JP2599500B2 - Superconducting element and fabrication method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting element and a method for manufacturing the same.
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 Josephson 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, in order to utilize the proximity effect, the superconducting FET has to be manufactured by bringing the superconducting source electrode 41 and the superconducting drain electrode 42 close to each other within several times the coherence length of the superconducting members. 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 less than 5nm 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.

According to the present invention, a superconducting channel formed of an oxide superconducting thin film formed on a substrate, and a superconducting source electrode and a superconducting drain electrode disposed on both ends of the superconducting channel, A superconducting element comprising a superconducting gate electrode disposed on the superconducting channel via a gate insulating layer to control a current flowing through the superconducting channel, wherein the superconducting channel and the gate insulating layer are integrally formed; Is composed of a c-axis oriented oxide superconducting thin film, wherein the gate insulating layer has the same constituent elements and crystal structure as the oxide superconductor constituting the c-axis oriented oxide superconducting thin film, A superconducting source electrode, a superconducting drain electrode, and a superconducting gate electrode. Pole is a
There is provided a superconducting element comprising an oxide superconducting thin film having an axial orientation.

Further, in the present invention, as a method for manufacturing the above-described superconducting element, a thickness corresponding to the sum of the respective thicknesses of the superconducting channel and the gate insulating layer is formed on an insulator substrate or a semiconductor substrate having an insulating film on the surface. A c-axis oriented oxide superconducting thin film is formed, and an a-axis oriented oxide superconducting thin film having a thickness corresponding to the thickness of the superconducting gate electrode is formed on the c-axis oriented oxide superconducting thin film, The a-axis oriented oxide superconducting thin film is processed to form a superconducting gate electrode, and the c-axis oriented oxide superconducting thin film is processed into the shape of a gate insulating layer and a superconducting channel. A method for manufacturing a superconducting element is provided, which comprises a step of removing oxygen in an oxide superconductor crystal constituting a part of the oxide superconducting thin film to form a non-superconductor.

Function In the superconducting element of the present invention, the gate insulating layer and the superconducting channel are formed integrally. The superconducting channel is c
The gate insulating layer is composed of an oxide superconductor composed of an axially oriented crystal, and the gate insulating layer has the same constituent elements and crystal structure as the oxide superconductor, and has a lower oxygen content than the oxide superconductor. Of oxides. In other words, the gate insulating layer and the superconducting channel have a single c
The axially oriented oxide superconducting thin film is processed into each shape, and the gate insulating layer portion is constituted by a non-superconductor formed by removing oxygen from the oxide superconducting thin film.

As described above, in the present invention, a part of the processed oxide superconducting thin film is made non-superconductor to form the gate insulating layer. Heat treatment is performed inside. Oxide superconductors, the properties of which are easily changed by the amount of oxygen in the crystal, especially when the oxygen content is small, the critical temperature is greatly reduced,
Loses superconductivity. By heating in an atmosphere having a low oxygen partial pressure, oxygen in the crystal is released, and the amount of oxygen decreases.

Therefore, in the method of the present invention, the oxide superconducting thin film is subjected to heat treatment in a high vacuum to remove oxygen, and a part of the thin film is converted into an insulator.
A gate insulating layer is formed. By adjusting the processing time, it is possible to remove oxygen in the crystal of an arbitrary thickness. Since the oxide superconductor has a large oxygen diffusion coefficient in the direction perpendicular to the c-axis of the crystal (oxygen easily moves in the direction perpendicular to the c-axis), the above-described oxide superconducting thin film is an oxide superconductor having a c-axis orientation. The gate insulating layer is formed of a crystal so that oxygen can escape from a side surface of a portion to be a gate insulating layer.

In the superconducting device of the present invention, the superconducting channel must have a thickness of 5 nm or less in the direction of the electric field generated by the gate electrode in order to open and close by the voltage applied to the gate electrode. The thickness of the gate insulating layer is about 10
The thickness must be such that the tunnel current of nm or more can be ignored. Therefore, in the method of the present invention, a thickness of about 20
An oxide superconducting thin film having a c-axis orientation of nm is formed.

In the method of the present invention, when processing the above oxide superconducting thin film to form a superconducting channel, the state of the oxide superconducting thin film is monitored and deteriorated under the influence of the substrate near the interface with the substrate. It is possible not to use the part that is.
The film forming method is preferably a sputtering method, MBE method, CVD method, vacuum evaporation method, etc. By setting the substrate temperature at the time of film formation to about 700 ° C., it is possible to form a c-axis oriented oxide superconducting thin film It is.

On the other hand, the superconducting source electrode, the superconducting drain electrode, and the superconducting gate electrode are preferably composed of an oxide superconducting thin film having an a-axis orientation. In the superconducting source electrode and the superconducting drain electrode, a main current flows in the thickness direction of the electrodes, and therefore, an a-axis oriented oxide superconducting thin film is preferable. In addition, when the superconducting gate electrode is composed of an oxide superconducting thin film having an a-axis orientation, since the crystal direction is different from that of the gate insulating layer portion disposed immediately below, the effect of removing oxygen from the gate insulating layer portion stops at the interface, The oxide superconductor of the superconducting gate electrode does not change.

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 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 superconducting element shown in the figure has a gate insulating layer 6 formed from a single c-axis oriented oxide superconducting thin film formed on a substrate 5.
And a superconducting channel 10. A superconducting gate electrode 4 composed of an a-axis oriented oxide superconducting thin film is arranged on gate insulating layer 6. The gate insulating layer 6 is made of a non-superconducting oxide having a smaller oxygen content than the oxide superconductor, and the superconducting channel 10 is made of c-axis oriented oxide superconductor crystal. The thickness of superconducting channel 10 is about 5 nm or less, and the thickness of gate insulating layer 6 is about 10 nm or more.

An insulating layer 7 is arranged around the superconducting gate electrode 4, and a surface protective film 8 is arranged on the superconducting gate electrode 4. The superconducting source electrode 2 and the superconducting drain electrode 3 are arranged on the superconducting channel 10 on both sides of the superconducting gate electrode 4 in contact with the insulating layer 7 so that the upper surface of the element becomes flat. The superconducting source electrode 2 and the superconducting drain electrode 3 are made of an oxide superconductor crystal having an a-axis orientation.

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, as shown in FIG. 2B, a surface of the substrate 5 as shown in FIG.
A c-axis oriented Y ₁ Ba ₂ Cu ₃ O _7-X oxide superconducting thin film 1 of a moderate thickness is formed by off-axis sputtering, reactive evaporation, MB
It is formed by a method such as the E method and the CVD method. The film forming conditions for forming the oxide superconducting thin film 1 by off-axis sputtering are shown below.

Sputtering Gas Ar: 90% O _2: as a 10% pressure 10Pa substrate temperature 700 ° C. substrate 5, MgO (100) substrate, SrTiO ₃ (100) substrate, CdNdAlO ₄ (001) an insulator, such as a substrate or an insulating the surface, A semiconductor substrate such as Si having a film 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.

As oxide superconductors, in addition to Y-Ba-Cu-O-based oxide superconductors, Bi-Sr-Ca-Cu-O-based oxide superconductors, Tl
—Ba—Ca—Cu—O-based oxide superconductor is preferred.

Next, an a-axis oriented oxide superconducting thin film 11 having a thickness of about 200 nm is laminated on the oxide superconducting thin film 1 as shown in FIG. The a-axis oriented oxide superconducting thin film 11 has a substrate temperature of about 650
The film can be formed using an off-axis sputtering method at a temperature of not more than ° C. The sputtering conditions are shown below.

Sputtering gas Ar: 90% O ₂ : 10% Pressure 10 Pa Substrate temperature 640 ° C. A surface protective film 8 is formed on the oxide superconducting thin film 11 at a position to be the superconducting gate electrode 4 as shown in FIG. The surface protective film 8 is preferably formed of a metal film such as Au, an insulating film, or the like, and preferably has a configuration in which the surface protective film 8 is laminated with a resist film as necessary. The oxide superconducting thin film 11 and the oxide superconducting thin film 1 are anisotropically etched by reactive ion etching, Ar ion etching or the like, and as shown in FIG. 2 (e), the superconducting gate electrode 4, gate insulating layer 16 and superconducting layer. A channel 10 is formed. The thickness of the gate insulating layer 16 is about 10 nm or more, and the length is shortened by promoting side etching as necessary. On the other hand, the superconducting channel 10 has a thickness of 5 nm or less, and, if necessary, is etched while monitoring the state of the oxide superconducting thin film when forming the superconducting channel 10,
Avoid using parts that have deteriorated due to the influence of the substrate.

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 is released from the side surface of the gate insulating layer portion 16, and the gate insulating layer 6 is formed below the superconducting gate electrode 4, as shown in FIG. 2 (f). As described above, the oxygen in the oxide superconductor crystal is released only from the side surface of the gate insulating layer portion 16 because the oxide superconductor has a large oxygen diffusion coefficient in a direction parallel to the a-axis and the b-axis of the crystal. Because.

After the above processing, as shown in FIG. 2 (g), an insulating film 7 is formed around the superconducting gate electrode 4 at the same height as the surface protective film 8. It is preferable to use a material such as SiN for preventing re-introduction of oxygen, for example, for the insulating film 7.

Finally, as shown in FIG.
A superconducting source electrode 2 and a superconducting drain electrode 3 made of an oxide superconducting thin film having an a-axis orientation are formed on both sides of the upper superconducting gate electrode 4 adjacent to the insulating film 7 and at the same height as the insulating film 7 and the protective film 8. Thus, the superconducting element of the present invention is completed. The a-axis oriented oxide superconducting thin film has a substrate temperature of about
It is possible to form a film at 650 ° C. or lower using an off-axis sputtering method. 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 manufactured by the method of the present invention, the restriction on the fine processing technology required when manufacturing a superconducting FET is relaxed. . 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 electrode, 3 ... Superconducting drain electrode, 4 ... Superconducting gate electrode, 5 ... Substrate

Claims (2)

(57) [Claims] 1. A superconducting channel formed of an oxide superconducting thin film formed on a substrate, a superconducting source electrode and a superconducting drain electrode disposed on both ends of the superconducting channel, and a gate insulating layer on the superconducting channel. In a superconducting element including a superconducting gate electrode arranged through a layer and controlling a current flowing through the superconducting channel, the superconducting channel and the gate insulating layer are integrally formed,
The superconducting channel is composed of a c-axis oriented oxide superconducting thin film, and the gate insulating layer has the same constituent elements and crystal structure as an oxide superconductor constituting the c-axis oriented oxide superconducting thin film, Composed of an oxide having a lower oxygen content than the superconductor, and the superconducting source electrode,
A superconducting element, wherein the superconducting drain electrode and the superconducting gate electrode are composed of an a-axis oriented oxide superconducting thin film. 2. A method for manufacturing a superconducting element according to claim 1, wherein the superconducting channel and the gate insulating layer have a thickness corresponding to the sum of the thicknesses of the superconducting channel and the gate insulating layer on an insulator substrate or a semiconductor substrate having an insulating film on the surface. An oxide superconducting thin film having a thickness of c-axis is formed on the oxide superconducting thin film having a thickness corresponding to the thickness of the superconducting gate electrode. Forming a superconducting gate electrode by processing the oxide superconducting thin film having the a-axis orientation;
The axially oriented oxide superconducting thin film is processed into the form of a gate insulating layer and a superconducting channel, and oxygen in the oxide superconducting crystal constituting the oxide superconducting thin film is removed from the portion to be the gate insulating layer to form a non-conductive layer. A method for manufacturing a superconducting element, comprising a step of forming a superconductor.