阅读说明：本技术 一种碳化硅晶体的二次退火方法 (Secondary annealing method of silicon carbide crystals ) 是由 陈荣坤 李文勇 马敬军 张冬 于 2021-07-26 设计创作，主要内容包括：本发明提供了一种碳化硅晶体的二次退火方法,包括：将碳化硅晶体放置在坩埚中在退火炉内进行退火处理；所述坩埚包括：大坩埚；设置在所述大坩埚内的小坩埚；设置在所述小坩埚内的环形石墨毡；所述晶体水平方向放置在环形石墨毡的中心。本发明在二次退火阶段使用“双坩埚”模式加“环形石墨毡”,减少温度梯度,使温度梯度保持在2～6℃,使晶体内部应力得到缓慢释放,同时避免晶体在退火过程中产生新的应力。本发明提供的二次退火的方法可对坩埚高度和晶体厚度进行合理配置,同时放置多块晶体,增加效率。(The invention provides a secondary annealing method of a silicon carbide crystal, which comprises the following steps: placing the silicon carbide crystal in a crucible and carrying out annealing treatment in an annealing furnace; the crucible includes: a large crucible; a small crucible disposed within the large crucible; the annular graphite felt is arranged in the small crucible; the crystal is horizontally placed in the center of the annular graphite felt. In the invention, a double-crucible mode and an annular graphite felt are used in the secondary annealing stage, so that the temperature gradient is reduced and kept at 2-6 ℃, the internal stress of the crystal is slowly released, and the new stress generated in the annealing process of the crystal is avoided. The secondary annealing method provided by the invention can reasonably configure the height of the crucible and the thickness of the crystal, and simultaneously place a plurality of crystals, thereby increasing the efficiency.)

1. A method of secondary annealing of a silicon carbide crystal, comprising:

placing the silicon carbide crystal in a crucible and carrying out annealing treatment in an annealing furnace;

the crucible includes:

a large crucible;

the small crucible is arranged in the large crucible, and the small crucible and the large crucible are coaxially arranged;

the annular graphite felt is arranged in the small crucible;

the silicon carbide crystals are horizontally placed in the center of the annular graphite felt.

2. The method of claim 1, wherein the silicon carbide crystal has a diameter of 4 to 8 inches.

3. The method as claimed in claim 1, wherein the annular graphite felt has an inner diameter of 1/3-2/3 of the diameter of the silicon carbide crystal, an outer diameter of not less than the diameter of the silicon carbide crystal, and a thickness of 0.3-2.5 times the thickness of the silicon carbide crystal.

4. The method according to claim 1, wherein the small crucible has an inner diameter 1.1 to 2.0 times the diameter of the silicon carbide crystal.

5. The method of claim 1, wherein the large crucible has an inner diameter 1.1 to 3.0 times the outer diameter of the small crucible.

6. The method according to claim 1, wherein the distance between the bottom of the small crucible and the bottom of the large crucible is 20 to 150 mm.

7. The method of claim 1, wherein the gap volume between the large crucible and the small crucible is 20-40% of the volume of the large crucible.

8. The method of claim 1, wherein the gap between the large crucible and the small crucible is filled with a thermal insulating material.

9. The method of claim 8, wherein the insulating material is selected from one or both of graphite felt or silicon carbide powder.

10. The method of claim 1, wherein the annealing process comprises:

heating to the annealing temperature, preserving heat, then carrying out primary cooling, and finally carrying out secondary cooling;

the temperature rise time is 10-20 hours;

the annealing temperature is 2000-2500 ℃;

the heat preservation time is 20-40 hours;

the primary cooling is to be carried out for 20-40 hours until the temperature is reduced to 1000-1250 ℃;

and the secondary cooling is to cool the mixture to room temperature within 10-15 hours.

Technical Field

The invention belongs to the technical field of silicon carbide crystals, and particularly relates to a secondary annealing method of a silicon carbide crystal.

Background

The silicon carbide crystal grown by using a physical vapor transport method (PVT) at present has high requirements on crucible materials, temperature, pressure, raw materials, seed crystals and the like due to high-quality crystals. The temperature field of the crucible has great influence on the stress and dislocation of the crystal. On one hand, the growth of the crystal depends on the temperature gradient of the thermal field, and the growth speed of the crystal in each direction is different according to the difference of the temperature gradients in the axial direction and the radial direction, so that the atoms in the silicon carbide crystal are mutually constrained, and stress is generated due to the fact that the silicon carbide crystal cannot freely expand and contract. On the other hand, the crystal growth surface is usually required to be slightly convex to reduce crystal defects, improve crystal quality and enlarge a crystal single crystal region, and the crystal surface shape also causes the internal stress of the crystal to be increased. Excessive stress causes the crystal to be easily broken, increases defects, reduces the processing yield, and also affects the surface type of the processed wafer, thereby affecting the quality of the epitaxial wafer. The proper in-situ annealing and secondary annealing process can effectively reduce the internal stress of the crystal.

In-situ annealing, namely annealing on the basis of not changing the original growth temperature field structure after the growth of the single crystal is finished, although the internal stress of the crystal can be released to a certain extent by slow cooling speed, annealing pressure improvement and other modes, the crystal still has larger stress after in-situ annealing due to the characteristic of large axial and radial temperature gradients inherent in the growth temperature field; the secondary annealing can reduce the annealing temperature gradient or change the gradient direction by changing the crystal placing mode, changing the temperature field structure, adjusting the pressure and temperature process curve and the like, thereby effectively eliminating the internal stress of the crystal.

The most common crystal annealing mode at present is that graphite paper is used for wrapping silicon carbide crystals and horizontally placing the silicon carbide crystals into a crucible which is fully paved with silicon carbide raw materials, and a plurality of crystals can be placed into the crucible at the same time; after the crucible is assembled, the crucible is placed into a high-temperature annealing furnace, the temperature is raised to the crystal annealing temperature under certain pressure, and then the temperature is slowly reduced. The method in the prior art uses graphite paper and a large amount of silicon carbide raw materials to prevent the crystal from being excessively graphitized, but the method cannot ensure that the radial temperature gradient of the crystal is controlled in a small range, and cannot avoid generating new stress in secondary annealing.

Disclosure of Invention

In view of the above, the present invention provides a secondary annealing method for silicon carbide crystals, which further reduces the temperature gradient in the axial and radial directions of the temperature field.

The invention provides a secondary annealing method of a silicon carbide crystal, which comprises the following steps:

placing the silicon carbide crystal in a crucible and carrying out annealing treatment in an annealing furnace;

the crucible includes:

a large crucible;

the small crucible is arranged in the large crucible, and the small crucible and the large crucible are coaxially arranged;

the annular graphite felt is arranged in the small crucible;

the silicon carbide crystals are horizontally placed in the center of the annular graphite felt.

Preferably, the diameter of the silicon carbide crystal is 4-8 inches.

Preferably, the inner diameter of the annular graphite felt is 1/3-2/3 of the diameter of the silicon carbide crystal, the outer diameter of the annular graphite felt is not smaller than the diameter of the silicon carbide crystal, and the thickness of the annular graphite felt is 0.3-2.5 times of the thickness of the silicon carbide crystal.

Preferably, the inner diameter of the small crucible is 1.1-2.0 times of the diameter of the silicon carbide crystal.

Preferably, the inner diameter of the large crucible is 1.1-3.0 times of the outer diameter of the small crucible.

Preferably, the distance between the bottom of the small crucible and the bottom of the large crucible is 20-150 mm.

Preferably, the volume of a gap between the large crucible and the small crucible is 20-40% of the volume of the large crucible.

Preferably, the gap between the large crucible and the small crucible is filled with a heat insulating material.

Preferably, the heat insulating material is selected from one or two of graphite felt or silicon carbide powder.

Preferably, the annealing treatment method comprises:

heating to the annealing temperature, preserving heat, then carrying out primary cooling, and finally carrying out secondary cooling;

the temperature rise time is 10-20 hours;

the annealing temperature is 2000-2500 ℃;

the heat preservation time is 20-40 hours;

the primary cooling is to be carried out for 20-40 hours until the temperature is reduced to 1000-1250 ℃;

and the secondary cooling is to cool the mixture to room temperature within 10-15 hours.

The invention provides a double-crucible structure and an annealing method of adding an annular graphite felt, which are used for adjusting the radial and axial temperature difference of a crystal, so that the temperature of each part of the crystal is basically the same, and the expansion and contraction of the crystal are not constrained by the inner parts. According to the invention, a double-crucible mode and an annular graphite felt are used in the secondary annealing stage, so that the radial and axial temperature gradients are reduced, the temperature gradient is kept at 2-6 ℃, the internal stress of the crystal is slowly released, and the new stress generated in the annealing process of the crystal is avoided. Because the edge of the crystal is easier to radiate heat compared with the center of the crystal, in order to keep the temperature consistency of the crystal and reduce the subsequent damage rate, the crystal is horizontally placed on the annular graphite felt, so that the heat preservation effect of the edge is higher than that of the center, and the method can simultaneously place a plurality of crystals according to the height of the crucible and the thickness of the crystal, thereby increasing the efficiency.

Drawings

FIG. 1 is a schematic structural diagram of an apparatus used in an annealing process according to an embodiment of the present invention;

fig. 2 is a cut SPI profile inspection diagram of a wafer obtained after annealing in embodiment 1 of the present invention;

fig. 3 is a diagram showing SPI surface type detection of a dicing sheet obtained without performing secondary annealing in comparative example 2 of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other examples, which may be modified or appreciated by those of ordinary skill in the art based on the examples given herein, are intended to be within the scope of the present invention. It should be understood that the embodiments of the present invention are only for illustrating the technical effects of the present invention, and are not intended to limit the scope of the present invention. In the examples, the methods used were all conventional methods unless otherwise specified.

The invention provides a secondary annealing method of a silicon carbide crystal, which comprises the following steps:

placing the silicon carbide crystal in a crucible and carrying out annealing treatment in an annealing furnace;

the crucible includes:

a large crucible;

the small crucible is arranged in the large crucible, and the small crucible and the large crucible are coaxially arranged;

the annular graphite felt is arranged in the small crucible;

the silicon carbide crystals are horizontally placed in the center of the annular graphite felt.

In the present invention, the silicon carbide crystal is preferably a conductive crystal and/or a semi-insulating crystal; the diameter of the silicon carbide crystal is preferably 4-8 inches; the thickness of the silicon carbide crystal is not particularly limited, and is preferably 10-60 mm, more preferably 20-50 mm, and most preferably 30-40 mm.

The preparation method of the silicon carbide crystal is not particularly limited, and the silicon carbide crystal prepared by adopting a physical vapor transport method (PVT) well known by the technical personnel in the field can be annealed again. The method of the present invention is not particularly limited, and the in-situ annealing method known to those skilled in the art may be adopted. In the present invention, the method for producing silicon carbide crystals preferably comprises:

slowly heating to the growth temperature of the silicon carbide crystal under high pressure by adopting a physical vapor transport method (PVT) to grow the silicon carbide crystal; and after the silicon carbide crystal growth is finished, carrying out primary annealing by naturally cooling to room temperature.

In the invention, the high pressure is preferably 1-10 kilopascals, more preferably 2-8 kilopascals, more preferably 3-6 kilopascals, and most preferably 4-5 kilopascals; the slow temperature rise speed is preferably 100-200 ℃/h, more preferably 120-180 ℃/h, and most preferably 140-160 ℃/h; the growth temperature of the silicon carbide crystal is preferably 2200-2400 ℃, more preferably 2250-2350 ℃, and most preferably 2300 ℃; the pressure in the growth process of the silicon carbide crystal is preferably 100-1000 Pa, more preferably 200-800 Pa, more preferably 300-600 Pa, and most preferably 400-500 Pa.

In the present invention, the primary annealing is preferably performed under pressure; the pressure is preferably 1-10 kilopascals, more preferably 2-8 kilopascals, more preferably 3-6 kilopascals, and most preferably 4-5 kilopascals; preferably, the pressure of the furnace chamber is increased to 1-10 kilopascals in the primary annealing process.

In the invention, the inner diameter of the small crucible is preferably 1.1 to 2.0 times, more preferably 1.3 to 1.7 times, and most preferably 1.5 to 1.6 times the diameter of the silicon carbide crystal. In the invention, the inner diameter of the small crucible is preferably 200-350 mm, more preferably 250-300 mm, and most preferably 260-280 mm; the height of the small crucible is preferably 300-500 mm, more preferably 350-450 mm, and most preferably 400 mm; the wall thickness of the small crucible is preferably 10-100 mm, more preferably 20-80 mm, more preferably 20-50 mm, more preferably 20-30 mm, and most preferably 20 mm. In the present invention, the material of the small crucible is preferably graphite.

In the present invention, the inner diameter of the large crucible is preferably 1.1 to 3.0 times, more preferably 1.5 to 2.5 times, and most preferably 2 times the outer diameter of the small crucible. In the invention, the inner diameter of the large crucible is preferably 250-400 mm, more preferably 300-350 mm, and most preferably 320-330 mm; the height of the large crucible is preferably 400-600 mm, more preferably 450-550 mm, and most preferably 500 mm; the wall thickness of the large crucible is preferably 10-100 mm, more preferably 20-80 mm, more preferably 30-50 mm, more preferably 30-40 mm, and most preferably 30 mm. In the present invention, the material of the large crucible is preferably graphite.

In the invention, the distance between the bottom of the small crucible and the bottom of the large crucible is preferably 20-150 mm, more preferably 50-120 mm, more preferably 80-100 mm, and most preferably 90 mm; the distance between the bottom of the small crucible and the bottom of the large crucible refers to the distance between the outer bottom of the small crucible and the inner bottom of the large crucible.

In the present invention, the volume of the gap between the large crucible and the small crucible is preferably 20 to 40%, more preferably 25 to 35%, and most preferably 30% of the volume of the large crucible.

In the invention, the small crucible and the large crucible are coaxially arranged, namely, the axes of the small crucible and the large crucible are superposed and are in concentric circle positions.

In the present invention, the gap between the large crucible and the small crucible is preferably filled with a heat insulating material; the heat insulating material is preferably selected from one or two of graphite felt or silicon carbide powder. In the invention, the granularity of the silicon carbide powder is preferably 100-3000 microns, more preferably 200-1500 microns, more preferably 400-1000 microns, and most preferably 500 microns. In the present invention, the method of assembling the large crucible and the small crucible preferably includes:

the filler is put into the large crucible and then the small crucible is put into the large crucible at a predetermined position.

In the present invention, the filler is preferably a graphite felt and/or a silicon carbide powder.

In the present invention, the method of assembling the large crucible and the small crucible more preferably includes:

and winding the outer surface of the small crucible by using a graphite felt, putting the graphite felt to a preset position of the small crucible on the bottom surface of the large crucible, and putting the small crucible into the large crucible to enable the small crucible and the large crucible to be in a coaxial position.

In the invention, the thickness of the graphite felt is preferably 5-20 mm, and more preferably 10-15 mm.

In the present invention, it is preferable that the outer diameter of the small crucible after winding the graphite felt is identical to the inner diameter of the large crucible.

In the present invention, it is preferable to put one or more layers of graphite felt on the bottom surface of the large crucible so that the thickness of the graphite felt is a desired distance between the inner bottom of the large crucible and the outer bottom of the small crucible.

In the present invention, the method of assembling the large crucible and the small crucible more preferably includes:

and silicon carbide powder is spread at the bottom of the large crucible until reaching the preset position of the small crucible, the small crucible and the large crucible are coaxially placed into the large crucible, and the gap between the large crucible and the small crucible is filled with the silicon carbide powder.

The invention adopts a double-crucible structure, and graphite felt or silicon carbide powder is filled in the middle of a large crucible and a small crucible for heat preservation to reduce the temperature gradient, so that the axial temperature gradient of the crystal is preferably less than 3 ℃.

In the invention, an annular graphite felt is arranged in the small crucible; the inner diameter of the annular graphite felt is preferably 1/3-2/3, more preferably 0.4-0.5 of the diameter of the silicon carbide crystal; the outer diameter of the annular graphite felt is preferably not less than the diameter of the silicon carbide crystal; the thickness of the annular graphite felt is preferably 0.3-2.5 times, more preferably 0.5-2 times, and most preferably 1-1.5 times that of the silicon carbide crystal.

In the invention, the outer diameter of the annular graphite felt is preferably 150-300 mm, more preferably 180-250 mm, and most preferably 200-230 mm; the inner diameter of the annular graphite felt is preferably 60-150 mm, more preferably 80-120 mm, and most preferably 100 mm; the thickness of the annular graphite felt is preferably 10-30 mm, more preferably 15-25 mm, and most preferably 20 mm.

In the invention, the annular graphite felt is preferably arranged at the bottom of the small crucible, and one or more layers of graphite felts can be arranged at the bottom of the small crucible according to the number of the silicon carbide crystals which are subjected to secondary annealing.

In the invention, the silicon carbide crystals are horizontally placed in the center of the annular graphite felt; the horizontal direction of the silicon carbide crystal is the axial direction of the silicon carbide crystal.

According to the invention, the silicon carbide crystal is horizontally placed in the center of the annular graphite felt, so that the center of the silicon carbide crystal can be better radiated, and the temperature difference between the edge and the center of the silicon carbide crystal is preferably delta T less than or equal to 3 ℃.

In the invention, the quantity of the silicon carbide crystals loaded into the small crucible can be reasonably prepared according to the height of the small crucible.

In the present invention, it is preferable that the assembled crucible is placed in a furnace chamber heating tube of an annealing furnace to perform annealing treatment.

Fig. 1 is a schematic structural diagram of an apparatus used in an annealing process according to an embodiment of the present invention, including:

a hole 1 is reserved at the top of the large crucible;

a medium frequency induction heating coil 2;

a heating cylinder 3;

big crucible 4

A small crucible 6;

a double crucible gap 5;

silicon carbide crystals 7;

an annular graphite felt 8.

In the invention, the medium-frequency induction heating coil is arranged outside the heating barrel; a large crucible is arranged in the heating tube; a small crucible is placed in the large crucible; a double-crucible gap is formed between the large crucible and the small crucible; the bottom surface of the small crucible is provided with an annular graphite felt; silicon carbide crystals are placed in the center of the annular graphite felt in the horizontal direction; a hole is reserved in the center of the top of the large crucible, the diameter of the hole is preferably 10-30 mm, more preferably 15-25 mm, and most preferably 20mm, and the hole is used for better introducing inert gas and extracting impurities in the large crucible.

In the invention, the annealing furnace in the annealing treatment process preferably adopts medium-frequency induction heating.

In the invention, the periphery of the heating cylinder of the annealing furnace is preferably provided with high-temperature resistant graphite felt in a surrounding mode for heat preservation.

In the invention, inert protective gas is preferably adopted for protection in the annealing treatment process; the inert shielding gas preferably comprises helium and/or argon; the pressure of the inert protective gas is preferably 3-8 kilopascals, more preferably 4-6 kilopascals, and most preferably 5 kilopascals.

In the present invention, the method of annealing treatment preferably includes:

and (4) heating to the annealing temperature, preserving heat, then carrying out primary cooling, and finally carrying out secondary cooling.

In the invention, the time for raising the temperature is preferably 10 to 20 hours, more preferably 12 to 18 hours, and most preferably 14 to 16 hours; the temperature rise time is more than or equal to 10 hours; the annealing temperature is preferably 2000-2500 ℃, more preferably 2100-2400 ℃, and most preferably 2200-2300 ℃; the heat preservation time is preferably 20 to 40 hours, more preferably 25 to 35 hours, and most preferably 30 hours.

In the invention, the primary cooling is preferably slow cooling, and is preferably cooled to 1000-1250 ℃ within 20-40 hours; the cooling time is preferably 25-35 hours, and more preferably 30 hours; the temperature of the temperature reduction is preferably 1050-1200 ℃, more preferably 1100-1150 ℃, and most preferably 1120-1130 ℃. In the invention, the cooling speed in the one-time slow cooling process is preferably 25-65 ℃/h, more preferably 30-50 ℃/h, and most preferably 40 ℃/h.

In the invention, the secondary cooling is preferably slow cooling, and is preferably cooled to room temperature within 10-15 hours; the cooling time is preferably 11-14 hours, and more preferably 12-13 hours; the room temperature is preferably 20-30 ℃, more preferably 22-28 ℃, and most preferably 24-26 ℃. In the invention, the cooling speed in the secondary cooling process is preferably 90-110 ℃/h, more preferably 95-105 ℃/h, and most preferably 100 ℃/h.

In the invention, secondary cooling is preferably carried out immediately after the primary cooling is finished; and the initial temperature of the secondary cooling is the finishing temperature of the primary cooling.

In the present invention, the annealing treatment preferably includes:

putting the assembled crucible into a heating barrel of an annealing furnace chamber, and filling inert gas as protective gas until the pressure is 3-8 kilopascals; raising the temperature to the annealing temperature of 2000-2500 ℃ within 10-20 hours, keeping the annealing temperature for 20-40 hours at the constant temperature, then slowly lowering the temperature for 20-40 hours to 1000-1250 ℃, and then slowly lowering the temperature for 10-15 hours to the room temperature.

In the present invention, the annealing treatment is preferably preceded by a furnace washing treatment, and the furnace washing treatment method preferably includes:

pumping the furnace chamber by using a mechanical pump until the pressure in the cavity is below 10Pa, heating to 200-500 ℃, and keeping pumping for 1-2 hours; then filling inert gas, and pumping again to below 10Pa to complete furnace washing.

In the invention, the heating temperature is preferably 300-400 ℃, and more preferably 350 ℃; the time for keeping the air suction is preferably 1.5 hours; the inert gas is preferably argon and/or helium; the pressure of the inert gas is preferably 1-10 kilopascals, more preferably 2-8 kilopascals, more preferably 3-6 kilopascals, and most preferably 4-5 kilopascals.

In the invention, the high annealing temperature and the longer heat preservation time are beneficial to the release of the stress of the silicon carbide crystal and the improvement of the internal defects of the crystal, the temperature rise rate is kept to be raised to the annealing temperature when the crystal grows, and the constant temperature is kept for more than 20 hours; in order to prevent the graphitization of the silicon carbide crystal, the invention increases the pressure of inert gas in the furnace. The invention uses a mode of 'double crucibles' and 'graphite felt' to carry out secondary heat preservation on the annealed crystal, thereby reducing the temperature gradient; and the temperature is slowly reduced in a sectional manner, the temperature reduction time is more than 40 hours, and the internal stress of the crystal can be slowly released in the annealing stage, and new stress is not generated any more, so that the safety of the crystal is ensured. The secondary annealing process of the silicon carbide crystal can effectively reduce the subsequent processing cracking of the crystal, improve the surface type of the cutting piece by reducing stress and further improve the surface type quality of the wafer.

In the process of epitaxy, the warping degree can influence the consistency of the nitrogen concentration of the epitaxial layer, the failure of a device is easily caused in the process of packaging test, and the yield is reduced. The improvement of the surface type of the silicon carbide substrate can reduce the crystal stress from the root and improve the surface type of the cutting piece. The annealed silicon carbide crystal obtained by the method provided by the invention has the advantages that the cutting processing cracking rate is effectively reduced, the face type of a cutting piece is effectively improved, the Warp value is reduced to be below 20, and the Bow value is below 5. After the cutting piece obtained by the method provided by the invention is subjected to subsequent processing, the warping degree of the product piece is further reduced.

The silicon carbide crystal adopted in the following embodiment of the invention is obtained by annealing the silicon carbide crystal grown by a PVT physical vapor transport method for one time: slowly heating (100-200 ℃/h) to the crystal growth temperature of 2200-2400 ℃ under 1-10 kilopascals, maintaining the pressure at 100-1000 Pa for crystal growth, increasing the pressure of the furnace chamber to 1-10 kilopascals after the crystal growth is finished, and naturally cooling to the room temperature.

Example 1

Annealing was carried out using the apparatus shown in FIG. 1: taking 6 4H-SiC conductive crystals with the diameter of 6 inches, horizontally putting the 6H-SiC conductive crystals into the center of an annular graphite felt in a double crucible, wherein the thickness of the annular graphite felt is 25mm, the outer diameter is 240mm, and the inner diameter is 80 mm; the inner diameter of the large graphite crucible is 300mm, the height is 400mm, and the wall thickness is 30 mm; the inner diameter of the small graphite crucible is 250mm, the height is 300mm, and the wall thickness is 20 mm; the distance between the bottom of the large graphite crucible and the bottom of the small graphite crucible is 50 mm; graphite felt is put into the gap of the double-crucible to preserve the heat of the small graphite crucible.

Assembling double crucibles (winding a graphite felt on the outer surface of a small graphite crucible to enable the outer diameter of the small graphite crucible to be consistent with the inner diameter of a large graphite crucible, paving a graphite felt with the thickness of 50mm on the bottom surface of the large graphite crucible, putting the small graphite crucible into the large graphite crucible, and enabling the small graphite crucible and the large graphite crucible to be in a coaxial position), putting the small graphite crucible into an annealing furnace, and performing furnace washing treatment on the annealing furnace, wherein the furnace washing treatment method comprises the following steps: pumping by using a mechanical pump until the pressure in the cavity is below 10Pa, heating to 300 ℃, and keeping pumping for 1 hour; then argon is filled to 3 kilopascals, and the furnace is cleaned after being pumped to below 10 Pa; and then, sequentially operating according to the requirements of annealing process temperature and time: argon gas is filled to 6 kilopascals as protective gas, and the pressure is kept unchanged during annealing; heating by adopting medium-frequency induction heating, heating to the annealing temperature of 2200 ℃ within 10 hours, and keeping for 25 hours; within 25 hours, the temperature is reduced to 1100 ℃ by adjusting the power, and the cooling speed is 40 ℃/h; subsequently, the temperature was lowered to room temperature at a rate of 100 ℃/h over a period of 10 hours.

Diamond multi-wire cutting is carried out on the annealed silicon carbide crystal in the embodiment 1 of the invention, and the surface type performance of the cutting piece is tested by adopting an SPI surface type detector, the detection result is shown in figure 2, and figure 2 is a 3D/2D surface type diagram generated by surface scanning of the cutting surface; in the subsequent cutting of 6 silicon carbide crystals annealed by the method provided by example 1, the 95% Warp value of the cut slice surface type is less than 20, and the Bow value is less than 5.

Example 2

Annealing was carried out using the apparatus shown in FIG. 1: taking 7 4H-SiC semi-insulating crystals with the diameter of 6 inches, horizontally putting the crystals into the center of an annular graphite felt in a double crucible, wherein the thickness of the annular graphite felt is 20mm, the outer diameter is 200mm, and the inner diameter is 60 mm; the inner diameter of the large graphite crucible is 300mm, the height is 400mm, and the wall thickness is 30 mm; the inner diameter of the small graphite crucible is 250mm, the height is 300mm, and the wall thickness is 20 mm; the distance between the bottom of the large graphite crucible and the bottom of the small graphite crucible is 25 mm; graphite felt is put into the gap of the double-crucible to preserve the heat of the small graphite crucible.

Assembling the double crucibles (winding a graphite felt on the outer surface of the small graphite crucible to enable the outer diameter of the small graphite crucible to be consistent with the inner diameter of the large graphite crucible, paving a graphite felt with the thickness of 25mm on the bottom surface of the large graphite crucible, and putting the small graphite crucible and the large graphite crucible in the large graphite crucible at the same axial position) and then putting the small graphite crucible and the large graphite crucible into an annealing furnace, and performing furnace washing treatment on the annealing furnace, wherein the furnace washing treatment method comprises the following steps: pumping by using a mechanical pump until the pressure in the cavity is below 10Pa, heating to 300 ℃, and keeping pumping for 1 hour; then argon is filled to 3 kilopascals, and the furnace is cleaned after being pumped to below 10 Pa; sequentially operating according to the temperature and time requirements of the annealing process: argon gas is filled to 6 kilopascals as protective gas, and the pressure is kept unchanged during annealing; heating by adopting medium-frequency induction heating, heating to the annealing temperature of 2100 ℃ within 10 hours, and keeping for 25 hours; within 30 hours, the temperature is reduced to 1000 ℃ by adjusting the power, and the cooling speed is 35 ℃/h; subsequently, the temperature was lowered to room temperature at a rate of 100 ℃/h over a period of 10 hours.

After the annealed silicon carbide crystal is cut according to the method in example 1, the surface shape detection is performed, and the detection result shows that in the subsequent cutting of 7 silicon carbide crystals annealed by the method in example 2, the surface shape of the cut piece is 96% Warp value less than 20, and the Bow value is less than 5.

Comparative example 1

Wrapping 6 blocks of 6-inch 4H-SiC conductive crystals by using graphite paper, putting the conductive crystals into a graphite crucible filled with silicon carbide powder, and annealing at the same annealing time and temperature: argon gas is filled to 6 kilopascals as protective gas, and the pressure is kept unchanged during annealing; heating by adopting medium-frequency induction heating, heating to the annealing temperature of 2200 ℃ within 10 hours, and keeping for 25 hours; reducing the temperature to 1100 ℃ within 20 hours by adjusting the power; subsequently, the temperature was lowered to room temperature at a rate of 100 ℃/h over a period of 10 hours.

According to the detection method in the embodiment 1, after the annealed silicon carbide crystal in the comparative example 1 is cut, the 83% Warp value of the cut piece surface type is less than 20, and the Bow value is less than 5; the profile was slightly inferior compared to the examples.

Comparative example 2

6 pieces of 6-inch diameter 4H-SiC conductive crystal were taken and were not subjected to secondary annealing.

According to the detection method in the embodiment 1, the silicon carbide crystal is cut without cracking, the cut piece surface type result is tracked, the Warp value of 65% of the cut pieces is less than 20, the Bow value is less than 5, the stress of the silicon carbide crystal is reduced by secondary annealing, and the improvement of the cut piece surface type is facilitated.

The invention provides a secondary annealing method of silicon carbide with a double-crucible structure and a novel crystal arrangement mode, which is used for adjusting the radial and axial temperature difference of the silicon carbide crystal, so that the temperature of each part of the silicon carbide crystal is basically the same, and the expansion and contraction of the crystal are not constrained by the interior. The invention uses a double-crucible mode and a secondary annealing method of the annular graphite felt, reduces the temperature gradient, keeps the temperature gradient at 2-6 ℃, and avoids the generation of new stress of the silicon carbide crystal in the secondary annealing process. Because the edge of the silicon carbide crystal is easier to dissipate heat compared with the center of the crystal, in order to keep the temperature consistency of the crystal and reduce the subsequent damage rate, the silicon carbide crystal is horizontally placed on the annular graphite felt, so that the heat preservation effect of the edge is higher than that of the center, and a plurality of crystals can be simultaneously placed according to the height of the crucible and the thickness of the silicon carbide crystal, thereby increasing the efficiency.

While only the preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.