Silicon nanowire sensor from electron beam lithography: design, fabrication and characterization

Abstract

This study demonstrates the process development of silicon nanowires (SiNWs) sensor requires both the fabrication of nanoscale diameter wires and standard integration to CMOS process. Prior to actual fabrication process, the SiNWs sensor is designed via Elphy Quantum GDS II Editor and AutoCAD. A total of four designs namely nanowire, alignment mark, electrode pad and test channel are designed in order to create a complete SiNWs sensor device. By using silicon-on-insulator (SOI) wafer as a starting material, the nanowires is fabricated using a top-down approach which involved Scanning Electron Microscope (SEM) based Electron Beam Lithography (EBL) method. The effect of line width and exposure dose on the pattern structure is investigated experimentally using the negative photoresist ma-N2403 for EBL. The exposure doses for the resist layer are varied in the range of 50μC/cm2 to 180μC/cm2 at 20 kV accelerating voltage with a beam current of 0.075nA. The nanowires resist masks are well developed with dimension less than 100nm in width for the dose exposure parameters of 80μC/cm2, 100μC/cm2 and 120μC/cm2. Subsequently, the two metal electrodes which are designated as source and drain are fabricated on top of individual nanowire using conventional lithography process. Morphological, electrical and chemical characteristics have been proposed to verify the outcome of the fabricated device. The major parts are to observe the nanowire profile in order to meet the nano-scale dimension by using Hawk 3D-nanoprofiler and to test the device performance electrically by using Semiconductor Parametric Analyzer (SPA) in terms of I-V relations. It is found that, the smallest SiNW with diameter of 65nm is well aligned with electrode pads have been obtained. Finally, the fabricated device is performed as pH level detection. Three types of standard aqueous pH buffer buffer solutions which are pH 4, pH 7 and pH 10 are used to test the electrical response of the device. The SiNWs sensor show the highest resistance value for pH 4 and the lowest resistance value for pH 10. In terms of sensitivity, the device with smaller nanowire is found to be more sensitive than larger nanowire as a result of the high surface-to-volume ratio.