What do you mean by 'contamination'? If you mean surface contamination, then the reason should be evident: it prevents direct contact between the metal surface and the oxygen atoms (that is why metals are generally painted).

Can you please clarify the question, in the sense that whether you are talking about some catalytic conversion process or any other application?

Sorry for any confusion. In microfabrication process, in oxidation (wet and dry process), metal contamination is avoided. I know metal contamination in oxide layer enhances leakage current, reduces dielectric properties and acts as an antenna to catch noise which reduces the device sensitivity. But I like to know more causes in depth about oxidation on Silicon wafer. Thanks for your time. I appreciate your expertise on any pros and cons in oxidation process in microfabrication process.