In recent years, the 3D NAND stack has continued to expand rapidly in the Z direction with tighter process windows, while deck-to-deck overlay faces increasing measurability and accuracy challenges. Meanwhile, thick opaque hard mask (HM) has been utilized in several layers, thus downgrades a lot of mark contrast and introduces large process variation (PV). To meet tightened overlay requirements, KLA’s Metrology Target Design (MTD) assisted by simulation analysis has become an important part of the holistic overlay improvement solution set. Compared to the conventional target design simulation process in which the target stack is built up with a standard template, in this work, a customized template was designed to fit actual mark behavior. Using this new template, more accurate simulation to measurement (S2M) matching was demonstrated, which makes the new target design more reliable. With this added benefit, the newly designed target achieved desirable residual improvement with better on-product overlay (OPO) performance. In the case of the challenging thick opaque HM layer, target segmentation and CD/Pitch optimization by simulation also significantly improved mark contrast and OPO using the newly designed mark. Along with contrast gain, the robustness against large PV of the simulated mark was increased by quantifying comprehensive PV on product wafers. Furthermore, with the verification of the measured data, simulation data can be used to establish a more thorough representation of the process characteristics and parametric sensitive virtual metrology, targeted to meet the goals of maximizing overlay accuracy in the 3D NAND process.
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.
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