Development of a fabrication instrument of high quality small tunnel junctions with the aid of room-temperature atomic layer deposition


The University of Electro-Communications, Tokyo Hiroshi Shimada

For the fabrication of small tunnel junctions, the electron-beam lithography with bi-layer resists and angle evaporation are used as the standard method. This method is very useful when one uses the Al electrode whose surface is easily oxidized by exposing to the oxygen gas so as to form a tunnel barrier. For instance, it is used for the realization of superconducting qubits. However, for most of metals for which the natural oxide layers are not good tunnel barriers, this method loses its usefulness.

We have constructed an E-gun-based fabrication instrument of small tunnel junctions; the sample chamber of which is equipped with a room-temperature atomic-layer deposition (ALD) of alumina films (photo). This enables one to combine the ALD-based tunnel-barrier formation with the standard bi-layer resists and angle evaporation method. Therefore, we can use most of metals as the base electrode of tunnel junctions and form alumina films with the ALD on it for the tunnel barrier.

The figure shows the observation of the superconducting gap as well as the Coulomb gap of the double small tunnel junctions made of vanadium with this method, (thus, we have succeeded in) demonstrating the effectiveness of this method.