the zero point, the test range of the axial test is that the Raman laser spot gradually moves upward from -100μm inside the substrate to 30μm above the substrate surface. Five points of the substrate are tested up and down, left and right. The five points on the substrate show the same trend, and there is only a difference in the maximum negative stress at the surface damage. When the laser spot gradually moves upward from -100μm in the substrate to -30μm, the stress fluctuates up and down along the horizontal line, indicating that the internal stress is uniform; when the laser spot is focused on the damaged stress extension layer, the stress curve begins to show a downward trend at -30μm, and the downward trend of stress gradually increases. When it approaches the zero point on the substrate surface, it reaches the negative stress maximum value, which is the stress value of the surface damaged layer; finally, the laser spot continues to move upward and gradually leaves the substrate surface into the air, and the stress shows a rapid rise to chaotic fluctuations. Therefore, the thickness of the surface damaged layer and the damaged stress extension layer of the substrate can be determined according to the axial test results of the substrate, thereby distinguishing the stress in the substrate into surface stress and internal stress, which provides a theoretical basis for the subsequent quantitative test of surface stress and internal stress in the substrate. Example 4 Quantitative Testing of Stress in the Substrate During the PVT crystal growth process, silicon carbide is affected by temperature gradient changes and the intrusion of inclusions, resulting in residual stress in the crystal. Different from the surface processing stress, the internal stress is located at a certain depth from the surface, reflecting the thermal stress and residual stress of crystal growth. In addition, silicon carbide undergoes a large amount of external mechanical stress in the process of preparing the substrate from the crystal through a series of processing, and it is difficult to ensure that the surface is completely horizontal, that is, there is a certain degree of curvature on the surface of the substrate. According to the measurement method of the curvature of the substrate surface obtained by Raman testing in Example 2, this embodiment, combined with the physical properties of silicon carbide, involves a method for quantitatively characterizing its internal stress that is not affected by surface curvature. According to the physical property of silicon carbide being light-transmissive, the method of using Raman to quantitatively characterize the internal stress of the substrate is divided into equal-spacing focusing test and equal-focusing plane test, as follows: 1. Equal spacing focus test The principle of the equidistant focusing test is shown in FIG1 . FIG1( b ) is a schematic diagram of the focus of the Raman laser inside the substrate, and FIG1( a ) is a schematic diagram of the test principle. Under this test mode, the focus depth (d) of the Raman laser focused inside the substrate at different test points remains unchanged. First, the penetration depth (Dp) of the Raman laser is calculated according to the following formula: In the above formula, Dp is the penetration depth of Raman excitation in the sample, α is the absorption coefficient, λ is the laser wavelength, and k is the extinction coefficient. The specific value of the penetration depth (Dp) is calculated according to the above formula, and a suitable focusing depth (d) is selected. The requirement that needs to be met is that the penetration depth (Dp) is greater than the focusing depth (d), and since the test is for internal stress, the focusing depth d needs to be greater than the thickness of the surface damage layer and the damage stress extension layer tested in Example 3. The Raman laser spot diameter (D) is then calculated according to the following formula: Where λ is the laser wavelength and Na is the numerical aperture of the device. The spot diameter (D) is the short axis diameter of the laser spot. The larger the value, the stronger the peak intensity of Raman at each test position, and the more impurity peaks there are, which affects the detection accuracy of Raman. When λ is determined, if the value of Na is large, the peak intensity is very weak, which will also affect the Raman detection accuracy. Therefore, it is necessary to select appropriate λ and Na to improve the accuracy of the test method. According to the characteristics of silicon carbide materials, the peak intensity reaches the highest when the laser is focused on the substrate surface. Therefore, the automatic focusing function of the Raman tester can be used to focus the laser on the substrate surface. Then, according to the focusing depth (d) determined above, the equally spaced focusing test is performed on different positions of the substrate, that is, the distance between the focusing plane and the substrate surface is always equal. This method is not affected by the bending and warping of the substrate surface, so that the obtained Raman peak intensity is always maintained at the axial maximum value of the test point. According to the stress quantitative testing method of Example 1, the in vivo stress test diagram of the substrate is obtained. 2. Equifocal plane test The principle of the equal-focusing plane test is shown in Figure 2. Figure 2(b) is a schematic diagram of the focus of the Raman laser inside the substrate, and Figure 2(a) is a test principle diagram. Under this test method, the focus depth (d) of the Raman laser spot focused on the inside of the substrate changes at different test points. The laser spot at different test points is focused on the same plane inside the substrate. The penetration depth (Dp) of the Raman laser is calculated according to the same formula as the equidistant focusing test, and the appropriate focus depth (d) is selected in the same selection method. Since the focus planes in this test method are equal, combined with the characteristics of silicon carbide, when the laser is focused on the substrate surface, the peak intensity reaches the highest and the signal gradually decays with the increase of the focus depth. And according to the effective signal-to-noise ratio tested by the Raman tester, it is found that the effective signal-to-noise ratio is inversely correlated with the Raman laser focus depth (d). Therefore, in this test method, the focus depth (d) at different test points needs to be combined with the curvature of the substrate to obtain the effective peak position offset through the stress quantitative test method of Example 1, thereby reducing the internal stress. The test error, the test method obtains the in vivo stress test result diagram of the substrate. Through the above two testing methods of equal spacing focusing and equal focusing plane, the peak position offset inside the substrate is obtained respectively, and the internal stress distribution of the substrate is calculated. The difference between the internal stress values ​​of the substrate calculated by the equal spacing focusing test and the equal focusing plane test is ≤0.2MPa, and the peak position difference between the equal spacing focusing test and the equal focusing plane test is <0.0005cm -1 The above two methods can verify the accuracy of the quantitative detection method of the present application, and the accuracy of the test method can be further improved according to the setting of the focus depth and the spot diameter. The internal stress includes the internal absolute stress and the internal relative stress, which is illustrated by taking 4H-SiC as an example: The absolute stress in the body is calculated by subtracting the measured 4H-SiC peak position from the standard peak position to determine the simplified wave vector peak position, such as FTO (2 / 4). The absolute stress in the body is calculated as: σabsolute=μ×(v 实际 -V FTO(2 / 4) ), σ is in MPa, v 实际 and V FTO(2 / 4) The unit is cm -1 , where v 实际 is the actual FTO(2 / 4) peak position at the substrate test point, V FTO(2 / 4) is the standard peak position of FTO (2 / 4) of the substrate; The relative stress in the body is calculated by subtracting the measured values ​​of the same simplified wave vector peak position at any two points in the SiC substrate surface, such as the FTO (2 / 4) peak position. The calculation formula for the relative stress in the body is: σrelative=μ×(v (x1,y1) -V (x2,y2) ), σ is in MPa, v (x1,y1) and V (x2,y2) The unit is cm -1 , where v (x1,y1) is the actual FTO(2 / 4) peak position at the substrate test point, V (x2,y2) is the actual FTO (2 / 4) peak position of the substrate relative point. The relative stress value of the test point is calculated according to the above calculation formula. If the relative value of σ is positive, it means that the stress of the test point relative to the relative point is tensile stress. If the relative value of σ is negative, it means that the stress of the test point relative to the relative point is compressive stress. Example 5 This embodiment relates to a method for preparing a graphite crucible for growing silicon carbide, comprising the following steps: 1) Soaking the graphite substrate in a tantalum pentachloride solution, wherein the concentration of tantalum pentachloride is 5%-95%, the porosity of the graphite substrate is 5%-55%, and gas pressure treatment is performed, the gas pressure is 1-2MPa, and the pressurization time is 0.5-1h. The soaked graphite substrate is dried and calcined to obtain a graphite substrate with Ta x C y Graphite substrate of the infiltration layer; 2) Prepare a slurry containing TaC, the slurry comprising: 25-40 parts of TaC powder, 20-35 parts of solvent, 15-25 parts of co-solvent, 1-3 parts of defoamer, 2-5 parts of thickener, 5-15 parts of straight-chain saturated aliphatic hydrocarbon, 1-3 parts of carbon powder, and spray the slurry onto the surface with TaC. x C yThe surface of the graphite substrate of the infiltration layer is dried and then sintered under an inert gas: sintering at 700-900 mbar and 500-700°C for 3-5 hours; then the pressure is increased to 1000-1100 mbar, the temperature is increased to 1600-1800°C, and sintering is performed for 8-10 hours; finally, the pressure is increased to 145-155 kPa, the temperature is increased to 2200-2400°C, and sintering is performed for 14-16 hours, and finally the temperature is reduced to room temperature and normal pressure within 15-25 hours to obtain the graphite crucible. Optionally, the calcination in step 1) is carried out under an inert gas, and the dried graphite substrate is placed in a vacuum furnace for stepwise heating and stepwise cooling: Stepwise heating: heating to 250-350℃, calcining for 4-6h, then heating to 850-950℃, calcining for 2.5-3.5h, and finally heating to 1200-1300℃, calcining for 3-5h; Step cooling: cool to 900-1100℃, maintain for 0.5-2h, then cool to 750-850℃, maintain for 0.5-1h, and finally cool to room temperature at a cooling rate of 15-25℃ / h. Optionally, the pressure in the vacuum furnace during calcination is 800-1000 mbar. Crucibles 1#-6# and comparative crucibles D1#-D3# were prepared according to the above preparation method, as follows: Crucible 1# 1) placing a tantalum pentachloride n-butanol solution soaked with a graphite substrate in a container, wherein the concentration of tantalum pentachloride in the tantalum pentachloride n-butanol solution is 70%, and the porosity of the graphite substrate is 45%. The container is placed in a pressure chamber for pressurization treatment at a pressurization rate of 0.1 MPa / min. When the pressure in the pressure chamber reaches 1 MPa, the pressure is maintained for 1 hour, and then the pressure is released. The soaked graphite substrate is transferred to a vacuum furnace, dried at 60°C for 7 hours, and then placed in a vacuum furnace. Argon gas was introduced to maintain the pressure in the furnace at 900 mbar, and then calcination was carried out. The temperature was increased step by step: increased to 300°C in 1 hour and calcined for 5 hours; then increased to 900°C in 2 hours and calcined for 3 hours; finally increased to 1250°C in 1 hour and calcined for 4 hours; then staged cooling began: cooled to 1000°C in 1 hour, maintained for 1 hour, then cooled to 800°C in 1 hour, maintained for 1 hour, and finally cooled to room temperature at a rate of 20°C / h to obtain a graphite substrate with a TaxCy infiltration layer. 2) 4N grade TaC was dissolved in an organic solvent to prepare a slurry containing TaC, the slurry comprising: 32.7 parts of TaC powder, 25 parts of ethanol, 20 parts of tetrahydrofuran, 1.5 parts of BYK defoamer, 3.5 parts of cellulose ether, 10 parts of straight-chain saturated aliphatic hydrocarbons, and 1.3 parts of carbon powder. After the above components were mixed and stirred for 2.5 hours, the slurry was sprayed onto the surface of the graphite substrate with the TaxCy penetration layer using a sprayer. The sprayed graphite substrate was placed in a heating furnace, heated to 200°C in an air environment and dried for 10 hours, and then argon gas was introduced into the heating furnace, sintered at 800 mbar and 600°C for 4 hours, then the pressure was increased to 1000 mbar, the temperature was increased to 1700°C, and sintered for 9 hours, and finally the pressure was increased to 150 kPa, the temperature was increased to 2300°C, and sintered for 15 hours, and finally cooled to room temperature and normal pressure after 20 hours, to obtain crucible 1#. Crucible 2# 1) placing a tantalum pentachloride n-butanol solution soaked with a graphite substrate in a container, wherein the concentration of tantalum pentachloride in the tantalum pentachloride n-butanol solution is 70%, and the porosity of the graphite substrate is 45%. The container is placed in a pressure chamber for pressurization treatment at a pressurization rate of 0.2 MPa / min. When the pressure in the pressure chamber reaches 1 MPa, the pressure is maintained for 1 hour, and then the pressure is released. The soaked graphite substrate is transferred to a vacuum furnace, dried at 80°C for 5 hours, and then a vacuum furnace is passed through the vacuum furnace. Argon gas was introduced to maintain the pressure in the furnace at 800 mbar, and then calcination was carried out with step-by-step heating: heating to 250°C for 1 hour and calcination for 6 hours; then heating to 850°C for 2 hours and calcination for 3.5 hours; finally heating to 1200°C for 1 hour and calcination for 5 hours; stage-by-stage cooling was started: cooling to 900°C for 1 hour and maintaining for 2 hours, then cooling to 750°C for 1 hour and maintaining for 1 hour, and finally cooling to room temperature at a rate of 15°C / h to obtain a graphite substrate with a TaxCy infiltration layer. 2) Dissolve 4N grade TaC in an organic solvent to prepare a TaC-containing slurry, which includes: 25 parts of TaC powder, 20 parts of ethanol, 25 parts of tetrahydrofuran, 3 parts of BYK defoamer, 2 parts of cellulose ether, 15 parts of straight-chain saturated aliphatic hydrocarbons and 3 parts of carbon powder are mixed and stirred for 2.5 hours, and then the slurry is sprayed onto the surface of the graphite substrate with the TaxCy penetration layer using a sprayer. The sprayed graphite substrate is placed in a heating furnace, heated to 180°C in an air environment and dried for 12 hours, and then argon is introduced into the heating furnace, and sintered at 700 mbar and 700°C for 3 hours, then the pressure is increased to 1000 mbar, the temperature is increased to 1800°C, and sintered for 8 hours, and finally the pressure is increased to 145 kPa, the temperature is increased to 2400°C, and sintered for 16 hours, and finally cooled to room temperature and normal pressure after 25 hours to obtain crucible 2#. Crucible 3# 1) placing a tantalum pentachloride n-butanol solution soaked with a graphite substrate in a container, wherein the concentration of tantalum pentachloride in the tantalum pentachloride n-butanol solution is 70%, and the porosity of the graphite substrate is 45%. The container is placed in a pressure chamber for pressurization treatment at a pressurization rate of 0.05 MPa / min. When the pressure in the pressure chamber reaches 2 MPa, the pressure is maintained for 0.5 h, and then the pressure is released. The soaked graphite substrate is transferred to a vacuum furnace, dried at 60° C. for 7 h, and then a vacuum furnace is introduced. Argon gas was used to maintain the pressure in the furnace at 1000 mbar before calcination. The temperature was increased step by step: from 350°C in 1 hour to calcine for 4 hours; then from 950°C in 2 hours to calcine for 2.5 hours; finally from 1300°C in 1 hour to calcine for 3 hours; the temperature was lowered in stages: from 1100°C in 1 hour to maintain for 0.5 hours, then from 850°C in 1 hour to maintain for 0.5 hours, and finally from 25°C / h to room temperature to obtain a graphite substrate with a TaxCy infiltration layer. 2) 4N grade TaC was dissolved in an organic solvent to prepare a slurry containing TaC, which included: 40 parts of TaC powder, 35 parts of ethanol, 15 parts of tetrahydrofuran, 1 part of BYK defoamer, 5 parts of cellulose ether, 5 parts of straight-chain saturated aliphatic hydrocarbons, and 1 part of carbon powder. After mixing and stirring the above components for 2.5 hours, the slurry was sprayed onto the surface of the graphite substrate with the TaxCy penetration layer using a sprayer. The sprayed graphite substrate was placed in a heating furnace, heated to 220°C in an air environment and dried for 8 hours, and then argon was introduced into the heating furnace. It was sintered at 900 mbar and 500°C for 5 hours, and then the pressure was increased to 1100 mbar, the temperature was increased to 1600°C, and sintered for 10 hours. Finally, the pressure was increased to 155 kPa, the temperature was increased to 2200°C, and sintered for 14 hours. Finally, it was cooled to room temperature and normal pressure after 15 hours to obtain composite coating 3#. Crucible 4# The difference between this embodiment and crucible 1# is that in step 1), the graphite substrate is not subjected to gas pressurization treatment during immersion in the tantalum pentachloride n-butanol solution, but is only immersed at room temperature and pressure for 1 hour, and then dried and calcined. The other conditions are the same as those of crucible 1#, and crucible 4# is obtained. Crucible 5# The difference between this embodiment and crucible 1# is that in step 1), the graphite substrate after soaking and drying is calcined in a vacuum furnace by maintaining the pressure in the furnace at 900 mbar, calcining at 700°C for 7 hours, and then calcining at 1200°C for 5 hours before starting staged cooling. The other conditions are the same as those of crucible 1#, and a composite coated crucible 5# is obtained. Crucible 6# The difference between this embodiment and crucible 1# is that in step 2), the graphite substrate after spraying the slurry and drying is sintered in the heating furnace as follows: sintering at 1000 mbar and 1200°C for 12 hours, and then the pressure is increased to 140kPa, the temperature is raised to 2300℃, sintered for 20h, and finally cooled to room temperature and normal pressure after 20h. The other conditions are the same as those of crucible 1#, that is, crucible 6# is obtained. Comparison crucible D1# Compared with Example 1, step 1) of this comparative example is the same, and step 2) adopts the CVD method, specifically: 2) will carry Ta x C y The graphite substrate of the infiltration layer is placed in a chemical vapor deposition furnace, evacuated to below 50Pa, and then heated to 1200°C. A mixed gas consisting of ethane, TaCl5, H2 and carrier gas argon is introduced. The mixed gas enters the deposition furnace at a stable flow rate, and the furnace pressure is maintained at 5000Pa. The molar ratio of TaCl5 and ethane gas is first controlled at 1.2:1 to form a transition layer with a thickness of 50μm; then the molar ratio of TaCl5 and ethane gas is controlled to 3:1 to form a TaC coating with a thickness of 60μm, thereby obtaining a comparative crucible D1#. Compare Crucible D2# Compared with the comparative crucible D1#, this comparative example does not perform step 1), and only performs step 2) on the graphite substrate. Step 2) forms a transition layer and a TaC coating on the graphite substrate by a CVD method. The specific conditions of the CVD method are the same as those of the comparative crucible D1#, that is, a comparative crucible D2# is prepared. Compare Crucible D3# Compared with Example 1, this comparative example does not perform step 1), and only performs step 2) on the graphite substrate. The operating conditions of step 2) are the same as those of Example 1, that is, a comparative crucible D3# is prepared. The crucibles 1#-6# and the comparative crucibles D1#-D3# prepared above were subjected to high temperature corrosion and thermochemical corrosion experiments to simulate the growth environment of the third generation semiconductor silicon carbide crystal using the PVT method. The crucibles 1#-6# and the comparative crucibles D1#-D3# were kept at 2300°C and etched in a silicon carbide atmosphere for 100 hours. The test results are shown in Table 1: Table 1 In Table 1, erosion weight loss rate = [(weight before experiment - weight after experiment) / weight before experiment] × 100%. The above high temperature erosion and thermochemical erosion experiments prove that the TaC coating of this embodiment can improve the resistance to the graphite substrate. The protective performance of the TaC coating can extend the tolerance and corrosion resistance of the TaC coating, thereby extending the service life of the crucible. The crucible 1# was subjected to SEM testing after multiple uses, and the results are shown in Figure 3. Figure 3(a) is a test image of the local composite coating on the crucible 1#, and Figures 3(b) and 3(c) are enlarged images of the cracks in Figure 3(a). It can be seen that even though the crucible 1# prepared in this embodiment has cracks after multiple uses, the width of the cracks can be controlled within 1μm, and no through cracks will be formed. It can still be used to produce high-quality silicon carbide crystals. The TaC coating involved in the above crucible x C y EDS mapping was performed on the permeation layer to characterize the Ta x C y The permeation layer is formed inside and on the surface of the graphite substrate, and interspersed with the inside of the graphite substrate, so that the bonding force with the graphite substrate is strong, thereby improving the durability of the crucible. x C y XRD test of the penetration layer, TaC coating and TaC x C y Ta of the permeation layer x C y The grains grow anisotropically. The TaC grains of the TaC coating grow along the

[0200] and

[0220] directions. The composite coating prepared in this embodiment is deposited on the surface of the graphite substrate, which can protect the graphite substrate and improve the temperature resistance and chemical corrosion resistance of the graphite substrate, thereby reducing the interference of the graphite substrate on the crystal growth environment and improving the stability of the crystal environment. The Ta can be controlled by calcining and sintering processes. x C y The oriented growth of grains in the infiltration layer and the TaC coating improves the compactness of the composite coating and can be used to prepare high-quality silicon carbide crystals. Example 6 This embodiment relates to a method for preparing a silicon carbide single crystal with a diameter of 150 mm, comprising the following steps: (1) Adding silicon carbide powder into a graphite crucible; (2) Evacuate the furnace to 10 -6 mbar, and then introduce high-purity inert gas to 300-500mbar, repeat this process 2-3 times, and finally evacuate the furnace to 10 -6 Below mbar; (3) introducing high-purity inert gas into the furnace body, raising the pressure to 10-100 mbar within 1-3 hours, and continuously introducing high-purity inert gas and maintaining the pressure constant; (4) Crystal growth stage: While maintaining the pressure unchanged, the temperature inside the furnace is raised to the single crystal growth temperature of 2200K-2800K within 3-5 hours, and the growth time is 30-150 hours; (5) After the single crystal growth is completed, open the furnace body and take out the graphite crucible to obtain the silicon carbide single crystal. Add equal amounts of silicon carbide powder to crucibles 1#-3# and comparative crucibles D1#-D3# prepared in Example 6, respectively; evacuate the furnace to 10 -6 mbar, and then introduce high-purity inert gas to 300 mbar. Repeat this process 3 times and finally evacuate the furnace to 10 -6 mbar or less; introduce high-purity inert gas into the furnace body, raise the pressure to 30 mbar within 1 hour, and continue to introduce high-purity inert gas to keep the pressure constant; crystal growth stage: while keeping the pressure constant, raise the temperature in the furnace body to the single crystal growth temperature of 2500K within 3 hours, and the growth time is 70 hours; after the single crystal growth is completed, open the furnace body, take out the graphite crucible, and obtain silicon carbide single crystals 1#-3# and comparison silicon carbide single crystals D1#-D3# respectively. The above-mentioned silicon carbide single crystals 1#-3# and comparative silicon carbide single crystals D1#-D3# were subjected to the same cutting, grinding, mechanical polishing and chemical polishing to obtain silicon carbide substrates 1#-3# and comparative silicon carbide substrates D1#-D3#. In vivo stress tests were performed using the methods of Examples 1-4. The results are shown in Tables 2 and 3. Table 2 Table 3 The surface shape of the silicon carbide substrates 1#-3# was detected. The local thickness deviation of the first gradient stress interval of the silicon carbide substrates 1#-3# was within the range of 0.1-0.3 μm, the local thickness deviation of the second gradient stress interval was within the range of 0.3-0.7 μm, and the local thickness deviation of the third gradient stress interval was within the range of 0.7-2 μm. The stress values ​​in Table 2-3 all refer to relative stress values. In this application, the positive and negative values ​​in the stress values ​​represent whether the substrate is subjected to tensile stress or compressive stress. In the prior art, when describing and evaluating the stress state of a substrate, the maximum value minus the minimum value in the above stress values ​​is often used to reflect the stress state of the substrate. For example, the relative stress in the axial body of the SiC substrate 1# first gradient stress interval is -13.3~15.7. When the relative stress in the axial body of the SiC substrate 1# first gradient stress interval is generally expressed in this field, it can also be described as a stress value of 29MPa. The data in the above Tables 2-3 are only the substrates prepared by crucibles 1#-3# prepared in Example 5 and comparative crucibles D1#-D3#. When the same crystal growth and processing methods in Example 6 are used and crucibles 4#-8# are used to prepare silicon carbide substrates, and the parameters in Tables 2-3 are characterized by absolute stress, the width of the third gradient stress interval extending inward from the edge of the substrate is 2.5-30 mm, the width of the second gradient stress interval extending inward from the edge of the third gradient stress interval is 10-50 mm; the relative stress in the body of the first gradient stress interval is Greater than or equal to the relative internal stress in the second gradient stress interval, and less than the relative internal stress in the third gradient stress interval; the radial internal relative stress in the first gradient stress interval is -15 to 15 MPa, the radial internal relative stress in the second gradient stress interval is -10 to 10 MPa, and the radial internal relative stress in the third gradient stress interval is -24.8 to 24.8 MPa; △S1≤△S2; 0MPa≤△S1≤10MPa, 0MPa≤△S2≤22MPa; △S3≤△S4; 0MPa≤△S3≤5MPa, 10MPa ≤△S4≤20MPa; 0MPa≤S1-S2≤14.5MPa, 0MPa≤S3-S1≤22.5MPa; the relative axial internal stress in the first gradient stress interval is -20~20MPa, the relative axial internal stress in the second gradient stress interval is -16~16MPa, and the relative axial internal stress in the third gradient stress interval is -25~25MPa; 0MPa≤△S5≤10MPa, 0MPa≤△S6≤10MPa, 0MPa≤△S7≤10MPa; the absolute internal stress in the second gradient stress interval is greater than or equal to The in-vivo absolute stress in the first gradient stress interval is less than the in-vivo absolute stress in the third gradient stress interval; the radial in-vivo absolute stress in the first gradient stress interval is -20 to 5 MPa, the radial in-vivo absolute stress in the second gradient stress interval is 5 to 15 MPa, and the radial in-vivo absolute stress in the third gradient stress interval is 15.2 to 50 MPa; the axial absolute stress in the first gradient stress interval is -30 to 10 MPa, the axial absolute stress in the second gradient stress interval is 10 to 30 MPa, and the axial absolute stress in the third gradient stress interval is 30.2 to 50 MPa. Each embodiment in this application is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the device, equipment, and non-volatile computer storage medium embodiments, since they are basically similar to the method embodiments, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. Finally, it should be understood that the above embodiments of the present application are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art should understand that the technical solutions described in the above embodiments can still be modified, or some of the technical features can be replaced by equivalents; and these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A silicon carbide substrate with a stepped stress distribution, wherein: The silicon carbide substrate has a diameter of 150 mm or more, and includes a first main surface and a second main surface; The first main surface has a first gradient stress interval, a second gradient stress interval surrounding the first gradient stress interval, and a third gradient stress interval surrounding the second gradient stress interval, wherein the width of the third gradient stress interval extending inward from the edge of the substrate is 2.5-30 mm, and the width of the second gradient stress interval extending inward from the edge of the third gradient stress interval is 10-50 mm; The internal relative stress in the first gradient stress interval is greater than or equal to the internal relative stress in the second gradient stress interval, and less than the internal relative stress in the third gradient stress interval. The internal relative stress is a stress value detected at a point extending vertically from the first main surface or the second main surface into the silicon carbide substrate by at least 3 μm.

2. The silicon carbide substrate with a stepped stress distribution according to claim 1, wherein: The in-vivo absolute stress in the second gradient stress interval is greater than or equal to the in-vivo absolute stress in the first gradient stress interval, and less than the in-vivo absolute stress in the third gradient stress interval, wherein the in-vivo absolute stress is a stress value detected at a position extending vertically from the first main surface or the second main surface into the silicon carbide substrate by at least 3 μm; and / or At any plane parallel to the first main surface and / or the second main surface, the radial internal absolute stress in the first gradient stress interval is -20 to 5 MPa, the radial internal absolute stress in the second gradient stress interval is 5 to 15 MPa, and the radial internal absolute stress in the third gradient stress interval is 15.2 to 50 MPa.

3. The silicon carbide substrate with a stepped stress distribution according to claim 2, wherein: An axial test is performed by extending vertically into the silicon carbide substrate from any point on the first main surface. The axial absolute stress in the first gradient stress interval is -30 to 10 MPa, the axial absolute stress in the second gradient stress interval is 10 to 30 MPa, and the axial absolute stress in the third gradient stress interval is 30.2 to 50 MPa.

4. The silicon carbide substrate with a step-wise stress distribution according to claim 1, wherein: At any plane parallel to the first main surface and / or the second main surface, the radial internal relative stress in the first gradient stress interval is -15 to 15 MPa, the radial internal relative stress in the second gradient stress interval is -10 to 10 MPa, and the radial internal relative stress in the third gradient stress interval is -24.8 to 24.8 MPa.

5. [Corrected 03.06.2024 according to Rule 91] The silicon carbide substrate with a step-by-step stress distribution according to claim 1, wherein: At any plane parallel to the first major surface and / or the second major surface, S max1 represents the maximum absolute value of the relative stress in the radial body within the first gradient stress interval, S max2 represents the maximum absolute value of the relative stress in the radial body within the second gradient stress interval, S max3 Represents the maximum absolute value of radial relative internal stress in the third gradient stress interval, △S1=S max1 -S max2 , △S2=S max3 -S max1 , △S1≤△S2.

6. The silicon carbide substrate with a step-wise stress distribution according to claim 5, wherein: 0MPa≤△S1≤10MPa, 0MPa≤△S2≤22MPa.

7. The silicon carbide substrate with a step-wise stress distribution according to claim 1, wherein: At any plane parallel to the first major surface and / or the second major surface, S min1 represents the minimum absolute value of the relative stress in the radial body within the first gradient stress interval, S min2 represents the minimum absolute value of the relative stress in the radial body within the second gradient stress interval, S min3 represents the minimum absolute value of the relative stress in the radial body within the third gradient stress interval, △S3=S min1 -S min2 , △S4=S min3 -S min1 , △S3≤△S4.

8. The silicon carbide substrate with a stepped stress distribution according to claim 7, wherein: 0MPa≤△S3≤5MPa, 10MPa≤△S4≤20MPa.

9. The silicon carbide substrate with a step-wise stress distribution according to claim 1, wherein: At any plane parallel to the first main surface and / or the second main surface, S1 represents the average absolute value of the radial internal relative stress in the first gradient stress interval, S2 represents the average absolute value of the radial internal relative stress in the second gradient stress interval, and S3 represents the average absolute value of the radial internal relative stress in the third gradient stress interval, 0MPa≤S1-S2≤14.5MPa, 0MPa≤S3-S1≤22.5MPa.

10. The silicon carbide substrate with a step-by-step stress distribution according to claim 1, wherein: An axial test is performed by extending vertically into the silicon carbide substrate from any point on the first main surface. The axial in-body relative stress in the first gradient stress interval is -20 to 20 MPa, the axial in-body relative stress in the second gradient stress interval is -16 to 16 MPa, and the axial in-body relative stress in the third gradient stress interval is -25 to 25 MPa.

11. The silicon carbide substrate with a step-wise stress distribution according to claim 10, wherein: An axial test is performed by extending vertically from any point of the first main surface into the silicon carbide substrate, S max5 It is recorded as the maximum relative stress in the axial body in the first gradient stress interval, S min5 It is recorded as the minimum relative stress in the axial body in the first gradient stress interval, △S5 = S max5 -S min5 , 0MPa≤△S5≤10MPa; S max6 It is recorded as the maximum relative stress in the axial body in the second gradient stress interval, S min6 It is recorded as the minimum relative stress in the axial body in the second gradient stress interval, △S6 = S max6 -S min6 , 0MPa≤△S6≤10MPa; S max7 S is the maximum relative stress in the axial body in the third gradient stress interval. min7 It is recorded as the minimum relative stress in the axial body in the third gradient stress interval, △S7 = S max7 -S min7 , 0MPa≤△S7≤10MPa.

12. The silicon carbide substrate with a step-wise stress distribution according to claim 1, wherein: The diameter of the silicon carbide substrate is greater than 200 mm, the width of the third gradient stress interval extending inward from the edge of the substrate is 21-30 mm, and the width of the second gradient stress interval extending inward from the edge of the third gradient stress interval is 10-39 mm; and / or The thickness of the silicon carbide substrate is not less than 0.1 mm.

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