The effect of metallic contamination on the dielectric breakdown of thermal oxide layers is investigated. Wafers were intentionally contaminated with Ca, Zn, Fe, Cu or Al. The oxidation behavior of the contaminants and their effect on the Si surface roughness were investigated and correlated with the oxide breakdown properties. It was observed that Ca interacts strongly with the Si substrate during ramp-up. This results in a large increase in the Si surface roughness and poor breakdown properties of the thermal oxide layer. Fe degrades the oxide integrity by the formation of defect spots during the oxidation and Al induces damage under the SiO2/poly-Si interface. Metals such as Cu and Zn diffuse easily in the Si substrate and, consequently, do not have a large impact on the oxide quality. Standard grade chemicals are the main source for the metallic impurities. After switching to ultrapure chemicals, the DI-water distribution system becomes the limiting factor. Some techniques to reduce the contamination and Si-surface roughening are presented.