An accurate and robust after-develop overlay measurement solution using YieldStar multi-wavelength optical metrology accompanied by a precise application strategy

Abstract

The technology evolution of 3D-NAND storage devices requires an extensive research and development (R and D) phase with frequent process changes during the film deposition, lithography and etching steps. These process changes might have an impact on the accuracy of overlay measurements, thereby influencing the on-product overlay performance. Besides, by increases of 3D-NAND storage density, the available space for overlay metrology error is significantly reduced. The combination of frequent process changes in R and D phase and tighter overlay metrology budget, increases the necessity of an accurate, yet robust overlay metrology solution that can be adopted for 3D-NAND development phase. In addition to that, such a metrology solution needs to have the ability of being smoothly transferred to the ramp and eventually to the high volume manufacturing (HVM) stage, where the metrology throughput is playing a significant role in terms of cost of ownership reduction. In this paper, we report the YieldStar multi-wavelength diffraction-based overlay (μDBO) metrology as a solution to address the above challenges. Unlike the conventional optical overlay metrology methods which use single light wavelength, this diffraction based technique uses multiple wavelengths to measure every single overlay metrology targets, which proves to be robust against process variation induced metrology errors. In order to demonstrate the advantages of this new metrology solution, the accuracy, robustness, and throughput performance of the multiwavelength μDBO metrology technique are evaluated in the YMTC 3D-NAND manufacturing process. In addition, a well-defined application strategy is developed for reducing the number of measurement wavelengths by the maturity level of process which results in a robustness gain without impacting the HVM throughput requirements.