Imaging breakdown spots in SiO2 films and MOS devices with a conductive atomic force microscope

Conductive Atomic Force Microscopy (C-AFM) is used to study the dielectric breakdown of ultrathin SiO₂ layers at a nanometric scale. First, bare oxide regions have been stressed and broken down using the tip as the metal electrode of a MOS structure. The results point out that the initial breakdown is electrically propagated to neighbour regions, affecting their dielectric strength. Moreover, the area affected by the initial breakdown depends on the breakdown hardness. In particular, it is shown that this area is smaller when the current through the structure is limited during the experiments. The effect of the current limitation is analysed in detail. Based on the results, a qualitative picture of the breakdown process is presented, which accounts for this effect. Finally, for the first time, the BD spots in standard MOS devices (with poly-Si gate) are electrically imaged with the C-AFM. The areas of the observed spots are in agreement with those obtained on bare oxides.