Abstract:
A dual-cathode arc plasma source is combined with a computer-controlled bias amplifier to synchronize substrate bias with the pulsed production of plasma. Accordingly, bias can be applied in a material-selective way. The principle has been applied to the synthesis metal-doped diamond-like carbon films, where the bias was applied and adjusted when the carbon plasma was condensing, and the substrate was at ground when the metal was incorporated. In doing so, excessive sputtering by too-energetic metal ions can be avoided while the sp 3 /sp 2  ratio can be adjusted. It is shown that the resistivity of the film can be tuned by this species-selective bias. The principle can be extended to multiple-material plasma sources and complex materials.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims benefit to the following U.S. Provisional Patent Application: 
         [0002]    U.S. Provisional Patent Application No. 60/970,855 entitled “FILTERED CATHODIC ARC DEPOSITION WITH ION-SPECIES-SELECTIVE BIAS”, by André Anders, filed Sep. 7, 2007, Attorney Docket No. NANO-01095US0. 
     
    
       [0003]    This invention was made with government support under Contract No. DE-AC03-76SF00098/DE-AC02-05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0004]    The present invention relates generally to film deposition. 
       BACKGROUND 
       [0005]    Substrate bias is a tool for controlling the energy of ions in plasma-based thin film growth. A space charge sheath forms between a substrate surface and the plasma bulk. Typically, the substrate bias is negative such that positive ions are extracted from the sheath-plasma edge and accelerated before they impact the substrate surface. Provided the sheath is much thinner than the mean free path between collisions, when traveling through the sheath ions gain the kinetic energy 
         [0000]      ΔE kin =QeΔV sheath    
         [0000]    where Q is the ion charge state number, e is the elementary charge, and ΔV sheath  is the voltage drop between plasma sheath edge and the substrate surface. By controlling the energy of ions, film properties such as density and stress are affected and can be tuned and optimized for the desired application. In the case of crystalline films, preferred orientation and texture can be influenced as well. 
         [0006]    Highly ionized plasmas are of interest to some coating processes because bias techniques can be effective, acting on ions but not on the neutral atoms. Examples of highly ionized plasmas are cathodic arc and high power impulse magnetron sputtering (HIPIMS) plasmas. 
         [0007]    Various forms of bias are known such as direct current (DC) bias, pulsed DC bias, radio frequency (RF) bias, etc. The bias acts on all ion species that enter the sheath from the plasma sheath edge. This characteristic of bias can be undesirable when ions of very different masses are involved because their sputter yields are different, even when their final energy at arrival on the substrate surface is the same. The issue can be aggravated when the heavier ion has a higher charge state. In this case, the energy is enhanced proportional to the charge state, and therefore the sputter yield is much higher by the combination of greater mass and higher energy. An excessive sputter yield can be detrimental to making the desired coating composed of light and heavy atoms. Therefore, it can be desirable to tune the bias in such a way as to adjust the value according to the species that are arriving. 
       SUMMARY 
       [0008]    A pulsed cathodic arc plasma source having two “triggerless” cathodes in a common anode body allows materials-selective bias by synchronizing the bias amplitude with the presence of the plasma of a specified material. The cathodes can comprise, in an embodiment, carbon and molybdenum. The bias voltage can be systematically adjusted for carbon only to obtain a target electrical resistivity of the growing film, while the metal deposition is not affect. Methods in accordance with the present invention using arc plasma sources with multiple cathodes and selective application of bias can allow a great variety of films and multilayers of mixed and complex composition to be formed. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]      FIG. 1  is a schematic of an embodiment of a system in accordance with the present invention using a dual-cathode cathodic arc plasma source and selective, synchronized initiation of arc pulses and bias pulses. 
           [0010]      FIG. 2  illustrates a cathodic arc plasma source with two cathodes in a water-cooled anode body. 
           [0011]      FIG. 3  is a flowchart for an embodiment of a method of forming a film in accordance with the present invention. 
           [0012]      FIG. 4  is a plot of resistivity of ta-C:Mo films with the pulsed bias acting only on carbon ions. 
           [0013]      FIG. 5  is a schematic of an alternative embodiment of a system in accordance with the present invention using a dual cathodic arc plasma sources and selective, synchronized initiation of arc pulses and bias pulses. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Metal-doped tetrahedral amorphous carbon films, usually designated as ta-C:Me, can be formed by filtered cathodic arc deposition. Cathodic arc deposition techniques generate an electrical arc that blasts ions from a cathode. Because ions are blasted from the surface of the cathode ballistically, it is common for not only single atoms, but larger clusters of atoms to be ejected. Cathodic arc deposition techniques use a filter to remove atom clusters from the beam before deposition. 
         [0015]    In order to produce a film rich in tetrahedral (i.e., diamond) bonding (also referred to herein as sp 3  bonding), carbon ions produced by a cathodic arc preferably have an energy of about 100-120 eV. Referring to Table 1, average ion charge state, particle charge state fractions, and the most likely “natural” kinetic energy of ions are shown for cathodic arc plasmas for a selection of elements. Because the “natural” kinetic energy of carbon ions is about 19 eV, negative bias of typically about 100 V is applied to produce a diamond-like film. If metal ions are produced by a cathodic arc, their most likely charge state is typically about 2 or 3 and their most likely “natural” energy typically exceeds 50 eV. When a negative bias of 100 V acts on the metal ions, their total kinetic energy can approach the range of 300-500 eV, which can cause severe sputtering. Moreover, the energetic impact of the metal ions on the surface can promote relaxation of sp 3  bonds to sp 2  bonds, thereby reducing the “diamond-likeness” of the film. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Material 
                 Q 
                 f 1   
                 f 2   
                 f 3   
                 f 4   
                 f 5   
                 f 6   
                 E kin  (eV) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 C 
                 1.00 
                 100 
                   
                   
                   
                   
                   
                 19 
               
               
                 Ti 
                 2.03 
                 11 
                 75 
                 14 
                   
                   
                   
                 59 
               
               
                 Cr 
                 2.09 
                 10 
                 68 
                 21 
                 1 
                   
                   
                 72 
               
               
                 Mo 
                 3.06 
                 2 
                 21 
                 49 
                 25 
                 3 
                   
                 149 
               
               
                 W 
                 3.07 
                 2 
                 23 
                 43 
                 26 
                 5 
                 1 
                 117 
               
               
                   
               
             
          
         
       
     
         [0016]    Referring to  FIG. 1 , embodiments of systems  100  for forming films on a substrate  120  in accordance with the present invention can comprise a cathodic arc plasma source  102  including a first cathode C 1  formed of carbon placed in close vicinity to a second cathode C 2  formed of a metal inside a common anode body  104 . A cathodic arc plasma source  102  is further shown in  FIG. 2 . The anode body  104  can be cooled to allow operation of the cathodic arc plasma source  102  with, in one implementation, pulses up to about 2 kA with duration of typically 1 ms, at a repetition rate of up to 10 pulses per second. At such operating parameters, the duty cycle is low (1% or less) and the average current and power are about 10 A and 200 W, respectively. Cathodic arc pulses can be initiated by applying an open-circuit voltage (typically 600 V), for example by an arc power supply  112 , to the selected cathode C 1 ,C 2 . Since no trigger electrode is required, the system can be said to employ “triggerless” triggering. 
         [0017]    The dual-cathode cathodic arc plasma source  102  can be used to inject streaming cathodic arc plasma into a plasma filter  108 , such as an open 90° filter, to remove unwanted macro-particles. A macro-particle “firewall”  118  within the chamber physically blocks particles emanating from the arc source and filter region. The cathodic arc plasma generated by the dual-cathode cathodic arc plasma source  102  can be selectively synchronized with a species-selective bias applied to an electrode  122  associated with the substrate  120  by a pulse generator  110 , which signal is amplified by a bias power amplifier  111 , to reduce excessive metal ion energy while having desirable carbon ion energy. Thus, for example, bias pulses can be applied when carbon ions arrive at the substrate  120  to have a desirable energy of about 100-120 eV to optimize the sp 3  content of film. The bias pulses can be reduced or omitted when metal ions arrive at the substrate  120 , thereby reducing sputtering and sp 3  bond relaxation. In an embodiment, synchronization can be controlled by a microcontroller such as an application specific integrated circuit (ASIC), or alternatively a general computing device  114  (e.g., a personal computer (PC)). Synchronization sequences can be defined by hardware and/or software. 
         [0018]    The plasmas paths inside the plasma filter  108  and at the plasma filter exit are offset because the injection points of plasma from the two cathodes C 1 ,C 2  are slightly different. If deposition occurs close to the filter exit without repositioning the substrate  120  onto which the film is deposited, two center regions of coatings are produced offset with respect to each other, with each center corresponding to one cathode material. The offset can be utilized if a combinatorial approach to materials research is desired, or reduced by increasing a distance from the filter exit to the substrate  120  (for example to more than 10 cm) and/or applying substrate motion. 
         [0019]    Referring to  FIG. 3 , an embodiment of a method in accordance with the present invention for forming a film on a substrate can comprise using a system including an electrode and a cathodic arc plasma source having an anode, a first cathode, and a second cathode (Step  100 ). A substrate is arranged on the electrode (Step  102 ) and a first arc plasma is generated from the first cathode (Step  104 ). A first bias is applied to the electrode to generate a desired energy of the ions in the first arc plasma (Step  106 ). A controller synchronizes the first bias and the first arc plasma, so that the appropriate bias is applied to the ions present in the arc plasma. A second arc plasma is generated from the second cathode (Step  108 ), and a second bias is applied to the electrode to generate a desired energy of the ions in the second arc plasma (Step  110 ). The controller synchronizes the second bias and the second arc plasma, so that the appropriate bias is applied to the ions present in the second arc plasma. A complex film can be built on the surface by sequentially performing the synchronized application of bias pulses and plasma generation, according to a sequence defined by a recipe, for example. 
         [0020]    Cathodic arc deposition was demonstrated using an experimental setup resembling the embodiment of  FIG. 1 . The experimental setup included a filtered dual-cathode cathodic arc system comprising a cathodic arc plasma source including a first cathode formed of carbon, a second cathode formed of molybdenum and a 90° open filter. Cathodic arc deposition was performed using the setup to produce a ta-C:Mo film on a substrate. An arc power supply was used to pulse the arc plasma between the two cathodes and a pulse generator connected with a bias power amplifier applied a bias to the substrate. A PC equipped with a National Instruments® virtual instrument with LabView® software was programmed to synchronize the bias pulses and arc pulses. The synchronization allowed writing recipes for the film composition and structure in which the number of arc pulses, their sequence, and the amplitude of bias were freely adjustable. The experimental setup allowed formation of a ta-C:Mo film where the carbon ions “saw” bias whereas the molybdenum ions arrived at an unbiased (grounded) substrate. In this sense, “species-selective” biasing was realized. 
         [0021]      FIG. 4  is a plot of resistivity of ta-C:Mo films with the pulsed bias acting only on the carbon ions (pulsed bias: 2 μs on and 6 μs off, for the duration of the presence of carbon plasma). The substrate was at ground when molybdenum plasma arrived. The molybdenum to carbon arc pulse ratio was kept constant at 1:20. The amplitude of the bias for carbon plasma was varied with the goal to tune the sp 3 /sp 2  ratio and thereby adjust the related optical and electrical properties, particularly the resistivity, while keeping the metal content at a constant level. Every 21 st  pulse was a metal pulse, and the metal was deposited with the substrate at ground. As can be seen, the resistivity was reduced as bias was increased to a level sufficient to provide desirable energy to the carbon ions, thereby forming a film rich in sp 3  bonding. 
         [0022]    Further embodiments of systems for forming films on a substrate in accordance with the present invention can include additional cathodes, thereby increasing the versatility and possibilities even further. In still further embodiments, additional anodes can be employed, so that multiple cathodic plasma arc sources  202 , 203  can be used. Such a system  200  is shown in  FIG. 5 . As shown, the system  200  includes two cathodes (C 1 ,C 2 ) with each cathode arranged in an anode body  204 , 205  and directed toward the substrate  220  through a corresponding filter  208 , 209 . Cathodic arc pulses can be initiated by applying an open-circuit voltage, for example by an arc power supply  212 , to the selected cathode C 1 ,C 2 . As above, the application of bias pulses to an electrode  222  by a pulse generator  210  is controlled by a PC  214  capable of sequencing or otherwise controlling the application of bias power to ions in the sheath. 
         [0023]    In still further embodiments, the background gas can be yet another source of material: the cathodic arc plasma sources can be used in reactive mode, producing compound films but utilizing the presence of reactive gases in the chamber (oxygen for oxides, nitrogen for nitrides, etc.). With these extensions, it is anticipated that a great variety of complex systems can be produced. For example, a system having three cathodes, an yttrium (Y) cathode, a barium (Ba) cathode, and a copper (Cu) cathode, operating in an oxygen background gas can be employed to synthesize YBCO high-T, superconducting films. Other examples of complex films that can be produced with embodiments of systems and methods in accordance with the present invention include transparent magnetic semiconductors such as ZnO:Cr, or transparent electronics based on ZnO:M, where “M” is a dopant that produces n-type or p-type conductivity and films including ternary oxides that show colossal magnetoresistance such as Nd 0.7 Sr 0.3 MnO 3 , or multiferroics such as BiFeO 3 , Bi 2 FeCrO 6 , BiCrO 3 , LaTiO 3 , and SrTiO 3 . 
         [0024]    Embodiments of systems for forming films on a substrate have been described herein as having bias pulses and cathodic arc plasma generation synchronized to enable sequentially deposition of materials to synthesize films with mixed material content or to deposit multilayers, as determined by the recipe of the process. In still further embodiment, systems and methods in accordance with the present invention can include simultaneous cathode operation using either a single power supply with a low-ohm distributing circuit, or by using two individual power supplies, each dedicated to a single cathode. Such simultaneous operation may be beneficial when the components need to react with each other rather than with the residual gas of the background vacuum.