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Applied Physics Letters

Microwave mixing with niobium variable thickness bridges

Li-Kong Wang, Alessandro Callegari, Bascom S. Deaver, D. W. Barr, Robert J. Mattauch

Published Aug 15, 1977• 15 citations
Publisher

Abstract

Niobium thin-film bridges 300 Å thick, 1 μm wide, and 0.5 μm long joining two bulk films 5000 Å thick and having normal resistance of ∼1 Ω have been fabricated and used for microwave mixing at 10 GHz. They exhibit Josephson, bolometric, and multiple-flux-flow mixing and have useful response at 100–200 GHz. The data show in a direct way limitations imposed by flux flow and heating.

DOI
10.1063/1.89679

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Niobium thin-film bridges 300 Å thick, 1 μm wide, and 0.5 μm long joining two bulk films 5000 Å thick and having normal resistance of ∼1 Ω have been fabricated and used for microwave mixing at 10 GHz. They exhibit Josephson, bolometric, and multiple-flux-flow mixing and have useful response at 100–200 GHz. The data show in a direct way limitations imposed by flux flow and heating.
Niobium variable thickness bridges are being fabricated for testing as millimeter wave mixers. Bridges have been fabricated on fused quartz, silicon and gallium arsenide substrates and characterized by measurements of the I−V curve, and dV/dI versus V and by mixing experiments at 80‐100 GHz. The bridges on Si and GaAs exhibit small Josephson steps but the response of all the bridges is limited by self‐heating, and mixing appears to be primarily bolometric. However the cooling of the bridges of Si is much better than the others.
To realize antenna-coupled Josephson detectors for microwave and millimeter-wave radiation, planar-type Nb nanobridge Josephson junctions were fabricated. Nb thin films whose thickness, the root mean square roughness and the critical temperature were 20.0 nm, 0.109 nm and 8.4 K, respectively were deposited using a DC magnetron sputtering at a substrate temperature of 700°C. Nanobridges were obtained from the film using 80-kV electron beam lithography and reactive ion-beam etching in CF 4 (90%) + O2 (10%) gases. The minimum bridge area was 65 nm wide and 60 nm long. For the nanobridge whose width and length were less than 110 nm, an I-V characteristic showed resistively-shunted-junction behaviour near the critical temperature. Moreover, Shapiro steps were observed in the nanobridge with microwave irradiation at a frequency of 6 – 30 GHz. The Nb nanobridges can be used as detectors in the antenna-coupled devices.
Amplification, down-conversion and detection of the microwaves in thin-film Josephson variable thickness bridges of submicron dimensions have been investigated experimentally. The results for indium and tin bridges are discussed taking into account the influence of the nonequilibrium electrons in a weak link region. The same experiments were carried out with SNS variable thickness Nb-Al-Nb bridges. They showed that the properties of SNS bridges agreed better with the theoretical predictions of the resistively shunted junction model in a wide temperature range. Moreover the parameters of the Nb-Al-Nb bridges have possessed better stability and reliability.
Several reactive dry etching methods; ion etching, sputter etching and plasma etching, were studied in order to fabricate a niobium variable-thickness type thin film bridge (VTB). Niobium was etched by CF4 reactive sputter etching with submicron accuracy and no significant influence on the superconducting transition temperature except very near the surface (<200 Å). The niobium VTB behaved as an S-S'-S bridge, and a reduced self-heating effect was realized by a variable-thickness geometry. The DC Josephson effect was observed in the temperature range without thermal hysteresis. The quality factor of the I c R n product obtained was at least 0.42 mV.
Thin film niobium nitride Josephson tunnel junctions have been used as harmonic mixers in experiments to measure the frequency of various submillimetre wavelength laser lines. The Josephson junctions were used as harmonic mixers with an 18-40 GHz microwave synthesiser providing the local oscillator signal. Harmonics up to the 140th of ∼18 GHz, generated within the junction, mixed with optically pumped laser lines up to 2.5 THz to produce output signals around 20 MHz. Results of frequency measurements and data on the performance of the mixing system, including signal to noise ratios and frequency response, will be presented.