Phenomenological classification of stress-induced leakage current and time-dependent dielectric breakdown mechanism

Abstract

The time-dependent dielectric breakdown (TDDB) of Cu damascene interconnects was investigated, noting the time variations in stress-induced leakage current. Copper interconnects normally have symmetric current-voltage curves, which suggests that defects are distributed symmetrically between two Cu lines. Although the impact damage model satisfies this requirement, as does the thermochemical E-model, the Cu diffusion model does not. Without the barrier metal, Cu+ ions rapidly penetrate the dielectric film and form unstable conduction filaments. The leakage current fluctuates greatly due to the rapid Cu movement in the last stage of bias temperature stressing. These current fluctuations also appear in the triangular voltage sweep so that a spurious peak emerges, which is unrelated to the ionic displacement current. The extrinsic TDDB has a small field acceleration parameter (0.5 cm/MV); however, it switches to a large one (4.5 cm/MV) at electrical fields that are higher than 2 MV/cm. Another type of degradation is the thermal reaction between Cu and low-k. High-temperature annealing (>200 °C) generates shallow-energy-level defects in the SiO forbidden-energy gap. The shallow-energy-level defects have less impact on the TDDB lifetime although they cause a large Poole–Frenkel type current.