Robust approach to determine the optimized illumination condition using process window analysis

Abstract

Several criteria are applied to optimize the best illumination and bias condition for a layer. Normalized image log-slope (NILS) and mask error enhancement factor (MEEF) are promising candidates to simply decide the optimized condition. NILS represents imaging capability and MEEF represents the mask uniformity influence on wafer image. MEEF has inversely relationship with NILS, but the optimized point of NILS does not exactly coincide with that of MEEF. Besides NILS and MEEF, the depth of focus (DoF) is an important factor for defining the process margin. The process window (PW) is expressed by DoF and exposure Latitude (EL). PW is general parameter used to determine the best lithographic condition. Large EL can be obtained at the condition with good image performance. In order to include mask uniformity effect in PW analysis, the common PW overlapping the final layout with positive and negative biased layouts is adopted. Starting with the minimum NA, sigma and threshold, OPC is performed to satisfy the target layout using aerial image model, and the final OPCed layout is obtained. The positive and negative biased layouts are generated from the final OPCed layout. The bias limit is determined considering mask uniformity. The common PW obtained by overlapping the final layout with positive and negative biased layouts is calculated. Then, NA, sigma and threshold are increased until the maximum values are reached. The common PW at each NA, sigma and threshold value is obtained using the same flow sequence. Comparing among calculated PWs, the NA, sigma and threshold of the maximum PW can be chosen as the best illuminator and bias condition. In this paper, the optimized illumination and bias condition is determined using PW for 60 nm memory device. The process flow is implemented by an OPC tool. By using the OPC tool for the illuminator optimization, the actual layout and multiple monitoring points can be measured. In spite of a large number of calculations, the fast calculation speed can be obtained by using the distributed process.