# Simulation-Based Analysis on Operational Control of Batch Processors in Wafer Fabrication
## 1 Introduction
However, if a partial load of a batch is waiting and a batch processor is available, a loading decision should be made as to whether to load the batch immediately or to wait for more lots to arrive to form a larger batch size. Starting a batch immediately with a partial batch size undermines the BP capacity, while delayed batch loading increases the waiting time for the jobs currently waiting at the BP station, and therefore, potentially increases lead times. Some research studies have addressed the BP control problem in semiconductor wafer fabs. Most of the BP control rules developed so far have focused on optimizing the local performance of batch processors. However, their application in multi-stage reentrant production systems may not result in as good a system-wide performance as expected [6]. For example, waiting time minimization at the BP stations may deteriorate the system-wide performance: It may lead to an unbalanced and excessive WIP level throughout the manufacturing system, resulting in increased system lead time. Hence, the decision making at batch machines should consider overall system performance, not only local performance. This paper examines the impact of BP control decisions on the system-wide performance of wafer fabs. In this paper, two system-wide performance measures are considered to examine the system performance: Throughput rate and lead times. The throughput rate may be the most important performance measure when production managers want to have a maximum throughput with given manufacturing facilities. Lead time is defined as the time needed for a wafer lot to go  
Figure 1: Schematic representation of a batch processor (BP) station with multiple batch processors. all the way through a wafer fab. Lead time is widely recognized as a key performance indicator in today's lean manufacturing environments [7], whose major objective is to keep high on-time delivery with minimum WIP levels while achieving a target throughput. [PERSON]'s law describes a relationship among throughput rate, lead time, and WIP inventory: \(L=\lambda W\) where \(L\) is the WIP inventory, \(\lambda\) is the throughput rate, and \(W\) is the lead time. This law states that given a specific production rate, the lead time is proportional to the WIP level. The objectives of this study include: (1) Identifying how much BP stations affect the behavior of the system when compared to discrete processor stations; (2) comparing the performance of the loading decisions with and without system status; and (3) examining the relationship between the local performance of the BP station and system-wide performance. A simulation model is constructed for a wafer fab, and the performance of BP operational controls is examined with simulation experiments. This paper is organized as follows: In the next section, existing BP control rules are reviewed. Section 3 describes the operational control rules for the batch processors that will be examined through simulation experiments. The simulation model and experimental results are presented in Section 4. Finally, in Section 5, conclusions are drawn, and some useful insights are discussed on BP control issues.