As design rule shrinks down, on-product focus control became more important since available depth of focus (DOF) is getting narrower and also required critical dimension uniformity (CDU) becomes tighter. Thus monitoring, control the scanner focus error and reducing the focus control budget of scanner are essential for the production. There are some critical layers which has so narrow DOF margin that hardly be processed on old model scanners. Our study mainly focused on the analysis of the scanner focus control budget of such layers. Among the contributors to the focus budget, inter-field focus uniformity was turned out to be the most dominant. Leveling accuracy and intra-field focus uniformity were also dominant.
KEYWORDS: Semiconducting wafers, Metrology, Reticles, Signal processing, Control systems, Lithography, Scanners, Process control, Scanning electron microscopy, Computer simulations
This paper introduces to improve inter-field CDU with on-product focus control by diffraction based focus (DBF) method. For DBF target selection, a robust focus metrology for focus control was obtained, and the selected DBF target was integrated on each seven spot of a product reticle. For on-product focus control, previously on-product focus monitoring was performed, and the monitored lots showed a stable focus fingerprint. Based on the result, Z and Z/ Rx/Ry corrections per field on wafers were applied. Focus uniformity of controlled wafers was improved up to 29% in comparison with non-corrected ones. To demonstrate the improvement of inter-field CDU, Full CDs on wafers were measured by SEM. As a result, inter-field CDU for controlled wafers was improved by 16% (3σ) compared with noncontrolled wafers.
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