Particle-in-cell simulation of electron and ion energy distributions in dc/rf hybrid capacitively-coupled plasmas


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A Particle-in-Cell simulation with Monte Carlo collisions was used to study electron and ion energy distributions (IEDs) in low-pressure (2.67 Pa) direct-current (dc)/radio-frequency (rf) hybrid capacitively-coupled Ar plasmas. One electrode (dc/rf electrode) of the parallel plate diode was powered by a 13.56 MHz source, and a negative dc bias voltage, whereas the opposite (substrate) electrode was grounded. Secondary electrons emitted from the dc/rf electrode accelerated in the adjacent sheath and entered the plasma, yielding a high-energy tail of the electron energy distribution. For given dc bias voltage, the plasma density increased as the secondary electron emission yield due to ion bombardment increased. A fraction of the secondary electrons were energetic enough to overcome the sheath potential barrier on the substrate electrode and bombard the substrate. The electron angular distribution on the substrate electrode had a peak of directional electrons superimposed on a typical cosine distribution. The mean energy and angular spread of directional electrons could be controlled by varying the dc bias voltage. However, as the dc bias became more negative, the dc/rf sheath expanded at the expense of the bulk plasma, reducing the plasma density, in agreement with published data. The IED on the substrate electrode exhibited a dominant bimodal feature with multiple shoulder peaks due to ion-neutral charge exchange collisions. The average ion energy decreased as the dc voltage became more negative, also in agreement with data. Pulsing the plasma power enhanced the tail of the electron energy distribution in the early activeglow (power ON), and yielded a distinct ballistic electron flux on the substrate with energy equal to the applied dc bias. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3214–3222, 2013