Alternative reticles have the potential to improve EPE for low-k1 EUV lithography on multiple aspects, by reducing mask 3D effects and improving optical contrast. We study the application of high-k masks and attenuated phase-shift masks at diffraction level and show that mitigation of mask 3D effects, such as contrast fading, is crucial for both good performance of both alternative-reticle types. We present optimum embodiments for both mask types. We find that the optimum attenuated phase-shift mask (PSM) results in a phase shift of 1.2 π. The extra 0.2 π phase shift required for the EUV mask compared to its DUV counterpart is needed to compensate the strong mask 3D effects; the 1.2 π phase shift is crucial for good performance at small pitch and was found for all 3 materials studied in this work: Ru, Pd, and Mo. We show that our Rubased attenuated PSM embodiment results in a strong gain in normalized image log slope (NILS). <30% NILS gain can be achieved compared to a Ta-based reference mask. To demonstrate the generic applicability of the mask, we show NILS gain using the same attenuated PSM embodiment for different use cases for 0.33 and 0.55-NA EUV lithography, including regular contacts, DRAM patterns, and contacts through pitch. We show that the optimum mask-type choice is application dependent and present our recommendations in a mask-decision tree. We discuss the implications of using new reticle absorbers for scanner integration.
EUV lithography is being used at relatively high-k1 Rayleigh factors. Advancing EUV to smaller resolution requires several technological advancements. The EUV reticle is a strong contributor that limits current EUV imaging performance. Improvements with advanced mask types are required to reduce mask 3D effects and to improve image contrast. This will enable low-k1 resolution with reduced stochastic defect rates. In this paper we discuss what the requirements of high-k absorber masks and attenuated phase shift masks are to achieve optimal imaging performance. Recommendations on the mask stack composition and the application of mask types to different use cases are based on the physical understanding of the mask diffraction spectrum.
EUV lithography is entering High Volume Manufacturing at relative high Rayleigh factor k<sub>1</sub> above 0.4. In comparison immersion lithography has been pushed to k<sub>1 </sub>values of 0.3 or below over the last two decades. One of the strong contributors determining the effective usable resolution is the mask absorber stack. The mask stack alters the diffraction by modifying the phase and intensity of the diffracted orders. In this paper we show the exposure results of a test mask having higher absorbance of EUV light and the advantages of reduced Mask 3D effects to imaging.
Mitigation of mask 3D effects is essential for EUV imaging of high resolution features. The 3D EUV masks give rise to phase effects over the diffracted orders and potentially distort the image on the wafer. These phase effects may reduce contrast, result in pattern shifts and result in best focus variations on wafer. Two variations on the current absorber are investigated to their impact on reduction of M3D effects and impact on image quality. Use of high-k absorber materials allows for thinner masks to be used and helps to reduce averse M3D effects. Attenuated phase shift masks work by allowing a higher optical transmission while giving a phase shift to the transmitted light, which further improves image contrast on wafer and also enables thinner absorbers to be used. Attenuated PSM absorbers show a stronger variation in imaging performance through incidence angle onto the reticle. It has been shown that this results in a variation in imaging performance for varying features and pitches. Specifically of interest is how NILS through focus is influenced by the different absorbers. Phase shift masks show better performance for NILS through focus on contact holes, and high-k masks work well for dense lines.