A Study On Time-Variable Multiple-Objectives Scheduling Of Wafer Fabrication System

The dissertation presents FUZZY based Time-Variable-Multiple-Objectives (TVMO) scheduling approach to solve some issues for the complex Semiconductor Wafer Fabrication System (SWFS). The fuzzy logic theory may assign membership values to the fuzzy variables and thus may more clearly describe the complex non-linear relations between time-variable membership set and weight of different objective. Therefore, fuzzy logic theory provides us with a potential way of dealing with the TVMO scheduling and real-time dispatching in the SWFS.SWFS is a large-scale, intensive capital investment, re-entrant process flows and various kinds of wafer products are processed simultaneously in the same fab. Which is a typically Large-Scale Complicated Re-entrant Manufacturing System (LSCRMS). In contemporary worldwide semiconductor market, since competitive advantage is achieved through offering a wide variety of IC products with high quality standards and in short lead-time, modern semiconductor manufacturers are all pursuing advanced scheduling strategies targeting at multiple objectives such as short cycle time, high equipment utilization and on-time delivery etc. Since complex interrelations and restrictions exist among different objectives for the scheduling of SWFS, weight of importance of each individual objective may vary along with time variables such as month, week and day. For example, at the beginning of each month, scheduling usually focus on the full utilization of bottleneck equipment, and reducing WIP level and cycle time are also taken into account. But at the end of a certain month, manufacturing planners have to pay more attention on timely delivery to avoid the penalty of over due dates. Consequently, scheduling covering multiple objectives with their individual weights varying along with time is regarded as the time-variable multiple objectives (TVMO) scheduling. To reach the best-compromised overall performance, qualified scheduling policies of SWFS must tackle the TVMO feature.Consequently, current multiple objectives scheduling approaches still cannot provide effective support to the real world wafer fabrication’s operations, and there is a need to improve exiting scheduling approaches in order to meet the satisfactory solution of multiple objectives under lot priority index changed in time. In order to increase a company’s competition edge and profitability, and effective schedule system must consider proper weights of multiple objectives with the ability of setting lot’s priority index under multiple objectives by time variable. Many practitioners had developed some popular types of software for wafer fabrication, for example, the ADEXA Company’s PP (Plant Planner) and SFS (Shop Floor Sequencer), they could be appropriated to optimize lot movement efficiency and contribute to wafer fabrication’s quality excellence, high throughput and competitive cycle times. However, no one currently had developed effective methodology to determine lot’s priority index and satisfy multiple objectives on commercial software packages. To cope with two major challenges in SWFS, the optimization methods of TVMO scheduling and real-time dispatching. This dissertation proposes a FUZZY based TVMO scheduling and real-time dispatching method. In this study, we present the method of determining production priority index by the multiple-objectives scheduling of combined dispatching rules are required. The fuzzy membership used to determine the weights for each objective according to imprecise semiconductor manufacturing experts knowledge. Through above method of integration, by which each WIP wafer lot in the fab can get a distinct priority index. The proposed algorithm presents a practical and useful way to find out lot priority index of TVMO feature, which is essential for real-time response to changes in SWFS. The study of production scheduling optimization provides positive support for related researches of production planning, scheduling, dispatching and control.The TVMO based autonomy & coordination-based real-time dispatching mechanism is an important prerequisite for real-time reflecting the dynamic stochastic events in the SWFS and related changes and weights setting of production objectives. The system describes a SWFS to be composed of a number of mutually communication physical objects and their interconnection relations, and the system models are built in a hierarchy consisting of“Work Station–Work Area–Fab”three levels, which accord with the organization of the real-world wafer fab. This dissertation extend time variable from“calendar day”detail to“dispatching time”through experienced planners’intelligence and understanding, above two membership functions are fundamental to the setup of weight of individual objective of time–variable characteristics. It is using dispatching criteria matrix table to perform real-time dispatcher for specific area. The study of dispatching optimization that provides effective solutions for related researches of complicated dispatching and control problems in the semiconductor manufacturing field. .