selective epitaxy

Selective epitaxy grows single-crystal silicon only on exposed silicon surfaces while suppressing growth on oxide or nitride areas. **Mechanism**: Chemistry is tuned so precursor molecules preferentially nucleate and grow on clean silicon surfaces. Growth on dielectric surfaces is suppressed by adding HCl or Cl2 which etches nuclei on non-silicon surfaces. **Chemistry**: SiH2Cl2 + HCl + H2 is common selective epi chemistry. Chlorine provides selectivity. **Temperature**: 600-900 C typical. Higher temperature improves selectivity and crystalline quality. **Applications**: Source/drain epitaxy in FinFET and GAA transistors - grow SiGe (PMOS) or SiP/SiC (NMOS) selectively in recessed S/D regions. **Raised S/D**: Selective epi builds up S/D regions above original surface for reduced resistance. **SiGe**: Selective SiGe epitaxy for compressive channel stress in PMOS. Ge fraction 20-50%. **Faceting**: Crystal growth rate varies with orientation, creating faceted surfaces at feature edges. **Loading effects**: Growth rate depends on local pattern density. Isolated features grow faster than dense arrays. **Defects**: Must avoid defects at epi/substrate interface and in grown film. Pre-epi clean is critical. **Equipment**: Single-wafer epitaxy reactors (ASM, Applied Materials).