Characterization of fluorinated silica thin films with ultra-low refractive index deposited at low temperature

• Low deposition temperature and organic precursor led to higher film fluorination.
• High fluorine and nanovoid structure led to drastic decrease in the refractive index.
• Silica based thin film with ultralow refractive index of 1.16 was produced.
• The produced ultralow-n film is highly stable against moisture absorption.

Structural and optical properties of low refractive index fluorinated silica (SiOxCyFz) films were investigated. The films were deposited on p-type silicon and polycarbonate substrates by radio frequency plasma enhanced chemical vapor deposition method at low temperatures. A mixture of tetraethoxysilane vapor, oxygen, and CF4 was used for deposition of the films. The influence of oxygen flow rate on the elemental compositions, chemical bonding states and surface roughness of the films was studied using energy dispersive X-ray analyzer, Fourier transform infrared spectroscopy in reflectance mode and atomic force microscopy, respectively. Effects of chemical bonds of the film matrix on optical properties and chemical stability were discussed. Energy dispersive spectroscopy showed high fluorine content in the SiOxCyFz film matrix which is in the range of 7.6–11.3%. It was concluded that in fluorine content lower than a certain limit, chemical stability of the film enhances, while higher contents of fluorine heighten moisture absorption followed by increasing refractive index. All of the deposited films were highly transparent. Finally, it was found that the refractive index of the SiOxCyFz film was continuously decreased with the increase of the O2 flow rate down to the minimum value of 1.16 ± 0.01 (at 632.8 nm) having the most ordered and nano-void structure and the least organic impurities. This sample also had the most chemical stability against moisture absorption.