Ion Energy Distribution in Collisionless and Collisional, Capacitive RF Sheath

by Ying Wang

Institution: University of California – Berkeley
Department: Nuclear Engineering
Year: 2012
Keywords: Electrical engineering; Nuclear engineering; capacitive rf discharge; ion energy distribution; particle in cell simulation; sheath
Record ID: 1960934
Full text PDF: http://www.escholarship.org/uc/item/0v9249t8


Capacitively driven radio frequency (rf) discharges are commonly used for plasma-assisted material processing. Because of the significant difference in the mobility of electrons and ions, a thin layer of sheath is always established at the boundary, which separates the discharge into two regions: quasi-neutral bulk plasma and positively charged sheath. Within the sheath, ions are accelerated by electric fields and therefore, bombard the electrode with significant energies. The ion energy distribution (IED) on the substrate is essentially important in the optimization of discharge operations. For plasmas sources typically operated at higher densities and lower pressures, the ion motion in the rf sheath is mainly collisionless since the ion mean free path is much larger than the sheath width. At high operating pressures and large sheath voltage drops, the sheaths are typically collisional.A fast and simple model consisting of a series of computational steps is of great value to predict the plasma parameters and IEDs, given the control parameters of the discharge. Respective models for IEDs in collisionless and collisional capacitive rf sheaths are developed in this dissertation, based on the sheath models developed in late 1980s. Both models do not rely on any intermediate parameters from simulation or experimental results and only take a few seconds (collisionless) or minutes (collisional) to get the final IEDs. Ion-neutral charge exchange reactions are considered for collisional rf sheaths. Energy dependent ion mean free path is taken into account. Particle-in-cell (PIC) simulations are used to verify the previous sheath models and the IED models.The PIC code OOPD1, being developed with a powerful capability as the development of the collisional IED model goes on, is introduced in this dissertation. Comparisons with XPDP1 are presented, which confirms OOPD1 as a trustable, friendly, and extensible simulation tool to observe the rf discharges.