JPH01205579A - Superconductive thin film and formation of superconductive thin film - Google Patents

Abstract

PURPOSE:To easily obtain an oxide superconductive thin film without giving a thermal influence on a foundation film by forming an amorphous film on a substrate followed by proceeding with crystallization through short time nonbalanced heat treatment by photoradiation. CONSTITUTION:Amorphous thin films are piled up on a substrate 4 at a low substrate temperature by a sputtering method or an electron beam evaporation method followed by irradiating laser light 6 on the film surface. At this time, by properly setting up irradiation time or power of laser light, the irradiated part alone can be heated in the film thickness direction up to prescribed depth. Heat treatment only of the film can be performed without heating the substrate 4 so that deterioration of a superconductive characteristic due to reaction of the film to the substrate can be prevented. Ristriction of heat resistance as to the substrate 4 is eliminated so that latitude of selection of a material of the substrate 4 is broadened. Thereby, an oxide superconductive thin film can be easily obtained having no thermal influence on the foundation film.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention is directed to a superconducting element that has the features of a superconducting thin film, ultra-high-speed switching, and low power consumption, and that operates even at liquid nitrogen temperatures (77 K) or higher. It concerns the production method.

1 Conventional technology 1 The conventional method for manufacturing oxide superconducting thin films requires a substrate temperature of approximately 400°C.
A thin film having a composition that will become an oxide superconductor is deposited on a substrate (SrTi03.MgO, sapphire, etc.) heated to above 0.0°C, and then heated to 800°C or more in 02 gas.
The film was crystallized by heat treatment at 900°C for one to several hours, resulting in a film exhibiting superconducting properties.

[Problem to be solved by the invention] However, in this method, the entire film and substrate are heated to high temperatures, so a reaction occurs at the interface between the substrate and the film, and the superconducting properties (for example, critical temperature, critical current, upper critical magnetic field) are deteriorated. to degrade. For this reason, the heat treatment temperature must be set at a low temperature to prevent reaction between the substrate and the film, and at a high enough temperature to sufficiently promote crystallization.

It is difficult to satisfy these contradictory temperature conditions at the same time, and there is a serious problem in that the reaction between the substrate and the film cannot be completely eliminated.

A similar problem also occurred in Josephson junctions using conventional oxide superconductors. Figure 5 shows the structure of a Josephson junction. Superconducting thin film 1 constituting the lower electrode
A tunnel barrier layer 3 is formed between the superconducting thin film 2 and the superconducting thin film 2 constituting the upper electrode. In order to form this Josephson junction, the lower electrode is first formed, then the tunnel barrier layer is formed, and then the upper electrode is formed, or the upper electrode is made of an oxide superconductor. In some cases, heat treatment is performed at around 900° C. after film formation. then)·
The barrier layer, which is thick on the order of tens of angstroms through which channel current can flow, is destroyed by the high temperature.

Therefore, in conventional bonding, the lower electrode is made of an oxide superconductor, the barrier layer is made of Ai oxide, and the upper electrode is made of a two-part electrode, Nb. There is no example in which both the lower electrode and the lower electrode are made of an oxide superconductor. In order to obtain a junction that operates at liquid nitrogen temperatures (77 K) or higher, it is necessary to have a junction in which all electrodes are made of oxide superconductors. In addition to bonding electrodes, superconducting wiring
In order to realize an element using a superconducting thin film, it is necessary to have a means to heat-treat the film without causing thermal deterioration to the underlying layer and obtain a film having superconducting properties.

Another example of a method for heat-treating a film in a short time is Applied Physics Letters Vol. 5IP, 1554.
Is there a method using flash lamp annealing as disclosed by Buhrman et al. of Cornell University? In this method, the heating time is very long, several seconds, and heating of the underlying film due to thermal diffusion is unavoidable. It is difficult to heat-treat only that part. Furthermore, this method cannot heat only the desired film by utilizing the difference in light absorption coefficient between the film and the underlying film. It is also impossible to pattern a superconducting film by locally heating only a desired portion within the film. Furthermore, the proposed method uses a film that is crystallized to some extent and is formed at substrate heating temperatures of 400° C. and 700° C. However, this method had the disadvantage that crystal growth would be random because there were seeds that had been randomly crystallized beforehand, and high-quality crystallization could not be expected with instantaneous heat treatment.

In an oxide superconductor, in the process of heat-treating a film to obtain a film having superconducting properties, the entire element reaches a high temperature when using a conventional heat-treatment method using a thermal equilibrium furnace, so that (1) it reacts with the substrate; (2) During bonding, the barrier layer is destroyed during formation of the upper electrode. (3) As a superconducting wiring, it reacts with the underlying layer. Significant difficulties arise in manufacturing such devices. An object of the present invention is to provide a novel method for forming a superconducting thin film and a method for forming a superconducting thin film element that solves the difficulties caused by this heat treatment.

[Means for Solving the Problems] In order to achieve such objects, the method for forming a superconducting thin film of the present invention converts a thin film having a composition capable of becoming an oxide superconductor into an amorphous state or a state with little crystallization. The method is characterized in that a thin film having superconducting properties is formed on the substrate by forming the thin film on the substrate and heat-treating the thin film to crystallize it to a predetermined depth in the film direction.

The method for forming the superconducting thin film element of the present invention includes the first
or on the tunnel barrier layer formed on the first superconductor, a thin film having a composition capable of becoming an oxide superconductor is placed in an amorphous or slightly crystallized state. The second superconductor is formed by forming a thin film and heat-treating the thin film to crystallize it to a predetermined depth in the film direction.

[Function] Conventionally, films deposited at low substrate temperatures become polycrystalline films with random crystal orientations, so even if they are subsequently heat-treated, crystallization occurs in random orientations, resulting in single crystals. It is not possible to obtain a high-quality crystal film close to that of . For this reason, only films with low superconducting critical temperatures could be obtained. However, as a result of detailed study on this point, we found that by keeping the substrate temperature low, an amorphous or nearly amorphous film can be obtained, and that by subsequent heat treatment of this film, crystallization progresses uniformly, even with short heat treatment. It was found that a film with a high critical temperature could be obtained. That is, in producing an oxide superconducting thin film, an amorphous thin film is formed by, for example, a sputtering method or an electron beam evaporation method, and then, for example, Nd:YA is deposited.
Heat treatment is performed by irradiating the film surface with laser light for a short time (about several nanoseconds) using a G laser or the like. In this case, the atmosphere around the sample is preferably an oxygen atmosphere, but may also be in the air. In this method, by appropriately selecting the power and wavelength of the laser beam, it is possible to perform heat treatment to a predetermined depth in the film thickness direction without affecting the underlying film. Furthermore, by focusing the laser beam and scanning it, or by not moving the substrate on which the thin film is deposited, it is possible to crystallize only the area irradiated with the laser beam, making it a superconducting thin film. In this way, in the present invention, by performing heat treatment using laser light, it is possible to obtain a superconducting thin film without affecting the underlying layer due to the heat treatment, thereby overcoming the above-mentioned difficulties.

According to the present invention, an oxide superconducting thin film can be obtained without causing thermal deterioration of the base and surroundings of the film.
Devices using Josephson junctions and superconducting wiring.
It is possible to fabricate devices that take advantage of the characteristics of oxide superconducting thin films, such as high critical temperature and high critical current.

[Example 1 The present invention will be explained in detail below by way of examples.

Example 1 FIG. 1 shows a first example of the present invention. In the figure, 5 is a thin film having a composition that can be an oxide superconductor, 6 is a laser beam, and 4 is a substrate. As the thin film 5, for example, Y:Ba:
A sputtered film or the like having a molar ratio of Cu=l:2:3 is used.

As the laser beam 6, a continuous laser, a pulsed ruby laser, a pulsed or continuous beam Nd:YAG laser,
A C02 laser or the like is used. A continuous light laser condenses light with a lens and scans the light for irradiation. In the case of a pulsed laser, the light is spread out using a lens and irradiated all at once. As the substrate 4, for example, SrTiO3, MgO, sapphire, etc. are used.

An amorphous thin film is deposited on the substrate 4 by sputtering or electron beam evaporation at a low substrate temperature, and then the film surface is irradiated with laser light 6. At this time, by appropriately setting the irradiation time or power of the laser beam, it is possible to heat only the irradiated portion to a predetermined depth in the film thickness direction. With this method, only the film can be heat-treated without heating the substrate 4, so that deterioration of superconducting properties due to reaction between the film and the substrate can be prevented. Furthermore, since there are no restrictions on heat resistance of the substrate, the range of selection of substrate materials is expanded.

Further, in this embodiment, the reaction between the substrate 4 and the film has been described, but the present invention is applicable not only to the substrate but also to the case where a superconducting layer is provided on an interlayer insulating layer. For example, when superconducting wiring is used in a semiconductor integrated circuit, it is possible to heat-treat only a desired film to make it a superconductor without affecting the underlying film and the underlying devices.

Example 2 As a second example, an example will be described in which the method for forming a superconducting thin film according to the present invention is used for the upper electrode of a Sigo-Sefson junction. Second
In the figure, 4 is a substrate, 2 is an upper electrode of a Josephson junction, 5A is a tunnel barrier layer, and 1 is a lower electrode.

To fabricate this bond, first place a superconducting thin film 1 on a substrate 4.
is deposited to form the lower electrode of the junction. For the heat treatment in this case, a conventional thermal equilibrium furnace may be used, or the laser annealing method according to the present invention described in Example 1 may be used to avoid reaction with the substrate. Next, a thin film 5 having a composition that is an oxide superconductor is deposited to form an upper electrode and a barrier layer. This thin film is formed at a low substrate temperature and is made into an amorphous or nearly amorphous state. The surface of this film is irradiated with laser light and heat treated to promote crystallization of the film and become a superconductor.At this time, the power of the laser
If the heat penetration distance is set to be slightly less than the thickness of the thin film 5 by appropriately adjusting the irradiation time, several nanometers of the lower layer of the film 5 can be left as the non-superconducting layer 5A.

Since this portion is usually an insulator, it can be used as the tunnel barrier layer 5A of the Josephson junction.

Embodiment 3 FIG. 3 shows a third embodiment of the wooden invention. The structure of the device is the same as that of Example 2 shown in FIG. 2, or is different in that a thin film other than the upper electrode is used as the tunnel barrier layer 3. The device fabrication procedure is the same as in Example 2 up to the formation of the lower electrode 1, or after that another thin film is deposited as the tunnel barrier layer 3. As the tunnel barrier layer 3, for example, 8 fl 203. MgO, Tag,
Possible materials include ZrO. Furthermore, Y:Ba:C is an insulator with the same constituent elements as the oxide superconductor but a different composition ratio.
A thin film of u-2:1:1 or the like can be used. After this, the two-part electrode 5 is deposited, and then the laser beam 6 is irradiated with the power and irradiation time appropriately adjusted, and the upper electrode 5 is deposited.
The material is heat-treated and made into a thin sheet with superconducting properties. At this time, a material having a smaller absorption coefficient with respect to the wavelength of the laser beam is selected as the tunnel barrier layer 3 compared to the upper electrode 5 and the lower type 8i1, or conversely, a material with a smaller absorption coefficient with respect to the wavelength of the laser beam is selected, or conversely, a material with a smaller absorption coefficient than the upper electrode 5 and the lower type 8i1 is selected. By adjusting the wavelength of the laser beam so that the absorption coefficient of the barrier layer is smaller than that of the barrier layer, it is also possible to heat-treat only the electrodes without heating the 1-channel harrier layer 3. Furthermore, an insulating layer is provided on the device shown in FIG. 3, and a superconducting film is further provided on the insulating layer by the method of Wood's invention, and the method of the present invention is repeatedly used, such as forming a control line of a magnetically coupled Josephson junction. This makes it possible to realize even more complicated technology structures.

Embodiment 4 FIG. 4 shows a fourth embodiment of the present invention. In this example,
Heat treatment is performed using laser light 6 focused by lens 7. In the figure, 5B is a portion of the thin film 5 that can become a superconductor that has been irradiated with laser light and has superconducting properties, and 5C is a portion of the thin film 5 that has not been irradiated with laser light and therefore remains an insulator. or a high resistance part close to it. The method of configuring the device is the same as in the second or third embodiment. By scanning the focused laser beam 6 and drawing a pattern on the thin film 5, the two-part electrode 5 is formed into an arbitrary shape.
B is formed. Therefore, a Josephson junction of arbitrary shape can be formed thereunder. At this time, the size of the bond can be reduced to about the beam diameter of a laser beam (1 micron or less), and a minute bond can be easily formed. Furthermore, by this method, soliton transmission lines made of long Josephson junctions used as logic elements can be easily formed. Furthermore, in this example, etching is performed.

Unlike processing methods such as riff 1-off, flattening can be achieved without removing (or patterning) the film constituting the upper electrode. This is a great advantage in terms of device structure.

[Effects of the Invention] As explained above, an amorphous film is first formed, and then crystallization is promoted by a single-time non-equilibrium heat treatment using light irradiation, thereby forming an oxide without affecting the base film. It has become possible to form superconducting thin films. In the example, 5rTi03. MgO.

By using an oxide crystal such as sapphire or by using a composite substrate in which the above-mentioned oxide is deposited on Sl, which is the most stable substrate material, the superconductor is deposited on this and the heat treatment according to the present invention is performed. It is clear that an element can be formed, and the effects of implementing the present invention can be fully exhibited. By using this method for the upper electrode of the Josephson junction,
Both the upper and lower electrodes are made of oxide superconductors, making it possible to form a Josephson junction that can operate at temperatures above liquid nitrogen temperature (77K). Furthermore, by applying the present invention to superconducting interconnects in semiconductor integrated circuits, it has become possible to heat-treat superconducting interconnects without adversely affecting underlying semiconductor devices, dramatically expanding the scope of application of superconducting interconnects. can be spread to

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a method for forming an oxide superconducting thin film according to the present invention, and FIGS. 2, 3, and 4 are Josephson junctions using the oxide superconducting thin film according to the present invention as electrodes, respectively. FIG. 5 is a cross-sectional view of a conventional Josephson junction. 1...lower electrode of Josephson junction, 2...second part N, h of Josephson junction, 3...tunnel barrier layer, 4...substrate, 5...becomes oxide superconductor. Vine thin film, 5A... Josephson junction (~ner barrier layer, 6... laser beam, 7... lens. Figure 2

Claims (2)

[Claims] (1) Forming a thin film with a composition capable of becoming an oxide superconductor on a substrate in an amorphous or minimally crystallized state, and heat-treating the thin film to crystallize it to a predetermined depth in the film direction. 1. A method for forming a superconducting thin film, which comprises forming a thin film having superconducting properties on a substrate by. (2) A thin film having a composition capable of becoming an oxide superconductor is formed on the first superconductor forming the lower electrode or on the tunnel barrier layer formed on the first superconductor. Alternatively, a method for forming a superconducting thin film element, characterized in that a second superconductor is formed by forming the thin film in a state with little crystallization and heat-treating the thin film to crystallize it to a predetermined depth in the film direction. .