Measurements of electron range and scattering in high voltage e-beam lithography

Abstract

Proximity effect from electron scattering is a major limitation to using e-beam lithography for high density, submicron patterns. Conventional beam writers operate near 20 keV where the range of the backscattered electron energy distribution is about 2 μ, comparable to the feature size in integrated circuits. Exposure from backscattered electrons creates complex interactions between the exposures of adjacent features and requires extensive computation for correcting the local dose. We report experimental measurements of both the magnitude and range of the backscattered electron energy distribution in resist on a Si substrate for beam energies (E) of 20, 40, 60, and 120 keV. The measurements were made using exposure of a 0.1 μ thick PMMA resist layer to determine constant deposited energy contours. The ratio of the integrated energy deposited in the resist by the backscattered electrons to that deposited by the primary electrons, ηE is found to be about 0.8 and is insensitive to energy. The range of the distribution increases from 2 μ at 20 keV to 43 μ at 120 keV following an E1.7 behavior. The long range at high energies greatly simplifies proximity calculations since the exposure can be averaged over areas much larger than the feature size. The proximity effect becomes a relatively uniform background dose without the local pattern distortions characteristic of conventional e-beam lithography at lower voltages.