Ultrathin poly(methylmethacrylate) resist films for microlithography

Abstract

To improve pattern fidelity of electron beam lithography in the nanometer regime a new class of ultrathin resist (less than 20 nm) has been investigated. Such films can be exposed with very low energy (<1 keV) electrons to virtually eliminate proximity effects or, at conventional electron energies, to allow easier proximity effect correction. In a previous study we investigated ultrathin (14 nm) poly(methylmethacrylate) (PMMA) films, prepared both by spin casting and Langmuir–Blodgett (LB) techniques, as high resolution electron beam resists. We reported that the pinhole density in 14-nm LB PMMA films was considerably lower than the density in spin-cast PMMA films of comparable thickness. 45-nm-wide lines were fabricated in 50-nm thick chromium films using LB PMMA films as resists. In this paper, we have investigated the lithographic performance of LB PMMA films with thicknesses ranging from 0.85 (one monolayer) to 7.7 nm (nine monolayer) exposed with different electron beam energies. Two distributions of pore-line defects, with diameter less than 20 nm, have been observed in the films after exposure and pattern transfer into chromium. The first distribution, observed only with PMMA resist films thinner than seven layers, was uniform over the sample, and the number of the defects increased dramatically as the film thickness decreased. The second type was only observed around the exposed regions in the films thinner than nine layers. For the second type, the number of defects was found to depend on the beam energy and substrate type. Sources of the two types of defects will be discussed, and the experimental results will be compared with a Monte Carlo simulation.