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 SF6, excited at either 2.45 or 5.85 GHz, has been applied to study the etching of SiO2 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 SiO2 surface, followed by SiF4 formation at, and desorption from the surface, and by assuming a constant density of adsorption sites for fluorine on the SiO2 surface. However, although this model provides the general trends of the etching kinetics of SiO2 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 SiO2 surface is mandatory prior to F-atom adsorption on the Si overlayer thus built up on SiO2. The model resulting from this hypothesis is in complete agreement with the experimental results obtained on the etching kinetics of SiO2 in SF6 plasmas. (C) 1999 American Vacuum Society. [S0734-211X(99)04101-3].