3D NAND Source Formation Etch
Source formation is the etch family that reaches the channel from a direction nothing else in the flow uses: sideways, through the slit, rather than straight down through the channel hole or straight through the slit from above the way CSL does.
It exists because the bottom punch from the channel-hole module cannot be trusted, by itself, to guarantee every one of 200 to 300-plus holes reaches the source plate cleanly.
A sacrificial source-poly strip opens the lateral path first. Removed through the slit, this strip clears a sacrificial fill out of the way near the bottom of the stack, giving the source-formation etches a route in toward the channel from the side instead of from directly below it.
Side-wall ONO removal, also called SWC (side-wall-contact etch), is the step that actually exposes the channel. It cuts the oxide-nitride-oxide charge-trap film off the channel's side face near its base — not at the bottom, where the earlier bottom-punch etch already worked, but along the wall, laterally, so the channel can meet the source plate from the direction the slit opened up.
A source-poly recess finishes the connection. Once the channel's side face is exposed, this recess shapes the final lateral joint between the channel and the source-line plate, completing a path that the bottom punch alone — a purely vertical etch — was never designed to guarantee on its own.
This is the array's second route to the same plate CSL reaches from above. CSL etch and contact fill close the slit vertically, straight down to the source-line plate; source formation closes the gap laterally, straight into the channel's side wall — two different etch families, run through the same trench, meeting at the same physical layer from two different directions.
Source formation is the step most easily mistaken for redundant, because the bottom punch already opened the ONO film once — but across Samsung's V-NAND, SK hynix's 4D NAND, Kioxia/WD's BiCS, Micron, and YMTC's Xtacking, a vertical punch alone has never been treated as sufficient; the lateral side-wall path through the slit is what actually closes the source connection with margin to spare.