Abstract:
A vacuum assembly for removing debris formed on the surface of a work chuck after a wafer grinding process by positioning a vacuum source above the work chuck and then activating the vacuum source.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method and apparatus for cleaning the porous ceramic grind chuck used in semiconductor wafer grinding machines. 
         [0003]    2. Background of the Invention 
         [0004]    U.S. Pat. No. 7,118,446, issued to Thomas A. Walsh and Salman Kassir and assigned to the assignee of the present invention exemplifies the status of prior art grinder apparatus technology. A chuck is provided in the apparatus to hold a work piece, such as a wafer, in place so that the work piece does not slip or otherwise move while being shaped by a grind wheel. 
         [0005]    The chuck is porous i.e. holes are drilled therethrough it or otherwise comprises a porous material; a partial vacuum being provided below the chuck to hold the work piece in place. Coolant is pumped directly onto an area of contact between a grind wheel and the workpiece surface, providing cooling and cleaning of grind debris (swarf) from the surface of the workpiece. 
         [0006]    During the grinding process, vacuum is applied through the porous portion of the work chuck to hold the wafer. Due to dimensional differences between the wafer and the porous portion of the work chuck, there can be a small section near the periphery of the porous portion that is exposed to the grinding swarf. The porous portion of the work chuck will “suck-up” the small particles in the grind swarf. Over time, the small particles from the grind swarf will clog the porous material closest to the perimeter of the wafer. The porous material may get clogged with small particles below the wafer surface. If the wafer edge is not pulled down to the work chuck, the edge of the wafer will rise during the grinding process removing too much material at the edge of the wafer. The clogging of the work chuck edge reduces the life of the work chuck due to the over grinding of the wafer edge. 
         [0007]    What is thus desired is to provide a work chuck cleaning procedure wherein the cleaning can be done either automatically or manually. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides method and apparatus for cleaning the surface of a chuck used to hold a workpiece, such as a wafer, in position during grinding and assisting in the removal of small particles in the work chuck allowing vacuum to flow again, the process being accomplished manually or automatically. In particular, a vacuum cleaner assembly is positioned within a wafer grinder apparatus adjacent the edge of the work chuck. The assembly comprises a vacuum device for pulling particles, or swarf, from the top surface of the chuck. The assembly further comprises a fluid source which, when activated, directs fluid to the chuck surface which in turn causes the assembly to hover above the work chuck surface. The vacuum source is then activated to remove particles from the chuck surface. A source of sonic energy may be positioned in the fluid path, the sonic energy loosening particles that may be tightly adhering to the chuck surface. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing therein: 
           [0010]      FIGS. 1A and 1B  are perspective and plan views, respectively, of a prior art grind apparatus assembly modified to incorporate the vacuum cleaning system of the present invention; 
           [0011]      FIG. 2  is a partial perspective view illustrating where the vacuum cleaner of the present invention is positioned relative to the work chuck; 
           [0012]      FIG. 3  is a partial perspective, sectional view illustrating the work chuck vacuum cleaner of the present invention; 
           [0013]      FIG. 4  is a sectional view of the cleaner shown in  FIG. 3  illustrating the cleaner positioned above the edge of the work chuck; 
           [0014]      FIG. 5  is a sectional view of the cleaner shown in  FIG. 3  illustrating the use of fluid flow to cause the cleaner assembly to hover above the top surface of the work chuck; 
           [0015]      FIG. 6  is a perspective view of another embodiment of the grind apparatus using an arm to move the vacuum assembly; and 
           [0016]      FIG. 7  is a more detailed view of the grind engine of  FIG. 6  modified to use an arm to move the vacuum assembly. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0017]    In order to put the present invention in proper perspective,  FIGS. 1A and 1B  illustrate a prior art grinder assembly, such as that disclosed in the &#39;446 patent, modified to incorporate the cleaner assembly of the present invention. 
         [0018]    Referring to  FIGS. 1A and 1B , shown are perspective and plan views respectively of the compact grinder assembly  100  disclosed in the &#39;446 patent. Shown is a grind spindle  102 , a spindle support column  104 , a work spindle  106 , a cabinet  108 , a splash pan  110 , a chuck  112 , a thickness probe  111 , a ball screw assembly  114 , a bed portion  118 , rails  120  and a ball screw  122 . 
         [0019]    The grind spindle  102  is coupled with the spindle support column  104 , and the spindle support column  104  is engaged with the rails  120  and the ball screw  122 . The cabinet  108  supports the rails  120 , ballscrew  122 , the work spindle  106  and the splash pan  110 . The thickness probe  111  is coupled with the work spindle  106  and is shown positioned above the chuck  112 . 
         [0020]    The grind spindle  102  is moved along a vertical axis by the ball screw assembly  114  and includes at least one grind wheel (not shown) in order to shape a work piece, for example, semiconductor wafers. 
         [0021]    The chuck  112  holds the work piece in place so that the work piece does not slip or otherwise move while being shaped by a grind wheel on the grind spindle  102 . For example, the chuck  112  is porous, e.g. it has holes drilled through it or otherwise comprises a porous material, and a partial vacuum is provided by a device (not shown) positioned below the chuck  112  to hold the work piece in place. 
         [0022]    The spindle support column  104 , supports the grind spindle  102 , and is moveably engaged with the support column  104 , and hence the grind spindle  102 , to translate back and forth in a horizontal direction. Specifically, the spindle support column  104 , and the grind spindle  102  move with respect to the cabinet  108 , the work spindle  106 , and thus a surface of a rotatable work piece mounted on the chuck  112 . 
         [0023]    The ability to translate the grinding assembly  104  allows shaping of a work piece to be achieved on both a face and an edge of the work piece with a single machine. Specifically, a grinding wheel of the grind spindle  102  is first positioned over an edge of the work piece and then moved into contact with the edge of the work piece until the edge is shaped as desired. The grind spindle  102  is then raised vertically above the work piece, translated horizontally over a face of the work piece so the grinding wheel is positioned over the face of the work piece, and then the grinding wheel is then placed in contact with the face of the work piece by lowering the grind spindle  102  until the grinding wheel is in contact with a portion of the face of the work piece. 
         [0024]    Referring to  FIG. 2 , the work chuck vacuum cleaner assembly  200  of the present invention is positioned above the top surface  202  of work chuck  204  and adjacent the edge  206  thereof. The backflush operation that is part of the process disclosed in the &#39;446 patent cooperatively operates with the assembly  200 , the former forcing trapped particles from near the bottom of the work chuck whereas assembly  200  removes trapped particles from the top surface of the work chuck. 
         [0025]      FIG. 3  shows details of the structure of assembly  200 . Specifically, a vacuum source  210  is attached to the hose mount and port  210  within housing  212 . Port  214 , also mounted within housing  212 , is supplied with distilled water or other liquid to enable assembly  200  to hover above the surface  202  of work chuck  204 . Channel  216  receives the hovering fluid. A seal  218  is positioned around a portion of the circumference of housing  212  as illustrated and a gimbal block  220  is secured within housing  212  and a porous material  222  is positioned below channel  216 . 
         [0026]    Referring to  FIG. 4 , as work chuck  204  rotates under assembly  200 , a side force, illustrated by arrow A, is induced on the assembly. In particular, the side force is generated by the relative motion of the chuck surface under the assembly or by the movement of the indexing table (not shown) that supports the chuck spindle mechanism. Since the projected gimbal point is below the top surface  202  of work chuck  204 , the leading edge of assembly  200  will not impact surface  202 . Specifically, the projected gimbal mechanism prevents the leading edge of assembly  200  from digging into the top surface of work chuck  204 . Since the projected gimbal point is below the surface of the work chuck, a side force (caused by friction between the moving parts) is exerted on assembly  200 , the leading edge will rotate upward instead of downward into the surface of the work chuck, allowing for smooth, vibration-free operation of the vacuum cleaner. 
         [0027]    Referring to  FIG. 5 , the basic operation of assembly  200  is illustrated. Particles of debris (represented by a single particle  230 ) need to be removed from the edge of chuck  204  for the reasons noted hereinabove. Fluid flow is initially introduced into inlet port  214 ; fluid represented by arrows  232  emitted through porous member  222  impinges upon the surface of the work chuck causing assembly  200  to hover above the surface of chuck  204  (the work chuck back flush could alternatively be utilized to hover assembly  200  above the work chuck surface). At this time, a vacuum flow is introduced to vacuum inlet  210  by the vacuum source used to hold the work piece on the chuck surface or by an independent vacuum source. The vacuum flow pulls the particles from the surface of chuck  204 , through vacuum port  210  and hence to a storage container. A movable arm positions assembly  200  above the surface of work chuck  204 . 
         [0028]    To provide a technique for ensuring that the particles are removed from the chuck surface for collection by the vacuum source, a sonic source (not shown) is placed in the fluid flow supply line, the sonic energy being directed to the chuck surface by the fluid flow itself. 
         [0029]    Note that although the cleaning assembly  200  of the present invention is shown positioned adjacent the edge portion  206  of work chuck  204  since the edge accumulates debris, or swarf, generated during the grinding process, the assembly can also be utilized to clean all portions of the chuck surface. 
         [0030]    The vacuum source can be manually initiated for most applications. However, if cleaning is required for every work piece, a control signal can be provided from the system control software to actuate the vacuum assembly after every wafer (or every N th  wafer) is ground. 
         [0031]    The operative cycle of assembly  200  is as follows: 
         [0032]    After the wafer is removed from the top surface of the work chuck  204 , the work chuck blow-off (air) and back flush (DI water) will be turned on to purge the majority of the particulates that were sucked into the porous work chuck material during the grinding cycle. During this process, the work chuck  204  will spin to push the particles off the edge of the work chuck. Now the majority of the particulates have been removed, the vacuum assembly will be actuated and placed on the work chuck. The majority of particles will be stuck where the perimeter of the wafer made contact with the porous section of the work chuck. Vacuum assembly  200 , the preferred embodiment, starts near the center of work chuck  204  and moves radially outward, as the chuck slowly rotates, until it reaches the location where the majority of the undesirable particulates are stuck. The vacuum assembly stays in this location as the work chuck rotates slowly. After the user defined vacuum time setting has been reached, the vacuum assembly will lift and rotate back to the home position. Note that vacuum assembly  200  never touches the surface of work chuck  204 ; there is always a layer of water present to cause the assembly to hover above the surface of the work chuck, the water being supplied either from assembly  200  or through the work chuck pores from below the porous chuck. 
         [0033]    Note that a movable arm or indexer positions assembly  200  above the surface of work chuck  204  and moves the assembly across the work chuck surface in the manner described hereinabove. A microcontroller is provided to move the arm or indexer in the desired sequence. 
         [0034]      FIG. 6  illustrates another grind engine (assembly)  300  in which the vacuum cleaning assembly  200  of the present inventor can be utilized (illustrated is the recently introduced Model 7AH grind engine manufactured by Strasbaugh, San Luis Obispo, Calif.). The components of particular interest with the respect to the invention illustrated are the porous grind chuck  204 , vacuum assembly  200 , base casting  302  and arm  304 . A controller (not shown) is utilized to position arm  304  adjacent the work chuck  204 . 
         [0035]      FIG. 7  is a different view of grind engine  300  and illustrates an alternate version of the vacuum assembly positioner. In particular, grind engine  300  is modified to incorporate a bearing  310  and rotary indexer  312 . Bearing  310  is seated in base casting  302  and rotary indexer  312  is seated in bearing  310 . 
         [0036]    The use of a controlled indexer to position a mechanical component is well known in the prior art and is not described herein in detail since it is not considered part of the present invention. Once the arm  304  has positioned cleaner  200  on chuck  204 , the indexer motion can be used to move cleaner assembly  200  across the top surface of chuck  204 . 
         [0037]    Although the cleaning process described hereinabove is preferably performed after the wafer grinding process, the cleaning process may be modified so that it occurs during the grinding process. In addition, for thin wafers, a second vacuum source may be added below the work chuck to ensure that the wafer edge is firmly held down in order to avoid lifting of the edge, thus allowing swarf to be pulled into the vacuum assembly. 
         [0038]    The process described hereinabove maintains the surface of the work chuck clean from grind swarf particles to avoid non-uniform thickness of the wafers and wafer star cracks in wafers generated from the vertical grinding force on the wafer being ground. 
         [0039]    While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.