Overhead Hoist Transport Research Articles

A fixed‐frequency second‐order generalized integrator phase‐locked loop‐based phase synchronization control in multisegment wireless power transfer systems

SummaryMultisegment wireless power transfer (MSWPT) technology provides a solution to the challenges of dust‐free environments and automation faced by automated material handling systems (AMHS) in current semiconductor wafer factories. However, variations in track current frequency and phase offset on the transmitting side of the MSWPT technology may lead to significant deficits in energy transmission capability. This paper introduces a method based on the fixed‐frequency second‐order generalized integrator phase‐locked loop (FFSOGI‐PLL) structure for frequency and phase synchronization among multiple transmitters in a MSWPT system. By synchronizing the track currents of multiple transmitters, this method significantly enhances the energy harvesting capacity of the H‐type pickup in the overhead hoist transport (OHT) system, ensuring stable voltage at the receiver (Rx) load. Both theoretical simulations and experiments confirm that the proposed method can effectively capture the phase difference of track current and achieve frequency locking and phase tracking. Building on this theoretical foundation, we constructed three transmitter (Tx) systems and a 1.5 kW Rx equipped with an H‐type pickup. This setup is capable of real‐time monitoring and phase‐synchronization control of track currents with varying phase offsets. The results convincingly demonstrate the strong performance and stability of the proposed method.

Visualization system to identify structurally vulnerable links in OHT railway network in semiconductor FAB using betweenness centrality.

In semiconductor fabrication (FAB), wafers are placed into carriers known as Front Opening Unified Pods (FOUPs), transported by the Overhead Hoist Transport (OHT). The OHT, a type of Automated Guided Vehicle (AGV), moves along a fixed railway network in the FAB. The routes of OHTs on the railway network are typically determined by a Single Source Shortest Path (SSSP) algorithm such as Dijkstra's. However, the presence of hundreds of operating OHTs often leads to path interruptions, causing congestion or deadlocks that ultimately diminish the overall productivity of the FAB. This research focused on identifying structurally vulnerable links within the OHT railway network in semiconductor FAB and developing a visualization system for enhanced on-site decision-making. We employed betweenness centrality as a quantitative index to evaluate the structural vulnerability of the OHT railway network. Also, to accommodate the unique hierarchical node-port structure of this network, we modified the traditional Brandes algorithm, a widely-used method for calculating betweenness centrality. Our modification of the Brandes algorithm integrated node-port characteristics without increasing computation time while incorporating parallelization to reduce computation time further and improve usability. Ultimately, we developed an end-to-end web-based visualization system that enables users to perform betweenness centrality calculations on specific OHT railway layouts using our algorithm and view the results through a web interface. We validated our approach by comparing our results with historically vulnerable links provided by Samsung Electronics. The study had two main outcomes: the development of a new betweenness centrality calculation algorithm considering the node-port structure and the creation of a visualization system. The study demonstrated that the node-port structure betweenness centrality effectively identified vulnerable links in the OHT railway network. Presenting these findings through a visualization system greatly enhanced their practical applicability and relevance.

Experimental Evaluation of AGV Dispatching Methods in an Agent-Based Simulation Environment and a Digital Twin

A critical part of Automated Material Handling Systems (AMHS) is the task allocation and dispatching strategy employed. In order to better understand and investigate this component, we here present an extensive experimental evaluation of three different approaches with randomly generated, as well as custom designed, environment configurations. While previous studies typically focused on use cases based on highly constrained navigation capabilities (e.g., overhead hoist transport systems), our evaluation is built around highly mobile, free-ranging vehicles, i.e., Autonomous Mobile Robots (AMR) that are gaining popularity in a broad range of applications. Consequently, our experiments are conducted using a microscopic agent-based simulation, instead of the more common discrete-event simulation model. Dispatching methods often are built around the assumption of the asynchronous evaluation of an event-based model, i.e., vehicles trigger a cascade of individual dispatching decisions, e.g., when reaching intersections. We find that this does not translate very well to a fleet of highly mobile systems that can change direction at any time. With this in mind, we present formulations of well known dispatching approaches that are better suited for a synchronous evaluation of the dispatching decisions. The formulations are based on the Stable Marriage Problem (SMP) and the Linear Sum Assignment Problem (LSAP). We use matching and assignment algorithms to compute the actual dispatching decisions. The selected algorithms are evaluated in a multi-agent simulation environment. To integrate a centralised fleet management, a digital twin concept is proposed and implemented. By this approach, the fleet management is independent of the implementation of the specific agents, allowing to quickly adapt to other simulation-based or real application scenarios. For the experimental evaluation, two new performance measures related to the efficiency of a material handling system are proposed, Travel Efficiency and Throughput Effort. The experimental evaluation indicates that reassignment mechanisms in the dispatching method can help to increase the overall efficiency of the fleet. We did not find significant differences in absolute performance in terms of throughput rate. Additionally, the difference in performance between SMP- and LSAP-based dispatching with reassignment seems negligible. We conclude with a discussion, where we consider potential confounding factors and relate the findings to previously reported results found in the literature.

Dynamic analysis and vibration control for overhead hoist transport

The OHT system is a transport device used for intra- or inter-plant transport in a fab or other factory, the load-bearing part of the goods stored under the trolley generates longitudinal vibration and lateral oscillation due to inertia force When the OHT in the air starts or stops. Firstly, the trolley dynamic model under the action of lateral thrust and longitudinal tensile force need to be established. secondly, the coupling analysis of horizontal oscillation and longitudinal oscillation on the model is carried out. Thirdly, aiming at the lateral swing and longitudinal vibration of the carriage, a double closed-loop vibration control strategy with tension control as inner loop and speed control as outer loop and carry out the simulation analysis of longitudinal vibration control is proposed. In the experiment of the ground trolley, the control strategy is applied. The simulation and experimental results show that the control method can quickly restrain the longitudinal vibration of the trolley bearing plate, reduce the pulling force of the trolley on the lifting steel belt, and avoid the large displacement of the bearing plate in the air.

Vehicle Look-Ahead Dispatching for Overhead Hoist Transport System in Semiconductor Manufacturing

This article investigates a look-ahead transport dispatching algorithm for the Automated Material Handling System (AMHS) of a semiconductor manufacturing facility. We consider two types of information: the estimated arrival time of upcoming transports of lots and the time required for an occupied vehicle to become idle. A Mixed Integer Linear programming model has been proposed to minimize the assignment (waiting) time of transport missions. Due to the complexity of the problem, three heuristics have been developed and tested on industrial data. The experiment simulation has been conducted by using a simulation model of a real semiconductor factory. The tests have shown the relevance and the effectiveness of the look-ahead dispatching algorithm.

Disturbance Rejection Control for Active Vibration Suppression of Overhead Hoist Transport Vehicles in Semiconductor Fabs

In modern semiconductor fabrication plants, automated overhead hoist transport (OHT) vehicles transport wafers in front opening unified pods (FOUPs). Even in a cleanroom environment, small particles excited by the mechanical vibration of the FOUP can still damage the chips if such particles land on the critical area of the wafers. To minimize the vibration excitation force transferred to the FOUP, this research focuses on controlling the vibration displacement level of an OHT hand unit interface between the OHT vehicle and the FOUP. However, since the OHT vehicle and the FOUP keep traveling, the target system is floating and there exists no external anchoring point for a controlling force source. In addition, no sensor attachments are permitted on mass-production FOUPs, which makes this vibration level suppression problem more challenging. In this research, a custom testbed is designed to replicate the acceleration profile of the OHT vehicle under its travel motion. Then, system modeling and identification is conducted using simulation and experiment to verify the fabricated testbed design. Finally, a disturbance observer-based controller (DOBC) is developed and implemented on a custom active vibration suppression actuator with inertia force-based counterbalancing to reduce peak vibration amplitude from 870 μm to 230 μm.

Traffic Flow Speed Prediction in Overhead Transport Systems for Semiconductor Fabrication Using Dense-UNet

To improve semiconductor productivity, efficient operation of the overhead hoist transport (OHT) system, which is an automatic wafer transfer device in a semiconductor fabrication plant (“fab”), is very important. A large amount of data is being generated in real time on the production line through the recent production plan of a smart factory. This data can be used to increase productivity, which in turn enables companies to increase their production efficiency. In this study, for the efficient operation of the OHT, the problem of OHT congestion prediction in the fab is addressed. In particular, the prediction of the OHT transport time was performed by training the deep convolutional neural network (CNN) using the layout image. The data obtained from the simulation of the fab and the actual logistics schedule data of a Korean semiconductor factory were used. The data obtained for each time unit included statistics on volume and speed. In the experiment, a layout image was created and used based on the statistics. The experiment was conducted using only the layout image without any other feature extraction, and it was shown that congestion prediction in the fab is effective.

Practical Q-Learning-Based Route-Guidance and Vehicle Assignment for OHT Systems in Semiconductor Fabs

We introduce and analyze vehicle-allocation algorithms for overhead hoist transport (OHT) systems in semiconductor wafer-fabrication facilities (fabs). OHT is the most widely used type of automated material-handling system in fabs, comprising hundreds of vehicles delivering lots between processing machines. Timely transport unit delivery by OHT systems are critical to the efficient overall operation of a modern fab. We first describe the limitations of current OHT vehicle-allocation algorithms, and then detail an improved system that can be implemented in practical environments to manage the operation of hundreds of OHT vehicles. Our proposed system is based on reinforcement learning. In particular, it uses Q-learning-based dynamic vehicle-allocation algorithms to examine traffic conditions and then allocate an OHT vehicle to a delivery job. We propose multiple algorithms based on the Q-learning approach and compare their performance with that of conventional allocation approaches, which reveals the appropriate algorithms for use in industry. We demonstrate that our algorithms are efficient and sufficiently fast to be used in a practical large-scale setting.

Predictive vehicle dispatching method for overhead hoist transport systems in semiconductor fabs

We propose to use information regarding the fab's future state for Overhead Hoist Transport (OHT) dispatching, which is named as ‘predictive dispatching’ in this paper. Unlike conventional dispatching methods, two kinds of information are additionally considered in our proposed methods: the expected arrival time of jobs in the near future and the time needed for occupied vehicles to become idle. We firstly develop Basic Predictive Dispatching (BPD) under the assumption that job arrival time prediction is error-free. We demonstrate that BPD consistently surpasses conventional benchmark dispatching methods, even when job arrival time prediction contains a certain level of error. However, as the level of error increases, the performance of BPD deteriorates. To improve BPD's performance in the environment with prediction error, we take the certainty level of job arrival time prediction into consideration in our second method called Certainty Weighted Predictive Dispatching (CWPD). Both BPD and CWPD formulate the OHT dispatching problem as a linear assignment problem, but two different matching cost functions are employed separately. By conducting experiments on a sample semiconductor fab, we validate the effectiveness of our proposed approaches. The superiority of CWPD over BPD in the environment with prediction error is also verified.

Q(λ) learning-based dynamic route guidance algorithm for overhead hoist transport systems in semiconductor fabs

A learning-based dynamic routing algorithm is proposed for the overhead hoist transport (OHT) systems of semiconductor fabrication facilities (fabs). An OHT system, which consists of multiple vehicles moving at high speeds on guided rails, is the primary automated material-handling system (AMHS) in a fab. Modern large-scale fabs have hundreds of vehicles moving lots between multiple processing machines. The dynamic routing method is a route guidance method that dynamically selects the best vehicle paths under given traffic conditions and congestion levels. Building on the learning method, we develop a reinforcement learning-based dynamic routing algorithm called QLBWR(λ), which consists of a Boltzmann softmax policy and a reward function. The proposed algorithm uses real-time information to effectively guide each vehicle so that it avoids congestion and finds an efficient path. The algorithm is also designed with a low computational burden, such that the efficient route can be found for hundreds of vehicles in real time. Simulation analyses on an actual fab layout are used to compare the performance of the proposed algorithm with common static and dynamic algorithms. The results show that the proposed algorithm outperforms the benchmarking algorithms.

Closed-Form Expressions on Lot Completion Time for Dual-Armed Cluster Tools With Parallel Processing Modules

Cluster tools, which consist of processing modules (PMs) and a transport robot, are used for wafer fabrication processes in semiconductor manufacturing fabs. Once processing of a wafer lot containing several wafers is completed in a tool, an overhead hoist transport (OHT) unloads the lot and transports it to another tool. Since it is not easy to estimate when a wafer lot is completed, an OHT is required to wait for the wafer lot to be completed near the tool, or tools become idle due to the late delivery of wafer lots. To reduce the resulting idle time, a closed-form expression of the lot completion time is derived, especially, for a dual-armed cluster tool consisting of parallel PMs, to reflect real applications. We even consider lot switching operations where two consecutive wafer lots are processed with an overlap in analyzing the completion time. We finally show that the formulas derived can be used when there are small processing time variations experimentally. Note to Practitioners —For a dual-armed cluster tool with parallel processing modules (PMs), a closed-form expression on the lot completion time is derived. For this, we also consider lot switching operations, where the previous and next lots are temporarily processed together in a tool. With the formulas, we can send overhead hoist transports (OHTs) just-in-time to tools to unload or load wafer lots in tools, which can significantly reduce the idle time of tools and OHTs. In addition, the formulas can be used to design the number of PMs for wafer lots and assign wafer lots to tools to reduce their flow time. We believe that more efficient planning and scheduling in semiconductor manufacturing can be achieved by utilizing our results.

Iterative two-stage hybrid algorithm for the vehicle lifter location problem in semiconductor manufacturing

Automated material handling systems (AMHSs) in semiconductor fabrication facilities (fabs) are inherently capital intensive because they are moving toward full automation. However, in addition to being capital intensive, full automation can come at the cost of compromised performance or instability when abnormal events occur. Vehicular congestion is one example of an abnormal event and is a recurring problem in fabs that reduces production efficiency. In this paper, motivated by a material handling system design problem when constructing a new semiconductor fabrication plant in practice, we present a model for optimizing the location of overhead hoist transport lifters, which have proven to be a suitable addition to AMHSs for resolving bottlenecks caused by heavy congestion. To do so, we study a capacitated facility location problem (CFLP) that incorporates real-life constraints and consider the interactions between lifters of differing types. We first propose a hybrid approach that combines a genetic algorithm with a depth-first search (DFS) based on memorization to approximate the optimum positions for the installation of the lifters. We then conduct a numerical experiment to compare the performance of our approach with optimal solutions in small- to medium-sized facilities and perform a sensitivity analysis for the important parameters involved. Finally, an experimental study based on real data from semiconductor fabs is conducted to demonstrate the applicability and usefulness of the proposed model.

Completion Time Analysis of Wafer Lots in Single-Armed Cluster Tools With Parallel Processing Modules

We analyze the completion time of wafer lots in single-armed cluster tools with parallel processing modules (PMs) by considering the lot switching operation. To effectively assign wafer lots and dispatch overhead hoist transports (OHTs) to manufacturing tools, it is crucial to obtain the completion time of wafer lots. However, estimating the completion time is not straightforward, due to the concurrent processing of two consecutive wafer lots during lot switching operation, which often increases wafer sojourn times in PMs. In this paper, we derive closed-form expressions of the completion time of wafer lots in single-armed cluster tools with parallel PMs. We assume that the robot unloads wafers in the order of their loading sequence. We then experimentally show that the formulas derived can be used even when processing time variation exists or another robot task sequence, which is of first-in first-out (FIFO), is assumed. Note to Practitioners —Due to the larger wafer size and circuit width reduction, cluster tools often perform the lot switching operation with each pair of consecutive wafer lots. In addition, since most tools are operated with parallel chambers, concurrent processing with two different wafer lots occurs frequently. Such transient periods in operating tools make it hard to estimate the completion time of wafer lots. In this paper, we derive closed-form expressions to obtain the completion time of wafer lots in single-armed cluster tools with parallel chambers. We further show that the formulas can be used with processing time variation or the FIFO rule. With the formulas, OHTs can be sent just-in-time to tools to load or unload wafer cassettes, and wafer lots can be assigned while minimizing the transient periods. In addition, the estimated completion time can be utilized in the planning and scheduling of wafer fabrication processes.

Real-time OHT Dispatching Mechanism for the Interbay Automated Material Handling System with Shortcuts and Bypasses

As a key to improve the performance of the interbay automated material handling system (AMHS) in 300 mm semiconductor wafer fabrication system, the real-time overhead hoist transport (OHT) dispatching problem has received much attention. This problem is first formulated as a special form of assignment problem and it is proved that more than one solution will be obtained by Hungarian algorithm simultaneously. Through proposing and strictly proving two propositions related to the characteristics of these solutions, a modified Hungarian algorithm is designed to distinguish these solutions. Finally, a new real-time OHT dispatching method is carefully designed by implementing the solution obtained by the modified Hungarian algorithm. The experimental results of discrete event simulations show that, compared with conventional Hungarian algorithm dispatching method, the proposed dispatching method that chooses the solution with the maximum variance respectively reduces on average 4 s of the average waiting time and average lead time of wafer lots, and its performance is rather stable in multiple different scenarios of the interbay AMHS with different quantities of shortcuts. This research provides an efficient real-time OHT dispatching mechanism for the interbay AMHS with shortcuts and bypasses.

Makespan Analysis of Lot Switching Period in Cluster Tools

Cluster tools each of which consists of processing modules, a transport robot, and loadlocks perform most wafer fabrication processes in semiconductor manufacturing. Recently, the lot size has been decreasing from 25 wafers to even 7–8 wafers due to the larger wafer size and circuit width reduction especially in large-scale integration manufacturing. Hence, the lot switching operation between two consecutive wafer lots occurs frequently. In this paper, we derive closed-form expressions of the makespan of the lot switching period in single-armed and dual-armed cluster tools with the given robot task sequences for the first time. With this research, the completion time of a wafer lot can be easily estimated even though its preceding or next lot is processed together during the lot switching period. Hence, the makespan can be used for integrating operation of overhead hoist transports and cluster tools and assigning wafer lots to cluster tools while minimizing the lot switching period.

The Preemptive Stocker Dispatching Rule of Automatic Material Handling System in 300 mm Semiconductor Manufacturing Factories

The integrated circuit (IC) manufacturing industry is one of the biggest output industries in this century. The 300mm wafer fabs is the major fab size of this industry. The automatic material handling system (AMHS) has become one of the most concerned issues among semiconductor manufacturers. The major lot delivery of 300mm fabs is used overhead hoist transport (OHT). The traffic jams are happened frequently due to the wide variety of products and big amount of OHTs moving in the fabs. The purpose of this study is to enhance the delivery performance of automatic material handling and reduce the delay and waiting time of product transportation for both hot lots and normal lots. Therefore, this study proposes an effective OHT dispatching rule: preemptive stocker dispatching (PSD). Simulation experiments are conducted and one of the best differentiated preemptive rule, differentiated preemptive dispatching (DPD), is used for comparison. Compared with DPD, The results indicated that PSD rule can reduce average variable delivery time of normal lots by 13.15%, decreasing average variable delivery time of hot lots by 17.67%. Thus, the PSD rule can effectively reduce the delivery time and enhance productivity in 300 mm wafer fabs.

Semiconductor FAB layout design analysis with 300-mm FAB data: “Is minimum distance-based layout design best for semiconductor FAB design?”

In this paper, one of the most common layout design approaches in general factory design – minimum flow-weighted distance (min-distance) layout – is verified for semiconductor fabrication facility (FAB) design. In min-distance layout design, machines or workstations with high flow rates are allocated close to each other to minimize the total distance of the material flow. This approach is widely used in various industries including semiconductor manufacturing. However, some research also discusses the drawbacks of this approach due to flow congestions and heavy traffic in material handling systems. We validate the min-distance approach with actual data from a modern 300-mm DRAM FAB. We logically generate 18 different cases with different layouts, overhead hoist transport track configurations, and production scenarios. The effectiveness and drawbacks of the min-distance approach in FAB design is investigated with these cases. From the simulation analysis using the Design of Experiments method, we logically show that the performance of material delivery is sensitive to the production volume in the min-distance layout. Also, the performance of the min-distance layout is significantly degraded when the volume is heavy; however, performance can be improved considerably with a few modifications to the bays. We also provide practical tips for an effective layout design method from the insight gained from the simulation analysis. This paper contributes to the critical analysis of the conventional layout design method and the identification of its effectiveness and limitations by using actual FAB data.

Using Rotacaster in The Heuristic Preemptive Dispatching Method for Conveyor-Based Material Handling of 450mm Wafer Fabrication

Semiconductor industry today is facing the transition from 300 to 450 mm wafer fabrication. However, the 450 mm wafers will pose challenges on handling and transport due to the increased size and weight. As the overhead hoist transport (OHT) system, which has been widely used in 300 mm wafer fabs, is likely to phase out, researchers have suggested using conveyor-based automated material handling systems (AMHSs) for 450 mm wafer fabrication. However, to best operate the AMHSs in a 450 mm wafer fab, effective dispatching methods that can best address the two critical problems: 1) traffic-jam and 2) lot prioritization, in the shop floor are required. In this research, we have proposed a heuristic preemptive dispatching method using Rotacaster (R-HPD) to operate the AMHS for 450 mm wafer fabrication. The R-HPD method includes some effective dispatching rules to control and dispatch wafer lots in the AMHS. Our simulation results showed that the R-HPD outperformed the differentiated preventive dispatching policy in terms of average lot delivery time (ALDT). The improvements of 49.8% and 61.9% of ALDTs for hot lots and normal lots were observed, respectively.

Model reformulation for conflict-free routing problems using Petri Net and Deterministic Finite Automaton

It is well known that routing problems are recast as a binary integer linear programming in overhead hoist transport (OHT) vehicle systems. Two kinds of discrete-time state space models of the routing problem are proposed in this paper that simplifies the constraints and binary variables. First, a Petri Net (PN) method is used to simplify the constraints. Second, a Deterministic Finite (DF) Automaton method is used for reducing the binary inputs of the state space equation in OHT systems. Finally, some numerical examples are shown to verify the validity of the proposed methods.

状態空間表現による自律無人搬送車のタスク割当てと経路計画の同時最適化

This paper proposes a new static simultaneous optimization of dispatching and conflict-free routing for unidirectional Overhead Hoist Transport (OHT) systems. The objective is to minimize the entire transportation time where the given transportation tasks are finished. A dynamical system modeling is applied for OHT system that is constituted by the OHT vehicles at the lower level and governed by the upper level scheduler. In the proposed modeling, it is clarified that the simultaneous optimization problems are recast as a binary integer linear problem. This paper presents a numerical example and shows that the proposed simultaneous optimization is better than the routing optimization in term of the entire transportation time.