Patent Publication Number: US-2005121321-A1

Title: Ignition device

Description:
The invention relates to an ignition device for igniting a high-current discharge of an electric arc vaporizer in a vacuum coating installation according to the preamble of claim  1 .  
      An arc vaporizer as described above, also referred to as arc or spark source, is employed for treating workpieces in high vacuum, in particular for plasma etching and/or for coating.  
      U.S. Pat. No. 3,783,231, U.S. Pat. No. 4,448,799 and U.S. Pat. No. 4,622,452 disclose various mechanical ignition fingers, in which the ignition of the arc discharge takes place by briefly placing the ignition finger onto the surface of the target, and, when withdrawing the ignition tip, a breaking spark is produced, which generates a sufficient number of electric charge carriers.  
      The disadvantage of such a device, currently utilized in many vacuum coating installations, is that at each arc source mechanically moved parts are disposed, which require a not inconsiderable adjustment and readjustment complexity as well as conventionally require dynamically loaded vacuum leadthroughs, which are frequently susceptible to failure. Furthermore, strong magnetic fields, such are frequently used in vacuum coating processes, can hinder the movement of the ignition finger. Due to the necessary robustness of the mechanical system, such devices are also conventionally implemented in a relatively large form, which necessitates an additional space requirement in the vacuum chamber.  
      EP 0 211 413 discloses another ignition mechanism. In this patent between the target surface (cathode) and the anode a ceramic insulator is disposed coated with a thin layer of electrically conductive material. After applying an ignition voltage between the cathode and the anode, the entire current flows over this thin conducting layer, whereby it vaporizes in one burst and charge carriers are liberated which make possible the igniting of the arc. After the coating process is switched off, a thin layer remains on the insulator, which allows the repeat ignition of the arc. Of disadvantage of such an ignition device is that it can only be employed in the production of layers which are sufficiently conductive.  
      The invention addresses the problem of providing an ignition device for an electric arc vaporizer which avoids the disadvantages of prior art.  
      This problem is solved through the characteristics according to the invention in the characterizing clause of claim  1 .  
      The ignition finger is installed such that the tip is permanently in contact with the target surface, which permits implementing the ignition device in a highly simple manner and without complex movable parts. In spite of the mechanically simple solution, in which a periodic readjustment of mechanical components is no longer required, the reliable ignition of the spark source is surprisingly successful even if, for example, high-ohmic layers are being deposited.  
      Moreover, in the case of such an ignition device no susceptibility to failure could be detected even in the presence of strong magnetic fields. 
    
    
      In the following the invention will be explained in further detail in conjunction with Figures illustrating several embodiment examples. In the drawing depict:  
       FIG. 1  section through a vacuum chamber with arc source and ignition device, and  
       FIG. 2  ignition device. 
    
    
       FIG. 1  shows schematically the functional principle of an arc source installed in a vacuum chamber  7  with an ignition device  1  according to the invention. In contrast to the above described prior art, the ignition finger is continuously in electrically conducting contact on the target surface even during the operation of the spark source. In order to ensure the continuous electrical contact in spite of the high mechanical stresses occurring during the operation due to temperature, and therewith to ensure a reliable ignition process, the tip must be pressed against the target surface, for example via a spring mechanism.  
      To apply the ignition potential, a switch  2  is closed, and an electric current flows supplied, in this case, from an ignition generator  3 , over a circuit I 2 . If a sufficiently high voltage is applied, due to the relatively high contact resistance between the ignition tip  10  and the target  9 , local melting and vaporization of the target surface  20  occurs at ignition point  5 . Additionally, during the ignition of the arc source, presumably through the tip effect occurring at high energy density and/or through microsparks, ionization of the metal vapor occurs, whereby the source circuit  11  fed from the generator of the arc supply  4 , between the target  9 , here connected cathodically, and the anode  6  is closed through the discharge plasma of the arc source.  
      The switch  2  should be selected such that switch-on times of a second or less are possible in order to avoid the thermal overloading of the ignition device  1 . During the tests the arc source could be ignited reliably at an ignition duration t zünd  between 0.05 to 1 s.  
      The application of the ignition potential can fundamentally also take place in simpler manner than through an additional ignition generator  3 , which, however, offers the advantage of a variable adjustable voltage. For example, in circuit I 2 , it is possible to switch to the zero potential or to the positive output signal of the arc supply  4 , which, as shown in  FIG. 1 , can also be connected to the anode  6 , instead of the ignition generator  3 . The setting of the desired ignition voltage can in this case take place via corresponding resistors R.  
      In preliminary tests, it was possible to generate the ignition of the arc at an ignition potential between +20 and +250 V relative to the cathode. For continuous tests the ignition potential was set between +100 and +180 V.  
      A further important point is the selection of a suitable material for the ignition finger  1 ′. Since the geometry of the ignition tip is to be approximately retained, in order to maintain the contact resistance to the target surface at a constant high level, it is important to vaporize the target surface and not the tip of the ignition finger. The material, of which at least the tip of the ignition finger must be comprised, must therefore have a melting point which is markedly higher, preferably by a few 100° C. than that of the target material and must not tend to form alloys with the target material at the ignition temperature. In order to permit universal applicability, the material should be thermally and mechanically stable at least at temperatures above 1000° C., but preferably still at temperatures of 2000° C. to 3000° C.; the material should consequently have a high melting point. In particular refractory metals are possible for use here, such as for example Zr, Nb, Mo, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, Pt or alloys of these metals or conductive compounds with nonmetals, such as for example tantalum or tungsten carbide.  
      Mechanically stabilized high carbon materials, such as for example carbon laminates, have also been found to be especially suitable.  
      In  FIG. 2 , a schematic example is shown of an embodiment in practice of an ignition device  1  according to the invention. It is comprised of a multi-part ignition finger  1 ′, fastened on a plug-in arm, as well as a plug connection  16 , fastened on the anode and receiving the plug-in arm  15  under insulation, which connection is connected with the feed line  17 . Therewith, for example for a target exchange, the ignition finger  1 ′ can simply be detached from the arc source in a short time.  
      In the present embodiment an industrial arc source, not further depicted in the drawing, with a target diameter of approximately 160 mm, such as is used in Balzers coating installations of type RCS or BAI 1200, was modified. In order to keep the space requirement of the ignition device  1  as low as possible, a bore was introduced into the encompassing anode  6  and the delimiting or confinement ring  18 . In the bore of the anode  6  was mounted the plug connection  16 , connected to the feed line  17 , for receiving the plug-in arm  15 . All parts  15 ,  16 ,  17  guided into the anode ring were electrically isolated from the anode  6  through an insulation  19 .  
      The diameter of the bore of the confinement ring  18 , electrically insulated with respect to the cathode, is laid out such that the gap between the outer diameter of the plug-in arm  15  and the inner diameter of the bore of the confinement ring  18  is minimally smaller than the dark space in conventional process conditions, i.e. approximately between 0.3 to 2 millimeters. Alternatively, here an insulation can also be provided, which, however, must withstand the high temperatures occurring here and the thermomechanical stresses resulting therefrom.  
      The ignition finger  1 ′ itself is structured in several parts. The housing  13  contains an advance device comprised of a spring  12  and a guide disk  11 , whereby the tip  10  presses with substantially constant force onto the target surface  20 . The spring  12  can additionally be preloaded through a setting screw  14 .  
      In the structural form realized here the housing  13  was implemented cylindrically with a diameter of 12 mm and a height of 25 mm.  
      The ignition tip  10  with a diameter of approximately 3 mm was fabricated of a readily workable laminated carbon composite material by SGL-Carbon and sharpened at the end in contact on the target  9 . It was subsequently plugged into the guide disk. The electric resistivity of the material at ambient temperature was between 20 and 30 Ohm×micrometer depending on the manufacture. In principle, the use of materials with lower or higher resistivity is also possible, since the contact resistance, as readily understood by a person skilled in the art, in the final analysis is the decisive factor is a result of the conductivity and the geometry of the tip. For high-carbon tips  10  a range of conductivity from 10 to 40 Ohm×micrometer can be covered.  
      Refractory metals indicate a lower resistivity of 0.04 to 0.12 Ohm×micrometer, which was the reason for selecting for such tests a narrower, to some extent needle-shaped, geometry of the tip  10 .  
      For the spring  12 , which during the operation of the target is located in the proximity of the plasma, the choice of the proper temperature-resistant material is essential. Especially suitable have been found to be springs  12  of chromium-nickel spring steel alloys, such as for example DIN X12CrNi177, 1.4310, with a spring rate between 0.2 to 3.0 N/mm, in particular between 0.5 to 1.0 N/mm.  
      The function tests were carried out with Ti, Cr or TiAlN targets. The electric resistance R was set in the range about 2 Ohms. At this value and at an ignition voltage about approximately 140 V briefly a current of approximately 70 A was measured. The switching mechanism was realized via a contactor, the closing and opening times being in the range of a few 100 milliseconds.  
      Under these conditions, an arc source could be ignited multiple times without encountering problems with an ignition device  1 , as described in detail in connection with  FIG. 2 , under an Ar or N 2  atmosphere at pressures around 5×10 −2  mbar.  
      A further structural form of the ignition device can be attained if the ignition mechanism is disposed laterally, thus at the edge region  8  of target  9 . However, here it is additionally necessary to ensure, for example through a magnetic guidance, that the spark is guided rapidly from the edge region  8  onto the surface  20  of the target in order to avoid damages of the insulating parts located between confinement or anode ring and target.  
      List of Reference Numbers  
     
         
           1  Ignition device  
           1 ′ Ignition finger  
           2  Switch  
           3  Ignition generator  
           4  Arc supply  
           5  Ignition point  
           6  Counterelectrode/anode  
           7  Vacuum chamber  
           8  Outer target region/edge region  
           9  Target  
           10  Tip  
           11  Guide disk  
           12  Spring  
           13  Housing  
           14  Setting screw  
           15  Plug-in arm  
           16  Plug connection  
           17  Supply line/feed line  
           18  Delimiting ring/confinement ring  
           19  Insulation  
           20  Target surface