Superconducting thin film and a method for preparing the same

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a superconducting thin film and a method for preparing the same. More particularly, it relates to a superconducting thin film of compound oxide having a high critical temperature and also possessing lasting stability for a long period and a method for preparing the same.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a phenomenon of a kind of phase change of electrons under which the electrical resistance becomes zero and the perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power.
Therefore, if the superconducting power cable is realized, the power loss of the order of 7% which is inevitable in the conventional power cables can be reduced greatly. Realization of superconducting coils for generating a very high magnetic field is expected to accelerate development in the field of fusion power generation in which the electric power is consumed beyond its output under the present technology, as well as in the field of MHD power generation or motor-generators. The development of superconductivity is demanded also in the other industrial fields such as in the field of electric power reservation; in the field of transportation for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; or in the field of science such as NMR or high-energy physics.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconducting bodies. Tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a highly sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
However, the critical temperature could not exceed 23.2K of Nb.sub.3 Ge which was the the highest Tc for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 �Z. Phys. B64 (1986) 189!.
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba--Pb--Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba--Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity. The new type compound oxide superconductor discovered by Bednorz and Muller is represented by �La, Sr!.sub.2 CuO.sub.4 which is called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30K which are extremely higher than known superconducting materials.
It was also reported in the newspaper that C. W. Chu et al discovered in the United States of America another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of in the order of 90K in February 1987. Still another type new superconducting material is a compound oxide of Bi--Sr--Ca--Cu--O system and Tl--Ba--Ca--Cu--O system which exhibit such high Tc as more than 100K and which are chemically much more stable than the abovementioned YBCO type compound oxide or the like.
And hence, possibility of existence of high-temperature superconductors have burst on the scene.
A thin film of superconducting compound oxide is prepared by sputtering technique. For example, in Japanese patent laid-open No. 56-109824 which discloses a method for preparing a thin film of superconductor represented by BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 0.05.ltoreq.x.ltoreq.0.35), high-frequency sputtering is carried out in an oxygen containing atmosphere and then the resulting film is further heated at 500.degree. to 550.degree. C. We also described sputtering conditions for preparing superconducting thin films of compound oxide by the sputtering technique in a co-pending U.S. patent Ser. No. 152,714 filed on May 2, 1988.
The abovementioned conventional sputtering technique itself is satisfactory but is helpless against the change in properties during storage.
In fact, the superconductivity or superconducting property of the abovementioned new type superconducting materials of compound oxide is influenced by the oxygen contents in the crystalline structure since the oxygen in the crystal is not stable and is apt to escape out of the crystal, resulting in that the superconductivity is lost or deteriorated with the passing of time.
Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a superconducting thin film improved in stability during storage or use and a method for preparing the same.
SUMMARY OF THE INVENTION
A superconducting thin film composed of compound oxide and deposited on a substrate according to the present invention is characterized in that an outer surface of the superconducting thin film is covered with a protective layer which is composed of any one of the following materials:
(i) oxide of metal,
(ii) carbide,
(iii) nitride or
(iv) the same material as the substrate.
The oxide of metal of (i) may be any oxide of an element selected from a group comprising a metal element of Ha group, a metal element of IIIa group, a metal element of IVa group, a metal element of Va group, a metal element of VIa group, a metal element of VIIa group, a metal element of VIIIa group, a metal element of Ib group, a metal element of IIb group, a semi-metal element of IIIb group and a semi-metal element of IVb group in the periodic table. More particularly, it can be mentioned one of oxides of following elements: Mg, Ca, Sr, Ba (IIa group); Y, La, lanthanoid (IIIa group); Ti, Zr (IVa group); V, Nb, Ta (Va group); Cr, Mo, W (VIa group); Mn (VIIa group); Fe, Co, Ni (VIIIa group); Cu, Ag (Ib group); Zn, Cd, Hg (IIb group); Al, Ga, In, Tl (IIIb group) and Si, Ge, Sn, Pb (IVb group).
According to the first aspect of the present invention, the protective layer of the present invention may be composed of oxide of at least one of elements selected from a group comprising elements of Ha group, IIIa group, IIIb group and IVb group, particularly, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga, In Tl, Si, Ge, Sn and Pb. Since the oxides of these elements are homologue of or possess very similar properties to components of the superconductor, no undesirable influence is produced when they are deposited on the surface of the superconductor. Still more, the oxides of these elements contain stable oxygen which may advantageously suppress liberation of oxygen from the crystalline superconducting thin film, so that the superconducting property of the thin film can be maintained.
According to second aspect of the present invention, the protective layer of the present invention may be composed of oxide of transition metal, particularly at least one metal selected from a group comprising Ti, Zr, V, Nb, Ta, Cr, Mo, W. Mn, Fe, Co, Ni, Cu and Ag. The oxides of these elements also contain stable oxygen which may advantageously suppress liberation of oxygen from the crystalline superconducting thin film to maintain the superconducting property of the thin film.
According to third aspect of the present invention, the protective layer of the present invention may be composed of oxide of such a metal element or elements that are contained in the superconducting thin film to be produced, such as Ba, Y, Sr or La. Each of these metal oxides is a component of the superconductor of compound oxide to be produced and have no undesirable effect on the superconductor.
The carbide of (ii) may be SiC or TiC each of which may contain other elements such as hydrogen. Since the carbide is chemically stable and hard and produce a protective layer having a high density which may advantageously suppress liberation of oxygen from the crystalline superconducting thin fin to maintain the superconducting property of the thin film.
The nitride of (iii) may be SiC or TiC each of which may contain other elements such as hydrogen. Since the nitride is chemically stable or inactive to the superconductor and produce a protective layer having a high density which may advantageously suppress liberation of oxygen from the crystalline superconducting thin film, the superconducting property of the thin film can be maintained for a long time.
In case of (iv), the protective layer may be composed of the same material as the substrate which is made of a single crystal of MgO or SrTiO.sub.3. When this material is deposited on the superconducting thin film, the deposited film grows epitaxially so that die crystalline axis of the resulting protective layer can be oriented to a predetermined direction. This means that the resulting protective film has a very high density and can prevent liberation of oxygen out of the superconducting thin film. Of course, this protective layer is inactive to the superconductor.
Selection of the abovementioned protective layers depend on type or system of the compound oxide used.
The superconducting thin film can be composed of compound oxide of an element a selected from IIa group of the Periodic Table, an element .beta. selected from IIIa group of the Periodic Table and at least one element .gamma. selected from a group comprising Ib, IIb, IIIb, IVb and VIIIa group of the Periodic Table. The element .gamma. is copper in general.
Particularly, the superconducting thin film is preferably a compound oxide represented by the general formula:
(.alpha..sub.1-x .beta..sub.x)Cu.sub.y O.sub.z
in which .alpha. and .beta. means the same definition as above, x is an atomic ratio of .beta. with respect to (.alpha.+.beta.) and has a range of 0.1.ltoreq.x.ltoreq.0.9, and y and z are respective atomic ratios with respect to (.alpha..sub.1-x .beta..sub.x) which is considered to be equal to 1 and each satisfy ranges of 0.4.ltoreq.y.ltoreq.3.0 and 1.ltoreq.z.ltoreq.5 respectively.
According to a preferred embodiment of the present invention, the element .alpha. is Ba or Sr and the element .beta. is at least one element selected from a group comprising Y, La, Gd, Dy, Ho, Er, Tm, Yb, Nd, Sm, Eu and Lu. From 10 to 80% of the element .alpha. may be substituted by one or more than one of elements selected from Ha group of the Periodic Table. The element .beta. may be a combination of more than two elements selected from IIIa group of the Periodic. A portion of these components can be substituted by at least one of elements selected from a group comprising Al, Fe, Ni, Zn, Ag and Ti.
An atomic ratio of .alpha. to .beta. depends on the type or system of the compound oxide or on the combination of .alpha. and .beta.. Following atomic ratios are preferably used in case of Ba--Y system, Ba--La system and Sr--La system respectively:
Y/(Y+Ba) 0.06 to 0.94, more preferably 0.1 to 0.4
Ba/(La+Ba) 0.04 to 0.96, more preferably 0.08 to 0.45
Sr/(La+Sr) 0.03 to 0.95, more preferably 0.05 to 0.1
Thus, the preferable compound oxides are Y--Ba--Cu--O system, La--Ba--Cu--O system and La--Sr--Cu--O system including the following special cases:
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Ho.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Lu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
Sm.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Nd.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Gd.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
Eu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Er.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Dy.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
Tm.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Yb.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, La.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,
(La, Sr).sub.2 CuO.sub.4-x
in which x is a number which satisfies a range of 0<x<1.
The abovementioned oxides possess preferably perovskite type or quasi-perovskite type crystal structure. The term of quasi-perovskite type means a structure which can be considered to have such a crystal structure that is similar to Perovskite-type oxides and includes an orthorhombically distorted perovskite or a distorted oxygen-deficient perovskite or the like.
The superconducting thin film may be also another type of superconductor consisting mainly of a compound oxide represented by the formula:
.THETA..sub.4 (.PHI..sub.1-q, Ca.sub.q).sub.m Cu.sub.n O.sub.p+r
in which .THETA. stands for Bi or Tl, .THETA. stands for Sr when .PHI. is Bi and stands for Ba when .PHI. is Tl, m and n are numbers each satisfying ranges of 6.ltoreq.m.ltoreq.10 and 4.ltoreq.n.ltoreq.8 respectively, p=(6+m+n), q is a number which satisfies a range of 0<q<1, and r is a number which satisfies a range of -2.ltoreq.r.ltoreq.+2. This system is considered to be a single phase of the following compound oxide or a mixed phase consisting mainly thereof:
Bi.sub.4 Sr.sub.4 Ca.sub.4 Cu.sub.6 O.sub.20-r, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-r,
Tl.sub.4 Ba.sub.4 Ca.sub.4 Cu.sub.6 O.sub.20-n, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-r,
in which r is a number which satisfies a range of -2.ltoreq.r.ltoreq.+2.
The substrate may be made of glass, quartz, silicon, sapphire, stainless, steel or other ceramics. Particularly, the substrate consists preferably of a single crystal of MgO or SrTiO.sub.3. Desirably, the superconducting thin film is deposited on a (001) plane or (110) plane of a single crystal of MgO or SrTiO.sub.3.
The present invention relates also to a method for preparing a superconducting thin film on a substrate, The method is characterized in that an outer surface of the superconducting thin film is covered with a protective layer which is composed of any one of the following materials:
(i) oxide of metal,
(ii) carbide,
(iii) nitride or
(iv) the same material as the substrate.
The thin film of superconductor can be prepared by the conventional physical vapour deposition technique, such as sputtering, vacuum deposition, ion plating, molecular beam epitaxial growth or the like. It is also possible to use chemical deposition technique (CVD) such as plasma CVD or photo CVD. Generally, the thin film of compound oxide is prepared by sputtering technique.
According to the present invention, more than two protective layers can be formed successively on the superconducting thin film layer. Each protective layer may be different in type and composition from adjacent layers.
According to a preferred embodiment of the present invention, the superconducting thin film is deposited on a substrate by sputtering technique and then a protective layer is deposited on a surface of the superconducting thin film in a common sputtering unit. Namely, the sputtering is preferably carried out by such a manner that, at first, both targets for the superconducting thin film and for the protective layer are set on respective target holders in a chamber, and then the target for the superconducting thin film is sputtered with argon gas or the like while the target for the protective layer is not sputtered. After a superconducting thin film is deposited on a substrate, the target for the protective layer is sputtered to deposit a protective layer on the superconducting thin film.
Such superconducting thin film can be prepared by sputtering technique described in our co-pending U.S. patent Ser. No. 152,714 filed on May 2, 1988.
The superconducting property can be improved by heat-treatment which is effected after the thin film of compound oxide is deposited on the substrate in oxygen containing atmosphere. The heat-treatment is preferably effected under a partial pressure of oxygen ranging from 0.1 to 150 atm at a temperature between 300.degree. and 1,500.degree. C. After this temperature is maintained for more than one hour, the resulting thin film is cooled slowly at a rate of less than 100.degree. C./min. Advantage of the heat-treatment can not be obtained if the heat-treatment is effected outside the abovementioned conditions. For example, if the in film is heated at a temperature which is higher than 1,500.degree. C., the abovementioned advantage can not be obtained but the superconductivity will be disappeared.
In case of the compound oxide type superconductor, oxygen deficiency in its crystal is a critical factor for realizing the superconductivity. Particularly, a heat-treatment under a relatively higher partial pressure of oxygen is very preferable and is considered to be indispensable to realize superior superconductivity.
However, it is impossible to prevent the thin film of compound oxide from deterioration which occur during storage even if the thin film is heat-treated completely. The deterioration of superconductivity, in other words a phenomenon that the superconducting property is lost gradually in time may be caused by disappearance or liberation of oxygen in the crystal because the superconductivity of compound oxide is observed in unstable or quasi-stable phase. This is a big problem of compound oxide type superconductors in their actual use.
This problem is solved by the present invention in which an outer surface of the thin film of superconductor is covered by a stable and hard protective layer which has a high density to prevent oxygen from escaping from its crystal structure.
In a stage of deposition of the protective layer on the superconducting thin film, the substrate on which the thin film is deposited should not be heated at a temperature which is higher than 500.degree. C. In fact, if the substrate is heated at a temperature which is higher than 500.degree. C., the superconducting property is damaged because oxygen in its crystal is apt to be lost.
The thin film of compound oxide is preferably deposited on a (001) plane or (110) plane of a single crystal of MgO or SrTiO.sub.3. In fact, the thin film of compound oxide possesses anisotropy in its crystal structure, so that a current passing along the direction which is in parallel with a plane defined by a-axis and b-axis of the crystal show a very high critical current density but the other current passing along the other directions is relatively lower. Therefore, when the c-axis is orientated to a direction which is perpendicular to the plane defined by bra-axis and b-axis of the crystal, a current which has much higher critical current density (Jc) can be delivered along a direction which is in parallel wit the plane.
According to a preferred embodiment of the present invention, the superconducting thin film composed of a single crystal or polycrystals of compound oxide whose c-axis is oriented to a direction which is perpendicular to the plane defined by by a-axis and b-axis of the crystal is deposited on a (100) plane of a substrate of single crystal of MgO or SrTiO.sub.3 which has such lattice constant or lattice constants in a-axis and/or b-axis that are proximate to those of the in film such as Ln.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 in order to realize a high critical current density (Jc) along the direction which Is in parallel with a surface of the substrate.
In case of (iv) where the protective layer is composed of the same material as the substrate which consist of a single crystal of MgO or SrTiO.sub.3, the deposited protective layer grows epitaxially so that the crystalline axis of the resulting protective layer is oriented to a predetermined direction, resulting in that protective film has a very high density and can prevent escape of oxygen out of the superconducting thin film.
It is apparent from the description abovementioned that the superconducting thin film of compound oxide according to the present invention have improved stability than conventional superconducting thin film and hence they can be utilized advantageously in a applications of thin film devices, such as Matisoo switching elements or Josephson device, Anacker memory device or Superconducting Quantum Interference Device (SQUID).
Now, the present invention will be described with reference to attached drawings which illustrate an apparatus used for carrying out the present invention. But, the present invention is not limited thereto.

BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an illustrative view of a magnetron sputtering unit which is used in the present invention.

Referring to FIG. 1 illustrating a magnetron sputtering unit which is used in the present invention, the sputtering unit includes, in a chamber 1, two target holders for a superconducting thin film 2 and for a protective layer 3, a pair of magnetron electrodes 4 each surrounding respective targets 2 and 3, high-frequency power sources 5 for actuating the magnetron electrodes 5 and a substrate holder on which a substrate is secured. The chamber 1 has an exhaust port 8 which is connected to a vacuum source (not shown) and a gas inlet port 7 for introducing an atmosphere gas. The substrate holder is provided with a heater 9 for controlling a temperature of the substrate 6. Such sputtering unit itself are known and used in a variety of applications.
In operation, after a substrate 6 is secured to the substrate holder and targets 2 and 3 are set on respective target holder in the chamber 1, vacuum is created in the chamber 1 and a suitable gas such as argon and/or oxygen at a predetermined pressure is introduced into the chamber 1. After the substrate 6 is heated to a predetermined temperature by the heater 9, high-frequency power is applied to one of the magnetron electrode 4 surrounding the target 2 for the superconducting thin film to start sputtering operation. Introduction of the atmosphere gas in the chamber 1 is controlled at a predetermined pressure during the sputtering operation. The deposition of the superconducting thin film of compound oxide is carried out by the conventional sputtering technique.
After the thin film of compound oxide is deposited, another the magnetron electrode 4 surrounding the target 3 for the protective layer is energized so that a thin film of the protective layer is deposited on the super conducting thin film.
Now, several embodiments of the method according to the present invention will be described by Examples, but the scope of the present invention should not be limited thereto.
In the following Examples, both of the superconducting thin film and the protective layer are prepared in the sputtering unit shown in FIG. 1. For comparison, two series of samples are prepared so that the first series possesses the protective layer while second series do not possess the same.
EXAMPLE 1
Preparation of a Target for Compound Oxide
At first, powders of Y.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu 1:2:3. Then, the resulting powder mixture was sintered at 950.degree. C. to obtain a sintered block of YBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting in film,
Deposition of Thin Films
The resulting target 2 and a target 3 composed of h-BN are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.-2 Torr are introduced and the substrate is heated at 620.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG./sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650.degree. C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that h-BN is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450.degree. C.
Measurement of Critical Temperature
Resistance of the resulting thin film is measured on such a sample that has vacuum-deposited aluminum electrodes at opposite sides of the thin film of compound oxide deposited on the substrate 6.
Measurement of the critical temperature Tc and Tcf was carried out by a conventional four probe method in which the sample was immersed in liquidized helium to cool the sample down to a temperature of 8K in a cryostat. Then, the temperature dependence of resistance of the sample was determined with rising the temperature gradually to determine a temperature of Tcf at which the perfect superconductivity start to be lost and resistance begin to appear and a temperature of Tc at which the superconductivity is lost and ordinary resistance begin to appear.
Changes of Tcf and Tc are determined by comparing two values observed on same sample just after the protective layer is deposited and after one month.
The result as well as main operational parameters are shown in Table 1.
EXAMPLE 2
The same procedure as Example 1 is repeated except that a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 is used as a target for a thin film of compound oxide which is prepared by following procedure.
Preparation of a Target for Compound Oxide
Powders of La.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of La:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970.degree. C. to obtain a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
The result as well as main operational parameters are shown in Table 1.
TABLE 1______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________1 1.0 .times. 10.sup.-2 620 BN 91 78 90 78 -- 89 77 36 --2 7.0 .times. 10.sup.-3 680 BN 58 47 59 48 -- 57 48 8 --______________________________________ (Note)--: Superconductivity is not observed in liquid helium
EXAMPLE 3
The same procedure as Example 1 is repeated except that the target of h-BN is replaced by a target of TiN.
The result as well as main operational parameters are shown in Table 2.
EXAMPLE 4
The same procedure as Example 2 is repeated except that the target of h-BN is replaced by a target of TiN.
The result as well as main operational parameters are shown in Table 2.
TABLE 2______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________3 8.0 .times. 10.sup.-3 730 TiN 96 85 95 86 -- 98 88 33 --4 5.0 .times. 10.sup.-3 690 TiN 51 38 53 37 -- 49 37 17 --______________________________________ (Note)--: Superconductivity is not observed in liquid helium
EXAMPLE 5
Preparation of a Target for Compound Oxide
At first, powders of Y.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950.degree. C. to obtain a sintered block of YBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
Deposition of Thin Films
The resulting target 2 and a target 3 composed of SiC are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.-2 Torr are introduced and the substrate is heated at 630.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG./sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650.degree. C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that SiC is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450.degree. C.
Measurement of Critical Temperature
Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as well as main operational parameters are shown in Table 3.
EXAMPLE 6
The same procedure as Example 5 is repeated except that a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 is used as a target for a thin film of compound oxide which is prepared by following procedure.
Preparation of a Target for Compound Oxide
Powders of La.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of La:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970.degree. C. to obtain a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
The result as well as main operational parameters are shown in Table 3.
TABLE 3______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________5 1.0 .times. 10.sup.-2 630 SiC 86 72 87 71 -- 85 72 33 --6 7.0 .times. 10.sup.-3 670 SiC 61 47 60 48 -- 62 50 -- --______________________________________ (Note)--: Superconductivity is not observed in liquid helium
EXAMPLE 7
The same procedure as Example 5 is repeated except that the target of SiC is replaced by a target of TiC.
The result as well as main operational parameters are shown in Table 4.
EXAMPLE 8
The same procedure as Example 6 is repeated except that the target of SiC is replaced by a target of TiC.
The result as well as main operational parameters are shown in Table 4.
TABLE 4______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________7 8.0 .times. 10.sup.-3 710 TiC 93 82 94 81 -- 91 80 31 --8 5.0 .times. 10.sup.-3 685 TiC 53 39 51 38 -- 52 38 -- --______________________________________ (Note)--: Superconductivity is not observed in liquid helium
EXAMPLE 9
Preparation of a Target for Compound Oxide
At first, powders of Y.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950.degree. C. to obtain a sintered block of YBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film,
Deposition of Thin Films
The resulting target 2 and a powder target 3 composed of MgO powder are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The substrate of MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.-2 Torr are introduced and the substrate is heated at 650.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG. /sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650.degree. C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that MgO is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450.degree. C.
Measurement of Critical Temperature
Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as welt as main operational parameters are shown in Table 5.
EXAMPLE 10
The same procedure as Example 9 is repeated except that a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 is used as a target for a thin film of compound oxide which is prepared by following procedure.
Preparation of a Target for Compound Oxide
Powders of La.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of La:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970.degree. C. to obtain a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
The result as well as main operational parameters are shown in Table 5.
TABLE 5______________________________________ Just after AfterPartial Temperature Deposi- 1pressure of Protec- tion month of O.sub.2 Substrate tive Jc Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (A/cm.sup.2) (K) (K) (K) (K)______________________________________ 9 1.0 .times. 10.sup.-2 650 MgO 3 .times. 10.sup.6 79 67 77 68 -- 1 .times. 10.sup.5 78 68 11 --10 7.0 .times. 10.sup.-3 620 MgO 5 .times. 10.sup.5 56 39 56 38 -- 8 .times. 10.sup.3 55 40 -- --______________________________________ (Note)--: Superconductivity is not observed in liquid helium
EXAMPLE 11
The same procedure as Example 9 is repeated except that the powder target of MgO is replaced by a target of SrTiO.sub.3 block.
The result as well as main operational parameters are shown in Table 6.
EXAMPLE 12
The same procedure as Example 10 is repeated except that the substrate of MgO is replaced by a substrate composed of a single crystal of SrTiO.sub.3 and that the target of powder MgO is also replaced by a target of SrTiO.sub.3. The thin film of compound oxide is deposited on (100) plane of the substrate of SrTiO.sub.3 block.
The result as well as main operational parameters are shown in Table 6.
TABLE 6______________________________________ Just after AfterPartial Temperature Deposi- 1pressure of Protec- tion month of O.sub.2 Substrate tive Jc Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (A/cm.sup.2) (K) (K) (K) (K)______________________________________11 8.0 .times. 10.sup.-3 700 SrTiO.sub.3 2 .times. 10.sup.6 89 76 88 77 -- 9 .times. 10.sup.4 90 78 25 --12 5.0 .times. 10.sup.-3 640 SrTiO.sub.3 7 .times. 10.sup.5 49 34 48 35 -- 3 .times. 10.sup.4 48 26 -- --______________________________________ (Note)--: Superconductivity is not observed in liquid helium
EXAMPLE 13
Preparation of a Target for Compound Oxide
At first, powders of Y.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950.degree. C. to obtain a sintered block of YBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
Deposition of Thin Films
The resulting target 2 and a target 3 composed of MgO are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.2 Torr are introduced and the substrate is heated at 600.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG./sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650.degree. C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that MgO is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450.degree. C.
Measurement of Critical Temperature
Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as well as main operational parameters are shown in Table 7.
EXAMPLE 14
The same procedure as Example 13 is repeated except that a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 is used as a target for a thin film of compound oxide which is prepared by following procedure.
Preparation of a Target for Compound Oxide
Powders of La.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of La:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970.degree. C. to obtain a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
The result as well as main operational parameters are shown in Table 7.
TABLE 7______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________13 1.0 .times. 10.sup.-2 600 MgO 89 71 90 72 -- 77 68 39 --14 7.0 .times. 10.sup.-3 700 MgO 54 36 55 37 -- 55 34 18 --______________________________________ (Note) -: Superconductivity is not observed in liquid helium
EXAMPLE 15
The same procedure as Example 13 is repeated except that the target of MgO is replaced by a target of SiO.sub.2.
The result as well as main operational parameters are shown in Table 8.
EXAMPLE 16
The same procedure as Example 14 is repeated except that the target of MgO is replaced by a target of SiO.sub.2.
The result as well as main operational parameters are shown in Table 8.
TABLE 8______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________15 8.0 .times. 10.sup.-3 650 SiO.sub.2 95 77 94 78 -- 92 75 34 --16 5.0 .times. 10.sup.-3 680 SiO.sub.2 47 35 48 37 -- 46 36 20 --______________________________________ (Note) -: Superconductivity is not observed in liquid helium
EXAMPLE 17
Preparation of a Target for Compound Oxide
At first, powders of Y.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950.degree. C. to obtain a sintered block of YBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
Deposition of Thin Films
The resulting target 2 and a target 3 composed of TiO.sub.2 are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.-2 Torr are introduced and the substrate is heated at 600.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG./sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650.degree. C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that TiO.sub.2 is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450.degree. C.
Measurement of Critical Temperature
Measurement of the critical temperature Tc and Tcf was carried out by the same method as Example 1. The result as well as main operational parameters are shown in Table 9.
EXAMPLE 18
The same procedure as Example 17 is repeated except that a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 is used as a target for a thin film of compound oxide which is prepared by following procedure.
Preparation of a Target for Compound Oxide
Powders of La.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of La:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970.degree. C. to obtain a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin foam.
The result as well as main operational parameters are shown in Table 9.
TABLE 9______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________17 1.0 .times. 10.sup.-2 600 TiO.sub.2 88 72 89 71 -- 79 67 41 --18 7.0 .times. 10.sup.-3 700 TiO.sub.2 57 39 56 41 -- 58 36 21 --______________________________________ (Note) -: Superconductivity is not observed in liquid helium
EXAMPLE 19
The same procedure as Example 17 is repeated except that the target of TiO.sub.2 is replaced by a target of ZrO.sub.2.
The result as well as main operational parameters are shown in Table 10.
EXAMPLE 20
The same procedure as Example 18 is repeated except that the target of TiO.sub.2 is replaced by a target of ZrO.sub.2.
The result as well as main operational parameters are shown in Table 10.
TABLE 10______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________19 8.0 .times. 10.sup.-3 650 ZrO.sub.2 93 77 95 76 -- 91 76 38 --20 5.0 .times. 10.sup.-3 680 ZrO.sub.2 48 36 49 37 -- 47 34 21 --______________________________________ (Note) -: Superconductivity is not observed in liquid helium
EXAMPLE 21
Preparation of a Target for Compound Oxide
At first, powders of Y.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950.degree. C. to obtain a sintered block of YBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
Deposition of Thin Films
The resulting target 2 and a target 3 composed of Y.sub.2 O.sub.3 are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.-2 Torr are introduced and the substrate is heated at 680.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG./sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650.degree. C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that Y.sub.2 O.sub.3 is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 430.degree. C.
Measurement of Critical Temperature
Measurement of the critical temperature Tc and Tcf was carried out by the same method as Example 1. The result as well as main operational parameters are shown in Table 11.
EXAMPLE 22
The same procedure as Example 21 is repeated except that a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 is used as a target for a thin film of compound oxide which is prepared by following procedure.
Preparation of a Target for Compound Oxide
Powders of La.sub.2 O.sub.3 and BaCO.sub.3 were mixed at an atomic ratio of La:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at 10% excessive proportion with respect to an atomic ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970.degree. C. to obtain a sintered block of LaBa.sub.2 Cu.sub.3 O.sub.7 which is used as a target 2 for a superconducting thin film.
Deposition of Thin Films
The resulting target 2 and a target 3 composed of La.sub.2 O.sub.3 are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane.
After the chamber 1 is vacuumed, argon gas at a partial pressure of 5.0.times.10.sup.-2 Torr and oxygen gas at a partial pressure of 1.0.times.10.sup.-2 Torr are introduced and the substrate is heated at 680.degree. C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm.sup.2 to prepare a thin film of compound oxide of 1 .mu.m on the substrate at a film forming rate of 0.50 .ANG./sec.
After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 730.degree. C. The deposited thin film is left under this condition for 20 hour and then cooled slowly at a cooling rate of 8.degree. C./min.
Then, another magnetron electrode for the protective layer 3 is energized so that La.sub.2 O.sub.3 is sputtered to deposit a protective layer of 2,000 .ANG. on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 380.degree. C.
The result as well as main operational parameters are shown in Table 11.
TABLE 11______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________21 1.0 .times. 10.sup.-2 680 Y.sub.2 O.sub.3 88 72 89 71 -- 79 67 41 --22 7.0 .times. 10.sup.-3 710 La.sub.2 O.sub.3 57 39 56 41 -- 58 36 11 --______________________________________ (Note) -: Superconductivity is not observed in liquid helium
EXAMPLE 23
The same procedure as Example 21 is repeated except that the target of Y.sub.2 O.sub.3 is replaced by a target of BaTiO.sub.3.
The result as well as main operational parameters are shown in Table 12.
EXAMPLE 24
The same procedure as Example 22 is repeated except that the target of Y.sub.2 O.sub.3 is replaced by a target of BaTiO.sub.3.
The result as well as main operational parameters are shown in Table 12.
TABLE 12______________________________________ OnePartial pressure Temperature Just after monthof of deposition later O.sub.2 Substrate Protective Tc Tcf Tc TcfNo. (Torr) (.degree.C.) Layer (K) (K) (K) (K)______________________________________23 8.0 .times. 10.sup.-3 640 BaTiO.sub.3 93 77 95 76 -- 91 76 38 --24 5.0 .times. 10.sup.-3 670 BaTiO.sub.3 48 36 49 37 -- 47 34 -- --______________________________________ (Note) -: Superconductivity is not observed in liquid helium

Number	Date	Country
62-168356	Jul 1987	JPX
62-168357	Jul 1987	JPX
62-172696	Jul 1987	JPX
62-172697	Jul 1987	JPX
62-172698	Jul 1987	JPX
62-178600	Jul 1987	JPX

Number	Name	Date
3852877	Ahn et al.	Dec 1974
3932315	Sleight	Jan 1976
4220959	Kroger	Sep 1980
4316785	Suzuki	Feb 1982
4692385	Johnson	Sep 1987
4753856	Haluska et al.	Jun 1988
4826808	Yurek et al.	May 1989
4908346	Strom et al.	Mar 1990
4925647	Kirchhofer	May 1990
4970197	Shiota et al.	Nov 1990
5001108	Taguchi	Mar 1991
5057484	Shiota et al.	Oct 1991
5086037	Hitotsuyangi et al.	Feb 1992
5232903	Yamazaki	Aug 1993
5266815	Sunami et al.	Nov 1993
5300487	Hed	Apr 1994
5447908	Itozaki et al.	Sep 1995
5587870	Anderson et al.	Dec 1996

Number	Date	Country
56-109824	Jan 1981	JPX
60-173885	Jan 1985	JPX

	Number	Date	Country
Parent	415763	Apr 1995
Parent	215497	Jul 1988

Superconducting thin film and a method for preparing the same

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Entry
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