Plasma Photoresist Stripper

A plasma photoresist stripper, or asher, removes resist that has already done its job — defined a pattern, survived an etch or implant — by converting it into volatile byproducts in an oxygen or hydrogen-based plasma, rather than dissolving it in solvent. The name "asher" comes from what the process looks like at a chemical level: the plasma oxidizes the resist's carbon backbone much like combustion would, turning solid polymer into gaseous CO2 and H2O that pump away, leaving no solid residue behind.

Resist that's just come out of an aggressive etch or implant is often chemically harder to strip than fresh resist, which is exactly why asher recipes differ so much from simple O2 cleaning. Implant-damaged resist in particular forms a hardened crust at the surface where ion bombardment has crosslinked and carbonized the top layer — a thin, dense, carbon-rich skin that resists a standard O2 plasma's oxidation chemistry. Strip recipes for post-implant resist typically add a hydrogen or fluorine component specifically to break through that crust before the bulk of the resist strips normally underneath.

Plasma strip and wet strip aren't competitors so much as sequential partners in most flows. A plasma asher is excellent at bulk resist removal and leaves no solvent to manage or dispose of, but it can leave trace ionic or metallic residue at the former resist-substrate interface that a wet clean removes more thoroughly; many process flows run a plasma ash first to remove the bulk of the resist quickly, followed by a brief wet clean to finish the surface, getting the asher's speed and the wet clean's residue removal in the same step sequence.

Plasma stripping turns photoresist's final job — surviving the process step it protected — into its last act, converting a hardened polymer mask into gas that simply pumps away, and the recipe details that separate a clean strip from a residue-leaving one usually trace back to exactly what that resist was asked to survive beforehand.