Search

We examine a cyclic scheduling problem of sequentially connected cluster tools with a single input and output module, which includes multi-cluster tools and linear cluster tools. Every component tool has a dual-armed robot, and chambers are parallelised for a long process step. An intermediate buffer between each pair of adjacent component tools has a limited capacity, and all processed wafers should return to the input and output module. To examine the scheduling problem, we first compute workloads of the process steps and robots to obtain a lower bound on the tool cycle time. We then identify a rule of assigning the chambers to the process steps that makes the tool cycle time independent of the order of using the parallel chambers. We also propose a simple robot task sequence which is modified from the well-known swap sequence for each component tool. We prove that the modified swap sequence is optimal when one of the process steps, not a robot, is the bottleneck. We also present a scheduling strategy which controls robot task timings to deal with interference of wafer flows between each pair of adjacent component tools. Finally, we perform numerical experiments to show the performance of the proposed sequence. [ABSTRACT FROM PUBLISHER]

In this study, we propose an effective real-time scheduling method based on the timetabling technique for the cluster tool. Instead of considering the no-wait assumption generally adopted in almost all of the previous literature, we take wafer transfer delay into consideration, which implies that a wafer can stay at the current chamber for a certain, limited time before proceeding to the next chamber. In order to improve the total cycle time, an additional pulling procedure to the timetabling is introduced and applied. Numerical examples show that the proposed method gives effective schedules in real time even for practical-sized problems. [ABSTRACT FROM AUTHOR]

1Modern cluster tools tend to clean the chambers each time a specified number of wafers have been processed. Such cleaning processes are intended to reduce the quality risk due to the residual chemicals within the chambers. When such cleaning processes are introduced, the conventional scheduling methods for cluster tools, such as the swap sequence for dual-armed cluster tools, are no longer effective. Therefore, we examine a cyclic scheduling problem for a dual-armed cluster tool that performs cleaning processes periodically. We first identify sufficient conditions for which the conventional backward and swap sequences give the minimum cycle time. We then develop a systematic way of generating all feasible robot task sequences, for which the cycle times can be computed. However, the number of feasible robot task sequences increases too fast to be enumerated as the cleaning cycle and the number of process steps increase. To address the complexity problem, we propose two heuristic scheduling strategies and compare them with the conventional scheduling methods and the lower bound of each schedule, respectively, to verify their effectiveness experimentally. [ABSTRACT FROM AUTHOR]

The wireless sensor network (WSN) has attracted much research interest due to its many potential applications in different fields. In this work, we have tried to improve energy efficiency at the node level and to increase the network lifetime by proposing routing model called energy-efficient clustering (ENEFC) based on a hierarchical routing scheme. The proposed ENEFC provides three different hierarchical routing schemes: hierarchical routing using cluster identification (HRCI), hierarchical routing using multi-hop (HRMH) and hierarchical routing using multilevel (HRML). The HRML assigns a level to each cluster head during the publicizing of the base station over the entire network, and when the route collapses the scheme efficiently establishes a new level for the route between the base station and cluster heads. The experimental results and analyses for the three schemes show that HRML is more effective in terms of energy efficiency than the other two schemes. [ABSTRACT FROM AUTHOR]

In semiconductor manufacturing, with the shrinking down of wafer circuit widths, a strict quality control is required for wafer fabrication processes, resulting in that after a wafer being processed and removed from a chamber, a cleaning operation that takes significant time is performed for eliminating the chemical residual. Such a cleaning operation makes a traditional backward strategy for single-arm cluster tools inefficient. By the existing studies, it is shown that the productivity can be improved if some numbers of chambers at a step are kept empty. With this idea, an extended backward strategy is proposed by deciding the optimal number of empty chambers. Based on such a strategy, this paper studies the challenging problem for scheduling a single-arm cluster tool with both chamber cleaning operations and wafer residency time constraint for the first time. By building a timed Petri net model for the system, two linear programs are proposed to determine the minimal cycle time and test the existence of a feasible schedule and find it if existing. At last, two industrial examples are used to demonstrate the obtained results. [ABSTRACT FROM AUTHOR]

A robotic cluster tool involves many activities whose time is subject to some disturbance, thus leading to the activity time variation. It results in wafer sojourn time fluctuation in a process module, which may in turn violate wafer residency time constraints. Some wafer fabrication requires a revisiting process. With wafer revisiting, the effect of activity time variation on wafer sojourn time fluctuation is so complicated that no analysis was reported to the best knowledge of the authors. It is vitally important to accurately analyze it. To do so, this paper adopts a Petri net model to describe the dynamical behavior of cluster tools. With this model, a real-time control policy is proposed to offset the effect of the activity time variation on wafer sojourn time fluctuation as much as possible. Then, the wafer sojourn time delay is analyzed and algorithms are developed to calculate its exact upper bound. With the proposed method, one can check if a given schedule is feasible under bounded activity time variation. Some practical examples are given to show the application of the proposed approach. [ABSTRACT FROM PUBLISHER]

In wafer fabrication, a robotic cluster tool is required to be closed down in order for engineers to perform its on-demand and preventive maintenance and switch between different wafer lots. They often deal with a close-down process subject to wafer residency time constraints, i.e., a wafer must exit from a processing chamber before its quality degradation within a certain time limit. To obtain higher yield, it is very important to optimize a close-down process for a cluster tool. Yet the existing literature pays no or little attention to this issue. By focusing on a time-constrained single-arm cluster tool, this paper intends: 1) to build its Petri net model to analyze its schedulability and 2) to develop computationally efficient algorithms to find an optimal and feasible schedule for its closing-down process under different workloads at its steps. Industrial examples are used to illustrate the application of the proposed method. [ABSTRACT FROM PUBLISHER]

The Ramadge–Wonham supervisory control paradigm has been shown effective in dealing with logic control. Nevertheless, time-related performance is always one of the major concerns in industry. Current methods for synthesizing time optimal supervisors are incapable of dealing with large discrete-event systems (DESs) with massive state spaces. This paper proposes an approach for finding a time optimal accepting trace for large DESs based upon sequential language projection, and pruning. The algorithms are tested on a linear cluster tool to show their effectiveness. [ABSTRACT FROM AUTHOR]

We consider a scheduling problem of a robotized cluster tool for semiconductor manufacturing, which should control wafer delays within a chamber so as not to exceed a specified limit under time variation. The prior research has proposed a p^+-time event graph, an extension of Petri nets, for modeling the scheduling problem and developed a method of verifying whether a cyclic p^+-time event graph or a cluster tool, which repeats identical work cycles, can satisfy the time constraints under time variation. In this paper, we extend and simplify the schedulability analysis method in the prior research for a noncyclic event graph or a cluster tool which performs start-up and close-down operation for a lot or lot switching, and obtain specialized results. We assume that a robot task sequence or firing sequence of transitions is given. Based on the schedulability analysis, we also propose a way of modifying a not always schedulable noncyclic p^+-time event graph with some qualifications to be always schedulable, that is, we prolong token holding times at some places or equivalently delay firings of some transitions so as to be always feasible. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Queueing models can be used to evaluate the performance of manufacturing systems. Due to the emergence of cluster tools in contemporary production systems, proper queueing models have to be derived to evaluate the performance of machines with complex configurations. Job cascading is a common structure among cluster tools. Because of the blocking and starvation effects among servers, queue time analysis for a cluster tool with job cascading is difficult in general. Based on the insight from the reduction method, we proposed the approximate model for the mean queue time of a cascading machine subject to breakdowns. The model is validated by simulation and performs well in the examined cases. [ABSTRACT FROM AUTHOR]

Cluster tools have been extensively used for many semiconductor manufacturing processes such as lithograph, etching, deposition, and testing. Most previous studies on cluster tool scheduling have focused on steady cycles in which cluster tools repeat identical work cycles. However, the proportion of noncyclic transient operation cycles such as start-up cycles and close-down cycles becomes larger as the lot size tends to be smaller. We examine optimal transient scheduling for a single-armed cluster tool, in which there are parallel chambers and a process chamber is a bottleneck, while minimizing the makespan of a lot. To do this, we first identify fundamental properties of noncyclic transient cycles in a tool by analyzing resource workloads. We then propose a simple robot task sequence, a generalized backward sequence, which performs backward operations incrementally for start-up cycles and decrementally for close-down cycles. We also develop workload-based conditions for which the generalized backward sequence has the minimum makespan for single-armed cluster tools with parallel chambers. Finally, we develop a linear programming model to find the minimum makespan of the generalized backward sequence for the cases in which the conditions are not met and show its effectiveness. [ABSTRACT FROM AUTHOR]

Dual-armed cluster tools for semiconductor manufacturing typically have had two arms fixed in opposite directions. Recently, new cluster tool robot systems with two independent robot arms have been introduced with the expectation that the arms’ flexibility will improve the throughput. However, the productivity gain has yet to be examined. Accordingly, we examine under which circumstances and the extent to which productivity gains can be achieved and how the robot task sequences should be scheduled to maximize the throughput. For this purpose, we develop a Petri net model that represents the tool behavior. We show that the well-known swap sequence, which is known to be optimal for conventional dual-armed tools, is not always optimal. Instead, we identify two other sequences that are optimal under certain conditions. We define the workloads for each process step and the transport module to derive conditions for optimality of the sequences, based on the Petri net model and the workload. We also develop a mixed integer programming (MIP) model to determine optimal sequences among one-cyclic schedules for the cases in which the proposed sequences are not optimal. Furthermore, we analyze and demonstrate how the two independent arms can increase the throughput in comparison to a conventional dual-armed robot. [ABSTRACT FROM AUTHOR]

Robotized cluster tools for semiconductor manufacturing have strict time constraints such that a wafer processed at a processing chamber should be unloaded within a specified time limit. Otherwise, it has a serious quality problem due to residual gases and heat within the chamber. Even though there have been studies on identifying a feasible tool operation schedule over such time constraints on wafer delays, such a schedule is subject to timing disruptions or time variation, and thus may violate the time constraints. In this study, we propose a more robust method of regulating wafer delays against timing disruptions not to exceed a specified limit. We first model the discrete-event behavior of a tool by a timed event graph. We then develop a feedback controller for single-armed and dual-armed cluster tools that can satisfy the time constraints by regulating wafer delays. To do this, we develop a feedback controller for the timed event graph by analyzing the timing behavior in a linear system model based on the max-plus algebra. The feedback controller postpones an event or firing of a transition, i.e., loading a wafer into a chamber, until a properly determined time elapses after an associated preceding event occurs. Finally, we present examples of feedback control and show that the feedback control is quite robust even under persistent time variation. [ABSTRACT FROM AUTHOR]

The paper investigates the options to increase the production yield of temperature compensated surface acoustic wave (SAW) devices with a defined range of operational frequencies. The paper focuses on the preparation of large wafers with SiO2 and AlN/Si3N4 depositions. Stability of the intermediate SiO2 layer is achieved by combining high power density UV radiation with annealing in high humidity environment. A uniform thickness of the capping AlN layer is achieved by local high-rate etching with a focused ion beam emitted by the FALCON ion source. Operation parameters and limitations of the etching process are discussed. [ABSTRACT FROM AUTHOR]

Some wafer fabrication processes require a wafer to visit some processing modules in a cluster tool multiple times, leading to a wafer revisiting process. They may pose wafer residency time constraints, i.e., a wafer can stay in a module for a limited time after it is processed. Although techniques exist for scheduling cluster tools with either wafer residency time constraints or wafer revisiting, it is much more challenging to schedule tools with both of them. Considering that atomic layer deposition is a typical wafer revisiting process, this paper intends to schedule a dual-arm cluster tool dealing with it. Based on the analysis of such a tool’s properties, a novel scheduling strategy called modified 1-wafer cyclic scheduling is derived. With this strategy, necessary and sufficient schedulability conditions are presented. If schedulable, highly efficient scheduling algorithms are developed to obtain a feasible and optimal schedule together with a way to implement the obtained one. Illustrative examples are given to show the application of the proposed approach. [ABSTRACT FROM PUBLISHER]

Semiconductor cluster tools are the integrated equipment to process a variety of silicon wafers for the fabrication of microelectronic components. The cluster tool system consists of several loadlock modules, processing chambers, and material handling armed robots for transferring wafers between them. Most scheduling problems for dual-armed cluster tools adopt cyclic scheduling with the assumption of swap sequence of the robot arm. A deadlock-free and non-cyclic scheduling is highly required to improve total throughput of cluster tools for various types of wafer flow patterns and various changes of equipment configurations. In this paper, we propose a Petri net decomposition approach to derive a near-optimal solution of deadlock-free and non-cyclic scheduling of dual-armed cluster tools to reduce the computational complexity. A timed Petri net model is introduced to represent a non-cyclic scheduling model for dual-armed cluster tools. In order to obtain a deadlock-free and non-cyclic schedule efficiently, we propose a deadlock avoidance control policy that restricts the markings to prevent unmarked siphons. Deadlock condition for the dual-armed cluster tools is characterized by the structure of Petri net model. The performance of the non-cyclic scheduling model by the proposed method is compared with that of the cyclic scheduling model. Computational results show the effectiveness of the non-cyclic scheduling model compared with the cyclic scheduling model for multiple wafer flow patterns. [ABSTRACT FROM PUBLISHER]

A multi-cluster tool is composed of a number of individual cluster tools linked by buffering modules (BMs). The capacity of a BM can be one or two. Aiming at finding an optimal one-wafer cyclic schedule, this paper explores the effect of two-space BMs on the performance of a multi-cluster tool. A Petri net (PN) model is developed to model it by extending resource-oriented PNs. The dynamic behavior of robot waiting and tasks, process modules, and buffers is well described by the model. This paper shows that there is always a one-wafer cyclic schedule that reaches the lower bound of the cycle time of a process-bound tool. Furthermore, a closed-form algorithm is revealed to find such a schedule for the first time for such multi-cluster tools. Illustrative examples are given to show the application and power of the proposed method. [ABSTRACT FROM AUTHOR]

Due to the different behavior of single-arm and dual-arm cluster tools, it is challenging to schedule a hybrid multi-cluster tool containing both of them. This paper aims to find an optimal one-wafer cyclic schedule for such a multi-cluster tool. It is assumed that the bottleneck individual cluster tool in it is process-bound, thereby making it process-dominant. To do so, this paper models a hybrid multi-cluster tool with Petri nets. With this model, it derives the conditions under which individual cluster tools can operate in a paced way. Based on these conditions, this paper shows that for any process-dominant hybrid multi-cluster tool there is always a one-wafer cyclic schedule. Then, it develops the algorithms to find the minimal cycle time and the optimal one-wafer cyclic schedule. It is computationally efficient and easy-to-implement in practice. Examples are given to show the application and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this work, cluster tool (CT) Plasma Assisted Molecular Beam Epitaxy (PA-MBE) grown AlGaN/GaN heterostructure on c-plane (0001) sapphire (Al2O3) were investigated by High Resolution X-ray Diffraction (HRXRD), Room Temperature Raman Spectroscopy (RTRS), and Room Temperature Photoluminescence (RTPL). The effects of strain and doping on GaN and AlGaN layers were investigated thoroughly. The out-of-plane ('c') and in-plane ('a') lattice parameters were measured from RTRS analysis and as well as reciprocal space mapping (RSM) from HRXRD scan of (002) and (105) plane. The in-plane (out-of plane) strain of the samples were found to be -2.5 x 10-3(1 x 10-3), and -1.7 x 10-3(2 x 10-3) in GaN layer and 5.1 x 10-3 (-3.3 x 10-3), and 8.8 x 10-3(-1.3 x 10-3) in AlGaN layer, respec-tively. In addition, the band structures of AlGaN/GaN interface were estimated by both theoretical (based on elastic theory) and experimental observations of the RTPL spectrum. [ABSTRACT FROM AUTHOR]

Cyclic scheduling and operation of a residency time-constrained single-arm cluster tool with failure-prone process modules are highly challenging. In some cases, when a failure occurs, there still exists a feasible cyclic schedule for the performance-degraded tool. In other cases, such a schedule no longer exists. For the latter, it is highly desired to respond to a process module failure properly such that the tool can continue working and the wafers in the tool can be completed in a feasible way. This work is the first one to study this important issue. The idea is to apply Petri nets to describe the dynamic behavior of a single-arm cluster tool. With the developed Petri net model, this paper formulates failure response policies to control the cluster tool such that it can keep working without violating any residency time constraint. The failure response policies are implemented via efficient real-time control laws. Illustrative examples are presented to show their usage. [ABSTRACT FROM AUTHOR]

The idle time between two consecutive tasks may seriously affect the wafer quality in some wafer fabrication processes requiring high temperature and pressure. Hence, a cluster tool provides high wafer quality and predictability when operated at a steady schedule, which means that every cycle repeats the same pattern considering not only a sequence but also timing. However, multiple process modules per step can cause the fluctuation of wafer quality even under the same sequence since different modules have different wafer delays. It is known that a dual-armed cluster tool can always have a steady schedule that can be established via a timed event graph and (max, +) algebra. We extend this result to a single-armed cluster tool and generalize the method to establish its steady schedule. Although a cluster tool is initially set on a steady schedule, the schedule can become unsteady after disruptive events, such as wafer alignment failures, and it cannot always return to it. We develop strategies that change the system dynamics of a cluster tool by appropriately delaying some tasks such that any disrupted schedule pattern is stabilized in a finite time. Finally, we examine how they help to flatten wafer delays after a disruption or when tasks have stochastic time fluctuation. [ABSTRACT FROM AUTHOR]

The study aims at identifying the personality factors which associate with the phenomena of academic underachievement and overachievement that the adolescents confront with. The research was performed on a batch of 284 students enlisted in the first year of study, their ages ranging from 18 to 25 years old. The main method of investigation consists on the inquiry based on a questionnaire, with the usage of two questionnaires as research tools. The results of the study have emphasized the fact that underachievement and overachievement associate with those personality traits that belong especially to the area of behavioral disorders and self-image, and are consequences of superficiality, nonconformity and negative self image (underachievement) and of conscientiousness, social conformism and positive self image (academic overachievement). The conclusions of the study may be of real use not only to the researchers in the area of education psychology but also to the practitioners in the domain of education who could improve the quality of the instructional-educational process, by observing the students' personalities and revaluating the maximum potential of each student. [ABSTRACT FROM AUTHOR]

We examine a scheduling problem of cluster tools that concurrently process two wafer types in a cyclic operational sequence. Whereas the process steps for different wafer types are assigned to different processing modules (PMs), the wafer loading and unloading tasks at the PMs are performed by a single robot. For a given cycle plan, which is a mix of different wafers for each cycle, we wish to determine the robot task sequence so as to minimize the tool cycle time. When a single wafer type is processed, the backward and swap sequences are optimal for single-armed and dual-armed tools, respectively. They are being prevalently used because of their simplicity and robustness. To maintain such advantages in concurrent processing, we introduce and define the concurrent backward and swap sequences (CBSs and CSSs, respectively). We then develop conditions on process times, robot task times, and the number of wafers produced in a cycle for which such CBSs and CSSs are still optimal for concurrent processing. We also show that, for some special cases, the two wafer types can achieve their maximum throughput rates as if each wafer type exclusively uses the tool regardless of other wafer types in progress. When the developed conditions do not hold, an effective mixed integer programming (MIP) model based on the CBSs and CSSs is used for robot task sequencing. Finally, we experimentally verify its efficiency and effectiveness by comparing to the existing scheduling methods for optimal scheduling of cluster tools. [ABSTRACT FROM PUBLISHER]

In operating a multi-cluster tool, it needs to coordinate the activities of multiple robots. Thus, it is very challenging to schedule it. This paper conducts a study on one-wafer cyclic scheduling for multi-cluster tools whose bottleneck cluster tool is process-bound. The system is modeled by a Petri net. With this model, conditions under which a one-wafer cyclic schedule exists are developed. Based on them, it is shown that, for any multi-cluster tool whose bottleneck cluster tool is process-bound, there is always a one-wafer cyclic schedule. Then, a method is presented to find the minimal cycle time and the optimal one-wafer cyclic schedule. It is computationally efficient. Illustrative examples are used to show the applications and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A cluster tool that consists of several processing modules, a transport robot, and loadlocks is widely used for wafer processing in the semiconductor industry. The cluster tool repeats an identical operational sequence for processing wafers in a lot, and such a cyclic operation sequence is determined by the wafer flow pattern and process times of a wafer lot. When a wafer lot changes, the tool operation sequence should be switched accordingly. Switching from a cyclic tool operation sequence to another is subject to deadlocks and unnecessary task delays to avoid scheduling complexity. Hence, it is necessary to have a scheduling method for such frequent lot switchings that prevents a deadlock and reduces the switching time. In this paper, we first develop a Petri net model for lot switching operations and then utilize a mixed integer programming model for an optimal schedule. However, since its practical value is limited, we develop effective heuristic methods for lot switching in single-armed and dual-armed cluster tools. The scheduling strategies adapt prevalent cyclic tool scheduling rules, such as backward and swap sequences. By computational experiments, we prove the efficiency and effectiveness of the proposed scheduling rules. [ABSTRACT FROM AUTHOR]

Highlight: [•] Microswitch-cluster programs were developed for one child and two adults with multiple disabilities. [•] The programs for two participants were to promote a constructive response and control inadequate posture. [•] The program for the third participant was to promote two constructive responses and control drooling effects. [•] Participants increased constructive responding and controlled inadequate posture or drooling effects. [Copyright &y& Elsevier]

Presents the table of contents for this issue of the periodical. [ABSTRACT FROM AUTHOR]