Nanopantography With Removable Membrane-Based Electrostatic Lens Arrays

Nanopantography is a method for massively parallel writing of nano-sized patterns using an ion beam. In this process, a broad area, collimated, nearly-monoenergetic ion beam is directed towards an array of micron-scale electrostatic lenses in direct contact with a substrate. By applying an appropriate DC voltage to the lens array with respect to the substrate, the ion beamlet entering each lens converges to a fine spot that can be 100 times smaller than the diameter of each lens. Previously, lenses fabricated directly on the silicon substrate were used to etch 3 nm diameter holes in silicon by exposure to a monoenergetic Ar+ ion beam and chlorine gas. This work reports on the development of removable and reusable free-standing membrane-based electrostatic lens arrays that are designed to pattern any conducting surface. The lens arrays are fabricated on a silicon wafer coated with PMGI, SU-8, gold, copper, and PMMA. Lens openings are lithographically defined, and an acrylic frame is placed over the array. The lens patterns are etched through the SU-8 and the membrane is released by dissolution of the PMGI layer. The applied voltage used to focus the ion beamlets also serves to electrostatically clamp the lens array to a conducting substrate, which is observed as a flattening of the membrane against the substrate surface and an increasing capacitance measured between the lens array and the substrate. An array with lens diameters between 0.8 μm and 1.5 μm was used to pattern features as small as 20 nm on a silicon substrate using a 70 eV Ar+ ion beam. Ion trajectory simulations were to understand the sensitivity of minimum feature size to the variation of lens potential, lens aspect ratio, and lens size. Simulations for this lens geometry agreed with the experimentally observed results when chromatic and spherical aberrations are considered. Based on the simulation results, it should be possible to print much smaller features via a step and repeat process with a thinner dielectric and narrow lens diameter. Diagnostics of a positive ion beam extracted from a pulsed oxygen plasma were conducted, confirming that the majority of the beam consists of 100 eV O2+ ions. Patterning of graphene with the nanopantography method was conducted with the O2+ ion beam, resulting in defect production in a focused spot 17 times smaller than the lens openings.