Abstract:
The disclosure is for an improved film coating useable on optical media molds and the apparatus for and method of making such a film. The film is a diamond-like carbon layer of 0.3 to 3.0 microns coated on a titanium underlayer of 0.1 to 1.0 microns. The method of making the diamond-like carbon film is to deposit a defect free underlayer coating on to the steel substrate of the mold using an electron beam coating apparatus that has a hollow cathode electron beam generator and a rotating crucible containing the coating material. The diamond-like carbon film is then produced on top of the underlayer coating.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention deals generally with the coating of metals and more specifically with a diamond-like carbon film on metal or ceramic substrates with improved adhesion and the apparatus and method for producing such a film.  
           [0002]    Perhaps the oldest method of depositing diamond-like carbon films is by breaking down hydrocarbon gases in the plasma of a radio frequency discharge. There are many references on this subject and some go back as much as 20 or 30 years. The term used to identify this method is radio frequency chemical vapor deposition, and the coatings it produces are commonly referred to as a:C—H coatings because of the presence of hydrogen in the film along with carbon. Such diamond-like carbon films are amorphous, meaning that they do not have long range repeatability of atomic orientation in their crystalline structure. These films are used for a variety of applications ranging from scratch resistant coatings on optical lenses to coatings on razor blades.  
           [0003]    However, diamond-like carbon films have poor adhesion when deposited on most metal substrates. One method of improving adhesion of the film to the substrate, particularly a steel substrate, is to deposit a thin layer of some other metal on the substrate before applying the diamond-like carbon film. This layer, which is called the underlayer, relieves the stresses in the diamond-like carbon film and prevents delamination. U.S. Pat. No. 5,827,613 by Nakayama et al discloses depositing a molybdenum underlayer by means of ion bombardment of a molybdenum grid in the vicinity of the substrate being coated.  
           [0004]    In the prior art the most common method of depositing the underlayer is sputtering. There are commercially available diamond-like carbon coating systems that utilize sputtering techniques to produce the underlayer. These sputtering systems work adequately for depositing the underlayer on the majority of two dimensional substrates where the coating is deposited onto a relatively flat surface, but depositing the underlayer on three dimensional parts by sputtering is often complicated and sometimes impossible.  
           [0005]    One reason is that the deposition rate drops dramatically as the distance increases between the target, the source of the material being sputtered, and the substrate. This results in thinner and more porous films on surfaces even slightly offset from the nearest surface of the substrate. For three dimensional substrates that means some surfaces will have compromised underlayer thickness and quality.  
           [0006]    The sputtering process is also characterized as a line of sight process, in which deposition occurs almost exclusively on those areas which can optically “see” the target. This limitation makes it difficult to sputter film onto complicated shapes such as concave or convex surfaces or holes. The productivity of the process also suffers since there is a limiting distance between the cathode and the substrate.  
           [0007]    Another problem with sputtered films is their inherent porosity which becomes a problem when producing optical quality surface finishes, referred to as zero-finish. The porosity of the underlayer results in a “hazy” appearance of the diamond-like carbon top coat, which is totally unacceptable in the optical disc molding industry.  
           [0008]    Since the underlayer is an integral part of a diamond-like carbon film, the limitations of the sputtering process described above restrict the use of diamond-like carbon films. It would be very beneficial to have a method of producing a metallic underlayer for diamond-like carbon films on three dimensional substrates and an underlayer which was free of haze.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention avoids the limitations of the prior art methods of producing the metallic underlayer for a diamond-like carbon film by using a hollow cathode method and apparatus for depositing a physical vapor deposition (PVD) coating which can be used independently or as an underlayer for a diamond-like carbon coating. The hollow cathode method uses a watercooled crucible acting as an anode and a hollow tube made of refractory metal acting as a cathode. The coating material is then placed in the crucible and an electron beam is generated between the hollow cathode and the crucible. The electron beam melts the coating material and vapor is therefore generated. The vapor is ionized by the electron beam with the aid of an injected inert gas, and the ions and neutral atoms migrate to the substrate that may also have a negative voltage relative to the chamber wall.  
           [0010]    This method which can be used to deposit individual coatings of materials such as titanium nitride, titanium carbo-nitride, and the like and also metallic underlayers for diamond-like carbon coatings has a far better ability to propagate a coating than does the sputtering technique. The difference is related to the way the vapor is generated and its vapor pressure. The hollow cathode method generates vapor with a higher partial pressure than does sputtering, and higher vapor pressure results in higher mobility of atoms yielding better coverage of three dimensional substrates and complicated shapes. The hollow cathode method produces very dense film with a surface finish that is as good as the original finish on the substrate. This eliminates problems with haze.  
           [0011]    Although the hollow cathode technology produces high quality, dense films, it has an inherent drawback that limits its application for depositing optical quality films onto compact disc molds. The problem is that conventional hollow cathode deposition methods produce splashes of molten metal which adhere to the surface of the treated parts and solidify. The size of such splashes may reach tens or even hundreds of microns, whereas the film thickness is only a few microns. Such splashes protrude from the underlayer appearing like mountains under a microscope, and they are poorly adhered to the rest of the underlayer. Any diamond-like carbon film deposited on top of a splash would repeat the shape of the splash and create a protrusion in the final film. High accuracy optical products such as optical media molds and optical lens molds require surface finishes for which imperfections are frequently measured in fractions of a microns, and splash defects are not acceptable for such molds.  
           [0012]    The present invention avoids splashes by rotating the crucible and placing the hollow cathode so that the vertical axes of the hollow cathode and the crucible are offset from each other. This geometry allows the electron beam generated by the hollow cathode to be directed to an area offset from the center of the crucible. Thus, the rotation of the crucible and the offset point of impact of the electron beam produce continuous movement of the location of the pool of melted material within the crucible. This action eliminates the creation of gas pockets which are the cause of the splashes that occur when films are produced without the present invention&#39;s offset beam and rotation of the crucible. The apparatus and method of the present invention thereby produces coatings and a diamond-like carbon film which are superior to any produced by the prior art, and which are completely satisfactory for surfaces on optical quality molds. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic diagram of the apparatus of the invention.  
         [0014]    [0014]FIG. 2 is an enlarged cross section view of the region of the invention including the hollow cathode and the rotating crucible. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 is a schematic diagram of coating apparatus  10  of the invention in which vacuum chamber  12  contains hollow cathode  14 , substrate holder  16 , and crucible  18 . Substrate  20  is held onto substrate holder  16  by conventional means such as mechanical clamping as substrate holder  16  is rotated. Radio frequency power is applied to substrate holder  16  by conventional means (not shown) through feedthrough  19 . The radio frequency is used to produce the diamond-like carbon top layer after the underlayer is coated onto the substrate.  
         [0016]    Vacuum chamber  12  is maintained at a suitable level of vacuum in the range of 1×10 −2  to 1×10 −5  Torr by vacuum port  22  which is connected to a vacuum system (not shown), and as in all such diamond-like carbon coating processes, a reactive gas such as acetylene is fed into vacuum chamber  12  at pipe  24 .  
         [0017]    Inert gas argon is supplied to vacuum chamber  12  through hollow cathode  14 . Hollow cathode  14  is maintained at a negative voltage of 20 to 100 volts relative to rotating crucible  18  by power supply  26 , and substrate holder  16  is maintained at a negative voltage relative to vacuum chamber  12  by power supply  46  which is connected to substrate holder  16  through radio frequency filter  48 . The DC voltage applied to substrate holder  16  is used for increasing the density of the underlayer and is not required for all applications.  
         [0018]    [0018]FIG. 2 is an enlarged cross section view of the region of the coating apparatus adjacent to rotating crucible  18  which is rotated by shaft  28  that passes through vacuum chamber wall  30 . Crucible  18  has a central cavity  32  which contains coating material  34 . The preferred coating material  34  for use as an underlayer for a diamond-like carbon film is titanium, which is placed in cavity  32  in the form of pellets and melted by the power supplied from electron beam  36  which is generated between hollow cathode  14  and crucible  18 . However, the same apparatus can be used to deposit not only the underlayer, but also bulk coatings such as titanium nitride and the like.  
         [0019]    Crucible  18  itself is water cooled by using shaft  28  to transport water to and from the crucible. As shown in FIG. 2, shaft  28  is hollow and has input pipe  29  located at its center. The return water path is in the annular space between input pipe  29  and the inner wall of hollow shaft  28 . Cooling cavity  31  within crucible  18  includes separator  33  to direct the cooling water against the walls of cooling cavity  31 . Water is furnished to and removed from hollow shaft  28  by a conventional rotating coupling (not shown).  
         [0020]    The benefit of the invention is attained by the rotation of crucible  18  and the offset orientation of hollow cathode  14  relative to axis of rotation  40  of crucible  18 . Crucible  18  is rotated by means of shaft  28  which is interconnected with a motor (not shown) external to vacuum chamber  12 . Shaft  28  passes through wall  30  of vacuum chamber  12  at sealed bearing  38 .  
         [0021]    As in all such hollow cathode systems, the electron beam melts the coating material in the crucible and forms liquid metal pool  43 , and metal vapor is therefore generated. Vapor  44  is also ionized by the electron beam as an inert gas, argon, is fed into the hollow cathode to maintain the ionization. The ions and neutral atoms migrate to substrate  20 , and when the ions and neutral atoms contact the substrate they form the desired coating. The ions and neutral atoms are actually deposited upon the substrate because the substrate is at a lower temperature than the liquid metal pool so that the metal vapor essentially condenses on the substrate.  
         [0022]    With the offset orientation, the electron beam created between hollow cathode  14  and crucible  18 , which acts as an anode, is directed to target material  34  at a location between axis  40  and sidewall  42  of cavity  32 . As crucible  18  rotates, the location at which electron beam  36  is directed creates liquid pool  43  within metal  34  in crucible cavity  32 , and the constantly changing location of liquid metal pool  43  prevents gas bubbles that would otherwise cause splashing. The criteria for the successful prevention of gas bubbles is that liquid pool  43  extends across axis  40  of crucible  18  and does not touch sidewall  42  of cavity  32 . The speed of rotation of crucible  18  must also be controlled to fall within the range between 1/20 and 3 revolutions per minute.  
         [0023]    When these specifications are met, the present invention prevents the splashing which results in imperfections in the coating deposited on the substrate.  
         [0024]    A film of diamond-like carbon can then be formed on top of the substrate coating when a hydrocarbon reactive gas such as acetylene is injected into vacuum chamber  12  at port  24  while radio frequency power is applied to substrate  20 . The resulting film has the required optical quality for compact disc molds because there are no splashes formed in the underlayer on the substrate.  
         [0025]    Thus, the method for producing the coating of the present invention is as follows:  
         [0026]    placing a substrate to be coated on a rotating substrate holder within a vacuum chamber;  
         [0027]    placing a material to be coated onto the substrate within a crucible rotating around an axis of rotation within the vacuum chamber;  
         [0028]    locating a hollow cathode with an axis within the vacuum chamber with the hollow cathode oriented so that the hollow cathode axis intersects the material within the crucible at a location offset from the axis of rotation of the crucible;  
         [0029]    producing a vacuum within the vacuum chamber;  
         [0030]    generating an electron beam between the hollow cathode and the material within the crucible to create a pool of melted material and to produce vapor of the material by feeding an inert gas into the hollow cathode and into the region of the electron beam to create ions to sustain the electron beam, and by applying a DC voltage between the cathode and the crucible;  
         [0031]    optionally, applying negative voltage to the substrate holder and substrate respectively; and  
         [0032]    maintaining the substrate at a temperature below the temperature of the melted material so that the material vapor deposits upon the substrate.  
         [0033]    Similarly, the method for producing the diamond-like carbon coating of the present invention on an optical media mold is as follows:  
         [0034]    placing a substrate to be coated on a rotating substrate holder within a vacuum chamber;  
         [0035]    placing a metal to be coated onto the substrate within a crucible rotating around an axis of rotation within the vacuum chamber;  
         [0036]    locating a hollow cathode with an axis within the vacuum chamber with the hollow cathode oriented so that the hollow cathode axis intersects the metal within the crucible at a location offset from the axis of rotation of the crucible;  
         [0037]    producing a vacuum within the vacuum chamber;  
         [0038]    generating an electron beam between the hollow cathode and the metal within the crucible to create a pool of melted metal and to produce vapor of the metal by feeding an inert gas into the hollow cathode and into the region of the electron beam to create ions to sustain the electron beam, and by applying a DC voltage between the cathode and the crucible;  
         [0039]    optionally, applying negative voltage to the substrate holder and substrate respectively;  
         [0040]    maintaining the substrate at a temperature below the temperature of the melted metal so that the metal vapor deposits upon the substrate as an underlayer;  
         [0041]    stopping the depositing of the underlayer on the substrate;  
         [0042]    supplying radio frequency power to the substrate; and  
         [0043]    feeding a reactive gas into the vacuum chamber after the underlayer is produced to form a diamond-like carbon film on top of the underlayer.  
         [0044]    It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.  
         [0045]    For example, although titanium is the preferred metal for the underlayer for a diamond-like carbon film, other metals can also be used.