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Applied Physics Letters

Fabrication of planar superconducting-normal-superconducting Josephson microbridges and SQUID’s using shadow evaporations

E. C. Jelks, G.L. Kerber

Published Jun 1, 1981• 1 citation
Publisher

Abstract

The technique of ’’shadow evaporation’’ has been used to fabricate superconducting-normal-superconducting Josephson microbridges and SQUID’s with bridge widths of about 0.2 μm and lengths of 0.1 μm or less. The devices consist of vanadium banks and gold bridges and have static current-voltage characteristics that are consistent with the resistively shunted junction model down to at least 0.4 TC. Refined versions of this technique may be useful for batch fabrication of single and multibridge Josephson devices.

DOI
10.1063/1.92187

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The technique of ’’shadow evaporation’’ has been used to fabricate superconducting-normal-superconducting Josephson microbridges and SQUID’s with bridge widths of about 0.2 μm and lengths of 0.1 μm or less. The devices consist of vanadium banks and gold bridges and have static current-voltage characteristics that are consistent with the resistively shunted junction model down to at least 0.4 TC. Refined versions of this technique may be useful for batch fabrication of single and multibridge Josephson devices.
Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current-voltage characteristics in the absence of any shunting resistance.
For the fabrication of nanoscale SQUIDs, for example having loop diameters of order 1 µm or smaller, the conventional trilayer window-type Josephson junctions are too large. Instead, we consider the use of nanobridge junctions with dimensions of order 100 nm. The problem of hysteresis in the junctions' current–voltage characteristic (IVC) is avoided by depositing a gold overlayer, which serves as both a thermal and electrical shunt. Noise measurement of dc SQUIDs which incorporate such Nb nanobridges as the Josephson junctions are reported. Devices having different IVCs were fabricated and characterized. The best device has a flux noise of 5 × 10 −6 Φ 0 Hz −1/2 at 4.2 K, and there is no significant increase in the noise level in a static magnetic field up to 2 mT. Techniques for increasing the voltage modulation depth, and hence improving the flux noise performance, are discussed.
We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by e-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in situ fabrication of junctions and capacitors without the use of the well-known suspended bridge (Dolan 1977 Appl. Phys. Lett. 31 337-9). The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 10(-2) µm(2) to more than 10(4) µm(2). We have fabricated Al/AlO(x)/Al Josephson junctions, phase qubit and capacitors using a 100 kV e-beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20 kV e-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillation decays and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this bridge-free technique.
We have studied the phase locking of two and eight microbridge arrays shunted by superconducting loops in a dc SQUID configuration. The difference in phase between adjacent bridges is controlled by the applied flux, and thus the radiated power is a periodic function of this flux. With adjustment of the external flux, the individual bridge voltages can be made to add in phase. The effect of additional normal shunts on the phase locking is also investigated. MICROBRIDGES COUPLED BY SUPERCONDUCTING LOOPS Experiment sly on two Josephson microbridges have shown that it is possible to obtain nearly complete in phase coherent radiation with narrowed linewidth by coupling the microbridges with external shunts. To obtain greater ouput power and narrower linewidth it is desirable to phase lock larger arrays of bridges. The narrow range of bias currents over which locking occurs, however, imposes strict tolerances on the uniformity of bridge characteristics if a single current bias is used. In this paper we discuss experiments which eliminate this difficulty by using a superconducting loop to couple the junctions, i.e., a dc SQUID configuration, so that the average frequency of each junction is always equal even for non-identical junctions. Also the phase difference between the junctions in the absence Of noise is set by the amount of externally applied flux due to the fluxoid quantization condition. With identical bridges and zero applied flux, the junctions oscillate in phase. In the geometry used in
We have fabricated two types of NbN nanobridge dc Superconducting QUantum Interference Devices (SQUIDs). From the analyses of the response to magnetic field, it is found that the current-phase relationship for the planar-type nanobridge and the nanobridge fabricated by field evaporation is close to sinusoidal. For V\gtrsim720 µV, The spectral densities of the voltage noise show linear dependence on the voltage for the SQUIDs with nanobridges whose length is about the superconducting coherence length of NbN. The SQUIDs with nanobridges with lengths of several times the NbN coherence length exhibit parabolic voltage dependence.