Method for annealing silicon carbide crystals and melt therefor

Abstract

The application discloses an annealing method of silicon carbide crystal, which comprises placing a silicon carbide crystal to be treated into a molten liquid containing elements Si and C for annealing. By controlling the annealing conditions, the internal defects of the crystal are moved to the surface, and the defects are repaired with the aid of Si and C in the molten liquid and specific metal or metal oxide. Meanwhile, the sublimation of Si elements on the surface of the crystal is compensated by Si in the molten liquid, so that new defects on the surface of the silicon carbide crystal can be avoided.

Description

Technical Field

[0001] This invention relates to the fields of semiconductor substrates and epitaxial processing. More specifically, this invention relates to an annealing method for silicon carbide crystals. Background Technology

[0002] Silicon carbide single crystal material is a representative of third-generation wide bandgap semiconductor materials, with characteristics such as wide bandgap, high thermal conductivity, high breakdown electric field, and high radiation resistance. Among them, N-type 4H silicon carbide can be used to make high-frequency devices such as MOSFETs, while P-type silicon carbide can be used to make bipolar devices such as IGBTs. High-purity semi-insulating silicon carbide will be applied in fields such as 5G mobile phone communication and military radar radio frequency.

[0003] However, both the grown silicon carbide single crystal and the processed silicon carbide wafer will have internal stress. Excessive internal stress will adversely affect further processing or epitaxial growth of silicon carbide. Annealing the crystal or substrate at 1000-2500℃ can reduce internal stress to some extent. However, such high temperatures can easily cause sublimation of Si in the silicon carbide crystal, resulting in new defects.

[0004] In view of this, there is an urgent need to provide an annealing method that can not only eliminate internal stress, but also repair defects in silicon carbide crystals and reduce the formation of new defects during the annealing process. Summary of the Invention

[0005] In order to at least solve one or more of the technical problems mentioned above, the present invention provides an annealing method for silicon carbide crystals, wherein the silicon carbide crystal to be treated is annealed in a melt containing elements Si and C.

[0006] Annealing in the molten metal allows internal defects to migrate to the surface, where elements in the molten metal repair them. Simultaneously, the immersion of the silicon carbide crystal in the molten metal during annealing isolates it from air, reducing secondary oxidation on the crystal surface. The presence of Si in the molten metal also minimizes new defects caused by S sublimation on the SiC crystal surface during annealing. Attached Figure Description

[0007] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent upon reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of the invention are illustrated by way of example and not limitation, and like or corresponding reference numerals denote like or corresponding parts, wherein:

[0008] Figure 1 A schematic diagram illustrating the steps of an annealing method for silicon carbide crystals according to an embodiment of the present invention is shown;

[0009] Figure 2 A schematic diagram of the steps of an annealing method for silicon carbide crystals, including pre-annealing preparation steps, is shown.

[0010] Figure 3 A schematic diagram of an apparatus suitable for implementing an annealing method for silicon carbide crystals according to embodiments of the present invention is shown. Detailed Implementation

[0011] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0012] It should be understood that the terms "comprising" and "including" as used in the specification and claims of this invention indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0013] It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used in this specification and claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations.

[0014] As used in this specification and claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if [described condition or event] is detected" may be interpreted, depending on the context, as "once determined," "in response to determination," "once [described condition or event] is detected," or "in response to detection of [described condition or event]."

[0015] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0016] Figure 1 A schematic diagram illustrating the steps of an annealing method for silicon carbide crystals according to an embodiment of the present invention is shown.

[0017] like Figure 1As shown, an annealing method for silicon carbide crystal includes, in step S300, placing the silicon carbide crystal to be processed in a melt containing elements Si and C for annealing.

[0018] The silicon carbide crystals to be processed include qualified silicon carbide single crystals grown using conventional techniques, or wafers obtained after processing such as wire cutting. Among existing technologies, the physical vapor transport (PVT) method is a relatively mature method for growing silicon carbide crystals. The PVT method for growing SiC crystals has specific requirements regarding crucible materials, temperature control precision, raw materials, seed crystals, and pressure, which fall within the scope of existing technology and will not be elaborated upon in this invention. Crystals grown using the PVT method contain varying degrees of defects, such as crystal defects within the wafer and surface defects at or near the wafer surface, including basal plane dislocations (BPD), stacking faults (SF), threaded edge dislocations (TED), threaded dislocations (TSD), micropipes, and grain boundaries. Furthermore, the crucible used in the PVT method for growing SiC crystals exhibits both longitudinal and radial temperature gradients, leading to inconsistent growth rates across different regions of the SiC crystal and significant internal stress. These defects and stresses affect the yield of SiC wafers and the quality of epitaxial wafers with SiC as the substrate. To repair these defects or release stress, the silicon carbide crystal is typically annealed. However, higher annealing temperatures can lead to the sublimation of Si elements on the surface of silicon carbide crystals, resulting in new defects.

[0019] Therefore, the inventors placed the annealing treatment of silicon carbide crystals under the protection of a melt containing specific elements, namely, a melt containing elements Si and C. For example, in step S300, the melt temperature is 1500℃-2500℃. During the annealing process, internal defects in the crystal move to the surface, and the defects are repaired with the help of Si and C in the melt, as well as specific metals or metal oxides. At the same time, the Si in the melt compensates for the sublimation of Si elements on the crystal surface, preventing the formation of new defects on the crystal surface.

[0020] Figure 2 A schematic diagram of the steps of an annealing method for silicon carbide crystals, including pre-annealing preparation steps, is shown.

[0021] like Figure 2 As shown, the preparation steps before annealing include steps S100 and S200. In the preparation step S100, the raw material of the melt is heated to a molten state, and the temperature of the molten state is controlled to be maintained at a suitable annealing temperature.

[0022] The proportions of each element in the melt mentioned in step S100 can be set according to the proportions of the Si alloy flux in the liquid-phase SiC crystal production process. The basic principle of liquid-phase silicon carbide crystal growth is that carbon is dissolved in a high-temperature liquid composed of silicon and flux. Due to supersaturation, carbon precipitates at the silicon carbide seed crystal, and at the same time, silicon atoms are carried out due to the effect of the coulomb field of the crystal lattice, thus realizing the growth of silicon carbide crystals.

[0023] One method for producing SiC crystals using the liquid phase method is as follows: Graphite is used as a crucible and a carbon source, and molten silicon is filled inside. A SiC seed crystal is placed on top of the graphite crucible, just in contact with the melt, and the seed crystal temperature is controlled to be slightly lower than the melt temperature. The temperature gradient is used as the growth driving force to achieve crystal growth. Another example is the Czochralski method: purified raw materials are placed in a crucible, which is then placed in an appropriate thermal field. During heating, the raw materials gradually melt in the crucible, and the pre-placed seed crystal is pulled up and rotated at a certain speed to grow a single crystal that meets the requirements. However, the liquid phase method for producing SiC crystals has drawbacks, such as slow growth rate and long cycle time.

[0024] The inventors discovered that during the annealing process of silicon carbide crystals, the repair of internal defects and the repair of new defects caused by Si sublimation can both be achieved using the SiC crystal growth process. However, if the PVT method is still used, the excessively fast crystal growth rate will cause new surface defects. Therefore, in the silicon carbide crystal annealing process of this invention, the inventors fully utilize the characteristics of slow growth rate and long cycle of SiC crystal production by the liquid phase method to repair crystal surface defects and simultaneously suppress Si sublimation. The slow growth rate, combined with the promotion of local defect repair and the suppression of Si sublimation, avoids the formation of new defects due to rapid growth.

[0025] According to one embodiment of the present invention, a fluxing agent may be added to the melt, for example: the melt contains elements Si, C and X, wherein X is a fluxing agent selected from one or more of Al, Ti, Cr, Y, Yb, Pr, Sn, La and Ce; the molar ratio of Si, C and X in the melt is a:b:c, wherein 0.30≤a≤0.60, 0.30≤b≤0.40, 0.05≤c≤0.50 and a+b+c=1.

[0026] According to one embodiment of the present invention, the annealing time is 1-72 hours, and the temperature of the melt can be selected from 1500℃-2000℃, 1800℃-2200℃, 1900℃-2100℃, or 1950℃-2050℃. Within the above temperature range, the melt maintains a stable molten state. The temperature control affects the liquid crystal growth rate, matching the compensation of defects. Moreover, the above temperatures are suitable for the annealing of silicon carbide crystals. The inventors have found that during the annealing process of silicon carbide crystals, high temperature helps to release the internal stress of the silicon carbide crystal and move the internal defects of the silicon carbide crystal to the surface. However, excessively high temperatures can cause damage to the surface of the silicon carbide crystal, causing a large amount of Si to escape from the surface of the silicon carbide crystal, resulting in carburization.

[0027] Based on this, the inventors utilized the annealing method in the solution, which reduces the escape of Si from the surface of silicon carbide crystals due to the presence of Si elements in the melt, thus allowing for the use of appropriate high temperatures.

[0028] According to an embodiment of the present invention, preferably, in the annealing process of step S3, the temperature of the melt is 2000°C and the annealing time is 5 hours. Similarly, the annealing time is also a very important control condition. Traditionally, under inert gas protection, annealing at 1450°C for 1-2 hours can improve the quality of SiC wafers. However, as the annealing time increases, for example, more than 3 hours, the quality of the silicon carbide crystal will decrease to varying degrees, indicating that prolonged annealing actually reduces the crystal quality of the wafer. This phenomenon can be explained by carbonization on the surface of the silicon carbide crystal. In fact, surface carbonization is unavoidable in traditional high-temperature annealing processes, and the carbonization rate is directly related to temperature. At higher temperatures, the carbonization rate is very fast, easily causing damage to the surface. At lower temperatures, although the carbonization effect is not obvious and can release the stress inside the silicon carbide crystal, as the annealing time increases, the carbonization effect accumulates, also causing surface damage.

[0029] In the embodiments of the present invention, the inventors used a relatively high temperature, preferably 2000°C, so that the raw materials of the melt are in a molten state. At the same time, a relatively long time was used, preferably 5 hours, so as to utilize the repairing effect of the melt and the suppression effect on Si escape to meet the annealing requirements of silicon carbide and improve the quality of silicon carbide crystals.

[0030] According to one embodiment of the present invention, during the annealing process, the silicon carbide crystal to be treated rotates in the molten metal. In step S200, when the silicon carbide crystal to be treated is slowly placed in the molten metal, the slow rotation can create a clear boundary between the silicon carbide crystal and the molten metal, and at the same time, the molten metal can more uniformly compensate for defects on the surface of the silicon carbide crystal, resulting in uniform heating of all parts of the silicon carbide crystal.

[0031] The heating and cooling rates of silicon carbide crystals have a significant impact on their performance in annealing processes. Excessive heating or cooling rates can increase residual stress within the crystals. To control the heating and cooling process, a gradual heating and cooling method can be used, for example, at a rate of 200-300℃ / h. For instance, the molten raw material and silicon carbide crystals can be placed together in an annealing apparatus, gradually heated to 2500℃, and then the silicon carbide crystals are immersed in the molten material. While immersed, the crystals are fixed to a rotating stirring rod, which drives their rotation. The rotation speed can be adjusted according to the size of the silicon carbide crystals and the annealing time.

[0032] According to one embodiment of the present invention, the silicon carbide crystal to be processed is a 4-8 inch N-type or P-type silicon carbide single crystal.

[0033] According to one embodiment of the present invention, the molten liquid is placed in a crucible and heated to 1500°C-2500°C by a heating device; the silicon carbide crystal to be processed is placed at the end of a rotating stirring rod; the heating device is a radio frequency induction heating furnace.

[0034] The crucible is preferably made of graphite, as the erosion of the graphite crucible by the molten metal can compensate for the reduction of carbon in the molten metal. A platform for fixing a silicon carbide crystal is provided at the end of the rotating stirring rod, perpendicular to the rotation axis of the stirring rod. The silicon carbide crystal is fixed on the platform at the end of the rotating stirring rod. The stirring rod is driven by external force and slowly descends into the molten metal. Simultaneously, the stirring rod rotates continuously, creating a boundary between the silicon carbide crystal and the molten metal, and ensuring uniform heating of all parts of the silicon carbide crystal. The radio frequency induction heating furnace maintains a constant temperature in the molten metal, providing a stable environment for the annealing of the silicon carbide crystal.

[0035] According to one embodiment of the present invention, the melt is formed by melting raw materials Si, SiC, and CeO2 in a molar ratio of Si:SiC:CeO2 = 1:1:1.

[0036] Si and SiC provide the Si and C sources, respectively, while CeO2 acts as a co-solvent to provide appropriate concentration control, thereby limiting the growth rate of SiC and controlling the erosion rate of the crucible. This maintains the ratio of Si to C sources, thus adapting to the repair of defects rather than leading to rapid crystal growth.

[0037] According to one embodiment of the present invention, the melt is formed by melting raw materials Si, SiC, CeO2, and Cr, with a molar ratio of Si:SiC:CeO2:Cr = 1:1:1:1 to 1:0.4:1:2. The addition of CeO2 and Cr to the melt, especially in the above-mentioned proportions, significantly enhances its ability to repair SiC crystals.

[0038] According to one embodiment of the present invention, the pressure during annealing of silicon carbide crystal is 0.8-1.2 bar, preferably 1 bar.

[0039] According to another aspect of the present invention, a melt for annealing silicon carbide crystals is provided, the temperature of the melt being 1500℃-2500℃; the melt contains elements Si, C and X, wherein X is one or more of Al, Ti, Cr, Y, Yb, Pr, Sn, La, and Ce; the ratio of Si, C, and X in the melt is a:b:c, wherein 0.30≤a≤0.60, 0.30≤b≤0.40, 0.05≤c≤0.50, and a+b+c=1.

[0040] According to one embodiment of the present invention, the method for preparing a melt includes: placing a composition containing elemental sources of Si, C and X in a crucible and heating it to a molten state to obtain a melt, wherein the elemental source of X is one or more of elemental X, a compound of element X.

[0041] Figure 3 A schematic diagram of an apparatus for annealing silicon carbide crystals applicable to embodiments of the present invention is shown.

[0042] like Figure 3 As shown, a silicon carbide crystal is fixedly mounted on a support platform at the end of a rotating stirring rod 2. The crucible 1 contains molten liquid. The molten liquid is heated to 1500℃-2500℃ by a heating device 3. The rotating stirring rod 2 is driven to descend, immersing the silicon carbide crystal in the molten liquid. The rotating stirring rod 2 is then driven to rotate slowly, allowing the silicon carbide crystal to complete annealing in the molten liquid. Finally, the rotating stirring rod 2 is driven to rise, resulting in the annealed crystal.

[0043] While numerous embodiments of the invention have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Many modifications, alterations, and alternatives will occur to those skilled in the art without departing from the spirit and essence of the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in the practice of the invention. The appended claims are intended to define the scope of protection of the invention and therefore cover equivalents or alternatives within the scope of these claims.

Claims

1. An annealing method of a silicon carbide crystal, characterized by, The application relates to a silicon carbide crystal annealing method. The silicon carbide crystal to be treated is annealed in a molten liquid containing elements Si and C; during the annealing process, the silicon carbide crystal to be treated is rotated in the molten liquid; The molten liquid further contains X; The X is one or more of Al, Ti, Cr, Y, Yb, Pr, Sn, La and Ce; in the molten liquid, the molar ratio of Si, C and X is a:b:c, wherein 0.30<=a<=0.60, 0.30<=b<=0.40 and 0.05<=c<=0.50, and a+b+c=1.

2. The silicon carbide crystal annealing method according to claim 1, wherein the temperature of the molten liquid is 1500-2500 DEG C, and the annealing time is 1-72 hours.

3. The silicon carbide crystal annealing method according to claim 1, wherein the raw material of the molten liquid is placed in a crucible, and is heated to 1500-2500 DEG C by a heating device to obtain the molten liquid; the silicon carbide crystal to be treated is placed at the end of a rotating stirring rod; and the end of the rotating stirring rod is immersed in the molten liquid.

4. The silicon carbide crystal annealing method according to claim 1, wherein the molten liquid is formed by melting raw materials Si, SiC and CeO2, and the molar ratio is Si:SiC:CeO2=1:1:

1.

5. The silicon carbide crystal annealing method according to claim 1, wherein the molten liquid is formed by melting raw materials Si, SiC, CeO2 and Cr, and the molar ratio is Si:SiC:CeO2:Cr=1:1:1:1-1:0.4:1:

2.

6. The silicon carbide crystal annealing method according to claim 1, wherein the pressure during the annealing is 0.8-1.2 bar.

7. A molten liquid for silicon carbide crystal annealing, wherein the temperature of the molten liquid is 1500-2500 DEG C; the molten liquid contains elements Si, C and X, wherein the X is one or more of Al, Ti, Cr, Y, Yb, Pr, Sn, La and Ce; in the molten liquid, the molar ratio of Si, C and X is a:b:c, wherein 0.30<=a<=0.60, 0.30<=b<=0.40 and 0.05<=c<=0.50, and a+b+c=1. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 0.30≤a≤0.60， 0.30≤b≤0.40， 0.05≤c≤0.50， ​

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