Wet Etch Process
Wet Etch Processes are the liquid-chemical-based material removal techniques used throughout semiconductor manufacturing for cleaning, thin film removal, and pattern transfer — providing high selectivity, low damage, and batch processing capability, though their isotropic (non-directional) etch profile limits them to applications where dimensional control is less critical than in plasma dry etching.
Key Wet Etch Chemistries
| Chemistry | Common Name | Targets | Selectivity |
|---|---|---|---|
| HF (dilute, 100:1 to 1000:1) | DHF | SiO2 | > 100:1 to Si, Si3N4 |
| NH4F + HF (6:1) | BHF (Buffered HF) | SiO2 (controlled) | Smooth etch, uniform rate |
| H2SO4 + H2O2 (4:1) | SPM / Piranha | Organics, metals | Strips photoresist |
| NH4OH + H2O2 + H2O | SC-1 / APM | Particles, organics | Standard RCA clean step 1 |
| HCl + H2O2 + H2O | SC-2 / HPM | Metallic contaminants | RCA clean step 2 |
| H3PO4 (hot, 160°C) | Hot Phos | Si3N4 | > 30:1 to SiO2 |
| KOH / TMAH | — | Silicon (anisotropic) | Crystal-plane selective |
RCA Cleaning Sequence (Industry Standard)
1. SC-1 (APM): NH4OH:H2O2:H2O (1:1:5) at 70-80°C.
- Removes: Particles, organics. Grows thin chemical oxide.
2. DHF Dip: Dilute HF (1:100) at room temp.
- Removes: Chemical oxide from SC-1. Leaves H-terminated Si surface.
3. SC-2 (HPM): HCl:H2O2:H2O (1:1:5) at 70-80°C.
- Removes: Metallic ions (Fe, Cu, Zn). Grows clean chemical oxide.
Wet Bench vs. Single-Wafer Processing
| Aspect | Wet Bench (Batch) | Single-Wafer Spin |
|---|---|---|
| Throughput | 50-100 wafers/batch | 1 wafer at a time |
| Chemical usage | High (large tanks) | Low (spray/puddle) |
| Uniformity | Good for simple cleans | Better for critical etches |
| Contamination | Cross-contamination risk | Clean per wafer |
| Use case | Standard cleans | Critical oxide strip, advanced cleans |
Wet Etch Characteristics
- Isotropic: Etches equally in all directions → lateral undercut equals vertical etch depth.
- Good selectivity: Chemical reactions are material-specific → stops on different films.
- No plasma damage: No ion bombardment or UV radiation.
- Batch capable: 50 wafers processed simultaneously → high throughput for non-critical steps.
Applications in Modern CMOS
- Pre-gate clean: Remove native oxide before gate dielectric deposition.
- SiGe selective etch: HCl vapor or dilute H2O2 selectively removes SiGe (nanosheet release).
- Sacrificial layer removal: Wet etch removes hard masks and spacers without damaging active structures.
- Post-etch residue removal: Fluorine-based or amine-based solutions clean etch polymer residue.
Wet etch processes are indispensable complementary techniques to dry etching — while plasma etch provides the anisotropic profiles needed for patterning, wet etch delivers the high selectivity, low damage, and cleaning capability essential for surface preparation and sacrificial layer removal throughout the CMOS integration flow.