time-dependent dielectric breakdown (tddb)

Time-dependent dielectric breakdown (TDDB) is a reliability failure mechanism where insulating dielectric materials gradually degrade under sustained electric field stress until catastrophic breakdown occurs. Physics: (1) Defect generation—electric field creates traps (broken bonds) in dielectric; (2) Trap accumulation—traps build up over time forming percolation path; (3) Breakdown—conducting path spans dielectric, causing short circuit. TDDB in BEOL (inter-metal dielectric): (1) Between adjacent metal lines at tightest pitch; (2) Electric field = V/spacing, increases as pitch shrinks; (3) Low-κ dielectrics more susceptible (weaker bonds, porosity); (4) Failure: leakage between lines → hard short. TDDB in FEOL (gate dielectric): (1) Across gate oxide under gate bias; (2) High-κ/metal gate TDDB involves trap generation in HfO₂ and interface; (3) Failure: gate-to-channel short, stuck transistor. Testing: accelerated stress at elevated voltage and temperature, measure time-to-breakdown statistics. Models: (1) E-model—field-driven, ln(tBD) ∝ -γE; (2) 1/E model—current-driven, ln(tBD) ∝ 1/E; (3) Power-law voltage model—for thin dielectrics, ln(tBD) ∝ -n×ln(V). Extrapolation: fit Weibull distribution to accelerated data, extrapolate to operating conditions for lifetime prediction (typically target 10-year reliability). Mitigation: (1) BEOL—increase line spacing on critical nets, improve dielectric quality; (2) FEOL—optimize gate dielectric process, minimize interface traps; (3) Design—voltage derating, spacing rules. TDDB margin is a critical limiter for BEOL pitch scaling and gate oxide thickness at advanced nodes.