Introduction to Automated Material Handling Systems in LCD Panel Production Lines

Transcription

1 Proceeding of the 2006 IEEE International Conference on Automation Science and Engineering Shanghai, China, October 7-10, 2006 Introduction to Automated Material Handling Systems in LCD Panel Production Lines Young Jae Jang Dept. of Mechanical Engineering Massachusetts Institute of Technology Cambridge, MA 02139, USA Gi-Han Choi FAS Division Shinsung ENG Co. Ltd. Sungnam, South Korea Abstract This paper introduces widely used current automated material handling systems in thin-film-transistor liquid-crystal-display (TFT-LCD) panel manufacturing systems. The automated material handling system (AMHS) in this paper refers to a hardware system that transports discrete parts from one processing machine to another. The TFT-LCD panel industry has been one of the fastest growing industries in the last decade. In particular, the process equipment for the TFT-LCD has undergone significant improvement, making technology innovations possible. However, the AMHS equipment has not had much improvement and in fact most of the current TFT-LCD factories use the same AMHS concept they used 10 years ago. Now, the role of the AMHS in TFT-LCD production lines becomes more important as production efficiency becomes a primary determinant of competitiveness. Therefore, TFT-LCD manufacturers are trying to increase their productivity by adopting an efficient material handling method and technology. Index Terms Automated material handling system, Liquid crystal display (LCD) manufacturing, Semiconductor industry, Production line design. I. INTRODUCTION The thin-film-transistor liquid-crystal-display (TFT- LCD 1 ) industry is one of the fastest-growing industries in the last decade. Since the first mass production LCD manufacturing system got started its in the early 90 s, the manufacturing technology, especially processing technology, has made significant innovations. The growth of the industry has been mainly led by the process technology innovation and the strong market demand. In particular, the process equipment for the LCD has undergone significant improvement, making technology innovations possible. However, the AMHS equipment has not seen much improvement and in fact most of the current LCD factories use the same AMHS concept they used 10 years ago. Generally, the AMHS includes an automated transportation system between processing machines, such as an automated guided vehicle (AGV), overhead shuttle (OHS), rail guided vehicle (RGV), or conveyor system. In this paper, we also include in the definition of the AMHS an automated storage and retrieval system (AS/RS, or automated buffer system) and an interface hardware. Manufacturing processes for LCD panels are similar to semiconductor device fabrication processes and therefore all process equipment is also similar. However, LCD 1 Since the TFT-LCD is the most popular technology that is produced in a mass production system among the various LCD technologies, unless otherwise mentioned, the LCD in this paper refers to the TFT-LCD. processes are performed on the surface of a glass substrate, called mother glass, instead of a wafer. During the early stage of the LCD industry, the size of the mother glass was about the size of the wafer. Since the processing equipment and the size of the parts moving around in the factory were alike, the semiconductor device manufacturers, such as IBM and NEC, were the first movers that entered the LCD industry [1]. Therefore, when they first built LCD factories, they used the same concepts in designing layouts and material handling systems that they used in their semiconductor fabrication facilities. LCD manufacturers have adopted the bigger size of the mother glass for the last ten years and now the standard mother glass used in 2006 is about 2m 2m. The size of the processing equipment has also become bigger and so has the size of the factory. However, the concepts of material handling and layout designs have changed little. Factories still use the same delivery methods and automation systems they copied from the semiconductor device facilities ten years ago. The only difference that has been made is the size of the hardware of the material handling systems. Since the material handling system concept was originally designed for 20 or 30 centimeter wafers, applying the concept to a system that handles almost 2m 2m mother glasses causes significant inefficiency. The role of the AMHS in LCD production lines is becoming more important as production efficiency becomes a primary determinant of competitiveness. Therefore, LCD manufacturers are trying to increase their productivity by adopting an efficient material handling method and technology. There is considerable research literature on AMHS used in semiconductor device fabrication facility. Although there is unique features and characteristic in AMHS used in the LCD industry, little research has been introduced. The purpose of this paper is to draw more attention from the research community by introducing the current issues in AMHS. To begin, we introduce the AMHS currently used in LCD production lines. Since the design choice and system selection in AMHS depend on the layout of processing machines, we first introduce a typical layout of an LCD manufacturing system. Hardware configurations and functions for each AMHS component are then explained. Last, we discuss some design issues in AMHS /06/$ IEEE 223

2 II. LCD MANUFACTURING A. Processes The LCD panel is composed of a front glass fitted with a color filter, a back glass that has transistors fabricated on it, and a light source located at the back of the panel. The front and back glasses are sandwiching a layer of liquid crystal and the transistor on the back glass uses the liquid crystal to control the passage of the light. Since the liquid crystal molecules respond to an external applied voltage, liquid crystals can be used as an optical switch, or light valve. That is, when voltage is applied to a transistor, the liquid crystal is bent allowing the light to pass through to form a pixel. The front glass, which is fitted with a color filter, gives each pixel its own color and the combination of these pixels in different colors forms the image on the panel. The Physics of the liquid crystal display is explained in [1]. In order to process LCD panels, the mother glasses need to go through more than 200 different processing steps. These steps are categorized into four different processing stages: the TFT, color filter (C/F), cell, and module stage. The TFT and C/F production stages, combined called the front-end or fabrication stage, are performed in parallel. During these stages, an array of pixels and circuitry are built on top of a glass. Next, in the cell stage, the two glasses are glued together and cut into individual cells. Then injection of liquid crystal into the cells is done. After that, assembly of cells into a customer-specified module is performed during the module stage. The front-end stage is considered more critical compared to the rest of the stages, since it includes very sensitive processing steps, such as photolithography, and also requires a high class of particle-free environment. Moreover, facilities in the frontend stages are more expensive and its process steps are more complex. For these reasons, the capital investment cost for a manufacturing facility in the front-end stage substantially surpasses that of the cell and module stages. Most LCD manufacturers, therefore, have focused on throughput improvement in the front-end stage. In this paper, we also concentrate on the AMHS used in the front-end stage. B. Manufacturing generations In industry, the LCD manufacturing generations are distinguished by the size of the mother glass. Table I shows the size of mother glass for each generation, indicating that the size of the mother glass increases with each generation. The bigger glass size can produce more panels. However, some process steps require more processing cycle time for bigger mother glasses. There is a trade-off between producing more panels and increasing cycle time. The front-end stage includes some sensitive processing steps that require a significant amount of machine setup time and machine calibration time. Note that these pre-processing times do not depend much on the size of a mother glass. Therefore, even if using a bigger mother glass increases the overall cycle time, the actual production rate for the panel also increases. That is why the manufacturers have been adopting a bigger mother glass. TABLE I MOTHER GLASS SIZE Generation Size 270x360mm 370x470mm 550x650mm Generation Size 730x920mm 1100x1300mm 1500x1850mm Generation 7 8 Size 2000x2350mm 2400x2600mm C. Transportation lot A widely used transportation lot is a cassette. The glassholding size of the cassette varies from 5 to 50 glasses depending on the generation and production capacity. This cassettebased transportation is common and the AGV, OHS, RGV, and stocker systems all use the cassette as a transportation lot. This is because if the glasses are contained in a cassette, no direct contact is required between an AMHS and glasses. Therefore, it can protect the glass effectively. Also, if the AMHS delivery speed is limited, lot-based transportation is necessary to meet a delivery demand. The downside of using the cassette is that another AMHS that can access an individual glass from a cassette is also needed, since most processing machines take a single glass as an input unit. This job, feeding a glass from a cassette to a processing machine, is performed by the interface machine called a loader or feeder. III. COMMON LAYOUT AND CONCEPT OF AMHS In order to understand the function and behavior of the AMHS, the part (glass) flow behavior in LCD processes has to be understood. Note that this flow behavior is highly dependent on the processing machine layout. In this section, we introduce the typical layout of the LCD production line, particularly for the front-end stage production system. As mentioned earlier, this production stage includes critical processing steps. Since these processing stages require a higher class of particle-free environment compared to the module stage, the front-end stage is done in a separate building or on a separate floor from the rest of the stages. Moreover, the re-entrant flow characteristic, which is common in the front-end stage, complicates the flow behavior in the system. That is, more efficient material flow is required, and therefore layout configurations are critical. On the other hand, module line productions are relatively simple and re-entrant flow behavior is not typically presented. Therefore, in this paper we focus on the layout of the TFT and C/F production floor. Since the processing steps in LCD panels are similar to those in semiconductor device fabrication, the machine layout and AMHS configuration for an LCD production line during the early age of the industry followed the semiconductor production systems. This layout still remains popular in the current LCD production line. The layout concept typically follows a job shop configuration with corridors as shown in Figure 1. The job shop configuration is a layout in which similar processing machines are placed together and sections of a floor are distinguished by the function of machine groups. 224

3 Fig. 1. A hypothetical layout of an AGV-based LCD production line Note that the machines are lined up and face each other across the central corridor, which is called a bay. The basic process steps in the front-end stage are cleaning, deposition, photolithography, after develop inspection (ADI), test, etching, strip, and after cleaning inspection (ACI). Usually one bay is assigned to each of these major operations. In each bay, a glass visits several machines to undergo all the required operations. The entire process is repeated several times to make the required number of layers. Glasses are shipped to other bays by a machine that takes input from one bay and generates output to a different bay using other separated interbay transportation systems. In the bay, the transporter, such as AGV or RGV, serves as the material transport between machines. Typically, the transporter in the bay serves only material movement within the bay. For example, if AGVs are used in a bay, then most likely the AGVs works only on delivery requests within the bay, never leaving it. Material movements between bays are performed by designated transporters. Some stockers are shown in the figure (marked as STK). These stockers temporarily store WIP. Note that some of the stockers are located between bays. That is, these bays are connected by the stockers. In this case, glass movements between the bays are done by the stocker robot inside of the stocker (details about the stocker system are explained in a later section). Material flow within a bay varies depending on the tools installed in the bay. In some bays, a glass may enter and be processed by only one or two machines and then leave. In some other bays, a glass goes through multiple machines sequentially before leaving. IV. AMHS In this section, the components of AMHS are introduced and their characteristics explained. AMHS is categorized by three components, based on their functionality: a transport system, buffer system, and interface system. The general function of the transport system is moving a part from one processing machine to another. A. Automated Guided Vehicle(AGV) Systems The AGV used to be the most popular transport mechanism for 3rd, 4th, and 5th generation LCD production lines. The AGVs are mainly used for the glass transportation within a bay. Multiple (usually two or three) invisible main path lanes with bi-path lanes on the floor guide the vehicles. A cassette of 5 to 20 glasses is used as a transportation lot and usually one vehicle has a one cassette load capacity. Loading and unloading a cassette are done by a robot arm installed on a vehicle. Figure 2 shows an example of an AGV system in a bay 2. The number of servicing vehicles in a bay depends on the size and the delivery request rate of the bay, and therefore, vehicles in a bay can be added or replaced depending on the amount of production capacity. Consequently, AGV systems are considered a cost-efficient transport system. Also, unlike other transportation systems, AGV systems can respond quickly to any layout or capacity of the production line, such as adding extra machines in a bay or expanding the size of bays. Therefore, AGV systems are considered the most flexible transport systems. The performance of AGV systems depends greatly on operation policies and guided path design. The operation system assigns a delivery job to a vehicle, directing it to the proper path on which the vehicle should travel. Therefore, a pre-defined dispatch rule or real-time scheduling is used. Determining the optimal number of vehicles in a bay is also a critical factor in the performance of the system. Fewer vehicles in a bay might lead to an over-utilization of the vehicles, resulting in not meeting the demand of delivery requests. In contrast, an excess number of vehicles could generate unnecessary traffic. There is research going on toward determining the optimal number of vehicles, configuring guide paths, and designing operation scheduling and dispatch rules. Due to their flexibility and cost efficiency, AGV systems used to be the most popular transport systems until the 5th 2 Screen capture from the Shinsung ENG AMHS solution animation clip with permission of Shinsung ENG Ltd. 225

4 Fig. 2. Picture of AGVs in a bay generation production line. However, as the size of the glass has become bigger, the AGV has been losing its popularity. Note that the AGV system requires at least two main path lanes in a bay. Therefore, the width of a bay also has to be increased. However, the amount of space has been a serious issue, and therefore the AGV system is being replaced by other transport systems. Common bay configurations and operations issues concerning AGV in LCD production lines are discussed in [2]. B. Overhead shuttle The overhead shuttle system (OHS) is a vehicle system running on guide tracks that are hung from the ceiling or supported by wall brackets. Since the system does not take up floor space, it reduces traffic congestion on a floor and improves the layout efficiency. Also, since it is a rail-guided system, travel speed is usually faster than that of the AGV system. Figure 3 shows a picture of an OHS system installed in a production line 3. Another advantage of the system is that it can respond easily to a process machine layout change or a facility expansion. The OHS system is mostly used for long-distance stocker-to-stocker delivery jobs. This is because the stocker can perform upward and downward movements to interface with ground and overhead material handling systems. Moreover, due to its high speed and flexible rail configurations, long-distance delivery jobs can be easily accomplished with the OHS system. The OHS system has been widely used up through the 5th generation. However, there are some concerns about using the system beyond the 5th generation because of the weight of the cassette. The guide rail configurations are investigated for the OHS system in [3]. C. Stocker system The stocker is the automated buffer system that temporarily stores glasses. As with the AGV system, the transportation lot for the stocker system is a cassette. The stocker usually consists of a moving carrier with robot arms handing a cassette and multi-level stocking shelves. Figure 4 shows a stocker system in service. In the figure, note that there is a guide rail on the ground and a carrier traveling on the rail. On the carrier, two towers support and guide the vertical motion of the robot arm (shown in black between the two towers). Notice also that, a cassette is loaded on top of the arm. In the picture, the beigecolored container is the stacking shelf for the cassettes. Figure 5 depicts the typical configuration of the stocker system. As shown in the figure, once a cassette enters through a load port of the stocker connected to a bay, it is handled by the robot and placed on the shelf. The two figures depict a stocker system with only one carrier. However, some stockers with long shelves often have two independent carriers in the system. Finally note also that this stocker is often used to connect a cassette flow from one bay to another bay. In this context, the stocker is not only a storage buffer but also a transport system for the inter-bay movement. 3 Screen capture from the Shinsung AMHS solution animation clip with permission of Shinsung ENG Ltd. Fig. 3. Picture of a OHS in service Fig. 4. Picture of a stocker in service 226

5 Fig. 5. Typical configuration of a stocker D. Inline stocker system Until the 4th generation, the AGV-based transport system and stocker were the mainstream of the AMHS in LCD production lines. However, during the transition from the 4th generation to the 5th generation, the concerns about the AMHS have been growing due to the bigger size of a glass. In order to use the AGV system, which requires more than two guided paths in parallel to the bay, the width of the bay has to be at least twice the width of the glass. In the 6th production generation, the glass size is mm. That is, the width of the bay has to be larger than 3000mm. However, due to space constraints, such a large size for the bay is rarely a preferred option. Another concern about using the AGV system is due to the buffer space. The AGV system has always required a stocker to store the WIP. As the glass size became bigger, a larger storage size was required. However, considering the number of stockers in a production line, assigning such a large space to the stockers was unlikely. In order to solve the bay size problem and stocker space problem, a new concept of the AMHS was introduced and partly applied from the 5th generation onward. Instead of assigning an extra space for the stocker, as shown in Figure 1, the stocker is placed on the bay and the stocker carrier performs the delivery job between the processing tools. This type of an AMHS is called the inline system in industry. The inline system is nothing new in the aspect of the hardware configuration. It is only a bigger stocker system for a bigger glass. However, using the stocker carrier as the main transporter generates several other advantages: First, since there are one or two carriers delivering cassettes on a bidirectional track, the traffic congestion is not an issue and the delivery time can be predicted more easily. Therefore, the inline system reduces the variability of the delivery time, which used to be the main issue in the AGV system. The decreased variability in delivery time eventually contributes to reduce the uncertainty in the overall production lead time of the product. The inline system was partly adopted in the 5th generation and it became the main AMHS in the 6th generation. However, there has been skepticism about the inline system applied to the later generation. The size of glass for the 7th generation is about mm. The volume of the cassette that holds glasses is about the volume of a mid-size sedan. The weight of the fully loaded cassette is more than 1, 000kg. In order to move such a huge cassette at a certain speed with a high movement precision, an expensive motor and controller are needed in the system. Therefore, the cost of the inline system rapidly increases as the size of the glass gets bigger. In addition, moving a cassette of such large volume at high speed generates turbulent flow in the system. Particle contamination is a big issue in the LCD production line, and therefore, the particles are controlled with carefully designed flow channels in the individual processing machine and in the production line. However, if there is turbulent flow, the flow control becomes very difficult, risking particle contamination on glasses. One last problem in using the inline system is the inventory holding cost. As the glass size increases, the dollar value of a single glass also increases. That means, for example, if the same number of WIP is maintained in the two different generations of production lines, the inventory holding cost for the later generation is more than that of the earlier generation. Consequently, the advantage of providing a lot of WIP space with multi-level stacking shelves in the inline system is lost when the glass size gets bigger. For these reasons, LCD manufacturers are actively investigating another AMHS that could replace the inline system in the 8th generation line. E. Conveyor system The conveyor system is one of the most widely used material handling systems in manufacturing systems in general. The conveyor system sends a part from one point to another point over a conveyor belt or rollers. Unlike a vehicle-based system, such as AGV or RGV, parts do not need to wait to be picked up by a moving entity. Once a part is placed on a belt or roller it will be sent to the destination within an expected delivery time. Therefore, this system is considered a more predictable system compared to the AGV and RGV. However, the conveyor system has never been a mainstream in the AMHS in the LCD production line. This is because the conveyor system transports a part only in uni-direction. Also, additional investment and system redesign may be required when a layout of the production line changes. This lack of flexibility limits its use in the LCD line. In the LCD production line, the transfer lot unit is a cassette. The conveyor systems in the LCD line are often used for a short distance delivery between one stocker and another stocker. F. Single glass transport system As the glass size has exceeded the more than 2 2m since the 7th generation, the industry began to have reservations about using the cassette as the transportation lot. Until the 7th generation, the size of a glass cassette has been the standard transportation lot size. The bigger glass size contains more panels in the glass, resulting in more value per glass. Therefore, the inventory holding cost also increases as the glass size increases. Furthermore, as mentioned in an earlier section, transporting such a large volume and weight of the cassette generates turbulent flow, causing particle contamination on a glass. Also more power in the material handling equipment as well as more effort in the precision control are needed. For these reasons, industry has actively 227

6 Fig. 6. Example of a single glass conveyor system investigated a new concept of an AMHS using a few glasses as a transportation lot. One of the results of this effort is the Single Glass Conveyor system (SGC). As the name implies, the conveyor system transports an single glass from one processing tool to another. The idea of using a conveyor system for a single glass transportation has been around for the last few years. In fact, some manufacturers have used the SGC for their cell and module stages. However, adopting the concept to the front-end stage requires a few technical challenges: an efficient layout of process tools and hardware design for glass movement. The conveyor system can be used effectively if all the process machines are installed linearly and the process flow is a linear single flow line. This is why the SGC is first applied to the cell and module stages whose process flow is almost linear. In contrast, the front-end stage has a lot of re-entrant flows and reworks. Therefore, in order to apply the SGC to the front-end stage, an efficient layout of machines as well as an optimized conveyor configuration are needed. The second challenge is the hardware design issue. Unlike the cassette-based system, in which the AMHS handles a cassette, not a glass, a direct contact between a glass and transportation system is required. An example of an SGC system is shown in Figure 6. In the figure, the upper drawing illustrates a three-dimensional view of the SGC system. This system consists of a double layer conveyor configuration the upper conveyor feeds a glass into a machine while the lower conveyor feeds out of a machine. In the figure, the lower drawing shows the top view of the system. Note that buffers are located in front of the machines. The diverter sends a glass from the main conveyor flow to a branched out conveyor. Usually, elevators are installed at the main entrance or exit side of the conveyor. The advantage of using the SGC system is that WIP can be reduced dramatically compared to the cassette based system. On the other hand, if there is any mechanical failure in any part of the conveyor system, the whole system may have to be stopped, causing a complete flow halt in the bay. Furthermore, if a process step or flow path changes, the whole system redesign may be required. a glass between the cassette and the processing machine. Figure 7 shows an interface system with two loading ports. As indicated in the figure, the port side faces a bay and the other side faces the processing tool. The figure also shows the robot handling a single glass. AGV or RGV delivers a cassette to a port in the interface system, the robot takes a glass from a cassette and then feeds the glass into the processing machine. Once the process is done, the glass comes out the processing machine and the robot receives the glass and puts it back into the cassette where the glass originally came from. Therefore, the cassette usually sits on the ports once it is delivered and waits until all the glasses finish the process. V. CHARACTERISTICS OF AMHS IN LCD LINES In this section AMHS design issues are addressed. Because of the high production rate, the physical dimension of the glass, and the particle-free requirement, AMHS in the LCD production line is not only a necessary system but also a system requiring special properties distinguished from those in the other industries. In an attempt to assist engineers during a concept design stage, system requirements, constraints, and variables are listed. A. Precision motion control The size of glass used in a recent generation exceeds 2m 2m. However, thinness of the glass has remained about mm. In order to handle the brittle material with high speed to meet the throughput requirement, a high precision mechanism in AMHS is needed minimizing glass vibration, making transfer work smooth and very precise, and designing optimal velocity and acceleration/decceleration profile. The precision robot development is explained in [4]. A primary constraint on glass handling is that no components should touch the upper surface of the glass as this will be the internal face of the panel. The multiple layers will be fabricated on the surface throughout the processing steps and the layers are very susceptible to damage from mechanical abrasion or chemical contamination. Also, due to the thinness of the glass, it should be handled securely without any impact or vibration. B. Particle-free environment Most of the LCD panel production is done in a clean room environment. Particularly, the TFT and color filter stages require a class 10 clean room environment. Therefore, the AMHS should not generate particles and should not be a source of particle contamination. For this reason, AMHS uses materials for preventing rust and dust, and a particular G. Interface system The interface system, also called the loading system, comprises one or more loading ports, a single glass handling robot, and an enclosure covering the system. As the name indicates, the function of the interface system is to hold cassettes in the ports while all the glasses finish the process and to transfer Fig. 7. A loading system and loading robot 228

7 surface finishing method. Also, using a special paint and a special bearing are common in AMHS. Furthermore, a sealed structure, in which all of the dust-causing components such as cables, belts, and motors are enclosed, is inside of the system. However, eliminating the sources of particles from AMHS is not enough to prevent particle contamination. AMHS should not interfere with the air flow in a production line. In the production line, air circulation is carefully controlled to prevent a particle from contaminating a glass. This air circulation control is usually done by generating vertical laminar airflow in the whole production line. Therefore, no moving system should disrupt this flow. If AMHS is moving too fast, or accelerating/decelerating too much, it can generate turbulent flow near the system and the particle control cannot be achieved. C. Space constraint As the size of a glass increases, the size of processing machines also increases. Although the overall footprint of production lines also has increased, there is always a limit in the size of the line due to cost, building structure, air circulation, fire regulations and so on. Therefore, efficient use of the floor space is also a requirement of AMHS. For this reason, some AMHS designers seriously consider a transporter holding a glass vertically. VI. CONCLUSION TFT-LCD manufacturing is complex, sensitive, and requires highly reentrant processes. Although the importance of AMHS has become increased, little attention has been given to the system. With the bigger size of mother glasses, there are new challenges and issues that have to be considered in order to design efficient AMHS. In this paper, we tried to draw more attention from the research community by introducing the current AMHS in TFT-LCD manufacturing systems and discussing the design issues of AMHS. REFERENCES [1] W. C. O Mara, Liquid Crystal Flat Panel Display. Van Nostrand Reinhold, [2] J. Jang, J. Suh, and P. M. Ferreira, An agv routing policy reflecting the current and future state of semiconductor and lcd production lines, International Journal of Production Research, vol. 39, no. 17, pp , [3] J.-H. Ting and J. M. A. Tanchoco, Optimal bidirectional spline layout for overhead material handling system, IEEE Transactions on Semiconductor Manufacturing, vol. 14, no. 1, pp , [4] C. Tsuzuku, The trend of robot technology in semi-conductor and lcd industry, The Industrial Robot, vol. 28, no. 5, pp , [5] B. Mahadevan and T. Narendran, Determination of unit load sizes in an agv-based material handling system for an fms, International Journal of Production Research, vol. 30, no. 4, pp , [6] S. Benjaafar, On production batch, tansfer batch, and lead times, IIE Transactions, vol. 28, no. 6, pp , D. Work-in-inventory In a LCD panel production line, a substantial portion of the work-in-inventory (WIP) is held by AMHS. Therefore, understanding the issue of WIP is crucial in analyzing and designing AMHS. WIP is one of the most important issues in virtually all manufacturing facilities. However, this WIP issue has become the most important issue as the size of glasses has become larger. The cost of WIP increases substantially because the larger glass can hold more panels. However, due to the random behavior of processing machines, such as an unsynchronized process flow, unexpected machine failure, unpredictable repair time, machine warm-up time, or process stop due to defect, a certain level of WIP is necessary. When a new AMHS is designed, the issue of WIP should be carefully considered. E. Transportation lot size issue As the size of the glass gets larger, there is skepticism about cassette-based transport systems. Since a larger glass contains more panels, this eventually increases the inventory holding cost. Therefore, the industry is actively searching for a technology to replace the cassette-based system. Generally, the economics of AMHS is influenced by the transportation load size. While large unit loads means fewer AGVs or RGVs with high payload rating, and fewer pallets with sophisticated handling and clamping devices and fixtures, small unit loads will reduce the cost per unit of AGVs, RGVs, and pallet but require more of them. It is important for the AMHS designers to identify the unit load sizes for all the jobs that are undergoing so that the cost of pallet and vehicles is minimized [5]. The issues about the transportation lot are described in [6] and [5]. 229