JPS592390B2 - 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 is easy to manufacture and has excellent magnetic field dependence while keeping the size of the junction element small, 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 responses such as microwaves and millimeter waves, and as detectors for the weak radiation emitted from the human brain and heart. It has been proposed to use it as a magnetic field detector or 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. (Japanese Patent Publication No. 7712/1983) proposed a quasi-planar Jossufson junction element.

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 . In practice, an insulating material such as SiO2 or a semiconductor is sputtered onto the superconductor thin film 2 with a thickness of several hundred to several thousand A, or the surface of the superconducting layer 2 is oxidized in an oxidizing atmosphere. By doing so, an insulator layer 4 having a thickness of about several hundred λ is formed. A suitable barrier material is deposited across the thickness of the insulator layer 4 and across the upper and lower superconducting thin films to a thickness of several hundred λ to several thousand λ to form the 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 λ to several thousand λ 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 ImR' product, allowing the use of various materials for the weak coupling part, and 4) The superconductor layer 3 can have a long life by using Nb, etc. other than a lead alloy, and 5 can be easily mass-produced using photorin graphy or electron beam phosphoryl. It became possible to do so.

The capacitance of this quasi-planar Josephson junction element is influenced by the area AXb of the superconductor layers 2 and 3 that face each other with the insulator layer 4 in between. The capacitance becomes smaller.

By the way, as is clear from Fig. 1, the superconductor layer 2
, 3 has a structure that directly affects the accuracy of the area AXb. Therefore, in order to reduce capacitance and variation, strict mask alignment accuracy is required when photolithography or electron beam lithography is used, making it difficult to manufacture. In addition, in the quasi-planar Josephson junction element, two band-shaped weak coupling parts 5
crosses the thickness side of the insulator layer 4 at only one place, and has no dependence on the magnetic field.

In order to have magnetic field dependence, it is sufficient to form two or more weak coupling parts 5 in parallel, but there is a problem that the width a of the superconductor layer becomes large, the size of the junction element becomes large, and the capacitance also becomes large. The object of the present invention is to provide a quasi-planar Josephson junction element that is easy to manufacture, maintains the size of the junction element small, and has a structure with excellent magnetic field dependence.

The purpose of this is to intersect two strip-shaped superconductor thin films through an insulator layer, and then cross the thickness sides of the insulator layers at each opposing edge of this crossing area to separate the upper and lower superconductor thin films into two layers. Embodiments 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 on a substrate 1, and another strip-shaped superconductor thin film 3 is placed on the superconductor thin film 2 below. It is disposed on the insulator layer 4 across the insulator layer 4. These upper and lower superconductor thin films 2 and 3
At each opposing edge of the intersection area, the thickness sides 4', 4'', 4'' of the insulator layer 4 exposed between the upper and lower superconductor layers
, 4'' 2 and connects the upper and lower superconductor thin films to form two linear barrier material films to form a weak coupling part 5. In this way, the two strip-shaped superconductor thin films are crossed through an insulating layer, so the capacitance of the device can be determined by the width a or b of the strip-shaped superconductor thin film, and this width can be formed to the correct size. If you keep it,
The area a×b of this intersection area does not change even if the positions of the superconductor thin films 2 and 3 are shifted, so the positional accuracy of mask alignment in photolinkage or electron beam linkage does not need to be very strict. .

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 superconducting current flows through ″, 4″′, 4″2,
This can be equivalently represented by four quasi-planar Josephson junction elements connected in parallel (FIG. 4).

In other words, the critical current of 1m of the device is four times the critical current of the quasi-planar Josephson junction element, and the junction resistance RJ is X times that of the quasi-planar Josephson junction element, so the 1mRj product is ultimately that of the quasi-planar Josephson junction element. ImRj of
It is the same as the product. Therefore, it maintains excellent characteristics equivalent to those of a quasi-planar Josephson junction element. Moreover, the Josephson junction element of the present invention behaves in the same manner as three junctions connected in parallel with a critical current ratio of 1:2:1 in response to a magnetic field in the direction of the arrow in FIG. do.

In other words, it operates as a three-junction interferometer, and the dependence of its current on the magnetic field is as shown in Figure 5A, and the
Compared to the dependence of the current on the magnetic field in the two-junction interferometer shown in Figure B, the resistance states between the peaks of the superconducting state become wider. Therefore, designing and manufacturing the switching element becomes easy. A method for 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 n, In, and Al, or alloys thereof. Please refer to FIG. A resist mask M, is made on the substrate 1 by photolithography or electron beam lithography (FIG. 6A), and a superconducting material is sputtered or evaporated to a thickness of several hundred to several thousand layers through this mask M1, and then The mask is removed and the strip pattern (first strip superconductor thin film 2
) (Figure 6B). Both ends of this pattern are enlarged to facilitate external connections. Next, a resist mask M2 having a strip-shaped window is placed across the first strip-shaped superconductor thin film 2 (FIG. 6C), and an insulating material or semiconductor material such as SIO2 is coated with a thickness of 50 λ to several thousand λ. Sputter or evaporate the superconducting material to a thickness of hundreds to thousands of λ.
After that, the mask is removed to leave a band-shaped pattern (second band-shaped superconductor thin film 3) (FIG. 6D). 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 thick. Next, a plurality of linear resist masks M3 extending in parallel at intervals d narrower than % of the width a of the first strip-shaped superconductor thin film 2 are formed in the intersection area of the strip-shaped pattern (FIG. 6E). ). Remove the unmasked barrier material by chemical etching (Figure 6F). Finally, the resist mask M3 is removed to complete the process (FIG. 6G). In addition, the 6th
The manufacturing steps shown in FIGS. E, F, and G may be replaced with the manufacturing steps shown in FIGS. 7A, B, and C.

A resist mask M4 having a plurality of linear windows 6 extending in parallel with an interval d narrower than the width a(7)% of the first strip-shaped superconductor thin film 2 is used as the first strip-shaped superconductor thin film 2. After sputter cleaning the entire surface, a weakly bonding material is sputtered or evaporated through the window 6 of the resist mask M4 (FIG. 7B). Next, when the resist mask is removed, the weak bonding material thereon is lifted off and the bonded element is completed. (7th
Figure G'). In the above manufacturing method, a resist mask M3 having a plurality of lines extending in parallel with an interval d narrower than % of the width a of the first strip-shaped superconductor thin film or a resist mask M4 having a window is used. (Figure 6E or Figure 7A)
This is very important. That is, if d<a/2, two weak coupling parts 5 are formed in the intersection area of both superconductor thin films 2 and 3.
The probability of forming (yield N) is, for example, a/2=
In the case of d+3c/2, it is expressed as f, so two weak coupling parts can be easily formed with good yield.

For example, as shown enlarged in FIG. 8, a=4μMld
= 1.7 μMlc = 0.2 μm, a yield of approximately 74% is obtained. The × mark indicates a case where three weak bonds are formed, and the ratio is 5/19, and in the case of two, it is 14
/19'70.74, which agrees with the yield obtained from the above formula. In this way, a plurality of linear resist masks M3 with d<a/2 or a resist master M having windows
By using No. 4, a weak coupling portion with a small width c can be easily formed with 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 superconductor thin films 2 and 3 are not bonded together, there is no effect on the characteristics of the bonded element.

Furthermore, in the above manufacturing method, a resist mask M2 is placed across the first strip-shaped superconductor thin film 2 (FIG. 6C), 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 manner. If, on the contrary, the mask M1 used to form the first strip-shaped superconductor thin film 2 is used to overlay an insulating material thin film over the entire surface of the first strip-shaped superconductor thin film 2, there will be significant problems in manufacturing the device. occurs. 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.
In order 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 is a graph showing an example of the dependence of the current of the Josefson junction element on the magnetic field. Figure 6 A, B, C, D, E
, F, G and FIGS. 7A, B, and C show the manufacturing process of the Josephson junction device of the present invention. FIG. 8 is an enlarged explanatory diagram for explaining the effects of the manufacturing method of the present invention. Symbols in the diagram,
1...Substrate, M1-M4...Resist mask, 2, 3...Superconductor thin film, 4...
...Insulator layer, 5... Thin film of weakly bonding substance, 6.
·····window.

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. A Josephson junction device characterized by comprising two linear weak coupling portions that connect and extend superconductor thin films. 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 Josephson junction characterized by creating a plurality of linear resist masks extending in parallel at intervals narrower than 1/2 of the width of the body thin film, and removing weakly bonded substances not covered by the resist masks. Method of manufacturing elements. 3. Create a first belt-shaped superconductor thin film on the substrate, create an insulator layer across this first belt-shaped superconductor thin film,
A second strip-shaped superconductor thin film is formed by overlapping this insulator layer, and the width is 1/2 that of the first strip-shaped superconductor thin film.
A resist mask having a plurality of linear windows extending in parallel at narrower intervals is placed on the first strip-shaped superconductor thin film, and after the surface is sputter-cleaned, a weakly bonding substance is applied through the windows. A method for manufacturing a Josephson junction element, comprising sputtering or vapor deposition.