JPH06260693A - Manufacture of superconducting element - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a superconducting element in which a crack, etc., is prevented in an oxide superconductor thin film having a c-axis in parallel with a board surface and its laminated structure and which is realized with excellent surface flatness. CONSTITUTION:After an oxide material having a cubic or tetragonal crystalline structure is cut out substantially in parallel with a crystal surface having its tetrad symmetry, the material is polished at an angle to the tetrad symmetrical crystalline surface along a direction of two equivalent crystalline main axes included in the tetrad symmetrical crystalline surface and perpendicular to each other. Thus, a board 1 having a screenlike atomic steps 2 on a surface is manufactured. Then, an oxide superconductor thin film 3 of Y-Ba-Cu-O is so formed on the surface having the steps of the board 1 by so orienting that an a-axis or a b-axis of unit lattice becomes substantially perpendicular to the board surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a superconducting element using an oxide superconductor thin film, and more particularly to a method of manufacturing a laminated superconducting element in which a direction in which a large superconducting current can flow is set in a direction perpendicular to a substrate. Regarding

[0002]

2. Description of the Related Art Conventionally, as a superconducting element, a tunnel-type Josephson junction element or the like having a laminated structure in which a metal superconductor such as Pb or Nb is used and a thin insulating layer capable of tunneling a superconducting conductor pair is sandwiched is used. Are known. Such a conventional tunnel type Josephson element is required to operate at an extremely low temperature close to the temperature of liquid helium. Further, since the tunnel-type Josephson junction exhibits a current-voltage characteristic having a hysteresis peculiar to the tunnel type Josephson junction, there is a problem that the circuit configuration becomes complicated, and it has not been widely put to practical use.

On the other hand, as a Josephson junction element using a metal superconductor and having no hysteresis characteristic, a so-called bridge type junction in which main electrodes made of a metal superconductor are connected by a thin metal laminated on the main electrodes is also developed. It is being advanced. However, such a bridge-type junction needs to operate at an extremely low temperature close to the temperature of liquid helium, has a small resistance in the bridge portion, and has a superconducting property of a metal superconductor, as in the case of the tunnel-type junction described above. Since the gap itself is small, it is difficult to obtain a large output voltage.

Under these circumstances, an oxide superconducting material which exhibits superconducting properties at a temperature higher than the liquid nitrogen temperature has recently been discovered and has been attracting a great deal of attention. The oxide superconductor is
The size of the superconducting gap is larger than that of conventional metal superconductors.
It has a possibility to be laminated with an oxide material that is larger than one digit and has a wide range of properties from metallic conduction to insulators. Using such an oxide superconductor, a stacked tunnel Josephson junction or a high output voltage,
SNS without hysteresis (superconductor / normal conductor /
If it becomes possible to fabricate a (superconductor) type junction, at least cryogenic operation will be unnecessary as compared with the Josephson junction configured using the above-mentioned conventional metal superconductor,
Moreover, since a larger output voltage can be obtained, a wide range of applications are expected.

[0005]

However, the oxide superconductor has a large anisotropy in its characteristics, and in the laminated structure in which the c-axis direction having a short coherence length is perpendicular to the substrate,
There is an essential problem that a superconducting current that is practically sufficient cannot flow through the laminated interface. On the other hand, in a laminated structure in which the c-axis of the oxide superconductor is parallel to the substrate and an interface parallel to this is provided, the oxide superconductor has a characteristic that the coefficient of thermal expansion in the c-axis direction is unusually large, so When cooling after the film, a large strain occurs with the substrate crystal,
There is a problem that cracks easily occur in the oxide superconductor thin film.

In other words, the development of the Josephson junction using an oxide superconductor having a high critical temperature is expected to bring a great industrial effect, but it is a method for producing a stable laminated structure for practical use. Had been a challenge.

The present invention has been made in order to solve such a problem, and prevents the generation of cracks and the like in the oxide superconductor thin film and its laminated structure in which the c-axis is parallel to the substrate surface. It can be realized in a form with excellent surface flatness, resulting in excellent controllability and a large output voltage.
It is an object of the present invention to provide a method for manufacturing a superconducting element that enables various superconducting elements including Josephson junctions to be manufactured with good reproducibility.

[0008]

According to the method for manufacturing a superconducting element of the present invention, an oxide material having a cubic or tetragonal crystal structure is cut out substantially parallel to a crystal plane having 4-fold symmetry. After that, by polishing along the directions of two equivalent crystal principal axes that are included in the 4-fold symmetry crystal plane and are orthogonal to each other, polishing is performed at an angle with respect to the 4-fold symmetry crystal plane. Of the substrate having the atomic steps in the form of a RE-AE-Cu-O system (where RE is at least one element selected from rare earth elements) , AE is at least one element selected from alkaline earth elements), and is formed by orienting an oxide superconductor thin film of (a) or (b) axis of its unit cell to be substantially perpendicular to the substrate surface. And a step of It is.

In the method for manufacturing a superconducting element of the present invention, first, a mesh-like atomic step, that is, a step close to a crystal lattice interval orthogonal to each other is formed on the surface of an oxide substrate having a cubic or tetragonal crystal structure. To form a micro stepped structure. In this reticulated atomic step, an oxide material having a crystal structure with cubic or tetragonal symmetry is first used as a substrate material, and it is cut out so as to have a principal plane substantially parallel to the crystal plane with 4-fold symmetry. . Then, along the directions of two equivalent crystal principal axes included in the above-mentioned four-fold symmetry crystal plane and orthogonal to each other, polishing is performed in each direction so as to form an angle with each of the four-fold symmetry crystal plane.

Specifically, first, polishing is performed along one crystal principal axis included in the 4-fold symmetry crystal plane so as to form an angle with the 4-fold symmetry crystal plane. By this first-step polishing, a micro-stepped structure is formed on the substrate surface in the crystal principal axis direction. Next, along the other major axis of the crystal, it is similarly polished so as to form an angle with the 4-fold symmetric crystal plane. The second-stage polishing is performed with the polished surface as seen as a macro in the first-stage polishing as a reference surface. By this second-step polishing, a micro step structure is formed in the crystal principal axis direction on each surface of the step structure that was previously formed. As a result, mesh-like atomic steps are formed on the substrate surface.

Since the four-fold symmetry planes are exposed between the atomic steps formed on the substrate surface (polished surface), the RE-AE-Cu-O-based oxide is oxidized on the substrate surface. When the superconducting thin film is formed, the c-axis of the oxide superconductor in the part where the 4-fold symmetry plane is exposed is aligned with one of the two equivalent crystal principal axes included in the 4-fold symmetry plane, Growth is performed with the a-axis or the b-axis standing in the direction perpendicular to the substrate surface. Here, since the c-axis of the oxide superconductor grows in conformity with two equivalent crystal main axes that intersect at right angles with almost equal probability, the peculiar coefficient of thermal expansion of the oxide superconductor in the c-axis direction is relaxed. can do.

By using a substrate having the 4-fold symmetry plane itself as a surface, the oxide superconductor thin film formed on the substrate starts random nucleation on the substrate surface, and a-axis or b-axis is formed. Although a structure in which the axis stands in the direction perpendicular to the substrate surface is obtained, the size of the crystal grains having different c-axis orientations is indefinite, and as a result, the height between the crystal grains greatly differs. As a result, the irregularities on the surface of the thin film become large, and in a significant case, a height difference of 50 nm or more may occur with respect to the average film thickness of 300 nm. On the other hand, in the manufacturing method of the present invention, by providing a mesh-shaped step structure (step-the crystal plane between steps is about several to several tens of crystal lattices) orthogonal to each other on the substrate surface, Since the step portions which are the starting points of the nucleus growth of the oxide superconductor are dispersed at substantially equal intervals, the initial growth process can be controlled. For this reason, the crystal grains having different c-axis orientations are dispersed in a small amount, so that the height difference between them can be suppressed to a significantly small level. As a result, an oxide superconductor thin film having excellent surface smoothness can be obtained.

The angles with respect to the 4-fold symmetry crystal plane in the above-mentioned substrate polishing are preferably 10 degrees or less.
If the polishing angle exceeds 10 degrees, other orientations, such as crystal grains in which the [110] direction of the oxide superconductor crystal is perpendicular to the substrate surface, may occur, increasing surface irregularities and superconducting properties. Will be reduced. That is, if the polishing angle is 10 degrees or less, it is possible to make the irregularities between the crystal grains extremely small even if a laminated structure having a practically sufficient thickness is formed. A more preferable polishing angle is in the range of 2 to 6 degrees.

The oxide superconductor thin film according to the present invention is RE
-AE-Cu-O-based oxide superconductor, chemical formula: REAE ₂ Cu ₃ O ₇ ……… (1) (where RE is Y, La, Sc, Nd, Sm , Eu, Gd, Dy, Ho, E
At least selected from rare earth elements such as r, Tm, Yb, and Lu
One of the elements is AE, and AE is at least one element selected from alkaline earth elements such as Ba, Ca, and Sr). The amount of oxygen is usually adjusted so as to obtain the optimum superconducting characteristics.
Further, the molar ratio of each element does not have to strictly satisfy the above ratio, and some variation is allowed as long as the superconducting characteristics can be obtained.

[0015]

[Function] Generally, in order to form an oxide superconductor thin film,
It is necessary to use a substrate made of a material that does not react with the oxide superconductor at a temperature of around 700 ° C required for thin film formation and has a crystal lattice size close to that of the oxide superconductor. One such substrate is S
Oxides with a perovskite structure similar to rTiO ₃ are widely used. As shown in FIG. 7, these substrates show good lattice matching with the c-axis of the oxide superconductor at room temperature, but the matching is poor at high temperatures where film formation is performed. This is because the coefficient of thermal expansion of the oxide superconductor in the c-axis direction exhibits a unique temperature dependence.

This peculiar temperature dependence is a phenomenon in which the copper and oxygen atoms in the oxide superconductor form a one-dimensional ordered structure, and the crystal system changes from tetragonal at high temperature to orthorhombic at low temperature. It corresponds to. Therefore, when the film is formed at a high temperature, the oxide superconductor film on the substrate grows while receiving a compressive force. Further, when the substrate is cooled after the film formation is completed, the oxide superconductor film contracts, and the compressive force changes to the tensile force. As a result, if the oxide superconductor has a c-axis parallel to the substrate surface and is grown with the c-axis aligned in one direction, cracks occur in the oxide superconductor film in the process of cooling to room temperature.

The above-mentioned cracks can be prevented by allowing the oxide superconductor film to grow microscopically in two directions such that the c-axes of the oxide superconductor film are orthogonal to each other at the same ratio and to relax the stress. However, if crystal grains having different c-axis orientations are randomly generated at the initial stage of growth, an imbalance between the crystal grains causes a large unevenness on the surface of the oxide superconductor film. In other words, by using a material having a cubic or tetragonal crystal structure as a substrate and forming an oxide superconductor thin film on the crystal plane having 4-fold symmetry,
Although it is possible to produce an oxide superconductor film in which the axis or the b-axis stands perpendicular to the substrate surface, its surface morphology must be insufficient for producing a laminated Josephson junction. I don't get it.

Therefore, in the manufacturing method of the present invention, first, a step having a crystal lattice spacing is formed on the substrate surface.
Many are formed in the direction of two equivalent crystal principal axes that are orthogonal to each other on the plane of rotational symmetry. When such a mesh-like step exists on the surface of the substrate, the oxide superconductor film starts to grow from the step. Therefore, by controlling the step density (controlling by the polishing angle), it is possible to artificially control the density of crystal grains having different c-axis orientations at the initial stage of growth. Then, by setting the step density sufficiently large, the crystal grains with different c-axis orientations can be made fine and
It is possible to grow by orienting so that the a-axis or the b-axis is substantially perpendicular to the substrate surface with almost equal probability with respect to one crystal main axis. This makes it possible to evenly grow the crystal grains, so that an oxide superconductor thin film having excellent surface smoothness can be obtained.

As described above, by using the method for manufacturing a superconducting element of the present invention, the RE-AE-Cu-O-based oxide is prepared by orienting the c-axis on the oxide substrate so as to be substantially parallel to the substrate surface. The superconducting thin film can be formed without leaving large stress in the film, and as a result, it is possible to prevent the occurrence of cracks, the decrease in the superconducting transition temperature, and the like.
In addition, even if a laminated structure having a thickness necessary for a practical Josephson junction is manufactured, the surface unevenness can be made sufficiently small, which causes a problem in the conventional laminated structure such as a short circuit between superconducting electrodes. It is possible to drastically reduce the defects.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a manufacturing process of an embodiment to which the method for manufacturing a superconducting element of the present invention is applied. This figure shows
YBa ₂ Cu ₃ O ₇ as superconducting electrode material, PrBa ₂ Cu ₃ O ₇ as intermediate barrier layer material, and SrTiO ₃ as substrate material
The manufacturing process of a three-layer stacked SNS type Josephson junction device using a single crystal is shown. The manufacturing process of the SNS type Josephson junction element will be described below with reference to FIG.

First, a SrTiO ₃ single crystal is cut into a wafer so as to have a main surface that is substantially parallel to a (100) crystal plane having 4-fold symmetry. Then, included in the (100) crystal plane,
Two equivalent crystal principal axes that are orthogonal to each other, ie [010]
By polishing the single crystal wafer in two steps by tilting 3 degrees with respect to the (100) crystal plane along the two directions of [001] and [001], as shown in FIG. An SrTiO ₃ substrate 1 having a mesh-shaped atomic step 2 is prepared on the substrate.

The manufacturing process of the substrate 1 will be described in detail with reference to FIGS. First, as shown in Fig. 2, SrT
With respect to the iO ₃ (100) wafer 11, one of two equivalent crystal principal axes included in the (100) crystal plane, for example, in the [010] direction
The (100) crystal plane is tilted 3 degrees (indicated by θ ₁ in the figure) and polished. The polished surface by this first-stage polishing is inclined by 3 degrees with respect to the (100) crystal plane. A microscopic view of this polished surface shows a staircase in the [010] direction as shown in Fig. 3. The atomic step 12a having a shape of a circle is formed. Reference numeral 11a in the drawing shows a polished surface in macroscopic view obtained by the first-stage polishing.

Next, polishing is performed with the (100) crystal plane tilted by 3 degrees (indicated by θ ₂ in the figure) in the other of the two crystal principal axes contained in the (100) crystal plane, that is, in the [001] direction. However, the second-stage polishing is performed using the polished surface obtained by the first-stage polishing as a reference surface. By this second stage polishing,
As shown in FIG. 4, stepwise atomic steps 12b are also formed in the [001] direction. Since the second-stage polishing is performed using the polished surface of the first-stage polishing as a reference surface, the step-like atomic steps 12a and 12b are formed in a mesh shape. These atomic steps 12
The plane between a and 12b is a (100) crystal plane.

By carrying out the two-step polishing in this way, as shown in FIG. 1A, SrTiO ₃ having a mesh-like atomic step 2 (12a, 12b) provided on the surface thereof.
The substrate 1 is obtained.

Next, on the surface 1a of the SrTiO ₃ substrate 1 provided with the mesh-shaped atomic steps 2, as shown in FIG. 1 (b), a thickness to be a lower electrode is formed by a multi-source reactive sputtering method. A YBa ₂ Cu ₃ O ₇ film 3 of 300 nm is formed. In forming this lower electrode, Y, Cu, and Ba ₂ CuO ₃ targets were used, and by controlling the high-frequency power applied to each target, the composition ratio of Y, Ba, and Cu was set to a predetermined 1: 2: 3
I am trying. In this example, the substrate temperature during film formation was maintained at 680 ° C. at which the a-axis of the oxide superconductor film was oriented perpendicular to the (100) plane of the SrTiO ₃ substrate 1, but at the beginning of film formation, The substrate temperature may be set low, or Pr-Ba-
It is also possible to provide a buffer layer such as a Cu-O film and use a method of making the orientation more reliable, together.

Then, on the lower electrode layer 3, a thickness of Pr, Cu, and Ba ₂ CuO ₃ was set as a target in the same vacuum chamber.
Intermediate barrier layer 4 consisting of a 25 nm PrBa ₂ Cu ₃ O ₇ film and upper electrode layer 5 consisting of a 100 nm thick YBa ₂ Cu ₃ O ₇ film.
Are sequentially grown as shown in FIG.

The superconducting transition temperature of the laminated film thus obtained was 83K. This transition temperature is ideal
The reason why the Y-Ba-Cu-O film has a temperature slightly lower than 92K is that the substrate temperature during film formation is low. The surface of the obtained laminated film is extremely smooth, no occurrence of cracks or the like was observed even by a high-resolution electron microscope, and the unevenness between crystal grains obtained by measurement with an atomic force microscope was The maximum was 3 nm in the scan range of 2 μm square.

Also, as a comparison with the present invention, as a substrate
When a laminated film was produced using the SrTiO ₃ (100) plane itself in the same manner as in the above-mentioned example, irregularities of 20 nm or more were observed between crystal grains, and the crystal grain size was also larger than that in the above-mentioned example. This clearly shows the effect of the present invention.

The processing steps after formation of the laminated film are the same as those used in the manufacture of ordinary semiconductor elements. That is,
As shown in Fig. 1 (c), the thickness of 100
After forming the gold vapor deposition film 6 having a thickness of nm, the pattern of the joint portion is transferred to the resist film by the optical exposure method, and then the upper electrode layer 5 and the intermediate barrier layer 4 are etched by the ion milling method using the resist as a mask. The joint area was 10 μm square.

Next, as shown in FIG. 1D, an insulating film 7 for insulating the wirings to the upper electrode layer 5 and the lower electrode layer 3 from each other is laminated, and a hole 7a is formed only in the upper portion of the joint. Process into shape. Various materials can be used as the insulating film 7, but in this embodiment, a negative resist is used for the purpose of simplifying the process. After that, the wiring 8 to the upper electrode layer 5 is formed of Au to complete the stacked Josephson element.

FIG. 5 shows current-voltage characteristics of the superconducting element obtained in this example at the temperature of liquid helium. There is no superconducting leakage current due to the micro short between the upper and lower superconducting electrodes, and it is considered as a critical current due to the Joffson junction.
1.4 mA, the I _c · R _n product, which is the output voltage of the device 4.1
mV was obtained. Output voltage expected for oxide superconductors 20
The reason why it is lower than mV is that the thickness of the intermediate barrier layer 4 is not optimized, and by making the Pr—Ba—Cu—O film 4 thinner, better characteristics can be obtained.

FIG. 6 is a diagram showing the dependence of the critical current on the applied magnetic field to confirm that the superconducting element of this example actually exhibits uniform Josephson characteristics. As can be seen from this figure, the fabricated device exhibited an ideal Fraunhofer pattern. Such Josephson characteristics were confirmed even at the liquid nitrogen temperature, and it was verified that the superconducting element manufactured by using the manufacturing method of the present invention could operate at high temperature.

In the above embodiments, the Y-Ba-Cu-O oxide superconductor was used as the superconducting electrode material, and the Pr-Ba-Cu-O normal conductor was used as the intermediate barrier layer material. , The production method of the present invention is not limited to the case of using these, even in the combination of other superconducting conductive electrode material and the intermediate barrier layer material, by controlling the film-forming conditions and appropriate selection of the film-forming method. It is feasible.

[0035]

As described above, according to the present invention,
Various superconducting devices with a laminated structure, including Josephson junction devices that can operate at high temperatures using oxide high-temperature superconductors, can be manufactured in a stable and reproducible manner, making a great contribution to the industry. Expected to do.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a manufacturing process of a superconducting element according to an embodiment of the present invention.

FIG. 2 is a diagram showing a main part of the manufacturing process shown in FIG.
SrTiO ₃ is a diagram for explaining a first stage of the polishing process with respect to a single crystal body.

FIG. 3 is a diagram showing a main part of the manufacturing process shown in FIG.
SrTiO ₃ is a diagram showing a state after the polishing process of the first stage of the single crystal body.

FIG. 4 is a diagram showing a main part of the manufacturing process shown in FIG.
SrTiO ₃ is a diagram showing a state after the polishing process of the second stage of the single crystal body.

FIG. 5 is a diagram showing current-voltage characteristics of a superconducting device obtained according to an example of the present invention.

FIG. 6 is a diagram showing an applied magnetic field dependency of a critical current of a superconducting device obtained according to an example of the present invention.

FIG. 7 is a diagram showing the temperature dependence of the coefficient of thermal expansion of a Y-Ba-Cu-O-based oxide superconductor and an oxide for a substrate.

[Explanation of symbols]

1 ... SrTiO ₃ substrate having a net-like atomic step 2, 12a, 12b ... Atomic step 3 ... Lower electrode layer 4 ... Intermediate barrier layer 5 ... Upper electrode layer 11 ... SrTiO ₃ (100) wafer

Claims (1)

[Claims] 1. An oxide material having a cubic or tetragonal crystal structure is cut out substantially parallel to a crystal plane having 4-fold symmetry, and then included in the 4-fold symmetry crystal plane,
Along the direction of two equivalent crystal principal axes that are orthogonal to each other,
Each of the steps of producing a substrate having a mesh-like atomic step on the surface by polishing at an angle with respect to the four-fold symmetric crystal plane, and RE on the surface provided with the mesh-like atomic step of the substrate, RE -AE-
Cu-O system (RE is at least one element selected from rare earth elements, AE is at least one selected from alkaline earth elements)
A superconducting oxide thin film of one element) is oriented such that the a-axis or the b-axis of its unit cell is substantially perpendicular to the surface of the substrate. Device manufacturing method.