Low-voltage aberration-corrected SEM metrology of thin resist for high-NA EUVL

Background: Lithography advancements require resist layer thickness reduction, essential to cope with the low depth of focus (DOF) characteristic of high numerical aperture extreme ultraviolet lithography (HNA EUVL). However, such a requirement poses serious challenges in terms of resist process metrology and characterization, as patterns in thin resist suffer from low contrast, which may affect the performance of the edge detection algorithms used for image analysis, ultimately impacting metrology. Aim: Investigate e-beam imaging using low landing energy (LE) settings as a possible way to address the thin resist film metrology issues. Approach: A low-voltage aberration-corrected SEM developed at Carl Zeiss is to image three thin resist thicknesses and two different underlayers, at various LE and number of frames. All images are analyzed using MetroLER software, to extract the parameters of interest [mean critical dimension (CD), line width roughness (LWR), and linescan signal-to noise-ratio (SNR)] in a consistent way. Results: The results indicate that mean CD and LWR are affected by the measurement conditions, as expected. Imaging through landing energy unravels two opposing regimes in the mean CD estimate, the first in which the mean CD increases due to charging and the second in which the mean CD decreases due to shrinkage. Additionally, the trend between LE and linescan SNR varies depending on the stack. Conclusion: We demonstrated the ability of low-voltage aberration-corrected SEM to perform thin resist metrology with good flexibility and acceptable performance. The landing energy proved to be an important knob for metrology of thin resist.