Abstract:
The invention is directed to an arrangement for providing target material for the generation of short-wavelength electromagnetic radiation, in particular EUV radiation. It is the object of the invention to find a novel possibility for providing target material for the generation of short-wavelength radiation based on an energy beam induced plasma which makes it possible to supply a reproducible successive flow of mass-limited targets in the interaction chamber in such a way that only the amount of target material needed for efficient generation of radiation achieves plasma generation. This object is met, according to the invention, in that the target generator opens into a selection chamber which precedes the interaction chamber and which has, along the target path, an outlet opening into the interaction chamber and in which a target selector is arranged. The target selector has elements for eliminating individual targets needed for the regular target sequence of the target generator, so that only the individual targets needed for efficient plasma generation and radiation generation corresponding to the pulse frequency of the energy beam are admitted to the interaction point.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of German Application No. 10 2004 037 521.6, filed Jul. 30, 2004, the complete disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The invention is directed to an arrangement for providing target material for the generation of short-wavelength electromagnetic radiation, in particular EUV radiation, based on an energy beam induced plasma. It is preferably applied in light sources for projection lithography in semiconductor chip fabrication. 
     b) Description of the Related Art 
     Reproducible mass-limited targets for pulsed energy input for plasma generation have gained acceptance, above all in radiation sources for projection lithography, because they minimize unwanted particle emission (debris) compared to other types of targets. An ideal mass-limited target is characterized in that the particle number at the interaction point of the energy beam is limited to the particles used for generating radiation. 
     Excess target material that is vaporized or sublimated or which, although ionized, is not excited by the energy beam to a sufficient degree for the desired radiation emission (marginal area or immediate surroundings of the interaction point) causes not only increased emission of debris but also an unwanted gas atmosphere in the interaction chamber which in turn contributes considerably to an absorption of the short-wavelength radiation generated from the plasma. 
     There are a number of embodiment forms of mass-limited targets known from the prior art. These are listed in the following along with their characteristic disadvantages:
         Continuous liquid jet, possibly also frozen (solid consistency) (EP 0 895 706 B1)
           Mass limiting can be realized only to a limited extent because of the large size of the target in one linear dimension, resulting in increased debris and an unwanted gas burden in the vacuum chamber.   The shock wave proceeding from the plasma expansion in the target jet in the direction of the target nozzle leads to a certain destruction of the target flow and, therefore, to a limiting of the pulse repetition rate of the laser excitation.   
           Clusters (U.S. Pat. No. 5,577,092), gas puffs (Fiedorowicz et al., SPIE Proceedings, Vol. 4688, 619) and aerosols (WO 01/30122 A1; U.S. Pat. No. 6,324,256 B1)
           lead to severe nozzle erosion with short distances between the interaction point and the target nozzle and, at large distances from the nozzle (due to dramatically decreasing average density of the target), to a low efficiency of the radiation emission of the plasma.   
           Continuous flow of individual droplets (EP 0 186 491 B1)
           requires precise synchronization with the excitation laser,   cold target material in the vicinity of the plasma (less than with the target jet, but still present) is vaporized and leads to absorbent gas atmosphere and increased debris.   
               

     All of the so-called mass-limited targets mentioned above have in common that there is more target material in the interaction chamber than is needed for generating the emitting plasma in spite of limiting the diameter of the target flow. With a continuous flow of droplets, for example, only about every hundredth drop is struck by the laser pulse. Apart from increased generation of debris, this leads to excess target material in the interaction chamber which causes an increased gas burden (particularly when xenon is used as target) and, therefore, an increased pressure in the interaction chamber. The increased gas burden leads in turn to an unwanted increase in the absorption of radiation emitted by the plasma. Further, the unused target material leads to increased material consumption and accordingly raises costs unnecessarily. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is the object of the invention to find a novel possibility for providing target material for the generation of short-wavelength radiation based on an energy beam induced plasma which makes it possible to supply a reproducible successive flow of mass-limited targets in the interaction chamber in such a way that only the amount of target material needed for efficient generation of radiation interacts with the energy beam and, therefore, debris generation and the gas burden in the interaction chamber are minimized. 
     In an arrangement for providing target material for the generation of short-wavelength electromagnetic radiation, in particular EUV radiation, in which a target generator for generating a regular succession of individual targets is arranged so as to open into an interaction chamber, wherein the generated target sequence advances along a target path, and an energy beam for generating a plasma emitting the desired radiation is directed to an interaction point on the target path, the above-stated object is met, according to the invention, in that the interaction chamber is preceded by a selection chamber into which the target generator opens and which has, along the target path, an outlet opening into the interaction chamber, and in that a target selector is arranged in the selection chamber, which target selector has means for eliminating individual targets from the regular target sequence of the target generator, so that only the individual targets necessary for efficient plasma generation corresponding to a given pulse frequency of the energy beam are admitted to the interaction point in the interaction chamber. 
     The target selector advantageously has a rotating chopper wheel in which the quantity of admitted individual targets and eliminated individual targets can be adjusted by means of a mark-to-space or duty cycle ratio of apertures to closed areas of the chopper wheel which cyclically or periodically cross the target path. 
     The target selector preferably comprises at least two chopper wheels that are arranged one after the other along the target path. The quantity of individual targets that are admitted and eliminated is adjusted by the duty cycle ratios of apertures to closed areas of the individual chopper wheels and by the phase position of the apertures of the chopper wheels with respect to one another. 
     The chopper wheels can be arranged on a common axis with fixed phase position relative to one another. However, they can also have separate, spatially separated axes or can be arranged coaxially on a solid shaft and at least one hollow shaft in order to make the phase position and the spacing of the chopper wheels variably adjustable. 
     In a variant with two chopper wheels, the first chopper wheel advisably has a duty cycle ratio of apertures to closed areas such that a column of individual targets from the target sequence provided by the target generator is admitted to the second chopper wheel. 
     The spacing of the chopper wheels along the target path is advisably adjusted in such a way that only one individual target from the target column entering through the first chopper wheel can pass through the second chopper wheel into the interaction chamber. 
     Because of the vaporization or sublimation of target material, particularly in target materials with a high vapor pressure (&gt;25 kPa) under process conditions (e.g., xenon), it is advantageous when the spacing of the chopper wheels along the target path is adjusted in such a way that at least two individual targets following one another in close succession from the target column entering through the first chopper wheel are admitted through the second chopper wheel, wherein at least a first target is a sacrifice target for forming a vaporization shield for at least one subsequent main target. 
     In another advisable constructional variant, the target selector has an open hollow cylinder which is arranged so as to be rotatable around its cylinder axis disposed orthogonal to the target path such that it is pierced by the target path at two points, and the quantity of admitted individual targets and eliminated individual targets can be adjusted by a duty cycle ratio of apertures to closed areas of the cylinder jacket and by the spacing of the cylinder axis relative to the target path. 
     The hollow cylinder advantageously has a duty cycle ratio of apertures to closed areas such that a column comprising a plurality of individual targets from the target sequence provided by the target generator is allowed to enter the hollow cylinder. 
     The spacing of the cylinder axis of the hollow cylinder relative to the target path can preferably be adjusted in such a way that only one individual target from the target column entering the hollow cylinder exits from the hollow cylinder into the interaction chamber. 
     Particularly for target materials with high vapor pressure which were mentioned above, the distance of the cylinder axis of the hollow cylinder from the target path is adjusted in such a way that at least two successive individual targets from the target column entering the hollow cylinder exit from the hollow cylinder into the interaction chamber, wherein at least a first target is a sacrifice target for forming a vaporization shield for at least one subsequent main target. 
     In another advantageous embodiment, the target selector has a deflecting unit based on a force field for deflecting a quantity of individual targets from their normal target path, wherein the force field is switchable in a pulsed manner so that only a determined number of individual targets generated by the target generator arrive in the interaction chamber through the outlet opening of the selection chamber and the wall next to the outlet opening is provided for intercepting the rest of the targets. The deflecting unit can be arranged in such a way that the deflected targets are caught in the selection chamber at the wall next to the outlet opening or in such a way that only the deflected targets reach the interaction point in the interaction chamber through the outlet opening of the selection chamber. 
     The target selector preferably comprises a ring electrode and a deflecting unit based on an electric field (similar to an oscillograph). However, the deflecting unit can also advisably be based on a magnetic field without changing the manner of operation described above. 
     The selection chamber advisably has a pump for differential pumping out of target material that is eliminated by the target selector. In addition, the selection chamber can have a heatable surface for faster vaporization of target materials with a lower vapor pressure under process conditions(&lt;25 kPa, e.g., tin compounds, particularly tin(IV) chloride or tin(II) chloride in alcoholic solution). A surface of this kind is advisably a wall of the selection chamber in the rotating direction of a chopper blade or the wall with the outlet opening or the surface of a chopper wheel. 
     Regardless of the type of means for target selection, it is advantageous for the adjustment of the target selector when it passes exactly one individual target into the interaction chamber from the target sequence provided by the target generator in order to bring this individual target, as mass-limited target, into interaction with the energy beam. However, it is preferable for the above-mentioned target materials with high vapor pressure under process conditions that the target selector is adjusted in such a way that it passes at least two successive individual targets of the target sequence provided by the target generator, wherein at least a first target of a target column of this kind is a sacrifice target for forming a vaporization shield for at least one subsequent main target. 
     The basic idea of the invention proceeds from the consideration that the desired short-wavelength electromagnetic radiation, particularly EUV radiation, that is radiated from an energy beam induced plasma is, according to the prior art, already partially absorbed again in the interaction chamber by vaporized target material. On the other hand, inefficiently excited target material results in increased debris generation. Therefore, the objective must be to select exactly as much target material from a reproducibly generated series of individual targets as is needed for efficient generation of short-wavelength electromagnetic radiation in the desired wavelength range. According to the invention, this is accomplished by means of adjustable selection of a conventionally provided individual target flow by eliminating excess individual targets before they enter the interaction chamber. Mechanical rotary elements with apertures or deflecting units based on electromagnetic fields for selectively passing individual targets in desired timed sequences are suitable for the required pulse frequencies of semiconductor lithography according to the invention. 
     The solution according to the invention makes it possible to provide reproducible successive flows of mass-limited targets in the interaction chamber for the generation of short-wavelength electromagnetic radiation based on an energy beam induced plasma in such a way that only the amount of targets needed for an efficient generation of radiation achieves interaction with the energy beam and, therefore, debris generation and the gas burden in the interaction chamber are minimized. Further, the consumption of target material is reduced and leads to a reduction in costs. 
     The invention will be described more fully in the following with reference to embodiment examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  shows a schematic view of the arrangement according to the invention with a target selector for providing individual targets for interaction with an energy beam in an interaction chamber, wherein the selection of individual targets from the target flow is carried out by means of a chopper wheel on which a suitable geometric ratio of apertures and closed areas is realized along a circular line; 
         FIG. 2  shows an embodiment of the invention for the selection of individual targets with two chopper wheels on a common axis, wherein initially defined columns of individual targets are generated for further selection; 
         FIG. 3  shows another embodiment example of the invention with two chopper wheels on separate axes rotating in opposite directions; 
         FIG. 4  shows a variant of the invention that is modified from  FIG. 2 , wherein two successive individual targets are provided for generating a radiation shield for one of the two individual targets; 
         FIG. 5  shows an embodiment form with two separately rotatable chopper wheels in which, in contrast to  FIG. 3 , the chopper wheels are arranged coaxially on a solid shaft and a hollow shaft; 
         FIG. 6  shows an embodiment example with a chopper wheel that is constructed as a hollow cylinder and which has an axis oriented orthogonal to the target path, wherein another isolation of targets is carried out analogous to  FIG. 2 ,  4  or  5  after a first preselection as a result of the target path piercing the hollow cylinder twice; 
         FIG. 7  shows a design variant with a target selector based on an electrical field for deflecting targets from the normal target path and intercepting the surplus individual targets at the selection chamber wall next to the outlet opening. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As is shown in  FIG. 1 , the arrangement for the generation of defined mass-limited targets for energy beam induced generation of short-wavelength electromagnetic radiation (preferably EUV radiation) basically comprises a target generator  1  which generates a discontinuous target flow  2  as a regular series  23  of individual targets  21  (droplets or pellets, i.e., solid target material, e.g., generated by frozen or solidified liquid droplets), and a target selector  3  which is arranged in a selection chamber  41  arranged in front of the interaction chamber  4 , wherein a plasma  6  is generated in the interaction chamber  4  by an energy beam  5  at an interaction point  61  given by the intersection of the target path  22  with the axis of an energy beam  5 . 
     The regular, discontinuous target flow which enters the selection chamber  41  as a close, regular target sequence  23  provided by the target generator  1  undergoes a cyclic or periodic elimination of a certain quantity of individual targets  21  of the target sequence  23  by means of the target selector  3 . An individual target  21 —as is shown in FIG.  1 —or a defined column  24  ( FIG. 4 ) can be passed. The selected individual targets  21  pass an outlet opening  43  of the selection chamber  41  which, at the same time, is an inlet opening into the interaction chamber  4 . They then arrive at the interaction point  61  with the energy beam  5  on their target path  22 . 
     In principle, the target selector  3  can periodically pass only an integral number of individual targets of the target flow  2  comprising individual targets  21  that are regularly delivered by the target generator  1  and laterally deflects the rest of the intervening target sequence  23 . In the basic variant shown in  FIG. 1 , the individual targets  21  admitted by the target selector  3  are spaced so as to be precisely adapted to the pulse sequence of the energy beam  5 . 
       FIG. 1  shows a particularly simple realization illustrating the principle of target selection in which a chopper wheel  31  is used as target selector  3 . The resulting duty cycle ratio of the individual targets  21  at the outlet opening  43  of the selection chamber  41  is given solely by the geometric ratio of the apertures  33  of the chopper wheel  31  to the closed areas between the apertures  33 . 
     The individual targets  21  provided in close succession from the target generator  1  initially impinge on the chopper wheel  31  which periodically allows a few individual targets  21  to pass depending on the number of revolutions and the aperture ratio (ratio of apertures  33  to closed areas in tangential direction between the apertures  33  of the preferably circular plate). 
     In this case, without limiting generality, only one individual drop target should be selected from a target sequence  23  of seven drops to collide with the energy beam  5  in the interaction chamber  4 . The trajectory  22  of the subsequent individual targets  21  (six individual targets are shown schematically for the sake of simplicity, but in reality there are 10 to 100 drops) is interrupted since they rebound on a closed area of the chopper wheel  31 . 
     At the point of interaction  61  of the individual target  21  and the energy beam  5  (which can preferably be a laser beam  52  or an electron beam), the frequency at which targets are prepared corresponds to the product of the rotating frequency and the quantity of apertures  33  which are arranged peripherally in the chopper wheel  31  (and which, aside from the bore holes shown schematically, can also have the shape of rectangles, trapezoids, slots or notches). 
     The design of the target selector  3  with one chopper wheel  31  is based on the following boundary conditions: The desired repetition frequency of a laser used as source for the energy beam  5  is, e.g., 10 kHz. A typical repetition rate of the close target sequence  23  of regularly reproduced individual droplets (generated, e.g., from a nozzle of 20 μm) is on the order of 1 MHz. Accordingly, only every hundredth droplet is necessary for the interaction with the laser beam  52  (shown only in  FIG. 4 ). 
     A technical solution that can satisfy this requirement for droplet isolation is a chopper wheel  31  with a duty cycle ratio of 1:99, as is shown schematically in  FIG. 1 . Assuming a size of the apertures  33  of 100 μm for an individual target  21  to be admitted, the period length is 10 mm. Consequently, for a chopper wheel  31  in which the apertures are arranged on a radius of 2.5 cm, about fifteen periods can be accommodated. The chopper wheel  31  must then run at a rotating frequency of 666 Hz. This corresponds to a speed of 40,000 RPM. It is technically difficult to achieve such rotational speeds and, therefore, the embodiment form shown in  FIG. 1  is only applicable for larger droplet diameters which are generally generated with a lower frequency (20 to 100 kHz). 
     The individual targets  21  of the close target sequence  23  of the target flow  2  that do not pass the target selector  3  are deflected by the chopper wheel  31  in the selection chamber  41 . They vaporize or sublimate at the surfaces in the selection chamber  41  (primarily at the surface of the chopper wheel  31  itself). The resulting target gas is pumped off differentially by a pump  41  and can be recovered and reused. 
     If required for the target material (e.g., with a low vapor pressure &lt;25 kPa), the chopper wheel  31  must be additionally heated so that the large number of eliminated targets of the target sequence  23  is sufficiently vaporized or sublimated in order to pump out the target gas by means of the pump  42 . With most current target materials (preferably xenon), however, the vapor pressure is already higher than the pressure inside the selection chamber  41  under process conditions. 
     There is a range of technical embodiment forms for the construction of the target generator  1 , vacuum pumps, of which only the pump  42  of the selection chamber  41  is shown, and for the target selector  3 . For example, aside from the vibration-controlled droplet generator, techniques such as the principle of the high-pressure liquid jet (continuous jet) known from ink printing technology, an embodiment variant of which is described with reference to  FIG. 7 , can be used for the target generator  1 . 
     Depending upon requirements given by the target material employed, useful embodiment forms for the pump  42  (as well as for the vacuum pumps of the interaction chamber  4 ) are cryopumps or scroll pumps. 
     Some special possibilities for realizing the target selector  3  will now be described more fully with reference to the following descriptions of the drawings ( FIGS. 2 to 7 ). 
     In the embodiment forms shown in  FIGS. 2 to 5 , the target selection is realized by means of two chopper wheels  31  and  32  which are arranged at a certain distance. Regardless of the desired target frequency at the interaction point  61 , each chopper wheel  31  and  32  can have a duty cycle ratio of 1:1. For example, about 750 apertures  33  can be arranged on the edge of every chopper wheel  31  or  32  with a radius of 2.5 cm and a period length of 200 μm. For the desired repetition frequency of 10 kHz of the laser beam  52  (only shown in  FIGS. 4 and 7 ), the two chopper wheels  31  and  32  must rotate at a frequency of about 13.3 Hz or 800 RPM. A solution of this kind can be controlled easily in technical respects considering that the entire arrangement must be operated under vacuum. 
     The frequency of a target column  24  is determined from the product of the speed and quantity of periods of the first chopper wheel  31  and the quantity of passed individual targets  21  per target column  24  is determined from the relative position (phase position) of the second chopper wheel  32  and the target frequency of the regular close target sequence  23 . 
     With the target selector  3  shown in  FIG. 2 , the individual targets  21  initially strike a first chopper wheel  31  which is rotatable around an axis  311  and which can pass cyclically defined columns  24  of individual targets  21  (four individual targets  21  are shown schematically in this case without limiting generality) depending on the rate of rotation and the duty cycle ratio (of apertures  33  to the closed areas located in between). The trajectory  22  of the subsequent individual targets  21  (also shown schematically as four) is interrupted because they collide with a closed area of the chopper wheel  31 . 
     A second chopper wheel  32  is located on the same axis  34  at a defined distance and a determined phase position relative to the chopper wheel  31  so that the second chopper wheel  32  can again pass only a predetermined quantity of individual targets  21  (in this case only one individual target  21 ) of the column  24  of individual targets  21  admitted by the first chopper wheel  31 . 
     The target sequences  23  or columns  24  that do not pass the two chopper wheels  31  and  32  vaporize and sublimate at warm surfaces in the selection chamber  41 . The resulting gas is pumped out through a pump  42  and can possibly be recycled. 
       FIG. 3  shows an embodiment form of a target selector  3  in which the second chopper wheel  32  is located on an axis  312  which is separate from axis  311  of chopper wheel  31 , these axes extending parallel to one another but so as to be spatially separated. The respective phase position between the chopper wheels  31  and  32  can accordingly be adjusted differently (e.g., individual target  21  or double-target comprising sacrifice target  25  and main target  27 ) for different speeds (target frequencies) and quantity of individual targets  21  still to be let in through the second chopper wheel  32  after the selection of a defined column  24  carried out by the first chopper wheel  31 . Also, it may be advantageous that the chopper wheels  31  and  32  move in opposite directions (as is shown in  FIG. 3 ) for target materials with a low vapor pressure (&lt;25 kPa) so that the target material that does not vaporize immediately is flung against a vaporization surface (not shown) inside the selection chamber  41 . 
     The functioning of the construction according to  FIG. 4  substantially corresponds to that shown in  FIG. 2 . However, the ratios of flight velocity of the individual targets  21 , distance and phase position of the chopper wheels  31  and  32  are adjusted in such a way that every two closely successive individual targets  21  reach the interaction chamber  4 . 
     The target closer to the plasma  6  has the function of a sacrifice target  25  for forming a vaporization shield  26  for the subsequent main target  27 . Accordingly, the sacrifice target  25  is completely or almost vaporized or sublimated corresponding to the absorbed radiation output from the plasma  6 . The subsequent main target  27  for interaction with the laser beam  52  arrives without considerable loss of mass at the interaction point  61  which is given by the intersection of the axis  51  of the laser beam  52  with the target path  22  and in which the plasma  6  emitting the desired radiation (e.g., EUV) is generated as a result of the input of energy into the main target  27 . 
     The functioning of the target selector  3  shown in  FIG. 5  corresponds in essence to the solution disclosed with reference to  FIG. 3 . The only difference is that collinear axes formed as a solid shaft  313  and hollow shaft  314  are used for the chopper wheels  31  and  32 . Accordingly, different speeds and—if required—a different rotating direction are possible with the same center of rotation. 
       FIG. 6  shows an appreciably modified embodiment example of a target selector  3 . This example shows an open hollow cylinder  34  which rotates around its cylinder axis  35  orthogonal to the target path  22 . 
     At the upper intersection of the hollow cylinder  34  and the target path  22 , target columns  24  are generated corresponding to the angular velocity and the duty cycle ratio of the apertures  33  of the hollow cylinder  34 . The quantity of individual targets  21  of the column  24  entering the interior of the hollow cylinder  34  is given by the product of the rotational speed of the hollow cylinder  34  and the quantity of apertures  33  in the outer surface. 
     At the lower intersection, a portion of the target column  24  is again obstructed in its trajectory  22  in that it is deflected by a closed area of the hollow cylinder  34 . The quantity of individual targets  21  that pass the target selector  3  designed in this way per time unit is adjustable by adjusting the cylinder axis  35  in x-direction. The initial phase can be adjusted by a y-displacement of the cylinder axis  35 . 
       FIG. 7  shows a second basic variant of the target selector  3  which diverges from the mechanical selection of excess individual targets  21  from the regular target sequence  23  of the target flow  23 . 
     As in the previous examples, the target flow  2  from the target generator  1  is generated in a regular target sequence  23  from individual targets  21 . In this case, however, it is assumed that a heterodyned high-pressure target generator  1  is used which can eject up to one million drops per second. Depending on the nozzle geometry, these drops have a size of only a few micrometers and fly at up to 40 m/s. Accordingly, this is a true liquid jet as is known from ink printing technology as a continuous jet or high-pressure system. 
     After the rapid disintegration of the initial high-pressure jet, the individual targets  21  fly through a ring electrode  36  which charges them electrically. The charged targets  27  then traverse a deflecting unit  37  in which the individual targets  21  that are not needed are deflected in the electrical field as in an oscillograph. Controlled by a trigger unit (not shown) for the defined generation of the laser beam  52  synchronous to the individual targets  21  entering the interaction point  61 , the electrical field between the electrodes of the deflecting unit  37  deflects a defined quantity of excess targets. The deflected targets  29  do not then fly through the outlet opening  43  of the selection chamber  41 , but rather are intercepted at the wall of the selection chamber  41  in which the outlet opening  43  to the interaction chamber  4  is located. The target material is then vaporized or sublimated at this wall of the selection chamber  41 , which thus serves as a simple catching device, and can be pumped out by means of the pump  42  and processed again. 
     In all of the examples described above, an additional amount of target material that is vaporized or sublimated due to the finite vapor pressure on the target path  22  from the inlet opening into the interaction chamber  4  to the interaction point  61  must be introduced for radiation generation in addition to the amount of target material that interacts directly with the energy beam  5  in order to generate a desired characteristic radiation in the plasma  6 . This process of vaporization or sublimation is reinforced by the radiation from the plasma  6  that is absorbed by the target material. 
     Therefore, the effective loss of mass must either be compensated by a corresponding increase in the initial size of the individual targets  21  or—as is shown in FIG.  4 —can be kept very small by means of one or more sacrifice targets  25  which serve as a vaporization shield  26 . The solution to the vaporization problem according to  FIG. 4  can accordingly be combined with all other embodiment forms of the invention. 
     Further, as was mentioned with reference to  FIG. 4 , target columns  24  with more than one main target  27  can also be realized when a laser beam  52  is used as energy beam  5 . Since it is known that the focus dimensions of the laser beam  52  cannot be adjusted to be infinitely small, but the smallest possible target diameter (with respect to the excitation depth) should be achieved for the sake of converting the individual targets  21  into radiating plasma  6  as completely as possible, it is useful to allow a plurality of main targets  27  to follow behind the radiation shield  26  of the sacrifice target  25  insofar as these main targets  27  can be excited simultaneously by a laser pulse (within the laser focus). In this connection, a plurality of target paths  22  located next to one another is also useful. 
     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 
     
         
           1  target generator 
           2  target flow 
           21  individual target 
           22  target path 
           23  target sequence 
           24  column 
           25  sacrifice target 
           26  vaporization shield 
           27  main target 
           28  charged target 
           29  deflected target 
           3  target selector 
           31  (first) chopper wheel 
           311  axis 
           312  (separate) axis 
           313  solid shaft 
           314  hollow shaft 
           32  second chopper wheel 
           33  aperture 
           34  hollow cylinder 
           35  cylinder axis 
           36  ring electrode 
           37  deflecting electrode 
           4  interaction chamber 
           41  selection chamber 
           6  plasma 
           61  interaction point