Focused ion beam fabrication and properties of nanoscale Josephson junctions for sensors and other applications

Abstract

PACS 74.50.+r, 74.81.Fa, 85.25.Cp, The methods of superconducting device fabrication by lithography and multilevel processing usually require a number of processing steps with lithographic resolution and alignment adequate for the scale of the device be fabricated. As an alternative, the focused ion beam (FIB) microscope is increasingly being used directly to fabricate devices. A major advantage of using a FIB compared to other lithography methods is its flexibility and high resolution. It allows in-situ, milling (~5 nm at a beam current of 1 pA) to a variety of depths, and imaging (2 nm) of the sample. In this paper we describe our development of junction fabrication techniques using the FIB and their application in creating a range of potential sensor devices and quantum electronics applications.1 Introduction Focused ion beam (FIB) systems offer the ease of use of a scanning electron microscope (SEM) but with sectioning and cross-sectional imaging capabilities which SEMs lack. Historically, their development was driven by the semiconductor production industry but their increasing availability within research institutions has enabled the development of new applications making widespread the use and development of these techniques.In a typical FIB system a focused ion-beam ejected from a liquid metal ion source (usually Ga), with a spot size of less than 10 nm, is scanned across a sample in a manner analogous to a SEM. Imaging using secondary electrons provides surface information with similar resolution to that obtainable from an SEM; however the main applications arise from the use of ions as the scanned species. These include compositional imaging via secondary ions, direct etching of material in selected regions for in-situ sectioning and imaging, microfabrication, transmission electron microscopy specimen preparation, and localised deposition and implantation of metal and insulator structures. The unique combination of 10 nm resolution imaging with the ability both to remove and to deposit material in selected areas provides a means of performing experiments which would otherwise be impossible or unreasonably timeconsuming.At its most general, a Josephson junction consists of two superconducting electrodes separated by a weak-link in which superconductivity is eliminated or suppressed: for example by an insulating tunnel barrier in a superconductor -insulator -superconductor (SIS) junction or by a thin non-superconducting metal layer in a superconductor -normal -superconductor (SNS) junction. All Josephson junctions for

Multiple-barrier and nanoscale superconducting devices

“…We have used this process to fabricate the devices described in the following section. For a general review of nanoscale device fabrication processes using FIB the reader is referred to [39].…”