In wafer fabs, the overhead hoist transport system operates through an overhead rail network, automated control, and precision mechanical coordination to move wafer cassettes quickly, accurately, and without manual intervention between process tools.
1. System Composition and Layout
The OHT is the core of the automated material handling system (AMHS), running on a precision rail network installed near the ceiling of the cleanroom. The rails are optimized according to production flow and equipment layout, covering key process areas such as lithography, etching, deposition, and storage zones to form an efficient material flow network. For non-standard precision parts used in OHT systems-such as gripping mechanisms, positioning brackets, and rail connectors-Hansheng Automation provides custom machining based on customer drawings, supporting single-piece or small-batch delivery.
2. Workflow
2.1 Task Reception and Planning
The OHT receives transport commands from the production control system through a communication interface. The commands include pickup and delivery addresses, priority levels, and other information.
2.2 Intelligent Movement and Navigation
After startup, the OHT relies on encoders or laser positioning sensors for precise positioning and navigation on the rail. It communicates in real time with the central dispatch system and other OHT units via a wireless network to avoid collisions and optimize routes, ensuring smooth traffic flow.
2.3 Precision Pickup
Upon arriving at the pickup point, the OHT lowers itself and uses a mechanical gripper or vacuum mechanism to grasp the wafer cassette (FOUP). At the same time, an integrated RFID or barcode reader verifies the wafer identity to ensure correct information.
2.4 Stable Transport
After pickup, the OHT rises to the running height and travels along the rail at high speed while maintaining stability. It continuously communicates and reports its status during transport.
2.5 Precision Placement
At the destination, the OHT lowers again and places the wafer cassette accurately into the equipment load port. After placement, it sends a confirmation signal to the control system.
2.6 Standby and Maintenance
After completing the task, the OHT automatically returns to an idle area or a designated standby position, runs a self-check procedure, and waits for the next command.
