Phase stability and in situ growth stresses in Ti/Nb thin films

The thin film growth evolution associated with changes in Ti's body-centered cubic (bcc) to hexagonal close-packed (hcp) phase transformations in Ti/Nb multilayered thin films is addressed. An in situ laser interferometer curvature measurement technique was used to monitor the intrinsic growth stresses for a series of these multilayers, with each multilayer having a different bilayer spacing but equivalent individual layer thickness. The initial Ti layer grows on Nb with a positive stress-thickness product slope up to ∼2 nm, whereupon it transitions to a slightly negative growth stress slope. This transition has been associated with the bcc to hcp Ti transformation. The Nb growth exhibited a significantly steeper stress-thickness product slope regardless of the Ti phase state. The decreasing interfacial stress between the two layers contributed to a collectively more compressive stress state for the multilayer. Atom probe tomography revealed Ti segregation to the columnar grain boundaries and significant Nb intermixing into the bcc Ti layer, which is rationalized as a surface exchange process driven by interfacial thermodynamic considerations. Using a molecular dynamics deposition simulation, this intermixing was found to be paramount in stabilizing the bcc Ti layer to larger layer thicknesses.