Topography effects and wave aberrations in advanced PSM technology

Both mask design and quality of the projection optics have a large impact on the performance of a phase shift mask (PSM). Topographic features on the reticle such as etched trenches in alternating PSM produce a spectrum of the diffracted light which differs from that one of an infinitely thin amplitude/phase object, as it is assumed in standard imaging algorithms. Many authors have investigated the consequences of this phenomenon with respect to aberration free imaging. However, the diffraction of light from topographic features implies also a modified interaction between the mask and wave aberrations of the projector. Rigorous simulation of the light diffraction from the mask is combined with standard lithography imaging algorithms to explore the interaction of topography effects and wave aberrations. For example, the nominal shift of a phase edge in the final resist profile can result both from topography effects and/or from odd-order wave aberrations such as tilt and coma. The sensitivity of typical lithographic parameters with respect to topography parameters and typical wave aberrations is investigated. PSM are also used for the monitoring of aberrations. Neglecting the topography of these phase objects may result in a misinterpretation of aberration phenomena. Consequences of rigorous diffraction defects for the design and interpretation of phase objects in aberration monitors will be discussed.