Ion dynamics in capacitively coupled argon–xenon discharges

It is recognized that in large-area, very-high-frequency capacitively coupled plasma (VHF CCP) reactors, the higher harmonics generated by nonlinear sheath motion can lead to enhanced standing wave excitation. In this work, a self-consistent electromagnetic model, which couples a one-dimensional, radial nonlinear transmission line model with a bulk plasma ﬂuid model, is employed to investigate the nonlinear standing wave excitation in a VHF driven, geometrically asymmetric capacitive argon discharge operated at low pressure. By considering a radially nonuniform plasma density proﬁle (case I ) calculated self-consistently by the nonlinear electromagnetic model and the corresponding radially-averaged, uniform plasma density proﬁle (case II ), we ﬁrst examine the eﬀect of the plasma density nonuniformity on the propagation of electromagnetic surface waves in a 3 Pa argon discharge driven at 100MHz and 90W. Compared to case II, the higher plasma density at the radial center in case I determines a higher plasma series resonance frequency, yielding stronger high-order harmonic excitations and more signiﬁcant central peak in the harmonic current density Jz,n and the harmonic electron power absorption pn proﬁles. Therefore, under the assumption of the radially uniform plasma density in a CCP discharge, the self-excitation of higher harmonics at the radial center should be underestimated. Second, using the self-consistent electromagnetic model, the eﬀect of the rf power on the excitation of nonlinear standing waves is investigated in a 3 Pa argon discharge driven at 100MHz. At a low power of 30W, the discharge is dominated by the ﬁrst two harmonics. The higher harmonic excitations and the nonlinear standing waves are observed to be enhanced with increasing the rf power, resulting in a more pronounced central peak in the radial proﬁles of the total electron power absorption density pe, the electron temperature Te, and the electron density ne. For all rf powers, the calculated radial proﬁles of ne show good agreement with the experimental data obtained by a floating double probe.

Section: Discussionmentioning

confidence: 99%

Simulation of nonlinear standing wave excitation in very-high-frequency asymmetric capacitive discharges: roles of radial plasma density profile and rf power

Zhou¹,

Zhao²,

Wen³

et al. 2021

“…The database is distributed via the LXCat project [45][46][47]. A graphical representation of the data is found in previous work [48].…”

Section: A Kinetic Model Of a Planar Dischargementioning

confidence: 99%

Validation of the smooth step model by particle-in-cell/Monte Carlo collisions simulations

Klich

Löwer

Wilczek

et al. 2022

Self Cite

Bounded plasmas are characterized by a rapid but smooth transition from quasi-neutrality in the volume to electron depletion close to the electrodes and chamber walls. The thin non-neutral region, the boundary sheath, comprises only a small fraction of the discharge domain but controls much of its macroscopic behavior. Insights into the properties of the sheath and its relation to the plasma are of high practical and theoretical interest. The recently proposed smooth step model provides a closed analytical expression for the electric ﬁeld in a planar, radio-frequency modulated sheath. It represents (i) the space charge ﬁeld in the depletion zone, (ii) the generalized Ohmic and ambipolar ﬁeld in the quasi-neutral zone, and (iii) a smooth interpolation for the transition in between. This investigation compares the smooth step model with the predictions of a more fundamental particle-in-cell/Monte Carlo collisions simulation and ﬁnds good quantitative agreement when the assumed length and time scale requirements are met. A second simulation case illustrates that the model remains applicable even when the assumptions are only marginally fulfilled.

“…High voltages of at least several hundreds of volts are required to ensure that highly energetic ions bombard the surface perpendicularly and high-aspectratio trenches can be etched. The gas composition, the neutral gas pressure, the frequency, the voltage and the electrode gap size are parameters which significantly affect the discharge [4][5][6][7]. Especially in industry, the reactor design and thus the choice of electrode gap size plays an essential role [8].…”

Section: Introductionmentioning

confidence: 99%

Nonlocal dynamics of secondary electrons in capacitively coupled radio frequency discharges

Noesges

Klich

Derzsi

et al. 2023

Self Cite

In capacitively coupled radio frequency (CCRF) discharges, the interaction of the plasma and the surface boundaries is linked to a variety of highly relevant phenomena for technological processes.One possible plasma-surface interaction is the generation of secondary electrons (SEs), which significantly influence the discharge when accelerated in the sheath electric field.However, SEs, in particular electron-induced SEs ($\updelta$-electrons), are frequently neglected in theory and simulations.Due to the relatively high threshold energy for the effective generation of $\updelta$-electrons at surfaces, their dynamics are closely connected and entangled with the dynamics of the ion-induced SEs ($\upgamma$-electrons).Thus, a fundamental understanding of the electron dynamics has to be achieved on a nanosecond timescale, and the effects of the different electron groups have to be segregated.This work utilizes $1d3v$ Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations of a symmetric discharge in the low-pressure regime ($p\,=\, 1\,\rm{Pa}$) with the inclusion of realistic electron-surface interactions for silicon dioxide.A diagnostic framework is introduced that segregates the electrons into three groups (``bulk-electrons'', ``$\upgamma$-electrons'', and ``$\updelta$-electrons'') in order to analyze and discuss their dynamics.A variation of the electrode gap size $L_\mathrm{gap}$ is then presented as a control tool to alter the dynamics of the discharge significantly.It is demonstrated that this control results in two different regimes of low and high plasma density, respectively.The fundamental electron dynamics of both regimes are explained, which requires a complete analysis starting at global parameters (e.g., densities) down to single electron trajectories.