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

Stefan Mertin, Tony Länzlinger, Cosmin S. Sandu, Jean Louis Scartezzini, Paul Muralt

Research output: Contribution to journalArticleScientificpeer-review

Abstract

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.

Original languageEnglish
Pages (from-to)170-177
Number of pages8
JournalApplied Surface Science
Volume435
DOIs
Publication statusPublished - 30 Mar 2018
MoE publication typeA1 Journal article-refereed

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Keywords

  • Low refractive index
  • Magnesium fluoride
  • Magnetron co-sputtering
  • Nano-composites
  • Optical properties
  • Thin-film deposition

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