Abstract:
A method of forming MgB 2  films in-situ on a substrate includes the steps of (a) depositing boron onto a surface of the substrate in a deposition zone; (b) moving the substrate into a reaction zone containing pressurized, gaseous magnesium; (c) moving the substrate back into the deposition zone; and (d) repeating steps (a)-(c). In a preferred embodiment of the invention, the substrate is moved into and out of the deposition zone and the reaction zone using a rotatable platen.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0001]     The U.S. Government may have a paid-up license in this invention and a right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00014-03-M-0005 awarded by the Office of Naval Research. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The field of the invention generally relates methods used to produce thin films. More specifically, the field of the invention relates to methods of forming films in-situ such as superconducting MgB 2  that do not require high temperature annealing.  
       BACKGROUND OF THE INVENTION  
       [0003]     Magnesium diboride (MgB 2 ) is a superconducting material having a superconducting transition temperature (T c ) of approximately 39 K. There is a significant interest in using MgB 2  to form superconducting wires, tapes, and films. Superconducting wire, for example, can be used for superconducting magnets, fault-current limiters, and power transmission. Films can be used to make Josephson junctions, SQUIDS (superconducting quantum interference devices), micro-electronic interconnects, RSFQ (rapid single flux quantum) devices, and other devices. Films can also be incorporated into RF and microwave devices in the form of resonators, filters, delay lines, and the like.  
         [0004]     With respect to film applications, growth of completely in-situ MgB 2  films is required in order to realize the multilayer technology necessary for electronics applications. The primary difficulty of depositing MgB 2  films is the very high vapor pressures of Mg required for the thermodynamic stability of the MgB 2  phase at elevated temperatures. A second problem relating to MgB 2  film formation is the high sensitivity of Mg to oxidation. Both of these concerns have made it difficult to grow MgB 2  films using conventional physical vapor deposition (PVD) techniques.  
         [0005]     In-situ MgB 2  films have been fabricated by annealing Mg-B or Mg-MgB 2  mixtures in situ in growth chambers. However, films produced by such techniques have shown a lower T c  and poor crystallinity. In-situ MgB 2  films have also been fabricated at low temperatures (&lt;350° C.) but these films are not epitaxial, their crystallinity is poor, their T c  values are low, and their resistivities are high. Zeng et al. have grown MgB 2  films in-situ by using HPCVD (hybrid physical-chemical vapor deposition) techniques. See X. H. Zeng et al.,  In situ epitaxial MgB   2    thin films for superconducting electronics,  Nature Materials 1, pp. 1-4 (2002). However, this method is not readily amenable to multilayer devices or applications requiring large-area film growth. Furthermore, the substrate temperature used in the method proposed by Zeng et al. is above 700° C., and growth has been successful only on a limited number of substrate materials.  
         [0006]     There thus is a need for a method of producing MgB 2  films in-situ within a temperature range of approximately 300° C. to approximately 700° C. A method is also needed that can grow MgB 2  films in-situ on a variety of substrate materials. A method is needed that can produce in-situ MgB 2  suitable for use in multilayer device fabrication. The method is also preferably applicable to superconducting films other than MgB 2 .  
       SUMMARY OF THE INVENTION  
       [0007]     In a first aspect of the invention, a method of forming MgB 2  films in-situ on a substrate comprises the steps of (a) depositing boron onto a surface of the substrate in a deposition zone; (b) moving the substrate into a reaction zone containing pressurized, gaseous magnesium; (c) moving the substrate back into the deposition zone; and (d) repeating steps (a)-(c).  
         [0008]     In a second aspect of the invention, a MgB 2  film is created using the method of the first aspect.  
         [0009]     In a third aspect of the invention, a method of forming a thin film of MgB 2  in-situ comprises the steps of providing a rotatable platen, the platen being rotatable within a housing having a reaction zone and a separate deposition zone, providing an evaporation cell coupled to the reaction zone, the evaporation cell containing magnesium. A source of boron is provided adjacent to the deposition zone and an electron beam is aimed at the source of boron. The substrate is loaded onto the platen and the platen is then rotated. The local environment around the substrate is heated. The evaporation cell is heated to produce gaseous magnesium in the reaction zone. Boron is evaporated using the electron beam gun.  
         [0010]     In a fourth aspect of the invention, a MgB 2  film is created using the method of the third aspect.  
         [0011]     In a fifth aspect of the invention, a method of forming a superconducting film of a known superconducting compound in-situ on a substrate comprising the steps: (a) depositing one or more elements of the superconductor onto a surface of the substrate in a deposition zone; (b) heating a non-gaseous element of the superconductor so as to produce a pressurized gaseous phase of the element inside a reaction zone; (c) moving the substrate into the reaction zone containing the pressurized gaseous element; (d) moving the substrate back into the deposition zone; and (e) repeating steps (a)-(d).  
         [0012]     It is an object of the invention to provide a method for making MgB 2  films in-situ on a substrate.  
         [0013]     It is a further object of the invention to provide a method for making MgB 2  film in-situ on a substrate in which the substrate is heated to a temperature within the range of about 300° C. to about 700° C.  
         [0014]     It is a further object of the invention to provide a method for making MgB 2  films in-situ on multiple sides of a substrate.  
         [0015]     It is another object of the invention to provide a method for making films in-situ of known superconductor.  
         [0016]     These and further objects of the invention are described in more detail below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  illustrates a preferred device used to form a film of MgB 2  in-situ.  
         [0018]      FIG. 2  illustrates a perspective view of the underside of the pocket heater shown in  FIG. 1 .  
         [0019]      FIG. 3  is a flow chart of a preferred process of making a film of MgB 2  in-situ.  
         [0020]      FIG. 4  is a flow chart of another preferred process of making a film of MgB 2  in-situ.  
         [0021]      FIG. 5  illustrates the resistivity of a MgB 2  film deposited on a polycrystalline alumina substrate.  
         [0022]      FIG. 6  illustrates the resistance of a MgB 2  film deposited on flexible stainless steel tape.  
         [0023]      FIG. 7  illustrates the resistivity of a MgB 2  film deposited on LSAT.  
         [0024]      FIG. 8  illustrates the resistivity of a MgB 2  film deposited on LaAlO 3 .  
         [0025]      FIG. 9  illustrates the resistivity of a MgB 2  film deposited on MgO.  
         [0026]      FIG. 10  illustrates the resistivity of a MgB 2  film deposited on SrTiO 3 .  
         [0027]      FIG. 11  illustrates the resistivity of a MgB 2  film deposited on YSZ.  
         [0028]      FIG. 12  illustrates the resistivity of a MgB 2  film deposited on r-plane sapphire.  
         [0029]      FIG. 13  illustrates the resistivity of a MgB 2  film deposited on c-plane sapphire.  
         [0030]      FIG. 14  illustrates the resistivity of a MgB 2  film deposited on m-plane sapphire.  
         [0031]      FIG. 15  illustrates the resistivity of a MgB 2  film deposited on 4H-SiC.  
         [0032]      FIG. 16  illustrates the resistivity of a MgB 2  film deposited on Si 3 N 4 /Si.  
         [0033]      FIG. 17  is a plot of estimated surface resistance values of MgB 2  film deposited on sapphire and alumina substrates.  
         [0034]      FIG. 18  illustrates a device according to one preferred aspect of the invention for depositing thin films on a long ribbon of tape. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]      FIG. 1  illustrates a preferred device  2  used to fabricate in-situ MgB 2  films. The device  2  includes a vacuum chamber  4  having a removable or openable lid  6  that permits a user to gain access to the interior of the vacuum chamber  4 . The vacuum chamber  4  is connected via a vacuum hose  8  to a vacuum pump  10 . During operation, the pressure inside the vacuum chamber  4  is a high vacuum—preferably less than 10 −6  Torr.  
         [0036]     A pocket heater  12  is provided inside the vacuum chamber  4  and is used to repeatedly move one or more substrates  14  between a deposition zone  16  in which boron is deposited on the one or more substrates  14  and a reaction zone  18  in which pressurized, gaseous magnesium reacts with the deposited boron to form an in-situ MgB 2  film. The pocket heater  12  includes a housing  20  which partially encloses the substrates  14  as described in more detail below. The housing  20  preferably includes heating coils (not shown) for heating substrates  14  contained within the interior of the pocket heater  12 . Preferably, there is a top heating coil, a side heating coil, and a bottom heating coil although other constructions may be used. The heating coils can heat the pocket heater  12  to a temperature exceeding 800° C., although growth temperatures between 300° C. and 700° C. are preferred.  
         [0037]     The housing  20  of the pocket heater  12  includes an upper portion  22  which completely covers an upper surface of the substrates  14 . The housing  20  also includes a lower portion  24  which partially encloses the underside surface of the substrates  14 . As best seen in  FIG. 2 , a pie-shaped wedge is removed from the lower portion  24  of the housing  20  to form the deposition zone  16 . The lower portion  24  of the housing  20  that does enclose the substrates  14  forms a reaction chamber  26  (i.e., reaction zone  18 ) between the underside of the substrates  14  and the interior surface of the lower portion  24  of the housing  20 . The reaction chamber  26  is disposed close enough to the underside of the substrates  14  such that a localized high pressure region of magnesium gas is created in the reaction chamber  26 . While this pressure has not been measured, it has been reported by others that at 750° C., the required vapor pressure of magnesium is about 10 mTorr in the thermodynamic growth window. It should be understood that “high pressure” in the context of the reaction chamber  26  is a relative term and the pressure within the reaction chamber  26  is above the pressure in the vacuum chamber  4  but significantly less than atmospheric pressure.  
         [0038]     A gap  28  is formed between the underside of the substrates  14  and the lower portion  24  of the housing  20 . Preferably, the size of this gap  28  can be adjusted by moving the lower portion  24  of the housing  20  towards or away from the substrates  14 . Preferably, the gap  28  formed has a width within the range of about 0.005 inch to about 0.015 inch.  
         [0039]     A rotatable platen  30  is disposed inside the housing  20  of the pocket heater  12  and is used to rotatably support one or more substrates  14 .  FIG. 2  illustrates a substrate  14  being held by the platen  30  in the deposition zone  16 . The substrate  14  held by the platen  30  may take any number of shapes and forms, including but not limited to, a wafer, chip, flexible tape, or the like. The present method has been used to deposit MgB 2  films onto up to three 2″ wafers at once, or a single 4″ wafer. In addition, the present method of fabricating in-situ MgB 2  films can be used with a wide variety of substrate  14  materials. These include by way of illustration and not limitation: LSAT, LaAlO 3 , MgO, SrTiO 3 , r-plane sapphire, c-plane sapphire, m-plane sapphire, yttria-stabilized zirconia (YSZ), silicon carbide, polycrystalline alumina, silicon, and stainless steel. With respect to silicon, a Si 3 N 4  buffer layer is first formed on the silicon substrate. It is believed that the present method can be used to deposit MgB 2  films on any substrate in which there is no chemical reaction with magnesium, boron, or MgB 2 .  
         [0040]     Consequently, the present method provides a novel way of forming MgB 2  films on a variety of technologically interesting and inexpensive substrates. Coated conductor applications are thus possible. In addition, MgB 2  films can be deposited on flexible tapes.  
         [0041]     Still referring to  FIG. 1 , the rotatable platen  30  is mounted on a rotatable shaft  32  that passes through the housing  20  of the pocket heater  12 . The rotatable shaft  32  is mechanically connected at one end to a motor or servo (not shown) that drives the shaft  32  and thus rotates the platen  30 . Preferably, the platen  30  is removable from the shaft  32  such that loading and unloading of the substrates  14  from the platen  30  can take place outside of the pocket heater  12  and vacuum chamber  4 .  
         [0042]     A magnesium evaporation cell  34  is provided inside the vacuum chamber  4 . The magnesium evaporation cell  34  contains heater coils  36  which are used to heat solid magnesium  38  contained within the evaporation cell  34 . The magnesium evaporation cell  34  is heated to a temperature of at least 550° C. and more preferably around 650° C. so as to create magnesium vapor. This temperature can be adjusted to control the pressure of gaseous magnesium within the reaction chamber  26 . A magnesium feed tube  40  connects the magnesium evaporation cell  34  to the reaction chamber  26  of the pocket heater  12 . The magnesium feed tube  40  is preferably heated by heater coils  42  so that magnesium does not condense on the inside of the feed tube  40 . Preferably, about 9 amps of power is supplied to the heater coils  42  so as to maintain the magnesium feed tube  40  at a higher temperature than the magnesium evaporation cell  34 . Of course a higher or lower amount of power may be used depending on the particular heating coils  42  used.  
         [0043]     As an alternative to the magnesium evaporation cell  34  and feed tube  40 , a source of magnesium can simply be placed inside the reaction chamber  26  of the pocket heater  12  wherein it will evaporate to form a high pressure gas inside the magnesium reaction chamber  26 .  
         [0044]     Still referring to  FIG. 1 , an electron beam crucible  44  is disposed inside the vacuum chamber  4  and beneath the deposition zone  16 . Boron  46  is placed inside the electron beam crucible  44 . An electron beam gun  48  is positioned inside the vacuum chamber  4  and is aimed at the electron beam crucible  44  containing the boron  46 . The electron beam gun  48  is used to heat the boron  46  to a sufficiently high enough temperature such that the boron  46  starts to evaporate.  
         [0045]     While the use of an electron beam gun  48  is preferred, the boron  46  may be deposited by any other method known to those skilled in the art.  
         [0046]     Two quartz crystal monitors (QCM)  50 ,  52  are optionally included in the vacuum chamber  4 . A first QCM monitor  50  is preferably aimed downward toward the electron beam crucible  44  and is used to monitor the evaporation rate of boron  46 . The second QCM monitor  52  is preferably aimed upward toward the underside of the substrates  14  and is used to monitor leakage of magnesium from the pocket heater  12  through the gap  28 .  
         [0047]     Still referring to  FIG. 1 , a moveable shutter  54  is positioned inside the vacuum chamber  4  between the deposition zone  16  of the pocket heater  12  and the magnesium evaporation cell  34 . The shutter  54  is used to prevent the boron  46  from depositing on the underside surface of the substrates  14 .  
         [0048]      FIG. 2  shows a perspective view of the underside of the pocket heater  12 . As seen in  FIG. 2 , the deposition zone  16  is in the shape of a pie-shaped wedge. During operation, the shaft  32  rotates the platen  30  containing one or more substrates  14 . The substrates  14  repeatedly move between the deposition zone  16  in which boron  46  is deposited and a reaction zone  18  in which pressurized, gaseous magnesium reacts to form MgB 2 .  
         [0049]      FIG. 3  is a flow chart illustrating one preferred method of forming MgB 2  on a substrate  14 . With reference to  FIG. 3 , one or more substrates  14  are loaded onto the platen  30 . The platen  30  is then attached to the shaft  32  of the pocket heater  12 . A source of magnesium  38  (preferably in the form of magnesium pellets) is loaded into the magnesium evaporation cell  34 . Boron  46  is then loaded into the electron beam crucible  44 . The lid  6  of the vacuum chamber  4  is then closed and the vacuum chamber  4  is pumped down to a low pressure (preferably less than about 10 −6  Torr).  
         [0050]     Rotation of the platen  30  is then initiated by turning the shaft  32 . The platen  30  is rotated at a rate within the range of about 100 rpm to about 500 rpm. Preferably, the rotation rate is about 300 rpm. Current is then supplied to heater coils (not shown) of the pocket heater  12  to heat the substrates  14  contained therein. Current is also supplied to the heater coils  42  on the magnesium feed tube  40  that connects the magnesium evaporation cell  34  and the reaction chamber  26  of the pocket heater  12 . Current is then supplied to the heater coils  36  surrounding the magnesium evaporation cell  34 . The typical temperature of the magnesium evaporation cell  34  needed for deposition is around 650° C.  
         [0051]     Once the temperature of the pocket heater  12 , magnesium evaporation cell  34 , and magnesium feed tube  40  have been established and maintained, the electron beam gun  48  is turned on and the supplied current is increased until the boron  46  melts and begins to evaporate. The current supplied to the electron beam gun  48  is adjusted until the desired deposition rate is achieved. A typical preferred deposition rate is about 0.1 nm/sec. This can be determined by use of QCM monitor  50 .  
         [0052]     The shutter  54  disposed between the electron beam crucible  44  and the deposition zone  16  is then opened. Deposition of boron  46  and film growth of MgB 2  on the underside of the substrates  14  proceed until the desired thickness of MgB 2  is reached. Once the desired thickness of MgB 2  is reached, the shutter  54  is closed and the current to the electron beam gun  48 , pocket heater  12 , magnesium evaporation cell  34 , and magnesium feed tube  40  is reduced to zero (the current to the magnesium feed tube  40  is left on for a little while in order to avoid condensation of magnesium and plugging of the feed tube  40 ). The substrates  14  are then removed from the platen  30  once the substrates have had time to cool down (typically a few hours).  
         [0053]     In one preferred embodiment of the invention, after the substrates.  14  have had time to cool down, the substrates  14  are turned over to expose the top side of the substrates  14  to the deposition zone  16  and reaction zone  18 . The process described above is then repeated to deposit a MgB 2  on the second side (formerly the top side) of the substrates  14 .  FIG. 4  illustrates the process of depositing MgB 2  on both sides of a substrate  14 . In this manner, double-sided deposition of MgB 2  can be performed which is required for some applications (e.g., microwave filters and microstrip transmission lines).  
         [0054]     The method described herein is particularly advantageous because it is compatible with multilayer deposition of other materials which is essential for various electronics applications. In addition, there is no need to maintain control of the magnesium/boron flux ratio because the magnesium vapor is produced independently of the boron deposition process. The pocket heater  12  used in the process is also beneficial in that MgB 2  films can be grown on multiple, varied substrates  14  simultaneously.  
         [0055]     The above-described method also effectively avoids MgO contamination because there are negligible amounts of oxygen and MgO in the reaction chamber  26  where MgB 2  is formed. In addition, any magnesium vapor that escapes the reaction chamber  26  should condense, getter, and will not be incorporated into the grown film.  
         [0056]      FIG. 5  illustrates the resistivity of a MgB 2  film deposited on a polycrystalline alumina substrate.  FIG. 6  illustrates the resistance of a MgB 2  film deposited on flexible stainless steel tape. For both substrates  14 , a T c  of approximately 38-39° C. is achieved.  
         [0057]      FIG. 7  illustrates the resistivity of a MgB 2  film deposited on LSAT.  FIG. 8  illustrates the resistivity of a MgB 2  film deposited on LaAlO 3 .  FIG. 9  illustrates the resistivity of a MgB 2  film deposited on MgO.  FIG. 10  illustrates the resistivity of a MgB 2  film deposited on SrTiO 3 .  FIG. 11  illustrates the resistivity of a MgB 2  film deposited on YSZ.  FIG. 12  illustrates the resistivity of a MgB 2  film deposited on r-plane sapphire.  FIG. 13  illustrates the resistivity of a MgB 2  film deposited on c-plane sapphire.  FIG. 14  illustrates the resistivity of a MgB 2  film deposited on m-plane sapphire.  FIG. 15  illustrates the resistivity of a MgB 2  film deposited on 4H-SiC.  FIG. 16  illustrates the resistivity of a MgB 2  film deposited on Si 3 N 4 /Si. In this case, a Si 3 N 4  buffer layer is first formed on silicon using conventional processes known to those skilled in the art. The Si 3 N 4  buffer layer may be formed on the substrate  14  prior to it being loaded into the device  2 . Alternatively, the pocket heater  12  may include a nitrogen feed in which the Si 3 N 4  buffer layer is formed inside the pocket heater  12 .  
         [0058]      FIG. 17  is graph of the estimated surface resistance at 10 GHz vs. the inverse reduced temperature (T c /T) of MgB 2  films deposited on sapphire and alumina substrates. The surface resistance values are estimated because extrinsic losses (R res  in  FIG. 17 ) in the measurements had to be estimated and subtracted off in order to arrive at the intrinsic surface resistance R s  of the MgB 2  film. Samples M 1  and M 4  of  FIG. 17  are MgB 2  films deposited onto a sapphire substrate. Samples M 2  and M 3  are MgB 2  films deposited onto an alumina substrate.  
         [0059]     In the particular preferred embodiment of the invention, magnesium is heated in an evaporation cell  34  to provide gaseous magnesium to the reaction chamber  26 . It should be understood, however, that other elements that are non-gaseous at standard room temperature and pressure may also be used with the present method. In this regard, the particular element (in its non-gaseous state) would be placed into an evaporation cell  34  and heated such that a gaseous form of the element is produced and delivered to the reaction chamber  26 .  
         [0060]     For example, TBCCO (Tl 2 Ba 2 CaCu 2 O 8  or other phases) may be produced in accordance with the invention. One or more of the non-gaseous elements (i.e., Tl, Ba, Ca, or Cu) may be placed into an evaporation cell  34  which is then connected to the reaction chamber  26  as described above. A separate reaction chamber  26  connected to a source of oxygen is also provided for the oxidation reaction. For example, U.S. Pat. No. 6,527,866 illustrates a pocket heater device having a reaction chamber  26  coupled to an oxygen source.  
         [0061]     With respect to TBCCO, one particular method employs placing Tl into the evaporation cell  34  and heating the evaporation cell  34  to form Tl vapor which then passes to a reaction chamber  26 . Another separate reaction chamber  26  containing oxygen is used to oxidize the film. The remaining metals (Ba, Ca, or Cu) are deposited onto the substrate  14  in the deposition zone  16 .  
         [0062]     Still other superconducting thin films may be formed in accordance with the method described above. These include, for example, bismuth strontium calcium copper oxide (BSCCO), mercury barium calcium copper oxide (HBCCO), and yttrium barium copper oxide (YBCO). Generally, the method described above can be used with any element that has a relatively high vapor pressure at the operating temperature of the pocket heater  12 . In addition, the reaction of the gaseous element inside the reaction zone  18  must be self-limiting. That is, using MgB 2  as an example, when magnesium reacts with boron, the reaction does not produce Mg 2 B or Mg 3 B 2 .  
         [0063]     It should also be understood that the present invention may be used to manufacture non-superconducting films. Again, the method described above can be used with any element that has a relatively high vapor pressure at the operating temperature of the pocket heater  12 . In addition, the reaction of the gaseous element inside the reaction zone  18  must be self-limiting. Examples of non-superconducting films include, by way of illustration and not limitation, dielectrics, ferroelectrics, semiconductors such as GaAs, InP, and GaN, magnetic materials, piezoelectric materials, and the like.  
         [0064]      FIG. 18  illustrates one alternative embodiment of the device  2  used to form a thin film on a ribbon of tape  60 . In this embodiment, the pocket heater  12  does not use a rotatable platen  30  as in the pocket heater  12  shown, for example, in  FIG. 1 . Instead, a conveyor arrangement is used to pass the substrate  14  (in this case a long ribbon of tape  60 ) through the pocket heater  12 . In  FIG. 18 , the pocket heater  12  has four different zones (A, B, C, and D) in which the film forming process takes place. As an example, zones B and D might take the form of deposition zones. In contrast, zones A and C might take the form of reaction zones in which a gaseous reactants are input via feeds  62 . Of course, the particular arrangement shown in  FIG. 18  is merely exemplary and other configurations can be used depending on the type of film produced.  
         [0065]      FIG. 18  illustrates the substrate  14  being unrolled and rolled on two rotatable drums  64 . In some applications, however, the nature of the thin film and/or substrate  14  may prevent the ribbon of tape  60  from being stored on drums  64 . In this case the ribbon of tape  60  is fed and stored in a linear format. In addition, while  FIG. 18  shows the ribbon of tape  60  making a single pass through the pocket heater  12 , the tape  60  may make several passes through the pocket heater  12 . In this regard, the ribbon of tape  60  may take the form of a single continuous ribbon of tape  60  that wraps around the rotatable drums  64 . The continuous ribbon of tape  60  is shown in dashed lines in  FIG. 18 .  
         [0066]     In the case of MgB 2 , the deposition of boron onto the tape substrate  14  may occur prior to the tape substrate  14  entering the magnesium pocket (e.g., zones A or C in  FIG. 18 ).  
         [0067]     While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.