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.

Time-Dependent Drift Example:Parameter ↑ UCL ───────────────────────────── │ ╱ │ ╱ Drift over time │ ╱ │ ────╱ │ Target │ │ LCL ───────────────────────────── └────────────────────────────────────→ PM 1 week 2 weeks 3 weeks Without intervention → out of spec by week 3

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.

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