JPH07120822B2 - Josephson device manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method of manufacturing an element using the Josephson effect which operates at a low temperature, particularly a high speed switching element and an element having a high magnetic field sensitivity.

(Prior Art) Conventionally, an element using a Josephson junction (a Josephson element) configured by using a metal such as niobium and an intermetallic compound superconductor such as niobium nitride is an insulator such as aluminum oxide or germanium. It is manufactured by forming a tunnel barrier made of a semiconductor between superconductors to form a Josephson junction.

To define the shape of the tunnel barrier, the exposure technology and processing technology used in the manufacture of semiconductors and the like are used. These Josephson devices are mainly manufactured on a silicon substrate. In some cases, an edge-junction type Josephson device having a tunnel barrier structure using a pattern edge of a superconductor thin film formed on a substrate is also used. Regarding the edge-junction structure, a structure in which the edge of one superconductor electrode film is in contact with the surface of the other superconductor electrode film is usually used. This is because the superconductivity of conventional superconductors of metals such as niobium and intermetallic compounds is isotropic regardless of the plane direction and crystal orientation of the film. These Josephson elements are manufactured by making full use of the exposure technology and processing technology used for manufacturing conventional silicon devices.

On the other hand, recently yttrium / barium / copper oxide (Y-Ba
-Cu-O) and the like have been found to have substances exhibiting superconducting properties. These oxide superconductors containing rare earths and copper are transformed to the superconducting state at an absolute temperature of 40K ~ 90K. Superconducting properties of these oxide high temperature superconductors,
That is, the material constants such as the critical current density, the critical magnetic field, the coherence length, etc. are significantly dependent on the crystal orientation.
There is a difference in the value even around one digit in the axial direction.

Furthermore, the oxide high-temperature superconductor thin film shows superconducting properties even without annealing when deposited on a strontium titanate (SrTiO ₃ ) crystal at a substrate temperature of 500 ° C to 700 ° C. It is known that the crystal axis of a film reflects the crystal orientation of the substrate. On the other hand, when the film is annealed at about 900 ° C., the critical temperature of the film rises and the superconducting property is improved. At this time, the film becomes polycrystalline and many crystal grains grow. Josephson devices and Squid (SQ
UID) is manufactured by the following method using oxide superconductor ceramics and thin films.

(Problems to be Solved by the Invention) A Josephson device using an oxide high-temperature superconductor is a method in which a ceramic rod of an oxide superconductor is cracked to form a minute weak bond inside the rod (Japan journal·
Of Applied Physics (Japan Journal of A
pplied Physics) Vol. 26, No. 5, pp. L701 to L703), or a method in which a grain boundary generated by annealing after film formation on a substrate of strontium titanate or magnesia is used as a Josephson junction. No. 249, pp. 73-78).

However, high-temperature oxide superconductors have a very small coherence length of about 2 to 4 nm, which is affected by superconductivity, desorption of oxygen that occurs during vacuum storage, and composition changes due to reaction with water vapor in the atmosphere. It is known that conductivity breakdown is likely to occur. Therefore, it was difficult to form a Josephson junction between oxide superconductors with good controllability by the same method of sandwiching an insulator when forming a conventional niobium / aluminum oxide film / niobium junction. That is, in the Josephson element manufactured by these conventional methods, the composition of the film deviates from the composition that becomes superconducting at the junction interface, so that a junction having sufficient performance cannot be obtained, and the critical current value of the junction is controlled. The reproducibility and reproducibility were extremely poor. On the other hand, in the Josephson device using the grain boundaries and cracks, it was difficult to control the positions of the grain boundaries and cracks, and thus it was difficult to define the bonding position. In particular, the conventional method could not form the Josephson junction orthogonal to the AB axis plane where the current density is high.

An object of the present invention is to solve the conventional problems and provide a Josephson element that can be formed at a designated position with good reproducibility.

(Means for Solving the Problem) A method for manufacturing a Josephson device according to the present invention is a Josephson device in which a grain boundary is formed in a superconductor film on a groove provided on a crystal substrate or a crystal film to form a Josephson junction. In the manufacturing of, after forming the groove, the same material as the crystal substrate or the crystal film or a material which is a base for the growth of the superconductor film is formed on the surface of the crystal substrate or the crystal film to form the width of the groove. It is characterized by including a step of reducing the size.

(Operation) According to the method for manufacturing a Josephson device according to the present invention, a Josephson device in which a grain boundary is formed in a superconductor film on a crystal substrate or a groove provided on a crystal film to form a Josephson junction, It is manufactured as follows.

First, fine grooves are formed in a crystal substrate on which a superconducting material such as an oxide high temperature superconductor is formed or in a region on the crystal film where a Josephson element is formed. The fine groove is formed by a method of digging a hole using an electron beam, a reactive ion etching (RIE) method which is usually used for processing a silicon device or the like. Then, a groove auxiliary is formed on the entire surface of the substrate by an electron beam evaporation method, a sputtering method, or the like to form a base auxiliary film which is a base for forming the above-mentioned crystalline material or another superconductor. In particular, a film forming method such as oblique vapor deposition in which a film is formed on the side wall of the groove is preferably used to reduce the groove width. This process is introduced because the processing accuracy for digging a groove is restricted by the accuracy of the exposure / etching technique and it is difficult to process a fine groove of about 0.5 μm or less with high accuracy.

Then, a superconducting material is formed on the entire surface of the substrate. The crystal structure of the formed superconducting thin film is a structure that matches the crystal structure of the base, reflecting the crystal structure of the base. Therefore, in the groove portion, a film having a structure different from that of the peripheral portion of the groove grows due to unevenness of the surface or destruction of the crystal structure during groove processing. The growth rate of this film is slowed by the effect of the trench sidewalls. For this reason, the crystal regions grown on both sides of the groove gradually expand, and eventually a crystal grain boundary is formed on the groove to come into contact with each other.

Finally, the superconductor film is processed into a desired shape to form a necessary circuit. That is, the superconducting film on the groove has a specified electrode width W.
To process. The area of the Josephson junction composed of the crystal grain boundaries produced as described above is approximately Wt, where t is the thickness of the superconductor film. Therefore, the electrode width W or the film thickness t
By changing the, the critical current value is controlled.

(Embodiment) FIG. 1 shows a manufacturing process of a Josephson device according to the first embodiment of the present invention.

First, a window 3 having a length of 6 μm and a width of 0.6 μm is opened in the photoresist 2 in a region where a Josephson element is formed on the crystal substrate 1 made of strontium titanate by using a normal exposure technique (FIG. 1 (a)). . Next, the groove 4 is formed by the reactive ion beam etching technique using chlorine gas (the 48th Japan Society of Applied Physics, Proceedings of the 48th Applied Physics Society, 1st volume, p. 67, lecture number 19p-D-2). The etching conditions at this time are: chlorine gas pressure 0.15Pa, extraction voltage 400V, ion beam 5 current +
μA / cm ² , etching rate is 10 nm / min for superconductor film,
The resist is 30 nm / min. Etching for 40 minutes with the chlorine ion beam 5 under the above conditions to dig a groove 4 with a depth of 400 nm (first
Figure (b)).

Then, the crystal substrate 1 made of strontium titanate is attached.
Base auxiliary film 6 made of strontium titanate heated at 600 ° C. and tilted by electron beam at a rate of 5 nm / min.
Is deposited to a thickness of 300 nm (Fig. 1 (c)). At this time groove 4
About 200 nm of strontium titanate grows on the sidewalls of. Therefore, the width of the groove 4 is reduced from about 0.6 μm to about 0.2 μm, and the depth of the groove 4 is reduced from 400 nm to 100 nm. As a result, the groove 4 is filled with the base auxiliary film 6 made of strontium titanate to a depth of 0.2 μm and a depth of 100 nm. At this time, in the regions on both sides of the groove 4, the base auxiliary film 6 made of strontium titanate having the same structure as the crystal substrate 1 grows.

Subsequently, the substrate 1 is heated to 650 ° C., and yttrium / barium / copper oxide is deposited at a deposition rate of 10 nm / min for 40 minutes in an oxygen atmosphere at a pressure of 10 Pascal to form a film. As a vapor deposition source, a sintered body of yttrium / barium / copper oxide is used. The composition of the sintered body is adjusted such that yttrium / barium / copper oxide formed by vapor deposition becomes a superconductor. The compositional deviation between the vapor deposition source and the grown film greatly changes depending on the apparatus and film forming conditions. For example, in this embodiment, a vapor deposition source in which barium and copper are increased by 2% and 4% respectively from a desired composition ratio is used. An electron beam accelerated at 10 KV is used to heat the vapor deposition source. As another method for supplying sufficient oxygen, a means for irradiating the entire surface of the sample with an oxygen ion beam accelerated at 100V may be used.

The crystal structure of the yttrium-barium-copper oxide superconductor having a film thickness of 400 nm formed as described above reflects the crystal structure of strontium titanate as the substrate. That is, when the strontium titanate substrate 1 whose C axis is perpendicular to the substrate surface is used, yttrium, barium, and
Copper oxide has a structure in which the C axis is perpendicular to the substrate surface.
In the upper part of the groove 4, grain boundaries 9 of the crystals of the first and second superconductor films 7 and 8 grown from both sides of the groove are formed to form a Josephson junction (FIG. 1 (d)). ). In the figure, the crystal grain boundaries 9 forming the Josephson junctions are schematically shown by lines, but more specifically, the grain boundary layers that are non-superconductors are formed at the temperature at which the Josephson device of the present invention is used. There is.

The grain boundary layer has a finite thickness, and the thickness is shorter than the coherence length. Therefore, a superconducting current, that is, a Josephson current flows through the grain boundary layer. In the yttrium-barium-copper oxide superconductor, the stoichiometric composition is Y = 1:
It is known that if it deviates from Ba = 2: Cu = 3, it will not become a superconductor. The part of the grain boundary layer sandwiched between the superconductor electrodes is an oxide having a composition ratio of yttrium / barium / copper different from 1: 2: 3. Therefore, since the critical temperature at which the grain boundary layer becomes superconducting is lower than the operating temperature, the superconducting material that exhibits normal conduction characteristics, semiconductor characteristics, or insulation characteristics at the operating temperature of the embodiment, or a normal conductor, a semiconductor, or an insulating material. One of the body.

The substance initially formed in the groove portion on the underlying auxiliary film 6 on the groove 4 is a superconductor or a non-superconductor described above in which the crystal axes are different. When the base auxiliary film 6 is formed on the portion above the groove 4, the crystal structure of the base auxiliary film 6 is different from that of the crystal substrate 1 in the groove portion due to the disorder of the surface structure of the crystal substrate 1 generated during processing of the groove 4. A substance having a crystal structure that reflects the disorder is formed. Due to the disorder of the crystal structure of the base auxiliary film 6 in the groove portion, the yttrium / barium / copper substance formed thereon is a superconductor or a substance having a composition ratio different from that of the superconducting phase. As the first and second superconductors grow on the base auxiliary film 6, the crystals of the first and second superconductor electrode films 7 and 8 cover the grooves, and through the crystal grain boundaries 9. Contact each other on the groove 4 to form a Josephson junction.

Subsequently, the first and second superconductor electrode films 7 and 8 are processed into a desired shape, for example, an electrode width W of 4 μm, by an exposure / etching technique using a photoresist, and necessary wiring and The Josephson device shown in the perspective view is formed in FIG. Processing using reactive ion beam etching at this time is performed as follows. Chlorine gas of 10 ^-3 Torr is ionized by high-frequency plasma, and the ionized chlorine gas is accelerated with an extraction voltage of 400 V to form an ion beam. The entire surface of the sample is irradiated with this ion beam to etch yttrium, barium, and copper oxide. For example, etching rate
The superconductor electrodes 7 and 8 are processed into a desired shape by etching at 4 nm / min for 80 minutes.

As a method of forming a superconductor film other than the above examples, a sputtering method using an electrode of a sintered body of yttrium / barium / copper oxide as a target, or a target or a metal or oxide of yttrium / barium / copper different from each other is used. Alternatively, a method of performing sputtering or vapor deposition simultaneously or in a time-division manner as the vapor deposition source can be used. Further, for processing the superconductor film, an ion beam etching method, a reactive plasma etching method using a gas other than chlorine, or the like is used. A good superconductor film is formed even when the sample temperature at the time of forming the superconductor film is set to a range of about 400 to 900 ° C other than 650 ° C.

According to the method for manufacturing the Josephson device of the present invention described above, the first and second superconductor electrode films and the crystal grain boundaries are formed on the groove provided in the crystal substrate. Therefore, the length of the junction is determined by the electrode width W of the superconductor electrode film, and the width of the junction is almost determined by the film thickness t of the superconductor electrode film. Therefore, the area of the junction becomes almost Wt and can be controlled by the film thickness t and the electrode width W. The position where the bond is formed can be controlled by the groove provided in the crystal substrate. That is, it becomes easy to control the shape and characteristics of the Josephson element.

The Josephson device having the grain boundaries of the first and second superconductor films formed as described above as a junction can form a junction having a large current density. That is, the critical current value of the Josephson element is determined as follows. When a base crystal having a C axis perpendicular to the substrate is used, the C axis of the superconductor film is perpendicular to the substrate. Therefore,
In the case of superconductor film, the direction (AB
The (axis) is the direction of the film surface, which coincides with the direction of current flowing through the junction. Therefore, as the current density of the Josephson device, the maximum current density of 10 ⁶ A / cm ² of the superconductor film can be obtained.
For example, the current density of the Josephson junction is 5 × 10 ⁵ A / cm ²
Assuming that, as the critical current value of the Josephson element,
The junction having 0.1 mA used in a normal logic circuit or the like is obtained by the first and second superconductor electrodes having an electrode width of about 1 μm when a 0.4 μm thick superconductor film is used. 0.
If the width of the superconductor electrode with a film thickness of 4 μm is 2 μm, it will be 0.2
With a mA of 4 μm, a Josephson device having a critical current value of 0.4 mA is formed.

In the above embodiment, the groove 4 has a rectangular cross section, but V
It may be a V-shape, a U-shape, a trapezoid, an inverted trapezoid or the like.

(Effect of the Invention) According to the method for manufacturing a Josephson device of the present invention, a Josephson device in which a grain boundary of a superconductor film is formed on a groove provided on a crystal substrate to form a junction is obtained. In particular, by reducing the width of the groove provided on the underlying crystal, formation of the grain boundary of the superconductor film formed on the groove is facilitated and control of the characteristics of the grain boundary junction is facilitated. Further, in the manufacturing method of the present invention, since the crystal structure of the superconductor film can be controlled by the crystal structure of the underlying layer, it is possible to form a Josephson element that allows a current to flow in the crystal axis direction with high current density with good controllability.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process represented by a cross-sectional structure of a Josephson device for explaining an embodiment of the present invention, and FIG. 2 is a perspective view of a Josephson device manufactured by a manufacturing method of the present invention. . 1 ... Crystal substrate, 2 ... Photoresist 3 ... Window, 4 ... Groove 5 ... Ion beam, 6 ... Base auxiliary film 7 ... First superconductor electrode film 8 ... Second superconductor Conductor electrode film 9 …… Crystal grain boundary, 10 …… Electrode width W

Claims (1)

[Claims] 1. A process for producing a Josephson element in which a grain boundary is formed in a superconductor film on a groove provided on a crystal substrate or a crystal film to form a Josephson junction, the crystal substrate or the crystal film being formed after the groove is formed. Manufacture of a Josephson device, characterized in that it includes a step of reducing the width of the groove by forming on the surface the same material as the crystal substrate or the crystal film or a material which is a base for the growth of the superconductor film. Method.