Effect of the HF Wave Frequency in the N-atom Production in \(N_2\) HF Plasmas: Application to CN Thin Films Deposition

Abstract

Variations of N atom density have been measured by emission spectroscopy, calibrated using NO titration, in the N₂ late and pink flowing afterglows as a function of the wave frequency f used to generate Surface Wave (SW) plasma. It is found that [N] increases with the absorbed power P_A but saturates above a threshold power which decreases with f. At 2450 MHz and a constant pressure of 4 Torr, [N] reaches 1.5 x 10¹⁵ cm ^-3 in the saturation regime while a saturated value of only 7 x10 ¹⁴ cm ^-3 is attained at 13.56 MHz. The density variations of [N] as a function of f and P_A correlate in the increasing region with that of the vibrational temperature as a result of electron collisions and in the saturated region with the gas temperature when it exceeds 500-600 K. Finally, [N] increases with pressure from 2 to 8 Torr (at constant P_A of 100 W) and decreases with the tube diameter (6 to 15 mm), at constant pressure of 4 Torr. These results show that the highest N-atom density is obtained at 2450 MHz.

A source of N-atoms produced by such N₂ afterglows was combined with laser ablation of graphite for CN_X thin films deposition. The XPS and IR analysis of the films have shown that the presence of nitrogen atoms in the post-discharge enhances the incorporation of N into the CNx layers. The N incorporation increases as pressure is increased through the formation of hydrogenated carbon nitride groups while at low pressure of about 10^-3Torr, unhydrogenated sp² and sp³ CN bonds are produced in amorphous forms.