Process for forming superconducting film

Abstract: Disclosed is a process for forming a superconducting film, in which multi-layer metal film (buffer film) is formed at a specific temperature on a ceramic substrate and a superconducting film is formed at a specific temperature on the multi-layer metal film. According to this process, a superconducting film having a high critical temperature can be formed over the ceramic substrate while controlling or suppressing the occurrence of a chemical reaction between the substrate and the superconducting film, and required superconducting performances can be manifested.

Description: The present invention relates to a process for forming a superconducting film. More particularly, the present invention relates to a process comprising forming a plurality of metal layers (buffer layers) on a ceramic substrate and forming a wiring pattern of an oxide superconductor on the buffer layers. An example of a method of forming a wiring pattern on a ceramic substrate for the circuit board (to interconnect between electronic elements) to be used for an electronic apparatus, is a method which comprises forming a thin film on the substrate by vacuum deposition or the like and forming a wiring pattern by an etching technique. Another example is a method comprising forming a wiring pattern on the substrate by using a paste composed mainly of a wiring conductor, by a printing technique, and firing the printed pattern to form a thick film pattern. The material of the film may be not only normal conductors such as Cu and Mo, but also superconductors such as those of Nb or Y-Ba-Cu-O type (Ref. M.K. Wu, L.R. Ashburn, C.J. Trong, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, and C.W. Chu: Phy. Rev. Lett., 58, (1987) 908), those of Bi-Sr-Ca-Cu-O type discovered by H. Maeda et al. (Ref. H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano: Jpn. J. Appl. Phys. 27 (1988) L209) and those of Bi-Pb-Sr-Ca-Cu-O reported by M. Takano et al. (Ref. M. Takano, J. Takada, K. Oda, H. Kitaguchi, Y. Miura, Y. Ikeda; Y. Tomii, and H. Mazaki: Jpn. Appln. Phys: 27 (1988) L 1041). Nevertheless, when a film of a conductor such as the above-mentioned normal conductor or superconductor is formed on the substrate, the intended substance does not have a homogeneous composition, due to a mutual reaction between the film and the substrate, for example, a chemical reaction, and thus sometimes the intended performance of the normal conductor and, more especially the superconductor is not obtained. A superconducting film device prepared by a method resembling the process of the present invention is known (Japanese Unexamined Patent Publication No. 63-279521), but this device is prepared by forming a metal film with a thermal expansion coefficient of at least 10 x 10 -6 /K in both directions on a ceramic substrate, and forming a covering film of a perovskite type metal oxide superconductor. This known technique is different from the process of the present invention in that, in this known technique, to prevent a formation of cracks in the coating film of the superconductor, a conductor layer of a metal having a specific thermal expansion coefficient is interposed between the ceramic substrate and the superconductor layer. Japanese Unexamined Patent Publication (Kokai) No. 63-305574 (Publication Date: December 13, 1988) discloses a superconducting substrate for forming a superconductor on a supporting substrate comprising a composite substrate with a stabilizing material which does not react chemically, interposed between the supporting substrate and the superconductor. Japanese Unexamined Patent Publication No. 63-305574 also discloses the formation of a superconducting film of the Y-Ba-Cu-O system, which comprises forming a metal (Pt, Pd, Ag, Au) layer by a deposition, splattering, flame spraying or plating process, and the like, on an Al 2 O 3 substrate, and forming the superconducting film on thus-obtained metal layer. There is no disclosure or suggestion therein that the double layer of the above metal layer is formed as a stabilizing material. Therefore, this conventional technique is clearly different from the present invention in object and structure. EP-A-0303083 and EP-A-0312015, which are a part of the state of the art according to Article 54(3) EPC, both disclose methods of manufacturing oxide superconductor devices in which one or more layers of metal are interposed between the substrate and the oxide superconductor layer. An object of the present invention is to form a uniform superconducting film on a ceramic substrate while controlling the occurrence of a chemical reaction between the ceramic substrate and the-superconducting film. According to the present invention, there is provided a process for forming a superconducting film containing copper oxide on a ceramic substrate, which process comprises: forming a multi-layered film on said substrate by printing a paste of a transition metal on the substrate and heating to a first temperature to form a first layer, and printing a paste of the same, or of a different, transition metal on the first layer and heating to said first temperature to form a second layer; and forming a superconducting film on said multi-layered film by applying onto said multi-layered film and heating to a second temperature, a superconducting film-forming substance containing copper oxide, said second temperature being lower than the first temperature and within a range in which a crystal of the superconducting film-forming substance can be grown. The present inventors carried out research with a view to realizing the foregoing object. As a result, they found that if, when forming a film of a superconductor on a ceramic substrate, a plurality of layers of a buffer substance which substantially does not react with the substance of the superconducting film are formed between the substrate and the superconducting film by a specific method, the occurrence of a chemical reaction between the substrate and the superconducting film can be effectively controlled, prevented or avoided. The present invention is based on this finding. In addition to enabling the aforesaid object to be met, as will be apparent from the following description, the process in accordance with the present invention can enable a superconducting film to be formed on a ceramic substrate through at least two intermediate layers, while preventing or controlling the formation of pinholes in the metal layers by forming the metal layers by a specific method. For a better understanding of the invention, and to show how the same can be carried into effect, reference will be made, by way of example only, to the accompanying drawings wherein: Fig. 1 is a block diagram illustrating the steps in a part of one process embodying the present invention; Fig. 2 is a block diagram illustrating the steps in a process embodying the present invention for preparing a metal film-formed substrate; Fig. 3 shows an X-ray diffraction pattern of a sample obtained according to a process embodying the present invention; and Fig. 4 is a graph illustrating the temperature dependency of the electric resistivity of a sample obtained according to a process embodying the present invention. As is apparent from the foregoing description, the process of the present invention comprises the step of forming a multi-layer metal film (buffer layer) having at least two layers on a ceramic substrate, such as an alumina substrate or Y 2 O 3 stabilized zirconia substrate, and the step of forming a film of an oxide superconductor on the metal film. These steps will now be described in detail, respectively. In the step of forming the multi-layer metal film having at least two layers, the following materials are preferably used as the metal film-constituting material. For the material of the first metal film layer formed on the ceramic substrate, transition metals, especially metals of Group Ib (copper group Cu, Ag, Au), and elements having the same crystal structure as that of the metals of Group Ib and having an atomic number adjacent to those of metals of the Group Ib, for example, elements of Group VIII, i.e., metals of the iron group (such as Fe, Co and Ni), metals of the platinum group (such as Pd and Pt), and Ru, Rh, Os and Ir can be used. A metal that can be bonded to a metal of Group lb through oxygen, for example, Cr, also can be used. The requirements of the present invention are met if the metal constituting the second metal film layer formed on the first metal film layer is a metal that can adhere closely to the metal of the first metal film layer. More specifically, this metal is selected from the above-mentioned metals and can be the same as or different from the metal of the first metal film layer. Furthermore, a metal that has little influence on the oxide superconductive material, or has no influence on the appearance of the superconducting phase, even if slightly substituted, can be used as the element of the metal layer. When employing Cu, Fe, Co, Ni and the like as the element of the metal layer, an inert gas atmosphere may be employed as a firing environment in which the metal layer is formed. Furthermore, an oxidizing environment, such as the atmosphere, can be employed as the firing environment in which the superconducting phase is formed. In this case, the upper metal layer might be oxidized, but even in such a case, an element of the metal layer which does not react with the superconductive material formed on or over the metal layer can be employed. In one embodiment the first metal film layer is formed on a ceramic substrate such as an alumina substrate by, for example, printing, drying and firing a paste (free of glass frit) composed only of an organic substance and a metal powder. After the firing, a paste of a metal which is the same as, or different from, the metal of the first metal film layer is printed and fired on the formed first metal film layer, whereby a ceramic substrate having at least two metal film layers formed thereon can be prepared. This multi-layered metal film acts as a buffer layer. An oxide superconductor, or a composition thereof, is deposited on the buffer layer and is then heat treated to obtain a superconducting film. It was found that, even if a metal film that substantially does not react with a superconducting substance is used, according to the state of the material texture of this metal film, the formation of a superconductor is sometimes inhibited. To realize the objects of the present invention, the firing temperature adopted for the formation of the metal film must be higher than the sintering temperature of the oxide superconductor. This is because, if the firing temperature for the formation of the metal film is equal to or lower than the firing temperature of the oxide superconductor, the energy state which is used mainly for sintering, in the vicinity of the surface of the metal constituting the metal film is often higher than the energy state obtained by a high-temperature firing. In this case, during the heat treatment of the oxide superconductor, the metal in the metal film causes a mass transfer, which causes a fluctuation in the composition of the superconducting film and a manifestation of the superconductivity is inhibited. Also the formation of the superconducting film must be carried out at temperatures within a range at which a crystal of the superconductor can be grown. If the sintering is carried out at a temperature outside the above-mentioned range, the growth of the crystal of the superconducting substance is inhibited, and the superconductivity having a predetermined critical temperature is not manifested. To prepare a high-temperature superconducting film having good superconducting characteristics, a temperature at which the superconducting film is melted is preferably adopted for the heat treatment. In this case, if pinholes exist in the buffer layer, a part of the molten composition for obtaining superconductivity migrates to the ceramic substrate, and as a result, the composition fluctuates and a good superconducting film cannot be obtained. According to the present invention, by forming a multi-layer metal film (buffer layer) comprising at least two metal film layers, and carrying out the formation of the metal film at a firing temperature higher than the firing temperature for the formation of the oxide superconductor, a diffusion of the superconducting substance into the buffer layer, and in consequence to the substrate, does not occur during the firing for forming the superconducting film (that is, the mass transfer to the buffer layer and to the substrate is controlled or prevented), and a good superconducting layer can be formed. It is considered that this is because the formation of pinholes in the buffer layer is prevented and the number of intergranular pores in the buffer layer is reduced. The firing temperature for forming the superconducting film on the substrate is selected from a range at which a crystal of the superconductor can be grown, as described hereinafter, and therefore, this firing temperature is changed in accordance with the superconductor-forming composition. The present invention will now be described in detail with reference to the following examples, which by no means limit the scope of the invention. Examples Examples 1 to 4 and Comparative Examples 1 and 2 The steps of one process embodying the present invention are illustrated in brief in Figs. 1 and 2. Figure 1 shows the steps of preparing a metal film-formed substrate, and Fig. 2 shows the step of forming a superconducting film over an alumina substrate. An Ag paste comprising Ag powder having an average particle size of 1 µm and a purity higher than 99.9%, and an organic substance (a solvent and an organic resin; the part by weight of the organic resin is 0.8 to 100 parts by weight of Ag powder) used as a vehicle, was printed on an Al 2 O 3 substrate having a purity of 99.7%. The printed substrates were dried at 110°C, and fired under various conditions at temperatures between 800 and 970°C for 10 minutes in air. The Ag paste was printed and dried on the fired Ag film in the same manner as described above and then fired to obtain a metal film-formed substrate (see Fig. 1.). The thickness of the metal film is not less than several µm, preferably more than 10 µm. This is to avoid the formation of pinholes in the metal layer, as an uneven metal layer is formed when the thickness of the metal layer is thicker than several µm. The total thickness of all of the metal layers obtained by a plurality of printing and firing processes is more than 20 µm. In this example, the thickness of the Ag layer formed by the first firing was 10 µm, and the total thickness of the Ag layer obtained by coating and firing the second Ag paste was 20 µm. By using the substrate fired at 800°C, when the firing of a superconducting film described hereinafter was conducted, the component of the superconducting substance migrated into the alumina substrate through the Ag film. When using the substrate fired at 900 to 955°C, diffusion of the component of the superconducting substance into the substrate was not detected at the firing of the superconducting film; and in the case of the substrate fired at a temperature higher than about 960°C, Ag melted and no film was formed on the substrate because of the surface tension. The substrate which had been fired twice at a high temperature for forming the metal layers on the substrate, as shown in Fig. 1, was used for the operations in the following steps. A high-temperature superconducting film was prepared in the following manner. In the case of the Bi-Pb-Sr-Ca-Cu-O system (the final amount of Pb was very small or not detectable), a composition for a high-temperature superconductor having a critical temperature of the 110 K class was prepared. As an example of the Bi-Pb-Sr-Ca-Cu-O system, Bi 0.7 Pb 0.3 Sr 1 Ca 1 CU 1.8 O 9.15 was used in this example. This composition was coarsely pulverized in a mortar and pestle and further pulverized in a ball mill to obtain a powder having an average particle size smaller than about 10 µm. The powder was mixed and kneaded with a vehicle composed of organic substances to prepare a paste composed mainly of a powder of the Bi-Pb-Sr-Ca-Cu-O system. This paste was printed and dried on an alumina substrate without buffer layers and a metal film-formed alumina substrate fired at about 900°C, and the substrates with the paste of the superconducting substances (or superconducting forming substance) printed thereon were fired in air to prepare superconducting film-formed substrate (see Fig. 2.). The firing temperature was within a range at which a crystal of a superconductor could be grown. In the case of the Bi-Pb-Sr-Ca-Cu-O system, this temperature range is from 830 to 860°C. The thickness of the superconducting film to be formed is 5 to 100 µm, although this depends on the mesh size during screen printing. In this example, a superconducting layer having a thickness of about 80 µm was formed. Accordingly, in Examples 1 to 4 of the present invention, as shown in Table 1 given hereinafter, the firing was carried out at 840°C, although the firing was carried out at 870°C in Comparative Example 1. Note, in Comparative Example 2, the metal film was not formed on the substrate. When the sample of Comparative Example 1 was analyzed by the X ray diffractometry using Cu Ka radiation, Ag, Bi-Sr-Cu-O superconducting phase with a Tc of about 10 K, CaO and CuO were detected on the film surface. The peak of the crystal of the high-Tc phase of the superconductor having the critical temperature of the 80 K or 110 K class of the Bi-Sr-Ca-Cu-O system was not detected. References on the Bi-Sr-Cu-O superconducting phase are as follows: (1) J. Akimitsu, A. Yamazaki, H. Sawa and H. Fujiki: Jpn. J. Appln. Phys. 26 (1987) L2080. (2) C. Michel, M. Hervier, H.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau: Z. Phys. B 68 (1987) 421. X-ray diffraction patterns of the samples of Examples 1 to 4 are shown in Fig. 3. In each sample, peaks of crystals of the low-Tc and high-Tc phases of the superconductor having the critical temperature of the 80 K or 110 K class were detected. From the results obtained when the firing was conducted for 1, 6 or 50 hours, it is seen that as the firing time is made longer, the peaks corresponding to the superconducting crystal having the critical temperature of 110 K class tend to increase, but when the firing time was 90 hours, the peaks decreased. Figure 4 shows the temperature dependency of the electric resistivity in the samples of Examples 2 to 4. In the sample of Example 3, a superconducting film in which the Tce (Tc end point) was about 90 K, was formed and in the sample of Comparative Example 2, the reaction with the substrate was observed but the transition to the superconductor was not observed. Sample Ag Metal Film Firing Temperature (°C) Firing Time (hours) Comparative formed 870 1 Example 1 Example 1 formed 840 1 Example 2 formed 840 6 Example 3 formed 840 50 Example 4 formed 840 90 Comparative not formed 840 1 Example 2 Example 5 An example using a Y 2 O 3 -partially-stabilized zirconia substrate as the ceramic substrate will now be described. An Ag paste composed of Ag powder having an average particle size of 1 µm and a purity higher than 99.9% and a vehicle formed of organic substances,was printed on a zirconia substrate and dried at 110°C. Firing was carried out at temperatures between 900 and 960°C in air for 10 minutes. The Ag paste was printed and dried on the fired Ag film and was then fired to obtain a zirconia substrate having two metal layers formed thereon. A composition for a high temperature superconductor of the Bi-Pb-Sr-Ca-Cu-O system having a critical temperature of the 110 K class was prepared. This composition was coarsely pulverized in a mortar and pestle and further pulverised in a ball mill to obtain a powder having an average particle size smaller than about 10 µm; the powder was mixed and kneaded with a vehicle composed of organic substances to form a paste comprising, as the main component, a powder of the Bi-Pb-Sr-Ca-Cu-O system. The paste was printed and dried on the zirconia substrate without metal layers and the zirconia substrate having two metal layers formed thereon, and firing was carried out at 840 to 860°C in air. The sample obtained by carrying out printing, drying and firing directly on the zirconia substrate had poor adhesion of the formed film to the substrate and the Bi-Pb-Sr-Ca-Cu-O film was easily peeled off. Furthermore, the resistivity remained at about 77 K. ln the sample obtained by carrying out printing the paste of superconducting substance (or superconductor forming substance), drying and firing on the metal layers, little peeling of the Bi-Pb-Sr-Ca-Cu-O film occurred, and a superconducting film having a Tco of about 110 K and a Tce of about 90 K was formed. The present invention has been described with reference to embodiments in which the intermediate metal film (buffer layer) comprises two layers. In the present invention, however, a method can be adopted in which a third metal film, composed of a metal having a good adherence to the metal of the above-mentioned second metal film, is formed on the second metal film to form a buffer layer with a three-layer structure. Example 6 Au paste instead of Ag paste was employed as the metal layer, and subjected the same process as in Example 1, except that the firing of Au paste printed substrate was carried out at 900°C, to obtain the same results as obtained in Example 1. As apparent from the foregoing description, according to the process of the present invention a thick superconducting film having a high critical temperature can be formed by using a ceramic material as a substrate. Furthermore, since a metal having a normal conductivity is used as the material of the buffer layer, this buffer layer can be used as a medium for maintaining a good connection between a normal conductor and a superconducting film, and accordingly, required conductor performances can be manifested.

Claims: A process for forming a superconducting film containing copper oxide on a ceramic substrate, which process comprises: forming a multi-layered film on said substrate by printing a paste of a transition metal on the substrate and heating to a first temperature to form a first layer, and printing a paste of the same, or of a different, transition metal on the first layer and heating to said first temperature to form a second layer; and forming a superconducting film on said multi-layered film by applying onto said multi-layered film and heating to a second temperature, a superconducting film-forming substance containing copper oxide, said second temperature being lower than the first temperature and within a range in which a crystal of the superconducting film-forming substance can be grown. A process as claimed in claim 1, wherein said ceramic substrate is an alumina substrate or a zirconia substrate. A process as claimed in claim 1 or claim 2, wherein the transition metal or metals is a metal/are metals of Group Ib (copper group) or Group VIII of the Periodic System of the Elements. A process as claimed in claim 3, wherein said first layer is formed by printing a paste of a metal of Group Ib or VIII on the ceramic substrate and heating to said first temperature, and said second layer is formed by printing on the first layer a paste of a second metal having good adhesion to the first metal and heating to said first temperature. A process as claimed in any preceding claim, wherein said multi-layered film is formed by additionally printing on the second layer a paste of a transition metal having good adhesion to the second layer and heating to said first temperature to form a third layer. A process as claimed in any one of claims 1 to 5, wherein the transition metal or metals is/are selected from Cu, Ag, Au, Ni, Pd and Pt. A process as claimed in any preceding claim, wherein the superconducting film is an oxide of the Bi-Pb-Sr-Ca-Cu-O system. A process as claimed in any preceding claim, wherein said first temperature is in the range 900 to 955 °C A process as claimed in any preceding claim, wherein said second temperature is in the range 830 to 860 °C.

Inventors: KAMEHARA NOBUO, NIWA KOICHI, UZUMAKI TAKUYA, YAMANAKA KAZUNORI, KAMEHARA, NOBUO, NIWA, KOICHI, UZUMAKI, TAKUYA, YAMANAKA, KAZUNORI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: H01L39
IPC codes: H01L39