EPL electron optics performance on test stand: 2. field distortion and stitching results

Nikon in collaboration with IBM has been developing electron projection lithography (EPL) as a promising candidate for 65 nm node technologies. In this paper, the latest performance of electron optics for a 65 nm R&D tool will be described with emphases on field stitching and distortions. For evaluation, data measured by a Nikon XY5i metrology tool, SEM pictures of visual vernier patterns, and other data will be presented to show field stitching results. These represent the first full main field stitching results achieved. To obtain stitching accuracy between any two adjacent subfields, it is critical to eliminate subfield distortion at each deflected position. Subfield distortion is mainly reduced by a combination of optimized curvilinear variable axis lens (CVAL) deflection optics and dynamic correction lenses adjustment. We will discuss an experimental method to minimize subfield distortion by adjusting the CVAL deflection optics at deflected positions. In addition, dynamic correction of the subfield image shape eliminates linear components of subfield distortion. Controllability of subfield shape will be discussed. As another important topic, we will present results of nonlinear distortion correction by a higher order dynamic correction lens system. By combining this with the above mentioned electron optical adjustment, we achieved less than 10 nm residual subfield distortion at the most deflected position. We will also present the wide 'tolerance' or 'margin' of nonlinear distortion correction adjustment.