Electromigration (EM) is a reliability failure mechanism where high-density electric current causes metal atoms to migrate along the electron flow direction, eventually creating voids (opens) or hillocks (shorts). Physics: momentum transfer from conducting electrons to metal atoms ("electron wind") at high current density. Cu EM: primarily along grain boundaries and interfaces (Cu/barrier and Cu/cap interfaces are dominant diffusion paths). Critical parameters: (1) Current density (J)—EM rate exponential with J; (2) Temperature—Arrhenius relationship, rate doubles every ~10-15°C; (3) Activation energy (Ea)—depends on diffusion path (interface ~0.8-1.0 eV, grain boundary ~0.7-0.9 eV for Cu). Black's equation: MTTF = A × J⁻ⁿ × exp(Ea/kT), where n ≈ 1-2 (current exponent). EM failure modes: (1) Void at via bottom—current divergence creates void, increases resistance; (2) Void under via—stress migration assisted; (3) Hillock—metal accumulation can bridge to adjacent line (short). Design rules: (1) Maximum current density per wire width (from foundry EM rules); (2) DC and AC (time-averaged) current limits; (3) Blech length—below critical length, back-stress prevents EM failure. EM improvement: (1) Cu cap—CoWP or CuSiN cap improves interface adhesion; (2) Metal liner—good barrier/Cu interface; (3) Bamboo structure—single-grain-width lines eliminate grain boundary paths; (4) Ru cap—better adhesion than dielectric cap. Testing: accelerated testing at high temperature and current density, extrapolate to use conditions. Advanced node concerns: smaller wire cross-sections mean higher current density for same current, making EM an increasingly critical constraint on interconnect reliability and design.