Membrane for physical vapor transport growth chambers

Abstract

The present invention relates to a physical vapor transport (PVT) growth chamber comprising: a source of silicon carbide for the sublimation of vapor species derived therefrom; a silicon carbide seed crystal; and a membrane disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation vapor species migrate through the membrane towards the silicon carbide seed crystal. The membrane has a D50 pore size in the range of 10 µm to 200 µm and the membrane consists of or comprises one or more of glassy carbon and pyrocarbon.

Description

[0001] 24PC04SC

[0002] 1

[0003] MEMBRANE FOR PHYSICAL VAPOR TRANSPORT GROWTH CHAMBERS

[0004] Field

[0005] The present disclosure relates to membranes for physical vapor transport (PVT) growth chambers; and PVT growth chambers comprising therein; and a method of producing said membranes.

[0006] Background

[0007] Crystalline silicon carbide is a semi-conductor valued for its fast, high-temperature and / or high voltage performance. A significant problem for single crystal SiC product in PVT growth chambers has been the elimination of defects: edge dislocations, screw dislocations (both hollow and closed core), triangular defects and basal plane dislocations.

[0008] The formation of single crystal silicon carbide is obtained from a source of silicon carbide, which is heated to elevated temperatures such that the source of silicon carbide undergoes sublimation to form silicon and carbon vapor. Through controlling the homogeneity of the ratio between the silicon vapor to carbon vapor, and the purity thereof, the growth of a single crystal silicon carbide from a seed crystal can be controlled and therefore defects minimized.

[0009] Membranes have been used to control the concentration and flow rate of vapor species. In the current state of the art, the membrane is graphite. Graphite has a relatively high reactivity to the sublimation vapors, especially Si vapor and this causes erosion of the porous member which reduces its filtering ability and generates carbon particles. These contaminants may become entrained in the vapor and disrupt the uniform growth of SiC crystals, causing dislocations within the crystal structure.

[0010] While the tantalum carbide coatings or the like can provide a protective layer in helping suppress impurity migration through and from the membrane, the chemical vapor deposition process used to coat the membrane is unable to completely coat all of the internal porous pathways within the membrane. While the use of a TaC coating may reduce carbon contamination and be less reactive, there is further scope to reduce carbon contamination and / or reactivity of the membrane in the growing of SiC crystals.24PC04SC

[0011] 2

[0012] Therefore, there is still a need for improved PVT growth chamber membranes to enhance the quality of single crystal silicon carbide that can be produced.

[0013] Summary

[0014] In a first aspect of the present disclosure, there is provided a physical vapor transport (PVT) growth chamber comprising:

[0015] a. a source of silicon carbide for the sublimation of vapor species derived therefrom;

[0016] b. a silicon carbide seed crystal; and

[0017] c. a membrane disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation vapor species migrate through the membrane towards the silicon carbide seed crystal, wherein the membrane has a Dso (median) pore size in the range of 10 pm to 200 pm and the membrane consists of, essentially consists of, or comprises, of one or both of glassy carbon (GC) and pyrocarbon (PyC).

[0018] The membrane may comprise a first surface in communication with a source of silicon carbide and a second surface in communication with the silicon carbide seed crystal; wherein one or both of the first and second surfaces consists of, or comprises, one or both of glassy carbon and pyrocarbon.

[0019] In some embodiments, the membrane comprises at least 10 wt% or at least 20 wt% or at least 30 wt% of at least 40 wt% or at least 50 wt% or at least 60 wt% of at least 70 wt% or at least 80 wt% or at least 90 wt% or 100 wt% of glassy carbon, pyrocarbon, or a combination thereof.

[0020] In some embodiments, the membrane (i.e. GC / PyC membrane) may comprise a substrate coated with one or both of glassy carbon and pyrocarbon. In these embodiments, the first and / or second surface may comprise or consist of one or both of glassy carbon, pyrocarbon and the substrate. This may be due to some of the substrate not being coated with one or both of glassy carbon and pyrocarbon. Preferably, the majority (e.g. > 50%) of the first and / or second surface is covered by glassy carbon and / or pyrocarbon. In some embodiments, the membrane comprises at24PC04SC

[0021] 3

[0022] least 90 wt% or at least 95 wt% or at least 98 wt% or essentially consists of glassy carbon and / or pyrocarbon.

[0023] The substrate may be an uncoated membrane or it may be a plurality of particles in which the coating functions to both coat the particles and bind the particles together to form a membrane.

[0024] The membrane may be directly or indirectly connected to a crucible. The crucible may form an internal surface of the PVT growth chamber. The crucible may comprise walls and a top and a base. The membrane may directly or indirectly sealingly connect to the crucible. When indirectly sealingly connected to the crucible, the membrane connects to another structure which is connected to the crucible. The other structure is preferably non-porous such that sublimation vapors are directed through the GC, PyC or GC / PyC membrane. Indeed, the membrane is preferably configured such that the sublimation vapor must pass through the membrane to reach the SiC seed crystal. This ensures that the membrane prevents particulates migrating onto the surface of the growing SiC crystal.

[0025] The membrane is typically in the shape of a planar disc when the membrane transverses across the walls of the crucible, separating a source of SiC (e.g. raw material zone) from a SiC seed crystal (e.g. final product zone). In other embodiments, the membrane is in the shape of a cylinder extending between the base and top of the crucible, separating a source of SiC from a SiC seed crystal. Other configurations are also possible depending upon the specific geometry of the PVT growth chamber.

[0026] The PVT growth chamber of the present disclosure comprises a membrane with excellent permeability, high thermal conductivity, durability and low friability, which is conducive to the efficient production of high-quality single crystal SiC.

[0027] Glassy carbon foam

[0028] In one embodiment, the membrane comprises or consists of an open cell foamed glassy carbon structure. Methods of producing an open cell foam of glassy carbon may be found in WO1998002382, RU2753654 and US4022875 which are herein disclosed by reference. A membrane comprising an open cell glassy carbon foam, which may have a reticulated structure, typically comprises a high porosity in the range of 80 to 98% v / v24PC04SC

[0029] 4

[0030] or 90 to 97% v / v. The pores in the foamed membrane may have an aspect ratio in the range of 0.5 to 2.0 or 0.7 to 1.5.

[0031] In another embodiment, the membrane comprises a substrate coated with glassy carbon. The substrate may comprise porous graphite or porous carbon or any other suitable material (e.g. graphite fibre / graphite felt). Other suitable materials need to be thermally stable at the operating conditions of the PVT growth chamber (e.g TaC or NbC or other metal carbides or nitrides).

[0032] Glassy carbon

[0033] Glassy carbon (GC) is generally produced by pyrolysis / carbonization of polymers. In the case of thermally stable membrane substrates such as porous graphite, the coating according to the disclosure may also be obtained by application of a polymer coating atop the porous graphite substrate as a precursor, followed by pyrolysis. Suitable processes for producing GC coatings are described for example in US 3,109,712, US 3,854,979 and US 4,816,338, which are incorporated herein by reference.

[0034] US 3,854,979 relates to the application of GC layers onto medical implants made of carbon or ceramic. The process disclosed therein includes heating and partially pyrolyzing a precursor polymer material selected from halogenated polymers such as PVC and natural organic materials such as petroleum pitch to obtain a pitch-like material having the approximate formula CnHn, mixing the pitch-like material with an aromatic solvent to obtain a slurry, coating the substrate with the slurry and baking the substrate. US 4,816,338 describes a similar process specifically for coating graphite.

[0035] Unless otherwise specified, the pyrolysis of glassy carbon precursor liquid shall reference the processes outlined in US 3,109,712, US 3,854,979 and US 4,816,338. In particular, the pyrolysis of a glassy carbon precursor liquid to form glassy carbon may be summarized as:

[0036] • Drying and / or curing the glassy carbon precursor liquid; and

[0037] • Heating the glassy carbon precursor such as cured organic polymers (e.g.

[0038] phenol aldehyde) to at least 800°C at a heating rate of between 1°C to 30°C / hr (or 2 to 20°C / hr); and maintaining a reached temperature for up to 24hr or until24PC04SC

[0039] 5

[0040] the glassy carbon has been formed. The maximum temperature reached may be up to 2000°C or up to 3000°C.

[0041] The glassy carbon precursor liquid or mixture may comprise the organic polymer or thermal decomposed component thereof and a solvent, (e.g. aromatic solvent, halogenated aliphatic solvents or water). The solvent is preferably selected to dissolve the organic polymer and form a miscible liquid. In some embodiments, the glassy carbon precursor may comprise a carbon source (e.g., furfural resin (e.g. furfuryl alcohol), phenolic resin (e.g. novolac-type resins), polyimide resin, polynaphthalene) and a polymerization agent (e.g. maleic anhydride).

[0042] In some embodiments, the glassy carbon precursors may be a carbon source comprising a carbon yield of at least 10% or at least 25% or at least 30% or at least 35% or at least 40% or at least 45% after carbonisation. Catalysts (e.g. NaOH or Na2COs) may be added to the glassy carbon precursor to increase carbon yields.

[0043] In some embodiments, the glassy carbon precursor may be a polyamide resin. Compared with phenolic and furfural resins, the glassy carbon obtained by polyimide carbonization is considered to have higher strength as well as a low thermal expansion coefficient.

[0044] In some embodiments, the pyrolysis / carbonization of the glassy carbon precursor liquid can be carried out in parallel with the graphitization of graphite precursor particles within the membrane.

[0045] Additives

[0046] The glass carbon precursor mixture may include additives such as curing agents, wettability agents, polymerization agents, filler materials, microstructure modifiers (e.g. porogens / pore forming agents) and processing aids such as viscosity modifiers. The microstructure modifiers may be in bead or fibre form. Specific additives may include, urotropine, (hexamethylenetetramine), methane sulfonic acid, sulfuric acid, benzoic acid, toluene sulfonic acid, trichloroacetic acid, phosphoric acid, sulfuric acid, dichloroacetic acid, fluorophosphoric acid, trifluoroacetic acid, hydrochloric acid, formic acid, hydrofluoric acid, hydrobromic acid, boric acid, benzenesulfonic acid, toluenesulfonic acid, and xylenesulfonic acid, boron trifluoride, aliphatic or cycloaliphatic24PC04SC

[0047] 6

[0048] polyamines (DETA, TETA, DACH, IPD), acid anhydrides (phthalic anhydride, maleic anhydride), activated carbon, carbon black, carbon nanoparticle, carbon nanotube, mesophase pitch powder, cokes, graphite powder, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), Poly(methyl-methacrylate) (PMMA) and polysaccharides like starch, sawdust, shell powder, ammonium carbonate, calcium carbonate, ammonium bicarbonate, ammonium chloride, moringa seed powder, chitosan, and other marine-derived polymers.

[0049] During processing, additives may be optionally added at up to about 50 to 60 wt%, with the majority of additives being in the form of polymerization agents, fillers and / or pore formers) relative to the total weight of the glassy carbon precursor mixture. When used, additives typically comprise at least 0.1 wt% or at least 0.2 wt% or at least 0.3 wt% relative to the total weight of the glassy carbon precursor mixture. For clarity, carrier solvents of the glassy carbon precursor are not considered an additive.

[0050] Within the final glassy carbon product, additives are typically filler materials up to about 10 wt% of the total weight of the glassy carbon product. Typically, the additives are in the range of 0.1 wt% to no more than 5 wt% or no more than 4 wt% or no more than 3 wt% of the total weight of the glassy carbon product.

[0051] Glassy carbon is an amorphous composition comprising non-graphitic carbon, which has properties consistent with that attributed to the carbonaceous material described in US 3,109,712. For the purposes of this disclosure, glassy carbon is deemed to be an amorphous carbon material; have resistance to oxidation (e.g. particles between 150 to 212 pm have less than 6.0 wt% or less than 5.0 wt% loss after exposure in air for 24 hours at 500°C); with isotropic morphology; or otherwise produced according to the methods in US 3,109,712, US 3,854,979 or US 4,816,338.

[0052]

[0053] Pyrocarbon (also referred to as pyrolytic carbon) coatings are applied using chemical vapor deposition (CVD), specifically through pyrolytic carbon deposition. In contrast to CVD coatings of TaC, pyrocarbon is able to more effectively coat a membrane due to its smaller gaseous precursor species which can more readily infiltrate fine pores. This process involves the thermal decomposition of hydrocarbon precursors at elevated24PC04SC

[0054] 7

[0055] temperatures in a controlled atmosphere, allowing carbon species to deposit on the substrate surface. The choice of precursor significantly influences the coating characteristics. Preferably, acetylene (C2H2) is employed when high deposition rates are desired. Additionally, common hydrocarbon precursors include alkanes such as methane (CH4), ethane (C2H6), and propane (C3H8); alkenes such as ethylene (C2H4) and propylene (C3H6); and aromatic hydrocarbons like benzene (C6H6), toluene (C7H8), and xylene (C8H10). Natural gas mixtures are common for large-scale processes.

[0056] The deposition environment is inert, such as argon or nitrogen, or reducing, such as hydrogen, to prevent oxidation during the coating process. CVD reactors can be hot-wall or cold-wall designs, operating under low pressure or atmospheric pressure. Deposition temperatures typically range from 1000 to 2200 °C, with the exact temperature chosen based on the desired microstructure. Lower temperatures, around 1000-1400 °C, tend to produce isotropic pyrocarbon, while higher temperatures, in the range of 1800-2200 °C, yield highly anisotropic, textured carbon. Microstructure control is achieved by adjusting temperature and deposition rate: isotropic pyrocarbon forms under lower temperatures and slower deposition, whereas anisotropic pyrocarbon forms at higher temperatures and faster deposition.

[0057] Preferably the pyrocarbon coating is isotropic. The pyrocarbon coating is also preferably smooth which would require a lower temperature and deposition rate. The coating will typically be subject to a graphitization temperature, just as the glassy carbon, to ensure stability at SiC growth temperatures. In some embodiments, the pyrocarbon coating is no less than 1 urn. In some embodiments, the substrate is porous graphite.

[0058] To produce a pyrocarbon coated membrane, a porous graphite part is placed in a CVD reactor under a flow of an inert carrier gas (e.g. argon) with the carbon precursor (acetylene) at a specified temperature (1000-1400 °C) until the desired coating thickness (e.g. >1 urn) is achieved (time dependent on growth conditions) to achieve the desired porosity. After coating, the sample may be finished with a graphitization heat treatment, preferably >=2300 °C.

[0059] An example of the coating of a membrane with pyrocarbon is provided in the journal article entitle “Fabrication of carbon coated ceramic membranes by pyrolysis of methane using a modified chemical vapor deposition apparatus”, Yuna-Yao Li, Tsuyoshi24PC04SC

[0060] 8

[0061] Nomura, Akiyoshi Sakoda, Motoyuki Suzuki; Journal of Membrane Science, Volume 197, issues 1-2, 15 March 2002, pages 23-35, which is incorporated herein by reference. Pyrocarbon / Pyrolytic carbon has a semicrystalline turbostratic carbon structure, with significant disorder between graphene layers, which are bound mostly by weak van der Waals forces. Pyrocarbon may be characterized by Raman spectroscopy and XRD analysis. Pyrolytic carbon exhibits a broad (002) diffraction peak in the range of 20 = 20°-26° (using a Cu X-ray source). This breadth indicates a low degree of graphitization and small average crystallite height (Lc). The key Raman spectroscopy markers include a D-band at around 1350 cm’1(for a 1.97 eV laser excitation), a G-band located around 1580 cm’1and a 2D band located around 2700 cm’1.

[0062] In some embodiments, the coating layer comprises or consists of glassy carbon. Glassy carbon is generally less reactive to silicon vapor than pyrocarbon. Therefore, glassy carbon is considered more durable than pyrocarbon. However, both coatings are significantly less reactive to silicon vapor than conventional porous graphite membranes. In some embodiments, the average thickness of the glassy carbon and / or pyrocarbon coating, when present, may be in the range of 500 nm to 50 pm. In some embodiments, the coating average thickness is at least 1.0 pm or at least 2.0 pm. In some embodiments, the coating average thickness is no more than 40 pm or no more than 30 pm or no more than 20 pm or no more than 10 pm. Lower thickness levels may result in significant portions of the membrane substrate which remain uncoated or exposed due to erosion during operation. Higher thickness levels may result in a decrease in permeability of the membrane.

[0063] In a second aspect of the present disclosure, there is provided a process of producing a membrane comprising:

[0064] A. applying a glassy carbon precursor mixture to a surface of a substrate to form a green product, said glassy carbon precursor mixture comprising an organic polymer, a solvent and optional additives;

[0065] B. allowing the green product to dry to evaporate the solvent;

[0066] C. curing the green product to form a cured product;

[0067] D. placing cured product in an oven with an inert atmosphere and heating the membrane to a sufficient temperature and for sufficient time to enable24PC04SC

[0068] 9

[0069] the organic polymer to thermally decompose and form a glassy carbon layer on the surface of the substrate; and

[0070] optionally, repeating steps A. to C. to increase the thickness of the glassy carbon layer on the surface of the substrate.

[0071] The green product comprises the substrate and glassy carbon precursor mixture.

[0072] The baking step D is preferably carried out at a temperature of at least 900°C or at least 1000°C or at least 1100°C or at least 1200°C or at least 1300°C or at least 1400°C. In some embodiments, the baking temperature is no more than 2800°C or no more than 2600°C or no more than 2400°C or no more than 2200°C or no more than 2000°C. While the higher temperatures assist in graphitization (when required), the quality of the glassy carbon material may be adversely affected at these higher temperatures. The time required to form the glassy carbon will depend upon the heating curve used. But it is typically at least 12 hrs or at least 24 hrs and typically no more than 14 days. In embodiments where graphitization of the substrate is not required (e.g. the substate is glassy carbon or TaC), the heat treatment cycle may or may not replicate graphitization conditions. If not replicating the graphitization cycle, temperatures and dwell times appropriate for purification purposes may be used.

[0073] In some embodiments, the substrate is a “uncoated” membrane, such as a porous graphite substrate. In other embodiments, the substrate is a plurality of particles and the glassy carbon precursor liquid coats and binds the particles together to form a membrane. The pores are created through gaps between coated particles. The wt% proportion of glassy carbon binder / coating to particles may in the range of 3:97 to 50:50. The particles may be any suitable material including metal carbides, metal nitrides, graphite; graphite precursor particles or glassy carbon particles. The particle size distribution of the particles is selected to obtain a targeted pore size distribution in the final product. The skilled artisan will be aware that the size distribution may be dependent upon the shape and the size of the particles. In one embodiment, the particle size distribution of the particles is in the range of 5 to 1500 pm or in the range of 8 to 500 pm or in the range of 5 to 250 pm and preferably in the range of 20 to 100 pm. In a preferred embodiment, the substrate is a plurality of glassy carbon particles. The glassy24PC04SC

[0074] 10

[0075] carbon particles may be formed from first producing larger components of glassy carbon and then is broken down to the desired particle size range. While the process typically produces a lower porosity membrane compared to a foamed GC membrane (e.g. in the range of 20 to 70% v / v or 30 to 60% v / v), the resultant glassy carbon membrane has been found to be highly durable due to the combination of glassy carbon’s inertness and mechanical properties. In addition, the pore size distribution is fine tunable through modification of the particle size distribution of the glass carbon particles used as the substrate.

[0076] In some embodiment, a method of producing a membrane includes the following steps:

[0077] • Mixing of metal carbides, metal nitrides, glassy carbon, graphite or graphite precursor particles with a binder. The binder system comprises glassy carbon precursors liquid, typically comprising an organic resin or (organic resin derived compound) diluted with a suitable solvent (e.g. an aromatic solvent such as toluene). Optional additives, including fillers and pore forming agents (e.g ammonium bicarbonate) may also be included in the mixture. Pore forming agents may include organic fibers, such as cotton fibres, carbon or polymeric fibres.

[0078] • Loading the mixture into a mold;

[0079] • Compacting the mixture;

[0080] • Curing the mixture under conditions to favor the formation of glassy carbon (e.g.

[0081] curing in air up to 180°C at a heating rate of < 5°C / min). The curing time may vary from about 6 to 100 hrs.

[0082] • Pyrolyzing the mixture to a temperature between 800 to 1200°C at a heating rate in the range of 1°C / hr to 200°C / hr (or 5°C / hr to 50°C / hr) in an inert atmosphere. In the pyrolyzing process, the temperatures may be optionally held at 300, 400, 500, 600, and 1000 °C for 60 min in order to drive away gaseous reaction products at different stages.

[0083] • If graphite precursor materials are used, roasting optionally involving several heating steps at temperatures in the range of about 600 to 1700 K (327 to 1427°C), may be performed over several days (e.g. up to 2 or 3 or 5 or 10 days). In some embodiments, the graphitization temperature may occur under vacuum24PC04SC

[0084] 11

[0085] (e.g. 50 to 150 Pa at temperatures up to about 2600 K (2327°C) for about 24 hours. Freon and / or chlorinated gases may be added during the heat treatment for purification purposes; and

[0086] • Machining the membrane into the desired shape.

[0087] Graphite precursor materials may be preferred over graphite material to avoid cracking of the glassy carbon binder and the graphite particles due to differences in shrinkage rates during cooling and thermal annealing. Graphite precursor material may be selected to better match the shrinkage rate of the glassy carbon material.

[0088] The graphite precursor materials may include carbon-rich materials, such as those derived from biomass (e.g. risk husks, wood), coal, or petroleum (e.g. petroleum coke) or any other carbon rich material (e.g. with a carbon yield of greater than 50% or greater than 80%), which upon being raised to temperatures typically between 2,200 to 3,000°C for sufficient time converts to graphite in a process which involves rearranging the carbon atoms into a crystalline lattice.

[0089] The viscosity of the coating / binder system preferably has a viscosity at room temperature in the range of 1 to 10,000 centipoise (cP) or 2 to 5000 cP or 3 to 3000 cP or 4 to 1000 cP or 5 to 500 cP or 6 to 200 cP or 7 to 100 cP. The viscosity of the coating / binder system may be altered by adjusting the proportion of solvent to glassy carbon polymer precursor (e.g. organic polymer). The viscosity of the coating / binder may be adjusted to adjust the thickness of the coating or binder layer, with lower viscosity generally resulting in lower coating / binder thicknesses. In some embodiments, the viscosity of the glassy carbon precursor liquid may be adjusted to control the ability of the glassy carbon precursor liquid to penetrate sufficiently into a membrane, in which dip-coating or vacuum impregnation techniques are employed. In general, the viscosity is lower for a coating system than a binder system. A coating system generally has a viscosity (at room temperature) of no more than 500 cP or no more than 200 cP or no more than 100 cP.

[0090] The membrane substrate is preferably cleaned to remove any surface contaminants. The pore size distribution of the membrane provides a balance between being sufficiently large to enable sufficient permeability through the membrane to achieve24PC04SC

[0091] 12

[0092] commercial growth rates, whilst being still able to prevent the migration of particulate matter through the membrane, thereby reducing the defects in the resultant SiC crystal. In one embodiment, the membrane substrate comprises a pore size distribution with a Dso in the range of 40 to 150 pm and the coated membrane comprises a D50 in the range of 30 to 70 pm.

[0093] In a third aspect of the present disclosure, there is provided a membrane as defined in the first aspect of the present disclosure or produced according to the second aspect of the present disclosure.

[0094] The glassy carbon precursor liquid may comprise the organic polymer and / or a decomposition product thereof. For the purposes of the present disclosure, the organic polymer is inclusive of monomers and decomposition (e.g. thermal decomposition) products thereof.

[0095] According to the present disclosure, a thermal decomposition product obtained by thermal decomposition of an organic polymer is used as the raw material for the preparation of glassy carbon. The carbonizable precursor organic polymers are not specifically limited according to the disclosure. Examples of the organic polymers include thermosetting resins, for example based on aromatic compounds, for example phenolic resins such as phenol / formaldehyde resins, alkyl phenol resin, oil-soluble phenol resin, resins based on furfuryl alcohol; halogenated polymers for example chlorinated paraffin, chlorinated polypropylene or the like. Carbonizable vinylic or acrylic polymers such as polyacrylonitrile or polyvinyl chloride vinyl, polyvinyl alcohol, vinyl acetate resin and polycarbonate resin may also be used.

[0096] The glassy carbon precursor liquid may comprise a thermal decomposition product of the organic polymers. The thermal decomposition of these organic polymers may preferably be conducted in such a manner that the polymers in the powdered or pelletized form is heated at 200°C. to 500°C. for at least 30 minutes in an inert atmosphere such as argon gas, no matter which kinds of the polymers is employed. The process of the thermal decomposition should be terminated before the organic polymer is completely carbonized. Although the heating temperature and time may vary depending on the heating equipment and the kind of the organic polymer employed, they may be determined by experiments so that the weight ratio of the carbon atoms to24PC04SC

[0097] 13

[0098] hydrogen atoms (C / H weight ratio) of the decomposition product be ultimately in the range of from 10 to 25:1.

[0099] The thermal decomposition product of the organic polymer thus produced is pitch-like and in the solid form at an ambient temperature. The thermal decomposition product of the organic polymer is then dissolved in a solvent to produce a solution, preferably with a concentration of 50 to 500 g / l or in the range of 100 to 400 g / L. The lower concentration limit may be dictated by the ability of the solution to form a continuous coating of the substrate; whilst upper concentration limit may be dictated by the target coating thickness and the propensity of the coating to crack. In view of solubility, the halogenated aliphatic solvents are preferred although other suitable solvents, such as an aromatic solvent (e.g. toluene) or water may also be used. Insoluble matter, if any, that is left in the solution is filtered off and removed.

[0100] In some embodiments, the glassy carbon precursor mixture may also comprise a heat resistance inorganic material, such as graphite (preferably with a long diameter of no more than 20 pm or no more than 10 pm or no more than 5 pm).

[0101] The raw material including the thermal decomposition product of the organic polymer and any heat-resistant inorganic material is attached or affixed to the graphite substrate and the substrate with the raw material thus affixed thereto is heated. Preferably, the graphite substrate has a difference in the thermal expansion coefficient of not larger than 1.5 x 10’6 / °C. from that of the glassy carbon coating. This assists in preventing the formation of cracks or peeling of the glassy carbon coating.

[0102] The glassy carbon precursor mixture may be applied using any suitable method such as ultrasonic impregnation, vacuum impregnation, brushing, spraying or dipping of the membrane substrate. After application of the coating, the membrane with the coating may be dried at a lower temperature of 50°C. to 100 °C. As mentioned hereinabove, the thickness of the coating is critical and should be controlled to be in the range of preferably 500 nm to 200 pm or 1.0 pm to 100 pm or 5 pm to 50 pm. The average coating thickness may be computed from the difference in weight of the coated membrane substrate before and after the application of the coating in combination with measuring the surface area of the membrane substrate prior to coating. Alternatively, the coating thickness may be determined via comparing the pore size distribution of the24PC04SC

[0103] 14

[0104] membrane substrate with the coated membrane product. A decrease in the Dso pore size from 100 pm to 60 pm may indicate a coating thickness of 20 pm. The thickness of the coating layer may be increased in any desired manner by repetition of the aforementioned application and drying / curing, applying the slurry admixed with the heat-resistant inorganic substance to an increased thickness, or by elevating the concentration of the organic polymer or the thermal decomposition product of the organic polymer.

[0105] In some embodiments, the glassy carbon coats at least 50% or at least 60% or at least 70% or at least 80% of a surface of one or more pores of the membrane. This may be determined from analysis of optical microscopic images of a cross-section of the coated membrane.

[0106] Carbonization / Pyrolysis may be carried out in a plurality of stages, for example, a first stage at about 500° C. to 800° C. to eliminate hydrogen, water or halogen, and a second stage at about 1000° C. to 1400° C. to effect further condensation of the typically largely aromatic hydrocarbon structures formed in the first stage. The temperature and the duration of the carbonization may be suitably chosen according to the type of precursor polymer and the substrate material. The maximum temperature may in some cases be limited by the stability and the melting point of the membrane substrate. The carbonization may be performed under reduced pressure and / or an inert gas atmosphere.

[0107] Membrane substrate

[0108] The membrane substrate may be any suitable which is able to withstand the temperatures within the PVT growth chamber during operation (e.g. in the range of 2400 to 2800 K (2127 to 2527°C)). While metal carbides (e.g. TaC and NbC) and metal nitrides may be suitable, porous graphite is generally preferred due to its good machinability and cost. A glassy carbon coating has the advantage of providing increased thermal conductivity, benefiting the ability of the PVT growth chamber to control the thermal gradient therein. The smoother surfaces of the glassy carbon compared to graphite also benefit improve vapor flow through the membrane.24PC04SC

[0109] 15

[0110] Suitable membrane substrates may be obtained commercially for coating with glassy carbon. Alternatively, a porous graphite membrane substrate may be manufactured according to the following process:

[0111] • Mixing of graphite or graphite precursor materials with a binder. The binder system should be chosen to give ample carbon yield upon carbonization (e.g. greater than 25% or greater than 30% or greater than 40%). Binders can be polymeric (e.g. phenolic resin), petroleum pitch, preferably coal-derived pitch or tar. Optional additives, such as fillers and pore forming agents (e.g ammonium bicarbonate) may also be included in the mixture. Pore forming agents may include organic fibers, such as cotton fibres, carbon or polymeric fibres or beads.

[0112] • Loading the mixture into a mold;

[0113] • Compacting the mixture;

[0114] • Heat treating the mixture in a furnace to roast the mixture to produce porous graphite and then further heat treat the porous graphite to induce graphitization and preferably remove contaminants to purify the graphitized porous graphite. Roasting may involve several heating steps at temperatures in the range of about 600 to 1700 K (327 to 1427°C), performed over several days. The graphitization temperature may occur under vacuum (e.g. 50 to 150 Pa at temperatures up to about 2600 K (2327°C) for about 24 hours. In some embodiments, the graphitization temperature may be up to 2800°C or up to 3000°C. Graphitization temperatures of at least 2200°C are preferred. Freon and / or chlorinated gases may be added during the heat treatment for purification purposes; and

[0115] • Machining the porous graphite into the desired shape.

[0116] CN116120079, which is disclosed herein by reference, discloses a representative example of how to produce a porous graphite grade with a specified permeability. By changing manufacturing process parameters, as would be known to the skilled artisan, porous graphite properties can be tuned to obtain the desired performance in the PVT SiC growth. CN118026684 and CN1186197043, which are disclosed herein by reference, also disclose suitable methods of producing porous graphite for the PVT SiC growth.

[0117] Membrane24PC04SC

[0118] 16

[0119] The membrane of the present disclosure comprises a first surface and an opposing second surface. The distance between the first and second surfaces being the thickness of the membrane. The SiC sublimation vapors (e.g. Si, Si2C and SiC2) flow in through the first surface of the membrane and flow out of the second surface of the membrane.

[0120] The membrane comprises the membrane substrate with one or both of a glassy carbon and pyrocarbon coating. The membrane may be coated on the first surface or the second surface. Preferably, the membrane is completely coated with one or both of glassy carbon and pyrocarbon or at least 10% or at least 20% or at least 30% or at least 50% of the membrane substate’s surface is coated with one or both of glassy carbon and pyrocarbon. In another embodiment, the extent of one or both of glassy carbon and pyrocarbon coating is such that the oxidation resistance of the membrane is closer to the oxidation resistance of glassy carbon and / or pyrocarbon to that of the underlying substrate (e.g. graphite). For example, if graphite has a 20 wt% weight loss after exposure in air at 500°C for 24 hrs; with glassy carbon having a 2 wt% weight loss, then a glassy carbon coated membrane with a weight loss of less than 9 wt% would meet this criteria. A similar criteria may also be used in regard to friability of the glassy carbon coated membrane relative to a graphite membrane. Pyrocarbon has similar properties to glassy carbon coated membranes.

[0121] The pore size distribution of the membrane compared to the uncoated membrane substrate is preferably such that the Dso (median pore size) has been reduced in the range of 1 to 100 pm or 5 to 50 pm.

[0122] The mechanical properties of the membrane should be sufficient to withstand the installation and operating environment. In some embodiments, the flexural strength of the membranes is in the range of 1.0 to 8.0 MPa. In a preferred embodiment, the flexural strength is at least 5.0 MPa or at least 6.0 MPa or at least 7.0 MPa. Preferably, in combination with high flexural strength the membrane also have good permeability (e.g. at least 5.0 Darcy or at least 10 Darcy or at least 20 Darcy or at least 30 Darcy or at least 40 Darcy).

[0123] Table 1 summarizes the different properties of the membrane.24PC04SC

[0124] 17

[0125] Table 1

[0126]

[0127] In some embodiments, the density of membrane ranges from 0.80 to 1.80 g / cc. The density of the membrane may be at least 0.85 g / cc or at least 0.90 g / cc or at least 0.95 g / cc or at least 1.00 g / cc or at least 1.05 g / cc or at least 1.10 g / cc. Lower membrane densities may negatively affect the mechanical strength of the membrane. The density of the membrane may be no more than 1.75 g / cc or no more than 1.70 g / cc or no more than 1.65 g / cc or no more than 1.60 g / cc or no more than 1.55 g / cc or no more than 1.50 g / cc or no more than 1.45 g / cc or no more than 1.40 g / cc or no more than 1.35 g / cc or no more than 1.30 g / cc or no more than 1.25 g / cc. Higher membrane densities may negatively affect the permeability of the membrane.

[0128] In one embodiment, the permeability of the membrane is between 5.0 and 100 Darcy. The permeability may be at least 7.0 Darcy or at least 10 Darcy or at least 15 Darcy or at least 20 Darcy or at least 25 Darcy or at least 30 Darcy. Lower levels of permeability may negatively affect the target growth rate of the single crystal SiC. The permeability may be no more than 80 Darcy or no more than 60 Darcy or no more than 55 Darcy or no more than 50 Darcy or no more than 45 Darcy or no more than 40 Darcy. Higher levels of permeability may negatively affect the ability of the membrane to block gas phase contaminants or particulates, thereby increasing defects in the single crystal SiC. In some embodiments, the porosity ranges between 20 and 70% or between 30 and 60% or between 35 and 55% of the total volume of the membrane. Porosity is measured by total volume occupied by void spaces relative to the total volume of the membrane.24PC04SC

[0129] 18

[0130] Pore sizes typically range from 0.1 to 500 pm. In some embodiments, the maximum pore sizes are limited. For example, less than 5% v / v or less than 4% v / v or less than 3% v / v or less than 2% v / v or less than 1 % v / v or less than 0.5% v / v of the pore volume is greater than 400 pm or greater than 300 pm or greater than 200 pm or greater than 100 pm or greater than 80 pm. The Dso pore size is typically at least 10 pm or at least 20 pm or at least 30 pm. Lower Dso pores sizes may negatively affect the permeability of the membrane. In some embodiments the membrane pore volume between 20 to 100 pm is at least 70% v / v or at least 80% v / v or at least 90% v / v or at least 95% v / v or at least 98% v / v.

[0131] In one embodiment, the Dso pore size is between 25 and 100 pm or between 30 and 80 pm or between 45 and 70 pm. Higher pores sizes may result in contaminants not being efficiently filtered by the membrane, resulting in the contamination of the single crystal structure. In one embodiment, the porous graphite membrane comprises two regions, wherein the two regions comprise a difference in Dso pore size of at least 5.0 pm or at least 8.0 pm or at least 10.0 pm or at least 15.0 pm or at least 20.0 pm. Typically, the difference in the Dso pore size between the two regions is no more than 50 pm or no more than 40 pm or no more than 30 pm or no more than 20 pm. In one embodiment, the pore size distribution comprises at least 60% or at least 65% or at least 70% or at least 75% or at least 80% or at least 85% of the pores being in the range of 20 and 80 pm or between 40 and 70 pm.

[0132] In one embodiment, the pore size distribution of is the following:

[0133] 2 to 20% or 9 to 15 % of pores smaller than 1 pm;

[0134] 1 to 20 % or 2 to 3 % of pores between 1 and 10 pm;

[0135] 65 to 90% or 75 to 87 % of pores between 10 and 100 pm; and

[0136] 1 to 15% or 2 to 7 % of pores larger than 100 pm.

[0137] The membrane preferably does not comprise any apertures which penetrate through the membrane, as this serves to by-pass the beneficial interaction between the SiC vapor species and the membrane as well as causing thermal “hot spots” due to the spikes of radiant heat that originate from the apertures.24PC04SC

[0138] 19

[0139] In one embodiment, the membrane may comprise porous graphite with a Dso pore size in the range of 20 to 140 pm or in the range of 25 to 120 pm or in the range of 30 to 100 pm or in the range of 35 to 80 pm or in the range of 40 to 60 pm. In a preferred embodiment, the membrane may comprise porous graphite with a Dso pore size in the range of 35 to 75 pm.

[0140] These pore size limitations effectively enable the membrane to filter out unwanted contaminants (e.g. carbon particulates or other particulate matter or impurities), whilst adjusting the vapor flow field to target a desired seed crystal growth rate and / or a desired seed crystal shape.

[0141] The pore size distribution of the membrane may comprise a D90 value of no more than 170 pm or no more than 160 pm or no more than 150 pm or no more than 140 pm or no more than 130 pm or no more than 120 pm or no more than 110 pm or no more than 100 pm or no more than 90 pm or no more than 80 pm or no more than 70 pm or no more than 60 pm or no more than 50 pm or no more than 45 pm. The pore size distribution of the membrane may comprise a difference between the D90 and Dso value of no more than 30 pm or no more than 25 pm or no more than 20 pm or no more than 18 pm or no more than 15 pm or no more than 12 pm or no more than 10 pm or no more than 8 pm or no more than 6 pm or no more than 5 pm. The narrow the pore size difference between Dso and D90 and narrower the pore size distribution and the more consistent the performance of the membrane is in terms of impurity removal and permeability. In some embodiments Dso - D10 > D90 - D10. In some embodiments D10 is in the range of 0.1 to 25 pm and preferably in the range of 0.2 to 10 pm or 0.5 and 3.0 pm.

[0142] For the purposes of the disclosure, an organic polymer is inclusive of degraded products thereof.

[0143] In some embodiment, the membrane comprises a continuous layer of porous material. The pores in the membrane provide a tortuous pathway through which vapor must travel from a first surface to a second surface of the membrane to reach the SiC seed crystal. A membrane is preferably exclusive of a layer of granules or particles.24PC04SC

[0144] 20

[0145] As used herein, the term "tortuous pathway" means a flow pathway having multiple branches, curves, angles, turns, etc., which prevent the straight path of flow. In some embodiments, the tortuous path increases the residence time of the vapor within the membrane to increase concentration uniformity. For clarity, the membrane preferably does not include through-holes which by-pass the tortuous pathway of the membrane. Reference to GC / PyC is reference to one or both of glassy carbon and pyrocarbon. The terms “glassy carbon precursor mixture” and “glassy carbon precursor liquid” are used interchangeably within the specification.

[0146] For clarity, the term “a first surface in communication with a source of silicon carbide and a second surface in communication with the silicon carbide seed crystal” means that the first surface is facing in the direction of, or has a pathway to, the source of the silicon carbide such that sublimation vapor species may flow from the source of silicon carbide to the first surface; and the second surface is facing in the direction of, or has a pathway to, the silicon carbide seed crystal, such that sublimation vapor species may flow from the second surface to the silicon carbide seed crystal.

[0147] Brief description of the Figure

[0148] Figure 1 is a schematic diagram of an apparatus for providing single crystal SiC comprising a PVT growth chamber wherein the GC / PyC membrane transverses across the walls of the chamber.

[0149] Figure 2 is a schematic diagram of an apparatus for providing single crystal SiC comprising a PVT growth chamber wherein the GC / PyC membrane extends from the base of the top of the chamber.

[0150] Figure 3 is an optical microscopic image of a cross-section of the membrane produced in Example 2.

[0151] Figure 4 is an optical microscopic image of a cross-section of the membrane produced in Example 4.

[0152] Figure 5 is a TGA spectra for a sample of glassy carbon and graphite.24PC04SC

[0153] 21

[0154] Figure 6A is an optical micrograph of a porous graphite sample of Example 5 after exposure to molten silicon.

[0155] Figure 6B is a profilometry micrograph of the porous graphite sample of Figure 6A. Figure 7A is an optical micrograph of a glassy carbon sample of Example 5 after exposure to molten silicon.

[0156] Figure 7B is a profilometry micrograph of the glassy carbon sample of Figure 7A.

[0157] Detailed description of a preferred embodiment

[0158] With reference to Figure 1 , there is provided an apparatus 10 for producing single crystal SiC comprising an insulation layer 20, typically made from graphite felt, encompassing PVT growth chamber 30. The growth chamber 30 comprises a graphite crucible 40. The graphite crucible may be permeable to gases such as nitrogen and argon, although the graphite crucible preferably does not have sufficient permeability to enable SiC sublimation vapors to pass through to any significant extent. Within the growth chamber there is provided a raw material zone 50 which comprises SiC or precursor thereof. At an opposing end of the growth chamber there is provided a silicon carbide seed crystal zone 60 including a seed crystal holder from which the single crystal silicon carbide is to grow. Located between the raw material zone and the silicon carbide seed crystal zone is positioned a GC / PyC membrane 70, which forms a transverse plane through which the sublimations vapors pass through. The GC / PyC membrane 70 comprises a first surface 72 in communication with the source of SiC 50 and a second surface 74 in communication with the SiC seed crystal zone 60. The membrane may directly or indirectly sealingly connect to the walls of the crucible. When the membrane indirectly sealingly connects to the walls of the crucibles, the membranes connects to another structure (not shown) which is sealingly connected to the crucible walls. The other structure is preferably non-porous such that sublimation vapors are directed through the GC / PyC membrane.

[0159] The GC / PyC membrane transverses the graphite crucible 40. In operation, induction heating coils 80 heats the contents positioned within depending upon their conductivity, including the crucible 40, raw materials 50 and the GC / PyC membrane 70. The temperature of the raw materials 50 may typically reach in the range of about 2600 to24PC04SC

[0160] 22

[0161] 2800 K, resulting in the sublimation of the raw materials into Si and SiC species. The temperature at the silicon carbide seed crystal zone 60 is typically approximately 2400 K to enable a sufficient temperature gradient for the sublimed raw materials vapors to flow towards the silicon carbide seed crystal zone 60.

[0162] The PVT growth chambers may have a number of different configurations. For example, Figure 2 illustrates an apparatus 210 comprising a GC / PyC membrane 270 which is cylindrical in shape and extends along an axial axis from a base 232 to a top 234 of the PVT growth chamber 230, with the silicon carbide seed crystal 260 on the interior side of the GC / PyC membrane cylinder and the source of silicon carbide 250 on the exterior of the GC / PyC membrane cylinder. The apparatus 210 also comprises an insulation layer 220 and heating coils 280. The membrane may directly or indirectly sealingly connect to the base and top of the crucible. When the membrane indirectly sealing connects to the base 232 and / or top 234 of the crucible 240, the membrane 270 connects to another structure (not shown) which is sealingly connected to the crucible top and / or base. The other structure is preferably non-porous such that sublimation vapors are directed through the GC / PyC membrane.

[0163] With reference to Figure 3, a cross section of a membrane comprises a substrate comprising porous graphite 300 and a plurality of pores 310. A glassy carbon coating 320 is disposed between the substate 300 and pores 310. The glassy carbon coating of pore (reference A) was determined to cover 85% of the cross-sectional perimeter of the pore, with only a small region of the pore’s perimeter 330 not coated with glassy carbon. The high degree of coverage by the glassy carbon coating 320 protects the underlying substrate from reacting with the sublimation vapors, thereby maintaining a more stable pore size distribution.

[0164] A glassy carbon (also referred to as vitreous carbon or glass-like carbon) based membrane may be produced by several methods. Coating methods may be applied to the surfaces of highly porous materials (e.g. carbon / graphite), or, entirely glassy carbon bodies may be prepared.

[0165] The preparation methods can be grouped as 1) dip coating a glassy carbon precursor over a bulk, porous substrate, 2) impregnation of a glassy carbon precursor onto a bulk, porous substrate, 3) coating a glassy carbon precursor over thermally resistant particles24PC04SC

[0166] 23

[0167] (e.g. carbon, TaC) and compacting, or 4) formation of an entirely glassy carbon porous body.

[0168] The first two methods coat the interior surfaces of the bulk, porous structures, without significantly reducing porosity. The third method forms a thin coating of glassy carbon precursor over the surface of particles to act as a binder and to hide the particle surfaces. Finally, the fourth method is composed entirely of glassy carbon with a glassy carbon binder used to bond together glassy carbon particles.

[0169] The GC interior coatings may be characterized by optical imaging of a sample cross section with polarized light. The glassy carbon exhibits a highly isotropic morphology. In addition to optical microscopy, temperature programmed oxidation (TPO), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), x-ray diffraction (XRD), Raman spectroscopy, and friability (or hardness) testing may be employed to characterize the glassy carbon. With reference to Figure 4, a cross section of a membrane comprising pores 400 encompassed by a number of glassy carbon particles 410, 420, 430, 440, 450, 460, 470, which are bound together with a glassy carbon binder phase 425, 435, 455. In comparison to the glassy carbon particles, the glassy carbon binder phase has a more motley appearance (e.g. compared to the appearance of particles 430 and 440 with binder phase 435 and particles 450 and 460 with binder phase 455).

[0170] Figure 5 shows Thermogravimetric Analysis (TGA) of glassy carbon prepared using a furfuryl alcohol precursor. Analysis was performed according to ASTM standard E2550 with first an inert atmosphere to 525°C, then an air atmosphere between 525°C and 570°C. Conditions were set to a heating rate of, an air flow of 10Oml / min, and a sample mass ~5mg that was sized to <45pm. Analysis demonstrates that the glassy material did not show either significant mass loss or gain, while the graphitized coke experienced an increase in weight up to almost 7%, presumably due to the uptake of oxygen in the graphitized material. The results confirm that a glassy carbon membrane would be less reactive than a graphite membrane.

[0171] Membrane methodologies24PC04SC

[0172] 24

[0173] The following methodologies are provided to form a membrane comprising a glassy carbon surface.

[0174] Dip coating: The glassy carbon precursor is applied by dipping the substrate material into the glassy carbon solution for in the range of 1 second to 1 hour e.g. for either 1 second dip or 10 second dip or 1 minute dip or 10 minute dip or 1 hour dip. The substrate is then removed from the solution, excess solution is allowed to drip off and may or may not be dried by heating and / or vacuum prior to curing. Drying under vacuum may aid in forming a more consistent coating thickness.

[0175] Impregnation

[0176] The glassy carbon precursor is applied via vacuum impregnation. Two methods are available.

[0177] a) Vacuum and then application - Vacuum is first pulled to a pressure in the range of -5 to -30 inHg. The glassy carbon precursor solution is then released to penetrate and coat the interior surface. The sample may be then soaked for between 1 second and 10 minutes.

[0178] b) Application and then vacuum - Samples are first submersed in the glassy carbon precursor and then vacuum is pulled to a pressure in the range of - 5 to -30 inHg, e.g. -5inHg or -10inHg or -15inHg or -20inHg or -25inHg or - 30inHg. The sample may be soaked for between 1 second and 10 minutes, e.g. 1 second or 10 seconds or 1 minute or 10 minutes.

[0179] Coating over thermally resistant particles

[0180] A glassy carbon precursor is applied to thermally resistant particles (such as carbon or other suitable particles e.g. TaC, which are suitable for use in PVT growth chambers) with sizes between 5 and 250 pm and preferably in ranges of 149-420 pm or 75-209 pm or 74-148 pm. The membrane may comprise in the range of 3 to 50 wt% glassy carbon precursor by weight. The remainder preferably comprises the thermally resistant particles. The sample is then molded and compacted.

[0181] Coating over glassy carbon particles24PC04SC

[0182] 25

[0183] A solid glassy carbon body is formed from curing and pyrolyzing of a cast glassy carbon precursor. The resulting solid body is crushed and ground to a desired particle size distribution. Glassy carbon particles may be sized between 5 and 250 pm or preferably between 20 and 100 pm. Glassy carbon precursor solution is added to the glassy carbon particles in a ratio of 3% or 5% or 10% or 20% or 30%, or 40%, or between 3 and 40 % of glassy carbon precursor by weight, with the glassy carbon particles comprising between 60 and 97% wt of the membrane.

[0184] For dip-coating and impregnation methods, samples may be removed from the solution and placed in empty vessels, where vacuum may be pulled to remove excess solution to retain open porosity. This method may also be used in the methods where the glassy carbon functions as both a binder and coating.

[0185] Samples may be subjected to curing in air up to 180°C under a heating rate of preferably less than 10°C / hr. Samples may be baked in the same cycle as curing or a separate cycle may be used. In some embodiments, samples are heated in a nitrogen atmosphere from 180°C to at least 1000°C at a rate of less than 5°C / min. Samples are then subjected to graphitization under a nitrogen atmosphere to a temperature above 2200°C.

[0186] Glassy Carbon precursor

[0187] Phenol formaldehyde (PF) resin (Plenco™ 14946) with water; or decomposed polyvinyl chloride (PVC) with toluene solvent with a room-temperature viscosity of 5-50 Pa s, may be used as a GC precursor for each of the methods above, with direct pyrolysis initiated after the coating / binder steps to produce glassy carbon or hybrid graphite / glassy carbon porous product.

[0188] Porous Graphite substrate

[0189] The porous graphite substrate used in the dip coating and impregnation methods are available in grade classified as PG25, PG45, PG60 and PG70 available from NEC-Morgan Porous Carbon and Graphite Products, 200 North Town Street, Fostoria, Ohio 44830, with a pore size distribution with a Dso in the range of about 35 to 100 pm. Membrane and GC Characterization:24PC04SC

[0190] 26

[0191] Oxidative resistance testing is performed on samples with a particle size distribution in the range of 150-212pm in air at a temperature of 500 or 550°C for 24 hours to measure weight loss (oxidation loss).

[0192] Friability or hardness testing can be performed according to ASTM standard D3802 with modified sieve sizes of 75pm, 150pm, and 212pm.

[0193] Flexural Strength testing is performed on rectangular bars for the purpose of assessing mechanical integrity. Flexural strength was measured following the guidance of ASTM C1161.

[0194] The temperature and intensity of chemical decomposition of carbon materials, characterized by TPO, is a suitable indicator for comparing relative reactivity. As such, decomposition intensity should be notably lower for glassy carbon compared to its substrate counterparts. TGA is also suitable. TGA should be performed in an air atmosphere in accordance with ASTM standard E2550.

[0195] An amorphous structure can be identified using XRD and / or Raman spectroscopy. The Lcand La peaks should still be easily identifiable under XRD, and the ID and IG peaks should be easily identifiable using Raman. These methods may be used for comparing crystallinity between materials.

[0196] Transmission Electron Microscopy (TEM) can be used to identify the cage-like structure which is characteristic of glassy carbon.

[0197] Pore Size Distribution (PSD) may be measured using mercury intrusion porosimetry. Coating thickness may be calculated from the PSD of the uncoated and coated membrane. For example, if the uncoated membrane had a Dso of 70 pm and the coated membrane had a Dso of 40 pm, then the average coating thickness will be deemed to be 70 - 40 / 2 = 15 pm. For membranes formed with a GC binder, the thickness of the binder may be determined by measurement of cross-sectional magnified images of the membrane.

[0198] Due to the structure of the glassy carbon coated porous carbons, reactivity measurements are conducted on the materials as two separate systems. That is, reactivity measurements are measured on glassy carbon only, and carbon substrate only. Because samples must be in particulate form for these tests, forming such samples24PC04SC

[0199] 27

[0200] from a glassy coated substrate would artificially inflate the exposed substrate surfaces, yielding non-representative reactivity values.

[0201] Method of producing membrane from petroleum coke particles and glassy carbon precursor liquid

[0202] Example 1

[0203] The glassy carbon composite membrane is formed by coating a petroleum coke with phenolic resin with 20 wt% water (phenolic resin solution). The petroleum particles are in the range of 74pm to 212pm. The petroleum coke particles are mixed with phenolic resin in a ratio of 200g petroleum coke and 75g of phenolic resin solution. The mixture is compacted, and pressure is maintained during baking and curing. Curing occurs in air up to 180°C, with a heating rate of no more than 0.1°C / min applied. The sample is held at temperature for 2 hours, and then further heated in a nitrogen atmosphere to 1000°C at a heating rate in the range of 0.1°C / min to 0.5 °C / min, then cooled at a rate of no more than 5°C / min. Finally, the sample is baked up to 2750°C in an inert atmosphere for sufficient time to graphitize the petroleum coke.

[0204] Example 2

[0205] A PG 70 grade, available from NEC-Morgan Porous Carbon and Graphite Products, 200 North Town Street, Fostoria, Ohio 44830, of porous carbon was used as the skeletal membrane which was dip coated with the furfuryl alcohol resin solution and heat treated (cured and graphitized) in accordance with Example 1.

[0206] Method of producing membrane from glassy carbon particles and glassy carbon precursor liquid

[0207] Example 3

[0208] A porous glassy carbon membrane is prepared from phenolic resin with 20 wt% water (phenolic resin solution) by curing in air to 180°C a heating rate of no more than 0.1 °C / min. The sample is held at temperature for 2 hours, and then heated in a nitrogen atmosphere to 1000°C at a heating rate in the range of 0.1°C / min to 0.5 °C / min of 0.1°C / min to to 0.5 °C / min, then cooled at a rate of no more than 5°C / min. The glassy carbon is then crushed to achieve a particle size distribution with a Dso in the range of24PC04SC

[0209] 28

[0210] 74pm to 212pm. Subsequently, the crushed material is mixed with phenolic resin solution in a ratio of 2.5g to 0.5g and pressed. The material is subject to the same curing and baking conditions as Example 1 , while pressure is maintained. The sample is baked up to 2750°C in an inert atmosphere to ensure the material is thermally stable at the operating temperature of the PVT growth chamber.

[0211] Example 4

[0212] The method of Example 3 was followed with some exceptions. Firstly, the particle size distribution is in the range of 50-500pm. Secondly, when the binding glassy carbon precursor is added to the already milled material, the material is subjected to minimal curing to no higher than 100°C, as to make the material able to be milled, but yet still flexible and can be hardened further after the milled material is compacted. An optical image of a cross-section of the resultant membrane is provided in Figure 4. The pore size distributions of the Examples 1 to 4 each possess a Dso in the range of 30 to 200 pm.

[0213] Results

[0214] Table 2

[0215]

[0216] As illustrated in Table 2, glassy carbon possesses superior oxidative resistance and low friability levels compared to graphite, which is conventionally used as membrane in PVT growth chambers for single crystal SiC production. The lower reactivity and friability of membranes comprising glassy carbon will inherently improve the quality of single crystal SiC produced in PVT growth reactors. The oxidation loss results of Table 2 are consistent with the TGA results obtained in Figure 5. When glassy carbon and graphite are heated in a TGA under air flow, graphite begins to gain weight between 5409C to 5659C when heated at a rate of 0.19C / min with air flow 100ml / min and sample mass ~5mg that was sized to <45pm. This demonstrates the more reactive atomic sites on graphite that readily chemisorb oxygen. In the case of glassy carbon, there are no24PC04SC

[0217] 29

[0218] available reactive sites for oxygen to bond and thus why glassy carbon does not change weight.

[0219] Additionally, the mechanical integrity of the glassy carbon-derived porous membranes has superior mechanical strength as compared to porous graphite membranes. The porous glassy carbon membrane prepared in Example 4 has a flexural strength of 18 MPa. This is 1.5 to 9 times higher than porous graphite with similar permeability (A PG 70 grade, available from NEC-Morgan Porous Carbon and Graphite Products, 200 North Town Street, Fostoria, Ohio 44830)

[0220] Example 5

[0221] A glassy carbon (GC) sample was compared with a porous graphite (PG) sample for degradation when exposed to silicon at high temperature.

[0222] The GC sample was prepared as followed:

[0223] Phenolic resin ( Plenco™ 14946) is placed into tray with a maximum resin depth of 1 inch. The heating apparatus is preheated to 95°C before placing in the resin loaded tray. Allow to dwell for 6 hours in air, then ramp the temperature up to 115°C over 16 hours in air before further ramping the temperature up to 250°C over 12 hours in air. The tray is removed from the heating apparatus and allowed to naturally cool down to room temperature. The heat-treated resin and then milled to a target particle size distribution between 5 and 1000 pm , with a Dso of 266 pm (Dio = 20 pm; D90 = 626 pm) The skilled artisan would be aware of variations and modification to the process and raw materials to change the particle size distribution as required. The milled material is then mixed with further phenolic resin (35 wt% resin; 65 wt% milled material) with the mixture then placed back into the heating apparatus, which is preheated to 100°C in air. The mixture is allowed to dwell under these conditions for 2 hours. The mixture is then removed from the heating apparatus and allowed to cool naturally to room temperature. The mixture is then filled into cylindrical molds of 13 mm diameter and 4 mm in height. The molded material is then baked through placing the molded material in a preheated heating apparatus at 95°C in an inert or reducing atmosphere. The temperature is then ramped to 200°C over 24 hours and then ramping to 1000°C over 136 hours before24PC04SC

[0224] 30

[0225] cooling down at a rate of 2°C per minute or less. The material is then baked up to 2750°C in an inert atmosphere to form the finished glassy carbon membrane disc. The porous graphite (PG) sample was machined into a cylindrical disc of 13 mm diameter and 4 mm height from a block of A PG 25 grade porous graphite, available from NEC-Morgan Porous Carbon and Graphite Products, 200 North Town Street, Fostoria, Ohio 4483.

[0226] The cylindrical disc samples were exposed to a molten silicon bath at 1800 °C for 3 hours to facilitate SiC formation, removed from the excess silicon, and then annealed at 2200 °C for 3 hours to sublimate the SiC. All heating was done in a nitrogen environment at 10 Torr. The samples, in addition to further porous graphite samples (PG 45 and PG 70) were also analyzed for permeability, pore size distribution and flexural strength.

[0227] Results

[0228] The results of the silicon reactivity experiment indicated a 52% loss of mass for the PG sample (Figure 6A & 6B). Moreover, the structural integrity of the PG sample was severely compromised after silicon exposure. This resulted in the sample cylindrical disc easily fragmenting and requiring special care to handle. In comparison, the GC sample experienced only a 10% loss of mass, and it remained robust post silicon exposure. The structural integrity of the cylindrical discs is depicted in Figures 6A and 7A (optical micrographs) of the PG and GC samples respectively. The profilometry micrographs (Figures 6B and 7B) illustrate the circular shape of the cylinder is fully retained for the GC sample (Figure 7B), with the profilometry data also indicating a relatively uniform and smooth topography in the GC sample. In contrast, the PG sample has become convex with the loss of height varying from about 1 to 2 mm in the central portion to 3 to 4 mm at the periphery (as illustrated by the higher color contrast in Figure 7B compared to Figure 6B). The results attest to enhanced resistance to silicon vapor degradation of the GC membrane in contrast to the conventional PG membrane.

[0229] As indicated in Table 3, the glassy carbon sample has a pore size distribution with a median size (Dso) of 72.7pm (72.5% v / v or pore volume within the range of 20 and 100 pm), which is in the preferred range for membranes within PVT growth chambers for24PC04SC

[0230] 31

[0231] producing single crystal SiC. Whilst the permeability is lower than that of the conventional porous graphite samples (PG 25 and PG45), the permeability of the glassy carbon membrane is still very much acceptable, being higher than the permeability of the PG 70 sample. The flexural strength of the glassy carbon sample is significantly higher all porous graphite samples (PG 25, PG 45 and PG 70) indicating that glassy carbon membrane has superior handleability properties, beneficial in the manufacturing process, to add to the superior chemical inertness as demonstrated by the silicon reactivity testing. These combined properties contribute to the improved performance of glassy carbon membranes relative to porous graphite membranes.

[0232] Table3

[0233]

[0234] Many variants, adaptations, product forms, uses, and applications of the present disclosure will be apparent to the person skilled in the art and are intended to be encompassed by this disclosure.

Claims

24PC04SC32Claims1. A physical vapor transport (PVT) growth chamber comprising:a. a source of silicon carbide for the sublimation of vapor species derived therefrom;b. a silicon carbide seed crystal; andc. a membrane disposed between the source of silicon carbide and the silicon carbide seed crystal, such that the sublimation vapor species migrate through the membrane towards the silicon carbide seed crystal, wherein the membrane has a Dsopore size in the range of 10 pm to 200 pm and the membrane comprises or consists of one or both of glassy carbon and pyrocarbon.

2. The PVT growth chamber of claim 1 , wherein the membrane comprises a first surface in communication with a source of silicon carbide and a second surface in communication with the silicon carbide seed crystal; wherein one or both of the first and second surfaces consists of or comprises one or both of glassy carbon and pyrocarbon.

3. The PVT growth chamber of claim 1 or 2, wherein the membrane directly or indirectly sealingly connects to a crucible, said crucible forming an internal surface of the growth chamber.

4. The PVT growth chamber of any one of the preceding claims, wherein the membrane comprises a substrate coated with one or both of glassy carbon and pyrocarbon.

5. The PVT growth chamber of any one of the preceding claims, wherein the membrane comprises a substrate coated with glassy carbon.24PC04SC336. The PVT growth chamber according to claims 4 or 5, wherein the substrate is selected from the group consisting of graphite, graphite precursor, glassy carbon, metal carbides and metal nitrides.

7. The PVT growth chamber of claim 6, wherein the substrate comprises porous graphite.

8. The PVT growth chamber of claim 6, wherein the substrate comprises graphite fibre or graphite felt.

9. The PVT growth chamber of claim 6, wherein the substrate comprises or consists of glassy carbon.

10. The PVT growth chamber according to any one of claims 4 to 8, wherein the average thickness of the glassy carbon and / or pyrocarbon coating is in the range of 5 nm to 50 pm.

11. The PVT growth chamber according to any one of the preceding claims, wherein the membrane comprises a Dso pore size in the range of 30 to 75 pm.

12. The PVT growth chamber according to any one of the preceding claims, wherein the membrane has a friability value in the range of 0.02 to less than 0.08 wt% loss.

13. The PVT growth chamber according to any one of the preceding claims, wherein the glassy carbon and or pyrocarbon coats at least 50% of a surface of one or more pores of the membranes.

14. A process of producing a membrane comprising:A. applying a glassy carbon precursor mixture to a surface of a substrate to form a green product, said glassy carbon precursor mixture comprising an organic polymer, a solvent and optional additives;B. allowing the green product to dry to evaporate the solvent;24PC04SC34C. curing the green product to form a cured product;D. placing cured product in an oven with an inert atmosphere and heating the membrane to a sufficient temperature and for sufficient time to enable the organic polymer to thermally decompose and form a glassy carbon layer on the surface of the substrate; andoptionally, repeating steps A. to C. to increase the thickness of the glassy carbon layer on the surface of the substrate.

15. The process according to claim 14, wherein the substrate comprises a pore size distribution with a Dso in the range of 40 to 150 pm and the membrane comprises a Dso in the range of 30 to 75 pm.

16. The process according to claim 14 or 15, wherein the glassy carbon precursor mixture comprises a concentration in the range of 50 to 500 g / L of organic polymer.

17. The process according to claim 16, wherein the heating of the glassy carbon precursor mixture results in the glass carbon layer coating the surface of the membrane.

18. The process according to any one of claims 14 to 17, wherein the glassy carbon precursor mixture is applied by dip coating or impregnation of a porous substrate.

19. The process according to any one of claims 14 to 18, wherein the substrate is a plurality of particles.

20. The process according to claim 19, wherein the heating of the glassy carbon precursor mixture results in the glassy carbon layer bonding the plurality of particles together to form the membrane.

21. The process according to claim 19 or 20, wherein the plurality of particles comprises have a particles size distribution with a D50 in the range of 5 to 40024PC04SC35pm; or a particle size distribution wherein 80 wt% of the particles comprise a particle size in the range of 5 to 400 pm.

22. The process according to any one of claims 19 to 21 , wherein the weight ratio of glassy carbon layer to the plurality of particles is in the range of 3:97 to 50:50.

23. The process according to any one of claims 19 to 22, wherein the particle size distribution of the plurality of particles and the proportion of glassy carbon precursor mixture is configured to obtain a coated membrane pore size distribution with a Dso in the range of 10 to 200 pm.

24. The process according to any one of claims 19 to 23, wherein the particles are composed of one or more of glassy carbon, graphite or graphite precursor.

25. The process according to any one of claims 19 to 24, wherein the plurality of particles are a graphite precursor and the green product is heated up to a temperature range of 600 (327°C) to 1700 (1427°C); and then raised to a temperature of up to 2800°C for sufficient time to form glassy carbon and optionally graphitize the substrate.

26. The process according to any one of claims 14 to 18, wherein the substrate is an uncoated membrane.

27. The process according to claim 26, wherein the glassy carbon precursor mixture has a viscosity at room temperature in the range of 1 to 10,000 cP.

28. A membrane as defined in any one of claims 1 to 13 or produced according to any one of claims 14 to 27.

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