JP2701736B2 - Superconducting element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting device exhibiting a specific performance by using superconductivity, such as a detecting device for a minute magnetic field, a voltage standard, a microwave or millimeter wave detecting circuit, a high-speed digital circuit, and an analog data processing circuit. The present invention relates to an element structure of a superconducting element suitable for high performance, high reliability, high durability, and the like, particularly related to the field of electronics, such as miniaturization and high output signal voltage or high current.

[0002]

2. Description of the Related Art Superconducting elements of the microbridge type exhibiting Josephson characteristics are used for superconducting quantum interference elements, voltage standards, microwave detecting elements, mixers and the like used in detecting devices for minute magnetic fields. That is, the superconducting current component reacts to the irradiation of the microwave by the Josephson effect, and a current step is generated with a cycle of a DC voltage proportional to the frequency. Such a response phenomenon to microwaves is used in microwave detection elements, mixers, and the like. In particular, the ratio between the microwave frequency and the generated voltage is a constant amount given as a ratio between the Planck constant, which is a physical constant, and the elementary charge. Based on this principle, a superconducting device is used as a voltage standard. Further, the voltage-current characteristics are modulated with respect to the magnetic field. One superconducting element in the superconducting loop
The superconducting quantum interference device in which one or two superconducting quantum interference elements are inserted is used for a highly sensitive magnetic field detector because the superconducting current changes periodically with a small period with respect to the applied magnetic field. The structure of a conventional microbridge type superconducting element is simple, and has a shape in which a part of the superconducting film is narrowed to a submicron size. The dimensions required for the microbridge are length and width,
This is the coherence length of the superconducting material constituting the microbridge, or a dimension several times as large as this. Since the coherence length of the metal-based superconducting material is several tens of nanometers or submicron, a microbridge-type superconducting element using the metal-based superconducting material is manufactured by using a submicron microfabrication technique.

[0003]

The above prior art has the following problems.

[0004] The first relates to the dimensions of the microbridge superconducting element. The superconducting coherence length of the oxide material, which is a high-temperature superconductor, is about 1 nm, which is extremely short as compared with a conventional metal-based superconducting material. In the superconducting element of the microbridge type, the phase of the superconducting waveguide function of each electrode is shifted at the microbridge part, and to show weak coupling characteristics, the length, width and thickness of the microbridge must be within several times the superconducting coherence length Need to be suppressed. Therefore, in order to obtain a microbridge type superconducting element using an oxide superconducting material, a fine processing technique of several times such a short coherence length, that is, at most 10 nm is required. Such a nanometer processing requires a very high technical level, and is very difficult even with an electron beam lithography technique.

The second problem relates to the current capacity of the microbridge superconducting element. The current capacity of the microbridge superconducting element is determined by the critical current density of the superconducting material used for the superconducting element and the cross-sectional area of the microbridge. When an oxide-based superconducting material is used, as described above, the cross-sectional area corresponding to a dimension exhibiting weak coupling characteristics is several nanometers square. Assuming that the critical current density at the operating temperature of the superconducting material is 10 6 amps, the current that can flow is only 0.1 μA at most. Normally, the superconducting current required for using the microbridge superconducting element for a superconducting quantum interference device or the like is several microamps to several hundred microamps. As long as the structure of a microbridge element using a conventional metal-based superconducting material is applied, it is not possible to obtain an oxide-based microbridge element that can operate practically.

An object of the present invention is to obtain a structure of a microbridge type superconducting element having a workable dimension by using an oxide superconducting material which can operate at a high temperature. Also,
An object of the present invention is to provide a structure of an oxide-based microbridge element having a current capacity that can be operated practically.

[0007]

In order to achieve the above object, the present invention has at least a pair of superconducting electrodes forming a microbridge structure on a substrate, wherein the superconducting electrodes are
It is composed of a laminated film composed of two or more films each of a superconducting film and a normal conducting film which is in a normal conducting state at an operating temperature.

In such a superconducting element, as a superconducting material in a laminated film forming a pair of superconducting electrodes and a microbridge, an oxide composed of Y, Ba, Cu,
oxides composed of i, Sr, Ca, and Cu, Tl, B
an oxide composed of a, Ca, and Cu, and as a normal conductive material, the constituent elements of these superconducting materials as main components;
An oxide having the same crystal structure as that of the superconducting material and substituting some of the constituent elements of the superconducting material to reduce the superconducting critical temperature or using an oxide having normal conduction characteristics is used.

In such a superconducting element, Y-Ba
-As a normal conductive film in the laminated film used together with the Cu oxide superconducting film, a part of Cu is Co, Fe, Ni, Zn, A
1, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,
Oxide substituted with a transition metal element such as W, or material in which a part or all of Y is replaced with a rare earth metal such as La, Pr, Ce, etc., to reduce the superconducting critical temperature or to make it a normal conducting property Is used.

Further, in such a superconducting element, B
i-Sr-Ca-Cu oxide or Tl-Ba-C
As a normal conductive film in a laminated film used together with a superconducting film such as an a-Cu oxide, a part of Ca is converted to Y, La, Ce, Pr, N
d, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb
Substitute with an element selected from rare earth elements to reduce the superconducting critical temperature or use a material having normal conduction characteristics.

[0011] As a laminated film constituting the superconducting element, the thickness of each superconducting film constituting the superconducting element is 10 nm or less and 2 n
m or more.

Further, in such a superconducting element, the length of the microbridge having the constricted structure is set to 0.2 μm or less and the width is set to 0.4 μm or less.

In the superconducting element, as the superconducting film and the normal conducting film constituting the laminated film, an oxide thin film having a crystal c-axis having a direction perpendicular to the film plane is used.

In the superconducting element, a normal conducting film independent of the microbridge is superposed on the microbridge part constituting the superconducting element via an insulating layer also made of an oxide thin film, and a voltage is applied to the normal conducting film. When applied, the structure is such that the electrical conduction characteristics of the microbridge portion are modulated.

[0015]

[Function] The superconducting weak coupling characteristics of the microbridge type superconducting element are due to the fact that the phase of the superconducting waveguide function is easily changed by making the microbridge dimension close to the coherence length. It is. Whether the coupling portion of superconductivity is a strong coupling whose phase is hard to change or a weak coupling can be best determined by the magnetic field dependence of the superconducting critical current. With strong coupling, the superconducting current flows up to the critical magnetic field of the bulk superconducting material. This is not due to a decrease in superconducting current due to a change in phase, but to a decrease in the amplitude of the wave function. If the coupling is weak, the application of a magnetic field causes a phase change, so that the superconducting current becomes zero at a low magnetic field depending on the size, regardless of the critical magnetic field.

The unit of the dimension where the phase of the superconducting waveguide function changes spatially is the coherence length in the case of a bulk sample. When the thickness of the superconducting thin film is reduced, the superconducting characteristics and the wave function that determines the superconducting characteristics are 2
Become dimensional. A feature of the two-dimensional superconducting characteristic is that phase fluctuation is likely to occur, and therefore, a phase change may occur in a dimension longer than the coherence length of the bulk sample. The thickness of such a bulk sample, which changes from the phase characteristic to the two-dimensional phase characteristic, is several unit cell thicknesses of the oxide superconducting crystal, and is 10 nm or less in film thickness.
If the thickness of the superconducting thin film is set to several nm, the phase can be changed at the microbridge even if the dimension in the in-plane direction is set to several tens nm. If the thickness of the superconducting thin film is set to a size close to one unit cell, the size of the microbridge can be set to a submicron size. However, if the film thickness is too small, a superconducting current required for an element cannot be obtained. If the superconducting thin film constituting the microbridge is composed of a laminated film composed of two or more layers each of a superconducting film and a normal conducting film which is in a normal conducting state at an operating temperature, the superconducting critical current changes its phase. It is not limited to the required dimensions. By increasing the number of superconducting layers in the laminated film, the critical current can be set according to the current level required from device design. In addition, in the layered structure of the superconducting film and the normal conducting film, superconducting coupling occurs between the superconducting layers via the normal conducting layer to form an integral superconductor, and the two-dimensional superconducting waveguide function is not lost.

In order for the two-dimensional wave function to be maintained even when the superconducting films are stacked, and for the superconducting critical current to be the product of the number of films, the crystal orientation and atomic arrangement rules must be extended to the film above the stacked film. It is necessary to maintain crystallinity such as properties. As the normal conductive material, use a material which has the same crystal structure as the superconductive material, and has a normal conductive characteristic by substituting a part of the constituent elements of the superconductive material. Is necessary for such a request. Furthermore, since the oxide-based superconducting material exhibits different superconducting properties in the ab plane of the crystal and in the c-axis direction, the c-axis of the crystal is in relation to the film plane in both the superconducting film and the normal-conducting film constituting the laminated film. By using an oxide thin film having a vertical orientation, uniform characteristics can be secured in a plane.

The phase stability of the wave function of the two-dimensional superconducting material varies depending on the carrier concentration. Therefore, by applying an electric field to the microbridge where the phase is apt to change, the carrier concentration is changed to control the superconducting characteristics such as the critical current and the voltage-current characteristics of the superconducting element.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on the embodiments described below.

(Embodiment 1) As shown in FIG.
0) On the oriented strontium titanate substrate 1, a film thickness of 6 n
m-Y-Ba-Cu oxide thin film 2 is formed by laser vapor deposition. The laser evaporation source is a KrF excimer laser, the substrate temperature during film formation is 700 degrees Celsius, and the oxygen partial pressure is 30 Pa. Y formed under such conditions
The -Ba-Cu oxide thin film shows an orientation in which the c-axis of the crystal is perpendicular to the film surface.

Next, a 6 nm-thick Pr-Ba-Cu oxide thin film 3 is formed by a laser vapor deposition method. The deposition conditions are the same as for the Y-Ba-Cu oxide thin film. Such Y
The steps of forming the -Ba-Cu oxide thin film and the Pr-Ba-Cu oxide thin film are each repeated five times to obtain a superconducting-normal conducting laminated film 4 having a thickness of 50 nm as a whole. The superconducting-normal-conducting laminated film has an orientation in which the c-axis of the crystal is perpendicular to the film surface as a whole.

As shown in FIG. 2, a pattern as a microbridge 12 and a pair of superconducting electrodes 11 is formed on the superconducting-normal conducting laminated film. The pattern is formed by forming a resist film pattern by electron beam lithography,
The resist film pattern is transferred to the superconducting-normal-conducting laminated film by an ion beam etching method using the method. Through such steps, a microbridge having a length of 50 nm and a width of 100 nm is obtained.

The micro-bridge type superconducting element thus manufactured is irradiated with microwaves to generate a voltage-
Current steps were detected at constant voltage intervals in the current characteristics. Further, as shown in FIG. 3, there is shown a so-called Fraunhofer pattern in which the superconducting critical current increases / decreases at a constant magnetic field period when a magnetic field is applied. The period of the magnetic field is about 100 Oe. These characteristics indicate that the microbridge superconducting element has a weak coupling characteristic.

Such a superconducting weak-coupling property indicates that the superconducting layer is made of Bi-Sr-Ca other than Y-Ba-Cu oxide.
It can also be obtained by producing a microbridge type superconducting element using other superconducting oxides such as -Cu oxide and Tl-Ba-Ca-Cu oxide. Further, the same applies to the case where a rare earth metal such as La or Ce is used for the Pr site in addition to the Pr—Ba—Cu oxide thin film as the normal conductive layer.
Further, as a normal conductive layer used together with the Y-Ba-Cu oxide superconducting layer, a part of Cu is changed to Co, Fe, Ni, Zn,
Al, Ti, Zr, Hf, V, Nb, Ta, Cr, M
Similar characteristics are exhibited even when an oxide substituted with a transition metal element such as o or W is used. As a normal conductive layer in a laminated film used together with a superconducting layer such as Bi-Sr-Ca-Cu oxide or Tl-Ba-Ca-Cu oxide, part of Ca is Y, L
a, Ce, Pr, Nd, Sm, Eu, Gd, Tb, D
Even when an oxide substituted with an element selected from rare earth elements such as y, Ho, Er, and Yb is used, the same weak coupling characteristics are exhibited.

By adopting the above-mentioned structure, the device dimensions can be formed by using a pattern forming processing technique such as an electron beam drawing method, and the device can be operated.
In addition, such microconducting superconducting elements having superconducting weak coupling characteristics are voltage standard and microwave detecting elements,
It can be used for a mixer and the like, and also for a superconducting quantum interference device.

Example 2 A superconducting quantum interference device having a microbridge superconducting device as an element device is manufactured.

A Bi-Sr-Ca-Cu oxide thin film having a thickness of 100 nm is formed on the (100) -oriented strontium titanate substrate 1 by a laser vapor deposition method. The laser evaporation source is a KrF excimer laser, the substrate temperature during film formation is about 700 degrees Celsius, and the oxygen partial pressure is 30 Pa. The Bi-Sr-Ca-Cu oxide thin film formed under such conditions exhibits an orientation in which the c-axis of the crystal is perpendicular to the film surface.

As shown in FIG. 4, a pattern as a superconducting wiring is formed on the Bi-Sr-Ca-Cu oxide thin film. The pattern is formed by forming a resist film pattern by an electron beam lithography method, etching by a chemical corrosion method, and changing the resist film pattern to Bi-Sr-Ca-C.
Transfer to u-oxide thin film. Since the side etching progresses in the etching by the chemical corrosion method, about 45
A pattern 15 having a degree of taper is obtained. By such a process, the superconducting loop 13 of the superconducting quantum interference device
Is obtained.

Next, a 6 nm-thick Bi-Sr-Ca-
Substituting a part of the Cu oxide thin film and the Ca site with Y;
Bi-Sr-Ca (Y) -Cu oxide thin films having normal conductivity characteristics are formed alternately by five layers by laser vapor deposition.
A pattern as a microbridge 12 and a pair of superconducting electrodes 11 is formed on the superconducting-normal conducting laminated film by using an electron beam drawing method and an ion beam etching method. Through such a manufacturing process, two microbridge superconducting elements are obtained.

In the superconducting quantum interference device manufactured by such a method, when a magnetic field is applied, the superconducting critical current has a value of 2 as calculated from the quantum flux and the size of the superconducting loop.
It shows a regular change with a milli-oersted period. When the element is biased to a voltage state, as shown in FIG. 5, when the applied magnetic field is proportional to an integral multiple of the flux quantum21,
A periodic change is shown between the cases 22 that are proportional to a half-integer multiple.

By adopting the above structure, the element dimensions can be formed by using a pattern forming processing technique such as an electron beam drawing method, and the element can be operated.
Further, the superconducting quantum interference device using the microbridge superconducting device as described above has a highly sensitive magnetic field characteristic and has sensor performance that can be used in fields such as medical measurement.

(Embodiment 3) A three-terminal element having a microbridge superconducting element having the structure shown in FIG. 6 as an element element is obtained by the following method.

On a (100) -oriented strontium titanate substrate 1, a 6-nm-thick Y-Ba-Cu oxide thin film and a portion of Cu were substituted with Co to obtain a YB having normal conductivity characteristics.
An a-Cu (Co) oxide thin film is formed alternately by five layers by laser vapor deposition. A pattern as the microbridge 12 and the pair of superconducting electrodes 16 and 17 is formed on the superconducting / normal conducting laminated film by using an electron beam drawing method and an ion beam etching method.

The strontium titanate thin film 18 is laminated as a gate insulating film by a laser vapor deposition method so as to cover the microbridge portion. KrF as laser evaporation source
Substrate temperature during film formation is 60 degrees Celsius
0 degree and the oxygen partial pressure is 30 Pa.

Next, a resist pattern as a gate electrode is formed in advance by an electron beam drawing method. Further, an Au film is formed by a vacuum evaporation method. The Au gate electrode film 19 is obtained by a lift-off method, that is, by removing the Au film portion on the resist together with the resist using a solvent.

The voltage-current characteristics of the microbridge channel portion are examined while applying a voltage signal to the gate electrode of the superconducting three-terminal device thus manufactured. The superconducting critical current increases as compared with 24 when a few volts is applied to the gate electrode and 23 when the gate voltage is zero. On the other hand, when a positive voltage of several volts is applied to the gate electrode, 2
5. The superconducting critical current decreases and approaches zero. Therefore, the present superconducting three-terminal element has a function of switching between a zero voltage state and a finite voltage state.

By adopting the above structure, the dimensions of the element can be formed by using a pattern forming technique such as an electron beam drawing method, and the element can be operated.
Further, the superconducting three-terminal element having such control characteristics can be used for a data processing device such as a logic circuit or a storage circuit. Further, it can be used as an analog device such as a superconducting quantum interference device or a digital-analog conversion device.

[0038]

As described above, the superconducting element according to the present invention has the following effects.

(1) By adopting the present element structure,
By using a pattern formation processing technique such as an electron beam lithography method, the device dimensions can be formed and the device can be operated.

(2) A microbridge-type superconducting element having an arbitrary superconducting current that meets the requirements of the circuit or device can be obtained.

Therefore, superconducting devices such as superconducting quantum interference devices, voltage standards and microwave detecting devices, and mixers,
Further, application to a superconducting data processing circuit or the like becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a film structure of a superconducting element.

FIG. 2 is a structural view of a microbridge superconducting element.

FIG. 3 is a diagram showing a critical magnetic field-current characteristic of a superconducting element.

FIG. 4 is a structural diagram of a superconducting quantum interference device.

FIG. 5 is a diagram showing voltage-current characteristics of a superconducting quantum interference device.

FIG. 6 is a structural diagram of a three-terminal element.

FIG. 7 is a diagram showing control characteristics of a three-terminal element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Superconducting layer, 3 ... Normal conducting layer, 4 ... Superconducting-
Normal conductive laminated film, 11: superconducting electrode, 12: microbridge, 13: superconducting loop, 14: superconducting wiring, 15 ...
Connection part, 21 ... applied magnetic field proportional to an integral multiple of the flux quantum,
22: applied magnetic field proportional to a half integral multiple, 23: gate voltage zero, 24: gate voltage negative, 25: gate voltage positive.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Tokukai Fukasawa 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo Inside the Hitachi, Ltd. Central Research Laboratory (72) Inventor Kazumasa Takagi 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo (56) References JP-A-6-13665 (JP, A) JP-A-64-28875 (JP, A) JP-A-1-161785 (JP, A)

Claims (3)

(57) [Claims] 1. A substrate having at least one pair of superconducting electrodes forming a microbridge structure on a substrate, each of said superconducting electrodes comprising at least two films of a superconducting film and a normal conducting film which is in a normal conducting state at an operating temperature. It is composed of a laminated film ,
The thickness of each superconducting film constituting the laminated film is 2 nm or more and 1
0 nm or less, and the superconducting film is made of Y, Ba, Cu
Oxide consisting of Bi, Sr, Ca and Cu
Oxide, oxide composed of Tl, Ba, Ca and Cu
And the normal conducting film is the same crystal as the superconducting film.
Having a structure, wherein some of the constituent elements of the superconducting film are transition metal
Normal conductivity characteristics by substitution with element or rare earth element
Superconductive element characterized in that which was turned into. 2. The superconducting element according to claim 1, wherein both the superconducting film and the normal conducting film constituting said laminated film have a c-axis of a crystal perpendicular to the film plane. 3. A superconducting element according to claim 1, wherein a voltage is applied to said normal conducting film by forming a normal conducting film on at least a microbridge portion of said superconducting electrode via an insulating film. A superconducting element, wherein the electric conduction characteristics of the microbridge portion are modulated.