HF dip uses dilute hydrofluoric acid to remove native oxide from silicon surfaces and etch oxide films. Concentration: Typically 1-2% HF (dilute HF or DHF), or buffered HF (BOE) for controlled etch rates. Native oxide removal: Silicon exposed to air grows thin native oxide (10-20 angstroms). HF strips this to expose bare silicon. Etch rate: Approximately 1 angstrom/second for thermal oxide in dilute HF. Higher for deposited oxides. Hydrogen termination: After HF, silicon surface is hydrogen-terminated (Si-H). Hydrophobic. Stable for short time. Uses: Pre-epitaxy clean, pre-gate oxide, contact opening, controlled oxide etch. Safety: HF is extremely hazardous - penetrates skin, causes systemic fluoride poisoning. Requires special training and safety protocols. Selectivity: High selectivity to silicon - etches oxide but not silicon. Buffered oxide etch (BOE): HF + NH4F - more stable etch rate and better oxide profile control. Process control: Timed dips, endpoint by hydrophobicity or ellipsometry. Modern usage: Still essential despite decades of optimization. No good replacement for native oxide removal.