Modes of low-pressure dual-frequency (27/2 MHz) discharges in hydrogen

190

178

This paper illustrates the application of particle simulation methods for the description of low-pressure discharges: Townsend discharges, cathode fall dominated dc glows and capacitively coupled radiofrequency discharges. The spatially and/or temporally varying electric field and the presence of boundaries (e.g. electrodes) in these plasma sources induce a non-hydrodynamic (or non-equilibrium) transport of some types of charged species, particularly of electrons. Particle-based methods provide, even under non-equilibrium conditions, a correct method of mathematical description of the particle transport and the determination of the distribution functions, which are crucial quantities in discharge modeling.

Section: Pic Simulation Of Radiofrequency Dischargesmentioning

confidence: 99%

Particle simulation methods for studies of low-pressure plasma sources

Donkó

2011

190

178

“…Instead, either hybrid discharges (RF-DC [6][7][8], capacitive-helicon [9,10], capacitive-inductive [10][11][12]) or dual-frequency (DF) CCRF discharges are used to decouple E i and i . There are two fundamentally different types of DF discharges: (i) classical discharges operated at substantially different frequencies [13][14][15][16][17][18][19][20][21][22][23][24], e.g. 2 and 27 MHz, and (ii) electrically asymmetric (EA) discharges operated at a fundamental frequency and its second harmonic, e.g.…”

Section: Introductionmentioning

confidence: 99%

Secondary electrons in dual-frequency capacitive radio frequency discharges

Schulze

Donkó

Schüngel

et al. 2011

122

151

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.

“…In contrast to single frequency discharges the ion energy and flux at the electrodes can be controlled separately (to some extent) in discharges driven at multiple, substantially different frequencies [7][8][9][10][11][12][13][14]. Typically, two substantially different frequencies, e.g.…”

Section: Introductionmentioning

confidence: 99%

The electrical asymmetry effect in capacitively coupled radio-frequency discharges

Czarnetzki

Schulze

Schüngel

et al. 2011

136

We present an analytical model to describe capacitively coupled radio-frequency (CCRF) discharges and the electrical asymmetry effect (EAE) based on the non-linearity of the boundary sheaths. The model describes various discharge types, e.g. single and multi-frequency as well as geometrically symmetric and asymmetric discharges. It yields simple analytical expressions for important plasma parameters such as the dc self-bias, the uncompensated charge in both sheaths, the discharge current and the power dissipated to electrons. Based on the model results the EAE is understood. This effect allows control of the symmetry of CCRF discharges driven by multiple consecutive harmonics of a fundamental frequency electrically by tuning the individual phase shifts between the driving frequencies. This novel class of capacitive radio-frequency (RF) discharges has various advantages: (i) A variable dc self-bias can be generated as a function of the phase shifts between the driving frequencies. In this way, the symmetry of the sheaths in geometrically symmetric discharges can be broken and controlled for the first time. (ii) Almost ideal separate control of ion energy and flux at the electrodes can be realized in contrast to classical dual-frequency discharges driven by two substantially different frequencies. (iii) Non-linear self-excited plasma series resonance oscillations of the RF current can be switched on and off electrically even in geometrically symmetric discharges. Here, the basics of the EAE are introduced and its main applications are discussed based on experimental, simulation, and modeling results.