Ruthenium Interconnects for Sub-3nm Semiconductor Energy Efficiency Gains
Ruthenium Interconnects for Sub-3nm Semiconductor Energy Efficiency Gains
The Challenge of Copper Interconnects in Ultra-Scaled Nodes
As semiconductor manufacturing advances beyond the 3nm node, traditional copper (Cu) interconnects face severe limitations. The increasing resistivity of copper due to electron scattering effects at shrinking dimensions, combined with rising power densities, creates significant bottlenecks in performance and energy efficiency.
Fundamental Physics Limitations
Copper's resistivity increases dramatically at nanoscale dimensions due to:
- Surface scattering (Fuchs-Sondheimer effect)
- Grain boundary scattering (Mayadas-Shatzkes model)
- Sidewall scattering in sub-30nm features
Ruthenium as a Potential Replacement
Ruthenium (Ru), a platinum-group metal, has emerged as a leading candidate to replace copper in advanced interconnects due to several advantageous properties:
Material Properties Comparison
| Property |
Copper (Cu) |
Ruthenium (Ru) |
| Bulk Resistivity (μΩ-cm) |
1.68 |
7.1 |
| Mean Free Path (nm) |
39 |
6.7 |
| Melting Point (°C) |
1085 |
2334 |
| Electromigration Resistance |
Moderate |
Excellent |
Size-Dependent Performance
While ruthenium has higher bulk resistivity than copper, its shorter electron mean free path makes it more favorable at extremely scaled dimensions:
- Below 20nm widths, Ru's resistivity advantage becomes apparent
- At 5nm dimensions, simulations show Ru interconnects can be 30-40% more conductive than Cu
- Ru maintains better conductivity uniformity across narrow features
Integration Challenges and Solutions
Implementing ruthenium interconnects requires addressing several technical hurdles:
Deposition Techniques
Current research focuses on atomic layer deposition (ALD) and chemical vapor deposition (CVD) methods:
- Ru ALD using (ethylbenzyl)(1,3-cyclohexadienyl)Ru precursors achieves conformal films
- CVD processes enable selective deposition on diffusion barriers
- Electrochemical deposition is being explored for void-free filling
Barrier Layer Requirements
Unlike copper which requires robust diffusion barriers, ruthenium's properties allow for:
- Thinner barrier layers (1-2nm vs. 2-4nm for Cu)
- Potential barrierless integration in some architectures
- Compatibility with novel 2D material barriers like graphene
Energy Efficiency Benefits
The transition to ruthenium interconnects offers multiple pathways for power reduction:
IR Drop Mitigation
Ru's superior scaling behavior reduces voltage drops across the interconnect network:
- Up to 25% reduction in dynamic power consumption
- More uniform power delivery across the die
- Reduced need for power grid overdesign
Thermal Management Advantages
The higher melting point and thermal conductivity of ruthenium enable:
- Lower operating temperatures at high current densities
- Reduced thermomigration risks
- Better heat dissipation from logic blocks
Reliability Considerations
Ruthenium's inherent material properties address key reliability challenges:
Electromigration Performance
Experimental data shows:
- 10-100x improvement in electromigration lifetime compared to Cu
- Lower activation energy for diffusion (1.0eV vs. 0.8eV for Cu)
- More stable grain structure under high current stress
Corrosion Resistance
Ru's chemical inertness provides:
- Superior resistance to oxidation and chemical attack
- Compatibility with advanced dielectric materials
- Long-term stability in harsh operating environments
Manufacturing Implications
The shift to ruthenium interconnects requires changes across the semiconductor ecosystem:
Process Flow Modifications
Key adjustments needed include:
- New CMP processes for Ru planarization
- Modified etch chemistries for pattern definition
- Alternative cleaning solutions post-processing
Supply Chain Considerations
The transition impacts material sourcing and costs:
- Ru is 5-10x more expensive than Cu by weight
- Global production capacity is more limited than copper
- Recycling infrastructure needs development
Future Research Directions
Ongoing investigations aim to further optimize ruthenium interconnects:
Alloying Approaches
Research into Ru-based alloys seeks to:
- Tune resistivity through controlled impurity addition
- Further improve electromigration resistance
- Optimize thermal expansion properties
3D Integration Schemes
Ru's properties make it attractive for:
- Monolithic 3D IC inter-tier vias
- Advanced packaging interconnects
- Hybrid bonding applications