Method of making a Bi-Sr-Ca-Cu-O superconductive film

Abstract: A Bi-Sr-Ca-Cu-O-type superconductive film is formed on an MgO (100) single crystal substrate by the chemical vapor deposition method at a film formation temperature of 780°C or less and a film formation speed of 1.0 nm/min or more, and exhibits an a-axis or b-axis preferential growth with respect to the substrate surface.

Description: The present invention relates to a method of fabricating a Bi-Sr-Ca-Cu-O superconductive film which exhibits an a-axis or a b-axis preferential growth with respect to a surface on which said film is formed by chemical vapor deposition. A method according to the preamble of claim 1 is known from the WP91-07236. Because of the high superconductivity critical temperature (Tc), oxide-type superconductive films are expected to be applied to electronic devices, such as transistors and Josephson junction devices. However, the superconductivity of an oxide-type superconductive film exhibits a strong anisotropy, so that it requires a film orientation control. In the case of device application, in particular, a device structure whose ab crystal surface with a large coherence length (ζ) allows a flow of electric current is advantageous from the viewpoint of the properties of the device to be obtained. For this purpose, it is important that the film should be of a-axis or b-axis orientation with respect to the substrate surface. Conventionally known methods of fabricating oxide-type superconductive films include the PVD (physical vapor deposition) method and the CVD (chemical vapor deposition) method. The lower limit of the film formation speed in the PVD method is relatively high as compared to that in the CVD method, and a reduction in speed in the PVD method will result in the film formation speed and the film composition being irregular, thereby making it difficult to obtain an ultra-thin film which is considered producible at low film formation speed ( Applied Physics Letters , Vol., 53, No. 7, 1988, pp. 624). Generally speaking, it is possible with the CVD method to maintain the saturated vapor of the material compound stable within a fixed temperature range. Further, before they are caused to react so as to effect film formation on a substrate, the vapor and the oxidizing gas are separate from each other, so that it is possible to cause the requisite reaction to take place exclusively on the substrate ( Journal of Applied Physics , Vol. 67, No. 3, 1990, pp. 1562). In this respect, the method is superior to the PVD method, in which it is difficult to control the reaction between the material gas and the oxidizing gas and the material vapor concentration in the system is unstable). Thus, it is more advantageous from the industrial point of view, to fabricate oxide-type superconductive films by the CVD method. As stated above, when using oxide-type superconductive films for various types of devices, an a-axis or b-axis film orientation is desirable. However, because of their strong anisotropy in crystal structure, oxide-type superconductive films have been inclined to c-axis orientation, which is particularly true of Bi-Sr-Ca-Cu-O-type superconductive films. There have conventionally been reported examples of a Bi-Sr-Ca-Cu-O-type superconductive film in which an a-axis or b-axis preferential growth is effected by sputtering, as well as examples of a Y-Ba-Cu-O-type superconductive film in which an a-axis or b-axis preferential growth is effected by the CVD method. However, there has been provided as yet no Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation prepared by the CVD method. From EP-A-446145 there is known a process for the preparing of high temperature, superconducting thin films wherein a thin film of yttrium-barium-copper-oxygen superconducting material is deposited on a magnesium oxide single crystal by magnetron sputtering and thereafter an upper layer of the same superconducting metal is deposited by vacuum deposition. According to an improvement, the chemical composition was changed to a sintered oxide material having a composition of Bi-Sr-Ca-Cu-O. From JP-A-2-252618 there is known a process for preparing a bismuth system superconducting thin film which, however, neither has an a-axis nor a b-axis preferential growth with respect to the surface of a substrate. Furthermore, there is known from WO91/07236 a method for the deposition by chemical vapor deposition wherein a substrate and a quantity of one or more reagents are provided and the substrate is placed within a furnace whereupon the reactants are introduced into the furnace resulting in the deposition of a coating consisting essentially of the reactant components on the substrate. There has been a request for the formation of a Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation with respect to the substrate surface by the CVD method. It is the object of the present invention to provide a Bi-Sr-Ca-Cu-O-type superconductive film fabrication method which makes it possible for a Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis preferential growth with respect to the substrate surface which is useful as a material for various types of devices, to be provided by the CVD method, which is advantageous from the industrial point of view in forming oxide-type superconductive films. The above object is achieved by the method as defined in claim 1. Preferred embodiments and further improvements of the inventive method are defined in the subclaims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail. Examples of the chemical composition of an Bi-Sr-Ca-Cu-O-type superconductor forming a Bi-Sr-Ca-Cu-O-type superconductive film according to the present invention include the following: Bi 2 Sr 2 Ca 1 Cu 2 O x Bi 2 Sr 2 Ca 2 Cu 3 O y To improve Tc (the critical temperature), the above compositions may further include Pb. A Bi-Sr-Ca-Cu-O-type superconductive film according to the present invention as described above is formed on an MgO single crystal substrate of a plane index of [100] by the CVD (chemical vapor deposition) method. In the following, a method of fabricating a Bi-Sr-Ca-Cu-O-type superconductive film according to the present invention will be described. A Bi-Sr-Ca-Cu-O-type superconductive film according to the present invention is fabricated by the CVD method, according to which a thin film is formed through deposition on an MgO (100) single crystal substrate, using material gases, a carrier gas, and an oxidizing gas in conformity with fixed chemical compositions. Examples of the material gases used in the present invention include organometallic complexes of Bi, Sr, Ca, Cu, or the like. Examples of the organic portions of the organometallic complexes, i.e., the complex ligands, include: acetylacetone (hereinafter abbreviated as "acac"), dipivaloylmethane (hereinafter abbreviated as "DPM"), cyclopentadiene, and further a compound as can be represented by the following formula: R-CO-CH 2 -CO-C(CH 3 ) 3 (where R indicates a fluorinated lower alkyl group of a carbon number of 1 ∼ 4.) Use of such ligands facilitates the synthesis and isolation of the metal complexes and enables the vapor pressure of the organometallic complexes to be low, so that it is very advantageous in preparing the material gases. Other examples of the ligands include phenyl group (hereinafter abbreviated as "ph"), alkyl group and aryl group. The alkyl group comprise methyl group (hereinafter abbreviated as "Me"), and ethyl group (hereinafter abbreviated as "Et"). Examples of the fluorinated lower alkyl group represented by R in the above formula include trifluoromethyl group, pentafluoroethyl group (hereinafter abbreviated as "PPM"), and heptafluoropropyl group. Examples of the organometallic complexes used as the material gases include the following: Bi(ph) 3 , Bi(DPM) 3 , BiMe 3 , BiEt 3 , Bi alcoholate, Sr(DPM) 2 , Sr(PPM) 2 , Ca(DPM) 2 , Ca(PPM) 2 , Cu(DPM) 2 , Cu(PPM) 2 , and Cu(acac) 2 Examples of the carrier gas used in the present invention when feeding the material gases to the reactor include inactive gases, such as Ar, He and Ne, and further, N 2 . Examples of the oxidizing gas used in the present invention include O 2 , O 3 , air, N 2 O and NO 2 . The partial pressure of the oxidizing gas in the entire gas is preferably 0.01 to 760 torr and, in particular, 1 to 10 torr. In film formation, the temperature of the MgO (100) single crystal substrate, i.e., the film formation temperature, is set at 780°C or less. A film formation temperature higher than 780°C would make it impossible to obtain a Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis preferential orientation with respect to the substrate surface. The film formation speed is set at between 1.0 nm/min and 2.0 nm/min. A film formation speed less than 1.0 nm/min would make it impossible to obtain a Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis preferential orientation with respect to the substrate surface. In the present invention, it is particularly desirable that the film formation be conducted at a film formation temperature of 780 to 750°C. Usually, a preferable thickness of a Bi-Sr-Ca-Cu-O-type superconductive film obtained in this way is in the range of approximately 100 to 1000 nm. The Bi-Sr-Ca-Cu-O-type superconductive film of the present invention is a high-property superconductor in which Tc is generally 50 to 100K, and exhibits an a-axis or b-axis preferential orientation in growth with respect to the substrate surface, so that it is very useful for various types of devices. By using an MgO (100) single crystal as the substrate and effecting film formation at a film formation temperature of 780°C or less and a film formation speed of between 1.0 nm/min and 2.0 nm/min it is possible for a Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation with respect to the substrate surface to be easily obtained by the CVD method. EXAMPLES The present invention will now be described more specifically with reference to examples and comparative examples. In the following, gas flow rate will be given in sccm, in terms of cc/min at 1 atm and 25°C. [Example 1] Using the material gases shown in Table 1, a Bi-Sr-Ca-Cu-O-type superconductive film was fabricated. The substrate used and the fabricating conditions are shown in Table 2. Material gas Temperature and flow rate Bi(ph) 3 87°C, 50 sccm Sr(DPM) 2 215°C, 50 sccm Ca(DPM) 2 183°C, 50 sccm Cu(DPM) 2 102°C, 50 sccm Oxidizing gas O 2 Substrate MgO single crystal (plane index: [100]) Substrate temperature 780°C Total pressure (Material gases + Ar + O 2 ) 10 torr O 2 partial pressure approximately 6.7 torr O 2 flow rate 400 sccm Film formation speed approximately 1 nm/min As a result, a film having a thickness of approximately 130 nm was formed after a film formation process of approximately 2 hours. The properties of the Bi-Sr-Ca-Cu-O-type superconductive film thus obtained were as follows: XRD spectrum: a crystal substrate of a-axis or b-axis orientation exhibiting peaks corresponding to the plane indexes of (100), (200), (300), and (400). Chemical composition: Bi 2 Sr 2 Ca 1 Cu 2 O x Critical temperature: approximately 50 to 60K [Examples 2 ∼ 6, Comparative Examples 1 ∼ 9] Film formation was conducted in the same manner as in Example 1, except that the film formation speeds and film formation temperatures (substrate temperatures) shown in Table 3 were adopted. The crystal orientation of each of the films obtained was measured by XRD and evaluated by the following criteria. The results are shown in Table 3 together with Example 1. Criteria for Evaluation: ○ = A Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis preferential orientation was obtained. Δ = A Bi-Sr-Ca-Cu-O-type superconductive film including a component of a-axis or b-axis orientation and a component of c-axis orientation in the same proportion was obtained. X = A Bi-Sr-Ca-Cu-O-type superconductive film of a c-axis preferential orientation was obtained. Film formation temperature (°C) Film formation speed (mm/min.) 2.5 1.0 0.5 720 ○ Example 2 ○ Example 3 Δ Comp.Ex.1 750 ○ Example 4 ○ Example 5 Δ Comp.Ex.2 780 ○ Example 6 ○ Example 1 Δ Comp.Ex.3 820 Δ Comp.Ex.4 X Comp.Ex.5 X Comp.Ex.6 850 X Comp.Ex.7 X Comp.Ex.8 X Comp.Ex.9 It is obvious from Table 3 that in accordance with the present invention, a high-property Bi-Sr-Ca-Cu-O-type superconductive film of an a-axis or b-axis preferential orientation can be obtained.

Claims: A method of fabricating a Bi-Sr-Ca-Cu-O superconductive film comprising, forming a Bi-Sr-Ca-Cu-O superconductive film on an MgO (100) single crystal substrate by a chemical vapor deposition method including using reaction gases of organometallic complexes of Bi, Sr, Ca, and Cu, a carrier gas and an oxidizing gas, characterized in that said chemical vapor deposition method is carried out at a substrate temperature of 780°C or less and a film formation speed of between 1.0 nm/min and 2.0 nm/min so that the Bi-Sr-Ca-Cu-O superconductive film has an a-axis or a b-axis perferential growth with respect to a substrate surface, and that partial pressure of said oxidizing gas in an entire gas being from 1.3 to 13.33 mbar (1 to 10 torr). A method according to claim 1, wherein said Bi-Sr-Ca-Cu-O superconductive film contains Pb to improve critical temperature. A method according to claim 1, wherein a ligand of said complexes is at least one selected from the group consisting of phenyl group, methyl group, ethyl group and aryl group. A method according to claim 1, wherein the organometallic complex of Bi is at least one selected from the group consisting of Bi(ph) 3 , Bi(DPM) 3 ;BiMe 3 , BiEt 3 , and Bi alcoholate, the organometallic complex of Sr is at least one selected from the group consisting of Sr(DPM) , and Sr(PPM), the organometallic complex of Ca is at least one selected from the group consisting of Ca(DPM) and Ca(PPM) the organometallic complex of Cu is at least one selected from the group consisting of Cu(DPM) , Cu(PPM) , and Cu(acac). A method according to claim 1, wherein the film formation temperature is 780 to 750°C. A method according to claim 1, wherein the thickness of said Bi-Sr-Ca-Cu-O-type superconductive film is 100 to 1000 nm.

Inventors: NAKAGAWA MIKIO, SHIOHARA YUH, SUGAWARA KAZUSHI, SUGIMOTO TSUNEMI, NAKAGAWA, MIKIO, SHIOHARA, YUH, SUGAWARA, KAZUSHI, SUGIMOTO, TSUNEMI, Nakagawa, Mikio, c/o Fujikura Ltd., Shiohara, Yuh, c/o Int.Supercond. Tech. Cent., Sugawara, Kazushi, c/o Int.Supercond. Tech. Cent., Sugimoto, Tsunemi, c/o Ube Industries, Ltd.
Applicants: FUJIKURA LTD, INT SUPERCONDUCTIVITY TECH, SHARP KK, UBE INDUSTRIES, FUJIKURA LTD., INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, SHARP CORPORATION, UBE INDUSTRIES, LTD.
Application year: 1992
Main classification: H01B12
IPC codes: H01B12, C04B35, C01G1, H01B13, H01L39, C30B29, C30B25, C01G29