JPH01211985A - Manufacture of josephson element - Google Patents

Abstract

PURPOSE:To facilitate the control the shape and characteristics of a Josephson element, by a method wherein, after a trench is formed on a crystal substrate or a crystal film, the width of the trench is narrowed by forming, on the surface, a film of the same material as the crystal substrate or the crystal film, or of the material turning to a base for growing a superconducting film. CONSTITUTION:A fine trench 4 is formed in the region to form a Josephson element on a crystal substrate 1. The width of the trench 4 is narrowed by forming, on the whole substrate 1 surface, a base supporting film 6 turning to a base for forming a superconductor. When a superconductor film is successively formed on the whole substrate 1 surface, the crystal structure of superconducting thin films 7, 8 reflect the crystal structure of the base, and a film having a structure different from the trench 4 periphery grows in the trench part 4, because of unevenness of the surface or destruction of crystal structure at the time of working the trench. Since the growing speed of said film becomes small by the effect of side walls of the trench 4, the crystal regions grown on both sides of the trench 4 extend gradually, and at last form a crystal grain boundary to come into contact with each other. Thereby the formation of crystal grain boundary of the superconducting films formed on the trench 4 is facilitated, and the control of characteristics of grain boundary junction is facilitated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a device using the Josephson effect that operates at low temperatures, particularly a high-speed switching device and a device having high magnetic field sensitivity.

(Prior art) Conventionally, devices using Josephson junctions (Josephson devices) constructed using metals such as niobium and intermetallic compound superconductors such as niobium nitride have been constructed using insulators such as aluminum oxide or germanium, It is fabricated by forming a tunnel barrier made of semiconductor between superconductors to form a Josephson junction.

Exposure technology and processing technology used in the manufacture of semiconductors and the like are used to define the shape of the tunnel barrier. These Josephson devices are mainly manufactured on silicon substrates. In some cases, an edge junction type Josephson device having a tunnel barrier structure using patterned edges 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 brought into contact with the surface of the other superconductor electrode film is usually used. This is due to the superconductivity of conventional superconductors of metals and intermetallic compounds such as niobium, or to the fact that they are isotropic, completely independent of the plane direction or crystal orientation of the film. These Josephson elements are manufactured using exposure and processing techniques that are used in the manufacture of conventional silicon devices.

On the other hand, recently, yttrium/barium/copper oxide (Y-B
It has been discovered that there are substances exhibiting superconducting properties such as a-Cu-0). These oxide superconductors containing rare earths and copper transition to a superconducting state at temperatures around 40 to 90 degrees absolute. The superconducting properties of these oxide high-temperature superconductors, that is, material constants such as critical current density, critical magnetic field, and coherence length, depend significantly on the crystal orientation, and vary by about an order of magnitude in the AB-axis in-plane direction and the C-axis direction. There are also differences in the values.

Furthermore, thin films of oxide high-temperature superconductors are deposited on strontium titanate (SrTt03) crystals at substrate temperatures of 500°C and up.
When deposited at 700°C, it exhibits superconducting properties even without annealing. It is known that the crystal axis of a film is oriented to reflect the crystal orientation of the substrate. On the other hand, when the film is annealed at around 900° C. after film formation, the critical temperature of the film increases and the superconducting properties are improved. At this time, the film becomes polycrystalline and many crystal grains grow. Josephson devices and SQUIDs using this oxide superconductor are manufactured by the following method using ceramics and thin films of the oxide superconductor.

(Problem to be solved by the invention) Josephson devices using oxide high-temperature superconductors are developed using a method (Japan journal·
of Applied Physics (Japan Jou
26, No. 5, pages L701-L703), and a method of forming Josephson junctions at grain boundaries produced by annealing after film formation on strontium titanate or magnesia substrates (
(IEICE Technical Report Vol. 87, No. 249, pp. 73-78).

However, the coherence length of oxide high-temperature superconductors, which is affected by superconductivity, is extremely small at about 2 to 4 nm, and the composition changes due to the release of oxygen during vacuum storage and the reaction with water vapor in the atmosphere, etc. It is known that conductive breakdown is likely to occur. For this reason, it is difficult to form a Josephson junction between oxide superconductors with good controllability using the same method of sandwiching an insulator when forming a conventional niobium-l-aluminum oxide film-l-niobium junction. In other words, in Josephson devices manufactured by these conventional methods, the composition of the film at the junction interface deviates from the composition that makes it superconducting, making it impossible to obtain a junction with sufficient performance, and it is difficult to control the critical current value of the junction. The accuracy and reproducibility were extremely poor. On the other hand, in Josephson elements using grain boundaries and cracks, it is difficult to specify the bonding position because the positions of the grain boundaries and cracks cannot be controlled. In particular, with conventional methods, it has not been possible to form a Josephson junction that is perpendicular 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 Problems) A method for manufacturing a Josephson device of the present invention is a Josephson device in which grain boundaries are formed in a superconductor film on a groove provided on a crystal substrate or a crystal film to form a Josephson junction. In manufacturing, after forming the groove, the width of the groove is increased by forming a film on the surface of the crystal substrate or crystal film with the same material as the crystal substrate or crystal film or a material that will serve as a base for the growth of the superconductor film. It is characterized by including a step of reducing the size.

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

First, fine grooves are formed in a region where a Josephson element is to be formed on a crystal substrate or crystal film on which a superconducting material such as an oxide high-temperature superconductor is to be deposited. The fine grooves are formed by a method of digging a hole using an electron beam, a reactive ion etching (RIE) method, which is generally used for processing silicon devices, and the like.

Subsequently, an auxiliary base film, which serves as a base for forming the crystalline material or other superconductor, is formed on the entire surface of the substrate by electron beam evaporation, sputtering, or the like, and the groove width is reduced. In particular, a film forming method such as oblique vapor deposition in which a film is formed on the side walls of the groove is preferably used to reduce the groove width. This process has been introduced because the processing accuracy for digging grooves is limited by the accuracy of exposure and etching techniques, making it difficult to process fine grooves of about 0.5 micrometers or less with high precision.

Next, a superconducting material is deposited over the entire surface of the substrate. The crystal structure of the deposited superconducting thin film reflects the crystal structure of the underlying layer and matches the crystal structure of the underlying layer. Therefore, in the groove portion, a film having a structure different from that in the peripheral portion of the groove grows due to surface irregularities or destruction of the crystal structure during groove processing. The growth rate of this film is slowed down by the effect of the trench sidewalls. Therefore, the crystal regions grown on both sides of the groove gradually expand and eventually form crystal grain boundaries on the groove and come into contact with each other.

Finally, the superconductor film is processed into a desired shape to form the necessary circuit. That is, the superconductor film on the groove has a specified electrode width W
Process it into The area of the Josephson junction formed by the grain boundaries created as described above is approximately Wt, where t is the thickness of the superconductor film. Therefore, by changing the electrode width W or film thickness t, the critical current value can be controlled.

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

First, a window 3 having a length of 6 pm and a width of 0.6 pm is formed in a photoresist 2 using a normal exposure technique in a region where a Josephson element is to be formed on a crystal substrate 1 made of strontium titanate.
(Figure 1(a)). Next, reactive ion beam etching technology using chlorine gas (Proceedings of the 48th Japan Society of Applied Physics, Vol. 1, page 67, lecture number 19p-D-2)
A groove 4 is formed by this. The etching conditions at this time were a chlorine gas pressure of 0.15 Pa, an extraction voltage of 400 V, and a current of several tens of pA/cm2 of the ion beam 5, and the etching speed was 10 nm/min for the superconductor film and 30 nm/min for the resist. Etching was performed for 40 minutes using the chlorine ion beam 5 under the above conditions to dig a groove 4 with a depth of 400 mm (FIG. 1(b)).

Subsequently, the crystal substrate 1 made of strontium titanate was heated to 600°C, and 5n
An auxiliary base film 6 made of strontium titanate is formed to a thickness of 300 mm at a speed of m/min (Fig. 1 (C)).
. Strontium titanate with a thickness of about 200 nm grows on the side wall of Shijo 4. Therefore, the width of groove 4 is 0.611m
The depth of groove 4 was reduced to about 0.2 1 m, and the depth of groove 4 was 400 m.
It becomes shallow from mm to 1100n. As a result, 0.2p
The groove 4 is filled with a base auxiliary film 6 made of strontium titanate to a width of m and a depth of 1100n. Groove 4 at this time
An auxiliary base film 6 made of strontium titanate and having the same structure as the crystal substrate 1 is grown on both sides of the substrate.

Subsequently, the substrate 1 is heated to 650°C and yttrium.
Barium/copper oxide was deposited at a deposition rate of 10 nm and 1 minute at a pressure of 10
The film is formed by vapor deposition for 40 minutes in a Pascal oxygen atmosphere.

A sintered body of yttrium, barium, and copper oxide is used as a vapor deposition source. The composition of the sintered body is adjusted so that the yttrium-barium-copper oxide formed by vapor deposition becomes a superconductor. The compositional deviation between the vapor deposition source and the grown film varies greatly depending on the equipment and film forming conditions. For example, in this embodiment, a vapor deposition source is used in which the amounts of barium and copper are increased by 2% and 4%, respectively, from the desired composition ratio. Here, an electron beam accelerated at 10 KV is used to heat the deposition source. Furthermore, as another method for supplying sufficient oxygen, a means of irradiating the entire surface of the sample with an oxygen ion beam accelerated at 100V may also be used.

The crystal structure of the 400 nm thick yttrium-barium-copper oxide superconductor formed as described above has a structure that reflects the crystal structure of the strontium titanate of the substrate. That is, when using the strontium titanate substrate 1 in which the C-axis is perpendicular to the substrate surface, yttrium-barium-copper oxide has a structure in which the C-axis is perpendicular to the substrate surface. In addition, in the upper part of the groove 4, grain boundaries 9 of the crystals of the first and second superconductor films 7.8 grown from both sides of the groove are formed, forming a Josephson junction (Fig. 1(d)). ).

Subsequently, the first and second superconductor electrode films 7.8 are processed into a desired shape, for example, with an electrode width W of 4 pm, by exposure and etching technology using photoresist, and the necessary wiring and wiring as shown in FIG. A Josephson element shown in a perspective view is formed. Processing using reactive ion beam etching at this time is performed as follows. Chlorine gas of 10'Torr is ionized with high frequency plasma, and the extraction voltage is 400v.
The ionized chlorine gas is accelerated to create an ion beam. The entire surface of the sample is irradiated with this ion beam to etch yttrium, barium, and copper oxide. For example, the superconductor electrode 7.8 is processed into a desired shape by etching for 80 minutes at an etching rate of 4 nm/min.

As a method for forming a superconductor film other than the above examples, a sputtering method using an electrode of a sintered body of yttrium, barium, and copper oxide as a target, or a method using different metals or oxides of yttrium, barium, and copper as targets. Alternatively, a method of simultaneously or time-sharing sputtering or vapor deposition as a vapor deposition source can also be used. Furthermore, ion beam etching, reactive plasma etching using a gas other than chlorine, etc. are used to process the superconductor film. The sample temperature during the formation of the superconductor film is not only 650°C but also 400°C to 9°C.
A good superconductor film can be formed even if the temperature is set in the range of around 00°C.

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

The Josephson element formed as described above, in which the grain boundaries of the first and second superconductor films are 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 using a base crystal whose C axis is perpendicular to the substrate,
The C-axis of the superconductor film is perpendicular to the substrate. Therefore, in a superconductor film, the direction where the critical current value is large (AB axis)
is the direction of the membrane surface, which coincides with the direction of the current flowing through the junction.

Therefore, the maximum current density of the superconductor film of 106 A/cm" can be obtained as the current density of the Josephson element. For example, the current density of the Josephson junction is 5 x 1.
Assuming 05A/am2, the critical current value of the Josephson element is 0, which is used in ordinary logic circuits, etc.
A junction with 1 mA is obtained by first and second superconductor electrodes with an electrode width of approximately 111 m using a 0.4 pm thick superconductor film. If the electrode width of a superconductor electrode with a film thickness of 0.4 pm is 2 pm, it will be 0.2 mA, and if it is 4 pm, it will be 0.
．． A Josephson device is formed using a critical current value of 4 mA.

In the above embodiment, the cross-sectional shape of the groove 4 is rectangular.
It may be a letter shape, a U-shape, a trapezoid, an inverted trapezoid, or the like.

(Effects of the Invention) According to the method for manufacturing a Josephson device of the present invention, a Josephson device can be obtained in which the grain boundaries of the superconductor film are formed on the grooves provided on the crystal substrate to form a junction. In particular, by reducing the width of the groove provided on the underlying crystal, it is easier to form grain boundaries in the superconductor film formed on the groove, and the characteristics of grain boundary junctions can be easily controlled. Furthermore, 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 with good controllability in which current flows in the direction of the crystal axis where the current density is high.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the manufacturing process as a cross-sectional structure of a Josephson device for explaining the present invention in detail, and FIG. 2 is a perspective view of the Josephson device manufactured by the manufacturing method of the present invention. 1... Crystal substrate 2... Photoresist 3
...Window 4...Groove 5...Ion beam 60. - Base auxiliary film 7...First superconductor electrode film 8...Second superconductor electrode film

Claims (1)

[Claims] In manufacturing a Josephson device in which grain boundaries are formed in a superconductor film on a groove provided on a crystal substrate or crystal film to form a Josephson junction, after the groove is formed, the crystal is placed on the surface of the crystal substrate or crystal film. A method for manufacturing a Josephson device, comprising the step of reducing the width of the groove by depositing the same material as the substrate or the crystal film, or a material that will serve as a base for the growth of the superconductor film.