Proton NMR Determination of Miscibility in a Bulk Model Photoresist System: Poly(4-hydroxystyrene) and the Photoacid Generator, Di(tert-butylphenyl)Iodonium Perfluorooctanesulfonate

The intimacy of component mixing in solvent-cast blends of poly(4-hydroxystyrene) (PHS) and a photoacid generator (PAG), di( tert -butylphenyl)iodonium perfluorooctanesulfonate (PFOS) were studied by solid-state proton NMR. These are simplified blends for chemically amplified photoresist formulations used in the micro-electronics industry. Multiple-pulse NMR techniques are used in both spin-diffusion and longitudinal relaxation experiments at ambient temperatures. It is deduced that PFOS is mixed with PHS on a molecular scale for the mass-ratio range investigated, namely, 91/9 to 55/45 PHS/PFOS; hence, the two components are thermodynamically miscible in this range. Control experiments involving a 91/9 physical mixture of PHS and PFOS as well as solvent-cast blends of polystyrene (PS) and PFOS serve to illustrate the contrast between data obtained for phase-separated blends and data for blends that display molecular level mixing. An attempt was also made to support the notion of intimate PFOS/PHS mixing by looking for evidence of 1 H− 19 F dipolar broadening in multiple-pulse blend spectra. This attempt was not successful owing to rapid polarization exchange between 19 F nuclei. These results, extendable to next-generation resist formulations, should also prove useful for characterizing photoacid/resist dispersions for sub-100 nm critical dimension control.