As Moore's law drives the semiconductor industry to tighter specifications, challenges are becoming real for overlay metrology. A lot of work has been done on the metrology tool capability to improve single-tool precision, tool-to-tool matching and Tool-Induced Shift (TIS) variability. But nowadays these contribute just a small portion of the Overlay Metrology Error (approximately 10% for 90nm technology). Unmodeled systematic, scanner noise and process variation are becoming the major contributors. In order to reduce these effects, new target design was developed in the industry, showing improvements in performance. Precision, Residual analysis, DI/FI (Develop Inspection / Final Inspection) bias and Overlay Mark Fidelity (OMF) are common metrics for measurement quality. When we come to measurement accuracy, we do not have any direct metric to qualify targets.
In the current work we evaluated the accuracy of different AIM (developed by Kla-Tencor) and Frame-In-Frame (FIF) targets by comparing them to reference “SEM” targets. The experiment was conducted using a special designed 65nm D/R reticle, which included various overlay targets. Measurements were done on test wafers with resist on etched poly printed on 248nm scanner.
The results showed that, for this "straight-forward" application, the best accuracy performance was achieved by the Non Segmented (NS) AIM target and was estimated in the order of 1.5 nm site-to-site. This is slightly more accurate than hole-based target and far more than NS FIF target in this particular case. When using the non-accurate NS FIF target, correctable parameters and maximum overlay prediction error analysis, showed up to 24nm overlay error at the edge of the wafer. We also showed that part of this accuracy error can be attributed to the non-uniformity of BARC deposition.