Effects of Substrate Bias and Ar Pressure on Growth of α-phase in W Thin Films Deposited by RF Magnetron Sputtering

It is commonly observed that tungsten (W) thin films deposited by sputtering at room temperature have a metastable beta-phase with relatively high resistivity and transform into a stable alpha-phase with relatively low resistivity under certain conditions. In this study, to obtain W films with low resistivity suitable for interconnect materials for semiconductors, we tried to identify deposition parameters for the formation of alpha-phase W by varying the substrate bias and argon (Ar) pressure in a radio-frequency (RF) magnetron sputtering system. In the initial stage for 1 s of deposition under 20 mTorr and a substrate bias of - 100 V, beta-phase W nanoparticles were observed using transmission electron microscopy (TEM). However, as the deposition time increased to 10 min under the same pressure and bias condition, the W film became a mixture of alpha- and beta-phases. The fraction of alpha-phase W increased further as the negative bias increased from - 100 to - 200 V. In addition, the film density increased and the surface roughness decreased as the bias changed from + 100 to - 100 V. These results indicate that the negative bias triggered the phase transformation of W from beta to alpha. The bias effect on the formation of alpha-phase W and film resistivity became more pronounced as pressure increased.