A new alignment mark implementation for color processing has been successfully tested in a joint activity between ASML and TOWER. Alignability and overlay performance have been proven by applying the combination of a pure high order mark design and a dual implementation using the ATHENATM alignment sensor and a PAS5500/400 machine.
By their very nature, the resists used in color filter processes will absorb light at various wavelengths. While this makes them useful as color filters, it can noticeably reduce the signal strength of pattern images as seen by lithography alignment systems. The pure high order mark design enhances the signal strength. Exposing two of such mark pairs in a Metal Last layer with two different metal plateaus (in previous metal layers: "Metal Last - 1" and "Metal Last -2") leads to two different optical mark depths and therefore mimics a four wavelength alignment system with ATHENA. In principle, the evaluated technique might be extended to more (than four) "wavelengths" as well as other process layers. Moreover, the use of scribe-line marks enhances productivity since no extra lithography step is required to expose Zero Layers. The performance of this implementation has been evaluated for 180-nm CMOS Image Sensor technology.
This paper discusses the overlay and alignment results of the evaluation. Alignment parameters such as absolute signal strength and signal strength variation were studied in detail. It is shown that such mark implementation shows good alignability and easily meets the product overlay requirements of Image Sensor devices.
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.
The benefits of using a run-to-run control system for overlay and CD control have been well documented. However, before any these benefits can be achieved, one must first integrate the run-to-run control system into the existing automation and manufacturing execution system (MES) environment. Integration details that are overlooked during the planning stages often times create unnecessary challenges down the road that can delay reaching advantageous control results. INFICON has developed a novel methodology of documenting process and integration requirements. This method, termed Use Case Review, congregates the appropriate resources from the supplier and the customer to review and customize a predetermined set of documents that describe the run-to-run controller. Each use case contains a flow diagram and a detailed sequence of transactions documenting the actors (Automation PC, Process Equipment, MES, etc.) and variables (Lot ID, Process Level ID, Recipe ID, etc.) involved. The combined set of use cases covers all aspects of integrating a lithography run-to-run controller. During the implementation of NVS ARGUS, TOWER Semiconductor Ltd. benefited from use case review and customization.