JP2976388B2 - Josephson junction element and method of forming Josephson junction - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Josephson junction device and a method for forming a Josephson junction in the device.
SQ used for measuring weak magnetic fields such as biomagnetism
The present invention relates to a Josephson junction element and a method of forming a Josephson junction that can be widely applied to any element having a Josephson junction, in addition to a UID, a millimeter wave detection element, and the like.

[0002]

2. Description of the Related Art Conventionally, two types of Josephson junctions are known: a tunnel type and a microbridge type. Of these, the tunnel type is the most commonly used type, but requires an extremely thin insulating film, requires advanced insulating film creation technology, and has a low junction size because it cannot be reduced in size. It is said that it is difficult to make noise.

On the other hand, the microbridge type is a type in which two superconducting electrodes are joined by a minute bridge (weakly connected portion), and is advantageous in reducing noise because the junction size is small in principle. But the cross-sectional area of the bridge,
In general, it is extremely difficult to form a joint because the length must be very small.

Here, a quasi-planar Josephson junction, which is a modification of the microbridge type, has a lower electrode 61 and an upper electrode 61 each formed of a superconducting thin film, as schematically shown in FIG. The electrode 62 is laminated with an insulating film 63 interposed therebetween, and the lower electrode 61 and the upper electrode 62 are stacked.
Has a structure in which a bridge 64 is formed so as to straddle both surfaces, and the lower and upper electrodes 61 and 62 are joined by the bridge 64. According to this structure, the length of the bridge (weak link length) ) Is the insulating film 6
3, the weak link length is determined by adjusting the film thickness, which is relatively easy to control, as compared with a general microbridge type (plane type) junction that relies on microfabrication on a flat surface. This is advantageous in that respect. Therefore, it can be said that this quasi-plane type Josephson junction is a particularly practical form among micro-bridge type junctions that can easily reduce noise.

[0005]

By the way, the following problems still remain in the quasi-plane Josephson junction having many advantages as described above.

That is, a bridge, which is a weak coupling portion in a quasi-planar type Josephson junction, is usually formed by forming a thin line using an electron beam exposure and then further narrowing the thin line using a means such as anodic oxidation. However, this method has problems in that the productivity is low, the characteristics of the completed junction are not uniform, and the characteristics of the weak bond are largely changed with time due to exposure to the atmosphere or the influence of the oxidation process. is there.

The present invention has been made in view of such circumstances, and a low-noise and high-performance Josephson junction element which has a small change with time in the characteristics of the junction, can easily shorten the weak link length, and a high-performance Josephson junction element. It is an object of the present invention to provide a method capable of producing a Josephson junction with good productivity and uniform characteristics.

[0008]

In order to achieve the above object, a Josephson junction device of the present invention comprises two superconducting thin films 1 and 2 as shown in FIGS. , Oxides or nitrides of metal superconductors, or
The superconducting thin film is laminated via an insulating film 3 made of an oxide or nitride of a substance exhibiting a proximity effect on the superconducting thin film, and the insulating film 3 has a locally low oxygen or nitrogen concentration region. 4 is formed and is characterized by the two superconducting thin films 1 and 2 being weakly coupled to each other by the low concentration region 4 of oxygen or nitrogen.

Further, in the method of forming a Josephson junction according to the present invention, the two superconducting thin films 1 and 2 are placed close to the oxide or nitride of the metal superconductor or the superconducting thin films 1 and 2. After laminating via an insulating film 3 made of an oxide or nitride of a substance exhibiting an effect, the insulating film 3
By locally injecting particles into the region, the oxygen or nitrogen atoms in the implanted portion are recoiled to create a low concentration region of oxygen or nitrogen, and the two superconducting thin films 1 and 2 are formed by the low concentration region of oxygen or nitrogen. It is characterized by forming a weak connection 4 between the two.

[0010]

In the method of the present invention, when particles are locally implanted into the insulating film 3, oxygen or nitrogen in the implanted portion recoils and is captured by the superconducting thin film 1 or 2 in the vicinity thereof.
In the insulating film 3, a low concentration region of oxygen or nitrogen is formed in that portion, and the insulation in that portion is broken. Since the insulating film 3 is an oxide or a nitride of a substance exhibiting a proximity effect to the superconductor or the superconducting thin films 1 and 2, the superconducting thin films 1 and 2 are weakly coupled by the dielectric breakdown region, and the weak coupling is performed. A part 4 is obtained.

In such a method, the weak bonding portion 4 is formed only by injecting particles after forming the laminate,
Moreover, since the dimensions (weak link length and width) of the obtained weak coupling portion 4 are controlled by the thickness of the insulating film 3 and the particle injection region, it is possible to obtain a uniform junction with good productivity under good productivity. Can be.

Further, in the Josephson junction device of the present invention, since the device is basically a weakly-coupled device, it is easy to reduce noise as compared with the tunnel type, and the weakly-coupled portion 4 is located inside the laminate. Since it is present, it is not exposed to the atmosphere, and its characteristics are less changed over time than a quasi-plane Josephson junction device.

[0013]

FIG. 1 is an explanatory view of the overall structure of a Josephson junction device according to an embodiment of the present invention. FIG. 1A is a schematic external view, and FIGS. It is a figure and the C arrow view.

A lower electrode 1 is formed on a substrate 10, and an upper electrode 2 is interposed over an insulating film 3 so as to extend over a part of the surface.
Are laminated. In this example, the lower electrode 1 and the upper electrode 2 are both Nb superconducting thin films, and the insulating film 3 is an oxide thereof, for example, Nb ₂ O ₅ .

All end surfaces of the lower electrode 1, the insulating film 3, and the upper electrode 2 are formed by slopes inclined at a predetermined angle with respect to the normal direction of the substrate 10, and among them,
As shown, one end face along the longitudinal direction of the lower electrode 1 is a slope E ₁ , and one end face along the longitudinal direction of the upper electrode 2 is a slope E _2.
Called. The thickness of the insulating film 3 is 10nm approximately on slopes facing on the flat upper surface of the lower electrode 1 40 nm, and on to the inclined surface E ₁ along the longitudinal direction.

FIG. 2 shows the device according to the present invention in FIG.
Taken along a plane indicated by A-A in an enlarged sectional view of the vicinity of the inclined surface E _2. The insulating film 3 on the slope E _1,
A low oxygen concentration region 4 is formed in a portion slightly inside the surface P exposed along the slope E _2, and the low oxygen concentration region 4 joins the lower electrode 1 and the upper electrode 2. A weak-coupling Josephson junction is formed. That is, in the low oxygen concentration region 4 in the insulating film 3, since the insulating film 3 is an oxide of Nb, the insulation is broken and the composition is dominated by Nb, and the lower electrode 1 and the upper electrode 2 Will be joined by a suitable Nb superconductor. Note that S is a step generated by a manufacturing process described later, and the step S may or may not be present.

Next, a method of manufacturing a Josephson junction device having the above structure will be described. First, an Nb superconducting thin film is formed on the substrate 10, the surface is oxidized by about 40 nm, and then the lower electrode 1 is patterned by using a photolithography technique or the like. Next, the surface is further oxidized by about 10 nm,
The entire surface of the lower electrode 1 is covered with an insulating film made of an Nb oxide film such as Nb ₂ O ₅ . At this time, the thickness of the insulating film becomes 10nm about each ramp portion comprising 40 nm, a slope E ₁ at the top surface flat portion of the lower electrode 1. Thereafter, an Nb superconducting thin film is uniformly formed thereon, and then the superconducting thin film and the unnecessary insulating film on the lower electrode 1 are removed by etching so that the pattern of the upper electrode 2 remains. Thereby, the lower electrode 1
The structure shown in FIG. 1 in which the upper electrode 2 is partially laminated on the insulating film 3 via the insulating film 3 is obtained. Further, the step S shown in FIG. 2 is generated by this final etching step.

[0018] Now, in this state, as shown in FIG. 3, the portion of the inclined surface E ₂ on the slope E _1, for example, the ions of Nb ions, from the normal direction of the substrate 10 at an energy of about 20~40keV inject. Thus, where the entered about 5~20nm from the surface to the inside of the inclined surface E ₂ (depending on the ionic species and implantation energy), the ion implantation layer 5 along the surface. At this time, in the region 4 where the ion-implanted layer 5 and the insulating film 3 overlap, the implanted ions cause oxygen ions to recoil, and the recoiled oxygen ions are the upper and lower Nb layers of the lower and upper electrodes 1 and 2. Captured by As a result, the insulation of the region 4 is destroyed due to the decrease in the oxygen concentration, and the composition of the region 4 is dominated by Nb, and a weak coupling portion is formed.

The Josephson junction thus obtained has a typical microbridge type having no hysteresis in the current-voltage characteristics, and can be clearly distinguished from the tunnel type.

In order to investigate the time-dependent change in the characteristics of the Josephson junction obtained in this way, the humidity was measured without protection at a humidity of 10%.
When the change in the critical current value was examined by leaving it in a 0% environment, it showed only a change of about 10% in 20 days, and it was confirmed that the junction was excellent in environmental characteristics with little change over time.

Here, the thickness of the insulating film 3 to be overlapped with the ion-implanted layer 5 is essentially important.
A tunnel current flows to form a tunnel-type Josephson junction, and if it is too thick (for example, 40 nm).
Degree) It has been confirmed that an effective weak connection is not formed.

The ions to be implanted are Nb and A
u, Cu, and Si ions may be used. FIG. 4 shows the relationship between these ion species and the amount of implantation per unit area and the obtained critical current value of the Josephson junction. The element structure into which the ions are implanted is shown in FIG. The implantation energy of these ions is 20 keV for monovalent ions and 40 keV for divalent ions.

As is apparent from FIG. 4, the critical current value and the ion implantation amount show a substantially linear relationship, confirming that the controllability and the reproducibility are excellent. Further, it has been confirmed that the inclination is slightly different depending on the ion species to be implanted.

The mechanism of the formation of the above-mentioned weakly-coupled portion can only be estimated at the present time. However, the junction obtained by the method of the embodiment of the present invention is a junction having no hysteresis in the current-voltage characteristic as described above. It has been confirmed that ions to be implanted may be Au, Cu, and Si in addition to Nb, and that a weakly coupled portion is not formed even when ion implantation is performed on a portion where the thickness of the insulating film 3 is thicker than a certain level. Based on such grounds, it can be said that the above guess is plausible. That is, if the thickness of the insulating film 3 is appropriate, weak coupling is surely formed by arbitrary ion implantation, and any ion implantation is effective when the insulating film 3 is too thick. The reason why a weak coupling portion is not formed is that oxygen ions recoil due to ion implantation, and the recoiled oxygen ions reach the upper and lower Nb layers if the film thickness of the insulating film 3 is appropriate, and are captured there. It is considered that if the film thickness is too large, the recoiled oxygen ions stay in the insulating film 3 without reaching the upper and lower Nb layers.

In view of the mechanism of the formation of such a weak bond, the ions to be implanted into the insulating film 3 are not only the above-mentioned ions actually confirmed, but also recoil oxygen or nitrogen as described later. Any ion can be used as long as the ion can be implanted, and not only ions but also high-speed neutral atoms can be implanted.

For the same reason, as the insulating film 3, in addition to the oxide of a metal superconductor such as Nb ₂ O ₅ , the insulating film 3 is close to the superconductor forming the lower and upper electrodes 1 and 2. It may be an oxide or a nitride of a substance exhibiting an effect, for example, a metal such as Al or a semiconductor such as Si, and the oxide or nitride may be formed on the insulating film 3 made of any of these materials. In this case, oxygen or nitrogen in the case of nitride is implanted with any ion or neutral atom capable of recoil, thereby forming a low-concentration region of oxygen or nitrogen to form a weak bond. be able to.

Further, in the above-described embodiment, in order to intersect the ion-implanted layer 5 with the insulating film 3, the portion of the insulating film 3 inclined with respect to the normal line of the substrate 10 is
Although the example in which the ions are implanted from the normal direction is shown, the present invention is not limited to this. For example, as schematically shown in FIG. 5, the insulating film 3 formed in parallel with the surface of the substrate 10 may be used. For example, the insulating film 3 and the ion-implanted layer 5 may intersect by injecting ions obliquely to the normal direction of the substrate 10 by, for example, tilting the substrate 10.

Further, in the above embodiments, in the structure of the superconductor, the insulating film, and the superconductor, a low-concentration region of oxygen or nitrogen is formed in the insulating film to form the weak coupling portion. Even if a low-concentration region of oxygen or nitrogen is formed in the insulating film in the structure of the normal conductor, the insulating film, and the superconductor exhibiting the proximity effect, the same effect can be obtained. It goes without saying that it also includes

[0029]

As described above, according to the present invention,
Two superconducting thin films are laminated via an insulating film made of an oxide or nitride of a metal superconductor, or an oxide or nitride of a substance exhibiting a proximity effect on the superconducting thin film. By locally injecting particles into the insulating film, a weak coupling portion between the two superconducting thin films is formed rather than forming a low-concentration region of oxygen or nitrogen in the insulating film. Compared to the conventional quasi-plane type Josephson junction, which is a practical element with a junction, the process of making the junction is simpler and the productivity is better, and the weak link weak link length and width are insulated. Since the thickness can be controlled by the thickness of the film and the region into which the particles are injected, a highly reproducible Josephson junction with uniform characteristics can be produced with high efficiency. In addition, the Josephson junction device obtained in this manner has a weak coupling portion inside the device, and is compared with a conventional device which is directly exposed to the air or adjusted by using anodic oxidation or the like. As a result, there is little change over time in the characteristics, and since it is basically a microbridge type and the size of the junction can be easily reduced,
A high-performance device with low noise and no hysteresis.

[Brief description of the drawings]

FIG. 1 is an explanatory view of the overall structure of a device according to an embodiment of the present invention. FIG. 1A is a schematic external view of the device, and FIG.

FIG. 2 is a partially enlarged cross-sectional view taken along the plane AA of FIG. 1 (B).

FIG. 3 is an explanatory diagram of a step of forming a Josephson junction according to an embodiment of the present invention.

FIG. 4 shows implanted ion species and their implanted amounts when a Josephson junction is produced by applying the production method of the present invention;
Graph showing the relationship between the obtained junction and the critical current value

FIG. 5 is an explanatory view of a method according to another embodiment of the present invention.

FIG. 6 is a schematic external view illustrating the structure of a conventional quasi-planar Josephson junction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Lower electrode 2 Upper electrode 3 Insulating film 4 Low oxygen concentration area 5 Ion implantation layer 10 Substrate

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Ueda 1 Nishinokyo Kuwabaracho, Nakagyo-ku, Kyoto-shi, Kyoto Shimazu Seisakusho Sanjo Plant (56) References JP-A-6-283775 (JP, A) (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 39/22 H01L 39/24

Claims (2)

(57) [Claims] 1. A method in which two superconducting thin films are interposed via an insulating film made of an oxide or nitride of a metal superconductor or an oxide or nitride of a substance exhibiting a proximity effect on the superconducting thin film. In addition, the low concentration region of oxygen or nitrogen is locally formed in the insulating film,
A Josephson junction device in which the two superconducting thin films are weakly coupled to each other by the low oxygen or nitrogen concentration region. 2. A method in which two superconducting thin films are interposed via an insulating film made of an oxide or nitride of a metal superconductor or an oxide or nitride of a substance exhibiting a proximity effect on the superconducting thin film. After lamination, the particles are locally implanted into the insulating film, so that oxygen or
Rebounds the nitrogen atom to create a low oxygen or nitrogen concentration region
A method for forming a Josephson junction, wherein a weakly-bonded portion between the two superconducting thin films is formed by the low-concentration region of oxygen or nitrogen .