Synthesis, characterization, and lithography of α-substituted 2-nitrobenzyl arylsulfonate photo-acid generators with improved resistance to post-exposure bake

Abstract

A new series of (alpha) -substituted-2-nitrobenzyl arylsulfonate photo-acid generators (PAG) was synthesized. A study of the thermal stability of the PAG's upon varying the (alpha) - substituent was done. The thermal stability was evaluated because there is a correlation between the PAG thermal stability and the post-exposure bake (PEB) temperature tolerance of resists formulated with 2-nitrobenzyl arylsulfonate PAG's and t-BOC polymers. The best thermal stabilities were obtained by having a bulky electron withdrawing group situated at both the (omicron) - and (alpha) -positions of the 2-nitrobenzyl chromophore. This substitution pattern enhances the thermal stability by suppressing the nucleophilic displacement of the sulfonate group by the 2-nitro group oxygen. Increasing the electron withdrawing ability of the (alpha) -substituent decreased the quantum yield for the photogeneration of acid from the 2-nitrobenzyl chromophore. However, one of these (alpha) -substituents, (alpha) -alkoxycarbonyl, was found to give PAG's with optimum thermal stability and quantum yield. These chromophores were used to synthesize PAG's based on strong arylsulfonic acids. The PAG's protected with these new chromophores allow for a higher PEB tolerance, in poly(4-(t-butyoxycarbonyloxy)styrene-sulfone) based resists, than was possible with the 2-(trifluoromethyl)-6-nitrobenzyl chromophore. It was possible to resolve small features (0.35 micrometers ) with a PEB at 135 degree(s)C with PAG's based on this new chromophore. In contrast, formulations based on a 2- (trifluoromethyl)-6-nitrobenzyl PAG of the same acid do not have this resolution at 135 degree(s)C because of poor thermal stability.