  In most PIC/MCC simulations of radio frequency capacitively coupled plasmas
(CCPs) several simplifications are made: (i) fast neutrals are not traced, (ii)
heavy particle induced excitation and ionization are neglected, (iii) secondary
electron emission from boundary surfaces due to neutral particle impact is not
taken into account, and (iv) the secondary electron emission coefficient is
assumed to be constant, i.e. independent of the incident particle energy and
the surface conditions. Here we question the validity of these simplifications
under conditions typical for plasma processing applications. We study the
effects of including fast neutrals and using realistic energy-dependent
secondary electron emission coefficients for ions and fast neutrals in
simulations of CCPs operated in argon at 13.56 MHz and at neutral gas pressures
between 3 Pa and 100 Pa. We find a strong increase of the plasma density and
the ion flux to the electrodes under most conditions, if these processes are
included realistically in the simulation. The sheath widths are found to be
significantly smaller and the simulation is found to diverge at high pressures
for high voltage amplitudes in qualitative agreement with experimental
findings. By switching individual processes on and off in the simulation we
identify their individual effects on the ionization dynamics and plasma
parameters. We conclude that fast neutrals and energy-dependent secondary
electron emission coefficients must be included in simulations of CCPs in order
to yield realistic results.
