Planar Josephson device

Abstract: The invention relates to a Josephson device and a method of manufacturing such a device which comprises two layers 11, 11′ of an oxidic superconducting material between which at least one non-­ superconducting layer 13 of silver sulphate is provided. The non-­ superconducting layer 13 is connected to the superconducting layers 11, 11′ through silver layers 12, 12′.

Description: The invention relates to a planar Josephson device and a method of manufacturing thereof, which device comprises two layers of an oxidic superconducting material between which at least one non-superconducting layer is provided. A method of manufacturing a Josephson device or Josephson junction is described in an article by M.G. Blamire et. al., in J. Phys. D: Appl. Phys. 20 , pages 1330-1335 (1987). In this article, a sapphire substrate is provided with a buffer layer of Y₂O₃ to which a first oxidic superconducting layer of YBa₂CU₃O 7-δ is applied. Subsequently, a thin layer of Y₂O₃ is provided which serves as a tunnel barrier. Next, a second layer of YBa₂Cu₃O 7-δ is applied, on which contacts of Nb may be provided. The thin layers are manufactured by means of dc magnetron sputtering using metallic target plates in an oxidizing atmosphere. Patterns are formed in the layers by means of plasma etching or ion etching. In the oxidic superconducting materials known so far, the superconducting properties, in particular the highest temperature T C at which superconductivity occurs, are negatively influenced by the presence of a large number of metallic elements, silver and gold being two of the few exceptions. In particular a disturbance of the oxygen content seems to have undesirable effects. In the manufacture of devices such as a Josephson junction, in which a tunnel barrier is required, it must therefore be precluded that diffusion takes place from the tunnel barrier towards the superconducting material or that a reaction between the tunnel barrier and the superconducting material takes place. To this effect, buffer layers of a metal which passes oxygen such as silver can be used, however, this is not always sufficient at a high temperature. An object of the invention is, inter alia , to provide a Josephson device and a simple method of manufacturing such a device, in which apart from silver no other metallic elements are used in the tunnel barrier. A further object of the invention is to provide a tunnel barrier for a Josephson device, which is stable at the high temperature necessary during firing the superconducting material. An additional object of the invention is to provide a Josephson device in which the two superconducting films are both formed from the same or two different oxidic superconducting materials. According to the invention, this object is achieved by a device and a method as described in the opening paragraph, in which the non-superconducting layer is manufactured from a silver salt which is connected to the superconducting layers through layers of silver. For example, silver chloride, silver bromide, silver chlorate and silver phosphate can be used as silver salt. In a preferred embodiment of the method according to the invention the silver salt is silver sulphate. In a suitable embodiment of the method according to the invention, the silver sulphate is manufactured from silver by a reaction with a mixture of sulphur dioxide and sulphur trioxide. In an alternative embodiment of the method according to the invention, the silver sulphate is manufactured from silver by a reaction with hydrogen sulphide followed by oxidation with oxygen. A particular advantage of this embodiment is the possibility to simultaneously carry out the oxidation with oxygen and a firing step of the oxidic superconducting material. A method of manufacturing a Josephson device in which a non-superconducting layer is connected to two layers of an oxidic superconducting material through layers of noble metal is described in the non-prepublished Netherlands Patent Application NL 8900405. Thin films of oxidic superconducting material can be obtained by means of various methods which are known per se . In general, a high-temperature treatment is necessary to obtain the desired composition, in particular, as regards the oxygen balance and the valence of the metal atoms, in particular copper atoms. According to M. Gurvitch et. al. in Appl. Phys. Lett. 51 (13), pages 1027-1029 (1987) thin films of the oxidic superconducting material YBa₂Cu₃O 7-δ are manufactured, for example, by means of dc magnetron sputter deposition using metal target plates in an oxidizing atmosphere, followed by firing at a high temperature, for example 800 to 900°C. Other commonly used methods are vacuum evaporation, laser ablation and chemical deposition from the vapour phase. In Appl. Phys. Lett. 52 (21), pages 1828-1830 (1988), C.E. Rice et.al. describe a method of manufacturing thin films of Ca-Sr-Bi-Cu oxides by applying CaF₂, SrF₂, Bi and Cu by means of vacuum evaporation, followed by oxidation steps at 725° and 850°C. The treatments at high temperature are necessary to obtain a material having a high critical temperature T C , i.e., the temperature below which superconducting behaviour occurs. The invention will be explained in greater detail by means of exemplary embodiments and with reference to the accompanying drawing, in which Fig. 1 diagrammatically shows a Josephson device according to the invention, and Fig. 2a-c diagrammatically shows a number of steps of a method according to the invention. Exemplary embodiment 1 . Fig. 2a shows a substrate 10 of strontium titanate SrTiO₃ on which an oxidic superconducting layer 11 of YBa₂Cu₃O 6.7 is provided, for example, by means of vacuum evaporation, the substrate being maintained at a temperature of 850°C. Other substrate materials, for example MgO, Y₂O₃ and substrates such as sapphire with protective buffer layers, for example Ta₂O₅, LaF₃ and ZrO₂ may also suitably be used in the method according to the invention. Substitutions in the superconducting material may be carried out in known manner without affecting the effectiveness of the method according to the invention. For example, Y may be completely or partly replaced by rare earth metal ions, Ba may be replaced by Sr or Ca, and O may be partly replaced by F. The invention may be alternatively applied using other oxidic superconducting materials such as Ca-Sr-Bi-Cu oxides, Bi being partly replaceable by Pb, (La, Sr)₂CuO₄ and cuprates containing Ca and/or Ba in addition to Tl. For the manufacture of the layer 11, sputtering using a target plate having the desired composition may for example be used instead of vacuum evaporation. In this process, the substrate is maintained at a high temperature but deposition may also take place at a lower temperature. In the latter case, an aftertreatment at a high temperature is required in order to obtain the desired superconducting properties. In a suitable embodiment of the method according to the invention, the layer 11 is manufactured by means of dc triode sputtering at a voltage of 1 kV in Ar at a pressure of approximately 0.5 Pa. Subsequently, this layer is subjected to a firing treatment at 700° to 800°C in oxygen for approximately 2 hours, as a result of which the layer 11 obtains the appropriate oxygen content and the desired superconducting properties. A layer 12 of silver having a thickness of 50 nm, see Fig. 2b, is applied to the layer 11, for example, by means of sputtering. Subsequently, the layers are subjected to a firing treatment at 450°C in oxygen for 0.5 hours, thus forming a superconducting contact between the silver 12 and the superconducting material 11. The surface of the silver layer is converted into silver sulphate Ag₂SO₄ by a reaction with a gas mixture of H₂S and N₂ (volume ratio 1 : 4) at room temperature and a pressure of 1 atmosphere for 10 minutes, followed by a post-firing treatment at 350°C in oxygen for 1 hour. In this manner, a layer 13 of silver sulphate having a thickness of approximately 2 nm is manufactured. A second superconducting layer 11′ which is applied to a substrate 10′ and on which a silver layer 12′ is provided, is pressed onto the silver sulphate layer 13 at a temperature of 200°C and a pressure of approximately 30 N/mm² in an oxygen atmosphere. Fig. 1 shows the Josephson device manufactured in this manner. Prior to pressing the two parts of the Josephson device together, the layers may be provided, in known manner, with patterns, for example, by means of photolithographic techniques followed by plasma etching or ion etching. The Josephson device may alternatively be manufactured in such a manner that after forming the silver sulphate layer 13 a silver layer 12′ is provided thereon by means of vacuum evaporation after which a superconducting layer 11′ is applied to the latter. It is particularly efficacious to ensure that the conversion of silver sulphide (formed by the reaction with H₂S) into silver sulphate takes place simultaneously with the firing treatment necessary for the superconducting layer or layers. Exemplary embodiment 2 . A Josephson device is manufactured in the manner described in exemplary embodiment 1, with this difference that the silver sulphate layer 13 is manufactured by a reaction of the silver layer 12 with a gas mixture of SO₂ and SO₃ (volume ratio 1 : 1) at room temperature and atmospheric pressure for 10 minutes.

Claims: A planar Josephson device comprising two layers of an oxidic superconducting material between which at least one non-superconducting layer is provided, characterized in that the non-superconducting layer consists of a silver salt which is connected to the superconducting layers through layers of silver. A method of manufacturing a Josephson device comprising two layers of an oxidic superconducting material between which at least one non-superconducting layer is provided, characterized in that the non-superconducting layer is manufactured from a silver salt which is connected to the superconducting layers through layers of silver. A method as claimed in Claim 2, characterized in that the silver salt is silver sulphate. A method as claimed in Claim 3, characterized in that the silver sulphate is manufactured from silver by a reaction with a mixture of sulphur dioxide and sulphur trioxide. A method as claimed in Claim 3, characterized in that the silver sulphate is manufactured from silver by a reaction with hydrogen sulphide followed by oxidation with oxygen. A method as claimed in Claim 5, characterized in that the oxidation with oxygen is carried out simultaneously with a firing step of the oxidic superconducting material.

Inventors: SCHOLTEN DAPHNE, SCHOLTEN, DAPHNE
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H01L39
IPC codes: H01L39