As semiconductor technology advances toward sub-2nm process nodes, traditional copper (Cu) interconnects face significant challenges in conductivity, electromigration resistance, and mechanical stability. The relentless scaling of transistor dimensions demands interconnects that maintain performance while mitigating reliability risks. Copper, the industry standard since the late 1990s, is reaching its physical limits due to increasing resistivity at reduced dimensions and susceptibility to electromigration-induced failures.
Ruthenium (Ru), a platinum-group metal, has emerged as a promising candidate to address these challenges. With its:
Ruthenium presents compelling advantages for next-generation interconnects.
| Property | Copper (Cu) | Ruthenium (Ru) |
|---|---|---|
| Bulk Resistivity (μΩ·cm) | 1.68 | 7.6 |
| Electromigration Activation Energy (eV) | 0.7-0.9 | 1.5-2.0 |
| Melting Point (°C) | 1085 | 2334 |
| Young's Modulus (GPa) | 130 | 447 |
The superior electromigration performance of ruthenium stems from several atomic-scale mechanisms:
Ruthenium's higher activation energy for atomic diffusion (1.5-2.0 eV vs. 0.7-0.9 eV for Cu) significantly reduces ion migration under current stress. This property is particularly valuable at elevated operating temperatures common in advanced nodes.
Unlike copper which suffers from grain boundary diffusion, ruthenium's hexagonal close-packed (HCP) crystal structure and strong Ru-Ru bonds (bond energy ~25 kJ/mol higher than Cu-Cu) provide inherent resistance to void formation and hillock growth.
While promising, ruthenium integration presents several technical hurdles:
Atomic layer deposition (ALD) has emerged as the preferred method for conformal ruthenium film growth in high-aspect-ratio structures. Recent advances in precursor chemistry (e.g., Ru(EtCp)₂ and RuO₄) have enabled deposition at temperatures compatible with back-end-of-line (BEOL) processing (150-300°C).
Ruthenium's weak adhesion to conventional barrier materials like Ta/TaN necessitates novel approaches:
Experimental results from leading semiconductor manufacturers demonstrate ruthenium's advantages at extreme scaling:
At 10nm line widths, ruthenium interconnects show only 30% resistivity increase compared to 150% for copper, owing to reduced surface scattering effects. This advantage becomes more pronounced below 5nm dimensions.
Accelerated lifetime testing reveals:
The transition to ruthenium interconnects requires addressing several manufacturing challenges:
Ruthenium's chemical inertness necessitates specialized slurry formulations for chemical-mechanical polishing (CMP). New oxidizer-based slurries with controlled redox potential have achieved removal rates comparable to copper CMP.
Dry etching of ruthenium requires chlorine-based chemistries at elevated temperatures (150-200°C). Recent developments in pulsed plasma etching have achieved sub-10nm features with vertical sidewalls and minimal roughness.
The implementation of ruthenium enables novel interconnect topologies:
Some manufacturers are exploring hybrid approaches where ruthenium is used for critical lower metal layers while retaining copper for upper levels, balancing performance and cost.
Ruthenium's superior barrier properties enable integration with ultra-low-k dielectrics (k < 2.0) that would be incompatible with copper, potentially reducing RC delay by 30-40%.
Ongoing research focuses on several key areas:
Tungsten-ruthenium (W-Ru) and molybdenum-ruthenium (Mo-Ru) alloys are being investigated to further improve electromigration resistance while maintaining acceptable resistivity.
The combination of ruthenium interconnects with 2D channel materials (e.g., MoS₂) could enable monolithic 3D ICs with reduced parasitics and improved thermal management.
Selective deposition techniques using supercritical fluids and electrochemical methods may enable single-digit nanometer interconnects without lithographic patterning.
The adoption of ruthenium interconnects introduces new considerations:
With annual ruthenium production approximately 35 tons (vs. 20 million tons for copper), supply chain strategies must account for potential volatility in platinum-group metal markets.
While ruthenium is more expensive per gram than copper, its superior reliability may offset costs through:
The semiconductor industry's roadmap suggests ruthenium adoption will progress through several phases:
The successful implementation of ruthenium interconnects will require continued collaboration between material scientists, process engineers, and device physicists to overcome remaining technical challenges while ensuring economic viability.