Copper Anneal is a thermal treatment applied to electroplated copper — to promote grain growth, reduce resistivity, stabilize the microstructure, and improve electromigration resistance before CMP planarization.

What Is Copper Anneal?

• Conditions: 100-400°C, 30 minutes to several hours, inert atmosphere (N₂ or forming gas).
• As-Plated Cu: Fine-grained (10-50 nm grains), high resistivity, metastable.
• After Anneal: Large grains (0.5-2 $mu m$), lower resistivity (~1.7 $muOmega$·cm), stable microstructure.
• Self-Annealing: Some Cu films undergo partial grain growth at room temperature over days (but controlled anneal is faster and more uniform).

Why It Matters

• Resistivity: Grain boundaries scatter electrons. Fewer grain boundaries (larger grains) = lower resistance.
• CMP Uniformity: Uniform grain structure improves CMP planarity and reduces dishing.
• Reliability: Large-grain, bamboo-like structure resists electromigration (no continuous grain boundary path for atom transport).

Copper Anneal is crystal healing for copper wires — growing the grains to reduce resistance and strengthen the metal against electromigration failure.