3D NAND Slit Etch

Slit etch, usually called gate-line slit or GLS, is the long trench that turns one continuous ONON stack into separate, electrically isolated memory blocks — and it is also the only path the replacement-gate module will later use to reach every word line in the array from the outside.

Stack and channel holes done
staircase already cut
Slit mask patterned
block, sub-block, select-gate zones
Slit depth familysame mask layer, different target depths
Block-boundary slit
full stack depth
Sub-block slit
partial depth, inside a block
TSG/SSL + BSG/GSL cut
shallow, top and bottom select gates
Slit bottom punch
opens source-line plate
Replacement gate next
runs back through this same trench

The same slit mask layer produces several very different trench depths depending on where in the array it falls.

Gate-Line Slit (GLS) — the Trench That Turns One Stack Into Blocks full-depth block cuts, a shallower sub-block cut, and even shallower select-gate cuts, all in the same stack ONON stack — same stack the channel holes and staircase etch run through Slit (block cut) channel holes (block interior) sub-block slit — partial depth TSG/SSL cut (drain select) BSG/GSL cut (source select) source-line plate — slit bottom punch opens this up from above Four depths, one trench-shaped etch block-boundary slit runs the full stack depth; a sub-block slit inside a block runs partial depth; TSG/SSL and BSG/GSL cuts near the very top and bottom are shallower still — only a few layers — splitting the drain-side and source-side select gates without touching the word lines below or above. next: replacement gate runs entirely back through this same slit — SiN pull-back, metal recess, CSL fill

The block-boundary slit runs the full depth of the stack, cutting all the way from the top of the array down to the source-line plate, and it is this cut that physically separates one memory block from its neighbor — every word line on one side of the slit is electrically isolated from the matching word line on the other side.

A sub-block slit, or partial-depth cut, divides a single block into smaller sub-blocks without running the full stack depth, trading a shallower etch for finer-grained erase and read control inside what would otherwise be one large block.

The select-gate cuts are the shallowest slits in the array, and the two are not interchangeable. A top select-gate cut (TSG/SSL cut) splits the drain-side select gate near the top of the stack; a bottom select-gate cut (BSG/GSL cut) splits the source-side select gate near the bottom — each only a few layers deep, precisely because cutting any deeper would sever word lines the design needs to keep intact.

A slit-bottom punch finishes the block-boundary and sub-block slits by opening the source-line plate underneath, which is what lets the common-source-line fill — and, far more importantly, the entire replacement-gate wet-etch chemistry — reach every layer of the stack from outside instead of only from the top.

The slit is the one etch in 3D NAND that exists purely to create access, not structure: Samsung's V-NAND, SK hynix's 4D NAND, Kioxia/WD's BiCS, Micron, and YMTC's Xtacking all cut this same family of depths — full block, sub-block, and the two select-gate trims — because the replacement-gate module that follows cannot pull a single sacrificial nitride layer out of a 200 to 300-plus layer stack any other way.

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