Photoresist Stripping
Photoresist Stripping
After lithography and etch, the photoresist must be completely removed (stripped) before proceeding. Methods:
- Oxygen plasma ashing: O₂ plasma converts organic resist into volatile CO₂ and H₂O. Fast and clean, but the plasma can damage sensitive underlying materials.
- Downstream plasma: The plasma is generated remotely, and only the reactive neutral species (O radicals) reach the wafer — no ion bombardment damage. Preferred for damage-sensitive applications.
- Wet strip: Solvent-based strippers or SPM (sulfuric-peroxide mix) for resist that plasma can't fully remove (e.g., heavily implanted, cross-linked resist).
Post-etch, post-implant, and post-CMP cleans often combine dry and wet steps in sequence to address different contamination types.
Key Concept: Implant-Hardened Resist
High-dose ion implantation creates a carbonized crust on the resist surface that resists oxygen ashing. This requires multi-step strip processes: first cracking the crust at low temperature, then stripping the bulk resist. This is one of the most challenging cleans in the fab.
Knowledge Check
Knowledge Check
1 / 1Why is downstream plasma preferred over direct plasma for resist stripping?