Parametric study of the electron temperature and density in dusty low-pressure RF plasmas with pulsed injection of hexamethyldisiloxane

Abstract

Optical emission spectroscopy (OES) is used to analyze the very-low-frequency cyclic evolution of the electron energy and density caused by repetitive formation and loss of dust nanoparticles in an argon RF asymmetric plasma reactor with pulsed injection of hexamethyldisiloxane (HMDSO, [CH3]6Si2O). All OES measurements are recorded in the dust cloud close to the cathode surface. A first wavelength range around 600 nm emanating from Ar excited states close to the ionization level is used to construct a Boltzmann diagram. On the second hand, by introducing a trace amount of rare gases (20% Xe, 20% Kr, 20% Ar, and 40% Ne) in the nominally pure Ar plasma, trace-rare gases OES measurements are performed for wavelengths between 750 and 900 nm. The measured emission intensities of Ar, Kr, and Xe are then compared to the predictions of a collisional-radiative model, with the electron temperature as the only adjustable parameter. These two methods provide temperatures characterizing the low and high-energy part of the electron population respectively. Relative electron densities are also estimated from relative line emission intensities. Both temperatures rise when dust occupation increases, and then decrease when dust is lost. The opposite trend was observed for the electron density [1]. Such cyclic behaviors of the electron energy and electron density in HMDSO-containing plasmas is in good agreement with the evolution processes in dusty plasmas, in which the formation of negative ions followed by electron attachment on nanoparticle's surfaces are critical phenomena driving dust growth. Evolution of the very-low frequency describing this cyclic behavior of the two electron temperatures and electron density with the operating parameters is also investigated. While the frequency increases with total pressure and HMDSO mass flow rate, it decreases with absorbed RF power. Further studies are in progress to relate such variations to those of the plasma characteristics, in particular the precursor fragmentation kinetics.