JP2001308399A - Superconducting joint element and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a superconducting joint element and its manufacturing method where manufacturing is easy with less variation in characteristics. SOLUTION: A Yba2Cu3O7-x film 11 (0<=x<=0.6) is formed on a substrate 10 by a liquid-phase epitaxy method as a first superconductor film. Then an insulating protection film 12 is formed on the Yba2Cu3O7-x film 11, over which an Nb film is formed as a mask, and then the Yba2Cu3O7-x film 11 is ion-milled to form a V-shape groove 11a. After the mask is removed, an NdBa2Cu3O7-y film is formed on the insulating protection film 12 by a sputtering method as a second superconductor. A non-superconducting layer is formed on the slope of the V-shape groove 11a due to difference in lattice constant of the superconductor film. Then the NdBa2Cu3O7-y film is pattered into a specified shape.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting junction element used in a superconducting circuit operating at a high speed and a method for manufacturing the same, and more particularly to a superconducting junction element having a Josephson junction and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, various types of Josephson devices using an oxide superconductor have been manufactured. Most of these are formed by depositing an oxide superconductor thin film on an oxide single crystal substrate such as MgO or SrTiO ₃ and exhibiting a Josephson effect by grain boundary bonding, ramp edge bonding, or lamination bonding (Joseph's effect). Song junction)
Has been realized. In ramp edge joining and laminated joining,
An extremely thin insulating film (tunnel barrier film) is formed between the two superconducting conductor thin films. In the Josephson device having the ramp edge structure, in addition to the advantages that the junction area is small and the parasitic inductance is small, the characteristic that the critical current distribution width is narrow recently has been obtained.

FIG. 14 is a sectional view showing an example of a ramp edge junction type Josephson device. In this type of Josephson element, a lower superconductor film 51 and an insulating film 52 are formed on a dielectric substrate 50, and these are ion-milled to form inclined surfaces. Then, a tunnel barrier film 53 made of an insulator is formed on the inclined surface, and an upper superconductor film 54 is formed on the opposite side of the lower superconductor film 51 with the tunnel barrier 53 interposed therebetween.

[0004]

In the above-described ramp-edge junction type Josephson device, at least three thin films of the lower superconductor film 51, the tunnel barrier film 53, and the upper superconductor film 54 must be deposited. It is difficult to produce an extremely thin tunnel barrier film 53 uniformly and with good reproducibility. In general, an ion milling method is used to form the inclined surface, but since the inclined surface is exposed to high-energy ions in a vacuum in the process, defects are basically easily introduced into the inclined surface, This is a cause of variations in tunnel characteristics.

Further, when the critical current value of the Josephson junction is set, a method of controlling by the junction area is used. In the ramp edge junction, one side of the junction area is determined by the thickness of the lower superconductor film 51. For this reason, if the critical current value is largely changed, the wiring pattern must be changed significantly, and circuit design becomes difficult. In fact, SF
The critical current value Ic of the Josephson junction in the Q (single flux quantum) circuit depends on the circuit,
(Josephson transmission line “Josephson Transmission L
ine "), the buffer requires a critical current value Ic that is at least twice as large as the critical current value Ic of the junction, while the buffer requires a critical current value Ic of about half, and a critical current value Ic of all the junctions requires a junction that differs by a factor of about 5 Is done.

[0006] The SFQ circuit is configured using a superconducting loop including a Josephson junction and an inductance as a basic unit. In the Josephson junction, good characteristics can be obtained by adopting the ramp edge structure as described above. On the other hand, it is necessary to prepare a wiring having a small inductance. In order to realize this, a microstrip line structure in which a wiring (upper wiring) is provided above a ground plane is preferable, and fabrication of a circuit constituted by such a Josephson junction and a microstrip line structure has been attempted. . For example, a structure is conceivable in which a ground plane (conductive film) is formed on the back surface of the substrate, and the lower superconductor film and the ground plane are electrically connected via via holes. However, such a structure has a disadvantage that circuit design becomes difficult because stray inductance is generated at the via hole. Further, in the Josephson element shown in FIG. 14, although the substrate temperature at the time of forming the upper electrode has a large effect on the bonding characteristics, there is a disadvantage that it is difficult to control the substrate temperature at the time of film formation.

Accordingly, it is an object of the present invention to provide a superconducting junction element which is easy to manufacture and has little variation in characteristics, and a method for manufacturing the same. Another object of the present invention is to
An object of the present invention is to provide a superconducting junction element having a small stray inductance between a ground plane and a Josephson junction, and a method for manufacturing the same.

[0008]

The superconducting junction element according to the first aspect of the present invention is characterized in that two kinds of oxide superconductors having mutually different lattice constants are connected to realize a tunnel barrier at an interface. I do. When two types of oxide superconductors having different lattice constants are connected, a lattice distortion occurs at the joint surface due to a difference in crystal lattice, and a non-superconducting layer is formed at that portion. The non-superconducting layer and two kinds of oxide superconductors sandwiching the non-superconducting layer constitute a Josephson element. Therefore, in the present invention, it is not necessary to form an extremely thin tunnel barrier film, the manufacture is easy, and the characteristics are uniform.

In this case, the two kinds of oxide superconductors are:
It is necessary that the difference between the lattice constants is small. As one oxide superconductor, for example, YBa ₂ Cu ₃ O _7-x ,
GdBa ₂ Cu ₃ O _7-x and YbBa ₂ Cu ₃ O
Any one selected from the group consisting of _7-x (provided that 0 ≦ x ≦ 0.6) can be used. As the other oxide superconductor, for example, NdBa ₂ Cu ₃ O _7-y , Sm
Any one selected from the group consisting of Ba ₂ Cu ₃ O _7-y and EuBa ₂ Cu ₃ O _7-y (where 0 ≦ y ≦ 0.6) can be used.

[0010] The superconducting junction element according to claim 3 of the present application comprises:
A first substrate having a substrate and an inclined surface formed on the substrate;
A superconducting film, an insulating protective film formed in a region on the first superconducting film excluding the inclined surface, and having a lattice constant different from a lattice constant of the first superconducting film; A second superconductor film formed in a region from above the insulating protective film to an inclined surface of the first superconductor film.

In the present invention, a second superconducting film is formed on an inclined surface of the first superconducting film, and a non-superconducting layer formed at a junction between the two superconducting films due to a difference in lattice constant. Is a tunnel barrier. In this case, when the second superconducting film is formed in a wiring pattern above the first superconducting film via an insulating protective film, the first superconducting film functions as a brand plane and has a microstrip line structure. . That is, a microstrip line structure can be obtained without forming a via hole. Thereby, it has a Josephson junction and a microstrip line structure,
A superconducting junction element with small stray inductance can be obtained.

According to a sixth aspect of the present invention, there is provided a method for manufacturing a superconducting junction element, comprising the steps of: growing a first superconducting film on a substrate by liquid phase epitaxy; and insulating the first superconducting film on the first superconducting film. Forming a protective film, forming a mask having a predetermined pattern on the insulating protective film, ion milling the insulating protective film and the first superconducting film to form the first superconducting film; Forming an inclined surface, removing the mask, forming a second superconductor film having a different lattice constant from the first superconductor film on the entire upper surface of the substrate, And 2) patterning the superconductor film.

In the present invention, a first superconductor film is grown on a substrate by a liquid phase epitaxy method. Thus, a single-crystal superconductor film having better crystallinity than the vapor-phase method is formed. Thereafter, an insulating protection film is formed on the first superconductor film, and a mask having a predetermined pattern is formed thereon. Then, the protection insulating film and the first superconductor film are ion-milled to form an inclined surface on the first superconductor film.
Next, after removing the mask, a second superconductor film is formed on the entire upper surface of the substrate. Thereby, two types of superconductor films having different lattice constants on the inclined surface are connected to each other, and a non-superconducting layer is formed.

As described above, in the present invention, it is not necessary to form an extremely thin insulating film between two superconductor films, and a Josephson junction can be easily formed.
Further, the first superconductor film is used as a ground plane,
A microstrip line structure using the superconducting film of FIG. Accordingly, it is not necessary to form a via hole connecting the upper wiring and the ground plane, and a superconducting junction element with small floating inductance is manufactured.

[0015]

Embodiments of the present invention will be described below with reference to the accompanying drawings. (First Embodiment) FIG. 1A is a sectional view showing a superconducting junction element according to a first embodiment of the present invention, and FIG. 1B is a schematic plan view thereof.

The dielectric substrate 10 is made of MgO single crystal,
A first superconductor film is formed on the entire upper side of the dielectric substrate 10.
YBa_TwoCu_ThreeO_7-xFilm 11 (where 0 ≦ x ≦ 0.
6) is formed. This YBa_TwoCu_ThreeO_7-xMembrane 1
The c-axis of 1 points in a direction perpendicular to the substrate surface. YBa_Two
Cu_ThreeO_7-xThe membrane 11 is provided with a step, and the lower step
On the surface, a lower superconductor electrode (hereinafter referred to as “lower electrode”)
Ud) as NdBa_TwoCu _ThreeO_7-yThe film 15b (however, 0
≦ y ≦ 0.6) is formed. Also, YBa_TwoCu
_ThreeO_7-xFor example, the upper surface of the film 11 has a width of 1 μm and a length of 1 μm.
V-shaped groove 11a having a depth of 20 μm and a depth of 0.4 μm is provided.
Have been.

And, YBa_TwoCu_ThreeO_7-xOn membrane 11
On the surface of the step, SrTiO_ThreeThe insulating protective film 12 made of
A second insulating protective film 12 is formed on the insulating protective film 12.
NdBa as a superconductor film_TwoCu_ThreeO_7-yThe film 15a is shaped
Has been established. This NdBa _TwoCu_ThreeO_7-yOf the film 15a
The c-axis points in a direction perpendicular to the substrate surface. NdBa_TwoC
u_ThreeO_7-yThe film 15a has an upper part as shown in FIG.
Superconductor electrodes (hereinafter referred to as “upper electrodes”) and wiring
It is patterned into a shape.

The insulating protective film 12 is made of YBa_TwoCu_ThreeO_7-x
A hole reaching the V-shaped groove 11a of the film 11 is formed,
YBa in the V-shaped groove 11a_TwoCu_ThreeO_7-xCrystal and Nd
Ba _TwoCu_ThreeO_7-yThe crystal is joined. This YB
a_TwoCu_ThreeO_7-xCrystal and NdBa_TwoCu_ThreeO_7-yWith crystals
Of the Josephson device due to the difference in lattice constant
Functions as a tunnel barrier. In addition, YBa_TwoCu_ThreeO
_7-xThe lattice constant of the crystal is 11.688 °, NdBa_TwoCu
_ThreeO_7-yHas a lattice constant of 11.794 °.

YBa ₂ Cu ₃ O _7-x film 1 serving as lower electrode
5b, two extraction electrodes 1 made of gold (Au)
6c and 16d are formed, and NdBa ₂ C as an upper electrode is formed.
Two extraction electrodes 16a and 16b made of gold are also formed on the u ₃ O _7-y film 15a. This enables measurement of current-voltage characteristics by a four-terminal method. That is, a current flows between the electrodes 16a and 16d, and the voltage between the electrodes 16b and 16c is measured.

FIG. 2 shows a superconducting junction element of the present embodiment.
FIG. 3 is a diagram showing a value circuit. YBa_TwoCu_ThreeO_7-xMembrane 11
The ground plane 21 is formed, and the V-shaped groove 11a is formed.
The formed Josephson junction 22 is the ground plane 2
1 and wiring (NdBa_TwoCu _ThreeO_7-yTo the membranes 15a) 23
It has a directly bonded microstrip line structure. This
As described above, in the superconducting junction element of the present embodiment, the Josephson
YBa whose Fuson junction is the first superconductor film_TwoCu_ThreeO
_7-xNdBa as film 11 and second superconductor film _TwoCu_Three
O_7-yRealized at the interface with the film 15a, YBa_TwoCu_ThreeO
_7-xFloating because the film 11 is the ground plane
Inductance is extremely small, making circuit design easy
You.

FIGS. 3 to 6 show a superconducting junction element of the present embodiment.
It is sectional drawing which shows the manufacturing method of a child in order of a process. First, FIG.
As shown in (a), a dielectric substrate made of MgO single crystal
10 was prepared, and by liquid phase epitaxy (LPE) method,
The surface of the dielectric substrate 10 has (001) -oriented YBa_TwoCu_Three
O_7-xYBa made of single crystal_TwoCu_ThreeO_7-xMembrane 1
Grow slightly thicker than 0 μm. By liquid crystal epitaxy
YBa on the dielectric substrate_TwoCu_ThreeO_7-xGrow thick film
The method is described in detail in, for example, JP-A-7-33590.
Described. Then, the film thickness is adjusted to about 10 μm.
To YBa_TwoCu_ThreeO_{7- x}The surface of the film 11 is polished and
Flatten.

Thereafter, an SrTiO ₃ single crystal is deposited on the YBa ₂ Cu ₃ O _{7 -x} film 11 by rf magnetron sputtering to form an insulating protective film 12. Insulating protective film 12
Is MgO, La _0.65 Sr besides SrTiO _3
It may be formed of _0.35 Al _0.5 Ta _0.4 O ₃ or a copper oxide PrBa ₂ Cu ₃ O _7-x (0 ≦ x ≦ 0.6) which does not exhibit metal conductivity. With any of the materials, a single crystal film with a uniform orientation direction can be obtained by setting the substrate temperature to 630 ° C. using rf magnetron sputtering or laser evaporation. The thickness of the insulating protective film 12 is sufficient for insulation.
0.3 μm because the surface flatness of the thin film is preserved.
m is preferable.

Next, as shown in FIG. 3B, an rf magnetron sputtering method
Nb (niobium) film 13 serving as a mask during ion milling
Is formed to a thickness of about 0.2 μm, and Au is formed on the Nb film 13.
The film 14 is formed to a thickness of about 0.1 μm. Next, FIG.
As shown in (c), the width of the Au film 14 is reduced to about 1 by ordinary photolithography or a focused ion beam (FIB).
A slit 14a having a thickness of about 20 μm and a length of about 20 μm is provided, and the Au film 14 in the lower electrode forming portion is removed.

Next, as shown in FIG.
4 is used as a mask, the Nb film 13 is selectively etched by a reactive ion etching (RIE) device using CF ₄ gas, and a slit 13 a reaching the insulating protection film 12 is formed.
And an insulating protective film 12 in a lower electrode forming portion.
To expose. Next, as shown in FIG. 4B, YBa ₂ Cu _{3 is applied} to the insulating protective film 12 by Ar ion milling.
A slit 12a reaching the O _7-x film 11 is formed, and the insulating protective film 12 in the lower electrode forming portion is removed to remove
exposing the a ₂ Cu ₃ O _7-x film 11.

Next, as shown in FIG. 4C, the remaining Nb film 13 is removed by reactive ion etching using CF ₄ gas. In this example, the Nb film is used as a mask during ion milling, but a photoresist film may be used instead of the Nb film. However, by using the Nb film, the inclination angle becomes substantially equal to the incident angle at the time of ion milling, and the inclined surface can be formed with high accuracy. As a mask used at the time of ion milling, a diamond film, a TiN film, or the like can be used.

Next, as shown in FIG. 5 (a), YBa ₂
The Cu ₃ O _{7 -x} film 11 is etched to form a groove 11 a having a V-shaped cross section, and a step is formed in a lower electrode formation portion. The inclined surface of the V-shaped groove 11a becomes a Josephson junction surface, and one side of the Josephson junction surface is determined by the depth. Therefore, in the case of a junction where a large critical current value is desired, the depth of the groove 11a may be increased. The depth of the groove 11a can be controlled by the width of the slit and the ion milling time. In this example, the depth of the groove 11a is about 0.4 μm.

Next, as shown in FIG.
Is heated to about 630 ° C, and the rf magnetron spa
NdBa is applied to the entire upper surface of the substrate 10 by the_TwoCu_Three
O_{7- y}Is deposited to a thickness of about 200 nm to form NdBa_TwoCu
_ThreeO_7-yA film 15 is formed. In addition, NdBa_TwoCu_ThreeO
_7-yBefore forming the film 15, a non-super
Conductive PrBa_TwoCu_ThreeO_7-yThin film about 10nm thick
May be formed.

Thereafter, an Au film 16 is formed on the NdBa ₂ Cu ₃ O _7-y film 15 to a thickness of 0.2 μm. next,
As shown in FIG. 5C, using a photoresist, A
A resist film 17 is formed on the u film 16 to cover the upper electrode forming portion and the lower electrode forming portion. Then, using the resist film 17 as a mask, as shown in FIG. 6A, the Au film 16 and the NdBa ₂ Cu ₃ O _{7 -y} film 15 are etched, and NdBa ₂ Cu ₃ O ₇ serving as an upper electrode and a wiring are formed. _-y film 15a and NdBa ₂ Cu ₃ O _7-y film 15 serving as a lower electrode
and b. After that, the resist film 17 is removed.

Next, a photoresist film (not shown) is formed on the Au film 16, a resist film of an extraction electrode pattern is formed through exposure and development steps, and the Au film 16 is patterned using the resist film as a mask. I do. Thus, as shown in FIG. 6B, the extraction electrodes 16a to 16a-1
6d is formed. Thus, the superconducting junction element of the present embodiment is completed.

In the above example, in the step shown in FIG. 4B, the hole 12a of the insulating protective film 12 is made of YBa ₂ Cu ₃ O.
_The ion milling was terminated when the film reached the _7-x film 11, but the Nb film 13 was removed and the YBa ₂ Cu ₃ O _7-x film 1 was removed.
1 until a V-shaped groove 11a and a step are formed (FIG. 5).
The ion milling may be continued (until the state of (a) is reached).

Hereinafter, the superconducting junction element will be described by the method described above.
Is actually manufactured and its characteristics are examined.
You. Use a MgO single crystal substrate as the dielectric substrate.
Liquid phase epitaxy on the single crystal substrate
YBa by taxi method_TwoCu _ThreeO_7-xForming a film,
SrTiO 3 is formed on the surface by rf magnetron sputtering._Threefilm
(An insulating protective film), an Nb film and an Au film were formed. Soshi
Using the Nb film as a mask_TwoCu_ThreeO_7-xV on membrane
A groove was formed. The depth of the V-shaped groove was 0.4 μm. Ma
At this time, the ion milling irradiation angle
The angle was set to 45 ° where the chin rate was large. Etch for each layer
Rate is 15 nm / min (SrTiO_ThreeMembrane), 5
0 nm / min (Au film), 25 nm / min (YBa_TwoC
u_ThreeO_7-xFilm) and 20 nm / min (Nb film).

The shape of the groove is changed by AFM (atomic force microsc
FIG. 7A shows the result of observation with an atomic force microscope (ope; atomic force microscope). For comparison, FIG. 7 shows the results obtained when a groove was formed using a photoresist as a mask instead of the Nb film.
(B). When the Nb film is used as a mask at the time of forming the groove, the inclination of the groove is almost equal to the incident angle of ion milling of 45 °, but when the photoresist is used, only a gentle angle of about 20 ° is obtained. Since the shape of the groove greatly affects the characteristics of the Josephson element, it is preferable to use an Nb film that can easily control the shape of the groove as a mask when forming the groove. Further, the Nb film has accumulated fine processing technology due to development aimed at manufacturing an integrated circuit having a low Tc, and it is a great advantage that such knowledge can be utilized.

NdBa serving as upper electrode_TwoCu_ThreeO_7-yfilm
Was deposited to a thickness of 200 nm. This NdBa_TwoCu
_ThreeO_7-yThe film is formed on the substrate by rf magnetron sputtering.
The film was formed at a temperature of 630 ° C. As a result,
NdBa_TwoCu_ThreeO _7-yA single crystal film was obtained.
Where NdBa_TwoCu_ThreeO_7-yThe substrate is used to form the film
A method of heating by placing on a substrate holder was adopted. The reason
The reason will be described below.

Conventionally, it has been difficult to set the substrate temperature to a specific value in a vacuum film forming apparatus. In particular, in the case of the substrate dropping method in which heating is performed by thermal radiation, it has been known that the radiation efficiency of the surface of a transparent substrate such as an MgO substrate changes due to deposition of an oxide superconductor, and the substrate temperature changes.
Therefore, conventionally, when a superconductor film is formed on a transparent substrate, a method of fixing the substrate to a substrate holder with a silver (Ag) paste having excellent heat conductivity has been used. However, in this method, there is a problem of contamination by Ag powder and a temperature distribution in the substrate, which occur when the substrate is attached and detached.

On the other hand, in the present embodiment, when the SrTiO ₃ film and the NdBa ₂ Cu ₃ O _7-y film are formed, the black YBa ₂ Cu ₃ O _7-x film is already formed thick on the substrate. The SrTiO ₃ film or NdBa ₂ Cu ₃ O _7-y
It is considered that the change in radiation efficiency when forming the film is small, and heating by radiation becomes possible. Table 1 below shows the results obtained by measuring the substrate temperature immediately before film formation (initial temperature) and the substrate temperature after film formation (end temperature) using a radiation thermometer. For comparison, MgO fixed to a substrate holder with silver paste was used.
The results of measuring the temperature of the transparent substrate are also shown. In Table 1, LPE is a substrate obtained by forming a YBa ₂ Cu ₃ O _7-x film on a MgO substrate by a liquid phase epitaxy method, LSAT
Indicates an unformed MgO substrate.

[0036]

[Table 1]

As is apparent from Table 1, in the method of bonding the substrate to the substrate holder with the silver paste, the initial temperature and the end temperature are different from each other. It can be seen that the controllability of the substrate temperature is improved. Further, since it is only necessary to place the substrate on the substrate holder, the handling is easy and the reliability of the process is improved.

FIG. 8 shows NdBa_TwoCu_ThreeO_7-yForm a film
The figure which shows the X-ray diffraction pattern of the surface of the board | substrate 10 after performing.
FIG. 9 is an enlarged view of the (006) peak. this
From these figures, it can be seen that the c-axis oriented upper superconductor electrode (NdBa_Two
Cu_ThreeO_7-yFilm) is formed. FIG.
In FIG. 7, the (006) peak is divided into four peaks. Each
These peaks show a tetragonal YBa_TwoCu_ThreeO
_7-x(C = 1.182 nm), its Kα and NdBa_TwoC
u_ThreeO_7-y(1.179 nm), oblique
Crystal YBa_TwoCu_ThreeO_7-x(1.169 nm), SrT
iO_Three(A = 0.387 nm). the above
Numbers in parentheses indicate lattice constants determined from diffraction angles
You. From this lattice constant, NdBa_TwoCu_ThreeO_7-yMembrane and Y
Ba_TwoCu _ThreeO_7-xAt the connection interface with the film, YBa_TwoCu_Three
O_7-xIs stretched to a lattice close to tetragonal and oxygen deficiency
Occurs, resulting in a tunnel barrier that does not exhibit superconductivity.
It is considered formed.

This result also indicates that in a composite material in which superconductors having different lattice constants are mixed, the interface may become a non-superconducting layer and become a pinning center. Next, when the inclined surface was observed by AFM, it was confirmed that the inclined surface was smooth even after the upper superconductor electrode was formed.
The crystallization temperature of NdBa ₂ Cu ₃ O _7-y is YBa ₂ Cu ₃
The temperature is higher than that of O _7-x , and the film forming temperature is set to be higher by about 50 ° C. Conventionally, when a thin film is deposited on an inclined surface provided on a thin film produced by a vapor phase method, the flatness of the inclined surface has been disordered and deteriorated due to recrystallization. However, in this embodiment,
Even after the NdBa ₂ Cu ₃ O _7-y film was formed, the smoothness was not particularly changed particularly on the inclined surface. This is considered to be due to the excellent crystallinity of the YBa ₂ Cu ₃ O _7-x film formed by liquid phase epitaxy. In addition, contamination by silver powder, which is a problem in the case of bonding with a silver paste, or variation in temperature distribution in a substrate can be solved, thereby improving the reliability of the process.

After forming the Au film for the extraction electrode,
The Au film and the NdBa ₂ Cu ₃ O _7-y film were patterned so that the width of the Josephson junction was 10 μm. afterwards,
After forming an extraction electrode for performing measurement by the four-terminal method, the electrode was attached to a ceramic chip carrier and connected with an Al wire by an ultrasonic bonder. The superconducting junction element of the present embodiment fabricated in this manner was cooled to about 50K, and the current-voltage characteristics were examined. The result is shown in FIG.
From this figure, the superconducting junction element of the present embodiment exhibits the characteristics of a weakly coupled Josephson element, and the critical current is 1.9 m.
In A, it can be seen that the upwardly convex characteristic indicates that a flux flow has occurred. This is 5 × 10 ⁴ A / cm ² in terms of critical current density. In addition, Shapiro steps by microwave irradiation could be observed.

In the present embodiment, a Josephson junction is realized at the junction interface between the first superconductor film and the second superconductor film by lattice strain based on the difference in lattice constant between the two.
As compared with the case where an extremely thin insulator film is formed between the first superconductor film and the second superconductor film, a Josephson junction can be easily realized, and the reliability of the junction is high.
Further, the first superconductor film is formed by a liquid phase epitaxy method,
Thereafter, since the second superconductor film is formed by the rf magnetron sputtering method, the fluctuation of the substrate temperature during the formation of the second superconductor film is small, and the in-plane uniformity of the second superconductor film is excellent. As a result, a Josephson junction having good characteristics can be manufactured with good reproducibility.

Further, since the characteristics of the Josephson junction are determined by the depth of the groove and the width of the wiring, an element having any characteristics can be formed at any position on the substrate. (Second Embodiment) FIGS. 11 to 13 show a second embodiment of the present invention.
FIG. 4 is a cross-sectional view showing a method for manufacturing the superconducting junction element according to the embodiment in the order of steps.

First, as shown in FIG.
A first superconductor film on a dielectric substrate 30 made of
YBa with a thickness of about 10 μm_TwoCu_ThreeO_7-xShape the membrane 31
After that, an insulating protective film 32 having a thickness of about 0.3 μm is formed.
To achieve. YBa_TwoCu_ThreeO_{7- x}The membrane 31 is the same as that of the first embodiment.
Similarly to the embodiment, Y is formed on the substrate 30 by liquid phase epitaxy.
Ba_TwoCu_ThreeO_7-xTo grow a (001) single crystal
Formed. The insulating protective film 32 is made of MgO, Sr
TiO_Three, La_0.65Sr_0.35Al_0.6Ta_0.4O_ThreeOr
PrBa_TwoCu_ThreeO_7-x(0 ≦ x ≦ 0.6)
The substrate temperature was reduced to about 6 by rf magnetron sputtering.
Formed at 30 ° C.

Next, as shown in FIG. 11B, a photoresist film 33 having a window at a portion corresponding to the upper electrode forming portion is formed on the insulating protection film 32. Then, using the resist film 33 as a mask, the insulating protective film 32 and the YB
The a ₂ Cu ₃ O _7-x film 31 was ion-milled, and FIG.
As shown in (c), a step is formed in the YBa ₂ Cu ₃ O _7-x film 31. The depth of this step determines the area of the Josephson junction. Therefore, when a large critical current value is desired, the height of the step may be increased. In this example,
It is assumed that the step is 0.4 μm. The inclination angle is 9
The vertical direction is preferably close to 0 °.
It is preferable to set the pattern edge as steep as possible and set the irradiation angle of ion milling to be vertical.

Next, as shown in FIG.
An insulating protective film 34 is formed on the entire upper surface of the "0". The same material as the insulating protective film 32 can be used as the material of the insulating protective film 34. Further, the thickness of the insulating protective film 34 is preferably about 0.3 μm from the viewpoint of securing sufficient insulating properties and preserving the surface flatness of the thin film.

Next, as shown in FIG. 12B, a resist film 35 is formed on the insulating protection film 34 by using a mask in which the above-mentioned resist film 33 is inverted in black and white. Then, using the resist film 35 as a mask, the insulating protective film 3 is used.
4 by ion milling, as shown in FIG.
The slope of the YBa ₂ Cu ₃ O _7-x film 31 at the step is exposed. In this case, ion milling is performed at a shallow irradiation angle.

Next, as shown in FIG.
NdBa ₂ C as a _second superconductor film
u ₃ O a _7-y film 36 to a thickness of approximately 200 nm. In this case, before forming the NdBa ₂ Cu ₃ O _7-y film 36, a PrBa ₂ Cu ₃ O _7-x film having a thickness of about 10 nm may be formed as a tunnel barrier film. When these films are formed, a single crystal film with a uniform orientation can be formed by maintaining the substrate temperature at 630 ° C. and forming the films by rf magnetron sputtering.

Next, as shown in FIG. 13C, the NdBa ₂ Cu ₃ O _7-y film 36 is formed by photolithography.
Is etched to form predetermined upper electrodes and wiring patterns. Thereafter, an Au film is formed on the entire upper surface of the substrate 30,
This Au film is patterned to take out the extraction electrodes 37a to 37a.
37d is formed. Thereby, the superconducting junction element of the present embodiment is completed. Also in the present embodiment, the same effect as in the first embodiment can be obtained.

According to the above method, the superconducting junction element is actually formed.
It was formed and the current-voltage characteristics were examined. However, Josephson
The width of the joint was 5 μm. As a result, the first embodiment
Characteristics similar to those of the state were observed. In addition, the above-mentioned first and second
In the second embodiment, the first superconductor film
YBa_TwoCu_ThreeO_7-x, NdB as the second superconductor film
a_TwoCu_ThreeO_7-yI explained the case where was used,
The present invention is not limited to this.
Substances can be used. For example, the first superconductor
As a film, YBa_TwoCu_ThreeO₇(Lattice constant 11.69
Å), GdBa_TwoCu_ThreeO₇(Lattice constant 11.63Å)
And YbBa_TwoCu_ThreeO₇(Lattice constant 11.63Å)
Using a superconducting oxide selected from the group consisting of
NdBa as a superconductor film_TwoCu _ThreeO₇(Lattice constant 1
1.74Å), SmBa_TwoCu_ThreeO₇(Lattice constant 11.
70Å) and EuBa_TwoCu_ThreeO₇(Lattice constant 11.7
0Å) using a superconducting oxide selected from the group consisting of
Can be

[0050]

As described above, according to the present invention,
Since a Josephson junction is realized by lattice distortion caused by a difference in lattice constant, manufacturing is easy and reliability is excellent. Further, by forming the first superconductor film by a liquid phase epitaxy method and then forming the second superconductor film,
Variations in the substrate temperature are avoided, the in-plane uniformity of the second superconductor film is good, and a Josephson junction with uniform characteristics can be formed with good reproducibility. Further, by realizing a microstrip line structure using the first superconductor film as a ground plane and the second superconductor film as an upper wiring,
The stray inductance can be reduced, and the circuit design becomes easier. Therefore, the present invention greatly contributes to the realization of an integrated circuit in which the SFQ circuit is integrated.

[Brief description of the drawings]

FIG. 1A is a sectional view showing a superconducting junction element according to a first embodiment of the present invention, and FIG. 1B is a schematic plan view thereof.

FIG. 2 is an equivalent circuit diagram of the superconducting junction element according to the first embodiment.

FIG. 3 is a cross-sectional view (part 1) illustrating the method for manufacturing the superconducting junction element of the first embodiment.

FIG. 4 is a sectional view (part 2) illustrating the method for manufacturing the superconducting junction element of the first embodiment.

FIG. 5 is a cross-sectional view (part 3) illustrating the method for manufacturing the superconducting junction element of the first embodiment.

FIG. 6 is a sectional view (part 4) illustrating the method for manufacturing the superconducting junction element of the first embodiment.

FIG. 7A is a diagram showing an AFM image of a groove formed using an Nb film as a mask, and FIG. 7B is a diagram showing an AFM image of a groove formed using a photoresist as a mask.

FIG. 8 is a view showing an X-ray diffraction pattern of the surface of the substrate after forming the NdBa ₂ Cu ₃ O _7-y film.

FIG. 9 is an enlarged view of the (006) peak in FIG. 8;

FIG. 10 is a diagram showing current-voltage characteristics of the superconducting junction element manufactured according to the first embodiment.

FIG. 11 is a sectional view (part 1) illustrating the method for manufacturing the superconducting junction element of the second embodiment.

FIG. 12 is a cross-sectional view (part 2) illustrating the method for manufacturing the superconducting junction element of the second embodiment.

FIG. 13 is a cross-sectional view (part 3) illustrating the method for manufacturing the superconducting junction element of the second embodiment.

FIG. 14 is a sectional view showing an example of a ramp edge junction type Josephson device.

[Explanation of symbols]

10, 30, 50: dielectric substrate, 11, 31: YBa ₂ Cu ₃ O _7-x film, 11a: groove, 12, 32, 34: insulating protective film, 13: Nb film, 14, 16: Au film, _{15a, 15b, 36 ... NdBa 2} Cu 3 O 7-y layer, 16 a to 16 d, 37a to 37d ... take-out electrode, 17,33,35 ... resist film, 51 ... upper superconductive film, 53 ... tunnel barrier film, 54 ... Lower superconductor film.

 ──────────────────────────────────────────────────続 き Continued on the front page (71) Applicant 000003078 Toshiba 1-1-1 Shibaura, Minato-ku, Tokyo (72) Inventor Yoichi Enomoto 1-14-3 Shinonome, Shinonome, Koto-ku, Tokyo Foundation International Superconductivity Inside the Superconductivity Engineering Research Center, National Institute of Advanced Industrial Science (72) Michiasa Iiyama 1-14-3 Shinonome, Koto-ku, Tokyo Foundation International Superconductivity Research Institute, Superconductivity Engineering Research Center (72) Inventor Hideo Suzuki Koto, Tokyo 1-14-3 Shinonome-ku, Tokyo, Japan Superconductivity Engineering Research Center, International Superconducting Technology Research Center (72) Inventor Saburo Hoshi 1-14-3, Shinonome, Shintomo, Koto-ku, Tokyo Superconductivity Engineering Inside the laboratory (72) Inventor Teruo Izumi 1-14-3 Shinonome, Koto-ku, Tokyo Foundation Law Inside the Superconductivity Engineering Research Center, International Superconducting Technology Research Center (72) Inventor Minoru Shiohara 1-14-3 Shinonome, Koto-ku, Tokyo Foundation F-term (Reference) 4M113 AA06 AA16 AA25 AD37 AD42 AD63 AD67 AD68 BA04 BA08 BA21 BC04 CA34

Claims (8)

[Claims] 1. A superconducting junction device wherein two types of oxide superconductors having different lattice constants are connected to each other to realize a tunnel barrier at an interface. 2. One of the two types of oxide superconductors comprises:
YBa_TwoCu_ThreeO_{7- x}, GdBa_TwoCu_ThreeO_7-xAnd Y
bBa_TwoCu_ThreeO_7-x(However, 0 ≦ x ≦ 0.6)
Oxide superconductor selected from the group consisting of
The other is NdBa_TwoCu_ThreeO_7-y, SmBa_TwoCu
_ThreeO_7-yAnd EuBa_TwoCu_ThreeO_7-y(However, 0 ≦ y ≦
0.6) any one type of oxidation selected from the group consisting of
2. The superconductor according to claim 1, wherein the superconductor is an object superconductor.
Conductive junction element. 3. A substrate, a first superconductor film having an inclined surface formed on the substrate, and an insulating protective film formed in a region on the first superconductor film excluding the inclined surface. A second superconductor having a lattice constant different from the lattice constant of the first superconductor film and formed at least in a region from above the insulating protective film to an inclined surface of the first superconductor film. A superconducting junction element comprising a film. 4. The first superconductor film forms a microstrip line ground plane, the second superconductor film forms an upper wiring of the microstrip line,
4. The superconducting junction element according to claim 3, wherein a joining surface between the first superconducting film and the second superconducting film forms a Josephson junction. 5. The method according to claim 1, wherein the first superconductor film is made of YBa._TwoCu
_ThreeO_7-x, GdBa _TwoCu_ThreeO_7-xAnd YbBa_TwoCu
_ThreeO_7-x(However, select from the group consisting of 0 ≦ x ≦ 0.6)
Made of any one of the above oxide superconductors,
2 is NdBa_TwoCu_ThreeO_7-y, SmBa
_TwoCu_ThreeO_7-yAnd EuBa_TwoCu_ThreeO_7-y(However, 0
≦ y ≦ 0.6) any one selected from the group consisting of
3. An oxide superconductor according to claim 1,
The superconducting junction element according to claim 4. 6. A first liquid phase epitaxy method on a substrate.
Growing a superconductor film, forming an insulating protection film on the first superconductor film, forming a mask of a predetermined pattern on the insulating protection film, Ion milling a film and the first superconducting film to form an inclined surface on the first superconducting film; removing the mask; and covering the entire upper surface of the substrate with the first superconducting film. A method for manufacturing a superconducting junction element, comprising: forming a second superconducting film having a lattice constant different from that of a body film; and patterning the second superconducting film. 7. The method according to claim 6, wherein the mask is formed of niobium (Nb). 8. A method according to claim 1, wherein the first exposed portion is removed between the step of removing the mask and the step of forming the second superconductor film.
7. The method for manufacturing a superconducting junction element according to claim 6, further comprising the step of forming a second insulating protective film on the superconducting film.