Reactivity of a DC-pulsed plasma: plasma diagnostics and nitrided sample analysis

The growing use of pulsed discharge in surface treatment processes requires time-resolved plasma diagnostics in order to determine the better conditions leading to the creation of a high density of active species in the vapour phase. A low frequency DC-pulsed nitriding plasma has been studied by means of optical emission spectroscopy and Langmuir probe measurements; both of these diagnostics are carried out with a time resolution of 1 mu s across the plasma period. The analysis of the plasma by an electrostatic probe allows us to determine the time variation of the electron density and energy distribution function. From these results we show the importance of the vibrational excitation of the nitrogen molecules during the afterglow and we explain some fundamental kinetic processes leading to an enhancement of the plasma reactivity. On the other hand, the spectroscopic measurements performed as a function of various plasma parameters (pressure: discharge and afterglow duration) lead to the determination of some plasma conditions that seem to improve the nitriding process. These favourable plasma conditions are discussed with regard to the characterization of nitrided samples. (C) 1997 Elsevier Science S.A.