Abstract:
The invention is directed to a method and arrangements for the suppression of debris in short-wavelength radiation sources based on a plasma, particularly for EUV sources for semiconductor lithography. The object of the invention is to find a novel possibility for suppressing the particle flow (debris) from a plasma which keeps the debris away from primarily optical components located downstream without excessive attenuation of the desired radiation emitted from the plasma. According to the invention, this object is met in that a buffer gas is injected inside the filter structure of the debris filter lateral to openings that are provided for passing the radiation. The filter structure generates a flow resistance in direction of the plasma and in direction of propagation of the radiation so that an increased gas pressure of buffer gas remains limited to a defined volume layer in the debris filter relative to the pressure in the vacuum chamber, and the buffer gas exiting from the filter structure of the debris filter is sucked out of the vacuum chamber by vacuum pumps.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of German Application No. 10 2005 020 521.6, filed Apr. 29, 2005, the complete disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     a) Field of the Invention  
         [0003]     The invention is directed to a method and arrangements for the suppression of debris in short-wavelength radiation sources based on a plasma in which the short-wavelength radiation that is emitted from the plasma and is in a vacuum chamber is directed through a debris filter with at least one mechanical filter structure before the short-wavelength radiation reaches the collector optics. It is preferably applied in EUV sources for semiconductor lithography.  
         [0004]     Besides the desired EUV radiation, light sources for EUV radiation that are based on a radiation-emitting hot plasma also generate debris, as it is called. By debris is meant any form of particles that are undesirable within the meaning of the present application and that exit the plasma and negatively influence the performance or stability of the application components, particularly the life of optical surfaces.  
         [0005]     Within the framework of EUV sources, the following types of debris can be distinguished: 
        a) fast, high-energy atomic particles/ions which damage optical surfaces through sputtering (energies are typically 1-10 keV);     b) slow particles which can deposit on optical surfaces (typical energies corresponding to a temperature of less than 5000 K or less than 1 eV);     c) large macroscopic particles from electrode material.        
 
         [0009]     In collector mirrors such as those used in microlithography for focusing the EUV radiation in an intermediate focus (multilayer mirrors or metallic mirrors with grazing light incidence), debris of type a) leads to a rapid loss of reflectivity through destruction of the sensitive coating. Since this effect is brought about by sputtering on the optical surface and the resulting removal of material, the decisive function of the arrangement according to the invention is either to eliminate the fast particles or to slow them down to kinetic energies significantly below 10 eV which no longer allow sputtering.  
         [0010]     Failure to completely eliminate debris of type a) is due to the lack of availability of suitable materials which must be equally resistant to the considerable loading by both radiation and debris and which, on the other hand, must be highly transparent for the EUV radiation. Therefore, many different methods for reducing the particle flow have already been presented and discussed in a great many publications.  
         [0011]     b) Description of the Related Art  
         [0012]     For example, DE 102 15 469 B4 describes a debris filter in which means for generating an electrical field orthogonal to the central propagation direction of the divergent EUV beam bundle are arranged downstream of the outlet opening of the vacuum chamber and in which means are provided for generating a gas sink resulting in a particle flow parallel to the direction of the electrical field.  
         [0013]     U.S. Pat. No. 6,359,969 B1 discloses a device containing a radiation source and a processing organ for processing the radiation of the radiation source. A filter containing a plurality of (planar) foils or plates oriented in radial direction from the punctiform radiation source is arranged between the radiation source and the processing organ.  
         [0014]     U.S. Pat. No. 6,683,936 describes an EUV-transparent interface structure in the form of a membrane or channel structure for optically connecting a first and a second chamber while suppressing the contaminating particle flow of the source. Inside the chamber, an inert gas flow is directed sideways to or onto the source in order to keep the particles away from the application.  
         [0015]     U.S. Pat. No. 6,566,668 B2 is directed to a high-energy photon source in which a pair of plasma pinch electrodes is arranged in a vacuum chamber. A debris collector between the pinch region and the radiation collector comprises a large number of passages, each of which is oriented toward the light beams, so that it passes the radiation and retards the debris which moves along more random paths. In addition, a flow of gas is directed toward the source location of the radiation in order to further slow the debris particles.  
         [0016]     Further, DE 102 37 901 B3 describes a debris filter comprising a plurality of blades. The blades have the shape of essentially flat surfaces with a pair of parallel edges, are arranged so as to be uniformly distributed radially with respect to a given optical axis of the radiation source, and are arranged orthogonally between an inner enclosing surface and an outer enclosing surface which are arranged parallel to one another and rotationally symmetric so as to arch over the plasma for a preselected solid angle around the optical axis.  
         [0017]     Similarly, the emission of debris from an x-ray source is prevented by a debris filter system according to U.S. Pat. No. 6,867,843 B2 in that an attraction unit has an attracting surface parallel to an axis passing through the emission point. Further, the debris filter has a rotation unit which rotates the attraction unit around said axis.  
         [0018]     The known arrangements have the disadvantage that they are not equally effective for all types of particles or absorb too much EUV radiation. For example, neutral particles from the plasma are not influenced by electromagnetic fields. High field strengths combined with high voltages are necessary for influencing fast particles. Parasitic gas discharges may be caused by applying these voltages. Apart from increased shadowing effects, rotating blade filters have the disadvantage that very fast particles are only deflected corresponding to the rotating direction. The necessary high pressures are not achieved in gas flows or gas curtains, but a considerable proportion of the desired EUV radiation is also absorbed.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0019]     The primary object of the invention is to find a novel possibility for suppressing the particle flow (debris) from a plasma which keeps the debris away from primarily optical components located downstream without excessive attenuation of the desired radiation emitted from the plasma.  
         [0020]     In a method for suppressing debris in short-wavelength radiation sources based on a plasma in which the short-wavelength radiation emitted in a vacuum chamber from the plasma as source location is guided through a debris filter having at least one mechanical filter structure before this radiation reaches collector optics for focusing the radiation in an intermediate focus, the above-stated object is met, according to the invention, in that a buffer gas is injected inside the filter structure of the debris filter lateral to openings that are provided for passing the radiation, wherein the filter structure generates a flow resistance in direction of the plasma and in direction of propagation of the radiation so that an increased gas pressure of buffer gas remains limited to a defined volume layer in the debris filter relative to the pressure in the vacuum chamber, and the buffer gas exiting from the filter structure of the debris filter is sucked out of the vacuum chamber by vacuum pumps.  
         [0021]     The buffer gas is preferably injected into an intermediate space between two filter structures.  
         [0022]     In a particularly advantageous manner, the buffer gas is injected into an intermediate space between two filter structures with constricted channels (openings for the passage of radiation).  
         [0023]     Further, it is advantageous when additional buffer gas is directed into an antechamber in front of the debris filter in order to achieve a pre-deceleration of particularly high-energy debris particles. The antechamber is preferably filled with residual gas which flows out of the discharge chamber for generating the plasma and places a substantial proportion of buffer gas in this region. The buffer gas can advantageously be sucked out of the antechamber into the vacuum chamber through outlet openings in order to minimize the buffer gas flowing out of the antechamber through the filter structure or can be sucked out of the antechamber directly by separate suction pumps in order to adjust a defined gas pressure in the antechamber on the order of 10 Pa.  
         [0024]     In commonly used mechanical filter structures which are outfitted with a central diaphragm, it is advisable to introduce buffer gas into the central diaphragm of the debris filter in order to suppress erosion of the diaphragm, particularly due to a self-focusing electron beam.  
         [0025]     In order to increase the gas pressure in the debris filter which drops continuously as a result of the buffer gas flowing out through at least one filter structure into the vacuum chamber, it is advantageous that the buffer gas pressure is increased by bursts synchronous to the pulsed radiation generation by means of a pulsed influx into the filter structure.  
         [0026]     Further, in an arrangement for the suppression of debris in short-wavelength radiation sources based on a plasma in which a debris filter having at least one mechanical filter structure is arranged in a vacuum chamber between the plasma, as source location of the emitted short-wavelength radiation, in front of collector optics for focusing the radiation in an intermediate focus, the above-stated object is met, according to the invention, in that inlet nozzles for injecting buffer gas into the filter structure are arranged inside the filter structure of the debris filter lateral to the openings that are provided for the passage of the radiation, wherein the filter structure has a flow resistance in direction of the plasma and in direction of propagation of the radiation so that a defined volume layer in the debris filter has an increased gas pressure of buffer gas compared to the pressure in the vacuum chamber, and in that at least one vacuum pump for evacuating the vacuum chamber is also provided for sucking out the buffer gas exiting from the filter structure of the debris filter.  
         [0027]     The debris filter advantageously has at least one blade structure. In this connection, the debris filter contains closely adjacent blades which are preferably oriented radial to the optical axis of the collector optics and at least two supporting rings for the blades which are oriented in a conical shape toward the source location.  
         [0028]     The debris filter can advisably have a different quantity of radially oriented blades between different pairs of supporting rings, wherein the quantity of blades increases outward from the optical axis. The debris filter advisably has at least two pairs of supporting rings, wherein the quantity of blades between a pair of supporting rings situated farther toward the outside is greater than the number of blades between the closest pair of supporting rings toward the inside. In a preferred construction, the number of blades between supporting rings is approximately twice that of the blades between the closest supporting rings toward the inside, so that the angular distance between adjacent blades is always on the order of 1 mm.  
         [0029]     For a particularly advantageous filtering of debris, the debris filter has two filter structures with a parallel volume layer situated therebetween for injecting buffer gas.  
         [0030]     The filter structures advisably have a greater flow resistance for the buffer gas because of a reduced diameter of the openings for the passage of radiation. In this way, a higher buffer gas pressure can be adjusted between the filter structures. The increased buffer gas pressure substantially improves the efficiency of the debris filter because the product of buffer gas density and path (path of interaction with the debris particles) is increased without having to increase the amount of buffer gas supplied for increasing gas pressure. Since the quantity of buffer gas (which is supplied to the debris filter and flows out under vacuum) does not increase, the vacuum system can also be dimensioned in a more economical manner.  
         [0031]     The filter structures advantageously have a smaller structural depth in direction of the radiation from the plasma, and a higher buffer gas pressure can be adjusted while maintaining the same supply of buffer gas between the filter structures so that the reduced structural depth does not result in a reduced product of debris particle path and buffer gas density or diminish the filtering action.  
         [0032]     To increase the flow resistance, the filter structure can advantageously be constructed as a hexagonal structure, wherein conical or cylindrical holes are introduced in the hexagonal elements.  
         [0033]     In another advantageous construction, the filter structure with increased flow resistance can be constructed as a filter plate, wherein holes are suitably arranged in the filter plate for passing the radiation emitted by the plasma.  
         [0034]     In a particularly advantageous manner, radially oriented elongated holes or slots are introduced in the filter plate.  
         [0035]     The filter structure of the debris filter advantageously has a circular central diaphragm. This central diaphragm is advantageously constructed as a diaphragm cup which opens toward the plasma and in which buffer gas is introduced in order to prevent erosion and destruction of the diaphragm by debris. In addition, an electric lens (generation of an electric field or magnetic field) can be arranged around the opening of the central diaphragm for deflecting charged particles, particularly electrons.  
         [0036]     In another advantageous construction, the central diaphragm is solid and is provided with a conical pocket hole that is introduced symmetric to the optical axis.  
         [0037]     Because of the heat acting upon the debris filter from the plasma (through radiation and debris), the blades are preferably rigidly fixed only in one supporting ring and are guided in a sliding manner in the other supporting ring so as to enable thermal expansion of the blades without warping. A cooling structure can also be arranged at the supporting ring at which the blades are rigidly fixed.  
         [0038]     Further, it is useful to actively cool the filter structure of the debris filter by means of a cooling circuit to further increase the density of the buffer gas.  
         [0039]     The filter structure of the debris filter can advantageously be cooled by means of the cooling circuit in such a way that temperature is maintained at a minimum value which lies above the condensation temperature of materials that are solid under normal conditions and are in at least partially vaporized state in the vacuum chamber. The minimum value to which the temperature is controlled can be meaningfully adjusted in such a way that the surface temperature of the filter structure lies above the condensation temperate (dew point) of a utilized target material that is solid under normal conditions.  
         [0040]     The filter structure of the debris filter is advisably made from heat-resistant material with good thermal conductivity because of the high thermal loading (from radiation energy and kinetic energy from debris particles). The filter structure is preferably made from at least one of the metals tungsten, molybdenum, or alloys thereof.  
         [0041]     In order to increase the buffer gas density in the debris filter without increasing the gas load in the vacuum chamber, at least one valve is advantageously provided for injecting the buffer gas into the filter structure of the debris filter. This valve is operated by bursts so as to be synchronized with a pulsed radiation generation from the plasma. The pulsed valve can either be synchronized to every individual radiation pulse from the plasma or is actively continuously switched in a synchronized manner by a burst from radiation pulses from the plasma.  
         [0042]     Inlet nozzles which are suitable for a supersonic nozzle jet expansion of the buffer gas into the intermediate spaces of the filter structure are preferably provided for injecting buffer gas laterally under high pressure.  
         [0043]     Inert gases, preferably noble gases, are used as buffer gas. The buffer gas is advantageously a mixture of at least a heavy gas and a light gas in order to achieve faster injection and better thermal conductivity.  
         [0044]     The blade structure of the debris filter is advantageously constructed so as to be insulated in such a way that an additional deflection of charged debris particles is possible by applying high-voltage pulses. In a particularly advantageous manner, the blade structure is insulated in such a way that the charging of the filter structure for deflecting charged debris particles is provided by a self-focused electron beam impinging on the central diaphragm.  
         [0045]     Further, the blade structure can be constructed in such a way that adjacent blades can be charged in a pulsed manner with different potential. Further, it is advisable for preventing parasitic discharges in the vacuum chamber to apply a defined potential (e.g., ground) to the blade structure at least occasionally.  
         [0046]     For applications with especially strict requirements with regard to absence of debris in the radiation source, it may be useful to arrange a similarly constructed debris filter in the vicinity of the intermediate focus in order to further reduce the debris.  
         [0047]     For purposes of adjusting and monitoring optimal filter operation, a sensitive sensor is advantageously arranged immediately downstream of the debris filter. This sensor can advisably be a film sensor with a tunable quartz resonator, a miniaturized mass spectrometer, or a time-of-flight spectrometer. The sensor is advantageously used for regulating active filter parameters for debris suppression, preferably of buffer gas pressures, injection angles, voltages, or pulse synchronization. However, it can also be used for generating an error report for the control of the plasma generation or of a superordinate lithography system.  
         [0048]     The basic idea behind the invention proceeds from the consideration that mechanical debris filters in the stationary state still pass a considerable proportion of debris and that when they are moved (preferably in rotation around the optical axis of the collector optics) in order to improve filtering action promote other unwanted contamination effects at the optical components. However, known flushing of the openings of the debris filter structure with counterflowing buffer gases causes considerable absorption of the emitted EUV radiation and shadowing effects because the effective area for particle deceleration extends far beyond the length of the blades.  
         [0049]     Therefore, the basis of the invention resides in that a buffer gas volume that is limited in a defined manner is introduced inside the known blade-like structure of the debris filter arrangement and decelerates and/or deflects the unwanted debris particles in such a way that they deposit on surfaces of the filter structure before reaching the optical components. Therefore, the reflectivity of the collector optics which intercept and focus the radiation emitted from the plasma is maintained over a longer period of time and maintenance costs and operating costs are accordingly reduced. In addition, the life of the debris filter is also prolonged.  
         [0050]     The solution according to the invention makes it possible to suppress the particle flow (debris) from the source location of a plasma-based radiation source allowing the destructive debris to be effectively kept away from primarily optical components arranged downstream without excessively attenuating the desired radiation emitted from the plasma.  
         [0051]     The invention will be described more fully in the following with reference to embodiment examples. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]     In the drawings:  
         [0053]      FIG. 1  schematically shows a plasma-based source with a debris filter, according to the invention, through which a buffer gas volume passes;  
         [0054]      FIG. 2  shows a construction of the debris filter with blade structure in a top view (viewed away from the radiation source) and in a side view in section;  
         [0055]      FIG. 3  shows a sectional view of the debris filter in which the buffer gas is introduced in the debris filter into each individual intermediate space between the radial blades central to the blade length (filter depth);  
         [0056]      FIG. 4  shows a schematic overall view of an EUV source with a debris filter comprising two blade structures which are arranged one behind the other and into whose intermediate spaces buffer gas is injected tangentially;  
         [0057]      FIG. 5  shows a variant of the debris filter with buffer gas injected between two concentric filter structures and with an additional higher-pressure antechamber which is fed, e.g., by a mixture of work gas and buffer gas of a gas discharge source;  
         [0058]      FIG. 6  shows a modified construction of a debris filter in which two filter plates with increased flow resistance due to constricted openings (channels) form a kind of gas cell for generating a substantially increased buffer gas pressure that is spatially limited in a defined manner;  
         [0059]      FIG. 7  shows a top view of a possible filter structure for a debris filter comprising two parallel metal plates;  
         [0060]      FIG. 8  shows schematic views of pressure profiles in different filter structures with injected gas: a) in a unitary blade structure according to  FIG. 3 , b) between two concentric blade structures according to  FIG. 4 , c) between two filter structures with an additional higher-pressure antechamber according to  FIG. 11 , d) with increased buffer gas pressure between two filter structures with constricted channels according to  FIG. 6 ;  
         [0061]      FIG. 9  shows a variant of the debris filter with a modified central diaphragm which has—primarily for absorption of a directed electron flow—a) a gas volume, b) an electric lens, or c) a conical pocket hole;  
         [0062]      FIG. 10  shows a debris filter with means for limiting the gas load in the vacuum chamber for pulsed injection of buffer gas and means of cooling the filter structure; and  
         [0063]      FIG. 11  shows a section from a debris filter structure in a (sectional) side view and radial section, wherein the blades are fastened in a cooled conical holding ring on one side and are guided in a sliding manner in another holder (or central diaphragm) on the other side in order to allow thermal expansion of the blades without tension states. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0064]     The basic principle of the invention—as is shown in  FIG. 1 —consists in that a debris filter  4  is arranged between the radiation-emitting plasma  1  and an intermediate focus  2  that is generated by collector optics  3  as a conjugate imaging of the source location (plasma  1 ). An increased gas pressure of a suitable buffer gas  5  is generated within the volume of the debris filter  4 . The flow resistance of a (mechanical) filter structure  41  of the debris filter  4  is used for limiting the gas load for the surrounding vacuum chamber  6  (shown only in  FIG. 4 ). Inside the filter structure  41 , the buffer gas  5  is injected lateral to the direction of the passage of radiation. Any noble gas, but preferably one with a heavy atomic weight (such as argon or krypton) because of their better braking effect on high-energy debris particles, can be used as buffer gas  5 . Other inert gases or mixtures of inert gases and/or noble gases can also be used, particularly to increase thermal conductivity.  
         [0065]     Through the use of the buffer gas  5  in a defined volume along the distance between the plasma  1  and collector optics, debris is effectively kept away in that the fast debris particles are increasingly decelerated by many collisions with the gas particles of the buffer gas  5  until they reach the average thermal speed of the buffer gas  5  and therefore become too weak for sputtering processes on optical surfaces (e.g., of the collector optics  3 ). The decisive parameter for deceleration is the integral of path times density of the buffer gas:  
         ∫     plasma   ⁢           ⁢   position     optics     ⁢     ρ   ·           ⁢     ⅆ   r           
 
 where ρ is the density of the buffer gas [particles/cm 3 ] and r is the distance traveled by the debris particles emitted from the plasma l. 
 
         [0066]     Since the plasma  1  is generated in a vacuum chamber  6  and vacuum atmospheres (usually &lt;2 Pa) are also needed to prevent radiation absorption along the optical path of the generated EUV radiation to the application, a buffer gas zone is created in a defined limited volume inside the debris filter  4 . Due to its flow resistance (even though it is small in conventional constructions), the filter structure  41  of the debris filter  4  constitutes the transition from a region B with high buffer gas density in the interior of the filter structure  41  to regions A and C of the vacuum chamber  6  (see  FIG. 4 ) which are located, respectively, to the left and right of the filter structure  41  (bottom of  FIG. 1 ). The pressure curve of the buffer gas is depicted in a purely qualitative (exaggerated) manner in  FIG. 1 .  
         [0067]     In the following, a number of arrangements for introducing a buffer gas volume of the type described above between the plasma  1  and collector optics  3  are suggested for optimizing the filter characteristics according to the invention.  
         [0068]     A combination of a buffer gas volume of sufficient density and a blade structure  42 , such as is already known in principle from German Patent DE 102 37 901 B3, is well suited for this purpose. In a blade structure  42  of this kind, as is shown again in modified form in  FIG. 1 , even a small deflection of the debris particles by means of buffer gas  5  that is injected laterally is sufficient to absorb the particles on the blade structure  42 . In this connection, the blade structure  42  assumes the task of keeping the buffer gas  5  with high density within a defined volume by means of its flow resistance.  
         [0069]     As is shown in  FIG. 2 , a suitably modified basic construction (based on DE 102 37 901 B3) comprises an arrangement of thin blades  43  (foils) which are oriented with respect to the (almost punctiform) radiation source (plasma  1 ) in such a way that the shadow cast by each blade  43  is minimal. It is essential for the effectiveness of the debris filter  4  that the blade structure  42  has a large surface which affords the possibility of condensation (adhesion) for the debris particles which are emitted from the plasma  1  and are then deflected or decelerated.  
         [0070]     In order to implement the invention—without limiting generality—a debris filter structure  42  in used, in which—as is shown in  FIG. 2  in a top view and side view—very thin (thickness of approximately 100 μm) blades  43  are oriented radial to the optical axis  31  (of the collector optics  3 ). This blade structure  42  is particularly advisable because it does not lead to additional shadowing in case the light source (e.g., a Z-pinch plasma) has a certain expansion along the axis of symmetry of the filter (which is identical to the optical axis  31  of the collector imaging).  
         [0071]     By means of a few supporting rings  44  which are shaped as outer conical surfaces, it is possible to insert a larger quantity of blades  43  in the outer area of the debris filter  4  (e.g., 180 blades  43  in the inner area compared to 360 blades  43  on the outside, or even  720  in a pair of supporting rings located farther outward) so that the ratio of filter depth l (=blade length) and blade spacing, which ratio is important for the suppression of debris, is also kept large in the outer region. A value of approximately 40-150 mm is selected for the filter depth  1 , preferably approximately 1 mm for the spacing of the blades  43 . This corresponds to an angle of approximately ½to 2° between adjacent blades  43 .  
         [0072]     Based on the above-described debris filters  4  which are known, per se, a defined buffer gas volume (which is relatively thin-layered with respect to the radiation propagation) is introduced ( FIG. 3 ). The buffer gas  5  is injected at suitable locations, preferably in the center of the blade length l tangential (circular) to the center of curvature of the blade structure  42 , into the intermediate spaces between the blades  43  from the outside along the line shown in dashes ( FIG. 3 ). The exact position and direction of the nozzles  51  (shown only in  FIG. 4 ) may vary; in general, it is most advantageous to inject exactly in the center of the blade depth l. When the blade structure  42  is not divided (radial to the plasma), the buffer gas is injected into each blade gap simultaneously. Since the area around the optical axis  31  is not permeable because of the geometry of the selected blade structure  42  and since the collector mirror  31  for grazing light incidence, as is assumed in  FIG. 3  (but shown only in  FIG. 4   a ), also does not collect radiation from this area, the buffer gas  5  can also be injected into the intermediate spaces of the blades  43  from the optical axis  31  (axis of symmetry of the debris filter) without an additional shadowing effect. The buffer gas  5  exits from the blade structure  42  radially with respect to the plasma position in both outward directions and is pumped out by the vacuum pumps  61  (both shown only in  FIG. 4 ) which are connected to the vacuum chamber  6 . Accordingly, a spherical shell layer of buffer gas  5  forms inside the blade structure  42  and pressure drops quite steeply outward from the center of the blade structure  42  on both sides corresponding to  FIG. 8   a.    
         [0073]     The advantage of this novel filter construction resides in the direction in which the buffer gas is injected lateral to the propagation of the EUV radiation so that only a very limited “layer thickness” of buffer gas  5  is formed inside the entire solid angle region that is used. Accordingly, a very good braking effect is achieved for fast, superthermal debris particles due to collisions with buffer gas atoms with hardly any noticeable radiation absorption.  
         [0074]     Another operative mechanism that takes place, according to the invention, by injecting the buffer gas  5  into the debris filter  4  in orthogonal direction to the openings of the filter structure  41  and, therefore, lateral to the movement direction of the debris particles improves the deposition of debris particles at the surfaces of the blades  43 . A third (smaller) component of the filtering effect of the buffer gas  5  occurs because of a counterflow of the buffer gas  5  flowing out through the openings of the filter structure  41  in direction of the source location of the radiation (plasma  1 ) so that the braking effect on fast debris particles is further increased.  
         [0075]     In a particularly improved variant of the invention shown in  FIG. 4 , the debris filter  4  comprises two concentric blade structures  42  and  42 ′ (structure depth of about ½ l of the type shown in  FIG. 2 ) so that a spherical shell layer remains free therebetween for filling with the buffer gas  5 . This free volume, intermediate space  52 , is used for filling with buffer gas  5 .  
         [0076]     As was already described above, the buffer gas nozzles  51  are arranged tangentially in the center to allow the buffer gas  5  to be injected along the line shown in dashes (representing a spherical shell three-dimensionally). When injecting the buffer gas  5 , care must be taken to inject it exclusively in the intermediate space  52  between the blades  43  so that it can only escape therefrom slowly into the vacuum chamber  6  through the openings of the blade structures  42  and  42 ′. In this way, a defined (quasi-static) pressure can be adjusted in the intermediate space  52  although buffer gas  5  flows off continuously over the flow resistance of the blade structures  42  and  42 ′. When the buffer gas nozzles  51  are directed to a substantial extent in direction of the openings of one of the blade structures  42 ,  42 ′, the buffer gas  5  contributes appreciably less to the effective absorption of debris particles during the unavoidable increase in the gas load (increased gas pressure) in the vacuum chamber  6 .  
         [0077]     Due to the flow resistance between the blades  43  relative to the free intermediate space  52 , there is an appreciable increase in the buffer gas pressure (of approximately 10 Pa) relative to the vacuum chamber pressure (typically &lt;2 Pa). The low pressure in the vacuum chamber  6  is required because a very high absorption of the generated EUV radiation would result otherwise (assuming typical optical path lengths of about 1 m in semiconductor lithography). The buffer gas  5  is pumped out radially through the blade structure  42  at the side of the plasma  1  and through the blade structure  42 ′ by the vacuum pumps  61  (both shown only in  FIG. 4 ) that are connected to the vacuum chamber  6 .  
         [0078]     The improvement in the filtering effect (for all debris particles) which is achieved in this variant according to  FIG. 4  is brought about by the (relatively thin) spherical shell layer of the buffer gas  5  inside the intermediate space  52  between the blade structures  42  and  42 ′. A higher buffer gas pressure and a higher pressure gradient can also be adjusted as can be seen from the schematic view in  FIG. 8   b.    
         [0079]     Due to the free volume (spherical shell layer between the blade structures  42  and  42 ′), a defined increase in buffer gas pressure (relative to the vacuum chamber  6 ) results in the intermediate space  52  because the buffer gas  5  is distributed homogeneously due to the flow resistance of the surrounding blade structures  42  and  42 ′. In the present case, with the surrounding filter structure  41 , the buffer gas pressure is defined by the following variables:  
         [0080]     mass flow of gas through the buffer gas line(s),  
         [0081]     suction power of the vacuum pumps (shown only in  FIG. 4 ), and  
         [0082]     conductance (as reciprocal of the flow resistance) of the blade structure.  
         [0083]     Generally, as regards forming the higher-pressure buffer gas layer within the surrounding filter structure  41 , the greater the ratio of suction power of the vacuum pumps  61  and conductance of the two blade structures  42  and  42 ′, the greater the ratio of the pressures within the debris filter  4  and within the vacuum chamber  6 . In other words, at a given pressure in the vacuum chamber  6 , a still higher pressure can be achieved in the debris filter  4  the greater the ratio of suction power to conductance. Since the suction power of the vacuum pumps  61  is limited, the ratio is essentially adjusted by reducing the conductance (increasing the total flow resistance of the filter structure  41 ).  
         [0084]     Accordingly, the arrangements described above can be further improved in that a complete filter structure  41 —as is shown at bottom in  FIG. 6   a —comprises two filter structures with increased flow resistance.  
         [0085]      FIG. 5  shows another modification of the invention which is based on the construction of the debris filter  4  according to  FIG. 4  and is suitable particularly for EUV sources based on a gas discharge (GDPP sources) which, in addition to the active work gas, also use an admixture of buffer gas in the discharge chamber.  
         [0086]     In this construction, buffer gas  63  which flows out of the discharge chamber (shown only schematically as an outlet in  FIG. 5 ) and which is used for the gas discharge is utilized for building an increased gas density for a first deceleration of debris particles in an antechamber  62  that is arranged in front of the debris filter  4 .  
         [0087]     In a variant designated as “A”, the antechamber  62  advantageously has outlet openings  64  by means of which the buffer gas  63  (proportion of residual gas from the discharge) originating in the gas discharge flows out into the vacuum chamber  6  in order to extensively prevent an exit through the blade structure  42  in the debris filter  4 .  
         [0088]     In a variant “B”, the antechamber  62  has at least one suction pump  65  which is connected directly to the outlet openings and which allows (separately from the vacuum pumps  61 ) a defined gas pressure between 1 and 20 Pa to be adjusted in the antechamber  62 . In this way, the action of the subsequent debris filter  4  can be reinforced in a monitored manner and optimized.  
         [0089]     The buffer gas  5  which is injected laterally within the two filter structures  42  and  42 ′ provides for the main deceleration of the debris particles in the debris filter  4  based on the principle of the defined volume layer inside the debris filter  4  in the same way as described above with reference to the construction shown in  FIG. 4 .  
         [0090]     In a preferred construction which is described with reference to  FIG. 6   a , the filtering action of the gas volume within two surrounding filter structures  41  can be further increased without increasing the gas load in the vacuum chamber  6  through exiting buffer gas  5 . For this purpose, two parallel filter plates  53  and  54  are provided which offer an increased flow resistance to the buffer gas  5 .  
         [0091]     These filter plates  53  and  53 , which need not necessarily be flat, have a distance from one another (as in the construction of the intermediate space  52  shown in  FIG. 4 ) which forms a parallel enclosed volume which is referred to in this example as a gas cell  55  because the buffer gas  5  located therein can be distributed more homogeneously due to the increased flow resistance of the filter plates  53  and  54  and an even higher buffer gas pressure can be adjusted.  
         [0092]     Referring to  FIG. 2  which was described above, filter plates  53  and  54  of the type mentioned above having an increased flow resistance can be produced in that the blade structure  42  used therein is constricted in that a plurality of supporting rings  44  which are shaped as conical shells and have (at least in part) the same (or similar) material thickness as that of the radially arranged blades ( FIG. 2 ) are constructed and arranged as is shown schematically in  FIG. 6   b . They can accordingly be arranged with a similarly close spacing (like the radially diverging blades, preferably 1-2 mm).  
         [0093]     In this case, the channels  57  formed in this way (as reduced openings of the blade structure  42 ) which transmit the radiation emitted from the plasma  1  to the collector mirror  32  (shown only in  FIG. 4 ) and which at the same time allow the buffer gas  5  to flow out of the interior of the blade structure  42  in its entirety are no longer defined only with respect to a plane by two divergent surfaces, but rather have an approximately quadratic cross section which increases with increasing distance from the source location (plasma  1 ) so that, in their entirety, they have the shape of a truncated pyramid. This leads to an appreciably increased flow resistance for the buffer gas  5  with a somewhat poorer optical transmission of the debris filter.  
         [0094]     As a result, a higher buffer gas pressure can be adjusted in the gas cell  55  with the buffer gas flow through the buffer gas nozzles  51  of the debris filter  4  remaining the same. This contributes (referring to  FIG. 8   d ) to the desired increase in the integral over the product of path length and gas density.  
         [0095]      FIG. 8  shows a comparison of the working profiles of the different filter structures of the filter variants shown in  FIGS. 2 and 3 ,  FIG. 4 ,  FIG. 5 , and  FIGS. 6 and 7 . The arrangement of  FIGS. 6 and 7  is distinguished by the best homogeneity of the buffer gas volume within the cell  55  (or intermediate space  52 ) of the debris filter  4 .  
         [0096]     In general, debris particles (regardless of whether they are charged or uncharged, fast or slow) are intercepted in an optimal manner in the buffer gas volume between the two filter structures  42 ,  42 ′ or  53 ,  54  of the debris filter  4 . In addition, the action of the buffer gas  5  remains spatially limited to the desired area (in front of the collector optics  3 ) and does not lead to an extinction of the EUV radiation as it continues over the optical beam path.  
         [0097]     A further limiting of the volume of the buffer gas  5  at high pressure can be achieved when desirable by increasing the flow resistance of the filter structure  41 .  
         [0098]     By abstracting to the hexagonal structure  56  shown in  FIG. 6   c , the total conductance L of the debris filter  4  can be estimated as follows by means of the known relationship for flow resistance W R  in thin tubes:  
       L   =       1     W   R       =       12.1   ·       d   3     l     ·     f   ⁡     (     p   ,   d     )         ⁢     l   ·       s     -   1       .               
 
         [0099]     The function ƒ(p,d) adopts the value of 1 in the case of molecular flow and is dependent upon pressure with values greater than 1 at higher pressures (transitional flow, viscous flow).  
         [0100]     With typical dimensions of the tubular channels  57  of the hexagonal structure  56  of 1 cm and typical pressures of 0.1 mbar, the system lies within the region of transition between molecular flow and laminar flow when p·d=0.1 mbar·cm. In this case, the conductance of the hexagonal structure  56  is already slightly pressure-dependent and is higher than would be the case with a purely molecular flow. Therefore, a reduction in conductance can be achieved by reducing the dimensions and accordingly changing them completely to the domain of molecular flow.  
         [0101]     However, when the channels  57  are reduced, the conductance is additionally changed by the altered geometry. In this case, L˜d 3 . Allowing for the increasing quantity of individual channels (˜l/d 2 ), there is, on balance, a reduction in conductance ˜d. This can be used to reduce the depth l (structural depth or thickness) of the structure plates  53  and  54 . Thus, taken as a whole, the entire filter structure  41  can be scaled to smaller channel dimensions with a slight reduction in conductance. This contributes to a stricter limiting of the buffer gas volume—to the gas cell  55 —and to an increase in the total integral of path times gas density.  
         [0102]     In a preferred construction—as is shown in a top view of one of the filter plates  53 ,  54  in  FIG. 7 —the function of a filter structure  41  of the kind described above is achieved in that the filter structure  41  is no longer composed of individual blades (as is  FIG. 2  or  FIG. 6   b ) or hexagonal structures  56  (as in  FIG. 6   c ), but rather is produced from a solid metal plate (preferably with a thickness of approximately 2 mm) by micromaterials machining methods (erosion, etching, laser drilling, laser cutting, etc.). In this connection, elongated holes  58  are preferably arranged instead of rotationally symmetric channels  57  ( FIG. 6   c ) in order to limit shadowing effects for the radiation emitted from the plasma  1 . The arrangement is carried out with a view to stability in order to retain sufficient supporting material between the elongated holes. Typical diameters of the elongated holes  58  or channels  57  are 100 μm (width of the elongated holes). For a reasonable wall thickness, there should remain about 10 μm.  
         [0103]     As is indicated in  FIG. 6   a , the implementation of narrowed channels  57  results in an intermediate space resembling a gas cell  55  which terminates on the inlet side and outlet side for the EUV radiation by correspondingly fine channels  57 . The buffer gas  5  introduced into this gas cell  55  is distributed uniformly and homogeneously because of the more strictly limited volume. Since the total drop in pressure between the cell  55  and the vacuum chamber  6  occurs over a path length of about 2 mm only, the integral over path times density can now be increased up to twofold compared to a conventional debris filter  4  (see  FIG. 8  for comparison) while maintaining the same gas load in the system.  
         [0104]     All of the assumptions discussed above relating to the orientation of the channels  57  and their defining walls on the source point (plasma  1 ) and the axis  31  of the collector optics  3  for minimized shadow effects on the emitted radiation still apply without exception to a filter structure  41  of this kind.  
         [0105]     Perforated plates in a hexagonal structure  56  of cylindrical or conical channels  57  afford the simplest topology for the filter structure  41  indicated in  FIG. 6   c  and a transparency of about 80% can be achieved in this way, e.g., where r 1 /r 2 =0.95.  
         [0106]     Compared to the hexagonal structure  56  shown in  FIG. 6   c , although the latter has cylindrical channels  57 , another useful topology for the filter plates  53  and  54 , shown in  FIG. 7 , which is provided by individual, radially oriented elongated holes (e.g., made by laser cutting) has the advantage of increased transmission, particularly when the light source (e.g., Z-pinch plasma  1 ) extends to a significant degree along the central surface normal (=axis of symmetry=optical axis  31 ) of the filter structure  41 .  
         [0107]     Since, as a rule, the maximum available suction capacity of the vacuum pumps  61  connected to the vacuum chamber  6  is limited, the buffer gas  5  is introduced into the debris filter  4 , as shown in  FIG. 10 , synchronous to the radiation pulses (of the plasma  1  which is normally generated in a pulsed manner) by a valve  59  that is operated in bursts. Temporarily higher pressures are realized in the debris filter  4  exclusively during the radiation emission of the plasma  1  so that the average pressure and the gas load in the vacuum chamber  6  are increased appreciably less and therefore do not require increased pumping capacity of the vacuum pumps  61 .  
         [0108]     Depending on the flow dynamics in the debris filter  4  and the repetition rate at which the EUV source is operated, the valve  59  can be triggered synchronous to each individual radiation pulse or synchronous to the pulse sequences (bursts) commonly used in lithography. The delay between the radiation pulse (or burst) and the triggering of the valve  59  is selected in such a way that the greatest possible debris reduction results.  
         [0109]     During operation, the buffer gas  5  is heated and (partially) ionized with every radiation pulse through radiation absorption. Both heating and ionization lead to an expansion, and therefore a loss in density, of the buffer gas cloud inside the filter structure  41 . The “lost” buffer gas must be replaced by new buffer gas  5 . When the buffer gas  5  (typically a heavy noble gas such as argon) is mixed with a light gas (e.g., helium), the expansion of this gas mixture from the inlet nozzles  51  takes place faster in case of a supersonic nozzle jet expansion. Therefore, the buffer gas  5  is more quickly available. At high repetition frequencies also, this has a beneficial effect on the absorption behavior of the debris filter  4  that is filled with buffer gas.  
         [0110]     The thermal conductivity of the gas mixture is also improved by adding a light gas (e.g., helium) to the heavy buffer gas (e.g., argon). This promotes cooling of the debris filter  4  which is likewise shown in  FIG. 10 .  
         [0111]     The need to cool the debris filter  4  arises for two reasons: The density of the buffer gas  5  (particles per volume) is the decisive quantity for the efficiency of the debris filter  4 . According to the general law of kinetic gas theory, density and temperature are inversely proportional. With the pressure remaining the same, twice the absolute temperature equals half of the particle density. The system is exposed to drastic heating at targeted outputs in commercial lithography systems. Therefore, cooling of the debris filter  4  is indispensable for prolonging the life of the system.  
         [0112]     Further, steps must be taken respecting the structural stability of the filter structure  41 . For this purpose, the filter structures  41  of the debris filter  4 , which—without limiting generality—are shown as blades  43  in  FIG. 11 , are connected on one side to an annular holder. This holding ring  46  is actively cooled by means of a suitable coolant (e.g., water) in an added cooling structure  47 . Accordingly, in addition to the cooling of radiation, the blades are cooled through thermal conduction from the blades to the ring. For this purpose, the filter structure  41  is made of a heat-resistant material with good heat-conducting properties such as tungsten, molybdenum or alloys thereof.  
         [0113]     As was already indicated above, the cooling of the debris filter is improved in that a light gas with good thermal conductivity, preferably helium or hydrogen, is mixed with the buffer gas of heavy atomic weight.  
         [0114]     In order to prevent warping of the filter due to thermal expansion of the blades  43 , the latter are fixedly connected to the holder of a cooled supporting ring  44  only on the cooled side and are guided (see  FIG. 11 ) into a slot of a slide guide  49  on the opposite side. The slide guide can be incorporated in another supporting ring  44  or in the central diaphragm  45  of the debris filter  4 .  
         [0115]      FIG. 9  shows further modifications of the debris filter  4  which also relate to the thermal stability of the filter structure  41 . Since the collector optics  3  arranged downstream in the beam path do not intercept EUV radiation on the axis  31  or at small angles to the axis  31 , the debris filter  4  generally terminates in the central region by a diaphragm  45 . As a result, this region is exposed to a large particle flow which must be absorbed through the central diaphragm  45 . This inevitably leads to sputtering effects, i.e., the diaphragm  45  is cut by the particle flow in that material is removed. This removed material, as secondary debris, poses an additional threat to optical components, etc., and subsequently leads to the uncontrollable destruction of the diaphragm  45 . It was found in experiments that after a fairly long time holes were formed in a plate of the diaphragm  45  having a thickness of 2 mm, since a self-focusing flow of electrons takes place on the optical axis of the radiation source. On the other hand, the ion flow from the plasma  1  is assumed to be approximately isotropic.  
         [0116]     The destruction of the material of the central diaphragm  45  described above can be prevented, according to  FIG. 9   a , by constructing the central region of the debris filter  4  in the form of a diaphragm cup  46  which opens toward the plasma  1 . When this diaphragm cup  46  is filled under sufficient pressure with a gas, preferably the buffer gas  5  that is provided for injecting into the filter structure  41 , the particle flow is absorbed and decelerated in the gas phase. The particles from the particle flow are subsequently mostly neutralized and have a low thermal speed so that the sputtering effect of the particles is reduced to zero.  
         [0117]     Another modification for the central diaphragm  45  according to  FIG. 9   b  shows the construction as diaphragm cup  46  with the input region being supplemented by an electric lens  47  (in the form of a coil or electrically charged pinhole diaphragm). The electron flow from the plasma  1  is expanded through this lens  47  and its sputtering effect at the bottom of the diaphragm cup  46  is substantially reduced.  
         [0118]      FIG. 9   c  shows another useful modification of the diaphragm  45  in which a conical pocket hole  48  is introduced in the latter. This can also considerably reduce the sputtering effect of the self-focusing electrons and other debris particles. In this case, the particles strike the body of the diaphragm  45  at a flat angle to the surface. Material that is removed from the walls of the pocket hole  48  settles again on the walls so that the material removed and the material deposited balance each other on the average and the life of the central diaphragm  45  is increased considerably.  
         [0119]     Many particles which are emitted from the plasma  1  and pass the debris filter  4  are electrically charged. As was already proposed in Patent DE 102 37 901 B3, it is useful to set the debris filter  4  or individual blades  43  to an electric potential.  
         [0120]     In an arrangement which can be realized according to  FIG. 11 , but which is not shown explicitly, differently charged blades  43  can also be used to improve the filtering of charged debris particles. For this purpose, the sliding members  49  are constructed so as to be electrically insulated and adjacent blades  43  of different polarity are fastened in a “meshing” manner (i.e., guided in the sliding member  49  in an insulated manner and alternately held in different supporting members  44  with opposite polarity). For this purpose, the schematic top view in  FIG. 11  must be arranged in additional interspersed guides in an offset manner by an arrangement comprising cooling structure  71 , supporting ring  44  and blades  43  which is mirrored at the sliding member  49 .  
         [0121]     Since kinetic energies of debris particles can reach several keV, corresponding DC voltages of up to several tens of kV would have to be applied in order to deflect or decelerate the particles. In order to prevent the expected parasitic discharges due to the residual gas within the vacuum chamber  6 , the voltage is applied only in a pulsed manner to the blades  43  which are guided so as to be insulated. Pulse lengths of less than one microsecond are advisably selected for applying voltage so briefly that no gas discharge takes place in the vacuum chamber  6  around the debris filter  4  (typical times are several hundred nanoseconds). Instead of the debris filter  4 , the collector optics  3  arranged downstream can also be provided with electrical contact in the same way.  
         [0122]     When the debris filter  4  is mounted in the vacuum chamber  6  so as to be electrically insulated, the self-centering electron beam from the plasma  1  (see remarks with reference to  FIG. 9 ) can be used to electrically charge the debris filter  4  synchronous to the radiation pulses.  
         [0123]     High-energy electromagnetic radiation which is absorbed on the collector optics  3  leads to the emission of photoelectrons. In this way, the collector optics  3  can be charged to high voltages resulting in parasitic discharges to other particles in the vacuum chamber  6 . Therefore, it is advisable to contact or ground the optics so that the electric charge can be dissipated in a targeted manner. This prevents discharges from the collector optics  3  to other components in the vacuum chamber  6 . The other components can also be grounded or connected to another defined potential if necessary.  
         [0124]     When target materials that are solid under normal conditions (e.g., tin) are used to generate the plasma  1 , the debris filter  4  and the collector optics  3  arranged downstream should be heated (temperature-controlled) to the extent that the surface temperature lies above the condensation temperature (dew point) of the material. This prevents target material from depositing at undesirable locations and therefore in particular prevents impairment of the operation of the collector optics  3  and debris filter  4 .  
         [0125]     The purpose of collector optics  3  in an EUV radiation source is to focus substantial portions of the radiation emitted from the plasma  1  in an intermediate focus  2 . Therefore, it may be advisable to arrange an (additional) debris filter  4  around this intermediate focus  2  in order to further reduce the transfer of debris for the application downstream.  
         [0126]     A further improvement in the long-term stability of the debris filtering is achieved by continuously monitoring the quality of the debris filtering. For this purpose, a sensor  8  for detecting the actual debris reduction is installed downstream of the debris filter. This sensor  8  picks up a measurement quantity representing the actual amount of debris that is passed. For this purpose, the sensor  8  can be a film sensor, as it is called, in which a quartz resonator whose resonant frequency can be measured very accurately is inserted into the beam path. Its resonant frequency is detuned when deposits are formed on its surface or sputtering takes place on its surface as a result of passed debris. Therefore, the change in its resonant frequency (df/dt) is a measure for the amount of debris emitted.  
         [0127]     Another type of sensor within the meaning of the invention is a sensor  8  that measures the energy (velocity), type (charge state), or amount of emitted particles. This type of sensor includes particle analyzers, e.g., mass spectrometers, time-of-flight spectrometers and related devices.  
         [0128]     When an unusually severe detuning takes place, the control of the radiation source and/or the superordinate lithography system is alerted to a defect in the debris filter (poor debris filtering) or to a defect in the source function (severe generation of debris).  
         [0129]     The signal of a sensor of this kind can then be used within the framework of the present invention so that parameters of the debris filter  4  (e.g., buffer gas flow or applied voltages, etc.) that can vary during operation can be tracked in the manner of a closed control loop.  
         [0130]     While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.  
       REFERENCE NUMBERS  
       [0000]    
       
           1  plasma  
           2  intermediate focus  
           3  collector optics  
           31  optical axis  
           32  collector mirror  
           4  debris filter  
           41  (any) filter structure  
           42 ,  42 ′ blade structures  
           43  blade  
           44  supporting ring  
           45  central diaphragm  
           46  cup diaphragm  
           47  electric lens  
           48  conical pocket hole  
           49  sliding guide  
           5  buffer gas  
           51  buffer gas nozzle  
           52  intermediate space  
           53  filter plate  
           54  filter plate  
           55  gas cell  
           56  hexagonal structure  
           57  channel  
           58  elongated hole  
           59  (pulsed) valve  
           6  vacuum chamber  
           61  vacuum pump  
           62  antechamber (in front of the debris filter)  
           63  buffer gas (flowing out of the discharge chamber)  
           64  outlet opening  
           65  suction pump  
           7  cooling circuit  
           71  cooling structure  
           8  sensor