Investigation into causes of resolution degradation in projection exposure using a gradient-index lens array

Abstract

Lithography is frequently used for fabrication of micro-components such as flexible print circuits, microfluidic devices and lens arrays. However, low-cost lithography tools with large exposure areas appropriate for such applications are not commercially available. It is thought that a scan-exposure system using a gradient-index lens array as a projection lens matches such requirements. However, in spite of adding back-and-forth sub-scans in the direction perpendicular to the main scan, unevenness of the light intensity was not completely removed, in the past research, and partial degradations of resolution were observed here and there in the field.For this reason, causes of intensity unevenness and resolution degradation were investigated here. In concrete, 30-μm line-and-space patterns were printed without any scans to confirm whether element lenses were arranged regularly and patterns were stitched smoothly. Pattern shifts between upper and lower element lenses of the 2-line gradient-index lens array were measured at every contact point of neighbored element lenses. As a result, it was clarified that patterns printed by neighbored lenses were not always stitched smoothly. It was considered that degradation of resolution and unexpected pattern-width distribution were caused by large pattern shifts observed in some places. It is necessary to adopt sub scans as fast and long as possible for improving the pattern-width homogeneity, and to limit the patterning area to where the pattern shifts are comparatively small for improving the resolution.