CN113564719B - Secondary annealing method of silicon carbide crystal - Google Patents
Secondary annealing method of silicon carbide crystal Download PDFInfo
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- CN113564719B CN113564719B CN202110843228.7A CN202110843228A CN113564719B CN 113564719 B CN113564719 B CN 113564719B CN 202110843228 A CN202110843228 A CN 202110843228A CN 113564719 B CN113564719 B CN 113564719B
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Abstract
The invention provides a secondary annealing method of silicon carbide crystals, 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; an annular graphite felt disposed within the small crucible; the crystal is horizontally arranged at the center of the annular graphite felt. The invention uses a double crucible mode and an annular graphite felt in the secondary annealing stage, reduces the temperature gradient, keeps the temperature gradient at 2-6 ℃, slowly releases the internal stress of the crystal, and simultaneously avoids the generation of new stress of the crystal in the annealing process. The secondary annealing method provided by the invention can reasonably configure the height of the crucible and the thickness of the crystals, and simultaneously place a plurality of crystals, thereby increasing the efficiency.
Description
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
At present, silicon carbide crystals grown by a Physical Vapor Transport (PVT) method have high requirements on crucible materials, temperature, pressure, raw materials, seed crystals and the like. Wherein the temperature field of the crucible has great influence on the stress and dislocation of the crystal. On one hand, the growth speed of the crystal in each direction is different according to the difference of the temperature gradients of the temperature fields in the axial direction and the radial direction, so that atoms in the silicon carbide crystal are mutually restrained, and stress is generated because the atoms cannot be freely expanded and contracted. On the other hand, it is generally required that the crystal growth surface be slightly convex to reduce crystal defects, improve crystal quality, and enlarge a single crystal region of the crystal, which also causes an increase in internal stress of the crystal. The excessive stress can cause easy breakage of the crystal, increase of defects and reduction of processing yield, and can influence the surface shape of the processed wafer, thereby influencing the quality of the epitaxial wafer. The proper in-situ annealing and secondary annealing processes can effectively reduce the internal stress of the crystal.
In-situ annealing is based on the original growth temperature field structure without changing after the single crystal growth is finished, and the stress in the crystal is released to a certain extent by means of slow cooling speed, annealing pressure improvement and the like, but the crystal still has larger stress after in-situ annealing due to the characteristic of large axial and radial temperature gradient inherent in the growth temperature field; the secondary annealing can reduce annealing temperature gradient or change gradient direction by changing crystal placement mode, changing temperature field structure, adjusting pressure and temperature process curve, etc., thereby effectively eliminating 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 put into a crucible fully paved with silicon carbide raw materials, and a plurality of crystals can be put into the crucible at the same time; after assembly, the crucible is put into a high-temperature annealing furnace, and the temperature is raised to the annealing temperature of the crystal under a certain pressure, and then the temperature is slowly lowered. The prior art method uses graphite paper and a large amount of silicon carbide raw materials for preventing excessive graphitization of the crystal, but the method cannot ensure that the radial temperature gradient of the crystal is controlled in a smaller range, and cannot avoid generating new stress in secondary annealing.
Disclosure of Invention
Accordingly, the present invention is directed to a method for secondary annealing of silicon carbide crystals, which further reduces the axial and radial temperature gradients of the temperature field.
The invention provides a secondary annealing method of silicon carbide crystals, 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 arranged in the large crucible, wherein the small crucible and the large crucible are coaxially arranged;
an annular graphite felt disposed within the small crucible;
the silicon carbide crystal is horizontally arranged at the center of the annular graphite felt.
Preferably, the silicon carbide crystal has a diameter of 4 to 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 is not smaller than the diameter of the silicon carbide crystal, and the thickness 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 to 2.0 times the diameter of the silicon carbide crystal.
Preferably, the inner diameter of the large crucible is 1.1 to 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 gap volume between the large crucible and the small crucible is 20-40% of the volume of the large crucible.
Preferably, a thermal insulation material is filled in a gap between the large crucible and the small crucible.
Preferably, the thermal insulation material is selected from one or two of graphite felt or silicon carbide powder.
Preferably, the annealing treatment method comprises the following steps:
heating to the annealing temperature, preserving heat, then carrying out primary cooling, and finally carrying out secondary cooling;
the heating time is 10-20 hours;
the annealing temperature is 2000-2500 ℃;
the heat preservation time is 20-40 hours;
the primary cooling is carried out for 20-40 hours to 1000-1250 ℃;
the secondary cooling is to cool 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 crystals to ensure that the temperatures of the crystals are basically the same, and the crystals are not constrained by each other when expanding and contracting. The invention uses a double crucible mode and an annular graphite felt in the secondary annealing stage, reduces radial and axial temperature gradients, keeps the temperature gradients at 2-6 ℃, slowly releases internal stress of the crystal, and simultaneously avoids new stress of the crystal in the annealing process. Because the crystal edge is easier to dissipate heat compared with the crystal center, in order to keep the temperature consistency of the crystal and reduce the subsequent breakage rate, the crystal is horizontally arranged on the annular graphite felt, so that the heat preservation effect of the edge is higher than that of the center.
Drawings
FIG. 1 is a schematic diagram of an apparatus used in an annealing process according to an embodiment of the present invention;
FIG. 2 is a chart showing the SPI profile test of a wafer obtained after annealing in example 1 of the present invention;
FIG. 3 is a graph showing the profile of SPI on a dicing sheet obtained in comparative example 2 of the present invention without secondary annealing.
Detailed Description
The following description of the technical solutions in the embodiments of the present invention will be clear and complete, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other examples of modifications and alterations will be apparent to those skilled in the art based on the examples herein, and are intended to be within the scope of the invention. It should be understood that the embodiments of the present invention are only used for illustrating the technical effects of the present invention, and are not used for limiting the scope of the present invention. In the examples, the methods used are conventional methods unless otherwise specified.
The invention provides a secondary annealing method of silicon carbide crystals, 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 arranged in the large crucible, wherein the small crucible and the large crucible are coaxially arranged;
an annular graphite felt disposed within the small crucible;
the silicon carbide crystal is horizontally arranged at 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, but is preferably 10 to 60mm, more preferably 20 to 50mm, and most preferably 30 to 40mm.
The method for preparing the silicon carbide crystal is not particularly limited, and the silicon carbide crystal prepared by a physical vapor transport method (PVT) well known to those skilled in the art is annealed again. The method of the primary annealing is not particularly limited, and the method can be performed by adopting an in-situ annealing technical scheme well known to those skilled in the art. In the present invention, the preparation method of the silicon carbide crystal preferably includes:
slowly heating to the growth temperature of the silicon carbide crystal under high pressure by adopting a Physical Vapor Transport (PVT) method to perform the growth of the silicon carbide crystal; and (3) performing primary annealing of naturally cooling to room temperature after the growth of the silicon carbide crystal is finished.
In the present invention, the high pressure is preferably 1 to 10Pa, more preferably 2 to 8 Pa, still more preferably 3 to 6 Pa, and most preferably 4 to 5 Pa; the slow heating 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 during the growth of the silicon carbide crystal is preferably 100 to 1000Pa, more preferably 200 to 800Pa, still more preferably 300 to 600Pa, and most preferably 400 to 500Pa.
In the present invention, the primary annealing is preferably performed under pressure; the pressure is preferably 1 to 10Pa, more preferably 2 to 8 Pa, more preferably 3 to 6 Pa, and most preferably 4 to 5 Pa; the pressure of the furnace chamber is preferably increased to 1-10 Pa in the primary annealing process.
In the present 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 present invention, the inner diameter of the small crucible is preferably 200 to 350mm, more preferably 250 to 300mm, and most preferably 260 to 280mm; the height of the small crucible is preferably 300-500 mm, more preferably 350-450 mm, and most preferably 400mm; the wall thickness of the small crucible is preferably 10 to 100mm, more preferably 20 to 80mm, still more preferably 20 to 50mm, still more preferably 20 to 30mm, and most preferably 20mm. 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 present invention, the inner diameter of the large crucible is preferably 250 to 400mm, more preferably 300 to 350mm, and most preferably 320 to 330mm; the height of the large crucible is preferably 400 to 600mm, more preferably 450 to 550mm, most preferably 500mm; the wall thickness of the large crucible is preferably 10 to 100mm, more preferably 20 to 80mm, more preferably 30 to 50mm, more preferably 30 to 40mm, and most preferably 30mm. In the present invention, the material of the large crucible is preferably graphite.
In the present invention, the distance between the bottom of the small crucible and the bottom of the large crucible is preferably 20 to 150mm, more preferably 50 to 120mm, still more preferably 80 to 100mm, and most preferably 90mm; 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 gap volume between the large crucible and the small crucible is preferably 20 to 40% of the large crucible volume, more preferably 25 to 35%, and most preferably 30%.
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 coincident and are in concentric circle positions.
In the invention, the gap between the large crucible and the small crucible is preferably filled with a thermal insulation material; the thermal insulation material is preferably selected from one or two of graphite felt or silicon carbide powder. In the present invention, the particle size of the silicon carbide powder is preferably 100 to 3000 micrometers, more preferably 200 to 1500 micrometers, still more preferably 400 to 1000 micrometers, and most preferably 500 micrometers. In the present invention, the assembling method of the large crucible and the small crucible preferably includes:
the filler is placed in a large crucible and then a small crucible is placed in the large crucible at a preset position.
In the present invention, the filler is preferably graphite felt and/or silicon carbide powder.
In the present invention, the assembling method of the large crucible and the small crucible more preferably includes:
winding the outer surface of the small crucible by using a graphite felt, placing the graphite felt at the bottom surface of the large crucible to the preset position of the small crucible, and placing the small crucible into the large crucible so that the small crucible and the large crucible are positioned at the coaxial position.
In the present invention, the thickness of the graphite felt is preferably 5 to 20mm, more preferably 10 to 15mm.
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 reaches the desired distance between the inner bottom of the large crucible and the outer bottom of the small crucible.
In the present invention, the assembling method of the large crucible and the small crucible more preferably includes:
spreading silicon carbide powder on the bottom of a large crucible until reaching the preset position of a small crucible, coaxially placing the small crucible and the large crucible into the large crucible, and filling the gap between the large crucible and the small crucible with the silicon carbide powder.
The invention adopts a double-crucible structure, graphite felt or silicon carbide powder is filled in the middle of the big crucible and the 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 of the diameter of the silicon carbide crystal, and more preferably 0.4-0.5; the outer diameter of the annular graphite felt is preferably not smaller than the diameter of the silicon carbide crystal; the thickness of the annular graphite felt is preferably 0.3 to 2.5 times, more preferably 0.5 to 2 times, and most preferably 1 to 1.5 times the thickness of the silicon carbide crystal.
In the present invention, the outer diameter of the annular graphite felt is preferably 150 to 300mm, more preferably 180 to 250mm, and most preferably 200 to 230mm; the inner diameter of the annular graphite felt is preferably 60-150 mm, more preferably 80-120 mm, and most preferably 100mm; the thickness of the annular graphite felt is preferably 10 to 30mm, more preferably 15 to 25mm, and most preferably 20mm.
In the invention, the annular graphite felt is preferably arranged at the bottom of the small crucible, and one or more layers of graphite felt can be arranged on the bottom surface of the small crucible according to the number of silicon carbide crystals subjected to secondary annealing.
In the invention, the silicon carbide crystal is horizontally arranged at 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 arranged in the center of the annular graphite felt, so that better heat dissipation is realized at the center of the silicon carbide crystal, and the temperature difference between the edge and the center of the silicon carbide crystal is preferably less than or equal to 3 ℃.
In the invention, the quantity of 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 to put the above-mentioned assembled crucible into a furnace chamber heating cylinder of an annealing furnace for annealing treatment.
The schematic structure of the device used in the annealing process in the embodiment of the invention is shown in fig. 1, which includes:
a hole 1 is reserved at the top of the large crucible;
an intermediate frequency induction heating coil 2;
a heating cylinder 3;
A small crucible 6;
a double crucible gap 5;
and an annular graphite felt 8.
In the invention, the medium-frequency induction heating coil is arranged outside the heating cylinder; a large crucible is arranged in the heating cylinder; a small crucible is arranged in the large crucible; a double-crucible gap is arranged 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 horizontally arranged at the center of the annular graphite felt; a hole is reserved in the center of the top of the large crucible, the diameter is preferably 10-30 mm, more preferably 15-25 mm, most preferably 20mm, and the hole is used for better feeding inert gas and extracting impurities in the large crucible.
In the invention, the annealing furnace is preferably heated by medium frequency induction during the annealing treatment.
In the invention, the periphery of the heating cylinder of the annealing furnace is preferably provided with a high-temperature-resistant graphite felt in a surrounding manner so as to keep warm.
In the invention, the annealing treatment process is preferably protected by inert protective gas; the inert shielding gas preferably comprises helium and/or argon; the pressure of the inert shielding gas is preferably 3 to 8 Pa, more preferably 4 to 6 Pa, and most preferably 5 Pa.
In the present invention, the annealing treatment method preferably includes:
heating to annealing temperature, maintaining the temperature, then cooling for the first time, and finally cooling for the second time.
In the present invention, the heating time 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 holding 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 to 35 hours, more preferably 30 hours; the temperature of the cooling is preferably 1050 to 1200 ℃, more preferably 1100 to 1150 ℃, and most preferably 1120 to 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, preferably cooling to room temperature within 10-15 hours; the cooling time is preferably 11 to 14 hours, more preferably 12 to 13 hours; the room temperature is preferably 20 to 30 ℃, more preferably 22 to 28 ℃, and most preferably 24 to 26 ℃. In the present invention, the cooling rate in the secondary cooling process is preferably 90 to 110 ℃/h, more preferably 95 to 105 ℃/h, and most preferably 100 ℃/h.
In the invention, the secondary cooling is preferably performed immediately after the primary cooling is finished; the initial temperature of the secondary cooling is the ending temperature of the primary cooling.
In the present invention, the annealing treatment preferably includes:
putting the assembled crucible into a heating cylinder of a furnace chamber of an annealing furnace, and filling inert gas as protective gas until the pressure is 3-8 Pa; heating to the annealing temperature of 2000-2500 ℃ within 10-20 hours, keeping the annealing temperature for 20-40 hours at constant temperature, slowly cooling to 1000-1250 ℃ for 20-40 hours, and slowly cooling to room temperature for 10-15 hours.
In the present invention, the annealing treatment is preferably preceded by a furnace washing treatment, and the method of the furnace washing treatment preferably includes:
pumping the furnace chamber to the pressure below 10Pa by using a mechanical pump, heating to 200-500 ℃, and keeping pumping for 1-2 hours; and then filling inert gas, and exhausting again to below 10Pa to finish furnace washing.
In the present invention, the heating temperature is preferably 300 to 400 ℃, more preferably 350 ℃; the time for maintaining the suction is preferably 1.5 hours; the inert gas is preferably argon and/or helium; the pressure of the inert gas is preferably 1 to 10Pa, more preferably 2 to 8 Pa, still more preferably 3 to 6 Pa, and most preferably 4 to 5 Pa.
In the invention, the high annealing temperature and the longer heat preservation time are favorable for releasing the stress of the silicon carbide crystal and improving the internal defects of the crystal, the temperature rising rate is increased to the annealing temperature during the growth of the crystal, and the constant temperature is maintained for more than 20 hours; in order to prevent graphitization of silicon carbide crystals, the invention increases the pressure of inert gas in the furnace. The invention uses a double crucible and graphite felt mode to carry out secondary heat preservation on the annealed crystal, thereby reducing the temperature gradient; the sectional type is slowly cooled, the cooling time is longer than 40 hours, the internal stress of the crystal can be slowly released in the annealing stage, and new stress and the safety of the crystal are not generated any more. The secondary annealing process of the silicon carbide crystal can effectively reduce subsequent processing cracking of the crystal, and improve the surface shape of the cut piece by reducing stress, thereby improving the surface shape quality of the wafer.
In epitaxy, the warping degree can influence the consistency of the nitrogen concentration of an epitaxial layer, the device is easy to lose efficacy in the packaging test process, and the yield is reduced. The improvement of the silicon carbide substrate surface shape can radically reduce the crystal stress and improve the cutting surface shape. 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 cut sheet type is effectively improved, the Warp value is reduced to below 20, and the Bow value is below 5. The cut sheet obtained by the method provided by the invention is subjected to subsequent processing, and the warping degree of the product sheet is further reduced.
The silicon carbide crystal adopted in the following examples of the invention is obtained by annealing a silicon carbide crystal grown by PVT physical vapor transport method once: slowly heating (100-200 ℃/h) to 2200-2400 ℃ at 1-10 Pa, maintaining the pressure at 100-1000 Pa for crystal growth, increasing the furnace chamber pressure to 1-10 Pa after the crystal growth is finished, and naturally cooling to room temperature.
Example 1
Annealing treatment was performed using the apparatus shown in fig. 1: 6 pieces of 4H-SiC conductive crystals with the diameter of 6 inches are taken and horizontally placed in the center of an annular graphite felt in a double crucible, the thickness of the annular graphite felt is 25mm, the outer diameter of the annular graphite felt is 240mm, and the inner diameter of the annular graphite felt is 80mm; the inner diameter of the large graphite crucible is 300mm, the height is 400mm, and the wall thickness is 30mm; the inner diameter of the small graphite crucible is 250mm, the height is 300mm, and the wall thickness is 20mm; the distance between the bottom of the large graphite crucible and the bottom of the small graphite crucible is 50mm; and placing graphite felt into the gap between the double crucibles to insulate the small graphite crucible.
Assembling a double crucible (winding graphite felts 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 to enable the small graphite crucible and the large graphite crucible to be in coaxial positions), then putting the small graphite crucible and the large graphite crucible into an annealing furnace, and carrying out furnace washing treatment on the annealing furnace, wherein the furnace washing treatment method comprises the following steps: pumping by 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 Pa, and the furnace is pumped down again to below 10Pa to finish furnace washing; then sequentially operating according to the annealing process temperature and time requirements: argon is filled to 6 Pa as a protective gas, and the pressure is kept unchanged during annealing; heating by adopting medium-frequency induction heating, heating to an annealing temperature of 2200 ℃ for 10 hours, and keeping for 25 hours; within 25 hours, the temperature is reduced to 1100 ℃ by adjusting the power, and the temperature reduction speed is 40 ℃/h; then cooled to room temperature at a rate of 100 c/h over 10 hours.
Diamond multi-wire cutting is carried out on the annealed silicon carbide crystal in the embodiment 1 of the invention, and an SPI (serial peripheral interface) surface type detector is adopted to test the surface type performance of the cutting piece, wherein the detection result is shown in figure 2, and figure 2 is a 3D/2D surface type graph generated by surface scanning of the cutting surface; the 6 silicon carbide crystals annealed by the method provided in example 1 were cut to a faceted 95% warp value of less than 20 and a bow value of less than 5 in the subsequent cuts.
Example 2
Annealing treatment was performed using the apparatus shown in fig. 1: 7 pieces of 4H-SiC semi-insulating crystals with the diameter of 6 inches are taken, horizontally placed in the center of an annular graphite felt in a double crucible, the thickness of the annular graphite felt is 20mm, the outer diameter of the annular graphite felt is 200mm, and the inner diameter of the annular graphite felt is 60mm; the inner diameter of the large graphite crucible is 300mm, the height is 400mm, and the wall thickness is 30mm; the inner diameter of the small graphite crucible is 250mm, the height is 300mm, and the wall thickness is 20mm; the distance between the bottom of the large graphite crucible and the bottom of the small graphite crucible is 25mm; and placing graphite felt into the gap between the double crucibles to insulate the small graphite crucible.
Double crucibles are assembled (graphite felts are wound on the outer surface of a small graphite crucible, the outer diameter of the small graphite crucible is consistent with the inner diameter of a large graphite crucible, graphite felts with the thickness of 25mm are paved on the bottom surface of the large graphite crucible, the small graphite crucible and the large graphite crucible are placed in the large graphite crucible at the coaxial positions), and then the double crucibles are placed in an annealing furnace, and the annealing furnace is subjected to furnace washing treatment, wherein the furnace washing treatment method comprises the following steps: pumping by 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 Pa, and the furnace is pumped down again to below 10Pa to finish furnace washing; the method sequentially comprises the following steps of: argon is filled to 6 Pa as a protective gas, and the pressure is kept unchanged during annealing; heating by adopting medium frequency induction heating, raising the temperature to an annealing temperature of 2100 ℃ for 10 hours, and keeping for 25 hours; within 30 hours, the temperature is reduced to 1000 ℃ by adjusting the power, and the temperature reduction speed is 35 ℃/h; then cooled to room temperature at a rate of 100 c/h over 10 hours.
The annealed silicon carbide crystal was cut according to the method of example 1, and then subjected to surface profile test, and as a result, the 7 annealed silicon carbide crystals according to the method of example 2 were cut in the subsequent cutting, the cut surface profile 96% warp value was less than 20, and the bow value was less than 5.
Comparative example 1
6 pieces of 6-inch 4H-SiC conductive crystals are wrapped by graphite paper and put into a graphite crucible filled with silicon carbide powder, and annealing is carried out at the same annealing time and temperature: argon is filled to 6 Pa as a protective gas, and the pressure is kept unchanged during annealing; heating by adopting medium-frequency induction heating, heating to an annealing temperature of 2200 ℃ for 10 hours, and keeping for 25 hours; reducing the temperature to 1100 ℃ by adjusting the power within 20 hours; then cooled to room temperature at a rate of 100 c/h over 10 hours.
After the silicon carbide crystal annealed in comparative example 1 of the present invention was cut by the method of example 1, the 83% warp value in the cut piece type was less than 20, and the bow value was less than 5; the surface shape is slightly inferior to the examples.
Comparative example 2
6 pieces of 4H-SiC conductive crystals with the diameter of 6 inches are taken and are not subjected to secondary annealing.
The method of example 1 was used to detect that the silicon carbide crystal was cut without cracking, and the cut sheet type was followed, wherein 65% of the cut sheets had a Warp value of less than 20 and a bow value of less than 5, and it was found that the secondary annealing reduced the stress of the silicon carbide crystal, which was more advantageous for improving the cut sheet type.
The invention provides a double-crucible structure and a novel silicon carbide secondary annealing method in a crystal placement mode, which are used for adjusting the radial and axial temperature difference of silicon carbide crystals, so that the temperatures of the silicon carbide crystals are basically the same, and the crystals are not constrained by each other when expanding and contracting. The invention uses a secondary annealing method of a double crucible mode and an 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 breakage rate, the silicon carbide crystal is horizontally arranged on the annular graphite felt, so that the heat preservation effect of the edge is higher than that of the center.
While the invention has been described with respect to the preferred embodiments, it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention and are intended to be within the scope of the present invention.
Claims (7)
1. A method of secondary annealing 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;
a small crucible arranged in the large crucible, wherein the small crucible and the large crucible are coaxially arranged;
an annular graphite felt disposed within the small crucible;
the silicon carbide crystal is horizontally arranged in the center of the annular graphite felt;
the inner diameter of the annular graphite felt is 1/3-2/3 of the diameter of the silicon carbide crystal, the outer diameter is not less than the diameter of the silicon carbide crystal, and the thickness is 0.3-2.5 times of the thickness of the silicon carbide crystal;
the distance between the bottom of the small crucible and the bottom of the large crucible is 20-150 mm;
and a gap between the large crucible and the small crucible is filled with a heat insulation material.
2. The method of claim 1, wherein the silicon carbide crystals have a diameter of 4 to 8 inches.
3. The method of claim 1, wherein the inner diameter of the small crucible is 1.1 to 2.0 times the diameter of the silicon carbide crystal.
4. The method of claim 1, wherein the inner diameter of the large crucible is 1.1 to 3.0 times the outer diameter of the small crucible.
5. The method of claim 1, wherein the gap volume between the large crucible and the small crucible is 20-40% of the large crucible volume.
6. The method of claim 1, wherein the insulating material is selected from one or both of graphite felt or silicon carbide powder.
7. 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 heating time is 10-20 hours;
the annealing temperature is 2000-2500 ℃;
the heat preservation time is 20-40 hours;
the primary cooling is carried out for 20-40 hours to 1000-1250 ℃;
the secondary cooling is to cool to room temperature within 10-15 hours.
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