JP2539584B2 - Superconducting quantum interference device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of superconducting electronic devices including Josephson junctions using oxide superconductors, and more particularly to a superconducting quantum interference device capable of operating above liquid nitrogen temperature (77K).

[0002]

2. Description of the Related Art A conventional element composed of an SNS junction using Ag as a normal conductive layer is a nature. Volume 354 (1991)
Pp. 513-515 (Nature, vol. 3)
54 (1991) pp 513-515). The above structure is shown in FIG. In the above-mentioned conventional technique, by depositing an oxide superconductor from a diagonally upper left direction on a substrate having a step formed on the surface in advance, a gap of several hundred nm is formed in the deposited oxide superconducting layer (S layer), and a space is formed. Form two superconducting electrodes that are electrically separated. The SNS junction is formed by evaporating Ag as normal conductive metal 3 (N layer) from the diagonally right upper side between the two superconducting electrodes without taking out the material from the device in which the superconducting electrode is formed. In addition, IEEE Transaction on Magnetics. Volume 27 (1991) pp. 3098-3101 (IEEE TRANSACTION MAGNE
TICS, vol. 27 (1991) pp3098-3
101) Forrester et al.
The structure of the NS junction is reported. The above structure is a structure in which the Au film 5 is formed on the oxide superconducting thin film, two superconducting electrodes are formed by microfabrication, and they are connected by the Au film 6.

[0003]

The above-mentioned prior art has a problem that it is difficult to integrate the elements because it is necessary to arrange the elements along the steps formed on the substrate.
Further, in order to control the superconducting current (Ic) and the junction resistance (Rn) flowing through the junction of the device by the distance between the electrodes and the film thickness of the normal conductive layer, the contact is sufficiently lower than the resistance of the normal conductive metal itself. I need resistance. However, the above-mentioned conventional SNS
In the joining, there is a problem that the joining resistance is about two orders of magnitude higher than the resistance of Au itself which is a normal conductor, and the joining resistance is dominated by the contact resistance.

An object of the present invention is a Josephson junction which can be operated at a temperature higher than the liquid nitrogen temperature, has excellent stability and reproducibility of the junction characteristics, and can be designed and controlled for the characteristics and the integration of elements, and It is an object to provide a superconducting quantum interference device (SQUID) having them.

[0005]

In order to solve the above-mentioned object, the present invention is a planar type SNS as shown in FIG.
Form a bond. In the above structure, a three-layer film composed of an oxide superconductor layer, a first normal metal layer made of a metal or an alloy, and a protective film layer made of a chemically stable metal is spatially arranged on the same plane. It is formed by connecting two superconducting electrodes with a normal conducting metal by separating them, forming two electrodes, and forming a second normal conducting metal layer made of a metal or an alloy so as to partially overlap them. There is. The first normal conductive metal layer and the second normal conductive metal layer may be different materials.

[0006]

The chemically stable protective film layer 9 such as Au and Pt used in the present invention acts as a protective film in the microfabrication process, and the first normal metal layer 8 is used in the microfabrication process. , Prevent it from changing. Therefore,
First material that can be connected to oxide superconductors with low contact resistance
Can be used for the normal conductive metal layer 8.

Further, by making the width of the second normal-conducting metal layer narrower than the width of the superconducting conductive electrode so that the distance between the two spatially separated superconducting conductive electrodes is constant, the superconducting current can be made uniform between the superconducting conductive electrodes. Can be flushed to. As a result, the characteristics and reproducibility of the element are improved, and the element can be easily designed and manufactured.

[0008]

EXAMPLES [Example 1] Examples of the present invention will be shown below.
The present invention will be further described.

The fabrication process will be described with reference to FIG. A SrTiO ₃ (100) single crystal is used as the substrate 11, and a HoBa ₂ Cu ₃ Ox (hereinafter abbreviated as HBCO) oxide superconducting thin film 12 is formed thereon. The film formation is a reactive deposition method in which Ho, Ba, and Cu metals are deposited on a substrate placed in oxygen plasma, and active oxygen in the oxygen plasma reacts with the metal to grow an oxide thin film on the substrate. The film formation temperature is 650 ° C., the film formation rate is 1.2 nm / min, and the film thickness is 50 to 200 nm. Also, as oxygen plasma, pure oxygen gas having a pressure of 0.01 Pa is applied with 875 gauss
ECR (electron cyclotron resonance) plasma discharged by irradiating the microwave of 2.45 GHz in the magnetic field is used, and the microwave power is 120 W. After the film formation, the film is cooled to 250 ° C. in oxygen plasma at a rate of 10 ° C./min. The oxygen plasma is stopped, the vacuum chamber is evacuated to a vacuum degree of 0.0001 Pa or less, and the Ag thin film 13, which is the first normal conductive metal, is vapor-deposited on the superconducting thin film at a substrate temperature of 250 ° C. The formation of the Ag thin film is continuously performed without taking out the material on which the superconducting thin film is formed from the forming apparatus. At this time, using a metal mask, an Ag thin film is formed only in the central portion of the sample, and the thickness of the Ag film is set to 10 to 50 nm. Further, similarly, an Au thin film 14 as a protective film is vapor-deposited thereon. The film thickness of the Au film is 50 to 100 nm.
By depositing Ag on the oxide superconductor without taking out the material to the outside, it is possible to prevent the normal metal / superconductor interface from being contaminated in a later step. Further, by chemically depositing Au which is chemically stable on Ag, it is possible to prevent alteration such as oxidation of Ag in the subsequent fine processing process.
The produced HBCO thin film was c-axis oriented, and the c-axis of HBCO was oriented perpendicular to the substrate surface. Tc of thin film is 85-9
0K and Jc are 77K and about 10 ⁶ A / cm ² at 0 magnetic field. Next, the sample is taken out from the film forming apparatus, an electron beam drawing resist 15 is applied to 300 nm, and a pattern is formed by an electron beam drawing method. A cross-sectional structure diagram at this point is shown in FIG. The structure of FIG. 4A is etched using the ECR plasma etching method using 0.01 Pa of Ar gas. The state after etching is shown in FIG. After the etching, the resist 15 is entirely removed by irradiation with oxygen plasma (FIG. 4 (c)). Next, a resist 15 for electron beam writing is applied again, and a lift-off pattern for producing the second normal conductive metal layer 16 is produced by the electron beam writing method (FIG. 4 (d)). After forming the lift-off pattern, an Au film, which is the second normal-conducting metal layer 16, is vapor-deposited around the junction (see FIG. 4).
(e)), and the film thickness is 200 nm. Excessive Au film is removed by a lift-off method using acetone, leaving only a portion necessary for bonding (FIG. 4 (f)). The completed joint has the structure shown in FIG. The lateral width (W1) of the oxide superconducting thin film and the first normal-conducting metal layer at the joint portion is 10 μm, the interelectrode distance (L) is 0.2 μm, and the lateral width (W) of the second normal-conducting metal layer (W).
2) is 5 μm.

The current of the SNS junction manufactured in this way-
The voltage characteristic (77K) is shown in FIG. Ic is 170 μA. In addition, I when irradiated with a microwave of 10 GHz
The -V characteristic can be clearly observed up to 10 or more Shapiro steps. Furthermore, by applying an external magnetic field, the Fraunhofer pattern peculiar to the Josephson junction can also be observed, and the Josephson junction having a uniform junction can be manufactured. In addition, the second Au containing 20% Ag is used.
When an SNS junction having the same structure as that of the normal conductive metal layer 16 was prepared, Ic was reduced to about 1/3. Therefore, it was confirmed that the joining characteristics can be controlled by alloying the normal conductive metal layer. Further, even when SrTiO ₃ (110) is used for the substrate, the Josephson junction can be manufactured using the above process.

Next, a dcSQUID is produced using the SNS junction produced in this way. Under the same conditions as the SNS junction manufacturing process described above, as shown in FIG.
After forming a three-layer film of protective film layer / normal conductive metal layer / oxide superconducting layer on a SrTiO ₃ (100) substrate 1 of m × 10 mm, the three-layer film was formed into a square film by electron beam drawing and dry etching. Process into a washer ring shape. Then, under the same conditions as SNS junction fabrication processes mentioned above, Wasshi
In the groove that reaches the substrate 1 in two places,
Normal metal that divides and connects the upper surface of the divided three-layer film
And a layer forming the SNS junction 18 to produce a d cSQUID. The outer diameter and the inner diameter of the washer ring 17 are 300 μm and 100 μm, respectively, and the joining width is 10 μm and the joining distance is 0.2 μm.

FIG. 7 shows the output voltage of the produced SQUID--
The magnetic field characteristics are shown. Practical characteristics can be obtained at 100 Hz or higher. Also, in the above process, 0.5 × 0.5 m
an area of m ^2, bonding width 10 [mu] m, the inter-electrode distance 0.2μm
100 SNS junctions are arranged in series. When these characteristics were examined, it was confirmed that all the junctions exhibited Josephson characteristics. Thus, the device structure of the present invention facilitates device integration. In this embodiment, the oxide superconducting thin film 12 is not limited to the HBCO thin film,
Of course, other high-temperature superconductor thin films such as YBa ₂ Cu ₃ 0x, Bi-based, Tl-based, etc. can be used, and as for the normal conductive metal, Au, Ag, Pt, Rh,
Re, Ir, Ru and alloys containing them can be used. The first normal conductive metal layer and the second normal conductive metal layer may be made of different materials. Also regarding the substrate 1,
If a superconducting thin film with good characteristics can be formed, M
Other materials such as gO and NdGaO ₃ can also be used, and the plane orientation may be other than the (100) plane.

[Second Embodiment] Next, a second embodiment of the present invention will be described with reference to FIG.

A film thickness of 2 is formed on the NdGaO ₃ (100) single crystal substrate 1 by a multi-target laser ablation method.
00 nm YBa ₂ Cu ₃ Ox (YBCO) superconducting thin film,
SrTiO ₃ thin films having a film thickness of 300 nm are sequentially manufactured. The manufacturing conditions are as follows for both YBCO and SrTiO _{3. The} targets are YBa ₂ Cu ₃ Ox sintered body and SrTi.
O ₃ single crystal is used. A KrF excimer laser with a wavelength of 248 nm is used at a frequency of 5 Hz and an energy density of 1.1 J / c.
Used at m ² . The substrate temperature is 700 ° C., and the oxygen partial pressure during film formation is 100 mTorr. This YBCO thin film is used as the ground plane 21, and the SrTiO ₃ thin film is used as the interlayer insulating film 20a between the ground plane 21 and the washer ring 17. Sr produced in this way
_On the two-layer film of TiO ₃ / YBCO, the _three- layer film of protective film layer / normal conductive metal layer / oxide superconducting layer by the method shown in Example 1.
23 is formed, and fine processing is performed by an electron beam drawing method, and the washer ring 17 and the SNS junction 18 (the upper surface of the protective film layer 22).
The second normal metal layer 10 is connected between the two) to form a dcSQUID. Ground plane 21
By producing the magnetic field, the magnetic flux that has passed through the center of the washer ring 17 always passes through the SrTiO ₃ thin film portion between the ground plane 21 and the washer ring 17 , so that a magnetic field is uniformly applied to the dcSQUID. As a result, SQUID
Improves sensitivity and stability, and simplifies design. Further, in order to form the input coil 19 on this washer ring, the SrTiO ₃ interlayer insulating film 20 with a film thickness of 200 nm is formed.
b is formed, and an HBCO thin film having a film thickness of 100 nm is formed thereon. This HBCO thin film is processed into a coil having a line width of 10 μm by electron beam drawing and ECR plasma etching. At this time, since the crystallinity is lowered on the joint portion, the HBCO thin film is processed so as to avoid the SNS junction 18 to form the input coil 19. Furthermore, a film thickness of 100 nm
SrTiO ₃ interlayer insulating film 20c , HBCO with a film thickness of 100 nm
A film was prepared and processed in the same manner, and the terminal 24 was taken out from the center of the input coil. FIG. 8 shows a plan view and a cross-sectional view of the SQUID element having the input coil. Even if the input coil 19 is manufactured, the SQUID
The characteristics of the element are not particularly deteriorated. The input coil 19 and the SQUID element are magnetically coupled. In addition,
In this embodiment, the oxide superconducting thin film and the ground plane are not limited to the YBCO thin film and the HBCO thin film, and needless to say, other high temperature superconductor thin films can be used. Regarding the substrate, other materials such as MgO and NdGaO ₃ can be used as long as a superconducting thin film having good characteristics can be formed, and the plane orientation may be other than the (100) plane. In this embodiment, SrTiO 3 is used as the interlayer insulating film.
3 was used, but the compatibility between the oxide superconductor and the lattice was good,
If the material does not easily react with the superconductor, NdGaO ₃ ,
LaAlO ₃ , YAlO ₃ , MgO, Y ₂ O ₃ , PrBa ₂
Even if another material such as Cu ₃ Ox is used, the same effect as in the above embodiment can be obtained.

[0015]

According to the present invention, the following effects can be obtained.

By forming a three-layered film of Au / normal conductive metal / superconductor in advance and using Au as a protective film for the normal conductive metal, it is possible to manufacture a bond using a normal conductive metal which is easy to react. Furthermore, Au, Ag, Pt, Rh, R are added to the normal conductive layer.
By using e, Ir, Ru, and alloys containing these, the resistance (Rn) of the normal conductive layer can be changed, whereby the bonding characteristics can be controlled and designed by the distance between the electrodes or the resistance value of the normal conductive metal itself. it can. As a result, the characteristics of the SNS junction and the characteristics of the SQUID can be designed and optimized.
The device could be integrated.

[Brief description of drawings]

FIG. 1 is a cross-sectional structure diagram of an SNS junction using a step formed on a substrate.

FIG. 2 is a cross-sectional structural diagram of a planar type SNS junction manufactured by a conventional technique.

FIG. 3 is a structural diagram of a planar SNS junction of the present invention.

FIG. 4 illustrates a fabrication process for a planar SNS junction of the present invention.

FIG. 5 is a current-voltage characteristic of the manufactured planar SNS junction.

FIG. 6 dc using the planar SNS junction of the present invention
It is a top view of SQUID.

FIG. 7 is an output voltage-magnetic field characteristic (77K) of the produced dcSQUID.

FIG. 8: dcSQUID with input coil
(a) A plan view and (b) a cross-sectional view.

[Explanation of symbols]

1 ... Substrate, 2 ... Superconducting electrode, 3 ... Normal conductive metal, 4 ... Step, 5 ... Au film, 6 ... Au film,
7 ... Oxide superconductor layer, 8 ... 1st normal conductive metal layer, 9 ... Protective film layer, 10 ... 2nd normal conductive metal layer, 11 ... Substrate, 12 ... ..Oxide superconducting thin films, 1
3 ... Ag thin film, 14 ... Au thin film, 15 ... Resist, 16 ... Second normal conductive metal layer, 17 ... Washer ring, 18 ... SNS junction, 19 ... Input coil, 20 ... Interlayer insulating film, 21 ... Ground plane.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ippei Imagawa 1-280 Higashi Koikeku, Kokubunji, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Uki, 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi, Ltd. Central Research Laboratory (72) Inventor Tokumi Fukasawa 1-280 Higashi Koigokubo, Kokubunji City, Tokyo Central Research Laboratory, Hitachi, Ltd. (56) Reference JP-A-2-298085 (JP, A) JP-A-2-46781 ( JP, A) JP-A-5-55646 (JP, A)

Claims (3)

(57) [Claims] 1. A facing the first and second three-layered film in which an oxide on a substrate superconductor layer and the first normal metal layer and the protective layer made of normal metal are laminated in this order to each other Superconducting-normal conducting-superconducting junction formed by connecting the first and second three-layer films with a second normal conducting metal film connected to the upper surface of each protective film layer. Part, and the normal conductive metal of the protective film layer is made of Au or Pt, and the first normal conductive metal layer is made of a material that can be connected to the oxide superconductor layer with lower contact resistance than the protective film layer. A superconducting quantum interference device characterized in that 2. The first normal metal layer is Ag or Ag.
The superconducting quantum interference device according to claim 1, wherein the superconducting quantum interference device is an alloy containing. 3. The superconducting quantum interference device according to claim 1, wherein two superconducting-normal-conducting-superconducting junctions are formed in a superconducting loop.