Combinatorial study of low-refractive Mg–F–Si–O nano-composites deposited by magnetron co-sputtering from compound targets

Deposition of nano-composite Mg–F–Si–O films on optical grade silica glass was studied employing RF magnetron co-sputtering from magnesium fluoride (MgF₂) and fused silica (SiO₂) targets. The aim was to obtain a stable and reliable sputtering process for optical coatings exhibiting a refractive index lower than the one of quartz glass (1.46 at 550 nm) without adding gaseous fluorine to the deposition process. The two magnetrons were installed in a confocal way at 45° off-axis with respect to a static substrate, thus creating a lateral gradient in the thin-film composition. The deposited Mg–F–Si–O coatings were structurally analysed by electron dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The obtained films consist of MgF₂ nanocrystals embedded in a SiO₂-rich amorphous matrix. Spectroscopic ellipsometry and spectrophotometry measurements showed that they are highly transparent exhibiting a very-low extinction coefficient k and a refractive index n in the desired range between the one of MgF₂ (1.38) and SiO₂ (1.46). Films with n = 1.424 and 1.435 at 550 nm were accomplished with absorption below the detection threshold.