New Photoresist Does Old Acid Chemistry

The mechanism of acid production in a molecular glass photoresist for extreme ultraviolet lithography involves the formation of an aryloxy radical, just as in traditional polymer resist materials (J. Phys. Chem. B, DOI: 10.1021/jp300677q). Researchers want to understand how this type of photoresist works because they believe it could yield better lithographic resolution on semiconductor chips. It is made from a ladder-type cyclic oligomer called noria, with 50% of its hydroxyl groups substituted with 2-acetyloxy-2-methyladamantyl ester groups (noria-AD₅₀). Both noria-AD₅₀ and traditional resist materials, such as poly(4-hydroxystyrene) (PHS), are mixed with triphenylsulfonium triflate to capture ionized electrons from light exposure; the lithographic image is developed through acid-catalyzed deprotection reactions. In the new work, a group led by David M. Bartels at the Notre Dame Radiation Laboratory used electron paramagnetic resonance spectroscopy to find that acid production in noria-AD₅₀ involves generation of an aryloxy radical, the same as for PHS. Surprisingly, the group found that noria-AD₅₀ produces nearly the same amount of acid as PHS even though noria-AD₅₀ has half the density of aryl alcohol groups. The researchers propose that proton transfers from the adamantyl groups prevent charge recombination and stabilize radical formation.