JP3028793B2 - Superconducting thin film and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a superconducting thin film. More specifically, the present invention relates to a superconducting thin film having a protective film on its surface and not deteriorating in air, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Oxide superconducting thin films, especially Y-Ba-Cu-O
The oxide superconducting thin film is easily decomposed by carbon dioxide gas or water vapor in the air, and its surface is covered with an amorphous layer formed by decomposing the surface layer. For this reason, even when a superconducting element is manufactured using the oxide superconducting thin film, the produced element does not exhibit expected characteristics or the superconductivity of the oxide superconducting thin film itself is lost.

A superconducting element considered to be extremely important in practical use is a superconducting field effect element. The superconducting field effect element is also called a superconducting field effect transistor, has a superconducting channel formed of an oxide superconductor, includes a gate electrode disposed on the superconducting channel via a gate insulating layer, and applies a voltage to the gate electrode. A three-terminal element that controls the current flowing through the superconducting channel according to the applied signal voltage. This element is a voltage-controlled element, has an action of amplifying a signal, and has practical characteristics such as a large current density.

The above superconducting field effect element has a thickness of 5
It has a superconducting channel formed of a very thin oxide superconducting thin film of about nm. As described above, in an element having an extremely thin oxide superconducting thin film, a portion corresponding to the element region may disappear as a result of the formation of the amorphous layer. In order to suppress this, the main process of device fabrication must be performed without taking out from an ultra-high vacuum, or the entire region where the oxide superconducting thin film is exposed must be covered with a protective film. It is not impossible to perform all of the device fabrication steps in a vacuum, but it is not possible to secure the degree of freedom of the process, the ease of processing, or the degree of freedom of the materials used. There are many advantages to processing outside or in other processing equipment.

[0005] However, there are also restrictions on the material and method of forming the protective film. First, at the time of forming the protective film, it is necessary not to deprive the underlying oxide superconducting thin film of oxygen or to cause mutual diffusion of constituent elements. Further, since the temperature difference during processing or operation is large, it is also required that there is no mechanical strain and that the coefficient of thermal expansion is close to that of the oxide superconductor.

When this protective structure is required even in the intermediate stage of device fabrication, it takes over the crystallinity of the oxide superconducting film and promotes crystal growth of the upper layer in order to enable fabrication of the upper device structure. Demands regarding crystallinity and lattice constant arise. In this case, workability is a new requirement.
For example, in order to make some electrical contact with the oxide superconducting film, it is necessary to selectively remove the formed thin film.
In this case, do not leave foreign matter on the oxide superconducting film,
It is necessary that there be a removal method with a high selectivity and that a patterning technique such as photolithography can be applied.

[0007]

Heretofore, emphasis has been placed on protecting the surface of an oxide superconducting thin film by suppressing changes over time in characteristics. There has been little discussion about a protective film that focuses on suppressing deterioration of characteristics caused by being taken out of a vacuum environment and coming into contact with air immediately after film formation.

[0008] Conventionally, the oxide superconducting thin film was removed from the vacuum environment after the film was formed, and a protective film was often formed in a different film forming apparatus or in the air. This includes formation of a silicon oxide film and formation of a resin layer. Materials that have also been used in semiconductors have generally been diverted. Therefore, the surface deterioration of the oxide superconducting film prior to the formation of the protective film cannot be prevented, and most of the oxide superconducting films have a configuration in which an amorphous layer is interposed between the oxide superconducting film and the protective film.

An object of the present invention is to provide an oxide superconducting thin film having a protective film which solves the above-mentioned problems of the prior art, and a method for producing the same.

[0010]

According to the present invention SUMMARY OF], peripheral covers the surface of the oxide superconductor layer and the oxide superconductor layer
_{SrTiO 3, Ba x Sr 1-} x TiO 3 to be protected from atmosphere circumference Oyo
And at least one oxide of BaTiO ₃ and a thickness of 1
A superconducting thin film having a protective layer having a thickness of 1 nm or less and thinner than the oxide superconducting layer is provided. In the present invention, it is preferable that the protective layer is formed of a laminated film of two or more oxide layers. Further, the protective layer,
Ba _x Sr _1-x TiO ₃ (where 0 ≦ x ≦ 1 and x is larger on the side in contact with the oxide superconducting thin film and decreases continuously as the distance from the oxide superconducting thin film increases) Is also preferred.

According to the present invention, a metal layer is provided on the protective layer, and the metal of the metal layer is thermally diffused in the protective layer.
And has a superconducting thin film and the metal layer and the oxide superconducting layer are electrically connected by the protective layer is also provided.

The superconducting thin film of the present invention is preferably used for a superconducting element, and the protective layer preferably forms part of the structure of the superconducting element. In this case, in a superconducting element having a laminated film in which the lower layer is an oxide superconductor layer and the upper layer is an oxide layer other than the oxide superconductor, the upper layer has a function of protecting the oxide superconductor layer. . More specifically, for example, in a superconducting field effect element, it is conceivable that the laminated film forms a superconducting channel and a gate insulating film of the superconducting field effect element.

[0013]

Further, the superconducting thin film of the present invention has a metal layer on a protective layer, the protective layer forms an alloy with the metal of the metal layer, and the metal layer and the oxide superconductor layer are formed of a protective layer. It is preferable that they are electrically connected to each other. This superconducting thin film is formed by forming an oxide superconducting thin film on a substrate and then continuously depositing the oxide thin film on the oxide superconducting thin film while maintaining a vacuum environment with the same film forming apparatus. It is preferable that a metal layer is formed while heating the substrate and alloyed with an oxide thin film. Immediately after the formation of the metal layer, the substrate may be heated in place and alloyed with the oxide thin film. Further, the laminated film having the metal layer formed as described above may be heated using lamp heating, laser heating, a heating furnace, or the like to further promote alloying.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The superconducting thin film of the present invention is mainly composed of an oxide superconductor layer whose surface is covered and protected by an oxide protective layer. This oxide protective layer prevents the oxide superconductor layer from being in contact with water vapor, carbon dioxide gas, and the like in the atmosphere and being decomposed by these. In the present invention, the oxide superconductor is a so-called Y-Ba-Cu-O-based (in this case, Y may be a lanthanoid element), Bi-Sr
-Ca-Cu-O system (may include Pb etc.), Tl-Ba-Ca
Any of -Cu-O type may be used. The protective layer of this oxide
After the formation of the oxide superconductor layer, it is preferable that the oxide superconductor layer is continuously formed on the oxide superconductor layer by using the same film forming apparatus while maintaining a vacuum environment. The meaning of maintaining a vacuum environment is
It is to maintain the pressure of the film forming apparatus as much as possible. If the pressure required for forming the protective layer is equal to the pressure for forming the oxide superconductor layer, it is to be maintained. Further, even when the two are different, the pressure at the time of forming the oxide superconductor layer is immediately changed to the pressure at which the protective layer is formed, so that no extra pressure change is performed during the process. By forming the protective layer in this way, an amorphous layer formed by decomposing the surface of the oxide superconductor does not exist between the oxide superconductor layer and the protective layer. That is, the oxide superconductor layer is covered with the protective layer in a state immediately after formation, and the dimensions and characteristics are maintained.

In the superconducting thin film of the present invention, the protective layer is made of Sr
An oxide of one of TiO ₃ , Ba _x Sr _1-x TiO ₃ and BaTiO ₃ , a laminated film of two or more oxide layers, or Ba
_x Sr _1-x TiO ₃ (where 0 ≦ x ≦ 1 and x is larger on the side in contact with the oxide superconducting thin film and decreases continuously as the distance from the oxide superconducting thin film increases) Is preferred. This is because each of these oxides has a lattice constant close to that of the oxide superconductor and does not deteriorate the oxide superconductor. The protective effect of the protective layer having a thickness of several nm is sufficient. However, when the protective layer has another function as described later, its thickness is determined based on the presence or absence of post-processing, suppression of mutual diffusion with the oxide superconductor layer, and the like. Usually 1 to 100
nm is preferred.

The superconducting thin film of the present invention is preferably used for a superconducting element, and each layer is preferably an element constituting the superconducting element. For example, the oxide superconductor layer may be a superconducting channel of the superconducting field effect element, and the protective layer may be a gate insulating film. In addition, a configuration in which the superconducting electrode of the tunnel type Josephson element and the insulator layer are used may be used.

The superconducting thin film of the present invention has a metal layer on a protective layer, the protective layer forms an alloy with the metal of the metal layer, and the metal layer and the oxide superconductor layer are electrically connected by the protective layer. May be connected to each other. In this case, the metal layer becomes a so-called ohmic electrode, and electrically connects the oxide superconductor layer to the outside. Also in this case, the protective layer sufficiently protects the oxide superconductor layer with a thickness of several nm, and this thickness provides sufficient electrical contact with the oxide superconductor layer. In this configuration, the protective layer is preferably SrTiO ₃ and the metal layer is Tr.
i, Nb, Ni, Ag, Au and the like are preferred. These metals are SrTi
This is because it is easy to form an alloy with O ₃ .

In order to produce the superconducting thin film of the present invention in which the protective layer forms an alloy with the metal layer, the oxide superconductor layer and the protective layer are formed by the above-described continuous film formation, and then the protective layer is formed on the protective layer. The metal layer is formed, for example, while heating the substrate. This heating is for promoting alloying. This heat treatment may be performed immediately after the formation of the metal layer. Since the metal layer is already formed on the protective layer, the oxide superconductor layer,
It can be formed in a device different from the protective layer. Also,
After the formation of the metal layer, lamp annealing, laser annealing, heating with a heating furnace, or the like may be performed to further promote alloying.

[0020]

[Embodiment 1] A superconducting thin film of the present invention was made of SrTiO ₃ (100)
A change in characteristics over time was compared with a conventional oxide superconducting thin film formed on a substrate and having no protective film. The film forming conditions are shown below.

[0021]

[Table 1] Oxide superconductor layer film formation method Pulsed laser deposition method (Y ₁ Ba ₂ Cu ₃ O _7-X ) Oxygen pressure 0.3 Torr Substrate temperature 690 ° C Film formation time 1 minute Film thickness 11 nm Protective layer film formation method Pulse Laser deposition (SrTiO ₃ ) Oxygen pressure 0.03 Torr Substrate temperature 600 ° C Deposition time 3 seconds Film thickness 3 nm

Next, each sample was divided into three parts, and immediately after the preparation, one day and four days later, the critical temperature was measured by a four-terminal method using an Ag electrode. The results are shown in FIG. 1 and FIG. FIG.
Is a measurement result of the critical temperature of the superconducting thin film of the present invention,
FIG. 2 shows the measurement results of the critical temperature of a conventional oxide superconducting thin film having no protective layer. ,, Indicate immediately after preparation, 1 day, and 4 days after, respectively. As can be seen from FIG. 1 and FIG. The critical temperature of the superconducting thin film of the present invention is 6
At 9.5K, there was almost no change at 65.2K after 1 day and 65.4K after 4 days. In contrast, conventional oxide superconducting thin films
Immediately after fabrication, 47.1K, 26.3K after 1 day, and no superconductivity after 4 days (3Ω at 16K), the critical temperature decreased significantly with time.

As described above, by forming the protective layer, even if the oxide superconductor layer is taken out of an ultra-high vacuum, its characteristic deterioration can be suppressed, and its aging can be prevented.

[0024]

Example 2 A superconducting thin film of the present invention was manufactured using different materials, and the change over time in characteristics was verified. A SrTiO ₃ (100) substrate was used as in Example 1. The film forming conditions are shown below.

[0025]

[Table 2] Oxide superconductor layer film formation method Pulsed laser deposition method (Y ₁ Ba ₂ Cu ₃ O _7-X ) Oxygen pressure 0.3 Torr Substrate temperature 690 ° C Film formation time 1 minute Film thickness 11 nm Protective layer film formation method Pulse Laser deposition method (Ba _0.5 Sr _0.5 TiO ₃ ) Oxygen pressure 0.03 Torr Substrate temperature 600 ° C Deposition time 3 seconds Film thickness 3 nm

Next, each sample was divided into two parts, and the critical temperatures immediately after preparation and seven days later were measured by a four-terminal method using an Ag electrode. As a result, the critical temperature of the above-mentioned superconducting thin film of the present invention was 73.2K immediately after the preparation, and hardly changed to 73K after 7 days. The superconducting thin film of the present invention has a protective layer,
Deterioration of the characteristics is suppressed, and changes over time in the characteristics are also prevented.

[0027]

Embodiment 3 On a SrTiO ₃ (100) substrate, an oxide superconductor layer was made of Y ₁ Ba ₂ Cu ₃ O _7-X oxide superconductor, and a protective layer was made of SrT.
The electrode was formed of TiO ₃ , an electrode of a Ti layer was formed thereon, alloyed with the protective layer, and the Ti layer and the oxide superconductor layer were electrically connected. The oxide superconductor layer had a thickness of 33 nm and was formed by a pulsed laser deposition method under the same conditions as in Example 1. Also, Sr
The TiO ₃ layer was also formed to a thickness of 11 nm by the pulse laser deposition method under the same conditions as in Example 1. The Ti layer was formed under the following conditions.

[0028]

[Table 3] Electrode film material Ti film forming method Heat evaporation method Film forming temperature Room temperature Film forming time 20 minutes Film thickness 150nm Contact resistance 15.8Ωcm ²

Next, in order to further reduce the contact resistance, a treatment for accelerating the alloying treatment was performed. The processing conditions are shown below.

[0030]

[Table 4] Electrode film heating method Lamp annealing method Heat treatment time 1 minute Heating atmosphere Vacuum atmosphere Contact resistance 10 ^-3 Ωcm ²

The electrode of the present invention is formed on the protective layer and does not directly contact the oxide superconductor layer, so that the oxide superconductor layer does not deteriorate. Also, the process of opening a window can be omitted. Furthermore, a general method for selectively forming an electrode material, for example, any method such as a selective etching method, a lift-off method, and a metal mask can be used, so that an electrode can be formed with high accuracy without being affected by a step. it can.

[0032]

Embodiment 4 The superconducting thin film of the present invention was formed by SrTiO _{3 by} MBE.
It was fabricated on a (100) substrate, and the change over time in the characteristics was measured. The deposition conditions are shown below:

[0033]

[Table 5] Oxide superconductor layer film forming method MBE method (Y ₁ Ba ₂ Cu ₃ O _7-X ) Pressure (O ₃ ) 5 × 10 ^-5 Torr (around substrate) Substrate temperature 700 ° C. Film forming time 20 minutes Film thickness 10 nm Protective layer Film forming method MBE method (SrTiO ₃ ) Pressure (O ₃ ) 5 × 10 ^-5 Torr (around substrate) Substrate temperature 500 ° C Film forming time 3 minutes Film thickness 3 nm

The sample of the superconducting thin film of the present invention was divided into three parts, and immediately after the preparation, one day and four days later, the critical temperature was measured by a four-terminal method using an Ag electrode. The critical temperature of the superconducting thin film of the present invention was 73.8K immediately after the preparation, and was almost unchanged at 73.5K after 1 day and 73.7K after 4 days.

[0035]

Example 5 A gate structure of a superconducting field effect element was produced using the superconducting thin film of the present invention produced by the MBE method. The process will be described with reference to FIG. First, FIG.
As shown in (a), Y ₁ Ba ₂ Cu _{3 is} deposited on SrTiO ₃ (100) substrate 5.
An O _7-X oxide superconducting thin film 1 was formed by MBE. The deposition conditions are shown below:

[Table 6] Oxide superconductor layer film forming method MBE method (Y ₁ Ba ₂ Cu ₃ O _7-X ) Pressure (O ₃ ) 5 × 10 ^-5 Torr (around substrate) Substrate temperature 700 ° C. Film forming time 10 minutes Film thickness 5 nm

Next, as shown in FIG.
E Keep the SrTiO ₃ protective layer 50 continuously without moving
A film was formed on the Y ₁ Ba ₂ Cu ₃ O _7-X oxide superconducting thin film 1 by the E method. The film forming conditions are shown below: Protective layer Film forming method MBE method (SrTiO ₃ ) Pressure (O ₃ ) 5 × 10 ^-5 Torr (around the substrate) Substrate temperature 500 ° C Film forming time 3 minutes Film thickness 3 nm

Subsequently, on the above protective layer, as shown in FIG.
A 400 nm SrTiO ₃ thin film 51 was laminated. Next, as shown in FIG. 3D, a photoresist film 6 was formed on a part of the SrTiO ₃ film 51. Using the photoresist film 6 as a mask, FIG.
As shown in (e), the SrTiO ₃ film 51 and the oxide superconducting thin film 1 were etched by Ar ion milling until the substrate 5 was exposed, thereby producing a gate insulating layer 52 and a superconducting channel 10.

As shown in FIG. 3F, a CeO ₂ film 8 was formed on the exposed portion of the substrate 5 at room temperature, and the portion on the photoresist film was removed by a lift-off method. Finally, FIG.
As shown in (g), the gate electrode 4 is deposited on the gate insulating layer 5 by vapor deposition.
The gate structure of the superconducting field effect device was formed by using Ag on 2.

The critical temperature of the superconducting thin film having the above gate structure was measured by a four-terminal method using an Ag electrode immediately after the SrTiO ₃ film 51 was formed and after the gate structure was formed. The critical temperature of the above-mentioned superconducting thin film of the present invention was 71.4K immediately after fabrication and 66.7K after processing, and the deterioration of the critical temperature was suppressed to 5K or less. Further, a source electrode and a drain electrode were provided in contact with both ends of the superconducting channel 10, and characteristics of the superconducting field effect element were measured.

FIG. 4 shows the gate voltage dependence of the drain current-voltage characteristics of the device. In the superconducting field effect element, the modulation degree of the superconducting current is 5% for positive polarity and 10% for negative polarity.
Met. Further, FIG. 5 shows those expressed as a function of temperature normalized to the value of the transconductance (transconductance) g _m compared to other device structures. The superconducting field effect element having the superconducting thin film of the present invention produced by the method of the present invention showed the most excellent value.

[0041]

As described above, according to the present invention, there is provided a superconducting thin film having an oxide superconductor layer covered with a protective layer and protected. The superconducting thin film of the present invention is used for a superconducting element, and both the protective layer and the oxide superconducting layer are elements constituting the superconducting element. In addition, by disposing a metal layer on the protective layer and forming an alloy with the protective layer, the metal layer can be an electrode electrically connected to the oxide superconductor layer.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in critical temperature of a superconducting thin film of the present invention.

FIG. 2 is a graph showing a change in critical temperature of a conventional oxide superconducting thin film.

FIG. 3 is a cross-sectional view illustrating a step of manufacturing a gate structure of a superconducting field effect element using the superconducting thin film of the present invention.

FIG. 4 is a graph showing gate voltage dependence of drain current-voltage characteristics of a superconducting field effect element using the superconducting thin film of the present invention.

FIG. 5 is a diagram showing a transconductance (transfer conductance) g _m value of a superconducting field effect element using the superconducting thin film of the present invention as a function of normalized temperature and comparing it with other element structures.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oxide superconducting thin film 2 Protective layer 4 Gate electrode 5 Substrate 6 Photoresist

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H01B 13/00565 H01B 13/00 565D H01L 39/22 ZAA H01L 39/22 ZAAG (56) References JP-A-7-45880 (JP, A JP-A-1-163918 (JP, A) JP-A-1-126205 (JP, A) JP-A-5-343754 (JP, A) JP-A-61-128545 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H01L 39/24 H01L 39/00 H01B 12/06 H01B 13/00 565 H01L 39/22 H01L 21/3205 H01L 21/321 H01L 21/3213 H01L 21/768

Claims (6)

(57) [Claims] 1. An oxide superconductor layer and SrTi for covering the surface of the oxide superconductor layer to protect it from the surrounding atmosphere.
At least one of O ₃ , Ba _x Sr _1-x TiO ₃ and BaTiO ₃
The oxide superconductor is composed of oxide and has a thickness of 11 nm or less.
A superconducting thin film having a protective layer thinner than a layer. 2. The method according to claim 1, wherein the protective layer is made of SrTiO ₃ , Ba _x Sr _1-x TiO.
₃ and superconducting thin film according to claim 1, characterized in that it is formed by a laminated film of two or more layers of oxide in the BaTiO _3. 3. The protective layer is made of Ba _x Sr _1-x TiO ₃ (where 0 ≦ x ≦ 1, x is larger on the side in contact with the oxide superconducting thin film, and decreases continuously as the distance from the oxide superconducting thin film increases). The superconducting thin film according to claim 1, wherein the superconducting thin film is formed of an oxide. 4. The method according to claim 1, further comprising a metal layer on the protective layer,
In the layer has a metal thermal diffusion of the metal layer, any one of claims 1 to 3, and the metal layer and the oxide superconductor layer is characterized by being electrically connected by said protective layer 1
The superconducting thin film according to the above item. 5. An oxide superconductor layer, an oxide protective layer for covering and protecting the surface of the oxide superconductor layer, and an oxide superconductor layer disposed on the protective layer with the protective layer interposed therebetween. In the method for producing a superconducting structure having a metal layer and a metal layer electrically connected to each other, after forming an oxide superconducting thin film, the oxide thin film is continuously formed while maintaining a vacuum environment in the same film forming apparatus. A method characterized in that a film is formed on an oxide superconducting thin film, a metal layer is formed on the oxide thin film while heating, and alloyed with the oxide thin film. 6. An oxide superconductor layer, an oxide protective layer for covering and protecting the surface of the oxide superconductor layer, and an oxide superconductor layer disposed on the protective layer via the protective layer. In the method for producing a superconducting structure having a metal layer and a metal layer electrically connected to each other, after forming an oxide superconducting thin film, the oxide thin film is continuously formed while maintaining a vacuum environment in the same film forming apparatus. A method comprising forming a film on an oxide superconducting thin film, forming a metal layer on the oxide thin film, heating the metal layer, and then alloying with the oxide thin film.