A novel vehicle assignment method for automated material handling system in semiconductor manufacturing

In semiconductor manufacturing, a diffusion tool is characterized by its long-processing time of 6 to 12 hours with the process flow being highly re-entrant, thus making it a form-batch operation. With the use of form-batch, wafer lots are temporarily stored in the stocker and are delivered to diffusion tools when the Work-In-Process (WIP) pool reaches 4 to 6 lots. As a result, diffusion tools wait for long wafer delivery time due to the low-efficiency AMHS vehicle assignment method, thus greatly impacting transportation and production efficiency. A novel vehicle assignment method is proposed to improve wafer lots´ transport time. Unlike the existing vehicle assignment method, which sequentially assigns empty vehicle to a load port, our proposed method calls several empty vehicles to move to a load port to simultaneously execute transport jobs with the goal of shortening a vehicle´s arrival time. To aid in our research, we performed simulation analysis of an AMHS for a diffusion area. The simulation outcome indicated that a substantial improvement in AMHS transport performance was achieved with implementation of the novel vehicle assignment method.