WO2004044275A2 - Process for removing metallic impurities from silicon carbide coated components of a silicon single crystal pulling apparatus - Google Patents

Abstract

A process for reconditioning a structural component of a crystal pulling apparatus for reuse therein. The structural component comprises a graphite substrate and a first protect layer of silicon carbide or glassy carbon the substrate, and optionally, a second protective layer comprising silicon covering the first protective layer. During the process, the structural component, while in a treatment chamber, is exposed to an iron-complexing gas comprising a halogen at a temperature and for a duration sufficient to reduce an iron concentration in the structural component.

Description

PROCESS FOR REMOVING METALLIC IMPURITIES FROM

SILICON CARBIDE COATED COMPONENTS OF A

SILICON SINGLE CRYSTAL PULLING APPARATUS

BACKGROUND OF THE INVENTION

[0001] The present invention is generally directed to a process for reconditioning parts or components of a silicon single crystal pulling device or apparatus by removing metallic impurities or contaminants from silicon-carbide or silicon-carbide/silicon coatings present thereon. In particular, the present invention is directed to a process for removing iron impurities from such coated components of a pulling apparatus, designed for growing single crystal silicon ingots in accordance with the Czochralski method, by means of contacting these coated components with a gas comprising an iron-complexing agent to form volatile iron-containing compounds or complexes therewith. In this way, iron may be vaporized and removed from the coating(s). The present process accordingly acts to extend the useful life of crystal puller components and, in particular, enables the preparation of single crystal silicon having low iron content.

[0002] Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication processes, is commonly prepared with the so-called Czochralski process. In this process, polycrystalline silicon ("polysilicon") is charged into a crucible, the polysilicon is melted, a seed crystal is immersed into the molten silicon and a single crystal silicon ingot is grown by slow extraction to a desired diameter. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter may be reduced gradually to form an end opposite the seed-end (e.g., an end-cone). Typically, the opposite end of the crystal is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. [0003] During the crystal growth process, iron and other metallic impurities may be incorporated in the crystal through the polycrystalline silicon charge, the quartz crucible, and graphite hot zone structural components (such as the susceptor, heaters, thermal shields and reflectors, or insulation which control the heat flow around the crucible and the cooling rate of the growing crystal). Impurities present in the polycrystalline charge and crucible diffuse throughout the melt and produce concentrations which do not vary along the radial direction of the ingot and/or wafer. In contrast, metallic impurities which evaporate out of graphite structural components diffuse into the growing crystal from the periphery. As a result, the concentration of metallic impurities in general, and iron in particular, increases radially outwardly from the central axis to the edge of the crystal. In addition to a radial variation, the concentration of iron within an ingot varies axially. In many circumstances, the iron concentration in the main body of an ingot decreases axially from the seed end to the tail end. This axially variation in iron is due in part to the fact that the earlier grown portions of the ingot are exposed to the evaporated iron for a longer period of time than later grown portions of the ingot.

[0004] Heavy metals strongly influence the electrical characteristics of silicon devices. The initial electrical effect is the introduction of energy levels near the center of the bandgap of silicon. These levels may act as recombination centers, thus decreasing the minority carrier recombination lifetime, a material parameter which strongly influences electrical characteristics such as leakage current, switching behavior, and storage time in metal oxide semiconductor (MOS) memories. Likewise, the role of the intermediate energy level as a generation center may affect, and thus distort, the ideal current-voltage characteristics of the p-n junction. Metallic impurities frequently cause various types of lattice defects such as metallic precipitates, stacking faults or dislocations that form in the active region on the surface of silicon substrates. These defects on the surface have a fatal influence on device performance and yield. In particular, it is known that iron and molybdenum reduce minority carrier lifetimes in silicon wafers, and copper and nickel can lead to oxygen induced stacking faults in the resulting crystal. [0005] In order to reduce the risk of crystal contamination with contaminants which can be out-gassed from graphite parts located around the growing crystal, it is common for graphite components within the hot zone to be coated with a protective barrier layer. Typically, the protective layer is silicon carbide ("SiC") because of its relatively high purity, chemical stability and heat resistance. (See, e.g., D. Gilmore, T. Arahori, M. Ito, H. Murakami and S. Miki, The impact of graphite furnace parts on radial impurity distribution in CZ grown single crystal silicon," J. Electrochemical Society, Vol. 145, No. 2, (Feb. 1998), pp. 621-628.) Silicon carbide coatings provide a barrier to impurity out-gassing by sealing the graphite surface, thus requiring impurities to pass through the coating by grain boundary and bulk diffusion mechanisms.

[0006] Although graphite substrates coated with a thin layer of silicon carbide have been used to overcome this problem to a certain extent, the introduction of "closed" hot zone configurations and increasingly stringent specifications for metal content in silicon wafers have rendered the existing graphite substrates coated with silicon carbide unsatisfactory. Closed hot zone configurations have been implemented to reduce the density of silicon lattice vacancy and/or silicon self-interstitial agglomerated intrinsic point defects (e.g., D- defects, Flow Pattern Defects, Gate Oxide Integrity Defects, Crystal Originated Particle Defects, crystal originated Light Point Defects and interstitial-type dislocation loops or defects, including both A-type and B-type defects) by controlling, among other things, the cooling rate of the growing silicon ingot during critical temperature ranges (e.g., between about the solidification temperature, i.e., about 1300 °C, and about 1050 °C). Typically, the cooling rate is controlled, at least in part, by including structural components such as upper, intermediate and lower heat shields above the melt surface. (See, e.g., U.S. Pat. No. 5,942,302.) As a comparison, for ingot temperatures from about solidification (i.e., about 1300 °C) to about a 1000 °C, a closed hot zone design typically limits the cooling rate to about 0.8 °C/mm to about 1.0 °C/mm, whereas a conventional open hot zone design cools the ingot at about 1.4 °C/mm to about 1.6 °C/mm.

[0007] In addition to using closed hot zone designs to avoid the formation of agglomerated intrinsic point defects, single crystal silicon ingots are allowed to dwell at a temperature between the temperature of solidification and a temperature within the range of about 1050 °C to about 900 °C for a period of hours which varies as a function of crystal diameter (e.g., (i) at least about 5 hours, 10 hours, 15 hours or more for 150 mm nominal diameter silicon crystals, (ii) at least about 5 hours, 10 hours, 20 hours, 25 hours, 30 hours or more for 200 mm nominal diameter silicon crystals, and (iii) at least about 20 hours, 40 hours, 60 hours, 75 hours or more for silicon crystals having a nominal diameter greater than 200 mm). It is to be noted, however, that the precise time and temperature to which the ingot is cooled is at least in part a function of the concentration of intrinsic point defects, the number of point defects which must be diffused in order to prevent supersaturation and agglomeration from occurring, and the rate at which the given intrinsic point defects diffuse (i.e., the diffusivity of the intrinsic point defects). Further details for growing single crystal silicon which is substantially free of agglomerated defects is provided in, for example, U.S. Patent Nos: 5,919,302; 6,254,672; 6,287,380, 6,328,795 and 6,312,516. All matter disclosed in the foregoing patents is incorporated herein by reference for all purposes.

[0008] Although closed hot zones effectively reduce agglomerated intrinsic point defects (e.g., single crystal silicon grown in an open hot zone design typically has about 1*10³ to about 1*10⁷ defects/cm³, whereas single crystal silicon grown in a closed hot zone typically has less than about 1*10³ defects/cm³), the increased amount of structural graphite, the higher temperatures, the closer proximity of structural components to the growing ingot and melt, and the longer duration of the pulling process can contribute to the increased amount of iron diffusing into the grown crystal. For example, crystals grown in a closed hot zone may have an average iron concentration which can potentially be as much as 5 to 10 times greater, and an edge iron concentration which can potentially be as much as 50 to 100 times greater than crystals grown in a typical hot zone (i.e., open hot zone), if proper steps are not taken. BRIEF SUMMARY OF THE INVENTION

[0009] Briefly, the present invention is directed to a process for reconditioning a structural component of a crystal pulling apparatus for reuse therein, the structural component comprising a substrate and a first protective layer covering at least a portion of the substrate, wherein the substrate comprises graphite, and the first protective layer comprises silicon carbide or glassy carbon. The process comprises placing the structural component in a treatment chamber, and exposing the structural component to an iron-complexing gas at a temperature and for a duration sufficient to reduce an iron concentration in the first protective layer, the iron-complexing gas comprising a halogen.

[0010] The present invention is also directed to a process for reconditioning a structural component of a crystal pulling apparatus for reuse therein, the structural component comprising a substrate and a first protective layer covering the substrate, wherein the substrate comprises graphite, and the first protective layer comprises silicon carbide. The process comprises placing the structural component in a treatment chamber, and exposing the structural component to an iron-complexing gas comprising a halogen under a vacuum pressure that is between about 15 and about 75 torr at a temperature that is between about 200 C and about 600 C for a duration that is between about 2 and 15 hours to reduce an iron concentration in the first protective layer to less than about 10 ppbw.

[0011] Additionally, the present invention is directed to a process for reconditioning a structural component of a crystal pulling apparatus for reuse therein, the structural component comprising a substrate, a first protective layer covering the substrate, and a second protective layer covering the first protective layer, wherein the substrate comprises graphite, the first protective layer comprises silicon carbide, and the second protective layer comprises silicon. The process comprising placing the structural component in a treatment chamber, and exposing the structural component to an iron-complexing gas comprising a halogen at a temperature and for a duration at least sufficient to remove the second protective layer. [0012] Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a graph illustrating the edge iron concentration (normalized) as a function of crystal length in the first and the eleventh single crystal silicon ingots prepared in a crystal pulling apparatus having a closed hot zone (as further described herein), in accordance with the Czochralski method (wherein the Y-axis is the iron concentration normalized to the lowest actual iron concentration).

[0014] Figure 2 is a graph illustrating the edge iron concentration (normalized) as a function of crystal length for a crystal grown in a crystal pulling apparatus comprising silicon-carbide-coated (diamond data points) or silicon- carbide/silicon-coated (box data points) parts, in accordance with the Czochralski method (wherein the Y-axis is the iron concentration normalized to the lowest actual iron concentration).

[0015] Figure 3 is a graph illustrating the edge iron concentration (normalized) as a function of crystal length for a crystal grown in a crystal pulling apparatus comprising silicon-carbide-coated (diamond data points) or silicon- carbide/silicon-coated (box data points) parts, in accordance with the Czochralski method (wherein the Y-axis is the iron concentration normalized to the lowest actual iron concentration).

[0016] Figure 4 is a graph illustrating the equilibrium compositions, and concentrations thereof, of various iron chlorides in the treatment chamber atmosphere as a function of temperature at atmospheric pressure (wherein the Y- axis is log scale).

[0017] Figure 5 is a graph illustrating the equilibrium compositions, and concentrations thereof, of various silicon chlorides in the treatment chamber atmosphere as a function of temperature at atmospheric pressure (wherein the Y- axis is log scale) [0018] Figure 6 is a graph illustrating the equilibrium composition, and concentration thereof, of FeCI₂ in the treatment chamber atmosphere as a function of temperature at a pressure of 15 torr.

[0019] Figure 7 is a graph illustrating the effect of pressure on the formation of FeCI₂ at a temperature of about 1000 °C.

[0020] Figure 8 is a diagram of a silicon single crystal pulling apparatus, illustrating in detail some of the various structural components which may be purified or cleaned in accordance with the present invention.

[0021] Figure 9 is a diagram of an apparatus used to diffuse iron from graphite and silicon-carbide coated graphite samples into a silicon wafer in order to determine the iron concentration in the samples.

[0022] Figure 10 is a graph which shows the concentrations of iron in four different graphite samples when uncoated and coated with two different silicon carbide layers.

DETAILED DESCRIPTION OF THE INVENTION

[0023] In accordance with the present invention, it has been observed that the outdiffusion of metallic contaminants (e.g., iron) during crystal growth can become more pronounced with time, particularly in the case of "closed" hot zones which are designed to maintain a grown crystal or crystal segment at an elevated temperature for an extended period of time (as further described elsewhere herein).

[0024] Without being held to a particular theory, it is generally believed that the increase in metallic contaminant concentration (e.g., edge iron concentration) is the result of diffusion of iron or other metallic contaminants from parts or structural components within the growth chamber, and more specifically, the hot zone of the crystal puller including parts or components having one or more protective coatings thereon to control or prevent the out-diffusion of iron or other metallic contaminants therefrom. In the case of such coated parts, the diminished performance thereof is believed to be attributable to cross- contamination from parts such as uncoated and extruded side heaters, and also from the handling of such parts after each crystal is grown. This increase in, for example, iron concentration is particularly acute for closed hot zones (wherein for example secondary heaters, heat shields or reflectors, insulation, etc. have been inserted to control the thermal gradient at the melt-solid interface and the cooling rate of the solidified silicon), due to the extended period of time the parts in the growth chamber are exposed to high temperatures.

[0025] Referring to Figure 2, the dependence of the iron concentration on growth process duration is illustrated by the decreasing amount of iron contamination with increasing axial length of the ingots. That is to say, the later grown portions of the ingots were exposed to the growth chamber atmosphere for shorter periods of time and, as a result, were less contaminated. The effect of duration on iron contamination, however, may be overshadowed by other conditions of the growth process. For example, referring to Figure 3, the concentration of iron in the ingot may increase with the axial distance due, at least in part, to a decrease in gas flow through the growth chamber which decreases the removal of evolved iron from the crystal puller.

[0026] In addition to the duration of the growth process, it has been discovered that the longer structural components are in service the more they tend to contaminate ingots. For example, referring to Figure 1 , the edge iron concentration in a first ingot is significantly lower than an eleventh ingot grown in the same crystal puller using the same "closed" hot zone components. Therefore, to combat this tendency of increasing contamination with use, structural components have typically been replaced with new parts before being broken or fully spent, which is uneconomical. It is to be noted that, unlike Figure 2, the ingots of Figure 1 had an increasing iron concentration as a function of ingot length that was due primarily to a different gas flow in crystal puller.

[0027] In accordance with the present invention, it has been discovered that metallic contaminants, such as iron, chromium, molybdenum, copper, titanium, sodium, calcium, cobalt, nickel, potassium, etc., and/or mixtures thereof, present in the coated surfaces of the crystal puller components, can be removed by means of exposing these components to a iron-complexing gas, or a gas comprising a iron-complexing agent (e.g., a halogen), for a given duration and at a given temperature, as further described herein below. In this way, these parts or components can be reconditioned and, therefore, the useful life of components may be extended because they can be reused, after optionally undergoing the re- application or deposition of a third protective layer.

/. Crystal Puller Components

[0028] Referring now to Figure 8, there is shown a crystal pulling apparatus indicated generally at 2. The apparatus comprises a crystal growth chamber 4 and a crystal chamber 6. Contained within crystal growth chamber 4 is a silica crucible 8 which contains molten polysilicon 26 for growing the silicon single crystal. A pulling wire (not shown) attached to a wire rotation device (not shown) is used to slowly extract the growing crystal during operation. Also contained within the crystal growth chamber 4 are several structural components which surround the crucible such as a susceptor 14 for holding the crucible in place, a melt heater 16 for heating the silicon melt, and a melt heater shield 18 for retaining heat near the crucible. A growth chamber with a closed hot zone design may also contain structural components such as a lower heat shield 31 that comprises an inner reflector 32, an outer reflector 33 and an insulation layer 34 sandwiched between coaxially positioned inner and outer reflectors 32 and 33, respectively. A closed hot zone design may also comprise an intermediate heat shield 35, and an upper heat shield 36. As previously stated, these structural components are typically constructed of graphite and control the heat flow around the crucible and the rate of cooling of the silicon single crystal. It should be recognized by one skilled in the art that other structural components such as the upper heater 37, upper insulation support 38, or upper insulation shield 39 may also be prepared for use in accordance with the present invention.

[0029] Figure 8 also depicts the iron contamination in the growing single crystal ingot 10 with iron emanating from structural components within the growth chamber (e.g., lower heat shield 31, intermediate heat shield 35, and upper heater shield 36). The portion of ingot 10 which is shaded 12 (not to scale), represents "edge" iron contamination of a silicon ingot grown in a closed hot zone constructed with conventional structural components. Edge iron is the common designation for iron contamination around the circumference of an ingot/wafer. Typically the extent of edge iron contamination is referred to as "edge iron concentration" which is the average iron concentration for the annular portion of a silicon wafer or main body of an ingot extending radially inward about 5 millimeters from the circumferential edge. The extent of edge iron contamination also affects the "average iron concentration" which is the average concentration of iron throughout an entire silicon wafer or main body of an ingot.

[0030] In accordance with one aspect of the present invention, structural components utilized in a growth chamber comprise a substrate and one or more protective layers, the substrate preferably having a low initial concentration of metallic contaminants (e.g., iron, chromium, copper, titanium, molybdenum, sodium, calcium, cobalt, nickel, potassium, etc., and/or mixtures thereof). (See, e.g., U.S. Patent No. 6,183,553 (Holder et al.) and U.S. Patent Application Serial No. 10/039,459 filed November 7, 2001 (H. Sreedharamurthy), the entire contents of both being incorporated herein by reference.) More specifically, the substrate of the components comprises graphite which is preferably at least about 99.9 percent by weight pure graphite, and more preferably at least about 99.99 percent by weight, or more pure graphite. Further, the graphite substrate preferably contains less than about 20 parts per million by weight ("ppmw") total metals (e.g., Fe, Mo, Cu, Ni), more preferably less than about 10 ppmw, still more preferably less than about 5 ppmw, still more preferably less than about 3 ppmw, and most preferably less than about 1.5 ppmw. In addition, while the concentration of iron in conventional hot zone graphite typically ranges from about 1.0 ppmw to about 0.05 ppmw, preferably the concentration of iron in a substrate used in accordance with one embodiment of the present invention is less than about 5 parts per billion by weight ("ppbw"), more preferably less than about 4 parts per billion by weight, still more preferably less than about 2 ppbw, and still more preferably less than about 1 ppbw.

[0031] In one embodiment, the structural components comprise at least a first protective layer which covers at least a portion of the surface of the substrate (e.g., the portion that is exposed to the atmosphere of the growth chamber). Preferably, the first protective layer covers the entire surface of the substrate. The first protective layer preferably comprises silicon carbide or glassy carbon. Preferably the amount of silicon carbide or glassy carbon in the first protective layer is between about 99.9 percent by weight to about 99.99 percent by weight, or greater. In one embodiment silicon carbide is selected to form the first protective layer, rather than glassy carbon. Structural components constructed of graphite and having a silicon carbide or glassy carbon coating are commercially available from, for example, Graphite Die Mold, Inc. (Durham, Conn.). Without being held to a particular theory, it is generally believed that the first protective layer or coating acts as a barrier layer which tends to seal in and contain contaminants which outdiffuse and/or are released from the graphite during exposure to high temperatures during crystal pulling.

[0032] In one preferred embodiment, the silicon carbide or glassy carbon protective coating contains less than about 5 ppmw total metals such as iron, chromium, molybdenum, copper, titanium, sodium, calcium, cobalt, nickel, potassium, more preferably less than 4 ppmw total metals, still more preferably less than 2 ppmw total metals and still more preferably less than about 1.5 ppmw. Further, while the concentration of iron in conventional hot zone silicon carbide coatings ranges from about 0.8 to about 0.5 ppmw, the concentration of iron in the protective coating is preferably no more than about 5 ppbw, more preferably no more than about 4 ppbw, still more preferably no more than about 2 ppbw, and still more preferably no more than about 1 ppbw. The thickness of the protective coating is generally at least about 75 micrometers, and preferably is between about 75 and about 125 micrometers, or more preferably about 100 micrometers.

[0033] In alternative embodiments, the component may additionally comprise a second protective layer, which comprises silicon, that is deposited upon and covering at least a portion of the first protective layer. Preferably, the second protective layer, if present, covers the entire first protective layer. Also, the amount of silicon in the second protective layer is preferably between about 99.9 percent by weight to about 99.99 percent by weight, or greater. The concentration of iron in the second protective layer is preferably no more than about 5 ppbw, more preferably no more than about 4 ppbw, still more preferably no more than about 2 ppbw, and still more preferably no more than about 1 ppbw. Typically, the second protective layer has a thickness which, in some embodiments, is less than about 200 micrometers (e.g., about 125 to about 150 micrometers), while in other embodiments is less than about 100 micrometers (e.g., ranging between about 40 and 80 micrometers, preferably between about 50 and 75 micrometers, and most preferably is about 60 micrometers).

[0034] Without being held to a particular theory, it is generally believed that the second protective layer or coating provides a protective chemical barrier which acts as a gettering sink to getter contaminants produced from the graphite components which are able to pass through the first protective coating, or contaminants such as iron evaporated from the first protective coating. Because of the high affinity of silicon for contaminants, the silicon in the second layer readily reacts with the contaminants to form stable suicides, which drastically reduces the diffusion capability of the contaminants to pass through the silicon layer. Referring again to Figures 2 and 3, the effectiveness of a second protective layer (indicated by Si/SiC) at reducing the amount of iron contamination in an ingot is readily apparent. Specifically, both figures indicate a significant decrease in the iron contamination of ingots grown in a growth chamber comprising structural components with silicon and silicon carbide layers compared to ingots grown in a growth chamber comprising structural components with only a silicon carbide layer (i.e., without a second protective layer comprising silicon).

[0035] It is to be noted that silicon-carbide and silicon-carbide/silicon coated parts are commercially available and/or may be prepared by means known in the art. (See, e.g., U.S. Patent No. 6,183,553.)

//. Component Cleaning

[0036] In accordance with the present invention, the above-described components may be treated or cleaned in order to remove iron and other metallic contaminants (e.g., chromium, molybdenum, copper, titanium, sodium, calcium, cobalt, nickel, and potassium), and thus are reconditioned for reuse in a crystal pulling apparatus. Generally speaking, the present process comprises exposing a used, coated component to a iron-complexing gas, or a gas comprising a iron- complexing agent, under conditions which are sufficient for a metal complex to form and then be removed (e.g., vaporized). Without being held to a particular theory, it is believed that the removal of metallic impurities from the first protective layer, the second protective layer (if present) and possibly even from the substrate is accomplished, at least in part, by outdiffusion of metal atoms (e.g., iron atoms) from the interior of the structural component (i.e., any portion of the structural component encompassed by the outermost surface) to outermost surface and/or outermost surface region (i.e., a region comprising the surface and extending inward some small distance to which the iron-complexing gas may diffuse) where the metal may be complexed by the iron-complexing gas and vaporized. Preferably, the process will be controlled to reduce the concentration of iron or other metallic impurities in the protective layer(s) or coating(s) to less than about 10 ppbw, more preferably less than about 5 ppbw, more still preferably less than about 2 ppbw, and still more preferably less than about 1 ppbw.

[0037] Still further, it is also believed that, the removal of metals from the structural component is due, at least in part, to the removal of the second protective layer, if present, from the structural component. Stated another way, the removal of metals, such as iron, may be accomplished, at least in part, by controlling the process conditions to remove at least a portion of the second protective layer, if present. Referring to Figure 5, the silicon of the second protective layer reacts with the halogen of the iron-complexing gas (e.g, chlorine) to form one or more silicon halide compounds (e.g., SiCI₄, SiCI₃, SiCI₂, and SiCI) that are gaseous at the process temperature employed. Thus, as with the iron impurities, silicon is removed from the structural component. Preferably, at least about 20% of the second protective layer is removed, more preferably at least about 40% is removed, still more preferably at least about 60% is removed, even still more preferably at least about 80% is removed, even more preferred at least about 90% is removed, still more preferred at least about 95%, and most preferably the entire second protective layer is removed in order to reduce the concentration of impurities. As such, in some embodiments (e.g., where the second protective layer is removed entirely), the metal atoms that are in the second protective layer prior to performing the process and the metal atoms that are diffused from the first protective layer and/or the substrate to the second protective layer during the process of the present invention are eliminated from the structural component.

[0038] In contrast to the second protective layer, the first protective layer is considered to be relatively stable or non-reactive with the iron-complexing gas throughout a range of temperatures that are typically used during the reconditioning process. For example, referring to Figure 5, which depicts relative equilibrium compositions of iron chloride gases as a function of temperature, the line representing solid silicon carbide in the system is substantially constant at temperatures from about 25 °C to about 800 °C. As such, in one embodiment the process of the present invention is preferably controlled so that substantially all or all of the first protective layer remains intact upon completion of the process. For example, less than about 1%, more preferably less than about 0.1%, still more preferably less than about 0.01%, and even still more preferably none of the protective layer is removed by the iron-complexing gas.

[0039] Although generally not preferred, it is possible to control the reconditioning process such that a portion or the entire first protective layer is removed by the iron-complexing gas. To remove the first protective layer, however, the reconditioning process is typically controlled so that the structural component is exposed to significantly higher temperatures. For example, in one embodiment a portion or all of the first protective layer may be removed by exposure to the iron-complexing gas at temperatures from about 1 ,500 °C to about 1 ,800 °C for a duration of about 6 to about 12 hours.

[0040] As set forth above, when two protective layers are present, a portion or all of the second (i.e., silicon) layer may be removed and a third protective layer may be deposited on the structural component by means known in the art before the structural component is reused in a crystal growth chamber. The deposition of a third protective layer, although preferred because of the increased metal gettering effect, is optional. Thus, a structural component having a portion or even the entire second protective layer removed by the reconditioning process may be reused in a crystal growth apparatus without the deposition of a third protective layer. In another embodiment the structural component having at least a portion of the second protective layer removed is subjected to a process to deposit a third protective layer comprising silicon prior to the structural component being reused in a crystal growth apparatus. More specifically, the third protective layer is deposited onto at least a portion of, and preferably the entire, remaining second protective layer (if present) or the first protective layer prior to the structural component being reused. That is to say, the entire structural component is preferably covered with the third protective layer. In one embodiment the third protective layer has a thickness such that a sum of the thicknesses of the third protective layer and the second protective layer, if present, is less than about 200 micrometers (e.g., between about 125 and about 150 micrometers). In another embodiment the third protective layer has a thickness such that a sum of the thicknesses of the third protective layer and the second protective layer, if present, is less than about 100 micrometers (e.g., between about 40 and about 80 micrometers, between about 50 and about 75 micrometers, or about 60 micrometers). Thus, in a particularly preferred embodiment in which the entire second protective layer is removed by the reconditioning process, a third protective layer comprising silicon is deposited on the first protective layer and the thickness of the third protective layer is about that of the second protective layer prior to performing the reconditioning process.

[0041] In one embodiment, the process of the present invention comprises removing a coated structural component from a crystal pulling apparatus and exposing the coated structural component to an iron-complexing gas that comprises a halogen in a treatment chamber or furnace at a temperature and for a time sufficient to remove metallic contaminants present to below some desired level or concentration.

[0042] This process is preferably carried out in a halogen purification furnace, which are widely known and used by those of skill in the crystal puller component fabrication art (e.g., Graphite Die Molde, Inc. of Durham, Connecticut). The halogen purification furnace typically comprises a treatment chamber constructed of high purity graphite and has ports for admitting and withdrawing the iron-complexing gas, structural component(s), and any other gases. Halogen purification furnaces also typically have means for drawing a vacuum in the treatment chamber. Still further, halogen purification furnaces typically comprise a means for heating the treatment chamber and items placed therein. Typically, the are heated either by resistance or induction heating.

[0043] The treatment chamber is preferably configured such that the iron- complexing gas can circulate of the entire surface of the structural component. One such configuration includes the use of graphite saggers (i.e., generally cylindrical baskets) that typically have holes in the bottom (or lower surface) and/or side wall and are open at the top (i.e., there is no top or upper surface), which allows for the iron-complexing gas to circulate over the entire surface of the structural component. Graphite saggers are preferable suspended at or near the center of the treatment chamber to maximize circulation. Alternatively, the structural component may be rotated or turned over and exposed to the iron- complexing gas as many times as necessary to facilitate thorough cleaning of all surfaces of the structural component. As the name implies, the graphite saggers are preferably made of high purity graphite, and able to fully support the structural component.

[0044] After loading the structural component into the treatment chamber, the structural component is exposed to the iron-complexing gas. The iron- complexing gas comprises a halogen such as fluorine, chlorine, bromine, and iodine, and mixtures thereof. Preferably, the halogen is fluorine, chlorine, or a mixture thereof. In one embodiment the halogen is chlorine. Fluorine is generally the most reactive of the halogens with respect to forming metal halides, and thus, in certain embodiments it may be preferred. Bromine, however, is tends to be preferred for forming a metal-halide complex with calcium and chromium contaminants. Although, chlorine and/or fluorine gas tend to be preferred sources of the halogen, other halogen-containing compounds may be used such as chlorocarbons, fluorocarbons, chlorofluorocarbons, and halogenated hydrocarbons. Generally, gases containing oxygen are avoided because the oxygen, at the typical process temperatures, is considered detrimental (i.e., it tends to oxidize) the carbon treatment chamber, the carbon saggers, and the carbon structural component substrates, and glassy carbon coatings because it . carbon oxides. Preferably, the concentration of oxygen in the atmosphere of the treatment chamber is less than about 0.5 parts per million atomic. As such, the iron-complexing gas typically comprises a halogen gas, in general, and preferably fluorine gas, chlorine gas, or a combination thereof.

[0045] In this regard it is to be noted that the complex-forming gas may comprise a pure halogen gas or halogen-containing compound gas, a mixture of a pure halogen gases and/or halogen-containing compound gasses, or a mixture of one or more halogen or halogen-containing compounds gasses with one or more non-halogenated gasses such as an inert gas such as a noble gas (e.g., helium and argon) and/or nitrogen. The inert gases are often referred to as carrier or diluent gases. The various combinations of gases may be used either simultaneously or sequentially. In one embodiment the iron complex-forming gas comprises a halogen gas such as fluorine gas, chlorine gas, or a combination thereof and an inert gas such as argon, nitrogen, or combination thereof. In another embodiment the iron-complexing gas comprises chlorine gas and argon gas or nitrogen gas.

[0046] The concentration of halogen in the iron-complexing gas is not overly critical and may vary widely (e.g., from 0.01 to about 100 percent by volume). However, the concentration of halogen is typically between about 20 and about 50 percent by volume of the iron-complexing gas with the remainder typically comprising argon or nitrogen. This concentration of halogen is typically sufficient to fully react with the amount of metallic contaminants in the used structural component and/or the silicon of the second protective layer, if present. Although the concentration of halogen affects the reactivity of the iron-complexing gas, a more or less reactive iron-complexing gas may be used provided the exposure time, exposure temperature, and/or atmospheric pressure in the treatment chamber are appropriately adjusted. In general, increasing temperatures, increasing exposure times, and/or decreasing atmospheric pressures enhance the removal of metal contaminants and/or silicon from the structural component.

[0047] Referring now to Figures 4 and 5, and without being held to any particular theory, it is generally believed that halogens readily form one or more complexes with the metallic contaminants present (e.g., Fe, Ca, Na). Typically, the formation of the metal complexes or metallic halides occurs at temperatures as low as about 25 °C or even lower. The removal or volatilization of the complexes, however, typically requires significantly higher temperatures (see, e.g., Table 2, below). Specifically, volatilization of metal halides is dependent upon the temperature of the surface of the structural component and the pressure of the atmosphere in which the structural component resides. If the temperature of the surface of the structural component is the same as or greater than the volatilization temperature of a metal halide at a given pressure, then the metal halide will volatilize. Conversely, if the temperature of the surface of the structural component is less than the volatilization temperature at a given pressure, then the metal halide will either not volatilize, or if already volatilized, will redeposit onto the surface of the structural component.

[0048] In addition to the temperature of the structural component, the pressure of the atmosphere surrounding the structural component plays a role in the volatilization of the metal complexes or metal halides. Referring now to Figures 6 and 7, it is to be noted that while the present process may be carried out at ambient pressure, in some embodiments the process may be carried out at a reduced pressure (i.e., under vacuum). Without being held to a particular theory, it is generally believed that reducing the atmospheric pressure within the treatment chamber or furnace enables lower temperatures and/or reduced reaction times to be employed, because the metallic impurities (i.e., iron) more readily out-diffuse from the contaminated components for complexation and removal. In addition, all other parameters being equal, a reduction in atmospheric pressure within the treatment chamber or furnace may, in some instances, enable lower concentrations of impurities (i.e., iron) to be achieved.

[0049] Table 2 shows the temperature and pressure at which a variety of metal halides of interest will volatilize. Without being held to theory, volatilization generally occurs as a sublimation process, but may also occur as an evaporation process. Table 2

Volatilization Temperatures and Pressures of Some Metal Halides

[0050] As such, the present process employs a temperature and a pressure which are sufficient to volatilize or vaporize the resulting complexes, and in particular iron complexes, for removal from the component, without, preferably, resulting in the decomposition of the first protective layer (e.g., the silicon carbide layer). More specifically, the structural component is preferably heated, while being exposed to the iron-complexing gas, to temperature that is greater than about 100 °C and no greater than about 2500 °C, preferably not greater than about 2000 °C, more preferably no greater than about 1500 °C, still more preferably no greater than about 1250 °C, and even more preferably no greater than about 1000 °C. In one embodiment the temperature is within the range of about 100 °C to about 1000 °C. In another embodiment the structural component is heated to a temperature or temperatures within the range of about 200 °C to about 900 °C. In yet another embodiment the temperature(s) is/are within the range of about 300 °C to about 800 °C. Additionally, in one embodiment the pressure within the treatment chamber ranges from about 0.1 to about 760 torr. In another embodiment the pressure within the treatment chamber ranges from about 1 to about 500 torr. In yet another embodiment the pressure ranges from about 10 to about 100 torr. In still another embodiment the pressure ranges from about 15 to about 75 torr. In still yet another embodiment the pressure ranges from about 25 to about 50 torr.

[0051] In this regard it is to be further noted that, generally speaking, at lower temperatures and/or higher pressures, longer durations of exposure are needed, and vice versa, to achieve the same or similar result. That is, higher temperatures and/or lower pressures typically result in completion of the process in shorter periods of time. Likewise, decreasing temperatures or increasing pressures typically result in completion of the process in longer periods of time. Typically, the process is controlled such that the component is exposed to the iron-complexing gas within the foregoing temperature ranges for a duration of at least about several tens of minutes (e.g., about 20, 30, 40, 50 or 60 minutes or more), to a few hours (e.g., about 2, 4, 6 or 8) and possibly up to several hours (e.g., about 10, 12, 15 or 20). Preferably, the duration is within the range of about 2 to about 15 hours. More preferably from about 3 to about 10 hours, and still more preferably from about 4 to about 6 hours. [0052] In addition to temperature and pressure, it is to be noted that the thicknesses of the protective layers may accounted for performing the process. Specifically, as the thickness of the layers increases the temperature and/or duration may be adjusted upward. For example, if the component comprises a first protective layer and/or a second protective layer that are/is relatively thin (e.g., about 75 micrometers and about 50 micrometers, respectively) the exposure temperature and duration may be between about 200 and about 600 °C for between about 2 to about 15 hours or between about 300 and about 500 °C for between about 4 and about 6 hours. Whereas, if the component comprises a first protective layer and/or a second protective layer that are/is relatively thick (e.g., about 125 micrometers and about 200 micrometers, respectively), the temperatures may be increased to between about 600 and about 900 °C or between about 700 and about 800 °C for the foregoing respective durations.

[0053] In one embodiment the structural component comprises a second protective layer and the process is controlled so that the second protective layer is removed at a rate ranging from about 10 to about 25 micrometers per hour. For example, at a pressure between about 25 and about 50 torr and at a temperature of about 800 °C the removal rate is about 20±5 micrometers per hour. At a pressure of between about 25 and about 50 torr and at a temperature of about 500 °C the removal rate is about 15±5 micrometers per hour. Thus, for a thickness of about 50 micrometers, the process preferably controlled so that the entire second protective layer is removed in about 2 to about 5 hours.

[0054] The volatilized metal complexes are purged from the atmosphere adjacent to the surface of the structural component. The metal complexes are preferably purged away from the surface of the structural component as soon as they are volatilized, to minimize redeposition onto the structural component. The purging can be effected by the action of the iron-complexing gas as it reacts with the metallic contaminants and circulates over the structural component. Generally, an iron-complexing/purge gas flow rate of at least about 5 standard cubic feet per minute (about 140 standard liters per minute) is sufficient. Preferably, the gas flow rate is between about 8 and about 20 standard cubic feet per minute (between about 230 and about 570 standard liters per minute). In order to minimize or prevent redeposition of metal halides on the structural component, the surface of the structural component is preferably not cooled below the minimum volatilization temperature at the particular pressure until after the all or substantially all of the volatilized metal complexes have been purged.

[0055] It is also desirable, in the preferred embodiment, to prevent deposition of the metal halides on the treatment chamber or elsewhere within the purge gas flow path, to avoid accumulation of such contaminants over time. The exposed surfaces of the treatment chamber and other portions of the purge gas flow path are preferably heated to a temperature sufficient to prevent deposition of the metal halides thereon. Additionally, the metal halides may be collected in a cold trap located in the purge gas flow path downstream of the treatment chamber. To be effective, the cold trap preferably comprises a rough surface, fins, and/or other means for obtaining a large surface area over which the purge gas can flow, and is preferably maintained at a temperature sufficiently low to allow deposition of the metal halides. In addition to the cold trap, the purged gas is also preferably passed through a halogen scrubber and/or filter.

[0056] Although generally not preferred, in one embodiment the iron- complexing gas as described above may be energized into a plasma state to react with the metallic contaminants forming metal-complexes or metal halides. Generally, the use of non-energized gases are preferred because they allow greater flexibility with respect to the type treatment device such as a heated tubular furnace, which is known to provide better control over the reaction temperature. Without being bound by theory, the high energy ionic and/or free radical halogen moieties present in the plasma react with the metallic contaminants to which they are exposed and form metal halides.

[0057] The plasma may be formed by the application of electromagnetic energy to the source-gas, such methods being well known to those of ordinary skill in the art. Preferably, the plasma is generated by energizing the iron- complexing gas in a microwave field having a frequency of from about 1.0 GHz to about 4.0 GHz. However, the plasma could alternatively be generated at other electromagnetic frequencies, as a RF plasma or UV plasma. Commercial units for microwave induced plasmas are available from Plasmatic Systems (North Brunswick, N.J.). Units for RF generator induced plasma are commercially available from Technics, Inc. (Dublin, CA). Other commercial sources of plasma etching units and supplies include Lam Research Corp. (Fremont, CA) and Applied Science and Technology, Inc (Woburn, MA).

[0058] The plasma is preferably a low pressure plasma (i.e. a plasma in which the temperature of the bulk of the source-gas is not in thermal equilibrium with the temperature of the ionic and/or free radical halogen moieties). The low pressure plasma results in only minimal heating of the structural component (i.e., the temperature of the structural component is primarily dependent upon the heating methods described above). Additionally, the lower pressures favor volatilization of the metal halides at lower temperatures. The pressure of the source-gas from which the low pressure plasma is generated most preferably ranges from about 0.01 torr (-1 Pa) to about 100 torr (-1 x 10⁴ Pa).

[0059] After the foregoing steps are completed, the structural component may have a silicon protective layer deposited or redeposited on the structural component. This deposited/redeposited silicon protective layer (also referred to as the third protective layer) may be grown on the silicon carbide or glassy carbon coated graphite component by chemical vapor deposition techniques known in the art such as ultra high vacuum chemical vapor deposition (UHVCVD) or atmospheric pressure chemical vapor deposition (APCVD). Suitable source gases for the protective silicon layer deposition include such gases as monochlorosilane, dichlorosilane, and trichlorosilane. These gases can be combined with a carrier gas such as hydrogen at a ratio of, for example, 30:1 carrier gas to silane source, to facilitate growth of the silicon layer.

[0060] The silicon layer can be grown at any temperature that facilitates silicon deposition through chemical vapor deposition. Examples of suitable temperature ranges include between about 900 °C and about 1300 °C. However, it will be recognized by one skilled in the art that other temperature ranges may be suitable, and may affect the rate of deposition of the silicon onto the silicon carbide coated graphite.

[0061] In a preferred embodiment utilizing the structural component is subjected to at least two separate deposition cycles of silicon to coat the surface of silicon carbide or glassy carbon with silicon to enable sufficient gettering. The first deposition cycle deposits about one half of the total thickness. Subsequently, the structural component is rotated so that all portions of the component are equally treated with the deposition of silicon. After rotation, a subsequent deposition cycle is initiated to deposit the second half of the third protective layer.

[0062] Regardless of whether a third protective layer is formed, after the reconditioning process the structural component is removed from the treatment chamber and preferably stored in a semiconductor grade container such as semiconductor grade plastic bags, which are widely known and used throughout the semiconductor industry, with a minimum of handling. After being placed in a bag, the component is then typically placed in cardboard box. When desired, the reinstallation of the reconditioned structural component in a crystal growing apparatus is performed in the same manner as a new structural components (i.e., the precautions for avoiding or minimizing contamination are identical and known to those of ordinary skill in the art).

///. Component Selection

[0063] Generally speaking, in one approach, wafers obtained from each single crystal silicon ingot grown in a given crystal puller are analyzed to determine the concentration of metallic impurities (e.g., iron) therein, as well as the radial location thereof within the wafer (e.g., proximate the edge). Once the edge iron concentration exceeds a given threshold (e.g., greater than about 1.5 x 10¹¹ atoms/cc, about 1 x 10¹¹ atoms/cc, or about even 5 x 10¹⁰ atoms/cc), production using the crystal puller is halted and all of the parts are removed for cleaning (e.g., coated parts), in accordance with the present invention, or replaced (e.g., uncoated parts). In this way, a given crystal puller may be used to produce a series (e.g., 5, 10, 15, 20, 25, etc.) of single crystal silicon ingots where, for example, the edge iron concentration therein is less than about 1.5*10¹¹ atoms/cc, about 1*10¹¹ atoms/cc, or about even 5*10¹⁰ atoms/cc.

[0064] However, if desired, in an alternative approach, parts or components of the crystal puller may be selectively removed, reconditioned, and reused for test crystal growth in order to identify the source(s) of the contaminant(s). In such instances, the concentration (e.g., average and/or edge) of metallic contaminants (e.g., iron) in the single crystal silicon may be reduced by cleaning at least one coated structural component in a location in the hot zone in which the component reaches at least about 950 °C for at least about 80 hours of the growth process and is within about 3 cm to about 5 cm from silicon melt or the ingot. Such parts or components include, for example, the upper heater, the upper heater shield, the intermediate heat shield, the inner reflector, the outer reflector, the insulation layer of the lower heat shield, the upper insulation support, and the upper insulation shield.

IV. Definitions

[0065] As used herein, the following phrases or terms shall have the given meanings: "agglomerated intrinsic point defects" mean defects caused (i) by the reaction in which vacancies agglomerate to produce D-defects, flow pattern defects, gate oxide integrity defects, crystal originated particle defects, crystal originated light point defects, and other such vacancy related defects, or (ii) by the reaction in which self-interstitials agglomerate to produce dislocation loops and networks, and other such self-interstitial related defects; "agglomerated interstitial defects" shall mean agglomerated intrinsic point defects caused by the reaction in which silicon self-interstitial atoms agglomerate; "agglomerated vacancy defects" shall mean agglomerated vacancy point defects caused by the reaction in which crystal lattice vacancies agglomerate; "substantially free of agglomerated intrinsic point defects" shall mean a concentration of agglomerated defects which is less than the detection limit of these defects, which is currently about 10³ defects/cm³; "radius" means the distance measured from a central axis to a circumferential edge of a wafer or ingot; "ppmw" means parts per million by weight; and, "ppbw" means parts per billion by weight.

[0066] The present invention is further illustrated by the following examples which are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner by which it may be practiced. Example 1

Determining an Acceptable Concentration of Iron Impurity in Closed Hot Zone Structural Components

[0067] A horizontal furnace tube was used to expose a monitor wafer via gas diffusion to four samples: 1) a standard graphite sample without any protective coating; 2) the standard graphite coated with silicon carbide from supplier A; 3) the standard graphite coated with silicon carbide from supplier B; and 4) the standard graphite coated with silicon carbide from supplier C. The samples were coupons about 50mm x 50mm x 25mm in size. A fused silica mask was utilized to separate the monitor wafer from each test sample. Four holes in the mask allowed the monitor wafer to be exposed to gases generated from the sample materials. Referring to Figure 9, each test stack consisted of a monitor wafer 50 for measuring the amount of iron transferred via diffusion, a fused silica mask 51 on top of the monitor wafer, and a sample 52 on top of a hole 53 in the mask. For each run, one wafer was used as a background sample and did not have a mask or samples on it.

[0068] Each of the samples were tested to measure iron diffusivity to the monitor wafer at three different temperatures: 800 °C, 950 °C and 1100 °C. The samples were held at atmospheric pressure throughout the two hour heat treatment, and a stream of argon gas over the wafers was maintained.

[0069] After each heat treatment, the wafer was sliced into quarter sections; each section containing the iron diffused from each sample. The minority carrier lifetime was determined for each wafer section and the background wafer. The minority carrier lifetime was used to determine the amount of iron present in the silicon wafer using the surface photovoltaic technique developed by G. Zoth and W. Bergholz described in the Journal of Applied Physics, vol. 67, (1990), pp. 6764-6771. The minority carrier lifetime was measured by injecting carriers into the silicon wafer sample by means of light and observing their decay by monitoring the change in the surface photovoltage effect. The surface photovoltage technique is the most sensitive method of measuring carrier diffusion length and is an accurate method for the quantitative evaluation of iron in silicon wafers. The method is based on the fact that, in silicon, iron atoms react with negatively charged boron acceptor atoms to form Fe-B pairs. Typically, the Fe-B pairs are generated by annealing the samples at about 70 °C for about 30 minutes. When heated, a portion of the Fe-B pairs disassociate and generate interstitial iron (Fβj) defects. All the Fe-B pairs disassociate, however, with illumination using a 250-Watt tungsten-halogen lamp. See, e.g., J. Lagowski, P. Edelman, O. Millie, W. Henly, M. Dexter, J. Jastrezebski and A. M. Hoff, Applied Physics Letters, vol. 63, (1993), pp. 3043- 3045. The concentration of iron in silicon is determined by comparing the minority carrier lifetime values at the two states set forth in the following equation:

[Fe]=(0.7/A)x(10¹⁶)x(1 /L^-1 /L₀ ²) (1).

L., and L₀ are minority carrier diffusion lengths in microns before and after the dissociation of Fe-B pairs, respectively, and A is the fraction of Fe-B pairs dissociated during thermal activation.

Table 3

Iron Evolved from a Structural Component as a Function of Temperature

[0070] The results listed in Table 3 indicate that the amount of iron evolved from a structural component increases with increasing temperature. At present, the maximum temperature that can be reached by this method is 1100 °C; during a typical closed hot zone growth process structural components can reach about 1250 °C for about 80 hours. Results to date, however, indicate that most of the iron present in the sample coupons come out in the form of vapor at 1100 °C. Thus, testing a sample at 1100 °C in accordance with the foregoing procedures provides an accurate measurement of the total concentration of iron impurity within the sample.

[0071] Using the foregoing procedures, the concentration of iron in the graphite of four suppliers was determined without a silicon carbide coating and with two different coatings. The results of the test, depicted in Figure 10, clearly indicate that there is significant variability in the concentration of iron in the graphite from the suppliers that were tested. Also, the results indicate that in some cases adding a coating may substantially increase the amount of iron evolved (see, graphite B, coating X and graphite D, coating X). On the other hand, the coating may decrease the amount of iron evolved (see, graphite A, coating Y; graphite C, coating Y; and graphite D, coating Y). The results clearly indicate that the silicon carbide coating designated X has a higher iron concentration than the Y coating. Thus, in contrast to Gilmore et al. at p. 626, to effectively control the amount of iron contamination in single crystal silicon initially grown (i.e., grown using a new or newly-coated component) in a growth chamber having a closed hot zone the concentration of iron in the graphite and the silicon carbide coating are preferably controlled.

Example 2 High Temperature Purification Process

[0072] Silicon-carbide/silicon-coated components or parts of a crystal pulling apparatus hot zone are placed in a water-cooled, stainless steel purification furnace equipped with an induction heating system and a vacuum pump (the furnace having a high degree of vacuum integrity (helium leak check of 10^"6 cc/minute) and having furnace components constructed of graphite with graphite felt thermal insulation). The furnace is sealed and, using the vacuum pump, the desired atmospheric pressure is established. The furnace is then heated to a temperature of about 200 °C, requiring about 6 to 8 hours, and then chlorine gas, or a chlorine/fluorine gas mixture, is introduced into the treatment chamber, an inert carrier gas optionally being used. The volatile metal complexes formed are removed from the treatment chamber using an effluent scrubbing system and treated in a neutralization station.

[0073] After about 4 to 12 hours the flow of chlorine gas is stopped and the furnace is cooled to room temperature, over a time period of a few hours up to about 24 hours, a flow of inert gas optionally being passed through the treatment chamber during the cool-down process. The purified parts are then removed from the furnace and sealed in containers, which ensure they remain essentially contaminant free, until they optionally undergo application of a new (or additional) protective coating layer.

[0074] In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. It is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMSWhat is claimed is:

1. A process for reconditioning a structural component of a crystal pulling apparatus for reuse therein, the structural component comprising a substrate and a first protective layer covering at least a portion of the substrate, wherein the substrate comprises graphite, and the first protective layer comprises silicon carbide or glassy carbon, the process comprising: placing the structural component in a treatment chamber; and, exposing the structural component to an iron-complexing gas at a temperature and for a duration sufficient to reduce an iron concentration in the first protective layer, the iron-complexing gas comprising a halogen.

2. The process of claim 1 comprising removing the structural component from a crystal growth apparatus prior to placing the structural component in the treatment chamber and placing the structural component in the crystal growth apparatus subsequent to the exposure to the iron-complexing gas.

3. The process of claim 1 wherein the exposure to the iron-complexing gas reduces an iron concentration in the substrate.

4. The process of claim 1 wherein the first protective layer comprises silicon carbide.

5. The process of claim 1 wherein the first protective layer covers the entire substrate.

6. The process of claim 1 wherein the first protective layer has a thickness that is at least about 75 micrometers.

7. The process of claim 1 wherein the structural component is selected from the group consisting of a susceptor, a heater, a thermal shield, and a heat reflector.

8. The process of claim 1 wherein the halogen is chlorine, fluorine, or a combination thereof.

9. The process of claim 1 wherein the iron-complexing gas comprises an inert carrier gas selected from nitrogen, argon, helium, or a combination thereof.

10. The process of claim 1 wherein the structural component is exposed to the iron-complexing gas under a vacuum pressure that is between about 0.1 and about 760 torr

11. The process of claim 1 wherein the structural component is exposed to the iron-complexing gas under a vacuum pressure that is between about 10 and about 100 torr.

12. The process of claim 1 wherein the structural component is exposed to the iron-complexing gas under a vacuum pressure that is between about 25 and about 50 torr.

13. The process of claim 1 wherein the iron-complexing gas is a plasma.

14. The process of claim 1 wherein the temperature is between about 100 °C and about 1000 °C.

15. The process of claim 1 wherein the temperature is between about 300 °C and about 800 °C.

16. The process of claim 1 wherein the duration is between about 20 minutes and 20 hours.

17. The process of claim 1 wherein the temperature is between about 200 °C and about 600 °C and the duration is between about 2 and about 15 hours.

18. The process of claim 1 wherein the temperature is between about 300 °C and about 500 °C and the duration is between about 4 and about 6 hours.

19. The process of claim 1 wherein the iron concentration in the first protective layer is reduced to less than about 10 ppbw.

20. The process of claim 1 wherein the iron concentration in the first protective layer is reduced to less than about 5 ppbw.

21. The process of claim 1 wherein the structural component, prior to being placed in the treatment chamber, comprises a second protective layer covering at least a portion of the first protective layer, the second protective layer comprising silicon, and the exposure to the iron-complexing gas reduces an iron concentration in the second protective layer.

22. The process of claim 21 wherein the temperature is between about 600 °C and about 900 °C, and the duration is between about 2 and about 15 hours.

23. The process of claim 21 wherein the temperature is between about 700 °C and about 800 °C, and the duration is between about 4 and about 6 hours.

24. The process of claim 21 wherein the second protective layer has a thickness that is less than about 200 micrometers.

25. The process of claim 21 wherein the second protective layer has a thickness that is between about 125 and about 150 micrometers.

26. The process of claim 21 wherein the second protective layer has a thickness that is less than about 100 micrometers.

27. The process of claim 21 wherein the second protective layer has a thickness that is between about 50 and about 75 micrometers.

28. The process of claim 21 wherein the iron-complexing gas removes at least a portion of the second protective layer.

29. The process of claim 28 comprising forming a third protective layer on at least a portion of the structural component after exposure to the iron- complexing gas, the third protective layer comprising silicon.

30. The process of claim 29 wherein the third protective layer has a thickness such that a sum of the thicknesses of the third protective layer and the second protective layer, if present, is less than about 200 micrometers.

31. The process of claim 24 wherein the entire second protective layer is removed and a third protective layer is formed on the entire structural component after exposure to the iron-complexing gas, the third protective layer comprising silicon and having a thickness that is between about 125 and about 150 micrometers or between about 40 and about 80 micrometers.

32. A process for reconditioning a structural component of a crystal pulling apparatus for reuse therein, the structural component comprising a substrate and a first protective layer covering the substrate, wherein the substrate comprises graphite, and the first protective layer comprises silicon carbide, the process comprising: placing the structural component in a treatment chamber; and exposing the structural component to an iron-complexing gas comprising a halogen under a vacuum pressure that is between about 15 and about 75 torr at a temperature that is between about 200 °C and about 600 °C for a duration that is between about 2 and 15 hours to reduce an iron concentration in the first protective layer to less than about 10 ppbw.

33. The process of claim 32 wherein the halogen is chlorine, the vacuum pressure is between about 25 and about 50 torr, the temperature is between about 300 °C and about 500 °C, the duration is between about 4 and about 6 hours, and the iron concentration is reduced in the first protective layer to less than about 5 ppbw.

34. A process for reconditioning a structural component of a crystal pulling apparatus for reuse therein, the structural component comprising a substrate, a first protective layer covering the substrate, and a second protective layer covering the first protective layer, wherein the substrate comprises graphite, the first protective layer comprises silicon carbide, and the second protective layer comprises silicon, the process comprising: placing the structural component in a treatment chamber; and exposing the structural component to an iron-complexing gas comprising a halogen at a temperature and for a duration at least sufficient to remove the second protective layer.

35. The process of claim 34 wherein the temperature is between about 600 °C and about 900 °C and the duration is between about 2 and 15 hours.

36. The process of claim 34 wherein the temperature is between about 700 °C and about 800 °C and the duration is between about 4 and 6 hours.

37. The process of claim 34 wherein the structural component is exposed to the iron-complexing gas under a vacuum pressure that is between about 25 and about 50 torr.

38. The process of claim 34 comprising forming a third protective layer covering the first protective layer, the third protective layer comprising silicon.

39. The process of claim 38 wherein the third protective layer has a thickness that is between about 125 and about 150 micrometers or between about 40 and about 80 micrometers.