<title>Photoresist film thickness for extreme ultraviolet lithography</title>

Abstract

The thickness of the photoresist directly impacts the etch stand off and may impact the number of defects in the spin- coated film. However, the maximum imaging layer thickness for extreme ultraviolet lithography (EUVL) is limited by absorption of the radiation. Attenuation in photoresist materials at relevant EUV wavelengths was calculated with atomic extinction coefficients provided from Henke et al. The calculations indicated that photoresist materials have an optical density (O.D.) of 4.0 micrometer<SUP>-1</SUP> (base e) so that 100 nm thick imaging layers have approximately 67% transmission at 13.4 nm wavelength. Using Prolith/3D<SUP>TM</SUP> (Finle Technologies, Austin, TX) simulations of the effect of highly attenuating materials on sidewall slope were done and shown to be small. Imaging experiments were performed in a commercially-available DUV resist material on the 10 X II microstepper and with an improved EUV resist formulation. The imaging results agreed well with the calculations. Top down and cross-section images showed good sidewall profiles in 95 nm thick films at the nominal dose because over 68% of the energy was transmitted through the film. When the thickness of the film was increased, the dose was increased slightly to compensate for the absorption while good sidewall profiles and linearity were maintained. Photoresist thicknesses as high as 145 nm were imaged with a 35% increase in dose. Results are also shown for a single layer resist exposed at 175 nm thickness with only slight sidewall degradation. It is shown that the imaging layer thickness for 13.4 nm lithography is likely to be 120 +/- 15 nm. If 11.4 nm wavelength radiation is chosen for EUV lithography, it is shown that thicknesses of 170 nm is possible.