Abstract:
An arc source filter is disposed between an arc cathode and a substrate in a vacuum arc deposition system. The filter includes a plurality of duct elements that surround the arc source. The duct elements have sufficient spatial dimensions to block particles. In addition, the duct elements have electrical and magnetic properties that are conducive for plasma transmission through the filter. On passing through the filter, the highly ionized arc plasma is essentially rid of particles making a source plasma for reacted as well as un-reacted coatings characterized by high density and near defect free quality. The design allows for flexibility in terms of filtering degree, length of coating zone as well as choice of arc source.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to arc deposition systems and, in particular, to methods of filtering particles from such arc deposition systems. 
         [0003]    2. Background Art 
         [0004]    Over the last 20 years, cathodic arc deposition has become established as a reliable source of highly ionized plasma for deposition of reacted as well as un-reacted coatings from conductive target materials such as zirconium, titanium, chrome, aluminum, copper and alloys thereof. The highly ionized plasma and the associated electron beam generated in the arc evaporation process is also used in such surface processing techniques as ion sputtering, etching, implantation and diffusion processes. 
         [0005]    An undesirable side effect of the arc evaporation process is the generation of macroscopically large particles (“macro-particles”) which tend to end up on substrates to be treated. These macro-particles potentially represent defects in the developing film, loosely adherent impurities, surface inhomogeneity and add roughness to the surface. The presence of macro-particles reduces the value and general applicability of the coating in demanding applications requiring superior properties such as corrosion performance, homogeneity, hardness, gloss or barrier performance. 
         [0006]    Filters that can lower macro content reaching the substrate from arc evaporated plasmas are known. Such prior art filters typically rely on the following mechanisms. Some filters provide a physical barrier that intercepts macro-particles as they propagate in line of sight from an arc spot on the cathode towards the substrate. Such barriers may be associated with features that partially prevent bouncing macro-particles from reaching the substrate. Other prior art filters use a shaped magnetic field that steers the arc electron beam in a trajectory clear of the physical barrier and a strength at least partially magnetically insulating the physical barrier from being an anode for the electrons. Finally, some prior art filters use a positive potential of the physical barrier repelling ions present in the arc generated plasma. Filters relying on a combination of these three filter principles are described in the scientific and patent literature. For an overview see Anders, A., “Approaches to rid cathodic arc plasmas of macro- and nanoparticles—a review,” S URFACE AND  C OATINGS  T ECHNOLOGY , volume 120, 1999, pages 319-330. 
         [0007]    Cathodes for filtered arc sources are typically point source, i.e. circular cathodes, while a few elongate configurations such as linear aligned multiple point sources and linear sources have been described. The linear arrangement of the arc source allows for elongated coating zones and greatly increases the large volume production potential of the filtered arc technology. Yet, cylindrical target filtered arc plasma sources are even more desirable for coating or ion processing of large substrates, sheet material in roll form, and for quantities of smaller substrates on a linear conveyor or circular carousel. 
         [0008]    Although the prior art methods for filtering macro-particles from arc deposition processes work reasonably well, these methods still suffer from a number of drawbacks. For example, deposition systems using the prior art filters tend to have a low net ion output current from the cathode target. The prior art methods also tend to have suboptimal utilization of the cathode surface. Finally, the coatings formed in such methods still tend to include an undesirable level of defects. 
         [0009]    Accordingly, there is a need for improved cathode arc deposition systems with improved macro-particle filtering. 
       SUMMARY 
       [0010]    The present invention solves one or more problems of the prior art by providing, in at least one embodiment, a filter for an arc deposition system which includes an elongated cathode, an anode, and at least one substrate. The filter includes an even number of duct assemblies symmetrically positioned around the elongated cathode. The duct assemblies define a magnetic field for guiding a plasma and have a baffle component for blocking macro-particles. 
         [0011]    In another embodiment, a filter assembly for use in an arc deposition system is provided. The filter assembly includes an even number of duct assemblies symmetrically positioned around the cathode target. The duct assemblies define paths through which positively charged ions are guided from a cathode target to a substrate. In order to accomplish such guidance, the duct assemblies are electrically biased such that positively charged ions are repelled. The duct assemblies also include components for generating a magnetic field that guide a plasma from the cathode target to the substrates. In particular, the magnetic field guides the movement of electrons which desirably results in ions (i.e., positively charged) moving in a manner to avoid collision with the filter. The duct assemblies also include baffles for blocking macro-particles from reaching substrates. Neutral and negatively charged particles are undesirable as they lead to imperfections and agglomerates in the coating. The neutral and negatively charged particles collide with the filter and are, therefore, removed and prevented from reaching the substrate. 
         [0012]    In still another embodiment, an arc deposition system for removing material from a cathode target which is deposited on a substrate is provided. The arc deposition system includes an elongated cathode target which is placed within a vacuum chamber. One or more substrates are positioned within a substrate zone that is a predetermined distance from the cathode target in the vacuum chamber. The filter assembly is also placed within the vacuum chamber such that the filter assembly is interposed between the cathode target and the substrate zone. Characteristically, the filter assembly includes an even number of duct assemblies symmetrically positioned around the cathode target. The duct assemblies define paths through which positively charged ions are guided from the cathode target to the substrates. In order to accomplish such guidance, the duct assemblies are electrically biased such that positively charged ions are repelled. The duct assemblies also include components for generating a magnetic field that guides a plasma from the cathode target to the substrates. The magnetic field guides the movement of electrons which desirably results in ions (i.e., positively charged) moving in a manner to avoid collision with the filter. The duct assemblies also include baffles for blocking macro-particles from reaching substrates. Neutrals and negatively charged particles are undesirable as they lead to imperfections and agglomerates in the coating. The neutral and negatively charged particles collide with the filter and are, therefore, removed and prevented from reaching the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0014]      FIG. 1A  is a schematic of an arc deposition system incorporating a filter; 
           [0015]      FIG. 1B  is a schematic cross section of the deposition chamber and components therein of an arc deposition; 
           [0016]      FIG. 2  is a top view of illustrating the positioning of the duct elements about a cathode target; 
           [0017]      FIG. 3  is a perspective view of a filter to be placed around a cathode in an arc deposition system; 
           [0018]      FIG. 4  is a perspective view of a filter to be placed around a cathode mounted on a vacuum flange; 
           [0019]      FIG. 5  provides an electromagnetic contour plot of filter when operated in ion transmission mode for a chromium cathode using a filter in accordance to  FIGS. 1-4 ; 
           [0020]      FIG. 6  provides magnetic flux density at a cross section through a filtered cylindrical cathode arc plasma source as depicted in  FIG. 4 ; 
           [0021]      FIG. 7A  is an optical micrograph at 200× magnification of a Cr coating without filtering; 
           [0022]      FIG. 7B  is an optical micrograph at 200× magnification of a Cr coating with filtering; 
           [0023]      FIG. 8  provides a plot of the frequency of particles as a function of particle size as realized from filter using chromium target; and 
           [0024]      FIG. 9  is a plot of relative horizontal angular distribution of deposit output from one of the four filter ports as realized with a chromium target 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0026]    Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
         [0027]    It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way. 
         [0028]    It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components. 
         [0029]    Throughout this application where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains. 
         [0030]    With reference to  FIGS. 1A and 1B , schematic illustrations of an arc deposition system incorporating a filter are provided. The arc deposition of this embodiment is an advance over the systems set forth in U.S. Pat. No. 5,269,898, the entire disclosure of which is incorporated by reference. In particular, the present embodiment provides a system in which a filter capable of significantly reducing the spoilage of a coating with macro-particles is adapted to the system of U.S. Pat. No. 5,269,898.  FIG. 1A  is a schematic of an arc deposition system incorporating a filter.  FIG. 1B  is a schematic cross section of the deposition chamber and components therein of an arc deposition. During operation of the arc deposition systems set forth above, an arc is struck which causes significant ionization of gas and cathode atoms thereby forming a plasma. The ionized metal cathode ions are directed from the region of the cathode towards the substrates which are coated with a metal layer. Arc deposition system  10  includes cathode target  12  which is placed within vacuum chamber  16 . Arc deposition system  10  includes at least one anode. In a refinement, wall  18  of vacuum chamber  16  is an anode. Characteristically, cathode target  12  is of an elongated design (e.g., cylindrical or rod-shaped). It should also be appreciated that the cross section of target  12  may be of virtually any shape, examples of which include, but are not limit to circular, triangular, square, pentagonal, hexagonal, elliptical or irregularly shaped. Arc deposition system  10  is maintained at a reduced pressure during coating of a substrate via suitable vacuum systems as is known in the field via port  19 . Typically, operating pressures are between 0.5 and 50 mT. It should be noted that fully reacted coatings can be realized above approximately 1 mT using (e.g., chromium target in mixed argon nitrogen atmosphere.) 
         [0031]    Although the present embodiment is not limited by the dimensions of cathode target  12 , typically cathode  12  has a diameter from 1 to 10 inches and a length from 6 inches to 5 feet. Substrates  20  are positioned within substrate zone  22  that is a distance d 1  from cathode target  12 . Filter assembly  24  is also placed with vacuum chamber  16 . In particular, filter assembly  24  is interposed between cathode target  12  and substrate zone  22 . Filter assembly  24  includes an even number of duct assemblies  26 ,  28 ,  30 ,  32  symmetrically positioned around cathode target  12 . Duct assemblies  26 ,  28 ,  30 ,  32  define ducts  34 ,  36 ,  38 ,  40  through which positively charged ions are guided from cathode target  12  to substrates  20 . In order to accomplish such guidance, duct assemblies  26 ,  28 ,  30 ,  32  are electrically biased such that positively charged ions are repelled from the duct assemblies via filter power supply  42 . A voltage of plus 10 volts or more effectively repels positively charged ions. Duct assemblies  24 ,  26 ,  28 ,  30  also include components for generating a magnetic field that guides a plasma (i.e., positively charged ions) from cathode target  12  to substrates  20 . In a refinement, duct assemblies  24 ,  26 ,  28 ,  30  also include baffles for blocking macro-particles from reaching substrates  20 . 
         [0032]    In a refinement, system  10  includes helical electromagnet coil  48  which is coaxially mounted about cathode target  12 . Helical electromagnet coil  48  is powered by a separate coil power supply  50 . Electromagnet coil  48  may be electrically isolated or it may be connected to the vacuum chamber  16 . 
         [0033]    Control system  52  is provided to vary the current input to each end of the cathode target  12  while maintaining the total arc current substantially constant, such that the current to each end of cathode target  12  may be varied between 0 and 100 percent of the total arc current supplied. Arc current is directly correlated to deposition rate and can be controlled between 50 A and, for example, 2000 A, the upper limit determined by cooling efficiency of target. This may be accomplished by using separate arc power supplies  54 ,  56  connected at each end of cathode target  12  with a controller  58  to provide complementary set point signals for the separate arc power supplies  54 ,  56 . In an alternative variation, a single arc power supply having two complementary current outputs may be employed. 
         [0034]    Still referring to  FIGS. 1A and 1B , the arc tends to be drawn toward whichever end of the cathode target  12  is receiving the larger fraction of the total current input, due to the self magnetic field of the arc current in the cathode target. The speed at which the arc moves in one direction along the cathode target  12  is partly determined by the degree of imbalance between the currents flowing into each end of cathode target  12 . The arc spot can, therefore, be scanned back and forth along the cathode target  12  by varying the division of current between the two ends of cathode target  12  in an oscillatory fashion. Cathode target  12  can, therefore, be uniformly eroded, and the arc can be maintained continuously on the cathode surface rather than being repeatedly restruck as taught by the prior art. A sensor may be conveniently located at each of the ends of the evaporable surface of cathode target  12  to provide a signal when the arc spot reaches one end of the evaporable surface of cathode target  12 , at which time the current division may be reversed, allowing automated scanning of the arc spot along the entire surface of cathode target  12 . 
         [0035]    Electromagnet coil  48  may be connected in series with the arc power supply  54 ,  56 , such that the arc current flows through electromagnet coil  48  to generate an axial magnetic field. Since the coil is connected between the positive output of arc power supply  54 ,  56  and the anode, and since the total arc current is constant, the current input to the electromagnet coil  48  is not affected by the variation in current to the two ends of cathode target  12 . This arrangement eliminates the necessity of a separate power supply for powering electromagnet coil  48 , but sacrifices independent adjustability of the strength of the applied magnetic field except through selection of the pitch of electromagnet coil  48 . 
         [0036]    With reference to  FIGS. 2 and 3 , a filter for an arc deposition system is provided. The filter of the present embodiment is positionable between the cathode and the substrates of the arc deposition system. Filter assembly  24  includes an even number of duct assemblies symmetrically positioned around elongated cathode  12 . The variation set forth in  FIGS. 2 and 3  includes four duct assemblies, i.e., duct assemblies  26 ,  28 ,  30 ,  32 . Duct assemblies  26 ,  28 ,  30 ,  32  define ducts  34 ,  36 ,  38 ,  40  through which positively charged ions are guided from cathode target  12  to substrates  20 . Duct assemblies  26 ,  28 ,  30 ,  32  define a magnetic field for guiding a plasma. Duct assemblies  26 ,  28 ,  30 ,  32  each include support component  60  and baffle component  62  for blocking macro-particles. In a refinement, baffle component  62  includes protrusions  64  for enhancing the ability of filtering out macro-particles. Electrical posts  66 ,  68  are used to connect to the filter power supply so that the duct assemblies are electrically biased for repelling positively charged ions. In such situations, baffle component(s) have an electrically positively charged surface. 
         [0037]    With reference to  FIG. 4 , a schematic perspective view of a filter assembly with peripheral substrate holders is provided. Filter assembly  80  includes duct assemblies  82 ,  84 ,  86 ,  88  which are mounted on vacuum flange  90 . Filter assembly  80  also includes sample holders  92  onto which samples to be coated are mounted. Note that in  FIG. 4 , several sample holders are removed to allow viewing of the duct assemblies. In general, sample holders  92  are distributed in a circle about the duct assemblies. Each of duct assemblies  82 ,  84 ,  86 ,  88  independently include structural component  94  which is formed from metal tubing. As set forth above, a current is passed through structural component(s)  94  to establish a magnetic field. Duct assemblies  82 ,  84 ,  86 ,  88  also independently include baffle component  98  for blocking macro-particles with protrusions  100  disposed thereon. Moreover, duct assemblies  82 ,  84 ,  86 ,  88  are linked together as set forth above via junctions  100 . 
         [0038]    It should be appreciated that dimensions and orientation of duct assemblies depicted above as well as characteristics of magnetic field elements the transmission and filter efficiency can be altered without departing from the underlying teaching of this invention. Filters allowing a tailoring of the amount of macro-particle content in the coatings are achievable. 
         [0039]    With reference to  FIGS. 2 ,  3 ,  4 ,  5 , and  6  a magnetic field is optionally created by passing a current through the duct assemblies so as to create a magnetic field. In particular, adjacent duct assemblies generate magnetic fields with opposite magnetic polarities. Arrows  70 ,  72 ,  74 ,  76  indicate an example of the directions that current may flow to create such magnetic field. To accomplish this alteration, the duct assemblies are electrically connected as shown by links  80 ,  82 ,  84  in a staggered manner at the top or bottom. The magnetic field generated in this manner has an orientation normal to an elongated cylindrical cathode surface and strength conducive to plasma guidance produced by passing current through the duct assemblies. Sufficient field strength for plasma guidance is design dependent. For the example, a useful range is provided between 6,000 and 12,000 ampere-turns. The lower limit is defined by target ion transmission, while the upper limit is defined by electron scattering phenomenon (magnetic electron mirror) blocking target ion transmission.  FIG. 5  provides a cross section of the magnetic field contour line for a filter having four duct assemblies as depicted in  FIG. 4 . The positions of duct assemblies  82 ,  84 ,  86 ,  88  in relation to the contour lines are shown in this figure. Similarly,  FIG. 6  provides magnetic flux density at a cross section through a filtered cylindrical cathode arc plasma source. It should be noted that duct assemblies  82 ,  84 ,  86 ,  88  may be biased (positively charge) as well as carrying a current to generate the requisite magnetic fields. In a variation, the structural component carries the current while the baffle component is biased. In this variation, structural and baffle components are electrically isolated. In another variation, the structural and baffle components are not electrically isolated. 
         [0040]    As set forth above, the filter of the present embodiment includes an even number of duct assemblies that are symmetrically placed around cathode target  12 . In a refinement, the number of duct assemblies is an even number from 2 to 8. In another refinement, the number of duct assemblies is an even number from 4 to 8. The inclusion of four duct assemblies is found to be particularly useful. Moreover, as depicted in  FIGS. 1A and 2 , baffle component  62  is positioned on a side of the duct assembly facing cathode target  12  such that a line of sight between the cathode and substrate zone is blocked. 
         [0041]    With reference to  FIGS. 1A and 1B , the arc deposition system may be operated as an electron beam system. During such operations, the filter may be operated without baffle bias. In this scenario, the magnetic field strengths may also be low. The operation of the filter in electron beam mode may aid in substrate cleaning and requires that the substrate is biased positively. The electron beam may also be used in aiding electron beam evaporation such as the deposition of aluminum using a secondary target source. 
         [0042]    Still referring to  FIGS. 1A and 1B , the arc deposition system may be operated as an ion beam system. During such operations, the pressures in the deposition chamber are at or above 1 mT and the filter is without baffle bias. In this scenario, escaping electrons ionize gas molecules which escape through the filter exits. The operation of a filter in ion beam mode may aid cleaning of substrate and requires that substrate is biased negative. Extended substrate treatment with ion beam may, at elevated temperature, facilitate nitriding of steel substrate (nitrogen containing plasma) and carburizing of steel (carbon containing plasma). 
         [0043]    It should further be apparent that alternative cathode configurations can be operated in the filter configuration of this invention. One such cathode configuration well known in the art, is the rotary cylindrical cathode described, for example, in U.S. Pat. No. 6,262,539 (the entire disclosure of which is incorporated herein by reference) which can be operated in present filter invention without departing from the scope and teachings of this invention. Another cathode configuration well known in the art, is the planar magnetron as described, for example, in U.S. Pat. No. 4,892,633 (the entire disclosure of which is incorporated herein by reference) which can be operated in present filter invention without departing from the scope and teachings of this invention. 
         [0044]    The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims. 
         [0045]    A filtered arc as disclosed with four duct elements was furnished with a chromium cylindrical cathode. The chamber was pumped down to 1 mTorr and maintained at pressure using argon as background gas. An arc was stricken and maintained at 900 A while passing 8000 A through duct elements biased at plus 30 volts, passing 100 A through steering coil and biasing the substrate at minus 50 volts. A current of 40 A was collected at the substrate and a Cr film deposited. The deposited film was documented for macros showing a reduction in macro content from 1% volume to less than 0.01% volume as compared to an unfiltered arc. The cross sectioned Cr film showed generally amorphous and isotropic properties without signs of defects. 
         [0046]      FIGS. 7A and 7B  provide a comparison of macro-particle content in a Cr coating obtained with filtering and without filtering as set forth above.  FIG. 7A  is a scanning electron micrograph of a Cr coating without filtering while  FIG. 7B  is a scanning electron micrograph of a Cr coating with filtering. It is readily observable that the coating with filter has significantly fewer macro-particles included therein.  FIG. 8  provides a plot of the frequency of particles as a function of particle size. This figure shows a very low distribution of particles of size about 1 micron. It should be noted that macro frequency density is underestimated for macro diameters under 0.5 microns due to resolution limitations of optical micrography used for macro documentation.  FIG. 9  provides the relative (%) thickness variation for coating deposited on substrates in the substrate zone as of function of angle of placement (0-90 degrees) with respect to a plane oriented through the cathode target along the long direction. Only zero to 90 degrees are documented due to symmetry. 
         [0047]    A filtered arc as disclosed with four duct elements was furnished with a chromium cylindrical cathode. The chamber was pumped down to 1 mTorr and maintained at pressure using argon and nitrogen in 1:1 ratio as background gas. An arc was stricken and maintained at 900 A while passing 8000 A through duct elements biased at plus 30 volts, passing 100 A through steering coil and biasing the substrate at minus 50 volts. A current of 40 A was collected at the substrate and a Cr film deposited. The deposited film was documented for macros showing a reduction in macro content from 1% volume to less than 0.01% volume as compared to an unfiltered arc. The cross sectioned CrN film showed generally amorphous and isotropic properties without signs of defects. 
         [0048]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.