JPS6272187A - Josephson tunnel junction device - Google Patents

Abstract

PURPOSE:To improve an energy voltage (Vg) and eliminate deviation of device characteristics by a method wherein a highly oriented magnesia thin film is employed as a foundation material and a tunnel barrier, composed of magnesia or aluminum oxide, and upper and lower electrodes, composed of niobium nitride, are made to grow by epitaxial growth to the same direction as the foundation material. CONSTITUTION:An MgO film 5 for epitaxial growth of NbN, which is to be a lower electrode 6 of a device, is formed. Successively, in the same vacuum, first the NbN 6 for the lower electrode of the device is deposited to the thickness of 100-150nm by reactive sputtering. Further, MgO 7 for a tunnel barrier is deposited to the thickness of 0.2-0.5mm which is several times of the coherence length of the NbN by sputtering. finally, NbN 8 for an upper electrode, is deposited to the thickness of 100-150nm. Then the upper electrode is processed by RIE after a resist stencil 9 is provided. Afther that, SiO2 10 for insulation is deposited by sputtering and the resist stencil 9 is lifted off. Finally, NbN 11 for wirings is deposited by sputtering and RIE and lifting-off are carried out to forme the device.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to Josephson tunnel junction devices. More specifically, the button has a high critical transition temperature and improves characteristic deterioration at the interface between the insulation barrier and the electrode material, improves energy voltage (Vg), eliminates variations in device characteristics, and improves reliability. The present invention relates to a Josephson tunnel junction device capable of improving performance.

[Conventional technology]

Josephson junction devices are used as low-power, ultra-high-speed switching logic circuit elements in microwave applications.

It has been proposed to be used as a detector for millimeter waves, submillimeter waves, etc., as a detector for weak magnetic fields emitted from living organisms, or as a voltage standard, and there are great expectations for its industrialization.

Up until now, Josephson junction elements have mainly been studied using lead alloy-based materials, which are easy to fabricate, and niobium-based materials, which have a high transition temperature and are resistant to thermal cycles.
Its structure is shown in Figure 3(a) and Figure 5(b).

The former has been studied for many years as a logic circuit element, and its technology has
1'□ Although it has been established, current research and development is shifting towards the latter due to the essential characteristics of the material, namely its vulnerability to temperature cycles.

Niobium nitride has a higher transition temperature (Tg': 16
K), the gap voltage vg, and
: A high sub-gap resistance R8° can be obtained, which is advantageous in terms of device application, and is also excellent in terms of stability of device characteristics against temperature changes. Furthermore, since it is chemically more stable than niobium, it is also superior in suppressing deterioration of device characteristics due to processing disturbances and interdiffusion at the tunnel barrier surface, and is expected to improve device characteristics and manufacturing process margins. From this point of view, the development of devices using &ll ab throats is being actively promoted, for example, using NbOx, α-3i:H, etc. as a tunnel barrier.
Excellent characteristics have been obtained using the element fabrication technology using the KP method. However, some problems still remain, such as a decrease in the gap voltage vg due to the decrease in the NbN film thickness and an increase in leakage current in the sub-gap region.

This is because the coherence length of the NbN film is short, about 2nll11, so deterioration of superconducting properties near the interface of the tunnel barrier, especially at the interface of the upper electrode (Figure 3) +21, has a significant effect on the device characteristics K1. It means. Generally, in the initial stage of film growth, KTa decreases because crystal growth is insufficient. Therefore, in order to improve the Tc of a thin film, it is important to develop a new thin film formation technology that can improve the Tc of the film during the initial growth process. Even in the case of a film, only a low value of about 12 or less was obtained.

As a method to solve this problem, it is possible to form a NbN film into a single crystal thin film or an epitaxial thin film and use it as an electrode material of a Josephson element. In the latter case, magnesia l, which has the same crystal structure as NbN (Na (type 1!)
There have been reports of epitaxial growth of NbN thin films on stretched planes of MgO) single crystals. In this case, a film with relatively good properties has been obtained, but there are problems with the method of stretching the MgO single crystal and the smoothness of the stretched plane, making it unsuitable for use in Josephson devices. I got angry.

[Purpose of the invention]

As a result of conducting research in search of a method to solve the above problems, the present inventor obtained the knowledge that an epitaxially grown NbN film can be used as an electrode material of a Josephson element, and MgO can be used as a tunnel barrier material. This is the completed version. That is, in the present invention, a highly oriented Magnequa thin film is used as the base material, and the tunnel barrier made of magnesia (MgO) or aluminum oxide (A1.Ox) and the upper and lower electrodes made of niobium nitride (NbN) are used as the base material. The present invention provides a Josephson tunnel junction device which is epitaxially grown in the same direction as .

[Technical means of invention]

The method for manufacturing the Sefson disodium tunnel junction device of the present invention is shown in FIG. To prepare the MgO thin film, Ar gas and It
) The N thin film was prepared by sputtering using Ar and a mixed gas, respectively. Further, the normal 5NEP method was used for preforming.

First, an MgO film (5) with a thickness of about 80 mm is formed for epitaxially growing NbN, which will become the lower electrode (6) of the device. The thicker the MgO film, the stronger the crystal orientation strength. Therefore, 1ibN is laminated on top of this.

As the film thickness becomes thinner, the dependence of Tc of NbN on the MgO film thickness becomes stronger. Therefore, in order to increase the To of NbN of the electrode material, it is advantageous to make the MgO film thicker, but approximately 801111
It is sufficient if it is 1 or more.

Next, in the same vacuum, H
b N +61 to 100 by reactive sputtering
150 nm to 150 nm. Further, a MgO single crystal for the tunnel barrier is formed by sputtering to a thickness of L2 to 15 nm, which is several times the N1)N coherence length. Finally, N b N +81 of the upper electrode is 1Q O! 11111
~150 nm. The thickness of the It)N film of the upper and lower electrodes was selected from the viewpoint of evaluating the characteristics of the superconducting thin film of this structure.

Next, a resist stencil (9) for patterning the first upper electrode is provided on this MgO/NbN film (MgO/Nba), and the upper electrode is processed by R-EK using an etching gas of OFa+Ox (5%). After that, 81N α (I) for insulation is sputtered, and the resist stencil is lifted off.Finally, NbNH for wiring is sputtered, and the R process and lift-off are performed in the same manner as for the upper electrode to complete the device.

Figures 2(a) and 2(b) show the current-voltage characteristics of the Josephson tunnel junction device fabricated by this method and the X-ray diffraction pattern of the MgO/NbN/MgO/NbN four-layer film, respectively.

[Effects of the present invention]

The device of the present invention has the following excellent features. That is, since it has become possible to epitaxially grow the upper and lower electrodes, it is possible to improve the Tc of the electrode material, especially the Tc of the upper electrode, which has been a problem. Therefore,
This has the advantage of improving the stability and reliability of the element. This is because the gap voltage of the current-voltage characteristic is high (approximately 4.5
m'V). This is a higher value than previously reported. Furthermore, by forming an epitaxial thin film of NbN, the resistivity of the film is reduced, and as a result, the magnetic field penetration depth can be made much shorter than the previously announced value.

This eliminates the drawbacks that conventionally required the use of Nb for ground planes and wiring, and makes it possible to use MbN as the superconducting material used in devices.
t) There is also the advantage that the original purpose of using N can be achieved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view at each step of an example of manufacturing a Josephson tunnel junction element of the present invention. FIG. 2(a) is a graph showing the current-voltage characteristics of a Josephson tunnel junction device, which is an example of the Josephson tunnel junction device of the present invention, and FIG. 2(b) is its X, 13 diffraction pattern. FIG. 5(a) is an external view of a conventional dicycefson junction element, and pA-3(b) is a cross-sectional view thereof. 1... Lower electrode, 2... Tunnel barrier 2
...Tc deterioration region 5 at the interface between the upper electrode and the tunnel barrier.
...Top electrode, 4...Substrate 5...Base material + Mg0), 6...Bottom electrode (NbN) 7.
...Tunnel barrier (MgO), B...Top electrode (
NbN)? ...Resist stencil, 10...Sift 11...Wiring (NbN) Patent applicant Toyo Soda Kogyo Co., Ltd. Figure-1 Figure-2(a) Figure-2(b) Figure-3 (a) Figure 3 (b) Procedural amendment (Touken February 14, 1986)

Claims (1)

[Claims] A Josephson tunnel formed by using a highly oriented magnesia thin film as the base material, and epitaxially growing the tunnel barrier made of magnesia (MgO) or aluminum oxide (AlOx) and the upper and lower electrodes made of niobium nitride in the same direction as the base material. Junction element.