why is first wafer after clean a qualification wafer

After a chamber clean, the chamber isn't stable immediately — the clean strips old buildup and changes the surface state, so the first wafer or two won't represent the steady-state process. That's why the FIRST wafer after clean is often treated as a QUALIFICATION wafer — you run it to confirm the chamber is back to baseline (correct deposition rate, uniformity, low particles) BEFORE you trust it for product wafers. Why GPS cares: it's part of process control — you don't want a contaminated or non-representative first wafer going to a paying customer. The key line: 'The wafer right after clean is the chamber's check-in — I qualify it to confirm baseline is restored before product runs.' It shows you understand the clean→passivate→qualify cycle, which is real field knowledge.