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Dual tone sub-resolution assist features to improve corner rounding

Corner rounding improvement is critical to device performance, yield, and cell area reduction. In this paper, we present a method to use dual tone sub-resolution assist feature (SRAF) to improve both the outer corner rounding and inner corner rounding which in turn enhance the pattern quality. The simulation data and wafer data are presented. A few parameters have been investigated, such as the position of the SRAF, the shape of the SRAF, resist type and mask tone. The preliminary results show that more than 40% reduction of both inner corner rounding and outer corner rounding can be achieved by placing sub-resolution assist features at appropriate locations. The limit of corner rounding improvement is determined by mask rule check (MRC) and resist sensitivities.