Hybrid methodology for on-product focus control using CD and diffraction-based focus marks

The semiconductor industry current standard of focus setup and control can be improved by the implementation of diffraction-based focus (DBF) marks and their applications. Determining best focus per scanner/reticle/ device/layer (SRDL) combination is currently done by exposing a focus/energy matrix (FEM) wafer and examining CD features. The drawback of this process of record (POR) method is that the accuracy is greatly influenced by inter- and intra-field effects, focus step size, and machine accuracy. However, DBF marks do not suffer from these drawbacks because they are measured on product and close to the CD features. Experiments confirm that when comparing Bossung curves on each scanner, the wafer-to-wafer variation is much lower using DBF. The setup time and accuracy of new SRDL combinations can also be greatly improved using DBF. DBF uses an asymmetry signal which is translated directly to focus values and is independent of any focus settings of the scanner. After accurately determining the best focus using DBF on only one SRDL combination, the focus setting can be applied to all other combinations and scanners will be matched. Instead of exposing a FEM for each SRDL combination, best focus only needs to be determined once. Experiments using five different machine/reticle combinations show that Bossung tops can be matched with significantly more accuracy compared to POR. Experiments also show a linear relation between energy and shift in Bossung top; both DBF and CD are sensitive to energy variation. When correcting for energy differences, the Bossung top scanner-to-scanner matching accuracy can be improved further. A method using DBF for scanner best focus matching saves up to 10 hours of CD-SEM and manpower setup time per SRDL combination. When a scanner needs to be requalified, the same DBF focus setup method can be used, reducing the scanner downtime.