Abrasive tool making process – material – or composition – Miscellaneous
Reexamination Certificate
1999-07-01
2001-05-15
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
Miscellaneous
C051S307000, C051S309000, C210S651000, C210S712000, C210S737000, C210S774000, C210S781000, C210S787000
Reexamination Certificate
active
06231628
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a method for the regeneration of an abrasive slurry utilized in the preparation of silicon wafers. More particularly, the present invention relates to a method of separating components of an exhausted slurry, used in slicing wafers from a single crystal or polycrystalline silicon ingot, such that the desirable abrasive grains and lubricating or cooling fluid therein can be reused.
Silicon wafers are obtained from a single crystal or polycrystalline silicon ingot by first slicing the ingot in a direction normal to its axis. Typically, the slicing operation is accomplished by means of a wire saw, wherein the silicon ingot is contacted with a reciprocating wire while a slurry containing abrasive grains is supplied to the contact area between the ingot and the wire. Conventional wire saw slurries typically comprise a lubricating or cooling fluid such as, for example, mineral oil or some water soluble liquid (e.g., polyethylene glycol, or “PEG”), and abrasive grains, such as silicon carbide.
During the slicing operation, silicon particulate are formed which become incorporated into the slurry. As the concentration of the silicon particulate in the slurry increases, the efficiency of the slicing operation decreases. Eventually, the slurry becomes ineffective, or “exhausted,” and must be discarded. Typically, the exhausted slurry is disposed of by incineration or treated by a waste water treatment facility. However, burning this slurry may generate carbon dioxide, while sending this slurry to a waste water treatment facility generally results in the formation of a sludge which must be disposed of in a landfill. Accordingly, both approaches of disposal are unfavorable from an environmental point of view. It is therefore desirable to eliminate, or significantly reduce the amount of, this waste which is generated.
In addition to the negative environmental consequences and the costs associated with the disposal of the exhausted slurry, silicon wafer production costs are also increased due to the fact that the slurry is generally disposed of prematurely. More specifically, the rate at which the abrasive grains in the slurry are “spent” (i.e., worn to a size too small for effective slicing) by the slicing operation is generally much less than the rate at which the slurry becomes contaminated by silicon particulate. Furthermore, the useful life of a typical lubricating or cooling fluid is dictated almost entirely by the build-up of silicon particulate; that is, the lubricating or cooling solution could be used for a much longer period of time, if it were not for the increasing concentration of silicon particulate in the slurry. As a result, the slurry is typically discarded once the level of silicon particulate in the slurry is too high, even though much of the abrasive grains and lubricating or cooling fluid are still usable.
In view of the foregoing, a need continues to exist for a method which may be utilized to separate silicon particulate from the slurry, thus enabling the reuse of abrasive grains and the lubricating or cooling fluid. Such a process would reduce the manufacturing costs associated with the slicing of silicon ingots. In addition, such a process would reduce the amount of waste product and/or waste byproduct emitted into the environment.
SUMMARY OF THE INVENTION
Among the several objects and features of the present invention may be noted the provision of a method for extending the useful life of the components of a slurry utilized in slicing a silicon ingot; the provision of such a method wherein these components are regenerated by separating them from an exhausted slurry; the provision of such a method wherein the lubricating or cooling fluid may be efficiently regenerated by separating it from solids in the slurry; the provision of such a method wherein abrasive grains may be regenerated by separating them from silicon particulate, spent grains and other particulate contaminants in the slurry; the provision of such a method wherein the regenerated lubricating or cooling fluid and the regenerated abrasive grains are reintroduced into the silicon wafer manufacturing process; the provision of such a method wherein the amount of waste which must be discarded is decreased; and, the provision of such a method wherein the cost of producing silicon wafers is reduced.
Briefly, therefore, the present invention is directed to a method for treating an exhausted slurry utilized in slicing silicon wafers from a silicon ingot, the slurry comprising an abrasive grain component and a lubricating fluid component, such that at least one of the components may be recovered and reused in preparing a fresh slurry. The method comprises heating the exhausted slurry to reduce the viscosity, and then separating the heated slurry into a liquid fraction and a solids fraction. The liquid fraction, which comprises the lubricating fluid component, is collected and reused in the preparation of a fresh slurry without further separation.
The present invention is further directed to a method for treating an exhausted slurry utilized in slicing silicon wafers from a silicon ingot, the slurry comprising an abrasive grain component and a lubricating fluid component, such that at least one of the components may be recovered and reused in preparing a fresh slurry. The method comprises heating the exhausted slurry to reduce viscosity and separating the heated slurry into a liquid fraction and a solids fraction, the solids fraction comprising silicon particulate and the abrasive grain component, the abrasive grain component further comprising a spent abrasive grain component fraction and an unspent abrasive grain component fraction. The solids component is mixed with water to form a solid/liquid mixture, which is then separated into a waste product fraction and a recycle fraction, the waste fraction comprising silicon particulate and a spent abrasive grain component fraction, the recycle fraction comprising the unspent abrasive grain component fraction. The unspent abrasive grain component fraction is then isolated from the recycle fraction for reuse in the preparation of a fresh slurry.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.
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Fragiacomo Guido
Zavattari Carlo
Marcheschi Michael
MEMC Electronic Materials , Inc.
Senniger Powers Leavitt & Roedel
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