Setting MRC rules: Balancing inspection capabilities, defect sensitivity and OPC

Abstract

One of the challenges associated with shrinking design dimensions is finding photomask inspection settings which achieve sufficient defect detection capabilities while supporting aggressive Optical Proximity Correction (OPC). The most recent technology nodes require very aggressive and advanced Resolution Enhancement Techniques (RETs) which involve printing small features that are challenging for mask inspection tools. We examine the problems associated with constraining Models-Based OPC with mask inspection driven rules. We give examples of a 45nm technology node contact layer design which will receive sub-optimal OPC treatment due to mask inspection constraints. We then take the mask defect specification typically used for this mask layer, and use Monte Carlo simulation methods to place minimum sized simulated defects in various locations in close proximity to these sensitive layouts. Simulations of the optimal OPC are compared to optimal OPC with defects, and to the sub-optimal constrained OPC. Using knowledge about the frequency of small defects on masks, one can compare the risks associated with small mask defects to the risks associated with sub-optimal OPC. This exercise demonstrates that there are some instances where mask rules based on inspection capabilities and defect sensitivity alone can be problematic, and that OPC requirements need to be taken into account when choosing a defect specification and an inspection strategy. We conclude by proposing a strategy for balancing these requirements in a practical manner.