Grain growth in thin Al films during deposition from partially ionized vapor

Abstract

Grain growth in thin Al films during deposition from partially ionized vapor flux with simultaneous self-ion bombardment was studied in this work. The films were deposited at constant ion energy of 940eV and total specific power of 0.4W/cm2 while the deposition time t of 6s to 246s and the resulting substrate temperature (Ts/Tm of 0.35–0.96) were varied. Thin continuous Al films exhibited normal grain growth through the entire experimental range of deposition time without limitation of grain growth by the film thickness effect. Three kinetic stages of the grain growth were observed within 100s of deposition time: the first one exhibits very slow grain growth, accelerated grain growth occurs in the second stage and then it rapidly changes to a retardation and stagnation mode in the third stage. Large average grain sizes Dg up to 11.3μm at film thickness of 1.4μm and integral grain growth rates up to 0.16μm/s were observed in this study. The experimental results were evaluated against various mechanisms of inhibition of grain growth. An estimate of the effective activation energy of the grain growth yields a value of 0.27eV which is lower than that of the bulk Al and much higher than the activation energy of surface self-diffusion on (111)Al monocrystal. The power law Dg=(k t)0.5 gives good match with experimental results in the initial deposition phase preceding the grain growth retardation, while another model that is based on the grain size dependent pinning force adequately explains the entire grain size dependence on time. It is deemed both ion enhanced film/surface interaction and impurities on one side and thermal grooves on another side contribute to the rapid retardation of the grain grooves commencing the second growth stage.