Brian D. Hunt

NbN/MgO/NbN SIS tunnel junctions for submm wave mixers

The authors report on the fabrication and testing of all-refractory NbN/MgO/NbN SIS (superconductor-insulator-superconductor) tunnel junctions for use as high-frequency mixers. Progress in the development of techniques for the fabrication of submicron-area tunnel junctions is described. Junction structures which have been investigated include mesa, crossline, and edge geometries. Using reactive sputtering techniques, NbN tunnel junctions with critical currents in excess of 10/sup 4/ A/cm/sup 2/ have been fabricated with V/sub m/ values as high as 65 mV and areas down to 0.1 mu m/sup 2/. Specific capacitance measurements on NbN/MgO/NbN mesa-type tunnel junctions give values in the range 60-90 fF/ mu m/sup 2/. These SIS tunnel junctions have been integrated with antennas and coupling structures for mixer tests in a waveguide receiver at 207 GHz. Preliminary mixer results are reported. >

Epitaxial YBa2Cu3O7 superconducting infrared microbolometers on silicon

Superconducting transition-edge infrared microbolometers have been fabricated by silicon micromachining using an epitaxial YLa.05Ba1.95Cu3O7-x (YBCO) film on a epitaxial yttria-stabilized zirconia buffer layer on silicon. The low thermal conductance of the micromachined structures combined with the sharp resistance change at the superconducting transition results in very sensitive infrared detectors. The broadband response of these thermal detectors makes them particularly useful at wavelengths longer than the typical operating range of semiconductor detectors ((lambda) greater than about 20 micrometers ) at moderately high temperatures (T approximately 70 K and higher). The use of standard silicon processing promises low-cost monolithic integration of the readout electronics for arrays of these devices. Preliminary measurements are reported here on a device 140 micrometers X 105 micrometers in size with a detectivity, D*, of 8 +/- 2 X 109 cm Hz1/2/Watt, and NEP of 1.5 X 10-12 Watts/Hz1/2 at 2 Hz and 80.7 K. This value of D* exceeds the highest previously reported D* for a YBCO transition-edge bolometer, and is comparable to the highest reported D* for a thermal detector operating at greater than about 70 K. The thermal time constant for this device was 105 +/- 20 msec.

Transition Edge YBa 2 Cu 3 O 7-x Microbolometers for Infrared Staring Arrays

High-temperature superconducting staring arrays are potentially important for both space and terrestrial applications which require the combination of high sensitivity over a broad wavelength range and relatively high temperature operation. In many such array applications sensitivity is more important than speed of response. Thus, it is desirable to design low- thermal-mass pixels that are thermally isolated from the substrate. To this end, Johnson, et al. at Honeywell have fabricated meander lines of YBa2Cu3O7-x (YBCO) sandwiched between layers of silicon nitride on silicon substrates. The silicon was etched out from under each YBCO meander line to form low-thermal-mass, thermally isolated microbolometers. These 125 micrometers X 125 micrometers devices are estimated to have a noise equivalent power of 1.1 X 10-12 W/Hz1/2 near 5 Hz with a 5 (mu) A bias (neglecting contact noise). A drawback of this original Honeywell design is that the YBCO is grown on an amorphous silicon nitride underlayer, which precludes the possibility of epitaxial YBCO growth. The YBCO therefore has a broad resistive transition, which limits the bolometer response, and the grain boundaries lead to excess noise. We discuss the potential performance improvement that could be achieved by using epitaxial YBCO films grown on epitaxial yttria-stabilized zirconia buffer layers on silicon. This analysis shows a significant signal to noise improvement at all frequencies in devices incorporating epitaxial YBCO films. Progress toward fabricating such devices is discussed.

Air Bridge and Vertical Carbon Nanotube Switches for High Performance Switching Applications

Carbon nanotubes are attractive for switching applications since electrostatically-actuated CNT switches have low actuation voltages and power requirements, while allowing GHz switching speeds that stem from the inherently high elastic modulus and low mass of the CNT. Our first NEM structure, the air-bridge switch, consists of suspended single-walled nanotubes (SWNTs) that lie above a sputtered Nb electrode. Electrical measurements of these air-bridge devices show well-defined ON and OFF states as a dc bias of a few volts is applied. The switches were measured to have switching times down to a few nanoseconds. Our second NEM structure, the vertical CNT switch, consists of nanotubes grown perpendicular to the substrate. Vertical multi-walled nanotubes (MWNTs) are grown directly on a heavily doped Si substrate, from 200 − 300 nm wide, ∼ 1 μm deep nano-pockets, with Nb metal electrodes to result in the formation of a vertical single-pole-double-throw CNT switch architecture.

Low-Noise Hybrid Superconductor/Semiconductor 7.4 GHz Receiver Downconverter for NASA Space Applications

A low-noise microwave receiver downconverter utilizing thin-film high-critical-temperature superconducting (HTS) passive circuitry and semiconductor active devices has been developed for use in space. It consists of an HTS pre-select filter, a cryogenic low-noise amplifier, a cryogenic mixer, and a cryogenic oscillator with an HTS resonator. The downconverter converts a 200 MHz wide band centered around 7.35 GHz to a band centered around 1.0 GHz. When cooled to 77 K, the downconverter plus cables inside a cryogenic refrigerator produced a noise temperature measured at the refrigerator port of approximately 50 K with conversion gain of 18 dB.

MEMS-based Force-Detected Nuclear Magnetic Resonance (FDNMR) spectrometer

The motivation for the work reported is portable NMR spectroscopy of liquids and solids with higher sensitivity than inductive detection and without the need for tuned elements specific to the frequency of each isotope observed. The fabrication and assembly of a BOOMERANG force-detected nuclear magnetic resonance (NMR) spectrometer is reported. The design is optimal for samples of ~ 50 micron diameter and realizes tolerances of ~1 micron in the Si and ferromagnetic parts. Optical lithography, electrodeposition, reactive ion etching, and release of the moving part by solution etching are key methods used. Resistance to delamination of the ferromagnetic material was achieved by Cr/Au deposition prior to electrodeposition of 85/15 Co:Ni.

High-temperature superconducting superconductor/normal metal/superconducting devices

We describe the fabrication and characterization of superconductor/normal metal/superconductor (SNS) devices made with the high-temperature superconductor (HTS) YBa2Cu3O(7-x). Structures of YBa2Cu3O(7-x)/Au/Nb on c-axis-oriented YBa2Cu3O(7-x) were made in both sandwich and edge geometries in order to sample the HTS material both along and perpendicular to the conducting a-b planes. These devices display fairly ideal Josephson properties at 4.2 K. In addition, devices consisting of YBa2Cu3O(7-x)/YBa2Cu3O(y)/YBa2Cu3O(7-x), with a normal metal layer of reduced transition temperature YBa2Cu3O(7-x) were fabricated and show a great deal of promise for applications near 77 K. Current-voltage characteristics like those of the Resistively-Shunted Junction model are observed, with strong response to 10 GHz radiation above 60 K.

Multilayer buffer system for fabrication of high-Tc edge-geometry superconductor/normal-metal superconductor (SNS) weak links on silicon-on-sapphire substrates

High frequency detector and circuit applications often require device fabrication on medium-to-low dielectric constant substrates ((epsilon) < 12). Silicon-on-sapphire (SOS) substrates have acceptably low dielectric constants and provide other important advantages, including the possibility of monolithic integration of silicon and superconducting circuitry. Our initial results with YBa2Cu3O7-x(YBCO) edge- geometry superconductor/normal-metal/superconductor (SNS) weak links fabricated on r-plane SOS substrates using cubic zirconia (YSZ) buffer layers revealed problems with grain boundary nucleation in the YBCO counterelectrode. These results motivated development of a new multilayer buffer system consisting of an epitaxial YSZ film grown on an SOS substrate, overlaid by a thin YBCO seed layer, and an epitaxial SrTiO3 (STO) layer. STO - YBCO bilayers grown over the YBCO seed layer show a remarkable improvement in epitaxial quality and in YBCO electrical properties relative to similar bilayers grown directly on the YSZ buffer. In addition, SNS weak links fabricated on SOS substrates using the multilayer buffer system exhibit dramatically improved electrical characteristics compared to devices produced on YSZ buffer layers. These are the first epitaxial edge-geometry SNS weak links produced on SOS substrates.

All-YBa2Cu3O(7-x) edge-geometry weak links

High quality YBa2Cu3O(7-x)/normal-metal/YBa2Cu3O(7-x) edge-geometry weak links are fabricated using nonsuperconducting Y-Ba-Cu-O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25-100-A-thick barrier layers exhibit current-voltage characteristics consistent with the resistively-shunted-junction model, as well as ac Josephson steps. Magnetic field modulation of the critical currents shows Fraunhofer-like behavior with a periodicity which scales with device area. The critical currents vary exponentially with barrier thickness indicating an effective normal metal coherence length of 20 A. The average resistances scale linearly with Y-Ba-Cu-O interlayer thickness and device area. The scaling behavior of the device parameters combined with the well-behaved magnetic field modulation of the critical currents demonstrates good barrier-layer uniformity with low pinhole densities. Near the transition temperature, the critical currents exhibit a temperature dependence proportional to (1 - T/Tc) squared.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Coplanar waveguide microwave filter of YBa2Cu3O7

Thin film low-pass microwave filters have been made with laser-ablated YBa2Cu3O(7-delta) (YBCO) deposited on LaAlO3 substrates, using a coplanar waveguide structure. The coplanar waveguide sections had dimensions suited for integrated circuits. Measured losses in liquid nitrogen were superior to the loss in a similar thin-film copper filter throughout the 0 to 9.5 GHz pass-band. A simple transmission-line model is adequate for filter design using YBCO films with repeatable characteristics. The measured filters demonstrate the performance of fully patterned YBCO after sealing in space-qualifiable hermetic packages. Five packaged filters of this design were delivered to the Naval Research Laboratory as candidates for space flight in the High Temperature Superconductivity Space Experiment.