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

Abstract

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 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).

Multiscale Simulation Approach on Sub-10 nm Extreme Ultraviolet Photoresist Patterning: Insights from Nanoscale Heterogeneity of Polymer

“…The time step for the MD runs was reduced to 0.5 fs for a stable relaxation of the system with non-neutral charge groups. The resulting position of the detached triflate anion corresponds to protonation site (i.e., acid generation site) because the migrating protons would be trapped at the sites, having the maximum proton affinity , (triflate anion: 12.90 eV in Table S3) among the available candidates (tf anion, HOSt, tBOCSt, and phenoxy radical).…”

Theoretical study on defect risks of chemically amplified resists used for extreme ultraviolet lithography

“…8,9) The protons of acids are typically supplied through the deprotonaion of oxidized polymer molecules. 10,11) Thus, the acids are generated with the combination of the oxidation and reduction of resist components upon exposure to EUV radiations. The generated acids diffuse and catalyze the deprotection of partially protected acidic polymer during post exposure baking (PEB).…”

Study on deprotonation from radiation-induced ionized acrylate polymers including acid-generation promoters for improving chemically amplified resists

“…Hole and proton transfer from the radical cations to additives in PMMA matrixes was observed. 20,21) Although the behavior of the radical cations of PHS formed by ionization has been observed by the pulse radiolysis method, [22][23][24][25] because of the extremely short lifetime of the radical cations of PMMA, direct evidence of their formation and deprotonation kinetics has not been confirmed. 21) Deprotonation from the side chain or mainchain radicals has been estimated from the analysis of radiation-induced neutral radicals (deprotonation products from radical cations); 17,26) it has also been shown that the deprotonation efficiency and proton diffusion depend on the protecting groups of methacrylate polymers.…”

“…The proton affinity of the proton acceptor is a good indicator of deprotonation from the radical cation. 24,27) Neutralization of quenchers with high proton affinity, such as amines, occurs when protons of the acid are trapped in the CAR. AGPs such as diphenyl sulfones with moderate proton affinity are considered to effectively increase the initial acid yield, because they enhance deprotonation from radical cations formed by ionizing radiation such as EUV light and electron beams, but they do not inhibit proton migration in the poly(hydroxystyrene-co-acrylate) resist polymer.…”