Fundamental processes in thin film growth The origin of compressive stress and the dynamics of the early growth stages

Abstract: The fundamental mechanisms behind the generation of compressive stresses in polycrystalline thin films, the effects of pulsed deposition fluxes on the dynamics of the early growth stages as well as the generation of energetic Ar+ ions in high power impulse magnetron sputtering (HiPIMS) discharges has been studied in this thesis.It was found that compressive film stresses in Mo films deposited using energetic vapor fluxes are correlated with high film densities while only a slight lattice expansion compared to relaxed Mo was found. This implies that the stress is caused by grain boundary densification and not defect creation in the grain bulk. The compressive stress magnitude should scale with the grain boundary length per unit area, or the inverse grain size, if the stress originates in the grain boundaries. This was found to be the case for dense Mo films confirming that the observed compressive stresses originate in the grain boundaries. Similarly to what has been suggested for conditions where adatoms are highly mobile we suggest that atom insertion into grain boundaries is the cause of the compressive stresses observed in the Mo films.Island nucleation, growth and coalescence are the dynamic processes that decide the initial microstructure of thin films growing in a three dimensional fashion. Using Ag on SiO2 as a model system and estimations of adatom life times and coalescence time it was shown that the time scales of island nucleation and coalescence are in the same range as the time scale of the vapor flux modulation in HiPIMS and other pulsed deposition methods. In situ real time measurements were used to demonstrate that it is possible to decrease the thickness at which a continuous film is formed from 21 to 15 nm by increasing the flux modulation frequency. A more in depth study where in situ real time monitoring was coupled with ex situ imaging and kinetic Monte Carlo simulations showed that this behavior is due to the interplay of the pulsed deposition flux and island coalescence where island coalescence is hindered at high pulsing frequencies.The generation of energetic Ar+ ions was investigated by ion mass spectrometry and Monte Carlo simulations of gas transport. It was shown that the energetic Ar+ ions originate from Ar atoms backscattered from the target that are ionized in the plasma by correlating the length of the high energy tail in the ion energy distribution functions with the atomic mass of the Cr, Mo and W sputtering targets.