Employing Ruthenium Interconnects for Next-Generation Semiconductor Miniaturization

The Challenge of Sub-5nm Interconnects

As semiconductor technology pushes beyond the 5nm node, traditional copper (Cu) interconnects face significant challenges. Increased resistivity due to electron scattering at reduced dimensions, electromigration issues, and difficulties in barrier layer scaling make copper less viable for ultra-thin interconnects.

Ruthenium: A Promising Alternative

Ruthenium (Ru), a platinum-group transition metal, emerges as a compelling alternative with several inherent advantages:

• Lower bulk resistivity: 7.1 μΩ·cm compared to copper's 1.68 μΩ·cm (but superior performance at nanoscale)
• Superior electromigration resistance: 10-100× better than copper at similar dimensions
• Thinner barrier requirements: Can be deposited directly without conventional barrier layers
• Better scalability: Maintains conductivity better than copper below 20nm widths

Size-Dependent Conductivity Advantage

While copper's resistivity increases dramatically below 20nm due to surface and grain boundary scattering, ruthenium shows more favorable scaling characteristics. At 10nm line widths, ruthenium interconnects demonstrate:

• 30-50% lower resistivity than equivalent copper lines
• More uniform grain structure in narrow dimensions
• Reduced line-edge roughness sensitivity

Material Properties and Integration Benefits

Crystallographic Advantages

Ruthenium's hexagonal close-packed (HCP) crystal structure contributes to its superior performance in nanoscale interconnects. Key attributes include:

• Higher melting point (2334°C vs. Cu's 1085°C) for better thermal stability
• Stronger interatomic bonds reducing electromigration
• Anisotropic conductivity favoring certain crystal orientations

Integration with Existing Fab Processes

Ruthenium offers several manufacturing advantages:

• Compatible with atomic layer deposition (ALD) for conformal thin films
• Does not require a diffusion barrier layer in many applications
• Good adhesion to low-k dielectric materials
• Potential for direct plating processes similar to copper electroplating

Performance Comparison in Advanced Nodes

Resistivity Scaling Trends

Comparative studies of ruthenium and copper interconnects show diverging performance as line widths shrink:

Line Width (nm)	Cu Resistivity (μΩ·cm)	Ru Resistivity (μΩ·cm)	Advantage Ratio
32	3.8	8.2	Cu favored
20	5.6	9.1	Neutral
14	8.9	10.3	Ru favored
10	12.7	11.8	Ru superior

Reliability Metrics

Ruthenium demonstrates remarkable reliability improvements:

• Electromigration lifetime improvement of 10-100× at 100°C operation
• Reduced thermomigration susceptibility at high current densities
• Better stress migration resistance during thermal cycling

Manufacturing Considerations

Deposition Techniques

The semiconductor industry is evaluating several ruthenium deposition methods:

• Atomic Layer Deposition (ALD): Enables conformal films down to sub-nm thicknesses with excellent step coverage
• Chemical Vapor Deposition (CVD): Higher growth rates suitable for thicker liners and seed layers
• Physical Vapor Deposition (PVD): Conventional sputtering approaches with modified process parameters
• Electrochemical Deposition: Emerging plating techniques for void-free gap fill

Patterning Challenges

The transition to ruthenium requires adaptation of patterning approaches:

• Dry etch chemistry development for anisotropic patterning
• Alternative chemical mechanical polishing (CMP) slurries
• Novel wet cleaning processes compatible with Ru surfaces
• Integration with extreme ultraviolet (EUV) lithography stacks

Thermal Management Aspects

Heat Dissipation Characteristics

Ruthenium's thermal properties impact chip design:

• Thermal conductivity: 117 W/(m·K) vs. Cu's 401 W/(m·K)
• Coefficient of thermal expansion: 6.4 μm/(m·K) vs. Cu's 16.5 μm/(m·K)
• Heat capacity: 24.0 J/(mol·K) at 25°C

The lower thermal conductivity requires careful thermal modeling but is partially offset by ruthenium's ability to operate at higher current densities without electromigration failures.

The Roadmap for Industry Adoption

Current Industry Status

The semiconductor ecosystem is making progress toward ruthenium adoption:

• IMEC: Demonstrated functional ruthenium interconnects at 14nm pitch
• Samsung: Evaluating Ru for sub-5nm nodes in research facilities
• Intel: Investigating hybrid Cu-Ru schemes for intermediate nodes
• TSMC: Exploring Ru for power delivery networks in advanced packaging

Remaining Technical Hurdles

Several challenges must be addressed before widespread adoption:

• Achieving comparable via resistance to copper interconnects
• Developing cost-effective bulk deposition processes
• Optimizing interfacial engineering with surrounding dielectrics
• Establishing comprehensive reliability qualification standards

The Future of Interconnect Materials

Beyond Ruthenium: Emerging Alternatives

The search for ideal interconnect materials continues with other candidates under investigation:

• Cobalt (Co): Already in use for local interconnects in some designs
• Tungsten (W): For specific applications requiring extreme stability
• Molybdenum (Mo): Another refractory metal with interesting properties
• 2D Materials: Graphene and MXenes as potential long-term solutions

The Hybrid Interconnect Approach

A likely transition path involves hybrid metallization schemes:

• Tiered implementation: Ruthenium for critical lower metal layers, copper for upper layers
• Composite structures: Ruthenium liner/barrier layers with copper fill
• Selective application: Ruthenium for high-current density signal paths only