Abstract:
The present invention improves wafer sampling methods by partitioning a semiconductor wafer into a set of sampling regions and calculating yield of a sampling region(s) of the semiconductor wafer.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
   This patent application is a divisional of U.S. patent application Ser. No. 10/711,201, entitled “Enhanced Sampling Methodology for Semiconductor Processing” filed on Sep. 1, 2004 now U.S. Pat. No. 6,998,867. 

   FIELD OF THE INVENTION 
   The present invention relates generally to semiconductor wafers, and more particularly to an enhanced sampling method for semiconductor wafers. 
   BACKGROUND OF THE INVENTION 
   Prior to shipment, a sampling of semiconductor wafers is tested for satisfaction of performance requirements of the die on the wafer. In accordance with prior art sampling methods, if the predicted yield of the entire semiconductor wafer does not meet yield requirements, the entire wafer, lot of wafers, e.g. batch, or even shipment of wafers will be labeled as “bad,” discarded, and not shipped. Prior art semiconductor wafer sampling methods are fraught with problems. 
   Prior art sampling methods waste fabrication resources, which consequently results in delayed or cancelled semiconductor shipments, by incorrectly presuming a random distribution of failures, e.g. bad die, on a semiconductor wafer. Because prior art sampling methods presume a random bad die distribution, if bad die are sampled, the entire semiconductor wafer, or worse yet lot or shipment, is labeled as “bad,” discarded, and not shipped. 
   Moreover, prior art sampling methods are inefficient, which also results in delayed or discarded semiconductor shipments. Because prior art sampling methods are time consuming, only a small population of semiconductor wafers, such as a single wafer or batch, is tested for predicted yield. One problem associated with such method is that such a small population rarely represents the predicted yield of an entire lot or shipment, which in turn leads to misguided shipment decisions based upon a misrepresented population of wafers. 
   Therefore, there remains a need in the art for an improved method for sampling semiconductor wafers, which improves the overall yield of the semiconductor processing line from semiconductor fabrication to shipment. 
   These and other deficiencies in the prior art are overcome through the present invention. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a method and program for sampling semiconductor wafers. Accordingly to the present invention, the semiconductor wafer comprises a set of sampling regions. The present invention then calculates the yield for at least one sampling region in the set of sampling regions. Based upon the calculated yield for at least one sampling region, the present invention predicts yield of the semiconductor wafer and/or adjusts the boundaries of the at least one sampling region such that the adjusted sampling region predicts a higher or lower yield for the semiconductor wafer. 
   The present invention saves semiconductor fabrication costs as well as improves fulfillment. The present invention presumes that bad die appear in grouped and generally isolated regions on the semiconductor wafer. Once a bad die is found, the present invention determines the boundaries of the bad die distribution and salvages the semiconductor wafer for the sampling regions with good die. Accordingly, semiconductor wafers that were labeled as bad and discarded using prior art methods are identified with good regions and salvaged for the same. In so doing, the present invention saves fabrication costs because fewer new semiconductor runs must be initiated. In addition, the present invention reduces shipment delays and cancellations because the present invention frees more semiconductors for commercial distribution. 
   Furthermore, the present invention improves the overall efficiency of semiconductor wafer testing. The present invention bypasses regions of the semiconductor wafers that are labeled “bad” in future tests. The present invention flags correction of the processing conditions that created the regions of bad die. Finally, the present invention predicts the outgoing quantity of good die and, if the prediction indicates failure to satisfy shipment commitments, initiates new wafer runs. 
   For at least the foregoing reasons, the present invention improves upon the overall yield of the semiconductor processing line from semiconductor fabrication to shipment. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The features and the element characteristics of the invention are set forth with particularity in the appended claims. The figures are for illustrative purposes only and are not drawn to scale. Furthermore, like numbers represent like features in the drawings. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows, taken in conjunction with the accompanying figures, in which: 
       FIG. 1  depicts a semiconductor wafer partitioned into a four quadrant sampling region in accordance with the present invention; 
       FIG. 2  depicts the same semiconductor wafer of  FIG. 1  with adjusted sampling region boundaries in accordance with the present invention; 
       FIG. 3  illustrates the statistical yield prediction of the present invention for the semiconductor wafer depicted in  FIGS. 1-2 ; and, 
       FIG. 4  illustrates the semiconductor wafer partitioned into overlaying circular and pie shaped sampling regions in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention will now be described with reference to the accompanying figures. In the figures, various aspects of the structures have been shown and schematically represented in a simplified manner to more clearly describe and illustrate the invention. 
   By way of overview and explanation, the present invention comprises a software program that calculates yield for a sampling region on a semiconductor wafer and based upon the calculated yield predicts yield of the semiconductor wafer and/or adjusts the boundary of at least one sampling region such that the adjusted sampling region predicts a higher or lower yield for the semiconductor wafer. Some sampling regions comprise shapes such as, but not limited to, quadrants, pie slices and rings, which can be symmetrical or asymmetrical. The software program iteratively chooses sampling region boundaries based upon a nearest neighbor approach such that the finally selected sampling region boundary encapsulates the maximum number of good die. Finally, the software program identifies bad sampling regions, i.e. the outer periphery of the semiconductor wafer that often contains partial die, from the beginning and automatically excludes such from wafer test runs. 
   In-line testable structures test the die in predetermined quantities and locations on the semiconductor wafer. Often the in-line testable structure test not each die, but instead a representative sampling of die. In accordance with the present invention, the software assigns the test data to a sampling region. The software then calculates the yield for the die in the assigned sampling region. Based upon the calculated yield for the sampling regions, the present invention either predicts the yield of the semiconductor wafer and/or adjusts the boundary of the sampling region such that the adjusted sampling region predicts either a higher or lower yield for the semiconductor wafer. Based upon such calculations, the software program identifies combinations of regions that have the highest predicted yield, suggests process modifications, and initiates new semiconductor wafer runs to satisfy shipment fulfillment. 
     FIG. 1  depicts a semiconductor wafer  100  partitioned into four quadrant sampling regions. The description below describes the situation where each sampling region has at least one tested die. It being understood, however, that each sampling region must not have a tested die. For example some sampling regions such as the outer periphery of the semiconductor wafer are known to have bad die. Accordingly, in such sampling regions the die must not be tested, but instead would be presumed bad. While all sampling regions have tested die in  FIG. 1 , not every die has been tested in any sampling region. Die  155  represents an untested die, while die  150  and  160  represent bad and good die respectively.  FIG. 1  depicts a wafer with a representative sampling of tested die. 
   With continued reference to  FIG. 1 , contrary to prior art knowledge, bad die  150  are not randomly distributed on the semiconductor wafer. With respect to the top half of the semiconductor wafer  100 , the fourth sampling region  116  comprises only bad die, while the third sampling region  114  comprises eight bad die and one good die  160 . Therefore, the top half of the semiconductor wafer with sampling regions  114 ,  116  comprises only one good die. With respect to the bottom half of the semiconductor wafer, the first sampling region  110  comprises only four bad die and two good die, while the second sampling region  112  comprises only three bad die and seven good die. Clearly, the bottom half of the wafer contains the most good die. In accordance with the inventive method of the present invention, the bottom half would be the recommended salvaged region of the wafer. Even with the improved sampling of the present invention, however, one good die  160  will be discarded. 
     FIG. 2  depicts the same semiconductor wafer  100  of  FIG. 1  with adjusted sampling region boundaries  220 ,  222 ,  224 ,  226 , and  228  in accordance with a further embodiment of the present invention. Note that the same number of good and bad die are present on the semiconductor wafer  100 , however in accordance with the present invention, the boundaries of the sampling regions have been adjusted. The software program recognized that at least one good die  160  could be salvaged in the upper right hand sampling region of the semiconductor wafer and iteratively moved the boundaries of the sampling regions to encapsulate the good die. Accordingly, based upon this adjusted boundary approach, if the sampling regions  220 ,  222  are salvaged, only bad die are discarded. 
     FIG. 3  illustrates the statistical yield prediction of the present invention for the semiconductor wafer  100  depicted in  FIGS. 1 and 2 . The predicted yield  380  for the combination of the first, second, and third quadrants  110 ,  112 ,  114  of  FIG. 1  is 70%. With reference to  FIG. 2 , the predicted yield  382  for the combination of the first and second slice  220 ,  222  is 45.5%. The software of the present invention also calculates yield capacities for a combination of sampling region types. As shown in  FIG. 3 , the predicted yield  384  for the combination of the third, fourth, and fifth slices  224 ,  226 ,  228  along with the second quadrant  112  is 34.5% 
     FIG. 1  depicts the predefined shape sampling method of the present invention, however as detailed above the present invention further comprises a dynamically chosen sampling region based upon a nearest neighbor method. In accordance with this method, a first region is tested for predicted yield. Next, a region neighboring that first region is tested for predicted yield. If the neighboring region has predicted yield characteristics similar to the first region, the first region expands to include that neighboring sampling region. Otherwise, the first region excludes the neighboring sampling region and chooses another neighboring sampling region for predicted yield comparison. In so doing, the present invention dynamically chooses sampling regions. 
     FIG. 4  illustrates the semiconductor wafer partitioned into overlaying circular and pie shaped sampling regions in accordance with the preferred embodiments of the present invention. It should be understood, that while the five circular sampling regions  170 - 174  and eight pie shaped sampling regions  140 - 147  are symmetrical, asymmetrical shapes are also within the spirit and scope of the present invention. The present invention calculates yield for any combination of pie shaped and/or circular sampling region. If a particular combination, such as for example circular sampling region  170  in combination with pie shaped sampling regions  144  and  145  predicted the most number of good die, such combination would be identified as the sampling region from which the die should be cut for shipment. 
   With continued reference to the nearest neighbor method of the present invention and  FIG. 4 , if the present invention tests sampling region  174 , which represents the outer periphery of the semiconductor wafer, and as is common with the outer periphery of a semiconductor wafer determines that the die are partial, and accordingly bad, the present invention would test the nearest neighbor, or in other words sampling region  173 . Sampling region  173  represents the ring closest to the outer periphery. If the present invention determines that the sampling region  173  has a similar deficient calculated yield, sampling regions  173  and  174  would be combined. Future testing of the combined sampling region  173  and  174  could then be prevented. Alternatively, die from the combined sampling regions  173  and  174  would be automatically discarded. 
   Once the present invention has calculated the yield for a given sampling as well as either identified the combination of sampling regions that maximizes predicted yield for the semiconductor wafer, such information can be used to predict semiconductor lot, and better still semiconductor shipment yield. If the predicted semiconductor lot or shipment yield does not meet a predetermined criteria, the present invention can initiate new semiconductor wafer runs. Alternatively, if the predicted semiconductor lot or shipment does not meet the predetermined criteria, the present invention can alter semiconductor fabrication process conditions. 
   While the present invention has been particularly described in conjunction with a specific preferred embodiment and other alternative embodiments, it is evident that numerous alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore intended that the appended claims embrace all such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.