Proceedings Article | 26 September 2019
KEYWORDS: Extreme ultraviolet lithography, Photoresist technology, Ions, Tin, Polymers, Photoresist materials, Metals, Sodium, Computational modeling, Integrated circuits
Extreme ultraviolet lithography is the newest technique to keep up with Moore’s law and create smaller integrated circuit feature sizes. However, novel photoresist materials must be used in order to withstand the high energy beam (λ=13.5nm). Metal-oxo clusters have been proposed as one photoresist solution, and specifically the most promising is a sodiumcentered tin-Keggin cluster. A simple one-step synthesis was developed to produce a Na-Sn Keggin cluster, without the need for heating, filtration, or recrystallization. However, the product was a mixture of the β-isomer (β-NaSn12) and the γ- isomer (γ-NaSn12), which share the formula [(MeSn)12(NaO4)(OCH3)12(O)4(OH)8]1+. For fundamental studies on the lithographic mechanisms occurring during exposure to be successful, a pure and stable isomer is desired. Computational modeling was recruited to determine the ground state energy of all five uncapped isomers in this Na-Sn Keggin system. Additionally, the inclusion of one or two tin atoms to the uncapped structure, called capping, altered which isomers were stabilized. Computations were also employed to evaluate the influence of this capping strategy for the single-capped β- isomer (β-NaSn13), the single-capped α-isomer (α-NaSn13), the single-capped γ-isomer (γ-NaSn13), and the doublecapped γ-isomer (γ-NaSn14). Density functional theory (DFT) was used to obtain the hydrolysis Gibbs free energy and HOMO-LUMO gap, which led to the stability ranking: β-NaSn12 < γ-NaSn12 < α-NaSn12 < δ-NaSn12 < ε-NaSn12 for uncapped clusters, which was consistent which experimental observations. The uncapped isomers were computationally evaluated to be more stable than their respective single-capped analogues. However, the double-capped γ-NaSn14 was more stable than either the uncapped or single-capped clusters. Therefore, capping has shown to be a useful tool in exploring the stability landscape of these Keggin clusters to promote a pure and stable material for the next generation EUV lithography photoresists. And noteworthy, this sodium-centered tin-Keggin ion represents the only Keggin ion family so far, that favors the isomers of lower symmetry.
