Achromatic Talbot lithography with partially coherent extreme ultraviolet radiation: mask and illumination parameter optimization

BackgroundCompact extreme ultraviolet (EUV) exposure tools utilizing partially coherent radiation are ideally suited for industrial EUV resist qualification regarding sensitivity, contrast, and resolution. For the broad applicability of the technology, the achievable resolution is crucial and requires thorough investigation.AimThe investigation of limiting factors for the achievable resolution for achromatic Talbot lithography is an important step in creating compact EUV exposure tools for industrial resist qualification and high-resolution patterning, accelerating the research and development of high-resolution EUV photoresists for next-generation microchips and processes.ApproachTo maximize the contrast of the resulting intensity distribution in the wafer plane for high-resolution patterns, both the illumination parameters and the utilized transmission masks are investigated by rigorous coupled-wave analysis simulations. The main influencing factors on the achievable resolution are identified and presented. In addition to the simulative optimization of the phase-shifting masks, the fabrication of the dense periodic nanopatterns becomes increasingly challenging for smaller periods. The mask fabrication process is therefore optimized to create stable and high-resolution periodic mask patterns.ResultsRigorous simulation of the achievable aerial image contrast in the wafer plane demonstrates the influence of partial coherence as well as the geometry and material selection of the mask. For the current mask design, the theoretical resolution is limited to a 14-nm half-pitch (wafer scale). The mask fabrication process is optimized leading to an experimental record resolution of 32.5-nm half-pitch for lines and spaces as well as 28-nm half-pitch for pinhole patterns with the presented demonstrator setup. Metal-based masks with optimized geometry will allow for the fabrication of 6.5-nm half-pitch patterns (wafer scale).ConclusionsAchromatic Talbot lithography can be used in compact EUV exposure tools with a theoretical resolution below 10-nm half-pitch (wafer scale). The main influencing factors on the achievable resolution are the mask material used and the degree of coherence of the utilized radiation.