JPH07297459A - Manufacture of quasi-planar josephson junction - Google Patents

Abstract

PURPOSE:To obtain a quasi-planar Josephson junction from which the adjustment of the critical junction current value can be more quantitatively recognized by forming a second superconducting film on the surface of a first superconducting film with an insulating film in between so that the end face of the second film can be positioned on the surface of the second film and vapor depositing a bridging material on the end face by using a focused ion beam decelerated to a specific speed. CONSTITUTION:After forming a first superconducting film l on the surface of a substrate 10 in a prescribed pattern, an insulating layer 3 and second superconducting film 2 are successively formed on the film 1 and the layer 3 and film 2 are similarly patterned. The films 1 and 2 are formed of 2.5 Nb, etc. Then a microbridge is formed between the first and second films l and 2 by mounting the substrate 10 on the sample stage 17 of a focused ion beam device with a decelerating function. Namely, the part proposed for the formation of the microbridge is suitably scanned with a deflection electrode 15 while the part is irradiated with a focused ion beam B1 decelerated to <=200eV. As a result an Nb (bridging material 40) is vapor deposited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a Josephson junction, and more particularly to a method for manufacturing a quasi-planar Josephson junction having a microbridge structure.

The present invention can be applied to the manufacture of various devices including Josephson junctions such as SQUIDs.

[0003]

2. Description of the Related Art There are various types of Josephson junction structures. Of these, a quasi-plane type microbridge structure is shown in FIG. A second superconducting film 2 is formed above the first superconducting film 1 formed on the substrate 10 with an insulating layer 3 interposed therebetween.
Are laminated so that the end surface portion of the is mounted. Then, above the end face portion, the first and second superconducting films 1 and 2 are formed.
The microbridge 4 is formed so as to extend over both of the above. The material of the microbridge 4 may be a superconductor such as Nb, or a normal conductor such as Au in a SNS structure utilizing the proximity effect.

In such a quasi-plane type microbridge structure, the bridge length, which needs to be minute for stabilizing the device operation, is determined by the thickness of the insulating layer 3 which is relatively easy to control. In addition, it is advantageous as compared with a flat type microbridge structure or the like that relies on fine processing on a flat surface.

The microbridge 4 in this Josephson junction having a quasi-plane type microbridge structure (hereinafter referred to as a quasi-plane type Josephson junction) is conventionally composed of the first and second microbridges.
The bridge material is vacuum-deposited on the surfaces of the superconducting films 1 and 2, and then a resist is applied. Then, the bridge material is patterned into a desired fine width by electron beam exposure and development. After that, the critical current value of the junction is adjusted by the anodic oxidation method.

[0006]

In the conventional manufacturing method of the quasi-planar type Josephson junction microbridge as described above, it is necessary to perform sub-micron order fine processing by an electron beam exposure apparatus, and at the same time, anodic oxidation is required. Since the adjustment of the critical current value by the method has poor reproducibility and is an irreversible reaction, the junction cannot be used when it is oxidized too much. Therefore, it is difficult to manufacture a plurality of junctions having a certain critical current value, and there is a problem in yield.

[0007]

According to the present invention, a quasi-planar type Josephson junction microbridge is used in a focused ion beam apparatus having a deceleration function as illustrated in FIG. ), The step (A) of depositing the bridge material 40 by the focused ion beam B ₁ decelerated to 200 eV or less, or the step (A) and the same focused ion beam as shown in FIG. Focused ion beam B ₂ from the device with higher energy than beam B ₁
It is obtained by combining with the step of etching the bridge material 40 already vapor-deposited.

[0008]

Operation: For example, when the bridge material itself is ionized to form a focused ion beam, which is decelerated below a certain energy and irradiated, the ions are vapor-deposited at the irradiation position to form a film.

On the other hand, when the same focused ion beam is irradiated with higher energy on the film of the bridge material which has already been vapor-deposited, the film is etched by sputtering.

The present invention utilizes this point. In other words, by using a focused ion beam made by the same focused ion beam device, it is possible to vapor-deposit and etch the bridge material by appropriately changing the energy, and if this is combined appropriately and repeated as necessary. The width or thickness of the microbridge, which governs the critical current value of the junction, can be adjusted reversibly.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of a characteristic process of an embodiment of the present invention, and FIG. 2 is an explanatory view of a structural example of a focused ion beam device having a deceleration function used in the process.

In advance, a first superconducting film 1 having a predetermined pattern is formed on the surface of a suitable substrate 10 by sputtering deposition, photolithography or the like, and then an insulating layer 3 is formed thereon and a second superconducting film is formed thereon. The superconducting film 2 is similarly formed by sputter deposition, and then similarly patterned by photolithography. The first and second
As the superconducting films 1 and 2, Nb or the like can be used, and as the insulating layer 3, Nb ₂ O ₅ or the like can be used.

The above may be exactly the same as the conventional manufacturing method.

Next, a microbridge 4 straddling the first and second superconducting films 1 and 2 is prepared. This preparation is carried out by using a substrate 10 as a sample stage of a focused ion beam apparatus with a deceleration function illustrated in FIG. It is performed by mounting it on 17.

This focused ion beam device with a deceleration function includes a liquid metal ion source 11 in a chamber (not shown),
An extraction electrode 12, electrostatic lenses 13a and 13b, a mass filter 14, a deflection electrode 15, a deceleration electrode 16, a sample stage 17 and the like are provided, and an accelerating power supply 18 that gives acceleration energy to ions, an ion source 11 and an extraction electrode. An extraction power source 19 that applies a potential difference between the two electrodes 12 and a deceleration power source 20 that applies deceleration energy to the ions are provided. With this configuration, the ion beam B extracted from the ion source 11 is focused by the electrostatic lenses 13a and 13b, and at the same time, only desired ions are selected by the mass filter 14 and guided to the sample stage 17. The energy when the ion beam B reaches the sample stage 17 is equal to the output voltage difference between the acceleration power supply 18 and the deceleration power supply 20. That is, by adjusting the output of the deceleration power supply 20, in principle, the energy of the focused ion beam B can be continuously changed from 0 to the output of the acceleration power supply 18.

When the substrate 10 is mounted on the sample stage 17 of such an apparatus and a Nb microbridge is formed, the chamber is evacuated and the ion source 11 is filled with Nb-Ni alloy, for example. , Nb ⁺ ions are guided to the substrate 10.

Then, first, a bridge material 40 as shown in FIG. 1 (a) is formed. That is, the deceleration power supply 2
The output voltage of 0 is increased to generate the focused ion beam B ₁ which is decelerated to the extent that Nb ⁺ can be directly deposited on the surfaces of the first and second superconducting films 1 and 2 (about 200 eV or less). While irradiating the position where the micro bridge is to be formed with this focused ion beam B ₁ , the deflection electrode 15 appropriately scans. Thereby, the Nb vapor deposition film (bridge material 40) as shown is formed.

When the desired junction critical current value is obtained by the bridge obtained in the above steps, the production is completed, but normally, one deposition does not bring about such an ideal state, Therefore, a step of adjusting the critical current value is provided next.

In this adjusting step, if the junction current value is larger than the desired value due to the bridge obtained in the previous step, etching of the already-deposited bridge material 40 shown below is performed, and if it is small, the above-mentioned vapor deposition is performed again. select.

As shown in FIG. 1 (b), the etching of the bridge material 40 lowers the output voltage of the deceleration power source 20 of the focused ion beam device as compared with that during vapor deposition, and generates a focused ion beam B ₂ of higher energy. It will be done. That is, while the substrate 10 is still attached to the sample stage 17, the ions from the same ion source 11 are guided to the substrate 10 with high energy, so that the already-deposited bridge material 4 can be obtained.
0 is sputter-etched. The ions used here may be the same Nb ⁺ as in the vapor deposition step, or when using an Nb-Ni alloy as the ion source as in this example, the setting of the mass filter 14 should be changed to guide Ni ^+. You may During this etching, the focusing degree of the ion beam can be adjusted as necessary to reduce the spot diameter at the irradiation position.

After this etching step, if the junction critical current value becomes too smaller than the desired value, the above vapor deposition may be performed again. In other words, the vapor deposition of the bridge material and the etching thereof may be appropriately repeated so that the critical current value matches the desired value, and the substrate 10 was mounted on the sample stage 17 of the same focused ion beam apparatus in both steps. Can be done in the state.

The vapor deposition of the bridge material is performed by irradiating the ions of the material itself as in the above embodiments, and by irradiating a focused ion beam of arbitrary ions in the atmosphere of the material or its compound gas. It is possible to use a so-called ion-assisted vapor deposition method in which molecules of the atmospheric gas are urged by the ion beam to deposit the molecules at a target position. When depositing an Nb film using this ion assisted vapor deposition, a gas of an organic compound of Nb (for example, niobium carbonate) can be used as an atmosphere gas, and Ga ⁺ or the like can be used for an ion beam.

In addition to the etching of the focused ion beam itself, the bridge material is also etched by irradiating the focused ion beam through a predetermined atmospheric gas to urge the molecules of the atmospheric gas to collide with the target position to perform etching. A so-called ion-assisted etching method can be adopted. At this time, F or its compound can be considered as the atmospheric gas, and Ga ⁺ or other appropriate gas can be used as the ions.

Therefore, in the present invention, ion beam vapor deposition or ion assisted vapor deposition can be adopted as the vapor deposition method, and ion beam (sputter) etching or ion assisted etching can be employed as the etching method, and these can be appropriately combined. However, with the substrate placed on the sample stage of the same focused ion beam device with the same deceleration function, by changing the conditions of the device, vapor deposition of the bridge material and its etching can be repeated as needed. Can be selected and executed.

The present invention can be applied not only to a superconductor such as Nb used as a microbridge but also to a Josephson junction having an SNS structure using a normal conductor such as Pt or Au for the microbridge. is there.

[0026]

As described above, according to the present invention,
By using a focused ion beam generated by a focused ion beam device with a deceleration function to deposit and etch the bridge material, it is possible to reversibly adjust the critical current value of the Josephson junction. It is possible to manufacture a large number of junctions having a constant critical current value and improve the yield.

At the same time, the process can be simplified as compared with the conventional manufacturing method using electron beam exposure or anodic oxidation, and
Not only can cost reduction be achieved due to the increased manufacturing speed, but bridge construction to adjustment can be performed consistently within the chamber of the focused ion beam device, preventing contamination from the atmosphere and ensuring reliable bonding. Moreover, the adjustment of the junction critical current value is mainly performed by adjusting the width and thickness of the bridge, so it is possible to grasp more quantitatively than the conventional anodizing method. And is also excellent in terms of reproducibility.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a characteristic process of an example of the present invention.

FIG. 2 is an explanatory diagram of a configuration example of a focused ion beam device with a deceleration function used in the present invention.

FIG. 3 is a structural explanatory view of a quasi-planar Josephson junction.

[Explanation of symbols]

1 ... 1st superconducting film 2 ... 2nd superconducting film 3 ... Insulating layer 4 ... Micro bridge 10 ... Substrate 40 ... Bridge material B ₁ , B ₂ ... Focused ion beam 11 ... Liquid metal ion source 12 ... Extraction electrodes 13a, 13b ... Electrostatic lens 14 ... Mass filter 15 ... Deflection electrode 16 ... Deceleration electrode 17 ... Sample stage 18 ... Acceleration power supply 19 ... Drawing power supply 20 ... Deceleration power supply

Claims (2)

[Claims] 1. A second superconducting film is laminated on the surface of the first superconducting film with an insulating layer interposed between the first superconducting film and the first superconducting film. A method for obtaining a Josephson junction by forming a bridge that spans both second superconducting films, comprising: 200e from a focused ion beam device having a deceleration function.
A method of manufacturing a quasi-planar Josephson junction, characterized in that the bridge is formed by a step of depositing a material for the bridge using a focused ion beam decelerated to V or less. 2. A second superconducting film is laminated on the surface of the first superconducting film with an insulating layer interposed therebetween such that the end face portion of the second superconducting film is located on the surface. A method for obtaining a Josephson junction by forming a bridge that extends over both of the second superconducting films, characterized in that the bridge is formed by a combination of the following steps (A) and (B). -Type Josephson junction manufacturing method. (A) 2 from a focused ion beam device having a deceleration function
A step of depositing the material for the bridge using a focused ion beam decelerated to 00 eV or less. (B) A step of etching the material of the bridge deposited in the step (A) using a focused ion beam having a higher energy than the above energy from the same focused ion beam device as in the step (A).