wip (work in progress)

WIP (Work in Progress) refers to the total inventory of wafers currently being processed within the semiconductor fab — encompassing all wafers at every stage of manufacturing from lot start through final test, including wafers actively being processed in tools, wafers waiting in queues, wafers in stockers and transport, and wafers on hold for engineering review. WIP management is a critical production metric because it directly impacts fab cycle time, throughput, tool utilization, and manufacturing cost. Little's Law provides the fundamental relationship: Cycle Time = WIP / Throughput, meaning for a given throughput level, higher WIP results in longer cycle time (wafers spending more time waiting in queues rather than being processed). WIP levels are managed at multiple granularities: total fab WIP (total wafer count — a major factory metric, typically measured in wafer starts per week or month), area WIP (inventory within specific process areas — lithography, etch, deposition, implant), tool group WIP (wafers queued for specific equipment groups), and lot-level WIP (tracking individual lots through the process flow). Optimal WIP balances competing objectives: too little WIP risks tool starvation (machines idle because no wafers are available — reducing throughput), while too much WIP increases cycle time (wafers spend excessive time in queues — increasing inventory cost and time-to-market). The relationship is nonlinear — as WIP increases beyond the optimal point, cycle time increases dramatically while throughput plateaus. Fab scheduling systems actively manage WIP through dispatch rules that prioritize lots based on factors including: due date urgency, process step criticality, hot lot priority, tool availability, and WIP balance targets. Advanced fab management uses WIP targets by technology node, product family, and process area to maintain optimal flow. Typical advanced logic fabs maintain WIP levels supporting 2-4 weeks of cycle time for commodity products and under 2 weeks for priority products.