Structural and electrical evolution of gate dielectric breakdown observed by conductive atomic force microscopy

To clarify the breakdown (BD) mechanism in the gate oxide of transistors, BD evolution from prebreakdown (pre-BD) stage to BD transient is investigated by conductive atomic force microscopy. High electric fields of both polarities are applied to induce pre-BD degradation. Structural hillocks are observed to be dependent on stress polarity. The height of hillocks increases with BD evolution while keeping a similar lateral size. Flatband shift caused by negative charge is observed independent of stress polarity. Lateral size of the electrical degradation is similar to that of the hillock throughout BD evolution. We attribute the hillocks to deformation of Si substrate at the pre-BD stage, where an electrothermal effect plays an important role.