To achieve higher resolution extreme ultraviolet lithography (EUVL) targeted toward sub-10 nm, reflective projection scanner image numerical apertures (NAi) are being increased beyond the current value of 0.33 to 0.55 and upward of 0.75 as a desirable target. Bragg reflectors using alternating silicon and molybdenum that have heretofore been coated as periodic multilayers cannot achieve desired reflected amplitudes as corresponding 0.25× mask numerical apertures (NAm) are accordingly increased. In addition, transverse magnetic-polarized image modulation decreases with NA, which becomes significant at 0.55 and above. We present here the optimization of non-regular alternating, or aperiodic, silicon-molybdenum multilayer reflective coatings that can achieve improved amplitude and polarization performance through angle as higher-NA EUVL lithography is pursued. Through the use of rigorous EM computation paired with a genetic optimization method, we show that amplitude apodization can be recovered to 60% peak reflectance for NAm values up to 0.2 (corresponding to NAi values of 0.8) while at the same time achieving a transverse electric degree of polarization exceeding 40%. In addition, aperiodic multilayers optimized for spectral bandwidth using refractory metals also show improvement over periodic designs.
To achieve higher resolution extreme ultraviolet lithography targeted toward sub10nm, reflective projection scanner image numerical apertures (NAi) are being increased beyond the current value of 0.33 to 0.55, and upwards of 0.75 as a desirable target. Bragg reflectors using alternating silicon and molybdenum that have heretofore been coated as periodic multilayers cannot achieve desired reflected amplitudes as corresponding 0.25X mask numerical apertures (NAm) are accordingly increased. Additionally, TM polarized image modulation decreases with NA, which becomes significant at 0.55 and above. We present here the optimization of non-regular alternating, or aperiodic, silicon-molybdenum multilayer reflective coatings that can achieve improved amplitude and polarization performance through angle as higher-NA EUVL lithography is pursued. Through the use of rigorous EM computation paired with a genetic optimization method, we show that amplitude apodization can be recovered to 60% peak reflectance for NAm values up to 0.2 (corresponding to NAi values of 0.8), while at the same time achieve a TE degree of polarization (DOP) exceeding 40%.
Layout designs are reaching the resolution limit for 0.33NA extreme ultraviolet lithography (EUVL) systems, with 0.55NA high-NA on the horizon. Alternative mask designs at reduced absorber thickness for higher image contrast have become necessary. Novel absorber candidates are classified as attenuated phase shifting mask (attPSM) absorbers, high-k mask absorbers and index matched absorbers (n ≈ 1) based on the complex refractive index (n – ik). We identify absorber candidates through effective media approximation (EMA) model and discuss design considerations for attPSM absorbers. Optimum phase shift for EUV attPSM is higher than π and it is influenced by the absorber material, diffraction angle at the mask, mask pattern, NA and absorber reflectivity. Index matched mask absorber designs with higher extinction coefficient are also proposed as promising candidates.
Next generation EUV scanners have been introduced with anamorphic, obscured multi-layer optics for operation at 0.55NA. Aberrations are of particular concern with high-NA EUVL, as the 13.5nm wavelength has returned wavefront phase errors to near I-line levels. With the central obscuration necessary so that additional lenses aren’t needed, the Zernike basis is no longer orthonormal, resulting in coefficient values which are dependent on the number of fitted terms. For an industry transition to a Fringe Tatian wavefront description to be successful, it is important to incorporate and carryover the intuitive understanding of the imaging effects of common aberrations. Using modifications to Prolith and Dr.LiTHO lithography simulators, this work defines a simulated lithography lens using the Fringe Tatian basis and includes simulations of common patterning conditions for next generation high-NA EUV nodes.
SMO sources with pupil fill values as low as 0.15 are targeted for use in High-NA EUV imaging to deliver improved NILS and contrast. These condensed source shapes in combination with high energy sources using 13.5nm light are placing ever increasing photo intensities onto EUV lens and masks. Furthermore, the highly focused diffraction energy of line space and contact patterns results in beam fluences that over time, may cause the multilayer performance to drift from the ideal. In this work a method for predicting imaging impacts in the high-NA, low pupil fill regime for simulated multilayer mirrors is presented.
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