Pad Nitride Strip
The pad nitride strip removes the silicon nitride layer that protected the active silicon all the way through the entire STI module — trench etch, fill, and oxide recess — using hot phosphoric acid in a wet process rather than any kind of plasma etch, specifically because the wet chemistry can be made extremely selective against silicon and against the oxide fill sitting right next to it. This pad nitride was never meant to be a permanent part of the chip; it existed purely to stop the STI trench etch and the later CMP or recess steps from attacking the active silicon directly, and once the entire STI sequence is finished, that protective layer has to come off completely without touching anything it was protecting.
Hot phosphoric acid earns its place here specifically because of how differently it treats nitride versus the materials surrounding it. At an elevated temperature, typically well above 150°C, phosphoric acid etches silicon nitride at a rate dramatically faster than it attacks either silicon or silicon dioxide, which means a timed wet strip can clear the pad nitride completely while leaving the STI oxide fill's carefully targeted recess depth essentially untouched — a plasma strip chemistry able to achieve the same selectivity against both of those competing materials simultaneously would be far harder to tune, which is why this particular step stays a wet process even in a flow built almost entirely around dry plasma etching.
What this strip leaves behind directly inherits whatever happened during every step that came before it, which makes it the natural point where accumulated process errors finally become visible. The pad nitride strip doesn't introduce new topography of its own, but the active silicon surface it exposes carries forward the exact result of the trench etch's profile, the fill's gap quality, and the recess etch's targeted depth — any residue, undercut, or recess-depth error from those earlier steps shows up clearly once this strip removes the layer that had been partially hiding it, which is why inspection after the pad strip is treated as a meaningful checkpoint for the whole STI module rather than a routine pass/fail on the strip itself.
Because this step is wet rather than dry, it runs on an entirely different class of equipment than every etch discussed earlier in this list. Wet processing tools from suppliers such as SCREEN Semiconductor Solutions and Tokyo Electron (which builds both dry etch and wet-clean platforms) handle hot-acid strip steps like this one in dedicated single-wafer or batch wet stations, physically separate from the plasma etch chambers used for the trench and recess steps that precede it — a reminder that a real DRAM process flow alternates between dry plasma and wet chemical steps constantly, choosing whichever chemistry actually delivers the selectivity a given step needs rather than defaulting to one processing technology throughout.
Read the pad nitride strip through a *protection-finished-remove-completely* lens rather than a *final-cleanup* lens — this layer did real, necessary work holding the active silicon safe through an entire sequence of trench, fill, and recess steps, and the strip's entire job is making sure that protection disappears without a trace exactly when its usefulness runs out.