Hot-Phosphoric-Acid Etch Station

Hot-phosphoric-acid, dilute-HF, and ozonated-water stations round out the standard wet-bench chemistry toolkit alongside RCA, piranha, and BOE — each targeting a specific film or contaminant that the others don't reach well. Hot phosphoric acid selectively strips silicon nitride; dilute HF is the gentler, lower-etch-rate cousin of buffered oxide etch; ozonated water replaces some of the harshest sulfuric/peroxide chemistry for final organic residue removal.

Nitride-over-oxide selectivity is the whole reason hot phosphoric acid earns its own dedicated station rather than sharing one with HF chemistries. Hot phosphoric acid at high temperature etches silicon nitride while barely touching silicon dioxide, which makes it the standard choice whenever a process needs to remove a nitride layer — a sacrificial spacer, a hard mask, a STI liner — without damaging an adjacent oxide film that HF-based chemistry would attack instead. That selectivity, not etch speed, is the property that makes it irreplaceable in these specific steps.

Ozonated water earns a place in modern fabs specifically because it's a safer substitute for part of what piranha used to do. Sulfuric-peroxide (piranha) clean is extremely effective at stripping organic contamination but comes with real handling hazards and generates a large volume of hazardous waste; ozonated DI water oxidizes much of the same organic residue with ozone gas dissolved in ultrapure water, at far lower chemical risk, which is why it has displaced some piranha steps in flows where the organic load doesn't require piranha's full strength.

Hot phosphoric acid, dilute HF, and ozonated water each exist to do one specific selective job that the fab's other standard wet chemistries handle poorly or not at all — nitride-selective strip, gentle native-oxide removal, and lower-hazard organic cleaning, respectively — rounding out the wet bench's chemistry toolkit rather than duplicating RCA or piranha.