Similarity scaling for low-pressure capacitive radio-frequency CF_{4} plasmas across operation modes.

Abstract

Similarity laws (SLs) describe how physical characteristics in plasma discharges remain consistent at different dimensional scales, and they have been verified in electropositive capacitively coupled radio-frequency (RF) plasmas across a wide range of discharge conditions. In this work, we explore the properties of similarity scaling in low-pressure CF_{4} capacitive RF plasma via fully kinetic particle-in-cell/Monte Carlo collision simulations. We find that under our conditions, the densities of all charged particles exhibit similarity. This property arises as the collision reactions governing the changes in particle density are all linear reactions, whose reaction rates satisfy the SL scaling. The similarity factor of electron density is consistent with classical similarity theory. However, the similarity factor of the density of each ion species deviates consistently from similarity theory. The reason for this deviation across all discharge modes, including striation, drift-ambipolar, and α-modes, is discussed. Furthermore, we show that this similarity property of ion densities constitutes a destruction of discharge similarity in the striation mode. This study improves the understanding of the similarity laws in electronegative RF discharges.