Abstract:
The invention is directed to an arrangement for the generation of intensive short-wavelength radiation based on a gas discharge plasma. It is the object of the invention to find a novel possibility for generating intensive short-wavelength radiation, particularly EUV radiation, based on a gas discharge plasma which achieves a long life of the electrode system along with a high total efficiency of the radiation source without substantially increasing the dimensions of the discharge unit. This object is met, according to the invention, in that exclusively suitably shaped vacuum insulation areas which have the shape of an annular gap and which are formed depending on the product of gas pressure (p) and interelectrode distance (d) between the cathode and anode are provided for insulating the cathode and anode from one another in a cylindrically symmetric electrode arrangement for reliable suppression of electron arcing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority of German Application No. 10 2005 025 624.4, filed Jun. 1, 2005, the complete disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     a) Field of the Invention  
         [0003]     The invention is directed to an arrangement for the generation of intensive short-wavelength radiation based on a gas discharge plasma, preferably as a source of EUV radiation. The invention is applied in particular in high-power radiation sources for ELV lithography which requires radiation sources with electrodes having a long life in the process of industrial fabrication of semiconductor chips.  
         [0004]     b) Description of the Related Art  
         [0005]     In semiconductor technology, there is a continuing trend toward increasingly smaller structures, and radiation with increasingly shorter wavelengths is required for lithographic generation of these structures. At present, EUV radiation sources, viewed as the most promising lithographic tool, are being developed. Basically, there are two different ways of generating the radiating plasma: by laser (LPP) and by gas discharge (GDPP).  
         [0006]     Various arrangements are known from the prior art relating to gas discharge-based EUV radiation sources, namely, Z-pinch, plasma focus, star pinch, hollow-cathode discharge arrangements, and capillary discharge arrangements. Further, there are variations in the above-named discharge types (e.g., hypocycloidal pinch discharge) and arrangements that combine elements of different discharge types. In all of these arrangements, a pulsed high-power discharge of &gt;10 kA is ignited in a gas of determined density, and a very hot (kT&gt;30 eV), dense plasma is formed locally as a result of the magnetic forces and dissipated power in the ionized gas.  
         [0007]     However, the radiation sources must satisfy precisely defined requirements for use in EUV lithography under production conditions:  
                                                       1. wavelength   13.5 nm ± 1%           2. radiation output in the intermediate focus   115 W           3. repetition frequency   7-10 kHz           4. Dose stability (averaged over 50 pulses)   0.3%           5. life of the collector optics   6 months           6. life of the electrode system   6 months.                      
 
         [0008]     It is standard for high-power EUV gas discharge sources of the type mentioned above to have a special ceramic disk or cylinder as an insulator between the electrodes. For example, U.S. Pat. No. 6,414,438 B1 discloses a method and arrangement by which short-wavelength radiation is generated from a gas-discharge plasma in that a pre-ionization of the work gas takes place between coaxial electrodes as a sliding discharge on ceramic surfaces which emits UV radiation and fast electrons, and the ionized gas is conducted through an axial aperture of one of the electrodes in the gas discharge area, where it ignites the main discharge.  
         [0009]     WO 03/087867 A1 discloses another high-energy photon source that generates EUV radiation in the range of 12-14 nm. In order to limit erosion of the electrodes, particularly of the center electrode and, therefore, to increase the lifetime of the electrodes, cylindrical insulators are arranged at the side walls of the center electrode so that the discharge current after pinch ignition is shifted over a larger area to another portion of the electrode. It is described as particularly advantageous that the center electrode is covered on the inner side and outer side with insulating tubing.  
         [0010]     DE 101 51 080 C1 describes similar tubular insulator configurations that are also added to the inner wall of the outer electrode. Further, different materials are also indicated for this purpose. It is evident that while all of these insulator tubes limit the erosion of the electrodes to determined surface zones, the lifetime of the insulator/electrode configurations is appreciably shortened through cracking and metallization, particularly with high pulse repetition frequencies of the EUV gas discharge source.  
         [0011]     For various reasons, the arrangements mentioned above always only meet the above-mentioned requirements (1-7) in a few respects. This can be explained using the example of star pinch discharge which, in itself, is advantageous. Because of the comparatively large distances between plasma and wall (which represents a severe problem in all of the other arrangements due to the otherwise small dimensions), the star pinch arrangement is characterized by a long electrode lifetime. However, the large dimensions of the star pinch discharges cause a luminous plasma with a length of more than 5 mm. This considerably reduces the efficiency of the collector optics and, therefore, the overall efficiency as a quotient of the output in the intermediate focus and the electric power introduced for the discharge. Electrode configurations which employ additional insulator tubes because of their short distances in order to improve the constant, stable generation of the plasma suffer in principle from premature failure of the insulator.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0012]     It is the primary object of the invention to find a novel possibility for generating intensive short-wavelength radiation, particularly EUV radiation, based on a gas discharge plasma which achieves a long life of the electrodes along with a high total efficiency of the radiation source without substantially increasing the dimensions of the discharge unit.  
         [0013]     In an arrangement for the generation of EUV radiation based on a gas discharge plasma in which a cathode and an anode are arranged in a cylindrically symmetric manner and a pre-ionized work gas is supplied to the cathode end, this object is met, according to the invention, in that exclusively suitably shaped vacuum insulation areas which have the shape of an annular gap and which are formed depending on the product of the gas pressure and the interelectrode distance of the cathode and anode for reliable suppression of electron arcing are provided for insulating the cathode and anode from one another.  
         [0014]     A device for pre-ionization of the work gas is advantageously provided within the centrally arranged cathode. The anode is preferably an annular electrode which encloses at least the cathode end with a small interelectrode distance and the discharge chamber.  
         [0015]     For pre-ionization of the work gas, a pre-ionization electrode with a projecting tubular insulator is advisably arranged in a centrally symmetric manner inside the cathode and opens into a cavity of the cathode. A surface sliding discharge can be generated at the insulator by a pre-ionization pulse between the pre-ionization electrode and the cathode so that the work gas which is ionized in this way flows out of the cavity via at least one through-channel at the cathode end into the discharge chamber, where it is converted into dense, hot plasma by a main discharge pulse. In this connection it should be noted that the ceramic insulator of the pre-ionization electrode needed for the surface sliding discharge is subject to comparatively very little electrical stress because the electrical energy that is dissipated per discharge (about 1 mJ) during pre-Ionization is only about one thousandth of the dissipated pulse energy of the main discharge (&gt;10 J).  
         [0016]     In a basic variant, only one through-channel is provided coaxial to the axis of symmetry of the discharge space. However, a plurality of uniformly distributed through-channels are directed along an outer conical surface through a common point on the axis of symmetry on an inner surface of the anode. The through-channels can also be combined to form an annular gap.  
         [0017]     The cathode end is advisably provided with a rounded electrode collar which projects into the interior of the anode that circles the discharge chamber. The vacuum insulation area located between the anode and cathode is protected against debris particles from the plasma and against electrode consumption by the electrode collar.  
         [0018]     Further, it is advantageous that the cathode end inside the electrode collar has a concave shape and is the location where the dense, hot plasma is formed. A pocket hole or a through-hole is advisably incorporated at the center of the concave curvature of the cathode to distribute the ion beam exiting from the plasma to a larger surface.  
         [0019]     The cathode advantageously has a small cavity as pre-ionization chamber and long through-channels which are arranged coaxially and shaped in such a way that, at the cathode end in the discharge chamber, primary electrically conducting ionization channels are directed through a common point on the axis of symmetry of the discharge chamber to a surface of the anode. The intersection point determines the preferred location of the luminous plasma.  
         [0020]     In another advantageous construction, the cathode has a large cavity and short through-channels. The cavity extends to the vicinity of a concave cathode end, and the through-channels are arranged in such a way that primary electrically conducting ionization channels are directed through a common point on the axis of symmetry of the discharge chamber to a surface of the anode from the ionized work gas flowing into the discharge chamber.  
         [0021]     In a first variant, a surface discharge used for the pre-ionization of the work gas is advisably provided at the inner side of the insulator, and the pre-ionization electrode is constructed so as to be shorter than the tubular insulator and with a central gas inlet inside the tubular insulator.  
         [0022]     In a second variant, the surface discharge used for the pre-ionization of the work gas is advantageously provided on the outer side of the insulator, and the pre-ionization electrode projecting into the cavity of the cathode is constructed with a central gas inlet and a tubular insulator located on the outer side.  
         [0023]     In another variant, the cavity of the cathode is expanded in width and, in the shape of a spherical hood, is provided with short through-channels over the concave cathode end which are directed to a common point of the axis of symmetry.  
         [0024]     In another advantageous construction, the cavity of the cathode is shaped so as to taper conically toward the cathode end and is provided directly with the gas inlet and has a circular opening at the concave cathode end. The pre-ionization electrode is inserted coaxially into this opening and leaves open an annular gap to the discharge chamber through which the work gas is directed in the shape of an outer cone surface to a point on the axis of symmetry in primary electrically conducting ionization channels.  
         [0025]     In this case, the pre-ionization electrode has a pocket hole at its surface facing the discharge chamber in the axis of symmetry and also advantageously has its own cooling channels.  
         [0026]     In another construction having a cavity that tapers conically toward the cathode end and a circular opening of the cathode, the pre-ionization electrode is advantageously snugly inserted into the opening with inner and outer insulators. The pre-ionization electrode has a plurality of gas inlets which are directed to the surface of the anode as through-channels between inner and outer insulators through a common point on the axis of symmetry of the discharge chamber.  
         [0027]     In another construction, an auxiliary electrode which is insulated from the cathode is advantageously inserted into the cavity of the cathode. The auxiliary electrode has the cavity provided for the pre-ionization of the work gas, and the pre-ionization electrode with outer insulator is arranged so as to project into the cavity of the auxiliary electrode, and at least one corresponding through-channel is provided in the cathode and auxiliary electrode for the exit of the pre-ionized work gas.  
         [0028]     For this purpose, a plurality of corresponding through-channels are advantageously arranged along an outer conical surface, whose tip lies on the axis of symmetry of the discharge chamber, from the cavity to the discharge chamber in the auxiliary electrode and the cathode to form primary ionization channels in the discharge chamber. In addition, the auxiliary electrode is insulated from the cathode end by another cavity.  
         [0029]     In order to increase the dielectric strength of the vacuum insulation, particularly with larger interelectrode distances, the vacuum insulation space (which has larger dimensions) has additional means for generating a magnetic field, and the flux lines of the magnetic field are oriented orthogonal to those of the electric field between the anode and cathode.  
         [0030]     For this purpose, concentric magnet rings are advantageously arranged inside and outside the vacuum insulation space between which the magnetic field is formed in radial direction. A body is formed at one of the electrodes (e.g., the anode) toward the transition area in order to prevent inhomogeneities in the electric field between the anode and cathode.  
         [0031]     In a second embodiment form, concentric magnet rings are arranged inside and outside the vacuum insulation space, around which are formed two opposed, circularly extending magnetic fields, and a body is likewise formed at the inner magnet ring to prevent inhomogeneities in the electric field in the transition area.  
         [0032]     In order to generate magnetic fields of suitable strength, the concentric magnet rings are advantageously constructed in the form of a plurality of individual, annularly arranged permanent magnets, preferably NdFeB magnets. However, the concentric magnet rings can also be constructed as a plurality of annularly arranged electromagnets.  
         [0033]     In another embodiment form, a pre-ionization unit has through-channels to a transition area between the vacuum insulation space and discharge chamber, and the work gas that is pre-ionized in this way is introduced into the discharge chamber through the narrow transition area of the vacuum insulation between the cathode and anode and is contracted by the main current pulse to form hot, dense plasma.  
         [0034]     In another advantageous construction of the invention, gas inlets are arranged at the outer vacuum insulation space which has a large interelectrode distance between the cathode and anode, and the gas pressure and interelectrode distance are adjusted in such a way that a spontaneous ignition can be carried out exclusively on the left-hand branch of the so-called Paschen curve, and the product of gas pressure and interelectrode distance is selected in such a way that the breakdown voltage exceeds a minimum value which depends upon the work gas that is used.  
         [0035]     In an advantageous manner, grooves or similar structures are additionally incorporated in the outer vacuum insulation space in at least one of the oppositely located electrode surfaces of the cathode and anode for locally increasing the interelectrode distance for the purpose of a local increase in the product of gas pressure and interelectrode distance and to initiate the spontaneous ignition in a plurality of primary ionization channels.  
         [0036]     In all of the preceding constructions of the invention, it is advantageous when at least the cathode and anode are outfitted with cooling channels for cooling. In arrangements in which additional auxiliary electrodes are provided for pre-ionization of the work gas, these auxiliary electrodes are also provided with cooling channels in an advantageous manner, at least when they extend directly up to the discharge chamber. Deionized water is preferably used as coolant.  
         [0037]     The arrangements for gas discharge-pumped generation of radiation in the range of 13.5 nm advantageously use xenon, lithium vapor or tin vapor, or gaseous tin compounds as work gas.  
         [0038]     The basic idea of the invention stems from the consideration that the lifetime of the electrode system of a radiation source based on gas discharge cannot be significantly increased by ceramic insulators which, while limiting the electrode consumption to certain areas, form cracks within a relatively short time due to the high thermal loading or acquire conductive surfaces because they are spattered by eroded electrode material so that the electrode system must be exchanged. Based on this fact, the invention provides a vacuum insulation of the electrodes; however, due to the gas supply lines, suitable pressures and interelectrode distances must be used because the breakdown voltage depends upon the product of the interelectrode distance and pressure level. A number of suitable forms of excitation for generating a pre-ionization in the form of primary (electrically conductive) ionization channels of ionized work gas which are directed into the discharge chamber are described in the following.  
         [0039]     The invention makes it possible to provide arrangements for generating intensive short-wavelength radiation, particularly EUV radiation, based on a gas discharge plasma which allow the lifetime of the electrode system to be increased appreciably with a high total efficiency of the radiation source and with comparable dimensions of the discharge unit.  
         [0040]     In the following, the invention will be described in more detail.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]     In the drawings:  
         [0042]      FIG. 1  shows a basic diagram of the arrangement according to the invention;  
         [0043]      FIG. 2  shows the prior art;  
         [0044]      FIG. 3  shows a variant of the invention with a pronounced cathode cavity serving to pre-ionize the work gas and with through-holes to the discharge chamber so that conducting channels which are oriented in a defined manner are formed in the discharge chamber for the main discharge;  
         [0045]      FIG. 4  shows a modified construction with respect to  FIG. 3  having an enlarged cathode cavity and shorter through-holes to the discharge space, wherein the surface discharge for pre-ionization is carried out already within the gas feed at the inner side of a ceramic tube;  
         [0046]      FIG. 5  shows a changed variant with respect to  FIG. 3  having a cathode cavity that is specially shaped in a spherically symmetric manner around the center of the discharge space and with very short through-holes to the discharge space;  
         [0047]      FIG. 6  shows a variant that is appreciably modified from  FIG. 3 , wherein the cathode cavity is formed as an annular chamber in which the pre-ionization electrode is inserted as a centrally symmetric rod into the cathode cavity, and the through-holes to the discharge space are accordingly transformed into a conical annular gap;  
         [0048]      FIG. 7  shows a modification of the construction according to  FIG. 6 , wherein the pre-ionization electrode with an outer ceramic tube generates a sliding discharge surface which is oriented as an outer cylindrical surface in direct line of sight to the center of the discharge space;  
         [0049]      FIG. 8  shows another modification of the construction in  FIG. 6 , wherein the pre-ionization electrode has a ceramic part with through-channels to the discharge space, the sliding discharge taking place along the surfaces of the thorough-channels in “visual contact” with the center of the discharge space;  
         [0050]      FIG. 9  shows a modification of  FIG. 5  with an auxiliary electrode which is arranged inside the cathode and forms the cathode cavity and enables a separation of the electrodes for the pre-ionization of the main discharge electrodes, anode and cathode;  
         [0051]      FIG. 10  shows a constructional variant in which the breakdown voltage is increased by providing a radially oriented magnetic field in the vacuum insulation area with a large interelectrode distance;  
         [0052]      FIG. 11  shows a constructional variant in which the breakdown voltage is increased by arranging two oppositely located, circularly oriented magnetic fields in the vacuum insulation area with a large interelectrode distance;  
         [0053]      FIG. 12  shows another construction of the invention in which the narrow transition area of the vacuum insulation is used for introducing the primary ionization channels into the discharge chamber, wherein the through-channels of the pre-ionization unit are introduced in the transition area; and  
         [0054]      FIG. 13  shows a construction of the invention without pre-ionization in which the vacuum insulation space (with a large interelectrode distance) is expanded locally in a deliberate manner so as to effect a spontaneous ignition of the work gas flowing into it. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]     As is shown in  FIG. 1 , the basic arrangement according to the invention contains a discharge chamber  1  which is formed by the main electrodes  2  (cathode  21  and anode  22 ) and a cooling jacket  15  through which a suitable coolant flows, a main pulse generator  3  for the high-voltage gas discharge, which main pulse generator  3  is connected to the main electrodes  2 , a pre-ionization pulse generator  4  for pre-ionization (for initiating the main discharge) which is connected between a pre-ionization electrode  51  and one of the main electrodes  2  (cathode  21  or anode  22  depending on the polarity of the main pulse generator  3 ), and a gas supply unit  6  for supplying work gas to the vacuum chamber  1 . The main pulse generator  3  has a low-inductance discharge circuit (not shown) which is constructed in such a way that the polarity at the cathode  21  and anode  22  can easily be changed.  
         [0056]     According to the invention, the insulation between the cathode  21  and anode  22  is achieved exclusively by an evacuated transition  14  which is arranged between the discharge space  12  and the vacuum insulation space  13  and is shaped as an outer surface of a cone. An interelectrode distance of &lt;1 mm is adjusted in the transition area  14 .  
         [0057]     Particles resulting from electrode consumption are prevented as far as possible from entering the evacuated zone leading up to the vacuum insulation space  13  by means of at least one rounded electrode collar  23  of the center electrode (cathode  21  or anode  22 ) which is rounded with a large radius in the discharge chamber  12  before the conical transition area  14 . This prevents excessive field strengths at the edges. The outer electrode (anode  22  or cathode  21 , depending on polarity) preferably also has rounded edges.  
         [0058]     The cathode  21  and anode  22  each contain at least one opening. The opening in the cathode  21  makes it possible for UV radiation, high-energy ions and electrons formed by the sliding discharge  53  (pre-ionization process), as well as other work gases, to enter the discharge space  12 , and the opening in the anode  22  forms a free solid angle for the outlet of the desired EUV radiation.  
         [0059]     The entire vacuum chamber  1  with the electrode configuration is constructed in a cylindrically symmetric manner with reference to an axis of symmetry  11  (of an axis arranged within the drawing plane).  
         [0060]     The current fed through the main pulse generator  2  generates a very hot (kT&gt;30 eV) and dense plasma  7  through resistance heating and through magnetic forces. This plasma  7  emits radiation in the desired spectral region (e.g., EUV region between 12.5 nm and 14 nm).  
         [0061]     The pre-ionization pulse generator  4  and the pre-ionization electrode  51  and a main electrode  2  (preferably cathode  21 ) can be used with any desired shapes of electrode analogous to the following examples. Xenon, tin vapor or lithium vapor, or gaseous tin compounds and lithium compounds can be used as work gas in all cases. Further, buffer gases are advisably mixed in to increase the efficiency of EUV radiation production on one hand and to achieve an advantageous deceleration of the fast particles from the plasma  7  on the other hand so as to improve the protection of the first collecting optics (not shown).  
         [0062]     After applying a pre-ionization voltage supplied by the pre-ionization pulse generator  4  to the pre-ionization electrode  51  and the cathode  21  for pre-ionization (for initiating the main discharge), a surface sliding discharge  53  takes place via a tubular ceramic insulator  52 . The surface discharge  53  is located on the inner side of the cylindrical insulator  52 . It generates high-intensity electron radiation, UV radiation, and x-ray radiation which pre-ionizes the gas in a through-channel  24  of the cathode  21  and transforms it into a conductive pre-plasma in the discharge chamber  12 .  
         [0063]     The conductive pre-plasma formed in the discharge chamber  12  is heated to the required temperature kT&gt;30 eV during the main discharge by magnetic compression and forms luminous plasma  7 .  
         [0064]     Total electrode insulation is ensured by the evacuated conical transition area  14  (pressure p&lt;15 Pa, interelectrode distance d&gt;0.5 mm) between the discharge chamber  12  and vacuum insulation space  13 .  
         [0065]     The rounded electrode collar  23  of the cathode  21  prevents excessive field strength at sharp edges due to its shape and prevents sputter particles of the cathode  21  from entering the evacuated conical transition  14  and the vacuum insulation space  13  of the vacuum insulation from the discharge chamber  12 .  
         [0066]     In both of the constructions shown in  FIG. 2  to  FIG. 5 , the cathode  21  has a cavity  25 . This cavity  25  serves to shape the electric flux lines in a suitable manner particularly in the through-channels  24  to the discharge chamber  12 . The through-channels  24  cause primary electrically conducting ionization channels  16  (shown in dashed lines), along which the main discharge current flows, to be formed in the discharge chamber  12 . In contrast to conventional hollow-cathode arrangements (e.g., according to WO 02/082871 A1 or WO 2004/019662), the connection between the cavity  25  and discharge space  12  is implemented in the present arrangements by means of through-channels  24  (e.g.,  FIG. 3 ) or by means of an annular gap  26  (see  FIG. 6 , for example) which create defined ionization channels  16  for the ignition of the main discharge pulse. These through-channels  24  are arranged on a sufficiently large circular circumference for reducing the thermal load per area unit. The same condition also applies to the shape of an annular gap  26  from the cavity  25  to the discharge chamber  12 .  
         [0067]     As was described with reference to  FIG. 1 , the cathode  21  and anode  22  are separated by a vacuum insulation comprising the vacuum insulation space  13  and evacuated transition area  14  leading up to the discharge chamber  12 , and the cathode  21  is provided with a rounded electrode collar  23  to prevent eroded electrode material from entering the transition area  14  and vacuum insulation space  13 .  
         [0068]      FIG. 3  shows a cathode  21  with long through-channels  24  from a relatively small cavity  25  to the discharge chamber  12 . After applying the pre-ionization voltage to the pre-ionization electrode  51 , a surface discharge  53  (sliding discharge) takes place between the pre-ionization electrode  51  and the cathode  21  on the outer surface of the cylindrical insulator  52 . It generates high-intensity electron radiation, UV radiation, and x-ray radiation which pre-ionizes the work gas in the through-channels  24  and the cavity  25 . An almost completely ionized pre-plasma is formed in the through-channels  24  during the main discharge. The electron beams which are generated in this way generate primary electrically conducting ionization channels  16  which intersect in the discharge chamber  12  at a point P on the axis of symmetry  11  and are directed to the opposite surface of the anode  22 .  
         [0069]     During the high-current phase of the main discharge, the current flows through these ionization channels  16  and generates the plasma  7  through heating of the pre-ionized work gas that flows in.  
         [0070]     The drawing in  FIG. 4  shows a cathode  21  in the discharge chamber  12  which is outfitted with a small cavity  25  and geometrically short through-channels  24 . In contrast to the second embodiment example described above, the surface discharge  53  takes place on the inner side of the cylindrical insulator  52 , since the pre-ionization electrode  51  is arranged inside the tubular insulator  52 . In other respects, its operation corresponds to that of the second embodiment example.  
         [0071]     In the embodiment form according to  FIG. 5 , the cathode  21  has a larger cavity  25  and a geometrically short annular gap  26  (as a special construction of a plurality of through-channels  24 ). In this case, webs S are arranged for holding the middle area of the cathode  21  and, at the same time, assist in improving the cooling of the highly thermally loaded central area of the cathode  21 . In other respects, the construction and operation correspond to the example according to  FIG. 3 .  
         [0072]     The embodiment example according to  FIG. 6  differs from the preceding embodiment examples (FIGS.  3  to  5 ) in that the connection of the cavity  25  of the cathode  21  to the discharge chamber  12  is formed as an annular gap  26  in such a way that the pre-ionization electrode  51  (with insulator  52 ) is inserted into a centrally symmetric conical bore hole of the cathode  21  to supplement the curved surface of the cathode  21 . Accordingly, due to the rotationally symmetric orientation of the pre-ionization electrode  51  in the bore hole of the cathode  21 , the uniform annular gap  26  can be accurately adjusted in any desired manner with respect to its gap width.  
         [0073]     The discharge sequence is carried out in exactly the same way as described with reference to  FIG. 3  and  FIG. 5 .  
         [0074]      FIG. 7  and  FIG. 8  refer to arrangements in which the surface discharge  53  (and the resulting electron beams) is made use of directly for generating primary ionization channels  16  in the discharge chamber  12  between the pre-ionization electrode  51  and the cathode  21  via the insulator  52 . For this purpose, it is necessary for the discharge chamber  12  to have “visual contact” with the surface discharge  53  at the insulator  52 . This means that the surface tangent of the insulator  52  must face the common point P.  FIG. 8  has the distinction that the through-channels  24  are formed by inner and outer insulators  56  and  55 , respectively, while the gas inlets  61  which are arranged individually in the pre-ionization electrode  51  are introduced directly in the ceramic through-channels  24  in order to generate the surface discharge  53  toward the cathode  21 .  
         [0075]     In  FIG. 9 , in contrast to  FIG. 5 , an additional auxiliary electrode  54  is arranged inside the cathode  21  in an enlarged cavity  25 . Another cavity  27  which works in exactly the same way as in the cathode  21  in  FIG. 4  is provided inside the auxiliary electrode  54 . This arrangement has three different high-voltage potentials: 
        1. Pulse voltage between the pre-ionization electrode  51  and the auxiliary electrode  54  for generating the surface discharge  53  via the ceramic insulator  52 .     2. Pulse voltage between the auxiliary electrode  54  and the cathode  21 . This pulse voltage accelerates the electrons starting in the through-channels  24  of the auxiliary electrode  54  toward the through-channels  24  in the cathode  21 .     3. Pulse high-voltage for the main discharge between the cathode  21  and anode  22 . The accelerated electrons generate primary ionization channels  16  for the main discharge which face in direction of the surface of the anode  22  and intersect at a point P on the axis of symmetry  11  of the discharge chamber  12 . The through-channels  24  in the auxiliary electrode  54  and cathode  21  can also be slit-shaped.        
 
         [0079]      FIGS. 10 and 11  show modifications of the arrangement shown in  FIG. 3 . At least one magnetic field having an orientation of the flux lines perpendicular to the direction of the electric field between the anode  21  and cathode  22  is additionally arranged in the vacuum insulation space  13 . The function of the magnetic field is explained in the following.  
         [0080]     If an ideal vacuum existed between the anode  22  and the cathode  21 , there would be no problems with electric arcing in the vacuum insulation. The breakdown voltage between the cathode  21  and anode  22  is dependent on a product p·d (gas pressure p times interelectrode distance d), and the breakdown voltage drops as the p·d values increase in all of the examples discussed herein (left-hand branch of the Paschen curve).  
         [0081]     Since a gas discharge source is additionally filled with gas (as work gas and/or as additional gas influx for debris mitigation), an effective p·d value is one in which the breakdown voltage decreases when gas pressure increases. However, for design-related reasons (e.g., because of the recipient connections for connecting to the vacuum pump  17 ), the increase in the p·d value cannot be compensated to an unlimited extent in the vacuum insulation space  13  (the area of the greatest interelectrode distance d) by reducing the interelectrode distance d. Initial experiments have shown that the limit of the dielectric strength is reached especially in the vacuum insulation space  13  under these conditions.  
         [0082]     However, by installing magnetic fields {right arrow over (B)} (electromagnets, permanent magnets of suitable material) in which the B-flux lines are perpendicular to the E-flux lines, the breakdown voltage for the present geometry (e.g., 5 mm interelectrode distance) and the existing work pressure of the gas (e.g., 15 Pa) can be increased by a factor of &gt;5. This is because electrons exiting from the cathode  21  which accelerate the electric field between the anode  22  and cathode  21  are decreased due to the magnetic field {right arrow over (B)} in such way that the acceleration path length of the electrons leading up to an interaction with a gas atom is sharply reduced in direction of the electric field. Therefore, the average kinetic energy of the electrons is comparatively low.  
         [0083]     Studies has shown that B-fields with field strengths on the order of 1 T (Tesla) are sufficient. These field strengths can also be achieved by permanent magnets (e.g., NdFeB magnets). Magnetic fields should advantageously be arranged at the locations with the greatest p·d values, e.g., in the vacuum insulation space  13 , that is, in areas with a large interelectrode distance or in the vicinity of gas inlet openings  61 .  
         [0084]      FIG. 10  shows a variant with two magnet rings  8 , between which a magnetic field {right arrow over (B)} is formed in radial direction to the axis of symmetry  11  of the discharge chamber  12  and of the entire electrode configuration. The magnetic field {right arrow over (B)} extends substantially over the entire vacuum insulation space  13  in this example.  
         [0085]     The areas around the inner and outer magnet rings  81  and  82  are not critical because the breakdown voltage in these locations is automatically increased due to the reduced distance d. However, it is useful to arrange a body  83  at the electrode (in this case, the anode  22 ) on the inner magnet ring  81  in order to prevent inhomogeneities in the electric field between the anode  22  and cathode  21  by adapting the interelectrode distance d from the transition area  14  to the magnet ring  81 . Alternatively, the magnet rings  81  and  82  can also be arranged at the cathode  21 . Electromagnets can also be used instead of permanent magnets.  
         [0086]     In the construction according to  FIG. 11 , two magnet rings  81  and  82  are arranged at the anode  22  so as to have an identical effect with respect to increasing the dielectric strength, but with circular orientation of the magnetic flux lines. In this variant, two circular magnetic fields {right arrow over (B)} 1  and {right arrow over (B)} 2  which are oriented opposite to one another are formed inside the magnet ring  81  and  82 , respectively. Magnetic field {right arrow over (B)} 2  is strengthened between the magnet rings  81  and  82  and, overall, is more homogeneous than in the radial shape shown in  FIG. 10 . The circular shape of field {right arrow over (B)} 2  also removes the charge carriers from the vacuum insulation space  13  more efficiently than with a radial magnetic field.  
         [0087]     The constructional variants according to  FIGS. 12 and 13  are characterized in that the ignition of the pre-plasma (generation of ionization channels  16 ) is carried out in the vacuum insulation space  13  and in the evacuated transition area  14  after applying the high-voltage main pulse to the cathode  21  and anode  22 . As in all of the preceding examples, the vacuum insulation space  13  has a larger interelectrode distance d compared to the transition area  14  of the vacuum insulation between the discharge chamber  12  and vacuum insulation space  13 .  
         [0088]     In the construction shown in  FIG. 12 , the annular pre-discharge (as was described with reference to  FIG. 3  to  FIG. 6 ) is initiated by pre-ionization, and the pre-ionized gas is introduced into the transition area  14  between the vacuum insulation space  13  and the discharge chamber  12  by means of the through-channels  24 . The vacuum-insulated transition area  14  which, in this example, takes over the function of shaping the primary insulation channels  16  for the main discharge is used for igniting the main discharge. In this case also, the conducting annular zone that is formed in this way contracts due to magnetic forces during the main current pulse in direction of the axis of symmetry  11  of the discharge space  12  to form the dense, hot plasma  7 .  
         [0089]     According to  FIG. 13 , the gas inlet  61  for the work gas is connected directly from the outside to the wide vacuum insulation space  13 . Since the vacuum chamber  1  is gas-tight and is evacuated in such a way that the gas discharge is carried out on the left-hand side of the Paschen curve, the discharge starts in the areas with the greater product of gas pressure p and interelectrode distance d when—as is the case in  FIG. 13 —there is no additional discharge initiation (e.g., through pre-ionization). The gas pressure is adjusted in such a way that a spontaneous ignition can be carried out only in the annular vacuum insulation space  13  for voltages above a defined value.  
         [0090]     In order to achieve a multiple-channel ignition by generating local, radially directed primary ionization channels  16 , additional, oppositely located grooves  29  are provided in the cathode  21  and anode  22 . These grooves  29  cause a further increase locally in the product of gas pressure p and interelectrode distance d at suitable positions in the vacuum insulation space  13  so as to enable a spontaneous ignition of the plasma especially in these grooves  29  at voltages above a defined value.  
         [0091]     The current ring or local ionization channels  16  in the grooves  29  formed in the vacuum insulation space  13  in this way are contracted due to the magnetic forces of the main discharge current radially in direction of the axis of symmetry  11  of the discharge chamber  12  through the conical transition  14  to the discharge space  12 . A conductive zone which is formed in this way and which occurs along the axis of symmetry  11  below the pocket hole  28  at the cathode end is then heated by the main current pulse to form the plasma  7  emitting EUV radiation.  
         [0092]     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  vacuum chamber  
           11  axis of symmetry  
           12  discharge chamber  
           13  vacuum insulation space  
           14  (evacuated) transition area  
           15  cooling channels  
           16  primary (electrically conducting) ionization channels  
           17  vacuum pump  
           2  electrodes  
           21  cathode  
           22  anode  
           23  rounded electrode collar  
           24  through-channel  
           25  cavity  
           26  annular gap  
           27  additional cavity  
           28  pocket hole  
           29  grooves  
           3  main pulse generator  
           4  pre-ionization pulse generator  
           5  pre-ionization unit  
           51  pre-ionization electrode  
           52  (tubular) insulator  
           53  surface discharge  
           54  auxiliary electrode  
           55 ,  56  inner, outer insulator  
           6  gas supply unit  
           61  gas inlet  
           7  plasma  
           8  magnet rings  
           81  inner magnet ring  
           82  outer magnet ring  
           83  body  
          {right arrow over (B)} magnetic field  
          {right arrow over (B)} 1 , {right arrow over (B)} 2  (oppositely oriented) magnetic fields  
          d interelectrode distance  
          p gas pressure  
          P common point (intersection of the ionization channels)  
          S web