JPH04105372A - Microbridge type Josephson junction device - Google Patents

Abstract

PURPOSE:To enable the title micro-bridge type Josephson junction element in excellent durability etc., to be obtained by a method wherein a superconducting film is formed on a substrate and then a metallic film s laminated on a bridge part. CONSTITUTION:An Er-Ba-Cu-O type superconducting film 2 is formed on a substrate 1 by vapor-depositing and heat-treating Er, Ba and Cu from three each of electron beam source at 10<-4>-10<-3>Torr in an oxygen containing atmosphere. Next, a metallic film 3 formed of Au by an RH vapor deposition process is resist-coated, exposed and developed and then the superconducting film 2 and the metallic film 3 are simultaneously etched away by ion-milling process using Ar to form the title micro-bridge type Josephson junction element. Besides, the width of the metallic film 3 is specified to exceed 1.5 times of the link length of the bridge part. Furthermore, the substrate 4 having the stepped part in the central part is used to form the superconducting film 2 and after performing the heat treatment process, the metallic film 3 is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a microbridge type Josephson junction element that can be used in superconducting transistors, infrared detectors, Josephson mixers, 5QUIDs, Josephson computers, and the like.

(Prior Art and its Problems) Conventionally, a microbridge type Josephson element made of an oxide superconducting material generally has the configuration shown in FIG. 4. That is, the superconducting film 2 has a microfabricated superconducting film 2 on a substrate 1, and an electrode 5 is placed on top of the superconducting film 2. The constricted part of the superconducting film in the center is the bridge part 4.

Typically, a microbridge type device is made of thin film layers, with two wide superconducting films (bank parts) connected by a narrow part. If the link length of the bridge section connecting the two bank sections is shorter than the coherent length, or at least about the same, then
Such a waist-type weak coupling in the film behaves as a Josephson device.

If it is difficult to create a short link length on a plane, the fifth
A method is also used in which a substrate having a step as shown in the figure is used and the bridge portion 4 is formed using the step (quasi-planar microbridge).

When making a microbridge-type Josephson junction device using an oxide superconductor, the link length of the bridge section should theoretically be about several tens of people, and even with current electron beam and X-ray lithography technology, The limit is a few hundred people at most.

Actually, microbridge type Josephson junctions using oxide superconductors such as YBa*Cu-0-a often have a link length of about 4 to 20 μm. This is extremely large compared to several tens of people, and it is thought that weak bonds are formed by grain boundaries. Therefore, it is a material that deteriorates much more easily than Nb-based materials that do not have bonding through grain boundaries. Since the bank part is wider than the bridge part, it is determined by the average sum of many grain boundaries.
Even if the edges are severely damaged, overall deterioration is not a problem.

However, since there are only a few grains in the joint with respect to the bridge width, the deterioration of individual grain boundaries directly affects the deterioration of the joint.

Furthermore, when etching a superconducting film using microfabrication technology, the microbridge portion is narrow in width and is therefore greatly affected by side etching. That is, in dry etching, it is easily affected by the impact of etching particles, and in wet etching, it is easily affected by a strong etching solution.

Therefore, there are problems in that the superconducting properties of the microbridge section tend to deteriorate and the bridge section properties tend to disappear due to compositional fluctuations and the like.

In particular, the structure of oxide superconductors tends to change depending on the amount of oxygen in them, and even if the ratio of other elements remains the same, even if the ratio of oxygen changes, it is a material that easily transforms into a semiconductor or an insulator.

Since the material is easily deteriorated in this way, there is also the problem that the durability of the element is poor.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above problems of the prior art and provide a microbridge type Josephson junction element with excellent durability.

(Means for solving problems) The above objects are achieved by the present invention as described below.

That is, the present invention is a microbridge type Josephson junction element characterized in that a superconducting film is formed on a base and a metal film is laminated on a bridge portion.

(Function) According to the present invention, since the superconducting film is formed on the base and the metal film is laminated on the bridge portion, it is most convenient when performing microfabrication or operating the device. It is possible to suppress deterioration of the bridge portion, which is prone to deterioration. This is particularly effective when using an oxide superconductor that utilizes weak bonds due to grain boundaries, since the bridge portion is more likely to deteriorate.

Especially the bridge part is made of Au, Pt, Pd, Re, I
It is also possible to suppress changes in the amount of oxygen by covering it with a metal that allows oxygen to pass through, such as n or Cu.

Further, by making the width of the metal film 1.5 times or more, preferably 2 to 2000 times, the link length of the bridge portion, deterioration of the bridge portion can be minimized.

Furthermore, even when a microbridge is efficiently formed using a step, the bridge can be protected in the same way.

Further, by making the step difference approximately 1 to 5 times the thickness of the superconducting film, it is possible to obtain a microbridge type Josephson junction element without impairing the protective effect of the bridge portion.

In the present invention, the superconducting film formed on the substrate is preferably an oxide, and when its chemical composition is expressed as A-B-C-O, A is La, Ce, Pr, Nd, P
m, Sm, Sc, Eu%Gd, Tb, Dy,
one or more elements selected from the group consisting of Ho, Er, Tm, Yb, Lu, Bi, Tl and Y, B is Ba
, Ce, one or more elements selected from the group consisting of Sr and Pb, C is ■, Ti%Cr, Mn, Fe, N
It is preferable that it is one or more elements selected from the group consisting of i, (:o, Ag, Cd, and Cu).

(Example) The present invention will be explained in more detail below using Examples.

Example 1 First, as shown in FIG. 1, a substrate 1 [5rTiO-(10
0) Er from three electron beam sources on the
Ba and Cu in an oxygen-containing atmosphere, 10-' to 10-
``Approximately 5,000 people deposited at Torr.The substrate temperature was approximately 4
The temperature was 00°C. Since the film was amorphous as deposited, it was heat-treated at approximately 900°C in oxygen gas to form Er-Ba-C.
A u-〇 type superconducting film 2 was fabricated (Fig. 1a).

As a result of X-ray diffraction and electron microscope observation, the superconducting film 2 thus obtained was found to be a polycrystalline oriented film in which the C axis of the orthorhombic structure was aligned perpendicular to the film surface.

Next, a metal film 3 (using Au) was formed using the RHMIF method.
After forming a film with a width of 5 mm, resist coating, exposure and development were performed, and the superconducting film 2 and metal film 3 were simultaneously etched by ion milling using Ar to form a microbridge-type Jiji Sefson element including a bridge portion 4 of 8 μm x 8 μm. (Figure 1b).

Next, a Cr--Au electrode 5 was attached (FIG. 1c), and the electrical resistance was measured by the four-probe method, and the electrical resistance became O at about 80°C. Jc was 20 and lO'AlCrd.

Furthermore, when 50 GHz millimeter waves were irradiated, a Shapiro step was observed, indicating that a Josephson junction was formed.

When a device without a metal film on the bridge part was created in the same way, Tc was set to 30, Jc was set to 20, and the output was 10'A/cm.
”, but no Shapiro step was observed even with millimeter wave irradiation.

Example 2 A device as shown in FIG. 2 was fabricated using an MgO single crystal substrate 1 having a step of 1 μm in the center of the substrate.

The superconducting film 2 is a Y-
It is a Ba-Cu-0 film. Conditions are substrate temperature 400℃
The acceleration voltage of the cluster ion beam gun for u is 4KV.
, ionization current 200mA, acceleration voltage of cluster ion beam gun for Ba is IKV, ionization current 50mA
, the acceleration voltage of the cluster ion beam gun for Y is 2K.
V, the ionization current is 100 mA, and the deposition rate is Y:
The molar ratio of Ba:Cu was controlled to be 1:2:3. The film thickness is soo.

As a person.

After heat treatment at 900° C. for 1 hour, a metal film 3 with a width of 1 mm was formed by ion beam sputtering. Material is PT
was used.

Fine processing was carried out in the same manner as in Example 1. The width of the bridge part is 101
．． Lm, length is 301. It was set to Lm. A 4-terminal measurement showed Tc=85K.

Next, in order to observe the photoresponse, this element was cooled to 4.5K and the bridge section was irradiated with a He-Ne laser of 5mW at a frequency of 100Hz for 10μm and 5ec for a period of time, a voltage signal was obtained and a light-voltage conversion of 8 was obtained. I understand.

Example 3 B1-3r-Ca-
CuII was formed. After post-heat treatment (820° C., 2 hours), Au was deposited by mask evaporation, etched and processed as shown in FIG. In this way, a superconducting quantum interference device consisting of two Josephson junctions can be realized.

This device is cooled to 77 K using liquid nitrogen and operated as a quantum interference device. At this time, signals are detected through the extraction electrodes 21 and 22.

(Effects of the Invention) As explained above, the present invention provides the following effects.

(1) Since metal is laminated on the bridge portion, deterioration of the bridge portion, which is most likely to deteriorate during microfabrication or when operating an element, can be suppressed.

(2) Since oxide superconductors are particularly susceptible to deterioration due to the amount of oxygen, changes in the amount of oxygen can be suppressed by covering them with a metal that allows oxygen to pass through.

(3) Since the deterioration of the superconductor is suppressed, the yield and durability of the device are improved.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of Example 1, Figure 2 is a conceptual diagram of Example 2, Figure 3 is a conceptual diagram of Example 3, and Figure 4 shows the form of a conventional microbridge Josephson junction. FIG. 5 is a diagram showing a quasi-planar microbridge. 1: Substrate 2. 21, 22: Superconducting film 3: Metal film 4 Bridge portion 5: Electrodes 6.6a, 6b: Steps Fig. 1 (a) Fig. 2 Fig. 3 Fig. 4 Fig. 5

Claims (6)

[Claims] (1) A microbridge-type Josephson junction device, characterized in that a superconducting film is formed on a substrate and a metal film is laminated on a bridge portion. (2) The superconducting film is an oxide and has a chemical composition of A-B-C.
When expressed as -O, A is La, Ce, Pr, Nd, Pm
, Sm, Sc, Eu, Gd, Tb, Dy, Ho, Er,
One or more elements selected from the group consisting of Tm, Yb, Lu, Bi, Tl and Y, B is Ba, Ca, Sr and Pb
One or more elements selected from the group consisting of, C is V, Ti
, Cr, Mn, Fe, Ni, Co, Ag, Cd and Cu
Claim 1 is one or more elements selected from the group consisting of
Microbridge type Josephson junction device described in . (3) Metal film is Au, Pt, Pd, Re, In and Cu
Claim 1 is one or more metals selected from the group consisting of
Or the micro bridge type Josephson junction element according to 2. (4) The microbridge type Josephson junction element according to any one of claims 1 to 3, wherein the width of the metal film is 1.5 times or more the link length of the bridge portion. (5) The microbridge type Josephson junction device according to any one of claims 1 to 4, wherein the base body is a substrate having a step, and the step portion includes a superconducting film having a superconducting weak bond. (6) The microbridge type Josephson junction device according to claim 5, wherein the height of the step is about 1 to 5 times the thickness of the superconducting film.