JPH077189A - Josephson junction element - Google Patents

Abstract

PURPOSE:To achieve voltage/current characteristics in which the leak current is suppressed by forming an aluminium film on a metal film, containing or not containing nitrogen, located between the substrate and the lower electrode of a Josephson junction element. CONSTITUTION:Niobium and aluminium are deposited continuously on a silicon substrate 1 by DC magnetron sputtering. A resist pattern for machining the lower electrode is formed on a three layer film. Micromachining is then conducted in the order of upper niobium film, aluminium oxide, and lower niobium film to form a lower electrode 5 and a tunnel barrier layer 6 followed by formation of an upper electrode 7 through machining of the upper niobium film. The surface of a ground plane 2 is made smooth by means of a coating film 3. A Josephson junction element 4 having such structure exhibits the voltage/current characteristics of a tunnel type Josephson junction element having low leak current in the liquid helium of 4.2 deg.K.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting device, and more particularly to a structure of a Josephson junction device suitable for obtaining a voltage-current characteristic with a small leak current.

[0002]

2. Description of the Related Art Conventionally, regarding the structure of a Josephson junction element, IEE Transactions on Magnetics, Vol. 25, No. 2, 1989.
March, 1131 to 1134 (IEEE Transaction
s on Magnetics, Vol.25, No.2, March, 1989, pp.1131
-1134), the lower electrode was formed on the SiO ₂ film.

[0003]

Generally, the tunnel barrier layer of the Josephson junction element is an ultrathin film having a thickness of several nm. Therefore, in order to manufacture an element with a small leak current, it is necessary that the surface of the lower electrode is smooth when the tunnel barrier layer is formed on the lower electrode. In the conventional technique, the underlying material of the lower electrode of the Josephson junction is Si.
It is an O ₂ film. Case in which the ground plane under the SiO ₂ film, since what is sandwiched between the ground plane and the lower electrode is only the SiO ₂ film, the surface roughness of the ground plane is reflected on the surface of SiO _2, Further, the surface roughness of SiO ₂ is reflected on the surface of the lower electrode film. Therefore, when the surface of the ground plane is rough and rough, the surface of the lower electrode is also rough and rough. For example, an aluminum oxide film is formed on the Nb lower electrode to form a tunnel barrier layer, and an Nb upper electrode is formed to form Nb.
In the case of making a / AlOx (aluminum oxide) / Nb junction, if the surface of the Nb lower electrode is rough, the aluminum oxide film cannot uniformly cover the lower electrode surface and the tunnel barrier layer cannot be formed uniformly. This causes a leak current in the voltage-current characteristics.

Further, when the Nb / AlOx / Nb Josephson junction is formed directly on the molybdenum resistance film or the molybdenum nitride resistance film, when the molybdenum resistance film or the molybdenum nitride resistance film has a rough surface, Similarly, the surface of the lower electrode is also roughened and roughened. Therefore, the tunnel barrier layer cannot be formed uniformly, which causes a leak current.

An object of the present invention is to provide a Josephson junction device,
Particularly, in a device having a metal film having a rough surface or a metal film containing nitrogen between a substrate and a lower electrode, a structure of a Josephson junction device suitable for obtaining a voltage-current characteristic with a small leak current is provided. Especially.

[0006]

The above object can be achieved by forming an aluminum film on a metal film or a metal film containing nitrogen between a substrate and a lower electrode of a Josephson junction element.

[0007]

The aluminum film is formed on the metal film or the metal film containing nitrogen between the substrate and the lower electrode of the Josephson junction element. For example, an aluminum film is formed over a niobium film, a molybdenum film, or a molybdenum nitride film.

FIG. 4 shows the dependence of the centerline average roughness of the Al surface in the Al / Nb bilayer film on the centerline average roughness of the Nb surface. FIG. 5 shows the dependence of the centerline average roughness of the Al surface in the Al / Mo bilayer film on the centerline average roughness of the Mo surface, and FIG. 6 shows the center of the Al surface in the Al / MoNx bilayer film. The dependence of the line average roughness on the center line average roughness of the MoNx surface was shown.

The film thickness of Al was all 10 nm. The centerline average roughness is measured by an atomic force microscope. N
When the center line average roughness of the surface of b, Mo or MoNx is 3 to 5 nm, the surface smoothness is increased by coating Al with 10 nm. When the center line average roughness of the surface of the niobium film, the molybdenum film, or the molybdenum nitride film between the substrate and the lower electrode is 3 to 5 nm, the surface of the aluminum film is It becomes smoother than the surface of the lower niobium film, molybdenum film, or molybdenum nitride film.

If the surface of the niobium film, the molybdenum film, or the molybdenum nitride film between the substrate and the lower electrode becomes smooth, the surface of the lower electrode, which reflects the smoothness of the underlying layer, also increases in smoothness. By increasing the smoothness of the lower electrode surface, the uniformity of the barrier layer is increased, and it becomes easy to manufacture a junction with a small leak current.

[0011]

1 is a sectional view showing a part of a Josephson junction element according to a first embodiment of the present invention. 300 nm of niobium and 10 nm of aluminum are continuously deposited on the silicon substrate 1 in the same vacuum by a DC magnetron sputtering method using argon gas at a pressure of 1.3 Pa.
The ground plane 2 and the smoothing coating film 3 are processed by the ion milling method using argon gas. After depositing SiO ₂ to a thickness of 300 nm by a high frequency magnetron sputtering method using a pressure of 1.3 Pa of argon gas, it is processed by a reactive ion etching method using a CHF ₃ gas to form a first interlayer insulating film 4.

Next, a lower niobium film (film thickness 200 nm),
An aluminum oxide film (film thickness 1 nm) and an upper niobium film (film thickness 100 nm) are successively deposited in this order without breaking the vacuum to form a three-layer film. The lower niobium film and the upper niobium film are formed by a DC magnetron sputtering method using argon gas at a pressure of 1.3 Pa. The aluminum oxide film is formed by depositing aluminum by a direct current magnetron sputtering method using an argon gas at a pressure of 1.3 Pa and then oxidizing it with oxygen at a pressure of 9.3 Pa for 10 minutes. A resist pattern for processing the lower electrode is formed on the three-layer film. Ion milling method using argon gas and C
By the reactive ion etching method using F ₄ gas,
The upper layer niobium film, the aluminum oxide film, and the lower layer niobium film are processed in this order to form the lower electrode 5 and the tunnel barrier layer 6. A resist pattern for defining the upper electrode is formed on the upper niobium film, and the upper niobium film is processed by the reactive ion etching method using CF ₄ gas to form the upper electrode 7.

Next, after depositing 400 nm of SiO ₂ by a high frequency magnetron sputtering method using an argon gas at a pressure of 1.3 Pa, it is processed by a reactive ion etching method using a CHF ₃ gas to form a second interlayer insulating film. 8
And

Finally, the amount of niobium was changed to 5 by the direct current magnetron sputtering method using an argon gas having a pressure of 1.3 Pa.
After being deposited to a thickness of 00 nm, it is processed by a reactive ion etching method using CF ₄ gas to form a superconducting wiring 9.

The Josephson junction device according to the first embodiment of the present invention can be realized by the above steps.
The center line average roughness of the surfaces of the ground plane 2 and the smoothing coating film 3 in the device of this example is about 3.0, respectively.
nm and about 2.2 nm, and the surface of the ground plane 2 is smoothed by the smoothing coating film 3.

The Josephson junction element shows the voltage-current characteristic of a typical tunnel type Josephson junction element with a small leak current as shown in FIG. 7 in liquid helium of 4.2K, and is higher than the gap voltage. The ratio of the resistance value (R _j ) at the voltage below the gap to the resistance value (R _nn ) at the voltage, R _j / R _nn value, is 12 to 14.
In contrast to this embodiment, the R _j / R _nn value of the element in which the smoothing coating film 3 in FIG. 1 is not used is 6 to 8, which shows a characteristic with a large leak current.

FIG. 2 is a sectional view showing a part of a Josephson junction element according to the second embodiment of the present invention. 300 nm of niobium and 10 nm of aluminum are continuously deposited on the silicon substrate 101 in the same vacuum by the direct current magnetron sputtering method using argon gas at a pressure of 1.3 Pa. The deposited niobium film and aluminum film are processed by an ion milling method using argon gas to form the ground plane 102 and the smoothing coating film 103. After depositing 300 nm of SiO ₂ by a high frequency magnetron sputtering method using a pressure of 1.3 Pa of argon gas, it is processed by a reactive ion etching method using a mixed gas of CHF ₃ and O ₂ to form a first interlayer insulating film. 104.

Next, after sputter cleaning the surface of the smoothing coating film 103 with plasma of argon gas, a lower niobium film (film thickness 200 nm), an aluminum oxide film (film thickness 1 nm), an upper niobium film (film thickness 100 nm). In this order, a three-layer film is formed by successively depositing without breaking the vacuum.
The lower niobium film and the upper niobium film are formed by a DC magnetron sputtering method using argon gas at a pressure of 1.3 Pa. The aluminum oxide film is formed by depositing aluminum by a direct current magnetron sputtering method using an argon gas at a pressure of 1.3 Pa and then oxidizing it with oxygen at a pressure of 9.3 Pa for 10 minutes. A resist pattern for processing the lower electrode is formed on the three-layer film. By the reactive ion etching method using CF ₄ gas and the ion milling method using argon gas, the upper layer niobium film, the aluminum oxide film, and the lower layer niobium film are processed in this order, and the lower electrode 105
And a tunnel barrier layer 106 is formed. A resist pattern for defining the upper electrode is formed on the upper niobium film, and CF ₄
The upper niobium film is processed by the reactive ion etching method using gas to form the upper electrode 107.

Next, after depositing 400 nm of SiO ₂ by a high frequency magnetron sputtering method using argon gas at a pressure of 1.3 Pa, it is processed by a reactive ion etching method using CHF ₃ gas to form a second interlayer insulating film. 1
08.

Finally, the amount of niobium was changed to 5 by the DC magnetron sputtering method using an argon gas having a pressure of 1.3 Pa.
After being deposited to a thickness of 00 nm, it is processed by a reactive ion etching method using CF ₄ gas to form a superconducting wiring 109. By this step, the Josephson junction device according to the second embodiment of the present invention can be realized. The center line average roughness of the surfaces of the ground plane 102 and the smoothing coating film 103 in the device of this example is about 3.0 n, respectively.
m and about 2.2 nm, and the surface of the ground plane 102 is smoothed by the smoothing coating film 103.

The Josephson junction element exhibits the voltage-current characteristics of a typical tunnel type Josephson junction element with a small leak current, similar to that shown in FIG. 7, in liquid helium at 4.2K. R _j / R _nn value 12-16 is obtained. Compared with the above-mentioned embodiment, the R _j / R _nn value of the element without the smoothing coating film 103 in FIG.
It exhibits a characteristic of large leakage current.

FIG. 3 is a sectional view showing a part of a Josephson junction element according to the third embodiment of the present invention. Niobium is deposited on a silicon substrate 201 by direct current magnetron sputtering using an argon gas at a pressure of 1.3 Pa.
After being deposited to a thickness of nm, the ground plane 202 is processed by a reactive ion etching method using CF ₄ gas. After depositing SiO ₂ to a thickness of 300 nm by a high frequency magnetron sputtering method using an argon gas at a pressure of 1.3 Pa, it is processed by a reactive ion etching method using a CHF ₃ gas to form a first interlayer insulating film 203.

Next, a molybdenum nitride film is deposited to 100 nm. The molybdenum nitride film is formed by a direct current magnetron sputtering method using a mixed gas of argon containing 2% of nitrogen and nitrogen at a pressure of 1.3 Pa. CF molybdenum nitride film
Resistor 204 is formed by processing by a reactive ion etching method using a ₄ + O ₂ mixed gas. After depositing SiO ₂ with a thickness of 350 nm by a high frequency magnetron sputtering method using an argon gas at a pressure of 1.3 Pa, it was processed by a reactive ion etching method using a CHF ₃ gas to form a second interlayer insulating film 205.

Next, the surface of the resistor 204 was sputter cleaned with plasma of argon gas, and then an aluminum film (film thickness 10 nm) and a lower niobium film (film thickness 200 n) were formed.
m), an aluminum oxide film (film thickness 1 nm), and an upper layer niobium film (film thickness 100 nm) are successively deposited without breaking the vacuum to form a four-layer film. The aluminum film, the lower niobium film, and the upper niobium film are formed by the DC magnetron sputtering method using argon gas at a pressure of 1.3 Pa. The aluminum oxide film is formed by depositing aluminum by a direct current magnetron sputtering method using argon gas at a pressure of 1.3 Pa, and then depositing it with oxygen at a pressure of 9.3 Pa.
It is made by oxidizing for 0 minutes. A resist pattern for processing the lower electrode is formed on the four-layer film. An upper niobium film, an aluminum oxide film, a lower niobium film, and an aluminum film are processed in this order by a reactive ion etching method using CF ₄ gas and an ion milling method using argon gas.
The smoothing coating film 206, the lower electrode 207, and the tunnel barrier layer 208 are formed. A resist pattern for defining an upper electrode is formed on the upper niobium film, and the upper niobium film is processed by a reactive ion etching method using CF ₄ gas to form an upper electrode 209.

Next, after depositing 400 nm of SiO ₂ by a high frequency magnetron sputtering method using an argon gas at a pressure of 1.3 Pa, it is processed by a reactive ion etching method using a CHF ₃ gas to form a third interlayer insulating film. Two
Set to 10.

Finally, the amount of niobium was changed to 5 by DC magnetron sputtering using an argon gas with a pressure of 1.3 Pa.
After being deposited to a thickness of 00 nm, it is processed by a reactive ion etching method using CF ₄ gas to form a superconducting wire 211. Through the above steps, the Josephson junction device according to the third embodiment of the present invention can be realized. The center line average roughnesses of the surfaces of the resistor 204 and the smoothing coating film 206 in the device of this example are about 4.0 nm and about 3.0, respectively.
It is 0 nm, and the surface of the ground plane 204 is smoothed by the smoothing coating film 206.

The Josephson junction device exhibits the voltage-current characteristics of a typical tunnel type Josephson junction device with a small leak current, similar to that shown in FIG. 7, in liquid helium at 4.2K. The values of R _j / R _nn are 11 to 16. In contrast to the above example, the R _j / R _nn value of the device in which the smoothing coating film 206 in FIG. 3 was not used was 5 to 9, showing a characteristic with a large leak current.

[0028]

According to the present invention, in a Josephson junction device having a metal film having a rough surface or a metal film containing nitrogen between a substrate and a lower electrode, a current with a small leak current-
It is possible to obtain voltage characteristics. Therefore, it becomes easy to use the Josephson junction element as an element forming the superconducting integrated circuit.

[Brief description of drawings]

FIG. 1 is a sectional view showing a part of a Josephson junction element according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a part of a Josephson junction element according to the second embodiment of the present invention.

FIG. 3 is a sectional view showing a part of a Josephson junction element according to a third embodiment of the present invention.

FIG. 4 is an explanatory diagram showing the dependence of the centerline average roughness of the Al surface in the Al / Nb bilayer film on the centerline average roughness of the Nb surface.

FIG. 5 is an explanatory diagram showing the dependence of the centerline average roughness of the Al surface in the Al / Mo bilayer film on the centerline average roughness of the Mo surface.

FIG. 6 is an explanatory diagram showing the dependence of the centerline average roughness of the Al surface in the Al / MoNx bilayer film on the centerline average roughness of the MoNx surface.

FIG. 7 is an explanatory diagram showing a voltage-current characteristic of the Josephson junction device according to the first embodiment of the present invention at a temperature of 4.2K.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Silicon substrate, 2 ... Ground plane, 3 ... Smoothing coating film, 4 ... First interlayer insulating film, 5 ... Lower electrode, 6 ... Tunnel barrier layer, 7 ... Upper electrode, 8 ... Second interlayer insulating film , 9 ...
Superconducting wiring.

Claims (4)

[Claims] 1. A substrate, a lower electrode formed above the substrate and made of a superconductor, a tunnel barrier layer formed on the lower electrode, and a tunnel barrier layer extending on the tunnel barrier layer. In a Josephson junction device having a body and an upper electrode, a laminated film sandwiched between the substrate and the lower electrode has a lower layer made of a metal film or a metal film containing nitrogen and an upper layer made of an aluminum film. A Josephson junction device including a layer film. 2. The Josephson junction device according to claim 1, wherein the metal film or the metal film containing nitrogen is a thin film selected from a niobium film, a zirconium film, a molybdenum film, and a molybdenum nitride film. 3. The Josephson junction device according to claim 1, further comprising a silicon oxide film between the aluminum film and the lower electrode. 4. The Josephson junction device according to claim 1, wherein the aluminum film is in direct contact with the lower electrode.