ORIGINAL COMPANY-CONTEXT PRACTICE
Design a process experiment when lots share a chamber
Original practice scenario: A hypothetical semiconductor process experiment compares two settings. Consecutive lots use the same chamber and residual effects may carry over. How should assignment and analysis reflect that?
Written for independent practice using company context; this is not a verified question from a company interview.
A REASONED APPROACH
Model answer.
Choose an experimental unit that accounts for shared equipment and carryover. Treating every wafer as independent can create narrow uncertainty estimates without additional independent evidence.
Work through it
- Specify the setting change, outcome window and chamber dependencies, identifying whether cleaning, stabilization or prior recipes can affect later observations.
- Use a reviewed blocked or clustered assignment with an appropriate sequence and washout policy, preserving product mix and operational constraints in the design.
- Analyze at the assignment level or with a justified dependence model, reporting uncertainty and checking order effects and chamber-specific results.
Tradeoffs to discuss
- Larger assignment blocks reduce cross-treatment contamination but provide fewer independent comparisons; frequent switching improves balance while increasing setup and carryover concerns.
Common pitfalls
- Do not remove inconvenient stabilization observations after inspecting outcomes.
- A statistical improvement in a controlled experiment is not by itself authorization to change an operational process.