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Chinese Physics B

Fabrication of Al/AlOx/Al Josephson junctions and superconducting quantum circuits by shadow evaporation and a dynamic oxidation process

Yulin Wu, Hui Deng, Haifeng Yu, Guangming Xue, Ye Tian, Jie Li, Yingfei Chen, Shi-Ping Zhao, Dongning Zheng

Published Jun 1, 2013• 18 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/22/6/060309

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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.
Abstract The most commonly used physical realization of superconducting qubits for quantum circuits is a transmon. There are a number of superconducting quantum circuits applications, where Josephson junction critical current reproducibility over a chip is crucial. Here, we report on a robust chip scale Al/AlO x /Al junctions fabrication method due to comprehensive study of shadow evaporation and oxidation steps. We experimentally demonstrate the evidence of optimal Josephson junction electrodes thickness, deposition rate and deposition angle, which ensure minimal electrode surface and line edge roughness. The influence of oxidation method, pressure and time on critical current reproducibility is determined. With the proposed method we demonstrate Al/AlO x /Al junction fabrication with the critical current variation (σ/⟨Ic⟩)(\sigma /\langle {I_{c} } \rangle )(σ/⟨Ic​⟩) ( σ / ⟨ I c ⟩ ) less than 3.9% (from 150 × 200 to 150 × 600 nm 2 area) and 7.7% (for 100 × 100 nm 2 area) over 20 × 20 mm 2 chip. Finally, we fabricate separately three 5 × 10 mm 2 chips with 18 transmon qubits (near 4.3 GHz frequency) showing less than 1.9% frequency variation between qubits on different chips. The proposed approach and optimization criteria can be utilized for a robust wafer-scale superconducting qubit circuits fabrication.
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.
The most commonly used physical realization of superconducting qubits for quantum circuits is a transmon. There are a number of superconducting quantum circuits applications, where Josephson junction critical current reproducibility over a chip is crucial. Here, we report on a robust chip scale Al/AlOx/AlAl/AlO_x/AlAl/AlOx​/Al junctions fabrication method due to comprehensive study of shadow evaporation and oxidation steps. We experimentally demonstrate the evidence of optimal Josephson junction electrodes thickness, deposition rate and deposition angle, which ensure minimal electrode surface and line edge roughness. The influence of oxidation method, pressure and time on critical current reproducibility is determined. With the proposed method we demonstrate Al/AlOx/AlAl/AlO_x/AlAl/AlOx​/Al junction fabrication with the critical current variation (σ/Icσ/I_cσ/Ic​) less than 3.9% (from 150×200150\times200150×200 to 150×600150\times600150×600 nm2nm^2nm2 area) and 7.7% (for 100×100100\times100100×100 nm2nm^2nm2 area) over 20×2020\times2020×20 mm2mm^2mm2 chip. Finally, we fabricate separately three 5×105\times105×10 mm2mm^2mm2 chips with 18 transmon qubits (near 4.3 GHz frequency) showing less than 1.9% frequency variation between qubit on different chips. The proposed approach and optimization criteria can be utilized for a robust wafer-scale superconducting qubit circuits fabrication.
Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.
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.
A minimal method to fabricate Al/AlOx_xx​/Al Josephson junctions (JJs) using photolithography and argon etching, before metallization and oxidation, is demonstrated. JJs with areas ranging from 1 to 6 μμμm2^22 can be fabricated and, with the appropriate oxidation conditions, the junction resistance can be varied by $\sim$2 orders of magnitude. Transmission electron microscopy reveals the successful fabrication of JJs with few grain boundaries suggesting reduced energy loss from two-level-systems. Superconducting QUantum Interference Devices (SQUIDs) fabricated from this methodology exhibit reduced resistance variation over multiple chips, compared with electron beam lithography, and the devices can sustain repeated thermal cycles to 10 mK with the excellent flux response remaining unchanged. The quantum applications of this technology are demonstrated by embedding a SQUID resonator into a 3D cavity and parametrically amplifying low photon numbers with gains of $\sim$40 dB. This work establishes the simplest approach to fabricating JJs to date, and it could prove pivotal to the widespread utilization of superconducting circuit-based quantum technologies.