Traditional semiconductor manufacturing relies on a fixed process recipe combined with classic statistical process control to monitor the production process. Leading edge manufacturing processes continue to require increasingly stringent critical dimension and overlay control, which in turn demands innovative methods for process control. Meeting tighter process specifications, while maintaining productivity, dictates implementation of Advanced Process Control (APC) methods. An active control method exercised in APC enables the user to modify recipe variables in order to compensate for various disturbances such as drift or step changes in tool operation, or in the conditions of incoming product. The automated version of this control methodology is termed Run-to-Run (R2R) control. R2R control systems compensate for many of the dynamic issues that stand in the way of high level tool dependability, leading to benefits such as compensation for process variation, improved overlay control, rework reduction, reduction in the use of send-ahead wafers, and increased exposure tool availability.
For R2R systems, the integrity of the data from metrology tools is critical. In an automated Fab environment, data is fed directly from measurement tools into databases, where it is used to generate feedback corrections on subsequent production material. Metrology measurements are often based on pattern recognition at the measurement site. Therefore, problems with pattern recognition can lead to flyer data, which in turn may impact the quality of data used in the feedback loop. Using operators to inspect and approve each measurement is costly.
In a foundry environment, where multiple products are manufactured, an additional challenge is introduced. Historical data used to generate feedback can often be out of date when the product is combined with tool status. Routine Preventive Maintenance (PM) procedures may require updating some machine constant values that are related to overlay performance. In these cases, the R2R controller should be “Reset” and a new send-ahead wafer should be used.
At Tower, a R2R control system, which provides overlay process corrections, was integrated into the production environment. Overlay performance metrics were monitored before and after system introduction to show the benefit of R2R control. Additional work was done to characterize the performance benefit of introducing advanced data filters and tool PM data into the same R2R control system. Results from the additional work show how effectively identifying and removing outliers can improve data integrity, and how tool PM data can be used to appropriately respond to step functions following exposure tool PM adjustments.