Characterization of a supersonic plasma source for nanostructured thin films deposition

The controlled growth of nanostructured thin films represents a challenging field of research which is related to many different applications of great scientific relevance. The properties of many materials can be greatly enhanced by optimizing the nanoscale assembly processes: by modelling the nanoparticles which create and assemble a film it is possible to achieve very promising results, although it requires a bottom-up approach capable of tailoring the properties with a high level of control or a complex set-up. Plasma-based synthesis processes have been widely developed and applied for an increasing number of technologies leading to important achievements and many industrial-scale applications, in particular in the field of nanoscience. Plasma Assisted Supersonic Jet Deposition offers a novel approach for nanostructured thin films deposition by combining a reactive plasma with a supersonic jet. An argon-oxygen inductively coupled plasma offers a reactive environment where a metalorganic precursor (titanium isopropoxide for TiO2 depositions) is dissociated and oxidized. The gas is then left to expand from a small orifice into a lower pressure vacuum vessel forming a supersonic jet, where the TiO2 nanoparticles are accelerated onto a substrate by the gas carrier mixture. This deposition technique has proven useful for the deposition of nanostructured thin film having a desired morphology at competitive deposition rates. In order to achieve an effective improvement of the synthesis process, an accurate knowledge of the expanding plasma jet chemistry and physics is of fundamental importance. In this PhD project a deep characterization of the supersonic plasma jet was performed using different diagnostics. The plasma discharge in the reactor was monitored by optical emission spectroscopy, Langmuir probes and the measurement of voltage and current across the antenna of the ICP source. The supersonic plasma jet was characterized using a quadrupole mass spectrometer to sample the gas from the jet at different positions along its axis of symmetry. The detection of neutral species, radicals, ion fluxes and their energy distribution functions led to an understanding of the expanding plasma properties, its composition and its influence on thin films deposition. In addition to this, based on experimental observations, a MATLAB code was developed to reproduce the ion energy distribution functions numerically from first principle calculations. During this project plasma assisted supersonic jet deposition was also operated for the deposition of nanostructured TiO2 samples whose chemical, physical and morphological properties were analysed by FTIR, profilometry, ellipsometry, AFM and SEM.