Characterization of local electrical properties of gate dielectrics by conductive atomic force microscopy

A conducting atomic force microscopy (C-AFM) in ultrahigh vacuum is used to directly observe the evolution of leakage path in HfO₂ gate dielectrics. Thanks to the UHV environment, reproducible results for both positive and negative tip biases are obtained without material formation on the surface, which has been a problem for atmospheric C-AFM. It is found that the density of leakage spots increases exponentially as a function of tip bias and that it is a large factor for leakage current increase. It is also found that these local leakage paths in HfO₂ films annihilate after applying a reverse tip bias. This process seems to be related to the initial stage of the forming process of resistive switching materials. The fact that these paths annihilate by a very small reverse bias suggests that this behavior is caused by local reduction and oxidation in the HfO₂ layer.