RU2538931C2 - METHOD OF FORMING HIGH CURRENT-CARRYING CAPACITY YBa2Cu3O7-x FILMS ON GOLD BUFFER UNDERLAYER - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: in the method of forming high current-carrying capacity YBa₂Cu₃O_7-x films on a gold buffer underlayer, a gold contact pad is formed on a dielectric substrate before depositing YBa₂Cu₃O_7-x films on the dielectric substrate. Targets made of gold and ceramic YBa₂Cu₃O₇ are sputtered using a laser with radiation wavelength of 1.06 mcm, pulse duration of 10-20 ns, pulse repetition frequency of 10 Hz and laser radiation power density of (5-7)·10⁸ W/cm². The method comprises preheating the gold target and substrate to temperature T=450-500°C, establishing pressure of 0.1-0.5 Pa, sputtering the gold target onto the substrate through a screen, placed 0.3-0.5 mm from the substrate, heating the YBa₂Cu₃O₇ target to T=600-700°C, heating the substrate to 800-840°C, establishing pressure of 50-100 Pa and sputtering the YBa₂Cu₃O₇ target onto the formed contact pads to thickness of 50-200 nm to form films with critical temperature of the superconducting junction T_s=88-89 K, width of the superconducting junction ΔT_s= 2-3 K and critical current density J_c>10⁵ A/cm².

EFFECT: obtaining superconducting films with high current-carrying capacity on a gold buffer underlayer, which provide high density of critical supercurrent density.

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Description

The invention relates to methods for forming, by laser spraying on a gold buffer sublayer, superconducting YBa ₂ Cu ₃ O _7-x (YBCO) films. The need to create reliable contact pads for superconducting films is due to the possibility of manufacturing elements of superconducting electronics from them.

Currently, there are various methods of forming contact pads for thin YBCO films for the electrical connection of thin-film elements of superconducting electronics with an electronic sample. Traditionally, metal contact pads for YBCO films are applied over the films [1, 2]. This technology for the manufacture of contact pads has a significant drawback, namely, that when applying the YBCO contact pads, the film is exposed to temperature under vacuum conditions, which leads to oxygen depletion of the film material and destruction of the superconductivity of the surface region associated with the formation of BaO and BaCO ₃ layers, which are dielectrics . To maintain an optimal oxygen index value, for example, plasma etching of the YBCO surface is used immediately before applying the metal in order to remove the degraded layer. After applying the contact pads, the samples are subjected to additional thermal annealing in an oxygen atmosphere. Despite the good results achieved using ion-plasma surface cleaning, such cleaning is accompanied by the destruction of superconductivity in the surface layer a few nanometers thick due to breaking of Cu-O bonds and diffusion of oxygen. A more effective cleaning method — cathodic surface spraying — also does not completely remove the degraded layer. For the manufacture of contact pads for YBCO materials, metals are suitable for which the binding energy is lower than that of Cu-O. Such metals include: Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Hg. Other metals actively interact with the YBCO material, forming poorly conductive or non-conductive layers. All contact materials used can be divided into the following groups: noble metals and their alloys (silver, gold, platinum group metals and alloys based on them), base metals and alloys based on them, cermet compositions [3]. For lightly loaded contacts, gold, rhodium, palladium, platinum and their alloys are used; for moderately loaded ones - palladium, platinum, silver, tungsten, nickel and their alloys; for highly loaded ones - silver, tungsten, nickel, copper, their alloys and cermet compositions, as well as mercury and graphite. At low contact loads and switching low currents, the working conditions of the contacts are very complex, and they are satisfied only with alloys based on platinum, palladium and gold. Gold has high electrical and thermal conductivity, corrosion resistance, does not oxidize and does not form oxide films, has a low and stable transition resistance in various atmospheric conditions at normal, elevated and low temperatures.

A known method of forming a superconducting thin film on the surface of a metal substrate [4], in which a buffer film is formed on the surface of silver, gold or platinum containing bismuth, lead, copper and a combination of alkaline earth calcium with strontium and 0-10% barium, which is subsequently thermally converted to superconducting thin film. The method is quite complicated technologically, as it involves a large number of operations and does not provide superconductors with a high critical current density.

A known method of forming superconducting thin films [5] on a metal substrate by magnetron sputtering of an oxide compound, in which the substrate and the target are placed in a vacuum chamber parallel to each other with the possibility of moving one relative to the other. The method allows to obtain a superconducting film coating, but with a small critical current value.

Closest to the claimed is a method of forming superconducting thin films of YBCO on a gold or platinum contact pad [6], in which to ensure better contact when the superconducting device is included in the electrical circuit and maintain the superconducting properties, the superconducting electrodes are sprayed over a sublayer of gold or platinum. The main disadvantages of this method are the following: 1) gold or platinum pads are applied to the dielectric substrate by thermal spraying at a temperature of 1700-1800 ° C, which does not provide the necessary adhesion of a gold or platinum film with a dielectric substrate; 2) the superconducting transport properties of YBCO films at the metal-superconductor interface are suppressed and the critical current density does not exceed 10 ³ A / cm ² , which leads to relatively rapid degradation of superconducting properties during storage and operation of superconducting sensors due to the formation of non-superconducting intergranular interlayers in the YBCO material films.

The objective of the present invention is to develop a method for forming by method of laser ablation on a gold buffer sublayer superconducting thin YBCO films with high current-carrying properties, providing a density of superconducting critical current of not less than 10 ⁵ A / cm ² characteristic of high-quality intergranular interlayers in the material of YBCO film. Improving the quality of intergranular layers in turn increases the stability of the superconductor-gold system to degradation. In addition, this method allows the formation of a gold contact pad with a thickness of up to 500 nm, which is sufficient for reliable welding of external electrodes.

The indicated technical result is achieved by the fact that a gold contact pad is formed on a dielectric substrate (for example, SiTiO ₃ , LaAlO ₃ , etc.) by laser spraying using a special nichrome mask, the adherence of which to the dielectric substrate is controlled by an additional thin nichrome plate with thickness d = 0 3-0.5 mm, as shown in figure 1. The gap between the mask and the substrate allows you to form a gold film of variable thickness, as shown in figure 2. In the shadow region, a sharp decrease in the thickness of the gold film from 500 to 50 nm is observed, as shown in Fig. 3, and then at a distance of several hundred micrometers a smooth decrease in the thickness of the gold film from 50 nm to zero occurs. The width of this thin section varies with the degree of fit of the mask. Next, a superconducting YBCO film is grown on a substrate with a gold film by laser spraying to form a YBCO / Au / substrate system using another nichrome mask that covers the areas of future contact pads on the gold film, as shown in Fig. 4. A superconducting film with a thickness of 50-200 nm is formed in thin sections of a gold film with a thickness of 0-50 nm and has sufficiently high superconducting parameters: critical temperature of the superconducting transition T _c ≈88-89 K, width of the superconducting transition ΔT≈2-3 K, critical current density J _c ≈10 ⁵ A / cm ² . The specific electrical resistance of the contact does not exceed the specific resistance of the gold film and is of the order of 10 ^-10 Ohm · cm ² . Figure 5 presents microphotographs of nucleation on a thin gold film of the islands of the YBCO film (a), their fusion (b) and further growth of the continuous YBCO film (c). According to the authors, high values of superconducting parameters, especially the critical current density, indicate the epitaxial growth of the YBCO film on a gold film, which, in turn, is a consequence of the long-range effect (the ability of single-crystal substrates to affect the orientation of thin films through buffer layers of various substances). The reason for the long-range at given values of the thickness of the buffer gold film can be elastic stresses between the substrate and the buffer layer [7], forming elastically stressed regions with a certain distribution of elastic energy, including a change in the density of elastic energy depending on the orientation angle. For example, in Fig.6. The photo of YBCO film with gold pads is presented.

To implement the method, we used an experimental setup with a vacuum spray chamber and a powerful pulsed Nd: YAG laser with a radiation wavelength of 1.06 μm, a pulse duration of 10–20 ns, and a pulse repetition rate of 10 Hz. The power density of the laser radiation on the surface of the gold target is (5-7) · 10 ⁸ W / cm ² . A laser beam hits a gold target, passing through a focusing lens and a quartz window of a vacuum chamber. The sprayed material of the gold target is deposited on the dielectric substrate through a nichrome mask with the desired geometry of the pattern. The nichrome mask is fixed at a distance of 0.3-0.5 mm from the substrate. The substrate with the mask is installed at a distance of 4-6 mm from the gold target with the working surface to the target. A pressure of 0.1-0.5 Pa is set in the vacuum chamber, which ensures the necessary size and shape of the plasma torch. The target and substrate are heated to a temperature of 450-500 ° C. After the deposition process, air is let into the vacuum chamber to normal atmospheric pressure and the sample is cooled to room temperature. For the deposition of a superconducting film, sections of the gold film intended for welding gold contact wires are covered with a nichrome mask. When sputtering YBCO films, the substrate temperature is set at 800-840 ° C, the target temperature is 600-700 ° C, the pressure in the vacuum chamber is 50-100 Pa, the target-substrate distance is 25-30 mm, and the laser radiation power density on the surface of the YBCO target (3- 5) · 10 ⁸ W / cm ² . After the deposition process, air is let into the vacuum chamber to normal atmospheric pressure and the sample is cooled to room temperature with a special annealing mode.

Literature

1. C. Peroz, C. Willard, D. Buzon and P. Tixador. Current limitation properties of YBCO / Au thin films // Supercond. Sci. Technol. 16 (2003), p. 54-59.

2. D. Liu, M. Zhou, X. Wang, H. Suo, T. Zuo, M. Schindl and R. Flukiger. Epitaxial growth of biaxially oriented YBCO films on silver // Supercond. Sci. Technol. 14 (2001), p.806-809.

3. Pyatin Yu.M. Materials in instrumentation and automation. M .: Engineering, 1982, 528 p.

4. Application EPO 0490784, cl. H01L 39/24, C 04 V 35/00, 1990.

5. Application EPO 0462906, cl. H01L 39/24, 1990.

6. RF patent No. 2133525 "Superconducting quantum interference sensor and method of its manufacture."

7. V.M. Ievlev, L.I. Trusov, V.A. Kholmyansky. Structural transformations in thin films. M .: Metallurgy, 1982, 248 p.

Claims (1)

A method of forming YBa ₂ Cu ₃ O _7-x films with high current-carrying ability on a gold buffer sublayer in which contact pads are formed before applying YBa ₂ Cu ₃ O _7-x films on a dielectric substrate, characterized in that for sputtering gold targets and YBa ₂ Cu ₃ O ₇ ceramics uses a laser with a radiation wavelength λ = 1.06 μm, a pulse duration of τ = 10 ÷ 20 ns and a pulse repetition rate of ν = 10 Hz, a laser radiation power density of P = (5 ÷ 7) · 10 ⁸ W / cm ² , while the gold target and the substrate are preheated to a temperature T = 450 ÷ 500 ° C, a pressure of p = 0.1 ÷ 0.5 Pa is set, after which a gold target, located at a distance of 4 ÷ 6 mm to the substrate, is sprayed onto the substrate through a mask located at a distance of 0.3 ÷ 0.5 mm from the substrate and the forming film of variable thickness 0 ÷ 500 nm, then the YBa ₂ Cu ₃ O ₇ target is heated to T = 600 ÷ 700 ° C, located at a distance of 25 ÷ 30 mm from the substrate, the substrate is heated to a temperature of T = 800 ÷ 840 ° C, set pressure p = 50 ÷ 100 Pa, after which the sprayed target YBa ₂ Cu ₃ O ₇ formed on the contact pads to a thickness L = 50 ÷ 200 nm to form a film with riticheskoy superconducting transition temperature T _c ≈88 ÷ 89 K, the superconducting transition width ΔT _c 2 ÷ 3 K, the critical current density J _c> 10 ⁵ A / cm ^2.

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