Investigation of maximum current sensing window for two-side operation, four-bit/cell MLC nitride-trapping nonvolatile flash memories

Localized charges in a nitride-trapping device provide two-bit/cell operations. Adding multilevel-cells (MLCs) to the physical bits produces a four-bit/cell device. However, it is difficult to get sufficient sensing windows for MLC operation because the left bit and right bit interfere with each other. This letter analyzes the effect of the second bit effect and investigates parameters affecting the sensing current window for physical four-bit/cell operations. The sensing window is found to increase with a higher reading bias, and also with a higher programmed V_t. However, severe second bit effects set in at high V_t, and decreased the sensing window again. An optimal sensing window is found at moderately high V_t.