Illumination optimization of periodic patterns for maximum process window

A rigorous computationally fast technique for optimizing the illumination is demonstrated based on Hopkins imaging formulation for partial coherent imaging. The technique optimizes the illumination by changing selecting areas of the illuminator, which enhance the pattern dependent diffraction orders. The illumination is also maximized for largest process window by increasing the normalized image log slope (NILS) and by increasing the depth of focus (DOF). For a 110-nm DRAM isolation pattern with 220-nm pitch the optimized illumination is an elliptical dipole element. This elliptical dipole element has the largest NILS through focus for the 110-nm main feature width and for the end of line. Simulation results with this elliptical dipole element show that the DOF is 0.6 μm. Experiments were done with a 35° dipole element, which approximates the elliptical dipole element. These experiments demonstrated that the 110-nm DRAM isolation pattern is resolvable with 0.4-μm DOF.