Influence of the anion in tin-based EUV photoresists properties

In this work we assess the effect of the change of counter-anions on the photolithography properties of butyl-Sn12 oxo hydroxo cages. The hydroxide anions were exchanged with tetrakis(pentafluorophenyl)borate (B(PFP)4)- and (phenyl) trifluoroborate (BF3Ph)- anions which exhibit a photoabsorption cross section at 92 eV that is similar to that of the butyl-Sn12 oxo hydroxo cages. The degradation of the EUV photoresist was monitored via in-situ EUV exposure followed by X-ray photoelectron spectroscopy (XPS) at the BEAR beamline (Elettra, Italy) at the C1s-edge. Both systems exhibit similar carbon losses of around 25% for 100 mJ/cm2 dose. The Sn12 cluster with acetate anions, as a reference compound, exhibit a loss of C1s XPS signal from the butyl chains of around 23% for the same 100 mJ/cm2 EUV exposure dose indicating a larger degradation of the Sn12 cluster for the latter. We also evaluated the patterning performance of the Sn12(B(PFP)4) resist via interference lithography at the XIL-II beamline (PSI, Switzerland) and found the positive tone character of the resist and its ability to write lines with 50 nm half pitch resolution for doses of 30 mJ/cm2. In contrast, Sn12(BF3Ph) acts as a sensitive negative tone resist, with doses of 12.5 mJ/cm2 sufficient to write 50 nm half pitch lines.