time-dependent variation
**Time-Dependent Variation** in semiconductor manufacturing refers to process parameter changes that occur over time due to equipment drift, consumable wear, or environmental factors.
## What Is Time-Dependent Variation?
- **Scale**: Hours to weeks of gradual shift
- **Sources**: Chamber seasoning, chemical aging, target erosion
- **Detection**: SPC trend analysis, drift alarms
- **Mitigation**: Scheduled maintenance, adaptive process control
## Why Time-Dependent Variation Matters
Without correction, gradual drift causes specification violations before the next scheduled maintenance window.
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**Common Drift Sources**:
| Equipment | Drift Mechanism | Rate |
|-----------|-----------------|------|
| PVD target | Erosion | ~1%/week |
| CVD chamber | Seasoning | Variable |
| Etch chamber | Polymer buildup | Days |
| Wet bench | Chemical depletion | Hours |
Run-to-run control (R2R) compensates automatically.