Parametric study of the etching of SiO2 in SF6 plasmas: Modeling of the etching kinetics and validation

The uniform distributed electron cyclotron resonance plasma of ${\mathrm{SF}}_{\mathrm{6}},$ excited at either 2.45 or 5.85 GHz, has been applied to study the etching of ${\mathrm{SiO}}_{\mathrm{2}}$ by F atoms as a function of the three relevant plasma parameters: neutral F-atom flux, ion flux, and ion energy. Three saturation effects are observed. At constant ion current density, the etch rate at first increases linearly with F-atom flux, but then it reaches a plateau, which rises when one raises the ion current density. Second, at constant F-atom flux, initially the etch rate also climbs linearly with ion current density, and again, levels out at larger ion current density, and is higher at larger F-atom flux; however, the initial increase is independent of the F-atom flux. Third, the etch rate evolves similarly as a function of bias voltage for constant F-atom flux and ion current density. These results are first interpreted by a simple mechanism of F-atom adsorption on the ${\mathrm{SiO}}_{\mathrm{2}}$ surface, followed by ${\mathrm{SiF}}_{\mathrm{4}}$ formation at, and desorption from the surface, and by assuming a constant density of adsorption sites for fluorine on the ${\mathrm{SiO}}_{\mathrm{2}}$ surface. However, although this model provides the general trends of the etching kinetics of ${\mathrm{SiO}}_{\mathrm{2}}$ as a function of each plasma parameter, it nevertheless fails explaining many details of the observed etch rates. In fact, ion induced desorption of oxygen from the ${\mathrm{SiO}}_{\mathrm{2}}$ surface is mandatory prior to F-atom adsorption on the Si overlayer thus built up on ${\mathrm{SiO}}_{\mathrm{2}}.$ The model resulting from this hypothesis is in complete agreement with the experimental results obtained on the etching kinetics of ${\mathrm{SiO}}_{\mathrm{2}}$ in ${\mathrm{SF}}_{\mathrm{6}}$ plasmas.