Investigation of nitrogen atom production in Ar/N{sub 2} and He/N{sub 2} surface wave plasmas

Full text: There is presently great interest in nitrogen plasmas for surface coating processes. Such as the deposition of nitride thin films and surface treatment of materials. Indeed, nitrogen plasmas have been used to nitride the surface of ferrous and non-ferrous materials in order to improve their surface properties such as resistance to corrosion and hardness. Moreover, the design and development of nitrogen atom sources could be essential for the synthesis of gallium nitride (GaN), a wide band-gap semiconductor whose properties have revolutionized the microelectronics and optoelectronics industries. Correlations have been established between the density of active species in the process, namely atomic nitrogen (N) produced by the discharge and GaN film properties. Thus, it is of fundamental importance to investigate the N-atom production mechanisms in such discharges. N-atom production has been studied in pure N{sub 2} surface-wave plasmas (SWP), as a function of operating parameters, namely gas pressure and electrical power. These studies indicate that the increase in the gas temperature (T{sub g}) limits the N-atom production. One possible way of enhancing the N{sub 2} dissociation rate ([N]/[N{sub 2}]) in the plasma could be the use of gas mixtures such as Ar/N{sub 2} or He/N{sub 2}. the aim of this paper is to characterize an Ar/N{sub 2} and He/N{sub 2} surface-wave discharge (SWD) by optical emission spectroscopy (OES), in order to determine the optimal plasma conditions in terms of [N]/[N{sub 2}]. The plasma is generated by a radio frequency (40.68 MHz) wave launcher. The effect of mixing N{sub 2} with Ar and He on the production of N-atoms in the plasma was investigated at varying experimental conditions, such as operating pressure (4.5 and 7.5 Torr), electrical power (40 to 120 W), at a total gas flow of 250 sccm. It was found that [N]/[N{sub 2}] increases with the partial pressure of Ar in the mixture by a factor of about 8 at 120W. Such an enhancement is reduced at lower incident powers. On the other hand, the effect of He on [N]/[N{sub 2}] is much different than for Ar, as also shown. A maximum in [N]/[N{sub 2}] is obtained for mixtures containing 50% of He in N{sub 2}. At higher He percentages, He has a detrimental effect on the nitrogen dissociation rate. Investigation of the effect of both Ar and He on T{sub g} is being conducted, in an attempt to explain the observed variations in [N]/[N{sub 2}]. The use of gas mixtures is observed to yield plasmas that have very different characteristics (electronic density and gas temperature) than those of a pure gas plasma, thus leading to changes in the kinetics of the active species production in the plasma.