The role of temporal coherence in imaging with extreme ultraviolet lithography optics

Abstract

Being based on reflective elements, extreme ultraviolet (EUV) lithography optics can support much larger spectral bandwidths then do modern refractive lithography optics. Moreover, EUV source power limitations dictate the use of the full bandwidth supported by the reflective EUV multilayers. A typical four mirror EUV optic supports an operating bandwidth of approximately 360 pm. This large bandwidth, combined with the off-axis nature of EUV lithography optics leading to significant pathlength differences across the field, raises the issue of temporal coherence effects on the imaging properties of EUV systems. Here we present analysis and computer simulation of the temporal coherence effect in EUV lithography optics. The analysis shows that under typical lithographic operating conditions, temporal coherence effects can still be safely ignored. However, non-negligible effects could arise in cases of higher-coherence imaging (small partial coherence factor, σ).