A plasma ashing process for removing photoresist, polymers and/or residues from a substrate comprises placing the substrate including the photoresist, polymers, and/or residues into a reaction chamber; generating a plasma from a gas mixture comprising oxygen gas (O2) and/or an oxygen containing gas; suppressing and/or reducing fast diffusing species in the plasma; and exposing the substrate to the plasma to selectively remove the photoresist, polymers, and/or residues from the substrate, wherein the plasma is substantially free from fast diffusing species.