High-Speed Multilevel nand Flash Memory With Tight $V_{\rm th}$ Distribution Using an Engineered Potential Well and Forward-Bias Adjusted Programming

This paper reports a high-speed multilevel-cell nand Flash memory device using a Si- SiO₂-TiN- TiO₂-SiO₂-TaN (SOTTOT) engineered potential well (EW). The SOTTOT EW Flash memory device has very fast cell programming speed and good data retention. A 16-kbit nand memory block using SOTTOT cells was programmed using a forward-bias-adjusted programming scheme, which enables bit adjustability during page programming to suppress the development of fast bits. The SOTTOT memory block shows fast programming speed (~40 μs/page), tight threshold voltage (V_th) distribution (~0.22 V/level), and clear V_th-level margins (~0.9 V) for the eight-level programming. The SOTTOT memory block also shows good resistance against pass/read disturbances as well as good ten-year data retention at an ambient temperature of 75 °C throughout 10⁵ programming/erasing cycling.