Long term time dependent variations in thermal wave monitor wafers

Implanted reference wafers, often referred to as "golden wafers", are generally used to verify the long term repeatability of metrology equipment and, by extension, the repeatability of the implantation process. This approach, while widely utilized, depends on the stability of the reference wafers themselves. The therma-wave therma-probe has become a standard monitoring tool for the ion implantation process. However, it is recognized that the thermal wave (TW) signal from implanted silicon wafers can significantly decrease over time. This is normally compensated for by an automatic correction routine in the TW measurement sequence referred to as "decay compensation". Over longer periods of time, months or longer, this correction may prove insufficient to completely alleviate the signal change. We present data collected on the therma-probe model TP420 from a series of wafers showing the residual time dependent signal change that may remain after this automatic correction has been applied.