WO2012094169A2 - Crystal ribbon fabrication with multi-component strings - Google Patents

Abstract

A string for growing ribbon crystal has a core and an outer cover, the cover composed of at least two different materials, chosen with the material of the core in amount and kind such that the CTE of the covered core matches in net, that of the silicon ribbon. The cover material is also chosen so that silicon readily wets significantly around the string, subtending an angle of at least about 55 degrees, up to a fully wetted string, resulting in a relatively thick strong ribbon adjacent the string, closer in thickness to the diameter of the sting. This prevents a thin, fragile ribbon near the string. For silicon ribbon, a cover may be an interspersed composition that is predominantly of silica, with some SiC. The core may also be composed of silica and SiC, in different proportions, and different geometry. Or, the core may be a single material, such as Carbon. SiC present in the cover in an amount as low as 10% by volume permits wetting around at least about 55 degrees of string circumference and does not excessively nucleate grain growth. Higher amounts of SiC are also beneficial. Using these same, or similar materials, the outer cover can be made fully dense and free of impurities that would harm silicon. The cover can be electrically non-conductive. Rather than silicon carbide, silicon nitride, and other materials can be used. It is also possible to intentionally mismatch the CTE of the string and the ribbon, such that the ribbon is in compression at the ends of the strings, which helps to prevent ribbon fracture.

Description

CRYSTAL RIBBON FABRICATION WITH MULTI-COMPONENT

STRINGS

INTRODUCTION

[0001] Techniques are described in U.S. Patent No.

4,689,109, entitled, STRING STABILIZED RIBBON GROWTH A METHOD FOR SEEDING SAME, issued August 25, 1987 in the name of

Emanuel M. Sachs, in which, as shown in Fig. 1 hereof, two partially wetted strings 102 stabilize the meniscus of a growing semiconductor ribbon 104. (See, in particular, Fig. 4 of the Sachs '109 patent.) The entire disclosure of the Sachs '109 patent is hereby fully incorporated herein by reference. These techniques are referred to herein in general as string ribbon fabrication techniques. This disclosure concerns using string ribbon techniques, especially to create substrates for photovoltaics . In particular, this application concerns fabricating string for such techniques. Further, this application concerns coatings on such strings.

[0002] It is known that certain advantages attend using silica as a material for these strings, because silicon ribbon can be grown with little or no nucleation of grains at the interface A06 between silicon and a silica string, due to the non-nucleating behavior of this interface. See

generally, Ciszek et al., Filament Material For Edge

Supported Pulling of Silicon Sheet Crystals, Solar Energy Research Institute, Golden Colorado (1982 and 1983) (Silica may also be referred to herein by its other common names of Si0₂, silicon dioxide, and quartz, each of which may be used interchangeably) . The full disclosure of Ciszek is

incorporated herein by reference. Ciszek notes that silica fibers break out of the ribbon because silica has a much lower coefficient of thermal expansion (CTE) than does silicon of the growing ribbon material. Hence, stresses develop during cooling. In general, the CTE of silicon is approximately (3.5-4) x 10^"6 1/°C, while that of silica is approximately .7 x 10^"6 1/°C.

[0003] A combination of two or more materials can be used to make a string which, in net, (as that term is discussed below) has a CTE that can be matched to that of silicon. At least one material must have a CTE higher than that of silicon and another must have a CTE lower than that of silicon, so that the proper ratio of CTEs may be matched to silicon. An example pair of materials is silicon carbide (SiC) and silica (Si0₂₎, where the CTE of silicon carbide, which is approximately (4-4.5) x 10^"6 1/°C, is higher, and that of silica is lower, than the CTE of silicon, set forth above.

[0004] This disclosure, discusses the CTE of strings that have regions with different compositions. For example, a string core may have a composition different from that of a cover on the core. In addition, the core may itself have regions of differing composition. The different compositions corresponding to different regions will, in general, have different CTEs. It is understood in the art that the net CTE of a body, such as a string, composed of two or more regions depends on at least: the CTEs of the compositions in the regions; the relative cross-sectional areas of the regions; and the elastic modulii of the compositions in the regions. Further, the behavior can be more complex when the string heats or cools through a temperature range where one or more of the material compositions undergoes some plastic flow.

Thus, while an estimate can be made of what the net CTE of an entire, covered string will be, from knowledge of the

compositions of the regions, the true arbiter of the CTE of a body is by experiment. Suitable experiments for evaluating the relative net CTEs of different bodies are discussed below.

[0005] Other researchers have sought to provide a multi part string, having a base portion, composed of an innermost core and a refractory layer that is supported by the core, and an externally exposed layer that is radially outward of the refractory layer. The externally exposed layer should be chosen to have a relatively low propensity to wet the ribbon material, and in particular, they warn against allowing the ribbon material to wet entirely around the string perimeter. They also propose to match the CTE of the combination base portion, to that of the silicon ribbon. These researchers propose a very thin externally exposed relatively non-wetting layer, so thin that its impact on the CTE of the entire string be minimal. These researchers also mention using an innermost core of either carbon or tungsten, and a refractory layer of silicon carbide. One material mentioned for the externally exposed layer is silica.

[0006] Regarding this just mentioned proposal, it is noted herein that if a silicon carbide un-covered string is covered with pure silica, the CTE abruptly changes from approximately 4 x 10^"6 1/°C to 0.7 x 10^"6 1/°C. Such a drastic change from one region to another results in high shear stresses at the interface between the two regions and these stresses can result in cracks and delaminations .

[0007] However, the present inventors have discovered that such a CTE matched string, with a silica externally exposed layer is used to grow string ribbon, the resulting crystal ribbon, while not typically fracturing of its own accord, as in Ciszek, is thinner adjacent the string, than it is over the rest of its span, for instance, further from the string at a location L. (Fig. 1 is not to scale. In general, the thin region extends much further along the width of the ribbon than is shown (as compared to the diameter of the string) . ) The inventors hereof have discovered that such a thin region is still too fragile to be useful. It is believed that the fragility stems from the small cross- sectional area of the thin neck region 112 between the main body of the silicon ribbon 104 and string 102.

[0008] Other problems arise using strings for ribbon production. Some string materials are deleterious to a silicon crystal, and thus it would be desirable to bar or minimize their diffusion into a growing crystal. Some string materials are porous, which porosity has several problems. One is that impurities from the porous region can diffuse to the silicon ribbon at a high rate, due to the high surface area associated with a porous core. Similarly, some

particulate materials that have other beneficial properties, such as SiC, cannot be made fully dense, which also then permits diffusion of impurities from and through the porous material. Another problem is that surface porosity of a porous string can act as crack initiation sites.

[0009] Thus, in some cases, the most desirable product is one with no porosity whatsoever. Such a product may have the maximum strength possible for a given material minimizing crack initiation. However, it is extremely difficult to densify a powder body to zero porosity. Often a few percent porosity provides satisfactory mechanical performance, because at this low level of porosity, pores tend to be isolated from each other (not connected) and therefore do not constitute major sites of crack initiation. Further, such a body will not allow liquids to pass through it, because the porosity is not interconnected. It is generally accepted that bodies of up to approximately 7% porosity can still be free of interconnected porosity. This absence of

interconnected porosity is therefore a common definition of acceptably high density. This is also a metric of importance for the coverings discussed herein, because a covering of density high enough to avoid interconnected porosity can constitute an effective barrier against migration of

impurities from within a core into the molten bath through which the string passes during String Ribbon crystal growth. Thus, when a covering is referred to herein as fully dense it is understood that this is a covering of sufficient density to be free of interconnected porosity, which in a typical case, would be approximately 7% porosity or less. However, for different types of particles, even a greater porosity may be free of interconnected porosity, and thus considered to be fully dense, as used herein.

[0010] Another problem with some strings relates to their electrical conductivity. If a solar cell is fabricated with a string whose outer surface is electrically conductive, then the junction of the solar cell must be isolated from this edge either by cutting out the edge, or performing a process such as laser junction isolation as is known in the art.

Either method results in the loss of energy generating area.

[0011] Thus, there is need for a string and ribbon system that results in pure, contaminant-free crystal ribbons having sufficient strength to withstand the fabrication process and subsequent use. There is further need for such a string and ribbon system that has sufficient strength and thickness adjacent the strings. There is further need for such a system in which neither the strings nor the crystal degrade or fracture or splinter during heating or cooling. There is also a need for such a system with strings that have a fully dense outside surface, which also acts as a diffusion barrier.

There is further a need for such ribbons having relatively large grain size. There is also a need for such a string that is electrically non-conductive.

[0012] Thus, the several objects of inventions disclosed herein include to provide a substantially pure silicon ribbon, grown upon strings, which has relatively large grains, and which is stable and robust throughout normal, typical manufacturing processes, resisting rupture, cracking and other degradation. It is further a goal to provide such crystal ribbons without undue expense or complexity. It is a further goal to provide such a string and ribbon system that can withstand typical handling and use. A further object of inventions hereof, is to provide a string that can be used with a crystal ribbon, which has a CTE matched to that of the crystal ribbon, which does not provide impurities to the crystal ribbon, and which has a fully dense outer layer to bar such impurity transport. Yet another object is to provide such a string that is electrically non-conductive.

SUMMARY

[0013] It has been discovered by the inventors hereof, that all of the foregoing objects can be satisfied by providing a string composed of a core and an outer cover layer, which outer cover is composed of at least two different materials. By judiciously choosing the identities and amounts of

materials in the core and the outer cover layer, compromises can be made that achieve acceptable impurity diffusion barring, net CTE matches, handling durability of both the string and the crystal ribbon, electrical insulation,

controlled wetting and a relatively large crystal ribbon grain size (due to minimal nucleation) . It is also possible that the core be composed of more than one material .

[0014] It has been discovered that a source of ribbon fragility with strings having an externally exposed surface of silica, for instance, is that the silicon wets only part way around the perimeter of the string, thereby leading to the ribbon being extremely thin and fragile near the string. As shown schematically in Fig. 1, the ribbon 104 is thinner at the neck 1112 than it is at a location L spaced away from the interface, because the ribbon material necks down to assume its required wetting angle with the outer layer of the string .

[0015] The ribbon material only wets around the string as far as permitted by the relatively large wetting angle a at the interface 106. The wetting angle is a consequence of the surface energies of the material being wetted and the wetting material. The undesirable weakness of the thin region is further exacerbated by etching the ribbon as a first step in cell fabrication, because such etching further thins this already thin region. (Such etching might be employed for cleaning the surface or for creating a light trapping texture on the surface to enhance the performance of a solar cell.)

[0016] Thus, it has been discovered by the inventors hereof that providing a material system in which the silicon more readily wets the string material, thereby allowing the silicon to wet more fully around the string, resulting in a thicker ribbon portion adjacent the string, closer in

thickness to the diameter of the sting, reduces the problem of a thin, fragile ribbon near the string.

[0017] It has also been discovered by the inventors hereof, that, not only can the fragility be reduced by providing a more fully wetted string, but also, CTE can be matched in net between a string and a ribbon, for instance a silicon ribbon, if the string is composed of a core, having at least one material, and an outer cover layer having at least two materials. The materials of each the core and the cover are chosen by identity and amounts, so that in net the CTEs of the string and ribbon match. When reference is made herein to a match between the CTE of any two bodies, for instance of the string and the CTE of the ribbon it will be understood that the CTE match applies within a temperature regime between the melting point of silicon and room temperature.

[0018] Other objects can also be achieved by adjusting the identities and amounts of the materials in the core and outer cover. The outer cover can be made to be fully dense and free of impurities that would harm the silicon. Thus, any such impurities that may be in the core (due to the CTE

requirements of the core, for instance, or other requirements of the core or methods of its manufacture) can be sequestered from diffusing to the ribbon. Similarly, using such a

composite structure, the nucleation potential of the outer cover, which the growing crystal ribbon contacts, can be kept to an acceptably low degree. Simultaneously, the outer cover may be made to be sufficiently highly wettable, so that the crystal ribbon material wets sufficiently around the circumference of the string, so that the ribbon does not thin down adjacent the string, from a larger thickness further along the ribbon, more distant from the string. It is also possible to achieve the above objectives and also have a sufficiently electrically non-conductive outer cover, for instance with an outer cover predominantly of silica, with some SiC.

[0019] In general, some properties of a string are

predominantly, or wholly governed by the outer covering. Such properties include the string's wettability, nucleation propensity, impermeability, and surface roughness. Other properties are governed more predominantly by the core, such as strength. Other properties are affected by both the cover and core. Such properties include the net CTE, and strength and flexibility. By using a string that has a core and a cover, composed differently, a designer has more flexibility to achieve all of these goals, than would the designer have with just a single monolithic structure. Similarly, by providing a cover composed of more than one material,

additional flexibility is provided.

[0020] Systems for use with growing crystal ribbons of materials other than silicon may also be designed with the same principles. In a preferred embodiment, the entire string and the growing ribbon can have substantially equal net CTEs. The outer surface of the string should inhibit nucleation of grains in the growing crystal. It should also facilitate wetting of the crystal significantly around the perimeter of the string, to a degree sufficient to provide adequate thickness of the ribbon, and thus strength at the interface between the ribbon and the string. Other embodiments are disclosed in which the CTE of the string and the ribbon are intentionally mismatched, as discussed below.

[0021] Inventions hereof include, but are not limited to: strings, so described, ribbons grown upon such strings, string and ribbon assemblies, methods of making ribbons using such strings, methods of making such strings, and products incorporating such grown ribbons.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 0022 ] Fig. 1 shows a schematic plan view in cross-section of a string of the prior art with a portion of ribbon grown thereon;

[ 0023 ] Fig. 2 shows a schematic cross-sectional view of a core of a string of an invention hereof having an inner layer and an outer layer of two different materials;

[ 0024 ] Fig. 3 shows a schematic cross-sectional view of a string of an invention hereof having a composition of two different materials interspersed with each other;

[ 0025 ] Fig. 4 shows, a schematic cross-sectional view of a core of a string of an invention hereof, having a single, central region composed of a single material;

[ 0026 ] Fig. 5 shows a schematic cross-sectional view of a core of a string of an invention hereof, having an outer cover having a composition of two different materials interspersed with each other;

[ 0027 ] Fig. 6 shows a schematic cross-sectional view of a string of an invention hereof, with a ribbon grown thereon, with a small degree of wetting of the ribbon material upon the string;

[ 0028 ] Fig. 7 shows a schematic cross-sectional view of a string of an invention hereof, with a ribbon grown thereon, with a moderate degree of wetting of the ribbon material upon the string;

[ 0029 ] Fig. 8 shows a schematic cross-sectional view of a string of an invention hereof, with a ribbon grown thereon, with a large degree of wetting of the ribbon material around approximately one half the perimeter of the string; [0030] Fig. 9 shows a schematic cross-sectional view of a string of an invention hereof, with a ribbon grown thereon, with a large degree of wetting of the ribbon material around substantially all of the perimeter of the string;

[0031] Fig. 10 shows, schematically, a slurry supply, which is agitated, for application to a string core, thereby forming a cover;

[0032] Fig. 11 shows, schematically, a wafer cut from a length of string ribbon crystal highlighting a region of the wafer adjacent the string;

[0033] Fig. 12 shows, schematically, the region of a string crystal, such as shown in Fig. 11, in which the string has been cut away from the ribbon, which string has a CTE that is higher than that of Si; and

[0034] Fig. 13 shows, schematically, the region of a string crystal, such as shown in Fig. 11, in which the string has been cut away from the ribbon, which string has a CTE that is lower than that of Si.

DETAILED DESCRIPTION

[0035] In the following, unless otherwise stated, the discussion centers on the specific case of growing string ribbon of silicon. The materials of the cover can be silica and silicon carbide. Material systems other than with silicon are also possible. The term core means the portion of the string that is subsequently covered with the cover (also called a coating) layer or layers. The core itself can consist of a single homogeneous material or a mixture of more than one material, or, it can be constructed of layers.

Thus, a core can have, within it, yet another core, which inner core, is covered. There may be any number of layers, and further inner cores, of such a core, as well as covering layers . [0036] For the purposes of this specification, a core is meant to be a string-like body made by any method, but which has one or more undesirable aspects that can be ameliorated with a cover. The cover is not necessary to the fabrication of a string ribbon per se, using such a core. For example, in the method, known in the art, discussed above, a small diameter inner-most core can be coated with refractory material by chemical vapor deposition as is currently

practiced by Specialty Materials, Inc. of Lowell, MA. For example, the inner most core can be carbon and the refractory material deposited by CVD can be silicon carbide. This two component, carbon and SiC string could be used to grow string ribbon silicon, but it has the undesirable properties of nucleating many grains and also of being electrically

conductive .

[0037] The inventors hereof have discovered that it would benefit from a cover provided in a subsequent step as taught in this specification. Thus, the two component carbon and SiC body can be considered itself to be a core, which is then covered by something else to deal with the nucleation and conductivity .

[0038] Other methods of fabricating string use ceramic powders as a precursor. For example, as is known in the art, a paste of ceramic particles and polymer binder can be extruded into a green fiber form. The binder is then burned out of the green body and the fiber is then sintered. This method results in fibers with some level of porosity, which is unacceptable. (It is also very slow and generally

unacceptably expensive.) A very economical method of

extrusion and firing is taught by Sachs, Serdy and Naiman in U.S. Patent No. 7,824,602, Ceramic Processing and Shaped Ceramic Bodies, the full disclosure of which is fully

incorporated herein by reference. This method results in very uniform, continuous fibers made from powder precursors.

However, the resulting body is generally porous. As powder precursors are not always available in acceptable levels of purity, powder based methods, especially when there is porosity present, may create unacceptable levels of chemical contamination. A covering can solve this problem. Thus, any of these bodies can be considered and used as a core, to which a cover is applied, as taught herein.

[ 0039 ] An invention hereof has a string with a core and a cover, also referred to herein in some places, as an outer cover or a coating. The composition of a cover may be

controlled to attain a balance of maximum desirable

properties and functions, including but not limited to:

forming a fully dense coating layer; acting as a diffusion barrier; matching in CTE, or deliberately mismatching;

achieving desirable and controllable wetting and nucleation properties; achieving mechanical durability and handle- ability of the string, as well as electrical non- conductivity. The CTE of string and silicon ribbon need not match each other precisely over the whole temperature range. Rather, when the CTE matched string is used to grow string ribbon, there will be no stress in the ribbon due to the string. An iterative, practical way to test for this

condition is discussed below.

[ 0040 ] Before discussing the cover characteristics, a brief discussion of core configuration would be helpful.

[ 0041 ] A core 220, as shown schematically with reference to Fig. 2, may have a central region 222 of one material, for instance carbon or tungsten, and an outer region 224 of another material, for instance SiC. A body 220 as shown, composed of carbon and SiC, can be designed to have a CTE that is in net, close to that of silicon. SiC is not suitable as an outer surface for making string ribbon, because it nucleates very readily, and it is electrically conductive.

[ 0042 ] Another form of core, as shown in Fig. 3 at 320, can have a body 322 that can be formed from a mixture of two different particles, for instance SiC, 323 and silica, 325. This mixture can be heat-treated to increase its density, and can be provided with a CTE that is sufficiently equal to that of silicon. But such a mixture with a matched CTE would need to be about 55% by volume SiC, and thus, will not be fully dense. Thus, it would not be suitable alone, as a string, for at least the reason that impurities could diffuse from it to the silicon ribbon.

[ 0043 ] Alternatively, as shown, schematically in Fig. 4, a basic core configuration can be a single material, in a homogeneous region 420. For instance, such a core can be of carbon, or other suitable material.

[ 0044 ] Fig. 5 shows a structure of a string of an invention hereof that can have all of the desired properties: net matched CTE, diffusion barrier, a nucleation inhibiting surface, electrical insulation and wettability. A core 520, which may be of either a dual layer construction, such as 220 shown in Fig. 2, an interspersed construction, such as 320 shown in Fig. 3, or a single material such as shown at 420 in Fig. 4, is surrounded by an outer cover 524.

[ 0045 ] The outer cover is a dual component body, which can be a composition formed from a particle mixture, with

particles of SiC 523 and silica 525. The more SiC that is present in the mixture, the greater will be the degree that silicon will wet around the string. The net CTE of the core 520 and the cover 524 can be adjusted to be substantially equal to that of silicon, because the relative amounts necessary for other considerations permit that. The silica provides a very low nucleation propensity. The amount of SiC that is needed to provide an adequate amount of wettability does not destroy this low nucleation propensity. Further, such a combination can be processed to full density, and it is also non-conductive.

[ 0046 ] For example, as shown schematically with reference to Fig. 10, the outer layer 524 may be formed by traversing a core 1020, as described above, through a slurry 1021 composed of a mixture of silica and silicon carbide powders and then firing this composite string 1003. In Figure 10, the slurry is contained in a vessel, which has a hole in its base. The slurry is retained by capillary attachment to orifice 1042 where it forms a liquid cap in the shape of a portion of a spherical surface. The height H of the slurry above the retention plane must be kept low, so as to guarantee that the slurry does not leak out. This method allows the passage of the core through the slurry without requiring that the core bend, and so eliminates a possible source of breakage during the covering process. It is useful to keep the slurry circulating using methods known in the art, to prevent settling of the particles in the slurry. It has been found that only a small amount of silicon carbide powder need be included with the silica to result in silicon wetting fully around the perimeter of the string, with the resulting larger cross sectional thickness of the silicon crystal, and a stronger silicon ribbon adjacent the string. Various example formulations are discussed below.

[0047] Due to the relatively good wettability of the outer cover, some very small amount of silicon may wet fully or partly around the circumference of the string. This amount is extremely thin and can be removed during any silicon etching that might be done to condition the surface, including to provide a light trapping texture on the surface.

[0048] While the presence of a second material in an outer covering, such as SiC, predominantly to enhance wettability, may introduce some amount of grain nucleation over that which would arise with silica, alone, the small quantity needed for wettability means that grain nucleation is less than it would be in the case of a string with an outer layer of only this wetting enhancing material. (As little as 5-10% by weight has been shown to be adequate . )

[0049] The propensity of a surface to nucleate grains may be assessed in several ways. The most direct way is to fabricate string with the material in question as a covering and to count the number of grains that are nucleated per unit length of ribbon edge. However, it may be useful to have a method to screen candidate materials without having to fabricate string. One suitable method is to measure the amount of undercooling possible with a candidate material. A small disk of the candidate material may be placed in a

Differential Scanning Calorimeter with a small piece of silicon on top of the disk. The silicon is melted and then the system allowed to cool while monitoring the temperature and comparing it to the temperature of a nearby dummy setup without silicon. From the difference in the temperature profiles, the undercooling can be inferred. The undercooling is the amount by which the temperature of of an interface may drop below the melting point of the silicon prior to the inititiation of solidification of the silicon. The inventors have found that the allowable undercooling at an interface of silica and silicon is over 100°C.

[0050] The foregoing has discussed, generally, a two part string, with a core and a cover, with the cover being

composed of at least two ingredients, typically being

fabricated from particulate mixtures. The core may be formed from one, or more materials. The identities of the

ingredients, and their relative amounts, can be adjusted to achieve a good compromise in matching the net CTE of the entire string to that of the growing ribbon crystal, as well as achieving sufficient wettability to provide a relatively thick crystal ribbon adjacent the string, and also

sufficiently large grain sizes. Other considerations are also important.

CREATING A COVERING

[0051] Mixtures of two or more materials, suitable for a cover as described above, may be created by mixing powders of the materials and applying the powder to the surface of a core. One effective method of application as shown with reference to Fig. 10, is to create a slurry 1021 of the powders and to pass the core 1020 through the slurry 1021 so that a uniform, thin coating 1024 is applied as discussed above. For example, a water-based slurry of fine powders can be created using dispersion methods known in the art,

preferably using particles in the size range of approximately 0.5 to approximately 5 microns. The slurry is kept in a pot 1040, which is agitated, as indicated by the circulation arrows. The coated slurry is then dried and fired at high temperature .

[0052] It is often advantageous that the cover layer be fully dense before it enters the silicon melt for string ribbon growth, or at least that it have a very small amount of open porosity. Such high-density coatings can be made when using powder mixtures that have a high silica content. Silica contents of greater than about 50% by volume will work. At the temperatures at which the coating is fired, the silica becomes very soft and this allows the mixture of silica and silicon carbide to densify by viscous phase sintering. Viscous phase sintering is dramatically faster than solid state sintering and thus, a fully dense cover layer can be created quickly, at relatively low temperature? and at low cost. In the case of some materials systems, this firing may be conducted in air, thereby reducing the cost of the process as compared to one having the need to fire in an inert atmosphere. For example the case of silica and SiC is well suited to firing in air. In fact, in this case, firing in air is advantageous because the surface of the SiC

particles oxidizes to silica and the result is then a strong interface between SiC powder and a silica matrix because silica bonds well to itself. The outer cover layer can thus be sintered to full density, forming a layer substantially impervious to the penetration of molten silicon. This layer also acts as a barrier to diffusion of impurities from within the string, outward into the silicon.

[0053] The methods of creating a cover layer with powders can be applied to a core that is fully dense, or to a core that has open porosity. It can be applied to cores that are made by a wide range of methods, including, but not limited to: chemical vapor deposition, extrusion, pulling, pultrusion and others including the methods disclosed in U.S. Patent No. 7,824,602, Ceramic Processing and Shaped Ceramic Bodies, in the names of Sachs, Naiman and Serdy, the full disclosure of which is fully incorporated herein by reference.

[0054] Creating the cover with mixtures of powders also provides a path toward matching the CTE of the string to the silicon. For example, if the core is itself CTE matched to silicon, then the covering can also be matched by varying the ratio of silica to silicon carbide appropriately. More typically, if the core has a CTE higher than that of silicon, then the covering can make the overall string matched in net CTE by providing a CTE lower than that of silicon, by using more silica and less silicon carbide in the coating, assuming that the other desired properties can be achieved with this other mixture.

[0055] Creating the cover of mixtures of two or more materials, including as mixtures of powders, also allows for the change in CTE between core and cover to be less abrupt than it otherwise might be — as discussed above in the case of a pure silica cover on a SiC core. In the limiting case, the core and cover can have the same CTE, as mentioned above. Minimizing the CTE mismatch between core and cover will help prevent the cover from detaching from the core and will reduce the rate of dissolution of the cover as it traverses through the molten silicon, because the state of stress will be lower. Similarly, the ability to handle the string before passing through the silicon melt may be enhanced.

[0056] In designing the mixture of silica and silicon carbide to be used, considerations of CTE match may dictate that the volumetric ratio of silicon carbide exceed 50%.

This will make viscous phase sintering much more difficult. Thus, to avoid such a case, the CTE of the core should be increased, so that the CTE of the cover may be decreased by using a lesser fraction of silicon carbide in the cover, thereby allowing for viscous phase sintering of the cover. [ 0057 ] Above, it has been discussed that it is important to balance the considerations of matching the CTEs of the core and its cover with each other, and in the aggregate, with that of the growing ribbon, for instance, silicon, with other considerations. The other considerations include to minimize nucleation of grains to an acceptable degree, and also to achieve a string that is wetted to a degree sufficient to avoid unacceptable fragility of the ribbon at its edges, and which is electrically insulating.

[ 0058 ] It is also possible to use powder, for instance, SiC, with particles that are small enough that it does not nucleate grains, or at least has a low probability of

nucleating grains. The continued growth of a grain requires that a critical radius of solid form. If the SiC particles are smaller than this critical size, the likelihood of grain nucleation is very small, or the nucleation of grains may even become impossible, resulting in the low nucleation behavior of silica (but, without using silica, or using only a relatively small amount) with increased wetting and

consequent elimination of ribbon breakage.

[ 0059 ] As noted above, the previous discussion centered on achieving a net CTE match between string and ribbon.

However, the same techniques can be used to attain a

deliberate and intended CTE mismatch. In particular, it may be advantageous for the string to have a CTE that is

deliberately lower than the CTE of the silicon ribbon. As above, this refers to the net CTE mismatch within a

temperature regime between the melting point of silicon and room temperature. Such a match or mismatch can be evidenced by a method of cutting a sliver of ribbon from the edge.

[ 0060 ] An iterative, practical way to evaluate the CTE match or mismatch condition of a string, as compared to its associated ribbon, is to use a laser to cut a thin sliver of ribbon from the edge. For example, methods known in the art can be used to cut 1 to 2 mm inward from the edge of a piece of grown string ribbon, typically for a length of at least 200 mm and preferably more. If there is no stress caused by the string, this piece will remain straight after separating from the larger portion of the string ribbon. If it curves so that it is concave toward the body of the ribbon from which it was separated, that means that the CTE of the string is lower than that of the silicon ribbon. If it curves so that the sliver is convex toward the body of the ribbon from which it was separated, that means that the CTE of the string is higher than that of the silicon ribbon, which would likely be too high for successful, typical use.

[0061] A deliberately lower CTE of a string, as compared to the ribbon, will beneficially result in the silicon ribbon in the very corners of a cut wafer to be in compression and therefore more resistant to failure than in a condition of zero stress.

[0062] Fig. 11 shows, schematically, a length of silicon ribbon 1104 grown between two strings 1102, pointing out the regions BC of the ribbon under discussion. Brittle materials, such as silicon, are stronger in compression than in tension. It is perhaps easiest to understand the potentially

beneficial effect of a string with a lower CTE than that of silicon ribbon, by first considering the detrimental effect of a string with a CTE higher than that of silicon ribbon.

[0063] Fig. 12 shows, schematically, this situation. As discussed above, when a sliver 1234 is cut out of the edge BC of the ribbon 1204 in a case where the string 1202 has a CTE higher than that of silicon. The sliver 1234 will be convex with respect to the body of ribbon from which it was removed. In order to have kept this sliver straight, while the ribbon was attached to it, the silicon ribbon 1204 adjacent to the ends of the sliver 1234 must have been in tension before the sliver was cut out, as shown by the arrows T, which represent the forces applied to the ribbon 1204. Note, the ribbon is in tension, with forces pulling outward at both edges 1204. This is detrimental, since brittle materials are weak in tension . [0064] Fig. 13 shows, schematically, the situation where the CTE of the string 1302 is lower than that of silicon ribbon 1304. Such a situation creates the opposite effect, where the silicon ribbon at the very edges is in compression, as indicated by the inward pointing arrows C. As the very corner of a cut wafer is the most common origin of cracks in a wafer, this is potentially very beneficial to mechanical yields during processing.

[0065] Note, that while the covering methods of inventions disclosed herein can be used to help tune the CTE of the string to be deliberately lower than that of the silicon ribbon, this aspect of inventions hereof also applies to the case of a string made even of a single, homogeneous material with no covering. By which, it is meant that it is an aspect of an invention hereof to provide a string ribbon system where the CTE of the string and the ribbon are mismatched, so that that of the string is lower than that of the growing ribbon, thereby providing the beneficial result that the ribbon will be in compression. This can be done with a simple, single component string, if other properties

necessary to use as a string exist.

[0066] Another important aspect of the covering methods described herein is that the surface of the string can be made electrically non-conductive, even if one of the

component materials of the cover is, itself conductive. For example, SiC is somewhat electrically conductive. If a solar cell is fabricated with a string whose outer surface is SiC, then the junction of the solar cell must be isolated from this edge either by cutting out the edge, or performing a process such as laser junction isolation as is known in the art. Either method results in the loss of energy generating area. The same is true of any string with a conductive surface, whether it be made of SiC, carbon, alloys of SiC and carbon, or other conductive materials. However, when the covering is made of a mixture of SiC and silica with a significant silica content, the coating material will be non- conductive, because individual SiC particles, or small aggregates of SiC particles, will be surrounded by silica. Further as noted above, if the SiC and silica powder layer is fired in air, the individual SiC particles are themselves coated with silica by oxidation, further guaranteeing a non- conductive string, even at higher SiC content. A non- conductive string eliminates the problem of shorting of the p-n junction of the solar cell, thereby eliminating the need for edge removal or junction isolation. Silica and SiC are not the only non-conductive combinations that can be used. Any combination that is non-conductive will have this

advantageous property, and is thus, an aspect of an invention hereof .

[0067] Another advantage of the disclosed methods of covering a core, using powders is that the cover can

mechanically interlock with any roughness on the surface of the core. For example, if the core is made from powders, it can be made with controlled roughness based on the particle size of powders used and also based on the degree of

dispersion of a slurry or paste used in making the core. In such a case, the cover made from powders will adhere to the core better than it might to a smooth core, due both to more surface area of contact and to mechanical interlocking.

[0068] Representative examples of ribbons formed with strings having different cover formulations are shown

schematically, with reference to Figs. 6, 7, 8 and 9.

[0069] For instance, Fig. 6 shows a core 620 and a cover 624 composed of approximately 90% silica and approximately 10% SiC. The silicon can be seen to have wetted the

composite string to a somewhat minimal degree, over a region of circumference subtended by angle S_f, that is approximately 55°, but more than would be the case for a pure silica outer cover layer. Thus, the thickness t of the ribbon 604 near the string 602 is relatively thin, and potentially fragile, but not as thin as would be the case for a pure silica outer cover, such as shown at Fig. 1. [0070] Fig. 7 shows a similar core 720 and a cover 724 composed of approximately 80% silica and approximately 20% SiC. As can be seen, the silicon has wetted the composite string to a greater degree, over a region of circumference subtended by angle S_g, that is approximately 90 degrees, thus resulting in a ribbon 704 with a larger thickness t near to the string 702. Thus, other things being equal, this ribbon will be less fragile than the one shown at Fig. 6.

[0071] Fig8. H shows a similar core a cover 824 composed of approximately 70% silica and approximately 30% SiC. As can be seen, the silicon can has wetted the composite string 802 to a greater degree, almost half-way around the perimeter, over a region of circumference subtended by angle S_h, that is approximately 175 degrees, thus resulting in a ribbon 804 with a larger thickness t near to the string. Thus, other things being equal, this ribbon will be less fragile than the two shown at Figs . 6 and 7.

[0072] Finally, Fig. 9 shows a similar core and a cover 924 composed of silica and more than 30% SiC. As can be seen, the silicon has wetted the composite string 902 fully around its perimeter, resulting in a ribbon 904 having a thickness t that is as thick, or slightly thicker than the diameter of the string 902.

[0073] Thus, it has been found that a multiple component string can be provided that has a combined net CTE that substantially matches that of the growing silicon ribbon.

The composite string has a core, predominantly for strength and to provide a major portion of CTE matching. The string has a multi-component cover, one component having nucleation inhibiting properties, and another component having wetting enhancing properties. (Both components may also have other properties.) The outer cover may for instance be between approximately 70% and approximately 90% Silica, and

correspondingly between approximately 30% and approximately 10% SiC. In general, the wettability of the outer cover is such as provides wetting of the ribbon around a portion of the circumference that is subtended by an angle of at least about 50-60 degrees, and which may be as large as the entire circumference .

Example 1

[0074] A core may be made by extrusion of a slurry that is a mixture of silica and silicon carbide powders according to the methods of US Patent 7,824,602, Sachs, Naiman and Serdy. In this method, a relatively low viscosity slurry is extruded at relatively low pressures directly into a bath containing a boron containing agent that rapidly cross links a polymer contained in the slurry and then dries to form a green fiber. Typically the green fiber is 150-500 microns in diameter, with a preferred range of 200-350 microns. Typically, the mixture is approximately 55-65% by volume SiC and 45-35% by volume silica. This mixture is chosen so that the CTE of the of the mixture is approximately equal to that of silicon, but generally slightly higher than that of silicon for reasons explained below. If the core is fired at this stage, it will have a diameter of between about 100 and about 400 microns preferably between about 150 and about 250 microns.

[0075] This core is then covered with a mixture of silica and silicon carbide powders. The covering is accomplished according to the method shown in Figure 10. Typically, the slurry for the covering will be a mixture of 95-55% by volume silica and 5-45% by volume SiC. A preferred range is 85-70% by volume silica and 15-30% by volume SiC. Typically both silica and SiC particles are .1-5 microns in size, with 1-3 microns being a preferred size range. In a preferred

embodiment, the covering is added to the core while the core is still green. The covering is dried, for example by the passage of warm air, and the core and covering are then fired together, typically at temperatures of approximately 1500°C for 1-10 minutes in an air environment. The firing may be performed in air as the presence of oxygen only helps to oxidize the surface of the SiC powder and promote bonding to the silica. The dried covering is typically 4-30 microns thick with a range of 6-20 microns being preferred. After firing, the covering is typically 2-20 microns thick with a range of 3-15 microns being preferred.

[0076] Note that it is also possible to fire the green core and then cover it and fire a second time, although an extra step is involved in this case. It is also possible to use pre-oxidized SiC particles, rather using a mixture of SiC and silica particles. By controlling the amount of pre-oxidation of the SiC particles, the correct ratio of SiC and silica can be achieved.

[0077] Since, the mixture used in the core has more SiC than silica, it is very difficult to cause it to fully densify in a practical and economical firing cycle and therefore retains significant porosity in the final, fired product. The covering, however, being high in silica

content, does fully densify, providing a impervious cover. It would be impossible to provide a core which was

sufficiently high in silica content so as to fire to full density, as such a core would have too low a CTE to allow either the core or a core with cover to have a NET CTE that matches silicon. However, the NET CTE of a core with more SiC than silica and a cover with more silica than SiC can be matched to that of silicon. As both core and cover have silica, and as the CTE of core and cover are not drastically different, there results an excellent bond between core and cover. For cores made using the boron cross-linking agent, it is important that the cover be fully dense to bar

diffusion of the boron to the silicon ribbon.

Example 2

[0078] The core is made by CVD of stoichimetric or non- stoichiometric SiC. As noted, such cores are known in the art and are made by depositing onto a very small diameter fiber made of carbon, tungsten, or other refractory material. Typically, the starting fiber is approximately 35 microns in diameter and the SiC is deposited up to a diameter of approximately 140 microns.

[0079] This core may be covered in much the same manner as described above in the context of Example 1. Again, the bond between cover and core will be strong because there is not a drastic difference in CTE between core and cover and because firing in air will cause the outside of the core to oxidize and bond well to the silica.

[0080] It should be noted, that while a string made by CVD of SiC that is intended to be used without a cover could have its CTE matched to the CTE of silicon, typically requiring that the SiC be non-stoichiometric , for example, having extra carbon. However, for the purposes of a covered string, where the covering is predominantly silica, the core made by CVD can be stoichimetric SiC, or closer to it and this material will have a slightly higher CTE and the NET CTE of the covered string can match that of silicon. It may be

especially advantageous to have the outer layer of the CVD core be stoichiometric SiC to allow for oxidation during firing and good bonding to the silica component of the covering.

[0081] A primary materials system discussed is that of SiC and silca. The SiC can be of several grain structures, including Alpha and Beta. The silica is typically amorphous silicon dioxide, however it can also be composed of some fractions of crystalline silicon dioxide, such as the

crystobalite phase.

[0082] Other materials systems can include silicon oxide (SiO) for the relatively less wetting material as well as some forms of silicon oxy-carbide, silicon oxy-nitride, silicon nitride and boron nitride. The more wetted material can include some forms of silicon nitride, and silicon carbo- nitride .

[0083] If crystal ribbons of other semiconductor materials are grown using strings, generally as described in the Sachs '109 patent, then the general principles set forth herein can be used to provide strings with cores and outer covers, which in net combination match the CTE of the crystal ribbon. The cover should also be one which does not promote grain

nucleation to an unacceptable degree, and which also permits wetting of the crystal ribbon material to a degree around the ribbon sufficient to promote adequate strength of the

interface. There should be a fully dense, electrically non- conductive outer covering, to bar impurities from diffusing or otherwise passing to the crystal ribbon. Such a system may be of a core and a cover, either of which may be one or two or more components . What is required is that the properties of CTE net match, nucleation inhibition and wettability are such that undue stresses and strains do not arise in the fabrication process, and that the ribbon may be grown to an acceptable thickness near to the string.

[0084] This disclosure describes and discloses more than one invention. The inventions are set forth in the claims of this and related documents, not only as filed, but also as developed during prosecution of any patent application based on this disclosure. The inventors intend to claim all of the various inventions to the limits permitted by the prior art, as it is subsequently determined to be. No feature described herein is essential to each invention disclosed herein. Thus, the inventors intend that no features described herein, but not claimed in any particular claim of any patent based on this disclosure, should be incorporated into any such claim.

[0085] Some assemblies of hardware, or groups of steps, are referred to herein as an invention. However, this is not an admission that any such assemblies or groups are necessarily patentably distinct inventions, particularly as contemplated by laws and regulations regarding the number of inventions that will be examined in one patent application, or unity of invention. It is intended to be a short way of saying an embodiment of an invention. [ 0086 ] An abstract is submitted herewith. It is emphasized that this abstract is being provided to comply with the rule requiring an abstract that will allow examiners and other searchers to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims, as promised by the Patent Office's rule .

[ 0087 ] The foregoing discussion should be understood as illustrative and should not be considered to be limiting in any sense. While the inventions have been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventions as defined by the claims.

[ 0088 ] The corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed.

[ 0089 ] The following aspects of inventions hereof are intended to be described herein, and this section is to ensure that they are mentioned. They are styled as aspects, and although they appear similar to claims, they are not claims. However, at some point in the future, the applicants reserve the right to claim any and all of these aspects in this and any related applications.

Claims

1. A string for use in a string ribbon crystal, the ribbon material having a coefficient of thermal expansion (CTE), the string comprising: a. a core comprising at least one component, each having a CTE; b. surrounding the core, a cover comprising: i. a first material, having a relatively low nucleation propensity with respect to the ribbon material, having a CTE and a wetting angle relative to the ribbon material; and ii. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone, and having a CTE; wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and in amount, such that a string composed of a core surrounded by a cover has a CTE that is in NET, substantially equal to the CTE of the ribbon material.

2. The string of aspect 1, the ribbon material

comprising silicon, the cover first material comprising silica, and the cover second material comprising silicon carbide .

3. The string of aspect 1, the core comprising carbon.

4. The string of aspect 1, the ribbon material

comprising silicon, the cover first material comprising silica, and the cover second material comprising silicon nitride .

5. The string of aspect 1, the core comprising a first component in a central location and a second component

substantially surrounding the first component.

6. The string of aspect 1, the core comprising a first component and a second component interspersed with each other.

7. The string of aspect 1, the first and second

materials of the cover being interspersed with each other.

8. The string of aspect 1, the cover comprising a barrier against the diffusion of material from within the core to the ribbon material.

9. The string of aspect 1, the cover comprising a barrier against the diffusion of metallic material from within the core to the ribbon material.

10. The string of aspect 1, the cover comprising a barrier against the diffusion of boron from within the core to the ribbon material.

11. The string of aspect 1, the cover comprising a substantially fully dense structure.

12. The string of aspect 1, wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and in amount, such that the CTE of the core, is in NET, substantially equal to the CTE of the cover.

13. The string of aspect 1, the cover layer being electrically substantially non-conductive.

14. A string for use in a string ribbon crystal, the ribbon material having a coefficient of thermal expansion (CTE), the string comprising: a. a core comprising at least one component, each having a CTE; b. surrounding the core, a cover comprising: i. a first material, having a CTE and a wetting angle relative to the ribbon material; and ii. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone, and having a CTE; wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and in amount, such that a string composed of a core surrounded by a cover has a CTE that is in NET, substantially equal to the CTE of the ribbon material.

15. A string for use in a string ribbon crystal, the ribbon material having a coefficient of thermal expansion (CTE), the string comprising: a. a core comprising at least one component; b. surrounding the core, a cover comprising: i. a first material, having a relatively low nucleation propensity with respect to the ribbon material, and a wetting angle relative to the ribbon material; and ii. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone.

16. A string and crystal ribbon assembly, comprising: a. a crystal ribbon comprising a material having a

CTE; b. adjacent and contacting two edges of the crystal ribbon, two strings, each string comprising: i. a core comprising at least one component, each having a CTE; and ii. surrounding the core, a cover, comprising:

A. a first material, having a relatively low nucleation potential with respect to the ribbon material, having a CTE and a wetting angle relative to the ribbon material; and

B. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone, and having a CTE; wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and in amount, such that a string composed of a core surrounded by a cover has a CTE that is in NET, substantially equal to the CTE of the ribbon material.

17. The assembly of claim 16, further, wherein each string is wetted by the ribbon material around between

approximately 55 degrees and 360 degrees of the string's circumference .

18. The assembly of aspect 16, wherein the ribbon, adjacent the string, is at least as thick as the diameter of the string.

19. A string and crystal ribbon assembly, comprising: a. a crystal ribbon comprising a material having a

CTE; b. adjacent and contacting two edges of the crystal ribbon, two strings, each string comprising: i. a core comprising at least one component, each having a CTE; and ii. surrounding the core, a cover, comprising:

A. a first material, having a CTE; and

B. a second material, having a CTE; wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and amount, such that a string composed of a core surrounded by a cover has a CTE that is in NET, less than the CTE of the ribbon material.

20. The assembly of claim 19, wherein: a. the first material has a relatively low nucleation potential with respect to the ribbon

material, and a wetting angle relative to the ribbon material; and b. the second material has a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone .

21. The assembly of claim 19, wherein at an end of its length, the ribbon is in compression in the direction along its width.

22. A string an crystal ribbon assembly, comprising: a. a crystal ribbon comprising a material having a

CTE; b. adjacent and contacting two edges of the crystal ribbon, two strings, wherein each string has a CTE that is in NET, less than the CTE of the ribbon material.

23. A method of making a crystal ribbon comprising the steps of: a. providing a quantity of molten ribbon material; b. providing a pair of strings, each string comprising : i. a core comprising at least one component; and ii. surrounding the core, a cover comprising:

A. a first material, having a relatively low nucleation propensity with respect to the ribbon material, and a wetting angle relative to the ribbon material; and

B. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone; and c. drawing the strings through the quantity of molten ribbon material.

24. The method of aspect 23, the step of drawing being conducted under conditions such that the molten ribbon

material wets around each string between approximately 55 degrees and 360 degrees of the string's circumference and such that a crystal ribbon forms between the two strings.

25. The method of aspect 23, further comprising the step of diffusing a junction, and allowing the junction to extend to the string, in a final device.

26. A method of making a string for use making a crystal ribbon of ribbon material, having a CTE, the method comprising the steps of: a. providing a core comprising at least one component, each having a CTE; and b. surrounding the core with a cover, the cover, comprising : i. a first material, having a relatively low nucleation propensity with respect to the ribbon material, having a CTE and a wetting angle relative to the ribbon material; and ii. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone, and having a CTE; wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and in amount, such that a string composed of a core surrounded by a cover has a CTE that is in NET, substantially equal to the CTE of the ribbon material.

27. A method of making a string for use making a crystal ribbon of ribbon material, having a CTE, the method comprising the steps of: a. providing a core comprising at least one component, each having a CTE; and b. providing a slurry, the slurry comprising: i. a first material, having a relatively low nucleation potential with respect to the ribbon material, having a CTE and a wetting angle relative to the ribbon material; and ii. a second material, having a wetting angle with respect to the ribbon material, that is lower than that of the first material, such that a combination of the first material and the second material has a wetting angle that is lower than that of the first material alone, and having a CTE; wherein the at least one component of the core and the first material and the second material of the cover are each chosen, in kind and amount, such that a string composed of a core surrounded by a cover comprising the first material and the second material has a CTE that is in NET, substantially equal to the CTE of the ribbon material; and c. applying the slurry to the core, such that a cover comprising the first and second materials

surrounds the core.

28. The method of aspect 27, the step of applying the slurry comprising the step of drawing the core through an opening in a vessel containing slurry, the opening being sufficiently small such that a cap of slurry is retained around the core and over the opening by capillary forces as the core is drawn through the opening.