## 3 Results and Discussion
### Comparative Study of RPS Plasma Cleaning Using NF\({}_{3}\) and F\({}_{2}\)-Gas Mixture  
Figure 2 summarizes the etch rates of the SiO\({}_{2}\) layer as a function of the fluorine mass in the total cleaning gas used by flowing into the chamber for one minute. An RPS unit was used to activate the desired cleaning gas. In our case, an F\({}_{2}\)-gas mixture and NF\({}_{3}\) were the cleaning gases to be tested. Several F\({}_{2}\)-gas mixtures of F\({}_{2}\), Ar, and N\({}_{2}\) were received from the Solvay company, having a maximum concentration of 30% of F\({}_{2}\). The percentage of the N\({}_{2}\) and Ar in the F\({}_{2}\)-gas mixture were varied to get the optimal mixture giving a high etch rate of the CVD chamber. The reason was to find the best F\({}_{2}\)-based clean gas mixture to be tested on site by our industrial partner.  
The RPS unit permits a high dissociation of NF\({}_{3}\), reaching more than 95%. From the reached etch rate based on the F\({}_{2}\)-gas mixture, we suppose that the dissociation of the F\({}_{2}\) molecules reached high values comparable to the NF\({}_{3}\) chemistry. Additionally, a certain amount of species will recombine on their way into the process chamber, independently of the cleaning chemistry used. The RPS unit is optimized and reprogrammed specifically to the use of NF\({}_{3}\) gas, which is widely employed in the industry.  
Figure 2 shows that the SiO\({}_{2}\) etch rates increase linearly by increasing the F\({}_{2}\) content in the total gas flow, whatever the cleaning gas used. For example, for an F\({}_{2}\)-gas mixture containing a total mass fluorine equal to 0.25 g reaches an etch rate of 1150 nm/min. However, by using NF\({}_{3}\) containing the same total mass of fluorine, a lower etch rate 750 nm/min is obtained.  
Figure 2: SiO\({}_{2}\) etch rates per minute as a function on atomic fluorine mass for an F\({}_{2}\)–gas mixture and NF\({}_{3}\) cleaning gas. The following parameters were kept constant: total flow, spacing for inner clean = 200 mils, spacing for outer clean = 650 mils, T = 400 \({}^{\circ}\)C, pressure for inner clean = 3 Torr, pressure for outer clean = 4 Torr, clean time = 45 s. The error bars are \(\pm\)10% and \(\pm\)4% for plasma cleaning using an F\({}_{2}\)–gas mixture and NF\({}_{3}\), respectively.  
The high etch rate obtained for the F\({}_{2}\)-gas mixture is explained by the presence of an optimal percentage of the N\({}_{2}\) gas. Another explanation is the much lower plasma energy needed to completely dissociate F\({}_{2}\)-gas mixtures compared to NF\({}_{3}\)[11]. It is important to take into account the recombination phenomena depending on the operating parameter and the hardware geometry [14].  
To gain more knowledge about the role of N\({}_{2}\) in the chamber cleaning, we represent in Figure 3 the SiO\({}_{2}\) etch rate as a function of the N\({}_{2}\) concentration in the F\({}_{2}\)-gas mixture for three different total masses of fluorine. A N\({}_{2}\) concentration between 35-45% in the F\({}_{2}\)-gas mixture is believed to be optimal. However, a concentration of F\({}_{2}\) higher than 30% in the mixture was not possible to study due to road traffic safety rules in Germany, which forbid the transportation of gas cylinders with such a concentration of F\({}_{2}\). According to the literature, N\({}_{2}\) takes the role of the oxygen catalyst by forming N\({}_{2}\)O during the SiO\({}_{2}\) cleaning, and thus increasing the removal rate of SiO\({}_{2}\) from the reactor surfaces [15; 16].