Study of sub-30nm thin film transistor (TFT) charge-trapping (CT) devices for 3D NAND flash application

Sub-30 nm TFT CT NAND flash devices have been extensively studied. Although TFT devices were often believed to have much worse performance than bulk devices, our results show that as devices scale down to sub-30 nm, the DC characteristics (such as read current and subthreshold slope (S.S.)) approach those of the bulk devices because sub-30 nm TFT devices often contain no grain boundaries. The memory window is also larger than the bulk planar devices due to the tri-gate structure that enhances the electric field during programming/erasing. However, a fair percentage of devices contain grain boundaries with poorer S.S. and g_m. Interestingly, this only affects the DC characteristics but does not impact the memory window. Furthermore, grain boundaries do not increase the random telegraph noise. The most serious drawback of grain boundaries is the impact on self-boosting window caused by junction leakage. A sub-30 nm TFT BE-SONOS NAND device with MLC capability and good retention is demonstrated.