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.