Fabrication of step-edge Josephson junctions for low-noise dc-SQUIDs

Abstract

We report the development of a fabrication process of high-resistance YBa 2 Cu 3 O 7 step-edge Josephson junctions on sharp argon ion beam etched SrTiO 3 substrate steps. For a reproducible fabrication, the redeposition of material at the step during the etch process has to be avoided. This is achieved by an alternating angle of ion incidence parallel to the edge of the photoresist mask. The reproducible straight step profile exhibits an angle around 60 • and is reflected in reproducible electrical properties of the junctions. Critical current I C and asymptotic normal state resistance R N both scale with the junction width w . The current-voltage (I -V ) characteristics depend on the ratio of film thickness t to step height h. For t/h > 0.85 they are flux-flow-like; for t/h < 0.8 they correspond to the resistively shunted junction (RSJ) model. Lower ratios of t/h correspond to higher specific resistances resulting at 77 K typically in asymptotic normal state resistances between 5 and 10 per junction, depending on w . At 77 K we achieve I C R N products of more than 150 µV. The junctions are applied to dc superconducting quantum interference devices (dc-SQUIDs) with inductances between 25 and 35 pH. At 77 K transfer functions of more than 100 µV −1 0 are achieved. 1/f noise of the step-edge dc-SQUIDs is observed, but using bias current modulation it is entirely suppressed resulting in a spectral density of flux noise of 8.4 µ 0 Hz −1/2 measured at 1 Hz and 77 K for a SQUID with an inductance of 30 pH.