# Simulation-Based Analysis on Operational Control of Batch Processors in Wafer Fabrication
## 5 Conclusions  
This paper attempts to provide managerial insights about the operational control policies at batch processing stations in wafer fabs. Batch processors have distinct characteristics different from discrete processors: The long batch processing time and non-smooth product flow caused by repetitive batching and splitting increase the flow variability, resulting in long lead times. Most previous studies on batch processors focus on a stand-alone BP station. Not much study has been done for batch processor control from a systems perspective. We have constructed a simulation model for a wafer fab and performed experiments with a variety of operational environments. From simulation studies, we have collected some interesting findings as follows:  
1. Batch processors are more sensitive than discrete processors in terms of capacity changes. A small change in capacity in the batch processors has more effect on the system performance than discrete processors. As a result, batch processors should have more excess capacity than discrete processors. 2. When the BP utilization is not very high, lower MBS threshold values perform better. In the case of very high BP utilization levels, the selection of the MBS level has little effect on the performance of the system. This claim is parallel to the same results studied in References [11; 32] in which only a stand-alone BP station is considered. 3. In the wafer fab with the reentrant product flow, the batch size of a full load may decrease the waiting time at the BP stations. However, it is seen that the decrease in the BP waiting time does not guarantee a good system-level performance. Our experiments show that the full load batching policy provides longer lead times than the MBS policy with lower threshold values,  
Figure 9: Performance of LKDN (Look-Downstream) and MBS6 (Minimum Batch Size 6) over different BP utilization levels in terms of (**a**) throughput, (**b**) production lead time, and (**c**) BOT (bottleneck) station utilization. especially in the cases of moderate and lower BP utilization. The result is contradictory to the common belief about batch size determination in industries where BP operation with a full load is widely used. It is believed that the result is one of the major contributions of this paper. 4. When a look-upstream control policy is used, it leads to the lower waiting time at the BP station. However, simulation experiments show that the look-upstream control policy does not lead to less system lead time. When the manufacturing system is characterized as a reentrant flow, the control schemes considering future job arrivals may not provide better system performance as expected. When the benefit of the look-upstream strategy is minimal from the systems perspective, it may be a good idea to apply simple rules like MBS without considering upcoming job arrivals. 5. The look-downstream control policy performs better for the system, especially with low or moderate BP utilization levels than the full load batching policy. The result is contradictory to the argument from Reference [19] insisting the control decision considering the state of downstream DP station has little effect on the system performance. The contradictory result is arguably due to the manufacturing environment with which the downstream DP station is a highly utilized bottleneck with 95% utilization in our system, while the utilization of downstream DP station is not that high, i.e., 80% in Reference [19]. When the utilization of the BP station is high, LKDN and MBS provide almost the same performance. The study in this paper may be improved on different levels. Firstly, a variety of testbeds may be modeled to examine the impact of batch processors with more scenarios. The limitation of our study in this paper is that we only consider a specific wafer fab model, and hence, the experimental analysis is done for a limited manufacturing case.