Ionic outgassing from photoacid generators upon irradiation at 13.5 nm

Photoresist outgassing is considered a possible source of contamination of optics in extreme ultraviolet (EUV) lithography at 13.5 nm. We measured the relative proportions of ionic outgassing from 18 commercially available photoacid generators (PAG), which is a key component of chemically amplified photoresists, upon irradiation at 13.5 nm. These PAG include 17 triarylsulfonium or diaryliodonium salts, which contain CF3SO3- or C4F9SO3- as the anion, and one PAG of molecular type. The overall outgassed ions in the range 10–200 u were counted in relative proportions. Outgassing of F+ is found to be dominant, and for most PAG the extent of F+ outgassing shows a satisfactory correlation with the ratio of F atomic photoabsorption to the overall PAG photoabsorption. Outgassed ions F+, CF+, CF2+ and CF3+ from PAG containing the CF3SO3- anion and additional CxFy+ such as C2F4+, C2F5+ and C3F5+ from those containing C4F9SO3- are identified. Triphenylsulfonium perfluoro-1-butanesulfonate is one PAG to emit the most abundant F+ and total ionic fragments, and a PAG of molecular type (N-hydroxy-5-norbornene-2,3-dicarboximide perfluoro-1-butanesulfonate) also emits abundantly both hydrocarbon ions and F+. Ionic outgassing of PAG cations includes (C6H5)2S+ from R(C6H5)2S+ salts and I+ from diaryliodonium salts. For PAG containing t-C4H9, significantly less F+ outgassing is observed; additional outgassing pathways are proposed. The pressure rise caused by PAG shows no dependence on the anion identity, but is correlated with cation photoabsorption, and ascribed to neutral aryl outgassing. Other minor outgassing species include HSO2+ from sulfonates; C11H9+ and ‘photostable’ PAH cations C12H8+ are identified for the first time and provide evidence of concurrent outgassing from, and polymerization of, PAG upon irradiation at 13.5 nm.