Advanced Process Control of Overlay with Optimal Sampling

摘要

This paper evaluates sampling plans for overlay metrology in the context of Advanced Process Control (APC). The relationship between APC opportunity (the maximum benefit achievable via APC) and correctable accuracy is investigated. The tradeoff between spatial and temporal sampling density is considered as well. This tradeoff expresses the relationship between temporal sampling needed to realize APC benefit and spatial sampling needed to achieve a level of total overlay error. The approach is in 3 phases. First, overlay correctables are derived based on a 10-field and 4-field sampling plan. Second, a model of the process variability associated with each plan is developed. Finally, the model is used to simulate APC benefit as a function of temporal sampling. The question explored is whether different sampling plans result in different process variability and thus different APC opportunity. Two features of process variability impact APC performance. One is the proportion of process disturbance and noise that make up the total measured variability. The other is the frequency distribution of the process disturbance. We find that the spatial sampling plan impacts both the proportion of process disturbance in the measured variability and the frequency distribution of the disturbance. As a result of a smaller magnitude and lower frequency disturbance in the 10-field plan, APC performance with this plan is substantially better than with the 4-field plan. Over a realistic range of temporal sampling, APC of correctables derived from the 10-field sample plan result in a 20 to 25% improvement over the baseline of no control on 4-field based correctables. When APC is applied to 4-field correctables, only about 8 to 10% improvement is achieved.