In the latest 3D NAND devices there is a larger focus on measurement accuracy control, coupled with more traditional minimization of Total Measurement Uncertainty (TMU). Measurement inaccuracy consumes an increasingly significant part of the overlay (OVL) budget, requiring control and optimization.
In this paper we will show the improvement in imaging OVL measurement accuracy using wave tuning (WT) capability combined with advanced algorithms to address 3D NAND process challenges. In addition to new OVL target designs that take advantage of WT capability, we also demonstrate improvement in OVL model residuals through optimization of measurement bandwidth, focus position and number of grab frames. Improvements in precision and tool-to-tool matching are also realized through both optimization of the region of interest (ROI) and splitting measurement areas using a dual-recipe technique.
Shrinking on-product overlay (OPO) budgets in advanced technology nodes require more accurate overlay measurement and better measurement robustness to process variability. Pupil-based accuracy flags have been introduced to the scatterometry-based overlay (SCOL) system to evaluate the performance of a SCOL measurement setup. Wavelength Homing is a new robustness feature enabled by the continuous tunability of advanced SCOL systems using a supercontinuum laser light source in combination with a flexible bandpass filter. Inline process monitoring using accuracy flags allows for detection, quantification and correction of shifts in the optimal measurement wavelength. This work demonstrates the benefit of Wavelength Homing in overcoming overlay inaccuracy caused by process changes and restoring the OPO and residual levels in the original recipe.