Best Capacity Scale of Wafer Fabrication Based on Production Performance and Cost

Semiconductor manufacturing companies have constructed large fabs to enhance their competitive advantage. A large fab has many advantages, such as low cost, short cycle time and flexibility. Nevertheless, the advantages from the expansion of production capacity are not infinite, and a large fab scale increases the risk of production management. This work presents a model to determine the best capacity scale, where the production performance and cost are the major factors. Previous simulation experiments demonstrated that increasing the capacity scale improves the production cycle time significantly only when the utilization of the bottleneck machine is fixed. Therefore, the product cycle time is the major production performance index to determine the capacity scale of a fab. The GI/G/m queuing model is applied to build up the queue time equations of the bottleneck machine. Moreover, differentiation is used to find the number of bottleneck machines with queue times below the minimum required. The optimal number of bottleneck machines can be set based on the calculation result. Based on the performance result, the equipment cost per wafer can be taken into account for the final capacity scale determination. Additionally, an example is described and a simulation model is developed to validate the proposed model.