oht (overhead hoist transport)

OHT (Overhead Hoist Transport) is an automated ceiling-mounted system that moves FOUPs between tools throughout the fab. **Design**: Vehicles travel on rails suspended from cleanroom ceiling. Hoist lowers to pick up and drop off FOUPs at tool load ports. **Coverage**: Network of rails connects all tools in fab. Routes programmed or optimized dynamically. **Capacity**: Each vehicle carries one FOUP. Fleet of vehicles managed by control system. **Integration**: MES (Manufacturing Execution System) dispatches OHT based on lot routing and tool availability. **Throughput**: Vehicles travel at 5-10 m/s. Optimize routing to minimize congestion and wait time. **Cleanliness**: Operates above wafer level, particles fall away from wafers. Enclosed tracks minimize particle generation. **Advantages over floor AGV**: No floor space consumed, no interference with personnel, cleaner operation. **Maintenance access**: Rail system designed for vehicle maintenance and recovery. **Interlocking**: FOUP handoff to load port interlocked with vehicle control. **Manufacturers**: Murata, Daifuku, Shinsung. Standard in 300mm fabs.