WHY SPC

Catch drift before it ruins a wafer

prevent bad wafers, don't just find them

Statistical process control, SPC, tracks a process's key measurements over time and flags it the moment it starts drifting away from normal, often before a single wafer is actually ruined. Waiting for final test to catch a problem means every wafer made in between is already at risk.

SPC's whole purpose is to catch a problem before it becomes expensive.

THE CONTROL CHART

A measurement, watched over time

A control chart plots a measurement in the order it was taken, with upper and lower control limits drawn from the process's normal variation. A point outside those limits, or a suspicious run of points drifting one direction, triggers an investigation immediately.

The limits aren't a spec; they're where this process normally lives.

WHAT GETS CHARTED

Anything worth watching continuously

📏

Critical dimension

Tracked after lithography and etch on nearly every layer

📐

Film thickness

Tracked after every deposition or growth step

🎯

Particle counts

Tracked to catch a tool quietly getting dirtier over time

If a step has a target, there's usually a control chart watching it.

OUT OF CONTROL

A signal, not a verdict

investigate first, don't just discard

A point outside the control limits, or several points trending the same direction, is called an out-of-control signal. It doesn't mean the wafer is necessarily bad; it means something changed, and an engineer needs to find out what before more wafers run through that same tool.

An out-of-control signal is a question, and the fab has to answer it before moving on.

UNIT 35 STUDY COMPLETE

📉

Ready for the Fab Challenge?

You've covered catching a process drifting out of spec before it ruins a wafer.