Helium ion beam milling for plasmonic nanoantennas

Abstract

The sub-20nm region is an emerging technology for the next generation of powerful microscopes, telecommunications, and high-performance computing devices. Such applications require pushing the limits of nanofabrication to new levels of precision and control. With an accuracy in the sub-5nm region, helium ion beam milling is one of the most advanced techniques of fabrication.1–3 The milling process is carried out in a helium ion beam microscope (see Figure 1), which is not only an imaging tool but also a nanofabrication device. Its imaging resolution is higher than that of a scanning electronmicroscope because of the larger mass, resulting in a smaller De Broglie wavelength of the helium ion beam. Further, its milling resolution is higher than that of a focused ion beam, which uses a gallium source, because of the reduced sputtering yield caused by the smaller mass of the helium ion compared to gallium. Our recent work shows the ability of the ultrafine control of the helium ion beam milling process to remove Au (gold) atoms layer by layer from the surface of a patterned Au film (see Figure 2). Because the technique is highly reproducible, we can have precise control at the 1nm level of the milling thickness by using the milling depth-dose curve. In addition, we can use the resistance of the milling area2 to detect how much material has been ground to a resolution of 2nm in depth. When the structure is milled close to the bottom (more than 85% of material removed), the resistance cannot reliably show the thickness of the material left due to the thin film scattering of electrons. However, at this point, it is possible to sensitively calibrate the presence of the remaining Au bridge through the optical properties of the metal nanostructure.3 We have fabricated metal nanorods with precisely controlled shapes using a combination of e-beam lithography and helium ion beam milling. Such metal nanorods show strong peaks in the optical spectrum known as surface plasmon resonances. Figure 1. A Carl Zeiss ORION PLUS helium ion beam microscope in the Nanofabrication Centre of the University of Southampton.