Novel high-contrast phase-shifting masks for EUV interference lithography

When it comes to the patterning of periodic structures like lines and spaces or contact hole arrays interference lithography can be effectively applied. By the use of ultrashort wavelengths in extreme ultraviolet (EUV) range, structure sizes can be pushed into the sub-100 nm region. Using advanced interference schemes, such as achromatic Talbot imaging, high-quality large-area patterning can be realized, e.g. for resist stochastics tests as well as for advanced fabrication processes in research and also small-batch production. Since the Talbot lithography only requires medium spatial coherence and accepts broadband emission, the approach can be used not only with coherent radiation as provided by synchrotron facilities but also with compact plasma-based EUV sources. In this contribution the theoretical resolution limit for the achromatic Talbot lithography is determined by simulations for the use of optimized phase-shifting transmission mask concepts illuminated by compact discharge-produced plasma EUV source operating at a main wavelength of 13.5 nm. The further effects that reduce a practical resolution limit, such as mask imperfections, positioner instabilities and resist contrast are also considered. With the realized EUV laboratory exposure tool and polymer-based contact hole phase-shifting masks 28 nm resolution has been demonstrated so far. Main limitations are found in the mask fabrication process that cannot be further down-scaled due to increasing aspect ratios and pattern degradation of the mask structures. To extend the developed nanopatterning technology to the sub-30 nm region, optimized phase-shifting transmission masks have to be designed and fabricated, enabling a contrast-rich intensity modulation in wafer plane. As size of the mask openings is approaching the exposure wavelength, mask geometry has to be optimized for every node. In this paper rigorous simulations of new mask designs optimized for the achromatic Talbot lithography are presented. For selected phase-shifting transmission masks, the influence of the mask material and geometry on the resulting aerial image is evaluated along with an analysis of the theoretical resolution limit.