Method for growing seed crystal assemblies and SiC single crystal Booleans

Abstract

To provide a seed crystal assembly for growing single-crystal Boules in a physical vapor transport (PVT) process, and a method for producing bulk SiC crystals in a physical vapor transport growth system. [Solution] The seed crystal assembly is a base structure connectable to a crucible, comprising: a base structure having a base diameter along the central axis of the seed crystal assembly and a base height along the central axis; and a single crystal seed crystal designed for growing a single crystal Boule on a growth surface. The single crystal seed crystal is attached to the base structure, and the seed crystal has a seed crystal diameter along the central axis and a seed crystal height along the central axis, and the specific thermal conductivity of the base structure differs from that of the seed crystal by 25% or less at growth temperatures between 2000°C and 2600°C.

Description

【Technical Field】 【0001】 The present invention relates to a seed crystal assembly for growing a single crystal boule in a physical vapor transport (PVT) process. Further, the present invention relates to a method for producing bulk SiC crystals in a physical vapor transport growth system. 【Background Art】 【0002】 Due to remarkable physical, chemical, and electrical properties, silicon carbide is used, inter alia, as a semiconductor substrate material for semiconductor components for power electronics, high-frequency components, and various special light-emitting semiconductor components. As a basis for these products, bulk SiC crystals with ideally high purity and defect-free quality are required. 【0003】 As is known in the art, bulk SiC crystals are generally produced by physical vapor deposition techniques, particularly the sublimation method. This process requires temperatures above 2000°C. Physical vapor transport (PVT) is essentially a process of sublimation and recondensation, in which raw materials and seed crystals are placed inside a growth furnace such that the temperature of the raw materials is higher than that of the seeds. The raw materials sublime, and the vapor species diffuse and accumulate on the seeds to form a single crystal. An example of the PVT process is disclosed in U.S. Patent No. 8,865,324. 【0004】 To produce wafer-shaped substrates, bulk SiC crystals are sliced thin, for example, by a diamond-impregnated wire saw. The surface is finished by subsequent multi-step polishing steps. To fabricate electronic components, thin single crystal layers (e.g., SiC or GaN) are epitaxially deposited on the polished wafers. The properties of these layers and thus the electronic components fabricated therefrom are highly dependent on the quality of the underlying SiC substrate. 【0005】 A major factor significantly impacting the quality of SiC substrates is stress in the crystal lattice that can occur during crystal growth. These stresses can cause bending (warping, deflection) of SiC substrates produced from bulk SiC crystals, or, in particular, basal plane dislocations (so-called BPDs), which can result from the relaxation of tensions within the crystal lattice. 【0006】 Stresses during SiC crystal growth can arise, in particular, from stresses originating from the seed crystal assembly, the temperature gradient required for crystal growth, or mechanical stresses between the crucible wall and the crystal during or after crystal growth. 【0007】 To progressively reduce the stress equilibrium during the growth of a SiC single crystal, all contributing components must be considered, and improvement and corrective measures must be found for each cause. 【0008】 In particular, this disclosure aims to reduce the adverse effects of seed crystals and holding systems (hereinafter also referred to as seed crystal assemblies) on stress equilibrium during the growth of SiC single crystals. It should be noted that the principles of this disclosure are also applicable to improving the bulk crystal quality of single crystals other than silicon carbide. However, SiC will always be used as an example below. 【0009】 Conventionally, single-crystal SiC seeds are fixed to a graphite seed crystal holder by adhesive. The seed crystal holder functions as a support for the seed crystal at the contact surface in the growth mechanism. The attachment of this sliding support to the crucible's support surface is shown in Figure 2. Alternatively, the seed crystal holder may be clamped axially or radially in the growth assembly. Axial and radial clamping are shown in Figures 3 and 4, respectively. The joining and connection of the two different materials in the seed holder-seed crystal system, coupled with the discontinuity of the transition from the edge of the SiC seed to the seed holder extended around the contact surface, causes non-uniform heat dissipation through the seed holder-seed crystal system. For growing high-quality SiC single crystals, it is crucial that the set temperature gradient, especially the radial temperature gradient at the growth front, is not disturbed or altered by non-uniform heat dissipation. Unpredictable temperature gradients and temperature deviations at the seed surface can suddenly induce localized lattice stress in the SiC seed, degrading the quality of the grown single crystal. 【0010】 As crystals grow and / or cool after growth, these stresses either remain within the crystal or are partially relieved by the generation of basal plane dislocations. Residual stresses can cause bending (warping, deflection) of SiC substrates produced from single-crystal Booleans. Dislocations also have a quality degradation effect on SiC substrates produced from single-crystal Booleans. 【0011】 Known seed crystal assemblies are shown, for example, in U.S. Patent No. 6,723,166 or U.S. Patent No. 9,590,046. According to U.S. Patent No. 6,723,166, by laterally enclosing the seed retainer, the seed retainer seed system should be designed so that the seed retainer is protected from SiC growth and low-defect SiC crystals can be deposited on the SiC seed. However, it has been shown that heterogeneous thermal bonding can occur, potentially leading to stress and dislocations within the SiC crystal. 【0012】 Furthermore, U.S. Patent No. 9590046 describes how the growth structure is designed to allow bending of the seed-retaining-seed-crystal system, taking into account the different thermal expansion coefficients (binary metal effect) of SiC and graphite. This prevents delamination of the adhesive. However, bending of the seed-retaining-germination system can cause uncontrollable stress and dislocations in the crystal. When the grown single-crystal boules are separated into the SiC substrate, these stresses cause the single-crystal wafer to bend and deflect (warpage and deflection). Excessive warpage and deflection directly lead to the rejection of the SiC substrate, as a SiC substrate with low values ​​of warpage and deflection is required for the subsequent epitaxy process. [Prior art documents] [Patent Documents] 【0013】 [Patent Document 1] U.S. Patent No. 8865324 [Patent Document 2] U.S. Patent No. 6,723,166 [Patent Document 3] U.S. Patent No. 9590046 [Overview of the project] 【0014】 Therefore, there is still a need for improved seed crystal assemblies and methods for growing at least one SiC single crystal boule in physical vapor transport (PVT) growth systems that overcome the problems in existing systems and result in higher-quality single crystal boules. 【0015】 This objective is achieved by the subject matter of the independent claim. Useful embodiments of the present invention are the subject matter of the dependent claim. 【0016】 This disclosure provides a seed crystal assembly for growing a single crystal boule in a physical vapor transport (PVT) process, the seed crystal assembly comprising a base structure connectable to a crucible, the base structure having a base diameter along the central axis of the seed crystal assembly and a base height along the central axis, and a single crystal seed crystal designed for growing a single crystal boule on a growth surface. The single crystal seed crystal is attached to the base structure, the seed crystal having a seed crystal diameter along the central axis and a seed crystal height along the central axis, wherein the specific thermal conductivity of the base structure differs from that of the seed crystal by 25% or less at growth temperatures between 2000°C and 2600°C. For example, the specific thermal conductivity of the base structure differs from that of the seed crystal by 20% or less, preferably 15% or less, at growth temperatures between 2000°C and 2600°C. 【0017】 By selecting materials in this way, the seed crystal can avoid being subjected to mechanical stress during the heating process, resulting in a lattice of the grown single-crystal Boolean having significantly fewer defects than when using conventional seed crystal assemblies. 【0018】 To improve the uniformity of the temperature profile at the growth interface, the seed crystal may form a step on the base structure, the step having a smaller diameter than the base structure. This improves heat dissipation, resulting in the desired temperature profile. For example, the diameter of the seed crystal may be at least 2 mm smaller than the diameter of the base structure, and may be at most 10 mm smaller. Considering the diameter ratio, the ratio of the base diameter to the seed crystal diameter may be in the range of 1.05 to 1.2. 【0019】 In a further example of this disclosure, the base structure has a first height along the central axis of the seed crystal assembly, the seed crystal has a second height along the central axis, and the base height is at least 1 mm greater than the seed crystal height. For example, the base height is at most 5 mm greater than the seed crystal height. Considering the ratio of the two heights, the ratio of the base height to the seed crystal height may be advantageously in the range of 1.0 to 2.0. 【0020】 According to a further advantageous example of the present disclosure, the surface of the base structure on the side opposite to the seed crystal includes a passivation layer including at least one of a carbon layer, a refractory metal layer, and a graphite layer. Such a passivation layer on the back surface of the base structure avoids evaporation of components from the base structure. For example, when the base structure is formed from SiC, evaporation of SiC at high growth temperatures can be avoided by the passivation layer. 【0021】 An advantageous similarity in the thermal conductivity of the seed crystal and the seed holder (also called the base structure) can be achieved by either a seed crystal assembly formed by at least two joined components or a seed crystal assembly formed as one integral part. 【0022】 For example, the seed crystal assembly may be fabricated by vertical erosion or grinding. 【0023】 According to another aspect of the present disclosure, a method for growing at least one SiC single crystal boule in a physical vapor transport (PVT) growth system is provided, the method comprising the steps of: placing SiC powder raw material in a raw material section; placing at least one seed crystal assembly according to one of the preceding claims in a growth section, the seed crystal including a SiC seed crystal, the raw material section being connected to the growth section for supplying sublimated gaseous components to the growth section; applying a high temperature between 2000 °C and 2600 °C to generate sublimated gaseous components that generate a SiC growth phase with the SiC seed crystal, a SiC monocrystalline boule being formed on the SiC seed crystal. 【0024】 Advantageously, the seed crystal assembly is attached to the growth system by a free suspension support or the seed crystal assembly is attached to the growth system by a clamped bearing. 【0025】 The accompanying drawings are incorporated in and form a part of the specification, showing some embodiments of the present invention. These drawings, together with the description, serve to explain the principles of the invention. The drawings merely illustrate preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Further, some aspects of the embodiments can be combined individually or in different combinations to form solutions according to the present invention. Further features and advantages will become apparent from the following more specific description of various embodiments of the invention shown in the accompanying drawings, in which like reference numerals refer to like elements, and the description of the figures is as follows. 【Brief Description of the Drawings】 【0026】 [Figure 1] It is a schematic cross-sectional view of a PVT growth apparatus. [Figure 2] It is a schematic cross-sectional view of a seed crystal assembly supported on a support surface. [Figure 3] It is a schematic cross-sectional view of a seed crystal assembly supported by axial clamping. [Figure 4] It is a schematic cross-sectional view of a seed crystal assembly supported by radial clamping. [Figure 5] It is a schematic cross-sectional view of a seed crystal assembly formed as an integral part. [Figure 6] It is a schematic cross-sectional view of a seed crystal assembly including another component. [Figure 7] It is a schematic cross-sectional view of a seed crystal assembly showing a temperature field at the growth front. [Figure 8] It is a schematic cross-sectional view of a seed crystal assembly according to a further example. [Figure 9] It is a schematic cross-sectional view of a seed crystal assembly according to a further example. [Figure 10] It is a schematic cross-sectional view of a seed crystal assembly according to a further example. 【Modes for Carrying Out the Invention】 【0027】 The present invention will be described in more detail here with reference to the drawings. Referring first to Figure 1, a growth apparatus 100 for producing a SiC bulk single crystal 102 (also called a single crystal boule or volume single crystal) by sublimation growth is shown. The growth apparatus 100 includes a growth crucible 104 which includes a SiC containment region 106 and a crystal growth region 108. The SiC containment region 106 contains, for example, powdered SiC raw material 110 which is introduced into the SiC containment region 106 of the growth crucible 104 as a starting material prepared in advance before the start of the growth process. 【0028】 The SiC seed crystal 112, which extends axially into the crystal growth region 108, is located in the region of the crucible end wall 116 of the growth crucible 104, which is opposite the SiC containment region 106. The SiC seed crystal 112 is a single crystal. The structure and arrangement within the growth crucible 104 will be described in more detail below with reference to Figures 2 to 10. 【0029】 The crucible end wall 116 serves as the crucible lid for the growth crucible 104 in the example shown in Figure 1. However, this is not essential. The SiC volume single crystal 102 to be grown is grown on a SiC seed crystal 112 by deposition from the SiC growth gas phase 114 formed in the crystal growth region 108. The growing SiC volume single crystal 102 and the SiC seed crystal 112 have approximately the same diameter. Even if there is a difference, it is at most 10%, and such a difference results in the seed diameter of the SiC seed crystal 112 being smaller than the single crystal diameter of the SiC volume single crystal 102. However, a gap not shown in Figure 1 may exist between the inside of one crucible side wall 118 and the growing SiC volume single crystal 102 and SiC seed crystal 112 on the other side. 【0030】 In the embodiment shown in Figure 1, the growth crucible 104 including the crucible lid 116 contains, for example, at least 1.75 g / cm³. 3 It may contain an electrically and thermally conductive graphite crucible material having a density of . An insulating layer 10 is placed around it. The insulating layer contains, for example, a foamed graphite insulating material, the porosity of which is significantly higher than that of the graphite crucible material in particular. 【0031】 The insulated growth crucible 104 may be placed inside a tubular container 122, which in some embodiments is designed as a quartz glass tube, to form an autoclave or reactor. To heat the growth crucible 104, an induction heating device in the form of a heating coil 124 may be placed around the container 122. Other suitable heating devices, such as resistance heaters, may also be used, of course. 【0032】 The growth crucible 104 is heated by a heating coil 124 to a growth temperature higher than 2000°C, preferably between 2000°C and 2600°C, particularly around 2200°C. The heating coil 124 inductively couples an electric current into the conductive crucible sidewall 118 of the growth crucible 104. This current flows substantially as a circulating current in the periphery direction within the circular, hollow cylindrical crucible sidewall 118, heating the growth crucible 104 in the process. If necessary, the relative position between the heating coil 124 and the growth crucible 104 can be changed axially, i.e., in the direction of the central longitudinal axis 126 of the growing SiC volume single crystal 102, also to optionally change the temperature or temperature profile within the growth crucible 104, in particular. 【0033】 The position of the heating coil 124, which can be changed axially during the growth process, is indicated by a double arrow 128 in Figure 1. In particular, the heating coil 124 is displaced in accordance with the growth progress of the growing SiC volume single crystal 102. The displacement is preferably in the downward direction, i.e., in the direction of the SiC raw material 110, and preferably by the same length as the length over which the SiC volume single crystal 102 grows, for example, about 20 mm in total. For this purpose, the growth apparatus 100 includes correspondingly configured monitoring, control, and adjustment devices, which are not shown in detail in the figure. 【0034】 The SiC growth gas phase 114 in the crystal growth region 108 is supplied by SiC raw material 110. The SiC growth gas phase 114 contains at least a gaseous component in the form of Si, Si2C, and SiC2 (i.e., SiC gas species). Transport of the SiC raw material 110 to the growth interface 130 in the growing SiC volume single crystal 102 occurs along an axial temperature gradient. 【0035】 An axial temperature gradient of at least 5 K / cm, preferably at least 10 K / cm, measured in the direction of the central longitudinal axis 126, is adjusted in particular at the growth interface 130. The temperature inside the growth crucible 104 decreases towards the growing SiC volume single crystal 102. This can be achieved by various means. Thus, axially varying heating can be performed by dividing the heating coil 124 into two or more axial component parts (details not shown). Furthermore, a stronger heating effect can be adjusted in the lower part of the growth crucible 104 than in the upper part of the growth crucible 104, for example, by the corresponding axial positioning of the heating coil 124. Also, the insulation of the two axial crucible end walls may differ. As schematically shown in Figure 1, the insulation layer 120 of the lower end wall of the crucible may have a greater thickness than that of the upper end wall of the crucible for this purpose. Furthermore, the insulation layer 120 adjacent to the upper end wall 116 of the crucible may have a central cooling opening 132 through which heat is dissipated, and the opening is positioned around the central longitudinal axis 126. The central cooling opening 132 is shown by a dashed line in Figure 1. 【0036】 The SiC volume single crystal 102 grows in a growth direction 134, which in the example shown in Figure 1 is directed from top to bottom, i.e., from the crucible lid 116 to the SiC containment region 106. The growth direction 134 extends parallel to the central longitudinal axis 126. When the growing SiC volume single crystals 102 in the shown embodiment are arranged concentrically within the growth apparatus 100, the central longitudinal axis 126 can also be allocated to the entire growth apparatus 100. 【0037】 Furthermore, the SiC growth gas phase 114 may contain doping substances, which are not shown in detail in Figure 1, but in this example are nitrogen (N2). In particular, alternative or additional doping substances such as aluminum (Al), vanadium (V), and / or boron (B) are also possible. The doping substances are supplied either in gaseous form or in the subsequently pre-treated SiC raw material 110. In this example, the SiC volume single crystal 102 has n-type doping with nitrogen. The polytype may be, for example, 4H-SiC. However, other doping or other SiC polytypes are also possible. 【0038】 Figures 2 to 4 provide a more detailed schematic illustration of how the seed crystal assembly 136 may be attached to the side wall 118 of the crucible 104. In these figures, the seed crystal assembly 136 is formed by a conventional seed retainer 138 (hereinafter also referred to as the base structure). However, the attachment principle described with reference to Figures 2 to 4 is also intended to be used for the seed crystal assembly, as will be explained in this disclosure and in more detail with reference to Figures 5 to 10. In conventional designs, the seed retainer 138 is fabricated from graphite. The connection between the seed retainer 138 and the single-crystal SiC seed crystal 112 is typically achieved by an adhesive layer 140. As shown in Figure 2, the seed retainer 138 is supported by a support surface 142 of the crucible side wall 118. Thus, the seed retainer is slidable to compensate for thermal expansion. 【0039】 According to the alternative example shown in Figure 3, the seed crystal assembly 136 is secured by an axial clamping bearing 144. Such an axial clamping bearing 144 applies a clamping force along the longitudinal axis 126 (see Figure 1). The axial clamping bearing 144 may include an annular fixing device or multiple smaller fixing devices arranged around the seed holder 138. A combination of one annular fixing device and several smaller fixing devices is also possible. 【0040】 Furthermore, as shown in Figure 4, the seed crystal assembly may be attached to the crucible by a radial clamp bearing section 146. The radial clamp bearing section 146 applies a force directed radially inward toward the center of the seed holding section 138. The radial clamp bearing section 146 may include a plurality of clamp elements arranged around the periphery of the seed holding section 138. Alternatively, the radial clamp bearing section 146 may include a clampable annular clamp element. 【0041】 Figure 5 shows a first example of a seed crystal assembly 136 according to the present disclosure. The object of the present disclosure is to produce stress-reduced or stress-free crystals. To solve this problem, firstly, a seed crystal / seed retainer system called a seed crystal assembly 136 is proposed, as shown in Figure 5. According to this aspect of the present disclosure, the seed crystal assembly 136 comprises a seed retainer (also called a base structure) 138 and a seed crystal 112, both of which are integrally formed from the same material, for example, SiC. 【0042】 By using such a three-dimensional SiC seed holding system, the crystal lattice of the SiC seed crystal remains stress-free during crystal growth. This means that stress and dislocations in the grown crystal may be reduced during growth. This disclosure is based on the use of a seed crystal holding system with optimal heat dissipation. In particular, by machining a monolithic SiC crystal to a predetermined shape, for example by die engraving or grinding, the holding portion 138 is generated to be supported on the support surface 148 or can be clamped by radial or axial clamps in the growth apparatus 100. Specific dimensions will be described in more detail with reference to Figure 7. 【0043】 As an alternative to the integrated design in Figure 5, the seed crystal assembly 136 may be fabricated by joining two or more separate parts. As shown in Figure 6, the seed holder 138 and the seed crystal 112 may also be joined together by bonding or special bonding techniques to form the seed crystal assembly 136 from at least two geometries made of SiC. Importantly, the seed holder (or base structure) 138 is fabricated from a material having a thermal conductivity close to that of the seed crystal 112. In particular, the thermal conductivity of the seed holder 138 at growth temperatures between 2000°C and 2600°C differs from that of the seed crystal 112 by 20% or less, preferably 15% or less. 【0044】 The interface layer 150 can be formed by various techniques. Bonding two SiC bodies or one SiC body to another body of a different material can be done primarily using carbon-based adhesives or adhesives that primarily form a carbon-containing adhesive layer upon curing. Examples include novolac resins, common phenolic resins, photoresists, or other organic carbon-based molecules. In addition, for bonding two SiC bodies, techniques utilizing adhesive force can be applied to connect the two SiC bodies. A further technique is diffusion bonding. Diffusion bonding is a high-temperature and high-pressure process that bonds two SiC components by atomic diffusion through pressing their surfaces together. It does not use filler metals and is highly reliable in applications where purity is critical. 【0045】 Essentially, this disclosure provides a seed crystal assembly 136 with optimized heat transport for retaining the seed crystal without (or at least with less) mechanical stress during a high-temperature PVT process. This aspect is particularly important at the start of the growing single crystal and during the initial growth. 【0046】 By controlling the thickness and diameter of each component of the seed crystal assembly 136, heat dissipation of the radial temperature gradient at the growth interface 130 of the seed crystal 112 can be optimized. In particular, by correctly selecting the height and diameter of the base structure 138 and the seed crystal 112, equilibrium heat dissipation can be achieved at the edges and central areas of the seed crystal. As shown in Figure 7, the seed crystal assembly 136 has a stepped structure, with the seed holder 138 having a first height H and the seed crystal 112 having a second height h. For example, the seed holder 138 has an essentially circular contour with a diameter D, and the seed crystal is formed as an essentially circular step with a smaller diameter d. It should be noted that these geometric considerations apply not only to the integrally formed example shown in Figure 5, but also to the composite seed crystal assembly 136. 【0047】 In the cross-sectional view of Figure 7, the dashed line 152 represents the temperature profile of the entire seed crystal assembly 136. As can be seen from this profile, the temperature across the diameter d of the seed crystal 112 remains constant over a fairly large portion of the entire growth interface 130. Therefore, mechanical stress is kept low within the lattice of the seed crystal 112. Furthermore, since the seed holder 138 and the seed crystal 112 are made of the same material, there is no difference in thermal expansion properties, further reducing the mechanical stress between the seed holder 138 and the seed crystal 112. By controlling the dimensions H / h and D / d, equilibrium heat dissipation can be achieved at the edges and central areas of the seed crystal 112. This is in contrast to the radial temperature field of the current level of technology, which can only be achieved in a non-equilibrium temperature field with a strong radial gradient. Curve 153 shows the temperature profile that can be achieved, for example, when using a conventional graphite seed holder to which a SiC seed crystal is attached with adhesive. The diameters D and d, and the heights H and h, are selected to control the optimal heat dissipation of the non-uniform temperature in the seed crystal 112 due to the radial temperature gradient. According to the present invention, controlled heat dissipation in the radial and axial directions is possible, particularly due to the geometry and material properties of the peripheral transition region 164 of the seed holding portion 138. This makes it possible to achieve a favorable temperature profile represented by the curve 152. In particular, the temperature can be kept constant even over a much larger radial area. 【0048】 Ideally, the ratio H / h is in the range of 1.0 to 2.0, and the ratio D / d is in the range of 1.05 to 1.20. The support surface 148, designed in this way, also functions as an equilibrium for controlling heat transfer through the seed crystal assembly 136. 【0049】 As mentioned above, conventional use of different materials in the seed crystal and seed holder, as well as interfaces (depending on the level of the art) resulting from bonding or clamping, can cause discontinuities in heat dissipation in the axial direction through the seed crystal assembly, but these can be avoided in the assembly according to this disclosure. 【0050】 Uniform heat dissipation or avoidance of hot spots at the seed edges or central areas means that the crystal lattice and seed-embryonic system are kept stress-free. Otherwise, the temperature-dependent coefficient of thermal expansion causes locally different expansions and / or compressions of the seed crystal lattice, which continue into the growing crystal. As a result, a strained single crystal is obtained. This can be avoided by using the seed crystal assembly 136 according to this disclosure. 【0051】 A further aspect of this disclosure relates to the passivation of the rear surface of the seed crystal assembly 136. This aspect will be described in detail with reference to Figures 8 to 10. Note that the figures in this disclosure are not to scale. All thin layers are shown with exaggerated thickness for clarity. 【0052】 In particular, when the seed crystal assembly 136 is formed by the SiC seed holding system, a passivation layer 154 can be provided to prevent evaporation of SiC. Passivation of the back surface of the SiC seed holding part 138 can be carried out by various methods. 【0053】 Firstly, the passivation layer can be formed as a coating from a fluid solution (e.g., novolac resin, phenolic resin, etc.). This solution is not shown in the drawings. 【0054】 Furthermore, as shown in Figure 8, a foil 156 (for example, made of expanded graphite) may be attached to the back surface of the seed-holding portion 138 by a suitable adhesive layer 158. Instead of graphite, the foil 156 may optionally be formed from a carbide-type refractory metal. The back surface of the seed-holding portion 138 may be provided with a foil 156 made of a refractory metal, such as tantalum, tungsten, niobium, molybdenum, rhenium, iridium, ruthenium, hafnium, zirconium, or a mixture of one or more refractory metals, or a foil made of one or more carbide-type refractory metals (e.g., TaC, WC, etc.). Alternatively, the foil 156 may be carbide-type after the foil 156 and the back surface of the seed-holding portion 138 have been joined. 【0055】 Furthermore, as shown in Figure 9, the back surface of the seed-holding portion 138 may be provided with graphite and / or (carbide-formed) refractory metal foil, regardless of whether the back surface of the SiC seed-holding portion is pre-coated with a carbon-containing compound, such as novolac, phenolic resin, or any suitable photoresist. To attach the foil to the back surface of the seed-holding portion, the foil can be secured by clamping it during the assembly of the growth assembly 100. As shown in Figure 9, the passivation layer 154 may include, for example, two or more foils 156 arranged alternately with the adhesive layer 158. 【0056】 A further example of the passivation layer 154 is shown in Figure 10. In this example, the carbon-containing layer 160 coating is deposited directly onto the back surface 162 of the seed-holding section 138. At least one foil 156 containing graphite and / or refractory metal is deposited on top of the coating 160. When the foils 156 are clamped by the crucible lid or any other suitable clamping device during the assembly of the growth apparatus 100, no additional adhesive needs to be applied between the foils 156. Two foils 156 are shown in Figure 10, but it is of course possible to include more layers. 【0057】 Any suitable combination of the examples described with reference to Figures 8 to 10 can be used to form a passivation layer 154 on the back surface 162 of the seed holding portion 138. It should also be noted that various passivation concepts may be used with a single seed crystal assembly 136 and any type of multi-part seed crystal assembly 136. 【0058】 In summary, the solution provided by this disclosure makes it possible to reduce the generation of stress that may arise from the interaction between the seed holder 138 and the seed crystal 112 during the growth of a single crystal, particularly a silicon carbide crystal, by using a stepped seed crystal assembly in which the seed holder 138 is made from a material having a thermal conductivity close to that of the seed crystal 112 itself. Advantageously, the seed holder 138 is made from the same material as the seed crystal 112, for example, SiC. The seed holder 138 may also be a single crystal, similar to the seed crystal 112, and in particular, the seed crystal assembly 136 may be formed as a single component. 【0059】 This directly improves the quality and yield of SiC substrates for use in downstream processes such as epitaxy, which require SiC substrates with low bending and / or deflection and low basal layer dislocation density. [Explanation of Symbols] 【0060】 100 growth equipment 102 Bulk single crystal 104 Growth Crucible 106 Containment area; raw material section 108 Crystal growth region; growth section 110 Raw materials 112 Seed Crystal 114 Growth gas phase 116 Crucible end wall 118 Crucible side wall 120 Insulation layer 122 Container 124 Heating coil 126 Central longitudinal axis 128 Variable position of the heating coil 130 Growth interface 132 Central cooling opening 134 Growth direction 136 Seed crystal assembly 138 Seed holding part; basal structure 140 Adhesives 142 Support surface of the crucible 144 Axial clamp bearing section 146 Radial clamping bearing section Support surface of 148 type crystal assemblies 150 Interface Temperature profile of 136 seed crystal assemblies (152 types) 153 Known temperature profiles of graphite species retaining sections 154 Passivation layer 156 Foil (graphite, refractory metal) 158 Adhesive layer 160 Coating with a carbon-containing layer 162 Back surface of the type holding section 164 Transition Area Height of the H base structure h Seed crystal height D Diameter of the base structure d Diameter of the seed crystal

Claims

[Claim 1] A seed crystal assembly for growing single-crystal Boule in a physical vapor transport (PVT) process, A base structure connectable to a crucible, having a base diameter along the central axis of the seed crystal assembly and a base height along the central axis, Includes a single crystal seed crystal designed for growing the single crystal Boolean on a growth surface, The single crystal seed crystal is attached to the base structure, and the seed crystal has a seed crystal diameter along the central axis and a seed crystal height along the central axis. A seed crystal assembly wherein the specific thermal conductivity of the base structure differs from that of the seed crystal by 25% or less at growth temperatures between 2000°C and 2600°C. [Claim 2] The seed crystal assembly according to claim 1, wherein the seed crystal forms a step on the base structure, and the step has a diameter smaller than the diameter of the base structure. [Claim 3] The diameter of the seed crystal is at least 2 mm smaller than the diameter of the base structure. The seed crystal assembly according to claim 2. [Claim 4] The seed crystal assembly according to claim 2, wherein the diameter of the seed crystal is at most 10 mm smaller than the diameter of the base structure. [Claim 5] The seed crystal assembly according to claim 1, wherein the ratio of the base diameter to the seed crystal diameter is in the range of 1.05 to 1.

2. [Claim 6] The seed crystal assembly according to claim 1, wherein the base height is at least 1 mm greater than the seed crystal height. [Claim 7] The seed crystal assembly according to claim 6, wherein the base height is at most 5 mm greater than the seed crystal height. [Claim 8] The seed crystal assembly according to claim 1, wherein the ratio of the base height to the seed crystal height is in the range of 1.0 to 2.

0. [Claim 9] The seed crystal assembly according to claim 1, wherein the specific thermal conductivity of the base structure differs from the specific thermal conductivity of the seed crystal by 20% or less, preferably 15% or less, at a growth temperature between 2000°C and 2600°C. [Claim 10] The seed crystal assembly according to claim 1, wherein the back surface of the base structure opposite to the seed crystal includes a passivation layer comprising at least one of a carbon layer, a refractory metal layer, and a graphite layer. [Claim 11] The seed crystal assembly according to claim 1, formed by at least two joined components. [Claim 12] The seed crystal assembly according to claim 1, formed as a single integrated component. [Claim 13] The seed crystal assembly according to claim 12, which is produced by vertical erosion or grinding. [Claim 14] A method for growing at least one SiC single crystal boule in a physical vapor transport (PVT) growth system, The steps include placing the SiC powder raw material in the raw material compartment, A step of placing at least one seed crystal assembly according to any one of claims 1 to 13 within a growth compartment, wherein the seed crystal includes a SiC seed crystal, and the raw material compartment is connected to the growth compartment to supply sublimated gaseous components to the growth compartment. A method comprising the steps of applying a high temperature between 2000°C and 2600°C to generate the sublimated gaseous components that generate the SiC growth phase in the SiC seed crystal, wherein a SiC volume single crystal boule is formed in the SiC seed crystal. [Claim 15] The method according to claim 14, wherein the seed crystal assembly is attached to the growth device by a free-suspension support, or the seed crystal assembly is attached to the growth device by a clamp-fastening bearing.

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