hf dip

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.