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Fabrication of metallic tunnel junctions for the scanning single electron transistor atomic force microscope

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Metallic tunnel junctions are important in the formation of high temperature single electron devices, which can act as the ultimate electrometer. We present a method for the fabrication of highly defined metallic tunnel junctions based on the step-edge cutoff process. Fabrication involves conventional electron beam lithography and lift-off of metallic thin films. Junctions scaling down to 50 nm linewidth have been achieved. The devices show a spread in impedance at low bias ranging from less than 10 MΩ to more than 100 GΩ. We have investigated the behavior of thin metallic films across a step forming a single tunnel junction. In the case of palladium we find that grain growth during deposition can give rise to multiple junctions across a single step. We illustrate this using one particular example of a junction, where isolated grains form a spontaneous Coulomb blockaded island with a charging energy of 20 meV at 77 K. The single and double junction fabrication process is compatible with our generic atomic force microscope probe technology, which has been shown to be capable of defining ultrasmall metallic structures on cantilevers and pyramidal tips. We demonstrate this by the fabrication of a lithographically defined device on a silicon cantilever with an integrated Ti/Au thin-film strain gauge.

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