Microscopic metallic air-bridge arrays for connecting quantum devices

Abstract

We present a single-exposure fabrication technique for a very large array of microscopic air-bridges using a tri-layer resist process with electron-beam lithography. The technique is capable of forming air-bridges with strong metal-metal or metal-substrate connections. This was demonstrated by its application in an electron tunneling device consisting of 400 identical surface gates for defining quantum wires, where the air-bridges are used as suspended connections for the surface gates. This technique enables us to create a large array of uniform one-dimensional channels that are open at both ends. In this article, we outline the details of the fabrication process, together with a study and the solution of the challenges present in the development of the technique, which includes the use of water-IPA (isopropyl alcohol) developer, calibration of the resist thickness, and numerical simulation of the development.

Semiconductor nanodevices as a probe of strong electron correlations

“…In 2015, the first observation of structure resembling higher-order excitations, as predicted by the mode-hierarchy model, was reported in Tsyplyatyev et al [65] in devices similar to that used by Jompol et al [28] but with air-bridges [67], see Fig. 14g.…”

“…(From [65] reprinted with permission.) (g) Scanning electron micrographs of a tunnelling device with air-bridges, showing how air-bridges are used not only to connect gates together across other gates, but also to link together all the finger gates defining the 1D wires, which avoids variation of the gate potential along each of these very short wires, as happens when the gates are joined together at one end [67,68].…”

Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor

“…Our 1D system consists of an array of ∼400 highly regular quantum wires formed in the upper layer by using a set of wire gates (WGs) fabricated on a Hall bar via standard electron beam lithography and connected by air bridges (see Fig. 1B and inset) ( 23 ). Use of an array averages out impurities, length resonances, and charging effects as well as increases the overall strength of the measured signal.…”

“…The bottom micrographs show, respectively, the lower and upper ends of the wire array (∼400 wires). To increase the uniformity of the 1D system, we developed a novel air-bridge technique to avoid having to use a connecting backbone structure [see ( 23 )]. ( B ) Gate operation and setting of tunneling conditions.…”

Aluminum air bridges for superconducting quantum devices realized using a single step electron-beam lithography process

“…Several methods have been developed to date to implement air bridge crossovers for superconducting circuits. These processes are carried out using techniques including using a photo resist scaffold followed by etching of the excess aluminum to separate the bridge 10,11 , employing resist stacks that use different resists to define both a scaffold as well as a resist window above the scaffold with an undercut for liftoff 12,13 , and forming a dielectric scaffold that is etched after Al deposition 14 . In this paper, we present a method to fabricate aluminum air bridges that uses a simple single-step fabrication process that relies on gradient exposure electron-beam lithography (EBL).…”