Overhead Hoist Transport Research Articles

Model reduction and composite control for overhead hoist transport system by singular perturbation technique

An innovative Overhead Hoist Transport (OHT) system is proposed and analyzed to transport fragile semi-finished products in factories. A triplet of double-link arm is used to carry the load, in replacement of the cables used conventionally. Unlike conventional OHT, the proposed OHT exhibits superior capability for high-speed transportation, flexible stiffness and is able to account for the inherent auto-sway characteristics and parameters uncertainties of the OHT system. The three-time-scale plant model of the OHT system, including the drive motors, flexible links and rigid links, is developed. By singular perturbation order-reduction technique, the highly nonlinear high-order dynamics of the OHT system can be modeled as a low-order linearized plant so that the synthesis of the feedback controller becomes simpler. The composite control, composed of sliding mode control and input shaping technique, is proposed. The sliding mode control is, as usual, employed to account for the system parameters uncertainties. On the other hand, to suppress theresidual vibration, i.e., auto-swaying, the input shaping technique is utilized by implementation of a finite-length sequence of impulses in the appropriate amplitude and time epoch. Finally, the efficacy of the proposal composite control strategy is examined and verified by intensive computer simulations.

Effective overhead hoist transport dispatching based on the Hungarian algorithm for a large semiconductor FAB

This paper considers the vehicle dispatching problem in large-scale overhead hoist transport (OHT) systems of semiconductor fabrication lines. We propose a Hungarian algorithm based OHT reassignment approach named HABOR. HABOR attempts to take advantage of simultaneous vehicle reassignment based on up-to-date system status using the formulation of the assignment problem. The effectiveness of HABOR is demonstrated using a sample OHT system of a semiconductor fabrication line with more than 130 vehicles, where the flow path of the line allows direct delivery so that an inter-bay wafer movement can be accomplished by a single vehicle without an intermediate storage step at a stocker. HABOR compares favorably with the shortest travel distance first rule and with the reassignment-based rule recently proposed by the authors.

Application of Petri Nets and Lagrangian Relaxation to Scheduling Automatic Material-Handling Vehicles in 300-mm Semiconductor Manufacturing

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper deals with vehicle-scheduling problem (VSP) in an automatic material-handling environment in 300-mm semiconductor wafer manufacturing. We adopt Petri nets (PNs) modeling techniques to model the complicated coupling dynamics among transport jobs and overhead hoist transport (OHT) vehicles in a 300-mm OHT loop. The congestion phenomenon among OHT vehicles is captured. With help of the PN models, we formulate the OHT VSP as an integer programming problem whose objective is to schedule OHT vehicles to transport jobs such that average job completion time is minimized. Instead of solving for the optimal solution, we develop a solution methodology to generate a feasible schedule efficiently. A Lagrangian relaxation step is first taken to decompose the PN-based, integer programming problem into individual job-scheduling subproblems. To reduce computation efforts in solving each subproblem optimally, we develop an approximation method to solve each job subproblem by utilizing a reduced PN model of the job. Lagrangian multipliers are then optimized by a surrogate subgradient method. A heuristic algorithm is developed to adjust the dual solution to a feasible schedule. Numerical results demonstrate that our solution methodology can generate good schedules within a reasonable amount of computation time for realistic problems. Compared to a popular vehicle-dispatching rule, our approach can achieve in average 32% improvements on the average delivery time in our realistic test cases. </para>

Effectiveness of vehicle reassignment in a large-scale overhead hoist transport system

This paper proposes an efficient vehicle reassignment dispatching rule and demonstrates the effectiveness of the rule using an overhead hoist transport (OHT) system for a semiconductor fabrication line. The OHT system has more than 150 OHT vehicles and its target vehicle utilization level is 70%. It allows direct delivery such that an inter-bay wafer movement can be accomplished by a single vehicle. Simulation analysis is used to compare the proposed rule with the shortest travel distance first (STDF) rule and existing reassignment-based rules. While STDF rule requires 170 vehicles for the target utilization level, the proposed vehicle reassignment rule requires only 161 vehicles. At the same time, the lead time and the variance of the lead time have been significantly reduced. The proposed rule also improves system performance compared to existing reassignment-based rules.

Differentiated preemptive dispatching for automatic materials handling services in 300 mm semiconductor foundry

Highly automated materials handling in 300 mm semiconductor manufacturing is one of the biggest concerns to foundry practitioners because effective coordination among efficient manual operations has been the core competence in their 200 mm manufacturing successful stories. It is still very challenging to provide almost no-wait transport to hot lots (high priority lots) in an automatic materials handling production environment. This paper proposes an effective overhead hoist transport (OHT) dispatching rule, differentiated preemptive dispatching (DPD) policy, to reduce the possible blocking effects during the transportation of hot lots in a 300 mm OHT system. The dispatching objective is to minimize the delivery time of hot lots while minimizing the impact to the transport of normal lots. Simulation experiments based on realistic data from a local 300 mm foundry fab are conducted. Numerical results demonstrate that the DPD rule can effectively expedite the movements of hot lots. The proposed DPD rule is very useful to hot lots management and shop floor control functions, like scheduling and dispatching, in a 300 mm fab-wide automated production environment.

Preemptive priority policy for hot lots transport services in 300mm semiconductor foundry

Highly automated material handling is one of the most concerns to foundry practitioners because efficient manual operations and coordination have been the core competence in 300mm manufacturing. Especially, one of the critical problems is how to provide an almost no-wait transport for hot lots in an automatic material handling environment. However, there is no effective method for hot lots handling in 300mm Overhead Hoist Transport (OHT) intrabay operation. This paper proposes an effective OHT dispatching method, Preemptive Priority Policy (PPP), to provide the transport services for hot lots against the normal lots transport requirements under 300mm frequently blocking transportation situation. The objective is to minimize the transport delay of hot lots and to convey our idea for hot lots transport services. Simulation experiments based on realistic data from a 300mm wafer foundry are conducted. Numerical results demonstrate that the PPP method can effective expedite the movement of the hot lots with acceptable time delay on normal lots, which is very useful to streamline the shop floor operations, like scheduling and dispatching, for eliminating time delays of hot lots in an automated environment. This can help fab managers for more precisely lots management and automated material handling system (AMHS) control.

Recent advances in semiconductor factory automation, part 2: equipment-level automation

This is the second of two special issues that reports some recent research results and industrial applications of semiconductor factory automation technologies with an emphasis on new trends and innovations. The two issues are the result of an effort made by the Technical Committee on Semiconductor Factory Automation. See the Web site at http://tc-sfa.ee.ntou.edu. tw for more information about this technical committee. The current issue focuses on equipmentlevel automation. “Scenario Normalization Techniques,” by Chung and Jeng, addresses the scenario normalization procedure for inline steppers (ILSs) and its implementation. The sequence diagram is used to highlight the operation details at the equipment level for modeling the operational scenarios of the stepper and track. Then, the overall operational scenarios are checked against the normalized operational flow to derive normalization rules, expressed in terms of “triggering rules,” before being implemented in an event-triggering programming platform as a coordinator between the equipment control system and the ILS. The design of an ILS coordinator for a real-world photolithographic cell controller is presented in detail. “Speedy Delivery,” by Liao and Fu, presents a simulation-based, two-phase approach for effective dynamic overhead hoist transport (OHT) allocation and dispatching in large-scale, 300-mm automated material handling systems (AMHS) management. Numerical results based on realistic data from a local 300-mm mass production fab demonstrate that this simulation-based, two-phase approach performs well both in minimizing the averages and variances of carrier delivery times as well as in achieving the target requirements. Joo and Lee, in the article “Virtual Control,” propose a virtual cluster tool for verifying a scheduler and a cluster tool controller. The authors explain the model architecture and modeling strategies. They also present the use of a formal model-based automatic error detection technique as well as experiences verifying a real cluster tool controller using the virtual cluster tool. Finally, we would like to thank all the authors for their contributions and the anonymous reviewers for helping improve the articles. We would also like to thank the editor-in-chief, Prof. Kimon P. Valavanis, for approving the two special issues and Ms. Rosalyn Snyder for great editorial assistance. H an d & b ac kg ro un d © C or bi s C or p. E le ct ro ni c ci rc ui t © D ig ita l S to ck , 1 99 6 F R O M T H E G U E S T E D I T O R S

Speedy delivery

In this article, we address the overhead hoist transport allocation and dispatching (OHTAD) problem in large-scale, 300-mm automated material handling system (AMHS) management. We propose a simulation-based, two-phase approach to solve the problem. At first, a discrete-event simulation model is built for each OHT loop. Without considering the OHT service requirements, the best OHT dispatching policy that efficiently controls OHT transports is selected among several OHT dispatching rules. For a given set of service requirements to an OHT loop, the required number of OHT vehicles in the loop can be calculated from the simulation results of this loop. However, as the total number of OHT vehicles in the fab is limited, the optimal number of OHT vehicles for each loop is then determined by leveraging the total requirements on OHT services from all the other loops. Our OHTAD objective is twofold: to meet the transport requirements of throughputs and to minimize carrier delivery times.

Behavior modeling and control of 300 mm fab intrabays using distributed agent oriented petri net

In a 300 mm integrated circuit (IC) foundry fab, the intrabay automation is more important than a 200 mm one. In general, the intrabay automation systems include the automated controls of the process tools, overhead hoist transports (OHTs), stockers, and rail shortcuts. These control systems are operated concurrently. They must cooperate seamlessly to increase the system performance and to avoid deadlocks. In the paper, the behaviors of the intrabay automation systems are modeled using the distributed agent oriented Petri net (DAOPN). The fab intrabay DAOPN models are categorized into the new-order generation, OHT vehicle dispatching, zone control, path decision, and tool processing agent models. Finally, the DAOPN is defined, and the modeling procedures of the intrabay behaviors are all illustrated. Consequently, an experimental intrabay OHT loop is discussed and evaluated using the proposed DAOPN models and the DAOPN simulator.

Modelling and Performance Evaluation of an Overhead Hoist Transport System in a 300 mm Fabrication Plant

In a 300 mm wafer fabrication plant, the automated wafer transport system is more important than in a 200 mm one. The weight of 25 pieces of 300 mm wafer in a front opening unified pod (FOUP) makes the frequent manual carrying and lifting impractical. An overhead hoist transport (OHT) provides the pod transport between process tools and stockers in an intrabay with high efficiency and safety. A modular-based coloured timed Petri net (CTPN) is used to model the dynamic behaviour of the OHT. The OHT system performance is evaluated using an object-based simulation technique. Performance measures are used to determine the number of OHT vehicles in the planning stage and also to control the dispatching of the OHT system in the operation stage. The CTPN token competition and conflict resolution rules are used to develop the intelligent controller. Finally, a simplified intrabay with four stations is constructed and evaluated.