Buffered Oxide Etch (BOE) is a mixture of HF and NH4F used to etch silicon dioxide with a controlled, stable etch rate. Chemistry: NH4F acts as buffer maintaining HF concentration and pH during reaction. Typical ratios 6:1 or 10:1 (NH4F:HF). Reaction: SiO2 + 6HF -> H2SiF6 + 2H2O. HF attacks Si-O bonds. Etch rate: Thermal oxide ~100nm/min at room temperature for standard BOE. Rate depends on oxide type and density. Selectivity: Very high selectivity to silicon (>100:1) and silicon nitride (~30:1 for thermal oxide vs stoichiometric nitride). Oxide type dependence: PECVD oxide etches faster than thermal oxide. Denser oxides etch slower. Advantages over pure HF: More stable etch rate over time. Better wetting of hydrophobic surfaces. Smoother etched surfaces. Applications: Pad oxide removal, sacrificial oxide release in MEMS, contact opening, pre-gate clean. Process control: Time-based etch with known rate. Endpoint by hydrophobic surface change (water beading on bare silicon). Safety: HF is extremely hazardous - causes deep tissue burns, systemic fluoride poisoning. Calcium gluconate antidote required on hand. Storage: Stored in HDPE containers. Attacks glass (quartz OK for short exposures).