Apparatus and method for growing SiC volumetric single crystals

Abstract

The invention relates to a device for growing SiC volumetric single crystals in a cavity in a growth direction by sublimation growth. The device comprises a base, wherein the base is used for heating a cavity; and an insulator surrounding the base, the insulator for thermally insulating the base from an exterior of the device. The insulator includes a thermal insulation wall surrounding the susceptor in a growth direction and a circumferential direction, the thermal insulation wall for reducing heat transfer from the susceptor to an exterior of the device in a radial direction, the radial direction being perpendicular to the growth direction; the heat conduction layer is arranged between the base and the heat insulation wall, has heat conductivity higher than that of the heat insulation wall and is used for distributing heat on the heat conduction layer; wherein the thermally conductive layer is spaced apart from the base to reduce heat transfer from the base to the insulator caused by heat conduction, and the thermally conductive layer is disposed on the thermally insulating wall to reduce waviness of the insulator to increase reflectivity of the insulator.

Description

Technical Field

[0001] The present application relates to an apparatus and a growth method for growing bulk single crystals of SiC. Background Art

[0002] Due to the excellent physical, chemical, electrical, and optical properties of silicon carbide (SiC), silicon carbide (SiC) is used as a semiconductor material for high-frequency components and special light-emitting semiconductor components, including as a raw material for power semiconductor components. For these components, SiC substrates with a large substrate diameter (equal to or greater than 200 mm) and high quality are required, such as those described in U.S. Patent No. 8,747,982 B2.

[0003] SiC single crystals are grown using suitable raw materials, for example, by physical vapor deposition (PVT) processes, such as those disclosed in U.S. Patent No. 8,865,324 B2. The crystals are then sliced into SiC substrates using a wire saw or similar slicing techniques, and the surfaces are refined using multi-stage polishing steps. In subsequent epitaxial processes, thin single crystal layers (such as SiC, GaN) are first deposited on the SiC substrates. The properties of these layers and the resulting devices depend on the quality of the SiC substrates.

[0004] Growing SiC crystals is very energy-consuming and thus an expensive process. Each energy saving with the same or increased crystal results in an increase in the economic efficiency of the resulting electrical components. The use of insulators has been proposed, such as those described in German Patent No. DE10 200 9 004 751 B4. In this way, energy can be saved and quality can be ensured.

[0005] At the same time, quality losses in crystal growth result in huge costs. To achieve high yields in crystal growth, it is necessary to be able to use raw materials and auxiliary materials with consistent quality, especially insulators, which must meet high requirements such as cleanliness, temperature stability, and transparency to electromagnetic fields. Summary of the Invention

[0006] In view of the above, the aim is to reduce costs and save energy. At the same time, an aim is to select materials that allow for setting a defined temperature distribution (which is consistent from growth to growth) to ensure quality. For the growth of high-quality SiC crystals, another aim is to have a defined temperature distribution prevail in the growth apparatus. Another aim is to use materials that can be used at high temperatures. Another aim is that the material can be used under vacuum conditions and meet the requirements of cleanliness.

[0007] At least one of these aims is solved by the subject matter of the independent claims. Advantageous embodiments are solved by the dependent claims.

[0008] According to a general aspect, there is provided an optimization of the surface of an insulator. More specifically, insulators that can be used in crystal growth applications typically have a wavy surface and a structure with varying density. This has a twofold negative impact on crystal growth.

[0009] More specifically, a fiber-based microstructure typically results in a surface with strong undulations and voids. Due to the non-optimal reflection of thermal radiation from the insulator to the surface, this leads to non-optimal and non-uniform energy return to the susceptor. Instead, the insulator gets heated due to unwanted absorption, or it is not possible to directly backscatter to the susceptor due to an unfavorable insulating surface. In regions of low fiber density (large voids), more radiant energy from the crucible is absorbed by the insulator, while in regions of high fiber density (small voids), less radiant energy is absorbed.

[0010] These local hotspots in the insulation create unwanted temperature non-uniformities that randomly and uncontrollably affect the temperature field during growth and can negatively impact the quality of the grown crystal. The way the insulation is produced further exacerbates this problem. Production-related gradual non-uniformities in the insulation due to fiber clumping or loss in the insulation also result in the described hotspots in the insulation, which are also difficult to control and affect.

[0011] In summary, modern insulation negatively affects crystal growth in two ways: insufficient heat return to the susceptor through thermal radiation due to the surface structure, and insufficient uniformity of the temperature field due to fiber non-uniformities and the formation of hotspots in the insulator.

[0012] By optimizing the insulator surface, the yield and quality of the crystal are increased by balancing the hotspots in the insulation (increasing uniformity, even from growth to growth) and by increasing the reflectivity of the insulation surface to the susceptor (energy efficiency).

[0013] The inventors have found that by surface treatment of the insulator, in particular by coating, infiltrating, and / or foiling the surface of the insulator, these two drawbacks can actually be eliminated. More specifically, coating, infiltrating, and foiling increase the energy returned to the susceptor, and the insulating effect can be significantly homogenized. Thermal radiation impinging on the optimized thermal insulation surface from the susceptor can be directly and directionally reflected back to the susceptor along the entire length of the thermal insulator. In addition, since the coating, infiltration, and foil each have a higher thermal conductivity than the original insulator, the temperature non-uniformities (hotspots) in the axial direction of the insulator can be compensated for.

[0014] Although the increased thermal conductivity slightly increases the energy absorption, since the electromagnetic coupling is typically higher in such materials, this effect is greater than the effect offset by the increased uniformity and its impact on the produced crystal.

[0015] More specifically, a first aspect relates to an apparatus for growing a bulk single crystal of SiC in a growth direction by sublimation in a cavity typically formed by a crucible. Silicon carbide crystals are generally grown using the physical vapor transport (PVT) method, in particular sublimation. Growth begins with a seed placed in the crucible, and the seed grows in the growth direction within the crucible, thereby forming a larger SiC crystal. Growth typically occurs in a crucible surrounding the cavity.

[0016] More specifically, an apparatus for growing a bulk single crystal of SiC generally includes a crucible, which is a container designed to withstand high temperatures and is used to hold materials at extreme temperatures, where the seed and source material are placed in the cavity. In addition, the apparatus includes a susceptor, which is a material that absorbs electromagnetic energy (usually in the form of microwaves or radio frequency waves) and converts it into heat. In short, while the crucible serves as a high-temperature container for holding and processing materials, the susceptor improves the efficiency of the heating process, especially in applications that utilize microwave or radio frequency energy.

[0017] Hereinafter, the susceptor specifically refers to the combination of the crucible and the susceptor. This has advantages for applications that are critical for precision, speed, and energy efficiency. Alternatively, the growth apparatus may also include a crucible in addition to the susceptor.

[0018] The container, i.e., the crucible or the susceptor that is part of the crucible, provides a cavity inside it, in which the bulk single crystal of SiC grows. The inner surface of the container faces the cavity. The outer surface of the container faces the insulator.

[0019] Advantageously, the container has a prismatic shape. A prism is a polyhedron that includes an n-sided polygonal base, a second base that is a translation replica (rigidly moved without rotation) of the first base, and n other faces that form the side walls, and the n other faces join the corresponding sides of the two bases. Thus, a closed cavity is achieved. This prismatic shape allows the bulk single crystal of SiC to be easily removed from the cavity, i.e., parallel to the faces forming the side walls. Generally, the container has a cylindrical shape, i.e., the base has a circular cross-section.

[0020] It is noteworthy that a geometric shape that can be used multiple times is advantageous because the cost of the container is high. Therefore, in particular, the geometric shape of the prism, especially the geometric shape of the cylinder, is advantageous because the bulk single crystal of SiC can be extracted along the growth direction and the dead space in the cavity can be reduced.

[0021] According to the first aspect, an insulator surrounds the susceptor. Thus, the insulator thermally insulates the susceptor from the outside of the apparatus. A thermal insulator is a material specifically designed to reduce or prevent heat transfer between objects at different temperatures (i.e., between the susceptor heated during growth and the outside at ambient temperature). Its main function is to block the flow of thermal energy through conduction, convection, and radiation.

[0022] As used herein, an insulator surrounds the surface of the pedestal, which means that the insulator encloses or encircles the outer portion of the pedestal, covering at least a portion of its entire exterior. In particular, the insulator is in very close proximity to the pedestal.

[0023] Furthermore, according to the first aspect, the insulator includes a thermally insulating wall that surrounds the pedestal in the growth direction and the circumferential direction. Thus, the insulator extends in two dimensions, namely the growth direction (i.e., the longitudinal direction) of the pedestal and the circumferential direction. As used herein, this means that the insulator has dimensions or components that are stretched along its length (longitudinally) and around its circumference (circumferentially).

[0024] Longitudinal refers to the longitudinal dimension of an object and is typically considered to be along its main axis, i.e., the growth direction of the SiC crystal. For example, in the case of a tube or a cylinder, the longitudinal direction is the direction from one end to the other end.

[0025] The circumferential direction refers to the direction around the outer boundary or surface of the pedestal. In the case where the container has a cylindrical shape, the circumferential direction would be a circular path around the exterior.

[0026] Therefore, the thermally insulating wall that extends in the longitudinal and circumferential directions has characteristics or dimensions that span its length and the perimeter around the outer surface of the pedestal. For example, in the case of a cylindrical or tubular object, where there are dimensions along the length and around the circumference, the insulator forms a hollow cylinder.

[0027] It is noted that the insulator may include additional sections for thermal insulation to completely surround the pedestal. For example, if the pedestal has a cylindrical shape, the insulator may include an insulating base, i.e., a section that extends in the radial dimension and the circumferential dimension.

[0028] In view of the above, the thermally insulating wall according to the first aspect reduces heat transfer in the radial direction from the pedestal to the outside of the device, which is perpendicular to the growth direction.

[0029] Particularly in the context of a growth device, an insulator that extends in the longitudinal and circumferential directions is advantageous because the surface area defined by these two dimensions is typically the largest surface area of the container. In addition, the temperature in the growth direction must be controlled particularly precisely. Furthermore, in the case of induction heating, the pedestal is typically heated in the circumferential direction, and thus, a thermally insulating wall that surrounds the pedestal in the circumferential direction is particularly advantageous.

[0030] Furthermore, according to the first aspect, a thermally conductive layer is provided between the pedestal and the thermally insulating wall, and the thermally conductive layer has a higher thermal conductivity than the thermally insulating wall for distributing heat over the thermally conductive layer.

[0031] As used herein, a thermal conductive layer is a material or coating designed to effectively transfer heat. Its primary purpose is to enhance the thermal conductivity in the layer, facilitating the rapid and efficient movement of heat from one point in the layer to another.

[0032] Furthermore, according to the first aspect, the insulator includes a thermal conductive layer that is spaced apart from the base to reduce heat transfer from the base to the insulator caused by thermal conduction.

[0033] Heat transfer can occur through various mechanisms, two of which are thermal conduction and thermal radiation. Thermal conduction is a mode of heat transfer that occurs through the direct microscopic interaction of particles within a material or between different materials in direct contact. During this process, thermal energy is transferred within the substance from a region of higher temperature to a region of lower temperature without any overall movement of the substance itself. Thermal radiation involves the transfer of heat through electromagnetic waves (such as infrared radiation). Unlike conduction, it does not require a material medium; heat is transferred through a vacuum or a transparent medium.

[0034] Therefore, by spacing the base apart from the insulator, typically more efficient thermal conduction is reduced, and heat transfer can be described by thermal radiation, especially since the device is used under vacuum conditions. Thus, by spacing the thermal conductive layer apart from the base, the thermal conductive layer can contribute to thermal insulation. However, a larger space increases the overall device. Since the device typically operates under vacuum conditions, space should be saved.

[0035] For example, the base and the thermal conductive layer are spaced apart (i.e., separated) in the radial direction by a distance equal to or greater than 0.1 mm, preferably equal to or greater than 0.5 mm, and even more preferably equal to or greater than 1 mm. Additionally or alternatively, the base and the thermal conductive layer are spaced apart in the radial direction by a distance equal to or less than 5 cm, preferably 1 cm, and even more preferably 5 mm.

[0036] Further, according to the first aspect, the thermal conductive layer is disposed on the thermally insulating wall, thereby reducing the corrugation of the insulator to increase the reflectivity of the insulator.

[0037] In the context of reflectivity, the term corrugation refers to the variations or irregularities in the reflective surface of a material or object. Corrugation can affect how light or electromagnetic waves interact with the surface and the resulting reflection characteristics. In particular, corrugation typically manifests as surface roughness on a reflective material. Microscopic irregularities or undulations on the surface can cause scattering and diffusion of the reflected light. Corrugation or roughness can affect the specular reflection (mirror-like reflection) and diffuse reflection (scattering reflection) properties of the surface. Generally, a higher corrugation value results in a lower reflection.

[0038] Therefore, by reducing the corrugation of the insulator, the reflectivity of the surface increases and the loss of thermal radiation is reduced because the amount of thermal radiation reflected or scattered back to the base (and absorbed by the base) increases.

[0039] In other words, the heat conducting layer increases the energy returned to the base, and the insulation effect can be significantly homogenized. The thermal radiation from the base impacting the optimized insulating surface can be directly and directionally reflected back to the base throughout the entire length of the insulation. Additionally, the higher thermal conductivity of the heat conducting layer balances the temperature non-uniformity (hot spots) in the axial direction of the insulation. Although the increased thermal conductivity slightly increases the energy absorption, since the electromagnetic coupling is usually higher in this material, this effect is compensated by the increase in uniformity and its impact on the resulting crystals.

[0040] Advantageously, the base has a higher absorption than the heat conducting layer. For example, based on the different types of materials used for the base and the insulating material, the absorption is different.

[0041] According to a second aspect, the thermal conductivity ratio of the heat conducting layer to the thermal conductivity of the thermal insulation wall is greater than or equal to 10. Advantageously, this ratio is greater than or equal to 14. Even more advantageously, this ratio is greater than or equal to 18.

[0042] The thermal conductivity ratio refers to the ratio of the thermal conductivity of one material (i.e., the thermal conductivity of the heat conducting layer) to the thermal conductivity of another material (i.e., the thermal conductivity of the thermal insulation wall).

[0043] Note that the thermal conductivity can vary with temperature, so the temperature relevant to a given application is considered here when comparing materials. Here, the reference temperature is considered to be the temperature at which the SiC bulk single crystal grows, in the range of 2000 - 2400 °C. It is worth noting that the exact temperature can vary depending on the specific SiC polytype required (e.g., 4H-SiC, 6H-SiC), the growth parameters, and the equipment used in the PVT process. In particular, a medium temperature of 2200 °C is considered relevant to the thermal conductivity.

[0044] Furthermore, different materials can have anisotropic thermal conductivities, meaning that the thermal conductivity can vary with direction (i.e., the circumferential direction and the growth direction). In this case, a medium anisotropic thermal conductivity is considered relevant.

[0045] The higher the thermal conductivity, the better the heat can be distributed. This is important, for example, in cases where local hot spots in the insulation result in undesirable temperature non-uniformities that have an uncontrollable impact on the temperature field during growth on a random basis and may have a negative impact on the quality of the grown crystal. The manner in which the insulation is produced further exacerbates this aspect. The production-related gradual non-uniformities in the insulation due to fiber clumping or losses in the thermal insulation material also result in the described hot spots in the thermal insulation material, which are also difficult to control and influence. Therefore, the thermal conductivity of the heat conducting layer is very important.

[0046] According to a third aspect, in addition to any of the foregoing aspects, the heat-conducting layer is arranged (i.e., provided) on the heat-insulating wall by at least one of coating, infiltration, and foil application. Thus, the heat-conducting layer and the heat-insulating wall can be made particularly close to each other, so that the heat transfer by conduction between the heat-insulating wall and the heat-conducting layer is increased. Therefore, the heat generated at the hot spots of the heat-insulating wall can be effectively distributed on the heat-conducting layer, thereby reducing the impact of the hot spots of heat.

[0047] To optimize the thermal conductivity, these layers are typically applied as thin and uniform layers. This ensures a tight interface between the layers and minimizes the thermal resistance.

[0048] In particular, surface coatings and infiltration can be applied to reduce the waviness and enhance the reflection characteristics. For example, reflective coatings are designed to maximize reflection by reducing surface irregularities. These coatings are crucial in applications where precise control of the reflected light is essential. The infiltration of the surface involves the process of a substance penetrating or permeating the surface of the material.

[0049] According to a fourth aspect, based on the third aspect, the heat-conducting layer is arranged on the heat-insulating wall by coating and / or infiltrating a metal carbide. The metal carbide coating has excellent hardness, wear resistance, and high-temperature stability. Preferably, the metal includes refractory metal carbides. These refractory metal carbides are selected based on specific requirements of the application, considering factors such as temperature resistance, wear resistance, and chemical compatibility. The coating and infiltration methods can include thermal spraying, physical vapor deposition, or chemical vapor deposition, depending on the material and the desired properties. In particular, these can be deposited from the gas phase and solution onto the insulating material and then produce a uniform and smooth surface.

[0050] It has been found that, in particular, tantalum carbide (TaC), tungsten carbide (WC), and hafnium carbide (HfC) are advantageous in terms of having a high melting point, wear resistance, and high hardness.

[0051] According to a fifth aspect, in addition to the third and fourth aspects, the heat-conducting layer is arranged on the heat-insulating wall by applying a foil to the heat-insulating wall. The foil can achieve a technical effect similar to that of coating or infiltrating the surface. In particular, the foil can include graphite or be made of graphite. Graphite foil is a thin and flexible material made of graphene sheets. Due to the structure of graphite, graphite has unique properties. Graphite is composed of carbon atoms arranged in a hexagonal pattern. In particular, graphite foil is an excellent heat conductor. In addition, graphite is chemically inert, which means it is resistant to many corrosive substances. This property makes graphite foil suitable for use in harsh environments where exposure to extreme temperatures, such as in a growth device, is a problem.

[0052] Preferably, the foil comprises exfoliated graphite. Exfoliated graphite refers to a form of graphite that undergoes a process called exfoliation, resulting in the expansion and separation of individual graphene layers. Exfoliated graphite exhibits anisotropy in thermal conductivity due to its layered structure. The thermal conductivity of exfoliated graphite is highly dependent on the direction in which heat is transferred relative to the plane of the graphene layers. Anisotropic thermal properties are a result of differences in heat conduction in a plane (in-plane) and perpendicular to the plane (out-of-plane).

[0053] Thus, anisotropic heat conduction can be used, so that parallel to the axis of symmetry of the growing device (i.e. in the growth direction) there is a higher thermal conductivity and is therefore ideal for equalizing temperature inhomogeneities along the insulation and providing a smooth surface for high reflection. In particular, as mentioned above, the in-plane direction shows a higher thermal conductivity, i.e. a hexagonal pattern. Therefore, this is beneficial for homogenizing inhomogeneities and hot spots in the insulation material.

[0054] According to a sixth aspect, in addition to any of the previous aspects, the thermal insulation wall includes or consists of felt.Felt is a common material that can be used for thermal insulation due to its fiber structure, which creates a network of air pockets that resists heat flow.

[0055] Advantageously, the felt may include at least one of soft felt, hard felt, and a combination of soft felt and hard felt.

[0056] A felt advantageous for the device has a fiber-based microstructure which results in a surface with strong undulations and cavities. This results in a non-optimal and non-uniform energy return towards the base due to non-optimal reflection of the thermal radiation from the thermally insulating walls. Conversely, the thermally insulating walls heat up due to undesired absorption or because direct backscattering towards the base is not possible due to an unfavorable thermally insulating wall surface. In areas with low fiber density (large cavities), more radiant energy from the crucible is absorbed in the thermally insulating walls, and in areas with high fiber density (small cavities), less radiant energy is absorbed. The negative effects are compensated by the thermally conductive layer discussed above.

[0057] The thermal insulation wall material may comprise or consist of short carbon fibers held together by a binder matrix (hard felt) and / or long carbon fibers consolidated by needling (soft felt).

[0058] The hard felt can be formed into the desired form, such as a hollow cylinder, during the manufacturing process, which then assumes a rigid shape. The soft felt is usually made into a sheet-like mat with a thickness of about 5 mm to 25 mm. The mat can then be cut as required, rolled and processed into the desired form, such as a hollow cylinder, by, for example, sewing. Hybrid solutions are also conceivable, in which, for example, a soft felt mat is rolled onto a hard felt cylinder and fixed thereto or a soft felt mat is rolled and enveloped by a hard felt cylinder.

[0059] According to a seventh aspect, in addition to the sixth aspect, the thermal insulation wall comprises or consists of a graphite thermal insulation material having short carbon fibers with a fiber length in the range between 1 mm and 10 mm and a fiber diameter in the range between 0.1 mm and 1 mm, wherein the short carbon fibers are bonded by a resin to form a rigid felt. Alternatively or additionally, the thermal insulation wall comprises a graphite thermal insulation material having long carbon fibers with a fiber length greater than 10, wherein the long carbon fibers are bonded by needling to form a soft felt.

[0060] According to an eighth aspect, in addition to any of the foregoing aspects, the thermal insulation wall is formed by a hollow cylinder that extends in a growth direction and circumferentially surrounds a base. Reference is made to the above description regarding the geometry of the hollow cylinder. Advantageously, a plurality of coaxial hollow cylinder layers are nested with each other, each cylinder layer having a different radius, and the cylinder having the smallest radius has a heat-conducting layer on its inner surface facing the base. Thus, different materials for different applications can be achieved.

[0061] According to a ninth aspect, in addition to any of the foregoing aspects, the ratio of the reflectivity of the heat-conducting layer to the reflectivity of the thermal insulation wall is greater than or equal to 1.3. Reflectivity, also known as the reflection coefficient, is a measure of the amount of light or other electromagnetic radiation reflected by a surface compared to the amount of light or other electromagnetic radiation incident on the surface. Thus, by virtue of the higher reflectivity of the heat-conducting layer, the insulation properties of the insulator can be increased compared to a device without a heat-conducting layer. Advantageously, the ratio is greater than or equal to 1.4. Even more preferably, the ratio is greater than or equal to 1.5.

[0062] According to a tenth aspect, in addition to any of the foregoing aspects, the corrugation height of the heat-conducting layer is less than or equal to 2 mm. Preferably, the corrugation height of the heat-conducting layer is less than or equal to 1 mm. Even more preferably, the corrugation height of the heat-conducting layer is less than or equal to 0.5 mm.

[0063] The corrugation height refers to the vertical distance between the highest peak and the lowest valley of the surface profile within a specified sampling length. It is a measure of the amplitude of the irregularities or undulations present on the surface. Waviness is different from roughness, where the former generally represents longer wavelength variations in the surface profile. The corrugation height is defined by irregularities with frequencies lower than those associated with roughness. These irregularities typically occur over longer distances on the surface. Thus, considering the longer wavelengths, i.e., radiation at a temperature of 2000 °C results in wavelengths in the micrometer range, the corrugation height is more suitable for defining the surface properties than surface roughness, which is more important for shorter wavelengths, i.e., wavelengths in the nanometer range. Techniques such as profilometry or interferometry can be used to measure the corrugation height. As a general rule, the smaller the corrugation height, the higher the reflectivity.

[0064] According to the eleventh aspect, in addition to any of the foregoing aspects, the thickness ratio of the thickness of the heat-conducting layer in the radial direction to the thickness of the heat-insulating wall in the radial direction is less than or equal to 1 / 10, preferably less than or equal to 1 / 15, and even more preferably less than or equal to 1 / 20.

[0065] The thin heat-conducting layer reduces the influence on the insulating performance of the heat-insulating wall. If the coating / permeate / foil is thick, too much electromagnetic radiation has been coupled into the coating and heated the insulating material too much so that it deteriorates significantly, and at the same time too little energy is coupled into the susceptor, which means it is more difficult to bring the device to the desired temperature.

[0066] In addition, in addition to the pure insulating effect, in the case of induction heating described below, the electromagnetic field of the coil should not be coupled into the insulating material and heat the insulating material itself. The insulating material must be permeable to the electromagnetic field and ensure as low losses as possible so that energy is directly coupled into the growth susceptor (which can be part of a crucible) and heats the source material inside. Therefore, a thin heat-conducting layer is particularly advantageous for induction heating.

[0067] The typical thickness of the heat-insulating wall is greater than or equal to 5 mm, and even more preferably 1 cm. In addition, the thickness of the heat-insulating wall is less than or equal to 15 cm, preferably less than 10 cm, and even more preferably less than 5 cm.

[0068] According to the twelfth aspect, in addition to any of the foregoing aspects, the insulator further includes at least one heat-insulating cover for covering the susceptor.

[0069] There are two different positions in the SiC growth setup where insulating materials are used. On the one hand, heat-insulating materials are arranged below and above the growth susceptor, and on the other hand, a hollow single-piece or multi-piece insulating cylinder is placed above the susceptor, which extends radially around the susceptor in the circumferential and longitudinal directions.

[0070] The thirteenth aspect relates to a system including the device according to any one of the foregoing aspects, wherein the system further includes at least one of the following: an induction heater, wherein the induction heater surrounds the insulator; a resistance heater, wherein the resistance heater is surrounded by the insulator; a reactor for accommodating the device; and a vacuum pump for evacuating the system, particularly the reactor.

[0071] Thus, heating is provided by at least one induction coil installed outside the reactor or by a resistance heater installed inside the reactor. Additionally, the device can be placed in the reactor where the actual crystal growth occurs. The device for growth includes different graphite and insulating materials, a seed crystal, and a source material, which are placed in a cavity formed in the reactor (i.e., in the crucible). Generally, the device includes carbon-based materials because these materials can withstand high temperatures and are not a source of impurities for SiC crystals at the same time.

[0072] The insulator is designed to ensure that as little energy introduced into the base is released to the outside, but can be used to heat the base as much as possible. The device including the base and the insulator is accommodated in the reactor, preferably in an inter-gas atmosphere of less than 50 mbar. According to the radiation law, the base emits some induction energy outward towards the insulator. As discussed above, the insulator is generally positioned away from the base, so the heat transfer from the base to the insulator is mainly through thermal radiation.

[0073] Additionally, the device can include means for measuring temperature, pyrometers, thermocouples, etc. The vacuum-sealed reactor can be evacuated by one or more vacuum pumps. Furthermore, an inert gas and / or a doping gas (such as nitrogen) can be supplied to the system via one or more gas supply lines, and the pressure in the system can be measured and controlled. All process parameters (pressure, temperature, gas flow...) can be set, monitored, and archived by a computer-controlled system controller. The system control communicates with all components involved (such as inverters, pyrometers, vacuum control valves, MFCs, pressure gauges).

[0074] A fourteenth aspect relates to a method for growing a bulk single crystal of SiC by sublimation growth in a growth direction in a cavity, the method comprising the following steps:

[0075] Providing a SiC seed crystal and a SiC source material in the cavity;

[0076] Heating a base to grow the bulk single crystal of SiC in the cavity;

[0077] Isolating the base with an insulator, wherein the insulator includes:

[0078] A thermally insulating wall that surrounds the base in the growth direction (Y) and the circumferential direction, the thermally insulating wall being used to reduce heat transfer in the radial direction (r) from the base to the outside of the device, the radial direction (r) being perpendicular to the growth direction (Y);

[0079] A heat-conducting layer provided between the base and the thermally insulating wall, the heat-conducting layer having a higher thermal conductivity than the thermally insulating wall and being used to distribute heat on the heat-conducting layer;

[0080] The heat-conducting layer is spaced apart from the base to reduce heat transfer from the base to the insulator caused by heat conduction, and the heat-conducting layer is disposed on the heat-insulating wall, thereby reducing the corrugation of the insulator to increase the reflectivity of the insulator.

[0081] For the description of this method, reference is made to the descriptions of Aspects 1 to 13 above. In particular, the same terms have been used, such as the cavity, the base, the insulator, the heat-insulating wall, and the heat-conducting layer, and thus the description of the terms used in this method is omitted. In particular, this method can be implemented by using the device of any one of Aspects 1 to 13 above.

[0082] Advantageously, according to the fifteenth aspect, in the method according to the fourteenth aspect, the heat-conducting layer increases the reflectivity such that the reduction in the thermal energy supplied to the base during heating is greater than or equal to 5%, preferably where the reduction in the thermal energy supplied to the cavity is greater than or equal to 10%. For example, the supplied energy is reduced by the measures described in any one of Aspects 2 to 12.

[0083] The present invention will now be described in more detail in an exemplary manner using the advantageous aspects and with reference to the accompanying drawings. The described aspects are merely possible configurations. However, in these configurations, the various features described above can be provided independently of each other or can be omitted.

[0084] The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings, together with the specification, are used to explain the principles of the present invention. The drawings are only for the purpose of showing how to manufacture and use the preferred and alternative examples of the present invention and should not be construed as limiting the present invention to the embodiments shown and described. In addition, several aspects of the embodiments can be considered separately or in different combinations to form a solution according to the present invention. Therefore, the embodiments described below can be considered separately or in any combination thereof. Description of the Drawings

[0085] The described embodiments are merely possible configurations, and it must be remembered that the various features described above can be provided independently of each other or can be completely omitted when implementing the present invention. As shown in the drawings, further features and advantages will become apparent from the following more detailed description of various embodiments of the present invention, where the same reference numerals denote the same elements, and where:

[0086] In the drawings:

[0087] Figure 1 shows a system having a device according to the example;

[0088] Figure 2 shows the heat-insulating wall interacting with the base;

[0089] Figure 3 showing a device with an additional heat-conducting layer; and

[0090] Figure 4 showing a flow chart for growing a bulk single crystal of SiC. Detailed Description

[0091] Reference is now made to the drawings and first to Figure 1 describe the present invention, Figure 1 A system 10 for growing a bulk single crystal of SiC in a chamber 110 is described. More particularly, Figure 1 a system 10 including a susceptor 100 is shown, and a reactor 11 forms a chamber in which the susceptor 100 is disposed. Further, an insulator 200 surrounds the susceptor 100. As described above, the susceptor 100 forms a crucible having a chamber for growing a bulk single crystal of SiC. Alternatively, according to an example not shown, the susceptor 100 may be provided separately from the crucible.

[0092] In the susceptor 100, a SiC seed 120 and a SiC source material 130 are provided at a first end.

[0093] The insulator 200 includes a thermal insulation wall 202 that surrounds the susceptor 100 in the growth direction Y and the circumferential direction C. In particular, as Figure 2 shown in the example of, the thermal insulation wall 202 forms a hollow cylinder extending along the growth direction Y and along the circumferential direction C. The thermal insulation wall 202 reduces heat transfer from the susceptor 110 to the outside of the device in the radial direction r. In particular, the radial direction r is perpendicular to the growth direction Y.

[0094] Further, the thermal insulator 200 includes two thermal insulation covers 204 for covering the susceptor. The thermal insulation covers 204 can be made of the same material as the thermal insulation wall 202.

[0095] According to one example, the thermal insulation wall 202 includes a felt, such as a soft felt, a hard felt, or a combination thereof. The hard felt includes short carbon fibers having a fiber length in the range between 1 mm and 10 mm and a fiber diameter in the range between 0.1 mm and 1 mm, wherein the short carbon fibers are bonded by a resin to form the hard felt. The soft felt contains long carbon fibers having a fiber length greater than 10, wherein the long carbon fibers are bonded by needling to form the soft felt.

[0096] As Figure 1 shown, the reactor 11 houses a device including the susceptor 100 and the insulator 200. A vacuum pump for evacuating the reactor 11 is not shown.

[0097] System 10 may further include a heater (not shown) disposed in reactor 11. For example, the system may include an induction heater, where the induction heater may surround insulator 200. Additionally or alternatively, and also not shown, system 10 may include a resistance heater, where the resistance heater is surrounded by insulator 200.

[0098] In Figure 2 is schematically shown Figure 1 the thermal insulation wall 202 of. In particular, in Figure 2 the example shown, induction heating 300 heats base 100. Alternatively, resistance heating (not shown) may be provided. In other words, base 100 absorbs electromagnetic energy from the heater and is thereby heated, and thus, the cavity surrounded by base 100 is heated.

[0099] As Figure 2 shown, thermal insulation wall 202 is spaced apart from base 100 to reduce heat transfer from base 100 to the insulator due to heat conduction. In other words, the heat transfer from base 100 to thermal insulation wall 202 is caused by radiation, which is described by the Stefan - Boltzmann law (1):

[0100] Q = ε * σ * A * T^4 (1)

[0101] In expression (1), the radiator is described. Q is the radiation power, ε is the (material - related) emissivity, σ is the Stefan - Boltzmann constant, A is the radiation area, and T is the absolute temperature of the radiator.

[0102] As Figure 2 shown, radiation 1 from base 100 is radiated onto thermal insulation wall 202. Given the corrugation of thermal insulation wall 202, the reflected radiation 2 is not directed to the wall of base 1100. Instead, the probability that the reflected radiation 2 interacts with thermal insulation wall 202 again is high, and thus, the probability that the reflected radiation 2 is absorbed by thermal insulation wall 202 increases.

[0103] Alternatively, as Figure 3 shown, a heat - conducting layer 210 is disposed between base 100 and thermal insulation wall 202. For example, heat - conducting layer 210 is disposed on thermal insulation wall 202 by at least one of coating, infiltration, and foiling.

[0104] As Figure 3As shown, the heat-conducting layer 210 reduces the corrugation of the insulator and increases the reflectivity of the insulator. In particular, after interacting with the surface of the heat-conducting layer 210, the radiation 1 is the redirected radiation 3 directed towards the base 100. Thus, the probability that the radiation is redirected to the base 100 is increased. Therefore, the heat-conducting layer 210 reduces the amount of the radiation power Q transferred from the base 100 to the insulator 200. It should be noted that the redirection may include processes such as reflection and scattering.

[0105] In addition, the heat-conducting layer 210 has a higher thermal conductivity than the heat-insulating wall 202. Therefore, the remaining part of the heat absorbed by the heat-conducting layer 210 is distributed in the heat-conducting layer 210 rather than being constrained by the heat-insulating wall 202, as can be derived from Fourier's law (2):

[0106] Q = λ * A * (dT / dx) (2)

[0107] Here, Q is the transferred heat output, λ is the thermal conductivity of the material, A is the cross-sectional area through which the heat is transferred, and dT / dx is the temperature gradient along the heat transfer direction.

[0108] Fourier's law describes the mechanism of heat conduction, which is here related to the homogenization of the temperature non-uniformity in the heat-conducting layer 210 of the insulator 200, and the Stefan-Boltzmann law describes the heat transfer by radiation, which describes the energy returning from the heat-conducting layer 210 of the insulator 200 to the base 100.

[0109] Therefore, the selection of the thermal conductivity offsets the development of hot spots in the heat-insulating wall 202, and providing the surface with the heat-conducting layer 210 reduces the corrugation, which in turn increases the energy return from the insulator 200 to the base 100.

[0110] As further schematically Figure 3 shown, the heat-insulating wall 202 is thicker than the heat-conducting layer 210. In particular, the thickness ratio of the thickness of the heat-conducting layer 210 in the radial direction r to the thickness of the heat-insulating wall 202 in the radial direction r is less than or equal to 1 / 10, preferably less than or equal to 1 / 15, and even more preferably less than or equal to 1 / 20. The thinner the thickness, the smaller the probability of absorbing electromagnetic radiation (i.e., the field emitted by the induction heater 300 and the thermal radiation emitted by the base 100).

[0111] According to an example, the thermal conductivity ratio of the thermal conductivity of the heat-conducting layer 210 to the thermal conductivity of the heat-insulating wall is greater than or equal to 10, preferably greater than or equal to 14, and even more preferably greater than or equal to 18. The higher the thermal conductivity ratio, the more effective the heat distribution by the heat-conducting layer 210.

[0112] According to one example, the heat-conducting layer 210 is arranged on the heat-insulating wall 202 by coating and / or infiltration of metal carbide. As Figure 3As shown, the grooves in the surface of the thermal insulation wall 202 can be filled with a coating and / or an infiltration material. In particular, the metal can comprise or consist of a refractory metal, such as at least one of TaC, WC, and HfC. As a result, the waviness is reduced and the reflectivity of the insulator is increased.

[0113] According to another example, the heat-conducting layer 210 is arranged on the thermal insulation wall 202 by applying a foil to the thermal insulation wall 202. In particular, the foil comprises graphite, such as exfoliated graphite. As a result, the waviness is reduced and the reflectivity of the insulator is increased.

[0114] In particular, the ripple height of the heat-conducting layer 210 is less than or equal to 2 mm, preferably less than or equal to 1 mm, and even more preferably less than or equal to 0.5 mm. To measure the ripple height, within a specified sampling length, here especially in the axial direction Y and / or the circumferential direction C, the vertical distance between the highest peak and the lowest valley of the surface profile is determined, i.e., the distance in the radial direction r.

[0115] According to another example, the ratio of the reflectivity of the heat-conducting layer to the reflectivity of the thermal insulation wall is greater than or equal to 1.3, preferably greater than or equal to 1.4, and even more preferably greater than or equal to 1.5.

[0116] In Figure 4 a method for growing such a bulk single crystal of SiC by sublimation growth in the growth direction Y is described. According to this method, in the first step S10, a SiC seed crystal can be provided in the cavity of a reactor as shown, for example, in Figure 1 The SiC seed crystal can be arranged at an end wall provided with a seed crystal holder for holding the SiC seed crystal. The end wall extends perpendicular to the growth direction. Additionally, SiC material can be provided in a storage area formed in the cavity.

[0117] The method further includes step S12: isolating the susceptor with an insulator comprising a thermal insulation wall and a heat-conducting layer, such as as described above in Figure 3 .

[0118] More specifically, the thermal insulation wall is arranged to surround the susceptor in the circumferential direction c and the growth direction Y, and the thermal insulation wall is used to reduce heat transfer from the susceptor to the outside of the apparatus in the radial direction r, the radial direction r being perpendicular to the growth direction Y. In addition, the insulator is provided with a heat-conducting layer disposed between the susceptor and the thermal insulation wall, and the heat-conducting layer has a higher thermal conductivity than the insulation wall for distributing heat on the heat-conducting layer. In addition, the heat-conducting layer is arranged to be spaced apart from the susceptor to reduce heat transfer from the susceptor to the insulator caused by heat conduction.

[0119] In addition, the method includes heating (step S14) the susceptor to grow a bulk single crystal of SiC in the cavity. In particular, a heat conducting layer is provided on the thermally insulating wall, thereby reducing the corrugation of the insulator and thus increasing the reflectivity of the insulator. During heating, the heat conducting layer redirects the radiation emitted from the susceptor back to the susceptor.

[0120] In particular, the heat conducting layer increases the reflectivity such that the thermal energy supplied to the susceptor during heating in S14 is reduced by greater than or equal to 5%, preferably where the thermal energy supplied to the cavity is reduced by greater than or equal to 10%.

[0121] It is noted that, according to the example discussed with reference to the drawings, the susceptor 100 forms a crucible. According to an example not shown, a susceptor separate from the crucible may be provided.

[0122] It is noted that, according to an example not shown, a plurality of coaxial hollow cylinder layers are nested with each other, each cylinder layer having a different radius, and the cylinder having the smallest radius has a heat conducting layer on the inner surface facing the susceptor.

Claims

1. A device for growing a SiC volume single crystal, for growing a SiC volume single crystal in a cavity (110) in a growth direction (Y) by sublimation growth, the device comprising a base (100) for absorbing electromagnetic energy, the base (100) being used to heat the cavity (110); and an insulator (200), the insulator (200) surrounding the base (100), the insulator (200) being used to thermally insulate the base (100) from the outside of the device, the insulator (200) comprising: a heat insulating wall (202), the heat insulating wall (202) surrounding the base (100) in the growth direction (Y) and the circumferential direction (c), the heat insulating wall (202) being used to reduce heat transfer from the base (100) to the outside of the device in a radial direction (r), the radial direction (r) being perpendicular to the growth direction (Y); A heat-conducting layer (210) is disposed between the base (100) and the heat-insulating wall (202), the heat-conducting layer (210) having a higher thermal conductivity than the heat-insulating wall (202) and being used to distribute heat on the heat-conducting layer (210); The heat-conducting layer (210) is spaced apart from the base (100) to reduce heat transfer from the base (100) to the insulator (200) caused by thermal conduction, and the heat-conducting layer (210) is arranged on the thermal insulation wall (202) to reduce the corrugation of the insulator (200) to increase the reflectivity of the insulator (200).

2. The device according to claim 1, wherein: The thermal conductivity ratio of the thermal conductivity of the thermal conductive layer (210) to the thermal conductivity of the thermal insulation wall (202) is greater than or equal to 10, preferably greater than or equal to 14, and even more preferably greater than or equal to 18.

3. A device according to any one of the preceding claims, wherein: The heat conductive layer (210) is arranged on the heat insulating wall (202) by at least one of coating, infiltrating and foiling the heat insulating wall (202).

4. The device according to claim 3, wherein: The heat conducting layer (210) is arranged on the heat insulating wall (202) by coating and / or infiltrating metal carbide, preferably, wherein the metal carbide comprises refractory metal carbide, optionally, wherein the refractory metal carbide comprises at least one of TaC, WC and HfC.

5. The device according to claim 3 or 4, wherein: The thermally conductive layer (210) is arranged on the thermally insulating wall (202) by applying a foil to the thermally insulating wall (202), preferably wherein the foil comprises graphite, optionally wherein the foil comprises exfoliated graphite.

6. A device according to any one of the preceding claims, wherein: The thermal insulation wall (202) comprises felt, preferably, the thermal insulation wall (202) comprises at least one of soft felt and hard felt.

7. The device according to claim 6, wherein: The thermal insulation wall (202) comprises a graphite insulation material, wherein the graphite insulation material comprises at least one of the following: Short carbon fibers having a fiber length in the range between 1 mm and 10 mm and a fiber diameter in the range between 0.1 mm and 1 mm, wherein the short carbon fibers are bonded by a resin to form a hard felt; and Long carbon fibers with a fiber length greater than 10 angstroms, wherein the long carbon fibers are bonded by needle punching to form a soft felt.

8. A device according to any of the preceding claims, wherein the thermal insulation wall (202) is formed by a hollow cylinder, which extends in the growth direction (Y) and surrounds the base (100) in the circumferential direction (c), preferably wherein a plurality of coaxial hollow cylinder layers are nested in each other, each cylinder layer having a different radius, and the cylinder with the smallest radius has the heat conductive layer (210) on the inner surface facing the base (100).

9. The device according to any one of the preceding claims, wherein: The ratio of the reflectivity of the heat conductive layer (210) to the reflectivity of the heat insulating wall (202) is greater than or equal to 1.3, preferably greater than or equal to 1.4, and even more preferably greater than or equal to 1.

5.

10. The device according to any of the preceding claims, wherein the corrugation height of the heat conducting layer (210) is less than or equal to 2 mm, preferably less than or equal to 1 mm, even more preferably less than or equal to 0.5 mm.

11. The device according to any one of the preceding claims, wherein: The thickness ratio of the thickness of the heat conductive layer (210) in the radial direction (r) to the thickness of the heat insulating wall (202) in the radial direction (r) is less than or equal to 1 / 10, preferably less than or equal to 1 / 15, and even more preferably less than or equal to 1 / 20.

12. The device according to any one of the preceding claims, wherein: The thermal insulator (200) further includes at least one thermal insulation cover (204) for covering the base (100).

13. A system (10) comprising an apparatus according to any one of the preceding claims, wherein the system (10) further comprises at least one of an induction heater (300), a resistance heater, a reactor (11) for accommodating the apparatus, and a vacuum pump for evacuating the reactor (11), wherein the induction heater surrounds the insulator (200), wherein the induction heater is surrounded by the insulator (200).

14. A method for growing a SiC bulk single crystal in a cavity (110) in a growth direction (Y) by sublimation growth, the method comprising the following steps: Providing a SiC seed crystal (120) and a SiC source material (130) in the chamber (110); heating a susceptor (100) to grow the SiC bulk single crystal in the cavity (130); The base (100) is isolated by an insulator (200), wherein the insulator (200) comprises: a heat insulating wall (202), the heat insulating wall (202) surrounding the base (100) in the growth direction (Y) and in a circumferential direction, the heat insulating wall (202) being used to reduce heat transfer from the base (100) to the outside of the device in a radial direction (r), the radial direction (r) being perpendicular to the growth direction (Y); a heat-conducting layer (210), disposed between the base (100) and the heat-insulating wall, the heat-conducting layer (210) having a higher thermal conductivity than the heat-insulating wall, and used for distributing heat on the heat-conducting layer (210); The heat-conducting layer (210) is spaced apart from the base (100) to reduce heat transfer from the base (100) to the insulator (200) caused by thermal conduction, and the heat-conducting layer (210) is arranged on the thermal insulation wall (202) to reduce the corrugation of the insulator (200) to increase the reflectivity of the insulator (200).

15. The method according to claim 14, wherein: The thermally conductive layer (210) increases reflectivity such that heat energy supplied to the susceptor (100) during heating is reduced by greater than or equal to 5%, preferably, wherein heat energy supplied to the cavity is reduced by greater than or equal to 10%.

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