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.

<svg viewBox="0 0 410 302" xmlns="http://www.w3.org/2000/svg" style="max-width:100%;height:auto" role="img"><rect x="0" y="0" width="410" height="302" rx="12" fill="#0d1117"/><g font-family="ui-monospace,SFMono-Regular,Menlo,Consolas,&quot;Liberation Mono&quot;,monospace" font-size="14"><text xml:space="preserve" x="20" y="31.7"><tspan fill="#c9d1d9">Time-Dependent Drift Example:</tspan></text><text xml:space="preserve" x="20" y="50.7"><tspan fill="#c9d1d9">Parameter</tspan></text><text xml:space="preserve" x="20" y="69.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">↑</tspan><tspan fill="#c9d1d9">    UCL </tspan><tspan fill="#6e7681">─────────────────────────────</tspan></text><text xml:space="preserve" x="20" y="88.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">         ╱</tspan></text><text xml:space="preserve" x="20" y="107.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">        ╱   Drift over time</tspan></text><text xml:space="preserve" x="20" y="126.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">       ╱</tspan></text><text xml:space="preserve" x="20" y="145.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">  </tspan><tspan fill="#6e7681">────</tspan><tspan fill="#c9d1d9">╱</tspan></text><text xml:space="preserve" x="20" y="164.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">    Target</tspan></text><text xml:space="preserve" x="20" y="183.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan></text><text xml:space="preserve" x="20" y="202.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">    LCL </tspan><tspan fill="#6e7681">─────────────────────────────</tspan></text><text xml:space="preserve" x="20" y="221.7"><tspan fill="#c9d1d9">   </tspan><tspan fill="#6e7681">└────────────────────────────────────→</tspan></text><text xml:space="preserve" x="20" y="240.7"><tspan fill="#c9d1d9">      PM    1 week    2 weeks    3 weeks</tspan></text><text xml:space="preserve" x="20" y="259.7"><tspan fill="#c9d1d9">   </tspan></text><text xml:space="preserve" x="20" y="278.7"><tspan fill="#c9d1d9">Without intervention </tspan><tspan fill="#6e7681">→</tspan><tspan fill="#c9d1d9"> out of spec by week 3</tspan></text></g></svg>

Common Drift Sources:

Run-to-run control (R2R) compensates automatically.