Utilizing a unique high NA optical system, a new methodology to measure device overlay accurately has been developed with a key differentiation. Historically, optical techniques to measure features below the image resolution require supporting measurement techniques to be used as a reference to anchor the optical measurement. This novel selfreference methodology enables accurate and robust optical metrology for device features after etch eliminating the need for external reference measurements such as Decap, x-sections or high landing energy SEMs. In this paper, we discuss how a high NA Optical Metrology system enables measurements on small area device replica targets, which enables the ability to create a reference target for device measurements. The methodology utilizes this reference target to enable accurate direct on device overlay measurements without the need for an external reference. Furthermore, the technique is expanded to improve the robustness of the measurement and monitor live in production the health of the recipe, ensuring accuracy overtime. This ultimately leads to a method to extend the recipes in real-time based on the health KPIs. The improved accurate and robust device overlay measurements have proven to improve the overlay performance compared to other techniques. This, combined with the speed of optical systems, enables unconstrained dense measurements directly on device structures after etch, allowing for improved overlay control.
The logic manufacturing process requires small in-device metrology targets to exploit the full dose correction potential of the modern scanners and process tools. A high-NA angular resolved scatterometer (YieldStar S-1250D) was modified to demonstrate the possibility of OCD measurements on 5x5µm2 targets. The results obtained on test wafers in a logic manufacturing environment, measured after litho and after core etch, showed a good correlation to larger reference targets and AEI to ADI intra-field CDU correlation, thereby demonstrating the feasibility of OCD on such small targets. The data was used to determine a reduction potential of 55% for the intra-field CD variation, using 145 points per field on a few inner fields, and 33% of the process induced across wafer CD variation using 16 points per field full wafer. In addition, the OCD measurements reveal valuable information on wafer-to-wafer layer height variations within a lot.