Process for cleaning a surface of thin film of oxide superconductor

Abstract: A process for removing contaminants from a surface of a thin film of oxide superconductor deposited on a substrate. The thin film of oxide superconductor is heat-treated in ultra-high vacuum at a temperature which is within a range of - 100 °C to + 100 °C of the temperature at which oxygen enter into the oxide superconductor. The process is used for removing photo-resist from a surface of thin film of oxide superconductor and for producing a layered structure containing at least one thin film of oxide superconductor such as Y₁Ba₂Cu₃O 7-x having a contaminated surface. On the cleaned surface, another thin film of oxide superconductor or non-superconductor is deposited. The resulting structure of layered thin films is used for fabricating superconducting transistor, Josephson junctions, superconducting circuits or the like.

Description: Background of the Invention Field of the Invention The present invention relates to a process for cleaning a surface of a thin film of oxide superconductor. The process according to the present invention may be used in connection with a process for preparing a layered structure containing at least one thin film of oxide superconductor, more particularly, for depositing more than two thin films of oxide superconductor each possessing a different crystal orientation successively on a substrate and for depositing, on a thin film of oxide superconductor, another thin film of different material. The process according to the present invention is used also for removing photo-resist from a thin film of oxide superconductor. Description of the related art Oxide superconductors are expected to be used in a variety of applications due to their higher critical temperatures than conventional metal superconductors. In fact, Y-Ba-Cu-O oxide superconductor possesses the critical temperature above 80 K and Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O oxide superconductors possesses that of above 100 K. These oxide superconductors, however, possess crystal anisotropy in their superconducting properties. In fact, the highest critical current density is observed in a direction which is perpendicular to c-axis of their crystal. From this fact, the direction of crystal must be considered in actual utilization of these oxide superconductors. When the oxide superconductors are used in superconducting electronics applications such as superconducting devices or integrated superconducting circuits, it is indispensable to prepare at least one thin film of the oxide superconductor and to laminate a plurality of thin films. The problem of crystal anisotropy becomes much server in such superconducting devices or integrated superconducting circuits. For instance, in order to realize high-performance superconducting devices or integrated superconducting circuits, it is requested to prepare two kinds of superconducting wiring lines: one part in which electric current flows in parallel with a surface of substrate and another part in which electric current flows perpendicularly to the surface of substrate. For example, in superconducting electrodes, current flows in parallel with the surface of substrate while, in interlayer connecting superconducting wiring lines which connect layers stratified on the substrate, current flows perpendicularly to the surface of substrate. Therefor, when oxide superconductor is used in high-performance superconducting devices or integrated superconducting circuits, it is requested to deposit both of c-axis orientated thin film of oxide superconductor in which the critical current density along the direction which is in parallel with the surface of substrate is higher than the other directions and of a-axis (or b-axis) orientated thin film of oxide superconductor in which the critical current density along the direction which is perpendicular to the surface of substrate is higher than the c-axis orientated thin film on a common surface of a substrate. Hereinafter, only a-axis oriented thin film is referred, since electric current flows equally along the direction which is perpendicular to the surface of substrate in a-axis orientated thin film and in b-axis orientated thin film. In the multi-layered structures for the superconducting devices or integrated superconducting circuits, two layers of a c-axis oriented thin film of oxide superconductor and of an a-axis oriented thin film of oxide superconductor must be deposited successively. Crystal orientation of the thin film of oxide superconductor can be controlled by selecting or adjusting film-forming temperature which is determined by substrate temperature. In fact, the a-axis oriented thin film can be realized at a substrate temperature which is lower by about 50 to 100 °C than a substrate temperature at which the c-axis oriented thin film grows. In a superconducting junction of so-called Josephson Junction realized with oxide superconductor, it is requested to deposit a bottom superconductor layer, am intermediate thin film of non-superconductor and a top superconductor layer on a substrate successively in this order. Josephson element is a two-terminals element, so that a logical circuit consisting of Josephson elements alone becomes complicated. In order to overcome this demerit of complexity, a variety of ideas of three-terminals elements are proposed. In the superconductor transistors consisting of superconductor and semiconductor which is a typical three-terminals element, it is also required to combine a thin film of semiconductor with a thin film of oxide superconductor and hence successive deposition of thin films each made of different material is required. In these superconducting elements, a superconducting current passes through a thin film of non-superconductor sandwiched between two adjacent layers of superconductors positioned close to each other. A distance between the two superconductors is determined by the coherence length of superconductor. In the case of oxide superconductor, the distance between two superconductors must be several nanometer because its coherence length is very short. On the other hand, from a point of view as performance of the superconducting devices, all thin films in the superconducting device must have high crystallinity, in other words, these thin films are preferably made of a single crystal or polycrystal having crystal orientation which is similar to single crystal. When the superconducting device has thin film(s) made of polycrystal whose crystal orientation is not well-ordered or has amorphous thin film(s), high-performance of the superconducting devices can not be expected and hence function thereof become unstable. When more than two thin films are deposited successively on a common substrate, it is usual practice to subject a surface of a bottom superconductor layer to cleaning operation before a top superconductor layer is deposited, otherwise electrical continuity between the bottom superconductor layer and the top superconductor layer is spoiled due to contaminants adsorbed on a surface of the bottom superconductor layer or undesirable oxides produced on the surface. Discontinuity of two layers result in formation of an undesirable junction between two layers. Superconducting devices or integrated superconducting circuits having such undesirable junction do not show desired performance and sometimes do not work. In particular, the surface condition of the bottom superconductor layer should be considered carefully because the coherence length of oxide superconductors is very short. Still more, oxygen of oxide superconductors is rather unstable and easily escape out of the thin film. Excessive oxygen deficient result in deterioration of superconducting properties and, in the worst case, loss of superconductivity. Therefore, the surface of bottom superconductor layer must be clean and also must have well-ordered crystallinity or superconducting property. In the field of semiconductor industries, surfaces are cleaned with ultra-pure water, by chemical washing, dry or wet etching or the like. In the case of oxide superconductors, however, these clearing technique can not be used due to high reactivity of oxide superconductors. If the surface of thin film of oxide superconductor is treated by these known techniques, undesirable reaction occur on the surface, resulting in that cleanness of the surface become worse and crystallinity and superconducting property are lost. It is also known to deposit the top superconductor layer, just after the bottom superconductor layer of oxide superconductor has been deposited, in an identical chamber. This technique, however, requires a big chamber and materials to be used for the top superconductor layer are limited. Therefore, an object of the present invention is to solve the problems and to provide an improved process for preparing a layered structure containing at least one thin film of oxide superconductor without deteriorating superconducting properties of the thin film of oxide superconductor. The process according to the present invention is applicable also to remove photo-resist residue which is remained on a thin film of oxide superconductor after pattering operation. Namely, patterning of a thin film of oxide superconductor can be effected by a variety of techniques. The most popular patterning technique is to protect a predetermined surface area on the thin film of oxide superconductor with photo-resist followed by etching. In the case of patterning of oxide superconductors which are chemically active, wetetching is not suitable and hence dry-etching such as reactive ion etching, electron beam etching or argon ion milling is used. Generally, patterning of thin films with photo-resist is carried out as following. At first, a surface of the thin film is cleaned and then photo-resist is coated thereon. The photo-resist is cured by pre-baking so that the photo-resist is fix to the thin film. The pre-baked photo-resist is irradiated with light through a mask. After development, the irradiated photo-resist is subject to post-baking and then is etched. Finally, remaining photo-resist is removed by wet-treatment with photo-resist remover or oxidizing reagent to clean the surface of the thin film. Patterning of thin film of oxide superconductor is carried out almost same sequence as above. In the case of thin film of oxide superconductor, however, serious deterioration of superconductivity is often observed after the patterning. The present inventors found such a fact that such deterioration of superconductivity of the thin film is caused by chemical reaction during removal of the photo-resist. In fact, because of relatively high reactivity, the oxide superconductor react with liquid photo-resist remover or oxidizing reagent at the removing stage of photo-resist, resulting in that a surface of the thin film is roughened and that composition of oxide superconductor is changed. Another thin film deposited on such deteriorated thin film does not show desired superconducting properties and hence it is difficult to obtain a layered structure of high-performance. It is also known to remove photo-resist by dry-process so-called "ashing" technique in which photo-resist is burnt in oxygen plasma. This ashing technique, however, reduce oxide superconductor during burning of photo-resist, resulting in that oxygen in oxide superconductor is lost. Oxygen-deficient oxide superconductor deviated from desired stoichiometry and lose sharply the superconductivity and often become non-superconductor. At the today's level of patterning technology, use of photo-resist is indispensable. This situation is same in the thin film of oxide superconductor. Therefore, still another object of the present invention is to solve the problems and to provide a process for removing photo-resist residue remained on a thin film of oxide superconductor. Summary of the Invention The present invention provide a process for removing contaminants from a surface of a thin film of oxide superconductor deposited on a substrate, characterized in that said thin film of oxide superconductor is a c-axis oriented thin film of Y 1 Ba 2 Cu 3 -O 7-x oxide and is heat-treated in ultra-high vacuum of lower than 133 x 10 -9 Pa (1 x 10 -9 Torr) at a temperature at which oxygen enters into said oxide superconductor. An essence of the process according to the present invention resides in that a surface of oxide superconductor is cleaned by heat-treatment in ultra-high vacuum. The heat-treatment is carried out preferably in ultra-high vacuum of lower than 133 x 10 -9 Pa (1 x 10 -9 Torr). Advantage of the present invention can not be expected at a vacuum of higher than 133 x 10 -9 Pa (1 x 10 -9 Torr). The heating temperature, at which oxygen enters into said oxide superconductor, is between 350 and 400°C for a thin film of Y 1 Ba 2 Cu 3 O 7-x . If the heating temperature is not higher than 350°C, countaminants on the surface of thin film oxide superconductor can not be removed and, if the heating temperature exceed 400 °C, oxygen escape out of the thin film of oxide superconductor and is lost. The substrate is preferably a single crystal of oxide such as MgO, StTiO 3 , PrGaO 3 or the like. The thin film of oxide superconductor can be a bottom superconductor layer which is deposited directly on a surface of the substrate. This bottom superconductor layer can be a thin film of oxide superconductor such as a c-axis oriented thin film of Y 1 Ba 2 Cu 3 O 7-x . The heat-treatment is effected just before a top supenconductor layer is deposited on the bottom superconductor layer so that contaminants such as hydrocarbons or metal carbides absorbed or deposited on the surface of the bottom superconductor layer are removed by the heat-treatment. Still more, local surface disorder of crystallinity in the bottom superconducting layer is repaired and enough oxygen is sustained during the heat-treatment, so that superconducting property is improved. The contaminants may be photo-resist residues remained on the surface of thin film of oxide superconductor and/or those produced when the thin film of oxide superconductor is exposed to air. The present invention may be advantageously used in connection with a process for depositing more than two thin films of oxide superconductor each possessing a different crystal orientation successively on a substrate. The present invention may also be advantageously used in connection with a process for depositing, on a thin film of oxide superconductor, another thin film of different material. In a preferred embodiment of the present invention, the process according to the present invention is used for removing photo-resist residue from a thin film of oxide superconductor. First application Thanks to the present invention, a top superconductor layer may be made of the same material as or different material from a bottom superconductor layer. The bottom and top superconductor layers can be a c-axis and an a-axis oriented thin films of Y 1 Ba 2 Cu 3 O 7-x respectively. Namely, in a process for depositing a first thin film of an oxide superconductor and a second thin film of oxide superconductor successively, crystal orientation of the first thin film being different from that of the second thin film, and the first thin film having a contaminated surface, a cleaning process according to the present invention may be carried out before the second thin film is deposited on the first thin film of an oxide superconductor. The present invention is applicable in connection with laminating or stratifying thin films of oxide superconductor each having a different crystal orientation successively and, more particularly, is advantageously applicable before depositing an a-axis oriented thin film of oxide superconductor on a c-axis oriented thin film of oxide superconductor. Second application The present invention may also be used in connection with a process for depositing, on a first thin film of oxide superconductor whose surface is contaminated, a second thin film made of different material from the oxide superconductor. In this case, the second thin film can be made of non-superconductor, for example insulator such as MgO or metal such as Ag. A third thin film of oxide superconductor or a top superconducting layer can be deposit additionally on the second thin film. The present invention is applicable in connection with stratifying a thin film of insulator or ordinary conductor on a thin film of oxide superconductor for fabricating superconducting devices. In both applications, the cleaning process according to the present invention is applicable, for example, to such a thin film of oxide superconductor which is exposed to air and hence whose surface is deteriorated, in order to clean the surface before another thin film of different material or the top superconductor layer having a different crystal orientation is deposited thereon, so that more than two thin film layers each having a different crystal orientation or a combination of a thin film of oxide superconductor and a non-superconductor can be deposited in different film forming apparatuses independently at their optimum conditions and hence the resulting layered thin films show an improved superconducting properties. The stratified thin films obtained by these applications show improved crystallinity, continuity and lattice matching at an interface which are requested for realizing superconducting elements or integrated superconducting circuits from which high-performance superconducting devices are fabricated. Preferred Embodiment In this preferred embodiment, the present invention provides a process for removing photo-resist from a thin film of oxide superconductor. The process according to the present invention used in this embodiment is a kind of ashing technology. The process, however, permits to remove photo-resist completely without deteriorating the thin film of oxide superconductor by selecting certain conditions including heating temperature and atmosphere so that oxygen atoms in the oxide superconductor do not move substantially and hence are not lost. Still more, photo-resist is removed by sublimation because the heat-treatment is carried out under ultra-high vacuum. The final pressure in a vacuum chamber is reduced lower than 133 x 10 -9 Pa (1 × 10 -9 Torr). In this embodiment of the process according to the present invention, photo-resist residue is not burnt but is sublimated so that oxide superconductor is not reduced and oxygen is not lost. From this fact, the superconducting property is not deteriorated. Still more, in the process according to the present invention, photo-resist is not subjected to chemical treatment with chemicals so that the surface of the thin film of oxide superconductor is not deteriorated or contaminated and is not roughened. This embodiment of the process according to the present invention permits to remove photo-resist without spoiling the superconducting properties of thin film of oxide superconductor. This merit come from such a fact that the surface of thin film of oxide superconductor is neither contaminated nor roughened by the process according to the present invention. Brief Description of the Drawing Fig. 1 illustrates successive steps for preparing a layered structure having at least one thin film of oxide superconductor as discussed in the first and second applications above. Fig. 1A shows a substrate 3 on which thin films are to be deposited successively. At first, a thin film of oxide superconductor (1) is deposited on the substrate (3) by off-axis sputtering method, laser abrasion method, reaction evaporation method, MBE or CVD technique. After deposition complete, the substrate (3) having the deposited thin film of oxide superconductor (1) is taken out of the sputtering chamber. A surface of the resulting thin film of oxide superconductor (1) has a deteriorated portion (10) thereon and is contaminated with hydrocarbons, BaCO 3 , BaCuO 2 or the like caused by a reaction with moisture in air shown in Fig. 1B. Then, the substrate (3) is placed in a ultra-high vacuum chamber which is then vacuumed lower than 133 x 10 -9 Pa (1 x 10 -9 Torr). After that, the thin film of oxide superconductor (1) is heat-teated under operational conditions determined by the present invention. The surface of the thin film of oxide superconductor (1) is monitored by a quadropole mass spectrometer (QMS) and is analyzed by means of a low energy electron diffraction analyzer (LEED) or a X-ray photoelectron spectrometer (XPS) after the heat-treatment, in such a manner that contaminants are removed, deteriorated portion (10) disappears and a crystalline surface is exposed. On the resulting cleaned surface (Fig. 1C), another thin film (2) (of oxide superconductor or of different material) is deposited in the same chamber by off-axis sputtering method, laser abrasion method, reaction evaporation method, MBE, CVD technique or the like. The same procedure as above is adopted in the case of embodiment in which the thin film of oxide superconductor has photo-resist residue in addition to the deteriorated portion (10). Description of the Preferred Embodiment Now, the present invention will be described with referring to Examples but the scope of the invention should not be limited thereto. Example 1 In this Example 1, an a-axis oriented thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x is deposited on a c-axis oriented thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x by the deposition process according to the steps illustrated in Fig. 1. At first, a c-axis oriented thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x (1) having a thickness of 300 nm is deposited on a substrate (3) of MgO (100) by off-axis sputtering method under following operational conditions: Sputtering gas Ar 90 % O 2 10 % Pressure 10 Pa Substrate temperature 700°C After deposition complete, the substrate (3) is taken out of the sputtering chamber. A surface of the resulting thin film of oxide superconductor (1) has a deteriorated portion (10) thereon and is contaminated with hydrocarbons, BaCO 3 , BaCuO 2 or the like caused by a reaction with moisture in air or the like. Then, the substrate (3) is placed in a ultra-high vacuum chamber which is then vacuumed lower than 133 x 10 -9 Pa (1 x 10 -9 Torr). After that, the heat-treatment of the thin film of oxide superconductor (1) is effected under following operational conditions: Pressure lower than 133 x 10 -9 Pa (1 x 10 -9 Torr) Heating temperature 350 to 400 °C (substrate temperature) Heating time 10 min The surface of the thin film of oxide superconductor (1) is monitored by a quadropole mass spectrometer (QMS) and is analyzed by means of a low energy electron diffraction analyzer (LEED) or a X-ray photoelectron spectrometer (XPS) after the heat-treatment, in such a manner that contaminants are removed, deteriorated portion (10) disappears and a crystalline surface is exposed. On the resulting cleaned surface (Fig. 1C), an a-axis oriented thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x (2) having a thickness of 200 nm is deposited by off-axis sputtering method under following operational conditions: Sputtering gas Ar 90 % O 2 10 % Pressure 10 Pa Substrate temperature 600 to 650 °C In the layered thin films prepared by this Example 1, it is confirmed that both of the bottom superconductor layer and the top conductor layer possess improved crystallinity, continuity and good matching of lattices at an interface. Example 2 Example 1 is repeated except that a thin film of MgO of the same thickness is deposited on the c-axis oriented thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x in place of the a-axis oriented thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x . In this Example 2, a thin film of MgO (2) having a thickness of 200 nm is deposited by evaporation method under following operational conditions: Pressure 10 Pa Substrate temperature 200 °C In the layered thin films prepared by Example 2 also, it is confirmed that both of the bottom superconductor layer and the MgO layer possess improved crystallinity and show a sharp boundary at an interface. Example 3 Example 2 is repeated except that the thin film of MgO is replaced by a thin film of Ag of the same thickness. The thin film of Ag having a thickness of 200 nm is deposited by evaporation method under following operational conditions: Pressure 10 Pa Substrate temperature 200 °C In the layered thin films prepared by Example 3 also, it is confirmed that both of the bottom superconductor layer and the Ag layer possess improved crystallinity and show good contact between two materials. Example 4 In this Example 4, thin film of oxide superconductor is patterned with photo-resist/etching technique and then, remaining photo-resist residue is removed by the process according to the present invention and by know method as following. A thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x deposited on a MgO substrate is used. This thin film of of Y 1 Ba 2 Cu 3 O 7-x possesses the initial values of the critical temperature (Tc) of 90 K and the critical current density (Jc) at liquid nitrogen temperature of 3.2 × 10 5 A/cm 2 . A surface of this thin film of oxide superconductor of Y 1 Ba 2 Cu 3 O 7-x is cleaned and is coated with positive-type photo-resist of quinodiazido type. The photo-resist is pre-baked at 80 °C for one minute to be dried. The pre-baked photo-resist film is irradiated with light through a mask and then developed. Finally, photo-resist film is post-baked at 150 to 200 °C for one minute. The reactive ion etching is carried out onto the obtained patterned photo-resist layer so that the thin film of oxide superconductor is etched. The photo-resist is also etched during this etching operation of the thin film of oxide superconductor. Of course, portions of the thin film of oxide superconductor to be protected against etching must not be exposed. Remained photo-resist residue is removed after this etching operation as following. In the process according to the present invention, the resulting substrate having a patterned thin film of oxide superconductor on which photo-resist is remained partly is placed in a vacuum chamber after the etching operation complete. The vacuum chamber is evacuated to a pressure of 133 x 10 -6 Pa (1 × 10 -6 Torr) and then heating of the substrate is initiated. The temperature of the substrate elevate up to 370 °C at the end of one hour heating. Photo-resist sublimate gradually. Evacuation of the vacuum chamber is continued to a pressure of 133 x 10 -10 Pa (1 × 10 -10 Torr). At this pressure, heating is ceased and then the substrate is left to be cooled down to ambient temperature in the vacuum chamber. For comparison, the remained photo-resist residue is burnt in oxygen plasma by the conventional ashing technique. The superconducting properties of the thin films of oxide superconductor treated by the process of the present invention and by the prior art are determined to obtain following results: Critical temperature (K) Critical current density (A/cm 2 , 77 K) Initial value 90 3.2 x 10 5 Invention 88 3.1 x 10 5 Prior art 42 --- The results reveal such a fact that the initial values of superconducting properties are substantially maintained in the case of the process according to the present invention but are seriously spoiled by the prior art. In other words, the process according to the present invention permits to remove photo-resist residue with minimum influence to the thin film of oxide superconductor.

Claims: A process for removing contaminants from a surface of a thin film of oxide superconductor deposited on a substrate, characterized in that said thin film of oxide superconductor is a c-axis oriented thin film of Y 1 Ba 2 Cu 3 O 7-x oxide and is heat-treated in ultra-high vacuum of lower than 133 x 10 -9 Pa (1 x 10 -9 Torr) at a temperature at which oxygen enters into said oxide superconductor. The process set forth claim 1 characterized in that the heating temperature, at which oxygen enters into said oxide superconductor, is between 350 and 400°C. The process set forth in any one of claim 1 to 2 characterized in that said contaminants are photo-resist residues remained on the surface of the thin film of oxide superconductor. The process set forth in any one of claim 1 to 2 characterized in that said contaminants are those produced when the thin film of oxide superconductor is exposed to air.

Inventors: IIYAMA MICHITOMO, INADA HIROSHI, NAKAMURA TAKAO, TANAKA SO, IIYAMA, MICHITOMO, INADA, HIROSHI, NAKAMURA, TAKAO, TANAKA, SO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01L39
IPC codes: H01L39