Transparent fluids for 157-nm immersion lithography

摘要

More than 50 fluorocarbon liquids are measured for transparency over the wavelength range 150 to 200 nm for the purpose of identifying a suitably transparent fluid for use in 157-nm liquid immersion lithography. Purification methods such as degasification, distillation, silica gel drying, and supercritical fluid fractionation are investigated to determine the impact of residual contaminants on absorbance. The purification processes are monitored by gas chromatography-mass spectrometry and Fourier-tranform infrared spectroscopy (for organics),19{sup left}F-nuclear magnetic resonance spectroscopy (for molecular structure), gel permeation chromatography (for molecular weight), Karl Fisher analysis (for water), and for residual dissolved oxygen. We find that in most cases, the absorbance is dominated by dissolved oxygen and water. Once the contaminant levels are reduced, the most transparent perfluoroether (PFE) measured is perfluoro-1,2-bis(2-methoxyethoxy)ethylene giycol (perfluorotriglyme) at 0.52 cm{sup}(-1), the most transparent perfluoroalkane (PFA) measured is perfluorohexane at 1.1cm{sup}(-1), the most transparent hydrofluoroether (HFE) was 1 -(1 H-tetrafluoro)ethoxy-2-(1 -trifluoromethyl)tetrafluoroethoxy-2-trifluoromethyl-1,1,2-trifluoroethane at 2.6 cm"1, and the lowest projected absorption coefficient for a hydrofluoroalkane (HFA) is decafluoro-2H,3H-pentane at <2 (cm){sup}(-1). Our chemical analysis shows that some impurities still remain in these materials, and further reductions in absorption are likely. Even so, our current absorption values should allow lens-to-wafer working distances (assuming 95% transmission) of 428, 203, 83, and 111 μm, respectively, for the four classes of fluids. The identification of these four classes (PFEs, PFAs, HFEs, and HFAs) of fluids for potential use as 157-nm immersion fluids, each with their own ranges of viscosity, vapor pressure, refractive index, dn/dT, synthetic routes, and cost, should allow for flexibility in performing tradeoff analyses for various 157-nm immersion lithography engineering designs and cost of ownership estimates.