JPH04202094A - Production of superconducting thin film - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing an oxide superconducting thin film. More specifically, the present invention relates to a novel method for producing a superconducting thin film using a Y-based Cu complex oxide, particularly using a Si wafer as a base substrate.

Conventional technology Superconductivity was once thought to be a unique phenomenon at extremely low temperatures that require cooling with liquid helium.

However, in 1986, Bednotes, Muller et al. showed a superconducting state at 30°C (La, Ba).
Since the discovery of 2CU○, in 1987, Chu et al.
Ba2CU30y was discovered, and subsequently, in 1988, the so-called Bi-based composite oxide superconducting material having a critical temperature of 100 degrees or higher was discovered by the aforementioned et al.

These series of composite oxide-based superconducting materials can achieve superconductivity by cooling with inexpensive liquid nitrogen, so the possibility of practical application of superconducting technology has suddenly begun to be discussed.

Initially, these composite oxide-based superconducting materials were synthesized as sintered bodies using a solid-state reaction method, but as a result of subsequent research progress, today it is possible to fabricate them as thin films of extremely high quality. I'm starting to get it.

However, each of these composite oxides has a complex crystal structure and forms a phase that exhibits effective superconducting properties only when it is formed on a specific substrate under specific conditions. That is,
As the underlying substrate for the oxide superconducting thin film, an MgO single crystal substrate, a 5rTiO+ single crystal substrate, etc. are used.

Problems to be Solved by the Invention However, the above-mentioned oxide single crystal substrates are generally expensive and in short supply, which is one of the extremely disadvantageous requirements when considering the practical use of oxide superconductors. It is considered. In addition, it is difficult to handle large-diameter oxide substrates manually, and considering that oxide superconducting thin films will have larger areas in the future, it is difficult to consider this material to be an appropriate material.

Therefore, it has been proposed to fabricate an oxide superconducting thin film using a Si wafer, which is currently stably supplied as an inexpensive and high-quality substrate material, as a base substrate. However,
There is a very large difference in thermal expansion coefficient between a Si wafer and an oxide superconductor. That is, while the coefficient of thermal expansion of the S1 single crystal is about 2.4, the coefficient of thermal expansion of the Y-based composite oxide, which is a typical oxide superconductor, is 14 or more. For this reason, when an oxide superconducting thin film is formed on a Si wafer, cracks occur due to thermal mismatch.

Therefore, the present invention solves the problems of the prior art and
The purpose of the present invention is to provide a novel method for producing a superconducting thin film that can form an oxide superconducting thin film on an I-wafer without causing cracks.

Means for Solving the Problems According to the present invention, in a method for producing an oxide superconducting thin film using a Si wafer as a base substrate, an oxide film having a smaller lattice constant than the target oxide superconducting thin film is formed on the S] wafer. A method for producing a superconducting thin film is provided, which includes the steps of forming a buffer layer using a material, and forming an oxide superconducting thin film on the buffer layer.

In the method for producing an oxide superconducting thin film according to the present invention,
Prior to forming the oxide superconducting thin film, the method includes a step of forming a buffer layer made of a material having a smaller lattice constant than the oxide superconducting thin film on the 81 substrate.

Here, the buffer layer refers to a specific material layer on which an oxide superconducting thin film can be formed.

In other words, the properties required of the base material in order to form a high-quality oxide superconducting thin film are: 1. Good lattice constant matching with the oxide superconducting thin film; 2. Good lattice constant matching between the base material and oxide superconducting thin film. The interdiffusion of the elements is small. However, although Sl has good matching with oxide superconductors in terms of lattice constant, at the normal deposition temperature of oxide superconducting thin films (around 700°C), interdiffusion of elements is severe, and Si wafers are used as base substrates. Even if we create an oxide superconducting thin film, the superconducting properties are extremely low, or
Or it is known to disappear.

Therefore, it has already been proposed to form a buffer layer using a material that has a crystal structure that serves as a base for the oxide superconducting thin film and that acts as a barrier to Si diffusion.

However, in such a conventional buffer layer, sufficient consideration has not been given to deterioration of the characteristics of the oxide superconducting thin film due to thermal mismatch with the Si wafer. That is, as a simple method, the thermal strain of the oxide superconductor thin film is alleviated by forming a buffer layer having a thermal expansion coefficient intermediate between that of Sl and that of the oxide superconductor. There is a way. However, in this method, since the thickness of the buffer layer is very thin compared to the thickness of the Si wafer, it has not been possible to effectively solve the problem of cracking.

On the other hand, in the method according to the present invention, by forming the buffer layer from a material having a smaller lattice constant than that of the oxide superconducting thin film, it is possible to offset the thermal strain by the lattice strain.

That is, in general, when materials with different lattice constants are epitaxially grown, strain is introduced due to the lattice mismatch. That is, for example, when the lattice constant of the substrate is a5 and the lattice constant of the thin film is af, compressive strain acts on the thin film when a5<af, and tensile strain acts on the thin film when as>af.

On the other hand, when forming an oxide superconducting thin film on a Si wafer, the thermal strain acting on the oxide superconducting thin film due to the difference in thermal expansion coefficient is in the direction of tensile force. Therefore, by forming the buffer layer with a material whose lattice constant is smaller than that of the oxide superconducting thin film, compressive strain due to lattice mismatch can be applied to the oxide superconducting thin film, and strain in the direction of the tensile force acting on the thin film can be alleviated. I can do it.

In addition, materials that can be used as the buffer layer in the method according to the present invention as described above include Y2O3, ZrO2,
Examples of Y include I○3, LaAlO3, and a stacked structure thereof.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following disclosure is merely an example of the present invention.
This is not intended to limit the technical scope of the present invention in any way.

Example A plurality of 81 single crystal wafers with a diameter of 2 inches were prepared, and Y-B
a-Cu composite oxide superconducting thin film, hereinafter referred to as “YBCO”
) was formed into a film.

First, a buffer layer having a lattice constant smaller than that of YBCO was formed using the materials shown in Table 1 below. The buffer layer was formed by sputtering. The film forming conditions are also shown in Table 1.

Table 1 A Y--Ba--Cu composite oxide thin film was formed on each Si wafer on which a buffer layer was formed as described above. Also, for comparison, M, which has a larger lattice constant than YBCO,
A Si wafer on which a buffer layer made of g O, SrT+ 03 was formed and a Si wafer without a buffer layer were prepared, and the same oxide superconducting thin film was formed on each comparative sample. The film formation method was the spankling method. The film forming conditions are as shown in Table 2 below.

The surface properties of the oxide superconducting thin films were observed for each sample prepared as shown in Table 2. The observation results are shown in Section 3 below.
As shown in the table.

Table 3 As shown in Table 3, the generation of cracks is effectively prevented by producing the oxide superconducting thin film according to the present invention.

As described in detail, according to the method of the present invention, an oxide superconducting thin film can be formed using a Si wafer as a substrate without generating cracks.

The use of Si wafers is advantageous not only because the substrates are inexpensive and stable in supply, but also because of the vast technological accumulation of microfabrication techniques and the like in the production of various devices. In addition, Si wafers are supplied with considerably large diameters, and can sufficiently accommodate large-area oxide superconducting thin films.

Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] A method for producing an oxide superconducting thin film using a Si wafer as a base substrate, comprising: forming a buffer layer on the Si wafer using an oxide having a smaller lattice constant than the target oxide superconducting thin film; A method for producing a superconducting thin film, the method comprising: forming an oxide superconducting thin film on the buffer layer.