Effect of secondary electron from the substrate in x-ray lithography using harder radiation spectra

Abstract

Proximity x-ray lithography using harder spectra has been proposed as a next generation lithography technology. The secondary electron from the substrate is one of the issues that should be solved to realize this technology. We investigated the effect of the electron from the silicon substrate on the pattern quality when using the shorter wavelength exposure. As a figure of merit for the effect of the electron from the substrate, we proposed a function R which is defined as the ratio of the energy absorption in the substrate to that in the resist. We confirmed the validity of R by comparing it with a Monte Carlo simulation (SPEED). Several exposure experiments, which corresponded to various R values, were carried out by changing mirror, filter, and mask membrane. We found an adequate range of R where the secondary effect can be ignored from these experiments. Furthermore, we showed that R could effectively be reduced by using an absorption edge of the resist material. Bromine is the best material to suppress the value of R in every wavelength region.