Secondary electrons in dual-frequency capacitive radio frequency discharges

Abstract

Two fundamentally different types of dual-frequency (DF) capacitively coupled radio frequency discharges can be used for plasma processing applications to realize separate control of the ion mean energy, E i , and the ion flux, i , at the substrate surface: (i) classical discharges operated at substantially different frequencies, where the low-and high-frequency voltage amplitudes, φ lf and φ hf , are used to control E i and i , respectively; (ii) electrically asymmetric (EA) discharges operated at a fundamental frequency and its second harmonic with fixed, but adjustable phase shift between the driving frequencies, θ. In EA discharges the voltage amplitudes are used to control i and θ is used to control E i. Here, we report our systematic simulation studies of the effect of secondary electrons on the ionization dynamics and the quality of this separate control in both discharge types in argon at different gas pressures. We focus on the effect of the control parameter for E i on i for different secondary yields, γ. We find a dramatic effect of tuning φ lf in classical DF discharges, which is caused by a transition from α-to γ-mode induced by changing φ lf. In EA discharges we find that no such mode transition is induced by changing θ within the parameter range studied here and, consequently, i remains nearly constant as a function of θ. Thus, despite some limitations at high values of γ the quality of the separate control of ion energy and flux is generally better in EA discharges compared with classical DF discharges.

Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation

“…A topic of particular importance in such plasma sources is the role played by surface interaction probabilities in determining the overall discharge dynamics and the electron heating. Voltage waveform tailoring has previously been predicted to be a more effective method of controlling the ion flux and ion bombardment energy than conventional dual frequency approaches in cases where the secondary electron emission probability at the electrodes is high and electron heating can be dominated by secondary electrons emitted from the electrodes [53]. Furthermore, there have been a number of investigations detailing the importance of secondary electron emission coefficients in such plasma sources and the problems and opportunities presented for plasma processing applications by their variation from material to material [54][55][56][57].…”

Electron heating modes and frequency coupling effects in dual-frequency capacitive CF4 plasmas

“…In such discharges <E i > is controlled by the low-frequency voltage amplitude (by varying the mean sheath voltage), while Г i is adjusted by the high-frequency voltage amplitude (due to the enhanced electron heating at high frequencies). However, a coupling effect of the driving frequencies was found to reduce the quality of the separate control of ion properties [46][47][48][49], which is further limited by the effect of secondary electrons [50][51][52].…”

Spectroscopy diagnostic of dual-frequency capacitively coupled CHF3/Ar plasma

“…5 However, investigations have shown that the separate control of electron density and ion energy is limited due to the coupling of both frequencies and the effect of secondary electrons by Turner and Chabert, Booth et al, Schulze et al, and so on. [4][5][6][7][8][9] Recently, a novel concept that a discharge is driven by multiple consecutive harmonics was proposed based on the Electrical Asymmetry Effect to solve the problem. [10][11][12] In order to understand the physical and chemical processes of fluorocarbon plasmas, both experimental and theoretical studies have been done over the past few decades.…”