wet chemical etch

Wet Chemical Etching is the selective removal of material by immersion in or spraying with liquid chemical etchants — an isotropic process widely used for cleaning, oxide removal, selective removal of films, and pre-diffusion surface preparation.

Key Wet Etch Chemistries

HF (Hydrofluoric Acid) / BHF (Buffered HF):
- Removes SiO2: SiO2 + 6HF → H2SiF6 + 2H2O
- Native oxide removal: 1:50 HF:H2O, < 30 seconds.
- BHF (NH4F:HF): More stable etch rate, used for gate oxide removal.
- Selectivity: SiO2 >> Si, Si3N4 (high selectivity).

H3PO4 (Phosphoric Acid):
- Selective Si3N4 etch at 160°C.
- Si3N4:SiO2 selectivity ~40:1 — used for STI nitride removal.

KOH / TMAH (Anisotropic Wet Etch):
- Crystallographic etching of Si: <111> planes etch 100x slower than <100>.
- Creates V-grooves, pyramids, (111) facets.
- TMAH: CMOS-compatible (no K+ contamination), used in MEMS.

HNO3:HF (Silicon Etch):
- Isotropic Si etch: HNO3 oxidizes Si, HF dissolves oxide.
- Used for edge polish, silicon sculpting.

SC-1 and SC-2 (RCA Clean):
- SC-1 (NH4OH:H2O2:H2O, 1:1:5 at 75°C): Removes particles and organic contamination.
- SC-2 (HCl:H2O2:H2O, 1:1:6 at 75°C): Removes ionic/metallic contamination.

Advantages vs. Dry Etch

• Low cost, high throughput, excellent selectivity.
• No plasma-induced damage.
• Uniform across wafer.

Disadvantages

• Isotropic: Lateral undercutting — not suitable for sub-100nm feature definition.
• Wet chemicals require careful handling (HF is extremely hazardous).
• EHS concerns: Waste disposal, worker safety.

Wet chemical etching is a fundamental and irreplaceable part of semiconductor processing — despite the dominance of dry etching for patterning, wet etching handles dozens of critical cleaning and selective removal steps in every device flow.