Abstract:
The invention is directed to an arrangement for generating EUV radiation based on a gas discharge plasma with high radiation emission in the range between 12 nm and 14 nm. It is the object of the invention to find a novel possibility for plasma-based radiation generation with high radiation output in the EUV spectral region (between 12 nm and 14 nm) which makes it possible to use tin as a work medium in EUV gas discharge sources for industrial applications. This object is met, according to the invention, in that a gas preparation unit is provided for defined control of the temperature and pressure of a tin-containing work medium and the flow thereof into the vacuum chamber in gaseous state. At least one thermally insulated reservoir vessel and a thermally insulated supply line are provided for transferring the gaseous tin-containing work medium from the gas preparation unit to the pre-ionization unit located inside the electrode housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of German Application No. 10 2005 041 567.9, filed Aug. 30, 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 an arrangement for the generation of EUV radiation based on a gas discharge plasma with a high radiation emission in the range between 12 nm and 14 nm. It is applied in industrial semiconductor fabrication and is conceived in particular for the process of EUV lithography under production conditions.  
         [0004]     b) Description of the Related Art and Problems Addressed by the Invention  
         [0005]     The generation of radiation from a gas discharge plasma has established itself in the field of plasma-based EUV radiation sources as a promising technology for excitation. Essentially, the following gas discharge concepts are known: Z-pinch arrangements with pre-ionization (e.g., U.S. Pat. No. 6,414,438 B1), plasma focus arrangements (e.g., WO 03/087867 A2), hollow-cathode discharge arrangements e.g., U.S. Pat. No. 6,389,106 B1), star pinch discharge arrangements (e.g., U.S. Pat. No. 6,728,337 B1), and capillary discharge arrangements (e.g., U.S. Pat. No. 6,232,613 B1).  
         [0006]     Further, there are variations of the above-named discharge types (e.g., hypocycloidal pinch discharge) and arrangements that combine elements of these different discharge types.  
         [0007]     In all of these arrangements, a pulsed high-power discharge of &gt;10 kA is ignited in a work gas of determined density, and a very hot (kT &gt;30 eV), dense plasma is generated locally as a result of the magnetic forces and dissipated power in the ionized work gas.  
         [0008]     Radiation sources must currently also satisfy the following specific requirements for use in semiconductor lithography under production conditions:  
                                                       1. wavelength   13.5 nm ± 1%           2. radiation output in the intermediate focus   115 W           3. repetition frequency   7-10 kHz           4. Dose stability (averaged over 50 pulses)   0.3%           5. life of the collector optics   6 months           6. life of the electrode system   6 months.                      
 
         [0009]     For sometimes different reasons, only certain aspects of these requirements are satisfied by the arrangements mentioned above. Above all, the radiation output, its stability, and the lifetime of the electrode system are generally insufficient.  
         [0010]     It has been shown especially that the required radiation outputs can only be achieved through an efficient emitter substance. Such substances which emit radiation in the desired spectral range between 13 nm and 14 nm in a particularly intensive manner are xenon, lithium, and tin.  
         [0011]     However, as described e.g. in WO 03/087867 A2, the latter two materials are difficult to manage in plasma generation because they are solid under normal conditions and, in addition, exhibit substantial debris emission. Further, the disadvantages of a successful handling of lithium and tin consist in the following difficulties: 
        in solid targets, discharge instabilities due to the formation of craters at the cathode;     formation of deposits at the electrodes (leads to a short-circuiting of the electrode system after prolonged operation);     with laser evaporation, poor dose distribution of the (preferably liquefied) target;     with gaseous targets, requirement for a high-power furnace for generating the necessary vapor pressure (with pure tin: temperatures T&gt;1000° C.).        
 
       OBJECT AND SUMMARY OF THE INVENTION  
       [0016]     It is the primary object of the invention to find a novel possibility for plasma-based radiation generation with high radiation output in the EUV spectral region (in particular between 12 nm and 14 nm) which makes it possible to use tin as a work medium in EUV gas discharge sources for industrial application.  
         [0017]     According to the invention, in an arrangement for the generation of EUV radiation based on a gas discharge plasma with high radiation emission in the range between 12 nm and 14 nm with two coaxial electrode housings enclosing a vacuum chamber, a first of which electrode housings is provided as a discharge chamber for the gas discharge for plasma generation and a second electrode housing having a pre-ionization arrangement for the generation of an initial ionization of a work gas that flows into the vacuum chamber, wherein a narrowed electrode collar of the second electrode housing projects into the first electrode housing, the above-stated object is met in that a gas preparation unit is provided for defined control of the temperature and pressure of a tin-containing work medium and the flow thereof into the vacuum chamber in gaseous state, wherein at least one thermally insulated reservoir vessel and a thermally insulated supply line are provided for transferring the gaseous tin-containing work medium from the gas preparation unit to the pre-ionization unit located inside the electrode housing.  
         [0018]     In a first variant, the gas preparation unit advantageously has a thermal vessel for cooled holding of a liquefied work medium with a tin compound that is gaseous under normal conditions.  
         [0019]     The gaseous tin compound that is used is preferably stannane (SnH 4 ). In this case, the thermal vessel is cooled to an internal temperature below −52.5° C., preferably to −100° C.  
         [0020]     For continuous supply of the EUV-emitting gaseous tin compound, a reactor is advisably employed for producing the tin compound, this reactor being connected to the cooled thermal vessel which serves to liquefy the gaseous tin compound and acts as a buffer storage.  
         [0021]     The gas preparation unit advantageously has, in addition, an inert-gas reservoir for mixing in an inert gas serving as an initiator for a homogeneous gas discharge of the gaseous tin compound. The inert-gas reservoir advisably contains at least one noble gas or nitrogen in order to generate a gas mixture of gaseous tin compound and inert gas.  
         [0022]     At least one mass flow control unit (mass flow controller) is preferably arranged in front of the gas inlet into the electrode housing for controlling the supplied quantity ratios of the gas mixture of gaseous tin compound and inert gas. The thermally insulated supply line for the gaseous work medium is advisably connected to the second electrode housing by a gas inlet.  
         [0023]     In order to minimize the debris emitted from the discharge chamber in direction of the first collector optics, it is also advantageous when the thermally insulated supply line for the gaseous work medium is connected to the first electrode housing via an annular gas inlet.  
         [0024]     In a second variant, the gas preparation unit advantageously has a thermal vessel in the form of a thermally insulated furnace which is preferably provided for evaporating a liquid tin compound. In another construction, the furnace is used for storing in liquid state a tin compound that is solid under normal conditions and for evaporating this tin compound.  
         [0025]     The furnace is advisably electrically heatable and has a thermostat for adjusting an evaporation temperature (adapted to the vacuum condition of the discharge chamber) of the utilized tin compound for a temperature range between 247° C. and 1400° C.  
         [0026]     The furnace for the evaporation of the work medium is advisably arranged in the immediate vicinity of the second electrode housing and the gas inlet is connected directly to the pre-ionization unit. The gas inlet of the pre-ionization unit is preferably designed in such a way that the evaporated tin-containing work gas is introduced into the pre-ionization chamber of the second electrode housing between an insulator tube enclosing the pre-ionization electrode and an outer insulator tube of the pre-ionization unit. In order to prevent condensation of the tin-containing work gas, a heat-conducting layer, preferably made of copper, is advisably arranged in the gas inlet at least in the initial area of the outer insulator tube. In addition, a heat-conducting layer can also be arranged in the gas inlet on the insulator tube.  
         [0027]     A tin compound suitable for the above-mentioned arrangement of the gas preparation unit is stannous chloride (SnCl 2 ). The furnace can advantageously be heated to a temperature between 247° C. and 623° C. to inject evaporated SnCl 2  into the vacuum chamber.  
         [0028]     The basic idea of the invention stems from the consideration that tin, by reason of its intensive spectral lines between 12 nm and 14 nm, is best suited for substantially increasing the yield of EUV radiation. On the other hand, there is reluctance to use tin primarily because elementary tin, as a target in solid form, does not permit stable plasma generation (due to crater formation), liquid tin requires a continuous high-temperature bath to generate a sufficient vapor pressure, and laser evaporation from the liquid phase is also very demanding with respect to technology.  
         [0029]     The invention overcomes these disadvantages in that the tin compounds, which can be changed to gaseous phase in a simple manner, are held in a temperature-managed, insulated manner prior to the pre-ionization of the work medium.  
         [0030]     The arrangements according to the invention make it possible to achieve a plasma-based generation of radiation based on a gas discharge with high radiation output in the EUV spectral range (between 12 nm and 14 nm) which permits the use of tin as a work medium in gas discharge sources for semiconductor lithography.  
         [0031]     The invention will be described more fully in the following with reference to embodiment examples. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     In the drawings:  
         [0033]      FIG. 1  shows a gas discharge source with a gas preparation unit for tin-containing work gas with a gas inlet on the cathode side and cooled electrode housings;  
         [0034]      FIG. 2  shows a construction of the gas discharge source according to the invention for tin-containing work gas with a gas inlet on the cathode side, “porous metal” cooling, and vacuum insulation between the electrode housings;  
         [0035]      FIG. 3  shows another construction of the gas discharge source according to the invention for tin-containing work gas with a gas inlet on the anode side, “porous metal” cooling, and ceramic insulation of the electrodes;  
         [0036]      FIG. 4  shows a constructional variant of the invention with a gas preparation unit for liquid or liquefied tin-containing substances, particularly stannane (SnH 4 ); and  
         [0037]      FIG. 5  shows another construction of the gas discharge source according to the invention with a gas preparation unit in the form of a cathode-side high-temperature gas inlet for solid tin-containing substances, particularly stannous chloride (SnCl 2 ). 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]      FIG. 1  shows the basic construction of the arrangement according to the invention. Without limiting generality, a Z-pinch gas discharge with pre-ionization is used, and a pulsed gas discharge takes place between the cathode and the anode. As in all of the other figures, the z-axis is identical to the axis of symmetry  6  of the discharge system extending vertically in the drawing plane. This discharge system is formed of a first electrode housing  1  (e.g., anode) and a second electrode housing  2  (e.g., cathode).  
         [0039]     The electrode housings  1  and  2  are shown in  FIG. 1  in a simplified schematic manner with a ribbed cooling arrangement. This type of cooling is usable only conditionally for the high-output EUV gas discharge sources described herein. The electrode housings  1  and  2  have rotationally symmetric cavities in the center, the pre-ionization chamber  71  for the pre-ionization of the work gas is located in the second electrode housing  2 , and the discharge chamber for the main gas discharge is located in the first electrode housing  1 . The two cavities are part of an entire vacuum chamber  4 , since the generation of a plasma  5  emitting the desired EUV radiation  51  is confined to a vacuum in the pressure range of several Pascals (e.g., 5 to 30 Pa).  
         [0040]     Since, in most cases the first electrode housing  1  for the main discharge and generation of the plasma  5  is connected as anode and the second electrode housing  2  for the pre-ionization is connected as cathode, the terms anode  1  and cathode  2  will be used for the sake of brevity in the following description without limiting generality.  
         [0041]     In  FIG. 1 , the work gas required for the gas discharge is injected into the pre-ionization chamber  71  of the vacuum chamber  4  through a gas inlet  82  in the cathode  2 . The vacuum chamber  4  is almost enclosed by the cathode  2  and has a narrowed outlet  21  into the interior of the anode  1 . The narrowed outlet  21  is formed by an electrode collar  22  which is shielded from the cylindrical inner wall of the anode  1  by a tubular insulator  13  so that the gas discharge can take place between the electrode collar  22  of the cathode  2  and an electrode collar  12  of the anode  1 , which electrode collar  12  is directed inward at the conical outlet  11 . Due to the strong magnetic forces, the pre-plasma generated during the gas discharge contracts in the axis of symmetry  6  to form a dense, hot plasma  5  (Z-pinch).  
         [0042]     A pre-ionization unit  7 , preferably for a sliding discharge  75 , is constructed in the cathode  2  to ionize the work gas that flows through a gas inlet  82 . The sliding discharge  75  takes place over the end area of an insulator tube  73  which encloses the pre-ionization electrode  72 . The pre-ionization electrode  72  on one side and the cathode  2  on the other side communicate with a pre-ionization pulse generator  74  for pulsed generation of the sliding discharge  75 . Further, the cathode  2  is connected to a high-voltage pulse generator  14  which triggers the main gas discharge in cooperation with the anode  1 .  
         [0043]     The supply of the work medium, according to the invention, is effected in that a tin-containing substance in gaseous state is streamed into the pre-ionization chamber  71  under defined pressure via a suitably arranged gas inlet  82 . The tin-containing work gas is made available by a gas preparation unit  8  in that a tin-containing substance in liquid phase is maintained close to the evaporation point in a thermal vessel, and a vapor pressure is accordingly generated through controlled temperature management and pressure regulation resulting in a sufficient flow of tin-containing work gas through the gas inlet  82  into the vacuum chamber  4  via a thermally and electrically insulated supply line  81 .  
         [0044]     The vacuum chamber  4  is maintained at a stationary vacuum level by means of a vacuum pump system  41  in spite of the work medium flowing in. To ensure continuous operation of pulsed plasma generation, the electrode housings  1  and  2  are cooled by means of heat exchanger structures  91  (shown in a simplified manner as ribs) in that the two electrode housings  1  and  2  are integrated in the cooling circuits of a heat removal system  9 .  
         [0045]     The construction according to  FIG. 2  shows an arrangement for an EUV gas discharge source which is modified from  FIG. 1  and in which the configuration of the electrode housings  1  and  2  is modified in such a way that the anode  1  no longer has an almost completely closed inner space but, rather, the vacuum chamber  4  completely encloses the latter and forms a vacuum insulation layer  31  between the anode  1  and cathode  2 . The preparation of gas and the supply of the tin-containing work gas initially remain unchanged, but all of the gas preparation variants described in detail in the following with reference to FIGS.  3  to  5  can be used.  
         [0046]     In this example, the heat removal system  9  is optimized by introducing porous material in the electrode housings  1  and  2  in the cooling circuit as heat exchanger structures  91 , which enables a faster transfer of heat and accordingly appreciably lowers the electrode temperatures in continuous operation.  
         [0047]     In the embodiment example according to  FIG. 3 , the tin-containing work medium for the gas discharge is provided as a gas mixture of tin compound and inert gas. For this purpose, the gas preparation unit  8  contains a thermal vessel  83  with the tin-containing compound and an inert-gas reservoir  86  which generate the suitable gas mixture as work medium by means of controllable valves.  
         [0048]     In the gas mixture, only the tin-containing component (e.g., SnH 4  gas) is the substance actually emitting the EUV radiation, and the inert gas which is mixed in additionally and which can be a noble gas (e.g., He, Ne, Ar) or nitrogen (N 2 ) serves as an initiator for a more homogeneous triggering of the gas discharge.  
         [0049]     The second special feature of this constructional variant consists in that the work medium generated in this way is streamed in through an annular gas inlet  82  at the anode  1  in direction of the cathode  2 , and an additional output to the vacuum pump system  41  is arranged at the back side of the cathode  2  which sucks in the gas mixture that is streamed in at the outlet  11  of the anode  1  in order to feed it into the pre-ionization chamber  71  of the pre-ionization arrangement. This has the advantage that when tin-containing work gases, e.g. SnH 4  or evaporated SnCl 2 , are used according to the invention, they are not blown in direction of the collector optics and therefore cannot lead to deposits.  
         [0050]     In the arrangement shown in  FIG. 4 , SnH 4  gas is used as work medium, and the gas preparation unit  8  is outfitted in the following manner for this purpose. The thermal vessel  83  described above is operated as a cooling vessel and is maintained at a suitable temperature (approximately −95° C. for SnH 4 ) to achieve the necessary vapor pressure over the liquefied SnH 4 . As is indicated in dashed lines as an option, the production of SnH 4  gas can be carried out continuously in a reactor  85  by methods known per se in order to ensure a continuous supply of SnH 4  gas. The cooled thermal vessel  83  is used for liquefaction and as a suitably temperature-controlled reservoir for maintaining the necessary vapor pressure for the tin-containing work gas component. An inert gas, preferably argon (or neon or nitrogen) is again mixed in as a second component of the work medium from an inert-gas reservoir  86 .  
         [0051]     The correct proportion of work gas components is adjusted by means of thermally insulated or suitably thermostatic lines  81  and mass flow controllers  84 . The mass flow controllers  84  are particularly advantageous when—as is shown in  FIG. 4 —gas recovery from the vacuum pump system  41  is carried out and gas is also fed in at the same time.  
         [0052]      FIG. 5  shows another embodiment example of the invention in which SnCl 2  is used as work medium. SnCl 2  is a crystalline white powder under standard conditions. This is deposited in the interior of a furnace  87  near the pre-ionization unit  7 . Due to the fact that sufficiently high vapor pressures of about 133 Pa do not occur, depending upon material, until defined high temperatures are reached, the furnace  87  must be heatable up to such temperatures and adequately thermally insulated on the outside. A temperature of about 623° C. is sufficient for SnCl 2  and a temperature of approximately 114° C. is sufficient for SnCl 4 , while a temperature of about 1400° C. is needed for metallic tin.  
         [0053]     The SnCl 2  vapor is introduced into the pre-ionization chamber  71  in the cathode  2  through an annular gas inlet  82  between the insulator tube  73  of the pre-ionization electrode  72  and an external insulator tube  76 . The outer insulator tube  76  is covered by a heat conduction layer  88  in the top part of its inner wall so that the vapor does not condense already before entering the pre-ionization chamber  71  of the cathode  2 . This heat conduction layer  88  is a copper layer, for example, which is vacuum-deposited on the outer insulator tube  76 . A heat conduction layer  88  of this kind can also be applied to the outer side of the inner insulator tube  73  to further reduce the cooling effect.  
         [0054]     All of the other elements in this construction of the invention are arranged in the same manner as in the preceding example and correspond to the basic functions described with reference to  FIG. 1 .  
         [0055]     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  first electrode housing  
           11  outlet opening  
           12  (first) electrode collar  
           13  tubular insulator  
           14  high-voltage pulse generator  
           2  second electrode housing  
           21  narrowed outlet  
           22  (second) electrode collar  
           3  electrically insulating layer  
           31  vacuum insulation gap  
           4  vacuum chamber  
           41  vacuum pump system  
           5  plasma  
           51  emitted radiation  
           6  axis of symmetry  
           7  pre-ionization unit  
           71  pre-ionization chamber  
           72  pre-ionization electrode  
           73  insulator tube  
           74  pre-ionization pulse generator  
           75  sliding discharge  
           76  outer insulator tube  
           8  gas preparation unit  
           81  thermally insulated supply lines  
           82  gas inlet  
           83  thermal vessel  
           84  mass flow controller  
           85  gas reactor  
           86  inert-gas reservoir  
           87  furnace  
           88  metal coating  
           9  heat removal system  
           91  heat exchanger structure (ribs)  
           92  porous material