3D NAND Staircase Landing Pad Etch

Step etch is easy to confuse with the trim-etch loop that shapes the staircase, but it is a separate etch with a separate job: once the staircase's shape is fully cut, step etch exposes the specific SiN surface on every one of those steps that a word-line contact will eventually need to land on.

Trim-etch loop finished
staircase shape fully cut
Step etch
exposes SiN pad on every stair
Oxide gap-fill
buries the whole staircase
CMP planarization
flattens the top surface
Staircase-oxide recess
re-exposes each pad at its depth
Word-line contact etch next
lands exactly here

That landing pad then gets buried, flattened, and re-exposed before anything ever contacts it.

Step Etch — Exposing the Landing Pad the Trim-Etch Shape Left Behind a separate etch from the trim-etch loop, run once the staircase shape itself is finished step etch exposes SiN landing here After the staircase shape 1. Step etch — exposes SiN pad on each stair, after trim-etch 2. Oxide gap-fill — buries the whole staircase in thick dielectric 3. CMP + oxide recess — flattens, then re-exposes every pad at correct depth Why step etch is its own step, not part of the trim-etch loop the trim-etch loop's job is shaping the staircase — cutting the stack back by one layer per cycle; step etch's job is exposing the specific SiN surface on each finished step that the later word-line contact etch will actually land on — a different target, run once shaping is done, not during it. buried again immediately by oxide fill, then re-exposed one more time by the staircase-oxide recess — three separate exposures of the same pad

The trim-etch loop shapes; step etch exposes. Every cycle of the trim-etch loop narrows the photoresist mask and removes one more pair of layers, building the staircase's physical shape one step at a time — but it is step etch, run once that shaping is complete, that clears away whatever thin film remains on top of each stair to expose the SiN the stair is actually meant to land a contact on.

Oxide gap-fill immediately buries the landing pad this etch just exposed. A thick dielectric fill covers the entire staircase structure right after step etch finishes, which sounds counterproductive until the next two steps are counted — this fill is what gives CMP a flat surface to plan to, and what the staircase-oxide recess will later cut back through.

CMP planarization and the staircase-oxide recess are what actually give the landing pad back. CMP flattens the gap-fill oxide across the whole staircase, and the recess that follows etches back down to each step's depth, re-exposing the same SiN pad that step etch originally uncovered — meaning this one landing surface gets exposed, buried, and re-exposed across three separate process steps before it is ever usable.

Getting the depth wrong at step etch shows up two modules later, not immediately. Because the pad gets buried again right after step etch runs, a landing pad left slightly too thick or too thin here will not surface as a problem until the staircase-oxide recess tries to re-expose it — by which point the error has already propagated through two more process steps.

Step etch is the staircase module's quiet middle step: the trim-etch loop gets the attention for carving dozens of depths into one stack, and the word-line contact etch gets the attention for landing on all of them, but step etch is the one actually responsible for making sure there is a clean, contactable surface sitting on top of every one of those steps in between.

Take 3d nand staircase landing pad etch further

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