JPH084157B2 - Josephson device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a Josephson element that can be applied to, for example, a high-speed switching element, SQUID, millimeter wave detection, etc., and a manufacturing method thereof. The present invention relates to a Josephson element of a type in which superconductors are connected by a weakly-bonded portion formed of a superconducting material and a manufacturing method thereof.

<Prior Art> The general structure of a Josephson element is as shown in FIGS. 5 to 9, and in each case, two superconductors 1 and 2 are connected by a weak coupling portion 3. The type shown in Fig. 5 is a type called superconducting microbridge.
1, 2 and the weakly coupled portion 3 are integrally formed of the same superconducting material. What is shown in FIG. 6 is a type called a plane type Josephson junction, and a weak coupling portion 3 made of a superconducting material is inserted between two superconductors 1 and 2. What is shown in FIG. 7 is called a thin film tunnel junction, in which a thin insulating film to be the weak coupling portion 3 is formed on the surface of one superconductor 1. Further, the one shown in FIG. 8 is called a sandwich type Josephson junction, and the weak coupling portion 3 is also formed by an insulator. Furthermore, what is shown in FIG. 9 is a type called a quasi-plane type Josephson junction, in which the surface of one superconductor 1 is superposed with the other superconductor 2 via an insulator 10, and these are made of a superconducting material. They are connected by the weakly connected portion 3.

<Problems to be Solved by the Invention> The superconductor 1 of each structure Josephson device as described above
When a current is applied between the two, its IV characteristic becomes as shown in FIG. Here, in order to make the characteristics of the Josephson device uniform, it is necessary to make the critical current value I _c in FIG. 10 constant. The value of I _c is determined by, for example, the cross-sectional area w × t of the weak coupling portion 3 in the type of FIG. 6, but in order to keep the value of I _c within a proper range, the dimensions of w and t are It is necessary to process with a precision on the order of submicrons, and conventional processing methods cannot suppress variations in I _c within a desired range, which is a factor that greatly reduces the yield of devices.

By the way, the recently discovered ceramics high-temperature superconductor has attracted attention because it can make a Josephson element that operates at liquid nitrogen temperature.However, since it is ceramics, it has poor workability and the above problems are more serious. It has become.

The present invention has been made in view of the above, and the critical current value I _c
Of the Josephson element, which can be changed as required, and the critical current value I _c without high-precision machining.
It is an object of the present invention to provide a method for manufacturing a Josephson device capable of setting a desired value.

<Means for Solving Problems> In order to achieve each of the above-mentioned objects, the first invention is, as shown in FIG. 1 corresponding to an embodiment, a superconducting material between two superconductors 1 and 2. In the element formed by connecting the weakly-bonded portions 3 formed in 1., a substance (for example, an Al ₂ O ₃ film) that diffuses into the weakly-bonded portions 3 by heating and reduces its critical current value I _c ) 4 is connected and arranged, and a heater (for example, Ta film) 5 for heating is thermally contacted.

A second invention is a method of manufacturing a Josephson element, which will be described with reference to FIG. 1 which also corresponds to the embodiment. Two superconductors 1 and 2 are connected by a superconducting material. Weakly bonded portion 3 is formed, and then weakly bonded portion 3 is formed.
When a substance (for example, an Al ₂ O ₃ film) 4 that diffuses into the weakly coupled portion 3 by heating and destroys the superconductivity of the diffusion region is brought into contact with the current, a current is passed between the two superconductors 1 and 2. Current is measured and the IV characteristic is measured, the weakly bonded portion 3 is locally heated to sequentially increase the diffusion region of the substance 4 to thereby increase the critical current value I _c in the IV characteristic. Is characterized by adjusting to a desired value.

<Action> In YBCO and LSCOS ceramic superconductors, the superconductivity is destroyed by diffusing Al, Al ₂ O ₃ , Si, SiO _{2 and the} like. It is known that

According to the Josephson element having the structure of the first invention, the heating by the heater 5 causes a substance such as Al ₂ O ₃ to diffuse into the weakly coupled portion 3 to increase the superconducting breakdown region thereof.
The cross-sectional area of the weakly coupled portion 3 having superconductivity can be reduced, and the I _c of the device can be appropriately reduced as necessary.

According to the manufacturing method of the second aspect of the present invention, the IV characteristic of the device is measured while diffusing the superconducting destructive substance in the weakly coupled portion 3 made of the superconducting material having an appropriate cross-sectional area, and the I _c value is a desired value. By stopping the diffusion at that time, a Josephson device having a constant Ic value can be obtained.

<Examples> Examples of the present invention will be described below with reference to the drawings.

 FIG. 1 is a perspective view showing the structure of the embodiment of the present invention.

An Al ₂ O ₃ film 4 is formed on the upper surface of the weakly-coupling portion 3 of the superconducting microbridge structure in which the superconductors 1 and 2 and the weakly-coupling portion 3 are integrally formed of the same superconducting material. Is formed with a Ta film 5. Lead wires 6 and 7 are connected to both ends of the Ta film 5 via electrodes (not shown).

The above structure can be manufactured by the following method.

A YBCO thin film was formed on a YSZ (yttria-stabilized zirconia) substrate by rf sputtering, and a superconducting microbridge structure consisting of superconductors 1 and 2 and weakly coupled portions 3 was formed by photolithographic wet etching. Formed. Here, the size of the weak coupling portion 3 is 10 μm × 10 μm,
The thickness was 2 μm. The pattern forming may be performed by an ion implantation method or the like.

Next, by rf sputtering as well, Al is formed on the weakly coupled portion 3.
_{A 2} O ₃ film 4 and a Ta film 5 were further laminated thereon. And Ta
Electrodes and lead wires 6 and 7 were connected to both ends of the membrane 5.

Now, when the element having the above structure is dipped in liquid nitrogen and a current is passed between the superconductors 1 and 2 to measure its IV characteristic, for example, it is as shown by the solid line in FIG. Then, in that state, when a current is passed between the lead wires 6 and 7 for several seconds to heat the Ta film 5, Al ₂ O ₃ is allowed to reach a predetermined depth in the Y-B-C-O film of the weak coupling portion 3. As a result of spreading and destroying its superconductivity,
Substantially, the thickness t of the weakly-bonded portion 3 became small, the cross-sectional area became small, the IV characteristic as shown by the broken line in FIG. 2 was obtained, and the critical current value decreased. When the Ta film 5 is further energized,
As shown by the alternate long and short dash line in the figure, the critical current value further decreased.
As described above, the magnitude of the critical current value can be controlled by the time of energizing the Ta film 5, and by stopping the energization at the time when the desired critical current value is obtained, the characteristics of the bar are obtained. A Josephson device is obtained.

The measurement of the IV characteristics and the energization for heating may be performed in parallel or alternately. When performing measurement and heating energization in parallel, the element with the structure shown in Fig. 1 is placed in a container filled with nitrogen gas, and the container is immersed in liquid nitrogen to prevent rapid boiling of liquid nitrogen due to heating. it can.

Further, as the structure of the element, in addition to the superconducting microbridge structure shown in FIG. 1, the planar Josephson junction type and the quasi-planar Josephson junction type shown in FIGS. 6 and 9 can be applied. In either case, a superconducting destructive substance may be placed in contact with the weakly bonded portion 3 made of superconducting material, and a heater for heating the contact portion may be thermally connected.

As the superconducting material of the weakly coupled portion 3, similar results could be obtained not only with the YBCO system ceramics but also with the LSCOS system ceramics. Further, as the superconducting destruction material, in addition to Al ₂ O ₃ mentioned above, SiO ₂ , Al and Si show almost the same result, and use of Sr, Ti, Fe and oxides thereof which are not efficient can do.

Next, a Josephson device having a superconducting microbridge structure and another example of the manufacturing method thereof will be described. FIG. 3 is an explanatory diagram thereof. In this example, first, a uniform Y-B-C-O film 11 is formed on a YSZ substrate or the like. Then that Y
On the -B-C-O film 11, as shown in FIG. 3, Al ₂ O ₃ films 4, 4 or other superconducting destructive substance films are formed. Then, by heating the Al ₂ O ₃ films 4 and 4, the YBCO film is formed.
11 is diffused in the film thickness direction, and Y-B- under the Al ₂ O ₃ films 4 and 4 is formed.
The CO film 11 is insulated to obtain a super electric microbridge structure. Then, by further heating the Al ₂ O ₃ films 4 and 4,
₂ O ₃ is diffused in the film surface direction, and the substantial width w of the weakly bonded portion 3 is
Is reduced to reduce the cross-sectional area, and heating is stopped when the desired IV characteristic is obtained.

This method is applicable to the planar and quasi-planar Josephson junction types shown in FIGS. 6 and 9. An example of application to a device having a quasi-planar Josephson junction is shown in FIG. In this example, the superconductor 1 and the insulator 10 thereon are
A uniform Y-B-on the superconductor 2 stacked via
A C—O film 11 is formed, and an Al ₂ O ₃ film 4 and a Ta film 5 are laminated above the Y—B—C—O film 11 except for the central portion, and further on both ends of the Ta film 5. Lead wires 6 and 7 for supplying a heating current are connected. In this structure, a current is passed through the lead wires 6 and 7 to heat the Ta film 5 to
₂ O ₃ diffuses in the film thickness direction of the Y-B-C-O film 11,
The portion of the —C—O film 11 other than the central portion is insulated. That is, the central portion where the superconducting property remains forms the weak coupling portion 3. Then, similarly, by further energizing and heating, Al ₂ O ₃ diffuses in the film surface direction of the Y-B-C-O film 11,
The substantial width w of the weakly coupled portion 3 can be appropriately narrowed.

<Effects of the Invention> As described above, according to the Josephson element of the present invention, the weak coupling portion made of a superconducting material destroys its superconductivity by heating, and the substantial cross-sectional area of the weak coupling portion is reduced. Since the substance for reducing the superconducting critical current is connected and arranged, and the heater for heating is thermally connected, its IV characteristic can be appropriately changed as needed, The versatility above increases.

Further, according to the manufacturing method of the present invention, the IV characteristics can be adjusted to a desired state without enhancing the processing accuracy of the weakly coupled portion, so that the yield is improved and the manufacturing cost is reduced. be able to.

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of an embodiment of the present invention, FIG. 2 is an explanatory view of its operation, FIGS. 3 and 4 are illustrations of another embodiment of the present invention, and FIGS. Is a perspective view showing a structural example of a conventional Josephson element, and FIG. 10 is a graph showing its IV characteristic. 1,2 …… Superconductor 3 …… Weak junction 4 …… Al ₂ O ₃ film 5 …… Ta film 6,7 …… Lead wire 10 …… Insulator 11 …… YBCO film

Claims (4)

[Claims] 1. A device in which two superconductors are connected by a weakly-bonded portion formed of a superconducting material, and the weakly-bonded portion is diffused into the weakly-bonded portion by heating, and its superconducting critical current value. Is placed in contact with a substance that lowers, and a heater for heating is thermally contacted,
Josephson device. 2. The superconducting material of the weakly bonded portion is a ceramics superconducting material, and the substance placed in contact with the weakly bonded portion is A.
The Josephson device according to claim 1, wherein the Josephson device is 1, Al ₂ O ₃ , Si or SiO ₂ . 3. A superconducting material is used to connect between two superconductors to form a weakly-bonded portion, and then the weakly-bonded portion is heated to diffuse into the weakly-bonded portion so that the superconductivity in the diffusion region is increased. In the state of contacting a substance that destroys, the current is passed between the two superconductors and the IV characteristics are measured, and the weakly coupled portion is locally heated to sequentially diffuse the diffusion regions of the substance. A method for manufacturing a Josephson device, characterized in that the critical current value in the IV characteristic is adjusted to a desired value by increasing the value. 4. The weakly-bonded portion is formed of a ceramics superconductor, and the substance is brought into contact with the weakly-bonded portion.
The method for manufacturing a Josephson device according to claim ₃ , wherein Al, Al ₂ O ₃ , Si or SiO ₂ is used.