Fabrication et analyse de revêtements de nitrure de silicium déposés par plasma pour de nouvelles applications optiques

In the present thesis, innovative methods and strategies are proposed for the plasma-enhanced chemical vapor deposition (PECVD) of thin films in order to tune their physical properties (e.g. optical constants, optical gap, etc.), by relying on the control and understanding of plasma characteristics and plasma-surface interactions rather than on the traditional variation of the gas mixture composition. The following paragraphs summarize the applied methodology and the main results, presented in the form of three articles at the core of this thesis, plus an additional chapter devoted to sensor fabrication and testing. (1) In the first article, we propose a new method of fabricating a-SiN_x:H alloys by pulsing the radiofrequency (RF) signal during the PECVD process. Spectroscopic ellipsometric analysis in the UV-VIS-NIR and FIR ranges, atomic force microscopy, and elastic recoil detection reveal strong variations in the optical constants (1.88 ≤ n ≤ 2.75, 10^-4 ≤ k ≤ 5 × 10^-2 at 550 nm), optical gap (4.01 eV ≥ E ₉ ≥ 1.95 eV), microstructural characteristics (1.3 nm ≤ surface roughness ≤ 8.3 nm), and chemical composition (0.47 ≤ x ≤ 1.35) of the coatings as a function of duty cycle. Using solely the control of duty cycle, we fabricate two types of a-SiN_x:H-based thin film devices, namely (i) a model Fabry-Perot optical filter deposited on a plastic substrate, and (ii) a superlattice structure displaying a photoluminescence signal four times as large as the reference single layer. (2) In the second article, transparent hydrogenated amorphous silicon nitride coatings are prepared by dual-mode microwave/radiofrequency PECVD. By controlling the effects of plasma density and ion energy on the film growth, it is possible to modify the microstructure of the coatings and hence the refractive index, n. Using this method, we are able to vary n from 1.6 to 2.0 at 550 nm, simply by adjusting the power levels of the radiofrequency and microwave components. (3) In the third article, surface treatments are performed on porous layers using argon and nitrogen RF plasmas in order to densify and flatten their surface, and hence to obtain an abrupt transition between porous and dense films. We show that besides the densification effect, preferential sputtering and annealing phenomena also occur during plasma treatments at high bias values ([special characters omitted] > 400 V), leading to silicon enrichment at the film surface and chemical stabilization of the film bulk. Using atomic force microscopy, we observe a significant reduction of the surface roughness after treatment of the single layers (≈ 70% reduction) and multilayer stacks (≈ 60% reduction). (4) In the next chapter, we demonstrate the concept of using nanoporous multilayer optical filters as optical gas sensors, based on the plasma-enhanced chemical vapor deposition of Si₃N₄ films with controlled porosity. In situ real-time spectroscopic ellipsometry, associated with the effective medium approximation is employed for characterizing the film microstructure, particularly the total volume fraction of porosity (f_v), and the fraction of open porosity (f_o). In order to do so, the effective refractive index (n_eff) of the as-grown films is evaluated both in vacuo and when exposed to ethanol vapor.