Patent Abstract:
Hands free removal of layers of material simultaneously from a number of dies is accomplished by temporarily positioning a plurality of die holding devices into different segmented open areas of a template mounted over the grinding surface. In one embodiment, frictional force imparted to each holding device by the grinding wheel serves to position the holding device against a stop within the confines of each opening. The stop in each segment could be positioned at a different radial distance from the center of the grinding wheel in order to use different portions of the grinding wheel to grind each of the dies. In some embodiments, the segments are offset from each other around the template in order to increase the effective working area of the grinding surface.

Full Description:
TECHNICAL FIELD 
     This disclosure relates to polishing semiconductors and more particularly to apparatuses and methods for hands free removal of layers of material simultaneously from a number of semiconductor dice. 
     BACKGROUND 
     It is often necessary to grind (polish) off layers of a semiconductor so that the inner structures can be made available for visual inspection, often by observation using an electron microscope. A polishing fixture useful for holding the semiconductor (or other device to be polished) against a polishing wheel for this purpose is the subject of U.S. Pat. No. 5,272,844. Some polishing operations are now performed using a positioning structure that is suspended over a grinding wheel. The positioning structure is a frame with an open center and a plurality of circumferentially spaced openings into which a die holding tool can be placed. The operator uses the side of the opening to help stabilize the holding tool while the die is being polished. Problems exist with the positioning device when it is desired to polish dies without requiring the operator to keep his/her hands on the tool. These problems primarily concern the fact that the tool is not held securely and thus “wobbles”. Stops are provided on the device for maintaining the tool within the openings. However, the stops are ineffective for their intended purpose. 
     Polishing in this manner is a manual process which can take anywhere from half a day to two or three days with an operator standing in front of the grinding wheel and holding the device while the wheel spins. The holding device is constructed such that it allows for the semiconductor to be positioned in various orientations depending upon the planar angle desired to be grinded away. This procedure is slow and tedious and often results in cramped hands and fingers. 
     BRIEF SUMMARY 
     Hands free removal of layers of material simultaneously from a number of dice is accomplished by temporarily positioning a plurality of die holding devices into different segmented open areas of a template mounted over the grinding surface. In one embodiment, frictional force imparted to each holding device by the grinding wheel serves to position the holding device against a stop within the confines of each opening. The stop in each segment could be positioned at a different radial distance from the center of the grinding wheel in order to use different portions of the grinding wheel to grind each of the dice. In some embodiments, the segments are offset from each other around the template in order to increase the effective working area of the grinding surface. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  illustrates a typical multi-layered semi-conductor; 
         FIG. 1B  shows a typical grinder used for polishing layers of a semi-conductor; 
         FIG. 2  shows a top view of one embodiment of a multi-segmented device for holding multiple die during polishing; 
         FIG. 3  is a perspective view of one embodiment of a support structure; and 
         FIG. 4  shows one embodiment of a flow chart of one method for using the polishing support concepts discussed herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  illustrates a typical multi-layered semi-conductor (die), such as semi-conductor die  10 . As shown, die  10  has passivation layer  101 , layers  102 ,  103 ,  104  and  105  in which active circuits can be constructed and vias  110 ,  111  and  112 . Electrical connections can be constructed to pass from one layer to another through one or more of the vias. 
     It could happen that during the design and/or manufacturing process a defect occurs within a die. Before that defect can be remedied for subsequently produced dice, the defect must be identified. For example, assume that a malfunction is detected in the circuit operation of a manufactured die, such as die  10 . In such a situation, die  10  would be delayered by grinding off successive planes of material, using, for example, grinder  100  shown in  FIG. 1B . A surface of die  10 , such as surface  101 , would be held in contact with moving grinding surface  12  of grinder  100  and over time surface  101  would be removed exposing layer  102  to view by, for example, electron magnification. If layer  102  is determined to be free of defects then via  110  is slowly polished away with the operator viewing, from time to time, via  110  for possible defects. As will be discussed, the IC package can be positioned within a tool (a T-tool) and the angle of attack of the IC package with respect to the plane of grinding surface  12  can be adjusted as desired. The tool that holds the die can be any of the well-known tools for holding dice for polishing, such are obtainable from TD Jam Precision. These tools have silicon feet that allow the tool to hold the work piece at various angles to the grinding surface. The work piece (die) can be fastened to the tool using fasteners, screws, epoxy, glue, springs or the like. 
     Sometimes the fault is pre-isolated to a certain layer or layers so that stopping the process before arriving at those layers is not necessary. Thus, if it is known (from electrical testing or otherwise) that a fault is contained in die  10  somewhere between layers  104  and  105  then layers  101  through  103  would be polished away without detailed observation. Timing may determine how deep (how many layers) the polishing has gone through. 
     In the example being discussed, the polishing continues, typically using a micron slurry (not shown), until the structure preceding level  130  has been removed (delayered). Polishing continues with observations being made after a particular amount has been renewed (for example, every ½ micron). Eventually, defect  120  will become visible and the operator will see that via  111  is shorted to via  112  by defect  120 . Often it is required to polish several IC devices in this fashion to find one or more defects. This might take a half day or even a full day (and sometimes longer) for each IC device. This process is known in the industry as P (as in polishing)-lapping. 
       FIG. 2  shows a top view of one embodiment of a multi-segmented device, such as device  20 , for holding multiple dice during parallel polishing of the die. Device  20  has an outer periphery  201  that is designed to mate with an outer periphery ( 142   FIG. 1 ) of grinder  100 . If desired, inserts (not shown) can be used to fit device  20  to grinders, such as grinder  100  ( FIG. 1A ), if the grinder has physical dimensions different from the physical dimensions of device  20 . The surface area of grinder  12  is seen looking down into the open area (“wings”)  202  of device  20 , which in the embodiment illustrated has pinwheel openings. In this embodiment, grinder surface  12  will be assumed to rotate or move counter-clockwise with respect to device  20  which is held stationary with the frame of grinder  20  ( FIG. 1A ). 
     Positioned within each slot  202  is at least one stop  24 . The stops  24  for each of the slots  202  can be at the same radius (as measured from the center of the grinder  100  outward) or preferably at different radii so that wear on the grinding surface  12  will be spaced radially outward as will be discussed. For 7 mm dies the stops  24  can be spaced 7 mm offset from each other. 
     In operation, a device to be polished is positioned on the bottom surface (not shown in  FIG. 2 ) of removable tool  26 . Once the die is secured in tool  26 , the tool  26  is then positioned within one of the slots  202  against grinding surface  12 . This positioning can be accomplished without regard to whether grinder  100  is operating or stopped. Friction force of the moving grinder surface  12  with respect to the substrate will force the tool  26  holding the substrate (die) against the side wall and against stop  24 . 
     Tool  26  will continue to rest against stop  24  while grinder surface  12  rotates there under without requiring the operator to maintain pressure or even touch the tool  26 . Using this arrangement, several tools  26  (in this embodiment three other tools) can be positioned in the other slots  202  of device  20  so that four dice can be simultaneously polished using the same grinder  100 , all without necessitating the use of the operator&#39;s hands to maintain the position of the respective tools  26 . Note that the die holding tools  26  need not be the same, since each slot  202  operates independently from each other slot  202 . Also note that while only one tool  26  is discussed as being positionable within a slot  202 , device  20  could be designed such that multiple tools  26  could be self-positioned within each slot  202  if the diameter of the grinding surface  12  is large enough to support multiple tools  26 . 
     By staggering the placement of each tool  26 , in one example, by 7 mm, the grinding surface  12  can be worn evenly and the slurry can also be positioned evenly because centripetal force will move the slurry from the center outward. 
     Stops  24  can be designed for a mating relationship with the end of the T tool  26 , so as to prevent wobble of the tool  26  and to maintain the tool  26  within the confines of the slot  202  in which the tool  26  is placed. In one embodiment, the stops  24  are rounded to fit the ends of the T tool  26 . In other embodiments, the stops  24  are at 90 degree angles with respect to the downstream side wall of the slot  202 . The height of the slot sides can be made to fit the tool height. In one embodiment this height is 15 mm. Also note that the slots  202  are not symmetrical about the center, and are offset from symmetrical by approximately 17.5 mm., such that the central openings of each slot  202  are not exactly opposite each other. This offset is so that a grinder surface  12  having a fixed diameter can handle more dice than it could be if the slots  202  were perfectly symmetrical about the center of the grinder  100 . The actual interior contour of each slot  202  is not critical but should be designed such that the T tool, or other dice holding device  26 , is easily positioned within the slot area  202 , both for placement and removal. 
     A cover, or partial cover, can be positioned above each slot  202  if desired. The cover, or partial cover can act as a splash guard to keep the slurry confined within the cover or dome. 
     Note that tool  26  could be designed with a number of indents along the top of T portion. The indents could mate with one or more tabs protruding from wall  210  thereby holding the tool  26  in a fixed relationship within each slot  202 . 
       FIG. 3  is a perspective view of one embodiment  30  of a template structure utilizing the concepts of this disclosure. In the embodiment shown, support (polishing template)  30  has outer periphery  31  designed to attach permanently or temporarily to a grinder  100 . This attachment can be, for example, by a skirt (not shown) around the periphery of the support where the skirt mates with the grinder  100 . The grinder attachment could also be feet, such as feet  32  which attach to the grinder  100  to prevent template  30  from moving while the grinder  100  is moving. The peripheral support could also mate with the fixed structure of the grinder  100  by using, for example, fasteners friction, notches, Velcro or the like. Top surface  36  can be thought of as a bridge suspended from periphery  31  (and supports  32 ) over the grinding surface. In the embodiment shown, surface  36  is relatively flat, but surface  36  can have portions curved upward over slots  33 - 1  to  33 - 4  to form a partial cover or dome over the slots to help prevent splashing of slurry from the grinder surface  12  when the template is being used. The template is open in the center to allow the die and its holding tool  26  to be placed on the grinding surface  12  as discussed above with respect to  FIG. 2 . 
     The template extends over the grinding surface  12  and has one or more slots  33 - 1  to  33 - 4  radiating outward from the center opening toward the peripheral support. Each slot area  33 - 1  to  33 - 4  would have at least one side wall  34  extending downward from the center to a point just above the grinding surface  12 . The exact distance above the grinding surface  12  where the bottom of the template wall  34  is positioned is controlled by the height of the skirt (or legs)  32  around the periphery  31  and is not critical so long as the side wall  34  has enough surface area, height ‘h’, to impart stability to the T tool  26 . The side wall  34  is constructed on what would be the down steam side of the slot area  33 - 1  to  33 - 4 . Down stream in this context is the side of the slot area  33 - 1  to  33 - 4  toward which the grinding surface  12  moves. If the grinding surface  12  goes in both directions, then two sidewalls  34  will be required for self-supporting polishing operation to be employed in both directions. In the embodiment of  FIG. 2 , since the grinding surface  12  is assumed to be counter-clockwise then the downstream side of the slot area  33 - 1  to  33 - 4  is as shown on the left side of the slot area  33 - 1  to  33 - 4 . 
     As discussed above, and in reference to  FIG. 3 , friction caused by the grinder  100  against the die surface under the T tool  26  forces a side (the top of the T tool) of the T tool  26  against the downstream side wall  34  and in mating relationship with the stop  35 - 1  to  35 - 4 , such as with stops  35 - 1  to  35 - 4 , mounted on the respective side walls  34 . Since the stops  35 - 1  to  35 - 4  are positioned closer to the center of the grinding surface  12  than is the T tool  26 , the movement of the grinder  100  pulls the T tool  26  against the stop  35 - 1  to  35 - 4 . Because the stop  35 - 1  to  35 - 4  is designed to mate with, or at least have complementary structures with, the side of the T tool  26 , the force of the friction from the grinding surface  12  on the die causes the T tool  26  to be held in a stable relationship with respect to the downstream side wall  34  and the stop  35 - 1  to  35 - 4 . 
     Once the holding tool  26  is placed in the slot  33 - 1  to  33 - 4 , friction holds the tool  26  in place. The operator is then free to place other dice in other slot areas  33 - 1  to  33 - 4 . At any time during the polishing process any tool  26 can be easily removed while the grinder  100  continues to polish the other dice. Thus, the operator can remove or replace any die at any time without disturbing the other dice which remain positioned relative to their respective downstream side walls  34 . 
       FIG. 4  shows one embodiment  40  of a flow chart of one method for using the polishing support concepts discussed herein. Process  401  determines if there is a die mounted in a holding tool ready to be polished. If not, then process  402  allows an operator or a machine to mount the die by selecting the desired orientation of the die to be presented to the grinding (polishing) surface. 
     Process  403  determines if a proper template is positioned on the proper grinder and if not process  404  selects the proper template and grinder and secures the template to the grinder. 
     Process  405  then positions the held die in an open slot of the selected template. The grinder is turned on (if it is not already on) and the bottom surface of the positioned die is polished for a period of time. The polishing results from the fact that the grinding surface moves relative to the die surface such that friction caused by the grinding surface against the die causes a side of the holding tool to move against the downstream side wall and in mating relationship with the stop mounted on the side wall. 
     After process  405  is complete, process  406  determines if additional dies are available for polishing. If so, processes  401  through  406  are reiterated and second, third and fourth tools can be positioned concurrently in other slots of the selected template. 
     Process  407  determines if it is time to inspect one of the positioned dice. This can be by elapsed time, or in some situations by other signals available to the operator. When it is time for inspection, process  408  removes the die from the slot while the grinding surface continues to move relative to the template support and the die is inspected. 
     Process  409  determines if polishing is complete with respect to the inspected die. If it is, the die is not returned to the template. If polishing is not complete then process  405  is reentered. If polishing is complete then process  410  ends the polishing with respect to the inspected die. Note that even though one die has been removed from the template the other dies continue to be polished in a hands-free manner. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Technology Classification (CPC): 1