An excursion is an unexpected deviation from normal process behavior or specifications that may affect product quality, requiring investigation and corrective action. Detection: Identified through SPC chart violations (out-of-control points, trends, shifts), metrology specification failures, defect inspection spikes, tool sensor anomalies, or parametric test failures. Types: Process excursion: Recipe deviation, tool malfunction, contamination event, chemical quality issue. Defect excursion: Sudden increase in defect density at a process step. Parametric excursion: Electrical parameters drifting or jumping outside control limits. Response protocol: 1) Detect and alert. 2) Hold affected lots. 3) Quarantine suspect tool. 4) Investigate root cause. 5) Assess material disposition. 6) Corrective action. 7) Resume production. Lot hold: Affected lots placed on engineering hold pending investigation. Cannot proceed to next process step until released. Material disposition: After investigation, lots may be: released (no impact), reworked (redo the step), scrapped (unrecoverable), or downgraded (sell at lower spec). Impact assessment: Determine which lots, wafers, and dies are affected. May require additional testing or inspection. Notification: Customers may need notification if shipped product could be affected. Documentation: Full excursion report documenting root cause, affected material, corrective actions, and preventive measures. Prevention: Robust FDC, APC, and SPC systems minimize excursion frequency and duration. Cost: Excursions are expensive - scrap cost, investigation time, lost throughput, potential customer impact.