Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B37/00—Lapping machines or devices; Accessories
  • B24B37/04—Lapping machines or devices; Accessories designed for working plane surfaces
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B55/00—Safety devices for grinding or polishing machines; Accessories fitted to grinding or polishing machines for keeping tools or parts of the machine in good working condition
  • B24B55/02—Equipment for cooling the grinding surfaces, e.g. devices for feeding coolant
  • B24B55/03—Equipment for cooling the grinding surfaces, e.g. devices for feeding coolant designed as a complete equipment for feeding or clarifying coolant
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B57/00—Devices for feeding, applying, grading or recovering grinding, polishing or lapping agents
  • B24B57/02—Devices for feeding, applying, grading or recovering grinding, polishing or lapping agents for feeding of fluid, sprayed, pulverised, or liquefied grinding, polishing or lapping agents

Definitions

• the present invention concerns a process and relative apparatus for treating exhausted abrasive slurries from the lapping process for the recovery of their reusable abrasive component. More specifically, the invention concerns a process, with the necessary equipment for implementing it, for recovering the reusable abrasive component contained in an abrasive slurry for lapping silicon, quartz or ceramic wafers when it is exhausted and enriched with undesired waste matter.
• the process enables the total recovery of the still reusable abrasive grains contained in the exhausted slurry coming from the lapping process that are the sole reusable component in this type of slurries, because the suspension liquid consists of water and the suspension additive is present in insufficient amount to make its recovery economically feasible.
• wafers which are obtained from either polycrystalline or monocrystalline silicon ingots by first cutting the ingot perpendicularly to its length.
• this slicing operation is carried out by means of wire saws or cutters in which a metal wire of considerable length and suitable mechanical resistance, wound in a system of rollers and spools, is contacted while moving with the ingot, perpendicularly to the ingot length, at the points where the cut is to be made.
• a slurry containing abrasive grains or particles is fed to the contact area between the cutter wire and the ingot.
• the conventional abrasive slurries used for cutting ingots of silicon, quartz or other ceramic material with a wire saw consist of a suspending, lubricant or cooling fluid such as a mineral oil or water-soluble organic liquids of high molecular weight (in particular, polyethylene glycol, PEG) in which abrasive particles of suitable hardness, generally of silicon carbide (SiC) 1 are suspended.
• a suspending, lubricant or cooling fluid such as a mineral oil or water-soluble organic liquids of high molecular weight (in particular, polyethylene glycol, PEG) in which abrasive particles of suitable hardness, generally of silicon carbide (SiC) 1 are suspended.
• PEG polyethylene glycol
• SiC silicon carbide
• the lapping operation is carried out with lapping machines essentially consisting of two large perfectly flat and round overlapping horizontal cast-iron plates that rotate in opposite directions to one another.
• the lower plate is uniformly covered with an abrasive slurry and the silicon wafers coming from the cutting operation are placed on it within special satellites that are thinner than the silicon wafers to be lapped.
• the satellites are made to rotate by the movement of the lower round plate by means of a cogged transmission system.
• the wafers are then uniformly covered with the abrasive slurry and the upper plate is lowered onto them.
• the whole system is thus made to rotate and the abrasive slurry is continuously fed into the space between the two plates throughout the lapping process. The operation continues until the wafer thickness required by the microelectronics technology is reached.
• the abrasive slurry used for the lapping process is composed of a mixture of de-ionised water (suspending liquid), the actual abrasive, usually alumina, AI 2 O 3 , with a target grain size between 4 and 20 ⁇ m, depending on the required characteristics, and a commercially available suspension additive which has the task of facilitating and stabilising the suspension of abrasive particles in water.
• This additive is present in the suspension in an amount ranging between 3 and 4% weight, and is normally a product of such low eco- nomic value that its recovery from exhausted abrasive slurry is not considered worthwhile.
• a typical composition of abrasive slurry for silicon wafer lapping is as follows:
• PW9 or PW15 20.0% by weight or mixed with other oxides 200-300 g/l - suspension viscosity 450 centipoise
• part of the abrasive grains lose their useful characteristics for the optimal functioning of the process itself because the alumina particles break down into smaller particles and are thus no longer suitable for the lapping operation: this is shown by the fact that the grain size distribution of the abrasive particles decreases to lower average values.
• the abrasive slurry becomes enriched with fine particulate coming from the silicon wafers when their thickness is reduced and from the cast iron plates (largely iron particulate) of the lapping machine that are abraded along with the silicon, as well as from the metal pipes of the machine itself.
• the percentage of fine particles (particles of a size ⁇ 4 ⁇ m of abrasive, iron and silicon) in an exhausted slurry is about 30% of the solid fraction.
• agglomerates of abrasive grains are created and accumulated during the lapping operation, especially owing to the deposition of fine particulate on the abrasive grains themselves, that are undesirable because they can potentially scratch and damage the surfaces undergoing the lapping process.
• the abrasive slurry coming out of the lower plate of the lapping machine no longer has the mechanical and chemical properties of fresh abrasive slurry: should it be reused as it is for another lapping cycle, it would negatively affect lapping process efficiency both in terms of outflow speed and the mechanical parameters of the silicon wafers obtained as well as in terms of metallic contamination.
• the exhausted abrasive slurry that is discarded may be disposed of via municipal and industrial waste treatment plants. However, the resulting sludge must then go to a landfill.
• the Japanese patent application JP 2003266309 ( Komatsu Denshi Kinzoku KK) describes a system for automating the operations for recovering, recycling and feeding the abrasive slurry to the lapping machines, and for improving the lapping yield by breaking down the abrasive grain agglomerates and eliminating the foreign contaminating particles.
• Decomposition of the abrasive grain agglomerates is carried out by means of a battery of tumbler screeners fed with the exhausted slurry coming from the lapping process; the abrasive grain agglomerates are thus broken down and the grains are then separated with the help of a recycling liquid.
• This liquid enables obtaining a suspension containing abrasive grains of suitable specific weight to be reused, which grains are recycled in the lapping machines.
• the recycling liquid itself enables obtaining a second suspension containing the non-reusable abrasive grains (that is, grains of low specific weight) and most of the contaminants, like iron and silicon, and this suspension can then be sent to a landfill for disposal.
• the non-reusable abrasive grains that is, grains of low specific weight
• the sorting system described allows eliminating the abrasive agglomerates but does not yield a high quality of recovered abrasive grains, because the recovered abrasive fraction still contains a certain amount of fine contaminating abrasive particles and fine powders (iron and silicon) which build up in the exhausted abrasive slurry as residues of the lapping process.
• the described process is based on the separation of abrasive grains by centrifugation: the exhausted slurry coming from the lapping machines is fed into a first centrifuge which enables separating the lapping process liquid from the abrasive grains and from the impurities.
• the abrasive grains of the first centrifuge are mixed with a liquid for cleaning and separating the fine particles and are then fed into two consecutive centrifuges which separate the reusable abrasive grains from the cleaning liquid, which will thus contain the fine abrasive grains and impurities.
• the abrasive grains obtained from the two consecutive centrifuges are mixed with the lapping liquid obtained from the first centrifuge and the recovery suspension thus obtained is fed to the lapping machines.
• the separation of the abrasive component and of the impurities from the lapping process liquid is carried out by a single centrifugation operation (first centrifuge).
• first centrifuge a single centrifugation operation
• an object of the present invention is thus to provide a method for treating exhausted abrasive slurries of the kind used in the lapping of silicon wafers by means of lapping machines, and enabling the virtually complete recovery of the abrasive grains that are still of a suitable size to be reused in the lapping process, with an economically feasible process that is easy to run.
• the method must also be able to guarantee the complete elimination, from the recovered and reusable component, of the waste matter coming from the lapping process (such as silicon and iron from the cast-iron plates of the lapping machines) and the broken-down grains of abrasive material that is no longer active.
• ⁇ abrasive pure alumina or in a mixture with other oxides, with grain size between 4 and 20 ⁇ m
• suspension liquid deionised water
• additive of a commercially available type with a high suspension capacity
• This technology which for the specific case of recovering all the useful components coming from the cutting of silicon wafers is certainly more complex, includes a section that can be adapted and advantageously used for recovering the abrasive component of the lapping process, wherein the exhausted abrasive slurry is subjected to separation by removing the reusable abrasive grains from the rest of the suspension by means of wet size-sorting in a hydrocyc- lone, wherein the said operation is optimally carried out not inside a single hydrocyclone but in a battery of hydrocyclones suitably connected in series and fed in counter-current.
• the feed to be treated is fed from the top at high speed and tangentially into the apparatus, so that the centrifugal force pushes the heavier particles towards the sides of the container. Moving in a spiral manner, the heavier particles are then collected in the container's conical bottom (underflow), while the clarified liquid comes out at the top from a central duct (overflow).
• the abrasive- grain enriched suspension obtained as an underflow (UF) from the bottom of the hydrocyclone goes to feed the next hydrocyclone in the series, and the bottom of the latter hydrocyclone yields a suspension of abrasive grains of suitable size for their reuse.
• the present invention specifically provides a process for treating exhausted abrasive slurries from the lapping process, comprising a suspending liquid essentially composed of water and an additive for stabilising the suspension, as well as reusable abrasive grains, fine abrasive grains, fine silicon particles and fine metallic particles, which process comprises the following steps: a) separating an exhausted slurry, by wet size-sorting treatment, into: i) a liquid suspension containing reusable abrasive grains and H) a liquid suspension containing the fine abrasive grains as well as the fine silicon particles and fine metal particles; b) eliminating the said liquid suspension containing the fine abrasive grains and the fine silicon particles and fine metal particles, and collecting the said liquid suspension containing the reusable abrasive grains in order to feed it back into the lapping process; the process being characterised by the fact that the said wet size-sorting treatment of operation a) is carried out by treating the mixture in a first battery of at least two hydrocyclones connected in
• the said first battery consists of between two and six hydrocyclones, but preferably four.
• the aforesaid described process represents the simplest version of the procedure according to the present invention, to be used when the final concentration of fine iron particulate in the recovered abrasive component is not critical.
• the liquid suspension containing the reusable abrasive grains " obtained from the bottom of the last hydrocyclone of the said first battery undergoes further treatment in order to obtain abrasive grains purified of traces of fine iron particles by chemical etching, by adding a solution of acid agent to the liquid suspension containing the reusable abrasive grains.
• the acid agent may be selected from the group consisting of nitric acid, hydrochloric acid, oxalic acid, sulphuric acid, citric acid and tartaric acid or their mixtures, and is used in excess so that the outflow liquid obtained from the treatment, which is discarded, contains the iron salts deriving from the acid leaching operation, along with the excess acid and any fine iron residues.
• the liquid suspension containing the reusable abrasive grains undergoes further wet size-sorting treatment, also carried out by treating the mixture in a battery of at least two hydrocyclones connected in series and fed in counter-current with water.
• a liquid suspension containing the reusable abrasive grains and essentially free of fine unusable abrasive grains is obtained from the bottom of the last hydrocyclone of the second battery.
• the top of the first hydrocyclone of the said second battery yields a liquid suspension of fine iron particulates, excess acid and iron salts, which is discarded.
• the number of hydrocyclones connected in series is normally between two and six, and is preferably four.
• the liquid suspension containing the reusable abrasive grains is subjected to a filtering operation by adding water, thereby obtaining - in the filtration cake - a solid phase containing the reusable abrasive grains and essentially devoid of fine abrasive grains.
• the filtrate of the said filtration operation yields a liquid suspension of excess acid and iron salts, and is discarded.
• the abrasive grains resulting from the process proposed according to the present invention are suitable to be reused in the lapping process, and to this end they are preventively placed in a suspension maturation tank with a shaker mechanism, after adding the necessary amount of water and fresh stabilising additive.
• the recovered abrasive suspension thus obtained may be mixed, before use, with a suitable quantity of fresh abrasive slurry and the whole lot is fed to the lapping process.
• the present invention concerns an apparatus for treating exhausted abrasive slurries coming from the lapping process according to the aforesaid described procedure, consisting of the following interconnected elements in sequence:
• a tank equipped with a shaker mechanism for the maturation of the recovered suspension Preferably, upstream of the said tank with shaker mechanism for the preparation of the lapping suspension liquid to be reused in the lapping process, there is also a tank with a shaker mechanism for the chemical etching of fine iron particles, and downstream of the said tank for the chemical etching of fine iron particles there is a second battery of at least two hydrocyclones con- nected in series, fed in counter-current with water, wherein the first hydrocyclone is fed with the exhausted abrasive slurries and the underflow of each hydrocyclone feeds the next hydrocyclone along with the recycling of the overflow of the next but one hydrocyclone.
• the suspension from the bottom of the last hydrocyclone feeds the said tank with shaker mechanism for the prepara- tion of the lapping suspension to be reused.
• the apparatus arrangement which adopts a filtering operation in place of the second battery of hydrocyc- lones, downstream of the said tank for the chemical etching of the fine iron particles there is a filtering apparatus from which the outflowing clarified liquid, consisting of a liquid suspension of excess acid and iron salts, is discarded.
• Figure 1 shows an overall block diagram of a process for treating exhausted abrasive slurries according to a first embodiment of the present invention
• Figure 2 shows an overall block diagram of a process for treating exhausted abrasive slurries according to a second embodiment of the present invention
• Figure 3 shows an overall block diagram of a process for treating exhausted abrasive slurries according to a third embodiment of the present invention
• Figure 4 shows a simplified apparatus arrangement of the first battery of hydrocyclones connected in series that represents the main section of the arrangement
• Figure 5 shows a simplified apparatus arrangement of the second battery of hydrocyclones connected in series that represents a preferred added section of the arrangement.
• the process according to the preferred variant of the present invention is essentially composed of three sections: 1 st hydrocyc- lone battery - chemical etching - 2 nd hydrocyclone battery.
• the illustrated arrangement is used when the final concentration of iron in the recovered abrasive component is critical, and is essentially com- posed of the following operations: a) collecting the exhausted abrasive slurry from the lapping machines inside a container called Tank 1 , which is necessary because the process may be carried out either in continuous or batch mode, depending on the desired process arrangement; in Figure 1 the process is continuous; b) separating the exhausted slurry, carried out by a wet size-sorting treatment in a first battery of hydrocyclones fed in counter-current, into two components: - an underflow (U. F.) suspension containing the reusable abrasive grains (> 4 ⁇ m) in the suspending liquid; the fine iron particles being still present up to a level of 1000-1500 ppm;
• U. F. underflow
• the same qualitative features of the final product can be obtained with a process carried out according to the arrangement illustrated in Figure 2 and essentially consisting of three sections: 1 st hydrocyclone battery - chemical etching - filtering.
• the arrangement illustrated in Figure 2 is used instead of the one illustrated in Figure 1 when the final concentration of iron in the reusable abrasive component is critical, and essentially involves the following operations: a) collecting the exhausted abrasive slurry from the lapping machines in- side a container called Tank 1 , which is necessary because the process may be carried out either in continuous or batch mode, depending on the desired process arrangement; in Figure 2 the process is in batch; b) separating the exhausted slurry, carried out by wet size-sorting treatment in a first battery of hydrocyclones fed in counter-current, into two components:
• a solid phase consisting of alumina grains of size > 4 ⁇ m and residual fine iron particles which are within the limits of 100-500 ppm; f) mixing the solid obtained by filtering in operation e) with water and an additive in order to obtain the suspension of reusable alumina particles
• the process consists of the following operations: a) collecting the exhausted abrasive slurry from the lapping machines inside a container called Tank 1 , which is necessary because the process may be carried out both in continuous mode or in batch, depending on the desired process arrangement; in Figure 3 the process is of the continuous type; b) separating the exhausted slurry, carried out by wet size-sorting treatment in a first battery of hydrocyclones fed in counter-current, into two components: ⁇ an underflow (U. F.) suspension containing the reusable abrasive grains
• Tank 2 which serves as a storage tank for the suspension containing the reusable abrasive component (> 4 ⁇ m); d) conveying the regenerated abrasive slurry thus obtained to a container called Tank 3, wherein the suspension is kept under shaking and can stabilise; e) mixing the suspension contained in Tank 3 with the suspension prepared with fresh abrasive material and then conveying the resulting abrasive slurry to the lapping machines for use.
• the hydrocyclone battery arrangement illustrated in Figure 4 can be used in order to separate the reusable abrasive grains (size > 4 ⁇ m) from the used and no-longer reusable abrasive grains (size ⁇ 4 ⁇ m) and from the fine particles of iron and silicon.
• the first hydrocyclone of this battery is fed with the exhausted abrasive slurry mixed with the overflow of the second hydrocyclone.
• the first hydrocyclone generates an underflow suspension, which goes to feed the second hydrocyclone, and an overflow suspension containing the abrasive grains of size ⁇ 4 ⁇ m, the fine particles of iron and silicon and the additive present in the exhausted slurry, which is discarded and sent to the waste treatment plant.
• the second hydrocyclone is fed with the underflow suspension of the first hydrocyclone mixed with the overflow suspension of the third hydrocyclone; the overflow suspension generated by the second hydrocyclone goes to feed the first hydrocyclone while the underflow suspension feeds the third hydrocyclone.
• the third hydrocyclone is fed with the underflow suspension of the second hydrocyclone mixed with the overflow suspension of the fourth hydrocyclone; the overflow suspension generated by the third hydrocyclone goes to feed the second hydrocyclone while the underflow suspension feeds the fourth hydrocyclone.
• the fourth hydrocyclone is fed with the underflow suspension of the third hydrocyclone preventively mixed with water; the overflow suspension generated by the fourth hydrocyclone goes to feed the third hydrocyclone while the underflow suspension containing the reusable abrasive grains of size > 4 ⁇ m, with an iron content within the limits of 1000-1500 ppm, is conveyed to collection Tank 2.
• the second battery of hydrocyclones is used in order to further reduce the iron content in the reusable abrasive slurry by combining the separating centrifugal action of the hydrocyclones with the chemical etching activity of an acid on the iron-based particles.
• the first hydrocyclone of the second battery is fed with the reusable abrasive suspension coming from Tank 2 mixed with the overflow of the second hydrocyclone.
• the first hydrocyclone generates an underflow suspension, which goes to feed the second hydrocyclone, and an overflow suspension containing the eliminated fine iron particles and the excess acid, that is discarded.
• the second hydrocyclone of the second battery is fed with the underflow suspension of the first hydrocyclone mixed with the overflow suspension of the third hydrocyclone; the overflow suspension generated by the second hydrocyclone goes to feed the first hydrocyclone while the underflow suspension feeds the third hydrocyclone.
• the third hydrocyclone is fed with the underflow suspension of the second hydrocyclone mixed with the overflow suspension of the fourth hydro- cyclone; the overflow suspension generated by the third hydrocyclone goes to feed the second hydrocyclone while the underflow suspension feeds the fourth hydrocyclone.
• the fourth hydrocyclone is fed with the underflow suspension of the third hydrocyclone preventively mixed with water; the overflow suspension generated by the fourth hydrocyclone goes to feed the third hydrocyclone while the underflow suspension containing the reusable abrasive grains of size > 4 ⁇ m, with an iron content within the limits of 100-500 ppm, is conveyed to collection Tank 2, where the regenerated suspension is prepared by adding water and an additive.
• the process according to the present invention aims to achieve the result of advantageously applying to the field of recovering abrasive grains from the lapping process a technology similar to the one described in the co-pending patent application RM2005
• the present invention is applied to exhausted slurries coming from a different process (lapping instead of cutting), which uses a different abrasive component (alumina or a mixture of oxides instead of silicon carbide), a different liquid support (water instead of polyethylene glycol or oil) and different additives;
• the process phases and relative arrangements have the following main differences: a.
• the underflow of the hydrocyclone battery of the previous process feeds a particular filtration system which, within the same apparatus, also carries out caustic etching, washing, chemical etching, washing and drying of the filtrate, while in the present process the underflow of the first hydrocyclone battery feeds one of the following process flows, depending on the level of iron contamination acceptable in the recovered abrasive component:
• the overflow of the hydrocyclone battery of the previous patent application is subjected to filtration, microfiltration, distillation and final filtration in order to recover the suspension fluid (polyethylene glycol or oil) and to completely eliminate the fine particles, while in the present invention the overflow of the first and second battery of hydrocyc- lones is directly discarded (the suspending fluid is water and is thus not recovered);
• the process described in the present invention can be run either in continuous or in batch mode, depending on the flow arrangement used, while the process in the previous patent application was only in continuous mode.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Grinding-Machine Dressing And Accessory Apparatuses (AREA)
• Treatment Of Sludge (AREA)
• Disintegrating Or Milling (AREA)

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• 2006
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  • 2007-12-04 EP EP07866781A patent/EP2094441B1/fr not_active Not-in-force
  • 2007-12-04 WO PCT/IT2007/000844 patent/WO2008078349A1/fr active Application Filing
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