  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 tunnelling 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 resist thickness and numerical simulation of
the development.
