JPS592391B2 - Josephson junction device and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a structure of a Josephson junction element that has uniform characteristics and is easy to manufacture, and a method for manufacturing the same.

Josephson junction devices have a wide range of applications, including as low-power, ultra-high-speed switching computer devices, as detectors for high-sensitivity, high-speed response to microwaves, millimeter waves, etc., and for use in weak magnetic fields emitted from the human brain and heart. It has been proposed to use it as a detector or as a voltage standard, and the demand for its industrialization is increasing day by day. The present inventor has previously discovered a configuration of a Josephson junction element that makes it possible to shorten the weak coupling part of the Josephson junction element to the utmost limit and improve its characteristics, and also to easily mass-produce elements with the same characteristics. proposed a quasi-planar Josephson junction device (Japanese Patent Publication No. 7712/1983).

As shown in FIG. 1, in a quasi-planar Josephson junction device, two superconductor layers 2 and 3 are placed on a substrate 1, partially facing each other with an insulator layer 4 in between, and extending across the insulator layer 4. A weak coupling portion 5 spans the upper and lower superconductor layers 2 and 3. With this configuration, the length of the weak bond is equal to the thickness of the extremely thin insulator layer 4, and therefore the length of the weak bond is determined by the sputtering or evaporation time of the insulating material when forming the insulator layer. can be extremely small and precisely controlled by adjusting the . Actually, by sputtering an insulating material or semiconductor such as 5102 onto the superconductor thin film 2 with a thickness of several hundred to several thousand λ, or by oxidizing the surface of the superconducting layer 2 in an oxidizing atmosphere. By doing so, an insulator layer 4 having a thickness of about several hundred layers is formed. A suitable barrier material is deposited across the thickness of the insulating layer 4 and across the upper and lower superconducting thin films to a thickness of several hundred to several thousand layers to form a weak coupling portion 5.
form. The length of the weak coupling portion 5 that connects the upper and lower superconductor layers 2 and 3 in this way is the thickness of the insulator layer 4, and ranges from several hundred to several thousand depending on the required impedance. An appropriate value can be selected. With this structure, the superconductor layers 2 and 3 of one electrode part
While maintaining a large film thickness, the length of the weak coupling part can be extremely shortened, thereby making it possible to significantly increase the ImRj product, allowing the use of various materials for the weak coupling part, and reducing electrostatic 4. The superconductor layer 3 can have a long life by using Nb, etc. other than lead alloy, and 5. It can be easily mass-produced using photolithography or electron beam lithography. It became like this.

Among the main characteristics of this quasi-planar Josephson junction device,
The capacitance depends on the area AXb of the superconductor layers 2 and 3 facing each other with the insulator layer 4 in between, and the smaller the area AXb, the smaller the capacitance becomes.

Further, the junction resistance Rj is influenced by the width C of the weak coupling portion 5 that crosses the thickness side of the insulating layer 4, and the smaller the width C is, the greater the junction resistance Rj becomes. In other words, the main variation (non-uniformity) in the characteristics of the quasi-planar Josephson junction element is the area a><. It can be said that it is determined by the precision of b and the precision of width C, that is, the precision of mask alignment when using photolithography or electron beam lithography. By the way, as is clear from FIG. 1, the quasi-planar Josephson junction element has a structure in which misalignment of the mask alignment (in the X or Y direction) directly affects the accuracy of the area AXb.
As a result, the characteristics (capacitance) of the elements tend to vary.

Furthermore, in order to reduce the capacitance and increase the junction resistance, for example, in order to form a weak coupling portion 5 with a width C of 1 μm or less in a minute area where a and b are about several μm, This requires extremely high precision in mask alignment and is extremely difficult to manufacture. It is an object of the present invention to reduce variations in characteristics of this quasi-planar Josephson junction element due to mask alignment accuracy and to facilitate fabrication.

The purpose of this is to intersect two strip-shaped superconductor thin films through an insulator layer, and at each opposing edge of this crossing region, cross the thickness side of the insulator layer to combine the upper and lower superconductor thin films into one. This is achieved by coupling with a linear weak bond.

In addition, for this purpose, two strip-shaped superconductor thin films are intersected with each other with an insulating layer interposed between them. This is achieved by a method of forming a weak bond using a mask or a resist mask with windows.

Examples of the present invention will be described in detail below.

FIG. 2 is an enlarged plan view showing one embodiment of the Josephson junction element of the present invention. FIG. 3 is a perspective view of FIG. 2.
As shown in FIGS. 2 and 3, an insulator layer 4 is disposed on a strip-shaped superconductor thin film 2 extending over a substrate 1, and another strip-shaped superconductor thin film 3 is placed on the superconductor thin film 2 below. It is disposed across the thin film 2 and on top of the insulator layer 4.

At each opposing edge of the intersection area of the upper and lower superconductor thin films 2 and 3, an insulator layer 4 exposed between the upper and lower superconductor layers
A linear barrier material film is formed which extends across the thickness sides 4' and 4'' of the upper and lower superconductor thin films to form a weak coupling part 5. In this way, two strip-shaped Since the superconductor thin films intersect with each other through the insulator layer, the capacitance of the device can be determined by the width a or b of the strip-shaped superconductor thin film. If formed to the same dimensions, the area AXb of this intersection region will not change even if the positions of the superconductor thin films 2 and 3 are shifted, so there is no need to be so strict about the positional accuracy of mask alignment in photolithography or electron beam lithography. Therefore, an element with uniform capacitance can be produced extremely easily.

In the operation of this Josephson junction element, the shortest distance between the upper and lower superconductor thin films is on both sides of the insulator layer 4', 4.
Since a superconducting current flows through 7'', this can be equivalently represented by two quasi-planar Josephson coupling elements connected in parallel (FIG. 4).

In other words, the critical current 1 m of the device is twice the critical current of the quasi-planar Josephson junction element, and the junction resistance Rj is % times the junction resistance of the quasi-planar Josephson junction element, so that the MRj product is ultimately the same as that of the quasi-planar Josephson junction element. ImRj of the element
It is the same as the product. Therefore, it maintains excellent characteristics equivalent to quasi-planetary Josephson junction elements. A method of manufacturing the Josephson junction device of the present invention will be described.

First, a first strip-shaped superconductor thin film 2 is formed on a substrate 1 using a mask.

This superconductor thin film consists of Nb, Ta, W, La, Pb, S
It is made of various superconducting materials that exhibit superconductivity, such as metals such as nJn and Al, or alloys thereof. Please refer to FIG. A resist mask M1 is made on the substrate 1 by photolithography or electron beam lithography (see Fig. 5A).
), the superconducting material is sputtered or evaporated to a thickness of several hundred to several thousand through this mask M1, and then the mask is removed to leave a band-shaped pattern (first band-shaped conductor thin film 2) (fifth Figure B). Both ends of this pattern are enlarged to facilitate external connections. Next, a resist mask M2 having a band-shaped window crossing this first band-shaped superconductor thin film 2 is formed.
(Figure 5C), sputtering or evaporating an insulating or semiconducting material such as SiO2 to a thickness of 58 to several thousand, and then sputtering or depositing a superconducting material to a thickness of several hundred to several thousand. Sputtering or vapor deposition is performed, and then the mask is removed to leave a band-shaped pattern (second band-shaped superconductor thin film 3) (FIG. 5D). Next, the entire surface is sputter cleaned to remove the oxide film from the superconductor surface. Then, a barrier material is sputtered or deposited over the entire surface to a thickness of several thousand λ. Next, a plurality of linear resist masks M3 extending in parallel at intervals d narrower than the width a of the first strip-shaped superconductor thin film 2 are formed in the intersecting regions of the strip-shaped patterns (FIG. 5E). Remove the unmasked barrier material by chemical etching (Figure 5F). Finally, the resist mask M3 is removed to complete the process (FIG. 5G). Furthermore, Figure 5E
, F, and G may be replaced with the manufacturing steps shown in FIGS. 6A, B, and C.

A resist mask M4 having a plurality of linear windows 6 extending in parallel at intervals d narrower than the width a of the first strip-shaped superconductor thin film 2 is applied to the first strip-shaped superconductor thin film 2. (FIG. 6A), and after sputter cleaning the entire surface, a weakly bonding material is sputtered or evaporated through the window 6 of the resist mask M4 (FIG. 6B). Next, when the resist mask M4 is removed, the weak bonding material on it peels off (
(lifted off) to complete the bonding element. (Figure 6C)
. In the above manufacturing method, a resist mask M3 having a plurality of lines extending in parallel at an interval d narrower than the width a of the first strip-shaped superconductor thin film or a resist mask pigeon having a window is used (fifth Figure E or Figure 6A) is important. That is, if d<a, both superconductor thin films 2
, 3, the probability (yield N) of forming one weak coupling part 4 in the intersection area of 3 is expressed as, for example, when a=d+2c, so one weak coupling part 4 can be easily formed with a high yield. I can do it.

For example, as shown enlarged in FIG. 7, a=4μMld
When =3.6μMlc=0.2μm, a yield of about 84% is obtained. When two weak bonds are formed, the ratio is 3/19, and when there is one, the ratio is 16/
19".0.84, which is consistent with the yield obtained from the soil method. In this way, by using a plurality of linear resist masks M3 with d<a or a resist mask M4 having windows, the yield can be improved. A weak coupling portion with a small width c can be easily formed at a high yield in the intersection region of both superconductor thin films without requiring much positional accuracy in mask alignment.

Therefore, mass production of junction elements with a large junction resistance Rj becomes easy. Note that the weak coupling portion 5 formed outside the intersection area is
Since both superconductors 2 and 3 are not bonded together, there is no effect on the characteristics of the junction element.

Furthermore, in the above manufacturing method, a resist mask M2 is placed across the first strip-shaped superconductor thin film 2 (FIG. 5C), an insulating material is sputtered or vapor-deposited, and then the same mask M2 is used to form a superconductor. Sputtering or vapor depositing a substance.

It is important to sputter or evaporate the insulating material across the first strip-shaped superconductor thin film 2 in this way. If, on the other hand, a thin film of insulating material is applied over the entire surface of the first strip-shaped superconductor thin film 2 using the mask M1 with which the first strip-shaped superconductor thin film 2 is made, significant inconveniences will occur in the manufacturing of the device. arise. In other words, it is extremely difficult to selectively remove only the insulating material from areas other than the intersection area where the upper and lower superconductor thin films overlap without damaging the underlying superconductor thin film surface by sputter etching before forming a weak bond. This is because it is difficult. Further, the entire surface is sputter cleaned to remove the oxide film from the superconductor surface, and at this time, the thickness side of the insulator layer is also shaped. An insulator layer is formed by sputtering or depositing an insulating or semiconductor material, but if this insulator layer is relatively thin (typically less than 200λ), pinholes may form and superconducting shorts may occur. To close this pinhole, the insulating layer may be oxidized by exposing it to an oxidizing atmosphere. Instead of sputtering or vapor depositing an insulating material or a semiconductor material to form an insulating layer, the surface of the superconductor thin film 2 exposed through the window of the mask M2 may be exposed to an oxidizing atmosphere and oxidized to form an insulating layer. .

[Brief explanation of the drawing]

FIG. 1 is an enlarged perspective view of a conventional quasi-planar Josephson junction element. FIG. 2 is an enlarged plan view of the Josephson junction device of the present invention. FIG. 3 is a perspective view of FIG. 2. FIG. 4 is an equivalent circuit of the Josephson junction device of the present invention. Figure 5A,
B, C, D, E, F, G and FIGS. 6A, B, and C show the manufacturing process of the Josephson junction device of the present invention. FIG. 7 is an enlarged explanatory diagram for explaining the effects of the manufacturing method of the present invention. Symbols in the figure, 1... Board, M1 ~ Pigeon...
・Lugist mask, 2, 3... superconductor thin film,
4...Insulator layer, 5...Thin film of weakly bonded substance, 6...Window of rugist mask.

Claims (1)

[Scope of Claims] 1. Two strip-shaped superconductor thin films intersecting with each other with an insulating layer interposed therebetween, and upper and lower layers crossing the sides of the thickness of the insulating layer at each opposing edge of the crossing region. 1. A Josephson junction device comprising: a linear weak coupling portion that connects and extends a superconductor thin film. 2. Create a first belt-shaped superconductor thin film on a substrate, create an insulating layer across this first belt-shaped superconductor thin film, and overlap this insulator layer to form a second belt-shaped superconductor thin film. The laminated thin film thus created is sputter-cleaned to form a thin film of a weakly binding substance on the entire surface of the laminated thin film, and the first belt-shaped superconducting film is formed on the thin film of the weakly binding substance. A method for manufacturing a Josephson junction device, characterized by creating a plurality of linear resist masks extending in parallel at intervals narrower than the width of a body thin film, and removing weakly bonded substances not covered by the resist masks. . 3 Create a first belt-shaped superconductor thin film on a substrate, create an insulator layer across this first belt-shaped superconductor thin film, and overlap this insulator layer to form a second belt-shaped superconductor thin film. A resist mask having a plurality of linear windows extending in parallel at intervals narrower than the width of the first strip-shaped superconductor thin film is applied to the first strip-shaped superconductor thin film. 1. A method of manufacturing a Josephson junction device, comprising sputtering or depositing a weakly bonding material through the window after sputter cleaning the surface of the thin film.