Copper penetration in thermal oxide was investigated using MOS capacitors by annealing at 450 °C and bias-temperature stress at 250 °C. Copper induces minority carrier generation lifetime decay and oxide leakage current increase. Degradation is enhanced by capacitor biasing, which confirms the role of Cu+ ions. The current-voltage characteristics are consistent with Poole-Frenkel model, showing that electron transport proceeds through traps created in the oxide bulk by copper. When a negative bias is applied, copper traps are removed from oxide near SiO2-Si interface and the leakage current is cancelled but the generation lifetime remains nil, copper contamination of silicon surface being not removed.
None of these effects are observed when the copper gate is separated from oxide by a 10 nm TiN layer, proving that this material is an efficient barrier against copper diffusion at 450°C.