SPIE Journal Paper | 17 November 2023
KEYWORDS: Line width roughness, Fourier transforms, Critical dimension metrology, Metrology, Signal to noise ratio, Film thickness, Extreme ultraviolet lithography, Scanning electron microscopy, Photoresist processing, Image analysis
BackgroundLithography advancements require resist layer thickness reduction, essential to cope with the low depth of focus characteristic of high numerical aperture extreme ultraviolet lithography (High NA 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.AimWe aim to investigate the impact of reducing the film thickness of the dry photoresist on the E-beam metrology and line local critical dimension uniformity (LCDU).ApproachThin dry resist films from 10 to 30 nm are patterned at pitches 24, 28, and 32 nm using EUVL. A standard critical dimension scanning electron microscope (CDSEM) is used to perform E-beam metrology. The captured SEM images are analyzed using software to extract the parameters of interest, such as the critical dimension (CD), power spectral density (PSD), unbiased line width roughness (uLWR), LWR correlation length, signal-to-noise ratio (SNR), and line LCDU for a range of features with lengths from 12 to 50 nm. Furthermore, the experimental results of the line LCDU of a range of feature lengths and roughness parameters are plotted for different process conditions of thickness and pitch in a normalized way.ResultsThe results indicate that the dry resist shows good CDSEM imaging contrast and high SNR even at the thinnest resist thickness. This enables extracting a reliable uLWR measurement without the need to increase the number of frames to modulate the SNR. Moreover, it has LWR correlation length of around 2.5 nm. By thinning the resist thickness, the uLWR and line LCDU increase, and the SNR and the LWR correlation length decrease. Finally, when normalizing the line LCDU to uLWR and the feature length to the correlation length, the resulting curves of the different process conditions of the thickness and pitch overlap with each other, following an analytical model that relates the line LCDU to the resist roughness parameters.ConclusionWe showed the impact of reducing the dry resist thickness on E-beam metrology and line LCDU for features foreseen for High NA EUVL. The dry resist has good imaging contrast, LWR correlation length of around 2.5 nm, and the line LCDU depends on the resist thickness (uLWR and LWR correlation length for each particular thickness). With such a dataset, we demonstrated an overlap between the normalized curves of the line LCDU to the uLWR and the feature length to the correlation length for the different process conditions of thickness and pitch. The uLWR and correlation length are the knobs to achieve good patterning uniformity for a given feature length.
