Plasma Ionization in Low-Pressure Radio-Frequency Discharges—Part II: Particle-in-Cell Simulation

Abstract—Plasma ionization in the low-pressure operation regime (< 5 Pa) of RF capacitively coupled plasmas (CCPs) is governed by a complex interplay of various mechanisms, such as field reversal, sheath expansion, and wave–particle interactions. In a previous paper, it was shown that experimental observations in a hydrogen CCP operated at 13.56 MHz are qualitatively well de-scribed in a 1-D symmetrical particle-in-cell (PIC) simulation. In this paper, a spherical asymmetrical PIC simulation that is closer to the conditions of the highly asymmetrical experimental device is used to simulate a low-pressure neon CCP operated at 2 MHz. The results show a similar behavior, with pronounced ionization through field reversal, sheath expansion, and wave–particle in-teractions, and can be exploited for more accurate quantitative comparisons with experimental observations. Index Terms—Capacitively coupled plasma (CCP), field rever-sal, heating, particle-in-cell (PIC) simulation, RF, wave particle interaction. UNDERSTANDING ionization mechanisms and plasmasustainment in low-pressure RF capacitively coupled plasmas (CCPs) is crucial in improving performances and further developing technological applications of such plasmas. The recent advance in optical diagnostics [1] made it possible to show that a complex interplay of ionization through field re-versal, sheath expansion, and, in particular, wave–particle inter-actions plays a fundamental role in sustaining such discharges ([2] and references therein). In this paper, the authors compared their experimental results with the results of a particle-in-cell (PIC) simulation developed by Vender and Boswell [3]. The results predicted by the simulation 15 years earlier turned out to be in good qualitative agreement with the new experimental results. However, the simulation represents a symmetrical sys-tem with an RF sheath on both sides, so it was not clear to what extent the observed ionization mechanisms were not due to the mutual interaction of the two sheaths. In this paper, we simulate a highly asymmetrical CCP with a lower excitation frequency (2 MHz) and another for a noble gas (neon), and we show tha