Forming Freeform Source Shapes by Utilizing Particle Swarm Optimization to Enhance Resolution in Extreme UV Nanolithography

This paper investigated pixel-based source shape optimization to enhance the resolution in a projection extreme UV (EUV) lithography system. The particle swarm optimization method was proposed to optimize the source shapes. The layout patterns of the masks were corrected using model-based optical proximity correction. The optimal source shape in an EUV lithography system was verified using numerical calculations, through which the aerial image of two types of mask pattern exposure was determined. The two types of mask patterns were a logic line/space (L/S) pattern with a critical dimension (CD) of 16 nm and a SRAM contact hole (CH) pattern with a CD of 45 nm. Two significant metrics were used to evaluate the modified source performance: the latent image intensity and process window. Results from the numerical calculations showed that for L/S target patterns, using the optimal source produced a latent image error of 13.02% after exposure that was superior to that of traditional off-axis source exposure. The latent image intensity of the mask between the lines and the gaps differed substantially, corner rounding situations had effectively improved, and no bridges were observed. Concerning CH target patterns, using the optimal source produced a latent image error of 0.04% after exposure that was also superior to that of traditional sources. The latent image intensity between the holes and the gaps also differed significantly, and the latent image intensity distribution at the four corners of the CH had straight angles. Because the source shapes calculated in this study comprised pixel-based sources, the controllable microelectromechanical system mirror array chip was used to match each mirror to each pixel source, and the calculated source shapes were successfully projected.