Controlled nanostructuration of polycrystalline tungsten thin films

Abstract

Nanostructured tungsten thin films have been obtained by ion beam sputtering technique stopping periodically the growing. The total thickness was maintained constant while nanostructure control was obtained using different stopping periods in order to induce film stratification. The effect of tungsten sublayers' thicknesses on film composition, residual stresses, and crystalline texture evolution has been established. Our study reveals that tungsten crystallizes in both stable α- and metastable β-phases and that volume proportions evolve with deposited sublayers' thicknesses. α-W phase shows original fiber texture development with two major preferential crystallographic orientations, namely, α-W⟨110⟩ and unexpectedly α-W⟨111⟩ texture components. The partial pressure of oxygen and presence of carbon have been identified as critical parameters for the growth of metastable β-W phase. Moreover, the texture development of α-W phase with two texture components is shown to be the result of a competition between crystallographic planes energy minimization and crystallographic orientation channeling effect maximization. Controlled grain size can be achieved for the α-W phase structure over 3 nm stratification step. Below, the β-W phase structure becomes predominant.