Systematic design of phase-shifting masks with extended depth of focus and/or shifted focus plane
An optimization based algorithm for designing phase-shifting masks is proposed. The approach is an extension of the previous work in the sense that the intensity image is optimized at a number of optical planes rather than just the focus plane. In addition, the algorithm can be used to design masks with shifted focus plane and/or extended depth of focus. The concept of a dual mask is introduced, and its consequences for practical phase-shifting mask design are shown. The proposed design techniques are applied to single line phase connectors, cross phase connectors, contact holes and bright lines. Simulation and experimental results verify the capability of the design technique for extending depth of focus and shift the focus plane. >