Abstract:
Semiconductor dice are electrically tested prior to final assembly. Dice failing the test are identified and not packaged. However, “good dice” (i.e., those dice that passed testing) in proximity to the failed dice frequently fail prematurely in the field. Therefore, in one embodiment, a method to identify those dice having a probability for early failure includes identifying a core die and a die cluster, adding the core die and at least one additional die from the die cluster to a weighted character map, and assigning a weighting value to each of the dice added to the weighted character map. At least one tier of buffer dice is then added to the weighted character map adjacent to each die on the weighted character map. Both the dice from the die cluster and the tier of buffer dice are marked, thereby preventing those dice from being packaged and consequently, shipped to customers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is a divisional of U.S. patent application Ser. No. 10/940,128, filed Sep. 14, 2004, now U.S. Pat. No. 7,105,364. 

   TECHNICAL FIELD 
   The invention relates to a method of increasing reliability of semiconductor integrated circuit dice and specifically to a method for anticipating any semiconductor device which may have a high probability of failure. 
   BACKGROUND ART 
   In a typical semiconductor fabrication process, completed integrated circuit dice fabricated on wafers (for example, wafer  101  of  FIG. 1 ) are subjected to an electrical testing (e.g., wafer probe or wafer sort) step prior to die singulation (i.e., dicing) and device packaging. The wafer probe will electrically test either a small sample of dice on a wafer or test each die separately by an electrical probe. 
   During the electrical probe, the wafer  101  is carefully mounted onto a movable plate (not shown) with at least three degrees of freedom (usually x-, y-, and z-axis movements). An electrical connection is made via a “probe card” (also not shown) which is a custom built circuit board designed to match a geometry of bonding pads on each die and connect the die to one or more pieces of test equipment. After making electrical contact between the probe card and the die, test programs are run to determine a pass/fail status of each tested die. If a die fails wafer probe, it is typically marked with an ink dot in the center of the die and the wafer  101  is moved into position for testing the next die. At assembly, an inked bad die  105  is discarded and a non-inked good die  103  is prepared for final packaging. 
   The non-inked die  103  which passed wafer probe could, however, due to its proximity to the inked bad die  105 , fail prematurely in the field. Therefore, it is desirable to have a method to anticipate which dice may have a high probability for failure. 
   SUMMARY OF THE INVENTION 
   The present invention is a method to increase a reliability of packaged semiconductor integrated circuit dice. In one embodiment, prior to final packaging, the dice on a semiconductor wafer are electrically tested. Those dice failing the electrical test are identified and/or marked in some way and therefore, not packaged. However, “good dice” (i.e., those dice that passed electrical testing) which are in proximity to the failed dice may still fail prematurely in the field. Therefore, the method presented herein to identify those dice that, due to their proximity to the failed dice, have a probability for an earlier than expected failure includes first identifying a die cluster where the die cluster consists of dice that failed the electrical test, identifying a core die from the die cluster, adding the core die from the die cluster to a weighted character map, adding at least one additional die from the die cluster to a weighted character map in accordance with a procedure given herein, and assigning a weighting value to each of the dice added to the weighted character map. Once the weighted character map is completed, a tier of buffer dice is added to the weighted character map adjacent to each die on the weighted character map. Buffer dice are added in accordance with an exemplary algorithm presented herein. Both the dice from the die cluster and the tier of buffer dice are marked (e.g., inked) or otherwise indicated, thereby preventing those dice from being packaged and, subsequently, being shipped to customers. 
   Optionally, additional tiers of buffer dice may be added to the weighted character map. The additional tiers of buffer dice are added in accordance with a separate exemplary algorithm for each tier. The additional tiers serve to increase the reliability of remaining, unmarked dice. 
   In another embodiment, a method to increase the reliability of packaged semiconductor integrated circuit dice includes identifying one or more dice having failed an electrical test, adding the one or more failed dice to a weighted character map, adding a first tier of buffer dice to the weighted character map adjacent to each die on the weighted character map, and noting which dice from the die cluster and tier of buffer dice, thereby indicating dice not requiring packaging. As with the first described embodiment, additional tiers of buffer dice may be added to the weighted character map in accordance with a separate exemplary algorithm for each tier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a typical inked semiconductor wafer with inked dice after electrical test. 
       FIGS. 2A and 2B  detail two types of clusters contained on semiconductor wafers in accordance with an embodiment of the present invention. 
       FIG. 3  is a flowchart defining a method for constructing weighted character and weighted number defect maps. 
       FIGS. 4A-4D  is an example of an original defect wafer map produced by electrical testing and followed by corresponding weighted character and weighted number defect maps developed in accordance with the method of  FIG. 3 . 
       FIGS. 5A-5F  is a further example of an original defect wafer map produced by electrical testing and followed by corresponding weighted character and weighted number defect maps developed in accordance with the method of  FIG. 3 . 
       FIG. 6  is a flowchart defining a method for constructing tiers of buffer dice onto the weighted character maps. 
       FIGS. 7A-7D  are examples of weighted character maps incorporating tiers of buffer dice produced in accordance with the method of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , the wafer  101 , containing a plurality of semiconductor integrated circuit dice may also contain clusters of bad dice. A cluster is defined, for example, as any single bad die that is surrounded by at least seven other bad dice. A first cluster type  107  and a second cluster type  109  are the cluster types possible under this definition when a total of eight bad dice are involved.  FIG. 2A  contains a primary bad die BD p    201  surrounded by seven other bad dice BD s    203 , forming the first cluster type  107 .  FIG. 2B  contains an alternative arrangement of seven additional bad dice BD s    203 , thereby forming the second cluster type  109 . A bad die  201  surrounded by more than seven bad dice  203  could therefore take a plurality of forms. The primary bad die  201  becomes the core cluster die and consequently, a locus point for building a weighted character map and a weighted number map, described herein. Optionally or in addition, a variable width exclusion zone, described infra, may be added to the weighted character map. A final weighted character map (not shown) is saved (e.g., in a wafer lot database) to indicate an x-y location of each of the bad dice  105 ,  201 ,  203 . 
   With reference to  FIG. 3 , an exemplary embodiment of a method  300  to anticipate which dice have a high probability for failure begins by selecting  301  a first die from an original wafer map. A determination  303  is made whether the first die is bad on the wafer. If the die is bad, a determination  305  next needs to be made whether the die is part of a die cluster. To determine  305  whether the die is part of a die cluster, by the exemplary definition given supra, at least seven additional bad dice must surround the die under consideration. 
   If the die is determined  303  not to be bad, the method will verify  311  whether all dice on the wafer have been considered. If all dice have not been considered, a next die is selected  313  and the process starts again with step  303 . 
   If a determination  305  is made that the first die is part of a die cluster, the die is added  307  to a weighted character map and a value is assigned  309  to the die on a weighted number map. The method  300  then verifies  311  whether all remaining dice on the wafer have been considered. 
   Once verified  311  that all dice on the wafer have been considered, a die from the original wafer map is again selected  315 . A determination  317  is made whether the die is bad on the wafer. If the die is bad, a determination  319  is made whether the die is touching another bad die with a weighting value of greater than or equal to 3 (i.e., ≧3). If the die is touching another bad die with a weighting value of ≧3, a further determination  321  is made whether the die is touching a weighted bad die edgewise. If so, the die is added  323  to a temporary character map and a quantitative value indicating the number of surrounding weighted dice in the weighted character map is added  325  to a temporary weighted number map. Referring back to  FIGS. 2A and 2B , each primary bad die  201  is surrounded by seven bad dice  203 . Hence, each primary bad die  201  would receive a quantitative value of “7.” 
   The method  300  then continues by verifying  327  whether all dice on the wafer have been considered. Notice also, that if a determination is made that either the die is not touching another bad die with a weighted value≧3 in step  319  or that the die is not touching a weighted bad die edgewise in step  321 , then the method  300  continues in each case to verify  327  whether all dice on the wafer have been considered. Further, if a determination  317  is made that the die is not bad, the method  300  also continues to step  327 . 
   If a determination  327  is made that not all dice on the wafer have been considered, a next die on the wafer is selected  329  and the method restarts at step  317 . However, if the determination  327  indicates that all dice have been considered, the die from the temporary character map is added  331  to a weighted character map and the die from the temporary number map is added  333  to a weighted number map. A determination  335  is then made whether new dice were added to the weighted character map. If so, another die is selected  315  from the original character map. If a result of the determination  335  indicates no new dice were added, the method  300  is complete. 
   A defect wafer map  400  of  FIG. 4A  generated by a wafer sort process, discussed supra, contains a cluster of bad dice B  401  surrounded by a field of good dice  403  (i.e., dice that passed wafer sort). In accordance with the exemplary method  300  described in conjunction with  FIG. 3 , a core cluster die B 0  is placed on a weighted character map  405 B ( FIG. 4B ) and a weighted value is assigned to the core cluster die B 0  on a weighted number map  407 B. In this example, the core cluster die is surrounded by eight bad dice (see defect wafer map  400 ) and thus receives a weighted value of “8.” 
   Next, adjacent bad dice B 1  that are touching the core cluster die B 0  on at least one edge are added to the weighted character map  405 C ( FIG. 4C ). Recall that bad dice  401  in the defect wafer map  400  must touch the core cluster die B 0  on at least one edge during the initial iteration to be added to the weighted character map  405 C. Here, the four bad dice on the corners of the cluster on the defect wafer map  400  are not touching the core cluster die B 0  edgewise and are, therefore, not added to the weighted character map  405 C. Since each of the newly added bad dice B 1  are in contact with only one other bad die (here, the core cluster die), each bad die B 1  is assigned a weighting value of “1” on the weighted number map  407 C. Recall that bad dice must be touching at least one other bad die on at least one edge to be added to the weighted character map, but may only be touching another bad die diagonally to add to a weighting quantity. 
   A further iteration examines the remaining four “corner dice.” Now the remaining bad dice  401  are touching the other bad dice, B 1 , on at least one edge (the first criterion for inclusion on the weighted character map) and are in contact, at least diagonally, with at least one bad die having a weighting of greater than or equal to three (the second criterion for inclusion on the weighted character map). The four “corner dice” are now added to the weighted character map  405 D ( FIG. 4D ) as bad dice B 2 , and each is assigned a weighting value of “3.” 
   A defect wafer map  500  of  FIG. 5  provides another example of a more complex defect cluster generated by a wafer sort process, discussed supra, contains a cluster of bad dice B  501  surrounded by a field of good dice  503  (i.e., dice that passed wafer sort). In accordance with the exemplary method described in conjunction with  FIG. 3 , core cluster dice B 0  are placed on a weighted character map  505 B ( FIG. 5B ) and a weighted value is assigned to each of the dice B 0  on a weighted number map  507 B. In this example, boundaries of regions contributing to the core cluster dice overlap. Therefore, each die in the overlapping boundary areas may contribute to the weighted value of more than one of the core cluster dice. Here, the core cluster dice are surrounded by either seven or eight bad dice (see wafer map  500 ) and thus receive weighted values of “7” or “8.” 
   Next, adjacent bad dice  501  that are touching the core cluster dice B 0  on at least one edge are added to the weighted character map  505 C ( FIG. 5C ). As with the example of  FIGS. 4A-4D , bad dice  501  in the defect wafer map  500  must each touch at least one of the core cluster dice on at least edge during the initial iteration to be added to the weighted character map  505 C. Here, nine bad dice  501  of the cluster on the defect wafer map  500  are not touching any of the core cluster dice and are, therefore, not added to the weighted character map  505 C. However, a determination will be made shortly on whether each of these dice will be included on a weighted character map. Each of the newly added bad dice B 1  are in contact with various numbers of other bad dice B 0  and are therefore each assigned an appropriate weighting value on the weighted number map  507 C. Again, bear in mind that bad dice  501  must be touching at least one other weighted bad die on at least one edge to be added to a weighted character map, but may only be touching another bad die diagonally to add to a weighting quantity for a particular die. 
   A further iteration examines the next tier of bad dice  501  from the defect wafer map  500 . This tier of bad dice are touching other bad dice, B 1 , on at least one edge and are in contact, at least diagonally, with at least one bad die, either B 1  or B 0 , having a weighting of greater than or equal to three. This tier of bad dice is added to the weighted character map  505 D as bad dice B 2 , and each is assigned an appropriate weighting value. 
   The iterative method  300  ( FIG. 3 ) continues until all bad dice  501  from the wafer map  500  are examined, adding bad dice, B 3 , to the weighted character map  505 E with an appropriate weighting value on weighted number map  507 E ( FIG. 5E ). In a final iteration, only a single bad die is added to the weighted character map  505 F ( FIG. 5F ) as bad die B 4 . Bad die B 4  is given a weighting value of “1” on the weighted character map  507 F. 
   Notice that two of the bad dice  501  from the defect wafer map  500  are not transferred onto a weighted character map, and consequently, neither are they assigned a weighting value on a weighted number map. The first of these bad dice is located in the second row and fourth column. The second bad die is in the fourth row and second column. The first bad die does not have at least one edge touching a bad die on any character map (e.g.,  505 F). The second bad die does have one edge touching a bad die, B 4 . However, the second criterion, being next to a bad die on a character map with a weighting value of greater than or equal to three, is not met—B 4  only has a weighting value of “1.” Therefore in this example, there are two bad dice that do not transfer to the character map. 
   Once weighted character and number maps are produced, an exemplary die buffer construction method  600  ( FIG. 6 ) serves to further reduce a probability of a semiconductor die failing in the field by preventing dice located proximately to known bad dice from being packaged as a final product. 
   The die buffer construction method  600  begins by utilizing  601  the final weighted character map (for example, weighted character map  505 F,  FIG. 5F ) and adding  603  a single die buffer tier next to each bad die in the final weighted character map. A single die buffer is added next to an edge and diagonally next to each bad die in the final character map. The buffer dice are inked or otherwise indicated to prevent packaging the buffer dice after wafer dicing operations. 
   After the single die buffer tier is added  603 , the method  600  may be complete for the local cluster (option  1 ) or additional buffer dice may be added (option  2 ). Under option  1 , the method  600  for the local cluster is complete and the method is applied to any other clusters on the wafer. Under option  2 , additional buffer tiers are added. A counter is initialized  605  to n=0 to determine how many additional buffer tiers are added. Next an additional die buffer tier is added  607  by placing a buffer die next to each edge of an inked die from a previous step. The counter is incremented  609  and a determination  611  is made whether n=2. (For this exemplary die buffer construction method  600 , a total of up to four buffer tiers are added. Alternatively, for more buffer tiers, n will need to be incremented to higher value.) If n is less than 2, the method  600  loops back to add  607  another buffer tier. Once three buffer tiers are added, a final buffer tier is added  613  next to each edge of an inked die as well as diagonally next to an inked die. The process is then repeated  615  for any remaining local clusters on the wafer. 
   With reference to  FIG. 7A , an example of using the exemplary die buffer construction method  600  begins by starting with the final weighted character map  505 F ( FIG. 5F ) and adding and inking dice, I 1 , for a first buffer tier. (Note that lower case “b&#39;s” indicate bad die from the original wafer map  500  that did not transfer to the final weighted character map  505 F. These dice tested bad at e-test and will therefore not be used for product, however, they are not determinative in adding buffer tiers. Further note that first tier inked dice I 1  are located both on an edge and diagonally adjacent to bad dice B from the weighted character map  505 F.) 
   In  FIG. 7B , a second tier of buffer dice I 2  has been added inked. Note that a buffer tier is not added for diagonally adjacent dice.  FIG. 7C  has a third tier of buffer dice I 3  added according to the same rules as the second tier. Finally, a fourth tier of buffer dice I 4  has been added and inked as indicated in  FIG. 7D . The fourth tier buffer dice I 4  are added according to the same rules as the first layer; that is, inked dice I 4  are located both on an edge and diagonally adjacent to bad dice I 3  from the weighted character map  505 F. 
   In the foregoing specification, the present invention has been described with reference to specific embodiments thereof. For example, elements such as the weighted character map and weighted number map need not actually exist or be formed to practice the method described herein, but are described merely as a construct to better aid in conveying an intent of the present invention. Alternatively, the weighted character map and weighted number map may be developed as part of a database used to construct one or more die buffer tiers. Additionally, neither bad dice nor buffer dice require physical inking. Another method to indicate that such dice should not go to final packaging, such as noting such dice on a database, is sufficient. It will, therefore, be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the present invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.