Abstract
Vapor-based metal film growth at conditions that promote high atomic mobility is typically accompanied by compressive stress formation after completion of island coalescence, while an apparent stress relaxation is observed upon deposition interruption. Despite numerous experimental studies confirming these trends, the way by which growth kinetics affect postcoalescence stress magnitude and evolution is not well understood, in particular, for sputter-deposited films. In this work, we study in situ and in real-time stress evolution during sputter-deposition of Ag and Cu films on amorphous carbon. In order to probe different conditions with respect to growth kinetics, we vary the deposition rate F from 0.015 to 1.27nm/s, and the substrate temperature TS from 298 to 413K. We find a general trend toward smaller compressive stress magnitudes with increasing TS for both film/substrate systems. The stress-dependence on F is more complex: (i) for Ag, smaller compressive stress is observed when increasing F; (ii) while for Cu, a nonmonotonic evolution with F is seen, with a compressive stress maximum for F=0.102nm/s. Studies of postdeposition stress evolution show the occurrence of a tensile rise that becomes less pronounced with increasing TS and decreasing F, whereas a faster tensile rise is seen by increasing F and TS. We critically discuss these results in view of ex situ obtained film morphology which show that deposition-parameter-induced changes in film grain size and surface roughness are intimately linked with the stress evolution.
Highlights
The evolution of stress—with respect to its type and magnitude—in vapor-deposited films is closely linked with the various film growth stages.[1,2,3,4] Isolated islands that form initially on the substrate surface exhibit a smaller-than-equilibrium lattice parameter due to the Laplace pressure they are subjected to.[5]
atomic force microscope (AFM) was used to study the morphology of Ag and Cu films deposited at various Th and F values, and the respective height–height correlation functions g(r) were calculated, as explained in Sec
Flötotto et al.[30] found faster tensile rise with increasing F and for larger grain sizes. They explained this trend in light of different shapes of grain boundary (GB) grooves during and after deposition: (i) during deposition, the driving force for adatom diffusion to the GB is large, allowing adatoms to overcome the accumulation of steps close to GB (i.e., Zeno effect65), which results in GB grooves that are more shallow than the equilibrium surface profile; (ii) while after deposition, the driving force for atom incorporation into GB decreases dramatically, the grooves deepen as adatoms attach to steps, and a shape closer to the equilibrium surface profile is attained.[21,63]
Summary
The evolution of stress—with respect to its type and magnitude—in vapor-deposited films is closely linked with the various film growth stages.[1,2,3,4] Isolated islands that form initially on the substrate surface exhibit a smaller-than-equilibrium lattice parameter due to the Laplace pressure they are subjected to.[5] An increase of the island size with continued deposition causes a reduction of the Laplace pressure, while the islands become less mobile and less prone to reshape As a result, their lattice cannot expand following the decrease of the Laplace pressure, and compressive stress emerges. We attribute the change in magnitude of the tensile rise to the decreasing grain size
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