JP2008188723A - Recycling method for used slurry - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To recycle used slurry by easily and inexpensively separating abrasive grains from the used slurry. <P>SOLUTION: In the recycling method for used slurry, alkali components are added to the used slurry, used in cutting of silicon ingots, and after melting silicon ingot cutting chips, abrasive grains are separated from the used slurry. It is preferable that decantation of the used slurry is carried out after sinking of the abrasive grains to separate the abrasive grains from the used slurry. It is also preferable that the abrasive grains before use in cutting are cleaned beforehand by an alkaline solution. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for recycling spent slurry used to cut single crystal, polycrystalline or amorphous silicon ingots to produce semiconductor and solar cell wafers.

Conventionally, for cutting a silicon ingot, a wire saw capable of cutting a large number of wafers at a time has been used. Cutting a silicon ingot using this wire saw is performed by supplying a cutting slurry containing abrasive grains to a traveling wire and pressing the wire against the silicon ingot. In cutting a silicon ingot, silicon particles generated as chips (hereinafter referred to as silicon ingot cutting scraps) are mixed into the slurry. When the amount of silicon ingot cutting scraps mixed increases as cutting progresses, the slurry viscosity changes and cutting efficiency decreases. As a result, the cutting state changes and the surface state of the resulting wafer also changes, which is not preferable. Finally, the slurry cannot be used and is discarded as a used slurry. In order to obtain a large number of silicon ingots at low cost, it is necessary to regenerate the slurry by separating silicon ingot cutting waste from the used slurry.
Therefore, as a countermeasure to such a problem, after heating the used slurry containing the abrasive grain component and the lubricating liquid component to reduce the viscosity, the solid portion is separated into the liquid portion, and water is poured into the solid portion. A method of adding a suspension and separating the suspension into a suspension containing reusable abrasive grains and a suspension containing silicon ingot cutting waste or the like has been proposed (for example, see Patent Document 1). ).
Further, the spent slurry is separated into a dispersion mainly containing abrasive grains and a dispersion mainly containing silicon ingot cutting waste by a method such as sedimentation separation, and then mainly containing silicon ingot cutting waste by a method such as centrifugation. A method for recovering a dispersion medium from a dispersion has been proposed (see, for example, Patent Document 2).

Special Table 2002-519209 JP 2003-340719 A

However, in the conventional method described above, it is necessary to heat the used slurry or to perform a plurality of separation operations, and there is a problem that the process is complicated and the cost is increased.
Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to separate abrasive grains from a used slurry and to recycle the used slurry at a simple and low cost.

Therefore, as a result of intensive research and development to solve the conventional problems as described above, the present inventors have added an alkali component to the used slurry in order to solve such problems. After dissolving the silicon ingot cutting waste, it was considered effective to separate the abrasive grains from the used slurry, and the present invention was completed.
That is, the present invention adds an alkali component to a used slurry containing abrasive grains used for cutting a silicon ingot, dissolves silicon ingot cutting waste, and then separates the abrasive grains from the used slurry. Is a method for recycling used slurry.

  ADVANTAGE OF THE INVENTION According to this invention, the method of isolate | separating an abrasive grain from a used slurry and recycling a used slurry can be provided simply and at low cost.

Hereinafter, embodiments of the present invention will be described in detail.
Embodiment 1 FIG.
In the used slurry recycling method according to Embodiment 1 of the present invention, only silicon ingot cutting waste is obtained by adding an alkaline component to the used slurry containing abrasive grains used for cutting the silicon ingot by a wire saw or the like. Is removed chemically. Next, the abrasive grains are separated from the used slurry from which silicon ingot cutting waste has been removed. Since the silicon ingot cutting scraps mixed in the slurry do not greatly differ in abrasive grains and particle diameter, it is usually difficult to separate them by a simple method. However, in this embodiment, the silicon ingot cutting to be removed By dissolving the scraps, the abrasive grains become the main solid content in the used slurry, and the abrasive grains can be easily separated. Silicon can exist stably in a neutral aqueous solution or in the air because it forms an oxide film on the surface, but is oxidized and dissolved in an alkaline aqueous solution. Therefore, silicon ingot cutting waste can be dissolved by adding an alkali component to the used slurry. Even if the slurry itself used for cutting is simply made alkaline, the oxidative dissolution of the silicon ingot cutting waste proceeds. However, due to the oxidative dissolution, the alkalinity of the slurry decreases, so the amount of dissolution is limited, and there is a large amount of alkaline components. If the pH of the slurry itself used for cutting is excessively increased by adding to the above, there is a problem that the cutting state of the silicon ingot changes and adversely affects the surface of the resulting wafer. In this Embodiment, since an alkali component is added with respect to a used slurry, such a problem can be avoided.

  Examples of the alkali component added to the used slurry include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, alkaline earth hydroxides such as magnesium hydroxide, calcium hydroxide and barium hydroxide, Ammonia, various amine compounds, quaternary ammonium salts, etc., or mixtures thereof can be mentioned. The addition of the alkali component can be performed by various methods such as a method of directly adding the alkali component itself, a method of adding it as an aqueous solution of the alkali component. In the method in which the alkali component itself is directly added, a temporary but highly alkaline region can be locally formed, so that silicon ingot cutting scraps whose surface is stabilized with an oxide or the like and the reaction is difficult to start are generated. Even if it exists, there exists an advantage that dissolution is easy to proceed. The method of adding as an aqueous solution of an alkali component has the advantage that it is easy to add while suppressing heat generation during the addition or monitoring pH and the like.

  What is necessary is just to adjust the quantity of the alkali component added to a used slurry according to the quantity of the silicon ingot cutting waste mixed in a slurry. In order to increase the dissolution rate of silicon ingot cutting scraps, it is necessary to increase the concentration (pH) of the alkali component to some extent, and the pH is preferably 11 or more, more preferably 11.5 or more, and most preferably 12 or more. Add the alkali component. If the pH is too low, the silicon ingot cutting chips will not dissolve. After adding an alkali component to a used slurry, an alkali component may be additionally added as necessary. In order to promote dissolution of silicon ingot cutting waste, the used slurry may be heated.

  In order to separate the abrasive grains from the used slurry from which the silicon ingot cutting waste has been removed, the used slurry is allowed to settle and the abrasive grains are allowed to settle, and then the supernatant is gently poured out, so-called decantation. It is preferable. Decantation removes most of the fine particles such as worn abrasive grains and unreacted silicon ingot cutting waste together with the supernatant liquid, so that liquid components, alkali components, silica, etc. in addition to the abrasive grains remain as impurities. This is because, however, precipitation without silicon ingot cutting waste is obtained. Impurities contained in the separated abrasive grains by reducing the viscosity of the slurry to make the abrasive grains easier to settle by adding water or the like to the used slurry before the abrasive grains are settled. Can be reduced. Instead of allowing the abrasive grains to settle down, a separation method such as centrifugation or filtration may be used.

  A regenerated slurry may be prepared using the abrasive grains obtained as described above (hereinafter referred to as recovered abrasive grains) as they are or after drying, or the recovered abrasive grains are washed with water or the like. Or you may prepare a reproduction | regeneration slurry using what was dried after washing | cleaning. The method of using the recovered abrasive grains as they are for the preparation of the regenerated slurry is the simplest. However, by cleaning the recovered abrasive grains, impurities are reduced, and the regenerated slurry is stable in characteristics (similar to the characteristics of the slurry before use). Obtainable. In the case of using the dried recovered abrasive grains, since there are few liquid components brought into the regenerated slurry from the recovered abrasive grains, there is an advantage that the slurry composition can be easily adjusted accurately.

  Further, the abrasive grains used to prepare the regenerated slurry may be only recovered abrasive grains, or may be those obtained by adding new abrasive grains to the recovered abrasive grains as necessary. If an alkali component remains as an impurity in the recovered abrasive grains, a neutralization operation may be necessary when preparing the regenerated slurry. Even if such a neutralization operation is performed, the ionic components present in the regenerated slurry may adversely affect the cutting of the silicon ingot. Therefore, if an alkaline material set so as to obtain an appropriate cutting property of the silicon ingot is used as the slurry used for cutting the silicon ingot, for example, one having a pH of 11 to 14, the alkali component remains as an impurity. In addition, it is not necessary to neutralize the recovered abrasive grains, and the above-mentioned problems do not occur.

  As an abrasive grain, what is generally used as an abrasive | polishing material and a low reactivity with an alkali component should just be used. Specific examples of the abrasive grains include silicon carbide, cerium oxide, boron nitride, aluminum oxide, and zirconium oxide, and these can be used alone or in combination of two or more. Compounds that can be used for such abrasive grains are commercially available. Specifically, as silicon carbide, trade names GC (Green Silicon Carbide) and C (Black Silicon Carbide) (manufactured by Fujimi Incorporated), Examples of aluminum oxide include trade names FO (Fujimi Optical Emery), A (Regular Fused Aluminum), WA (White Fused Aluminum), and PWA (Platelet Calcined Alumina) (manufactured by Fujimi Incorporated). Some of these commercially available abrasive grains partially react with alkali such as SiC. In recycling such a slurry containing abrasive grains, the surface state of the abrasive grains changes before and after recycling, so the slurry containing new abrasive grains and the abrasive grains recovered from the used slurry are blended. Slurry characteristics such as fluidity and cutting performance may be different from the regenerated slurry. In order to suppress such a change in slurry characteristics, it is preferable to remove a portion that reacts by previously washing new abrasive grains with an alkaline aqueous solution or the like. Examples of the alkaline aqueous solution used for this treatment include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, alkaline earth hydroxides such as magnesium hydroxide, calcium hydroxide and barium hydroxide, ammonia, Examples include various amine compounds, quaternary ammonium salts, and aqueous solutions containing a mixture thereof.

  Although the average particle diameter of an abrasive grain is not specifically limited, Preferably it is 1 micrometer-60 micrometers, More preferably, it is 5 micrometers-20 micrometers. If the average particle diameter of the abrasive grains is less than 1 μm, the cutting speed is extremely slow and is not practical. If the average particle diameter of the abrasive grains exceeds 60 μm, the surface roughness of the wafer surface after cutting increases. This is not preferable because the wafer quality may deteriorate.

  The content of the abrasive grains is not particularly limited, but is preferably 30% by mass to 70% by mass with respect to the mass of the entire slurry. When the content of abrasive grains is less than 30% by mass, the cutting speed becomes slow and the practicality may become poor. When the content of abrasive grains exceeds 70% by mass, the viscosity of the slurry becomes excessive. In some cases, it may be difficult to introduce the slurry into the cutting interface of the silicon ingot.

As the liquid component of the slurry, water, alcohols, polyhydric alcohols such as glycerin and ethylene glycol, polar organic solvents such as ethanolamines, DMF, DMA, and NMP, and mixtures thereof can be used.
The slurry includes alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, alkaline earth hydroxides such as magnesium hydroxide, calcium hydroxide and barium hydroxide, ammonia, various amine compounds, An alkali component such as a quaternary ammonium salt may be contained. In this case, as described above, in addition to the effect of reducing the influence of alkali components brought as impurities from the abrasive grains obtained by separation into the recycled slurry, the resistance between the wire and the silicon ingot in the wire saw is reduced. There is also an effect. The slurry may contain various anionic, cationic and nonionic surfactants. In this case, there is an effect of improving the stability of the slurry viscosity and improving the familiarity between the slurry and the wire or silicon ingot. Furthermore, the slurry may contain a polymer, a low molecular thickener, or the like. In this case, there is an effect that the slurry can be efficiently introduced into the cutting portion of the silicon ingot and the settling of the abrasive grains in the slurry can be suppressed.

  The slurry can be prepared by mixing each of the above components in a desired ratio. The method of mixing each component is arbitrary, for example, it can carry out by stirring with a wing-type stirrer. About the mixing order of each component, it is arbitrary. Further, for the purpose of purification or the like, the prepared slurry may be further treated, for example, filtration treatment, ion exchange treatment or the like.

EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to these.
[Example 1]
A slurry was prepared by mixing 40 kg of water, 20 kg of glycerin, and 50 kg of SiC abrasive grains (manufactured by Fujimi Incorporated, GC # 1200). The pH of the slurry was neutral and the viscosity was about 100 mPa · s. Using this slurry, cutting was performed under the cutting conditions shown below until a polycrystalline silicon ingot (150 mm square, 25 mm long) was cut.

<Cutting conditions>
Cutting device: Slurry circulation type multi-wire saw Wire diameter: 100 μm
Cutting allowance: 0.13mm
Cutting pitch: 0.39 mm
Cutting speed: 0.35 mm / min Wire traveling speed: 600 m / min

  After cutting the silicon ingot, the whole amount of the used slurry and 20 kg of water used for wire saw washing were put into a container, and sodium hydroxide was added little by little until the pH of the used slurry became 12.5 while stirring the inside of the container. . Stirring was stopped and left for about 2 hours until the abrasive grains settled. Thereafter, the container was tilted and only the supernatant liquid was poured out of the container. When the recovered abrasive grains taken out as a precipitate were observed with an optical microscope, no silicon ingot cutting waste was present, and only abrasive grains were observed. The recovery rate of the abrasive grains was 90% by weight. As described above, the abrasive grains containing no silicon ingot cutting waste could be recovered at a high recovery rate of 90% by weight by a simple operation of adding an alkali component to the used slurry.

  Similar to the slurry using only new abrasive grains, a regenerated slurry was prepared using recovered abrasive grains (90 wt%) and new abrasive grains (10 wt%) supplemented as a deficiency. Since the pH of the regenerated slurry was 9, it was adjusted to neutral using hydrochloric acid. Moreover, the viscosity of the regenerated slurry was 90 mPa · s, which was about 10 mPa · s lower than that of a slurry prepared using only new abrasive grains.

  For comparison, when a slurry was prepared using only new abrasive grains washed with an aqueous sodium hydroxide solution having a pH of 12.5, the pH was 8.7, and the viscosity after neutralization was 90 mPa · s. It was equivalent to the regenerated slurry.

  Using any of the above slurries, the silicon ingot was cut without any problem. As can be seen from these results, the regenerated slurry prepared using the recovered abrasive grains can be used without problems for cutting silicon ingots, and new abrasive grains before being used for cutting can be preliminarily washed with an alkaline aqueous solution. By washing, changes in slurry characteristics before and after recycling can be suppressed.

[Example 2]
A slurry was prepared by mixing 40 kg of water, 20 kg of glycerin, 50 kg of SiC abrasive grains (manufactured by Fujimi Incorporated, GC # 1200) and sodium hydroxide. The amount of sodium hydroxide used for slurry preparation is such that the pH of the slurry is 13.5. Using this slurry, a silicon ingot was cut in the same manner as in Example 1. Although the pH of the used slurry was 11.1, a large amount of silicon ingot cutting waste was observed in the used slurry. Sodium hydroxide was added in small portions until the spent slurry had a pH of 12.5. As in Example 1, when the recovered abrasive grains taken out as a precipitate were observed with an optical microscope, no silicon ingot cutting waste was present, and only abrasive grains were observed. Moreover, when the reproduction | regeneration slurry was prepared using the recovery abrasive grain similarly to Example 1 and the silicon ingot was cut | disconnected using this, there was no problem.
As can be seen from this result, it was confirmed that the recycling method according to the present invention was effective even when the silicon ingot was cut using an alkaline slurry. Naturally, in this case, neutralization operation or the like is not necessary when preparing the regenerated slurry.

Claims (5)

  A used slurry characterized in that after adding an alkali component to a used slurry containing abrasive grains used for cutting a silicon ingot and dissolving silicon ingot cutting waste, the abrasive grains are separated from the used slurry. Slurry recycling method.   The method for recycling used slurry according to claim 1, wherein the separation is performed by decanting the used slurry in which abrasive grains have settled.   The recycled slurry according to claim 1 or 2, wherein a regenerated slurry is prepared using the separated abrasive grains.   The used slurry recycling method according to any one of claims 1 to 3, wherein the abrasive grains before being used for cutting are previously washed with an alkaline aqueous solution.   The method of recycling a used slurry according to any one of claims 1 to 4, wherein the slurry used for cutting the silicon ingot is alkaline.