Wet Strip Process is the chemical removal of photoresist and process residues using liquid solvents or oxidizing chemistries — complementary to plasma ashing, applied where plasma damage to sensitive underlying layers must be avoided.
Wet Strip Chemistries
Sulfuric Acid + Hydrogen Peroxide (SPM / Piranha):
- H2SO4:H2O2 (4:1), 120–140°C.
- Extremely aggressive organic removal — destroys resist in seconds.
- Generates heat (exothermic) — "piranha" solution.
- Used before gate oxidation, after implant strip.
- Leaves surface clean of organic material but may require additional metal clean.
NMP (N-Methyl-2-Pyrrolidone) / EKC:
- Polar aprotic solvents with amine additives.
- Swell and dissolve unexposed or lightly-exposed resist.
- Used for post-etch residue removal (organic + some inorganic).
- Temperature: 70–90°C, immersion or spray.
- Cu compatible: Does not corrode copper.
ACT / EKC 265:
- Hydroxylamine + solvent + corrosion inhibitor.
- Removes post-metal-etch residue — works with Cu, Al, TiN.
- Low pH, compatible with most metals.
DS-20 (Dilute H2SO4):
- Dilute sulfuric acid strip — gentler than piranha.
- Strips implanted resist without leaving metal contamination.
When to Use Wet vs. Plasma Strip
| Situation | Preferred Strip |
|---|---|
| Post-implant resist | Plasma ash first, then SPM |
| Hardened crust resist | Plasma ash first |
| Cu BEOL post-etch | Wet (EKC/ACT) — no O2 plasma on Cu |
| Sensitive III-V surface | Wet only |
| Organic residue on oxide | Plasma ash |
Process Integration
- Most flows use two-step: O2 plasma ash bulk + wet strip final clean.
- Single-step wet: Reserved for Cu-incompatible-plasma applications.
Wet strip process selection is a critical integration decision — choosing the wrong chemistry damages underlying films or leaves unacceptable residues that cause device failures or contamination of subsequent process steps.
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