JPH07307498A - Superconducting element - Google Patents

Abstract

PURPOSE:To obtain a structure of a microbridge-type superconducting element which uses an oxide-based superconducting material capable of being operated at high temperature and whose size can be worked by a method wherein a superconducting electrode is constituted of a laminated film composed of two or more films each of superconducting films and of normal conduction films which are in a normal conduction state at an operating temperature. CONSTITUTION:At least one pair of superconducting electrodes 11 which form a microbridge structure 12 are provided on a substrate 1, and the superconducting electrodes 11 are constituted of a laminated film 4 composed of two or more films each of superconducting films 2 and of normal conduction films 3 in a normal conduction state at an operating temperature. For example, a Y-Ba-Cu oxide thin film 2 in a film thickness of 6nm is formed on a (100)-orientation strontium titanate substrate 1, and a Pr-Ba-Cu oxide thin film 3 in a film thickness of 6nm is then formed. A film formation process for the Y-Ba- Cu oxide thin film 2 and that for the Pr-Ba-Cu oxide thin film 3 are repeated respectively five times, and a superconducting-normal conduction laminated film 4 in a film thickness of 50nm is obtained as a whole.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art A microbridge type superconducting element exhibiting Josephson characteristics is used for a superconducting quantum interference element, a voltage standard or a microwave detecting element, a mixer, etc. used in a device for detecting a minute magnetic field. That is, the superconducting current component reacts to microwave irradiation by the Josephson effect, and a current step is generated with a direct current voltage proportional to the frequency as a cycle. 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 the ratio of the Planck constant, which is a physical constant, to the elementary charge. Based on this principle, superconducting devices are used as voltage standards. Furthermore, the voltage-current characteristic is modulated with respect to the magnetic field. 1 superconducting element in the superconducting loop
The superconducting quantum interference device with one or two inserted therein is used for a highly sensitive magnetic field detector because the superconducting current changes periodically with a minute period with respect to the applied magnetic field. The structure of a conventional microbridge type superconducting element is simple, and a part of the superconducting film is formed into a submicron size. The dimensions required for the micro bridge part are the length and width,
The coherence length of the superconducting material that constitutes the microbridge, or several times this size. Since the coherence length of the metal-based superconducting material is several tens of nm, or the dimension of submicron, the microbridge type superconducting element using the metal-based superconducting material is manufactured by using the submicron microfabrication technique.

[0003]

The above-mentioned prior art has the following problems.

The first is related to the dimensions of the microbridge type superconducting element. The oxide material, which is a high-temperature superconductor, has a superconducting coherence length of about 1 nm, which is extremely short as compared with the conventional metal-based superconducting material. In a microbridge type superconducting element, the phase of the superconducting waveguiding function of each electrode is shifted at the microbridge part and weak coupling characteristics can be shown by setting the microbridge length, width and thickness within several times the superconducting coherence length. Need to be kept to. Therefore, in order to obtain a microbridge type superconducting element using an oxide superconducting material, a microfabrication technique of several times such a short coherence length, that is, at most 10 nm is required. Such nanometer processing requires an extremely high technical level and is very difficult even with the electron beam drawing technology.

The second is related to the current capacity of the microbridge type superconducting element. The current capacity of the microbridge type 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 portion. When the oxide-based superconducting material is used, as described above, the cross-sectional area corresponding to the dimension exhibiting the weak bonding property is a few nanometer square. If the critical current density at the operating temperature of the superconducting material is 10 6 amperes, the current value that can be passed is only 0.1 microamperes at most. Usually, the superconducting current required to use the microbridge type superconducting device in a superconducting quantum interference device or the like is several microamperes to several hundred microamperes. As long as the structure of the conventional microbridge type element using a metal-based superconducting material is applied, it is not possible to obtain an oxide type microbridge type element that can operate practically.

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

[0007]

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

In such a superconducting device, as a superconducting material in a laminated film forming a pair of superconducting electrodes and a microbridge, an oxide composed of Y, Ba and Cu, B
Oxide consisting of i, Sr, Ca, and Cu, Tl, B
An oxide composed of a, Ca, and Cu is used, and as a normal conductive material, the constituent elements of these superconducting materials are the main components,
An oxide having the same crystal structure as that of the superconducting material and reducing the superconducting critical temperature by substituting a part of the constituent elements of these superconducting materials or having normal conductivity characteristics is used.

In such a superconducting device, Y-Ba
As a normal conductive film in a laminated film used together with a Cu oxide superconducting film, a part of Cu is Co, Fe, Ni, Zn, A
l, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,
Oxide substituted with a transition metal element such as W, or a material with a reduced superconducting critical temperature or a normal conduction characteristic by substituting a part or all of Y with a rare earth metal such as La, Pr, Ce To use.

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 a-Cu oxide, a part of Ca is Y, La, Ce, Pr, N.
d, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb
A material whose superconducting critical temperature is reduced by substitution with an element selected from rare earth elements such as, or whose normal conduction characteristic is used is used.

As a laminated film forming a superconducting element, the film thickness of each superconducting film to be formed is 10 nm or less, and 2n
The size should be at least m.

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

In the superconducting device, both of the superconducting film and the normal conducting film that form the laminated film use oxide thin films having a crystal c-axis oriented perpendicular to the film surface.

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

[0015]

[Function] The superconducting weak-coupling characteristic of the microbridge type superconducting element is due to the fact that the phase of the superconducting waveguiding function is easily changed by making the dimension of the microbridge close to the coherence length. Is. Whether the coupling of the superconducting cup ring is a strong coupling or a weak coupling in which the phase is hard to change can be best judged 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 due to the reduction of the amplitude of the wave function, not the reduction of the superconducting current due to the spatial change of the phase. In the case of weak coupling, 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 dimension, regardless of the critical magnetic field.

The unit of the dimension in which the phase of the superconducting waveguide function spatially changes is the coherence length in the case of the 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. The characteristic of the two-dimensional superconducting property is that phase fluctuation easily occurs, and therefore, the phase change can occur at a dimension longer than the coherence length of the bulk sample. The film thickness at which the phase characteristic of such a bulk sample changes to a two-dimensional phase characteristic is several unit cell thickness of the oxide superconducting crystal, and the film thickness is 10 nm or less.
If the thickness of the superconducting thin film is set to several nm, even if the dimension in the in-plane direction is set to several tens of nm, the phase change can be caused in the microbridge portion. If the thickness of the superconducting thin film is set to a size close to that of one unit cell, the size of the microbridge can be set to a submicron size. However, if the film thickness is too thin, the superconducting current required for the device cannot be obtained. If the superconducting thin film that constitutes the microbridge is composed of a laminated film consisting of two or more of each of the superconducting film and the normal conducting film that is in the normal conducting state at the operating temperature, the phase of the superconducting critical current changes. It is not limited to the required dimensions. By increasing the number of superconducting layers of the laminated film, the critical current can be set according to the current level required from device design. Moreover, 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-dimensionality of the superconducting waveguiding function is not lost.

Even if the superconducting films are laminated, the two-dimensionality of the wave function is maintained and the superconducting critical current is the product of the number of films. It is necessary to maintain the crystallinity such as sex. As the normal conductive material, use a material that has the constituent elements of the superconducting material as a main component, has the same crystal structure as the superconducting material, and has the normal conductivity characteristic by substituting a part of these constituent elements of the superconducting 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, in both the superconducting film and the normal conducting film constituting the laminated film, the c-axis of the crystal is relative to the film surface. By using an oxide thin film having a vertical orientation, it is possible to secure uniform characteristics in the plane.

The phase of the wave function of the two-dimensional superconducting material changes in spatial stability depending on the carrier concentration. Therefore, superconducting characteristics such as critical current and voltage-current characteristics of the superconducting element can be controlled by changing the carrier concentration by applying an electric field to the microbridge portion where the phase is apt to change.

[0019]

EXAMPLES The present invention will be described based on the following examples.

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

Next, a Pr-Ba-Cu oxide thin film 3 having a film thickness of 6 nm is formed by a laser deposition method. The film forming conditions are the same as those for the Y-Ba-Cu oxide thin film. Y like this
The steps of forming the —Ba—Cu oxide thin film and the Pr—Ba—Cu oxide thin film are repeated 5 times, respectively, to obtain a superconducting-normal conductive laminated film 4 having a thickness of 50 nm as a whole. The superconducting-normal conducting laminated film as a whole exhibits an orientation in which the crystal c-axis is perpendicular to the film surface.

As shown in FIG. 2, a pattern as a microbridge 12 and a pair of superconducting conductive electrodes 11 is formed on this superconducting-normal conductive laminated film. To form the pattern, a resist film pattern is formed by an electron beam drawing method and Ar is used.
The resist film pattern is transferred to the superconducting-normal conductive laminated film by an ion beam etching method using. By such a process, a microbridge having a length of 50 nm and a width of 100 nm is obtained.

The microbridge type superconducting element thus manufactured is irradiated with microwaves to generate a voltage-
A current step was detected at a constant voltage interval in the current characteristic. Further, as shown in FIG. 3, a so-called Fraunhofer pattern is shown in which the superconducting critical current increases and decreases with a constant magnetic field cycle in response to the application of a magnetic field. The period of the magnetic field is about 100 Oersted. These characteristics show that the microbridge type superconducting element has weak coupling characteristics.

Such a superconducting weak-coupling characteristic can be obtained by using Bi-Sr-Ca as a superconducting layer in addition to Y-Ba-Cu oxide.
It can also be obtained by producing a microbridge type superconducting element using another superconducting oxide such as -Cu oxide or Tl-Ba-Ca-Cu oxide. Further, as the normal conductive layer, 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.
Further, as a normal conductive layer used together with the Y-Ba-Cu oxide superconducting layer, a part of Cu is 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. Bi-Sr-Ca-Cu oxide, Tl-Ba-Ca-Cu oxide, etc. As a normal conductive layer in a laminated film used with a superconducting layer, a 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 binding property is exhibited.

By adopting the above-mentioned structure, the element size can be formed and the element can be operated by using the pattern forming processing technique such as the electron beam drawing method.
Further, such a microbridge type superconducting element having a superconducting weak coupling characteristic is a voltage standard or a microwave detecting element,
It can also be used in a mixer or the like, and also in a superconducting quantum interference device.

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

On the (100) oriented strontium titanate substrate 1, a Bi-Sr-Ca-Cu oxide thin film having a film thickness of 100 nm is formed by a laser 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 has an orientation in which the crystal c-axis is perpendicular to the film surface.

On the Bi-Sr-Ca-Cu oxide thin film, as shown in FIG. 4, a pattern as a superconducting wiring is formed. The pattern is formed by forming a resist film pattern by an electron beam drawing method and etching by a chemical corrosion method to form a resist film pattern of Bi-Sr-Ca-C.
Transfer to u oxide thin film. Since side etching progresses in etching by the chemical corrosion method, about 45
A pattern 15 having a degree taper is obtained. Through these steps, the superconducting loop 13 of the superconducting quantum interference device 13
The wiring 14 including is obtained.

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

In the superconducting quantum interference device manufactured by such a method, when the magnetic field is applied, the superconducting critical current is calculated as 2 from the quantum flux and the dimensions of the superconducting loop.
It shows a regular change with a period of Millierstead. When the element is biased in the voltage state, as shown in FIG. 5, when the applied magnetic field is proportional to an integral multiple of the magnetic flux quantum 21,
In the case of being proportional to a half-integer multiple, a periodic change is shown between 22.

By adopting the above-mentioned structure, it becomes possible to form the element by using the pattern forming processing technique such as the electron beam drawing method, and the element can be operated.
Further, the superconducting quantum interference device using the microbridge type superconducting device as described above has magnetic field characteristics with high sensitivity and has sensor performance applicable to the fields of medical measurement and the like.

(Embodiment 3) A three-terminal element having a microbridge type 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 Y-Ba-Cu oxide thin film having a film thickness of 6 nm and a part of Cu were replaced by Co to make a normal-conducting YB.
Five a-Cu (Co) oxide thin films are alternately formed by a laser deposition method. A pattern as a microbridge 12 and a 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.

A strontium titanate thin film 18 is laminated as a gate insulating film by laser deposition so as to cover the microbridge portion. KrF as a laser evaporation source
Substrate temperature at the time of film formation is 60 degrees Celsius using the excimer laser of
The oxygen partial pressure is 30 Pa.

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

The voltage-current characteristics of the microbridge type channel portion are examined while applying a voltage signal to the gate electrode of the superconducting three-terminal element thus manufactured. The superconducting critical current increases as compared with 24 when a negative voltage of several 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 the zero voltage state and the finite voltage state.

By adopting the above-mentioned structure, the element size can be formed and the element can be operated by using the pattern forming processing technique such as the electron beam drawing method.
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 memory 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 device according to the present invention has the following effects.

(1) By adopting this element structure,
By using a pattern forming processing technique such as an electron beam drawing method, the size of a device that can be formed becomes and the device can operate.

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

Therefore, a superconducting quantum interference device, a voltage standard or microwave detecting device, a superconducting device such as a mixer,
Furthermore, it can be applied to a superconducting data processing circuit.

[Brief description of drawings]

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

FIG. 2 is a structural diagram of a microbridge type superconducting element.

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

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]

1 ... Substrate, 2 ... Superconducting layer, 3 ... Normal conducting layer, 4 ... Superconducting-
Normal conductive laminated film, 11 ... Superconducting conductive electrode, 12 ... Micro bridge, 13 ... Superconducting loop, 14 ... Superconducting wiring, 15 ...
Connection part, 21 ... Applied magnetic field proportional to integer multiple of magnetic flux quantum,
22 ... Applied magnetic field proportional to half-integer multiple, 23 ... Zero gate voltage, 24 ... Negative gate voltage, 25 ... Positive gate voltage.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Tokumi Fukasawa, Tokumi Fukazawa, 1-280 Higashi Koikeku, Kokubunji, Tokyo (72) Central Research Laboratory, Hitachi, Ltd. (72) Kazumasa Takagi, 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi Central Research Laboratory

Claims (7)

[Claims] 1. A substrate having at least one pair of superconducting electrodes having a microbridge structure, wherein each of the superconducting electrodes comprises two or more films of a superconducting film and a normal conducting film in a normal conducting state at an operating temperature. A superconducting element comprising a laminated film. 2. The superconducting element according to claim 1, wherein the superconducting film forming the superconducting conductive electrode comprises an oxide composed of Y, Ba and Cu, an oxide composed of Bi, Sr, Ca and Cu, Tl, A superconducting element, which is selected from oxides of Ba, Ca, and Cu. 3. The superconducting element according to claim 1, wherein the normal conducting film has the same crystal structure as the superconducting film,
A superconducting element characterized by normalizing its characteristics by substituting a transition metal element or a rare earth element for some of the constituent elements of the superconducting film. 4. The superconducting element according to claim 1, wherein the thickness of each superconducting film constituting the laminated film is 10 nm or less and 2 nm or more. 5. The superconducting element according to claim 1, wherein the length of the twisted portion of the microbridge structure is 0.2 μm or less and the width is 0.4 μm or less. Superconducting element. 6. The superconducting element according to claim 1, wherein the c-axis of the crystal of each of the superconducting film and the normal conducting film forming the laminated film is perpendicular to the film surface. 7. The superconducting element according to claim 1, wherein a voltage is applied to the normal conducting film by forming a normal conducting film on at least the microbridge portion of the superconducting conductive electrode via an insulating film. A superconducting device, wherein the electric conduction characteristic of the microbridge portion is modulated.