We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17-μm-wide weak links over a ∼1-μm-thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.
A15 superconductors are distinguished by their high critical temperatures, magnetic fields, and current-carrying capabilities. Among them, Nb
3Al is of particular interest for superconducting quantum circuits as a means to extend device operating temperatures, provided that its electrodynamic properties are well understood. Here, we report on the synthesis of Nb
3Al thin films by magnetron co-sputtering followed by rapid thermal processing, yielding superconducting transition temperatures above 16~K. Microwire devices patterned from these films exhibit a coherence length of
3.2nm and superfluid densities as low as
1.1×1026m−3, suggesting that Nb
3Al may enable high kinetic inductance in thinner films. Coplanar waveguide resonators fabricated on Nb
3Al demonstrate single-photon internal quality factors up to
2.26×105. These results establish Nb
3Al as a promising material platform for the development of superconducting quantum circuits operating at elevated temperatures, contingent on appropriate control of interfacial chemistry and surface morphology.
Superconducting Hot-electron Bolometric (HE.B) mixers employing thin films ofNbN have become the devices of choice for heterodyne detection at tera,hertz frequen- cies. HEB mixers are inherently insensitive to the bandgap frequency of the superconducting material used in the device, in contrast to SIS mixers,. We have developed a fabrication pro- cess for thin NbN films at the National Institute of Standards and Technology in Boulder, CO. A two-fold approach can be taken. The first is to maximize the critical temperature of the superconducting device by growing thicker films at the expense of IF bandwidth, whereas the second approach focuses on maximizing the IF bandwidth at the expense of the critical tem- perature. So far, we have developed a film-deposition process utilizing our DC reactive mag- netron sputtering chamber. By current-biasing the RF plasma in a mixture of Ar and N2, while using a Nb target, we can control the film stoichiometry and produce films with thicknesses of 5 nm. The films are deposited on MgO substrates that are heated to about 800 °C during depo- sition. A typical critical temperature (TO is about 10 K and the transition width is very small (0.5 K). The films are evaluated by measuring their superconducting characteristics as well as their thickness and surface roughness by means of AFM analysis. Phonon-cooled HEB devices are fabricated from the films to study their performance as HEB mixers. The device fabrication process at UMass/Amherst involves lift-off lithography of the antenna (gold), and Reactive ion etching (R1E) or wet etching of the NbN. The I-V char- acteristics and noise temperatures of the devices were measured in order to determine their quality for PHEB applications.
A novel process suitable for fabricating superconducting circuitry based on the all refractory material NbN is described. In this process, an in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 5mV have been achieved for high quality junctions. In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators.