Abstract:
A magnetically coupled roller assembly is provided. A magnet is positioned on the roller and couples to a roller positioned on a drive assembly. The roller and driver are separated by a chamber wall and magnetically coupled therethrough. A shaft coupled to the roller may be positioned in a pocket formed in the chamber wall, and the roller and driver magnets may be axially offset so as to “pull” the roller toward the interior of the pocket.

Description:
This application claims priority from U.S. provisional application Serial No. 60/136,909, filed Jun. 1, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the rotation of thin disks. More specifically, the present invention relates to an improved roller for rotating semiconductor substrates within a tank of fluid. 
     BACKGROUND OF THE INVENTION 
     A conventional method for cleaning particles from semiconductor wafers submerged within a tank of fluid is known as megasonic cleaning. During megasonic cleaning, a transducer oscillates between compressed and strained states at a rate near 1 MHz. The megasonic oscillation output by the transducer is coupled to the fluid contained within the tank, causing pressure oscillation therein. As the pressure in the fluid oscillates between positive and negative, cavitations or bubbles form in the liquid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation gently cleans the surface of the wafer. 
     In practice, megasonic cleaners experience a number of limitations. For instance, the intensity of the cleaning energy experienced across the wafer&#39;s surface often decreases with increased distance from the transducer. This energy gradient necessitates wafer rotation in order to achieve uniform cleaning across the entire wafer surface. 
     Conventionally, to achieve rotation, a wafer is positioned on a pair of rollers each of which are mounted to a shaft that extends through the wall of the megasonic tank. Outside the megasonic tank each shaft is coupled to a motor that when energized causes the shaft, and the roller attached thereto, to rotate. As the rollers rotate, so does the wafer positioned thereon. 
     Although such roller assemblies facilitate uniform wafer cleaning across the entire surface of the wafer, they also experience several drawbacks, such as causing cleaning fluid leakage. In an effort to prevent cleaning fluid leakage, rubber seals are used to seal between the roller shaft and the wall of the tank. In practice, however, the heat and chemistry of the tank fluid degrade the seals and leaking inevitably occurs. Fluid leaking undesirably increases the systems operating costs. To reduce this cost, the leaking fluid is sometimes collected, filtered and recirculated to the tank. The degrading seals and leaking chemistry present a potential particle source, and, because leaking rates are random, introduce variables to the cleaning process. Eventually the seals have to be replaced causing downtime costs, as well as labor and replacement parts costs. 
     Accordingly, a need exists for an improved method and apparatus for rotating a substrate as it is processed within a fluid tank. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems described above, by providing a roller which magnetically couples a driver for rotating the roller (i.e., an inventive roller/driver assembly). Because the roller and driver are magnetically coupled, no openings are required in the walls of the fluid filled processing tank, and no leakage occurs. In one aspect, an outwardly extending pocket is formed in or sealed to a wall of the processing tank, and a shaft of the inventive roller/driver assembly is supported by the pocket. This configuration has been found to provide excellent positional accuracy. 
     Because of the magnetic coupling between the driver and the roller, the roller may be easily installed and removed, often requiring no tools whatsoever. Further, in the pocket embodiment a magnet positioned on the roller is axially offset from a magnet positioned on the driver, the driver magnet being positioned outside (i.e., in a direction away from the tank) relative to the roller magnet. The axial offset causes the roller magnet to be attracted outwardly, away from the tank, and thus continuously pulls the roller shaft into the pocket. Accordingly, in certain embodiments, no rings or locking devices are required to lock either the roller assembly or the driver assembly in place, and no metal parts are exposed to the cleaning fluid chemistry, therefore corrosion may be avoided. Moreover, cleaning fluid leakage may be completely eliminated resulting in less wafer to wafer processing variations and less particle generation. Fluid filled processing tanks which employ the inventive roller/driver system may be able to provide higher throughput with lower consumable costs and higher, more consistent quality yields. 
     Other features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational view of a megasonic tank that employs the inventive roller assembly; and 
     FIG. 2 is a side elevational view of one aspect of the inventive roller assembly which eliminates fluid leakage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a front elevational view of a megasonic tank system  11 . The megasonic tank system  11  comprises a tank  13  (e.g., a plastic tank), having a transducer  14  positioned along the bottom thereof (either inside or outside the tank  13 ). The transducer  14  may be equal in length to the diameter of a wafer W to be cleaned thereby. Above the transducer  14   a  first inventive roller assembly  15   a  and a second inventive roller assembly  15   b  are positioned to vertically support a wafer W in line with the transducer  14 . A stabilizing mechanism  17  is positioned outside of the point along the wafer W&#39;s edge which is closest to the transducer  14 , and is therefore outside of the transducer  14 &#39;s highest energy field. The stabilizing mechanism  17  may be positioned so as to contact the wafer W at any point that will sufficiently reduce or prevent the wafer W from wobbling. Typically the inventive roller assemblies  15   a| ,  15   b  and the stabilizing mechanism  17  each comprise a groove for supporting a wafer W with minimal contact. The inventive roller assemblies  15   a ,  15   b  which support the wafer W are described in detail with reference to FIG.  2 . 
     FIG. 2 is a side elevational view of an embodiment of the inventive roller assembly  15  which may eliminate fluid leakage. The inventive roller assembly  15  comprises a wafer support  19  having a groove  21  in which the wafer W (FIG. 1) is positioned. To enhance friction between the wafer support  19  and the wafer W, the groove  21  may be bead blasted, knurled or may have a plurality of small holes drilled therein, along the portion of the wafer support  19  which contacts the wafer W. The inventive roller assembly  15  further comprises a shaft  23  having a magnet  25  coupled thereto. The shaft magnet  25  may be shaped like a sleeve which extends around the entire circumference of the shaft  23 , although the shaft magnet  25  may be any size or shape as long as sufficient coupling exists between the shaft magnet(s)  25  and an external drive magnet  27 , to achieve rotation of the shaft  23 . In one aspect, the shaft magnet  25  is enclosed in a plastic enclosure  29  which is sealed to the shaft  23 . 
     The shaft  23  of the inventive roller assembly  15  is mounted in an outwardly extending housing or pocket  31 . The pocket  31  may be integral with a wall  33  of the megasonic tank  13 , or may be a separate housing, preferably plastic, which seals around an opening in the wall  33  of the megasonic tank  13 , as shown in FIG.  2 . The pocket  31  and the inventive roller assembly  15  are sized so that the inventive roller assembly  15  is firmly supported by the pocket  31 , but is able to rotate therein. 
     The pocket  31  also supports a driver assembly  35  which is external to the pocket  31 . The driver assembly  35  comprises the drive magnet  27  which may comprise a sleeve which extends around the entire circumference of the pocket  31 , although the drive magnet(s)  27  may be any size or shape as long as sufficient coupling exists between the drive magnet  27  and the shaft magnet  25 . The shaft magnet  25  and the drive magnet  27  are of opposite polarity so that an attractive force exists therebetween. The attractive force is strong enough to rotate the roller assembly  15  when the driver assembly  35  rotates. The drive magnet  27  may be mounted between an outer metal housing  37  and an inner plastic housing  39 , which encircles the pocket  31 , and the inner plastic housing  39  may be sized so as to be firmly supported by the pocket  31  but able to rotate therearound. The driver assembly  35  is further coupled to a driving mechanism such as a motor  41 . 
     In operation, the motor  41  is energized and begins rotating the driver assembly  35 . The magnetic coupling between the drive magnet  27  and the shaft magnet  25  causes the inventive roller assembly  15  to rotate as the driver assembly  35  rotates. Friction between the groove  21  and the wafer W positioned therein causes the wafer W to rotate. The transducer  14  is energized and begins oscillating at a megasonic rate. Megasonic energy is therefore coupled to the fluid and travels upward therethrough to contact the surface of the wafer W. As the wafer W rotates, the stabilizing mechanism  17  passively rotates therewith, while reducing and preferably preventing wafer wobble. Energy from the transducer  14  cleans the rotating wafer W as is known in the art. The level of cleaning fluid contained within the megasonic tank  13  varies less than that experienced by megasonic tanks which employ conventional rollers. Accordingly, the inventive roller assembly  15  may add significant value to the megasonic tank  13 , and to any automated cleaning/processing system in which the megasonic tank  13  is employed. 
     The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. The configuration and operation of the megasonic tank  13  and the location of the inventive roller assemblies are merely exemplary, as are the specific shapes, sizes and materials of the roller assembly components pictured herein. The roller assembly groove may or may not have a roughened surface. 
     Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.