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

A lateral charge-profiling technique based on local-flatband-voltage (V_FB)monitoring using optical-substrate-current spectroscopy (I_{Sub, photo} spectroscopy) in nitride-based charge-trap flash (CTF) memories is proposed. Under optical illumination by photons with above-bandgap energy E_ph(>E_g,Si), I_{Sub, photo} is abruptly increased at the gate voltage V_G=V_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_{Sub, 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_{Sub, 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_TC of the region, over which localized trapped charges are distributed, and its width to the density Q_nit(x)=qN_nit(x) [C/cm²] in the nitride storage layer. Based on the proposed I_{Sub, photo} spectroscopy, lateral charge profiling is demonstrated in a programmed NROM cell. The validity of the proposed I_{Sub, photo} spectroscopy is confirmed by comparing the measured I_D-V_G characteristics with TCAD simulation incorporating the extracted N_nit(x) by I_{Sub, 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.