Journal of Micro/Nanolithography, MEMS, and MOEMSDesign and analysis of a compact EUV interferometric lithography system
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A small-scale interferometric EUV exposure system is presented, based on Talbot interferometric imaging. In this approach, illuminated EUV grating mask orders are recombined remotely through reflection, resulting in interference with spatial preservation across the image plane. The approach is achromatic for NA=λ/p using parallel recombination mirrors and is superior to other methods when efficiency and alignment tolerances are considered. The system design is compatible with both a compact EUV plasma source, which can yield exposure times on the order of several minutes per field, and a compact resonant cavity EUV laser source, which can produce exposure times on the order of several seconds. The system is capable of resolution to 15 to 30-nm half-pitch and is adjustable to accommodate targeted values in this range. Field size is approximately 1 mm, which is consistent with the current field size offered on 193-nm immersion interferometric systems. Wafer stage technology allows for nanometer positioning control across the field and through a field depth of over 50 µm. EUV mask grating fabrication requirements are within the current state of the technology, where no grating component requires resolution better than 2× the image plane pitch (a full-frequency doubled system). Reflection technology has been utilized for each component of the interferometer unit. While the grating mask employs (SiMo)40 multilayers, the interferometric turning mirrors are metallic coatings at angles sufficient to achieve reflection efficiencies greater than 80% for 15-nm half-pitch resolution. The efficiency of the interferometric head is between 7 and 29%, much higher than that which can be achieved using other interferometric lithography (IL) methods. Vibration control for the EUV-IL15 is achieved through a state-of-the-art piezoelectric active isolation system to achieve vibration criteria levels at 1/3 vibration control level E.