Fabricating Microlenses on Photodiodes to Increase the Light-Current Conversion Efficiency

This letter presents a microlenses (MLs) fabrication process for enhancing the photocurrent (PC) generation efficiency of photodiodes (PDs). The concept was demonstrated with a plano-convex spherical ML fabricated directly on top of a PD. The PDs are of type n⁺-p-substrate junction fabricated in a standard CMOS process. The base and sag height of the MLs measure 26 and 5 μm, respectively. The PC generation was measured inside a dark chamber under normally incident red light illuminance both with and without the ML on top of the PD. The illuminance was linearly incremented from 0 to 2000 lux. The ML forces more light to impinge into the PDs active area and, therefore, promotes more PC generation. The measurements show that the PD generates, at an approximately constant rate of 14%, more PC with the MLs than without the MLs.