Novel ultrahigh-density flash memory with a stacked-surrounding gate transistor (S-SGT) structured cell

In order to overcome the limitation of cell area of 4F² per bit in conventional NAND flash memory cells, stacked-surrounding gate transistor (S-SGT) structured cell is proposed. This newly structured cell achieves a cell area of 4F²/N per bit, where N is the number of stacked memory cells in one silicon pillar, without using multibit per memory cell technology. The S-SGT structured cell consisting of two stacked memory cells in one silicon pillar achieves a cell area per bit of less than 50% of the smallest reported NAND structured cell. The novel S-SGT structured cells are fabricated by vertical self-aligned processes using a 0.2 μm design rule. The S-SGT structured cell can be programmed and erased by uniform injection and uniform emission of Fowler-Nordheim (F-N) tunneling electrons over the whole channel area of the memory cell, respectively, which is the same program and erase mechanism as in conventional NAND structured cell. This high performance S-SGT structured cell is applicable to high-density nonvolatile memories for 16 G/64 G bit Flash memories and beyond.