Experimental Investigation of Hysteresis Effects in Capacitively Coupled Radio-Frequency Plasmas Operated in CF₄

Capacitively coupled plasmas (CCPs) operated in fluorine-containing gases (e.g., CF₄, SF₆) are widely used for etching processes in the semiconductor industry. Previous studies on this type of discharge have shown a mode transition from the α-/drift-ambipolar mode to the α-mode in CCPs operated in CF₄, induced by increasing the applied voltage. This transition is accompanied by pronounced changes in key plasma parameters such as electron density, ion flux, and radical density. Importantly, the mode transition exhibits a hysteresis: when the voltage is decreased, the discharge stays in the α-mode at voltages where it was in the α-/drift-ambipolar mode at increasing voltage. Hence, for the same external control parameters, two significantly different sets of discharge parameters are possible. In this work, we experimentally investigate this mode transition in a geometrically symmetric CCP using minimally invasive phase-resolved optical emission spectroscopy through an optical fiber (Fiber PROES [1]), a Retarding Field Energy Analyzer, and hairpin probe diagnostics. The measurements cover a wide range of pressures, driving voltages, and Ar/CF₄ mixtures. The results are in good qualitative agreement with previous simulations and emphasize the importance of plasma diagnostics for revealing nonlinear effects in plasma processes.

[1] F. Beckfeld et al., Rev. Sci. Instrum. 96, 033507 (2025)