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.