JPH0645443B2 - Method for manufacturing structure having superconducting thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconductor material, and more particularly to a method for producing an oxide superconductor material.

[Prior Art] Recently, superconducting materials having essentially zero electric resistance at temperatures of 40 K or higher have been attracting attention. These materials are oxides composed of at least one rare earth element, at least one Group IIA element and copper, and a typical example thereof is Y.
Ba ₂ Cu ₃ O ₇ , Y ₃ Ba ₃ Cu ₆ O ₁₄ , and YSr ₂ Cu ₃
There is O ₇ (these structural formulas represent general stoichiometric ratios, but these are merely approximate nominal vacancies as occur in the oxygen lattice). These oxide materials are typically considered for applications such as power distribution in electronic components or in their interconnections. Although not necessarily required for electronic components, the essentially zero resistance is above the boiling point of liquid nitrogen, that is, 7
Achieving Tc at temperatures above 7K is desirable for applications such as electronic components. here,
Essentially zero resistance is a resistance of about 10 ^-9 Ω-cm or less.

Oxide superconductors are typically manufactured by sintering oxide or carbide powders. For example YBa
_In the case of ₂ Cu ₃ O ₇ , yttrium oxide, barium carbide and copper oxide are mixed in an appropriate ratio, and the temperature is adjusted to 700 ° C. to 10 ° C.
Sintering is performed in an oxygen atmosphere at a temperature in the range of 00 ° C. The material thus obtained is reported to have essentially zero resistance at temperatures in the range 65-98K. There are also some unique reports. For example, in Physical Review Letters (58, 2579, 1987), Ovshinsky et al. Reported that barium fluoride was exceptionally sintered at 155 K by sintering it with yttrium metal and copper oxide. It is said that a material that obtains low resistance was obtained. However, there is no report that it was reproduced.

Most superconducting material samples are bulky, but some attempts have been made to form thin films on substrates. Thickness 5μ
Thin films of m or less are particularly suitable for applications such as electronic components where bulk materials cannot be used.

These thin film formation methods are described in the Materials Research Society of Anaheim, California.
81 pages of Extended Abstracts High Temperature in the Proceedings of the Spring 987 Spring Meeting
R. Superconductors. H. Also on page 169 by R. Koch and others. H. Hammond
nd), etc., as well as physical review letters,
58, 2684 (1987); Chaudha
ri) et al., such a thin film forming method is a method in which yttrium, barium and copper are vapor-deposited in an oxygen atmosphere, and a film formed by this is deposited at 850 to 91.
It includes a step of annealing at a temperature in the range of 0 ° C.
R. E. Somekh et al., Nature, 3
26,857 (1987), a thin film forming method is reported in which sputtering from Cu, BaO and Y ₂ O ₃ sources is performed on a sapphire substrate at 1050 ° C., followed by annealing at 500 ° C. in oxygen. The formation of the film by sputtering is described in M. It is also described on page 85 of the conference proceedings by Moriwaki et al., But the details of the process are not described.

The thin films thus obtained are mostly promising, but the films themselves are not strong. Its superconductivity has also been reported to decrease over time and upon 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 problems of the conventional thin film described above and has desired electrical properties and mechanical durability is obtained by using a vapor deposition technique and is contained in the final material IIA. It is obtained by using a Group IIA element fluoride as a source of a Group element. For example, when making YBa ₂ Cu ₃ O ₇ , barium fluoride is used with a suitable copper source such as metallic copper and a suitable yttrium source such as yttrium oxide. The film obtained by annealing this is
It exhibits essentially zero resistance at a temperature as high as 92K, and can maintain those properties for a sufficiently longer period than before. Further, the material is superior in mechanical durability to the conventional one and is relatively unaffected by moisture in the atmosphere.

EXAMPLES Examples according to the present invention will be described in detail below.

A thin film is typically deposited on the substrate, which results in a mechanically superior structure. In addition, a substrate should be used that prevents substantial degradation of the superconducting material due to chemical reactions and diffusion. Here, the substantial deterioration means that Tc is 4
Lower than 0K or critical current is 1 × 10 ³ amp / c
It is lower than m ² .

In summary, the substrate selection criterion is to substantially prevent undesired diffusion from the superconducting material to the substrate or from the substrate to the superconducting material as well as the chemical reaction between the substrate and the superconducting material. For example, when the superconducting film of the present invention is directly deposited on silicon, the superconducting film reacts with silicon to cause deterioration. In this case, a protective film may be provided between the reactive substrate and the superconducting film.

Suitable substrate materials include sapphire coated with SrTiO ₄ or zirconium oxide film. Strontium titanate does not excessively degrade the superconducting material, and zirconium dioxide prevents the reaction between the superconducting material and sapphire. Unlike the bulk materials, gold films are not preferred as reaction barriers. This is because gold diffuses rapidly into the superconducting material, and the protective function rapidly declines.

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

At least one non-radioactive rare earth element other than terbium (praseodymium and cerium tend to degrade superconducting properties, but at low concentrations it is applicable and not excluded for less demanding applications) and calcium, barium or At least one Group IIA element such as strontium, and copper.

Typical materials are US patent application No. 001,682, No. 021,2
No. 29, No. 024,046, and No. 027,371. For the present invention, yttrium, lanthanum and scandium are included as rare earth elements.

Such materials include, for example, the nominal formula YBa ₂ Cu.
Some are represented by ₃ O ₇ , Y ₃ Ba ₃ Cu ₆ O ₁₄ , and YSr ₂ Cu ₃ O ₇ . In general, the nominal formula approximates the stoichiometric ratio, and is the nominal deviation (n) that occurs as a result of oxygen vacancies and / or inhomogeneities introduced with or without intention.
ominal deviations) are also possible.

Regardless of the type of superconducting oxide, the present invention is directed to rare earth materials, copper, and Group IIA element fluorides (barium fluoride,
(Strontium fluoride and / or calcium fluoride)
Of the simultaneous vapor deposition or sequential vapor deposition. In addition,
“Vapor deposition” according to the present invention includes all techniques for depositing a material from the vapor phase. That is, techniques such as thermal evaporation by resistance heating or electron beam heating, dc or rf plasma sputtering, and secondary ion beam sputtering are included.

It does not mean that a source of rare earth elements is needed. The rare earth element is generally vapor-deposited using a simple metal or a metal oxide. However, these materials do not have a substantial vapor pressure at the temperatures obtained by resistively heating the crucible, so it is not desirable to use resistive heating for vapor deposition.
Therefore, a technique such as electron beam vapor deposition is preferable for vapor deposition of the rare earth composition. However, this does not exclude other technologies. The flux of rare earth materials (eg, yttrium) produced by vapor deposition techniques is typically 1-10 Angstroms per second.

Resistance heating is convenient for depositing copper. In general,
A source of copper, such as copper or copper oxide, is placed in a pyrolysis boat such as a tungsten boat and resistively heated to produce a vapor deposition flux in the range of 1-10 Angstroms per second. Although resistance heating is convenient, it does not exclude other techniques.

To obtain the desired properties of the materials in the present invention, the source of the Group IIA element should be supplied from fluoride. The vapor deposition of such a material can be easily performed by resistance heating. Practically, the flux of barium fluoride is 3 to 30 angstroms per second. Lower flux rates are practical for all vapor deposition materials if the relative proportions can be maintained to some extent. The flux rate (angstrom / sec) is obtained by measuring the amount of flux deposited on the crystal quartz velocity monitor and calculating the deposition rate on the substrate having the bulk density as a whole. However, assuming a single coefficient, the correction is performed by taking into consideration the difference in distance and angle between the speed monitor and the substrate of the source. This correction method is described in "Thin Film Technology Handbook" (by Meissel and Grang
It is described in the item "Vacuum evaporation" of McGraw-Hill Inc. 1970). The desired property is obtained from the fluoride of the IIA group element, but it does not affect the superconducting property.
It is also possible to use small amounts of other sources such as group metals. Incidentally, a powder source such as barium fluoride powder degrades properties because the powder tends to adsorb ambient moisture. For this reason, bulk anhydrous materials are preferred, but anhydrous powders are not excluded.

The relative flux of each element forming superconducting materials determines the final stoichiometric composition of these materials. In particular, excellent conductive properties can be achieved with a wide range of stoichiometric compositions associated with the above-mentioned fluxes, but effective materials are also obtained outside this range. The adjusted sample can be used to determine the appropriate flux rate to obtain the desired properties.

The oxygen needed to form the superconducting material is the process of vapor deposition,
It is added in 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 under the assumption of vapor deposition. Generally, it is convenient to use a pure oxygen stream, but the pure oxygen stream may be diluted with an inert gas such as argon or another noble gas. Similarly, the oxygen added in the annealing process does not have to be pure oxygen. Oxygen at a considerably high pressure is also inconvenient, but oxygen at a pressure much lower than atmospheric pressure in the annealing process causes oxygen deficiency in the superconducting material, resulting in deterioration of superconducting properties.

The vapor-deposited film is annealed at 800 to 920 ° C. in an oxygen-containing atmosphere. A perovskite-like crystal structure is formed in this annealing step. In general, low temperatures do not cause the necessary changes in the crystal structure, but long periods of high temperatures reduce the superconducting properties due to phase separation and reaction with the substrate. Annealing time is 3 to 6 hours at a temperature in the preferred temperature range. (This annealing time depends to some extent on the film thickness, and this temperature range is 1000 to
For 2000 Å thick films, thicker films require longer anneal times. The conditioned sample can be used to determine the anneal time and temperature of a film of a certain thickness. ) Generally, the resistance of a superconducting film varies depending on the direction. For example, the resistance in the c crystal axis direction is about three orders of magnitude higher than that in the ab plane. For most applications, it is preferred that the c-crystal axis direction be orthogonal to the major plane of the substrate. In general, thick films are rich in material that has a c-axis along the surface. Because of this, the thin film is T
It becomes low resistance when it is more than c.

The superconducting material of the present invention can be patterned using conventional lithographic techniques, lift-off and etching. For example, the deposition substrate may be a conventional resist material AZ411.
0 (a product of Hoechst, USA) is patterned to open the resist material at an area on the substrate where a superconducting material is required. A vapor deposition process is performed on the substrate coated with the resist material except the openings. Then, after the resist material is removed, it is annealed to leave the vapor deposition film in the required shape. This subsequent step of annealing transforms the film portion into a superconducting material that is patterned at the required locations. Since the superconducting materials of the present invention are not water labile, the resist material is stripped with a conventional stripper such as acetone, which contains water as a mixture. Alternatively, the resist material is deposited before and after the annealing step, the resist material is drawn, and the portion of the superconducting material is exposed.
The composition is then immersed in an etching solution of water and hydrochloric acid in a ratio of 10: 1. This drawing technique is used to draw the required patterns in applications such as electronic components.

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

Example 1 A superconducting film is produced on a strontium titanate substrate. This board is 1 / 2mm thick and about 1cm
, And the main surface is orthogonal to the [100] crystal axis direction. The surface of the substrate as delivered by the manufacturer has been mechanically and chemically polished. The substrate is washed by dipping it in trichloroethane, acetone, methanol. The substrate is then blown dry in dry nitrogen and placed on a sample holder in a vacuum container to expose the main surface of the substrate. The sample holder is heated to 550 ° C.

There are three vapor deposition sources in the vacuum vessel, the first one is a conventionally commercially available electron beam vapor deposition apparatus, and this vapor deposition apparatus uses an yttrium target. 2nd, 3rd
The thing uses the resistance heating tungsten ship type dish. One hull is filled with copper metal and the other hull is filled with 99.9% pure barium fluoride. The vacuum chamber also has three crystal quartz velocity monitors located at positions where the flux from each evaporation source can be measured.

The vacuum vessel is initially about 1 x 10 ^-6 torr (7.5 x 10 ^-9
Pascal) is evacuated. The electron beam flux of the electron beam evaporator and the temperature of the two hulls were adjusted to give an evaporation flux of yttrium of 2.2 Å / sec and 2.0 Å / sec respectively.
Second copper evaporative flux and 6.5 Å /
Generates an evaporative flux of barium fluoride in seconds.

Oxygen is blown through a 1/8 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 traverse the evaporative flux. A sufficient amount of oxygen flow is blown from the stainless tube to increase the pressure in the vacuum chamber to about 5
Maintain at × 10 ⁻⁵ Torr (37.5 × 10 ⁻⁸ Pascal). The shutter originally placed between the evaporation source and the substrate is then released. Deposition is continued until a thickness of 1/2 micron is obtained. The shutter then blocks the flack. The vapor deposition is complete, the oxygen flow is stopped, the vacuum chamber is filled with oxygen to 100 mmTorr, the substrate is cooled, and the vacuum chamber is replaced with dry nitrogen.

The sample is taken out of the vacuum chamber and placed in a vertical quartz tube furnace. This vertical quartz tube furnace has an outer diameter of 2.4 cm and 1
A quartz tube with a wall thickness of mm. The sample is placed in a furnace and oxygen flows through a quartz tube. The velocity of this oxygen stream is about 1 cc / sec. After that, the temperature in the furnace is from room temperature to 30 ° C
The temperature can be increased to 910 ° C. at a rate of / minute. The furnace temperature is 9
Hold at 10 ° C for 2 hours. After that, the temperature in the furnace is lowered to about 550 ° C at a rate of 5 ° C / minute. This temperature is maintained for 30 minutes, after which the heating is stopped and the sample is taken out when the temperature inside the furnace reaches 200 ° C.

Four spring contacts are attached to the surface of the film with a spacing of about 0.24 mm. The board is placed so that its backside is on a copper block, which has a resistance heater and
A silicon diode thermometer is embedded. The block on which the substrate is placed is placed in a can made of phenol, and this whole is placed on the liquid level in a liquid helium dewar container filled with helium gas. Resistance measurements were made with an alternating current of 1 μA. The results are shown in the figure.

Example 2 The method of Example 1 is also performed in Example 2, except that a film having a thickness of 0.2 μm is vapor-deposited on the substrate kept at room temperature. ) Then the peak temperature 85
Annealed at 0 ° C. for 3 hours, cooled the substrate to room temperature,
This annealing process is repeated. Tc of this film
Is 90 K and its maximum current density is about 1 × 10 ⁶ amp / cm ² at temperatures between 77 and 82 K. However, this maximum current density is the maximum current per unit cross-sectional area where the film retains superconductivity. This measurement was made in the same manner as described in Example 1, except that two scratches were made on the film so that the current passed through a narrow area and the insulation displacement electrode was deposited on the superconducting film. Connected to the four gold dots that were made.

Example 3 The manufacturing method of Example 1 was also performed in Example 3, except that barium fluoride was vapor-deposited at a rate of 6 Å / sec and the annealing step was performed at a peak temperature of 920 ° C. for 3 hours.
It was done in 0 minutes. The deposited thickness was about 4000 Å and the Tc was 84K.

Example 4 The process of Example 2 is also carried out in Example 4, except that barium fluoride is vapor-deposited at a rate of 7 Å / sec and the deposition thickness is 0.5 μm and the initial 600
The Angstrom deposition was done only from barium fluoride. Annealing was performed in both cycles at a peak temperature of 910 ° C. Other vapor depositions were performed as described in Example 2. This Tc was 80K.

Example 5 The process of Example 2 is performed in Example 5 as well, except that the deposition of barium fluoride is 6.5 angstroms /
The copper deposition was performed at a rate of 1.8 Å / sec with a total deposition thickness of about 4000 Å. Each annealing step was performed at 920 ° C. for 6 hours. This Tc was 85K.

Example 6 The process of Example 5 is also performed in Example 6, except that the total deposition thickness is about 4000 Å and the annealing step is performed once at 920 ° C. for 6 hours. Further, after cleaning, the substrate before the evaporation process was coated with a 1 μm thick AN4110 resist material. This resist material layer has a wavelength of 3
After exposing to 65 nanometer UV through a conventional mask and developing with a developer diluted with deionized water at a ratio of 4: 1, opposite terminals are formed. After removing the evaporated film from the evaporator and before the annealing step, the resist material is removed with acetone. The remaining film forms the desired pattern before and after the annealing step. A 2 μm wire was formed on a similar substrate using the corresponding technique. The Tc of this film was 92K.

After performing this measurement, a pattern (1 μm thick) was formed on the same film with the resist material of AZ4110, and the entire substrate was drawn, and then a 1 mm wide line (150 mm) was formed over the entire substrate.
(except for the mid range of μm length).
The substrate is immersed for 5 seconds in a solution of 10 hydrochloric acid to 1 hydrochloric acid (by volume). The resist material is removed by soaking in acetone for about 5 minutes. The sample was dried with dry nitrogen. Wide range Tc of 90K
And, the part was 82K.

Example 7 To measure the underwater resistance of the material, the evaporation process of Example 5 was performed for a total thickness of 3000 Å and the film was immersed in water for 5 minutes before the annealing step. The Tc of the film after performing the annealing step once at 920 ° C. for 6 hours was 83K.

Example 8 The process of Example 4 is performed in Example 8 as well, except that the deposition of barium fluoride is 6.5 Å /
The deposition of copper was performed at a rate of 1.8 Å / sec and the substrate was replaced with single crystal sapphire with a zirconium oxide coating. The zirconium oxide coating is deposited in an oxygen atmosphere (5 × 10 ⁻⁵ Torr) by electron beam evaporation of zirconium and has a thickness of about 1000 Å. The one obtained has a Tc of about 80K.

Example 9 The process of Example 2 is performed in Example 9 as well, except that the total deposited thickness is about 1000 Å. An annealing process was performed once at a peak temperature of 800 ° C. for 6 hours. The Tc of the obtained film was 91K.

[Brief description of drawings]

The figure is a diagram showing the characteristics of the superconducting object obtained by the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical indication location H01B 13/00 HCU Z 7244-5G H01L 39/24 ZAA B 9276-4M (72) Inventor John Henry Scofield United States, 44074 Ohio Oberlin, North Professor 250 (72) Inventor William John Scockpol United States, 08520 New Jersey East Windsor, Overton Road 21 (56) Reference Japanese Patent Laid-Open No. 63-274027 (JP, A) Kai 63-297217 (JP, A) JP 63-225528 (JP, A)

Claims (3)

[Claims] 1. A method of manufacturing a structure having a superconducting thin film region, comprising: depositing a material on a substrate; and annealing the deposited material in an oxygen atmosphere, the vapor depositing step comprising: , A source made of yttrium,
A method for producing a structure having a superconducting thin film, which is carried out using a source made of a copper composition and a source made of barium fluoride. 2. The annealing is performed at about 800 ° C. to 920 ° C.
The manufacturing method according to claim 1, wherein the temperature is in the range. 3. The manufacturing method according to claim 1, wherein the substrate is made of strontium titanate.