E-beam assisted fabrication of a subwavelength aluminum mesh

Abstract

Using e-beam lithography on a single layer of polymethylmethacrylate (PMMA) we designed a relatively thick subwavelength aluminum mesh on top of sapphire. The 100 nm thick mesh consisted of two perpendicularly oriented sets of 100 nm wide parallel metal lines with a center to center distance as low as 260 nm. Due to the large proximity effect during e-beam exposure and the small spacing between metallic lines the use of an adhesion promoting layer appeared necessary to avoid premature peeling of the photoresist. Using a monoatomic layer of hexamethyldisilazane (HMDS) as an adhesion promoter between the sapphire and the PMMA, a 500 nm thick photoresist layer could be exposed and developed with excellent control over the features sizes. Line spacing distances from 500 nm down to 160 nm were achieved. An oxide plasma etch was found to be necessary for metal adhesion during the lift-off process. Due to the small spacing between the aluminum lines, use of a bi-layer photoresist technique to achieve undercut was not possible. Thermal evaporation of aluminum was performed and e-beam evaporation didn't help smoothing the metal surface. An additional ultrasonic bath in acetone was found necessary to ease the lift-off process.