SPC Control Chart for Clustered Defects

Control chart is one of the most important tools in SPC(Statistical Process Control). The C control chart, which has been successfully used in traditional industry for many years, requires that the number of defects in a sample follows the Poisson distribution. But the defects in IC fabrication tend to cluster and the Negative binomial distribution or Neyman distribution should be used to describe the number of defects and calculate the control limits. If the ordinary C control chart is used for the clustered defects, we could get a wrong conclusion from the control chart about whether the process is out of control.