Novel hexameric tin carboxylate clusters as efficient negative-tone EUV photoresists: high resolution with well-defined patterns under low energy doses

Synthesis of two novel tin carboxylate clusters (RSn)₆(R'CO₂)₈O₄Cl₂ is described, and their structures have been characterized by X-ray diffraction. These clusters have irregular ladder geometry to form very smooth films with small surface roughness (RMS <0.7 nm) over a large domain. EUV lithography can be used to resolve half pitches (HPs) in the order of 15-16 nm with line width roughness (LWR = 4.5-6.0 nm) using small doses (20-90 mJ cm^-2). Cluster 1 (R = n-butyl; R'CO₂ = 2-methyl-3-butenoate) contains only a radical precursor and cluster 2 (R = vinyl, R'CO₂ = 2-methylbutyrate) bears both a radical precursor and an acceptor; the latter is much better than the former in EUV and e-beam photosensitivity. For these clusters, the mechanisms of EUV irradiation have been elucidated with high resolution X-ray photoelectron spectroscopy (HRXPS) and reflective Fourier-transform infrared spectroscopy (FTIR). At low EUV doses, two clusters undergo a Sn-Cl bond cleavage together with a typical decarboxylation to generate carbon radicals. The n-butyl groups of cluster 1 are prone to cleavage whereas the vinyl-Sn bonds of species 2 are inert toward EUV irradiation; participation of radical polymerization is evident for the latter.