Dear MEMSNet colleagues, I am currently having an RIE issue. I am using Cu (copper) hard mask to etch patterned polymer with oxygen gas, but the etch rate is very low. When I try to etch the same material with Au mask it works fine (high etch rates). The same has happened when using Ni (nickel) hard mask and oxygen gas. Is this some sort of catalytic reaction? If so, can anybody explain a bit more in detail (just the principle, no mathematical models). Thank you. Ghazal Hakemi PhD candidate Microsystems and Nanotechnology Centre Cranfield University Bedord, U.K.