stress migration

Stress Migration Overview Stress migration (stress voiding) is a reliability failure mechanism where mechanical stress in metal interconnects drives atomic diffusion, creating voids that increase resistance or cause open-circuit failures—even without electrical current flowing. Mechanism - Source of Stress: Thermal expansion mismatch between copper (CTE ~17 ppm/°C) and surrounding dielectric/barrier (CTE ~1-3 ppm/°C). After high-temperature processing and cool-down, Cu is under tensile stress. - Void Formation: Atoms migrate from high-stress to low-stress regions along grain boundaries and interfaces. Material depletion creates voids. - Critical Locations: Vias connecting wide metal lines to narrow lines (stress gradient at via base), under via connections, and at metal line corners. Risk Factors - Wide Metal Lines: More stressed than narrow lines (higher total stress volume). Lines > 10μm wide are most vulnerable. - Storage Temperature: Void growth fastest at 150-250°C (enough thermal energy for diffusion, but not enough to relax stress by plastic deformation). - Long Vias: Single-via connections to wide metals are highest risk. - Bamboo Grain Structure: Large grains spanning the full line width block grain-boundary diffusion paths, redirecting stress to interfaces. Testing - JEDEC JESD22-A174: Standard stress migration test. - Bake at 150-200°C for 500-1000 hours. - Monitor via chain resistance for increases indicating void formation. Mitigation - Redundant vias (use 2+ vias instead of single via for critical connections). - Metal slot rules (add slots to wide metal to reduce stress volume). - Optimized barrier/liner to improve Cu adhesion and block diffusion paths. - Cap layer engineering (SiCN, SiN) to control interface diffusion.