The chemical interaction of Al on a SiO2 dielectric layer and the addition of Al into Ru thin films on SiO2 for use as copper diffusion barrier layers are assessed in situ using X-ray photoelectron spectroscopy. Thin (∼1-2 nm) Al films were deposited on a SiO2 substrate and in a separate experiment on a 3 nm Ru liner layer on SiO 2, and both Al/SiO2 and Al/Ru/SiO2 structures were subsequently thermally annealed. Results indicate the reduction of SiO 2 and the subsequent formation of Al2O3 with the release of Si from the dielectric. The Al/Ru/SiO2 structure showed evidence for the diffusion of Al through the Ru layer and the subsequent interaction of the Al with the underlying SiO2 dielectric to form Al2O3. In this case, the reduction of SiO2 leads to the release of Si from the dielectric and the subsequent chemical interaction of Ru with Si. © 2014 Published by Elsevier B.V.