JP2776004B2 - Method of manufacturing Josephson device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting device, and more particularly to a method for manufacturing a Josephson device using a metal oxide superconducting thin film having a junction.

Description of the Related Art Conventionally, such a niobium nitride (NbN) and niobium 3 germanium as A15 type binary compound as a superconductor (Nb ₃ Ge) has been known. Various superconducting elements using these materials have also been proposed. However, the superconducting transition temperature Tc of these materials was at most 24K.

As perovskite compounds, Ba-Pb-Bi-
An O type (JP-A-60-173885) is known, and a number of superconducting elements using this type of material have been studied.
However, the Tc of this material was as low as about 13K, and practical use was difficult.

However, a Ba-La-Cu-O-based high-temperature superconductor having a Tc of 30 to 40K has been proposed [JGBednorz and KAMulle.
r, Zeit Shrift Fair Physik
hrift fur physik B) −Condensed Matter 64,189−193
(1986)]. Furthermore, Tc exceeding 90 K has been reported in the Y-Ba-Cu-O system [MKWu et al., Physical Review Letters, Vol. 58, No.
9,908-910 (1987)], and its practical use has become promising because it has become higher than the boiling point of liquid nitrogen (77 K).

To actually apply superconductors to functional devices,
It is essential to make a Josephson junction, and various techniques have been proposed for that purpose. In a Josephson junction using a metal oxide superconducting thin film, a so-called grain boundary Josephson junction that effectively utilizes a grain boundary formed during crystal growth is generally used. This type
SNS (Superconductor-Normalconductor-Superconduct
It is considered that this is a weakly-coupled Josephson junction. In addition, many attempts have been made to form a SIS (Superconductor-Insulator-Superconductor) junction using a metal or another non-superconducting material for the barrier layer, and further using an insulator for the barrier layer. .

However, a Josephson element using a metal oxide superconducting thin film and having a grain boundary junction type has extremely poor reproducibility during fabrication. Also, a tunnel-type Josephson junction requires two superconductors to be very close to each other (several nanometers or less) via an insulator, and it is extremely difficult to manufacture using high-temperature oxide superconductors with current technology. is there.

On the other hand, the so-called weak-coupling Josephson junction causes the superconductors to weakly couple by approaching each other via normal conductors, weak superconducting parts, or small bridges, thus exhibiting the Josephson effect. It is to let.
The weak-coupling type Josephson junction has an advantage in that the interval between the superconductors can be relatively widened (several hundred nm or more) as compared with the tunnel-type Josephson junction, so that the restriction on the manufacturing accuracy is relaxed.

Problems to be Solved by the Invention However, metal oxide superconductors are extremely unstable chemically, and in particular, such as chemical reaction between the superconductor and moisture in the air and desorption of oxygen which is a constituent element of the superconductor. As a result, superconducting properties may be degraded on the surface or interface of the superconductor, or superconductivity may be lost. Therefore, when making a junction using a laminated structure, not only a tunnel-type Josephson junction but also a weakly-coupled Josephson junction has an insulator or semiconductor-like layer at the interface between the superconductor and the barrier layer. In some cases, making the fabrication of the element difficult. In order to manufacture a Josephson device with good reproducibility and controllability, it is important to prevent the influence of characteristic deterioration at the interface of the superconductor.

The present inventors have paid attention to a change in characteristics when a metal oxide superconductor is irradiated with ultraviolet rays and a subsequent oxidation treatment, and have studied in detail. As a result, it has been found that the superconducting property of the metal oxide superconducting thin film is improved by performing the oxidation treatment during or after the irradiation of the ultraviolet rays as compared with the unirradiated portion.

The present inventors have invented a method for manufacturing a Josephson device using a novel metal oxide superconducting thin film based on this new discovery regarding ultraviolet irradiation.

Means for Solving the Problems A method for manufacturing a Josephson device according to the present invention is directed to a method for manufacturing a metal oxide superconducting thin film formed on a substrate, the method comprising: The method includes a step of irradiating ultraviolet rays from a surface on which the thin film is not formed, and performing an oxidation treatment during or after the irradiation of the ultraviolet rays.

Operation With this configuration, a Josephson junction can be manufactured only by irradiation with ultraviolet light and oxidation treatment, so that the process is simplified as compared with the conventional manufacturing using a laminated structure. Further, since the interface between the superconductor and the bonding layer is not exposed to the air during the process, deterioration of the superconductivity at the superconductor interface due to moisture in the air does not occur. Further, a processing process such as etching, a high-temperature heat treatment, or the like is not required.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

1 (a) to 1 (c) are basic conceptual diagrams of a method for manufacturing a Josephson device according to an embodiment of the present invention.

First, as shown in FIG. 1A, a metal oxide superconducting thin film 2 is formed on a substrate 1 by, for example, a sputtering method. Next, as shown in FIG. 1B, the surface of the thin film 2 and the back surface of the substrate 1 are irradiated with ultraviolet rays 3 and then oxidized by, for example, exposure to oxygen plasma. Since the decay constant of the ultraviolet light 3 in the metal oxide superconducting thin film is relatively large, the range in which the irradiation effect of the ultraviolet light 3 occurs is limited to a portion from the irradiated surface to a certain depth. The present inventors have confirmed that in this portion, superconductivity is improved by performing oxidation treatment during or after irradiation. Therefore, due to these processes, as shown in FIG. 3C, a portion having excellent superconductivity exists in the metal oxide superconducting thin film 2 from the front surface and the back surface to a certain depth. If these portions are called electrode portions 4 and 5, it is possible to form a bond having a structure in which the bonding portion 6, which is a portion having inferior superconducting properties because the ultraviolet irradiation effect is not exerted, is sandwiched between the electrode portions 4 and 5.

The junction 6 can be made sufficiently thin by adjusting the irradiation amount (for example, irradiation intensity and irradiation time) of the ultraviolet light 3, and at an appropriate irradiation amount, the joint 6 becomes a weakly-coupled Josephson junction and becomes a Josephson junction. The present inventors have confirmed that the Son effect is exhibited.

Although the cause of the improvement in superconductivity due to such ultraviolet irradiation and oxidation treatment is not yet clear,
The present inventors have investigated the crystal structure, electronic state or magnetic susceptibility of the metal oxide superconducting thin film after irradiation, and also the characteristics in a magnetic field, and found that oxygen ions in the crystal, which influence the superconducting characteristics, are more ordered. It seems to be correctly arranged. The present inventors also believe that ultraviolet irradiation has an effect of, for example, recovering the crystallinity of a portion having poor crystallinity to some extent and generating a pinning center having an effect of stopping movement of magnetic flux.

FIG. 2 shows the results of the present inventors examining the wavelength dependence of the light absorption coefficient using a bismuth-based superconducting thin film as an example. FIG. 2 is a characteristic diagram showing the wavelength dependence of the light absorption coefficient of the bismuth-based superconducting thin film.

As shown in FIG. 2, the absorption peak is around 250 nm. This wavelength corresponds to the emission spectrum of mercury, and a normal low-pressure mercury lamp can be used for irradiation. The absorption coefficient is about 5 × 10 ⁵ cm ⁻¹ near 250 nm, which is 100 n
It shows that the transmittance is about 0.01 for the m-thick film. This indicates that the UV irradiation effect is relatively limited near the surface, and it is clear that the UV intensity in the superconducting thin film decays exponentially from the surface,
It is possible to calculate in advance how far from the surface the influence of the irradiation depends on the irradiation amount of the ultraviolet light.

FIG. 3 is a characteristic diagram showing the wavelength dependence of the light absorption coefficient of a magnesium oxide (MgO) crystal. The MgO crystal has a small absorption around 250 nm. Therefore, when the MgO crystal is used as the substrate 1 for forming the metal oxide superconducting thin film 2, when the ultraviolet light 3 is irradiated from the substrate 1 side, the ultraviolet light 3 transmits well. It can be seen that the irradiation effect can be given to the metal oxide superconducting thin film 2 efficiently.

As an oxidation treatment method, it is effective and convenient to irradiate oxygen ions generated by the discharge of a gas containing oxygen or neutral oxygen atoms in an excited state, or to perform treatment in a gas containing ozone. Was found. Neither of these oxidation treatments requires heating the sample. However, a method of heating the sample to about 300 to 400 ° C. for 1 to 2 hours in an oxidizing atmosphere is also effective as the oxidizing treatment. At such a temperature, the metal oxide superconducting thin film 2 does not recrystallize and does not change its shape.

 Hereinafter, specific examples will be described.

First, using a (100) plane MgO single crystal as a substrate 1 and sintering YB by a high frequency planar magnetron sputtering method.
An a ₂ Cu _4.5 O _x target was sputter-deposited in a mixed gas atmosphere of Ar and O ₂ , and a metal oxide superconducting thin film 2 made of crystalline YBa ₂ Cu ₃ O ₇ was deposited on the substrate 1. In this case, the gas pressure was 0.5 Pa, the sputtering power was 150 W, the sputtering time was 20 minutes, the film thickness was 0.2 μm, and the substrate temperature was 700 ° C.
The metal oxide superconducting thin film 2 thus obtained exhibited superconductivity, and its transition temperature was about 90K.

Further, the metal oxide superconducting thin film 2 was irradiated with ultraviolet light 3 from the surface thereof and the substrate 1 side for about 3 hours using a low-pressure mercury lamp. Thereafter, the substrate was left in an oxygen atmosphere at 400 ° C. for 2 hours to perform an oxidation treatment.

However, in this case, since a voltage must be applied to the junction 6, the characteristics of the Josephson junction cannot be measured with the structure of FIG. 1 (c).

FIG. 4 is a sectional view of an element for measuring characteristics of a Josephson junction. As shown in FIG. 4, the ultraviolet irradiation from the surface of the metal oxide superconducting thin film 2 formed on the substrate 1 is performed only on a portion of about 1 mm square limited by a metal mask. The electrode section 4 was formed. Substrate 1
The other electrode portion 5 was formed by irradiating the entire surface with ultraviolet rays from the back surface of the substrate. The region sandwiched between the electrode portions 4 and 5 constitutes the joint 6. After the oxidation treatment, a conductive wire was connected on the electrode portion 4 with a conductive paste 7. The connection to the lower electrode portion 5 is made by partially removing the surface of the joint portion 6 by ion beam etching or the like and then conducting the conductive wire with a conductive paste 9.
10 connected.

An AC voltage of 85 Hz was applied between the conductor 8 and the conductor 10, and the current flowing through the joint 6 was measured. As a result, the current-voltage characteristics had a nonlinearity peculiar to the Josephson junction. Further, when the junction 6 was irradiated with a microwave of 20 GHz, a voltage step was observed on the curve of the current-voltage characteristics, and this step was unique to the Josephson junction from the relationship between the position of this step and the frequency of the microwave. It turned out to be the so-called Shapiro step. From these results, it was found that a weakly-coupled Josephson junction was formed in the prototype device.

By this method, a Josephson element could be formed with good controllability and a simple process.

In addition, for forming the metal oxide superconducting thin film 2, Y-Ba-
In addition to Cu-O, AB-Cu-O, Bi-Sr-Ca-Cu-O or Tl-Ba-Ca-Cu-O composite compound is physically deposited by thermal evaporation such as electron beam evaporation or laser beam evaporation. It may be attached on the substrate 1 by a vapor growth method. Although the composition formula of these metal oxide superconductors has not been clearly determined, A-
For B-Cu-O, oxygen-deficient perovskite (A,
B) ₃ Cu ₃ O _7-x , and for this type of material we consider A to be Y, La and La series elements (atomic number 57,
At least one of 59-60, 62-71), B is Ba, Sr, etc.
If at least one of the group IIa elements and the element ratio of the prepared thin film is in the range of 0.5 ≦ (A + B) /Cu≦2.5, a superconducting phenomenon can be found even if there is a slight difference in the critical temperature. It was confirmed. Bi-Sr-Ca-Cu-
O or Tl-Ba-Ca-Cu-O superconductor has a critical temperature of 100K
And very useful in practice, but it was relatively difficult to form an excellent superconducting film due to the large number of elements contained. The present inventors have confirmed that, if the production conditions and the like are strictly controlled, these materials can be formed into a thin film with good reproducibility by the same method as that of the AB—Cu—O superconductor. The method for forming the metal oxide superconducting thin film 2 is not limited to the physical vapor deposition method, but may be a chemical vapor deposition method such as a normal pressure or reduced pressure chemical vapor deposition method, or a plasma chemical vapor deposition method. The present inventors have confirmed that the photochemical vapor deposition method is also effective if the ratio of the component elements is matched.

EFFECT OF THE INVENTION The method for manufacturing a Josephson device according to the present invention has a great feature in that a Josephson junction can be manufactured only by irradiating ultraviolet rays and oxidizing a thin metal oxide superconducting thin film having good uniformity. . The treatment by ultraviolet irradiation does not raise the temperature of the thin film, has good controllability, and is easy to process. Therefore, it is a very simple process compared to the conventional fabrication using a laminated structure. An element can be manufactured.

In addition, since the interface between the superconductor and the bonding layer is not exposed during the process, the deterioration of the superconductivity at the interface of the superconductor due to moisture in the air, which is a conventional problem, does not occur. Further, since a processing process such as etching or a high-temperature heat treatment is not required, there is no change in the shape, and it is particularly effective for lamination and integration with other electronic devices and other materials.

Further, the Josephson device and the Si
Alternatively, integration with a device such as GaAs becomes possible. Further, the method for manufacturing a Josephson device of the present invention can be used for manufacturing various superconducting devices such as SQUID. In particular, since the transition temperature of this type of metal oxide superconductor may be room temperature, the practical range is wide and the industrial value of the present invention is high.

[Brief description of the drawings]

1 (a) to 1 (c) are basic conceptual diagrams of a method for manufacturing a Josephson device in one embodiment of the present invention, and FIG. 2 is a characteristic diagram showing the wavelength dependence of the light absorption coefficient of a bismuth-based superconducting thin film. FIG. 3 is a characteristic diagram showing the wavelength dependence of the light absorption coefficient of a magnesium oxide (MgO) crystal, and FIG. 4 is a sectional view of an element for measuring the characteristics of a Josephson junction. 1 ... substrate, 2 ... metal oxide superconducting thin film, 3 ... ultraviolet light.

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kentaro Seto 1006 Kazuma Kadoma, Kadoma City, Osaka Inside Matsushita Electric Industrial Co., Ltd. (56) References JP-A-64-50578 (JP, A) JP-A-1 -1447878 (JP, A) JP-A-1-205579 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01L 39/00 H01L 39/22-39/24

Claims (5)

(57) [Claims] An ultraviolet ray is applied to a metal oxide superconducting thin film formed on a substrate from a surface of the metal oxide superconducting thin film and a surface of the substrate on which the metal oxide superconducting thin film is not formed. A method for manufacturing a Josephson device, comprising irradiating and oxidizing during or after irradiation of the ultraviolet light. 2. The method according to claim 1, wherein the metal oxide superconducting thin film is made of a material containing a copper element. 3. The method according to claim 1, wherein the metal oxide superconducting thin film containing copper element is A-
3. The method for producing a Josephson device according to claim 2, comprising a B-Cu-O composite compound. Here, A is Y, La and La series elements (atomic numbers 57 and 59).
-60, 62-71), B is Ba, Sr, etc. II
The composition ratio of at least one of the group a elements and the A, B and Cu elements is 0.5 ≦ (A + B) /Cu≦2.5. 4. The metal oxide superconducting thin film containing a copper element is a Bi-
3. The method for manufacturing a Josephson device according to claim 2, comprising a composite compound of Sr-Ca-Cu-O or Tl-Ba-Ca-Cu-O. 5. The method according to claim 1, wherein the substrate is a single crystal of MgO.