Focused ion beam (FIB) allows direct milling of nanostructures on working materials without the need for additional masks. However, the ion beam tends to damage the surface morphology, redeposit working materials, and cause surface roughness that deteriorates the fidelity of pattern transfer, resulting in optical loss and degradation of device performance.

Take surface plasmonic devices, for instance. Blunt-edged device structures caused by low-definition pattern transfer can reduce the electric field hotspot intensity and alter the spectral response. We have recently observed that the efficiency of surface plasmon enhanced quantum-dot emission in a FIB-fabricated device was deteriorated by ion-beam-induced surface roughness and structure deformation.

These issues can be overcome by simply covering the working materials with a sacrificial metallic protective layer during milling. We demonstrate the technique using a 50 nm-thick aluminium layer. The protective layer prevents the working material from exposure to low-energy peripheral ion beam tails, reduces lateral milling of the substrate and therefore avoids deformation of the pattern edges and the surface morphology.

High-fidelity nanostructure patterning can be achieved under a broad range of ion beam milling conditions. It is possible to create tapered grooves with tunable open angles with a proper control of the ion beam dose and current. The technique is also suitable for high-fidelity fabrication of nanoimprint templates.

Full details are reported in Nanotechnology.