Discharge regimes and emission characteristics of capacitively coupled radio frequency argon plasma with a square wave input

This work presents a computational and experimental investigation of capacitively coupled radio frequency argon plasma in a roll-to-roll chemical vapor deposition system for carbon nanostructure growth. The system operates at moderate pressures (less than 30 mbar) with an 80 kHz square wave voltage input. The computational model aids the understanding of plasma properties and α– γ transition parameters which strongly influence the nanostructure deposition characteristics in the system. A 1D plasma model is developed to characterize the effects of input voltage, gas pressure, frequency, and waveform on the plasma properties. A hybrid mode which displays the characteristics of both α and γ discharges is found to exist for the low cycle frequency 80 kHz square wave voltage input due to the high frequency harmonics associated with a square waveform. The modeled gas temperatures are 22.5% higher than the experimentally measured values due to the presence of molecular species in the experiments. Collision radiative modeling is performed to predict the argon emission intensity in the discharge gap. The results are found to lie within 16% of the optical emission spectroscopy measurements with better agreement at the center of the discharge, where the measurement uncertainty is low and the emission by ions is not significant.