Index of refraction of high-index lithographic immersion fluids and its variability

We have developed a number of second-generation high-index candidate immersion fluids that exceed the 1.6 refractive index requirement for immersion lithography at 193 nm to replace the water used in first-generation immersion systems. To understand the behavior and performance of different fluid classes, we use spectral index measurements, based on the prism minimum deviation method, to characterize the index dispersion. In addition to fluid absorbance and index requirements, the temperature coefficient of the refractive index is a key parameter. We have used a laser-based Hilger-Chance refractometer system to determine the thermo-optic coefficient (dn/dT) by measuring the index change versus temperature at two different laser wavelengths, 632.8 and 193.4 nm. Also, we determined the batch-to-batch (within a 6-month period), before and after irradiation (at 193.4 nm), before and after air exposure, and before and after resist exposure (image printing test) variations of index and Delta n/Delta lambda. The optical properties of these second-generation immersion fluids mostly compare favorably to water; the ratio of index of refraction at 193.4 nm is 1.644/1.437, the dispersion from d-line (Delta n(193-d)) is 0.160 versus 0.103 and dn/dT at 193.4 is -550 x 10(-6)/K vs. -93 x 10(-6)/K, respectively. (c) 2009 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3124189]