Process for preparing thin film of oxyde superconductor

Abstract: A process for preparing a thin film of oxide superconductor on a single crystal substrate of semiconductor by physical vapour deposition. At first, an under-layer of an oxide having a thickness of 50 to 200 Å is deposited on the single crystal substrate of semiconductor at a substrate temperature of lower than 500 °C, and secondly an upper-layer of superconducting oxide material is deposited on said under-layer at a substrate temperature of higher than 600 °C.

Description: Background of the Invention Field of the invention The present invention relates to a process for preparing a thin film of oxide superconductor on a single crystal substrate of semiconductor by physical vapour deposition. Description of the related art Superconducting oxide materials such as Y-Ba-Cu-O system, Bi-Sr-Ca-Cu-O system, Tl-Ba-Ca-Cu-O system or the like are expected to be utilized in electronics devices such as Josephson element or superconducting transistors due to their high critical temperatures (Tc). In order to realize such electronics devices, it is indispensable to prepare thin films of these oxide superconductors. Recently, it has become possible to prepare single crystal thin films of these oxide superconductors of good quality on a single crystal substrate of MgO, SrTiO 3 or the like by physical vapour deposition technique including sputtering, vacuum evaporation and laser abrasion or by chemical vapour deposition technique. It is known from EP-A-0 341 148 a semiconductor substrate having a superconducting thin film and a process producing the same. According to the teaching of this reference the structure obtained consists of an oxide superconductor upper-layer/zrO 2 or MgO/semiconductor substrate. Further it is known from US-A-4 880 770 a metalorganic deposition process for preparing superconductive oxide films. The structure obtained consists of an upper layer of oxide superconductor/amorphous metal oxide/semi-conductor substrate. However, in the case of the superconducting transistors in which a single crystal thin film of oxide superconductor must be deposited on a single crystal substrate of semiconductor, it is impossible or very difficult to prepare a thin film of oxide superconductor having good crystallinity. In fact, when an oxide superconducting material is deposited on a single crystal substrate of semiconductor such as a silicon single crystal, separated island portions of the oxide superconducting material which are separated from each other are formed initially on the substrate until a thin film of oxide superconducting material becomes to a thickness of about 20 nm (200 Å). Therefore, even if deposition of the oxide superconducting material is continued further on such separated islands of the oxide superconducting material, the resulting thin film can not be a uniform and smooth thin film having good crystallinity. Although a uniform and smooth thin film of oxide superconductor may be obtained by lowering a substrate temperature during deposition, the thin film prepared at low temperatures is poor in crystallinity in such extent that the thin film can not be used in actual applications. Therefore, an object of the present invention is to solve the problems and to provide a process for preparing a single crystal thin film of oxide superconductor of high quality on a single crystal substrate of semiconductor such as silicon. SUMMARY OF THE INVENTION The present invention provides a process for preparing a thin film of oxide superconductor on a single crystal substrate of semiconductor, which comprises depositing firstly by RF sputtering an under-layer of a superconducting oxide material having a thickness of 5 to 20 nm (50 to 200 Å) on said single crystal substrate of semiconductor at a substrate temperature between 300° and 500°C, and depositing secondly by RF sputtering an upper-layer of a superconducting oxide material on said under-layer at a substrate temperature of higher than 600°C, said upper-layer of superconducting oxide material being thicker than said under-layer wherein both said under-layer and said upper-layer are deposited in a same vacuum chamber. of superconducting oxide material on the resulting under-layer at a substrate temperature of higher than 600 °C. The under-layer may be made of any superconductive oxide material but is preferably made of the same superconducting oxide material as the upper layer. The upper-layer of superconducting oxide material can be made of any known superconducting oxide including Y-Ba-Cu-O system, Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system. In particular, the present invention is advantageously applicable for Y-Ba-Cu-O system and Bi-Sr-Ca-Cu-O system. The single crystal substrate of semiconductor can be a single crystal of silicon or compound semiconductor such as GaAs, InP. According to the process of the present invention as claimed, the under-layer is deposited at a substrate temperature of lower than 500 °C. Lower limit of this substrate temperature depends on kinds of under-layer used but is between 300 and 500 °C, more preferably between 400 and 500 °C. If the substrate temperature exceeds 500 °C, vapour or particles of the oxide superconducting material arrived at and deposited on a surface of the substrate coagulate in a form of islands which are separated from each other in the first stage of deposition during which deposited particles of the oxide superconducting material form a thin film of oxide superconducting material having a thickness of about 20 nm (200 Å). When the substrate temperature is lower than 500 °C according to the present invention as claimed, the vapour or particles of the oxide superconducting material arrived at the surface of the substrate are distributed uniformly over the surface of substrate and form a uniform under-layer. Although this under-layer prepared at this stage is not composed of a single crystal, crystallinity of the under-layer is not important at this stage because such under-layer can be changed to a single crystal in next stage. In other words, a purpose of the under-layer is to form a uniform layer which is not separated like islands. In order to achieve this purpose, it is preferable to prepare an under-layer of amorphous which can be easily changed to single crystal at next stage. The thickness of the under-layer is important and is limited to a range between 5 to 20 nm (50 to 200 Å). If the thickness of the under-layer is not thicker than 50 Å, the upper-layer deposited on such under-layer in next stage can not be a uniform layer but will be composed of separate islands. If the thickness of the under-layer increase over 200 Å, such under-layer can not be changed to perfect single crystal in next stage. A preferable thickness of the under-layer is about 10 nm (100 Å). According to the process of the present invention as claimed, the upper-layer of oxide superconducting material is secondly deposited on the resulting under-layer at a substrate temperature of higher than 600 °C. In this stage, the single crystal substrate of semiconductor is heated above 600 °C, resulting in that the under-layer having poor crystallinity or amorphous which has been prepared in previous stage is re-crystallized to single crystal. The upper-layer of the oxide superconducting material thus prepared on the under-layer becomes a thin film of single crystal. The substrate temperature for depositing the upper-layer of oxide superconducting material should be selected in such a range that the under-layer is re-crystallized to single crystal and also no damage is given to the single crystal substrate of semiconductor. The substrate temperature depends on kinds of oxide superconductor to be deposited but must be above 600 °C. More precisely, when an upper-layer of Y-Ba-Cu-O system oxide is deposited, a preferable substrate temperature is about 640 °C, and when an upper-layer of Bi-Sr-Ca-Cu-O system oxide is deposited, a preferable substrate temperature is about 700 °C. The under-layer and the upper-layer are prepared successively in the presence of oxygen gas by RF sputtering in a same vacuum chamber. Apparatus and deposition conditions of RF sputtering themselves are known. The process according to the present invention as claimed has such a merit that a thin film of oxide superconducting material having improved crystallinity can be prepared on a single crystal substrate of semiconductor by a simple modification of existing technology only by controlling the substrate temperature. Now, the present invention will be described with reference to Examples, but the scope of the present invention as claimed should not be limited thereto. Description of the Preferred Embodiments In following Example, a thin film of oxide superconductor was prepared on a single crystal substrate of semiconductor by RF magnetron sputtering according to the process of the present invention as claimed. Example 1 A thin film of Y-Ba-Cu-O system superconducting oxide was prepared on a (100) plane of a single crystal substrate of silicon. A target used in this Example was a sintered oxide of Y, Ba and Cu (atomic ratio : Y : Ba : Cu = 1 : 2 : 2.6). By using this target, an under-layer of Y-Ba-Cu-O system oxide was firstly deposited on the single crystal substrate of silicon and then an upper-layer of Y-Ba-Cu-O system oxide was deposited on the resulting under-layer in a same vacuum chamber successively. Deposition conditions for the under-layer and the upper-layer are summarized in Table 1 (1 Torr = 133,32 Pa; 10Å = 1 nm). Under-layer Upper-layer Substrate temperature (°C) 420 640 Gas pressure (Torr) 0.1 0.1 Sputtering gas (Ar+O 2 ) O 2 /(Ar+O 2 ) (vol %) 20 20 RF power density (W/cm 2 ) 2 2 Deposition rate (Å/sec) 0.3 0.3 Thickness of a layer (Å) 100 4,000 For comparison, a comparative sample was prepared by the same method as above but the substrate temperature was maintained at a constant value of 640 °C for both of the under-layer and the upper-layer. In this comparative sample, the total thickness of under-layer + the upper-layer was adjusted to a same value as above (namely, 4,100 Å). Surface conditions of the resulting superconducting thin films were observed by SEM (scanning electron microscopy) and the critical temperature (Tc) and the critical current density (Jc) at 77 K were determined by usual methods. The results are summarized in table 2. Tc (K) Jc (A/cm 2 ) Surface condition Invention 82 1 x 10 5 uniform and smooth Comparative 40 ---- uneven Example 2 The same procedure as Example 1 was repeated but the single crystal substrate of silicon was replaced by a single crystal substrate of GaAs to prepare a thin film of Y-Ba-Cu-O system superconducting oxide on a (100) plane of the GaAs substrate. Deposition conditions for the under-layer and the upper-layer are summarized in Table 3 (1 Torr = 133,32 Pa; 10 Å = 1 nm). Under-layer Upper-layer Substrate temperature (°C) 380 630 Gas pressure (Torr) 0.1 0.1 Sputtering gas (Ar+O 2 ) O 2 /(Ar+O 2 ) (vol %) 20 20 RF power density (W/cm 2 ) 2 2 Deposition rate (Å/sec) 0.3 0.3 Thickness of a layer (Å) 100 4,000 For comparison, a comparative sample was prepared by the same method as above but the substrate temperature was maintained at a constant value of 630 °C for both of the under-layer and the upper-layer. In this comparative sample, the total thickness of under-layer + the upper-layer was adjusted to a same value as above (namely, 4,100 Å). Surface conditions of the resulting superconducting thin films were observed by SEM (scanning electron microscopy) and the critical temperature (Tc) and the critical current density (Jc) at 77 K were determined by usual methods. The results are summarized in table 4. Tc (K) Jc (A/cm 2 ) Surface condition Invention 85 3 x 10 5 uniform and smooth Comparative 45 ---- uneven Example 3 The same procedure as Example 1 was repeated but the single crystal substrate of silicon was replaced by a single crystal substrate of InP to prepare a thin film of Y-Ba-Cu-O system superconducting oxide on a (100) plane of the InP substrate. Deposition conditions for the under-layer and the upper-layer are summarized in Table 5 (1 Torr = 133,32 Pa; 10 Å = 1 nm). Under-layer Upper-layer Substrate temperature (°C) : 390 710 Gas pressure (Torr) 0.1 0.1 Sputtering gas (Ar+O 2 ) O 2 /(Ar+O 2 ) (vol %) 20 20 RF power density (W/cm 2 ) 2 2 Deposition rate (Å/sec) 0.3 0.3 Thickness of a layer (Å) 100 4,000 For comparison, a comparative sample was prepared by the same method as above but the substrate temperature was maintained at a constant value of 710 °C for both of the under-layer and the upper-layer. In this comparative sample, the total thickness of under-layer + the upper-layer was adjusted to a same value as above (namely, 4,100 Å). Surface conditions of the resulting superconducting thin films were observed by SEM (scanning electron microscopy) and the critical temperature (Tc) and the critical current density (Jc) were determined by usual methods. The results are summarized in table 6. Tc (K) Jc (A/cm 2 ) Surface condition Invention 87 5 x 10 5 uniform and smooth Comparative 50 ---- uneven Example 4 A thin film of Bi-Sr-Ca-Cu-O system superconducting oxide was prepared on a (100) plane of a single crystal substrate of silicon. A target used in this Example was a sintered oxide of Bi, Sr, Ca and Cu (atomic ratio : Bi : Sr : Ca : Cu = 2 : 2 : 2 : 2.5). By using this target, an under-layer of Bi-Sr-Ca-Cu-O system oxide was firstly deposited on the single crystal substrate of silicon and then an upper-layer of Bi-Sr-Ca- Cu-O system oxide was deposited on the resulting under-layer in a same vacuum chamber successively. Deposition conditions for the under-layer and the upper-layer are summarized in Table 7 (1 Torr = 133,32 Pa; 10 Å = 1 nm). Under-layer Upper-layer Substrate temperature (°C) 360 680 Gas pressure (Torr) 0.1 0.1 Sputtering gas (Ar+O 2 ) O 2 /(Ar+O 2 ) (vol %) 20 20 RF power density (W/cm 2 ) 2 2 Deposition rate (Å/sec) 0.3 0.3 Thickness of a layer (Å) 100 4,000 For comparison, a comparative sample was prepared by the same method as above but the substrate temperature was maintained at a constant value of 680 °C for both of the under-layer and the upper-layer. In this comparative sample, the total thickness of under-layer + the upper-layer was adjusted to a same value as above (namely, 4,100 Å). Surface conditions of the resulting superconducting thin films were observed by SEM (scanning electron microscopy) and the critical temperature (Tc) and the critical current density (Jc) at 77 K were determined by usual methods. The results are summarized in table 8. Tc (K) Jc (A/cm 2 ) Surface condition Invention 85 2 x 10 5 uniform and smooth Comparative 60 ---- uneven Example 5 The same procedure as Example 4 was repeated but the single crystal substrate of silicon was replaced by a single crystal substrate of GaAs to prepare a thin film of Bi-Sr-Ca-Cu-O system superconducting oxide on a (100) plane of the GaAs substrate. Deposition conditions for the under-layer and the upper-layer are summarized in Table 9 (1 Torr = 133,32 Pa; 10 Å = 1 nm). Under-layer Upper-layer Substrate temperature (°C) 380 690 Gas pressure (Torr) 0.1 0.1 Sputtering gas (Ar+O 2 ) O 2 /(Ar+O 2 ) (vol %) 20 20 RF power density (W/cm 2 ) 2 2 Deposition rate (Å/sec) 0.3 0.3 Thickness of a layer (Å) 100 4,000 For comparison, a comparative sample was prepared by the same method as above but the substrate temperature was maintained at a constant value of 690 °C for both of the under-layer and the upper-layer. In this comparative sample, the total thickness of under-layer + the upper-layer was adjusted to a same value as above (namely, 4,100 Å). Surface conditions of the resulting superconducting thin films were observed by SEM (scanning electron microscopy) and the critical temperature (Tc) and the critical current density (Jc) at 77 K were determined by usual methods. The results are summarized in table 10. Tc (K) Jc (A/cm 2 ) Surface condition Invention 90 3 x 10 5 uniform and smooth Comparative 67 ---- uneven Example 6 The same procedure as Example 4 was repeated but the single crystal substrate of silicon was replaced by a single crystal substrate of InP to prepare a thin film of Bi-Sr-Ca-Cu-O system superconducting oxide on a (100) plane of the InP substrate. Deposition conditions for the under-layer and the upper-layer are summarized in Table 11 (1 Torr = 133,32 Pa; 10 Å = 1 nm). Under-layer Upper-layer Substrate temperature (°C) 410 670 Gas pressures (Torr) 0.1 0.1 Sputtering gas (Ar+O 2 ) O 2 /(Ar+O 2 ) (vol %) 20 20 RF power density (W/cm 2 ) 2 2 Deposition rate (Å/sec) 0.3 0.3 Thickness of a layer (Å) 100 4,000 For comparison, a comparative sample was prepared by the same method as above but the substrate temperature was maintained at a constant value of 670 °C for both of the under-layer and the upper-layer. In this comparative sample, the total thickness of under-layer + the upper-layer was adjusted to a same value as above (namely, 4,100 Å). Surface conditions of the resulting superconducting thin films were observed by SEM (scanning electron microscopy) and the critical temperature (Tc) and the critical current density (Jc) at 77 K were determined by usual methods. The results are summarized in table 12. Tc (K) Jc (A/cm 2 ) Surface condition Invention 93 4 x 10 5 uniform and smooth Comparative 70 - --- uneven

Claims: A process for preparing a thin film of oxide superconductor on a single crystal substrate of semiconductor, which comprises depositing firstly by RF sputtering an under-layer of a superconducting oxide material having a thickness of 5 to 20 nm (50 to 200 Å) on said single crystal substrate of semiconductor at a substrate temperature between 300° and 500°C, and depositing secondly by RF sputtering an upper-layer of a superconducting oxide material on said under-layer at a substrate temperature of higher than 600°C, said upper-layer of superconducting oxide material being thicker than said under-layer wherein both said under-layer and said upper-layer are deposited in a same vacuum chamber. The process set forth in claim 1, wherein said under-layer has a thickness of about 10 nm (100 Å). The process set forth in claim 1 wherein the superconducting oxide material of said under-layer is different from the superconducting oxide material of said upper-layer. The process set forth in claim 5 wherein the superconducting oxide material of said under-layer has a similar crystal structure or lattice constant to that of said upper-layer. The process set forth in claim 1 wherein said single crystal substrate of semiconductor is made of silicon or compound semiconductor. The process set forth in claim 1 wherein said upper-layer of the superconducting oxide material is selected from the group consisting of Y-Ba-Cu-O system, Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system. The process set forth in claim 6 wherein said upper-layer of the superconducting oxide material is Y-Ba-Cu-O system and is deposited at a substrate temperature about 640°C. Process set for in claim 6 wherein said upper-layer of the superconducting oxide material is Bi-Sr-Ca-Cu-O system and is deposited at a substrate temperature about 700°C.

Inventors: HIGAKI KENJIRO, ITOZAKI HIDEO, MATSUURA TAKASHI, HIGAKI, KENJIRO, ITOZAKI, HIDEO, MATSUURA, TAKASHI, HIGAKI, KENJIRO, C/O ITAMI WORKS, ITOZAKI, HIDEO, C/O ITAMI WORKS, MATSUURA, TAKASHI, C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: H01L39
IPC codes: H01L39