Paola Diomede

Hybrid simulation of a dc-enhanced radio-frequency capacitive discharge in hydrogen

A PIC-MCC/fluid hybrid model was employed to study a parallel-plate capacitively coupled radio-frequency discharge in hydrogen, under the application of a dc bias voltage. When a negative dc voltage was applied to one of the electrodes of a continuous wave (cw) plasma, a ?beam? of secondary electrons was formed that struck the substrate counter-electrode at nearly normal incidence. The energy distribution of the electrons striking the substrate extended all the way to VRF?+?|Vdc|, the sum of the peak RF voltage and the absolute value of the applied dc bias. Such directional, energetic electrons may be useful for ameliorating charging damage in etching of high aspect ratio nano-features. The vibrational distribution function of molecular hydrogen was calculated self-consistently, and was found to have a characteristic plateau for intermediate values of the vibrational quantum number, v. When a positive dc bias voltage was applied synchronously during a specified time window in the afterglow of a pulsed plasma, the ion energy distributions (IEDs) of positive ions acquired an extra peak at an energy equivalent of the applied dc voltage. The electron energy distribution function was slightly and temporarily heated during the application of the dc bias pulse. The calculated IEDs of and ions in a cw plasma without dc bias were found to be in good agreement with published experimental data.

Particle-in-cell simulation of ion energy distributions on an electrode by applying tailored bias waveforms in the afterglow of a pulsed plasma

A Particle-in-Cell simulation with Monte Carlo Collisions (PIC-MCC) was conducted of the application of tailored DC voltage steps on an electrode, during the afterglow of a capacitively-coupled pulsed-plasma argon discharge, to control the energy of ions incident on the counter-electrode. Staircase voltage waveforms with selected amplitudes and durations resulted in ion energy distributions (IED) with distinct narrow peaks, with controlled energies and fraction of ions under each peak. Temporary electron heating at the moment of application of a DC voltage step did not influence the electron density decay in the afterglow. The IED peaks were “smeared” by collisions, especially at the higher pressures of the range (10–40 mTorr) investigated.

Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: : excitation dynamics and ion flux asymmetry

Parallel plate capacitively coupled plasmas in hydrogen at relatively high pressure (n1 Torr) are excited with tailored voltage waveforms containing up to five frequencies. Predictions of a hybrid model combining a particle-in-cell simulation with Monte Carlo collisions and a fluid model are compared to phase resolved optical emission spectroscopy measurements, yielding information on the dynamics of the excitation rate in these discharges. When the discharge is excited with amplitude asymmetric waveforms, the discharge becomes electrically asymmetric, with different ion energies at each of the two electrodes. Unexpectedly, large differences in the H2+ fluxes to each of the two electrodes are caused by the different H3+ energies. When the discharge is excited with slope asymmetric waveforms, only weak electrical asymmetry of the discharge is observed. In this case, electron power absorption due to fast sheath expansion at one electrode is balanced by electron power absorption at the opposite electrode due to a strong electric field reversal.

Kinetic simulation of capacitively coupled plasmas driven by trapezoidal asymmetric voltage pulses

A kinetic Particle-In-Cell simulation with Monte Carlo Collisions was performed of a geometrically symmetric capacitively coupled, parallel-plate discharge in argon, driven by trapezoidal asymmetric voltage pulses with a period of 200 ns. The discharge was electrically asymmetric, making the ion energy distributions at the two electrodes different from one another. The fraction of the period (α), during which the voltage was kept at a constant (top-flat) positive value, was a critical control parameter. For the parameter range investigated, as α increased, the mean ion energy on the grounded electrode increased and the ions became more directional, whereas the opposite was found for the ions striking the powered electrode. The absolute value of the DC self-bias voltage decreased as α increased. Plasma instabilities, promoted by local double layers and electric field reversals during the time of the positive voltage excursion, were characterized by electron plasma waves launched from the sheath edge.

Kinetic modelling of atom production and thermalization in CCRF discharges in H2

Two kinetic models are applied to H atoms in capacitively coupled, radio-frequency H2 discharges. A multicomponent particle-in-cell with Monte Carlo collisions kinetic model of the plasma phase, including several ionic species, electrons, molecules in specific internal states, is used to determine the atom source term in the whole discharge volume, while a Monte Carlo model is applied to H atoms. Using these models it is possible to treat the whole cycle of H atoms including production, slowing-down and development of the thermalized component and to account for the total atomic balance in the gas phase. The relevance of different atom production channels is quantitatively discussed, and results for the energy distribution and energy flux distribution on surfaces are given. Recommendations for H atom kinetics data are provided.

Rapid calculation of the ion energy distribution on a plasma electrode

A model was developed to rapidly calculate the ion energy distribution (IED) on an electrode immersed in plasma, for a given voltage waveform applied to the electrode through a blocking capacitor. The model combined an equivalent circuit representation of the system, with an equation for a damped potential to which ions respond, during their transit through the sheath. Predicted IEDs on both conducting and insulating surfaces for a variety of applied voltage waveforms (spike, staircase, square wave, etc.) agreed with published experimental data. For these comparisons with experiments, peak broadening due to the resolution of the ion energy analyzer was also taken into account. Using “tailored” waveforms of the applied voltage, desired IEDs may be obtained in terms of peak energies and fraction of ions under each peak.

Instabilities in Capacitively Coupled Plasmas Driven by Asymmetric Trapezoidal Voltage Pulses

Instabilities in Ar capacitively coupled plasmas driven by asymmetric trapezoidal voltage pulses were observed using particle-in-cell simulations. Despite its geometric symmetry, the discharge was electrically asymmetric. The instabilities were electron plasma waves launched from the sheath edge at the time the voltage pulse was applied and were accompanied by local electric field reversal.