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.
Highlights
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
An alternative approach is variable exposure: the electron-beam penetration depth into the resist can be controlled by varying the acceleration voltage and dose on a single layer of PMMA resist in a one-stage exposure,[13] and polyimide and double-layer PMMA can be combined in a two-stage technique.[14]
We have optimized the process by using a water/IPA mixture to develop the PMMA
Summary
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. We present a process with the combined advantages of the single-stage exposure and multipleresist methods, in order to fabricate large numbers of fine-feature air-bridges with very high yield. An air-bridge structure is formed when gate metal is evaporated on top (Figure 1d) and remains on the sample after the resists are stripped off (lift-off, Figure 1e).
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