Nanoscale Dynamics of Buried Charge Trap in Oxide-Nitride-Oxide Stacks Investigated Using Kelvin Probe Force Microscopy

Understanding charge trapping and diffusion behavior is crucial for optimizing nonvolatile memory (NVM) devices, which commercially involve a silicon oxide-silicon nitride-silicon oxide (ONO) stack structure. However, studies on charge traps in ONO stack structures using Kelvin probe force microscopy (KPFM) have been limited. Here, we examine trapped charge dynamics in ONO devices using KPFM, especially trapped electrons and holes in the buried silicon nitride layer and on the exposed silicon oxide. Furthermore, we observe diffusion of trapped holes on the exposed oxide and electron diffusion within the buried nitride during the program process. As a result, the diffusion coefficients are quantified as 5.20 × 10^-13 cm²/s for holes trapped in the exposed oxide layer and 1.22 × 10^-14 cm²/s for electrons trapped in the subsurface nitride layer. Hence, we identify distinct charge dissipation in buried and surface layers, demonstrating the capability of KPFM to characterize charge-trap materials and improve NVM devices.