Superconducting Contact Structure

The present invention relates to a superconducting contact structure formed by an oxide superconductor and a metal superconductor in a superconductor IC or the like.

An oxide represented by yttrium-barium-copper oxide (YBCO) as described in Physical Review Letters, Vol. 58, P. 908 serves as a superconductor at a relatively high critical temperature. YBCO as a superconductor has a critical temperature of 97 K. A circuit combining this oxide superconductor and a metal superconductor represented by niobium may be arranged. In this case, a superconducting contact between these two superconductor materials must be established.

FIG. 1 shows a typical conventional example of a superconducting contact structure made of two metal superconductors. A superconductor 14 is brought into contact with a superconductor 12 through a contact hole in an insulating film 13, thereby establishing a superconducting contact.

A critical, largely anisotropic, superconducting current in YBCO is described in Japanese Journal of Applied Physics, Vol. 26, P. L1248. The superconducting critical current density in a direction perpendicular to the layer is about one percent of that in a direction parallel to the layer. When an oxide superconductor is formed so that the layer is parallel to a substrate 11, it is necessary to supply a large amount of current in a direction parallel to the substrate 11. A small superconducting current is not supplied in the vertical direction, and therefore good superconducting contact cannot be obtained. In order to prevent a decrease in the critical current density, the contact area is 100 times the cross-sectional area of the superconductor 12. However, such a large contact hole is not preferable in view of a need to make things smaller in order to achieve a greater; integration density.

It is an object of the present invention to provide a good superconducting contact between an oxide superconductor and a metal superconductor.

In order to achieve the above object of the present invention, a superconducting contact structure is provided between an oxide superconductor and a metal superconductor. The oxide superconductor has a recess in a surface which is in contact with the metal superconductor.

FIG. 1 is a cross-sectional view showing a conventional superconducting contact structure;

FIG. 2 is a cross-sectional view showing a superconducting contact structure according to an embodiment of the present invention;

FIG. 3 is a cross-sectional view showing a superconducting contact structure according to another embodiment of the present invention; and

FIG. 4 is a cross-sectional view showing a superconducting contact structure according to still another embodiment of the present invention.

FIG. 2 shows a superconducting contact structure according to an embodiment of the present invention. Referring to FIG. 2, a 500-nm thick and 5 .mu.m wide superconductor film 12 consisting of YBCO as an oxide superconductor and having a layer structure parallel to a substrate 11 is formed on the substrate 11. A recess 15 having a width of 2 .mu.m, length of 5 .mu.m, a depth of 450 nm and a direction perpendicular to a longitudinal direction of the superconductor film 12 is formed in the superconductor film 12. A 500-nm thick insulating film 13 composed of silicon dioxide is formed on the superconductor film 12. A contact hole 16 having a width of 3 .mu.m and a length of 5 .mu.m is formed in the insulating film 13 so as to include the recess 15 of the superconductor film 12. A superconductor film 14 consisting of niobium, as a metal superconductor is formed on the surface of the insulating film 13 and over the inner surfaces of the contact hole 16, and down into the recess 15 so as to fill the entire recess 15 with the metal superconductor and is brought into contact with the superconductor film 12.

With the above structure, superconductor electrons flowing along the oxide superconductor film 12 can flow directly into the metal superconductor film 14. Since critical current density anisotropy is not present in the metal superconductor film 14, the superconducting critical current density is increased at the contact portion between the superconductor films 12 and 14.

At least 90% or more of the horizontal critical current of the superconductor film can be obtained at the contact portion, although the small contact hole 16 has a size of 5.times.3 .mu.m. When the width of the recess 15 of the superconductor film 12 is increased to dimensions close to the width of the contact hole 16, the current densities of the structure of the embodiment and is further increased.

When the structure of the above embodiment is used, the critical current of the superconducting contact portion between the oxide superconductor film 12 and the metal superconductor film 14 can be increased and the contact area can be reduced.

FIG. 3 shows a superconducting contact structure according to another embodiment of the present invention. In this embodiment, a recess 17 in of a superconductor film 12 is tapered. Other arrangements of this embodiment are the same as those in FIG. 2. Since the recess 17 of the superconductor film 12 is tapered, a contact area is increased between the superconductor film 12 and a superconductor film 14 thus obtaining a more rigid superconducting contact structure.

In order to taper the hole 17, for example, a mask pattern is tapered. Etching is performed by using a Cl.sub.2 gas or the like so as to transfer the taper to the underlying superconductor film 12.

FIG. 4 shows a superconducting contact structure according to still another embodiment of the present invention. Unlike in FIGS. 2 and 3, FIG. 4 is a cross-sectional view of the superconductor film 12 in a direction perpendicular to the longitudinal direction thereof. In this embodiment, a plurality of recesses 18 are formed in a superconductor film 12. Since the superconductor film 12 has the plurality of recesses 18, the contact area between the superconductor film 12 and a superconductor film 14 can be increase in he horizontal direction. Therefore, a critical current density of the superconducting contact portion can be further increased.

In the superconducting contact structure according to the present invention, as has been described above, good superconducting contact having a large superconducting critical current can be achieved between a metal superconductor film and an oxide superconductor having a high critical current anisotropy level.