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A novel method for the fabrication of sub-1-/spl mu/m Nb/AlO/sub x//Nb tunnel junctions has been developed, that is based on spin-on glass planarization. The Nb/AlO/sub x//Nb sandwich and the Nb wiring layer are structured by reactive ion etching using e-beam lithography. The insulation between the base electrode and the wiring layer is realized by planarised spin-on glass. Single electron transistors with junction areas of 0.3 /spl mu/m/spl times/0.3 /spl mu/m and linear arrays of junctions with sizes down to 0.5 /spl mu/m/spl times/0.5 /spl mu/m have been fabricated and measured.