Ming-Hung Lin

Petri-net and GA based approach to modeling, scheduling, and performance evaluation for wafer fabrication

A significant amount of risk is involved in the wafer fabrication due to huge investment costs, long production cycle time, and short production life cycle. In this paper, a genetic algorithm (GA) embedded search strategy over a hybrid color-timed Petri-net (HCTPN) for wafer fabrication is proposed. Through the HCTPN model, all possible behaviors of the wafer manufacturing systems such as WIP status and machine status can be completely tracked down by the reachability graph of the net. The chromosome representation of the search nodes in GA is constructed directly from the HCTPN model, recording the information about the appropriate scheduling policy for each workstation in the fabrication. A better chromosome found by GA is received by the HCTPN based schedule builder, and then a near-optimal schedule is generated.

A virtual factory based approach to on-line simulation and scheduling for an FMS and a case study

The recent years have witnessed a revolution in computing capabilities. At the same time, modern manufacturing systems are becoming increasingly complex and capital-intensive. Here, we propose a virtual factory wherein an efficient prototyping testbed will be provided. It will be possible to develop detailed models to support system design and operation, to test different system configurations. Besides, It will be possible to manipulate all system features which impact performance, and to see the results of these manipulations, all without disrupting the actual system. This paper is one among very few that tries to represent the virtual factory in an analytic form so that many existing mathematical analyses can be applied. New pseudo resources can be added to form a new virtual environment, and control policy designed by engineers will be evaluated before being issued. Since a state-transition sequence of the virtual factory from the initial state to the final state can be seen as a schedule of the modeled system. An effective schedule of the processing can be obtained by using an A* based search algorithm, namely, Limited-Expansion A algorithm.

Modelling, control and simulation of an IC wafer fabrication system: A generalized stochastic coloured timed Petri Net approach

This study presents a generalized stochastic coloured timed Petri net(GSCTPN) to model an IC wafer fabrication system. According to the GSCTPN, it models the dynamic behaviours of the IC fabrication system, such as loading, reentrant processing, unloading and machine failure. Furthermore, modular and synthesis techniques are used to construct a large and complex system model. The two major sub-models are the Process-Flow Model and the Transportation Model. The Transportation Model incorporates a simple motion-planning rule and a collision avoidance strategy to solve the variable speed and traffic jam problems of vehicles. This work also describes a simulation based performance analysis and schedule adjustment. To demonstrate the promise of the proposed work, this study makes actual Taiwanese IC wafer fabrication systems the target plant layout for implementation.

Modeling, analysis, simulation and control of semiconductor manufacturing systems: a generalized stochastic colored timed Petri net approach

A generalized stochastic colored timed Petri net (GSCTPN) is used to model an IC wafer fabrication system. There are two major sub-models: a process flow model and transportation model. There are two different automated guided vehicle systems, namely, the interbay system and intrabay system. For multiple-load and variable-speed AGV systems, we embed a simple motion-planning rule and introduce a collision avoidance strategy in the model to solve the variable speed and traffic jam problems of vehicles. The simple control policies for AGV visiting and AGV routing are discussed. The heuristic rules for lot release and lot scheduling are also studied. To obtain performance measures, simulation is used. To show the promising potential of the proposed work, a real-word IC wafer fabrication system is used as a target plant layout for implementation.

Variable structure control of constrained dynamic systems

A modified computed torque controller has been developed by N.H. McClamroch and D. Wang (1987). If the mathematical model of the robot is exact, the modified computed torque can simultaneously control the robot motion and contact force in an accurate way. However, there may exist uncertainties in the model, such as flexibility of joints and links, joint friction, and an inexact surface model. It is shown that the modified computed torque controller may result in an unstable closed-loop system for the system with uncertainties. This difficulty can be overcome by using a variable structure controller. The controller is robust in that it is insensitive to variations in the plant parameters and to external disturbances of contact force.<<ETX>>

Modeling, scheduling, and prediction for wafer fabrication: queueing colored Petri-net and GA based approach

We propose a modeling tool named QCPN (queueing colored Petri net). The main idea of this tool is to combine the original CTPN (colored timed Petri net) with the queueing systems. With the QCPN model, we can predict the delivery date of any specific product under some scheduling policies efficiently and rather precisely. In the scheduling phase, we use the GA based approach to search for the optimal combination of a number of heuristic rules. This QCPN based GA scheduler can greatly reduce the computation time so as to meet the need for a rapidly changing environment.

Systematic creation and application of virtual factory with object oriented concept

Proposes a systematic method of creation and application of a virtual factory. Basic definition of a virtual factory is first given, followed by its foundation principles and architectural overview, and then a systematic method based on object technologies is proposed to create the virtual factory. Specially, for a real manufacturing environment, a corresponding virtual factory can be created via the polymorphism parameterized universal virtual factory with several interactions. The virtual factory can be linked to the real factory and enable simulation-based control by means of the switching architecture we proposed. Its reliable predictions can not only prove the production scenarios but also improve the decision making process of acquisition managers in the applications for flexible automated production. The present work is being applied to the experimental robotized flexible assembly system developed by Intelligent Robotics and Automation Laboratory in National Taiwan University.

An effective search strategy for wafer fabrication scheduling with uncertain process requirements

We can decide the operation order with unknown potential order requirements using the scheduling architecture proposed in the paper. First, the branch-and-bound search based on a Markov chain method is proposed. The Markov chain gets the service rate records and arrival rate records from the manufacturing execution system. We can get the possible beginning times of operations for each job via the Markov chain. The information of the possible beginning time can help us to approximate the solution space. Thus, by the information of the possible beginning times of operations, a branch-and-bound search scheduler can be used to find a sub-optimal scheduling.

Optimum design and investigation on diffuser polymethylmethacrylate (PMMA) peristaltic micropumps

Utilizing micro-electro-mechanical-systems (MEMS) techniques and a solvent-assisted bonding process, a new generation of diffuser peristaltic polymethylmethacrylate (PMMA) micropumps was optimized and fabricated. The main purpose of this study is to compare the performance of optimized and un-optimized micopump which have the same diffuser throat/inlet area (i.e. 16000 µm2). Furthermore, an additional optimized design which has smaller diffuser inlet area was considered to validate and analyze the effect of diffuser inlet area to the micropump performance. The experimental results were validated by comparing with previous generation which had not been optimized the diffuser element. Specifically, the experimental results showed that, with similar diffuser element inlet area (i.e. 160000 µm2), with and without optimized micropumps yield maximum flow rates of 246.4 µL/min and 194.8 µL/min, respectively. Furthermore, it is shown that the back pressure in the optimized micropump is 6.9 kPa, while that in the un-optimized pump is 5.69 kPa. The effect of diffuser element throat/inlet area to pump flow rate and back pressure was investigated by comparing the experimental results of two optimized designs, one with 80 µm × 80 µm and the other with 127 µm × 127 µm cross-sectional area. The results indicated that, the design with larger inlet area gave higher flow rate. However, the rate of reduction in the maximum flow rate with increasing back-pressure increases at the higher inlet area design, which is due to the greater pressure dissipation/loss associated with a larger channel cross-sectional area.

New approach combining numerical technique and simulation for analysis of large discrete event systems based on Petri nets

We propose a method that combines simulation and numerical techniques and directly integrated using interval arithmetic techniques for the analysis of large discrete event systems. A system is first divided into several layers. Identifying subsystems that can be modeled in isolation solves a system for each layer. In each subsystem, the Markovian assumption allows us to establish a set of linear equality constraints among the expectation of state variables in the Petri nets, such as token numbers in the places. A pseudo random process is responsible for the timing of the model, event times are completely determined by simulation. Thus, linear equality constraints are computed according to the pre-simulated event times, and it is possible to know the probabilities of interactions between subsystems.