Rapid fabrication of semiellipsoid microlens using thermal reflow with two different photoresists

Abstract

A simple and rapid fabrication method for a semiellipsoid microlens array has been proposed in this study. Low-loss coupling can be achieved between a laser diode and a single-mode fiber. The single-mode fiber is tipped with a newly developed semiellipsoid microlens in this technique. Average array coupling loss is as low as 15%. Two different photoresist layers are firstly coated on a silicon wafer in sequence. After the lithography process, the elliptical photoresist column with two different photoresists can be obtained. During the thermal reflow processing, upper photoresist layer (AZ-4620) reaches the glass transition temperature (Tg), which is transformed from a glassy state into a rubbery state. Due to their high surface tension, liquid photoresist tend to minimize the structural energy and reduce their surface trying to achieve a lens shape. Since the glass transition temperature of lower photoresist layer (AZ-5214E) is higher than the temperature of thermal reflow, the lower photoresist layer still maintained solid state. The elliptical base can precisely define the bottom of the liquid photoresist as an elliptical shape. Electroforming technology is then used to convert the photoresist patterns into a metallic mold for PDMS semiellipsoid microlens. A coupling efficiency of up to 85% can be obtained using a semiellipsoid microlens fabricated by our proposed method.