Fabrication of large area nanogap electrodes for sensing applications
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Fabrication of large area nanogap electrodes for sensing applications A. Bendavid1, L. Wieczorek1, R. Chai1, J. S. Cooper1 and B. Raguse1 1
CSIRO Materials Science and Engineering, PO Box 218, Lindfield, NSW 2070, Australia.
ABSTRACT A large area nanogap electrode fabrication method combinig conventional lithography patterning with the of focused ion beam (FIB) is presented. Lithography and a lift-off process were used to pattern 50 nm thick platinum pads having an area of 300 μm × 300 μm. A range of 30-300 nm wide nanogaps (length from 300 μm to 10 mm ) were then etched using an FIB of Ga+ at an acceleration voltage of 30 kV at various beam currents. An investigation of Ga+ beam current ranging between 1-50 pA was undertaken to optimise the process for the current fabrication method. In this study, we used Monte Carlo simulation to calculate the damage depth in various materials by the Ga+. Calculation of the recoil cascades of the substrate atoms are also presented. The nanogap electrodes fabricated in this study were found to have empty gap resistances exceeding several hundred MΩ. A comparison of the gap length versus electrical resistance on glass substrates is presented. The results thus outline some important issues in lowconductance measurements. The proposed nanogap fabrication method can be extended to various sensor applications, such as chemical sensing, that employ the nanogap platform. This method may be used as a prototype technique for large-scale fabrication due to its simple, fast and reliable features. INTRODUCTION A nanogap electrode is a pair of electrodes with nanometers separation. Nanogap electrodes are important structures for the investigation of material properties at the nanometer scale. There is great interest in nanogap electrodes because of their use in macroscopic electronic devices and in sensing applications. They are also important tools for the examination of material properties at the nanometer scale, even at the molecular scale. The attractions of using nanogap electrodes are because of their potential for increasing the detection limit with scaling down of the sensor area. In sensing applications it is desirable to miniaturize the sensor area in order to reduce dead volume, reduce the mass of the sample required for analysis, and, thereby, to reduce the dimensions of the other system components [1]. The limit of detection (LOD) in terms of the mass of the analyte is expected to scale as A , where A is sensor area [1]. Smaller values for the space, width and length parameters of the electrodes generally lead to higher sensor response sensitivity [2]. The sensitivity of chemiresistors can be improved by using nanostructured electrodes, such as those based on gold nanoparticles, graphene and nanotubes. Several techniques for fabricating narrow gap electrodes have been reported and are extensively used in nano-devices and sensing applications. Some of these techniques include electron beam
lithography (EBL) [3], mechanical break junctions [4], surface-catalyzed chemical deposit
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