Analysis of EUVL mask effects under partially coherent illumination

In extreme ultraviolet lithography (EUVL) a reflective mask is illuminated obliquely, and the illumination is partially coherent. Due to the small NA (0.25) and sigma (0.5) the incident angles do not vary too much throughout the source distribution, but, unlike in the optical case, the topography is rather pronounced. Moreover, the multilayer reflectivity varies significantly even for small variations of the incident angle. So as a result the object spectrum will not only be shifted as a function of the source point, but amplitudes and phases will also vary significantly. On the way to more advanced technology nodes, NA needs to increase up to 0.5, and effects induced by partially coherent illumination could be critical and must be appropriately modeled and investigated. In this paper the impact of the real source distribution on EUVL imaging is investigated. For this a rigorous electro-magnetic field solver is used to predict the subtle effects associated with the three-dimensional topography of the mask absorbers. We introduce the advanced topographical mask illumination concept for rigorous and fast simulation of EUVL mask under partially coherent illumination. Rigorous simulations are performed for line and spaces with an outlook to future technology nodes.