Abstract:
The present invention is directed toward an apparatus and method for the cross flow filtration of polishing slurry compositions used in semiconductor wafer planarization. In one aspect, an apparatus according to the invention includes an elongated cylindrical filter element adapted to be rotated at predetermined angular velocities that is disposed within a cylindrical housing. The housing has an inlet that is fluidly connected to a source of polishing slurry through a pump, an outlet to provide filtered slurry to a planarization machine, and a bypass outlet that is fluidly connected to the source of polishing slurry to allow refiltration of the bypass fluid. A motor is also included to impart rotational motion to the cylindrical filter element. By rotating the filter element in the housing while slurry is flowing through the housing, a fluid shear layer develops on the filter surface that repels larger particles suspended in the slurry from the filter surface, while admitting those of acceptable size to generate a filtered slurry. A portion of the slurry not subject to filtration is routed to the bypass outlet for refiltration. 
     In an alternate aspect of the invention, the apparatus includes an inner cylindrical filter element and an outer cylindrical filter element that are concentrically disposed within a housing. The filter elements may be independently rotated to achieve multistage filtration of the slurry.

Description:
TECHNICAL FIELD 
     This invention relates to chemical-mechanical planarization of semiconductor wafers, and more particularly to slurry filtration systems used in such machines. 
     BACKGROUND OF THE INVENTION 
     Chemical Mechanical Planarization (CMP) is the preferred technique for globally planarizing semiconductor wafers at high levels of integration. In CMP, the semiconductor wafer is generally mounted in a wafer carrier disposed above a polishing pad that is attached to a rotatable platen. The exposed surface of the wafer is then pressed against the polishing pad with a prescribed down force, and the polishing pad and/or the wafer are then independently rotated while the wafer carrier is translated across the pad surface. While the semiconductor wafer is moved across the polishing pad, a polishing slurry is distributed across the surface of the pad to facilitate planarization of the wafer. The slurry is generally comprised of a combination of chemical etchants and very highly abrasive particles in a liquid suspension to simultaneously etch and abrade the wafer surface as it moves across the pad. Polishing slurry compositions commonly used in wafer planarization are generally comprised of abrasive compounds such as colloidal silicon dioxide or a dispersoid of alumina with particle sizes in the 0.01-0.3 micron range. Suitable chemical agents for etching the wafer are generally chemical compounds such as potassium hydroxide or ammonium hydroxide. 
     A significant problem encountered in CMP is surface damage to the wafer due to relatively large abrasive particles that scratch the surface of the wafer. This problem is partiaily addressed during the slurry manufacturing process, since the abrasive particles that comprise the slurry are sized so that abrasive particles of sufficient size to cause wafer scratching are eliminated. In a typical polishing slurry, however, abrasive particles that are unacceptably large may still be encountered, since the sizing procedure may not exclude all of the abrasive particles of unacceptable size. These abrasive particles, commonly referred to as “tails”, generally exist in slurry formulations in proportion to the cost of the formulation because a reduction in the number of tail particles requires that the abrasive particle sizing be more rigidly controlled when the slurry is formulated. 
     The occurrence of surface scratching particles also results from abrasive particles that combine, or agglomerate, in the slurry to form particles that have an effective size significantly outside the range of acceptability. Thus, surface-scratching agglomerations may form even where the population of tail particles is very rigidly controlled. The absence of sufficient fluid motion in the slurry has been identified as a significant contributor to the formation of abrasive particle agglomerations, since the abrasive particles tend to settle out of the suspension unless the slurry is subjected to some fluid movement. 
     To minimize the possibility of wafer surface scratching due to the presence of tails or agglomerated particles, prior art polishing slurry distribution systems have used fluid filters to trap particles of unacceptable size before the slurry is deposited on the polishing pad of the planarization machine. FIG. 1 shows a typical polishing slurry distribution system according to the prior art. As shown therein, the polishing slurry  12  is retained within a storage tank  11 . The slurry  12  is then pumped from the storage tank  11  by a pump  14 , and delivered to the fluid filter  16 , where the polishing slurry  12  passes through a filter element  17  in a flow direction substantially perpendicular to the surface of filter element  17 . After flowing through the filter element  17 , the slurry  12  emerges as the filtered slurry  13  that is devoid of either tails or agglomerated abrasive material. The filtered slurry  13  may then be supplied to the wafer planarization machine  18  to be consumed during the wafer planarization process. 
     A principal shortcoming inherent in the prior art slurry distribution system  10  resides in the fluid filter  16 . As shown in FIG. 1, the filter element  17  is generally comprised of a fine interwoven network of polypropylene fibers with open flow areas between the fibers to allow the passage of particles up to a prescribed particle size, and to retain those of larger size. As trapped particles steadily occlude the open areas in the filter element  17 , it becomes increasingly restrictive to the flow of slurry, thus limiting the flow of slurry to the planarization pad. Because the flow of slurry continuously deteriorates as slurry flows through the filter  16 , continual readjustments to the wafer planarization procedure must be made in order to achieve consistent planarization results. When the flow of slurry is restricted to minimally sufficient levels, the fluid filter  16  (or more generally, the filter element  17 ) must be removed and replaced. As a consequence, frequent replacement of the filter element  17  commonly occurs in order to achieve a reasonably uniform flow of slurry to the pad over successive wafer planarizations, and to avoid the occurrence of an insufficient slurry flow during any wafer planarization process. 
     Accordingly, the frequent removal and replacement of the fluid filter  16  makes the wafer more expensive to produce, due to the occurrence of equipment downtime required for servicing the filter, in addition to the cost of the replacement filters. 
     The problems associated with particle occlusion of the fluid filter  16 , as described above, are further exacerbated when slurry compositions are used which have large numbers of tail particles. As briefly described above, slurry compositions that contain a greater number of tail particles are generally less expensive to manufacture. Consequently, an economic incentive exists to utilize these compositions in wafer planarization. Prior art slurry distribution systems, however, have not fully permitted the use of these lower cost slurry formulations, since the useful life of the fluid filter would be substantially shortened. Accordingly, the full economic benefit to be derived from the use of these slurry formulations in wafer planarization has not been realized. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward an apparatus and method for the cross flow filtration of polishing slurry compositions used in semiconductor wafer planarization. An apparatus according to one aspect of the invention includes an elongated cylindrical filter element adapted to be rotated at predetermined angular velocities that is disposed within a housing. The housing has an inlet that is fluidly connected to a source of polishing slurry through a pump, an outlet to provide filtered slurry to a planarization machine, and a bypass outlet that is fluidly connected to the source of polishing slurry to allow refiltration of the bypass fluid. A motor is also included to impart rotational motion to the cylindrical filter element. By rotating the filter element in the housing while slurry is flowing through the housing, a fluid shear layer develops on the filter surface that repels larger particles suspended in the slurry from the filter surface, while admitting those of acceptable size to generate a filtered slurry. A portion of the slurry not subject to filtration is routed to the bypass outlet for refiltration. 
     In an alternate aspect of the invention, the apparatus includes an inner cylindrical filter element and an outer cylindrical filter element that are concentrically disposed within a housing. The filter elements may be independently rotated to achieve multistage filtration of the slurry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a slurry distribution system in accordance with the prior art. 
     FIG. 2 is a schematic view of a cross flow slurry filter apparatus in accordance with an embodiment of the invention. 
     FIG. 3 is a cross sectional view of the cross flow slurry filter apparatus in accordance with an embodiment of the invention. 
     FIG. 4 is a schematic view of the cross flow slurry filter in accordance an embodiment of the invention. 
     FIG. 5 is a cross sectional view of a cross flow slurry filter apparatus in accordance with another embodiment of the invention. 
     FIG. 6 is a schematic view of a cross flow slurry filter apparatus in accordance with another embodiment of the invention. 
     In the drawings, like reference numbers identify similar elements or steps. For ease in identifying the discussion of any particular element, the most significant digit in a reference number refers to the Figure number in which the element is first introduced (e.g., element  24  is first introduced and discussed with respect to FIG.  2 ). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally directed to an apparatus and method of slurry flow filtration. Many of the specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 2 through 6 to provide a thorough understanding of such embodiments. One skilled in the art will understand, however, that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description. In addition, it is understood that terms of art such as “polishing slurry” or “slurry” pertain to fluids that contain abrasive particles that are used in semiconductor wafer planarization. 
     FIG. 2 is a schematic representation of a polishing slurry filter  20  according to one embodiment of the invention. As shown therein, the polishing slurry filter  20  is comprised of a housing  200  to sealably contain slurry, and a fluid filter element  210  in the housing  200  that is capable of rotational movement relative to the housing  200 . The housing  200  is further comprised of an inlet port  220 , an outlet port  240 , and a bypass port  230 . An actuator  26  rotates the internal fluid filter element  210 . 
     Still referring to FIG. 2, the operation of the filter  20  will now be described. A source of unfiltered polishing slurry  21  is propelled by a pump  23  through a supply line  22 . Slurry is then transported to the inlet port  220  of the housing  200  by a line  27  to fill the housing  200  with slurry. When the filter element  210  is rotated by the actuator  26 , a fluid shear layer (not shown) develops on a surface of the filter element  210 , which, in turn, allows the larger particles to be repelled away from the filter surface. In response to the pressure imposed by pump  23 , the small particles in suspension that are adjacent to the filter element  210  pass though the element, while the larger particles are collected on the surface of the filter element  210 . Accordingly, filtered slurry moves through the element  210  into the annular space  205 . Due to the pressure gradient imposed by the pump  23 , the filtered slurry leaves the housing  200  at an outlet port  240 , and is transported to the planarization machine  24  through a line  29 . Slurry that contains the larger, wafer scratching abrasive particles leaves the housing  200  through the bypass port  230 . The slurry leaving the housing  200  at the bypass port  230  may be discarded, or alternatively, it may be routed through a bypass line  25  to be reintroduced into the supply line  22 . This alternative configuration is regarded as particularly advantageous since the bypassed slurry contains potentially filterable polishing slurry. 
     An embodiment of the polishing slurry filter  20  will now be described in greater detail. Referring to FIG. 3, the filter  20  includes a cylindrical housing  300  that is a substantially cylindrical in shape with an inlet port  342 , an outlet port  344 , and a bypass port  346 . The inlet port  342  is fluidly connected with a source of polishing slurry through a pump (not shown), and the bypass port  344  is fluidly connected to the source of polishing slurry through a bypass line (also not shown). The cylindrical housing  300  also includes a drive shaft opening  353  that allows the input of motive power to a filter assembly, which will be described in greater detail below. 
     The cylindrical housing  300  may be constructed of metal, or any substantially rigid thermoplastic material, such as nylon, and may be comprised of a casting that is subsequently subjected to machining processes to form he structures identified, or it may be formed entirely by machining processes. 
     Positioned within the cylindrical housing  300  is a rotating filter assembly  310  of substantially cylindrical shape that is contained within the interior of the housing  300 . The filter assembly  310  further includes a filter element  311  positioned on the filter assembly  310  to define a cylindrical filtration surface  312  and an interior fluid cavity  325 . The filter element  311  is preferably comprised of a polypropylene filter material suitable for the filtration of CMP slurry materials. Polypropylene filter materials that may be used in this embodiment are available from the Millipore Corporation of Bedford, Mass., although other filtration materials may be equally suitable. 
     The upper end of the cylindrical filtration surface  312  is enclosed by a circular upper end plate  320  that includes a plurality of flow openings  321 . Attached to the upper end plate  320  is a drive shaft  350  that is rigidly and concentrically connected to the upper end plate  320 . A fluid seal  324  that is capable of rotation is positioned about the upper end plate  320  to prevent the flow of fluid thereby. Although the fluid seal  324  may be comprised of an elastomeric o-ring disposed within a retaining groove in the upper end plate  320 , a number of alternative fluid sealing devices may be used. For example, the fluid seal  324  may be a fluid labyrinth seal. 
     The lower end of the cylindrical filtration surface  312  is enclosed by a lower end plate  323  that is substantially similar in size and shape to the upper end plate  320 , and also includes a plurality of flow openings  325 . A fluid seal  326  prevents the flow of fluid past the lower end plate. The upper end plate  320  and the lower end plate  323  may be constructed of metal, or alternatively, from a substantially rigid thermoplastic material such as nylon, or some other equivalent material. 
     The upper end plate  320  and the lower end plate  323  are connected by a plurality of support members  322  that extend longitudinally along the cylindrical filtration surface  312  to provide support for the filter element  311 . The supports are positioned between the upper end plate  320  and the lower end plate  323  at selected radial locations about end plates  320  and  323 . 
     The rotating filter assembly  310  is supported within the cylindrical housing  300  by a top bearing assembly  331  that retains drive shaft  350 , and is capable of supporting the thrust load due to the weight of the filter assembly  310 . A shaft seal  352  is located adjacent to drive shaft opening  353  to prevent the migration of polishing slurry into the bearing assembly  331 . The bearing assembly  331  may be comprised of a simple journal bearing, or alternatively, more complex bearings, such as antifriction bearings may be used. 
     A motor  351  is attached to drive shaft  350  to rotate the filter assembly  310 . The motor  351  is preferably an electric motor, but other alternative means are equally applicable. For example, the motor  351  may be a fluid operated motor, or alternatively, the rotating filter assembly  310  may be remotely driven by magnetically coupling an external drive apparatus to a ceramic radial disk magnet fixedly attached to elongated filter assembly. Moreover, the motor  351  may be dedicated to providing motive power to more than a single device. For example, the motor  351  may be used to simultaneously drive the rotating filter assembly  310  and the pump  23  that transports slurry to the cross flow filter  30 . 
     The operation of the cross flow slurry filter  20  will now be discussed in connection with FIGS. 3 and 4. Turning to FIG. 4, the cross flow filter  20  is shown in fluid communication with a storage tank  11  that contains a volume of polishing slurry  12 . The filter  20  is also in fluid communication with a planarization machine  48 . As shown therein, a volume of polishing slurry  12  is drawn from a storage tank  11  through the line  22  by the pump  23 , which transports slurry to the cross flow filter  20  though the line  27 . Turning now to FIG. 3, the slurry enters input port  342  in the cylindrical housing  300  and then enters the interior cavity  325  through flow openings  325  in the lower end plate  323 . A shear layer develops on the interior surface of filter element  310  due to the rotation of the element  310 , and the flow of slurry through the interior cavity  325 . Accordingly, the particle concentration gradient described previously is established on the interior surface of filter element  310 , and abrasive particles of selected size will pass through the filter element  310  and into the annular space  313  in response to the pressure imposed by pump  23 . The filtered slurry then leaves the annular space  313  through the exit port  344  for delivery to a planarization machine  48  through line  29  (as shown in FIG.  4 ). The portion of the slurry not subjected to filtration leaves the rotating filter assembly  310  through fluid passages  321 , leaving the cylindrical housing  300  through the bypass opening  346 . 
     In contrast to conventional slurry filters, the filter  20  does not filter all of the slurry that enters the filter because only a portion of the fluid entering the interior cavity  325  is capable of interacting with the fluid shear layer that develops on the inner surface of the filter element  310 . As a result, the slurry that leaves the filter  20  through bypass opening  346  contains slurry that is potentially filterable, together with the larger particles that were excluded by the filtration process. This slurry may accordingly be subjected to repeated filtration within the filter  20  to recover the filterable slurry. 
     Referring again to FIG. 4, the repeated filtration of slurry will be described in greater detail. According to the foregoing description, a mixture of filterable slurry containing elevated amounts of larger abrasive particles emerges from the housing  300  at bypass port  346  and enters the line  25 . The mixture may be selectively discarded through valve  450 , but preferably, the mixture is routed to the inlet of pump  23  through line  25  to be refiltered to recover additional amounts of filterable slurry. A valve  420  is optionally included in line  25  so that the fluid pressure and residence time of slurry within the cross flow filter  20  may be controlled. 
     An additional advantage in recirculating the slurry around the bypass line  25  is that it may be advantageously employed as a recirculation loop for maintaining fluid motion within the system when the flow of slurry to the planarization machine  48  is interrupted by closing valve  440 . Turning to FIG. 4, it is seen that when valve  440  is closed, slurry will continue to circulate about the continuous loop formed by lines  25  and  27 , with fluid motion provided continuously by pump  23 . As mentioned above, maintaining fluid motion within a slurry distribution system significantly inhibits the formation of abrasive agglomerations, and such recirculation loops are well known in the art. For example, U.S. Pat. No. 5,993,647 to Huang, et al. describes a flow recirculation loop that contains a slurry filter. However, the filter disclosed in the Huang reference is not a cross flow filter and is thus incapable of providing the operational advantages as described herein. 
     FIG. 5 is a partial cross-sectional view of a cross flow filter  50  in accordance with an alternate embodiment of the invention. In this embodiment, an apparatus capable of multiple stages of slurry filtration is described, although for clarity of presentation, an apparatus capable of only two successive stages of slurry filtration will be shown. As shown in FIG. 5, the cross flow filter  50  includes a housing  500  that is substantially cylindrical in shape with an inlet port  501 , an outlet port  502 , a bypass port  572 , an intermediate inlet port  503  and an intermediate outlet port  571 , which are integrally formed with the housing  500 . The inlet port  501  is fluidly connected to a source of polishing slurry through a pump (not shown), and the bypass port  572  is fluidly connected to the source of polishing slurry through a bypass line (not shown). The intermediate inlet port  503  and the intermediate outlet port  571  are also fluidly connected in this embodiment. The cylindrical housing  500  further includes a drive shaft opening  573 , and additional internal flow openings  580 - 583  to allow fluid flow from the internal filtration assemblies to the ports  501 ,  503 ,  571  and  572 . 
     Positioned within the cylindrical housing  500  is an inner rotating filter assembly  510  that is contained within an outer rotating filter assembly  530 . The inner rotating filter assembly  510  and the outer rotating filter assembly  530  are constructed in a manner substantially similar to the rotating filter assembly  310  described in the previous embodiment, and will now be described more fully. The inner rotating filter assembly  510  includes a filter element  511  positioned on the inner rotating filter assembly  510  to define an inner cylindrical filtration surface  512 , and an inner interior fluid cavity  513 . As in the previous embodiment, the filter element  511  is preferably comprised of a polypropylene filtration material, although other materials capable of fluid filtration may be used. 
     The upper end of the inner cylindrical filtration surface  512  is enclosed by an inner upper end plate  514  that is circular in shape and includes a plurality of flow openings  515 . Attached to the inner upper end plate  514  is an inner drive shaft  575  that is rigidly and concentrically connected to the inner upper end plate  514 . A fluid seal  518  is disposed about the outer periphery of the inner upper plate  514  to prevent fluid passage between the inner rotating filter assembly  510  and the outer rotating filter assembly  530 . The lower end of the cylindrical filtration surface  512  is enclosed by an inner lower end plate  516  that is substantially similar in size and shape to the inner upper end plate  514 , and which includes flow openings  517 . A fluid seal  519  is similarly disposed about the outer periphery of the inner lower plate  514  to prevent fluid passage between the inner and outer rotating filter assemblies. The inner upper end plate  514  and the inner lower end plate  516  are connected by a plurality of support members  521  that extend longitudinally along the inner cylindrical filtration surface  512  to support the filter element  511 . 
     The outer rotating filter assembly  530  is similar in construction to the inner rotating filter assembly  510 , and also includes a filter element  531  positioned on the outer rotating filter assembly  530  to define an outer cylindrical filtration surface  532 , and an outer interior space  533 . 
     The filter element  531  is also preferably comprised of a polypropylene filtration material, however, the porosity of filter elements  511  and  531  may be different. For example, filter element  511  may be comprised of a material with larger open flow areas to permit the passage of larger abrasive particles, while filter element  531  is comprised of a material with somewhat more restricted open flow areas, to inhibit the passage of a portion of the abrasive particles passed by filter element  511 . 
     The upper end of the outer cylindrical filtration surface  532  is enclosed by an outer upper end plate  533  that includes a plurality of outer flow openings  534  and inner flow openings  535 . Attached to the outer upper end plate  533  is an outer drive shaft  574  that is rigidly and concentrically connected to the outer upper end plate  533  which also allows the inner drive shaft  575  to rotate therein. A shaft fluid seal  536  is located in the outer upper end plate  533  to prevent the migration of fluid into the clearance space between the outer drive shaft  574  and the inner drive shaft  575 . An inner upper fluid seal  590  is disposed on the outer upper plate  533  to prevent fluid passage into the outer interior space  533 . An outer upper fluid seal  591  is similarly disposed on the outer upper plate  533  to restrict fluid passage between the outer interior space  533  and the outer annular space  596 . The outer drive shaft  574  and inner drive shaft  575  may be supported by a bearing  595  attached to the housing  500 . 
     The lower end of the outer cylindrical filtration surface  532  is enclosed by an outer lower end plate  538  that is substantially similar in size and shape to the outer upper end plate  533 , that includes a plurality of outer flow openings  539  and inner flow openings  540 . The outer lower end plate  533  also includes an inner lower fluid seal  593  and an outer lower fluid seal  592  to similarly restrict fluid passage between the outer interior space  533  and the outer annular space  596 . 
     As in the previous embodiment, a motor  351  is attached to the apparatus  50 . In this embodiment, however, the inner rotating filter assembly  510  and the outer rotating filter assembly  530  must be capable of simultaneous rotation, and preferably be rotated in opposite directions and at differing rotational speeds. To achieve simultaneous counter-rotation of assembly  510  and assembly  530 , a transmission  576  capable of providing the required rotational directions and speeds is interposed between the motor  351  and the outer drive shaft  574  and inner drive shaft  575 . Alternatively, the motor  351  may be comprised of a first radial disk magnet fixedly attached to the inner filter assembly  510 , and a second radial disk magnet fixedly attached to the outer filter assembly  530  which magnetically couple with a first and second magnetic drives, respectively. 
     The operation of the multistage cross flow filter  50  will now be discussed with reference to FIGS. 5 and 6. Turning to FIG. 6, the multistage cross flow filter  50  is shown in fluid communication with a storage tank  11  that contains a volume of polishing slurry  12 . The filter  50  is also in fluid communication with a planarization machine  48 . Polishing slurry  12  is drawn from the storage tank  11  through line  22  by the pump  23 , which transports slurry to the multistage cross flow filter  50  though the line  27 . Returning now to FIG. 5, the slurry filtration processes internal to the filter  50  are described. The slurry  12  enters the input port  501  and passes through internal opening  580  in the cylindrical housing  500 . The slurry then proceeds through opening  540  in the outer lower end plate  538  and into the inner interior cavity  513 . Since the inner rotating filter assembly  510  is in motion, fluid filtration in the manner previously described takes place across the inner cylindrical filtration surface  512  and enters the outer interior space  533 . Slurry that is not filtered at the inner cylindrical filtration surface  512  passes out of the inner rotating filter assembly  510  through the flow opening  515 , and proceeds through the flow opening  535  in the outer upper end plate  533  to the flow opening  582  to exit the filter  50  at bypass port  572 . Briefly turning to FIG. 6, the unfiltered slurry that leaves the filter  50  at bypass port  572  may be recycled through filter  50  by transporting the slurry along bypass line  25  to the inlet of pump  23 . A valve  420  is provided in line  25  to control the fluid pressure and residence time of slurry in the inner interior space  513 . Returning now to FIG. 5, the slurry that has passed through the inner cylindrical filtration surface  512  and occupies the outer interior space  533  is of intermediate quality, and is subject to an additional stage of filtration at the outer rotating filter assembly  530 , since the outer rotating filter assembly  530  is simultaneously in motion. Slurry that passes through the outer cylindrical filtration surface  532  is thus fully filtered, and leaves the filter  50  though the exit port  502 , whereupon it may delivered to a planarization machine  48  through line  29  (as shown in FIG.  6 ). Slurry that is not filtered at the outer cylindrical filtration surface  532  leaves the outer interior space  533  through flow opening  534  in the outer upper end plate  533  and through flow opening  583  to exit the filter  50  at the intermediate outlet port  571 . Referring again to FIG. 6, the slurry that leaves intermediate outlet port  571  may be transported to the intermediate inlet port  503  along the line  600  for recycling through the filter  50 . Line  600  further includes a valve  601  to control the pressure and residence time of the slurry in the outer interior space  533 . A portion of the slurry at this intermediate stage may also be removed through valve  450  for use in other planarization processes. 
     Still referring to FIG. 6, the line  600  may be incorporated as an integral part of the cylindrical housing  500  to eliminate the external flow path depicted in FIG.  6 . Further, an optional pump in line  600  may be used to augment the pressure difference across the outer cylindrical filtration surface  532 . 
     The above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed. While specific embodiments of, and examples of, the invention are described in the foregoing for illustrative purposes, various equivalent modifications are possible within the scope the invention, as those skilled in the relevant art will recognize. Moreover, the various embodiments described above can be combined to provide further embodiments. For example, a plurality of filter devices capable of a single stage of filtration, as described in an embodiment of the invention as the filter  20 , may be combined with other similar devices in a series flow arrangement to achieve multiple stages of slurry filtration. In addition, the housing and the filter assembly may be tapered, rather than having a cylindrical form. Finally, the unfiltered slurry may first be introduced into the annular space, and emerge as filtered slurry from the interior volume of the filter assembly. Accordingly, the invention is not limited by the disclosure, but instead the scope of the invention is to be determined entirely by the following claims.