“Virtual IED sensor” at an rf-biased electrode in low-pressure plasmaстатья
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Дата последнего поиска статьи во внешних источниках: 6 августа 2019 г.
Аннотация:Energy distribution and the flux of the ions coming on a surface are considered as the keyparameters
in anisotropic plasma etching. Since direct ion energy distribution (IED) measurements
at the treated surface during plasma processing are often hardly possible, there is an opportunity for
virtual ones. This work is devoted to the possibility of such indirect IED and ion flux measurements
at an rf-biased electrode in low-pressure rf plasma by using a “virtual IED sensor” which represents
“in-situ” IED calculations on the absolute scale in accordance with a plasma sheath model containing
a set of measurable external parameters. The “virtual IED sensor” should also involve some
external calibration procedure. Applicability and accuracy of the “virtual IED sensor” are validated
for a dual-frequency reactive ion etching (RIE) inductively coupled plasma (ICP) reactor with a
capacitively coupled rf-biased electrode. The validation is carried out for heavy (Ar) and light (H2)
gases under different discharge conditions (different ICP powers, rf-bias frequencies, and voltages).
An EQP mass-spectrometer and an rf-compensated Langmuir probe (LP) are used to characterize
plasma, while an rf-compensated retarded field energy analyzer (RFEA) is applied to measure IED
and ion flux at the rf-biased electrode. Besides, the pulsed selfbias method is used as an external
calibration procedure for ion flux estimating at the rf-biased electrode. It is shown that pulsed selfbias
method allows calibrating the IED absolute scale quite accurately. It is also shown that the
“virtual IED sensor” based on the simplest collisionless sheath model allows reproducing well
enough the experimental IEDs at the pressures when the sheath thickness s is less than the ion
mean free path ki (s < Li). At higher pressure (when s > Li), the difference between calculated and
experimental IEDs due to ion collisions in the sheath is observed in the low energy range. The
effect of electron impact ionization in the sheath on the origin and intensity of low-energy peaks in
IED is discussed compared to ion charge-exchange collisions. Obviously, the extrapolation of the
“virtual IED sensor” approach to higher pressures requires developing some other sheath models,
taking into account both ion and electron collisions and probably including even a model of the
whole plasma volume instead of plasma sheath one.