Development of measurement technique for carbon atoms employing vacuum ultraviolet absorption spectroscopy with a microdischarge hollow-cathode lamp and its application to diagnostics of nanographene sheet material formation plasmas

This study describes the development of a compact measurement technique for absolute carbon (C) atom density in processing plasmas, using vacuum ultraviolet absorption spectroscopy (VUVAS) employing a high-pressure CO₂ microdischarge hollow-cathode lamp (C-MHCL) as the light source. The characteristics of the C-MHCL as a resonance line source of C atoms at 165.7 nm for VUVAS measurements of the absolute C atom density are reported. The emission line profile of the C-MHCL under typical operating conditions was estimated to be the Voigt profile with a Δν_L/Δν_D value of 2.5, where Δν_L is the Lorentz width and Δν_D is the Doppler width. In order to investigate the behavior of C and H atoms in the processing plasma used for the fabrication of two-dimensional nanographene sheet material, measurements of the atom densities were carried out using the VUVAS technique. The H atom density increased with increasing pressure, while the C atom density was almost constant at 5×10¹² cm^-3. The density ratio of C to H atoms in the plasma was found to influence the morphology of carbon nanowalls (CNWs). With increasing H/C density ratio, the growth rate decreased and the space between the walls of the CNWs became wider.