Mechanistic studies of the acidolysis reactions occurring in silicon-containing bilayer photoresists

As the feature sizes of semiconductor devices continue to shrink, there is an increasing interest in thin film imaging approaches such as silicon-based bilayer resists. We have developed such a resist based on a copolymer of 4-hydroxystyrene with a silicon-containing monomer, which functions simultaneously as the acid-sensitive component and a source of O2 etch resistance. In an attempt to understand the reactions that occur in the photoresist film, the acidolysis reactions of the 2-[tris(trimethylsilyl)silyl]ethyl moiety have been studied in solution. Acid-catalyzed cleavage of the model 2-trimethylsilylethyl acetate in solution proceeds via a nucleophilic attack on the silicon atom of the protonated acetate. Protonation of 2-[tris(trimethylsilyl)silyl]ethyl acetate is postulated to lead to a bridged siliconium cation, which reacts with nucleophiles along three pathways and yields products in which a nucleophile is attached to a silicon atom. This mechanism is consistent with the silylation of phenolic hydroxyl groups in the photoresist film consisting of a copolymer of 4-hydroxystyrene with 2-[tris(trimethylsilyl)silyl]ethyl methacrylate, observed during photolithographic processing.