Spectroscopie d'émission VUV-visible provenant de plasmas basse pression: Applications aux traitements de surfaces polymériques

The main objective of this thesis is to make a contribution to the spectroscopic study of low pressure glow discharges in the vacuum ultraviolet and to reveal the effect of this radiation on polymeric materials. This research considers the following important aspects relevant to low pressure glow discharges: (i) characterization of vacuum ultraviolet (VUV) emission of several plasmas in the wavelength range available through crystalline fluoride windows, (ii) analysis of the emission dependence on several external parameters, and (iii) study of the VUV effect, in the accessible range, on polymeric materials of interest with respect to plasma treatments.

The plasmas investigated, namely, hydrogen, oxygen and their mixtures with argon, were chosen by taking into consideration the composition of the gases used in technological plasmas, and the conditions needed for polymeric material irradiation. Two separate spectrophotometric instruments, a VUV monochromator (ARC VM-502) and an optical multichannel analyzer (OMA), were used.

For the chosen plasmas, we measured the variation of the intensity of the atomic lines for different gas pressures and absorbed power levels. In the case of pure gas plasmas, the pressure dependence of the atomic line intensities was attributed to changes in the electron energy distribution function (EEDF).

The argon emission spectra are very sensitive to the presence of impurities. The mixture plasmas we have studied (Ar-H2 and Ar-O₂) show a strong emission of atomic fines in the VUV region, stronger than those resulting from the pure molecular gases. The above mixtures show promise as photon sources for the treatment of polymer surfaces.

An important external parameter for the characterization of glow discharges is the excitation frequency. The goal of the specific experiments was to directly investigate the frequency-dependence effect.

The frequency-related effects on plasma emission were investigated for two plasmas: pure hydrogen and 7% H₂ in Ar mixture. In the case of pure hydrogen, we observed a change from a non-stationary to a stationary electron energy distribution function. For the Ar-H₂ mixture, we have shown that the EEDF is stationary and that the dissociation rate for hydrogen molecules does not depend on frequency. A difference between the excitation mechanisms of different atomic lines was proven.

Among the most sensitive materials with respect to plasma generated UV radiation are polymers. We have optimized the gaseous composition for various types of VUV radiation fluxes for the photochemical treatments of polymers.

We have developed a system containing a thermo-stabilized quartz crystal microbalance (QCM) and chambers for sample irradiation and gas absorption measurements. In this way we have separately measured the effects of both the active oxygen species (AO) and the VUV, as well as their synergistic actions. (Abstract shortened by UMI.)