A sawtooth appears when a parameter drifts steadily between maintenance events and snaps back when the event happens: etch rate decaying between wet cleans, removal rate falling as a polishing pad wears and jumping when it is replaced, an ion source aging until it is swapped. Each tooth's slope gives the drift rate and its length the maintenance interval. Charting it against a fixed centerline creates runs and false alarms, so engineers often chart the residual after removing the expected drift.
Removal rate is a perfect sawtooth with the pad changes. Plot it against pad life instead of date and every tooth lines up.
Also heard as
- sawtooth pattern
- PM sawtooth
Related terms
Drift
Slow, steady creep of a process or gauge reading in one direction as something wears, depletes or ages.
Shift
Sudden, sustained step change in a process mean, often after a maintenance event, material change or recipe edit.
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
EWMA
Exponentially weighted moving average: a chart or controller that blends each reading with the past to catch small, slow shifts.