Investigation of O-atom kinetics in O2, CO2, H2O and O2/HMDSO low pressure radiofrequency pulsed plasmas by time-resolved optical emission spectroscopy

Abstract

In the present paper, O-atom kinetics is investigated in low pressure (around 0.7 Pa) O2/hexamethyldisiloxane pulsed plasmas by time-resolved optical emission spectroscopy. In order to distinguish direct O-atom excitation by electron impact from dissociative excitation of O2, CO, CO2 and H2O molecules formed in O2/HMDSO plasmas, measurements were first carried out in pure O2, CO2 and H2O plasmas. From these measurements it was concluded that direct O-atom excitation by electron impact was the dominant mechanism and that actinometry could be used to monitor O-atom kinetics provided molecular oxygen dissociative excitation is taken into account. Second, the surface O-atom loss coefficient measured in oxygen pulsed plasmas is in agreement with the literature (10−1) and is five times higher than the one measured in CO2 pulsed plasma (2 × 10−2). This latter result is attributed to competition effect at surface sites between CO and O. Finally, we point out the influence of water molecule wall adsorption on O-atom kinetics in H2O and O2/HMDSO pulsed plasmas. During the post-discharge, H2O and/or OH molecules adsorbed at the reactor walls strongly reduce the O-atom loss coefficient. During the discharge, ion bombardment enhances desorption of molecules leading to an increase in the O-atom loss coefficient.