Abstract:
Method of forming an as-grown film of a superconductor composed of the MgB 2  compound which is made by simultaneous evaporation of magnesium and boron is provided. The as-grown film is superconductive without an annealing process to make the film superconductive. The present invention can be applied to fabricate an integrated circuit of the superconductor film, because the high temperature annealing process to make the as-grown film superconductive is not needed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority Japanese Patent Application No. 2001-356803, filed Nov. 22, 2001 in Japan, the contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to method of forming a superconductor film. Especially the present invention relates to method of forming a superconductor film of a compound of magnesium and boron formed on a substrate by vacuum evaporation of magnesium and boron.  
           [0004]    2. Description of the Related Art  
           [0005]    A MgB 2  compound features in its high transition temperature to a superconductor. It is reported that the coherent length ξ(0)=54 Å, magnetic penetration depth of MgB 2  is 140-180 nm. A thin film of MgB 2  can be applied to an integrated circuit of a superconductor film of MgB 2  at a high temperature, thus it is expected to apply the thin film MgB 2  to electronics devices.  
           [0006]    It is well known as method of making the superconductor of MgB 2  to form a bulk of MgB 2  by compression molding of powder of single crystals of MgB 2  and anneal the bulk at a high temperature. As another method of forming the superconductor of MgB 2 , it is known to form a thin film of a MgB 2  compound by using PLD (pulse laser deposition), and anneal the thin film of MgB 2  in the temperature of 600° C.-1200° C. to make the film superconductive. The PLD method is as follows. A boron target is irradiated with a laser beam to evaporate boron atoms. The boron atoms are deposited on the substrate. The boron atoms layer on the substrate is exposed to high temperature magnesium vapor in a reaction room to form the MgB 2  film on the substrate. Further the film is annealed at a high temperature so as to make the film superconductive. Another method of forming the superconductor film of a MgB 2  compound by using the PLD is as follows.  
           [0007]    Magnesium atoms and boron atoms are deposited on a substrate in magnesium vapor and boron vapor originated respectively from laser irradiation to a magnesium target and a boron target in a reaction room. Moreover the layers of magnesium and boron on the substrate are exposed to vapor of magnesium in the reaction room. The magnesium and boron on the substrate react in the magnesium vapor to form a thin film of the MgB 2  compound on the substrate. The film is annealed at a high temperature to make the film superconductive.  
           [0008]    All prior arts to fabricate the superconductor of the MgB 2  compound need high temperature annealing of 600° C.-1200° C. The necessity of the annealing process of the MgB 2  film to produce features of a superconductor makes difficult to fabricate a device such as a thin film integrated circuit on a substrate. Further it is not known that an as-grown film of MgB 2  on a substrate by vacuum evaporation of magnesium and boron has features of a superconductor without annealed.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of the present invention is to provide method of forming a film of superconductor composed of the MgB 2  compound. An as-grown film of the superconductor has features of a superconductor without annealed.  
           [0010]    The present invention provides the method forming an as-grown thin film of a superconductor composed of the MgB 2  compound which is made by simultaneous evaporation of magnesium and boron. The as-grown film composed of the compound of magnesium and boron can be a superconductor without annealed. The as-grow film can be composed of a MgB 2  compound, or the film can be composed of one or plural of compounds of magnesium and boron different from the MgB 2  compound, a magnesium element and a boron element in addition to the MgB 2  compound.  
           [0011]    As mentioned above, an as-grown film of a superconductor film of MgB 2  made by the present invention has features of a superconductor without annealed. Thus, the present invention can be applied to fabricate an integrated circuit of the superconductor thin film, because the high temperature annealing process to make the as-grown film superconductive is not needed.  
           [0012]    The objects, advantages and features of the present invention will be more clearly understood by referencing the following detailed disclosure and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 shows an embodiment of an apparatus of vacuum evaporation for implementing the present invention.  
         [0014]    [0014]FIG. 2A shows temperature-resistance characteristics of a MgB 2  compound of an example  1  made with the method of the present invention.  
         [0015]    [0015]FIG. 2B shows temperature-resistance characteristics of which axis of abscissas is enlarged and corresponds to the region of 23K-28K on the axis of abscissas in FIG. 2A.  
         [0016]    [0016]FIG. 3 shows substrate temperature dependence of the temperature-resistance characteristics of the film formed by the present invention.  
         [0017]    [0017]FIG. 4 shows film forming rate dependence of critical temperature and residual-resistance ratio.  
         [0018]    [0018]FIG. 5 shows temperature-resistance characteristics of MgB 2  of an example  2  made by the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 shows an apparatus of vacuum evaporation for implementing the present invention. An as-grown film of a compound of magnesium and boron is formed by simultaneous evaporation of magnesium and boron using the apparatus. The apparatus is composed of a reaction room  1 , element containers  3 ′ and  4 ′. an electron gun  5 , film thickness gauges  7  and  8 , a film monitor  9 , vacuum pumps  14 , a rotary pump  15 , a vacuum gauge  16 , a power source  17  of the electric gun, a power source  18  of the element holder  3 ′, a substrate holder  10 , SiC heater  11 , a CA thermocouple  12 , and a substrate heating controller  13 .  
         [0020]    The method forming the film of the MgB 2  compound using the vacuum evaporator shown in FIG. 1. Boron  4  and magnesium  3  are put respectively in the container  4 ′ and the container  3 ′. A substrate  2  is held on the substrate holder  10 . The material of the substrate  2  is for example Al 2 O 3 , MgO, SiO 2  etc. The reaction room is exhausted into vacuum by the vacuum pump  14  and the rotary pump  15 . Electron beams generated by the electric gun  5  are curved by magnetic field (magnet is not shown) and the surface of the boron  4  is irradiated with the electron beams to heat it.  
         [0021]    The boron heated by the irradiation of electron beams evaporates into vapor. The container  3 ′ containing the magnesium  3  is heated by the power source  18  to evaporate the magnesium  3 . The substrate  2  is heated by the SiC heater  11 . The temperature of the substrate  2  is controlled to a temperature of 100° C.-350° C. with the substrate heating controller  13 . The boron vapor originated from the boron  4  is deposited on the substrate  2 . The thickness of the boron film is measured with the film thickness gauge  8  and monitored with the film thickness monitor  9 . The magnesium vapor originated from the magnesium  3  is deposited on the substrate  2 . The thickness of the film of the magnesium is measured with the film thickness gauge  7  and monitored with the film thickness monitor  9 .  
         [0022]    The magnesium and boron deposited simultaneously on the substrate  2  is reacted chemically on the substrate  2  to produce the compound of magnesium and boron. Or the vapor of the magnesium and boron reacts in the reaction room  1  to produce the compound of magnesium and born. Molecules of the compound of the magnesium and boron are deposited on the substrate  2 .  
         [0023]    The as-grown film is usually polycrystalline, amorphous or single crystal, and features in a superconductor without annealed. The as-grow film can be composed of a MgB 2  compound, or the film can be composed of one or plural of compounds of magnesium and boron different from the MgB 2  compound, a magnesium element and a boron element in addition to the MgB 2  compound. Polycrystal or amorphous as-grown films as follow are explained. In the explanation in the followings, the temperatures of the substrates are measured with the thermocouple. Thus the real temperatures of the substrates may be a little more than those measured.  
         [0024]    In the embodiment as follows, the substrates temperatures are in a temperature of 100° C.-350° C., the film forming rate of boron is 0.5-1.5 Å/sec, the film forming rate of magnesium is about 5 Å/sec, and the film forming pressure is about 10 −6  Torr, which is a degree of vacuum in the reaction room. An as-grown film of MgB 2  is formed on a substrate with the above condition.  
         [0025]    [0025]FIG. 2A and 2B show resistance-temperature characteristics of MgB 2  film of an example  1  made by the present invention. The axis of abscissas in FIG. 2B is enlarged and corresponds to the region of 23K-28K on the axis of abscissas in FIG. 2A. The as-grown film of MgB 2  is formed on a substrate using the vacuum evaporation apparatus shown in FIG. 1. The substrate temperature is at a temperature of 200° C., and the film forming rate of magnesium is 5 Å/sec, the film forming rate of boron is 0.5 Å/sec. The film forming pressure is about 10 −6  Torr. The temperature-resistance characteristics are those of the as-grown film without annealed.  
         [0026]    Moreover as the critical temperature and Residual Resistance Ratio (RRR), the critical temperature Tc on of 26K, Tc off of 23.3K and RRR of 2.78 are measured. The Tc on and Tc off show Tc of onset and Tc of offset respectively. Residual Resistance Ratio is a ratio of resistance at 300K to resistance at 40K. It shows a film quality. Greater the RRR, the film is nearer to a single crystal. Moreover the RRR, the specific resistance is less and the film is nearer to a metal.  
         [0027]    It is shown from the characteristics in FIG. 2A and 2B that an as-grown film of superconductor of its critical temperature Tc of about 25K can be formed by the simultaneous evaporation of magnesium and boron, and it has features of the superconductor without annealed.  
         [0028]    [0028]FIG. 3 shows the substrate temperatures dependence of resistance of films which are made on various substrate temperatures by the present invention. The film forming rate of magnesium is 5 Å/sec, the film forming rate of boron is 0.5 Å/sec, the substrate temperature is in a temperature of 100° C.-250° C. The films which are formed on the substrates of temperature lower than about 180° C. are metallic, and the films which are formed on the substrates of temperature higher than about 210° C. are semiconductor. Moreover, it is found that the films which are formed on the substrates of temperature between 190-210° C. are superconductive.  
         [0029]    [0029]FIG. 4 shows the boron film forming rate dependence of critical temperature, and the temperature-resistance characteristics of films made by the present invention. The substrate temperature is 200° C. and the magnesium film forming rate is 5 Å/sec for all data. Black marks show the boron film forming rate of 0.5 Å/sec, white marks show the boron film forming rate of 1.0 Å/sec, and double black circle marks show the boron film forming rate of 1.5 Å/sec. In the FIG. 5, of numbers corresponding to each mark, the number  1  shows characteristics of Tc on, the number  2  shows characteristics of Tc off and the number  3  shows RRR characteristics.  
         [0030]    When the film forming rate is 0.5 Å/sec, films of MgB 2  of critical temperature of about 25K and RRR of 2.7 are formed. When the film forming rate is 1.0 Å/sec, MgB 2  films of critical temperature of about 21K and RRR of 2.15 are formed. When the film forming rate is 1.5 Å/sec, MgB 2  films of critical temperature of about 22K and RRR of 2.05 are formed. Films of the film forming rate of 0.5 Å/sec are the best two of the films of MgB 2  shown in FIG. 4 because the critical temperature is higher and RRR is greater than those of another film forming rate.  
         [0031]    The composition of boron and magnesium in the MgB 2  compound films of which the film forming rate is 0.5 Å/sec in FIG. 4 is measured with WDS (an analyzer of a composition with irradiation of electron beam). The measured composition of boron and magnesium is B:Mg=2:2.74. The ideal composition of boron and magnesium is B:Mg=2:1. So the MgB 2  films of the example  1  are magnesium rich films. Considering magnesium is vaporizable easily, it is better to deposit the film with the film forming rate so as to form a compound of composition of boron and magnesium of B:Mg=1:about 10. However can as grown film of MgB 2  which has features of a superconductor without annealed ca be formed with the film forming rate other than the above mentioned film forming rate.  
         [0032]    [0032]FIG. 5 shows resistance-temperature characteristics of MgB 2  film of an example  2  made by the present invention. The as-grown film of MgB 2  is formed on a substrate using the vacuum evaporation apparatus shown in FIG. 1. The film forming rate is different from the example  1  shown in FIG. 2A and 2B. The substrate temperature is at a temperature of 200° C., and the film forming rate of magnesium is 5 Å/sec, the film forming rate of boron is 1.0 Å/sec. The temperature-resistance characteristics are those of the as-grown film without annealed. Critical temperature Tc and specific resistance ρ of the film are about 21K and 8.0 μΩcm (at 30K) respectively as shown in FIG. 5. A superconductor film of MgB 2  can be formed with the film forming rate of the example  2  using the present invention.  
         [0033]    Polycrystalline or amorphous films formed by the present invention are explained above. However an as-grown film of single crystal which is a superconductor without annealed can be made by the simultaneous evaporation of magnesium and boron using the apparatus shown  1  FIG. 1. The single crystal film of MgB 2  is included in a scope of the present invention.  
         [0034]    The many features and advantages of the present invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modification and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modification and equivalents falling within the scope of the invention may be included in the present invention.