Visualization of charges stored in the floating gate of flash memory by scanning nonlinear dielectric microscopy

By applying scanning nonlinear dielectric microscopy (SNDM), we succeeded in clarifying that electrons existed in the poly-Si layer of the floating gate of a flash memory. The charge accumulated in the floating gate can be detected by SNDM as a change in the capacitance of the poly-Si (floating gate) by scanning the surface of the SiO₂-SiN₄-SiO₂ (ONO) film covering the floating gate. There was a clear black contrast region in the SNDM image of the floating gate area, where electrons were injected. However, no clear contrast appeared in the floating gate where electrons were not injected. We confirmed that SNDM is one of the most useful methods of observing the charge accumulated in flash memory.