Advanced Interconnects
Low-k dielectrics, air gaps, and future materials
Low-k Dielectrics
Low-k Dielectrics
To reduce capacitance between wires, the dielectric constant (k) of the insulator between wires must be lowered:
| Material | Dielectric Constant (k) | Status |
|---|---|---|
| SiO₂ | 4.0 | Traditional |
| Fluorinated SiO₂ (FSG) | 3.5 | 130–90 nm |
| SiOCH (CDO) | 2.5–3.0 | 65–14 nm |
| Porous SiOCH | 2.0–2.5 | 10 nm and below |
| Air gaps | ~1.0 | Emerging at tight pitches |
Lower k = lower capacitance = faster signals and less power. But low-k materials are mechanically weak and thermally poor, creating integration challenges.
Future Interconnect Materials
Future Interconnect Materials
As copper wires shrink below ~15 nm width, their resistivity increases dramatically due to electron scattering from surfaces and grain boundaries. Potential alternatives being researched:
- Ruthenium (Ru): Doesn't need a barrier layer (saves space), lower resistivity than Cu at very small dimensions. Being adopted for via fills.
- Cobalt (Co): Already used for M0/M1 contact layers at some foundries. Good gap-fill properties.
- Molybdenum (Mo): Being explored for local interconnects at 2nm and beyond.
- Graphene/carbon nanotubes: Theoretical promise but manufacturing challenges remain significant.
Key Concept: The Resistivity Wall
At dimensions below ~15nm, copper's bulk resistivity (1.7 µΩ·cm) is no longer achievable — surface and grain boundary scattering can triple the effective resistivity. This is forcing the industry to explore new conductor materials for the tightest layers, even though copper will remain for upper layers.
Knowledge Check
Knowledge Check
1 / 2Why are low-k dielectrics used between metal wires?