what causes deposition rate drift and how do you catch it

Deposition RATE slowly drifts over time — the film gets thicker or thinner than the target for the same recipe. Common causes: chamber wall buildup (Lesson 58), target wear (PVD — target thins as it erodes), temperature drift, gas flow/pressure changes, component aging. How you CATCH it: monitor thickness/uniformity over time (you see a trend before it's out of spec), track process data, watch for drift after cleans or between PMs. How you correct it: clean/condition, replace the target, adjust recipe, PM. The key line: 'Rate drift is the slow thief of uniformity — I catch it by monitoring trends, not reacting to one bad wafer.' Your measurement background (thickness/uniformity) makes you well-suited to catch drift early. Show the trend-tracking mindset.