Sub-micron AlN Films with High Thermal Conductivity and Electrical Resistivity for Efficient Heat Dissipation in 3D ICs

This article explores the potential of polycrystalline aluminum nitride (poly-AlN) films as an efficient material for thermal management in 3D integrated circuits (ICs). It focuses on the deposition, optimization, and performance of sub-500 nm thick poly-AlN films, which show high thermal conductivity, electrical resistivity, and excellent surface smoothness, making them promising candidates for heat dissipation in advanced semiconductor devices.

Key Points from the Article:

1. Thermal Challenges in 3D ICs:

• 3D integration of logic and memory increases performance and density, but it also creates significant thermal management challenges due to higher power density and poor heat dissipation.

• Traditional packaging solutions, such as external cooling, do not address intra-chip temperature variations, which are critical for system reliability.

2. Poly-AlN Films for Thermal Management:

• Poly-AlN films are sputter-deposited below 200°C and show promising thermal properties, with cross-plane thermal conductivity (κ⊥) up to 100 W/m·K and in-plane thermal conductivity (κ∥) reaching 30 W/m·K.

• The films are highly resistive, with electrical resistivity exceeding 10¹⁴ Ω·cm, comparable to silicon dioxide (SiO₂), making them suitable for integration in 3D ICs where electrical isolation is crucial.

• The films are also ultra-smooth, with surface roughness reduced to below 5 Å after chemical mechanical polishing (CMP), ensuring compatibility with bonding processes.

3. Optimization and Post-Processing:

• Post-deposition rapid thermal annealing (RTA) at temperatures up to 950°C enhances crystallinity and reduces oxygen-related defects, significantly improving thermal conductivity.

• Oxygen content in the films plays a significant role in their properties. High oxygen concentrations lead to defects that reduce thermal conductivity, while annealing in forming gas (FG) reduces oxygen content, enhancing performance.

• Cathodoluminescence (CL) spectra show that reducing oxygen-related defects boosts thermal conductivity by minimizing the formation of defect complexes.

4. Electrical Resistivity and Dielectric Properties:

• The electrical resistivity of poly-AlN films increases with annealing, reaching up to 7 × 10¹⁴ Ω·cm at high temperatures. This is consistent with the reduction in ionic conduction due to the mitigation of Al vacancies.

• The dielectric constant of AlN is measured to be ~8.5 for 1.8 µm-thick films, which remains relatively stable across a frequency range of 3-43 GHz, ensuring its suitability for high-frequency applications.

5. Thermal Simulations for 3D ICs:

• Thermal simulations show that replacing traditional low-κ dielectric materials (e.g., SiO₂) with poly-AlN in stacked chips reduces hotspot temperatures by approximately 15% to 17%, significantly improving heat dissipation.

• Simulations further reveal that increasing the thermal conductivity of bonding and interlayer dielectric materials is key to reducing temperature in hotspots, with poly-AlN proving particularly effective for heat management.

6. Conclusion:

• The study demonstrates that poly-AlN films are a scalable and BEOL-compatible material with high thermal and electrical performance, ideal for managing heat in 3D ICs.

• The films' ability to mitigate hotspot temperatures and their low surface roughness make them a promising candidate for advanced thermal management solutions in next-generation semiconductor devices.

This research positions poly-AlN films as a promising material for enhancing the thermal performance of 3D ICs, addressing the growing challenges of heat dissipation in high-performance integrated systems.