Field note — etch
CD on the first production cassette after wet clean walked 4 nm. Chamber matching blamed RF hours. The tool had skipped seasoning; the wall was still “clean†in the wrong way.
After wet PM, chamber surfaces are not the steady-state chemistry the recipe was qualified against. Polymer and byproduct films that stabilize rate and selectivity rebuild over seasoning wafers (or a defined plasma recipe). Skip them to save takt and the first product lot becomes the seasoning lot—with customer silicon.
This is adjacent to load-lock leak-up and ESC helium stories, not a repeat. The patient is wall condition after open-chamber work.
What to run before product
Log: wet-clean ticket, seasoning recipe ID, wafer count, etch-rate monitors on first vs. fifth cassette, and who waived seasoning. A first-wafer cliff that vanishes by cassette three is wall memory, not a new gas bottle.
Rule
No product after wet PM without completed seasoning—or a signed risk accept with a hold on the first N wafers for metrology. “Base pressure good†is necessary and not sufficient.