copper interconnect

**Copper Interconnects** — the metal wiring that connects billions of transistors within a chip, replacing aluminum since the 180nm node (IBM, 1997) due to lower resistance. **Why Copper?** - 40% lower resistivity than aluminum (1.7 vs 2.7 $\mu\Omega$·cm) - Better electromigration resistance at same current density - Lower RC delay = faster signal propagation **Damascene Process (How Copper Is Patterned)** Copper cannot be dry-etched like aluminum, so the "damascene" process is used: 1. Deposit dielectric, etch trenches and vias 2. Deposit barrier layer (TaN/Ta) to prevent Cu diffusion into silicon 3. Deposit thin Cu seed layer by PVD 4. Electroplate copper to fill trenches 5. CMP to remove excess copper, leaving Cu only in trenches **Dual Damascene**: Etch both via and trench before metallization — reduces process steps. **Scaling Challenges** - At sub-10nm wire widths, resistivity increases sharply due to electron scattering at surfaces and grain boundaries - Barrier layer consumes proportionally more of the wire cross-section - Alternative metals being explored: Ruthenium, cobalt, molybdenum for narrowest layers **10-15 copper metal layers** are stacked in a modern processor, carrying signals and power across the chip.