A process for preparing a superconducting thin film of compound oxide

Abstract: In order to prepare a well-crystalline thin film of compound oxide superconductor a silicon wafer, before the thin film of compound oxide superconductor is deposited on the silicon wafer, the silicon wafer is heated firstly at a temperature of higher than 900°C in high vacuum of lower than 10⁻⁶ Torr, then a thin film of ZrO₂ is deposited secondly on the silicon wafer, and finally, the thin film of ZrO₂ deposited on the silicon wafer is annealed thirdly in air at a temperature of 800 to 850°C.

Description: Background of the Invention Field of the Invention The present invention relates to an improvement in a process for preparing a well-ordered crystalline thin film of copper-containing superconducting compound oxide such as YBCO on a silicon wafer substrate, more particularly, to a process for pre-treating a surface of the silicon wafer substrate. Description of the related art Superconducting phenomenon had been explained as a unique phenomenon which can be observed only at extremely low temperatures which can be realized with liquid helium. However, a new type superconducting compound oxide of (La, Sr)₂CuO₄ which exhibits the superconductivity at 30 K was reported in 1986 by Bednorz and Müller (Z. Phys. B64, 1986, 189). Then, another superconducting material of YBa₂CU₃O y having higher critical temperature of about 90 K was reported in 1987 by C. W. Chu et al. (Physical Review letters, Vol. 58, No. 9, p. 908) and then Maeda et al. reported so-called bismuth type compound oxide superconductor having the critical temperature of about 100 K in 1988 (Japanese Journal of Applied Physics. Vol. 27, No. 2, p.1209 to 1210). These superconducting, compound oxides are expected to be utilized in actual uses of superconductors because the superconducting phenomenon can be realized with relatively cheap cryogen of liquid nitrogen. In early stage of development, these superconducting compound oxides were obtained as sintered bodies by solid reaction but it becomes possible to prepare thin films of these superconducting compound oxide materials. When thin films of oxide superconductor are used in electronics, it is requested to prepare their thin film whose crystals are well-oriented. In fact, oxide superconductors possess high anisotropy in their superconducting properties and hence it is necessary to control crystal orientation at film-forming stage so as to adjust to applications. In the thin films of oxide superconductors, higher current flows along a direction which is perpendicular to c-axis. For example, when the thin film of oxide superconductor is used as superconducting wiring lines in a circuit, it is required to pass superconducting current along a direction which is in parallel with a surface of substrate. In this case, the c-axis of crystals of the thin film must be oriented perpendicularly with respect to the surface of substrate. In other words, (110) oriented thin films of oxide superconductor are required in these applications. In another application such as layered superconductor-insulator-superconductor (SIS) type device, it is required to pass superconducting current along a direction which is perpendicular to the surface of substrate. In this case, the c-axis of crystals of the thin film prepared must be oriented in parallel with the surface of substrate. In other words, (100) oriented thin film or (110) oriented thin films of oxide superconductor is required in these applications. Importance of selection of deposition plane in substrate or optimization of deposition conditions, particularly substrate temperature for realizing a desired crystal orientation in a thin film of oxide superconductor was reported by many papers including Enomoto et al., Japanese Journal of Applied Physics, Vol. 26, No. 7, July 1987, pp.L1248-L1250, Asano et al. in Japanese Journal of Applied Physics, Vol. 28, No. 6, June 1989, pp.L981 -L983 or the like. Predetermined crystal orientation of oxide superconductor can be realized only under predetermined condition and on predetermined substrate material. In fact, the substrate on which a thin film of oxide superconductor is deposited must be selected from those that have well-matched lattice constants and do not diffuse or migrate into the superconductor such as MgO (100) single crystal or SrTiO₃ (100) or (110) single crystal. Use of these substrates in industrial applications, however, is limited because they are costly material and are not produced in mass production. Still more, since diameters of these oxide single crystal substrates are limited, it is impossible to produce a thin film of oxide superconductor having a large area which will be requested in near future. Therefore, it is proposed to used, as substrate, a silicon single crystal (silicon wafer) which is less expensive and is available stably on market so as to prepare a thin film of oxide superconductor thereon. However, when a thin film of oxide superconductor is deposited on the silicon wafer, superconducting properties of the resulting thin film are seriously deteriorated or lost due to chemical reaction between oxide of which the superconducting thin film is made and silicon of which the substrate is made. In order to solve this problem, it is proposed to interpose a buffer layer between the silicon wafer and the thin film of oxide superconductor so as to prevent the reaction. However, known buffer layers are not so satisfactory as to control or adjust crystal orientation of a thin film of oxide superconductor which is deposited thereon. The present applicant proposed to interpose two intermediate layers between a silicon wafer and a thin film of oxide superconductor in U.S. patent application serial No. 07/806,329 which corresponds to EP patent application No. 91 403 398.0. Furthermore, each of the documents US-A-4 882 312; Applied Physics Letters vol. 52, no. 14, 4 April 1988, New York, US, pp. 1185 - 1186, A. Mojro-Campero et al.; Solid State Technology vol. 33, no. 2, Feb. 1990, pp. 113-117, A. Inam et al.; and Applied Physics Letters vol. 53, no. 25, 19 Dec. 1988, New York, US, pp. 2566 - 2568, A Mojro-Campero et al. discloses the deposition of a thin film of ZrO₂ as a buffer layer on a silicon wafer before a thin film of compound oxide superconductor is deposited thereon. Therefore, an object of the present invention is to solve the problem and to provide a pre-treatment process of the silicon wafer, which permits to prepare a thin film of oxide superconductor having a desired crystal orientation on a silicon wafer. Summary of the Invention The present invention provides a process for preparing a thin film of compound oxide superconductor on a substrate of silicon wafer, characterized by a first step of heating the silicon wafer at a temperature of higher than 900 °C in high vacuum lower than 1.33·10⁻⁴ Pa (10⁻⁶ Torr), a second step of depositing a thin film of ZrO₂ on the silicon wafer, a third step of annealing the thin film of ZrO₂ deposited on the silicon wafer in air at a temperature of 800 to 850 °C, and the final step of depositing the thin film of compound oxide superconductor. An essence of the present invention resides in that, in a process for depositing a thin film of oxide superconductor on a silicon wafer, a surface of the silicon wafer is subjected to a predetermined pretreatment before deposition of a buffer layer on which the thin film of oxide superconductor is deposited directly. All of said steps are carried out preferably in a same vacuum chamber successively. In the process according to the present invention, a silicon wafer to be used is heat-treated in high-vacuum so that the surface of the silicon wafer is highly cleaned. The degree of vacuum is lower than 1.33·10⁻⁴ Pa (10⁻⁶ Torr) and the silicon wafer is heated at a temperature above 900 °C. Advantageously, the silicon wafer can be washed also with hydrogen fluoride (HF) or the like before the heat-treatment. When a (110) oriented or (001) oriented thin film of oxide superconductor is desired, deposition is effected on a (100) plane of the silicon wafer. On the resulting cleaned silicon wafer, a thin film of ZrO₂ which functions as a buffer layer for a thin film of oxide superconductor to be deposited directly thereon is deposited by any known technique including sputtering and molecular beam epitaxial technique (MBE). The thin film of ZrO₂ has preferably a thickness between 10 and 1,000 Å. If the thickness of the buffer layer is not thicker than 10 Å, it is difficult to prevent migration or diffusion of silicon from the substrate to the thin film of oxide superconductor. On the contrary, if the thickness of the buffer layer is thicker than 1,000 Å, the surface condition of the buffer layer is spoiled or becomes worse so that the properties of the thin film of oxide superconductor deposited thereon become worse. The annealing of the thin film of ZrO₂ deposited on the silicon wafer can be carried out in air at a temperature of 800 to 850 °C. A surface of the thin film of ZrO₂ becomes smooth by this annealing and hence crystallinity of the thin film of oxide superconductor to be deposited directly thereon is improved. The annealing is carried out preferably for time duration between 1 and 5 hours. If the annealing is effected lower than 800 °C, advantages of the annealing can not be expected. On the contrary, if the annealing is effected higher than 850 °C, silicon diffuse excessively as well as properties of the thin film of ZrO₂ are also deteriorated. On thus pre-treated silicon wafer, a thin film of compound oxide superconductor including Y-type, Bi-type and Tl-type can be grown epitaxially. The thin films of oxide superconductors can be prepared by any known technique such as sputtering. The process according to the present invention is applicable to any known oxide superconductors and is advantageously applicable to Y-Ba-Cu-O oxide superconductor such as Y₁Ba₂Cu₃O 7-x (x= ± 1), Bi-Sr-Ca-Cu-O oxide superconductor such as Bi₂Sr₂Ca₂CU₃O 10-x (x= ± 1) and Tl-Ba-Ca-Cu-O oxide superconductor such as Tl₂Ba₂Ca₂Cu₃O 10-x (x= ± 1) which have the most attractive properties including their high critical temperatures. As is stated above. it is known that effective thin films of all oxide superconductors can be deposited only on predetermined crystalline surfaces and that diffusion of silicon into the thin film of oxide superconductor result in deterioration and loss of superconducting properties. Several materials for the buffer layer are proposed. Known materials for the buffer layer are selected from those that can produce crystalline surfaces which are suitable for realizing thin films of oxide superconductor thereon and which function as a diffusion barrier to silicon and constituent elements of oxide superconductor. In fact, known buffer layers are satisfactory to prevent diffusion of silicon but contribute little to crystal growth of the thin film of oxide superconductor to be deposited directly thereon. According to the present invention, the buffer layer on which a thin film of oxide superconductor is deposited is made of a material which does not or little react with superconductor, has well-matched lattice constants with crystals of the superconductor and also has a predetermined crystal orientation, so that the thin film of oxide superconductor deposited on the buffer layer grows epitaxially and possesses a well-ordered crystal orientation, and hence, thin films of compound oxide superconductors prepared by the process according to the present invention have improved crystallinity and show improved critical current density (Jc). Use of silicon single crystal wafer as a substrate is very advantageous in the field of electronics devices since a variety of fine processing technology of silicon wafer are known and applicable in addition to such merits that single crystal wafer is less expensive and is available stably on market. And also, a thin film having a large area can be prepared because a silicon wafer having a large diameter is available on market. Description of the Preferred Embodiments Now, the present invention will be described with reference to Examples, but the scope of the present invention should not be limited thereto. Example 1 A (001) oriented thin film of Y-Ba-Cu compound oxide superconductor was prepared by the process according to the present invention. Ten silicon wafers were used. The thin film was deposited on (100) plane of the silicon wafer. Five silicon wafers were washed with 50 % solution of hydrogen fluoride and were left in a furnace at 930 °C under a vacuum of lower than 1.33·10⁻⁴ Pa (1 x 10⁻⁶ Torr) for one hour. A thin film of ZrO₂ was deposited on each of thus treated five silicon wafers and non-treated five silicon wafers by MBE technique by using metal Zr under the conditions shown in Table 1. Then, a thin film of Y-Ba-Cu compound oxide was deposited on each of ten silicon wafers each having thus deposited ZrO₂ thin film by magnetron sputtering by using a target of a sintered block having a composition of Y:Ba:Cu = 1 : 2: 3.8 under the conditions shown in Table 2. The critical temperature (Tc) and the critical current density (Jc) were determined. The results are summarized in Table 3. The value of the critical current density (Jc) was determined at 77 K. Example 2 Example 1 was repeated but the Y-Ba-Cu compound oxide was replaced by Bi-Sr-Ca-Cu compound oxide. In the Example 2, a thin film of the Bi-Sr-Ca-Cu compound oxide was deposited on a silicon wafer having the ZrO₂ thin film by magnetron sputtering by using a target of a sintered block having a composition of Bi:Sr:Ca:Cu = 2:2:2:3 under the conditions shown in Table 4. The critical temperature (Tc) and the critical current density (Jc) were determined. The results are summarized in Table 5. The value of the critical current density (Jc) was determined at 77 K.

Claims: A process of preparing a thin film of compound oxide superconductor on a substrate of silicon wafer, comprising a first step of heating said silicon wafer at a temperature of higher than 900 °C in high vacuum lower than 1.33·10⁻⁴ Pa (10⁻⁶ Torr), a second step of depositing a thin film of ZrO₂ on said silicon wafer, a third step of annealing said thin film of ZrO₂ deposited on said silicon wafer in air at a temperature of 800 to 950 °C, and the final step of depositing said thin film of compound oxide superconductor. The process set forth in claim 1, wherein all of said steps are carried out in a same vacuum chamber successively. The process set forth in claim 1 or 2, wherein said compound oxide superconductor is Y-Ba-Cu-O oxide superconductor. The process set forth in claim 1 or 2, wherein said compound oxide superconductor is Bi-Sr-Ca-Cu-O oxide superconductor. The process set forth in any one of claim 1 to 4, wherein said thin film of ZrO₂ is prepared by MBE technique. The process set forth in any one of claim 1 to 5, wherein said thin film of compound oxide superconductor is prepared by sputtering method.

Inventors: ITOZAKI HIDEO C O ITAMI WORKS, NAKANISHI HIDENORI C O ITAMI W, SHIKATA SHIN-ICHI C O ITAMI WO, ITOZAKI, HIDEO, C/O ITAMI WORKS OF SUMITOMO, NAKANISHI, HIDENORI, C/O ITAMI WORKS OF SUMITOMO, SHIKATA, SHIN-ICHI, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01L39
IPC codes: H01L39, C30B29, C23C14