In order to improve resolution and depth of focus (DOF) in reduction projection aligner, we investigate annular illumination method, in which the center portion of light source is screened.
This paper describes the relationship between resist pattern formation capability and the screened ratio of condenser aperture. And also, the pattern deformation induced by annular illumination is investigated.
First, on the basis of simulation analysis of this illumination method, the optimum optical parameters were selected to obtain high resolution and enough DOF. The effect of annular illumination was confirmed experimentally in i-line exposure. Secondly, to explore optical contrast dependence, we compared the resolution capabilities of both positive and negative resists, having different dissolution characteristics each other. It was found that annular illumination is more effective in low contrast region of light intensity. Thirdly, the dependence on pattern periodicity (L/S and isolated line), and pattern deformation were also examined. This method is effective in periodic pattern but not in isolated pattern, and induces a little pattern deformation in the edge region.
Annular illumination that is a simple method compared with phase shift mask, is a promising for expanding the process margin of 0.35 ?m resist patterning. Furthermore, this method becomes more effective, if adapting thin resist or new high resolution resist, which can be resolved even in low contrast light condition.