KR100267974B1 - method for fabricating josephson junction device operating on high temperature - Google Patents

Abstract

PURPOSE: A method for making a high-temperature superconductive SNS Josephson junction element is provided to reduce an allowable error between products by easily changing a thickness of a conductive oxide material making a line-width of junction or SNS. CONSTITUTION: A first high-temperature superconductive layer of YBCO and a first insulating layer are sequentially deposited on a monocrystal substrate. The first insulating layer and the first high-temperature super conductive layer are partially etched, and a lower electrode is formed to have a predetermined slope. NdBa2Cu3O7-X conductive oxide layer, a second high-temperature superconductive layer and the second insulating layer are sequentially deposited on the exposed lower electrode and the total substrate. A second insulating layer, the second high-temperature superconductive layer and a conductive oxide layer are sequentially etched to form an upper electrode pattern. A contact hole is formed to interconnect the upper electrodes, a metal layer is deposited on the contact hole, is patterned thereby forming an electrode pad.

Description

Method for fabricating josephson junction device operating on high temperature

The present invention relates to a method of manufacturing a Josephson junction element.

The superconductor is an interesting field from an academic point of view because it has unique electrical and magnetic characteristics such as the Messner effect, the Josephson effect, and the flux quantization, in addition to the resistive properties. It has a tremendous potential for application throughout the industry, including medical devices and computers. The biggest obstacle to the practical use of the superconductor is that the critical temperature at which the superconductor phenomenon occurs is extremely low, and therefore, expensive and intractable liquid helium must be used as the refrigerant. In 1987, an oxide superconductor with a critical temperature above the boiling point of liquid temperature (77K) was first discovered, making practical use of superconducting more realistic.

Since the high temperature superconductor is a ceramic material, there is little mechanical processability or ductility, so the research is focused on small-scale electronic engineering applications using thin films rather than large-scale applications using wire rods.

The Josephson effect is a very unusual nonlinear phenomenon in which a pair of electrons is tunneled without a voltage drop when a thin insulating film is sandwiched between two superconductors (Josephson junction).

Josephson junctions made of high-temperature superconductors are very important for studying the properties of high-temperature superconductors that are not yet known, as well as microwave active devices such as sensors, oscillators and mixers, such as superconducting quantum interference devices (SQIDs). It is an essential element for small scale applications of high temperature superconductors, such as ultrafast switching elements. However, because the superconductor has a very short coherence length of ~ 10Å and the large anisotropy of the material, it is difficult to make a conventional tunnel barrier type junction, that is, a SIS (superconductor-insulator-superconductor) junction. Therefore, Josephson junctions are manufactured by several different methods. The methods designed up to now include grain boundary junctions using granular properties of high temperature superconductors, and high temperature superconductor Josephson junctions have been manufactured so far. Bicrystal junctions, step edge junctions, bi-epitaxial junctions, and the like, as shown in FIGS. 1A-1C, respectively.

In the bicrystal bonding, as shown in FIG. 1A, an epitaxial thin film 13 is deposited on a substrate formed by bonding two single crystals 11 and 12 of the same type having different crystal axes, and then epitaxial. By forming the thin film 13 by patterning in a predetermined pattern, an artificial crystal boundary is formed along the joint surface 14 to form a Josephson junction.

As shown in FIG. 1B, the step age junction is formed by forming a steep step 15 on the substrate 10 and epitaxially depositing a high temperature superconductor, thereby forming two grain boundaries on the upper and lower step surfaces 16 and 17. ) And Josephson junction was made.

The bi-epitaxial junction then deposits a seed layer having a suitable lattice constant, such as a seed layer 21 of MgO, on a portion of the substrate 20 as shown in FIG. 1C. On top of that, a buffer layer 22 of SrTiO ₃ is deposited on the entire surface, whereby a 45 ° crystal rotation occurs between the seed layer 21 and the buffer layer 22 on the substrate 20. 23) Epitaxial deposition results in a 45 ° grain boundary 24 forming the Josephson junction.

Although the bi-crystal bonding is superior to the bonding having different characteristics, the bi-crystal bonding requires a special type of substrate, so the manufacturing cost is high, the type of substrate is limited, and the Josephson bonding is formed only within the boundary formed on the substrate. Because of this possibility, the place where the Josephson junction can be manufactured is limited, and there is a drawback that it is not easy to adjust the junction characteristic variables such as critical current (I _c :) and junction resistance (R _n : normal resistance).

In addition, in the case of the step edge junction, there is a problem in that it is difficult to design the device because the junction structure is unnecessary, low reproducibility, and difficult to control the characteristics of the junction.

In addition, in the case of bi-epitaxial bonding, there are problems such as a large manufacturing process, difficult process control, and difficult to control characteristics, resulting in large tolerances between products and low reproducibility.

Therefore, the present invention has been invented in view of the above-described conventional problems, and an object of the present invention is to manufacture a high temperature superconductor Josephson junction device having a simple manufacturing process, not only easy process control but also easy to adjust characteristics and small tolerances between products. It is to provide a method.

Another object of the present invention is to provide a method for manufacturing a high-temperature superconductor Josephson junction can be manufactured by selecting a portion of the Josephson junction is low in cost, easy to control the bonding properties.

Yet another object of the present invention is to provide a method for manufacturing a high temperature superconductor Josephson junction device which is easy to design and has high reproducibility.

1A to 1C are a perspective view and a cross-sectional view of various conventional high temperature superconducting Josephson junction elements,

2A to 2E are cross-sectional views of high-temperature superconducting Josephson junction elements in each manufacturing process of the present invention;

3 is a graph showing the temperature-resistance characteristics of an NBCO thin film fabricated under the same conditions as when bonded on a separate SrTiO ₃ (STO) single crystal substrate;

4 is a view showing the current-voltage characteristics (measurement temperature 77K) of the high-temperature superconducting SNS Josephson junction element of the edge structure by the manufacturing method of the present invention,

5a and 5b are views showing the critical current and the junction resistance characteristics (measurement temperature 97K) according to the thickness change of the high temperature superconducting SNS Josephson element of the edge structure according to the present invention,

6 is a graph comparing the threshold current change with the temperature predicted by the proximity effect theory of the high temperature superconducting SNS Josephson junction element of the edge structure according to the present invention;

7 is a graph showing the magnetic flux-voltage characteristics (measurement temperature 77K) of the high temperature superconducting SNS Josephson junction element of the edge structure according to the present invention.

Explanation of symbols for main parts of the drawings

10, 20, 30: substrate 11, 12: single crystal

13: epitaxial thin film 14: bonding surface

15: step 16, 17: step surface

21: seed layer 22: buffer layer

23: high temperature superconductor layer 24: grain boundary surface

31, 34: superconductor layer 32: insulator layer

34 conductive oxide layer 35 STO thin film

36, 37: gold electrode

Method of manufacturing a high temperature superconductor Josephson junction device of the present invention for achieving the above object comprises the steps of sequentially depositing a first high temperature superconductor layer and a first insulating layer of the YBCO series used as a lower electrode on a single crystal substrate; Etching a portion of the deposited first insulator layer and the first high temperature superconductor layer to form a lower electrode having one end exposed and having a predetermined slope, and including the exposed lower electrode and the insulator layer on the front surface of the substrate. Sequentially depositing a second high superconductor layer and a second insulator layer of the YBCO series used as the upper metal layer and the upper electrode, and forming the lower high electrode on the first high superconductor layer and the first insulator layer. Sequentially etching the second insulating layer, the second high temperature superconductor layer, and the metal layer to form an upper electrode pattern; And forming a contact hole for electrically connecting the lower electrode and the upper electrode, depositing a metal layer on the front surface including the contact hole, and patterning the electrode pad.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2A to 2E schematically illustrate the cross section of each manufacturing process according to an embodiment of the present invention. First, as shown in FIG. 2A, a lower electrode is disposed on a single crystal substrate 30 such as SrTiO ₃ (abbreviated STO) or MgO. A sputtering method, a thermal, or a YBa ₂ Cu ₃ O _7-x (hereinafter abbreviated as YBCO) layer 31 and a 2000 Å thick insulator (STO) layer 32, which are 1000 Å to 2500 Å thick superconductors, are used. It is formed by sequentially depositing using a high temperature superconducting thin film manufacturing method such as e-beam deposition method, or pulsed laser deposition method.

Subsequently, as shown in FIG. 2B, portions of the insulator (STO) layer 32 and the YBCO layer 31 are sequentially etched using a photoetching method and an ion milling etching method to form a lower electrode pattern having an inclined edge of 10 to 30 °. After formation, ion milling washing is performed on the sample top with low energy to remove contaminants. At this time, the slope or surface fine structure of the edge at which the junction is formed when forming the lower electrode pattern is an important factor that determines the characteristics of the junction. Particularly, when the slope becomes more than 30 °, the grain boundary juction is unnecessary for the SNS junction. If the inclination is less than 10 ° it is difficult to manufacture.

Next, as shown in FIG. 2C, an NdBa ₂ Cu ₃ O _7-x layer (hereinafter abbreviated as NBCO) layer having a thickness of 50 μs to 500 μs, which is a metal material of a high temperature superconductor, is formed on the entire surface of the substrate 30 including the lower electrode pattern and the STO layer. ) Is mounted in an evaporation chamber (not shown) to form epitaxial growth, and the YBCO layer 34 having a thickness of 1500 Å to 3000 Å used as the upper electrode and the insulator STO thin film 35 for protecting a device having a thickness of 500 1 to 1500 Å are used. Are sequentially in-situ deposited.

In this case, the NBCO layer 33 may be any of RBa ₂ Cu ₃ O _7-x series high temperature superconductor substituted with a rare earth element (R) instead of YBCO, and the structure of the NBCO layer is the same as that of YBCO. The lattice constants are a = 3.86Å, b = 3.92Å and c = 11.77Å, which are almost the same as YBCO.

As shown in FIG. 2D, the upper electrode pattern is formed by etching the STO thin film 35, the YBCO layer 34, and the NBCO layer 33 formed on the upper side where the lower electrode pattern is formed by the photolithography method and the ion milling method. do.

Next, as shown in FIG. 2E, gold electrode pads 36 and 37 are formed on the lower electrode pattern and the upper electrode pattern by using a lift off process and a sputtering deposition method.

The high temperature superconducting Josephson junction element manufactured by the method of the present invention has a conductive oxide (N) instead of the insulator (I) of the SIS structure, unlike a junction using a conventional SIS (superconductor + insulator + superconductor) form or a grain boundary surface. The superconductor-conductive oxide-superconductor (SNS) -type junction, which inserts the intercalation layer, utilizes the proximity effect that the superconductivity of the superconductor penetrates into the conductive oxide when the superconductor (CS) approaches the conductive oxide (N). The distance between these proximity effects is generally much longer than the tunnel effect length, making it easier to fabricate the joint. The Josephson junction due to the proximity effect has almost the same characteristics as the Josephson junction due to the tunnel effect, and follows the RSJ (Resistively shunted juction) model, so that the applicability is good and the junction structure of the superconductor has an edge structure as shown in FIG. 2E. Since the edge formation position can be changed as needed and the thickness of the junction side or the conductive oxide can be arbitrarily changed, it is easy to control the junction characteristic variables such as the critical current and the junction resistance of the junction, as well as the coherence length. As the coherence length is formed in the ab plane with a large coherence length in the high-temperature superconductor whose coherence length is different depending on the crystal direction, it is easier to manufacture the joint and the joint characteristics are improved.

In addition, the high-temperature superconductor is YBa ₂ Cu ₃ O _7-x (YBCO), and the conductive oxide is RBa ₂ Cu ₃ O _7-x (R is a rare earth element) series of high temperature superconductor, for example, NdBa ₂ Cu ₃ O _{Since 7-x} (NBCO) is used, the structure of the conductive oxide (NBCO) is the same as that of the YBCO structure, and the lattice constants are almost the same. Therefore, the conductive oxide can change the critical temperature (Tc) according to the deposition conditions, and since the NBCO material having a lower threshold temperature (Tc) than the operating temperature of the junction was used, as shown in FIG. 3, a separate SrTiO ₃ (STO NBCO thin film fabricated on the single crystal substrate under the same conditions as the fabrication of the NBCO thin film, that is, it is a metallic material of the NBC conductive oxide used for SNS bonding, as can be seen from the fact that the critical temperature is 26K and exhibits metallic properties. Able to know.

And as can be seen from the graph of Figure 4 shows the current-voltage characteristics of the high-temperature superconducting SNS Josephson junction element of the edge structure according to the present invention, Josephson junction characteristics due to the proximity effect of the Josephson junction element of the present invention is a tunnel effect It can be seen that it is almost the same as Josephson's junction characteristic by, and is in good agreement with the RSJ (resistively shunter juction) model.

In addition, as can be seen from Figure 5 showing the critical current junction characteristics of the high temperature superconducting Josephson junction of the edge structure of the present invention, the high temperature superconducting Josephson junction device of the present invention can arbitrarily adjust the threshold current according to the thickness change of the conductive oxide have.

6 is a graph comparing the predicted value of the critical current change with the temperature of the edge structure of the edge structure according to the present invention according to the temperature and the proximity effect, the measurement results and these values are in good agreement with the present invention. It can be seen that the characteristics of the high temperature superconducting Josephson junction element are due to the proximity effect.

Figure 7 shows the magnetic flux-voltage characteristics of the SQUID of the high-temperature superconducting SNS Josephson junction element of the edge structure according to the present invention can be seen that it works well at 77K, the liquid nitrogen temperature.

As described above, the high temperature superconducting Josephson junction element according to the manufacturing method of the present invention can change the edge formation position as needed and can arbitrarily change the line width of the junction or the thickness of the conductive oxide forming the SNS. It is easy to adjust the bonding characteristics such as resistance, so that the tolerance between products is small and the coherence length is formed in ab plane with large coherence length in high temperature superconductor whose coherence length is different according to the crystal direction. At the same time, there is an effect that the bonding characteristics are improved.

Claims (3)

Sequentially depositing a first high temperature superconductor layer of YBCO series and a first insulating layer to be used as a lower electrode on the single crystal substrate; Etching a portion of the deposited first insulator layer and the first high temperature superconductor layer to form a lower electrode having one end exposed and having a predetermined slope; Sequentially depositing a conductive oxide layer of NdBa2Cu3O7-X, a YBCO-based second high temperature superconductor layer and a second insulator layer, which are used as upper electrodes, on the front surface of the substrate including the exposed lower electrode and insulator layer; Sequentially etching the first high temperature superconductor layer and the second insulating layer, the second high temperature superconductor layer, and the conductive oxide layer forming the lower electrode to form an upper electrode pattern; A high temperature superconducting SNS Josephson junction, comprising: forming a contact hole for electrically connecting the upper electrode and the upper electrode, and depositing a metal layer on the front surface including the contact hole and patterning the electrode pad; Method of manufacturing the device. The method of claim 1, The slope is a method of manufacturing a high temperature superconducting SNS Josephson junction device, characterized in that formed in 10 ~ 30 °. The method of claim 1, The first and second high temperature superconductors of the YBCO series are YBaCu3O7-x method for manufacturing a high temperature superconducting SNS Josephson junction device.

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