Instabilities in a capacitively coupled oxygen plasma

Abstract

Periodic fluctuations in the frequency range from 0.3 to 3 kHz were experimentally investigated in capacitively coupled radio frequency (13.56 MHz) oxygen plasma. The Gaussian beam microwave interferometry directly provides the line integrated electron density fluctuations. A system of two Langmuir probes measured the floating potential spatially (axial, radial) and temporally resolved. Hence, the floating potential fluctuation development is mapped within the discharge volume and provides a kind of discharge breathing and no wave propagation. Finally, it was measured the optical emission pattern of atomic oxygen during the fluctuation as well as the RF phase resolved optical emission intensity at selected phase position of the fluctuation by an intensified charge-coupled device camera. The deduced excitation rate pattern reveals the RF sheath dynamics and electron heating mechanisms, which is changing between low and high electronegativity during a fluctuation cycle. A perturbation calculation was taken into account using a global model with 15 elementary collision processes in the balance equations for the charged plasma species (O2+, e, O−, O2−) and a harmonic perturbation. The calculated frequencies agree with the experimentally observed frequencies. Whereby, the electron attachment/detachment processes are important for the generation of this instability.

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

“…A more in-depth understanding of the fundamental processes in CCRF plasmas can improve the performance of plasmas sources used in these practical applications, which could ultimately create enormous social benefits. During the past few years, many researchers studied electronegative CCRF plasmas [4][5][6][7][8][9][10][11][12][13][14][15][16]. Investigations of the charged species' dynamics are most important to obtain a detailed understanding of the fundamental mechanisms of plasma generation as a basis for improving the performance of such plasma sources.…”

“…In these two modes, the electron power deposition, as well as the ionization/excitation rate is typically high around the sheath edges and low in the plasma bulk [4]. By contrast, in plasmas operated in electronegative gases, such as CF 4 , O 2 , SiH 4 , etc., electrons can also gain energy inside the bulk region [5][6][7][8][9][10][11]. This is made possible by the drift and ambipolar electric fields inside the bulk plasma and at the edges of the collapsing sheaths, which effectively heat electrons [drift-ambipolar (DA) mode] [12][13][14].…”

Electron power absorption in capacitively coupled neon-oxygen plasmas: a comparison of experimental and computational results

“…Thermalization of fast neutrals due to collisions with atoms/molecules of the background gas contributes to the heating of the gas (see section 3.3). Elastic collisions of fast Ne atoms (Ne f ) and fast O 2 molecules (O f 2 ) with background O 2 molecules and Ne atoms (processes [42][43][44][45] are considered in the model in the following way: (i) the scattering cross sections are derived from σ V , the viscosity cross section and (ii) isotropic scattering in the COM frame is assumed. As reliable cross section data for Ne f and O f 2 for neonoxygen mixture are not available in the literature, the cross sections are calculated for processes 42-45 based on the pair-potential between the particles, for which the Lennard-Jones (LJ) type is assumed:…”

“…The a ↔ b symmetry of these equations results in the same cross sections for Ne f +O 2 and O f 2 +Ne collisions. These computed cross sections (for processes [42][43][44][45] are plotted in figure 4. The calculation of the cross sections of fast neutrals was validated by comparing our results for argon gas with those given in [70].…”

“…As a consequence of this, it is more likely that the highly energetic SEs within the sheaths induce ionization rather than excitation. Due to their high relevance in material processing, oxygen CCPs have been studied extensively, both experimentally and by simulations [13,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. In pure oxygen CCPs, transitions between the α-mode and the DA-mode have been found by changing the gas pressure [13,14], the gap distance [14,17], the driving frequency [13,47], the driving voltage waveform [13,15,18,46,51,52], and the external magnetic field [49].…”