Patent Publication Number: US-2006013942-A1

Title: Method for improving electrical conductivity of metals, metal alloys and metal oxides by ion beam implantation

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to the art of treating metals, metal alloys and metal oxides, and more particularly to a new and improved method for enhancing the electrical conductivity of metals, metal alloys and metal oxides.  
      One area of use of the present invention is in the manufacturing of electrodes for capacitors, batteries and the like, although the principles of the present invention can be variously applied. Metals and metal alloys have a native oxide present on the surface. This is an insulating layer and, hence, if the material is to be used as a substrate for an electrode, the oxide has to be removed or made electrically conductive.  
      If the oxide is removed by chemical treatment, such as by etching with an acid or electrolytic etching to expose the underlying metal, special steps must be taken in order to complete the electrical contacts before the native oxide can be regenerated and interfere with the electrical contacts. Such measures require special apparatus and extremely careful handling of the materials. All of this adds cost to fabrication of these materials into electrical devices to which electrical contact must be made.  
      Another approach involves removing the oxide layer and plating the bare substrate metal with an expensive noble metal, such as silver, gold, or alloys of silver, gold and platinum, or the formation of an electrically conducting compound on the bare substrate surface. The overcoating materials are expensive and the steps required to plate the substrate are costly and time consuming. In addition, the metal plating or electrically conductive compound must be disposed on the substrate as a continuous film for maximum performance. Therefore, the plating or compound formation typically is carried out after the substrate metal is formed into its final shape for the intended electrical device in order to avoid damage to the coating. This, in turn, adds to the cost and complexity of the manufacturing process.  
      2. Prior Art  
      U.S. Pat. No. 5,098,485 to Evans proposes a solution to the oxide problem by altering the native oxide from an electrically insulating to an electrically conducting condition. This is done without removal of the native oxide layer to expose the underlying metal or alloy. A solution containing ions of an electrical material is applied to the native oxide layer, and then the substrate, oxide and applied ions are heated to an elevated temperature for a time sufficient to incorporate the ions into the oxide layer, changing it from an electrical insulator to an electrical conductor.  
      U.S. Pat. No. 5,599,580 to Muffoletto et al. relates to a method for improving the electrical conductivity of a substrate by depositing a small amount of metal thereon and then directing a high-energy beam onto the substrate. This intermixes the deposited material with the native oxide of the metal substrate, changing the native oxide from being electrically insulating to electrically conductive. The high-energy beam is an ion beam from a high-energy ion source or a laser beam. The substrate with the electrically conductive native oxide layer is useable in the manufacture of electrodes for capacitors and batteries.  
     SUMMARY OF THE INVENTION  
      It would, however, be highly desirable to provide a new and improved method for enhancing the electrical conductivity of metals, metal alloys and metal oxides that does not require additional heat treatment, provides control over the density and depth of the material introduced to the treated surface, can be performed in a manner preventing substrate degradation and deformation, and improves the quality of the treated surface.  
      The present invention provides such a method for improving the electrical conductivity of a substrate of a metal, metal alloy or metal oxide by doping the native oxide with a suitable material causing the oxide to become electrically conductivity. This is done with ions embedded into the metal, metal alloy and/or metal oxide causing the oxide layer to become electrically conductive. The method can be used for metal, metal alloys and/or oxide and mixed oxides from Groups IVA, (Ti, Zr, and Hf), VA, (V, Nb, and Ta), VIA, (Cr, Mo, and W), Al, Mn, Ni, and Cu). The ions or mixtures of ions to be implanted are from Group VIIIA metals (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt) elements. These ions are implanted using an ion implantation process. This includes elements from Group VIIIA being in the form of charged, energetic particles that are directed into the substrate metal, metal alloy and/or oxide from Groups IVA, VA, and VIA. The doped ions reside in the substrate as impurities.  
      The method of the present invention advantageously provides a doping method that does not require additional heat treatment and also provides control over the dose and depth of the ions implanted in the treated surface. The method can be performed at low temperatures preventing substrate degradation and deformation. It is believed that the quality of the treated surface is improved by the method of the present invention. An additional advantage of the present invention is that the surface can be treated to passivate it from chemical reaction while still providing adequate electrical conductivity. Stainless steels having native insulating oxides can also be treated to provide an electrically conductive oxide layer.  
      A substrate treated by the method of the present invention is ready for further processing in the manufacture of an electrode for use in a capacitor, a battery, and the like. Typically, in the case of a capacitor, an appropriate electrode material is deposited on the treated substrate surface by techniques well known to those skilled in the art. Examples of electrode materials are redox pseudo capacitance materials such as, but not limited to, oxides and mixed oxides of ruthenium, iridium, manganese, nickel, cobalt, tungsten, niobium, iron, molybdenum or under potential deposition systems such as palladium, platinum, lead dioxide or electro-active conducting polymers such as polyaniline, polypyrole and polythiophene.  
      The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description together with the included drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagrammatic view illustrating a substrate having a native oxide layer that is treated by the method of the present invention.  
       FIG. 2  is aschematic view of an ion beam implanter  18  useful in carrying out the process of the invention.  
       FIG. 3  is a diagrammatic view of a substrate after treatment by the method of the present invention.  
       FIG. 4  is a diagrammatic view of the substrate of  FIG. 3  having electrode material deposited thereon for use in manufacture of a capacitor electrode.  
       FIGS. 5, 6  and  7  are graphs depicting impedance spectroscopy scans on substrates of the type shown in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Metals and metal alloys have a native oxide present on the surface that is electrically insulating and must be removed or made electrically conductive if the metal or metal alloy is to be used as an electrode in devices such as capacitors and batteries. Referring to  FIG. 1 , there is shown a substrate  10  having an electrically insulating native oxide layer  12  on a surface thereof. In accordance with the present invention, oxide layer  12  is made more electrically conductive, i.e. changed from electrically insulating to electrically conductive. Substrates treated by the method of the present invention include metals and alloys thereof selected from the groups consisting of Group IVA metals (Ti, Zr, and Hf), Group VA metals (V, Nb, and Ta), Group VIA metals (Cr, Mo, and W), aluminum, manganese, nickel, copper and stainless steel. They typically have a thickness in the range from about 0.001 mm. to about 2.0 mm.  
      In accordance with the present invention, the native oxide layer  12  is doped with a suitable material causing it to become electrically conductive. Doping is by ions embedded therein. The ions or mixtures of ions to be implanted are from Group VIIIA (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt) elements. These ions are implanted using an ion implantation process such as those used in the manufacture of semiconductors. In this process, elements from Group VIIIA in the form of charged, energetic particles are directed into the above substrate metals, metal alloys and/or oxides as impurities.  
      Referring to  FIG. 2 , the doping process is preferably carried out in a suitable doping or implantation chamber  14  of a specially designed end station  16  of a suitable high current ion generating apparatus  18 , such as a Varian-Extrion 200 kV implanter, an Eaton-Nova implanter, and a like instrument. A fixture  20  supported on a base  22  is mounted within the implantation chamber  14 . The base  22  is designed to rotate and cool a base plate  22 , such as of titanium. A plurality of appropriately shaped substrate holders  26 , also made of titanium, is mounted on the base plate  24 . The holders  26  are designed to securely hold a plurality of substrates  10  and directly expose them to an incoming ion beam  28 . It is to be understood that the shape of the particular substrate holders secured to the base plate  24  depends upon the shape of the particular substrate being worked on. Also, the holders  26  have been conditioned so that they do not give off ions during ion implantation into the native oxide  12  on the substrates. If this were to happen and the holders were of a different material than that of the intended doping ions, the native oxide  12  would be contaminated by an unintended material, which could affect its conductivity.  
      The substrates  10  first are introduced into the implantation chamber  14  and held by the holders  26  supported on the base plate  26  of the base  22  of the fixture  20 . The next step involves the creation of a proper vacuum environment within the implantation chamber  14 . To this end, a vacuum within the implantation chamber  14  must be created which is less than about 5×10 −4  torr, and preferably is about 1×10 −7  torr. After a plurality of substrates  10  is introduced into the chamber  14 , the proper vacuum is established therein  14  by vacuum pump  30 . Preferably, the vacuum pump  30  should be of an oil-free type so as to avoid the possibility of introducing surface contamination onto the part to be ion implanted.  
      With the substrates  10  secured in the fixtures  20  within the implantation chamber  14 , the outer surfaces containing the native oxide layer  12  are exposed to a direct line of the incoming ion beam  28 . In order to achieve such a direct line, the fixture  20  is rotatable on its base  22  by motors (not shown), as indicated by an arrow  32 . The ion beam  28  is of at least one of the elemental doping materials that have been introduced into an ion production chamber  34 . The thusly-created ions pass through an ion extraction pre-accelerator  36 , then to separating magnets  38  before traveling through an ion accelerator  40 . Careful attention must be paid to having the proper ion beam power density acting on the surfaces of the substrates  10 . For, if it is too high in certain areas, localized heating may occur. Consequently, the peak ion beam power density of a 50 keV beam should not exceed about ten microamperes per square centimeter.  
      Control of the ion beam power density can be achieved in a number of ways. Preferably, and as herein illustrated, this low power ion beam current density is affected by expanding the spot size of the incoming ion beam  28  by a magnetic Quadra pole or an electro-static lens system  42 . The surfaces of the substrates  10 , now secured in the fixture  20  within the implant chamber  14 , are then exposed to the incoming ion beam  28 , properly modified, if need be, by the lens system  42 , for a period from about one minute to about one hour, and preferably for about five to ten minutes. A preferred ion beam particle energy is about  50  keV, and implants a dose of about 1×10 14  ions/cm 2  to about 1×10 21  ions/cm 2 , preferably about 3×10 17  ions/cm 2 , to a depth of about 100 to 10,000 angstroms, preferably about 200 angstroms, below the outer surfaces of the substrates  10 .  
      The result is that the ion implanted native oxide  12  surface of each substrate  10  has had its properties modified, i.e., rendered electrically conductive, without leaving any discrete layer on its surface. Since the ion implantation process is typically carried out at room temperature, it does not create toxic or hazardous waste, and is readily compatible with clean room and high technology processing environments.  
      Doping by means of the ion implantation method of the present invention converts the electrically insulating native oxide layer  12  to a doped layer  44  on substrate  10 , as shown in  FIG. 3 . This doped layer  44  has a degree of electrical conductivity sufficient to render the substrate  10  useable as an electrode in a device such as a capacitor or battery. In other words, native oxide layer  12  has been converted from being essentially non-conductive, i.e. insulating, to having an increased and improved degree of electrical conductivity. Thus, the quality of the treated surface of substrate  10  is improved in that the surface layer  12  is changed from an insulating, semiconducting or dielectric state to a doped, electrically conducting state  44 .  
      The substrate shown in  FIG. 3 , treated by the method of the present invention, is ready for further processing in the manufacture of an electrode for use in capacitors, batteries and the like. Typically, in the case of a capacitor, an appropriate electrode material  46 , as shown in  FIG. 4 , is deposited on the substrate treated surface by techniques well known to those skilled in the art. Examples of electrode material  46  are redox pseudo capacitance materials such as, but not limited to, oxides and mixed oxides of ruthenium, iridium, manganese, nickel, cobalt, tungsten, niobium, iron, molybdenum, or under potential deposition systems such as palladium, platinum, lead dioxide or electro-active conducting polymers such as polyaniline, polypyrole, and polythiophene.  
      The present invention is illustrated further by the following examples.  
     EXAMPLE I  
      A tantalum substrate similar to substrate  10  shown in  FIG. 1  was abraded on one side using a 3M Scotch-brite pad of a very fine type. This produced a rough surface on the side to be coated. It was then degreased and cleaned. This was accomplished by cleaning the foil in an ultrasonic bath using acetone as a solvent for about 10 minutes. Next, the substrate was washed in an ultrasonic methanol bath and then blow-dried, using dry, clean compressed air.  
      The substrate was now ready to have the native oxide layer at its outer surface doped with palladium. This was done by first loading the substrate into an ion beam generating apparatus evacuated to a pressure of about 1×10 −5  torr. Palladium ions were introduced into the ion production chamber. Then, an energetic ion beam of Pd ions with an energy level of about 50 keV bombarded the deposited surface causing the palladium to mix with the substrate. This continues until the palladium ions had penetrated to a depth of about 200 angstroms at a concentration of about 3×10 17  ions/cm 2 . On completion, the substrate surface was ready to be coated with ruthenium oxide for use in making a capacitor electrode.  
     COMPARATIVE EXAMPLE I  
      A tantalum substrate was prepared according to the previously discussed U.S. Pat. No. 5,098,485 to Evans. In particular, a brush made from palladium wires was brought into contact with and brushed across a dry tantalum surface. It is believed that palladium atoms were thereby mechanically transferred to the tantalum and tantalum oxide. The brushed tantalum was heated to 900° C. for 15 minutes, producing an electrically conductive surface that was then coated with ruthenium oxide in a similar manner as in Example I.  
       FIG. 5  compares the capacitance of ruthenium oxide coating a bare tantalum substrate to the capacitance of ruthenium oxide coating tantalum treated according to Example I and Comparative Example I. Curve  50  in  FIG. 5  is for bare or unprocessed tantalum. Curves  52  and  54  are for tantalum treated according to the present invention (Example I) and according to the prior art Evans patent (Comparative Example I), respectively.  
       FIG. 6  compares the resistance of ruthenium oxide coating a bare tantalum substrate to the resistance of ruthenium oxide coating tantalum treated according to Example I and Comparative Example I. Curve  60  in  FIG. 6  is for bare or unprocessed tantalum. Curves  62  and  64  are for tantalum treated according to the present invention (Example I) and according to the prior art Evans patent (Comparative Example I), respectively. The relatively lower capacitance and higher resistance of the untreated tantalum (curves  50  and  60 ) is due to the presence of the insulating native oxide. The prior art treatment according to the Evans patent (curves  54  and  64 ), while an improvement over an untreated substrate, does not render the native oxide as electrically conductive as the present invention doping technique (curves  52  and  62 ).  
       FIG. 7  shows the coating behavior of unprocessed tantalum in curve  70 , and processed tantalum where doping is by way of ion beam in curve  72  according to the present invention of Example I and where doping is by way of the treatment technique of the Evans patent in curve  74  according to Comparative Example I. The curves of  FIG. 7  also were obtained by performing impedance spectroscopy scans on the electrodes. They show the same results as the curves of  FIGS. 5 and 6 , but from a different perspective. They show the relative ideal behavior of the electrodes when used as capacitors. An ideal capacitor has no resistance and the spectroscopy scan would be a vertical line, perpendicular to the horizontal. Consequently, the closer the scan is to vertical, the more ideal the capacitor, or in this case the electrode. The scan for ruthenium oxide coating on unprocessed tantalum, shown by curve  70  in  FIG. 7 , is the farthest away from the vertical and, thus, is less ideal than the coatings on processed tantalum.  
      It is therefore apparent that the present invention accomplishes its intended objects. While embodiments of the present invention have been described in detail, that is for the purpose of illustration, not limitation.