Synthesis of acrylate polymers with acid labile organosilicon protecting groups and their application as dry-developable photoresists for ArF lithography

Abstract

Poly(2-trimethylsilyl-2-propyl methacrylate) was synthesized and evaluated as a potential dry-developable chemically amplified photoresist. The deprotection mechanism of new protecting group, 2-trimethylsilyl-2-propyl ester, was evaluated. Deprotection of poly(2-trimethylsilyl-2-propyl methacrylate) was studied for various photogenerated acids. Deprotection of 2-trimethylsilyl-2-propyl (TMSP) ester did not occur in a catalytic manner when the photogenerated acid possessing a fluoride ion source such as $SbF_6^-$ or $AsF_6^-$ was used. Because the cleaved product, α-methyl-α-(trimethylsilyl)ethyl cation, undergoes rearrangement through methyl migration from silicon to carbon with nucleophilic attack on the silicon by the fluoride ion, the acid was consumed in the reaction. Fortunately, when the counterion of the acid does not provide a fluoride ion, e.g., sulfonate, the carbonium ion undergoes elimination to produce 2,2,3-trimethyl-2-silabut-3-ene, and regenerates another acid. The thermal stability of TMSP ester was very good up to 200℃ and similar to t-butyl ester, and acid catalyzed cleavage of the TMSP began at 80℃. The deprotection of 2-trimethylsilyl-2-propyl group of the polymer took place in the exposed region after post-exposure bake. The difference of silicon content between the unexposed region and exposed regions was large enough to form patterns using oxygen reactive-ion etching. γ-Butyrolactone-2-yl methacrylate was introduced to increase adhesion property and glass transition temperature of the matrix polymer. Poly(2-trimethylsilyl-2-propyl methacrylate-co-γ-butyrolactone-2-yl methacrylate) was evaluated as a resist for ArF excimer laser lithography. Sub-micron pattern profiles were obtained using dry development process with $O_2$-RIE as well as conventional development process with 2.38 wt% TMAH solution.