Abstract:
The invention is directed to an arrangement for metering target material for the generation of short-wavelength electromagnetic radiation from an energy beam induced plasma, in particular X radiation and EUV radiation. The object of the invention is to find a novel possibility for metering target material for the generation of short-wavelength electromagnetic radiation from an energy beam induced plasma which makes it possible to provide reproducibly supplied mass-limited targets in such a way that only the amount of target material for plasma generation that can be effectively converted to radiating plasma in the desired wavelength region arrives in the interaction chamber and, therefore, debris generation and the gas burden in the interaction chamber are minimized. This object is met, according to the invention, in that an injection device is provided for target generation, wherein means are arranged upstream of the nozzle in a nozzle chamber for a defined, temporary pressure increase in order to introduce an individual target into the interaction chamber exclusively when required, and an antechamber is arranged around the nozzle for generating a quasistatic pressure upstream of the interaction chamber, wherein an equilibrium pressure in the antechamber prevents the escape of target material as long as there is no pressure increase in the nozzle chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of German Application No. 10 2004 036 441.9, filed Jul. 23, 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 and a method for metering target material for the generation of short-wavelength electromagnetic radiation from an energy beam induced plasma. It is applied in particular in EUV radiation sources for projection lithography in semiconductor chip fabrication. 
   b) Description to 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 in the focus 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 EUV 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 (with slight damping) 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 metering target material for the generation of short-wavelength electromagnetic radiation, in particular X radiation and EUV radiation, from an energy beam induced plasma which makes it possible to provide reproducibly supplied mass-limited targets in such a way that only the amount of target material for plasma generation that can be effectively converted to radiating plasma in the desired wavelength region arrives in the interaction chamber and, therefore, debris generation and the gas burden in the interaction chamber are minimized. 
   In an arrangement for metering target material for the generation of short-wavelength electromagnetic radiation, in particular EUV radiation, in which a target generator is arranged for providing target material along a given target path and an energy beam for generating a radiation emitting plasma is directed to the target path, the above-stated object is met, according to the invention, in that the target generator has an injection device which contains a nozzle chamber with nozzle and which is connected with a reservoir, wherein means are provided at the nozzle chamber for a defined, temporary pressure increase in order to introduce an individual target into the interaction chamber at the interaction point exclusively when required for the generation of plasma, and in that means are arranged for adjusting an equilibrium pressure in the nozzle in order to compensate for a pressure drop at the nozzle of the injection device resulting from the pressure difference between the vacuum pressure in the interaction chamber and the pressure exerted on the target material in the reservoir, wherein the adjusted equilibrium pressure prevents the escape of target material as long as there is no temporary pressure increase in the nozzle chamber. 
   A piezo element is advantageously provided as means for the pressure increase in the nozzle chamber. The piezo element causes a reduction in the volume of the nozzle chamber by means of inward displacement of a wall of the nozzle chamber. For this purpose, the nozzle chamber preferably has a membrane wall which is pressed into the interior of the nozzle chamber when voltage is applied to the piezo element. However, a piezo stack can also advisably be arranged inside the nozzle chamber for reducing the volume of the chamber. 
   In another advantageous variant, a constriction is provided in the nozzle chamber and a heating element is arranged around this constriction, wherein the target material is heated inside the constriction and a defined target volume is thrust into the nozzle chamber as a result of thermal expansion and leads to the temporary pressure increase. A portion of a connection line to the reservoir close to the nozzle chamber can also advisably be used as a constriction of the nozzle chamber. 
   Additional pressure is advantageously applied in the reservoir for liquefaction for a target material that is liquid at the process temperature only at pressures above 50 mbar. A target material that can be used for this embodiment variant is preferably xenon. 
   With a target material that is liquid at the process temperature at pressures of less than 50 mbar, the gravitational pressure of the target material in the reservoir can advisably be used for adjusting pressure. Target materials using tin are preferably used for this purpose. Various tin alloys and tin chlorides have proven particularly suitable for the generation of EUV radiation. Tin(IV) chloride (SnCl 4 ), which is already in liquid form under process conditions for plasma generation, and tin-II-chloride (SnCl 2 ) are suitable as preferred target material when used in aqueous or alcoholic solution. 
   When using this kind of target material which is liquid at pressures below 50 mbar under process conditions for plasma generation, the hydrostatic or gravitational pressure of the target material can be used to minimize the equilibrium pressure at the outlet of the nozzle. For reducing pressure, a height difference between the liquid level of the target material at the nozzle and in the reservoir must be adjusted in such a way that the liquid level in the reservoir lies below the outlet of the nozzle in the direction of the force of gravity. For this purpose, the nozzle of the nozzle chamber can advisably be arranged in direction of the force of gravity so that the individual targets are subject to the acceleration due to gravity along the target path. On the other hand, it can be advantageous for the desired reduction of the pressure drop in the target nozzle when the nozzle is arranged at the nozzle chamber opposite to the direction of the force of gravity. 
   The means for generating an equilibrium pressure are preferably realized in that an antechamber having an opening along the target path for the exit of the individual targets is arranged around the nozzle of the injection device in front of the interaction chamber, wherein a quasistatic pressure is present in the antechamber which, as equilibrium pressure, prevents target material from escaping as long as there is no temporary pressure increase in the nozzle chamber. 
   A buffer gas is preferably fed to the antechamber as a moderator for high kinetic energy particles from the plasma. The buffer gas supplied to the antechamber can be an inert gas or a noble gas. Nitrogen, helium, neon, argon and/or krypton are preferably used. 
   The energy beam required for introducing energy into the individual target according to the invention is preferably a focused laser beam. A pulse of the energy beam in the interaction chamber is advisably synchronized with the ejection of exactly one individual target. 
   However, it has proven advantageous particularly when using a laser beam as energy beam that a pulse of the energy beam in the interaction chamber is synchronized with the ejection of at least two individual targets from the nozzle of the injection device, wherein at least a first target is a sacrifice target for generating a vapor screen for at least one main target to be struck by the energy beam. 
   In a first modified construction variant, a pulse of the energy beam in the interaction chamber is synchronized with the ejection of at least two individual targets from a plurality of nozzles of the injection device, wherein the nozzles are arranged in at least one plane that forms an angle between 3° and 90° (depending on the target diameter and spacing of the nozzles) with a plane defined by the axis of the energy beam and a mean target path. In this connection, nozzles of the same size can be arranged at a shared nozzle chamber or at separate nozzle chambers. 
   In a second preferred embodiment, a pulse of the energy beam in the interaction chamber is synchronized with the ejection of a plurality of individual targets following one another in close succession from every nozzle of the injection device, wherein at least a first individual target from each nozzle is a sacrifice target for generating a vapor screen for at least one main target to be struck by the energy beam. 
   The changes in pressure in every nozzle chamber of the injection device are advantageously synchronized with the pulse of the energy beam in such a way that a target column comprising at least one sacrifice target and two main targets is prepared for every pulse of the energy beam from every nozzle. The nozzle chambers of the injection device for the ejection of targets can have an in-phase synchronization or an alternating phase-delayed synchronization of the means for temporarily increasing pressure. The latter variant has the added advantage that the individual targets move to the interaction point (e.g., the laser focus) so as to be offset relative to one another and results in a kind of “target curtain” when the nozzles are correspondingly arranged in a plurality of rows close together. 
   Further, in a method for metering target material for the generation of short-wavelength electromagnetic radiation, in particular EUV radiation, in which target material is provided from a nozzle of a target generator along a given target path and an energy beam for generating a radiation-emitting plasma is directed to the target path, the above-stated object is met by the following steps:
         generation of a quasistatic equilibrium pressure at the nozzle so that no target material exits from the nozzle in the inoperative state of the target generator;   generation of a temporary pulsed pressure increase in a nozzle chamber located fluidically upstream of the nozzle, so that target material is shot out of the nozzle chamber through the nozzle and is accelerated as an individual target in direction of an interaction point with the energy beam; and   synchronization of the pulsed pressure increase in the nozzle chamber with a pulse of the energy beam so that every individual target is struck precisely by a pulse of the energy beam.       

   Accordingly, the invention is based on the fundamental consideration that only precisely as much target material as is needed for efficient generation of short-wavelength electromagnetic radiation in the desired wavelength range may reach the interaction point because any excess amount of target material, even if only located in the area surrounding the interaction point, leads to the generation of unwanted target gas and additional debris. Also, it must be prevented that any target material at all passes the interaction point between the pulses of the energy beam in order to minimize the gas burden from vaporized or sublimated target material in the evacuated interaction chamber and to minimize the consumption of target material. 
   For this purpose, an injection device operating in a pulsed manner is used, according to the invention, for dispensing the individual targets in metered amounts, which injection device provides individual targets only when required, i.e., on demand (through pulse control), by means of an adjusted equilibrium pressure at the nozzle opening during pauses between injections. 
   The arrangement according to the invention makes it possible to introduce target material into the interaction chamber in the exact amount needed for efficient radiation generation at a desired repetition rate of the energy beam and to minimize debris generation and radiation absorption through vaporized target material in the interaction chamber. Further, the consumption of target material is reduced so that costs are appreciably lowered. Further, it is possible to increase the pulse repetition frequency. 
   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 schematic view of the arrangement according to the invention; 
       FIG. 2  is a schematic illustrating the method according to the invention; 
       FIG. 3  shows a variant of the injection device with piezo element; 
       FIG. 4  shows a variant of the injection device with heating element; 
       FIG. 5  is a schematic phase diagram for xenon; 
       FIG. 6  illustrates an advantageous synchronization of individual targets as columns of sacrifice targets and main targets; 
       FIG. 7  shows two constructions of the injection device for generating target fields a) with a plurality of nozzles at a nozzle chamber and b) with one nozzle at each separate nozzle chamber; 
       FIG. 8  shows a variant of the target generator with a special construction of the reservoir for reducing the equilibrium pressure at the nozzle for target materials with low vapor pressure (&lt;50 mbar); 
       FIG. 9  shows a special variant of the target generator for target materials with low vapor pressure (&lt;50 mbar) in which the equilibrium pressure at the nozzle can be adjusted by means of its ejection direction opposed to the force of gravity and to the gravitational pressure of the target material relative to the interaction chamber. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic view showing a portion of a radiation source for generating short-wavelength electromagnetic radiation based on a plasma induced by the input of energy. The drawing shows an interaction chamber  1  in which individual targets  3  are prepared along a target path  31  by a target generator  2 . The target path  31  is intersected by the axis  41  of an energy beam  4  at an interaction point  51 , wherein a plasma  5  emitting the desired radiation is generated by the energy beam  4  impinging on a respective individual target  3 . 
   The target generator  2  comprises an injection device  21  with a nozzle  211  and a nozzle chamber  212  which is able to cause a temporary change in volume ΔV and, therefore, a change in pressure of the nozzle chamber pressure P Dk . The principle is similar to that of conventional inkjet nozzles and will be described in more detail ( FIG. 3  and  FIG. 4 ) in the following. Further, the injection device  21  of the nozzle chamber  212  is connected to a reservoir  22  for the target material  32  which is maintained in liquid state at a defined process temperature with a suitable pressure p 1 . 
   The nozzle  211  opens into an antechamber  23  in which an antechamber pressure p 2  is maintained. The antechamber  23  has at least one gas feed  231  that supplies an additional gas for adjusting a uniform (quasistatic) pressure around the nozzle  211 . Further, the antechamber  23  has, along the target path  31 , an opening  232  for the individual targets  3  that are shot in a pulsing manner from the nozzle  211  for passing into the interaction chamber  1 . The opening  232  presents a defined flow resistance for the gas that is fed into the antechamber  23 . Depending on the amount of gas supplied to the antechamber  23 , the antechamber pressure p 2  can be adjusted approximately statically, i.e., there is a stationary gas flow. The supply of gas is regulated by the gas feed  231  in such a way that an equilibrium pressure is adjusted on the liquid target material  32  at the nozzle  211  so that no target material  32  can exit without a change in pressure in the nozzle chamber  211 . An individual target  3 , i.e., a defined amount of target material  32 , is not shot out of the nozzle  211  until there is a temporary change in pressure in the nozzle chamber  212  (represented by volume change ΔV). The individual target  3  flies through the antechamber  23 , passes through the opening  232  of the latter into the interaction chamber  1  and is available as a mass-limited individual target  3  for generation of the plasma  5 . 
   The gas which is fed into the antechamber  23  and which likewise reaches the interaction chamber  1  through the opening  232  is pumped out in the interaction chamber  1 . One or more vacuum pumps (not shown) that are connected to the interaction chamber  1  are dimensioned in such a way that a vacuum pressure p 3  is maintained at which the desired radiation is absorbed as little as possible (&lt;100 Pa). 
   Further, the gas supplied to the antechamber  23  can serve in addition as a moderator (buffer gas/moderator) for high kinetic energy particles (debris) from the plasma  5  which are decelerated and absorbed by the buffer gas so as to prolong the service life of the optical and mechanical components, particularly the collector mirror (not shown) for the radiation emitted from the plasma  5 , and the nozzle  211 . 
     FIG. 2  schematically illustrates the method according to the invention, wherein an individual target  3  is generated from the nozzle  211  only when this individual target  3  can also be converted (at a later time) by the energy beam into radiating plasma  5  at the interaction point  51 . This means that individual targets  3  are generated only on demand. Since only liquid target material  32  can exit through the nozzle  211 , this is referred to as Drop On Demand. Accordingly, corresponding to the desired pulse frequency of the energy beam  4 , individual targets  3  are generated which arrive at the interaction point  51  with a period of the pulse frequency of the energy beam  4 . Therefore, there are no individual targets  3  or other excess residual target components that continue along the target path  32  beyond the interaction point  51 . 
   A possibility for metering very small volumes (up to the picoliter range) at frequencies of several kilohertz based on the so-called drop-on-demand method for nozzles of inkjet printers is described in the following with reference to  FIG. 3  (piezo principle) and  FIG. 4  (bubble jet principle). 
   All embodiment forms for realizing the drop-on-demand method have the same fundamental functional features with limited liquid ejection which are generalized, according to the invention, as follows. Upstream of the nozzle  211  there is a nozzle chamber  212  that is completely filled with a liquid (target material  32 ). By reducing the volume of the nozzle chamber  212 , a defined amount of target material  32  corresponding approximately to the amount of the change in volume ΔV of the nozzle chamber  212  is ejected through the nozzle  211  and accordingly generates a mass-limited individual target  3 . 
   The difference between the various embodiments of the drop-on-demand method employed by the invention consists only in the specific technique for achieving the volume reduction of the nozzle chamber  212  and temporary pressure increase at the nozzle  211 . However, the specific way in which the volume change ΔV is carried out is not essential to the functioning of the principle of generation of mass-limited individual targets  3  according to the invention, so that any other principles (techniques) for temporary defined changes in pressure in the nozzle chamber  212  are also comprehended by the teaching of the invention. 
   Common to all methods of this type is that the static pressure on the reserved liquid and the pressure p 2  at the nozzle  211  are virtually equal in the inoperative state, i.e., when no individual target  3  (liquid droplet) is to be generated. The liquid target material  32  can be prevented from exiting through capillary forces with small pressure differences. 
   Two boundary conditions must be taken into account for metering small volumes of target material  32  in individual targets  3  for the generation of energy beam induced plasmas  5  that emit their radiation in the extreme ultraviolet spectral range. First, the target material  32  must be under vacuum for the excitation by the energy beam  4  in the interaction chamber  1 , wherein—in order to prevent or minimize reabsorption of the desired radiation—the pressure p 3  ( FIG. 1  and  FIG. 8 ) in the interaction chamber  1  is typically less than 100 Pa (1 mbar). Second, the liquid pressure p Dk  in the case of xenon (as preferred target material) must be at least approximately 80 kPa (0.8 bar) so that xenon is in a liquid state of aggregation, as can be seen from the phase diagram in  FIG. 5 . 
   If the outlet of the nozzle  211  were located directly in the interaction chamber  1  (see  FIG. 1 ), the (large) pressure gradient in the nozzle  211  would necessarily lead to the continuous outflow of the liquid target material  32  into the vacuum of the interaction chamber  1 , wherein one of the known target forms, i.e., jet target (continuous target flow according to EP 0 895 706 B1), discontinuous droplet flow (regularly exiting droplets according to EP 0 186 491 B1), dense droplet mist (from gas puff according to WO 01/30122 A1, or spray according to U.S. Pat. No. 6,324,256 B1) would occur, depending on the nozzle shape, liquid pressure and liquid temperature. 
   The injection device  21  based on the piezo effect is shown schematically in  FIG. 3 . A piezo element  213 , whose dimensions and volume increase when voltage is applied to it and which accordingly temporarily reduces the chamber volume by means of a change in volume ΔV of the nozzle chamber  212 , is located in the nozzle chamber  212  or at a membrane forming a wall of the nozzle chamber  212 . At the same time, the pressure in the nozzle chamber  212  increases above the equilibrium pressure p 2  in the antechamber  23 . Therefore, when a voltage pulse is applied to the piezo element  213  a drop of liquid target material  32  is shot from the nozzle  211  into the antechamber  23 . This process leads to the generation of individual targets  3  capable of synchronization with the desired or given pulse frequency of the energy beam  4  which can advantageously be a laser beam  42  ( FIG. 7   a ). 
     FIG. 4  shows the schematic of an embodiment form of the injection device  21  based on the so-called bubble jet principle which is likewise known, per se, from inkjet printing technology. In this embodiment, a heating element  215  is arranged around a (preferably cylindrical) constriction  214  of the nozzle chamber  212 . When a defined amount of target material  32  is to be dispensed through the nozzle  211 , the heating element  215  is intensively heated temporarily. The constriction  214  for the heating element  215  can also be a segment of the connection line leading to the reservoir  22  so that the nozzle chamber  212  can be kept small and compact. 
   Due to the pulsed heating of the heating element  215 , the liquid target material  32  vaporizes locally in the constriction  214  so that a vapor bubble  33  is formed. This vapor bubble  33  causes an increase in the volume of the target material  32  at a constant volume of the nozzle chamber  212  and, as a result of the pressure increase which therefore occurs in the nozzle chamber  212 , presses an amount of liquid target material  32  out of the nozzle  211  in an explosive manner. The vapor bubble  33  collapses as a result of the ejection and subsequent cooling of the liquid, and target material  32  flows out of the reservoir  22 . 
     FIG. 5  shows the phase diagram of xenon, which is a preferred target material  32 . The diagram shows the typical temperature-pressure range for a xenon jet which detaches, possibly actively or passively, in droplets. This range occurs at temperatures between approximately 163 K (−111° C.) and 184 K (−90° C.) and at a pressure of about 0.1 MPa (1 bar) to 2 MPa (20 bar). Below a pressure of 80 kPa (0.8 bar), xenon is no longer liquid at any temperature. Therefore, it is necessary to charge a reservoir  22  with liquid xenon at a pressure p 1  of at least 0.8 bar. Xenon is advantageously liquefied at a temperature of 165 K under a pressure of 200 kPa in the reservoir. Approximately the same pressure is adjusted as antechamber pressure p 2  in a quasistatic (i.e., fluidically stationary) manner in the antechamber  23  by means of the gas feed  232  (according to the view in  FIG. 1 ). 
   With other target materials  32 , e.g., water or aqueous solutions of preferred characteristic EUV radiators (for example, tin alloys, tin(II) chloride SnCl 2  or tin(IV) chloride SnCl 4 ), as well as for aqueous or alcoholic solutions thereof, the phase diagram in  FIG. 4  is very similar qualitatively, but the pressure-temperature range lies at appreciably different values. A target generator  2  that is somewhat modified in that the gravitational pressure of the liquid column in the reservoir  22  is used for reducing the equilibrium pressure at the nozzle  211  can be used with this group of target materials  32 , as will be described below with reference to  FIG. 8  and  FIG. 9 . 
   An exactly timed, metered injection of target material  32  (e.g., according to  FIG. 1 ) is achieved in that the nozzle  211  opens into an antechamber  23  having increased pressure relative to the interaction chamber  1 , so that in the passive state of the injection device  21  an equilibrium exists between the liquid pressure p Dk  in the nozzle chamber  212  and a quasistatic pressure p 2  in the antechamber  23  through which gas flows. Target material  32  is shot out as a mass-limited individual target  3  only by means of a temporary pressure increase in the nozzle chamber  212  (according to the so-called drop-on-demand method), wherein the individual target  3  passes through the antechamber  23  virtually unchanged because of the increased pressure (at least the vapor pressure of the target material) and begins to vaporize only after exiting through an opening  232  in the vacuum of the interaction chamber  1 . 
   The pressure p 2  in the antechamber  23 , which has an opening for the passage of the individual target  3  along its predetermined target path  31 , is adjusted in that gas flows in via comparatively large feed lines  231  and escapes into the interaction chamber  1  through the opening  232  which must be somewhat larger than the individual target  3  itself. The opening  232  constitutes flow resistance for the supplied gas. Therefore, the pressure at the gas feeds  231  is regulated in such a way that a quasistatic pressure p 2  almost identical to pressure p 1  ( FIG. 1 ) is adjusted in the antechamber  23  and acts on the reserved liquid in the reservoir  22 . The inactive condition and the thermodynamic condition for a target material  32  (e.g., xenon) that is liquefied (gaseous under normal pressure) in the reservoir  22  are accordingly met. 
   When it is required to dispense an individual target  3 , the pressure of the liquid p Dk  ( FIG. 1 ) is temporarily increased above the pressure p 2  of the antechamber  23  in the injection device  21  for changing the volume ΔV in the nozzle chamber  212 . A certain amount of target material  32  is accordingly pressed out of the nozzle  211  and accelerated. 
   The individual target  3  that is formed in this way flies through the antechamber  23  which is at pressure p 2  and enters the interaction chamber  1  through its opening  232 , wherein a plasma  5  is generated by the introduction of energy (e.g., a laser pulse) in the individual target  3  arriving at the interaction point  51 . Vacuum pumps (not shown) at the interaction chamber  1  are designed in such a way that a correspondingly low vacuum pressure p 3  (&lt;100 Pa) is adjusted. 
   When the individual target  3  has entered the interaction chamber  1 , a vaporization and sublimation process takes place—in a particularly intensive manner in the case of xenon—at the target surface, which reduces and cools the injected target material  32 . This cooling is accompanied by a phase conversion, depending on the target volume and length of the target path  31 , so that an individual target  3  of liquid target material  32  can also be frozen at the interaction point  51  (solid state of aggregation). 
   In addition to the amount of target material  32  for an individual target  3  which interacts directly with the energy beam  4  at the interaction point  51 , an additional amount of target material  32  must be introduced for an efficient generation of radiation because of the vaporization and sublimation of the target material. This additional amount of target material  32  is vaporized and sublimated in the interaction chamber  1  along its target path  31  from the opening  232  of the antechamber  23  to the interaction point  51 . This latter process is reinforced by the radiation from the plasma  5  that is absorbed by the target material  32  when a close succession of individual targets  3  is required because of high pulse repetition frequency of the energy beam  4 . 
   Therefore, it is useful to shoot a column of (at least) two liquid drops out of the nozzle  211  at a very short interval as is illustrated in  FIG. 6 , wherein the first drop(s) is (are) sacrifice targets  34  and the last drop is the main target  35  (remaining individual target  3  for the interaction with the energy beam  4 ). 
   In this connection,  FIG. 6  shows, after a volume change ΔV (time t 0 ), a column of initially two targets  34  and  35  in the time segment from t 1  to t 4 , of which only the main target  35  is left at the interaction point  51  because the sacrifice target  34  is vaporized or sublimated along the target path  31 . The advantage of this procedure for generation of the final individual target  3  (main target  35 ) at the interaction point  51  is that metering is simpler because the main target  35  traverses the interaction chamber  1  behind the vaporization screen  36  of the sacrifice target(s)  34  virtually without loss of mass. 
   In order to reduce the evaporation or sublimation of target material  32  from the individual targets  3  shot from the nozzle  211 , the gas flowing out into the antechamber  23  and interaction chamber  1  is selected in such a way that it acts, in addition, as a moderator for high kinetic energy particles from the plasma  5  (also called buffer gas). For this purpose, a gas is used which, on one hand, has the lowest possible absorption for the desired wavelength of the radiation from the plasma  5  and which, on the other hand, by pulsing, provides for good energy transmission and energy distribution of the high-energy atoms and ions (debris) emitted from the plasma  5 . Gases of this kind are, e.g., inert gases such as nitrogen or most noble gases with a low atomic number such as helium, neon, argon or krypton. Argon (possibly mixed with helium in order to improve the flow behavior) is preferably used. 
   The radiation conversion from the plasma  5  is more efficient when the individual target  3  has a smaller depth than the energy beam  4 , i.e., the target diameter is small. This is conflicts with the fact that, e.g., a laser beam  42  (as preferred realization of the energy beam  4 , e.g.,  FIG. 6   a ) cannot be focused as small as desired and the efficiency of the radiation generation could therefore be increased by means of a “flat” target. A solution which approaches this ideal and which can actually be realized consists in one or more rows of droplets as is shown in  FIGS. 6   a  and  6   b.    
   For this purpose, as is shown in  FIG. 7   a , a plurality of nozzles  211  are arranged closely adjacent to one another in a nozzle chamber  212 , each of which ejects an individual target  3  simultaneously. These individual targets  3  which are lined up in one or more straight lines ( FIG. 7   b ) arrive at the focus  43  of the laser beam  42  after a defined time of flight along the separate target paths  31  and are illuminated simultaneously during a laser pulse and converted into radiating plasma  5 . 
     FIG. 7   b  builds upon the same principle as  FIG. 7   a , but in this case each nozzle  211  is associated with a separate nozzle chamber  212 . The separate volume changes ΔV in the individual nozzle chambers  212  can preferably be carried out by separate piezo elements (not shown) synchronously or—as is shown in  FIG. 7   b —with a time offset. 
   According to  FIG. 7   a , the nozzles  211  are arranged along a straight line which has an angle α, clearly diverging from 90°, with the optical axis  41  of the laser beam  42 . Alternatively, the nozzles  211  can also be arranged so as to be offset relative to one another in a plurality of rows (according to DE 103 06 668 A1) in order to increase the density of the individual targets  3  (e.g., without substantial gaps or overlapping). 
   Further, the “flat” target according to  FIG. 7   a  or  7   b  can be combined with the droplet column according to  FIG. 5 , wherein a plurality of main targets  35  follow the sacrifice targets  34  that are included for vaporization, so that there occurs in all almost a “carpet” of droplets which is struck by a pulse of the laser beam  42 . In combination with the above-mentioned plurality of rows of nozzles (not shown), the intervals between the individual targets  3  arriving in the laser focus  43  can also be narrowed when the nozzles  211  of different rows have ejection times that are slightly delayed with respect to one another. 
   Another special construction of the invention for target materials  32  with low vapor pressure is shown in  FIG. 8 . 
   When the target material  32  is a liquid having a low vapor pressure (&lt;50 mbar) under process conditions, e.g., tin(IV) chloride (SnCl 4  has a vapor pressure of about 25 mbar at room temperature), or tin(II) chloride (SnCl 2  has a vapor pressure of about 24 mbar in aqueous or alcoholic solution at room temperature), or simply water (H 2 O vapor pressure approximately 25 mbar), the gas pressure in the antechamber  23  can be minimized and the gas burden in the interaction chamber  1  can accordingly be reduced. For this purpose, as is shown in  FIG. 8 , the pressure of the target material  32  in the nozzle chamber  212  is reduced in that the gas pressure p 1  in the reservoir  22  is suitably adjusted by evacuating the gas volume over the target material  32  by means of a vacuum pump  221  outfitted with a regulating valve. 
   In addition or alternatively, the liquid pressure p Dk  at the nozzle  211  can be adjusted by a height difference h 1  between the levels of the target material  32  in the reservoir  22  and in the nozzle  211  to
 
 p   Hd   =ρ·g·h   1 ,
 
where ρ is the density of the target material  32  and g is the acceleration due to gravity. The pressure p 2  in the antechamber need then—at the minimum, when p 1  corresponds to the vapor pressure of the target material  32 —compensate only the gravitational pressure
 
 p   Sd   =ρ·g·h   2 
 
of the target material  32  along the nozzle  211  in the nozzle chamber  212  in addition in order to prevent target material  32  from flowing out of the nozzle  211  in the passive state of the injection device  21 .
 
     FIG. 9  shows another modification of the arrangement according to  FIG. 8  for target materials  32  with low vapor pressure (&lt;50 kPa) in which the ejection direction of the nozzle(s)  211  is oriented against the acceleration due to gravity. Accordingly, another reduction of the required equilibrium pressure p 2  at the nozzle  211  can be achieved. 
   If the gravitational pressure
 
 p   Hd   =ρ·g·h   1 
 
of the liquid column of the target material  32  can be successfully adjusted through the selection of target material  32  and of the (negative) height difference h 1  (between the outlet of the nozzle  211  and the liquid level in the reservoir  22 ) in such a way that the pressure difference between the pressure p 1  (at a minimum, the vapor pressure of the target material  32 ) in the reservoir  22  and the vacuum pressure p 3  (e.g., 100 Pa) in the interaction chamber  1  can be compensated, an antechamber  23  is not required in theory. For this reason, it is shown in dashes in  FIG. 9 .
 
   However, in this configuration also, it has proven advisable to use an antechamber  23  in order, on the one hand, to avoid unnecessarily large lengths of the connection line between the reservoir  22  and the nozzle chamber  212  and, on the other hand, to decelerate highly kinetic particles (debris) from the plasma  5  and additionally stabilize the target path  31  of the individual targets  3 . 
   The object in the embodiment variants according to  FIG. 8  and  FIG. 9 , as in all of the variants of the invention, is to adjust the sum of all of the pressure components acting at the outlet of the nozzle  211  to zero in the inoperative state of the injection device  21 , i.e., to compensate the pressure p 1  (at least the vapor pressure of the target material  32 ) which is substantially higher in the reservoir  22  than in the interaction chamber  1 . However, aside from the antechamber arrangement that is primarily suggested for this purpose with dynamic pressure p 2  (as counter-pressure to the minimum adjusted vapor pressure of the target liquid) that is adjusted so as to be quasistatic (fluidically stationary), other equivalent means for pressure compensation clearly belong to the technical teaching of the invention, for example, the variants without an antechamber  23  which were described with reference to  FIG. 9 . 
   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  vacuum chamber 
         2  target generator 
         21  injection device 
         211  nozzle 
         212  nozzle chamber 
         213  piezo element 
         214  constriction 
         215  heating element 
         22  reservoir 
         221  vacuum pump 
         23  antechamber 
         231  gas feed 
         232  opening 
         3  individual target 
         31  target path 
         32  target material 
         33  vapor bubble 
         34  sacrifice target 
         35  main target 
         36  vaporization screen 
         4  energy beam 
         41  axis 
         42  laser beam 
         43  focus 
         5  plasma 
         51  interaction point 
       h 1 , h 2  height difference 
       p 1 , p 2 , p 3  pressure 
       p Dk  liquid pressure (in the nozzle chamber) 
       ΔV volume change 
       α angle