Browsing by Subject "Impurities"

One of the main goals of materials research is to find the link between the properties of materials and their fundamental structures. The distinct properties of thin films, categorized as materials from a few single layers of atoms to some hundreds of nanometers, have enjoyed an unparalleled demand in modern device manufacturing, and thus the investigation of factors which determine thin film structure and morphology is a vital area of research. In the case of thin films, their final structures can often be connected back to the initial film formation stages, such as in the crystallographic growth competition during island growth and coalescence. In this thesis, thin film growth stages are studied from the perspective of how they are affected by impurities. From the initial diffusion of adatoms on a bare substrate; to the formation of islands, their growth and coalescence; to the mobility of grain boundaries and bulk diffusion leading to the formation of a fully continuous layer; impurities influence each of these thin film growth processes in a multifaceted way, either acting as growth inhibitors, promoters or potentially neutral agents. To this end, Ag and Cu thin films were synthesized by magnetron sputtering onto SiO_2/Si substrates, with thicknesses ranging from 3 nm to 30 nm using varied deposition conditions, with the addition of a 3 nm amorphous carbon layer to limit further restructuring and oxidation. Impurities were let into the deposition atmosphere via a controlled opening of a leak valve, corresponding to a step-wise increase of base pressure from 10^(-8) Torr to 10^(-6) Torr and finally 10^(-5) Torr. The full range of thin films was deposited with each base pressure (except for 10^(-5) Torr for Cu) using two deposition rates, around 0.1 Ås^(-1) and 2 Ås^(-1). Each film was characterized ex situ with ellipsometry, 4PP, XRD and AFM to map the morphological and microstructural evolution during film growth. It is found that impurities tend to inhibit island coalescence and initial grain growth, resulting in a reduction of continuous film formation thickness and average grain size, leading to the formation of flatter films with, in most cases, less surface roughness. In later stages, it is found that impurities may allow for more grain growth by their incorporation into the growing facets. In terms of crystal structure, it is shown that impurities have a more pronounced effect on (111) oriented grains, inhibiting their growth, thus altering the preferred growth orientations of Ag and Cu by allowing (200) grains to grow larger. Grain radii and equivalent ellipse distributions showed the different responses of Ag and Cu to impurities. Ag films showed more prominent effects when a lower deposition rate was used, highlighting the impact of impurities on diffusive processes, while Cu films exhibited more effects with the use of higher deposition rates, indicating that the role of impurities, in this case, was more significant after the formation of a continuous layer.