PDMS microlenses coating with a SiO2 ultra-thin film deposited by atomic layer deposition

The optical characteristics of microlenses are significantly influenced by their surface roughness, which is determined by the fabrication process. High-surface roughness increases light scattering, reducing the optical efficiency of the microlenses. The atomic layer deposition (ALD) technique, known for its high conformity, can minimize the roughness of the substrate, filling the surface voids and smoothing its surface. This study presents a polydimethylsiloxane (PDMS) microlens coated with a 30 nm ultra-thin film of silicon dioxide (SiO2) deposited by plasma-enhanced ALD (PEALD) at a low temperature (50 °C). The main aim was to decrease the surface roughness of a polymeric microlens by passivating it. Atomic force microscopy (AFM) measurements were taken before and after the SiO2 deposition, to assess the average surface roughness (Ra). Due to the conformity of PEALD, Ra was reduced by a factor of 3.2 (Figure 1). The microlenses were observed by scanning electron microscopy (SEM) after the SiO2 deposition. Through X-ray photoelectron spectroscopy (XPS), an elemental composition analysis of the SiO2 ultra-thin film was also performed, confirming the formation of SiO2 and a stoichiometric ratio of silicon to oxygen (Si:O) of approximately 27:58.