Dynamic laser speckle in optical projection lithography: causes, effects on CDU and LER, and possible remedies

Abstract

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, fundamentally limiting the dose control in the lithographic system. Further, the spatial distribution of dynamic speckle fluctuations is shown to depend on illumination conditions, contributing to unwanted effects such as line edge roughness (LER). In this work we show, theoretically and by numerical simulation, how the choice of the illuminator intensity distribution influences LER. In particular, it is noted that speckle-induced LER is a prominent cause of the long-range changes in the position of the line edge, as evidenced in the calculations of the power spectral density (PSD) of the LER.