Abstract:
The invention is directed to a method and an arrangement for the plasma-based generation of intensive short-wavelength radiation, particularly EUV radiation. The object of the invention, to find a novel possibility for plasma-based generation of intensive soft x-radiation, particularly EUV radiation, which permits efficient energy conversion in the desired spectral band with high repetition frequency (several kHz) of the plasma excitation, minimized emission of debris and low erosion of the nozzle of the target generator, is met according to the invention in that an additional energy beam is directed on the target flow spatially in advance of its interaction with the high-energy beam, the target flow being acted upon by this additional energy beam with substantially weaker energy pulses compared to the high-energy beam in order to divide the target flow into a first portion and at least one second portion, wherein the target flow is excited at an interaction point within the second portion by the high-energy beam for generating a hot, radiating plasma, and the second portion is decoupled from the first portion and therefore from the target generator in such a way that a hydrodynamic disturbance generated in the second portion by the pulse of the high-energy beam is transmitted into the first portion only negligibly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of German Application No. 10 2004 005 242.5, filed Jan. 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 a method and an arrangement for plasma-based generation of intensive short-wavelength radiation in which a target flow comprising defined portions which is made available in a reproducible manner interacts with a pulsed energy beam for exciting radiation-emitting plasma. The invention is particularly suitable for the generation of soft x-radiation, preferably EUV radiation, for the exposure of very small structures in semiconductor lithography. 
   b) Description of the Related Art 
   In the prior art relating to energy beam pumped plasma-based radiation sources, mass-limited targets for plasma generation have become increasingly accepted because they minimize unwanted particle emission (debris) compared to other types of targets. A mass-limited target is wherein the particle number in the focus of an energy beam is limited to the order of magnitude of the ions used for generating radiation. In this connection, EP 0 186 491 B1 describes the excitation of individual droplets, i.e., exactly one droplet is impinged upon per energy pulse. The droplet size is of the same order of magnitude as the laser focus. When a pulsed energy beam is directed on a series of individual droplets, it is necessary to synchronize both events with one another with respect to space and time. However, the generation of droplets in a vacuum chamber depends upon the characteristics of the target material and is not possible for every target material. For example, xenon cannot be used to generate individual droplets under the process conditions of EUV lithography. 
   Further, targets in the form of clusters (U.S. Pat. No. 5,577,092), gas puffs (H. Fiedorowicz, SPIE Proceedings, Vol. 4688, 619) or aerosols (WO 01/30122) have been described for plasma generation. However, the average density of such targets in the focus volume is substantially less than in liquid targets or solid targets because the target comprises microscopic particles or is in gaseous form. Further, the target divergence is generally so great (opening angle of several degrees) that the average target density decreases rapidly with increasing distance from the nozzle. Therefore, the energy beam can be coupled in efficiently only in the immediate vicinity of the nozzle, which leads to a high thermal loading of the nozzle and inevitably results in nozzle erosion. 
   While arrangements with a continuous target jet (liquid or frozen jet) such as is described in WO97/40650, for example, allow a relatively large working distance from the nozzle, they are susceptible to shock waves. This means that the radiation-generating energy pulse that is coupled in causes hydrodynamic disturbances extending relatively far along the jet axis and the characteristics of the continuing jet for optimal plasma generation and radiation generation are impaired. These disturbances prevent a high pulse repetition frequency because it is necessary to wait for the disturbances to die down before the next pulse. 
   Further, the prior art includes plasma-based radiation sources with optimized energy conversion in a determined spectral range. For example, it is known from U.S. Pat. No. 4,058,486 to use a defined pre-pulse prior in time to a main excitation pulse generating the radiating plasma in order to increase the efficiency of the energy conversion of the excitation radiation into emitted radiation of the plasma in the EUV range. 
   OBJECT AND SUMMARY OF THE INVENTION 
   It is the primary object of the invention to find a novel possibility for plasma-based generation of intensive short-wavelength radiation, particularly soft x-radiation, which permits efficient energy conversion in a desired spectral band with high repetition frequency (several kHz) of the plasma excitation, minimized emission of debris and low erosion of the nozzle of the target generator. 
   In a method for the plasma-based generation of soft x-radiation, particularly for the generation of extreme ultraviolet (EUV) radiation, in which defined portions of a target flow that is provided in a reproducible manner are made to interact with a pulsed high-energy beam for exciting a radiation-emitting plasma, wherein the interaction results in the generation of a radiation-emitting plasma, the above-stated object is met, according to the invention, in that an additional energy beam is directed on the target flow spatially in advance of its interaction with the high-energy beam, the target flow being acted upon by this additional energy beam with substantially weaker energy pulses compared to the high-energy beam in order to divide the target flow into a first portion and at least one second portion, wherein the target flow is excited at an interaction point within the second portion by the high-energy beam for generating a hot, radiating plasma, and in that the first portion is generated by the target generator as a continuous target jet with a low divergence and the second portion is decoupled from the first portion and therefore from the target generator at least in such a way that a hydrodynamic disturbance generated in the second portion by a pulse of the high-energy beam at the interaction point is transmitted into the first portion as a negligible disturbance compared to the disturbance of the additional energy beam. 
   Depending on whether the division of the target flow into two portions is carried out partially or completely, the energy pulse of the additional energy beam can advantageously impinge on the target flow before, simultaneously with, or after the pulse of the high energy beam. When the target flow is repeatedly acted upon by the additional energy pulse in a regular sequence in order to generate a series of defined second portions, each of these portions is a mass-limited individual target. In semiconductor lithography, the pulses of the additional energy beam and of the high-energy beam are directed—so as to be synchronized with one another—on the target flow preferably with repetition frequencies of several kilohertz. The target flow is advantageously provided as a continuous target flow in liquid or solid aggregate state at least at the location of the impinging additional energy beam. A liquefied or frozen gas, preferably an inert gas, e.g., xenon, is preferably used as target flow. 
   The additional energy beam is advantageously split off from the high-energy beam, and the pulses of the high-energy beam and additional energy beam are electrically and/or optically synchronized by means of at least one synchronizing unit. 
   However, the additional energy beam can also advisably be prepared from a separate radiation source and its pulses can be synchronized with those of the high-energy beam by suitable triggering. 
   Further, in an arrangement for the plasma-based generation of soft x-radiation, particularly for the generation of extreme ultraviolet (EUV) radiation, with a target generator for providing a low-divergence target flow which is provided in a reproducible manner in a vacuum chamber and with a pulsed high-energy beam that is focused on defined portions of the target flow at an interaction point for generating a radiation-emitting plasma, the above-stated object is met, according to the invention, in that an additional pulsed energy beam is directed on the target flow spatially in front of the interaction point for dividing the target flow into a first portion and at least one second portion, the additional energy beam has a substantially lower pulse energy compared to the high-energy beam, the first portion has a connection to the target generator that is characterized by a continuous, low-divergence target flow and the second portion is decoupled from the continuous target flow and, therefore, from the target generator at least in such a way that a hydrodynamic disturbance generated in the second portion by a pulse of the high-energy beam at the interaction point is transferred into the first portion at most as a disturbance that is negligible compared to the disturbance of the additional energy beam, and in that means are provided for synchronizing the pulses of the high-energy beam and additional energy beam. 
   The target flow from the target generator to a dividing point defined by the impingement of the additional energy beam is advantageously a continuous target flow of liquefied or frozen gas, preferably an inert gas, particularly xenon. 
   In order to generate the additional dividing energy beam, a separate beam source is advisably provided. However, the additional energy beam can also be diverted from (coupled out of) the high-energy beam. 
   An electron beam, an ion beam or a laser beam is preferably applied as an additional energy beam. In the latter case, the high-energy beam is likewise advantageously a laser beam for exciting the radiating plasma. However, it can also be a particle beam, e.g., an electron beam or an ion beam. 
   As an alternative to the possibility of providing the dividing laser beam from a separate laser source, preferably optical means in the form of a beam guiding device are provided for coupling out a portion from the excitation laser beam. 
   In order to couple in the excitation laser beam and dividing laser beam at different locations of the target flow, a focusing device is advisably provided which has either separate focusing lenses or a common focusing lens for the excitation laser beam and dividing laser beam; in the latter case, the excitation laser beam and dividing laser beam are coupled in and focused at different angles as beam bundles for economizing on space. 
   A beam guiding device containing at least one beam-deflecting element is advantageously provided for directing the beam bundles at defined different angles relative to one another. Further, a polarization-selective or wavelength-selective element or a tilting mirror can advisably be provided for adjusting the angle of incidence of at least one of the beam bundles on the focusing lens in the beam guiding device. 
   In order to minimize the return of hydrodynamic disturbance (shock waves due to the generation of the hot plasma at the interaction point) into the continuous target flow and/or to improve the conversion efficiency of desired emitted radiation in the hot plasma, a synchronizing device is provided for adjusting the time position of the pulses of the high-energy beam and dividing beam (additional energy beam). The synchronizing device has a trigger unit and/or a delay element for synchronizing two separate energy beam sources for the high-energy beam and the additional energy beam. The delay element can be an optical or electronic delay loop. When a portion is to be coupled out of the high-energy beam for the dividing beam, the synchronizing device advantageously has a delay element in only one of the beam paths of the high-energy beam or dividing beam. 
   The basic idea of the invention starts from the problem that, under the required process conditions (near the triple point of the target material), xenon, which is presently favored as a target material for EUV sources in semiconductor lithography, can only be provided as a continuous target flow which is very susceptible to hydrodynamic disturbances. However, particularly in radiation sources for semiconductor lithography in which the highest possible pulse repetition frequency is required, the hydrodynamic instability of the target is the limiting factor for the desired increase in pulse energy and repetition frequency. 
   The invention resolves this conflict in that in order to provide the target in a highly repetitive manner the energy pulse generating the radiating plasma impinges on a target volume that is decoupled from the target volume flowing after it by a separation process that takes place ahead of it spatially (division or at least spatially limited thinning of the continuous target flow over the entire target diameter). By means of a small (compared to the high-energy pulse) introduction of energy, the hydrodynamic disturbances caused by the radiation-generating energy pulse cannot propagate in the subsequently flowing target volume or is at least greatly attenuated. The resulting advantages are potentially high pulse repetition frequencies and—in contrast to conventional droplet generation—a volume that can be adjusted relatively simply over the length of the decoupled target portion which results in low debris emission by way of a mass-limited target. Further, the synchronization of the separating process with the radiation-generating energy pulse is also substantially simpler than in “natural” droplet formation in which the droplet frequency is not completely free from fluctuations. 
   Moreover, due to the low divergence of the target flow that is initially provided in a continuous manner (jet), a relatively large working distance from the nozzle can be selected (order of magnitude of several centimeters). Accordingly, in conjunction with the inventive separation of the continuous target flow, the erosion of the nozzle of the target generator and the thermal loading per unit of area are reduced. 
   The invention makes it possible to realize a radiation source with a high average output based on a plasma for generating an intensive short-wavelength radiation which permits an efficient energy conversion (with excitation pulse energies of some 10 mJ) in a desired spectral band and a high repetition frequency (several kHz) of the plasma excitation with a liquid target flow that is provided in a reproducible manner. This appreciably reduces erosion of the target nozzle and/or emission of debris. 
   In the following, the invention will be described more fully with reference to embodiment examples. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  shows a basic view of the arrangement according to the invention for generating radiation with a target flow that is provided in a reproducible manner, wherein a dividing pulse that lies ahead of the plasma-generating high-energy beam spatially is provided for suppressing shock waves running back into the target flow; 
       FIG. 2  shows an embodiment form of the invention with a dividing laser beam that is coupled out of an excitation laser beam, wherein the two laser beams are focused on the target flow by the same focusing optics; 
       FIG. 3  shows a construction of the embodiment form of  FIG. 2  in which the excitation laser beam and the dividing laser beam have an optical delay element for adjusting the time position of the two laserpulses; 
       FIG. 4  shows an embodiment form of the invention with two triggered, unpolarized lasers for generation a radiating plasma and for separating target portions, wherein the two laser beams are focused on the target flow by the same focusing optics and one of the laser beams has an optical delay element; 
       FIG. 5  shows an embodiment form with two triggered lasers that are polarized orthogonal to one another for separating the target portions and for generating a radiating plasma, wherein the two laser beams are focused on the target flow by the same focusing optics and one of the laser beams has an optical delay element; 
       FIG. 6  shows a variant of the invention analogous to  FIG. 5  with two triggered, polarized lasers, wherein an electronic delay element is provided in one of the laser beams for adjusting the time position of the two laserpulses relative to one another; and 
       FIG. 7  shows an embodiment form of the invention with laser beams that are polarized orthogonal to one another and which are focused on the target flow by separate focusing optics, and an optical delay element. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The method, according to the invention, that can be derived in principle from  FIG. 1  comprises the following steps:
     generating a continuous target flow  21 , preferably a liquid jet (or, simply, jet), by means of a suitably shaped nozzle  11  of a target generator  1 ;   radiating an energy pulse on a dividing point  25  which leads to a spatially limited, at least partial vaporization (thinning) of the continuous target flow  2  for generating a shock wave barrier relative to the continuous target jet  21 ;   radiating a high-energy beam  31  on an interaction point  24  that follows the dividing point  25  spatially in such a way that the volume (portion  22 ) of the target flow  2  downstream of the dividing point  25  is converted at least partially into radiation-emitting plasma.   

   A preferably emitted wavelength range of the radiation plasma  4  is adjusted by varying at least one of the following parameters: diameter of the target flow  2 ; pulse energy of the high-energy beam  31 . 
     FIG. 1  shows the target flow  2  divided into a continuous target jet  21  and at least one portion  22  that is separated in a defined manner. When using a target material that is gaseous under normal conditions (xenon, in the present example), the gas is liquefied in the target generator  1  at suitable pressure and suitable temperature and a liquid jet is subsequently injected into the vacuum chamber  12  through a target nozzle  11  (and possibly is even frozen when exiting from the target nozzle  11  into the vacuum chamber  12 ). 
   The target volume of portion  22  which is to be excited by an energy pulse of the high-energy beam  31  is at least substantially divided from the originally continuous target flow  2  on the axis  23  spatially in front of the interaction point  24  by means of the pulsed action of the additional energy beam  32  that is provided from a dividing beam source  33 . This means that the high-energy beam  31  impinges on a target flow  2  that is at most still somewhat continuous. By dividing (thinning) the target flow  2  at least partially, it is ensured that hydrodynamic disturbances (shock waves  27 ) are minimized in the subsequent continuous target jet  21  connected to the target nozzle  11 . The radiation-generating pulse of the high-energy beam  31  is accordingly focused on a target volume that is not directly connected to the target nozzle  11 . The pulse parameters are selected in such a way that the efficiency of the radiation generation is optimized in the desired wavelength region (e.g., EUV region around 13.5 nm). 
   The radiation-generating pulse of the high-energy beam  31  and the additional energy pulse of the additional energy beam  32  lying ahead of it in the direction of the target nozzle  11  both cause shock waves that propagate along the axis  23  of the target flow  2 . Shock waves  26  caused by the dividing pulse of the additional energy beam  32  are identified by dashed arrows in  FIG. 1  and shock waves  27  caused by the radiation-generating pulse of the high-energy beam  31  are indicated by solid black arrows. The amplitudes of the shock waves  26  and  27  are represented by the arrow lengths. 
   However, the amplitude of the shock wave  26  proceeding from the additional energy pulse of the dividing beam source  33  is substantially smaller than that of the hot plasma  4  caused by the high-energy beam  31 . 
   By means of the dividing pulse of the dividing beam source  33 , the target material is changed to a vapor phase over the entire cross section (in most cases, this is an expanding cold plasma, i.e., a plasma that hardly emits radiation in the relevant spectral region, e.g., at 13.5 nm). 
   When the shock wave  27  proceeding from the (hot) plasma  4  at the interaction point  24  reaches the location of the cold plasma  41 , the density of the originally continuous target jet  21  has decreased so sharply at that location at the dividing point  25  that the shock wave  27  running back into the target flow  2  is greatly attenuated. 
   The portion  22  of the target flow  2  can be considered sufficiently decoupled (separated) when the amplitude of the shock wave  27  of the hot plasma  4  generated by the radiation-emitting energy pulse is smaller in the continuous target jet  21  communicating with the target generator  1  than the amplitude of the shock wave  26  of the dividing pulse (shock wave barrier). 
   The adjustment of the volume of portion  22  (as mass-limited target) is carried out at a constant diameter of the target flow  2  through the timing and magnitude of the introduced energy of the dividing pulse and through the timing and distance of the high-energy pulse of the energy beam source  3  relative to the dividing pulse generated by the dividing beam source  33 . 
   When the shock wave  27  at the interaction point  24  proceeding from the hot plasma  4  reaches the location of the cold plasma  41 , the density of the continuous target jet  21  at that location has decreased so much that the shock wave  27  running upstream in the target flow  2  is transmitted in a highly attenuated manner at most. Assuming that the target flow  2  is recurrently acted upon synchronously by the dividing pulse and high-energy pulse, a portion  22  of the target flow  2  that is impinged upon by the high-energy beam  31  can be considered as divided on both sides and is therefore considered to be a mass-limited individual target. 
   In this connection, the timed sequence of the pulses of the high-energy beam  31  and additional energy beam  32  (dividing beam) is adjusted in such a way that the amplitude of the shock wave  27  of the pulse of the high-energy beam  31  running back into the continuous target jet  21  is smaller than the shock wave  26  of the dividing pulse initiated by the dividing beam  32 . This preferably takes place by means of a dividing pulse that precedes in time. In  FIG. 1 , a synchronizing device  7  is provided for this purpose between the energy beam source  3  and the dividing beam source  33 , which synchronizing device  7  determines the time position of the two pulses and possibly varies them depending on a diagnostic unit (not shown) in order to extensively suppress registered hydrodynamic disturbances in the continuous target jet  21 . Depending on the diagnosis which may also include observation of the conversion efficiency of the excitation radiation and emitted radiation, it is also possible to adjust a simultaneous or subsequent dividing pulse relative to the high-energy pulse. 
   Further, by suitably selecting the pulse energy of the energy beam source  3  and the volume (length) of the separated portion  22  of the target flow  2  (mass-limited target), the radiation conversion of excitation energy of the high-energy beam  31  can be optimized in the desired wavelength region of the hot plasma  4 . 
   It is assumed in the following—without limiting generality—that a pulsed excitation laser beam  34  is used as high-energy beam  31  and a dividing laser beam  35  is used as dividing beam  32  and that an inert gas, preferably xenon, is used for the target flow  2 . However, the same effect of plasma generation can also be achieved by an electron beam (energy&gt;10 eV). In an analogous manner, lithium, fluorine, gallium to selenium, indium to strontium, or compounds thereof, particularly saline solutions or fluoro-fomblin, can be used as target material. 
   The apparatus which is shown in a simplified manner in  FIG. 2  basically comprises—analogous to FIG.  1 —a target generator  1  which injects a target flow  2  of liquid target material with a low divergence into a vacuum chamber  12  (p&lt;1 mbar) by means of a target nozzle  11  (none of the elements mentioned above is shown in  FIG. 2 , with the exception of the target nozzle  11  and target flow  2 ), an individual energy beam source (not shown) which supplies a laser beam for dividing into an excitation laser beam  34  and a dividing laser beam  35 , and a common focusing lens  51  that focuses the two laser beams  34  and  35  on different locations of the target flow  2 . The target flow  2  is struck by the excitation laser beam  34  at the interaction point  24  and by the dividing laser beam  35  at the dividing point  25  which lies along the axis  23  of the target flow  2  spatially in front of the interaction point  24 . 
   The linearly polarized energy beam generated by a laser (serving as the energy beam source  3 ) enters a beam guiding device  6  and is converted at that location, initially by a half-wave plate  61 , into a laser beam with nonvanishing orthogonal polarization components. A polarizing beam splitter  62  then carries out a division (in this case, unequal) into a high-energy excitation laser beam  34  (solid line and polarization vector perpendicular to the drawing plane) and a weaker, dividing laser beam  35  (dashed lines and polarization vector parallel to the drawing plane) by splitting the polarization directions. 
   The energy of the dividing laser beam  35  is to be selected in such a way that the target material at the dividing point  25  evaporates over the cross section of the target jet  21 , so that at least a sufficient thinning is carried out over the entire cross section of the target flow  2  and, at most, the generation of an expanded cold plasma  41  causes a complete separation of the portion  22  of the target flow  2 . The radiation-generating energy pulse of the excitation laser beam  34  and the pulse of the preceding dividing laser beam  35  both cause shock waves  26  and  27  which propagate along the axis  23  of the target flow  2 . The pulse of the dividing laser beam  35 , which has a substantially lower pulse energy than the pulse of the excitation laser beam  34  impinging at the interaction point  24  (&lt;25% of the pulse energy of laser beam  35 ), thins the continuous target jet  21  at the dividing point  25  to the extent that a portion  22  is separated at least with respect to the shock wave transmission in the direction of the target nozzle  11 . 
   In the case of the locally thinned target flow  2 , the decoupling of the portion  22  from the area of the continuous target jet  21  is considered to be sufficient and the target flow  2  is considered to be “separated in principle” when the hydrodynamic disturbances caused by the pulse of the excitation laser beam  34  are attenuated to the extent that they are no longer perceived as troublesome in the continuous target jet  21  that flows afterward, i.e., when they are smaller in the continuous target jet  21  than the shock waves  26  generated by the (substantially smaller) pulse of the dividing laser beam  35 . 
   After polarization splitting, the dividing laser beam  35  is guided by a deflecting mirror  63  to another polarizing splitter  64  which is oriented in the passing direction for the polarization vector of the dividing laser beam  35  and which joins the two separated laser beams  34  and  35  again for focusing on the target flow  2 . 
   The excitation laser beam  34  is adjusted by means of a tilting mirror  65  in such a way that it strikes the additional splitter  64  in the reflection direction for its polarization vector. Due to the position of the tilting mirror  65 , the excitation laser beam  34  diverges slightly from the orthogonal direction with respect to the passing direction of the dividing laser beam  35  so that two beam bundles  52  and  53  which are polarized orthogonal to one another and inclined relative to one another strike a common focusing lens  51 . 
   As a result, the two inclined beam bundles  52  and  53  are focused through the focusing device  5  on different locations, the dividing point  25  and the interaction point  24 . Accordingly, the excitation laser beam  35  again strikes a portion  22  which—as was described above—is sufficiently decoupled from the latter area of the target flow  2  at least with respect to the shock wave propagation in the continuous target jet  21 . 
   As will be explained more fully in the following examples, the timed sequence in which the pulses of the excitation laser beam  34  and dividing laser beam  35  impinge on the target flow  2  can be controlled in different ways. The arrangement according to  FIG. 2  primarily makes use of a delay due to the different optical path lengths as will be shown in detail in the following examples. 
   However, laser pulses of the excitation laser beam  34  and dividing laser beam  35  that are offset in time can also be generated directly by means of suitable pulse shaping in an individual laser and the pulses that are then always offset in time are correspondingly divided through the use of an optical switch (not shown) in the beam path of the laser. 
   In the following five examples, different variants of the beam guiding device  6  for optical beam guiding and positioning of the excitation laser beam  34  and dividing laser beam  35  are described in order to effectively influence the target flow  2  with the smallest possible space requirement of the beam input-coupling into the vacuum chamber  12 . For this purpose—without limiting generality—two laser beams with beam bundles  52  and  53  that are inclined relative to one another (i.e., that are not axially parallel with respect to the focusing lens  51 ) are focused on two different target locations along the axis  23  of the target flow  2  in the first four examples by a common focusing lens  51 . 
   In order to illustrate the basic optical variants for coupling in the two laser beams—the excitation laser beam  34  and dividing laser beam  35 —and to include the different possibilities for the timed sequence of the two different laser pulses on the target flow  2 , the target flow  2  is shown in the following without specifying the direction and the excitation laser beam  34  and dividing laser beam  35  are not expressly designated. When two separate lasers are used (Examples 2 to 4), they are referred to as laser A and laser B so that both lasers A and B can be used alternatively for generating the excitation laser beam  34  or the dividing laser beam  35 . 
   EXAMPLE 1 
     FIG. 3  is based on the diagram in  FIG. 2  in that a polarized laser beam enters a beam guiding device  6 , where it is converted by a half-wave plate  61  into a laser beam with nonvanishing orthogonal polarization components and is divided by means of a polarizing beam splitter  62  into different portions with polarization vectors ∥ and ⊥, respectively, that are parallel to or perpendicular to the drawing plane. 
   The laser beam with the parallel polarization vector ∥ arrives at the focusing lens  51  via a deflecting mirror  63  and a polarizing splitter  64  for this polarization direction located in the passing direction. 
   The laser beam with the perpendicular polarization vector ⊥ is directed by a tilting mirror  65  to a retroreflector  66  which reflects in parallel the beam deflected by the tilting mirror  65  and directs it to the polarizing splitter  64 . The beam path along the tilting mirror  65  and the retroreflector  66  to the splitter  64  is an optical delay element  72  by which the time sequence of the two laser beams can be adjusted relative to one another when the retroreflector  66  is moved corresponding to the direction indicated by the arrow. 
   Due to the orientation of the tilting mirror  65 , the laser beam with the perpendicular polarization vector ⊥ is reflected at the splitter  64  in such a way that it strikes the focusing lens  51  at different angles relative to the laser beam with the parallel polarization vector ∥ that is likewise guided to the focusing lens  51  and different locations of the foci are accordingly formed on the target flow  2 . 
   EXAMPLE 2 
   In the variant according to  FIG. 4 , the excitation laser beam  34  and dividing laser beam  35  are generated in two triggered lasers, laser A and laser B. The trigger unit  71  provided for this purpose normally triggers both lasers A and B simultaneously so that a delay element, which is realized in this case as an optical delay element  72  as in  FIG. 3 , is required for adjusting a time shift of the laser pulses. However, controlling by means of offset trigger pulses from the trigger unit  71  is also possible. 
   In this example and in all of the following examples, the portions  22  that are divided (not necessarily completely) from the continuous target jet  21  are generated in that the beam bundles  52  and  53  focused by the lasers A and B are focused on different locations (located one after the other along the target axis  23 ) which—assuming a target diameter of 20 μm—lie at a distance from one another of some 10 μm to several millimeters or in special cases (as is described later in Example 6) up to several centimeters depending on the pulse energy delivered by the excitation laser beam  34 . 
   The embodiment form in  FIG. 4  shows the focusing for two unpolarized lasers A and B of the same wavelength, whose beam bundles  52  and  53  impinging on the focusing lens  51  have beam axes that are inclined relative to one another in that one of the laser beams (laser B in this case) is geometrically guided past a deflecting mirror  63  by means of which the other laser beam (laser A in this case) is coupled into the focusing lens  51 . The beam bundles  52  and  53  of the two lasers A and B intersect in front of and behind the focusing lens  51 . 
   A desired time delay of the pulse of laser A relative to that of laser B can be realized in a simple manner through optical means by the tilting mirror  65  and the retroreflector  66  (right-angle mirror). When the retroreflector  66  is movable from and toward the mirrors  65  and  63  parallel to the incident and emergent beam axes, the optical path length and therefore the time delay between the laser pulses can be varied in any manner desired. 
   EXAMPLE 3 
   In the arrangement shown in  FIG. 5 , two lasers A and B which are polarized linearly orthogonal to one another are combined by a polarizing splitter  64 . The beam bundles  52  and  53  striking the focusing lens  51  are tilted slightly with respect to one another. The inclination of the beam axis directed to the splitter  64  is adjusted by the tilting mirror  65 . As is described with reference to  FIG. 4 , a time delay of the pulses of the two lasers A and B, which is again optional, is realized by optical means by an additional retroreflector  66 . In this example, the adjustment of the pulse delay of laser A is again carried out by means of a movable retroreflector  65 , but could be realized alternatively in the same way in the beam path of laser B. 
   When using two lasers A and B with different wavelengths, the beam splitter  64 —in contrast to the principle shown in FIG.  5 —can also be realized as an edge filter (dichroic mirror) for combining the two laser beams with a slight tilt relative to the optical axis of the focusing lens  51 . 
   The beam bundles  52  and  3  of the two lasers A and B can at least partly overlap in the common focusing lens  51  and are again focused on the desired two different locations of the target flow  2  at different angles. 
   In the preceding examples 1 to 3, the delay element  72  with the tilting mirror  65  and retroreflector  65  is always considered optional. Optical paths of equal length can also be used by dispensing with the retroreflector  66  and by directing the tilting mirror  65  (rotated by 90°) to the polarization-optical beam splitter  64  so as to be inclined in a defined manner relative to the laser beam. A delay can also be realized electronically. 
   EXAMPLE 4 
   The two lasers A and B shown in  FIG. 6  have orthogonal linear polarization directions relative to one another and are (as in  FIG. 5 ) coupled in on the common focusing lens  51  by means of a polarization-optical beam splitter  64 . As in the preceding example, the beam bundles  52  and  53  strike the focusing lens  51  at different angles and partially overlap. 
   A delay loop is realized in this example by an electric delay element  73  and is represented in  FIG. 6  as a delay line of corresponding length. The optical paths of the beam paths of laser A and laser B are of approximately equal length up to the focusing lens  51 . The trigger unit  71  provides the same trigger pulses for both lasers A and B (simultaneously). However, the electric delay element  73  could also be replaced by a trigger unit  71  that is capable of sending phase-shifted trigger pulses to lasers A and B. 
   EXAMPLE 5 
   In  FIG. 7 , a laser beam used as energy beam  31  is divided in the same way as is shown in  FIG. 2  and  FIG. 3 . However, after the division, which is shown in this example as polarization-optical beam splitting (but which can also advantageously be a dichroic division), it can be guided as completely separate beam paths. 
   The two beam paths are not subsequently joined by a polarizing splitter  64 , but are focused on two different locations of the target flow  2  separately by two separate focusing lenses  51 . 
   For reasons of space, the beam bundles  52  and  53  to be focused are to be oriented in different radial directions around the axis  23  of the target flow  2 . In the present example, they can both impinge orthogonal to the axis  23  of the target flow  2 . As is shown in  FIG. 7 , their diametrically opposed incident direction was selected only by way of example because of the simpler two-dimensional view. The optical axes of the two focusing lines  51  preferably enclose an angle of less than 90° around the axis  23 . 
   The optical delay element  72  formed by deflecting mirror  63  and retroreflector  66  is again considered optional and is only required when plasma generation and division are to be carried out with a delay in time with respect to one another. 
   The essential advantage of a mass-limited target generated according to the preceding examples in the form of a defined portion  22  consists in a potentially very high repetition frequency of the pulses for generating the radiating plasma  4 . 
   Further, in contrast to individual droplets, whose volume can be varied only slightly when the nozzle diameter is fixed, the volume of the separated portion  22  can be adjusted relatively simply by means of the spatial distance between the dividing point  25  of the dividing beam  25  (advantageously the dividing laser beam  35 ) and the interaction point  24  of the energy beam  31  (advantageously laser beam  34 ). 
   The synchronization of the energy beam  31  and dividing beam  32 , particularly when both are laser beams, is also substantially simpler than synchronizing an excitation laser to separate droplet targets that are provided through oscillating mechanical generation in which the frequency of the droplet formation is not totally free from fluctuations. 
   Due to the low divergence of a continuous target jet  21 , a relatively large working distance (on the order of several centimeters) from the target nozzle  11  of the target generator  1  can be selected. 
   EXAMPLE 6 
   With reference to  FIG. 1 , a special construction of the invention is described in the following in which a particularly long portion  22  can be separated from the target jet  21  exiting continuously from the target nozzle  11  by the dividing beam  32 . 
   A freely progressing, long portion  22  of this kind is then repeatedly acted upon successively along its length by the high-energy beam  31  when passing the interaction point  24 . The length of the portion  22  and the quantity of the high-energy pulses “fired off” on it is limited by the stability of the target material after the effect of the shock waves  27  starting with the first pulse of the high-energy beam  31 . 
   In this embodiment form, the transmission of the shock waves  27  and their disruptive effect when the target flow  2  exits from the target nozzle  11  of the target generator  1  is prevented in all cases, so that the highly sensitive process of injecting the target flow  2  into the vacuum chamber  12  can be better controlled and stabilized. 
   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 
         11  target nozzle 
         12  vacuum chamber 
         2  target flow 
         21  continuous target jet (first portion) 
         22  second portion 
         23  axis (of the target flow) 
         24  interaction point 
         25  dividing point 
         3  energy beam source 
         31  high-energy beam 
         32  dividing beam (additional energy beam) 
         33  dividing beam source 
         34  excitation laser beam 
         35  dividing laser beam 
         4  hot plasma 
         41  cold plasma 
         5  focusing device 
         51  focusing lens 
         52 ,  53  beam bundles 
         6  beam guiding device 
         61  half-wave phase plate 
         62  polarization-optical splitter 
         63  deflecting mirror 
         64  (combining) beam splitter 
         65  tilting mirror 
         66  retroreflector 
         7  synchronizing device 
         71  trigger unit 
         72  optical delay element 
         73  electric delay element