3D NAND

# One Vertical String Replaces Many Planar Cells

## 8. 8. One Vertical String Replaces Many Planar Cells

Put the macaroni channel and the layer-by-layer word lines together and the result is a 3D NAND string: a single vertical polysilicon tube, passing through dozens or hundreds of stacked word-line layers, with one memory cell formed wherever the tube crosses a word line. This is the structural payoff of going vertical. A planar NAND string achieved its serial, page-and-block organization by laying dozens of discrete floating-gate transistors in a row across the wafer surface, connected source-to-drain, with lithography defining every individual cell's position along that row. A 3D NAND string achieves the identical electrical organization — the same serial string of transistors sharing a channel, each independently addressable by its own control gate — by building one hole, one macaroni tube, and letting that single vertical structure cross as many word-line layers as the stack contains. Adding capacity no longer means drawing more discrete transistors across more wafer area; it means adding more word-line layers to the same stack and letting the same channel hole simply run a little taller through it. This is also the core reason 3D NAND's scaling metric became layer count rather than lithographic feature size: the lithography that matters most is the hole itself, drilled once, not the cell pitch repeated across the wafer.

Same String Organization, Different Direction 8. One Vertical String Replaces Many Planar Cells PLANAR STRING Many discrete floating-gate transistors in a row Lithography places and isolates each cell separately More cells per string means more wafer area used 3D VERTICAL STRING One macaroni channel crossing many word-line layers Each crossing forms one cell, no separate placement step More cells per string means more layers, same footprint the serial string survives the pivot, only its geometry changes
Why Layer Count Became the Scaling Metric That Matters 8. One Vertical String Replaces Many Planar Cells WHAT USED TO SCALE CAPACITY Lithographic cell pitch, shrunk node after node This is the knob that hit the walls in steps 1 and 2 WHAT SCALES CAPACITY NOW Number of stacked word-line layers one hole crosses The hole itself is drilled once per string, not per cell add a layer, add a cell to every string at once, no new lithography per cell

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