The critical role of substrate bias for the sputter deposition of molybdenum thin films

Abstract

In this study, the critical role of substrate bias during sputter deposition of Mo thin films is discussed. Two sets of Mo thin films having a nominal thickness of 300 nm were prepared at sputtering pressures of 0.55 and 0.33 Pa, with applied negative substrate bias in the range of 0–250 V. While the lateral grain size of ~15 nm was observed for all the Mo films, grain structure, texture, residual stress and resistivity were greatly altered by the combination of bias magnitudes and sputtering pressures. The sputtering pressure of 0.55 Pa in the absence of substrate bias produced substantially high film resistivity of 56.9 μΩ-cm, which can be attributed to the relatively open grain boundary structure formed by the shadow effect for low-mobility Mo atoms. Applied substrate bias reduces the density of open grain boundaries resulting in an abrupt (by more than a factor of two) resistivity reduction to 24.5 μΩ-cm, with the concurrent rise in tensile film stress from 0.2 to 1.3 GPa. The films deposited at 0.33 Pa contain dense grain boundaries and only a modest dependence of film resistivity in the range of 13.8–20.8 μΩ-cm on the substrate bias was observed, for which grain boundary scattering is the dominant scattering mechanism for the resistivity increase relative to the known bulk resistivity. The films deposited at 0.33 Pa possess compressive stress of 0.4 GPa in the absence of substrate bias, which increases up to 2.2 GPa with the applied substrate bias of 250 V.