Target of superconductive oxide having low resistivity

Abstract: For fabrication of a target of a superconductive oxide of the bismuth-calcium-strontium-copper oxide system, ingredient powders of a calcium carbonate, a strontium carbonate and a copper oxide are mixed to a predetermined proportion and, then, calcined to produce a bulk solid of a complex oxide, followed by pulverizing the solid bulk, then being shaped into a disk configuration, then sintering to produce a porous bulk body (11) of the complex oxide, and the porous bulk body is impregnated with molten bismuth (12) to produce the target which contains metal bismuth, so that the target is improved in thermal conductivity, electric resistivity and mechanical strength.

Description: FIELD OF THE INVENTION This invention relates to a target used in sputtering system and, more particularly, to a process of fabricating a target of a superconductive oxide of the bismuth-calcium-strontium-copper oxide system. BACKGROUND OF THE INVENTION Recently, a superconductive oxide of the bismuth-calcium-strontium-copper oxide system was proposed, and research and development efforts have been made for target formed of the superconductive oxide, because the sputtering technique is an attractive process for formation of a superconductive thin film in various practical application. A typical example of the process of fabricating the target starts with preparation of ingredient powders of the bismuth oxide (represented by the molecular formula of Bi₂O₃), the calcium carbonate (represented by the molecular formula of CaCO₃), the strontium carbonate (represented by the molecular formula of SrCO₃) and a copper oxide (represented by the general formula of CuO). These ingredient powders are mixed in a predetermined proportion to produce a mixture, and the mixture is calcined at 700 degrees to 800 degrees centigrade for a predetermined time period. The product is then pulverized. These steps, i.e., the calcining and the pulverizing are repeated twice or three times, and a powder of the superconductive oxide of the bismuth-calcium-strontium-copper oxide system is finally obtained. The powder of the superconductive oxide is molded by using a usual pressing technique and, thereafter, sintered to produce a target. The powder of the superconductive oxide may be alternatively shaped by using a hot pressing technique to form the target. However, a problem is encountered in the target produced through the prior art process in that cracks are much liable to take place in the target during the sputtering. This is because of the fact that the superconductive oxide of the bismuth-calcium-strontium-copper oxide system is small in thermal conductivity. Since the target is cooled off at one surface thereof and subjected to the bombardment of plasma at the opposite surface during the sputtering, a large difference in temperature takes place between both surfaces due to the small thermal conductivity. The large difference in temperature is causative of a thermal stress and, accordingly, the cracks. Thus, the prior art target suffers from a relatively short life time, which increases the production cost of a thin superconductive film. Another problem inherent in the prior art target is a low electric resistivity. This results in that an expensive RF sputtering system is necessary for the prior art target, and a DC diode sputtering system is hardly usable. The expensive RF sputtering system also increases the production cost of the thin superconductive film. Still another problem is inherent in the target fabricated through the prior art process in small mechanical strength, and, for this reason, the prior art target is liable to be broken by ill usage. This also decreases the life time of the target and, accordingly, increases the production cost of the thin superconductive film. The prior art target has still another problem in a relatively low critical temperature or a zero-resistivity temperature. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide a process through which a target with a large thermal conductivity is fabricated. It is also an important object of the present invention to provide a process through which a target with a low resistivity is fabricated. It is also an important object of the present invention to provide a process through which a target with a large mechanical strength is fabricated. It is still another important object of the invention to provide a process through which a target with a relatively high critical temperature is fabricated. To accomplish these objects, the present invention proposes to impregnate a complex oxide of the calcium-strontium-copper oxide system with molten bismuth. The target thus fabricated contains the metal bismuth which is operative to increase the thermal conductivity as well as the electrical conductivity. In accordance with the present invention, there is provided process of fabricating a target used in a sputtering system, comprising the steps of: a) preparing ingredient powders of a calcium carbonate, a strontium carbonate and a copper oxide; b) producing a porous bulk body of a complex oxide from the ingredient powders, the complex oxide being of the calcium-strontium-copper oxide system; and c) impregnating molten bismuth into the porous bulk body of the complex oxide. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of a process of fabricating a target according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which Figs. 1 to 6 are views showing, in a modeled form, a process sequence for fabricating a target according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS First embodiment Description is hereinunder made for a process of fabricating a target formed of a superconductive oxide of the bismuth-calcium-strontium-copper oxide system with reference to Figs. 1 to 6. Figs. 1 to 6 merely show the process sequence in a modeled form. The process sequence starts with preparation of ingredient powders 1, 2 and 3 of the calcium carbonate (represented by the molecular formula of CaCO₃, the strontium carbonate (represented by the molecular formula of SrCO₃) and a copper oxide (represented by the general formula of CuO). These ingredient powders 1, 2 and 3 are about 7 microns in average diameter and not smaller in purity than about 99.9 %. The ingredient powders 1, 2 and 3 are mixed to respective proportions indicated in Table 1 as shown in Fig. 1. Each of the mixing operations are carried out in moisture by using a ball-mill for three hours, and each mixture 4 of the ingredient powders is dried thereafter. Each of the mixtures 4 is poured into an alumina vessel 5, and the alumina vessel 5 is placed in a furnace chamber 6 where a high temperature atmospheric ambient is created. The mixtures are calcined at respective high temperatures indicated in Table 1 for about 5 hours to produce bulk solids of complex oxides as shown in Fig. 2, and the complex oxides are of the calcium-strontium-copper oxide system. The bulk solids of the complex oxides are respectively pulverized into powders 7 by using a ball-mill as illustrated in Fig. 3, and the powders 7 of the complex oxides are respectively referred to as "samples 1 to samples 6" in Table 1 depending upon the proportion of the mixtures. The average diameters of the powders are shown in Table 1. However, if the average diameter ranges between about three microns and about 15 microns, the powders 7 is acceptable. The temperature for the calcining should be within a range between about 850 degrees and about 1050 degrees centigrade. If the mixture is calcined at a low temperature below about 850 degrees in centigrade, the carbonates are not effectively decomposed, and, accordingly, a substantial amount of carbon is left in the complex oxides. If the complex oxides contain the carbon, a superconductive oxide produced therefrom merely shows a relatively low critical temperature. On the other hand, if the calcining is carried out above about 1050 degrees in centigrade, the copper oxide tends to be melted, and, for this reason, the calcined product is out of the calcium-strontium-copper oxide system. As illustrated in Fig. 4, the powders 7 are pressurized at about 2 tons per square centimeter by using a pressing machine 8 to produce bulks of compressed powders 10, respectively, and each bulk of the compressed powder 10 is placed in a furnace chamber 9 where a high temperature atmospheric ambient is created. The bulks of the compressed powders 10 are heated to respective temperatures indicated in Table 1 for about five hours, thereby being subjected to sintering as shown in Fig. 5. The sintered products 11 have a generally disk shaped configuration which is about 50 millimeters in diameter and about 10 millimeters in thickness. The sintered products 11 are porous and of the calcium-strontium-copper oxide system, and, for this reason, the sintered products 11 are hereinunder referred to as "porous oxide". Bismuth films 12 are mounted on the bulk bodies of the porous oxides 11, respectively, and placed in a furnace chamber 13 as shown in Fig. 6. The furnace chamber 13 is heated to about 300 degrees centigrade and maintained for about three hours. In the high temperature furnace chamber 13, the bismuth films 12 are melted and impregnate into the porous oxides 11, respectively. Upon the impregnation with the molten bismuth, the porous oxides 11 are converted into superconductive oxides of the bismuth-calcium-strontium-copper oxide system, respectively, and, accordingly, targets of the superconductive oxides are finally produced. In order to melt the metal bismuth, the furnace chamber is heated to about 300 degrees, however, it is acceptable to heat up the furnace chamber to equal to or greater than the melting point of bismuth which is about 272 degrees in centigrade. Second embodiment Another process sequence according to the present invention is similar to that of the first embodiment with the exception of the target formation stage, and, for this reason, no description is made for the stages from the preparation to the formation of the powders of the complex oxides 7 which correspond to Figs. 1 to 3. The powders of the complex oxides 7 are subjected to hot pressing treatments at about 150 kilograms per square centimeter for about three hours in high temperature atmospheric ambients, respectively, and the temperatures of the atmospheric ambients are indicated in Table 1. The powders of the complex oxides are shaped into a generally disk shaped configuration which is about 50 millimeters in diameter and about 10 millimeters in thickness, thereby producing bulk bodies of the complex oxides, respectively. The bulk bodies of complex oxides are also porous and of the calcium-strontium-copper oxide system. Then, bismuth films are mounted on the bulk bodies of the complex oxides and heated to not less than about 272 degrees centigrade. Then, the metal bismuth films are melted and impregnate into the porous complex oxides, respectively. Thus, targets of the superconductive oxides are fabricated through the process sequence. In this instance, the hot pressing is carried out in the atmospheric ambient, however, another example may be hot pressed in a vacuum or an argon ambient. Sample Proportion (wt.%) Calcine (°C ) Average diameter (micron) Process Temp. ( °C ) CaCO₃ SrCO₃ CuO 1 24.5 36.3 39.1 850 7.1 sintering 800 hot press 550 2 28.3 41.7 30.0 920 5.5 sintering 850 hot press 600 3 27.5 40.2 32.5 990 3.0 sintering 1040 hot press 980 4 24.6 36.5 39.1 1050 6.0 sintering 1000 hot press 800 5 27.3 40.2 32.5 960 13 sintering 980 hot press 900 6 24.6 36.3 39.1 950 6.5 sintering 900 hot press 700 Evaluation For comparison use, a target is fabricated through a prior art process as follows. The ingredient powders 1, 2 and 3 as well as a powder of the bismuth oxide (represented by the molecular formula of Bi₂O₃) are prepared and mixed to the proportion of 15.6 % (CaCO₃), 23.1 %(SrCO₃), 24.9 % (CuO) and 36.4 %(Bi₂O₃) by weight. The mixing operation is continued over five hours, and the mixture is dried thereafter. The mixture is calcined in an atmospheric ambient at about 800 degrees centigrade for about ten hours and, then, pulverized into a powder. The calcining and the pulverization are repeated five times to form a powder of a superconductive oxide of the Bismuth-calcium-strontium-copper oxide system. The powder of the superconductive oxide is shaped at 2 tons per square centimeter to produce a bulk body of a compressed powder by using a press machine, and the bulk body of the compressed powder is sintered in an atmospheric ambient at about 850 degrees centigrade for about ten hours, then a disk shaped target is produced in about 50 millimeters in diameter and about 10 millimeters thick. The thermal conductivities are measured for the targets produced through the processes of the present invention as well as the prior art example, and the amount of metal bismuth impregnated is also measured for each of the targets of the present invention. The electric resistivities and the flexural strengths are measured for all of the targets. The results are summarized in Table 2. In order to decide the respective critical temperatures, all of the targets are successively set into a dc magnetron sputtering system, an argon ambient is created in the sputtering chamber and regulated to about 1,33 Pa (10⁻² torr). Substrates of magnesium oxide (MgO) with (100) orientation are faced to the respective targets at a distance of about 50 centimeters, and the sputtering is carried out at 0.5 amperes and about 500 volts. On the respective substrates are deposited thin superconductive oxide films each of which is about two microns thick and of the bismuth-calcium-strontium-copper oxide system. The critical temperatures are measured by using four probe method. The results are also indicated in Table 2. Sample Bi (volume %) Thermal conductivity, (Cal/cm.sec°C) Electric resistivity, (10⁻⁶ohm.cm) Flexural strength (kg/cm²) Critical temp. (Kelvin) 1 (S) 41 8 x 10⁻³ 300 560 76 (H) 41 10 x 10⁻³ 330 560 73 2 (S) 36 7 x 10⁻³ 350 300 77 (H) 35 8 x 10⁻³ 330 310 75 3 (S) 16 3 x 10⁻³ 750 250 78 (H) 16 2 x 10⁻³ 900 260 76 4 (S) 26 5 x 10⁻³ 460 300 72 (H) 26 6 x 10⁻³ 500 290 74 5 (S) 37 8 x 10⁻³ 330 350 76 (H) 37 7 x 10⁻³ 300 350 77 6 (S) 34 7 x 10⁻³ 360 330 76 (H) 34 6 x 10⁻³ 400 320 74 PA None 1 x 10⁻⁵ 7800 116 Impossible by the DC system S and H in Table 2 stand for the sintering and the hot pressing, respectively, and PA indicates the target produced through the prior art process. As will be understood from Table 2, the targets produced in accordance with the present invention are improved in the thermal conductivity, the electric resistivity and the flexural strength. These desirable properties are achieved by the metal bismuth impregnated. By virtue of the low electric resistivity, the targets of the present invention are sputtered by the DC magnetron sputtering system, however, the prior art target is hardly sputtered by the DC system. The targets of the present invention are usable for prolonged time period without any cracks, so that the thin superconductive film is decreased in the production cost. The thin superconductive films produced with the target of the present invention are high in critical temperature due to a negligible amount of carbon contained.

Claims: A process of fabricating a target used in a sputtering system, comprising the steps of a) preparing ingredient powders of a calcium carbonate, a strontium carbonate and a copper oxide, and b) producing a porous bulk body of a complex oxide from said ingredient powders, said complex oxide being of the calcium-strontium-copper oxide system, characterized by the step impregnating molten bismuth into the porous bulk body of said complex oxide.

Inventors: OHUCHI YUKIHIRO C O MITSUBISHI, SUGIHARA TADASHI C O MITSUBISH, TAKESHITA TAKUO C O MITSUBISHI, OHUCHI, YUKIHIRO C/O MITSUBISHI METAL CORP., SUGIHARA, TADASHI C/O MITSUBISHI METAL CORP., TAKESHITA, TAKUO C/O MITSUBISHI METAL CORP.
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1989
Main classification: H01L39
IPC codes: C04B41, H01L39, C01G1, C04B35, C01G29, H01B13, H01B12, C23C14