Low $f$ -Number Microlenses for Integration on Optical Microsystems

This paper presents microlenses (MLs) with low $f$ -number made of AZ4562 photoresist for integration on optical microsystems. The fabrication process was based on the thermal reflow and rehydration. Large series of MLs were fabricated with a width of 35 $\mu \text{m}$ , a thickness of $5~\mu \text{m}$ , and spaced apart by 3 $\mu \text{m}$ . The MLs were fabricated directly on the surface of a die with type $n+/p$ -substrate junction photodiode fabricated in a standard CMOS process. The measured focal length was 49 $\mu \text{m}$ with a tolerance of $\pm 2~\mu \text{m}$ (maximum error of ±4%), resulting in a numerical aperture of $33.6 \times 10^{\mathrm {-2}}$ ( $\pm 1.3 \times 10^{\mathrm {{-}2}})$ . The measurements also revealed an $f$ -number of 1.4.