JP2594934B2 - Weakly coupled Josephson device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weak-coupled Josephson device operated near liquid helium temperature, and more particularly to a weak-coupled Josephson device suitable for high-speed switching operation.

[Conventional technology]

For the conventional weakly-coupled Josephson device,
EEE, Transactions on Electron Devices, Eday -28, Issue 11 (198
1) Pages 1394 to 1397 (IEEE, TRANSACTIONS ON ELEC
TRON DEVICES, VOL.ED-28, No.11 (1981) pp.1394-139
7). The formation of a weak bond using a step portion of a substrate is described in Japanese Patent Application Nos. 54-122376 and 55-16923. However, the weak coupling portion described in the above-mentioned conventional example is made of a superconductor, and is made of a constricted portion that weakly connects the two superconducting films.

[Problems to be solved by the invention] As shown in FIG. 6, this element is a normal conductor made of Si.
Superconducting electrodes 65 and 66 consisting of Pb and In alloy thin films on 64
Are opposed to each other in a planar structure. Generally Jiyosefuson current I flowing between the superconductor electrodes of weak coupling type Jiyosefuson element having such a structure is given by the _{^{Iα (T 2 / ξ n)}} · exp (-L / ξ n) ... ( Equation 1). Here, T is the operating temperature of the element, which is lower than the superconducting transition temperature of the superconducting electrode, and is generally a liquid helium temperature (4.2 K). ξ _n in the electron pair coherence length in the normal conductor, by connexion determined the nature of the normally conducting. L is the distance between the two superconductor electrodes facing each other and is referred to as the coupling length. To obtain the number μA or more Jisefuson junction current sufficient to practical use in this device, the coupling length L less than 10 times the electron pair coherence length xi] _n, i.e. should be about 0.5μm or less,
In addition, in order to fabricate a large number of devices having the same Josephson junction current, it is necessary to increase the dimensional accuracy of each coupling length. In the prior art, first, a superconductor thin film made of a Pb-In alloy is formed on a Si substrate, and then the device pattern and the coupling length are processed by etching using a photoresist or an electron beam resist as a mask. Two superconductor electrodes were formed. As described above, in the element having the planar structure, a process of processing the coupling length portion with an extremely fine pattern dimension of 0.5 μm or less with high accuracy and high reproducibility is required, but this processing is not always easy. For this reason, the device manufactured by the conventional method has a Josephson junction current depending on the coupling length of about ± 10
% Or more, and a distribution of about ± 30% between substrates, resulting in poor current uniformity and poor reproducibility.

An object of the present invention is to provide a weak-coupling-type Josephson element configured by connecting two superconductors via a normal conductor,
It is an object of the present invention to provide a new device structure which facilitates formation of a fine coupling between two superconductors and enhances its dimensional accuracy, thereby improving uniformity and reproducibility of device characteristics.

[Means for solving the problem]

In order to achieve the above object, in the present invention, instead of the conventional planar element structure in which the coupling length portion is formed by pattern processing, two superconductors are separated by a step portion formed of a normal conductor or an insulator, In addition, a first element structure in which the coupling length is determined based on the size of the step in the step portion is adopted.

In the present invention, the normal conductor has a three-layer structure in which the normal conductor is sandwiched between two superconductors. And a second element structure for electrically separating the two superconductors.

[Action]

According to the first element structure, the steps can be formed by a thin film deposition method such as vacuum deposition or a method of etching a normal conductor. In the thin film deposition method and the etching method, dimensional control can be performed with good reproducibility in a unit of several nm, and the size of the step, that is, the dimensional accuracy of the coupling length and the reproducibility can be remarkably improved. Therefore, the uniformity and reproducibility of the device characteristics can be improved.

According to the second element structure, an element having an arbitrary coupling length can be manufactured by controlling the thickness of the normal conductor serving as a weak coupling material. The normal conductor can be formed by a thin film deposition method such as a vacuum evaporation method or a sputtering method, and these thin film deposition methods can control the film thickness, that is, the bond length with a reproducibility of several nm.
Therefore, the uniformity and reproducibility of device characteristics depending on the coupling length can be improved.

〔Example〕

Hereinafter, the first element structure of the present invention will be described in detail using Examples 1 to 5.

(Example 1) (1) First, as shown in FIG.
Si (0.5 μm thick) was formed as a step member 12 on a 2-inch diameter Si substrate 11 on which a pattern 13 (manufactured by Hoechst, trade name: AZ-1350J) was formed by vacuum evaporation. This Si is reduced to such an extent that the acceptor or the donor freezes out at an extremely low temperature at which the weak-bond type Josephson junction of the present invention operates, that is, 1 × 10 ²⁴ m ⁻³ or less.

(2) Then, by dissolving and removing the photoresist 13 with acetone, as shown in FIG.
At 12, a step having a height of 0.5 μm was formed.

(3) Next, as shown in FIG. 1 (c), a Cu film having a thickness of 1 μm is formed as a normal conductor 14 by a vacuum evaporation method, and then the superconductor electrodes 15 and 16 are formed in the same vacuum chamber.
A 0.3 μm Nb film was formed on the Cu film by a vacuum evaporation method. At this time, Nb is deposited from a direction where the step surface cannot be seen,
Nb was not deposited on the step surface. This gives Nb
The superconductor electrode 15 and the superconductor electrode 16 were separated by a gap of 0.2 μm, that is, a coupling length 17 at the step.

Here, when the Nb superconductor electrode film was formed by the sputtering method, Nb jumped onto the step surface, and the electrodes 15 and 16 were short-circuited. However, by by connexion removed etching thickness 10~20nm the Nb film by reactive plasma Etsu quenching method using this after CF ₄ gas, can be removed Nb adhering to the stepped surface, the electrodes 15 and 16 Separation was possible at the step.

(4) Finally, the Nb superconductor electrode films 15 and 16 are etched using a photoresist as a mask material by a plasma etching method using CF ₄ gas to form a weakly-coupled Josephson as shown in FIG. The device was completed.

In this embodiment, the coupling length 17 can be set to an arbitrary length by changing the height (film thickness) of the step member 12 or the film thickness of the superconductor electrode 15. Therefore, the Si step material 12
Is fixed at 0.5 μm, and the film thickness of the Nb superconductor electrode 15 is added to the aforementioned 0.3 μm, and 0.2 μm, 0.25 μm, 0.35 μm, 0.
As a result of 4 μm, the bond length is 0.1 μm, 0.15 μm, 0.2 μm, 0.2
5 μm and 0.3 μm junctions could be produced. The dimensional accuracy of these bonding lengths is determined by the film thickness controllability and the spatial distribution of the film thickness when vacuum depositing the Si step material and the Nb superconducting electrode. In the case of the present embodiment, this dimensional accuracy is ±
At 1% or less, the reproducibility was very good at ± 2% or less. As a result, the junction current, which was distributed about ± 10% or more in the substrate by the conventional method, is reduced to ± 5% or less, and the junction current, which was about ± 30% scattered between prototype lots, is reduced to ± 10% or less. As a result, the uniformity and reproducibility of the junction current were improved.

(Example 2) (1) First, as shown in FIG. 2 (a), a Cu film having a thickness of 1 μm is formed as a normal conductor 24 on the entire surface of a Si substrate 21 by a vacuum deposition method, and then a pattern of a photoresist 23 is formed. Was formed.

(2) Next, using the photoresist as a mask material, the Cu film was etched to a depth of 0.5 μm by an ion milling method using Ar gas. The photoresist was dissolved and removed with acetone to form a step of 0.5 μm on the Cu film as shown in FIG. 2 (b).

(3) Next, the surface of the sample is subjected to high frequency sputtering in an Ar gas atmosphere of 1.33 Pa to remove impurities and oxides on the surface of the sample. An Nb film was formed by a vacuum evaporation method as shown in FIG.

(4) Then, using the photoresist (AZ-1350J) as a mask material and etching the Nb film by a plasma etching method using CF ₄ gas, a weak coupling type Josephson device as shown in FIG. completed. As in Example 1, the uniformity and reproducibility of the Josephson junction current could be improved.

In this embodiment, the normal conductor formed on the Si substrate
Steps were provided by etching the Cu film, but instead of the Si substrate
It is also possible to use a Cu substrate and etch it to form a step.

Example 3 (1) After forming a pattern of a photoresist 33 (AZ-1350J) on a Si substrate 31 as shown in FIG. 3 (a), 0.5 μm thick SiO was used as a step member 32. It was formed by a vacuum evaporation method.

(2) Then, by dissolving and removing the photoresist with acetone, as shown in FIG.
2, a step having a height of 0.5 μm was formed.

(3) Next, as shown in FIG.
As 35 and 36, an Nb film having a thickness of 0.4 μm was formed by a vacuum evaporation method, and subsequently, 0.8 μm thick Cu was deposited as a normal conductor 34 on the Nb film in the same vacuum. As a result, the two Nb superconductor electrodes 35 and 36 were connected to each other at the step formed by the SiO step member 32 via the Cu normal conductor 34 with a bond length of 0.1 μm.

(4) Then, using the photoresist as a mask material, the Cu normal conductor 34 and the Nb were formed by ion milling using Ar gas.
After etching the superconductor electrodes 35 and 36, FIG. 3 (d)
An element pattern was formed as shown in FIG.

(5) Using the photoresist (AZ-1350J) as a mask material, the Cu normal conductor 34 other than the stepped portion was etched by an ion milling method using Ar gas, and FIG. 3 (e) and (f). ), A weakly-coupled Josephson device was completed.

As in the present embodiment, even when the weak coupling portion is formed with a normal conductor on the superconductor electrode, the same characteristics as those of the first embodiment can be obtained.
The uniformity and reproducibility of the Josephson junction current were improved.

It should be noted that the step member 32 of the present embodiment may be replaced with Cu as a normal conductor from SiO, and a weak coupling portion may be formed by sandwiching the superconductor electrode above and below with a normal conductor.

(Example 4) Next, a control electrode is provided at a step portion of the weakly-coupled Josephson device of the present invention, and a voltage is applied to the control electrode to apply an electric field to the normal conductor between the two superconductor electrodes. An embodiment in which the state is changed to produce a superconducting transistor for controlling the Josephson junction current will be described with reference to FIG.

(1) Nb having a thickness of 0.5 μm as a control electrode 48 on a Si substrate 41
Is formed by a vacuum deposition method and then etched by a plasma etching method using CF ₄ gas using a photoresist as a mask material to form a step of 0.5 μm in height with an Nb film as shown in FIG. 4 (a). did.

(2) Next, as shown in FIG.
The wiring pad portion of the Nb control electrode 48 was covered with 43, and an Nb oxide insulating film 49 having a thickness of about 30 nm was formed on the surface of the Nb by anodization.

(3) Then, after dissolving and removing the photoresist 43 with acetone, the sample was set in a vacuum device and subjected to a heat treatment at a temperature of about 200 ° C. for 30 minutes in a vacuum of 5 × 10 ⁻⁴ Pa or less. Thereafter, the surface of the sample was subjected to high frequency sputtering in a 1.33 Pa O ₂ gas atmosphere to remove impurities on the sample surface. Then, as a normal conductor 44, a polycrystalline Si film having a thickness of about 0.1 μm is formed from an oblique direction through the step surface by a vacuum evaporation method,
Subsequently, an Nb film having a thickness of 0.4 μm was formed as the superconductor electrodes 45 and 46 from a direction in which the cross section could not be seen by a vacuum evaporation method. Thereby, as shown in FIG. 4 (c), the superconductor electrodes 45 and 46 could be separated at the step portion with a coupling length of 0.1 μm.

(4) Finally, using the photoresist as a mask material and ion milling using Ar gas, the Nb superconductor electrode 45 and
The superconducting transistor as shown in FIG. 4 (d) was completed by etching the 46 and the Si normal conductor 44.

In this embodiment, the uniformity and reproducibility of the Josephson junction current can be improved as in the first embodiment, and by applying a voltage of 200 to 200 mV to the control electrode, the Josephson junction current can be reduced to 0 to 80 μA. The range could be controlled.

As described above, the present embodiment provides a superconducting transistor having a new structure in which a control electrode function is added to the step member of the device according to the present invention described in the first embodiment.

Fifth Embodiment In the fourth embodiment, the example in which the control electrode is provided below the Josephson element has been described. However, the control electrode may be formed on the Josephson element. Therefore, Embodiment 1
Although the control electrode is provided on the element (shown in FIG. 1 (d)) described above with an insulating film interposed therebetween, there is a disadvantage that the control voltage is increased. In this case, it is considered that the element was configured in the order of the normal conductor, the superconductor electrode, the insulating film, and the control electrode, so that it was difficult for the electric field to be applied to the conductive conductor. Therefore,
An embodiment in which elements are formed in the order of superconductor electrode-normal conductor-insulating film-control electrode will be described below with reference to FIG.

(1) First, as shown in FIG. 5 (a), a 0.5 μm thick SiO film was formed as a step member 52 on a Si substrate 51.

(2) Nb having a thickness of 0.4 μm was used as the superconductor electrodes 55 and 56 in a direction in which the step surface could not be seen by vacuum evaporation.
After forming a film, subsequently as a normal conductor 54, a thickness of 0.15μ
m of polycrystalline Si is formed by a vacuum deposition method from the direction looking through the step surface, and the surface is dried in an O ₂ gas atmosphere at 100
Oxidized by heat treatment at ℃ 20 minutes, about 25 nm thick silicon oxide
An insulating film 59 was formed to have a structure as shown in FIG.

(3) Next, using the photoresist as a mask material, the silicon oxide insulating film 59 is formed by ion milling using Ar gas.
After etching the Si normal conductor 54 and the Nb superconducting electrodes 55 and 56 to form an element pattern, the ion-milling method also uses the Si oxide insulating film 59 and the Si normal conductor 54 except for the cut portion.
Only this was etched to obtain a structure as shown in FIG. 5 (c).

(4) Then, as shown in FIG. 5 (d), an SiO interlayer insulating film 60 having a thickness of 0.7 μm is formed by a vacuum evaporation method, and a control electrode 58 is formed on the Si oxide insulating film 59 as a control electrode 58 having a thickness of 1.0 μm. μm P
A superconducting transistor was completed by forming a b-In deposited film.

In this embodiment, similarly to the fourth embodiment, the Josephson junction current could be controlled by applying a voltage to the control electrode.

In Examples 1 to 5 above, was used with Nb superconducting electrodes, NbN Besides this, Nb ₃ Al, Nb compound such as Nb ₃ Ge, Pb
Needless to say, a Pb alloy such as -In, Pb-Bi, Pb-Au-In, or MoN can be used. As for the normal conductor material, the example of Cu, Si has been described, but Al, Ge, GaAs,
Similar effects were obtained when InP, InAs, InSb, GaSb, GaP, etc. were used.

Hereinafter, the present invention will be described in detail using Examples 6 and 7.

(Example 6) (1) First, a 200-nm-thick Si substrate 111 having a thickness of 2
An Nb film was formed by a DC magnetron sputter method, and a first superconductor electrode pattern was formed on the Nb film by a light exposure method using a photoresist (trade name: AZ-1350J, manufactured by Hoechst). The photoresist is used as a mask material, and the Nb film is etched by a reactive plasma etching method using a mixed gas of CF ₄ and O ₂ to form a first Nb superconductor as shown in FIG. An electrode 112 was formed.

(2) Next, after cleaning the surface of the Nb superconductor electrode 112 by a high-frequency sputter cleaning method using Ar gas, the thickness of the normal conductor 113 is reduced to 50 nm by a vacuum deposition method.
(FIG. 7 (b)). At this cryogenic temperature at which the weakly-coupled Josephson device of the present invention operates,
The impurity concentration was set so as to freeze out the acceptor or the donor, that is, 1 × 10 ²⁴ m ⁻³ or less. In this deposition, the thickness of the deposited film was controlled by using a quartz crystal film thickness meter.

(3) After coating the weakly bonded portion of the Si film with a photoresist (the AZ-1350J) 114, the exposed surface of the Si film is oxidized by a high-frequency plasma oxidation method using O ₂ gas. Then, a SiOx insulator layer 115 having a thickness of about 5 nm or more was formed (FIG. 7C), and the photoresist was dissolved and removed with acetone.

(4) Next, after the weak coupling portion of the Si normal conductor 113 and the surface of the SiOx insulator layer 115 are cleaned by a high frequency sputter cleaning method using Ar gas, the Nb film having a thickness of 200 nm is subjected to direct current. It was formed by a magnetron sputter method (FIG. 7 (d)). Then, a second superconductor electrode pattern is formed using the photoresist, and the Nb film is etched by a reactive plasma etching method using CF ₄ to form a second superconductor electrode pattern.
The Nb superconductor electrode 116 was formed to complete a weakly-coupled Josephson device as shown in FIG. 7 (e).

By the method as described above, the thickness of the normal conductor 113, that is, the coupling length in addition to the aforementioned 50 nm, 100 nm, 150 nm, 200 nm
The dimensional accuracy was ± 2% or less of the set value, and the reproducibility was extremely good at ± 5% or less. As a result, it is possible to reduce the junction current, which was distributed about ± 10% or more in the substrate by the conventional method, to ± 5% or less, and to reduce the junction current, which was distributed about ± 30% between prototype lots, to ± 10% or less. The uniformity and reproducibility of the device characteristics could be improved.

(Embodiment 7) In Embodiment 6 described above, since an electrode pattern is formed for each layer, a step of removing impurities such as photoresist remaining in the weakly bonded portion by a high frequency sputter cleaning method and cleaning the surface is necessary. It was. In order to prevent the two superconductor electrodes from being short-circuited by the step of the first superconductor electrode, the cross section of the first superconductor electrode 112 must be trapezoidal as shown in FIG. It was difficult to increase the density of the device without being narrowed. Therefore, an embodiment in which the above-described cleaning step is unnecessary in the present invention and the element is flattened to facilitate high density will be described below with reference to FIG.

(1) First, as shown in FIG. 8A, an NbN film having a thickness of 200 nm is formed as a first superconductor electrode 122 on a Si substrate 121, and then a Si film having a thickness of 100 nm is formed as a normal conductor 123. An NbN film having a thickness of 100 nm was continuously formed as the second superconductor electrode 126. Here, the NbN film was formed by a DC magnetron sputter method, and the Si film was formed by a vacuum deposition method, and the film thickness was controlled using a quartz crystal film thickness meter. By continuously forming the three-layer film in advance in this way, the step of cleaning the film surface described in Example 6 is not required.

(2) Next, a photoresist (the AZ-1350)
In J), a first superconductor electrode pattern was formed by a light exposure method. Then, using this photoresist 124 as a mask material, the two NbN superconductor electrodes 1 were formed by a reactive plasma etching method using CF ₄ gas as shown in FIG.
The ordinary conductor 123 was etched with 22,126 and Si.

(3) Subsequently, an SiO insulating film 127 is formed to a thickness of 300 nm on the substrate by a vacuum evaporation method, the resist 124 is dissolved with acetone, and the SiO film is deposited thereon to remove the SiO film. 8 As shown in FIG. 8C, the surface of the normal conductor 123 was back-filled with the SiO insulator 127.

(4) Further, a photoresist (the AZ-
At 1350 J), a pattern of a weakly bonded portion was formed by a light exposure method. Then, using this photoresist 128 as a mask material, C
After etching the second NbN superconductor electrode 126 by reactive plasma etching using F ₄ gas, the exposed surface of the Si normal conductor 123 is oxidized by high-frequency plasma oxidation using O ₂ gas. Then, an SiOx insulator layer 125 having a thickness of about 5 nm or more was formed as shown in FIG. 8 (d).

(5) Next, as shown in FIG.
A 100 nm SiO insulating film 129 was formed by a vacuum evaporation method. Then, the resist 128 was dissolved in acetone, and the SiO film deposited thereon was removed, thereby filling the surface of the second superconductor electrode 126 with the SiO insulator 129 to flatten the device.

(6) Finally, an NbN film having a thickness of 150 nm is formed by a DC magnetron sputter method, and then the NbN film is etched by a reactive plasma etching method using CF ₄ gas using a photoresist as a mask material to form an NbN superconductor. Extraction electrode 130
Was formed, and a flattened weak coupling type Josephson device was completed as shown in FIG. 8 (f). As in the case of the sixth embodiment, the uniformity and reproducibility of the Josephson junction current can be improved. Further, compared with the sixth embodiment, the cleaning step is omitted, and the element structure is flattened to facilitate the high density. .

Above in Examples 6-7, it was used superconducting electrodes NbN, this addition to Nb, Nb ₃ Ge, MoN or Pb-an In, can of course be used, such as Pb alloy such as Pb-Bi. In addition, the normal conductor material has been described using the example of Si,
Similar effects were obtained by using Al, Ge, and Cu.

〔The invention's effect〕

As described above, an element having the first element structure according to the present invention, that is, a weakly-coupled element in which two superconductor electrodes are separated at a step and the two superconductor electrodes are connected by a normal conductor disposed at the step. In the case of the Josephson device, the step which becomes the bond length can be controlled by a thin film deposition method such as a vacuum evaporation method or an etching method which is excellent in both dimensional controllability and reproducibility. Therefore, set the dimensional accuracy of the bond length to ±
The reproducibility can be reduced to 1% or less, the reproducibility can be reduced to ± 2% or less, and the Josephson junction current distributed by ± 10% or more in the conventional device can be reduced to ± 5%.
In the following, the variation of the junction current between the prototype lots can be reduced from about ± 30% to ± 10% or less of the prior art, and the uniformity and reproducibility of the device characteristics can be improved.

According to the second element structure of the present invention, the control of the film thickness of the normal conductor, which is the coupling length of the weakly-coupled Josephson element, is performed by a thin film deposition method such as a vacuum evaporation method which is excellent in both dimensional control and reproducibility. Can be performed. Therefore, the dimensional accuracy of the bond length,
The reproducibility can be greatly improved, and the dependent Josephson junction current varies from ± 10% or more on the substrate by the conventional method to ± 5% or more, and also varies by about ± 30% between prototype lots. Can be reduced to ± 10% or less, and the uniformity and reproducibility of element characteristics can be improved.

[Brief description of the drawings]

1 to 5 are views showing a device fabrication process in each embodiment of the present invention, FIG. 6 is a diagram showing a conventional device structure, and FIGS. 7 and 8 are devices in an embodiment of the present invention. It is a figure showing a manufacturing process. 11,21,21,41,51 …… Si substrate, 12,32,52 …… Step material, 13,2
3,33,43 …… Photoresist, 14,24,34,44,54,64 …… Normal conductor, 15,16,25,26,35,36,45,46,55,56,65,66 …… Superconductor electrode, 17,67 …… Coupling length, 48,58 …… Control electrode, 4
9,59 ... insulating film, 60 ... interlayer insulating film, 111,121 ... substrate, 112,116,122,126,132,136 ... superconductor electrode, 113,1
23,133 …… Normal conductor, 115,125 …… Insulator layer, 114,124,1
28 Photoresist, 127, 129 Insulator, 130 Superconducting extraction electrode.

Claims (1)

(57) [Claims] 1. A substrate, an insulating film formed on a part of the substrate, a first superconductor electrode formed on the substrate adjacent to the insulating film, and a first superconducting electrode formed on the insulating film. A second superconductor electrode formed, a normal conductor layer formed from the first superconductor electrode to the second superconductor electrode, and an insulating film formed on the normal conductor layer via an insulating film. And the first superconductor electrode and the second superconductor electrode are separated by a step formed by the insulating film, and the normal conductor layer is weak at the step portion. A weak-coupling type Josephson device, wherein a coupling portion is formed, and the control electrode is formed so as to apply an electric field to a weak-coupling portion of the normal conductor layer.