Investigation of stochastic roughness effects for nanoscale grating characterization with a stand-alone EUV spectrometer

Abstract

Fast and non-destructive non-imaging metrology of nanostructures is crucial for the development of integrated circuits and for the corresponding in-situ metrology within fabrication processes. Stochastic variations related to the gratings local period (line edge roughness, LER) and line width (line width roughness, LWR) are of special interest due to their key role for the minimal achievable structure size. Non-imaging metrology approaches taking these statistic variations into account are quite limited. For scatterometry, models predict a change of the grating’s diffraction efficiency according to a DebyeWaller factor but only in the non-zeroth diffraction orders. The authors perform simulations of nanoscale gratings that suggest an influence of LER and LWR on the reflectance (zeroth diffraction order efficiency) which motivate an extended study on LER and LWR measured by spectrally resolved EUV reflectometry here described as EUV spectrometry. The authors present reconstruction results of nanoscale gratings measured with a compact spectrometer utilizing extreme ultraviolet (EUV) radiation emitted by a discharged-produced plasma (DPP) EUV source. The use of two sequential spectrographs, one for the reference measurement of the source spectrum, the other one for the measurement of the spectrum after sample interaction, combined within the experimental setup allows to measure the broadband reflectance with 2% relative uncertainty of samples under various grazing incidence angles. The method offers a proven sub-nm reconstruction accuracy for critical grating parameters. Within the presented study, the measured samples are dedicated test samples, fabricated to exhibit well-defined LER and LWR at different grating periods and linewidths. In addition, the samples are also cross-characterized by the Physikalisch-Technische Bundesanstalt (PTB, Berlin). Experimental and simulative results are discussed to derive approaches to include LER and LWR as parameters in the physical model for reconstruction.