Fabrication of planarized small area Nb/Al,AlO_x/Al/Nb Josephson tunnel junctions using reactive ion etching in SF₆

High quality Nb/Al,AlO_x/Al/Nb Josephson tunnel junctions have been made with the help of a fabrication process based on reactive ion etching of Nb in SF₆. The V_m value of these junctions is typically 60–70 mV at 4.2 K. At 1.6 K, a V_m of 4.1 V has been measured, which is the highest value that has ever been reported for this type of junction. The area of the junctions ranges from 1 to 25 μm². By burying the Nb/Al,AlO_x/Al/Nb trilayer in the substrate, a planarized junction configuration has been obtained. Reactive ion etching of Nb in SF₆ plasmas has been studied in detail. Anisotropic etch profiles can be obtained because of the formation of a resistant layer during etching, which prevents etching of Nb under the photoresist. The etching process has been monitored with a spectrometer. The fluorine emission at 703.7 nm is shown to be suitable for end point detection.