A molecular dynamics study on stress generation during thin film growth

XY Zhou and XX Yu and D Jacobson and GB Thompson, APPLIED SURFACE SCIENCE, 469, 537-552 (2019).

DOI: 10.1016/j.apsusc.2018.09.253

Molecular Dynamics (MD) simulations have been employed to model the growth stresses of body-centered cubic (BCC) metal thin films, with tungsten being the primary case study, as a function of various embryonic island textures, grain sizes, grain morphologies, deposition rates, and deposition energies. Depending on the shape and size of the islands, the tensile stress varied as a function of the available contact area. If the adatoms were sufficiently confined to the surface, the tops of these islands initiated the elastic strain for coalescence. This was particularly relevant for island morphologies that had varied curvature gradients near the contact points. Depending on the texture of the film, the roughness changed, with the <1 1 1> orientation being the roughest and <0 0 1> orientation being the smoothest. These topologies are explained by differences in surface diffusivities. The injection energy of adatoms was found to have a dramatic effect on film stress. Species with injection energies in excess of 50 eV resulted in a notable increase in the structural disorder at the grain boundary-free surface intersection which corresponded to a reduction of the tensile stress. Upon ceasing deposition, these disordered regions experienced a recovery to the BCC structure with an increase in the tensile stress. Upon resuming deposition, at the same energies, the disordered structure re- developed and the stress became less tensile and matched the prior deposited stress evolution. Finally, reducing the grain size resulted in an increase in tensile stress up to a critical size, whereupon it decreased. This reversion is explained in terms of grain growth and grain boundary structure during deposition. Through these series of systemically controlled MD simulations, the paper addresses the significance of different microstructures on the evolution of thin film stress.

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