High performance source optimization using a gradient-based method in optical lithography

Recently, source and mask optimization (SMO) has been proposed as an effective solution to help extending the life time of conventional 193 nm lithography. However, SMO is very computationally intensive. To mitigate this issue, we propose a highly effective and efficient method for source optimization in this paper. Based on the gray-level pixel based source representation, the gradient of the cost function is calculated to guide optimization to improve the wafer image fidelity and depth of focus (DOF). This method is demonstrated using two mask patterns with critical dimension of 45 nm, including a periodic array of contact holes and an asymmetric mask pattern from an SRAM layout. Comparing with two recently proposed methods, our method can provide greater improvements in image quality and over 10× running speed enhancement. The robustness of our method is verified using several different initial source patterns. Results show that similar final optimized source patterns and image quality have been achieved.