WO2006135208A1

WO2006135208A1 - Apparatus for recycling the disposed sludge produced in the manufacturing process of the silicon wafer

Info

Publication number: WO2006135208A1
Application number: PCT/KR2006/002301
Authority: WO
Inventors: Young-Chul Chang
Original assignee: Young-Chul Chang
Priority date: 2005-06-17
Filing date: 2006-06-15
Publication date: 2006-12-21
Also published as: KR100703963B1; KR20060132333A

Abstract

An apparatus for recycling disposed sludge produced in a process of manufacturing silicon wafers is provided. The apparatus includes a heating unit, a first centrifugal separator, and a second centrifugal separator. The heating unit reduces viscosity of the disposed sludge by heating the disposed sludge to have a temperature in a range from about 65°C to the boiling point. The first centrifugal separator centrifuges the heated sludge from the heating unit into a solid matter and a liquid matter. The second centrifugal separator centrifuges the liquid matter from the first centrifugal separator into sawdust and cutting oil.

Description

APPARATUS FOR RECYCLING THE DISPOSED SLUDGE PRODUCED IN THE MANUFACTURING PROCESS OF THE

SILICON WAFER

Technical Field

[1] The present invention relates to an apparatus for recycling disposed sludge produced in a process of manufacturing silicon wafers and, more particularly, to an apparatus for recycling disposed sludge by effectively separating and recovering abrasive and cutting oil from the disposed sludge.

[2]

Background Art

[3] As information technologies and semiconductor industries have made incredible progress, the demands for single crystal silicon wafers have been explosively increasing. Single crystal silicon wafers are manufactured by cutting a single crystal silicon ingot into wafers using an abrasive-coated wiresaw while supplying cutting oil. Then, the wafers are polished using a polisher. Generally, the cutting and polishing processes produce a large amount of sawdust as much as about 20% to 30% of the single crystal silicon ingot.

[4] In order to remove the by-products such as the abrasive (silicon carbide, aluminum oxide and silicon dioxide), the sawdust, and the cutting oil, a cleaning process is performed. Therefore, disposed sludge is produced from the silicon wafer manufacturing process, and the disposed sludge includes the abrasive, the Si sawdust, and the cutting oil.

[5] Such disposed sludge, which is produced from the semiconductor wafer manufacturing process, is classified as industrial wastes. Since the sludge contains the abrasive and the cutting oil, the disposed sludge cannot be simply incinerated. Also, the disposed sludge cannot be buried under the ground because the cutting oil seriously pollutes the soil. In order to dispose of the sludge produced from the silicon wafer manufacturing process, the sludge was solidified with cement, and the solidified sludge was buried under the ground.

[6] However, such a conventional disposal method is incongruent from environmental, economical and temporal points of views. In order to overcome the problems of the conventional disposal method that solidifies and buries the sludge, a method for recycling sludge by separating and recovering abrasive, sawdust and cutting oil from the sludge was introduced.

[7] As a conventional method for recycling sludge, a centrifuge based recycling method was widely used rather than a method for recycling sludge through solvent extraction. The sludge is easily dissolved by water or oil, and thus, the cutting oil can be easily separated from the sludge by using water or oil. However, the cutting oil includes emulsifying additive, and the emulsifying additive is easily transformed in a dry or a distillation process. The transformed emulsifying additive causes abrasives to be de posited without being dispersed in the cutting oil when the cutting oil is reused.

[8] The centrifuge based recycling method includes two centrifugation processes. In the first centrifugation process, the sludge is separated into a solid matter including a plenty of abrasives and a liquid matter including sawdust and cutting oil, by which the abrasives are recovered. In the second centrifugation process, the liquid matter obtained in the first centrifugation process is separated into the sawdust and the cutting oil, by which the cutting oil is recovered.

[9] In the conventional centrifuge based recycling method, a cutting fluid is added into the disposed sludge to reduce the viscosity of the disposed sludge before performing the first centrifugation process. The disposed sludge has very high viscosity because the disposed sludge contains a large amount of Si sawdust. Due to the large amount of Si sawdust, it is impossible to centrifuge the disposed sludge without adding the cutting fluid into the disposed sludge. Generally, the cutting oil recovered from the disposed sludge in the second centrifugation process is used as the cutting fluid.

[10] According to conventional technology, the disposed sludge may be heated at the normal temperature or a temperature slightly higher than the normal temperature, for example, about 3O⁰C ± 15⁰C after adding the cutting fluid into the disposed sludge. Heating the sludge is only for smoothly mixing the disposed sludge with the cutting fluid. That is, the heating has been recognized as a supplementary way for improving the efficiency of adding the cutting fluid.

[11] The conventional method for recycling sludge by centrifuging the sludge after adding the cutting fluid has following shortcomings.

[12] At first, adding the cutting fluid inevitably increases the amount of sludge to be processed. This lengthens a processing time, raises a processing cost, and requires large-sized apparatuses. According to an examination, adding the cutting fluid increase the amount as much as about 4 to 5 times.

[13] Also, adding the cutting fluid requires an additional device for returning the cutting fluid recovered from the second centrifugation process and another additional device for keeping the mixing ratio of the disposed sludge and the cutting fluid constant. Those devices make a sludge recycling apparatus complicated.

[14]

Disclosure of Invention Technical Problem

[15] It is, therefore, an object of the present invention to provide an alternative apparatus for recycling disposed sludge to adding a cutting fluid for dilution to improve recycling efficiency

[16] It is another object of the present invention to provide an apparatus for recycling disposed sludge, which minimizes a processing time and a processing cost and has a simple and compacted structure.

[17] It is further another object of the present invention to provide an apparatus for recycling disposed sludge, which has a simple structure that requires a minimum installation cost.

[18]

Technical Solution

[19] In accordance with one aspect of the present invention, there is provided an apparatus for recycling disposed sludge produced in a process of manufacturing a semiconductor wafer, the apparatus including: a heating unit for reducing viscosity of the disposed sludge by heating the disposed sludge to have a temperature in a range from about 65⁰C to the boiling point; a first centrifugal separator for centrifuging the heated sludge from the heating unit into a solid matter and a liquid matter; and a second centrifugal separator for centrifuging the liquid matter from the first centrifugal separator into sawdust and cutting oil.

[20] The heating unit may heat the disposed sludge to have a temperature in a range of about 65⁰C to 9O⁰C.

[21] The apparatus may further include a recycling unit for recycling the disposed sludge to a recycled sludge by mixing the solid matter from the first centrifugal device and the cutting oil separated at the second centrifugal device.

[22]

Advantageous Effects

[23] An apparatus for recycling disposed sludge according to the present invention has following advantages.

[24] The apparatus for recycling disposed sludge according to the present invention provides a high recycling efficiency by controlling the viscosity of the disposed sludge through heating the disposed sludge instead of adding a cutting fluid for diluting the disposed sludge.

[25] Since the apparatus for recycling disposed sludge according to the present invent ion dose not add the cutting fluid into the disposed sludge, the amount of the sludge to be processed is not increased. Therefore, the apparatus for recycling disposed sludge according to the present invention can minimize a processing time and a processing cost, and has a compact structure.

[26] Also, the apparatus for recycling disposed sludge according to the present invention has a simple structure that requires a minimum installation cost. The present invention makes the apparatus for recycling disposed sludge have the high efficiency of recycling the disposed sludge using a simple operation principle and structure.

[27]

Brief Description of the Drawings

[28] The above objects, other objects and features of the present invention will become more apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

[29] Fig. 1 is a diagram illustrating a method of recycling abrasive, sawdust and cutting oil from disposed sludge according to an embodiment of the present invention;

[30] Fig. 2 is a view for inducing an equation for calculating the processing capacity of a centrifugal separator;

[31] Figs. 3 through 10 show results of analyzing the components of a solid matter separated from disposed sludge in the first centrifugation process; and

[32] Fig. 11 is a diagram illustrating an apparatus for recycling disposed sludge according to an embodiment of the present invention.

[33]

Best Mode for Carrying Out the Invention

[34] Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter.

[35] Fig. 1 is a diagram illustrating a method of recycling abrasive, sawdust and cutting oil from disposed sludge according to an embodiment of the present invention.

[36] In order to recycle the disposed sludge by separating and recovering abrasive and cutting oil from the disposed sludge, two centrifugation processes are performed.

[37] The first centrifugation process is performed to separate a solid matter having a plenty of abrasive such as SiC and a liquid matter having a plenty of cutting oil from the disposed sludge. Then, the second centrifugation process is performed to remove fine Si sawdust from the liquid matter by driving a centrifugal separator at a highspeed so as to recycle the cutting oil to be close to new cutting oil that has the concentration of, for example, 0.98g/cc.

[38] In a centrifugal field, a centrifugal force, a buoyant force, and a frictional force are applied to particles as shown in Eq. 1.

[39]

= φπ α³p ω²R - φπ α³p₀ω²R - C^^ ^•■• (1)

[40] The particles are fine grains having a several μ diameter. Since a liquid where the particle floats has very high viscosity, it may assume that Stake's Law is applied for the frictional force.

[41] r - 24 = 12μ (2)

Re apυ

[42] If Eq.2 is substituted for Eq.1, Eq.3 can be obtained.

[43] m^ = (A) π α³p CU ²R - (A) π α ³P _oω ²R - 6π μ a^ - (3)

(it O O

[44] Since all forces applied to the particle in the centrifugal field of the high viscosity liquid keep equilibrium, the acceleration become zero. Therefore, the left term of Eq.3 may become zero and thus an equation for velocity may be expressed as Eq.4.

[45]

2a(ρ — p_o)w²R v - (4)

9μ

[46] In Eq.4, v can be expressed as dRldt because it denotes a radial velocity from the center. Therefore, it may be expressed as Eq.5. [47] dR = [ ²<P-P« >^2R ]_dt ... (5)

R 9μ

[48] If Eq.5 is integrated, Eq.6 is obtained.

[49]

ln(- R|-₀ ) = ^2α2( 9μ ^oK* - (6)

[50] If Eq.6 is simplified for t, Eq.7 is obtained.

[51]

[52] A time required for moving a fine grain from a starting point R to a predetermined radius R can be calculated through Eq. 7.

[53] The linear velocity of the raw sludge dl/dt may be expressed as below Eq. 8.

[54]

AL = R. = ^Q • ■ • (8) dt A _π (R? - R? ) [55] Therefore, the retention time τ of the raw sludge in the centrifugal separator can be calculated by Eq. 9. [56]

[57] In Eq. 7, if the retention time of Eq. 9 is applied into Eq. 7 as the time required for centrifugation, Eq. 10 can be obtained. [58]

[59] In the left term of Eq. 10, terms R , R and L denote the volume of centrifugal separator as shown in Fig. 2, and Q is the velocity of liquid flowing into the centrifugal separator, which denotes the processing capacity of the centrifugal separator.

[60] If Eq. 10 is simplified based on the capacity of the centrifugal separator, it can be expressed as below Eq. 11.

[61]

[62] In Eq. I I, R , R and L are terms related to the volume of the centrifugal separator, and t, a, p,

P o and μ are terms related to the properties of particles and liquid, ω is a term related to the driving condition of the centrifugal separator. [63] The processing capacity of a centrifugal separator can increase by enlarging the volume of the centrifugal separator or accelerating the rotation speed of the centrifugal separator. Among the methods for increasing the processing capacity with the volume of the centrifugal separator and driving conditions fixed, the most effective method for increasing the processing capacity is to reduce the viscosity of liquid.

[64] The viscosity of the liquid may be reduced by adding solvent into the liquid or by increasing the temperature of the liquid.

[65] If the viscosity of the liquid is reduced by adding the solvent into the liquid, the apparent viscosity can be reduced and the processing capacity of the centrifugal separator can increase. However, the dilution is not matched with the object of the cen- trifugation since the object of the centrifugation is the concentration of the particles. Furthermore, since the liquid has to be diluted with a solvent before centrifuging the liquid, the volume of the disposed sludge increases. Consequently, the total processing time for centrifuging the disposed sludge increases.

[66] On the contrary, a method of increasing a temperature can reduce the viscosity of the liquid without increasing the volume of the disposed sludge. Therefore, the method of increasing the temperature has higher efficiency compared to the method of adding the solvent.

[67] Therefore, in the present invention, the viscosity of the disposed sludge is reduced by increasing the temperature of the sludge. Such a method of reducing the viscosity of the disposed sludge is matched with the object of the present invention for increasing the recycling efficiency.

[68] According to a simulation result, it is preferable that the concentration of the recycled cutting oil must be less than 0.93g/cc. If the concentration of the recycled cutting oil is higher than the 0.93 g/cc, the recycled cutting oil may influence the quality of the wafer such as surface roughness and flatness, and requires new cutting oil to be added. Therefore, the related cost thereof increases.

[69] If the temperature of the disposed sludge is lower than 65⁰C when the disposed sludge flows into a first centrifugal separator, the separation efficient of the first centrifugal separator is degraded. Therefore, it is impossible for a second centrifugal separator to refine the cutting oil to have a required concentration, and causes the pipe to be clogged. Therefore, it is hard to apply this method for mass production.

[70] Therefore, the temperature of the disposed sludge must be kept higher than about

65⁰C. If the temperature of the disposed sludge is kept higher than about 65⁰C, the separation efficiency of the first centrifugation is improved and the second centrifugation can produce the cutting oil with the concentration less than about 0.93g/cc which is reusable for wire-sawing.

[71] However, if the temperature of the disposed sludge becomes higher than 9O⁰C, the processing cost for sustaining the temperature increases although the separation efficiency hardly changes. Therefore, it is preferable to maintain the temperature of the sludge in a temperature range from about 65 ⁰C to 9O⁰C.

[72] Figs. 3 to 10 are graphs showing results of analyzing a solid matter separated from the sludge in the above temperature range in the first centrifugation. Those graphs show that the components of the solid matter separated in the first centrifugation are influenced by the heating temperature of the disposed sludge which is heated before the first centrifugation.

[73] Fig. 3 is a table showing results of analyzing the solid matter according to temperatures, and Figs. 4 through 10 are graphs showing the results shown in Fig. 3. Fig. 4 is a diameter analysis graph (granularity 6%), Fig. 5 is a diameter analysis graph (granularity 50%), and Fig. 6 is a diameter analysis graph (granularity 97%). Fig. 7 is a content analysis graph (cutting oil), Fig. 8 is a content analysis graph (Sic), and Fig. 9 is a content analysis graph (Si). Fig. 10 is a density analysis graph.

[74] Referring to Figs. 3 to 10, as the temperature of the disposed sludge increases higher, the Si content and cutting oil content are reduced and the SiC content in the solid matter increases. Some of result data shows different aspects, but it seems to be occurred by the inaccuracy of samples.

[75] Fig. 11 is a schematic diagram illustrating an apparatus for recycling disposed sludge according to an embodiment of the present invention.

[76] Referring to Fig. 11, when the disposed sludge flows into a tank lorry or a drum 1, the disposed sludge is conveyed to a storing tank 3 by a pump P. Since the concentration of the cutting oil may change according to a wire sawing device and collecting conditions such as a number of recycling, a period of reserving, and an external temperature, the conveyed sludge is stirred to be uniform.

[77] In the storing tank 3, the disposed sludge is heated to a predetermined temperature.

That is, the storing tank 3 functions as a heating unit. It is preferable to heat the disposed sludge to have a temperature in a range from about 65⁰C to the boiling point. More preferably, the disposed sludge is heated to have a temperature in a range from about 65⁰C to 9O⁰C.

[78] The sludge heated and stirred at the storing tank 3 flows into the first centrifugal separator 5. The first centrifugal separator separates the sludge into the solid matter having abrasive (SiC) and a liquid matter having sawdust and cutting oil.

[79] If the disposed sludge in the first centrifugal separator 5 is lower than 65⁰C, the first centrifugal efficiency is abruptly degraded and this makes the operation of the second centrifugal separator impossible. That is, if the first centrifugation efficiency is degraded, a plenty of the abrasive flows into the second centrifugal separator and causes the second centrifugal separator to be interrupted.

[80] Sine the cutting fluid is not required to be added, the amount of sludge does not increase. Therefore, the processing time and cost does not increase and the large-sized recycling apparatus is not required.

[81] The solid matter separated from the first centrifugal separator 5 is conveyed to a recycling tank 15 through the storing tank 7, and the liquid matter flows into the second centrifugal separator 11 through the storing tank 9.

[82] The second centrifugal separator 11 separates the liquid matter into sawdust and cutting oil.

[83] The cutting oil separated at the second centrifugal separator 11 is conveyed to the recycling tank 15 through the storing tank 13. The cutting oil is mixed with the abrasive conveyed from the storing tank 7 at a predetermined ratio to produce a recycled sludge. That is, the recycling tank 15 functions as a recycling unit.

[84] While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

[85]

Claims

[1] An apparatus for recycling disposed sludge produced in a process of manufacturing a semiconductor wafer, the apparatus comprising: a heating unit for reducing viscosity of the disposed sludge by heating the disposed sludge to have a temperature in a range from 65⁰C to the boiling point; a first centrifugal separator for centrifuging the heated sludge from the heating unit into a solid matter and a liquid matter; and a second centrifugal separator for centrifuging the liquid matter from the first centrifugal separator into sawdust and cutting oil.

[2] The apparatus of claim 1, wherein the heating unit heats the disposed sludge to have a temperature in a range of 65⁰C to 9O⁰C.

[3] The apparatus of claim 1 or 2, further comprising a recycling unit for recycling the disposed sludge to be a recycled sludge by mixing the solid matter from the first centrifugal device and the cutting oil separated at the second centrifugal device.