stress migration in copper

**Stress Migration (SM) in Copper** is a **reliability failure mechanism where copper atoms diffuse due to mechanical stress gradients** — typically tensile stress that develops during cooling from processing temperatures, causing void nucleation and growth near via connections. **What Is Stress Migration?** - **Cause**: CTE mismatch between Cu ($alpha approx 17$ ppm/°C) and dielectric ($alpha approx 0.5$ ppm/°C). Cu wants to contract more than the dielectric allows -> tensile stress in Cu. - **Voiding**: Atoms migrate toward free surfaces (via bottoms, grain boundaries) to relieve stress, leaving voids behind. - **Temperature**: Worst case at intermediate temperatures (~150-250°C) where diffusion is active but stress is not fully relaxed. **Why It Matters** - **Wide Lines**: Counterintuitively, SM is *worse* in wider metal lines (more total stress, more atoms available to migrate). - **Burn-In**: Can be triggered or accelerated by burn-in testing conditions. - **Design Fix**: Redundant vias and via-array rules reduce SM risk. **Stress Migration** is **thermal contraction pulling copper apart** — a mechanical stress-driven failure where the mismatch between copper and glass tears the metal from within.