# Simulation-Based Analysis on Operational Control of Batch Processors in Wafer Fabrication
## 2 Literature Review on BP Control Rules
Experimental results show that the use of upstream and downstream information in batching decisions may lead to additional improvements at the light to moderate traffic intensity, but the improvement vanishes under high traffic conditions. A BP control heuristic for a \(\delta\)\(\rightarrow\)\(\beta\)\(\rightarrow\)\(\delta\) manufacturing network is presented in Reference [19], in which the states of both upstream and downstream machines are considered. It is claimed through simulation experiments that the benefit of utilizing information about the state of an upstream discrete machine appears to be an order of magnitude larger than that of utilizing information about the state of a downstream discrete machine. An extension of the NACH policy is developed in Reference [20] for a two-stage \(\beta\)\(\rightarrow\)\(\delta\) system that incorporates knowledge about future arrivals and the status of critical machines in downstream processing in the control decision process. The idea is to balance the time for the lots to spend waiting at a BP station with the time spent in the setup at a downstream DP station, thus improve the overall lead time. A rolling horizon BP control strategy is proposed in Reference [21] for a \(\delta\)\(\rightarrow\)\(\beta\) network which incorporates the sequence decisions on the upstream processor through a re-sequencing approach to improve the mean lead time performance of the batch processor. Simulation experiments show that the re-sequencing approach improves the lead time performance of the \(\delta\)\(\rightarrow\)\(\beta\) network as compared to the NACH and MBS-based policies, especially when the number of product types is large, and the BP traffic intensity is low or moderate. Very few pieces of research work deal with the control problems of the batch processors from a systems perspective. The effect of multiple operational decisions, including job release, mask scheduling, and batch loading on wafer fab performance (throughput and lead time) is examined in Reference [22]. For a batch loading decision, BFQL (back and front queue leveling) heuristic is proposed in which possible starvation of the immediate downstream station is incorporated in an MBS-based loading policy. Appropriate combinations of the lot release, lot dispatching, and batching decisions are examined through simulation experiments. One possible drawback of the BFQL rule is that the control decision only considers the immediate downstream station regardless of its scheduled workload. If the scheduled workload of the downstream station is low, the system performance is not much affected by the downstream station so that the downstream status is not that important to be included in the batching decision process. The effect of dispatching rules and job release policies on system performance in wafer fabs is examined in References [23; 24]. Several composite dispatching rules have been suggested, and the effect of the combination of dispatching rules and job release policies is compared through simulation experiments. However, in these studies, the effect of batch processors on system performance is not explicitly examined as only an MBS-based loading rule, or a full-batch loading rule is assumed. To the best of our knowledge, little work has been done to use simulation analysis to examine the effect of different BP operational decisions on system-wide performance where the BP processors are explicitly considered. Our study in this paper is to examine the impact of BP control decisions on system-wide performance through simulation experiments with different BB control rules.