Pressure-dependent transition from atoms to nanoparticles in magnetron sputtering: Effect on WSi2 film roughness and stress

We report on the transition between two regimes from several-atom clusters to much larger nanoparticles in Ar magnetron sputter deposition of WSi2, and the effect of nanoparticles on the properties of amorphous thin films and multilayers. Sputter deposition of thin films is monitored by in situ x-ray scattering, including x-ray reflectivity and grazing incidence small angle x-ray scattering. The results show an abrupt transition at an Ar background pressure Pc; the transition is associated with the threshold for energetic particle thermalization, which is known to scale as the product of the Ar pressure and the working distance between the magnetron source and the substrate surface. Below Pc smooth films are produced, while above Pc roughness increases abruptly, consistent with a model in which particles aggregate in the deposition flux before reaching the growth surface. The results from WSi2 films are correlated with in situ measurement of stress in WSi2/Si multilayers, which exhibits a corresponding transition from compressive to tensile stress at Pc. The tensile stress is attributed to coalescence of nanoparticles and the elimination of nano-voids.