Fabrication of Ultrasmooth, High-Aspect Ratio Nanostructures

Deterministic and versatile approaches to sample preparation on nanoscopic scales are important in many fields including photonics, electronics, biology and material science. However, challenges exist in meeting many nanostructuring demands─particularly in emerging optical materials and component architectures. Here, we report a nanofabrication workflow that overcomes long-standing challenges in deterministic and top down sample preparation procedures. The salient feature is a carbon mask with a low sputter yield that can be readily shaped using high resolution electron beam processing techniques. When combined with focused ion beam processing, the masking technique yields structures with ultrasmooth, near-vertical sidewalls. We target different material platforms to showcase the broad utility of the technique. As a first test case, we prepared nanometric gaps in evaporated Au. Gap widths of 7 ± 2 nm, aspect ratios of 17, and line edge roughness values of 3σ = 2.04 nm are achieved. Furthermore, the gap widths amount to an order of magnitude improvement on system resolution limits. As a second test case, we designed and fabricated dielectric resonators in the ternary compounds MnPSe₃ and NiPS₃; a class of van der Waals material resistant to chemical etch approaches. Nanoantenna arrays with incrementally increasing diameter were fabricated in crystalline, exfoliated flakes. The optical response was measured by dark field spectroscopy and is in agreement with simulations. The workflow reported here leverages established techniques in material processing without the need for custom or specialized hardware. It is broadly applicable to functional materials and devices, and extends high speed focused ion beam milling to true sub-10 nm length scales.