BEOL (Back-End-of-Line) Integration is the fabrication of the multi-level metal interconnect stack above the transistors — building 10-15+ layers of copper wires and vias in low-k dielectric that route signals, power, and clock across the chip.

BEOL Process Sequence (per layer)

• Dielectric Deposition: Deposit low-k ILD (SiCOH, $k$ ≈ 2.5-3.0).
• Patterning: Lithography and etch of trenches (wires) and vias (vertical connections).
• Barrier/Seed: Deposit TaN/Ta barrier + Cu seed layer by PVD.
• Cu Fill: Electroplate copper to fill trenches and vias.
• CMP: Planarize excess copper — dual-damascene process.

Why It Matters

• RC Delay: BEOL wire RC delay increasingly dominates total chip delay at advanced nodes.
• Power Delivery: Power distribution network through BEOL must deliver >100A at <1V with minimal IR drop.
• Reliability: Electromigration, stress migration, and TDDB in BEOL are critical reliability concerns.

BEOL is the highway system of the chip — building layer upon layer of copper highways that carry signals and power across billions of transistors.