New High Index Fluids: Exploiting Anomalous Dispersion for Immersion Lithography

ABSTRACT In immersion lithography a fluid with a high refractive index is used to enable increases in the numerical aperture (NA) of the imaging system and therefore decr ease the minimum feature size that can be patterned. Water has been used in first generation immersion lithography at 193 nm for the 45 nm node. To generate still smaller features, fluids with a higher index than water are needed. Both saturated hydrocarbons and a new class of salts with incorporated alkane groups have been studied. Both of these types of fluids possess the “adjustable” absorbance edge behavior needed to provide a fluid with a high index and low absorbance at 193 nm . Since alkanes have physical properties that are difficult to integrate into current fluid handling systems, the aqueous solutions are particularly attractive as more semiconductor-friendly fluids. A full characterization of the optical properties of these fluids will be reported, as well as physical property results and confirmation of the feasibility of 32 nm l/s imaging with 1.5 NA using the salt solutions. Keywords: Immersion lithography, high index fluid, alkanes, ammonium salts, absorbance, 193 nm