Plasmonic ‘top-hat’ nano-star arrays by electron beam lithography Shaoli Zhu a,b,⇑ , Han-Hao Cheng b , Idriss Blakey b,c , Nicholas Stokes d , Kostya (Ken) Ostrikov e,f , Michael Cortie a a Institute for Nanoscale Technology, University of Technology Sydney, PO Box 123, Broadway, NSW 2007, Australia b Australian Institute for Bioengineering and Nanotechnology, University of Queensland, QLD 4072, Australia c Centre for Advanced Imaging, University of Queensland, QLD 4072, Australia d Institute for Nanocale Technology, Sydney, Australia e Plasma Nanoscience Laboratories, CSIRO Manufacturing Flagship, PO Box 218, Lindfield, NSW 2070, Australia f Institute for Future Environments and School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000, Australia article info Article history: Received 2 April 2015 Accepted 14 April 2015 Available online 18 April 2015 Keywords: Nano-star Electron beam lithography Exposure dose Optical properties abstract Lithography techniques play an important role in the fabrication of nanoscale functional devices. In elec- tron beam lithography (EBL) the optimum dose of electron irradiation is a critical parameter. In this paper, we first identify suitable EBL fabrication parameters by writing patterns with different sizes, per- iods and electron radiation doses. After finding suitable fabrication parameters, we show how five- pointed gold nanostructures with electric field-enhancing ‘top hats’ can be fabricated using EBL. Reflectance data of these arrays is measured in order to assess their potential applications in biosensing arrays. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Nanostructures with well-controlled shapes and sizes may be fabricated using focused ion beam (FIB) [1], electron beam litho- graphy (EBL) [2,3] or nanoimprint lithography (NIL) [4]. Gold is a particularly suitable material for such structures [5] especially using EBL which is capable of fabricating patterns down to about 10 nm resolution under ideal conditions [6]. Nanostructures made this way have potential applications in nano-electronic and photonic devices [7]. Poly(methylmethacrylate) (PMMA) is a common positive tone resist for EBL because it has a high res- olution, good reproductivity, and stability [8,9]. Typically, PMMA has sensitivity of <100 lC/cm 2 varying with the resist heights, the acceleration voltages, and the development procedures used [10]. Resolution limits [11], surface chemical modification [12] and photoresist [13] effect on the EBL fabrication process have been previously explored for simple shapes such as gold nanor- ods or deep trenches. Unfortunately EBL in thick resist layers still suffers from the inevitable problem of low resolution due to broadening of the energy deposition profile of the electron beam inside the resist by electron scattering in the resist and the substrate [13]. Therefore it is still challenging to fabricate com- plex nanostructures using EBL. In the present work, we investigate the optimum techniques to produce arrays of gold nano-stars for potential use in biosensing. In addition, we demonstrate a reliable technique to place a ‘top hat’ disk on top of a central hole within the nanostar. Such ‘top- hats’ have recently been reported to enhance the localised electric field on circular shapes [14] and they should also be effective on star shapes. Two layers of PMMA were used to improve the lift-off process. Patterns with different sizes, periods and doses are designed. The effects of the spin coating parameters on the photoresist thickness and the substrate’s effect on the dose requirements are also established. Finally, an array of complex gold nanostars is fabricated using EBL and the optical properties are assessed. 2. Experimental 2.1. Dose test patterns Patterns with different shapes, periods and doses were designed using ELPHY Quantum design software (Dortmund, Germany). As shown in Fig. 1, stars, star-holes, and star-star holes with different sizes, different periods and different doses (60–360 lC/cm 2 ) were created. The writing field size was 100 lm  100 lm. The dose http://dx.doi.org/10.1016/j.mee.2015.04.084 0167-9317/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Institute for Nanoscale Technology, University of Technology Sydney, PO Box 123, Broadway, NSW 2007, Australia. Microelectronic Engineering 139 (2015) 13–18 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee