Lateral Trapped-Charge Profiling Based on the Extraction of the Flatband Voltage by Using the Optical Substrate Current in Nitride-Based Charge-Trap Flash Memories

摘要

A lateral charge-profiling technique based on local-flatband-voltage $(V_{ rm FB})$ monitoring using optical-substrate-current spectroscopy ( $I_{{rm Sub}, {rm photo}}$ spectroscopy) in nitride-based charge-trap Flash (CTF) memories is proposed. Under optical illumination by photons with above-bandgap energy $E_{rm ph} (≫ E_{g, {rm Si}})$, $I_{{rm Sub}, {rm photo}}$ is abruptly increased at the gate voltage $V_{G} = V_{rm FB}$ due to a sudden increase of the excess minority-carrier diffusion current. As expected in this principle, while the single-step feature of the $I_{{rm Sub}, {rm photo}}$ –$V_{G}$ curve is observed in the case of N-MOSFETs, a multistep response is clearly observed in nitride-based CTF memories. The mechanism of steplike $I_{{rm Sub}, {rm photo}}$ spectroscopy is analyzed, supported by analytical models, and verified by comparison with TCAD simulation results. The results show that the height of the step corresponds to the lateral length $L_{rm TC}$ of the region, over which localized trapped charges are distributed, and its width to the density $Q_{rm nit}(x) = qN_{rm nit}(x) [hbox{C/cm}^{2}]$ in the nitride storage layer. Based on the proposed $I_{{rm Sub}, {rm photo}}$ spectroscopy, lat-neral charge profiling is demonstrated in a programmed NROM cell. The validity of the proposed $I_{{rm Sub}, {rm photo}}$ spectroscopy is confirmed by comparing the measured $I_{D}$– $V_{G}$ characteristics with TCAD simulation incorporating the extracted $N_{rm nit}(x)$ by $I_{{rm Sub}, {rm photo}}$ spectroscopy. The proposed lateral charge-profiling technique is expected to be a useful technique for extracting the trapped-charge distribution in NROM and/or multibit CTF memories. This extraction technique has advantages of electrical stress free, exclusion of the effect from interface traps, and the applicability to devices with large gate leakage current.