JPH03104177A - Squid element - Google Patents

Abstract

PURPOSE:To improve characteristics by bringing a thin film having proximity effect with a superconducting thin film closely into contact with the surface of the superconducting thin film except a Josephson junction. CONSTITUTION:On the surfaces of thin films (electrode part 2a, 2b) of high temperature superconductor except Josephson junction parts (3a, 3b) of a SQUID ring made of a polycrystalline thin film, thin films (e.g. Au thin films 4a, 4b) composed of material exhibiting proximity effect with the above superconducting thin film are formed in a close contact sate. When normal conductor like Au, Cu, etc., is made to approach superconductor, the normal conductor exhibits superconductivity by proximity effect. Hence, in the case where a grain boundary I exists in the electrode part of the SQUID ring, a superconducting path is formed in the thin films 4a, 4b adjacent to the grain boundary I, so that a shielding current flows also in this part, and the permeating of magnetic flux into the grain boundary is blocked.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a SQUID device used for measuring ultra-weak magnetic fields, and more specifically to a SQUID device using a high-temperature superconducting thin film.
Regarding D element.

<Prior Art> An RF-SQUID or DC-SQUID element has a structure in which one or two Josephson junctions are provided within a superconducting ring, and the inflow and outflow of magnetic flux at this Josephson junction is observed.

FIG. 4 is a diagram schematically showing the structure of a conventional DC-SQUID element, in which two Josephson junctions 43a, 43b are provided in a loop formed of superconducting material. The other parts are electrode parts 42a, 4
Connect the current terminal and voltage terminal as 2b.

By the way, many reports have already been made regarding SQUID devices using high-temperature superconducting thin films such as YBCO, but all of the Josephson junctions are essentially grain boundary junctions, and criticality occurs at the grain boundaries. By taking advantage of the low current density, a Josephson junction, which is a weak coupling, is obtained.

For this reason, in SQUID devices made of high-temperature superconducting thin films that have already been announced, electrode portions and Josephson junctions are obtained by patterning a polycrystalline thin film of high-temperature superconducting material.

<Problem to be solved by the invention> By the way, in high-temperature superconducting thin films, in general, the problem shown in FIG.
As shown in the perspective view and side view in (b), when a grain boundary I exists, magnetic flux penetrates into the thin film from that portion.

Therefore, S using the conventional high temperature superconducting thin film as described above
According to the QUID element, magnetic flux also enters the electrode portions 42a, 42b, which means that the electrode portions 42a, 42b also enter the magnetic flux.
This is equivalent to the existence of a weak coupling portion within 2b, which causes deterioration of device characteristics such as unstable magnetic field-output voltage characteristics.

<Means for Solving the Problems> The present invention has been made for the purpose of improving the characteristics of a SQUID element using a high-temperature superconducting thin film, and its structure will be described with reference to FIG. 1 corresponding to an embodiment. To explain,
In the present invention, SQUID made of polycrystalline thin film of high temperature superconductor
A thin film made of a material that exhibits a proximity effect with the superconducting thin film (for example, Au
The thin films 4a and 4b) are formed in close contact with each other.

<Effect> When a normal conductor such as Au or Cu is brought close to a superconductor, the normal conductor exhibits superconductivity due to the proximity effect, which causes SQU to
Even if a grain boundary I exists in the electrode part of the ID ring, a superconducting bus is formed in the adjacent thin films 4a and 4b, and a shielding current flows through this part, preventing magnetic flux from entering the grain boundary I. be done.

Embodiment> FIG. 1 is a structural explanatory diagram of an embodiment of the present invention, in which (a) shows a front view and (b) shows a BB sectional view thereof.

A DC-SQUID ring is formed on the upper surface of a substrate 1 made of MgO or the like by patterning a YBCO polycrystalline thin film and connecting two electrode parts 2a and 2b with two digocefson junction parts 3a and 3b.

Josephson junction 3a of this DC-SQUID ring
The upper surfaces of electrode portions 2a and 2b, excluding electrode portions 2a and 3b, are covered with Au thin films 4a and 4b, respectively.

According to the above embodiment of the present invention, the electrode portion 2a is shown in FIG.
As shown in the enlarged cross-sectional view of the portion of the electrode section 2a (2b
) adjacent to the Au films 4a and 4b become superconducting due to the proximity effect, and even if there are grain boundaries
! Since a superconducting path is formed in the films 4a and 4b and a shielding current flows through this portion, magnetic flux does not enter the grain boundary (2).

Note that the embodiments of the present invention as described above can be manufactured, for example, by the procedure shown below.

First, a YBCO thin film is uniformly formed on a substrate 1 by RF magnetron sputtering or the like, and then this is annealed to crystallize the YBCO thin film and form grain boundaries therein. Next, a photoresist is applied on the YBCO thin film, and the resist is removed by a photolithography process, leaving the pattern shown in FIG. 1. Then, using the resist film left by the development as a mask, a SQUID ring having the pattern shown in FIG. 1 is obtained by dry etching such as Ar ion etching. The above is publicly known.

Next, the Josephson junction 3a of the SQUID ring
A mask is formed above 3b and 3b, and Au is deposited or ion coated from above. Finally, by removing the mask, an element having the structure shown in FIG. 1 is obtained.

The present invention provides the Josephson joint 3 as in the above embodiment.
The structure is not limited to a structure in which an Au thin film or the like is not formed above the Josephson junction 3a or 3b, and for example, as shown in a cross-sectional view of the main part in FIG. Formed, and uniformly spread Au! over the entire element from above! A film 4 may also be formed. However, the thickness of this mask 5 is set to be greater than or equal to the coherence length of the superconductor forming the element.

In the structure shown in FIG. 3, the Au thin film 4 is separated from the superconductor above the digocefson junctions 3a, 3b by the presence of the mask 5, so that no proximity effect occurs in that part, and the digocefson junctions 3a, 3b It does not prevent magnetic flux from entering. Note that as the material of the mask 5, MgO or a resin-based insulator can be used.

Incidentally, in addition to Au, Cu and the like are known as materials that exhibit the proximity effect, and the present invention is applicable to any type of material that exhibits such a proximity effect.

Furthermore, it goes without saying that the present invention is not limited to DC-SQUIDs, but can also be applied to RF-SQUIDs in exactly the same way.

Effects of the Invention> As explained above, according to the present invention, a thin film such as Au that exhibits the proximity effect is closely formed on the surface of the high temperature superconducting thin film (electrode part) excluding the Josephson junction of the SQUID ring. Therefore, even if a grain boundary exists in the superconducting thin film in that part, magnetic flux from the outside does not enter, and the grain boundary in this part is apparently erased, improving the characteristics of the SQUID element.

Furthermore, the Au thin film of the present invention has the effect that it can serve as a protective film for high temperature superconducting thin films that are susceptible to moisture, such as YBCo thin films.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the structure of the embodiment of the present invention, and FIG. 2 is an explanatory diagram of the operation of the embodiment of the present invention shown in an enlarged sectional view of the electrode portion 3a.
FIG. 3 is a sectional view of main parts of another embodiment of the present invention, FIG. 4 is a schematic structural explanatory diagram of a conventional DC-SQUID element, and FIG.
The figure is an explanatory diagram of the state of magnetic flux penetration in a polycrystalline high-temperature superconducting thin film. 1... Substrates 2a, 2b... Electrode parts 3a, 3b... Josephson junction parts 4a, 4b, 4
・-・Au3 film 5・・・Insulator mask

Claims (1)

[Claims] In an element having at least one Josephson junction in a superconducting ring formed of a polycrystalline thin film of a high-temperature superconductor, a layer between the superconducting thin film and the superconducting thin film is provided on the surface of the thin film excluding the Josephson junction. A SQUI characterized by closely forming a thin film made of a material that exhibits a proximity effect.
D element.