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摘要
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. Note to Practitioners-The scheduling of dual-armed cluster tools has been attracted by many researchers recently with growing semiconductor manufacturing. Most of the conventional studies have been concentrated on cyclic scheduling to minimize the cycle time with the swap strategy for robot arm operations. The swap strategy has been widely used to analyze the steady-state operation with cyclic schedules in order to simplify the model with less computational complexity. However, dual-armed cluster tool operations for the swap strategy are restricted because possible states are limited. Non-swap strategy outperforms the swap strategy in the transport-bound region. In order to deal with lot switching, frequent wafer flow pattern changes, periodic maintenance, and processing time variations, a deadlock-free and non-cyclic scheduling without swap strategy is highly required. In this paper, we propose a Petri net decomposition approach with a deadlock avoidance policy that can derive a near-optimal solution for much larger size of non-cyclic scheduling problems of dual-armed cluster tools. A timed Petri net model for non-cyclic scheduling is developed for the dual-armed cluster tools with different flow pattern without assuming the swap strategy. Deadlock is analyzed with minimal siphons of the proposed model and the deadlock-free conditions are derived. Computational experiments show the effectiveness of the non-cyclic scheduling solution derived by the proposed method for multiple wafer flow patterns.
- 出版日期2015-1
