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>Sub,

photo</sub>
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>Sub,

photo</sub>
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>Sub,

photo</sub>
-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>Sub,

photo</sub>
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/cm
²
] in the nitride storage layer. Based on the proposed I
<sub>Sub,

photo</sub>
spectroscopy, lateral charge profiling is demonstrated in a programmed NROM cell. The validity of the proposed I
<sub>Sub,

photo</sub>
spectroscopy is confirmed by comparing the measured I
_D
-V
_G
characteristics with TCAD simulation incorporating the extracted N
_nit
(x) by I
<sub>Sub,

photo</sub>
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.