Optimization of the focus monitor mark in immersion lithography according to illumination type

With the shrinking of critical dimension, the demand for a process window has reached a new level, which is denoted as the depth of focus at certain exposure latitudes. Therefore, high-quality monitoring and controlling of focus shift are becoming more and more critical. With the purpose of providing an optimal focus monitoring mark, which can be applied in freeform or off-axis illumination with a big sigma and hypernumerical aperture (NA) scheme, a global optimization method combined with the idea of a genetic algorithm is developed. For illustration, two optimal mask structures under quasar and freeform illumination conditions are given by the optimized method. The numerical simulations with the lithography simulator PROLITH are provided to demonstrate the performances of these two structures. In addition, the robustness of these optimized structures is analyzed by considering the phase-shift error in mask manufacturing. The above simulation results verify the effectiveness and validity of the proposed optimization methodology and also show that the mask structure provided by the optimized method has the potential to be an efficient candidate for measuring the defocus of scanners in the immersion lithography with hyper NA.