Critical queue time is the high-priority waiting-time window where approaching expiration requires immediate scheduling escalation to avoid process violation - it represents near-term risk state for time-sensitive lots.

What Is Critical queue time?

• Definition: Queue-time condition where remaining allowable wait margin is low and urgent action is needed.
• Risk Signal: Lot transitions from normal priority to elevated priority as deadline proximity increases.
• Scheduling Role: Drives dispatch override logic and contingency routing decisions.
• Operational Context: Common in clean-to-furnace, etch-to-clean, and other sensitive sequence loops.

Why Critical queue time Matters

• Violation Prevention: Early escalation reduces probability of max-queue breaches.
• Yield Protection: Avoids quality loss from aging-sensitive intermediate states.
• Flow Coordination: Forces alignment between upstream start decisions and downstream capacity.
• Resource Prioritization: Focuses attention on lots with highest immediate quality risk.
• Excursion Control: Prevents clustered failures when bottlenecks disrupt time-critical routes.

How It Is Used in Practice

• Risk Tiering: Define warning bands based on remaining queue-time margin.
• Dispatch Escalation: Promote critical lots to priority classes with protected transport and tool access.
• Precheck Rules: Block upstream release when downstream readiness cannot support time-window compliance.

Critical queue time is an essential real-time risk signal for fab scheduling - proactive escalation around shrinking queue margins is necessary to maintain quality and flow reliability.