Improvements in Cpk using real-time feedback control

The usual approach to bring the value of the process capability index, C_pk, to an acceptable level is to design equipment and develop a process in which the process variable is robust to external disturbances. However, an alternative approach involves the use of in situ sensors and real-time feedback control. Currently this approach is not widely implemented in the semiconductor industry. In this paper we provide analytic justification to quantify the potential improvement in C_pk if a real-time feedback control scheme is used instead of the usual open-loop approach. We show that for the case of feedback control the level of C_pk is only limited by the accuracy and reproducibility of the sensor provided that the target values are indeed achievable by the processing equipment. This result also holds in the presence of disturbances and nonlinearities. C_pk values for open-loop and real-time feedback control strategies are compared for two experimental applications: single-wafer CVD nitride and polysilicon processes