Abstract:
The present invention is directed to an arrangement for switching high electric currents by way of a gas discharge at high voltages or for generating gas discharge plasma emitting EUV radiation. It is the object of the invention to find a novel possibility for generating a hollow cathode plasma that permits a longer life of the cathodes of short wavelength-emitting gas discharge radiation sources and pseudospark switches, also in high-power operation. This object is met in that the metal wall between the hollow cathode space and the discharge space has a thickness on the order of the centimeter range so that the openings of the metal wall change into relatively long channels and in that substantially radially extending cooling channels are introduced in the metal wall to reduce the ion erosion of the metal wall of the hollow cathode through efficient cooling.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority of German Application No. 10 2007 020 742.7, filed Apr. 28, 2007, the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    a) Field of the Invention 
         [0003]    The present invention is directed to an arrangement for switching high electric currents by way of a gas discharge at high voltages or for generating gas discharge plasma emitting EUV radiation, comprising an anode and a cathode which are both shaped so as to be hollow in a rotationally-symmetric manner and through which a discharge space is formed in the interior of the anode, wherein the cathode has a hollow cathode space for pre-ionization of a work gas and the hollow cathode space is delimited relative to the discharge space by a metal wall with a plurality of openings for streaming pre-ionized work gas into the discharge space, these openings being arranged at regular spatial intervals. It is applied particularly in gas discharge arrangements for generating plasma that emits EUV radiation in radiation sources for semiconductor lithography and pseudospark switches. 
         [0004]    b) Description of the Related Art 
         [0005]    Special gas discharge arrangements for generating short-wavelength radiation are operated by electrically pulsed high-power sources. In the simplest case, they are capacitors which are charged by line voltage equipment and then discharged when an electric contact is closed by suitable switches by means of a gas discharge arrangement. Peak currents of up to 50 kA at voltages of more than 5 kV with rates of current rise greater than 1 kA/ns must be handled. Pseudospark switches which are described, e.g., in U.S. Pat. No. 6,417,604 B1, U.S. Pat. No. 5,502,356 A, U.S. Pat. No. 5,126,638 A and U.S. Pat. No. 5,399,941 A are suitable for this purpose. 
         [0006]    Pseudospark switches are gas-filled discharge arrangements with electrodes comprising one or more discharge openings arranged in a suitable geometric manner. These openings cause directed, stable discharges. The purpose of using a plurality of discharge channels is to reduce the local current density. The gas pressure and electrode spacing are selected in such a way that the operating point lies on the left-hand side of the Paschen curve. The cathode is preferably shaped as a hollow cathode, and one or more trigger openings in an intermediate wall of the cathode make it possible to ignite a hollow cathode plasma. 
         [0007]    By abstracting from the physical functional principle, it can be seen that the essential difference between pseudospark switches and gas discharge radiation sources merely consists in that the latter has an additional anode opening for radiation emission. Therefore, the functionality (useful life) can be prolonged in both cases of application by improving the design of the hollow cathode. 
         [0008]    The conventional arrangements of gas discharge radiation sources and pseudospark switches have two substantial disadvantages which severely limit the life of the current-loaded electrodes:
   a) The geometry of known pseudospark switches and radiation sources based on hollow cathode gas discharges does not permit a high-power cooling of the cathode. High-power operation of such switches (repetition frequencies of greater than 4 kHz) requires the dissipation of an average heat output of several tens of kW.   b) The thickness of the metal wall which separates the discharge space from the hollow cathode space is usually about 1 to 3 mm. This severely limits the life of the cathode, which is exacerbated by the poor dissipation of heat.   
 
         [0011]    It is easily recognized that the cause of the short life of the cathode is the functionally important metal wall between the hollow cathode space and the main discharge space because, on one hand, it is the quickest to become worn in such a way as to impair function due to the ion erosion and, on the other hand, is simply too thin for a high-power cooling for reducing erosion. However, increasing the wall thickness, which would obviously substantially prolong the useful life of the wall against erosion, would bring about a change in the discharge behavior due to the substantial lengthening of the discharge holes in the cathode wall. 
         [0012]    In contrast to the conventional hollow cathode structure in which the cathode wall—as described, e.g., in U.S. Pat. No. 2006/0138960 A1—has a plurality of uniformly arranged openings in a sieve-like manner, the attainable current strengths are diminished when the wall thickness is increased because of the relatively long through-openings so that the hollow cathode plasma no longer leads to the desired stable gas discharge in the discharge space. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0013]    It is the primary object of the invention to find a novel possibility for generating a hollow cathode plasma that also permits a longer life of the cathodes of pseudospark switches and short wavelength-emitting gas discharge radiation sources in high-power operation, i.e., at a high average output of the pulsed gas discharge. 
         [0014]    In an arrangement for switching high electric currents by way of a gas discharge for generating gas discharge plasma emitting EUV radiation, comprising an anode and a cathode which are both shaped so as to be hollow in a rotationally-symmetric manner and through which a discharge space is formed in the interior of the anode, wherein the cathode has a hollow cathode space for pre-ionization of a work gas and the hollow cathode space is delimited relative to the discharge space by a metal wall with a plurality of openings for streaming pre-ionized work gas into the discharge space which are arranged at regular spatial intervals in order to provide spatially distributed base points of gas discharge paths through the openings for a high current flow through the discharge space, the above-stated object is met according to the invention in that the metal wall between the hollow cathode space and the discharge space has a thickness on the order of one centimeter so that the openings of the metal wall change into relatively long channels and the ends of the channels are directed to the discharge space on a common intersection point (S) in the discharge space, and in that substantially radially extending cooling channels are introduced into the metal wall to reduce the ion erosion of the cathode through efficient cooling. 
         [0015]    The openings of the channels to the discharge space are advantageously arranged in a uniformly distributed manner on at least one concentric circular line along the curved metal wall. Further, the channels in the metal wall have a consistent diameter which is substantially smaller in relation to the length of the channels at least within a portion converging at a common intersection point of the discharge space and which presents a discharge channel for orienting a plasma channel to be generated in the discharge space. 
         [0016]    In a construction which is particularly advantageous from the view point of manufacture, the channels are formed of channel portions which are collinear and channel portions which converge in the discharge space, the collinear channel portions proceeding from the hollow cathode space and passing into converging discharge channels. 
         [0017]    The collinear channel portions which start in the hollow cathode space advantageously have a greater diameter than the converging discharge channels, and only the converging discharge channels are formed with a defined ratio of diameter (D) and length (L). The ratio of diameter and length of the discharge channels is preferably between 0.1 and 0.15. 
         [0018]    Further, in an arrangement for switching high electric currents by way of a gas discharge in pseudospark switches comprising an anode and a cathode which are both shaped so as to be hollow in a rotationally-symmetric manner and through which a discharge space is formed in the interior of the anode, wherein the cathode has a hollow cathode space for pre-ionization of a work gas and the hollow cathode space is delimited relative to the discharge space by a metal wall with a plurality of openings for streaming pre-ionized work gas into the discharge space which are arranged at regular spatial intervals in order to provide spatially distributed base points of gas discharge paths through the openings for a high current flow through the discharge space, the above stated object is met in that the metal wall between the hollow cathode space and the discharge space has a thickness on the order of one centimeter so that the openings of the metal wall change into relatively long channels and the ends of the channels are oriented to the discharge space in a collinear to divergent manner in order that the gas discharge paths through the channels in the discharge space are spatially distributed as strictly directed plasma channels, and in that substantially radially extending cooling channels are introduced into the metal wall to reduce the ion erosion of the metal wall of the hollow cathode through efficient cooling. 
         [0019]    The openings of the channels to the discharge space are advisably arranged in a uniformly distributed manner on at least one concentric circular line along the curved metal wall. 
         [0020]    At least within a defined portion presenting a discharge channel opening into the discharge space, the channels in the metal wall advantageously have a uniform diameter which is substantially smaller than the length of the channels. 
         [0021]    When the metal wall is especially thick or when the inlet directions into the discharge space diverge, the inlet channels are advisably formed of collinear channel portions and channel portions which diverge toward the discharge space, wherein the collinear channel portions proceed from the hollow cathode space and pass into discharge channels diverging toward the discharge space. The collinear channel portions starting in the hollow cathode space have a greater diameter than the diverging discharge channels to the discharge space, and only the diverging discharge channels are formed with a defined ratio of diameter and length. The ratio of diameter and length of the discharge channels is advantageously between 0.1 and 0.15 in the uniform inlet channels as well as in the combined inlet channels. 
         [0022]    In both of the basic arrangements for switching high electric currents by way of a gas discharge, the cooling channels are advantageously arranged centrally between the discharge channels and mutually intersect for the purpose of reducing the ion erosion of the metal wall between the hollow cathode space and the discharge space. The coolant supply and coolant outlet are formed so as to be located opposite one another in a semicircular shape. 
         [0023]    For this purpose, the coolant supply and the coolant outlet are preferably formed as oppositely located grooves which are recessed into the rear end face of the cathode along a cylinder surface area. 
         [0024]    Each of the channels for streaming in work gas is advantageously enclosed by cooling channels which are arranged symmetrically in pairs, and all of the center axes of such coolant channel pairs intersect in the axis of symmetry of the hollow cathode. The cooling channels of a cooling channel pair are preferably introduced into the metal wall parallel to one another. 
         [0025]    The cathode is advisably made of a high-melting metal, preferably tungsten or molybdenum. 
         [0026]    But the cathode can also advantageously be composed of a cathode base body and an electrode collar, wherein only the electrode collar comprises the high-melting metal and the cathode base body is made of a metal with high thermal conductivity, preferably copper or a copper alloy. The boundary between the metal with high thermal conductivity and the high-melting metal advisably extends within the metal wall of the cathode. The cooling channels can be arranged inside the cathode collar as well as inside the cathode base body. 
         [0027]    The invention makes it possible to realize an arrangement for generating a hollow cathode plasma which permits a comparatively long life of the cathodes of short wavelength-emitting gas discharge radiation sources and pseudospark switches also in high-power operation, i.e., at a high average output of the gas discharge that is generated in a pulsed manner. 
         [0028]    The invention will be described more fully in the following with reference to embodiment examples. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    In the drawings: 
           [0030]      FIG. 1  shows a schematic view of the arrangement according to the invention in which the wall between the hollow cathode space and main discharge space is appreciably thicker in order to receive a cooling system; 
           [0031]      FIG. 2  shows a special construction of the cooling system of the hollow cathode with intersecting parallel double-channels in cross section (A-A) through the intermediate wall of the hollow cathode and in axial section (B-B) through the hollow cathode; and 
           [0032]      FIG. 3  shows a construction of the invention as a pseudospark switch with simplified cooling channel system analogous to  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    As is shown in  FIG. 1 , the arrangement for switching high electric currents, which is suitable for high-power current switching or for generating EUV radiation, has an anode  1 , which surrounds an anode interior  11  in a rotationally symmetric manner and is temperature-regulated in a conventional manner by an anode cooling system  12 , and a cathode in the form of a hollow cathode  2 . The hollow cathode space  21  is separated by a metal wall  22  from a discharge space  3  formed in the anode interior  11 . The metal wall  22  has a thickness in the centimeter range (preferably in the range of ≧1 cm) and is made of a high-melting material such as, e.g., tungsten or molybdenum, in view of the high thermal loading (at least at the surface facing the discharge space  3 ). 
         [0034]    The gas discharge arrangement is peripherally connected to a pre-ionization unit  4  which is arranged in the hollow cathode space  21  of the hollow cathode  2 , a pre-ionization generator  5 , and a main discharge pulse generator  6 . A gas supply unit  7  provides for the supply of a work gas to the hollow cathode space  21 , preferably via the pre-ionization unit  4 , and a vacuum system  8  provides a sufficient vacuum at least for the discharge space  3  or also for the environment of the entire electrode arrangement. 
         [0035]    Inlet channels  23  from the hollow cathode space  21  to the discharge space  3  are provided in the metal wall  22  for streaming in work gas that is ionized in the hollow cathode space  21  and are arranged in the metal wall  22  so as to be uniformly distributed, preferably symmetrically around the axis of symmetry  13 , in order to provide the most symmetric possible distribution of the base points F for current to exit from the hollow cathode  2  into the discharge space  3  during the main discharge inside the discharge space  3 . 
         [0036]    For an optimal discharge in the discharge space  3  in which plasma channels  31  are formed from ionized work gas streaming in in a directed manner, a determinately small ratio of diameter D and length L of about 0.1 to 0.15 is adjusted at least in some portion of the inlet channels  23  provided in the metal wall  22 . This dimensioning of the inlet channels  23  must be maintained obligatorily only for the portion of the channel which, as discharge channel  231 , determines the respective flow-out direction of the ionized work gas in the discharge space  2  and through which the (incipient) gas discharge initialized therein predetermines the forming of the directed “plasma channels”  31  in the discharge space. That is, the ratio of the dimensions D and L only concerns the (portion of the) discharge channel  231  oriented in the discharge space  3 . The “thicker” collinear input portions  232  of the inlet channels  23  (preferably constructed as collinear bore holes) which start in the hollow cathode space are to be attributed to the hollow cathode space  21  in terms of function. These input portions  232  are designed at the appropriate locations for connecting the hollow cathode space  21  to the discharge channels  231  so that—given a fixed position and length L of the discharge channels  231 —the metal wall  22  can be constructed with any thickness (e.g., also &gt;1 cm) for introducing the cooling lines  24 . 
         [0037]    With a thickness d in the centimeter range, the metal wall  22  constructed in this way between the hollow cathode space  21  and discharge space  3  substantially increases the usable life against erosion caused by ions occurring during the main discharge and has the advantage that suitable cooling channel geometries can be introduced into a metal wall  22  which allow metal wall thicknesses of 3 cm that successfully reduce erosion in continuous operation. According to the invention, the wall openings that are conventional in the prior art change into channels  23  of varying length depending on the thickness of the metal wall  22 . 
         [0038]    The primary aim of designing the metal wall  22  between the discharge space  3  and the hollow cathode space  21  to be thicker is to make available sufficient material for a known erosion rate (␣1 g cathode material/10 8  discharges). But at the same time this step can provide a previously unavailable material thickness for a direct cooling through cooling channels  24  inside the metal wall  22 . However, initial experiments with this hollow cathode shape with a thick metal wall  22  exhibited an appreciably reduced current flow through the discharge space  3 . 
         [0039]    Surprisingly, it was found that the cause of this was that the discharge channels  23  in the metal wall  22  of the hollow cathode  2  behave like individual tubular hollow cathodes without an intermediate wall and with a surface anode arranged at the front. For the latter configuration, NIKULIN (e.g., Tech. Phys. 44 6 (1999) 641) published the findings of extensive basic experiments in which a determined ratio of diameter and length of a tubular cathode shape was indicated as the condition for an optimal discharge behavior. 
         [0040]    With respect to the cathode shape according to the invention, it was proven that a different type of discharge takes place within the discharge space  3  for hollow cathodes  2  having an intermediate metal wall  22  when this metal wall  22  is constructed with a thickness d in the centimeter range, this discharge type changing from a discharge shape which is spatially distributed (through defined base points F at the openings in the metal wall  22 ) to a defined quantity of stable, strictly oriented channel discharges (plasma channels  31 ) of long tubular hollow cathodes (without an intermediate wall) which must be considered separately. Based on the tube dimensioning indicated by NIKULIN for the “free hollow cathode”, a way was found to adapt the discharge conditions to a hollow cathode plasma generated through long inlet channels  23  in which a high (pulsed) current flow via a defined quantity of very stably forming plasma channels  31  is achieved within the discharge space  3  by precise spatial orientation of discharge channels  231  having defined dimensions. 
         [0041]    Without loss of generality—particularly because of a diverging construction in pseudospark switches (see FIG.  3 )—the determinately dimensioned portions of the inlet channels  23 , i.e., the discharge channels  231 , are directed to a common intersection point S in an arrangement for generating EUV radiation in  FIG. 1  in order that the plasma which contracts during the discharge as a result of the current-induced magnetic field formation is focused for a high radiation yield in the spectral region of soft x-ray radiation (EUV) from the start. (For pseudospark switches, the principal goal at this point is a broad spatial distribution in the discharge space  3  according to  FIG. 3  in order to minimize the thermal heating). 
         [0042]    In an electrode arrangement according to  FIG. 1 , the ratio between diameter D and length L of the discharge channels  231  for plasma generation at intersection point S of the discharge space  3  can be optimized both with and without the pre-ionization unit  4  in the hollow cathode space  21 . 
         [0043]    To generate the dense, hot (radiating) plasma—as is shown in FIG.  1 —the inlet channels  23  are bent out so as to direct them to the common intersection point S in the axis of symmetry  13  of the discharge space  3 . Consequently, they are formed of different portions, a collinear portion  232  being formed (preferably drilled) from the hollow cathode space  21  into the metal wall  22  parallel to the axis of symmetry  13  and a converging portion, serving as discharge channel  231 , being oriented to the common intersection point S of all of the discharge channels  231  in the discharge space  3 . 
         [0044]    As can be seen particularly clearly in the bottom part of  FIG. 2  from the cross-sectional view through the hollow cathode  2  along plane A-A, cooling channels  24  for reducing the ion erosion of the metal wall  22  are arranged in the center between the inlet channels  23  which are arranged so as to be uniformly distributed (preferably on a circular line) around the axis of symmetry  13 . 
         [0045]    In a particularly advantageous construction which is shown in  FIG. 2  in the bottom cross-sectional view (along plane A-A of the upper axial section B-B), the cooling channels  24  are parallel to one another in pairs and enclose an inlet channel  23 , respectively, along their center line. The parallel pair of cooling channels  24  arranged in this way intersect a number of times, first between the inlet channels  23  and then within the circle formed by the inlet channels  23 , so that a maze of intersecting portions of the cooling channels  24  is formed inside the circle of the inlet channels  23 . 
         [0046]    Regardless of whether or not the cooling channels  24  cross or intersect one another as parallel pairs within a plane or in different planes (not shown) or extend as individual cooling channels  24  ( FIG. 3 ) crossing, e,g., in the axis of symmetry  13 , between the inlet channels  23 , the cooling channels  24  are substantially radially oriented and are connected at the periphery of the hollow cathode  2  to a semi-circular coolant supply  25  and a semicircular coolant outlet  25  which lie symmetrically opposite from one another. 
         [0047]    In the special construction according to  FIG. 2 , the coolant supply  25  is connected by a cylindrically-shaped connection groove  27  to one end of the cooling channels  24 , and the coolant outlet  26  is connected to its other end by a cylindrically-shaped connection groove  27  which is located opposite from it symmetric to the axis of symmetry  13 . The connection groove  27  is preferably cut into the hollow cathode  2  from the back side. 
         [0048]    An alternative variant for introducing the cooling channels  24  as intersecting individual channels—as is shown at bottom in  FIG. 3  for the construction of a pseudospark switch—can be used in an equivalent manner for the hollow cathode  2  shown at the top in  FIG. 2 . 
         [0049]    In order to improve the cooling power, the hollow cathode  2  can be composed of two different materials, a cathode base body  28  and a cathode collar  29  as is shown in axial section at top in  FIG. 2 . The electrode collar  29 , which is the current outlet surface of the hollow cathode  2  to the discharge space  3 , is manufactured from a high-melting material (e.g., tungsten, molybdenum, etc.) and the cathode body  28  which is preferably fixedly connected to the cathode collar  29  by the manufacturing technique of back-casting, is produced from a very highly heat-conducting material (e.g., copper, silver, etc., or alloys thereof). 
         [0050]    The cooling channels  24  advisably extend inside the cathode base body  28 , but can also be introduced (preferably additionally) in the cathode collar  29 . 
         [0051]      FIG. 3  shows a construction of the invention as a pseudospark switch. All of the fundamental principles and constructions according to  FIGS. 1 and 2 , with the exception of the open anode shape and the plasma channels  31  intersecting in the discharge space  3 , apply in this case. In this case, the anode  1  is designed so as to be closed and can be constructed in a pot-shaped manner. 
         [0052]    In this case, the inlet channels  23  of the hollow cathode space  21  to the discharge space  3  do not need to be divided into collinear portions and converging portions, but rather are discharge channels  231  considered as a whole, since a concentrated hot (radiating) plasma column need not be generated. The discharge channels are preferably constructed so as to diverge or—as is shown at top in FIG.  3 —in a collinear manner. For a divergent orientation, however, it may be necessary to provide “thicker” collinear input portions  232  in the metal wall  22  so as to adjust the required ratios of diameter D and length L of the discharge channels  231  proceeding from this metal wall  22  so as to curve outward. A corresponding curvature of the metal wall  22  must also be provided in this case. 
         [0053]    While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention. 
       REFERENCE NUMBERS 
       [0000]    
       
           1  anode 
           11  anode interior 
           12  anode cooling system 
           13  axis of symmetry 
           2  hollow cathode 
           21  hollow cathode space 
           22  metal wall 
           23  discharge channel 
           24  cooling channel 
           25  coolant supply 
           26  coolant outlet 
           27  connection groove 
           28  cathode base body 
           29  cathode collar 
           3  discharge space 
           31  plasma channel 
           4  pre-ionization unit 
           5  pre-ionization pulse generator 
           6  main discharge pulse generator 
           7  gas supply unit 
           8  vacuum system 
         F base point 
         d thickness (of the metal wall) 
         D diameter (of the discharge channel) 
         L length (of the discharge channel) 
         S (common) intersection point