Alternative loading and dispatching policies for furnace operations in semiconductor manufacturing: a comparison by simulation

In semiconductor manufacturing, furnaces are used for diffusion and deposition operations. A furnace is a batch processing machine, which can simultaneously process a number of lots together as a batch. Whenever a furnace becomes available, scheduling the next batch involves decisions on both which operation to process next (dispatching policy) and how many lots to put into the batch (loading policy). A simulation model of a wafer fabrication facility is used to examine the effects of different loading and dispatching policies for diffusion operations. Results indicate that the loading policy has a significant effect on the average diffusion flow time as well as the overall cycle time of the products, whereas dispatching policy has a less significant effect. We show that the production volume of a product should be considered in setting the minimum number of lots needed to start a batch. We suggest that the diffusion flow time for a low volume product can be reduced by releasing the product in batches or by setting the minimum batch size such that the work-in-process of the product can be moved faster