Conductive atomic force microscopy as a tool to reveal high ionising dose effects on ultra thin SiO₂/Si structures

The electrical stress behaviour of non-irradiated and irradiated (2 to 7-nm thick) SiO₂/Si structures is investigated using conductive-atomic force microscopy. A protocol based on the successive application of two ramped-voltage stresses (RVS) on each test point is performed. The environmental implementation conditions of such an experiment are then investigated. A statistical approach based on the use of Weibull distributions is also adopted. Before irradiation, for the thinnest samples, it is shown evidence of stress-induced trap-assisted tunnelling leading to a high decrease in threshold voltages on the second RVS. After high-dose X-ray irradiations, the first RVS exhibit voltage-shift effects increasing with the oxide film thickness, whereas for the second RVS, no additional effect is observed. The high locality of these measurements, sensitive to a few tens of trapped charges, is therefore demonstrated and constitutes a new step towards a better understanding of oxide degradation mechanisms due to radiation effects.