A Conductive Atomic Force Microscope (C-AFM) has been used to investigate the nanometer scale electrical properties of Metal-Oxide-Semiconductor (MOS) memory devices with Silicon nanocrystals (Si-nc) embedded in the gate oxide. This study has been possible thanks to the high lateral resolution of the technique, which allows to characterize areas of only few hundreds of nm2 and, therefore, the area that contains a reduced number of Si-nc. The results have demonstrated the capability of the Si-nc to enhance the gate oxide electrical conduction due to trap assisted tunneling. On the other hand, Si-nc can act as trapping centers. The amount of charge stored in Si-nc has been estimated through the change induced in the barrier height measured from the I-V characteristics. The results show that only ~20% of the Si-nc are charged. These nanometer scale results are consistent with those obtained during the macroscopic characterization of the same structures. Therefore, C-AFM has been shown to be a very suitable tool to perform a detailed investigation of the performance of memory devices based on MOS structures with Si-nc at such reduced scale.
The effect of current limited stresses (CLS) on the breakdown (BD) SiO2 gate oxides has been analyzed at a nanometric scale with a Conductive Atomic Force Microscope (C-AFM). Bare oxide regions have been stresed and broken down using the tip of the C-AFM as the metal electrode of a metal-oxide-semiconductor (MOS) structure. Afterwards, post-BD I-V characteristics and topographical and current images of the affected areas have been obtained to analyze the post-BD conduction, the structural damage induced in the oxide and the BD propagation. The results shwo that BD phenomenon, although triggered at one point, is electrically propagated to neighbor regions. Moreover, the area affected by BD, the structural damage and the post-BD conduction depend on the breakdown hardness. In particular, it is shown that these magnitudes are smaller when the current through the structure is limited during BD transient.