Resolution improvement with annular illumination

In order to improve resolution and Depth Of Focus (DOF) in reduction projection aligner, authors investigated an annular illumination method in which the center portion of light source is screened. This paper describes the improved resist pattern resolution and DOF with annular illumination. The pattern deformation induced by annular illumination was also investigated. Furthermore, an optimization method of light source shape and its results were described. First, basic annular illumination characteristics were analyzed using simulation, and then its effect was confirmed experimentally in i-line exposure