Seed unit and apparatus for growing bulk silicon carbide single crystals

Abstract

The invention provides a seed unit (1) for growing bulk SiC single crystals, the seed unit (1) having a wafer-like SiC single crystal seed (2), the SiC seed (2) having a growth surface (4a) arranged on a wafer front surface (4) for growing bulk SiC single crystals to be grown. The SiC seed crystal (2) has a crystal longitudinal center axis (6) extending in the axial direction. The radial direction is oriented perpendicular to the axial direction. The seed unit (1) also has a back layer assembly (3) arranged on the wafer back side (5) of the SiC seed (2), the structure of which varies radially starting from the longitudinal centre axis (6) of the crystal such that the radial temperature gradient is adjusted during the growth of the bulk SiC single crystal in the SiC seed (2).

Description

[0001] The content of European patent application EP 24 162 679.5 is incorporated herein by reference. Technical Field

[0002] The present invention relates to a seed unit for growing a bulk silicon carbide single crystal and an apparatus for growing a bulk silicon carbide single crystal. Background Art

[0003] Due to its excellent physical, chemical, electrical, and optical properties, the semiconductor material silicon carbide (SiC) is also used as a starting material for (including but not limited to) power switching semiconductor components, high-frequency components, and special light-emitting semiconductor components. These components require SiC substrates (SiC wafers) with the largest possible substrate diameter and the highest possible quality. These are all based on high-value bulk SiC single crystals.

[0004] Such bulk silicon carbide single crystals are usually grown by means of physical vapor transport (PVT), for example by means of the sublimation method described in US Pat. No. 8,865,324 B2. In this case, a single-crystal silicon carbide wafer, serving as a silicon carbide seed crystal, is introduced into a growth crucible together with a suitable source material. Under controlled temperature, pressure, and gas conditions, the source material sublimates. The gaseous material (=SiC, Si2C, SiC2) is transported to the SiC seed crystal by means of an axial temperature gradient and deposited from the SiC growth vapor onto the SiC seed crystal, thereby growing a bulk SiC single crystal.

[0005] Wafer-shaped SiC single-crystal substrates are cut from these bulk SiC single crystals (e.g., using a wire saw). After their surfaces undergo multiple polishing steps (as part of component manufacturing), they are provided with at least one epitaxial layer, also typically composed of SiC. Defects are often transferred from the SiC substrate to the applied epitaxial layer, resulting in impaired component performance. Therefore, component quality depends fundamentally on the quality of the grown bulk SiC single crystal and the resulting SiC substrate.

[0006] The geometry of the silicon carbide substrate used is crucial for the production of the component's epitaxial layers. Therefore, good thermal coupling in the epitaxial reactor is essentially only possible if the silicon carbide substrate exhibits no significant bowing, which is crucial for uniform, high-quality epitaxial layer growth. Conversely, poor silicon carbide substrate geometry (e.g., particularly excessive bowing and / or warping) inevitably leads to poor epitaxial process quality and / or low yield.

[0007] During the growth of bulk SiC single crystals, the axial temperature gradient plays a decisive role in the material transport from the source material to the SiC seed crystal. On the one hand, the axial temperature gradient should be high enough to achieve an economically favorable growth rate, but on the other hand, it should not be too high to avoid the formation of greater thermal stresses in the crystal volume, which can lead to the aforementioned problems in the SiC substrate produced from the bulk SiC single crystal. Due to the axial temperature gradient in the growth crucible, a temperature difference also occurs on the back side of the SiC seed crystal, between the hotter wafer back side of the SiC seed crystal and the cooler crucible area behind the SiC seed crystal. In order to avoid material evaporation from the back side of the SiC seed crystal wafer to the back side, and to avoid the formation of related defects in the bulk SiC single crystal being grown on the wafer front side of the SiC seed crystal, the SiC seed crystal needs to be back-side passivated.

[0008] For example, CN 116121855 A describes a SiC seed crystal having a multilayer protective layer of carbon (in particular graphite) on the back side of the wafer.

[0009] US2011 / 0229719 A1 describes a SiC seed crystal having a single or multiple protective layers on the back side of the wafer. The protective layers are composed of hard carbon, diamond-like carbon, diamond, tantalum, or tantalum carbide to prevent material evaporation. US2011 / 0229719 A1 also describes how the SiC seed crystal, which may have a backside protective layer, is fastened to the crucible lid of a crucible used for growth using an adhesive layer or a mechanical support.

[0010] CN 218860954 U describes another method for making the temperature in the SiC seed crystal uniform. Therefore, the temperature difference and the resulting internal stress in the silicon carbide seed crystal should be reduced. For this reason, a multi-part thermal insulation material is placed on the back of the SiC seed crystal, which includes a displaceable adapter. The displacement of the adapter means that the back of the SiC seed crystal also has a cavity, the size of which can be adaptively adjusted according to the corresponding (heat) conditions. However, this structure is complicated and therefore expensive.

[0011] CN 218175203 U describes a multi-part additional heating device arranged on the back side of a SiC seed crystal, the individual parts of which can be displaced in order to change the temperature field in a targeted manner even during the growth. In practice, such a structure is both complex and expensive. Summary of the Invention

[0012] It is an object of the present invention to provide a seeding unit and a device of the kind mentioned in the introduction, with which the temperature field of a SiC seed can be influenced in a simpler and better manner than with known methods.

[0013] In order to achieve the objectives related to a seed crystal unit, a seed crystal unit according to the features of claim 1 is provided. The seed crystal unit according to the present invention comprises a wafer-shaped single-crystalline SiC seed crystal, whose growth surface is arranged on the front side of the wafer, for growing a bulk SiC single crystal to be grown, wherein the SiC seed crystal has a crystal longitudinal center axis extending along the axial direction and a radial direction perpendicular to the axial direction, and a back layer assembly is arranged on the back side of the wafer of the SiC seed crystal, whose structure changes radially starting from the crystal longitudinal center axis, so that a radial temperature gradient is generated in the SiC seed crystal during the growth of the bulk SiC single crystal.

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

[0015] The wafer-shaped SiC seed crystal has a particularly substantially cylindrical geometric shape. The peripheral edge surface of the SiC seed crystal has a particularly substantially cylindrical outer surface shape.

[0016] In particular, the back layer assembly completely covers the SiC seed crystal. Preferably, the back layer assembly consists of a single layer or a plurality of single layers. The term "structure of the back layer assembly" is understood herein to refer in particular to the geometric dimensions of the back layer assembly and / or the material composition of the back layer assembly.

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

[0018] It has been recognized that the lateral and spatial distribution of temperature differences or temperature gradients within the growth region directly influences the growth rate, phase boundary morphology and thermal stresses in the growing bulk SiC single crystal, but is not limited thereto. The stress distribution in the bulk SiC single crystal, for its part, influences the dislocation equilibrium and the curvature that occurs in the subsequent process steps of component production of wafer-shaped SiC substrates produced from the bulk SiC single crystal. In order to take into account the influence of the three-dimensional temperature field, it can be divided into an axial component along the longitudinal center axis of the crystal (preferably an axial temperature gradient) and a radial component perpendicular to the longitudinal center axis of the crystal or parallel to the particularly flat growth surface of the SiC seed crystal (preferably a radial temperature gradient). Both components have an impact on the achievable crystal quality and should therefore be taken into account.

[0019] For example, radial temperature gradients can affect the morphology of the phase boundaries and thus the surface of the growing bulk SiC single crystal. Ideally, the phase boundary is curved in a slightly convex manner so that the disruption in the edge region does not migrate inward, but it should not be too convex to avoid excessive stress in the bulk SiC single crystal due to excessive curvature. The stress in the bulk SiC single crystal mainly leads to the formation of basal plane dislocations (BPDs), which have an adverse effect on the long-term stability of electronic components produced from SiC substrates obtained from bulk SiC single crystals. In addition, excessive stress can cause the SiC substrate to bend during further production, thereby reducing the yield of usable components.

[0020] It has also been recognized that optimizing the heat flow from one axial end of the growth crucible to the opposite axial end of the growth crucible (i.e., from the SiC source material through the SiC seed crystal to the crucible lid) can improve the temperature profile of the growing bulk SiC single crystal, thereby reducing stress and, consequently, minimizing the density of basal plane dislocations. Furthermore, bulk SiC single crystals produced using this improved temperature profile can yield SiC substrates with significantly lower curvature, thereby improving the yield of subsequent components produced thereon. The improved temperature profile is primarily based on the fact that a backside layer assembly is provided on the backside of the SiC seed crystal wafer, which can be used to control the heat transfer from the bulk SiC single crystal growing on the front side of the wafer. Due to the backside layer assembly, the axial and radial temperature gradients can be largely separated and, in particular, can be adjusted largely independently of each other. For example, the radial temperature gradient can be affected by radial variations of the backside layer assembly, while the axial temperature gradient does not change significantly. In addition to regulating the radial temperature gradient, the backside layer assembly preferably also serves to provide backside passivation, so that no material evaporates from the backside of the SiC seed crystal wafer. In this respect, it has an advantageous dual function, in particular also serving as a rear-side passivation protective layer.

[0021] An important aspect of growing bulk SiC single crystals with the aid of a seed unit is to regulate the temperature flow from the hottest point to the coldest point of the growth apparatus. The hottest point is located in the region of the SiC storage area where the SiC source material (in powder or solid form) is introduced into the growth crucible, and the coldest point is located on the axial end side of the growth crucible opposite the SiC storage area, i.e., in particular, on the crucible lid arranged on the axial end side of the seed unit facing away from the SiC storage area. After reaching the corresponding temperature and pressure combination, the target material begins to be transported and the crystal begins to grow, as a result of the corresponding temperature difference being adjusted between the SiC seed crystal and the SiC source material of the seed unit. The phase boundary morphology (=growth boundary surface) of the growing bulk SiC single crystal also depends primarily on the heat dissipation through the seed unit in the direction of the crucible lid. This heat dissipation is advantageously influenced in a desired manner by the back layer assembly having a radially varying structure.

[0022] The physical principle that forms the basis of heat transfer is Fourier's law (1):

[0023]

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

[0025] Q=ε*σ*A*T 4 (2),

[0026] Where Q is the radiant power emitted by the object, ε is the object's emissivity, σ is the Stefan-Boltzmann constant, A is the area of ​​the object, and T is the absolute temperature. Because SiC growth occurs above 2000K, the Stefan-Boltzmann law, which describes heat transfer via radiation, is applicable to the production of bulk SiC single crystals, as opposed to Fourier's law, which describes the heat transfer mechanism via conduction.

[0027] The seed unit according to the present invention is preferably based on a locally defined combination of two heat transfer mechanisms, which facilitates homogenization or optimal adjustment of local temperature differences. As a result, the growing bulk SiC single crystal can develop very good phase boundary morphology. Furthermore, internal stresses and defect densities in the crystal microstructure are significantly reduced. In particular, on the one hand, a good bulk SiC single crystal has only very slightly curved phase boundaries, which significantly reduces stresses within the crystal microstructure. On the other hand, a slightly convex curvature is particularly desirable to prevent defects from forming in the edge regions of the growing bulk SiC single crystal and from penetrating into the high-quality inner region, which is particularly important for further processing in component production. To achieve this, the seed unit according to the present invention particularly comprises a single-layer or multi-layer back layer assembly with a radially varying structure. The individual layers of the back layer assembly may have different chemical and / or physical properties and a radially constant or variable thickness. Heat transfer via radiation and heat conduction is thus adjusted to achieve nearly ideal values ​​for the radial temperature gradient and, preferably, the axial temperature gradient. A cavity arranged on the back side of the SiC seed crystal can also produce the aforementioned effects. These measures can be used individually or in any combination.

[0028] Previous backside coatings on SiC seeds have the function of preventing material from evaporating backward and forming related defects, but have no targeted effect on the temperature distribution or temperature gradient in the SiC seed and other areas of the growth crucible used to grow bulk SiC single crystals.

[0029] Furthermore, to date, the temperature field during bulk SiC crystal growth has been adjusted geometrically via the growth crucible and / or the insulation surrounding the growth crucible. However, the axial and radial temperature gradients are almost always coupled and cannot be adjusted separately.

[0030] According to the seed crystal unit of the present invention, the radial temperature gradient can be adjusted by appropriately modifying the back side of the wafer of the SiC seed crystal (i.e., placing a back layer component with a structure that changes radially on the back side of the wafer), so that the radial and axial temperature gradients can be optimized in a targeted manner, and are basically independent of each other, and will not have other adverse effects on other conditions in the growth crucible used to grow bulk SiC single crystals. In an advantageous manner, such modification of the back side of the wafer of the SiC seed crystal does not require a large expansion conversion on the growth crucible. On the contrary, the growth crucible remains as compact as before. Similarly, other variation options for optimizing the overall structure of the growth equipment are also retained. In addition, since the back layer component that affects the temperature gradient is arranged on the back side of the wafer of the SiC seed crystal, in particular, close to or directly adjacent to the back side of the wafer, the temperature gradient is adjusted at the position closest to the bulk SiC single crystal being grown on the SiC seed crystal during growth. Therefore, the thermal influence is very direct and effective.

[0031] Due to the improved directional influence on the temperature field during growth, the seed unit allows for a significant reduction in internal mechanical stresses and dislocations in the grown bulk SiC single crystal. This has a positive impact on the quality and yield of subsequent process steps. Furthermore, the seed unit allows for the design of very compact and flexible growth equipment.

[0032] Advantageous embodiments of the seed unit according to the invention are apparent from the features of the claims which are dependent upon (but not limited to) claim 1 .

[0033] An embodiment in which the back layer assembly consists of a single layer is advantageous. The back layer assembly is then designed, in particular, as a single layer. This results in a particularly simple and cost-effective design. Preferably, the axial layer thickness of the single layer, i.e., the layer thickness measured in the axial direction, is between 0.5 μm and 10 μm, in particular between 1 μm and 5 μm. If the layer thickness varies, for example, in the radial direction, this layer thickness refers in particular to the maximum thickness or expansion in the axial direction.

[0034] According to a further advantageous embodiment, the back layer assembly consists of a plurality of individual layers. The back layer assembly is then designed, in particular, to have a plurality of layers. This allows for a design that can be very precisely adapted to the respective application. Preferably, at least some of the plurality of individual layers are arranged one above the other in the axial direction and / or radially adjacent to one another. Preferably, the total axial layer thickness of all individual layers is between 0.5 μm and 20 μm, in particular between 1 μm and 10 μm. In particular, all individual layers together have a common total axial layer thickness. In the case of a total layer thickness that varies, for example, in the radial direction, this total layer thickness refers in particular to the maximum thickness or expansion in the axial direction. Preferably, the individual layers at least partially comprise mutually different layer materials. In particular, they at least partially consist of mutually different layer materials.

[0035] According to a further advantageous embodiment, each individual layer of the back layer assembly consists of a layer material that is a material from the group of carbon and carbides or at least includes a material from this group. In particular, the carbide is a metal carbide.

[0036] According to another advantageous embodiment, the back layer assembly has an axial assembly thickness that increases radially from the longitudinal center axis of the crystal. In particular, the axial assembly thickness increases continuously or in steps. In particular, the axial assembly thickness increases radially from the longitudinal center axis of the crystal to the transverse edges, for example to the peripheral edges of the back layer assembly, by a factor of 1.5 to 20, preferably 1.75 to 15, more preferably 2, 5, or 10.

[0037] According to another advantageous embodiment, the backside layer assembly is located directly on the backside of the SiC seed crystal wafer. Therefore, during the growth process, the thermal influence on the SiC seed crystal and the bulk SiC single crystal grown thereon is very direct and effective. In particular, the radial temperature gradients that prevail during the growth process can be very effectively influenced and regulated.

[0038] According to another advantageous embodiment, the back layer assembly includes a layer cavity. The layer cavity is particularly embedded, preferably completely embedded. Preferably, the axial expansion of the layer cavity reaches 5 mm. The layer cavity also facilitates influencing and regulating the radial temperature gradients prevailing in the SiC seed crystal and the bulk SiC single crystal grown thereon during crystal growth. The maximum assembly thickness of the back layer assembly with the layer cavity, particularly when measured in the axial direction, is preferably between 2 mm and 15 mm, more preferably between 3 mm and 10 mm.

[0039] In order to achieve the object relating to an apparatus, an apparatus is provided according to the features of claim 14. The apparatus according to the invention has a heatable growth crucible having a SiC storage area arranged in a first portion for receiving SiC source material and a crystal growth area arranged in a second portion, a seed unit or according to one of the advantageous embodiments described above, and a seed holder for supporting the seed unit within the growth crucible so that at least the growth surface of a SiC seed of the seed unit is arranged in the crystal growth area.

[0040] The first portion and the second portion are each arranged in particular within the growth crucible and are preferably spaced apart from one another. Preferably, the first portion is adjacent to a first axial end side (eg lower) boundary wall of the growth crucible.

[0041] The device according to the invention and its embodiments provide essentially the same advantages as 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 in particular from the features of the claims which are dependent on claim 14 (but not limited thereto).

[0043] In an advantageous embodiment, a rear-side equipment cavity is arranged on the rear side of the seed unit facing away from the SiC storage area, with an axial expansion of the equipment cavity of up to 5 mm. The equipment cavity is formed in particular by the free space between the rear side of the seed unit and the second axial end side (e.g., upper) boundary wall of the growth crucible (preferably the crucible lid). BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Other features, advantages and details of the present invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings. In the drawings:

[0045] Figure 1 A first exemplary embodiment of a seed unit for growing bulk SiC single crystal is shown,

[0046] Figure 2 Shows the use of Figure 1 An exemplary embodiment of an apparatus for growing bulk SiC single crystals using a seed unit, and

[0047] Figures 3 to 7 Another exemplary embodiment of a seed unit is shown, each for growing a bulk SiC single crystal. DETAILED DESCRIPTION

[0048] exist Figures 1 to 7 In the drawings, corresponding parts have the same reference symbols. The details of the exemplary embodiments explained in more detail below may also constitute inventions themselves or be part of the subject matter of the invention.

[0049] Figure 1 An exemplary embodiment of a seed unit 1 for sublimation growth of a bulk SiC single crystal (not shown) is shown. The seed unit 1 has a wafer-shaped single-crystalline SiC seed crystal 2 and a back layer assembly 3. The SiC seed crystal 2 has a wafer front side 4 and a wafer back side 5 opposite to the wafer front side 4, and a growth surface 4a located on the wafer front side 4 is used to grow the bulk SiC single crystal to be grown. On the wafer back side 5, the SiC seed crystal 2 is directly coated with the back layer assembly 3. In this regard, the SiC seed crystal 2 and the back layer assembly 3 are directly adjacent to each other. The back layer assembly 3 completely covers the SiC seed crystal 2 on the wafer back side 5 of the SiC seed crystal 2.

[0050] The silicon carbide seed crystal 2 and the seed crystal unit 1 as a whole have a central longitudinal axis 6. The longitudinal axis 6 coincides with the central axis of symmetry of the silicon carbide seed crystal 2 (particularly in the case of a cylindrical geometry). Directions along or parallel to the longitudinal axis 6 are referred to herein as axial directions. Directions perpendicular to the longitudinal axis 6 are referred to as radial directions. Directions extending around the longitudinal axis 6 are referred to as tangential directions.

[0051] The back layer assembly 3 has an assembly thickness D measured in the axial direction, which varies radially from the longitudinal center axis 6 of the crystal, i.e., in the direction of the peripheral edge of the silicon carbide seed crystal 2. Figure 1 In the exemplary embodiment shown, the component thickness D increases radially with a total of three discrete steps 7. The geometric structure of the back layer component 3 changes radially starting from the longitudinal center axis 6 of the crystal. Due to this radially varying structure, the radial temperature gradient is adjusted during the growth of the bulk silicon carbide single crystal within the SiC seed crystal 2 (especially the area around the SiC seed crystal 2 and the growing bulk SiC single crystal) so that no significant internal mechanical stress is generated in either the SiC seed crystal 2 or the growing bulk SiC single crystal. Preferably, if internal stress already exists, this radial temperature gradient even helps to reduce such internal stress. As a result, a completely defect-free bulk SiC single crystal is finally produced, from which a SiC substrate with few defects can be obtained for producing high-quality components with a high yield.

[0052] The different component thicknesses D of the back layer component 3 are between 1 μm and 5 μm. In particular, the component thickness D increases five-fold starting from the center of the longitudinal center axis 6 of the crystal towards the peripheral edge (i.e. in the radial direction). At the peripheral edge, the component thickness D reaches a maximum value, in particular 5 μm measured at this location. Figure 1 In the exemplary embodiment of the present invention, the back layer component 3 is formed as a single layer. Therefore, the back layer component 3 only comprises a single layer. The back layer component is composed of metal carbide. Figure 1In an exemplary embodiment of the present invention, the back layer component 3 is tantalum carbide (TaC). In another exemplary embodiment not shown, the back layer component 3 can also be composed of another layer of material (such as graphite, or another carbon material or another carbide), in particular a metal carbide with a high melting point metal (such as tungsten or another refractory metal).

[0053] Figure 2 An exemplary embodiment of a growth apparatus 8 for producing bulk SiC single crystals (also not shown) by sublimation growth is shown. The growth apparatus 8 comprises a growth crucible 9 including a SiC storage region and a crystal growth region 11. The SiC storage region comprises, for example, SiC powder feedstock 12.

[0054] Growth crucible 9 has a crucible container 13 and a crucible cover 14. Growth crucible 9 has a first axial end wall 15 and a second axial end wall 16. First axial end wall 15 is adjacent to the SiC storage area, and second axial end wall 16 is formed by crucible cover 14. Growth crucible 9 also has a peripheral wall 17, which, like first axial end wall 15, is an integral part of crucible container 13. Figure 1 The seed crystal unit 1 is placed in the growth crucible 9 by means of a seed crystal holder 18, so that the wafer front side 4 of the SiC seed crystal 2 is arranged with the growth surface 4a in the crystal growth zone 11. In the exemplary embodiment, the wafer front side 4 of the SiC seed crystal 2 is loosely located on the annular seed crystal holder 18 in the region of the peripheral edge.

[0055] A peripheral gap 19 is defined between the inner side of the peripheral wall 17 and the seed unit 1 .

[0056] Figure 2 The growth crucible 9 is made of graphite crucible material which is both electrically and thermally conductive. Figure 2 In addition, an induction heating device in the form of a heating coil is provided to heat the crucible 9, which is also not shown. This heating device heats the crucible 9 to a temperature above 2100° C. required for growth.

[0057] The SiC growth gas phase in the crystal growth zone 11 is provided by the SiC source material 12. The SiC growth gas phase contains at least gas components in the form of SiC, Si2C and SiC2 (=SiC gas species). The material transport from the SiC source material 12 to the growth surface 4a is carried out along an axial temperature gradient, which is set by means of a heating device and extends parallel to the longitudinal center axis 6 of the crystal. At the growth surface 4a, the growth temperature is relatively high, at least 2100°C, in particular even at least 2200°C or 2300°C. At this position, the gas components of the SiC growth gas phase are deposited, thereby achieving the growth of a bulk SiC single crystal. The temperature in the growth crucible 9 decreases axially from the SiC source material 12 to the crucible lid 14, thereby forming the above-mentioned axial temperature gradient.

[0058] An equipment chamber 20 is disposed on the back side of the seed unit 1, away from the SiC storage area 10. The equipment chamber 20 is located between the crucible lid 14 and the back layer assembly 3 of the seed unit 1. This equipment chamber 20 also facilitates the radially varying structure of the back layer assembly 3, similarly used to adjust the temperature gradient described above. The equipment chamber 20 is advantageous but optional. Another exemplary embodiment may also not include such an equipment chamber 20.

[0059] Figure 3 A further exemplary embodiment of a seed unit 21 is shown. The seed unit 21 also comprises a SiC seed 2 with a wafer front side 4 and a wafer back side 5. Compared to the seed unit 1, the seed unit 21 has a back layer assembly 22 of a different design. The back layer assembly 22 is also of single-layer design and also completely covers the SiC seed 2, and its assembly thickness D also increases radially starting from the longitudinal center axis 6 of the crystal up to the peripheral edge, in the exemplary embodiment in particular increasing by a factor of 2. In this case, the increase is not step-like, but continuous. At the peripheral edge, the assembly thickness D reaches a maximum value, which is in particular 3 μm measured at this location. In this respect, the back layer assembly 21 also has a geometry that changes radially starting from the longitudinal center axis 6 of the crystal, whereby the radial temperature gradient is adjusted again at least within the SiC seed 2 during the growth of the bulk SiC single crystal.

[0060] Figure 4A seed unit 23 of another exemplary embodiment is shown. The seed unit 23 comprises a SiC seed 2 and a back layer assembly 24, which in this exemplary embodiment is designed as a multilayer. The back layer assembly 24 comprises a total of five individual layers 25, 26, 27, 28 and 29. The first four individual layers 25 to 28 are applied one above the other directly to the wafer back 5 of the silicon carbide seed 2, each individual layer having the shape of a planar wafer and, in this respect, in particular at each position of the relevant individual layer 25 to 28, having the same axial expansion. In the exemplary embodiment shown, the four individual layers 25 to 28 each consist of a graphite-based thermal insulation material and form an alternating layer system. The outermost individual layer 29 is located on the side of this layer system of the four individual layers 25 to 28 facing away from the SiC seed 2 and is designed similarly to Figure 1 The back layer assembly 3. The individual layers 29 have an axial expansion (= thickness of this individual layer) that increases radially in three steps starting from the longitudinal center axis 6 of the crystal. In this respect, the outermost individual layer 29 and the back layer assembly 24 as a whole also have a radially varying geometry. Thus, the radial temperature gradient is adjusted. Figure 4 In an exemplary embodiment, the outermost individual layer 29 is composed of a metal carbide (e.g., tungsten carbide WC). The backing layer assembly 24 has an axial assembly thickness D, which represents the axial expansion of all individual layers 25 to 29 arranged one above the other. Due to the radial variation of the outermost individual layer 29, the assembly thickness D increases radially from the longitudinal center axis 6 of the crystal toward the peripheral edge, in an exemplary embodiment, by a factor of 2. At the peripheral edge, the assembly thickness D reaches a maximum, with a value of 10 μm being measured at this location.

[0061] Figure 5 Another exemplary embodiment of a seed crystal unit 31 is shown. The seed crystal unit 31 has a SiC seed crystal 2 and other back layer components 32. The back layer component 32 is designed as a multi-layer, wherein Figure 4 Compared with the back layer component 24, Figure 5In the exemplary embodiment, the three single layers 33, 34 and 34 are not arranged one above the other in the axial direction, but are radially adjacent to each other and concentric with respect to the longitudinal center axis 6 of the crystal. The central single layer 33 is formed as an annular body and has a minimum (uniform) single layer thickness in the axial direction, in particular 1 μm in the exemplary embodiment. The intermediate single layer 34, which is directly radially adjacent to the central single layer 33, is formed as an annular body and has a (uniform) single layer thickness that is greater than the thickness of the inner single layer 33, in particular 2 μm in the exemplary embodiment. The outer single layer 35 is directly adjacent to the intermediate single layer 34 and has an even greater (uniform) single layer thickness than the intermediate single layer, in particular 5 μm in the exemplary embodiment. The component thickness D of the back layer component 32 also increases radially from the longitudinal center axis 6 of the crystal to the peripheral edge, in particular increasing by a factor of 5 in the exemplary embodiment. For the back layer component 32, a structure is again produced as a whole, the geometry of which changes radially from the longitudinal center axis 6 of the crystal. Figure 5 In the exemplary embodiment of FIG. 5 , the individual layers 33 , 34 and 35 are each composed of a different insulating layer material based on graphite.

[0062] Figure 6 Another exemplary embodiment of a seed crystal unit 36 ​​is shown, comprising a SiC seed crystal 2 and an additional backside layer assembly 37. Backside layer assembly 37 is also designed as a multilayer structure, consisting of a lower single layer 38, which contacts and covers the entire wafer backside 5 of the SiC seed crystal 2, and a two-part upper single layer 38 having a wafer-shaped middle single layer 39 and an edge-side annular single layer 40 concentrically surrounding the middle single layer. Backside layer assembly 37 has the same axial expansion at every point. In this exemplary embodiment, the assembly thickness D is uniform throughout. For example, assembly thickness D is 4 μm. However, in particular, at least two single layers 39 and 40 of the upper or outer layer are each composed of different layer materials. Thus, in this exemplary embodiment, the structure of backside layer assembly 37 varies in material composition radially from the longitudinal center axis of the crystal. This, in this exemplary embodiment, achieves desired radial temperature regulation during the growth of the bulk SiC single crystal.

[0063] Figure 7Another exemplary embodiment of a seed crystal unit 41 is shown, comprising a SiC seed crystal 2 and a single-layer backside layer assembly 42. The backside layer assembly 42 has a non-uniform profile on the side of the wafer backside 5 facing away from the SiC seed crystal 2, similar to backside layer assemblies 3 and 24, with three concentric steps 43. Consequently, the assembly thickness D of backside layer element 42 also increases radially from the longitudinal center axis 6 of the crystal to the peripheral edge, specifically increasing by a factor of 1.5 in the exemplary embodiment. At the center, the assembly thickness D is, for example, 4 mm, while at the peripheral edge (where the axial expansion is greatest), the assembly thickness D is 6 mm. Furthermore, a layer cavity 44 is located within backside layer assembly 42, with an axial cavity height H of, for example, 3 mm. Due to the backside surface profile, backside layer assembly 42 has a radially varying structure. Consequently, the presence of layer cavity 44 also enables adjustment of the radial temperature gradient during the growth of the bulk SiC single crystal.

[0064] exist Figure 2 In the apparatus 8 of FIG. 1 , seed units 21, 23, 31, 36, and 41 can be used instead of the seed unit 1 for growing bulk SiC single crystal. All of the seed units 1, 21, 23, 31, 36, and 41 are characterized in that they can individually adjust, in particular, the radial temperature gradient and the axial temperature gradient within the growth apparatus 8, thereby facilitating the growth of bulk SiC single crystal with particularly few defects.

Claims

1. A seed crystal unit for growing bulk silicon carbide SiC single crystal, comprising a) a wafer-shaped single-crystal SiC seed crystal (2), the SiC seed crystal (2) having a growth surface (4a) arranged on the wafer front side (4), the growth surface (4a) being used to grow the bulk SiC single crystal to be grown, a1) wherein the SiC seed crystal (2) has a crystal longitudinal center axis (6) extending along the axial direction and a radial direction perpendicular to the axial direction, as well as b) a back layer assembly (3; 22; 24; 32; 37; 42) arranged on the back side (5) of the wafer of the SiC seed crystal (2), b1) The structure of the back layer assembly varies radially from the longitudinal center axis (6) of the crystal, so that a radial temperature gradient is adjusted during the growth of the bulk SiC single crystal within the SiC seed crystal (2).

2. The seed crystal unit according to claim 1, characterized in that: The back layer assembly (3; 22; 42) consists of a single layer.

3. The seed crystal unit according to claim 2, characterized in that: The axial layer thickness of the individual layers is between 0.5 μm and 10 μm, in particular between 1 μm and 5 μm.

4. The seed crystal unit according to claim 1, characterized in that: The back layer assembly (24; 32; 37) is composed of a plurality of single layers (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40).

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

6. The seed crystal unit according to claim 4 or 5, characterized in that: The total axial layer thickness of all the individual layers ( 25 , 26 , 27 , 28 , 29 ; 33 , 34 , 35 ; 38 , 39 ; 40 ) is between 0.5 μm and 20 μm, in particular between 1 μm and 10 μm.

7. The seed unit according to any one of claims 4 to 6, characterized in that The individual layers (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40) at least partially comprise mutually different layer materials.

8. The seed unit according to any one of claims 2 to 7, characterized in that Each individual layer (25, 26, 27, 28, 29; 33, 34, 35; 38, 39; 40) of the back layer assembly (3; 22; 24; 32; 37; 42) consists of a layer material from the group of carbon and carbide or includes at least one material from said group.

9. A seed unit according to any one of the preceding claims, characterized in that The axial component thickness of the back layer component (3; 22; 24; 32; 42) increases radially starting from the longitudinal center axis (6) of the crystal.

10. The seed crystal unit according to claim 9, characterized in that: The axial component thickness increases continuously or in discrete steps.

11. A seed unit according to any one of the preceding claims, characterized in that The back layer assembly (3; 22; 24; 32; 37; 42) is directly adjacent to the wafer back side (5) of the SiC seed crystal (2).

12. A seed unit according to any one of the preceding claims, characterized in that The back layer assembly (42) has a layer cavity (44).

13. The seed crystal unit according to claim 12, characterized in that: The axial expansion of the layer cavity (44) reaches 5 mm.

14. An apparatus for growing bulk SiC single crystals, comprising a) a heatable growth crucible (9) having a SiC storage area (10) arranged in a first portion for receiving SiC source material (12), and a crystal growth area (11) arranged in a second portion, b) A seed unit (1; 21; 23; 31; 36; 41) according to any one of the preceding claims, and c) a seed crystal support (18) for supporting a seed crystal unit (1; 21; 23; 31; 36; 41) in the growth crucible (9) such that at least the growth surface (4a) of the SiC seed crystal (2) of the seed crystal unit (1; 21; 23; 31; 36; 41) is arranged in the crystal growth zone (11).

15. The device according to claim 14, characterized in that An equipment cavity (20) is arranged on the back side of the seed crystal unit (1; 21; 23; 31; 36; 41) facing away from the SiC storage area (10), wherein the axial expansion of the equipment cavity can be up to 5 mm.

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