Process Improvement in the Microelectronic Industry by State Space Modelling

Abstract

AbstractThe exponentially weighted moving average (EWMA) model can be applied to a process controlling the thickness of nitride layers in the manufacture of microelectronic devices, involving the standard use of the notions of EWMA control chart, EWMA full adjustment and EWMA deadband adjustment charts for process monitoring and process improvement. We suggest that a dynamic step forward to process improvement is gained by considering basic state space models, which are known as local‐level models. The EWMA predictor is a limiting form of forecast for the local‐level model, so we propose replacing the EWMA by a local‐level model and thus develop feedback adjustment schemes. Under Gaussian assumptions, the entire forecast distribution is obtained routinely. A detailed development of the proposed state space adjustment scheme is given to the nitride layers process and a number of conclusions and recommendations are made. The proposed scheme is found to perform better than the EWMA if there is evidence of a shift in the level of the output process. Copyright © 2005 John Wiley & Sons, Ltd.