The impact of lot-to-lot and wafer-to-wafer variations on SPC

In-line process control using SPC (statistical process control) charts is very well known for a long time. A vast amount of work has been done and extended in the area of optimal process control with sampling and control charts. However, the usage of standard process control charts for monitoring the defects during wafer fabrication could lead to serious errors. Specifically, this error arises due to the difference in the lot-to-lot and wafer-to-wafer variances of the in-line defect measurements. The purpose of this paper is to present the results of a new SPC model which explicitly accounts for the lot-to-lot and wafer-to wafer variations. We illustrate that the application of traditional policies could lead to suboptimal results by as much as 17%. Note that the traditional SPC charts are based on the assumption that the measurements are independent and identically distributed