## 3 Results and Discussion  
Comparative Study of RF Plasma Cleaning Using C\({}_{2}\)F\({}_{6}\) and F\({}_{2}\)-Gas Mixture  
The purpose of this experiment is to compare the cleaning effect of F\({}_{2}\)-gas mixture to the standard used cleaning process based on C\({}_{2}\)F\({}_{6}\)/O\({}_{2}\) chemistry. A Taguchi Type L9 design of experiments (DOE) was adapted for conceptualizing the experiment. Etch rate data generated with the conventional 13.56-MHz PECVD system (FOGALE Nanotech, Nimes, France) were stable and well repeatable within a \(\pm\)3% range [9]. Susceptor temperature was set to 400 \({}^{\circ}\)C. The temperature of the chamber wall was set constant at 55 \({}^{\circ}\)C by continuous water cooling. Figure 1a-d shows the SiO\({}_{2}\) cleaning rates, under RF plasma cleaning, dependent on varying four parameters: chamber pressure, total flow amount of gas, RF power, and spacing between showerhead and the susceptor. The etching uniformity of SiO\({}_{2}\) films was below \(\pm\)5%. The etch rate is an average value of at least four identical wafers with an SiO\({}_{2}\) layer. These results are in good agreement to previously reported data [9; 10]. The addition of Ar gas in the F\({}_{2}\)-gas mixture is needed, on one side, for a safe and stable plasma ignition, and on the other side, to keep the etch non-uniformity of SiO\({}_{2}\) below \(\pm\)8%. In the absence of an argon element, the etch non-uniformity of SiO\({}_{2}\) rose up to \(\pm\)40%. As can be seen in Figure 1, for all four varied parameters, the curves of the SiO\({}_{2}\) etch rate show the same trend for both gas chemistries under the same conditions. However, the cleaning rate with the F\({}_{2}\)-gas mixture is higher in comparison to the one obtained for the cleaning gas based on C\({}_{2}\)F\({}_{6}\). It was observed that the distance between the parallel plate reactor, showerhead and susceptor is an important factor, as are the RF power and working pressure, which influence the cleaning rate of the PECVD chamber. The SiO\({}_{2}\) etch rate for the F\({}_{2}\)-gas mixture increases by a factor \(\sim\)1.3 due to the lower bond dissociation energy of 155 kJ/mol at 298 K needed to break the F\({}_{2}\) molecules [11]. In comparison to C\({}_{2}\)F\({}_{6}\) molecules, a higher dissociation energy, which is approximately 402 kJ/mol at 298 K, is needed [12]. Therefore, F atoms form more easily from the F\({}_{2}\)-gas mixture at comparably low plasma energy. In contrary, to reach the same density of F atoms from C\({}_{2}\)F\({}_{6}\) molecules, a high excitation energy is required. Figure 1: **a–d**. SiO\({}_{2}\) etch rates using F\({}_{2}\)/Ar/N\({}_{2}\) mixture and C\({}_{2}\)F\({}_{6}\)/O\({}_{2}\) as a function of (**a**) pressure, (**b**) plasma power, (**c**) spacing between the showerhead and the susceptor, and (**d**) gas flow. The error bars are \(\pm\)8% and \(\pm\)3% for plasma cleaning using F\({}_{2}\)/Ar/N\({}_{2}\) mixture and C\({}_{2}\)F\({}_{6}\)/O\({}_{2}\), respectively. The concentration of F atoms is correlated to the etch rate for capacitively-coupled plasma discharge. A kinetic study showed that SiO\({}_{2}\) removal by spontaneous chemical reaction is dominant at high temperatures of 300 \({}^{\circ}\)C-400 \({}^{\circ}\)C, while SiO\({}_{2}\) removal by ion-enhanced removal is dominant below 100 \({}^{\circ}\)C [13]. Consequently, the increase of the flow rate of the F\({}_{2}\)-gas mixture presumably increases the atomic F concentration present inside the CVD chamber, resulting in an increase of the oxide layer's etch rate. However, a high flow rate of the clean gas had no time to reach a complete dissociation. The gas mixture (20% F\({}_{2}\)/ 70% Ar + N\({}_{2}\)) provides a very high efficiency of cleaning in conventional CVD reactors. The total amount of fluorine consumption can be reduced by a factor of four to 10, depending on the chosen process parameters and the PECVD chamber type.