JP7691102B2 - Method for cleaning a slurry concentrator using a separation membrane for cross-flow filtration - Google Patents

Description

The present invention relates to a method for cleaning a slurry concentrator, and in particular to a method for cleaning a slurry concentrator that uses a separation membrane used in cross-flow filtration for regenerating used CMP slurry discharged from a CMP process.

In the manufacture of semiconductor integrated circuits, in addition to planarizing the surface of a wafer substrate, in recent years, methods such as planarizing conductor metals embedded in a substrate in the Damascene method, planarizing insulating materials with a lower relative dielectric constant than _SiO2 , and planarizing via holes to increase the number of stacked cells, as well as methods for manufacturing high-density cells and manufacturing finer patterns of 100 nm or less, have become important. Chemical mechanical polishing (hereinafter referred to as CMP process) is widely adopted for planarizing wafers and members formed on the substrate, and is becoming more important in the semiconductor manufacturing process. As planarization of semiconductor materials becomes more important, the amount of CMP slurry used in the CMP process increases, and the proportion of CMP slurry in the manufacturing cost of semiconductor integrated circuits also increases. For this reason, there is a demand to reduce the price of CMP slurry while maintaining its quality.

One method for reducing the price of CMP slurry is known to be the regeneration of used CMP slurry. For example, a method for regenerating CMP slurry is known that includes a concentration step for removing part of the dispersion medium from used CMP slurry containing abrasive grains, dispersion medium, and impurities, a pH adjustment step for adjusting the pH of the concentrated slurry, and a recovery step for recovering abrasive grains and dispersion medium having a predetermined particle size or less from the pH-adjusted slurry (Patent Document 1). Patent Document 1 aims to efficiently recover abrasive grains (polishing agent) by solid-liquid separation.

In addition, a method for regenerating CMP slurry is also known in which used CMP slurry collected in a tank is circulated and concentrated while removing water through a cross-flow type ultrafiltration unit, and the pH of the concentrate is adjusted to regenerate the CMP slurry (Patent Document 2). A membrane filter such as a ceramic filter is used as the ultrafiltration unit.

When circulating concentration using a cross-flow type ultrafiltration unit or the like is performed for a long period of time, contaminants such as aggregates adhere to the filter unit, and in such cases the filter unit is washed. The filter unit can be washed by disassembling the concentration device to physically remove aggregates adhering to the filter section or the connecting flow path, by circulating a washing liquid without disassembling the concentration device, or by backwashing by running a washing liquid from the permeate side to the concentrate side (Patent Document 3). From the viewpoint of the washing effect, it is most preferable to disassemble the device for washing, but this can be very costly. Therefore, in consideration of the burden and cost required for washing, a washing method that does not require disassembly of the device is preferable. However, depending on the type and amount of contaminants, washing methods that do not require disassembly of the concentration device may not be sufficient to remove them.

Patent Document 4 discloses a method for cleaning a filter unit (hollow fiber membrane module) that includes a primary air cleaning step in which air is supplied to the raw water chamber and the air and some of the water in the raw water chamber are discharged from the top of the raw water chamber, a backwashing step in which the permeated water is pushed into the raw water side after the primary air cleaning step, and a drainage step in which cleaning wastewater is discharged from the bottom of the container after the backwashing step, and describes that this method can thoroughly remove turbid matter adhering to the hollow fiber membrane.

JP 2002-331456 A Special Publication No. 2013-535848 JP 2005-88008 A JP 2020-157250 A

Air washing is a known cleaning method that does not require disassembly of the device, in which a liquid containing air is passed through the filter to remove deposits on the filter surface. However, in order to perform air washing, it is necessary to install a separate gas generator to generate air bubbles. This may lead to increased device costs and maintenance burdens. Furthermore, if the inside of the filter unit has an uneven or bent shape, or if the inner diameter of the flow path is not constant, aggregates and the like are likely to accumulate, and cleaning may be insufficient.

The present invention was made in consideration of these circumstances, and aims to provide a method for cleaning a slurry concentrator that uses a separation membrane used in cross-flow filtration, which allows filter units with complex internal shapes to be easily cleaned without adding any special mechanisms.

The cleaning method of the present invention is a method for cleaning a slurry concentrating device (hereinafter also simply referred to as "concentrating device") that includes a liquid storage tank, a cross-flow type filter unit through which slurry is pumped from the liquid storage tank, and a circulation flow path that connects the liquid storage tank and the filter unit in a ring shape and enables the slurry to circulate, the filter unit having a plurality of filter parts and a connecting flow path disposed between adjacent filter parts, and the cleaning method is characterized in that after forming an air pool in the internal flow path of the concentrating device, a gas-liquid mixture fluid containing the slurry or other liquid and the air in the air pool is passed through the filter parts and the connecting flow path. Hereinafter, the cleaning method of the present invention is also referred to as "this air cleaning".

The above-mentioned slurry may be any of a variety of liquids commonly referred to as suspensions, but is characterized by being a slurry containing abrasive grains such as silica or ceria used in the semiconductor manufacturing process, or a slurry for manufacturing semiconductor integrated circuits. The above-mentioned slurry is preferably used CMP slurry discharged from a CMP process for manufacturing high-density semiconductor integrated circuits.

The air reservoir is formed by closing at least two valves provided before the flow path of the filter unit and draining the slurry or other liquid between the closed valves. It may also be formed by closing a valve provided before the flow path of the filter unit and returning all or part of the slurry in the connecting flow path, the filter section, and the circulation flow path in the filter unit to the liquid storage tank.

The filter section is characterized by having a ceramic filter.

There is no limitation on the shape of the connecting flow path, but a bent shape is often used to facilitate the arrangement of multiple filter units, and when carrying out this air cleaning, the flow rate inside the connecting flow path is more likely to change. This makes it easier for aggregates adhering to the inner wall of the connecting flow path, which has a bent shape like a U-shaped tube, to peel off, resulting in even better cleaning effects.

In this air cleaning, after forming an air pool in the internal flow path of the concentrator, a gas-liquid mixed fluid containing slurry or other liquid and the air from the air pool is passed through the filter section and connecting flow path. Since the filter unit has multiple filter sections and connecting flow paths arranged between adjacent filter sections, it is possible to effectively clean the entire filter unit without adding a special gas generation device. This makes it possible to remove aggregates that are attached inside the filter unit, especially to the bent connecting flow paths between the filter sections.

When concentrating and regenerating used CMP slurry discharged from a CMP process, gels such as pad debris and slurry agglomerates tend to accumulate in the connecting flow paths within the filter unit. The slurry concentrated by the concentrating device cleaned by this air cleaning is used CMP slurry discharged from a CMP process for manufacturing semiconductor integrated circuits. In the present invention, it is possible to remove the agglomerates when concentrating such used CMP slurry, and it is possible to prevent a decrease in the amount of permeated water when viewed as the entire filter unit. This makes it possible to improve the efficiency of regeneration of used CMP slurry.

The air pocket is formed by closing at least two valves provided before the flow path of the filter unit and draining the slurry or other liquid between the closed valves, so that an air pocket can be formed in the flow path in the area closed by the valves before the flow path of the filter unit without using a special gas generating device. This makes it easy to form a gas-liquid mixture fluid for cleaning the filter unit.

The most preferred case of the present invention is when the filter unit has a ceramic filter as the separation membrane. The strength of the filter part is higher than when it is made of organic materials, and even if it is subjected to sudden changes in flow rate or pressure, there is little risk of the pore size of the micropores changing or the filter part being damaged.

There are no limitations on the shape of the connecting flow path, but a bent shape is often used to facilitate the arrangement of multiple filter units, and when liquid containing air is passed through the connecting flow path, the flow rate inside the connecting flow path is more likely to change. This makes it easier for aggregates adhering to the inner wall of a connecting flow path with a bent shape such as a U-shaped tube to peel off, resulting in even better cleaning effects.

FIG. 2 is a schematic diagram of a concentrating device to be cleaned by the method of the first embodiment. FIG. 13 is a schematic diagram of a concentrating device cleaned by the method of the second embodiment. FIG. 13 is a graph showing the transition of the daily average transmembrane pressure. FIG. 1 is a graph showing the change in the daily average permeate amount during normal rotation operation. FIG. 13 is a graph showing the change in the average permeate amount per day during reverse rotation operation.

The following describes the slurry concentrating device that is the subject of this air washing. Figure 1 is a schematic diagram showing an example of a concentrating device that is washed using the washing method of the first embodiment of the present invention. In Figure 1, P01 is a pump that applies pressure to a fluid such as a slurry to cause it to flow, and V01 to V04 are valves for adjusting the flow direction and flow rate of the fluid.

As shown in FIG. 1, the concentrating device 1 includes a liquid storage tank 2, a cross-flow type filter unit 3 through which slurry pumped from the liquid storage tank 2 by a pump P01 passes, and a circulation flow path 4 that connects the liquid storage tank 2 and the filter unit 3 in a ring shape and enables the slurry to circulate. The filter unit 3 has a plurality of filter sections 31 and a connecting flow path 32 disposed between adjacent filter sections 31. The concentrating device 1 includes a plurality of valves V01 to V04. The connecting flow path 32 and the circulation flow path 4 may be provided with separate flow paths and pumps for drawing the slurry out of the concentrating device 1 and for introducing a washing liquid.

As an example of a method for concentrating a slurry using the concentrating device, a method for concentrating used CMP slurry discharged from a CMP process for manufacturing semiconductor integrated circuits will be described. In the method for concentrating CMP slurry, the used CMP slurry is first stored in the storage tank 2. The stored used CMP slurry is pumped by the pump P01 to pass through the filter unit 3. The slurry flows into the filter unit 3 from the inlet 3a and is concentrated (filtered) through a filtration membrane provided on the side wall of the slurry flow path (filter channel) inside each filter part 31. The permeated water that has permeated the filtration membrane is discharged to the outside of the concentrating device 1 from the outlet 31d. While the permeated water is discharged to the outside of the concentrating device 1, the concentrated slurry (effluent) is returned to the storage tank 2. By repeating this circulation, the concentration of the used CMP slurry increases. The concentration may be performed by reverse operation in which the slurry flows in from the outlet 3b side of the filter unit 3 and flows out from the inlet 3a.

When the above-mentioned slurry concentration is carried out over a long period of time, changes in the concentration, acidity, composition, etc. of the slurry occur, which may result in the formation of aggregates of organic and inorganic particles in the liquid. The aggregates may become excessively large as the aggregation progresses (for example, aggregates of 1 μm or more in size). Here, the size of the aggregate refers to the maximum length (major axis) of the aggregate. If the number of excessively large aggregates increases, the risk of clogging the filter unit increases. In such cases, the cleaning method of the present invention can be used to easily clean the concentrator and reduce the possibility of the filter unit becoming clogged.

Details of carrying out the cleaning method of the first embodiment in forward rotation operation will be described below. When performing forward rotation operation, valves V02 and V03 are closed in advance, and these valves remain closed even while the device is being cleaned. As a result, depending on the opening and closing of valves V01 and V04, the gas-liquid mixture fluid flows into the inlet 3a of the filter unit and out of the outlet 3b, flowing in the forward direction through the circulation flow path 4.

The operator performing the cleaning stops the operation of the pump P01 and closes the valve V04 installed in front of the flow path of the filter unit 3 (on the side of the inlet 3a). The operator returns part of the slurry in the connecting flow path 32 and the filter part 31 of the filter unit 3 to the liquid storage tank 2, thereby forming an air pocket in the flow path inside the concentrating device 1. The operator may also form an air pocket with the valve V04 open. The operator may also return all or part of the slurry in the connecting flow path 32 and the filter part 31 of the filter unit 3 and the circulation flow path 4 to the liquid storage tank 2. The operator may not only return part of the slurry to the liquid storage tank 2, but may also form an air pocket by introducing air generated by a special gas generator.

Specifically, the operator stops the operation of pump P01 during the slurry concentration operation, and returns the slurry in the connecting flow path 32 and the filter part 31 of the filter unit 3 to the liquid storage tank 2 through the recovery flow path (not shown). This causes liquid and air to be present in the flow paths inside the concentration device 1. At this time, valves V01 and V04 may be in either an open or closed state. Note that the connecting flow path 32 or the circulation flow path 4 may be used as the recovery flow path, or piping connected separately to each part may be used.

Before forming the air pocket, cleaning with a liquid may be performed in addition to the slurry concentration operation. Therefore, the flow path of the concentration device may be filled with liquid such as pure water or an alkaline aqueous solution, not limited to slurry. For example, in the case of a silica abrasive grain slurry, the liquid may be slurry, pure water, an alkaline aqueous solution, tap water, ion-exchanged water, or permeated water. From the viewpoint of cleaning properties, pure water, an alkaline aqueous solution, ion-exchanged water, or permeated water is preferred, and from the viewpoint of workability, slurry is preferred.

After forming an air pocket in the flow path inside the concentrating device 1, the operator passes a gas-liquid mixture fluid containing slurry and air from the air pocket through the filter section 31 and the connecting flow path 32. Specifically, the pump P01 is operated again to circulate the gas-liquid mixture fluid through the flow path inside the concentrating device 1. If the valve V04 is closed, the valve is opened. In this case, the valve V04 may be opened before or after the operation of the pump P01. The operator may pass a gas-liquid mixture fluid containing not only slurry but also other liquids and air from the air pocket.

By passing a mixed fluid of air and liquid through the entire filter unit, which is composed of multiple filter parts and connecting flow paths arranged between adjacent filter parts, effective cleaning is performed as it passes through the filter unit. This reduces the particle size of aggregates (sediments) that cannot pass through the filter channels and are attached to the connecting flow paths between the filter parts, allowing them to be removed from the connecting flow paths.

The cleaning of the concentrator may be performed by forward rotation, in which the fluid flows from the inlet 3a side of the filter unit to the outlet 3b side, or by reverse rotation, in which the fluid flows from the outlet 3b side to the inlet 3a side. From the viewpoint of cleaning, it is preferable to perform cleaning by combining forward and reverse rotation. Furthermore, the cleaning method of the present invention may be combined with a backwash cleaning method in which the fluid flows from the permeate side to the concentrate side.

Next, we will explain how to clean the device shown in Figure 1 by reverse rotation operation. In reverse rotation operation, valves V02 and V03, which are closed during forward rotation operation, are opened for cleaning. Valves V01 and V04 are closed in advance, and these valves remain closed while the device is being cleaned. Other operations can be performed in the same way as in forward rotation operation. Forward rotation and reverse rotation during cleaning may be operated alternately, or only one of them may be operated continuously. This air cleaning is very effective and can be performed in a short time, but even if the concentrator is operated continuously, sufficient effects can be obtained by performing it once every one to three months.

The details of the concentrator shown in FIG. 1 are described below. The filter unit 3 has three filter parts 31 and two bent connecting flow paths 32 arranged to connect adjacent filter parts 31. The filter unit 3 has filter parts 31a, 31b, and 31c arranged in order from the inlet 3a side of the filter unit 3 to the outlet 3b side.

The shape of the connecting flow path 32 is not limited, but a bent shape such as a U-shaped tube is often used, which makes it easy to arrange multiple filter units. The connecting flow path 32 may be a straight tube shape instead of a bent shape. Also, connecting flow paths with different inner diameters may be used.

The filter section has a ceramic filter inside as a filtration membrane. Note that the filtration membrane inside the filter section is not limited to a ceramic filter and can be freely selected from various filters.

The filtration membrane used in the concentrator is a separation membrane used in cross-flow filtration, and may be an ultrafiltration membrane or a reverse osmosis membrane as long as it has a hollow fiber shape. When used to concentrate and regenerate used CMP slurry, an ultrafiltration membrane is preferably used from the viewpoint of most efficiently recovering the abrasive particles in the concentrated liquid after recovery.

The separation membrane used in the cross-flow filtration to which this cleaning method is applied may be an organic membrane made of an organic material, or an inorganic membrane (ceramic filter) made of an inorganic ceramic. The organic membrane is preferably made of any of polyethylene, tetrafluoroethylene, polyvinylidene fluoride, polypropylene, cellulose acetate, polyacrylonitrile, polyimide, polysulfone, polyethersulfone, aromatic polyamide, and polyvinyl alcohol, and the inorganic membrane is preferably made of ceramic materials such as aluminum oxide, zirconium oxide, and titanium oxide, stainless steel, and glass.

A preferred filtration membrane for the filter section of the concentrator cleaned by this air cleaning is a ceramic filter. This membrane has about 30 to 40 holes (filter channels) with an inner diameter of 3 mm to 4 mm, like a lotus root, in one ceramic module, and when stretched out, it is about 1 m long, and about 20 of these are bundled together and housed in the filter section. The flow rate can be freely set, but for example, when the liquid flows through the internal flow path of the concentrator at a flow rate of 60 m ³ /h, it flows through the flow path inside the filter channel at about 3.4 m/s.

The number of filter parts is not limited to three, and any number can be provided. The number of filter parts can be, for example, 1 to 10. From the standpoint of concentration processing speed, equipment costs, maintenance burden, etc., it is preferable that the number of filter parts is 2 to 8, and more preferably 3 to 6.

By using a ceramic filter in the filter section, the strength of the filter section is higher than when it is made of organic materials, and even if it is subjected to sudden changes in flow rate or pressure, there is little risk of the pore size of the micropores changing or the filter section being damaged. This allows a higher pressure to be applied to the filter unit than during normal concentration operation, resulting in a greater cleaning effect.

As the gas-liquid mixture that flows into the filter unit passes through many small-diameter filter channels, the gas is divided even finer than when it flowed in, and the gas becomes a gas-liquid mixture that contains bubbles of about 0.5 mm to 1 mm. This is thought to make it even easier to peel off and remove deposits that have adhered to the inside of the connecting flow path.

Next, a cleaning method according to a second embodiment of the present invention will be described with reference to FIG. 2. Note that detailed descriptions of the matters described with reference to FIG. 1 will be omitted.

Figure 2 is a schematic diagram showing an example of a concentrating device cleaned by the cleaning method of the second embodiment. In Figure 2, P01 is a pump that pressurizes the fluid to flow, and V01 to V04' are valves for adjusting the flow direction and flow rate of the fluid. As shown in Figure 2, the circulation flow path 4 extends from the pump P01 toward the filter unit 3 and branches into two. In one of the branched circulation flow paths 4, two valves V04 and V04' are provided on the side of the inlet 3a, which is in front of the flow path of the filter unit 3 in the case of forward rotation operation. In the other branched circulation flow path 4, two valves V02 and V02' are provided on the side of the outlet 3b, which is in front of the flow path of the filter unit 3 in the case of reverse operation. Here, "in front of the flow path of the filter unit" refers to the side into which the fluid actually flows when it flows into the filter unit, and in forward rotation operation, the side of the inlet 3a of the filter unit 3 is in front of the flow path of the filter unit, and in reverse operation, the side of the outlet 3b of the filter unit 3 is in front of the flow path of the filter unit.

In the second embodiment, when only forward operation is performed, only one valve V02 may be provided on the outlet 3b side of the filter unit 3, and when only reverse operation is performed, only one valve V04 may be provided on the inlet 3a side of the filter unit 3. When cleaning with forward operation, the two valves V04 and V04' are closed before forming an air pocket and are opened before passing the gas-liquid mixture fluid to the filter unit. When cleaning with reverse operation, the two valves V02 and V02' are closed before forming an air pocket and are opened before passing the gas-liquid mixture fluid to the filter unit.

Details of carrying out the cleaning method of the second embodiment in forward rotation operation will be described below. When performing forward rotation operation, valves V02, V02', and V03 are closed in advance, and these valves remain closed even while the device is being cleaned. As a result, depending on the opening and closing of valves V01, V04, and V04', the gas-liquid mixture fluid flows into the inlet 3a of the filter unit and out of the outlet 3b, flowing in the forward direction through the circulation flow path 4.

The operator performing the cleaning stops the operation of pump P01, closes the two valves V04 and V04' installed in front of the flow path of the filter unit 3, and extracts the slurry between the closed valves. This causes an air pocket to form in the flow path between the two valves V04 and V04'. Note that the operator may close more than two valves installed in front of the flow path of the filter unit, and the liquid to be extracted is not limited to slurry and may be pure water, permeated water, or other liquid.

Specifically, the operator stops the operation of pump P01 during the slurry concentration operation, closes the two valves V04 and V04', and extracts the slurry in the flow path between the two valves V04 and V04' through a recovery flow path (not shown) and returns it to the liquid storage tank 2. This leaves liquid and air in the flow path inside the concentration device 1. At this time, pump P01 may be stopped or may remain operating. Also, the extracted slurry does not have to be returned to the liquid storage tank 2.

After forming an air pocket in the internal flow path of the concentrating device 1, the operator passes the gas-liquid mixture fluid, which contains the slurry and the air from the air pocket, through the filter section 31 and the connecting flow path 32. Specifically, the operator operates the pump P01 again and opens the valves V04 and V04' to circulate the gas-liquid mixture fluid through the internal flow path of the concentrating device 1. Note that the valves V04 and V04' may be opened before or after the operation of the pump P01.

By cleaning using the above-mentioned method, an air pocket can be formed in the flow path in front of the filter unit flow path in the area closed by the valve without the need for a special gas generating device. This makes it easy to create a gas-liquid mixture fluid for cleaning the filter unit.

In addition, the greater the pressure applied when the valves V04 and V04' are opened, the greater the sudden pressure change, and the greater the vibration and impact that the flow path in the filter unit receives. Therefore, from the viewpoint of cleaning, it is preferable to open the valves V04 and V04' that form the air reservoir and operate the pump P01 at the same time to pressurize the gas-liquid mixture fluid, and it is more preferable to operate the pump P01 before opening the valves V04 and V04' to pressurize the air reservoir. When a sudden pressure change occurs, the gas-liquid mixture fluid flows suddenly into the connecting flow path, and greater vibration and impact are applied to the connecting flow path than when the pump is operated after the valves are opened, so that the cleaning effect is superior. Note that when the valve V01 is closed together with the valves V04 and V04', the valve V01 is also opened together with the valves V04 and V04' in order to pass the gas-liquid mixture fluid through the filter section 31 and the connecting flow path 32. When multiple closed valves are opened, it is preferable to open the closed valves at the same time from the viewpoint of causing a sudden pressure change.

When pressure suddenly drops in the internal flow path of the concentrator, if an aqueous liquid is used as the fluid, water vapor bubbles, a phenomenon known as cavitation, are generated. The cleaning method of the present invention creates a sudden pressure drop by operating the pump and opening and closing the valve, which generates bubbles that can be used for cleaning as a gas-liquid mixed fluid.

The cleaning method of the second embodiment may be operated in either the forward or reverse direction, following the first embodiment. When operated in reverse, the gas-liquid mixture flows into the connecting flow path in the opposite direction to when the connecting flow path is operated in forward direction. This allows the gas-liquid mixture to apply vibrations and impacts to the connecting flow path from a direction that is less likely to receive flow from the fluid during normal operation (forward operation), and is expected to provide excellent cleaning effects.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

The concentrator shown in FIG. 1, which is equipped with a ceramic filter in the filter section, was operated for 200 days, during which cleaning tests were conducted using the methods of the Examples, Comparative Examples, and Reference Examples described below, and the changes in the transmembrane pressure and permeate volume before and after each cleaning were evaluated. Here, the transmembrane pressure is the pressure difference between the pressure (pressure 1) at the end of the filter section 31a on the inlet 3a side and the pressure (pressure 3) at the end of the filter section 31c on the outlet 3b side (see FIG. 1). The transmembrane pressure in normal rotation operation is pressure 1 - pressure 3, and in reverse rotation operation is pressure 3 - pressure 1. In addition, the transmembrane volume is the F1 transmembrane volume, the F2 transmembrane volume, and the F3 transmembrane volume, respectively, of the filter section 31a, the F2 transmembrane volume, and the F3 transmembrane volume of the filter section 31c.

[Example]
In the concentrator 1 shown in Fig. 1 equipped with a ceramic filter in the filter section, the operation of the pump P01 was stopped during the concentration operation in which the slurry was circulated at a flow rate of about 60 ^m3 /h. The slurry inside the filter section 31, the connecting flow path 32, and the circulation flow path 4 was returned to the liquid storage tank 2 to form an air pocket in the flow path inside the concentrator 1. At this time, the valves V01 and V04 were in an open state. Next, the stopped pump P01 was operated again to circulate a gas-liquid mixed fluid containing the slurry and air from the air pocket in the flow path inside the concentrator 1 for cleaning. The slurry was circulated for 9 hours at a flow rate of about 30 ^m3 /h by normal rotation (air cleaning).

[Comparative Example]
The permeated water was pumped from the outlet 31d provided in the filter section 31 into the filter unit 3 to replace the slurry, thereby filling the flow path of the concentrator 1 with the permeated water. Thereafter, the permeated water was circulated for 9 hours at a flow rate of about 60 ^m3 /h by normal rotation without forming any air pockets (liquid washing). Here, the permeated water had a very low concentration of components and was in a state close to pure water.

[Reference example]
As a conventional cleaning method, the concentrator was disassembled and the inside of the storage tank, circulation flow path, and connecting flow path were cleaned, and the ceramic filter was replaced (disassembly and cleaning).

Figure 3 shows the daily average transmembrane pressure (pressure 1 - pressure 3) during forward operation and the daily average transmembrane pressure (pressure 3 - pressure 1) during reverse operation when the concentrator was operated for 200 days. During the concentration operation, forward and reverse operation were alternated approximately every hour. Note that no measurements were taken during cleaning or when the device was stopped for equipment reasons. Figure 4 shows the daily average permeate volume during forward operation when the concentrator was operated for 200 days, and Figure 5 shows the daily average permeate volume during reverse operation. In the figures, the locations shown as examples, comparative examples, and reference examples indicate the time when cleaning was performed using each cleaning method.

As shown in Figure 3, the transmembrane pressure difference hardly changes over the entire period, in both forward and reverse operation, regardless of the operating conditions or before and after cleaning. The transmembrane pressure difference during forward operation is approximately 430 kPa, while the transmembrane pressure difference during reverse operation is approximately 380 kPa. This is thought to be due to differences in the positional relationships (distances) of the piping length, valves, filter units, etc., as viewed from the pump side and the storage tank side.

It was confirmed that the amount of permeated water recovered after cleaning and gradually decreased over time. In the case of normal rotation operation, the decrease in the amount of F1 permeated water due to operation after cleaning was relatively small, while the decrease in the amount of F3 permeated water was significant. Not only the amount of F3 permeated water, but also the amount of F2 permeated water tended to gradually decrease with continued operation (Figure 4). In the case of reverse rotation operation, in contrast to normal rotation operation, the decrease in the amount of F3 permeated water due to operation after cleaning was relatively small, and the decrease in the amount of F1 permeated water was the largest. Not only the amount of F1 permeated water, but also the amount of F2 permeated water tended to gradually decrease with continued operation (Figure 5).

Comparing the degree of recovery of the permeate volume by each cleaning method, the Reference Example (disassembly and cleaning) has a large degree of recovery of the permeate volume after cleaning and a large cleaning effect because each component of the concentrator is physically cleaned using cleaning tools and the ceramic filter is also replaced. In the Example (air cleaning), a significant recovery of the permeate volume was confirmed, although not as much as in the Reference Example, despite the fact that the concentrator was not disassembled or the filter was not replaced. In contrast, a slight recovery of the permeate volume was confirmed in the Comparative Example (liquid cleaning), but the degree of recovery was small compared to the Example, and a rapid decrease in the permeate volume was observed in the initial operation after cleaning.

As shown in Figures 4 and 5, a decrease in the amount of permeated water over time due to long-term concentration operation was confirmed in both forward and reverse rotation. If the slurry stored in the storage tank were uncontaminated pure water, it is thought that there would be no decrease in the amount of permeated water over time. However, in reality, pad debris generated by CMP polishing, aggregates of particulate components in the slurry, or gelled products of these substances gradually accumulate, and it is thought that this causes a decrease in the amount of permeated water.

In general, micropores of about 50 nm formed on the side walls of a filter channel with an inner diameter of 3 mm in a ceramic filter are likely to be considered the cause of filter clogging. However, in such concentrating devices equipped with cross-flow type filter units, a backwash mode is executed in which permeate water is stored in a backwash tank at set intervals during concentration operation and returned to the filter channel at high pressure to prevent clogging of the micropores. For this reason, clogging is unlikely to occur during normal concentration operation.

The F1 permeate volume, which is the volume of water that passes through the filter section on the inlet side during forward operation, and the F3 permeate volume, which is the volume of water that passes through the filter section on the outlet side during reverse operation, were both approximately 20 L/min and did not decrease over time, so it is believed that there is no significant difference in the clogging of the three ceramic filters themselves.

Considering factors other than the filter section as causes of the decrease in the amount of permeate over time, one possibility is that aggregates have adhered to the connecting flow path of the filter unit. Because the connecting flow path used in this test was a U-shaped tube, the flow inside the tube was prone to turbulence, and areas where the flow was prone to stagnation were likely to occur. Furthermore, because the inner diameter of the connecting flow path was not constant, the flow speed differed depending on the location inside the U-shaped tube, and it is thought that aggregates were likely to accumulate in areas on the inner wall of the tube where the flow speed was slow.

When a fluid leaves a filter section and flows through a connecting flow path into the next adjacent filter section, the flow rate is slowest in the sections with a large pipe diameter or in concave sections. For example, in the case of a connecting flow path with an inner diameter of about 200 mm at both ends, which are connected to the filter section, and an inner diameter of about 100 mm at the middle section, the flow rate of the fluid passing through the flow paths at both ends is significantly slower, at about 1/4 of the flow rate at the middle section. Because the connecting flow path is sandwiched between filter sections equipped with ceramic filters with a channel diameter of 3 mm, if the particle size of the aggregates exceeds 3 mm, they tend to remain and accumulate in the connecting flow path. It is thought that if this deposit accumulates on the surface of the filter housing, it will clog the filter channel.

Coagulation can also accumulate on the housing surface on the side connected to the circulation flow path of each filter section located on the inlet and outlet sides of the filter unit. However, in the backwash mode, which is performed periodically during concentration operation, the liquid is sent at a flow rate about five times faster than the flow rate during normal concentration operation, so even if deposits temporarily form on the housing surface, they are likely to be washed away and removed by the strong backwash flow.

The amount of permeated water from each filter part when the fluid flows ideally, such as in a state without deposits or filter clogging immediately after decomposition and cleaning, will be described. Under normal operation conditions, when the inlet flows in at 60 m ³ /h, the amount of permeated water F1 is 20 L/min, the amount of permeated water F2 is 19.6 L/min, the amount of permeated water F3 is 19.2 L/min, and the amount of permeated water flows out from the outlet at 56.5 m ³ /h. However, in reality, as shown in FIG. 4, only the amount of permeated water from the two filter parts on the storage tank side (the amount of permeated water F2 and the amount of permeated water F3) decreased over time. This is thought to be due to the fact that the flocculated matter was deposited in the two U-shaped tubes by continuing the concentration operation for a long period of time, causing an imbalance in the membrane pressure difference of each filter.

Specifically, the transmembrane pressure ideally applied to each of the three filter sections is approximately 1/3 of the transmembrane pressure applied to the entire filter unit. When concentration operation is performed over a long period of time, causing aggregates to accumulate in the connecting flow path and increasing the load required to flow through the connecting flow path, it is believed that the downstream filter section connected to the connecting flow path is subjected to a transmembrane pressure in which the above load is lost as pressure loss from the ideal transmembrane pressure. It is believed that this is why the amount of permeated liquid from the two downstream filter sections decreased over time, even though the micropores of each ceramic filter were not clogged.

The concentrator cleaning method of the present invention can easily clean filter units with complex internal shapes, and is therefore suitable for cleaning concentrators used to regenerate used CMP slurry discharged from CMP processes.

REFERENCE SIGNS LIST 1 Concentration device 2 Storage tank 3 Filter unit 3a Inlet 3b Outlet 31, 31a, 31b, 31c Filter section 31d Outlet 32 Connecting flow path 4 Circulation flow path P01 Pump V01 to V04, V02', V04' Valve

Claims (3)

A method for cleaning a slurry concentrating apparatus using a separation membrane used for cross-flow filtration, the apparatus comprising: a liquid storage tank; a cross-flow type filter unit through which a slurry is passed from the liquid storage tank by a pump; and a circulation flow path that connects the liquid storage tank and the filter unit in a ring shape and enables the slurry to circulate, the method comprising the steps of:
The filter unit has a plurality of filter parts and a connecting flow path disposed between adjacent filter parts,
the cleaning method is a method of forming an air pocket in a flow path inside the concentrating device, and then passing a gas-liquid mixture fluid containing the slurry or other liquid and the air in the air pocket through the filter unit and the connecting flow path ,
a filter unit that is provided with a filter section that is connected to the filter unit through a passage for filtering the slurry or other liquid from the passage; a filter unit that is connected to the filter section through the passage for filtering the slurry or other liquid from ... The method for cleaning a slurry concentrator using a separation membrane used in cross-flow filtration according to claim 1, characterized in that the slurry is used CMP slurry discharged from a CMP process for manufacturing semiconductor integrated circuits. 3. The method for cleaning a slurry concentrating apparatus using a separation membrane used for cross-flow filtration according to claim 1, wherein the filter portion comprises a ceramic filter.

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