Experimental study and kinetic modelling of chemical vapor deposition process of silicon oxide and oxynitride thin films for aqueous corrosion barriers
The deposition of silica-based materials is widely used in numerous industrial sectors, including microelectronics, food packaging, gas separation and pharmaceutics. Depending on the targetapplication, these materials are required to fulfil specific requirements in terms of mechanical properties, durability, and composition. The implementation of such coatings in pharmaceutics applications require, more specifically, good aqueous barrier and anti-diffusion properties, as well as effective corrosion resistance. The hydrolytic resistance and the durability of the coatings is directly linked to the level of densification of the ceramic network. In the case of amorphous SiO2, an improved network cross-linking, and by consequence densification, can be induced through the partial replacement of the O2- anions by N3- (or C4-) ones, producing denser amorphous silicon oxynitride (SiOxNy) or silicon oxycarbide (SiOxCy) coatings that can meet the performance requirements dictated by the various pharmaceutical applications. However, very little information is available in the literature concerning the deposition of SiOxNy coatings in accordance to the application-specific constraints: namely the production of chemically inert films on complex, 3D substrates, deposited at atmospheric pressure and at moderate temperatures (<570°C), with high deposition rates. To achieve the above goals, the deposition of amorphous silica-based films is undertaken via the utilization of a thermal CVD process defined around reactive, novel chemistries. The reactive chemical pathways aids in the production of SiOxNy at temperatures lower than the conventional ones, and more importantly, compatible with thermosensitive substrates. The gradual increase in N and C contents in the deposits is carried out through carefully selected precursor molecules and reagent gas compositions. Innovative deposition routes, based on single or dual-precursor combinations of classic silicon-containing precursors such as tetraethylorthosilicate (TEOS) or hexamethyldisilazane (HMDS), and more novel compounds such as tris(dimethylsilyl)amine (TDMSA) are explored. Since the progressive incorporation of nitrogen and carbon in the films is at the core of this work, the resulting evolution of the silicate network is extensively studied through physicochemical, structural and mechanical characterization protocols. A multidisciplinary approach is embraced, combining materials science, analytical chemistry and process engineering in a way that involves the simultaneous development of resistant barrier films through CVD experiments, alongside gas phase analysis, simulation and numerical computation. The experimental information obtained through this approach is utilized in order to enrich previous literature models or define completely novel deposition mechanisms. Through 3D representation of the reactor spatial domain, the developed chemical models are implemented in order to replicate the deposition process via simulation. Computational fluid dynamics (CFD) calculations are used to understand the particularities of film formation in confined spaces and complex 3D-parts, obtain predictions in terms of gas phase and solid phase composition, as well as investigate potentials and solutions for optimization of the deposition process. The established correlations between process conditions, films structure, composition and properties, alongside the integration of a coupled computational and experimental approach, are used to arrive at durable silica-based materials with sustainable barrier performance, flexible for utilization in diverse application domains
