Mask Blank Inspection with Multi-wavelength High Harmonic Source(Invited)
The fabrication of defect-free Extreme Ultraviolet(EUV)lithography mask blanks for future nodes remains a pressing challenge,driving the demand for high-sensitivity mask inspection tools for smaller defects.A typical EUV lithography mask blank is composed of more than 40 molybdenum/silicon(Mo/Si)bilayers coated on low thermal expansion glass,even minor imperfections in the EUV mask blank may lead to significant negative effects on the printing process.Such imperfections also called defects can be generally categorized into two types:amplitude defects and phase defects,posing extreme difficulty for non-EUV inspection systems.Therefore,in recent years,Actinic Blank Inspection(ABI)tools utilizing Microscopic Scattering Dark-field Imaging(MS-DFI)system have gained prominence;however,challenges for smaller nodes persist.These have spurred exploration of alternative sources,notably the High-Harmonic Generation(HHG)source.This work investigated the detection capabilities with HHG source using the rigorous finite-difference time-domain(FDTD)method.Normalized scattering signal intensity of different orders of HHG source are compared in order to identify the wavelength promising for enhanced detectability.The wavelength range we choose is 13.5～38 nm(i.e.,59～21th order).To investigate the basic features of different defects,three structural models have been employed.Surface defect model refer to the amplitude defect.Phase defects are divided into two categories:shallow defect model and deep defect model.The simulation results indicate that for surface defect,defects with specific diameters exhibit pronounced signals at wavelengths distinct from 13.5 nm.For defects below 10 nm,the scattering signal intensity of all wavelengths diminishes with decreasing defect size.However,the decline rate for sources around 30 nm is much slower than that for 13.5 nm,their intensity can be even higher for a defect of 2 nm diameter than that of 13.5 nm.The example is the 38 nm wavelength source:as the defect diameter decreases,the normalized signal intensity at 13.5 nm significantly diminishes,while that of 38 nm becomes comparable to that of 13.5 nm.For 2 nm defect,the normalized signal intensity of 38 nm is even higher than that of 13.5 nm.Additionally,because the 38 nm wavelength is longer than 13.5 nm,the yield of the 38 nm source in HHG can be much higher than that of 13.5 nm.This suggests that for smaller defects,the longer wavelength of 38 nm might be a better choice.To investigate the signal intensity of phase defect,penetration depth of various wavelengths in the EUV mask blank are first calculated employing the transfer matrix.The superior penetration depth of 38 nm wavelength leads us to inquire about its capability to detect phase defects.So,the scattering signal intensity at 13.5 nm and 38 nm wavelength are compared using the same defect parameters.The results of shallow defect suggest a stronger dependence of signal intensity on FWHM width than on height,indicating that width might be a more important factor in this context.And it also implies that the aspect ratio(the ratio of the width to the height of an object)could potentially play an important role in signal intensity.The signal intensity of shallow defects at 38 nm wavelength exhibits a flatter distribution compared to that of 13.5 nm.This characteristic endows the 38 nm wavelength with enhanced detectability for defects across different heights and widths,particularly when the signal intensity at 13.5 nm is decreased.For example,shallow defects with heights between 7～20 nm and widths between 20～80 nm can be detected much more efficiently by the longer wavelength source of 38 nm.For deep defects,the results suggests that the scattering signal primarily originates from the top 20 bilayers'deformation.like the 13.5 nm signals,the scattering intensity of the 38 nm source signals is highly dependent on defect heights.In both simulations,for particles located 40 bilayers beneath the mask surface,there's a pronounced drop in signal intensities for defect heights less than 10 nm.Moreover,the signals for these defects diminish nearly an order of magnitude for every 1 nm reduction in height.These fully planarized defects,which cause no surface protrusion,pose significant detection challenges.By comparing the signals at 13.5 nm and 38 nm for deep defects of the same height and width,we found that the benefits of using a 38 nm wavelength for deep defects are relatively modest.Nevertheless,considering the ease and potentially higher efficiency of generating a 38 nm wavelength HHG source,this result is still valuable,particularly for defects where the 38 nm signal is comparable to that of 13.5 nm.
High harmonic generationExtreme ultraviolet lithographyDark-field imagingDefect inspectionFinite-difference time domain
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