Abstract:
The invention is directed to a device for generating flows of gas for filtering the radiation emitted in plasma-based radiation sources. It is the object of the invention to find a novel possibility for generating a gas curtain in the immediate vicinity of a radiating plasma so as to permit a simple arrangement and design and a long life of the device for generating the gas curtain under extreme thermal stress. According to the invention, this object is met in that a slit nozzle is formed of a plurality of partial bodies comprising different materials to form a supersonic nozzle profile for the generation of a broad gas curtain in order to accommodate the slit nozzle to different thermal and precision-mechanical requirements in the gas inlet region and in the gas outlet region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of German Application No. 10 2007 023 444.0, filed May 16, 2007, 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 device for generating flows of gas for filtering the radiation emitted in plasma-based radiation sources in which at least one supersonic slit nozzle is provided for generating a gas curtain in the beam bundle that can be coupled out. 
         [0004]    b) Description of the Related Art 
         [0005]    The invention is preferably applied in semiconductor chip fabrication in radiation sources for EUV lithography for protecting the collector optics and other optics downstream of the latter from debris. 
         [0006]    As per the current state of the art, the structuring of semiconductor chips is carried out by means of optical lithography. To this end, the desired structure contained on a mask is imaged onto a semiconductor wafer. A light-sensitive coating on this semiconductor wafer undergoes chemical changes through exposure and accordingly allows further selective working of exposed and unexposed surfaces. The spatial resolution that can be achieved by this method is limited by the wavelength of the light that is used. Progressive miniaturization requires a continual increase in resolution which can only be achieved through a reduction in the light wavelength. Extreme ultraviolet (EUV) radiation with a wavelength of 13.5 nm is provided as a substitute for the formerly used DUV excimer lasers with a wavelength of 193 nm. Plasma-based radiation sources must be resorted to for generating these wavelengths. Because every known material is highly absorbent for EUV radiation, the entire process from generation of radiation to exposure must be carried out under high vacuum using reflecting optics. 
         [0007]    High-power radiation sources for EUV radiation are based on a luminous plasma that is generated by a laser pulse (LPP-Laser-Produced Plasma) or a gas discharge (GDP-Gas Discharge Plasma). In so doing, a material, e.g., xenon, tin or lithium, is heated to temperatures of several hundred thousand degrees so that the occurring plasma is emitted in the EUV region. This radiation is collected by means of collector optics and imaged in an intermediate focus which constitutes the interface to the adjoining exposure module. Aside from achieving the highest possible radiation output in the wavelength region around 13.5 nm, the life of the plasma-generating components of the radiation source and of the collector optics is of primary importance. The plasma emits not only the desired radiation but also high-energy particles and larger clusters, collectively referred to as debris. Depending on the operating parameters, the emitted debris leads to abrasion of reflecting layers, deposits of impurities, or a coarsening of the optical surface. All of these processes reduce the reflectivity of the optics. Therefore, devices for protecting the collector optics from debris play a key role in the successful use of high-power EUV sources. 
         [0008]    The following basic principles are known for protecting the collector optics:
   a) mechanically moving apertures and shutters that pass the radiation pulse and seal against slower debris particles   b) stationary or rotating arrangements of fins or plates disclosed, e.g., in EP 1 274 287 A1 and EP 1 391 785 A1 having fins or plates which are oriented substantially exclusively in radial direction toward the locus of emission and which capture the debris particles by adhesion   c) electric and/or magnetic fields (usually in combination with filters for uncharged particles) such as are described, e.g., in U.S. Pat. No. 6,881,971 B2   d) spaces which are filled with buffer gas and in which debris particles are decelerated by collision with gas particles (and are eliminated through suction or other filter means as described, e.g., in DE 10 2005 020 521 A1), and   e) buffer gas curtains forming a fast, flat lateral gas flow which decelerate and deflect debris particles (see WO 2003/26363 A1).   
 
         [0014]    Basically, these same debris filter concepts are used for GDP sources and LPP sources, but with the difference that in GDP sources the plasma occurs in the immediate spatial vicinity of an electrode system so that only a limited solid angle of emitted radiation is available, whereas in LPP sources substantially larger radiation angles must be covered. 
         [0015]    It has long been known to employ streaming buffer gases in the evacuated chambers for generating plasma in combination with mechanical and field-coupled debris filters because the gas particles decelerate and/or deflect the debris particles by colliding with them and appreciably improve the action of additional debris filters. However, an efficient evacuation system must be used to maintain a reduced extinction effect of the buffer gas for the emitted EUV radiation. 
         [0016]    The use of a gas curtain, as is mentioned above (e) and described in WO 2003/26363 A1, is a particularly efficient way to filter debris. When implemented with supersonic nozzles, a gas curtain of this kind can even deflect larger debris clusters adequately and hardly interferes with the vacuum needed to generate plasma. Further, it requires little space and can be used in the immediate vicinity of the plasma. 
         [0017]    However, the problem with using a gas curtain is that its extension (within a plane) is spatially limited so that the generating slit nozzle must be moved as close as possible to the plasma in order to cover a large solid angle of the EUV radiation. This results in a high thermal stress on the nozzle. However, supersonic slit nozzles comprise extremely precise, highly engineered shapes which are difficult to cool and extremely difficult to produce from high-melting materials (e.g., tungsten or molybdenum). 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0018]    It is the primary object of the invention to find a novel possibility for generating a gas curtain in the immediate vicinity of a radiating plasma so as to permit a simple arrangement and design and a long life of the device for generating the gas curtain under extreme thermal stress. 
         [0019]    In a device for generating gas flows for filtering the emitted radiation in plasma-based radiation sources in which at least one supersonic slit nozzle is provided for generating a gas curtain in the radiation bundle that can be coupled out, the above-stated object is met according to the invention in that the slit nozzle is formed of a plurality of partial bodies comprising different materials to form a supersonic nozzle profile for the generation of a broad gas curtain in order to accommodate the slit nozzle to different thermal and precision-mechanical requirements. 
         [0020]    The slit nozzle advantageously comprises a gas inlet part and a gas outlet part to form the nozzle profile. The gas inlet part is made of a metal that is easily workable with respect to high-precision mechanics and the gas outlet part is made of a high-melting metal. The gas inlet part is preferably manufactured from steel or stainless steel, while the gas outlet part is advisably fashioned from molybdenum or tungsten. The reason is that the gas outlet is located closer to the center of the plasma, so it is exposed more heat load. 
         [0021]    The slit nozzle advantageously contains a gas distribution pipe for streaming the gas into the nozzle profile in a defined direction. The gas distribution pipe has, along a surface line, a gas inlet row which is formed so as to be longitudinally uniform for radial passage of gas. 
         [0022]    The gas inlet row is advisably formed as a series of equidistantly disposed circular holes, elongated holes, or as a continuous slot. Circular holes of the gas inlet row preferably have a diameter of between 10 μm and 500 μm. For elongated holes or a continuous slot, the width is advantageously between 30 μm and 300 μm, the length of the elongated holes can preferably be from 1 mm to 20 mm. The distances between the individual circular holes or elongated holes are advisably selected in the range of 1 mm to 5 mm. 
         [0023]    The dimensioning of the openings of the gas inlet row is basically dictated by:
   a) the type of gas (smaller holes for lighter gases, larger holes for heavier gases),   b) the desired properties of the gas curtain (pressure and density in the gas flow, directional focusing, and Mach number of the gas flow),   c) the permissible through-flow quantities (capacity of the vacuum pumps in the flow range of the gas curtain and realizable pressures in the gas feed line), and   d) geometric factors (width of the gas curtain, size of the vacuum chamber, etc.).   
 
         [0028]    The gas distribution pipe can be made of a metal material or a ceramic material. The gas feed for supplying the gas is advantageously arranged at a front side of the gas distribution pipe. 
         [0029]    The partial bodies of the slit nozzle such as the gas inlet part, gas outlet part or gas distribution pipe are advisably joined together by a detachable connection. Pins, screws, rivets, clamps or a sleeve can be used for this purpose. 
         [0030]    The partial bodies of the slit nozzle can also be put together by means of a permanent connection, e.g., material bonds (welding, soldering or gluing) or at least one detachable connection (e.g., for the gas distribution pipe) and one permanent connection (for connecting the partial bodies). 
         [0031]    Further, in an arrangement for generating EUV radiation based on a gas discharge in which an electrode arrangement for generating a gas discharge plasma and collector optics for collecting along an optical axis the EUV radiation emitted from the plasma are arranged in a vacuum chamber and a slit nozzle for generating a gas curtain is arranged orthogonal to the optical axis between the plasma and collector optics, the above-mentioned object is met in that the slit nozzle has at least one gas outlet part made of high-melting material and additional parts for the gas inlet are produced from material which is easily workable with respect to high-precision mechanics, and the slit nozzle is fastened at least partially in a radiation shadow of the electrode arrangement provided for generating plasma, and a suction device for the gas curtain is arranged opposite to the optical axis of the collector optics, likewise in the radiation shadow. 
         [0032]    The collector optics are advantageously grazingly reflecting reflection optics comprising nested collector mirrors, and a mechanical fin arrangement is arranged between the gas curtain, which is generated by the slit nozzle and suction device, and the collector optics. 
         [0033]    The gas curtain which is generated between the slit nozzle and suction device preferably comprises a buffer gas for decelerating debris particles which achieves a high level of debris suppression in cooperation with the mechanical fin arrangement. The gas curtain can also comprise a gas mixture that is provided for spectral filtering of the EUV radiation generated by the plasma in order to achieve a required spectral purity (elimination of out-of-band radiation) of the emitted EUV radiation, 
         [0034]    Further, in an arrangement for generating EUV radiation based on a laser-generated plasma in which a laser beam for generating the EUV radiation-emitting plasma is directed in a vacuum chamber to targets supplied along a target path, and collector optics for collecting along an optical axis the EUV radiation emitted from the plasma and a slit nozzle for generating a gas curtain are arranged orthogonal to the optical axis between the plasma and collector optics, the above-mentioned object is met in that the slit nozzle has at least one gas outlet part of high-melting material and additional parts for the gas inlet are produced from materials which are easily workable with respect to high-precision mechanics, and the slit nozzle is arranged outside a focusable radiation cone, and a suction device for the gas curtain is likewise arranged outside a focusable radiation cone opposite to the optical axis of the collector optics. 
         [0035]    In this case, a collector mirror that is formed in reflection optics for normal incidence of radiation is advisably used as collector optics and is arranged behind the plasma, an additional mechanical fin arrangement for suppressing debris being arranged between the gas curtain generated by the slit nozzle and suction device and the collector optics. The gas curtain generated between the slit nozzle and the suction device preferably comprises a buffer gas for decelerating debris particles. However, it can also comprise a gas mixture which brings about a spectral filtering of the emitted EUV radiation at the same time. 
         [0036]    An additional slit nozzle for generating another gas curtain downstream of the plasma is advantageously provided in direction of the beam bundle that is focused by the collector mirror, a suction device being arranged opposite to the optical axis. This additional gas curtain generated by the slit nozzle and suction device can advantageously be a buffer gas for decelerating debris particles from the plasma but can also be a gas mixture for spectral filtering of the EUV radiation generated by the plasma. 
         [0037]    The solution according to the invention makes it possible to realize a gas curtain in the immediate vicinity of a radiating plasma, which permits a simple arrangement and design and a long life of the slit nozzle under extreme thermal stresses. Further, the solution is suitable for different modes of generating EUV radiation-emitting plasma and, in addition to suppression of debris, enables a spectral filtering of the generated radiation in the same way. 
         [0038]    The invention will be described more fully in the following with reference to embodiment examples. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    In the drawings: 
           [0040]      FIG. 1  shows the basic construction of a slit nozzle according to the invention in cross section (at left) and in longitudinal section (at right); 
           [0041]      FIG. 2  shows views of the slit nozzle comprising a gas inlet part and a gas outlet part in disassembled state (at left) and in assembled state (at right); 
           [0042]      FIG. 3  shows two further embodiment forms of the distribution pipe of the slit nozzle a) with elongated holes and b) with a continuous slot as gas inlet row; 
           [0043]      FIG. 4  shows an arrangement of a slit nozzle according to the invention in a gas discharge EUV source; and 
           [0044]      FIG. 5  shows an arrangement of slit nozzles according to the invention in a laser-based EUV source. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0045]      FIG. 1  shows the construction of a slit nozzle  1  according to the invention in cross section and in longitudinal section. The slit nozzle  1  for forming a supersonic nozzle with a slit-shaped nozzle profile  11  is divided with respect to the gas outlet direction and comprises at least one gas inlet part  14  and a gas outlet part  15 . The gas outlet part  15  is manufactured from a heat-resistant material (such as tungsten, molybdenum, or the like) in view of the high thermal loading because of its immediate proximity to the plasma. The gas inlet part  14  of the slit nozzle  1  is made of a material which is not quite as resistant to heat but which can be machined more easily (e.g., stainless steel). Both parts  14  and  15  must be adapted to one another in such a way that the gas inlet part  14  and gas outlet part  15  form a nozzle profile  11  with a joint  16  that is as closed as possible so as not to generate a tear-off edge for the gas flow. The slit nozzle  1  has an entrance slit  12  for the gas to be streamed out, adjoined by a nozzle profile  11  which has a parabolic cross section and which is symmetric to a center plane  13  of the nozzle (section plane B-B). The center plane  13  of the nozzle defined by the nozzle profile  11  and the entrance slit  12  is also the (center) plane of the gas curtain  18  to be generated. The nozzle profile  11  in each plane orthogonal to the center plane  13  of the nozzle has the same cross section shown on the left-hand side of  FIG. 1  (section plane A-A). A cylindrical recess in which a gas distribution pipe  2  is inserted for introducing the buffer gas is arranged in front of the nozzle profile  11  in the gas inlet part  14  of the slit nozzle  1  along the entrance slit  12 . 
         [0046]    The gas distribution pipe  2  has a closed end face  27  at one side and adjoins a gas feed  28  on the opposite side. The gas distribution pipe  2  has a gas inlet row  22  along a surface line  21 . This gas inlet row  22  is oriented centrally to the entrance slit  12  of the slit nozzle  1  and can comprise a plurality of small circular holes  23  (see  FIG. 1 ) or elongated holes  24  ( FIG. 3   a ) or a continuous slot  25  ( FIG. 3   b ). 
         [0047]    The dimensioning of the holes  23  according to  FIG. 1  may range from several tens to several hundreds of micrometers. The width of an elongated hole  24  or of a slot is on the order of 30 μm to 300 μm. The spacing between the circular holes  23  or elongated holes  24  may reasonably range from 1 to 5 mm. A length between 1 mm and 10 mm is preferably selected for elongated holes. 
         [0048]    The entrance slit  12  of the nozzle profile  11  in the slit nozzle  1  is two- to ten-times wider than the dimensioning of the gas inlet row  22  of the distribution pipe  2 . This ensures a simple alignment of the gas distribution pipe  2  so that the gas inlet row  22  can easily be centered in the entrance slit  12  of the slit nozzle  1 . If not positioned in the center, the stream could tear off at the edges of the nozzle profile  11  or generate turbulence resulting in the destruction of the gas curtain  18 . The gas distribution pipe  2  is stopped by a screw  26  after the centric, symmetric alignment of the gas inlet row  22  relative to the nozzle profile  11  of the slit nozzle  1 . Besides the simple alignment, this also allows the gas distribution pipe  2  to be changed quickly and simply. Other fastening techniques include clamping, soldering or welding are also possible, of course. The gas feed  28  for introducing gas into the gas distribution pipe  2  is preferably arranged at an end face  27  of the distribution pipe  2 . However, it could also be arranged, e.g., centrally, at the outer surface of the distribution pipe  2 . 
         [0049]      FIG. 2  shows a perspective view of the slit nozzle  1 . In the view on the left-hand side, the slit nozzle  1  is shown disassembled into the gas inlet part  14  and the gas outlet part  15  and is shown assembled in the view on the right. 
         [0050]    In the present example, the gas inlet part  14  is made of stainless steel and the gas outlet part  15  is made of molybdenum. The gas inlet part  14  and gas outlet part  15  are held together by pins  17 . However, other possible fastenings are also possible, e.g., frictionally engaging connections by means of metal sleeves or clamping brackets (e.g., of stainless steel) or material-bond connections by soldering or welding. 
         [0051]      FIG. 4  shows the use of the slit nozzle  1  according to the invention in a gas discharge EUV source  3 . A plasma  5  which emits primarily EUV radiation  51  is generated within an electrode system  31  by means of a gas discharge in a suitable work gas (e.g., xenon). All of the radiation-generating components are located in a vacuum chamber  4  that is maintained at a low pressure (several 10 Pa) by a vacuum pump system  41  to ensure suitable conditions for the generation of a plasma  5  and a low extinction of the EUV radiation  51  emitted from the plasma  5 . 
         [0052]    The geometry of the electrode system  31  limits the solid angle of the emitted EUV radiation  51  to an exit cone  32 . Accordingly, the debris to be intercepted in the form of high-energy particles which are emitted from the plasma  5  on one hand and jump off of the hot surfaces of the electrode system  31  on the other hand is limited to this exit cone  32 . A radiation shadow  33  is located in the vacuum chamber  4  outside the exit cone  32  of the EUV radiation  51 . 
         [0053]    The slit nozzle  1  is arranged in the vacuum chamber  4  in such a way that it remains in the radiation shadow  33 , i.e., outside the exit cone  32 , at least with the gas inlet part  14 . The gas outlet part  15  is resistant to heat because of the choice of material and can therefore be located (partially) in the exit cone  32 . Of course, geometry permitting, the slit nozzle  1  can also be mounted entirely in the radiation shadow  33 . 
         [0054]    An inert buffer gas exiting from the slit nozzle  1  generates a flat gas curtain  18  due to its supersonic speed. A suction device  42  is arranged opposite to the latter so as to interfere as little as possible with the high vacuum of the vacuum chamber  4 . The gas curtain  18  is characterized by a local pressure increase and by a uniform flow direction of all of the buffer gas particles. 
         [0055]    Due to the pressure increase within the gas curtain  18  relative to the high vacuum of the vacuum chamber  4 , atoms and ions (debris) emitted by the plasma  5  undergo a large number of collisions with buffer gas atoms, which leads to their deceleration (energy release) and accordingly to a reduction in their destructive potential. Larger particles receive additional impulses in the flow direction of the gas curtain  18  due to the many collisions with the directed buffer gas particles. Depending on the size and energy of the particles, they are deflected at least far enough to adhere to a fin arrangement  6  located downstream. The fins  61  and the intermediate spaces  62  therebetween are oriented radial to the plasma  5  or to an optical axis  71  of the collector optics  7 . The collector optics  7 , which comprise a plurality of nested collector mirrors  72  in this construction, images the EUV radiation  51  emitted in the exit cone  32  in an intermediate focus  73  (shown only in  FIG. 5 ) which, as an intersection of the converging lines of the focused beam bundle  74 , lies in the extension of the optical axis  71  outside of  FIG. 4 . 
         [0056]    The particles emitted by the plasma  5  and electrode arrangement  31  can traverse the fin arrangement  6  only in a straight line in radial directions within the exit cone  32  of the EUV radiation  51 . A change in direction of the debris particles within the gas curtain  18  leads to an altered trajectory on which they cannot pass the fin structure  6  because they come into contact with one of the fins  61 . 
         [0057]    Together with the radial fin structure  6 , the gas curtain  18  presents an almost impassable obstacle for individual atoms and ions as well as for larger particle clusters and is therefore an efficient method for extensive elimination of harmful debris. 
         [0058]    An advisable arrangement of the described slit nozzle  1  in an EUV source  8  based on laser-generated plasma  5  is shown schematically in  FIG. 5 . Without limiting generality, the target flow required for the generation of plasma was selected as individual droplets along a target path  81 . The target path  81  is shown parallel to the flow direction of the gas curtain  18  for purposes of a clearer depiction, but it would also be reasonable for the direction of the target path  81  to diverge from the flow direction of the gas curtain  18  because the incident direction of the laser beam  82  extends as orthogonal as possible to the target path  81 . 
         [0059]    To protect additional imaging optics (not shown) from direct EUV radiation  51  (that is, EUV radiation  51  not. focused by the collector mirror  71 ), another gas curtain  19  is arranged parallel to the first gas curtain  18  and is generated by a slit nozzle  1  of identical construction and is intercepted by a shared suction device  42 . 
         [0060]    The pulsed laser beam  82  provided for generating plasma  5  was shown in perspective in the foreground, although the arrangement of the rest of the components of the laser plasma EUV source  8  is strictly a side view (sectional view along the optical axis  71  of the collector mirror  72 ). This view was chosen in order to make it clear that the laser beam  82 —like the target path  81 —which is directed to the optical axis  71  (in the focus) of the collector mirror  72  substantially parallel to the two gas curtains  18  and  19  encloses, together with the target path  81 , an angle diverging from 180°. 
         [0061]    Since the solid angle in which the plasma  5  emits EUV radiation  51  is not limited by an electrode system  32  (as in  FIG. 3 ), there is, a priori, also no suitable radiation shadow  33  available for protective positioning of the slit nozzle  1 . However, a shadow area can be generated artificially by means of additional diaphragms (not shown) so that the slit nozzle  1  is arranged in a shielded manner at least outside the focusable radiation cone  83  and outside the beam bundle  84  that is focused in the intermediate focus  73 . 
         [0062]    Additional debris filters, e.g., a fin arrangement  6  (as is shown in  FIG. 4 ), can also be used in this type of laser-generated EUV source  8  to prevent debris from reaching collector optics and other imaging optics. 
         [0063]    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  nozzle 
           11  nozzle profile 
           12  entrance slit 
           13  center plane of the nozzle 
           14  gas inlet part 
           15  gas outlet part 
           16  joint 
           17  pin 
           18  gas curtain 
           19  additional gas curtain 
           2  distribution pipe 
           21  surface line 
           22  gas inlet row 
           23  circular holes 
           24  elongated holes 
           25  (continuous) slot 
           26  screw 
           27  end face 
           28  gas feed 
           3  gas discharge EUV source 
           31  electrode system 
           32  exit cone 
           33  radiation shadow 
           4  vacuum chamber 
           41  vacuum pump 
           42  suction device 
           5  plasma 
           51  emitted EUV radiation 
           6  fin arrangement 
           61  fins 
           62  intermediate spaces 
           7  collector optics 
           71  optical axis 
           72  collector mirror 
           73  intermediate focus 
           74  focused beam bundle 
           8  laser radiation EUV source 
           81  target path 
           82  laser beam 
           83  focusable radiation cone