Depth Dependence of Threshold Voltage Shift in 3-D Flash Memories Exposed to X-Rays

The effects of total ionizing dose on 3-D NAND floating gate cells with vertical architecture are analyzed as a function of the cell depth in the pillar. The focus of this work is on TID-induced threshold voltage shifts, but the implications on the raw bit error rates are also discussed. Underlying mechanisms are elucidated, concluding that due to the manufacturing process and the geometry of the pillars, the effects of total dose are larger at the bottom than at the top of the cell array.