Process for the formation of silicon carbide

Abstract

Process for the formation of silicon carbide, comprising the steps i) Providing a sublimation oven (200) comprising an oven jacket (201), at least one heating element (202) arranged outside the oven jacket (201), and a hot zone (203) arranged inside the oven jacket (201) surrounded by insulation (204), wherein the hot zone (203) comprises: a) a crucible (210) having an upper region (220) and a lower region (240); b) a crucible cover (215) that tightly seals the crucible (210); c) a seeding module arranged in the upper region (220) of the crucible (210), the seeding module comprising a silicon carbide seeding crystal (100) having an upper surface (150) and a lower surface (160) exposed to the upper region of the crucible (210), ii) Producing, outside the crucible (210), a source module (235) containing an outer annular chamber (231) and a hollow inner chamber, iii) Arranging a solid silicon carbide precursor in the outer annular chamber of the source module (235) while the source module (235) is located outside the crucible (210), iv) After arranging the silicon carbide precursor in the outer annular chamber (231) of the source module (235), while the source module (231) is located outside the crucible (210), arranging the source module (231) containing the silicon carbide precursor inside the crucible (210) such that the silicon carbide precursor is positioned in the lower region (240) of the crucible (210), which faces the lower surface (160) of the silicon carbide seed crystal. v) Heating the hot zone (203) with the heating element (202) to sublime the silicon carbide precursor, and vi) Formation of silicon carbide on the lower surface (160) of the silicon carbide seed crystal (100).

Description

Cross-reference to related registrations

[0001] This application is related to the preliminary US patent application serial number 617874,620, which was filed on September 6, 2013. The entire contents of that patent application are incorporated herein by reference. Background of the invention 1. Field of the invention

[0002] The present invention relates to a process for producing bulk silicon carbide with low defect density. 2. Description of the state of the art

[0003] Silicon carbide (SiC) has gained significant interest in recent years due to its outstanding chemical, physical, and electrical properties. In particular, it has been found that bulk single-crystal SiC is useful in semiconductor applications, including, for example, as a substrate for components in power electronics and LEDs. Other applications for this material are also emerging.

[0004] Silicon carbide can be produced by a variety of methods known in the field. For example, large single crystals of silicon carbide are produced using a physical vapor transport (PVT) method. In this method, a source, such as powdered silicon carbide, is placed in a high-temperature region of a crystal growth furnace and heated. A seed crystal, such as a silicon carbide single-crystal wafer, is also placed in a lower-temperature region. The silicon carbide is heated, subliming, and the resulting vapors reach the cooler silicon carbide seed crystal, onto which material is deposited. Alternatively, the source can be a mixture of silicon and carbon particles that reacts upon heating to form SiC, which subsequently sublimes and recrystallizes on the seed crystal.

[0005] Although large silicon carbide single crystals can be produced using a crystal growth furnace, the process is often difficult to control. For example, it is critical that the process conditions, such as the temperature gradient between the source and the seed crystal, are kept constant throughout the crystal growth process, which typically takes place over several days at temperatures above 2000°C, in order to produce a single crystal with consistently uniform properties. Small variations in the process conditions can lead to large changes in the quality of the grown silicon carbide single crystals. Furthermore, as growth progresses, sublimation of the seed crystal and / or the growing crystal can occur if the process conditions are not properly controlled.Furthermore, product quality can be affected by the types of components used in the crystal growth chamber, as some can decompose depending on the growth conditions and thus chemically disrupt growth. As a result, silicon carbide grown in a sublimation furnace often contains defects in the crystals, such as low-angle grain boundaries, dislocations, Si and C second-phase inclusions, various polytype inclusions, and microtubes, which impair the material's performance properties. Even when specific conditions for a single-crystal growth process are met, these defects persist.While single-crystal growth processes can maintain a high-quality product, variability from run to run is typically observed, as any fluctuation in the source, seed crystal, or components of the device can produce inconsistencies in the product.Methods and apparatus for the production of silicon carbide and related technologies are described, for example, in the publications US 5 944 890 A, US 6 451 112 B1, US 2002 0 083 892 A1, US 2003 0 094 132 A1, US 2006 0 102 068 A1, US 2007 0 283 880 A1, US 2008 0 026 591 A1, US 2011 0 214 606 A1, US 2012 0 006 255 A1, US 2012 0 103 249 A1, US 2014 0 220 298 A1, US 2014 0 158 042 A1, US 2011 0 111 171 A, CN 1 02 414 349 A, JP 2001-114599 A, JP 2008-115033 A, JP 2009-256159 A, JP 2011-020860 A, JP 2013-136494 A, JP 2013-124196 A, JP 4 860 164 B2, EP 2 954 101 B1, EP 2 954 100 B1, KR 10 2012 0 139 398 A, WO2001 / 063020 A1, WO 97 / 07265 A1 and WO2014 / 123636 A1.

[0006] For this reason, until now there has been no furnace or process for reliable and repeatable silicon carbide sublimation that can efficiently and cost-effectively produce high-quality large silicon carbide single crystals. Therefore, there is a need in industry for an improved silicon carbide growth apparatus and an improved silicon carbide growth process. Summary of the invention

[0007] The present invention relates to a method for producing silicon carbide. The method comprises the steps of providing a sublimation furnace comprising a furnace jacket, at least one heating element arranged outside the furnace jacket, and a hot zone arranged inside the furnace jacket, which is surrounded by insulation. The hot zone comprises a crucible having an upper region and a lower region, a crucible cover that tightly seals the crucible, a silicon carbide precursor arranged in the lower region of the crucible, and a seed crystal module arranged in the lower region of the crucible, wherein the seed crystal module has a silicon carbide seed crystal having an upper surface and a lower surface exposed to the upper region of the crucible, the lower surface facing the silicon carbide precursor.The process further comprises the steps of heating the hot zone with the heating element to sublime the silicon carbide precursor and forming silicon carbide on the lower surface of the silicon carbide seed crystal. The seed crystal module preferably comprises a seed crystal holder having at least one vapor release opening, and the silicon carbide seed crystal is positioned in the seed crystal holder. The silicon carbide seed crystal preferably also comprises at least one seed crystal protective layer on its upper surface. Various embodiments of the process are described.

[0008] The present disclosure further relates to a sublimation furnace for the formation of silicon carbide, comprising a furnace shell, at least one heating element arranged outside the furnace shell, and a hot zone arranged inside the furnace shell, surrounded by insulation. The hot zone comprises a crucible having an upper region and a lower region, a crucible cover that tightly seals the crucible, a silicon carbide precursor arranged in the lower region of the crucible, and a seed crystal module comprising a silicon carbide seed crystal arranged in the upper region of the crucible, the silicon carbide seed crystal having an upper surface and a lower surface, the lower surface facing the silicon carbide precursor.

[0009] The present disclosure also relates to a process for producing a seed crystal module for the formation of silicon carbide in a sublimation furnace. The process comprises the steps of: providing a silicon carbide seed crystal having an upper surface and a lower surface, and positioning the silicon carbide seed crystal in a seed crystal holder having at least one vapor release port to produce the seed crystal module. The present disclosure also relates to the seed crystal module itself and to the silicon carbide seed crystal.

[0010] It should be understood that both the foregoing general description and the following detailed description are merely exemplary and explanatory and are intended to provide a further explanation of the present invention as claimed. Brief description of the drawings Fig. 1a and Fig. Figure 1b shows perspective views of a silicon carbide single crystal holder used in various embodiments of the present invention. Fig. Figure 2 is a schematic view of a sublimation oven used in various embodiments of the present invention. Fig. 3a and Fig. Figure 3b shows different views of a hot zone of a sublimation oven used in different embodiments of the present invention. Fig. Figure 4 shows a cross-sectional view of a silicon carbide single crystal boule produced by an embodiment of the present invention. Detailed description of the invention

[0011] The present invention relates to a process for the production of silicon carbide.

[0012] In the process according to the invention for the formation of silicon carbide, a sublimation furnace is provided, comprising a furnace shell, a hot zone, and insulation surrounding the hot zone within the furnace shell. The furnace shell can be any type known in the field and used for high-temperature crystallization furnaces, including a quartz shell comprising an outer wall and an inner wall that define a cooling channel for circulating a cooling fluid, for example, water. Alternatively, the furnace shell can also be a single-walled quartz shell with air cooling, for example, from the bottom of the shell upwards. The furnace shell is surrounded by at least one heating element that provides heat to promote and control crystal growth.

[0013] The hot zone comprises a crucible with a crucible cover or lid and both a silicon carbide precursor (sometimes referred to herein as the silicon carbide source) and a silicon carbide seed crystal arranged within the crucible. Each of these is described in more detail below. Insulation surrounds the hot zone when arranged inside the furnace shell and may be any material known in the field to have low thermal conductivity and to withstand the temperatures and conditions inside the furnace, including, for example, graphite. Preferably, the insulation comprises multiple layers of fibrous insulation, for example, graphite felt, and the number of layers may vary depending on, for example, the thickness of the layer, the size of the furnace shell, the size and shape of the crucible, the crystal growth conditions, and the cost.Preferably, the shape and dimensions of the insulating layers correspond to the shape and size of the crucible used and provide a sufficiently low thermal conductivity to maintain the desired temperature gradient for crystal growth. For a cylindrical crucible, for example, the insulation preferably comprises layers of fibrous insulating material in a donut shape, stacked to enclose the crucible. Preferably, the hot zone surrounded by insulation is enclosed in a holding container or quartz container to simplify handling and to maintain a consistently low thermal conductivity. Any space between the outside of the holding container and the inside of the furnace shell may be filled with an inert gas or gas mixture, for example, a combination of argon and nitrogen.

[0014] For the present method, the furnace jacket, the hot zone, and the insulation can be combined in a variety of ways. In one embodiment, for example, the insulation is provided within an open-topped holding container and positioned within the furnace jacket (for example, on a movable or stationary base), which has heating elements around the outside of the jacket. The hot zone, comprising the crucible, which is tightly sealed with the crucible cover and contains the silicon carbide precursor and the silicon carbide seed crystal, is arranged inside the insulation, such that the insulation surrounds the hot zone. Alternatively, in another embodiment, the insulation is arranged within the furnace jacket, preferably in a holding container, and the crucible is positioned within the insulation.The silicon carbide source and the silicon carbide seed crystal are then placed inside the crucible, which can be tightly sealed with the crucible cover. In another embodiment, the insulation is positioned around the hot zone, either with or without the source and seed crystal, and together the hot zone and the insulation are arranged inside the furnace jacket, preferably using a holding container. Regardless of the order, the hot zone is preferably positioned horizontally (for example, axially) in the center of the furnace jacket along a vertical central axis of the jacket. A vertical positioning along this central axis depends, for example, on the type and location of the heating elements described below, as well as on the desired temperature gradient to be produced.Preferably, the hot zone is positioned vertically in or above the center of the furnace shell, as well as the center of the heating elements surrounding the furnace shell. A specific vertical positioning can be adjusted for optimal growth performance. After the crucible has been equipped with the silicon carbide source and the silicon carbide seed crystal, it can then be tightly sealed with the cover. Other combinations are also possible and would be known to a person skilled in the art. Optionally, a porous filter, for example, a porous graphite filter, can be placed between the silicon carbide precursor and the silicon carbide seed crystal.

[0015] Once the sublimation furnace has been assembled, the method of the present invention further comprises the step of heating the hot zone with the heating elements to sublimate the silicon carbide precursor, thereby forming silicon carbide on the silicon carbide seed crystal. The heating elements can be any known elements in the field that are suitable for changing the temperature within the furnace shell and, more specifically, within the crucible, to effect sublimation of the source. The heating element can be a single element or can comprise several elements, which may be preferred for increased control. For the present invention, the heating elements are preferably induction heating devices that wrap around the outside of the furnace shell and are capable of inductively coupling with components inside the furnace shell, specifically the crucible.To measure, maintain, and / or regulate the temperature within the crucible, the hot zone can also include at least one temperature sight tube positioned above the crucible. Preferably, the tube extends through the crucible cover to a position above the silicon carbide seed crystal. From there, temperature values ​​can be measured, and if necessary, the energy supplied to the heating elements can be adjusted to ensure that the desired crystal growth conditions are maintained.

[0016] As described above, the hot zone comprises a crucible, a crucible cover, a silicon carbide precursor, and a silicon carbide seed crystal. The crucible can be any crucible known in the field that can withstand the conditions prevailing in the sublimation furnace. Preferably, the crucible and the crucible cover comprise graphite. Furthermore, the crucible can have any shape or size, depending, for example, on the shape and size of the furnace shell, the amount of silicon carbide precursor used, and the desired shape and size of the silicon carbide product to be formed. For example, the crucible can have a substantially cylindrical shape.The crucible has an upper region (which is a region at the top of the crucible) and a lower region (which is a region at the bottom of the crucible), and these regions can have the same or different shapes and / or the same or different cross-sectional areas. For example, both the upper and lower regions of the crucible can have a substantially cylindrical shape, with the upper region having a larger diameter than the lower region. In this example, the insulation can conform closely to both the upper and lower regions, thus being in contact with the entire outer surface of the crucible, or preferably, the insulation can conform closely to the upper region but not to the lower region of the crucible, leaving a gap between the bottom of the crucible and the insulation.The silicon carbide precursor is located in the lower region of the crucible, while the silicon carbide seed crystal is located in the upper region. When the hot zone is heated by the heating elements surrounding the furnace shell, the silicon carbide precursor reacts and / or sublimates, forming silicon- and carbon-containing vapors. These vapors then rise through the hot zone towards the silicon carbide seed crystal, where they condense and solidify, forming the silicon carbide product.

[0017] The silicon carbide precursor can be positioned directly in the lower region of the crucible or, alternatively, provided in a separate, self-supporting container located in the lower region. The silicon carbide precursor is contained within a source module arranged inside the crucible. The source module, which has multiple precursor chambers, can be any container suitable for withstanding the conditions required for silicon carbide formation and which does not interfere with or introduce contaminants into the product. Preferably, the source module comprises graphite. The shape of the module can vary depending on, for example, the amount of precursor supplied and the shape of the crucible. The source module has an outer annular chamber and a cylindrical inner chamber.For example, the source module can be a cylindrical insert arranged to form an annular chamber in the lower region of the crucible. In this way, the outer annular chamber, which is closer to the heating elements, can contain the silicon carbide precursor, while the inner cylindrical chamber provides space for the passage of sublimation products to reach the silicon carbide seed crystal. In this embodiment, the cylindrical source module preferably comprises porous graphite capable of allowing diffusion of vapors produced by the sublimation of the silicon carbide precursor.

[0018] The silicon carbide precursor comprises silicon carbide and can be in any form, including powder, granules, solid, or a combination thereof. Preferably, the silicon carbide precursor is substantially solid and has a very low concentration of particulate material, including, for example, less than 10%, less than 5%, and less than 1% particulate material. Most preferably, the silicon carbide precursor is a solid material that is essentially free of particulate material. Furthermore, the precursor can also be either a low-density porous solid or a high-density non-porous solid compared to the density of silicon carbide.

[0019] The silicon carbide precursor can be prepared in a variety of ways. For example, a reagent mixture containing silicon and carbon, such as graphite, can be prepared and then heated to form an essentially solid silicon carbide precursor mixture containing silicon carbide. Alternatively, particulate silicon carbide can be provided and also heated to be converted to an essentially solid silicon carbide precursor. The extent of the conversion can be varied, for example, by the degree of heating, the temperature, and, for a reagent mixture, the silicon-to-carbon ratio and the shape of the mixture. For instance, silicon particles and carbon particles can be combined to form a particulate mixture, which increases the surface area available for reaction to form the silicon carbide precursor.Preferably, a molar excess of carbon is used to drive the conversion of silicon to silicon carbide. For example, the molar ratio of carbon to silicon used to prepare the silicon carbide precursor is preferably greater than 1.0, more preferably from about 1.05 to about 1.5, and most preferably from about 1.1 to about 1.3.

[0020] Any known particulate silicon carbide or silicon and carbon particles can be used to prepare the silicon carbide precursor. For example, if a particulate mixture is used, the silicon particles preferably have an average particle size of about 0.1 mm to about 10 mm, more preferably of about 0.5 mm to about 5 mm, and most preferably of about 1 mm to about 4 mm. Furthermore, the carbon particles preferably have an average particle size in the range of about 50 micrometers to about 1000 micrometers, more preferably of about 75 micrometers to about 750 micrometers, and most preferably of about 85 micrometers to about 500 micrometers. Particles in these ranges can be prepared using any method available in the field, including, for example, sieving.

[0021] The particulate mixture can be formed either as a homogeneous or nearly homogeneous mixture of silicon and carbon particles, for example by combining reagent particles and agitation or mixing, or as a heterogeneous mixture by combining reagent particles or intentional mixing. For example, the silicon and carbon particles can be combined in alternating layers of silicon and carbon, with each carbon layer having substantially equal amounts of carbon and each silicon layer having substantially equal amounts of silicon. The particulate mixture is provided in a source module, as described above, which can either be positioned and heated within the crucible or heated separately and then positioned in the lower region of the crucible.

[0022] The hot zone also includes a silicon carbide seed crystal positioned in the upper region of the crucible where silicon carbide is formed. The seed crystal is monocrystalline to ensure the formation of essentially single-crystal silicon carbide. Any seed crystal known in the field can be used. Preferably, the silicon carbide seed crystal is a silicon carbide wafer, in particular a circular silicon carbide wafer such as can be obtained, for example, by slicing a silicon carbide single-crystal body into wafers. The diameter of the silicon carbide seed crystal wafer can vary depending on a variety of factors, such as the size of the crucible and the desired size of silicon carbide to be produced.For example, a silicon carbide seed crystal can be a circular silicon carbide wafer with a diameter greater than approximately 75 mm (for example, from approximately 80 mm to approximately 85 mm), greater than approximately 100 mm (for example, from approximately 105 mm to approximately 115 mm), greater than approximately 150 mm (for example, from approximately 160 mm to approximately 170 mm), and greater than approximately 200 mm (for example, from approximately 210 mm to approximately 230 mm). The thickness of the silicon carbide seed crystal can also vary depending on factors such as cost and availability. For example, the silicon carbide seed crystal can be a circular silicon carbide wafer with a thickness of approximately 0.5 mm to approximately 3.5 mm, for example, from approximately 0.6 mm to approximately 1.3 mm, and from approximately 0.7 mm to approximately 1.1 mm.

[0023] The silicon carbide seed crystal has an upper and a lower surface and is positioned in the upper region of the crucible, with its lower surface facing the silicon carbide precursor located in the lower region. Since the seed crystal is monocrystalline silicon carbide, it has a silicon face and a carbon face, and the seed crystal can be positioned so that each face is oriented towards the precursor, depending on the silicon carbide polytype to be formed. For example, 6H silicon carbide can be formed if the silicon side of a silicon carbide seed crystal wafer is the lower side, while 4H silicon carbide can be grown from the carbon side of the seed wafer.

[0024] Preferably, the silicon carbide seed crystal is provided in a separate, independent seed crystal module, and the seed crystal module is positioned in the upper region of the crucible. The seed crystal module can be any container capable of withstanding the silicon carbide crystal growth conditions and can also contain the silicon carbide seed crystal, holding the seed crystal in position without disturbing the silicon carbide growth. The seed crystal module preferably comprises a seed crystal holder containing the seed crystal, with the lower surface of the seed crystal, on which silicon carbide grows, exposed to the upper region of the crucible. Preferably, up to 75% of the lower surface is exposed, more preferably 80%, and most preferably 90%. In this way, the crystal growth surface exposure is maximized.

[0025] Furthermore, the upper surface of the silicon carbide seed crystal contained in the seed crystal holder is preferably exposed to the upper region of the crucible. In particular, more than about 75% of the upper surface of the seed crystal is exposed within the seed crystal holder, including more than about 80% and more than about 90%. This is contrary to what is typically the case in the field. A person skilled in the art would adhere the upper surface of the seed crystal to a protective barrier, for example, a thick block of graphite that can act as a lid for the crucible. It would be expected that leaving the upper surface of the seed crystal exposed would decompose the seed crystal and create defects within the growing silicon carbide crystal. However, attaching orAdhering the seed crystal to a block has significant disadvantages, particularly because a thermal mismatch between the block and the seed crystal creates stresses and defects in the growing crystal. Furthermore, adhesives capable of withstanding the growth conditions add further costs to the process and can introduce contaminants into the seed crystal growth environment.

[0026] Surprisingly, it was found that the upper surface of the silicon carbide seed crystal can be left exposed to the upper region of the crucible, with the exposed surface being unattached to or suspended from any component above the seed crystal in the hot zone, including the crucible or crucible lid, and that as a result, silicon carbide can be formed on the seed crystal with reduced overall crystal defects. To ensure that the upper surface of the silicon carbide seed crystal does not degrade during silicon carbide growth, it is preferred that the upper surface of the seed crystal includes a seed crystal protective layer that can act as a barrier to the crystal growth environment in the upper region of the crucible. Preferably, the entire upper surface of the silicon carbide seed crystal comprises the seed crystal protective layer.This layer can be a single layer or may comprise several layers, but is overall very thin compared to the thickness of the seed crystal. Preferably, the seed crystal protective layer has a thickness of less than about 250 µm, and more preferably, it is less than about 100 µm thick, for example, about 10 µm to about 90 µm, about 30 µm to about 80 µm, and about 50 µm to about 70 µm. The seed crystal protective layer can comprise any material capable of preventing a reaction with the upper surface of the seed crystal without undesirable thermal stress, particularly due to a mismatch in the coefficient of thermal expansion. The seed crystal protective layer can, for example, comprise one or more carbon-containing layers, such as a coating comprising graphite or a photoresist layer that has been cured at high temperature.If the seed crystal protective layer comprises at least two coating layers, the thickness of the layers can vary, as long as the total thickness preferably remains below about 250 micrometers. For example, each layer can be about 1 micrometer to about 100 micrometers thick, including about 2 micrometers to about 5 micrometers if the layer is a cured photoresist, and about 20 micrometers to about 30 micrometers if the layer is a graphite coating layer.

[0027] The seed crystal protective layer can be applied to the silicon carbide seed crystal surface using any coating technique known in the field that is capable of producing a thin surface layer. Thus, in one embodiment of the method of the present invention, a seed crystal module is provided by a process that includes the step of applying at least one coating to a surface of the silicon carbide seed crystal and optionally hardening the resulting coating, depending on the material used. The surface can be either the silicon side of the seed crystal or the carbon side of the seed crystal.Once coated, the surface encompassing the seed crystal protective layer is used as the top surface of the seed crystal, and the seed crystal, preferably contained in a seed crystal module, is positioned in the crucible with this protected surface exposed to the upper region of the crucible.

[0028] A variety of different types of seed crystal holders can be used, and the holder can be made of any metal capable of withstanding the high-temperature conditions necessary for silicon carbide crystal growth, including, for example, graphite. For the method of the present invention, the seed crystal holder preferably comprises one or more vapor release openings, which are spaces provided in the holder to allow vapors produced, for example, from the sublimation of the silicon carbide precursor, to escape from the crucible.The vapor release openings are preferably located below the lower surface of the silicon carbide seed crystal when contained within the seed crystal holder, and are adjacent to or outside the circumference of the seed crystal. This allows excess vapor to escape upwards towards and around the seed crystal(s) and then out of the crucible, which preferably also includes one or more ventilation holes to allow vapor to pass through. Furthermore, a vapor release ring may also be included, either within the seed crystal holder or between the outer surface of the seed crystal holder and the crucible wall. The ring comprises one or more holes, which may be aligned with the ventilation holes in the crucible. Upper vapor release holes in the upper part of the seed crystal holder may also be used.Adjusting the ring allows the amount of vapor discharged from the crucible and into the surrounding insulation to be varied by changing the orientation of the holes.

[0029] The number of vapor release vents in the seed crystal holder can be varied, as can their shape and size, and the vents can be located at different positions on the seed crystal holder. For example, the seed crystal holder can include a multitude of vapor release vents positioned around a central axis of the holder that is perpendicular to the bottom surface of the silicon carbide seed crystal. For a silicon carbide seed crystal that is a circular wafer, the multitude of vapor release vents can be positioned symmetrically around the outside of the seed crystal at equal distances from the central axis. The shape of the vents can be, for example, circular, oval, rectangular, or square, and the vents can be positioned within the body of the seed crystal holder or along an edge.As a special example, the seed crystal holder can include a multitude of square vapor release openings positioned symmetrically around the lower edge of the seed crystal holder facing the source, essentially forming the shape of a castle tower.

[0030] Thus, the silicon carbide formation process of the present invention comprises the steps of providing a sublimation furnace comprising a furnace jacket, insulation, and a hot zone, wherein the hot zone comprises a crucible having a silicon carbide precursor in the lower region and a silicon carbide seed crystal in the upper region. According to the invention, the silicon carbide precursor is contained in a source module, and the silicon carbide seed crystal is contained in a seed crystal module. These modules are prepared outside the crucible in separate steps and then subsequently positioned within it. In this way, the source, the seed crystal, or both can be provided as pre-packaged components separate from the sublimation furnace. The source is consumed during silicon carbide production, and the silicon carbide seed crystal becomes part of the grown or removed silicon carbide.The silicon carbide product is grown and removed along with the produced material. Accordingly, one method of the present invention is a consumption method for producing silicon carbide in the hot zone of a sublimation furnace. The furnace is provided, the consumable source module and / or the consumable seed crystal module are arranged in the lower or upper region of the hot zone, silicon carbide is formed, and both the source and the seed crystal are removed to be replaced by another source and another seed crystal for a subsequent run of silicon carbide production.

[0031] A specific embodiment of the method of the present invention is described below. However, it should be clear to those skilled in the art that this embodiment is merely illustrative and not limiting, being given only as an example. Numerous modifications and other embodiments are within the scope of the expert knowledge of a person skilled in the art and are considered to fall within the scope of the present invention. Furthermore, those skilled in the art should recognize that the specific conditions and configurations are exemplary and that actual conditions and configurations will depend on the specific system. Those skilled in the art will also be able to recognize and identify equivalents to the specific elements shown without using more than routine experimentation.

[0032] In a first example of the process of the present invention, a substantially solid silicon carbide precursor mixture was prepared by placing 660 g of particulate silicon (sieved to a particle size range of 1.0 to 3.35 mm, average particle size 2.18 mm) and 340 g of particulate carbon (sieved to a particle size range of 90 to 500 micrometers, average particle size 294 micrometers) into the outer annular chamber of a cylindrical graphite source module having a central annular wall of porous graphite positioned at the bottom of a cylindrical graphite crucible. The silicon and carbon particles were arranged in alternating layers, each layer having approximately equal amounts of material (6 layers of carbon particles alternating with 5 layers of silicon particles).A porous graphite filter was positioned over the several layers, but not in contact with the precursor mixture; rather, it was placed at a distance of approximately 20 mm. The crucible was then tightly sealed with a lid containing a cylindrical temperature sight tube. The partially filled crucible was placed inside a cylindrical quartz container surrounded by several layers of soft graphite felt approximately 0.635 cm thick. The container was filled and placed inside a furnace jacket. It was heated by induction heating devices positioned around the outside of the furnace jacket to a temperature of approximately 1400°C for 2 hours, followed by a temperature of approximately 2240°C for another 2 hours. The resulting essentially solid silicon carbide precursor mixture was used as is for the production of silicon carbide.

[0033] Separately, a seed crystal module was fabricated by applying a seed crystal protective layer to the silicon side of a polished 80 mm silicon carbide wafer (thickness between 0.8 and 1.0 mm) and placing the protected seed crystal in a seed crystal holder. Specifically, a first photoresist layer (Megaposit SPR, a viscous cresol-novolak resin solution, available from Rohm and Haas Electronic Materials) was applied to the polished silicon side of the silicon carbide wafer and cured to form a first layer 2 to 4 micrometers thick. Then, a second photoresist layer was applied to the first layer and also cured (the thickness was 2 to 4 micrometers). Subsequently, a carbon-containing layer was applied to the second photoresist layer by spraying graphite (Aerodag G, a non-aqueous graphite spray dispersion) at approximately 120°C (the thickness was 20 to 30 micrometers).The composite material was then heated under vacuum at 1150°C for two hours to ensure complete curing and coverage. A third photoresist layer was also applied to the porous carbon layer, adding minimal thickness as the photoresist layer filled the porous layer. This was followed by a spray application of a second graphite layer (20 to 30 micrometers thick) as a sealant. Finally, the entire composite material was heated under nitrogen at atmospheric pressure at 300°C for eight hours to produce a silicon carbide seed crystal wafer with multilayer seed crystal protection on its silicon side.

[0034] The protected silicon carbide seed crystal was placed in a graphite seed crystal holder comprising several vapor release ports, thereby forming the seed crystal module. This is in Fig.1a, which is a view of the top side of a seed crystal module, and Figure Fig. Figure 1b, which is a view of the underside, shows the silicon carbide seed crystal 100, which comprised the seed crystal protective layer 110 on the silicon side, prepared as described above, and was positioned in seed crystal holder 120. This holder comprised a plurality of rectangular vapor release openings 130 symmetrically arranged around the lower circumference of the holder in a turret configuration. Furthermore, seed crystal holder 120 also includes additional vapor release openings 140 in the upper part of the holder, which can be aligned with ventilation holes in the crucible, as shown in Figure 1b. Fig.2 is shown and discussed below. Vapors produced by sublimation of a silicon carbide precursor can therefore approach silicon carbide seed crystal 100 and escape through vapor release opening 130 and additional vapor release openings 140 via ventilation holes in the crucible, entering the surrounding insulation, as illustrated, for example, by arrow A. Silicon carbide seed crystal 100 is placed in seed crystal holder 120 with the seed crystal protective layer 110 facing upwards, which allows 4H silicon carbide to grow on the lower surface of the seed crystal, the carbon side. As shown in Fig. 1a and Fig. As can be seen in Figure 1b, the upper surface 150 and lower surface 160 of the silicon carbide seed crystal 100 are exposed or freestanding in seed crystal holder 120.

[0035] Fig.Figure 2 is a schematic view of sublimation furnace 200, comprising furnace shell 201 surrounded by induction heating elements 202, and further comprising hot zone 203 within the furnace shell 201, positioned along a central axis 215 of furnace shell 201 slightly above the center of the induction heating elements 202 and surrounded by insulation 204 comprising multiple layers of graphite felt. Hot zone 203 and insulation 204 are enclosed in quartz containers, with a space between them filled with a mixture of argon and nitrogen. As shown in Fig. 3a and Fig.As shown in Figure 3b, hot zone 203 comprises crucible 210, which consists of graphite having a generally cylindrical shape, the diameter in the upper region 220 being larger than the diameter in the lower region 240. Seed crystal holder 120, comprising silicon carbide seed crystal 100, is positioned in the upper region 220 of crucible 210, with both the upper surface 150 and the lower surface 160 of the silicon carbide seed crystal 100 exposed to the upper region 220. The lower surface 160 faces the substantially solid silicon carbide precursor mixture 230, which is positioned in the lower region 240 of crucible 210 and is produced, as described above, in the outer annular chamber 231 of source module 235, which has an open annular region 232 formed by the porous graphite wall 233.Essentially solid silicon carbide precursor mixture 230 is separated from seed crystal holder 120 by a porous graphite filter 225, which, as shown, is not in contact with the source or the seed crystal. Seed crystal holder 120 comprises rectangular vapor release openings 130 (in the figure shown in ). Fig. (3 not visible in the provided view), which provide a path for the passage of vapors from crucible 210 through ventilation holes 260 and aligned holes 270 in vapor release ring 280. Crucible 210 is tightly sealed with crucible cover 215, into which a temperature sight tube 290 is inserted, positioned above the silicon carbide seed crystal 100 for monitoring the temperature in the hot zone 203.

[0036] In this exemplary process of the present invention, after preparation of the sublimation oven 100, the hot zone 203 is heated to a temperature between 2080°C and 2110°C by induction heating elements 202, while simultaneously the reaction pressure is reduced to about 0.5 Torr, and these conditions are maintained for about 100 hours (with the temperature kept within a range of 50°C). A temperature gradient develops within the crucible, with the lower region being about 20 to 40°C hotter than the upper region 220. In this way, substantially solid silicon carbide precursor mixture 230 is sublimed, and bulk silicon carbide is formed on the lower surface 160 of silicon carbide seed crystal 100, with excess vapors escaping through vapor release ports 130.

[0037] Similarly, in a second example, 1050–1150 g of particulate SiC (poly-alpha-SiC, available from Washington Mills) was placed in the outer annular chamber of a cylindrical graphite source module and positioned in a graphite crucible and within a furnace jacket, as described in the preceding example. The SiC was then heated to a temperature of about 1400°C for two hours, followed by a temperature of about 2240°C for another two hours, to produce an essentially solid silicon carbide precursor with a density of about 1.1 g / L. Separately, a seed crystal module was also prepared, comprising a 100 mm silicon carbide wafer with a seed crystal protective layer applied to its silicon side, contained in a seed crystal holder, using a procedure similar to that described in the previous example.In particular, a first photoresist layer (Megaposit SPR) was applied to the polished silicon side of a silicon carbide wafer and cured, forming a first layer 2–4 micrometers thick. A carbon-containing layer was then applied to the first photoresist layer by spraying graphite (Aerodag G) (total thickness 20–30 micrometers). The composite material was heated under vacuum at 1150°C for two hours to ensure complete curing and coverage. This process was repeated to produce a silicon carbide seed crystal wafer with a multilayer seed crystal protection layer on its silicon side. The protected seed crystal was then placed in the graphite seed crystal holder used in the previous example and positioned in the upper region of the graphite crucible, with both sides of the seed crystal exposed to the upper region.were exposed, as in . Fig. 3a and Fig. 3b shown. The resulting hot zone was surrounded by insulation comprising multiple layers of graphite felt approximately 0.635 cm thick and with very low thermal conductivity (0.15 W / (°K·m) at 1000°C), and the combination was placed in a quartz container and axially heated in the sublimation furnace located in Fig. Figure 2 shows the induction heating elements centered above the middle. Bulk silicon carbide was formed on the lower surface of the silicon carbide seed crystal, as described above.

[0038] The resulting silicon carbide can vary in shape and size depending, for example, on the size of the silicon carbide precursor, the size of the exposed area of ​​the lower surface of the silicon carbide seed crystal, and the relative shape of the crucible. For instance, the grown silicon carbide may have the shape of a boule or single-crystal body with a substantially circular cross-sectional shape in a direction parallel to the lower surface of the silicon carbide seed crystal. The single-crystal body may increase in diameter perpendicular to the seed crystal surface up to a maximum diameter and also decrease in diameter to a rounded point, forming a generally conical lower surface or bottom surface.The maximum diameter of the silicon carbide single crystal can vary depending on the time the silicon carbide has grown and the growth conditions. Generally, the maximum diameter ranges from about 70 mm to about 250 mm. For example, the maximum diameter can be greater than 75 mm (e.g., from about 80 mm to about 85 mm), including greater than about 100 mm (e.g., from about 105 mm to about 115 mm), greater than about 150 mm (e.g., from about 160 mm to about 170 mm), and greater than about 200 mm (e.g., from about 210 mm to about 230 mm). The single crystal weight depends on the growth conditions but is generally about 60% to 80% of the initial weight of the source material.

[0039] The bulk silicon carbide produced in the second example was, for instance, a single crystal body with a circular horizontal cross-section and a shape that was in Fig.Figure 4 shows that the single-crystal body has an upper section A with a curved outer surface that increases in diameter from the initial silicon carbide seed crystal wafer at the top to the approximate center of the middle section B, which is the widest section of the single-crystal body, with a maximum diameter of about 112 mm. The rounded outer surface of the middle section B can be ground to a flat surface (as shown) for wafer production. The single-crystal body is covered by a domed section C, which has a shiny outer surface when grown. For this example, the grown silicon carbide single-crystal body weighs about 748 g and is about 33 mm high, with section A being about 10 mm and sections B and C being about 12 mm.

[0040] Therefore, the present disclosure also relates to a bulk silicon carbide single-crystal body produced by the process of the present invention. Since the single-crystal body was grown on a silicon carbide seed crystal, as described above, the grown single-crystal body comprises this seed crystal. Thus, in a specific embodiment of the present invention, a silicon carbide single-crystal body grown on a circular silicon carbide wafer has a generally bulbiform shape with a substantially circular horizontal cross-sectional shape and comprises a flat outer surface which encompasses the circular silicon carbide wafer and therefore has one or more, preferably at least two, for example, 2-5, carbon-containing coatings.The bulk silicon carbide single-crystal body also has a central section with a substantially circular cross-sectional shape in a direction parallel to the flat surface, with a diameter slightly larger than the diameter of the flat surface, and a conical or domed outer surface opposite the flat outer surface. The conical outer surface preferably has a mirror-like finish.

[0041] The silicon carbide of the present disclosure, produced by the process of the present invention, was found to have improved properties, in particular a generally low total number of defects, including, but not limited to, edge dislocations, screw dislocations, and base-plane defects (dislocations and stacking faults). For example, the silicon carbide was found to have a total defect number of less than about 8000 / cm² and, in particular, less than about 6000 / cm². Furthermore, the edge dislocation density of the silicon carbide was found to be low, for example, about 4000 / cm² and, in particular, less than about 2500 / cm². It was also found that the silicon carbide has a screw dislocation density of less than about 3500 / cm², including less than about 3000 / cm².Furthermore, the base plane defect density of silicon carbide produced according to the invention is also low, for example less than about 500 / cm² or less than about 200 / cm². Preferably, the silicon carbide produced by the process of the present invention has a step dislocation density of less than about 2500 / cm², a screw dislocation density of less than about 3000 / cm², and a base plane defect density of less than about 200 / cm².

[0042] From the grown silicon carbide single crystal body, which has low defect levels, a large number of silicon carbide wafers can be removed, and the number of wafers will depend on the size of the central section (with the maximum diameter), the target wafer thickness, and the removal method used. For the in Fig.From the 4 single-crystal bodies shown, it would be expected that 7-10 wafers could be cut from the central section B, which could be polished if necessary. Each of these wafers, resulting from a single-crystal body which, on average, has a low number of defects, would be expected to have improved physical and performance characteristics, especially for LED or high-energy device applications. For example, 1.25 mm silicon carbide wafers were produced from section (B) of the single-crystal body shown in Fig. Figure 4 shows sections – one from the upper part (W1) and one from the lower part (W2). These were polished to a thickness of approximately 1.00 mm and analyzed for defect levels. The results are shown in Table 1 below: Table 1 Wafer Screw displacements Step changes Basic level Total defects W1 915 1001 143 2058 W2 44 1210 477 1732

[0043] As the data show, both wafers were found to have surprisingly low total defect counts. Furthermore, the total number of defects decreased through Section B, indicating that defects decreased as growth progressed. Screw dislocations, step dislocations, and base-plane defects were also all surprisingly low. With such low defect levels in each wafer, one would expect all wafers from Section B to exhibit enhanced performance, for example, in LED and high-power electronics applications.

[0044] The present disclosure also relates to the sublimation furnace used in the process described above. The sublimation furnace comprises a furnace jacket, at least one heating element arranged outside the furnace jacket, and a hot zone arranged inside the furnace jacket, surrounded by insulation. The jacket, heating elements, insulation, and heating zone can be any of those described above. The hot zone particularly comprises a crucible, a crucible cover that tightly seals the crucible, a silicon carbide precursor arranged in the lower region of the crucible, and a silicon carbide seed crystal arranged in the upper region of the crucible.In the sublimation furnace of the present disclosure, any of these components described above can be used, including, for example, an substantially solid silicon carbide precursor mixture such as can be produced by heating a particulate mixture comprising silicon and carbon particles, and a seed crystal module comprising the silicon carbide seed crystal, specifically a seed crystal having an upper surface and a lower surface which are exposed or exposed to the upper region of the crucible, respectively, and wherein the upper surface of the silicon carbide seed crystal comprises a seed crystal protective layer.

[0045] The foregoing description of preferred embodiments of the present invention has been presented for illustrative and descriptive purposes. It is not intended to be exhaustive or to limit the invention to the exact form disclosed. In light of the above teachings, modifications and variations are possible or may be derived from the practical application of the invention. The embodiments have been chosen and described to explain the principles of the invention and its practical application, enabling a person skilled in the art to use the invention in various embodiments and with various modifications suitable for the particular use under consideration. The scope of the invention is to be defined by the appended claims and their equivalents. Reference symbol list 100 Silicon Carbide Seed Crystal 110 Seed crystal protective layer 120 seed crystal holders 130 steam release ports 140 additional steam release ports 150 upper surface 160 lower surface 200 Sublimation oven 201 Oven jacket 202 induction heating elements 203 hot zone 204 Insulation 215 Center axis 210 crucibles 215 Crucible cover 220 upper region 225 Graphite filters 230 Silicon carbide precursor mixture 231 annular chamber 232 ring-shaped region 233 Graphite wall 235 Source module 240 lower region 260 ventilation holes 270 aligned holes 280 steam release ring 290 Temperature sight tube A upper section B middle section C Dome section

Claims

A method for the formation of silicon carbide, comprising the steps of: i) providing a sublimation furnace (200) comprising a furnace jacket (201), at least one heating element (202) arranged outside the furnace jacket (201), and a hot zone (203) arranged inside the furnace jacket (201) surrounded by insulation (204), wherein the hot zone (203) comprises: a) a crucible (210) having an upper region (220) and a lower region (240); b) a crucible cover (215) tightly sealing the crucible (210); c) an inoculation module arranged in the upper region (220) of the crucible (210), wherein the inoculation module comprises a silicon carbide inoculation crystal (100) having an upper surface (150) and a lower surface (160) facing the upper region of the crucible (210). is exposed, has,ii) manufacture, outside the crucible (210), a source module (235) containing an outer annular chamber (231) and a hollow inner chamber,iii) Arranging a solid silicon carbide precursor in the outer annular chamber of the source module (235) while the source module (235) is located outside the crucible (210), iv) After arranging the silicon carbide precursor in the outer annular chamber (231) of the source module (235) while the source module (231) is located outside the crucible (210), arranging the source module (231) containing the silicon carbide precursor inside the crucible (210) such that the silicon carbide precursor is positioned in the lower region (240) of the crucible (210), which faces the lower surface (160) of the silicon carbide seed crystal, v) Heating the hot zone (203) with the heating element (202) to sublime the silicon carbide precursor, and vi) Forming of Silicon carbide on the lower surface (160) of the silicon carbide seed crystal (100)., Method according to claim 1, wherein the seed crystal module comprises a seed crystal holder (120) having at least one vapor release opening (130), and the silicon carbide seed crystal (100) is positioned in the seed crystal holder (120). Method according to claim 2, wherein the seed crystal holder (120) comprises a plurality of vapor release openings (130). Method according to claim 3, wherein the seed crystal holder (120) has a central axis (215) perpendicular to the lower surface (160) of the silicon carbide seed crystal (100) and wherein the plurality of vapor release openings (130) are arranged symmetrically around the central axis (215). Method according to claim 3, wherein the silicon carbide seed crystal (100) is a circular silicon carbide wafer and wherein the plurality of vapor release openings (130) are at the same distance from the central axis (215). Method according to claim 2, wherein the crucible (210) comprises a ventilation hole or several ventilation holes (260), the hot zone (203) further comprises at least one vapor release ring (280) which comprises a hole or several holes (270), and wherein the vapor release ring (280) is arranged above the seed crystal holder (120) with at least one of the holes (270) aligned with at least one of the ventilation holes (260) of the crucible (210). Method according to claim 2, wherein the vapor release opening (130) is located at a position below the lower surface (160) of the silicon carbide seed crystal (100). Method according to claim 1, wherein the upper surface (150) of the silicon carbide seed crystal (100) comprises a seed crystal protective layer. Method according to claim 8, wherein the seed crystal protective layer has a thickness of less than 250 micrometers. Method according to claim 8, wherein the seed crystal protective layer has a thickness of less than 100 micrometers. Method according to claim 10, wherein the thickness of the seed crystal protective layer is 10 micrometers to 90 micrometers. Method according to claim 10, wherein the thickness of the seed crystal protective layer is 30 micrometers to 80 micrometers. Method according to claim 10, wherein the thickness of the seed crystal protective layer is 50 micrometers to 70 micrometers. Method according to claim 8, wherein the seed crystal protective layer comprises at least two coating layers. Method according to claim 14, wherein at least one coating layer is a hardened photoresist layer. Method according to claim 15, wherein the hardened photoresist layer has a thickness of 2 micrometers to 5 micrometers. Method according to claim 14, wherein the at least one coating layer is a graphite coating layer. Method according to claim 17, wherein the graphite coating layer has a thickness of 20 micrometers to 30 micrometers. Method according to claim 8, wherein the silicon carbide seed crystal (100) has a silicon side and a carbon side and wherein the upper surface is the silicon side. The method of claim 8, wherein the silicon carbide seed crystal (100) has a silicon side and a carbon side and wherein the upper surface is the carbon side. Method according to claim 8, wherein the seed crystal protective layer is a hardened multilayer composite material comprising 2-5 coatings.

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