Lithography with 157 nm lasers

Projection photolithography at 157 nm was studied as a possible extension of current 248-nm and planned 193-nm technologies. At 157 nm, lasers are available with ∼8 W average power. Their line width is narrow enough as to enable the use of catadioptric, and maybe all-refractive optics similar to those used at 248 and 193 nm. The practicality of such designs is further enhanced by measurements of calcium fluoride, which show that its absorption is sufficiently small (∼0.004 cm^-1) at 157 nm. Binary masks with chromium and chromeless phase shifting masks were fabricated on calcium fluoride as the transparent substrate. Robust photoresists at 157 nm still need to be developed, and they probably will be of the top surface imaging or bilayer type. Indeed, a silylation resist process was shown to have characteristics at 157 nm similar to those at 193 nm. The calcium fluoride based masks were integrated with the silylation process and a home-built, small-field, 0.5-numerical aperture stepper to provide projection printing with features as small as 80 nm. These initial results indicate that 157-nm lithography has the potential to become a manufacturing technology at dimensions well below 100 nm.