Total-Ionizing Dose Effect in 3-D Charge-Trap nand Flash Memory Cells

초록

Total-ionizing dose (TID) effects on programmed cells in 3-D charge-trap NAND flash memory are investigated using technology-computer-aided design (TCAD) simulations calibrated to a commercial off-the shelf (COTS) 176-layer 3-D NAND flash memory. During retention, the surrounding electric field (E-field) enhances electron-hole pair (EHP) generation and induces hole trapping in the charge-trap layer (CTL), resulting in a reduction in the threshold voltage (V-T ). The blocking oxide (BOX) is more strongly affected by radiation due to its larger volume compared to the bandgap engineered (BE)-tunneling oxide (TOX). V-T degradation becomes more pronounced with increasing total dose, particularly at higher program verifying (PV) levels. Moreover, V-T state reliability by TID becomes more vulnerable on continued scaling of the gate and spacer lengths, below the word line pitch of 54 nm. Furthermore, BOX thickness introduces a tradeoff between retention and TID-induced V-T degradation, where a thinner BOX improves the radiation hardness with a cost of time dependent retention loss

키워드