  Ion-assisted surface processes are the basis of modern plasma processing. Ion
energy distribution (IED) control is critical for precise material
modification, especially in atomic-level technologies such as atomic layer
etching. Since this control should be done in real time, it requires real-time
feedback using fast process sensors. In the general case of an industrial
plasma reactor, when direct IED measurement is not possible, the IED can be
estimated using the concept of a virtual IED sensor. In this paper, a similar
virtual IED sensor is considered using an asymmetric dual-frequency (df) rf CCP
discharge as an example. It is based on a fast calculation method of the IED at
an rf-biased electrode. This approach uses the experimentally measured sheath
voltage waveform and plasma density (or ion flux) as input data, and also
includes Monte-Carlo simulation of ion motion in the sheath to take into
account the effect of ion-neutral collisions. To validate this approach,
experiments were carried out using various plasma diagnostics in several gases:
argon and xenon as examples of plasma with atomic ions and nitrogen as an
example of plasma with molecular ions. It is shown that in all cases it is
possible to obtain an adequate IED estimation, close to the experimental one,
in a reasonably short time (~ tens of seconds when using a modern PC). The
results obtained demonstrate the possibility of using a virtual IED sensor in
real plasma processing.
