Evolution of leakage paths in HfO2∕SiO2 stacked gate dielectrics: A stable direct observation by ultrahigh vacuum conducting atomic force microscopy

A conducting atomic force microscopy (C-AFM) in ultrahigh vacuum (UHV) is used to directly observe the evolution of leakage path in HfO2∕SiO2 stacked 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.