Cobalt Fill Process is the CVD and electroless-plating technique for filling contact holes and vias with cobalt metal as an alternative to tungsten — offering lower resistivity for narrow features (< 15 nm diameter), eliminating the thick TiN barrier requirement, and enabling lower contact resistance at advanced nodes where the barrier metal consumes an unacceptable fraction of the available plug volume.

Why Cobalt Instead of Tungsten?

• At 15 nm contact diameter: 5 nm TiN barrier leaves only 5 nm of W → most of the plug is barrier.
• Cobalt can be deposited with minimal or no barrier → more metal, lower resistance.

Cobalt CVD Process

1. Barrier (optional): Ultra-thin TiN or TaN (~1-2 nm) — if needed for adhesion. 2. Co CVD nucleation: Cobalt precursor (Co2(CO)8 or similar) + H2 at 150-200°C. 3. Co CVD fill: Continue deposition to fill contact/via. 4. Anneal/Reflow: 300-400°C causes cobalt grain growth and void elimination. 5. CMP: Polish back excess cobalt.

Cobalt Reflow Advantage

• Unlike tungsten, cobalt can be reflowed at moderate temperature.
• Reflow fills small voids and seams that form during initial CVD fill.
• Result: Void-free fill even in features with re-entrant profiles.
• This is cobalt's key differentiator over tungsten for the smallest features.

Where Cobalt Is Used

• Intel 10nm (Intel 7): Introduced cobalt for M0/M1 (thinnest interconnect layers).
• Contact level: Some foundries use Co for source/drain contacts (replacing W).
• Via0: The via connecting contact to M1 — critical for resistance.
• Cobalt cap (CoWP): Selective cobalt deposition on Cu lines — improves EM resistance.

Challenges

• Oxidation: Cobalt oxidizes readily — must maintain reducing atmosphere during processing.
• Precursor cost: Cobalt CVD precursors more expensive than WF6.
• Selectivity: Achieving selective cobalt deposition (only inside features, not on field) is difficult.
• Reliability: Cobalt EM behavior different from W — characterization needed per integration scheme.

Beyond Cobalt: Ruthenium

• At < 10 nm dimensions, even cobalt resistivity becomes limiting.
• Ruthenium (Ru): Lower electron scattering at nanoscale → potentially lower effective resistivity.
• Ru does not need a barrier at all — deposited directly on dielectric.
• Active R&D at 2nm/1.4nm nodes.

The cobalt fill process is a key materials innovation for the most advanced semiconductor nodes — by solving the barrier-thickness overhead problem that plagued tungsten contacts at sub-15nm dimensions, cobalt enables the lower contact resistance essential for maintaining transistor drive current at each new generation.