Method and apparatus for fabricating superconductive thin films

Abstract: Chemical reactions of superconducting raw materials (10) with active oxygen atoms and their charged particles are accelerated by using at least oxygen plasma in the fabrication process of a superconductive body (1). Thereby an ionic crystal is grown in a short time, which provides stable superconducting materials of high quality such as high critical temperature and low resistivity. In another aspect, a substrate (2) is irradiated simultaneously with streams (13) of vapor of metal elements, of which a superconductive body is to be composed, and a stream (13) of gas of ions generated in a plasma chamber (20) and film growth is effected while keeping the substrate (2) at a temperature higher than 400°C to produce a ceramic type superconductive thin film.

Description: The present invention relates to a method and apparatus for fabricating superconductive oxide films, and in particular high temperature oxide superconducting films. A superconducting material (La, Ba)₂CuO₄ having a K₂NiF₄ type structure, which Bednorz and Muller have discovered in 1986, has shown a critical temperature as high as 30K. In 1987, it has been found that YBa₂Cu₃O 7-x having a perovskite type structure has a still higher critical temperature T c = 90K and it has shown the superconductive state even at the liquid nitrogen temperature. By a prior art fabrication method, as disclosed in Japanese Journal of Applied Physics, Vol. 26, No. 5 (1987) pp. L676-L677, e.g. in the case of the Y-Ba-Cu-O oxide superconductive body, powders of Y₂O₃, BaCO₃ and CuO were mixed at a suitable ratio and calcinated in the atmosphere or in an oxygen gas medium at a temperature of 950°C for one hour. Thereafter, it was shaped in a suitable form such as pellets, etc. and sintered in the atmosphere or in oxygen at a temperature of 900°C for about 18-19 hours by means of an electric furnace 9, as indicated in Fig. 2, in which a reference numeral 1 represents a high temperature superconductive body obtained by sintering a pellet made of the superconducting material 10 having the composition described above and 2 indicates a substrate. The prior art technique stated above had problems that fabrication took a long time, that no attention was paid to mixing of impurities, etc., that productivity and stability were unsatisfactory, etc. The present invention relates to superconducting materials used for high output magnets, Josephson devices, SQUID, etc. and in particular to a method for fabricating oxide superconductive thin films working at temperatures higher than the boiling point of liquid introgen. A representative method for fabricating a superconductive thin film is published in Japanese J. of Appl. Phys., Vol. 26, No. 5 (1987) pp. L709-L710. Japanese Journal of Applied Physics, Vol. 26, No. 7, July 1987, pp. L1221-L1222 discloses a method for manufacturing superconductive oxide thin films by sputtering, wherein the sputtering gas contains oxygen. A further method for manufacturing superconductive oxide thin films is described in Physical Review B, Vol. 35, No. 16, June 1987, pp. 8821-8823. In accordance with this method, the films are formed by metal vapour deposition in a partial pressure of oxygen. EP-A-0 282 839 (prior art according to Art 54(3) EPO) describes a method for fabricating a superconductive oxide thin film by vapour deposition of metal elements in an atmosphere containing oxygen, which may be provided as an oxygen ion beam. The known methods have the disadvantage that it is difficult to control the ratio between the metal elements and oxygen in the thin film. Controlling this ratio is essential for achieving a high critical temperature Tc. SUMMARY OF THE INVENTION It is an object of the invention to provide an efficient method and apparatus for producing a superconductive oxide thin film having a high critical temperature Tc. This object is achieved by the method of claim 1 and the apparatus of claim 6. The subclaims are directed to preferred embodiments of the invention. One embodiment of the present invention provides a method for forming ceramic oxide superconducting thin films, in which a stream of vapor of metallic elements constituting a superconductive body e.g. made of a compound such as YBa₂Cu₃O 7-x , (La 1-x Ba x )CuO₄, etc. and a gas stream of ions produced in a plasma chamber are projected or applied simultaneously on a substrate and the film growth is effected by crystallizing them. As a plasma source it is preferable to use a plasma of ions generated in an electron cyclotron resonance microwave plasma generator or an RF-plasma generator. Various kinds of ions generated in this plasma source are not accelerated, but reach the surface of the substrate by diffusion and react with the stream of metal vapor. Consequently, the energy of the ions is very low and therefore, the ions give the substance no damage due to the energy of the ions. In order to obtain a uniform film, it is preferable that each of streams of vapor of plural metal elements is flow-rate-controlled by means of a film thickness monitor. An evaporation chamber is kept at a vacuum, which is higher than that in the reaction chamber, in which gas is introduced, a differential pumping mechanism comprised of a vacuum pump and a throttle plate, disposed on both the two chambers. In this way the metal source of plural elements is not contaminated by the plasma gas. For this reason the evaporation speed is controlled to be constant and it is possible to fabricate films of high quality. The superconducting materials according to the present invention mean ceramic oxide superconducting materials in general, such as (La, Ba)₂CuO₄ compounds of K₂NiF₄ structure having a critical temperature of 40K, YBa₂Cu₃O 7-x compounds having a critical temperature of 90K, ErBa₂Cu₃O 7-x compounds, HoBa₂Cu₃O 7-x compounds, EuBa₂Cu₃O 7-x compounds, Bi₂Sr₂Ca₂Cu₃O x compounds having a critical temperature of 120K, Tl₂Ba₂Ca₂Cu₃O x compounds, etc. The drawings show embodiments of the present invention as well as comparative examples and a scheme of a prior art technique. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a scheme for explaining a method for fabricating superconducting materials according to a comparative example, illustrating a device for realizing same; Fig. 2 is a scheme showing a prior art technique; Fig. 3 shows an example of characteristics of a superconducting material; Figs. 4A, 4B and 4C are schemes showing other comparative examples; and Fig. 5 is a scheme showing an embodiment of the present invention for fabricating thin films. DESCRIPTION OF THE DRAWINGS Fig. 1 is a scheme illustrating an experimental apparatus. A vacuum chamber 4 made of glass, etc. can be evacuated to a high vacuum (about 1.3 x 10⁻⁶ Pa) by means of an evacuating device, within which chamber a superconductive body is formed. Eventually, a substrate 2 on which superconducting materials are set, and a heater 3 for heating them are disposed therein. On the other hand, oxygen plasma is generated while supplying oxygen gas of simple substance or oxygen gas, in which freon gas, argon gas, etc. are mixed, to the vacuum chamber 4 and at the same time feeding a coil 5 disposed outside of the vacuum chamber 4 (capacitive coupling is also possible) with RF power (several 100 W to several 100 MW) from an RF power source 6. In order to facilitate generation of the plasma and confinement thereof, it is preferable to superpose an external magnetic field on the RF magnetic field by means of a field coil 7 (it is also possible not to use this magnetic field). The arrangement of this magnetic field can be any of dispersion type, Miller type, CASP type, etc. Further, in the case where the superconductive body 1 is formed into a thin film, the raw material 10 constituting the superconductive body 1 is powdered and injected into the vacuum chamber 4. On the contrary, in the case where the superconductive body 1 is a pellet, the superconducting material 10 described above is formed into a pellet, which is set on the substrate 2 described above. Now the working mode of the apparatus stated above and the method for fabricating the superconductive body will be explained. In the case where the superconductive body 1 is formed into a thin film, the substrate 2 (on which the thin film is formed) is heated by means of the heater 3 (0 to 1000°C), after having evacuated the vacuum chamber 4 to a high vacuum. Then an active gas such as oxygen, oxygen + freon, etc. is introduced thereinto (10⁻³ - 10⁷ Pa) and a plasma having a high temperature (10 - 10⁴ eV) and a high density (about 10¹⁰ - 10¹⁶/cc) is generated. Then the superconducting raw material, e.g. fine powder of Y₂O₃, BaCO₃ and CuO (weight ratio ≃ 0.6 : 1.5 : 1.0) is injected into the plasma. At this time active oxygen atoms, those constituting freon, and their charged particles react chemically with atoms constituting the superconducting raw material and are deposited on the substrate 2 so as to grow ionic crystals, forming a superconductive thin film. When a thin film having a predetermined thickness is obtained, the injection of the superconducting row material 10 is stopped. At this time it is desirable to anneal it in the plasma by means of the heater (not longer than several hours at a temperature between 300 and 1500°C). Fig. 3 indicates the temperature dependence of the resistivity of a high temperature oxide superconductive body Y-Ba-Cu-O fabricated in the manner described above. It can be understood therefrom that the thin film fabricated by the above method has a higher critical temperature and a lower resistivity with respect to that fabricated by the prior art method using an electric furnace. Further the former has a stability and a critical current density, which are higher than those for the latter. On the other hand, in the case where the superconductive body 1 is formed into a pellet, for example the superconducting raw material 10 stated previously is set on the substrate 2 after having been formed into a pellet. After the vacuum chamber has been evacuated to a high vacuum and the pellet has been heated (to a temperature, at which it is not sintered), it is sintered in the plasma so as to form the superconductive body 1. Further, the position, where the substrate 2 is set in this case, differs in general from the position for the formation of the thin film described previously (it is set positively in the plasma). Figs. 4A, 4B and 4C are schemes illustrating other experimental apparatuses. In the apparatus indicated in Fig. 4A the superconducting raw material 10 acts as the cathode and the desired superconductive body 1 is produced on the anode side. As the power supply an RF, DC or pulse source is used and at least an oxygen plasma is used, as described previously. Further, at this time, the substrate 2 may be heated by means of the heater 3 (omitted in the figure), as stated above. Fig. 4B is a scheme illustrating an apparatus, in the case where microwave power (frequency higher than 1 GHz, 10 W to 10² kW) is used as the RF power. Fig. 4C indicates an apparatus, in which arc discharge is used. The apparatus indicated in Figs. 4A, 4B and 4C are essentially the same as that indicated in Fig. 1. However, since physical quantities (temperature, density, etc.) of the plasma vary, depending on the method for generating the plasma, the working gas pressure and the position of the substrate 2 differ therefrom. In any case at least effectiveness of a plasma at the surface of the superconductive body 1 remain unchanged. According to this invention, since ions produced in the plasma are in an active state, they are easily combined with evaporated metal atoms and thus crystallization of the oxide takes place at a relatively low temperature. This effect will be described in detail, referring to the embodiment of Fig. 5, in which a thin film of YBa₂Cu₃O 7-x is formed. A metal evaporation chamber 11, by which a ternary simultaneous evaporation of Y, Ba and Cu was possible, was disposed in a vacuum chamber 4, whose ultimate vapor pressure was 10⁻⁴ Pa. Streams 13 of vapor of the elements are generated by irradiating the sources thereof one after another with an electron beam 12. The speed of each of the streams of vapor of the elements was inputted in an evaporation speed controller 15 through an electron stimulated light emission photodetector 14 and fed back to a power supply for an electron gun 16 so that it reached a predetermined value and that it didn't vary during the evaporation. In the vacuum chamber there was disposed a differential pumping mechanism consisting of a pump mounted for each of the evaporation chamber 11 and the reaction chamber 20 and a throttle plate 17 separating them apart from an aperture, through which the streams of metal vapor pass. O₂ gas was introduced in the reaction chamber from an O₂ gas bomb 18 to a pressure of 10⁻² Pa. After that, in the upper chamber a plasma of O ions was generated by means of an electron cyclotron resonance microwave (ECR) plasma generator 7 or an RF wave generator 5, 6. The deposition speed of each of the metal elements of Y, Ba and Cu was obtained by using the relative relation between a thin film deposited previously and a film thickness monitor. The evaporation speed was so controlled to be 30 nm/s, and they were separately evaporated so as to obtain a molar ratio thereof of 1 : 2 : 3. At the same time the substrate 2 was irradiated with the O₂ plasma. The evaporation chamber was kept at a high vacuum of 10⁻⁴ Pa owing to the differential pumping mechanism. For this reason the melts of Y, Ba and Cu were formed stably and thus it was possible to keep the evaporation speed to be constant. The O⁺ plasma was generated in the plasma generating portion, as indicated in Fig. 5, and they were not accelerated intentionally, but they reached the surface of the substrate by diffusion. For this reason the ion energy was as low as a value, which was smaller than 10 eV and the ions gave therefore the thin film no damage due to ion impact. For the substrate a Si monocrystal having a crystallographical orientation of (100) was used. The heater 3 mounted on the substrate was fed with electric current so that the temperature of the substrate was varied from the room temperature to 800°C. The thickness of deposited films was kept at 500 nm and the film quality was examined by using an X-ray diffraction device. In this way it was found that it was amorphous for the substrate kept at the room temperature, but a superconductive state of YBa₂Cu₃O 7-x was obtained already at a temperature of the substrate of 400°C. No reaction phase was found between the YBa₂Cu₃O 7-x film and the Si monocrystal. For the conventional fabrication of superconductive bodies oxide substrates such as MgO, Al₂O₃, SrTiO₃ and YSZ (Yttria stabilized Zirconia) were used principally. This is due to the fact that the YBa₂Cu₃O 7-x film and the Si substrate react with each other, because a temperature higher than 700°C is necessary for the substrate temperature or annealing after the film formation. The superconductive film formed on the Si substrate according to this invention was a film of high quality, whose critical temperature was 87K, whose critical current density was about 10⁵ A/cm² and whose characteristics were not deteriorated, even if it was left in the atmosphere for about two months. The oxygen plasma used in the present invention may be generated in normal and/or pulse form using at least one of RF power (including a microwave) and direct-current power. Alternatively, the oxygen plasma may be generated using an external magnetic field. The application of this invention is not restricted to the YBa₂Cu₃O 7-x superconductive body, but it can be applied as well to other ternary superconductive bodies, e.g. (La, Ba)₂CuO₄. According to the present invention, at least oxygen plasma is used in the fabrication process of superconducting material including a thin film, whereby a chemical reaction and/or a physical reaction of a superconducting raw material such as a stream of metal vapor and active oxygen atoms such as radical oxygen atoms and/or charged particles thereof are promoted, so that a single crystal of superconducting material (film) is grown at a low temperature in a short time. Thus, a superconducting material with good characteristics such as high critical temperature and high critical current density and with high stability i.e. long life can be produced. According to the present invention, since the combination of Si devices with superconductive bodies is easy, the economical merit thereof is very great.

Claims: A method for fabricating a superconductive oxide thin film, comprising the following steps: generating a vapour (13) of metal elements, and applying said vapour (13) to a substrate (2) in the presence of oxygen, to grow a superconductive thin film on said substrate, characterised by the step of generating a plasma gas (8) containing oxygen ions, and in that said vapour (13) is generated in an evaporation chamber (11) and is applied together with said plasma gas (8) to said substrate (2) in a reaction chamber (20) to cause the vapour and the plasma gas to react with each other and to grow the superconductive thin film on the substrate by crystallisation, said evaporation chamber (11) being separate from said reation chamber (20) and being evacuated to a higher vacuum than said reaction chamber (20). The method of claim 1, wherein said evaporation chamber is evacuated by a differential pumping mechanism (17, 19). The method of claim 1 or 2, wherein said ions in said plasma gas (8) are generated by an electron cyclotron resonance microwave generator or a high-frequency plasma generator. The method of any of claims 1 to 3, wherein the flow rate of said vapour (13) and of said plasma gas (8) is separately controlled. The method of any of claims 1 to 4, wherein the temperature of said substrate is kept at 400 °C during growth of said superconductive thin film. An apparatus for fabricating a superconducting oxide thin film, comprising: an evaporation chamber (11) specifically designed to generate a stream of vapour of metal elements, and characterised in that it further comprises : a plasma generator (7) specifically designed to generate an oxygen plasma gas (8), a reaction chamber (20) specifically designed to let said stream of vapour (13) and said oxygen plasma gas (8) react on a substrate (2), and a throttle plate (17) separating said evaporation chamber (11) from said reaction chamber (20) and specifically designed to let said evaporation chamber have a higher vacuum than said reaction chamber. The apparatus of claim 6, wherein said plasma generator (7) includes an electron cyclotron resonance microwave generator or a high-frequency plasma generator.

Inventors: AIDA TOSHIYUKI, FUKAZAWA TOKUUMI, MIYAUCHI KATSUKI, OKAMOTO YUKIO, TAKAGI KAZUMASA, TAKAYAMA SHINJI, AIDA, TOSHIYUKI, FUKAZAWA, TOKUUMI, MIYAUCHI, KATSUKI, OKAMOTO, YUKIO, TAKAGI, KAZUMASA, TAKAYAMA, SHINJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: H01L39
IPC codes: C23C14, H01L39, C23C16, C23C4