JPH02264486A - Superconductive film weakly coupled element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a high-speed, high-output superconducting element that uses weak bonding between grain boundaries of microcrystals whose crystal axes are randomly oriented. .

<Prior art> Recently, La-based, Y-containing lanthanide-based, Bi-based or T
Superconducting materials with a high critical temperature Tc have been developed, such as those based on metals, and are expected to be put to practical use because they have the advantage of being able to be cooled with liquid nitrogen compared to conventional metal-based or alloy-based superconducting materials.

On the other hand, the Josephson effect has traditionally been used in devices using superconducting properties, and tunnel-type and weak-coupling structures that have this effect have been used. A tunnel junction has a structure in which an ultra-thin insulating film with a thickness equal to or less than the coherence length of superconducting electrons is interposed between two superconductors. The bonding characteristics and reliability can be greatly affected by the thickness. In particular, oxide superconductors are difficult to fabricate because their coherence length is short, on the order of a few λ, and each insulating film needs to be made uniform on the order of one person. No bond could be obtained.

On the other hand, a weakly coupled junction has a pre-fuji type structure in which the cross-sectional area of the superconductor of the element is partially reduced between its electrodes. The cross-sectional area of this bridge also depends on the coherence length of the superconductor used, so creating a bond that works well with an oxide superconductor required tens to thousands of people to perform microfabrication. .

<Problems to be Solved by the Invention> As explained above, when creating a Josephlin effect junction using a superconductor, tunnel junctions have problems with the uniform production of insulating films and reliability. The weakly bonded junction has a problem in microfabrication of the bridging portion, and it has not been possible to obtain a detection element that makes effective use of oxide superconductivity with a high Tc.

The present invention solves the problems of conventional Josephrine junctions, and provides a superconducting membrane weak-coupling device that achieves high-speed switching operation due to superconducting weak coupling without requiring special microfabrication, and has higher output than conventional ones and is easier to handle. is intended to provide.

Means for Solving the Problems> In order to achieve the object of the present invention, the bridging portion of a superconductor that constitutes a weak bond is formed of a conductive film composed of fine particles in which the crystal axes of the particles are randomly oriented. It was made into a superconducting element.

The formation of a superconducting film made of randomly oriented fine particles as described above can be achieved by selecting the crystallinity, coefficient of thermal expansion, and thermal conductivity of the substrate, or by controlling the growth rate, temperature, etc. for producing the superconducting film.

<Function> The weak bond formed by the bridging part of the superconductor in which the crystal axes of the fine particles of the present invention are oriented in a disorderly manner is similar to the weak bond formed by the bridging part of the conventional superconductor made of particles in which the crystal axes are oriented. In comparison, the critical current is small and the output voltage is large, so the present invention eases the microfabrication conditions for the bridging portion of the superconducting weak junction and facilitates handling.

The effects of the present invention can be obtained when a superconducting film grown on a polycrystalline metal substrate at a relatively low temperature is used. This effect occurs when the directions of the crystal axes of adjacent superconducting particles differ, resulting in a layer of grain boundaries that lowers the critical current density and increases the output voltage. It is thought that the seeping quasiparticles reduce the number of superconducting electron pairs (Cooper pairs) in the bridging part of the superconductor, and have the effect of lowering the critical current density in the bridging part.

<Example> Embodiments of the present invention will be described with reference to the drawings.

In the example, an oxide superconductor having a composition ratio of Y1'Ba2Cu30y-a was used. For the final and final parts of the superconductor to be fabricated, heat-resistant and oxidation-resistant platinum (Pt) is deposited on a substrate at a temperature of 450°C or lower (preferably 200°C or lower).
A film consisting of fine particles with randomly oriented crystal axes was formed.

If the above film forming conditions are changed to raise the substrate temperature to 450° C. or higher, a film will be obtained in which the crystal axes of the particles are oriented.

For example, even if the substrate is a single crystal of MgO and the substrate temperature is 200°C, when a PT film is formed, almost amorphous P is formed.
It becomes a thin film. When this Pt thin film substrate is maintained at 600°C and a film of the above-mentioned Y-based oxide superconductor is formed, the crystal axes are oriented randomly, and the Y-based film is composed of fine particles with a diameter of about 0.1 to 1 μm. However, if the Y-based superconducting film is formed directly on the MgO substrate, the C axis of the Y-based superconducting film is oriented perpendicular to the MgO substrate.

FIG. 6 shows the characteristics 81 of the weak coupling element made from the film on the Y-based Pt film prepared above and the characteristics of the weak coupling element made from the film on the MgO substrate. In Figure 6, the vertical axis shows the bias current applied to an element with a superconducting weak junction, converted into current density (A/c♂), and the horizontal axis shows the voltage (mV) generated in the element. It is.

This figure 6 shows this. As shown, the film 81 on MgO is
Compared to the film 82 on Pt, the film 81 has a higher critical current density, and the film 81 of the present invention, which is made of particles with randomly oriented crystal axes, generates a high output voltage at the same bias current and has improved sensitivity as a detection element. , indicating that it is easy to handle.

First example FIG. 1 shows the configuration of a first embodiment of the present invention.

In this figure, (al is a front view, (b) is [al x-x'
FIG.

This figure 1 fat is partially made of metal on the substrate 1
The films 2 were laminated, and a superconducting bridge part 4 was formed on the Pt film, and superconducting films 8 for attaching electrodes were formed on the MgO substrates on both sides of the superconducting bridge part 4. Subsequently, the bias current electrodes 5, 5' of this device and the voltage electrode 6.degree. 6' for detecting the voltage generated by the device were formed on the superconductor film 3 by vacuum evaporation of Ti.

FIG. 1 [b) is an xx' cross-sectional view of this [a],
In the produced superconducting film, the C-axis of the film a on the MgO substrate 1 was oriented in a direction perpendicular to the substrate 1, and the upper film 4 on the Pt film was a film composed of randomly oriented fine particles.

Figure 2 shows a perspective view of the manufacturing process of the device shown in Figure 1.
It is a diagram. In Fig. 2, [al] is a groove with a width W of 500 μm and a film thickness αl of 0.2 μm, which is formed by electron beam evaporation using magnesia (MgO) on a (100) plane single crystal substrate 1 heated to 200° C. and using a lift-off method. This is a state in which a fine PT film 2 has been formed.

That board 1.

Heated to 600°C, and the vicinity of the substrate 1 was heated to 5 mTor.
The elements Y, Ba and Cu are kept in an oxygen atmosphere of
A superconducting thin film 3.4 having a thickness of 0.5 μm was produced by electron beam evaporation controlled to have a composition ratio of :Ba:Cu=1:2:8. Critical temperature T of the fabricated superconducting film
C is 85 and critical current Jc is 77 when film 3 on the substrate is 2
×105A/12. Film 4 on Pt film is 2X103A/J
' had the characteristics shown in Figure 6. The vertical axis of FIG. 6 is the bias current density applied to the superconducting film, and the horizontal axis is the voltage generated by the film. A dotted curve 81 in the figure indicates a superconducting film 41 consisting of fine particles with disordered crystal axis orientation on a Pt film. A solid curve 32 shows the characteristics of the superconducting film 8 grown on the MgO substrate in the C-axis direction.

The superconducting film fabricated as described above was shaped on Pt so that the center part became narrower by electron beam photolithography and reactive sputtering using chlorine (C) gas, so that the width W at the center part was 5 μm.
It was made into the shape shown in Fig. 2 fbl with a brifuge of . continue,
A Ti film with a thickness of 0.0mm was deposited by electron beam evaporation using a metal mask.
A 5 μm current electrode 5°5′ and a voltage electrode 6.6′ were prepared.

The device fabricated as described above was cooled to 77K, and the relationship between the bias current and the voltage generated is shown in FIG. 8 as a graph. The vertical axis of this figure is the bias current value applied to the current electrode 5.5' of the element, and the horizontal axis is the generated voltage measured at the voltage electrode 6.6'.

In the measurement state shown in FIG. 3, 10.
When 8 GHz microwave was irradiated, Shapiro steps approximately every 20 μV were observed in the voltage-current curve, indicating that this bridge portion had a Josephson junction effect.

Second Embodiment FIG. 4 is a front view showing a second embodiment of the present invention. This second embodiment differs from the first embodiment in the substrate configuration for controlling the orientation of superconducting particles. Figure 4 shows
The superconducting film 4 of the bridging part is deposited on the non-oriented metal film 2, and the superconducting film 3 which becomes the electrical part is a crystallized metal film 21.22.
The composition deposited on top is shown.

The manufacturing process of the device of the above second example will be explained with reference to FIG.

[al] in FIG. 5 is a Si single crystal substrate 1 with a (100) surface, on which a 1 μm thick Al film (not shown) is formed by vacuum evaporation.
Only the band width W1 is left in the part. Subsequently, this substrate was heated to 550° C., and a Pt film having a thickness α1 of 0.5 μm was formed by electron beam evaporation. Formed Pt
At least the films 21 and 22 on the substrate 1 were crystallized and oriented.

In the above state, the band-shaped A film formed in the previous step is
If the Pt film on the A film is removed by dissolving it in an OH solution, the Pt film on the A film is also removed at the same time, resulting in the lift-off method, resulting in the configuration shown in Figure 5 (al).Next, this substrate 1 is heated to 200°C and the Pt
There is no orientation and the film thickness is 0.
A 5 μm thick Pt film 2 was formed to have the shape shown in FIG. 5 [bl].

After that, in the same manner as in the first embodiment, the substrate 1 having the above structure is heated to 600° C., the surrounding area is maintained in an oxidizing atmosphere of about 5 mTorr, and Y is used as an evaporation source.

Each element of Ba and Cu is Y:Ba:Cu=1:2:
The superconducting film was deposited at a ratio of 8 to form a superconducting film 3.4.

The Y-based oxide superconducting film formed on the above Pt film is Pt
The film becomes a film 4 of randomly oriented particles on the film, and the Pt film 21.2
2 is a film in which the C axis is oriented perpendicular to the surface of the substrate.

Subsequently, the shape shown in (C) of this figure was formed by reactive sputtering using carbon gas. This bridge part has a width W
2 was set to 5 μm.

Finally, the current electrodes [5, 5' and voltage electrodes 6, 6' shown in (dl) in Figure 1 were formed of a Ti thin film with a thickness of 0.5 μm by electron beam evaporation using a metal mask.

The device fabricated in the second example 2 also showed a similar relationship between the generated voltage and the bias current value when measured in the same manner as in the first example, and also showed that a Shapiro step occurred in response to microwave irradiation. Furthermore, when irradiated with infrared light, which has a shorter wavelength, the occurrence of a Shapiro step corresponding to the wavelength of the irradiated light was observed. This device also exhibited magnetoresistive effects against magnetic fields as a superconductor weakly junction aggregated device.

The above is an example of the present invention, and the sensitivity of the element can be changed by changing the film thickness, width, length, etc. of the bridging part of this superconducting film detection element, and miniaturization can be achieved by applying microfabrication. is also possible.

In Examples 1 and 2, the width of the bridging portion of the superconductor was set to 6
Although the width of this bridge part was set to 0.
As a result of experiments with grooves varying in the range from 5 μm to 100 μm, it was confirmed that the relationship between bias current and generated voltage was similar to that in the example, and the occurrence of a Shapiro step caused by microwave irradiation, and the wavelength of the measured electric hood wave was confirmed. It was also found that the sensitivity could be controlled by adjusting the film thickness to correspond to the .

Furthermore, in the example, YIB a2 Cu30) is used as the superconducting film.
Although the oxide superconductor having the composition -X was used, it is not limited to this composition, and other oxide superconductors (La1-xMX
)2CuOa-x (M is Ba, Sr, Ca, etc.)
LnBa2Cu307-X' (Ln is Nd.

Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm or yb), Bi25r2Ca2Cu301o or a part of this Bi is replaced with Pb, for example (Bioy Pbo3)z 5rzCaxCu301
6, etc., Ti2B a3 Ca2 Cu3oi01, Ba1-xKxBi03-8, etc., it is possible to produce an element having the same characteristics as described in the embodiment.

Furthermore, the production of the superconducting film constituting the element of the present invention is not limited to the electron beam evaporation method using an oxygen gas atmosphere as in the embodiment, and other sputtering methods may be used.

It may be manufactured by a CVD method, a laser vapor deposition method, or the like.

<Effects of the Invention> In the present invention, the superconducting film serving as the detection part is composed of fine particles whose crystal axes are randomly oriented, and the grain boundary effect lowers the critical current density and critical magnetic field, thereby increasing the sensitivity to electromagnetic waves and magnetic fields. The present invention is intended to constitute a superconducting membrane weakly coupled device that improves the performance and has a large output voltage.

The above-mentioned weak coupling element is thought to be due to the grain boundary structure in which the grain boundaries of the superconducting particles constituting the detection section are in contact with the crystal axes of the grains on both sides at different planes.

Therefore, it is possible to fabricate a superconducting film with a simple structure, high precision, and high speed characteristics without requiring any special microfabrication technology for the bridging portion of the weakly coupled element.

[Brief explanation of drawings]

FIG. 1 is a front view (a) and a cross-sectional view (b) of a first embodiment of the present invention, and FIG. 2 is a perspective view showing the process of the element of the first embodiment. Fig. 3 is an electrical characteristic diagram of the element of the first embodiment, Fig. 4 is a front view of the second embodiment of the present invention, Fig. 5 is a process perspective view of the element of the second embodiment, and Fig. 6 is FIG. 3 is a diagram showing differences in electrical characteristics depending on the structure of a superconducting film. 1 is a substrate, 2 is a Pt film, 3 is a superconducting film in the electrode section, 4 is a superconducting film in the detection section, 5 and 5' are current electrodes, and 6 and 6' are voltage electrodes. Agent Patent attorney Takeshi Sugiyama (1 other person) (Q) Figure (α) Cb) Figure (α) Cb)

Claims (1)

[Scope of Claims] 1. An element using weak coupling of a superconducting film, characterized in that a superconducting film made of fine particles in which crystal axes are randomly oriented is used at least in the operating portion of the superconducting film. Coupling element. 2. The superconducting film weakly coupled device according to claim 1, wherein the superconducting film is an oxide superconductor. 3. The superconducting film weakly coupled device according to claim 1 or 2, wherein the superconducting film made of fine particles whose crystal axes are randomly oriented is a film deposited on a heat-resistant metal substrate.