Depth-of-focus enhancement in high-numerical aperture EUV lithography by source and mask optimization

Recently, ASML's EXE:5000 scanner has become operational, which uses a Numerical Aperture (NA) of 0.55, which is 67% higher than the 0.33 NA of the current generation of EUV scanners. This higher NA results in the printing of smaller features, which is essential for advancing semiconductor technology and sustaining Moore's Law. However, as NA increases, the incidence angles on wafer also increase, leading to a stronger variation of the aerial image through focus, thereby reducing the Depth-of-Focus (DOF). Due to a limited focus control in the scanner and the finite resist thickness, this reduced DOF could result in a pattern failure. This study explores the enhancement of DOF in high NA EUV lithography, particularly within the framework of logic metal clips and their building blocks, i.e. dense and isolated lines, and tip-to-tip (T2T) features, using a Dark Field mask and a negative tone resist. We explain by simulations and experiments that the overlapping DOF of Pitch 20, 40 and 60 nm horizontal Lines and Spaces can be significantly enhanced by using three strategies: illumination source optimization, mask feature corrections, and mask absorber stack optimization. Additionally, we clarify how these three strategies significantly improve T2T printing performance in terms of CD control and Local CD Uniformity. In conclusion, we show that by a rational application of the three optimization strategies, an acceptable overlapping DOF can be achieved.