SITE-SELECTIVE ATMOSPHERIC PRESSURE PLASMA-ENHANCED CHEMICAL VAPOUR DEPOSITION PROCESS FOR MICROMETRIC DEPOSITION – SIMULATION AND EXPERIMENTAL STUDY

Field: Chemical Engineering (Physical Sciences)

Atmospheric Pressure Plasma has been used to enhance and/or initiate the Chemical Vapour Deposition (AP-PECVD) to deposit thin films or functional layer coatings over a large surface area on a large range of substrates. Now an ability to localise the AP-PECVD coating on an area of interest and control the deposition’s dimension showed its potential application as a viable technique to perform Additive Manufacturing (AM). Additive Manufacturing (AM) is a bottom-up approach in which 2-D patterning or 3-D structures are built using a layer-by-layer deposition. AM allowed easy design optimization and quickly provided the customized parts on demands, thus making itself a very popular technique in the mainstream manufacturing process. As such, it has a wide application in automotive, optics, electronics, aeronautics, medical and biotechnology fields. However, the existing AM printing techniques have some limitations regarding high-resolution printing deposition in a wide variety of substrates and very often get restricted to the types of precursors that could be printed. Whereas, due to the high energetic/reactive species in non-thermal plasma, the AP-PECVD deposition has been obtained using a wide range of precursors on a versatile surface. Thus, there has been a growing interest in performing an area selective localised AP-PECVD coating, mainly by adapting the design of the PECVD reactor. Hence, this thesis aims to design, optimize and study a one-step mask-free AP-PECVD plasma process that could locally deposit the material of interest with high precision to perform AM. In the thesis, the technical approach undertaken by the home-built prototype “plasma torch” is to decouple the plasma generator annular tube and the precursor injector central capillary. This approach has allowed a way to tune the diameter of the deposited dot by changing the dimension of the precursor injector, which has been demonstrated by the deposition of the micro-dot as small as 400µm in diameter. Further, the flexibility to move the capillary tube without significant changes in the plasma torch's overall geometry has also allowed for selectively injecting the precursor (Methylmethacrylate, MMA) in the spatial plasma post-discharge region. Thanks to this setting, the deposited dot has high retention of monomer's chemistry (functional group) and unprecedented molecular weights (oligomeric chain up to 18 MMA units). Hence, initially, a novel area selective AP-PECVD plasma torch design has been demonstrated, and its performance has been defined to obtain the micro resolution coating. During the research work, gas flow rates have been identified as a crucial parameter in obtaining the localised coating; three kinetic regimes with different coating morphology have been discovered. By performing a thorough computational fluid dynamic (CFD) simulation of the torch phenomena, it has been possible to establish a parallel between the fluid behaviour and the deposition size. The deposition was found to be confined in a zone created by the dynamical behaviour of gas, i.e., re-circulating vortices between the torch and substrate. Hence, later the gas flow rate was used to tune the diameter of the confinement zone, which in return changed the diameter of the deposited dot. The gas flow dynamic impacts the involved species, i.e., reactive plasma species, precursor molecules, and the open-air interaction and distribution on the surface of the substrate. When organosilicon precursors with the presence or absence of vinyl bond and/or ethoxy groups are used, it results in different depositional chemical reactions and depositional patterns. The correlation between the depositional patterns and the mass fraction distribution of involved species has been obtained thanks to the performed CFD simulation done in parallel. Further, the likelihood of deposition mechanisms like "vinyl group opening by free radical" for vinyl containing precursor resulting in silicon oxycarbide-like (SiOxCyH) structural deposition, and the Reactive Oxygen Species (ROS)-induced "fragmentation and adsorption" deposition mechanism resulting in silica SiOx like structural deposition for siloxane containing precursor has been suggested and discussed. The understanding gained from this systematic case study implies the importance of reactive plasma species in the underlying deposition mechanisms; hence, it has been suggested that tuning/tailoring its distribution can alter the chemical nature of deposition and its pattens. Overall, this thesis work provides insight into area selective AP-PECVD coating (plasma printing) and demonstrates that plasma technology is a viable option for additive manufacturing. The findings would be helpful in both designing the AP-PECVD plasma torch and selecting precursors for the desired organic/inorganic deposition. Thanks to the insight gained during the thesis work, the home-designed prototype of the plasma torch has been upgraded to implement in a commercial 3-D printer.