Study of Thermal and Plasma Enhanced Atomic Layer Deposition of AlN and Al2O3/TiO2 Films for Diverse Applications

Atomic layer deposition (ALD) has become widely utilized and researched technique for highly conformal and precise thickness controlled thin film fabrication. Due to these inherent advantages, ALD has been applied for diverse applications such as memories, logic and solar cells etc. This thesis presents research of certain plasma enhanced ALD (PEALD) and thermal ALD thin films with a focus on the effect of process parameters on the material properties. Mechanical properties of PEALD aluminum nitride films (mainly trimethylaluminum (TMA) and ammonia processes) were studied and supported with structural and compositional analysis. The study showed that higher deposition temperature and capacitively coupled plasma bias voltage improved density and impurity incorporation of the films, consequently positively influencing coating hardness and elastic modulus. Also, tensile stress magnitude was found to increase with temperature and high bias voltage lead to slightly compressive film. Moreover, adhesion and tribological behavior were determined but no marked correlation to process conditions were established. PEALD AlN was also studied with an inductively coupled plasma source and TMA/N2:H2 precursor chemistry with a focus on structural and compositional annealing (400–1000 °C) behavior of the films. Results showed outgassing of hydrogen, slight oxidation despite the vacuum environment and likely hydrolysis of the AlN films due to annealing. The films maintained their amorphous structure and their thickness reduced. The third studied AlN PEALD chemistry of AlCl3/NH3 employed a high process temperature (~500 °C). The developed process produced (002) preferential crystallinity, very low impurity concentration and high mass density films. In addition, the residual stresses of the films were found controllable by plasma exposure time and varied from slightly tensile to strongly compressive. ALD of consecutive bilayers of Al2O3/TiO2 (aka a nanolaminate) was investigated together with a physical vapor deposition (PVD) hard coating for wear resistant anticorrosive protection of steel. First, the corrosion resistance capability of the ALD nanolaminate improved with increased thickness and with plasma pretreatments which increased coating adhesion. Endurance against detrimental corrosion current was increased with two orders of magnitude with these hybrid films in comparison to the bare steel. In the follow-up study, the hybrid film samples were exposed to subsequent wear-corrosion environments showing that the anticorrosion protection was maintained at a high level despite the top-most ALD nanolaminate was completely removed. It can be concluded that the ALD nanolaminate encapsulates conformally the PVD coatings pinholes and defects which act as the main pathways for corrosion currents.