wet etch

Wet etching is a material removal process in semiconductor manufacturing that uses liquid chemical solutions to dissolve target materials through chemical reactions, as opposed to plasma (dry) etching which uses ionized gases. Wet etching is one of the oldest and simplest patterning techniques in microfabrication, involving immersion of wafers in an etchant solution held at controlled temperature and concentration. The process is predominantly isotropic — etching proceeds at approximately equal rates in all directions (laterally and vertically), creating undercut beneath the mask edge. While this isotropy limits wet etching for critical patterning at sub-micron dimensions, wet etch remains indispensable in modern semiconductor manufacturing for specific applications. Common wet etch chemistries include: buffered oxide etch (BOE or BHF, typically 6:1 NH4F:HF) for SiO2 removal at ~100 nm/min; hot phosphoric acid (H3PO4 at 160-180°C) for selective Si3N4 etching over SiO2; dilute HF for native oxide removal and surface preparation; piranha (H2SO4:H2O2) for organic residue stripping; SC-1 (NH4OH:H2O2:H2O) and SC-2 (HCl:H2O2:H2O) for RCA cleaning; KOH or TMAH for anisotropic crystallographic silicon etching along crystal planes; and mixed acids (HF:HNO3:CH3COOH) for silicon polishing. Wet etching advantages include extremely high selectivity (>1000:1 achievable for many material pairs), no plasma-induced damage, low equipment cost, batch processing capability, and excellent surface quality. Disadvantages include isotropic profile limiting resolution, handling of hazardous chemicals, large chemical consumption, environmental waste treatment requirements, and potential for particle contamination. Modern wet etch systems use single-wafer spin processing with megasonic agitation for improved uniformity, reduced chemical consumption, and better process control compared to traditional batch immersion tanks.