JPS6386486A - Josephson junction device and manufacture thereof - Google Patents

Abstract

PURPOSE:To have a gap voltage proper to NbN and reduce the sub-gap leakage current without interposing a low-Tc layer or normal layer by providing auxiliary layers made of Nb and having a film thickness not greater than the penetration depth of cooper pain between the superconducting layers made of NbN and the tunnel barrier layer. CONSTITUTION:A title device is constructed by a first superconducting layer 12 made of NbN and formed on an insulating substrate or a substrate 11 having as insulator on the surface thereof, a first auxiliary layer 13 made of Nb and formed on the first superconducting layer 12, a tunnel barrier layer 14 formed on the first auxiliary layer 13, a second auxiliary layer 15 made of Nb and formed so as to be opposed to the first auxiliary layer 13 through the tunnel barrier layer 14, and a second superconducting layer 16 made of NbN and formed on the second auxiliary layer 15. With this, the reactions between the first and second superconducting layers 12, 16 and the tunnel barrier layer 14 are prevented by the first and second auxiliary layers 13, 15. Further, as the first and second auxiliary layers 13, 15 are superconductors each having a thickness not greater than penetration depth of cooper pair (about 50Angstrom ), a junction device can be obtained which has a gap voltage proper to NbN and a small sub-gap leakage current.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a tunnel-type Josephson junction device and a method for manufacturing the same.

(Prior Art) Logic circuits and memory circuits composed of Josephson junction elements have the great advantage of consuming less power and operating at high speed compared to semiconductor circuits.

As circuits become more integrated, there is a need to develop Josephson junction devices that can take full advantage of these advantages. The main characteristics required of Josephson junction devices for high-speed operation are high gap voltage, low junction capacitance, and low sub-gap leakage current. These characteristics can be achieved by using a superconductor layer with a large energy gap, that is, a superconductor layer with a large superconducting transition temperature Tc, and a tunnel barrier layer with a small dielectric constant, and by using a superconductor layer with a large energy gap, that is, a tunnel barrier layer with a small dielectric constant, This is achieved by making the transition region exhibiting low Tc or normal properties as thin as possible. A Josephson junction element that uses an NbN film as a superconductor layer and a metal or semiconductor oxide film as a tunnel barrier layer is attracting attention as a Josephson junction element most suitable for this purpose.

In the case of NbN films, films with high Tc (
Tc-15K) can be obtained at a relatively low substrate temperature of 1000C or less, so even when depositing the superconductor layer after forming the tunnel barrier layer, thermal reaction between the superconductor layer and the tunnel barrier layer will not occur. It progresses more slowly than other high Tc superconductor films, and can be expected to have good bonding properties.

As a conventional example, there is a paper published in 1985 by Ueda et al. in the Technical Report of the Institute of Electronics and Communication Engineers, pages 2.7 to 12 of CPM85. As shown in FIG. 3, the Josephson junction device proposed here includes a first superconductor layer 32 made of NbN formed on a substrate 31, and a superconductor layer 32 formed on the first superconductor layer 32. a tunnel barrier layer 33 formed, and a second superconductor layer 34 made of NbN formed opposite to the first superconductor layer 32 with the tunnel barrier layer 33 interposed therebetween.
It is made up of. First, the tunnel barrier layer 33 has a number of 10
It is formed by depositing an aluminum film and then oxidizing its surface. As described above, the method of forming the tunnel barrier layer 33 by depositing a metal layer on the first superconductor layer 32 and then thermally oxidizing or plasma oxidizing its surface in an oxygen-containing gas atmosphere is an oxide film. This method is generally used because it provides better uniformity and controllability of the oxide film thickness within the substrate surface than the method of depositing the oxide film itself.

(Problem to be solved by the invention 9 However, in the structure of the Josephson junction element described above,
First superconductor layer 32 made of NbN and tunnel barrier layer 3
The A1 films 3 and 3 are in direct contact with each other. Therefore, a reaction occurs in which AI, which has a low nitriding energy, takes N atoms from NbN, and a low Tc layer or a normal layer containing fewer N atoms than the stoichiometric composition is formed on the surface of the first superconductor layer 32. .

In this way, the transition region between the first superconductor layer 32 and the tunnel barrier layer 33 widens, resulting in problems such as a decrease in the gap voltage of the Josephson junction element and an increase in sub-gap leakage current. occurs. Also,
Also between the tunnel barrier layer 33 and the second superconductor layer 34, depending on the NbN deposition conditions, the same nitriding reaction of AI as described above occurs, deteriorating the junction characteristics. Many of the metals applied to the tunnel barrier layer for the same purpose as in the conventional example described above have a nitriding energy smaller than the value of Nb, and therefore cause the same problem as Al.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Josephson junction device and a method for manufacturing the same that eliminates such conventional drawbacks.

(Means for Solving the Problems) According to the present invention, the first
a superconductor layer, a tunnel barrier layer formed on this first superconductor layer, and NbN formed opposite to the first superconductor layer via this tunnel barrier layer. Second
In the Josephson junction device having a superconductor layer as a basic component, there is a junction between the first superconductor layer and the tunnel barrier layer or between the tunnel barrier layer and the second superconductor layer. A Josephson junction element is obtained, which is characterized in that an auxiliary layer is interposed in at least one of the corners, the depth of which the Cooper pair penetrates, and the thickness of the film below.

Furthermore, according to the present invention, NbN is deposited on the substrate by sputtering an Nb target in a mixed gas atmosphere of argon and nitrogen.
After removing the nitrogen-excess layer on the surface of the first superconductor layer, depositing a first superconductor layer on the surface of the first superconductor layer; oxidizing, nitriding or fluoridating the metal or semiconductor layer to form a tunnel barrier layer; and depositing the first superconductor layer on the tunnel barrier layer. A method for manufacturing a Josephson junction element is obtained, which is characterized by including the step of forming a second superconductor layer made of NbN by a method similar to the above.

(Function) In the structure according to the present invention, between the first superconductor layer made of NbN and the tunnel barrier layer, or between the tunnel barrier layer and the NbN
An auxiliary layer made of Nb and having a thickness equal to or less than the penetration depth of the Cooper pair is provided at least on one side between the superconductor layer and the second superconductor layer made of N. Therefore,.

Even if the formation energy of nitriding per nitrogen atom is lower in the metal or semiconductor that is a constituent atom of the tunnel barrier layer than in the corner, the metal or semiconductor will take away N atoms from the first and second superconductor layers. Reactions are prevented by the auxiliary layer. Furthermore, since Nb, which is a constituent atom of the first and second superconductor layers, is used in the auxiliary layer, the reaction between them is difficult to proceed. Furthermore, since the auxiliary layer is a superconductor with a film thickness that is less than the penetration depth of Cooper pairs, the superconducting properties of NbN, which is the material of the first and second superconductor layers, are maintained due to the proximity effect. Moreover, a Josephson junction element with small sub-gap leakage current can be formed.

After depositing the metal or semiconductor on the first superconductor layer,
A tunnel barrier layer formed by a chemical reaction such as oxidation, nitridation, or fluorination has excellent film thickness uniformity and controllability within the substrate surface. In addition, the production energy per atom of oxygen, nitrogen, and fluorine in the above chemical reaction is Nb
The reaction between the auxiliary layer and the tunnel barrier layer can be significantly reduced by using a metal or semiconductor compound that has a smaller reaction value than the same reaction value for the auxiliary layer.

Furthermore, in the manufacturing method according to the present invention, the first superconductor layer is formed by sputtering a medium target in an argon-nitrogen atmosphere.
After depositing the bN film, the nitrogen-rich surface layer is removed. Although the sputtering method is excellent in forming a high Tc NbN film, nitrogen is adsorbed on the NbN film surface immediately after sputtering, and a low Tc layer with excess nitrogen is likely to be formed. By removing this layer, the transition region between the first superconductor layer and the auxiliary layer can therefore be thinned. As a result, N
A Josephson junction element having a gap voltage inherent to bN and a small sub-gap leakage current can be obtained (Example) An example of a Josephson junction element of the present invention will be described in detail with reference to the drawings.

This Josephson junction element, as shown in FIG.
a first auxiliary layer 13 made of Nb formed on the superconductor layer 12; a tunnel barrier layer 14 formed on the first auxiliary layer 13;
A second auxiliary layer 15 made of Nb formed opposite to the auxiliary layer 13 of
The second superconductor layer 16 is made of N. The first and second superconductor layers 12 and 16 are formed by sputtering a Nb target in an argon-nitrogen atmosphere and have a thickness of 20 mm.
It is a NbN film with 0OA. The Tc of this film is 15-16. The first and second auxiliary layers 13 and 15 are sputtered films with a thickness of approximately 50 mm. The tunnel barrier layer 14 is an Al2O3 film formed by first sputtering an Al film several tens of times thick and then thermally oxidizing its surface in a pure oxygen atmosphere.

According to this embodiment, the first and second superconductor layers 12°16
Since the first and second auxiliary layers 13.15 are provided between the first and second superconductor layers 12.16 and the tunnel barrier layer 14, the reaction between the first and second superconductor layers 12.16 and the tunnel barrier layer 14 is as follows. 1,
This is prevented by the second auxiliary layer 13.15. Furthermore, since Nb, which is a constituent atom of the first and second superconductor layers, is used in the first and second auxiliary layers 13.15, the reaction between them is difficult to proceed. Furthermore, since the first and second auxiliary layers are superconductors with a depth below the Cooper pair penetration depth (approximately 50 layers), a Josephson junction element with a gap voltage inherent to NbN and a small sub-gap leakage current can be obtained. can get.

The AI used to form the tunnel barrier layer 14 in this example has a lower oxidation energy than Nb, and even after the oxide film is formed, it remains between the first and second superconductor layers 12.16 and the tunnel barrier layer. No reaction with 14 occurs. Also, AI is Nb
It has good wettability and a homogeneous oxide film can be formed with good controllability through thermal oxidation.

In this example, the auxiliary layer was provided both between the first superconductor layer 12 and the tunnel barrier layer 14 and between the tunnel barrier layer 14 and the second superconductor layer 16. Either one can have a great effect depending on the conditions. In particular, when the tunnel barrier layer 14 is formed by oxidation, nitridation or fluoridation after deposition of a metal or semiconductor, an auxiliary layer may be provided between the first superconductor layer 12 and the tunnel barrier layer 14. is very effective in preventing reactions between the two. moreover,
In this example, an AI oxide film formed by oxidizing after depositing A1 was used for the tunnel barrier layer 14, but insulating films formed by oxidizing, nitriding or fluoriding other metals or semiconductors, A similar effect can be obtained with a deposited insulating film or semiconductor film. However, if the metal or semiconductor atoms constituting the tunnel barrier layer 14 have a smaller generation energy for each of the above reactions than Nb, the tunnel barrier R14 will be more stable.

Next, an embodiment of the method for manufacturing a Josephson junction element of the present invention will be described in detail with reference to the drawings.

First, as shown in FIG. 2(a), an Nb target is sputtered onto an insulating substrate or a substrate 21 having an insulating material on its surface in an argon-nitrogen atmosphere by high-frequency magnetron sputtering to give a film thickness of 2,000 yen. A first superconductor layer 22 made of NbN is formed. At this time, a large amount of nitrogen is adsorbed on the surface of the first superconductor layer 22, resulting in a low Tc layer with excess nitrogen.
Layer 23 is formed. The first one by Auger electron spectroscopy
According to the depth analysis of the composition of the superconductor layer 22, the composition of the nitrogen-rich low Tc layer 23 is approximately 50% nitrogen in excess of the stoichiometric value, and the thickness of this layer is 10%. It is about the size of a person. Therefore, the nitrogen-excess layer 23 is replaced by an auxiliary layer 24' made of Nb with a thickness of about 5 OA, and a metal layer 25 made of AI with a thickness of about 100 Å.
When continuously deposited, a structure as shown in FIG. 2(b) is obtained.

Next, as shown in FIG. 2(c), the surface of the A1 layer 25 is thermally oxidized in a pure oxygen atmosphere to form a tunnel barrier layer 26. As shown in FIG. 2(d), the first superconductor layer 2
A second superconductor layer 27 made of NbN with a thickness of 2,000 yen is deposited in the same manner as in case 2 to complete the formation of the bonding layer. The feature of the method for manufacturing the present Josephson junction element is the method of forming the above-mentioned junction constituent layers, but in order to realize an actual element, it is necessary to add the following process as an example.

First, Freon 12 (CC12F2) and Freon 13 (CF)
4) is used as an etching gas to pattern the bonding constituent layer as shown in FIG. 2(d) to form the lower wiring, and then the second layer as shown in FIG. 2(e) is patterned. An etching mask 28 is formed on the superconductor layer 27 at a location where the joint will be formed, and the second superconductor layer 27 is patterned by reactive etching using CCl2F2 or CF4 to define the joint. Next, as shown in FIG. 2(f), an insulating layer 29 made of silicon monoxide (Sin) or the like is deposited with the etching mask 28 left in place. After lifting off the etching mask 28 to obtain the structure shown in FIG. 2(g), the surface of the second superconductor layer 27 is sputter-cleaned, and a second layer is formed as shown in FIG. 2(h). A third superconductor layer 30 is deposited and patterned in the same manner as superconductor layer 27 to form upper wiring.

In this embodiment, the auxiliary layer 24 is deposited after removing the nitrogen-rich layer 23 on the surface of the first superconductor layer 22 produced in the step of FIG. 2(a). Therefore, the first superconductor layer 22
The transition region exhibiting low Tc or normal properties that exists between the auxiliary layer 24 and the auxiliary layer 24 can be thinned. Further, as shown in FIG. 2(b), the metal layer 25 made of AI is
is formed on the superconductor layer 22 with an auxiliary layer 24 made of Nb interposed therebetween. Therefore, reaction between the first superconductor layer 22 and the metal or semiconductor R25 is prevented by the auxiliary layer 24. Since the AI oxide forming the tunnel barrier layer 26 is more stable than the Nb oxide as described above, no chemical reaction occurs between the auxiliary layer 24 and the tunnel barrier layer 26. moreover,
AI has good wettability to the dental membrane, and a homogeneous oxide film is formed in a controlled and comfortable manner through thermal oxidation. From the above, N
A Josephson junction element having a gap voltage inherent to bN and a small sub-gap leakage current can be obtained. In this example, an A1 oxide film formed by oxidizing after depositing AI was used for the tunnel barrier layer 14, but other metals or semiconductors could be oxidized or
Even insulating films formed by nitriding or fluoriding reactions,
If the absolute value of the production energy for each reaction of the metal or semiconductor is larger than the value for the similar reaction of Nb, results similar to those of this example can be obtained.

(Effects of the Invention) As explained above, according to the present invention, an auxiliary layer made of Nb having a film thickness equal to or less than the penetration depth of Cooper pairs is provided between the superconductor layer made of NbN and the tunnel barrier layer. As a result, a Josephson junction device with the original gap voltage of NbN and small sub-gap leakage current can be obtained without the intervening low Tc layer or normal layer generated by the reaction between the superconductor layer and the tunnel barrier layer. It will be done. In addition, by using a compound formed by oxidizing, nitriding, or fluoridating a metal or semiconductor whose reaction energy is lower than that of Nl for the tunnel barrier layer, we can improve its chemical stability, uniformity of film thickness, and A Josephson junction element with further improved controllability is obtained.

Furthermore, according to the present invention, since the auxiliary layer is formed after removing the nitrogen-excess low Tc layer on the surface of the first superconductor layer, the auxiliary layer is formed between the first superconductor layer and the auxiliary layer. A Josephson junction device with excellent characteristics without a transition region with poor superconducting characteristics can be manufactured.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a Josephson junction element of the present invention;
FIGS. 2(a) to 2(h) are cross-sectional views showing the method of manufacturing a Josephson junction element according to the present invention in order of steps, and FIG. 3 is a cross-sectional view showing a conventional Josephson junction element. In the figure, 11, 21.31 are the substrates, 12, 22.3
2 is the first superconductor layer made of NbN, xa, 24+im
a first auxiliary layer consisting of or an auxiliary layer consisting of Nb, 14,2
6.33 is a tunnel barrier layer, 15 is a second auxiliary layer consisting of a corner, 16, 27, 34 is a second superconductor layer made of NbN, 23 is a nitrogen-excess low Tc layer, 25 is a metal layer, 28 is an etching mask, 29 is an insulator layer, and 30 is a third superconductor layer.

Claims (1)

[Claims] 1. A first superconductor layer made of NbN formed on a substrate, a tunnel barrier layer formed on this first superconductor layer, and In the Josephson junction element, the basic component is a second superconductor layer made of NbN formed opposite to the first superconductor layer, wherein the first superconductor layer and the tunnel An auxiliary layer made of Nb and having a thickness equal to or less than the penetration depth of the Cooper pair is interposed between the tunnel barrier layer and the second superconductor layer or between the tunnel barrier layer and the second superconductor layer. Josephson junction element. 2. The tunnel barrier layer is a compound formed by a chemical reaction such as oxidation, nitridation, or fluorination of a metal or semiconductor, and the value of production energy per atom of oxygen, nitrogen, or fluorine in the chemical reaction is relative to Nb. The Josephson junction device according to claim 1, characterized in that the value is smaller than that of the same reaction. 3. Sputtering a Nb target in a mixed gas atmosphere of argon and nitrogen to deposit a first superconductor layer made of NbN on the substrate, and reducing excess nitrogen on the surface of the first superconductor layer. After removing the above layer, there is a step of successively depositing an auxiliary layer made of Nb and a metal or semiconductor layer with a thickness less than the penetration depth of the Cooper pair, and then oxidizing, nitriding or fluoridating this metal or semiconductor layer to form a tunnel. Joseph's method, comprising the steps of: forming a barrier layer; and forming a second superconductor layer made of NbN on the tunnel barrier layer in the same manner as the first superconductor layer. A method for manufacturing a Son junction element.