Copper Annealing
Copper Annealing is the controlled thermal treatment of electroplated copper interconnects to promote grain growth and recrystallization — transforming the as-deposited fine-grained microstructure into large-grained copper with lower electrical resistivity, improved electromigration resistance, and more uniform CMP removal, directly impacting interconnect performance and reliability at every technology node.
Why Copper Needs Annealing
- As-deposited electroplated Cu: Fine grains (20-50 nm diameter), high grain boundary scattering.
- Resistivity of as-deposited Cu: ~2.5-3.0 μΩ·cm (vs. bulk Cu: 1.67 μΩ·cm).
- After annealing: Grains grow to 0.5-2 μm → resistivity drops 10-20%.
- Large grains have fewer grain boundaries → better EM resistance (atoms pile up at boundaries).
Self-Annealing Phenomenon
- Electroplated Cu undergoes spontaneous recrystallization at room temperature over hours to days.
- Driven by: High internal stress from the plating process provides energy for grain growth.
- Self-annealing is variable and uncontrolled → fabs use deliberate thermal anneal for consistency.
Anneal Process
| Condition | Typical Range | Effect |
|---|---|---|
| Temperature | 100-400°C | Higher T → faster, larger grains |
| Time | 30 sec - 30 min | Longer → more complete recrystallization |
| Atmosphere | Forming gas (N2/H2) or N2 | Prevents Cu oxidation |
| Timing | After plating, before CMP | Ensures uniform CMP removal |
- Standard recipe: 200-350°C for 1-5 minutes in forming gas.
- Must anneal BEFORE CMP: Non-uniform grain structure causes dishing and erosion variation during polish.
Grain Size and Resistivity
- Resistivity contribution from grain boundaries: $\Delta\rho_{GB} \propto \frac{1}{d}$ (d = grain diameter).
- At advanced nodes (Cu line width < 30 nm): Wire width < grain size → grains span the entire wire cross-section (bamboo structure).
- Bamboo structure: Actually beneficial for EM — atoms cannot diffuse along grain boundaries down the wire length.
Impact on CMP
- Non-annealed Cu: Mix of small and large grains → different polish rates → surface roughness.
- Properly annealed Cu: Uniform large grains → smooth, predictable CMP.
- Without anneal before CMP: 10-30% increase in dishing and erosion defects.
Impact on Electromigration
- Large grains: Fewer grain boundaries for atomic diffusion → 2-5x improvement in EM lifetime.
- Combined with proper barrier (TaN/Ta): Cu interconnects meet 10-year reliability targets at elevated temperatures.
Copper annealing is a critical but often overlooked step in the BEOL process — this simple thermal treatment fundamentally transforms the electrical and mechanical properties of the interconnect metal, ensuring that the billions of copper wires in a modern chip perform reliably throughout the product lifetime.