# Simulation-Based Analysis on Operational Control of Batch Processors in Wafer Fabrication
## 3 Operational Control Rules for Simulation Analysis  
The control problems in wafer fab can be classified into two major problems, lot release and dispatching [24]. Lot release involves a fab-level decision that specifies when and how many wafer lots should be released to the wafer fab for the first process, while dispatching is a workstation-level decision that determines which job should be loaded next on an idle machine. There are two typical control strategies in terms of lot release: Open-loop release and closed-loop release. The open-loop release policies release wafer lots into the fab based on static production scheduling regardless of the current system status, while the closed-loop release policies consider wafer fab status in release decision making. It is widely recognized that the performance of closed-loop release policies is better compared with that of open-loop release policies [24]. One of the simple open-loop release policies is CONTIME (CONstant TIME) where wafer lots are released into the fab at a constant time interval (for example one wafer lot is released every 2 h). With the CONTIME lot release rule, the production rate can be anticipated by the release interval, while the WIP inventories are variable depending on the operational control decisions at the workstations. A typical closed-loop release policy is CONWIP (CONstant WIP) where a constant number of WIPs are maintained in the fab [25]. In the CONWIP release rule, whenever a wafer lot leaves the wafer fab, a new wafer lot is introduced into the system. Contrary to the open-loop control systems, the throughput rates are variable depending on the operational control decisions at the workstations, while WIP levels are constantly maintained. The other widely known closed-loop release policies include the workload regulating (WR) rule [26] and the starvation avoidance (SA) rule [27] where a new lot is released to the wafer fab when the capacity load of the bottleneck station drops to a specified level. In both the WR and SA rules, the throughput is controlled by changing the critical values. Various closed-loop release rules have been proposed subsequently, which are mostly variations of the WR and SA rules [24; 28]. Once the wafer lots are released into the wafer fab, they visit a number of workstations where a dispatching decision should be made as to which lot should be loaded to be processed next. Most of the existing dispatching approaches are based on priorities that are set by using product information, such as FCFS (first come first served), SRPT (shortest remaining processing time), and EDD (earliest due date), as well as system status including workload and WIP levels in downstream and/or upstream workstations. A wide variety of dispatching policies are reviewed extensively in Reference [15]. For the batch processors, the BP control decisions on batch forming and batch loading time should be made. In this paper, three control schemes, MBS-based rule, Look-upstream rule, and Look-downstream rule are examined: A control rule only with local information, a control rule with upstream information, and a control rule with downstream bottleneck station information, respectively (See Figure 3). The BP control rules are based on the existing BP control policies because our objective is not to develop new BP control rules, but investigate the impact of BP control schemes from the systems' viewpoint. (1) MBS-based rule (MBS#): Processing is initiated when the batch size of a job type is greater than or equal to a predetermined minimum batch size (MBS). The notation MBS# is used to represent the MBS rule with minimum batch size #. For example, suppose we have a batch processor whose batch capacity is six wafer lots. Then, MBS4 is an MBS rule where a batch of wafer lots is loaded when there are at least four wafer lots in the waiting queue. Two extreme cases are MBS1 and MBS6. In MBS1, whenever an idle BP finds at least one wafer lot waiting in the queue, it immediately starts loading and  
Figure 3: Three BP control rules under consideration.