Comprehensive Characterization Method for Modeling Retention Transients in NAND Flash Memory
Wen-Chien Liu, Yung-Yueh Chiu, Toshiaki Takeshita, R. Shirota · IEEE Transactions on Electron Devices · 2024
A comprehensive characterization method and a physically based model for profiling the evolution of the trapped oxide charge (${Q}_{T}\text {)}$during data retention (high-temperature baking after program/erase (P/E) cycling) was developed. This method involves monitoring the transients of threshold-voltage (${V}_{\text {th}}\text {)}$shift and transconductance (${G}_{m}\text {)}$recovery simultaneously. It is observed that the scatter relation between${G}_{m}$recovery and the corresponding${V}_{\text {th}}$reduction for different baking temperatures can be effectively merged over a universal curve. This curve consists of two stages. In the first stage of retention,${V}_{\text {th}}$decreases but${G}_{m}$remains almost constant. In the subsequent stage,${G}_{m}$increases in proportion to the decrease in${V}_{\text {th}}$. To describe this characteristic, two different${Q}_{T}$distribution regions were indispensably introduced; one is a near-interfacial (NI) region close to the Si surface and the other is a bulk oxide (BO) region. During retention,${Q}_{T}$in the NI region tunnels out, and simultaneously${Q}_{T}$in the BO region supplements it via transport mechanisms. The transition point between the first and second stages occurs when all the${Q}_{T}$in the BO region dissipated. Results show good agreement between measured and simulated retention transients.