Integrating assist feature print fixing with OPC

A challenge in model-based assist feature placement is to find optimal placements while satisfying mask rules and preventing AF printing. There are numerous strategies for achieving this ranging from fully rule-based methods to pixel-based inversion. Our proposed solution is to identify the optimal locations of assist features using modeling information based strictly on optics and resist stack optical characteristics. Once these positions have been found, preliminary AFs can be placed. At this point suggested sizes and shapes can be identified, although these can later be modified. In a later step, MRC cleanup, printability fixing, and main-pattern OPC can be performed simultaneously. This has the advantage of allowing the use of the full process model to predict the location of OPC edges accurately, and use calibrated or 3d mask models to determine assist feature printing behavior. This correction is done while maintaining MRC constraints. In this flow, an AF placement field, generated from the pre-OPC target patterns, can be used to provide accurate guidance on how to move assist features to get the most benefit while keeping other constraints in mind. Using this method, a range of printability fixing strategies, guided by placement benefits, is available. We present data showing that the benefit of AF placements can be determined from optical parameters, on target (non-OPC) data, and that this method leads to beneficial yet compliant masks.