precursor

A precursor is a chemical compound delivered as vapor that reacts on the wafer surface to form the desired thin film during CVD or ALD. **Requirements**: Must be volatile enough for vapor delivery, reactive enough for deposition, yet stable enough for safe handling and storage. **Types**: **Metal-organic**: Metal atoms bonded to organic ligands (TDMAT for TiN, TMA for Al2O3). **Halide**: Metal halide compounds (WF6 for tungsten, TiCl4 for TiN). **Hydride**: Simple hydrogen compounds (SiH4 for silicon, NH3 as reactant). **TEOS**: Tetraethyl orthosilicate for SiO2 deposition. **Delivery**: Liquid precursors vaporized in heated bubblers or direct liquid injection systems. Gas precursors from cylinders. **Purity**: Semiconductor-grade precursors require extreme purity (ppb levels of metallic impurities). **Decomposition**: In thermal CVD, precursors decompose at hot surface. In ALD, precursors chemisorb without decomposing. **Safety**: Many precursors are pyrophoric (ignite in air), toxic, or corrosive. Specialized handling required. **Cost**: Advanced precursors (high-k, metal-organic) can be very expensive. Significant consumable cost. **Development**: New processes often require novel precursor chemistry. Active area of research.