Ion energy and angular distributions in low-pressure capacitive oxygen RF discharges driven by tailored voltage waveforms

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A thorough understanding of the energy transfer mechanism from the electric field to electrons is of utmost importance for optimization and control of different plasma sources and processes. This mechanism, called electron power absorption, involves complex electron dynamics in electronegative capacitively coupled plasmas (CCPs) at low pressures, that are still not fully understood. Therefore, we present a spatio-temporally resolved analysis of electron power absorption in low pressure oxygen CCPs based on the momentum balance equation derived from the Boltzmann equation. Data are obtained from 1d3v Particle-In-Cell / Monte Carlo Collision simulations. In contrast to conventional theoretical models, which predict 'stochastic/collisionless heating' to be important at low pressure, we observe the dominance of Ohmic power absorption. In addition, there is an attenuation of ambipolar power absorption at low pressures due to the strong electronegativity, and the presence of electropositive edge regions in the discharge, which cause a high degree of temporal symmetry of the electron temperature within the RF period.

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Electron power absorption in low pressure capacitively coupled electronegative oxygen radio frequency plasmas

Vass¹,

Wilczek²,

Lafleur³

et al. 2020

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“…For example, in electropositive discharges, the 'α-mode' [26] and the 'γ-mode' [27], where electrons are accelerated by electric fields during the times of sheath expansion within each RF period and the strong electric field inside sheaths, respectively, are the most common electron power absorption modes. However, in electronegative discharges, the depletion of the electron density plays an important role for the electron power absorption [7,[28][29][30]. In strongly electronegative discharges, the low electron density leads to the presence of strong drift electric fields in the bulk region as well as of strong ambipolar fields near the sheath edges.…”

Section: Introductionmentioning

confidence: 99%

Electron power absorption dynamics in magnetized capacitively coupled radio frequency oxygen discharges

Wang

Wen

Hartmann

et al. 2020

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The influence of a uniform magnetic field parallel to the electrodes on radio frequency capacitively coupled oxygen discharges driven at 13.56 MHz at a pressure of 100 mTorr is investigated by one-dimensional Particle-in-Cell/Monte Carlo collision (1D PIC/MCC) simulations. Increasing the magnetic field from 0 to 200 G is found to result in a drastic enhancement of the electron and the O + 2 ion density due to the enhanced confinement of electrons by the magnetic field. The time and space averaged O − ion density, however, is found to remain almost constant, since both the dissociative electron attachment (production channel of O − ) and the associative electron detachment rate due to the collisions of negative ions with oxygen metastables (main loss channel of O − ) are enhanced simultaneously. This is understood based on a detailed analysis of the spatio-temporal electron dynamics.The nearly constant O − density in conjunction with the increased electron density causes a significant reduction of the electronegativity and a pronounced change of the electron power absorption dynamics as a function of the externally applied magnetic field. While at low magnetic fields the discharge is operated in the electronegative Drift-Ambipolar (DA) mode, a transition to the electropositive α-mode is induced by increasing the magnetic field. Meanwhile, a strong electric field reversal is generated near each electrode during the local sheath collapse at high magnetic fields, which locally enhances the electron power absorption. A model of the electric field generation reveals that the reversed electric field is caused by the reduction of the electron flux to the electrodes due to their trapping by the magnetic field.The consequent changes of the plasma properties are expected to affect the applications of such discharges in etching, deposition and other semiconductor processes.

“…Tailored voltage waveforms comprise the superposition of two or more harmonics of a fundamental voltage frequency; by adjusting the relative phase or amplitude between harmonics, it is possible to introduce amplitude and slope asymmetries into the voltage waveform [21][22][23][24] . Commonly employed tailored waveforms include 'sawtooth' waveforms 25,26 , which exhibit a substantial slope asymmetry, and 'peak' or 'valley' waveforms [27][28][29][30] , which exhibit a substantial amplitude asymmetry, i.e. the maximum positive and negative applied voltages within one period of the fundamental applied frequency are not equal.…”

Section: Introductionmentioning

confidence: 99%

Decoupling ion energy and flux in intermediate pressure capacitively coupled plasmas via tailored voltage waveforms

Doyle

Gibson

Boswell

et al. 2020

The discrete control of ion energy and flux is of increasing importance to industrially relevent plasma sources. The ion energy distribution functions (IEDFs) and net ion flux incident upon material surfaces in intermediate pressure (≈ 133 Pa, 1 Torr) radio-frequency capacitively coupled plasmas (rf CCPs) are coupled to the spatio-temporal sheath dynamics and resulting phaseaveraged sheath potential. For single frequency driven discharges this co-dependence of ion energy and flux on the sheath potential limits the range of accessible operating regimes. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate quasiindependent control of the ion flux and IEDF incident upon plasma facing surfaces in a collisional (≈ 200 Pa, 1.5 Torr argon) rf hollow cathode discharge driven by multi-harmonic (n ≥ 2) tailored voltage waveforms. The application of variable phase offset n = 5 tailored voltage waveforms affords a significant degree of control over the ion flux Γ Ar + and mean ion energy ǫAr + , modulating each by 1 factors of 2.9 and 1.6, respectively as compared to 1.8 and 1.6, achieved via n = 2 dual-frequency voltage waveforms. The disparate modulations achieved employing n = 5 tailored voltage waveforms demonstrate a significant degree of independent control over the mean ion energy and ion flux for collisional conditions, enabling access to a wider range of operational regimes. Maximising the extent to which ion energy and flux may be independently controlled enables improvements to plasma sources for technological applications such as plasma assisted material manufacture and spacecraft propulsion.