Thermal and chemical stability of reflowed-photoresist microlenses

We have investigated the effect of heat treatment on the thermal and chemical stability of photoresist microlenses which were made by a reflow method. The microlenses were formed by patterning a novolac-based photoresist (PR) to pillar shapes and by reflowing it at 140 °C. After reflowing, the microlenses were heat treated at a relatively high temperature between 250 °C and 350 °C. After the heat treatment, the fundamental functions as a lens were maintained for infrared laser beams with wavelengths above 800 nm, except volume shrinkage and increment of the refractive index. The heat-treated microlenses also were not attacked by methanol and acetone. Our results suggest wide application of the PR as a simple, cost effective and stable lens medium.