Abstract:
A ganged saw blade assembly for dicing of wafers includes a plurality of circular saw blades positioned along a common central axis and erodible pitch spacers positioned along the common central axis between adjacent saw blades. The pitch spacers are eroded to a desired diameter relative to the common central axis to maintain a desired saw exposure, e.g. by sawing into an abrasive material with the saw blade assembly. The saw blade assembly thus permits use of the saw blades over longer periods notwithstanding erosion of the blades.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to assemblies of rotary saw blades for the semiconductor industry. 
       BACKGROUND 
       [0002]    In the making of electrical components from semiconductor materials and the like, multiple patterns are produced on a substrate that is subsequently sawed into smaller portions. The saws on which this is done are generally termed “dicing saws”. The circular blades used in such a saw are made of abrasive materials, for example, pieces of diamond held in a resin, CBN abrasive, or electrolytically deposited (electroformed) nickel bond matrix. The saw blades are thin so that they have a small width of cut, and thereby waste less material and generate less heat. But when a saw blade is thin it needs support near the outer edge where it is cutting. This support must be supplied without interfering with the planned cutting task. One way to provide support is to use a round disk of a smaller diameter than the blade. As an example, a saw blade of 2 inch diameter may be adjacent to and in contact with a support of 1.7 inch diameter, so that the blade can cut into a substrate to a depth of 0.1 inch, while still having a 0.05 inch clearance between the support and the substrate. In the example: (2−1.7)/2−0.1=0.05. In this example, 0.15 inches is known as the “blade exposure” or “cutting edge”. 
         [0003]    As an example of prior art,  FIG. 1  illustrates a ganged cutter assembly  2  having a hub body  4  with a bore  6  that precisely fits on a pilot  8  of a rotating spindle  10  and is secured by a spindle nut  12 . In this example, there is no key-way or similar feature for torque transmission. The friction between the squeezed faces of the cutter assembly  2  and the spindle  10  is sufficient to prevent slippage. The ganged cutter assembly  2  has blades  14  and spacers  16  arranged and squeezed in place on a shoulder  18  by an outer flange  20  fastened with bolts  22 . The spacers  16  provide the support previously described. Multiple copies of the cutter assembly  2  would typically be assembled and stocked by a tool room of a manufacturing facility, so they could be brought to a dicing machine as the previous cutter assembly becomes worn and needs replacement. It is not usually blade dullness that dictates blade cutter assembly replacement. The blades are usually made of diamonds or other particles impregnated into a bonding material. As a diamond particle becomes dull, the cutting forces acting on it increase and it is pulled from the bonding material so that new diamond particles become exposed to keep the blades sharp. However, this process means a continual decrease in the diameter of the blades. As the abrasive blade wears, its diameter becomes smaller while the adjacent spacer diameter remains the same. The spacers are typically manufactured from hard materials that resist wear, for example hardened stainless steel or aluminum-oxide ceramic. Eventually, in attempting to perform the intended cut, the spacer contacts the substrate surface. This can damage the substrate and the saw. Prior to this degree of wear, the sawing process must be stopped and a new cutter assembly put on the spindle and the old one returned to the tool room. Back in the tool room, the blade exposure can be returned to a sufficient depth by either replacing the abrasive blade with a new one, or installing a smaller diameter spacer and continuing with the same blades. This process causes down time for the saw, increased labor costs, and the need to stock spacers of various diameters. 
         [0004]    There are at least two kinds of dicing saws. The kind with a single supported blade, for example those disclosed in U.S. Pat. No. 5,261,385 to Kroll, that must do multiple passes across a substrate to achieve multiple cuts, and a ganged set of blades (as seen in  FIG. 1 ). Ganged sets of blades are blades that are on a common axis and spaced apart from one another by spacers. In many cases, the spacers also provide support. In this application, the nouns “support” and “spacer” may be used interchangeably to refer to the same piece of hardware. While gang blade assemblies provide a multi-fold increase in machine efficiency and throughput, a notable portion of tool cost is the labor involved to stack the assembly. This cost reduces price-competitiveness compared with a single-blade process model, which does not require a stacking procedure. 
         [0005]    When a gang blade assembly is stacked for the first time, using freshly manufactured blades and spacers, the parts are usually free of warpage and stacking is not too difficult. However, after use, when re-stacking is needed with smaller spacers, the blades and previously used spacers are often found to be warped, making stacking more difficult. In an effort to get the least measured run-out, it is often necessary to angularly change (this process is commonly known as clocking) the blades and spacers relative to each other many times while measuring the run-out. 
         [0006]    If the frequency of re-stacking procedures can be reduced then the gang blade assemblies will require less labor to use and will be more cost competitive. 
         [0007]    One way to reduce the frequency of re-stacks is to machine or erode the spacers to a smaller diameter without unstacking them from the blades. If the spacers are of a hard material this could be difficult. But, it is known, to provide spacers of a softer material that can be eroded away relatively easily, by purposefully contacting it with a hard dressing material. Further, if the dressing material is substantially softer than the blades, then the blades can cut into the dressing material with no harm done. If desired, the dressing material can be specifically chosen to “dress” the blades, (i.e., selectively remove the blade bonding material to expose fresh particles). Such a process of eroding the spacers is described in U.S. Pat. No. 5,261,385 to Kroll, for single blade dicing saws. However, it is desirable to use erodible spacers in a ganged saw assembly to achieve even greater efficiencies. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with principles of the present invention, a marked improvement is accomplished in the allowable blade wear in a ganged saw blade assembly, such as a gang saw for dicing of wafers. This is accomplished by constructing the gang saw of saw blades separated by erodible pitch spacers. As the saw blades erode during use, the pitch spacers may be eroded in a controlled, matching fashion, e.g. by pausing the use of the tool and sawing into an abrasive material with the saw blade assembly at a controlled cut depth that abrades the pitch spacers so as to return the saw blades to the desired exposure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
           [0010]      FIG. 1  is an exploded perspective view of a ganged dicing saw assembly of the prior art and a spindle to which it attaches. 
           [0011]      FIGS. 2A-2C  illustrate an embodiment of a ganged dicing saw assembly being stacked on an assembly tool, and then in its final form. 
           [0012]      FIG. 3  illustrates an assembled view of the ganged dicing saw embodiment of  FIG. 2C  in cross-section on a partially cross-sectioned spindle assembly on a spindle. 
           [0013]      FIG. 3A  is a detailed view as indicated in  FIG. 3 . 
           [0014]      FIG. 4  illustrates the embodiment of  FIGS. 2C and 3  being brought to an abrasive block for a blade dressing and erosion sequence. 
           [0015]      FIG. 5A  is a detail view as indicated in  FIG. 4 , of a ganged assembly before erosion and dressing. 
           [0016]      FIG. 5B  is the detail view of  FIG. 5A  after erosion and dressing. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIGS. 2 and 2B  illustrate an embodiment of a gang cutter  40  of the current design being assembled from blades  42 , pitch spacers  44 , and end spacers  46 . The embodiment does not use a hub body  4 , flange  20 , or bolts  22 . Instead the blades  42  and spacers  44 ,  46  are held together by adhesive  48 . The adhesive  48  need not be strong enough to transmit the cutting torque during use because as illustrated in  FIG. 3 , the gang cutter  40  will be squeezed on the spindle  10  by the spindle nut  12  acting through a clamp spacer  50 , so mechanical friction between adjacent spacers  44 ,  46  and blades  42  will transmit torque even without the adhesive  48  remaining bonded. However, the invention is not so limited, and in some embodiments adhesives or other bonding techniques may provide the only torque transmission path. 
         [0018]    As seen in  FIGS. 3 and 3A , the gang cutter  40  has blades  42  of an outside diameter designated D, and the pitch spacers  44  and flange spacers  46  have an outside diameter designated S. A gang cutter  40  setup like this is said to have an exposure (E) calculated as E=(D−S)/2. As seen in  FIG. 3A , as the gang cutter  40  is brought in to a cutting relationship with a workpiece  52  it can theoretically cut through, or cut a groove to a depth equal to E. However in practice, clearance for coolant flow and debris flushing should be considered, so that actual maximum cut depth may be less than E. 
         [0019]    As explained in the background, as the blade  42  wears, D decreases while S remains constant, except for minor erosion from cutting fluid and debris. This causes E to decrease, and it is a limiting factor as to how long production can continue. In the present invention, the spacers  44 , 46  are made of an easily abraded material such as a plastic composite, or a molded/extruded/pressed graphite, or a pressed graphite, or a combination of these or any suitable material. They may be bisque-fired (i.e. partially-fused) ceramic. When E decreases to a limit, for example an E 1  ( FIG. 5A ), before the next workpiece  52  is cut a dressing block  54  is put in place of the workpiece  52  as seen in  FIG. 4 . The dressing block  54  is made of an abrasive material that is harder than the pitch spacers  44  and the flange spacers  46 , but significantly softer than the blades  42 . By simply cutting into the dressing block  54  to a depth, for example E 2 , the pitch spacers  44  and the flange spacers  46  will be eroded away and the exposure will at that time be E 2  ( FIG. 5B ). Then production cutting may resume. Advantageously, if the blades  42  require a dressing pass to sharpen or hone them, as many do, the dressing block  54  may be chosen to accomplish the sharpening or honing at the same time. 
         [0020]    The exposure E is a critical parameter in thin-blade, precision slicing/dicing operations. Maximum exposure to thickness ratios have been empirically developed by the assignee of this application, based upon the bond composition (and stiffness) of the blade. 
         [0021]    There are three fundamental bond types used in the dicing saw industry (in order of increasing stiffness—i.e. elastic modulus): resinoid, sintered metal and electroformed Ni. Experience has led to the use of the following maximum aspect ratios: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Maximum 
               
               
                   
                 Blade Bond Type 
                 Exposure:Thickness ratio 
               
               
                   
                   
               
             
             
               
                   
                 resin 
                 10:1 
               
               
                   
                 Metal 
                 20:1 
               
               
                   
                 Ni 
                 30:1 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    To assure that the above concept is understood, an example calculation is as follows: A blade that is 0.010 inches thick and 2 inch diameter is adjacent spacers that are 1.700 diameter, of any thickness. Then, E=(2−1.7)/2=0.150, and the ratio is 0.150/0.010=15:1, so this combination would be acceptable blade exposure for blades  42  made of metal or Ni, but not for blades  42  made of a resin. 
         [0023]    An advantage of the erodible spacer concept is that maximization of initial exposure will not be required in order to maximize blade life. For example, it will only be necessary to erode the spacers enough to expose an additional 0.010″−0.015″ beyond the required cut depth. This means that cuts can be more precise and cutting speeds and production increased on a consistent basis. It also means that whereas the economics of cutter assembly life may have previously led to the use of a long blade exposure and therefore a metal or Ni blade, now it is possible to use the less expensive resin blades  42 . 
         [0024]    Referring again to  FIGS. 2A-2C , the stacking fixture  56  and its method of use will be explained. A tapered post  58  receives an expanding mandrel  60 . A stacking spacer  62 , blades  42 , pitch spacers  44  and the flange spacers  46 , are stacked to surround the expanding mandrel  60 . Adhesive  48 , for example a polyvinyl acetate (PVA) is put between the blades  42 , pitch spacers  44 , and flange spacers  46  as they are stacked. The adhesive  48  is sized to spread out in a fine layer and enter the porous areas of the blades  42 , pitch spacers  44  and flange spacers  46  so that excess adhesive  48  does not affect the overall stack of the gang cutter  40 . If necessary, relief areas (not shown) may be included in the pitch spacers  44  and the flange spacers  46  to make space for excess adhesive  48 . As an alternative, a suitable adhesive may be pre-applied to some or all of the spacers or blades, and then activated in a suitable process, such as for example, by heat, pressure, radiation, etc. A downward force on the stack moves the expanding mandrel  60  down the tapered post  58  so that the expanding mandrel  60  expands and engages the inside diameter of the flange spacers  46 , pitch spacers  44 , and blades  42  to align them. The stacking spacer  62  is sized to limit the diametral expansion of the expanding mandrel  60  so that excess force cannot be applied to the inside diameters of the blades  42 , pitch spacers  44 , and the flange spacers  46 . A weight  64  or another way to supply compression is left in place while the adhesive  48  cures.  FIG. 2C  shows the gang cutter  40  after it has been removed from the stacking fixture  56 , ready to be installed as in  FIG. 3 . 
         [0025]    Although the embodiments described have pitch spacers  44  of all the same thickness, and flange spacers  46  that are flanged, any combination and quantity of spacers may be used. The flanged spacers provide another surface to grip while handling the gang cutter  40 , which is especially beneficial for small sizes. 
         [0026]    The embodiment described in  FIGS. 2A-5B  uses adhesive  48  rather than the hub body  4 , flange  20 , and bolts  22  of the prior art  FIG. 1 . However, it is contemplated that erodible spacers can also be used in embodiments that use the hub body  4  and flange  20  and bolts  22  of  FIG. 1 . This may be for new designs, or for existing equipment already in use. Any limitations caused by the largest diameters of the hub body  4  and forward flanges must be considered because the smaller the blades  42  and spacers become, the more the hub body  4  and flange  20  will protrude. 
         [0027]    The table below lists some typical blade OD/ID/thickness dimensions. However, note that these are examples, and the possible OD/ID/thickness combinations are not limited to this list. The table also includes the associated pitch spacer thicknesses that might be included in a gang. Spacer OD would associate with required exposure and that exposure could range from zero to approximately the max ratio allowed by the blade bond type, which may change as materials and processes improve. The last column, containing a special symbol, is to identify some sizes that are expected to be a commonly used size. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Blade 
                 Pitch Spacer 
                   
               
             
          
           
               
                 OD 
                 ID 
                 Thks 
                 Thks 
               
               
                   
               
               
                 2.000-2.188″ 
                  .750″ 
                 .0006-.012″ 
                 .020-.080″ 
                 * 
               
             
          
           
               
                 50.80-55.56 mm 
                 19.05 
                 mm 
                 .015-.300 
                 mm 
                 .500-2.000 
                 mm 
                   
               
             
          
           
               
                 3.000″ 
                 1.250″ 
                 .0006-.012″ 
                 .020-.080″ 
                   
               
             
          
           
               
                 76.2 
                 mm 
                 31.75 
                 mm 
                 .015-.300 
                 mm 
                 .500-2.000 
                 mm 
                   
               
             
          
           
               
                 3.000″ 
                 1.575″ 
                 .0006-.012″ 
                 .020-.080″ 
                 * 
               
             
          
           
               
                 76.2 
                 mm 
                 40.00 
                 mm 
                 .015-.300 
                 mm 
                 .500-2.000 
                 mm 
                   
               
             
          
           
               
                 4.000″ 
                 2.047″ 
                 .0012-.050″ 
                 .040-.500″ 
                   
               
             
          
           
               
                 101.6 
                 mm 
                 52.00 
                 mm 
                 .030-1.270 
                 mm 
                 1.000-12.700 
                 mm 
                   
               
             
          
           
               
                 4.300″ 
                 3.500″ 
                 .0024-.050″ 
                 .040-.500″ 
                   
               
             
          
           
               
                 109.22 
                 mm 
                 88.90 
                 mm 
                 .060-.270 
                 mm 
                 1.000-12.700 
                 mm 
                   
               
             
          
           
               
                 4.400″ 
                 3.500″ 
                 .0024-.050″ 
                 .040-.500″ 
                   
               
             
          
           
               
                 111.76 
                 mm 
                 88.90 
                 mm 
                 .060-1.270 
                 mm 
                 1.000-12.700 
                 mm 
                   
               
             
          
           
               
                 4.500″ 
                 3.500″ 
                 .0024-.050″ 
                 .040-.500″ 
                 * 
               
             
          
           
               
                 114.30 
                 mm 
                 88.90 
                 mm 
                 .060-1.270 
                 mm 
                 1.000-12.700 
                 mm 
                   
               
             
          
           
               
                 4.600″ 
                 3.500″ 
                 .0024-.050″ 
                 .040-.500″ 
                   
               
             
          
           
               
                 116.84 
                 mm 
                 88.90 
                 mm 
                 .060-1.270 
                 mm 
                 1.000-12.700 
                 mm 
                   
               
             
          
           
               
                 5.000″ 
                 3.500″ 
                  .005-.050″ 
                 .040-.500″ 
                   
               
             
          
           
               
                 127.00 
                 mm 
                 88.90 
                 mm 
                 .127-1.270 
                 mm 
                 1.000-12.700 
                 mm 
               
               
                   
               
             
          
         
       
     
         [0028]    The invention has been described herein with reference to specific embodiments, and those embodiments have been explained in substantial detail. However, the principles of the present invention are not limited to such details which have been provided for exemplary purposes.