The study of the anomalous thermomechanical effect of fluorine-doped silicon dioxide (FSG) films using temperature dependent FTIR measurements

Fluorine-doped silicon dioxide (FSG) is a material of interest in surface acoustic wave (SAW) technology due to its anomalous thermomechanical behavior. Therefore, it can be used in SAW devices for improved temperature compensation compared to undoped SiO₂. However, up to now there is still no generally accepted theory, which explains this phenomenon. We investigate the reason for its anomalous thermomechanical behavior using Fourier transform infrared spectroscopy (FTIR) measurements of a fluorine-doped silicon dioxide thin films with about 3.4 at.% of fluorine at different temperatures. Geometrical interpretations of the FTIR measurements reveal that the decreasing intertetrahedral Si-O-Si bond angle θ with increasing temperature overcompensates the increasing Si-O bond length, which results in a very low thermal expansion coefficient. The fluorine doping of SiO₂ leads to an increase in the bond angle and therefore a less dense network.