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Chinese Science Bulletin (Chinese Version)

Fabrication of superconducting flux quantum bits and the feature parameters of Al/AlOx/Al tunnel junctions

Lin Wang, Weiwei Xu, Peiheng Wu, GuoZhu SUN, XiaoHu LI, Ji-Quan Zhai

Published Sep 1, 2014• 0 citations
Publisher

Abstract

As one of the candidates for realizing quantum bits, superconducting quantum bits, based on Josephson junction, have the advantages of easy integrating, coupling with the outside environment and so on. Double-layer photoresist was used to prepare suspended shadow mask by deep ultra-violet (DUV) lithography. We fabricated the sample of superconducting flux quantum bits by electron beam evaporation and static oxidization with aluminum as superconducting material. According to the dependence of the flatness on the thickness of aluminum film, we select 60 nm as the thickness of the bottom electrode of tunnel junction. By increasing the distance between the evaporation source and the substrate, we have improved the smoothness of film and thus reduced the leakage current of tunnel junction. The tri-layer structure of aluminum tunnel junction was observed by transmission electron microscopy (TEM), and the relationship between the thickness of barrier layer and the oxidation pressure was obtained.

DOI
10.1360/n972014-00168

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As one of the candidates for realizing quantum bits, superconducting quantum bits, based on Josephson junction, have the advantages of easy integrating, coupling with the outside environment and so on. Double-layer photoresist was used to prepare suspended shadow mask by deep ultra-violet (DUV) lithography. We fabricated the sample of superconducting flux quantum bits by electron beam evaporation and static oxidization with aluminum as superconducting material. According to the dependence of the flatness on the thickness of aluminum film, we select 60 nm as the thickness of the bottom electrode of tunnel junction. By increasing the distance between the evaporation source and the substrate, we have improved the smoothness of film and thus reduced the leakage current of tunnel junction. The tri-layer structure of aluminum tunnel junction was observed by transmission electron microscopy (TEM), and the relationship between the thickness of barrier layer and the oxidation pressure was obtained.
Besides serving as promising candidates for realizing quantum computing, superconducting quantum circuits are one of a few macroscopic physical systems in which fundamental quantum phenomena can be directly demonstrated and tested, giving rise to a vast field of intensive research work both theoretically and experimentally. In this paper we report our work on the fabrication of superconducting quantum circuits, starting from its building blocks: Al/AlO x /Al Josephson junctions. By using electron beam lithography patterning and shadow evaporation, we have fabricated aluminum Josephson junctions with a controllable critical current density ( j c ) and wide range of junction sizes from 0.01 μm 2 up to 1 μm 2 . We have carried out systematical studies on the oxidation process in fabricating Al/AlO x /Al Josephson junctions suitable for superconducting flux qubits. Furthermore, we have also fabricated superconducting quantum circuits such as superconducting flux qubits and charge-flux qubits.
The superconductive Josephson junction is the key device for superconducting quantum computation. We have fabricated Al/Al2O3/Al tunnel junctions using a double angle evaporation method based on a suspended shadow mask. The Al2O3 junction barrier has been formed by introducing pure oxygen into the chamber during the fabrication process. We have adjusted exposure conditions by changing either the oxygen pressure or the oxidizing time during the formation of tunnel barriers to control the critical current density J c and the junction specific resistance R c . Measurements of the leakage in Al/Al2O3/Al tunnel junctions show that the devices are suitable for qubit applications.
Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.
An ultra-thin aluminum oxide layer is a key component for Josephson junctions (JJ) in superconducting quantum bits (qubits). This layer serves as a barrier layer for Cooper pairs tunneling between the superconducting electrodes and significantly influences the overall performance of the junction. In this study, we investigate the impact of aluminum deposition rates on the microstructure and chemical variation of the aluminum oxide layer, as well as the device’s yields and qubits’ lifetimes. Surprisingly, although the oxide layer roughness and thickness variation improve noticeably with an increasing Al deposition rate from 0.5 Å/s to 5 Å/s, the qubit’s coherence time is almost unchanged. We attribute this to the fact that Cooper pairs tunnel only through the thinnest region of the barrier. Additionally, we revealed a stress-induced grain boundary sliding short-circuit failure mechanism in the JJ. Our discovery provides a vital understanding of oxide microstructure and JJ functionality, which are critical for improving the coherence time of superconducting qubits.
We have developed a combined photolithography and electron-beam lithography fabrication process for sub-\mum to \mum-size Nb/Al-AlOx/Nb Josephson junctions. In order to define the junction size and protect its top electrode during anodic oxidation, we developed and used the new concept of an aluminum hard mask. Josephson junctions of sizes down to 0.5 \mum2 have been fabricated and thoroughly characterized. We found that they have a very high quality, which is witnessed by the IV curves with quality parameters Vm > 50 mV and Vgap = 2.8 mV at 4.2 K, as well as IcRN products of 1.75-1.93 mV obtained at lower temperatures. In order to test the usability of our fabrication process for superconducting quantum bits, we have also designed, fabricated and experimentally investigated phase qubits made of these junctions. We found a relaxation time of T1 = 26 ns and a dephasing time of T2 = 21 ns.
Abstract This work is concerned with Al/Al-oxide(AlOx)/Al-layer systems which are important for Josephson-junction-based superconducting devices such as quantum bits. The device performance is limited by noise, which has been to a large degree assigned to the presence and properties of two-level tunneling systems in the amorphous AlOx tunnel barrier. The study is focused on the correlation of the fabrication conditions, nanostructural and nanochemical properties and the occurrence of two-level tunneling systems with particular emphasis on the AlOx-layer. Electron-beam evaporation with two different processes and sputter deposition were used for structure fabrication, and the effect of illumination by ultraviolet light during Al-oxide formation is elucidated. Characterization was performed by analytical transmission electron microscopy and low-temperature dielectric measurements. We show that the fabrication conditions have a strong impact on the nanostructural and nanochemical properties of the layer systems and the properties of two-level tunneling systems. Based on the understanding of the observed structural characteristics, routes are suggested towards the fabrication of Al/AlOx/Al-layers systems with improved properties.