JP2691065B2 - Superconducting element and fabrication method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting element and a manufacturing method. More specifically, the present invention relates to a superconducting element having a novel configuration and a method for manufacturing the same.

2. Description of the Related Art A typical element using superconductivity is a Chosefson element. The Josephson element has a configuration in which a pair of superconductors are coupled via a tunnel barrier, and can perform high-speed switching operation. However, the Josephson element is a two-terminal element, and requires a complicated circuit configuration to realize a logic circuit.

On the other hand, examples of a three-terminal element utilizing superconductivity include a superconducting base transistor and a superconducting FET. In FIG.
1 shows a conceptual diagram of a superconducting base transistor. The superconducting base transistor shown in FIG. 3 comprises an emitter 21 composed of a superconductor or a normal conductor, a tunnel barrier 22 composed of an insulator, a base 23 composed of a superconductor, a semiconductor isolator 24, and a normal conductor. The collector 25 is stacked. The superconducting base transistor is an element that performs high-speed operation with low power consumption using high-speed electrons that have passed through the tunnel barrier 22.

FIG. 4 shows a conceptual diagram of a superconducting FET. The superconducting FET shown in FIG. 4 has a superconducting source electrode 4 composed of a superconductor.
1 and the superconducting drain electrode 42 are arranged on the semiconductor layer 43 close to each other. The lower portion of the semiconductor layer 43 between the superconducting source electrode 41 and the superconducting drain electrode 42 is largely shaved and thin. Also, the semiconductor layer
A gate insulating film 46 is formed on the lower surface of 43, and a gate electrode 44 is provided on the gate insulating film 46.

The superconducting FET is an element that performs high-speed operation with low power consumption by controlling the superconducting current flowing through the semiconductor layer 43 between the superconducting source electrode 41 and the superconducting drain electrode 42 by the gate voltage by the proximity effect.

Further, a three-terminal superconducting element in which a channel is formed by a superconductor between a source electrode and a drain electrode and a current flowing through the superconducting channel is controlled by a voltage applied to a gate electrode has been disclosed.

PROBLEM TO BE SOLVED BY THE INVENTION Superconducting base transistor and superconducting FET described above
Have a portion where a semiconductor layer and a superconductor layer are laminated. However, it is difficult to produce a stacked structure of a semiconductor layer and a superconductor layer using an oxide superconductor that has been studied in recent years. In addition, even if this structure can be manufactured, it is difficult to control the interface between the semiconductor layer and the superconductor layer,
The device did not operate satisfactorily.

Further, since the superconducting FET utilizes the proximity effect, the superconducting source electrode 41 and the superconducting drain electrode 42 must be made close to each other within about several times the coherence length of the superconductors constituting them. In particular, since the oxide superconductor has a short coherence length, when an oxide superconductor is used, the distance between the superconducting source electrode 41 and the superconducting drain electrode 42 must be several tens nm or less. Such microfabrication is very difficult, and conventionally, a superconducting FET using an oxide superconductor could not be produced with good reproducibility.

Further, in the conventional superconducting element having a superconducting channel, a modulation operation was confirmed, but complete on / off operation could not be performed due to a high carrier density. Since the oxide superconductor has a low carrier density, the possibility of realizing the above-mentioned element which performs a complete on / off operation by using it for a superconducting channel is expected. However,
The superconducting channel must be about 5 nm thick,
It has been difficult to realize such a configuration.

Therefore, an object of the present invention is to provide a superconducting element having a novel configuration and a method of manufacturing the superconducting element, which has solved the above-mentioned problems of the related art.

Means for Solving the Problems According to the present invention, a superconducting channel formed in an oxide superconducting thin film formed on a substrate, a superconducting source region and a superconducting drain region arranged on both sides of the superconducting channel, and In a superconducting device comprising a gate electrode for controlling a current flowing through the superconducting channel, the c-axis having a flat upper surface formed on a flat substrate, wherein the oxide superconducting thin film is formed on the substrate. An oriented thin film, at least a part of the superconducting source region and the superconducting drain region is formed of an a-axis oriented oxide superconducting thin film, and the insulating layer is the same as the oxide superconductor forming the oxide superconducting thin film. A superconductor characterized by being composed of an oxide having an element and a crystal structure and having a lower oxygen content than the oxide superconductor. Element provided.

Further, in the present invention, as a method for producing the above-mentioned superconducting element, a step of forming a c-axis oriented oxide superconducting thin film having a total thickness of a superconducting channel and a gate insulating layer on a substrate, and the oxide superconducting thin film The step of forming a gate electrode on top, and etching this oxide superconducting thin film using the gate electrode as a mask to make the thickness of the part not masked by the gate electrode suitable for the superconducting channel, and the part directly under the gate electrode. The step of exposing the side surface of the oxide superconducting thin film of 1. and heating the oxide superconducting thin film in a vacuum to release oxygen of the oxide superconductor directly under the gate electrode, thereby losing the superconducting property and gate insulating. Forming a layer.

The superconducting element of the present invention includes a superconducting channel made of an oxide superconductor, a source electrode and a drain electrode for flowing a current through the superconducting channel, and a gate electrode for controlling a current flowing through the superconducting channel. In the superconducting element of the present invention, each electrode does not necessarily need to be a superconducting electrode.

Further, while a conventional superconducting FET uses a superconducting proximity effect to flow a superconducting current through a semiconductor, in the superconducting element of the present invention, a main current flows through the superconductor. Therefore, the sine of the microfabrication technology required when manufacturing the conventional superconducting FET is relaxed.

The superconducting channel must be about 5 nm thick in the direction of the electric field generated by the gate electrode in order to open and close with the voltage applied to the gate electrode. The main point of the present invention is to realize such an ultra-thin superconducting channel.

In the method of the present invention, first, an oxide superconducting thin film having a thickness of about 20 nm is formed on a substrate. In order to form such a thin oxide superconducting thin film, a method of strictly controlling the growth rate of the thin film and the film forming time is generally used, and this method is preferable when a sputtering method or the like is used. However, since the oxide superconductor crystal has a crystal structure in which the respective constituent elements are stacked in layers, a method of stacking an appropriate number of unit cells of the oxide superconductor by MBE (molecular beam epitaxy) is also preferable.

A portion having a thickness of about 10 nm or more from above the oxide superconducting thin film is changed to an insulator, and a portion having a thickness of about 5 nm below is used as a superconducting channel. In order to convert a part of the oxide superconducting thin film into an insulator, heat treatment is performed in a high vacuum. The properties of oxide superconductors are apt to change depending on the amount of oxygen in the crystal, and particularly when the oxygen content is small, the critical temperature is greatly reduced or the superconductivity is lost. Also, by heating in an atmosphere with a low oxygen partial pressure, oxygen in the crystal escapes,
The amount of oxygen decreases.

Therefore, in the method of the present invention, the oxide superconducting thin film is heat-treated in a high vacuum to remove oxygen, and a part of the thin film is converted into an insulator. By adjusting the processing time, it is possible to remove oxygen in the crystal of an arbitrary thickness. Further, since oxygen easily moves in the oxide superconductor in a direction perpendicular to the c-axis of the crystal, it is also preferable to form a groove parallel to the c-axis of the crystal in a portion of the oxide superconducting thin film from which oxygen is to be removed, and to perform heat treatment.

Further, in the superconducting device of the present invention, the thickness of the insulating layer of the gate electrode should be such that a tunnel current of about 10 nm or more can be ignored. Therefore, in the method of the present invention, an insulating layer having a thickness of about 15 nm is formed from an oxide superconducting thin film having a thickness of about 20 nm.

The above-mentioned oxide superconducting thin film is thick enough to form an insulating layer and a superconducting channel, but insufficient for a source region and a drain region. Therefore, in the method of the present invention, an oxide superconducting thin film is further grown on the source region and the drain region to form a superconducting source region and a superconducting drain region.

In the superconducting element of the present invention, MgO, SrTiO
_3. An oxide single crystal substrate such as CdNdAlO ₄ can be used.
On these substrates, an oxide superconducting thin film composed of highly oriented crystals can be grown, which is preferable. In addition, S whose surface is coated with MgAl ₂ O ₄ , BaTiO _3, etc.
It is also preferable to use a semiconductor substrate such as an i substrate.

The superconducting element of the present invention includes a Y-Ba-Cu-O-based oxide superconductor, a Bi-Sr-Ca-Cu-O-based oxide superconductor, a Tl-Ba
Any oxide superconductor such as -Ca-Cu-O-based oxide superconductor can be used.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following disclosure is merely an example of the present invention, and does not limit the technical scope of the present invention.

Embodiment FIG. 1 shows a sectional view of a superconducting element of the present invention. First
The illustrated superconducting element has an oxide superconducting thin film 1 formed on a substrate 5. Nearly the center of the oxide superconducting thin film 1 is
It is a superconducting channel 10, and a gate electrode 4 is formed on the superconducting channel 10 with an insulating layer 6 made of an oxide superconductor crystal having a small number of oxygen. A superconducting source region 12 and a superconducting drain region 13 having a thickness of about 200 nm are formed on both sides of the superconducting channel 10 of the oxide superconducting thin film 1.

With reference to FIG. 2, a procedure for manufacturing the superconducting element of the present invention by the method of the present invention will be described. First, the surface of the substrate 5 as shown in FIG. 2 (a) has a thickness of about 20 as shown in FIG. 2 (b).
Off-axis sputtering method, reactive evaporation method, MBE method, CVD of Y ₁ Ba ₂ Cu ₃ O _7-x oxide superconducting thin film 11 as thin as nm
It is formed by a method such as a method. The film forming conditions when forming the oxide superconducting thin film 11 by off-axis sputtering are shown below.

Sputtering gas Ar: 90% O ₂ : 10% Pressure 10 Pa Substrate temperature 700 ° C. Substrate 5 includes MgO (100) substrate, SrTiO ₃ (100) substrate, CdNdAlO ₄ (001) insulator substrate, Alternatively, a semiconductor substrate such as Si having an insulating film on the surface is preferable. It is preferable that a MgAl ₂ O ₄ film formed by a CVD method and a BaTiO ₃ film formed by a sputtering method are laminated on the surface of the Si substrate.

Examples of the oxide superconductor include a Y-Ba-Cu-O-based oxide superconductor, a Bi-Sr-Ca-Cu-O-based oxide superconductor, and a Tl-
A Ba-Ca-Cu-O-based oxide superconductor is preferable, and a c-axis oriented thin film is preferable. This is because the c-axis oriented oxide superconducting thin film has a large critical current density in a direction parallel to the substrate.

Next, as shown in FIG.
A normal conductor film 17 is formed on the upper surface by a CVD method, a sputtering method, or the like. For the normal conductor film 17, it is preferable to use Au, Ti, a high melting point metal such as W, or a silicide thereof. The normal conductor film 17 is etched by reactive ion etching, Ar ion etching or the like to form the gate electrode 4 and the oxide superconducting thin film 11 of about 10 nm as shown in FIG. 2 (d).
The above is etched. Next, the substrate temperature is heated to 400 ° C. or higher in a vacuum of about 10 ⁻⁵ Pa. Then, oxygen in the oxide superconductor crystal escapes from the side surfaces 18 and 19 exposed by the etching of the oxide superconducting thin film 11, and the insulating layer 6 is formed below the gate electrode 4 as shown in FIG. 2 (e). It is formed. The portion below the insulating layer 6 of the oxide superconducting thin film 11 becomes the superconducting channel 10. Thus, the oxygen in the oxide superconductor crystal escapes from the side surfaces 18 and 19 because the oxide superconductor has a large oxygen diffusion coefficient in the direction parallel to the a-axis and the b-axis of the crystal.

After the above process, as shown in FIG.
16 is formed on the whole surface. For the insulating film 16, for example, MgO can be used, and it is preferable to form it by a sputtering method or the like. Anisotropic etching is performed on this insulating film 16, and an insulating member is formed on the side surface of the gate electrode 4 as shown in FIG.
14 and 15 are formed.

Finally, as shown in FIG. 2 (h), a superconducting source region 2 and a superconducting drain region 3 made of an a-axis oriented oxide superconducting thin film are formed on both sides of the gate electrode 4 to complete the superconducting element of the present invention. . The a-axis oriented oxide superconducting thin film can be formed using an off-axis sputtering method at a substrate temperature of about 650 ° C. or lower. The sputtering conditions are shown below.

Sputtering gas Ar: 90% O ₂ : 10% Pressure 10 Pa Substrate temperature 640 ° C. When the superconducting element of the present invention is produced by the method of the present invention, the fine processing technology required for producing a superconducting FET is limited. Is alleviated. Further, since the surface can be flattened, it becomes easy to form wiring later if necessary. Also, the constituent elements of the insulating layer between the gate electrode and the superconducting channel are:
Since it is equal to the constituent element of the oxide superconductor forming the superconducting channel, it is mechanically stable and the interface is clean. The superconducting element of the present invention is easy to manufacture, has stable performance, and has good reproducibility.

Effect of the Invention As described above, the superconducting element of the present invention has a configuration in which the superconducting current flowing in the superconducting channel is controlled by the gate voltage. Accordingly, unlike the conventional superconducting FET, the superconducting proximity effect is not used, so that a fine processing technique is unnecessary. Further, since there is no need to stack a superconductor and a semiconductor, a high-performance element can be manufactured using an oxide superconductor.

The present invention further promotes the application of superconducting technology to electronic devices.

[Brief description of the drawings]

FIG. 1 is a schematic view of a superconducting element of the present invention, FIG. 2 is a schematic view showing a process for producing a superconducting element of the present invention by a method of the present invention, and FIG. FIG. 4 is a schematic diagram of a base transistor, and FIG. 4 is a schematic diagram of a superconducting FET. [Main reference numbers] 1 ... Oxide superconducting thin film, 2 ... Superconducting source region, 3 ... Superconducting drain region, 4 ... Gate electrode, 5 ... Substrate

Continuation of the front page (56) References JP-A-1-170080 (JP, A) JP-A-1-120866 (JP, A) JP-A-1-161789 (JP, A)

Claims (3)

(57) [Claims] 1. A superconducting channel formed in an oxide superconducting thin film formed on a substrate, a superconducting source region and a superconducting drain region arranged on both sides of the superconducting channel, and an insulating layer on the superconducting channel. A superconducting device having a gate electrode arranged to control the current flowing through the superconducting channel, wherein the oxide superconducting thin film is a flat c-axis oriented thin film formed on a flat substrate, At least one of the superconducting source region and the superconducting drain region is composed of an a-axis oriented oxide superconducting thin film, and the insulating layer has the same constituent elements and crystal structures as those of the oxide superconductor forming the oxide superconducting thin film, A superconducting element comprising an oxide having an oxygen content lower than that of the oxide superconductor. 2. The superconducting element according to claim 1, wherein the superconducting superconducting source region and the superconducting drain region are thicker than the superconducting channel. 3. A step of forming a c-axis oriented oxide superconducting thin film having a total thickness of a superconducting channel and a gate insulating layer on a substrate, a step of forming a gate electrode on the oxide superconducting thin film, and a gate. Etching this oxide superconducting thin film using the electrode as a mask to make the thickness of the part not masked by the gate electrode suitable for the superconducting channel, and exposing the side surface of the oxide superconducting thin film directly under the gate electrode And heating this oxide superconducting thin film in vacuum,
3. A step of forming a gate insulating layer by losing oxygen in the oxide superconductor immediately below the gate electrode to lose superconductivity, and manufacturing the superconducting element according to claim 1 or 2. Method.