Experimental Observation and Computational Analysis of Striations in Electronegative Capacitively Coupled Radio-Frequency Plasmas

Abstract

Self-organized spatial structures in the light emission from the ion-ion capacitive RF plasma of a strongly electronegative gas (CF4) are observed experimentally for the first time. Their formation is analyzed and understood based on particle-based kinetic simulations. These "striations" are found to be generated by the resonance between the driving radio-frequency and the eigenfrequency of the ion-ion plasma (derived from an analytical model) that establishes a modulation of the electric field, the ion densities, as well as the energy gain and loss processes of electrons in the plasma. The growth of the instability is followed by the numerical simulations.Plasmas in electronegative gases exhibit complex physical and chemical kinetics [1][2][3][4][5][6][7][8][9][10][11]. Their main constituents are typically positive and negative ions, and electrons are only present as a minor species. Such a composition leads to unique effects, e.g., the dominant mechanism of electron energy gain is typically due to the ambipolar and drift electric fields within the ion-ion plasma bulk [12][13][14][15][16][17][18], in sharp contrast with the mechanisms in (more common) electropositive (electron-ion) plasmas where the dynamics of the boundary sheaths conveys energy to the electrons.Being complex dynamical systems, plasmas are susceptible to various instabilities. Strong modulations of the plasma density and light emission -termed as "striations" -have extensively been studied in electropositive DC discharges [19][20][21][22], wherein ion-acoustic or ionization waves form the basics of these features. Striations also occur in electropositive inductively coupled plasmas [23], plasma display panels [24], and in plasma clouds in the ionosphere [25,26]. In these system the appearance of striations is explained by theories based on the electron kinetics. Little is known, however, about the nature of striations in electronegative plasmas where the ion kinetics may play the dominant role: observations have been limited to DC plasmas [27,28], and striations have never been observed in electronegative capacitively-coupled RF (CCRF) plasmas, to our best knowledge. Here we report the observation of striations, that form in the bulk of electronegative CCRF plasmas. The experimental observations are compared with simulation data, which allow a detailed investigation of the underlying physics.In the experiment (described in detail in [18]) the plasma is produced in CF 4 between two parallel electrodes made of stainless steel with a diameter of 10 cm. The gap between the electrodes is L=1.5 cm. The bottom electrode and the chamber walls are grounded. A sinusoidal voltage of a function generator is amplified and applied to the top electrode via a matching network. The generator is also connected to a pulse delay generator that triggers in a synchronized manner an intensified charge-coupled device (ICCD) camera for Phase Resolved Optical Emission Spectroscopy (PROES) measurements. The ICCD camera is equipped with an objective lens and an interfer...

Plasma stratification in radio-frequency discharges in argon gas

“…However, the number of striations per unit length remained constant at different interelectrode distances. For driving frequencies of 70, 29,6, 4.2, and 3.7 MHz, they observed 8,9,10,11,and 12 striations at L=30 cm. Light emission oscillated at the second harmonic of the applied voltage.…”

“…Although discussions of surface-wave and kinetic effects can be found in the literature, no satisfactory explanations of these striations have been proposed so far. Standing striations have been also observed in Inductively Coupled Plasma (ICP) for certain geometries and discharge conditions [8,9].Striations appeared also in Particle-in-Cell (PIC) simulations of plasmas in different gases and gas mixtures [10,11,12]. Plasma stratification, as an example of selforganization at the kinetic level, remains a great challenge for the plasma science and physics of gas discharges [13].Previously, standing striations in argon CCP have been observed in a range of frequencies 3-80 MHz and gas pressure 2 Torr, in a tube with radius R=1.25 cm [7].…”

Striations in electronegative capacitively coupled radio-frequency plasmas: analysis of the pattern formation and the effect of the driving frequency

“…Multilayer structures, appearing as alternating bright and dark areas along the length of a single plasma column correspond to the second category. They are usually termed as "striations", which have extensively been studied in atmospheric/low pressure and DC/RF glow discharges [6][7][8][9]. These structures are also expected to play an important role in a variety of plasma-based applications such as plasma enhanced chemical vapor 4 / 31 Striations in electronegative capacitively coupled radio-frequency plasmas: analysis of the pattern formation and the effect of the driving frequency…”

“…The visual appearance of such discharges typically shows a homogeneous bright zone in the central bulk region surrounded by two dark sheath regions adjacent to the electrodes.At conditions when the ion plasma frequency becomes comparable to or higher than the driving frequency, positive and negative ions may respond to the RF electric field with an oscillating motion, generating space charges. This may lead to the formation of stable periodic structures ("striations") as explained below [8].The electric field caused by the space charges enhances or attenuates the local drift electric field in the bulk, resulting in a spatially modulated electric field profile. The total field reinforces the response of the positive and negative ions to the alternating RF electric field by "focusing" them into the striations.Consequently, the space charges, as well as the striated electric field are enhanced due to a positive feedback.…”

Striations in electronegative capacitively coupled radio-frequency plasmas: Effects of the pressure, voltage, and electrode gap

“…In most previous studies, the (positive and negative) ions in this drift field have generally been considered to be at rest (being non-responsive to the drift electric field, which oscillates at the driving frequency, e.g., 13.56 MHz, due to their heavy masses), in contrast to the mobile electrons that instantaneously follow the rapidly alternating electric field. However, when the ion plasma frequencies, ± = √ 2 ± 0 ± ⁄ , where e is the elementary charge, ± is the (positive or negative) ion density, 0 is the permittivity in vacuum, and ± are the ion masses, becomes comparable to, or higher than the driving frequency, positive and negative ions may respond to the RF alternating electric field, leading to the formation of space charges [31,32]. The electric field caused by the space charges enhances or attenuates the local drift electric field in the bulk, resulting in at first a slight spatial modulation of the electric field and of the ion densities.…”