Etching Kinetics and Surface Conditions for KNb_xO_y Thin Films with Fluorine- and Chlorine-Based Plasma Chemistries

The investigation of etching kinetics and surface conditions for KNb_xO_y thin films in CF₄ + Ar and Cl₂ + Ar inductively coupled plasmas was carried out. The variable processing parameters were Ar content in a feed gas (0–75% Ar), gas pressure (4–10 mTorr) and input power (400–700 W) at constant bias power of 100 W. The combination of plasma diagnostics by Langmuir probes and plasma modeling provided the data on internal plasma parameters, gas-phase chemistry and steady-state densities of plasma active species. The compositional changes of the etched surfaces were investigated using X-ray photoelectron spectroscopy (XPS). It was found that the fluorine-based etching chemistry provides the much lower halogen atom flux together with the much higher KNb_xO_y etching rate under the condition of quite close ion momentum fluxes in both gas systems. The correlations between measured etching rates and model-predicted fluxes of active species suggested the neutral-flux-limited etching regime with the much lower effective reaction probability between KNb_xO_y and Cl atoms. The last effect may be related to lower volatility of NbCl_x compared with NbF_x (that is confirmed by XPS data) as well as to the higher energy threshold for KNb_xO_y + Cl reaction.