3D NAND Slit Bottom Punch Etch
Slit bottom punch is a short, separate etch run right after the main slit cut, and it exists because stopping a deep trench etch exactly on a thin film is a different, harder problem than cutting the trench itself.
The main slit etch is deliberately left incomplete by a few nanometers, on purpose, so this punch can finish the job with its own tuned chemistry.
The main slit etch stops just short of the source-line plate, not because the etch cannot go deeper, but because it should not. A single continuous etch run all the way to the plate risks either stopping early, leaving blocking film in place, or over-etching and damaging the plate itself — so the main cut is tuned to clear the bulk of the trench and deliberately leaves a thin oxide or ONO film in place as a safety margin.
The punch etch then removes exactly that thin remaining film, with chemistry suited to it rather than to the bulk oxide above it. Because the target here is thin and the plate underneath is not meant to be touched, this etch runs with tighter control and different selectivity than the trench etch that preceded it — a short step, but one with very little room for error in either direction.
Two modules downstream cannot start without this punch. Replacement gate's wet pull-back chemistry and the CSL contact fill both reach the source-line plate exclusively through the slit, which means neither one has a usable path into the array until the bottom punch finishes opening that last film.
This is the slit's version of the same problem the channel hole etch solves from above. The channel-hole bottom punch opens the ONO film at the base of each hole so the channel reaches the source plate vertically; the slit bottom punch opens the last film at the base of the trench so the slit reaches the same plate from the side — two separate punches, cut from two different directions, converging on one shared target.
Slit bottom punch is proof that the hardest part of an etch is sometimes not depth or aspect ratio but precision at the very end of a cut: the main slit trench across Samsung's V-NAND, SK hynix's 4D NAND, Kioxia/WD's BiCS, Micron, and YMTC's Xtacking all leave this last film deliberately uncut, because finishing it separately is safer than trying to land the whole trench in one continuous pass.