Tool ‘occupancy’ commences once a lot or batch has captured a tool. The first step is to check if the tool requires a setup for the impending operation. The setup requirement of the tools is based on two mechanisms;
- If the last lot processed on the tool is from the same process flow as the current lot, then a setup based on the Time per Spec Setup time is required,
- If the last lot processed on the tool was from a different setup group ID, then a setup based on the Time Per Group Setup is required.
Next, the lots are loaded onto the tool. Loading may involve an operator for a
certain time fraction of the loading process. Once the lot/tool pairing has captured an appropriate operator, the operator is then occupied for the Operator Loading
Fraction of that particular operation. Once the loading fraction of time has been
reached the operator is released.
Loading Processing Unloading
Operator Loading Fraction Operator Process Fraction Operator Unload Fraction
Figure 5.4: Portion of time the operator is occupied during the process step.
The next step, processing, operates in a similar way as loading in that an operator is required for a fraction of the process time. Unloading the lot from the tool follows the same pattern again. In this way, an operator may be called up to three times for one process step as shown in Fig. 5.4. It is also assumed, (because it is otherwise un- documented), that the fraction of time that the operator is occupied for either loading, processing or unloading, is at the beginning of the task. An argument could be made that the unloading fraction of the operators time is towards the end of the unloading process, however, this would be far more difficult to implement in the model. Further- more, unloading time was generally very small in comparison to the processing step, and
the operator unloading fraction was even smaller. Based on this, and due to a lack of clarification in the dataset documentation, it was decided that it was sufficient to allocate the proportion of operator unloading time to the beginning of the activity.
Equation (5.4) is used to calculate mean process time per lot pt, which is then used to calculate the time until tool becomes free tf in Eq.(5.5), and total lot cycle time through an operation ct in Eq.(5.6):
pt = Time per Batch ∗ No. of batches required for the lot + Time per Lot
+ Time per Wafer in Process ∗ No. of wafers in the lot + Product Setup (if appropriate)
+ Group Setup (if appropriate) (5.4)
tf = Load Time + pt + Unload Time (5.5)
ct = Load Time + pt + Wafer Travel Time + Unload Time (5.6)
The total time the lot is occupied (ct ) differs from the time the tool is occupied (tf ) by the addition of the Wafer Travel Time which is effectively the cascade time on the tool. A consequence for the model strategy used here, where lot and tool items are paired together, is that the lot and tool items must be separated for the Wafer Travel
Time period. The only way to account for this difference between tool occupancy and
lot occupancy is to unpair the items and allow the tool to become available for other lots, while holding the lot for the Wafer Travel Time period. This is one of the disadvantages of using an entity-centric modelling approach. However, given that only a small proportion of the tools have a Wafer Travel Time, it was seen to have a negligible impact on the factory operating curve.
After the operation, the lot or batch is divided and transported to its next step, a task which may require a transport operator. The following steps summarise the interaction between lots, batches, tools and operators;
1. Lot begins a new operation step,
2. If it is a batch operation then lot waits to form a maximum or minimum batch size depending on the user selection,
3. Lot/batch queues for a tool from the required toolset,
4. If a tool is available then the lot/batch and tool are paired, otherwise the lot/batch queue for the next available tool,
5. Once a tool is captured, the lot/batch/tool request a loading operator, if one is available then the lot/tool/batch grouping are paired with the operator, otherwise the it waits for the next available operator from the operator set to become available, 6. The lot/batch/tool/operator grouping is held for the operator loading fraction, 7. The operator is then released and the lot/batch/tool grouping is held for the re-
maining loading fraction,
8. Steps 5-7 are repeated for the processing step and the operator processing fraction, 9. Steps 5-7 are repeated for the unloading step and the operator unloading fraction, 10. The tool is then released from the lot/batch grouping,
11. The lot/batch is held for the wafer travel time component to simulate the cascading time on the tool,
12. The batch (if one was formed) is then split into its original lots,
13. Each lot then carries out steps 1-12 for its subsequent operations until all of its required operations have been fulfilled.