JPH0328075B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a tunnel-type Josephson device, and more particularly to a method for manufacturing a minute tunnel-type Josephson device suitable for integrated circuits.

(Prior art) As a typical conventional example, H. Kroger et al. published Applied Physics Letters in August 1981.
There is a method proposed in a paper published in vol. 39, no. 3, pages 280-282 of Letters). This method will be explained step by step using cross-sectional views of FIGS. 2a to 2c. As shown in FIG. 2a, a first superconductor electrode 22 made of niobium (Nb), a tunnel barrier layer 23, and a second superconductor electrode 24 made of Nb are disposed on a substrate 21.
A three-layer film is successively formed. First superconductor electrode 2
2 and the second superconductor electrode 24 are deposited by direct current magnetron sputtering. Tunnel barrier layer 23
is formed by depositing a silicon-hydrogen (Si-H) alloy and thermally oxidizing it. After patterning the three-layer films 22, 23, and 24 to form the lower wiring, as shown in FIG. Then, using the first and second superconductor electrodes 22 and 24 as anodes, the exposed portion of the second superconductor electrode 24 is anodized to the tunnel barrier layer to form an insulator layer 26. After removing the etching mask 25, the exposed surface of the second superconductor electrode 24 is sputter cleaned, and the third superconductor electrode 24 is deposited using the same film formation method as the first and second superconductor electrodes 22, 24. After application of superconductor electrode 27 and subsequent processing, a Josephson element as shown in FIG. 2c is obtained.

(Problems to be Solved by the Invention) In this method, in the anodic oxidation step shown in FIG.
Partially oxidized up to 4. Moreover, it is not easy to control the width of penetration of the oxide layer into the lower part of the etching mask 25 on the order of submicrons. Therefore, 1~
When manufacturing an integrated circuit that includes a large number of Josephson devices with micro junction dimensions of about 2 μm, there are problems such as not being able to obtain the target critical current value of the Josephson devices, and the uniformity of this value within the wafer. This raises the question of sufficiency.

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

(Means for Solving the Problems) According to the present invention, the step of successively forming a first superconductor electrode, a tunnel barrier layer, and a second superconductor electrode on a substrate; a step of forming an etching mask on the body electrode at a location that will become the junction, and removing the second superconductor electrode and the tunnel barrier layer by dry etching, and removing the first insulator layer while leaving the etching mask. the process of adhering;
dry etching the first insulator layer to selectively cover the sidewalls of the joint with the first insulator layer; and anodizing the exposed surface of the first superconductor electrode to form a second insulator layer. and, after removing the etching mask, forming a third superconductor electrode in electrical contact with the second superconductor electrode. A method for manufacturing a tunnel-type Josephson device is obtained.

(Function) In the present invention, first, the dimensions of the joint are defined by dry etching the second superconductor electrode and the tunnel barrier layer, and then the side walls of the joint are covered and protected with the first insulating layer. , anodizing the exposed first superconductor electrode to form a second insulator layer for electrical isolation between the first and third superconductor electrodes. Therefore, during anodic oxidation, there is no problem that the oxidation extends to the joints as in the conventional example. As a result, it becomes possible to manufacture a Josephson element that has micro-junctions with high dimensional accuracy defined by dry etching technology and has small local variations in the dimensions of these junctions.

(Example) Next, the present invention will be described in detail with reference to an example shown in cross-sectional views of FIGS. 1a to 1f.

First, as shown in FIG.
superconductor electrode 12, tunnel barrier layer 13, second
A three-layer film consisting of superconductor electrodes 14 is formed.
The first and second superconductor electrodes 12 and 14 are Nb films with a thickness of 300 nm and 150 nm, respectively, deposited by sputtering or electron beam evaporation. The tunnel barrier layer 13 is formed by thermally oxidizing an aluminum Al film with a thickness of about 5 nm deposited by sputtering or vapor deposition in a pure oxygen O ₂ atmosphere. The above three-layer film was patterned using a normal photolithography process using Freon 12 (CCl ₂ F ₂ ) and Freon 13
A reactive sputter etching method using (CF ₄ ) as an etching gas is used. Next, as shown in FIG. 1b, after forming a resist mask 15 on the second superconductor electrode 14 at a location that will become the bonding part, CCl ₂ F ₂
The _second method is a reactive sputter etching method using
The superconductor electrode 14 and tunnel barrier layer 13 are sequentially removed to define a junction. Next, as shown in FIG. 1c, silicon dioxide (SiO ₂ ) is deposited to a thickness of 150 nm over the entire surface of the sample by plasma CVD or sputtering while leaving the etching mask 15 to form a first insulating layer 16. do. Next, Freon 23
The first insulator layer 16 is etched by a reactive sputter etching method using (CHF ₃ ) or the like or an ion beam etching method until the flat surface of the first superconductor electrode 12 is exposed. In these anisotropic etching methods, etching mainly proceeds in a direction perpendicular to the substrate surface, so that etching remains in this direction around the junction where the initial film thickness of the first insulating layer 16 is thick; As shown in FIG. 1d, a structure is obtained in which the side walls of the joint are covered with the first insulating layer 16. Thereafter, the first superconductor electrode 12 is placed in an aqueous solution of ammonium pentaborate and ethylene glycol.
When the exposed surface of the first superconductor electrode 12 is anodized using the superconductor as an anode, a second insulating layer 17 made of niobium oxide (Nb ₂ O ₅ ) is formed as shown in FIG. 1e. The film thickness of Nb ₂ O ₅ is controlled by the anodic oxidation voltage V in a relationship of approximately 2 nm/V. In this example, a 20 nm Nb ₂ O ₅ film was grown at V=100 (v). Since the volume expansion from Nb to Nb ₂ O ₅ is approximately 2.6 times, the thickness of the first superconductor electrode 12 consumed by anodic oxidation is approximately 80 nm. Here, the first superconductor electrode 12 was used as the anode for anodic oxidation, but a conductor layer previously provided under the first superconductor electrode 12 while maintaining electrical contact may also be used. . Finally, after removing the etching mask 15, the surface of the second superconductor electrode 14 is sputter cleaned, and as shown in FIG.
A third superconductor electrode 18 made of a Nb film of m is formed.

In this embodiment, in the anodization step shown in FIG. 1e, the side wall of the joint is covered and protected by the first insulating layer 16, so the anodic oxidation layer does not penetrate into the joint. . Therefore, even if the anodic oxide film of the first superconductor electrode 12 is used as the electrical insulating layer between the first superconductor electrode 12 and the second superconductor electrode 14, the bonding dimension is anisotropic. Since it is defined by a dry etching method, Josephson elements with high dimensional accuracy and small local variations can be formed.

In this example, Nb films were used as the first, second, and third superconductor electrodes, but the first superconductor electrode was made of niobium nitride (NbN), which can be anodized.
Various superconductor materials can be used for the second and third superconductor electrodes.
In addition to the Al oxide film, other metal oxide films, semiconductor films, insulator films, etc. can also be used as the tunnel barrier layer. Moreover, there is no problem in using an insulating film other than the SiO ₂ film for the first insulating layer.

(Effects of the Invention) As explained above, according to the present invention, the anodic oxide film of the first superconductor electrode is formed on the electrical insulating layer between the first superconductor electrode and the second superconductor electrode. Even when using the method, a Josephson element with high dimensional accuracy and small local variations can be formed because the bonding dimensions are determined by an anisotropic dry etching method.

[Brief explanation of drawings]

FIGS. 1a to 1f are cross-sectional views showing a method for manufacturing a tunnel-type Josephson device according to the present invention in order of steps, and FIGS. 2 a-c are sectional views showing a conventional method for manufacturing a tunnel-type Josephson device in order of steps. In the figure, 11, 21 are substrates, 12, 22 are first superconductor electrodes, 13, 23 are tunnel barrier layers, 14, 24 are second superconductor electrodes, 15, 2
5 is an etching mask, 16 and 26 are first insulator layers or insulator layers, 17 is a second insulator layer, 1
8 and 27 are third superconductor electrodes.

Claims (1)

[Claims] 1 Step of sequentially forming a first superconductor electrode, a tunnel barrier layer, and a second superconductor electrode on a substrate, forming an etching mask at a location on the second superconductor electrode that will become a joint. , removing the second superconductor electrode and the tunnel barrier layer by dry etching, depositing a first insulator layer while leaving the etching mask, and dry etching the first insulator layer. selectively covering a side wall of the joint portion with the first insulating layer;
anodizing the exposed surface of the superconductor electrode to form a second insulator layer; after removing the etching mask, forming a third insulator layer in electrical contact with the second superconductor electrode; 1. A method for manufacturing a tunnel-type Josephson device, comprising the step of forming a superconductor electrode.