Grain Growth in Copper

Grain Growth in Copper is the microstructural evolution process where small copper grains coalesce into larger ones — driven by the reduction of grain boundary energy, occurring during thermal annealing or even at room temperature (self-annealing) in electroplated copper films.

What Drives Grain Growth?

• Driving Force: Reduction of total grain boundary energy (minimizing surface area).
• Normal Growth: Average grain size increases uniformly. Rate $propto$ exp($-E_a/kT$).
• Abnormal Growth: A few grains grow at the expense of many (secondary recrystallization). Common in thin Cu films.
• Factors: Temperature, film thickness, impurities (S, Cl from plating bath), stress, texture.

Why It Matters

• Resistivity: Grain boundary scattering dominates at narrow linewidths (< 50 nm). Larger grains = lower resistivity.
• Electromigration: The "bamboo" grain structure (grain spanning the full wire width) blocks mass transport along grain boundaries — the #1 EM failure path.
• Variability: Uncontrolled grain growth leads to resistance variation between wires.

Grain Growth is the metallurgy of nanoscale wires — controlling crystal evolution to optimize the electrical and reliability properties of copper interconnects.