queue time

Queue time is the **non-productive waiting period** between consecutive process steps in semiconductor manufacturing. It's one of the largest contributors to overall cycle time—often **60-80%** of total fab cycle time is queue time, not actual processing. **Why Queue Time Matters** Queue time isn't just about efficiency. Some films **oxidize or absorb moisture** if wafers wait too long, directly impacting yield. For example, gate oxide pre-clean to oxidation must happen within **2-4 hours** or native oxide regrows. Reducing queue time also cuts time-to-market and WIP inventory costs. **Critical Q-Time Sequences** • Pre-clean → Gate oxidation: **< 4 hours** (native oxide regrowth) • Metal deposition → CMP: **< 24 hours** (copper oxidation/corrosion) • Wet etch → Diffusion: **< 2 hours** (surface contamination) • Litho coat → Expose → Develop: **< 8 hours** (resist aging) **How to Reduce Queue Time** The most effective strategies include adding capacity at bottleneck tools, optimizing dispatching rules to prioritize critical lots, smoothing WIP flow to reduce variability, and co-locating tools to minimize transport time between critical process pairs.