A proximity ion beam lithography process for high density nanostructures

Image contrast in proximity ion beam lithography is limited by scattered ions which enter the opaque regions of the mask and exit through the sidewalls of the mask windows. The scattering angles are widely distributed resulting in a ‘‘proximity effect’’ whose range is on the order of the mask-to-wafer gap. This problem becomes more severe with increasing pattern density and sets the resolution limit for high density patterns such as interdigital transducers. The only way to counteract this effect is to limit the ion range to a fraction of the mask thickness so that the scattered ions can be recaptured by adjacent sidewalls. This article explores the dependence of image contrast on resolution, pattern density, and beam energy in proximity ion beam lithography. Patterns with feature sizes in the range from 20 to 50 nm and 0.4 μm pitch have been printed with a linewidth change of only 3 nm for a 10% change in dose.