SEED UNIT AND APPARATUS FOR GROWING BULK SiC CRYSTAL

Abstract

To provide a seed unit and an apparatus which allow the temperature field in the SiC seed crystal to be influenced in a simpler and better manner than the known prior art approaches.SOLUTION: A seed unit (1), intended for growing a bulk SiC single crystal, has a wafer-like single crystalline SiC seed crystal (2) with a growth surface (4a) arranged on a wafer front side (4) for growing the bulk SiC single crystal to be grown. The SiC seed crystal (2) has a crystal longitudinal mid-axis (6) extending in an axial direction. A radial direction is oriented perpendicularly to the axial direction. The seed unit (1) also has a rear side layer component (3) arranged on a wafer rear side (5) of the SiC seed crystal (2), the structure of which changes starting from the crystal longitudinal mid-axis (6) in the radial direction. Thus, the radial temperature gradient is adjusted during the growth of the bulk SiC single crystal within the SiC seed crystal (2).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a seed unit for growing bulk SiC crystals and an apparatus for growing bulk SiC crystals. [Background technology]

[0002] Due to its excellent physical, chemical, electrical and optical properties, the semiconductor material silicon carbide (SiC) is used as a starting material, inter alia, for power switching semiconductor components, for high-frequency components and for special light-emitting semiconductor components. These components require SiC substrates (=SiC wafers) with the largest possible substrate diameter and of the highest possible quality. These are based on high-value bulk SiC single crystals.

[0003] Such bulk SiC single crystals are typically produced by physical vapor transport (PVT), for example by the sublimation method described in US Pat. No. 6,233,999. In this process, a single-crystal SiC wafer as a SiC seed crystal is introduced into a growth crucible together with suitable source materials. Under controlled temperature, pressure, and gas conditions, the source materials are sublimated. Gas species (=SiC, SiC, SiC) are transported to the SiC seed crystal due to the axial temperature gradient, where they are precipitated from the SiC growth vapor phase onto the SiC seed crystal, resulting in the formation of a bulk SiC single crystal.

[0004] From these bulk SiC single crystals, wafer-shaped single-crystal SiC substrates are cut, for example by wire sawing, and then—in particular—after a multi-stage polishing process of their surface as part of the component manufacturing process, are provided with at least one epitaxial layer, in particular consisting of SiC. Defects are generally propagated from the SiC substrate to the applied epitaxial layer, thus resulting in a deterioration of the component properties. Therefore, the quality of the component fundamentally depends on the quality of the grown bulk SiC single crystal and the SiC substrate obtained therefrom.

[0005] The geometry of the SiC substrates used is crucial for the production of epitaxial layers of components. Therefore, good thermal coupling in epitaxial reactors, which is crucial for homogeneous and high-quality epitaxial layer growth, can essentially only be achieved with SiC substrates that do not have significant warpage. In contrast, SiC substrates with poor geometrical properties, i.e., excessive amounts of warpage and / or bowing, inevitably result in poorer quality and / or lower yields from the epitaxial process.

[0006] During the growth of bulk SiC single crystals, the axial temperature gradient is a crucial factor in the transport of material from the source material to the SiC seed. On the one hand, it is likely to result in an economically beneficial growth rate, but on the other hand, it is unlikely that significant thermal stresses will occur in the crystal volume, which would lead to the aforementioned problems in SiC substrates made from bulk SiC single crystals. Due to the axial temperature gradient prevailing in the growth crucible, a temperature difference between the warmer rear wafer side of the SiC seed and the cooler crucible region behind the SiC seed is also created at the rear side of the SiC seed. To avoid material evaporation from the rear side of the SiC seed wafer to the rear and the associated defect formation in the bulk SiC single crystal growing on the front side of the SiC seed wafer, rear-side passivation is performed on the SiC seed.

[0007] For example, US Pat. No. 5,629,999 describes that a SiC seed crystal can be provided on its rear wafer side with a multilayer protective layer made of carbon, in particular graphite.

[0008] Patent Document 3 describes that in order to avoid material evaporation, the SiC seed crystal can be provided on its wafer rear side with a single or multiple protective layer made of hard carbon, in particular diamond-like carbon, diamond, tantalum, or tantalum carbide. Patent Document 3 also describes that the SiC seed crystal, optionally provided with a rear protective layer, is fixed to the crucible cover of the crucible used for growth by an adhesive layer or by a mechanical holder.

[0009] Patent document 4 describes another approach in which the temperature within the SiC seed crystal is made uniform. In this way, temperature differences in the SiC seed crystal and the resulting internal stresses are reduced. For this purpose, a multi-part insulation containing movable adaptive elements is arranged on the rear side of the SiC seed crystal. The movement of the adaptive elements means that a cavity provided on the rear side of the SiC seed crystal can adapt its dimensions to the respective (thermal) conditions. However, this structure is complex and therefore expensive.

[0010] Patent document 5 describes a multi-part additional heating device placed behind the SiC seed crystal, the single part of which can be moved to allow targeted changes in the temperature field even during the growth process, but even this structure is complex and expensive. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] US8865324B2 [Patent Document 2] CN116121855A [Patent Document 3] US2011 / 0229719A1 [Patent Document 4] CN218860954U [Patent Document 5] CN218175203U Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to provide a seed unit and device of the kind mentioned in the introduction, which allows to influence the temperature field in the SiC seed crystal more easily and better than known approaches. [Means for solving the problem]

[0013] To achieve the object regarding the seed unit, a seed unit is provided according to the features of claim 1. The seed unit according to the invention comprises a wafer-shaped monocrystalline SiC seed crystal having a growth surface arranged on the front side of a wafer on which the bulk SiC single crystal to be grown grows, and a rear layer part arranged on the rear side of the wafer of the SiC seed crystal, the SiC seed crystal having a crystal longitudinal medial axis extending in the axial direction and a radial direction oriented perpendicular to the axial direction, the structure of the rear layer part varying in the radial direction starting from the crystal longitudinal medial axis, so that a radial temperature gradient is created inside the SiC seed crystal during the growth of the bulk SiC single crystal.

[0014] The seed unit may also be specifically referred to as a seed system.

[0015] The wafer-shaped SiC seed crystal has, in particular, a substantially cylindrical geometric shape, the peripheral edge surface of the SiC seed crystal having, in particular, the shape of an essentially cylindrical outer surface.

[0016] In particular, the rear layer component completely covers the SiC seed crystal. It preferably consists of a single layer or may consist of several single layers. The term "structure of the rear layer component" is understood here to mean in particular the geometric dimensions of the rear layer component and / or the material composition of the rear layer component.

[0017] During growth, the bulk SiC single crystal grows axially on the SiC seed crystal and has the same crystal longitudinal medial axis as the SiC seed crystal or seed unit, where "axial" is understood to mean in particular a direction parallel to or along the central crystal longitudinal medial axis, "radial" is understood to mean a direction perpendicular thereto, and "tangential" is understood to mean a circumferential direction extending around the crystal longitudinal medial axis.

[0018] It has been determined that the lateral and spatial distribution of the temperature difference or temperature gradient in the growth region influences the growth rate, phase boundary shape, and thermal stress of the growing bulk SiC single crystal. The stress distribution in the bulk SiC single crystal, on the other hand, influences the dislocation balance and, further, the subsequent warpage of wafer-shaped SiC substrates produced from the bulk SiC single crystal during further process steps of component fabrication. To account for their effects, the three-dimensional temperature field can be divided into an axial component in the direction of the crystal's longitudinal medial axis (preferably the axial temperature gradient) and a radial component perpendicular to the crystal's longitudinal medial axis or parallel to the particularly flat growth surface of the SiC seed crystal (preferably the radial temperature gradient). Both components influence the achievable crystal quality and should therefore be taken into account.

[0019] For example, radial temperature gradients affect the shape of the phase boundary and therefore the surface of the growing bulk SiC single crystal. Ideally, the phase boundary is slightly convexly curved so that fractures at the edge region do not migrate inward, but not too convex so that excessive stress does not build up in the bulk SiC single crystal due to excessive curvature. Stress in the bulk SiC single crystal primarily leads to the formation of basal plane dislocations (BPDs), which adversely affect the long-term stability of electronic components made from SiC substrates obtained from the bulk SiC single crystal. Furthermore, excessive stress can lead to warping of the SiC substrate during further manufacturing, resulting in a reduced yield of usable components.

[0020] It has also been confirmed that optimized control of the complete heat flow from the axial end of the growth crucible to the other opposite axial end of the growth crucible, i.e., from the SiC source material through the SiC seed crystal to the crucible cover, can improve the temperature profile of the growing bulk SiC single crystal, thereby reducing stress and thus minimizing the density of basal plane dislocations. Furthermore, SiC substrates with significantly less warpage can be obtained from bulk SiC single crystals produced with this improved temperature profile, resulting in increased productivity of components subsequently produced thereon. The improved temperature profile is essentially based on the fact that a rear layer component is installed on the wafer rear side of the SiC seed crystal, thereby enabling controllable heat transport in the bulk SiC single crystal growing on the wafer front side. Thanks to the rear layer component, the axial and radial temperature gradients can be largely separate and, in particular, can be adjusted largely independently of each other. For example, the radial temperature gradient can be affected by, for example, radial changes in the rear layer component, while the axial temperature gradient is not significantly altered. The rear layer component can also preferably be used for rear-side passivation, in addition to adjusting the radial temperature gradient, so that material does not evaporate from the rear side of the SiC seed wafer. In this respect, it has an advantageous dual function and is therefore also a protective layer that passivates the rear side in particular.

[0021] An important aspect for growing bulk SiC single crystals using a seed unit is the adjustment of the temperature flow from the warmest to the coldest point of the growth apparatus. The warmest point is in the area of ​​the SiC source material (in powder or solid form) introduced into the growth crucible in the SiC storage area, and the coldest point is on the axial end side of the growth crucible opposite the SiC storage area, i.e., on the axial end side of the crucible cover, located in particular on the side of the seed unit away from the SiC storage area. After achieving the corresponding combination of temperature and pressure, the target material transport occurs and crystal growth begins due to the corresponding temperature difference between the SiC seed crystal of the seed unit and the SiC source material. The phase boundary shape (=growth interface) of the growing bulk SiC single crystal also depends primarily on the heat dissipation through the seed unit in the direction of the crucible cover. This heat dissipation is advantageously influenced by a rear layer component with a structure that varies radially in a desired manner.

[0022] The physical principle underlying heat transport is Fourier's law (1):

[0023]

number

[0024] where Q is the heat transferred by conduction, T1 is the temperature of the warm surface, and T2 is the temperature of the cold surface. A is the area through which the heat flows, λ is the thermal conductivity (= temperature dependent material variable), and d is the thickness of the body between the cold and warm surfaces. The Stefan-Boltzmann law (2) is:

[0025]

number

[0026] where Q is the radiation intensity emitted by a body, ε is the emissivity of the body, σ is the Stefan-Boltzmann constant, A is the area of ​​the body, and T is the absolute temperature. Since SiC growth occurs above 2000 K, the Stefan-Boltzmann law is valid for the production of bulk SiC single crystals, and the law describes heat transport via radiation, as opposed to the heat conduction mechanism described by Fourier's law.

[0027] The seed unit according to the present invention is preferably based on a locally defined combination of two heat transport mechanisms, which favors uniformity or optimized regulation of local temperature differences. Consequently, a very good phase boundary shape of the growing bulk SiC single crystal can be generated. Furthermore, internal stresses and defect densities in the crystal microstructure can be significantly reduced thereby. In particular, good bulk SiC single crystals have only very slightly curved phase boundaries, which result in significantly reduced stresses in the crystal microstructure. In particular, a slightly convex curvature is desirable to prevent defects occurring in the edge regions of the growing bulk SiC single crystal from penetrating into the high-quality internal region, which is particularly important for further processing to produce the component. To achieve this, the seed unit according to the present invention has a single-layer or multi-layer rear layer component with a radially varying structure, where the single layers can have different chemical and / or physical properties with radially constant or radially varying thicknesses. Consequently, heat transport by radiation and heat conduction is regulated so that substantially ideal values ​​for the radial temperature gradient and, preferably, the axial temperature gradient are achieved. A cavity placed on the rear side of the bulk SiC single crystal may also favor the desired effect. These measures may be used individually or in any combination with each other.

[0028] Previous backside coatings of SiC seeds functioned to avoid defect formation associated with back evaporation of material, but did not have a targeted effect on the temperature distribution or temperature gradients within the SiC seed and in other regions of the growth crucible used to grow bulk SiC single crystals.

[0029] Furthermore, the temperature field has hitherto been adjusted during the growth of bulk SiC single crystals via geometrical means in the growth crucible and / or in the thermal insulation surrounding the growth crucible, but ultimately the axial and radial temperature gradients are almost always coupled and cannot practically be adjusted separately from each other.

[0030] The seed unit according to the present invention, in which the radial temperature gradient can be adjusted by appropriate modifications on the rear side of the SiC seed wafer, i.e., by the placement there of a rear layer component with its structure varying in the radial direction, allows the radial and axial temperature gradients to be optimized in a targeted manner and substantially independently of each other, without undesirably affecting other conditions within the growth crucible used to grow the bulk SiC single crystal. Advantageously, this modification to the rear side of the SiC seed wafer does not require a major expansion transformation of the growth crucible. Rather, the growth crucible remains as compact as before. Similarly, other modification options for optimizing the overall structure of the growth apparatus are maintained. Furthermore, thanks to the fact that the rear layer component affecting the temperature gradient is positioned particularly close to or directly adjacent to the rear side of the SiC seed wafer, the temperature gradient is adjusted closest to the bulk SiC single crystal growing on the SiC seed during growth. Therefore, the thermal influence is very direct and efficient.

[0031] The seed unit therefore allows for a significant reduction in internal mechanical stresses and dislocations in the grown bulk SiC single crystal due to an improved and targeted influence of the temperature field during the growth process, which has a beneficial effect on the quality and productivity of subsequent process steps. Furthermore, the seed unit allows for a very compact design, making the growth apparatus very flexible to use.

[0032] Advantageous embodiments of the seed unit according to the invention are evident, inter alia, from the features of the claims dependent on claim 1 .

[0033] An advantageous embodiment is one in which the rear layer part consists of a single layer. In this case, the rear layer part is designed in particular with one layer. As a result, a particularly simple and cost-effective design is realized. Preferably, the single layer has an axial layer thickness, i.e., a layer thickness measured in particular in the axial direction, of 0.5 μm to 10 μm, in particular 1 μm to 5 μm. In the case of a layer thickness that varies, for example, in the radial direction, this layer thickness refers in particular to the maximum thickness or extent in the axial direction.

[0034] According to a further advantageous embodiment, the rear layer part consists of several monolayers. In this case, the rear layer part is especially designed with several monolayers. Consequently, a design that can be very precisely adapted to the respective application is realized. Preferably, at least some of the monolayers are arranged axially one above the other and / or radially next to each other. Preferably, the overall axial layer thickness of all monolayers is 0.5 μm to 20 μm, especially 1 μm to 10 μm. In particular, all monolayers considered together have a common overall axial layer thickness. In the case of an overall layer thickness that varies, for example, in the radial direction, this overall layer thickness refers in particular to the maximum thickness or extent in the axial direction. Preferably, the monolayers at least partially comprise layer materials that differ from one another. In particular, the monolayers at least partially consist of layer materials that differ from one another.

[0035] According to a further advantageous embodiment, each monolayer of the rear layer component consists of a layer material which is a material selected from the group consisting of carbon and carbides or which comprises at least one material from this group, in particular the carbide being a metal carbide.

[0036] According to a further advantageous embodiment, the rear layer part has an axial part thickness that increases radially starting from the crystal longitudinal medial axis. In particular, the axial part thickness increases continuously or in discrete steps. In particular, the axial part thickness increases radially starting from the crystal longitudinal medial axis to the outer edge, for example the peripheral edge, of the rear layer part by a factor of 1.5 to 20, preferably 1.75 to 15, preferably 2, 5 or 10.

[0037] According to a further advantageous embodiment, the rear layer component rests directly on the wafer rear side of the SiC seed crystal. Consequently, the thermal influence on the SiC seed crystal and on the bulk SiC single crystal growing thereon during growth is very direct and efficient. In particular, the radial temperature gradient prevailing during growth can be influenced and adjusted very efficiently.

[0038] According to a further advantageous embodiment, the rear layer part comprises a layer cavity, which is in particular buried, preferably completely buried. The layer cavity preferably has an axial extent of up to 5 mm. The layer cavity also favors the influence and regulation of the radial temperature gradient prevailing during crystal growth in the SiC seed crystal and in the bulk SiC single crystal grown thereon. The rear layer part with the layer cavity preferably has a maximum part thickness, in particular measured in the axial direction, of 2 mm to 15 mm, preferably 3 mm to 10 mm.

[0039] To achieve the object of the present device, an apparatus is provided according to the features of claim 14. The apparatus according to the invention comprises a heatable growth crucible with a SiC storage area arranged in a first part for accommodating SiC source material and a crystal growth area arranged in a second part, a seed unit according to the previous description or one of its advantageous embodiments, and a seed holder for holding the seed unit in the growth crucible such that at least a growth surface of a SiC seed crystal of the seed unit is arranged in the crystal growth area.

[0040] The first and second parts are preferably arranged in a growth crucible, preferably spaced apart from each other, with the first part preferably adjacent to a boundary wall (e.g., lower) of the growth crucible at a first axial end thereof.

[0041] The device according to the invention and its embodiments offer substantially the same advantages as those already described in connection with the seed unit according to the invention and its embodiments.

[0042] Advantageous embodiments of the device according to the invention are evident, inter alia, from the features of the claims dependent on claim 14.

[0043] In particular advantageous is an embodiment in which a rear device cavity is arranged on the rear side of the seed unit away from the SiC storage area, said device cavity having an axial extent of in particular up to 5 mm. The device cavity is in particular formed by the free space between the rear side of the seed unit and a (e.g. upper) boundary wall of the growth crucible at the second axial end, preferably the crucible cover.

[0044] Further features, advantages and details of the invention will become apparent from the following description of exemplary embodiments taken in conjunction with the drawings. [Brief explanation of the drawings]

[0045] [Figure 1] FIG. 1 illustrates a first exemplary embodiment of a seed unit for growing bulk SiC single crystals. [Figure 2] 2 shows an exemplary embodiment of an apparatus for growing bulk SiC single crystals with a seed unit according to FIG. 1; [Figure 3] FIG. 1 illustrates another exemplary embodiment of a seed unit for growing bulk SiC single crystals. [Figure 4] FIG. 1 illustrates another exemplary embodiment of a seed unit for growing bulk SiC single crystals. [Figure 5] FIG. 1 illustrates another exemplary embodiment of a seed unit for growing bulk SiC single crystals. [Figure 6] FIG. 1 illustrates another exemplary embodiment of a seed unit for growing bulk SiC single crystals. [Figure 7] FIG. 1 illustrates another exemplary embodiment of a seed unit for growing bulk SiC single crystals. DETAILED DESCRIPTION OF THE INVENTION

[0046] Corresponding parts are provided with the same reference numerals in Figures 1 to 7. Details of the exemplary embodiments described in more detail below may constitute inventions in their own right or may be part of the inventive subject matter.

[0047] FIG. 1 shows an exemplary embodiment of a seed unit 1 for sublimation growing a bulk SiC single crystal (not shown). The seed unit 1 includes a wafer-shaped single-crystal SiC seed crystal 2 and a backside layer component 3. The SiC seed crystal 2 has a wafer front side 4 on which a growth surface 4a for growing the bulk SiC single crystal to be grown is located, and a wafer back side 5 opposite the wafer front side 4. On the wafer back side 5, the SiC seed crystal 2 is directly coated with the backside layer component 3. In this respect, the SiC seed crystal 2 and the backside layer component 3 are located directly adjacent to each other. The backside layer component 3 completely covers the SiC seed crystal 2 on its wafer back side 5.

[0048] The SiC seed crystal 2, and also the seed unit 1 as a whole, has a central crystal longitudinal medial axis 6, which coincides with the central axis of symmetry of the SiC seed crystal 2, in particular of its cylindrical geometry. Directions along or parallel to the crystal longitudinal medial axis 6 are referred to herein as axes. Directions perpendicular to the crystal longitudinal medial axis 6 are referred to as radial directions. Circumferential directions extending around the crystal longitudinal medial axis 6 are tangential directions.

[0049] The rear layer component 3 has a component thickness D measured in the axial direction, which varies radially starting from the crystal longitudinal medial axis 6, i.e., toward the peripheral edge of the SiC seed crystal 2. This component thickness D increases radially in the exemplary embodiment shown in FIG. 1 , which has a total of three distinct steps 7. The rear layer component 3 has a geometric structure that varies radially starting from the crystal longitudinal medial axis 6. Due to this radially varying structure, a radial temperature gradient is adjusted within the SiC seed crystal 2 (and particularly in the region surrounding the SiC seed crystal 2 and within the growing bulk SiC single crystal) during the growth of the bulk SiC single crystal, so that no appreciable internal stresses arise in either the SiC seed crystal 2 or the growing bulk SiC single crystal. Preferably, this radial temperature gradient also contributes to reducing such internal stresses, even if they already exist. Consequently, a highly defect-free bulk SiC single crystal is ultimately produced, from which SiC single crystals with low defects can also be obtained for the production of high-quality components with a high yield.

[0050] The variable part thickness D of the rear layer part 3 has a value between 1 μm and 5 μm. In particular, the part thickness D increases five-fold from the center to the peripheral edge, i.e., in the radial direction, at the crystal longitudinal medial axis 6. At the peripheral edge, the part thickness D is at its maximum value, specifically 5 μm. In the exemplary embodiment of FIG. 1, the rear layer part 3 is formed of one layer. It therefore comprises only a single monolayer. The rear layer part consists of a metal carbide, in the illustrated exemplary embodiment of tantalum carbide (TaC). In other exemplary embodiments not shown, the rear layer part 3 may also consist of another layer material (e.g., graphite or another carbon material) or another carbide (e.g., a metal carbide with a high-melting-point metal, such as tungsten (W) or another refractory metal).

[0051] 2 illustrates an exemplary embodiment of a growth apparatus 8 for producing bulk SiC single crystals (also not shown) by sublimation growth. Growth apparatus 8 includes a growth crucible 9 that includes a SiC storage area and a crystal growth area 11. The SiC storage area contains SiC source material 12, e.g., a powder.

[0052] The growth crucible 9 has a crucible vessel 13 and a crucible cover 14. The growth crucible 9 has a first axial end wall 15 arranged adjacent to the SiC storage region and an opposite second axial end wall 16 formed by the crucible cover 14. The growth crucible 9 further has a peripheral wall 17 which, like the first axial end wall 15, is a component of the crucible vessel 13. The seed unit 1 of FIG. 1 is positioned in the growth crucible 9 by a seed holder 18 so that the wafer front side 4 of the SiC seed crystal 2 is located in the crystal growth region 11 with the growth surface 4 a. In the illustrated exemplary embodiment, the wafer front side 4 of the SiC seed crystal 2 in the region of the peripheral edge rests loosely on the annular seed holder 18.

[0053] A peripheral gap 19 is provided between the inner wall of the peripheral wall 17 and the seed unit 1 .

[0054] The growth crucible 9 in Fig. 2 is made of an electrically and thermally conductive graphite crucible material. A thermal insulator (not shown in Fig. 2) is arranged around it. Furthermore, an induction heater (also not shown) in the form of a heating coil is provided to heat the growth crucible 9. The growth crucible 9 is heated by this heater to the high temperatures of 2100°C or more required for growth.

[0055] The SiC growth gas phase in the crystal growth region 11 is supplied by means of a SiC source material 12. The SiC growth gas phase contains at least gaseous components in the form of SiC, SiC, and SiC (=SiC gas species). Material transport from the SiC source material 12 to the growth surface 4a occurs along an axial temperature gradient, set by a heating device, extending parallel to the crystal's longitudinal medial axis 6. A relatively high growth temperature of at least 2100°C, in particular at least 2200°C or even 2300°C, prevails at the growth surface 4a. At this location, the gaseous components of the SiC growth gas phase precipitate, resulting in the growth of a bulk SiC single crystal. The temperature decreases axially within the growth crucible 9 from the SiC source material 12 to the crucible cover 14, creating the aforementioned axial temperature gradient.

[0056] An apparatus cavity 20 is arranged at the rear side of the seed unit 1, away from the SiC storage area 10. It is located between the crucible cover 14 and the rear layer part 3 of the seed unit 1. This apparatus cavity 20 also contributes to the radially varying structure of the rear layer part 3 to adjust the temperature gradients mentioned above. The apparatus cavity 20 is advantageous, but is nevertheless only optional. There are other exemplary embodiments that do not have such an apparatus cavity 20.

[0057] FIG. 3 shows another exemplary embodiment of a seed unit 21. The seed unit 21 again includes a SiC seed crystal 2 with a wafer front side 4 and a wafer rear side 5. Unlike the seed unit 1, the seed unit 21 has a differently designed rear layer component 22. It is again designed in one layer and again completely covers the SiC seed crystal 2. Its component thickness D also increases radially from the crystal longitudinal medial axis 6 to the peripheral edge, particularly by a factor of two in the exemplary embodiment. In this case, the increase is not stepped but continuous. At the peripheral edge, the component thickness D is at its maximum value, specifically 3 μm. In this regard, the rear layer component 21 also has a geometric structure that varies radially from the crystal longitudinal medial axis 6, so that a radial temperature gradient is again adjusted at least within the SiC seed crystal 2 during the growth of the bulk SiC single crystal.

[0058] FIG. 4 shows another exemplary embodiment of a seed unit 23. The seed unit 23 includes a SiC seed crystal 2 and a rear layer component 24, which in this exemplary embodiment has a multi-layer design. The rear layer component 24 has a total of five monolayers 25, 26, 27, 28, and 29. The first four monolayers 25-28, which are applied directly above and below the wafer rear side 5 of the SiC seed crystal 2, each have the same axial extension / bulge at the specific location of each of the monolayers 25-28. In the illustrated exemplary embodiment, each of these four monolayers 25-28 consists of a graphite-based insulating material and forms an alternating layer system. The outermost monolayer 29 is placed on the side of this layer system of the four monolayers 25-28 facing away from the SiC seed crystal 2 and has a design similar to that of the rear layer component 3 of FIG. 1. The monolayer 29 has an axial extent (i.e., thickness) that increases radially in three steps starting from the crystal longitudinal medial axis 6. In this respect, the outermost monolayer 29, and therefore the rear layer component 24 as a whole, again has a radially varying geometry. Consequently, the radial temperature gradient is adjusted. In the exemplary embodiment of FIG. 4, the outermost monolayer 29 is made of a metal carbide, e.g., tungsten carbide (WC). The rear layer component 24 has an axial component thickness D that increases radially starting from the crystal longitudinal medial axis 6 to the peripheral edge due to the axial bulging of all the monolayers 25-29 arranged axially one above the other and the radial variation of the outermost monolayer 29. In the exemplary embodiment, this increase is, in particular, twofold. At the peripheral edge, the component thickness D is at its maximum value, specifically 10 μm.

[0059] FIG. 5 shows another exemplary embodiment of a seed unit 31 comprising a SiC single crystal 2 and another rear layer component 32. It has a multi-layer design. Unlike the rear layer component 24 of FIG. 4, in the exemplary embodiment of FIG. 5, three monolayers 33, 34, and 35 are not arranged axially one above the other, but radially adjacent to each other and concentrically with the crystal longitudinal medial axis 6. The central monolayer 33 is formed as a cylindrical complete wafer and has the smallest (uniform) monolayer thickness in the axial direction, specifically 1 μm in the exemplary embodiment. The middle monolayer 34, directly radially adjacent to the central monolayer 33, is formed as an annular body and has a (uniform) monolayer thickness greater than that of the inner monolayer 33, specifically 2 μm in the exemplary embodiment. The outermost monolayer 35 is directly adjacent to the middle monolayer 34 and has a (uniform) monolayer thickness greater than that of the middle monolayer, specifically 5 μm in the exemplary embodiment. The component thickness D of the rear layer component 32 also increases radially from the crystal longitudinal medial axis 6 to the peripheral edge, in particular by a factor of 5 in the exemplary embodiment. For the rear layer component 32 as a whole, a structure is again created whose geometry changes radially from the crystal longitudinal medial axis 6. In the exemplary embodiment of Figure 5, the individual layers 33, 34 and 35 each consist of a different thermal insulation layer material based on graphite.

[0060] FIG. 6 shows a further exemplary embodiment of a seed unit 36 ​​including a SiC seed crystal 2 and a further rear layer component 37. The rear layer component 37 also has a multi-layer design and consists of a lower monolayer 38 that contacts and covers the entire wafer rear side 5 of the SiC seed crystal 2, a two-component upper layer with a wafer-like central monolayer 39, and an annular peripheral monolayer 40 that concentrically surrounds the central monolayer. The rear layer component 37 has the same axial extent at all points. The component thickness D is the same throughout in this exemplary embodiment, e.g., 4 μm. However, in particular, at least the two upper or outermost monolayers 39 and 40 each consist of different layer materials, so that in this exemplary embodiment, the structure of the rear layer component 37 varies radially from the crystal longitudinal medial axis in terms of material composition, so that the desired radial temperature adjustment is also achieved in this exemplary embodiment during the growth of the bulk SiC single crystal.

[0061] FIG. 7 shows a further exemplary embodiment of a seed unit 41 comprising a SiC seed crystal 2 and a rear layer component 42 for a further single layer. The side of the rear layer component 42 facing away from the wafer rear side 5 of the SiC seed crystal 2 has a stepped (uneven) contour with three concentric steps 43 corresponding to the rear layer components 3 and 24. Therefore, the component thickness D of the rear layer component 42 also increases radially from the crystal longitudinal medial axis 6 to the peripheral edge, in particular by a factor of 1.5 in the exemplary embodiment. In the center, the component thickness D is, for example, 4 mm, while at the peripheral edge—where it has its greatest axial extent—it is, for example, 6 mm. Furthermore, a layer cavity 44 is provided inside the rear layer component 42, with an axial cavity height H of, for example, 3 mm. Due to its rear surface contour, the rear layer component 42 has a radially varying structure. Therefore, and due to the layer cavity 44, a radial temperature gradient is adjusted during the growth of the bulk SiC single crystal.

[0062] Seed units 21, 23, 31, 36 and 41 can be used in the apparatus 8 of Figure 2 in place of seed unit 1 used therein for growing bulk SiC single crystals. All seed units 1, 21, 23, 31, 36 and 41 are characterized in that they allow particularly separate adjustment of the radial and axial temperature gradients inside the growth apparatus 8 and thus contribute to particularly low-defect growth of bulk SiC single crystals. [Explanation of symbols]

[0063] 1;21;23;31;36;41 seed units 2 SiC seed crystal 3;22;24;32;37;42 Rear layer parts 4 Front side of wafer 4a Growth surface 5 Back side of wafer 6 Crystal Longitudinal Medial Axis 9. Growth Crucible 10 SiC storage area 11 Crystal growth region 18 Seed Holder 20 Device cavity 25,26,27,28,29;33,34,35;38,39;40 Single layer 42 Rear layer parts 44-layer cavity

Claims

1. 1. A seed unit for growing bulk SiC single crystals, comprising: a) a wafer-shaped SiC seed crystal (2) of monocrystalline structure having a growth surface (4a) arranged on the wafer front side (4) for growing a bulk SiC single crystal to be grown; b) a rear layer component (3; 22; 24; 32; 37; 42) arranged on the wafer rear side (5) of the SiC seed crystal (2), a1) the SiC seed crystal (2) has a crystal longitudinal medial axis (6) extending in an axial direction, with the radial direction oriented perpendicular to the axial direction; b1) the structure of the rear layer part varies in the radial direction starting from the crystal longitudinal medial axis (6), so that a radial temperature gradient is adjusted within the SiC seed crystal (2) during the growth of the bulk SiC single crystal; Seed unit.

2. 2. A seed unit according to claim 1, characterized in that the rear layer part (3; 22; 42) consists of a single layer.

3. 3. The seed unit according to claim 2, wherein the monolayer has an axial layer thickness of 0.5 μm to 10 μm.

4. 2. A seed unit according to claim 1, characterized in that the rear layer part (24; 32; 37) consists of a plurality of single layers (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40).

5. The plurality of monolayers (25, 26, 27, 28, 29; ３３，３４，３５；３８，３９； 5. The seed unit according to claim 4, characterized in that at least some of the nozzles 40 are arranged axially one above the other and / or radially adjacent to one another.

6. 6. Seed unit according to claim 4 or 5, characterized in that the total layer thickness in the axial direction of all the monolayers (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40) is between 0.5 μm and 20 μm.

7. 6. Seed unit according to claim 4 or 5, characterized in that the single layers (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40) at least partially have layer materials which differ from one another.

8. 6. A seed unit according to claim 2, characterized in that each of the sub-layers (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40) of the rear layer part (3; 22; 24; 32; 37; 42) consists of a layer material which is a material selected from the group consisting of carbon and carbides or which comprises at least one material from this group.

9. 6. The seed unit according to claim 1, wherein the rear layer part (3; 22; 24; 32; 42) has an axial part thickness that increases radially starting from the crystal longitudinal medial axis (6).

10. 10. The seed unit according to claim 9, wherein the axial part thickness increases continuously or in discrete steps.

11. 6. The seed unit according to claim 1, wherein the rear layer part (3; 22; 24; 32; 37; 42) is directly adjacent to the wafer rear side (5) of the SiC seed crystal (2).

12. 6. The seed unit according to claim 1, wherein the rear layer part (42) has a layer cavity (44).

13. 13. A seed unit according to claim 12, characterized in that the bed cavity (44) has an axial extent of up to 5 mm.

14. 1. An apparatus for growing bulk SiC single crystals, comprising: a) a heatable growth crucible (9) comprising a SiC storage area (10) located in a first portion containing SiC source material (12) and a crystal growth area (11) located in a second portion; b) a seed unit (1; 21; 23; 31; 36; 41) according to any one of claims 1 to 5, and c) a seed holder (18) for holding the seed unit (1; 21; 23; 31; 36; 41) in the growth crucible (9) such that at least the growth surface (4 a) of the SiC seed crystal (2) of the seed unit (1; 21; 23; 31; 36; 41) is positioned in the crystal growth region (11); An apparatus comprising:

15. 15. The device according to claim 14, characterized in that a device cavity (20) is arranged on the rear side of the seed unit (1; 21; 23; 31; 36; 41) away from the SiC storage area (10), said device cavity having an axial extent of up to 5 mm.

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