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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.

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Room temperature photoluminescence (PL) from plasma-polymerized hexamethyldisiloxane (PP-HMDSO) thin films deposited on silicon wafers has been investigated as a function of both the applied RF power and the monomer flow rate. Films were deposited in a low pressure–low temperature remote plasma ignited in a 13.56 MHz hollow cathode discharge reactor, using pure HMDSO as a monomer and Ar as a feed gas. The substrate temperature during the deposition was as low as 40 °C and the total pressure was about 0.03 mbar. Optical emission spectroscopy (OES) has been used as in situ tool for monitoring the different chemical species present in the plasma during deposition processes. The deposited PP-HMDSO films showed a strong, broad ‘green/yellow’ PL band. The RF power and the flow rate of the HMDSO monomer are found to have a significant impact on the PL intensity of the deposited film. The changes in the chemical bonding of the film as a function of deposition parameters have been investigated by using the Fourier transform infrared (FTIR) spectroscopic analysis and are related to PL and OES results. The ‘green/yellow’ PL band is ascribed to chemical groups and bonds of silicon, hydrogen and/or oxygen constituting the films, in particular, SiH, SiO bonds and silanol Si–O–H groups.

Plasma diagnostics and Si etching were carried out in 13.56 MHz SF6 remote plasma generated in a hollow cathode discharge system. The plasma diagnostics were performed in the remote zone at a constant pressure of 40 Pa, at a constant applied RF power of 300 W and as a function of the SF6 flow rate (40–2000 sccm), where absolute concentrations of fluorine atoms were measured using actinometry optical emission spectroscopy; electron density, electron temperature and plasma potential were determined using single Langmuir probe, positive ion flux and negative ion fraction were determined using an electrostatic planar probe. The silicon etching process was studied at two distant values of flow rate, 80 and 1800 sccm and for three conditions of the substrate holder, namely the substrate is grounded, the substrate is negatively biased and the substrate is positively biased. The etched silicon was characterized for etch rate, optical reflectance and photoluminescence properties. It was found that the etch rate is relatively high (about 15 mg cm−2 min−1) and it is controlled mainly by the ratio of the ion flux over the reactive atomic fluorine flux, the highest etch rate was obtained at the higher flow rate (1800 sccm) and for positively biased substrate. The reflectance of the silicon surface was significantly reduced after etching and a reflectance as low as 0.2% was measured. A visible photoluminescence from the etched surface was recorded; it is centred at about 600 nm, and its intensity is inversely proportional to the measured reflectance.

Silicon (Si) nanostructures were prepared in the downstream of radiofrequency SF 6 /O 2 mixture plasma generated in 13.56 MHz hollow cathode discharge system. Depending on the oxygen percentage in the mixture, the obtained Si nanostructures were characterized for their different properties: etching rate, morphology, optical reflectance, photoluminescence, spectral response and humidity sensing. It is found that the etching rate exhibits a maximum value when the O 2 ratio reaches 5%. An interesting defect-induced "violet" luminescence is reported from the Si nanostructures, whose intensity depends on their density. The obtained Si nanostructures have shown to induce a spectral response (SR) enhancement, in comparison with a smooth Si substrate, of about 100 times at 1100 nm wavelength. A very short response time (1 sec) to the humidity was measured for 5% O 2 in the SF 6 /O 2 plasma mixture, which was found to be well-correlated with the porosity of the Si nanostructures.

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