WO2013001308A2

WO2013001308A2 - Purified silicon

Info

Publication number: WO2013001308A2
Application number: PCT/GB2012/051522
Authority: WO
Inventors: Erlend F. NORDSTRAND; Ursula GIBSON
Original assignee: Norwegian University Of Science And Technology (Ntnu)
Priority date: 2011-06-29
Filing date: 2012-06-29
Publication date: 2013-01-03
Also published as: WO2013001308A3; GB201111042D0

Abstract

A process for purifying Si comprising (I) obtaining a silicon fibre, ribbon or silicon microplate in which one dimension thereof is less than 75 microns; (II) oxidising said fibre, ribbon or microplate to provide a silicon dioxide skin thereon; (III) annealing said silicon dioxide coated fibre, ribbon or microplate; (IV) etching the silicon dioxide skin; and optionally repeating steps (II) to (IV) above.

Description

Purified Silicon

This invention relates to a new process for the formation of purified Si. In particular, the invention relates to the formation of highly pure microplates, ribbons or fibres of Si which can be used commercially in fields where high purity Si is desired.

The use of highly pure Si for various applications is well known. The electronics industry and solar industry use large amounts of highly pure silicon to make their products. Highly pure Si is not naturally occurring so the industry has devised many methods of preparing Si with a sufficient level of impurity. Typically, single crystals of Si are prepared with very low levels of impurity in the form of boules or ingots that can then be formed into wafers. To achieve the desired level of impurity requires, however, high energy processes and a lot of time. There is also significant wastage of Si in conventional processes for making purified Si. Purified Si is therefore a very expensive product.

The current favoured method for the formation of highly pure Si crystals involves refinement by chemical means, followed by bulk solidification/

crystallisation. A rather complex chemical process is used to develop a highly pure form of Si. The process starts from metallurgical grade Si, involves treatment with HC1 and various distillation and vaporisation steps.

Metallurgical Grade Silicon is first pulverized and reacted with anhydrous hydrogen chloride (HC1) to form trichlorosilane (S1HCI3). The reaction creates products like silicon tetrachloride and the chlorides of impurities. The purification process is carried out by a fractional distillation method as the products

trichlorosilane and unwanted chlorides are liquids at room temperature.

The purified S1HCI3 is subjected to chemical vapour deposition (CVD). The chemical reaction is a hydrogen reduction of S1HCI₃ (2S1HCI₃ + 2H₂ = 2Si + 6HC1)

Pure Si can also be produced by pyrolysis method in which silane (S1H₄) is reacted with heat following the equation:

SiH₄ + HEAT = Si + 2H₂ In this process the reactor is operated at about 900°C and supplied with silane instead of trichlorosilane.

Alternatively, the metallurgical grade may be purified to a level suitable for solar cells using a combination of slag treatment and leaching, followed by directional recystallization.

The resulting purified Si is still polycrystalline however. The Czochralski process is often then used to grow single crystal Si. The Czochralski process involves the solidification of atoms from a liquid phase at an interface.

The poly-crystalline silicon starting material is put into a quartz crucible, which is then placed inside a crystal growth furnace. The material is then heated to a temperature that is slightly in excess of the silicon melting point. A small single- crystal rod of silicon is then dipped into the silicon melt. The conduction of heat up the seed crystal will produce a reduction in the temperature of the melt in contact with the seed crystal to slightly below the silicon melting point. The silicon will therefore freeze onto the end of the seed crystal, and as the seed crystal is slowly pulled up out of the melt it will pull up with it a solidified mass of silicon that will be a crystallographic continuation of the seed crystal. Both the seed crystal and the crucible are rotated but in opposite directions during the crystal pulling process in order to produce crystalline ingots of circular cross section.

The ingot length will generally be of the order of 3 m, and many hours or even days are required for the "pulling" of a complete ingot. The crystal pulling is done in an inert-gas atmosphere (usually argon or helium), and sometimes a vacuum is used. This is done to prevent oxidation.

This process is obviously very expensive. It involves high temperatures, inert atmospheres and vacuums. Also, Si found at the edge of the ingot can be contaminated by the vessel and is thus often sliced out and thrown away. Defects in the Si can cause further wastage and there is also significant wastage of Si in the impure top layer which is cut away.

The process is also a batch process. After each ingot is formed, it has to be removed from the vessel, the vessel may need to be cleaned and a new process begun. It is obvious that ideally, purification of Si would take place continuously. Even for the production of "solar grade" materials where purity levels less than for electronics applications can be tolerated, the process is energy intensive, slow and involves purification steps at high temperatures. The resultant material, when cast and wafered, results in further waste, and variability in the product wafers, as the impurities segregate differently. High levels and uneven distribution of these impurities leads to regions of "grit" which are unusable are cut off and discarded.

It is obvious that ways of reducing the cost and waste of these products are highly desirable.

The present inventors have devised a new process for purifying Si starting from fibres, ribbons or microplates of Si. These fibres, ribbons or microplates can be formed from metallurgical grade silicon, the cheapest form of Si on the market, which typically presents in irregular lumps. It can therefore be considered to be particulate in form. In the claimed process, the lumpy metallurgical grade Si is converted into fibres, ribbons or microplates and processed to increase purity. This technique can replace the bulk chemical and zone refinement purification of Si as discussed above and instead provides highly pure Si in the form of microplates, ribbons or fibers. It is envisaged that these fibres, ribbons and microplates will have their own uses but of the course these microplates, ribbons or fibers could be used as starting stock for the growth of single crystal boules or multicrystalline ingots.

We propose a purification method that takes advantage of the increased surface area of microplates, ribbons or fibers and the important fact that microplates, ribbons and fibres have one or more very small dimensions. This provides a reduced distance for out-diffusion of impurities. This method may be applied to the production of all grades of silicon, e.g. solar grade or electronic grade silicon.

The process of the invention requires the formation of an oxide skin on the Si microplate, ribbon or fibre, annealing thereof and subsequent etching of the oxide skin. A similar purification method has been demonstrated in small spheres of silicon, with formation of electronic grade material possible after a few refining cycles. In US 5069740 therefore, the inventors take metallurgical grade silicon and melt it in an oxygen atmosphere. During the heating step an oxide layer is provided on the silicon and when the Si within that layer melts, the Si substrate spheroidises (i.e. forms spheres). Impurities diffuse out to the skin from the Si and after cooling and etching, a purer Si sphere is formed.

The present inventors have realised that if one dimension of an Si substrate is made very small and such a material is used as the Si substrate, much better results can be achieved than using spheres. In a substrate with one or more minute dimensions, impurities in the Si have a very short distance to travel to the skin. The skin acts as a getter for the impurities and if the migration distance over which those impurities have to travel is minimised, the rate of purification can be maximised. Moreover, as the impurities have at least one very small dimension along which to migrate to the getting skin, that allows the other dimension(s) of the substrate to be essentially of any size. Very long fibres or ribbons can therefore be produced and refined in enormous quantities very rapidly. As long as those fibres or ribbons are narrow, the impurities can migrate to the skin and be removed.

The present inventors have realised that the technique of the invention can be used much more effectively on fibers of silicon (which we consider one dimensional substrates) or on ribbons or microplates (which we consider two dimensional substrates) as the geometric construct for carrying out the

oxidation/annealing/etching process than on spheres which are perfectly three dimensional. The use of the claimed purification process therefore opens the door to fully continuous purification of Si.

Moreover, the invention also relates to a new process for pulling fibres, e.g. for purification. The inventors have found that when using a fibre pulling method involving a glass sheath with Si within the sheath, improvements are possible if an interface layer is introduced between the glass sheath and Si contained therein. This improves the processing of the Si fibres by reducing mechanical strain and improving wetting between the silicon core and the glass sheath. By this method, longer continuous lengths and thinner core fibres can be drawn.

Summary of invention

Thus, viewed from one aspect the invention provides a process for purifying Si comprising (I) obtaining a silicon fibre, ribbon or microplate in which one dimension thereof is less than 75 microns in size;

(II) oxidising said fibre, ribbon or microplate to provide a silicon dioxide skin thereon;

(III) annealing said silicon dioxide coated fibre, ribbon or microplate;

(IV) etching the silicon dioxide skin; and optionally

repeating steps (II) to (IV) above.

Viewed from another aspect the invention provides a continuous process for the purification of Si comprising:

(I) passing a silicon fibre or ribbon in which one dimension thereof is less than 75 microns into an oxidation zone to provide a silicon dioxide skin thereon;

(II) passing said coated Si fibre or ribbon into an annealing zone in which said Si is heated such that impurities therein migrate to said silicon dioxide coating;

(III) passing said annealed Si fibre or ribbon to a cooling zone; and (IV) passing said cooled fibre or ribbon to an etching zone where the silicon dioxide coat is removed. These steps can, of course, be repeated.

Viewed from another aspect the invention provides a purified Si fibre, ribbon or microplate made by the process of the invention.

Viewed from another aspect the invention provides the use of a purified Si fibre, ribbon or microplate made by the process of the invention in electronic, optical or solar applications.

Viewed from another aspect the invention provides an electronic device, optical device or solar cell comprising a purified Si fibre, ribbon or microplate made by the process of the invention.

Viewed from another aspect the invention provides a process for pulling a Si fibre, e.g. a Si fibre for purification using the methods hereinbefore defined, comprising heating a cylindrical composition comprising

(I) a Si core such as a Si rod;

(II) an interface layer of a metal oxide, nitride or sulphide or mixtures thereof surrounding longitudinally said core;

(III) a glass sheath surrounding longitudinally said interface layer;

and pulling a Si fibre from said core once said Si melts. The use of an interface layer, with the glass sheath and Si core construct also forms an aspect of the invention. Thus, viewed from another aspect the invention provides a cylindrical composition of matter comprising a Si core (e.g. a Si rod), surrounded longitudinally by an interface layer of a metal oxide, nitride or sulphide or mixtures thereof which is in turn surrounded by a glass sheath. All layers are preferably adjacent.

Detailed Description of Invention The starting material for the process of the invention can be metallurgical grade silicon, or alloys of silicon with metals that can easily be separated from the material during later processing. These grades of Si typically contains a variety of problematic impurities which need to be reduced in concentration or removed before the Si is of sufficient quality to be used in relevant industrial applications. Common impurities are Al, Fe, Cr, Ni and other transition metals in general. Impurities of particular concern are iron and aluminium. In order to have a commercially viable Si material, the levels of these impurities must be of the order of parts per million if not parts per billion.

The present invention seeks to remove those impurities by providing the Si in a form where at least one dimension thereof, preferably at least two dimensions thereof, are less than 75 microns in length. Ideally, that is provided by the use of a fibre of diameter of 75 microns or less or ribbon of thickness 75 microns or less. In order to withstand the oxidation, annealing, etching process described below the fibres, ribbons or microplates do however, need a certain minimum dimension to avoid being too weak. For example, fibres should not be nanofibres. The smallest dimension of the fibres, ribbons and microplates of the invention should be at least 10 microns, preferably at least 20 microns such as at least 30 microns.

The term fibre is used herein to cover substantially cylindrical forms of Si in the form of a fibre. The term fibre is intended to cover therefore Si rods, wires, and whiskers for example. Ribbons of Si are those where one dimension is less than 75 microns and the transverse direction is from 75 microns to 25 or more cm. The ribbon has any length. At least one dimension of the fibres, ribbons or microplates of the invention should be less than 75 microns in size. As noted below, once one dimension is less than 75 microns, the size of the other dimensions is less critical. In a fibre, it may be that the second smallest dimension is preferably less than 100 microns, especially less than 75 microns. Preferably two dimensions of the fibres of use in the invention should be less than 75 microns in size. That can obviously be achieved by using a cylindrical fibre of diameter of less than 75 microns.

The length of the fibre used in the invention is however, essentially unrestricted. The fibre should be at least 200 microns in length, preferably at least 500 microns, such as 1 mm or more in length. In theory however, fibres of the invention can be formed in very long lengths such as more than 1 mm, more than 1 cm, or more than 1 m. If fibres are drawn continuously then the fibre can be essentially of limitless length as we envisage that this fibre can then be treated continuously in our process.

Fibres should be generally circular in cross section although it will be appreciated that oval cross sections and the like are possible. As the other two dimensions in a fibre are preferably less than 75 microns, the fibres of the invention can be considered pseudo "one dimensional".

Ribbons of the invention have at least one dimension less than 75 microns in size. The transverse direction may be 75 microns to 10 cm or perhaps to 25 cm or more. The length of the ribbon used in the invention is however, essentially unrestricted. The ribbon should be at least 200 microns in length, preferably at least 500 microns, such as 1 mm or more in length. In theory however, ribbons of the invention can be formed in very long lengths such as more than 1 mm, more than 1 cm, or more than 1 m.

Microplates

Whilst the use of ribbons or especially fibres is preferred, the invention can also be applied to microplates. A microplate is considered to be a structure in which one dimension thereof is reduced to less than 75 microns but the other two dimensions are larger. The term microplate is intended to cover therefore flat sheet type structures although there is no requirement for the sheet to be regular in its flat shape.

The other dimensions in a microplate might be at least 200 microns, preferably at least 500 microns, such as 1 mm or more. The other two dimensions in a microplate are independent of each other and there is no requirement to form regular microplates.

The invention works by reducing one or two dimensions of the Si substrate to less than 75 microns. This then allows rapid segregation of impurities during the annealing step leading to purer Si.

Fibre/Ribbon/Microplate formation

Any convenient method can be used to form the fibres, ribbons or microplates required to effect the invention. A convenient method for producing short fibres, ribbons and microplates is melt spinning. In the melt spinning process molten Si is provided in an inert vessel, such as a graphite crucible. The crucible is provided with a hole in the bottom through which molten Si can be dropped onto a clean spinning disk. Fibres, ribbons and microplates form on the disk as the molten Si cools. The melt could also be fired onto a rotating disc from a nozzle and so on.

The size of the hole in the crucible and the pressure at which the material is forced through that hole can be used to manipulate the size of the microplates, ribbons and fibres formed by this process. Larger holes obviously allow more molten Si to fall onto the disk and allow more rapid processing of the material. The rotational speed of the disk, and the wetting properties thereof can also be used to control the dimensions and form of the product.

Microplates, ribbons and fibres made by melt spinning might have dimensions of 75 microns or less in thickness or diameter and lengths of mm to several cm.

In order to avoid contamination of the Si with material from the disk, and to provide good wetting characteristics, it is important that the disk has a suitable surface before use. The disk can be formed from a substance that does not contaminate the cooling Si such as clean copper or graphite. The use of graphite might be especially preferred because it is wetted by the Si.

The disk can be warmed or cooled to affect the solidification process and hence the nature of the microplates, ribbons and fibres formed. In order to maximise microplate/fibre/ribbon formation, especially in a melt spinning process, the inventors believe a high cooling rate is preferable. Such a cooling rate might be hundreds of degrees Celsius per second.

Alternative fibres could be made by encapsulation of Si in a silicon dioxide sheath, i.e. glass encapsulation. It is known in the optical fibre industry to insert crystalline Si into hollow glass fibres. Since Si has a lower melting point than glass, it will melt during the process of pulling a glass fiber. Si can be inserted into the hollow preform before it is drawn. The Si can form a fibre by passing through the narrowing region of the silicon dioxide sheath under gravity. The cavity may be lined with a glass of different composition to improve the compatibility between the silicon and the glass, or surfactant chemicals such as aluminium or chromium or other metals or oxides or nitrides or carbides may be introduced at the interface. This preform is then placed into a furnace at the top of a fibre drawing tower, and procedures normal for the production of glass fibre are used to melt, elongate and form a fiber. The temperatures depend on the glass used in the process, but must be above the melting point of silicon.

In particular, the inventors have found that the introduction of a "surfactant type" layer or interface layer between the silica glass sheath and the Si within the sheath modifies the interface between glass and Si and improves the fibre production process. In particular, the use of metal oxides, nitrides and sulfides might be useful in this regard. It will also be possible to use a mixture of these components, e.g. a mixture of two metal oxides. The glass transition or melting point of the material is such that it will harden at a temperature lower than the melting point of silicon, thus reducing thermal stresses.

Preferred metals of use in the interface layer are group (I) or (II) metals or Zn. Most preferred are group (I) or (II) metal oxides or Zn oxide such as MgO,

ZnO, SrO, CaO, BaO and Na₂0. These materials can also be produced in situ from the hydroxide, the carbonate and other compounds. A specific process which can be used involves coating the inside of a silica glass tube with a slurry of metal hydroxide or metal oxide particles, for example, Ca(OH)₂ before adding the charge of silicon.

The hydroxide/oxide slurry may be applied in a rotating tube to make a uniform layer. The suspension may use water, ethanol, or other volatile liquids as solvent or carrier. The layer is then dried, either at room temperature (preferred method), or by heating. The amount of hydroxide/oxide etc (i.e. metal salt) relative to the amount of silicon may be in the weight ratio of 1 : 10 up to 1 :500, with the preferred range depending on the uptake of impurities in the oxide, but being around 1 :40.

The silicon, in particulate or rod form, can then be added to the silica tube. The tube is heated in a rotating holder (perhaps vertical or horizontal). This heating process may allow conversion of a precursor of an oxide such as a hydroxide to an oxide. When the silicon is molten and the glass begins to soften, force is applied to draw out a thin fiber.

In a further surprising development, the inventors have found that it may not even be necessary to apply the interface layer as a coating on the glass sheath.

Instead, this might be provided simply by mixing the interface layer compounds with the Si charge. This modifies the interface between Si and sheath as required and if the surface energies are appropriate, the Si separates from the interface layer compounds during the drawing process.

Thus Si rods, powder or pellets could be mixed with the interface modifier compound, e.g. a metal oxide, and placed in a glass sheath for drawing.

Thus viewed from another aspect the invention provides a process for pulling a Si fibre, e.g. a Si fibre for purification using the methods hereinbefore defined, comprising heating a cylindrical composition comprising

(I) a Si core such as a Si rod mixed with a metal oxide, nitride or sulphide or mixture thereof; and

(II) a glass sheath surrounding longitudinally said core;

and pulling a Si fibre from said core once said Si melts.

The glass sheath, Si core construct also forms an aspect of the invention. Thus, viewed from another aspect the invention provides a cylindrical composition of matter comprising a Si core such as a Si rod mixed with a metal oxide, nitride or sulphide or mixture thereof which is surrounded longitudinally by a glass sheath. All layers are preferably adjacent.

The interface layer does not need to be very thick to have a useful effect. Layers of 0.5 to 10 microns are useful.

The amount of a metal oxide, nitride or sulphide or mixture thereof added to the Si core in the second embodiment can range from a tenth of a percent to over 10 wt percent.

In a further embodiment, rather than using metal oxides directly, the inventors envisage the use of precursor compounds to the oxide which might convert to an oxide under appropriate conditions, e.g. heating in air. Such compounds include carbonates, nitrides and hydroxides e.g. of the metal ions hereinbefore defined.

It is believed that there is a synergy between the use of an interface layer and the formation of purified silicon fibres/ribbons/flakes. The presence of the interface layer improves the processing of the Si fibres by reducing mechanical strain and improving wetting between the silicon core and the glass sheath. This may allow longer continuous lengths and thinner core fibres to be drawn. The interface modifier also serves as a repository for impurities that diffuse to the surface of the fibre.

In theory the length and diameter of the fibre formed can be manipulated by adjusting the length and diameter of the silicon dioxide preform and the fibre drawing conditions. Silicon core diameters of 35 to 100 microns and lengths of many meters to km are are envisaged.

Techniques for making coaxial glass fibres are well known in the optic fibre industry.

Whilst the encapsulating material is preferably silicon dioxide, in order to make the material more etchable or to make the softening temperature similar to a silicon alloy being used as the core, it make be possible to add soda lime or calcium oxide or alumina or borosilicates to the silica to form more conventional glass therefore. Care has to be taken that any additives in the glass introduced to the Si can be removed by the subsequent purification steps. Once the fibre is formed within the casing, the casing can be used to transport the silicon for subsequent annealing and can then be etched away. It is stressed that this casing is not the same as a skin or coating formed during oxidation. The skin will be very much thinner than a sheathing in which a fibre can be formed. An encapsulating glass sheath might be 100 to 300 microns in thickness for example. Etching of the silicon dioxide sheathing can be achieved using the etching techniques below, especially with the use of HF. Fiber-drawing techniques that produce multiple silicon cores with thin silica or other glass separators might be used to speed the production process. This process is known as the formation of photonic crystal fiber drawing, and is well known. The use of glass interface layers which can be etched by other chemicals is anticipated.

Production of limited length fibers has been demonstrated by others for optical applications.

A similar structure might be produced by coaxial extrusion of glass and Si layers, using a process similar to that used by the fibreglass industry. In this process, a crucible containing molten silicon would be placed above and coaxially with the nozzle used for the expulsion of glass, to form a coaxial structure.

Another embodiment involves the coating of a glass fibre or other carrier structure such as a wire or tube with a thin sheath of silicon by passing the fibre through a melt of silicon, thus creating a coaxial structure with silica or similar material on the inside, and silicon metal on the outside. Flat carrier structures would result in the formation of fibres in the form of silicon ribbons. A fibre or ribbon may thus be formed by drawing a template material through molten silicon. Oxidation

Before oxidation, it might be useful to carry out an etching procedure as herein described to remove any impurities already located on the surface. Also, melting and remelting of the Si structure can encourage the formation of larger grains reducing the amount of in-plane grain boundaries. This might make it harder for impurities to be sequestered distant from the surface of the sample during subsequent annealing processes.. Once a fibre, ribbon or microplate is formed, a silicon dioxide skin or coating needs to be provided thereon.

We use the term skin and coating interchangeably herein. In order to introduce a silicon dioxide skin onto the Si fibres, ribbon or microplates, these can be heated to below their melting temperature in an atmosphere containing oxygen. Preferably, a wet oxidation atmosphere is used. The atmosphere here should be saturated, water vapour, with argon or similar inert gas or oxygen as the carrier gas. This oxidation can be performed in an optically heated "rapid thermal processor" or in a conventional tube or box furnace.

The temperature used for this step should not exceed 1400°C. Preferably, the temperature should not exceed 1300°C. It is preferred therefore if the Si is not melted during this step. Melting the Si microplate, ribbon or fibre without the silicon dioxide skin might lead to loss of its form and perhaps spheroidisation, which is to be avoided.

Preferred ranges for heating are 800 to 1200°C, preferably 900 to 1150°C, especially 1000 to 1100°C.

The duration of this step can vary, perhaps from 1 minute to 5 hours or more.

There is a clear correlation between the length of the oxidation reaction, the temperature of the reaction and the thickness of the silicon dioxide skin which forms on the Si. In general, longer oxidation times and higher temperatures encourage thicker skins.

The length and temperature of the step can therefore be tailored depending on the desired thickness of the silicon dioxide skin.

The silicon dioxide skin may be 0.2 to 3 microns in thickness, e.g. 1 to 2 microns. Conveniently, the skin is about 1 micron in thickness. Skins that are too thin, do not provide stability during annealing. To achieve a 1 micron thickness skin may require heating for around 1-2 hr at appropriate temperature.

Preferably the skin should be less than 5% of the thickness of the fibre, ribbon or microplate.

The silicon dioxide skin stabilizes the fibres, ribbons or microplates for the annealing step. The skin also reduces agglomeration of microplates, ribbons or fibres during the subsequent steps of the process. It is preferred if this oxidation step is carried out as a distinct step, separate from the annealing step. It will be appreciated, however, that the oxidation step could be carried out in the same heating operation as the annealing step. The oxidation step could form a dwell during an overall heating step up to annealing temperature. Thus, heating could be stopped and the temperature held at a particular temperature for a period whilst the silicon dioxide skin forms. Once this is formed, heating can resume and the annealing step can be initiated.

It may be that a dwell is not necessary. If a suitable heating rate is used, it may be possible to form a suitable skin simply as the heating process goes from 800 to l lOO°C.

To stop any further skin forming, the oxygen containing atmosphere can be removed or at least the water or oxygen content of the atmosphere can be reduced

Annealing Step

In order to remove impurities, the coated Si is then annealed. This means the coated Si is heated to a temperature close to or just above the melting point of Si but below the melting point of the silicon dioxide skin. For example, the material may be heated to a temperature in the range 1400 to 1500°C. By heating to this temperature, the impurities present in the Si migrate to the silicon dioxide skin layer. The Si is therefore purified. In particular, Al, B and Fe impurities have been shown to migrate to the skin layer.

It is preferred if an oxygen atmosphere is maintained for this step. The presence of oxygen encourages the healing of the silicon dioxide skin and hence limits the interaction of the flake, ribbon or fibre with the container. The

atmosphere used for annealing might contain a lower percentage of oxygen than used in the oxidation step, such as air. Alternatively, an inert atmosphere might be used, e.g. nitrogen.

The duration of this step can be varied but if the sample is melted, there is little advantage to a long anneal. Typical annealing times are a few minutes, e.g. up to 10 minutes. The coated silicon fibres, ribbons or microplates are then allowed to cool. This can occur naturally or cooling can be controlled by the use of an appropriate device such as an oven. Typically, cooling occurs down to ambient temperature. The cooling Si can reform as a single crystal as it cools but there is no requirement in the invention that a single crystal material forms. The present invention also envisages the formation of multicrystalline Si materials. These still have valuable commercial properties.

Etching

Etching can then be used to remove the skin and hence also impurities that have migrated into it. Any etching process can be used as is well known in the art. The use of HF as an etching compound is preferred. The etching process may take a few minutes and is obviously dependent on the thickness of the skin formed during the oxidation step. Thicker skins take longer to etch but are capable of gettering more impurity. The etching properties of silica glass are well known, and the etching can be monitored by measuring the optical reflection of microplates or fibres. Other glass materials can be calibrated.

The etching solution used is typically a 5 to 49% HF solution This step can take place at room temperature.

Once the skin is removed, the remaining fibre, ribbon or microplate is purer than before the process was carried out as impurities have moved into the skin. It will be appreciated that the fibre, ribbon or microplate will be slightly thinner than before the process owing to the removal of the skin layer. Thickness reduction is envisaged to be less than 5% per cycle.

The whole procedure can then be repeated until a sufficient level of purity is achieved. It may be that for solar grade silicon, two cycles are required. For electronics grade silicon, more than two cycles may be required. It is envisaged that each cycle will improve the purity of the Si by at least one, preferably at least two orders of magnitude, though this will be dependent to some extent on the starting material, as some impurities are known to be more difficult to remove. A major benefit of the invention is that the materials on which the process is carried out are two dimensional or even one dimensional. By this is meant that one of the dimensions of the material is so small (i.e. less than 75 microns) that it is de minimis in comparison to the other dimensions.

As noted above, in US 5069740, a similar purification method using oxidation, annealing and etching is used on spherical substrates. Spheres are of course, the most perfect three dimensional shape as they have equal dimensions in all directions. As the maximum distance from the centre is the same in all directions, impurities in a spherical Si particle may have no small dimension to reach the skin. For this reason, US 5069740 suggests the use of small spheres as objects on which to carry out the purification process. Nevertheless, the spheres of use in US 5069740 are still a minimum of 250 microns in diameter.

The present inventors have realised that if a two or especially one dimensional substrate is used as the Si material, a much preferred process is possible. In a two or one dimensional substrate, impurities have a very short distanced to travel to the skin. That maximises purification potential. Moreover, as impurities can be removed in the small dimension, that allows the other

dimension(s) of the substrate to be essentially of any size. This is of particular relevance in fibre and ribbon applications.

Very long fibres or ribbons can therefore be produced and refined in enormous quantities using this method. As long as those fibres/ribbons are narrow, the impurities can migrate to the skin and be removed.

This invention therefore opens the door to a continuous process for Si refining. A Si fibre or ribbon can be formed and passed through an oxidation zone, annealing zone and etching zone. By passing the fibre or ribbon at an appropriate speed, this process can be carried out continuously. The treated fibre or ribbon may be one that has just been drawn and so on.

Moreover, the process can be carried out on a multitude of fibres or ribbons simultaneously. The fibre or ribbon can then be collected on a spool for example for transportation and so on. Alternatively, that spool can be returned to the start of the process and purified further and so on. The insight that the use of a one dimensional substrate enables continuous production offers massive potential in this industry where still today, batchwise bulk purification is used.

Moreover, because at least one dimension of the fibers/microplates/ribbons is so small, the diffusion of impurities from the Si into the skin is quick. It takes minutes or hours rather than days to achieve equivalent levels of purity. It is much faster, for example, than the time taken for impurities to migrate to the top of a cooling Si ingot in conventional refining.

The fibre or ribbon embodiment of the invention also results in a product that has a linear geometry, which may have significant advantages in some applications.

Applications

It will be appreciated that the purified silicon made by the process of the invention has applications in any field where high purity Si might be needed such as in the electronics industry, radar detectors, solar cells and so on. Fibres from pure silicon drawn using this invention are of particular interest in optical applications such as in the formation of optical fibres.

The invention will now be described with reference to the following non limiting examples and figures.

Figure 1 is a drawing of the apparatus used to form fibres and microplates of Si in example 1. A graphite crucible (1) provided with a hole in the bottom (2) of size 1 mm. The crucible is surrounded by a silica sheath (3) for the embodiment where heating of the crucible is provided by induction heating. Radiative heating would not need this. Si is present inside the crucible. On melting, the Si passes through hole (2) onto spinning copper plate (4).

Figure 2 is a graph of temperature and oxidation time vs skin thickness.

Figure 3 is an analysis of impurities in a Si fibre before the process of the invention.

Figure 4 is an analysis of impurities in a Si fibre after one cycle of the invention. The concentration of impurities is measured as a function of the distance from the surface of the fibre. As impurities should migrate from within the fibre centre to the skin, it will be clear that the concentration of impurities within the fibre should reduce after purification the further from the fibre surface the measurement is taken. In figure 3 it can be clearly seen that the concentration of impurities is substantially equal at all distances from the surface. That simply reflects therefore an even distribution of impurities within the starting metallurgical grade Si.

In figure 4 after one cycle, it is also clear that the further from the surface the measurement is taken, the lower the impurity content. This reflects therefore the fact that impurities from the centre of the fibre have migrated towards the surface. It will be seen that for Al, the impurity measurement 8 microns into the fibre has reduced by three orders of magnitude. By repeating the purification process of the invention, the levels of impurity in the Si will reduce still further. The reduction in impurity level is particularly marked for Al and Fe. This invention therefore represents a particular preferred method for reducing these impurities.

Figure 5 is a micrograph showing the morphology of the fibres of the invention before and after one cycle of the invention.

Figure 6 shows a Si fibre core within a glass sheath.

Figure 7 shows the results of repeating the process of the invention with multiple annealing and etching processes.

Figure 8 is an SEM image showing the presence of a CaO layer between glass and Si layers in the coated tube.

Figure 9 is an SEM image showing a cross section of a glass sheath, interface layer and Si made analogously to example 8 but using BaO.

Example 1

Melt-spinning

A graphite crucible with an opening in the bottom was held in a fused silica sheath as shown in figure 1. Metallurgical grade silicon was employed. The graphite crucible was heated with an induction coil in an argon atmosphere in order to melt the Si in the crucible. Molten silicon runs out (or can be forced out with pressurized gas) of the 1-3 mm hole in the bottom of the graphite crucible onto a spinning clean copper wheel. The heating rate of the silicon was between 50 and 500°C per second. The cooling rate on contact with the spinning wheel is much higher.

Small-dimension silicon microplates and fibres are formed. Fibres are typically 50 microns in diameter and microplates may be 4-8 mm x 4-8 mm x 50 microns thick. Fibre ribbons also form have a thickness of 50 microns, but are long and of similar width to microplates. Fibres and microplates typically have a rough surface morphology that may be associated with partial crystallization during cooling. Example 2

Fiber-drawing / fibre sheathing

A silicon rod typically 3-8 mm in diameter is introduced into a cavity of a glass preform rod, which may have a diameter of 25 to 75 mm. The cavity may be lined with a glass of different composition to improve the compatibility between the silicon and the glass, or surfactant chemicals such as aluminium or chromium or other metals or oxides or nitrides or carbides may be introduced at the interface. This preform is then placed into a furnace at the top of a fibre drawing tower, and procedures normal for the production of glass fibre are used to melt, elongate and form a fiber. The temperatures depend on the glass used in the process, but must be above the melting point of silicon.

A sheath Si fibre made by such a process is shown in figure 6.

Example 3

Coating

Silicon fibres made by the process in example 1 were oxidised in a wet oxidation process for a period of time at 800 to 1100°C. Fibres were placed into a silica tube which was placed in the hot zone of a tube furnace. Argon gas was bubbled through water to provide a flow of saturated water vapour around the fibres. In this example, the fibres were heated for 2 hours at 1100°C. Other times and temperatures could be used. The oxide layer formed on the fibres varys in thickness depending on the oxidation time and temperature. Target thickness was about 1 micron thick. Figure 2 shows the correlation between temperature and oxidation time. Longer oxidation times and higher temperatures encourages thicker layers. An oxidation at 1100 for 2 hrs was used for further study.

This procedure was applied to four fibers and a witness chip of polished silicon wafer as control. All samples showed similar oxidation coloration.

Example 4 Annealing

After oxidation, samples were loaded into a quartz ampoule, and annealed at 1450°C for 5 minutes in a box furnace. The material was allowed to cool naturally back to ambient temperature. In some tests the samples were removed from the hot zone when the temperature in the furnace dropped to 200-300°C.

Example 5 Etching

Samples were removed from the quartz ampoule and placed in a beaker of concentrated (49%) HF for a period of two to ten minutes. Agitation of the solution was performed and the samples were rinsed with deionized water after etching. Analysis

The morphology of the samples was significantly altered by annealing, with the fibers adopting a smooth surface texture. These fibres are shown in Figure 5.

Auger Electron spectroscopy testing showed that there were high

concentrations of impurities in the slag areas, suggesting that the remaining silicon has higher than original purity. Testing of these fibers by SIMS was performed and results presented in figures 3 and 4.

Example 6 The coating , annealing and etching procedure described above was repeated up to 4 times. Figure 7 shows impurity levels after 1 to 4 etches. Example 7 - Stripping of glass-clad fibers

Glass sheaths were produced as described above, using both a pure silica cladding and a cladding containing other elements. Silicon rods were placed in the preforms and drawn down to a total fiber thickness of 200-300 microns with a silicon core of 50-75 microns.

Etching was effected using 49% HF with agitation. Etching proceeded rapidly at the interface of the fiber and the silicon core, and demonstrated the possibility for reduced process time versus etching the entirety of the glass. The glass sheath can also be removed by thermal shock.

Example 8 - Interface layer

A glass preform rod as described in example 2 was employed. The inside of the silica tube was coated with a mixture of CaO and water. The slurry was applied with the glass rod in a rotating tube to ensure the layer applies uniformly. The layer was dried at room temperature.

Silicon particles as described in example 2 were then added to the CaO coated silica tube. The tube was heated in a rotating holder and when the silicon melts and the glass begins to soften, force was applied to draw out a thin fiber.

SEM image figure 8 shows the presence of a CaO layer between glass and Si layers in the coated tube.

SEM figure 9 shows a cross section of a tube, interface layer and Si made analoguosly to example 8 but using BaO.

Claims

1. A process for purifying Si comprising

(I) obtaining a silicon fibre, ribbon or microplate in which one dimension thereof is less than 75 microns in size;

(III) annealing said silicon dioxide coated fibre, ribbon or microplate;

(IV) etching the silicon dioxide skin; and optionally

repeating steps (II) to (IV) above.

2. A continuous process for the purification of Si comprising:

(III) passing said annealed Si fibre or ribbon to a cooling zone; and

(IV) passing said cooled fibre or ribbon to an etching zone where the silicon dioxide coat is removed.

3. A process as claimed in any preceding claim wherein the fibre, ribbon or microplate is formed by melt spinning.

4. A process as claimed in claim 1 or 2 wherein the fibre is formed by encapsulation within a silica sheath and etching of that sheath.

5. A process as claimed in claim 1 or 2 wherein the fibre or ribbon is formed by drawing a template material through molten silicon.

6. A process as claimed in claim 1 or 2 wherein the fibre or ribbon is formed by extrusion through coaxial nozzles to form a silicon core in glass.

7. A process as claimed in claim 1 or 2 wherein the fibre is drawn from Si within a silica sheath.

8. A process as claimed in claim 7 wherein a metal oxide, sulphide or nitride or mixture thereof is used as an interface layer between the silica sheath and the Si within the sheath.

9. A process as claimed in claim 7 wherein a metal oxide, sulphide or nitride or mixture thereof is admixed with the Si within the sheath.

10. A process as claimed in any preceding claim wherein the oxidation step takes place at between 800 and 1100°C.

11. A process as claimed in any preceding claim wherein the silicon dioxide skin is 0.2 to 3 microns in thickness.

12. A process as claimed in any preceding claim wherein the annealing step is carried out at a temperature of 1400 to 1500°C.

13. A process as claimed in any preceding claim wherein the etching step takes place in HF.

14. A purified Si fibre, ribbon or microplate made by the process of claims 1 to 13.

15. Use of a purified Si fibre, ribbon or microplate made by the process of claims 1 to 13 in electronic, optical or solar applications.

16. An electronic device, optical device (e.g. optical fibre) or solar cell comprising a purified Si fibre, ribbon or microplate made by the process of claims 1 to 13.

17. A process for pulling a Si fibre, e.g. a Si fibre for purification using the methods hereinbefore defined or for other applications,, comprising heating a cylindrical composition comprising

(I) a Si core such as a Si rod

(III) a glass sheath surrounding longitudinally said interface layer;

and pulling a Si fibre from said core once said Si melts.

18. A cylindrical composition of matter comprising a Si core (e.g. a Si rod), surrounded longitudinally by an interface layer of a metal oxide, nitride or sulphide or mixtures thereof which is in turn surrounded longitudinally by a glass sheath.

19. A process for pulling a Si fibre, e.g. a Si fibre for purification using the methods hereinbefore defined or for other applications,, comprising heating a cylindrical composition comprising

(I) a Si core such as a Si rod mixed with a metal oxide, nitride or sulphide or mixture thereof;

(II) a glass sheath surrounding longitudinally said core;

and pulling a Si fibre from said core once said Si melts.

20. A cylindrical composition of matter comprising a Si core such as a Si rod mixed with a metal oxide, nitride or sulphide or mixture thereof which is surrounded longitudinally by a glass sheath.