Positive and negative photoresists for ArF lithography using bile acids

Abstract

Novel negative resists for ArF excimer laser lithography based on the crosslinking reaction between acetal and hydroxyl groups were evaluated with copolymers of Cholic acid methacrylate and 1,3 Dioxolane-2-yl-ethyl methacrylate. The work of adhesion increases as the amount of 1,3 Dioxolane-2-yl-ethyl methacrylate in the copolymer increases, which means cyclic ethers enhance adhesion. The polymers have a good transmittance at 193nm and possess good thermal stability up to 230 ℃. But due to low hydrophile-lipophile balance, they were not developed by TMAH solution. To promote developability in TMAH solution, we introduced itaconic anhydride into the copolymer and the resist formulated with this material shows the resolution of 5μm pattern at a dose of 45 mJ $cm^{-2}$ using a mercury-xenon lamp in a contact printing mode and TMAH as a developer. Two kinds of homopolymers derived from cholic and lithocholic acids containing cyclic acetal as a pendant crosslinker, poly(1,3dioxolane-2-yl-ethyl cholate methacrylate) and poly(1,3dioxolane-2-yl-ethyl lithocholate), were synthesized for negative photoresists. Cyclic acetals were confirmed to crosslink with hydroxyl groups of bile acids. From the high values of work of adhesion of homopolymers with pendant cyclic acetals, cyclic ethers are proved to be a good adhesion enhancer. Negative photoresist using poly(dioxolane cholate methacrylate) showed a resolution of 5 ㎛. Alicyclic polymers have the necessary transparency and etch resistance, but their incorporation gives rise to poor adhesion to a silicon substrate. Therefore, hydrophilic moiety such as a hydroxyl group should be incorporated in the matrix polymer. However, the matrix polymer containing hydroxyl groups showed foot profiles due to cross-linking during a lithographic process. Poly (norbornene-2-carboxylic acid tert-butyl lithocholate ester-co-succinic acid 2-(5-norbornene-2-carbonyloxy)-ethyl ester ethyl ester-co-maleic anhydride) was synthesized. The succini...