DE102010025606A1 - Reprocessing saw liquid used in preparing silicon wafers comprises sawing silicon ingots by wire separation honing-lapping and/or wire abrasive cuttings, suspending silicon abrasive particles and separating the abrasives by sedimentation - Google Patents

Abstract

Reprocessing saw liquid used in preparing silicon wafers comprises: sawing silicon ingots by wire separation honing-lapping and/or wire abrasive cuttings, e.g. liquid or solid particles, where the solid particles are silicon abrasive particles and their fragments and sawing by saw wire is carried out using optionally supplementary adjuvants; separating a part of the liquid from the solid particles, suspending the silicon abrasive particles and liquid in a liquid suspension medium, e.g. alcohol, fatty acids or ionic surfactants; and separating the sedimented abrasive particles.

Description

The invention relates to the treatment of spent cutting fluid or sawing fluid from the sawing process of silicon wafer production, in particular the preparation and recovery of the solid particles contained therein, as well as liquid mixtures or their constituents.

Silicon wafers serve as a carrier plate for electronic components and in particular for photovoltaic elements. They therefore form the basis for the production of individual solar cells, which are further processed into ready-to-use solar modules. The wafers are themselves usually obtained by sawing larger blocks, so-called ingots, wherein the sawing is done by means of wire saws, in which a plurality of parallel, closely spaced wires are moved along their longitudinal direction over the material to be sawn at a sufficiently high speed. Such a wire saw is for example in the US A 2006/0249134 described.

This separation, usually referred to as sawing, is actually a separating lapping in a first variant of the process control. This cutting or separating tools, d. H. the saw wires, at least at the respective cutting point (cutting front) brought into contact with a fluid cooling or Trennslurry containing Abrasions- or grinding or sawing particles, which create a cutting gap on the cutting front by material removal and so by deepening des- cutting gap cause the dividing or separating the individual wafer slices. In such sawing processes, different slurries or slurries (so-called slurries) are used as the cutting or sawing liquid, which slurries are made of a liquid component serving as a carrier, such as, for example, slurries. As oil or different glycols, and a so-called. Loose grain as abrasive medium, such as. As silicon carbide, are composed, and in which the pure liquid component usually additionally contains additives, the z. B. prevent premature settling of the abrasive particles. A consumed cutting or sawing liquid (slurry) arising in this separation lapping process consists of the pure liquid used, the abrasive used, chipped or abraded Si material (Si abrasion) and metallic wire abrasion as well as possibly added additives.

In another variant of the process comes as a sawing wire with hard abrasive particles such. B. diamond saw wire used. The adhered to the surface of the wire abrasive particles, so-called. Bound grain cause a grinding process through which also forms a cutting gap.

The consumed cutting fluid resulting from this wire cutting grinding (such as the Diamond Wire Cutting (DWC) process) consists of the used cooling lubricant, abraded Si material (Si abrasion), few broken abrasive grains, and a very small amount of metallic wire abrasion , An overview of the different types of wafer orders is available in http://en.wikibooks.org/wiki/Silicon_processing__Wafer_setting described.

Due to the increasing demand for electronic components, in particular wafers for photovoltaic elements, microelectronic components, as well as for light-emitting diodes (LEDs) are increasingly larger amounts of so consumed slurries of which previously only individual components can be recycled and a currently not recyclable remnant consuming must be disposed of. Many attempts have therefore already been made to recycle slurries consumed in this way and to reuse their constituents or to supply them with another technically useful use.

So describes z. B. the DE 10 2007 048 879 A1 a method of recycling such sawing fluids, wherein abrasive particles and silicon material removed (silicon abrasion) are removed by filtration through a core-tracking filter. However, such isolated fluids still contain very fine abrasive particles and silicon particles. If such fluids freed of spent abrasive particles and larger chipped silicon particles are reused, then wafers having a particularly smooth and even surface can be obtained.

Typical process steps in the processing of spent Sägeslurries or the recovery of the fluids contained therein and large abrasive particles include collecting or circling spent slurries from the sawing process or the Trennläppen, especially the Drahttrennläppen, and separating the spent slurry into a fluid and a solids fraction ( eg by means of a cyclone or a filter). Since a liquid cyclone has a limited selectivity, only the reusable coarse-grained abrasive is separated in the first stage (1st cyclone), and in the second stage (2nd cyclone), the reusable fluid is now separated from the remainder of the first stage , In the second stage, a residual sludge remains, which is either landfilled or further processed for use in the steel industry. Such a procedure is described in the US 6,929,537 B2 described, in which such a residual sludge is introduced into containers. If the carrier liquid contains glycols and if the recycled slurry constituents contain or contain residues from the wire sawing of semiconductor wafers, metallic impurities can be separated with the method described there, since these would cause rejects in the process of microchip production. This leaching said method is z. B. in the EP 1 893 385 described. Also in this method, a filtration residue of filtrate or microfiltration consisting of fine SiC particles, fine Si particles and metal particles is obtained, which can be typically disposed of or used only for inferior purposes.

Used cutting fluid from the Trennläppprozess consists of the pure liquid component (eg., Polyethylene glycol), which may optionally contain other additives, Abrasivteilchen (eg SiC particles), strained silicon material and metallic abrasion from the saw wire. Used cutting fluid from the cut-off process (eg Diamond Wire Cutting Process DWC) consists of the pure cutting fluid (eg, additized water or a polyethylene glycol), swept silicon material, abrasive particles erupted from the surface of the saw wire, and metal abrasion.

In general, such residual sludge or filter residues consist of mixtures of the pure liquid component with a remainder of fine abrasive particles used for cutting or Trennläppen abrasive particles such. As silicon carbide or diamond, fragments thereof, abrasive particles which have chipped from the saw wire (eg., In the DWC process), and silicon particles, which are caused by sawing a Siliziumbricks or -ingots. Optionally, small amounts of detached metal particles of the saw wire are also included. Both the silicon contained in the residual sludge or the filter residue, the sawing or separating particles or flaking of these and the liquid are by themselves quite useful, industrially utilizable materials, provided that they are present in a sufficient purity separately.

The solids contained in the cutting fluid include in addition to Si and metal particles of saw wire especially loose d. H. unbound abrasive particle (loose grain) and abrasive particles broken off by saw wire (detached, bound grain). Typical abrasive particles from the separation lapping, as well as abrasive particles from wire cutting grinding include SiC, as well as diamond.

Such residual sludge or filter residue usually has a solids content of at least 50% by weight, in particular at least 60% by weight, whereby residual sludges having a solids content of at least 70, in particular at least 80%, are obtained due to the small residual proportion of fluid , The solids content consists of 70-95% sawing or abrasive particles (eg SiC), 5-30 or 29% silicon abrasion and up to 3% saw wire abrasion.

One of the biggest problems currently encountered in using such saw slurries is their incomplete recyclability. In particular, the residual sludge is discarded, as in the US 6,929,537 is described. So far, the viscous carrier liquid, usually an oil, is separated from the remaining residual sludge by the extractive and abrasive materials and the abraded material for the most part by extraction, for the most part after prior filtration or centrifugation. For extraction, organic solvents are used in particular. These organic solvents then have to be separated from the oil in a further step. The separation of oils and organic solvents, however, is very costly, since for this separation thermal processes such as rectification, distillation, evaporation or a combination of the aforementioned methods are carried out. Such separation processes are carried out in large distillation plants, which are classified as particularly dangerous in the highest explosion hazard classes. The construction and operation of such large facilities is extremely expensive, so that only a few of such facilities exist and the slurries or liquid mixtures to be processed across Europe must be transported, whereby the already high overall costs are further increased by the high-risk transport of these highly combustible substances. Therefore, many attempts have been made to solve this problem.

Furthermore, for example, describes the EP 0 791 385 A1 a method for recovering a slurry in the sawing of ingots in semiconductor manufacturing, in which first the viscosity of the carrier or cooling liquid contained therein is reduced by adding a solvent and then the abrasive grains contained in the slurry are separated. Thereafter, the water-soluble cooling fluid is extracted from the residual liquid. Subsequently, the remaining finely divided Si-containing suspended solids are discarded and the separated abrasive grains and the extracted water-soluble coolant used again.

In the EP 0 147 541 A2 is a process for the purification of circulating fluids in the fine and ultra-fine machining of materials, such as For example, described during glass processing. The used liquids or slurries are enriched with the finest abrasion. In order to regenerate such a liquid for reuse, it is proposed to add a liquid or dry flocculant, whereby the fine dust introduced by abrasion may clump and then be separated by means of a simple filter.

Furthermore, the describes US 6,231,628 B1 a method in which the viscosity of the liquid is reduced in spent slurries, such as those produced when silicon ingots are cut for silicon wafers, and the solid components are subsequently separated from the liquid. In this case, the separated lubricant liquid is used without a further separation step. For the separation itself, in addition to the use of Cyclonseparatoren, sedimentation centrifuges and filtration centrifuges and the use of a filter is proposed.

From the DE-A 19507584 a method for the separation of low molecular weight compounds from organic solvents is known in which by means of a modified silicone membrane, which is applied to microporous support membranes, the low molecular weight fractions are separated by nanofiltration on such a membrane. However, it has been found that the use of such nanomembranes for recycling such separation media or separation fluids is not possible, since these still after extraction or dilution with an organic solvent and after separation of the solid particles contained still, albeit to a small extent residues of minute suspended particles contains, which destroy such a membrane immediately.

In the DE 10 2007 033 258 A The same applicant also describes a method for reprocessing spent release media for reuse in which the solids are inexpensively separated from the oily viscous carrier liquid by means of a ceramic membrane. However, such Sägeslurries usually contain other, also expensive additives such as viscosity modifying agents such. B. thixotropic agents, which are also separated in the aforementioned methods. In this case, spent slurries are first diluted by means of a solvent, and after separation of the solids, the dilute carrier fluid thus obtained is separated from the diluent by means of a ceramic nanofiltration membrane.

The WO 2009/084068 A1 describes a process by which spent slurries from the Trennläppverfahren, used cutting fluid from the diamond wire cutting process or slurry residual sludge or residual sludge from the DWC process are recycled so that the cutting fluid is almost completely recovered and the second treatment product is a dry Powder is formed. This dry powder is either a mixture of SiC, Si and metal abrasion or a mixture of Si, diamond and metal abrasion. The separation of the powder into its chemical constituents and a classification into desired particle size fractions are not the subject of this patent application.

However, it has been shown that with all the processes of the prior art, the dissolved or finely suspended additives present in the sawing liquid are likewise separated off, so that they are either extracted from the retentate or the filter in a complicated process or discarded with them Need to become. Furthermore, the silicon abrasion contained in the used sawing liquid, the residual sludge or the filter residue has not yet been available for recycling.

The invention therefore has the task of reprocessing used sawing or cutting fluids both saw slurries and spent sawing or cutting fluid from wire cutting grinding (such as from the DWC process) in such a way that both the liquid phase, as well as the solid phase, in particular the abrasive or abrasive particles contained therein (loose or formerly bound grain) and the Si-abrasion in a sufficiently high purity can be separated from each other so that they can be reused industrially and as a pure product and not must be disposed of costly.

This object is achieved by the features defined in the claims.

It has now surprisingly been found that in the used sawing or cutting fluid from the Trennläppprozess (Sägeslurry) or in the used sawing or cutting fluid of the wire cutting grinding present mixtures of solid components, can be easily separated from each other when the used saw or cutting fluid with a liquid suspension medium solution or suspension fluid containing alcohol, fatty acids and / or ionic surfactants as suspending agent is suspended. In an alternative embodiment of the invention, in addition to or instead of the alcohol, the fluid may contain other liquids of lower density than the pure cutting fluid, such as, for example, As water and / or other alcohols. Alcohols which can be used according to the invention include monoalcohol, in particular an alkyl alcohol, specifically methanol or C ₂ -C ₅ -alcohol, such as ethanol, propanol, etc. It has been found that in such a suspension medium, the silicon particles can be suspended and thus remain floating in the liquid phase, whereas the grinding or sawing particles , such as B. SiC or diamond sediment rapidly. The terms sawing fluid, sawing slurry and cutting fluid are used synonymously below.

In this way it is therefore possible, silicon particles of grinding, sawing and cutting articles such. B. silicon carbide particles or diamond particles to separate. In principle it is possible to carry out the separation by means of simple sedimentation, but it is preferred to accelerate the sedimentation by centrifugation.

Preferred suspending agents are methanol, fatty acids, in particular C ₁ -C ₅ -, preferably C ₁ -C ₃ -fatty acids, ionic surfactants, both cationic and preferably anionic surfactants, and soaps. It has been found that it is expedient to add the suspending agents in an amount sufficient to ensure that all possible silicon particles are kept in suspension. It is expedient to use it in excess, based on the weight of the solid particles, in particular the silicon particles.

When an alcohol, especially methanol, is used as the suspending agent, it has been found desirable to use it in at least twice the amount (by weight) based on the total weight of the spent sawing liquid or sludge to be treated. At least three times the amount by weight is particularly preferred, with at least four times being particularly preferred.

The suspension can easily be produced by conventional methods such as, for example, stirring, ultrasound or by means of an electric field.

In a preferred form of the invention, only the residual sludge is treated further by the sawing liquid. Residual sludge occurs when the used or used cutting fluid from the Trennschleifprozess already pure liquid was withdrawn, that is residual sludge is in this case a thickened used cutting fluid. Residual sludge also arises when the used cutting fluid (used slurry) from the separation leach process has been removed from a first recycling step reusable large abrasive particles of typically greater than 4 or greater than 5 microns in diameter and reusable pure liquid. As already described above, residual sludge has a solids content of more than 50% by mass. Whether this residual sludge is the result of centrifugation or filtration does not matter.

In this preferred embodiment, this residual sludge or the filter residue prior to the addition of the suspending solution metallic admixtures such. As iron, copper, nickel, manganese, chromium, etc. withdrawn. Such metal particles arise z. B. by abrasion of the metal saw, especially the saw wire. This can be done by means of magnetic deposition, a deposition as in EP 1 893 385 for the purification of the SiC by means of oxalic acid + H ₂ SO ₄ + HCC or only with oxalic acid. The separation of the metallic admixtures by the addition of oxalic acid is carried out with the aim that form metal oxalates and these are then washed out. In the latter case, it is a particularly preferred embodiment when the suspending solution has a slightly acidic pH. It has been shown that by separating any other metal particles, such as iron, the suspension of the pure silicon or the sedimentation of silicon carbide and thus the separation is improved. The complexing agent is preferably added to the solid particles in an amount of at least 0.1%, preferably of at least 0.5% by weight, based on the solids content. In individual cases, however, it may be expedient to add at least 1 or at least 3% by weight or at least 5% by weight, if appropriate at least 10% by weight. After oxalates have formed, they must be washed out. Suitable solvents are any in which oxalates are soluble. If PEG has been used as the liquid component of the sawing liquid, PEG is of course preferred as the solvent. Water, lye or other glycols are also suitable. Thereafter, the solid particulate mixtures, such as either the mixture of silicon particles and silicon carbide particles from the separation leach process, or the mixture of silicon particles and chipped bonded grain from the abrasive cut process, are separated from the fluid, such as by filtration or centrifugation.

In a preferred embodiment, the suspension medium has a slightly acidic pH, preferably from 4 to 6.5, with minimum values of 4.5, in particular of 5 being preferred. Preferably, the maximum pH is not more than 6.

In a further preferred embodiment, the used optionally further treated sawing liquid (for example by thickening or leaching) for producing the suspension additionally contains a carrier fluid in addition to the suspending agent. Typical carrier fluids are water and / or the Components of the liquid components from the sawing liquid. Conveniently, therefore, the suspension medium contains at least 10% by weight of the liquid component or partial component thereof used in the sawing liquid, but preferably at least 15% by weight, with at least 20% by weight being particularly preferred. Very particular preference is given to amounts of at least 25% by weight.

Typical Sägeflüssigkeiten are water, glycols such. As polyethylene glycol or polypropylene glycol and water-based, additierte cooling lubricants. If the sawing fluid (cutting fluid) contains no glycols, these can be added to the suspending fluid.

In a preferred embodiment according to the invention, the liquid suspension medium has a density between 1.55 and 1.8 g / cm ³ . Preferably, the minimum density is 1.6, especially 1.63. Preferred maximum densities are 1.75, especially 1.7.

It has proven expedient to remove a large part of the liquid component of the used sawing liquid by means of conventional sedimentation and / or filtering techniques before suspending the silicon. It is expedient to remove at least 50% by weight of the pure liquid, but preferably at least 70% by weight. It is particularly preferred to remove at least 80 or 90% by weight of the fluid present in the cutting fluid.

Moreover, it has proven expedient, after removing the at least larger part of the cutting fluid from the Si and abrasive particles, to add to the sediment or filtrate a viscosifying agent, such as a glycol, for example polyethylene glycol, so that the resulting slurry contains at least 20, in particular at least 30 wt .-% of this agent. It is particularly preferable to add this agent in an amount containing a slurry of 40 to 60% by weight (based on the liquid) of this viscosity agent. Only then is the suspending agent preferably added in excess (based on the weight of the particles). Typical glycols are diethylene glycol (DEG), diethylene glycol monomethyl ether (DEGME), ethylene glycol (EG), triethylene glycol (TEG), propylene glycol (PEG), polyethylene glycol (PEG), especially PEG-200 and PEG-300, and dipropylene glycol (DPG).

The abrasive particles such as SiC and diamond tend to sediment quickly. Centrifugation can accelerate this sedimentation process. In the case of the suspension remaining in the supernatant, only the suspended silicon abrasion remains as a solid. This Si is z. B. separated by filtration and has a high purity. The permeate contained in the filtration can be separated by means of typical distillation or rectification. In this way it is possible, for example, not only the suspending agent such. As methanol but also z. B. an optionally contained the carrier fluid of the suspension medium such. B. PEG with a water content of less than 0.1%.

The inventive method is particularly suitable for the recovery of pure silicon. Such purified silicon is particularly suitable for further processing into electronic components.

According to the inventive method, it is possible to obtain silicon carbide in a purity of at least 95%, in particular at least 96%, with purities of at least 98 or 99% being readily achieved. The thus recovered silicon particles have a purity of at least 98, especially at least 99%. Such recycled silicon carbide as well as silicon can be readily used for metallurgical processes and thus constitutes an important utilizable raw material. In addition, the resulting silicon is pure enough to be optionally used after further purification as a solar grade silicon for the growth of multicrystalline solar ingots can.

The invention is characterized by the 1 exemplified. 1 shows a technological scheme of the recycling of recyclables according to the invention cutting fluid, SiC abrasive and silicon from previously discarded Slurryrestschlämmen, DWC residual sludge or filter residues such residual sludge on the example of recycling Slurryrestschlamm.

Residual sludge is obtained after reusable large-grain abrasive from spent cutting fluid and a portion of the cutting fluid, e.g. As glycol, mostly by two cascaded centrifuges were separated. Residual sludge is also obtained after, from spent cutting fluid from the DWC (Diamond Wire Cutting) process, the solid contained therein, such as Si abrasion and abrasive grains agglomerated by the wire, e.g. B. was separated by centrifugation.

Residual sludge, which is usually stored in a tank, is filtered in a first step for further separation of carrier fluid, usually PEG. (This step is advantageous but not mandatory). This results in a concentrated residual sludge, which is about 70% Solids and 30% of the fluid (PEG). The residual sludge solids consist of ultrapure silicon, silicon carbide (only in the case of residual sludge from spent slurry), chipped abrasive grains (DWC only), and metal abrasion.

To remove the metal abrasion is in step 2 this residual sludge is treated with oxalic acid at a pH of 5 to 6. After further addition of PEGs, alkalis or other oxalate solvent such. B. Water, the mixture is in step 4 subjected to a further filtration. The resulting liquid filtrate is freed from the water and the oxalic acid and can then the step 3 be fed again. The residual sludge recovered in the last filtration is now in step 5 according to the invention with a fluid, in particular with as much PEG, added until the mixture contains 50% solids and about 50% fluid. The resulting pumpable sludge is in the step 6 then mixed with four times the amount (based on the total sludge) with methanol. The mixture of silicon abrasion, abrasive particles and liquid, which in this example consists of PEG and methanol, is suspended in such a way that the silicon abrasion is suspended and the sedimentation-prone abrasive particles are separated by sedimentation.

This sedimentation takes place in step 7 by centrifugation. The remaining in the supernatant remaining silicon abrasion is then in the step 8th separated from the suspension fluid. This is done z. B. via a filter z. B. via a chamber filter press, which receives the silicon particles back. The liquid filtrate obtained through the chamber filter press is removed by means of distillation (step 9 ) into its constituents, here methanol and PEG, decomposed, which can be used accordingly again. The sediment of centrifugation (step 7 ) contains essentially SiC as well as small amounts of Si. This is therefore again mixed with methanol and a second centrifugation (step 10 ). Thereafter, as in the first centrifugation, the supernatant is passed through a chamber filter press to obtain the silicon and the fluid filtrate is treated accordingly. The resulting Si is very pure and can be used not only in metallurgy but also for the cultivation of Si ingots for photovoltaic applications after a further purification process. After filtration of the mixture of the 2nd centrifugation (step 11 ) becomes either pure SiC for use in metallurgy or diamond granules are obtained for reuse.

The invention will be explained in more detail by the following examples:
100 g of a residual sludge are subjected to a filtration, from which the cutting fluid contained in the residual sludge, in this specific case PEG 300, was separated off. The filter cake thus obtained was admixed with 2.5 g of oxalic acid. There were implemented in the filter cake metal impurities in oxalates. These oxalates were dissolved with 500 g of PEG 300. Filtration was used to separate the added PEG 300, including the oxalates dissolved therein. Thereafter, 150 g of PEG 300 was added to the filter cake again to obtain a flowable mixture consisting of about 50% PEG and about 50% Si and SiC particles. To this mixture was added 4 times the mass of methanol, and the mixture was stirred for 120 minutes. From the resulting suspension, the SiC already settles by gravity even without additional force. However, this process was accelerated by centrifuging the suspension formed by stirring. The sediment was 99% SiC. In the overflow of the centrifuge was a suspension of PEG, methanol and Si particles. This overflow was filtered. The filter cake was washed with acetic acid for analysis and analyzed. It was found to be 99% Si. The permeate of the filtration was separated by distillation into its components PEG and methanol, which are thus available for reuse.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2006/0249134 A [0002]
• DE 102007048879 A1 [0007]
• US 6929537 B2 [0008]
• EP 1893385 [0008, 0029]
• US 6929537 [0013]
• EP 0791385 A1 [0014]
• EP 0147541 A2 [0015]
• US 6231628 B1 [0016]
• DE 19507584 A [0017]
• DE 102007033258 A [0018]
• WO 2009/084068 A1 [0019]

Cited non-patent literature

• http://en.wikibooks.org/wiki/Silicon_processing__Wafer_setting [0005]

Claims (10)

A method for recycling sawing liquid consumed in the production of silicon wafers by sawing silicon blocks by wire-cutting and / or wire-cutting, comprising at least one liquid and solid particles, wherein the solid particles are silicon abrasion, abrasive particles used for sawing, and fragments thereof, abrasion from a saw wire used for sawing , and optionally additional auxiliaries, comprise, by separating at least part of the liquid from the solid particles, characterized in that the mixture of silicon abrasives, abrasive particles and liquid by suspending in a liquid suspension medium containing at least one compound selected from alcohol, fatty acids and ionic surfactants such is suspended, that the silicon abrasion is kept in suspension and the sedimentation tending abrasive particles are separated by sedimentation. A method according to claim 1, characterized in that metal particles contained are removed by means of an oxidizing and / or complexing treatment. Method according to one of the preceding claims, characterized in that the removal of metal particles by means of oxalate and / or a magnet is performed. Method according to one of the preceding claims, characterized in that for suspending the suspension fluid in excess, based on the weight of the solid particles, is added. Method according to one of the preceding claims, characterized in that after separation of the sedimentation-prone abrasive particles by sedimentation, the remaining in the supernatant in suspension silicon abrasion separates from the fluid suspension. Method according to one of the preceding claims, characterized in that the suspension fluid contains at least 20 wt .-% polyethylene glycol. Method according to one of the preceding claims, characterized in that the suspension fluid is an aqueous fluid. Method according to one of the preceding claims, characterized in that the fluid of the sawing sludge and / or the suspension fluid is separated by separation of the solid particles by distillation into its constituents. Method according to one of the preceding claims, characterized in that the sedimentation is carried out by centrifugation. Use of the method according to any one of claims 1-9 for the recovery of pure silicon.

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