Method for fabricating oxide superconducting device

Details Method for fabricating oxide superconducting device Method for fabricating oxide superconducting device

1998-09-29

2001-03-27

Gulakowski, Randy (Department: 1746)

Etching a substrate: processes

Forming or treating josephson junction article

C505S325000, C505S329000, C505S410000, C505S728000

Reexamination Certificate

active

06207067

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for fabricating an oxide superconducting device (a Josephson Junction device) using an oxide superconductor.
2. Description of the Related Art
Characteristics of a Josephson Junction are high-speed processing and low power consumption. Among these benefits the low power consumption provides a great benefit when a lot of devices are integrated. Reproducibility and uniformity of the device characteristics are necessary in order to realize a desired design performance.
As methods for fabricating a Josephson Junction as the basics of an oxide superconducting device the following methods are known:
(a) a method for fabricating graded-step-type junctions, in which a graded-step with several hundreds of nm height is formed on a substrate using a conventional photolithography technique and an ion beam such as Ar, or using a reactive ion etching, and in which a weak link of an oxide superconducting thin-film introduced at the graded-step portion is utilized;
(b) a method for fabricating grain boundary-type junctions, using a bi-crystal substrate;
(c) a method for fabricating ramp-edge-type junctions, in which a lower superconducting thin-film is first formed and a graded-step is constructed thereon using a photolithography technique; in the graded-step portion, a barrier layer is formed between the lower thin-film and an upper superconducting thin-film formed afterward; and
(d) a method for fabricating plane-type junctions, in which a deteriorated region is formed on a substrate using a converging ion beam (See for example, Japanese Patent Laid-Open No. 6-151986), or a micro groove is formed on a substrate, to utilize a weak link region of an oxide superconducting thin-film generated in the beam irradiation region.
Of these fabricating methods, the method for fabricating grain boundary-type junctions using a bi-crystal substrate mentioned in (b) is not suited for producing an integrated circuit because the position where the Josephson Junction is constructed is limited to the region where the bi-crystal joins together, so that it is difficult to arrange freely a lot of junctions on a substrate. With regard to the method for fabricating ramp-edge-type junctions mentioned in (c) there are disadvantages such as complexity of the fabricating procedure and difficulty in avoiding deterioration in the superconductivity during the procedure. With regard to the method for fabricating graded-step-type junctions mentioned in (a) and the method for fabricating plane-type junctions using a converging ion beam mentioned in (d) the procedure is simple and the degree of freedom for arrangement of junctions is high. The method mentioned in (d) using a converging ion beam is more advantageous than the method mentioned in (a) using photolithography when realizing an integrated circuit, because the method mentioned in (d) makes it possible to construct junctions in a smaller micro region than the method mentioned in (a).
Referring to the drawings, a method for fabricating junctions by forming a groove on a micro region of a substrate using a conventional converging ion beam will be described.
FIG. 9
is a plan view showing a diagrammatic sketch of the construction of a conventional junction.
FIG. 10
is a cross-sectional view of the conventional junction taken along the line A-A′ of FIG.
9
. In
FIGS. 9 and 10
numeral
1
indicates a single crystal substrate such as MgO, numeral
2
indicates an oxide superconducting thin-film (device) such as YBa
2
Cu
3
O
7−x
(0≦X≦0.5) or NdBa
2
Cu
3
O
7−x
(0≦X≦0.5), numeral
3
indicates an irradiation region of a converging ion beam, numerals
4
,
5
, and
6
indicate a grain boundary of the oxide superconducting thin-film
2
, numerals
7
and
7
′ indicate electrodes for extraction. In the irradiation region of a converging ion beam
3
the ion beam etches the substrate
1
to form a V-shaped groove. When the oxide superconducting thin-film is formed on the MgO substrate so as to be oriented in the direction of the c-axis, the film oriented in the direction of the c-axis will grow in such a manner that the c-axis is perpendicular to the substrate on the plane surface of the MgO substrate, and the c-axis is also perpendicular to an inclined surface of a V-shaped groove portion. As a result, a grain boundary in which orientations of the adjacent crystal grains are different from each other is formed, and this grain boundary portion forms weak links at the regions
4
, and
6
shown in
FIG. 10
, where the crystal grain grown on the substrate comes into contact with the crystal grain grown on the inclined surface of the groove, and also forms a weak link at the region
5
shown in
FIG. 10
, where the crystal grains grown on the inclined surfaces of the groove come into contact with each other, so that a junction is formed.
FIGS. 11A
,
11
B, and
11
C are sectional views showing the fabricating processes of a conventional method for fabricating an oxide superconducting device. Referring to
FIG. 11A
, a gold thin-film
8
having a thickness of about 100 nm is formed on the substrate
1
and a Ga ion beam is radiated at a junction fabricating portion by means of a converging ion beam apparatus. The gold thin-film
8
is a film to inhibit electrification due to the ion beam. Then, as shown in
FIG. 11B
, the gold thin-film
8
is completely removed. Since the beam intensity distribution of an ion beam has a Gaussian distribution, as shown in
FIG. 11B
, a V-shaped groove is formed in the ion beam irradiation region
3
of the substrate
1
. Next, as shown in
FIG. 11C
the oxide superconducting thin-film
2
is formed. Then, a pattern is formed so as to cross over the ion beam irradiation region
3
and the device shown in
FIG. 9
is fabricated.
Since the method for fabricating junctions using a converging ion beam has a high degree of freedom for arrangement of junctions and it provides an advantage in its feasibility in micromachining as described above, the feasibility in fabricating an integrated circuit and the like is high. Under the present circumstances, however, because of a wide range of variations in device characteristics such as a critical current of each device and because of poor reproducibility, there is a problem that it is difficult to prototype with high reproducibility a circuit combined with a plurality of devices.
It is necessary to form with high reproducibility an optimized groove configuration for constructing a junction in order to form a device having high reproducibility and uniformity. In a conventional method for fabricating junctions using a converging ion beam, however, it is difficult to optimize the groove configuration, because the beam intensity distribution determines the groove configuration and only a limited configuration is obtained under performance specifications of the converging ion beam apparatus.
Further, since the beam intensity distribution differs according to the apparatus and to the ion source, and the ion source varies over time, a problem arises in the reproducibility of the groove configuration, such as variation in the groove configuration due to a change of the apparatus or replacement of the ion source.
FIG. 12
shows measured data of the sectional configuration of the groove portion of an MgO substrate in a conventional method for fabricating junctions using a converging ion beam. A gold thin-film with a thickness of 100 nm is formed on the MgO substrate and the junction forming portion is irradiated with a Ga ion beam using a converging ion beam apparatus. Acceleration voltage is 30 KeV, beam current is 6 pA, and amount of ion irradiation is 5.12×10
17
/cm
2
.
FIG. 12
shows data measuring the sectional configuration of the MgO substrate by means of AFM after removing the gold thin-film.
FIG. 13
illustrates variations of the inclination angle of the inclined portion according to the position of the beam within the groove in FIG.
12
.
When the width of the groove is

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