Modelling and performance evaluation of an overhead hoist transport system in a 300 mm fabrication plant

In a 300 mm wafer fabrication plant, the automated wafer transport system is more important than in a 200 mm one. The weight of 25 pieces of 300 mm wafer in a front opening unified pod (FOUP) makes the frequent manual carrying and lifting impractical. An overhead hoist transport (OHT) provides the pod transport between process tools and stockers in an intrabay with high efficiency and safety. A modular-based coloured timed Petri net (CTPN) is used to model the dynamic behaviour of the OHT. The OHT system performance is evaluated using an object-based simulation technique. Performance measures are used to determine the number of OHT vehicles in the planning stage and also to control the dispatching of the OHT system in the operation stage. The CTPN token competition and conflict resolution rules are used to develop the intelligent controller. Finally, a simplified intrabay with four stations is constructed and evaluated.