RESOLUTION IMPROVEMENT WITH ANNULAR ILLUMINATION

In order to improve resolution and Depth Of Focus (DOF) in reduction p rojection aligner, authors investigated an annular illumination method in which the center portion of light source is screened. This paper d escribes the improved resist pattern resolution and DOF with annular i llumination. The pattern deformation induced by annular illumination w as also investigated. Furthermore, an optimization method of light sou rce shape and its results were described. First, basic annular illumin ation characteristics were analyzed using simulation, and then its eff ect was confirmed experimentally in i-line exposure. Secondly, to expl ore optical contrast dependence, authors made a comparison between the resolution capabilities of both positive and negative resists, having different dissolution characteristics from each other. It was found t hat annular illumination was more effective in low-contrast region of light intensity. Thirdly, the dependence on pattern periodicity (L/S a nd isolated line) and pattern deformation were also examined. This met hod was effective in periodic pattern but not in isolated pattern, and induced a little pattern deformation in the edge region. Finally, opt imization of annular illumination was investigated in KrF excimer lase r lithography for 0.25 mum pattern formation. Practical DOF will be ob tained in the near future, when optimum annular illumination is applie d to improved resist material. It was found that optimum light source shape (NA of source outer radius = 0.35 and annular shield ratio = 70% ) did not depend on resist resolving performance or defocus. Annular i llumination, which was a simple method compared with phase shift mask, is a promising method for expanding the process margin of resist patt erning. Furthermore, this method becomes more effective when if adapti ng thin resist or new high resolution resist, which can be resolved ev en in low-contrast light conditions.