Ion energy distributions for collisional ion sheaths at an rf-biased plasma electrode
Abstract
In plasma etching for materials processing, ions are often accelerated towards
the substrate via a sheath electric field generated by a bias voltage applied to the substrate
electrode. By applying at tailored bias waveform to the electrode the resulting ion energy
distribution can be manipulated to have a single narrow peak at a specified energy. For low
neutral pressures, ion motion through the sheath is collisionless, and nearly all the incident
ions strike the substrate with the full ion energy dictated by the bias waveform. However,
as the plasma conditions are adjusted and the sheath thickness becomes long compared to
the ion mean free path, the sheath becomes collisional. As ions undergo charge-transfer
collisions while traversing the sheath, they strike the substrate with a lower ion energy than
dictated by the bias waveform. As the system becomes more collisional, the peak of the ion
energy distribution is suppressed, and the ion flux of lower-energy ions increases. Ion energy
distributions at the substrate are measured with a retarding field energy analyzer for many
systems of various collisionality, and the aforementioned effects of the collisional sheaths are
observed.