The origin of broad distribution of breakdown times in polycrystalline thin film dielectrics

The distribution of breakdown times of thin film dielectrics, stressed in a constant voltage mode, is generally interpreted in terms of percolation theory of dielectric breakdown. The percolation model suggests that relative distribution of failure times (normalized to the mean) should narrow down considerably for thicker dielectrics. Explicitly contradicting this prediction, we find a larger distribution of failure times even for relatively thick polycrystalline oxides. We use atomic force microscopy and conductive AFM measurements to confirm that breakdown in these films are primarily localized in the grain boundaries, decorated with large number of pre-existing defects. The classical percolation model—adapted to this specific situation of spatially localized trap generation—offers an intuitive explanation of the breadth of the failure time distribution in thick polycrystalline dielectric. The theory offers an opportunity to optimize the intrinsic trade-off between variability and reliability in polycrystalline films.