FEOL (Front End of Line) — the portion of chip fabrication that creates the transistors themselves, from bare silicon wafer through completed gate and source/drain structures.

FEOL Process Sequence

1. Well formation: Ion implant p-well and n-well regions 2. STI (Shallow Trench Isolation): Etch + fill trenches to isolate transistors 3. Gate stack formation: Grow gate dielectric (SiO₂ + HfO₂), deposit gate electrode (poly-Si or metal) 4. Gate patterning: Lithography + etch to define gate length (critical dimension) 5. Halo + LDD implants: Control short-channel effects 6. Spacer formation: Define S/D offset from gate 7. Source/drain implant: Heavy doping for low-resistance S/D 8. Activation anneal: Activate dopants and repair implant damage 9. Silicide formation: Reduce contact resistance on S/D and gate 10. Contact etch stop layer (CESL): Deposit stressed SiN for strain engineering

Key Metrics

• Gate length: 5–30nm depending on node
• Gate oxide (EOT): 0.5–1.0nm
• Junction depth: 5–15nm
• All dimensions controlled to sub-nanometer precision

FEOL at Different Nodes

• Planar MOSFET: Through ~22nm
• FinFET: 22nm–3nm
• GAA/Nanosheet: 3nm and beyond

FEOL defines the intrinsic transistor performance — everything in BEOL is just connecting what FEOL built.