resist strip / ashing

Resist stripping or ashing removes photoresist after etching is complete, using plasma or wet chemical processes. **Plasma ashing**: Oxygen plasma converts organic resist to CO2 and H2O. Downstream asher common. **Wet strip**: Chemical stripping with solvents (NMP, DMSO) or piranha (H2SO4 + H2O2). **When to use which**: Plasma for most stripping, wet for sensitive structures or when plasma damage is concern. **Post-etch residue**: Etch leaves polymer residues that must also be removed. Ash alone may not be sufficient. **Strip chemistry**: N2/H2 and forming gas reduce metal oxidation. O2 for straight organic removal. **Implanted resist**: Ion implant hardens resist crust. More aggressive strip needed. May require wet chemistry. **Complete removal**: Any resist residue causes defects in subsequent processing. Verification required. **Equipment**: Barrel ashers (batch), downstream plasma (single wafer), wet benches. **Temperature**: Elevated temperature (150-300C) increases ash rate. **Strip rate**: nm/minute. Depends on resist type, process history, strip chemistry.