Molecular-ion-beam-assisted deposition of stable SiOF films

Abstract

Recently 1 we reported on low refractive index fluorinated silica (SiOF) films obtained by a Molecular Ion Beam Assisted Deposition (MIBAD) process, using a fluorocarbon precursor in an End Hall ion source during evaporation of silica grains. These films were unstable due to moisture absorption and a subsequent chemical reaction, leading to an irreversible change in composition and increase of refractive index. In this paper we investigate an inert gas enhanced MIBAD process, involving the simultaneous use of a fluorocarbon precursor and an inert gas in the ion gun. Ion beam composition was analyzed by energy selective ion mass spectrometry. Addition of inert gas to the fluorocarbon precursor not only increases the total ion current, but also enhances the dissociation of the fluorocarbon precursor into smaller fragments. The films were characterized by ellipsometry, IR transmission spectroscopy and in situ and ex situ stress measurements. Time evolution of film properties was followed for several weeks. For a given fluorocarbon precursor flow rate, refractive index measured directly after deposition increases with increasing inert gas flow rate, while refractive index change with time is reduced. Fluorine content in the films is maximum for low inert gas flow rates, but decreases for higher flow rates to reach approximately the same level as for pure fluorocarbon ion beam. The observed refractive index stabilization for high inert gas flow rates can be explained by a change in film structure rather than film composition, i.e. by reduced film porosity due to high inert gas ion bombardment. With carefully chosen deposition conditions, stable films with low refractive index can be obtained.