DE102007050483A1 - Mixture of a thixotropic dispersion medium and abrasive grains as abrasive - Google Patents

Abstract

The present invention is the use of a mixture of a thixotropic dispersion medium and abrasive grains as abrasives for chip-removing machining of workpieces, in particular of semiconductor crystals. In this way, the mixture is particularly easy to reprocess and a particularly functional separation process for the workpiece to be machined is provided.

Description

The The invention relates to the mixture of a thixotropic dispersion medium and abrasive grains as abrasive.

Such mixtures are well known and are used in practice as, for example, drilling mud formulations. In this connection, the thickening of water-based systems using clays is widely exploited. In fact, these mixtures tend to thixotropic thickening, which is advantageously used for the described applications, for example in connection with geological drilling. That's how it deals DE 43 02 462 A1 with the use of alkoxylates of water-insoluble alcohols for controlling the rheological properties of flowable and pumpable aqueous preparations of finely divided minerals, which are used as working materials in the field of digestion of geological formations.

Comparable Bohrspülzusätze are the subject of DE 29 18 683 A1 , Here, a drilling mud additive for effectively dispersing clays in an aqueous medium is described. The drilling fluid in question is used in rotary drilling and, due to its viscosity, makes it easy to carry away rock fragments. Due to the thixotropic nature of the drilling fluid, it tends to gel upon interruption of the drilling process and prevents splinters from settling around the drill bit.

The machining of workpieces and in particular of semiconductor crystals, which include silicon monocrystals, is carried out quite independently thereof with special aqueous compositions. These usually comprise a dispersion medium which uses organic components, namely composed of a hydrophilic polyhydric alcohol compound, a lipophilic polyhydric alcohol compound and water. In the dispersion medium, colloidal silica particles prepared from a silicate are dispersed, like the DE 699 11 549 T2 describes in detail.

The the aforesaid chip removing machining processes and the associated aqueous composition or the aqueous cutting fluid has disadvantages. This also applies to polishes on this basis, which are widely used in semiconductor electronics to be sawn and processed before subsequent wafers Smooth process steps on the surface.

Indeed it has turned out in this context that the sedimentation of the dispersed in the organic dispersion medium abrasive grains by the grain size of the Abrasive and thus the grains is determined. Increases the size of the grains, so must the dispersion medium be set to a higher viscosity, unchanged to ensure proper transport of the abrasive to ensure and a sedimentation of the abrasive and prevent any blockages.

Around this increased viscosity of the mixture respectively Counteract cutting suspension is usually the concentration of the abrasive is reduced to the total viscosity including to maintain the grains. However, this slows down the chip removal Process or one performed at this point mostly Sawing process. Another systemic disadvantage of such known organic dispersion media is that they over have a low heat capacity. As a result, there is little protection against overheating and the dispersion medium in question can only be hard to break down and recover.

Also A disadvantage of such organic dispersion media is the complicated Manufacturing process and the necessary disposal after use. In many cases, a reuse is not possible because in the Dispersion medium dissolved ultrafine particles from used Abrasive grains and additional abrasion not with financially acceptable expenditure of technology and time discharged can be. This is where the invention starts.

Of the Invention is the technical problem underlying a mixture of a dispersion medium and abrasive grains as an abrasive for the chip-removing machining of workpieces, especially of semiconductor crystals, so on, that with improved heat capacity and impeccable Processing result a cost-effective reprocessing respectively disposal succeeds.

to Solution of this problem is the subject of the invention the use of a mixture of a thixotropic dispersion medium and Abrasive grains as abrasive for chip removal Processing of workpieces, in particular of semiconductor crystals. These semiconductor crystals are preferably silicon monocrystals. In addition, of course, also workpieces be processed from polysilicon. Likewise those from other semiconductor materials, such as for example, gallium arsenide, gallium indium phosphite, etc.

As a rule, the dispersion medium with the abrasive grains dispersed therein, ie the aforementioned mixture, does not form used for machining. As a rule, this means chip removal processes, which include, for example, 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 spanab carrying separation process in slices of desired thickness (wafer), which are then usually still polished to order for further process steps prepare.

The The invention has surprisingly found that a Known per se thixotropic dispersion medium advantageous with dispersed therein and abrasive grains as abrasives for the described chip removal of workpieces, in particular of semiconductor crystals, can be used. Here comes the thixotropic effect of the dispersion medium for the described purpose of particular importance. Because that Dispersion medium has charged according to an advantageous embodiment colloidal particles on. These charged colloidal particles form with sufficient concentration in the dispersion medium to each other Networks and at best an elastic gel, which the abrasive grains permanently in itself wearing. As a result, sedimentation becomes abrasive effective grains prevent and leave any with it prevent associated negative effects within the scope of the invention. That is, the transport of the mixture by one in the Normally used ripper will not be affected by any sedimentation hampered, making the processing particularly easy and reliable he follows.

at the same time is the strength of the bond between the individual colloids However, particles are low, so at sufficiently high shear rates the network can be reversibly reversed again. As a result of which the suspension becomes thinner, or decreases the viscosity, so that in total the thixotropic Behavior explained. As colloidal particles in the dispersion medium Smectites, that is phyllosilicates with a three-layer structure, are preferred for use. These are usually in water as a dispersant suspended. In this context, a decisive factor is a high yield point of the dispersant at the same time low viscosity under shear stress.

It has generally proven that the dispersion medium about 1 wt .-% to 10 wt .-%, in particular 1 wt .-% to 5 wt .-% and preferably 2 wt .-% to 3 wt .-% of a dissolved in water Tonminerals. That is, the dispersion medium is predominantly water as the dispersant and the already mentioned clay mineral in the specified concentration together. The clay minerals used are preferred to smectite-containing clays, such as bentonite but also Hectorite. In addition, other smectites such as corrensite, Rectorite, saponite, stevensite, etc. conceivable. These are for the described thixotropic behavior known. You can do that basically both synthetic and natural Use clay minerals.

Also certain starches as well as organic polymers can advantageously dissolved in water and as inventive Dispersion medium are used. Furthermore, in addition to clay minerals also needle-shaped chain silicates such as sepiolite in a position to form thixotropic aqueous suspensions. These require However, very high solids content and the yield point is less pronounced compared to the viscosity. As a result, the invention thus not only clay mineral comprises colloidal particles.

in the Detail is the thixotropic behavior of the usually in the water dissolved clay mineral thereby causes, that in one such clay mineral suspension without movement bridges between form the individual dissolved particles or smectites. These bridges represent the already mentioned network which is the abrasive dispersed in the dispersion medium acting grains and their sedimentation prevented. This is particularly easy because the grains with grain sizes below 100 μm, preferably less than 50 microns and preferably less than 20 microns are equipped.

First if this mixture or generally the described dispersion medium subjected to a shearing motion, break the described Bridges on. As a rule, for example, 1 kg to the abrasive grains with 1 l of the dispersion medium is mixed, are within the mixture enough grains available, so even at minor microscopic shear rates break the mentioned bridges and the viscosity of the mixture decreases. This is generally stated but not a problem, because a corresponding fluid movement the mixture is associated with a mixture transport, which either flows into the desired chip-removing machining or afterwards in a reprocessing or a re-use the mixture.

It is particularly preferred if the abrasive grains are dispersed in the thixotropic dispersion medium, for example, with an average particle 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 in the described network easily record and hold. 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 grain sizes of significantly less than 500 microns.

In As a rule, the particle size of the used Clay minerals less than 200 microns and is particularly preferred below 100 μm. In this way, a total of one relatively low viscosity of the mixture from the abrasive acting grains and the dispersion medium of water and the clay mineral additive. Actually According to the invention viscosities of mostly clear less than 1 Pas (1 pascal-second) 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 are particularly preferred for the production of slices of silicon monocrystals (silicon wafers). 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 with a low feed rate of about 0.1 mm / min. worked and set the cutting width in the range of below 0.2 mm. An example of such a separation device is shown in FIG DE 698 24 655 T2 described.

As a result the low viscosity of the invention Mixture does not come at this point to the training of so-called "taper". Because the mixture is convenient because of their low viscosity with uniform liquid thickness around the Wire along its entire length and it comes in handy not or hardly to the mixture at the beginning of the cutting process dams and at the end of the cutting process is no longer sufficient is available. That is, the danger that is in the Sägefuge a wedge (taper) forms, becomes clear reduced. All the more so, as it is recommended, the wire or in general, the cutting tool with alternating cutting direction to lead the semiconductor crystal to be separated.

Thereby, that the described "wedge effect" (taper) within the Sägefuge is reduced and due to the reduced Viscosity at the same time a better material transport at this Place takes place, also the roughness of the produced slice (silicon wafer) reduced compared to previous approaches. As a result can be dispensed with elaborate reworking for polishing partly or are these reworking measures much less expensive as in the prior art, which with the previously mentioned organic Dispersion media works.

in addition comes that the invention and used Especially easy to recycle. It can First, the desired coarse fraction of the abrasive for example, by filtering or in a cyclone from the suspension or mixture. Alternatively, the dispersant For example, be diluted with water to the extent that The network described above no longer forms and it too a sedimentation comes. This sedimentation leads to a separation according to grain sizes.

Of the unwanted fines from the cut material or silicon and the grains respectively silicon carbide can then by the already contained clay minerals as well as possibly with the addition of further clay minerals and optionally a flocculant be flocculated. Alternatively, can also be the Change the pH or the electrolyte concentration and the Flocculation and consequently sedimentation. Always the Advantage achieved that the grains of the abrasive practically impossible or hardly lost. The same applies the dispersion medium. That is, the proportion of non-reusable Waste is clear over the prior art reduced. In addition, the flocculated material is health is harmless and fed to a secondary use can be. The disposal of a normal landfill is possible. That means, special disposal measures must be not be taken.

As already explained, the dispersion medium settles out the dispersant (mostly water) and an additive together, the regular for the desired Thixotropy provides. This addition (usually the clay mineral) can with the abrasive grains of, for example, silicon carbide dry mixed and marketed. The dry mix from the additive and the abrasive grains we then only immediately completed before processing with the dispersing agent to the mixture. This saves transportation costs because the dispersant (Water) usually anyway at the place of the chip-removing processing is available.

Example:

• 1. In the prior art, in connection with the wire sawing of silicon, for example, a mixture of polyethylene glycol and Silicon carbide grains used with a mean grain diameter of 10 microns. 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.

by virtue of of during the sawing inevitably embedded semiconductor particles increases the viscosity and there is a risk that feed channels can clog in the separator.

2. Inventive mixture

It It has been shown that already a suspension of 2 wt .-% very finely ground bentonite (with a grain size of less than 100 μm) in water at comparable mixing conditions itself a period of 60 hours no visible sedimentation of the Silicon carbide grains has shown. Indeed The mixture was prepared so that the aforementioned dispersion medium with the proportion of 2 wt .-% bentonite in water to 1 l with 1 kg of silicon carbide grains same grain size as in Example 1 was mixed to the Compare results.

The is called, while in the prior art already significant sedimentation phenomena are observed, the mixture according to the invention even after 60 Not hours. This can essentially be attributed to that the bentonite dissolved in water itself is in the set Concentration of 2% by weight with its colloid and charged smectite platelets the previously mentioned network forms and therein the silicon carbide grains be held so that they can not sediment. In addition, the mechanical interlocking of the pointed silicon grains reduced among each other because they are spaced in the network are. This allows the mixture with low mechanical Solve effort and transport easily.

• 3. Eine erfindungsgemäße Mischung aus 1 kg Silizium-Carbidkörnern mit einer mittleren Körnung im Bereich von ca. 10 μm mit 1 l des Dispersionsmediums, welches 2,5 Gew.-% Bentonit enthält, führt auf eine dynamische Viskosität von 150 mPas.

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

• 3. A mixture according to the invention of 1 kg of silicon carbide grains having an average grain size in the range of about 10 .mu.m with 1 l of the dispersion medium containing 2.5 wt .-% bentonite, leads to a dynamic viscosity of 150 mPas.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• DE 4302462 A1 [0002]
• DE 2918683 A1 [0003]
• - DE 69911549 T2 [0004]
• - DE 69824655 T2 [0020]

Claims (10)

Using a mixture of a thixotropic Dispersion medium and abrasive grains as abrasive for the machining of workpieces, in particular of semiconductor crystals. Use according to claim 1, characterized that the semiconductor crystals are silicon single crystals is. Use according to claim 1 or 2, characterized that the dispersion medium with the dispersed therein as well as abrasive acting grains for non-forming chip-removing Machining, for example for cutting the workpiece, is used. Use according to one of claims 1 to 3, characterized in that the dispersion medium contains about 1% by weight to about 10 wt .-%, in particular about 1 wt .-% to about 5 wt .-% and preferably about 2 wt .-% to about 3 wt .-% of a dissolved in water Tonminerals. Use according to claim 4, characterized that the clay mineral in low grain with a medium Grain size of below 500 microns, preferably less than 200 microns, and more preferably less than 100 microns is used. Use according to claim 4 or 5, characterized that as a clay mineral a layer mineral, in particular three-layer clay mineral, For example, bentonite is used. Use according to one of claims 1 to 6, characterized in that the mixture has a viscosity less than 1 Pas, in particular less than 500 mPas, and preferably such in the range of 30 to 350 mPas depending of the proportion of the abrasive grains. Use according to one of claims 1 to 7, characterized in that from the mixture used, the grains the abrasive is separated for reuse become. Use according to one of claims 1 to 8, characterized in that the mixture used after addition of additional dispersant and / or a flocculant is sedimented. Use according to one of claims 1 to 9, characterized in that the abrasive grains in a particle size below 100 microns, preferably less than 50 μm.