Plasma Ashing

Plasma Ashing is the dry removal of photoresist using oxygen plasma — converting organic resist material to volatile CO2, H2O, and N2 by-products through chemical reactions with reactive oxygen species, without wet chemistry.

Why Plasma Ashing?

• Post-etch resist is hardened ("crust") from ion bombardment — wet strippers (acetone, NMP) struggle to remove it.
• Implanted resist contains embedded ions — wet strip leaves contamination.
• Ashing is dry, clean, and selective to underlying inorganic layers.

Mechanism

1. O2 plasma generates atomic oxygen (O*) and ozone (O3).
2. O* reacts with organic polymer: CxHy + O* → CO2 + H2O.
3. Nitrogen-containing resists: also produces N2, NOx.
4. Net result: Resist oxidized to volatile gases — pumped away.

Process Conditions

• Temperature: 200–300°C (standard), 100–150°C (FEOL) to avoid dopant redistribution.
• Pressure: 0.5–5 Torr for downstream (remote) ashing.
• Power: 500–2000W RF or microwave.
• Additive gases: CF4 or forming gas (H2/N2) to remove Si-rich residues.

Ashing Types

• Barrel Asher: Wafer in O2 plasma — uniform but damages underlayer.
• Downstream (Remote) Ashing: Plasma generated upstream, only radicals reach wafer — less damage.
• UV-Ozone: UV-generated ozone at room temperature — gentle, for fragile structures.

Challenges

• Photoresist poisoning: Organic base/acid contamination from resist blocks PMOS implant activation — requires high-T ashing before implant anneal.
• Underlayer oxidation: O plasma can oxidize metal lines — add forming gas.
• Cu incompatibility: O plasma oxidizes Cu — require H2 or forming gas for Cu BEOL.

Plasma ashing is an indispensable step in semiconductor processing — performed 10–30 times per device flow, it is as critical as the etch or deposition steps it serves.