WO2009053007A1 - Method and device for recovering a mixture of a thixotropic dispersant and abrasive grains as abrasives - Google Patents

Abstract

The invention relates to a method and a device for recovering a mixture of a dispersant and abrasive grains as abrasives. The mixture is used for machining, especially for separating by machining, workpieces, preferably semiconductor crystals. The invention is characterized in that a thixotropic and substantially water-based dispersant is used.

Description

 Method and apparatus for treating a mixture of a thixotropic dispersion medium and abrasive grains as abrasives

Description:

The invention relates to a process for the preparation or separation of grains from a mixture of a dispersion medium and the abrasive particles concerned in the course of their use as abrasive for chip removal, in particular for the chip-removing separation of workpieces, preferably semiconductor crystals. The core therefore is to separate the abrasive grains from the mixture. These grains act or act as a whole the mixture as an abrasive in chip removal machining.

In the processing of semiconductor crystals, in particular silicon, and the chip-removing separation of such crystals into wafers, wire saws or wire machines are generally used, which use a mixture (slurry) of the dispersion medium and the abrasive grains as abrasive , With the aid of this mixture, the cutting process is made possible only because the particles which are freely movable and abrasive in the mixture cause a lapping process associated therewith (see DE 195 38 612 A1).

In the prior art, a dispersion medium which consists of organic components, namely a hydrophilic polyhydric alcohol compound, a lipophilic polyhydric alcohol compound and water, is generally used. In the dispersion medium, colloidal silica particles prepared from a silicate are dispersed, as described in detail in DE 699 11 549 T2.

In addition, the subject matter of WO 2006/137098 A1 is a method for the reuse of such abrasively used in the lapping process. kender grains. For this purpose, hydrocyclones or centrifuges are used, which is relatively expensive. This is where the invention starts.

The invention is based on the technical problem of further developing such a method so that the procedural complexity is reduced and the costs of an associated device are minimized.

To solve this technical problem is in a generic method for preparing a mixture of a dispersion medium and abrasive Kommer in the course of their (the grains) use as abrasive for chip removal, especially for chip removal of workpieces, preferably semiconductor crystals, provided that as a dispersion medium a thixotropic and substantially water-based dispersion medium is used.

Thus, according to the invention, the dispersion medium used is based primarily on water as the dispersion medium, which as a rule represents a proportion of more than 85% by weight in the dispersion medium. It is furthermore of particular importance that the dispersion medium has a thixotropic behavior, ie a viscosity or viscosity which changes depending on the mechanical loading of the dispersion medium. In this case, the viscosity of the thixotropic and substantially water-based dispersion medium according to the invention is generally adjusted so that the concentration of colloids or colloidal particles is dimensioned in the dispersion medium so that sedimentieren at dormant dispersion medium, the abrasive grains of desired grain size for their separation, whereas mostly Fine components are retained in the dispersion medium.

The thixotropic dispersion medium and the abrasive grains as abrasives form the overall mixture, which is used for chip-removing machining of workpieces and in particular for chip-removing separation of semiconductor crystals. These semiconductor crystals are preferably rod-shaped or rod-shaped silicon monocrystals, which in the course of the chip-removing separation into slices desired size and previously set thickness (wafer) to be separated. In addition, of course, workpieces made of polysilicon can be processed. Likewise those of other semiconductor materials, such as gallium arsenide, gallium indium phosphite, etc ..

As a rule, the dispersion medium with the abrasive grains dispersed therein, ie the abovementioned mixture, is used for non-forming chip-removing machining. This refers to the previously mentioned chip-removing separation processes, which include, for example, wire sawing, wire cutting, etc. of the relevant workpiece. This workpiece is, as already described and preferred, a semiconductor crystal, which is subdivided with the aid of the mixture in a corresponding chip-removing separation process in the desired thickness wafers (wafers), which are then usually still polished to order them for further process steps prepare.

The invention has surprisingly recognized that a thixotropic and substantially water-based dispersion medium known per se can advantageously be used with the grains dispersed therein as an additional phase and having abrasives as abrasives for the described chip-removing machining of workpieces, in particular of semiconductor crystals. The thixotropic effect of the dispersion medium for the described application is of particular importance. For the dispersion medium has, according to an advantageous embodiment, charged colloidal particles. These charged colloidal particles form networks with sufficient concentration in the dispersion medium to each other and in the best case an elastic gel, which carries the abrasive grains permanently in itself. As a result, sedimentation of the abrasive grains during normal operation or chip removal processing is prevented and any adverse effects associated therewith can be prevented within the scope of the invention. That is, the transport of the mixture through a usually used separation machine (wire saw machine) is not affected by any sedimentation hindered, so that the processing is particularly simple and reliable.

At the same time, however, the strength of the bond between the individual colloidal particles is low, so that at sufficiently high shear rates, the network can be reversibly resolved again. As a consequence of this, the suspension again becomes less viscous, or the viscosity decreases with a mechanical shear stress, so that overall the thixotropic behavior is explained. The colloidal particles in the dispersion medium are smectites, ie layer silicates with a three-layer structure, which are preferably used. These are usually suspended in water as a dispersant. In this context, a high yield strength of the dispersant is crucial, with low viscosity under shear stress. This is basically known from DE 1 023 732.

In contrast to this prior publication, however, according to the invention, the buoyancy of the particles to be sedimented is not changed by increasing the density of the dispersion medium in the mixture. Rather, the density and also the density of the dispersion medium remain essentially the same and the gel strength is merely adjusted by the addition of more or less colloidal particles. In fact, the gel strength is a measure of the gelling ability of the gel, which can be determined using, for example, a Bousher gelling tester. The gel strength is generally measured in g / cm ² or N / m ² or in Pa or mPa. In any case, according to the invention, the gel strength is varied in order to selectively retain or sediment the abrasive grains in the gel which forms the abrasive grains, as will be explained in more detail below.

It has generally been proven that the dispersion medium about 1 wt .-% to 10 wt .-%, in particular about 1 wt .-% to 5 wt .-% and preferably about 2 wt .-% to 3 wt. -% of a clay mineral dissolved in water. That is, the dispersion medium is composed predominantly of water as the dispersant and the already mentioned clay mineral in the indicated concentration. The proportion of water is the dispersion medium usually more than 85% by weight. The clay minerals used are preferably smectite-containing clays, for example bentonite, but also hectorite. In addition, other smectites such as corrensite, rectorite, saponite, stevensite, etc. are conceivable. These are known for the described thixotropic behavior. Basically both synthetic and natural clay minerals can be used.

Certain starches as well as organic polymers can advantageously be dissolved in water and used as the dispersion medium according to the invention. Furthermore, in addition to clay minerals also acicular chain silicates such as sepiolite are able to form thixotropic aqueous suspensions. However, these require very high solids content and the yield point is less well compared to the viscosity. As a result, not only clay mineral colloidal particles are encompassed by the invention.

In detail, the thixotropic behavior of the clay mineral, which is generally dissolved in the water, is brought about by the formation of bridges between the individual dissolved particles or smectites in such a clay mineral suspension without movement. These bridges represent the already mentioned network, which carries the abrasive grains dispersed in the dispersion medium and prevents their sedimentation. This is achieved particularly simply by equipping the abrasive grains with particle sizes below 100 .mu.m, preferably less than 50 .mu.m and preferably below 20 .mu.m.

Only when this mixture or generally the described dispersion medium is subjected to a shearing motion, the bridges described break. As a rule, for example, 1 kg of the abrasive grains is mixed with 1 l of the dispersion medium, enough grains are available within the mixture, so that at slight microscopic shear rates, the addressed bridges break open and the viscosity of the mixture decreases. However, this is generally not a problem, because a corresponding liquid movement of the mixture is associated with a mixture transport, which is either in the desired Span- eroding processing or thereafter in a reprocessing or reuse of the mixture.

It is particularly preferred if the abrasive grains are dispersed in the thixotropic dispersion medium, for example, with an average grain diameter of less than 100 μm, in particular below 50 μm, and preferably in the range of approximately 20 μm. Because such grain sizes can be easily record and hold in the described network.

In addition, the invention recommends in the dispersion medium in addition to the dispersant water as an additive, the clay mineral in low grain size with

Use grain sizes of significantly less than 500 microns.

In a further preferred embodiment of the method according to the invention, alternatively or additionally, Na and / or Li bentonites dissolved in water for the gelation can be used. Particularly preferred synthetically produced hectorite has proven to be favorable, which although relatively expensive, but shows a very good thixotropic gelation.

In general, the grain size of the clay mineral used is less than 200 microns and is more preferably below 100 microns and usually even below 50 microns or even below 20 microns. In this way, overall a relatively low viscosity of the mixture of the abrasive grains and the dispersion medium of water and the clay mineral additive is provided. In fact, inventively viscosities of mostly significantly less than 1 Pas (1 pascal-second) are observed. The viscosity is thus always below that of, for example, glycerol (1, 5 Pas).

It is particularly preferred if the abovementioned described dynamic viscosity of the mixture is in the range of less than 500 mPas and is preferably below 400 mPas. Mostly, a range between 30 and 350 mPas is observed depending on the proportion of abrasive grains for the mixture. As a result, low viscosities are available, which in particular for the production of slices of silicon single crystals (Silicon wafers) are particularly preferred. Because the mixture in question is usually used to rinse a wire, which cuts through the said silicon single crystal or generally the semiconductor crystal. It is usually worked with a low feed rate of about 0.1 mm / min, and set the cutting width in the range of below 0.2 mm. An example of such a separating device is described in DE 698 24 655 T2.

The method according to the invention is particularly suitable for first employing abrasively acting grains based on silicon carbide, boron carbide and / or diamond for the chip-removing machining and subsequently separating them from the mixture. This applies in particular to the use of an abrasive produced in such a way or a relevant mixture in connection with the wire sawing in the processing of mono- or polycrystalline silicon. But even quartz, glass, ceramics, etc. can be processed in this way chip removal and in particular be subjected to a chip-removing separation.

Due to the low viscosity of the mixture according to the invention, there is no formation of so-called "taper" in the preceding chip-removing machining. Because the mixture settles due to their low viscosity with virtually uniform liquid thickness around the wire in its entire length and it is virtually impossible or hardly to the fact that the mixture at the beginning of the cutting process jams and is no longer sufficiently available towards the end of the cutting process. This means that the risk of a taper forming in the saw joint is significantly reduced. All the more so, as it is recommended to lead the wire or generally the cutting tool with an alternating cutting direction through the semiconductor crystal to be separated.

The fact that the described "wedge effect" (taper) is reduced within the Sägefuge and at the same time a better material transport takes place at this point due to the reduced viscosity, also the roughness of the produced slice (silicon wafer) is compared to previous approaches ver Ringert. As a result can be dispensed with elaborate post-processing for polishing partially or these reworking measures are much less expensive than in the prior art, which works with the previously mentioned organic dispersion media.

In this case, the mixture according to the invention and used can be processed very easily. Here, the invention is based on the recognition that generally sedimentation processes, including the sedimentation of abrasive grains, obeys Stokes's law, according to which the rate of descent in a liquid (the dispersion medium) depends on the grain diameter, the density difference between the grain and the liquid and the viscosity of the liquid is dependent. By changing (increasing) the viscosity of large and heavy abrasive grains, their sinking speed can be reduced.

If the dispersion medium according to the invention is not subjected to shear stress or mechanical loading, the already mentioned gel is formed. Such a gel is an E-modulus solid. The modulus of elasticity of the gel of the thixotropic and substantially water-based dispersion medium depends on the number and strength of the bonds of its colloids per unit volume. Thus, the type and concentration of the colloid in the water is responsible for the number and strength of the bonds and thus the modulus of elasticity. This modulus represents or is synonymous with the gel strength already referred to in the introduction. The modulus of elasticity or the gel strength is time-dependent, because the formation of the network from the colloids is time-dependent, as the following example makes clear:

In Fig. 1 the time course of gel formation according to Fann for a 2.5 wt .-% aqueous bentonite solution with the brand name "tribomont" is shown. Here, the Y-axis represents the gel strength in divisions, where the respective Y-value multiplied by 0.48 indicates the gel strength in Pa. On the X-axis, on the other hand, the time in seconds is removed. Only when shear stresses or general mechanical forces act on the gel, the network described above breaks and then larger volumes of the gel can be shifted against each other, which then results in a (previously not observed, of course) certain viscosity of the gel.

Example 1: Adjustment of the carrying capacity of a gel

The load on a gel caused by a particle, in the present case the abrasive grains, is proportional to the quotient of the weight and the projection area of the particle. Since the volume of the respective particle (s) in an assumed spherical appearance increases with the cube of the radius, but the projection surface only increases with the square of the radius, a large particle (abrasive grain) in a given gel becomes more likely than sink a small sphere of equal density.

To determine the carrying capacity of a gel, usually the gel point and the gel strength are determined. For this purpose, the pendulum devices according to WEISS (DIN 4127) and the ball harp according to SOOS (DIN V 4126-100, Section 6.1.2 ff.) Are available as test devices.

The ball harp according to SOOS consists of spheres of the same density with different radii, which are connected via threads of different lengths to a frame, which is lowered into the gel to be examined. One then determines which sphere radius no longer sinks into the gel. This spherical radius of known density allows a statement about the bearing capacity of the gel and consequently both the gel point and the gel strength.

With the so-called Fann viscometer, the gel strength of a thixotropic suspension or of the water-based thixotropic dispersion medium according to the invention can be determined. It makes use of the fact that a gel can transmit shear forces. So if the viscous Simeter between two nested cylinders with a distance x a gel, so at the beginning of a slow rotational movement of the cylinder to another torque is transmitted, which can be measured. The gel strength is given in the already indicated scale parts, the scale parts being converted into the gel strength with the unit Pa by multiplying the scale values by the factor 0.48, as already explained with reference to FIG.

As already stated, the gel strength depends on the time in which the gel is not subjected to any shear stress. In the context of FIG. 1, 25 g

Tribomont (bentonite) in one liter of deionized water at 3000 rpm per

Minute dispersed for 30 minutes. The suspension was allowed to stand for 24 hours

25 ° C allowed to swell. The time course of the gel strength is in the said

Fig. 1 shown. Already after about 10 sec. Resting time showed the aqueous suspension or the thus prepared thixotropic and in

Substantially water-based dispersion medium has a shear modulus of about one Pa. So that a particle can sink in this gel, must therefore

Pressure below the particle exceeds a Pa. If the applied pressure is lower, the particle (the abrasive grain in the invention) remains permanently embedded in the gel, it floats.

In order to separate the abrasive or the abrasive grains from the mixture in the preparation or separation process according to the invention, ideally no mechanical shear stresses are exerted on the gel, but this acts almost exclusively as an elastic gel. As a consequence, sedimentation occurs in a solid. In sedimentation, only the grains of the "right" grain size exert a shear stress on the gel, breaking the bond and causing said grain to slip through until the next bond, etc. Smaller grains, on the other hand, are retained in the gel. It is immediately apparent that the rate of descent is very low, whereby the sinking movement thereby induces no turbulence and thus shear stress in the gel. Once sedimented in this way sedimented particle or the abrasive grain, the bond is formed in the gel again. This basic relationship is shown in the following Fig. 2.

In the context of FIG. 2, the solid line generally shows the sedimentation behavior of particles in a liquid. It is on the Y-

Each axis the rate of descent v of the particle in question and on the

X-axis be represented radius r. As expected, there is no fixed point in a liquid in which the particle in question remains in suspension. Without the ever observable Brownian motion, all particles eventually sink.

The dashed line in FIG. 2 now shows the sedimentation behavior of particles in a gel in general and in particular the sedimentation behavior of the abrasive grains in the thixotropic and substantially water-based dispersion medium according to the invention, which rests and has formed the mentioned gel. It can be seen that the rate of descent v decreases to zero, depending on the gel strength and the particle below a critical grain size r. Smaller particles thus remain permanently in suspension (sinking speed = zero).

When resorting to abrasive grains having a particle size below 100 microns, preferably having a diameter of less than 50 microns, has a viscosity of the mixture of less than 1 Pas, in particular less than 500 mPas and preferably in the range of 30 to 350 mPas proved to be particularly favorable depending on the proportion of the grains in question (measured after two minutes with a Brookfield viscometer). These design rules apply in any case for the approach of the mixture in the course of its use in the subsequent wire cutting or wire sawing.

If the previously briefly characterized mixture is to be treated after use or the abrasive grains are to be separated from the mixture, the dispersion medium is diluted with water as a rule until a sedimentation of the abrasive effect occurs Grains is effected, namely at a suitable gel strength of the resulting from the thus prepared dispersion medium gel (see Fig. 2). In this case, the dilution is carried out until the desired abrasive particles to be sedimented actually sink through the stationary dispersion medium (gel) to the bottom of an associated container and deposit themselves here.

It comes to sedimentation or separation. The addition of water for dilution increases the proportion of the dispersing agent (water) in the dispersion medium as a whole. It has proved to be advantageous if the proportion of clay mineral dissolved in the dispersion medium at about 1 wt .-% to 1, 5 wt .-% after dilution.

It is understood that the process of preparing the mixture of the abrasive grains and the thixotropic and substantially water-based dispersion medium takes place only when a certain degree of contamination of the mixture is achieved here by additionally dissolved fine particles or fines (mostly abrasion). This degree of contamination can be determined optically, for example. If the mixture is soiled that it has to be treated, it is completely or partially removed from the actual chip-removing separation process. Only then is the dispersion medium diluted and dispersed, to such an extent that the abrasive grains (for the first time) sediment (can). Because in the previously completed separation process sedimentation should not take place so as not to clog the separator. The sedimentation of the abrasive grains can be done by diluting the dispersion medium with water and / or by the fact that the mixture is no longer - as in the separation process - kept moving or reassured. In principle, the sedimentation can also be achieved by introducing a shearing action.

If, for example, a 2.5% strength by weight suspension of the clay mineral is used in one liter of water, ie 25 g of the clay mineral are added to one liter of water and dispersed, then the dispersion medium formed in this way will be approx. 1 min. Rest period without shear forces a gel strength of about 1 N / m ² according to a Pa. Consequently, a particle sinking and sedimenting in this gel, ie the abrasive grain in the example, must produce a pressure of more than 1 Pa in order to be able to sediment in such a gel. If one starts from a spherical volume of the abrasive grain, the corresponding minimum grain diameter is calculated to be approximately 60 μm. That is, the thixotropic dispersion medium thus set retains abrasive grains in its quiescent state or does not sediment grains having a grain diameter of less than 60 μm. For abrasives or abrasive grains of other density such. As boron nitride, corundum, diamond, etc., of course, other grain diameter, as the previously explained diagram corresponding to FIG. 2 makes clear.

If such a dispersion medium is diluted, that is, the proportion of the clay mineral in the example to the already mentioned about 1 wt .-% to 1, 5 wt .-% is reduced, so decreases accordingly, the viscosity and consequently the gel strength, so that now abrasive particles with particle diameters well below 60 microns can still sediment. In this case, therefore, predominantly fines are retained in the dispersion medium, that is, grains in the mixture having a grain diameter of perhaps 20 μm or 10 μm and below. These fines are generally flocculated according to the invention after separation of the suspension above the sediment. Most of these are unwanted fines from the cut material or silicon and abrasion of the grains or the silicon carbide or other abrasive. These fines are flocculated by the already contained clay minerals and optionally with the addition of further clay minerals and optionally a flocculant. Alternatively or additionally, the pH or the electrolyte concentration of the mixture can also be changed and the flocculation and consequently sedimentation can be achieved. The silicon or pure silicon contained in the suspension or the dispersion medium can be separated as valuable material by other methods, for. B. discharged by flotation of the other solids become. The silicon thus obtained can be recycled back into the material cycle, for example reflowing and processing into polysilicon or, in the best case, monocrystalline silicon.

It is always ensured within the scope of the invention that the grains of the abrasive or the abrasive grains are virtually impossible or hardly lost. The same applies to the dispersion medium. Because in addition to the abrasive grains additionally contained in the mixture to be processed fine fractions are known to z. B. flocculated or otherwise separated and can be skimmed off. The proportion of non-reusable waste is therefore significantly reduced compared to the prior art. In addition, the flocculated material is harmless to health and can be used for secondary use. Disposal via a normal landfill is possible. This means that special disposal measures do not have to be taken.

As already explained, the dispersion medium is composed of the dispersion medium (usually water) and an additive which regularly provides the desired thixotropy. This additive (usually the clay mineral) can be dry blended and marketed with the abrasive grains of, for example, silicon carbide. The dry mix of the additive and the abrasive grains is then completed to the mixture just prior to processing with the dispersant. As a result, transport costs are saved, because the dispersion medium (water) is usually already present at the place of chip removal machining.

Always first, the mixture of the dispersion medium and the abrasive grains is prepared, then the said mixture for chip removal machining, in particular for the chip-removing separation of workpieces and here preferably Haibieitermaterialien used. During or generally after this procedure, the soiled mixture is wholly or partly treated as described. In particular, the abrasive grains are separated from the (soiled) mixture. Comparative example

1. State of the art

In the prior art, in connection with the wire sawing of silicon, for example, a mixture of polyethylene glycol and silicon carbide grains having an average grain diameter of 10 μm is used. The silicon carbide grains are added in a ratio of 1 kg to 1 l of the polyethylene glycol. This results in a total density of the suspension of about 1, 6 kg / l. The viscosity of this known dispersion is in the range of about 350 mPas.

Due to semiconductor particles inevitably embedded during the sawing process, the viscosity increases and there is the risk that supply channels in the separating device can become clogged. In addition, the cutting speed of the wire sawing process decreases.

2. Mixture according to the invention

It has been shown that even a suspension of 2% by weight of very finely ground bentonite (with a grain size of less than 100 μm) in water under comparable mixing conditions, even over a period of 60 hours, does not show any visible sedimentation of the silicon carbide grains showed. In fact, the mixture was prepared by mixing the aforesaid dispersion medium containing 2% by weight of bentonite in water to 1 liter with 1 kg of the same grained silicon carbide grains as used in Example 1 to compare the results ,

That is, while significant sedimentation phenomena are already observed in the prior art, the mixture of the present invention does not show such even after 60 hours. This can essentially be attributed to the fact that the bentonite suspended in water, even at the adjusted concentration of 2% by weight with its colloid and charged smectite platelets, forms the previously mentioned network and contains therein the Silicon carbide grains (or other abrasive grains) are kept so that they can not sediment. In addition, the mechanical interlocking of the pointed silicon grains with each other is reduced because they are spaced in the network. As a result, the mixture can be solved with little mechanical effort and transport easily.

The dynamic viscosity is in the range of about 40 mPas. As a result, the piping of the separator can be easily cleaned by rinsing with water. Mechanical cleaning is not required. The rinse water can be disposed of via the sewage system.

General embodiment

A mixture according to the invention of 1 kg of silicon carbide grains having a mean grain size in the range of about 10 μm with 1 l of the dispersion medium which contains 2.5% by weight of bentonite leads to a dynamic viscosity of 150 mPas. In this case, after the sawing process in the polluted mixture all solids with a particle size smaller than 5 microns from the reusable solids content greater than 5 microns (abrasives) are separated in the example.

In the treatment of the above-described (polluted) mixture, it has proved to be particularly favorable that a clay mineral or generally phyllosilicate is used for producing the thixotropic properties of the dispersion medium. Because such clay minerals are due to their extremely large, especially inner surfaces of several 100 m ² / g in a position to store in the mixture to be treated, in particular metal abrasion, store or store. Such metal abrasion, in particular iron abrasion, results from the used separation device or the wire used in this Sieiie in a wire saw. According to the invention, the iron particles are now absorbed by the clay mineral present in the dispersion medium and consequently can not adversely affect the cutting properties of the abrasive particles, as has hitherto been the case. This ensures that the abrasive particles practically without additional (acid) treatment after their sedimentation can be fed directly to a recycling or can be re-introduced into the circulation for a subsequent separation process.

It should be noted that the iron present in the polluted dispersion medium or the iron particles as such can not be adsorbed by the clay mineral in the dispersion medium. Rather, only the adsorption of iron cations takes place. For this reason, the metallic iron in the (polluted) thixotropic and substantially water-based dispersion medium according to the invention must be oxidized. This is generally ensured by oxygen or air injection using a compressed air line.

Incidentally, such iron particles or resulting water-insoluble compounds generally belong to the fine particles and are flocculated together with the other fine particles. A further factor contributing to this is the fact that a sedimentation device for the treatment of the mixture is generally equipped with the already mentioned connected gas source, in particular an air source. With the help of this air source, the aufaufende mixture is applied to the interior and foamed. At the same time, the air introduced into the mixture to be treated ensures that any iron still present in the mixture undergoes oxidation and is brought into a filterable form by the possible addition of flocculants.

Furthermore, the gas or air introduced into the mixture to be treated ensures that premature sedimentation of the abrasive or abrasive grains is prevented. Because due to the described dilution of the dispersion medium in the course of treatment (dilution of the dispersion medium until the Bentonitanteii is about 1 wt .-% to 1, 5 wt .-%) this is regularly no longer able to abrasive grains of the grains Be able to carry particles. The air supply produces a total mixing, but does not result in shear forces, Therefore, after the application of air, the desired gel forms relatively quickly and a good separation result is observed.

The tank for the mixture as well as any sedimentation device may also be equipped with a perforation release agent. This perforation release agent is, according to an advantageous design, a microperforated floor or a microperforated partition wall. In this micro-perforated plate or partition, the compressed air or generally a gas can be introduced under pressure. As a result, on the one hand the foaming and flocculation of the ultrafine particles is made possible and on the other hand admitted after elimination of gas or air admission, the sedimentation of the abrasive grains or set depending on gas or Luftbeaufschlagung in motion.

After the sedimentation of the abrasive grains in the sedimentation device, the fine particles of clay mineral, silicon abrasion of the grains, etc., which collect above them, are first separated by suction or pouring off the abrasive grains. The sediment from the abrasive grains is then overcoated with water or generally a liquid and again brought with the aid of compressed air from the microperforated base plate or the perforation release agent with the water layered over into a homogeneous suspension to this suspension from the sedimentation or a To pump appropriately designed container and, for example, in the circuit to transfer directly into the tank for the mixture through which the wire of the wire saw is guided or with the help of which the wire of the wire saw undergoes wetting by the mixture.

After this purification step, it is conceivable to dilute the sediment again and to mix it up in a suspension. This is problematic insofar as the clay mineral present in the dispersion medium has already been flocculated with the fines in the first step within the sedimentation device. This allows the volume of water to continue in this subsequent and possibly additional purification step increase. In the process, the abrasive grains can be treated again in a sedimentation device with a microperforated bottom or a microperforated intermediate wall and with recourse to an introduced gas source or air.

Embodiment for recovering or preparing the mixture

First, the dispersion medium is prepared from 1 kg of water as a dispersing agent and 25 g of bentonite as a dispersed phase or thixotropic additive.

The proportion of water is thus about 97.6 wt .-% based on the

Dispersion medium (1 kg / 1, 025 kg kg 97.6 wt .-%). About 1 kg abrasive grains of silicon carbide are then added to this dispersion. The mixture thus obtained is used for lapping silicon single crystals in the course of their separation into wafers.

It can be seen in the following Fig. 3, a wire saw machine 2, which is passed through a tank 1, in which the aforementioned mixture is located. Because the mixture is applied to a wire of the wire saw machine 2, a silicon monocrystal 3 can be cut into the desired disks or wafers.

After use of the mixture or exceeding a certain and predetermined degree of contamination, the proportion of bentonite formerly about 2.5 wt .-% in the dispersion medium by diluting the mixture or the dispersion medium with water to about 1, 2 wt. -% reduced. The dispersion medium is redispersed with the introduction of shear forces. This is done in a sedimentation device or a dilution tank 4. For this purpose, the mixture to be treated is pumped from the tank 1 into the already mentioned sedimentation device or the dilution tank 4 and diluted here.

The sedimentation device 4 is provided with a microperforated intermediate wall or, in general, a perforation separator, in the present case a bottom 5. equipped, in which in the example, a pressure line 6 is connected. In this way, compressed air can be injected into the microperforated floor or the intermediate wall 5.

On the one hand, with the aid of the injected air, oxidation of the iron in the used (polluted) mixture is favored and the addition of iron to the silicon carbide grains is prevented or the adsorption of the iron cations on the bentonite is made possible. On the other hand, the supply of air ensures that the abrasive grains or silicon Carbidkömer can not sediment before the complete filling. In order to initiate the sedimentation phase, the compressed air is switched off or the compressed air line 6 is closed.

Due to the thixotropic nature of the mixture, a gel forms relatively quickly from the dispersion medium because no more mechanical shear forces are introduced (compare FIG. 2). As a result, the abrasive grains or silicon carbide grains sink onto the microperforated partition wall 5 and form a sedimentation layer here. Depending on the viscosity set by the dilution, a distinction can be made between the coarse fractions deposited in the sedimentation layer and their grain diameter and the fine fractions remaining in the used mixture. As a consequence of this, the fine fraction of water, bentonite, silicon and finely ground silicon carbide grains existing above the depositing sediment is separated from the sediment by the grains which are actually abrasive or silicon carbide grains.

The supernatant fines from abrasion, silicon, clay mineral (bentonite), iron, etc. are removed from the dilution tank or the sedimentation device 4 and treated separately. This can be z. B. by the addition of flocculants. The fumigated components can easily be skimmed off or filtered out by sieves, or they are given time for re-sedimentation, which can be accelerated by a centrifuge or a hydrocyclone. It is also possible to recover the silicon content from the fines, z. B. by flotation. Subsequently, the sediment from the abrasive grains is covered with pure water. By applying the sediment again with compressed air via the compressed air line 6, the sediment or the previously deposited abrasive particles are again transferred to a suspension. This suspension can be pumped into another tank in order to experience a renewed dilution, or the remaining iron can be oxidized by the addition of oxidants and further blown-in air.

In a further supplementary purification step, this suspension of water and abrasives can be thickened by a FesWlüssig separation. In this case, processes are preferably used again in which the abrasive grains are separated from the liquid phase by gravity, since this prevents clogging of sieves. Particularly advantageous is the use of a lamellar thickener, which has a high separation efficiency with little space and without moving parts.

The thickened abrasives or thickened abrasive grains can then be dispersed in a further step with water and clay mineral (bentonite) or a suitable clay mineral suspension (bentonite suspension) with the addition of fresh abrasives or fresh abrasive grains. A drying of the abrasive is not necessary when using water as a dispersant.

In principle, however, the suspension prepared in this way can also be returned immediately and again to the tank 1 and is available again for the described lapping process in conjunction with the wire saw machine 2. For this purpose, it is only necessary to add the required amount of bentonite or Tonrninerai to the suspension of the abrasive grains and water in order to set the desired thixotropic properties again. Comparative example

A particular advantage in the treatment of the mixture according to the invention based on the aqueous colloidal suspension of clay minerals and preferably smectites or montmorilonite as the main constituent of regularly used Betonits over other thixotropic colloidal suspensions z. B. based on organic colloids is in the prevention of contamination of the abrasive or the abrasive grains with iron abrasion. In fact, the aforesaid iron abrasion results primarily from wear of the billet during a cutting operation with, for example, a wire-sawing machine. This iron abrasion or a resulting contamination of the mixture reduces the cutting performance of the abrasive. For this reason, in the prior art, the abrasive obtained in the recycling, preferably the Siliziumcarbidkörner, again treated with acid to bring the iron in solution. The disposal of the used acid is associated with additional costs. The separation of iron from the thus contaminated Siliziumcarbid- by means of magnetic separators is expensive, since in this case also the adhering to the iron silicon carbide is removed from the suspension.

There are now two reasons why an aqueous suspension of clay minerals and consequently the mixture according to the invention of the dispersion medium prepared therefrom reduce or completely prevent the adhesion of iron to generally the abrasive grains and in particular the lithium carbide.

In fact, the iron from the wire abrasion is in the form of very small particles, which are equipped with grain diameters in the range of about 1 micron. These ultrafine iron particles have a very large surface area. In an organic dispersion medium such. B. Poiyethyiengiykol or oil according to the prior art, the aforementioned iron particles are largely protected from attack by oxygen and thus can not oxidize. That is, in a prior art blend, this would be according to the invention performed pressure or gas application using the compressed air line 6 is not successful or would show no effect.

In contrast, the water constituting the major part of the dispersion medium according to the invention can absorb oxygen, especially at low temperatures. This then leads to an oxidation of the metallic iron or the iron particles present in the mixture as soiling. In the case of mixtures based on organic dispersion media based on the prior art, this effect is difficult to achieve even with the addition of water. This can essentially be attributed to the fact that such mixtures become warm due to their low heat capacity during cutting and thus oxygen can no longer be sufficiently dissolved.

In addition to the described chemical effect of the dissolution of the metallic iron in the mixture according to the invention additionally leads to a physical effect that precipitates less or no iron on the abrasive grains in the inventive mixture Thus, the dissolved in the mixture iron particles in an organic Disperstonsmedium according to the prior art, an extremely large surface area. So they are very unstable due to the surface tension. Therefore, the iron particles will try to degrade the surface, which may be due to the iron particles precipitating on the silicon carbide or, generally, the abrasive grains. In fact, in the context of blends of the prior art, the abrasively acting particles are the only component in this mixture which has a solid surface. If the attachment described has occurred on the said grains, further iron particles can accumulate. The effect is thus comparable to the crystal growth at a nucleus and the formation of such a germ. This applies in any case to mixtures of the prior art based on organic dispersion mediums.

If now such an organic dispersion medium by an inventive Dispersioπsmedlum so an aqueous suspension based on For example, Beπtoπit replaced, thereby the available in the suspension and thus the dispersion medium surface drastically increased contribute significantly to the Montmorilonitkristalle, which form the thixotropic suspension and have a surface area of several 100 m ^a / g. Since these surfaces of the montmorilonite crystals are negatively charged, they offer themselves as (inner) surface for precipitation of the iron or dissolved in the mixture used iron particles. Alternatively, it is also conceivable that the Montmorilonitkristalle attach with their positively charged edges to the respective iron particles and thus shield it.

It does not matter if the processes run simultaneously or at different times. In any case, the number of iron particles is significantly reduced in the mixture according to the invention, which can be reflected or precipitate on the abrasive Komömem. In this way, the iron or can be the iron particles that do not affect the abrasive grains, in the purification or processing of the mixture according to the invention together with the other fines of example, the abrasive grains and the silicon from the mixture remove. This explains why a treatment of the thus obtained abrasive Kömem with acid is unnecessary.

test description

2 kg of a mixture according to the invention of 1 kg of deionized water, 25 g of bentonite and 1 kg of Siliziumcarbidkörnem is used to demonstrate the effects described above for lapping iron plates. The

Iron content is determined by determining the weight loss of the lapped

Plates determined The mixture thus obtained is then diluted by the addition of deionized water so far in the course of their treatment that the Betonitgehalt in the suspension is reduced to 1.25 wt .-%. Subsequently, a sedimentation, wherein the remaining fines of the

Sediment is separated by decantation. Subsequently, both the fine fraction and the sediment are dried at about 105 ° C to determine the solids content. As a reference, a sample of the unpurified original and unpolluted mixture is also dried.

All samples are digested with nitric acid to have all the iron present as trivalent ions. The analysis of the iron content is done by atomic absorption spectroscopy.

This results in the following results:

The amount of iron in the original mixture is taken as a reference of 1. All data relate to the proportions of iron in the dry matter analyzed.

Untreated original mixture: 1

Fresh silicon carbide: 0.64

Recycled silicon carbide (sediment): 0.3 Decanted fines: 28

The amount of iron in the separated fines of the treated mixture is thus about 100 times greater than the iron content in the sedimented recycled silicon carbide, which is virtually free of iron. Thus, the particular success in the preparation of the mixture according to the invention becomes clear.

Diagrams showing the success of the separation according to grain sizes:

Finally, 4 diagrams are shown, which make clear the previously described in detail separation of the abrasive grains according to their respective particle size in the course of the treatment process according to the invention. For this purpose, 500 g of silicon carbide were dispersed with 500 g of distilled water, 12.5 g of bentonite and 60 g of Silitin N85 (Hoffmann Mineral) as Feinaπteil and subsequently in a flask for static sedimentation decanted. After 2 hours, the liquid fines in the gel were separated from the solid silicon carbide sediment by decantation. The starting materials, the mixture and the recycled sediment were wet subjected to the subsequent particle size analysis. For this the light scattering was used.

Claims

claims: 1. A method for preparing a mixture of a dispersion medium and abrasive grains in the course of their use as abrasive for machining, in particular for the chip-removing separation of workpieces, preferably of semiconductor crystals, d a d c h e c e n e c e s in that a thixotropic and substantially water-based dispersion medium is used as the dispersion medium. 2. The method according to claim 1, characterized in that the abrasive grains undergo sedimentation by diluting the dispersion medium or the mixture with water and / or calming the mixture and / or introduced targeted shear. 3. The method according to claim 1 or 2, characterized in that undesired fine fraction is flocculated in the mixture to be treated. 4. The method according to any one of claims 1 to 3, characterized in that the concentration of colloids in the dispersion medium for varying the viscosity or gel strength of the dispersion medium in dependence on the grain size auszuschcheidender each abrasive grains is set. 5. The method according to any one of claims 1 to 4, characterized in that the dispersion medium about 1 wt .-% to about 10 wt .-%, in particular about 1 wt .-% to about 5 wt .-% and preferably about 2 wt .-% to about 3 wt .-% of a water-soluble clay mineral. 6. The method according to claim 5, characterized in that the clay mineral contained in the mixture to be processed abrasion, in particular metal abrasion of an inserted Trennvorrichtüng or wire saw (2), einlagert. 7. The method according to any one of claims 1 to 6, characterized in that the fine fraction in the mixture to be prepared by adding further Ton- minerals and / or a flocculant and / or flocculated by compressed air. 8. The method according to any one of claims 1 to 7, characterized in that the flocculation is favored by changing the pH and / or the electrolyte concentration within the mixture. 9. The method according to any one of claims 1 to 8, characterized in that the clay mineral of the mixture to be treated up to a proportion of about 1 wt .-% to about 1, 5 wt .-%, based on the dispersion medium, by adding water is diluted. 10. An apparatus for preparing a mixture of a dispersion medium and abrasive grains, which are used as abrasives for the chip-removing machining, in particular chip-removing separation of workpieces, preferably semiconductor crystals, with a separation device (2), a tank (1) for the mixture and a sedimentation device (4), characterized in that the dispersion medium of the mixture is formed thixotropic and substantially water-based. 11. The device according to claim 10, characterized in that on the sedimentation device (4) a compressed air line (6) is connected, which acts on the sedimentation device (4). 12. The device according to claim 10 or 11, characterized in that the sedimentation device (4) has a perforated intermediate wall (5). 13. Device according to one of claims 10 to 12, characterized in that the mixture to be treated in the sedimentation device (4) for Sedimentation remains mechanically unaffected and rests or at most undergoes a laminar flow.