Mitigating the forbidden pitch of extreme ultraviolet lithography using mask optimization based on genetic algorithm

BackgroundIn advanced technology nodes, forbidden pitches (FPs) can reduce the depth of focus and limit the overlapped process window of lithography. In extreme ultraviolet (EUV) lithography, one pattern in a different orientation or different position of arc slit field will have a different shadowing effect, due to the chief ray angle and azimuthal angle of the incident light. Therefore, it is necessary to mitigate the FP effect of EUV lithography.AimWith the purpose of mitigating the phenomenon of FPs in the arc slit exposure field, we propose an optimization method based on a genetic algorithm (GA).ApproachThe optimization method is derived from the basic flow of the GA. Three mask parameters are selected as the variables, including the mask bias and the width and position of the assist feature. The cost function is designed to evaluate the process window, normalized image log slope, and contrast of different fields in the arc slit. After multiple iterations, an optimal combination of three variables is obtained.ResultsThe simulation results show that the optimized mask structure given by the proposed method in the arc slit exposure field can improve the process window in the horizontal and vertical orientations, respectively.ConclusionsThe limitation of FPs to the process window is removed using mask optimization based on the GA. The proposed optimization method has the potential to be an effect candidate for FPs mitigation.