Low-T_c ramp-type superconductor-insulator-superconductor Josephson junctions

Superconductor–insulator–superconductor Josephson junctions with an area smaller than 1 µm2 are increasingly used for many applications because of their low capacitances and short switching times. The characteristics of analogue and digital SQUIDs, for example, can be improved using small-area Josephson junctions. We have developed Nb/Al, AlO_x/Al/Nb junctions based on an edge geometry. These low-T_c ramp-type junctions allow sub-µm² junction areas using micrometre resolution photolithography. The experimental I–V characteristics of the ramp-type junctions showed non-hysteretic behaviour. Using these ramp-type junctions, the first series of dc SQUIDs have been successfully developed. The characteristics of the ramp-type junctions and the dc SQUIDs have been investigated both experimentally and by using numerical simulations. The white flux noise level of a dc SQUID based on ramp-type junctions with an area of 0.2 µm × 12 µm was measured to be √S_φ = 1.2μφ0Hz^-1/2 .