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Nanostructures for Technical Applications

Working Group 2.44

Four-Angle Evaporation Method

To improve the performance of single electron tunneling (SET) devices, it is desirable to minimize the charge island dimensions. This allows higher operating temperatures to be achieved and the background charge noise to be reduced. Using four evaporation steps in combination with a dry etching process the island size can minimised by variation of the angles.

The resist system, consisting of a bottom layer of copolymer, germanium, and a top layer of PMMA, is exposed with an e-beam system. The top resist is developed and afterwards the pattern is transferred into the germanium layer by a dry etching process. To get a large undercut in the copolymer layer another dry etching process is necessary. This results in a free-standing germanium bridge (a).

Onto this resist mask, 4 evaporation steps under different angles are performed. With the first two evaporation steps of aluminium, a gap between two electrodes is produced (b, c). The tunnel barrier is generated by an in situ oxidation of the aluminium layer. During the third and forth evaporation step Au/Pd is evaporated (d, e). Due to the larger tilting angles, the gap is closed by an Au/Pd dot with twice the thickness of a single layer. In a final dry etching process, Au/Pd is thinned. During this step the Au/Pd dot in the centre of the electrodes was separated from the Au/Pd layer on the substrate (f, g). Samples have been produced using this method with island sizes < 30 nm in diameter.

SET-Transistor prepared by Four-Angle Evaporation Method
f) Schematic view; g) SEM-Image.