JPS6156875B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for manufacturing a bridge-type Josephson element.

1A and 1B are plan views each showing an example of a bridge-type Josephson element, and FIG. 2 is a - line in FIG. 1A or B of the Josephson element shown in FIG. 1A or FIG. 1B. FIG. 3 is a perspective view of the Josephson device shown in FIG. 1B. In FIGS. 1, 2, and 3, 1 and 2 are superconducting films made of superconductors such as niobium (Nb) and lead (Pb). The superconducting film 1 and the superconducting film 2 may be made of the same type of superconductor or may be made of different types of superconductors. 3 is a joining part (neck part) made of a superconductor and weakly connects the two superconducting films 1 and 2; 4 is an insulator such as glass or sapphire, or a semiconductor such as silicon which becomes an insulator at the operating temperature of the Josephson device. It is a board consisting of.

If superconducting films 1 and 2 are weakly coupled to the extent that their states can be expressed by wave functions ψ ₁ and ψ ₂ with different phases (the coupling strength is expressed by a small coefficient ε), then ψ ₁ , ψ The equation followed by ₂ adds the superconducting electrons entering through the bond as a perturbation term, This is the Schrödinger equation that can be expressed as

Here, μ ₁ and μ ₂ are superconducting films 1 and 2, respectively.
is the energy of the electrons in the superconductor.

If the density and phase of superconducting electrons of the superconductors of superconducting films 1 and 2 are n ₁ , n ₂ and θ ₁ , θ ₂ respectively, then It can be written as

Substituting equation () into equation (), setting n ₁ = n ₂ = n, and treating the real part and imaginary part separately, we get ∂n ₁ /∂t=(2/h)εnsin(θ ₂ −θ ₁ )=-∂n ₂ /∂t () (∂/∂t) (θ ₂ −θ ₁ )=−(1/h)(μ ₂ −μ ₁ ) () is derived.

The current flowing from superconducting film 2 to superconducting film 1 is equal to 2e∂n ₁ /∂t (e is the charge of electrons), so if the cross-sectional area and volume of the superconductor at junction 3 are S and V, respectively,
The current density i can be expressed by the following equations () and ().

i = i ₀ sin (θ ₂ - θ ₁ ) () i ₀ = 4eVεn ₁ /hS () In equation (), if phase difference θ ₁ - θ ₂ = θ is not zero, a superconducting current proportional to sin θ will flow. . This superconducting current is the Josephson current.

In the Josephson elements shown in FIGS. 1, 2, and 3, microfabrication of the joint portion 3 is difficult.
It is necessary to control the length l, width w, and thickness d of the joint portion 3. Although fine processing with a length l and a width w is possible by electron beam exposure, etc., it is difficult to make the thickness d thinner than the electrode portion made of the superconducting films 1 and 2. In other words, it is difficult to thin only the joint portion 3 by etching or to thinly deposit it by conventional vapor deposition methods. Therefore, it has been difficult to form the bonding portion 3 with good controllability through fine processing in order to weakly bond the two superconducting films 1 and 2 together.

This invention was made in view of the above points, and after controlling the thickness of two superconducting films, which are electrode parts, and the bonding part between them by vertical evaporation method, the thickness of the two superconducting films is controlled to be thin, and then by diagonal evaporation method. It is an object of the present invention to provide a method of manufacturing a Josephson device in which the thickness of the bonding portion can be controlled with high precision by thickening only the superconducting film in the electrode portion.

The present invention will be explained below based on examples.

4A to 4G are cross-sectional views showing the main steps of an embodiment of the method for manufacturing a Josephson device according to the present invention, and FIGS. 5A to 5D are sectional views, respectively.
FIG. Note that FIG. 4 is a cross-sectional view taken along the - line shown in FIG. 3.

First, as shown in FIG. 4A, a substrate surface insulating film 5 such as a silicon dioxide (SiO ₂ ) film is formed on a silicon substrate 4a, and a silicon nitride (Si ₃ N ₄ ) film of a different type from the substrate surface insulating film 5 is deposited on a silicon substrate 4a. An intermediate film 6 made of an insulating film such as a film or a semiconductor film such as polycrystalline silicon, and an upper film 7 made of an insulating film are sequentially formed. The upper coating 7 may be of the same type as the substrate surface insulating film 5. Next, as shown in FIG. 4B, a resist film 8 made of a photoresist (e.g. AZ-1350), an electron beam resist (e.g. PMMA, PBS), etc. is coated on the upper coating 7, and Draw the desired Josephson element pattern by lines. Subsequently, as shown in FIG. 4C, the resist film 8 is developed to form an opening 81 having the above pattern.
, and has this opening 81.
Using as a mask, an opening 71 corresponding to the opening 81 is formed in the upper coating 7 by wet etching and dry etching. Next, as shown in FIG. 4D and FIG. 5A, the intermediate film 6 is etched by wet etching and dry etching using the resist film 8 or the upper film 7 as a mask to form an opening having a Josephson element pattern. Section 71
An opening 61 having a periphery outside the periphery of the opening 61 is formed. Subsequently, as shown in FIGS. 4E and 5B, after removing the resist film 8, niobium (Nb), lead (Pb), etc. A superconductor is vapor-deposited by electron beam evaporation or the like to form a first vapor-deposited film 9 such that the first vapor-deposited film 91 has a desired thickness at the joint portion. By this vapor deposition, the first
The thickness of the deposited film 91 can be controlled from several tens of angstroms to several hundreds of angstroms. Furthermore, as shown in FIG. 4F and FIG. 5C, by the same vapor deposition method,
A superconductor of the same or different nature is obliquely deposited so as not to be deposited on the first deposited film 91 at the joint part, and is integrated with the first deposited film 9 to a thickness of several thousand Å.
A second vapor deposited film 9a having a certain thickness is formed. Finally, as shown in FIGS. 4G and 5D, the intermediate coating 6 is etched away by wet etching or dry etching to form the upper coating 7.
When the second vapor deposited film 9a thereon is lifted off, at the position shown by the dashed line in FIG.
The second vapor deposited film 9a is removed, and the first vapor deposited film 91 remains at the joint portion, forming the joint portion 3. A thicker second vapor deposited film 9a also remains in the direction perpendicular to the dashed-dotted line shown in FIG. 3, resulting in a desired Josephson device in which the bonding portion 3 is thin and the superconducting films 1 and 2, which are electrode portions, are thick. be able to. superconducting film 1,
If superconducting films 2 are made as thin as bonding portion 3, there will be problems such as not being able to obtain stable characteristics due to pinholes that occur during film formation, or peeling off from substrate 4a, so superconducting films 1 and 2 should be made thick. This is necessary, and from this point of view, the method according to the present invention is effective.

The degree of bonding between the two superconducting films 1 and 2 at the junction 3 needs to be weak enough that the superconducting films 1 and 2 exhibit different phases from each other, and strong enough to allow superconducting electrons to pass back and forth. Therefore, the joint portion 3 requires highly precise micromachining. The length l and width w of the joint portion 3 can be controlled by electron beam exposure, but the thickness d must be controlled by vapor deposition. In recent years, advanced vapor deposition technology has made it possible to control film thicknesses of several tens to 100 Å. This invention utilizes oblique vapor deposition and vertical vapor deposition to
The superconducting films 1 and 2, which are the electrode portions, are characterized in that a thin film thickness of several tens to several hundred angstroms can be obtained, and a thick film thickness of several thousand angstroms can be obtained. By the method of the present invention, the cross-sectional area S of the bonding portion 3 and the bonding coefficient ε can be determined with high accuracy, and a Josephson element with stable characteristics can be formed.

In the above embodiment, a case has been described in which the substrate surface insulating film 5 made of an SiO ₂ film is formed on the silicon substrate 4a, but this substrate surface insulating film is not necessarily required.

Furthermore, in the above embodiments, a case has been described in which the intermediate film 6 is made of an insulating film or a polycrystalline silicon film, and the upper film is made of an insulating film. may be formed.

As described in detail above, in the method of manufacturing a Josephson device according to the present invention, a thin first layer is formed by vapor deposition in a direction substantially perpendicular to the main surface of a substrate, using a film having openings in the pattern of a Josephson device as a mask. A superconducting film that will become the electrode portion and a superconducting film that will become the bonding portion are formed by the vapor-deposited film, and a thick second vapor-deposited film by diagonal vapor deposition that is not deposited on the bonding portion increases the thickness of the electrode portion. Since the thickness is increased, the joint portion can be formed sufficiently thin and with high dimensional accuracy, and the electrode portion can be formed thick, so a Josephson element with good characteristics can be manufactured.

[Brief explanation of the drawing]

Figures 1A and B are plan views of examples of bridge-type Josephson elements, Figure 2 is a sectional view of the Josephson element shown in Figure 1A or B, and Figure 3 is a perspective view of the Josephson element shown in Figure 1B. 4A to 4G are cross-sectional views showing the main steps of an embodiment of the method for manufacturing a Josephson device according to the present invention,
FIGS. 5A to 5D are plan views corresponding to FIGS. 4D to G, respectively. In the figure, 1 and 2 are superconducting films that are electrode parts, 3 is a bonding part, 4 is a substrate, 4a is a silicon substrate, 5 is a substrate surface insulating film, 6 is an intermediate coating,
61 is an opening, 7 is an upper coating, 71 is an opening, 8
81 is a resist film, 81 is an opening, 9 is a first vapor deposited film, 91 is a first vapor deposited film 9, 9 in a joint part
a is the second deposited film. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. An intermediate film is formed on the substrate which is an insulator at the operating temperature of the Josefson element, and the intermediate film is made of a material that is not etched or has a slow etching rate depending on the etching agent for the intermediate film. a step of forming an upper film, a step of forming a resist film on the upper film, and a step of forming an opening having a pattern of Josephson elements in which two electrode parts and a joint part connecting the electrode parts are arranged in a plane using the resist film. a step of etching using the resist film as a mask to form an opening in the upper film that matches the opening of the resist film; etching using the resist film or the upper film as a mask; forming an opening in the intermediate film having a peripheral wall outside the periphery of the opening in the upper film; a step of forming a superconductor in a direction substantially perpendicular to the main surface of the substrate from above the upper film from which the resist film has been removed; forming a first deposited film consisting of a first part deposited on the substrate and a second part deposited on the upper film; forming a second vapor-deposited film by vapor-depositing a superconductor in a direction oblique to the main surface of the substrate so that the superconductor is not vapor-deposited on a portion of the first portion of the vapor-deposited film that will be a bonding portion of the Josefson element; and a step of sequentially etching away the intermediate film. 2. The method for manufacturing a Josephson device according to claim 1, wherein the intermediate film is a polycrystalline silicon film and the upper film is a silicon dioxide film. 3. The method for manufacturing a Josephson device according to claim 1 or 2, wherein the substrate is made of a silicon substrate and silicon dioxide deposited on the surface of the silicon substrate.