JPS6120377A - Superconductive circuit - Google Patents

Abstract

PURPOSE:To obtain the titled device which has a high integration degree and stable action and can be manufactured with high yield, by a method wherein a thin film of crystal with a (1,0,0) plane or polycrystal oriented mainly to a (1,0,0) plane is selectively formed and used for a superconductive electrode, in the crystal of a superconductor having a crystal structure of NaCl structure. CONSTITUTION:An NbN thin film is formed on an MgO single crystal substrate 1 of (1,0,0) plane by reactive sputtering. The NbN thin film is made of polycrystal of (1,0,0) plane strongly oriented to the (1,0,0) plane or to this direction by following the crystallinity of the MgO substrate. This NbN thin film is processed into the superconductive electrode 2 by Ar ion etching with a mask of a photo resist pattern. Next, an interlayer insulation film 3 made of an SiO2 thin film and having an aperture is formed. The surface of the superconductive electrode 2 exposed in this aperture is oxidized with Ar plasma into a tunnel barrier layer 4; successively, a superconductive electrode 5 made of Pb-5wt% In alloy is formed.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a superconducting circuit that operates at extremely low temperatures where a superconducting state can be obtained, and involves producing a highly integrated superconducting circuit with stable operation at a high manufacturing yield. The present invention relates to a superconducting circuit with a structure that can be used.

[Background of the invention]

In superconducting circuits that operate at extremely low temperatures, especially circuits using Josephson junctions, it is recommended, for example, to use a superconducting thin film with an NaCΩ crystal structure as the material for superconducting electrodes or wiring, as described in IEEE Trans.

Magnetics Wag-15, 314 (198
“Fabrica” by Osaka et al.
For example, NbN has a superconducting transition temperature of 14~
15, which is higher than other superconductors, has the advantage that circuit operation is stable against temperature fluctuations. NbN thin film is published in Journal of Applied Physi
Bac in @s 54.6509 (1983)
"Properties of NbN th"
It is known that it can be manufactured by an in fi 1 ms sputtering method. However, since it has not been possible to obtain thin films with good crystallinity, there is no knowledge of the relationship between the crystallinity and orientation of superconducting thin films with the NaCμ structure and the surface condition of the thin film, especially the relationship with the surface natural oxide film thickness. It wasn't.

On the other hand, although superconducting circuits using NbN or the like have the above-mentioned features, there are many variations in characteristics, problems with reproducibility, and manufacturing yields are not high.

[Purpose of the invention]

An object of the present invention is to provide a superconducting circuit that has a high degree of integration, stable operation, and can be manufactured at a high yield.

[Summary of the Invention] In the present invention, the surface of a superconductor crystal having an NaCQ crystal structure, which is easily obtained as a thin film, is a (1°0.0) plane and a (1°0.0) plane.
Among the thin films parallel to the 1,1,1) plane, crystals with the (1,0,0) plane, which has a particularly slow oxidation rate, or mainly the (1,0,
0) By selectively forming a plane-oriented polycrystalline thin film and using it as a superconducting electrode, it is possible to easily form interconnections between superconductors and to form a Josephson junction tunnel barrier layer with good controllability. It is something.

The present inventors investigated the variations in superconducting circuits using NbN, etc., and found that the natural oxide film on the surface of superconducting thin films is
It has been newly discovered that it is necessary to control the properties well because it greatly affects the controllability of manufacturing when forming superconducting connections between superconductors and tunnel barrier layers for Josephson junctions. The present invention was obtained as a result of further research based on this discovery.

[Embodiments of the invention]

Hereinafter, the present invention will be described in detail with reference to Examples. (1
, 0, 0) plane, an NbN thin film with a thickness of about 200 nm was formed on an MgO single crystal substrate 1 with a reactive sputtering method. The substrate temperature at this time was 400° C., and Ar + 30% N2 gas at a pressure of 2 Pa was used for sputtering. This NbN thin film inherits the crystallinity of the MgO substrate and is made of a (1,0°0) plane or a polycrystal strongly oriented in this direction. This NbN thin film was processed by Ar ion etching using the photoresist pattern as a mask.
300 nm thick SiO as superconducting II2
An interlayer insulating film 3 made of a thin film and having an opening was formed. The surface of the superconducting electrode 2 exposed in this opening is pressed 1
A tunnel barrier layer 4 having a thickness of 3 to 4 nI11 is formed by oxidation by plasma using Ar+5%02 gas of pa.
Subsequently, a superelectric Josephson junction made of a Pb-5wt%In alloy with a thickness of about 50 nm is formed by the N tunnel barrier layer 4 because the crystal orientation of the lower superconducting electrode 1 is aligned.
b The oxide film is uniform, and the N on the (1,0,0) plane
Since the bN crystal has a slow oxidation rate and the natural oxide formed during the process before forming the tunnel barrier layer 4 is uniform and has a thickness of at most 1 nm, it was possible to improve the controllability of the manufacturing process.

FIG. 2 shows a second embodiment of the invention. A superconducting electrode 2 is formed on a substrate 1 similar to the first example. Next, the surface of the superconducting electrode 2 was etched by about 10 nm using Ar ions, and then a superconducting electrode made of NbN was formed. When the connection between the two superconducting electrodes was formed in this way, the superconducting electrode 2 mainly consisted of NbN in the (1, 0, 0) plane.
Since it is made of crystal, the natural oxide film formed before forming the connection is as thin as 1 nm at most, and therefore the surface cleaning treatment using Ar ions can be performed with extremely good reproducibility. As a result, we were able to improve the reproducibility of the superconducting critical current flowing through the superconducting connection.

In these examples, an MgO crystal with a (1,0,0) plane was used as the substrate, but Si with a (1,0,0) plane was used.

Similar results could be obtained using N a CQ.

Furthermore, although NbN was used as the superconductor in the above example, similar results were obtained using MoN.

〔Effect of the invention〕

As described above, according to the present invention, in a superconducting circuit that operates at extremely low temperatures, it is possible to suppress the natural oxide film thickness that occurs on the surface of the superconducting electrode constituting the circuit during manufacturing to 1.0 nm at most. Since the thickness is uniform because the crystal planes are aligned, the cleaning process for the surface can be reduced to about half of the conventional cleaning process, and the reproducibility can be increased. When forming a tunnel barrier layer, the uniform crystal orientation of the superconducting electrode surface improves the uniformity of the oxide tunnel barrier layer, thus improving the uniformity and reproducibility of the Josephson junction characteristics. I was able to do it.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a part of a superconducting circuit in one embodiment of the present invention, and FIG. 2 is a sectional view showing a part of a superconducting circuit in another embodiment of the invention.

Claims (1)

[Claims] 1. A superconducting thin film that operates at extremely low temperatures and has a crystal structure of NaCl (rock salt) as part or all of the wiring or electrode, and the superconducting thin film has a crystal structure of (1,0 , 0)-plane crystal or a polycrystal mainly oriented in the (1, 0, 0) plane. 2. In the superconducting circuit according to claim 1,
The superconducting thin film with the NaCl (rock salt) structure is made of NbN, Mo
A superconducting circuit characterized in that it is made of at least one material selected from N. 3. In the superconducting circuit according to claim 1 or 2, the base material forming the superconducting thin film is M
The superconducting thin film is made of one material selected from gO, NaCl, and Si, and the superconducting thin film is formed using a single crystal whose plane is the (1, 0, 0) plane or a polycrystalline body oriented in the (1, 0, 0) plane. A superconducting circuit characterized by: