Metrology
Beginner
Measuring the wafer during manufacturing to make sure each layer came out the right size and in the right place.
Novice
Measurement inside the fab: the thickness of each deposited film, the width of the smallest printed shapes (the critical dimension), and how well each layer lines up with the one below (overlay). Inspections for defects run alongside. The results keep each step within its limits.
Expert
The data feed statistical process control (charts that flag a tool drifting outside its normal range) and run-to-run correction (adjusting a tool’s settings for the next batch from the last one’s measurements). Measuring every wafer would cost tool time and delay, so fabs sample, trading cost against how quickly they catch a problem.