Dynamic laser speckle as a detrimental phenomenon in optical projection lithography

Dynamic speckle is caused by the finite pulse length and limited spectral linewidth of the partially coherent radiation from the excimer lasers used in optical projection lithography. One effect of the dynamic speckle is that the energy delivered to a certain position at the wafer is a stochastic quantity and cannot be precisely controlled, which fundamentally limits the dose control in the lithographic system. Further, the spatial distribution of the dynamic speckle is shown to depend on illumination conditions, contributing to unwanted effects such as line edge roughness (LER). We present a formulation for the dynamic speckle of the partially coherent optical field, based on a temporal degrees of freedom approach. We show theoretically and by numerical simulation how the choice of the illuminator intensity distribution influences LER. We also point to some pitfalls in the struggle to combat dynamic speckle. It is fundamentally important to realize that instantaneously any combination of uncorrelated fields adds to the total field on a complex amplitude (not intensity) basis just like correlated fields. Therefore, any spatial redistribution of light, e.g., with the help of microlens arrays, does not reduce the dynamic speckle.