A pivotal issue is the identification of appropriate robustness, tuning, and adjustment factors. Our approach to factor identification is based on both qualitative and quantitative analysis of the equipment and process. Qualitatively, we consider the symmetry of the equipment and use engineering judgment to identify candidate controllable and non-tunable responses. Circumferential uniformity, then, is expected to be hard to control and is therefore treated as a robustness issue to be handled via off-line optimization. We thus treat radial uniformity as a tuning issue and seek to control it on-line.
Quantitative analysis complements consideration of the equipment configuration.
An ideal tuning factor is one with index such that robustness is not adversely affected by process tuning. The robustness measures circumferential uniformity. Based on a quality loss analysis, for a single tuning factor case, is where and are the group mean and standard deviation of etch rate at each wafer radius. Designed experiments are performed to study the effects of candidate tuning factors. For oxide etch in the AutoEtch 590, previous experiments indicated that gap spacing and CHF flow rate were candidate tuning factors.