JPH03283680A - Squid device using proximity effect - Google Patents

Abstract

PURPOSE:To reduce I/f noise by forming a stage at the center of a substrate to divide the substrate into an upper level and a lower level with the stage as a border wherein high temperature this superconductive films are adhered on the entire surface including the stage while the thin films are weakly jointed with each other and thin metallic films exhibiting a proximity effect are provided on the plane thin films respectively. CONSTITUTION:A stage 1a for dividing a substrate 1 into a right and a left parts at the center of the MgO substrate 1, while YBCO high temperature thin superconducting films 2, 3 are respectively adhered to an upper level and a lower level with the stage interposed. At this time the films 2, 3 form a narrow part of YBCO type similar to that formed at the stage 1a wherein interconnection is constituted of junctions 4a, 4b. Then thin Au films 5, 6 are respectively adhered to the thin films 2, 3 on the upper and lower levels. Thus the thin films 5, 6 provided on the thin films 2, 3 forming an electrode of a DC-SQUID device become superconductive due to the proximity effect while coherent length of the electrode is appromixately 1000 angstroms and entrance of magnetic flux can be prevented to eliminate I/f noise.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a SQUID element used for detecting extremely small magnetic fields.

<Conventional technology> In order to obtain a Josephson junction with good performance, the bridge length (length of the weak junction) should ideally be about 3 to 5 times the coherent length of the bridge material used. It is also known that even if this ideal bridge length cannot be obtained, the bridge length should be shortened so as to get as close to it as possible.

In a Josephson junction using Nb, the ideal bridge length is about several hundred. In order to obtain such an extremely short bridge length with good reproducibility, a quasi-planar Josephson junction is advantageous.

In a quasi-planar Josephson junction, another superconducting thin film is layered on top of a superconducting thin film formed on a flat substrate surface, with an insulating layer interposed between the two superconducting thin films. The bridge length can be determined by the film thickness dimension, which can be controlled relatively easily, which is extremely advantageous compared to the planar Josephson junction, which requires microfabrication on the plane. be.

By the way, when making a Josephson junction device using an oxide high-temperature superconductor such as YBCO, it is difficult to make a good quasi-planar junction device as in the case of using a conventional superconductor such as Nb type, for the following reasons. It is difficult to obtain a Josephson junction.

First, high-temperature superconducting thin films such as YBCO are generally unstable and easily diffused, and it is not easy to stack them in good condition.

Furthermore, high-temperature superconducting thin films obtained using current film-forming techniques have a surface flatness that discolors by about 100 degrees, and pin contact occurs when they are stacked on each other with an insulating layer interposed between them.

Furthermore, oxide high-temperature superconductors are subject to processing limitations such as being susceptible to deterioration due to water, and processes cannot be selected relatively freely as in the case of Nb-based superconductors.

In view of these points, the present inventor, in collaboration with others, provided a step on the substrate, formed a high-temperature superconductor thin film on the upper and lower planes of the step, and formed both high-temperature superconductor thin films. A Josephson junction element in which a high temperature superconductor thin film bridge is used to cross the stepped portion has already been proposed (Japanese Patent Application No. 1-12185).

According to this proposal, a quasi-planar Josephson junction can be obtained without stacking superconducting thin films, so high-performance devices can be obtained with good reproducibility even using high-temperature superconducting thin films. Since it is only necessary to form a film of m, there is an advantage that the manufacturing process is simplified.

<Problems to be Solved by the Invention> By the way, as mentioned above, high-temperature superconductor thin films have a short coherence length, and due to the presence of grain boundaries, even if a Josephson junction is successfully formed, magnetic flux will not be present in the electrode part. There are problems in that 1/f noise due to the intrusion of the magnetic field and hysteresis in response to the magnetic field occur.

The present invention was made to improve these points, and has the following points:
/f SQU using a high-temperature superconductor thin film that is less prone to noise and hysteresis in magnetic field response! The purpose is to provide D elements.

Means for Solving the Problems> The structure for achieving the above object will be described with reference to FIG. 1 corresponding to the embodiment. High-temperature superconductor thin films 2 and 3 are formed on the planes on both sides of the part 1a, and the high-temperature superconductor thin films 2 and 3 on the upper and lower sides are formed on the step part 1.
In a, a constriction part (joint part) 4a of the high temperature superconductor thin film,
4b, and on top of the high temperature superconductor thin films 2 and 3 on the upper and lower planar sides,
Metal thin films 5 and 6 are formed, each of which exerts a proximity effect on this high temperature superconductor thin film.

<Function> A layer A is placed on the high temperature superconductor thin films 2 and 3 forming the electrode part.
By forming thin metal films rm5 and 6 such as u, AI, PL, etc., the coherent length of the films 2 and 3 in the electrode portion becomes longer. This eliminates magnetic flux from entering the electrode section,
The accompanying magnetic flux fluctuations eliminate 1/f noise and the like.

<Embodiment> Fig. 1 is a perspective view of an embodiment of the present invention.
shows an example in which the present invention is applied.

A step is provided on the surface of the MgO base Fi,1,
High-temperature superconductor thin films 2 and 3 made of YBCO are formed on the upper and lower surfaces of the step portion 1a, respectively.

The high temperature superconductor thin films 2 and 3 have a stepped portion 1a.
They are mutually joined by joint parts 4a and 4b formed by forming narrow parts in the same high-temperature superconductor thin film made of YBCO.

Aug films 5 and 6 are formed on the high temperature superconductor thin films 2 and 3 on the upper and lower plane parts, respectively.

According to the above-described embodiments of the present invention, the A u ) 39 films 5 and 6 formed above the high temperature superconductor thin films 2 and 3 forming the electrode portion of the DC-3QU ID are made superconducting by the proximity effect, The coherent length of the electrode part is 1000
About a person. As a result, magnetic flux is prevented from entering the electrode portion, and noises such as magnetic flux fluctuation, l/f noise, or magnetic flux flow caused by this are eliminated.

Next, a manufacturing method of the above embodiment of the present invention will be described.

FIG. 2 is an explanatory diagram of the manufacturing procedure.

First, as shown in FIG. 2(a), a flat substrate 10 of an appropriate thickness is prepared, and a resist is uniformly applied to its surface. Leave one side and remove the other side with approximately the center as the border.

Next, by etching the surface of the substrate 10 by Ar ion milling using the remaining resist film 11 as a mask, the substrate 10 having the stepped portion 1a as shown in FIG. 2(C) is etched.
get.

Then, as shown in FIG. 2(d), YBC is formed by sputtering or the like from above the substrate 1 having such a stepped portion 1a.
An O thin film 12 is uniformly formed. At this time, as shown in Fig. 3, which shows an enlarged view of the main part in this state, the obtained YBCOI
The upper and lower membranes of the membrane 12 are weakly connected at the stepped portion 1a of the membrane plate 2.

In this state, YBCO)! An Au thin film is formed on the film 12 by sputtering or the like. At this time, as shown in FIG. 4, the substrate 1 is tilted with respect to the traveling direction of sputtered particles from the target 41.

As a result, as shown in FIG. 2(e), the step 1a becomes a shadow, and the Au thin film becomes YBC in the step 1a.
There are parts that are not formed on the O thin film 12, and the Au thin films 5 and 6 are formed on the YBCO thin film 12 in the upper and lower planar parts.

Next, as shown in the plan view in FIG. 2(f), a pattern of C-3QUID is formed on the resist 15 so that the upper and lower YBCO thin films 12 are bonded at two places at the stepped portion 1a. Using this as a mask, the Au thin film and YBCOI film are etched by Ar ion milling. As a result, the DC-3QUID element shown in FIG. 1 is obtained.

Note that in the above embodiments, the high temperature superconductor thin films 2 and 3
Of course, it is possible to use not only the YBCO thin film but also other high-temperature superconductor thin films, and the material of the substrate 1 may be any material that has good compatibility with the high-temperature superconductor used. whatever. In addition to Au, there are other metals that exhibit the proximity effect, such as AI and Pt, and it goes without saying that these may also be used.

<Effects of the Invention> As explained above, according to the present invention, the high-temperature superconductor thin films on the upper and lower sides with the stepped portion in between are weakly bonded to each other at the stepped portion, and the upper and lower side Since a metal thin film exhibiting a proximity effect was formed on each high-temperature superconductor thin film in the plane part of the metal thin film, the coherent length of the high-temperature superconductor thin film in the electrode part was
If you use , it will be about 1000 times longer due to discoloration.

As a result, hysteresis due to the intrusion of magnetic flux into the film of the electrode portion is eliminated, and 1/f noise due to fluctuations in the magnetic flux trapped within the film is reduced.

Furthermore, when resistance appears due to magnetic flux movement or heat generation occurs, the metal thin film on the high-temperature superconductor thin film acts as a heat sink and has the effect of suppressing self-heating.

[Brief explanation of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is an explanatory diagram of its manufacturing procedure, and FIGS. 3 and 4 are supplementary explanatory diagrams of steps in the manufacturing procedure. 1...Substrate 1a...Step part 2.3...High temperature superconductor thin film 4a, 4b...Joint part 5.6...Au thin film

Claims (1)

[Claims] A high-temperature superconductor thin film is formed on the surface of a substrate provided with a step, on both planes across the step, and the high-temperature superconductor thin film on the upper and lower sides of the step is formed at the step. Metal thin films are formed on the high-temperature superconductor thin films in the upper and lower plane parts, which are connected to each other by the narrowed portions, and exert a proximity effect on the high-temperature superconductor thin films, respectively. ,
SQUID device using proximity effect.