Driven by the ever shrinking feature size in the chip design and the complexity of the heuristic subresolution assist feature (SRAF) placement rules, more wafer data for calibration and more effort on the part of design engineers are necessary for each process node. The main purpose of placing assist features around the main feature is to improve the through focus robustness on the target edge of the main feature. Therefore, it is desirable to study the effect of assist feature placement on the main feature in terms of the focus sensitivity with a quick and accurate method. In this paper, an innovative way of calculating the signal dependence on the locations of the SRAF placed in the isoline pattern is proposed. The results indicated that the optimal location for the assist feature placement to increase the depth of focus is about 380 nm from the isoline center and is independent of the isoline structure for an annular illumination system. The results are verified by using independent tools and measured before and after optical proximity correction. Consistency of the optimal assist feature location is observed, and a qualitative explanation of the observations is presented. (c) 2007 American Vacuum Society.