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A novel type of photoresist microlenses is conceived. The characteristic of these lenses is their mushroom shape: the lens dome is located at a certain controllable distance from the substrate, and the curvature of the lens is larger than that of traditional truncated-sphere lenses. These mushroom microlenses are equilibrium-state, stable products of reflow that are obtained when starting from a multilayer photoresist stack. Integration on light-emitting diodes results in enhanced output efficiency and directivity: a vertical-to-surface emission enhancement of up to a factor of 8 has been achieved.
Heremans et al. (1997) studied this question.