Visualization on charge distribution behavior in thickness-scalable HfO2 trapping layer by in-situ electron holography and Kelvin Probe Force Microscopy technology

In this paper, charge transportation and storage characteristics of HfO₂ charge-trapping layer with various thicknesses were systematically investigated for three dimension (3D) memory device application. For the first time, charge transportation behavior is visualized by introducing in-situ electron holography technology. Under programming transient state mode, charge distribution near blocking oxide is clearly observed followed by lateral charge spreading through HfO₂ grain boundary. Meanwhile, steady state behaviors are also given by Kelvin Probe Force Microscopy technique. With HfO₂ trapping layer is scaled from 10nm into 3nm, although enough charge storage capability guaranteed mainly by Interface trap sites, severe degradation in data retention characteristics becomes bottleneck. Also, it is proved that the interfaces provide dominate trap sites and vertical decay plays a superior role in charge loss. Finally, nano-scale effects of HfO₂ trapping layer are demonstrated from experiment which is well agreed with our characterization results. The study may provide optimization for charge trapping structures, especially for high-density 3D NAND flash applications.