A coupled two-sheath simulation of RF bias at high electronegativities

Abstract

In this paper, a 1D numerical, time-dynamic, fluid model (SHEATHSIM) has been developed to study the effect of RF chuck bias on high density electronegative discharges especially under conditions of sheath inversion and negative ion extraction. In each sheath, ion momentum conservation and ion continuity equations, closed by Poisson's equation, are solved for a given set of bulk densities, temperatures and mean free path lengths of each ionic species. Two such sheaths are closed by a common bulk potential and current continuity. Qualitatively, high electronegativities (≥100), low electron temperatures (≤2.5 eV), high asymmetries (≥10), DC coupling of bias power and low frequencies (≈100 kHz) tend to favour sheath inversion and subsequent negative ion extraction. Quiescent floating sheath simulations show the self-consistent rise of the plasma regions and the Bohm criterion. Such modelling in conjunction with source antennae electromagnetics and chemical kinetics can lead to computer aided process and reactor design with a view to reducing charge-up damage through negative ion extraction.

Negative ion extraction from hydrogen plasma bulk

“…In this case, negative ions cannot be extracted from plasma. [21][22][23] On the opposite, for extremely high electronegative plasma a ) ffiffiffiffiffiffiffi ffi c=l p (c ¼ T e /T À is the electron to negative ion temperature ratio), negative space-charge sheaths are formed and the positive ion flux that leaves the plasma is almost fully neutralized by negative ions, like in ion-ion plasma. [18][19][20] For intermediate regimes, the sheath in front of the most positively biased electrode passes from classical positive sheath to negative sheath with the increase of a.…”

Alternating fluxes of positive and negative ions from an ion–ion plasma

“…For example, we had simulated sheath field inversion ͑inversion allows negatively charged particles to be accelerated out of the glow center͒ and found that extraordinarily large electronegativities (N Ϫ /Neу1000) were required to extract negative ions with any significant flux density. 8 It turns out that pulsing the plasma excitation in the presence of an attaching gas can enable such electronegativities and even form a nearly electron-free ''positive-ion-negative-ion'' plasma. 9,10 It is firmly believed that these ion-ion plasmas can be biased to extract either positive or negative ions, unfortunately, experimental proof is not yet published.…”

Characterization of electronegative plasmas

“…Such a formula, arising from the Bohm sheath criterion in the presence of negative ions [3], is appropriate under the conditions T e >> T − and n + n − > n e , which are met in most of the experiments in plasmas of electronegative gases. When negative ions dominate, a presheath is created into which positive ions enter from the plasma bulk with a kinetic energy of the order of the thermal energy of the dominant negative charged specie (k B T − ) [4]. The electron temperature T e is extracted from the slope of the linear fit of the logarithmic Langmuir characteristic.…”