Drift is gradual change over time: an etch rate falling as chamber walls coat, a polishing pad glazing, a sensor aging, a filament thinning, a reference standard degrading. On a control chart it shows up as a trend or a long run before any single point breaks a limit. Run-to-run controllers and EWMA charts are designed to track it, and some scheduled maintenance exists purely to reset it.
Deposition rate drifts down about half a percent a day until the next wet clean, so the controller bumps time to compensate.
Gauge drift looks exactly like process drift on a chart. Run the golden wafer before you retune the tool.
Also heard as
- creep
- trend
- walk
Related terms
Shift
Sudden, sustained step change in a process mean, often after a maintenance event, material change or recipe edit.
EWMA
Exponentially weighted moving average: a chart or controller that blends each reading with the past to catch small, slow shifts.
Sawtooth
Chart pattern of gradual drift and a sudden reset, repeating with each maintenance cycle or consumable change.
Run of seven
Seven consecutive chart points on one side of the centerline, or steadily rising or falling, read as a process shift.