Real-time stress evolution during early growth stages of sputter-deposited metal films: Influence of adatom mobility

Using a multi-beam optical stress sensor, the real-time stress evolution during the early growth stages of a large class of sputter-deposited metal (Me) films is studied with monolayer sensitivity. For high-mobility fcc (Ag, Au, Pd) metals, a typical compressive-tensile-compressive (CTC) behavior is observed, characteristic of a Volmer-Weber growth mode. A correlation between the homologous temperature (T-s/T-m), tensile stress peak position, grain size and steady-state compressive stress in the post-coalescence stage is presented. For low-mobility bcc (Mo, W, Ta) metals (T-s/T-m <= 0.10) deposited on a-Si, kinetic limitations result in a 2D growth mode highly influenced by interfacial effects. The film force is initially dominated by change in surface stress, which scales with the surface energy difference Delta gamma = gamma(Me) - gamma(a-Si). For both Mo and W, a stress transient is observed in the 2-4 nm range, followed by the development of unexpectedly large tensile stress, ascribed to a phase transition towards their equilibrium alpha-Mo and alpha-W structure. Such transient is not evidenced during Ta growth for which a compressive stress regime is steadily established and related to the growth of its metastable beta-Ta structure. For all low-mobility metals, the final stress regime is controlled by the energetics of the incoming species and intrinsic mechanical properties of the material. (c) 2013 Elsevier Ltd. All rights reserved.