A comprehensive constraint programming approach for the rolling horizon-based scheduling of automated wet-etch stations

This contribution introduces a novel and comprehensive way of dealing with the automated wet-etch station (AWS) scheduling problem. It first analyzes the field and points out some very important problem aspects that have not been properly addressed yet. Then, an expressive constraint programming (CP) formulation that considers all types of AWS transfer device movements is proposed. The model accounts for both (i) loaded trips the robot makes to transfer wafer lots between consecutive baths, and (ii) empty movements that take place when the device moves itself from a bath, where it has dropped a wafer lot, to another bath where it is required to pick up a different lot. The CP approach is afterward generalized in order to implement an innovative rolling horizon methodology. The proposed method allows the continuous operation of the wet-etch station, minimizing the unproductive intervals that would otherwise appear between the scheduling periods that correspond to different wafer lot sets that are fed by the previous manufacturing step. The formulation has been extensively tested with problem instances of various dimensionalities in productivity maximization scenarios, in which makespan was chosen as the performance measure. The results have demonstrated that robot unloaded movements cannot be neglected; otherwise, they may lead to incorrect schedules. Furthermore, they have shown that AWS scheduling needs to be addressed as an on-line activity.