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AIP conference proceedings

Alternatives in fabricating submicron niobium Josephson junctions

G. Daalmans, J. Zwier

Published Jan 1, 1978• 0 citations
Publisher

Abstract

This paper deals with the fabrication of submicron niobium microbridges and submicron niobium tunnel junctions. Several techniques are described to fabricate niobium microbridges. The most important of them is a technique in which niobium is evaporated through a bakable submicorn mask. The technique to fabricate submicron tunnel junctions is basically a shadow evaporation technique. In contrast to earlier attempts it is a reproducible method suitable for large scale application.

DOI
10.1063/1.31356

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This paper deals with the fabrication of submicron niobium microbridges and submicron niobium tunnel junctions. Several techniques are described to fabricate niobium microbridges. The most important of them is a technique in which niobium is evaporated through a bakable submicorn mask. The technique to fabricate submicron tunnel junctions is basically a shadow evaporation technique. In contrast to earlier attempts it is a reproducible method suitable for large scale application.
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.
The authors have developed two distinct processes for the fabrication of mesoscopic Josephson junction qubits that are compatible with conventional CMOS processing. These devices, based on superconducting Al/Al2O3/Al tunnel junctions, are fabricated by electron beam lithography using single-layer and multi-layer resists. The new single-layer resist process is found to have significant advantages over conventional fabrication methods using suspended tri-layer shadow masks. It is simpler and more accurate to implement, and avoids the significant areas of redundant metallisation that are an unavoidable by-product of the tri-layer shadow mask method.
A novel method of fabrication, as described in the following, was developed that ensures a good reproducibility of the junction. Namely, poly-Si was deposited over a Si substrate whose surface had been previously thermally oxidized. The poly-Si overlaid surface was then thermally oxidized. The bridge configuration of the junction was microprocessed by means of electron beam lithography. The fabricated configuration served as a mask in depositing the Nb layer on the substrate by means of rf sputtering to conclude the fabrication of a microbridge Josephson junction. The final process of the formation of the variable thickness microbridge configuration was readily carried out by placing another properly shaped mask over the masking bridge or by forming an SiO2 layer over the poly-Si layer. The SiO2 layer was processed by selective plasma etching so that it would act as a mask over the bridge portion. The stabilization of the characteristics of the junction has been achieved due to the passivation which takes place during the heat treatment in atmosphere by forced oxidation of the Nb film surface and to the annealing of the film itself.
We present two possible methods for the fabrication of sub-micron sized Josephson junctions, namely the shadow-evaporation technique and the cross-type technology. Their importance for the field of modern super-conducting technology is discussed. As examples we present measurement results of a two-qubit sample and a prototype of a microwave detector fabricated each by one of the described methods. We review potential applications of superconducting quantum circuits based on the developed methods.
Josephson tunnel junctions are the centerpiece of almost any superconducting electronic circuit, including qubits. Typically, the junctions for qubits are fabricated using shadow evaporation techniques to reduce dielectric loss contributions from the superconducting film interfaces. In recent years, however, sub-micrometer scale overlap junctions have started to attract attention. Compared to shadow mask techniques, neither an angle dependent deposition nor free-standing bridges or overlaps are needed, which are significant limitations for wafer-scale processing. This comes at the cost of breaking the vacuum during fabrication, but simplifies integration in multi-layered circuits and implementation of vastly different junction sizes and enables fabrication on a larger scale in an industrially standardized process. In this work, we demonstrate the feasibility of a subtractive process for the fabrication of overlap junctions. In an array of test contacts, we find low aging of the average normal state resistance of only 1.6% over 6 months. We evaluate the coherence properties of the junctions by employing them in superconducting transmon qubits. In time domain experiments, we find that both, the qubit life- and coherence time of our best device, are, on average, greater than 20 μs. Finally, we discuss potential improvements to our technique. This work paves the way toward a more standardized process flow with advanced materials and growth processes, and constitutes an important step for the large scale fabrication of superconducting quantum circuits.
All hard Josephson tunnel junctions, whose base and counter electrodes are composed of double-layered niobium nitride (NbN) and niobium (Nb) films, have been successfully fabricated by isolating a junction sandwich formed on a whole silicon wafer with a reactive ion etching technique. The reactive ion etching technique has been used for patterning both base and counterelectrodes, and self-aligning definition of junction areas has been performed. The fabricated junctions show good quality single-particle tunneling characteristics and excellent uniformity in critical currents.