Using Carbon Nanotube Vias for Ultra-High-Density Interconnects in Next-Generation Neuromorphic Computing Chips

Introduction to Scaling Challenges in Neuromorphic Hardware

As the demand for neuromorphic computing systems intensifies, the limitations of conventional copper interconnects become increasingly apparent. Traditional metallization techniques struggle to meet the ultra-high-density requirements of next-generation neuromorphic architectures, which aim to emulate the synaptic density and energy efficiency of biological brains. Carbon nanotube (CNT) vias emerge as a promising solution, offering superior electrical conductivity, thermal stability, and scalability.

The Case for Carbon Nanotube Vertical Interconnects

The integration of CNT-based vertical interconnects addresses three critical bottlenecks in neuromorphic hardware:

• Resistive-Capacitive (RC) Delay: Copper interconnects suffer from increasing resistance at scaled dimensions, whereas CNTs maintain ballistic conduction even at nanometer scales.
• Electromigration: CNTs demonstrate exceptional current-carrying capacity (theoretical limit ~10⁹ A/cm²) compared to copper (~10⁶ A/cm²).
• Thermal Management: With thermal conductivity exceeding 3000 W/mK, CNTs outperform copper (385 W/mK) in dissipating heat from dense 3D architectures.

Material Properties Comparison

Property	Copper	Carbon Nanotubes
Electrical Conductivity (S/m)	5.96×107	1×108 (ballistic)
Current Density Limit (A/cm2)	~1×106	~1×109
Thermal Conductivity (W/mK)	385	>3000

Fabrication Techniques for CNT Vias

The successful implementation of CNT vias requires precise control over several fabrication parameters:

Chemical Vapor Deposition (CVD) Growth

Plasma-enhanced CVD enables the vertical growth of CNT bundles within predefined via holes. Key process parameters include:

• Temperature range: 400-800°C (lower than traditional BEOL compatible processes)
• Catalyst materials: Fe, Co, Ni nanoparticles with controlled size distribution
• Growth rate: Typically 1-10 μm/min depending on process conditions

Contact Resistance Optimization

The interfacial resistance between CNTs and metal electrodes remains a critical challenge. Current approaches include:

• End-bonded contacts using carbide-forming metals (Ti, W)
• Doping with potassium or iodine to reduce Schottky barriers
• Plasma treatment to functionalize CNT ends prior to metallization

Integration with Neuromorphic Architectures

The unique properties of CNT vias enable novel neuromorphic circuit designs:

3D Crossbar Arrays

CNT vias facilitate the vertical stacking of memristive crossbar arrays, overcoming the area limitations of planar designs. Experimental implementations have demonstrated:

• Via pitch scaling below 50 nm (compared to ~100 nm for copper)
• Reduced sneak paths in 3D memory architectures
• Improved thermal management in stacked configurations

Spiking Neural Networks

The high-speed signal propagation in CNT interconnects (group velocity ~1×10⁶ m/s) matches the temporal requirements of biologically plausible spiking neural networks. This enables:

• Synchronous operation across large-scale neural arrays
• Reduced latency in spike transmission between layers
• Lower energy per spike compared to metal interconnects

Reliability Considerations

The long-term stability of CNT interconnects must address several factors:

Environmental Degradation

While individual CNTs are chemically stable, practical implementations require protection against:

• Oxidation at elevated temperatures (>300°C in air)
• Moisture absorption in porous CNT bundles
• Mechanical deformation during packaging processes

Statistical Variations

The stochastic nature of CNT growth leads to variations that must be accounted for in circuit design:

• Diameter distribution (typically 1-3 nm for single-wall CNTs)
• Chirality variations affecting metallic/semiconducting behavior
• Packing density fluctuations in via holes

Performance Benchmarks

Recent experimental results demonstrate the potential of CNT vias:

Electrical Characteristics

• Resistance values: 1-10 kΩ per via for typical 100 nm diameter structures
• Current density: >5×10⁸ A/cm² demonstrated in laboratory conditions
• Capacitance: 0.1-1 fF per via, enabling RC delays below 1 ps for short interconnects

Thermal Performance

• Thermal resistance: 10^-9-10^-8 K·m²/W for individual CNTs
• Heat dissipation: Demonstrated 3× improvement over copper in 3D test structures

Future Development Pathways

The roadmap for CNT via implementation includes several critical milestones:

Manufacturing Scale-up

Transitioning from laboratory demonstrations to production requires:

• Development of wafer-scale uniform growth processes
• Integration with existing semiconductor fabrication tools
• Yield improvement strategies for defect management

Heterogeneous Integration

Combining CNT interconnects with emerging device technologies:

• Monolithic 3D integration with oxide-based memristors
• Co-integration with 2D material transistors (MoS₂, WSe₂)
• Hybrid quantum-classical computing architectures

The Competitive Landscape of Alternative Solutions

While CNT vias show exceptional promise, competing technologies must be objectively evaluated:

Graphene Interconnects

Graphene ribbons offer similar benefits but face challenges in:

• Edge scattering effects degrading conductivity at nanoscale widths
• Higher contact resistance compared to CNT end contacts
• More complex patterning requirements for vertical interconnects

Optical Interconnects

Photon-based solutions provide immunity to electromagnetic interference but suffer from:

• Larger footprint of optoelectronic components
• Higher energy per bit compared to projected CNT performance
• Challenges in dense 3D integration

Theoretical Limits and Projections

Fundamental physics establishes the ultimate boundaries for CNT interconnect performance:

Quantum Conductance

A single metallic CNT channel exhibits conductance quantization at G₀ = 2e²/h ≈ 77.5 μS. Practical implementations must consider:

• Theoretical minimum resistance: ~6.5 kΩ per conducting channel
• Multi-channel conduction in bundles or multi-wall CNTs
• Tunneling effects at sub-nanometer scales

Scaling Projections

The International Roadmap for Devices and Systems (IRDS) projects:

• Aspect ratios >20:1 for sub-10 nm vias by 2030
• Current density requirements exceeding 10⁸ A/cm²
• Tolerance for sub-nm alignment accuracy in 3D stacking