JPH0193424A - Manufacture of structure having superconductive thin film - Google Patents

Abstract

PURPOSE: To produce an oxide superconducting material having desired electric property and mechanical durability by vapor-depositing the material on a substrate by using sources respectively consisting of a rare-earth composition, a copper composition and fluoride of a IIA group element, and annealing the material in an oxygen atmosphere.

CONSTITUTION: In a production method of a structure having a superconducting thin film region, the material is vapor-deposited on the substrate consisting of e.g. strontium titanate. The vapor deposition is realized by using the source consisting of the rare-earth composition such as Y, the source consisting of the copper compound and the source consisting of the fluoride of a IIA group element such as Ba. Next, the vapor-deposited material is annealed at about 800-920°C in the oxygen atmosphere.

COPYRIGHT: (C)1989,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to superconductor materials, and particularly to a method for producing oxide superconductor materials.

[Prior Art] Recently, superconducting materials whose electrical resistance becomes essentially zero at temperatures above 40°C have attracted attention. These materials are oxides consisting of at least one rare earth element, at least one Group IIA element, and copper, a typical example of which is Y Ba 2 Cu a O□.

YBa2Cu3O7 (although these structures represent general stoichiometry, these are simply approximate nominal vacancies as occur in the oxygen lattice). These oxide materials are typically considered for applications such as power distribution lines in electronic components or in their interconnections.

Although it is not necessarily necessary for electronic parts,
It is desirable for applications such as electronic components that essentially zero resistance be achieved at Tc above the boiling point of liquid nitrogen, ie, at temperatures above 77°C. Here, essentially zero resistance refers to a resistance of about 10-9 Ω-el or less.

Oxide superconductors are typically produced by sintering oxide or carbide powders.

For example, in the case of YBa2Cu3O7, yttrium oxide, barium carbide and copper oxide are mixed in appropriate proportions, and 7
Sintering is carried out in an oxygen atmosphere at a temperature ranging from 00°C to 1000°C. The material thus obtained is reported to have essentially zero resistance at temperatures in the range 65-98°C.

There are also some unique reports. For example, Physical Review Letters
According to a report by Ovshlnsky et al. in Letters, 5g, 2579° 1987), a material that obtains exceptionally low resistance at 155 by sintering barium fluoride with metal yttrium and copper oxide. was obtained. However, there are no reports of it being reproduced yet.

Although most superconductor material samples are in bulk, some attempts have been made to form thin films on substrates. Thickness 5μ
Sub-m films are particularly suitable for applications such as electronic components where bulk materials cannot be used.

These thin film formation methods were developed by the Materials Research Society (Materials Re5ear) in Anaheim, California.
In the minutes of the 1987 spring meeting of chSociety, R, H, Kotsuho, et al.
Also on page 169 is R.H. Hammon.
d) etc. and Physical Review Letters.

58.2684 (1987) by P. Schohari (Ch.
This method of forming a thin film involves the steps of depositing yttrium, barium, and copper in an oxygen atmosphere and annealing the resulting film at a temperature in the range of 850 to 910°C. Contains. R, E, SoIIekh, etc.
Nature, 326.

857 (1987), reported a thin film formation method in which sputtering from Cu, BaO, and Y2O3 sources was performed on a sapphire substrate at 1050°C, followed by annealing at 500°C in oxygen.

Incidentally, the formation of a film by sputtering is also described by M. Moriwaki et al. on page 85 of the above meeting minutes, but the details of the process are not described.

Although the thin films thus obtained are mostly promising, the films themselves are not durable. Its superconductivity is also reported to decrease with time and with exposure to the atmosphere.

Moreover, it is very difficult to always reproduce the desired superconductivity. This is because the properties of the film are very sensitive to process parameters such as deposition rate and annealing time.

[Summary of the Invention] An oxide superconductor material that solves the above problems of conventional thin films and has desired electrical properties and mechanical durability can be produced by using vapor deposition technology and containing IIA in the final material. It is obtained by using a fluoride of a Group IIA element as a source of a Group element. For example, when producing YBa2Cu3O7, barium fluoride is used with a suitable copper source, such as metallic copper, and a suitable yttrium source, such as yttrium oxide. The resulting annealed film exhibits essentially zero resistance at temperatures as high as 92°C, and can maintain these properties for a significantly longer period of time than previously possible. Additionally, the material has superior mechanical durability and is relatively unaffected by atmospheric moisture.

[Example code] Examples according to the present invention will be described in detail below.

Typically, a thin film is deposited on a substrate, resulting in a mechanically superior structure. Furthermore, a substrate should be used that prevents substantial degradation of the superconducting material due to chemical reactions or diffusion. Here, substantial deterioration means that Tc is 4
0 or the critical current is 1×10aIlp
/cII12.

In short, the criterion for selecting the substrate is to substantially prevent undesired diffusion from the superconducting material to the substrate or from the substrate to the superconducting material, as well as chemical reactions between the substrate and the superconducting material. For example, if the superconducting film of the present invention is deposited directly on silicon, the superconducting film and silicon will react and degrade. In this case, a protective film may be provided between the reactive substrate and the superconducting film.

Suitable substrate materials include: ■ S r T io 4 or ■ sapphire coated with a zirconium oxide film. Strontium titanate does not unduly degrade the superconducting material, and zirconium dioxide prevents reactions between the superconducting material and sapphire. Unlike bulk materials, gold films are not preferred as reaction barriers. This is because gold rapidly diffuses into superconducting materials, rapidly reducing its protective function.

A wide range of materials can be manufactured with the present invention. Generally, these materials are classified as oxide superconducting materials, and are the following oxides.

- At least one non-radioactive rare earth element other than terbium (praseodymium and cerium tend to degrade superconducting properties, but at low concentrations they can be used in less demanding applications and are not excluded.

); ■ at least one nA group element such as calcium, barium or strontium; and ■ copper.

The representative material is US Patent Application No. 001.882.

No. 021,229, No. 024,048, and No. 02
No. 7.371. For the present invention, rare earth elements include yttrium, lanthanum and scandium.

Examples of such materials include those expressed by the nominal formula 7 and Y S r 2 Cu a O7.

In general, the nominal formula approximately expresses the stoichiometric ratio, and the nominal deviation (nominal
Irrespective of the type of superconducting oxide, the present invention provides a method for simultaneously combining rare earth materials, copper, and fluorides of Group IIA elements (barium fluoride, strontium fluoride, and/or calcium fluoride). It involves vapor deposition or sequential vapor deposition steps. Note that "vapor deposition" in the present invention includes all techniques for depositing a material from a vapor phase. That is, techniques such as thermal evaporation with resistance heating or electron beam heating, DC or RF plasma sputtering, as well as secondary ion beam sputtering, etc. are included.

A source of rare earth elements is not required.

Rare earth elements are generally vapor-deposited using simple metals or metal oxides. However, these materials do not have substantial vapor pressure at the temperatures obtained by resistively heating the crucible, making it undesirable to use resistive heating for deposition. Therefore, techniques such as electron beam evaporation are preferred for depositing rare earth compositions. However, this does not exclude other technologies. The flux of rare earth materials (e.g. yttrium) produced by vapor deposition techniques is typically 1 to 1 per second.
0 angstrom.

Resistance heating is convenient for performing copper vapor deposition.

Generally, a source of copper, such as copper or copper oxide, is placed in a pyrolytic boat-shaped dish, such as a tungsten boat-shaped dish, and resistance heating produces a deposition flux in the range of 1 to 10 angstroms per second. It should be noted that the preference for resistive heating does not exclude other techniques.

In order to obtain the desired properties of the material in the present invention, IIA
The source of group elements should come from fluoride.

Deposition of such materials can be easily accomplished by resistance heating. A practical barium fluoride flux is 3 to 30 angstroms per second. Lower flux rates are practical for all deposited materials, provided the relative proportions can be maintained to some extent. The flux rate (7 angstroms) is obtained by measuring the amount of flux deposited on a crystalline quartz velocity monitor and calculating the deposition rate on the substrate with the entire bulk density. However, a single coefficient is assumed and modified to take into account the difference in distance and angle between the speed monitor and the source board. This modification method is described in the "Vacuum Evaporation" section of the "Thin Film Technology Handbook" (Meisel and Brang, McGraw-Hill, 1970). Although the thermal properties are obtained from the fluorides of Group IIA elements, it is also possible to use small amounts of other sources, such as Group IIA metals, without affecting the superconducting properties. Incidentally, powder sources such as barium fluoride powder degrade the properties since the powder tends to adsorb ambient moisture. As such, bulk anhydrous materials are preferred, although anhydrous powders are not excluded.

The relative flux of each element forming superconducting materials determines the final stoichiometry of these materials. In particular, excellent conductive properties can be achieved over a wide range of stoichiometric compositions associated with the aforementioned fluxes, although effective materials can be obtained outside this range. Adjustment samples can be used to determine the appropriate flux rate to obtain the desired properties.

The oxygen needed to form superconducting materials is obtained through the process of vapor deposition,
It is added during either or both of the annealing steps. For example, a stoichiometric excess of oxygen is added to the deposition substrate by conventional techniques, such as oxygen flow through a conduit, upon deposition.

Although it is generally convenient to use a pure oxygen stream, it may be diluted with an inert gas such as argon or other noble gas. Similarly, the oxygen added in the annealing process does not need to be pure oxygen. Oxygen at fairly high pressures is also inconvenient, but oxygen at far lower pressures than atmospheric during the annealing process will cause oxygen vacancies in the superconducting material, reducing its superconducting properties.

The deposited film is annealed at 800-920° C. in an oxygen-containing atmosphere. This annealing process forms a perovskite-like crystal structure. In general, low temperatures do not cause the necessary changes in crystal structure, but prolonged exposure to high temperatures reduces superconducting properties, due to phase separation and reactions with the substrate.

The annealing time is 3 to 6 hours at a preferred temperature range. (This annealing time depends to some extent on the film thickness, and this temperature range is between 1000 and 200°C.
For films with a thickness of 0 angstroms, thicker films require longer annealing times. The prepared sample can be used to determine the annealing time and temperature for a film of a certain composition and thickness. ) Generally, the resistance of superconducting films differs depending on the direction. For example, the resistance in the C crystal axis direction is about three orders of magnitude larger than that in the ab plane. In most applications, it is preferred that the C crystal axis direction be perpendicular to the main plane of the substrate. Typically,
Thick films contain more material with C crystal axis orientation along the surface.

For this reason, a thin film has a low resistance above Tc.

The superconducting materials of the present invention can be patterned using conventional lithography techniques, lift-off and etching. For example, the deposition substrate may be coated with conventional resist material AZ411.
0 (a product of Hoechst, USA), and the locations where superconducting material is needed on the substrate are opened by creating openings in the resist material. A deposition process is performed on the substrate coated with resist material except for the openings.

After the resist material is removed, it is annealed, leaving a deposited film in the desired shape. This next step of annealing transforms this film section into a superconducting material patterned at the required locations. Since the superconducting material of the present invention is not water-stable, the resist material is removed with conventional removal agents such as acetone (which contain water as a mixture). Alternatively, either before or after the annealing step, a resist material is deposited and the resist material is written to expose portions of the superconducting material. The structure is then immersed in an etching solution of 10:1 water and hydrochloric acid. This drawing technique is used to draw the necessary patterns in applications such as electronic components.

The following specific examples illustrate suitable conditions for producing the superconducting materials of the present invention.

Example 1 A superconducting film is produced on a strontium titanate substrate. This substrate has a thickness of 1/2 mm, a diameter of about 1 cm, and its main surface is perpendicular to the [100] crystal axis direction. As delivered from the manufacturer, the surface of the substrate is polished both mechanically and chemically. The substrate is cleaned by soaking in trichloroethane, acetone, and methanol. The substrate is then blown dry in dry nitrogen and placed on a sample holder in a vacuum container to expose the major surface of the substrate. This sample holder is heated to 550°C.

There are three deposition sources in the vacuum vessel, the first being a conventional commercially available electron beam evaporator using a yttrium target. 2nd, 3rd
One uses a resistance-heated tungsten boat-shaped dish. One boat-shaped pan is copper metal and the other boat-shaped pan is filled with 99.9% pure barium fluoride. The vacuum vessel also has three crystalline quartz velocity monitors positioned to determine the flux from each source.

The vacuum vessel is initially approximately 5 x 10' torr (7,5 x 10-
9 pascals). The electron beam flux of the electron beam evaporator and the temperature of the two ship-shaped dishes were adjusted to achieve a deposition flux of yttrium of 2.2 Å/sec and 2.0 Å/sec, respectively.
The evaporative flux of copper in seconds and 6.5 angstroms/
Generate an evaporative flux of barium fluoride in seconds.

Oxygen is blown through a 178 inch (0.32 mm) diameter stainless steel tube. The orifice of this tube is about 3 cm from the surface of the substrate, and the tube is angled so that it does not cross the evaporative flux. A sufficient flow of oxygen is blown through the stainless steel tubing to maintain the pressure in the vacuum chamber at approximately 5×10' Torr (37,5×10' Pascals).

The shutter, which was initially placed between the evaporation source and the substrate, is then released. Deposition continues until a thickness of 172 microns is obtained. The shutter then screens out the fluff. The deposition is complete, the oxygen flow is stopped, the vacuum chamber is filled with oxygen to 100+u+)-L, the substrate is cooled, and the vacuum chamber is replaced with dry nitrogen.

The sample is removed from the vacuum chamber and placed in a vertical quartz tube furnace. This vertical quartz tube furnace is a quartz tube with an external diameter of 2.4c11 and a wall thickness of 1 mm. The sample is placed in a furnace and oxygen flows through a quartz tube. The rate of this oxygen flow is approximately 1 cc/sec. Thereafter, the temperature inside the furnace is raised from room temperature to 910°C at a rate of 30°C/min. The temperature inside the furnace is maintained at 910° C. for 2 hours. The furnace temperature is then lowered to about 550°C at a rate of 5°C/min. This temperature is maintained for 30 minutes, after which heating is discontinued and the sample is removed when the furnace temperature reaches 200°C.

Four spring pressure contacts are attached to the surface of the film spaced approximately 0.24 mm apart. The substrate rests on a copper block with its back side embedded with a resistance heater and a silicon diode thermometer. The block with the substrate on it is placed inside a phenol can, and the entire block is placed in a liquid helium dewar filled with helium gas.
placed above the liquid level in the container. Resistance measurements were made using 1 μA alternating current. The results are shown in the figure.

Example 2 In Example 2, the manufacturing method of Example 1 is followed, except that a 0.2 μm thick film is deposited on a substrate at room temperature. ) Next, the peak temperature 85
Annealed at 0 °C for 3 hours, the substrate was cooled to room temperature,
This annealing step is repeated. Tc of this film
is 90°C and its maximum current density is about 1 x 10Bamp/(J) at temperatures between 77°C and 82°C. Maximum current per cross-sectional area. This measurement was made in a manner similar to that described in Example 1, except that two scratches were made in the film so that the current passed through a narrow area, and the pressure Electrodes were connected to four gold dots deposited on the superconducting film.

Example 3 In Example 3, the manufacturing method of Example 1 was also carried out, except that barium fluoride was deposited at a rate of 6 angstroms/second, and the annealing step was performed at a vinyl temperature of 920° C. for 3
It lasted 0 minutes. The deposit thickness was approximately 4000 angstroms and the Tc was 84.

Example 4 In Example 4, the manufacturing method of Example 2 is carried out, except that barium fluoride is deposited at a rate of 7 angstroms/second, the deposition thickness is 0.5 μm, and the first 600 μm
Depositions of one Angstrom were made only from barium fluoride. Annealing was performed at a peak temperature of 910° C. for both cycles. Other depositions were performed as described in Example 2. The Tc was 80.

Example 5 In Example 5, the manufacturing method of Example 2 is carried out, except that the barium fluoride is evaporated at 6.5 angstroms/m/m.
Copper deposition was performed at a rate of 1.8 angstroms/second, with a total deposition thickness of about 4000 angstroms.

Each annealing step was performed at 920° C. for 6 hours. This Tc
It was 85 years ago.

Example 6 In Example 6, the method of Example 5 was followed, except that the total deposited thickness was about 4000 angstroms and a single annealing step was performed at 920° C. for 6 hours. Furthermore, after cleaning and before the evaporation step, a 1 μm thick AZ4110 resist material was applied to the substrate. This resist material layer has wavelength 3
After exposure to 65 nanometer ultraviolet light through a conventional mask and development with a developer solution diluted 4:1 in deionized water, the opposing terminals are formed. After removing the evaporation film from the evaporator and before the annealing step, the resist material is removed using acetone. The remaining film is subjected to an annealing process to form the desired pattern. A 2 μm wire was formed on a similar substrate using a comparable technique. The Tc of this film was 92.

After making this measurement, the same film was patterned (1 μm thick) with a resist material of AZ4110, and the entire substrate was drawn with 1 mm wide lines (15 μm thick) across the entire substrate.
(except for the intermediate range of length 0 μm). The substrate is immersed for 5 seconds in a solution of 1 part hydrochloric acid to 1 part water (by volume). This resist material is removed by soaking in acetone for about 5 minutes. This sample was dried with dry nitrogen. Over a wide area the Tc was 90 and in some parts 82.

Example 7 To measure the underwater resistance of the material, the evaporation process of Example 5 was performed for a total thickness of 3000 Angstroms, and the film was soaked in water for 5 minutes before the annealing step. After one 6 hour annealing step at 920° C., the film had a Tc of 83.

Example 8 In Example 8, the manufacturing method of Example 4 is also carried out, except that the barium fluoride is evaporated at 6.5 angstroms/m/m.
Copper deposition was performed at a rate of 1.8 Angstroms/second, and the substrate was replaced with single crystal sapphire with a coating of zirconium oxide. The zirconium oxide coating was deposited by electron beam evaporation of zirconium in an oxygen atmosphere (5 x 10-5 Torr);
It has a thickness of approximately 1000 angstroms. The resultant has a Tc of about 80.

Example 9 In Example 9, the method of Example 2 was followed, except that the total deposited thickness was about 1000 Angstroms. One 6-hour annealing step was performed at a peak temperature of 800°C. The Tc of the obtained film was 91.

[Brief explanation of the drawing]

The figure is a diagram showing the characteristics of a superconducting object obtained by the present invention. Applicant: American Telephone &T (K)

Claims (11)

[Claims] (1) A method of manufacturing a structure having a superconducting thin film region, comprising the steps of depositing a material on a substrate and annealing the material in an oxygen atmosphere, the deposition step comprising the steps of: depositing a material on a substrate; and annealing the material in an oxygen atmosphere. A method for manufacturing a structure having a superconducting thin film, characterized in that the method is carried out using a source consisting of a copper composition, a source consisting of a fluoride of a group IIA element, and a source consisting of a fluoride of a group IIA element. 2. The method of claim 1, wherein the annealing is performed at a temperature in the range of about 800°C to 920°C. (3) The manufacturing method according to claim 1, wherein the rare earth composition comprises yttrium. (4) The method according to claim 3, wherein the Group IIA composition comprises barium. (5) The method according to claim 1, wherein the Group IIA composition comprises barium. (6) The manufacturing method according to claim 1, wherein the substrate is made of strontium titanate. The manufacturing method according to claim 1, further comprising the step of (7) patterning the superconducting thin film. (8) The manufacturing method according to claim 1, wherein the copper vapor deposition is performed by resistance heating. (9) The manufacturing method according to claim 8, wherein the vapor deposition is performed by resistance heating of copper. (10) The manufacturing method according to claim 1, wherein the copper vapor deposition is performed by sputtering. (11) The manufacturing method according to claim 1, wherein the structure is made of an electronic component.