phosphoric acid etch

Phosphoric acid (H3PO4) etching is a critical wet chemical process in semiconductor manufacturing used primarily for the selective removal of silicon nitride (Si3N4) films over silicon dioxide (SiO2). The process uses concentrated phosphoric acid (approximately 85-86% H3PO4 by weight) heated to 155-165°C at its boiling point, where it achieves selectivity ratios of silicon nitride to thermal oxide exceeding 30:1 to 50:1 under optimized conditions. The etch rate of LPCVD Si3N4 in hot H3PO4 is typically 4-6 nm/min, while thermal SiO2 etches at only 0.1-0.2 nm/min. This exceptional selectivity makes hot phosphoric acid indispensable in processes requiring precise nitride removal without attacking oxide — the most prominent application being the LOCOS (Local Oxidation of Silicon) process and modern STI (Shallow Trench Isolation) integration flows where a sacrificial nitride hardmask must be stripped selectively over pad oxide. The etch mechanism involves hydrolysis of silicon nitride by water molecules dissolved in the phosphoric acid solution at elevated temperature. The reaction produces silicic acid and ammonium phosphate as byproducts. Maintaining precise boiling point temperature and water concentration is critical — the etch rate and selectivity are extremely sensitive to the H2O:H3PO4 ratio. As etching proceeds, water evaporates and dissolved silicon byproducts accumulate, changing the bath chemistry and requiring replenishment or replacement. Modern single-wafer phosphoric acid etch systems provide superior control through precise temperature regulation, continuous acid concentration monitoring, and fresh chemistry delivery for each wafer. Bath lifetime management is critical as silicon-containing byproducts can precipitate as particles if concentration exceeds saturation. The process also etches deposited oxides (TEOS, HDP oxide) faster than thermal oxide, so selectivity ratios depend on the specific oxide type. Phosphoric acid processing requires careful safety controls due to the high temperature and corrosive chemistry.