  One-dimensional particle-in-cell simulation is used to simulate the
capacitively coupled argon plasma for a range of driving frequency from 13.56
MHz to 100 MHz. The argon chemistry set can, selectively, include two
metastable levels enabling multi-step ionization and metastable pooling. The
results show that the plasma density decreases when metastable atoms are
included with higher discrepancy at higher excitation frequency. The
contribution of multistep ionization to overall density increases with
excitation frequency. The electron temperature increases with the inclusion of
metastable atoms and decreases with excitation frequency. At lower excitation
frequency, the density of Ar** (3p5 4p, 13.1 eV) is higher than Ar* (3p5 4s,
11.6 eV), whereas, at higher excitation frequencies the Ar* (3p5 4s, 11.6 eV)
is the dominant metastable atom. The metastable and electron temperature
profile evolve from a parabolic profile at lower excitation frequency to a
saddle type profile at higher excitation frequency. With metastable, the
electron energy distribution function (EEDF) changes its shape from Druyvesteyn
type, at low excitation frequency, to bi-Maxwellian, at high frequency plasma
excitation, however a three-temperature EEDF is observed without metastable
atoms.
