Cobalt and Ruthenium Interconnect Metallization — As copper interconnect dimensions shrink below 15nm, alternative metals such as cobalt and ruthenium are being adopted to overcome the resistivity scaling limitations and reliability challenges that plague copper at nanoscale line widths.

Motivation for Alternative Metals — The transition away from copper at the tightest pitches is driven by fundamental physical limitations:

• Copper resistivity increases dramatically at narrow line widths due to electron scattering at grain boundaries, surfaces, and interfaces
• Barrier volume fraction in copper lines consumes an increasingly large percentage of the total cross-section, further reducing effective conductivity
• Mean free path of copper electrons (~39nm at room temperature) exceeds the line dimensions at advanced nodes, triggering severe size effects
• Cobalt and ruthenium have shorter electron mean free paths (~10nm and ~6nm respectively), resulting in less resistivity degradation at small dimensions
• Crossover dimension where alternative metals match or outperform copper occurs at approximately 10–15nm line width depending on barrier requirements

Cobalt Metallization — Cobalt has been adopted for local interconnect and contact levels at leading-edge nodes:

• CVD cobalt using Co2(CO)8 or cobalt amidinate precursors provides conformal fill of narrow features with good step coverage
• Barrierless integration is possible because cobalt does not diffuse into silicon dioxide as readily as copper, eliminating the need for thick TaN/Ta barriers
• Selective deposition of cobalt on metal surfaces enables bottom-up fill of vias and contacts, reducing void formation
• Grain structure optimization through anneal conditions improves bulk resistivity and electromigration performance
• Contact resistance at the cobalt-silicide interface must be minimized through careful surface preparation and liner engineering

Ruthenium Metallization — Ruthenium offers unique advantages for semi-damascene and subtractive patterning approaches:

• Subtractive etch of ruthenium is feasible using oxygen-based plasma chemistries, enabling patterning approaches not possible with copper
• ALD ruthenium from metalorganic precursors provides atomic-level thickness control for thin liner and seed applications
• Oxidation resistance of ruthenium simplifies integration by eliminating the need for protective capping layers after patterning
• Low-resistivity ruthenium films with resistivity approaching 8 μΩ·cm can be achieved with optimized deposition and anneal conditions
• Hybrid schemes combining ruthenium liners with copper fill leverage the advantages of both metals at intermediate dimensions

Integration and Reliability — Adopting new metals requires comprehensive process development and reliability qualification:

• Electromigration performance of cobalt and ruthenium lines shows different failure mechanisms compared to copper, often with improved lifetimes at narrow dimensions
• Stress migration behavior must be characterized under thermal cycling and constant temperature stress conditions
• CMP processes for cobalt and ruthenium require different slurry chemistries and removal rate selectivities compared to copper
• Etch and clean processes must be adapted to handle the different chemical properties of these metals without introducing contamination

Cobalt and ruthenium metallization represent a paradigm shift in interconnect technology, enabling continued scaling of local interconnects beyond the practical limits of copper through barrierless integration and alternative patterning approaches.