Hot-Phonon Controlled MicroSQUIDs With Independently Controlled Junctions

The microSQUID is a particular type of DC-SQUID notable both for its small loop area and the use of microbridges as Josephson junctions. Hot phonons can be used to reduce the critical current in a superconducting microbridge and hence allow electrical control of the SQUID parameters. A normal metal layer, isolated from the superconductor by a thin dielectric film, generates non-equilibrium hot phonons that travel a short distance before thermalisation. These hot phonons are very effective in breaking Cooper pairs, thereby suppressing the order parameter and reducing the junction critical current. Phonon control of a microSQUID has previously been demonstrated in which both junctions were influenced by the same control current, allowing control of the magnetic hysteresis. Here we report a modified fabrication process, allowing independent control of the microbridge junctions. The fabrication of the devices involves several stages of electron beam lithography, deposition and reactive ion etching. The superconductor used is a sputtered niobium film, isolated from the normal metal, phonon-producing layer by a dielectric SiO₂ film. Separate lithography stages form the normal metal wires and the rest of the SQUID loop, which then act as an etch mask for a CF₄ and O₂ reactive ion etch used to define the SQUID in the niobium layer.