2019-Meta
The two fundamental problems in the production of metallic optical meta-surfaces by electron beam lithography on transparent non-conductive substrates are the problem of lateral resolution on the order of tens of nanometers and the problem of guiding the accumulated charge off the substrate during the fabrication. Therefore, we have developed and optimized a fabrication method which utilizes two layers of resist: the bottom layer is the thicker one, formed by non-conductive resist enabling high resolution, and the top layer is formed by a thin conductive resist that does not reach such a high a resolution as the bottom resist but can dissipate the excess charge.
The whole process consists of five steps (Fig. 1): (1) cleaning and heating the substrate, (2) applying two resist layers by spin coating, (3) electron beam exposure, (4) deposition of the active metal layer and (5) removing excess resist and metal.
The substrate is cleaned sequentially in acetone and isopropyl alcohol for 2 min, while the beaker with the sample and solution is placed in an ultrasonic cleaner. Subsequently, the substrate is rinsed under demineralized water and the surface is dried with a stream of nitrogen and heated up to 100 °C on a hotplate. By spin coating at 4000 rpm for 60 s, two electron resist layers are successively applied onto the substrate, first AR-P 6200.07 (CSAR 62) and then AR-PC 5090.02 (Electra 92) as a conductive film. Electron beam exposure parameters were: 30 keV energy, 25 pA current, 5 nm trace size, and 140 µC/cm2 dose. The development was carried out in AR 600-546 developer for 1 min and then was stopped in demineralized water for 30 s. The deposition of the metal layer was performed by evaporation, where 3 nm Ti layer was deposited first as an adhesive layer, followed by the 50 nm Au layer as an optically active metal. In the last step, the excess metal together with resist residues were removed by AR 600-71 for several hours (i.e. lift-off method) followed by cleaning in isopropyl alcohol and demineralized water.
The topography of the resulting fabricated metal nanostructures was inspected by scanning electron microscopy (Fig. 2). The optical response of metasurfaces building blocks (different diameters of nano-disks) was then checked by confocal optical spectroscopy, and the obtained optical spectra were compared with the spectra obtained from numerical simulations using finite-difference time-domain method (Fig. 3).
