in-situ doping

In-situ doping introduces dopant atoms during CVD film deposition for precise, uniform doping without separate implantation. **Mechanism**: Dopant precursor gas added to CVD gas mixture. Dopant atoms incorporate into growing film simultaneously with silicon. **Precursors**: PH3 (phosphine) for n-type, B2H6 (diborane) for p-type, AsH3 (arsine) for n-type arsenic doping. **Advantages**: Uniform doping throughout film thickness. No implant damage. Immediate electrical activation. Sharp doping profiles. **Concentration control**: Dopant concentration controlled by precursor gas flow ratio. Wide range from lightly to heavily doped. **Applications**: Doped polysilicon gates, doped epitaxial layers, contact regions, resistors, emitters. **Profile control**: Can vary dopant concentration during deposition by changing gas ratios, creating graded profiles. **Polysilicon**: In-situ doped poly has more uniform doping than implanted poly, especially for thin films. **Limitations**: Dopant incorporation can affect growth rate and film properties. High doping levels may change grain structure. **Activation**: Dopants are electrically active as-deposited for substitutional incorporation. No anneal needed in some cases. **Selectivity interaction**: Dopant gases can affect selective epi selectivity. Process optimization required.