Diagnostics of N2–Ar plasma mixture excited in a 13.56 MHz hollow cathode discharge system: application to remote plasma treatment of polyamide surface

Abstract

N2–x% Ar plasma gas mixture, generated in a hollow cathode RF discharge system, has been characterized by both optical emission spectroscopy (OES) and double Langmuir probe, as a function of experimental parameters: total pressure (5–33 Pa), and different fractions of argon (7 ≤ x ≤ 80), at a constant applied RF power of 300 W. N2 dissociation degree has been investigated qualitatively by both the actinometry method and the ratio of the atomic nitrogen line emission intensity at 672.3 nm to the vibrational band (0–0) of the N2 second positive system at 337.1 nm. Both methods showed that the increase in argon fraction enhances the dissociation of N2, with a maximum at x = 50 for the pressure of 5 Pa, although the two methods give two opposite trends as a function of total pressure. Spectroscopic measurements showed that the vibrational temperature of the N2 second positive system increases with both argon fraction and total pressure increase, it lies between 4900 and 12 300 K. Langmuir probe measurements showed that, in the remote zone, the electron temperature falls in the range 1.57–1.75 eV, the density varies between 5 × 109 and 1.4 × 1010 cm−3 and that both the plasma ionization degree and electron temperature increase towards the source. In addition, the process of plasma–polyamide (PA) surface interaction, in the remote plasma zone, has been studied through OES analysis during plasma treatment of PA to monitor the possible emissions due to the polymer etching. An increase in atomic nitrogen line (672.3 nm) intensity is obtained, atomic carbon line (833.52 nm) and the band emission (0–0) from the CN (B 2Σ+–X 2Σ+) violet system were observed. The PA surface modification has been confirmed through the improvement of its hydrophilic character as the water contact angle measured after the plasma treatment significantly decreased.