Synthesis, copolymerization studies and 157 nm photolithography applications of 2-trifluoromethylacrylates
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Advances in microelectronic devices have relied heavily on improved photolithographic imaging capabilities. The resolution limit of optical lithography can be improved by lowering the wavelength of exposure light. The latest reduction in exposure wavelength is from 193 nm to 157 nm. The focus of this work is the synthesis, copolymerization studies and lithographic imaging capabilities of 2-trifluoromethylacrylates. Model calculations and gas phase absorbance measurements of model compounds first suggested that these materials would provide suitable transparency at the 157 nm wavelength. Methyl 2-trifluoromethylacrylate was synthesized and aniocically polymerized and variable angle spectroscopic ellipsometry showed that this material had an absorbance that was 1,000 times more transparent than its non-fluorinated analogue. A variety of relatively transparent resist materials based on a 2- trifluoromethylacrylate backbone were synthesized by anionic polymerization, and these materials were successfully imaged at 157 nm. While 2- trifluoromethylacrylates do not undergo homopolymerization with radical initiators, they do radically copolymerize with various norbornenes. Interestingly, these materials exhibit a 2:1 (2-trifluromethacrylate:norbornene) monomer incorporation. This phenomenon was exploited to produce a number of relatively transparent materials that produced positive-tone structures when imaged at the 157 nm wavelength. Kinetic studies were performed to show that the copolymerizations of 2-trifluormethacrylates and norbornene derivatives deviate from the terminal model and follow the penultimate model. Competitive reaction studies using the “mercury method” were performed to demonstrate that substitution of a trifluoromethyl group can indeed effect the reactivity of a propagating radical, lending support to the proposed penultimate model. The structure of the 2-trifluoromethylacrylate propagating radical will also be investigated by electron spin resonance spectroscopy.