Hyper NA EUV lithography: an imaging perspective

Background: To print ever smaller features at high contrast projection lithography technology has evolved to shorter wavelength light and larger numerical aperture (NA). After enabling the extreme-ultraviolet (EUV) wavelength, the industry is looking into increasing the NA. At NA's much higher than 0.55, new effects such as polarization will start to play a role, and larger impact of ultimate mask resolution and material interactions is expected. Already at NA 0.55, a small contrast loss is predicted due to the use of unpolarized light in the scanner. Further increasing the NA beyond 0.55 will exacerbate the contrast loss. In addition, the larger incidence angles on mask when the NA increases above 0.55, will further enhance the mask 3D (M3D) effects forcing additional mask changes.Aim: Study the potential imaging challenges regarding NA scaling beyond 0.55 and identify if the imaging can still work at this higher NA and whether specific changes to the mask stack are required.Approach: We study the polarization effects in detail and assess their impact quantitatively for a set of generic building blocks. To enable proper imaging at high incidence angles, new mask architectures that include changes in the EUV mask absorber and multilayer will have to be tested using rigorous simulations.Results: The current periodic Mo/Si multilayer concept is adequate to support 0.75 NA performance. For NA 0.85 aperiodic multilayers would be beneficial. Novel mask absorber materials currently being developed for the 0.33 and 0.55 NA systems are adequate to support 0.75 NA imaging. M3D phase effects slightly increase with NA but can be compensated by adjusting absorber n / k and thickness. Regions of interest in the absorber space remain low-n and high-k. NA 0.75 promises significant normalized image log slope (NILS) gain over NA 0.55 in the L/S pitch range of 20 to 30 nm even when considering M3D effects, existing multilayer periodicity, and no polarizer. For contact holes (CHs), NILS > 2.4 without polarizer can be achieved in both dark and bright mask tonalities for pitches 19 nm and above, even when considering M3D effects, existing multilayer periodicity, and no polarizer. Even if polarization control could provide significant NILS gain for lines and spaces, EUV polarizer may not be worth it in terms of the balance between contrast and throughput. Further validation should be done using a full stochastic resist model in the future. For CHs, the NILS gain possible by polarization is too small to warrant the lost dose.Conclusions: We identified clear advantages of high NA imaging but found no significant blocking factors for the imaging of the investigated patterns (single pitch lines/spaces and CHs).