Abstract:
A method of applying a fluid to a brush is provided. The method includes outputting a flow of fluid from a shaft to an area between the shaft and a distributor where the flow of fluid is restricted by the distributor to generate a uniform pressure buildup inside of the distributor. The method further includes delivering the fluid from the area through at least one opening in the distributor to an outer surface of the distributor where the outer surface of the distributor abuts an inner surface of a housing. The method additionally includes dispensing the fluid from between the outer surface of the distributor and the inner surface of the housing to an outer surface of the housing through at least one perforation in the housing, the housing being attached to a brush. The method also includes applying the fluid through the brush where the fluid is received from the outer surface of the housing. The uniform pressure buildup inside of the distributor enables the brush from end to end to receive an approximate equal amount of liquid.

Description:
This is a Divisional of application of copending prior application Ser. No. 09/112,666 filed on Jul. 9. 1998, now U.S. Pat. No. 6,247,197. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor processing and more particularly to a brush assembly for cleaning wafers. 
     BACKGROUND OF THE INVENTION 
     Semiconductor manufacturing processes demand wafers, typically silicon wafers, which are substantially particulate free. As the semiconductor industry moves towards processing larger diameter wafers, for example 300 mm diameter wafers, it becomes increasingly difficult to remove particulates from the wafers. In particular, wafer cleaning processes must effectively remove particulates from the larger wafer surface area associated with the larger diameter wafers. Further, wafer cleaning processes must clean the wafers without exerting undue force on the wafers since larger diameter wafers have less mechanical strength than smaller diameter wafers. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a brush assembly includes a distributor having a slot matrix formed in an outer surface of the distributor, the slot matrix including a plurality of longitudinal slots intersecting a plurality of annular slots. The distributor is mounted on a hollow shaft having a plurality of perforations. The brush assembly further includes an outer housing having an inner surface abutting the outer surface of the distributor and a brush mounted on the housing. 
     During use, liquid flows from inside of the shaft through the shaft perforations to the distributor. The liquid then flows through a plurality of perforations in the distributor, one perforation being located in each longitudinal slot between adjacent annular slots. After flowing through the perforations in the distributor, the liquid flows through the longitudinal slots to the annular slots. The liquid then flows through the annular slots in the distributor to and through annular columns of perforations in the housing. The liquid flowing through the annular columns of perforations in the housing flushes the brush from the inside out. 
     Of importance, the flow of liquid from the shaft to the housing is readily controlled by appropriately selecting the dimensions of the longitudinal slots and annular slots in the distributor through which the liquid must flow. Generally, increasing the cross-sectional area and, to a lesser extent, decreasing the length of a slot increases the flow of liquid through the particular slot and vice versa. Thus, the flow of liquid from the shaft to the housing is readily controlled (restricted) by selecting the cross-sectional area of the longitudinal slots and annular slots of the distributor. As an illustration, a first distributor having longitudinal slots and annular slots with greater cross-sectional areas than those of a second distributor will allow a greater amount of liquid to flow from the shaft to the housing than the second distributor. 
     In one embodiment, the annular slots are formed closer together near the ends of the brush than in the center of the brush. As a result, a greater amount of liquid is provided to the ends of the brush than to the center. This is a particular advantage in wafer cleaning operations where a greater effective wafer surface area near the ends of the brush must be cleaned. 
     The distributor also restricts the liquid flow from the shaft to the housing. This allows the number of perforations in the housing to be increased without significantly increasing the overall amount of liquid used. This is particularly advantageous since increasing the number of perforations in the housing reduces localized nonuniform flushing of the brushes. Further, by restricting the flow of liquid, the distributor causes a uniform pressure buildup inside of the distributor. This, in turn, ensures that both ends of the brush receive the same amount of liquid and are uniformly flushed which improves particulate removal from the brush and reduces or eliminates uneven wear of the brush. 
     In accordance with the present invention, a method of removing particulates from a brush is provided. The method includes creating a liquid flow from a shaft to a housing, the brush being mounted on the housing. This liquid flow is redistributed by a distributor. In one embodiment, the liquid flow is redistributed to restrict the flow of liquid. In another embodiment, the liquid flow is redistributed to cause a greater amount of the liquid flow to flush the ends of the brush than the center of the brush. 
     These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the various embodiments set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of wafer cleaning system including a pair of wafer cleaners. 
     FIG. 2 is a partial top view of one of the wafer cleaners. 
     FIG. 3 is a partial frontal view of the wafer cleaner. 
     FIG. 4 is a partial perspective view of the wafer cleaner. 
     FIG. 5 is a partial side view of the wafer cleaner. 
     FIG. 6 is a frontal view of wafer cleaner during use in accordance with the present invention. 
     FIG. 7 is a side view, partially cutaway, of a brush assembly which provides a desired liquid flow distribution in accordance with the present invention. 
     FIG. 8 is a side view of a region of the distributor of FIG. 7 in accordance with the present invention. 
     FIG. 9 is a cross-sectional view of the distributor along the line IX—IX of FIG. 8 in accordance with the present invention. 
     FIG. 10 is a side view of a distributor in accordance with one embodiment of the present invention. 
     FIG. 11 is a cross-sectional view of the distributor of FIG. 10 in accordance with this embodiment of the present invention. 
     FIGS. 12 and 13 are end plan views of the distributor of FIG. 10 in accordance with this embodiment of the present invention. 
     FIG. 14 is a cross-sectional view of a housing for use with the distributor of FIGS. 10-13 in accordance with this embodiment of the present invention. 
     FIG. 15 is a cross-sectional view of the housing of FIG. 14 mounted on the distributor of FIGS. 10-13 in accordance with this embodiment of the present invention. 
     FIG. 16 is a cross-sectional view of the housing and distributor along the line XVI—XVI of FIG. 15 in accordance with this embodiment of the present invention. 
     FIG. 17 is an end plan view of the housing and distributor of FIG. 15 in accordance with this embodiment of the present invention. 
     FIG. 18 is an exploded perspective view of a brush assembly without a brush in accordance with this embodiment of the present invention. 
     FIG. 19 is a cross-sectional view of a cap in accordance with this embodiment of the present invention. 
     FIG. 20 is an end view of the cap of FIG. 19 in accordance with this embodiment of the present invention. 
     FIG. 21 is a side view, partially cross-sectioned, of a shaft in accordance with this embodiment of the present invention. 
     FIG. 22 is a cross-sectional view of a hub in accordance with this embodiment of the present invention. 
     FIGS. 23 and 24 are end plan views of the hub of FIG. 22 in accordance with this embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Several elements shown in the following Figures are substantially similar. Therefore, similar reference numbers are used to represent similar elements. 
     FIG. 1 is a perspective view of wafer cleaning system  8  including wafer cleaners  14 ,  16 . Wafer cleaning system  8  includes a robotic arm  10 , a wet buffer unit  12 , wafer cleaners  14 ,  16 , a spin drying unit  18 , and a finish cassette  20 . 
     Robotic arm  10  has an end-effector  11  which uses a vacuum to hold a wafer. End-effector  11  can be rotated from the horizontal position in which arm  11 A is located horizontally from arm  11 B, as shown in FIG. 1, to a vertical position in which arm  11 A is located above arm  11 B. Wet buffer unit  12  includes a plurality of horizontal slots in which to hold wafers. Typically, wet buffer unit  12  has sprayers which spray liquid on the wafers to keep the wafers wet from previous wafer processing, such as wafer polishing. Wafer cleaners  14  and  16 , which are described in detail below, are substantially identical with the exception, in this example, that a different scrubbing solution is used in wafer cleaner  14  than in wafer cleaner  16 . Spin drying unit  18  dries the wafer by spinning the wafer at high speeds, thereby removing any liquid from the surface of the wafer. Spin drying unit  18  is further described in Jones, application Ser. No. 08/680,739, filed Jul. 15, 1996, now U.S. Pat. No. 5,875,507, herein incorporated by reference in its entirety. Finish cassette  20  has a plurality of slots for holding the finished wafers. 
     During use, robotic arm  10  removes a wafer which is oriented horizontally from wet buffer unit  12  (the perimeter  22  of the wafer as it is removed from wet buffer unit  12  is indicated in FIG.  1 ). Robotic arm  10  then rotates the wafer 90° to a vertical orientation and inserts the wafer into vertical slot  24  of wafer cleaner  14 . After processing of the wafer in wafer cleaner  14  (described below), robotic arm  10  removes the wafer from wafer cleaner  14  through slot  24 . This sequence is repeated with wafer cleaner  16 . The wafer is then rotated 90° by robotic arm  10 . The wafer is then loaded horizontally into spin drying unit  18  and finally is loaded from spin drying unit  18  to finish cassette  20 . 
     FIG. 2 is a partial top view of wafer cleaner  14 . As shown, wafer cleaner  14  includes a housing  23  which includes slot  24  through which a wafer is inserted into wafer cleaner  14 . Slot  24  is fitted with a door  27  which opens and closes slot  24 . Wafer cleaner  14  further includes a first rotary brush  26  and a second rotary brush  28 . Brushes  26 ,  28  are made of polyvinyl alcohol (PVA) foam although other materials such as nylon, mohair or a mandrel wrapped with polishing pad material can be used. In one embodiment, brushes  26 ,  28  are PVA foam manufactured by KANEBO of Japan. Brushes  26 ,  28  are located horizontally from one another. 
     Located between brushes  26 ,  28 , and defined by brushes  26 ,  28 , is a region  30 . Located vertically below region  30  is a first roller  32  and a second roller  34 . Rollers  32 ,  34  have V-grooves  36 ,  38 , respectively, extending around the periphery of the rollers. 
     Brushes  26 ,  28  are mounted to first ends of shafts  40 ,  42 , respectively. Rotary unions  41 ,  43  are mounted to second ends of shafts  40 ,  42 , respectively. Shafts  40 ,  42  have central cavities formed therein which allow liquid to flow from rotary unions  41 ,  43  through shafts  40 ,  42 , respectively. Further, shafts  40 ,  42  have perforations in the regions of shafts  40 ,  42  to which brushes  26 ,  28 , respectively, are mounted. The perforations allow liquid to be distributed from shafts  40 ,  42  to brushes  26 ,  28 . 
     Wafer cleaner  14  further includes a plurality of spray nozzles. In particular, located proximate to and above brush  26  is a first set of spray nozzles  56 . Similarly, located proximate to and above brush  28  is a second set of spray nozzles  58 . During use, first and second sets of spray nozzles  56 ,  58 , spray liquid towards a wafer located between brushes  26 ,  28 , respectively. In one embodiment, first and second sets of spray nozzles  56 ,  58 , each comprise three individual spray nozzles, although other numbers of spray nozzles can be used, e.g. four. 
     Servo motors  44 ,  46  are connected to pulleys on the second ends of shafts  40 ,  42  by drive belts  45 ,  47 , respectively. Shaft  40  is mounted into bearings  48  and  50 . Similarly, shaft  42  is mounted into bearings  52  and  54 . 
     FIG. 3 is a partial front view of wafer cleaner  14 . As shown in FIG. 3, bearings  52 ,  54  are mounted to an upper movable plate  80 . Bearings  48 ,  50  are mounted to a lower movable plate  82 . Motors  46 ,  44  are also mounted to movable plates  80 ,  82 , respectfully. During use, motors  44 ,  46  rotate shafts  40 ,  42  in opposite directions, thereby rotating brushes  26 ,  28  in opposite directions, respectively. Generally, brushes  26 ,  28  are rotated between 50 to 1500 revolutions per minute. 
     Further, upper plate  80  is coupled to a first end  84 A of a pivot  84  and lower plate  82  is coupled to a second end  84 B of pivot  84 . Pivot  84  is coupled at its center  84 C to a section  23 A of housing  23  (or alternatively to a plate  23 A connected to housing  23 ). Also coupled to section  23 A is an air cylinder  86 . Air cylinder  86  has a piston  88  coupled by a pressure transducer  89  to upper plate  80 . 
     By controlling pressurized air flow into and out of air cylinder  86 , the position of piston  88  can be controlled, and hence the position of brushes  26 ,  28  can be controlled. In particular, when piston  88  is partially extended as in FIG. 3, brushes  26 ,  28  are located at a distance from one another. However, when piston  88  is retracted into air cylinder  86  (moved in the direction towards section  23 A as indicated by the arrow in FIG.  3 ), upper plate  80  is also moved towards section  23 A. Since shaft  42  is mounted to upper plate  80 , shaft  42  and brush  28  are also moved towards section  23 A. 
     The movement of upper plate  80  towards section  23 A causes first end  84 A of pivot  84  to also move towards section  23 A. Since pivot  84  is coupled at its center  84 C to section  23 A, the motion of first end  84 A causes an equal and opposite motion of second end  84 B of pivot  84 . Thus, as upper plate  80  moves towards section  23 A, lower plate  82  moves away from section  23 A. Since shaft  40  is mounted to lower plate  82 , shaft  40  and brush  26  are also moved away from section  23 A. The net result is that when piston  88  is retracted, brushes  26 ,  28  are moved towards one another and when piston  88  is extended (moved away from section  23 A), brushes  26 ,  28  are moved away from one another. Further, the pivot  84  ensures that the perpendicular component of force (further described below) of each brush ( 26 ,  28 ) is equal and opposite to that of the other brush ( 28 ,  26 ). 
     FIG. 4 is a partial perspective view of wafer cleaner  14 . As shown in FIG. 4, mounted to upper plate  80  are bearings  90 ,  92  and  94 . Running through bearings  90 ,  92  is a first immobilized shaft and running through bearing  94  is a second immobilized shaft (these shafts are not illustrated in FIG. 4 for purposes of clarity). As piston  88  of air cylinder  86  is extended and retracted and upper plate  80  moved, upper plate  80  slides along the shafts running through bearings  90 ,  92 , and  94 . In this manner, plate  80  is prevented from moving in any direction except perpendicular to the plane of section  23 A. Similar bearings and shafts are mounted to plate  82  which also prevent plate  82  from moving in any direction except perpendicular to the plane of section  23 A. 
     FIG. 5 is a partial side view of wafer cleaner  14 . As shown in FIG. 5, a drive belt  60  couples rollers  32 ,  34  to roller motor  62 . An idler pulley  61  maintains a proper tension in drive belt  60 . During use, motor  62  causes drive belt  60  to move thereby rotating rollers  32 ,  34 . Also shown in FIG. 5 are shafts  96  and  98  which run through bearings  90 ,  92  and  94 , respectively. 
     FIG. 6 is a partial frontal view of wafer cleaner  14  during use. As shown in FIG. 6, initially brushes  26 ,  28  are at positions  66 ,  68 , respectively (indicated by phantom circles). Wafer  64  is then inserted vertically through slot  24  into region  30  by robotic arm  10  (not shown). While the wafer is held by end-effector  11  (not shown), brushes  26 ,  28  are moved towards each other to positions  70 ,  72 , respectively. Typically, brushes  26 ,  28  travel approximately 0.5 inches between positions  66  and  70 ,  68  and  72 , respectively. At positions  70 ,  72 , brushes  26 ,  28  contact first and second surfaces  74 ,  76 , respectively, of wafer  64 . The perpendicular component of force (force exerted perpendicular to planes formed by surfaces  74 ,  76  of wafer  64 ) exerted by brush  26  (and brush  28 ) on to wafer  64  is measured and controlled. For example, by measuring and controlling the force exerted by piston  88  on pressure transducer  89  (FIG.  3 ), the perpendicular component of force exerted by brushes  26 ,  28  on to wafer  64  is measured and controlled. Generally, the perpendicular component of force exerted by each brush on wafer  64  is less than 50 pounds per square inch (PSI) and preferably is 5 PSI. 
     End-effector  11  then releases wafer  64 , robotic arm  10  removes end-effector  11  from wafer cleaner  14  and door  27  over slot  24  is closed. As best seen in FIG. 5, wafer  64  is held by brushes  26 ,  28  at a first position  64 A. Brushes  26 ,  28  are then caused to rotate by servo motors  44 ,  46  (FIGS. 2,  3 ), respectively. Servo motors  44 ,  46  rotate brushes  26 ,  28  at substantially the same speed. As shown in FIG. 6, brush  26  is rotated clockwise and brush  28  is rotated counterclockwise. This rotation of brushes  26 ,  28 , forces wafer  64  (to a position  64 B in FIG. 5) into V-grooves  36 ,  38  of rollers  32 ,  34 , respectively. This engages wafer  64  to rollers  32 ,  34 . Motor  62  then causes rollers  32 ,  34  to rotate which, in turn, cause wafer  64  to rotate. Generally, the wafer is rotated at less than 500 RPM. 
     Referring back to FIG. 6, brushes  26 ,  28  are then flushed from the inside out by liquid supplied to brushes  26 ,  28  from shafts  40 ,  42 . Substantially simultaneously, first and second sets of spray nozzles  56 ,  58 , spray liquid on brush  26 , first surface  74  of disk  64  and brush  28 , second surface  76  of disk  64 , respectively. 
     In one embodiment, wafer cleaner  14  further includes third and fourth sets of spray nozzles  57 ,  59  located below first and second sets of spray nozzles  56 ,  58 , respectively. During a first stage of the wafer cleaning cycle, a first liquid is sprayed from sets of spray nozzles  57 ,  59  (or  56 ,  58 ). During a second stage of the wafer cleaning cycle, a second liquid is sprayed from sets of spray nozzles  56 ,  58  (or  57 ,  59 ). For example, the first liquid can be a surfactant and the second liquid can be de-ionized water. Alternatively, the same liquid can be sprayed from sets of spray nozzles  56 ,  57 ,  58 ,  59  simultaneously. Further, additional liquids can be sprayed during various stages of the wafer cleaning cycle by adding additional sets of spray nozzles. 
     Alternatively, only first and second sets of spray nozzles  56 ,  58  are used, but individual nozzles of each of the sets of spray nozzles are plumbed to different liquids. In this manner, selective nozzles can spray different liquids at various stages in the wafer cleaning cycle. 
     The flow of liquid to brushes  26 ,  28  and first and second sets of spray nozzles  56 ,  58  is controlled by opening and closing valves coupled to feed lines (not shown) which are plumbed to shafts  40 ,  42  via rotary unions  41 ,  43 , respectively and sets of spray nozzles  56 ,  58 . Further, the operation of wafer cleaner  14  is controlled by a conventional programmable logic controller (PLC), for example by a PLC model #2600 manufactured by Control Technology Corp. located in Hopkinton, Mass. 
     The combination of the scrubbing action on the surfaces  74 ,  76  of wafer  64  caused by the rotation of brushes  26 ,  28  along with liquid supplied through brushes  26 ,  28  and by sets of spray nozzles  56 ,  58 , removes particulates from surfaces  74 ,  76  of wafer  64 . In particular, particulates are scrubbed from surfaces  74 ,  76  by brushes  26 ,  28 , respectively. These particulates are flushed from brushes  26 ,  28  by the liquid supplied to brushes  26 ,  28  through shafts  40 ,  42 . 
     Further, particulates which are loosened by the scrubbing action of brushes  26 ,  28 , but remain on surfaces  74 ,  76  of wafer  64 , are flushed from surfaces  74 ,  76  by liquid sprayed from sets of spray nozzles  56 ,  58 . By orienting wafer  64  vertically instead of horizontally, the removal of particulates from the surfaces  74 ,  76  is enhanced. In particular, by orienting wafer  64  vertically, liquid sprayed on to surfaces  74 ,  76  of wafer  64  and particulates trapped in the liquid have a tendency to fall from surfaces  74 ,  76  due to gravity. In contrast, if wafer  64  were oriented horizontally, particulates would tend to be moved around on surfaces  74 ,  76  and would not be as readily removed. Thus, wafer cleaner  14  is particularly well suited for larger diameter wafers in which particulates must be removed from a larger surface area. For example, wafer cleaner  14  is particularly well suited for cleaning 200 mm and 300 mm diameter wafers. 
     Further, by orienting wafer  64  vertically and by scrubbing both surfaces  74 ,  76  simultaneously, mechanical stress on wafer  64  is minimized. This is because the perpendicular component of the force exerted by brush  26  on wafer  64  is offset by the perpendicular component of the force exerted by brush  28  on wafer  64 . (The perpendicular components of force exerted by each brush of the wafer is equal and opposite to that of the other brush.) Thus, the net force which is exerted on wafer  64  by brushes  26 ,  28  is substantially parallel to the plane formed by surface  74  (or surface  76 ). Since wafer  64  has the greatest mechanical strength in this plane, wafer cleaner  14  is well suited for larger diameter disks. (Larger diameter disks generally flex when force is exerted in a plane perpendicular to side  74 .) 
     After wafer  64  has been scrubbed for a predetermined period of time, generally 30 to 120 seconds and typically 45 seconds, the flow of liquid to brushes  26 ,  28  and sets of spray nozzles  56 ,  58 , is shut off. Substantially simultaneously, the rotation of rollers  32 ,  34  and brushes  26 ,  28  is stopped. Door  27  over slot  24  is opened, robotic arm  10  inserts end-effector  11  into slot  24  and the end-effector  11  engages wafer  64 . Then, Brushes  26 ,  28  are moved back to positions  66 ,  68 , respectively, and robotic arm  10  removes wafer  64 . Wafer cleaner  14  is now ready to process another wafer. 
     As described in Jones et al., U.S. application Ser. No. 09/113,811, now U.S. Pat. No. 6,230,753 cofiled herewith and incorporated herein by reference in its entirety, wafer  64  can be held in place during loading/unloading by a finger and can also have its edge scrubbed simultaneous with surfaces  74 ,  76 . 
     Referring to FIG. 1, by using two wafer cleaners  14 ,  16 , sequentially, a wafer can be scrubbed and rinsed with two different solutions. In one embodiment, for example, the scrubbing liquid in wafers cleaner  14  is an ammonia solution or a surfactant available from Valtec or Allied. The scrubbing liquid in wafer cleaner  16  is de-ionized water. This arrangement is particularly advantages since surfactant residue on the wafer from wafer cleaner  14  is readily removed by the water rinse in wafer cleaner  16 . However in alternative embodiments, other scrubbing liquids are used, for example acid or caustic solutions are used in either wafer cleaner  14  or  16 . Further, it is understood that only a single wafer cleaner can be used, or that several wafer cleaners can be used. 
     Referring again to FIG. 2, as the art moves to larger diameter wafers, e.g. 300 millimeter (mm) diameter wafers, the length of brushes  26 ,  28  is correspondingly increased, where the length is measured along the longitudinal axis from ends  230 ,  232  to ends  234 ,  236  of brushes  26 ,  28 , respectively. To flush this greater brush length from the inside out with a sufficient amount of liquid, a greater amount of liquid must be provided from rotary unions  41 ,  43  to the central cavities of shaft  40 ,  42  and to brushes  26 ,  28 , respectively, than with shorter length brushes. 
     As set forth above, shafts  40 ,  42  have perforations in the regions where brushes  26 ,  28  are mounted to shafts  40 ,  42 , respectively. Typically, brushes  26 ,  28  are mounted to shafts  40 ,  42 , by mandrel assemblies, i.e. brushes  26 ,  28  are mounted to mandrel assemblies which are mounted to shafts  40 ,  42 , respectively. Perforations in shafts  40 ,  42  and the mandrel assemblies allow liquid to be distributed from shafts  40 ,  42  to brushes  26 ,  28 , respectively. However, as the length of brushes  26 ,  28  becomes greater to accommodate larger diameter wafers, the flow of liquid to brushes  26 ,  28  may become non-uniform. In particular, as the length of brushes  26 ,  28  increases, a pressure drop within shafts  40 ,  42  may exist from ends  234 ,  236  to ends  230 ,  232  of brushes  26 ,  28 , respectively. This pressure drop, in turn, causes a greater amount of liquid to pass through perforations in shafts  40 ,  42  near ends  234 ,  236  of brushes  26 ,  28  than near ends  230 ,  232  of brushes  26 ,  28 , respectively. This non-uniform flushing of brushes  26 ,  28  can cause various undesirable effects such as insufficient particulate removal near ends  230 ,  232  of brushes  26 ,  28 , or uneven wear of brushes  26 ,  28 . 
     In addition to the difficulty of providing a uniform flow to brushes  26 ,  28 , as the length of brushes  26 ,  28  increases it also becomes increasingly difficult to prevent localized nonuniform flushing of brushes  26 ,  28 . To illustrate, assume the case where the length of brushes  26 ,  28  increases but the overall number of perforations in shafts  40 ,  42  and the associated mandrel assemblies remains the same. In this case, the distance between adjacent perforations correspondingly increases. Accordingly, the portions of brushes  26 ,  28  proximate a perforation receives a large amount of liquid flow but the portions located between adjacent perforations receives a significantly reduced liquid flow. Thus, the portions of brushes  26 ,  28  between adjacent perforations may not be flushes sufficiently to remove undesirable particulates. 
     To reduce localized nonuniform flushing of brushes  26 ,  28 , the number of perforations can be increased. However, increasing the number of perforations correspondingly increases the liquid flow resulting in a larger overall amount of liquid which must be filtered and otherwise handled. To reduce the overall amount of liquid which must be handled, the diameter of the perforations can be reduced. However, there are practical manufacturing limitations which limit the minimum diameter of the perforations. Accordingly, an improved brush assembly is needed which reduces or eliminates end-to-end and localized liquid flow nonuniformities without substantially increasing the overall amount of liquid which must be handled. 
     FIG. 7 is a side view, partially cutaway, of a brush assembly  300  which provides a desired liquid flow distribution in accordance with the present invention. Brush assembly  300  includes a brush  26 A mounted on an outer mandrel housing  331 . Located within housing  331  is an inner mandrel flow distributor  333 . Distributor  333  in combination with housing  331  form mandrel assembly  335 . Within distributor  333  is a shaft  40 A. In FIG. 7, brush  26 A, housing  331 , distributor  333  and shaft  40 A are partially cutaway for purposes of clarity and discussion. 
     Brush  26 A is formed of a permeable material such as PVA foam, nylon, mohair or polishing pad material to allow liquid to readily pass from the inner surface  336  to the outer surface  338  of brush  26 A. Outer surface  338  includes a plurality of protuberances  340  which, during use, contact and scrub the wafer. Illustratively, housing  331  and distributor  333  are polyvinylidene fluoride (PVDF) and shaft  40 A is  316  stainless steel although it is understood that other materials can be used. 
     Inner surface  336  of brush  26 A forms a pressure fit with outer surface  342  of housing  331 . Housing  331  includes a plurality of perforations  344  which extend from inner surface  346  to outer surface  342  of housing  331 . During use, liquid is supplied from inside of housing  331  through perforations  344  to brush  26 A. 
     Inner surface  346  of housing  331  abuts outer surface  348  of distributor  333 . Outer surface  348  of distributor  333  has a slot matrix  350  formed therein. As shown in FIG. 7, slot matrix  350  includes a plurality of longitudinal slots  352  parallel to the longitudinal axis of distributor  333  and plurality of annular slots  354  circling distributor  333  perpendicular to the longitudinal axis of distributor  333 . Each annular slot  354  corresponds with a radial column  345  of perforation  344  in housing  331  as further described below. Further, located between adjacent annular slots  354  in each longitudinal slot  352  is a perforation  356  extending from inner surface  358  of distributor  333  to the associated longitudinal slot  352 . During use, liquid is supplied from inside distributor  333  though perforations  356  to longitudinal slots  352 . The liquid flows in longitudinal slots  352  to annular slots  354 . From annular slots  354 , liquid is provided through perforations  344  in housing  331  to brush  26 A. 
     Located within distributor  333  is shaft  40 A. Shaft  40 A includes a plurality of perforations  360 . During use, liquid provided to shaft  40 A from a rotary union (e.g. see rotary unions  41 ,  43  of FIG. 2) flows from the cavity inside of shaft  40 A through perforations  360 . The liquid flows through distributor  333 , through housing  331  and to brush  26 A. As discussed further below, distributor  333  redistributes the flow of liquid between shaft  40 A and housing  331  by causing the liquid to flow through longitudinal slots  352  and annular slots  354 , i.e. through slot matrix  350 . 
     FIG. 8 is a side view of a region  370  of distributor  333  of FIG. 7 in accordance with the present invention. As set forth above, during use liquid flows through perforations  356  and into longitudinal slots  352 . Since outer surface  348  of distributor  333  abuts inner surface  346  of housing  331  (see FIG.  7 ), liquid exiting perforations  356  is contained in longitudinal slots  352  and generally in slot matrix  350 . (A small amount of liquid may leak between outer surface  348  of distributor  333  and inner surface  346  of housing  331  but for practical purposes this leakage is negligible.) 
     As indicated by the arrows in FIG. 8, liquid flows from each perforation  356  through the associated longitudinal slot  352  to the adjacent annular slot  354 . At annular slot  354 , the liquid flow is diverted from longitudinal slot  352  into annular slot  354  by an opposite flow of liquid through the particular longitudinal slot  352  from the adjacent perforation  356 . The liquid then flows through annular slot  354  to and through perforations  344  in housing  331  (the location of a single perforation  344  is indicated by the dashed circle in FIG.  8 ). 
     Of importance, the flow of liquid from perforations  356  in distributor  333  to perforations  344  in housing  331  is readily controlled by appropriately selecting the number and dimensions of longitudinal slots  352  and annular slots  354 . In particular, by appropriately selecting the cross-sectional area and, to a lesser extent, the length of longitudinal slots  352  and annular slots  354 , the liquid flow is controlled. For example, a greater liquid flow can selectively be provided to one slot over another slot, or to a first portion of a slot over a second portion of the slot, by appropriately selecting the dimensions of the slot(s). Generally, increasing the cross-sectional area and, to a lesser extent, decreasing the length of a slot increases the flow of liquid through that particular slot and vice versa. 
     As illustrated in FIG. 8, annular slot  354  has a depth D AS  and a width W AS . By increasing (decreasing) depth D AS  and/or width W AS , the resistance to liquid flow through annular slot  354  is decreased (increased) and, correspondingly, the amount of liquid which flows to perforation  344  in housing  331  is increased. 
     FIG. 9 is a cross-sectional view of distributor  333  along the line IX—IX of FIG. 8 in accordance with the present invention. As shown in FIG. 9, longitudinal slot  352  has a depth D LS  and a width W LS . By increasing (decreasing) depth D LS  and/or width W LS , the resistance to liquid flow through longitudinal slot  352  is decreased (increased) and, correspondingly, the amount of liquid which flows to perforation  344  in housing  331  is increased. Further, although the flow through longitudinal slot  352  is primarily determined by the cross-sectional area, the resistance to liquid flow through longitudinal slot  352  can be decreased (increased) to some extent by decreasing (increasing) the length L LS  of longitudinal slot  352  between perforation  356  and annular slot  354  (see FIG.  8 ). 
     FIG. 10 is a side view of distributor  333 A in accordance with one embodiment of the present invention. Referring to FIGS. 7 and 10, since a radial column  345  of perforations  344  in housing  331  is associated with each annular slot  354 A- 354 F and annular slots  354 A- 354 F are closer to one another near ends  365 A,  365 B of distributor  333 A, a greater number of perforation  344  per unit area of housing  331  is provided near the ends of the brush assembly. Accordingly, a greater amount of liquid is provided to clean the wafer near the ends of the brush assembly where a greater effective wafer surface area must be cleaned than at the center. 
     By causing the liquid to flow through longitudinal slots  352  and annular slots  354 A- 354 F to reach perforations  344 , the liquid flow to perforations  344  is restricted. This allows a large number of perforations  344  to be used, e.g. eight perforations  344  per radial column  345 , to be used without a significant increase in the overall amount of liquid used. Further, by using distributors with longitudinal slots  352  and annular slots  354 A- 354 F having different cross-sectional areas, the overall amount of liquid can readily be controlled. As an illustration, a first distributor having longitudinal slots  352  and annular slots  354 A- 354 F with greater cross-sectional areas than those of a second distributor will allow a greater amount of liquid to flow from shaft  40 A to housing  331  than the second distributor. 
     Further, by restricting the flow of liquid, distributor  333 A causes a uniform pressure buildup inside of distributor  333 A. This, in turn, ensures that both ends  230 A,  234 A of brush  26 A receive the same amount of liquid improving particulate removal from brush  26 A and reducing or eliminating uneven wear of brush  26 A. Thus, brush assemblies in accordance with the present invention are well suit for use in wafer cleaners such as the wafer cleaner illustrated in FIG.  2 . 
     Illustrative specifications for the various characteristics of distributor  333 A shown in FIG. 10 are set forth below in Table 1. In the tables which follow, dimensions are in inches unless otherwise indicated. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 A1  
                 8 × .78 
               
               
                   
                 A2  
                 .91 
               
               
                   
                 A3  
                 1.41 
               
               
                   
                 A4  
                 2.16 
               
               
                   
                 A5  
                 3.16 
               
               
                   
                 A6  
                 4.66 
               
               
                   
                 A7  
                 6.91 
               
               
                   
                 A8  
                 9.16 
               
               
                   
                 A9  
                 10.66 
               
               
                   
                 A10 
                 11.66 
               
               
                   
                 A11 
                 12.41 
               
               
                   
                 A12 
                 12.91 
               
               
                   
                 A13 
                 8 × 13.10 
               
               
                   
                 A14 
                 ¾-16 UNF-2A 
               
               
                   
                 A15 
                 Min Thread Relief Permissible 
               
               
                   
                 A16 
                 8 × @ 45° .062 × .011 Deep Longitudinal Slot 
               
               
                   
                 A17 
                 10 × .062 × .011 Deep Annular Slot 
               
               
                   
                 A18 
                 12.90 
               
               
                   
                 A19 
                 .60 
               
               
                   
                 A20 
                 Ø2.00 
               
               
                   
                   
               
             
          
         
       
     
     As set forth in Table 1, annular slots  354 A,  354 B,  356 C,  354 D,  354 E,  354 F and longitudinal slots  352  have equal depths D AS , D LS  and equal widths W AS , W LS , respectively. Accordingly, the resistance to liquid flow and thus the liquid flow through annular slots  354 A,  354 B,  356 C,  354 D,  354 E,  354 F and longitudinal slots  352  for any given length of the particular slot is approximately equal. However, to some extent the liquid flow is affected by the length L LSi  (i=1 through 5 in this embodiment) through longitudinal slot  352  which the liquid must flow, where length L LSi  is the length between a perforation  356  and the corresponding annular slot  354 A,  354 B,  354 C,  354 D,  354 E,  354 F. Of importance, this length L LSi  varies to provide a somewhat greater flow of liquid to annular slots  354 E,  354 F near ends  365 A,  365 B of distributor  333 A than annular slot  354 A near the center of distributor  333 A. 
     In particular, length L LS1  between perforations  356  and annular slots  354 A,  354 B is greatest with distances L LS2 , L LS3 , L LS4  respectively decreasing to the minimum length L LS5 . Thus, the greatest resistance to liquid flow (and the least amount of liquid flow) is to annular slot  354 A with the resistances to annular slots  354 B,  354 C,  354 D respectively decreasing (and the liquid flow respectively increasing) to the minimum resistance (and the greatest liquid flow) to annular slots  354 E,  354 F. Thus, the spacing of annular slots  354 A- 354 F further ensures that a greater amount of liquid is provided to clean the wafer near the ends of the brush assembly where a greater effective wafer surface area must be cleaned than at the center. 
     FIG. 11 is a cross-sectional view of distributor  333 A of FIG. 10 in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristics of distributor  333 A shown in FIG. 11 are set forth below in Table 2. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 B1  
                 .200 
               
               
                   
                 B2  
                 .750 
               
               
                   
                 B3  
                 1.19 
               
               
                   
                 B4  
                 1.81 
               
               
                   
                 B5  
                 2.69 
               
               
                   
                 B6  
                 3.94 
               
               
                   
                 B7  
                 5.81 
               
               
                   
                 B8  
                 8.06 
               
               
                   
                 B9  
                 9.94 
               
               
                   
                 B10 
                 11.19 
               
               
                   
                 B11 
                 12.06 
               
               
                   
                 B12 
                 12.69 
               
               
                   
                 B13 
                 80 × Ø.062 Thru 0.C. of Longitudinal Slots 
               
               
                   
                 B14 
                 Ø1.125 
               
               
                   
                 B15 
                 .39 
               
               
                   
                 B16 
                 (2×)Ø1.025 × .093 Groove 
               
               
                   
                 B17 
                 Ø1.010 × .187 Groove 
               
               
                   
                 B18 
                 13.75 
               
               
                   
                 B19 
                 14.120 
               
               
                   
                 B20 
                 15.39 REF 
               
               
                   
                 B21 
                 .87 
               
               
                   
                 B22 
                 .82 
               
               
                   
                 B23 
                 Ø1.750 
               
               
                   
                 B24 
                 Ø1.313 
               
               
                   
                   
               
             
          
         
       
     
     As shown in FIG. 11, distributor  333 A has an O-ring groove  398  in inner surface  358 A in which an O-ring is seated to form a seal between shaft  40 A (not shown) and distributor  333 A. In this manner, liquid is prevented from leaking out of distributor  333 A at end  365 A. 
     FIGS. 12 and 13 are plan views of distributor  333 A taken from ends  365 A,  365 B, respectively, of FIG. 10 in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristic shown in FIGS. 12 and 13 are set forth in Table 3 below. 
     
       
         
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                 C1 
                 Ø.755 
               
               
                 C2 
                 4 ×  2-56 UNC-2B .25 Min Full Thd On Ø1.500 B.C. 
               
               
                 D1 
                 Ø.501 Thru 
               
               
                   
               
             
          
         
       
     
     FIG. 14 is a cross-sectional view of a housing  331 A for use with distributor  333 A of FIGS. 10-13 in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristic of housing  331 A shown in FIG. 14 are provided in Table 4 below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 E1  
                 Ø1.375 
               
               
                   
                 E2  
                 20°  All Around 
               
               
                   
                 E3  
                 (88×) Ø.062 Holes Thru 
               
               
                   
                 E4  
                 15° All Around 
               
               
                   
                 E5  
                 Ø1.20 
               
               
                   
                 E6  
                 14.130 
               
               
                   
                 E7  
                 12.94 
               
               
                   
                 E8  
                 12.44 
               
               
                   
                 E9  
                 11.69 
               
               
                   
                 E10 
                 10.69 
               
               
                   
                 E11 
                 9.19 
               
               
                   
                 E12 
                 6.94 
               
               
                   
                 E13 
                 4.69 
               
               
                   
                 E14 
                 3.19 
               
               
                   
                 E15 
                 2.19 
               
               
                   
                 E16 
                 1.44 
               
               
                   
                 E17 
                 .94 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 15 is a cross-sectional view of housing  331 A of FIG. 14 mounted on distributor  333 A of FIGS. 10-13 in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristics shown in FIG. 15 are provided in Table 5 below. 
     
       
         
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
             
             
               
                 F1 
                 Flush 
               
               
                 F2 
                 .42 
               
               
                 F3 
                 Ø.125 PVDF Pins Thru Both Sides. 
               
               
                 F4 
                 Ø.755 
               
               
                 F5 
                 Internal Chamfer On Distributor 331A To Feed 
               
               
                   
                 Over Housing 333A And O-Ring 368 To Rest 
               
               
                   
                 Position Shown. Holes (88×) In Distributor 
               
               
                   
                 331A To Be Positioned +11.25° Or −11.25° From 
               
               
                   
                 Holes (80×) In Housing 333A. 
               
               
                 F6 
                 O-Ring 1″ ID 1⅛″ OD BUNA 
               
               
                 F7 
                 13.94 
               
               
                   
               
             
          
         
       
     
     As illustrated in FIGS. 10,  11  and  15 , distributor  333 A has O-ring grooves  362 ,  364  in which O-rings  366 ,  368 , respectively, are placed. O-rings  366 ,  368  form a seal between housing  331 A and distributor  333 A which prevents liquid flowing between housing  331 A and distributor  333 A from leaking out at the ends. Further, referring to FIG. 15, housing  331 A and distributor  333 A have mounting holes  370  through which pins  374  are inserted to fixedly mount housing  331 A on distributor  333 A. 
     FIG. 16 is a cross-sectional view of housing  331 A and distributor  333 A along the line XVI—XVI of FIG. 15 in accordance with this embodiment of the present invention. As shown in FIG. 16, perforations  344  in housing  331 A are radially offset from perforations  356  in distributor  333 A. Further, this radial offset changes in adjacent radial columns  345  of perforations  344 . Illustratively, the radial offset Ø 1  between perforations  356  in distributor  333 A and perforations  344  of a first radial column  345  in housing  331 A is 11.25° and the radial offset Ø 2  between perforations  356  and perforations  344 A of a second radial column  345  in housing  331 A is 33.75°. By having radial columns  345  of perforations  344  offset from one another, liquid distribution and flushing of the brush (not shown) mounted on housing  331 A is enhanced. 
     FIG. 17 is a plan view of housing  331 A and distributor  333 A of FIG. 15 taken from end  365 B in accordance with this embodiment of the present invention. An illustrative specification for the feature shown in FIG. 17 is provided in Table 6 below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
             
               
                   
                 G1 
                 Ø1.375 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 18 is an exploded perspective view of a brush assembly  300 A without brush  26 A in accordance with this embodiment of the present invention. As shown in FIG. 18, end  365 B of mandrel assembly  335 A is sealed with an O-ring  380  and cap  382 . Generally, cap  382  threads on end  365 B and compresses O-ring  380  against a flat seal surface  384  of mandrel assembly  335 A. In this manner, liquid is prevented from leaking out of end  365 B of mandrel assembly  335 A. 
     As described above in reference to FIG. 11, as mandrel assembly  335 A forms a seal with shaft  40 A (not shown) by an O-ring  386  shown in FIG.  18 . Mandrel assembly  335 A is engaged to shaft  40 A by a hub  388  which is mounted to mandrel assembly  335 A with screws  390 . As further described below, pins in shaft  40 A are seated in slots  392  in hub  388  by a spring washer  394  and washer  396 , where spring washer  394  provides a spring force between a lip  400  (see FIG. 11) of distributor  333 A and the pins in shaft  40 A. 
     FIG. 19 is a cross-sectional view of cap  382  in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristics of cap  382  shown in FIG. 19 are provided in Table 7 below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
             
             
               
                   
                 H1  
                 .25 × .25 BRK All Around 
               
               
                   
                 H2  
                 ¾-16 UNF-2B 
               
               
                   
                 H3  
                 .050 
               
               
                   
                 H4  
                 .101 
               
               
                   
                 H5  
                 Ø.501 
               
               
                   
                 H6  
                 Ø1.125 
               
               
                   
                 H7  
                 Ø1.376 
               
               
                   
                 H8  
                 .250 
               
               
                   
                 H9  
                 .437 Min Full Thd 
               
               
                   
                 H10 
                 .737 Max 
               
               
                   
                 H11 
                 1.062 
               
               
                   
                 H12 
                 1.125 
               
               
                   
                 H13 
                 .23 
               
               
                   
                   
               
             
          
         
       
     
     As shown in FIG. 19, cap  382  has an O-ring groove  402  in which O-ring  380  (see FIG. 18) is seated. 
     FIG. 20 is a end view of cap  382  taken from end  382 A of FIG. 19 in accordance with this embodiment of the present invention. An illustrative specification for the characteristic illustrated in FIG. 20 is provided in Table 8 below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
             
             
               
                   
                 I1 
                 Ø2.00 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 21 is a side view, partially cross-sectioned, of shaft  40 A in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristic of shaft  40 A shown in FIG. 21 are provided in Table 9 below. 
     As shown in FIG. 21, shaft  40 A is hollow and has a plug  410  which seals end  412  of shaft  40 A. The opposite end  415  of shaft  40 A is threaded to allow attachment of a rotary union (e.g. see rotary union  41  of FIG.  2 ). Further, extending through shaft  40 A is a pin  414 . Pin  414  is seated in slots  392  of hub  388  (see FIG.  18 ). During use, shaft  40 A is rotated by a motor (e.g. see motor  44  of FIG.  2 ). Referring now to FIGS. 18 and 21, since pin  414  is engaged with hub  388 , rotation of shaft  40 A causes brush assembly  300 A to rotate. 
     FIG. 22 is a cross-sectional view of hub  388  in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristics of hub  388  shown in FIG. 22 are provided in Table 10 below. 
     FIG. 23 is a plan view from end  388 A of hub  388  of FIG. 22 in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristics of hub  388  shown in FIG. 23 are provided in Table 11 below. 
     As shown in FIG. 23, hub  388  has two through-slots  420 . Referring to FIGS. 21 and 23, hub  388  can be inserted over end  412  of shaft  40 A and slid along the length of shaft  40 A to pin  414 . Pin  414  then fits through through-slots  420 . Hub  388  is then rotated 90° and slid back towards end  412  to engage pin  414  is slots  392 . 
     FIG. 24 is a plan view from end  388 B of hub  388  of FIG. 22 in accordance with this embodiment of the present invention. Illustrative specifications for the various characteristics of hub  388  shown in FIG. 24 are provided in Table 12 below. 
     Although the present invention has been described with reference to various embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, the wafer can be a generally circular silicon wafer, glass wafer, ceramic wafer, oxide wafer, tungsten wafer although other types of wafers can be used. Further, although various values, materials and dimensions have been provided, it is understood that these values, materials and dimensions are only illustrative and not limiting and that other values, materials and dimension can be used. For example, instead of slots having rectangular cross-sections, slots having other cross-sectional shapes such as semicircular slot can be used. Further, although various liquids have been set forth, it is understood that substantially any liquid or chemical can be used with a wafer cleaner and brush assembly in accordance with the present invention. For example, various alcohols, surfactants, ammonia based solutions, buffer solutions, high PH solutions and low PH solutions can be used. Thus, the invention is limited only by the following claims.