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A good production planning in ramp-up fab is difficult to be built due to the variation of production environment, such as the increase of capacity and throughput target, the changes of product mix and the development of new process technology. In this study, we design a complete planning system containing three models with the hierarchical conception.
First, the order-accepting model is used to determine the optimal product mix with the considerations of the gained profit and the capacity limitation. Second, we define each planning period as a planning stage, different from others, in which the production environment is unchanged. The length of every planning period may be not the same. For each stage, we design a heuristic with mathematical programming to derive the utilization rate of each workstation and the ideal system WIP level so as to achieve the throughput target. Then, we scheme the wafer start time under the fixed-WIP releasing policy with the dynamic capacity requirement plannign (DCRP). Besides, according to the layer-splitting heuristic, production cycle time for each photolithography layer of each product type can be estimated exactly. The wafer out time thus can be computed for setting the due-date. At last, we use the sub-layer as the planning object in the detail production scheduling model. With this design, the daily moves for each sub-layer of each product type can be planned to level the production flow.
To satisfy the requirement of a ramp-up fab, we have to deal with not only the variation of production environment but also the function of quick planning. From the experiments, we find that the calculation of utilization rate for each workstation is very close to simulation results, about 0.78% difference only. Besides, the expected system WIP deviate by 7.89% from those predicted by simulation model, even 1.63% only in the third planning period. These results mean that the planning system we proposed above is effective and efficient for a ramp-up fab without any simulation tool.
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