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IEEE Transactions on Magnetics

Microwave properties of variable thin-film Josephson bridges

V. Gubankov, V. P. Koshelets, G. A. Ovsyannikov, Alexander N. Vystavkin

Published Jan 1, 1979• 6 citations
Publisher

Abstract

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.

DOI
10.1109/tmag.1979.1060206

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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.
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.
We report here numerical calculations on the properties of superconducting weak links as mixers with external local oscillators (LO). Our calculations are based on the resistively shunted-junction model. We assume that the LO and the signal are ideal current sources of amplitudes ILO and IS and of frequencies ωLO and ωs, respectively. We calculate the IF voltage across the weak link as a function of the bias current, the LO current amplitude ILO, and the normalized signal frequency Ωs = ħωs/(2 e Ic R), where Ic is the critical current and R is the shunt resistance. If ωIF/ωLO and Is/Ic are small, we find a linear IF response. Using this method we find the frequency dependence of the IF response, estimate the conversion efficiency for the matched case in both the absence and the presence of noise, and calculate the required LO power. We have fabricated thin-film constrictions by using electron lithography. We have made constrictions of indium films with thicknesses of ~ 1 200 A with widths as narrow as 2 000 A and lengths as short as 1 000 A and have studied their I-V characteristics in the presence of RF radiation and measured their X-band mixing properties. We have measured an NEP of 10-13 W/Hz1/2 and a dynamic range of 23 dB (3 dB deviation from linearity) and estimate that, with optimum matching, these would become 3 × 10-18 W/Hz1/2 and 74 dB, respectively.
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.
Measurements at 3 GHz of the microwave properties of a superconducting thin-film microbridge coupled to a narrowly tuned strip-line resonator (coupled Q approximately 10 3 ) indicate that such a device displays the same variations in small-signal impedance as predicted by the resistively shunted junction model of a Josephson weak-link. Whenever this impedance becomes negative the microbridge is found to generate microwave radiation whose frequency is determined not by the junction bias voltage but by the natural resonance of the strip-line.
In this paper, we present our latest results on the properties of superconducting weak links prepared in Mo films by ion-implantation and in Nb films by electron-beam lithography. In the case of the ion-implanted weak links, which structurally resemble proximity-effect bridges, we find that the static properties are quite well described by a recent theory of Likharev and Yakobson. We further find that the rf response of these weak links is in excellent agreement with quasistatic resistively-shunted-junction model calculations based on a sinusoidal current-phase relation at those temperatures for which such a current-phase relation is predicted by the static theory of Likharev and Yakob-son. For the e-beam fabricated Nb bridges, which have the Dayem bridge geometry, our sample preparation techniques have allowed us to produce bridges and two-junction interferometers in which the link dimensions are as small as about 2000 Angstroms. We find that Josephson effects are observable in these bridges even when the bridge dimensions exceed the temperature-dependent coherence length by about an order of magnitude, but are still less than the effective thin-film penetration depth.
We report on the characteristics of thin film microbridge Josephson junctions fabricated using electron beam lithography and discuss the techniques used to fabricate these junctions, which are typically 0.3μ square. Extensive measurements have been made on both individual junctions of tin and indium and on small arrays of these junctions. Synchronization of current biased series arrays by an applied microwave field has been achieved. This synchronization appears as a zero differential resistance across the entire array at a voltage of MVJwhere M is the number of elements in series and VJis the Josephson voltage hv/2e. Furthermore we have observed a systematic variation with microbridge dimension of the microwave enhancement of the critical current.