WO1998048085A1

WO1998048085A1 - Chemical reaction barriers for use with silica containers and/or graphite support vessels in production of single crystal silicon

Info

Publication number: WO1998048085A1
Application number: PCT/US1998/007835
Authority: WO
Inventors: John D. Holder
Original assignee: Memc Electronic Materials, Inc.
Priority date: 1997-04-23
Filing date: 1998-04-16
Publication date: 1998-10-29

Abstract

A chemical barrier is disclosed for use in minimizing the extent of reaction between silica, SiO2, originating from a silica container and carbon, C, originating from a graphite support vessel supporting the container during production of single crystal silicon by a Czochralski-type process. The chemical barriers are preferably substantially non-reacting with respect to both silica and carbon. To the extent the chemical barriers react with silica and/or carbon, the reaction products include only stable, substantially non-reactive solids or alternatively, such solids with a relatively small amount of gases. Preferred materials suitable for use in the chemical barriers of the present invention (alone, as heterogeneous mixtures or as composite layers) include metals and/or metal oxides in contact with the outer surface of a silica container and metals, metal oxides and/or metal carbides in contact with the outer surface of a graphite support vessel. The chemical barriers of the present invention can be applied by forming a layer of barrier material, for example, by depositing or coating the barrier material to the inner surface of the silica container and/or to the outer surface of the graphite support vessel. Alternatively, the barrier can be applied as a thin foil, film or other sheet placed between the appropriate surfaces of the silica container and graphite support vessel.

Description

CHEMICAL REACTION BARRIERS FOR USE WITH SILICA CONTAINERS AND/OR GRAPHITE SUPPORT VESSELS IN PRODUCTION OF SINGLE CRYSTAL SILICON

BACKGROUND OF THE INVENTION The present invention generally relates to the production of single crystal silicon, and specifically, to silica containers and graphite support vessels used for the production of single crystal silicon.

Most single crystal silicon used for microelectronic circuit fabrication is prepared by the Czochralski (CZ) process. In this process, a single crystal silicon ingot is produced by melting polycrystalline silicon in a crucible, dipping a seed crystal into the molten silicon, withdrawing the seed crystal to initiate single crystal growth and growing the single crystal ingot.

Polycrystalline silicon is typically melted in vitreous silica crucibles or other silica-lined containers. Vitreous silica is the amorphous form of Si0₂, and crucibles made of vitreous silica are commonly referred to as quartz crucibles or fused quartz crucibles.

While vitreous silica is the material of choice for containing molten silicon during crystal growth in Czochralski-type processes, vitreous silica becomes less viscous with increasing temperature and becomes soft enough to flow under an applied stress at temperatures exceeding about 1815 K. Hence, vitreous silica containers are susceptible to a loss of structural integrity, including sagging and/or other deformation during production of single crystal silicon. As such, graphite support vessels such as susceptors or crucibles are typically used to support the vitreous silica crucibles, liners or other containers in which the polycrystalline silicon is melted. Additionally, several treatments are directed to enhancing the strength of the silica container, including for example lining the inner surface of the silica container with aluminum nitride (JP 93035119 B; JP 62216994 A), lining the inner and/or outer surfaces thereof with a uniform devitrified layer of crystalline silica (U.S. Patent No. 4,429,009 to Pastor et al . ) or coating the inner and/or outer surfaces thereof with a devitrification promoter (EP 0753605A) . The support of silica containers using graphite support vessels leads to a variety of problems which can influence the quality of single crystal silicon being prepared. For example, silica in contact with hot graphite can decompose while carbon can oxidize according to reactions which include:

Si0₂ + C → SiO(g) + CO(g); and Si0₂ + 3C → SiC + 2CO(g) , such reactions being collectively referred to herein as Si0₂-C reactions. The resulting carbon monoxide can become incorporated into and contaminate a single crystal silicon ingot drawn from the silicon melt contained within the container/support vessel system. Moreover, the oxidation of the graphite support vessel and diffusion of silicon from the silica container to the graphite support vessel contributes to a loss of structural integrity of the support vessel. To minimize carbon contamination of the silicon crystal and/or the impact of the Si0₂-C reactions and Si diffusion on the structural integrity of the support vessel, it has been proposed that the inner surface of graphite support vessels and/or the outer surface of silica containers be coated with SiC, TiC, NbC, TaC, ZrC (JP 7089789 A) or, alternatively, with glasseous carbon (U.S. Patent No. 5,476,679 to Lewis et al . ) . The use of a carbon-fiber- mesh lining for the graphite support vessel has also been suggested. (SU 1248333 A) . However, these approaches are not sufficiently adequate for preventing carbon contamination and for ensuring the structural integrity of the container/support vessel system. The proposed metal carbide coatings and carbon linings are generally too reactive with silica to be particularly useful in commercial silicon crystal growth. For example, an SiC coating between the container and support vessel rapidly deteriorates, probably due to reactions between SiC and Si0₂, which can occur even at relatively modest temperatures. Similar reactions between ZrC and Si0₂ and between TaC and Si0₂ are also thermodynamically favored to occur at such temperatures. Moreover, while much attention has been focused on preserving the structural integrity of the multiple-use support vessel, little effort has been directed to the structural degradation of the silica container caused by the Si0₂ reactions, and to the associated adverse effects of such degradation.

Hence, despite the aforementioned approaches, the Si0₂-C reactions and their adverse effects on the structural integrity of silica containers and on zero- defect crystal growth remain problematic . Because the rate of these adverse reactions increases dramatically with increasing temperature, this problem is becoming particularly acute as the industry shifts to the production of larger-diameter single crystal silicon ingots using larger polycrystalline silicon charges in larger capacity silica containers. The higher temperatures required in such applications increase the reaction rate of Si0₂-C reactions, causing silica container deformation and, ultimately, decreased zero- defect growth of the single crystal silicon ingot.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to prepare single crystal silicon of better quality, and particularly, to prepare single crystal silicon from larger charges while minimizing the extent of reactions occurring between silica and carbon, thereby improving the structural integrity of the silica container and the graphite support vessel, and ultimately, increasing the length of zero-dislocation growth of a silicon crystal grown therein. It is also an object of the invention to achieve single crystal silicon of such improved quality in a cost effective manner with minimum impact on existing commercial production methods.

Therefore, the invention is directed to an apparatus suitable for containing a pool of molten silicon during production of single crystal silicon. The apparatus comprises a silica container, a graphite support vessel and a chemical barrier. The silica container has an inner surface which defines a cavity capable of containing the pool of molten silicon and an outer surface which is in contact with and supported by the graphite support vessel. The graphite support vessel has an inner surface which defines a cavity adapted to receive the outer surface of the silica container, thereby providing support thereto during production of single crystal silicon. The support vessel also has an outer surface adapted for use in a Czochralski-type crystal puller. The chemical barrier is situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel. The chemical barrier preferably has a melting temperature greater than about 1500 °C. The chemical barrier can comprise an element or compound in contact with the outer surface of the silica container which does not substantially react with silica during production of single crystal silicon at temperatures ranging from about 1550 °C to about 1800°C. The chemical barrier can alternatively comprise an element or compound in contact with the outer surface of the silica container which is thermodynamically favored to react with silica to form one or more reaction products at temperatures ranging from about 1550 °C to about 1800°C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid- phase reaction products formed being less than about 3:1. The chemical barrier can also be characterized as comprising a metal or an oxide of a metal in contact with the outer surface of the silica container. Preferred metals and metal oxides are selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum oxide, zirconium, zirconium dioxide, hafnium, hafnium dioxide, titanium, and titanium dioxide.

Any of the aforementioned chemical barriers can further comprise an element or compound in contact with the inner surface of the graphite support vessel which does not substantially react with carbon during production of single crystal silicon at temperatures ranging from about 1550 °C to about 1800 °C.

Alternatively, any of the aforementioned chemical barriers can further comprise an element or compound in contact with the inner surface of the graphite support vessel which is thermodynamically favored to react with carbon to form one or more reaction products at temperatures ranging from about 1550 °C to about 1800°C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid-phase reaction products formed being less than about 3:1. Any of the aforementioned chemical barriers can also be characterized as further comprising a metal, a carbide of a metal or an oxide of a metal in contact with the inner surface of the graphite support vessel . Preferred metals, metal carbides and metal oxides are selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide. In any and/or in all of the aforementioned chemical barriers, the chemical barrier material and solid reaction product formed, if any, via reaction with silica most preferably does not form a eutectic composition with silica. Where eutectics do form, high-temperature eutectics are preferred. That is, eutectics preferably do not form at temperatures less than about 1525 °C, more preferably at temperatures less than about 1575 °C, and even more preferably at temperatures less than about 1625°C. The chemical barrier can, in one embodiment, consist essentially of one of the aforementioned barrier materials .

In an alternative embodiment, the chemical barrier can comprise a heterogeneous mixture of elements or compounds. A preferred mixture is non-uniformly dispersed solid-solid solutions of (i) a metal carbide and a metal oxide with the concentration of metal oxide in contact with silica being greater than the concentration of metal carbide in contact therewith, or alternatively, (ii) a metal and a metal oxide with the concentration of metal oxide in contact with silica being greater than the concentration of metal in contact therewith. In a further embodiment, the chemical barrier can be a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer in contact with the outer surface of the silica container. The first and second layers are also adjacent each other or, alternatively, are adjacent one or more intermediate layers situated between the first and second layers.

The invention is directed, as well, to a silica container suitable for holding a pool of molten silicon formed therein during production of single crystal silicon, typically in a Czochralski-type process. The container comprises a body consisting essentially of silica and a chemical barrier. The silica container has inner and outer surfaces with the inner surface defining a cavity capable of containing a pool of molten silicon. The shape or form of the outer surface is not narrowly critical, but is generally adaptable for use with the graphite support vessel. The chemical barrier is as set forth above and covers at least a portion of the outer surface of the silica body.

The invention is directed, moreover, to a graphite support vessel, such as a susceptor or crucible, for use in supporting a silica container during production of a single crystal silicon ingot from a silicon melt formed within the container. The support vessel comprises a graphite body consisting essentially of graphite and a chemical barrier. The support vessel has an inner surface which defines an open cavity adapted to receive the silica container and an outer surface adapted for use in a Czochralski-type crystal puller. The chemical barrier is as set forth above and covers at least a portion of the inner surface of the body. Another aspect of the invention is directed to a process for producing single crystal silicon from polycrystalline silicon. In one process, polycrystalline silicon is loaded into a silica container having an outer surface and an inner surface which defines a cavity capable of containing a pool of molten silicon. The silica container is supported with a graphite support vessel having an inner surface adapted to receive the silica container and an outer surface adapted for use in a Czochralski-type crystal puller. A pool of molten silicon is formed in the silica container, and a single crystal silicon ingot is drawn from the molten silicon. A chemical barrier as set forth above is applied to at least a portion of the outer surface of the silica container or to at least a portion of the inner surface of the graphite support vessel, such that the chemical barrier is situated therebetween during formation of the silicon melt and during production of single crystal silicon. In an exemplary specific process, at least about 100 kg of polycrystalline silicon is loaded into a silica container having a diameter of at least about 22 inches (about 56 cm) , and a pool of molten silicon is formed in the silica container by heating the silica container and support vessel to achieve a corner temperature or a wall temperature of at least about 1550 °C for a period ranging from about 6 hours to about 18 hours .

In another process, an apparatus comprising a silica container, a graphite support vessel and a chemical barrier as described above is assembled. Polycrystalline silicon is loaded into the cavity of the silica container and a pool of molten silicon is formed therein. A single crystal silicon ingot is then drawn from the molten silicon. Other features and objects of the present invention will be in part apparent to those skilled in the art and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a section view of an empty Czochralski crucible.

FIG. 2 is a section view of an empty graphite susceptor suitable for use in supporting the crucible of Figure 1.

FIG. 3 is a schematic of a Czochralski-type crystal puller which includes a section view of the crucible of Figure 1 situated in and supported by the susceptor of Figure 2 for a configuration typically used in a batch process .

FIG. 4 is a schematic of a Czochralski-type crystal puller which includes a section view of an inner crucible and an outer crucible with the outer crucible situated in and supported by a susceptor for a configuration typically used in a continuous process.

FIGS. 5A through 5C are graphs showing thermodynamic equilibria data for combinations of materials which model a silica-container/chemical-barrier/graphite-support- vessel system using tungsten, W, as the barrier material, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 5A shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and W (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 5B shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and W (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 5C shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole), W (0.1 mole) and varying amounts of C (ranging from about 5xl0^"4 mole to about 0.01 mole) at a temperature of about 1600 °C.

FIGS . 6A through 6G are graphs showing thermodynamic equilibria data for combinations of materials which model a silica-container/chemical-barrier/graphite-support- vessel system using tantalum, Ta, tantalum carbide, TaC, tantalum oxide (e.g. Ta₂0₅) , and/or composite mixtures or layers of the same as the barrier material, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 6A shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Ta (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 6B shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and Ta (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 6C shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Ta₂0₅ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 6D shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and TaC (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 6E shows equilibria constituents and quantities (log moles) for a system starting with TaC (0.1 mole) and Ta₂0₅ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figures 6F and 6G show equilibria constituents and quantities (log moles) for systems starting with Si0₂ (1 mole), Ta (0.1 mole) and varying amounts of TaC (ranging from about lxl0^"4 mole to about 0.1 mole) at a temperature of about 1500 °C (Fig. 6F) and 1600 °C (Fig. 6G) , respectively. FIGS. 7A through 7D are graphs showing thermodynamic equilibria data for combinations of materials which model a silica-container/chemical-barrier/graphite-support- vessel system using molybdenum, Mo, molybdenum carbide, MoC, and/or composite mixtures or layers of the same as the barrier material, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 7A shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and Mo (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 7B shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Mo (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 7C shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole), Mo (0.1 mole) and varying amounts of C (ranging from about lxlO^"4 mole to about 0.01 mole) at a temperature of about 1600 °C. Figure 7D shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and MoC (0.1 mole) at temperatures ranging from about 800 °C to about 1600 °C.

FIGS. 8A through 8H are graphs showing thermodynamic equilibria data for combinations of materials which model a silica-container/chemical-barrier/graphite-support- vessel system using zirconium, Zr, zirconium carbide, ZrC, zirconium dioxide, ZrC-Zr0₂, and/or composite mixtures or layers of the same as the barrier material, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 8A shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and Zr (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 8B shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Zr (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 8C shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Zr0₂ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 8D shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and ZrC (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 8E shows equilibria constituents and quantities (log moles) for a system starting with ZrC (0.11 mole) and Zr0₂ (0.09 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 8F shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and Zr0₂ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 8G shows equilibria constituents and quantities

(log moles) for a system starting with Si0₂ (1 mole) , Zr0₂ (0.1 mole) and varying amounts of C (ranging from about 5xl0^"4 mole to about 0.01 mole) at a temperature of about 1600 °C. Figure 8H shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole), Zr0₂ (0.05 mole) and Zr0₂*Si0₂ (0.05 mole) at temperatures ranging from about 800 °C to about 1800 °C.

FIGS . 9A through 9E are graphs showing thermodynamic equilibria data for combinations of materials which model a silica-container/chemical-barrier/graphite-support- vessel system using hafnium, Hf, hafnium carbide, HfC, hafnium dioxide, HfC-Hf0₂, and/or composite layers or mixtures thereof as the barrier material, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 9A shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and Hf (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 9B shows equilibria constituents and quantities (log moles) for a system starting with Hf (0.1 mole) and C (1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 9C shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Hf0₂ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 9D shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and HfC (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 9E shows equilibria constituents and quantities (log moles) for a system starting with HfC (0.11 mole) and Hf0₂ (0.09 mole) at temperatures ranging from about 800 °C to about 1800 °C.

FIGS. 10A through 10E are graphs showing thermodynamic equilibria data for combinations of materials which model a silica-container/chemical- barrier/graphite-support-vessel system using titanium, Ti, titanium carbide, TiC, titanium dioxide, Ti0₂, or composite layers or mixtures thereof as the barrier material, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 10A shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and Ti (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 10B shows equilibria constituents and quantities (log moles) for a system starting with Ti (0.1 mole) and C (1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 10C shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Ti0₂ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 10D shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and TiC (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 10E shows equilibria constituents and quantities (log moles) for a system starting with TiC (0.09 mole) and Ti0₂ (0.11 mole) at temperatures ranging from about 800 °C to about 1800 °C. FIGS. 11A through 11F are section views of an empty silica crucible situated in and supported by a graphite susceptor with a chemical barrier situated between various portions thereof. As illustrated in the various figures, the chemical barrier is situated between the lower halves of the corners (Fig. 11A) , the bottoms and the lower halves of the corners (Fig. 11B) , the entire corners (Fig. 11C) , the bottoms and the entire corners (Fig. 11D) , the corners and the bottom and middle portions of the walls (Fig. HE) , and the bottom portion of the walls (Fig. HF) .

FIGS. 12A though 12D are graphs showing thermodynamic equilibria data for combinations of materials which model different regions of a silica- container/graphite-support-vessel system, without a chemical barrier, at a pressure of 0.018 bar (1800 Pa) and in the presence of argon (1 mole) . Figure 12A models the no-contact region and shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole), SiO(g) (0.01 mole) and varying amounts of CO(g) (ranging from about 0.01 mole to about 1 mole) at a temperature of about 1400 °C. Figures 12B and 12C model the temporary-contact region. Figure 12B shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Si0₂ (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 12C shows equilibria constituents and quantities (log moles) for a system starting with Si0₂ (1 mole) and C (0.1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 12D models the continuous-contact region and shows equilibria constituents and quantities (log moles) for a system starting with C (1 mole) and Si0₂ (1 mole) at temperatures ranging from about 800 °C to about 1800 °C. The invention is described in further detail below with reference to the figures, in which like items are numbered the same in the several figures.

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a chemical barrier is used to minimize the extent of reaction between silicon dioxide, Si0₂, originating from a silica container and carbon, C, originating from a graphite support vessel supporting the container during production of single crystal silicon by a Czochralski-type process. (Example

1) . The concurrent decomposition of the silica container and oxidation of the graphite support vessel, particularly problematic at the higher temperatures associated with larger-sized charges and larger-diameter crystals, contribute to a loss of structural integrity of the container/support system, and directly effect zero- dislocation growth of single crystal silicon. Without being bound by theory, at temperatures greater than about 1400 °C, Si0₂ and C are converted entirely to gaseous reaction products according to the reaction:

Si0₂ + C → SiO(g) + CO(g) . Void spaces are created by the formation of the gaseous reaction products at the silica-container / graphite- support -vessel interface. The silica container, under the hydrostatic pressure of the silicon melt, fills these void spaces and becomes stretched and thinned in areas adjacent thereto. Such stretching and thinning appears to cause the formation of cracks in the silica container, and ultimately, the release of silica particles from the container into the melt. The meltborne silica can be incorporated in the single crystal ingot and initiate dislocations in the crystalline structure of the silicon, thereby making the crystal unsuitable for the production of silicon wafers used in the manufacture of semiconductor devices. Because the chemical barriers of the present invention are substantially non-reacting with respect to both silica and carbon, or, alternatively, react with silica and/or carbon to form as reaction products, only stable, substantially non-reactive solids or alternatively, such solids with a relatively small amount of gasses, the chemical barriers help minimize the extent of Si0₂-C reactions and the deleterious consequences associated therewith.

As used herein, the term "container" is intended to include crucibles, liners or other vessels in which a pool of molten silicon can be formed for use in preparing a single crystal silicon ingot in a Czochralski-type crystal puller. The term "silica" or "Si0₂" is intended to include both vitreous silica and/or crystalline silica, individually or in combination. The term

"support vessel" is intended to include susceptors, crucibles or other receptacles used to support a container. The term "polycrystalline silicon" is intended to include polycrystalline silicon without limitation as to shape, form or method of production.

Exemplary polycrystalline silicon includes "chunk" polycrystalline silicon typically prepared by a Siemens- type process and "granular" polycrystalline silicon typically prepared by a fluidized-bed reaction process. Polycrystalline silicon is loaded into a silica container suitable for use in conjunction with a graphite support vessel to prepare single crystal silicon by a Czochralski-type process. In general, the silica container can be an untreated silica container consisting essentially of a silica body, without regard to crystalline or non-crystalline form. In one embodiment, the silica body can consist essentially of vitreous silica. Alternatively and preferably, the silica body can comprise a vitreous silica vessel having a layer of substantially devitrified silica formed on its inner and/or outer surfaces and/or having a devitrification promoter coated on its inner and/or outer surfaces . The silica container is preferably substantially free of alkaline-earth metal, alkali metal and other impurities, both within the bulk of the container and on its inner or outer surfaces. Vitreous silica containers of suitable quality are commercially available from a variety of sources, including for example, General Electric Co., Quartz Products Department (Cleveland, Ohio) .

While the particular geometry of the silica container is not narrowly critical, the container will generally have inner and outer surfaces which define an at least partially open structure capable of containing or otherwise holding a liquid such as molten silicon. Referring to Figure 1, an exemplary silica container is an untreated silica crucible 10. The crucible 10 generally has an inner surface 12, an outer surface 14, a centerline 15 and a top edge 16. The inner surface 12 defines an open cavity 24 into which polycrystalline silicon is loaded. The crucible 10 includes a bottom portion 17, a corner portion 18 and a side or wall portion 19, referred to hereinafter as the bottom 17, corner 18 and wall 19, respectively of the crucible 10. In the illustrated embodiment, the wall 19 is substantially vertical and the bottom 17 is substantially horizontal. More precisely, the wall 19 defines a substantially vertical circumferential area which includes a top portion 19a, a middle portion 19b and a bottom portion 19c, the top, middle and bottom portions 19a, 19b and 19c each including about 1/3 of the total surface area of the wall 19. The approximate divisions between the top, middle and bottom portions 19a, 19b and 19c are generally illustrated by phantom lines 21. The bottom 17 is parabolic, having a slope with a substantially greater horizontal component than vertical component. The corner 18 is a curved annular boundary region in the vicinity of the intersection of the wall 19 and bottom 17. The corner 18 intersects the wall 19 where the curvature of the corner 18 ceases, illustrated generally as line 22. The corner 18 has a radius of curvature which is less than the radius of curvature of the bottom 17 and generally intersects the bottom 17 where the radius of curvature changes. The corner 18 has upper and lower halves each of which include about half of the total surface area of the corner 18, with the upper half being closer to the wall 19 and the lower half being closer to the bottom 17. The centerline 15 of the crucible 10 is substantially parallel to the wall 19 and intersects a geometric centerpoint of the bottom 17.

A graphite support vessel supports the silica container during formation of silicon melt therein and during production of single crystal silicon from the molten silicon. The graphite support vessel can be an untreated graphite support vessel consisting essentially of carbon in its graphite form. The graphite support vessel is preferably substantially free of alkaline-earth metal, alkali metal and other impurities, both within the bulk of the support vessel and on its inner or outer surfaces. Graphite support vessels of suitable quality are commercially available from a variety of sources, including for example, UCAR (Clarksburg, WVa) . While the particular geometry of the graphite support vessel is not narrowly critical, the support vessel will generally have an inner surface adapted to receive the silica container. That is, the inner surface of the support vessel defines a shape which substantially conforms to the shape of the silica container which it is designed to support . The outer surface of the support vessel is adapted for use in a Czochralski-type crystal puller. Referring to Figure 2, an exemplary graphite support vessel is an untreated graphite susceptor 30. The susceptor 30 has an inner surface 32, an outer surface 34, a centerline 35 and a top edge 36. The inner surface 32 of the susceptor 30 defines an open cavity 44 and includes a bottom portion 37, a corner portion 38 and a side or wall portion 39, referred to hereinafter as the bottom 37, corner 38 and wall 39, respectively, of the susceptor 30. In the illustrated embodiment, the susceptor bottom 37 is substantially horizontal and the susceptor wall 39 is substantially vertical. More specifically, the bottom 37 is approximately parabolic and has a slope with a substantially greater horizontal component than vertical component. The susceptor wall 39 defines a circumferential area, which includes a top portion 39a, a middle portion 39b and a bottom portion 39c, the top, middle and bottom portions 39a, 39b and 39c each including about 1/3 of the total surface area of the wall 39. The approximate divisions between the top, middle and bottom portions 39a, 39b and 39c are generally illustrated by phantom lines 41. The susceptor wall 39 can be tapered open slightly relative to the true vertical, such that a diameter of the wall 39 measured at the top edge 36 of the susceptor 30 is slightly greater than a diameter of the wall 39 measured at point lower in the wall 39, for example, at the middle portion 39b of the wall 39. While not depicted as such in the illustrated embodiment, the susceptor 30 is more typically configured as two or three separate pieces to facilitate receipt of a silica crucible 10 or other silica container into the open cavity 44. The susceptor corner 38 is a curved annular boundary region in the vicinity of the intersection of the wall 39 and bottom 37. The corner 38 intersects the wall 39 where the curvature of the corner 38 ceases, illustrated generally as line 42. The corner 38 has a radius of curvature which is less than the radius of curvature of the bottom 37 and generally intersects the bottom 37 where the radius of curvature changes. The corner 38 has upper and lower halves each of which include about half of the total surface area of the corner 38, with the upper half being closer to the wall 39 and the lower half being closer to the bottom 37. The centerline 35 of the susceptor 30 is substantially parallel to the wall 39 and intersects a geometric centerpoint of the bottom 37. The susceptor 30 further includes a base 46 located underneath the bottom portion 37 of the inner surface 32. The base 46 is adapted for connection to a movable pedestal 52 of a Czochralski-type crystal puller. A chemical barrier is situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel . For example, Figure 3 shows a chemical barrier 70 separating crucible 10 and susceptor 30 in a Czochralksi-type crystal puller 50, with the base 46 of the susceptor 30 attached to a movable pedestal 52. This configuration is typically associated with a batch-type process. An alternative design for the silica container and the graphite support vessel, typically associated with a continuous-type process, is shown in Figure 4. In this design, a double container system comprising an inner crucible 10' and an outer crucible 10'' is supported by a graphite support vessel (susceptor 30'). The outer crucible 10'' and the susceptor 30' are separated by chemical barrier 70 situated therebetween. The geometries of the crucible 10 and susceptor 30 illustrated in Figures 1, 2, 3 and 4 are exemplary. The geometries of the silica container and the graphite support vessel may vary substantially from the illustrated embodiments and still fall within the scope of the invention. Moreover, the silica container / chemical barrier / graphite support vessel apparatus and methods involving the application of a chemical barrier thereto are not limited with respect to the type or nature of the process. Batch-type, continuous-type and recharge-type processes, as well as variations thereof, are within the scope of the present invention.

The chemical barrier preferably comprises or consists essentially of a material which does not substantially react with silica and/or carbon during production of single crystal silicon in a Czochralski- type process at a temperature of the high- temperature/high-heat flux region -- typically the bottom portions 19c, 39c of the walls 19, 39 and corners 18, 38 -- ranging from about 1550 °C to about 1800 °C. A chemical barrier material is considered to be substantially non-reactive with silica and/or carbon based on the lack of definitive evidence of the formation of reaction products. More specifically, where one or more solid-phase reaction products are thermodynamically favored to be formed, a chemical barrier material is considered to be substantially non-reactive with silica and/or carbon if definitive visual detection of the reaction product is not possible with the naked eye of a person whose vision is or is corrected to be 20/20. Where only gas-phase reaction products are thermodynamically favored to be formed, a chemical barrier material is considered to be substantially non- reactive with silica and/or carbon if definitive formation of such products is not detected using instrumentation and methods known in the art, including for example gas chromatographic analysis of a sample drawn from near the top edge of the silica container / graphite support vessel system. The chemical barrier can also comprise or consist essentially of a material which reacts with silica and/or carbon to form a substantially non-reactive solid reaction product without forming a gaseous reaction product. While substantially non- reactive barriers or barriers which are converted entirely to solid reaction products are preferred, the chemical barrier can alternatively comprise a barrier material which, upon contact with silica and/or carbon, forms at least one stable solid reaction product and at least one gaseous reaction product -- provided, that relatively less gaseous reaction products are formed at typical processing temperatures than would form due to the high-temperature Si0₂-C reactions occurring in the absence of the barrier. Minimizing the extent of gaseous reaction product formation ensures an advantage over systems lacking an appropriate chemical barrier. The amount of undesirable gaseous reaction products which are thermodynamically favored to form during in-si tu reaction of the chemical barrier material with either silica or graphite can be characterized by the stoichiometric molar ratio of gas-phase reaction products formed to solid- phase reaction products formed. Table 1 summarizes some reactions which are thermodynamically favored to occur between several barrier materials and silica or carbon at temperatures ranging from about 1550 °C to about 1800 °C. Where appropriate, the stoichiometric molar ratio of gaseous reaction products formed to solid reaction products formed are listed in the table. For the chemical barriers of the present invention, the stoichiometric molar ratio of gaseous reaction products formed to solid reaction products formed by in-si tu reaction of the chemical barrier material with either silica or graphite at temperatures ranging about 1550 °C to about 1800 °C is preferably less than about 3:1, more preferably equal to or less than about 2:1, and most preferably equal to or less than about 1:1. As shown in Table 1, metals and metal oxides are preferred barrier materials for use in contact with the silica container. For contact with the graphite support vessel, preferred barrier materials include, in order of preference, metal carbides, metals and metal oxides. In addition, the chemical barrier material and solid-phase reaction product formed via in si tu reaction with silica and/or carbon, if any, most preferably do not form a eutectic composition with silica, with graphite, and/or with each other. Where eutectics do form, it is preferred that they do not form at temperatures less than about 1525 °C, preferably at temperatures less than about 1575 °C, and more preferably at temperatures less than about 1625 °C. Most preferably, no eutectic at all is formed at any temperature. While the formation of eutectics at temperatures greater than about 1525 °C provides an additional upper-limit temperature constraint, operability at less than such high-temperature eutectics may still be advantageous in some circumstances.

Moreover, the chemical barrier preferably has some or all of the following characteristics: is non-toxic, has a melting point greater than about 1500 °C, more preferably greater than about 1600 °C and most preferably greater than about 1700 °C, has a hardness and toughness which allows for multiple use in a container/support vessel system, has a thermal emissivity which is about the same as the emissivity of carbon at temperatures above about 1500 °C, has conductive heat transfer characteristics which, in consideration of the thickness of the barrier, do not substantially adversely affect heat transfer across the barrier, has a sufficiently low vapor pressure / diffusivity so as not to contaminate the melt, and has a segregation coefficient which is sufficiently low such that contamination of the melt with the barrier material will not result in significant incorporation thereof into the growing crystal. TABLE 1

Thermodynamically Favored Reactions at T>1550°C

Barrier Material w/carbon (ratio)* w/silica (ratio)*

W - Stable - (N/A) - Stable - (N/A)

Mo - Stable - (N/A) - Stable - (N/A)

Ta Ta + C → TaC (N/A) 2Ta + 5Si0₂ → Ta₂0₅ + 5SiO(g) (5:1)

Ta₂0₅ Ta₂0₅ + 7C → 2TaC + 5C0(g) (5:2) - Stable - (N/A)

TaC - Stable - (N/A) 2TaC + 7Si0₂ → Ta₂0₅+7SiO (g) +2C0 (g) (9:1)

MoC - Stable - (N/A) MoC + Si0₂ → Mo + CO(g) + SiO(g) (2:1)

Zr Zr + C → ZrC (N/A) Zr + 2Si0₂ → Zr0₂ + 2SiO(g) (2:1)

Zr0₂ Zr0₂ + 3C → ZrC + 2C0(g) (2:1) - Stable - (N/A)

ZrC - Stable - (N/A) ZrC + 3Si0₂ → Zr0₂ + CO(g) + 3SiO(g) (4:1)

Hf Hf + C → HfC (N/A) Hf + 2Si0₂ → Hf0₂ + 2SiO(g) (2:1)

Hf0₂ Hf0₂ + 3C → HfC + 2CO(g) (2:1) - Stable - (N/A)

HfC - Stable - (N/A) HfC + 3Si0₂ → Hf0₂ + 3SiO(g) + CO(g) (4:1)

Ti Ti + C → TiC (N/A) Ti + 2Si0₂ → Ti0₂ + 2SiO(g) (2:1) τio₂ Ti0₂ + 3C → TiC + 2C0(g) (2:1) - Stable - (N/A)

TiC - Stable - (N/A) TiC + 3Si0₂ → Ti0₂ + 3SiO(g) + CO (g) (4:1)

Notes * (ratio) = stoichiometric molar ratio of gaseous reaction products formed to solid reaction products formed N/A = not applicable

Hence, the chemical barrier of the present invention can, in one embodiment, consist essentially of an element or compound which, at temperatures greater than about 1550 °C, and preferably at temperatures ranging from about 1550 °C to about 1800 °C: is substantially non- reactive to both silica and carbon; is substantially non- reactive to silica but reacts with carbon to form a solid reaction product without forming a gaseous reaction product or to form solid and minimal gaseous reaction products, as described above; is substantially non- reactive to carbon but reacts with silica to form a solid reaction product without forming a gaseous reaction product or to form solid and minimal gaseous reaction products, as described above; or reacts with silica and, independently, with carbon to form a solid reaction product without forming a gaseous reaction product and/or to form both solid and minimal gaseous reaction products, as described above. Elements and/or compounds having such properties include various metals and metal oxides, and selected metal carbides. The chemical barrier preferably consists essentially of one of the following elements or compounds: tungsten, W, molybdenum, Mo, molybdenum carbide, MoC, tantalum oxide, Ta₂0₅, zirconium, Zr, zirconium dioxide, Zr0₂, hafnium, Hf, hafnium dioxide, Hf0₂, titanium, Ti, and titanium dioxide, Ti0₂.

Particularly preferred chemical barriers of this type include: a barrier consisting essentially of tungsten, W, or a barrier consisting essentially of molybdenum, Mo. While the aforementioned elements and compounds are preferred, the invention can also include others compounds and/or elements, including for example, other transition metals, rare-earth metals, heavy metals, their oxides and/or their carbides. Some metals, such as rhenium and platinum are attractive based on their physical properties, but are presently too expensive to be preferred. In an alternative embodiment, the chemical barrier can comprise or consist essentially of a mixture of elements or compounds. As used herein, the term "mixture" is intended to mean a heterogeneous association of at least two different elements and/or compounds regardless of whether such elements and/or compounds are uniformly dispersed. Preferred chemical barriers can comprise or consist essential of a mixture of a metal, a metal carbide and/or a metal oxide, in each of the various combinations, including for example, a mixture of elements or compounds selected from the group consisting of tungsten, W, tantalum, Ta, tantalum carbide, TaC, tantalum oxide, Ta₂0₅, molybdenum, Mo, molybdenum carbide, MoC, zirconium, Zr, zirconium carbide, ZrC, zirconium dioxide, Zr0₂, hafnium, Hf , hafnium carbide, HfC, hafnium dioxide, Hf0₂, titanium, Ti, titanium carbide, TiC and titanium dioxide, Ti0₂. Mixtures selected from the following combinations of metals and/or carbides with oxides are particularly preferred: Ta and/or TaC with Ta₂0₅; Zr and/or ZrC with Zr0₂; Hf and/or HfC with Hf0₂; and Ti and/or TiC with Ti0₂. While the chemical barrier can be a heterogeneous mixture which is a solid/solid solution of elements and/or compounds uniformly dispersed with respect to the other elements and/or compounds in the solution, in a preferred heterogenous mixture used as a chemical barrier situated between a graphite support vessel and a silica container, the aforementioned combinations of metals, carbides and oxides are non- uniformly dispersed in a solid/solid solution such that a higher concentration of elemental metal and/or metal carbide is adjacent the graphite support vessel and a higher concentration of metal oxide is adjacent the silica container. The advantage of employing such a non- uniformly-dispersed mixture as a chemical barrier lies in the observation that, as discussed in detail below, the metal carbide is substantially non-reactive with graphite, the elemental metal reacts with graphite in- si tu to form the metal carbide without forming gaseous reaction products, and the metal oxide is substantially non-reactive with silica. Hence, such a chemical barrier minimizes the extent of in-si tu gaseous reaction product formation, thereby resulting in less void formation between the graphite support vessel and silica container.

The chemical barrier can, in a further embodiment, be a composite chemical barrier comprising at least two distinct layers of materials, with each layer consisting essentially of one element, compound, or discrete heterogeneous mixture of elements or compounds . Each of the layers in contact with the silica container or, independently, with the graphite support vessel are preferably substantially non-reactive therewith or react therewith in si tu to form stable, solid reaction products without forming or with minimum formation of gaseous reaction products. For example, the layer adjacent to and in contact with the outer surface of the silica container preferably consists essentially of a metal or a metal oxide, or heterogeneous mixtures thereof, and preferably consists essentially of a metal, a metal or a metal oxide selected from the group consisting of tungsten, W, molybdenum, Mo, tantalum oxide, Ta₂0₅, zirconium, Zr, zirconium dioxide, Zr0₂, hafnium, Hf, hafnium dioxide, Hf0₂, titanium, Ti, and titanium dioxide, Ti0₂. The layer adjacent to and in contact with the inner surface of the graphite support vessel preferably consists essentially of a metal, a metal carbide, or a metal carbide or heterogeneous mixtures thereof, and preferably consists essentially of a metal, a metal carbide or a metal oxide selected from the group consisting of tungsten, W, tantalum, Ta, tantalum carbide, TaC, tantalum oxide, Ta₂0₅, molybdenum, Mo, molybdenum carbide, MoC, zirconium, Zr, zirconium carbide, ZrC, zirconium dioxide, Zr0₂, hafnium, Hf, hafnium carbide, HfC, hafnium dioxide, Hf0₂, titanium, Ti, titanium carbide, TiC and titanium dioxide, Ti0₂. Adjacent layers within the composite chemical barrier are preferably substantially non-reactive with each other or react with each other in si tu to form stable, solid reaction products without forming or with minimum formation of gaseous reaction products.

A particularly preferred composite chemical barrier comprises a first layer consisting essentially of a metal or a metal carbide and a second layer consisting essentially of a metal oxide adjacent the first layer.

For example, the particularly preferred composite barrier can comprise first and second adjacent layers consisting essentially of, respectfully: tantalum and tantalum oxide, Ta-Ta₂0₅, tantalum carbide and tantalum oxide, TaC- Ta₂0₅, zirconium and zirconium dioxide, Zr-Zr0₂, zirconium carbide and zirconium dioxide, ZrC-Zr0₂, hafnium and hafnium dioxide, Hf-Hf0₂, hafnium carbide and hafnium dioxide, HfC-Hf0₂, titanium and titanium dioxide, Ti-Ti0₂, and titanium carbide and titanium dioxide, TiC-Ti0₂. A graphite-support-vessel / composite-chemical-barrier / silica-container system employing the aforementioned particularly preferred composite chemical barrier is configured with one surface of the first layer (comprising a metal or metal carbide) being adjacent graphite support vessel and one surface of the second layer (comprising the metal oxide) being adjacent the silica container. The other surfaces of the first and second layers are adjacent each other, or alternatively, adjacent one or more intermediate layers situated between the first and second layers. Such intermediate layers can be of a material suitable for use in the high- temperature process conditions required for single crystal growth, including for example other metals, carbides, oxides, nitrides, etc. An exemplary silica- container / composite-chemical-barrier / graphite- support-system can be configured as "crucible / Hf0₂ / HfC / susceptor", such that: the Hf0₂ layer has a first surface adjacent the outer surface of the silica container and a second surface adjacent the HfC layer; and the HfC layer has first surface adjacent the Hf0₂ layer and a second surface adjacent to the inner surface of the graphite support vessel . The advantage of the particularly preferred system is appreciated by observing, as discussed in detail below, that the metal carbide is substantially non-reactive with graphite, the elemental metal reacts with graphite in-si tu to form the stable metal carbide without forming gaseous reaction products, the metal and metal carbide are both substantially non-reactive with the metal oxide and the metal oxide is substantially non-reactive with silica. A slightly less preferred composite chemical barrier can comprise a first layer consisting essentially of a metal (in elemental form) and a second layer consisting essentially of a metal carbide adjacent the first layer. For example, such a composite barrier can comprise first and second adjacent layers consisting essentially of, respectfully: tantalum and tantalum carbide, Ta-TaC, zirconium and zirconium carbide, Zr-ZrC, hafnium and hafnium carbide, Hf-HfC, and titanium and titanium carbide, Ti-TiC. When employed in a graphite-support- vessel / composite-chemical-barrier / silica-container system, such a composite chemical barrier is preferably configured with the first layer (comprising a metal) being adjacent the silica container and the second layer (comprising the metal carbide) being adjacent the graphite support vessel. As discussed in detail below, the metal carbide is substantially non-reactive with both the graphite and the elemental metal . The elemental metal reacts with silica in-si tu to form a stable metal oxide, but with concurrent formation of SiO(g). Despite the formation of some gaseous reaction products in this system, the stoichiometric molar ratio of gaseous reaction products formed to solid reaction products formed is less than about 3:1 for each of the Ta~TaC, Zr- ZrC, Hf-HfC and Ti-TiC composite barrier materials at temperatures greater than about 1550 °C. As such, this system offers a substantial advantage over other chemical barrier systems and over a support-vessel / silica- container system which lacks a chemical barrier.

As noted above and further exemplified by the following detailed discussion of the thermodynamic properties of the particularly preferred barrier materials, it is generally preferable to use a barrier consisting essentially of an element or compound when that element or compound is substantially non-reactive with both silica and graphite and/or when the in-si tu reaction with silica and/or graphite forms only stable, solid reaction products. In contrast, when the thermodynamics at the process conditions required for single-crystal-silicon growth favor the reaction of an element or compound with silica to form an oxide and/or the reaction of the element or compound with graphite to form the carbide, wherein at least one of the reactions includes the concurrent formation of gaseous reaction product, then it is generally preferable to use a chemical barrier comprising a non-uniformly dispersed heterogeneous mixture or a composite chemical barrier, as described. Note, however, that exceptions to the aforementioned general preferences may exist in individual situations -- for example, due to kinetic considerations . To determine whether a potential barrier material would be acceptable for use in a particular crystal growth process, thermodynamic analyses can be performed using microprocessor-based software, such as "HSC Chemistry" available from Outokumpu Research (Pori, Finland) . The following discussion presents the results of such thermodynamic analyses for barrier materials which include tungsten, tantalum, molybdenum, zirconium, hafnium, titanium, and where appropriate, oxides and/or carbides thereof. A similar approach can be used to identify and/or evaluate other elements or compounds which would be suitable for use as chemical barriers. Tungsten, W, is a most preferred chemical barrier material. As shown in Figures 5A and 5B, thermodynamic calculations indicate that: W (0.1 mole) is not favored to react appreciably with Si0₂ (1 mole) at temperatures ranging from about 800 °C to about 1800 °C. (Fig. 5A) ; and although W (0.1 mole) is somewhat favored to react with C (1 mole) to form WC at temperatures less than about 1100 °C, no significant conversion to WC is favored to occur at temperatures greater than about 1200 °C (Fig. 5B) . Hence, these equilibrium data indicate that at the operating temperatures of interest in silicon crystal production -- typically at or greater than about 1550 °C -- tungsten is extremely thermodynamically stable with both graphite and silica. Moreover, no reactions are favored to occur between W, C and/or Si0₂ where they are in mutual, tertiary, contact with each other -- for example, if the W barrier had a structural defect which allowed C to contact the silica container. As shown in Figure 5C, a system which includes W (0.1 mole), Si0₂ (1 mole) and C (amounts ranging from about 5x10^"4 mole to about lxlO^"2 mole) at a temperature of about 1600 °C is thermodynamically stable. While W could be combined with other barrier materials (e.g. as a mixture or as part of a composite layer) , its stability with respect to both silica and graphite demonstrate that a preferred chemical barrier consists essentially of W. Tantalum, Ta, and tantalum oxide (e.g. Ta₂0₅) are also suitable barrier materials based on thermodynamic data and experimental observations. When the chemical barrier comprises or consists essentially of tantalum in its elemental form, in si tu conversion of Ta to its carbide, TaC, and to its oxide, Ta₂0₅, is thermodynamically favored, at least at the surface-most regions of the Ta barrier which are in contact with the graphite and silica, respectively. Figure 6A shows that Ta (0.1 mole) conversion to TaC is favored in the presence of C (1 mole) at temperatures above about 800 °C up to at least about 1800 °C. Fig. 6B shows that when Ta (0.1 mole) is present with Si0₂ (1 mole) : conversion to Ta₂Si and Ta₂0₅ is somewhat favored at temperatures ranging from about 800 °C to about 1400 °C; and Ta conversion to Ta₂0₅ and SiO(g) is favored at temperatures greater than about 1400 °C. The tantalum carbide and oxide -- favored to form in-si tu adjacent the graphite support vessel and the silica container, respectively, -- are relatively stable against graphite and silica, respectfully, as seen in Figures 6A and 6B, respectfully. As such, a suitable chemical barrier could consist essentially of Ta in its elemental form. Experimental Ta foils performed acceptably as a chemical reaction barrier. (Example 1) . The chemical barrier could, alternatively, comprise or consist essentially of Ta₂0₅. As noted, the Ta₂0₅ would be stable against the silica container ( See Fig. 6B) . As shown in Figure 6C, Ta₂0₅ (0.1 mole) is favorably converted in-si tu to TaC in the presence of C (1 mole) at temperatures greater than about 850°C. The resulting TaC is, as noted, stable against the graphite support vessel . While a chemical barrier consisting essentially of TaC would be stable against the graphite support vessel (See Fig. 6A) , the data of Figure 6D shows that TaC (0.1 mole) is thermodynamically favored to react in si tu with Si0₂ (1 mole) at temperatures greater than about 1525 °C to form Ta₂0₅, SiO(g) and CO(g) according to the reaction:

2TaC + 7Si0₂ → Ta₂0₅ + 7SiO(g) + 2CO (g) . The relatively high molar ratio of gaseous reaction product formed per mole of solid, stable reaction product formed makes the use of a barrier consisting essentially of TaC less desirable than a barrier consisting essentially of Ta or essentially of Ta₂0₅. Nonetheless, TaC may be used more advantageously in a chemical barrier comprising a mixture of TaC with other chemical barrier materials and/or in a composite chemical barrier, as discussed below.

The use of a composite chemical barrier (e.g. TaC- Ta₂0₅, Ta-Ta₂0₅ or Ta-TaC) would also be suitable and may be preferred relative to elemental tantalum because the extent of in-si tu formation of gaseous reaction products is reduced. The composite chemical barrier can, for example, comprise composite layers consisting essentially of TaC and Ta₂0₅. For the TaC-Ta₂0₅ composite chemical barrier, the TaC is thermodynamically stable against the graphite support vessel, and the Ta₂O_s is thermodynamically stable against the silica container, as noted. Moreover, the TaC and Ta₂0₅ are thermodynamically stable against each other, thereby being suitable for use together in a composite. Figure 6E shows that TaC (0.1 mole) is stable in the presence of Ta₂0₅ (0.1 mole) at temperatures ranging from about 800 °C to about 1600 °C. At temperatures greater than about 1600 °C, TaC (0.1 mole) is favorably converted to Ta and CO(g) . Alternatively, the composite chemical barrier can comprise composite layers consisting essentially of Ta and Ta₂0₅, or in a further, slightly less preferred alternative, composite layers consisting essentially of Ta and TaC. For the Ta-Ta₂0₅ composite chemical barrier, the Ta₂0₅ is stable against the silica container and the Ta adjacent the graphite is converted in-si tu to stable TaC. Advantageously, such in-si tu conversion of Ta to TaC does not concurrently produce gaseous reaction products. For the Ta-TaC composite chemical barrier, the TaC is stable against the graphite support vessel and the Ta adjacent the Si0₂ container is converted in-si tu to stable Ta₂0₅.

In view of a Ta₂0₅-Si0₂ eutectic which forms at a temperature of about 1570 °C, a chemical barrier consisting essentially of Ta or Ta₂0₅, a chemical barrier comprising a heterogeneous mixture of Ta or TaC with

Ta₂0₅, and a composite barrier of Ta-Ta₂0₅, of Ta-TaC or of Ta₂0₅-TaC is, in general, preferably used at temperatures of less than about 1570 °C. Comparison of the data presented in Figures 6F and 6G suggest, however, that the presence of TaC reduces the thermodynamic potential for conversion of Ta to Ta₂0₅. This may be advantageous in offsetting the potential effects of a Si0₂-Ta₂0₅ eutectic. Figures 6F and 6G show that in systems in which Ta (0.1 mole) and TaC (amounts ranging from about lxlO^"4 mole to about 0.1 mole) are present with Si0₂ (1 mole) at temperatures of about 1500 °C (Fig. 6F) and 1600 °C (Fig. 6G) , respectively, the tantalum carbide and tantalum oxide are substantially thermodynamically stable.

Molybdenum is also a suitable reaction barrier. Mo is stable at the temperatures of interest to both silica and graphite. Figure 7A shows that Mo (0.1 mole) is not thermodynamically favored to react with Si0₂ (1 mole) at temperatures ranging from about 800 °C to about 1800 °C. Figure 7B shows that Mo (0.1 mole) is thermodynamically stable with C (1 mole) at temperatures ranging from about 1130 °C to about 1800 °C, but that at temperatures of less than about 1130 °C, conversion of Mo to molybdenum carbide, MoC, is thermodynamically favored. Moreover, where Mo, C and/or Si0₂ are in mutual contact with each other -- for example, if the Mo barrier had a structural defect which allowed C to contact the silica container -- the Mo (0.1 mole) is stable in the presence of Si0₂ (1 mole) and C (amounts ranging from about IxlO^"4 mole to about lxlO^"2 mole) at a temperature of about 1600 °C (Fig. 7C) . Hence, as Mo is stable against both silica and graphite at the temperatures of interest, Mo is preferably used as a chemical barrier in its elemental form. Mo could also be used, however, in conjunction with other elements and/or compounds. Furthermore, molybdenum carbide, MoC, could also be used as a barrier material. Figure 7D shows that in a system in which MoC (0.1 mole) is in contact with Si0₂ (1 mole), in si tu conversion of the MoC to Mo is thermodynamically favored at temperatures above about 1180 °C. The thermodynamic data discussed above is consistent with experimental observations based on tests in which Mo foils were used as a chemical barrier: whereas Mo did not react with the silica container, porous MoC flakes which did not adhere to the graphite were observed. (Example 1) .

Zirconium, Zr, and zirconium oxide, Zr0₂, are also suitable chemical barrier materials. A chemical barrier can comprise or consist essentially of zirconium in its elemental form. In this embodiment, thermodynamic preferences indicate that elemental Zr would react, in si tu, with Si0₂ to form Zr0₂ and with C to form ZrC. Specifically, as shown in Figure 8A, Zr (0.1 mole) preferentially reacts with Si0₂ (1 mole) to form: Zr0₂ and SiO(g) at temperatures greater than about 1300 °C; and Zr0₂*Si0₂ and Si at temperatures less than about 1300 °C. Figure 8B shows that Zr (0.1 mole) in the presence of C (1 mole) is preferentially converted to ZrC over a temperature ranging from about 800 °C to about 1600 °C. The zirconium oxide and carbide -- formed in-si tu adjacent the silica container and graphite support vessel, respectively, are relatively stable against silica and graphite, as shown in Figures 8A and 8B, respectfully. The stability of Zr0₂ (0.1 mole) in the presence of Si0₂ (1 mole) is further shown in the absence of C (Fig. 8F) and in the presence of increasing amounts of C (Fig. 8G) . As such, the chemical barrier could consist essentially of Zr in its elemental form. The chemical barrier could, alternatively, comprise or consist essentially of Zr0₂. As noted, the Zr0₂ would be stable against the silica container (See Figs. 8A, 8F and 8G) . As shown in Figure 8C, Zr0₂ (0.1 mole) is favorably converted in-si tu to ZrC in the presence of C (1 mole) at temperatures greater than about 1200°C. ZrC is similarly formed in-si tu at greater than about 1200 °C when Zr0₂*Si0₂ (0.05 mole) and Zr0₂ (0.05 mole) are both present with C (1 mole) . The resulting ZrC is, as noted, stable against the graphite support vessel . In experimental tests in which Zr0₂ cloth was placed between the outer surface of a silica container having a uniform layer of devitrified silica formed thereon and the inside surface of a graphite support vessel, the devitrified silica layer was slightly fluxed, most probably due to the formation of a Zr0₂-Si0₂ eutectic at a temperature of about 1690 °C. (Example 1) . While a chemical barrier consisting essentially of ZrC would be stable against the graphite support vessel (See Fig. 8B) , the data of Figure

8D shows that ZrC (0.1 mole) is favored to react in-si tu with Si0₂ (1 mole) at temperatures greater than about 1350 °C to form Zr0₂, CO(g) and SiO(g), according to the reaction: ZrC + 3Si0₂ → Zr0₂ + 3SiO(g) + CO (g) .

The relatively high molar ratio of gaseous reaction product formed per mole of solid, stable reaction product formed makes the use of a barrier consisting essentially of ZrC less desirable than a barrier consisting essentially of Zr or essentially of Zr0₂. Nonetheless,

ZrC may be used more advantageously in a chemical barrier comprising a mixture of ZrC with other chemical barrier materials and/or in a composite chemical barrier, as discussed below. The use of a composite chemical barrier (e.g. ZrC-

Zr0₂, Zr-Zr0₂ or Zr-ZrC) would also be suitable and may be preferred relative to elemental zirconium because the extent of in-si tu formation of gaseous reaction products is reduced. The composite chemical barrier can, for example, comprise composite layers consisting essentially of ZrC and Zr0₂. For the ZrC-Zr0₂ composite chemical barrier, the ZrC is thermodynamically stable against the graphite support vessel, and the Zr0₂ is thermodynamically stable against the silica container, as noted. Moreover, the ZrC and Zr0₂ are thermodynamically stable against each other, thereby being suitable for use together in a composite. Figure 8E shows that ZrC (0.11 mole) is stable in the presence of Zr0₂ (0.09 mole) at temperatures ranging from about 800 °C to about 1800 °C. The use of a composite chemical barrier comprising composite layers consisting essentially of ZrC and Zr0₂, would, desirably, reduce the extent of in-si tu conversion of zirconium and the corresponding extent of gas and void formation associated therewith. Alternatively, the composite chemical barrier can comprise composite layers consisting essentially of Zr and Zr0₂, or in a further and slightly less preferred alternative, composite layers consisting essentially of Zr and ZrC. For the Zr-Zr0₂ composite chemical barrier, the Zr0₂ is stable against the silica container and the Zr adjacent the graphite is converted in-si tu to stable ZrC. Advantageously, such in-si tu conversion does not involve the formation of gaseous reaction products. For the Zr-ZrC composite chemical barrier, the ZrC is stable against the graphite support vessel and the Zr adjacent the Si0₂ container is converted in-si tu to stable Zr0₂.

In view of the potential formation of a Zr0₂-Si0₂ eutectic at a temperature of about 1690°C, a chemical barrier comprising Zr, ZrC or Zr0₂ and/or a composite chemical barrier comprising Zr-ZrC, Zr-Zr0₂, or Zr0₂-ZrC is preferably used at temperatures of less than about 1690 °C.

Hafnium, Hf, and hafnium dioxide, Hf0₂, are also suitable chemical barrier materials . A chemical barrier can comprise or consist essentially of Hf in its elemental form. The use of elemental Hf between surfaces of silica and carbon results in the in si tu formation of thermodynamically stable Hf0₂ and HfC at the silica/Hf and carbon/Hf surfaces, respectively. Figure 9A shows that elemental Hf (0.1 mole) is thermodynamically preferentially converted in the presence of Si0₂ (1 mole) to its stable oxide, Hf0₂, and Si at temperatures ranging from about 800 °C to about 1300 °C, and to Hf0₂ and SiO(g) at temperatures ranging from about 1300 °C to about 1800 °C. Figure 9B shows that Hf (0.1 mole) is preferentially converted to HfC in the presence of C (1 mole) at temperatures ranging from about 800 °C to about 1600 °C. The hafnium oxide and carbide -- formed in-si tu adjacent the silica container and graphite support vessel, respectively, are relatively stable against silica and graphite, as shown in Figures 9A and 9B, respectfully.

Hence, the chemical barrier could consist essentially of Hf in its elemental form. The chemical barrier could, alternatively, comprise or consist essentially of Hf0₂. As noted, the Hf0₂ would be stable against the silica container (See Fig. 9A) . As shown in Figure 9C, Hf0₂ (0.1 mole) is favorably converted in-si tu to HfC in the presence of C (1 mole) at temperatures greater than about 1200°C. The resulting HfC is, as noted, stable against the graphite support vessel . While a chemical barrier consisting essentially of HfC would be stable against the graphite support vessel (See Fig. 9B) , the data of Figure 9D shows that HfC (0.1 mole) is favored to react in-si tu with Si0₂ (1 mole) at temperatures greater than about 1350 °C to form Hf0₂, CO(g) and SiO(g), according to the reaction: HfC + 3Si0₂ → Hf0₂ + 3SiO(g) + CO(g) .

The relatively high molar ratio of gaseous reaction product formed per mole of solid, stable reaction product formed makes the use of a barrier consisting essentially of HfC less desirable than a barrier consisting essentially of Hf or essentially of Hf0₂. Nonetheless, a chemical reaction barrier consisting essentially of HfC can be advantageously used in a silica container / graphite support system relative to a system lacking any chemical barrier. Moreover, HfC may be used more advantageously in a chemical barrier comprising a mixture of ZrC with other chemical barrier materials and/or in a composite chemical barrier, as discussed below.

The use of a composite chemical barrier (e.g. HfC- Hf0₂, Hf-Hf0₂ or Hf-HfC) would also be suitable and may be preferred relative to elemental hafnium because the extent of in-si tu formation of gaseous reaction products is reduced. The composite chemical barrier can, for example, comprise composite layers consisting essentially of HfC and Hf0₂. For the HfC-Hf0₂ composite chemical barrier, the HfC is thermodynamically stable against the graphite support vessel, and the Hf0₂ is thermodynamically stable against the silica container, as noted. Moreover, the HfC and Hf0₂ are thermodynamically stable against each other, thereby being suitable for use together in a composite. Figure 9E shows that HfC (0.11 mole) is stable in the presence of Hf0₂ (0.09 mole) at temperatures ranging from about 800 °C to about 1800 °C. The use of a composite chemical barrier comprising composite layers consisting essentially of HfC and Hf0₂, would, desirably, reduce the extent of in-si tu conversion of hafnium and the corresponding extent of gas and void formation associated therewith. Alternatively, the composite chemical barrier can comprise composite layers consisting essentially of Hf and Hf0₂, or in a further and slightly less preferred alternative, composite layers consisting essentially of Hf and HfC. For the Hf-Hf0₂ composite chemical barrier, the Hf0₂ is stable against the silica container and the Hf adjacent the graphite is converted in-si tu to stable HfC -- advantageously, without the formation of gaseous reaction products. For the Hf-HfC composite chemical barrier, the HfC is stable against the graphite support vessel and the Hf adjacent the Si0₂ container is converted in-si tu to stable Hf0₂.

In view of the formation of a Hf0₂-Si0₂ eutectic at a temperature of about 1680 °C, however, a chemical barrier comprising Hf, HfC or Hf0₂ or composite chemical barrier comprising Hf-Hf0₂, Hf-HfC or HfC-Hf0₂ is preferably used at temperatures of less than about 1680 °C.

Titanium, Ti, and titanium dioxide, Ti0₂, are likewise suitable as chemical barrier materials. A chemical barrier can comprise or consist essentially of Ti in its elemental form. The use of elemental Ti between surfaces of silica and carbon results in the in si tu formation of thermodynamically stable Ti0₂ and TiC at the silica/Hf and carbon/Hf surfaces, respectively. Figure 10A shows that elemental Ti (0.1 mole) is preferentially converted in the presence of Si0₂ (1 mole) to its stable oxide, Ti0₂, at temperatures ranging from about 800 °C to about 1800 °C. Figure 10B shows that Ti (0.1 mole) is preferentially converted to TiC in the presence of C (1 mole) at temperatures ranging from about 800 °C to about 1600 °C. The titanium oxide and carbide -- formed in-si tu adjacent the silica container and graphite support vessel, respectively, are relatively stable against silica and graphite, as shown in Figures 10A and 10B, respectfully. Hence, the chemical barrier could consist essentially of Ti in its elemental form. The chemical barrier could, alternatively, comprise or consist essentially of Ti0₂. As noted, the Ti0₂ would be stable against the silica container (See Fig. 10A) . As shown in Figure 10C, Ti0₂ (0.1 mole) is favorably converted in-si tu to TiC in the presence of C (1 mole) at temperatures greater than about 900°C. The resulting TiC is, as noted, stable against the graphite support vessel. While a chemical barrier consisting essentially of TiC would be stable against the graphite support vessel (See Fig. 10B) , the thermodynamic data presented in Figure 10D shows that TiC (0.1 mole) is favored to react in-si tu with Si0₂ (1 mole) at temperatures greater than about 1450 °C to form Ti0₂, SiO(g) and CO(g) according to the reaction: TiC + 3Si0₂ → Ti0₂ + 3SiO(g) + CO(g) .

The relatively high molar ratio of gaseous reaction product formed per mole of solid, stable reaction product formed makes the use of a barrier consisting essentially of TiC less desirable than a barrier consisting essentially of Ti or essentially of Ti0₂. Nonetheless, a chemical reaction barrier consisting essentially of TiC can be used more advantageously in a chemical barrier comprising a mixture of TiC with other chemical barrier materials and/or in a composite chemical barrier, as discussed below. The use of a composite chemical barrier (e.g. TiC- Ti0₂, Ti-Ti0₂ or Ti-TiC) would also be suitable and may be preferred relative to elemental titanium because the extent of in-si tu formation of gaseous reaction products is reduced. The composite chemical barrier can, for example, comprise composite layers consisting essentially of TiC and Ti0₂. For the TiC-Ti0₂ composite chemical barrier, the TiC is thermodynamically stable against the graphite support vessel, and the Hf0₂ is thermodynamically stable against the silica container, as noted. Moreover, the TiC and Ti0₂ are thermodynamically stable against each other, thereby being suitable for use together in a composite. Figure 10E shows that TiC (0.09 mole) is stable in the presence of Ti0₂ (0.11 mole) at temperatures ranging from about 800 °C to about 1800 °C. The use of a composite chemical barrier comprising composite layers consisting essentially of TiC and Ti0₂, would, desirably, reduce the extent of in-si tu conversion of titanium and the corresponding extent of gas and void formation associated therewith. Alternatively, the composite chemical barrier can comprise composite layers consisting essentially of Ti and Ti0₂, or in a further and slightly less preferred alternative, composite layers consisting essentially of Ti and TiC. For the Ti-Ti0₂ composite chemical barrier, the Ti0₂ is stable against the silica container and the Ti adjacent the graphite is converted in-si tu to stable TiC -- advantageously, without the formation of gaseous reaction product. For the Ti-TiC composite chemical barrier, the TiC is stable against the graphite support vessel and the Ti adjacent the Si0₂ container is converted in-si tu to stable Ti0₂.

Similar to other metal-oxide/silicon-dioxide systems, a Ti0₂-Si0₂ eutectic forms at a temperature of about 1550 °C. As such, a chemical barrier comprising Ti , TiC or Ti0₂ or a composite chemical barrier comprising Ti0₂-TiC, Ti-TiC, or Ti-Ti0₂ is preferably used at temperatures of less than about 1550 °C.

The above-described chemical barriers, whether configured as a single element or compound (e.g. W, Mo, MoC, Ti0₂) , as a heterogeneous mixture of elements or compounds, or as a composite chemical barrier comprising at least two distinct layers of elements or compounds (e.g. ZrC-Zr0₂, Hf-HfC), may be satisfactorily employed independently (ie, alone) as a chemical barrier. Moreover, it is likewise within the scope of the invention to use these barrier configurations in other, unspecified combinations with each other or in combination with other barriers presently known in the art or later developed. For example, while the chemical barrier of the present invention is, as described above, preferably used in conjunction with an untreated silica container consisting essentially of silica and with an untreated graphite support vessel consisting essentially of carbon, the chemical barrier of the present invention may also be used in conjunction with other surface treatments, coatings, deposits and/or films presently known in the art or developed in the future . For example, the chemical barrier could be applied to a treated silica container comprising a silica core vessel having inner and outer surfaces where at least a portion of the inner and/or the outer surface of the silica vessel has been treated according to various methods presently known or later developed in the art. Such treatment methods include, for example, forming external coatings on the surface, growing coatings in si tu at the surface, etc. Moreover, the chemical barrier could be applied to a treated graphite support vessel comprising a graphite core vessel which consists essentially of carbon and has inner and outer surfaces where at least a portion of the inner and/or the outer surface of the core vessel has been treated (e.g. coated, etc.) according to various methods presently known or later developed in the art. Such treatment methods include, for example, forming coatings on the surface, growing such or other coatings in si tu at the surface of the core vessel, or plasma spraying such or other surface treatments onto the surface .

The portions of the outer surface of the silica container and the inner surface of the graphite support vessel between which the chemical barrier is situated will depend, generally, on the area of contact between these surfaces during single crystal growth, the duration of such contact, and the temperature of the interface region defined by such contact. In general, the chemical barrier is situated to cover at least the portion of these surfaces having relatively high-temperature contact for relatively long periods. As explored in more detail below, the area of high-temperature, long-duration contact between the silica container and the support vessel being used therewith will depend on the particular designs for the silica container and the graphite support vessel, on hot-zone design considerations, and on other generally known operation considerations.

The area of contact between the silica container and graphite support vessel will, for batch processes, vary with time during crystal growth. With reference to

Figure 3, after a silicon melt is formed in the silica crucible 10, the molten silicon 48 which is below the melt surface 58 exerts a hydrostatic force on the wall 19, corner 18 and bottom 17 of the crucible 10. Because the heated silica container is softened at the melt temperatures, the hydrostatic (gravitational) force operates to push the softened silica container against the graphite support vessel. In a batch process, however, the level of the molten silicon decreases as the silicon ingot forms, and therefore, the hydrostatic pressure exerted on the wall 19, corner 18 and bottom 17 decreases as the single crystal ingot is drawn, thereby allowing the upper portions 19a, 39a of the walls 19, 39 to release from each other, and resulting in a decrease in the amount of contact between the outer surface 14 of crucible wall 19 and the inner surface 32 of the support vessel wall 39 as the crystal is pulled. In general, therefore, it is possible to define regions of contact based upon the time of contact between the walls of the silica container and the support vessel: (1) a no-contact region which is located above the initial melt line (ie, above the initial level of the surface 58 of the molten silicon 48) ; (2) a temporary-contact (or limited-contact) region which is located below the initial melt line but substantially above the final melt line; and (3) a continuous-contact region located where the hydrostatic pressure and/or the weight of the silica container forces the softened container against the support vessel during substantially the entire crystal-pulling process. For the container/support-vessel system illustrated in Figures 1 through 4, the no-contact region correlates roughly with the top one-third of the interface region between the walls of the silica container and the graphite susceptor, illustrated as the top portions 19a and 39a, of the container and support vessel walls, respectively. The temporary-contact region generally correlates with the bottom two-thirds of the walls and the upper-half of the corner element, shown in Figures 1 through 4 as including the wall portions 19b, 19c, 39b, 39c and the upper halves of the corners 18, 38. The continuous contact region generally correlates with the bottoms 17, 37 and the lower halves of the corners 18, 38. However, the aforementioned correlations are exemplary only. The correlation of time-of-contact- defined regions to particular portions of the silica container / support vessel interface will vary with design thereof and with variations in heating profile. Observations suggest that the continuous contact region is of most concern with respect to the detrimental effects of the Si0₂-C reactions (Example 2) . Accordingly, the chemical barrier is preferably situated at least between the portions of the silica container and graphite support vessel which define the hottest portions or areas of the continuous contact region or of temporary contact regions adjacent thereto. Typically, the hottest temperatures of the container/support vessel system are found in the bottom portion of the walls thereof and in the corners thereof; the upper portions of the walls and the bottoms of the container/support vessel are relatively cooler. As such, with reference to Figure HA, for example, the chemical barrier 70 is preferably situated between the lower halves of the corners 18, 38 of the crucible 10 and susceptor 30. Alternatively, the chemical barrier can be situated between the entire interface area defined by the continuous contact region. With reference to Figure HB, for example, the chemical barrier 70 can be located between the lower halves of the corners 18, 38 and the bottoms 17, 38 of the crucible 10 and the susceptor 30. The chemical barrier can also be situated to cover areas which include portions of both the continuous contact and the temporary contact regions, and which are characterized by relatively high temperature and high heat flux for long periods relative to other portions thereof. For example, with reference to Figure HC, the chemical barrier 70 is situated to cover at least the entire corners 18, 38 of the crucible 10 and susceptor 30. Referring to Figure HD, the chemical barrier 70 is situated between and separates the portions of the silica container and graphite support vessel which define the bottoms 17, 37 and the entire corners 18, 38 of the crucible 10 and susceptor 30. As another example, with reference to Figure HE, the chemical barrier 70 can be located between the corners 18, 38, and the bottom portions 19c, 39c and middle portions 19b, 39b of the walls 19, 39. As a further example, the chemical barrier 70 can be located between the bottom portions 19c, 39c of the walls 19, 39 (Figure HF) , or alternatively, between the bottom portions 19c, 39c of the walls 19, 39 and the upper halves of the corners 18, 38 (not shown) or the entire corners 18, 38 (not shown) . As another example, shown in the system illustrated in Figure 4, the chemical barrier is situated between the portion of the inner surface 32 of the susceptor 30 defined by the bottom 37, corner 38, bottom portion 39c and middle portion 39b of the wall 39 and the outer surface 14 of the corresponding portions 17, 18, 19b and 19c of the crucible 10. Alternatively, the chemical barrier can be situated between the entire outer surface of silica container and inner surface of the graphite support vessel (ie, including the no-contact, temporary-contact and continuous-contact regions) . While the presence of a chemical barrier in the no-contact region is not narrowly critical to the invention, it may be economically preferable to apply the chemical barrier over the entire surfaces rather than selected portions thereof. The several embodiments discussed above are considered exemplary; the chemical barrier can likewise be situated in other areas or regions or combinations thereof as dictated by particular structural and thermal designs .

Several approaches may be used to construct the container/barrier/support system of the present invention. In general, the chemical barrier can be applied between the appropriate surfaces of the container and support vessel by methods which provide the requisite coverage at an appropriate thickness. While the barrier can be applied to the interfacing surfaces of either or both the silica container and/or the graphite support vessel, the barrier is, in general, preferably applied to the inner surface of the graphite support vessel rather than to the outer surface of the silica container. Because the silica containers are typically single-use, whereas the graphite support vessels are typically capable of multiple use, this approach affords the possibility of reuse of the chemical barrier. However, it may be preferably in certain circumstances to apply the barrier material to the silica container. For example, application to the silica container may be preferred where the barrier material is capable only of a single use and application to the silica container is economically and/or technically preferred. Also, as discussed below, application of the chemical barrier to the outer surface of the silica container can be used in preparation of a composite chemical barrier comprising more than one barrier material .

The chemical barrier can, in one approach, be applied between the outer surface of the silica container and the inner surface of the graphite support vessel by forming a layer of a barrier material on the outer surface of the silica container and/or on the inner surface of the graphite support vessel . The chemical barrier layer can be formed on the silica or graphite surfaces by a number of methods, including for example, depositing or coating the chemical barrier on the appropriate silica and/or graphite surfaces. The deposition may be carried out by plasma spraying methods known in the art. (Example 3) . Other deposition methods, such as vapor deposition or chemical vapor deposition may also be used to form a chemical barrier layer on the silica or graphite surfaces. Application of the chemical barrier by formation of a layer of the barrier material is particularly suited for single-layer or composite- layer barrier materials . A layer of the chemical barrier may also be formed on the appropriate surface of the silica container and/or the graphite support vessel by coating the appropriate surface with a composition which includes the barrier material of interest. For example, the coating composition could include an elemental barrier material (e.g. W) , preferably in powder form, suspended in a volatile carrier such as an alcohol (e.g. isopropyl alcohol) or a mixture of an alcohol and water. The coating composition can be in the form of a slurry or otherwise sufficiently viscous form to allow for application of the coating to the appropriate surface. Application of the coating composition could be carried out in a manner analogous to painting by using a brush, roller, sprayer, dipping or other means suitable for applying such coatings. After the coating is applied, the coated silica container and/or graphite support vessel can be dried to help drive off volatile constituents of the coating composition. Application of the chemical barrier by coating the appropriate surface is particularly suited for applying heterogeneous mixtures of barrier materials. The simplicity of the application may also make this approach favored for reapplication of single-use or limited-use chemical barriers, or for touch-up of locally-damaged barriers.

Additionally, the chemical barrier layer can be formed on the silica or graphite surfaces by in si tu growth from another previously applied material (e.g. elemental Ta, Zr, Hf or Ti, as discussed above) . In another approach, a chemical barrier can be applied in the form of a thin foil, film or woven cloth (generally and collectively referred to herein as a "sheet") by placing or laying out the sheet between the respective surfaces of the silica container and graphite support vessel. (Example 1) . This approach is particularly suited to barrier materials which are commercially available in such forms, including for example, Zr0₂ and Hf0₂. Modifications and variations of the aforementioned approaches and other approaches known in the art or later identified could also be used to establish a container/barrier/support system.

Moreover, variations and combinations of these approaches and other approaches known in the art or later identified could be used to establish a container/barrier/support system having a composite chemical barrier (e.g. ZrC-Zr0₂) . Such a composite barrier may be created by forming a layer of a first chemical barrier material (e.g. Zr0₂) on the outer surface of the silica container and forming a layer of a second chemical barrier material (e.g. ZrC) on the inner surface of the graphite support vessel used to support the silica container during crystal growth. In another exemplary approach, a composite chemical barrier can be created by forming a layer of a first chemical barrier material on the appropriate surface of either the silica container or the graphite support vessel and applying (ie, placing, laying or otherwise situating) a sheet of a second barrier material between the graphite support vessel and the silica container. While less preferred, a composite barrier can also be created by using sheets (e.g. foil or film or cloth) for both the first and second barrier materials . Regardless of the particular chemical barrier configuration or the approach used to apply the chemical barrier between the outer surface of the silica container and the inner surface of the graphite support vessel, the barrier may be reapplied if the layer subsequently becomes too thin or becomes damaged during handling. The thickness of the chemical barrier is not narrowly critical. The barrier can generally be of a thickness sufficient to substantially avoid the release of a commercially significant amount of silica particles into the silicon melt and the incorporation thereof into the silicon crystal grown therefrom. As used herein, the term "commercially significant amount" of silica particles is intended to mean an amount which results in a commercially significant decrease in the structural integrity of the silica container (e.g. cracking, bulging, bowing or other deformation) and/or a commercially significant decrease in the zero-dislocation length upon growth of a single crystal silicon ingot from a silicon melt formed in the silica container. While a decrease in zero-dislocation length of more than about 10% can presently considered commercially significant, the decrease in zero-dislocation growth is preferably less than about 5% and more preferably less than about 1%. Even smaller decreases may be significant in the future. The preferred thickness of the chemical barrier may also be dependent on the composition of the chemical barrier. In general, the thickness of the chemical barrier is preferably at least about 50 μm, more preferably at least about 100 μm and most preferably at least about 200 μm. Chemical barriers having such thicknesses are achievable using the application approaches described above. The maximum thickness of the chemical barrier is not narrowly critical, and is generally dictated by considerations which include the expense of the barrier material, the difference in coefficients of thermal expansion between the barrier material and the silica container and/or graphite support vessel to which it is applied, heat transfer considerations, etc. A thickness of about 1 mm will generally be sufficient for most applications. For particular applications, thicknesses of about 700 μm, 500 μm and about 400 μm will be suitable. Hence, the thickness of the chemical barrier preferably ranges from about 50 μm to about 1000 μm, and can also range from about 100 μm to about 700 μm, from about 200 μm to about 500 μm and from about 300 μm to about 400 μm.

Once the polycrystalline silicon is loaded into a suitable silica-container/chemical-barrier/graphite- support-system, the crucible can be placed in a conventional CZ silicon crystal growth apparatus and the polycrystalline silicon can be heated to melt the polycrystalline silicon until a pool of molten silica forms in the silica container. The heating profile is not narrowly critical, and will generally vary depending on the type of loading (ie, chunk, granular or mixed loadings), the size and design of the crucible, the size and type of crystal grower, etc. In a typical arrangement, referring to the crucible / susceptor system illustrated in Figure 3, the pedestal 52 supporting the base 46 of the susceptor 30 is positioned such that the bottom 17 of the crucible 10 is near the top of the heater 54. The crucible 10 is gradually lowered into the space inside the heater 54. The speed at which the crucible 10 is lowered into closer proximity of the heater 54 and the value of other factors affecting melting of the polycrystalline silicon, such as heater power, crucible rotation and system pressure, are generally known in the art.

Ordinarily, in a batch-type process configuration, the temperature of the region of contact between the crucible 10, chemical barrier 70 and susceptor 30 at corners 18, 38 and/or at the bottom portions 19c, 39c of the walls 19, 39 is at least about 1550 °C for an 18" (about 46 cm) diameter crucible charged with about 70 kg polycrystalline silicon during a meltdown period ranging from about 4 hours to about 6 hours. Higher temperatures -- preferably at least about 1575 °C, at least about 1600 °C or at least about 1625 °C -- can also be used in such an 18 "/70 kg system to effect a shorter meltdown period without detrimental effect on the structural integrity of the silica container or graphite support vessel. The advantage of the present invention is particularly significant using larger diameter silica containers and/or larger charge sizes. For example, the temperature of the high heat-flux corner and/or bottom portion of the wall region is preferably at least about 1650 °C for a 22" (about 56 cm) diameter crucible charged with about 100 kg of polycrystalline silicon during a melt-down period ranging from about 8 hours to about 10 hours, or with a larger, 120 kg, charge and a melt-down period of about 10 hours. However, even higher temperatures can be used -- at least about 1675 °C or at least about 1700 °C -- with such a 22"/120 kg system to effect a commensurately shorter meltdown period (e.g. ranging from about 6 hours to about 8 hours) . For a larger silica container, such as a 24" (about 61 cm) diameter crucible charged with about 140 kg of polycrystalline silicon, the corner and/or bottom portion of the wall temperature is preferably at least about 1650 °C, and can also be at least about 1675 °C or at least about 1700 °C. The meltdown period for such a 24"/140 kg system will vary with temperature, but can typically range from about 10 hours to about 12 hours at about 1650 °C and from about 8 hours to about 10 hours at temperatures ranging from about 1675 °C to about 1700 °C. As another example, a 32" (about 81 cm) diameter crucible charged with an amount of polycrystalline silicon ranging from about 160 kg to about 200 kg can be heated to a corner and/or bottom portion of the wall temperature of at least about 1650 °C, or more preferably, at least about 1675 °C or at least about 1700 °C. These higher temperatures are preferred to allow for a melt-down period for such a

32"/l60-200 kg system which is commercially reasonable -- for example, ranging from about 12 hours to about 15 hours .

The aforementioned temperatures, charge sizes and melt-down periods are exemplary of batch-type processes. The lower temperature limits specified therein are to be considered applicable as well to continuous-type or recharge-type process. The upper temperature of the region of contact between the crucible 10, chemical barrier 70 and susceptor 30 at corners 18, 38 and/or at the bottom portions 19c, 39c of the walls 19, 39 is not narrowly critical, however, regardless of whether the system is employed in a batch-type, continuous-type, or recharge-type process. Appropriate upper limits, dictated, for example, by material melt temperatures, eutectic temperatures, etc, are in part pointed out herein and will be otherwise apparent to those of skill in the art . When employing the barrier materials of the present invention in a crystal-growing process, the operational temperatures of such a process are not to be constrained by the temperature ranges used to define preferred barrier materials (e.g. 1550 °C to about 1800 °C) . A skilled artisan will also appreciate that other parameters set forth herein, including charge size and melt-down period, will vary appropriately for non-batch- type processes such as continuous-type and/or recharge- type processes .

Hence, in a preferred application, the chemical barriers of the invention can be applied to the corner and/or the bottom portion of the wall region of the outer surface of a silica container having a diameter of at least about 22 inches (about 56 cm) , or alternatively, to the inner surface of a graphite support vessel used in conjunction therewith. Preferred chemical barriers for high-temperature applications with larger-diameter silica containers include a chemical barrier consisting essentially of tungsten or molybdenum, or a composite chemical barrier with the first layer (adjacent the inner surface of the graphite support vessel) and the second layer (adjacent the outer surface of the silica container) consisting essentially of, respectfully, zirconium carbide and zirconium oxide, zirconium and zirconium oxide, hafnium carbide and hafnium oxide, or hafnium and hafnium oxide. An apparatus comprising the silica container, the graphite support vessel and the chemical barrier is assembled. Polycrystalline silicon charges of at least about 100 kg of are loaded into the silica container, and a pool of molten silicon is formed in the silica container by heating the silica container and support vessel to achieve a corner temperature of at least about 1650 °C for a period of at least about 3 hours, more preferably for at least about 6 hours, and preferably for a period of less than about 18 hours, more preferably for a period of less than about 12 hours, whereafter a single crystal silicon ingot may be drawn therefrom.

The aforementioned silica container diameters, charge sizes and melt-down periods, and particularly, the combination of such parameters with the various temperatures, are recited as being exemplary of the advantages of the present invention, and are not intended to be limiting as to the scope of the invention. A person skilled in the art will readily appreciate that the use of the aforedescribed chemical barrier between the silica container and the support vessel allows, in general, for the silica container to be heated to higher temperatures without the detrimental effects on crystal growth which are associated with known systems. The capability of using higher temperatures translates into increased productivity through either larger charge sizes and/or reduced melt-down periods.

The polycrystalline silicon is typically exposed, while heating, to a purge gas to flush out undesirable gasses such as SiO(g) and CO(g) originating from the reaction of SiO(g) with hot graphite. The purge gas is typically an inert gas such as argon and typically flows at a rate ranging from about 10 1/min to about 300 1/min, depending on the type and size of the crystal puller.

Once the silicon melt is formed, a single crystal silicon ingot can be drawn the molten silicon using a conventional Czochralski-type process.

The following examples illustrate the principles and advantages of the invention.

EXAMPLES Example 1: Use of Thin-Foil Chemical Barriers to Reduce the Extent of Si0₂-C Reactions

Thin-foils of Ta and Mo were applied, independently, between a graphite susceptor and silica crucible during preparation of single crystal silicon in a Czochralski- type crystal puller. Briefly, Ta and Mo test-barrier foils (each 2 cm x 10 cm x 250 μm) were placed between the corners of a silica crucible and a graphite susceptor used to prepare single crystal silicon from polycrystalline charges of 120 kg. The polycrystalline silicon was melted by heating the crucible/susceptor system to a corner temperature not exceeding about 1650 °C for a melt -down period of about 12 hours.

Both the Ta and Mo chemical barriers were effective in minimizing the extent of Si0₂-C reactions. Following the experiment, the corner regions of the crucible and susceptors were inspected. While reaction between the silica crucible and graphite susceptor was observed to have occurred at surface regions around the outer periphery of the test-barrier, little, if any evidence of reaction was observed in the areas of the crucible and susceptor surfaces between which the test barrier was situated. The Ta foil remained intact and, following the experiment, was removed without damage to the foil. As such, it would have been possible to reuse the Ta foil. The Mo foil became brittle, flaky and somewhat porous, and as such, could not have been removed without damage thereto. Based on qualitative observation, the Ta foil appeared to be a relatively better barrier than the Mo foil. Similar experiments were carried out using a test- barrier of woven zinc-oxide, Zn0₂, Y₂0₃-stabilized cloth. (Zircar Inc., Florida, N.Y.). While the Zn0₂ test barrier was hard and brittle, it was, nonetheless, effective in preventing loss of graphite and silica material.

Example 2: Si0₂-C Reactions in the Continuous-Contact, Temporary-Contact and No-Contact Regions

To gain a better understanding of the Si0₂-C reactions which occur between a silica container and graphite support vessel in the absence of a chemical barrier, thermodynamic equilibria data was calculated for a silica/carbon system modeled to represent the no- contact, temporary-contact and continuous-contact regions. For each of these regions, the model included the presence of Argon gas (1 mole) and a pressure of 0.018 bar (1800 Pa) . The no-contact region was modeled by considering the potential for reaction between Si0₂ (1 mole) and CO(g) (at various concentrations) at a temperature of 1400 °C and in the presence of SiO(g) (0.01 mole). CO(g) and SiO(g) are product gasses from the Si0₂-C reactions occurring in the temporary-contact and continuous-contact regions. As shown in Figure 12A, no thermodynamically favored reactions occur in such a system when the amount of C0(g) present therein is less than about 0.35 moles. While some production of SiC is thermodynamically favored when the amount of CO(g) in the modeled system is greater than about 0.4 moles, the amount of CO present in an actual Czochralski-type system is expected to be sufficiently low so as to make the effects of such a reaction negligible.

The temporary-contact region was modeled in two cases -- from the perspective of the graphite support vessel and from the perspective of the silica container. In the first case, the thermodynamic potential was evaluated for reaction between C (1 mole) and Si0₂ (0.1 mole) at temperatures ranging from 800 °C to 1800 °C. The data in Figure 12B indicates that at temperatures greater than about 1100 °C, the Si0₂ present in the system is favored to react with C to form stoichiometric amounts of SiC and CO (g) . The calculations also indicate that conversion of SiC to SiO(g) and CO(g) is not appreciably favored even at temperatures as high as 1700 °C. Hence, although the Si0₂ and C are converted to initially to SiC, the SiC is thermodynamically stable with the graphite support vessel and can remain in contact therewith. In the second case, the thermodynamic potential was evaluated for reaction between Si0₂ (1 mole) and C (0.1 mole) at temperatures ranging from 800 °C to 1800 °C. The data in Figure 12C indicates that at temperatures greater than about 1000 °C, the C present in the system is favored to react with Si0₂ to form stoichiometric amounts of SiC and C0(g) . However, at temperatures greater than about 1325 °C, the SiC reacts further with Si0₂ to form SiO(g) and CO (g) . Hence, contact between the silica container and the graphite support vessel results in the decomposition of the silica container and the oxidation of the graphite support vessel to form gaseous reaction products .

The continuous-contact region was modeled to evaluate the thermodynamic potential for reaction between C (1 mole) and Si0₂ (1 mole) at temperatures ranging from 800 °C to 1800 °C. The equilibrium data of Figure 12D shows that as temperatures increase, conversion of C and Si0₂ to SiC and CO(g) is predominantly favored at temperatures ranging from about 1150 °C to about 1400 °C. At temperatures greater than about 1400 °C, the conversion of C and Si0₂ to SiO(g) and CO(g) is thermodynamically favored. At temperatures less than about 1150 °C, conversion of Si0₂ and C to CO(g) and SiC is not appreciably favored. Based on this model, it can be appreciated that because high temperature (>1400 °C) contact between C and Si0₂ is maintained throughout the period during which the melt is formed and the crystal is pulled, conversion of the graphite support vessel and silica container to gaseous reaction products is favored during this entire period.

The reactions which are predicted to be favored based on thermodynamic calculations for the no-contact, temporary-contact and continuous contact regions are in substantial agreement with observations made of graphite support vessels and silica containers used without a chemical barrier in actual preparation of single crystal silicon. When a post-crystal-growth graphite susceptor was evaluated, very little graphite or silica reaction was observed above the initial melt line (ie, no contact region) , as indicated by the presence of only light Si0₂ and SiC coatings. However, below the initial melt line and above the corner radius (ie, the approximate temporary-contact area) , deposits of yellow SiC crystals were routinely observed. Moreover, in the region of the wall below the SiC deposit band, in the corners and in the bottom of the susceptor, a significant loss of graphite was observed, but no SiC crystals were observed. Hence, the physical observations correlate directly with the thermodynamic calculations discussed above.

Example 3 : Formation of Chemical Barrier Layer on a Graphite Support Vessel

Chemical barrier layers were applied to a graphite support vessel by depositing, in separate experiments, Ta and ZrC on the inner surface thereof . Powders of these barrier materials were plasma sprayed onto the inner surface of graphite susceptors using a fixed-position sprayer while rotating the susceptors during application. The portion of the susceptor to which the chemical barrier was applied included the entire bottom, the entire corner and about 1" upwards on the wall thereof. Standard spray velocities were used without substantial complications. The plasma spray was Ar shielded to limit reaction with air. The Ta and ZrC barrier materials demonstrated very good sprayability . The chemical barrier layers formed as described above were visually uniform, and were deposited at thicknesses ranging from about 10 mils (about 250 μm) to about 15 mils (about 380 μm) .

In light of the detailed description of the invention and the examples presented above, it can be appreciated that the several objects of the invention are achieved. The explanations and illustrations presented herein are intended to acquaint others skilled in the art with the invention, its principles, and its practical application. Those skilled in the art may adapt and apply the invention in its numerous forms, as may be best suited to the requirements of a particular use. Accordingly, the specific embodiments of the present invention as set forth are not intended as being exhaustive or limiting of the invention.

Claims

I CLAIM :

1. An apparatus for containing a pool of molten silicon during production of single crystal silicon in accordance with a Czochralski-type method, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising an element or compound in contact with the outer surface of the silica container which does not substantially react with silica during production of single crystal silicon at temperatures ranging from about 1550 °C to about 1800°C.

2. The apparatus of claim 1 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel which does not substantially react with carbon during production of single crystal silicon at temperatures ranging from about 1550 °C to about 1800°C.

3. The apparatus of claim 1 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel which is thermodynamically favored to react with carbon to form one or more reaction products at temperatures ranging from about 1550 °C to about 1800°C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid-phase reaction products formed being less than about 3:1.

4. An apparatus for containing a pool of molten silicon during production of single crystal silicon according to a Czochralski-type method, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising an element or compound in contact with the outer surface of the silica container which is thermodynamically favored to react with silica to form one or more reaction products at temperatures ranging from about 1550 °C to about 1800°C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid-phase reaction products formed being less than about 3:1.

5. The apparatus of claim 4 wherein the element or compound in contact with the outer surface of the silica container is thermodynamically favored to react with silica to form solid-phase reaction products without forming gas-phase reaction products.

6. The apparatus of claim 4 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel which does not substantially react with carbon during production of single crystal silicon at temperatures ranging from about 1550 °C to about 1800°C.

7. The apparatus of claim 4 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel which is thermodynamically favored to react with carbon to form one or more reaction products at temperatures ranging from about 1550 °C to about 1800 °C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid-phase reaction products formed being less than about 3:1.

8. An apparatus for containing a pool of molten silicon during production of single crystal silicon, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising a metal or an oxide of a metal in contact with the outer surface of the silica container.

9. The apparatus of claim 8 wherein the chemical barrier further comprises a metal, a carbide of a metal or an oxide of a metal in contact with the inner surface of the graphite support vessel .

10. The apparatus of claim 8 wherein the chemical barrier consists essentially of a metal.

11. The apparatus of claim 8 wherein the chemical barrier consists essentially of a metal oxide.

12. The apparatus of claim 8 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer in contact with the outer surface of the silica container, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers .

13. The apparatus as set forth in claim 12 wherein the second layer consists essentially of a metal.

14. The apparatus as set forth in claim 12 wherein the second layer consists essentially of a metal oxide.

15. The apparatus as set forth in claim 12 wherein the first layer consists essentially of a metal.

16. The apparatus as set forth in claim 12 wherein the first layer consists essentially of a metal carbide.

17. The apparatus as set forth in claim 12 wherein the first layer consists essentially of a metal oxide.

18. The apparatus as set forth in claim 12 wherein the first layer consists essentially of a metal carbide and the second layer consists essentially of a metal oxide .

19. An apparatus for containing a pool of molten silicon during production of single crystal silicon, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising an element or compound in contact with the outer surface of the silica container selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum oxide, zirconium, zirconium dioxide, hafnium, hafnium dioxide, titanium, and titanium dioxide.

20. The apparatus of claim 19 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide.

21. The apparatus of claim 19 wherein the chemical barrier consists essentially of tungsten.

22. The apparatus of claim 19 wherein the chemical barrier consists essentially of molybdenum.

23. The apparatus of claim 19 wherein the chemical barrier consists essentially of zirconium.

24. The apparatus of claim 19 wherein the chemical barrier consists essentially of hafnium.

25. The apparatus of claim 19 wherein the chemical barrier consists essentially of zirconium dioxide, hafnium dioxide or molybdenum carbide .

26. The apparatus of claim 19 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer in contact with the outer surface of the silica container, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers.

27. The apparatus as set forth in claim 26 wherein the second layer consists essentially of tungsten or molybdenum.

28. The apparatus as set forth in claim 26 wherein the second layer consists essentially of zirconium, hafnium or titanium.

29. The apparatus as set forth in claim 26 wherein the second layer consists essentially of a compound selected from the group consisting of tantalum oxide, zirconium dioxide, hafnium dioxide, titanium dioxide and molybdenum carbide .

30. The apparatus as set forth in claim 26 wherein the first layer consists essentially of tungsten or molybdenum.

31. The apparatus as set forth in claim 26 wherein the first layer consists essentially of an element selected from the group consisting of tantalum, zirconium, hafnium and titanium.

32. The apparatus as set forth in claim 26 wherein the first layer consists essentially of a compound selected from the group consisting of molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide and titanium carbide.

33. The apparatus as set forth in claim 26 wherein the first layer consists essentially of a compound selected from the group consisting of tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

34. The apparatus as set forth in claim 26 wherein the first and second layers consist essentially of a compounds selected from group consisting of, respectively, tantalum carbide and tantalum oxide, zirconium carbide and zirconium dioxide, hafnium carbide and hafnium dioxide, and titanium carbide and titanium dioxide .

35. A silica container for holding a pool of molten silicon formed therein during production of single crystal silicon in a Czochralski-type process, the container comprising a body consisting essentially of silica and having inner and outer surfaces, the inner surface defining a cavity capable of containing the pool of molten silicon, and a chemical barrier covering at least a portion of the outer surface of the body, the chemical barrier comprising a metal or an oxide of a metal in contact with the outer surface of the silica container.

36. The silica container of claim 35 wherein the metal or oxide in contact with the outer surface of the silica container is selected from the group consisting of tungsten, molybdenum, tantalum oxide, zirconium, zirconium dioxide, hafnium, hafnium dioxide, titanium and titanium dioxide.

37. The silica container of claim 35 wherein the chemical barrier further comprises a metal, a carbide of a metal or an oxide of a metal capable of contacting an inner surface of a graphite support vessel when the silica container is supported therein.

38. The silica container of claim 35 wherein the chemical barrier further comprises an element or compound capable of contacting an inner surface of a graphite support vessel when the silica container is supported therein, the element or compound being selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide.

39. The silica container of claim 35 wherein the chemical barrier consists essentially of a metal or an oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

40. The silica container of claim 35 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the outer surface of the silica container and a second layer capable of contacting an inner surface of a graphite support when the silica container is supported therein, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers.

41. The silica container as set forth in claim 40 wherein the first layer consists essentially of a metal or a metal oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

42. The silica container as set forth in claim 40 wherein the second layer consists essentially of a metal, a metal carbide or a metal oxide selected from the group consisting of tungsten, molybdenum, tantalum, zirconium, hafnium, titanium, molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide, titanium carbide, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

43. The silica container as set forth in claim 40 wherein the first layer consists essentially of a metal oxide and the second layer consists essentially of a metal carbide .

44. A graphite support vessel for supporting a silica container during production of a single crystal silicon ingot from a silicon melt formed within the container, the support vessel comprising a body consisting essentially of graphite and having an inner surface which defines an open cavity capable of receiving the silica container, and a chemical barrier covering at least a portion of the inner surface of the body, the chemical barrier comprising a metal, a carbide of a metal or an oxide of a metal in contact with the inner surface of the graphite support vessel and a metal or an oxide of a metal capable of contacting the outer surface of the silica container when the silica container is supported in the graphite support vessel .

45. The graphite support vessel of claim 44 wherein the chemical barrier consists essentially of a metal or a metal oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

46. The graphite support vessel of claim 44 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer capable of contacting the outer surface of the silica container when the silica container is supported by the graphite support vessel, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers.

47. The graphite support vessel as set forth in claim 46 wherein the first layer consists essentially of a metal or a metal carbide or a metal oxide selected from the group consisting of tungsten, molybdenum, tantalum, zirconium, hafnium, titanium, molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide, titanium carbide, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

48. The graphite support vessel as set forth in claim 46 wherein the second layer consists essentially of a metal or a metal oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

49. The graphite support vessel as set forth in claim 46 wherein the first layer consists essentially of a metal carbide and the second layer consists essentially of a metal oxide.

50. A process for producing single crystal silicon from polycrystalline silicon, the process comprising assembling an apparatus into which polycrystalline silicon can be loaded and subsequently heated to form a pool of molten silicon, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of receiving the polycrystalline silicon and containing the pool of molten silicon, a graphite support vessel for supporting the silica container, the support vessel having an inner surface which defines a cavity capable of receiving the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising a metal or an oxide of a metal in contact with the outer surface of the silica container, loading polycrystalline silicon into the cavity of the silica container, forming a pool of molten silicon in the silica container, and drawing a single crystal silicon ingot from the molten silicon.

51. The process as set forth claim 50 wherein the chemical barrier is situated between the outer surface of the silica container and the inner surface of the graphite support vessel by (1) applying a barrier material, before assembling the apparatus, to the outer surface of the silica container or to the inner surface of the graphite support vessel, or, alternatively or additionally, by (2) placing or laying out one or more sheets of a barrier material between the outer surface of the silica container and the inner surface of the graphite support vessel.

52. The process as set forth claim 50 wherein at least about 100 kg of polycrystalline silicon is loaded into a silica container having a diameter of at least about 22 inches (about 56 cm) , and a pool of molten silicon is formed in the silica container by heating the silica container and support vessel to achieve a corner temperature or a wall temperature of at least about 1550 °C for a period ranging from about 6 hours to about 18 hours .

53. The process of claim 50 wherein the metal or oxide in contact with the outer surface of the silica container is selected from the group consisting of tungsten, molybdenum, tantalum oxide, zirconium, zirconium dioxide, hafnium, hafnium dioxide, titanium and titanium dioxide.

54. The process of claim 50 wherein the chemical barrier further comprises a metal, a carbide of a metal or an oxide of a metal in contact with the inner surface of the graphite support vessel.

55. The process of claim 50 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of a graphite support vessel, the element or compound being selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide.

56. The process of claim 50 wherein the chemical barrier consists essentially of a metal or an oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

57. The process of claim 50 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the outer surface of the silica container and a second layer in contact with the inner surface of a graphite support vessel, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers .

58. The process as set forth in claim 57 wherein the first layer consists essentially of a metal or a metal oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

59. The process as set forth in claim 57 wherein the second layer consists essentially of a metal, a metal carbide or a metal oxide selected from the group consisting of tungsten, molybdenum, tantalum, zirconium, hafnium, titanium, molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide, titanium carbide, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

60. The process as set forth in claim 57 wherein the first layer consists essentially of a metal oxide and the second layer consists essentially of a metal carbide.

AMENDED CLAIMS

[received by the International Bureau on 24 September 1998 (24.09.98); original claims 1-60 replaced by amended claims 1-57 (15 pages)]

1. An apparatus or containing a pool of molten silicon during production of single crystal silicon in accordance with a Czochralaki-type method, the apparatus comprising

5 a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal

10 silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the Bilica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and

15 the inner surface of the graphite support vessel, the chemical barrier comprising (a) an element or compound in contact with the outer surface of the silica container which does not substantially react with silica during production of single crystal silicon at temperatures

20 ranging from about 1550 °C to about 1B00°C, and (b) an element or compound in contact with the inner surface of the graphite support vessel which is thermodynamically favored to react with carbon to form one or more reaction products at temperatures ranging from about 1550 °C to

25 about 1800°C, the stoichiometric molar ratio of the thermodynamically favored gas -phase reaction products formed, if any, to the thermodynamically favored solid- phase reaction products formed being less than about 3:1.

2. An apparatus or containing a pool of molten silicon during production of single crystal silicon according to a Czochralski-type method, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite Bupport vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising an element or compound in contact with the outer surface of the silica container which is thermodynamically favored to react with silica to form one or more reaction products at temperatures ranging from about 1550 °C to about 1800°C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid-phase reaction products formed being lees than about 3:1.

3. The apparatus of claim 2 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel which does not substantially react with carbon during production of single crystal silicon at temperatures ranging from about 1550 °C to about 1800°C.

4. The apparatus of claim 2 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel which is thermodynamically favored to react with carbon to form one or more reaction products at temperatures ranging from about 1550 °C to about 180O°C, the stoichiometric molar ratio of the thermodynamically favored gas-phase reaction products formed, if any, to the thermodynamically favored solid-phase reaction products formed being less than about 3:1.

5. An apparatus for containing a pool of molten silicon during production of single crystal silicon, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising a metal oxide in contact with the outer surface of the silica container.

6. The apparatus of claim 5 wherein the chemical barrier further comprises a metal, a carbide of a metal or an oxide of a metal in contact with the inner surface of the graphite support vessel.

7. The apparatus of claim 5 wherein the chemical barrier consists essentially of zirconium dioxide. B . The apparatus of claim 5 wherein the chemical barrier consists essentially of a metal oxide.

9. The apparatus of claim 5 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer consisting essentially of a metal oxide in contact with the outer surface of the silica container, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers.

10. The apparatus as set forth in claim 12 wherein the second layer consists essentially of zirconium dioxide.

11. The apparatus as set forth in claim 9 wherein the first layer consists essentially of a metal.

12. The apparatus as set forth in claim 9 wherein the first layer consists essentially of a metal carbide .

13. The apparatus as set orth in claim 9 wherein the first layer consists essentially of a metal oxide ,

1 . An apparatus for containing a pool of molten silicon during production of single crystal silicon, the apparatus comprising a silica container having inner and outer surfaces , the inner surface defining a cavity capable of containing a pool of molten silicon, a graphite support vessel for supporting the silica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the silica container, and a composite chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the composite chemical barrier comprising a first layer consisting essentially of a metal carbide or a metal oxide in contact with the inner surface of the graphite support vessel, and a second layer consisting essentially of a metal in contact with the outer surface of the silica container, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers.

15. The apparatus of claim 14 wherein the first layer consists essentially of a metal carbide.

16. The apparatus of claim 14 wherein the first layer consists essentially of a metal oxide.

17. An apparatus for containing a pool of molten silicon during production of single crystal silicon, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of containing a pool of molten silicon,

AMENDED SHEET (ARTICLE 19) a graphite support vesεcl for supporting the θilica container during production of single crystal silicon from the molten silicon contained within the silica container, the support vessel having an inner surface which receives the Bilica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surf ce of the graphite support vessel , the chemical barrier comprising an element or compound in contact with the outer surface of the silica container selected from the group consisting of molybdenum carbide, tantalum oxide, zirconium, zirconium dioxide, hafnium, hafnium dioxide, titanium, and titanium dioxide.

IB, The apparatus of claim 17 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of the graphite support vessel selected from the group consisting of molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide.

19. The apparatus of claim 19 wherein the chemical barrier consists essentially of zirconium dioxide .

20. The apparatus of claim 17 wherein the chemical barrier consists essentially of zirconium.

21. The apparatus of claim 17 wherein the chemical barrier consists essentially of hafnium.

AMENDED SHEET (ARTICLE 19)

22. The apparatus of claim 17 wherein the chemical barrier consists essentially of zirconium dioxide, hafnium dioxide or molybdenum carbide.

23. The apparatus of claim 17 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer in contact with the outer surface of the silica container, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers .

24. The apparatus as set forth in claim 26 wherein the second layer consists essentially of zirconium dioxide.

25. The apparatus as set forth in claim 23 wherein the second layer consists essentially of zirconium, hafnium or titanium.

26. The apparatus as set forth in claim 23 wherein the second layer consists essentially of a compound selected from the group consisting of tantalum oxide, zirconium dioxide, hafnium dioxide, titanium dioxide and molybdenum carbide .

27. The apparatus as set forth in claim 23 wherein the first layer consists essentially of tungsten or molybdenum.

28. The apparatus as set forth in claim 23 wherein the first layer consists essentially of an element selected from the group consisting of tantalum, zirconium, hafnium and titanium.

29. The apparatus as set forth in claim 23 wherein the first layer consists essentially of a compound selected from the group consisting of molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide and titanium carbide.

30. The apparatus as βet forth in claim 23 wherein the first layer consists essentially of a compound selected from the group consisting of tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide .

31. The apparatus as set forth in claim 23 wherein the first and second layers consist essentially of a compounds selected from group consisting of, respectively, tantalum carbide and tantalum oxide, zirconium carbide and zirconium dioxide, hafnium carbide and hafnium dioxide, and titanium carbide and titanium dioxide .

32. A silica container for holding a pool of molten silicon formed therein during production of single crystal silicon in a CzochralBki-type process, the container comprising a body consisting essentially of silica and having inner and outer surfaces, the inner surface defining a cavity capable of containing the pool of molten silicon, and a chemical barrier covering at least a portion of the outer surface of the body, the chemical barrier comprising a metal oxide or a metal selected from the group consisting of zirconium, hafnium and titanium in contact with the outer surface of the silica container.

33. The silica container of claim 32 wherein the metal oxide in contact with the outer surface of the silica container is selected from the group consisting of tantalum oxide, zirconium dioxide, hafnium dioxide, and titanium dioxide.

34. The silica container of claim 32 wherein the chemical barrier further comprises a metal, a carbide of a metal or an oxide of a metal capable of contacting an inner surface of a graphite support vessel when the silica container is supported therein.

35. The silica container of claim 32 wherein the chemical barrier further comprises an element or compound capable of contacting an inner surface of a graphite support vessel when the silica container is supported therein, the element or compound being selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide.

36. The silica container of claim 32 wherein the chemical barrier consists essentially of a metal or an oxide selected from the group consisting of zirconium. hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

37. The silica container of claim 32 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the outer surface of the silica container and a second layer capable of contacting an inner surface of a graphite support when the silica container is supported therein, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers .

38. The silica container as set forth in claim 37 wherein the first layer consists essentially of a metal or a metal oxide selected from the group consisting of zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

39. The silica container as set forth in claim 37 wherein the second layer consists essentially of a metal, a metal carbide or a metal oxide selected from the group consisting of tungsten, molybdenum, tantalum, zirconium, hafnium, titanium, molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide, titanium carbide, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

40. The silica container as set forth in claim 37 wherein the first layer consists essentially of a metal oxide and the second layer consists essentially of a metal carbide.

41. A graphite support vessel for supporting a silica container during production of a single crystal silicon ingot from a silicon melt formed within the container, the support vessel comprising a body consisting essentially of graphite and having an inner surface which defines an open cavity capable of receiving the silica container, and a chemical barrier covering at least a portion of the inner surface of the body, the chemical barrier comprising a metal carbide, a metal oxide, or a metal selected from the group consisting of zirconium, hafnium or titanium in contact with the inner surface of the graphite support vessel and a metal or an oxide of a metal capable of contacting the outer surface of the silica container when the silica container is supported in the graphite support vessel .

42. The graphite support vessel of claim 41 wherein the chemical barrier consists essentially of a metal or a metal oxide selected from the group consisting of zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

43. The graphite support vessel of claim 41 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the inner surface of the graphite support vessel and a second layer capable of contacting the outer surface of the silica container when the silica container is supported by the graphite support vessel, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers.

4 . The graphite support vessel as set orth in claim 43 wherein the first layer consists essentially of a metal or a metal carbide or a metal oxide selected from the group consisting of zirconium, hafnium, titanium, molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide, titanium carbide, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

45. The graphite support vessel as set forth in claim 43 wherein the second layer consists essentially of a metal or a metal oxide selected from the group consisting of tungsten, molybdenum, zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

46. The graphite support vessel as set forth in claim 43 wherein the first layer consists essentially of a metal carbide and the second layer consists essentially of a metal oxide.

47. A process for producing single crystal silicon from polycrystalline silicon, the process comprising assembling an apparatus into which polycrystalline silicon can be loaded and subsequently heated to form a pool of molten silicon, the apparatus comprising a silica container having inner and outer surfaces, the inner surface defining a cavity capable of receiving the polycrystalline silicon and containing the pool of molten silicon, a graphite support vessel for supporting the silica container, the support vessel having an inner surface which defines a cavity capable of receiving the silica container, and a chemical barrier situated between at least a portion of the outer surface of the silica container and the inner surface of the graphite support vessel, the chemical barrier comprising a metal oxide or a metal selected from the group consisting of zirconium, hafnium and titanium in contact with the outer surface of the silica container, loading polycrystalline silicon into the cavity of the silica container, forming a pool of molten silicon in the silica container, and drawing a single crystal silicon ingot from the molten silicon.

48. The process as set forth claim 47 wherein the chemical barrier is situated between the outer surface of the silica container and the inner surface of the graphite support vessel by (1) applying a barrier material, before assembling the apparatus, to the outer surface of the silica container or to the inner surface of the graphite support vessel, or, alternatively or additionally, by (2) placing or laying out one or more sheets of a barrier material between the outer surface of the silica container and the inner surface of the graphite support vessel.

49. The process as set forth claim 47 wherein at least about 100 kg of polycryatalline silicon is loaded into a silica container having a diameter of at least about 22 inches (about 56 cm) , and a pool of molten silicon is formed in the silica container by heating the silica container and support vessel to achieve a corner temperature or a wall temperature of at least about 1550 °C for a period ranging from about 6 hours to about 18 hours .

50. The process of claim 47 wherein the metal or oxide in contact with the outer surface of the silica container is selected from the group consisting of tantalum oxide, zirconium, zirconium dioxide, hafnium, hafnium dioxide, titanium and titanium dioxide.

51. The process of claim 47 wherein the chemical barrier further comprises a metal, a carbide of a metal or an oxide of a metal in contact with the inner surface of the graphite support vessel .

52. The process of claim 47 wherein the chemical barrier further comprises an element or compound in contact with the inner surface of a graphite support vessel, the element or compound being selected from the group consisting of tungsten, molybdenum, molybdenum carbide, tantalum, tantalum carbide, tantalum oxide, zirconium, zirconium carbide, zirconium dioxide, hafnium, hafnium carbide, hafnium dioxide, titanium, titanium carbide and titanium dioxide.

53. The process of claim 47 wherein the chemical barrier consists essentially of a metal or an oxide selected from the group consisting of zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide.

54. The process of claim 47 wherein the chemical barrier is a composite chemical barrier comprising a first layer in contact with the outer surface of the silica container and a second layer in contact with the inner surface of a graphite support vessel, the first and second layers also being adjacent each other or, alternatively, being adjacent one or more intermediate layers situated between the first and second layers .

55. The process as set forth in claim 54 wherein the first layer consists essentially of a metal or a metal oxide selected from the group consisting of zirconium, hafnium, titanium, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide,

5G. The process as set forth in claim 54 wherein the second layer consists essentially of a metal, a metal carbide or a metal oxide selected from the group consisting of tungsten, molybdenum, tantalum, zirconium, hafnium, titanium, molybdenum carbide, tantalum carbide, zirconium carbide, hafnium carbide, titanium carbide, tantalum oxide, zirconium dioxide, hafnium dioxide and titanium dioxide .

57. The process as set forth in claim 54 wherein the first layer consists essentially of a metal oxide and the second layer consists essentially of a metal carbide.