PRODUCTION METHOD FOR BULK SiC CRYSTAL

Abstract

To provide a method that makes it possible to produce a bulk SiC single crystal with fewer stresses.SOLUTION: A method for producing a bulk SiC single crystal by sublimation growth is provided. On a wafer-shaped single crystalline SiC seed crystal 2, an internal initial stress measurement is carried out, and the seed crystal has a wafer front side 4 with a surface 3 intended for SiC growth, a wafer rear side 5, and a crystal longitudinal mid-axis 6 extending in an axial direction, and a radial direction is oriented perpendicularly to the axial direction. The SiC seed crystal is classified, using the stress measurement result, into a first class when the initial stresses are below a first stress boundary value, into a second class when the initial stresses are from the first stress boundary value to a second stress boundary value, and into a third class when the initial stresses exceed the second stress boundary value. Only when the SiC seed crystal has been classified into the first class or the second class, the sublimation growth is carried out with the SiC seed crystal, and when it is classified into the second class, at least one stress-reducing measure is carried out.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing at least one bulk SiC crystal by sublimation growth. [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 U.S. Patent No. 5,627,493. 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, and they precipitate from the SiC growth vapor onto the SiC seed crystal, resulting in the growth 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] A further aspect that can reduce the quality of SiC substrates is stress within the material. Stress present within a grown bulk SiC crystal can specifically lead to the formation of crystalline defects (e.g., dislocations, in particular), breakage during further processing, or deformation (see above) in SiC substrates made from such bulk SiC crystals. This significantly impairs the economical manufacturability of SiC substrates. Furthermore, SiC substrates with such stress-induced deformations can only be used for limited alternative uses.

[0007] US Patent No. 5,949,999 describes the measurement of these stresses in SiC substrates in order to estimate the warpage that the SiC substrate will exhibit during further processing. Suitable measurement methods for this purpose are based on the detection and evaluation of Raman scattering or Raman shift in the SiC crystal lattice or the detection and evaluation of X-ray diffraction in the SiC crystal lattice.

[0008] Unwanted stresses can occur during SiC crystal growth due to the temperature gradients required for crystal growth. These thermal effects are created, on the one hand, by the need for a transfer force for material transport (i.e., the temperature difference from the SiC source material to the SiC seed crystal) and, on the other hand, by the fact that the growth boundary (i.e., the growth surface) of the growing bulk SiC single crystal must have a certain convex curvature to prevent the proliferation of defects from the radial edge region toward the center of the crystal. However, stresses in bulk SiC single crystals can also be caused by forces that arise between the crucible wall and the growing bulk SiC single crystal during cooling after the actual growth process, for example, due to the different cooling behavior of the growth crucible and the bulk SiC single crystal. Thus, in this case, wall effects are the culprit. Wall effects and thermal effects that cause stress in the material are triggered or determined by process parameters during SiC growth. A proper structure of the growth apparatus and / or appropriate process guidance can at least to some extent counteract thermal and wall effects as sources of stress.

[0009] Patent document 3 describes that the temperature inside the SiC seed crystal is made uniform. Thus, temperature differences and therefore internal stresses inside the SiC seed crystal resulting from thermal effects are reduced. For this purpose, a multi-part heat insulator containing a movable adapting element is arranged on the rear side of the SiC seed crystal. The movement of the adapting element means that the 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] US Patent No. 5,949,999 describes how, in order to avoid stresses caused by the wall effect, a two-layer insert with porous graphite in the inner layer and hard graphite in the outer layer is introduced between the inner wall of the growth crucible and the SiC seed crystal and bulk SiC single crystal growing therein.

[0011] Furthermore, in most SiC growth processes, the SiC seed crystal is securely bonded to the structure used for growth. Such a firm adhesive bond of the SiC seed crystal to a crucible cover, which also functions as a seed holder, is described, for example, in U.S. Pat. No. 6,275,999. Additionally, another embodiment is disclosed in which the SiC seed crystal is fastened to the crucible cover by lateral mechanical holders. The SiC seed crystal only loosely rests on the lateral holders and the crucible cover. U.S. Pat. No. 6,275,999 describes the use of a crucible cover made of polycrystalline SiC. The SiC seed crystal is attached by a firm adhesive bond to this crucible cover, which again functions as a seed holder. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] US8865324B2 [Patent Document 2] EP3048641A1 [Patent Document 3] CN218860954U [Patent Document 4] CN116334748A [Patent Document 5] US2011 / 0229719A1 [Patent Document 6] CN110306239A Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention is to provide a method of the type described in the introduction, which makes it possible to produce bulk SiC single crystals with less stress than known methods. [Means for solving the problem]

[0014] This object is achieved by a method according to the features of claim 1. In the method according to the invention, stress measurements for detecting internal physical initial seed stresses are carried out on wafer-shaped monocrystalline SiC seeds, the SiC seeds having a front side (particularly a rear side opposite the front side) with a growth surface intended for growing a bulk SiC single crystal, and a crystal longitudinal medial axis extending in the axial direction, with the radial direction oriented perpendicular to the axial direction. Furthermore, the stress measurements are used to classify the SiC seeds, so that the SiC seeds are classified into a first class if the detected initial seed stress is lower than a first stress boundary value, into a second class if the detected initial seed stress is between the first and second stress boundary values, or into a third class if the detected initial seed stress exceeds the second stress boundary value. Furthermore, only when the SiC seed crystal is classified into the first or second class is actual sublimation growth performed using the SiC seed crystal, in which a bulk SiC single crystal grows on the SiC seed crystal, and when classified into the second class, at least one stress reduction measure is performed.

[0015] The term "sublimation growth" is to be understood here generally, in particular it includes steps that pre- and / or post-treating the actual growth process, also referred to herein as actual sublimation growth.

[0016] Stress measurements to detect the internal physical initial seed stress can be performed on wafer-shaped monocrystalline SiC seeds, especially when they are not configured or installed as a component of a seed unit, or even when they are fixedly installed. Furthermore, the seed unit can include, in particular, a seed holder and a connection medium, such as an adhesive layer, between the SiC seed and the seed holder. A seed unit is also referred to as a seed system. During measurements on an uninstalled SiC seed, the measured initial seed stress is the stress generated in the microstructure of the SiC seed during its manufacture. During measurements on a SiC seed installed in a fixed seed unit, the measured initial seed stress consists of two stress components: on the one hand, as a first stress component, the stress generated in the crystalline microstructure of the SiC seed during its manufacture, and on the other hand, a second stress component resulting from the different expansion coefficients of the SiC seed and other components of the seed unit, e.g., the seed holder, and the connection medium.

[0017] The at least one stress reduction measure implemented in the case of classification into the second class is in particular an additional measure that is not normally implemented in the case of classification into the first class, for example.

[0018] The wafer-shaped SiC seed crystal in particular has a substantially cylindrical geometric shape, and the peripheral edge surface of the SiC seed crystal in particular essentially has the shape of the cylindrical outer surface.

[0019] 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.

[0020] It has been determined that the unwanted stress in the bulk SiC single crystal can be caused by an initial seed stress present before the start of growth in a seed unit (=seed system) formed in or with the SiC seed crystal, and transferred from the SiC seed crystal or seed unit to the bulk SiC single crystal growing thereon. Thus, the seed effect is the cause in this case.

[0021] Stresses in the growing bulk SiC single crystal due to such a seed effect are more difficult to influence other stresses. Furthermore, the initial seed stress from the SiC seed crystal or seed unit during the growth process has a much greater influence on the growing bulk SiC single crystal than is known from other growth processes used for the production of other technologically important crystals. For SiC growth, SiC seed crystals with diameters comparable to the bulk SiC single crystals to be grown therein are typically used. During SiC growth, only a small increase in diameter, if any, can be achieved, which distinguishes SiC growth from narrow-neck crystal growth methods used, for example, in the growth of pure silicon (Si) crystals.

[0022] In order to be able to counteract the influence of the initial seed stresses from the SiC seed crystal or seed unit on the stress balance in the growing bulk SiC single crystal, these initial seed stresses must be known, and for that reason they are measured in the method according to the present invention.

[0023] The findings from this stress measurement, which is performed specifically and individually for each SiC seed crystal, are then used to first determine whether the tested SiC seed crystal is suitable for growing bulk SiC single crystals (=class 1 and 2), and if so, whether stress reduction measures should be implemented for this purpose (=class 2) or not (=class 1). If the tested SiC seed crystal falls into the second class due to its stress balance and therefore requires stress reduction measures, the growth process is adapted to the stress conditions present in the SiC seed crystal to react to the initial seed stress during growth. In this way, the initial seed stress is weakened, and very low-stress bulk SiC single crystals can be grown. In the case of low-stress SiC seeds classified into the first class, it is possible to grow such very low-stress bulk SiC single crystals without using stress reduction measures.

[0024] The method according to the present invention makes it possible for the first time to individually determine the initial seed stress in each SiC seed crystal and, if necessary, to react to it using appropriate customized stress reduction measures during SiC crystal growth. In this way, bulk single crystal stress in the growing bulk SiC single crystal can be significantly reduced, which also leads to a definite improvement in the quality of the SiC substrate obtained from the bulk SiC single crystal produced in this way. The substrate quality is improved directly through the greatly reduced propagation of stress from the SiC seed crystal.

[0025] Advantageous embodiments of the method according to the invention are evident, inter alia, from the features of the claims dependent on claim 1.

[0026] In an advantageous embodiment, stress measurements are used to determine the particularly complete and / or particularly relative stress distribution of the initial seed stress in the SiC seed crystal, thus providing very accurate knowledge of the stress conditions in the SiC seed crystal, so that decisions regarding the further use of the examined SiC seed crystal can be made in a well-founded manner.

[0027] According to a further advantageous embodiment, a stress difference measured between the center of the SiC seed crystal arranged on the crystal's longitudinal medial axis and the radial edge region of the SiC seed crystal, the stress difference being in the range of 5 MPa to 15 MPa, in particular in the range of 7.5 MPa to 12.5 MPa, and preferably 10 MPa, is used as the first stress boundary value. According to a further advantageous embodiment, a stress difference measured between the center of the SiC seed crystal arranged on the crystal's longitudinal medial axis and the radial edge region of the SiC seed crystal, the stress difference being in the range of 400 MPa to 600 MPa, in particular in the range of 450 MPa to 550 MPa, and preferably 500 MPa, is used as the second stress boundary value. In particular, to determine the stress difference, a first differential stress measurement is performed at the center of the SiC seed crystal and a second differential stress measurement is performed at the radial edge region. In particular, the second differential stress measurement is performed in a radial edge region of an edge measurement location radially spaced apart from a sidewall laterally defining the SiC seed crystal or from a lateral edge of the SiC seed crystal by an edge distance, the edge distance having a value between 0.5 mm and 6 mm, preferably a value between 1 mm and 5 mm, preferably a value between 1 mm and 3 mm or a value of 5 mm.

[0028] According to a further advantageous embodiment, stress measurements on the SiC seed crystal are carried out by X-ray diffraction measurements, neutron diffraction measurements, Raman shift measurements, or photoelastic measurements. In particular, any combination of these measurements is possible. By these measurement methods, the initial seed stress in the material microstructure of the SiC seed crystal can be determined in a non-destructive, non-contaminating, and very accurate manner. Equivalent results are not possible using other methods, such as computer simulation of the growth equipment used. At best, these other methods provide an unsatisfactory, rough estimate of the stress conditions within the SiC seed crystal.

[0029] According to a further advantageous embodiment, at least one stress reduction means comprises contacting the rear side of the SiC seed wafer with a plurality of stress reduction elements, the stress reduction elements having different levels of thermal conductivity. In particular, the stress reduction elements are in direct contact with the SiC seed wafer, which is preferably done in the case of a SiC seed wafer whose rear side is freely accessible. Alternatively, however, the rear side of the SiC seed wafer may only be indirectly contacted with the stress reduction elements, which is preferably done when another component already rests directly on the rear side of the SiC seed wafer, such as, for example, when the SiC seed is bonded to or installed in a seed unit.

[0030] According to a further advantageous embodiment, the at least one stress reduction means comprises the use of an apparatus for sublimation growth, the apparatus including at least one transverse cavity in a region axially adjacent to the rear side of the SiC seed wafer introduced into the apparatus. The transverse cavity is preferably arranged directly adjacent to the rear side of the SiC seed wafer, thereby making it possible to particularly effectively and efficiently influence the seed stress. The transverse cavity is preferably arranged in the axial front end wall of a growth crucible used for sublimation growth as a component of the apparatus. This results in a very compact yet efficient structure, the outer and inner contours and dimensions of which do not differ substantially from those of a SiC growth apparatus without the stress reduction element.

[0031] According to a further advantageous embodiment, the at least one stress reduction means comprises an apparatus having a growth crucible used for sublimation growth, the axial front end wall of which, arranged adjacent to the SiC seed crystal introduced into the apparatus, has at least one, in particular outer, recess, again resulting in a very compact and efficient structure.

[0032] According to a further advantageous embodiment, the at least one stress reduction means comprises an apparatus having a growth crucible used for sublimation growth, the side wall of which has at least one longitudinal cavity surrounding the SiC seed crystal and the crystal growth region, in particular tangentially or concentrically with the crystal longitudinal medial axis. Again, this results in a very compact and efficient structure, the outer and inner contours and dimensions of which do not differ substantially from those of a SiC growth apparatus without the stress reduction element.

[0033] According to a further advantageous embodiment, at least one stress reduction means comprises coating the rear side of the SiC seed wafer with a rear layer component, in particular a single layer or a multilayer. The rear layer component may in particular only have a single layer or, preferably, may consist of several individual layers or monolayers, which may be arranged radially adjacent to one another and / or axially overlapping one another. In particular, the rear layer component completely covers the SiC seed. Preferably, at least two mutually different layer materials, in particular with different levels of thermal conductivity, are provided for the rear layer component. In this way, a particularly good stress reduction effect and a very good level of compensation for the seed stress initially contained in the SiC seed can be achieved.

[0034] According to further advantageous embodiments, any combination of the advantageous stress reduction means described above may be used.

[0035] According to a further advantageous embodiment, the internal physical bulk single-crystal stress, particularly the complete stress distribution, in the produced bulk SiC single crystal can be determined, particularly after determining the actual sublimation growth. Determining the complete stress distribution is also particularly called stress mapping. Preferably, the determined stress distribution can be used to determine the further use of the bulk SiC single crystal. Preferably, the stress distribution of the bulk single-crystal stress is determined by X-ray diffraction measurement, neutron diffraction measurement, Raman shift measurement, or photoelastic measurement. In particular, any combination of these measurement methods is possible.

[0036] 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]

[0037] [Figure 1] 1 shows a schematic diagram of an exemplary embodiment of a measurement device for detecting seed stress in a wafer-like SiC seed crystal. [Figure 2] 2 shows a plan view of the wafer front side of a SiC seed crystal inspected using the measurement technique according to FIG. 1; [Figure 3] 3 shows the stress distribution determined for the SiC seed crystal tested according to FIGS. [Figure 4] 2 shows an exemplary embodiment of a growth apparatus for growing bulk SiC single crystals on SiC seed crystals inspected using the measurement technique according to FIG. 1; [Figure 5] 2 shows an exemplary embodiment of a growth apparatus for growing bulk SiC single crystals on SiC seed crystals inspected using the measurement technique according to FIG. 1; [Figure 6] 2 shows an exemplary embodiment of a growth apparatus for growing bulk SiC single crystals on SiC seed crystals inspected using the measurement technique according to FIG. 1; [Figure 7] 2 shows an exemplary embodiment of a growth apparatus for growing bulk SiC single crystals on SiC seed crystals inspected using the measurement technique according to FIG. 1; [Figure 8] 2 shows an exemplary embodiment of a growth apparatus for growing bulk SiC single crystals on SiC seed crystals inspected using the measurement technique according to FIG. 1; [Figure 9] 2 shows an exemplary embodiment of a growth apparatus for growing bulk SiC single crystals on SiC seed crystals inspected using the measurement technique according to FIG. 1; DETAILED DESCRIPTION OF THE INVENTION

[0038] Corresponding parts are provided with the same reference numerals in Figures 1 to 9. Details of the exemplary embodiments described in more detail below may also independently constitute the invention or may be part of the subject matter of the invention.

[0039] FIG. 1 is a highly schematic illustration of an exemplary embodiment of a measurement device 1 for inspecting a single-crystal, wafer-like SiC seed crystal 2 .

[0040] A SiC seed crystal 2 intended for growing a bulk SiC single crystal is shown in side view in Figure 1 and in plan view in Figure 2. The SiC seed crystal 2 has a growth surface for growing the bulk SiC single crystal to be grown, which surface is located on a wafer front side 4 of the SiC seed crystal 2. Furthermore, the SiC seed crystal 2 has a wafer back side 5 opposite the wafer front side 4.

[0041] Furthermore, the SiC seed crystal 2 has a central crystal longitudinal medial axis 6, which coincides with the central axis of symmetry of the SiC seed crystal 2, particularly 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.

[0042] The measuring device 1 shown in FIG. 1 is intended to detect seed stresses that are initially present in the crystalline microstructure of a SiC seed crystal 2, i.e., before the start of the actual SiC growth process. In the exemplary embodiment shown in FIG. 1, the SiC seed crystal 2 to be tested is not installed. In another exemplary embodiment, the SiC seed crystal 2 may be installed during testing using the measurement technique, e.g., as a component of a seed unit. It is then fixedly connected, in particular, to other components of the seed unit in question. The seed stresses detected in the case of such an installed SiC seed crystal 2 consist of two stress components. The first stress component relates to the already mentioned stresses that exist in the crystalline microstructure of the SiC seed crystal 2 due to its manufacturing reasons. The second stress component results from different thermal expansion coefficients of the different installed components of the seed unit in question.

[0043] The measurement device 1 comprises a control / evaluation device 7, a transmitter device 8 and a receiver device 9. After appropriate activation by the control / evaluation device 7, the transmitter device 8 directs an interrogation signal 10 to the SiC seed crystal 2 where it interacts with its stressed crystal microstructure. The interrogation signal 10 is affected by this interaction and contains information about the seed stress present in the SiC seed crystal 2. It then reaches the receiver device 9 as a response signal 11, where it is detected. An evaluation is then performed by the control / evaluation device 7, which is connected to both the transmitter device 8 and the receiver device 9.

[0044] Furthermore, the measuring device 1 is designed to determine the complete stress distribution of the seed stress in the SiC seed crystal 2. For this purpose, an interrogation signal 10 is directed to various positions of the growth surface 3 on the wafer front side 4 of the SiC seed crystal 2. In the case of an alternative inspection method, the interrogation signal 10 is also directed to the wafer rear side 5 on the SiC seed crystal 2. After the end of the inspection sensing the growth surface 3, a complete picture of the stress distribution in the SiC seed crystal 2 is obtained in the control / evaluation device 7. With this, conclusions can be drawn as to how the SiC seed crystal 2 can continue to be used. For this purpose, two stress boundary values ​​σ G1 and σ G2 The classification is performed using the first stress boundary value σ G1 When the seed stress is lower than the first stress boundary value σ, the SiC seed crystal 2 is classified into the first class. G1 to the second stress boundary value σ G2 When the detected seed stress is equal to or less than the second stress boundary value σ, the classification into the second class is performed. G2 , the SiC seed crystal 2 is assigned to the third class.

[0045] Two stress boundary values ​​σ G1 and σ G2and are each formed as the difference between two detected stress values. The first stress value is determined at the center of the crystal longitudinal medial axis 6 of the SiC seed crystal 2, and the second stress value is determined in the radial edge region 12 (shown by hatching in FIG. 2 ). The radial edge region 12 does not directly contact the lateral peripheral edge 13 of the SiC seed crystal 2, but rather begins at a further radial distance from this lateral peripheral edge 13. The second stress value is detected at an edge measurement location 12a in the radial edge region 12. Its radial edge distance x from the lateral peripheral edge 13 is in particular in the range of 1 mm to 5 mm. The radial extent Δx of the radial edge region 12 is therefore 4 mm. In the illustrated exemplary embodiment, the second stress value required for the stress difference is detected at an edge distance x of approximately 3 mm. In the exemplary embodiment, the stress boundary value thus defined for the classification of the SiC seed crystal is the first stress boundary value σ G1 The second stress boundary value σ G2 The detected seed stress and the two stress boundary values ​​σ G1 and σ G2 For comparison, a difference is formed between the seed stress determined in the center of the SiC seed crystal 2 examined at the position of the crystal longitudinal medial axis 6 and the second stress detected in the radial edge region 12 at the edge measurement position 12 a at an edge distance x of 3 mm from the lateral peripheral edge 13 in the case of the illustrated exemplary embodiment, resulting in two stress boundary values ​​σ G1 and σ G2 It is compared to.

[0046] The further use of the SiC seed crystal 2 inspected in this way is governed by classification. If classified into the first class, the inspected SiC seed crystal 2 has only a very small initial seed stress. It can be used without additional measures to grow a low-stress bulk SiC single crystal. If classified into the second class, the SiC seed crystal 2 has a large initial seed stress, but if additional stress-reducing measures are taken to weaken the initial seed stress during SiC crystal growth, these stresses still allow the SiC seed crystal 2 to be used for growing a bulk SiC single crystal, and in this respect, allow the growth of a low-stress bulk SiC single crystal. If classified into the third class, the SiC seed crystal 2 has too much initial seed stress, and therefore it cannot be used to grow a bulk SiC single crystal. Instead, it is rejected (rejected).

[0047] The measurement principle on which the measurement device 1 is based can vary. Stress measurements on the SiC seed crystal 2 can be carried out in particular by means of Raman shift measurements, photoelastic measurements, X-ray diffraction measurements and / or neutron diffraction measurements.

[0048] The stress measurement by measuring the Raman shift will be described below.

[0049] Due to the strong Si-C covalent bond, the effect on the Raman shift caused by stress is very small, for which reason absolute measurement of stress is difficult. Therefore, relative measurements are used, in which the Raman shift value is determined with respect to a reference position (typically the center of the growth surface 3) by means of a dot matrix covering the entire growth surface 3 of the SiC seed crystal 2 being examined. Stress can also be estimated from the following equation:

[0050]

number

[0051] where ψ denotes the stress-stretch-frequency shift coefficient of the material, Δω denotes the frequency shift increment, and σ denotes the stress.

[0052] Stress measurement using photoelasticity measurements will be described below.

[0053] The optical properties are also affected by the effect of stress on the atomic arrangement in the crystalline structure of the material. A ray of light passing through the material is split into two rays with different propagation speeds; thus, in the case of polarized light, a phase delay occurs, from which conclusions can be drawn regarding the stress in the material. The following equation is applicable:

[0054]

number

[0055] where Δ denotes a phase delay, C denotes a stress optical coefficient, t denotes a thickness of the sample, λ denotes an optical wavelength in a vacuum, and λ1 and λ2 denote the first and second principal stresses, respectively.

[0056] Stress measurement using X-ray diffraction measurement and neutron diffraction measurement will be explained below.

[0057] The effect of stress on the atomic arrangement in the crystal structure also influences and can be adjusted for the results of diffraction experiments. Due to stress, the lattice spacing of the crystal structure is altered, which can be measured by X-ray or neutron diffraction experiments. It is possible to draw conclusions regarding the stress and the direction of the stress in the material. The following equation is applicable to this:

[0058]

number

[0059] where ε hkl indicates the observed distortion of the diffraction plane, and d hkl indicates the actual lattice spacing, and d hkl,0 denotes the ideal strain-free lattice spacing, E denotes the elastic modulus, and σ hkl indicates stress.

[0060] After determining the complete stress distribution (=stress mapping) within the SiC seed crystal 2 to be examined, for example using one of the methods described above, the wafer-shaped SiC seed crystal 2 can be divided into suitable zones with various stress value ranges that are generally radially symmetric due to the conditions prevailing in SiC crystal growth (see left image in Figure 3).

[0061] As already described, the SiC seed crystals 2 classified into the second class are used for growing bulk SiC single crystals, but additional measures (see the image on the right of Figure 3 and Figures 4 to 9) are taken to reduce stress during the actual SiC growth process.

[0062] 3 shows, in the left image, a stress zone diagram of the inspected SiC seed crystal 2 derived from the determined stress distribution, where the seed crystal is classified as class 2. According to the stress zone diagram, the SiC seed crystal 2 includes an inner central region 14 with small strain and initial seed stress σ1, a transition region 15 with a somewhat larger initial seed stress σ2, and a highly strained edge region 16 with a relatively high initial seed stress σ3. The three regions 14, 15, and 16 are arranged radially symmetrically and concentrically with respect to each other. The seed stresses σ1, σ2, and σ3 may also be stress derivative values, and in each case the centrally determined values ​​are used as reference values ​​for subtraction.

[0063] The stress distribution of the examined SiC seed crystal 2, which has a more strongly distorted edge region 16, as shown schematically in the left image of FIG. 3, is primarily due to the convexly curved isotherms used during the manufacture of the SiC seed crystal 2 in question. To weaken the initial seed stresses σ1, σ2, and σ3 during the SiC sublimation growth of a bulk SiC single crystal grown on this SiC seed crystal 2, heat removal from the SiC seed crystal 2 in the strongly distorted edge region 16 is increased relative to the central region 14, which has a smaller strain. In this way, the curvature of the isotherms is locally reduced during the SiC sublimation growth of the bulk SiC single crystal. This is achieved, for example, by the use of stress reduction elements 17 and 18, shown in the right image of FIG. 3. The stress reduction elements 17 and 18 have different levels of thermal conductivity and are in direct contact with the SiC seed crystal 2 on the wafer rear side 5. The two stress reduction elements 17 and 18 are designed to be radially symmetrical to each other and are arranged concentrically. The radius r, which determines the boundary between the central stress reduction element 17 and the contacting outer annular stress reduction element 18, is selected to determine the complete distribution of the initial seed stress within the SiC seed crystal 2. In this way, the growth conditions during the SiC sublimation growth of the bulk SiC single crystal are individually adapted to the stress conditions prevailing within the SiC seed crystal 2 used for this purpose. In the illustrated exemplary embodiment, the inner stress reduction element 17 has a lower level of thermal conductivity than the outer stress reduction element 18. Due to the additionally provided stress reduction means, the bulk SiC single crystal grown in this way has only very low internal stresses. In this respect, very high-quality SiC substrates and, as well as high-quality SiC components, can be produced therefrom with high productivity.

[0064] FIG. 4 shows an exemplary embodiment of a growth apparatus 19 for producing bulk SiC single crystals (not shown) using a SiC seed crystal 2 using sublimation growth and tested using the measurement technique according to FIG.

[0065] Stress reducing elements 17 and 18 according to Figure 3 are used in a growth apparatus 19. They are loosely mounted on the wafer rear side 5 of a SiC seed crystal 2 held in a growth crucible 20 by a seed holder 21.

[0066] Growth crucible 20 includes a SiC storage region 22 and a crystal growth region 23. SiC storage region 22 contains SiC source material 24, for example, powder.

[0067] The growth crucible 20 has a crucible vessel 25 and a crucible cover 26. The growth crucible 20 has a first axial front end wall 27 disposed adjacent the SiC storage region 22 and an opposite second axial front end wall 28 formed by the crucible cover 26. The growth crucible 20 further has a tangential peripheral side wall 29 which, like the first axial front end wall 27, is a component of the crucible vessel 25. The SiC seed crystal 2 is positioned in the growth crucible 20 by a seed holder 21 such that the wafer front side 4 of the SiC seed crystal 2 is located in the crystal growth region 23 with the growth surface 3. 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 21.

[0068] The growth crucible 20 is made of an electrically and thermally conductive graphite crucible material. A thermal insulator (not shown in FIG. 4) 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 20. The growth crucible 20 is heated by this heater to the high temperatures required for growth, above 2100°C.

[0069] The SiC growth gas phase in the crystal growth region 23 is supplied by means of a SiC source material 24. 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 24 to the growth surface 3 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 3. 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 20 from the SiC source material 24 to the crucible cover 26, creating the aforementioned axial temperature gradient.

[0070] FIG. 5 shows a further exemplary embodiment of a growth apparatus 30 for growing bulk SiC single crystals (again not shown), in which a SiC seed crystal 2 inspected using the measurement technique according to FIG. 1 and thus classified as belonging to the second class is used. Unlike the growth apparatus 19 according to FIG. 4, in the case of the growth apparatus 30, a further structure for mounting the SiC seed crystal 2 in the growth crucible 20 is provided as a stress reduction measure. In this exemplary embodiment, the SiC seed crystal 2 is firmly fixed to the crucible cover 26 by a spacer ring 31. The spacer ring 31 is glued to the SiC seed crystal 2 at the wafer rear side 5. A further adhesive connection exists between the spacer ring 31 and the crucible cover 26, thereby forming an entirely tightly bonded seed unit 32 including the SiC seed crystal 2, the spacer ring 31, and the crucible cover 26 as components. The spacer ring 31 is arranged concentrically with the crystal longitudinal mid-axis 6 and surrounds a central transverse cavity 33 axially adjacent to the wafer rear side 5 of the SiC seed crystal 2. This transverse cavity 33 is empty. Process gases can collect there during SiC growth. However, the transverse cavity 33 has a different level of thermal conductivity than the spacer ring 31, which may be made of, for example, a graphite material, and thus provides a stress reduction effect similar to that of the growth apparatus 19 due to the stress reduction elements 17 and 18. Furthermore, the transverse cavity 33 also has an inherent stress reduction effect. Overall, the initial seed stress present in the SiC seed crystal 2 mounted in the seed unit 32 is at least largely compensated for during the SiC sublimation growth of the bulk SiC single crystal and preferably is not transferred to the growing bulk SiC single crystal. Again, the result is a bulk SiC single crystal with very low stress.

[0071] FIG. 6 shows a further exemplary embodiment of a growth apparatus 34 for growing bulk SiC single crystals (again not shown), in which the SiC seed crystal 2 inspected by the measuring device 1 is directly coated on the wafer rear side 5 with a rear layer component 35 as a stress-reducing measure to reduce the initial seed stress detected in the crystalline microstructure of the SiC seed crystal 2. The rear layer component 35 has two layers. It has two layers or individual layers 36 and 37 arranged radially next to each other, both of which are arranged concentrically with the crystal longitudinal medial axis 6. Both individual layers 36 and 37 consist of different layer materials with different levels of thermal conductivity, which results in a stress-reducing effect as a whole similar to that of the growth apparatuses 19 and 30. The SiC seed crystal 2 and the rear layer component 35, which is directly adjacent to the wafer rear side 5, form a seed unit 38 that rests loosely on the annular seed holder 21 with the wafer front side 4 of the SiC seed crystal 2 in the region of its peripheral edge.

[0072] A transverse cavity 39 is located on the rear side of the seed unit 38, away from the SiC storage region 22. It is located between the crucible cover 26 and the rear layer part 35 of the seed unit 38. This transverse cavity 39 constitutes a further stress reduction means for the growth apparatus 34. In this respect, it is advantageous, but merely optional. There is another exemplary embodiment that does not have such a transverse cavity 39 between the crucible cover 26 and the seed unit 38.

[0073] 7 shows a further exemplary embodiment of a growth apparatus 40 for sublimation growth of bulk SiC single crystals (again not shown), in which a SiC seed crystal 2 classified into a second class according to the determination of stress distribution using measurement techniques is directly bonded to the inner cover 41 of a crucible cover 42. This tightly bonded unit consisting of the SiC seed crystal 2 and the crucible cover 42 forms a seed unit 43 which includes a recess 45 as a stress reducing means in the outer cover 44 of the crucible cover 42.

[0074] FIG. 8 shows a further exemplary embodiment of a growth apparatus 46 for sublimation growth of bulk SiC single crystals (again not shown), in which a SiC seed crystal 2 classified as a second class according to the determination of stress distribution using measurement techniques is glued to the inner cover 41 of a crucible cover 47. The tightly bonded unit consisting of the SiC seed crystal 2 and the crucible cover 47 forms a seed unit 47a. The growth apparatus 46 differs from the growth apparatus 40 according to FIG. 7 essentially by the design of the crucible cover 47. It has in its interior a transverse cavity 48 of approximately cylindrical shape, completely embedded in the growth apparatus 40, which replaces the recess 45 of the growth apparatus 40. Both the internal transverse cavity 48 of the growth apparatus 46 and the recess 45 of the growth apparatus 40 are stress reducing means, which in this respect weaken the initial seed stress detected in the SiC seed crystal 2.

[0075] 9 shows a further exemplary embodiment of a growth apparatus 49 for sublimation growth of bulk SiC single crystals (again not shown), which includes a seed unit 50 consisting of a crucible cover 51 and a SiC seed crystal 2 directly bonded to the inside of the cover 41. The SiC seed crystal 2 is classified into a second class according to the stress distribution determined using measurement techniques, and the growth apparatus 49 therefore has stress reduction means for compensating for the initial seed stress contained in the SiC seed crystal 2. This is the case for a crucible vessel 52 which includes in its side wall 53 a longitudinal cavity 54 completely embedded therein. The longitudinal cavity 54 extends axially and is axially disposed within the side wall 53 so as to tangentially surround the SiC seed crystal 2 and the crystal growth region 23.

[0076] After the growth process is completed, the bulk SiC single crystals produced by the growth apparatuses 19, 30, 34, 40, 46, and 49 may be further tested, particularly using stress measurement techniques. The same measurement methods as those used to measure the stress of the SiC seed crystal 2, i.e., Raman shift measurements, photoelastic measurements, X-ray diffraction measurements, and / or neutron diffraction measurements, can be used. Thus, a complete distribution of the bulk single crystal stress contained within the grown bulk SiC single crystal is obtained. Based on this, a decision can be made regarding further use. Preferably, particularly low-stress, otherwise high-quality bulk SiC single crystals can be used to produce new SiC seed crystals. However, the bulk SiC single crystals can, of course, alternatively be used to produce SiC substrates for component manufacturing.

[0077] Overall, the sublimation growth methods or growth apparatuses 19, 30, 34, 40, 46 and 49 described above, with appropriate stress reduction measures, allow for the production of very low stress bulk SiC single crystals. [Explanation of symbols]

[0078] 2 SiC seed crystal 3 Growth surface 4 Front side of wafer 5 Wafer rear side 6 Crystal Longitudinal Medial Axis 12 Radial Edge Region 17,18 Stress reduction elements 23 Crystal growth region 30;34;46 Growth equipment 33;39;48 Transverse Cavity 35 Rear layer parts

Claims

1. 1. A method for producing bulk SiC single crystals by sublimation growth, comprising: a) performing stress measurements to detect internal physical initial seed stresses on a wafer-shaped monocrystalline SiC seed crystal (2), the SiC seed crystal (2) having a wafer front side (4) with a growth surface (3) intended for the growth of the bulk SiC single crystal to be grown, a wafer rear side (5) and a crystal longitudinal medial axis (6) extending in an axial direction, with the radial direction oriented perpendicular to the axial direction; b) using the stress measurement, classifying the SiC seed crystal (2), which is b1) being classified into a first class when the detected initial seed stress is lower than a first stress boundary value; b2) when the detected initial seed stress is between the first stress boundary value and the second stress boundary value, the stress is classified into a second class; b3) when the detected initial seed stress exceeds the second stress boundary value, the stress is classified into a third class; and c) performing actual sublimation growth using the SiC seed crystal (2), in which the bulk SiC single crystal grows on the SiC seed crystal (2), only when the SiC seed crystal (2) is classified into the first class or the second class; c1) A manufacturing method in which, if classified in said second class, at least one stress reduction measure is implemented.

2. 2. The method of claim 1, wherein stress measurements are used to determine the stress distribution of the initial seed stress within the SiC seed crystal (2).

3. 3. The method according to claim 1, wherein a stress difference measured between the center of the SiC seed crystal (2) arranged on the crystal longitudinal medial axis (6) and a radial edge region (12) of the SiC seed crystal (2) is used as a first stress boundary value, the stress difference being in the range of 5 MPa to 15 MPa.

4. 3. The method according to claim 1, wherein a stress difference measured between the center of the SiC seed crystal (2) arranged on the crystal longitudinal medial axis (6) and a radial edge region (12) of the SiC seed crystal (2) is used as a second stress boundary value, the stress difference being in the range of 400 MPa to 600 MPa.

5. 3. The method according to claim 1, wherein the stress measurement of the SiC seed crystal (2) is performed by X-ray diffraction measurement, neutron diffraction measurement, Raman shift measurement, or photoelastic measurement.

6. 3. The method of claim 1, wherein the at least one stress reduction means comprises contacting the wafer rear side (5) of the SiC seed crystal (2) with a plurality of stress reduction elements (17, 18), the stress reduction elements (17, 18) having different levels of thermal conductivity.

7. 3. The method according to claim 1, wherein the at least one stress reduction means comprises the use of an apparatus (30; 34; 46) for the sublimation growth, the apparatus comprising at least one transverse cavity (33; 39; 48) in a region axially adjacent to the wafer rear side (5) of the SiC seed crystal (2) introduced into the apparatus (30; 34; 46).

8. 8. The method of claim 7, wherein the transverse cavity (33) is positioned directly adjacent to the wafer rear side (5) of the SiC seed crystal (2).

9. 8. The method according to claim 7, characterized in that the transverse cavity (48) is arranged in the axial front end wall (28, 47) of a growth crucible (20) used for the sublimation growth as a component of the device (46).

10. 3. The method according to claim 1, wherein the at least one stress reducing means comprises an apparatus (40) having a growth crucible (20) used for the sublimation growth, the axial front end wall (28, 42) of the growth crucible arranged adjacent to the SiC seed crystal (2) introduced into the apparatus (40) having at least one recess (45).

11. 3. The method according to claim 1, wherein the at least one stress reducing means comprises an apparatus (40) having a growth crucible (20) used for the sublimation growth, the sidewall (53) of the growth crucible (20) surrounding the SiC seed crystal (2) and a crystal growth region (23) having at least one longitudinal cavity (54).

12. 3. The method according to claim 1, wherein the at least one stress reducing means comprises coating the wafer rear side (5) of the SiC seed crystal (2) with a rear layer component (35).

13. 13. The method according to claim 12, characterized in that the rear layer part (35) is provided with at least two mutually different layer materials having different levels of thermal conductivity.

14. 3. The method of claim 1, wherein a stress distribution of internal physical bulk single crystal stresses in the bulk SiC single crystal to be produced is determined, and the determined stress distribution is used to determine further use of the bulk SiC single crystal.

15. 15. The method of claim 14, wherein the stress distribution of the bulk single crystal stress is determined by X-ray diffraction measurement, neutron diffraction measurement, Raman shift measurement, or photoelasticity measurement.

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