Sheath impedance effects in very high frequency plasma experiments

TLDR

The study measured the ion energy distribution at the ground electrode across 13.56–70 MHz in a symmetrical capacitive argon discharge and summarized the results with simple physical models compared to particle‑in‑cell simulations. At very high frequencies, reduced sheath impedance permits high plasma power and ion flux while keeping ion energy and electrode voltage low, with the lower energy limit governed by the sheath floating potential and thinner sheaths amplifying impedance reduction—explaining the key characteristics of very high‑frequency plasmas.

Abstract

The frequency dependence (13.56–70 MHz) of the ion energy distribution at the ground electrode was measured by mass spectrometry in a symmetrical capacitive argon discharge. Reduced sheath impedance at very high frequency allows high levels of plasma power and substrate ion flux while maintaining low levels of ion energy and electrode voltage. The lower limit of ion bombardment energy is fixed by the sheath floating potential at high frequency, in contrast to low frequencies where only the radio frequency voltage amplitude is a determinant. The capacitive sheaths are thinner at high frequencies which accentuates the high frequency reduction in sheath impedance. It is argued that the frequency dependence of sheath impedance is responsible for the principal characteristics of very high frequency plasmas. The measurements are summarized by simple physical descriptions and compared with a particle-in-cell simulation.