# Impact of Ar/CF4 Mixed Gas Flow Rate on Silicon Etching Using Surface Discharge Plasma
## 4 Discussion  
In this study, surface discharge plasma under atmospheric pressure conditions was utilized with Ar and CF4 mixed gas to etch monocrystalline silicon substrates. The study focused on examining the etching characteristics of silicon substrates as a function of changes in the flow rate of Ar/CF4 mixed gas. The findings indicate that within a flow rate range of 0 to 6000 sccm, higher flow rates of the Ar/CF4 mixed gas are associated with increased etching of silicon in both depth and width. The etching mechanism primarily involves the bombardment of the substrate by Ar ions produced in the plasma, along with a chemical reaction involving active species such as Fx and CFn. These species are generated as CF4 dissociates, a process facilitated by collisions between electrons, which are accelerated by an electric field, and metastable excited Ar species (11.6 eV, 11.8 eV) interacting with CF4 molecules. The specific dissociation reactions of CF4 and the energies required for these reactions are outlined in Equations (1)-(8). \[\text{CF}_{4}\rightarrow\text{CF}_{3}+\text{F}\] 5.60 [eV] (1) \[\text{CF}_{4}\rightarrow\text{CF}_{2}+\text{F}_{2}\] 7.81 [eV] (2) \[\text{CF}_{4}\rightarrow\text{CF}_{2}+2\text{F}\] 9.46 [eV] (3) \[\text{CF}_{3}\rightarrow\text{CF}_{2}+\text{F}\] 3.80 [eV] (4) \[\text{CF}_{2}\rightarrow\text{CF}+\text{F}\] 5.40 [eV] (5) \[\text{CF}_{2}\rightarrow\text{C}+2\text{F}\] 11.00 [eV] (6) \[\text{CF}\rightarrow\text{C}+\text{F}\] 5.60 [eV] (7) \[\text{F}_{2}\rightarrow\text{2F}\] 4.34 [eV] (8)  
Figure 7: Dependency of (**a**) etched depth and (**b**) etched width on the silicon substrate surface during etching using surface discharge plasma generated at various Ar/CF4 gas flow rates. Active species such as F, F\({}_{2}\), and CF\({}_{n}\), generated through the reactions specified in Equations (1)-(8), are thought to play a crucial role in facilitating the etching process. Figure 8 displays the emission spectrum of the atmospheric pressure surface discharge plasma, measured using a multichannel spectrometer (USB4000, Ocean Optics Inc., Orlando, FL, USA, wavelength resolution 1.5 nm). Within this figure, peaks attributed to F, Ar, and CF\({}_{n}\) were identified in the plasma produced by the Ar/CF\({}_{4}\) mixed gas [17]. Line spectra originating from Ar and F typically appeared at wavelengths of 763.5105 nm and 634.851 nm, respectively [18]. In the current measurement, a line spectrum due to Ar was detected at a wavelength of 763.78 nm, along with a band spectrum attributed to F centered at approximately 630 nm. Accordingly, this study focused on a wavelength of 631.45 nm, corresponding to the peak emission intensity within the F-band spectrum. Figure 9 illustrates the emission spectrum intensities of the excited species stimulated by Ar (763.78 nm), F (631.45 nm), and CF\({}_{n}\) (327.54 nm) at various gas flow rates. As the presence of CF\({}_{n}\) was confirmed through its band spectrum, the study particularly noted the emission intensity at 327.54 nm, representing the peak of this band spectrum. As the gas flow rate increased, the emission intensities of F and CF\({}_{n}\), the key species in the etching process, also increased. Conversely, the variation in the emission intensity of the Ar peak with increasing gas flow rate remained negligible. Given the inert nature and low reactivity of Ar, minimal changes due to variations in the gas flow rate are expected. Therefore, an increase in the gas flow rate is presumed to enhance the generation of active species, such as F\({}_{x}\) and CF\({}_{n}\), thereby promoting the progression of silicon etching. Figure 8: Emission spectrum of surface discharge plasma generated at different Ar/CF\({}_{4}\) gas flow rates (measured wavelength: 180–900 nm, discharge-sustaining voltage: 9.4 kV). \[\text{CF}_{4}\rightarrow\text{CF}_{2}+\text{F}_{2}\]0 Changes in Ar, F, and CF\({}_{n}\) emission intensities in surface discharge plasma at various Ar/CF\({}_{4}\) gas flow rates (F peak: 631.45 nm, Ar peak: 763.78 nm, CF\({}_{n}\) peak: 327.54 nm).