REAL-TIME DISPATCHING SYSTEM OF GIGA-FAB

REAL-TIME DISPATCHING SYSTEM OF GIGA-FAB YING-MEI TU Department of Industrial Management, Chung-Hua University, Taiwan, R.O.C. E-mail: amytu@chu.edu.tw Abstract Semiconductor manufacturing companies have constructed large fabs to enhance their competitive advantage.normally, a modern Giga-fab has several production phases associated with AMHS (Automatic Material Handling System), and monthly capacity is greater than 100K [tsmc website].since there are many production phases connected and the demand uncertainties particularly for make-to-order environment, the process management of shop floor is capricious. Therefore, how to properly manage and control the shop floor of each phase is very important indeed. In this work, a real-time dispatching (RTD) system is proposed. Two major functions of RTD are Where s next and What s next separately.the concept of safety stock isapplied to Where s next function to assign the correct location for a lot. Regarding to What s next function, the transportation time is considered for the lot with time constraint. Besides, a new algorithm is applied for Look ahead function which can increase the accuracy of What s next assignment. Keywords Giga-fab, Automatic Material Handling System, Real Time Dispatching. I. INTRODUCTION In order to fulfill market demand and increase the competitive advantages, the semiconductor manufacturers will expand the capacity in the existed fab or build up new fabs. Normally, there are two characteristics of new fab: more advanced technology and larger scale than existed fab. Moreover, there will have several production phases integrated by AMHS (Automatic Material Handling System) at fabis called as Giga-fab. A Giga-fab has many benefits, such as low cost, short cycle time and flexibility. Accordingly, Giga-fab is now produced on a large scale to maximize the benefits and create economies of scale. Nevertheless, there are many issues need to be resolved particularly for shop floor control management [1]. As every knows, the wafer fabrication is one of the most complicated industries in the world. It consists of thousands of process steps, re-entrant flows, and batch processing. Besides, there are many uncertainties within the production activities, such as machine breakdown, the variance of process time, hold/release activities, etc. Although a perfect plan has existed, plan never keeps up with changes.agiga-fab consists of several phases by usinga same MES (Manufacturing Execution System), which is regarded as a fab. It means all equipment of the Giga-fab can support the production activities no matter the equipment located. If the past concepts and methods of shop floor control are applied to such a Giga-fab, the products will be transported among phases. It not only results the heavy load of transportation equipment but also increases the cycle time of products. Hence, a new shop floor control model for Giga-fab is necessary indeed. Generally, there are two functions, wafer release control and real time dispatching, included in the shop floor control system of wafer fabrication. Although there is a prefect release plan, the variances of production environment, such as machine breakdown, hold/release activities, time constraint issues, etc. will result to the ineffectiveness of shop floor. Besides, there are several different characteristicsbetween Giga-fab and single fab, for example, a Giga-fab consists of several phases and shares with the same MSE. Generally, a phase of Giga-fab can be regarded as a single fab not only from the scale but also the capability of production. The capacity of Giga-fab will be several times of single fab, and it is more complicated than single fab particularly for shop floor control. Furthermore, the equipment with the same function will be located in different phase. It means more factors should be considered in the dispatching. Therefore, the mechanism of real time dispatchingof Giga-fab is important and should be reviewed and redesigned.in this study, a real-time dispatching system of Giga-fab is proposed. In order to fully take the advantages of Giga-fab, some concepts are added in Where s next and What s next functions. This paper is structured In section 2, we present a literature review. The concept and algorithm of real-time dispatching is proposed in section 3, and the final is conclusions. II. LITERATURE REVIEW Over the past decades, abundant researches with methods to propose the shop floor control. Hung and Chang [2] proposed the modified least slack rule and the shortest remaining flow time rule to reduce the flow times in wafer fabrication facilities. Bowman [3] developed a release control rule to maximize the throughput by not starving a bottleneck machinebased on the concepts of JIT. Louw and Page [4]developed an open queuing network modelling approach to estimate the size of the time buffers in production systems controlled by the Theory of Constraints philosophy. Nevertheless,the multi-phases configuration and time constraint issues 15

have not been considered in the previous researches. Based on these control policies, the advantages of Giga-fab will be neglected and the shop floor control of Giga-fab will be more difficult challenge. Furthermore, thereal-time dispatching system has become a common component of MES (Manufacturing Execution System) in wafer fabrication. There were also many management frameworks and concepts included. Toba et al. [5]proposed a load balancing method which balanced all processing operations of products among multiple semiconductor wafer fabrication lines (fabs) by using predictive scheduling results.lee and Hsiao[6]developed a feasible dispatching algorithm shortest inter cycle timeto balance WIP and shorten lots' cycle time among production areas.chang et al.[7] focused on the enhancement of RTD system in two-phase twin-fab. These researches definitelyachieved two primary objectives, first one is to balance the twin-phase tools loading to decrease the count of cross-phase reservation, and the second one is to decrease the number of inefficient cross-phase transfer. Nonetheless, these were lacking of full consideration in the parameters of tools loading calculation. Lee et al. [8]proposed an off-line multiple-objective scheduling module and an on-line real-time dispatching module to improve the production performance of semiconductor manufacturing system. It absolutely is questionable that the interrelated efforts between two modules might be decreased due to the complications of executable flows. Choi et al.[9]proposed a real-time scheduling mechanism with a decision tree to decide the appropriate dispatching rules. The data mining techniques was adopted accordingly to eliminate the computational burden required to carry out simulation runs to select dispatching rules. Furthermore, the genetic algorithm was also applied to the real time dispatching in several researches. Lee&Ni[10]proposedan optimization method which would lead to the best wafer release policy in the chamber tool to maximize the overall yield of the wafers in semiconductor manufacturing system. Pickardtet al. [11]developed a two-stage hyperheuristic for the generation of a set of work center specific dispatching rules, which combined a genetic programming (GP) algorithm that evolves a composite rule from basic job attributes with an evolutionary algorithm (EA) that searches for a good assignment of rules to work centers. The above researches revealed that most of real-time dispatching systems use the current status of shop floor to make decisions. Unfortunately, the function of look ahead is still scanty. These approaches can be adopted under a single fab conditions, it will be ineffective when applied to agiga-fab. III. REAL TIME DISPATCHING SYSTEM The purpose of real time dispatching is to arrange the production activities,basedon the current status of shop floor, including the status of WIP (Working In Process) and tools. Generally, it can be roughly divided into two parts, one is Where snext that means when a lot has been completed and then would be transferred to the next operation s nearby location. The other is What snext that means when the tool s load-port (L/P) is available and then reserves a lot to process. These two functions have been applied to a single fab for several years and proved worked well. A Giga-fab consists of several phases and shares with the same MES. If the past concepts and methods of shop floor control are applied to such a Giga-fab, the products will be transported among phases. It not only results the heavy load of transportation equipment but also increases the cycle time of products. Hence, a new dispatching system for Giga-fab is necessary indeed. As mentioned above, a modern Giga-fab has several production phases associated with AMHS (Automatic Material Handling System) and shares with the same MES. Hence, from the dispatching point of view, all equipment with the same function can be regards as the same workstation to increase the flexibility of production control. Generally, due to the speediness and convenience of AHMS, the Giga-fab can be regarded as a single fab. However, it can be found that AMHS is usually overloaded after a period of time. The reason why AMHS overloaded is because of the poor dispatching of shop floor. Since the RTD system of single fab ignores the phase information, the Where s next function will assign the completed lot to the other phase no matter it will exist available n the near future of current phase. Next moment, the What s next function has to assign the lot of the other phase to the available equipment. AMHS is always under such a poor assignment and leads to the overloaded situation. Furthermore, due to the more advancedprocess technology, the process limitation is tighter. If the transportation time of AMHS is out of consideration, the selection of What s next will make wrong decision. The root cause is due to the concepts and information of phases not included in RTD system. Although the Look ahead function has been implemented, it is still a big issue in practice. The Look ahead function in current RTD system will look ahead about four steps and need around two minutes to calculate. From the system point of view, to reduce the calculation time of huge data is really difficult. Therefore, a pre-schedule concept is proposed in this RTD system. The detailed description of RTD system for Giga-fab is as follows. 3.1. Function of Where s next From the manufacturing point of view, material 16

handling is necessary but it does not have any value added for products, moreover, it may make products damage during material handling period. Based on this concept, the target of Where s next function is to transfer the lot to the real next operation s location. There is a situation always occurred in the fab that a lot is transferred to its next operation s location by the order of Where s next function, however, this lot is transferred to another next operation s location by the order of What s next function before it processed. Therefore, there are many ineffectivetransportationsoccurred. It is obvious that the objective of Where s next function is to find out a shortest route for lot production. When material shortage occurs in other phases or other rules of What s next is selected, the lot which just transferred to this location will be transferred to other location. In order to avoid this situation, the concept of safety stock is applied to the Where s next function. The procedure of Where s next function is Step 1. Assign the equipment to different group by function and location In Giga-fab, the equipment with the same function will be located in different phase or in the same phase but different bay. In order to make the accurate assignment in Where s next function, the location of equipment should be grouped and identified its location. Step 2.Setup the safety stock of equipment group The purpose of safety stock is to prevent the starvation of equipment. Generally, the major cause of machine starvation is the upstream machine breakdown. In order to avoid over estimation, the safety stock will be calculated by Giga-fab and then to distribute to each individual equipment group defined by step 1. The detailed calculation procedure is Step 2.1.Calculate the safety stock of equipment group The calculation the safety stock quantity of each equipment group is based on the research result of our previous study [12]. The following is the detailed procedure. Step 2.1.1 Calculate the expected time of capacity loss The purpose of WIP is to provide for resources to be put into full economical use and prevents unpredictable events from disturbing maximum output rate. Based on this concept, the capacity of workstation will be lost if the upstream machine breakdown under a balance line. The time of upstream machine breakdown is called as time of capacity loss in this study. Under this definition, the time of capacity loss by equipment can be calculated 17 CL = CL CL = P MTTR P = 1 A MTBF A = MTBF + MTTR CL : total expected time of capacity of CL :expected time of capacity of machine jat M :numbers of machine of P :probability of breakdown of machine jat A :availability of machine jat MTBF : mean time between failure of machine jat MTTR :mean time to repair of machine jat Step 2.1.2 Calculate the expected capacity loss The major task of this step is to calculate the expected capacity loss of equipment when upstream equipment failed. Because the equipment will process multiple products, the capacity loss can be calculated by average service rate. The equations are SS = EC V EC = CL V V = V V = T PT PT = (PT r ) SS :safety stock of EC :total expected capacity loss of equipmenti V :average service rate of V : average service rate of machine jat PT :average processing time of machine jat PT : processing time of product p in machine jat r :production rate of product p n:numbers of product T:unit time Step 2.2.Calculate the safety stock of defined workstation group

SS = SS G M SS :safety stock of workstation group g G :numbers of machine at workstation group g 3.2. Function of What s next There are two major functions included in the What s next function, the factor of transportation time and improvement of Look ahead function. The transportation time should be taken into account especially for the product with time constraint processing in the cross phase. The calculation of transportation time is TT = ETT + LTT TT :transportation time of lot l from current location to its destination. ETT :empty travel time of OHT for the transportation of lot l LTT :loaded travel time of OHT for the transportation of lot l Regarding to the Look ahead function, it is applied to the batch machine or some specific equipment to provide the information of lot arrival time. Based on the arrival information, the dispatching system can make a correct decision of run or wait. In this module, the concept of pre-scheduling is applied. Whole WIP in the Giga-fab will be scheduled to the assigned equipment and rescheduled every x time period (x is defined by user). Because the data is calculated in advance, it can meet the requirement of RTD system. Besides, the data may be out of date, but it can be overcome by reducing the time period of reschedule. The detailed procedure of system is Step 1.Define the workstation which needs Look ahead data Step 2.Get required data The required data include current WIP of each phase and the turn ratio (TR) of technology. Step 3. Schedule current WIP The purpose of this step is to get the get the arrival time of lot to the first assigned workstation. Therefore, the forward scheduling is applied and technology TR is used. The equation is T = 1 TR T :arrival time of lot lto the workstationi TR :turn ratio of step k of technology t 18 S :current step number of lot l S :the step number of the first assigned workstation i Step 4.Back to step 2 to recalculate and revise data every ten minutes CONCLUSIONS Giga-fab is the main stream of semiconductor industry. Nonetheless, the benefits of Giga-fab will be decreased significantlyif lacking of a suitable management and control. In this work, a real time dispatching system of Giga-fab is developed.the major functions of real-time dispatching system are Where s next and What s next. In order to assign the correct location for a lot, the concept of safety stock is used to Where s next function. For the What s next function, the transportation time is considered for the lot with time constraint. Besides, a new algorithm is applied to Look ahead function which can increase the accuracy of What s next assignment. The shop floor of Giga-fab will be more effective under the implementation of this real-time dispatching system. ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology, R.O.C. for financially supporting this research under Contract No. MOST 103-2221-E-216-021 REFERENCES [1] Y.M. Tu, Wafer Release Decision Model of Giga-fab, Proceedings of the Second European Academic Research Conference on Global Business, Economics, Finance & Social Sciences, Zurich, Switzerland, July-3-5, 2015, Paper ID. Z562. [2] Y. F. Hung and C. B. Chang, Dispatching Rules Using Flow Time Predictions For Semiconductor Wafer Fabrications,Journal of the Chinese Institute of Industrial Engineers, 19(1),pp 61-74, 2002. [3] R. A. Bowman, Job Release Control Using a Cyclic Schedule, Production and Operations management, 11(2), pp274-286, 2002. [4] L. Louwand D. C. Page, Queuing network analysis approach for estimating the size of the time buffers in Theory of Constraints-controlled production systems, International Journal of Production Research, 42(6), pp1207-1226, 2004. [5] H. Toba, H. Izumi, H. Hatada and T. Chikushima, Dynamic load balancing among multiple fabrication lines through estimation of minimum inter-operation time, IEEE Transactions on Semiconductor Manufacturing, 18(1),pp 202-213, 2005. [6] Y. Y. Lee and C. C. Hsiao, Cross area lot arrangement shortest inter cycle time, 2007 International Symposium on Semiconductor Manufacturing Conference, PP1-3, 2007. [7] C. S. Chang, H. L. Lo, C. C. Pan and D. L. Wu, Advanced effective dispatching method for 300mm Twin-Phase Twin-Fab, 2009Advanced Semiconductor Manufacturing Conference, pp76-79, 2009. [8] Y. F. Lee, Z. B. Jiang and H. R. Liu, Multiple-objective scheduling and real-time dispatching for the semiconductor manufacturing system, Computers &

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