Applicability of projection lithography using a gradient-index lens array to patterning of 50–150 micron thick resist

Abstract

Scan projection lithography using a gradient index lens array as a projection lens is advantageous for printing rough patterns more than 15 μm in large areas at a stretch. In addition to this wonderful performance, it was demonstrated in this paper that patterning in very thick resist films with thicknesses of 50–150 μm was practicable. Since imaging light rays came to a wafer with the maximum inclination angle θ of 12° depending on the property of selected lens array, pattern images formed in the thick resist film with a thickness of t had blurs of b = 2t tan12° = 0.435t at the resist surface. For this reason, the b value became the resolution limit of 1:1 lines-and-spaces patterns printed using a resist with the thickness of t. It was actually demonstrated that the resolution limits for 50- and 100-μm thick resist SU-8 almost corresponded to b. Aspect ratios were also coincided with the estimated value of t/b = 2.35. If the aspect ratios were tolerated to be smaller, patterns with thicknesses of more than 150 μm were printable. In addition, means for improving the patterning performances were discussed. Utilization of a lens array with a smaller inclination angle of imaging light rays, optimization of the exposure-light wavelength, and intentional defocusing of the wafer may be effective.