Deprotonation of Poly(4-hydroxystyrene) Intermediates: Pulse Radiolysis Study of Extreme Ultraviolet and Electron Beam Resist

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Poly(4-hydroxystyrene) (PHS) has been used in current lithography as a backbone polymer and is also a promising material for EUV and electron beam (EB) lithography. PHS is efficiently deprotonated after the ionization of its radical cation at a low pK_{\text{a}} ({<}0). Thus, a hydroxystyrene unit is incorporated in the chemically amplified resist formula as a proton source. The deprotonation mechanism after ionization can be characterized by using pulse radiolysis techniques. In this study, the dynamics of PHS radical cations generated upon exposure to EB were investigated in various solvents such as ketones, acetates, and nitriles. The observed deprotonation rate is controlled by intramolecular interaction in the solvents with low proton affinities. However, it is suggested that the formation of the hydrogen bonding complex between a hydroxyl group of PHS and a solvent molecule increases the proton transfer rate in dimethyl sulfoxide and pyridine with higher proton affinities ({>}850 kcal/mol).
2013-06-25