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Japanese Journal of Applied Physics

Nb/AlOx/Nb Josephson Tunnel Junctions Using Electron Beam Evaporation

Atsuki Inoue, Akiyoshi Nakayama, Yoichi Okabe

Published Jul 1, 1988• 11 citations
Publisher

Abstract

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%.

DOI
10.1143/jjap.27.1234

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Chinese Science Bulletin (Chinese Version)•Sep 1, 2014•0 citations
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%.
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.
Niobium nitride-niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a-MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a-MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.
We present the latest development of an Nb/Al-AlOx/Nb Josephson junction fabrication process for superconducting integrated circuits at SIMIT. Our fabrication process has four niobium superconducting layers and one Ti/Pd resistor layer with 1.0 Ω/square sheet resistor for junction shunting and circuit biasing. Critical current density Jcof the junctions is controlled from 30 A/cm2to 10 kA/cm2by exposing aluminum layer to pure or mixed O2atmosphere. Correlation between measured Jcand exposure conditions has been established. Average quality factor (Rsg/Rn) of the junctions in our process is larger than 20 with subgap leakage factor (Vm) beyond 30 mV. Critical current uniformity is obtained according to the switching current (Is) distribution of a 1000-JJ array. Measured standard deviation sigma of Isis less than 4% over the 100-mm wafer. Plasma resonance in a large Josephson junction is used to measure specific capacitance (Cs) of the junction. Sheet inductances of superconducting niobium strip-lines are extracted by a SQUID diagnose circuit and the measured results show good agreement with our design values.
All-NbN Josephson tunnel junctions with sputter-deposited magnesium oxide barriers have been fabricated using single-crystal NbN films for the base electrodes. Fabricated Josephson junctions have shown good tunneling characteristics with large gap voltages (5.6-5.8 mV), narrow gap widths (0.1-0.2 mV, from 30 to 70%), and small subgap leakage currents (V/sub m/=20-30 mV, measured at 3 mV). The results of a measurement of a subgap structure for a fabricated junction suggest that the excess leakage currents of fabricated junctions are due to multiparticle tunneling through locally thin areas in the MgO barriers.
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.