Source and Mask Co-Optimization Using a Gradient Based Method in Optical Lithography

Abstract

With the technology node entering 45nm and below and the delay of the EUV lithography, the resolution enhancement techniques (RET) has been considered indispensable to extend the life time of conventional 193nm lithography. Traditional RETs, for example, optical proximity correction (OPC), phase-shifting mask (PSM), inverse lithography technique (ILT) and so on, use a fixed source thus limiting the degrees of freedom during the mask pattern optimization. To overcome the restriction, recently, source and mask co-optimization (SMO) has been proposed as a strong contender in RET. However, SMO is very computationally intensive. To mitigate the issue, we adopt a method which has demonstrated in good effectivity and efficiency in illumination source optimization. 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). Moreover, the similar method has been used in mask optimization to form a complete SMO algorithm. The SMO algorithm is demonstrated using two mask patterns with critical dimension (CD) of 45nm, including a periodic array of contact holes and a tri-bar pattern. Both the results showed high image quality have been achieved using our SMO algorithm.