JPH02267103A - Production of superconducting thin film - Google Patents

Abstract

PURPOSE:To obtain at low cost the title thin film capable of accomplishing large aperture and high critical current density and of suppressing grain boundary barrier by exposing the surface of a fluoride to a plasma atmosphere to effect activation followed by forming a superconducting thin film on the resultant activated surface. CONSTITUTION:The surface of a fluoride such as a CaF2 film formed on a silicon wafer substrate is exposed to a plasma atmosphere to effect activation. This atmosphere can be formed by introducing, after vacuuming. Ar or O2 followed by conversion into a plasma by microwave. Thence, a superconducting thin film such as of Bi2Sr2CaCu2Odelta base is formed on the resulting activated surface, This thin film can be fixed at a proper position in good wettability on the fluoride surface. Thereby, large aperture. high critical current density and the suppression of grain boundary barrier can be accomplished, thus obtaining the objective superconducting thin film suitable for e.g. superconducting devices in favorable mass productivity at low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a superconducting thin film used for 5QUID, Josephson device, superconducting transistor, electromagnetic wave sensor, device wiring, electrode, antenna, etc.

[Prior art] When applying oxide superconducting materials, which are currently a hot topic, to thin film devices, it is difficult to epitaxially grow superconducting materials because a high critical current density is required and it is necessary to suppress potential barriers such as grain boundaries. It can be said to be essential. For epitaxial growth, it is necessary to match the lattices of the substrate and superconducting material, and generally PHYSICA
L REVIEW B VOL, 38 No.
1 (1988) 765-767, APPLIED
FHYSIC8LETTER3VOL, 53N
O, 17 (1988) 1654-1656, oxide single crystal substrates including MgO were used.

Recently, a method has been studied in which a buffer layer such as a CaF2 film having a similar lattice constant is formed on a silicon wafer or a Garahiso wafer, and then an oxide superconducting thin film is formed thereon.

[Problems to be solved by the invention] However, when using a single crystal substrate, it is not possible to increase the diameter, so efficient production is not possible as with semiconductors, the shape of the element is limited, and the manufacturing cost (especially of the substrate) There were problems such as high price.

Furthermore, the method of forming a buffer layer CaF2 film using a single-crystal silicon wafer (which can be obtained up to about 20 cm in diameter) or a Garihiso wafer, which can have a large diameter, has a significantly better superconductivity than the method of directly depositing an oxide superconducting thin film on the wafer. Although it exhibits conductive properties, it has not yet reached the level of properties that can be achieved using a single crystal substrate.

The reason for this is thought to be that the wettability between CaF2 and the superconducting material elements is poor, so that the elements are not fixed at appropriate positions during the film growth process.

The present invention is intended to solve these problems, and its purpose is to create an ultra-high-performance fabric that enables large diameters, high critical current densities, and suppression of grain boundary barriers, and has no limitations on applications and is excellent in mass production. The purpose is to obtain a conductive thin film at low cost.

[Means for Solving the Problems] In order to solve the above problems, the method for producing a superconducting thin film of the present invention involves exposing and activating the fluoride surface in a plasma atmosphere when forming a superconducting thin film on a fluoride. After that, a superconducting thin film is formed.

[Example] The present invention will be described in detail below with reference to Examples.

First, a CaF2 film with a thickness of 400 to 500 nm is formed on a single crystal silicon wafer substrate by a reactive vapor deposition method.

The film formation conditions were: Ca metal was used as the evaporation source, and the substrate temperature was 480°C.
1 Initial vacuum degree 2*1O-6Torr, film formation rate 17
~20 nm/min. Fluorine is supplied to the film by irradiating the substrate with CFa gas turned into plasma during film formation. After film formation, an annealing treatment is performed as necessary to improve and stabilize the crystallinity of the CaF2 film.

Next, the CaF2
After setting the substrate on which has been formed and evacuation, the surface of the CaF2 film is activated by introducing and irradiating the surface of the CaF2 film with argon, oxygen, etc., which have been turned into plasma by microwaves outside the chamber, near the substrate. The irradiation time is 20 to 120 seconds. The appropriate irradiation time varies depending on the gas used, and care must be taken as too long will roughen the surface. Then on the active surface B i2
A s r2Calcu20δ superconducting film was formed to a thickness of 150 nm. The film formation conditions were as follows: Bi, Sr, Ca, and Cu metals were used as evaporation sources (evaporation was performed using a Knudsen cell).
, vacuum degree 3-6*1O-5Torr, substrate temperature 6800
The C2 film formation rate is 20 to 35 nm/min, and oxygen is supplied by irradiating oxygen plasma activated by microwaves during film formation.

Next, the difference in crystallinity due to the presence or absence of plasma irradiation before film formation was investigated using RHEED, X-ray diffraction, electron microscopy, etc. The results showed that the film after plasma irradiation showed better epitaxial growth.

Next, the critical temperature and critical current density of this oxide superconducting thin film were measured using a four-probe method. The measurement temperature was 60K, and the measurement atmosphere was helinium gas. For cooling, a cryogenic refrigerator UV204SR manufactured by Daikin Industries was used.

The results are shown in Table 1 together with comparative examples. Comparative examples include cases in which no plasma exposure was performed before film formation, cases in which a single crystal substrate (MgO) was used, and cases in which an oxide superconducting film was directly formed on a silicon wafer.

As can be seen from the table, the superconducting thin film of the present invention shows a remarkable improvement in critical current density, which is equal to or higher than that using an MgO single crystal substrate.

The reason why the critical current density is higher than that using an MgO single crystal substrate is because the lattice constant of CaF2 is closer to the lattice constant of the superconducting material than that of MgO, and the matching is originally better.

Table 1 In other words, by improving the wettability between CaF2 and the deposited material, which inhibited epitaxial growth, CaF2 was added to the base layer.
It can be said that the original characteristics of the rolling conductive film using this method have been brought out.

In this example, plasma is generated using microwaves, but plasma may also be generated using RF.Although plasma is generated outside the chamber and then introduced into the chamber for irradiation, plasma may be generated within the chamber. This is explained in terms of superconducting materials, but this is because oxide superconducting materials have strong anisotropy and are the materials in which the present invention is most effective. However, since the crystallinity can be improved, there is no problem.

Table 2 Table 2 shows the price of one single crystal substrate. Although the single crystal silicon wafer substrate has a large diameter, about twice that of the MgO single crystal substrate, the price is about 1/20th that of the MgO single crystal substrate. If it becomes possible to use a single-crystal silicon wafer substrate in this way, it becomes possible not only to increase the diameter but also to significantly reduce costs.

[Effects of the Invention] As described above, according to the present invention, an epitaxially grown film can be obtained that can bring out the inherent matching property of CaF2 with a superconducting substance. Therefore, it becomes possible to increase the critical current density and suppress the barrier of grain boundaries, making it a superconducting film suitable for superconducting devices and the like. Since silicon wafers or wafers can be used as substrates, there are fewer restrictions on the shape, ⑤ productivity is improved, and the price of the substrate is much lower, making it possible to reduce costs.

The oxide superconducting thin film obtained according to the present invention may be used as it is, or after being subjected to microfabrication, formation of a protective film, lamination of other materials, etc., to 5QUID, Josephson device, superconducting transistor, electromagnetic wave sensor, magnetic sensor, element wiring, It can be applied to current control elements, magnetic flux quantum memories, optical switch elements, magnetic shields, antennas, etc.

that's all Applicant: Seiko Epson Corporation Representative Patent Attorney Kizobe Suzuki and 1 other person

Claims (1)

[Claims] A method for producing a superconducting thin film, which comprises forming a superconducting thin film on a fluoride after activating the surface of the fluoride by exposing it to a plasma atmosphere.