JP2768276B2 - Oxide superconducting junction element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic device using an oxide superconductor, and more particularly to a magnetic flux quantum superconducting device using magnetic flux quanta as an information carrier.

[0002]

2. Description of the Related Art Many electronic devices made of superconductors utilize the Josephson effect, and the design and fabrication of Josephson junctions are extremely important in device fabrication. There are two types of Josephson junctions: tunnel type and weak coupling type. The tunnel junction has a structure in which an extremely thin insulator 21 is sandwiched between superconducting thin films 20 as shown in FIG. 2, and a hysteresis is seen in the current-voltage characteristics as shown in FIG. On the other hand, the weak-coupling type junction has a normal conductor or semiconductor 30 sandwiched between superconductors 31 as shown in FIG. 3, and has no hysteresis in current-voltage characteristics. Many superconducting elements have been made using metal-based superconductors such as Pb and Nb. In each case, each layer is formed in the thickness direction to produce a Josephson junction, a resistor, and a wiring. When manufacturing a junction that requires current-voltage characteristics without hysteresis,
In many cases, a structure in which a tunnel junction is shunted by a resistor is used.

On the other hand, many oxide superconductors discovered in recent years have a critical temperature at which superconductivity becomes higher than the temperature of liquid nitrogen, and are expected to greatly expand the field of application of superconducting electronic elements.

Many attempts have been made to apply oxide superconductors to electronic devices, but in many cases there are few attempts to fabricate logic devices using SQUIDs (superconducting quantum interference devices) that detect weak magnetic fields. In the case of the SQUID, the bonding utilizes the crystal grain boundary of the superconducting thin film formed on the interface by providing a step on the substrate or changing the plane orientation of the substrate.

[0005]

As described above, a tunnel type junction has been often used in a conventional superconducting element as seen in a Josephson integrated circuit. However, in manufacturing an ideal tunnel junction using an oxide superconductor, the coherence length of the oxide superconductor is as short as several nm, and the thickness of the tunnel barrier layer must be extremely thin (up to 1 nm). It has been difficult to produce such a thin insulating layer that is homogeneous with an oxide. Therefore, in order to form an oxide superconducting logic integrated circuit, it is necessary to use a weak coupling type junction such as a microbridge, a superconducting / normal conducting / superconducting (SNS) junction using a crystal grain boundary or a metal as a normal conducting layer. In designing a logic element, it is necessary to employ an element which can be manufactured by this type of junction. Many weak-coupling junctions manufactured so far use a crystal grain boundary created in the film by providing a step on the substrate or bonding substrates with different crystal orientations and epitaxially growing a superconducting thin film on it. ing. As described above, the method of processing a substrate is greatly restricted in element fabrication, and cannot be adopted particularly in a logic circuit requiring integration.
Therefore, in a logic element, a weak coupling type junction which can be integrated in a planar type is desired. Also, as a logic circuit formed by a weak-coupling type junction, an element using quantum flux as a carrier of information is appropriate instead of a normal voltage transfer type Josephson element which determines 1,0 of information in a superconducting state and a voltage state.

Further, there are the following problems to be solved in manufacturing a superconducting element applicable to a circuit. An element including an electrode layer, a wiring layer, an interlayer insulating layer, and the like has a laminated structure including several types of materials. When an oxide superconductor and another oxide material are stacked, the film formation temperature is 650.
° C or higher, and the superconducting properties of the lower superconducting layer may be degraded as the number of layers increases and the time for exposure to high temperatures increases. Therefore, while the metal-based superconducting element has a laminated structure of ten or more layers, it is desired to reduce the number of layers in the oxide superconducting element as much as possible. In an element utilizing magnetic flux, a superconducting ground plane is essential so that the magnetic flux is not trapped by the superconductor.
The need to reduce the number of layers and the need for a ground plane are contradictory.

Another problem is the thickness of the superconducting layer. The magnetic penetration depth of the oxide superconductor is approximately 140 nm
It is characterized in that it is longer than 80 nm of the metallic superconductor. Devices using quantum flux as a carrier of information are generally SQ
UID is a constituent element. The SQUID requires a superconducting loop, and to achieve this, the film thickness, including the wiring layer, must be greater than the magnetic field penetration length.

In order to fabricate a planar-type weak-coupling junction suitable for integration, the length and width (distance between superconducting electrodes) of a superconducting bridge and a normal conductive layer connecting superconducting electrodes must be on the order of 0.1 μm. Condition. When such a fine processing (generally groove-shaped etching processing) is performed on the superconducting film,
In order to reduce the aspect ratio during processing, the thickness of the superconducting electrode layer must be 0.1 μm or less. This is inconsistent with the thickness condition of the superconducting layer including the wiring layer described above.

An object of the present invention is to solve the above-mentioned problems and to manufacture an integrable logic element using an oxide superconductor.

[0010]

In order to achieve the above object, according to the present invention, the thickness of the superconducting layer is changed depending on the location. That is, the junction portion is thin, and the superconducting loop portion and the wiring portion are thick (ideally, thicker than the magnetic field penetration length).
Is what you do. Since the film thickness ratio is as large as 1:10, it is difficult to control the film thickness accurately by etching. For this purpose, in the present invention, the formation of the superconducting layer is performed in two stages, the entire pattern is processed with a layer having a large thickness, and the joint is removed. Thereafter, an extremely thin film required for the production of the junction is formed on the thick superconducting layer, and the thick portion of the underlying superconducting layer is joined, and this portion is subjected to fine processing on the order of 0.1 μm to form a junction. .

On the other hand, in order to reduce the number of layers to be stacked, the ground plane and the superconducting electrode layer are shared, and the distance between the shared electrode and the other electrode is set to a distance equal to or less than the magnetic field penetration length.

[0012]

The superconducting state required for the magnetic flux quantum superconducting element can be maintained by forming the wiring and the electrode with a superconducting film thicker than the magnetic field penetration length. On the other hand, since the thickness of the bonding portion is set to 0.1 μm or less, it is possible to simultaneously process the inter-electrode distance (up to 0.1 μm) required for producing a planar weak coupling. As a result, a superconducting logic element including the SQUID as a component can be manufactured.

[0013]

EXAMPLES The contents of the present invention will be described with reference to examples.

(Example 1) A magnetic flux quantum parametron (QFP) was manufactured using an oxide superconductor. The manufacturing process will be described with reference to a side view and a top view in FIG. The side view is a cross section taken along the line AA 'indicated by a dashed line in the top view. (1)
Strontium titanate having a plane orientation of (110) (S
rTiO ₃ ) Y having a thickness of 300 nm on a substrate 40
A Ba ₂ Cu ₃ O _X thin film 41 was produced by a laser deposition method. The substrate temperature during the formation of this thin film was 700 ° C., and the oxygen partial pressure was 0.2.
Torr. The thin film showed superconducting properties when formed. (2) This thin film was processed to form an element pattern. In this method, after applying an electron beam resist, a relatively large pattern is formed by electron beam drawing and ion beam etching. The electron beam lithography method was used for the pattern production because the miniaturization was possible and a process in which the electron beam resist used did not use water could be adopted. This is effective in preventing the characteristic deterioration of the superconducting thin film. The moat 42 was formed over the width of 1 μm on the thin film corresponding to the joint. (3) Next, SrTiO _{3 of the} insulating film
The thin film was similarly formed by a laser deposition method as an interlayer insulating film. The thickness was 300 nm. Thereafter, a pattern was formed on the insulating film 43 by electron beam lithography and ion beam etching as in the case of the superconducting film. (4) On top of this, the film thickness 30 for the two excitation lines 44 and the input / output lines 45 of the QFP
A 0 nm YBa ₂ Cu ₃ O _X thin film was formed by a laser deposition method. The pattern processing process is the same as the above process. (5) Next, YBa ₂ Cu ₃ O having a thickness of 25 nm
_{The X} thin film 46 was produced by a laser deposition method. The manufacturing conditions are the same as those of the lower superconducting film. (6) The thin superconducting film has a width of 0.1 μm by electron beam lithography and ion beam etching.
m, a bridge having a length of 0.1 μm. As shown in FIG. 1, the cross-sectional structure of the joint portion manufactured in this manner has a different thickness.

The flux quantum parametron produced in this way was confirmed to operate in liquid nitrogen at 77 K, although at a low speed.

Example 2 A microbridge type junction was manufactured by the same manufacturing process as in Example 1, except that the width of the junction was 0.1 μm and the length of the junction was 0.2 μm. Although the dimensions of the junction were changed, it exhibited characteristics as a superconducting junction and could be applied to a logic element.

(Embodiment 3) The same flux quantum parametrons 51 as those manufactured in Embodiment 1 were combined to form a 1/2 frequency dividing circuit. FIG. 5 shows the circuit configuration. DC SQUID5 using the junction of the present embodiment for detecting the output signal
2 was used. In the 1/2 frequency dividing circuit, many wiring intersections are required, and an intersection having a structure as shown in FIG. 6 was manufactured. In the figure, the superconducting wiring 6 and the superconducting layer 6 serving also as a ground plane are shown.
Only 2 is shown, and the description of the insulating layer is omitted. As a result, it was confirmed that the liquid helium was operating at a clock of 1 GHz (frequency of the excitation power supply) from the presence of a peak of 500 MHz in the output analysis by the spectrum analyzer. Although there was no ground plane at the intersection, no operational failure was caused by the magnetic flux trap.

(Embodiment 4) In Embodiments 1 to 3, the superconducting electrode is formed directly on the substrate. In this embodiment, a YBa ₂ Cu ₃ O _X superconducting thin film having a thickness of 300 nm is first formed directly on the substrate. This was used as a ground plane. A QFP was formed thereon in the same manner as in Example 1. Since the number of laminated layers increased, the characteristics of the superconducting thin film used for the ground plane and the laminated superconducting thin film were deteriorated, and the operation of the QFP became possible at 20K.

Using a magnesium oxide (MgO) having a plane orientation of (100) as a substrate, a microbridge type junction was produced by the method of Example 1. Under the influence of the substrate, the plane orientation of the superconducting thin film was (001), that is, the crystal orientation was such that the c-axis was perpendicular to the substrate. In this case, the width of the junction is 0.1 μm
When the thickness is set to 0.2 μm, there is no large change in the magnitude of the superconducting current, and it is difficult to design the device by changing the size of the junction. Therefore, it is necessary to incline the c-axis of the crystal with respect to the substrate.

In the above embodiment, the YBa ₂ Cu ₃ O _X superconducting thin film is used as the superconductor. However, it is clear that the present invention can be applied to Bi-based materials other than Y-based, ie, 1-2-3-based materials. . In addition, although QFP was manufactured as a magnetic flux quantum element, an RSFQ (Rapid), which is also composed of SQUID in addition to QFP and uses magnetic flux quantum as an information carrier,
It is also applicable to Single Flux Quantum.

[0021]

As described above, by changing the film thickness of the bonding portion and the wiring portion as described above, the width and length of the bonding portion can be processed on the order of 0.1 μm, and the microbridge type bonding can be manufactured. Was. Micro-bridge type bonding is different from bonding using steps, because it has a planar structure,
Suitable for integration. This means that the QF shown in this embodiment
This means that it can be widely applied not only to the application as P but also to an element utilizing a weakly-coupled superconducting junction. On the other hand, since the thickness of the wiring portion was made larger than the magnetic field penetration length, the SQUID
A superconducting loop for D could be formed.

Deterioration of superconducting characteristics due to lamination during device fabrication, which was a problem when using an oxide superconductor, could be solved by substituting the ground plane with one electrode.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view and a top view showing a micro-bridge type junction of the present invention, and the cross-sectional view is a view showing a cross section of a central portion of the top view.

FIG. 2 is a diagram showing a current-voltage characteristic in the case of a tunnel junction structure.

FIG. 3 is a diagram showing current-voltage characteristics in the case of a weak-coupling junction structure.

FIG. 4 is a schematic diagram illustrating a manufacturing process of the present invention, and is a diagram illustrating a cross section (a portion indicated by a symbol AA ′ in a top view) and a top surface of an element in each process.

FIG. 5 is a diagram illustrating a circuit configuration of a 1/2 frequency divider circuit in which four flux quantum parametrons are combined.

FIG. 6 is a diagram showing an intersection of superconducting wiring.

[Explanation of symbols]

20 ... superconducting thin film, 21 ... insulator, 30 ... semiconductor, 31 ... superconductor, 40 ... strontium titanate _{substrate, 41 ·· YBa 2 Cu 3 O} X film, 42 ... Hori,
43 ·· insulating film, 44 ·· excitation line, 45 ·· input / output line,
46 ・ ・ YBa ₂ Cu ₃ O _X thin film, 47 ・ ・ Junction, 51 ・
・ Flux quantum parametron, 52 ・ ・ DC SQUID, 6
1. superconducting wiring, 62 superconducting layer.

Continuing on the front page (72) Inventor Tokukai Fukasawa 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo Inside the Hitachi, Ltd. Central Research Laboratory (72) Inventor Akira Tsukamoto 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo Hitachi, Ltd. In-house (72) Inventor Shoichi Akamatsu 1-280 Higashi-Koikekubo, Kokubunji-shi, Tokyo Inside the Hitachi, Ltd.Central Research Laboratories (72) Inventor Uki Kabazawa 1-280, Higashi-Koikekubo, Kokubunji-shi, Tokyo In-house Hitachi Research Center (56 References JP-A-63-263780 (JP, A) JP-A-4-162577 (JP, A) JP-A-1-293581 (JP, A) JP-A-1-205481 (JP, A) 58-23391 (JP, A) Electronic Materials, Vol. 26, January Issue (Showa 62-1) PP. 83-89 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 39/00 ZAA H01L 39/22-39/24 ZAA G01R 33/035 ZAA

Claims (4)

(57) [Claims] And 1. A pair of opposing superconducting electrodes, have a, an oxide superconductor film formed and between the superconducting electrodes on the top and the pair of the pair of superconducting electrodes, the pair of Superconducting electrode
The oxide superconducting junction element, characterized in that the oxide superconducting film formed therebetween has a constricted portion . 2. The oxide superconducting junction device according to claim 1, wherein the superconducting electrode is a Y-Ba-Cu-based oxide of a perovskite-type oxide high-temperature superconductor or Bi-S
An oxide superconducting junction element comprising an r-Ca-Cu-based oxide. 3. The oxide superconducting junction element according to claim 1, wherein a direction of a current flowing through a portion of the oxide superconducting film formed between the pair of superconducting electrodes in the oxide superconducting film. Is an oxide superconducting junction element parallel to the ab plane perpendicular to the c-axis of the oxide superconducting film portion. 4. The oxide superconducting junction element according to claim 1, wherein said oxide superconducting junction element is used to constitute a flux quantum parametron or a carrier of flux quantum information. Oxide superconducting junction element.