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.
SiCoNi and Other Dry Cleans
SiCoNi and Other Dry Cleans
Beyond resist stripping, dry chemistries handle the most damage-sensitive surface preparations in the fab — places where even dilute HF would attack adjacent low-k or metal.
| Process | Chemistry | Targets | Notes |
|---|---|---|---|
| SiCoNi (Applied Materials) | NH₃ + NF₃ → (NH₄)₂SiF₆ | Native SiO₂ on Si | Forms a sublimating salt; gentle, no liquids |
| Certas (Tokyo Electron) | HF gas + NH₃ vapor | SiO₂ at via bottoms | Selective to nitride and metal |
| COR (Sony / TEL) | HF + NH₃ at low T | Oxide / nitride bumps | Self-limiting reactant layer + sublimation |
| Plasma H₂ / Ar | H₂ or Ar plasma | Native CuO, organic residue | Pre-clean for PVD/CVD on Cu |
The common theme: a reactive gas forms a thin solid salt or layer on the target film at low temperature, the wafer is then warmed (often just to ~100–150 °C), and the salt sublimes away. No liquid means no surface tension, no watermarks, and no damage to fragile 3D structures.
Key Concept: Why Dry Cleans Matter for 3D Devices
FinFETs and nanosheets have nearly vertical sidewalls and re-entrant geometries. A wet clean's surface tension can collapse adjacent fins (pattern collapse). Dry cleans avoid that risk entirely — increasingly the only viable option below 5 nm.
Knowledge Check
Knowledge Check
1 / 2Why is downstream plasma preferred over direct plasma for resist stripping?