There are many works on extension of KrF lithography for 90 nm logic generation, especially for those back end of line (BEOL) layers. High cost and immaturity of ArF tools and photoresists are the major factors that make IC manufacturers try to seek for the possibility of KrF lithography. For mass production of 130 nm node, KrF lithography has been pushed hard to achieve 160 nm contact holes with 320 nm pitch. However, with pushing KrF lithography further, printing of 140 nm via holes with the minimum pitch of 280 nm was required by the tight 90 nm design rules. Optimizing illumination settings is one way to obtain reasonable process windows through all the pitches for mass production of 90 nm node logic devices, and maintaining exposure tools in good conditions is the other. The control of pattern deformation becomes more and more significant when the critical dimension is drove to the limit. In this paper, oval shaped via holes were found for symmetrical pitch patterns. Lens aberration and synchronization errors of scanners are always the first considerations when pattern deformation happened. But after investigations, improvement of via pattern deformation control has been demonstrated by reducing the low frequency resonance of scanner projection lens. The via deformation is investigated in combination of different scanning and stepping speed of scanner stages, which will cause different amplitude of projection lens resonance. Low frequency region of projection lens resonance spectra showed less amplitude while scanning or stepping speed was slow. Pattern distortion was also reduced as amplitude of low frequency project lens resonance went low. Common process window was then improved due to the elimination of via cd difference between x and y direction. With this improvement, reasonable process window (DOF ~ 0.3 um) can be achieved for mass production of 90 nm devices on KrF lithography tools.