DE102010029755A1 - Producing silicon carbide volume single crystal, useful for producing semiconductor device, comprises e.g. producing silicon carbide growth gas phase in crystal growth region of crucible, and growing silicon carbide volume single crystal - Google Patents

Abstract

Producing silicon carbide volume single crystals, comprises: (a) producing a silicon carbide growth gas phase in a crystal growth region (5) of a growing crucible (3), growing the silicon carbide volume single crystal (2) exhibiting a central middle longitudinal axis; (b) supplying silicon carbide growth gas phase at least partially from a silicon carbide source material (6), which is located in a storage area of the growing crucible, and at least one dopant; and (c) bounding the crystal growth region by silicon carbide-surface portions and carbon-surface portions. Producing silicon carbide volume single crystals, comprises: (a) producing a silicon carbide growth gas phase in a crystal growth region (5) of a growing crucible (3), and growing the silicon carbide volume single crystal (2) exhibiting a central middle longitudinal axis by deposition from a silicon carbide growth gas phase, where the deposition takes place at a growth boundary surface (16) of the silicon carbide volume single crystal; (b) supplying silicon carbide growth gas phase at least partially from a silicon carbide source material (6), which is located in a storage area of the growing crucible, and at least one dopant comprising nitrogen, aluminum, vanadium or boron; and (c) bounding the crystal growth area by silicon carbide-surface portions and carbon-surface portions, and selecting these surface portions so that a silicon carbide/carbon surface portion ratio, which is formed by dividing the surface proportion of silicon carbide surface portion by the carbon surface portion, always has a value of less than 1. An independent claim is also included for a single-crystal silicon carbide substrate comprising a substrate main surface and a substrate thickness, where the determined local specific electrical resistance for an arbitrary first 4 mm 2>large, preferably quadratic partial area of the substrate main surface and based on the substrate thickness differs by not > 4 momega cm from the local specific electrical resistance of an arbitrary particular second 4 mm 2>large, preferably quadratic partial area.

Description

The invention relates to a method for producing a SiC bulk single crystal and a monocrystalline SiC substrate.

Due to its outstanding physical, chemical, electrical and optical properties, the semiconductor material silicon carbide (SiC) is also used, inter alia, as a starting material for power electronic semiconductor components, for high-frequency components and for special light-emitting semiconductor components. For these components, SiC substrates (= SiC wafers) with the largest possible substrate diameter, the highest possible quality and the highest possible uniform electrical resistivity are required.

Basis for the SiC substrates are high-quality SiC bulk single crystals, which are usually produced by means of physical vapor deposition, in particular by means of a z. B. in the US 6,773,505 B2 and in the DE 199 31 332 C2 described sublimation process. From these bulk SiC crystals, the disk-shaped monocrystalline SiC substrates are cut out, which are then provided in the context of component manufacturing with at least one particular of SiC existing epitaxial layer. The quality and reliability of the components also depends, among other things, on how homogeneously distributed the local electrical resistance set by dopant addition during the sublimation growth is. Lateral variations in dopant concentration in the SiC substrate can lead to nonuniform component properties and even total component failure, depending on where in the SiC substrate that component is placed. Since the SiC substrates z. B. are involved because of a back-side contact in the active part of the components in the SiC substrate existing defects and / or inhomogeneities, such as doping inhomogeneities, ie fluctuations in the resistance distribution, the properties of the components. Their quality thus depends essentially on that of the grown SiC bulk single crystal and the SiC substrates obtained therefrom.

During conventional sublimation growth, different subregions are formed at the growth interface of the growing SiC bulk single crystal. In a central region or at least close to the center, there is a largely flat and smooth surface structure (= surface morphology) with a very large ratio of step depth (or width) to step height of the crystal growth stages. This facet region is surrounded by a transition region, followed by an edge region with a curved and rough surface structure with an approximately balanced ratio of step depth to step height of the crystal growth stages. The subsections mentioned also differ in some cases considerably in their respective dopant concentration and thus in their local electrical resistances. Since the edge region occupies the largest part of the growth interface, the cultivation conditions are currently selected such that the edge region has the desired dopant concentration. As a result, the facet region is over-doped, and components processed there may be of lower quality or, in the worst case, even unusable.

The effect of the facet formation and the uselessness of the facet region for the component production are e.g. B. also in the abstract to the JP 2008 290 895 A described.

The object of the invention is therefore to provide a method for producing a SiC bulk single crystal and a monocrystalline SiC substrate so that a better suitability for component manufacturing is given.

To solve the problem relating to the method, a method according to the features of claim 1 is given.

According to the invention, the previously unfavorable threefold division of the growth interface leading to an inhomogeneous lateral resistance distribution into a facet region, a transition region and an edge region is largely avoided. With the sublimation growth method according to the invention, a SiC bulk single crystal without a facet region or at least without a significant facet region is produced instead. This is achieved by creating similar or even the same growth conditions in the center around the center longitudinal axis as at the edge. In particular, the growth interface of the growing SiC bulk single crystal in the center has at least comparable step geometry of the crystal growth steps as in the peripheral region. This is especially true for the entire duration of SiC volume single crystal fabrication. In the center there is preferably also a rough surface structure with crystal growth stages whose step height is similar to their step depth. In particular, the step depth is at most five times as large as the step height. The ratio of step depth to step height is thus in the center, as well as everywhere else at the growth interface, preferably in the range between one and five. As a result, identical or at least similar conditions for the incorporation of the dopants are present everywhere at the growth interface, so that a substantially uniform dopant concentration occurs throughout the growing SiC bulk single crystal and as a consequence of which adjusts an equal or similar large local electrical resistivity practically everywhere. The distribution of resistance in the lateral (and preferably also in the axial) direction is very homogeneous and, above all, has no abrupt changes, as is the case with conventional SiC bulk single crystals at the lateral transition from the facet to the edge region.

In the method of the invention, the dopant addition to the SiC growth gas phase during growth will be adjusted to provide the desired dopant concentration and thus the desired resistivity substantially throughout the growing SiC bulk single crystal. Since, as explained above, no facet region is formed, the previous inefficient practice of eliminating the (to date too heavily doped) facet region for the production of high-quality components and leaving untapped substrate surface unnecessary makes it unnecessary.

In the context of the invention, it was also recognized that the advantageous elimination of the facet region can be achieved by means of a carbon excess set in the SiC growth gas phase. The excess carbon is given in particular during the entire duration of SiC bulk single crystal preparation and is preferably present directly at or before the growth interface. The higher proportion of carbon (C) in the SiC growth gas phase compared to silicon (Si) is achieved in accordance with the invention in that the crystal growth range is limited by a larger proportion of carbon surfaces than at SiC surfaces. These surfaces are to be understood generally.

They include not only the surfaces detectable by means of the geometrical external dimensions, but also "inner" surfaces with which the SiC growth gas phase can also come into contact or interact and which can be particularly relevant in particular in the case of a pulverulent or very porous material. Thus, in particular, the SiC area ratio comprises the interface of the preferably powdery SiC source material to the crystal growth area, and also the growth interface of the bulk SiC bulk single crystal. On the other hand, the carbon area fraction may, in particular, comprise the wall of the growth crucible, which preferably laterally delimits the crystal growth region, and preferably consists of a graphite material.

Overall, substantially facet-free SiC bulk single crystals can be produced with the cultivation method according to the invention, so that a largely homogeneous lateral distribution of the local electrical resistance is provided. Strong resistance fluctuations as in conventionally produced SiC bulk single crystals do not occur in a SiC bulk single crystal produced according to the invention. Thus, SiC bulk single crystals produced according to the invention are characterized by a higher quality and can be reused more efficiently, in particular for the production of semiconductor components.

In the embodiment according to claim 2, the carbon excess according to the invention in the SiC growth gas phase is set on the basis of structural measures which can be derived essentially from a geometrical observation of the partial areas bounding the crystal growth region. The SiC area fraction is formed by two approximately circular boundary surfaces, namely the surface of the SiC source material and the growth interface. The crystal diameter D of the growing SiC bulk single crystal can change in the course of growth, in particular increase. In the axial direction, the respective crystal diameter D can vary, for example, by about 20%. The upper limit for the crystal diameter D is given by the cross-sectional area of the crucible interior in the crystal growth region. Assuming a cylindrical crucible interior, its interior diameter is therefore the relevant geometry parameter both for the maximum cross-sectional area of the SiC bulk single crystal and for the surface of the SiC source material. Accordingly, the total surface area of the two circular SiC boundary surfaces is given by π · (D / 2) ² + · π · (D / 2) ² = 2 · π · (D / 2) 2, where D is the maximum crystal diameter and on the other hand, the crucible interior diameter is designated. The carbon area fraction is determined by the cylindrical inner wall of the crucible made of graphite material and is calculated as 2 · π · (D / 2) · L, where D denotes the crucible interior diameter and L denotes a growth region length of the crystal growth region. Taking into account the condition according to the invention that the carbon area fraction is greater than the SiC area fraction, the condition L> (D / 2) results from the two area calculation regulations. The growth region length L is therefore greater than half the (maximum) crystal diameter D set. With increasing growth region length L, the carbon surplus thereby achieved increases. However, the growth region length L should preferably not be greater than 250 mm, since then the material transport from the SiC source material to the growth interface can no longer be ensured to a sufficient extent or only with considerable additional effort.

In the further embodiment according to claim 3, the carbon area fraction is much larger than the SiC surface portion, resulting in a particularly advantageous very high carbon excess in the SiC growth gas phase, and the growth interface in the center is particularly strong curved or particularly pronounced growth stages includes. During growth, a very Si rich gas phase is transported from the hot SiC source material to the crystal growth region. So that the silicon reacts with carbon and grows up as SiC, in particular the largest possible carbon-containing surface, for. A graphite surface, are provided on the walls of the crystal growth region. This can be realized in particular by the use of a porous or powdery material having a larger "inner" surface than its actual (geometric) outer surface. In particular, when using a powder having a small grain size, the "inner" surface may be several orders of magnitude higher than its outer surface, so that an SiC / C area ratio smaller than 0.01 can be easily achieved.

With the further embodiment according to claim 4 is achieved a particularly high proportion of carbon area. This is particularly due to the large "inner" surfaces of the inner and especially the middle layer. Although the second graphite material of the inner layer is more porous and less dense than the first graphite material of the outer layer. Nevertheless, the inner layer is mechanically stable and fixes the loose carbon powder of the middle layer to the wall. In addition, the inner layer, due to its porosity of the Si-rich gas phase, allows access to the carbon powder of the middle layer. The carbon powder saturates the Si-rich gas phase with carbon due to its large "internal" surface.

The second graphite material provided according to claim 5 for the inner layer offers on the one hand a sufficient mechanical stability for fixing the carbon powder of the middle layer and on the other hand a sufficient porosity for mass transport from and to the middle layer.

In the further embodiment according to claim 6, the carbon powder of the middle layer has an advantageous particularly small grain size. The smaller the particle size, the greater the "inner" surface of the material of the middle layer which can be achieved for the gas species of the Si-rich gas phase.

In the further embodiment according to claim 7, the SiC source material is covered with a double layer of carbonaceous materials, wherein a lower layer of loose carbon powder is fixed by means of an overlying layer of porous graphite material. This measure further increases the carbon area fraction.

In the further embodiment according to claim 8, the formation of a carbon excess in the SiC growth gas phase is supported by the fact that even in the SiC source material of the carbon content outweighs the silicon content, in particular by 20 mol% to 40 mol%, typically by about 25 mol %.

According to the further embodiment mentioned in claim 9, the growth interface in particular everywhere has a substantially uniform step geometry, so that the formation of a facet region is suppressed particularly efficient and a largely homogeneous incorporation of the dopants in the growing SiC bulk single crystal is given.

To solve the object concerning the monocrystalline SiC substrate, an SiC substrate according to the features of claim 10 is given. The monocrystalline SiC substrate according to the invention is one having a substrate main surface and a substrate thickness, with a local specific electrical resistance determined for any first 4 mm ² large, in particular square, partial surface of the substrate main surface and based on the substrate thickness of at most 4 mΩcm the local electrical resistivity of any second 4 mm ² large particular square partial area deviates.

In particular, the SiC substrate may have a global (= mean) specific resistance value of at most 20 mΩcm, for example 15 mΩcm or 16 mΩcm, determined with respect to the entire substrate main surface. For this purpose, the SiC substrate is preferably doped with at least one dopant, wherein the at least one dopant is preferably an element from the group of nitrogen, aluminum, vanadium and boron.

Overall, the SiC substrate according to the invention is characterized in the substrate surface main area by a particularly homogeneous distribution of the local electrical resistivity. Accordingly, it is outstandingly suitable for the efficient production of preferably high-quality components with a low reject rate. In contrast, result in the device manufacturing using existing SiC substrates due to the inhomogeneously distributed in conventional SiC substrates electrical resistance lower yield and / or higher rejects. The SiC substrate according to the invention can therefore be used with particular advantage, for example as a substrate for the production of semiconductor components.

Monocrystalline SiC substrates with such evenly distributed and in particular low electrical resistance have not existed so far. They can first be produced from bulk SiC crystals which have been produced according to the above-described method according to the invention or its embodiments, for example by successive and disk-wise cutting or sawing of such bulk SiC crystals. The substrate main surface of such a SiC substrate is in particular oriented substantially perpendicular to the direction of growth of the SiC bulk single crystal.

The SiC substrate according to the invention fulfills the industrial requirements with regard to an insert for the production of semiconductor components. A substrate thickness of such an SiC substrate measured perpendicular to the substrate main surface lies in particular in the range between about 100 μm and about 1000 μm and preferably in the range between about 200 μm and about 500 μm, the substrate thickness being a global thickness variation of preferably not more than the complete substrate main surface 20 microns. Furthermore, at least one of the two mutually opposite substrate main surfaces has a surface roughness of preferably at most 3 nm. The SiC substrate has a certain mechanical stability and is in particular self-supporting. It preferably has a substantially circular disc shape, i. H. the substrate main surface is practically round. Optionally, there may be a slight deviation from the exact circular geometry due to at least one provided on the peripheral edge marking mark.

In the embodiment according to claim 11, an extremely homogeneous lateral resistance distribution is present, so that the SiC substrate also meets the requirements for a particularly low-jaggle component production or for the production of particularly high-quality components.

In the further embodiments according to claim 12, the substrate main surface has a particularly large substrate diameter, which in particular can assume values of at least 76.2 mm, 100 mm, 150 mm, 200 mm and 250 mm. The larger the substrate diameter, the more efficiently the monocrystalline SiC substrate can be used, for example, for the production of semiconductor devices. This reduces the manufacturing costs for the semiconductor components. An SiC substrate with such a large diameter can advantageously also for the production of relatively large semiconductor devices, the z. B. have a footprint of about 1 cm ² can be used. However, as the crystal or substrate diameter increases, it becomes more difficult to realize the excess carbon provided in the production process of the invention. The SiC / C area ratio decreases with increasing crystal or substrate diameter. This is also because the distance between the SiC source material and the growth interface, that is, the growth region length L of the crystal growth region, should not be arbitrarily increased. In particular, this length should always be about the same size wherever possible irrespective of the respective crystal or substrate diameter, for example in order to ensure proper transport from the SiC source material and to the growth interface. In this respect, the excess carbon in the production of SiC bulk single crystals with a large crystal diameter should not be increased by an increase in the growth region length L, but by otherwise increasing the carbon area fraction, such. By enlarging or additionally providing "internal" surfaces of carbon material. In this respect, it is therefore particularly favorable for large SiC substrates, which also have as homogeneous a distribution of resistance as possible, if during the crystal growth process the above-described measures for suppressing a facet region are carried out.

Further features, advantages and details of the invention will become apparent from the following description of exemplary embodiments with reference to the drawing. It shows:

1 an embodiment of a cultivation arrangement for producing a SiC bulk single crystal by means of sublimation breeding,

2 and 3 a conventional SiC bulk single crystal in a cross-sectional view or in a plan view,

4 and 5 two further embodiments of breeding arrangements for the sublimation growth of a SiC bulk single crystal with an additional carbon lining in the culture crucible,

6 to 8th an embodiment of a means of one of the Züchtungsanordnungen according to 1 . 4 and 5 produced SiC bulk single crystal in a cross-sectional view, in a plan view and in an enlarged central section, and

9 an embodiment of a monocrystalline SiC substrate, which consists of a means of one of the cultivation arrangements according to 1 . 4 and 5 grown SiC bulk single crystal, in a cross-sectional view.

Corresponding parts are in the 1 to 9 provided with the same reference numerals. Also details of the embodiments explained in more detail below may constitute an invention in itself or be part of an inventive subject matter.

In 1 is an embodiment of a breeding arrangement 1 for producing a SiC bulk single crystal 2 shown by sublimation breeding. The breeding arrangement 1 contains a breeding pot 3 , which is a SiC storage area 4 and a crystal growth region 5 includes. In the SiC storage area 4 For example, there is powdery SiC source material 6 , as a ready-made starting material before the start of the breeding process in the SiC storage area 4 of the breeding knob 3 is filled.

At one of the SiC storage area 4 opposite inner wall of the breeding knob 3 namely on its crucible lid 7 , is in the crystal growth range 5 a seed crystal 8th appropriate. On this seed crystal 8th grows to be grown SiC bulk single crystal 2 by deposition from one in the crystal growth region 5 forming SiC growth gas phase 9 on. The growing SiC bulk single crystal 2 and the seed crystal 8th have about the same diameter. If at all, this results in a deviation of at most 10%, around the germination diameter of the seed crystal 8th smaller than is a single crystal diameter of SiC bulk single crystal 2 ,

The breeding pot 3 including the crucible lid 7 exists in the embodiment according to 1 of an electrically and thermally conductive graphite crucible material having a density of z. B. at least 1.75 g / cm ³ . Around it is a thermal insulation layer 10 arranged. The latter consists for. Example of a foam-like graphite insulation material whose porosity is particularly significantly higher than that of the graphite crucible material.

The thermally isolated breeding crucible 3 is inside a tubular container 11 placed, which is executed in the embodiment as a quartz glass tube and forms an autoclave or reactor. To heat the breeding crucible 3 is around the container 11 is an inductive heating device in the form of a heating coil 12 arranged. The breeding pot 3 is by means of the heating coil 12 at cultivation temperatures of more than 2000 ° C, in particular to about 2200 ° C, heated. The heating coil 12 inductively couples an electrical current into an electrically conductive crucible wall 13 of the breeding knob 3 one. This electrical current flows essentially as a circular current in the circumferential direction within the circular and hollow cylindrical crucible wall 13 and heats the breeding pot 3 on. If necessary, the relative position between the heating coil 12 and the breeding pot 3 axially, ie in the direction of a central longitudinal axis 14 of the growing SiC bulk single crystal 2 , be changed, in particular to the temperature or the temperature profile within the breeding crucible 3 adjust and if necessary to change. The position of the heating coil which can be changed axially during the growth process 12 is in 1 through the double arrow 15 indicated. In particular, the heating coil 12 on the growth progress of the growing SiC bulk single crystal 2 adjusted shifted. The displacement preferably takes place downwards, that is to say in the direction of the SiC source material 6 , and preferably by the same length as that of the SiC bulk single crystal 2 wake up, z. B. in total by about 20 mm. For this purpose, the breeding arrangement includes not shown in detail appropriately designed control, control and adjustment.

The SiC growth gas phase 9 in the crystal growth area 5 becomes through the SiC source material 6 fed. The SiC growth gas phase 9 contains at least gas components in the form of Si, Si ₂ C and SiC ₂ (= SiC gas species). The transport from the SiC source material 6 to a growth interface 16 on the growing SiC bulk single crystal 2 takes place along an axial temperature gradient. At the growth interface 16 in particular one in the direction of the central longitudinal axis 14 measured axial temperature gradient of at least 5 K / cm, preferably at least 10 K / cm set. The temperature within the breeding knob 3 increases to the growing SiC bulk single crystal 2 down. This can be achieved through various measures. Thus, about a not shown detail distribution of the heating coil 12 in two or more axial sections of an axially varying heating are provided. Furthermore, in the lower section of the breeding leg 3 , z. B. by a corresponding axial positioning of the heating coil 12 , a stronger heating effect can be set than in the upper section of the breeding knob 3 , In addition, the thermal insulation may be formed differently on the two axial Tiegelstirnwänden. As in 1 indicated schematically for this purpose, the thermal insulation layer 10 at the lower crucible end wall have a greater thickness than at the upper crucible end wall.

The SiC bulk single crystal 2 grows in a growth direction 18 in the in 1 shown embodiment from top to bottom, so from the crucible lid 7 towards the SiC storage area 4 . is oriented. The growth direction 18 runs parallel to the central central longitudinal axis 14 , As the growing SiC bulk single crystal 2 in the embodiment shown concentrically within the breeding arrangement 1 can be arranged, the central center longitudinal axis 14 also the breeding arrangement 1 be assigned in total.

In addition, the SiC growth gas phase contains 9 also in the illustration according to 1 Dopants not shown in detail, which in the exemplary embodiment is nitrogen (N ₂ ). Alternative or additional dopants such as in particular aluminum (Al), vanadium (Va) and / or boron (B) are also possible. The dopant supply is either gaseous or via the then pretreated SiC source material 6 , The nitrogen content in the SiC growth gas phase 7 is adjusted so that the nitrogen doping of the growing SiC bulk single crystal 2 so great is that the growing SiC bulk single crystal 2 has a relatively low averaged (= global) resistivity of at most about 20 mΩcm.

In 2 and 3 is a conventional SiC bulk single crystal 19 shown in a cross-sectional view and in a plan view. The SiC bulk single crystal 19 has a crystal structure according to the 4H modification whose crystallographic [0001] major axis in the embodiment shown is parallel to the growth direction 18 and thus also to the middle longitudinal axis 14 runs. The SiC bulk single crystal 19 has three sections 20 to 22 , Around the middle longitudinal axis 14 is a central facet area 20 arranged by a border area 21 is surrounded, being between the two subregions 20 and 21 a transition area 22 is formed.

The facet area 22 has a largely flat and smooth surface structure (= surface morphology). In the 2 schematically with marked crystal growth stages have one in the growth direction 18 , ie in the [0001] direction, measured step height B, that in the facet region 20 compared with one perpendicular to the growth direction 18 measured step depth A is much smaller. Here, the quotient N = A / B has a very large value. The step height B is typically less than 1 .mu.m, whereas the step depth A is generally about 100 .mu.m.

The border area 21 On the other hand, it has a curved surface structure which largely corresponds to the typically convex temperature field in front of the growth interface 16 equivalent. The curved surface structure leads to an approximately balanced ratio of step depth A to step height B. The quotient N = A / B here has approximately the value one. As a result, the surface is in the edge area 21 significantly rougher than in the facet area 20 , The step height B and the step depth A are here typically each about 1 mm.

Since the incorporation of dopants varies widely for differently oriented crystal surfaces, and in the facet region 20 the proportion of areas perpendicular to the growth direction 18 are oriented (ie areas in the direction of the step depth A) strongly dominated, it comes in the facet area 20 to a much higher dopant insertion into the crystal structure than in the edge region 21 , Overall, the SiC bulk single crystal 19 the dopant concentration and thus also the local electrical resistivity perpendicular to the direction of growth 18 distributed inhomogeneous. This is unfavorable because the resistivity is either in the facet range 20 or in the border area 21 does not meet the desired specifications, so that the sub-area concerned not or only partially z. B. can continue to be used for component manufacturing.

The facet area 20 does not necessarily have to be concentric with the central longitudinal axis 14 be arranged. In particular, when the SiC bulk single crystal with a low slope of z. B. 1 ° to 10 ° relative to the crystallographic [0001] major axis grows, then the facet area then adjusting asymmetric or acentric with respect to the central longitudinal axis 14 be placed. However, this does not change the inhomogeneous distribution of the dopant concentration and the specific electrical resistance.

In order to achieve better usability, that of SiC bulk single crystals 2 so bred that he has virtually no facet range 20 having. To achieve this, the growth conditions become in the crystal growth range 5 adjusted so that the growth interface 16 also in the central area around the central longitudinal axis 14 is curved and there in particular has at least a similar step geometry as in the edge region. This is done in the SiC growth gas phase 9 in the area in front of the growth interface 16 set an excess of carbon, which is due to an appropriate design of the crystal growth region 5 limiting surfaces is accomplished. The latter have a SiC area fraction and a carbon area fraction. These surface portions are dimensioned so that an SiC / C area ratio formed by dividing the SiC area fraction by the carbon area fraction always, ie, throughout the manufacturing period, has a value of less than one.

At the in 1 shown breeding arrangement 1 the SiC / C area ratio <1 is achieved by moving in the direction of the central longitudinal axis 14 extending Growth region length L of the crystal growth region 5 is greater than the perpendicular to the central longitudinal axis 14 oriented radius (= half the interior diameter) of the cylindrical interior of the breeding crucible 3 , The inner diameter is in particular equal to the maximum crystal diameter of the growing SiC bulk single crystal 2 , which is perpendicular to the central longitudinal axis 14 has a substantially round cross-sectional area. As the growth progresses, the growth region length L decreases due to the increasing length of the growing SiC bulk single crystal 2 , The said condition for the SiC / C area ratio is valid until the end of the growth process, ie also for the then present minimum growth area length L. The SiC area fraction is formed by the surface of the SiC source material 6 and through the growth interface 16 , The carbon area fraction is formed by the crystal growth region 5 adjacent part of the consisting of the graphite crucible material crucible wall 13 ,

In the 4 and 5 shown further embodiments of breeding arrangement 23 and 24 Each contains a modified structure in order to set a particularly low value for the SiC / C area ratio, which is in particular smaller than 0.01. The breeding orders 23 and 24 are similar to the breeding arrangement 1 , In the following, only the differences will be discussed.

In the breeding arrangement 23 according to 4 is the crystal growth area 5 bounded laterally by a three-layer wall construction. This three-layered wall structure comprises, as an outer layer, the crucible wall consisting of the graphite crucible material (= first graphite material) 13 , as a middle layer 25 a carbon powder and as an inner layer 26 a graphite fixation material (= second graphite material) which is more porous than the graphite crucible material and which in particular has a density of less than 60% of a theoretical maximum density of graphite. This maximum density is 2.3 g / cm ³ . The carbon powder of the middle layer 25 is composed of relatively small grains of powder. At least 90% of the powder grains have an average grain size of at most 250 μm, preferably even at most 50 μm. The designed as a hollow cylinder inner layer 26 is mechanically stable. She holds the loose carbon powder of the middle layer 25 at the position provided in the three-layer wall construction.

Due to the porosity of the inner layer 26 and above all, the small grain size of the carbon powder of the middle layer 25 this increases the crystal growth area 5 limiting carbon area fraction drastically. In fact, the carbon surface area is also occupied by "inner" surfaces, which also have the SiC growth gas phase 9 interact and therefore also contribute to the intended carbon surplus. The porous inner layer 26 and the carbon powder of the middle layer 25 have very large such "inner" surfaces, resulting in a particularly low SiC / C area ratio.

In the breeding arrangement 24 according to 5 is in addition to the three-layer wall construction as in the breeding arrangement 23 also a two-layer covering of the SiC source material 6 intended. This is the SiC source material 6 immediately with a first source capping layer 27 covered, in particular, from the same carbon powder as the middle layer 25 consists of the three-layer wall construction. On the side away from the SiC source material is the carbon powder-containing source capping layer 27 in turn, with a porous second source capping layer 28 covered graphite material, in particular from the same porous graphite-fixing material as the inner layer 26 consists of the three-layer wall construction.

The second source capping layer 28 in turn fixes the carbon powder of the first source capping layer 27 , Otherwise, the coverage of the SiC source material is reduced 6 the SiC / C area ratio in two ways. On the one hand, the carbon area fraction is increased by the additionally provided cover made of carbon materials. At the same time, on the other hand, the SiC area ratio becomes due to the coverage of the SiC source material 6 reduced, with the two-layer carbon cover for from the SiC source material 6 sublimated gas species is passable to the mass transfer to the growing SiC bulk single crystal 2 continue to guarantee. In any case, the carbon coverage of the SiC source material contributes 6 in addition to forming the carbon excess in the SiC growth gas phase 9 at.

Another optional measure, the formation of carbon excess in the SiC growth gas phase 9 serves, is already in the introduced SiC source material 6 provided excess carbon, which may in particular be between 20 mol% and 40 mol%, and is typically about 25 mol%.

In 6 to 8th is a SiC bulk single crystal prepared according to the invention 2 shown in a cross-sectional view, in a plan view and in an enlarged section. It has in contrast to the conventional SiC bulk single crystal 19 according to 2 and 3 no facet area 20 , In the illustrations according to 6 and 7 is to illustrate this difference that in the conventional SiC bulk single crystal 19 always available facet area 20 dashed with registered. The growth interface 16 is in the SiC bulk single crystal produced according to the invention 2 also in a central area 29 around the middle longitudinal axis 14 no longer exactly as in the conventional SiC bulk single crystal 19 but curved.

This curvature is accompanied by a step geometry of the crystal growth stages in the central area 29 similar pronounced as in the edge area 21 , In particular - are like 6 apparent - the step height B of the crystal growth stages of the growth interface 16 and also the ratio of step depth A to step height B, ie the quotient N = A / B, in the central region 29 almost as big as in the border area 21 , The growth interface 16 has one in the direction of the center longitudinal axis 14 and in the growth direction 18 most prominent growth front point 30 around which is the advantageously faceted central area 29 the growth interface 16 is arranged. The also in the central area 29 around the growth front point 30 essentially, ie in particular apart from the course of the steps, curved shape of the growth interface 16 goes out of the 8th shown enlarged section VIII of 6 out. The step geometry in the central area 29 is set so that at any 1 mm long, substantially perpendicular to at least one of the central area 29 oriented to existing local growth stages, ie in particular oriented substantially perpendicular to the step height B defining step side surface, and on the growth interface 16 part of the route 31 between the starting point 32 and the endpoint 33 the leg 31 one in the direction of the center longitudinal axis 14 measured axial height difference Δh of at least 10 microns, typically of at least 100 microns, is given.

Any slight curvature or step geometry differences between the central area 29 and the edge area 21 are of minor importance. In particular, they play no role in the incorporation of the dopants, so that there is a very homogeneous distribution of the dopant concentration and thus the electrical resistance. The SiC bulk single crystal 2 is characterized in particular by a largely uniform in the lateral direction of the distribution of the local electrical resistivity. In the embodiment, the SiC bulk single crystal 2 an n-doping with nitrogen. It is also 4H-SiC. In principle, however, a different doping or another SiC polytype is possible.

From a facet-free SiC bulk single crystal produced according to the invention 2 can be monocrystalline SiC substrates 34 generate, which also have no facet range and also have very favorable mechanical and electrical properties. All such monocrystalline SiC substrates 34 of which in 9 an embodiment is shown in a cross-sectional view are made of the SiC bulk single crystal 2 obtained by axially successive slices perpendicular to the direction of growth 18 or to the center longitudinal axis 14 be cut off or sawn off. The SiC substrate 34 is tall and thin.

In the embodiment has its substrate main surface 35 a large substrate diameter D of 150 mm, whereas a substrate thickness t is at the low value of only about 500 μm.

In addition, the SiC substrate has 34 like the SiC bulk single crystal 2 a very homogeneously distributed resistivity, with respect to the entire substrate main surface 35 and the (global) average determined on the substrate thickness t is about 16 mΩcm. One for any first 4 mm ² large area of the substrate main surface 35 and based on the substrate thickness t determined local resistivity deviates by at most 4 mΩcm, preferably by at most 2 mΩcm, from the local electrical resistivity of any second 4 mm ² large face. The resistance distribution is therefore extremely homogeneous. Also, the SiC substrate in the embodiment is the SiC bulk single crystal 2 n-doped and of 4H polytype.

Due to the very uniform electrical resistivity, the SiC substrate is suitable 34 equally well at any point for further processing in the context of the production of semiconductor devices. Accordingly, the yield is high and the reject rate is low.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 6773505 B2 [0003]
• DE 19931332 C2 [0003]
• JP 2008290895 A [0005]

Claims (12)

Method for producing a SiC bulk single crystal ( 2 ), where a) in a crystal growth region ( 5 ) of a breeding knob ( 3 ) a SiC growth gas phase ( 7 ) is generated, and a central central longitudinal axis ( 14 ) having SiC bulk single crystal ( 2 ) by means of deposition from the SiC growth gas phase ( 9 ), wherein the deposition at a growth interface ( 16 ) of the growing SiC bulk single crystal ( 2 ), b) the SiC growth gas phase ( 9 ) at least partially from a SiC source material ( 6 ) located in a storage area ( 4 ) of the breeding knob ( 3 ), and at least one dopant from the group consisting of nitrogen, aluminum, vanadium and boron, characterized in that c) the crystal growth region ( 5 ) is limited by a SiC area ratio and by a carbon area ratio, and these area ratios are selected so that a SiC / C area ratio formed by dividing the SiC area ratio by the carbon area ratio always becomes smaller as one has. Process according to Claim 1, characterized in that the SiC bulk single crystal ( 2 ) perpendicular to the central longitudinal axis ( 14 ) has a round cross-sectional area with a crystal diameter (D), and the crystal growth area ( 5 ) in the direction of the central longitudinal axis ( 14 ) over a growth region length (L), wherein the growth region length (L) is selected to be greater than half the crystal diameter (D), and more preferably at most 250 mm. Method according to one of the preceding claims, characterized in that a value of less than 0.01 is selected for the SiC / C area ratio. Method according to one of the preceding claims, characterized in that a crystal growth region ( 5 ) bounding sidewall of the breeding knob ( 3 ) is constructed in three layers, wherein for the outer layer ( 13 ) a first graphite material, for the middle layer ( 25 ) a carbon powder and for the inner layer ( 26 ) a second graphite material which is more porous than the first graphite material is used. A method according to claim 4, characterized in that for the second graphite material of the inner layer ( 26 ) a density of less than 60% of a theoretical maximum density of graphite, which is at 2.3 g / cm ^3, is provided. A method according to claim 4 or 5, characterized in that for the carbon powder of the middle layer ( 25 ) Grains of which at least 90% have a mean grain size of at most 250 μm, preferably of at most 50 μm. Method according to one of the preceding claims, characterized in that the SiC source material ( 6 ) with a layer ( 27 ) is made of carbon powder, the carbon powder in turn on that of the SiC source material ( 6 ), in particular with a porous graphite material covering a density of less than 60% of a theoretical maximum density of graphite, which is 2.3 g / cm ³ . Method according to one of the preceding claims, characterized in that in the initially introduced SiC source material ( 6 ) a carbon surplus between 20% and 40% is provided. Method according to one of the preceding claims, characterized in that the growth interface ( 16 ) of the growing SiC bulk single crystal ( 2 ) local growth stages and one in the direction of the central longitudinal axis ( 14 ) most prominent growth front point ( 30 ), and for the growth interface ( 16 ) also in the central area ( 29 ) around the growth front point ( 30 ) is provided in a substantially curved shape, so that at any 1 mm long, perpendicular to at least one of the central area in this ( 29 ) local growth stages and at the growth interface ( 16 ) ( 31 ) between the starting point ( 32 ) and the endpoint ( 33 ) of the leg ( 31 ) in the direction of the central longitudinal axis ( 14 ) measured axial height difference (Δh) of at least 10 microns, preferably of at least 100 microns, is given. Single-crystal SiC substrate with a substrate main surface ( 35 ) and a substrate thickness (t), wherein a for any first 4 mm ² large particular square partial surface of the substrate main surface ( 35 ) and based on the substrate thickness (t) determined local electrical resistivity deviates by at most 4 mΩcm from the local electrical resistivity of any second 4 mm ² large particular square partial area. SiC substrate according to claim 10, characterized in that the local electrical resistivity of the arbitrary first partial area differs by at most 2 mΩcm from the local resistivity of any second partial area. SiC substrate according to claim 10 or 11, characterized in that the substrate main surface ( 35 ) has a diameter (D) of at least 100 mm, in particular of at least 150 mm, preferably of at least 200 mm, and most preferably of at least 250 mm.

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