Optimizing experimental design for fast and accurate CD metrology of interconnects using coherent extreme-ultraviolet scatterometry

Abstract. Background Metrology techniques with high accuracy and throughput are needed to address critical challenges in the characterization of periodic nanostructured materials. Copper interconnects are structures for which precise topography characterization is needed. Although common methods include atomic force and scanning electron microscopy, extreme-ultraviolet (EUV) scatterometry has unique capabilities for the characterization of structure parameters and material properties. Aim We study the effect of experimental design parameters (wavelength and incidence angle) on EUV scatterometry. We explore the fundamental sensitivity capabilities of EUV scatterometry for interconnect metrology when data are collected such that the information content in a single EUV diffraction pattern is maximized by tuning the wavelength and incidence angle. Approach We applied experimental design methods based on the Fisher information matrix to EUV scatterometry of an interconnect to obtain the optimized EUV wavelength and angles at which to obtain a single diffraction pattern. Using rigorous coupled wave analysis simulations and Monte Carlo uncertainty quantification, the accuracy of the parameter reconstructions under an optimal design is quantified. Results The uncertainty of the sample parameters depends strongly on wavelength and incidence angle, necessitating experimental design methods to achieve maximal sensitivity. When data are collected under an optimal experimental design, simulations indicate that sub-angstrom accuracy can be obtained from only a single diffraction pattern for the dishing depth of interconnect pads (0.02 nm standard deviation), and the substrate density can additionally be extracted (0.015  g/cm3 standard deviation). Decorrelation of the sample parameters is observed with optimal design, and 13.8 nm EUV light provides better sensitivity than longer wavelengths such as 30 nm. Conclusions Experimental design methods are an essential tool for ensuring that experimental data are collected under an optimal design that maximizes the information in the data. Implementing these methods opens further capabilities for fast and precise interconnect metrology with EUV scatterometry.