Cobalt Interconnect

Cobalt Interconnect — using cobalt instead of copper for the narrowest local metal layers, addressing the resistivity scaling challenge where copper's advantage disappears at very small wire widths.

The Problem with Copper at Small Widths

• Bulk Cu resistivity: 1.7 μΩ·cm
• But at <15nm wire width: Effective resistivity rises to 5–10+ μΩ·cm due to:
• Electron scattering at grain boundaries (grains are small in narrow wires)
• Surface scattering at wire sidewalls
• Barrier liner (TaN/Ta) occupies 30–40% of wire cross-section

Why Cobalt?

• No barrier needed (Co doesn't diffuse into dielectric like Cu)
• Better gap fill in narrow trenches (CVD cobalt flows into small features)
• Resistivity comparable to Cu at very narrow widths (barrier-free advantage)
• Better electromigration resistance

Current Usage

• Intel: Cobalt for M0/M1 (finest pitch local wires) since 10nm node
• TSMC: Cobalt contacts (not yet for wires)
• Cu remains dominant for wider intermediate and global wires

Future Metal Candidates

• Ruthenium (Ru): Even shorter mean free path than Co → potentially better at <10nm width. Barrierless. Active research
• Molybdenum (Mo): Very low resistivity at narrow widths. Intel exploring for future nodes

The shift from copper to alternative metals at the finest pitches is inevitable — the physics of electron scattering at nanoscale dimensions demands it.