JPH01109780A - Formation of tunnel junction of oxide superconductor and nb superconductor - Google Patents

Abstract

PURPOSE:To improve superconductivity of the surface of an underlay electrode by arranging, after forming a pair of counter electrodes by a Nb superconductive thin film, an insulating layer on such underlying electrode and providing a wiring layer so that the underlay electrode may superconductively be in conduction with the overlay electrode. CONSTITUTION:An underlay electrode 2 composed of an oxide superconductive material is formed on a substrate 1, then a layer 3 to become a tunnel barrier is formed by covering the pattern of such underlying electrode 2. Then a Nb superconductive thin film 4 is formed by covering the layer 3 to become the tunnel barrier, and a pair of counter electrodes 4A being formed by removing by reactive ion etching a part other than the tunnel junction in each of Nb superconductive thin film 4 and the layer 3 to become the tunnel barrier. Subsequently, after forming an insulating layer 6 on the underlay electrode 2, a wiring layer 7 which will superconductively be in conduction with the overlay electrode of the counter electrodes 4A. According to the constitution, there is no resist pattern on the underlying electrode before forming the tunnel barrier, whereby no thermal treatment in an oxygen atmosphere imposes problem when patterning the counter electrodes, and superconductivity of the surface of the underlying electrode can be improved.

Description

[Detailed description of the invention] [Industrial application field] The invention is based on the use of an oxide superconductor in the base electrode and Nb in the counter electrode.
This invention relates to a method for forming superconducting tunnel junctions using superconductors.

[Prior Art] Studies have been underway on methods for fabricating Josephson junctions using high Tc (transmuting temperature) oxide superconductors as electrode materials, but no satisfactory results have been obtained so far. do not have. Conventional Josephson junctions using oxide superconductors have only been of the point contact type using bulk materials or the bridge type using grain boundary effects in thin films.

Although bridge-type Josephson junctions can be applied to magnetometers and submillimeter wave detectors, the characteristics of bridge-type Josephson junctions formed by conventional techniques cannot be controlled at all. Moreover, since the distribution of the Josephson current is extremely non-uniform, even if the value of the Josephson current could be controlled, the operation would be unstable due to external noise.

In order to control the Josephson current value and to ensure stable operation against external noise, it is essential to create a tunnel-type Josephson junction. When a high Tc oxide superconductor is used as the base M electrode, the counter electrode is made of a pb gold alloy or Nb, Nb in order to reduce leakage current below the gap voltage.
N and Nb based A15 compounds are used. This is because a high-quality thin film can be formed on the ultra-thin tunnel barrier while suppressing interdiffusion with the tunnel barrier while the substrate is cooled. However, since the PB gold alloy has low mechanical strength and insufficient reliability, Nb, NbN or Nb
A system A15 compound is used.

According to the prior art, Nb, a Nb-based A15 compound, or a NbN thin film formed on a water-cooled substrate is patterned using a resist stencil or by a foot-off process. Problems with patterning the counter electrode by lift-off processing include the following. The resist stencil is usually formed using AZ 1350J or AX1470 photoresist, but the heat resistance temperature of these resists is about 130° C. at most.

In addition, the resist stencil is formed immediately after buttering the base electrode, then the surface of the base electrode is cleaned, a tunnel barrier is formed, and finally a counter electrode thin film is formed, followed by lift-off. It will be held in Static purification of the surface of the base electrode may require heating in an atmospheric gas containing oxygen, and the temperature is 3.
It is carried out at 00-500°C. However, if a resist stencil is present, this heat treatment will not be possible and the surface of the underlying electrode will not be sufficiently cleaned.

In order to avoid hardening of the resist due to heating or ashing due to oxygen, if the counter electrode is patterned by chemical etching using an acidic solution, the following problem arises.

High Tc oxide superconductors are easily corroded even by very dilute acids and have no etching selectivity with respect to counter electrode Nb, Nb-based A15 compounds or NbN. When etching the counter electrode,
A portion of the base electrode is exposed to the etching solution and is lost, making it impossible to obtain the desired bonding pattern. For this reason, patterning of the counter electrode by chemical etching cannot be applied.

[Problems to be Solved by the Invention] As described above, it is difficult to fabricate a tunnel-type Josephson junction using a high Tc oxide superconductor as a base electrode using the conventional techniques.

The present invention was made to improve the above-mentioned drawbacks, and aims to provide a method that can form a good tunnel junction without damaging the oxide superconductor.

[Means for Solving the Problems] In order to achieve such an object, the present invention includes a step of forming a base electrode pattern made of an oxide superconductor on a substrate, and a process of forming a tunnel over the base electrode pattern. a step of forming a layer to serve as a barrier; a step of forming an Nb-based superconductor thin film to cover the layer to serve as a tunnel barrier and serve as a counter electrode;
A step of forming a counter electrode by removing the portions of the Nb-based superconductor thin film and the tunnel barrier layer other than the tunnel junction by reactive ion etching, and a step of forming an insulating layer on the base electrode. and forming a wiring layer superconductingly connected to the upper surface of the counter electrode.

[Function] The manufacturing process of the Josephson junction according to the present invention is shown in FIG. As shown in FIG. 1(a), a base electrode pattern 2 is formed on a substrate l. The base electrode 2 is (M1+-xM
2x)ycuoz (0<X <1.1≦Y≦2.2≦
2≦4. Ml is a group metal (Sc, Y, La, Ce, Pr
, Nd, Pm, Yb.

LLl, B, AJZ, Ga, In, Tj2), M2
is a group II metal (Be, Mg, 11:a.

It is a high TC oxide superconducting material made of one selected from Sr, Ba, Zn, (:d, l1g)). Next, as shown in FIG. 1(b), a layer 3 serving as a tunnel barrier is formed to cover the underlying electrode pattern. tunnel barrier 3
The base electrode surface is artificially treated with a metamorphic layer and Al1.
203. Different materials such as MgO and ZrO2 are used. After forming the tunnel barrier, a Nb-based superconductor thin film 4 is formed on the entire surface of the substrate as shown in FIG. 1(c).Next, a junction resist pattern 5 is formed as shown in FIG. 1(d). Form. After that, as shown in FIG. 1(e), using the resist 5 as a mask, the Nb-based superconductor thin film is coated with CF4+O□.
Reactive ion etching is performed using mixed gas plasma to form 73 opposing electrodes 4A and tunnel barrier 3A.

At this time, since the base electrode material is not etched at all by CF4 + O2, a pattern can be obtained by selectively etching only the Nb-based superconductor and the layer that will serve as the tunnel barrier.

After patterning the counter electrode thin film, the resist 5 is used as a lift-off stencil without removing it as shown in FIG. Pattern 6. Finally, as shown in FIG. 1(g), the wiring layer 7
is formed to complete the fabrication of the Josephson junction.

According to the manufacturing method of the present invention, since there is no resist pattern on the surface of the base electrode before the tunnel barrier is formed, heating in an oxygen atmosphere or plasma Even if the treatment is performed, there is no problem in patterning the counter electrode. Furthermore, since it is based on reactive ion etching, unlike etching with acid, it does not damage the underlying electrode. Due to the above effects, the present invention can provide a tunnel-type Josephson junction in which the superconducting properties of the surface of the underlying electrode directly under the tunnel barrier are improved.

[Example] Specific examples of the present invention will be described below.

Example 1 A tunnel junction using a high TC oxide superconductor as an electrode was fabricated on a sapphire substrate.

That is, on the sapphire substrate, La-Ba-Cu-0
Using a target (100mmφ, 5mm thickness),
2CUO for the base electrode (Lao9Ba6.
A tetraoxide superconducting thin film was formed at a H of 200 nII+. Next, a base electrode pattern was formed through resist coating, exposure, development, and chemical etching with dilute nitric acid.

As a pretreatment for tunnel barrier formation, the base electrode surface is
．． 8 atm Ar + 10 Vol [4 in 02 mixed scum
By heating to 00° C. and annealing for 30 minutes, the deterioration of the surface of the underlying electrode was recovered. Next, the tunnel barrier was formed by replacing the atmospheric gas with 0.2 atm of ^r+5 2 and leaving it at 400° C. for 15 minutes to form an extremely thin layer on the surface of the base electrode.

Next, a counter electrode Nb thin film was formed on the entire surface of the substrate to a thickness of 300 r+m by DC magnetron sputtering (using a target with a diameter of 100 mm and a thickness of 5 mm, Ar gas pressure of 2 Pa, sputtering current of 1.2 A). Thereafter, a resist pattern for the bonding portion was formed, and reactive ion etching was performed using a parallel plate dry etching device so that only the counter electrode thin film at the bonding portion remained. 1-) Ching conditions are CF4+ 5 zero 021QPa, RF power 50
It is W. Mg was used as a stencil to lift off the insulating layer while leaving the resist pattern at the junction intact.
The O thin film was deposited at 400 nm by RF magnetron sputtering.
The insulating layer was patterned by dissolving the resist in acetone.

Finally, a 500 nm thick Nb wiring layer was formed to complete the fabrication of the junction. The characteristics of this Josephson junction in No. 5 were good as shown below. Figure 2 shows the current-voltage characteristics.

Gap voltage 8.5mV, R(at 5mV)/R(a
t lomV) -14°Ij umbrella TI (at lomV)
-5.2 mVIj is the maximum Josephson current.

Performance example 2 Prepare an Mg0 substrate and use it as an M-5r-Cu-0 target (M: Sc, Y, La,
II: e, Pr, Nd, Pm, Yb.

Lu, B, AI! , , Ga, In, Tl1),
By f-magnetron sputtering method, (Mo, 7Sr
o, 3) 13cu03 (M:Sc, Y.

La, Ce, Pr, Nd, Pm, Yb, Lu, B, AJ
2, Ga, In, Tl) oxide superconducting thin film
It was deposited to a thickness of 100 nm (Ar gas pressure of 10 Pa, RF power of 500 W), and the pattern of the base electrode was formed by chemical etching in the same manner as in Example 1. Next, as a pretreatment for tunnel barrier formation, in 02 gas at 2600 Pa,
The substrate was annealed at 350°C for 30 minutes, evacuated to vacuum again, and an ultra-thin Zr film was formed to a thickness of Znm by DC magnetron sputtering (using a target with a diameter of 75 mm and a thickness of 3 mm, Ar gas pressure of 4 Pa, sputtering current 0.1
A) Oxidation in oxygen at 50 Pa to form a zrO2 tunnel barrier, then counter electrode Nb3Ge,
Nb5AJ2, Nb3Sn, and Nb3Ga thin films were each formed to a thickness of 300 nm (diameter 100 mm, 5
Use mm-thick alloy target, substrate temperature 200℃, A
(Gas pressure: 5 Pa, sputtering current: 0.8 A). After that, a resist pattern for the bonding area is formed, and using a parallel plate type tri-etching device, reactive ions are etched so as to leave only the thin film of the counter electrode at the bonding area. Etched. Etching conditions were CF, + + 5!j; 0210Pa,
The rf power is 50W. As a stencil for lifting off the insulating layer while leaving the resist pattern at the junction intact, MgO thin film was formed to a thickness of 400 nm by RF magnetron sputtering (75 mm in diameter, 3 mm in diameter).
Use thick target, Ar gas pressure 4 Pa, RF power 3
00W) L, the resist was dissolved in acetone and the insulating layer was patterned.

Finally, a wiring layer of Nb3Sn with a thickness of 500 nm was formed to complete the fabrication of the junction. Table 1 shows the 10K of these joints.
The characteristics of

Table-1 Tunnel junction characteristics TJ2 when changing Group 1II metals
12 15 8 Nb3Ge Here, Rj is the resistance value at a voltage 8 mV lower than the gap voltage, and fin is the resistance value at a voltage 5 mV higher than the gap voltage.

Example 3 A Y-M-Cu-0 target (M: Be, Mg, Ca,
Sr, Ba, Zn, Cd.

11g) by RF magnetron sputtering method,
(Yo, Mo3)13CuO, (M: Be, Mg, Ca
, Sr, Ba, Zn, Cd.

11g) Deposit oxide superconducting R film to a thickness of 300 nm (
(Ar gas pressure: 6 Pa, RF power: 400 W) The underlying electrode was patterned through the steps of resist coating, exposure, development, and etching. Subsequently, as a pretreatment for tunnel barrier formation, in an oxygen atmosphere of 1300 Pa, 5
Annealing was performed for 60ml at a temperature of 00°C. After evacuation again, an extremely thin film of AfL was formed to a thickness of 3 nm by DC magnetron sputtering, and thermally oxidized in oxygen at 50 Pa to form an Au203 tunnel barrier.

Next, a counter electrode NbN thin film was formed on the entire surface of the substrate to a thickness of 300 nm by DC magnetron sputtering (using a target with a diameter of 100 mm and a thickness of 5 mm, Ar+
30 zero N2 gas pressure 2 Pa, sputtering current 1.28)
L/ta. Thereafter, a resist pattern for the bonding portion was formed, and reactive ion etching was performed using a parallel plane dry etching device so that only the counter electrode thin film at the bonding portion remained. Etching conditions are CF4+ 5%'0210
Pa.

The power was 50 W. A thin MgO film was formed to a thickness of 400 nIn by magnetron sputtering as a stencil for lifting off the insulating layer while leaving the resist pattern at the junction intact (diameter 75 nm, 3
A mm thick target was used, Ar gas pressure was 4 Pa, and RF power was 300 W.) The resist was dissolved in acetone and the insulating layer was patterned.

Finally, a 500 m thick NbN wiring layer was formed to complete the fabrication of the junction. Table 2 shows the characteristics of 10 of these joints. As is clear from Table 2, these joints exhibited good characteristics.

Table 2 Characteristics of tunnel junction when changing Group II metal Group II metal Gap voltage (mV) 117 n I j
(mA) [Effects of the Invention] As explained above, according to the present invention, since there is no resist pattern on the surface of the base electrode before the tunnel barrier is formed, Even if heating or plasma treatment is performed in an oxygen atmosphere, there is no problem in patterning the counter electrode. Furthermore, since it is based on reactive ion etching, unlike etching with acid, it does not damage the underlying electrode. Due to the above effects, the present invention can provide a tunnel-type Josephson junction in which the superconducting properties of the surface of the underlying electrode directly under the tunnel barrier are improved.

[Brief explanation of the drawing]

FIG. 1 is a process diagram showing the procedure for forming a tunnel junction according to the present invention, and FIG. 2 is a characteristic diagram showing an example of current/voltage characteristics of a Josephson tunnel junction. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Base electrode of Josephson tunnel junction, 3... Tunnel barrier, 4... Counter electrode of Josephson tunnel junction, 5... Resist pattern of junction part, 6... - Insulating layer thin film, 7... Wiring layer, Ij... Maximum Josephson current, Vg... Gap voltage, Rj... Subgap resistance, Rn... Normal conduction tunnel resistance. 1 (flowing) Ji:? [Death Fusontonne] Shirei's electric current: L = Voltage characteristic diagram Figure 2

Claims (2)

[Claims] (1) A step of forming a base electrode pattern made of an oxide superconductor on a substrate, a step of forming a layer to serve as a tunnel barrier covering the base electrode pattern, and a step facing the layer covering the layer to serve as a tunnel barrier. Nb as an electrode
forming a Nb-based superconductor thin film, and removing portions of the Nb-based superconductor thin film and the tunnel barrier layer other than the tunnel junction by reactive ion etching to form a counter electrode. An oxide superconductor comprising the steps of: forming an insulating layer on the base electrode; and forming a wiring layer superconductingly connected to the upper surface of the counter electrode. Method for forming Nb-based superconductor tunnel junction. (2) The oxide superconductor is (M1_1_−_XM2_
X)_YCuO_Z(0<X<1, 1≦Y≦2, 2≦Z
≦4, M1 is a group III metal (Sc, Y, La, Ce, Pr
, Nd, Pm, Yb, Lu, B, Al, Ga, In, T
l), M2 is a group II metal (Be, Mg, Ca, Sr, Ba
, Zn, Cd, Hg)),
Nb-based superconductors include Nb, Nb_3Ge, Nb_3Al,
The method for forming an oxide superconductor/Nb-based superconductor tunnel junction according to claim 1, wherein the tunnel junction is one selected from Nb_3Sn, Nb_3Ga, and NbN.