3D NAND

# Word-Line and Control-Gate Layering

## 9. 9. Word Line and Control Gate Layering

Each horizontal layer in a 3D NAND stack is built from an alternating pair of thin films: one film that eventually becomes a conductive word line, wrapping around every channel hole that passes through that layer, and one insulating film that separates it from the word line above and below. In the most common manufacturing approach, the stack is first deposited as alternating layers of silicon oxide and silicon nitride (a sacrificial nitride here, distinct in role from the charge-trap nitride described earlier, even though it is chemically the same material) — oxide for insulation, sacrificial nitride standing in for where the conductive word line will eventually go. After the channel holes are etched and the ONO/channel structure is formed inside them (the next two steps cover this etch and deposition sequence), a separate process removes the sacrificial nitride layers through narrow slits cut into the stack and replaces them with a conductive metal, typically tungsten, forming the actual word lines — a sequence often called the “replacement gate” or “gate-last” process. Every word line that results wraps fully around every channel hole at its layer, which is exactly the gate-all-around geometry described in step 7, repeated independently at each of the dozens or hundreds of layers the stack contains.

Alternating Layers: Insulator, Sacrificial Film, Repeat 9. Word Line and Control Gate Layering AS FIRST DEPOSITED Alternating silicon oxide and sacrificial silicon nitride Oxide insulates, sacrificial nitride marks a future word line Different role than the charge-trap nitride, same material the stack starts as a placeholder sandwich, not yet functional word lines
The Replacement-Gate Step: Sacrificial Nitride Becomes Tungsten 9. Word Line and Control Gate Layering REMOVE Narrow slits cut into the finished stack Sacrificial nitride etched out through those slits REPLACE Conductive tungsten fills the resulting gaps Forms the real word line at every layer at once RESULT Gate-all-around word line wraps each channel hole Repeated independently at every one of many layers often called the replacement-gate or gate-last process

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