# Impact of Ar/CF4 Mixed Gas Flow Rate on Silicon Etching Using Surface Discharge Plasma
## 3 Experimental Results  
This report details the etching process of a monocrystalline silicon substrate using surface discharge plasma generated at varying gas flow rates of Ar/CF4 mixed gas ranging from 0 to 6000 sccm. The resulting etching profile illustrates the impact of these variations on the material's etching. Figure 2 displays a photograph of the surface discharge plasma, highlighting its generation near the triple junction point of the silicon substrate, discharge gas, and quartz glass (dielectric), and extending along the quartz surface. Notably, the plasma generated along the dielectric (quartz glass) consists of numerous filament-like surface streamers, indicating that etching occurs near the triple junction point where the plasma is generated, the extent of which depends on the shape of the dielectric and the diameter of the filamentary surface streamer. Figure 1: Schematic diagrams depicting (**a**) an overview of the atmospheric pressure surface discharge plasma generation system and (**b**) a layout diagram viewed from above, illustrating the lower electrode with the positioned silicon substrate. Figure 2: Photo of surface discharge plasma generated under Ar/CF4 mixed gas: (**a**) Side view of the concave dielectric captured from the front. (**b**) Side view of the concave dielectric captured diagonally from the front. Figure 3 depicts the waveforms of discharge current and voltage during the generation of surface discharge plasma with a flow rate of Ar and CF\({}_{4}\) mixed gas at 6000 sccm and a discharge-sustaining voltage (\(V_{pp}\)) of 8.4 kV. The mixture of Ar and CF\({}_{4}\) maintains a ratio of 7:3, corresponding to flow rates of 4200 sccm for Ar and 1800 sccm for CF\({}_{4}\). The discharge voltage waveform is characterized as a pulse with a repetition frequency of 9.0 kHz. The discharge current consists of a displacement current, which arises due to a minor spatial gap between the metal electrode and the dielectric and the capacitance of the dielectric, as well as a pulse current generated by the initiation of surface streamers. Figure 4 presents a three-dimensional representation of a groove formed by etching using surface discharge plasma. The specified flow rates of Ar and CF\({}_{4}\) mixed gas are 0 sccm and 3000 sccm, respectively. A gas flow rate of 0 sccm indicates a condition in which the discharge-generated gas is confined within the discharge vessel, maintaining a gas mixture ratio of Ar to CF\({}_{4}\) at 7:3. Conversely, at a flow rate of 3000 sccm, the respective flow rates for Ar and CF\({}_{4}\) are 2100 sccm and 900 sccm. The discharge-sustaining voltage and the duration of the etching treatment are maintained at 9.4 kV and 5 min, respectively. Notably, etching by the surface discharge plasma progresses linearly along the concave quartz glass. The etching rate is at its maximum at the center of the filamentary surface streamer and gradually decreases with increasing distance from the streamer's center, resulting in a gently sloping valley-shaped etching cross-section. Figure 5 clarifies the definition of groove width induced by surface discharge. Assessing the etching scenario along the width direction poses a challenge in determining the starting point of groove formation. Therefore, in this study, groove widths at depths of 0.2 \(D\) and 0.8 \(D\) relative to the total etching depth (\(D\)) are designated as \(W_{0.2}\) and \(W_{0.8}\), respectively. Figure 4: Surface structure of silicon substrate obtained after etching by surface discharge plasma generated under Ar/CF\({}_{4}\) mixed gas. (**a**) Ar/CF\({}_{4}\) gas flow 0 sccm; (**b**) Ar/CF\({}_{4}\) gas flow 3000 sccm. Figure 3: (**a**) Discharge current waveform and (**b**) discharge voltage waveform due to surface discharge plasma generated at an Ar/CF\({}_{4}\) gas flow rate of 6000 sccm and a discharge-sustaining voltage (\(V_{pp}\)) of 8.4 kV. Subsequently, the etching progressions along the depth (\(D\)) and width (\(W_{0.2}\) and \(W_{0.8}\)) directions via surface discharge plasma were evaluated.