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.